4. Protein Aggregation Diseases Flashcards
What are the two types of protein aggregates?
What is the difference between them?
- Non-amyloid aggregates
- Non cross-ß structure
- Causes sickle-cell anemia
- Amyloid aggregates
- Cross-ß structure
- Causes Alzheimer’s, Parkinson’s, cataractsm, etc…
sickle-cell anemia
- Disease characterized by low number of RBCs
- RBCs have a lifespan of 10-20 days (compared to a regular lifespan of 120 days)
- A single mutation in hemoglobin causes sickle-cell disease
- Causes non-amyloid aggregates
- Africa has the highest prevalence of sickle-cell disease in the world
- In the United States, African-Americans are more likely to have sickle-cell disease
What is the mutation that causes sickle-cell anemia?
The most common mutation that causes sickle cell anemia is a single nucleotide change from GAG to GTG at amino acid position 6 in the β-chain of Hb.
Mutation from GAG to GTG replaces Glu by Val at amino acid position 6 in the β-chain of Hb. Mutated β-chain is called as sickle(s) chain and mutated Hb as HbS.
What is the phenotype and genotype of an individual who inherits both sickle-cell disease genes?
A person will be born with sickle cell disease only if two genes are inherited—one from the mother and one from the father.
All hemoglobin molecules in patients consist of 2 alpha and 2 s (sickle or mutated beta) chains.
What is the genotype of an individual who is a carrier of the sickle cell trait?
A person who inherits just one gene is healthy and said to be a “carrier” of the disease or sickle cell trait. A carrier has an increased chance of having a child with sickle cell disease if he or she has a child with another carrier.
In these persons, half of their hemoglobin molecules consist of 2 alpha and 2 beta chains, and half consist of 2 alpha and 2 s chains.
Describe the mutation that causes sickle-cell anemia?
The mutation is from Glu to Val at 6th amino acid in the ß-chain, which is a charged residue to hydrophobic residue mutation.
Mutation makes Hb more hydrophobic at the site of mutation. Individual sickle Hb (Hb S) molecules stick to one another forming long rod-like fibers, twined as a helical bundle.
What structure do the aggregates look like in sickle-cell anemia? What does this ultimately cause the tertiary structure of Hb to look like?
- Aggregates are of α-helical structure.
- The content of α-helical structure is similar between sickle and adult hemoglobins.
- The interface between four polypeptide chains is more open.
What disease is the 5th leading cause of death?
- 5.3 million Americans have AD, 5th leading cause of death in those ≥65 years old. Following AD diagnosis, survival is typically 4-6 yrs.
Alzheimer’s disease is the most common cause of dementia in older people.
What is the typical age that Alzheimer’s disease presents?
- The prevalence of AD increases exponentially with age.
- Therefore, most cases present in people older than 65 years but about 5% of cases occur younger than age 65 and can be as young as 40 years.
What are the four risk factors of AD?
- Age
- In general, the autosomal dominant form of Alzheimer’s disease begins between age 40-60 years, and ‘late onset’ familial or sporadic Alzheimer’s disease begins between 70-85 years.
- Genetic Factors
- Alterations on chromosome 1, 14, or 21
- Head injury
- Gender
- Alzheimer’s disease is more common in women.
Note: Prevalence is independent of the race.
What are the cognitive clinical presentations of AD?
- Memory loss (poor recall and losing items)
- Language impairment
- Impaired executive function (unable to perform tasks or movements when asked)
- Disorientation (Impaired perception of time and unable to recognize familiar people)
What are the noncognitive clinical presentations of AD?
- Depression, hallucinations, delusions
- Behavioral disturbances (physical and verbal aggression, motor hyperactivity, uncooperativeness, wandering etc)
What are the functional clinical presentations of AD?
- Inability to care for self (dressing, bathing, toileting, and eating)
What is considered to be the fundamental cause of AD?
Provide evidence.
- Postulated that amyloid beta (Aβ) deposits are the fundamental cause of the disease.
- Support for this postulate comes from the location of the gene for the amyloid beta precursor protein (APP) on chromosome 21
- People with trisomy 21 (Down Syndrome) who thus have an extra gene copy almost universally exhibit AD by 40 years of age.
What is the pathophysiology that characterizes AD?
Alzheimer’s disease is characterized by loss of neurons and synapses in the cerebral cortex and certain subcortical regions.
In AD, misfolding occurs in what two proteins?
- Amyloid Plaques*-deposits of the beta-amyloid protein generated from Alzheimer amyloid precursor protein (APP) which accumulates in spaces between nerve cells. This interferes with the neuron to communicate with one another resulting in abnormal brain function and apoptosis.
- Neurofibrillary Tangles*-accumulate inside of nerve cells. They are deposits of tau proteins that are involved in microtubule formation.
Describe amyloid precursor proteins in a patient with AD.
Amyloid precursor protein (APP) is cleaved by β-secretase and γ-secretase to form the neurotoxic Aβ42 fragment that aggregates into the amyloid plaque that is a hallmark feature of AD. The gene for APP is located on chromosome 21. The exact function of APP is not known. The Aβ fragment has no structure in its native state.
Describe the role of Tau in a healthy neuron.
- Tau is a major microtubule associated protein (MAP) of a mature neuron.
