Human Immunodeficiency Virus & Acquired Immune Deficiency Syndrome (AIDS) Part
I: Nuts and Bolts
The uniqueness of AIDS
AIDS is a new disease to our species and in our society
AIDS in humans originated from the mutation(s) of a virus common to another species
AIDS changed the tropisms of several pathogens common to human
AIDS allowed the reemergence in human of pathogens that were nearly eradicated
AIDS continues to demonstrate to role of cellular community in the evolution of several
human disease
Acquired Immune Deficiency Syndrome (AIDS) is a severe depression of cellular
immunity caused by the pathogenic human retrovirus human immunodeficiency virus
(HIV)
Human immunodeficiency virus is a pathogenic human retrovirus
Retrovirus
From reverse transcription, characteristic of these viruses
Virion:
Enveloped
Icosahedral or conical capsid
Enveloped acquired at plasma membrane
Contains reverse transcriptase
Diameter 100nm
Genome:
Linear single -stranded RNA
7 -10 kilobases
Encodes for 9 structural and regulatory proteins
Distinctive Characteristic:
Two identical copies of the RNA genome per virion
Genome RNA converted to DNA by reverse transcriptase
A DNA copy of the genome (provirus) is integrated into host cell DNA
Historical Perspective
Among retroviruses, human immunodeficiency virus is classified as a lentivirus
HIV Type
HIV type 1(HIV -1)
Prevalent throughout the world
Slow but progressive deterioration of the immune system (5 -8 years)
HIV type 2
Found primarily in West Africa
Slow but progressive deterioration of the immune system (11 -19 years)
HIV -1 Subtypes (Clades)
A subtype nomenclature for classification of HIV -1 clinical isolates (individual
HIV -1 strains)
Determined by genetic diversity
Subtypes cluster in specific geographical areas
The major (M) group contain 9 subtypes (A-D, F-H, J, K)
The B subtype is predominant in United States and Europe
Outlier (O) group, non-M, non-O (N) group, and P group also recognized
The existence of subtypes throughout the world must be considered as we attempt to
develop safe and effective AIDs vaccines
Origin of the AIDS Virus
Sequence homology studies provide evidence that the AIDS virus originated
through mutation of simian immunodeficiency virus (SIV)
SIV of chimpanzee  HIV -1
SIV of sooty mangebey monkey HIV -2
HIV -1 thought to have appeared in humans between 1884 and 1924, more
focused estimate at 1908
Fragment of HIV -1 genome recovered from blood sample collected from a
man in Africa in 1959 (Kinshasa, the Congo)
Fragments of HIV -1 genome recovered from blood sample collected from
a woman in Africa in 1960 (Kinshasa, the Congo)
Replication of retroviruses (including HIV -1)
What is the fine structure of the retrovirus genome?
Retrovirus Genome (other than lentiviruses)
Gag (group -specific antigen)
Capsid protein
Matrix protein
Nucleocapsid protein
Pol (DNA polymerase)
Reverse transcriptase
Protease
Integrase
Env (envelope proteins)
Gps120
Gps41
Gp160 (precursor)
Lentivirus Genome
Gag (group -specific antigen)
Pol (DNA polymerase)
Env (envelope proteins)
Several regulatory genes…
Vif
Vpu
Tat
Rev
Nef
Vpr (structural regulatory protein)
What is the replication scheme for HIV -1?
Receptors & Tropisms
T -cell -tropic strains of HIV -1
CD4 molecule
CXCR4 chemokine receptor
Macrophage -tropic strains of HIV -1
CD4 molecule
CCR5 chemokine receptor
Reverse Transcriptase
RNA -dependent DNA polymerase activity of reverse transcriptase copies the entire
single -stranded RNA genome to make RNA -DNA molecule
RNase H activity of reverse transcriptase removes the template genome RNA probably
as genome RNA is copies into single -stranded complementary DNA strand
DNA -dependent DNA polymerase activity of reverse transcriptase uses single -stranded
DNA as a template to produce a linear double -stranded DNA copy (proviral DNA) of
original single -stranded RNA
Reverse transcriptase functions as three enzymes
RNA -dependent DNA polymerase
Uses single -stranded RNA genome as template to make a single -stranded DNA
copy of genome
RNAse H activity
Selectively dissolves the RNA part of the RNA -DNA hybrid
DNA -dependent DNA polymerase
Uses single -stranded DNA copy of genome to make a double -stranded DNA
copy of genome (provirus or proviral DNA)
Proviral DNA Integration
Integrase binds to two ends of the linear proviral DNA, brings them together and in
close proximity to cellular DNA
Integrase removes tow 3’ terminal nucleotides of each strand of the proviral DNA
Proviral DNA is inserted into host DNA by concerted cleavage and ligation reaction
Host enzymes carry out repair synthesis of gap
Regulatory Proteins of HIV
Tat (Transactivator of transcription)
Vastly increase the level of transcription of HIV provirus (dsDNA)
Released by infected cells in culture and found free I the blood of HIV -infected
persons
Can be absorbed by uninfected cells and act directly as a toxin to produce cell
death
Nef (Negative effector)
Markedly elevates virus replication
Induces changes in protein cell surface expression and signal transduction
Decreases levels of MHC Class I expression of host cell surface
Protects infected cell from apoptosis
Maturation
Full -length transcript from proviral DNA can severe as genome RNA
Env protein (gp160) travels to lumen of the ER for glycosylation where it is cleaved in
gp120 and gp41 by protease, and ultimately trans -located to the plasma membrane
where they are inserted
Gag and pol travel to the inner surface of plasma membrane where they bind to newly
synthesized genome RNA
Assembly around genome RNA causes curvature of plasma membrane as the virion
grows
A roughly spherical particle is formed as the envelopment process proceeds
When the enveloped particles are release, the protease cleaves the gag and pol
polyproteins to yield individual structural and enzymatic proteins
Structural proteins rearrange to form the viral core of mature virions that are now
infectious
Epidemiology and Transmission of HIV -1
Transmission of HIV -1 occurs through direct contact with infected body fluids (Not
transmitted via aerosols)
Body fluids infected with HIV -1…
Blood and blood products
Semen and cervicovaginal secretions
Amniotic fluid
Saliva
Tears
Urine
Cerebrospinal fluid
HIV -1 not detected in sweat
HIV -1 is not transmitted by casual contact
Handshaking
Hugging
Kissing on the cheek
Deep kissing
Sharing of household items
Toilet seats, door knob, hot tubs
Routes of HIV -1 Transmission
Sexual route
Blood route
Mother to child
Target cells, receptors and tropisms
Target Cells and Receptor
T -lymphocytes (T -cells) and macrophages are primary targets for HIV -1 infection
CD4 molecule is expressed on their surfaces
CD4 molecule serves as primary receptor for HIV -1
CD4 molecule binds to distinct domains on envelope gp120 molecule of HIV -1
Set of co -receptor identified of T -cells and macrophages
Serve as natural receptors for chemokines
Co -receptors bind to distinct domains on envelope gp120 molecule of HIV -1
(CXCR4 for T cells; CCR5 for macrophages)
Determines tropism of HIV -1 strains
Dendritic cells represent an important target of HIV -1 infection during primary
infection
Dendritic cells are macrophage -like antigen -presenting cells found in the blood and
skin/ genital tract (Langerhans cells)
Because of their location in genital tracts, dendritic cells considered to be initial target
for HIV -1 infection during sexual transmission
Dendrites cells express CD4 molecule
Dendritic cells initially express CCR5 co -receptor, but can express CXCR4 co -receptor
upon maturation
HIV -1 Strains and Tropisms
R5 Strains
Use CCR5 chemokine receptor
Tropism for macrophages /(immature) dendritic cells
X4 Strains
Use CXCR4 chemokine receptor
Tropism for CD4+ T cells /(mature) dendritic cells
R5X4 Strains
Use either CCR5 or CXCR4 chemokine receptors
Tropism for macrophages /dendritic cells and CD4+ T cells
All use CD4 molecule as primary receptor
What is the sequence of events whereby HIV -1 infection evolves into AIDS?
