The genome variation and developmental transcriptome maps reveal genetic differentiation of skeletal muscle in pigs.

Natural and artificial directional selections have resulted in significantly genetic and phenotypic differences across breeds in domestic animals. However, the molecular regulation of skeletal muscle diversity remains largely unknown. Here, we conducted transcriptome profiling of skeletal muscle across 27 time points, and performed whole-genome re-sequencing in Landrace (lean-type) and Tongcheng (obese-type) pigs. The transcription activity decreased with development, and the high-resolution transcriptome precisely captured the characterizations of skeletal muscle with distinct biological events in four developmental phases: Embryonic, Fetal, Neonatal, and Adult. A divergence in the developmental timing and asynchronous development between the two breeds was observed; Landrace showed a developmental lag and stronger abilities of myoblast proliferation and cell migration, whereas Tongcheng had higher ATP synthase activity in postnatal periods. The miR-24-3p driven network targeting insulin signaling pathway regulated glucose metabolism. Notably, integrated analysis suggested SATB2 and XLOC_036765 contributed to skeletal muscle diversity via regulating the myoblast migration and proliferation, respectively. Overall, our results provide insights into the molecular regulation of skeletal muscle development and diversity in mammals.

CRISPR Screen Identifies the RNA-Binding Protein Eef1a1 as a Key Regulator of Myogenesis.

Skeletal muscle myogenesis hinges on gene regulation, meticulously orchestrated by molecular mechanisms. While the roles of transcription factors and non-coding RNAs in myogenesis are widely known, the contribution of RNA-binding proteins (RBPs) has remained unclear until now. Therefore, to investigate the functions of post-transcriptional regulators in myogenesis and uncover new functional RBPs regulating myogenesis, we employed CRISPR high-throughput RBP-KO (RBP-wide knockout) library screening. Through this approach, we successfully identified Eef1a1 as a novel regulatory factor in myogenesis. Using CRISPR knockout (CRISPRko) and CRISPR interference (CRISPRi) technologies, we successfully established cellular models for both CRISPRko and CRISPRi. Our findings demonstrated that Eef1a1 plays a crucial role in promoting proliferation in C2C12 myoblasts. Through siRNA inhibition and overexpression methods, we further elucidated the involvement of Eef1a1 in promoting proliferation and suppressing differentiation processes. RIP (RNA immunoprecipitation), miRNA pull-down, and Dual-luciferase reporter assays confirmed that miR-133a-3p targets Eef1a1. Co-transfection experiments indicated that miR-133a-3p can rescue the effect of Eef1a1 on C2C12 myoblasts. In summary, our study utilized CRISPR library high-throughput screening to unveil a novel RBP, Eef1a1, involved in regulating myogenesis. Eef1a1 promotes the proliferation of myoblasts while inhibiting the differentiation process. Additionally, it acts as an antagonist to miR-133a-3p, thus modulating the process of myogenesis.

A comprehensive epigenome atlas reveals DNA methylation regulating skeletal muscle development.

DNA methylation is important for the epigenetic regulation of gene expression and plays a critical role in mammalian development. However, the dynamic regulation of genome-wide DNA methylation in skeletal muscle development remains largely unknown. Here, we generated the first single-base resolution DNA methylome and transcriptome maps of porcine skeletal muscle across 27 developmental stages. The overall methylation level decreased from the embryo to the adult, which was highly correlated with the downregulated expression of DNMT1 and an increase in partially methylated domains. Notably, we identified over 40 000 developmentally differentially methylated CpGs (dDMCs) that reconstitute the developmental trajectory of skeletal muscle and associate with muscle developmental genes and transcription factors (TFs). The dDMCs were significantly under-represented in promoter regulatory regions but strongly enriched as enhancer histone markers and in chromatin-accessible regions. Integrative analysis revealed the negative regulation of both promoter and gene body methylation in genes associated with muscle contraction and insulin signaling during skeletal muscle development. Mechanistically, DNA methylation affected the expression of muscle-related genes by modulating the accessibly of upstream myogenesis TF binding, indicating the involvement of the DNA methylation/SP1/IGF2BP3 axis in skeletal myogenesis. Our results highlight the function and regulation of dynamic DNA methylation in skeletal muscle development.

