The Rho GTPase signaling pathway modulates Moraxella catarrhalis invasion into human respiratory epithelial cells by regulating actin polymerization

<i>Moraxella catarrhalis</i> invasion of host respiratory epithelial cells is a critical mechanism driving acute exacerbations of chronic obstructive pulmonary disease (AECOPD). Although previous studies have extensively demonstrated that dynamic changes in the actin cytoskeleton are central to the invasion of host cells by <i>Moraxella catarrhalis</i>, the detailed mechanisms underlying the specific upstream signaling pathways and key regulators driving this process remain incompletely understood. Our study identifies and validates the essential roles of key Rho GTPase regulators (CDC42, Rac1, ArpC2, ArpC4) in actin polymerization during <i>M. catarrhalis</i> infection, thereby elucidating a more comprehensive and specific molecular mechanism. Invasion assays and Transmission electron microscopy (TEM) showed that the Rho GTPase signaling pathway modulates <i>M. catarrhalis</i> bacterial load in A549 cells by regulating macropinosome volume. Further experiments used <i>M. catarrhalis</i> strains 73-OR and ATCC 25238 to invade wild-type A549 cells, CDC42^-/- A549 cells, Rac1^-/- A549 cells, ArpC2^-/- A549 cells and ArpC4^-/-A549 cells respectively. Invasion assays and TEM were performed to quantify internalized bacteria, macropinosome volume changes, and bacterial distribution; Western blot analyses and cellular immunofluorescence were used to measure F-actin/G-actin ratios and microfilament fluorescence intensity. These results indicate that Rho GTPase signaling pathway modulates <i>M.catarrhalis</i> invasion by regulating actin polymerization dynamics. Specifically, CDC42 and Rac1 are essential for actin polymerization and bacterial internalization. ArpC4 contributes to actin remodeling without influencing invasion, while ArpC2 is uninvolved in both processes. These findings provide a theoretical basis for targeting innate immunity to prevent and treat <i>M. catarrhalis</i>-induced AECOPD.