The animal body plan develops during the early stages of embryogenesis, when cells that will form internal tissues become positioned in the interior of the embryo. This reorganization, called gastrulation, occurs through the movement of early embryonic cells into layers called germ layers. Subsequently, cells within a germ layer organize into primordia that differentiate into organs. Our laboratory uses the model organism C. elegans to investigate the mechanisms of gastrulation and organogenesis. During C. elegans gastrulation, specific cells ingress from the surface of the embryo into its interior. We are focusing on several questions related to these movements: How do early embryonic cells acquire polarity such that cytoskeletal proteins needed for cell ingression become properly localized? What are the mechanisms of cell ingression movements? How do cells of different types come to occupy an appropriate position for organ formation? And finally, how do cells within an organ communicate with each other to link together adhesively and form functional units? We use microscopy to visualize cell polarization and morphogenesis live in the transparent C. elegans embryo, and perform genetic and cell biological manipulations to determine the molecular mechanisms that drive these events.
Polarization of early embryos by cell-cell contact
PAR proteins, a group of cortically enriched and asymmetrically localized polarity regulators, polarize early embryonic cells to prepare them for gastrulation. Cell-cell contact causes PAR-3, PAR-6 and PKC-3 to localize exclusively to contact-free surfaces (Nance and Priess, 2002). PAR-3 is required to concentrate non-muscle myosin, which accumulates on contact-free surfaces and induces an apical constriction that promotes ingression movements of endodermal cells (Nance et al., 2003). We identified the PAC-1 protein as a critical regulatory protein that connects cell contact polarization signals to PAR-3 asymmetry (Anderson et al., 2008). PAC-1 is a RhoGAP protein that negatively regulates Rho GTPases. Rho GTPases are signaling proteins have conserved roles in cell polarization and cytoskeletal organization. PAC-1 is recruited to sites of cell contact and locally excludes PAR proteins by inhibiting the Rho GTPase CDC-42, which is otherwise activated constitutively by multiple RhoGEF proteins (Chan and Nance, 2013). We showed that PAC-1 is recruited to cell contacts through interactions with the transmembrane adhesion protein HMR-1/E-cadherin (Klompstra et al., 2015). HMR-1 must be present on both touching cells to localize, suggesting that homophilic interactions between HMR-1 molecules across cell contacts stabilize the protein at these sites, concentrating PAC-1 at contacts to trigger polarization.
The two primordial germ cells in C. elegans embryos are transcriptionally quiescent, similar to PGCs in many other animals. Since endodermal cells require transcription to ingress, PGCs must use a different mechanism to internalize during gastrulation. We found that PGCs ingress by hitchhiking into the embryo on the backs of endodermal cells (Chihara and Nance, 2013). However, far from a passive process, PGCs actively upregulate the adhesion protein HMR-1/E-cadherin using a post-transcriptional mechanism mediated by the hmr-1 3′ UTR, allowing them to stick to the endodermal cells. Unexpectedly, we found that HMR-1/E-cadherin is required only in PGCs, suggesting that HMR-1 does not function as a homophilic adhesion protein linking PGCs to endoderm. Rather, we speculate either that strong PGC-PGC adhesion is needed for association with endodermal cells, or that HMR-1 on the PGC surfaces is binding to a different adhesion protein on endodermal cells.
Polarization, cell junction formation, and tube formation in epithelia
PAR proteins also function in the genesis of epithelial cells during organogenesis. Epithelial cells have a pronounced apicobasal polarity that is important for their function and their connection to each other. We have shown that PAR-3 defines the apical domain of epithelial cells during polarization (Achilleos, et al., 2010), and that the PAR-6 protein is required after polarization to assemble the junctions that connect epithelial cells to each other (Totong et al., 2007). Surprisingly, the master polarity regulator CDC-42 is not required for epithelial polarity in the C. elegans embryo, but rather regulates actin and adheres junctions to facilitate asymmetric cell shape changes during epidermal morphogenesis (Zilberman et al., 2017). We performed genetic screens to identify PAR (PKC-3) effector proteins that regulate epithelial cell junction formation, and identified both predicted (LGL-1) and novel (an aECM protein) proteins (Montoyo-Rosario et al., 2020). In a related project, we are examining how PAR proteins and the vesicle-tethering exocyst complex participate in lumen formation within a single-celled tube (the excretory cell canal) (Armenti et al., 2014a). The canal lumen forms via the targeted delivery of cytoplasmic vesicles. We found that PAR proteins and exocyst enrich at the lumenal surface, that exocyst activity is needed for vesicle fusion events at the lumenal surface, and that PAR proteins are required for to asymmetrically position exocyst components at the membrane. For these studies, we developed a method to conditionally and rapidly degrade tagged proteins from specific cells, allowing us to examine the function of essential genes after their acute depletion (Armenti et al., 2014b). Using targeted protein degradation, we were able to build a pathway connecting the EXC-5 RhoGEF, CDC-42, PAR-6, PKC-3 and exocyst to excretory cell intracellular lumen extension (Abrams and Nance, 2021).
Stem cell-niche interactions and germ cell development
We are using the primordial gonad, which consists of only four cells, as a model to determine how cells of different types communicate to assemble together during organ formation. The primordial gonad is a well-defined example of a stem cell-niche interaction, as it contains two stem cells (primordial germ cells) and two niche cells (somatic gonad precursors). We have identified roles for regulated cell-cell adhesion and basement membrane components in primordial gonad formation (Rohrschneider and Nance, 2013). In a surprising discovery, we showed that endodermal cells cannibalize large protrusions that primordial germ cells form, using a non-mitotic contractile ring (Maniscalco et al., 2019), altering their contents (Abdu et al., 2016). We identified a scission complex containing actin, dynamin, and a sorting nexin that functions in endodermal cells to actively bite lobes off of primordial germ cells, defining a novel mechanism for cellular remodeling. Somatic gonad precursor cells function as a niche by preventing endodermal cells from cannibalizing the PGC cell bodies and by templating a basement membrane that maintains PGC quiescence until embryogenesis is complete (McIntyre and Nance, 2020).