Analysis of muscle adaptations to terrestrial stepping in the Urodelan amphibian Pleurodeles waltlii

The changes of myosin isoform pattern and of its associated light chains in relation to the myosin ATPase profile were analysed in different muscles of the hypothyroidian amphibian Pleurodeles waltlii submitted to terrestrial stepping, using electrophoretic and histochemical techniques. These changes were specific to the muscle type but appeared globally characterized by a type-IIB to type-IIA/I fibre transition associated with a transition from fast to intermediate and/or slow myosin isoforms. These results are similar to the effects of endurance training on locomotor muscles of mammals. The diaphragm of experimental animals was also characterized by a complete disappearance of the larval myosin isoforms which were detected in the diaphragm of control animals. The myosin pattern of ventricular muscle did not change following terrestrial stepping. This work indicates that thyroid hormone does not regulate the muscle adaptations that occur following terrestrial stepping and suggests a more complex mechanism of regulation in which innervation could be implicated.

The emergence of an adult muscle phenotype in urodelan amphibians: an immunohistochemical study

Electrophoretic techniques adapted for the analysis of muscles of lower invertebrates reveal four myosin heavy chain isoforms in the dorsalis trunci of Pleurodeles waltlii: two fast (MHC-IIA, MHC-IIB), and one slow (MHC-I) in the adult and one isoform (MHC-La) in the larvae. Polyclonal antibodies were prepared against the larval (anti-MHC-La) and one of the fast myosin (MHC-IIA) isoforms and their specificity was confirmed by western blot analysis. An immunohistochemical analysis was then carried out on frozen sections of the dorsalis trunci of P. waltlii at different stages of development. From stage 44 it was possible to demonstrate the presence of MHC-IIA in the small diameter fibers at the periphery of the muscle; the number and diameter of these fibers increased from stage 44 to stage 56 when anatomical metamorphosis had finished. By stage 56 these fibers could also be readily identified using standard histochemical techniques as type IIA fibers. We conclude that fast IIA myosin is expressed well before the final adult muscle phenotype has been established and its expression is therefore independent of thyroid hormone.

Pituitary-thyroid axis controls the final differentiation of the dorsal skeletal muscle in urodelan amphibians

A histoenzymological study of the ATPase activity of myosin in the dorsal axis muscle (dorsalis trunci) was carried out on two species of urodelan amphibians: Pleurodeles waltlii, a euthyroid species with spontaneous metamorphosis and Ambystoma mexicanum, a neotenic hypothyroid species. P. waltlii and A. mexicanum underwent an operation after which cytological analysis of the remaining pituitary were carried out in parallel. The muscle phenotype of urodelan amphibians varies according to the thyroid status of the species. In euthyroid adults, IIA fibers are dominant whereas in hypothyroid adults, IIC fibers are dominant. The number of type IIB (fast) and type I fibers (slow) are similar in both species. Physiological or experimental modulation of the concentration of circulating thyroid hormones results in a modification of the muscle fiber type profile pertaining to the considered species. We found that pituitary (TSH) plays a dominant role in the maturation of type IIC fibers in both species. Moreover, it seems to modulate the development of IIA fibers in P. waltlii and that of IIB fibers in A. mexicanum. Its action is thus species specific. Through partial or total hypophysectomy experiments, we have been able to demonstrate the influence of the hypophysothyroidian axis on the appearance of the adult muscle phenotype during metamorphosis.

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.

White muscle differentiation in the eel (Anguilla anguilla L.): changes in the myosin isoforms pattern and ATPase profile during post-metamorphic development

Myosin isoforms and their light and heavy chains subunits were studied in the white lateral muscle of the eel during the post metamorphic development, in relation with the myosin ATPase profile. At elver stage VI A1 the myosin isoforms pattern was characterized by at least two isoforms, FM3 and FM2. The fast isomyosin type 1 (FM1) appeared during subsequent development. It increased progressively in correlation with the increase in the level of the light chain LC3f. FM1 became predominant at stage VI A4. At the elver stage VI A1, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed at least two heavy chains, namely type II-1 and II-2. The type II-1 heavy chain disappeared in the yellow eel white muscle, and V8-protease peptide map showed the appearance of a minor heavy chain type II-3 as early as stage VI B. Comparison of myosin heavy chains and myosin isoforms patterns showed the comigration of different myosin isoforms during white muscle development. The myosin ATPase profile was characterized by a uniform pattern as far as stage VI A4. A mosaic aspect in white muscle was observed as early as stage VI B, showing the appearance of small acid labile fibers. This observation suggests that the type II-3 heavy chain is specific to the small fibers.

