Effect of miR-493-5p on proliferation and differentiation of myoblast by targeting ANKRD17

The hypertrophy and conversion of postnatal muscle fibers largely determine the yield and quality of meat, which is closely related to the economic value of pigs. MicroRNA (miRNA), as a kind of endogenous noncoding RNA molecule, is widely involved in myogenesis of livestock and poultry. The longissimus dorsi tissues of Lantang pigs at 1 and 90 days (LT1D and LT90D) were collected and profiled by miRNA-seq. We found 1871 and 1729 miRNA candidates in LT1D and LT90D samples, and 794 miRNAs were shared. We identified 16 differentially expressed miRNAs between two tested groups and explored the function of miR-493-5p inmyogenesis. The miR-493-5p promoted the proliferation and inhibited the differentiation of myoblasts. Using GO and KEGG analyses of 164 target genes of miR-493-5p, we found that ATP2A2, PPP3CA, KLF15, MED28, and ANKRD17 genes were related to muscle development. RT-qPCR detection showed that the expression level of ANKRD17 was highly expressed in LT1D libraries, and the double luciferase report test preliminarily proved that miR-493-5p and ANKRD17 have a directly targeting relationship. We established miRNA profiles for the longissimus dorsi tissues of 1-day-old and 90-day-old Lantang pigs and found that miR-493-5p was differentially expressed and associated with myogenesis by targeting ANKRD17 gene. Our results should serve as a reference for future studies on pork quality.

Aldehyde dehydrogenase 2 deficiency promotes skeletal muscle atrophy in aged mice

Aldehyde dehydrogenase 2 (ALDH2) detoxifies acetaldehyde produced from ethanol. A missense single nucleotide polymorphism (SNP) rs671 in ALDH2 exhibits a dominant-negative form of the ALDH2 protein. Nearly 40% of people in East Asia carry an inactive ALDH2*2 mutation. Previous studies reported that ALDH2*2 is associated with increased risk of several diseases. In this study, we examined the effect of ALDH2 deficiency on age-related muscle atrophy and its underlying mechanisms. We found that ALDH2 deficiency promotes age-related loss of muscle fiber cross-sectional areas, especially in oxidative fibers. Furthermore, ALDH2 deficiency exacerbated age-related accumulation of 4-hydroxy-2-nonenal (4-HNE), a marker of oxidative stress in the gastrocnemius muscle. Similarly, mitochondrial reactive oxygen species (ROS) production increased in aged ALDH2-knockout mice, indicating that ALDH2 deficiency induced mitochondrial dysfunction. In summary, ALDH2 deficiency promotes age-related muscle loss, especially in oxidative fibers, which may be associated with an increased accumulation of oxidative stress via mitochondrial dysfunction.

Ca2+ leak through ryanodine receptor 1 regulates thermogenesis in resting skeletal muscle

The evolution of mammals to use skeletal muscle as a source of heat allowed them to spread to all parts of the globe. The generation of heat requires increased adenosine triphosphate (ATP) hydrolysis in the resting muscle in a regulated manner, but how this mechanism works is unknown. The results suggest that mammals increase their RyR1 Ca²⁺ leak rate to amplify a basal ATP turnover rate at the sarcoplasmic reticulum Ca²⁺ pump that is higher than that of lower vertebrates. Muscle-based thermogenesis allows regulation of body temperature that is essential for life in mammals and provides a potential pathway for manipulating body weight or temperature by altering metabolic rate.

Mammals rely on nonshivering thermogenesis (NST) from skeletal muscle so that cold temperatures can be tolerated. NST results from activity of the sarcoplasmic reticulum (SR) Ca²⁺ pump in skeletal muscle, but the mechanisms that regulate this activity are unknown. Here, we develop a single-fiber assay to investigate the role of Ca²⁺ leak through ryanodine receptor 1 (RyR1) to generate heat at the SR Ca²⁺ pump in resting muscle. By inhibiting a subpopulation of RyR1s in a single-fiber preparation via targeted delivery of ryanodine through transverse tubules, we achieve in-preparation isolation of RyR1 Ca²⁺ leak. This maneuver provided a critical increase in signal-to-noise of the SR-temperature-sensitive dye ER thermoyellow fluorescence signal from the fiber to allow detection of SR temperature changes as either RyR1 or SR Ca²⁺ pump activity was altered. We found that RyR1 Ca²⁺ leak raises cytosolic [Ca²⁺] in the local vicinity of the SR Ca²⁺ pump to amplify thermogenesis. Furthermore, gene-dose-dependent increases in RyR1 leak in RYR1 mutant mice result in progressive rises in leak-dependent heat, consistent with raised local [Ca²⁺] at the SR Ca²⁺ pump via RyR1 Ca²⁺ leak. We also show that basal RyR Ca²⁺ leak and the heat generated by the SR Ca²⁺ pump in the absence of RyR Ca²⁺ leak is greater in fibers from mice than from toads. The distinct function of RyRs and SR Ca²⁺ pump in endothermic mammals compared to ectothermic amphibians provides insights into the mechanisms by which mammalian skeletal muscle achieves thermogenesis at rest.

