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Gastrulation is a time during development when cells destined to produce internal tissues and organs move from the surface of the embryo into the interior. It is critical that the cell movements of gastrulation be precisely controlled, and coordinated with cell specification, in order for the embryo to develop normally. C. elegans gastrulation is relatively simple, can be observed easily in the transparent embryo, and can be manipulated genetically to uncover important regulatory mechanisms. Many of these cellular and molecular mechanisms – including cell shape, cytoskeletal, and cell cycle changes – appear to be conserved from flies to vertebrates. Here we review gastrulation in C. elegans, with an emphasis on recent data linking contact-induced cell polarity, PAR proteins, and cell fate specification to gastrulation control.
Many cells migrate during development to reach their proper location within the embryo. The first major cell rearrangements occur during gastrulation, a crucial time in development when cell movements separate the embryo into distinct layers called germ layers. The outer ectodermal layer produces the nervous system and the skin, the inner endodermal layer forms the digestive tract, and the intermediate mesodermal layer makes muscle and other tissues. Gastrulation must be exquisitely regulated to ensure that specific cells move to the correct position at the appropriate developmental time. Studies in model organisms such as flies, fish, frogs, and mice have begun to reveal details of how gastrulation movements are controlled and effected (reviewed by Keller et al., 2003; Leptin, 2005; Solnica-Krezel, 2005; Heisenberg and Solnica-Krezel, 2008). Recently, the nematode C. elegans has emerged as a new model system to investigate the cell biological and genetic control of gastrulation. Here, we review studies in C. elegans highlighting the importance of cell shape changes, contact-induced cell polarity, cell fate regulators, and extracellular signals in promoting gastrulation movements.
Gastrulation can occur through a variety of different strategies, including invagination of cell sheets or ingression of cells through a superficial structure such as the blastopore or primitive streak (reviewed by Keller et al., 2003; Leptin, 2005; Solnica-Krezel, 2005; Shook and Keller, 2008). In C. elegans and most other nematodes, gastrulation involves the ingression of pairs or small groups of cells from various locations on the ventral surface (Sulston et al., 1983; Nance and Priess, 2002; reviewed by Schierenberg, 2006). Most of the cells that ingress during gastrulation are born on the ventral surface, although a few cells are born in more dorsal positions and migrate ventrally before internalizing. As we describe below, at least some of the mechanisms of nematode gastrulation appear to be conserved in flies and vertebrates, suggesting that the wide variety of gastrulation movements observed in different species might be controlled by a set of common cellular mechanisms.
The early cleavages in C. elegans produce cells with predictable identities and characteristic positions, including the endoderm precursor cells (EPCs), which are born on the posterior ventral surface and will eventually produce the entire digestive tract. Gastrulation begins at the 26-cell stage when the two EPCs ingress from the surface of the embryo into a small blastocoel cavity (Fig. 1; Sulston et al., 1983). The blastocoel cavity begins to form during the 4-cell stage when small separations appear between cell contacts in the central region of the embryo. By the 26-cell stage, the blastocoel reaches its maximum size, which is approximately half the volume of an ingressing EPC (Nance and Priess, 2002). Because of the small size of the blastocoel, other cells must shift their positions to make room for the ingressing EPCs in the center of the embryo (see below). After the EPCs complete their ingression during the 44-cell stage (taking less than 30 minutes to ingress), mesodermal cells from several different lineages ingress over the next three hours from various regions of the ventral surface. Mesodermal cells can still ingress when EPC ingression is prevented by laser-killing of the E cell (parent of the EPCs), suggesting that the ingression of different groups of cells is largely independent (Nance and Priess, 2002). The final step in gastrulation occurs about an hour later when dorsal epidermal cells migrate and stretch to meet at the ventral midline, sealing the embryo in a layer of skin. Epidermal migration – also called ventral enclosure – has been the subject of several recent reviews and will not be discussed further here (reviewed by Piekny and Mains, 2003; Ding et al., 2004; Chisholm and Hardin, 2005).
