SPERMATOGENESIS is a complex developmental process that occurs in several phases. A large number of genes have been identified that are expressed during spermatogenesis1,2, but the biological significance of many of these is not yet known. We have used gene targeting to selectively eliminate the transcription factor CREM (cyclic AMP-responsive element modulator), which is thought to e important for mammalian spermatogenesis3–5. Male mice deficient for all CREM proteins are sterile, as their developing spermatids fail to differentiate into sperm, and postmeiotic gene expression in the testis declines dramatically. The cessation of sperm development is not accompanied by decreases in the levels of follicle-stimulating hormone or testosterone. Our findings indicate that the CREM gene is essential for spermatogenesis, and mice deficient for this transcription factor could serve as a model system for the study of idiopathic infertility in men.

Severe impairment of spermatogenesis in mice lacking the CREM gene

The nuclear envelope links the cytoskeleton to structural components of the nucleus. It functions to coordinate nuclear migration and anchorage, organize chromatin, and aid meiotic chromosome pairing. Forces generated by the cytoskeleton are transferred across the nuclear envelope to the nuclear lamina through a nuclear-envelope bridge consisting of SUN (Sad1 and UNC-84) and KASH (Klarsicht, ANC-1 and Syne/Nesprin homology) proteins. Some KASH-SUN combinations connect microtubules, centrosomes, actin filaments, or intermediate filaments to the surface of the nucleus. Other combinations are used in cell cycle control, nuclear import, or apoptosis. Interactions between the cytoskeleton and the nucleus also affect global cytoskeleton organization. SUN and KASH proteins were identified through genetic screens for mispositioned nuclei in model organisms. Knockouts of SUN or KASH proteins disrupt neurological and muscular development in mice. Defects in SUN and KASH proteins have been linked to human diseases including muscular dystrophy, ataxia, progeria, lissencephaly, and cancer.

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Interactions Between Nuclei and the Cytoskeleton Are Mediated by SUN-KASH Nuclear-Envelope Bridges

https://onlinelibrary.wiley.com/doi/pdf/10.1042/BC20060086

Current knowledge of the large RhoGAP family of proteins

During spermatogenesis, diploid stem cells differentiate, undergo meiosis, and transform into haploid spermatozoa. As this precisely timed series of events proceeds, chromosomal ploidy is reduced, and the nucleosomes of the chromatin are replaced by a transcriptionally quiescent protamine-containing nucleus. The premature termination of transcription during the haploid phase of spermatogenesis necessitates an especially prominent role for posttranscriptional regulation in the temporal and spatial expression of many testis-specific proteins and isozymes. In this review article, discussion will focus on novel mechanisms regulating gene expression in mammalian male germ cells from genome to protein. BioEssays20:555–561, 1998. © 1998 John Wiley & Sons Inc.

Molecular mechanisms of male germ cell differentiation

Targeted disruption of the Akap4 gene causes defects in sperm flagellum and motility.

Spag4, a novel sperm protein, binds outer dense-fiber protein Odf1 and localizes to microtubules of manchette and axoneme.

Interactional cloning of the 84-kDa major outer dense fiber protein Odf84. Leucine zippers mediate associations of Odf84 and Odf27.

Outer dense fiber protein 2, Odf2, is a major component of the outer dense fibers, ODF, in the flagellum of spermatozoa. ODF are associated with microtubule doublets that form the axoneme. We recently demonstrated that tyrosine phosphorylation of Odf2 is important for sperm motility. In the course of a study of Odf2 using Odf2 mouse knockout lines we observed that males of a high percentage chimaerism, made using XL169 embryonic stem cells, were infertile, whereas mice of low-medium percentage chimaerism were fertile. XL169 ES cells have a β-geo gene trap cassette inserted in the Odf2 gene. To determine possible underlying mechanisms resulting in infertility we analyzed epididymal sperm and observed that >50% displayed bent tails. We next performed ultrastructural analyses on testis of high percentage XL169 chimaeric mice. This analysis showed that high percentage XL169 chimaeric mice produce elongating spermatids that miss one or more entire outer dense fibers in their midpiece and principal piece. In addition, we observed elongating spermatids that show thinning of outer dense fibers. No other obvious abnormalities or defects are present in elongating spermatids. Spermatozoa from the caput and cauda epididymis of XL169 mice of high percentage chimaerism show additional tail defects, including absence of one or more axonemal microtubule doublets and bent tails. Sperm with bent tails display abnormal motility. Our results document the possible impact of loss of one Odf2 allele on sperm tail structure and function, resulting in a novel sperm tail phenotype.

/pdf/gene-trap-mutation-of-murine-outer-dense-fiber-protein-2-37n8kwzojh.pdf

Gene trap mutation of murine Outer dense fiber protein-2 gene can result in sperm tail abnormalities in mice with high percentage chimaerism

Kinesins are tetrameric motor molecules, consisting of two kinesin heavy chains (KHCs) and two kinesin light chains (KLCs) that are involved in transport of cargo along microtubules. The function of the light chain may be in cargo binding and regulation of kinesin activity. In the mouse, two KLC genes, KLC1 and KLC2, had been identified. KLC1 plays a role in neuronal transport, and KLC2 appears to be more widely expressed. We report the cloning from a testicular cDNA expression library of a mammalian light chain, KLC3. The KLC3 gene is located in close proximity to the ERCC2 gene. KLC3 can be classified as a genuine light chain: it interacts in vitro with the KHC, the interaction is mediated by a conserved heptade repeat sequence, and it associates in vitro with microtubules. In mouse and rat testis, KLC3 protein expression is restricted to round and elongating spermatids, and KLC3 is present in sperm tails. In contrast, KLC1 and KLC2 can only be detected before meiosis in testis. Interestingly, the expression profiles of the three known KHCs and KLC3 differ significantly: Kif5a and Kif5b are not expressed after meiosis, and Kif5c is expressed at an extremely low level in spermatids but is not detectable in sperm tails. Our characterization of the KLC3 gene suggests that it carries out a unique and specialized role in spermatids.

Heide A. Tarnasky

Papers

Spag4, a novel sperm protein, binds outer dense-fiber protein Odf1 and localizes to microtubules of manchette and axoneme.

Interactional cloning of the 84-kDa major outer dense fiber protein Odf84. Leucine zippers mediate associations of Odf84 and Odf27.

Gene trap mutation of murine Outer dense fiber protein-2 gene can result in sperm tail abnormalities in mice with high percentage chimaerism

Kinesin light-chain KLC3 expression in testis is restricted to spermatids.

A new rat gene RT7 is specifically expressed during spermatogenesis.