Abstract
After denervation of muscle, motor axons reinnervate original synaptic sites. A recombinant fragment of the synapse specific laminin beta 2 chain (s-laminin) was reported to inhibit motor axon growth. Consequently, a specific sequence (leucine-arginine-glutamate, LRE) of the laminin beta 2 chain was proposed to act as a stop signal and to mediate specific reinnervation at the neuromuscular junction (Porter, B.E., J. Weis, and J.R. Sanes. 1995. Neuron. 14:549-559). We demonstrate here that native chick laminin-4, which contains the beta 2 chain and is present in the synaptic basement membrane, does not inhibit but rather promotes motor axon growth. In native heterotrimeric laminin, the LRE sequence of the beta 2 chain is found in a triple coiled-coil region that is formed by all three subunits. We show here that the effect of LRE depends on the structural context. Whereas a recombinant randomly coiled LRE peptide indeed inhibited outgrowth by chick motoneurons, a small recombinant triple coiled-coil protein containing this sequence did not.
Highlights
After denervation of muscle, motor axons reinnervate original synaptic sites
They were isolated from chick heart by sequential immunoaffinity chromatography to Monoclonal antibody (mAb) specific for individual chick laminin subunits (Brandenberger and Chiquet, 1995)
A laminin isoform mixture was obtained from Sepharose-coupled mAb llB7 directed against the common ~/1 chain
Summary
We demonstrate here that native chick laminin-4, which contains the 132chain and is present in the synaptic basement membrane, does not inhibit but rather promotes motor axon growth. The L R E sequence of the [32 chain is found in a triple coiled-coil region that is formed by all three subunits. EGENERATING motor axons preferentially reinnervate original synaptic sites that cover only ~0.1% of the muscle fiber surface (Letinsky et al, 1976). Factors responsible for this selectivity are likely to be components of the synaptic basement membrane, because, if the muscle fibers have been killed as well, motor axons still grow back to former synaptic sites on basement membrane "ghosts" (Sanes et al, 1978). Brandenberger is Howard Hughes Medical Institute, University of California School of Medicine, Parnassus and Third Avenues, San Francisco, CA 94143-0724
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