Mammary branch initiation and extension are inhibited by separate pathways downstream of TGFβ in culture

Abstract

During the branching morphogenesis process that builds epithelial trees, signaling from stimulatory and inhibitory growth factors is integrated to control branch initiation and extension into the surrounding stroma. Here, we examined the relative roles played by these stimulatory and inhibitory signals in the patterning of branch initiation and extension of model mammary epithelial tubules in culture. We found that although several growth factors could stimulate branching, they did not determine the sites at which new branches formed or the lengths to which branches extended. Instead, branch initiation and extension were defined by two separate signals downstream of the inhibitory morphogen, transforming growth factor (TGF)-β. Branch initiation was controlled by signaling through p38 mitogen-activated protein kinase, whereas branch extension was controlled by Smad-mediated induction of a second diffusible inhibitor, Wnt5a. These data suggest that mammary epithelial branching is patterned predominately by repulsive signaling, and that TGFβ activates multiple inhibitory pathways to refine the architecture of the tree.

A self-avoidance mechanism in patterning of the urinary collecting duct tree

“…An obvious candidate signalling system for inhibiting epithelial advance is the TGFβ-superfamily: cells from other branching systems such as mammary gland show a reduced motility from shaped wells in the presence of autocrinesecreted, accumulating TGFβ [13,15]. Furthermore, treatment of ureteric bud/collecting duct-derived cell lines with TGFβ itself inhibits advance and branching of tubules in 3-dimensional collagen gel culture [16] and intact kidney rudiments [17].…”

Extracellular matrix and cytoskeletal dynamics during branching morphogenesis

“…47 Moreover, glycosaminoglycans (GAG) are thought to bind to growth factors and have been shown to accumulate specifically in the BM surrounding the ducts, 48 which might serve as a reservoir for TGFβ to inhibit the formation of new branches during mammary gland development. [49][50][51] Because the branching epithelial cells are directly in contact with ECM via transmembrane receptors like integrins, the role of β1-integrin has been explored in mammary branching morphogenesis. β1-integrin associates with numerous a-integrin subunits to bind to ECM proteins including collagens, laminins and fibronectin, and conditional deletion of β1-integrin in the luminal epithelium of mouse mammary glands revealed defects in alveolar morphogenesis that were associated with disorganized epithelial aggregates which did not clear properly.…”

SLIT/ROBO1 Signaling Suppresses Mammary Branching Morphogenesis by Limiting Basal Cell Number

“…It functions by inhibiting cellular proliferation, but how it restricts cell growth, especially in a cell type specific manner, is not well defined. In LECs, non-canonical WNT5A acts downstream of TGF-β1 (Pavlovich et al, 2011; Roarty and Serra, 2007), and inhibits cell growth by antagonizing canonical WNT signaling (Roarty et al, 2009). In cap cells or MECs, no downstream mediators of TGF-β1 have been identified to date.…”