AUF1 gene transfer increases exercise performance and improves skeletal muscle deficit in adult mice

Muscle function and mass begin declining in adults long before evidence of sarcopenia and include reduced mitochondrial function, although much remains to be characterized. We found that mRNA decay factor AU-rich mRNA binding factor 1 (AUF1), which stimulates myogenesis, is strongly reduced in skeletal muscle of adult and older mice in the absence of evidence of sarcopenia. Muscle-specific adeno-associated virus (AAV)8-AUF1 gene therapy increased expression of AUF1, muscle function, and mass. AAV8 AUF1 muscle gene transfer in 12-month-old mice increased the levels of activated muscle stem (satellite) cells, increased muscle mass, reduced markers of muscle atrophy, increased markers of mitochondrial content and muscle fiber oxidative capacity, and enhanced exercise performance to levels of 3-month-old mice. With wild-type and AUF1 knockout mice and cultured myoblasts, AUF1 supplementation of muscle fibers was found to increase expression of Peroxisome Proliferator-activated Receptor Gamma Co-activator 1-alpha (PGC1α), a major effector of skeletal muscle mitochondrial oxidative metabolism. AUF1 stabilized and increased translation of the pgc1α mRNA, which is strongly reduced in adult muscle in the absence of AUF1 supplementation. Skeletal muscle-specific gene transfer of AUF1 therefore restores muscle mass, increases exercise endurance, and may provide a therapeutic strategy for age-related muscle loss.

Metabolic fingerprinting of cell types in mouse skeletal muscle by combining TOF-SIMS with immunofluorescence staining

Skeletal muscle tissue is composed of various muscle cell types which differ in physiological functions. Changes in cell type composition of skeletal muscle are associated with the development of metabolic diseases. Skeletal muscle cell types are currently distinguished by immunofluorescence (IF) staining based on myosin heavy chain (MHC) isoform difference. However, it remains a challenge to provide metabolic fingerprints of different muscle cell types by IF staining. Therefore, in this study, we proposed a method to examine metabolite distribution within different cell types by time-of-flight secondary ion mass spectrometry (TOF-SIMS) with high spatial resolution. Skeletal muscle samples from C57/BL6 mice were obtained by slicing. Cell types in TOF-SIMS images were labelled corresponding to IF images from the same region of serially cut sections. Mass spectra corresponding to individual muscle cells were extracted to compare metabolic fingerprints among cell types. Skeletal muscle cells were classified into two clusters based on the mass spectra of individual cells. Unsaturated diacylglycerol (DG) and fatty acid (FA) species were found to be distributed in a cell-type dependent manner. Moreover, relative quantification showed that the content of unsaturated DGs, oleic acid and linoleic acid was higher in type I and type IIA cells than in type IIB cells. TOF-SIMS in combination with IF enables us to directly visualize metabolite distribution in different cell types, to find potential biomarkers for cell type classification. TOF-SIMS imaging coupled with IF staining has been proved to be a promising tool for metabolic fingerprinting of different skeletal muscle cell types.

Transitional Hybrid Skeletal Muscle Fibers in Rat Soleus Development

Skeletal muscles comprise hundreds of individual muscle fibers, with each possessing specialized contractile properties. Skeletal muscles are recognized as being highly plastic, meaning that the physiological properties of single muscle fibers can change with appropriate use. During fiber type transitions, one myosin heavy chain isoform is exchanged for another and over time the fundamental nature of the fiber adapts to become a different fiber type. Within the rat triceps surae complex, the soleus muscle starts out as a muscle comprised of a mixture type IIA and type I fibers. As neonatal rats grow and mature, the soleus undergoes a near complete transition into a muscle with close to 100% type I fibers at maturity. We used immunohistochemistry and single fiber SDS-PAGE to track the transformation of type IIA into type I fibers. We found that transitioning fibers progressively incorporate new myofibrils containing type I myosin into existing type IIA fibers. During this exchange, distinct type I-containing myofibrils are segregated among IIA myofibrils. The individual myofibrils within existing muscle fibers thus appear to represent the functional unit that is exchanged during fiber type transitions that occur as part of normal muscle development.

