SYGL-1 and LST-1 link niche signaling to PUF RNA repression for stem cell maintenance in Caenorhabditis elegans

Abstract

Central questions in regenerative biology include how stem cells are maintained and how they transition from self-renewal to differentiation. Germline stem cells (GSCs) in Caeno-rhabditis elegans provide a tractable in vivo model to address these questions. In this system, Notch signaling and PUF RNA binding proteins, FBF-1 and FBF-2 (collectively FBF), maintain a pool of GSCs in a naïve state. An open question has been how Notch signaling modulates FBF activity to promote stem cell self-renewal. Here we report that two Notch targets, SYGL-1 and LST-1, link niche signaling to FBF. We find that SYGL-1 and LST-1 proteins are cytoplasmic and normally restricted to the GSC pool region. Increasing the distribution of SYGL-1 expands the pool correspondingly, and vast overexpression of either SYGL-1 or LST-1 generates a germline tumor. Thus, SYGL-1 and LST-1 are each sufficient to drive “stemness” and their spatial restriction prevents tumor formation. Importantly, SYGL-1 and LST-1 can only drive tumor formation when FBF is present. Moreover, both proteins interact physically with FBF, and both are required to repress a signature FBF mRNA target. Together, our results support a model in which SYGL-1 and LST-1 form a repressive complex with FBF that is crucial for stem cell maintenance. We further propose that progression from a naïve stem cell state to a state primed for differentiation relies on loss of SYGL-1 and LST-1, which in turn relieves FBF target RNAs from repression. Broadly, our results provide new insights into the link between niche signaling and a downstream RNA regulatory network and how this circuitry governs the balance between self-renewal and differentiation.

A PUF hub drives self-renewal inC. elegansgermline stem cells

“…LST-1 or SYGL-1 was then induced by transferring L4s to OP50-seeded NGM plates 254 and passaging for several generations (SHIN et al 2017). The assay for Figure 6A was 255 performed at 25°.…”

“…S3). The control fbf-1 fbf-2 double mutants made more than 100 germ cells and 388 maintained a PZ, as described previously(MERRITT et al 2008;SHIN et al 2017; this 389…”

Niche maintenance of germline stem cells inC. elegansmales

“…The DTC and its processes extend around the distal-most germ cells, forming a plexus that correlates roughly with the distal pool (CINQUIN et al 2010;BYRD et al 2014;LEE et al 2016) and that contacts most of those germ cells (LEE et al 2016). Cells in the distal pool express self-renewal markers, LST-1 and SYGL-1 (KERSHNER et al 2014;SHIN et al 2017), but lack differentiation markers (CINQUIN et al 2010;FOX AND SCHEDL 2015). By contrast, cells in the proximal pool lack self-renewal markers (SHIN et al 2017), but increasingly express differentiation markers, including both meiotic and sexual fate regulators (HANSEN et al 2004;THOMPSON et al 2005;CINQUIN et al 2010;FOX AND SCHEDL 2015;BRENNER AND SCHEDL 2016).…”

“…Cells in the distal pool express self-renewal markers, LST-1 and SYGL-1 (KERSHNER et al 2014;SHIN et al 2017), but lack differentiation markers (CINQUIN et al 2010;FOX AND SCHEDL 2015). By contrast, cells in the proximal pool lack self-renewal markers (SHIN et al 2017), but increasingly express differentiation markers, including both meiotic and sexual fate regulators (HANSEN et al 2004;THOMPSON et al 2005;CINQUIN et al 2010;FOX AND SCHEDL 2015;BRENNER AND SCHEDL 2016). Moreover, cells in the distal pool are essentially uniform in terms of cell cycle and respond similarly to cell cycle arrest (CRITTENDEN et al 2006;CINQUIN et al 2010;ROSU AND COHEN-FIX 2017), whereas the more proximal cells undergo one or two mitotic divisions as they progressively differentiate and enter meiosis (CRITTENDEN et al 2006;MACIEJOWSKI et al 2006;CINQUIN et al 2010; FOX AND SCHEDL 2015; ROSU AND COHEN-FIX 2017) ( Fig.…”

“…The molecular response to GLP-1/Notch signaling has been assessed in hermaphrodites (LEE et al 2016;SHIN et al 2017). Whereas the receptor is found throughout the progenitor zone ( Fig.…”