Masticatory function and properties of masseter muscle fibers in microphthalmic (mi/mi) mice during postnatal development

Development and Regeneration of Muscle, Tendon, and Myotendinous Junctions in Striated Skeletal Muscle

Owing to a rapid increase in aging population in recent years, the deterioration of motor function in older adults has become an important social problem, and several studies have aimed to investigate the mechanisms underlying muscle function decline. Furthermore, structural maintenance of the muscle–tendon–bone complexes in the muscle attachment sites is important for motor function, particularly for joints; however, the development and regeneration of these complexes have not been studied thoroughly and require further elucidation. Recent studies have provided insights into the roles of mesenchymal progenitors in the development and regeneration of muscles and myotendinous junctions. In particular, studies on muscles and myotendinous junctions have—through the use of the recently developed scRNA-seq—reported the presence of syncytia, thereby suggesting that fibroblasts may be transformed into myoblasts in a BMP-dependent manner. In addition, the high mobility group box 1—a DNA-binding protein found in nuclei—is reportedly involved in muscle regeneration. Furthermore, studies have identified several factors required for the formation of locomotor apparatuses, e.g., tenomodulin (Tnmd) and mohawk (Mkx), which are essential for tendon maturation.

Factors Involved in Morphogenesis in the Muscle-Tendon-Bone Complex

A decline in the body's motor functions has been linked to decreased muscle mass and function in the oral cavity and throat; however, aging of the junctions of the muscles and bones has also been identified as an associated factor. Basic and clinical studies on the muscles, tendons and bones, each considered independently, have been published. In recent years, however, research has focused on muscle attachment as the muscle-tendon-bone complex from various perspectives, and there is a growing body of knowledge on SRY-box9 (Sox9) and Mohawk(Mkx), which has been identified as a common controlling factor and a key element. Myostatin, a factor that inhibits muscle growth, has been identified as a potential key element in the mechanisms of lifetime structural maintenance of the muscle-tendon-bone complex. Findings in recent studies have also uncovered aspects of the mechanisms of motor organ complex morphostasis in the superaged society of today and will lay the groundwork for treatments to prevent motor function decline in older adults.

Morphological classification and comparison of suboccipital muscle fiber characteristics

In an attempt to clarify the function of the suboccipital muscles, we performed morphological observation of the suboccipital muscles for variations in the muscle belly and compared the morphology of their muscle fibers in terms of cross-sectional area by immunostaining with anti-myosin heavy chain antibodies. The cadavers of 25 Japanese individuals were used: 22 for morphological examinations and three for histological examinations. Among samples of the rectus capitis posterior major muscle (RCPma) and rectus capitis posterior minor muscle (RCPmi), 86.4% had a typical muscle appearance with a single belly, and 13.6% had an anomalous morphology. None of the samples of the obliquus capitis superior (OCS) or obliquus capitis inferior (OCI) muscles had an anomalous appearance. Measurement of cross-sectional area revealed that fast-twitch muscle fibers in the RCPma and OCI had a significantly greater cross-sectional area than those of the RCPmi and OCS. The cross-sectional area of intermediate muscle fibers was also significantly greater in the OCS than in the RCPma, RCPmi, and OCI. The cross-sectional area of slow-twitch muscle fibers was significantly greater in the OCS than in the RCPma, RCPmi, and OCI, and the RCPmi showed a significantly greater cross-sectional area for slow-twitch muscle fibers than did the RCPma, and OCI. Our findings indicate that the RCPmi and OCS exert a greater force than the RCPma and OCI, and act as anti-gravity agonist muscles of the head. Prolonged head extension in individuals with anomalous suboccipital muscle groups could result in dysfunction due to undue stress.

