N6-methyladenosine modification of a parvovirus-encoded small noncoding RNA facilitates viral DNA replication through recruiting Y-family DNA polymerases.

Human bocavirus 1 (HBoV1) is a human parvovirus that causes lower respiratory tract infections in young children. It contains a single-stranded (ss) DNA genome of ~5.5 kb that encodes a small noncoding RNA of 140 nucleotides known as bocavirus-encoded small RNA (BocaSR), in addition to viral proteins. Here, we determined the secondary structure of BocaSR in vivo by using DMS-MaPseq. Our findings reveal that BocaSR undergoes N6-methyladenosine (m6A) modification at multiple sites, which is critical for viral DNA replication in both dividing HEK293 cells and nondividing cells of the human airway epithelium. Mechanistically, we found that m6A-modified BocaSR serves as a mediator for recruiting Y-family DNA repair DNA polymerase (Pol) η and Pol κ likely through a direct interaction between BocaSR and the viral DNA replication origin at the right terminus of the viral genome. Thus, this report represents direct involvement of a viral small noncoding RNA in viral DNA replication through m6A modification.

Vitamin a potentiates sheep myoblasts myogenic differentiation through BHLHE40-modulated ID3 expression.

BACKGROUND: Vitamin A and retinoic acid (RA, a metabolite of vitamin A), are inextricably involved to the development of skeletal muscle in animals. However, the mechanisms regulating skeletal muscle development by vitamin A remain poorly reported. The current study designed to investigate the underlying mechanism of vitamin A affecting myogenic differentiation of lamb myoblasts through transcriptome sequencing (RNA-Seq) and gene function validation experiments. It provides a theoretical basis for elucidating the regulation of vitamin A on skeletal muscle development as well as for improving the economic benefits of the mutton sheep industry. RESULTS: Newborn lambs were injected with 7,500 IU vitamin A, and longissimus dorsi (LD) muscle tissue was surgically sampled for RNA-Seq analysis and primary myoblasts isolation at 3 weeks of age. The results showed that a total of 14 down-regulated and 3 up-regulated genes, were identified between control and vitamin A groups. Among them, BHLHE40 expression was upregulated in vitamin A group lambs. Furthermore, BHLHE40 expression is significantly increased after initiation of differentiation in myoblasts, and RA addition during differentiation greatly promoted BHLHE40 mRNA expression. In vitro, RA inhibited myoblasts proliferation and promoted myoblasts myogenic differentiation through BHLHE40. Moreover, BHLHE40 was proved to inhibit the expression of the DNA binding inhibitor 3 (ID3), and meanwhile, ID3 could effectively promote myoblasts proliferation and inhibit myoblasts myogenic differentiation. CONCLUSIONS: Taken together, our results suggested that vitamin A inhibited myoblasts proliferation and promoted myoblasts myogenic differentiation by inhibiting ID3 expression through BHLHE40.

Recognition of single-stranded nucleic acids by small-molecule splicing modulators.

Risdiplam is the first approved small-molecule splicing modulator for the treatment of spinal muscular atrophy (SMA). Previous studies demonstrated that risdiplam analogues have two separate binding sites in exon 7 of the SMN2 pre-mRNA: (i) the 5'-splice site and (ii) an upstream purine (GA)-rich binding site. Importantly, the sequence of this GA-rich binding site significantly enhanced the potency of risdiplam analogues. In this report, we unambiguously determined that a known risdiplam analogue, SMN-C2, binds to single-stranded GA-rich RNA in a sequence-specific manner. The minimum required binding sequence for SMN-C2 was identified as GAAGGAAGG. We performed all-atom simulations using a robust Gaussian accelerated molecular dynamics (GaMD) method, which captured spontaneous binding of a risdiplam analogue to the target nucleic acids. We uncovered, for the first time, a ligand-binding pocket formed by two sequential GAAG loop-like structures. The simulation findings were highly consistent with experimental data obtained from saturation transfer difference (STD) NMR and structure-affinity-relationship studies of the risdiplam analogues. Together, these studies illuminate us to understand the molecular basis of single-stranded purine-rich RNA recognition by small-molecule splicing modulators with an unprecedented binding mode.

CRISPR-Mediated Enzyme Fragment Complementation Assay for Quantification of the Stability of Splice Isoforms.

