Expression of slow and fast myosin heavy chains in overload muscles of the developing rat

Molecular and cellular mechanisms involved in the generation of fiber diversity during myogenesis

Skeletal muscles have a characteristic proportion and distribution of fiber types, a pattern which is set up early in development. It is becoming clear that different mechanisms produce this pattern during early and late stages of myogenesis. In addition, there are significant differences between the formation of muscles in head and those found in rest of the body. Early fiber type differentiation is dependent upon an interplay between patterning systems which include the Wnt and Hox gene families and different myoblast populations. During later stages, innervation, hormones, and functional demand increasingly act to determine fiber type, but individual muscles still retain an intrinsic commitment to form particular fiber types. Head muscle is the only muscle not derived from the somites and follows a different development pathway which leads to the formation of particular fiber types not found elsewhere. This review discusses the formation of fiber types in both head and other muscles using results from both chick and mammalian systems.

Differential expression of myosin heavy chain mRNA and protein isoforms in four functionally diverse rabbit skeletal muscles during pre- and postnatal development

Myosin heavy chains (hcs) are the major determinant in the speed of contraction of skeletal muscle, and various isoforms are differentially expressed depending on the functional activity of the muscle. Using the rapid amplification of cDNA ends (3' RACE) method, we have characterised the 3' end of the embryonic, perinatal, type 1, 2a, 2x, and 2b myosin hc genes in rabbit skeletal muscle and used them as probes in RNase protection assays to quantitatively monitor their expression in different type of skeletal muscles just before and after birth. SDS PAGE was used to study the changes in the expression level of their respective protein and to determine the relative abundance of each myosin hc isoform in the muscles studied. The results show that for each anatomical muscle, the developmental changes in myosin hc gene expression at the mRNA level correlate strongly to those observed at the protein level. By studying their developmental expression in four functionally diverse skeletal muscles (semimembranosus proprius, diaphragm, tibialis anterior, and semimembranosus accessorius), it was shown that all muscles express the embryonic, perinatal, and type 1 isoform during prenatal development up to the E27 stage. In the diaphragm, low levels of the type 2a and 2x transcripts, which are adult fast isoforms, were also detected at the E27 stage. During the first week of postnatal growth the myosin hc transition leading to the expression of the adult isoforms is complex, and as many as five different myosin heavy chains are concurrently expressed in some muscles at around birth. As the animal matures, individual muscles become adapted to perform highly specialised functions, and this is reflected in the myosin hc composition within these muscles. Accordingly, the expression of the type 1 isoform, and the sequence of appearance and the expression levels of the type 2 isoforms, were exclusively dependent on the muscle type and largely reflect the functional activity of each muscle during the postnatal growth period.

Contractile properties and myosin heavy chain composition of newborn rat soleus muscles at different stages of postnatal development

This study was undertaken to correlate some of the functional characteristics with the myofibrillar composition in myosin heavy chain isoforms on newborn and adult rat soleus muscles. The following postnatal ages were chosen in order to determine the role of innervation in the establishment of the mature muscle phenotype: before (postnatal day 6), when (postnatal day 12), and after (days 17 and 23) the monosynaptic innervation appeared. The steady state of definitive innervation was controlled on adult muscles (i.e. approximately 13 weeks). Muscle maturation was followed by ATPase staining and fibre diversity was observed at postnatal day 12. The functional properties of skinned bundles isolated from newborn rats were determined by Calcium/Strontium activation characteristics (Tension/pCa and pSr relationships). From postnatal days 6 to 17, the Soleus bundles exhibited Calcium/Strontium activation characteristics intermediate between slow and fast fibre populations previously described in muscles. At day 23, the Calcium/Strontium activation characteristics of the soleus were closer to those of a slow type. Moreover, we observed a decrease in Ca affinity concomitant with the installation of the monosynaptic innervation, and an increase of the slow type I during postnatal development. Finally, this work reported a greater correlation between the Calcium/Strontium activation parameters and the myosin heavy chain isoform composition at the postnatal days when the mature monosynaptic innervation pattern occurred.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

