Transcript Protein-Misfolding Diseases

Protein-Misfolding Diseases
PHY6940
April 8th 2009
Jessica Nasica
Overview of the presentation
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General facts on protein folding
Causes of misfolding and protein aggregation
Cellular consequences of protein aggregation
Protein-misfolding diseases
Amyloidoses
Amyloid fibril formation
Neurodegenerative diseases and Alzheimer’s disease
Therapy solutions
Curing Alzheimer’s disease
General facts on protein folding
only 1 amino acid sequence
General facts on protein folding
Possible starting configurations : 10^16
The “new view”
Compact configurations : 10^10
• A stochastic search of the many conformations
accessible to one particular sequence
• A polypeptide chain searches for the lowest-energy state
by a process of trial and error
Transition states : 10^3
• Native-like interactions between residues are more
stable than non-native ones
• To undergo correct folding, the native structure
must
Lowest energy
state go
Native conformation : 1
through a transition state
General facts on protein folding
The Protein Quality Control (PQC) system
found in ER
and cytosol
General facts on protein folding
Role of chaperones
• To help proteins in their folding process
• To unfold misfolded proteins before their degradation by
the proteasome unit
• To protect proteins from interfering interactions during
folding
General facts on protein folding
Chaperones
The concentration of chaperones is genetically selfregulated and increases with the presence of misfolded
proteins.
They possess an ATPase domain that reversibly binds
with the hydrophobic parts of partially folded proteins
General facts on protein folding
• Important elements for protein folding:
– The amino acid sequence
– The right cellular environment
(T ,P ,pH , etc …)
– The perfect balance between the various folding
states
– A fully functional Protein Quality Control (PQC)
system
(Chaperones, proteasome unit )
α-synuclein gene mutations (Parkinson’s)
Causes of misfolding and protein aggregation
3 main causes leading to aggregation :
• Genetic mutations resulting in a substitution, deletion or
addition of amino acids
• Changes in environmental conditions of the cell leading
to dysfunction of the PQC system or the mitochondria
and/or the creation of unwanted interactions with folding
proteins
• Post-translational accidental changes to the polypeptide
after RNA-translation into amino-acids
Causes of misfolding and protein aggregation
How protein aggregates form
• Change in cellular conditions  more misfolded proteins
 the PQC system is overwhelmed -> aggregation is
favored
• Aggregation is thought to be set in by protein segments
containing hydrophobic amino acids residues, β-sheet
predisposition and low net charge.
Causes of misfolding and protein aggregation
States accessible to a protein molecule
Free-energy folding landscape for
chaperone-mediated protein folding
Causes of misfolding and protein aggregation
Protein aggregation is a 2-stage event
1.
2.
The nucleation  proteins start attaching reversibly to
a growing nucleus
Proteins attach irreversibly to the nucleus until it
becomes a larger aggregate.
Cellular consequences of protein aggregation
• Loss-of-function pathogenesis: if misfolded proteins are
prematurely degraded by PQC system
 protein deficiency disease
• Gain-of-function pathogenesis: if misfolded proteins are
not eliminated but accumulated instead
 disease pathology
 toxicity
Some diseases display both pathogenic mechanisms.
Cellular consequences of protein aggregation
+
the slowing down of polypeptides translation
Protein-misfolding diseases
include conditions where a protein:
• fails to fold correctly
(cystic fibrosis, Marfan syndrome, amyotonic lateral sclerosis)
• is not stable enough to perform its normal function
(many forms of cancer)
• fails to be correctly trafficked
(familial hypercholesterolemia, α1-antitrypsin deficiency)
• forms insoluble aggregates that deposit toxically
(neurodegenerative diseases: Alzheimer’s, type II diabetes,
Parkinson’s and many more)
Protein-misfolding diseases
Protein-misfolding diseases
Conditions may be :
• Familial  the disorder is genetically inherited and symptoms
appear during childhood ( e.g., Huntington )
• Sporadic  patternless and characterized by a late onset. Primarily
due to aging or to an incorrect lifestyle.
Not associated with gene mutations. (e.g., most of Alzheimer’s and
Parkinson’s cases and many more)
• Transmissible  (e.g., prion disease, spongiform encephalopathies
and fatal familial insomnia)
A closer look at amyloidoses
Main family of protein-misfolding diseases  most
clinically relevant due to the high occurrence of
neurodegenerative diseases and type II diabetes
2 types:
• Systemic amyloidoses  large amounts of fibrils
accumulate everywhere.
