Models, Concepts, and Mechanisms of Tissue Regeneration
How and why tissue regeneration does (or does not) occur are critical questions. The biology of regeneration remains both challenging and fascinating, and new discoveries have the potential to impact clinical outcomes of many diseases of organ damage, including heart failure, Alzheimer’s disease, and diabetes.
It has been known for centuries that salamanders and fish regenerate complex tissues much more effectively than mammals. Zebrafish have emerged as a central model system for studying regeneration, due to their ability to regenerate myriad tissues and to the availability of molecular genetic tools. Over the past decade, our laboratory has spearheaded the use of zebrafish to reveal concepts and mechanisms of regeneration.
We study the initial morphogenesis and injury-induced regeneration of several tissues in zebrafish. Our student and postdoc projects investigate adult hearts, fins, spinal cord, skin, scales, and other tissues. We have also begun to test ideas in mammalian models.
There is little natural regeneration of the major structural cells of the adult mammalian heart, the cardiomyocytes, after injury. This regenerative shortcoming is highly relevant to human disease, given the high prevalence in the United States of ischemic myocardial infarction and heart failure. Many years ago, we introduced a model system approach to heart regeneration, by showing that adult zebrafish can regenerate new muscle lost after major cardiac injury. Since then, we have found that cardiac regeneration is not based on stem cells, but rather involves activation and proliferation of spared cardiac myocytes. We also have demonstrated multiple roles during heart regeneration for the epicardium, a thin epithelial layer enveloping the cardiac chambers, and the inner endothelial lining of the chambers called the endocardium. Together, models like zebrafish and neonatal mice have great potential to reveal methods to gauge and stimulate human heart regeneration. We are continuing to investigate how muscle and non-muscle cells respond to injury and orchestrate regeneration. Key issues include the identification of cardiomyocyte mitogens, the definition of gene regulatory elements that activate regeneration programs, and how to use fundamental mechanistic data in platforms to boost cardiac regenerative capacity in mammals. Our methods are exploratory and rely heavily on generation of new mutant and transgenic animals.
Zebrafish fins are transparent, intricately patterned structures, making them tractable for asking fundamental questions about complex tissue regeneration. Within two weeks after amputation of a fin, a series of healing, proliferation, and patterning events replaces bone, epidermis, blood vessels, nerves, and connective tissue mesenchyme. Our work on fin regeneration has helped establish the cellular origins of regenerated fin tissue, and it has identified new concepts and molecular mechanisms of appendage regeneration. We are pursuing informative mutants in fin regeneration, both by forward genetic screens and targeted gene editing. Also, we are developing new methods for imaging of key cellular and molecular events during regeneration, to acquire and quantify live cellular and subcellular events in regenerating complex tissues.
Spinal Cord Regeneration
Primary and secondary tissue damage from spinal cord injury permanently impairs sensory and motor functions, causing irreversible paralysis. Developing therapies to treat and reverse spinal cord injury is an urgent need in regenerative medicine and remains an enormous research challenge. The path to an effective cure requires a combination of molecular, cellular, electrostimulatory, and engineering approaches, and must be guided by a deeper understanding of the inherent regenerative capacity of spinal cord tissue. We have developed a new program to harness the remarkable, innate ability of zebrafish to regenerate a severed spinal cord.
Following spinal cord injury, nerve cell death and scar formation inhibit regeneration. To date, attempts to alleviate the negative effects of scarring, and to support cell survival and nerve regrowth after injury have not achieved mammalian spinal cord regeneration. For many years, scientists have considered glial tissue, a major supporting component of the spinal cord, to be a scar-causing roadblock to regeneration. Yet, attempts to inhibit glial scarring have worsened repair. Indeed, there is growing evidence that some mammalian glial cells have pro-regenerative properties in addition to or instead of anti-regenerative effects.
Remarkably, only a handful of groups worldwide use adult zebrafish to investigate the innate ability for spinal regeneration. Just 6 to 8 weeks after a paralyzing injury that completing severs their spinal cord, zebrafish form new neurons, regrow axons, and recover the ability to swim. Importantly, these regenerative events proceed without massive scarring. Instead, following injury, specialized glial cells assist in forming a tissue bridge over the two severed ends, allowing axons to grow across the wound and reestablish crucial connections. We recently reported a factor called connective tissue growth factor (ctgf) that is induced in an apparent new population of glial cells that participate in the earliest bridging events. We found that zebrafish require the ctgf gene for normal glial bridging and effective spinal cord regeneration, and that providing extra ctgf accelerates glial bridging, visible regeneration, and functional recovery from spinal cord injury.
We are exploring the underlying gene regulatory mechanisms of spinal cord regeneration, with the goals of finding key factors and regulatory sequences that initiate regeneration programs in spinal cord cell types. Ultimately, this information could be employed in novel strategies to boost regeneration in mammals.