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.

Sox8 encodes a high-mobility group transcription factor that is widely expressed during development. Sox8, -9 and -10 form group E of the Sox gene family which has been implicated in several human developmental disorders. In contrast to other SoxE genes, the role of Sox8 is unclear and Sox8 mouse mutants reportedly showed only idiopathic weight loss and reduced bone density. The careful analysis of our Sox8 null mice, however, revealed a progressive male infertility phenotype. Sox8 null males only sporadically produced litters of reduced size at young ages. We have shown that SOX8 protein is a product of adult Sertoli cells and its elimination results in an age-dependent deregulation of spermatogenesis, characterized by sloughing of spermatocytes and round spermatids, spermiation failure and a progressive disorganization of the spermatogenic cycle, which resulted in the inappropriate placement and juxtaposition of germ cell types within the epithelium. Those sperm that did enter the epididymides displayed abnormal motility. These data show that SOX8 is a critical regulator of adult Sertoli cell function and is required for both its cytoarchitectural and paracrine interactions with germ cells.

Sperm utilize glycolysis to generate ATP required for motility, and several spermatogenic cell-specific glycolytic isozymes are associated with the fibrous sheath (FS) in the principal piece of the sperm flagellum. We used proteomics and molecular biology approaches to confirm earlier reports that a novel enolase is present in mouse sperm. We then found that a pan-enolase antibody, but not antibodies to ENO2 and ENO3, recognized a protein in the principal piece of the mouse sperm flagellum. Database analyses identified two previously uncharacterized enolase family-like candidate genes, 64306537H0Rik and Gm5506. Northern analysis indicated that 64306537H0Rik (renamed Eno4) was transcribed in testes of mice by Postnatal Day 12. To determine the role of ENO4, we generated mice using embryonic stem cells in which an Eno4 allele was disrupted by a gene trap containing a beta galactosidase (beta-gal) reporter (Eno4(+/Gt)). Expression of beta-gal occurred in the testis, and male mice homozygous for the gene trap allele (Eno4(Gt/Gt)) were infertile. Epididymal sperm numbers were 2-fold lower and sperm motility was reduced substantially in Eno4(Gt/Gt) mice compared to wild-type mice. Sperm from Eno4(Gt/Gt) mice had a coiled flagellum and a disorganized FS. The Gm5506 gene encodes a protein identical to ENO1 and also is transcribed at a low level in testis. We conclude that ENO4 is required for normal assembly of the FS and provides most of the enolase activity in sperm and that Eno1 and/or Gm5506 may encode a minor portion of the enolase activity in sperm.