Small-molecule splicing modulators exemplified by an FDA-approved drug, risdiplam, are a new pharmacological modality for regulating the expression and stability of splice isoforms. We report a CRISPR-mediated enzyme fragment complementation (EFC) assay to quantify the splice isoform stability. The EFC assay harnessed a 42 amino acid split of a ß-galactosidase (designate α-tag), which could be fused at the termini of the target genes using CRISPR/cas9. The α-tagged splice isoform would be quantified by measuring the enzymatic activity upon complementation with the rest of ß-galactosidase. This EFC assay retained all the sequences of introns and exons of the target gene in the native genomic environment that recapitulates the cell biology of the diseases of interest. For a proof-of-concept, we developed a CRISPR-mediated EFC assay targeting the exon 7 of the survival of motor neuron 2 (SMN2) gene. The EFC assay is compatible with 384-well plates and robustly quantified the splicing modulation activity of small molecules. In this study, we also discovered that a coumarin derivative, compound 4, potently modulated SMN2 exon 7 splicing at as low as 1.1 nM.

AMPKα1 regulates Idh2 transcription through H2B O-GlcNAcylation during brown adipogenesis.

AMP-activated protein kinase (AMPK) is indispensable for the development and maintenance of brown adipose tissue (BAT), and its activity is inhibited due to obesity. The isocitrate dehydrogenase 2 (IDH2) is a mitochondrial enzyme responsible for the production of α-ketoglutarate, a key intermediate metabolite integrating multiple metabolic processes. We previously found that AMPKα1 ablation reduced cellular α-ketoglutarate concentration during brown adipocyte differentiation, but the effect of AMPKα1 on Idh2 expression remains undefined. In the present study, mouse C3H10T1/2 cells were transfected with Idh2-CRISPR/Cas9, and induced to brown adipogenesis. Our data suggested that brown adipogenesis was compromised due to IDH2 deficiency in vitro, which was accompanied by down-regulation of PR-domain containing 16. Importantly, the IDH2 content was reduced in brown stromal vascular cells (BSVs) separated from AMPKα1 knockout (KO) BAT, which was associated with lower contents of histone 2B (H2B) O-GlcNAcylation and monoubiquitination. Furthermore, both GlcNAcylated-H2B (S112) and ubiquityl-histone 2B (K120) contents in the Idh2 promoter were decreased in AMPKα1 KO BSVs. Meanwhile, ectopic O-linked N-acetylglucosamine transferase (OGT) expression was positively correlated with Idh2 expression, while OGT (T444A) mutation abolished the regulatory effect of AMPKα1 on Idh2. In vivo, reduced AMPKα1 activity and lower IDH2 abundance were observed in BAT of obese mice when compared with those in control mice. Taken together, our data demonstrated that IDH2 is necessary for brown adipogenesis and that AMPKα1 deficiency attenuates Idh2 expression, which might be by suppressing H2B O-GlcNAcylation modification.

Astragalus polysaccharide regulates brown adipogenic differentiation through miR-1258-5p-modulated cut-like homeobox 1 expression.

Astragalus polysaccharide (APS) is the major natural active component of Astragalus membranaceus, which has been recognized as one of the most popular herbal medicines worldwide. Enhancing the formation and function of brown adipose tissue increases energy expenditure and hence may potentially be used against obesity and type 2 diabetes. The aim of the present study was to explore the effect and mechanism of APS on brown adipocyte formation. Mouse C3H10T 1/2 cells were subject to APS, and both proliferation and brown adipogenic differentiation were determined. The results showed that APS exhibits a decreased proliferation ability, which is accompanied by downregulated proliferating cell nuclear antigen, cyclin D1, and cyclin-dependent kinase 4. APS promotes the differentiation of C3H10T 1/2 cells into brown adipocytes and induces the expressions of key brown adipogenic transcriptional factors, including CCAAT/enhancer-binding protein ß, uncoupling protein 1, and PR domain-containing 16. Importantly, APS enables insulin sensitization in brown adipocytes, which may proceed through activation of the canonical phosphatidylinositol-3-kinase (PI3K)/protein kinase B (AKT) signaling pathway and AMP-activated protein kinase (AMPK). Furthermore, the level of cut-like homeobox 1 (CUX1) is positively related to brown adipogenic differentiation, while APS regulates Cux1 expression through interaction with miR-1258-5p. Notably, the promotional effect of APS on brown adipogenic differentiation was abolished by Cux1 knockout. Collectively, our results suggest that APS enhances the differentiation of C3H10T 1/2 cells into brown adipocytes through regulating Cux1 via miR-1258-5p.

RNA-Targeting Splicing Modifiers: Drug Development and Screening Assays.

