The Molecular Chaperone Hsp90α Is Required for Meiotic Progression of Spermatocytes beyond Pachytene in the Mouse

Abstract

The molecular chaperone Hsp90 has been found to be essential for viability in all tested eukaryotes, from the budding yeast to Drosophila. In mammals, two genes encode the two highly similar and functionally largely redundant isoforms Hsp90α and Hsp90β. Although they are co-expressed in most if not all cells, their relative levels vary between tissues and during development. Since mouse embryos lacking Hsp90β die at implantation, and despite the fact that Hsp90 inhibitors being tested as anti-cancer agents are relatively well tolerated, the organismic functions of Hsp90 in mammals remain largely unknown. We have generated mouse lines carrying gene trap insertions in the Hsp90α gene to investigate the global functions of this isoform. Surprisingly, mice without Hsp90α are apparently normal, with one major exception. Mutant male mice, whose Hsp90β levels are unchanged, are sterile because of a complete failure to produce sperm. While the development of the male reproductive system appears to be normal, spermatogenesis arrests specifically at the pachytene stage of meiosis I. Over time, the number of spermatocytes and the levels of the meiotic regulators and Hsp90 interactors Hsp70-2, NASP and Cdc2 are reduced. We speculate that Hsp90α may be required to maintain and to activate these regulators and/or to disassemble the synaptonemal complex that holds homologous chromosomes together. The link between fertility and Hsp90 is further supported by our finding that an Hsp90 inhibitor that can cross the blood-testis barrier can partially phenocopy the genetic defects.

Heat Shock Protein 90 Functions to Stabilize and Activate the Testis-specific Serine/Threonine Kinases, a Family of Kinases Essential for Male Fertility

“…In the absence of this constant sampling and stabilization by HSP90, the kinases are targeted for ubiquitination and degradation. The male germ cells in HSP90␣ null mice do not progress through meiosis (34), and based upon our work reported here, it appears likely that HSP90 is also critical to spermiogenesis, the differentiation of spermatids into mature spermatozoa. It is very possible that during the post-meiotic differentiation of male germ cells, HSP90 functions to coordinate the temporal and spatial activation of the TSSKs.…”

“…However, it remained to be determined whether the role of HSP90 in activation was limited to TSSK6 or if the other TSSKs were also regulated by HSP90. Interestingly, male germ cells express the HSP90␣ (gene Hsp90aa1) isoform and targeted disruption of HSP90␣ in mice resulted in male infertility without any other obvious somatic defects (34). The spermatogenic cells failed to develop beyond meiosis in the HSP90␣ null mice, demonstrating that HSP90 is critical for meiosis in the male germ cells.…”

Hsp90 as a therapeutic target in endocrinology: current evidence

“…34 Extended exposure of pubertal male mice to high doses of the Hsp90α/β N-terminal domain inhibitor, pochoxime A, a new radicicol derivative with efficacy favorable for entry into brain and testes, 28,165,166 caused testicular atrophy in some of the animals similar to the phenotype seen in mice with genetic disruption of Hsp90α. 33 These results suggest that such drugs could be used to generate pharmacological infertility in male mice. In this context, it is of considerable interest that gamendazole, a new class of Hsp90 inhibitor selective for Hsp90β, which binds to the Hsp90 C-terminal region in …”

“…32 Hsp90α-deficient male mice, whose Hsp90β levels remain unaltered, have been shown to be sterile because of an inability to produce sperm. 33 The Hsp90α isoform has been further linked to male fertility through evidence of its specific chaperoning of the androgen receptor (AR) of spermatogonia, a key factor for the initiation and maintenance of spermatogenesis and for the survival of spermatocytes in adult testis. 34 Since the Hsp90β null mutation in mouse was early embryonic lethal, this isoform has specific developmental functions that cannot be fully supported by Hsp90α.…”

“…In studies of Hsp90α-deficient male mice, Grad et al 33 demonstrated testicular atrophy and infertility caused by apoptosis of spermatocytes, indicating that the Hsp90α isoform is required for spermatogenesis. Subsequent conditional deletion of Hsp90α, in adult mice, showed that the chaperone is essential for germ cell development in mature adult testes.…”