Endothelial Expression of β1 Integrin Is Required for Embryonic Vascular Patterning and Postnatal Vascular Remodeling

The largest subgroup of integrins is that containing the β1 subunit (5, 6). β1 integrins have been implicated in a wide array of biological processes, including cell migration, adhesion, formation of basement membrane, and control of the cell cycle (5, 6, 11, 13, 20, 25, 31). They have also been purported to play an essential role in angiogenesis, although the data supporting this role have been obtained in an indirect manner using techniques such as blocking antibodies or RNA interference (15, 22, 30). The role of β1 integrin in angiogenesis has also been inferred by the finding that tumor formation by β1 null cells is defective, with absence of tumor vascularization (4). Further, β1 integrin expression is highly upregulated in the central nervous system vasculature during maturation, and it has thus been proposed that this integrin plays an important role in maintaining the integrity of the mature central nervous system vasculature (26). Interestingly, cerebral cavernous malformations (CCMs) have been linked to defects in the KRIT-1 gene, the protein product of which interacts with the integrin cytoplasmic domain-associated protein 1 alpha (ICAP-1α) (24, 36-38). ICAP-1α directly binds the cytoplasmic tail of β1 integrin. Further, the ICAP-1α binding sites for both KRIT-1 and β1 integrin are similar, and both contain a crucial NPXY motif (36). It has thus been postulated that KRIT-1 and β1 integrin compete for binding to ICAP-1α and that this competition modulates β1 integrin signaling and function. Therefore, the vascular defects observed in patients with KRIT-1 gene mutations may involve abnormal β1 integrin regulation via ICAP-1α. Global homozygous deletion of the β1 integrin gene in the mouse resulted in an early embryonic lethal phenotype with a defect in implantation and failure to organize the inner embryonic mass. This phenotype prevented a definitive assessment of the role of β1 integrin in vascular development (9, 32). Murine knockouts of some known β1 binding partners have defective vascularization, further pointing to a potentially important role of β1 integrin in angiogenesis (27). Among these are α1 integrin-deficient mice which display elevated levels of angiostatin and suppressed tumor vascularization (27), α5 integrin deletion resulting in severe vascular defects and embryonic lethality (35), and α4 integrin deletion resulting in defective vascularization and abnormal distribution of pericytes (12). In order to address more specifically the role of β1 integrin in vascular development and vascular remodeling, we used a conditional Cre-lox approach to delete the β1 integrin gene in the endothelium. Here we show that there is an absolute requirement for at least one functional β1 integrin allele in endothelial cells (EC) and that homozygous β1 integrin deletion in the endothelium causes embryonic death at approximately embryonic day 9 (E9). We show that endothelial β1 integrin plays a crucial role in vascular development and patterning. Further, we show that β1 integrin effects on the vasculature are dose dependent and that heterozygous deletion of β1 integrin in the endothelium leads to abnormal patterns of flow-mediated vascular remodeling in the mature murine vasculature.