Decreased parkin solubility is associated with impairment of autophagy in the nigrostriatum of sporadic Parkinson’s disease

Parkinson’s disease (PD) is a motor disorder that involves death of dopaminergic neurons in the substantia nigra pars compacta. Parkin is an autosomal recessive gene that is mutated in early onset PD. We investigated the role of parkin and autophagic clearance in postmortem nigrostriatal tissues from 22 non-familial sporadic PD patients and 15 control samples. Parkin was insoluble with altered cytosolic expression in the nigrostriatum of sporadic PD. Parkin insolubility was associated with lack of degradation of ubiquitinated proteins and accumulation of α-Synuclein and parkin in autophagosomes, suggesting autophagic defects in PD. To test parkin’s role in mediating autophagic clearance, we used lentiviral gene transfer to express human wild type or mutant parkin (T240R) with α-Synuclein in the rat striatum. Lentiviral expression of α-Synuclein led to accumulation of autophagic vacuoles, while co-expression of parkin with α-Synuclein facilitated autophagic clearance. Subcellular fractionation showed accumulation of α-Synuclein and tau hyper-phosphorylation (p-Tau) in autophagosomes in gene transfer models, similar to the effects observed in PD brains, but parkin expression led to protein deposition into lysosomes. However, parkin loss of function mutation did not affect autophagic clearance. Taken together, these data suggest that functional parkin regulates autophagosome clearance, while decreased parkin solubility may alter normal autophagy in sporadic PD.

► Parkin is insoluble and accumulates in the nigrostriatum of sporadic PD brains.
► Decreased parkin solubility may alter normal autophagy in sporadic PD brains.
► Functional, not mutant, parkin gene transfer enhances autophagic clearance.
► Aging may reduce parkin solubility and function to regulate autophagic clearance.