Extracellular leucine-rich repeat proteins are required to organize the apical extracellular matrix and maintain epithelial junction integrity inC. elegans

Abstract

SUMMARYEpithelial cells are linked by apicolateral junctions that are essential for tissue integrity. Epithelial cells also secrete a specialized apical extracellular matrix (ECM) that serves as a protective barrier. Some components of the apical ECM, such as mucins, can influence epithelial junction remodeling and disassembly during epithelial-to-mesenchymal transition (EMT). However, the molecular composition and biological roles of the apical ECM are not well understood. We identified a set of extracellular leucine-rich repeat only (eLRRon) proteins in C. elegans (LET-4 and EGG-6) that are expressed on the apical surfaces of epidermal cells and some tubular epithelia, including the excretory duct and pore. A previously characterized paralog, SYM-1, is also expressed in epidermal cells and secreted into the apical ECM. Related mammalian eLRRon proteins, such as decorin or LRRTM1-3, influence stromal ECM or synaptic junction organization, respectively. Mutants lacking one or more of the C. elegans epithelial eLRRon proteins show multiple defects in apical ECM organization, consistent with these proteins contributing to the embryonic sheath and cuticular ECM. Furthermore, epithelial junctions initially form in the correct locations, but then rupture at the time of cuticle secretion and remodeling of cell-matrix interactions. This work identifies epithelial eLRRon proteins as important components and organizers of the pre-cuticular and cuticular apical ECM, and adds to the small but growing body of evidence linking the apical ECM to epithelial junction stability. We propose that eLRRon-dependent apical ECM organization contributes to cell-cell adhesion and may modulate epithelial junction dynamics in both normal and disease situations.

The Extracellular Matrix Protein Artichoke Is Required for Integrity of Ciliated Mechanosensory and Chemosensory Organs inDrosophilaEmbryos

“…LRR are protein-protein interaction motifs of 20-30 amino acids enriched in leucines (Dolan et al 2007). In C. elegans, the LRR proteins LET-4 and EGG-6 and the secreted LRR protein SYM-1 are required for the organization of the apical ECM in the excretory duct and pore (Mancuso et al 2012). In Drosophila, the LRR-secreted protein Convoluted (Conv) is necessary for apical matrix organization during tracheal tube morphogenesis (Swanson et al 2009).…”

Role of the extracellular matrix in epithelial morphogenesis

“…Now, Sundaram and colleagues identified three related apically located proteins bearing leucine-rich repeats (LET-4, EGG-6 and SYM-1) with potential characteristics of embryonic sheath components. 32 Consistent with this idea, the distance between the vitelline membrane and the sheath is increased in let-4 mutants and there is cytoplasmic material in the eggshell of both egg-6 and let-4 mutants. However, a definitive association of EGG-6/LET-4/SYM-1 with the embryonic sheath would require confirmation by immuno-electron microscopy.…”

“…The combined loss of LET-4 and SYM-1 causes embryos to rupture towards the end of the elongation process, which often occurs in areas where the embryo is deformed and could reflect either local adherens junction weakening or poor epidermis integrity. 32 Likewise, single egg-6 or let-4 fail to maintain the junction between two cells of the excretory system, which form unicellular tubes. 32 LET-4 and EGG-6 bear transmembrane and cytoplasmic domains, which at least for LET-4 appear dispensable for function, 32 whereas SYM-1 has no transmembrane domain.…”

“…32 Likewise, single egg-6 or let-4 fail to maintain the junction between two cells of the excretory system, which form unicellular tubes. 32 LET-4 and EGG-6 bear transmembrane and cytoplasmic domains, which at least for LET-4 appear dispensable for function, 32 whereas SYM-1 has no transmembrane domain. 33 The authors suggest that in the epidermis as well as in the excretory system the junctional defects are likely to be indirect, since they occur at a late stage and are not accompanied by the type of polarity defects observed in mutants affecting junction assembly.…”

“…33 The authors suggest that in the epidermis as well as in the excretory system the junctional defects are likely to be indirect, since they occur at a late stage and are not accompanied by the type of polarity defects observed in mutants affecting junction assembly. 32 Instead, the presence of cytoplasmic material in the eggshell of let-4 mutants could mean that these proteins help maintain epidermis plasma membrane integrity and/or hydrostatic pressure. While let-4;sym-1 double mutants cause herniae, like enzymatic removal of the embryonic sheath, their elongation phenotype appears less severe and also completely different from the complete elongation collapse observed after loss of the cuticle collagen SQT-3.…”