Deletion of the DNA/RNA-binding protein MSY2 leads to post-meiotic arrest

Abstract

Y-box proteins are a well-characterized family of nucleic acid binding proteins that are expressed from bacteria to human. This review will focus on MSY2, a member of the Y-box gene family that is exclusively expressed in male and female germ cells. MSY2 is the mouse ortholog of FRGY2, the Xenopus germ cell-specific protein and the human germ cell protein, Contrin. MSY2 functions as a co-activator of transcription in male germ cells and plays an important role in the translational repression and storage of both paternal and maternal mRNAs in spermatocytes, spermatids and oocytes. Following gene targeting, matings of heterozygotes produce a normal Mendelian ratio with equal numbers of phenotypically normal males and females. However, males and females lacking Msy2 are infertile. In Msy2-null males, spermatogenesis is disrupted in post-meiotic germ cells with many misshapen and multinucleated spermatids. No spermatozoa are found in the epididymis. The germ cell specificity and the critical functions played by this multifunctional DNA- and RNA-binding protein during spermatogenesis make Contrin, the human ortholog of MSY2, an attractive and novel target for male contraception.

Introduction

In light of the rapidly increasing rate of world population growth, it is essential to offer humankind a wide variety of safe, effective and reversible fertility regulation methods. Excellent advances have been made in male contraception research focusing primarily on vas occlusion and suppression of spermatogenesis by hormonal methods (reviewed in Waites, 1993, van Look and Pérez-Palacios, 1994, Rajalakshmi and Griffin, 1999, Nieschlag and Henke, 2005). Numerous agents including androgens, and GnRH agonists and antagonists with androgens, have been employed for hormonal suppression of spermatogenesis. Such approaches take advantage of the major role the brain plays in producing spermatozoa. Disruption of the finely tuned hypothalamus-pituitary axis by the addition of exogenous testosterone successfully and reversibly suppresses spermatogenesis. However, failure to achieve azoospermia in some men, the high cost of the contraceptive agents, and the substantial lag time before infertility commences argue for a need to develop additional male contraceptives.

A common goal of many researchers seeking to develop male contraceptives is to identify specific targets whose inactivation selectively inhibits spermatogenesis and/or sperm function. Unfortunately, deletion or inactivation of many of the target proteins that perturb spermatogenesis also show phenotypic change in somatic cells, a consideration that prevents their development as contraceptive agents. In contrast to traditional drug design methodologies in which molecules are identified and efforts are subsequently made to minimize the molecule's toxicological side effects by structural changes, many drug design efforts now utilize structure-based inhibitor design to directly create maximally effective and specific compounds. To apply this methodology to male contraception, several criteria must be met. Target molecules need to be identified that are essential for spermatogenesis and are solely found in the testis. Ideally, the proteins should be expressed in the germ cells, but not somatic cells of the testis, because germ cell proteins are likely to provide a better target for contraception reversibility.

Human male infertility that is caused by severe defects in sperm production is estimated to affect about 2% of men. The complexity of spermatogenesis suggests that many types of perturbations ranging from hormonal to genetic to behavioral will contribute to these infertilities. A growing number of gene mutations or deletions that cause male infertility are now known (reviewed in Matzuk and Lamb, 2002). In many cases, these mutations map to genes that encode DNA- or RNA-binding proteins, some of which appear specific to spermatogenesis. Nucleic acid-binding proteins are important cellular proteins because they play crucial roles in transcription, mRNA processing and transport and translational regulation. RNA-binding proteins are prominent in the germ cells of the mammalian testes, especially in post-meiotic cells where a large number of “paternal” mRNAs are processed, stored and require temporally regulated activation. The need for post-transcriptional regulation in male germ cells exists because RNA synthesis terminates during mid-spermiogenesis necessitating special mechanisms to stabilize and store mRNAs (reviewed in Hecht, 1998). Subsequently, there is selective temporal activation of a large group of mRNAs that encode proteins needed during the terminal phases of spermatogenesis. Among the proteins undergoing such activation over a period of more than a week in mice and men are DNA-binding proteins such as the transition proteins 1 and 2 and the protamines 1 and 2, sperm mitochondrial proteins such as a selenoprotein present in the sperm midpiece, axonemal proteins and numerous sperm fibrous sheath and outer dense fiber proteins. Several RNA-binding proteins, including a putative germ cell-specific member of the Y-box family of proteins, regulate the post-transcriptional expression of these translationally delayed mRNAs (Chennathukuzhi et al., 2003, Yang et al., 2005a).

Here we shall focus primarily on one member of the Y-box family of proteins, MSY2, a mouse germ cell expressed protein that is the ortholog of Contrin, a human germ cell Y-box protein. Gene targeting of MSY2 produces phenotypically normal, but sterile mouse homozygotes. Spermatogenesis is disrupted during spermiogenesis with many misshapen spermatozoa and no spermatozoa are found in the epididymis. We propose that germ cell-specific Y-box proteins such as the human protein, Contrin, are good targets to develop into a male contraceptive.