ABSTRACT: Skeletal development and growth are complex processes regulated by multiple microenvironmental cues, including integrin-ECM interactions. The ?1 sub-family of integrins is the largest integrin sub-family and constitutes the main integrin binding partners of collagen I, the major ECM component of bone. As complete ?1 integrin knockout results in embryonic lethality, studies of ?1 integrin function in vivo rely on tissue-specific gene deletions. While multiple in vitro studies indicate that ?1 integrins are crucial regulators of osteogenesis and mineralization, in vivo osteoblast-specific perturbations of ?1 integrins have resulted in mild and sometimes contradictory skeletal phenotypes. To further investigate the role of ?1 integrins on skeletal phenotype, we used the Twist2-Cre, Osterix-Cre and osteocalcin-Cre lines to generate conditional ?1 integrin deletions, where Cre is expressed primarily in mesenchymal condensation, pre-osteoblast, and mature osteoblast lineage cells respectively within these lines. Mice with Twist2-specific ?1 integrin disruption were smaller, had impaired skeletal development, especially in the craniofacial and vertebral tissues at E19.5, and did not survive beyond birth. Osterix-specific ?1 integrin deficiency resulted in viable mice which were normal at birth but displayed early defects in calvarial ossification, incisor eruption and growth as well as femoral bone mineral density, structure, and mechanical properties. Although these defects persisted into adulthood, they became milder with age. Finally, a lack of ?1 integrins in mature osteoblasts and osteocytes resulted in minor alterations to femur structure but had no effect on mineral density, biomechanics or fracture healing. Taken together, our data indicate that ?1 integrin expression in early mesenchymal condensations play an important role in skeletal ossification, while ?1 integrin-ECM interactions in pre-osteoblast, odontoblast- and hypertrophic chondryocyte-lineage cells regulate incisor eruption and perinatal bone formation in both intramembranously and endochondrally formed bones in young, rapidly growing mice. In contrast, the osteocalcin-specific ?1 integrin deletion had only minor effects on skeletal phenotype.
