Expression of the myosin heavy chain IIB gene in porcine skeletal muscle: the role of the CArG-Box promoter response element

Research Progress on the Regulating Factors of Muscle Fiber Heterogeneity in Livestock: A Review

The type of muscle fiber plays a crucial role in the growth, development, and dynamic plasticity of animals' skeletal muscle. Additionally, it is a primary determinant of the quality of both fresh and processed meat. Therefore, understanding the regulatory factors that contribute to muscle fibers' heterogeneity is of paramount importance. Recent advances in sequencing and omics technologies have enabled comprehensive cross-verification of research on the factors affecting the types of muscle fiber across multiple levels, including the genome, transcriptome, proteome, and metabolome. These advancements have facilitated deeper exploration into the related biological questions. This review focused on the impact of individual characteristics, feeding patterns, and genetic regulation on the proportion and interconversion of different muscle fibers. The findings indicated that individual characteristics and feeding patterns significantly influence the type of muscle fiber, which can effectively enhance the type and distribution of muscle fibers in livestock. Furthermore, non-coding RNA, genes and signaling pathways between complicated regulatory mechanisms and interactions have a certain degree of impact on muscle fibers' heterogeneity. This, in turn, changes muscle fiber profile in living animals through genetic selection or environmental factors, and has the potential to modulate the quality of fresh meat. Collectively, we briefly reviewed the structure of skeletal muscle tissue and then attempted to review the inevitable connection between the quality of fresh meat and the type of muscle fiber, with particular attention to potential events involved in regulating muscle fibers' heterogeneity.

Disuse-Induced Muscle Fatigue: Facts and Assumptions

Skeletal muscle unloading occurs during a wide range of conditions, from space flight to bed rest. The unloaded muscle undergoes negative functional changes, which include increased fatigue. The mechanisms of unloading-induced fatigue are far from complete understanding and cannot be explained by muscle atrophy only. In this review, we summarize the data concerning unloading-induced fatigue in different muscles and different unloading models and provide several potential mechanisms of unloading-induced fatigue based on recent experimental data. The unloading-induced changes leading to increased fatigue include both neurobiological and intramuscular processes. The development of intramuscular fatigue seems to be mainly contributed by the transformation of soleus muscle fibers from a fatigue-resistant, "oxidative" "slow" phenotype to a "fast" "glycolytic" one. This process includes slow-to-fast fiber-type shift and mitochondrial density decline, as well as the disruption of activating signaling interconnections between slow-type myosin expression and mitochondrial biogenesis. A vast pool of relevant literature suggests that these events are triggered by the inactivation of muscle fibers in the early stages of muscle unloading, leading to the accumulation of high-energy phosphates and calcium ions in the myoplasm, as well as NO decrease. Disturbance of these secondary messengers leads to structural changes in muscles that, in turn, cause increased fatigue.

Active vitamin D increases myogenic differentiation in C2C12 cells via a vitamin D response element on the myogenin promoter

Background: Skeletal muscle development during embryogenesis depends on proliferation of myoblasts followed by differentiation into myotubes/multinucleated myofibers. Vitamin D (VD) has been shown to affect these processes, but there is conflicting evidence within the current literature on the exact nature of these effects due to a lack of time course data. With 20%-40% of pregnant women worldwide being VD deficient, it is crucial that a clearer understanding of the impact of VD on myogenesis is gained. Methods: A detailed 8-day differentiation time course was used where C2C12 cells were differentiated in control media (2% horse serum) or with different concentrations of active VD, 1,25 (OH)2D3 (10-13 M, 10-11 M, 10-9 M or 10-7 M), and measurements were taken at 6 time points. DNA, creatine kinase and protein assays were carried out as well as quantitative PCR to determine expression of Myf5, MyoD, myogenin, MHC I, and MHC neonatal, MHC embryonic, MHC IIa, MHC IIx, and MHC IIb mRNAs. Transfections were carried out using one vector containing the myogenin promoter and another containing the same promoter with a 3 base mutation within a putative vitamin D response element (VDRE) to determine effects of 1,25 (OH)2D3 on myogenin transcription. Finally, a ChIP assay was performed to determine whether the VD receptor (VDR) binds to the putative VDRE. Results: 1,25(OH)2D3 caused an inhibition of proliferation and an increase in differentiation in C2C12 cells. Myf5, myogenin, MHC I, and MHC neonatal, MHC embryonic, MHC IIa, MHC IIx, and MHC IIb expression were all increased by 1,25(OH)2D3. Myotube size was also increased by VD. When the putative VDRE on the myogenin promoter was mutated, the increase in expression by VD was lost. ChIP analysis revealed that the VDR does bind to the putative VDRE on the myogenin promoter. Conclusion: Active VD directly increases myogenin transcription via a functional VDRE on the myogenin promoter, resulting in increased myogenic differentiation, increased expression of both the early and late MHC isoforms, and also increased myotube size. These results highlight the importance of VD status during pregnancy for normal myogenesis to occur, but further in vivo work is needed.

