The mitogen-activated protein kinase (MAPK) signaling pathway regulates numerous cellular processes in the C. elegansgerm line. In young hermaphrodites, di-phosphorylated MAPK is detected at high levels in late pachytene and in the proximal oocytes; however, when meiotic maturation is arrested for extended time in fog-2 females, activated MAPK is not detected. Interestingly, the activation of MAPK is inversely correlated with the assembly of large RNP granules in oocytes. In this study, we are addressing the hypothesis that MAPK inhibits the condensation of RNA-binding proteins (RBPs) into RNP granules. It is critical to understand how the assembly of RNP granules is regulated as they are hypothesized to maintain oocyte quality during extended meiotic arrest and other stress conditions. First, we used RNAi to knockdown mpk-1 in GFP::MEX-3 worms, and we detected ectopic condensation of MEX-3 into granules in oocytes. Since CAR-1 condenses into large granules in response to heat stress, we next asked if activated MAPK levels decrease in proximal oocytes after 1.5 hours at 31°C. We observed reduced levels of anti-diP-MAPK and the assembly of CAR-1 granules in oocytes. In parallel, to examine the effects of decreased mpk-1 expression on CAR-1 protein condensation, we used the temperature-sensitive allele mpk-1(ga111). At the permissive temperature, MAPK levels were high in oocytes, similar to wild-type worms, and CAR-1 was diffusely distributed in the oocyte cytoplasm. At the restrictive temperature, we observed reduced MAPK levels and increased CAR-1 condensation into granules. Wild-type worms at the restrictive temperature of 25°C had a similar phenotype as mpk-1(ga111) at the permissive temperature; therefore, the assembly of CAR-1 granules in mpk-1(ga111) is not due to mild heat stress. Current experiments are testing the effect of mpk-1(RNAi) in GFP::CAR-1 and GFP::CGH-1 worms. Our findings to date, support the hypothesis that MAPK activity inhibits the assembly of RNP granules; however, the mechanism by which MAPK may modulate the condensation of RNA binding proteins is not known. We plan to investigate if MAPK directly phosphorylates one or more RBPs, or if MAPK acts indirectly on RBP condensation, perhaps via the CCT chaperonin.

Affiliation:
- Central Michigan University