DIFFERENTIAL EXPRESSION OF MYOGENIC REGULATORY GENES AND MSX-1 DURINGDEDIFFERENTIATION AND REDIFFERENTIATION OF REGENERATING AMPHIBIAN LIMBS

An amputated limb of an adult urodele amphibian is capable of undergoi ng regeneration, The new structures form from an undifferentiated mass of cells called the regenerative blastema. The cells of the blastema are believed to derive from differentiated tissues of the adult limb. However, the exact source of these cells and the process by which they undergo dedifferentiation are poorly understood. In order to elucidat e the molecular and cellular basis for dedifferentiation we isolated a number of genes which are potential regulators of the process. These include Msx-1, which is believed to support the undifferentiated and p roliferative state of cells in the embryonic limb bud; and two members of the myogenic regulatory gene family, MRF-4 and Myf-5, which are ex pressed in differentiated muscle and regulate muscle-specific gene act ivity, As anticipated, we find that Msx-1 is strongly up-regulated dur ing the initiation of regeneration, It remains expressed throughout re generation but is not found in the fully regenerated limb. The myogeni c gene MRF-4 has the reverse expression pattern, It is expressed in ad ult limb muscle, is rapidly shut off in early regenerative blastemas, and is only reexpressed at the completion of regeneration. These kinet ics are paralleled by those of a muscle-specific Myosin gene. In contr ast Myf-5, a second member of the myogenic gene family, continues to b e expressed throughout the regenerative process, Thus, MRF-4 and Myf-5 are likely to play distinct roles during regeneration, MRF-4 may dire ctly regulate muscle phenotype and as such its repression may be a key event in dedifferentiation. Myf-5 may play a role in maintaining a di stinct myogenic lineage during regeneration. (C) 1995 Wiley-Liss, Inc.