The DSL ligand APX-1 is required for normal ovulation in C. elegans

Abstract

DSL ligands activate the Notch receptor in many cellular contexts across metazoa to specify cell fate. In addition, Notch receptor activity is implicated in post-mitotic morphogenesis and neuronal function. In C. elegans, the DSL family ligand APX-1 is expressed in a subset of cells of the proximal gonad lineage, where it can act as a latent proliferation-promoting signal to maintain proximal germline tumors. Here we examine apx-1 in the proximal gonad and uncover a role in the maintenance of normal ovulation. Depletion of apx-1 causes an endomitotic oocyte (Emo) phenotype and ovulation defects. We find that lag-2 can substitute for apx-1 in this role, that the ovulation defect is partially suppressed by loss of ipp-5, and that lin-12 depletion causes a similar phenotype. In addition, we find that the ovulation defects are often accompanied by a delay of spermathecal distal neck closure after oocyte entry. Although calcium oscillations occur in the spermatheca, calcium signals are abnormal when the distal neck does not close completely. Moreover, oocytes sometimes cannot properly transit through the spermatheca, leading to fragmentation of oocytes once the neck closes. Finally, abnormal oocytes and neck closure defects are seen occasionally when apx-1 or lin-12 activity is reduced in adult animals, suggesting a possible post-developmental role for APX-1 and LIN-12 signaling in ovulation.

Tension-dependent RHGF-1 recruitment to stress fibers drives robust spermathecal tissue contraction

“…Once the oocyte enters the spermatheca, premature contraction before the eggshell is synthesized, will pinch the oocyte, and cause embryonic lethality ( Hegsted et al, 2016 ; Tan and Zaidel-Bar, 2015 ). On the other hand, delayed or lack of contraction creates a “traffic jam” in the fertility process, with embryos accumulating in the spermatheca and oocytes becoming endomitotic ( Kovacevic et al, 2013 ; McGovern et al, 2018 ).…”

A Genome-Wide RNAi Screen for Enhancers of a Germline Tumor Phenotype Caused by Elevated GLP-1/Notch Signaling inCaenorhabditis elegans

“…In this case, the activity of DSL ligands APX-1 and ARG-1 in the proximal somatic gonad inappropriately interact with GLP-1 on the surface of persistent undifferentiated germ cells and thereby drive proximal tumor formation. These ligands are expressed in cells born in late stages of somatic gonad development ( Nadarajan et al 2009 ) and act to promote ovulation ( McGovern et al 2018 ). Since initial meiotic entry (also termed “initial meiosis”; Pepper et al, 2003b )), the time when the first germ cells overtly differentiate by entering prophase of meiosis I, occurs in the third larval stage (L3), these later-appearing DSL ligands normally have no access to GLP-1 -expressing germ cells.…”

Activation and organization of the actomyosin cytoskeleton in the C. elegans spermatheca

“…This may have evolved as a mechanism to maintain different tissue tone in the spermathecal distal neck and valve compared to the bag. Tight closure of the spermathecal distal neck during ovulation is required to prevent the embryo from sliding backward into the proximal gonad arm (Gissendanner et al, 2008;McGovern et al). However, hyperconstriction of the spermathecal bag cells can damage embryos (Tan and Zaidel-Bar, 2015), supporting the idea that distal neck and bag cells constrict to different degrees during ovulation.…”