The response of adult and developing rat plantaris muscle to overload

The effect of overload on the rat plantaris muscle was studied in animals of different ages. Overload was induced by removal of gastrocnemius and soleus muscles. As expected, when the operation was carried out in adults, the plantaris muscle became heavier and stronger. These changes occured within 30 days after the operation. In animals in which the operation was carried out 1-12 days after birth and the muscle examined 6-20 weeks later, different results were obtained. In the group operated at 1-9 days of age, the muscles developed a lower maximal twitch and tetanic tension than the contralateral plantaris muscle. There was no difference in the time to peak or muscle weight between the overloaded and the contralateral muscles. Similar changes were observed in animals where the overload was induced at 11 or 12 days of age except for the weight which was significantly higher than that of the control plantaris muscles. The number of slow fibers increased in animals where overload was induced 11-12 days postnatally or in adults, but not when muscles were overloaded at 9 days of age. The possible reasons for the different response of adult and neonatal muscles to overload are discussed.

Are region-specific changes in fibre types attributable to nonuniform muscle hypertrophy by overloading?

Muscle fibre composition was compared among the proximal (25%), middle (50%) and distal (75%) regions of the muscle length to investigate whether compensatory overload by removal of synergists induces region-specific changes of fibre types in rat soleus and plantaris muscles. In addition, we evaluated fibre cross-sectional area in each region to examine whether fibre recruitment pattern against functional overload is nonuniform in different regions. Increases in muscle mass and fibre area confirmed a significant hypertrophic response in the overloaded soleus and plantaris muscles. Overloading increased the percentage of type I fibres in both muscles and that of type IIA fibres in the plantaris muscle, with the greater changes being found in the middle and distal regions. The percentage of type I fibres in the proximal region was higher than that of the other regions in the control soleus muscle. In the control plantaris muscle, the percentage of type I and IIA fibres in the middle region were higher than that of the proximal and distal regions. With regard to fibre size, type IIB fibre area of the middle and distal regions in the plantaris increased by 51% and 57%, respectively, with the greater changes than that of the proximal region (37%) after overloading. These findings suggest that compensatory overload promoted transformation of type II fibres into type I fibres in rat soleus and plantaris muscles, with the greater changes being found in the middle and distal regions of the plantaris muscle.

Analysis of Ca2+ and Sr2+ activation characteristics in skinned muscle fibre preparations with different proportions of myofibrillar isoforms

To understand how the coexistence of fast and slow contractile and regulatory systems within single skeletal muscle fibres might affect contractile behaviour, fibre segments from the fast-twitch extensor digitorum longus and predominantly slow-twitch soleus muscle of the adult rat were tied together, either in parallel or in series, and then activated in Ca(2+)- and Sr(2+)-buffered solutions. Experimental force-pCa and force-pSr relations were compared with theoretical force-pCa and force-pSr curves predicted by a model for composite fibres, which accounted for the coexistence of fast and slow myosin within the contractile unit and enabled an estimate to be made of the relative contribution of fast- and slow-twitch elements within the tied-fibre combinations. The contractile behaviour of a fast-twitch and a slow-twitch muscle fibre tied either in series or in parallel, were compared with the force-pCa and force-pSr data predicted from the composite fibre model. Interestingly, the resultant force-pCa(-pSr) curves of the parallel-tied fibre combinations were well fitted with those predicted by the composite model. However, the experimental force-pCa(-pSr) curves of the series-tied fibres were not well fitted by a composite curve based on the known proportion of fast- and slow-twitch fibre components. A total force-length diagram was devised to take into account changes in the length of the fibre segments tied in series during activation, as well as possible differences in fibre diameter. Using this diagram it was possible to explain accurately the Ca2+ and Sr2+ activation curves of known fast- and slow-twitch segments tied in series. The results from this study are important for the interpretation of contractile data obtained from single muscle fibres exhibiting mixed fast- and slow-twitch contractile characteristics. Such muscle fibres have previously been identified in animals affected by muscular diseases (e.g. dystrophy), in mammalian extraocular muscles and in animals subjected to long-term exercise training.

