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?