Extracellular Leucine-Rich Repeat Protein Let-4 is Required to Organize the Extracellular Matrix and Maintain Junctions in C. Elegans Epithelia

Epithelial cells line the interior of many organs, therefore a better understanding of how these cells are maintained could offer insights into many human diseases. This work focuses on two aspects of epithelia: cell junctions and the apical Extracellular Matrix (ECM). Epithelial cell junctions consist of conserved junction proteins that connect cells to each other, serve as a barrier, and separate the apical and basal domains of the cells. The specialized apical ECM of epithelial cells serves to protect the cells and interact with the outside environment. The apical ECM is present in many epithelia, but is poorly studied. I have characterized the apical domains of epithelial cells and their junctions in the Caenorhabditis elegans excretory system in order to develop the organ as a model for epithelial development and maintenance. With my colleagues, I studied the development of the epithelial cells of the excretory system, and described roles for Ras and Notch in a biased competition model of cell fate determination. I have characterized the localization of conserved proteins of the polarity PAR complex, and the junctional cadherin-catenin complex using molecular markers and immunohistochemistry. This has established that the excretory system shares common features with other epithelia, which supports the C. elegans excretory system as a good epithelial model system. I have also shown that extracellular leucine-rich repeat protein LET-4 has a role in organization of the apical ECM, and junction maintenance in both the excretory system and epidermis of C. elegans. Characterization of the LET-4 protein has indicated that it localizes apically in epithelial cells, but is not enriched at junctions. I characterized the let-4 loss of function phenotype with molecular markers, electron microscopy, and genetic interaction experiments, and have detected no defect in initial formation of junctions. These data suggest that it is not initial junction formation, but junction integrity that is affected in let-4 mutants. The let-4 analysis suggests that epithelial junction maintenance requires an intact apical ECM. The continuation of this work will involve further characterization of the molecular components of the apical ECM and identification of LET-4 interacting proteins