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Protein fraction isolated from epididymal fluid re‐associates sperm in vitro: Possible role of serpins in rat rosettes assembly

In many mammalian species, sperm associate as a consequence of the epididymal transit. From the classic Rouleaux in guinea pig to the most recent work in mouse and echidna, authors have focused mainly on a detailed morphological description of this phenomenon. Some of these articles have also begun to describe the nature of the material present between sperm heads. Here, we try to better understand the factor/s involved in rat sperm association (Rosette). Based on previous work describing the appearance of Rosettes in the distal segments of the rat epididymis, we consider that sperm during their transit must be in contact with factor/s present in the caudal lumen in order to associate with each other. By an in vitro sperm re‐associating assay, we try to determine the in vivo phenomenon observed in the lumen. The assay consists of co‐incubating non‐associated sperm with several protein fractions obtained from epididymal caudal fluid. After establishing the most active fraction, the proteins were characterized by MALDI‐TOF mass spectrometry. Among the proteins we found two members of the serine protease inhibitors family; an α‐1 antitrypsin and a new protein with an α‐1 antitrypsin like domain which includes a sequence compatible with the serpins' reactive center loop. These serpins may play a role in the assembly/disassembly process of Rosettes by modulating lumenal protease activity. Finally, a biochemical‐morphological model which explains the sperm–proteases interaction was proposed. Mol. Reprod. Dev. 77: 410–419, 2010. © 2010 Wiley‐Liss, Inc.