Three Stages of HIV -1 Infection
Primary Infection
Asymptomatic infection
Symptomatic disease progression (AIDS)
Primary HIV -1 Infection
HIV -1 (R5 strains) infects dendritic cells and/ or macrophages at the mucosal
epithelium (CCR5 co- receptor)
HIV -1 travels to lymph nodes enters the blood within 2 days of infection to establish a
viremia
Explosive virus replication associated with viremia (107 HIV -1 /ml)
~80% of persons develop flu-like symptoms (fever, headache, lymphadenopathy, night
sweats) associated with skin rash on the trunk that abates within 2 weeks onset, but
persistent lymph-adenopathy and headache (meningitis) can continue of months
Marked reduction of viremia due to HIV -1 specific immunity
HIV -1 infected macrophages persist as reservoir (provirus)
Asymptomatic HIV -1 Infection
Period of variable duration measured in years with few clinical manifestations
Low levels of HIV -1 detected within peripheral blood
Extensive HIV -1 replication within lymph nodes that is associated with progressive
deterioration of immune function (immune dysregulation)
High mutation and antigenic modulation results in the generation of numerous HIV -1
strains
Antigenic Modulation
HIV -1 exhibits high mutation rate
Mutation rate 5x greater than influenza
10,000 to 100,000 mutation per day
High mutation rate generates numerous antigenically distinct strains of HIV -1
Numerous strains may show spectrum of virulence
X4 strains appear that show tropism for CD4+ T cells (Latent HIV -1 infection in resting
CD4+ T cells)
Asymptomatic HIV -1 infection
Virus replication is controlled by cellular immunity via CD8+ T cell responses
Cellular immunity is enhanced by a dominant Th1 CD4+ T cell cytokine profile
Many peripheral blood resting CD4+ T cells harbor virus in a latent state
Asymptomatic period is usually 5 -8 years
Persons remains infectious and can transmit virus
Gradual loss of CD4+ T cells function and number (immune dysfunction and beginning
of immune deficiency)
Symptomatic disease progression (AIDS)
HIV -1 levels increase within peripheral blood with concomitant increase in diversity of
HIV -1 strains
X4 strains of HIV -1 predominate with tropism of CD4+ T cells (CD4+ T helper cells)
Hypergammaglobulinemia appears due to chronic activation of B cells (polyclonal B cell
activation)
Type 1 type cytokine production shifts to Type 2 cytokine production (Th1/Th2 shift)
Th1 and TH2 CD4+ T Cells
CD4+ T helper cell can be categorized into 2 groups according to pattern of cytokines
synthesis
Th1 CD4+ T cells
Synthesize cytokines IL-2, IL-12 and INF-y
Stimulate cellular immunity
Th2 CD4+ T cells
Synthesize cytokines Il-4, IL-6 and IL-10
Stimulate antibody synthesis
Symptomatic Disease Progression (AIDS)
Absolute numbers if CD4+ T cells decline (immune deficiency)
DTH T-cell functions decline and the patient become anergic
Protective CD8+ T-cell function decline
Appearance of numerous opportunistic infections
CD4+ T-cell numbers…
Persons without HIV -1 infection normally have 700 -1000 CD4+ T cells per microliter
(ul) of peripheral blood
HIV -1 -infected persons are considered to have “normal” CD4+ T -cell counts if the
number is > 500 CD4+ T cells per ul of peripheral blood
A person is classified as having AIDS if the CD4+ T -cell count drops to <200 CD4+ T
cells per ul of peripheral blood (CDC classification of AIDS)
Spectrum of Outcomes
Typical progressors
Develop AIDS 8 -10 years after primary infection
Account of 75 -80% of HIV -1 -infected persons
Rapid progressors
Develop AIDS 2 -3 years after primary infection
Account for ~10% of HIV -1 infected persons
Nonprogressors (long -term nonprogressors)
Healthy 15 -30 years after HIV -1 -infection
Account for 10 -17% of HIV -1 -infected person (1:500)
Nonprogressors (long -term survivors)
Virus load in blood is far less than in progressors
Strong immune response
Broadly reactive neutralizing antibodies
Potent antiviral CD8+ T -cell activity
Lymph nodes show normal architecture
HIV -1 isolates replicate poorly (attenuated strains?)
Exhibit mutation in gene(s) for CCR5 (highly resistant to R5 strains)
Clinical Manifestations
HIV -1 infection is a continuum of disease that leads to AIDS
Symptomatic HIV -1 Disease
Cutaneous manifestations
Respiratory complications
Gastrointestinal complication
Hepatitis
Hematologic disorders
Ophthalmologic abnormalities
Neurologic manifestations
AIDS wasting syndrome
Pediatric manifestations
Diagnosis of HIV -1 Infection
Once a pathogenic retrovirus had been shown to be the causative agent of AIDS
And the virus could be transmitted by blood or blood product
It was necessary to develop a simple yet sensitive test to screen the blood supply of HIV
-1 infection and protect the general population from HIV -1 -contaminated blood
Laboratory Diagnosis
Serologic tests (detection of IgG)
ELISA
Western Blot assay
Antigen detection /capture assay (detection of p24 antigen)
Polymerase chain reaction (PCR) assay (Detection of HIV -1 provirus DNA)
Management of HIV -1 Infection (Antiviral Chemotherapy)
Antiretrovirus Drugs
Goals
Slow HIV -1 -induced immunosuppression and thereby affect clinical
manifestations caused by opportunistic infection and malignancies
Affect clinical manifestations caused by direct HIV -1 infection (wasting disease,
retinal disease, HIV -associated dementia)
Antiviral Drugs
The preparation of a safe and effective antiviral drug is not an easy task
Often difficult to prevent virus replication without killing the cell
There is a certain degree of toxicity associated with an antiviral drug
Reverse Transcriptase Inhibitors
Azidothymidine (AZT) [Zidovudine, Retrovir] was first FDA -approved antiretroviral
drug to show benefit for management of HIV -1 infection in AIDS patients (March 1987)
Transiently increased CD4+ T -cell counts
Reduced serum p24 levels
Reduced opportunistic infections
Increased survival time
United States clinical trail results challenged by European clinical trial (Concorde Trail)
Provided proof-of-concept that antiretroviral drugs effective
Azidothymidine Maternal -Infant HIV -1 Transmission
AZT is well -tolerated during pregnancy
No malformations
No premature births
No fetal distress
AZT reduces HIV -1 transmission from 25% to 8% (60 to 70% reduction)
Reverse Transcriptase Inhibitors
Toxicities
Toxic to bone marrow (severe anemia /neutropenia)
Rapid weight loss (anorexia)
Painful peripheral neuropathy and numbness
Pancreatitis
Abnormalities of the liver (hepatitis)
Generalized clinical symptoms of fever, rash, and/or diarrhea
Protease Inhibitors
Prevents cleavage of the virus gag and pol polypeptide precursors to produce mature