Integration of multi-omics data reveals cis-regulatory variants that are associated with phenotypic differentiation of eastern from western pigs.

BACKGROUND: The genetic mechanisms that underlie phenotypic differentiation in breeding animals have important implications in evolutionary biology and agriculture. However, the contribution of cis-regulatory variants to pig phenotypes is poorly understood. Therefore, our aim was to elucidate the molecular mechanisms by which non-coding variants cause phenotypic differences in pigs by combining evolutionary biology analyses and functional genomics. RESULTS: We obtained a high-resolution phased chromosome-scale reference genome with a contig N50 of 18.03 Mb for the Luchuan pig breed (a representative eastern breed) and profiled potential selective sweeps in eastern and western pigs by resequencing the genomes of 234 pigs. Multi-tissue transcriptome and chromatin accessibility analyses of these regions suggest that tissue-specific selection pressure is mediated by promoters and distal cis-regulatory elements. Promoter variants that are associated with increased expression of the lysozyme (LYZ) gene in the small intestine might enhance the immunity of the gastrointestinal tract and roughage tolerance in pigs. In skeletal muscle, an enhancer-modulating single-nucleotide polymorphism that is associated with up-regulation of the expression of the troponin C1, slow skeletal and cardiac type (TNNC1) gene might increase the proportion of slow muscle fibers and affect meat quality. CONCLUSIONS: Our work sheds light on the molecular mechanisms by which non-coding variants shape phenotypic differences in pigs and provides valuable resources and novel perspectives to dissect the role of gene regulatory evolution in animal domestication and breeding.

Influence of an L-type SALMFamide neuropeptide on locomotory performance and muscle physiology in the sea cucumber Apostichopus japonicus.

Neuropeptides in the SALMFamide family serve as muscle relaxants in echinoderms and may affect locomotion, as the motor behavior in sea cucumbers involves alternating contraction and extension of the body wall, which is under the control of longitudinal muscle. We evaluated the effect of an L-type SALMFamide neuropeptide (LSA) on locomotory performance of Apostichopus japonicus. We also investigated the metabolites of longitudinal muscle tissue using ultra performance liquid chromatography and quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) to assess the potential physiological mechanisms underlying the effect of LSA. The hourly distance, cumulative duration and number of steps moved significantly increased in sea cucumbers in the fourth hour after injection with LSA. Also, the treatment enhanced the mean and maximum velocity by 9.8% and 17.8%, respectively, and increased the average stride by 12.4%. Levels of 27 metabolites in longitudinal muscle changed after LSA administration, and the increased concentration of pantothenic acid, arachidonic acid and lysophosphatidylethanolamine, and the altered phosphatidylethanolamine/phosphatidylcholine ratio are potential physiological mechanisms that could explain the observed effect of LSA on locomotor behavior in A. japonicus.

Landscape of tissue-specific RNA Editome provides insight into co-regulated and altered gene expression in pigs (Sus-scrofa).

RNA editing generates genetic diversity in mammals by altering amino acid sequences, miRNA targeting site sequences, influencing the stability of targeted RNAs, and causing changes in gene expression. However, the extent to which RNA editing affect gene expression via modifying miRNA binding site remains unexplored. Here, we first profiled the dynamic A-to-I RNA editome across tissues of Duroc and Luchuan pigs. The RNA editing events at the miRNA binding sites were generated. The biological function of the differentially edited gene in skeletal muscle was further characterized in pig muscle-derived satellite cells. RNA editome analysis revealed a total of 171,909 A-to-I RNA editing sites (RESs), and examination of its features showed that these A-to-I editing sites were mainly located in SINE retrotransposons PRE-1/Pre0_SS element. Analysis of differentially edited sites (DESs) revealed a total of 4,552 DESs across tissues between Duroc and Luchuan pigs, and functional category enrichment analysis of differentially edited gene (DEG) sets highlighted a significant association and enrichment of tissue-developmental pathways including TGF-beta, PI3K-Akt, AMPK, and Wnt signaling pathways. Moreover, we found that RNA editing events at the miRNA binding sites in the 3'-UTR of HSPA12B mRNA could prevent the miRNA-mediated mRNA downregulation of HSPA12B in the muscle-derived satellite (MDS) cell, consistent with the results obtained from the Luchuan skeletal muscle. This study represents the most systematic attempt to characterize the significance of RNA editing in regulating gene expression, particularly in skeletal muscle, constituting a new layer of regulation to understand the genetic mechanisms behind phenotype variance in animals.Abbreviations: A-to-I: Adenosine-to-inosine; ADAR: Adenosine deaminase acting on RNA; RES: RNA editing site; DEG: Differentially edited gene; DES: Differentially edited site; FDR: False discovery rate; GO: Gene Ontology; KEGG: Kyoto Encyclopaedia of Genes and Genomes; MDS cell: musclederived satellite cell; RPKM: Reads per kilobase of exon model in a gene per million mapped reads; UTR: Untranslated coding regions.