Nonuniform changes in fibre types in the soleus muscle of the developing rat

Muscle fibre composition among the proximal (25%), middle (50%) and distal (75%) regions of the rat soleus muscle at various ages were compared to investigate whether the region-specific changes in fibre types known to occur under nonphysiological conditions (i.e. electrical stimulation along with immobilization in a lengthened position) also occur in the developing muscle. In addition, we attempted to detect fibres with nonuniform histochemical properties in the different segments, processing with myofibrillar actomyosin adenosine triphosphatase after pre-incubation at pH 10.3 against successive cryosections (200 microns apart). Samples were obtained from 66 Wistar rats of both sexes ranging in age from 13 to 85 days and subdivided into age groups of 2, 3, 4, 5 and 12 weeks. The mass and length of the soleus muscle increased most significantly at age 3-4 and 2-4 weeks, respectively. The distal region had a significantly lower percentage of type I fibres than the middle region at age 3 weeks, than the proximal and middle regions at age 4 weeks. In addition, some fibres [0.7 (SD 0.5)% n = 19 +/- 13] possessing nonuniform histochemical properties in different segments were observed from the middle and distal region at age 4 weeks. These findings would suggest that transformations from type II to type I fibres in the soleus muscle of the developing rat occur from the proximal or middle segments.

The effect of exercise on the contractile properties of single skinned fast- and slow-twitch skeletal muscle fibres from the adult rat

The effects of long-term endurance exercise on the contractile properties of single skinned muscle fibres from adult rats, were investigated. Adult (4-month-old) male rats were subjected to a 16-week, high-intensity endurance swimming programme, where animals carried a load (corresponding to 2% of body wt), during all 2-h training sessions. At the conclusion of the training period, muscle fibres isolated from the extensor digitorum longus (EDL), and soleus (SOL), could be classified into distinct classes or fibre types on the basis of their Ca(2+)- and Sr(2+)-activated contractile characteristics. The fast-twitch EDL comprised two fibre populations, while the slow-twitch SOL was found to be composed of three distinct fibre types. Endurance swimming modified the contractile characteristics of fibres from both the EDL and SOL, but exerted greater influence on those of the SOL. This was illustrated by significant increases in the sensitivity to Ca2+ and Sr2+, and a lower threshold for contraction by these activating ions, in the exercised group. Not one of the total of 272 fibres sampled, exhibited mixed fast- and slow-twitch contractile characteristics, often associated with exercise-induced fibre type transformations. Thus, high-intensity endurance swimming induced changes in some single muscle fibre contractile properties of adult rats, but did not cause major changes in fibre type distribution.

Adaptation of rat extensor digitorum longus muscle to gamma irradiation and overload

The right extensor digitorum longus (EDL) muscle of growing male rats was overloaded by ablation of its synergist tibialis anterior (TA) muscle. Four weeks later, the overloaded muscle was heavier and contained larger type IIA, IIX and IIB fibres than either untreated contralateral muscle or control muscle from an untreated animal. The myonuclear-to-myoplasmic volume ratio was maintained in the overloaded muscle. Overloaded EDL muscle, previously subjected to a dose of irradiation sufficient to sterilise satellite cells, and EDL muscle which had been only irradiated, were significantly lighter and contained significantly smaller fibres than controls, though a significant amount of normal EDL muscle growth did occur following either treatment. The myonuclear-to-myoplasmic volume ratio of the irradiated muscles was smaller than in controls. Overloaded muscle, with or without prior irradiation, possessed a smaller proportion of fibres containing IIB myosin heavy chain (MHC) and a larger proportion of fibres containing IIA and IIX MHC; a significant percentage of these fibres coexpressed either type IIA and IIX MHC or type IIX and IIB MHC. Thus in the absence of satellite cell mitosis, muscles of young rats possess a limited capacity for normal growth but not for compensatory hypertrophy. Adaptations in MHC gene expression to chronic overload are completely independent of satellite cell activity.