The GTPase Rac1 selectively regulates Salmonella invasion at the apical plasma membrane of polarized epithelial cells

The bacterial pathogen Salmonella typhimurium colonizes its animal hosts by inducing its internalization into intestinal epithelial cells, This proces s requires reorganization of the actin cytoskeleton of the apical plasma me mbrane into elaborate membrane ruffles that engulf the bacteria, Members of the Rho family of small GTPases are critical regulators of actin structure , and in nonpolarized cells, the GTPase Cdc42 has been shown to modulate Sa lmonella entry. Because the actin architecture of epithelial cells is organ ized differently from that of nonpolarized cells, we examined the role of t wo Rho family GTPases, Cdc42 and Rad, in invasion of polarized monolayers o f MDCK cells by S. typhimurium. Surprisingly., we found that endogenous Rad , but not Cdc42, was activated during bacterial entry at the apical pole, a nd that this activation required the bacterial effector protein SopE, Furth ermore, expression of dominant inhibitory Rac1 but not Cdc42 significantly inhibited apical internalization of Salmonella, indicating that Rad activat ion is integral to the bacterial entry process. In contrast, during basolat eral internalization, both Cdc42 and Rad were activated; however, neither G TPase was required for entry, These findings, which differ significantly fr om previous observations in nonpolarized cells, indicate that the host cell signaling pathways activated by bacterial pathogens may vary with cell typ e, and in epithelial tissues may further differ between plasma membrane dom ains.