A Sawtooth Pattern of Cadherin 2 Stability Mechanically Regulates Somite Morphogenesis

Abstract

SUMMARY Differential cadherin (Cdh) expression is a classical mechanism for in vitro cell sorting [1]. Studies have explored the roles of differential Cdh levels in cell aggregates and during vertebrate gastrulation, but the role of differential Cdh activity in forming in vivo tissue boundaries and boundary extracellular matrix (ECM) is unclear [2–6]. Here, we examine the interactions between cell-cell and cell-ECM adhesion during somitogenesis, the formation of the segmented embryonic precursors of the vertebral column and musculature. We identify a sawtooth pattern of stable Cdh2 adhesions in which there is a posterior-to-anterior gradient of stable Cdh2 within each somite, while there is a step-like drop in stable Cdh2 along the somite boundary. Moreover, we find that the posterior somite boundary cells with high levels of stable Cdh2 have the most columnar morphology. Cdh2 is required for maximal cell aspect ratio and thus full epithelialization of the posterior somite. Loss-of-function analysis also indicates that Cdh2 acts with the fibronectin (FN) receptor integrin α5 (Itgα5) to promote somite boundary formation. Using genetic mosaics, we demonstrate that differential Cdh2 levels are sufficient to induce boundary formation, Itgα5 activation, and FN matrix assembly in the paraxial mesoderm. Elevated cytoskeletal contractility is sufficient to replace differential Cdh2 levels in genetic mosaics, suggesting that Cdh2 promotes ECM assembly by increasing cytoskeletal and tissue stiffness along the posterior somite boundary. Throughout somitogenesis, Cdh2 promotes ECM assembly along tissue boundaries and inhibits ECM assembly in the tissue mesenchyme.

PAPCcouples the Segmentation Clock to somite morphogenesis by regulating N-cadherin dependent adhesion

“…Our data suggest that in the chicken embryo, PAPC prevents epithelialization of cells in the rostral somite compartment by controlling CDH2 endocytosis, thus negatively regulating its function. This results in a sawtooth pattern of CDH2 resembling that recently described in zebrafish (McMillen et al, 2016). The action of PAPC on CDH2 endocytosis leads to the establishment of an interface between cells expressing high levels of CDH2 (posterior S0) and lower CDH2 levels (anterior S-I) at their cell membrane at the forming somite border.…”

“…The action of PAPC on CDH2 endocytosis leads to the establishment of an interface between cells expressing high levels of CDH2 (posterior S0) and lower CDH2 levels (anterior S-I) at their cell membrane at the forming somite border. In zebrafish, CDH2 inhibits integrin alpha5 activation in adjacent PSM and such an interface is required to allow the activation of integrin alpha5 by the EPHA4-EphrinB2 system (Julich et al, 2009;Koshida et al, 2005;McMillen et al, 2016). Integrin activation at this interface results in the local assembly of fibronectin fibrils in the forming intersomitic fissure.…”

PAPC couples the segmentation clock to somite morphogenesis by regulating N-cadherin-dependent adhesion

“…The action of PAPC on CDH2 endocytosis leads to the establishment of an interface between cells expressing high levels of CDH2 ( posterior S0) and lower CDH2 levels (anterior S-I) at their cell membrane at the forming somite border. In zebrafish, CDH2 inhibits Integrin alpha5 activation in adjacent PSM and such an interface is required to allow the activation of Integrin alpha5 by the EphA4-Ephrin-B2 system (Julich et al, 2009;Koshida et al, 2005;McMillen et al, 2016). Integrin activation at this interface results in the local assembly of fibronectin fibrils in the forming intersomitic fissure.…”

“…Hence, overexpression of PAPC-S in anterior PSM cells reduces CDH2 distribution at the membrane of the expressing cells. CDH2 plays an important role in the maintenance of the epithelial structure in anterior PSM (Duband et al, 1987;Horikawa et al, 1999;Linask et al, 1998;Radice et al, 1997) and in boundary formation (McMillen et al, 2016). Thus, the reduction of CDH2 at the membrane of cells overexpressing PAPC could explain their loss of epithelial polarity, their acquisition of a mesenchymal fate and their segregation in the rostral compartment of the forming somite.…”

“…In zebrafish, Ephrin-B2/EphA4 signaling in the PSM is also implicated in the activation of Integrin alpha5 at the forming somite boundary leading to the restriction of fibronectin fibril assembly in the intersomitic fissure (Koshida et al, 2005;Julich et al, 2009). The type I cadherin N-cadherin (CDH2), which is the major cadherin expressed in the paraxial mesoderm, plays an important role in somite epithelialization and boundary formation (Duband et al, 1987;Horikawa et al, 1999;Linask et al, 1998;Radice et al, 1997;McMillen et al, 2016). In zebrafish, Cdh2 is also required in adjacent PSM cells to maintain Integrin alpha 5 in an inactive conformation, thus inhibiting the formation of fibronectin fibrils inside the PSM (Julich et al, 2015).…”