structural proteins and enzymes, thereby halting virus replication
Clinical trials have shown therapeutic efficacy for management of HIV -1 infection
Reduces virus load within peripheral blood
Increases CD4+ T -cell counts
Reduced opportunistic infections
Extends survival time
Lipodystrophy, a redistribution of fat from the face and limbs to the gut
Concern that long -term use will lead to coronary disease /stroke due to adverse effect
on lipid metabolism
Combination Antiretroviral Therapy (cART or ART) [Highly Active Antiretroviral
Therapy] (HAART)
Combination therapy using protease inhibitor in combination with two reverse
transcriptase inhibitors (or other Antiretrovirus drugs)
Clinical trials have demonstrated additive / synergistic therapeutic efficacy (HIV -1 infected persons and AIDS patients)
Decrease HIV -1 load within the peripheral blood
Improves immune functions (CD4+ T cells)
Prevents onset of many opportunistic infections and malignancies
A dramatic decrease in AIDS -related mortality and morbidity
Transformed AIDS into a chronic disease HIV -1 infection
Has not reduced new cases of HIV -1 infection
Reasons for Caution
Toxicities
Reverse transcriptase inhibitors cause moderate to severe toxicities (bone marrow
toxicity, peripheral neuropathy, pancreatitis, liver abnormalities
Protease inhibitors cause lipodystrophy
Emergence of drug -resistant HIV -1 strains
Antiretroviral drugs select for drug -resistant variants of HIV-1 through mutation events
cART (ART) -resistant strains of HIV -1 recognized in 5 -15% of HIV -1 infected patients
Decline in immune function
Appearance of opportunistic infection
Progression from HIV -1 infection to AIDS
Treatment failure (breakthrough)
Antiretroviral drugs are virostatic and fail to clear virus from HIV -1 -infected cells and
tissues
Antiretroviral drugs fail to eliminate reservoirs of HIV -1 infection (resting CD4+ T cells)
Antiretroviral drugs fail to affect HIV -1 strains within the central nervous system
(blood-brain barrier)
HIV -1 replication commences, and disease progression continues when drugs
disconnected or when drug doses lowered due to patient noncompliance
Integrase Inhibitors
~12 years were needed to develop clinically usable inhibitors of integrase due to
difficulty in crystallization of molecule
Four integrase inhibitors approved /licensed
May be taken in combination with other HIV -1 drugs to minimize adaptation of the
virus
May offer hope to HIV -1 -infected persons who have developed resistance to HAART/
cART or ART
Truvada
First FDA -approved drug for adults who are not HIV -1 -infected but who are risk for
becoming HIV -infected
A combination of two reverse transcriptase inhibitors
Contained within one tablet that is taken once a day
Can be used for HIV -1 treatment in combination with other HIV -1 drugs
Vaccines
A safe and effective vaccine to protect us against HIV -1 infection has not yet been
developed
Human Immunodeficiency Virus and Acquired Immune Deficiency Syndrome (AIDS)
Part II Neurologic Diseases
Neurologic disease arising during HIV/AIDS originate from two sources:
Opportunistic infections of the brain /spinal cord
Direct HIV infection of the brain /spinal cord
Neurologic Disease Associated with Opportunistic Infections
The severe depression of cellular immunity during AIDS results in increased
susceptibility to opportunistic infections that may invade the central nervous system
and cause neuropathologic changes and neurologic signs and symptoms
The frequency of neuropathologic changes of all types in the brains of HIV -infected
patients as found at time of autopsy ranges from 55% to 95%
Opportunistic infections are the most common complications during AIDS with a
frequency between 25% and 65%
Opportunistic virus infections are responsible for 20% to 33% of neurologic
manifestation during AIDs (11 autopsy series; n=1,736)
The central nervous system during AIDS is affected by two groups of opportunistic
viruses
Papovavirus
Progressive multifocal leukoencephalopathy  JC virus
Herpesviruses
Variable neurologic manifestations  Cytomegalovirus
Herpes simplex virus
(This list excludes AIDS -associated tumors causes by viruses)
Progressive Multifocal Leukoencephalopathy (PML)
Progressive multifocal leukoencephalopathy (PML) is a subacute demyelinating disease
associated with suppression of cellular immunity
First described in 1958 as neuropathologic complication of lympho -proliferative
disorders
Only ~200 cases of PML has been recorded worldwide in over 25 years prior to the
appearance of AIDS
More than 30 cases of AIDS -related PML were reported on only a 4 -month period
during appearance of AIDS in 1982
Progressive multifocal leukoencephalopathy (PML) is causes by the JC virus
The JC Virus
1965 -Electron microscopic studies showed virus particles in brains of patients dying
from PML
1971 -Virus cultured and designated JC virus (initials of patients)
JC virus is a SV40 -like virus classified I the family polyomaviridae
JC virus is ubiquitous worldwide and acquired during childhood
~70% of adults are seropositive worldwide
40 -60% of children seropositive by age 10
JC virus resides in extraneutral sites (kidneys, spleen, bone marrow)
~5 -10% of population excretes JC virus in urine
Remain latent /persistent in kidney and possible the brain
BK virus (another SV40 -like virus classified in the family Polyoma-viridae) is found in
renal tissue, but is not a cause of PML
Polyomaviridae
From Greek poly (many) and -oma (tumor)
Virion:
Naked capsid with icosahedral symmetry
Diameter 45nm
Genome:
Circular Double -Stranded DNA (5.3 kilobases)
DNA packaged as “minichromosome” (cellular histones)
Encodes for two transcription units (early and late)
Early proteins (T antigens) regulate cell cycle and direct DNA replication
The Polyomaviruses
Polyomavirus  mouse
Simian virus 40 (SV40)  Rhesus monkey
BK virus  Human
JC virus  Human
PML Clinical Manifestations
Insidious onset of progressive neurologic deficits that are usually focal
Substantial /rapidly advancing cognitive changes
Memory loss
Personality /behavioral changes
Global dementia is rare
Headache are infrequent and patients remain afebrile
Rapid disease progression often leads to cortical blindness and paralysis (hemiparesis)
Death occurs within 1 years (mean of 4 months, but as early as 1.5 months)
PML Clinical and Histopathologic Findings
Multiple lesions develop in the white matter of the cerebrum and at times in brainstem
or cerebellum
Cerebrospinal fluid remains normal with no pleocytosis, no elevation of protein content,
no increase in immunoglobulins
Histopathologic findings include.
