A new approach of urodele amphibian limb regeneration: study of myosin isoforms and their control by thyroid hormone.

In P. waltlii, an urodele amphibian species which undergoes spontaneous metamorphosis, study of native myosin in pyrophosphate gels at various stages of normal development demonstrates a complete larval to fast myosin isoforms transition, which occurs more precociously in forelimb muscles than in the dorsal and ventral muscles. In the neotenic species A. mexicanum, forelimb muscles development also presents a complete myosin isoforms transition which is in contrast with the partial myosin isoforms transition observed in the dorsal muscle. In metamorphosed or neotenic animals of both species aged 1 year, forelimb regeneration is characterized by a complete transition from larval to fast myosin isoforms, that occurs earlier and more rapidly than in normal forelimb development. When forelimb regeneration is studied in P. waltlii aged 4 years, the adult fast and slow isomyosins are expressed very early in the regeneration process. In experimental hypothyroidian P. waltlii, the larval to fast isoforms transition in regenerating forelimb muscles is slightly delayed. Experimental hyperthyroidism accelerates the disappearance of larval isomyosins in regenerating forelimb muscles, both in P. waltlii and A. mexicanum aged 1 year. This work demonstrates that changes in myosin isoform pattern during forelimb regeneration in adult urodele amphibians are different from changes occurring in the normal forelimb development. They take place without any thyroid hormone influence, as opposed to normal development, and appear to be age-dependent.