Martin P T, Kaufman S J, Kramer R H, Sanes J R
Department of Anatomy and Neurobiology, Washington University Medical School, St. Louis, Missouri 63110, USA.
Dev Biol. 1996 Feb 25;174(1):125-39. doi: 10.1006/dbio.1996.0057.
Differentiation of both pre- and postsynaptic structures at the skeletal neuromuscular junction is organized by the basal lamina that occupies the synaptic cleft. As beta1 integrins are a major class of receptors for basal lamina components, we stained muscles with antibodies to the 10 integrin alpha subunits known to form dimers with beta1, to determine if any of these molecules were concentrated at synaptic sites on muscle fibers. In both developing and adult muscle, the integrin alpha1 chain was selectively associated with presynaptic cells (Schwann cells and/or nerve terminals), while alpha7 was present on both synaptic and extrasynaptic portions of the muscle fiber surface. Thus alpha1 and alpha7 integrins are present in synaptic membranes. Expression of the alpha7 chain was analyzed further by staining with antibodies specific for three alternatively spliced products of the alpha7 gene (A, B, and C), all of which were expressed in muscle. The alpha7A and alpha7B isoforms were confined to synaptic sites in adult muscle, while alpha7C was present both synaptically and extrasynaptically. In developing muscle, alpha7A appeared postnatally and specifically at the synapse; alpha7B was present throughout the muscle fiber perinatally, becoming confined to the synapse in the second postnatal week; and alpha7C was present extrasynaptically both perinatally and in adulthood. Thus, two of the alpha7 integrins are synapse-specific, and all three show distinct spatiotemporal patterns of expression within a single cell type. Finally, we asked whether perturbation of laminin expression affected the distribution of the alpha7 integrins. In normal mice, laminin beta2 is concentrated in synaptic basal lamina. In beta2-null mutant mice, alpha7A was still present at synaptic sites, but alpha7B was absent. This result provides genetic evidence that basal lamina composition is a determinant of integrin distribution.
骨骼神经肌肉接头处突触前和突触后结构的分化是由占据突触间隙的基膜组织的。由于β1整合素是基膜成分的主要受体类型，我们用针对已知可与β1形成二聚体的10种整合素α亚基的抗体对肌肉进行染色，以确定这些分子中是否有任何一种在肌肉纤维的突触部位富集。在发育中和成年肌肉中，整合素α1链选择性地与突触前细胞（施万细胞和/或神经末梢）相关联，而α7则存在于肌肉纤维表面的突触和突触外部分。因此，α1和α7整合素存在于突触膜中。通过用针对α7基因三种可变剪接产物（A、B和C）的特异性抗体染色，进一步分析了α7链的表达，所有这些产物都在肌肉中表达。α7A和α7B异构体局限于成年肌肉的突触部位，而α7C则在突触和突触外均有存在。在发育中的肌肉中，α7A在出生后出现并特异性地出现在突触处；α7B在围产期存在于整个肌肉纤维中，在出生后第二周局限于突触；α7C在围产期和成年期均存在于突触外。因此，两种α7整合素是突触特异性的，并且所有三种在单一细胞类型中都表现出不同的时空表达模式。最后，我们询问层粘连蛋白表达的扰动是否会影响α7整合素的分布。在正常小鼠中，层粘连蛋白β2集中在突触基膜中。在β2基因敲除突变小鼠中，α7A仍存在于突触部位，但α7B不存在。这一结果提供了遗传学证据，表明基膜组成是整合素分布的一个决定因素。
