MAP-1B/TAU functional redundancy during laminin-enhanced axonal growth.

When cultured cerebellar macroneurons develop attached to a laminin-containing substrate or after the acute addition of laminin to the tissue culture medium, there is an acceleration in the rate and extent of axonal elongation. Furthermore, laminin is capable of inducing axonal formation and microtubule stabilization in neurons arrested at stage II of neuritic development by tau suppression (Caceres and Kosik, 1990; Caceres et al., 1991). Laminin-enhanced or induced axonal extension is paralleled by a selective and dramatic incorporation of phosphorylated MAP-1b into axonal microtubules. Axonal formation in neurons growing in the presence of laminin is prevented by treatment of the cultures with a mixture of MAP-1b and tau antisense oligonucleotides, but not by the single suppression of any one of these MAPs. However, suppression of MAP-1b, but not of tau, greatly reduces the increase in the rate and extent of axonal elongation induced by laminin. No such effects are elicited by MAP-1b antisense oligonucleotides in neurons growing in the absence of laminin, e.g. polylysine alone, where most of the MAP-1b present in the cells is dephosphorylated and not associated with the cytoskeleton. Taken collectively, these data suggest that, with regard to axonal elongation, MAP-1b and tau can be functionally substituted, and that extracellular matrix molecules, such as laminin, affect axonal extension by promoting the in vivo utilization of MAP-1b.