Bi / CNS 150
Lecture 24
Friday November 22, 2013
Two neurodegenerative diseases
Henry Lester
Kandel, Chapters Chapter 43, 44 (p. 1002 - 1012), 59
1
Alzheimer’s disease
1. Clinical description
2. Genetics
3. Pathophysiology
4. Biomarkers and animal models
5. Heterozygote advantage: none known
6. Therapeutic approaches
We’ll follow this general organization for all neural diseases
2
1. Symptoms of Alzheimer’s Disease
1.
2.
AD begins with a “pure” impairment of cognitive function.
“mild cognitive impairment” does not always lead to dementia.
Progression
A.
AD begins slowly. At first, the only symptom may be mild forgetfulness.
In this stage, people may have trouble remembering recent events, activities, or
the names of familiar people or things.
They may not be able to solve simple math problems.
They may begin to repeat themselves every few minutes in conversation.
B.
In the middle stages of AD, individuals may forget how to do simple tasks, like
brushing their teeth or combing their hair.
They can no longer think clearly.
They begin to have problems speaking, understanding, reading, or writing.
C.
Late stage: AD patients may become anxious or aggressive, or wander away
from home.
Eventually, patients need total care.
3
Incidence of Alzheimer’s disease (AD)
AD is the most common degenerative brain disease (est. 5 million USA, 25 million globally)
Risk Factors: age
65-74
75
80
>85
~5%
~10%
~20%
~50%
(However, AD is not considered a normal part of aging).
The more common form of AD, known as late-onset or sporadic AD, occurs later in life, with no
obvious inheritance pattern.
However, several risk factor genes may interact with each other to cause the disease.
Most common risk factor gene identified so far for late-onset AD, is a gene that makes one form of
apolipoprotein E (apoE). ApoE4 (prevalence ~ 16%) is the risk factor gene, 3-4 fold dominant
increase.
Familial AD, which is rarer, usually starts at age 30 - 60.
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The Anatomical Hallmark of Alzheimer’s Pathology:
Amyloid Plaques and Neurofibrillary Tangles in Brain
Amyloid Plaques
contain large amounts of a 42 amino acid
peptide termed “b-amyloid”, or Ab42
In the next lecture, we note that bamyloid itself is the initial cause of the
pathophysiology that leads to dementia.
Amyloid plaques probably contribute to
the later stages of pathology
Neurofibrillary tangles: rich in cytoskeletal proteins,
especially the microtubule-associated protein, “tau”.
In the tangles: heavily phosphorylated proteins,
which may cause aggregation and precipitation of the
cytoskeleton.
Also generally reduced brain volume, especially in entorhinal cortex and hippocampus
5
Red circles: presenilin 1 and APP mutations associated with familial AD
presenilin 1 is part of
-secretase, a membrane-associated protease
Hardy and Selkoe, 2002, Science
6
There are “tauopathies” as well,
Mutations in tau protein.
These cause frontotemporal dementia with parkinsonism,
linked to Chromosome 17
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3. Pathophysiology. Aβ40 and Aβ42 are proteolytic products formed from APP
β-APP = amyloid precursor protein. APP proteins: 110 to 140 kDal.
APP expressed by most tissues, especially neurons; reaches axon terminals and dendrites.
APP is also found in glial cells.
Overproduction of Ab40 and Ab42 results from
altered ratio of proteolytic cleavages at sites termed a, b, and .