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.

Response of the porcine MYH4-promoter and MYH4-expressing myotubes to known anabolic and catabolic agents in vitro

Myosin heavy chain-IIB (MyHC-IIB; encoded by MYH4 or Myh4) expression is often associated with muscle hypertrophic growth. Unlike other large mammals, domestic pig breeds express MyHC-IIB at both the mRNA and protein level.

Aim

To utilise a fluorescence-based promoter-reporter system to test the influence of anabolic and catabolic agents on increasing porcine MYH4-promoter activity and determine whether cell hypertrophy was subsequently induced.

Methods

C2C12 myoblasts were co-transfected with porcine MYH4-promoter-driven ZsGreen and CMV-driven DsRed expression plasmids. At the onset of differentiation, treatments (dibutyryl cyclic-AMP (dbcAMP), Des(1–3) Insulin-Like Growth Factor-1 (IGF-I), triiodo-l-thyronine (T3) and dexamethasone (Dex)) or appropriate vehicle controls were added and cells maintained for up to four days. At day 4 of differentiation, measurements were collected for total fluorescence and average myotube diameter, as indicators of MYH4-promoter activity and cell hypertrophy respectively.

Results

Porcine MYH4-promoter activity increased during C2C12 myogenic differentiation, with a marked increase between days 3 and 4. MYH4-promoter activity was further increased following four days of dbcAMP treatment and average myotube diameter was significantly increased by dbcAMP. Porcine MYH4-promoter activity also tended to be increased by T3 treatment, but there were no effects of Des(1–3) IGF-I or Dex treatment, whereas average myotube diameter was increased by Des(1–3) IGF-I, but not T3 or Dex.

Conclusion

Porcine MYH4-promoter activity responded to dbcAMP, Des(1–3) IGF-I and T3 treatment in vitro as observed previously in reported in vivo studies. However, we report that increased MYH4-promoter activity was not always associated with muscle cell hypertrophy. The fluorescence-based reporter system offers a useful tool to study muscle cell hypertrophic growth.

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In vitro porcine MYH4-promoter-reporter system to test anabolic & catabolic agents.

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Changes in porcine MYH4-promoter activity & myotube diameter measured in tandem.

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MYH4-promoter activity responded to dbcAMP, Des(1–3) IGF-I and T3 as seen in vivo.

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Increased MYH4-promoter activity was not always associated with cell hypertrophy.

Regulation of myosin heavy chain antisense long noncoding RNA in human vastus lateralis in response to exercise training

Alterations to muscle activity or loading state can induce changes in expression of myosin heavy chain (MHC). For example, sedentary individuals that initiate exercise training can induce a pronounced shift from IIx to IIa MHC. We sought to examine the regulatory response of MHC RNA in human subjects in response to exercise training. In particular, we examined how natural antisense RNA transcripts (NATs) are regulated throughout the MHC gene locus that includes MYH2 (IIa), MYH1 (IIx), MYH4 (IIb), and MYH8 (Neonatal) in vastus lateralis before and after a 5-wk training regime that consisted of a combination of aerobic and resistance types of exercise. The exercise program induced a IIx to IIa MHC shift that was associated with a corresponding increase in transcription on the antisense strand of the IIx MHC gene and a decrease in antisense transcription of the IIa MHC gene, suggesting an inhibitory mechanism mediated by NATs. We also report that the absence of expression of IIb MHC in human limb muscle is associated with the abundant expression of antisense transcript overlapping the IIb MHC coding gene, which is the opposite expression pattern as compared with that previously observed in rats. The NAT provides a possible regulatory mechanism for the suppressed expression of IIb MHC in humans. These data indicate that NATs may play a regulatory role with regard to the coordinated shifts in MHC gene expression that occur in human muscle in response to exercise training.

