STRATEGIES FOR TESTIS SPECIFIC GENE EXPRESSION
The mammalian testis is a unique organ programmed for both endocrine and germ cell production, functions that are clearly interdependent. The germ cell component displays distinctive developmental properties illustrated by programmed molecular events that occur with the onset and during spermatogenesis and include activation and inactivation of numerous genes yielding protein products with distinct or modified properties. Genes expressed during spermatogenesis can be classified as “housekeeping” or structural. Both categories include testis specific isozymes and isoforms. One such example is LDH-C4, a member of the lactate dehydrogenase gene family that is transcribed only during prophase of the first meiotic division. We have cloned and sequenced the promoter of this gene and demonstrated functionality. Even though this gene and protein are well-studied, there remains the question of why LDH-C4 supplants the other lactate dehydrogenases in testis and sperm metabolism.
A second example of an unique protein is provided by calpastatin. This protein is the endogenous inhibitor of calpain, a cytoplasmic cysteine protease. The calpastatin gene, unlike ldhc, is the product of alternative promoter usage by which a truncated testis specific isoform of the somatic calpastatin is produced. Testis calpastatin (tCAST) is transcribed and translated in round spermatids. The promoter region and coding exon is located within an intron of the somatic gene. We have co-localized the testis calpastatin and calpain to the region of the sperm between the plasma membrane and outer acrosomal membrane where presumably it may be a player in the events associated with the acrosome reaction and/or with sperm-egg fusion.
A third example is UDP-N-acetylhexosamine pyrophosphorylase, described originally as AgX, the product of an alternatively spliced mRNA. A 16 amino acid deletion in the protein product results in a change in substrate specificity.
The large number of testis specific and testis abundant isozyme and protein isoforms suggests that they are not a biological curiosity, but rather are required for both full and complete spermatogenesis and for sperm function. Mechanisms regulating testis-specific gene expression, and structure/function aspects of testis gene expression will be addressed in this report.
