Synaptic activity and activity-dependent competition regulates axon arbor maturation, growth arrest, and territory in the retinotectal projection.
In the retinotectal projection, synapses guide retinal ganglion cell (RGC) axon arbor growth by promoting branch formation and by selectively stabilizing branches. To ask whether presynaptic function is required for this dual role of synapses, we have suppressed presynaptic function in single RGCs using targeted expression of tetanus toxin light-chain fused to enhanced green fluorescent protein (TeNT-Lc:EGFP). Time-lapse imaging of singly silenced axons as they arborize in the tectum of zebrafish larvae shows that presynaptic function is not required for stabilizing branches or for generating an arbor of appropriate complexity. However, synaptic activity does regulate two distinct aspects of arbor development. First, single silenced axons fail to arrest formation of highly dynamic but short-lived filopodia that are a feature of immature axons. Second, single silenced axons fail to arrest growth of established branches and so occupy significantly larger territories in the tectum than active axons. However, if activity-suppressed axons had neighbors that were also silent, axonal arbors appeared normal in size. A similar reversal in phenotype was observed when single TeNT-Lc:EGFP axons are grown in the presence of the NMDA receptor antagonist MK801 [(+)-5-methyl-10,11- dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate]. Although expansion of arbor territory is prevented when neighbors are silent, formation of transient filopodia is not. These results suggest that synaptic activity by itself regulates filopodia formation regardless of activity in neighboring cells but that the ability to arrest growth and focusing of axonal arbors in the target is an activity-dependent, competitive process.
Ben Fredj
Naila
N
Medical Research Council Centre for Developmental Neurobiology, King's College London, London, SE1 1UL, United Kingdom.
Hammond
Sarah
S
Otsuna
Hideo
H
Chien
Chi-Bin
CB
Burrone
Juan
J
Meyer
Martin P
MP
D000704:Analysis of Variance; D000818:Animals; D030801:Animals, Genetically Modified; D001369:Axons / Q000502:physiology*; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000235:genetics; D016291:Dizocilpine Maleate / Q000494:pharmacology; D004622:Embryo, Mammalian; D004625:Embryo, Nonmammalian; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D018507:Gene Expression Regulation, Developmental / Q000187:drug effects / Q000235:genetics; D049452:Green Fluorescent Proteins / Q000235:genetics; D020439:Growth Cones / Q000502:physiology; D006624:Hippocampus / Q000166:cytology; D008666:Metalloendopeptidases / Q000235:genetics; D009474:Neurons / Q000502:physiology*; D017120:Nystagmus, Optokinetic / Q000187:drug effects / Q000502:physiology; D011554:Pseudopodia / Q000502:physiology; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; D051381:Rats; D017207:Rats, Sprague-Dawley; D012160:Retina / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D013477:Superior Colliculi / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D013569:Synapses / Q000502:physiology*; D016708:Synaptophysin / Q000378:metabolism; D013744:Tetanus Toxin / Q000235:genetics; D013997:Time Factors; D014157:Transcription Factors / Q000235:genetics; D014162:Transfection / Q000379:methods; D014795:Visual Pathways / Q000187:drug effects / Q000502:physiology; D015027:Zebrafish; D029961:Zebrafish Proteins / Q000235:genetics
D004268:DNA-Binding Proteins; D018691:Excitatory Amino Acid Antagonists; C092350:FM 4-64; C542436:Gal4 protein, zebrafish; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; D016708:Synaptophysin; D013744:Tetanus Toxin; D014157:Transcription Factors; D029961:Zebrafish Proteins; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins; D016291:Dizocilpine Maleate; D008666:Metalloendopeptidases; C093556:zinc-endopeptidase, tetanus neurotoxin; D002118:Calcium