Kandel et al.,Principles of Neural Science
© McGraw-Hill Professional Publishing
8
All known genetic risk factors predisposing to Alzheimer’s disease
Increase accumulation of Aβ peptides
Chromosome
Gene defect
Phenotype
21
β-APP mutations
↑All Aβ peptides, or Aβ40 peptides
A673T↓ Aβ peptides, AD, cognitive decline
19
ApoE4 polymorphism
(ε4 allele)
↑Density of Aβ plaques & vascular deposits
14
Presenilin 1 mutation
↑Production of Aβ42 peptides
1
Presenilin 2 mutation
↑Production of Aβ42 peptides
6
TREM2
↑Density of Aβ plaques
“The precise meaning of the amyloid hypothesis changed over the years, and differs among
scientists. Originally, it was thought that the actual amyloid is pathogenic—hence the term
“amyloid hypothesis”. The more current version of this hypothesis posits that Aβ (especially
Aβ42) microaggregates—also termed “soluble Aβ oligomers” or “Aβ-derived diffusible
ligands” (ADDLs)—constitute the neurotoxic species that causes AD” – Sheng, 2012
Abnormal states of tau mediate some effects of β-amyloid. This stage may be distal to the
more toxic dimers and oligomers.
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There are no convincing data suggesting the presence of a single “Aβ receptor”
Soluble Oligomers of Aβ42
Aβ42 peptides form soluble oligomers of ~4 to 40 peptides.
These oligomers interact with other proteins and precipitate to form amyloid plaques.
Soluble oligomers of Aβ42 (containing ~12 to 40 peptides) are toxic to neurons.
The 12-mer is the most significant toxic form.
Misfolded Aβ42 may spread from one cell to another, like a prion.
10
Normal function
of presenilin’s -secretase
activity:
Notch signaling?
APP cleavage
would be a
“side effect”
Components of the core γ-secretase
proteolytic complex:
Presinilin (either PS1 or PS2),
anterior pharynx-defective 1 (APH-1),
nicastrin (NCT), and
presenilin enhancer 2 (PEN-2)..
Barthet et al, Progress in Neurobiology 2012
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General Cellular Processes that Could Account for aβ Pathology
Protein homeostasis and turnover
Accumulation of erroneous, misfolded, or partially processed proteins
continues as a major theme in neurodegenerative disease.
When proteins accumulate in the endoplasmic reticulum, this causes ER
stress and the unfolded protein response. An unchecked unfolded protein
response can eventually cause cell death.
Excitotoxicity, excess Ca2+ influx
Excess excitation overwhelms the cell’s ability to maintain ion gradients
via ion-coupled transporters.
Various metabolites accumulate in wrong compartments (extracellular, cytosol, organelle).
Ca2+ also accumulates in the cytosol and organelles, activating transduction systems,
overactivating enzymes, and generally leading to cell death.
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Pathogenic hypotheses for mostly synaptic toxicity in Alzheimer’s disease
Loss of synapses correlates better than plaques or tangles with cognitive deficits
Sheng et al. Cold Spring Harb Perspect Biol 2012
See also Figure 44-6
13
Specific example of synaptic malfunction:
soluble oligomers block induction of long-term potentiation
Wild Type
CM
(Injected into ventricles 10 min
before high frequency stimulation of
the rat Schaffer collateral pathway).
CM + Aβ oligomers
CM = “conditioned medium” from a
cell line engineered to express Aβ
CM + Antibody against Aβ
CM + nonspecific
“control” antibody
Walsh et al., Nature 2002 14
Animal models:
mice overexpressing APP, especially with AD-associated mutations.
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4.
Biomarkers for AD.
Only an autopsy is conclusive, but progress in two areas:
1. Cerebrospinal fluid analyses of tau, phospho-tau at position 181, and Aβ42.
Individual values, or ratios among these.
2. A positron emission tomography (PET) marker, [18F]Florbetapir,
binds to plaques containing β-APP.
Negative
Positive
“A negative scan indicates sparse to no neuritic plaques and is inconsistent with a
neuropathological diagnosis of AD at the time of image acquisition; a negative scan result
reduces the likelihood that a patient’s cognitive impairment is due to AD. A positive scan
indicates moderate to frequent amyloid neuritic plaques; neuropathological examination has
shown this amount of amyloid neuritic plaque is present in patients with AD, but may also be
present in patients with other types of neurologic conditions as well as older people with
normal cognition. [Florbetapir] is an adjunct to other diagnostic evaluations.”
