NPB 173 Midterm 2

neurodegenerative disorders

involve the progressive loss of neuronal function and/or neuronal cell death; they ALL involve dysfunction at the level of individual neurons and molecular abnormalities; certain brain regions/circuits are more vulnerable to degeneration in different disorders

most promising topic of study to treat neurodegenerative disorders

the molecular mechanisms for cell death, which can be highly debated and investigated

Inheritance of Huntington's Disease

autosomal dominant; almost all affected individuals have an affected paren; if one parent is affected, there is a 50% chance that offspring will inherit the disease

note: equal probability in males and females

Progression of Huntington's Disease

a progressive disease that manifests in adulthood, although it has no singular point of onset and it worsens over time

symptoms of Huntington's disease

early symptoms: first chorea, then motor impairment (rigidity, inability to initiate movements)

later symptoms: cognitive impairments (dementia) and a tendency towards psychiatric problems, and eventually terminal effects

chorea

a neurological disorder that results in abnormal involuntary movements that occurs when trying to walk/move; usually the first prominent indication of Huntington's disease (key factor in diagnosis); worsens over time as huntington's progresses

three stages of huntington's disease

1. premanifest

2. presymptomatic

3. prodromal

(1-3 all occur before diagnosis based on motor symptoms instead of genetics)

4. manifest - the stage after diagnosis, also based on motor symptoms

neuron structure

high specialized with common organelles (nucleus, mitochondria, ribosomes, proteins, etc)

proteins in neurons

participate in EVERY aspect of neuronal function; the site of molecular abnormalities; folded chains of amino acids that form polypeptides

Central Dogma of Molecular Biology

DNA-transcription-RNA-translation-protein

DNA in central dogma

DNA stores info about protein structure through genetic instructions; can be copied into identical replicas to allow this info to be preserved in all cells after cell division

transcription

the first stage of gene expression, where specific genes within the DNA are copied into RNA

DNA and RNA both consist of

chains of connected nucleotides

translation

the second stage of gene expression, where proteins are synthesized based on instructions carried by the transcribed RNA

codons

triplets of three nucleotides that code for a specific amino acid

chromosomes

in humans, DNA is packaged into 46 chromosomes (23 homologous pairs, one from each parent); each chromosome carrying DNA for hundreds of individual genes.

alleles

each gene has two alleles (variants of the gene), one for each chromosome in the pair; homozygous vs heterozygous

Homozygous

two alleles are identical for a gene

Heterozygous

two alleles differ for a gene

Huntington's disease is cause by

an autosomal dominant allele for the huntingtin gene

huntingtin gene (HD/HTT)

everyone has this gene on the short arm of chromosome 4; the gene has a CAG nucleotide repeat near its beginning

CAG codon

corresponds to the amino acid glutamine (thus cag repeats produce lots of glutamine)

Normal (unaffected) number of CAG repeats

26 or less

Intermediate (unaffected) number of CAG repeats

-don't develop symptoms, but children are at risk

27-35

Reduced penetrance (affected) number of CAG repeats

-may or may not develop symptoms at any age (many in old age)

36-39

Full penetrance (affected, have Huntingtin's) number of CAG repeats

40 or more

CAG repeats

CAG repeats are prone to expansions that causes polyglutamine (polyQ) tracts in proteins; number of CAG repeats in Huntingtin gene determines disease occurrence and penetrance

note: everyone has CAG repeats

penetrance

in genetics, the proportion of individuals with a particular genotype who will develop an associated phenotype

genetic test for huntington's disease

genetic testing can be performed long before symptoms develop; rarely done in children

neurodegeneration in Huntington's disease

causes wide scale brain atrophy (reduced brain size), resulting in the enlargement of the ventricles; occurs most prominently in the striatum, which takes place before symptoms are readily apparent

eventually extends past striatum and includes the cortex (cortical degeneration likely contributes to cognitive impairements)

loss of function in Huntington's disease

the huntingtin protein has high levels of expression in the CNS that may play an important role in scaffolding and interacting with other proteins; removal of this gene is lethal in embryos

Note: loss of function is NOT the primary cause of Huntington's neurodegeneration

loss of function mutation

results in a reduction in the normal function of a protein encoded by that gene

neural inclusions in Huntington's disease

visible aggregates of protein inside neurons that contain polyQ (repeated glutamine) portion of huntingtin protein (but not the C-terminal end); inclusions are not thought to cause cell death; instead they protect the cell, perhaps be hiding otherwise harmful abnormal huntingtin protein

what causes neurodegeneration in Huntington's disease

cause by fragments of the abnormal Huntingtin protein that contain the PolyQ tract encode by the long CAG repeat

gain of function mutations

result in an acquired new function in a protein encoded by that gene; new functions may be beneficial or harmful

gain of function effects of huntingtin mutation

hypothesis 1- fragments of mutant Huntingtin protein cause dysregulation of genetic transcription (alters proteins)

hypothesis 2- fragments of mutant huntingtin protein alter proteostasis

thus, a harmful gain of function in huntingtin may result in a loss of function of other proteins

