Molecular basis of neural control of ontogenetic allometric growth in the regenerating Axolotl limb

Abstract

Urodele amphibians such as the Mexican axolotl have the amazing ability to regenerate missing limbs. Limb regeneration is dependent on the formation of a transient regenerative organ known as the blastema, which is composed of limb progenitor cells that proliferate, pattern, and re‐differentiate into the missing limb tissues. Initially the limb regenerate is small in size and undergoes a period of rapid ontogenetic allometric growth until it reaches the proportionally appropriate size. The underlying mechanisms that regulate this growth in the late staged regenerate are not known. We have recently shown that the limb nerves are required to maintain the growth of the regenerating limb, and that the size of the regenerated limb positively correlates with the abundance of nerves that are connected to it. To identify the molecular signals that could regulate the growth of the regenerate, we evaluated the expression of previously established nerve dependent growth factors in the limb nerves and the regenerating limb tissues during the late stages of limb regeneration. We found that the expression of multiple growth factors including FGFs and BMPs are elevated in both tissues during the stages of regeneration that exhibit the most rapid growth rates. Removing nerve signaling from the late staged regenerate, which attenuates growth, alters the expression of these growth factors in the regenerating tissues. Last, we found that pharmacological inhibition of BMP signaling inhibits the growth of the regenerating limb and mimics the effect of denervation on the expression of some, but not all of the growth factors that we previously evaluated. These studies constitute the first steps in identifying the molecular underpinnings of the neural regulation of ontogenetic allometric growth of the regenerating amphibian limb.