Autofocusing of ring Airy beams embedded with off-axial vortex singularities.

We report the autofocusing behaviors of ring Airy beams (RABs) embedded with two kinds of off-axial vortex singularities. The influences of embedded positions and topological charges of point and r vortices on the autofocusing dynamic are numerically and experimentally investigated. The results show that, for the first-order vortex, the embedded position significantly affects the focal field, and once the singularity is located on the main ring of RAB, the symmetric Bessel profile of the focal field will be broken, otherwise the Bessel-like focus can self-heal at the focal plane. However, for the higher-order vortex embedded near the main ring, it will split into several fundamental vortices and then separate with each other along the radial direction under the interaction with the RAB background. Our results hold potential for the practical application of RABs in the atmosphere and other propagation systems with perturbation and even singularities.

Identification of Gli1-interacting proteins during simvastatin-stimulated osteogenic differentiation of bone marrow mesenchymal stem cells.

Simvastatin has been shown to promote osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). Our study aimed to illuminate the underlying mechanism, with a specific focus on the role of Hedgehog signaling in this process. BMSCs cultured with or without 10-7 mol/L simvastatin were subjected to evaluation of osteogenic differentiation capacity. Osteogenic markers such as type 1 collagen (COL1) and osteocalcin (OCN), as well as key molecules of Hedgehog signaling molecules, were examined by Western blot and real-time polymerase chain reaction (PCR). Co-immunoprecipitation and mass spectrometry assays were applied to screen for Gli1-interacting proteins. Cyclopamine (Cpn) was used as a Hedgehog signaling inhibitor. Our results indicated that simvastatin increased alkaline phosphatase (ALP) activity; mineralization of extracellular matrix; mRNA expression of ALP, COL1, and OCN; and expression and nuclear translocation of Gli1. Contrasting effects were observed in Cpn-exposed groups, but were partially rescued by the simvastatin treatment. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses indicated that Gli1-interacting proteins were primarily associated with mitogen-activated protein kinase (MAPK) (P = 7.04E-04 ), hippo, insulin, and glucagon signaling. Further, hub genes identified by protein-protein interaction network analysis included Gli1-interacting proteins such as Ppp2r1a, Rac1, Etf1, and XPO1/CRM1. In summary, the current study showed that the mechanism by which simvastatin stimulates osteogenic differentiation of BMSCs involves activation of Hedgehog signaling, as indicated by interactions with Gli1 and, most notably, the MAPK signaling pathway.

Shaping vector fields in three dimensions by random Fourier phase-only encoding.

Simultaneously controlling the spatial distribution of multiple parameters of a light field in a three-dimensional (3D) space is highly desirable because of its prominent applications in the areas of optical imaging, microscopy, and manipulation. Phase-only encoding techniques that use a phase-only computer-generated hologram (CGH) to reshape and efficiently reconstruct target fields have fostered substantial interests. In this paper, we propose a convenient encoding method to construct vector fields with spatially structured multiple parameters in a 3D space by integrating the Fourier phase-only encoding technique into a modified Sagnac polarization conversion system. Without spatial filtering, various vector fields are constructed instantly at the image plane. Furthermore, utilizing a macro-pixel encoding approach, we demonstrate the possibility of a simultaneous and an independent construction of multiple vector fields in a 3D space. This method can also benefit the design of a metasurface to implement a polarization hologram.

