title: Reciprocal regulation of axonal Filopodia and outgrowth during neuromuscular junction development.
abstract: BACKGROUND
  The assembly of the vertebrate neuromuscular junction (NMJ) is initiated when nerve and muscle first contact each other by filopodial processes which are thought to enable close interactions between the synaptic partners and facilitate synaptogenesis. We recently reported that embryonic Xenopus spinal neurons preferentially extended filopodia towards cocultured muscle cells and that basic fibroblast growth factor (bFGF) produced by muscle activated neuronal FGF receptor 1 (FGFR1) to induce filopodia and favor synaptogenesis. Intriguingly, in an earlier study we found that neurotrophins (NTs), a different set of target-derived factors that act through Trk receptor tyrosine kinases, promoted neuronal growth but hindered presynaptic differentiation and NMJ formation. Thus, here we investigated how bFGF- and NT-signals in neurons jointly elicit presynaptic changes during the earliest stages of NMJ development.
  
  
  RESULTS
  Whereas forced expression of wild-type TrkB in neurons reduced filopodial extension and triggered axonal outgrowth, expression of a mutant TrkB lacking the intracellular kinase domain enhanced filopodial growth and slowed axonal advance. Neurons overexpressing wild-type FGFR1 also displayed more filopodia than control neurons, in accord with our previous findings, and, notably, this elevation in filopodial density was suppressed when neurons were chronically treated from the beginning of the culture period with BDNF, the NT that specifically activates TrkB. Conversely, inhibition by BDNF of NMJ formation in nerve-muscle cocultures was partly reversed by the overexpression of bFGF in muscle.
  
  
  CONCLUSIONS
  Our results suggest that the balance between neuronal FGFR1- and TrkB-dependent filopodial assembly and axonal outgrowth regulates the establishment of incipient NMJs.
authors.affiliation: Division of Life Science, State Key Laboratory of Molecular Neuroscience, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China.
authors.forename: Pan P
authors.initials: PP
authors.lastname: Li
authors.forename: Jie J
authors.initials: JJ
authors.lastname: Zhou
authors.forename: Min
authors.initials: M
authors.lastname: Meng
authors.forename: Raghavan
authors.initials: R
authors.lastname: Madhavan
authors.forename: H Benjamin
authors.initials: HB
authors.lastname: Peng
mesh_terms: D000818:Animals; D001369:Axons / Q000378:metabolism*; D049109:Cell Proliferation; D018076:DNA, Complementary / Q000378:metabolism; D005786:Gene Expression Regulation; D057809:HEK293 Cells; D006801:Humans; D009132:Muscles / Q000378:metabolism; D009469:Neuromuscular Junction / Q000196:embryology / Q000378:metabolism*; D009474:Neurons / Q000378:metabolism; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D011554:Pseudopodia / Q000378:metabolism*; D051496:Receptor, Fibroblast Growth Factor, Type 1 / Q000378:metabolism*; D020813:Receptor, trkB / Q000378:metabolism*; D013572:Synaptic Vesicles / Q000378:metabolism; D014982:Xenopus laevis / Q000378:metabolism*
chemical_list: D018076:DNA, Complementary; D051496:Receptor, Fibroblast Growth Factor, Type 1; D020813:Receptor, trkB