Several cell biological changes within the EPCs combine to drive EPC ingression. Prior to gastrulation, each cell in the embryo has a contact-free apical surface that faces the eggshell, and basolateral surfaces that face other cells or the blastocoel cavity. As the EPCs begin to ingress, their apical surfaces flatten and constrict, resulting in a movement of the cytoplasm towards the inner side of the cell (Nance and Priess, 2002; Lee and Goldstein, 2003). Apical constriction is thought to occur when non-muscle myosin II at the apical surface causes cortical microfilaments to locally contract (Fig. 1; Nance and Priess, 2002; Lee and Goldstein, 2003; Nance et al., 2003). Non-muscle myosin II (hereafter myosin) consists of two heavy chains, two light chains, and two regulatory light chains that must be phosphorylated for actomyosin-mediated contraction to occur (reviewed by Matsumura, 2005; Conti and Adelstein, 2008). Both the heavy chain (NMY-2; see Fig. 1; Guo and Kemphues, 1996) and phosphorylated regulatory light chain (p-rMLC) accumulate at the apical surfaces of the EPCs as these cells ingress (Nance and Priess, 2002; Lee et al., 2006). Drugs that depolymerize microfilaments or inhibit myosin activity block EPC ingression (Lee and Goldstein, 2003). However, as described below, preventing the apical enrichment of activated myosin in the EPCs during gastrulation stages slows but does not block EPC ingression (Nance et al., 2003), raising the possibility that actomyosin-dependent forces other than apical constriction contribute to EPC internalization. Activated myosin also accumulates at the constricting apical surfaces of gastrulating amphibian bottle cells and of Drosophila mesodermal cells, suggesting that apical constriction mediated by asymmetric myosin activity is a conserved mechanism for internalizing cells during gastrulation (Young et al., 1991; Fox and Peifer, 2007; Lee and Harland, 2007).
As the EPCs ingress, neighboring cells migrate over to cover them (Nance and Priess, 2002; Lee and Goldstein, 2003). One result of these spreading movements is the liberation of space within the interior of the embryo. Since a confining eggshell surrounds the embryo and the blastocoel cavity is quite small relative to the EPCs, the redistribution of embryonic mass created by spreading movements might be essential to free interior space for the ingressing cells. Spreading cells include the MS cells (mesodermal precursors), which extend wedge-shaped processes in the direction of migration, and the P4 cell (germline precursor), which migrates toward the MS cells to cover the apical surfaces of the ingressing EPCs. Although the MS and P4 cells migrate toward one another, elegant blastomere recombination experiments have shown that the MS cells can migrate over the surfaces of the EPCs even when the P4 cell is removed (Lee and Goldstein, 2003). When the EPCs are separated, rotated relative to each other, and rejoined, MS and P4 both still migrate over the surface of the adjacent EPC, but they no longer migrate towards each other. These experiments indicate that the MS and P4 cells do not simply chemotax toward one another, but rather converge in response to unknown cues from the EPCs (Lee and Goldstein, 2003). Later mesodermal cell ingressions can occur without spreading of neighboring cells, presumably because the volume of late-ingressing cells is comparatively small (due to rounds of reductive cell division), and additional space for their internalization is not needed (Nance and Priess, 2002).