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5. Heterozygote advantage: none known
6. Therapeutic approaches
Acetylcholinesterase inhibitors
(because cholinergic basal forebrain neurons are among the first to die in AD)
donepezil, galantamine, rivastigmine
NMDA inhibitors
memantine
Still in development
β-secretase inhibitors
Failed: latrepirdine = dimebolin (unknown mechanism)
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γ-secretase inhibitors such as semagecestat have failed so far
Two types of candidates target this protease complex:
"Notch-sparing" γ-secretase inhibitors, which block cleavage of APP selectively
over that of Notch.
γ-secretase modulators, which shift the proportion of Aβ peptides produced in favor
of shorter, less aggregation-prone species.
Antibodies against β-APP or amyloid-β have given disappointing results
Gantenerumab (penetrates BBB)
Solanezumab (mildly successfully in mild stages of AD)
Bapineuzumab
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Parkinson’s disease
1.
Clinical description
2.
Genetics
3.
Pathophysiology
4.
Biomarkers and non-human models
5.
Heterozygote advantage: none known
6.
Therapeutic approaches: a, Symptomatic relief; b. Protection
James Parkinson, apothecary surgeon
1817, An Essay on the Shaking Palsy, described "paralysis agitans",
from observations of 6 individuals during his daily walks in London
Parkinson’s disease
(tremor at rest 3-5 Hz, “pill-rolling”,
slow movements, particularly when starting;
short, rapid steps)
but most Parkinson patients are either medicated or electrically stimulated
Excellent dramatization of the large motor problems in a PD patient
http://www.youtube.com/watch?feature=endscreen&v=j86omOwx0Hk&NR=1x
19
Dopaminergic neurons in the human brain: Saggital view
Substantia Nigra:
Dopaminergic neurons die in PD
Rodent brain section, stained for tyrosine hydroxylase: coronal view
20
Symptoms appear
when “most” (50% -80%)
SNc neurons are lost.
Most people lose
dopaminergic neurons
throughout life; PD
patients have lost more.
Remaining dopaminergic
neurons may form
additional “sprouts” which
partially compensate for
the loss.
Nestler, Hyman, Malenka,
Molecular Neuropharmacology,
© McGraw-Hill
Professional Publishing
21
A hallmark of PD pathology:
Intracellular “Lewy bodies”, especially in dopaminergic neurons
(Lewy bodies also occur in other diseases, especially some dementias)
22
PD patients often have additional symptoms and degeneration
Constipation. Detailed surveys show that most PD patients have constipation long
before the clinical symptoms.
Constipation does not predict PD.
Intestinal biopsies show Lewy bodies in the neurons of the intestinal wall.
Braak staging: various neurons with long axons through the brain show Lewy
bodies before dopaminergic neurons; but there are few or no symptoms.
(Braak & del Tredici, Neurology 2008)
Sleep disorders, especially in rapid eye movement sleep.
Olfactory problems (patients have “anosmia”).
Depression.
Dementia.
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3. Genetics. Familial Parkinson’s disease provides a good review of biochemistry
(~ 10% of patients). Onset 30’s to 50’s (rarely earlier or later)
Chromosome
location
Gene or protein name
Inheritance
pattern
PARK1 &
PARK4
4q21–q23
a-synuclein
AD
PARK2
6q25.2-q27
Parkin
E3 Ubiquitin ligase
AR
PARK3
2p13
Unknown
AD, IP
PARK5
4p14
UCH-L1
ubiquitin-C-terminal hydrolase-L1
AD
PARK6
1p35-p36
PINK1, PTEN-Induced Putative Kinase 1
AR
PARK7
1p36
DJ-1
Uknown function
AR
PARK8
12cen
LRRK2
leucine-rich repeat kinase 2
AD
PARK9
1p36
ATP13A2
AD
PARK10PARK16
Various
Much less is known
various
Locus
(Membrane fusion in various organelles)
AD, autosomal dominant; AR, autosomal recessive; IP, incomplete penetrance
See also Table 44-3
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dopamine
Parkinson’s disease: pathophysiology
reactive: HO
1.
