Myosin heavy chain profiles in regenerated fast and slow muscles innervated by the same motor nerve become nearly identical

Plasticity of mature muscles exposed to different activation patterns is limited, probably due to restricted adaptive range of their muscle fibres. In this study, we tested whether satellite cells derived from slow muscles can give rise to a normal fast muscle, if transplanted to the fast muscle bed. Marcaine-treated rat soleus and extensor digitorum longus (EDL) muscles were transplanted to the EDL muscle bed and innervated by the 'EDL' nerve. Six months later expression of myosin heavy chain isoforms was analysed by areal densities of fibres, binding specific monoclonal antibodies, and by SDS gel electrophoresis. Both regenerated muscles closely resembled each other. Their myosin heavy chain profiles were similar to those in fast muscles although they were not identical to that in the control EDL muscle. Since not even regenerated EDL was able to reach the myosin heavy chain isoform profile of mature EDL muscle, our experimental model did not permit studying the adaptive capacity of satellite cells in different muscles in its whole extent. However, the results favour the multipotential myoblast stem cell population in rat muscles and underline the importance of the extrinsic regulation of muscle phenotype.

Changes in myosin heavy chain profile of mature regenerated muscle with endurance training in rat

The objective of the present study was to examine the response of fast-twitch muscle to endurance training long after the muscle had regenerated from toxin injury. Seventeen male Wistar rats were randomly assigned to a sedentary (S, n = 10) or a trained group (T, n = 7). Endurance training by treadmill running (5 days week(-1), 30 m min(-1), 7% grade, 2 h day(-1) for 5 weeks) was initiated 5 weeks after myofibre degeneration was induced in the right extensor digitorum longus muscle (EDL) by two injections of 0.2 mL of the unfractionated venom from Naja nigricollis snake. Gel electrophoresis analyses showed that training alone resulted in a 140% increase in type IIX myosin heavy chain (MHC) (P < 0.01) and a slight decrease in type IIB MHC (-14% P < 0.05). Regeneration alone induced an increase in both type IIA and IIX MHC expression (103%, P < 0.05, and 131%, P < 0.01, respectively), and a concomitant decrease in the percentage of type IIB MHC (P < 0.05). The shift from type IIB toward type IIA MHC composition observed in regenerated muscles of T rats resulted not only from an additive, but from a cumulative effect of training and regeneration. Immunohistochemical analysis of MHC content in individual fibres showed similar changes. These data suggest that the impact of endurance training on fast-type MHCs was more marked in mature regenerated muscles than in regenerating ones, and provide evidence of the heightened plasticity of fully regenerated muscles to repeated exercise.

Transformation of slow- or fast-twitch rabbit muscles after cross-reinnervation or low frequency stimulation does not alter the in vitro properties of their satellite cells

We previously showed that satellite cells isolated from rabbit fast-twitch and slow-twitch muscles presented different behaviours in culture; cells from slow muscle differentiated more quickly and fused into more numerous myotubes than those from fast muscle. Moreover, only slow-muscle derived satellite cells expressed in vitro the slow type I myosin heavy chain isoform (MyHC). We wanted to investigate whether the properties of satellite cells originating from different muscles were under the influence of the adult fibre type on which they were located. For this purpose, we transformed the properties of the adult rabbit fast-twitch semimembranosus accessorius (SMa; approximately 100% type II fibres) and the slow-twitch semimembranosus proprius (SMp; 100% type I fibre) muscles by (1) cross-reinnervating the SMp with the main branch of the fast SMa nerve; or (2) electrical stimulation at 10 Hz of the SMa muscle. We studied their satellite cells in vitro. Five-month cross-reinnervation of the SMp induced a large shift of its MyHC type characteristics towards those of a fast muscle, and three-month electrical stimulation at low frequency transformed the fast-twitch SMa into a slow-twitch muscle, as shown by SDS-PAGE of MyHC. In spite of the transformation of their muscle characteristics, satellite cells in culture kept their original properties. Indeed, as shown by MyoD and myogenin gene expression as markers of fusion, satellite cells isolated from cross-reinnervated and from control SMp began to fuse by eight days of culture, and expressed MyoD and myogenin at that stage. Later they differentiated into numerous myotubes. Satellite cells isolated from electrically stimulated and control SMa presented a similar behaviour in culture: they did not express MyoD and myogenin at eight days, and fused by ten days into only a few myotubes. Moreover, MyHC gene expression showed that, in contrast with slow-muscle derived satellite cells, the type I MyHC gene was not expressed by satellite cells isolated from the stimulated SMa in spite of its homogeneous type I fibre composition. Taken together, these data support the idea that once constituted, muscle fibre types per se do not influence the properties of their associated satellite cells.

Regenerated rat fast muscle transplanted to the slow muscle bed and innervated by the slow nerve, exhibits an identical myosin heavy chain repertoire to that of the slow muscle

The hypothesis that the limited adaptive range observed in fast rat muscles in regard to expression of the slow myosin is due to intrinsic properties of their myogenic stem cells was tested by examining myosin heavy chain (MHC) expression in regenerated rat extensor digitorum longus (EDL) and soleus (SOL) muscles. The muscles were injured by bupivacaine, transplanted to the SOL muscle bed and innervated by the SOL nerve. Three months later, muscle fibre types were determined. MHC expression in muscle fibres was demonstrated immunohistochemically and analysed by SDS-glycerol gel electrophoresis. Regenerated EDL transplants became very similar to the control SOL muscles and indistinguishable from the SOL transplants. Slow type 1 fibres predominated and the slow MHC-1 isoform was present in more than 90% of all muscle fibres. It contributed more than 80% of total MHC content in the EDL transplants. About 7% of fibres exhibited MHC-2a and about 7% of fibres coexpressed MHC-1 and MHC-2a. MHC-2x/d contributed about 5-10% of the whole MHCs in regenerated EDL and SOL transplants. The restricted adaptive range of adult rat EDL muscle in regard to the synthesis of MHC-1 is not rooted in muscle progenitor cells; it is probably due to an irreversible maturation-related change switching off the gene for the slow MHC isoform.

