title: Live Imaging of Calcium Dynamics during Axon Degeneration Reveals Two Functionally Distinct Phases of Calcium Influx.
abstract: Calcium is a key regulator of axon degeneration caused by trauma and disease, but its specific spatial and temporal dynamics in injured axons remain unclear. To clarify the function of calcium in axon degeneration, we observed calcium dynamics in single injured neurons in live zebrafish larvae and tested the temporal requirement for calcium in zebrafish neurons and cultured mouse DRG neurons. Using laser axotomy to induce Wallerian degeneration (WD) in zebrafish peripheral sensory axons, we monitored calcium dynamics from injury to fragmentation, revealing two stereotyped phases of axonal calcium influx. First, axotomy triggered a transient local calcium wave originating at the injury site. This initial calcium wave only disrupted mitochondria near the injury site and was not altered by expression of the protective WD slow (WldS) protein. Inducing multiple waves with additional axotomies did not change the kinetics of degeneration. In contrast, a second phase of calcium influx occurring minutes before fragmentation spread as a wave throughout the axon, entered mitochondria, and was abolished by WldS expression. In live zebrafish, chelating calcium after the first wave, but before the second wave, delayed the progress of fragmentation. In cultured DRG neurons, chelating calcium early in the process of WD did not alter degeneration, but chelating calcium late in WD delayed fragmentation. We propose that a terminal calcium wave is a key instructive component of the axon degeneration program.
  
  
  CONCLUSIONS
  Axon degeneration resulting from trauma or neurodegenerative disease can cause devastating deficits in neural function. Understanding the molecular and cellular events that execute axon degeneration is essential for developing treatments to address these conditions. Calcium is known to contribute to axon degeneration, but its temporal requirements in this process have been unclear. Live calcium imaging in severed zebrafish neurons and temporally controlled pharmacological treatments in both zebrafish and cultured mouse sensory neurons revealed that axonal calcium influx late in the degeneration process regulates axon fragmentation. These findings suggest that temporal considerations will be crucial for developing treatments for diseases associated with axon degeneration.
authors.affiliation: Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, California 90095, Jules Stein Eye Institute and Department of Ophthalmology, David Geffen School of Medicine, University of California, Los Angeles, California 90095, and m.vargas@jsei.ucla.edu sagasti@mcdb.ucla.edu.
authors.forename: Mauricio Enrique
authors.identifier: http://orcid.org/0000-0002-7410-6938
authors.initials: ME
authors.lastname: Vargas
authors.affiliation: Laboratory of Brain Development and Repair, The Rockefeller University, New York, New York 10065.
authors.forename: Yuya
authors.identifier: http://orcid.org/0000-0001-5855-7168
authors.initials: Y
authors.lastname: Yamagishi
authors.affiliation: Laboratory of Brain Development and Repair, The Rockefeller University, New York, New York 10065.
authors.forename: Marc
authors.initials: M
authors.lastname: Tessier-Lavigne
authors.affiliation: Department of Molecular, Cell, and Developmental Biology, University of California, Los Angeles, California 90095, m.vargas@jsei.ucla.edu sagasti@mcdb.ucla.edu.
authors.forename: Alvaro
authors.initials: A
authors.lastname: Sagasti
mesh_terms: D000818:Animals; D030801:Animals, Genetically Modified; D001369:Axons / Q000378:metabolism* / Q000473:pathology*; D002118:Calcium / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology*; D002478:Cells, Cultured; D005727:Ganglia, Spinal / Q000166:cytology / Q000502:physiology; D051379:Mice; D009410:Nerve Degeneration / Q000378:metabolism* / Q000473:pathology*; D015027:Zebrafish
chemical_list: D002118:Calcium
keywords: Wallerian degeneration; WldS; axon; calcium; mitochondria; zebrafish