Most cases are unexplained.
oxidative damage?
H2
C
C
H2
NH3+
HO
2.
Dopaminergic neurons may be selectively vulnerable because the cell body must
maintain large amounts of axoplasm and presynaptic proteins.
3.
Dopaminergic neurons may be selectively vulnerable because dopamine is highly
reactive.
4.
Pesticides (Wang et al, Eur J Epidemiol. 2011).
5.
The “frozen addict”. An impurity in synthetic heroin.
Taken up by the dopamine transporter (expressed only in dopaminergic cells).
Kills cells.
6.
The “encephalitis lethargica” pandemic (worldwide epidemic) of 1918 killed 20
million people. Some people experienced selective damage to dopaminergic
neurons. Presumably an autoimmune reaction was provoked by an still-unknown
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infectious agent (virus or bacteria).
(“Awakenings”, O. Sacks).
a-synuclein has an unknown function; it’s an “intrinsically disordered protein”.
Mutant a-synuclein forms fibrils.
The hallmark of PD pathology:
Intracellular “Lewy bodies”, especially in dopamine neurons
Improper mitochondrial fission / fusion
may be one of the early events
(Prof. D. Chan, Caltech)
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4. Animal Models for Parkinson’s Disease:
Drosophila that overexpress synuclein
1. The 4 dopaminergic neurons
die preferentially!
We don’t know why.
(2. The cells show dense structures
like Lewy bodies)
3. The flies show a
“movement disorder”
See also work of Prof. Bruce
Hay, Caltech
27
More Models for Parkinson’s Disease
a. Toxin-treated mice, rats, and monkeys
b. Mice with altered PARK genes
c. Yeast that harbor synuclein mutations (Cooper et al, Science 2006)
b. Cultured cells from people carrying PD mutations (“disease in a dish”)
human embryonic stem cells (hESCs)
human induced pluripotent stem cells (iPSCs)
But there are still major technical issues in generating dopaminergic neurons
that behave like the vulnerable neurons.
Distinction between SNc and VTA?
Antibody stains for tyrosine hydroxylase and other markers are not yet
sufficient to reveal a true phenotype.
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Biomarkers for Parkinson’s Disease
a. Experienced neurologist is the best judge. No effective blood test for PD,
Responsiveness to L-dopa is a good criterion.
a. Imaging: [125I]ioflupane, high-affinity dopamine transporter (DAT) ligand.
Single-photon emission computed tomography (SPECT)
May help to differentiate Parkinsonian
symptoms from conditions with similar
symptoms, such as essential tremor.
Normal striatum
Effectiveness as a screening or
confirmatory test and for monitoring
disease progression or response to
therapy has not been established.
http://en.wikipedia.org/wiki/File:Datscan.JPG
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5. Heterozygote advantage: none known
6. Therapeutic approaches to Parkinson’s disease
a. Symptomatic relief: L-dopa
30
HO
H2
C
NH3+
CO2-
enzyme:
decarboxylase
HO
levodopa, “L-dopa”
zwitterion
permeates into brain
via a transporter
HO
H2
C
C
H2
NH3+
HO
dopamine
does not enter brain
Used with carbidopa,
which inhibits decarboxylase.
Prevents hydrolysis in the blood and in the peripheral
nervous system.
D2 receptor agonist is often added. This seems to reduce dyskinesias (next slide).
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L-dopa remains effective only as long as sufficient dopaminergic neurons
remain to take up and secrete dopamine.
Side effects of L-dopa
Dyskinesias (“bad movements”, very common in people who have
used L-dopa for many years, often seen in TV appearances of medicated PD
patients).
Mechanism is unknown.
coupled pathways?
“Outside-in” continued activation of Gi-
Visual hallucinations, other psychotic symptoms, sleep disturbances,
and confusion.
“On-off” phenomenon: abrupt and transient fluctuations in the
severity of parkinsonism at intervals during the day; such fluctuations are
unrelated to ongoing drug dosage.
