Neurotrophins and Netrins Require Calcineurin/NFAT Signaling to Stimulate Outgrowth of Embryonic Axons (original) (raw)

Abstract

Shirasaki and Pfaff, 2002), but their subsequent choice 4 Howard Hughes Medical Institute of major axonal pathways (correlating with their colum-5 Beckman Center for Molecular and Genetic Medicine nar identity in the spinal cord) is directed by distinct Stanford University Medical School combinations of LIM homeodomain transcription factors 300 Pasteur Drive (Kania et al., 2000; Sharma et al., 1998; Thor et al., 1999; Stanford, California 94305 Tsuchida et al., 1994). Another example of transcription factors regulating later aspects of neuronal morphogenesis is provided by the homeodomain transcription fac-Summary tor Otx1, which is required for regulating stereotyped pruning of layer 5 cortical neuron branches (Weimann Axon outgrowth is the first step in the formation of et al., 1999). Genetic screens in Drosophila have also neuronal connections, but the pathways that regulate identified the zinc finger protein Sequoia as an important axon extension are still poorly understood. We find regulator of dendrite development (Brenman et al., 2001; that mice deficient in calcineurin-NFAT signaling have Gao et al., 1999) and the zinc finger protein Brakeless dramatic defects in axonal outgrowth, yet have little as critical for axon targeting during visual system develor no defect in neuronal differentiation or survival. In opment (Rao et al., 2000; Senti et al., 2000). Although vitro, sensory and commissural neurons lacking calsome of these transcriptional processes may be acticineurin function or NFATc2, c3, and c4 are unable vated autonomously in neurons as a consequence of to respond to neurotrophins or netrin-1 with efficient an early specification event, in other cases their action axonal outgrowth. Neurotrophins and netrins stimumay be regulated by late environmental signals. Coulate calcineurin-dependent nuclear localization of pling neuronal transcription to extracellular signals NFATc4 and activation of NFAT-mediated gene tranwould allow fine tuning the timing of their activation. For scription in cultured primary neurons. These data indiexample, expression of various ETS family transcription cate that the ability of these embryonic axons to refactors, which appear to control late aspects of neuronal spond to growth factors with rapid outgrowth requires morphogenesis (Arber et al., 2000; Livet et al., 2002), is activation of calcineurin/NFAT signaling by these facregulated by the receipt of target-derived signals (Haase tors. The precise parsing of signals for elongation turnet al., 2002; Lin et al., 1998). Thus, the emerging picture ing and survival could allow independent control of is that specific aspects of neuronal morphogenesis may these processes during development. be controlled by dedicated transcriptional programs, some of which are regulated by environmental cues. Introduction However, the range of neuronal properties that are governed by changes in gene expression and the identity The complex yet stereotyped morphologies of neurons of key transcriptional regulators of such events remain arise during embryonic development through the growth largely unknown. of axons and dendrites from neuronal cell bodies. Extrin-NFAT transcription complexes are appealing candisic and intrinsic factors contribute to shaping these exdates for regulating aspects of neuronal morphogenesis tensions (Edlund and Jessell, 1999; Gao et al., 1999). because they integrate extracellular signals (Crabtree, A variety of extracellular cues, including netrins and 1989). Cell membrane signaling results in the assembly neurotrophins, stimulate, inhibit, and guide process exof NFAT transcription complexes in the nucleus and the tension and branching by binding receptors present on activation of genes that are dependent on the cell type axonal and dendritic growth cones and along the axonal in which the signal is received (Crabtree and Olson, and dendritic shafts (Giger and Kolodkin, 2001; Huang 2002; Graef et al., 2001b, 1999; Shaw et al., 1988). A and Reichardt, 2001; Tessier-Lavigne and Goodman, rise in intracellular Ca 2ϩ activates the serine/threonine 1996). How neurites respond to these cues is determined phosphatase calcineurin (Klee et al., 1979) and rapidly by developmental programs that control the repertoire dephosphorylates the four cytoplasmic subunits of expressed receptors and signal transduction mole-NFATc1-4 (http://www.gene.ucl.ac.uk/nomenclature/ cules. genefamily/NFAT/NFAT.shtml) (Clipstone and Crabtree, There is mounting evidence for dedicated transcrip-1992; Flanagan et al., 1991). Dephosphorylation of sertional programs, acting after the initial specification of ines in the amino-termini of NFATc proteins by calneurons into generic classes that regulate later aspects cineurin exposes nuclear localization sequences leading to their rapid nuclear import. NFATc cytoplasmic subunits require other transcription factors for DNA binding,