C. elegans GLA-3 is a novel component of the MAP kinase MPK-1 signaling pathway required for germ cell survival

Abstract

During oocyte development in Caenorhabditis elegans, approximately half of all developing germ cells undergo apoptosis. While this process is evolutionarily conserved from worms to humans, the regulators of germ cell death are still largely unknown. In a genetic screen for novel genes involved in germline apoptosis in Caenorhabditis elegans, we identified and cloned gla-3. Loss of gla-3 function results in increased germline apoptosis and reduced brood size due to defective pachytene exit from meiosis I. gla-3 encodes a TIS11-like zinc-finger-containing protein that is expressed in the germline, from the L4 larval stage to adulthood. Germ stem cells are the precursors to all subsequent generations in a species, and, therefore, germ cell formation is tightly monitored to ensure high-fidelity transfer of the genetic material. Germ cell genome integrity is monitored at several checkpoints, allowing for DNA repair, cell cycle arrest, and apoptosis when required. In mammals, inactivation of genes with checkpoint function frequently results in aberrant cell death and infertility (Lim and Hasty 1996;Bender et al. 2002). Germ cell development is also characterized by either massive waves or low but constant levels of apoptosis that are not caused by genetic defects. For example, >99.9% of oocytes undergo apoptosis in response to hormonal changes that occur at several stages during the female life cycle in mammals (Tilly 2001;Kim and Tilly 2004). Apoptosis is also the fate of ∼50% of germ cells undergoing oogenesis in the gonad of Caenorhabditis elegans hermaphrodites. (Gumienny et al. 1999). Characterization of the pathways that regulate germ cell death will contribute to our understanding of the cell suicide decision and might allow for more efficient therapeutic manipulation of the apoptotic program.C. elegans is a good model to study the signaling cascades involved in the decision between germ cell survival and germ cell death (Hengartner 1997;Gumienny et al. 1999). The adult hermaphrodite gonads consist of two U-shaped tubes that are connected at a common uterus. At the distal end of each gonad, mitotic germ stem cells proliferate in response to the Notch ligand LAG-2. Cells beyond the influence of LAG-2 enter meiosis and progress through the pachytene stage of meiosis I; this transition requires activation of the RAS/MAPK (MAP kinase) signaling cascade (Hubbard and Greenstein 2000;Seydoux and Schedl 2001). Following transition through pachytene, germ cells can either enter diakinesis of meiosis I and differentiate into oocytes or undergo apoptosis. We previously suggested that these cell deaths are the result of a physiological, homeostatic control mechanism that limits the number of germ cells permitted to differentiate into oocytes (Gumienny et al. 1999).

The phosphoinositide phosphatase MTM-1 regulates apoptotic cell corpse clearance through CED-5–CED-12 in C. elegans

“…gla-3 and gla-1 function in the hermaphrodite germline (Kritikou et al, 2006;Lettre et al, 2004). Loss of gla gene function superficially results in a similar germ cell corpse phenotype as observed in mtm-1(RNAi) treated animals: in gla-3 mutants, all germ cell corpse observed localized inside sheath cells (Fig.…”

Germ Cell Apoptosis and DNA Damage Responses

“…Therefore, MAP kinase signalling defective germlines show apoptosis competency and the lack of apoptosis in MAP kinase signalling defective mutants might be a genuine lack of physiological germ cell death. Consistent with this view, excessive apoptosis occurs when MAP kinase signalling is elevated in mutants of the lip-1 MAP kinase phosphatase (at high temperature) or the gla-3 RNA binding protein which can affect MAP kinase signalling (Kritikou et al 2006 ) . However, there is also emerging evidence that MAP kinase signalling might be needed to repress germ cell apoptosis.…”

Noncanonical control of C. elegans germline apoptosis by the insulin/IGF-1 and Ras/MAPK signaling pathways

“…27 The Ras/MAP kinase pathway also promotes DNA damage-induced germ cell apoptosis by cep-1-dependent and cep-1-independent mechanisms. 28,29 Given that insulin/IGF-1 has been shown to promote Ras/MAPK signaling in C. elegans vulval development and lifespan, 30,31 we wondered if they also cooperate in promoting germline apoptosis. Using antibodies to phosphorylated ERK, which has been shown to crossreact with phospho-MPK-1 in C. elegans, 29 we stained germlines isolated from wild-type, daf-2(lf) and pdk-1(lf) mutants treated with IR ( Figure 6).…”