L1 Verbeek: Mechanisms Parkinson’s Disease
Neuropathology of PD: deficit in dopamine production.
Alpha-synuclein: aggregation & propagation
Monoamine neurotransmitters (metabolism)
Phenylalanine is hydroxylated to tyrosine, tyrosine is
hydroxylated to LevoDopa.
L-Dopa is converted into Dopamine by AADC.
Dopamine is finally converted into HVA (homovanillic
acid).
Excess L-Dopa can also be converted into VLA (waste
product), this pathway is only relevant when there is
an excess of L-Dopa.
Dopamine is stored in NT vesicles, which
will be released in the synaptic cleft.
Dopamine binds to dopamine receptors,
molecular signals will be active in the
postsynaptic cell.
Dopaminergic neurons are located in the Substantia Nigra.
There are many mechanisms for PD, the most important/relevant:
*Alpha synuclein
Alpha synuclein accumulation lewy bodies: intracellular. But also Lewy neurites:
accumulation in dendrites/axons.
Alpha synuclein accumulation leads to neuronal cell loss in Substantia Nigra; loss of
dopaminergic neurons which leads to loss of motor functions.
Alpha synuclein: protein. The normal function is not well understood, but it might play a role in
clustering SVs in synapses and might help to regulate the release of dopamine.
(Alternative) splicing could lead to
different synuclein variants, the full
length synuclein has 140 AA.
1
,Lewy bodies form by pathological aggregation of alpha-synuclein:
Monomeric protein natively unfolded in solution.
Monomers adopt a misfolded state.
Aggregates into amyloid structures.
(proto)Fibrils are enriched in Beta-structures.
Oligomers and protofibrils are thought to be cytotoxic.
-lag phase
small amount of monomeric protein is converted to nucleation competent
oligomers
-elongation phase
at critical concentration rapid association of monomers to the nuclei occurs
elongation of aggregates into amyloid fibrils
-equilibrium
aggregation is complete
-Detection assay of aggregation: Thioflavin T
only fluorescent when bound to B-sheets. Increase in fluorescence = increase in fibril formation.
-Detection of secondary structure of proteins: Circular dichroism
Secondary structures: alpha helix, beta sheet, random coil.
-Visualization of fibrils: EM (electron microscopy). Example: amyloid beta fibrils
-Visualization of (proto)fibrils: Atomic force microscopy
2
, Clinical diagnosis happens in stage 3 (midbrain)
*Propagation of protein aggregation
Propagation of Tau vs. Alpha-synuclein proteins:
Temporo-spatial spreading of: tau-positive neurofibrillary lesions in the
process of Alzheimer’s disease (left) and alpha-synuclein-positive lesions
(Lewy bodies and neurites, right) in the process of Parkinson’s disease.
Mechanisms of propagation:
- Protein aggregation
- Cell-to-cell transfer of alpha synuclein
- Fluid transport in CSF
Cell-to-cell propagation
Proposed routes of intracellular transfer:
1) Synaptic transfer: misfolded proteins may be released
from cells through synaptic vesicle fusion with the plasma
membrane and thereafter be taken up by the
postsynaptic neuron.
2) Tunnelling nanotubes (TNTs) have been implicated in
the transfer of prion, alpha-synuclein etc.
3) Phagocytosis: demonstrated for Amyloid-beta
4) Exocytosis of exosomes: The fusion of multivesicular
bodies (MVB) with the plasma membrane may release
exosomes containing intraluminal aggregate-containing
vesicles (exosomes) that can be internalized by
neighbouring cells.
5) Lysosomal exocytosis: fusion of lysosomes with the
plasma membrane can release naked aggregates into the
extracellular space.
Model of prion-like protein aggregate-induced formation
of TNTs and intracellular spreading:
3
Neuropathology of PD: deficit in dopamine production.
Alpha-synuclein: aggregation & propagation
Monoamine neurotransmitters (metabolism)
Phenylalanine is hydroxylated to tyrosine, tyrosine is
hydroxylated to LevoDopa.
L-Dopa is converted into Dopamine by AADC.
Dopamine is finally converted into HVA (homovanillic
acid).
Excess L-Dopa can also be converted into VLA (waste
product), this pathway is only relevant when there is
an excess of L-Dopa.
Dopamine is stored in NT vesicles, which
will be released in the synaptic cleft.
Dopamine binds to dopamine receptors,
molecular signals will be active in the
postsynaptic cell.
Dopaminergic neurons are located in the Substantia Nigra.
There are many mechanisms for PD, the most important/relevant:
*Alpha synuclein
Alpha synuclein accumulation lewy bodies: intracellular. But also Lewy neurites:
accumulation in dendrites/axons.
Alpha synuclein accumulation leads to neuronal cell loss in Substantia Nigra; loss of
dopaminergic neurons which leads to loss of motor functions.
Alpha synuclein: protein. The normal function is not well understood, but it might play a role in
clustering SVs in synapses and might help to regulate the release of dopamine.
(Alternative) splicing could lead to
different synuclein variants, the full
length synuclein has 140 AA.
1
,Lewy bodies form by pathological aggregation of alpha-synuclein:
Monomeric protein natively unfolded in solution.
Monomers adopt a misfolded state.
Aggregates into amyloid structures.
(proto)Fibrils are enriched in Beta-structures.
Oligomers and protofibrils are thought to be cytotoxic.
-lag phase
small amount of monomeric protein is converted to nucleation competent
oligomers
-elongation phase
at critical concentration rapid association of monomers to the nuclei occurs
elongation of aggregates into amyloid fibrils
-equilibrium
aggregation is complete
-Detection assay of aggregation: Thioflavin T
only fluorescent when bound to B-sheets. Increase in fluorescence = increase in fibril formation.
-Detection of secondary structure of proteins: Circular dichroism
Secondary structures: alpha helix, beta sheet, random coil.
-Visualization of fibrils: EM (electron microscopy). Example: amyloid beta fibrils
-Visualization of (proto)fibrils: Atomic force microscopy
2
, Clinical diagnosis happens in stage 3 (midbrain)
*Propagation of protein aggregation
Propagation of Tau vs. Alpha-synuclein proteins:
Temporo-spatial spreading of: tau-positive neurofibrillary lesions in the
process of Alzheimer’s disease (left) and alpha-synuclein-positive lesions
(Lewy bodies and neurites, right) in the process of Parkinson’s disease.
Mechanisms of propagation:
- Protein aggregation
- Cell-to-cell transfer of alpha synuclein
- Fluid transport in CSF
Cell-to-cell propagation
Proposed routes of intracellular transfer:
1) Synaptic transfer: misfolded proteins may be released
from cells through synaptic vesicle fusion with the plasma
membrane and thereafter be taken up by the
postsynaptic neuron.
2) Tunnelling nanotubes (TNTs) have been implicated in
the transfer of prion, alpha-synuclein etc.
3) Phagocytosis: demonstrated for Amyloid-beta
4) Exocytosis of exosomes: The fusion of multivesicular
bodies (MVB) with the plasma membrane may release
exosomes containing intraluminal aggregate-containing
vesicles (exosomes) that can be internalized by
neighbouring cells.
5) Lysosomal exocytosis: fusion of lysosomes with the
plasma membrane can release naked aggregates into the
extracellular space.
Model of prion-like protein aggregate-induced formation
of TNTs and intracellular spreading:
3
