Regulation of tyrosine phosphorylation of the nicotinic acetylcholine receptor at the rat neuromuscular junction

Agrin induces phosphorylation of the nicotinic acetylcholine receptor

Agrin causes acetylcholine receptors (AChRs) on chick myotubes in culture to aggregate, forming specializations that resemble the postsynaptic apparatus at the vertebrate skeletal neuromuscular junction. Here we report that treating chick myotubes with agrin caused an increase in phosphorylation of the AChR beta, gamma, and delta subunits. H-7, a potent inhibitor of several protein serine kinases, blocked agrin-induced phosphorylation of the gamma and delta subunits, but did not prevent either agrin-induced AChR aggregation or phosphorylation of the beta subunit. Experiments with anti-phosphotyrosine antibodies demonstrated that agrin caused an increase in tyrosine phosphorylation of the beta subunit that began within 30 min of adding agrin to the myotube cultures, reached a plateau by 3 hr, and was blocked by treatments known to block agrin-induced AChR aggregation. Anti-phosphotyrosine antibodies labeled agrin-induced specializations as they do the postsynaptic apparatus. These results suggest that agrin-induced tyrosine phosphorylation of the beta subunit may play a role in regulating AChR distribution.

An extended family of protein-tyrosine kinase genes differentially expressed in the vertebrate nervous system

We have used PCR to identify 13 novel protein-tyrosine kinase genes (tyro-1 to -13), six of which (tyro-1 to -6) are preferentially expressed in the developing vertebrate nervous system. The tyro-2 and tyro-9 genes encode kinase domains that exhibit strong amino acid sequence similarity to the equivalent regions of the receptors for EGF and FGF, respectively, and may encode novel receptors for these or related polypeptide ligands. The tyro-1 to -6 genes are all expressed during central nervous system neurogenesis and exhibit distinct and highly regionalized patterns of expression in the adult brain. Together with recent studies in invertebrates, these data are consistent with the hypothesis that protein-tyrosine kinases play a central role in neural development.

The mechanism of agrin-induced acetylcholine receptor aggregation

Agrin, a protein isolated from the synapse-rich electric organ of Torpedo californica, induces the formation of specializations on myotubes in culture which resemble the post-synaptic apparatus at the vertebrate skeletal neuromuscular junction. For example, the specializations contain aggregates of acetylcholine receptors and acetylcholinesterase. This report summarizes the evidence that the formation of the post-synaptic apparatus at developing and regenerating neuromuscular junctions is triggered by the release of agrin from motor axon terminals and describes results of recent experiments which suggest that agrin-induced tyrosine phosphorylation of the beta subunit of the acetylcholine receptor may play a role in receptor aggregation.

trkB, a neural receptor protein-tyrosine kinase: evidence for a full-length and two truncated receptors

We have screened an adult rat cerebellar cDNA library in search of novel protein tyrosine-kinase (PTK) cDNAs. A cDNA for a putative PTK, trkB, was cloned, and its sequence indicates that it is likely to be derived from a gene for a ligand-regulated receptor closely related to the human trk oncogene. Northern (RNA) analysis showed that the trkB gene is expressed predominantly in the brain and that trkB expresses multiple mRNAs, ranging from 0.7 to 9 kb. Hybridization of cerebral mRNAs with a variety of probes indicates that there are mRNAs encoding truncated trkB receptors. Two additional types of cDNA were isolated, and their sequences are predicted to encode two distinct C-terminally truncated receptors which have the complete extracellular region and transmembrane domain, but which differ in their short cytoplasmic tails.

Induction of synaptic development in cultured muscle cells by basic fibroblast growth factor

The role of basic fibroblast growth factor (bFGF) in signaling the development of the neuromuscular junction was examined. Beads coated with bFGF induced the formation of acetylcholine receptor (AChR) clusters in cultured Xenopus myotomal muscle cells. Tyrphostin, a tyrosine kinase inhibitor, abolished AChR clustering induced by bFGF beads, suggesting a role of tyrosine kinase activation in AChR clustering. Using specific antibodies, we demonstrated the presence of both bFGF and its receptor in the myotomal muscle in vivo during the period of neuromuscular connection. However, similar tissue from older animals with mature neuromuscular junctions showed an apparently truncated form of the bFGF receptor. These data suggest that bFGF may play a role in signaling synaptogenesis in skeletal muscle.

trkB, a neural receptor protein-tyrosine kinase: evidence for a full-length and two truncated receptors

We have screened an adult rat cerebellar cDNA library in search of novel protein tyrosine-kinase (PTK) cDNAs. A cDNA for a putative PTK, trkB, was cloned, and its sequence indicates that it is likely to be derived from a gene for a ligand-regulated receptor closely related to the human trk oncogene. Northern (RNA) analysis showed that the trkB gene is expressed predominantly in the brain and that trkB expresses multiple mRNAs, ranging from 0.7 to 9 kb. Hybridization of cerebral mRNAs with a variety of probes indicates that there are mRNAs encoding truncated trkB receptors. Two additional types of cDNA were isolated, and their sequences are predicted to encode two distinct C-terminally truncated receptors which have the complete extracellular region and transmembrane domain, but which differ in their short cytoplasmic tails.