Role of the extracellular matrix in epithelial morphogenesis: a view from C. elegans

Abstract

The extracellular matrix (ECM) plays an essential role in organizing tissues, defining their shapes or in presenting growth factors. Their components have been well described in most species, but our understanding of the mechanisms that control ECM remodeling remains limited. Likewise, how the ECM contributes to cellular mechanical responses has been examined in few cases. Here, I review how studies performed in C. elegans have brought several significant advances on those topics. Focusing only on epithelial cells, I discuss basement membrane invasion by the anchor cell during vulva morphogenesis, a process that has greatly expanded our knowledge of ECM remodeling in vivo. I then discuss the ECM role in a novel mechanotransduction process, whereby muscle contractions stimulate the remodeling of hemidesmosome-like junctions in the epidermis, which highlights that these junctions are mechanosensitive. Finally, I discuss progress in defining the composition and potential roles of the apical ECM covering epidermal cells in embryos.

Figure 1. Integrin contribution to BM invasion by the anchor cell (AC). (A and B) During the late 2nd larval stage and early 3rd larval stage, a netrin signal originating from the ventral neurons (A), in parallel to a signal of unknown nature secreted by the first two daughters of the central vulval precursor P6.p (black arrows in A′) polarize the AC (graded red in A′). INA-1/PAT-3 integrin activity in the AC is essential to achieve AC polarization (B). (C–D) Once polarized, the AC starts to breach and remove the BM between the somatic gonad and the vulva (green and blue lines). Two features of AC polarity correspond to the formation of ventral finger-like protrusions that start to breach the BM, and the secretion of hemicentin (orange ovals; A′). During the mid-3rd larval stage, the AC dissolves the BM at the interface with P6.p grand-daughters (C′). INA-1/PAT-3 integrin activity in the AC is essential to achieve BM removal (D). (E–F) Vulval cells (letter A–F) invagination and toroid formation causes the BM to slide laterally, further expanding the gap between the somatic gonad and the vulva (pink arrows in E) until INA-1/PAT-3 activity acting in VulD stabilizes it. In the absence of INA-1/PAT-3 integrin, BM sliding continues (F). The utse corresponds to a small syncytium resulting from the fusion of the AC and several ventral uterine cells.

Figure 2. Contribution of the basal and apical ECM to embryonic elongation. (A) Side representations of wild-type embryos at three stages of elongation (early to late), showing individual epidermal cells (green, yellow, blue). These cells lengthen 4x along the A/P axis, while maintaining contacts with the same neighbors, and reduce their dorso/ventral dimension resulting in a 2.5x reduction of embryonic diameter. The drawings also show the apical ECM (black) and C. elegans hemidesmosome-like junctions (CeHDs, red). CeHDs evolve over time from a dotted and disorganized to a well-organized striped pattern. The aECM starts to form early on; muscles become active slightly before the 2-fold stage. Elongation most likely involves two distinct phases, since the Rho-kinase is dispensable after mid-elongation, when muscles start contracting. (B) Anatomy of the muscle-epidermis connection showing a cross-section through the embryo (left). The BM found at the muscle-epidermis interface contains Perlecan (UNC-52) which acts to anchor muscle myofilaments through integrins, and the epidermis most likely through myotactin (a large single-pass transmembrane nematode specific protein) which organizes C. elegans hemidesmosome-like (CeHD) junction at the basal side of the cell. A distinct set of receptors is found at apical CeHDs (dotted rectangles, right). Muscles are A/P-oriented and anchored to the cuticle through basal and apical CeHDs, which are bridged by intermediate filaments. Some CeHD components were omitted for clarity. GIT-1 anchoring to CeHDs depends on a muscle tensional input; we hypothesize that tension induces a conformational change within some CeHD components (symbolized here by VAB-19 and VAB-10 becoming more squarish, although the identity of the actual CeHD component that would undergo this change is unknown). (C) Three mutant situations with consequences on embryonic elongation. The mechanotransduction pathway was identified through a synthetic lethal screen conducted in a weak vab-10 mutant (with no elongation defect on its own); combining it with a strong pak-1 mutant, which on its own only slows down elongation, induces strong elongation and CeHD organization defects (like in a vab-10 null background, left). In strong pat mutants elongation is blocked at the 2-fold stage, and CeHDs are mildly affected (middle). In a double mutant combination for two putative aECM components (right), embryos elongate but rupture at the end of elongation with herniae and epidermis integrity defects (green cytoplasm in eggshell).