On Arms and Legs: How to Rebuild Different Limb Structures

Hans-Georg Simon

Northwestern University Medical School
CMIER-Developmental Biology Program
2300 Children's Plaza M/C 204
Chicago, IL 60614

The relationship between embryonic limb development and limb regeneration is not yet understood. The T-domain transcription factors Tbx4 and Tbx5 have been implicated, by virtue of their limb-type specific expression, in controlling the identity of vertebrate legs and arms, respectively. To study the roles these genes have in developing and regenerating limbs, we have cloned Tbx4 and Tbx5 cDNAs from the newt, and generated antisera that recognize Tbx4 or Tbx5 proteins. We show that in two urodele amphibians, newts and axolotls, the regulation of Tbx4 and Tbx5 is different from higher vertebrates. At the mRNA and protein level, both Tbx4 and Tbx5 are expressed in developing hindlimbs as well as in developing forelimbs. The co-expression of these genes argues that additional factors are involved in the control of limb-type specific patterns. While Tbx4 and Tbx5 are co-expressed in the limbs during development, they display a different expression during regenerative pattern formation: Tbx4 is exclusively re-activated in hindlimb blastemas, and Tbx5 is exclusively re-activated in forelimb blastemas. Different from higher vertebrates, Tbx4 and Tbx5 remain expressed at low levels in adult newt tissue, which may suggest a requirement for genes that control limb-specific patterns to maintain positional information in the event of regeneration. Growth and differentiation of the regeneration blastema resemble that of the embryonic bud; however, we present clear molecular evidence that the patterning mechanisms in developing and regenerating structures are controlled by different gene activities.