During development and tissue homeostasis, patterns of cellular organization, proliferation, and

During development and tissue homeostasis, patterns of cellular organization, proliferation, and movement are highly choreographed. receptors in controlling cell decision-making is underscored by the prevalence with CB7630 which they are causally implicated in cancer1. RTKs are under tight control by several modes of regulation, the best-studied of which are production C both transcriptional and post-transcriptional C and ligand availability. As such, aberrant RTK activation in cancer is often caused by gene amplification, receptor overexpression, autocrine activation, or gain-of-function mutations. However, mounting evidence suggests that RTKs are also subject to exquisite spatial control, in both individual cells and multicellular tissues. Indeed, RTKs first appeared evolutionarily during the transition to multicellularity as cells developed more complex and compartmentalized ways of interfacing with their environment2, 3. Box 1 Receptor tyrosine kinases The mammalian receptor tyrosine kinase (RTK) superfamily of transmembrane receptors includes at least 58 members that share a conserved architecture (reviewed in Refs. #1,6). Epidermal growth factor receptor (EGFR) was the first RTK discovered and the first found to be directly mutated in human cancer130. As such it has served as the prototype for understanding RTKs. Early studies led to the canonical view that EGFR and other RTKs are activated via ligand-induced dimerization, kinase activation and was first recognized in all cells in a given tissue are in contact; therefore this process must be overridden during development and tissue homeostasis. In non-confluent endothelial cells, vascular endothelial growth factor (VEGF) induces activation and internalization of VEGF receptor-2 (VEGFR2) yielding continuous mitogenic signaling45, 46, 48. In contrast, confluent cells do not proliferate in response to VEGF; instead, VEGFR2 associates with vascular endothelial cadherin (VE-cadherin) at adherens junctions and is not internalized. It has been proposed that the density-enhanced phosphatase-1 (DEP-1), which is also recruited to adherens junctions, mediates dephosphorylation of VEGFR2, preventing internalization and continuous proliferative signaling45, 46. Consistent with these findings, blocking VE-cadherin function or expression in 3D endothelial cultures enhances VEGFR2-dependent sprouting49. In response to cell-cell contact, EGFR can also be restricted to a non-signaling, non-internalizing plasma membrane CB7630 compartment47, 50C53. This property is dependent upon E-cadherin engagement and, importantly, seems to reflect the cells ability to sense the amount of cadherin-mediated contact with which they are engaged. For example, cadherin levels, cell junction status (ovary provide a compelling example of the role of spatial RTK localization during directed cell migration (reviewed in Ref. #56). The anterior follicular epithelium within the fly ovary contains a group of border cells that invade the underlying germline tissue and migrate to the posterior-localized oocyte. Studies from several Mouse monoclonal antibody to MECT1 / Torc1 groups have revealed that two RTKs expressed on border cells, EGFR and platelet-derived growth factor (PDGF)/VEGF-related receptor (PVR), sense ligands expressed by the oocyte, and direct the border cells to them56. During this process, the level of RTK signaling is not critical; instead, spatially localized RTK activity is required for proper guidance (Fig. 4a). Jekely including the embryonic ectoderm, eye and ovary61, 62. A similar use of this principle results in the distinction between tip and stalk cells that orchestrates both tracheal branching in and angiogenic sprouting in vertebrates. In each case, tip cells define themselves as distinct from stalk cells, in part, by the expression of a specific RTK (FGFR/Breathless in and VEGFR in vertebrate endothelial cells) that modulates the Notch pathway, a key regulator of cell fate and patterning processes in a variety of organisms54. For example, VEGF signaling in tip cells upregulates the expression of the Notch ligand Delta-like 4, increasing Notch signaling and downregulating the expression of VEGFR2 in neighboring stalk cells63, 64 (Fig. 4c). During angiogenic sprouting, the tip cell itself localizes VEGFR2 and VEGFR3 specifically to filopodia, where they detect an extracellular gradient of VEGF ligand to direct filopodial extension and migration/sprouting65, 66. Control of intracellular RTK localization While conventional models depict RTKs that signal from the plasma membrane before undergoing endocytosis to attenuate signaling, mounting evidence indicates that RTKs remain active in endosomal storage compartments (examined in Refs #67C69). In truth, for some RTKs internalization is definitely required for a total signaling response. Moreover, particular RTKs can activate unique effectors from the plasma membrane versus the endosome, yielding yet another level of spatial control (axial). The idea that endosomes sustain and localize RTK signaling CB7630 within the cell instead of merely attenuating it was originally put forth in the mid-1990s by Bergeron and colleagues, who observed that soon after EGF excitement, most activated EGFR colocalized with its connected signaling substances SHC, GRB2, and mSos in early endosomes, suggesting that signaling continues.