Skap2 Directs Integrin-Stimulated Cytoskeletal Rearrangement, Modulates Macrophage Effector Functions, And Regulates Atherogenesis

Background: Integrins mediate macrophage functions, such as migration and phagocytosis, that are critical for atherogenesis. We have previously shown that the myeloid-specific adaptor protein Skap2 controls integrin-induced macrophage migration and actin remodeling, and we hypothesized that, through this role in macrophage function, Skap2 could modulate atherogenesis. Methods & Results: On a standard diet, 24-week old Skap2 -/- ;apoE -/- C57BL/6 mice develop increased atherosclerotic plaque in their aortic roots compared to their Skap2 +/- ;apoE -/- counterparts (plaque area by Oil Red O stain: 135±20 vs. 44±10 x10 3 μ m 2 , n=8 per genotype) with no detectable difference in weight, lipid profile, development, or other organ pathology. Moreover, the Skap2 -/- mice have specific increased circulating cytokine levels (IL-1α 14.9±8.6, IL-6 6.4±2.4, IL-8 12.0±4.1 pg/ml, n=8) compared to Skap2 +/- mice (IL-1α 2.0±0.8, IL-6 2.1±0.8, IL-8 4.4±2.5 pg/ml, n=8), supporting a macrophage-centric inflammatory process. Consistent with our previous finding that Skap2 binds Sirpα, bone marrow derived macrophages (BMMs) lacking either Skap2 or Sirpα exhibit similarly impaired local actin polymerization upon ligating αV integrins with antibody-coated microbeads. Furthermore, BMMs containing Skap2 mutants reveal that this cytoskeletal response requires the lipid binding activity of the Skap2 PH domain, likely by disrupting an auto-inhibitory interaction with the dimerization domain. Downstream of Skap2 and Sirpα, activation of the RacGEF Vav1 followed by PAK and LimK phosphorylation comprise the events leading to cofilin-mediated actin polymerization. While Skap2 does not impact baseline nonspecific macropinocytosis, Skap2 -/- BMMs demonstrate impaired acetylated LDL uptake, a process mediated by the scavenger receptor and requiring actin reorganization. Conclusion: We find that Skap2 deficiency increases atheromatous burden in apoE -/- mice, and we detail a pathway by which Skap2, in conjunction with Sirpα, directs integrin-mediated actin reorganization with functional consequences in macrophages. These findings provide new insight into the role of integrin-mediated intracellular signaling in the inflammatory component of atherosclerosis.