Deficiency of RAB39B Activates ER Stress-Induced Pro-apoptotic Pathway and Causes Mitochondrial Dysfunction and Oxidative Stress in Dopaminergic Neurons by Impairing Autophagy and Upregulating α-Synuclein

Deletion and missense or nonsense mutation of RAB39B gene cause familial Parkinson's disease (PD). We hypothesized that deletion and mutation of RAB39B gene induce degeneration of dopaminergic neurons by decreasing protein level of functional RAB39B and causing RAB39B deficiency. Cellular model of deletion or mutation of RAB39B gene-induced PD was prepared by knocking down endogenous RAB39B in human SH-SY5Y dopaminergic cells. Transfection of shRNA-induced 90% reduction in RAB39B level significantly decreased viability of SH-SY5Y dopaminergic neurons. Deficiency of RAB39B caused impairment of macroautophagy/autophagy, which led to increased protein levels of alpha-synuclein and phospho-alpha-synuclein(Ser129) within endoplasmic reticulum (ER) and mitochondria. RAB39B deficiency-induced increase of ER alpha-synuclein and phospho-alpha-synuclein(Ser129) caused activation of ER stress, unfolded protein response, and ER stress-induced pro-apoptotic cascade. Deficiency of RAB39B-induced increase of mitochondrial alpha-synuclein decreased mitochondrial membrane potential and increased mitochondrial superoxide. RAB39B deficiency-induced activation of ER stress pro-apoptotic pathway, mitochondrial dysfunction, and oxidative stress caused apoptotic death of SH-SY5Y dopaminergic cells by activating mitochondrial apoptotic cascade. In contrast to neuroprotective effect of wild-type RAB39B, PD mutant (T168K), (W186X), or (G192R) RAB39B did not prevent tunicamycin- or rotenone-induced increase of neurotoxic alpha-synuclein and activation of pro-apoptotic pathway. Our results suggest that RAB39B is required for survival and macroautophagy function of dopaminergic neurons and that deletion or PD mutation of RAB39B gene-induced RAB39B deficiency induces apoptotic death of dopaminergic neurons via impairing autophagy function and upregulating alpha-synuclein.

Automated summary

Deletion and mutation of RAB39B gene cause Parkinson's disease by inducing degeneration of dopaminergic neurons through impaired autophagy function and upregulation of alpha-synuclein. RAB39B deficiency leads to activation of ER stress pro-apoptotic pathway, mitochondrial dysfunction, and oxidative stress, resulting in apoptotic death of dopaminergic cells. Mutant forms of RAB39B do not prevent the increase of neurotoxic alpha-synuclein and activation of pro-apoptotic pathway, unlike the wild-type RAB39B.