Inclusion bodies in large distorted nuclei of oligodendrocytes surrounding a focus
of demyelination
Bizarre changes in astrocytes within areas of demyelination
PML Pathogenesis
Mechanism (s) by which JC virus enters brain may involve…
Circulating JC -virus -infected macrophages
Direct JC virus infection of astrocytes which in turn infects oligodendrocytes
JC virus does not infect neurons, and their axons are intact throughout the
demyelinating process
JC virus persistently infects oligodendrocytes that leads to demyelination in the cerebral
white matter
JC virus is present in high amount (>1010 particles /gm brain)
PML Diagnosis
Brain CT and brain MRI often show multiple white matter lesion most commonly
located in the parieto -occipital regions (MRI >CT)
PML confirmed by histopathologic examination of brain tissue obtained at time of
biopsy (or autopsy)
JC virus DNA detected by PCR assay in samples of cerebrospinal fluid, but negative is
~30% of HIV -infected persons
PML Treatment and Prevention
Cytosine arabinoside (Ara -C) given intravenously or intrathecally failed to affect clinical
progression or confer survival benefit in clinical trails
Cidofovir failed to affect the clinical course in clinical trails
Combination Antiretroviral Therapy (cART) (combination reverse transcriptase
inhibitors + protease inhibitor) for management of HIV infection and therefore
management of retrovirus -induced immunosuppression has extended PML survival
time to more than 2 years
Cytomegalovirus Encephalopathy
Cytomegalovirus
Cytomeglovirus (CMV) is a ubiquitous B-herpesvirus that infects ~80% of adults
worldwide
Primary infection occurs via infected saliva (respiratory route) that produces a viremia
Primary infection is usually asymptomatic
Virus shed in saliva and urine
CMV -specific immune responses clear active virus infection
Virus establishes a life -long latent infection in circulating monocyte /macrophages and
bone marrow cells
Circulating monocytes /macrophages that harbor latent virus serve as vehicles to
transport virus to tissue throughout the body
Spectrum of Cytomegalovirus Disease
Asymptomatic infection
Infectious mononucleosis (rare and distinct from Epstein -Barr virus -induced infectious
mononucleosis)
Cytomegalic inclusion disease (congenital disease)
A number of diseases in immunosuppressed patients populations including AIDS
patients
Spectrum of Cytomegalovirus Disease During AIDS
Pneumonitis
Hepatitis
Colitis
Retinitis
Neurologic disease
AIDS appears to alter the tropism of CMV making it more neurotropic
Cytomegalovirus neurologic disease during AIDS present when <50 CD4+ T cells per ul
of peripheral blood
Cytomegalovirus Neurologic Diseases During AIDS
Cytomegalovirus invades the brain by two possible routes
Disseminates from the blood via infected monocytes /macrophages
Disseminates through the cerebrospinal fluid
No specific cell type of the central nervous system in susceptible selectively to
cytomegalovirus infection because virus -specific molecules have been detected in
Neurons, astrocytes, Oligodendrocytes, ependymal cells endothelial cells, cells of
the leptomeninges
18% to 33% of AIDS patients have evidence of cytomegalovirus infection of the central
nervous at the time of autopsy
Two distinct patterns of cytomegalovirus encephalopathy
Subacute and slowly progressive panencephalopathy that leads to death in 1 to 2 years
Ependymal cell infection that leads to increase in ventricular size and rapid progression
to death in week in weeks to months
Herpes Simplex Virus Neurologic Diseases
Herpes simplex virus type 1 and 2 causes atypical patterns of neurologic disease during
AIDS not seen in immunologically normal adults
Atypical Pattern of Herpes Simplex Encephalitis
Herpes simplex encephalitis in the immunologically normal adult is caused by herpes
simplex virus type 1 and manifest as a temporal lobe disease
Herpes Simplex Encephalitis During AIDS
Three distinct patterns recognized
Acute necrotic and hemorrhagic neurologic disease confined to temporal lobes, but
caused by herpes simplex virus type 2
Chronic subacute panencephalitis not involving the temporal lobes (brainstem,
cerebellum, and subependymal region) but caused by herpes simplex virus type 1
Subclinical infection of the brain parenchyma associated with herpes simplex virus type
1 infection
Neurologic Diseases Associated with Direct HIV -1 Infections
Scrapie and visna of sheep have become protypes of slow virus infections
Scrapie is caused by unconventional infectious agent (prion)
Visna is caused by conventional infectious agent (retrovirus)
Slow Virus Infections (Conventional Viruses)
Sheep
Visina  Visna virus
Human
Subacute sclerosing panencephalitis  Measles virus
Progressive rubella panencephalitis  Rubella virus
Progressive multifocal leukoencephalopathy  JC virus
Visna and Visna Virus
Visna virus is classified as a retrovirus
Among retroviruses, the visna virus is classified as a lentivirus
Visna Virus Pathogenesis
Primary infection can occur by horizontal or ventrical transmission
Horizontal transmission by inhalation of respiratory secretion
Vertical transmission by congenital infection (cross placenta)
Tropism for monocyte /macrophage
Blood monocytes carry virus genome (provirus) without replication
CD4+ T cells not a target for virus infection (unlike HIV)
Characteristized by long incubation period (2 -7 years)
Immune response fails to efficiently neutralize virus infection
Humoral immune response markedly slow
Circulating precursors of cytotoxic CD8+ T cells evident, but cell -mediated
immune response remains ineffective for virus clearance
Immunosuppression is not a feature of visna virus infection (unlike HIV)
Visna (Meningoencephalitis)
Invasion of CNS is thought to occur via infected circulating blood monocytes that cross
intact blood -brain barrier
Evidence for virus replication in variety of cell types, but microglial cells appear to be
primary target
Histopathology localized to white matter of brain and spinal cord
Massive infiltration of mononuclear cells around blood vessels
Generalized gliosis of astrocytes
Development of large irregular collections of mononuclear -microglial cells
termed microglial nodules
Areas of diffuse demyelination
Insidious, afebrile onset of neurologic signs and symptoms that includes progressive or
intermittent course of paralysis over many months
Cerebrospinal fluid changes include chronic pleocytosis and local antibody synthesis
(intrathecal virus -specific IgG synthesis)
Visna virus is responsible for neurologic disease but….
Visna virus can also cause a fatal respiratory disease in sheep called Maedi
Visna -Maedi Virus
“Maedi” is lcelandic for respiratory distress
Lesion consist of widespread infiltration of mono -nuclear cells into the interstitial
spaces of the lung and around bronchi
In lceland, affected sheep have either visna or maedi or both types of disease, although
maedi is more prevalent
HIV -Associate Dementia (HAD) [AIDS -dementia complex] [HIV Encephalopathy]
When is the course of HIV infection does the brain become infected with HIV?
HIV (R5 strains) can recovered from CSF within 2 weeks of primary infection
Primary infection associated with flu -like symptoms
Also, headache, photophobia, nuchal rigidity (meningitis)
Brain infection established early in infection, but kept in check until end -stage disease
(AIDS)
HIV DNA detected in brain throughout course of infection
HIV RNA/ protein detected only in first few weeks after infection but again in
end stage disease (AIDS)
In late HIV infection (AIDS), immune control of central nervous system infection is lost,
and brain infection proceeds as does activation of central nervous system mononuclear
cells that leads to neurologic injury and dementia
HIV -Associated Dementia Clinical Presentation
~20% of AIDS patients develop dementia before death
Clinical course usually abrupt over weeks /months, not insidious over year
Initial presentation included forgetfulness, and loss of concentration followed by
behavioral changes (apathy, social withdrawal, irritability)
Disease progresses to loss of fine motor control, unsteadiness of gait, and tremors
Most deteriorate into global cognitive dysfunction, mutism, incontinence, and
generalized spasticity although patient is awake
Death occurs 3 to 6 months after onset of dementia
HIV -associated dementia can occur despite use of combination Antiretrovirus therapy
(cART)
HIV -Associated Dementia Neuropathology
Gross inspection of brain at time of autopsy reveals signs of tissue wasting as evidenced
by lower than expected brain weight and significant enlargement of brain sulci, fissures,
and ventricles
All typical cases of HIV encephalopathy share…
Multinucleated cells (cell fusion or syncytia)
Large irregular collection of mononuclear -microglial cells termed microglial
nodules
Foci of myelin loss
HIV -Associated Myelopathy (HAM)
Most common cause of spinal cord dysfunction in HIV -infected persons
Activated macrophages found in the posterior and lateral columns of spinal cords, so
pathogenesis may be similar to HIV -associated dementia (HAD)
Patients complain of slowly progressive lower extremity weakness and stiffness, trouble
walking, urinary frequency /incontinence
Cerebrospinal fluid is typically acellular with mild to moderate increase in protein; HIV
-1 RNA amounts not increased
No proven treatment for HIV -associated myelopathy, but improvement has been noted
after beginning combination Antiretrovirus therapy (cART)
How does HIV enter the brain /spinal cord? How does HIV cause neuropathology?
