Notch signaling pathway

Notch signaling mediates cell-cell interactions in a multitude of animal developmental and physiological processes. These interactions have been shown to be key determinants of cell fate decisions as well as important for tissue homeostasis. In C. elegans the Notch pathway has been most studied in relation to cell fate determination. Core components of the Notch pathway in C. elegans include receptors lin-12 and glp-1, which respond to ligands lag-2, apx-1, arg-1 and dsl-1. The LIN-12 Notch pathway directs somatic cell fate decisions during gonadogenesis, in particular, during vulval development. GLP-1 directs cell fate decisions during germline proliferation. A number of direct and indirect modulators of LIN-12 and GLP-1 have been identified, in one case, the same regulator, HECD-1, has been shown to positively modulate LIN-12 in the somatic gonad while negatively modulating GLP-1 in the germ line.

Wnt signaling pathway

Wnt glycoproteins are signaling molecules that control a wide range of developmental processes and is a conserved feature of metazoan development. In C. elegans Wnt signaling has been shown to play a role in cell fate specification and determination of cell polarity, cell migration, and axis determination during axon outgrowth. A 'canonical' Wnt signaling pathway has been elucidated in vertebrate and invertebrate model systems where Wnt binding leads to the stabilization of the transcription factor beta-catenin, which then enters the nucleus to regulate Wnt pathway target genes. Like other species, the C. elegans genome encodes multiple genes for Wnt ligands, EGL-20, LIN-44, MOM-2, CWN-1, CWN-2) and Wnt receptors (LIN-17, MOM-5, MIG-1, CFZ-2, LIN-18). Canonical Wnt signaling in C. elegans, utilizes the beta-catenin BAR-1 to convert POP-1 into an activator and controls the expression of several homeobox genes. However, unlike vertebrates or Drosophila, the C. elegans genome encodes multiple beta-catenin genes (HMP-2, BAR-1, SYS-1, WRM-1), which give rise to noncanonical Wnt signalling pathways: for example, the endoderm induction pathway requires the beta-catenin WRM-1 and parallel input from a mitogen-activated kinase (MAPK) pathway to downregulate POP-1.

EGF signaling pathway

The Epidermal Growth Factor (EGF) signaling pathway was initially identified by Nobel Prize winner Stanley Cohen who found EGF to be involved in numerous cellular signaling events in mouse development. The single C. elegans EGF ligand, lin-3 and its receptor, lin-28 were initially found in screens for vulval development mutants. Further analysis of similar mutants, led to the identification of the C. elegans core EGFR signaling pathway components, SEM-5/GRB2, SOS-1, LET-60/RAS, LIN-45/RAF, MEK-2/MEK, and MPK-1. Positive regulators of this pathway were subsequently found in suppressor screens, and include KSR-1, SUR-8/SOC-2, and SUR-6. Negative modulators were found in further forward and reverse genetic screens and include UNC-101, SLI-1, GAP-1, SUR-5 and ARK-1 (in conjunction with GRB-2). LIN-3/EGF acts as a signal from the single anchor cell to induce neighboring undifferentiated epithelial cells to adopt the vulval fate during larval development. Impairing this signaling event results in a failure of vulval fate specification, and eventually a vulvaless animal that cannot lay eggs (the "bag of worms" phenotype). As in the mouse, EGF signaling was found to be involved in a number of other processes, although perhaps most importantly it is required for larval survival; homozygous let-23 null worms die at the L1 stage. Further, EGF signaling has been shown to function in coordinating vulva to uterus communication during development, promoting male spicule development, inducing P12.p cell fate in the epithelia, at the same time acting as a physiological signal, promoting ovulatory contractions of the gonad sheath cells, inducing a reversible nervous system quiescence during each larval molt, and regulating protein and calcium homeostasis in the adult worm.

RTK/Ras/MAPK signaling pathway

Receptor Tyrosine Kinase (RTK)/Ras GTPase/MAP kinase (MAPK) activated signaling pathways are used repeatedly during metazoan development to control many different biological processes. C. elegans contains two different RTKs (LET-23/EGFR and EGL-15 /FGFR) that are known to stimulate LET-60/Ras and a MAPK cascade consisting of the kinases LIN-45/Raf, MEK-2/MEK and MPK-1/ERK. This Ras/MAPK cascade is required for multiple developmental events, including induction of vulval, uterine, spicule, P12 and excretory duct cell fates, control of sex myoblast migration and axon guidance, and promotion of germline meiosis. Studies in C. elegans have provided much insight into the basic framework of this RTK/Ras/MAPK signaling pathway, its regulation, how it elicits cell-type specific responses, and how it interacts with other signaling pathways such as the Wnt and Notch pathways.

