The expression and distribution of laminin in the developing nervous system

The extracellular matrix glycoprotein laminin exerts profound effects on the survival and differentiation of neurons in vitro. Although principally confined to the basement membranes of the adult extracellular matrix, during development laminin immunoreactivity may be found both within the interstitial extracellular matrix and on the membranes of neural cells, in which location it may be expected to affect their development in vivo. To investigate the reasons for the occurrence of laminin outside basement membranes, the expression of genes coding for laminin subunits has been analysed in mouse sciatic nerves at different postnatal ages. The results show that the expression of genes coding for laminin subunits decreases from high levels at birth, very low steady state levels of the individual mRNAs being found in the adult. Thus, the predicted high rate of laminin synthesis during development might exceed the rate at which it may be incorporated into basement membranes. The effective change in distribution of laminin into the basement membrane is therefore likely to be simply a consequence of the down-regulation of gene expression, rather than being caused, for example, by the differential expression of laminin variants.