ICAM-1 signaling pathways associated with Rho activation in microvascular brain endothelial cells

Endothelium of the cerebral blood vessels, which constitutes the blood-brain barrier, controls leukocyte adhesion and trafficking to the brain. Investigating signaling pathways triggered by the engagement of adhesion molecules expressed on brain endothelial cells, we report here that ICAM-1 cross-linking induces tyrosine phosphorylation of three cytoskeleton-associated proteins: focal adhesion kinase, paxillin, and p130Cas (Cas), which are found to associate as complexes. Tyrosine-phosphorylated Cas associates with the adaptor protein Crk and the GTP exchange factor C3G. In the same conditions the small G protein Rho was activated, as shown by the increase in its GTP loading. In addition, tyrosine phosphorylation of focal adhesion kinase, paxillin, and Cas as well as triggering of the Crk signaling pathway are blocked by pretreatment of the cells with the exoenzyme C3, a specific Rho inhibitor. C3-sensitive activation of the c-Jun N-terminal kinase in response to ICAM-1 cross-linking is also observed, whereas no significant activation of Ras or of the extracellular signal-regulated kinase was detected. In conclusion, these results suggest that through coupling to Rho activation and phosphorylation of cytoskeletal proteins and transcription factors, ICAM-1 cross-linking participates in the cell shape changes and gene regulation that may accompany lymphocyte migration through the blood-brain barrier.