SAS1B protein [ovastacin] shows temporal and spatial restriction to oocytes in several eutherian orders and initiates translation at the primary to secondary follicle transition

Background: Sperm Acrosomal SLLP1 Binding (SAS1B) protein (ovastacin) is an oolemmal binding partner for the intra‐acrosomal sperm protein SLLP1. Results: Immunohistochemical localization revealed that SAS1B translation is restricted among adult tissues to the ovary and oocytes, SAS1B appearing first in follicles at the primary‐secondary transition. Quiescent oocytes within primordial follicles and primary follicles did not stain for SAS1B. Examination of neonatal rat ovaries revealed SAS1B expression first as faint signals in postnatal day 3 oocytes, with SAS1B protein staining intensifying with oocyte growth. Irrespective of animal age or estrus stage, SAS1B was seen only in oocytes of follicles that initiated a second granulosa cell layer. The precise temporal and spatial onset of SAS1B expression was conserved in adult ovaries in seven eutherian species, including nonhuman primates. Immunoelectron micrographs localized SAS1B within cortical granules in MII oocytes. A population of SAS1B localized on the oolemma predominantly in the microvillar region anti‐podal to the nucleus in ovulated MII rat oocytes and on the oolemma in macaque GV oocytes. Conclusions: The restricted expression of SAS1B protein in growing oocytes, absence in the ovarian reserve, and localization on the oolemma suggest this zinc metalloprotease deserves consideration as a candidate target for reversible female contraceptive strategies. Developmental Dynamics, 242:1405–1426, 2013. © 2013 Wiley Periodicals, Inc. Key findings SAS1B protein is highly conserved in primary sequence and the ASTL gene is tightly regulated across eutherians, showing an identical immunohistochemical profile of ovarian expression and oocyte restriction among adult tissues, and a similar ∼44 to 55 kDa range of apparent protein masses. SAS1B is first translated in ooplasm of follicles undergoing the primary‐secondary follicle transition; quiescent germ cells in primordial follicles and early growing oocytes in primary follicles show no evidence of SAS1B protein translation in any species studied. SAS1B is concentrated in the microvillar domain on the egg plasma membrane in species in which this feature is well developed. In species such as macaque, SAS1B shows a uniform distribution on the plasma membrane of GV oocytes. A population of SAS1B is concentrated in cortical granules within mature oocytes indicating that the protein reaches the cell surface and redistributes on the oolemma and in the perivitelline sp