Association between the lack of teeth and the expression of myosins in masticatory muscles of microphthalmic mouse

The purposes of the present study were to elucidate the influences of the deficiency of teeth on masticatory muscles, such as the masseter, temporalis and digastric muscles and compare the influence among masticatory muscles. We analysed the expressions of myosin heavy chain (MyHC) isoform messenger RNA (mRNA) and protein in these muscles in the microphthalmic (mi/mi) mouse, whose teeth cannot erupt because of a mutation in the mitf gene locus. The expression levels of MyHC mRNA and protein in the masseter, temporalis, digastric, tibialis anterior and gastrocnemius muscles of +/+ and mi/mi mice were analysed with real-time polymerase chain reaction and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, respectively. The mi/mi masseter muscle at 8 weeks of age expressed 4.1-fold (p < 0.05) and 3.3-fold (p < 0.01) more MyHC neonatal mRNA and protein than that in the +/+, respectively; the expression level of MyHC neonatal protein was 19% of the total MyHC protein in the masseter muscle of mi/mi mice. In the digastric muscle, the expression levels of MyHC I mRNA and protein in the mi/mi mice were 4.7-fold (p < 0.05) and 5-fold (p < 0.01) higher than those in the +/+ mice. In the temporalis, tibialis anterior and gastrocnemius muscles, there was no significant difference in the expression levels of any MyHC isoform mRNA and protein between +/+ and mi/mi mice. These results indicate associations between the lack of teeth and the expression of MyHC in the masseter and digastric muscles but not such associations in the temporalis muscle, suggesting that the influence of tooth deficiency varies among the masticatory muscles.

Changes in the expression of myosins during postnatal development of masseter muscle in the microphthalmic mouse

In the present study, to elucidate the influences of the deficiency of teeth on the masseter muscle, we analyzed changes in the expression of MyHC isoform mRNAs during postnatal development in mi/mi mice using real-time PCR. By 8 weeks of age, MyHC I had nearly disappeared in the +/+ mice, while it was still present in the mi/mi, and the level of MyHC I mRNA in the mi/mi was 5.1-fold higher than that in the +/+ (p<0.01). The levels of MyHC IIx mRNAs in the mi/mi mice were 41 ~ 55% lower than those in the +/+ at both 3 weeks and 4 weeks of age (p<0.05). No significant difference in the expression of MyHC IIa and IIb mRNAs in the masseter muscle was found between the mi/mi and +/+. From these results, we speculate that the deficiency of teeth affects the masseter muscles during the postnatal development.

Contribution of occlusal activity to synaptogenesis in masticatory muscles

Synaptogenesis in the neuromuscular junction involves a nicotinic acetylcholine receptor (nAChR) switch and elimination. The microphthalmic mouse (mi/mi) with a mutation in the mitf gene cannot perform occlusal activity, because its teeth do not erupt. The present study attempted to elucidate the contribution of occlusal activity to synaptogenesis in masticatory muscles. In the masseter of the mi/mi, the nAChR elimination initiated, but did not progress normally, after 3 weeks of age, when the occlusal activity emerged in the +/+ mouse, whereas the nAChR switch progressed normally during the entire period of synaptogenesis. The mRNA expression patterns of nAChR subunits in the temporalis and digastric of the mi/mi differed from those in its masseter. These findings suggest that, in the masseter, occlusal activity is essential for the completion of nAChR elimination, but not for the nAChR switch, and that the contribution of occlusal activity to synaptogenesis varies among the masticatory muscles.

Relationship between function of masticatory muscle in mouse and properties of muscle fibers

Mammals exhibit marked morphological differences in the muscles surrounding the jaw bone due to differences in eating habits. Furthermore, the myofiber properties of the muscles differ with function. Since the muscles in the oral region have various functions such as eating, swallowing, and speech, it is believed that the functional role of each muscle differs. Therefore, to clarify the functional role of each masticatory muscle, the myofiber properties of the adult mouse masticatory muscles were investigated at the transcriptional level. Expression of MyHC-2b with a fast contraction rate and strong force was frequently noted in the temporal and masseter muscles. This suggests that the temporal and masseter muscles are closely involved in rapid antero-posterior masticatory movement, which is characteristic in mice. Furthermore, expression of MyHC-1 with a low contraction rate and weak continuous force was frequently detected in the lateral pterygoid muscle. This suggests that, in contrast to other masticatory muscles, mouse lateral pterygoid muscle is not involved in fast masticatory movement, but is involved in functions requiring continuous force such as retention of jaw position. This study revealed that muscles with different roles function comprehensively during complicated masticatory movement.

