In today’s open access paper, researchers argue that the regeneration of outer, visible ear tissue is a useful area of focus for understanding why mammals are limited in their regenerative capacity. Species such as salamanders and zebrafish can regenerate limbs and internal organs, and researchers would like to understand how to enable this capability in mammals. The ear is interesting in this respect because some mammals are capable of regeneration of ear tissue, while others are not, giving a starting point for a closer comparison of the relevant biochemistry between more similar species. Mice are incapable of ear tissue regeneration, which is why ear notching is a common means of animal identification used in laboratories. Interestingly, this is how the exceptional regenerative capacity of MRL mice was discovered – the ear notches healed.
This leads to the advance noted today, in which researchers identified mechanisms that allow some mammals to regenerate ear tissue. They succeeded in reproducing this outcome in mice via upregulation of ALDH1A2 and consequent changes in fibroblast behavior in injured tissues. In most mammals, scarring forms in place of complete regeneration of lost tissue following injury. Fibroblasts are the cells responsible for depositing the extracellular matrix that forms scar tissue. Other lines of work have pointed to differences in the behavior of macrophages and senescent cells in species with different regenerative capacities, and all of these cell populations interact in complex ways following injury and during regeneration. A complete picture remains to be established, but this ALDH1A2 overexpression research has practical implications for human regenerative medicine; there may be a basis for forms of therapy here.
Reactivation of mammalian regeneration by turning on an evolutionarily disabled genetic switch
Regeneration is well maintained in some animal lineages but has been lost in many others during evolution and speciation. Identification of the causal mechanism underlying the failure of regeneration in mammals through comparative strategies is usually entangled by the large phylogenetic distance from highly regenerative species (mostly lower vertebrates). Exploration of principles in the evolution of regeneration demands an organ with easy accessibility and diverse regenerative capacities. One such mammalian organ is the ear pinna, which evolved to funnel sound from the surrounding environment for better distinguishing between ambient noise and predators or prey. The ear pinna possesses complex tissues such as skin and cartilage and exhibits remarkable diversity in the ability to regenerate full-thickness holes punched through this organ in placental mammals.
By performing a side-by-side comparison between regenerative species (rabbits, goats, and African spiny mice) and nonregenerative species (mice and rats), we found that the failure of regeneration in mice and rats was not due to the breakdown of tissue-loss triggered blastema formation and proliferation. Single-cell RNA sequencing and spatial transcriptomic analyses of rabbits and mice identified the response of wound-induced fibroblasts (WIFs) as a key difference between the regenerating and nonregenerating ear pinna.
Gene overexpression studies discovered that Aldehyde Dehydrogenase 1 Family Member A2 (Aldh1a2), encoding a rate-limiting enzyme for the synthesis of retinoic acid (RA) from retinaldehyde, was sufficient to rescue mouse ear pinna regeneration. The activation of Aldh1a2 upon injury was correlated with the regenerative capacity of the tested species. Furthermore, we demonstrated that the deficiency of Aldh1a2 expression, together with the augmented activity of the RA degradation pathway, contributed to insufficient RA production after injury and eventually the failure of regeneration. An exogenous supplement of RA – but not the synthetic precursor retinol – was sufficient to induce regeneration by directing WIFs to form new ear pinna tissues. The inactivation of multiple Aldh1a2-linked regulatory elements accounted for the injury-dependent deficiency of Aldh1a2 in mice and rats. Importantly, activation of Aldh1a2 was sufficient to promote ear pinna regeneration in transgenic mice.