In summary, this study analyses the changes in the concentration and localization of the actin filament-severing protein scinderin, during testicular development, and in spermatozoa during and following the epididymal transit. Immunoblot analyses showed one band at 80 kDa in chromaffin cells, fetal and adult tubules, interstitial cells, aorta and vena cava, and spermatozoa. Scinderin immunoreactiv-ity was associated with a region corresponding with the subacrosomal space and with the acrosome in the spermatids. In epididymal spermatozoa, scinderin was located in the anterior acrosome and in the equatorial segment, but in ejaculated spermatozoa, the protein appeared in the acro-some and the post-equatorial segment of the head. In Sertoli cells, the protein was detected near the cell surface during all the stages of the cycle and within the cytoplasm, where it accumulated near the base in a stage-specific manner.
The changes reported here in the distribution of the Ca2+-dependent actin filament-severing protein scinder-in in the spermatozoa during and following epididymal transit may represent a different response of the gametes to local conditions that altered the [Ca2+]i. Scinderin is a calcium-dependent actin filament-severing protein that has been shown to have a major role in the regulation of exo-cytosis of secretory vesicles during neurotransmitter release by controlling cortical actin disassembly. Perhaps during their transit through the excretory ducts of the testis, the spermatozoa need to modify the state/form of their actin so as to allow emergence of the acrosome reaction, which has been called ‘‘a sperm exocytosis’’.
Scinderin immunoreactivity in epididymal cells also coincides with the presence of the cortical perijunctional actin filament network reported along the principal cells’ lateral plasma membranes. Whether scinderin accompanies the cell junctions in the epididymal cells or in the Sertoli cells, the protein may conceivably have similar effects on cellular contacts.
The sequestration of [Ca2+]i in the cisternae of ER accompanying the cortical or peripheral actin in the Sertoli cells could induce modifications in affinity of the Ca2+-dependent actin-interacting protein scin-derin for either the cytosol, the phospholipids of the membrane, or actin; these modifications, in turn, could prove significant in the physiology of the Sertoli cell junctions that are responsible for the maintenance of the blood-testis barrier or for the release of mature spermatids. In Ma-din-Darby canine kidney (MDCK) confluent cell cultures, tight junctions failed to develop when extracellular Ca2+ levels were low.
In Sertoli cells. In the testis, Sertoli cells share a morphological feature found in cells of most epithelia: they possess a layer of subsurface or cortical actin that occupies the peripheral cytoplasm and surrounds the perimeter of the cell. In the Sertoli cells, this cortical or peripheral actin is typically sandwiched between cisternae of endoplasmic reticulum (ER) on the intracellular side and the plasma membrane on the extracellular side. Within the Sertoli cell, the monomeric form of actin has been reported in the base, the middle, and the apex, while the filamentous form has been shown in the base and the apex of the cell. Thus, G-actin was localized in the same sites as F-actin, but, in addition, the monomeric actin was found in sites where F-actin was not detectable.
Actin, particularly F-actin, has been identified within the subacrosomal space of developing spermatids. Collectively, these authors emphasize that F-actin decreases with the completion of the spermatid’s development until it becomes virtually undetectable by the time mature spermatids are released from the seminiferous tubules. This suggests that the older the spermatid, the more abundant the G-actin and the less abundant the F-actin. Assuming that the principal role of cortical F-actin is to act as a physical barrier that could rigidify the structure that it accompanies, namely, cell membranes, the predominance of one form of actin over the other within the subacrosomal space could influence the shaping of the acrosome to meet specific physiological and structural requirements.
The present work is the first to document the variations in the concentration and the localization of the Ca2+-depen-dent actin filament-severing protein scinderin or adseverin, which reportedly shares a similar sequence, in testicular and epididymal cells during development, and in spermatozoa during and after their epididymal transit. It is also the first successful attempt to localize the protein in tissue sections.
Scinderin in the Testis
In the germ cells. Scinderin immunoreactivity is associated principally with a region corresponding with the sub-acrosomal space in the round spermatids and with the remodeling acrosome in the elongated spermatids. Because scinderin is a Ca2+-dependent actin filament-severing protein, these observations should be viewed in relation to other reports of actin in developing spermatids and spermatozoa.