Marsupial cardiac myosins are similar to those of eutherians in subunit composition and in the correlation of their expression with body size

Developmental, physiologic and phylogenetic perspectives on the expression and regulation of myosin heavy chains in mammalian skeletal muscles

The kinetics of myosin controls the speed and power of muscle contraction. Mammalian skeletal muscles express twelve kinetically different myosin heavy chain (MyHC) genes which provides a wide range of muscle speeds to meet different functional demands. Myogenic progenitors from diverse craniofacial and somitic mesoderm specify muscle allotypes with different repertoires for MyHC expression. This review provides a brief synopsis on the historical and current views on how cell lineage, neural impulse patterns, and thyroid hormone influence MyHC gene expression in muscles of the limb allotype during development and in adult life and the molecular mechanisms thereof. During somitic myogenesis, embryonic and foetal myoblast lineages form slow and fast primary and secondary myotube ontotypes which respond differently to postnatal neural and thyroidal influences to generate fully differentiated fibre phenotypes. Fibres of a given phenotype may arise from myotubes of different ontotypes which retain their capacity to respond differently to neural and thyroidal influences during postnatal life. This gives muscles physiological plasticity to adapt to fluctuations in thyroid hormone levels and patterns of use. The kinetics of MyHC isoforms vary inversely with animal body mass. Fast 2b fibres are specifically absent in muscles involved in elastic energy saving in hopping marsupials and generally absent in large eutherian mammals. Changes in MyHC expression are viewed in the context of the physiology of the whole animal. The roles of myoblast lineage and thyroid hormone in regulating MyHC gene expression are phylogenetically the most ancient while that of neural impulse patterns the most recent.

Expression of masticatory-specific isoforms of myosin heavy-chain, myosin-binding protein-C and tropomyosin in muscle fibers and satellite cell cultures of cat masticatory muscle

We test the hypothesis that cat jaw satellite cells belong to a distinct lineage preprogrammed to express masticatory-specific isoforms of myosin heavy-chain (m-MyHC), myosin-binding protein-C (m-MBP-C), and tropomyosin (m-Tm) during myogenesis in vitro. A monoclonal antibody (MAb) against m-MyHC and MAbs raised here against cat m-MBP-C and m-Tm were used to stain cryostat sections of cat masseter muscle and cultured myotubes derived from satellite cells of cat temporalis and limb muscles, using peroxidase immunohistochemistry. MAbs against m-MBP-C bound purified m-MBP-C in Western blots. MAbs against m-Tm failed to react with m-Tm in Western blots, but reacted with native m-Tm in gel electrophoresis-derived ELISA. In cat masseter sections, MAbs against m-MyHC, m-MBP-C, and m-Tm stained all masticatory fibers, but not the jaw-slow fibers. Cat jaw and limb muscle cultures mature significantly more slowly relative to rodent cultures. However, at 3 weeks, all three MAbs extensively stained temporalis myotubes, whereas they apparently stained isolated myotubes weakly in cat limb and rat jaw cultures. We conclude that satellite cells of masticatory fibers are preprogrammed to express these isoforms during myogenesis in vitro. These results consolidate the notion that masticatory and limb muscle allotypes are distinct.

Effects of hypothyroidism on myosin heavy chain composition and fibre types of fast skeletal muscles in a small marsupial, Antechinus flavipes

Effects of drug-induced hypothyroidism on myosin heavy chain (MyHC) content and fibre types of fast skeletal muscles were studied in a small marsupial, Antechinus flavipes. SDS-PAGE of MyHCs from the tibialis anterior and gastrocnemius revealed four isoforms, 2B, 2X, 2A and slow, in that order of decreasing abundance. After 5 weeks treatment with methimazole, the functionally fastest 2B MyHC significantly decreased, while 2X, 2A and slow MyHCs increased. Immunohistochemistry using monospecific antibodies to each of the four MyHCs revealed decreased 2b and 2x fibres, and increased 2a and hybrid fibres co-expressing two or three MyHCs. In the normally homogeneously fast superficial regions of these muscles, evenly distributed slow-staining fibres appeared, resembling the distribution of slow primary myotubes in fast muscles during development. Hybrid fibres containing 2A and slow MyHCs were virtually absent. These results are more detailed but broadly similar to the earlier studies on eutherians. We hypothesize that hypothyroidism essentially reverses the effects of thyroid hormone on MyHC gene expression of muscle fibres during myogenesis, which differ according to the developmental origin of the fibre: it induces slow MyHC expression in 2b fibres derived from fast primary myotubes, and shifts fast MyHC expression in fibres of secondary origin towards 2A, but not slow, MyHC.

Cross-bridge kinetics of fast and slow fibres of cat jaw and limb muscles: correlations with myosin subunit composition

