In the C. elegans hermaphrodite germline, over half of the developing germ cells are eliminated through “physiological” germ cell apoptosis. This elimination is hypothesized to provide the surviving germ cells both the cytoplasmic nutrients and space required to undergo the growth necessitated to form a healthy, fertilization-competent oocyte. While this elimination involves the core apoptotic machinery of CED-9, CED-4 and CED-3, how this machinery is activated in “physiological” germ cell apoptosis remains unknown. Recently, we identified an unknown role for the ELAV-type RNA binding protein ETR-1 in germline apoptosis. This study aims to investigate the role of ETR-1 in both reproduction and “physiological” germ cell apoptosis.
ETR-1 is known to play a role in muscle development, as etr-1(RNAi) animals arrest at the two-fold stage of embryogenesis. Our fertility assays conducted in wild-type, a somatic defective RNAi mutant (rrf-1(pk1417)) and a germline defective RNAi mutants (ppw-1(pk1425)) implicate a reproductive role of ETR-1. etr-1(RNAi) animals have a reduced brood size compared to control animals in all three mutant lines tested. Additionally, the ability of ETR-1 depletion to suppress the published WEE-1.3-depletion infertility phenotype is dependent on ETR-1 being depleted in the soma. While investigating the cause of the reduced brood size of etr-1(RNAi) animals, we observed an increase in the number of germ cells undergoing apoptosis.
We utilized Transmission Electron Microscopy (TEM) to investigate the germlines of ETR-1-depleted animals. TEM confirms the increase in apoptotic germ cells and reveals significant defects in the structure of the somatic gonadal sheath cells encasing the germline compared to controls. Since the gonadal sheath cells are involved in the engulfment of apoptotic germ cells, and we had previously localized ETR-1 to the sheath, this implies a defect in the engulfment of dying germ cells. We will demonstrate that ETR-1 depletion in ced-1(e1735) engulfment mutants rescues the etr-1(RNAi) reduced brood size, and co-depletion of ced-1(RNAi) and etr-1(RNAi) suppresses the increase in the number of apoptotic bodies observed in etr-1(RNAi) animals.
Combined this data reveals a novel role for ETR-1 in reproduction and “physiological” germ cell apoptosis, potentially in the process of engulfment. These studies will increase our understanding of the apoptotic pathway and how ETR-1, a highly conserved CELF/BRUNOL protein, influences an organism’s reproductive capability.