GASZ Is Essential for Male Meiosis and Suppression of Retrotransposon Expression in the Male Germline

Lang Ma,Gregory M Buchold,Derek Y Han,Martin M Matzuk,Wei Yan,Angshumoy Roy,Cristian Coarfa,Preethi H Gunaratne,Huifeng Zhu,Kathleen H Burns,Michael P Greenbaum,R Alan Harris,Michael T Mcmanus

doi:10.1371/journal.pgen.1000635

Abstract

Nuage are amorphous ultrastructural granules in the cytoplasm of male germ cells as divergent as Drosophila, Xenopus, and Homo sapiens. Most nuage are cytoplasmic ribonucleoprotein structures implicated in diverse RNA metabolism including the regulation of PIWI-interacting RNA (piRNA) synthesis by the PIWI family (i.e., MILI, MIWI2, and MIWI). MILI is prominent in embryonic and early post-natal germ cells in nuage also called germinal granules that are often associated with mitochondria and called intermitochondrial cement. We find that GASZ (Germ cell protein with Ankyrin repeats, Sterile alpha motif, and leucine Zipper) co-localizes with MILI in intermitochondrial cement. Knockout of Gasz in mice results in a dramatic downregulation of MILI, and phenocopies the zygotene–pachytene spermatocyte block and male sterility defect observed in MILI null mice. In Gasz null testes, we observe increased hypomethylation and expression of retrotransposons similar to MILI null testes. We also find global shifts in the small RNAome, including down-regulation of repeat-associated, known, and novel piRNAs. These studies provide the first evidence for an essential structural role for GASZ in male fertility and epigenetic and post-transcriptional silencing of retrotransposons by stabilizing MILI in nuage.

Highlights

The differentiation program of the germline is distinct from somatic cells in that resetting of the epigenome by demethylation of DNA and histones must take place for proper post-fertilization development of the embryo [1]
Protection of the germline from genetic intruders may require the association of PIWI-interacting RNA (piRNA)-synthesizing enzymes and other components of this nuage structure through direct or indirect associations with GASZ
Consistent with compromised transposable element DNA methylation seen in MILI and MIWI2 null testes, we found significant hypomethylation of presumed germ cell-derived long interspersed nuclear element 1 (Line L1) and intracisternal A particle (IAP) sequences in Gasz2/2 testes compared to controls at P14 (Figure 6H)

Summary

Introduction

The differentiation program of the germline is distinct from somatic cells in that resetting of the epigenome by demethylation of DNA and histones must take place for proper post-fertilization development of the embryo [1]. The resetting of the epigenetic state of the germline followed by the acquisition of male-specific methylation imprints, while a necessary component for postfertilization development, exposes the germline to potential risk from retrotransposon mobilization [3]. Repeat-associated piRNAs limit expression of retrotransposons at the post-transcriptional level and through epigenetic silencing by the recruitment of DNA methyltransferases including DNMT3A and DNMT3L [11,12,13,14,15]. In the absence of these small RNAs, retrotransposon expression is dramatically increased in the germline, leading to DNA damage and cell death

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Conclusion