Cell cycle expansion is another cell biological change that likely contributes to efficient EPC ingression. Each cell in the early embryo has a cell cycle length that is characteristic of the lineage from which it arises. Compared to cells in other somatic lineages, the cell cycle length of the endoderm lineage is considerably longer (Sulston et al., 1983; Edgar and McGhee, 1988). Consequently, the two EPCs complete their ingression entirely within a single cell cycle, dividing only after they are fully internalized. Studies on gad-1 mutant embryos suggest that the extended EPC cell cycle is important for EPC ingression (Knight and Wood, 1998; Lee et al., 2006). Some gastrulation-defective mutants, such as gad-1 (encoding a WD repeat protein), fail to expand the E cell cycle, and the EPCs divide prematurely and remain on the surface of the embryo (Knight and Wood, 1998). EPC ingression in gad-1 mutant embryos can be rescued by artificially extending the EPC cell cycle using low-level laser irradiation, suggesting that the shortened E cell cycle in gad-1 mutants contributes to EPC ingression defects (Lee et al., 2006). EPCs that cannot constrict apically and divide while still on the embryo surface have a normal cell cycle, indicating that the expanded EPC cell cycle is preprogrammed rather than coordinated with ingression and delayed until ingression is completed (Nance et al., 2003). Similar expansions in cell cycle length occur in invaginating Drosophila mesoderm and amphibian bottle cells, and have been shown to be important for ingression (Grosshans and Wieschaus, 2000; Mata et al., 2000; Seher and Leptin, 2000; Kurth, 2005). One appealing theory is that cell cycle expansion allows cytoskeletal proteins – such as actin and myosin – to finish facilitating cell ingression movements before relocalizing to the cytokinetic furrow.
In order for myosin to become enriched at the apical surfaces of ingressing cells, and for the blastocoel cavity to form only in the central regions of the embryo, cells must acquire polarities that distinguish their different surfaces. Polarization begins at the 4-cell stage, when cell contacts induce the asymmetric localization of a group of conserved polarity proteins called PAR proteins. PAR proteins are required to polarize a variety of cells in both invertebrates and vertebrates, including epithelial cells, oocytes, the C. elegans zygote, and at least some neurons (reviewed by Nance, 2005; Solecki et al., 2006; Dow and Humbert, 2007; Goldstein and Macara, 2007). In each of these cases, PAR proteins polarize cells by developing a cortical asymmetry, which limits their interactions with downstream effectors to one region of the cell. Early in the 4-cell stage, PAR-3 (a PDZ domain scaffolding protein), PAR-6 (a PDZ and semi-CRIB domain scaffolding protein) and PKC-3 (an atypical protein kinase C) localize uniformly around the cortex of somatic cells (Etemad-Moghadam et al., 1995; Tabuse et al., 1998; Hung and Kemphues, 1999; Nance and Priess, 2002). By the late 4-cell stage, PAR-3, PAR-6, and PKC-3 are excluded from the ‘inner’ cortex (adjacent to sites of cell contact; synonymous with basolateral) and become restricted to the ‘outer’ (contact-free; synonymous with apical) cortex of cells. Two other PAR proteins – the kinase PAR-1 and the RING finger protein PAR-2 – localize specifically to the inner cortex at the late 4-cell stage (Guo and Kemphues, 1995; Boyd et al., 1996; Nance and Priess, 2002). We refer to this asymmetric localization of PAR proteins as ‘inner-outer’ rather than ‘apical-basolateral’ to distinguish the non-epithelial early embryonic cells from epithelial cells that form later during embryogenesis and develop an analogous apical-basolateral PAR asymmetry (Leung et al., 1999; Bossinger et al., 2001; McMahon et al., 2001; Totong et al., 2007). Inner-outer PAR asymmetry is maintained through early embryogenesis. When two early embryos are joined together in vitro to create new contacts, PAR-2 localizes to the ectopic sites of cell contact as well as the original cell contacts, and PAR-3 and PAR-6 are excluded from the ectopic cell contacts and restricted to the remaining contact-free cortex (Nance and Priess, 2002). These experiments demonstrate that the polarized localization of PAR proteins at the 4-cell stage is determined by the pattern of cell contacts.
