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Abstract:
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Synucleins represent a conserved family of small proteins that include α - , β - , and
γ - isoforms , which are highly expressed in neurons of the vertebrate nervous system . The
normal function of these proteins is not well understood . However , in humans α -
synuclein dysfunction is causatively linked to Parkinson’s Disease (PD ) , where it
abnormally accumulates in neuronal cell bodies as protein aggregates that are associated
with neuronal death . Although the associations between synuclein accumulation and
cellular death are established in PD , the extent to which this occurs in other contexts ,
such as neuronal injury , is unknown . Furthermore , the effects of synuclein aggregation
on the function of synapses , where synuclein is normally localized , are not well
understood . To address these questions I took advantage of the experimentally accessible
nervous system of the sea lamprey (Petromyzon marinus ) . I used molecular cloning and
phylogenetic analyses to characterize three lamprey synuclein orthologues , one of which
is highly expressed within a class of neurons called the giant reticulospinal (RS ) neurons .
Spinal cord injury induces the accumulation of synuclein protein only within a population
of poor surviving RS neurons , and this accumulation is correlated with cellular death .
Thus , similar to PD , the abundance of synuclein protein is associated with neuronal
toxicity . In a related project , I demonstrated that elevating synuclein levels at synapses , such as occurs in PD , is deleterious to synaptic function through an inhibition of synaptic
vesicle (SV ) recycling . By injecting excess synuclein protein directly into the axons of
giant RS neurons , and analyzing the ultrastructural morphology of synapses , I have
shown that clathrin -mediated synaptic vesicle endocytosis was greatly inhibited . The
conserved N -terminal domain was sufficient to inhibit vesicle recycling , and injecting
synuclein mutants with disrupted N -terminal α -helices caused reduced defects in SV
recycling . Therefore the α -helical structure of the N -terminus is necessary to inhibit SV
recycling at early stages of clathrin -mediated endocytosis . Binding interactions with
clathrin -mediated endocytosis components , such as the phosphoinositide lipid PI (4 )P
support this hypothesis . These studies provide a better understanding of the mechanisms
by which synuclein dysfunction leads to neuronal death after injury and synaptic
dysfunction in PD and other synuclein -associated diseases . |