Diverse Roles of PUF Proteins in Germline Stem and Progenitor Cell Development in C. elegans

Abstract

Stem cell development depends on post-transcriptional regulation mediated by RNA-binding proteins (RBPs) (Zhang et al., 1997; Forbes and Lehmann, 1998; Okano et al., 2005; Ratti et al., 2006; Kwon et al., 2013). Pumilio and FBF (PUF) family RBPs are highly conserved post-transcriptional regulators that are critical for stem cell maintenance (Wickens et al., 2002; Quenault et al., 2011). The RNA-binding domains of PUF proteins recognize a family of related sequence motifs in the target mRNAs, yet individual PUF proteins have clearly distinct biological functions (Lu et al., 2009; Wang et al., 2018). The C. elegans germline is a simple and powerful model system for analyzing regulation of stem cell development. Studies in C. elegans uncovered specific physiological roles for PUFs expressed in the germline stem cells ranging from control of proliferation and differentiation to regulation of the sperm/oocyte decision. Importantly, recent studies started to illuminate the mechanisms behind PUF functional divergence. This review summarizes the many roles of PUF-8, FBF-1, and FBF-2 in germline stem and progenitor cells (SPCs) and discusses the factors accounting for their distinct biological functions. PUF proteins are conserved in evolution, and insights into PUF-mediated regulation provided by the C. elegans model system are likely relevant for other organisms.

Keywords: C. elegans; RNA regulation; germline; pumilio and fem-3-binding factor; stem cells.

FIGURE 1

Schematic of C. elegans hermaphrodite germline and RNA binding protein network downstream of GLP-1/Notch. (A) C. elegans germline development is supported by continuous SPC proliferation promoted by GLP-1/Notch signaling from the DTC (Pazdernik and Schedl, 2013). Progenitors enter meiosis when they reach the transition zone, and later differentiate into sperm and oocytes. Several types of RNA granules reside in germ cells and facilitate germ cell development and embryogenesis. (B) Downstream of GLP-1/Notch, FBFs maintain SPC proliferation by repressing the expression of GLD-1, GLD-2, and GLD-3 that inhibit SPC proliferation and promote differentiation (Kimble and Crittenden, 2007 and references in sections “RNA-Binding Protein Network Downstream of GLP-1/Notch” and “PUF Function in Maintaining Germline SPCs”).

FIGURE 2

The multiple functions of FBFs and PUF-8 in C. elegans germline SPCs. (A) PUF-8 acts redundantly with MEX-3 to facilitate GLP-1 signaling (Ariz et al., 2009). Downstream of GLP-1/Notch, FBFs promote germline SPC proliferation by repressing cell cycle regulators, meiotic mRNAs, and mpk-1 MAP kinase (Crittenden et al., 2002; Lee et al., 2007a; Kalchhauser et al., 2011). (B) FBFs act with GLD-2, GLD-3 complex to promote SPC meiosis by activating GLD-1 expression (Suh et al., 2009). PUF-8 facilitates meiosis by repressing LET-60/RAS (Vaid et al., 2013), while FBFs repress mpk-1. The contribution of mpk-1 repression by FBFs to regulation of SPC proliferation and differentiation is discussed in section “PUF Function in Inhibiting Mitotic Cell Fate of SPCs and Promoting Differentiation.” (C) PUF-8 controls the sperm/oocyte switch by acting redundantly with FBF-1 to repress fog-2 (Bachorik and Kimble, 2005). FBF proteins control the sperm/oocyte switch by acting with NOS-3 to repress fem-3 (Kraemer et al., 1999; Arur et al., 2011) as well as by repressing fog-1 and possibly fog-3 (Thompson et al., 2005). Both PUF-8 and FBF-1 cooperate with LIP-1 to repress MPK-1 activity in SPCs, dpMPK-1 refers to a diphosphorylated active form of MPK-1 (discussion and references in section “PUF Function in Controlling the Sperm/Oocyte Decision in Germline Mitotic Zone”). dpMPK-1 promotes spermatogenesis, although specific relevant substrates are yet unknown. (D) PUF-8 maintains germ cell fate by repressing somatic transcription factor PAL-1 (Mainpal et al., 2011).

FIGURE 3

Modification of FBF biological activity though interactions with protein partners. (A) On its own, FBF PUF domain binds to target mRNAs containing a canonical 9-nt motif (Wang et al., 2009; Bhat et al., 2019; Qiu et al., 2019). (B) FBF PUF domain’s RNA-binding specificity can be influenced by interactions with protein partners such as CPB-1 (Menichelli et al., 2013) and LST-1 (Qiu et al., 2019). (C) FBFs can repress target mRNAs by recruiting deadenylase complex (Goldstrohm et al., 2006; Suh et al., 2009). (D) FBFs can promote mRNA polyadenylation by interacting with the poly(A) polymerase complex (Eckmann et al., 2002; Suh et al., 2009).