The protein alpha-synuclein has been linked to Parkinson’s disease (PD). The prevailing hypotheses state that this protein is directly toxic to neurons and thus the removal of alpha-synuclein from neurons is considered a viable therapeutic option. However, we have compelling evidence that show that alpha-synuclein is required for dopamine neuron survival and that its removal results in neurodegeneration. The objective of this proposal is to test the novel hypothesis which states that the role of alpha-synuclein in PD pathogenesis can be explained as a toxic loss-of-function.
Gene-therapy mediated delivery of alpha-synuclein to pre-clinical model neurons results in the formation of alpha-synuclein aggregates and subsequent neurodegeneration. In this proposal we will examine whether the toxicity observed in these models of PD is the result of the recruitment of endogenous alpha-synuclein into these aggregates, which effectively depletes the neuron of this crucial protein. We will utilize a mutated form of alpha-synuclein, which is incapable of forming aggregates. We will first confirm that this novel form of alpha-synuclein retains its function despite these mutations. Secondly, in order to directly test whether maintaining alpha-synuclein levels can protect against the neurodegeneration observed in these models, we will utilize viral gene-therapy vectors to deliver this non-aggregatable form of alpha-synuclein during the toxic insult. Finally, we will determine whether endogenous alpha-synuclein is indeed depleted in these models.
Relevance to Diagnosis/Treatment of Parkinson’s Disease:
This study aims to confirm that alpha-synuclein-mediated neurodegeneration is, in fact, due to a toxic loss-of-function. If proven true, therapeutic approaches aimed at removing this protein would no longer be consider safe. Rather, it would guide future therapeutic efforts toward maintaining alpha-synuclein homeostasis in the neuron.
Upon the successful completion of this project we aim to have established that A) Endogenous alpha-synuclein gets recruited into experimentally induced aggregates, B) Our non-aggregatable form of alpha-synuclein retains function, and C) Maintaining alpha-synuclein homeostasis in the neuron is protective, thereby demonstrating that neurodegeneration is due to loss of functional alpha-synuclein.
These studies have yielded the following findings: Gene therapy-mediated overexpression of the soluble form of alpha-synuclein results in partial protection against neurodegeneration in the substantia nigra (SN) following our toxic loss-of function paradigm (gene therapy-mediated removal of endogenous pre-clinical model alpha-synuclein). This tells us that alpha-synuclein retains its function despite the insertion of the mutations that render it soluble. In contrast, overexpression of human wildtype alpha-synuclein (normally toxic) did not rescue neurons in this paradigm. We thereafter tested our central hypothesis by co-expressing alpha-synuclein together with wt human alpha-synuclein. Alpha-synuclein treated pre-clinical models exhibited improved motor behavior and upon a histological analysis we found that treated neurons displayed a healthier appearance than that seen in control pre-clinical models despite the appearance of aggregates. Our findings suggest that maintaining levels of soluble alpha-synuclein partially protects against the insult that occurs with alpha-synuclein aggregation.