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Other drugs for relief of PD symptoms.
Monoamine oxidase (MAO type B) inhibitors
Muscarinic antagonists may prolong the action of certain key interneurons in the striatum
Amantadine (blocks NMDA Receptors, may decrease excitotoxicity)
Mitochondrial stabilizers, such as coenzyme Q10 and creatine
Adenosine receptor antagonists (Gs coupled, A1 or A2A), mechanism uncertain
33
Deep brain stimulation
for Parkinson’s Disease
Developed from ablation
Activate neurons?
Silence neurons?
Axons
passing through
Nestler, Hyman, Malenka,
Molecular Neuropharmacology,
© McGraw-Hill
Professional Publishing
34
6. Therapeutic approaches to Parkinson’s disease
b. Arrest the degeneration Goal: intervene in early-stage PD with drug
taken from that point.
Some degeneration has already occurred.
Good example of the interplay between diagnosis & therapy.
35
Does deep brain
stimulation for
Parkinson’s disease
delay degeneration?
Data are unclear,
both in humans
and animal models.
See also work of
Prof. Gradinaru,
Caltech
Nestler, Hyman, Malenka,
Molecular Neuropharmacology,
© McGraw-Hill
Professional Publishing
36
Inverse correlation between
a person’s history of tobacco use & susceptibility to Parkinson’s disease
3
Pools
1. Meta-analysis of 45 studies
case
cohort all
2
Relative
risks
1
0
Hernan, 2002
individual studies: case-control
cohort
2. Tanner, 2002: 33 discordant identical twins including at least one smoker. The twins
without PD smoked more than their brothers smoked (37.1 vs 25.3 pack-years, p=0.077)
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A.
Tobacco use protects against PD, vs PD patients use less tobacco
B. Nicotine itself is probably a part of smoking’s apparent neuroprotective action
1.
In rodents and monkeys, nicotine protects dopaminergic neurons against toxins
(M. Quik, several papers)
2.
α4 nicotinic receptor knockout mice lack this protection
(Ryan et al, 2001)
3.
Inverse correlation persists for chewing tobacco in epidemiological studies
C. We need additional human data, e. g., PD among Swedish snus users
D. Simply use nicotine patches?
1.
Clinical trial under way for early-stage PD patients given nicotine patches
2.
Nicotine itself is a perfect addictive drug but a sub-optimal therapeutic drug.
3.
Nicotine itself activates several nicotinic receptors; many people cannot tolerate
patches
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PD Therapeutic approaches: protection by proteins or gene therapy
The “limiting neurotrophic factor” hypothesis for neuronal survival
See lectures on development
Usually, more
neurons initially
appear in a ganglion
or nucleus than are
required to innervate
the target cells.
The excess neurons die
This occurs because
the target secretes a
limiting amount of a
trophic factor
Figure 53-14
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GDNF, a neurotrophic factor
Since the early 1990’s, several pharmaceutical companies have experimented with pumps that
infuse glial cell derived neurotrophic factor (GDNF) in or near the substantia nigra or its target,
the striatum.
Several companies abandoned, despite anecdotal stories of success; others continue.
artemin ( a GDNF homolog)
Part of a growth factor receptor
PDB: 2GHO
40
Attempts at Gene therapy for PD
Using adeno-associated viruses modified to express the protein of interest.
“AAV2” is a viral “vector”.
Lentivirus vectora also show promise.
Further attempts with GDNF
Attempts with neurturin. Insufficient encouraging results
41
Gene therapy with
glutamate decarboxylase
in
subthalamic nucleus
“AAV2-GAD”
Safety established;
effectiveness unknown.
Phase II shows initial
Encouraging results
(LeWitt et al.,
Lancet Neurol, 2011)
Nestler, Hyman, Malenka,
Molecular Neuropharmacology,
© McGraw-Hill
Professional Publishing
42
Henry Lester’s “office” hours,
1:15-2 Red Door
End of lecture
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