Distribution of the ten known laminin chains in the pathways and targets of developing sensory axons

Laminins are heterotrimers of α, β, and γ chains. At present, five α, three β, and two γ chains have been described. The best characterized laminin (laminin 1 = α1, β1, γ1) promotes neurite outgrowth from virtually all classes of developing neurons, implying that laminins may serve as axon guidance molecules in vivo. Moreover, different laminin trimers exert distinct effects on subsets of laminin-1-responsive cells, suggesting that isoform diversity may underlie some axonal choices in vivo. As a first step toward evaluating these hypotheses, we have documented the expression patterns of all 10 known laminin chains in the peripheral nervous system and spinal cord of the murine embryo. The α2, α4, β1, and γ1 chains are expressed in peripheral axonal pathways by embryonic day (E) 11.5, when sensory and motor axonal outgrowth is underway. Thus, laminins (but not laminin 1) may promote peripheral axonal outgrowth. By E 13.5, laminin chains are differentially expressed in the limb-bud, with prominent expression of α2 and α4 in muscle and of α3 and α5 in skin. This pattern raises the possibility that laminin isoform diversity contributes to the ability of cutaneous and muscle sensory axons to distinguish their targets. Later in development, some chains (e.g., α2, α4, and β1) are downregulated in peripheral nerve while others (e.g., γ1), continue to be expressed by Schwann cells into adulthood. In contrast to peripheral nerves and ganglia, laminin chains are expressed at low levels, if at all, in the developing spinal cord gray matter.