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

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.

Transposable elements in the mammalian germline: a comfortable niche or a deadly trap?

“…TE reactivation also perturbs the self-renewal program of spermatogonial stem cells, which results in spermatogenic tubules completely devoid of any germ cells within a few weeks. This phenotype was originally described for the methylationdeficient Dnmt3L mutant males (Bourc'his and Bestor, 2004), observed again in PIWI mutants (Carmell et al, 2007;Aravin et al, 2007b) and more recently recapitulated in mutations for proteins supporting the piRNA/ PIWI pathway, such as the Tudor domain-containing proteins TDRD1 and TDRD9 (Ollinger et al, 2008;Soper et al, 2008;Ma et al, 2009;Reuter et al, 2009;Shoji et al, 2009). Tudor domain-containing proteins assemble specialized RNA processing platforms in the cytoplasm of germ cells, to which they recruit PIWI proteins through their ability to bind symmetrically dimethylated arginines, a post-translational modification undergone by both MILI and MIWI2 (Aravin et al, 2009;Vagin et al, 2009;Wang et al, 2009;Shoji et al, 2009).…”

“…Tudor domain-containing proteins assemble specialized RNA processing platforms in the cytoplasm of germ cells, to which they recruit PIWI proteins through their ability to bind symmetrically dimethylated arginines, a post-translational modification undergone by both MILI and MIWI2 (Aravin et al, 2009;Vagin et al, 2009;Wang et al, 2009;Shoji et al, 2009). As a whole, directed mutagenesis in mice has allowed the identification of nine proteins with a key role in safe guarding the male germline against TEs (Table 1 and Figure 1): Dnmt3L, Dnmt3A, MIWI2, MILI, Maelstrom (MAEL), TDRD1, TDRD9, GASZ and Tex19.1 (Testis expressed 19.1) (Ollinger et al, 2008;Soper et al, 2008;Ma et al, 2009). All of these proteins, except Tex19.1, have been involved in the same TE restricting pathway, initiated by Figure 1 Expression pattern and mutant phenotype of proteins involved in TE repression in the developing male germline in mice.…”

“…Contrary to flies and worms, which rely on the piRNA pathway but are devoid of genomic methylation, the mammalian germline combines both piRNA-induced degradation and DNA methylation to silence TEs. Their epistatic relationship explains why mutations in genes required for piRNA production, such as Mili, Miwi2, Tdrd1, Tdrd9 and Gasz, also affect the process of de novo methylation of TE elements in prospermatogonia (Carmell et al, 2007;Aravin et al, 2007b;Ma et al, 2009;Reuter et al, 2009;Shoji et al, 2009).…”

piRNA profiling during specific stages of mouse spermatogenesis

“…The murine transposon-derived piRNAs share features with the Drosophila repeat-associated piRNAs and are also proposed to control transposable elements (Klattenhoff and Theurkauf 2008). In addition to PIWI proteins, other proteins such as GASZ, MOV10L1, Sun1, Tdrd9, and Tdrd1 are also required for biogenesis and/or stability of mouse piRNAs (Chi et al 2009;Ma et al 2009;Reuter et al 2009;Shoji et al 2009;Zheng et al 2010).…”

The RNA helicase MOV10L1 binds piRNA precursors to initiate piRNA processing

“…Recently, it has been shown that a Tudor domain-containing (Tdrd) protein known as Tdrkh promotes the 3 ′ trimming process (Honda et al 2013;Saxe et al 2013). In addition to Piwi proteins and Zuc, other factors essential for piRNA processing, including GASZ (germ cell protein with ankyrin repeats, sterile α motif, and leucine zipper) and Tdrd proteins (Hosokawa et al 2007;Ma et al 2009), are localized on the cytoplasmic surface of mitochondria in electron-dense granules known as intermitochondrial cement (ICM) in mammalian spermatocytes or more generally as nuage in Drosophila and other animals (Siomi et al 2011;Pillai and Chuma 2012).…”