The important role for PAR proteins in regulating myosin localization and blastocoel positioning was demonstrated by removing PAR-3 and PAR-6 specifically from early embryonic cells. Because PAR proteins are provided maternally and are required at an earlier stage to polarize the zygote along its anterior-posterior axis (Etemad-Moghadam et al., 1995; Guo and Kemphues, 1995; Boyd et al., 1996; Watts et al., 1996; Tabuse et al., 1998; Hung and Kemphues, 1999), it was necessary to use a novel protein degradation technique to determine the function of PAR proteins in gastrulation. Maternally expressed proteins, when tagged with the PIE-1 Zinc finger 1 (ZF1) domain, are present in the zygote but then degrade rapidly from early embryonic somatic cells (Reese et al., 2000). Expression of ZF1-tagged PAR-3 protein rescues the zygote polarity defects in par-3 mutant embryos, but then the fusion protein degrades from cells before gastrulation begins (Nance et al., 2003). In these PAR-3-depleted embryos, cell separations similar to those that form the blastocoel appear between more superficial cell contacts, and in some cases the blastocoel is contiguous with the embryo surface. Additionally, EPCs in PAR-3-depleted embryos fail to accumulate myosin apically, and gastrulation movements are slowed significantly but still occur. Similar blastocoel and gastrulation phenotypes are observed in embryos when PAR-6 is depleted by the same method (Nance et al., 2003). These results indicate the importance of PAR proteins in the contact-induced polarization of early embryonic cells that prepares the embryo for gastrulation.
How do cell contacts promote the inner-outer asymmetry of PAR proteins in early embryos? Anderson et al. (2008) began to address this question by screening for mutations that prevent inner-outer PAR asymmetry from developing. Mutations in a gene they named pac-1 (‘PAR-6 at contacts’) prevent PAR-3, PAR-6, and PKC-3 from becoming restricted to the outer cortex of early embryonic cells; instead, these proteins remain associated with both the inner and outer cortex. The failure to establish inner-outer PAR asymmetry causes phenotypes similar to those seen in embryos depleted of PAR-3 or PAR-6: activated myosin regulatory light chain (p-rMLC) fails to accumulate at the apical surfaces of the EPCs, and EPC ingression is delayed significantly though still occurs (Anderson et al., 2008). GFP-PAC-1 localizes to the inner but not the outer cortex in both wild-type and par-6 mutant embryos and also can localize to ectopic cell contacts generated by combining two embryos in vitro (Anderson et al., 2008). These results demonstrate that PAC-1 is recruited to the inner cortex by cell contacts and functions upstream of PAR-6 to establish inner-outer PAR asymmetry.
PAC-1 contains a RhoGAP domain, which functions to inhibit Rho GTPases (Chen et al., 1994). Rho GTPases are a group of signaling proteins that regulates the actomyosin cytoskeleton (reviewed by Etienne-Manneville and Hall, 2002; Lundquist, 2006; Cowan and Hyman, 2007); Rho GTPases cycle between active GTP-bound and inactive GDP-bound states, and RhoGAP proteins inhibit Rho GTPases by promoting GTP hydrolysis (reviewed by Etienne-Manneville and Hall, 2002; Lundquist, 2006; Tcherkezian and Lamarche-Vane, 2007). In early embryos lacking the Rho GTPase CDC-42, PAC-1 localizes normally but PAR-6 is found in the cytoplasm rather than at the outer cortex. Conversely, expressing a constitutively active form of CDC-42 (Aceto et al., 2006), which cannot be inactivated by PAC-1, causes PAR-6 to associate with both the inner and outer cortex (Anderson et al., 2008). The opposite phenotypes of pac-1 mutant and CDC-42-depleted embryos, and the similar phenotypes of pac-1 mutant embryos and embryos expressing constitutively active CDC-42, suggest that CDC-42 is a target of PAC-1 inhibition. Since CDC-42 localizes uniformly around the entire cell cortex while PAC-1 is found only at the inner cortex, Anderson et al. (2008) proposed that PAC-1 inactivates CDC-42 at cell contacts, leaving CDC-42 active at the contact-free outer cortex where PAC-1 is absent. CDC-42 at the outer cortex likely recruits or stabilizes PAR-6 directly through interaction with the PAR-6 semi-CRIB (CDC-42/Rac interactive binding) domain, since mutations in this domain prevent PAR-6 from associating with the cortex (Aceto et al., 2006; Anderson et al., 2008). These findings provide a molecular link between cell contacts and the inner-outer PAR asymmetries they induce in order to promote blastocoel formation and efficient cell ingressions during gastrulation (Fig. 2). As cells ingress, they form new cell contacts and eventually lose their contact-free surface. Because PAC-1 appears to be rapidly recruited to new cell contacts (Anderson et al., 2008), it could function to quickly adjust cell polarity and cytoskeletal asymmetries in response to the dynamic changes in cell contacts that occur during gastrulation.