HIV -Associated Dementia Neuropathogenesis
HIV entry into brain
HIV neurotropism
Toxic effects of HIV patients
Toxic cellular factors produced by macrophages /microglia and astrocytes
Transactivation during co -infection with other viruses
HIV entry into the brain
Cellular sources include HIV -infected macrophages and CD4+ T lymphocytes
Free virus (virus in blood plasma) can infect endothelial cells and astrocytes of the blood
-brain barrier
Circulating HIV from blood first infects vascular endothelium of brain and
subsequently infects astrocytes
Alternatively, astrocytes can be infected or by HIV passage through endothelial
cells by transcytosis (transcellular transport involving vesicles) without infecting these
cells
HIV infection of endothelial cells /astrocytes of blood -brain barrier can disturb barrier
in many ways
Endothelial cell infection can result in release of TNF -a that induces endothelial
cell leakage
Tat production can upregulate adhesion molecules on infected endothelial cells
and facilitate entry of HIV -infected cells
Tat and gp41 production by infected endothelial cells alter structural integrity of
blood -brain barrier; increases permeability
Secretion of proinflammatory molecules (IL-6 and IL-10) by activated immune cells can
contribute to transendothelial migration of infected or uninfected monocytes
/macrophages
HIV neurotropism
After reaching the brain, a neurotropic HIV strain emerges with macrophage tropism
(R5 strains)
HIV infection can be detected in macrophage /microglia, astrocytes, and
oligodendrocytes but infection of neurons remains controversial
Infection could occur via CD4+ molecule, but alternative receptor
D6 (another chemokine receptor) may serve as co -receptor on astrocytes
HIV strains of the CSF and the brain are often different from those of the blood
Correlation between high level of HIV -1 RNA in CSF and development of
dementia
CSF strains of HIV remain macrophage -tropic (R5 strains)
CSF and brain isolate in contrast to blood isolates from the same person can be
resistant to antiviral therapy (cART)
A virus strain in the same host can evolve independently in brain and blood, and
distinguished by molecular /biological characteristics
Toxic effects of HIV protein
One consequence of macrophage /microglia HIV infection is the production of high
levels of HIV proteins within the brain parenchyma
A large body of evidence suggests that many HIV proteins are toxic to neurons and
astrocytes
Gp120
Tat
Gps41 and Nef
HIV -Associated Dementia Neuropathogenesis
The envelope glycoprotein gp120 appears to be especially involved in a number of
processes that are detrimental to neurons, astrocytes, and endothelial cells, often
leading to apoptosis
Gps120 kills cultured human brain cells
Intracerebral injection of gp120 induces cell death in rat neurons
Gp120 activated tyrosine kinase activity that is toxic to brain cells
Gps120 induces production of nitrate oxide that is cytotoxic
Transgenic mouse expression of gps120 causes astrocytosis and neuron loss
Tat protein
Toxic to brain -derived cells in culture
Neurotoxic after intracerebral injection of mice
Binds to lipoprotein receptor -related (LPR) and enters neurons to cause cell death
Neuroexcitatory (via NMDA receptor) and induces decrease in cell membrane resistance
Induced MCP -1, IL -8, and IP -10 expression in astrocytes, all chemotactic proteins for
lymphocytes and monocytes
Enhances amyloid-B accumulation within brain that is associated with dementia
Gp41 and Nef proteins
Gp41 and Nef share sequence homology with scorpion toxin
Gps41 linked to increased expression of nitric oxide synthase and toxic effects of nitric
oxide
Nef selectively expressed on astrocytes, affects normal transmembrane conduction, and
compromises electrical potential and function of neurons
Nef affects differentiation of oligodendrocytes
Toxic cellular factors produced by macrophages /microglia and astrocytes
HIV infection of the brain induces high production of toxic cellular products that have
neuropathic effects
Apoptosis of neurons and astrocytes is detected in brain tissue of AIDS patients
Most apoptotic cells are found near HIV -infected cells, but are not infected with
HIV
Provides evidence for a role of soluble cell factors in brain cell dysfunction and
death
Mostly notably, TNF -a mRNA has been found in high levels in astrocytes and brain
macrophage /microglia in patients with dementia
Toxic to astrocytes and causes astrocytic proliferation
Induces IL -1 secretion that is cytotoxic and induces astrocyte proliferation
Cytotoxic to neurons and oligodendrocytes
Causes degeneration of myelin
Transactivation during co -infection with other viruses
Human herpesviruses (cytomegalovirus) and papovaviruses (JC virus) may exacerbate
neuropathology and serve as cofactors during neuropathogenesis of dementia
Double -labeling in situ techniques show that CMV and HIV can co -infected same cell
in brain of AIDS patients
Co -infection of cultures of primary astrocytes with CMV and HIV:
Enhances HIV replication due to trans activation of retrovirus LTR promotor
HIV trans activator Tat gene also enhances CMV replication
Transactivation is bidirectional
Transmissible Spongiform Encephalopathies (Human Prion Diseases)
A group of closely related fatal neurodegenerative disease that affect humans and
several other mammals that share common characteristics
Transmissible in nature and/or experimentally
Invariably lead to brain dysfunction or “encephalopathy” which manifests
clinically as cognitive, behavioral, sensory and/or motor dysfunction
Uniformly fatal
Currently diagnosed definitively only by neuropathology (or genetically in certain
cases)
Transmissible spongiform encephalopathies are caused by prions (“proteinaceous
infectious particles”)
Misshapen forms of an endogenous protein normally found within the central nervous
system
Scrapie
“Rida”, a degenerative disease of the brain is now known as scrapie
The name scrapie derives from the observation that sheep develop pruritus and
compulsively rub against objects until there is loss of wool
Scrapie exist in many countries, but there is no evidence scrapie has ever spread to
humans
The infection mostly commonly appears to be transmitted from ewe to lambs as a result
of postnatal exposure
Sheep develop motor difficulties, and continued neuro -degeneration that ultimately
leads to death within 4 -6 weeks of first appearance of symptoms
Scrapie also affects goats in nature, and hamster and mice are affected in the laboratory
as animal models of the disease
Bovine Spongiform Encephalopathy (“mad cow disease”)
In 1986, cows in the United Kingdom were noted to have behavioral and gait changes
that progressed to considerable incoordination
Behavioral changes (apprehension and mental status decrease) coined the term mad
cow disease
A transmissible spongiform encephalopathy not previously recognized in cattle
All cattle breed susceptible
No evidence for direct cattle -to -cattle transmission
Evidence points to unused cow tissues and cow bone from slaughterhouses (offal) used
to feed cattle (cow cannibalism)
Tissues and bone from sheep with scrapie suspected in the preparation of offal
Outbreak traced to 1981 decision that changed the process to collect melted fat and
pulverizes residual solids of offal
Outbreak peaked in 1992 with ~37,000 confirmed cases that year
Kuru
“trembling from fear and cold”
Some afflicted persons did shake uncontrollably
Others could barely walk
Small children jerked spastically
Adults leaned on heavy poles to support themselves
Those lying helplessly on the ground showed signs of starvation
Eye muscles lost function
Victims of Kuru could hear and understand questions, but they could not reply because
they could not speak
Kuru and the Fore Tribe
Kuru had been known to the tribe for ~50 years, first appearing in ~1990
Adult men rarely developed Kuru
Kuru was a disease primarily of women and children (both boys and girls)
The fore tribe believe it was under a sorcerer’s curse
200 persons (1% of tribe) were dying each year
Ratio of men to women was becoming 3:1
50% of adult men had no wife or children
Unmarried men forced to assume tasks usually done by women
The Fore tribe practiced cannibalism
Practice not unusual to New Guinea
Provide an avenue for passage of an infectious agent
Fore tribe ate tissues (especially the brain) of relatives as expression of sorrow,
respect, and love for decreased
A religious ceremony to maintain the spirit of the decreased in the community
The adult women were primarily involved in cannibalism
Adult men and adolescent boys lived largely separate live living together in huts
Adult men and adolescent boys hunted in the forest, but kept meat for themselves, not
sharing with the women
Women looked after the domestic pigs, but rarely ate them (although the men ate the
pigs)
Food for the women and young children consisted mostly of vegetable, insects, frog, and
small rodents
The women prepared the body of the decreased for the ceremony
It was during this preparation that women consumed body tissues
Practice introduced by women in ~1900
Women suffered from lack of protein in their diet
Practice allowed a way to obtain protein disguised as religious ceremony Women