Diffusion of fluorescently labeled macromolecules in cultured muscle cells

Myotubes were obtained from culture of satellite cells. They had a sarcomeric organization similar to that of muscle. The diffusion in the direction perpendicular to the fibers of microinjected fluorescein isothiocyanate-dextrans of molecular weight ranging from 9500 to 150,000 was examined by modulated fringe pattern photobleaching. On the time scale of the observation, 10-30 S, all of the dextrans were completely mobile in the cytoplasm. The diffusion coefficients were compared to the values obtained in water. The ratio D(cytoplasm)/D(w) decreased with the hydrodynamic radius R(h) of the macromolecules. The mobility of inert molecules in muscle cells is hindered by both the crowding of the fluid phase of the cytoplasm and the screening effect due to myofilaments: D(cytoplasm)/D(w) = (D/D(w)) protein crowding x (D/D(w))(filament screening). The equation (D/D(w))filament screening = exp(-K(L)RCh) was used for the contribution of the filaments to the restriction of diffusion. A free protein concentration of 135 mg/ml, a solvent viscosity of cytoplasm near that of bulk water, and a calculated K(L) of 0.066 nm(-1), which takes into account the sarcomeric organization of filaments, accurately represent our data.

Expression of myosin isoforms in denervated, cross-reinnervated, and electrically stimulated rabbit muscles

The expression of myosin heavy (MyHC) and light (MyLC) chain isoforms was analyzed after denervation and cross-reinnervation by a fast nerve of the slow-twitch Semimembranosus proprius (SMp) muscle, and after denervation and electrical stimulation at low frequency of the fast-twitch Semimembranous accessorius (SMa) muscle of the rabbit. The control SMp (100% type I fibers) expressed 100% type I MyHC and 100% slow-type (1S', 1S and 2S) MyLC isoforms. Five month denervation did not alter significantly the MyHC expression of the muscle, but induced the expression of a new type 1 MyLC corresponding most probably to an embryonic MyLC. Five-month cross-reinnervation of the SMp by the fast SMa nerve induced a large change of its fiber type properties. As shown by immunocytochemistry, almost all fibers were stained by fast myosin antibody, but a high proportion of them co-expressed slow myosin. This result was in agreement with biochemical data showing that fast MyHC and MyLC isoforms became predominant. The control SMa (nearly 100% type II fibers) expressed almost 100% type II MyHC (70% type IIb and 22% IIx/d) and 100% fast-type (1F, 2F and 3F) MyLC isoforms. Five month denervation of the SMa induced a shift in its MyHC, with 98% type IIx/d and 2% type IIb isoforms, and no change in the proportions of its MyLC. Three month electrical stimulation at 10 Hz of the SMa transformed its fiber type composition. All fibers reacted with the slow myosin antibody and a minor proportion of them were stained by the fast myosin antibody. These observations were in agreement with the biochemical analysis showing a large predominance of the slow-type MyHC and MyLC isoforms. Taken together, these results obtained from rabbit muscles which are normally homogeneous in either fast-twitch or slow-twitch fiber types, further support the idea that the different myosin isoforms, particularly the MyHC, are differentially regulated by motor innervation. Type I MyHC is maintained in denervated SMp muscle, but is not expressed in denervated SMa. Type IIb isoform is the most sensitive to neural influence, as it disappears rapidly in denervated and electrically stimulated fast-twitch SMa muscle, and is barely expressed in cross-reinnervated slow-twitch SMp muscle. In contrast, type IIa and type IIx/d are less dependent upon motor innervation. In addition to the previous studies of d'Albis et al. analysis of these results leads us to conclude that, in the rabbit, sensitivity to motor innervation increases from the glycolytic to the oxydative types of fibers, in the order IIB > IIX/IID > IIA > I.

Relationship between muscle myosin isoforms and contractile features in rabbit fast-twitch denervated muscle

The effects of 8-day-old rabbit fast-twitch gastrocnemius denervation on the type of myosin isoforms and on contractile features (maximum velocity Vmax and contraction time (CT) of the muscle were followed between 15 and 60 days postnatal. The myosin isoforms and the Vmax and CT values of the denervated gastrocnemius displayed large changes during this period. These changes, which led at 2 months postnatal to a muscle displaying the properties of a slow-twitch muscle did not occur in synchrony: complete conversion to slow-type myosin isoforms occurred only at 60 days postnatal, whereas complete conversion to slow-twitch Vmax and CT values occurred as soon as 35 days postnatal. The results address a new question concerning the relationship between muscle myosin and contractile features.

