If you trace our evolutionary tree way back to its roots — long before the shedding of gills or the development of opposable thumbs — you will likely find a common ancestor with the amazing ability to regenerate lost body parts.
Researchers have built a running list of the genes that enable regenerating animals to grow back a severed tail or repair damaged tissues. Surprisingly, they have found that genes important for regeneration in these creatures also have counterparts in humans. The key difference might not lie in the genes themselves but in the sequences that regulate how those genes are activated during injury.
A Duke study appearing April 6 in the journal Nature has discovered the presence of these regulatory sequences in zebrafish, a favored model of regeneration research. Called “tissue regeneration enhancer elements” or TREEs, these sequences can turn on genes in injury sites and even be engineered to change the ability of animals to regenerate.
Kang discovered that the element could be separated into two distinct parts: one that activates genes in an injured heart, and, next to it, another that activates genes in an injured fin. He fused these sequences to two regeneration genes, fibroblast growth factor and neuregulin 1, to create transgenic zebrafish whose fins and hearts had superior regenerative responses after injury.
They tested whether these “tissue regeneration enhancer elements” or TREEs could have a similar effect in mammalian systems like mice. Collaborator Brian L. Black, PhD, of the University of California, San Francisco attached one TREE to a gene called lacZ that produces a blue color wherever it is turned on. Remarkably, he found that borrowing these elements from the genome of zebrafish could activate gene expression in the injured paws and hearts of transgenic mice.
“We are just at the beginning of this work, but now we have an encouraging proof of concept that these elements possess all the sequences necessary to work with mammalian machinery after an injury,” said Poss. He suspects there may be many different types of TREEs: those that turn on genes in all tissues; those that turn on genes only in one tissue like the heart; and those that are active in the embryo as it develops and then are reactivated in the adult as it regenerates.
Eventually, Poss thinks that genetic elements like these could be combined with genome-editing technologies to improve the ability of mammals, even humans, to repair and regrow damaged or missing body parts.
LEN activity in neonatal mice.
The green signal in these images of an injured zebrafish heart and a fin indicate the activity of a gene that enhances tissue regeneration. (Image: Junsu Kang, Duke University.)
How tissue regeneration programs are triggered by injury has received limited research attention. Here we investigate the existence of enhancer regulatory elements that are activated in regenerating tissue. Transcriptomic analyses reveal that leptin b (lepb) is highly induced in regenerating hearts and fins of zebrafish. Epigenetic profiling identified a short DNA sequence element upstream and distal to lepb that acquires open chromatin marks during regeneration and enables injury-dependent expression from minimal promoters. This element could activate expression in injured neonatal mouse tissues and was divisible into tissue-specific modules sufficient for expression in regenerating zebrafish fins or hearts. Simple enhancer-effector transgenes employing lepb-linked sequences upstream of pro- or anti-regenerative factors controlled the efficacy of regeneration in zebrafish. Our findings provide evidence for ‘tissue regeneration enhancer elements’ (TREEs) that trigger gene expression in injury sites and can be engineered to modulate the regenerative potential of vertebrate organs.
SOURCES – Duke University, Nature