Lack of glutamate transporter EAAC1 in the epididymis of infertile c-ros receptor tyrosine-kinase deficient mice.

Transgenic male mice carrying inactive mutations of the receptor tyrosine kinase c-ros lack the caput epididymidis initial segment and are infertile because sperm volume regulation is compromised. Complementary DNA arrays were used to detect differences in gene expression in the caput epididymidis of heterozygous fertile and homozygous infertile males. The glutamate transporter excitatory amino acid carrier 1 (EAAC1) was expressed in all epididymal regions with high expression in the initial segment and cauda epididymidis. Homozygous knockout mice did not express EAAC1 messenger RNA (mRNA) in the caput but they did express the gene in the corpus and cauda. Immunohistochemical staining for EAAC1 confirmed regional mRNA expression and demonstrated an adluminal location on stereocilia/microvilli of principal cells. The glutamate transporter-associated protein (GTRAP) 3-18 was detected in all epididymal regions independent of genotype, but a highly abundant novel transcript of 4.2 kilobases was found only in the initial segment of heterozygous c-ros mice. High-performance liquid chromatography measurement of glutamate revealed a significantly higher content in the proximal caput of infertile mice than fertile mice, and tissue glutamate content decreased distally in both genotypes. Because glutamate is used as an osmolyte in somatic cells, the lack of EAAC1 reported here may disturb normal osmolyte balance in the proximal epididymal lumen and compromise sperm maturation, in particular the development of sperm volume regulatory mechanisms.