Wnt signaling: a promising target for osteoarthritis therapy.

Osteoarthritis (OA) is the most common joint disease worldwide and a leading cause of disability. Characterized by degradation of articular cartilage, synovial inflammation, and changes in periarticular and subchondral bone, OA can negatively impact an individual's physical and mental well-being. Recent studies have reported several critical signaling pathways as key regulators and activators of cellular and molecular processes during OA development. Wnt signaling is one such pathway whose signaling molecules and regulators were shown to be abnormally activated or suppressed. As such, agonists and antagonists of those molecules are potential candidates for OA treatment. Notably, a recent phase I clinical trial (NCT02095548) demonstrated the potential of SM04690, a small-molecule inhibitor of the Wnt signaling pathway, as a disease-modifying oseoarthritis drug (DMOAD). This review summarizes the role and mechanism of Wnt signaling and related molecules in regulating OA progression, with a view to accelerating the translation of such evidence into the development of strategies for OA treatment, particularly with respect to potential applications of molecules targeting the Wnt signaling pathway.

Polypeptide Preparation by ß-Lactone-Mediated Chemical Ligation.

Native chemical ligation (NCL) represents a cornerstone strategy in accessing synthetic peptides and proteins, remaining one of the most efficacious methodologies in this domain. The fundamental requisites for achieving a proficient NCL reaction involve chemoselective coupling between a C-terminal thioester peptide and a thiol-bearing N-terminal peptide. However, achieving coupling at sterically congested residues remains challenging. In addition, while most NCLs proceed without epimerization, ß-branched (e.g., Ile, Thr, Val) and Pro-derived C-terminal thioesters react slowly and can be susceptible to significant epimerization and hydrolysis. Herein, we report an epimerization-free NCL reaction via ß-lactone-mediated native chemical ligation which constructs sterically congested Thr residues. The constrained ring from the ß-lactone allows rapid peptide ligation without detectable epimerization. The method has a broad side-chain tolerance and was applied to the preparation of cyclic peptides and polypeptidyl thioester, which could be difficult to obtained otherwise.

Production of constrained L-cyclo-tetrapeptides by epimerization-resistant direct aminolysis.

The synthesis of constrained 12-membered rings is notably difficult. The main challenges result from constraints during the linear peptide cyclization. Attempts to overcome constraints through excessive activation frequently cause peptidyl epimerization, while insufficient activation of the C-terminus hampers cyclization and promotes intermolecular oligomer formation. We present a ß-thiolactone framework that enables the synthesis of cyclo-tetrapeptides via direct aminolysis. This tactic utilizes a mechanism that restricts C-terminal carbonyl rotation while maintaining high reactivity, thereby enabling efficient head-to-tail amidation, reducing oligomerization, and preventing epimerization. A broad range of challenging cyclo-tetrapeptides ( > 20 examples) are synthesized in buffer and exhibits excellent tolerance toward nearly all proteinogenic amino acids. Previously unattainable macrocycles, such as cyclo-L-(Pro-Tyr-Pro-Val), have been produced and identified as µ-opioid receptor (MOR) agonists, with an EC50 value of 2.5 nM. Non-epimerizable direct aminolysis offers a practical solution for constrained peptide cyclization, and the discovery of MOR agonist activity highlights the importance of overcoming synthetic challenges for therapeutic development.

Haplotype-resolved assembly of a pig genome using single-sperm sequencing.

Single gamete cell sequencing together with long-read sequencing can reliably produce chromosome-level phased genomes. In this study, we employed PacBio HiFi and Hi-C sequencing on a male Landrace pig, coupled with single-sperm sequencing of its 102 sperm cells. A haplotype assembly method was developed based on long-read sequencing and sperm-phased markers. The chromosome-level phased assembly showed higher phasing accuracy than methods that rely only on HiFi reads. The use of single-sperm sequencing data enabled the construction of a genetic map, successfully mapping the sperm motility trait to a specific region on chromosome 1 (105.40-110.70 Mb). Furthermore, with the assistance of Y chromosome-bearing sperm data, 26.16 Mb Y chromosome sequences were assembled. We report a reliable approach for assembling chromosome-level phased genomes and reveal the potential of sperm population in basic biology research and sperm phenotype research.

