Mouse Spam1 (PH-20) is a multifunctional protein: evidence for its expression in the female reproductive tract

Sperm adhesion molecule 1 (Spam1) is a widely conserved sperm surface protein with multiple roles in mammalian fertilization. Although the gene for this protein has been thought to be testis specific based on Northern blot analysis, there is evidence for nontesticular expression when transcripts are analyzed by more sensitive techniques. In the present investigation, results of a reverse transcription polymerase chain reaction assay, an RNase-protection assay (RPA), and an in situ transcript hybridization assay revealed that the murine Spam1 gene is transcribed in the female genital tract. RPA revealed that Spam1 transcripts are synthesized in a region-dependent manner, with the oviduct having lower transcript levels than the uterus and vagina. The transcripts levels were 3- to 10-fold lower in the female genital tract than in the testis. In situ transcript hybridization assay revealed RNA in the luminal epithelium in all three regions of the genital tract and in the uterine myometrium and the oviductal mesothelium. Western blot analysis and immunohistochemistry demonstrated that the protein concentration is 1.5- to 3-fold lower in female tissues than in sperm, and localization is similar to that of the transcripts. The protein has hyaluronidase activity at neutral pH, which is unique for sperm hyaluronidase, but not at acidic pH. In the uterus, Spam1 expression fluctuated during the estrous cycle. Its localization suggests that in addition to functioning as a secretory protein, it may be involved in hyaluronic acid metabolism or turnover in the female genital tract. Our results provide further evidence that Spam1 is a multifunctional protein and that it is less restricted in its expression than previously reported.