RNA splicing is an essential step in producing mature messenger RNA (mRNA) and other RNA species. Harnessing RNA splicing modifiers as a new pharmacological modality is promising for the treatment of diseases caused by aberrant splicing. This drug modality can be used for infectious diseases by disrupting the splicing of essential pathogenic genes. Several antisense oligonucleotide splicing modifiers were approved by the U.S. Food and Drug Administration (FDA) for the treatment of spinal muscular atrophy (SMA) and Duchenne muscular dystrophy (DMD). Recently, a small-molecule splicing modifier, risdiplam, was also approved for the treatment of SMA, highlighting small molecules as important warheads in the arsenal for regulating RNA splicing. The cellular targets of these approved drugs are all mRNA precursors (pre-mRNAs) in human cells. The development of novel RNA-targeting splicing modifiers can not only expand the scope of drug targets to include many previously considered "undruggable" genes but also enrich the chemical-genetic toolbox for basic biomedical research. In this review, we summarized known splicing modifiers, screening methods for novel splicing modifiers, and the chemical space occupied by the small-molecule splicing modifiers.

Identification of a cellularly active SIRT6 allosteric activator.

SIRT6, a member of the SIRT deacetylase family, is responsible for deacetylation of histone H3 Nε-acetyl-lysines 9 (H3K9ac) and 56 (H3K56ac). As a tumor suppressor, SIRT6 has frequently been found to have low expression in various cancers. Here, we report the identification of MDL-800, a selective SIRT6 activator. MDL-800 increased the deacetylase activity of SIRT6 by up to 22-fold via binding to an allosteric site; this interaction led to a global decrease in H3K9ac and H3K56ac levels in human hepatocellular carcinoma (HCC) cells. Consequently, MDL-800 inhibited the proliferation of HCC cells via SIRT6-driven cell-cycle arrest and was effective in a tumor xenograft model. Together, these data demonstrate that pharmacological activation of SIRT6 is a potential therapeutic approach for the treatment of HCC. MDL-800 is a first-in-class small-molecule cellular SIRT6 activator that can be used to physiologically and pathologically investigate the roles of SIRT6 deacetylation.

miR-124-3p affects the formation of intramuscular fat through alterations in branched chain amino acid consumption in sheep.

Intramuscular fat is used to determine meat quality in animals; however, factors affecting branched chain amino acid (BCAA) catabolism, which fuels adipogenesis and lipogenesis, remain unclear. To better understand the post-transcriptional influence on BCAA catabolism during adipogenesis, we investigated the role of miR-124-3p. Stromal vascular fraction (SVF) cells were isolated from skeletal muscle of sheep, and induced to differentiate. We determined the roles of miR-124-3p and its predicted target, branched chain keto acid dehydrogenase E1, alpha polypeptide (BCKDHA), in adipogenic differentiation and lipogenesis of SVFs after overexpressing or inhibiting miR-124-3p or BCKDHA, respectively. miR-124-3p altered the luciferase activity of constructs containing 3'-UTR of BCKDHA and the formation of lipid droplets, along with the adipogenic markers and BCAA consumption. Besides, the adipogenic performance and BCAA consumption in BCKDHA-overexpressing or knocked-down SVFs and the expression of adipogenic marker genes were altered. We demonstrate that miR-124-3p is an important factor for adipogenesis and provide insights into the formation of intramuscular fat in animals.

Identifying the SUMO1 modification of FAM122A leading to the degradation of PP2A-Cα by ubiquitin-proteasome system.

FAM122A is a highly conserved protein in mammals. Here, we identify that FAM122A can be sumoylated at lysine 89, which can be de-conjugated by SENP1. Furthermore, the sumoylation of FAM122A reduces the PP2A-Cα protein level together with the reduced phosphatase activity of PP2A, which suppresses cell proliferation. Collectively, our results suggest that the sumoylation of FAM122A may have a significant role in cellular function.

Moderate alcohol intake induces thermogenic brown/beige adipocyte formation via elevating retinoic acid signaling.