used bamboo blades to prepare the tissues and packed the brain in small bamboo tubes
to be steamed over an open fire before consumption
NIH experiments duplicated those conditions and found steamed brain to
contain 10 million infectious agents per gram of steamed brain
Infections probably took place through cuts, open sores, scratches of the skin, or
mucous membranes of the mouth
Small children (boys and girls) playing in the area also given pieces of brain tissue to
consume
Initial histopathologic examination of brain tissue sections taken from victims of Kuru
were negative
No signs of inflammation
No signs of infection
More rigorous histopathologic examination by neuropathologist revealed abnormalities
Loss of neurons (spongiform encephalopathy)
Activation of astrocytes
Abnormalities similar to that seen for another neurologic disease, Creutzfeldt -Jakob
disease, a medical curiosity at the time with only two cases reported in the literature
Kuru of the Fore tribe was known worldwide as Creutzfeldt -Jakob
Kuru was an epidemic of Creutzfeldt -Jakob disease within the Fore people
Rhesus monkeys and chimpanzees were inoculated with fresh brain tissue taken from a
victim of Kuru
Two chimpanzees ~20 months after inoculation developed clinical signs and symptoms
of Kuru that included trembling
Histopathologic analysis of brain tissues taken from chimpanzees showed changes
consistent with Kuru and Creutzfeldt -Jakob disease (spongiform encephalopathy)
First evidence that a transmissible agent is involved in the pathogenesis of Kuru and
Creutzfeldt -Jakob disease
Creutzfeldt -Jakob Disease
Rapidly progressive global dementia, myoclonus, and marked progressive motor
dysfunction (tremors)
Normal blood, cerebrospinal fluid, CT, and MRI findings
Death occurs within 1 year of onset of symptoms
Definitive diagnosis depends on brain pathology at time of biopsy /autopsy
Gross pathology of brain shows no change or some limited cerebral cortical or cerebellar
atrophy
Histopathologic examination of fixed brain sections shows
Neuronal loss
Astrocytic gliosis /proliferation
Spongiform degeneration (large multilobulated vacuoles)
There is an absolute absence of inflammatory responses
Creutzfeldt -Jakob disease (CJD)
Sporadic form
Familial form
Latrogenic form
Variant form of Creutzfeldt -Jakob disease (vCJD)
Sporadic Form Creutzfeldt -Jakob Disease
Accounts for ~85% of all cases of CJD
Bulk of cases occur between ages 50 and 79
By age 80 years, incidence drops
Suggests an exposure to infectious agent at specific age followed by very long
incubation period
Presents as a presenile dementia characterized by rapidly progressive mental
deterioration, myoclonic jerking, deterioration of motor functions, ataxia, and death
within one year of onset of symptoms
Familial Form Creutzfeldt -Jakob Disease
Accounts for 5% to 15% of all cases of CJD
A genetic disorder due to point mutations /insertions in a gene (PRNP gene) found on
short arm of chromosome 20
Onset takes place at a younger age when compared with sporadic CJD
Diseases progression characterized by progressive insomnia, autonomic dysfunction,
endocrine changes, decline in motor /cognitive functions, and death within one year of
onset of symptoms
Gertsmann -Straussler -Scheinker disease is a phenotype of familial Creutzfedt -Jakob
disease
Presents clinically as a progressive cerebellar ataxia
Mean survival time of ~4.5 years
Latrogenic Form Creutzfeldt -Jakob Disease
Accounts for ~1% of all cases of CJD
Medical transmission of infectious agent
Corneal transplantation
Electrodes used in neurosurgical procedures
Human growth hormone
Human gonadotropin
Human cadaveric dura mater in neurosurgical procedures
Since 1974, multiple cases have been reported
Of 168 cases, median incubation period of 13 years
Disease progression similar to sporadic CJD
Variant Creutzfeldt -Jakob Disease
First recognized in 1995 in Great Britain as a form of CJD whose clinical picture differs
greatly from that of sporadic CJD
Cases occur in younger age group (mean age =26 years)
Early stages of illness present with psychiatric symptoms (depression, withdrawal,
anxiety and irritability)
Sensory symptoms develop (limb pain, numbness, cold feelings) that progress to ataxia,
dementia, and death usually 13 months after onset of symptoms
Variant Creutzfeldt -Jakob Disease (Mad Cow Disease in Human)
Origin unclear
Transmission of infectious agent responsible for bovine spongiform encephalopathy that
was recognized in Great Britain in 1988
Identified in patients who received blood transfusions from donors who later died of
vCJD
What is a prion?
Scrapie Agent
Scrapie in sheep has been recognized by shepherds for >200 years
Infectious nature of scrapie agent was confirmed in 1936 when scrapie was transmitted
by injection of material from brain of infected sheep to another sheep
Scrapie agent passed through filters that do not transmit bacteria but transmit viruses
“Scrapie agent” infectivity not completely eliminated by ultraviolet light, nucleases,
standard fixation of tissue samples, and many kinds of sterilization procedures
including standard autoclaving
The unusual characteristics of the scrapie agent led to speculations that it might lack
nucleic acid, but could be a self -replicating protein
Purification of scrapie -infected tissue revealed a 27 -30 kd protein identified by a
procedure that used proteinases and various detergents
This protein was designated PrP (proteinase -resistant protein) and postulated to be a
protein that is infectious
PrP
Sequencing of PrP followed by cloning its gene revealed that PrP is a protein found in
normal hosts including humans (immunological tolerance)
Entire ORF resides in single exon, so no RNA splicing
A 33 -35 Kd protein encoded by the PRNP human gene found on chromosome 20
Made as precursor protein with signal region at N -terminus that can cleave by
signal peptidase
C -terminus has extension that can be removed by another proteinase
C -terminus region has two glycosylation sites and disulfide
Attached to membrane “rafts” (areas rich in cholesterol and sphingomyelin) and found
on cell surface (but can be released)
An analogous gene and protein have been identified in mice, other mammals, birds, and
fungi
Why do we all not develop a transmissible spongiform encephalopathy?
There are two isoforms of PrP
PrPC is the cellular isoform of the prion protein
PrPSC or (PrPCJD) is the abnormal pathogenic isoform of the prion protein that causes
illness
PrPC versus PrPSC
PrPC and PrPSC are encoded by same gene and share amino acid composition
(immunological tolerance)
PrPC is sensitive to proteinase K, whereas PrPSC is resistant
PrPC is soluble in nondenaturing detergents, whereas PrPSC insoluble
PrPSc (but not PrPC) aggregates and forms amyloid plagques in brain with TSE
PrPC has an a -helix content of 42% and little B -sheet, whereas PrPSC has an a -helix
content of ~30% and 45% B -sheet content
Explains differences in sensitivities to proteinase K /detergents
Explains ability of PrPSC to aggregate and form amyloid plaques
It is therefore hypothesized that after infection with the PrPSC isoform of the PrP
protein, the PrPSC isoform causes the PrPC isoform to be coverted into the PrPSC
isoform
PrP
PrP is a normal cellular protein (PrPC)
Proteinase K sensitive
Alpha helix conformation
PrpC is converted to abnormal form (PrPSC)
Proteinase K resistanr
Beta helix conformation  abnormal beta -sheets of protein  transmissible
spongiform encephalopathy
PrPSC can induce other PrPC to become PrPSC
This conversion takes place in vitro
PrPC Expression
PrPC mRNA is expressed mostly in the brains of healthy animals and highly regulated
during development
Highest levels of PrPC mRNA found in neurons
PrPC associated with N -CAM (Neural cell adhesion molecule) on surface of
neurons
PrPC is also expressed in leukocytes, heart, skeletal muscles, lung, intestinal tract,
spleen, and reproductive organs
PrPC is a cell -surface glycoprotein that forms part of cell membrane structures known
as lipid rafts
Function of PrPC
The role of PrPC in the nervous system remains elusive and controversial
Most cellular functions attributed to PrPC related to a wide range of signal transduction
pathway involving molecular interactions with multiple cellular proteins of which some
are vital for normal neuronal function
Overexpression of PrPC in mouse neuroblast brain cells stimulates cell
proliferation and cell cycles whereas silencing of PrPC slows down the cell cycle
PrPC has been shown to interact with metal ions (copper, zinc, iron) in different
signaling pathways
Disruption of the PrPC gene in knockout mice (Prnp-/- mice) develop normally and do
not show signs of neurologic disease suggesting the absence of PrPC is not a sufficient
cause of neurodegenerative or neurodevelopmental abnormalities
Arguments Against Prions
Goats with scrapie exhibit two different clinical syndromes
Goats become hyperactive
Goats become drowsy
At least 20 different strains of PrPSC have been recognized in mice
Differences in incubation times
Difference in vacuole distribution within the CNS
Could structural variations distinguish one prion strain from another?
