Coordination of epithelial branching and salivary gland lumen formation by Wnt and FGF signals

Abstract

Branching morphogenesis is a molecularly conserved mechanism that is adopted by several organs, such as the lung, kidney, mammary gland and salivary gland, to maximize the surface area of a tissue within a small volume. Branching occurs through repetitive clefting and elongation of spherical epithelial structures, called endbuds, which invade the surrounding mesenchyme. In the salivary gland, lumen formation takes place alongside branching morphogenesis, but in a controlled manner, so that branching is active at the distal ends of epithelial branches while lumen formation initiates at the proximal ends, and spreads distally. We present here data showing that interaction between FGF signaling and the canonical (β-catenin dependent) and non-canonical branches of Wnt signaling coordinates these two processes. Using the Axin2(lacZ) reporter mice, we find Wnt/β-catenin signaling activity first in the mesenchyme and later, at the time of lumen formation, in the ductal epithelium. Gain and loss of function experiments reveal that this pathway exerts an inhibitory effect on salivary gland branching morphogenesis. We have found that endbuds remain devoid of Wnt/β-catenin signaling activity, a hallmark of ductal structures, through FGF-mediated inhibition of this pathway. Our data also show that FGF signaling has a major role in the control of lumen formation by preventing premature hollowing of epithelial endbuds and slowing down the canalization of presumptive ducts. Concomitantly, FGF signaling strongly represses the ductal marker Cp2l1, most likely via repression of Wnt5b and non-canonical Wnt signaling. Inhibition of canonical and non-canonical Wnt signaling in endbuds by FGF signaling occurs at least in part through sFRP1, a secreted inhibitor of Wnt signaling and downstream target of FGF signaling. Altogether, these findings point to a key function of FGF signaling in the maintenance of an undifferentiated state in endbud cells by inhibition of a ductal fate.

Involvement of Wnt, Eda and Shh at defined stages of sweat gland development

“…In contrast to hair follicles and sweat glands, salivary gland early stage development depends instead on Fgf and Pitx2 pathways (Jaskoll et al, 2004b;Tucker, 2007). Eda and Shh pathways are then required for latestage branching morphogenesis (Häärä et al, 2011;Patel et al, 2011). In early stage mammary gland development, yet another pattern is seen.…”

ROCK1-directed basement membrane positioning coordinates epithelial tissue polarity

“…Our results demonstrate that PAR-1b can regulate the orientation of an apical membrane, consistent with results in MDCK cells (Cohen et al, 2004;Cohen et al, 2011;Cohen and Müsch, 2003), and that it does so indirectly through regulation of basement membrane deposition. Although the mechanism by which an apical membrane forms is not completely understood in the salivary gland, it occurs first in the developing ducts (Patel et al, 2011;Walker et al, 2008) and might be independent of basement membrane as apical proteins appear first in cells that do not contact basement membrane, consistent with data showing that the establishment of an axis of polarity can be a cell-autonomous process (Baas et al, 2004). Nevertheless, our results indicate that basement membrane can promote the establishment of an apical membrane in salivary gland epithelial cells.…”

The Wnt-Myb pathway suppresses KIT expression to control the timing of salivary proacinar differentiation and duct formation

“…At the early stage of SMG development (E12-E15), WNT/β-catenin-dependent signaling is active in mesenchyme around end buds and around the parasympathetic ganglion, and after E15 WNT/β-catenin signaling downregulates in mesenchyme and is concomitantly upregulated in epithelium of SMG main ducts (Knosp et al, 2015;Patel et al, 2011). Opposing reports exist for the role of the WNT signaling pathway in salivary gland development.…”

“…Opposing reports exist for the role of the WNT signaling pathway in salivary gland development. SB415286, a glycogen synthase kinase 3 (GSK-3) inhibitor that activates the β-catenindependent pathway (Coghlan et al, 2000), suppresses branching morphogenesis of SMG rudiments (Patel et al, 2011), whereas XAV939, a tankyrase inhibitor that inhibits the β-catenin-dependent pathway, also inhibits epithelial branching and growth of SMG rudiments (Haara et al, 2011). Thus, the precise function and mechanism of WNT signaling in SMG development remain to be determined.…”