June 22, 2017          In this developing backbone of a zebrafish, collapsed inner    cells (green) are replaced by newly fluid-filled sheath cells    (red) from the outer layer. The remaining cellular debris    clumps together in the center of the structure, which is called    a notochord. Credit: Jennifer Bagwell, Duke University    
      Duke researchers have discovered a unique repair mechanism in      the developing backbone of zebrafish that could give insight      into why spinal discs of longer-lived organisms like humans      degenerate with age.    
    The repair mechanism apparently protects the fluid-filled cells    of the notochord, the precursor of the spine, from mechanical    stress as a young fish begins swimming. Notochord cells go on    to form the gelatinous center of intervertebral discs, the    flat, round cushions wedged between each vertebrae that act as    shock absorbers for the spine.  
    The disappearance of these cells over time is associated with    degenerative disc disease, a major    cause of human pain and disability worldwide.  
    "It is not difficult to speculate that these same mechanisms of    repair and regeneration are present in humans at very early    stages, but are lost over time," said Michel Bagnat, Ph.D.,    senior author of the study and assistant professor of cell    biology at Duke University School of Medicine. "If we are going    to think about techniques that foster intervertebral disc    regeneration, this is the basic biology we need to understand."  
    The study appears June 22, 2017, in Current Biology.  
    Bagnat likens the notochord to a garden hose filled with water.    The hardy structure consists of a sheath of epithelial cells surrounding a collection of    giant fluid-filled or "vacuolated" cells. During development,    these vacuolated cells rarely pop, despite being under constant    mechanical stress. Recent research has    suggested that tiny pouches known as caveolae (Latin for    "little caves") that form in the plasma membrane of these cells    can provide a buffer against stretching or swelling.  
    To see whether the caveolae protected vacuoles from bursting,    his team and collaborators from Germany generated mutants of    three caveolar genes in their model organism, the zebrafish.    Because these small aquarium fish are transparent as embryos,    the scientists could easily visualize any spinal defects    triggered by the loss of caveolae.  
    The researchers found that when the mutant embryos hatched and    started swimming, exerting pressure on their underdeveloped    backbones, their vacuolated cells started to break up. While    the finding confirmed their suspicions, it turned up a puzzling    discovery. "In the caveolar mutants, you see these serial    lesions up and down the notochord, and yet the mature spine    formed normally," said Bagnat. "That was very puzzling to us."  
    To figure out how that was possible, lead authors Jamie Garcia    and Jennifer Bagwell took a closer look at the notochord of    mutant fish. They marked the vacuolated cells green and the    surrounding epithelial sheath cells red and then filmed the    fish shortly after they hatched and started swimming. First,    they could see an occasional vacuolated cell break and spill    its contents like a water balloon. Then, over the course of    fifteen hours, a nearby epithelial sheath cell would move in,    crawl over the detritus of the collapsed cell, and morph into a    new vacuolated cell.  
    They performed a few more experiments and found that the repair    response was triggered by the release of the cell contents,    specifically the basic molecular building blocks known as    nucleotides. The researchers then isolated live epithelial    sheath cells and treated them with nucleotide analogs to show    that they turned into vacuolated cells.  
    "These cells, which reside in the discs of both zebrafish and    man, seem capable of controlling their own repair and    regeneration," said Bagnat. "Perhaps it is a continuous release    of nucleotides that is important for keeping the disc in good    shape."  
    The study may offer insight not only into the development of    back and neck pain, but also into the origins of cancer. Their    data suggests that chordomas, rare and aggressive notochord    cell tumors, may begin when epithelial sheath cells leave the notochord and invade the skull and other tissues.  
     Explore further:        Stem cells therapy for naturally occurring intervertebral disc    disease  
    More information: "Sheath cell invasion and    trans-differentiation repair mechanical damage caused by loss    of caveolae in the zebrafish notochord," Jamie Garcia, Jennifer    Bagwell, Brian Njaine, James Norman, Daniel S. Levic, Susan    Wopat, Sara E. Miller, Xiaojing Liu, Jason W. Locasale, Didier    Y.R. Stainier and Michel Bagnat. Current Biology, June    22, 2017. DOI: 10.1016/j.cub.2017.05.035
      Journal reference: Current      Biology    
      Provided by: Duke      University    
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