proteostasis

homeostatic regulation of protein synthesis, folding, trafficking, and degredation

striatum

a cluster of neurons composed of distinct neuronal cell types/pathways within the basal ganglia (a subcortical part of the cerebrum) that consists of the caudate nucleus and putamen; important for control of voluntary movements, action planning, decision making, learning, reward processing, and motivation

neural circuits in the striatum

direct pathway- facilitates movements (neurons have D1 dopamine receptors)

indirect pathway- inhibits movements (neurons have D2 dopamine receptors)

loss of indirect pathway neurons in the striatum

causes LESS inhibition of movement, therefore causing more unintended movements; these neurons degenerate first in Huntington's disease, causing chorea

loss of direct pathway neurons in striatum

causes LESS facilitation of movement, therefore causing reduced ability to control intended movements; these neurons degenerate second in huntington's disease, causing rigidity and motor impairment

huntington's disease from molecule to behavior:

1. autosomal dominant allele for huntingtin gene is inherited with abnormally large number of CAG repeats

2. expression of abnormal huntingtin protein with long polyQ tract

3. fragments of abnormal protein lead to neuronal degeneration

4. degeneration disproportionately affects circuits vulnerable to the abnormality in the striatum

5. affected circuits in the striatum play a critical role in motor control

6. motor control impairments manifest at a behavioral level

neurotransmitter mediated synaptic transmission

1. synaptic vesicles in the terminal buttons of a sending neuron release neurotransmitters into the synaptic cleft

2. neurotransmitters cross the synaptic cleft to the receiving neuron

3. neurotransmitters fit into receptor sites located on the receiving neuron

chemical synapse

a type of synapse at which a chemical (a neurotransmitter) is released from the axon of a neuron into the synaptic cleft, where it binds to receptors on the next structure (receptors can be ion channels or metabotropic receptors)

metabotropic receptors

act through second messengers

Life Cycle of a Neurotransmitter

1. Synthesis via chemical reactions 2. Storage in synaptic vesicles 3. Synaptic release via exocytosis 4. Receptor binding 5. clearing and reuptake/break down

neurotransmitter classification

bases on structure, mode of action, and location

glutamate and GABA

common neurotransmitters for ionotropic signaling in the brain

neuromodulatory neurotransmitters

act via metabotropic signaling (ex: biogenic amines)

for the most part, a neuron has ________ neurotransmitter that it releases, but can be affected by __________ neurotransmitters.

one; multiple (via many different types of receptors to have different downstream effects)

neurotransmitter synthesis

starts from precursor molecules (e.g. amino acids) and is regulated by enzymes expressed by neurons

neurotransmitter clearance

neurotransmitters are cleared from the synaptic cleft via:

1. diffusion- passive movement of neurotransmitters to other areas (relatively slow)

2. active transport- transporters on neurons and/or glia remove neurotransmitters

3. degradation- enzymes in synaptic cleft or intracellular enzymes break down neurotransmitters

dopamine life cycle

dopamine is synthesized in two steps from the amino acid tyrosine through intermediary molecule DOPA (both steps catalyzed by specific enzymes); the dopamine transporter mediates re-uptake of free dopamine in the synaptic cleft back into the presynaptic terminal. dopamine is degraded by monoamine oxidase (MAO) and catechol-o-methyl transferase (COMT)

dopamine acts as a

neuromodulator

neuromodulators

act via metabotropic receptors with effects that are slower and longer lasting than ionotropic effects

ascending neuromodulatory systems

multiple systems in the brain that use distinct neurotransmitters, including dopamine, norepinephrine, serotonin, and acetylcholine; these systems have different brain stem areas as their sources and have highly divergent projection targets

glutamate

excitatory neurotransmitter

GABA

inhibitory neurotransmitter

GPe

globus pallidus external segment

STN

subthalamic nucleus

GPi

globus pallidus internal segment

SNr

substantia nigra pars reticulata

SNc

substantia nigra pars compacta; provides dopamine input to striatum

D1 vs D2 dopamine receptors

both are metabotroic; D1 is excitatory and D2 is inhibitory ; note: dopamine is a neuromodulator

Parkinson's disease effect on basal ganglia

leads to cell death that is most severe in dopamine neurons of the SNc, reducing dopamine neuromodulation of the striatum

bradykinesia

slowness of moevement

parkinson's disease signs/symptoms

bradykinesia, resting tremor, muscle rigidity, sleep disorders, control of internal organs (autonomic nervous system), and cognitive impairment; greater prevalence and progression with old age