Mechanical properties of the jaw-closing muscles of the cat are poorly understood. These muscles are known to differ in myosin and fibre type compositions from limb muscles. This work aims to correlate mechanical properties of single fibres in cat jaw and limb muscles with their myosin subunit compositions. The stiffness minimum frequency, f(min), which reflects isometric cross-bridge kinetics, was measured in Ca(2+)-activated glycerinated fast and slow fibres from cat jaw and limb muscles for temperatures ranging between 15 and 30 degrees C by mechanical perturbation analysis. At 15 degrees C, f(min) was 0.5 Hz for limb-slow fibres, 4-6 Hz for jaw-slow fibres, and 10-13 Hz for limb-fast and jaw-fast fibres. The activation energy for f(min) obtained from the slope of the Arrhenius plot for limb-slow fibres was 30-40% higher than values for the other three types of fibres. SDS-PAGE and western blotting using highly specific antibodies verified that limb-fast fibres contained IIA or IIX myosin heavy chain (MyHC). Jaw-fast fibres expressed masticatory MyHC while both jaw-fast and jaw-slow fibres expressed masticatory myosin light chains (MLCs). The nucleotide sequences of the 3' ends of the slow MyHC cDNAs isolated from cat masseter and soleus cDNA libraries showed identical coding and 3'-untranslated regions, suggesting that jaw-slow and limb-slow fibres express the same slow MyHC gene. We conclude that the isometric cross-bridge cycling kinetics of jaw-fast and limb-fast fibres detected by f(min) are indistinguishable in spite of differences in MyHC and light chain compositions. However, jaw-slow fibres, in which the same slow MyHCs are found in combination with MLCs of the jaw type, show enhanced cross-bridge cycling kinetics and reduced activation energy for cross-bridge detachment.

Heterogeneity of alpha-cardiac myosin heavy chains in a small marsupial, Antechinus flavipes, and the effect of hypothyroidism on its ventricular myosins

The effect of drug-induced hypothyroidism on ventricular myosin gene expression was explored in a small marsupial, Antechinus flavipes. Pyrophosphate gel electrophoresis, SDS-PAGE and western blotting were used to analyse changes in native myosin isoforms and myosin heavy chains (MyHCs) in response to hypothyroidism. In some animals, five instead of the normal three native myosin components were found: V(1a), V(1b),V(1c), V(2) and V(3), in order of decreasing mobility. In western blots, V(1a), V(1b), and V(1c) reacted with anti-alpha-MyHC antibody, but not with anti-beta-MyHC, whereas V(2) and V(3) reacted with anti-beta-MyHC antibody. SDS-PAGE of the unusual ventricular myosins revealed three MyHC isoforms, two of which bound anti-alpha-MyHC antibody while the third bound anti-beta-MyHC antibody. We conclude that V(1a), V(1b), V(1c) are triplets arising from the dimerization of two distinct alpha-MyHC isoforms. Hypothyroidism, verified by metabolic studies, decreased alpha-MyHC content significantly (t-test, P < 0.001) from 91.6 +/- 5.9% (SEM, n = 4) in control animals to 67.2 +/- 5.7% (SEM, n = 4) in hypothyroid animals, with a concomitant increase in beta-MyHC content. We conclude that in adult marsupials, ventricular myosins are also responsive to changes in the thyroid state as found in eutherians, and suggest that evolution of the molecular mechanisms underlying this thyroid responsiveness predate the divergence of marsupials and eutherians.

Developmental changes in ventricular myosin isoenzymes of the tammar wallaby

Ventricular myosin in eutherian mammals undergoes a perinatal change in response to a sharp rise in thyroid hormone levels during development. In this investigation, changes in ventricular myosin heavy chains (MyHCs) of the tammar wallaby (Macropus eugenii) from early pouch life to adulthood were analysed using native gel electrophoresis, SDS-PAGE and western blotting. Adult wallaby ventricle showed three myosin isoenzymes, V(1), V(2) and V(3); western blots using specific anti-alpha-MyHC and anti-beta-MyHC antibodies showed their MyHC compositions to be alphaalpha, alphabeta and betabeta, respectively. Ventricular muscle in early pouch joeys expressed predominantly beta-MyHC. Up to 200 days, the time of initial pouch exit, alpha-MyHC content was around 5%. Thereafter, there was a sharp increase of alpha-MyHC expression to 35% by 242 days of age, eventually falling back to 23% in the adult. These changes correlate with known surges in plasma levels of thyroid hormones around pouch exit. The results suggest that ventricular myosins in a marsupial mammal also undergo a developmental change, and that marsupial ventricular myosins are thyroid responsive as in eutherians. The increased alpha-MyHC expression empowers the heart to meet the enhanced cardiovascular demands of out-of-pouch activity and the thermogenic action of thyroid hormones.

Fiber types in canine muscles: myosin isoform expression and functional characterization

This study was aimed to achieve a definitive and unambiguous identification of fiber types in canine skeletal muscles and of myosin isoforms that are expressed therein. Correspondence of canine myosin isoforms with orthologs in other species as assessed by base sequence comparison was the basis for primer preparation and for expression analysis with RT-PCR. Expression was confirmed at protein level with histochemistry, immunohistochemistry, and SDS-PAGE combined together and showed that limb and trunk muscles of the dog express myosin heavy chain (MHC) type 1, 2A, and 2X isoforms and the so-called “type 2dog” fibers express the MHC-2X isoform. MHC-2A was found to be the most abundant isoform in the trunk and limb muscle. MHC-2X was expressed in most but not all muscles and more frequently in hybrid 2A-2X fibers than in pure 2X fibers. MHC-2B was restricted to specialized extraocular and laryngeal muscles, although 2B mRNA, but not 2B protein, was occasionally detected in the semimembranosus muscle. Isometric tension (Po) and maximum shortening velocity ( Vo) were measured in single fibers classified on the basis of their MHC isoform composition. Purified myosin isoforms were extracted from single muscle fibers and characterized by the speed ( Vf) of actin filament sliding on myosin in an in vitro motility assay. A close proportionality between Vo and Vf indicated that the diversity in Vo was due to the different myosin isoform composition. Vo increased progressively in the order 1/slow < 2A < 2X < 2B, thus confirming the identification of the myosin isoforms and providing their first functional characterization of canine muscle fibers.