We previously identified solute carrier 22a14 (Slc22a14) as a spermatogenesis-associated transmembrane protein in mice. Although Slc22a14 is a member of the organic anion/cation transporter family, its expression profile and physiological role have not been elucidated. Here, we show that Slc22a14 is crucial for sperm motility and male fertility in mice. Slc22a14 is expressed specifically in male germ cells, and mice lacking the Slc22a14 gene show severe male infertility. Although the overall differentiation of sperm was normal, Slc22a14−/− cauda epididymal spermatozoa showed reduced motility with abnormal flagellar bending. Further, the ability to migrate into the female reproductive tract and fertilise the oocyte were also impaired in Slc22a14−/− spermatozoa. The abnormal flagellar bending was thought to be partly caused by osmotic cell swelling since osmotic challenge or membrane permeabilisation treatment alleviated the tail abnormality. In addition, we found structural abnormalities in Slc22a14−/− sperm cells: the annulus, a ring-like structure at the mid-piece–principal piece junction, was disorganised, and expression and localisation of septin 4, an annulus component protein that is essential for the annulus formation, was also impaired. Taken together, our results demonstrated that Slc22a14 plays a pivotal role in normal flagellar structure, motility and fertility in mouse spermatozoa.

Solute carrier 22a member 14 (SLC22A14) plays a critical role in male infertility in mice. We previously revealed that one of the causes of infertility is impaired capacitation. However, the molecular mechanism remained unclear. Here, we show that the influx of HCO3–, a trigger of capacitation, is impaired and intracellular pH is decreased in the sperm of Slc22a14 knockout (KO) mice. While intracellular cAMP concentration did not increase during capacitation in Slc22a14 KO spermatozoa, HCO3–-dependent soluble adenylate cyclase activity was normal, and the addition of 8-bromo cAMP rescued the decreased protein tyrosine phosphorylation. In addition, the intracellular pH of Slc22a14 KO sperm was lower than that of wild-type sperm and did not increase after the addition of HCO3–. Although its relationship to the regulation of intracellular pH is unknown, TMEM225, a possible protein phosphatase inhibitor, was found to be decreased in Slc22a14 KO sperm. The decreased in vitro fertilization rate of Slc22a14 KO sperm was partially rescued by an increase in the intracellular pH (pHi) and the addition of 8-bromo cAMP. These results suggest that SLC22A14 is involved in capacitation through the regulation of HCO3– transport and pHi.