Gene expression profiling of porcine skeletal muscle satellite cells after poly(I:C) stimulation

Porcine satellite cells (PSCs) play a vital role in the construction, development and self-renewal of skeletal muscle. In this study, PSCs were exposed to poly(I:C) stimulation to mimic viral infection during the proliferation and differentiation phases at 0, 12, 24 and 48 hours (h) of the stimulation. The untreated and treated PSCs were analyzed by the RNA-Seq technology. There were 88, 119, 104 and 95 genes being differentially expressed in 0 h vs 12 h treated, 12 h vs 24 h treated, 0 h vs 24 h treated and 24 h vs 48 h untreated comparison libraries, respectively. The GO terms analysis results showed that during the proliferation phase of treated PSCs, the up-regulated genes related to the immune system were highly expressed. In addition, the gene expressions associated with muscle structure development in response to growth factor emerged during the differentiation phase of untreated PSCs. The biological pathways associated with Influenza A, Toll-like receptor and chemokine signaling were revealed in PSCs following poly(I:C) stimulation. The differentially expressed genes were confirmed by quantitative real-time PCR. These findings expanded our understanding of gene expressions and signaling pathways about the infiltrated mechanism of the virus into PSCs.

Effect of sodium 4-phenylbutyrate on Clenbuterol-mediated muscle growth

Previously, we highlighted induction of an integrated stress response (ISR) gene program in skeletal muscle of pigs treated with a beta-adrenergic agonist. Hence we tested the hypothesis that the ER-stress inhibitor, sodium 4-phenylbutyrate (PBA), would inhibit Clenbuterol-mediated muscle growth and reduce expression of genes that are known indicators of an ISR in mice. Clenbuterol (1mg/kg/day) administered to C57BL6/J mice for 21 days increased body weight (p<0.001), muscle weights (p<0.01), and muscle fibre diameters (p<0.05). Co-administration of PBA (100mg/kg/day) did not alter the Clenbuterol-mediated phenotype, nor did PBA alone have any effects compared to that of the vehicle treated mice. Clenbuterol increased skeletal muscle mRNA expression of phosphoserine amino transferase 1 (PSAT1, p<0.001) and cyclophillin A (p<0.01) at day 3, but not day 7. Clenbuterol decreased mRNA expression of activating transcription factor (ATF) 4 and ATF5 at day 3 (p<0.05) and day 7 (p<0.01), X-box binding protein 1 (XBP1) variant 2 mRNA at day 3 only (p<0.01) and DNA damage inducible transcript 3 (DDIT3/CHOP) mRNA at day 7 only (p<0.05). Co-administration of PBA had no effect on Clenbuterol-induced changes in skeletal muscle gene expression. In contrast, treatment of C2C12 myotubes with 5mM PBA (8hr) attenuated the thapsigargin-induced ISR gene program. Prolonged (24-48hr) treatment with PBA caused atrophy (p<0.01), reduced neoprotein synthesis (p<0.0001) and decreased expression of myogenin and fast myosin heavy chain genes (p<0.01), indicating an inhibition of myogenic differentiation. In summary, Clenbuterol did not induce an ISR gene program in mouse muscle. On the contrary, it reduced expression of a number of ISR genes, but it increased expression of PSAT1 mRNA. Co-administration of PBA had no effect on Clenbuterol-mediated muscle growth or gene expression in mice, whereas PBA did inhibit thapsigargin-induced ISR gene expression in cultured C2C12 cells and appeared to inhibit myogenic differentiation, independent of altering ISR gene expression.

microRNAs: Modulators of the underlying pathophysiology of sarcopenia?

Skeletal muscle homeostasis depends on an intricate balance between muscle hypertrophy, atrophy and regeneration. As we age, maintenance of muscle homeostasis is perturbed, resulting in a loss of muscle mass and function, termed sarcopenia. Individuals with sarcopenia exhibit impaired balance, increased falls (leading to subsequent injury) and an overall decline in quality of life. The mechanisms mediating sarcopenia are still not fully understood but clarity in our understanding of the precise pathophysiological changes occurring during skeletal muscle ageing has improved dramatically. Advances in transcriptomics has highlighted significant deregulation in skeletal muscle gene expression with ageing, suggesting epigenetic alterations may play a crucial and potentially causative role in the skeletal muscle ageing process. microRNAs (miRNAs, miRs), novel regulators of gene expression, can modulate many processes in skeletal muscle, including myogenesis, tissue regeneration and cellular programming. Expression of numerous evolutionary conserved miRNAs is disrupted in skeletal muscle with age. Given that a single miRNA can simultaneously affect the functionality of multiple signaling pathways, miRNAs are potent modulators of pathophysiological changes. miRNA-based interventions provide a promising new therapeutic strategy against alterations in muscle homeostasis. The aim of this review is two-fold; firstly to outline the latest understanding of the pathophysiological alterations impacting the deregulation of skeletal muscle mass and function with ageing, and secondly, to highlight the mounting evidence for a role of miRNAs in modulating muscle mass, and the need to explore their specific role in sarcopenia.