- Its primary role is to stimulate tubulin assembly into microtubules and to stabilize neuronal cytoskeleton by interacting with microtubules.
- Tau binds to axonal microtubules and result in the formation of microtubule bundles.
Describe Tau in a patient with AD.
- Phosphorylation level of tau isolated from autopsied AD brain is 3-4 fold higher than normal human brains.
- Disruption in the balance of protein kinases and phosphatases contributes to abnormal phosphorylation of tau observed in AD.
- Results in the destabilization of microtubules and self-aggregation of tau into neurofibrillary tangles (NFTs).
- Break down of microtubule network leads to a compromised axonal transport and ultimately loss of synapses and degeneration.
Describe the kinetics of amyloid formation.
- Native protein, if structured, partially unfolds leading to the exposure of hydrophobic residues.
- More than one unfolded molecule with exposed hydrophobic patches stick with one another to form circular aggregates known as oligomers.
- Oligomers stack to form long protofibrils.
- Protofibrils twine around one another to form long amyloid fibrils.
- Amyloid fibrils precipitate resulting in large amyloid plaques.
Are oligomers or protofibrils toxic?
Oligomers.
Describe the difference between a normal ß-hairpin structure and a cross-ß helical structure.
- Cross-ß helical structure.
- Long ß-sheet is between two different molecules.
- ß-strands from the same molecule perpendicular to the long axis, but no H-bonds.
- H-bonds between ß-strands from different molecules parallel to the long axis.
Where do the side chains of the ß-amyloid structure protrude?
- Dry interface between the two sheets; Wet interface on the outside surfaces.
- Complementarity of side chains protruding into the dry space.
What are the two processes of amyloid growth involved in AD?
- Initial formation of small nuclei (nucleation)
- Fibril growth starting from nuclei (propagation)
What is the order of pathophysiological characteristics that occur before the onset of AD symptoms?
What protein becomes misfolded in Parkinson’s disease?
- A natively unstructured protein, a-synuclein, becomes misfolded.
- Forms amyloid-like filaments
- Dopaminergic neurons develop characteristic filamentous inclusions called Lewy Bodies
- Problem: accumulation of protein aggregates rich in β-pleated sheet conformation conferring toxic properties
What characterizes Parkinson’s Disease?
- Progressive neurodegenerative disorder characterized by tremor at rest, rigidity, bradykinesia, stooped posture, loss of postural reflexes, and shuffling gait.
- Autonomic dysfunction and cognitive deficits may occur with disease progression.
Describe the wild-type Huntington peptide.
- Necessary for normal embryonic development
- Helps with cell regulatory processes
- Gene is located on Chromosome 4
- Huntington gene is widely distributed throughout the brain and many tissues throughout the body
- Wild Type Huntington contains 6-34 Cytosine-adenine-guanine (CAG) trinucleotide repeats in the HTT gene near the amino terminus.
- CAG trinucleotide sequence codes for the amino acid glutamine
What is the genetic determinant of HD?
- Number of CAG repeats important in diagnosis, disease onset and severity.
- Toxicity of the Huntington gene occurs with expanded trinucleotide repeats
- More than 35 repeats: expanded, unstable, disease-causing
- 36-39 repeats: possibly disease causing
- More than 40 repeats: disease causing
In the picture below, what brain correlates to the toxicity of HD?
Neither; toxicity of Huntington does not correlate with degree of cell/ neuronal atrophy.
What are the four causes of ALS?
superoxide dismutase 1 (SOD1)
- Protects cells from superoxide toxicity by converting to harmless water.
- SOD1 is a homodimer in which each subunit binds one copper atom and one zinc atom. Cu carries out the chemical change. Zn binding promotes dimerization and stabilizes the native form.
- Mutations in SOD1 cause familial ALS.
What causes SOD1 to fold?
- ALS-causing SOD1 mutations promote production of Copper-deficient misfolded species.
- The aggregation is from a well-folded, structured protein, unlike the case of Alzheimer’s where the aggregation is from a natively unfolded protein.
How do amyloid aggregates cause toxicity? That is, what cell machinery is affected by misfolded proteins?
- Coaggregation of essential cytoplasmic components
- Poisoning of the proteasome thereby inhibiting timely degradation of many cellular proteins
- Saturation of cytoplasmic chaperones that catalyze essential protein folding and refolding
- Damaging mitochondria by aggregation onto the cytoplasmic surface and/or transport into the mitochondrial intermembrane space
prion disease
- Transmissible spongiform encephalopathies (TSEs)
- Fatal, degenerative neurological disease
- Replicate by binding to normal prion protein (PrPC) which converts to disease-causing prion protein (PrPSC)
- PrPC – rich in α-helices
- PrPSC – rich in ß-sheets
- Aggregation of PrPSC leads to disease
What misfolded protein causes cataracts?
- The lens is made of crystallin proteins
- Must be well-ordered for vision clarity
- Any disruption in the arrangement of the crystallins causes cataract.
- Crystallins also act as chaperones. Defective crystallins result in more protein aggregates.
What are the two types of crystallins that are relevant to cataracts?
Human γ D-crystallin and Human γ S-crystallin are monomeric two-domain major proteins of the human eye lens and significant components of mature-onset cataracts.
Mature-onset cataracts are thought to be an aggregated state of modified, damaged, or partially unfolded states of lens crystallins.
What are the five different types of cataracts?