It is likely that the molecular links between cell contacts, PAR polarity, and myosin localization are utilized to facilitate the ingression of multiple types of cells. For example, the later-ingressing MS cells also depend on PAR-3 and PAR-6 for their timely ingression (Nance et al., 2003), and like the EPCs, ingressing MS cells accumulate myosin apically and appear to flatten and constrict their apical surfaces as they ingress (Nance and Priess, 2002). Although pac-1 is conserved in flies and vertebrates (its mammalian homologs are ARHGAP10 and ARHGAP23; Anderson et al., 2008), it has not yet been determined if pac-1 homologs regulate gastrulation movements in other species. However, spatial restriction of Rho GTPase activity is a conserved feature of gastrulation. For example, the guanine nucleotide exchange factor RhoGEF2, which can activate Rho1 GTPase, is required for apical myosin accumulation and invagination of Drosophila mesodermal cells (Barrett et al., 1997; Hacker and Perrimon, 1998; Nikolaidou and Barrett, 2004; Rogers et al., 2004; Grosshans et al., 2005). RhoGEF2 accumulates at the apical surfaces of invaginating mesodermal cells at the time of gastrulation, and mutations that interfere with RhoGEF2 apical localization disrupt mesodermal cell invagination (Grosshans et al., 2005; Fox and Peifer, 2007; Kolsch et al., 2007).
Because C. elegans endodermal and mesodermal cells ultimately internalize in embryos lacking PAR-3 or PAR-6, it is likely that PAR-independent mechanisms also contribute to cell internalization. One possibility is that the spreading movements of adjacent cells, in addition to freeing space within the embryo, also help to push the ingressing cells inward. An intriguing possibility is that differences in cell adhesion between ingressing cells and their neighbors induce these spreading movements. While it is not known if such adhesion differences exist, removing the cell surface adhesion proteins HMR-1/E-cadherin and LAD-1/SAX-7/L1CAM together can disrupt EPC ingression (J. Hardin, pers. comm.). One group of cells that might utilize a different ingression mechanism is the germline precursor cells (daughters of P4), which ingress with other mesodermal cells during later stages of gastrulation. Unlike somatic cells, the germline precursor cells (called P0 through P4 at successive developmental stages) do not develop contact-induced PAR asymmetries (Etemad-Moghadam et al., 1995; Guo and Kemphues, 1995; Boyd et al., 1996; Tabuse et al., 1998; Hung and Kemphues, 1999; Nance and Priess, 2002). Additionally, while embryonic transcription is required for EPC ingression (Powell-Coffman et al., 1996), the germline precursor cells appear to be transcriptionally quiescent (reviewed by Schaner and Kelly, 2006). This suggests that germline precursor cells may utilize distinct ingression mechanisms, or that these cells may be carried passively into the embryo by adjacent ingressing cells.