Wnt5a deficiency in osteocalcin-expressing cells could not alleviate the osteoarthritic phenotype in a mouse model of post-traumatic osteoarthritis.

Objectives: Wnt5a, which regulates the activities of osteoblasts and osteoclasts, is reportedly overexpressed in osteoarthritis (OA) tissues. The purpose of this study was to elucidate its role in the development of OA by deleting Wnt5a in osteocalcin (OCN)-expressing cells. Materials and Methods: Knee OA was induced by anterior cruciate ligament transection (ACLT) in OCN-Cre;Wnt5afl/fl knockout (Wnt5a-cKO) mice and control littermates. Eight weeks after surgery, histological changes, cell apoptosis, and matrix metabolism of cartilage were evaluated by toluidine blue, TUNEL staining, and im-immunohistochemistry analyses, respectively. In addition, the subchondral bone microarchitecture of mice was examined by micro-computed tomography (micro-CT). Results: Histological scores show substantial cartilage degeneration occurred in ACLT knees, coupled with decreased collagen type II expression and enhanced matrix metalloproteinase 13 expression, as well as higher proportions of apoptotic cells. Micro-CT results show that ACLT resulted in decreased bone mineral density, bone volume/trabecular volume, trabecular number, and structure model index of subchondral bones in both Wnt5a-cKO and control littermates; although Wnt5a-cKO mice display lower BMD and BV/TV values, no significant difference was observed between Wnt5a-cKO and control mice for any of these values. Conclusion: Our findings indicate that Wnt5a deficiency in OCN-expressing cells could not prevent an osteoarthritic phenotype in a mouse model of post-traumatic OA.

Rapid and Direct Assessment of Asphalt Volatile Organic Compound Emission Based on Carbon Fiber Ionization Mass Spectrometry.

Due to the complicacy of asphalt fumes, the analytical methods for investigating volatile organic compounds (VOCs) are very limited. In this study, a direct and real-time analysis method based on carbon fiber ionization mass spectrometry (CFI-MS), an ambient mass spectrometric technique, was established and successfully applied in the analysis of asphalt VOCs. The asphalt VOCs can be directly detected in the open atmosphere without the collection step of asphalt fumes, and the mass spectra of one asphalt sample can be obtained in a few seconds in both positive and negative ion modes. By investigating the mass spectral changes of asphalt fumes at different heating temperatures ranging from 50 to 200 °C, the temperature factor of asphalt fume emission was demonstrated in this work. The research results demonstrate that the complexity of asphalt fumes is positively related to the applied temperature. Moreover, the VOCs of saturates, aromatics, resins, and asphaltenes fractions were also analyzed by the direct analysis method. The result shows that aromatics contribute most to the emission of VOCs. In addition, the obtained mass spectra combined with the principal component analysis method show the great potential to quickly screen VOC inhibitors of asphalt materials.

Circular RNA transcriptome across multiple tissues reveal skeletal muscle-specific circPSME4 regulating myogenesis.

There are significant differences in meat production, growth rate and other traits between Western commercial pigs and Chinese local pigs. Comparative transcriptome approaches have identified many coding and non-coding candidate genes associated various traits. However, the expression and function of circular RNAs (circRNAs) in different pig tissues are largely unknown. In this study, we conducted a comprehensive analysis of the genome-wide circRNA expression profile across ten tissues in Luchuan (a Chinese local breed) and Duroc (a Western commercial breed) pigs. We identified a total of 56,254 circRNAs, of which 42.9 % were not previously annotated. We found that 33.7 % of these circRNAs were differentially expressed. Enrichment analysis revealed that differentially expressed circRNAs might contribute to the phenotypic differentiation between Luchuan and Duroc pigs. We identified 538 tissue-specific circRNAs, most of which were specifically expressed in the brain and skeletal muscle. Competitive endogenous RNA network analysis suggested that skeletal muscle-specific circPSME4 was co-expressed with MYOD1 and targeted by ssc-miR-181d-3p. Functional analysis revealed that circPSME4 knockdown could promote the proliferation and differentiation of myoblasts. Together, our findings provide valuable resources of circRNAs for animal breeding and biomedical research. We demonstrated that circPSME4 is a novel regulator of skeletal muscle development.

