The Meis homeoprotein regulates the axolotl Prod 1 promoter during limb regeneration

During limb regeneration in salamanders the blastemal cells give rise only to structures distal to the level of amputation. This proximodistal identity can be regulated by ectopic expression of Meis homeoproteins or the three finger protein Prod 1 which acts at the cell surface. It has been suggested that Meis acts by regulating the transcription of Prod 1. We have sequenced the axolotl Prod 1 promoter and selected two candidate sites for binding Meis homeoproteins. The sites were mutated in various combinations in promoters expressing a luciferase reporter gene. The promoter activity was assayed by nucleofecting AL1 cells, a cultured axolotl limb cell line that expresses both Prod 1 and Meis 1 and 2. The activity of the promoter was inhibited by 60% after mutation at Meis site 1, but not at Meis site 2. The promoter constructs were electroporated into axolotl limb blastemas and the wild type promoter was more active in a proximal blastema than in a contralateral distal blastema. The wild type promoter was significantly more active than a (site 1+site 2) mutant promoter in contralateral proximal blastemas, but the promoters were equivalent in contralateral distal blastemas. The separate site 1 or site 2 mutants were not significantly different from wild type in contralateral proximal blastemas, thus contrasting with the site 1 results in AL1 cells. These data provide strong support for the hypotheses that the Prod 1 promoter is regulated on the proximodistal axis, and that Meis homeoproteins directly regulate the promoter on this axis during limb regeneration in addition to cultured cells.