Identification of β-synuclein on secretory granules in chromaffin cells and the effects of α- and β-synuclein on post-fusion BDNF discharge and fusion pore expansion

alpha-Synuclein is strongly implicated in the pathogenesis of Parkinson's disease as well as in other neurodegenerative diseases. However, its normal function in cells is not understood. The N-termini of alpha-, beta-, and gamma-synuclein contains six to seven 11-amino acid repeats that are predicted to form amphipathic helices. Membrane-binding and membrane-curving abilities of synuclein raise the possibility that synuclein could alter cellular processes that involve highly curved structures. In the present study we examined the localization of endogenous synuclein in bovine chromaffin cells by immunocytochemistry and its possible function to control protein discharge upon fusion of the granule with the plasma membrane by regulating the fusion pore. We found with quantitative immunocytochemistry that endogenous beta-synuclein associates with secretory granules. Endogenous alpha-synuclein only rarely co-localizes with secretory granules. Overexpression of alpha-synuclein but not beta-synuclein quickened the post- fusion discharge of BDNF-pHluorin by approxinately 30%. However, neither alpha- nor beta-synuclein significantly altered curvature dynamics associated with fusion pore expansion that were measured by the combination of polarization and total internal reflection fluorescence microscopy (pTIRFM). Whatever the mechanism, the physiological significance of the small increased rate of post-fusion protein discharge caused by alpha-synuclein remains to be demonstrated, especially since endogenous beta-, but not alpha-synuclein is the predominant synuclein isoform associated with chromaffin granules.