Myofiber properties of mouse mylohyoid muscle in the growth period

The mouse mylohyoid muscle belongs to the mastication-related suprahyoid muscle group. It shows a plate-like morphology and forms the mouth floor. There have been no reports on the characteristics of the mouse mylohyoid muscle fibers, and especially on their functional role during ingestion action, and many points remain unclear. We examined the mouse mylohyoid muscle at both the transcriptional and protein levels by RT-PCR, immunohistochemistry, and Western Blotting. MyHC-2b, which is expressed in almost all head and neck muscles and is thought to play a role in rapid mastication movement, was not detected in the mouse mylohyoid muscle. This result suggests that the mouse mylohyoid muscle has a special function and does not directly function during ingestion.

Changes in the myosin heavy chain 2a and 2b isoforms of the anterior belly of the digastric muscle before and after weaning in mice

During the process of growth and development, the digastric muscle is subjected to marked functional changes, including the change from suckling to mastication. In particular, because the anterior belly of the digastric muscle, which is one of the suprahyoid muscles, plays an important role in mastication. Therefore, this muscle seems to undergo a marked functional change before and after weaning. However, the details remain unknown. Here, to clarify the changes in the muscle fibre characteristics of the anterior belly of the digastric muscle before and after weaning, we examined myosin heavy chain isoforms at the protein (immunohistochemistry) and mRNA (transcription) levels. As a control, the changes in the muscle fibre characteristics of the sternohyoid muscle, which is anatomically aligned in the same direction as the anterior belly of the digastric muscle, were analyzed. The results showed that, in the anterior belly of the digastric muscle that is involved in mandibular movements in mice, the ratio of a fast-contraction isoform with strong contractile force increased after weaning. We believe that this occurred in response to a functional change from suckling to mastication. On the other hand, there was little change in the composition of sternohyoid muscle.

Changes in the Muscle Fibre Properties of the Mouse Temporal Muscle after Weaning

To clarify changes in the muscle fibre properties of the temporal muscle related to the start of masticatory movement, we immunohistochemically investigated myosin heavy chain (MyHC) isoform protein expression using pre-weaning and post-weaning mice. In addition, we examined the expression of a gene coding for those MyHC proteins. Immediately after weaning, isoforms with fast and potent contractility were frequent. This suggests that the temporal muscle plays an important role in a marked functional change in the oral cavity from lactation to mastication, contributing to oral function in cooperation with other masticatory muscles.

Effects of stretching stimulation with different rates on the expression of MyHC mRNA in mouse cultured myoblasts

In vivo studies have shown that changes in the characteristics of skeletal muscle fiber are determined by type of exercise or training. These earlier studies on mechanical stimulation, however, have all employed stimulation applied at a constant intensity, and no studies appear to have investigated change with variation of intensity of stimulation. In this study, we investigated the characteristics and differentiation of myoblasts stretched at different rates. Myoblasts were stimulated at 3 different rates, and the numbers of cells and nuclei on days 1, 3, and 5 were compared. The myosin heavy chain (MyHC) mRNA expression level was also compared. We investigated expression of MyHC-perinatal to determine speed of differentiation of myoblasts, and expression of MyHC-2b, 2d, and 2a to ascertain muscle cell characteristics. Counting cells and nuclei of myoblasts revealed clear promotion of differentiation with stretching. With rapid stretching, expression of MyHC-perinatal was high at first, but then showed a decrease. In terms of effect on muscle fiber characteristics, MyHC-2b, MyHC-2d, and MyHC-2a were high with rapid, medium, and slow stretching, respectively. This indicated that myoblast differentiation was promoted regardless of difference in stretching speed, with the myoblasts acquiring the muscle-fiber characteristics appropriate to each rate of stretching.