Myosin structure in the eel (Anguilla anguilla L.). Demonstration of three heavy chains in adult lateral muscle

Myosin extracts from central white fibers and peripheral red fibers of the lateral muscle of eel (Anguilla anguilla) were analysed by electrophoresis under non-dissociating conditions, which demonstrated a polymorphism of myosin isoforms. The light and heavy subunit content of the isomyosins was established using SDS-PAGE and two-dimensional electrophoresis. In the central white muscle, 3 myosin isoforms FM3, FM2, FM1, were characterized by 3 types of fast light chain and one fast heavy chain HCf; the existence of a fourth isomyosin is discussed. In the peripheral red muscle, two myosin isoforms were found, SM1 and SM2, each characterized by a specific heavy chain, HCs1 or HCs2, and containing the same slow light chain content. This work demonstrates for the first time the existence of 3 heavy chains in the skeletal muscle of a fish.

Myosin structure and thyroidian control of myosin synthesis in urodelan amphibian skeletal muscle

Electrophoretic analysis in non-dissociating conditions reveals three types of myosin in adult urodelan amphibian skeletal muscles: 3 isoforms of fast myosin (FM), one isoform of intermediate myosin (IM) and one or two isoforms of slow myosin (SM). Each type is characterized by a specific heavy chain HCf (FM), HCi (IM) and HCs (SM), respectively. In all urodelan species, as in mammals, fast isomyosins associate HCf and the three fast light chains LC1f, LC2f, and LC3f. In most urodelan species the intermediate myosin contains LC1f and LC2f and can be considered as an homodimer of the alkali LC1f. However, in Euproctus asper, IM is characterized by the association of both slow and fast LC with HCi. Slow myosin is a hybrid molecule associating HCs with slow and fast LC. During metamorphosis, a myosin isoenzymic transition occurs consisting in the replacement of three larval myosins (LM) characterized by a specific heavy chain (HCI), by the adult isomyosins with lower electrophoretic mobilities. At the same time there is a change in the ATPase myofibrillar pattern, with the larval fiber types being replaced by adult fibers of types I, IIA and IIB. In the neotenic and perennibranchiate species, which do not undergo spontaneous metamorphosis, sexually mature larval animals present a change in the myosin isoenzymic profile, but no complete transition. The coexistence of larval and adult isomyosins and the persistence of transitional fibers of type IIC in the skeletal muscle are demonstrated. Experimental hypo- and hyperthyroidism indicate that thyroid hormone stimulates the regression of the larval isomyosins, possibly through indirect pathways. In contrast, the appearance and the persistence of the adult isomyosins seem to be independent of thyroid hormone. Thus, the control of the isoenzymic transition in the skeletal muscle of urodelan amphibians appears to imply indirect mechanisms, operating differently on each of the two phases of the complete transition.

Thyroidal status and myosin isoenzymic pattern in the skeletal dorsal muscle of urodelan amphibians--the perennibranchiate Proteus anguinus

In the perennibranchiate Proteus anguinus, larval myosin isoforms were shown to coexist for life with the adult isomyosins that appeared at the end of the larval stage. Analysis of the myofibrillar ATPase profile also revealed that a high percentage of immature fibers persisted in adults. A long-term treatment with large amounts of T3 had no effect on juvenile individuals. Applied to subadult animals it promoted a regression of larval myosin isoforms and a reduction in the percentage of immature fiber types. The regulative effect of T3 in the myosin isoenzymic transition may be delayed and depends on metabolic conditions, which suggests it is indirect.

[Quantitative and qualitative analysis of the effects of various doses of beta-irradiation (50-5000 rads) on epidermal cells of embryos and larvae of Pleurodeles waltlii Mich. (Amphibia, Urodela)]

Embryos of Pleurodeles waltlii at the hatching stage were irradiated with doses of 50 to 5 000 rad. From 70 to 500 rad chromosomal aberrations appear; they are studied respectively 24,48 hours and 3 weeks after the treatment. Breakages are observed, that may be followed by rearrangements, i.e. acentric, telocentric and dicentric fragments, chromatid translocations and chromosome translocations. With time, the cells showing the most severe abnormalities are eliminated by the developing larvae. From 1 000 rad cytoplasmic structures (membrane systems and mitochondria) are alterated .