YEXNR-12228; No. of pages: 13; 4C: 3, 4, 7, 8 Experimental Neurology xxx (2016) xxx–xxx Contents lists available at ScienceDirect Experimental Neurology journal homepage: www.elsevier.com/locate/yexnr Review Article Learning to swim, again: Axon regeneration in fish Jeffrey P. Rasmussen, Alvaro Sagasti ⁎ Department of Molecular, Cell and Developmental Biology, 621 Charles E. Young Drive S., University of California, Los Angeles, CA 90095, USA article info abstract Article history: Damage to the central nervous system (CNS) of fish can often be repaired to restore function, but in mammals re- Received 21 January 2016 covery from CNS injuries usually fails due to a lack of axon regeneration. The relatively growth-permissive envi- Received in revised form 25 February 2016 ronment of the fish CNS may reflect both the absence of axon inhibitors found in the mammalian CNS and the Accepted 27 February 2016 presence of pro-regenerative environmental factors. Despite their different capacities for axon regeneration, Available online xxxx many of the physiological processes, intrinsic molecular pathways, and cellular behaviors that control an axon's fi Keywords: ability to regrow are conserved between sh and mammals. Fish models have thus been useful both for identifying fi Axon factors differing between mammals and sh that may account for differences in CNS regeneration and for charac- Regeneration terizing conserved intrinsic pathways that regulate axon regeneration in all vertebrates. The majority of adult Spinal cord injury axon regeneration studies have focused on the optic nerve or spinal axons of the teleosts goldfish and zebrafish, Optic nerve which have been productive models for identifying genes associated with axon regeneration, cellular mechanisms Synapse of circuit reestablishment, and the basis of functional recovery. Lampreys, which are jawless fish lacking myelin, fi Zebra sh have provided an opportunity to study regeneration of well defined spinal cord circuits. Newer larval zebrafish fi Gold sh models offer numerous genetic tools and the ability to monitor the dynamic behaviors of extrinsic cell types reg- Lamprey ulating axon regeneration in live animals. Recent advances in imaging and gene editing methods are making fish Wallerian degeneration models yet more powerful for investigating the cellular and molecular underpinnings of axon regeneration. © 2016 Elsevier Inc. All rights reserved. Contents 1. Introduction............................................................... 0 2. Why do axons regenerate so well in the fishCNS?.............................................. 0 2.1. Are inhibitory factors present in the fishCNS?............................................ 0 2.2. InhibitorsofinhibitoryfactorsintheCNS?.............................................. 0 2.3. Do positive cues promote axon regeneration in the fishCNS?...................................... 0 3. Intrinsic pathways regulating axon regeneration in fish............................................ 0 3.1. Physiologicalregulatorsofaxonregeneration............................................. 0 3.2. Conservedmoleculesandpathwaysregulateintrinsicaxongrowthpotential.............................. 0 3.3. Identifyingnewmoleculesregulatingaxongrowthpotential...................................... 0 4. Howdocircuitsrewire?Regeneratingsynapsesandrestoringfunction.................................. 0 4.1. Functionalmeasuresofregeneration................................................. 0 4.2. Axonguidanceduringregeneration................................................. 0 4.3. Synapticregeneration....................................................... 0 4.4. Reestablishment of synaptic specificity................................................ 0 5. Dynamiccellbehaviorsduringaxonregeneration.............................................. 0 5.1. Coordinationofaxonregenerationwithwoundhealing........................................ 0 5.2. Cleaningupafteraxondegeneration................................................. 0 5.3. Bridgingcellspromoteaxonregeneration.............................................. 0 5.4. More cellular behaviors to explore in fish?.............................................. 0 Acknowledgments............................................................... 0 References.................................................................. 