C. Elegans Somatic And Germ Cell Fate Decisions And Maintenance Of Cell Autophagy, Lsy-2 And Sumo Modification Pathway Synergy

Germ cells are set aside from other somatic cells at early embryonic stage in most sexually reproducing animals and formation of germ cells in many organisms, including in C.elegans and Drosophila,involves the segregation of specialized germ plasma from oocyte to germ precursor cells.Establishment of somatic and germ precursor cells during early embryogenesis in C.elegans requires maternal distributed proteins,such as PIE-1 and MEX-1,while maintenance of the soma/germ distinction requires the activities of multiple chromatin remodeling complexes, including the 1in-35 Rb pathway and the NURD complex,whose mutants lead to the ectopic expression of germline traits by somatic cells.How the germ/soma distinction is established? Whether other critical factors or pathways are involved in this process besides PIE-1 and MEX-1? How the lin-35 Rb pathway and the NURD complex are targeted to and the repressive state of germline specific genes is maintained in somatic cells? So,the mechanisms that specify and maintain the distinct somatic and germ cell fates during animal development are poorly understood.To understand how the somatic and germ cell fate is specified and maintained, we screened a library of bacterial clones expressing dsRNA designed to individually inactivation of 16,540 genes(targeting about 86%of the genome) for mutants with ectopic expression of the P granule specific reporter,pgl-1::gfp,in somatic cells. In this study,we show that a basal level of autophagy is required for eliminating the germ cell-specific gene products in somatic cells in C.elegans. Mutations in the autophagy pathway lead to the accumulation of germ P granules in various somatic tissues during embryogenesis and early larvae.Autophagy is one of the major degradative pathways in eukaryotic cells,and it is the only one with the capacity to degrade entire organelles.This process is ubiquitous among eukaryotes and has been discovered from yeast to man.But the exact molecular mechanism and the physiological function of autophagy are still unclear and we lack convenient tool to answer these questions.It is generally believed that autophagy is a non-selective,bulk degradation system of long-lived proteins and organelles.However,this general belief has been challenged recently.Emerging evidence indicates that autophagic degradation is highly selective,at least in certain cases.Right now,how autophagy specifically selects cargo is still largely unknown.Identification of more selectively degraded cargoes will help determine the molecular basis of the selective action and functions of autophagy.In our study we found that C.elegans P granules were selectively degraded in somatic cells in early embryo by autophagy.Mutations in the autophagy pathway lead to the accumulation of germ P granules in various somatic tissues during embryogenesis and early larvae.So this discovery provides a good marker(pgl-1::gfp) to monitor autophagy process in C.elegans.If function of autophagy machinery is normal,pgl-1::gfp will not accumulated in somatic tissues in embryo and early larvae. If the autophagy machinery was disrupted,pgl-1::gfp would be accumulated in somatic tissues.So we can use GFP to monitor autophagy in C.elegans easily.We found that mutations in lsy-2,encoding a zinc finger protein,result in various somatic cells adopting characteristics of germ cells including ectopic expression of P granules and this transformation requires the activitieds of multiple chromatin remodeling complexes.Moreover,lsy-2 functions synergistically with the lin-35 Rb pathway in specifying larval development and the maintaining the soma/germ distinctions.We further show that a basic domain in LSY-2 directly binds to RNA,suggesting a role for RNA in repressing germ cell-specific genes in somatic cells.We also found that mutations in sumoylation pathway,including ubc-9, uba-2/aos-1 and smo-1,lead to the expressin of germ cell fate by somatic cells.Our analysis reveal that the establishment and maintenance of the soma/germ distinction in C.elegans requires the concerted action of autophagy,chromatin remodeling complexes,the RNA binding protein LSY-2 and the post-translational sumoylation pathway.That autophagy pathway involes in soma/germ distinction provides a convenient tool(pgl-1::gfp) to monitor autophagy in C.elegans.