The immune system participates in regeneration, particularly the innate immune cells called macrophages. The behavior of these cells also appears to be an important part of the differences between (a) proficient regeneration, exhibited by salamanders, zebrafish, and to a lesser degree by a few mammals such as spiny mice, and (b) the limited regenerative capacities of the rest of the vertebrate kingdom. Cut off a finger or an arm, and we do not regrow that limb. Our hearts do not regenerate well from damage. Our nerves do not restore themselves from injury. Salamanders accomplish all of these things, and a number of groups in the life science research community are working towards an explanation for that difference. The research results noted here represent the latest incremental gain in understanding, one of many such steps since the turn of the century.
The presently emerging picture of regeneration is one of a coordinated dance of biochemistry involving temporarily present senescent cells, macrophages, and the various populations of cells and stem cells resident in a tissue. Take away the macrophages and it all falls apart; that much has been demonstrated in the studies of recent years. When researchers look at aspects of this dance in salamanders, it appears to be a lot more efficient than is the case in most mammals - but still, as demonstrated in the research here, take away the macrophages and salamanders heal as poorly as we do. Further, spiny mice, that unlike other mammals can regenerate several tissue types without scarring, have salamander-like macrophage behavior during regeneration.
Despite the intriguing examples of tissue regeneration in spiny mice and engineered MRL mice, there is more going on in salamanders and zebrafish than just greater efficiency in the activities of senescent cells and macrophages in tissue regrowth. Salamander and zebrafish cells reprogram themselves into pluripotent states in response to injury, building a blastema, a mass of cells capable of generating all of the necessary replacement parts. Limb regeneration in those species bears a great deal of resemblance to embryonic development. Mammals do not do this, and it seems quite plausible that the reasons why mammals do not do this go far beyond macrophage behavior. One plausible theory is that most species lost the ability to regenerate in this way due to the evolution of cancer suppression mechanisms: inserting the human tumor suppressor gene ARF into zebrafish shuts down their ability to regrow fins and organs, for example.
So it seems very plausible at this point that adjusting macrophage activity is a path to some degree of enhanced human regeneration. Indeed, simple demonstrations in mammals have been carried out involving alterations of macrophage polarization, the balance between pro-inflammatory and pro-regeneration populations of these cells. However, the full salamander package with cellular reprogramming and blastemas recapitulating embryonic development seems likely to require an earnest reengineering of mammalian cellular biochemistry, and as such is probably not a near-term prospect. In the near term, the plausible goal is the enhanced regeneration of MRL and spiny mice, not the limb regrowth of salamanders and zebrafish. In the long term, of course, everything is possible, but we have other battles to fight before that comes to pass.
The answer to regenerative medicine's most compelling question - why some organisms can regenerate major body parts such as hearts and limbs while others, such as humans, cannot - may lie with the body's innate immune system, according to a new study of heart regeneration in the axolotl, or Mexican salamander. Researchers found that the formation of new heart muscle tissue in the adult axolotl after an artificially induced heart attack is dependent on the presence of macrophages, a type of white blood cell. When macrophages were depleted, the salamanders formed permanent scar tissue that blocked regeneration.
The goal is to activate regeneration in humans through the use of drug therapies derived from macrophages that would promote scar-free healing directly, or those that would trigger the genetic programs controlling the formation of macrophages, which in turn could promote scar-free healing. The team is already looking at molecular targets for drug therapies to influence these genetic programs. "If humans could get over the fibrosis hurdle in the same way that salamanders do, the system that blocks regeneration in humans could potentially be broken. We don't know yet if it's only scarring that prevents regeneration or if other factors are involved. But if we're really lucky, we might find that the suppression of scarring is sufficient in and of itself to unlock our endogenous ability to regenerate."
The prevailing view in regenerative biology has been that the major obstacle to heart regeneration in mammals is insufficient proliferation of cardiomyocytes, or heart muscle cells. But researchers found that cardiomyocyte proliferation is not the only driver of effective heart regeneration. The findings suggest that research efforts should pay more attention to the genetic signals controlling scarring. When a human experiences a heart attack, scar tissue forms at the site of the injury. While the scar limits further tissue damage in the short term, over time its stiffness interferes with the heart's ability to pump, leading to disability and ultimately to terminal heart failure. The next step is to study the function of macrophages in salamanders and compare them with their human and mouse counterparts. Ultimately, researchers would like to understand why macrophages produced by adult mice and humans don't suppress scarring in the same way as in axolotls and then identify molecules and pathways that could be exploited for human therapies.
In dramatic contrast to the poor repair outcomes for humans and rodent models such as mice, salamanders, and some fish species are able to completely regenerate heart tissue following tissue injury, at any life stage. This capacity for complete cardiac repair provides a template for understanding the process of regeneration and for developing strategies to improve human cardiac repair outcomes. Using a cardiac cryo-injury model we show that heart regeneration is dependent on the innate immune system, as macrophage depletion during early time points post-injury results in regeneration failure.
In contrast to the transient extracellular matrix that normally accompanies regeneration, this intervention resulted in a permanent, highly cross-linked extracellular matrix scar derived from alternative fibroblast activation and lysyl-oxidase enzyme synthesis. The activation of cardiomyocyte proliferation was not affected by macrophage depletion, indicating that cardiomyocyte replacement is an independent feature of the regenerative process, and is not sufficient to prevent fibrotic progression. These findings highlight the interplay between macrophages and fibroblasts as an important component of cardiac regeneration, and the prevention of fibrosis as a key therapeutic target in the promotion of cardiac repair in mammals.