Repeated extrinsic and anisotropic mechanical inputs promote polarized adherens junction elongation

A key challenge in development is to understand how complex organisms physically coordinate the morphogenesis of multiple tissues. Here, using biophysical approaches, we investigate how muscles located under the epidermis specifically stimulate the extension of anterior-posterior (AP-oriented) epidermal adherens junctions during late C. elegans embryonic elongation. First, light-sheet imaging shows that asynchronous patterns of muscle contractions drive embryo rotations. In turn, junctions between the lateral and dorso-ventral epidermis repeatedly oscillate between a folded, hypotensed state, and an extended, hypertensed state. Second, FRAP (Fluorescence Recovery After Photobleaching) analysis of an E-cadherin::GFP construct shows that muscle contractions stimulate E-cadherin turnover. Moreover, a mechano-chemical model recapitulating in vivo observations predicts that enhanced accumulation of E-cadherin along AP-oriented junctions lowers their line tension and further favors their elongation. Altogether, our results illustrate how muscle contractions fluidize epidermal adherens junctions, which, combined with anisotropic tension within the epidermis, drives their polarized extension.