POLYDACTYLY AND ECTOPIC ZPA FORMATION IN ALX-4 MUTANT MICE

Correct development of the limb is dependent on coordination between t hree distinct signaling centers, Recently, fibroblast growth factor-4 has been identified as a crucial determinant of AER function, which di rects limb bud outgrowth, and Sonic hedgehog has been identified as a signaling molecule that mediates ZPA function, which specifies anterio r-posterior patterning in the developing limb bud, In addition, Shh an d FGF-4 reciprocally reinforce each other's expression via a positive feedback loop, providing a molecular basis for the coordination of lim b bud outgrowth and anterior-posterior patterning, The mechanisms by w hich these signaling centers come to occupy their normal positions in the posterior limb bud during development are not understood, Here we identify and characterize Alx-4, a gene that encodes a paired-type hom eodomain protein, Alx-4 is expressed in several populations of mesench ymal cells, including mesenchymal cells in the anterior limb bud, and mice homozygous for targeted disruption of the Alx-4 gene have multipl e abnormalities, including preaxial polydactyly, The polydactyly is as sociated with the formation of an ectopic anterior ZPA, as indicated b y anterior expression of Sonic hedgehog, HoxD13 and fibroblast growth factor-4. The expression of other candidate regulators of anterior-pos terior positional information in the limb bud, including HoxB8 and Gli 3, is not altered in Alx-4 mutant embryos, By chromosomal mapping expe riments, Alx-4 is tightly linked to Strong's luxoid, a polydactylous m ouse mutant, The results identify Alx-4 as a determinant of anterior-p osterior positional identity in the limb and a component of a regulato ry program that restricts ZPA formation to the posterior limb bud mese nchyme.