Muscle creatine kinase-deficient mice. I. Alterations in myofibrillar function

The regulation of contractile activity in mice bearing a null mutation of the M-isoform of creatine kinase gene, has been investigated in tissue extracts and Triton X-100-treated preparations of ventricular, soleus, and gastrocnemius muscles of control and transgenic mice. Skinned fiber experiments did not evidence any statistical difference in the maximal force or the calcium sensitivity of either muscle type. Rigor tension development at a low MgATP concentration was greatly influenced by phosphocreatine in control but not in transgenic mice as should be expected. In calcium-activated ventricular preparations, although the force developed by each cross-bridge was the same in control and transgenic animals, the rate constant of tension changes appeared to be markedly slowed in transgenic animals. As the ventricular isomyosin pattern was not altered, we suggested that, in transgenic animals, cross-bridge cycling was hindered by a local decrease in the MgATP to MgADP ratio, due to lack of a local MgATP regenerating system. Myokinase activity was not significantly changed while activities of pyruvate kinase or glyceraldehyde-3-phosphate dehydrogenase were found to be increased in transgenic animals. These results show that no fundamental remodelling occurs in myofibrils of transgenic animals but that important adaptations modify the bioenergetic pathways including glycolytic metabolism.

The effect of denervation on myosin isoform synthesis in rabbit slow-type and fast-type muscles during terminal differentiation. Denervation induces differentiation into slow-type muscles

The soleus and gastrocnemius medialis of eight-day-old rabbits were denervated and the effects were examined after fifty-two days by biochemical, cytochemical and mechanical methods. The contralateral soleus exhibited the properties of slow-type muscle, namely a predominance of slow-type myosin isoforms and slow-type oxidative fibers, slow twitch and low maximal velocity for shortening. The contralateral gastrocnemius exhibited the properties of fast-type muscle, namely a predominance of fast-type myosin isoforms and fast-type non-oxidative fibers, fast twitch and high maximal velocity of shortening. Denervation of muscles caused the differentiation of the two muscles towards slow-type muscles. Both denervated soleus and gastrocnemius muscles exhibited a predominance of slow-type myosins (either the normal type, made up of slow heavy and light chains, or the hybrid type, made up of slow heavy and regulatory light chains and fast essential light chains), a predominance of slow-type fibers, and slow mechanical properties. Thus, innervation in rabbit appears to be a determining factor for differentiation into fast-type muscle, but it is not necessary for differentiation into slow-type muscle. This conclusion contradicts the findings of previous studies in rat and thus raises new questions concerning the role of nerves in controlling the expression of myosin isoforms and the differentiation of muscle fibers.

Rabbit masseter expresses the cardiac alpha myosin heavy chain gene. Evidence from mRNA sequence analysis

The presence of myosin alpha heavy chain in the rabbit masseter has been previously suggested at the protein level [(1991) Basic App. Myol. 1, 23-34; (1991) Histochem. J. 23, 160-170]. To confirm this finding, we cloned most of the mRNA corresponding to the myosin heavy chain S2 subfragment. PCR analysis and subsequent nucleotide sequence determination of the amplified cDNA demonstrates the presence of a myosin alpha heavy chain mRNA in rabbit masticatory muscles.

Opposite regulations by androgenic and thyroid hormones of V1 myosin expression in the two types of rabbit striated muscle: skeletal and cardiac

The finding that V1 cardiac myosin is expressed in masticatory skeletal muscles of the rabbit provided a unique opportunity for comparing the hormonal regulation of V1 in skeletal and cardiac muscles. Thyroid hormones had no significant effect on the postnatal expression of V1 in masticatory muscles, but increased this expression in cardiac ventricles. In contrast, androgenic hormones reduced V1 expression in masticatory muscles, but did not affect it significantly in cardiac ventricles. Modulation of V1 gene transcription in striated muscle is thus shown here to depend both on the target muscle and on the hormone.

A model system of coupled activity of co-immobilized creatine kinase and myosin

Myosin and creatine kinase were co-immobilized onto Immunodyne films to mimic the behaviour of creatine kinase bound to the M-line of myofilaments. The Mg-ATPase activity of bound myosin was studied by a coupled enzymatic assay, which detects Mg-ADP in the bulk solution by means of pyruvate kinase and lactate dehydrogenase. The competition for Mg-ADP between pyruvate kinase and creatine kinase either free in solution or co-immobilized with myosin was studied at various creatine phosphate concentrations. Bound creatine kinase competed efficiently when present in very low amounts, corresponding to an activity ratio higher than 1:20,000 between creatine kinase and pyruvate kinase and a molar ratio higher than 1:1000 between creatine kinase and myosin. The Mg-ADP produced by myosin ATPase in the vicinity of the film did not diffuse into the bulk solution but, in the presence of creatine phosphate, was recycled into Mg-ATP by the neighbouring creatine kinase. The existence of an unstirred layer near the surface of the film is sufficient to explain the channeling of ADP (or ATP) between co-immobilized myosin and creatine kinase, without direct interaction or 'intimate coupling' between the enzymes. The problem now is to determine the importance of this kind of facilitated diffusion in the myofilaments in vivo.

Reinnervation of denervated extensor digitorum longus of the rat by the nerve of the soleus does not induce the type I myosin synthesis directly but through a sequential transition of type II myosin isoforms

The fast-contracting extensor digitorum longus (EDL) muscle of 1-month-old rats was denervated and reinnervated by the nerve innervating the slow-contracting soleus muscle. After variable periods of time, the myosin isoform content of the EDL was analyzed by sensitive electrophoretic techniques, which allowed to discriminate between the slow-type I and the three, IIA, (IID or IIX) and IIB, fast-type II myosin isoforms. Compared to the control EDL, which contains predominantly the IIB isoform, the operated muscles contained variable proportions of all the isoforms. Analysis of the results leads us to conclude that reinnervation of EDL induces a sequential transition of myosin isoforms: IIB----(IID or IIX)----IIA----I.

