Small Rho GTPases play a fundamental role in the control of morphogenesis and cell proliferation of several epithelial organs

INTRODUCTION Coordination of tissue growth and morphogenesis is crucial for generating adult functional organs (Hogan and Kolodziej, 2002; Lu and Werb, 2008). How the two processes are integrated during the development of many organs is still a major unresolved question. During pancreatic development, branching morphogenesis coincides with growth and differentiation (Jensen, 2004; Pan and Wright, 2011; Puri and Hebrok, 2010; Spagnoli, 2007). As different pancreatic cell types become specified, they organise themselves into discrete domains along the axis of the branches, delineating, for instance, a multipotent progenitor cell domain at the distal tips of the branching epithelium (Zhou et al., 2007). Importantly, recent observations have shown that the initial number of these progenitors pre-determines the final pancreas organ size (Stanger et al., 2007). Thus, establishment of a proper progenitor pool size during branching phase [between embryonic stage (E)12 and E14 in the mouse embryo] is crucial for normal pancreas formation and function, including digestion and blood sugar regulation. However, the molecular mechanisms underlying the coordinated action of branching and progenitor proliferation in the developing pancreas are largely unknown. Small Rho GTPases play a fundamental role in the control of morphogenesis and cell proliferation of several epithelial organs (Van Aelst and Symons, 2002; Etienne-Manneville and Hall, 2002; Kesavan et al., 2009). GTPases act as molecular switches, cycling between active GTP-bound and inactive GDP-bound states, a process that is tightly regulated by distinct classes of proteins, including the Rho GTPases-activating proteins (GAPs) (Tcherkezian and Lamarche-Vane, 2007). Specifically, GAPs inactivate GTPases by accelerating their intrinsic GTPase activity thereby converting them into the inactive GDP-bound form (Sordella et al., 2003; Tcherkezian and Lamarche-Vane, 2007). Because of the ubiquitous distribution of small GTPases, a restricted expression of GAPs appears to be fundamental for the precise spatial-temporal regulation of their activity. Here, we have identified the RhoGAP Stard13 [also called Shirin in Xenopus embryos (Spagnoli and Brivanlou, 2006) and deleted liver cancer (DLC) 2 in humans (Leung et al., 2005)], as a tissue-specific GTPase regulator of mammalian pancreas development. Genetic ablation of Stard13 disrupts epithelial branching and distal tip-domain morphogenesis, resulting in hampered proliferation of pancreatic progenitors and subsequent organ hypoplasia. We show that Stard13 acts by regulating Rho signalling spatially and temporally during pancreas development. Finally, our results suggest a reciprocal interaction between the Rho-actin and mitogen-activated protein kinase (MAPK) signalling pathways to regulate progenitor cell proliferation locally. Together, these results identify Stard13 as a molecular integrator of growth and morphogenesis that acts by restricting Rho-actin activity in the pancreas.