Mouse Macrophages Completely Lacking Rho Subfamily GTPases (RhoA, RhoB, and RhoC) Have Severe Lamellipodial Retraction Defects, but Robust Chemotactic Navigation and Altered Motility

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Abstract
RhoA is thought to be essential for coordination of the membrane protrusions and retractions required for immune cell motility and directed migration. Whether the subfamily of Rho (Ras homolog) GTPases (RhoA, RhoB and RhoC) is actually required for the directed migration of primary cells is difficult to predict. Macrophages isolated from myeloid-restricted RhoA/RhoB (conditional) double knockout (dKO) mice did not express RhoC and were essentially pan-Rho deficient. Using real-time chemotaxis assays, we found that retraction of the trailing edge was dissociated from advance of the cell body in dKO cells, which developed extremely elongated tails. Surprisingly, velocity (of the cell body) was increased, while chemotactic efficiency was preserved, compared to wild-type (WT) macrophages. Randomly migrating RhoA/RhoB dKO macrophages exhibited multiple small protrusions and developed large branches due to impaired lamellipodial retraction. A mouse model of peritonitis indicated that monocyte/macrophage recruitment was, surprisingly, more rapid in RhoA/RhoB dKO mice than in WT mice. In comparison to dKO cells, the phenotypes of single RhoA or RhoB deficient macrophages were mild due to mutual compensation. Furthermore, genetic deletion of RhoB partially reversed the motility defect of macrophages lacking the RhoGAP (Rho GTPase-activating protein) myosin IXb (Myo9b). In conclusion, the Rho subfamily is not required for front end functions (motility and chemotaxis), although both RhoA and RhoB are involved in pulling up the rear end and resorbing lamellipodial membrane protrusions. Macrophages lacking Rho proteins migrate faster in vitro, which, in the case of the peritoneum, translates to more rapid in vivo monocyte/macrophage recruitment.