Effect of testosterone and thyroid hormone on the expression of myosin in the sexually dimorphic levator ani muscle of rat

During postnatal development, the myosin transition from embryonic and neonatal isoforms to adult isoforms has been shown to occur with half-transition times of about 20 and 32 days in the male and female levator ani muscles, respectively. We show that this difference could not be attributed to the testosterone male hormone, since treatment of newborn females by testosterone did not modify the half-transition time. However, treatment of females by thyroid hormone accelerated the myosin transition of the female muscle, which then occurred at almost the same time as the transition of the male muscle. This suggests that the difference between the half-transition times of the male and female levator ani muscles may be largely attributed to different sensitivities of the male and female muscles to thyroid hormone. This is the first example of sexually dimorphic muscle response to thyroid hormone.

Muscle-specific response to thyroid hormone of myosin isoform transitions during rat postnatal development

Transitions from embryonic and neonatal to adult-type-II isomyosins are known to be related to the increase in the thyroid hormone plasma concentration during postnatal development. These transitions have been shown, however, to occur at different times, depending on the muscle, suggesting that each muscle responds differently to the thyroid hormone. We have investigated quantitatively the effects of experimental hypothyroidism and hyperthyroidism on isomyosin transitions from birth until the 45th postnatal day in eight rat muscles: diaphragm, intercostals, gastrocnemius medialis, soleus, plantar muscles of the foot, tongue muscle, levator ani and bulbocavernosus complex, and masseter. Hypothyroidism delayed the isomyosin transitions in all the muscles examined, particularly in the sexually dimorphic muscles (levator ani and bulbocavernosus complex and masseter). However, it did not eventually inhibit isomyosin transitions, indicating that the thyroid hormone was not an absolute requirement for these to occur. Hyperthyroidism had only a slight effect on isomyosin transition in the diaphragm, and accelerated such transitions in the other muscles. The transition curves of all the muscles investigated, except those of the sexually dimorphic muscles, became similar to that of the diaphragm, demonstrating that the various muscles did not display the same sensitivity to the thyroid hormone but were regulated by it in the same way. The isomyosin transitions in the sexually dimorphic muscles remained late in comparison to that in the diaphragm, which suggests a more complex regulation. The effect of hyperthyroidism was not permanent and could be reversed, by interruption of the treatment, to a greater or lesser extent depending on the muscle. In all muscles containing slow-type-I isomyosin, hypothyroidism had no effect on this isomyosin synthesis, whereas hyperthyroidism inhibited it. This inhibition ceased rapidly after the interruption of the treatment.

Comparative structural and enzymatic properties of skeletal muscle myosin from neonatal and adult rabbits

Several structural and enzymatic properties of myosin from skeletal muscles of neonatal and adult rabbits were compared. Electrophoretic analyses and proteolysis experiments indicated that differences between the two myosin types could be attributed to their heavy subunits. Circular dichroism measurements of subfragment-1 species, and trypsin-digested derivatives showed that the neonatal protein contained less alpha-helices than the adult form. The Mg2(+)-ATPase activity of neonatal myosin was lower than that of adult myosin, especially in the presence of actin. In comparison with adult subfragment-1, it was found that the binding of ATP analogues such as adenosine 5'-[beta, gamma-imino]triphosphate and PPi, or that of ATP (as deduced from the apparent KmATP) to neonatal subfragment-1 in the presence of actin was enhanced, while that of ADP was decreased. On the other hand, the association of actin with the ADP - neonatal-subfragment-1 complex was weaker. These features must be expressed in the cyclical actin-myosin association/dissociation steps occurring in ATP hydrolysis, and more particularly in the reassociation of actin with the ATP-hydrolysis-products - myosin complex.

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.

Myosin isoform transitions in regeneration of fast and slow muscles during postnatal development of the rat

Regeneration of rat fast (gastrocnemius medialis) and slow (soleus) muscles was examined after degeneration of myofibers had been achieved by injection of cardiotoxin into the hindleg during the first week after birth. Myogenesis in the regenerating muscles was compared to postnatal myogenesis in the contralateral and in control muscles. Synthesis of embryonic and neonatal myosin isoforms was initiated 3 days after injury. These forms were gradually replaced by the intermediate and fast adult isoforms (type II fiber myosins), whose synthesis followed the same curve in regenerating, contralateral, and control muscles. In contrast, synthesis of the slow myosin isoform (type I fiber myosin) was greatly delayed in injured muscles, but eventually became equal to its synthesis in contralateral and control muscles. It therefore appears that synthesis of type II fiber myosins is similarly regulated, probably by thyroid hormone, in developing regenerating and normal muscles, while synthesis of type I fiber myosin depends on other factor(s).

Specific programs of myosin expression in the postnatal development of rat muscles

The expression of myosin during postnatal development was studied in a dozen muscles of the rat. All muscles displayed the usual sequential transitions from embryonic to neonatal and to adult isomyosins. However, we observed that these transitions did not take place uniformly. Thus, half-transition times for the appearance of the adult intermediate and fast myosin extended from seven days for diaphragm, the most precocious muscle of all those examined, to 23 days for male rat masseter. Besides the large differences between their half-transition times, we noticed that the transition curves displayed different slopes, covering different periods. Differences between muscles mainly affected the neonatal-to-adult transition rather than the embryonic-to-neonatal transition, since the embryonic-type myosin disappeared from all muscles examined except for one, at about the same time, by the end of the first week after birth. In addition, the appearance of slow myosin varied for each muscle and did not follow curves parallel to those for intermediate and fast myosins. These results indicate that each muscle of the rat is subjected to a specific program of myosin isoform transitions during postnatal development.

