Epigenetic regulation of testis-specific gene expression.

Cellular differentiation is mediated by differential gene expression. The cells of the testis are no exception. Indeed, recent studies based on microarray and expressed sequence tag analyses have revealed dynamic changes in gene expression during spermatogenesis. The autosomal phosphoglycerate kinase gene Pgk2 is an example of a gene that is tightly regulated during spermatogenesis, with transcription initiating in primary spermatocytes and ceasing in postmeiotic spermatids. Our studies show that this tissue-specific transcription is regulated at multiple levels, including binding of ubiquitous and testis-specific transcription factors, reconfiguration of nucleosomes, decondensation of chromatin, modifications of specific residues in histones, and demethylation of DNA. Our ability to isolate relatively pure populations of different spermatogenic cell types has allowed us to determine the order of these events that lead up to initiation of transcription of the Pgk2 gene. By examining similar parameters in transgenic mice carrying various portions of Pgk2 regulatory sequence, we have localized specific signals responsible for each of these regulatory events. Together these results demonstrate that activation of testis-specific transcription of the Pgk2 gene involves a multifaceted, ordered cascade of epigenetic events that potentiates this locus in preparation for the initiation of transcription.