αB-Crystallin Is Found in Detergent-resistant Membrane Microdomains and Is Secreted via Exosomes from Human Retinal Pigment Epithelial Cells

alpha B-crystallin (alpha B) is known as an intracellular Golgi membrane- associated small heat shock protein. Elevated levels of this protein have been linked with a myriad of neurodegenerative pathologies including Alzheimer disease, multiple sclerosis, and age-related macular degeneration. The membrane association of alpha B has been known for more than 3 decades, yet its physiological import has remained unexplained. In this investigation we show that alpha B is secreted from human adult retinal pigment epithelial cells via microvesicles (exosomes), independent of the endoplasmic reticulum-Golgi protein export pathway. The presence of alpha B in these lipoprotein structures was confirmed by its susceptibility to digestion by proteinase K only when exosomes were exposed to Triton X-100. Transmission electron microscopy was used to localize alpha B in immuno-gold- labeled intact and permeabilized microvesicles. The saucer- shaped exosomes, with a median diameter of 100-200 nm, were characterized by the presence of flotillin-1, alpha-enolase, and Hsp70, the same proteins that associate with detergent-resistant membrane microdomains (DRMs), which are known to be involved in their biogenesis. Notably, using polarized adult retinal pigment epithelial cells, we show that the secretion of alpha B is predominantly apical. Using OptiPrep gradients we demonstrate that alpha B resides in the DRM fraction. The secretion of alpha B is inhibited by the cholesterol-depleting drug, methyl beta-cyclodextrin, suggesting that the physiological function of this protein and the regulation of its export through exosomes may reside in its association with DRMs/lipid rafts.