Clinically, low and moderate alcohol intake improves human health with protection against metabolic syndromes, including type 2 diabetes; however, mechanisms that are associated with these effects remain to be elucidated. The aims of this study were to investigate the effects of moderate alcohol intake on thermogenic brown/beige adipocyte formation and glucose and lipid homeostasis, as well as the involvement of retinoic acid (RA) signaling in the entire process. C57BL6 male mice were supplemented with 8% (w/v) alcohol in water for 1 or 4 mo. Alcohol intake prevented body weight gain, induced the formation of uncoupling protein 1-positive beige adipocytes in white adipose tissue, and increased thermogenesis in mice, which is associated with decreased serum glucose and triacylglycerol levels. Mechanistically, alcohol intake increased RA levels in serum and adipose tissue, which was associated with increased expression of aldehyde dehydrogenase family 1 subfamily A1 (Aldh1a1). When RA receptor-α signaling was conditionally blocked in platelet-derived growth factor receptor-α-positive adipose progenitors, the effects of alcohol on beige adipogenesis were largely abolished. Finally, moderate alcohol prevented high-fat diet-induced obesity and metabolic dysfunction. In conclusion, moderate alcohol intake induces thermogenic brown/beige adipocyte formation and promotes glucose and lipid oxidation via elevation of RA signaling.-Wang, B., Wang, Z., de Avila, J. M., Zhu, M.-J., Zhang, F., Gomez, N. A., Zhao, L., Tian, Q., Zhao, J., Maricelli, J., Zhang, H., Rodgers, B. D., Du, M. Moderate alcohol intake induces thermogenic brown/beige adipocyte formation via elevating retinoic acid signaling.

Astragalus Polysaccharide Improves Insulin Sensitivity via AMPK Activation in 3T3-L1 Adipocytes.

Astragalus polysaccharide (APS) is an important bioactive component of Astragalus membranaceus which is used as an anti-diabetes herb in traditional Chinese medicine. The objective of this study was to investigate the effects and mechanisms of APS on insulin-sensitizing of adipocytes. Mouse 3T3-L1 preadipocytes were used as a model. The results showed that APS increased preadipocytes proliferation in a dose dependent manner, and 0.1 µg/mL APS sufficiently increased Proliferating Cell Nuclear Antigen (PCNA) content (p < 0.01). Moreover, APS enhanced intracellular lipid accumulation and mRNA expression of proliferator-activated receptor γ (PPARγ, p < 0.01), CCAAT/enhancer binding protein α (C/EBPα, p < 0.01) and fatty acid binding protein (aP2, p < 0.01). As expected, corresponding protein contents were elevated. Importantly, APS increased 2-(N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)Amino)-2-Deoxyglucose (2-NBDG) uptake (p < 0.01). Meanwhile, both mRNA and protein content of glucose transporter 4 (Glut4) were elevated by APS (p < 0.01). The APS treatment enhanced tyrosine phosphorylation of insulin receptor substrate 1 (IRS1, p < 0.05) and phosphor-Akt content (p < 0.01). Besides, phosphorylated AMP-activated protein kinase (AMPK) content was increased in the APS treated cells (p < 0.01). Taken together, APS improved insulin sensitivity by enhancing glucose uptake, possibly through AMPK activation. These results suggested that APS might be a therapeutic candidate for insulin resistance.

AMPKα1 deficiency suppresses brown adipogenesis in favor of fibrogenesis during brown adipose tissue development.

Brown adipose tissue (BAT) dissipates energy for thermogenesis which reduces or prevents obesity and metabolic dysfunction. AMP-activated protein kinase (AMPK) is a master regulator of energy metabolism and its activity is inhibited in the developing BAT due to obesity. We previously found that AMPK is required for brown fat development and thermogenic function, but the non-brown adipogenic differentiation of progenitor cells due to AMPKα1 deficiency has not been defined. We found that, in vivo, the thermogenic capacity and morphology of BAT were compromised due to AMPK deficiency, which was correlated with decreased progenitor density in BAT. In addition, the expression of fibrogenic markers was higher in AMPK deficient compared to wild-type mice. Furthermore, we transplanted AMPKα1 wild-type (WT) and floxed BAT into the same recipient mice; following tamoxifen induced AMPKα1 knockout in floxed BAT, the fibrogenesis was enhanced compared to WT mice. Taken together, our data demonstrated that AMPKα1 deficiency suppressed brown adipogenesis in favor of fibrogenesis during BAT development.

Maternal obesity exacerbates insulitis and type 1 diabetes in non-obese diabetic mice.