Correlation among the proton charges and molecular masses of myosin subunits

The molecular masses and isoelectric points of myosin light and heavy chains were calculated from their known primary sequences and their respective distribution in a two-dimensional graph is displayed. Implications for the electrophoretic study of myosin subunits are inferred from this analysis.

Myosin light chains of guinea-pig striated muscles. Similarities and differences with rat myosin light chains

1. Myosin light chains of guinea-pig striated muscles have been screened by two-dimensional gel electrophoresis and compared to rat myosin light chains. 2. The fast type light chains 1F and 3F, slow type light chains 1S and 2S, and embryonic type light chain 1E are shown to differ in the two rodents; only the fast type light chains 2F co-electrophorese on the gel. 3. In guinea-pig, as in rat, ventricle muscle light chains appear the same as the 1S and 2S light chains and atrial light chain type 1 the same as the 1E light chain. We show that this embryonic light chain of guinea-pig myosin is difficult to identify and may be confused with the adult 1F light chain.

Regeneration after cardiotoxin injury of innervated and denervated slow and fast muscles of mammals. Myosin isoform analysis

The regeneration of adult rat and mouse slow (soleus) and fast (sternomastoid) muscles was examined after the degeneration of myofibers had been achieved by a snake venom cardiotoxin, under experimental conditions devised to spare as far as possible the satellite cells, the nerves, and the blood vessels of the muscles. Three days after the injury, no myosin was detectable in selected portions of the muscles. New myosins of embryonic, neonatal, and adult types started to be synthesized during the following two days. Adult myosins thus appeared more precociously than in development, which implies that the synthesis of myosin isoforms during regeneration does not entirely 'recapitulate' the sequence of myosin transitions observed during normal development. Two weeks after the injury, the isomyosin electrophoretic pattern displayed by regenerated muscles was already the same as that of control muscles; the normal adult pattern was therefore expressed more rapidly in regenerating than in developing muscles. Except for the synthesis of the slow isoform which was generally inhibited in denervated muscles, the same types of myosins were expressed during the early stages of regeneration in denervated as in innervated muscles; long-term denervation prevented however the qualitative and quantitative recovery of the normal myosin pattern.

Regulation by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. I. Neonate mouse

Changes both in the ATPase myofibrillar profile and in the electrophoretic pattern of myosin isoforms were examined in the mouse dorsal skeletal muscle (longissimus) during postnatal development. In the newborn, only type II C and a few type I fibers were present; differentiation into type II A and II B fibers took place during the 3 weeks following birth. During the same period, a transition from three neonatal isomyosins to four adult isoforms was observed. The two phenomena were related to a marked increase in the serum thyroid hormones levels. Hypothyroidism and hyperthyroidism experiments were performed. Hypothyroidism produced by propylthiouracil treatment of pregnant females and thiourea injections of the litters was shown to induce a complete inhibition of postnatal muscular differentiation. Hyperthyroidism produced by triiodothyronine treatment of the neonate mice significantly accelerated the myosin transition and the switch in the myofibrillar pattern. Our results suggest a primordial role for thyroid hormones in directly regulating the appearance of myosin and fiber adult types and in modulating directly or indirectly the disappearance of the neonatal types.

Regulation by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. II. Urodelan amphibians

In the urodelan amphibian Pleurodeles waltlii, spontaneous anatomical metamorphosis was correlated with an increase in the serum level of thyroxine (T4). It was also accompanied by a change in the myofibrillar ATPase profile of the dorsal skeletal muscle; fibers of larval type were gradually replaced by the adult fiber types I, II A, and II B. Likewise, a myosin isoenzymic transition was observed in dorsal muscle, larval isomyosins were replaced by adult isoforms. In a related species, Ambystoma mexicanum, in which no spontaneous external metamorphosis occurs under standard conditions, the serum T4 level was shown to remain low. During further development, the myofibrillar ATPase profile acquired the adult fiber types, but a high percentage of immature fibers of type II C persisted. Myosin isoenzymic transition was also incomplete; larval isoforms were still distinguished in the neotenic adults. In experimental hypothyroidian P. waltlii, no external metamorphosis occurred; the myofibrillar ATPase profile was of the immature type, and the larval isomyosins persisted. Triiodothyronine induced experimental anatomical metamorphosis in A. mexicanum; only limited changes in the myofibrillar ATPase profile resulted from the treatment, but a complete myosin isoenzymic transition was observed. These results tend to indicate that a moderate increase in the level of thyroid hormone is sufficient to induce the differentiation of adult fiber types, together with the production of adult myosin isoforms in the skeletal dorsal muscle of amphibians, while a pronounced increase would be necessary for repressing the initial larval features.

Pig platelet tropomyosin: interactions with the other thin-filament proteins

Pig platelet tropomyosin exhibits many of the functional activities of skeletal tropomyosin. At low ionic strength it forms end-to-end aggregates similar to those formed by skeletal tropomyosins. It forms a 1:1 complex with muscle troponin or with a troponin I-pig brain calmodulin complex, as well as a 1:6 association with platelet filamentous actin. Electron microscopy of paracrystals shows that the troponin binding site is slightly C-terminal of the unique cysteine, corresponding to position 190 of the rabbit skeletal alpha-tropomyosin sequence. The effect of a complex comprising platelet actin and tropomyosin on the ATPase activity of rabbit skeletal muscle myosin subfragment-1 was similar to that displayed by its skeletal muscle counterpart. Platelet tropomyosin decreased the activity by roughly half in a calcium-independent manner. Addition of troponin to the actin-tropomyosin in the absence of calcium results in further inhibition and allows the full activity of the complex to be restored by Ca2+. These results differ from those obtained by Côté & Smillie for horse platelet tropomyosin and this may reflect the different isomeric nature of pig platelet tropomyosin. These results suggest that the functional properties of non-muscle tropomyosins may differ when comparisons are made between proteins isolated from the same type of cell but in different species. Differences in self-association and actin-binding properties may be finely graded between different isoforms.