Familial vs sporadic Parkinson's Disease

familial: monogenic (recessive and dominant)/inherited (earlier onset)

sporadic: non-genetic (majority of cases)

likelihood of parkinson's disease

depends on genetic and environmental factors that interact to increase/decrease risk (canyon metaphor)

lewy body inclusions in parkinson's disease

a specific type of protein aggregate within cells that displace other components, composed of multiple proteins with alpha-synuclein prevalent

note: lewy bodies are not the toxic element; they sequester toxic intermediates

alpha-synuclein

the protein encoded by the SNCA gene; it is abundant in the brain and has prominent alpha helix motifs in its normal structure; point-mutations in the protein can cause Parkinson's with autosomal dominance and full penetrance

alpha-synuclein role in protein degradation

the alpha helix motifs can attach to the cell membrane; this motif can take an alternate configuration that promotes beta-sheets formed from alpha-synuclein aggregates, leading to lewy body formation

rate model of Parkinson's disease

states that its effects are due to an imbalance of activity in direct and indirect pathways of the basal ganglia caused by dopamine loss

decreased dopamine effects

-reduces activity in the direct pathway via less D1 receptor excitation

-increases activity in the direct pathway via less D2 receptor inhibition

-more bursts of action potentials, increased oscillations, and increased synchronous action potentials across neurons

(both effects inhibit movement, explaining bradykinesia in parkinson's disease)

pattern hypothesis of parkinson's disease

suggests that changes in response patterns due to dopamine loss play a role in its effects, beyond just changes in overall balance between direct and indirect pathways

progression of parkinson's pathology

the most severe neurodegeneration occurs in the SNc, but it usually begins elsewhere (braak staging); parkinson's affects the motor loop of the basal ganglia most severely, then progresses to other loops

Braak staging

degeneration progressing from more peripheral to more central areas in parkinson's disease

molecular treatments for parkinson's

most effective treatment: medication with L-DOPA, the intermediate precursor to dopamine in its synthesis pathway (can cross the blood-brain barrier); often paired with peripheral blockers of amino acid decarboxylase that doesn't cros the blood-brain barrier, to counteract side effects

note: dopamine release, receptor effects, clearance, and degradation are all potential targets for treatments, but non are as effective for parkinson's as L-DOPA

cellular treatments for parkinson's

goal is to slow, stop, or prevent disease progression. the best potential target depends on the molecular cascade that leads to toxicity (but these approaches have not yet been successful)

neural circuit treatments for parkinson's

1. replacement- artificially replace SNc neurons that are lost (not yet successful)

2. removal- artificial lesioning of STN mitigates symptoms, but is highly invasive

3. deep brain stimulation- most effective surgical approach (often used after L-DOPA); regular, artificial high frequency stimulation to STN that may block or override abnormal STN activity

RNA modification

RNA is transcribed from DNA into pre-mRNA that includes introns (removed) and exons (remain). Removal of introns is RNA splicing and produces a mature mRNA that can then be translated into a protein

multiple exons allow for

alternative splicing (the inclusion of different exons in the mature mRNA) that results in different proteins

protein post-translational modifications

reversible and irreversible ways to alter proteins; useful for protein function/regulation

reversible protein modifications

phosphorylation/dephosphorylation; a phosphoryl group is added or removed from amino acids in a protein

irreversible protein modification

protein cleavage, where a protein is cut into separate parts

protein targeting

the control of protein localization (vis ER, golgi, etc); protein function depends on proteins being in the right place

axon length effect on protein targeting

proteins are synthesized in cell bodies, so they may have to be transported great distances along the axon. Diffusion would take years, thus proteins travel via active transport with microtubules

axonal transport via microtubules

microtubules (cylindrical cytoskeleton polymers of the protein tubulin) extend along the length of axons and regulate transport (kinesin and dynein); this is a fast transport mechanism essential for neuron function

protein kinesin

molecular motors that allow for anterograde transport (cell body towards axon terminal) that move proteins, organelles, etc

protein dynein

molecular motors that allow for retrograde transport (towards cell body from axon terminal) that move proteins, organelles, etc

protein-mediated toxicity in neurodegeneration

leads to cell death in both huntington's and parkinson's disease; reinforces the importance of proteostasis for neurodegenerative disorders

mild cognitive impairment (MCI)

the first symptoms of Alzheimer's disease, which involves noticeable cognitive decline beyond expectations from normal aging (individuals have preserved activities of daily living); often leads to dementia

dementia

impairment in thinking and memory severe enough to interfere with daily abilities (not necessarily alzheimers)

familial vs sporadic alzheimers

familial- genetic, more likely early onset

sporadic- not inherited, more common

two distinguishing features of alzheimer's disease via neuropathology

amyloid plaques and neurofibrillary (tau) tangles

amyloid plaques (AB)

extracellular aggregates of amyloid beta within brain gray matter; often contain degenerative neural structures and glia

neurofibrillary tangles

intracellular aggregates of tau protein in cell bodies, dendrites, and axons

Amyloid Precursor Protein (APP)

a transmembrane protein (mainly extracellular) with a high abundance at synapses; thought to be involved in synaptic formation and plasticity; the precursor of amyloid beta (AB), which is a segment of APP partially embedded in the membrane in uncleaved APP

cleavage of APP into amyloid beta

cleavage occurs when protein is bound to the membrane; APP has multiple sites of cleavage near AB: an alpha site near the middle of AB, a beta site on the N terminal side of AB, and two gamma sites on the C terminal side of AB

Note: the production of AB depends on the order of cleavage

secretases

membrane bound enzymes that mediate cleavage events; thus, cleavage events only occur when APP or segments of it are bound to the membrane