Genetic basis for an evolutionary shift from ancestral preaxial to postaxial limb polarity in non-urodele vertebrates

• Published in: Current biology Vol. 31; no. 22; pp. 4923 - 4934.e5
• Main Authors: Trofka, Anna, Huang, Bau-Lin, Zhu, Jianjian, Heinz, William F., Magidson, Valentin, Shibata, Yuki, Shi, Yun-Bo, Tarchini, Basile, Stadler, H. Scott, Kabangu, Mirindi, Al Haj Baddar, Nour W., Voss, S. Randal, Mackem, Susan
• Format: Journal Article
• Language: English
• Published: England Elsevier Inc 22-11-2021
• Subjects: 5'Hoxd genes; Animals; axolotl Gli3 CRISPR knockdown; Biological Evolution; Extremities - anatomy & histology; Gli3; limb axis formation polarity; Limb Buds; Mammals; Mice; postaxial limb polarity in mutant axolotl; preaxial limb polarity in mutant mice; Transcription Factors - genetics; Urodela - anatomy & histology; vertebrate limb development; limb axis formation polarity; axolotl Gli3 CRISPR knockdown; postaxial limb polarity in mutant axolotl; preaxial limb polarity in mutant mice; 5'Hoxd genes; vertebrate limb development; Gli3
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2021.09.010

In most tetrapod vertebrates, limb skeletal progenitors condense with postaxial dominance. Posterior elements (such as ulna and fibula) appear prior to their anterior counterparts (radius and tibia), followed by digit-appearance order with continuing postaxial polarity. The only exceptions are urodele amphibians (salamanders), whose limb elements develop with preaxial polarity and who are also notable for their unique ability to regenerate complete limbs as adults. The mechanistic basis for this preaxial dominance has remained an enigma and has even been proposed to relate to the acquisition of novel genes involved in regeneration. However, recent fossil evidence suggests that preaxial polarity represents an ancestral rather than derived state. Here, we report that 5′Hoxd (Hoxd11-d13) gene deletion in mouse is atavistic and uncovers an underlying preaxial polarity in mammalian limb formation. We demonstrate this shift from postaxial to preaxial dominance in mouse results from excess Gli3 repressor (Gli3R) activity due to the loss of 5′Hoxd-Gli3 antagonism and is associated with cell-cycle changes promoting precocious cell-cycle exit in the anterior limb bud. We further show that Gli3 knockdown in axolotl results in a shift to postaxial dominant limb skeleton formation, as well as expanded paddle-shaped limb-bud morphology and ensuing polydactyly. Evolutionary changes in Gli3R activity level, which also played a key role in the fin-to-limb transition, appear to be fundamental to the shift from preaxial to postaxial polarity in formation of the tetrapod limb skeleton.
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•Gli3 repressor (Gli3R) activity level governs tetrapod limb axis formation polarity•5′Hoxd-Gli3 balance modulates cell-cycle exit to determine mouse limb axis polarity•Alternating A-P digit appearance in mammals is linked to primary limb axis polarity•Axolotl Gli3 knockdown shifts the ancestral preaxial dominance to postaxial
Uniquely in salamanders, the tetrapod primary limb axis forms with ancestral preaxial dominance, but the underlying basis is unknown. Here, Trofka et al. show Gli3 has a central role; mice with elevated Gli3 repressor activity revert to preaxial dominance. Conversely, Gli3 knockdown shifts the axolotl limb axis from preaxial to postaxial polarity.