Changes in Muscle-fiber Properties of the Murine Digastric Muscle Before and After Weaning

The digastric muscle is one of the suprahyoid muscles and consists of the anterior and posterior bellies. Because muscle fiber alignments in these two bellies are different, the functional roles are said to be different. Since the digastric muscle relates to mastication, its functions may change markedly before and after weaning, but many details remain unknown. The aim of this study was to clarify changes in muscle fiber properties of the anterior and posterior bellies of the digastric muscle in mice before and after weaning. Expressions of myosin heavy chain (MyHC) isoforms were assessed at the protein and transcriptional levels. Expression of the MyHC-2b isoform, an isoform displaying fast, strong contraction, was greater in the anterior belly than in the posterior belly after weaning. This suggests that, in mice, the anterior belly of the digastric muscle needs to move rapidly anteroposteriorly for mastication, compared with the posterior belly.

Myosin heavy chain composition of tongue muscle in microphthalmic (mi/mi) mice before and after weaning

To elucidate the effects of teeth on muscle fibers in the tongue during the developmental process, we examined the expression of muscle contractile proteins and the genes for those proteins in normal mice and microphthalmic (mi/mi) mice with impaired tooth eruption. The mice were observed during the growth period, including weaning, which is when feeding movements undergo major changes. Expression of the myosin heavy chain (MyHC)-2a protein, whose contraction speed is relatively slow, disappeared after weaning in normal mice, while it remained in high concentrations even after weaning in mi/mi mice. The presence of MyHC-2a after weaning in mice with no tooth eruption was attributed to a compensation for lack of proper masticatory function and sucking-like movements, as MyHC-2a is necessary for these movements.

Characteristics of muscle fibers reconstituted in the regeneration process of masseter muscle in an mdx mouse model of muscular dystrophy

Mdx mice, which lack dystrophin, were examined for changes in the properties of muscle fibers in the growth process of the masseter muscle at the morphological, protein and transcriptional levels. The slow-type isoform, MyHC-1, and the fast-type isoforms, MyHC-2a, MyHC-2d and MyHC-2b, were examined at the protein and the transcriptional level. Morphological examination showed that in the mdx mouse masseter muscle, degeneration, necrosis, and regeneration occurred, particularly at the age of 4 weeks, and many regenerated muscle fibers with centrally located nuclei were observed at the age of 9 weeks. The results of examination at the protein and the transcriptional level showed that in the process of muscle fiber degeneration, necrosis, and regeneration, the mdx mouse masseter muscle acquires muscle fiber characteristics entirely different from those in the normal mouse masseter muscle. In particular, MyHC-1, which is rarely found in normal mice, was very strongly expressed.

Superficial and deep layer muscle fibre properties of the mouse masseter before and after weaning

To clarify changes in the properties of the masseter muscle superficial and deep layer muscle fibres, which initiate masticatory movement, myosin heavy chain isoforms were evaluated based on immunohistochemistry at the transcription level in male mice both before and after weaning. In the results, MHC-2b isoforms, the isoforms with the fastest contraction speed, were observed in the superficial layer after weaning. However, MHC-2a isoforms with slower contraction speeds were not apparent. By contrast, in the deep layer, MHC-2a isoforms were present, as were MHC-2b isoforms, however, there were fewer MHC-2b isoforms present than in the superficial layer. The most rapid movement in the mouse mandible was observed anteroposteriorly during mastication. As the superficial layer of the masseter muscle runs parallel to the direction of mandibular movement, the presence of MHC-2b isoforms in it is consistent. The presence of MHC-2a isoforms in the deep layer, lying at right angles to the direction of mastication movement, is consistent with the positional adjustment of the mandible contributed by the deep layer muscle fibres during masticatory movement. We therefore conclude that complicated masticatory movement is achieved by the presence of various muscle bundles within the masseter, each carrying out different roles.