Unraveling the mechanisms behind organ regeneration in zebrafish

Nov 02, 2009
Unlike humans, zebrafish are able to regenerate amputated appendages. Credit: Courtesy of the Salk Institute

The search for the holy grail of regenerative medicine -- the ability to "grow back" a perfect body part when one is lost to injury or disease -- has been under way for years, yet the steps involved in this seemingly magic process are still poorly understood.

Now researchers at the Salk Institute for Biological Studies have identified an essential cellular pathway in that paves the way for limb regeneration by unlocking patterns last seen during embryonic development. They found that a process known as histone demethylation switches cells at the site from an inactive to an active state, which turns on the genes required to build a copy of the lost limb.

"This is the first real molecular insight into what is happening during limb regeneration," says first author Scott Stewart, Ph.D., a postdoctoral researcher in the lab of Juan Carlos Izpisúa Belmonte, Ph.D., who led the Salk team. "Until now, how amputation is translated into gene activation has been like magic. Finally we have a handle on a process we can actually follow."

Their findings, which will be published in a forthcoming issue of Proceedings of the National Academy of Sciences, U.S.A., help to explain how epimorphic regeneration—the regrowing of morphologically and functionally perfect copies of amputated limbs—is controlled, an important step toward understanding why certain animals can do it and we cannot.

"Our experiments show that normal development and limb regeneration are controlled by similar mechanisms," explains Izpisúa Belmonte, a professor in the Gene Expression Laboratory. "This finding will help us to ask more specific questions about mammalian limb regeneration: Are the same genes involved when we amputate a mammalian limb? If not, what would happen if we turned them on? And if we can affect these methylation marks in an amputated limb, what effect would that have?"

The Belmonte lab uses zebrafish, a small fish from the minnow family, to study limb regeneration. "If you amputate the tail of the zebrafish, it regenerates in about a week, seemingly with no effort and leaving no scar," explains Stewart. "What's more, it regenerates a perfect copy and—like growing grass—it will do this over and over again."

Since regeneration recapitulates in broad strokes , during which a complex multi-cellular organism develops from a handful of embryonic stem cells, the researchers began by comparing gene expression patterns between the two processes. During development, genes within specific cell types are turned on and off to trigger the necessary expression patterns that create a whole animal. Once their job is done, they lie silently till they are turned on once again following amputation.

Top: Treating the amputation site with demethylase-inhibitors blocks regeneration. Bottom: The fully regrown tail fin seven days after amputation without treatment. Credit: Courtesy of Dr. Scott Stewart, Salk Institute for Biological Studies

Based on these similarities, Stewart reasoned that genes involved in regeneration may share silencing mechanisms with the ones active in embryonic stem cells. Embryonic stem cells are maintained in a ready-to-go state, "poised" for action to become whatever cell type is needed. The key to this "poised" state are histones, DNA packaging proteins that are also used as carriers for chemical modifications, such as methylation and acetylation. These chemical marks serve as "on" and "off" switches for specific genes.

Stewart discovered that the histone modifications that poise embryonic stem cell-specific genes for activation are also found on the histones near genes involved in regeneration, putting them into a ready-to-go state. "This suggests that two different gene expression programs may exist; one for normal cellular activity and one for regeneration," explains Stewart. To test this hypothesis, the team looked at the histone marks during regeneration. As suspected, they saw a reduction in "off" switches and an increase in "on" switches in regenerating tissue, tipping the balance toward gene expression.

Delving deeper, the researchers found that enzymes that remove the "off" mark, so-called demethylases, are present in high levels in regenerating tissue. One enzyme in particular, called Kdm6b.1, is found exclusively in that are undergoing the regeneration process. Without Kdm6b.1, zebrafish failed to regenerate amputated fins, meaning removal of the "off" mark is a prerequisite for fin regeneration.

In the long term, the Salk researchers hope that these findings will help them understand whether we can affect the outcome of mammalian limb regeneration. In the more immediate future, the team plans to use global approaches to identify all the targets of Kdm6b.1 during regeneration, and to find out what gives the signal to turn these genes off when regeneration is complete.

Source: Salk Institute (news : web)

Explore further: Fighting bacteria—with viruses

add to favorites email to friend print save as pdf

Related Stories

Forsyth scientists discover early key to regeneration

Dec 13, 2006

Science may be one step closer to understanding how a limb can be grown or a spinal cord can be repaired. Scientists at The Forsyth Institute have discovered that some cells have to die for regeneration to occur. This research ...

Liver regeneration may be simpler than previously thought

Apr 11, 2007

The way the liver renews itself may be simpler than what scientists had been assuming. A new study, appearing in the April 13 issue of The Journal of Biological Chemistry, provides new information on the inner workings of cel ...

MicroRNAs help zebrafish regenerate fins

Mar 14, 2008

Biologists have discovered a molecular circuit breaker that controls a zebrafish's remarkable ability to regrow missing fins, according to a new study from Duke University Medical Center.

Newts which Regrow their Hearts

Dec 05, 2006

When a newt loses a limb, the limb regrows. What is more, a newt can also completely repair damage to its heart. Scientists at the Max Planck Institute for Heart and Lung Research in Bad Nauheim have now started ...

Recommended for you

Fighting bacteria—with viruses

19 hours ago

Research published today in PLOS Pathogens reveals how viruses called bacteriophages destroy the bacterium Clostridium difficile (C. diff), which is becoming a serious problem in hospitals and healthcare institutes, due to its re ...

Atomic structure of key muscle component revealed

20 hours ago

Actin is the most abundant protein in the body, and when you look more closely at its fundamental role in life, it's easy to see why. It is the basis of most movement in the body, and all cells and components ...

Brand new technology detects probiotic organisms in food

Jul 23, 2014

In the food industr, ity is very important to ensure the quality and safety of products consumed by the population to improve their properties and reduce foodborne illness. Therefore, a team of Mexican researchers ...

Protein evolution follows a modular principle

Jul 23, 2014

Proteins impart shape and stability to cells, drive metabolic processes and transmit signals. To perform these manifold tasks, they fold into complex three-dimensional shapes. Scientists at the Max Planck ...

Report on viruses looks beyond disease

Jul 22, 2014

In contrast to their negative reputation as disease causing agents, some viruses can perform crucial biological and evolutionary functions that help to shape the world we live in today, according to a new report by the American ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

gmurphy
5 / 5 (1) Nov 02, 2009
This is excellent work but will be difficult to exploit until the mechanisms which coordinate the development of spatially segregated stem cells (such as around the circumference of a stump of an arm) are fully understood