PBRM-1/PBAF-regulated genes in a multipotent progenitor

The Caenorhabditis elegans somatic gonadal precursors (SGPs) are multipotent progenitors that generate all somatic cells of the adult reproductive system. The two SGPs originate in the mesodermal layer and are born through a division that produces one SGP and one head mesodermal cell (hmc). One hmc terminally differentiates and the other dies by programmed cell death. The PBAF chromatin remodeling complex promotes the multipotent SGP fate. Complete loss of PBAF causes lethality, so we used a combination of Cre/lox recombination and GFP nanobody-directed protein degradation to eliminate PBRM-1, the signature subunit of the PBAF complex, from 83 mesodermal cells, including SGPs, body muscles, and the hmc. We used RNA sequencing to identify genes acting downstream of PBAF in these cells and identified 1955 transcripts that were significantly differentially expressed between pbrm-1(-) and pbrm-1(+) in the mesoderm of L1 larvae. We found that genes involved in muscle cell function were overrepresented; most of these genes had lower expression in the absence of PBRM-1, suggesting that PBAF promotes muscle differentiation. Among the differentially expressed genes were 125 genes that are normally expressed at higher levels in SGP vs. hmc and positively regulated by pbrm-1 and 53 that are normally expressed at higher levels in hmc vs. SGP and are negatively regulated by pbrm-1; these are candidate regulators of the SGP/hmc fate decision. We validated one candidate gene using a fluorescent reporter; the hsp-12.3 reporter was derepressed in SGPs in pbrm-1 mutants, suggesting that hsp-12.3 expression is normally repressed by pbrm-1 in SGPs.