Secretion of αB-Crystallin via exosomes: New clues to the function of human retinal pigment epithelium.

αB-crystallin (αB) is an archetypical small heat shock protein whose physiological function is not clearly defined. The interest in this protein arises from its well-established but poorly understood association with a myriad of neurodegenerative diseases, cancer and cardiomyopathies. The discovery of the secretion of αB from human adult retinal pigment epithelial cells (ARPE19) via exosomes not only points to the involvement of this protein in lateral transfer of information between cells in the visual system but also to the status of this protein as a potential ligand that may activate or modulate immune and stress responses, normal growth and oncogenic pathways. Retinal pigment epithelium (RPE) is a single layer of polarized cells that supports photoreceptor physiology and function. We have initiated investigations on understanding the origin of the elevated levels of αB in extracellular sub-retinal proteolipid deposits (known as "drusen") associated with the death of photoreceptor neurons in age-related macular degeneration (AMD). Here we discuss the potential implications of the presence and transport of αB in exosomes across cell membranes in RPE.