Fibronectin promotes brain capillary endothelial cell survival and proliferation through alpha 5 beta 1 and alpha v beta 3 integrins via MAP kinase signalling

We showed previously that blood vessel maturation in the CNS is associated with a developmental switch in brain capillary endothelial cells (BCEC), from fibronectin signalling during angiogenesis to laminin signalling in the adult. To investigate the functional significance of this switch, we have examined the response of BCEC to different extracellular matrix (ECM) proteins. This showed that BCEC proliferation was significantly promoted by fibronectin (28.2 +/- 4.0%) and by vitronectin (14.8 +/- 2.1%) compared with uncoated glass (7.2 +/- 0.7%), while BCEC survival was significantly promoted by fibronectin (1130 +/- 131 cells), vitronectin (830 +/- 63 cells), collagen IV (703 +/- 77 cells) and laminin (680 +/- 34 cells) compared with the uncoated glass (367 +/- 48 cells). Biochemical studies showed that BCEC express a limited repertoire of integrins, including the beta 1 integrins, alpha 3 beta 1, alpha 5 beta 1 and alpha 6 beta 1, and the alpha v beta 3 integrin. Function-blocking studies showed that the response to fibronectin was mediated equally by the alpha 5 beta 1 and alpha v beta 3 integrins. Analysis of signalling pathways revealed that fibronectin stimulated activation of the p44/p42 MAP kinase signalling pathway and pharmacological inhibitors of this pathway blocked BCEC proliferation on fibronectin. Taken together, these findings show that fibronectin exerts a strong angiogenic influence on endothelial cells (EC) in the CNS, and that this is mediated through the alpha 5 beta 1 and alpha v beta 3 integrins via MAP kinase signalling. In addition to a fundamental role in development, these findings may also have implications in pathological conditions of the CNS where fibronectin is re-expressed.