Differences in glycosylation pattern
Extent of protease resistance
Conformational stability
Does peptide recombination operate during generation of strains?
Some investigators argue that generation of strains of prions must be encoded by a
small nucleic acid
Prion Purity Concerns
Protein aggregates are very difficult to purify completely, and they tap other molecules
Ratio of infectious units PrPSC molecules is from 1:105 to 1:106, consistent with a
small infectious agent trapped in the PrPSC aggregate or perhaps bound to PrPSC
Only a fraction of infectious activity is extremely resistant to ultraviolent light
and disinfectants, consistent with a small infectious agent with nucleic acid being
protected by PrPSC aggregate
Prion Inactivation
Incineration
Treatment with mercaptoethanol, SDS
Treatment with sodium hypochlorite (20,000 ppm)
Immersion for 60 min in 2N sodium hydroxide
Autoclaving for 90 min at 132 -136 C (versus 15 -30 min at 121 C)
Gloves must be worn when handling biopsy /autopsy tissues even after fixation!
Alzheimer’s Disease
Alzheimer’s disease is the most common neurodegenerative disease in the United States
where it results in a progressive and age -associated dementia
The word ‘dementia’ first appeared in ~600 A.D. in a book written by Saint Isidore, the
archbishop of Seville
The term has its origin in Latin
The prefix ‘de’ means a deprivation or loss
The root ‘ment’ means mind
The suffix ‘ia’ indicates a state
Dementia = a state out of mind
Alzheimer’s Disease Risk Factors
Age
Most persons develop Alzheimer’s Disease after the age of 65
Risk for development of Alzheimer’s Disease reaches 50% for persons above the age of
85
Alzheimer’s Disease is not a normal part of aging
Family History and Genetics
Genetic differences have been identified in 1% to 5% of cases of Alzheimer’s Disease
where genetic differences have been detected, so the vast majority of cases are not
genetically inherited
There are two from of Alzheimer’s Disease
Early onset familial Alzheimer’s Disease
Sporadic Alzheimer’s Disease
Both have a basis in genetics
~0.1% of cases Alzheimer’s Disease are familial forms of autosomal dominant
inheritance
Those with a parent, brother, or sister with Alzheimer’s Disease are at risk for
developing the disease (genetic heritability 49% to 79%)
This form is known as “early onset familial Alzheimer’s Disease” and occurs before age
65
Attributed to mutations in one of three genes”
APP gene that encodes for amyloid precursor protein
PS1 gene that encodes for presenilin 1
PS2 gene that encodes for presenilin 2
Most cases of Alzheimer’s Disease do not exhibit familial autosomal dominant
inheritance and termed “sporadic Alzheimer’s Disease”
The best-known genetic risk factor is inheritance of the Apolipoprotein E (APOE -e4)
gene
Between 40% and 80% of persons with Alzheimer’s Disease possess the APOE -e4 gene
The APOE -e4 gene increases risk of disease development by 3 to 15 times
Other Risk Factors
Epidemiology studies have identified environmental factors that help to maintain brain
health during aging and perhaps protect against Alzheimer’s Disease
Healthy eating
Social and physical activity
Avoiding excess alcohol and tobacco
The risk for developing Alzheimer’s Disease appears to increase with heart disease,
diabetes, high blood pressure, and high cholesterol
Stages of Alzheimer’s Disease
Mild (Pre -dementia) Alzheimer’s Disease
Onset is gradual, involving short -term memory loss and other problems with thinking
Family members may dismiss memory problems as a normal part of aging
Presents as short -term memory loss and mild personality changes (loss of
spontaneity, apathy, tendency to withdrawal from social interactions)
The brain areas involved in memory include the cortex, especially the hippocampus
Pre -dementia disease begins in the entorhinal cortex, located in the medial
temporal lobe, which connects the hippocampus (memory formation) to the cerebral
cortex, a hub for memory and navigation
Neuronal loss may begin years before signs of memory loss emerges
CSF fills in the space previously occupied by brain tissue as the brain atrophies
Moderate Alzheimer’s Disease
Longest stage that can last for years, but different from person to person
Problems in abstract thinking and in order intellectual functions (difficulty
working with numbers, understanding what is being read, daily organization,
diminishing ability to dress appropriately)
Person becomes confused or disorientated as to time and place (disoriented as to
month or year, cannot describe location of home that progresses to wandering)
Significant behavioral changes (agitation, irritability, quarrelsomeness, violence,
suspicions of other lying, cheating, stealing, hallucination of both sight and sound)
Brain atrophy extends to other areas of the cerebral cortex
Advanced Alzheimer’s Disease
Person unable to care for himself /herself
Unable to speak more than a few dozen words
Cannot recognize familiar faces
Develop habits like wringing of the hands or shedding tissues
Loss of bladder and bowel control
Brain seems unable to tell body what to do (person holds food in mouth and
forgets to swallow)
The cortex atrophies in area that controls speech, reasoning, sensory processing, and
conscious thought
Muscle mass and mobility deteriorates to the point where the person is bedridden and
unable to feed themselves
Death
Death is the final outcome of Alzheimer’s Disease
Death is usually an external factor such as pneumonia, heart disease, infection of
pressure ulcers, but not the disease itself
Alzheimer’s Disease Treatment
There is no cure for Alzheimer’s Disease
Current approaches focus on treatment of cognitive problems and attempt to slow or
delay the symptoms of disease
Reduction in the activity of cholinergic neurons is a key feature of Alzheimer’s Disease
Acetylcholinesterase inhibitors are used to reduce the rate at which acetylcholine is
degraded, thereby increasing the concentration of acetylcholine in the brain and
combating loss of acetylcholine caused by the death of cholinergic neurons
These drugs do not change the underlying disease process
Five drugs have been approved by the FDA to the treat the symptoms of Alzheimer’s
Disease
Four drugs are acetylcholinesterase inhibitors
Tacrine to treat mild to moderate disease
Donepezil to treat mild to moderate disease
Rivastigmine to treat mild to moderate disease
Galantamine to treat mild to moderate disease
One drug is an antagonist to the NMDA -receptor involved in memory function
Memantine to treat moderate to severe disease
The Pathogenesis of Alzheimer’s Disease
Alzheimer’s disease is characterized by two neuropathologic features that lead to
neuronal degeneration
Extracellular plaques of amyloid -B proteins (amyloid plaque formation)
Intracellular neurofibrillary tangles
Plaques and neurofibrillary tangles occur with normal aging and in some other
neurodegenerative disorders
Plaques and neurofibrillary tangles in Alzheimer’s disease are localized to areas in the
brain that correspond to clinical symptoms
Plaque Formation (Amyloid -B protein plaques)
Proteolytic Processing of APP
Amyloid -B plaques are c lumps of insoluble peptides that result from the abnormal
cleavage of amyloid precursor protein (APP)
Amyloid Precursor Protein
Amyloid precursor protein (APP) is a transmembrane protein that penetrates through
the neuron’s membrane and critical for neuron growth, survival, and post -injury repair
APP is produced naturally in large quantities in neurons and metabolized very rapidly
