Background: Epithelial cyst and tubule formation represent critical processes for the development of many mammalian organs and involve transient, highly choreographed changes in cell polarity. The Rho family of small GTPases, whose prototypes are RhoA, Rac1, and Cdc42, regulate many biologic processes, including cell polarization and morphogenesis. The exocyst is a conserved eight-subunit protein complex involved in the biogenesis of polarity; in yeast, it is a downstream effector for several Rho family proteins, and, in mammals, plays a central role in cystogenesis and tubulogenesis.
Methods: Inducible cell lines expressing mutant forms of RhoA, Rac1, and Cdc42 and an in vitro model of cystogenesis and tubulogenesis were used to examine the effects of Rho family proteins on cyst and tubule formation. A series of pulse-chase assays, using basolateral, apical, and secretory proteins, were performed to examine the synthesis and membrane trafficking profile of the various Rho family mutant proteins.
Results: We show that expression of mutant RhoA, Rac1, and Cdc42 proteins all result in abnormal cyst and tubule formation. Furthermore, with respect to cystogenesis and tubulogenesis, the phenotypic effects of expressing each mutant Rho family protein are different. Specifically, cyst and, therefore, tubule formation is completely inhibited in the presence of constitutively active RhoA and tubulogenesis is inhibited in the presence of dominant negative Rac1. Reversal of cyst polarity is seen in the presence of dominant negative RhoA, dominant negative Rac1, and both dominant negative and constitutively active Cdc42. The series of synthesis and delivery assays, using basolateral, apical, and secretory proteins, revealed that Rho family mutant proteins display an exocyst-like trafficking profile.
Conclusion: The differential effects suggest that RhoA, Rac1, and Cdc42 all act to control cyst and tubule formation and may act in concert to control these higher-order processes. The exocyst-like membrane trafficking profile displayed by the Rho family mutant proteins raises the possibility that Rho family proteins interact, either directly or indirectly, with the exocyst to control cyst and tubule formation.