0 ⁎ Corresponding author. E-mail address: [email protected] (A. Sagasti). http://dx.doi.org/10.1016/j.expneurol.2016.02.022 0014-4886/© 2016 Elsevier Inc. All rights reserved. Please cite this article as: Rasmussen, J.P., Sagasti, A., Learning to swim, again: Axon regeneration in fish, Exp. Neurol. (2016), http://dx.doi.org/ 10.1016/j.expneurol.2016.02.022 2 J.P. Rasmussen, A. Sagasti / Experimental Neurology xxx (2016) xxx–xxx 1. Introduction 2. Why do axons regenerate so well in the fish CNS? Fish, our distant vertebrate cousins, are at least as vulnerable as What underlies the disparate regenerative abilities of mammalian we are to injuries, but their nervous systems have a greater capacity and fish CNS axons? Exposing mammalian axons to cells of the fish to regrow axons, repair circuits, and recover function. Despite the CNS, or fish axons to cells of the mammalian CNS, can distinguish difference in regenerative ability between mammals and fish, many whether differences in regeneration are attributable to neurons them- of the molecular and cellular pathways that regulate axon regenera- selves or to their surrounding environment. Regenerating axons of tion are conserved. Fish models have already provided insight into both mammalian and fish neurons are repelled by mammalian oligo- shared mechanisms of axon regeneration and new techniques prom- dendrocytes and myelin (Bandtlow et al., 1990; Bastmeyer et al., ise to make them even more powerful systems for investigating how 1991; Fawcett et al., 1989) but both can grow in the presence of fish ol- molecules and cells regulate neural repair. igodendrocytes or fish CNS conditioned media (Bastmeyer et al., 1991, Adult fish regeneration models, which have been established for 1993; Schwalb et al., 1995; Schwartz et al., 1985; Wanner et al., 1995). decades, and the more recently developed larval zebrafish model, Thus, just as factors in the environment of the mammalian CNS and PNS have distinct experimental advantages (Table 1). The robust regen- account for differences in the success of axon regeneration (David and eration of optic nerve (ON) and spinal cord axons in larval lamprey Aguayo, 1981), distinct factors in the mammalian and fish CNS environ- and adult goldfish and zebrafish has been exploited to identify fac- ment account for much of their difference in axon regeneration. Three ex- tors that promote successful regeneration in the central nervous sys- planations for the more growth-permissive environment of the fish CNS tem (CNS). By contrast, most studies using the larval zebrafish model have been proposed: the absence of inhibitory cues, the presence of fac- have focused on axon regeneration in the peripheral nervous system tors that block inhibitory cues, and the presence of pro-growth factors. (PNS). The amenability of larval zebrafish to live imaging and genetic manipulation makes them ideal for studying dynamic behaviors of 2.1. Are inhibitory factors present in the fish CNS? regenerating axons and extrinsic cell types. Adult and larval fish both have well-defined circuits and stereotyped behaviors, facilitat- The adult mammalian CNS contains multiple molecules inhibito- ing studies of the cell biology underlying axon regrowth and synapse ry to axon growth, including myelin proteins, ECM proteins such as reestablishment, and making it possible to address how anatomical chondroitin sulfate proteoglycans (CSPGs) and Tenascins, and regeneration relates to functional recovery. chemorepulsive guidance cues (reviewed by Giger et al., 2010). The Here we review four aspects of axon regeneration studies in adult mammalian myelin proteins RTN4-A/Nogo-A, myelin-associated and larval fish models. First, we discuss efforts to answer one of the glycoprotein (MAG), and oligodendrocyte myelin glycoprotein most fascinating questions about axon regeneration in the adult (OMgp)—all of which have homologs in the zebrafish genome fish CNS—why is it so much more successful than axon regeneration (Lehmann et al., 2004; Shypitsyna et al., 2011; our unpublished in the mammalian CNS? Second, we describe studies of intrinsic observations)—activate the growth-inhibiting Nogo receptor (NgR) growth pathways in fish, which have demonstrated that the molecu- complex on regenerating axons (Giger et al., 2010). Although func- lar basis of axon growth is conserved between fish and mammals. tional studies of fish MAG and OMgp have not been reported, These studies have also identified new molecules associated with re- zebrafish Nogo-A homologs have been studied biochemically and ge- generative axon growth, providing candidate targets for therapeutic netically. Purified mammalian Nogo and MAG inhibit the growth of interventions. Third, we review studies in both adults and larvae that fish axons (Abdesselem et al., 2009; Chen et al., 2013), but the fish assessed the success of functional recovery and mechanisms of cir- homolog of the mammalian Nogo-66 domain, which is responsible cuit re-establishment. Finally, we discuss what has been learned for growth inhibition in mammals, does not affect growth of fish or from fish models about interactions of non-neuronal cells with mammalian axons (Abdesselem et al., 2009). Zebrafish Nogo-66