The proSAAS chaperone provides neuroprotection and attenuates transsynaptic α-synuclein spread in rodent models of Parkinson's disease

Parkinson’s disease is a devastating motor disorder involving the aberrant aggregation of the synaptic protein synuclein (aSyn) and degeneration of the nigrostriatal dopaminergic tract. We previously showed that proSAAS, a small secreted chaperone protein widely expressed in neurons within the brain, is able to block aSyn‐induced dopaminergic cytotoxicity in primary nigral neuron cultures. We show here that coinjection of proSAAS‐encoding lentivirus profoundly reduced the motor asymmetry caused by unilateral nigral AAV‐mediated human aSyn overexpression. This positive functional outcome was accompanied by significant amelioration of the human aSyn‐induced loss of both nigral tyrosine hydroxylase‐positive cells and striatal tyrosine hydroxylase‐positive terminals, demonstrating clear proSAAS‐mediated protection of the nigro‐striatal tract. ProSAAS overexpression also reduced the content of human aSyn protein in both the nigra and striatum and reduced the loss of tyrosine hydroxylase protein in both regions. Since proSAAS is a secreted protein, we tested the possibility that proSAAS is able to block the transsynaptic spread of aSyn from the periphery to the central nervous system, increasingly recognized as a potentially significant pathological mechanism. The number of human aSyn‐positive neurites in the pons and caudal midbrain of mice following administration of human aSyn‐encoding AAV into the vagus nerve was considerably reduced in mice coinjected with proSAAS‐encoding AAV, supporting proSAAS‐mediated blockade of transsynaptic aSyn transmission. We suggest that proSAAS may represent a promising target for therapeutic development in Parkinson’s disease.