Deadenylase depletion protects inherited mRNAs in primordial germ cells

Abstract

A crucial event in animal development is the specification of primordial germ cells (PGCs), which become the stem cells that create sperm and eggs. How PGCs are created provides a valuable paradigm for understanding stem cells in general. We find that the PGCs of the sea urchin Strongylocentrotus purpuratus exhibit broad transcriptional repression, yet enrichment for a set of inherited mRNAs. Enrichment of several germline determinants in the PGCs requires the RNA-binding protein Nanos to target the transcript that encodes CNOT6, a deadenylase, for degradation in the PGCs, thereby creating a stable environment for RNA. Misexpression of CNOT6 in the PGCs results in their failure to retain Seawi transcripts and Vasa protein. Conversely, broad knockdown of CNOT6 expands the domain of Seawi RNA as well as exogenous reporters. Thus, Nanos-dependent spatially restricted CNOT6 differential expression is used to selectively localize germline RNAs to the PGCs. Our findings support a 'time capsule' model of germline determination, whereby the PGCs are insulated from differentiation by retaining the molecular characteristics of the totipotent egg and early embryo.

Differential Nanos 2 protein stability results in selective germ cell accumulation in the sea urchin

“…This exclusivity may be explained by unique Nanos transcription in the sMics although preliminary results show that it is transcribed more broadly (Yajima, Swartz and Oulhen, personal communication). Alternatively, a broadly transcribed Nanos 2 mRNA may only be retained in the sMics since many mRNAs appear to be degraded selectively in the soma by a CNOT6-dependent mechanism, one that is lacking in the sMics (Gustafson and Wessel, 2010; Oulhen and Wessel, 2013; Swartz et al, 2014). Indeed, Nanos 2 contains an element in its 3′UTR, GNARLE (Global Nanos Associated RNA Lability Element), essential for the selective enrichment of its mRNA in the small micromeres (Oulhen and Wessel, 2014; Oulhen et al, 2013).…”

“…Although only a limited number of mRNAs have been identified as Nanos/Pumilio targets, these mRNAs encode proteins with key cellular and developmental functions. These Nanos-targeted mRNAs include cyclin B (Asaoka-Taguchi et al, 1999; Dalby and Glover, 1993; Kadyrova et al, 2007; Lai et al, 2011), hid (Hayashi et al, 2004; Sato et al, 2007), hunchback (Murata and Wharton, 1995; Wreden et al, 1997), fem 3 (Ahringer and Kimble, 1991; Zhang et al, 1997), VegT (Lai et al, 2012), and CNOT6 (Swartz et al, 2014). This list may expand significantly; using a database from Xenopus laevis , a series of additional candidates of Nanos/Pumilio targeted mRNAs in the germline have been generated based on scanning for PRE consensus sequences in the 3′UTR sequences (Lai and King, 2013).…”

A quiet space during rush hour: Quiescence in primordial germ cells

“…It functions through its interaction with Pumilio, which binds RNAs containing a conserved motif usually found in the 3′UTR of mRNAs; this motif is referred to as the Pumilio Response Element (PRE) (Sonoda and Wharton, 1999; Wharton and Struhl, 1991). Only a few mRNAs have been identified as nanos/pumilio targets: cyclin B (Asaoka-Taguchi et al, 1999; Dalby and Glover, 1993; Kadyrova et al, 2007; Lai et al, 2011), hid (Hayashi et al, 2004; Sato et al, 2007), hunchback (Murata and Wharton, 1995; Wreden et al, 1997), fem 3 (Ahringer and Kimble, 1991; Zhang et al, 1997), VegT (Lai et al, 2012) and CNOT6 (Swartz et al, 2014). In the sea urchin Strongylocentrotus purpuratus (the purple sea urchin), three nanos orthologs are present in the genome, but Sp nanos 2 (mRNA and protein) is the only nanos that accumulates specifically in the PGCs at the blastula stage when the quiescence phenotype is detected (Juliano et al, 2010).…”

“…Its absence contributes to low/no mitochondrial activity in the PGCs). Over 150 mRNAs are preferentially depleted (FDR<0.05) in the PGCs relative to the somatic cells, and many of these are also candidates for nanos – mediated mRNA degradation selectively in the PGCs (Swartz et al, 2014). Clearly nanos is capably of transitioning an early embryonic cell with broad maternal mRNA populations into a phenotypically distinct cell with broad quiescence.…”

“…Both Nanos mRNA and protein appear to accumulate selectively in the PGCs, yet this is accomplished largely post-transcriptionally and even post-translationally, respectively (e.g. (Oulhen and Wessel, 2016a; Oulhen et al, 2013; Swartz et al, 2014). The gene encoding Nanos 2 appears to be broadly transcribed throughout the embryo in most, if not all, cell types.…”