An investigation on the pathogenic mechanism of mutant PINK1-induced hereditary Parkinson's disease

Parkinson’s disease (PD) affects ~ 1 % of the population above the age of 60 and is the most common neurodegenerative motor disorder. Recent studies indicated that numerous missense or truncating mutations of phosphatase and tesin homologue (PTEN)-induced kinase 1 (PINK1) gene are implicated in the pathogenesis of familial type 6 of Parkinson’s disease (PARK6) and that PINK1 is the second most frequent causative gene in early-onset Parkinson’s disease (PD)). PINK1 is believed to exert a neuroprotective effect on dopaminergic neurons in substantia nigra pars compacta (SNpc). PINK1 mutations associated with PARK6 could cause the loss of neuroprotective function and resulting degeneration of SNpc dopaminergic neurons and parkinsonism. The elucidation of molecular mechanism underlying PINK1 neuroprotective function is essential for understanding the pathogenesis of PARK6 caused by PINK1 mutations. PINK1 mutations have also been found in patients affected with sporadic early-onset parkinsonism. PARK6 and sporadic PD patients exhibit similar clinical features. Therefore, it is very likely that common molecular mechanisms are involved in the pathogenesis of both PARK6 and sporadic PD. Therefore, elucidation of the pathogenic mechanism underlying PARK6 is expected to shed a light on the molecular pathogenesis of more common sporadic PD. In the present study, we will investigate molecular mechanisms underlying the PINK1 neuroprotective function and pathogenic mechanism of PARK6 by performing the following investigations: (1) Our investigation using 293 cell lines expressing PINK1 strongly suggests that wild-type PINK1 produces the neuroprotective and anti-apoptotic effects by inhibiting mitochondrial release of apoptogenic proteins and that PARK6 mutant PINK1 loses its anti-apoptotic effect and causes the apoptotic death of SNpc dopaminergic neurons. PARK6 mutant PINK1 ubiquitously expressed causes the selective neurodegeneration of substantia nigra, suggesting that a cell-specific mechanism is responsible for mutant PINK1-induced neurotoxicity. In the present study, wild-type or PARK6 mutant PINK1 is expressed in primary neuronal culture of substantia nigra, which is vulnerable to mutant PINK1-induced neurotoxicity in vivo. Subsequently, we will study the molecular mechanisms by which PINK1 exerts its neuroprotective effect on SN dopaminergic neurons and mutant PINK1 induces substantia nigra neuronal death. (2) Our recent study indicates that PINK1 protein is mainly expressed in the mitochondria and that mitochondrial expression is necessary for PINK1-mediated protective effect. The Ser/Thr kinase domain is the only functional domain in PINK1 protein. Therefore, PINK1 functions as a mitochondrial Ser/Thr protein kinase and exerts its neuroprotective effect on SNpc dopaminergic neurons by phosphorylating unknown substrates in the mitochondria. Interestingly, most PINK1 mutations associated with PARK6 are found in the Ser/Thr kinase domain of PINK1. These findings strongly suggest that an impairment of mitochondrial Ser/Thr kinase activity caused by PINK1 mutations leads to the neurodegeneration of SNpc dopaminergic cells and PARK6. Identification of mitochondrial substrate(s) of PINK1-mediated phosphorylation is required to understand the molecular mechanism of PINK1 neuroprotective effects. In the present study, proteomic analysis will be performed to identify mitochondrial PINK1 substrate(s). (3) In the present study, we will study molecular mechanisms of PINK1 neuroprotective effect and mutant PINK1-induced neurotoxicity with aid of cultured substantia nigra neurons and in vitro identification of mitochondrial PINK1 substrate(s). Although in vitro cellular model has been widely used to study pathogenic mechanisms of neurological disorders, important findings observed from in vitro cellular model have to be corroborated by in vivo animal model. Furthermore, better understanding the molecular basis of mutant PINK1-induced degeneration of SNpc dopaminergic neurons in vivo is essential for the development of effective therapy for PARK6 or sporadic PD. The autosomal recessive inheritance mode indicates a loss-of-function caused by PINK1 mutations is involved in the pathogenesis of PARK6. Thus, we will investigate the molecular pathogenesis of PARK6 and physiological functions of PINK1 in vivo using mice lacking PINK1 expression (PINK1-/- mice).

Project ID:PG9603-0291
External Project ID:NHRI-EX96-9619NI