Proteasome regulation of the chromodomain protein MRG-1 controls the balance between proliferative fate and differentiation in theC. elegansgerm line

Abstract

The level of stem cell proliferation must be tightly controlled for proper development and tissue homeostasis. Multiple levels of gene regulation are often employed to regulate stem cell proliferation to ensure that the amount of proliferation is aligned with the needs of the tissue. Here we focus on proteasome-mediated protein degradation as a means of regulating the activities of proteins involved in controlling the stem cell proliferative fate in the C. elegans germ line. We identify five potential E3 ubiquitin ligases, including the RFP-1 RING finger protein, as being involved in regulating proliferative fate. RFP-1 binds to MRG-1, a homologue of the mammalian chromodomain-containing protein MRG15 (MORF4L1), which has been implicated in promoting the proliferation of neural precursor cells. We find that C. elegans with reduced proteasome activity, or that lack RFP-1 expression, have increased levels of MRG-1 and a shift towards increased proliferation in sensitized genetic backgrounds. Likewise, reduction of MRG-1 partially suppresses stem cell overproliferation. MRG-1 levels are controlled independently of the spatially regulated GLP-1/Notch signalling pathway, which is the primary signal controlling the extent of stem cell proliferation in the C. elegans germ line. We propose a model in which MRG-1 levels are controlled, at least in part, by the proteasome, and that the levels of MRG-1 set a threshold upon which other spatially regulated factors act in order to control the balance between the proliferative fate and differentiation in the C. elegans germ line.

Determination of differentially regulated proteins upon proteasome inhibition in AML cell lines by the combination of large‐scale and targeted quantitative proteomics

“…We quantified the accumulation of three well‐known interaction partners, i.e , two chromatin regulatory proteins, MORF 4L1 and L2, and MRFAP1, in both cell lines after proteasome inhibition (Supporting Information figure S6B). Recent studies have shown that MRFAP1 proteins have fast turnover rates, and are rapidly degraded via the ubiquitin‐proteasome system . This protein and its interaction partner MORF4L1 were found amongst the most up‐regulated proteins after NEDD8 inhibition, in multiple human cell lines .…”

Germline Stem Cell Differentiation Entails Regional Control of Cell Fate Regulator GLD-1 in Caenorhabditis elegans

“…Post-translational mechanisms including ubiquitin-mediated proteolysis have been implicated in the control of germline stem cell differentiation (Macdonald et al 2008;Jeong et al 2011;Gupta et al 2015). GLD-1 expression is qualitatively recapitulated by translational reporters (Merritt et al 2008), and our quantitative analysis of reporter GFP accumulation suggests that most of the quantitative control of GLD-1 levels in the distal germline is mediated through regulation of the gld-1 39 UTR ( Figure S4), indicating that post-translation control of GLD-1 accumulation plays only a minor role in regulating GLD-1 levels in the proliferative zone.…”

Analysis of Germline Stem Cell Differentiation Following Loss of GLP-1 Notch Activity in Caenorhabditis elegans

“…However, the use of DAPI staining in this study did not allow detection of PZ cells remaining at hours 6-8 (Figure 4) because chromosome morphology does not allow an unambiguous classification of individual germ-cell fate [see also Figure S2 in which we analyze glp-1(q224)]. In contrast, the use of REC-8/HIM-3 costaining can distinguish between PZ cells and meiotic prophase cells, even when they are intermixed, as often occurs in the meiotic entry region in wild type and in certain incompletely penetrant germline tumor mutants (MacQueen and Villeneuve 2001;Hansen et al 2004a;Fox et al 2011;Gupta et al 2015). To illustrate the distinction between antibody markers and DAPI morphology, Figure 2A (arrowhead) shows a cell that has entered meiosis (HIM-3 positive; REC-8 negative) whose nucleus shows sphericalrather than crescent-shaped DAPI morphology.…”