Phosphoglycerate kinase (PGK) (ATP:3-phospho-D-glycerate 1-phosphotransferase, EC 2.7.2.3) is a metabolic enzyme functioning in the Embden-Meyerhof pathway that converts glucose (or fructose) to pyruvate. Two functional loci for the production of PGK have been identified in the mammalian genome. PGK-1 is an X-linked gene expressed constitutively in all somatic cells and premeitotic germ cells. The human PGK-1 gene consists of 11 exons and 10 introns encompassing a region approximately 23 kilobases (kb) in length. PGK-2 is an autosomal gene expressed in a tissue-specific manner exclusively in the late stages of spermatogenesis. In the present study, a molecular analysis of a human genomic clone of PGK-2 originally isolated by Szabo et al. has revealed that this autosomal sequence completely lacks introns and contains characteristics of a processed gene, or 'retroposon', including the remnants of a poly(A)+ tail and bounding direct repeats. Typically such processed sequences form non-functional pseudogenes that have evolved multiple genetic lesions which preclude translation of any transcript into a functional polypeptide. For example, an X-linked processed pseudogene of PGK-1 (psi PGK-1) in humans has been identified and shown to contain premature termination codons in all reading frames. It was therefore unexpected to find that the intronless autosomal PGK sequence reported here is not a pseudogene, but is rather a functional gene that has retained a complete open reading frame, and is actively expressed in mammalian spermatogenesis. Both the unusual conservation of function in this processed PGK-2 gene and its tissue-specific expression in spermatogenesis are best explained as a compensatory response to the inactivation of the X-linked PGK-1 gene in spermatogenic cells before meiosis.

Human testis-specific PGK gene lacks introns and possesses characteristics of a processed gene

Spermatocytes and round spermatids were isolated from rat testis and the effects of addition of 3.3 mM-glucose and 6 mM-DL-lactate to the incubation medium on the morphology, oxygen consumption and incorporation of uridine and amino acids of these cells were investigated. Addition of lactate to isolated germ cells increased O2 consumption 1.8-fold and incorporation of precursors of RNA and protein by at least 5-fold. The amino acid incorporation into spermatocytes and spermatids was irreversibly decreased after a preincubation for at least 5 h or 70 min respectively in the absence of lactate, indicating degeneration of the germ cells. In the presence of lactate, however, spermatocytes maintained their morphological integrity for at least 24 h. Addition of glucose to isolated germ cells had no effect on any of the parameters investigated. It is concluded that isolated spermatocytes and round spermatids may utilize lactate, perhaps secreted by Sertoli cells, as the main source of energy.

https://rep.bioscientifica.com/downloadpdf/journals/rep/62/2/jrf_62_2_010.pdf

Exogenous lactate is essential for metabolic activities in isolated rat spermatocytes and spermatids.

H-2, the major his tocompatibi l i ty complex of the mouse, is a cluster of loci posit ioned in the middle por t ion of ch romosome 17 (Fig. 1). Present da t a indicate that the cluster is composed of ten loci grouped into regions and subregions (Fig. 2). Tradit ionally, the borders of the H-2 complex are thought to lie in the K region at the centromeric end of the cluster and the D region at the telomeric end. At the lat ter portion of chromosome 17 lies another cluster of seven loci compris ing the T region, originally identified by the Tla locus. The loci in the T region show certain similarities to some of the tl-2 loci, and it is possible that , genetically, the region is, in fact, par t of the H-2 complex. Fur ther down at the telomeric por t ion o f ch romosome 17 is a group of i sozyme loci, which are becoming useful markers in his tocompatibility studies. The H-2 loci occur in many v a r i a n t s a l l e l e s a n d the combinat ions of certain [1-2 alleles in a single ch romosome form many different haplotypes . F o r example,

H-2 haplotypes, genes, regions, and antigens: First listing

Phosphoglycerate kinase 2 (PGK2), an isozyme that catalyzes the first ATP-generating step in the glycolytic pathway, is encoded by an autosomal retrogene that is expressed only during spermatogenesis. It replaces the ubiquitously expressed phosphoglycerate kinase 1 (PGK1) isozyme following repression of Pgk1 transcription by meiotic sex chromosome inactivation during meiotic prophase and by postmeiotic sex chromatin during spermiogenesis. The targeted disruption of Pgk2 by homologous recombination eliminates PGK activity in sperm and severely impairs male fertility, but does not block spermatogenesis. Mating behavior, reproductive organ weights (testis, excurrent ducts, and seminal vesicles), testis histology, sperm counts, and sperm ultrastructure were indistinguishable between Pgk2(-/-) and wild-type mice. However, sperm motility and ATP levels were markedly reduced in males lacking PGK2. These defects in sperm function were slightly less severe than observed in males lacking glyceraldehyde-3-phosphate dehydrogenase, spermatogenic (GAPDHS), the isozyme that catalyzes the step preceding PGK2 in the sperm glycolytic pathway. Unlike Gapdhs(-/-) males, the Pgk2(-/-) males also sired occasional pups. Alternative pathways that bypass the PGK step of glycolysis exist. We determined that one of these bypass enzymes, acylphosphatase, is active in mouse sperm, perhaps contributing to phenotypic differences between mice lacking GAPDHS or PGK2. This study determined that PGK2 is not required for the completion of spermatogenesis, but is essential for sperm motility and male fertility. In addition to confirming the importance of the glycolytic pathway for sperm function, distinctive phenotypic characteristics of Pgk2(-/-) mice may provide further insights into the regulation of sperm metabolism.

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Phosphoglycerate Kinase 2 (PGK2) Is Essential for Sperm Function and Male Fertility in Mice

The downpour of information about the H-2 complex has hit immunology with the force of a tropical rainstorm. It is flooding whole fields, uprooting carefully cultivated orchards, displacing cottages, houses, and palaces, threatening to drown their inhabitants. This and the following article (Klein et al. 1982) are meant to serve as kinds of life-savers for those who have not learned how to swim in the rapid and murky waters. The First Listing, published four years ago (Klein et al. 1978), consisted only of tables; to this Second Listing we have added a brief description of all the loci residing, or thought to be residing, on the mouse chromosome 17, the chromosome carrying the H-2 complex. This arrangement necessitated the division of the Second Listing into two parts, the first part concerned with loci on chromosome 17 outside of the H-2 complex and the second part dealing with the complex itself. The discussion of the individual loci is prefaced by a description of the chromosome.

H-2 haplotypes, genes, and antigens: second listing. I. Non-H-2 loci on chromosome 17.

Mammals possess two genetically distinct forms of phosphoglycerate kinase (PGK). Only the X-linked PGK-A isozyme is detectable by electrophoresis of somatic and ovarian tissues of the mouse and of most other eutherian mammals studied to date. The PGK-B isozyme is highly active in testes and epididymides which contain spermatozoa, as well as in semen. PGK-B activity is first detectable in testes of 22-day-old mice and shows rapid increase concomitant with germ cell proliferation until day 40, after which activity increases only slightly. Two allelic isozymes (F and S) in mice express equal enzyme activity and are inherited in an autosomal codominant manner. One is equally as heat stable as PGK-A; the other is heat labile. Two of the 33 strains tested express neither F nor S, but do express a faintly staining PGK band which is of slightly faster mobility than F and which is absent from the other stains. This may represent a third allelic isozyme of PGK-B with reduced enzyme activity.

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Papers

Testis specific phosphoglycerate kinase B in mouse

Mammalian testis phosphoglycerate kinase.