Snapshots of Life: Coming Face to Face with Development

Zebrafish larva

Credit: Oscar Ruiz and George Eisenhoffer, University of Texas MD Anderson Cancer Center, Houston

Zebrafish (Danio rerio) is a favorite model for studying development, in part because its transparent embryos make it possible to produce an ever-growing array of amazingly informative images. For one recent example, check out this Federation of American Societies for Experimental Biology’s 2016 BioArt winner, which shows the developing face of a 6-day-old zebrafish larva.

Yes, those downturned “lips” are indeed cells that will go on to become the fish’s mouth. But all is not quite what it appears: the two dark circles that look like eyes are actually developing nostrils. Both the nostrils and mouth express high levels of F-actin (green), a structural protein that helps orchestrate cell movement. Meanwhile, the two bulging areas on either side of the fish’s head, which are destined to become eyes and skin, express keratin (red).

Oscar Ruiz, who works in the lab of George Eisenhoffer at The University of Texas MD Anderson Cancer Center, Houston, used a confocal microscope to create this image. What was most innovative about his work was not the microscope itself, but how he prepared the sample for imaging. With traditional methods, researchers can only image the faces of zebrafish larvae from the side or the bottom. However, the Eisenhoffer lab has devised a new method of preparing fish larvae that makes it possible to image their faces head-on. This has enabled the team to visualize facial development at much higher resolution than was previously possible.

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Regenerative Medicine: New Clue from Fish about Healing Spinal Cord Injuries

Zebrafish Spinal Cord

Caption: Tissue section of zebrafish spinal cord regenerating after injury. Glial cells (red) cross the gap between the severed ends first. Neuronal cells (green) soon follow. Cell nuclei are stained blue and purple.
Credit: Mayssa Mokalled and Kenneth Poss, Duke University, Durham, NC

Certain organisms have remarkable abilities to achieve self-healing, and a fascinating example is the zebrafish (Danio rerio), a species of tropical freshwater fish that’s an increasingly popular model organism for biological research. When the fish’s spinal cord is severed, something remarkable happens that doesn’t occur in humans: supportive cells in the nervous system bridge the gap, allowing new nerve tissue to restore the spinal cord to full function within weeks.

Pretty incredible, but how does this occur? NIH-funded researchers have just found an important clue. They’ve discovered that the zebrafish’s damaged cells secrete a molecule known as connective tissue growth factor a (CTGFa) that is essential in regenerating its severed spinal cord. What’s particularly encouraging to those looking for ways to help the 12,000 Americans who suffer spinal cord injuries each year is that humans also produce a form of CTGF. In fact, the researchers found that applying human CTGF near the injured site even accelerated the regenerative process in zebrafish. While this growth factor by itself is unlikely to produce significant spinal cord regeneration in human patients, the findings do offer a promising lead for researchers pursuing the next generation of regenerative therapies.

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Cool Videos: Regenerating Nerve Fibers

If you enjoy action movies, you can probably think of a superhero—maybe Wolverine?—who can lose a limb in battle, yet grow it right back and keep on going. But could regenerating a lost limb ever happen in real life? Some scientists are working hard to understand how other organisms do this.

As shown in this video of a regenerating fish fin, biology can sometimes be stranger than fiction. The zebrafish (Danio rerio), which is a species of tropical freshwater fish that’s an increasingly popular model organism for biological research, is among the few vertebrates that can regrow body parts after they’ve been badly damaged or even lost. Using time-lapse photography over a period of about 12 hours, NIH grantee Sandra Rieger, now at MDI Biological Laboratory, Bar Harbor, ME, used a fluorescent marker (green) to track a nerve fiber spreading through the skin of a zebrafish tail fin (gray). The nerve regeneration was occurring in tissue being spontaneously formed to replace a section of a young zebrafish’s tail fin that had been lopped off 3 days earlier.

Along with other tools, Rieger is using such imaging to explore how the processes of nerve regeneration and wound healing are coordinated. The researcher started out by using a laser to sever nerves in a zebrafish’s original tail fin, assuming that the nerves would regenerate—but they did not! So, she went back to the drawing board and discovered that if she also used the laser to damage some skin cells in the tail fin, the nerves regenerated. Rieger suspects the answer to the differing outcomes lies in the fact that the fish’s damaged skin cells release hydrogen peroxide, which may serve as a critical prompt for the regenerative process [1]. Rieger and colleagues went on discover that the opposite is also true: when they used a cancer chemotherapy drug to damage skin cells in a zebrafish tail fin, it contributed to the degeneration of the fin’s nerve fibers [2].

Based on these findings, Rieger wants to see whether similar processes may be going on in the hands and feet of cancer patients who struggle with painful nerve damage, called peripheral neuropathy, caused by certain chemotherapy drugs, including taxanes and platinum compounds. For some people, the pain and tingling can be so severe that doctors must postpone or even halt cancer treatment. Rieger is currently working with a collaborator to see if two protective molecules found in the zebrafish might be used to reduce or prevent chemotherapy-induced peripheral neuropathy in humans.

In recent years, a great deal of regenerative medicine has focused on learning to use stem cell technologies to make different kinds of replacement tissue. Still, as Rieger’s work demonstrates, there remains much to be gained from studying model organisms, such as the zebrafish and axolotl salamander, that possess the natural ability to regenerate limbs, tissues, and even internal organs. Now, that’s a super power we’d all like to have.

Reference:

[1] Hydrogen peroxide promotes injury-induced peripheral sensory axon regeneration in the zebrafish skin. Rieger S, Sagasti A. PLoS Biol. 2011 May;9(5):e1000621

[2] Paclitaxel-induced epithelial damage and ectopic MMP-13 expression promotes neurotoxicity in zebrafish. Lisse TS, Middleton LJ, Pellegrini AD, Martin PB, Spaulding EL, Lopes O, Brochu EA, Carter EV, Waldron A, Rieger S. Proc Natl Acad Sci U S A. 2016 Apr 12;113(15):E2189-E2198.

Links:

Chemotherapy-Induced Peripheral Neuropathy (National Cancer Institute/NIH)

Learning About Human Biology From a Fish (National Institute of General Medical Sciences/NIH)

Sandra Rieger (MDI Biological Laboratory, Bar Harbor, ME)

NIH Support: National Institute of Dental and Craniofacial Research; National Institute of General Medical Sciences; National Institute of Neurological Disorders and Stroke