Suppression of astrocytic autophagy by αB-crystallin contributes to α-synuclein inclusion formation

Background: Parkinson's disease (PD) is characterized by a chronic loss of dopaminergic neurons and the presence of proteinaceous inclusions (Lewy bodies) within some remaining neurons in the substantia nigra. Recently, astroglial inclusion body has also been found in some neurodegenerative diseases including PD. However, the underlying molecular mechanisms of how astroglial protein aggregation forms remain largely unknown. Here, we investigated the contribution of alpha B-crystallin (CRYAB), a small heat shock protein, in alpha-synuclein inclusion formation in astrocytes. Methods: Small interfering RNA (siRNA)-mediated CRYAB (siCRYAB) knockdown or CRYAB overexpression was performed to investigate the impact of CRYAB on the autophagy in human glioblastoma cell line U251 cells. Coimmunoprecipitation (co-IP) and immunoblotting were used to dissect the interaction among multiple proteins. The clearance of alpha-synuclein in vitro was evaluated by immunocytochemistry. CRYAB transgenic mice and transgenic mice overexpressing A30P mutant form of human alpha-synuclein were used to examine the influence of CRYAB to alpha-synuclein accumulation in vivo. Results: We found that knockdown of CRYAB in U251 cells or primary cultured astrocytes resulted in a marked augmentation of autophagy activity. In contrast, exogenous CRYAB disrupted the assembly of the BAG3-HSPB8HSC70 complex via binding with BAG3, thereby suppressing the autophagy activity. Furthermore, CRYAB-regulated autophagy has relevance to PD pathogenesis. Knockdown of CRYAB remarkably promoted cytoplasmic clearance of alpha-synuclein preformed fibrils (PFFs). Conversely, selective overexpression of CRYAB in astrocytes markedly suppressed autophagy leading to the accumulation of alpha-synuclein aggregates in the brain of transgenic mice expressing human alpha-synuclein A30P mutant. Conclusions: This study reveals a novel function for CRYAB as a natural inhibitor of astrocytic autophagy and shows that knockdown of CYRAB may provide a therapeutic target against proteinopathies such as synucleinopathies.