PAR proteins polarize all of the somatic cells in the early C. elegans embryo. Although inner-outer PAR asymmetry is required to facilitate gastrulation, only a subset of cells actually ingress. Therefore, gastrulation must be triggered by some other more cell-specific signal. Because gastrulation must be intimately coupled with cell fate specification, an attractive idea is that cell fate regulators also direct cell movements during gastrulation. The establishment of endoderm cell fate begins at the 4-cell stage, when MOM-2/Wnt signals originating from the P2 germline precursor cell polarize the adjacent EMS cell so that it produces daughter cells with different fates—the anterior MS cell (mesodermal precursor) and the posterior E cell (endodermal precursor and parent of the ingressing EPCs; Goldstein, 1992; Goldstein, 1993; Rocheleau et al., 1997; Thorpe et al., 1997; reviewed by Maduro, 2006). The outcome of WNT signaling, which works in conjunction with the maternally supplied transcription factor SKN-1 present within EMS, is that end-1 and end-3 are expressed exclusively in the E cell and its immediate descendants (Bowerman et al., 1993; Zhu et al., 1997; Maduro et al., 2005; reviewed by Maduro, 2006). END-1 and END-3 are redundant GATA transcription factors that function as master inducers of endoderm fate (Zhu et al., 1997; Zhu et al., 1998; Maduro et al., 2005). Mutations that transform the fate of the E cell and its descendants, such as skn-1, mom-2/Wnt, and chromosomal deficiencies removing both end-1 and end-3, can disrupt ingression of the EPCs (Bowerman et al., 1992; Thorpe et al., 1997; Zhu et al., 1997; Maduro et al., 2005; Lee et al., 2006). Ectopic expression of the end genes can also trigger ingression-like movements. For example, mutations in pie-1, which encodes a transcriptional repressor that prevents SKN-1 and other transcription factors from operating in the germline precursor cell, transform the germline precursor P3 to an E cell-like fate, resulting in the ectopic ingression of the transformed P3 daughter cells (Mello et al., 1992; Mello et al., 1996; Lee et al., 2006). Therefore, EPC ingression appears to be controlled, at least in part, by the same transcription factors that induce endoderm fate. Analysis of later-ingressing mesodermal cells shows that cells with the same fate often ingress at the same time, even if they are located at different positions on the surface of the embryo (Nance and Priess, 2002). These observations indicate that it is the identity of cells, rather than their position within the embryo, that determines whether and when they will ingress.
Although the secreted MOM-2/Wnt ligand and its cell-surface receptor MOM-5/Frizzled are important for endoderm specification (see above), Wnt signaling appears to have an additional role in triggering EPC ingression. Many mom-2 mutant embryos and a few mom-5 mutant embryos fail to produce endoderm (Rocheleau et al., 1997; Thorpe et al., 1997; Lee et al., 2006). However, most mom-2 and mom-5 mutant embryos that do express endoderm markers in the EPCs still fail to gastrulate normally, suggesting that MOM-2/Wnt and MOM-5/Fz are separably required for both endoderm specification and for gastrulation (Lee et al., 2006). To determine when Wnt/Fz signaling is needed for normal gastrulation, Lee et al. (2006) used in vitro cell recombination experiments to manipulate the presence of MOM-2/Wnt signaling (from P2 to EMS) both during and after endoderm specification. During the 4-cell stage, a mom-2 EMS cell was exposed to a wild-type P2 cell to allow endoderm specification, and then the P2 cell was quickly replaced with either a wild-type or a mom-2 mutant P2 cell. The wild-type P2 cell was more effective than the mom-2 mutant P2 cell at rescuing gastrulation defects, suggesting that while Wnt signaling contributes to endoderm specification, Wnt signaling is also required for gastrulation after endoderm specification begins at the 4-cell stage.
To determine how Wnt signaling contributes to gastrulation, Lee et al. (2006) examined several of the cell biological aspects of gastrulation in Wnt signaling mutants. Like gad-1 (see above), many mom-2 and mom-5 mutant embryos also exhibit a shortened E cell cycle; however, unlike in gad-1, laser-induced extension of the short E cell cycle in mom-2 and mom-5 mutant embryos fails to rescue EPC ingression (Lee et al., 2006). This suggests that Wnt signaling is required for both E cell cycle expansion and for other aspects of gastrulation. Lee et al. (2006) observed that PAR polarity is normal and myosin (NMY-2) accumulates apically in the EPCs of mom-5 mutant embryos. However, myosin activation (p-rMLC) is defective in mom-5 mutant embryos, and the apical surfaces of the EPCs do not appear to constrict. Thus Wnt/Fz signaling, in addition to specifying the fates of the EPCs (including the E cell cycle length), plays a secondary role in activating myosin within these cells to allow apical constriction.