Accompanying the dramatic increase in maternal obesity, the incidence of type 1 diabetes (T1D) in children is also rapidly increasing. The objective of this study was to explore the effects of maternal obesity on the incidence of T1D in offspring using non-obese diabetic (NOD) mice, a common model for TID. Four-week-old female NOD mice were fed either a control diet (10% energy from fat, CON) or a high-fat diet (60% energy from fat) for 8 weeks before mating. Mice were maintained in their respective diets during pregnancy and lactation. All offspring mice were fed the CON to 16 weeks. Female offspring (16-week-old) born to obese dams showed more severe islet lymphocyte infiltration (major manifestation of insulitis) (P<0.01), concomitant with elevated nuclear factor kappa-light-chain-enhancer of activated B cells p65 signaling (P<0.01) and tumor necrosis factor alpha protein level (P<0.05) in the pancreas. In addition, maternal obesity resulted in impaired (P<0.05) glucose tolerance and lower (P<0.05) serum insulin levels in offspring. In conclusion, maternal obesity resulted in exacerbated insulitis and inflammation in the pancreas of NOD offspring mice, providing a possible explanation for the increased incidence of T1D in children.

Transcription Factor SATB2 Regulates Skeletal Muscle Cell Proliferation and Migration via HDAC4 in Pigs.

Skeletal muscle development remarkably affects meat production and growth rate, regulated by complex regulatory mechanisms in pigs. Specific AT sequence-binding protein 2 (SATB2) is a classic transcription factor and chromatin organizer, which holds a profound effect in the regulation of chromatin remodeling. However, the regulation role of SATB2 concerning skeletal muscle cell fate through chromatin remodeling in pigs remains largely unknown. Here, we observed that SATB2 was expressed higher in the lean-type compared to the obese-type pigs, which also enriched the pathways of skeletal muscle development, chromatin organization, and histone modification. Functionally, knockdown SATB2 led to decreases in the proliferation and migration markers at the mRNA and protein expression levels, respectively, while overexpression SATB2 had the opposite effects. Further, we found histone deacetylase 4 (HDAC4) was a key downstream target gene of SATB2 related to chromatin remodeling. The binding relationship between SATB2 and HDAC4 was confirmed by a dual-luciferase reporter system and ChIP-qPCR analysis. Besides, we revealed that HDAC4 promoted the skeletal muscle cell proliferation and migration at the mRNA and protein expression levels, respectively. In conclusion, our study indicates that transcription factor SATB2 binding to HDAC4 positively contributes to skeletal muscle cell proliferation and migration, which might mediate the chromatin remodeling to influence myogenesis in pigs. This study develops a novel insight into understanding the molecular regulatory mechanism of myogenesis, and provides a promising gene for genetic breeding in pigs.

Guanidinoacetic acid ameliorates hepatic steatosis and inflammation and promotes white adipose tissue browning in middle-aged mice with high-fat-diet-induced obesity.

Guanidinoacetic acid (GAA) is a naturally occurring amino acid derivative that plays a critical role in energy metabolism. In recent years, a growing body of evidence has emerged supporting the importance of GAA in metabolic dysfunction. Hence, we aimed to investigate the effects of GAA on hepatic and adipose tissue metabolism, as well as systemic inflammatory responses in obese middle-aged mice models and attempted to explore the underlying mechanism. We found that dietary supplementation of GAA inhibited inguinal white adipose tissue (iWAT) hypertrophy in high-fat diet (HFD)-fed mice. In addition, GAA supplementation observably decreased the levels of some systemic inflammatory factors, including IL-4, TNF-α, IL-1ß, and IL-6. Intriguingly, GAA supplementation ameliorated hepatic steatosis and lipid deposition in HFD-fed mice, which was revealed by decreased levels of TG, TC, LDL-C, PPARγ, SREBP-1c, FASN, ACC, FABP1, and APOB and increased levels of HDL-C in the liver. Moreover, GAA supplementation increased the expression of browning markers and mitochondrial-related genes in the iWAT. Further investigation showed that dietary GAA promoted the browning of the iWAT via activating the AMPK/Sirt1 signaling pathway and might be associated with futile creatine cycling in obese mice. These results indicate that GAA has the potential to be used as an effective ingredient in dietary interventions and thus may play an important role in ameliorating and preventing HFD-induced obesity and related metabolic diseases.

Neonatal vitamin A supplementation improves sheep fertility potential.