• Organ-limited amyloidoses  fibrils accumulate locally in
one organ ( e.g., brain )
A closer look at amyloidoses
Characterized by the formation, accumulation and
deposition of similar highly-organized insoluble fibrillar
aggregates  amyloid fibrils
AFM image of β2-microglobulin amyloid fibrils
Causes of the formation of amyloid fibrils
Amyloid fibril formation
Structural properties
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Insoluble fibrous aggregates
Specific optical behavior (binds to dye “Congo red”)
Highly organized macrostructure a few nm in diameter
Characterized by a cross-β quaternary structure
Cross-β composed of 2-β sheets facing each other,
closely interacting
• β-sheets have their β-strands perpendicular to fibril axis
Laser scanning confocal microscopy
Amyloid fibril formation
Mechanism of formation  3 steps:
1.
Alignment of the molecules to form β-sheets  fastest
stage  involves H-bonds
2.
Formation of the cross-β structure  slower than step
1  involves Van-der-Waals forces  interdigitation of
residues side chains  “steric zipper” structure
3.
Fibril formation  involves non-covalent bonds
Amyloid fibril formation
Amyloid fibril formation is a nucleated-growth process
 presence of a pre-formed nucleus
 rapid growth: additional proteins added more
easily once the large entropy barrier is overcome
Amyloid fibrils are stabilized by the protein concentration
and by the formation of steric zippers
Aggregation rates depend on the charge, secondary
structure propensities, hydrophobicity and length of the
proteins
The efficiency of the PQC system is also very important
Toxicity of amyloid fibrils
annular pores
Schematic representation of structural species during amyloid formation.
Protofibrils may form spherical, chain-like or annular pore-like structures to go
through the cell membranes
Consequences of amyloid fibril
formation
Neurodegenerative diseases
They are localized amyloidoses affecting the brain
& the most-occurring type of age-related PMDs
Neurodegenerative diseases
Aggregation happens in neurons in the brain & spinal cord
deterioration of neurons or their myelin sheath
dysfunction
movement coordination
impaired
memory loss
Schematic view of a neuron
Alzheimer’s disease
 most common progressive neurodegenerative disorder
 massive loss of neurons
 Accumulation of Aβ-amyloid protein (extracellular)
 Accumulation of tau protein (intracellular)
 Formation of amyloid plaques (Aβ )
 Formation of neurofibrillar tangles or NFTs ( tau protein )
Plaques
NFTs
Alzheimer’s disease
Effects on the brain
Brain shrinkage
Alzheimer’s disease
Resulting symptoms
• Memory impairment
• Inability to think by themselves
• Inability to function independently
Parkinson’s disease
 2nd most common neurodegenerative disease due to
aging
 Degeneration of specific dopaminergic neurons in the
substantia nigra
 Accumulation of α-synuclein protein in presence of
dopamine
Α-synuclein accumulating neurons
 Other neurons are not affected
Parkinson’s disease
Resulting symptoms
• Muscular rigidity  extreme pain
• Postural instability
• Resting tremor
Huntington’s disease
• Polyglutamine disease  mutation encoding for an
addition of Q amino-acids
• Accumulation of the misfolded protein Huntingtin
• Formation of toxic inclusions in brain cells
• Degeneration of glutamatergic striatal neurons
polyQ inclusion
In neocortex
Therapeutic solutions
3 main approaches:
1.
Inhibition of protein aggregation
2.
Interference with post-translational peptide changes
before the misfolding/aggregation step
3.
Upregulation of molecular chaperones or aggregateclearance mechanisms
Therapeutic solutions
1.
Inhibition of amyloid ß (Aß) fibril neurotoxicity by laminin.
Therapeutic solutions
2.
• Targeting Aβ-formation by inhibiting β- and γ- secretase
proteins responsible for the formation of amyloid plaques
• Inhibition of tau protein phosphorylation
(hyperphosphorylation of tau protein is responsible for its
aggregation)
Therapeutic solutions
3.
Clonidine and Minoxidil enhance the clearance of
aggregate prone proteins, including mutant
Huntingtin and mutants of α-synuclein
Curing Alzheimer’s disease
 Disease affecting 37 million people worldwide
 In 40 years, 1 in 85 people will develop AD
 Current drug research aims at preventing Aβ-formation
and blocking the formation of amyloid plaques to slow
the progression of the disease
 A few programs targeting the tau protein
Curing Alzheimer’s disease
Preventing Alzheimer’s disease
Thank you !