Shekaran Asha, Shoemaker James T, Kavanaugh Taylor E, Lin Angela S, LaPlaca Michelle C, Fan Yuhong, Guldberg Robert E, García Andrés J
Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 313 Ferst Drive, Atlanta, GA 30332, USA; Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, 315 Ferst Drive, Atlanta, GA 30332, USA.
Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, 313 Ferst Drive, Atlanta, GA 30332, USA.
Bone. 2014 Nov;68:131-41. doi: 10.1016/j.bone.2014.08.008. Epub 2014 Aug 27.
Skeletal development and growth are complex processes regulated by multiple microenvironmental cues, including integrin-ECM interactions. The β1 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 β1 integrin knockout results in embryonic lethality, studies of β1 integrin function in vivo rely on tissue-specific gene deletions. While multiple in vitro studies indicate that β1 integrins are crucial regulators of osteogenesis and mineralization, in vivo osteoblast-specific perturbations of β1 integrins have resulted in mild and sometimes contradictory skeletal phenotypes. To further investigate the role of β1 integrins on skeletal phenotype, we used the Twist2-Cre, Osterix-Cre and osteocalcin-Cre lines to generate conditional β1 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 β1 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 β1 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 β1 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 β1 integrin expression in early mesenchymal condensations play an important role in skeletal ossification, while β1 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 β1 integrin deletion had only minor effects on skeletal phenotype.
骨骼发育和生长是由多种微环境信号调控的复杂过程，包括整合素与细胞外基质（ECM）的相互作用。整合素的β1亚家族是最大的整合素亚家族，并且是骨的主要细胞外基质成分——I型胶原蛋白的主要整合素结合伴侣。由于完全敲除β1整合素会导致胚胎致死，因此对β1整合素体内功能的研究依赖于组织特异性基因缺失。虽然多项体外研究表明β1整合素是成骨和矿化的关键调节因子，但β1整合素在体内成骨细胞中的特异性扰动却导致了轻微的、有时甚至相互矛盾的骨骼表型。为了进一步研究β1整合素对骨骼表型的作用，我们使用Twist2-Cre、Osterix-Cre和骨钙素-Cre品系来产生条件性β1整合素缺失，其中Cre分别在这些品系的间充质凝聚、前成骨细胞和成熟成骨细胞谱系细胞中主要表达。Twist2特异性β1整合素缺失的小鼠体型较小，骨骼发育受损，尤其是在E19.5时的颅面和椎骨组织，并且出生后无法存活。Osterix特异性β1整合素缺乏导致存活的小鼠出生时正常，但在颅盖骨化、门齿萌出和生长以及股骨骨矿物质密度、结构和力学性能方面表现出早期缺陷。虽然这些缺陷持续到成年期，但随着年龄增长会变得更轻微。最后，成熟成骨细胞和骨细胞中缺乏β1整合素导致股骨结构有轻微改变，但对矿物质密度、生物力学或骨折愈合没有影响。综上所述，我们的数据表明，早期间充质凝聚中的β1整合素表达在骨骼骨化中起重要作用，而前成骨细胞、成牙本质细胞和肥大软骨细胞谱系细胞中的β1整合素-ECM相互作用调节幼龄、快速生长小鼠膜内和成软骨形成的骨骼中的门齿萌出和围产期骨形成。相比之下，骨钙素特异性β1整合素缺失对骨骼表型只有轻微影响。