APP is delivered to the axons where it is transported by fast axonal transport to synaptic
terminals
Transgenic mice that overexpress AA have enlarged neurons
Intracerebral injections of APP into adult mice improves synaptic density and cognitive
function
Knockout mice in which the APP gene is deleted show no phenotype changes suggesting
that loss of APP or its function is not deleterious to the adult animal
Proteolytic Processing of APP
Nonamyloidogenic Pathway
APP cleavage involves three different types od serine proteases a -secretase, B secretase, y -secretase
The functionally most important proteolytic progressing of APP is mediated through the
“nonamyloidogenic pathway” that involves cleavage of APP by the action of a -secretase
and y -secretase
This result in a soluble 40 amino acid peptide (amyloid -B40; AB40)
Proteolytic Processing of APP Amyloidogenic Pathway
APP cleavage involves three different types of serine proteases a -secretase, B -secretase,
y -secretase
In Alzheimer’s disease, cleavage of APP by the action of B -secretase and y -secretase
cleaves APP at an incorrect place that creates a 42 amino acid peptide called amyloid B42 (AB42) or amyloid -B
Amyloid -B is not soluble and aggregates into identifiable clumps termed amyloid
plaques
The Genetics of Alzheimer’s Disease
Three genes have been identified in familial Alzheimer’s Disease
APP is the gene that encodes for APP (the protein)
APP is located on chromosome 21
People with Down’s syndrome (trisomy 21; 3 copies of chromosome 21) develop
Alzheimer’s Disease at a younger age (late 20s) than the general population
There are at least 20 known mutation of APP
PS1 and PS2 are genes that encode for presenilin 1 and presenilin 2, the catalytic subunit
of y -secretase
Cleavage by a -secretase followed cleavage by y -secretase yields a soluble 40
amino acid peptide (amyloid -a)
There are >140 known mutations in PS1 and ~10 known mutations in PS2
Amyloid B plaques
Amyloid -B plaques are clumps of insoluble peptides that result from the abnormal
cleavage of amyloid precursor protein (APP)
Amyloid -B (AB42)
Initial work in tissue culture showed that amyloid -B (AB42) fibrils are toxic to neurons,
resulting in complete death of all cells within 24 hours
Intracellular injection of AB42, but not AB40, kill neurons
Transgenic mice that overexpress mutant human APP develop AB42 deposition by 4 to
6 months and show evidence for subsequent neuronal injury, loss of synaptic terminals,
synaptic dysfunction, abnormalities on spatial memory tests, inflammation (microglia
activation), and activation of multiple caspases (apoptosis)
Amyloid -B (AB42)
Amyloid -B generation leads to several outcomes that all contribute to onset and
progression of Alzheimer’s Disease
Loss of cholinergic neurons that make acetylcholine (Cholinergic neurons <25% in late stage disease)
Neurotransmitter deficiency (acetylcholine, serotonin and norepinephrine levels are
greatly diminished)
Caspase 3 and caspase 6 are activated that leads to death of neurons by apoptosis
Apoptosis leads to death of oligodendrocytes that results in loss of myelin
Microglia are activated that leads to inflammation and release of reactive oxygen
species, proteinases, and complement proteins
Neurofibrillary Tangles (Tau)
Tau and Neurofibrillary Tangles
Neurofibrillary tangles are seen as dying neurons or dead neurons when viewed after
histologic staining
Neurofibrillary tangles result from the destruction of neuronal microtubules caused by
the modification of their supporting protein, tau
Microtubules (made if tubulin) are essential components of neuronal cells structure
because they act as tracks along which nutrients are delivered and neuronal
transmission is propagated in the neuronal axon
During Alzheimer’s Disease, tau proteins become phosphorylated, disrupting their
bonds to microtubules, thereby collapsing microtubule structure and destroying the
neuron’s transport and communication systems
Collapse of the microtubule system results in neuronal death
Soluble AB40 can control cleavage and phosphorylation tau
Both amyloid -B protein plaques and neurofibrillary (tau) tangles contribute to the
pathophysiology of Alzheimer’s Disease
Two forms of Alzheimer’s Disease
Early onset familial Alzheimer’s Disease
Sporadic Alzheimer’s Disease
The pathology of both forms involves development of amyloid -B protein plaques and
neurofibrillary (tau) tangles
Is Alzheimer’s Disease a Human Prion Disease?
Both amyloid -B protein plaques and neurofibrillary (tau) tangles begin in the
entorhinal cortex and spread in a consistent pattern throughout the brain
A prion is a misshapen form of an endogenous protein normally found within the
central nervous system
Alzheimer’s Disease is increasing being identified as a protein misfolding disease
(proteopathy) caused by plaque accumulation of abnormally folded beta protein and tau
protein in the brain
Similarities in properties of amyloid -B and PrPSC in Alzheimer’s disease and
Cruetzfeldt -Jakob disease, respectively, suggest a prion -like pathogenesis
Do AB42 “prions” move from neuron to neuron as AB42 accumulates in the
extracellular space to form plaques?
Key Commonalities Between Amyloid -B “seeds” and PrPSC
The active seeding agent is a form of the protein itself
Amyloid -B seeds are rich in B -sheet secondary structure
Misfolded amyloid -B seed can manifest as structurally and functionally variant strains
Amyloid -B seeds vary in sensitivity to proteinase K
Amyloid -B seeds retain bioactivity after boiling for 5 min and fixation for years in
formaldehyde
Amyloid -B seeds spread throughout the brain via interconnected region
Amyloid -B seeds can be transmitted from seeded mice to subsequent hosts
Amyloid -B and tau versus PrPSC
Oligomers of amyloid -B stimulate neurons to induce a structural transition in tau that
leads to its aggregation and subsequent deposition in neurofibrillary tangles
Amyloid -B dimers isolated from the cortex of Alzheimer’s disease patients induced tau
hyperphosphorylation and degeneration in cultured neurons
Neurofibrillary (tau) tangles have been observed to form in nerve cell bodies and around
nerve cell bodies in Gertsmann -Straussler -Scheinker disease
Creutzfeldt -Jakob disease -like pathology has been observed in some cases of
Alzheimer’s Disease
Does Herpes Simplex Virus Type 1 Serve as Cofactor in the Pathogenesis of Alzheimer’s
Disease?
Alzheimer’s Disease Risk Factors
Family History and Genetics
Most cases of Alzheimer’s Disease do not exhibit familial autosomal dominant
inheritance and termed “sporadic Alzheimer’s Disease”
The best-known genetic risk factor is inheritance of the Apolipoprotein E (APOE -e4)
gene
Between 40% and 80% of persons with Alzheimer’s disease possess the AOP -e4 gene
The APOE -e4 gene increases risk of disease development by 3 to 15 times
Herpes Simplex Virus Type 1 and Alzheimer’s Disease
Herpes simplex virus type 1 DNA is present in a high proportion of brain of elderly
normal persons and Alzheimer’s disease patients
The presence of herpes simplex virus type 1 DNA and possession of an APOE E4 allele
together confer a major risk for Alzheimer’s disease that is greater than virus DNA and
APOE E4 allele alone
~60% of Alzheimer’s disease cases harbor herpes simplex virus type 1 in brain and were
APOE E4 allele carriers, so do other herpesvirus (herpes simplex virus type 2,
cytomegalovirus, human herpesvirus 6) play a role in the other 40% of cases?
Does a kinase encode by herpes simplex virus type 1 plays a role in
hyperphosphorylation of tau?