This study aimed to explore the effects of neonatal vitamin A (VA) supplementation on testis development and spermatogenesis. A total of 32 newborn lambs were intramuscularly injected with corn oil (control group) or corn oil + 2500 IU/kg BW VA (VA group). They were slaughtered and sampled at 3 weeks and 8 months of age to analyze spermatogenesis, cell proliferation, hormone secretion, antioxidant status of the testis, and adult sheep sperm parameters. Compared with the control group, the expression of spermatogonial differentiation-related genes in VA group was up-regulated (P < 0.05). Testis weight, seminiferous tubule diameter, number of spermatogonium and spermatocyte, and sperm density increased significantly in VA group at 8 months of age (P < 0.05). Neonatal VA injection upregulated the expression of the cell proliferation marker PCNA and cell cycle-related genes in the testis (P < 0.05). VA increased the concentrations of testosterone (T), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) in the serum and upregulated steroidogenesis-related genes in the testis (P < 0.05). The antioxidant levels in the VA group were maintained at high levels. The total antioxidant capacity (T-AOC), antioxidant enzyme content and antioxidant-related genes were increased in the testis (P < 0.05). Furthermore, neonatal VA injection activated retinoic acid (RA) signaling to maintain the blood-testosterone barrier (BTB) in the testis of 3-week-old sheep. AMP-activated protein kinase (AMPK) and protein kinase B (AKT) signaling were also modulated in the sheep testis (P < 0.05). Taken together, VA supplementation in newborn rams promotes testis development and spermatogenesis to improve fertility.

Sea buckthorn oil regulates primary myoblasts proliferation and differentiation in vitro.

Skeletal muscle is the main edible part of meat products, and its development directly affects the yield and palatability of meat. Sea buckthorn oil (SBO) contains plenty of bioactive substances and has been recognized as a potential functional food product. The study aimed to explore the effects and possible mechanisms of SBO on sheep primary myoblast proliferation and myogenic differentiation. The results implied that SBO exhibited a pro-proliferative effect on primary myoblasts, along with up-regulated proliferating cell nuclear antigen (PCNA) and Cyclin D1/cyclin-dependent kinase 4 (CDK4) abundances. And, SBO promoted myotube formation by increasing the expression of myogenin. Meanwhile, we found that SBO inhibited the expression of miRNA-292a. Moreover, the regulatory effect of SBO on myogenic differentiation of myoblasts was attenuated by miRNA-292a mimics. Of note, SBO activated protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling pathway and augmented glucose uptake and glucose transporter 4 (GLUT4) content, which might be attributed to AMP-activated protein kinase (AMPK) activation. Additionally, the results were shown that SBO increased the abundance of antioxidative enzymes, including glutathione peroxidase 4 (Gpx4) and catalase. In summary, these data suggested that SBO regulated the proliferation and myogenic differentiation of sheep primary myoblasts in vitro, which might potentiate the application of SBO in muscle growth.

Vitamin A regulates mitochondrial biogenesis and function through p38 MAPK-PGC-1α signaling pathway and alters the muscle fiber composition of sheep.

BACKGROUND: Vitamin A (VA) and its metabolite, retinoic acid (RA), are of great interest for their wide range of physiological functions. However, the regulatory contribution of VA to mitochondrial and muscle fiber composition in sheep has not been reported. METHOD: Lambs were injected with 0 (control) or 7,500 IU VA palmitate into the biceps femoris muscle on d 2 after birth. At the age of 3 and 32 weeks, longissimus dorsi (LD) muscle samples were obtained to explore the effect of VA on myofiber type composition. In vitro, we investigated the effects of RA on myofiber type composition and intrinsic mechanisms. RESULTS: The proportion of type I myofiber was greatly increased in VA-treated sheep in LD muscle at harvest. VA greatly promoted mitochondrial biogenesis and function in LD muscle of sheep. Further exploration revealed that VA elevated PGC-1α mRNA and protein contents, and enhanced the level of p38 MAPK phosphorylation in LD muscle of sheep. In addition, the number of type I myofibers with RA treatment was significantly increased, and type IIx myofibers was significantly decreased in primary myoblasts. Consistent with in vivo experiment, RA significantly improved mitochondrial biogenesis and function in primary myoblasts of sheep. We then used si-PGC-1α to inhibit PGC-1α expression and found that si-PGC-1α significantly abrogated RA-induced the formation of type I myofibers, mitochondrial biogenesis, MitoTracker staining intensity, UQCRC1 and ATP5A1 expression, SDH activity, and enhanced the level of type IIx muscle fibers. These data suggested that RA improved mitochondrial biogenesis and function by promoting PGC-1α expression, and increased type I myofibers. In order to prove that the effect of RA on the level of PGC-1α is caused by p38 MAPK signaling, we inhibited the p38 MAPK signaling using a p38 MAPK inhibitor, which significantly reduced RA-induced PGC-1α and MyHC I levels. CONCLUSION: VA promoted PGC-1α expression through the p38 MAPK signaling pathway, improved mitochondrial biogenesis, and altered the composition of muscle fiber type.