Expression of S‐laminin and laminin in the developing rat central nervous system

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Abstract
The extracellular matrix component, s‐laminin, is a homologue of the B1 subunit of laminin. S‐laminin is concentrated in the synaptic cleft at the neuromuscular junction and contains a site that is adhesive for motor neurons, suggesting that it may influence neuromuscular development. To ascertain whether s‐laminin may also play roles in the genesis of the central nervous system, we have examined its expression in the brain and spinal cord of embryonic and postnatal rats. S‐laminin was not detectable in synapse‐rich areas of adults. However, s‐laminin was present in discrete subsets of three laminin‐containing structures: (1) In the developing cerebral cortex, laminin and s‐laminin were expressed in the subplate, a transient layer through which neuroblasts migrate and cortical afferents grow. Both laminin and s‐laminin disappeared as embryogenesis proceeded; however, laminin was more widely distributed and present longer than s‐laminin. (2) In the developing spinal cord, laminin was present throughout the pia. In contrast, s‐laminin was concentrated in the pia that overlies the floor plate, a region in which extracellular cues have been postulated to guide growing axons. (3) In central capillaries, s‐laminin appeared perinatally, an interval during which the blood‐brain barrier matures. In contrast, laminin was present in capillary walls of both embryos and adults. To extend our immunohistochemical results, we used biochemical methods to characterize s‐laminin in brain. We found that authentic s‐laminin mRNA is present in the embryonic brain, but that brain‐derived s‐laminin differs (perhaps by a posttranslational modification) from that derived from nonneural tissues. We also used tissue culture methods to show that glia are capable of synthesizing “brain‐like” s‐laminin, and of assembling it into an extracellular matrix. Thus, glia may be one cellular source of s‐laminin in brain. Together, these results demonstrate that s‐laminin is present in the developing central nervous system, and raise the possibility that this molecule may influence developmental processes.