Active-site titration of enzymes at high concentration. Application to myosin ATPase

The number of active sites of soluble and filamentous myosin and of its subfragments, heavy meromyosin and subfragment-1, has been determined. The titration involves steady-state kinetic measurements at a high enzyme concentration and varying substrate concentrations (or vice versa), in the presence of a substrate-regenerating system. Some practical and theoretical conditions for its execution are given, and, in particular, the effect of a possible heterogeneity of the active sites on the titration curves is analysed. Under the experimental conditions of the study, the number of active sites is close to that of myosin heads, and the heads seem to be functionally identical; the catalytic constants kcat and Km characterizing each active site are similar within some limits (1-2 for the ratio of kcat values; 1-5 for that of Km values).

Isoforms of myosin and actin in human, monkey and rat myometrium. Comparison of pregnant and non-pregnant uterus proteins

Using several electrophoretic procedures, we have compared the forms of myosin and actin in pregnant and non-pregnant uterus of woman, monkey (Macaca fascicularis) and rat. On non-dissociating gels, native myosin of the three species migrates as a single band, of identical mobility independently of the physiological state. Remigration of this band in dissociating conditions shows that it is constituted of two heavy chains of respectively 201 kDa and 205 kDa; the relative proportions of these two bands are different for the three animal species but do not vary during pregnancy. Using two-dimensional gel electrophoresis, we found that the 17-kDa light chain of purified uterus myosin exists under two isoelectric forms, the more acidic one becoming progressively predominant at the end of pregnancy in the human as in the monkey uterus, while we observed no changes in the rat. In two-dimensional gel electrophoresis, actin of human, monkey and rat uterus is present under three isoforms, the most basic one (the gamma form) increasing early in pregnancy in the two primate species but being always the most abundant form in the rat. The ATPase activity of human uterus myosin was found to be similar for the protein extracted from both pregnant and non-pregnant uterus. The changes observed in the 17-kDa light chain and in the actin isoforms might nevertheless participate in the modifications of contractility of the uterus during pregnancy of the primates.

Effect of tropomyosin on the interactions of actin with actin-binding proteins isolated from pig platelets

Several non-muscle tropomyosins have been reported to lack the ability to polymerize in a head-to-tail manner [Dabrowska, R. et al. (1983) J. Muscle Res. Cell Motil. 1, 83-92; Côté, G.P. (1983) Mol. Cell. Biochem. 57, 127-146]. Unlike rabbit skeletal muscle tropomyosin, these proteins could therefore not protect the F-actin microfilaments neither from disassembly or from cross-linking by the other actin-associating factors. However, we have provided evidence that, in vitro, pig platelet tropomyosin, although shorter in molecular length, exhibits the same properties as the muscle protein: it self-associates and forms a 1:6 complex with platelet filamentous actin under physiological conditions [Prulière et al. (1984) J. Muscle Res. Cell Motil. 6, 126]. In this paper, we examine the effects of several other actin-binding proteins on the microfilaments saturated with this non-muscle tropomyosin. Since contractile proteins often vary with the cell type and may require different conditions for their interactions, we have developed a procedure which allows the parallel purification of actin-binding protein (ABP), vinculin, alpha-actinin, gelsolin as well as actin and tropomyosin from the same batch of cells. Thus, using an homogeneous system, we show by viscometry, sedimentation and densitometry, and by electron microscopy, that pig platelet tropomyosin can protect the structure of the microfilaments from the action of the modulating factors to the same extent as rabbit skeletal muscle alpha-tropomyosin. Our data suggest that interaction of ABP, vinculin or alpha-actinin can occur only with the ends of the filaments when F-actin is saturated with tropomyosin, while cross-linking takes place by interactions with sites localized along the entire length of F-actin in the absence of tropomyosin. Moreover, the presence of tropomyosin on F-actin leads to the total inhibition of gelsolin severing activity, although it did not prevent the binding of gelsolin to the F-actin--tropomyosin complex. This suggests that pig platelet as well as skeletal muscle tropomyosins have the ability to increase the strength of the interaction between actin monomers within the filament. This also suggests that the binding sites of gelsolin along the filaments are not localized in the groove of the F-actin helix.

Comparison of myosins from the masseter muscle of adult rat, mouse and guinea-pig. Persistence of neonatal-type isoforms in the murine muscle

Adult rat, mouse, and guinea-pig masseter muscles display distinct myosin electrophoretic patterns. The rat muscle contains four main forms which by reference to the myosins of the IIB tensor fasciae latae, of the IIA mylohyoid, and of the red and white portions of the sternomastoid muscles, correspond respectively to the intermediate-type and to the three fast-type isoforms. The mouse masseter muscle contains only three main myosins, the intermediate-type and two fast-type isoforms. The guinea-pig muscle also displays only three bands, whose assignment is, however, less straightforward than in the murine species; their electrophoretic mobilities are not strictly the same as those of their homologous forms in rat and mouse. Comparison with the myosins of the tensor fasciae latae and of the sternomastoid muscles of guinea-pig allows their identification as intermediate and fast-type myosins. In addition to these typical adult-type forms, adult murine masseter muscles are observed to contain between zero and 30% of neonatal-type myosins. The comparison of the developmental transitions of myosins in the rat masseter with those in the skeletal muscles of the same animal indicates a delay in the appearance of the adult as well as in the disappearance of the neonatal-type myosins in the masseter muscle. Both the variability in myosin types with the animal species and the atypical presence of neonatal forms in the murine adults suggest that myosin expression in the masseter muscle is subjected to unusual regulations.