Vulval development

The C. elegans vulva connects the hermaphrodite uterus to the outside of the nematode. The development of this organ is an intensively studied biological process making it a useful model for animal organogenesis. C. elegans vulval development involves the temporal and spatial coordination of intercellular signaling, evolutionarily conserved signal transduction pathways, and transcriptional regulation. Among the pathways involved in vulval development are the WNT, epidermal growth factor (EGF), and LIN-12/NOTCH signaling pathways. These pathways work in temporal succession to prime epidermal cells to first be competent to take on vulva cell fates and later to solidify their vulval cell specification. These pathways also work antagonistically to specify the precise pattern of cell fates needed to form the vulva. A key cell in vulval development is the anchor cell. Signals from this cell initiate the transition from epidermal to vulval precursor cell. This cell also influences the specification of surrounding uterine cells, which is required to connect the uterus to the vulva. The development of the final vulval organ requires the anchor cell to invade between terminally differentiated vulval cells in a behavior analogous to that of metastatic tumor cells.

Germline apoptosis

Germline apoptosis is an integral part of oogenesis in many animals, including humans. Germline apoptosis relies on key components of the core apoptotic machinery, including CED-3 and CED-4 and on the same engulfment mechanism as for somatic corpses. The Ras pathway might directly or indirectly regulate germline PCD as shown, unlike somatic apoptosis, cell death in the germline is not developmentally programmed by cell lineage. Germline apoptosis can be divided into two categories, physiological or stress-induced. Physiological germline apoptosis reduces the number of cells that complete oogenesis, and its induction is independent of the BH3-only apoptosis effecter EGL-1, differing from other apoptotic pathways. Stress-induced germline apoptosis is triggered by a genomic integrity checkpoint. In addition, exposure to certain environmental insults or pathogens induces germ cell apoptosis. Germ cell apoptosis in C. elegans occurs only during oocyte production, and is further restricted to the gonadal loop region, where developing oocytes exit the pachytene stage of meiotic prophase and transition into diplotene. let-60 (ras), lin-45 (raf), mek-2 (MAPKK), and mpk-1 (MAPK) have been show to be required for exit from pachytene arrest, as well as for PCD in the female germline, suggesting the Ras pathway might directly or indirectly regulate germline PCD.

miRNA pathway

microRNA (miRNA) elements are short (20-25 nucleotides (nt)), noncoding, single-stranded RNAs that bind to complementary sequences on target messenger RNA (mRNAs), which can result in translational repression or target mRNA degradation. microRNAs are critical components of developmental timing, responses to stress, and aging. Regulating the biogenesis and metabolism of miRNAs thus has an enormous impact on the health of the organism. In the genome, miRNAs reside in intergenic regions as well as within introns and exons of protein-coding genes. miRNAs are transcribed as long primary miRNAs (pri-miRNA). These pri-miRNA transcripts form hairpin-like structures with a double stranded stem and a terminal loop. pri-miRNAs are cleaved into smaller ~65-nt long precursor miRNA (pre-miRNA) by terminal-loop binding proteins and a microprocessor complex. This complex is composed of DRSH-1/DROSHA and PASH-1/DGCR8. In humans and yeast, the pre-miRNA product is exported to the cytoplasm by Exportin-5/XPO5 and RanGTP, respectively. The closest homologs of these proteins in C. elegans include xpo-1 and Y105E8A.1/yrs-2. Once in the cytoplasm, the pre-miRNA is cleaved at the top of the terminal loop by DCR-1/DICER, producing a duplex of two mature ~22nt miRNAs.An alternative, DRSH-1/DROSHA-independent, miRNA biogensis pathway has been identified in C. elegans and Drosophila. In this pathway pre-miRNAs are created through mRNA spliced introns. These introns form Mirtron lariats, which are debranched and refolded into pre-miRNA species. Mirtron generated pre-miRNAs are exported and processed in the same way as pre-miRNAs generated through the canonical miRNA biogenesis pathway. As mentioned previously, miRNA activity functions in many biological processes and hence needs to be temporally and spatially regulated. Much of that regulation occurs at the level of miRNA biogenesis and processing through Drosha binding/associated proteins, Dicer binding proteins, or proteins that bind to pri- and/or pre-miRNAs terminal loops. These regulators control expression or activity of miRNAs on specific miRNAs or families of miRNAs. For example, DCR-1 interaction proteins, ALG-1 and ALG-2, are required for lin-4 and let-7 miRNA formation and maturation. miRNA processing can also be influenced by ADAR editing, cellular localization, proteins known to regulate transcription (such as p53, SMADs) and proteins known to regulate mRNA stability (such as KSRP).