title: A developmental timing switch promotes axon outgrowth independent of known guidance receptors.
abstract: To form functional neuronal connections, axon outgrowth and guidance must be tightly regulated across space as well as time. While a number of genes and pathways have been shown to control spatial features of axon development, very little is known about the in vivo mechanisms that direct the timing of axon initiation and elongation. The Caenorhabditis elegans hermaphrodite specific motor neurons (HSNs) extend a single axon ventrally and then anteriorly during the L4 larval stage. Here we show the lin-4 microRNA promotes HSN axon initiation after cell cycle withdrawal. Axons fail to form in lin-4 mutants, while they grow prematurely in lin-4-overexpressing animals. lin-4 is required to down-regulate two inhibitors of HSN differentiation--the transcriptional regulator LIN-14 and the "stemness" factor LIN-28--and it likely does so through a cell-autonomous mechanism. This developmental switch depends neither on the UNC-40/DCC and SAX-3/Robo receptors nor on the direction of axon growth, demonstrating that it acts independently of ventral guidance signals to control the timing of HSN axon elongation.
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, Connecticut, USA.
authors.forename: Katherine
authors.initials: K
authors.lastname: Olsson-Carter
authors.forename: Frank J
authors.initials: FJ
authors.lastname: Slack
mesh_terms: D000818:Animals; D001369:Axons / Q000378:metabolism*; D017173:Caenorhabditis elegans / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D029742:Caenorhabditis elegans Proteins / Q000235:genetics / Q000378:metabolism*; D015815:Cell Adhesion Molecules / Q000235:genetics / Q000378:metabolism*; D002454:Cell Differentiation; D005786:Gene Expression Regulation; D035683:MicroRNAs / Q000235:genetics / Q000378:metabolism; D009046:Motor Neurons / Q000166:cytology* / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism; D011971:Receptors, Immunologic / Q000235:genetics / Q000378:metabolism*; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D000097905:Roundabout Proteins
chemical_list: D029742:Caenorhabditis elegans Proteins; D015815:Cell Adhesion Molecules; D035683:MicroRNAs; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins; D011971:Receptors, Immunologic; D012097:Repressor Proteins; D000097905:Roundabout Proteins; C102669:LIN-14 protein, C elegans; C452195:LIN-28 protein, C elegans; C102174:UNC-40 protein, C elegans; C452194:lin-4 microRNA, C elegans