Myosin switches in skeletal muscle development of an urodelan amphibian, Pleurodeles waltlii. Comparison with a mammalian, Mus musculus

The isomyosins from dorsal axial muscle, which appear successively through metamorphosis of P.waltlii, are shown to be composed of identical fast-type light chains but of distinct heavy subunits. We observe that this modification goes with a change in ATPase activity as also in the case of mouse. Metamorphosis in amphibian as well as birth in mammalian are thus both accompanied by the synthesis of new myosins of higher catalytic efficiency.

Effect of the filamentous structure of myosin on the actomyosin ATPase activity

The ATPase activities of acto-heavy meromyosin and of acto-myosin minifilaments have been compared under the same conditions at low ATP (0.1 mM) and at several KC1 concentrations. The activities, which are strongly salt-dependent in both systems, have been found to be similar at high ionic strength (about 0.16 M) but different at lower ionic strength (0.06-0.07 M). Under this last condition, the catalytic constants kcat and Km are lower for acto-myosin minifilaments than for acto-heavy meromyosin ATPase. In addition, at low ionic strength, any decrease in the concentration of any of the ionic species (ATP, citrate, etc.) induces an increase in the interaction strength between myosin and actin filaments, as revealed by the Km changes. The presence of the troponintropomyosin complex and of Ca2+ also enhances the strength of this interaction. On the other hand, the occurrence of particular interactions between F-actin and myosin minifilaments is further substantiated by the phenomenon of superprecipitation which occurs when the ATP concentration decreases. The favourable effect of the organized structure of the myosin minifilaments on the ATPase activity of actomyosin is discussed.

Purification of a troponin I-like factor from pig platelet

A troponin I-like factor has been purified from pig platelet by G150 Sephadex filtration of a low ionic strength extract, acidification at pH 4.2, ion exchange on DE-52 cellulose, and affinity chromatography on calmodulin-Sepharose. This protein (Mr 17000), together with pig brain calmodulin and platelet tropomyosin, is able to participate to the reconstitution in vitro of a thin filament-like complex which modulates with 55% calcium sensitivity the platelet actin-activated Mg2+-dependent ATPase activity of rabbit skeletal muscle myosin.

Myosin isoenzymes in adult rat skeletal muscles

The myosin isoenzymic content of several adult rat muscles has been analyzed by electrophoresis under non-dissociating conditions. Fast-twitch fibres, whether of the oxidative type, such as the red masseter, or of the glycolytic type, such as the white tensor fasciae latae, are all shown to contain three isomyosins with respective mobilities identical in both types of muscles. These three isoenzymes, which, as in the case of myosins from avian fast muscles, represent alkali light chain hetero-and homodimers with similar large subunits, occur in somewhat variable relative proportions depending on the muscle. No obvious correlation was established between the type of the fast fibres--either oxidative or glycolytic--and the type of the myosins. Besides the three fast isoenzymes, other muscles, such as the predominantly fast latissimus dorsi and the mixed diaphragm, are shown to contain one myosin species of lower electrophoretic mobility; this supplementary isoenzyme comigrates with the major component of the predominantly slow-twich soleus muscle, but differs from the avian slow-tonic isoform. Ca2+ ATPase determinations on gel indicate that the fast isomyosins all display similar activity, which is five to ten times higher, depending on the experimental conditions of the assay, than the activity shown by the slow isoenzymes. Altogether, at least five isoenzymes, corresponding to one "slow-twich", one "slow-tonic", and three "fast-twitch" myosin species, were detected in rat skeletal muscles.U

Comparison of myosin isoenzymes from slow-tonic and fast-twitch fibers of frog muscle

Myosin from distal cruralis bundle of triceps femoris composed of slow-tonic and fast-twitch fibers in an about 1 to 1 ratio, from rectus abdominis containing a lower proportion of slow-tonic fibers, and from a fast-twitch sartorius muscle of the frog, was characterized by means of polyacrylamide gel electrophoresis in the presence of sodium dodecylsulphate (SDS), electrophoresis in non-dissociating conditions, and determination of the ATPase activity. SDS-polyacrylamide gel electrophoresis failed to reveal the presence of any specific light chain in myosin from slow-tonic fibers. Light chains having the same mobilities as fast-twitch LC1 and LC2 were observed; as previously described by Focant and Reznik [15], myosin from tonic fibers appears, on the other hand, devoid of LC3. By electrophoresis in non-dissociating conditions myosin from sartorius muscle was resolved into three components which comigrated with the three myosin isoenzymes from the fast-twitch posterior latissimus dorsi muscle of the chicken. Preparations from rectus abdominis and from cruralis bundle were shown to contain an additional component of lower electrophoretic mobility. Its relative proportion in these two muscles suggests that it represents the slow-tonic fiber myosin isoenzyme. Analysis of proteolytic digestion patterns revealed differences in the heavy chain structure of the isoenzymes from slow and fast fibers, respectively. As indicated by ATPase measurements, fast-twitch myosin exhibits a higher catalytic activity than myosin from slow-tonic fibers, the difference being of the same order as that reported for myosins from slow and fast muscles of higher vertebrates.

