By David N. Leff
In the news last week was the case of a man who had lost his hand and forearm in an accident. When a donor's replacement member became available, surgeons and microsurgeons united the donor's bones, muscles, ligaments, nerves, blood vessels and skin to the stump of the recipient's former limb. They and the recipient won't know for many months the extent of function, if any, this heroic transplant will achieve.
It's quite a different story with amphibians. When a salamander loses an arm or a leg, it quickly grows a new one, complete with fingers or toes. To be sure, the replacement has rigid cartilage instead of bone, but it works fine.
Frogs are also amphibians, but they don't have this ability to regenerate a lost limb. A lizard, on the other hand, when a predator chomps down on its tail, will cast off that appendage and quickly replace it.
Mammals, of course, are left out of this regeneration gift entirely, or almost entirely. If as much as three-fourths of a human liver is removed, the organ will grow back all of the lost tissue, and more — functional but not structurally identical to the original.
Male deer drop their antlers in the spring, and regrow them in time to lock horns with other male deer during the fall rutting season. And if a rabbit gets a full-thickness wound on its ear, that lesion will heal over perfectly, without a scar or other sign of the injury.
When immunologist Ellen Heber-Katz asked a speaker at a wound-healing meeting some years ago if mice shared this innate ear-healing ability, he replied, "No, unfortunately, mice don't close ear-holes."
In fact, that's not a total misfortune.
Heber-Katz, a professor at the Wistar Institute, in Philadelphia, told BioWorld Today that ear-punching is "a standard method researchers use in numbering their laboratory mice. There is a code for making multiple holes in varied patterns. Essentially, they will identify one mouse in the animal colony for its entire lifetime, because the holes don't close."
With one serendipitous exception.
In a strain of mouse they had procured from the Jackson Laboratory, Heber-Katz and her post-doctoral fellow were testing an antigen that seemed to inhibit the autoimmune process in multiple sclerosis and lupus. "We were looking for an animal model of spontaneously occurring disease," she recalled, "and we ear-punched those mice."
When she checked on the colony three weeks later, all of their ears were as intact as the day they were born.
"We assumed that someone had goofed," she observed, "so we re-ear-punched them, and sure enough by 30 days later the holes had closed again. We were very excited. There was no scarring. No evidence of the holes at all. So we thought, 'Well, this is incredible wound-healing!'"
Punched-Out Ear-Holes Make Total Comeback
"Although mouse ears are semi-translucent, and almost paper-thin, those 2.1-millimeter-diameter [three thirty-seconds of an inch] punch-outs had regenerated the normal architecture of collagen structure, angiogenesis, hair follicles, sebaceous glands and cartilage, without any scarring or contracture," she reported.
Her report appears in the current Proceedings of the National Academy of Sciences (PNAS), dated Sept. 29. Its title is, "Genetic analysis of a mammalian wound-healing trait." Heber-Katz is the paper's senior author.
This total healing, she pointed out, is an apparent exception to the dogma that lower forms of life regenerate, while higher forms repair.
Her team's point of departure in analyzing the genetic and molecular basis of this maverick trait was to cross-breed those healer mice with non-healers, and identify their respective progeny with the classical Mendelian inheritance pattern.
"Our initial screen of the entire mouse genome," Heber-Katz observed, "was 101 mice. We now have about 500 more to analyze," comparing the genomic sequences of healer and non-healer animals, and correlating the healers' chromosomal DNA with their trait.
Hybrid mice, with one parent a healer and the other not a healer, tend to display an intermediate regenerative ability, for example, partial or incomplete closure of the punch hole. "Our findings so far indicate," she said, "that in inbred mice, this type of healing is a heritable trait.
"We already know from the genetic analysis," Heber-Katz went on, "that the healer trait is multigenic. We don't know the exact genes yet, but we do have regions that we're looking at in terms of fine mapping, and which we will be sequencing. Loci on mouse chromosomes 7, 8, 12, 13 and 15," she continued, "play a role in the mammalian regeneration process.
Acne Medicine A Candidate Gene
"Those genes also regulate other genes," she said. "So we have a whole cascade of gene products that we're identifying. And we've found various molecules involved. One candidate is the retinoic acid receptor gamma. We think it could be a gene. Retinoic acid is very important in induction of differentiation during embryogenesis, and in skin growth. It's used in the treatment of acne and psoriasis. So it is a strong candidate gene. Interestingly, retinoic acid is also involved in amphibian regeneration."
She sees potential for this genetic research in future wound healing. "If there is something unusual about the healer gene or promoter, if gene therapy becomes incredibly viable, then one might be able to transfer these genes into mammals, and help regeneration to occur," she said.
"Regeneration involves a lot of the things that probably occur during development," she went on. "All these processes should be there already. But they don't seem to be turned on in the adult. What exactly that means we don't know. But there may be factors that we will be able to isolate from these animals that will promote regeneration. Or cells that may be transplanted, to allow this process to occur.
"It could have very great potential." Heber-Katz concluded, "for wound and burn healing, organ replacement, spinal-cord regeneration and controlling tissue growth." *