Mixing it up: the biological significance of hybrid skeletal muscle fibers

Skeletal muscle fibers are classified according to the myosin heavy chain (MHC) isoforms and other myofibrillar proteins expressed within these cells. In addition to 'pure' fibers expressing single MHC isoforms, many fibers are 'hybrids' that co-express two or more different isoforms of MHC or other myofibrillar proteins. Although hybrid fibers have been recognized by muscle biologists for more than three decades, uncertainty persists about their prevalence in normal muscles, their role in fiber-type transitions, and what they might tell us about fiber-type regulation at the cellular and molecular levels. This Review summarizes current knowledge on the relative abundance of hybrid fibers in a variety of muscles from different species. Data from more than 150 muscles from 39 species demonstrate that hybrid fibers are common, frequently representing 25% or more of the fibers in normal muscles. Hybrid fibers appear to have two main roles: (1) they function as intermediates during the fiber-type transitions associated with skeletal muscle development, adaptation to exercise and aging; and (2) they provide a functional continuum of fiber phenotypes, as they possess physiological properties that are intermediate to those of pure fiber types. One aspect of hybrid fibers that is not widely recognized is that fiber-type asymmetries - such as dramatic differences in the MHC composition along the length of single fibers - appear to be a common aspect of many fibers. The final section of this Review examines the possible role of differential activities of nuclei in different myonuclear domains in establishing fiber-type asymmetries.

Muscle Fiber Type Changes in Lumbrical Muscles at Early Stages of Chronic Nerve Compression

Chronic nerve compression (CNC) neuropathy is a common disease in the clinic and provokes paraesthesia, or numbness at early stage. The changes in muscle fiber composition and motor nerve terminal morphology in distal muscles were studied in this study. A well-established CNC model was used to assess the changes in the muscles. Behaviors were measured by von Frey filament test. The myosin heavy chain isoforms and neuromuscular junctions (NMJs) were stained by immunofluorescence to show the muscle fiber types composition and motor nerve terminals morphologic changes in the flexor digitorum longus (FDL) and lumbrical muscle. The fiber cross-sectional areas of different muscle fiber types were measured. The small-fiber degeneration of cutaneous nerve fibers was examined by detecting the protein gene product 9.5 (PGP9.5) with immunofluorescence. At 2nd month after compression, the proportion of type I and type II B fibers was markedly decreased, and that of type II A fibers was increased in the lumbrical muscle. There was no significant change in composition of muscle fiber types in FDL and NMJ morphology of FDL and lumbrical muscles. Intra-epidermal nerve fibre density (IENFD) declined at 2nd month after the compression. Our study reveals the morphological changes of the FDL and lumbrical muscle at an early stage of CNC. These findings may be helpful to understand muscle damage and pathophysiological development of the nerve compression, and provide new evidence for early treatment of CNC.

Effect of carcinogen 1,2-dimethylhydrazine treatment on fiber types in skeletal muscles of male Wistar rats

The cancerogen 1,2-dimethylhydrazine (DMH), widely used in the experimental animal model of carcinogenesis, affects various organs, but its effect on muscle fibers is unknown. To evaluate the effect of 15-week DMH treatment on the fiber size and myosin heavy chain (MyHC) isoforms, which substantially determine fiber types and their contractile characteristics, pure and hybrid fiber types were immunohistochemically determined according to the MyHC isoform expression in soleus, extensor digitorum longus, gastrocnemius medialis and lateralis muscles of DMH-treated and control male Wistar rats. Whereas the size of fibers was mostly unaffected, the MyHC isoform expression was partially affected in both gastrocnemius samples, but not in the soleus and extensor digitorum longus of DMH-treated rats. The lower proportions of hybrid fiber types and especially that of type 1/2x in most gastrocnemius samples of DMH-treated rats resulted in a shift towards a single MyHC isoform expression, but the extent and pattern of the MyHC isoform shift varied across the different gastrocnemius samples. Such variable response to DMH treatment across muscles indicates that each muscle possesses its own adaptive range. These findings are essential for an accurate evaluation of skeletal muscle characteristics in DMH animal model.

Myosin Heavy Chain Expression Can Vary over the Length of Jaw and Leg Muscles

Muscle fiber type classification can be determined by its myosin heavy chain (MyHC) composition based on a few consecutive sections. It is generally assumed that the MyHC expression of a muscle fiber is the same over its length since neural stimulation and systemic influences are supposed to be the same over its length. We analyzed this in detail in three muscle types: the temporalis (closer) and digastricus (opener; both first brachial arch), and the medial gastrocnemius (somite). Sections of the muscles were incubated with monoclonal antibodies against various MyHC isoforms, and the distribution of these isoforms within individual fibers was followed over a distance of approximately 1 mm. The staining intensity of a fiber was measured and compared with the other fibers in the section. In the temporalis, digastricus, and gastrocnemius, 46, 11, and 15%, respectively, of their MyHC-I fibers showed a variation in the staining intensity over the length of their fibers, as well as 47, 87, and 22%, respectively, of their MyHC-IIA fibers. Most variable fibers were found amongst those with an overall relative intermediate staining intensity, which are presumably hybrid fibers. We conclude that different parts of a muscle fiber can have different fiber type compositions and, thus, contractile properties. Some muscle parts might reach their maximum contraction peak sooner or later than a muscle part a few microns further away. Next to stimulation by the nerve and systemic influences, local influences might also have an impact on the MyHC expression of the fiber.