The overall goal of the UCLA UDALL center is to elucidate mechanisms of early dysfunction in cellular and animal models of Parkinson disease (PD) that can eventually be translated into novel neuroprotective treatments for PD in humans. Basic and clinical research evidence is accumulating that synaptic and cellular dysfunctions not only precede dopaminergic cell loss but also affect non-dopaminergic neurons. The identification of genetic mutations responsible for familial forms of PD offers the potential to glean new insight into the mechanisms underlying the more common sporadic forms of the disease. Mutations in the parkin gene cause an early-onset autosomal recessive form of PD, but are also found in patients with late- onset forms of PD indistinguishable from idiopathic PD. This project builds upon our discovery of four novel parkin-binding proteins, synaptotagmin (syt)1 and syt11, a synapsin-like-protein (SLP), and ataxin-2, a polyQ domain protein. SLP, syt11, and ataxin-2 are found in Lewy bodies of PD patients. The interaction of parkin with ataxin-2 is of particular importance in light of recent findings that polyQ expansion in the SCA2 (ataxin-2) gene are found in patients with late-onset tremor-predominant PD without ataxia or slow saccadic eye movements. We will test the following hypotheses: 1) Parkin ubiquitinates and facilitates degradation of these parkin interactors. 2) Over-expression of these parkin interactors causes neurotransmitter homeostatic dysfunction that progresses to cell death in vitro. 3) Dysfunction and death are prevented by co-expression of parkin, but not by co-expression of parkin mutants. 4) These effects are not restricted to primary dopaminergic neurons, but are also seen in hippocampal glutamatergic neurons.
Three specific aims are proposed to test these hypotheses: 1) We will characterize the interaction of parkin with these novel proteins and determine whether parkin accelerates degradation of the proteins in cell-lines. We will also investigate whether mutant parkins lose the ability to bind these proteins or to ubiquitinate them. 2) In cultured cell-lines, we will determine whether exogenous expression of parkin interactors results in alterations in dopamine release and metabolism and cell death and whether co-expression of parkin can prevent these changes. 3) We will examine the mechanism by which these parkin interactors may regulate synaptic transmission and cell survival in primary dopaminergic and glutamatergic neurons utilizing measures of synaptic vesicle recycling. This project is closely integrated with Projects 2 and 3, in which parkin-mediated synaptic dysregulation will be studied in vivo, in brain slices and in primary cultured neurons. Our experiments will be particularly informative for the translation of our basic research findings to the diagnosis and treatment of PD (Project 5), hopefully preventing significant neuronal loss.
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