Loss of SED1/MFG-E8 results in altered luminal physiology in the epididymis

SED1/MFG-E8, herein referred to as SED1, is a bimotif adhesive protein with ascribed functions in a range of cell-cell interactions, including sperm-egg binding. In the male reproductive tract, SED1 is secreted by the initial segment of the epididymis, where it coats sperm and subsequently facilitates binding to the egg zona pellucida. We have recently reported that SED1-null epididymides show an unexpected incidence of spermatic granulomas, reflecting breakdown of the epithelium and a consequent autoimmune response against sperm antigens. However, spermatic granulomas are most often manifest in the distal segments of the epididymis, whereas the bulk of SED1 is expressed in the proximal epididymis. In some models, the presence of granulomas in the distal epididymis is associated with an underlying defect in the maintenance of luminal fluid homeostasis. Herein, we report that SED1-null epididymal fluid is both hypo-osmotic and alkaline, relative to wildtype epididymal fluid. Furthermore, the SED1-null epididymal epithelium exhibits various hallmarks of disrupted fluid reabsorption and pH regulation, including altered morphology of clear cells, increased intracellular vesicles, and apical distribution of VATPase. Results indicate that the SED1-null epididymal pathologies are not the secondary consequences of defective testes or efferent ducts or of improper epididymal differentiation, unlike that seen in other epididymal models. The expression and distribution of various ion exchangers, channels, and enzymes that mediate fluid transport and pH regulation are examined in wildtype and SED1-null epididymides, and models to account for how SED1 functions in luminal fluid dynamics are discussed.