Duke: A freshwater zebrafish costs less than two
bucks at the pet store, but it can do something priceless: Its spinal
cord can heal completely after being severed, a paralyzing and often
fatal injury for humans.
While watching these fish repair their own
spinal cord injuries, Duke University scientists have found a particular
protein important for the process. Their study, published Nov. 4 in the
journal Science, could generate new leads into tissue repair in humans.
“This is one of nature’s most remarkable
feats of regeneration,” said the study’s senior investigator Kenneth
Poss, professor of cell biology and director of the Regeneration Next
initiative at Duke. “Given the limited number of successful therapies
available today for repairing lost tissues, we need to look to animals
like zebrafish for new clues about how to stimulate regeneration.”
When the zebrafish’s severed spinal cord
undergoes regeneration, a bridge forms, literally. The first cells
extend projections into a distance tens of times their own length and
connect across a wide gulf of the injury. Nerve cells follow. By 8
weeks, new nerve tissue has filled the gap and the animals have fully
reversed their severe paralysis.
To understand what molecules were potentially
responsible for this remarkable process, the scientists conducted a
molecular fishing expedition of sorts, searching for all of the genes
whose activity abruptly changed after spinal cord injury.
Of dozens of genes strongly activated by
injury, seven coded for proteins that are secreted from cells. One of
these, called CTGF or connective tissue growth factor, was intriguing
because its levels rose in the supporting cells, or glia, that formed
the bridge in the first two weeks following injury.
“We were surprised that it was expressed in
only a fraction of glial cells after the injury. We thought that these
glial cells and this gene must be important,” said lead author Mayssa
Mokalled, a postdoctoral fellow in Poss’s group. Indeed, when they tried
deleting CTGF genetically, those fish failed to regenerate.
Humans and zebrafish share most
protein-coding genes, and CTGF is no exception. The human CTGF protein
is nearly 90% similar in its amino acid building blocks to the zebrafish
form. When the team added the human version of CTGF to the injury site
in fish, it boosted regeneration and the fish swam better by two weeks
after the injury.
“The fish go from paralyzed to swimming in the tank. The effect of the protein is striking,” Mokalled said.
CTGF is a large protein, made of four smaller
parts, and it has more than one function. But the second half of the
CTGF protein seems to be the key to the healing, the group found. That
might make it easier to deliver and more specific as a therapy for
spinal injuries.
Poss said that unfortunately, CTGF is
probably not sufficient on its own for people to regenerate their own
spinal cords. Healing is more complex in mammals, in part because scar
tissue forms around the injury. Poss’s group expects studies of CTGF to
move into mammals like mice.
“Mouse experiments could be key,” Mokalled said. “When do they express CTGF, and in what cell types?”
These experiments may reveal some answers to
why zebrafish can regenerate whereas mammals cannot. It may be a matter
of how the protein is controlled rather than its make-up, Poss said.
The group also plans to follow up on other
proteins secreted after injury that were identified in their initial
search, which may provide additional hints into the zebrafish’s secrets
of regeneration.
“I don’t think CTGF is the complete answer,
but it’s a great thing to have in hand to inform new ways to think about
the real challenge of trying to improve regeneration,” Poss said.