The present study based on comparisons between wild type and trophinin null spermatozoa showed that trophinin proteins are expressed on mouse sperm tails and that the trophinin-binding peptide GWRQ-MAPS can promote mouse sperm motility. We previously observed similar motility-enhancing activity of GWRQ-MAPS on human sperm . Therefore we anticipate that GWRQ-MAPS can promote sperm motility in numerous mammals, which could be addressed in future studies.
Although trophinin null male mice did not exhibit infertility , when we isolated sperm from 4–5 month old trophinin null male mice from twice back-crossed or the second generation of the C57BL/6 strain, we observed that sperm were often immotile (LW and MNF, unpublished observations). However, in trophinin null male mice from subsequent back-crosses, trophinin nulls showed no apparent spermatozoa motility defect, suggesting that the sperm motility defect seen in trophinin null males is dependent on genetic background. It is possible that defects in tail motility associated with trophinin null male mice are more pronounced in the 129/SvJ background in which the trophinin null ES line was generated  than in a C57BL/6 background.
Here, we found using CASA that GWRQ-MAPS peptide activated tail motility of wild-type mouse spermatozoa (Figure 3 and 4), confirming our previous study of human sperm showing that GWRQ-MAPS enhanced sperm motility .
Sperm tail motility requires movement produced by the energy of the microtubule-associated ATPase, dynein, in an evolutionally conserved manner [18–21]. Trophinin associates with bystin and tastin in trophoblastic cells [1, 2]. We previously found that dynein binds to tastin  and that tastin binds to trophinin through bystin in the cytoplasm [1, 17, 22, 23]. In human sperm tail, trophinin, bystin, and tastin proteins are apparently co-expressed . It is likely that the trophinin N-terminal cytoplasmic domain associates with dynein in the mouse sperm tail, providing a potential mechanism for GWRQ-mediated hyperactivation of sperm tails.
Sperm motility requires high ATP levels to support coordinated movement of the central axoneme and surrounding flagellar structures . ATP is generated mainly by oxidative phosphorylation in mitochondria located in the midpiece. However, ATP generated there must be supplied to the principal piece of the tail, requiring energy . Recent findings in mice deficient in the sperm-specific glycolytic enzyme, glyceraldehyde 3-phosphate dehydrogenase-S (GAPDS), suggest that ATP generated by glycolysis at the principal piece is essential for sperm motility . GAPDS localizes to the principal piece of the mouse sperm tail , where trophinin is localized (Figure 1B). GAPDS and other germ cell-specific glycolytic enzymes may be critical to ensure a sufficient localized supply of ATP along the length of the sperm flagellum for sperm motility. The close proximity of trophinin and GAPDS suggests that the trophinin/bystin/tastin complex may arrest dynein ATPase in a manner similar to that we reported for ErbB4 in trophoblastic cells [12, 13]. In silent trophectoderm cells, trophinin arrests ErbB4 through bystin in the cytoplasm and therefore HB-EGF (heparin-binding EGF-like growth factor) bound to ErbB4 does not promote autophosphorylation of the ErbB4 cytoplasmic domain. When GWRQ binds the trophinin extracellular domain, bystin is dissociated from ErbB4, allowing ErbB4 to undergo autophosphorylation . Since it is known that tastin directly binds dynein ATPase , we hypothesize that GWRQ binding to the trophinin extracellular domain releases tastin from trophinin in the cytoplasm, enabling it to activate dynein ATPase. We speculate that a trophinin, bystin and tastin complex arrests dynein, so that dynein cannot hydrolyze ATPs locally supplied by glycolysis. Thus trophinin may play a role in conserving ATP in the principal piece during long distance travel toward the oocyte, where sperm need to move vigorously to achieve fertilization.