Translation repression by maternal RNA binding protein zar1 is essential for early oogenesis in zebrafish

Abstract

A large amount of maternal RNA is deposited in oocytes and is reserved for later development. Control of maternal RNA translation during oocyte maturation has been extensively investigated and its regulatory mechanisms are well documented. However, translational regulation of maternal RNA in early oogenesis is largely unexplored. In this study, we generated zebrafish zar1 mutants that result in early oocyte apoptosis and fully penetrant male development. Loss of p53 suppresses the apoptosis in zar1 mutants and restores oocyte development. zar1 immature ovaries show upregulation of proteins implicated in endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). More importantly, loss of Zar1 causes marked upregulation of zona pellucida (ZP) family proteins, while overexpression of ZP proteins in oocytes causes upregulation of stress-related activating transcription factor 3 (atf3), arguing that tightly controlled translation of ZP proteins is essential for ER homeostasis during early oogenesis. Furthermore, Zar1 binds to ZP gene mRNAs and represses their translation. Together, our results indicate that regulation of translational repression and de-repression are essential for precisely controlling protein expression during early oogenesis.

Zebrafishdazlregulates cystogenesis upstream of the meiotic transition and germline stem cell specification and independent of meiotic checkpoints

“…Like dazl mutant GCs, vasa, a Dazl target (Haston et al, 2009b;Kee et al, 2009;Li et al, 2019;Reynolds et al, 2005) and zili mutant GCs become vacuolated (Houwing et al, 2008). In contrast, brca, zar1, and fancl mutant oocytes do not and reach later meiotic stages (Miao et al, 2017;Rodriguez-Mari et al, 2010;Shive et al, 2010). Failure of tp53/chk2 to suppress GC loss in dazl -/is consistent with dazl function in a premeiotic GC program that acts before and independent of these meiotic checkpoint pathway regulators.…”

“…Loss of germline has been reported to occur either by cell death or failure to maintain GC fate and differentiation as somatic cell types (Gross-Thebing et al, 2017;Kossack and Draper, 2019;Leerberg et al, 2017;Rodriguez-Mari et al, 2010;Slanchev et al, 2005;Uchida et al, 2002;. During normal development, apoptosis of GCs is associated with male-specific differentiation (Pradhan et al, 2012;Rodriguez-Mari et al, 2010;Siegfried and Nusslein-Volhard, 2008;Slanchev et al, 2005;Tzung et al, 2015;Uchida et al, 2002;, and mutation of the apoptosis regulator, tp53, can suppress cell death and loss of oocytes in several zebrafish mutants, including zar1, brca2, and fancl (Miao et al, 2017;Rodriguez-Mari et al, 2010;Shive et al, 2010).…”

“…Similarly, we determined that mutation of zebrafish chek2 does not interfere with meiosis or fertility in zebrafish. Because p53 and chk2, though not essential for normal development can suppress GC loss in other contexts, we expected that simultaneous mutation of chk2 or p53 mutation might similarly suppress germline loss in dazl mutants much as tp53 mutation suppresses oocyte loss and sex reversal of brca, zar1, fancl mutants (Miao et al, 2017;Rodriguez-Mari et al, 2010;Shive et al, 2010). In contrast, although loss of chk2, but not p53, could prolong survival of abnormal dazl mutant GCs, these cells did not progress further in meiosis and eventually were not maintained, resulting in sterility.…”

The RNA-binding protein Igf2bp3 is critical for embryonic and germline development in zebrafish

“…Mis-regulation of RNAs through abnormal RBP functions often lead to developmental consequences. For instance, knockout of mouse Igf2bp1 leads to gross defects in development (Hansen et al, 2004) and knockout of the RBP Zar1 in zebrafish leads to abnormalities in sex determination, resulting in male-only fish (Miao et al, 2017). Several questions remain regarding how individual RNA-binding proteins contribute to development and the mechanism for their actions in specific cell types.…”

“…Germplasm RNAs are post-transcriptionally regulated by factors including microRNAs and RNA-binding proteins. RNA-binding proteins have a wide range of roles in processes such as oocyte maturation (Miao et al, 2017;Sun et al, 2018) and axis formation (Bontems et al, 2009;Kumari et al, 2013).…”