Myosin isoenzyme redistribution in chronic heart overload

Since the first observation by Spann et al., it has become clear that in cardiac hypertrophy induced by a mechanical overloading, the velocity of shortening of the cardiac muscle (Vmax) is reduced (see ref. 2 for review). Most authors agree that this mechanical alteration is accompanied by a decrease in the Ca2+-dependent ATPase activity of myosin (see ref. 3 for review). The molecular basis of such changes was unknown because the structural modifications of the myosin molecule were ill-defined. Nevertheless, it has recently been shown that, like skeletal muscle myosin, cardiac myosin is composed of several polymorphic forms, comparable to isoenzymes. In the skeletal muscle, new functional requirements can induce changes in both contractile activity and type of myosin isoenzyme synthesised. We now report that an increase in cardiac work produced by mechanical overloading in rats induces the preferential synthesis of a cardiac myosin isoenzyme characterised by specific immunological and electrophoretic properties and exhibiting a lower ATPase activity. This adaptive change could account for the reduced shortening speed of this hypertrophied cardiac muscle.

Magnesium ion dependent adenosine triphosphatase activity of heavy meromyosin as a function of temperature between +20 and -15 degrees C

The hydrolysis of Mg2+-adenosine 5'-triphosphate (ATP) by heavy meromyosin has been studied between +20 and -15 degrees C, especially in the low-temperature range, in a medium containing 30% (v/v) ethylene glycol by fluorometric, spectrophotometric, and potentiometric measurements. The time course of the fluorescence changes of the enzyme during the reaction depends markedly on the temperature in consequence of large differences between the activation energies of the various steps. The observed kinetics have been analyzed according to the simplified scheme of Bagshaw & Trentham [Bagshaw, C. R., & Trentham, D. R. (1974) Biochem. J. 141, 331-349]. The following results have been obtained. (1) The rate-limiting step of the reaction changes in this temperature range; at 20 degrees C M**.ADP.Pi is the predominant steady-state complex, and M*.ADP predominates at -15 degrees C, with a half-life of approximately 10 min. (2) As expected, on the basis that it is the dissociation of the M*.ADP complex which becomes rate limiting at low temperature, one observes, in the pre-steady-state below 0 degrees C, both a proton burst and a lag phase in ADP release. (3) At low temperature, the equilibrium M*.ATP in equilibrium M**.ADP.Pi is displaced to the left All the kinetic data obtained in this study are compatible with a simple pathway for the Mg2+-ATP hydrolysis by myosin and with sequential release of the reaction products.

An electrophoretic study of native myosin isozymes and of their subunit content

Myosin polymorphism in muscles has been studied by a variety of electrophoretic techniques, in non-dissociating and in dissociating conditions. The analysis of myosin isozymes in the native state was achieved in pyrophosphate buffer and required only minute amounts of protein; identical results were obtained with purified or crudely extracted myosin. The determination of the subunit content of each isozyme was done in the presence of sodium dodecyl sulphate or urea for light chain, and in a phenol, acetic acid and urea system for heavy chain screening. Electrophoresis in non-dissociating conditions has led to the separation of up to a dozen of myosin isozymes, differing in mobilities by as much as 30%. Muscle specificity of myosin was clearly established. Apart from a few exceptions, all the muscles tested were shown to contain more than one myosin species; fast-twitch muscles for instance all contained the same three isozymes, but in variable ratios. Class specificity of myosin appeared related to the relative proportions of isozymes in a given muscle. A second electrophoresis in dissociating solvents of the myosin bands first resolved in pyrophosphate buffer has then allowed a further characterization of the various isozymes. The differences in mobilities observed in the native state were shown to come either from the light chains, or from the heavy chains, or from both. The first case was illustrated by the three species present in fast muscles, which were shown to correspond to three alkali light-chain isozymes, the heterodimer representing in some instances up to 40% of the total. Next to light-chain muscle type specificity, electrophoresis in the phenol, acetic acid, urea system has led to the detection of differences in the heavy chains of fast, slow and cardiac myosins. The application of these various electrophoretic techniques to the analysis of the modification of myosin isozymes during development or in pathology studies can be considered.

Optical spectroscopic study of the ADP-myosin interaction

The binding of ADP to heavy meromyosin, and the separated subfragment 1 components S-1(A1) and S-1 (A2), has been observed by ultraviolet spectrophotometry. The results are compatible with the presence of spectroscopically equivalent and independent sites, one per head, at both 10 degrees C and 25 degrees C. We do not observe the heterogeneity of binding and of the spectroscopic response that has been reported. The binding has also been followed by other methods sensitive to the effect of ligand on the aromatic residues of the protein, viz. intrinsic fluorescence of heavy meromyosin and changes in the near-ultraviolet Cotton effects of myosin, and its active fragments. Within the limits of our experimental precision, the binding profiles, based on concentration of myosin heads, are the same for myosin as for subfragment 1. A perturbation in the circular dichroism is also generated by pyrophosphate, which competes with ADP. The spectra suggest that subsites for the purine ring and the diphosphate can be recognized. The sensitivity of binding profiles obtained by methods of the kind used here to cooperative or antagonistic interactions between the binding sites has been analysed. It is clear that sizeable effects of this nature could be concealed by the binding curves, even for high experimental precision.