In C. elegans, there are three homologs of Dishevelled (Dsh), a cytoplasmic protein that functions downstream of Wnt/Fz signaling (reviewed by Eisenmann, 2005; Wallingford and Habas, 2005). These three Dsh homologs are redundantly required for normal E cell cycle expansion and for normal EPC ingression (Walston et al., 2004; Lee et al., 2006). Downstream of Dsh, Wnt/Fz signaling can function through several different pathways, some of which lead to changes in gene expression (reviewed by Eisenmann, 2005; Gordon and Nusse, 2006). Whether Wnt/Fz signaling leads to p-rMLC accumulation through a transcriptional or non-transcriptional pathway is not yet known. In either case, it will be interesting to identify the Wnt target genes or proteins that are important for ingression and learn how they control myosin activation.
Control of cell internalization by cell fate regulators appears to be a common theme of gastrulation. For example, in flies the mesoderm-inducing transcription factor Twist also regulates the expression of genes needed for cytoskeletal changes that trigger mesoderm invagination (Leptin and Grunewald, 1990; Costa et al., 1994; Kolsch et al., 2007). Similarly, FGF and Nodal signaling are required in vertebrates for both mesoderm specification and for gastrulation movements (Carmany-Rampey and Schier, 2001; Ciruna and Rossant, 2001; Nutt et al., 2001; Branford and Yost, 2002; Feldman et al., 2002; Ninomiya et al., 2004; reviewed by Heisenberg and Solnica-Krezel, 2008). Extensive work in vertebrate model systems has established a dual role for Wnt signaling (through different downstream pathways) in cell fate specification and in the polarized movements of gastrulating cells (reviewed by Wallingford and Habas, 2005; Heisenberg and Solnica-Krezel, 2008). The coordinate regulation of both cell fate and gastrulation genes ensures that cells of the proper identity end up in the proper place within the embryo.
Recent work on C. elegans gastrulation has expanded our knowledge of the control of gastrulation in several different ways. Cell contacts initiate inner-outer cell polarity via PAC-1, CDC-42, and PAR proteins, leading to the apical accumulation of activated myosin and the restriction of the blastocoel to the inner-most region of the embryo. By contrast, endoderm fate regulators, including secreted Wnt signals, are required for the normal E cell cycle expansion, and for activation of myosin at the apical surfaces of the EPCs. Both of these pathways combine to drive the apical constriction and ingression of the EPCs (Fig. 3).
Clearly there are still many gaps to fill in our understanding of how C. elegans gastrulation is controlled. Proper PAC-1 localization is critical for inducing inner-outer PAR asymmetry in embryonic cells, but how is PAC-1 recruited specifically to sites of cell contact? How does inner-outer PAR asymmetry promote myosin localization? How do end-1, end-3, and Wnt/Fz signaling trigger cell cycle expansion, myosin activation, and apical constriction within the EPCs? Finally, what is the role of cell adhesion in gastrulation? The fact that several mechanisms of C. elegans gastrulation – such as apical constriction and control of gastrulation movements by cell fate regulators – are shared with flies and vertebrates suggests that the worm will continue to provide a fruitful model for understanding conserved molecular features of gastrulation control.
We thank Jeff Hardin for sharing data before publication, members of the Nance lab for discussions, and Ann Wehman for helpful comments on the manuscript. Support for M.R.R. and J.N. was provided by National Institutes of Health grant GM078341.
Grant Sponsor: National Institutes of Health; Grant number GM078341