Parkinson's Disease and Mechanisms of Fast Axonal Transport

Parkinson’s disease (PD) is characterized by loss of dopaminergic neurons in the substantia nigra and their connections to the striatum as well as accumulation of intracellular inclusions of α-synuclein called Lewy bodies and Lewy neurites. Mutations of α-synuclein have also been identified in cases of early onset familial PD. Taken together this suggests there may be a common pathogenic mechanism for both familial and sporadic PD. It has been established that misregulation of axonal transport leads to loss of synaptic function and previous results from our laboratory demonstrate that perfusion of squid axoplasm with α-synuclein increases retrograde and decrease anterograde transport. Additional experiments in the squid suggested that α-synuclein effects on transport are mediated via kinases such as non-receptor tyrosine kinases Src, Fyn and protein kinase C μ (PKCμ) consistent with an effect on the phosphorylation state of the molecular motors. We hypothesized that specific domains within α-synuclein alter regulatory pathways of fast axonal transport in affected neurons through a mechanism involving these kinases. To test this hypothesis specific domains within α-synuclein with a potential role in pathogenesis were identified, and the mechanism demonstrated in squid axoplasm, and tested in a cellular model. Squid axoplasm experiments confirmed that the C-terminus of pathogenic forms of α-synuclein activate non-receptor tyrosine kinases such as Src and Fyn. These in turn activate PKCμ, which through direct or indirect mechanisms, affects the phosphorylation of the motors. Tests of this pathogenic mechanism in SH-SY5Y cells stably expressed with wild type or A53T α-synuclein revealed that α-synuclein does not affect the expression of the molecular motors but may affect their phosphorylation status and therefore their ability to walk on microtubules or to carry cargo. In addition, the experiments revealed that PKCμ is tightly regulated in SH-SY5Y cells, which makes this cell line unsuitable for the study of this kinase in the context of α-synuclein expression. However, a protein that is immunologically related to PKCμ is dependent upon α-synuclein expression. Its identity, which may be key in PD, has yet to be discovered