GENE EXPRESSION IN SPERMATOGENIC CELLS

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The long-term goal of these studies is to determine the mechanisms that regulate development of germ cells in the male. This is being approached by identifying genes that are expressed specifically in male germ cells and are up-regulated during particular phases of male germ cell development. These are referred to as chauvinist genes because male germ cells favor their expression with strong prejudice. They include germ cell homologue genes, unique genes, and genes expressing unique transcripts. Germ cell homologue genes are expressed only in spermatogenic cells, but are closely related to other genes expressed in somatic cells. One of these is glyceraldehyde 3-phosphate dehydrogenase (Gapd-s), likely to be important in male fertility and reproductive toxicology. The GAPD-S enzyme is "off" in spermatids and "on" in sperm and has a key role in regulating the generation of ATP required for fertilization. The expression of the Gapd-s mRNA and GAPD-S protein have been determined, the minimum promoter region required for transcription in transgenic mice has been identified, and the phenotype produced by a knockout of the Gapd-s gene is being determined. Studies are also underway to produce targeted mutations in three unique genes that are expressed only in spermatogenic cells and lack somatic cell isoforms. These genes encode proteins believed to have critical roles in sperm-egg binding (fertilin beta), spermatid nuclear condensation (protamine 1), and flagellar structural integrity (Fscl). The fertilin beta protein is a sperm surface component that has metalloprotease and disintegrin domains, and binds specifically to alpha1beta6 integrin on the egg surface. Protamine 1 is a highly basic protein that replaces histones during the compaction of DNA in the forming sperm head. Fscl is a major cytoskeletal component of the sperm flagellum. Mutations in the genes are expected to produce disruption of sperm formation or fertilization in homozygous male mice. The predicted phenotypes are similar to conditions observed in some infertile men and the results will be valuable both for understanding the development and function of sperm and for determining the possible roles of mutations in these genes in human infertility.