Expression, localization, and regulation of inhibitor of DNA binding (Id) proteins in the rat epididymis.

The epididymis is the site in which spermatozoa are matured and stored. Regional differences along the epididymis are essential for the establishment of the microenvironment required for germ cell maturation. Inhibitor of DNA binding (Id) proteins are transcription factors that modulate the functions of basic helix-loop-helix (bHLH) transcription factors and act by binding to and sequestering bHLH proteins; the latter act to regulate cellular proliferation and differentiation. The objectives of this study were to determine the mRNA expression and the immunocytolocalization of Id1, Id2, Id3, and Id4 in the epididymis of adult rats and to determine the Id3 protein expression profile in orchidectomized and in aged animals. We found that, at the mRNA level, Id proteins are expressed in a unique, region-specific manner along the epididymis. Id1 immunoreactivity is specific to myoid cells; the presence of Id2 is observed in clear cells, myoid cells, and in the apical region of principal cells. Id3 immunoreactivity is essentially confined to the nuclei of principal cells and myoid cells, whereas Id4 is observed mainly in myoid and narrow cells. Thus, Ids are localized to different cell types and are differentially expressed, at both the mRNA and protein levels, along the epididymis. Expression of Id3 is differentially regulated in response to orchidectomy along the epididymis. The fact that these regulators of gene expression are expressed in this manner may provide some insight into the differential expression of other genes that lead to region-specific differences along the epididymis, a hallmark of this tissue.