CD55 and CD59 protein expression by Apodemus (field mice) sperm in the absence of CD46.

The mammalian female reproductive tract has an abundance of complement components, which play a vital role in protection against genital pathogens. Sperm may be protected against complement-mediated damage by complement regulatory proteins, including membrane cofactor protein (CD46), decay accelerating factor (CD55) and CD59. However, sperm from Apodemus (field mice) do not express CD46 protein. The aim of the present study was to determine whether Apodemus sperm may be protected against complement-mediated damage by expression of CD55 and CD59 in the absence of CD46. We demonstrate here that, like Mus musculus mice (house mice), wild-caught Apodemus flavicollis, Apodemus microps and Apodemus sylvaticus mice express both glycosylphosphatidylinositol (GPI)- and transmembrane (TM)-anchored testicular CD55 mRNA transcripts. In Mus, testicular GPI- and TM-CD55 transcripts are generated by two distinct but closely related genes. We show that in contrast to Mus, CD55 isoforms in A. sylvaticus are generated by alternative splicing of a single copy gene. Testicular CD59 mRNA transcripts were also identified in A. flavicollis, A. microps, A. sylvaticus and M. musculus. CD55 and CD59 proteins are broadly distributed on epididymal sperm from wild-caught Apodemus and Mus mice as well as BALB/c mice, with expression on the acrosome, neck and tail. Thus, despite not expressing CD46 protein, Apodemus sperm may be protected against complement-mediated injury in the female genital tract by CD55 and CD59.

Absence of spermatozoal CD46 protein expression and associated rapid acrosome reaction rate in striped field mice (Apodemus agrarius).

BACKGROUND: In rodents, the cell surface complement regulatory protein CD46 is expressed solely on the spermatozoal acrosome membrane. Ablation of the CD46 gene is associated with a faster acrosome reaction. Sperm from Apodemus flavicollis (yellow-necked field mice), A. microps (pygmy field mice) and A. sylvaticus (European wood mice) fail to express CD46 protein and exhibit a more rapid acrosome reaction rate than Mus (house mice) or BALB/c mice. A. agrarius (striped field mice) belong to a different Apodemus subgenus and have pronounced promiscuity and large relative testis size. The aim of this study was to determine whether A. agrarius sperm fail to express CD46 protein and, if so, whether A. agrarius have a faster acrosome reaction than Mus. METHODS: Reverse transcription polymerase chain reaction (RT-PCR) was used to assess whether A. agrarius transcribe testicular CD46 mRNA. RT-PCR was supplemented with 3'- and 5'-rapid amplification of cDNA ends to determine the complete nucleotide sequence of A. agrarius CD46. Fluorescence microscopy was used to assess whether CD46 protein is expressed by A. agrarius sperm. The acrosome status of A. agrarius sperm was calculated over time by immunocytochemistry using peanut agglutinin lectin. RESULTS: We demonstrate that A. agrarius mice transcribe two unique alternatively spliced testicular CD46 mRNA transcripts, both lacking exon 7, which differ from those described previously in other Apodemus species. The larger A. agrarius CD46 transcript has an insert between exons 10 and 11 which, if translated, would result in a novel cytoplasmic tail. In addition, A. agrarius CD46 transcripts have an extended AU-rich 3'-untranslated region (UTR) and a truncated 5'-UTR, resulting in failure to express spermatozoal CD46 protein. We show that A. agrarius has a significantly faster spontaneous acrosome reaction rate than A. sylvaticus and Mus. CONCLUSION: Absence of CD46 protein expression is associated with acrosomal instability in rodents. A. agrarius mice express novel CD46 transcripts, resulting in the trade of spermatozoal CD46 protein expression for a rapid acrosome reaction rate, in common with other species of field mice. This provides a strategy to increase competitive sperm advantage for individuals, leading to faster fertilisation in this highly promiscuous genus.

Rapid sperm acrosome reaction in the absence of acrosomal CD46 expression in promiscuous field mice (Apodemus).

There is pronounced promiscuity and sperm competition in long-tailed field mice (Apodemus sylvaticus). These mice have evolved unusual sperm behaviour favouring rapid fertilisation, including dynamic formation of sperm trains and their subsequent dissociation. The cell surface complement regulatory (CReg) protein CD46 is broadly expressed in eutherian mammals other than rodents, in which it is expressed solely on the spermatozoal acrosomal membrane. Ablation of the CD46 gene has been associated with a faster acrosome reaction (AR) rate in inbred laboratory mice. Here, we demonstrate that wild-caught field mice of three species, A. sylvaticus, A. flavicollis and A. microps, exhibit a more rapid AR than wild-caught house mice Mus musculus or inbred laboratory BALB/c mice. We also demonstrate that wild-caught field mice of these three species, unlike house mice, produce alternatively spliced transcripts of testicular CD46 mRNA lacking exons 5-7 or 6-7, together with an extended 3' - and often truncated 5'-utr, leading to failure to express any sperm CD46 protein in both the testis and epididymis. Male field mice may therefore have traded expression of this CReg protein for acrosomal instability, providing a novel genus-specific strategy to favour rapid fertilisation and competitive advantage in the promiscuous reproductive behaviour of wild field mice.