Published December 22, 2014 Text by Ben Short [email protected] In Focus Osmotic gradient is just the tonic for wounded epithelia E In Focus • THE JOURNAL OF CELL BIOLOGY Downloaded from jcb.rupress.org on February 5, 2015 pithelial layers protect organisms FOCAL POINT from their external environment, and any damage to these tissues must therefore be repaired as quickly as possible. Wound repair is generally thought to be initiated by intrinsic cues, such as changes in the structure or mechanics of damaged tissues. But Gault et al. reveal that extrinsic signals can also trigger tissue repair. In particular, differ- (Left to right) William Gault, Balázs Enyedi, and Philipp Niethammer reveal that differences in osmotic pressure between epithelial tissues and their external environment help initiate the tissue’s wound ences between the osmolarity of zebrafish response by stimulating ATP release and epithelial cell migration. In a normal, hypotonic environment, tissues and their external environment can zebraﬁsh tailﬁn epithelium closes a wound within 20 minutes of the initial injury (left). In an isotonic stimulate wound closure by inducing epi- environment, however, cell migration is impaired and the wound remains open (right). thelial cell migration (1). “Then we wanted to know what hapZebrafish live in fresh water, which has attached to the underlying basement meman osmolarity much lower than that of the brane and an outer, suprabasal layer that pens if we perturb ATP metabolism inside animal’s interstitial fluids. Philipp Nietham- acts as the tissue’s permeability barrier. the tissue,” Niethammer says. “To do that, mer and colleagues at Memorial Sloan Ket- Gault et al. found that, after injury in hypo- we inhibited the enzymes that break down tering Cancer Center in New York previously tonic media, suprabasal cells assembled a ATP in the extracellular space.” Raising demonstrated that the drop in interstitial multicellular actomyosin “purse string” ATP levels by inhibiting these extracellular osmolarity that occurs when zebrafish around the wound margin, while the basal enzymes pharmacologically, or knocking tailfins are wounded and exposed to their cells migrated as a sheet toward the center down a particularly abundant ATPase environment stimulates the recruitment of of the wound, apparently dragging the called ENTPD3, increased the extent and suprabasal cells along for duration of basal cell motility upon woundleukocytes that mediate the the ride. In isotonic media, ing. In contrast, lowering ATP levels by tissue’s inflammatory response over the following “By abrogating the suprabasal cells still adding the soluble ATPase apyrase to few hours (2). Niethammer the osmolarity formed a purse string, but wounded tissues decreased cell migration. the basal cells didn’t mi- “So we concluded that ATP is an endogewondered whether osmolardifference… grate and therefore failed to nous wound signal in the zebrafish tailfin,” ity differences might also the fish close the wound and restore says Niethammer. regulate the more immedithe epithelial barrier. “So This ATP signal is released when woundate wound response of the didn’t know tailfin epithelial cells, which it was injured.” there are different wound ed tissues experience the lower osmolarity closure mechanisms in the of their external environment. A similar quickly act to seal any holes two layers that have a differ- wound detection and response mechanism in the tissue. “Because epithelial barriers are so important for protect- ent dependency on the environmental might operate in mammalian tissues exposed to hypotonic environments, such as ing the organism, it’s essential that wound tonicity,” Niethammer explains. Gault et al. then turned their attention the esophageal epithelium. Niethammer, responses are initiated within seconds of to how osmolarity differences might in- however, still has questions about the mechthe injury,” Niethammer explains. Niethammer and colleagues, led by post- duce basal cell migration into wounds. anism in zebrafish: “How is the ATP redoc William Gault, therefore examined the Many cell types respond to osmotic leased, and how is it sensed by the cells? immediate wound response of zebrafish lar- shocks by secreting nucleotides such as That’s what we’re currently working on.” vae surrounded by an isotonic, rather than a ATP (3), and ATP has been shown to stimhypotonic, environment (1). “The sheet of ulate epithelial cell migration and wound 1. Gault, W.J., et al. 2014. J. Cell Biol. http:// dx.doi.org/10.1083/jcb.201408049. epithelial cells didn’t react to the presence healing in vitro (4). 2. Enyedi, B., et al. 2013. Nat. Cell Biol. The researchers found that ATP was of the wound,” Niethammer says. “So by 15:1123–1130. abrogating the osmolarity difference be- released from zebrafish tailfin wounds in 3. Hoffmann, E.K., et al. 2009. Physiol. Rev. tween the inside and the outside of the fish, hypotonic, but not isotonic, environments. 89:193–277. Moreover, adding exogenous ATP stimu- 4. Boucher, I., et al. 2010. Am. J. Physiol. Cell the fish didn’t know it was injured.” Physiol. 299:C411–C421. The zebrafish tailfin epithelium consists lated the migration of basal cells toward of two layers of cells, an inner basal layer wounds even in isotonic media. PHOTOS COURTESY OF MEMORIAL SLOAN KETTERING CANCER CENTER Researchers describe how environmental signals trigger wound repair in zebrafish. 679