Dielectric Metalens for Superoscillatory Focusing Based on High-Order Angular Bessel Function.

The phenomenon of optical superoscillation provides an unprecedented way to solve the problem of optical far-field label-free super-resolution imaging. Numerous optical devices that enable superoscillatory focusing were developed based on scalar and vector diffraction theories in the past several years. However, these reported devices are designed according to the half-wave zone method in spatial coordinates. In this paper, we propose a dielectric metalens for superoscillatory focusing based on the diffraction of angular Bessel functional phase modulated vector field, under the inspiration of the tightly autofocusing property of a radially polarized high-order Bessel beam. Based on this kind of metalens with a numerical aperture (NA) of 0.9, the linearly polarized light is converted into a radially polarized one and then focus into a superoscillating focal spot with the size of 0.32λ/NA. This angular spectrum modulation theory involved in this paper provides a different way of designing superoscillatory devices.

MiR-743a-5p regulates differentiation of myoblast by targeting Mob1b in skeletal muscle development and regeneration.

The microRNAs (miRNAs) play an important role in regulating myogenesis by targeting mRNA. However, the understanding of miRNAs in skeletal muscle development and diseases is unclear. In this study, we firstly performed the transcriptome profiling in differentiating C2C12 myoblast cells. Totally, we identified 187 miRNAs and 4260 mRNAs significantly differentially expressed that were involved in myoblast differentiation. We carried out validation of microarray data based on 5 mRNAs and 5 miRNAs differentially expressed and got a consistent result. Then we constructed and validated the significantly up- and down-regulated mRNA-miRNA interaction networks. Four interaction pairs (miR-145a-5p-Fscn1, miR-200c-5p-Tmigd1, miR-27a-5p-Sln and miR-743a-5p-Mob1b) with targeted relationships in differentiated myoblast cells were demonstrated. They are all closely related to myoblast development. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated cell cycle signals important for exploring skeletal muscle development and disease. Functionally, we discovered that miR-743a targeting gene Mps One Binder Kinase Activator-Like 1B (Mob1b) gene in differentiated C2C12. The up-regulated miR-743a can promote the differentiation of C2C12 myoblast. While the down-regulated Mob1b plays a negative role in differentiation. In addition, the expression profile of miR-743a and Mob1b are consistent with skeletal muscle recovery after Cardiotoxin (CTX) injury. Our study revealed that miR-743a-5p regulates myoblast differentiation by targeting Mob1b involved in skeletal muscle development and regeneration. Our findings made a further exploration for mechanisms in myogenesis and might provide potential possible miRNA-based target therapies for skeletal muscle regeneration and disease in the near future.

circRNAome profiling reveals circFgfr2 regulates myogenesis and muscle regeneration via a feedback loop.

BACKGROUND: Circular RNAs (circRNAs) represent a novel class of non-coding RNAs formed by a covalently closed loop and play crucial roles in many biological processes. Several circRNAs associated with myogenesis have been reported. However, the dynamic expression, function, and mechanism of circRNAs during myogenesis and skeletal muscle development are largely unknown. METHODS: Strand-specific RNA-sequencing (RNA-seq) and microarray datasets were used to profile the dynamic circRNAome landscape during skeletal muscle development and myogenic differentiation. Bioinformatics analyses were used to characterize the circRNAome and identify candidate circRNAs associated with myogenesis. Bulk and single-cell RNA-seq were performed to identify the downstream genes and pathways of circFgfr2. The primary myoblast cells, C2C12 cells, and animal model were used to assess the function and mechanism of circFgfr2 in myogenesis and muscle regeneration in vitro or in vivo by RT-qPCR, western blotting, dual-luciferase activity assay, RNA immunoprecipitation, RNA fluorescence in situ hybridization, and chromatin immunoprecipitation. RESULTS: We profiled the dynamic circRNAome in pig skeletal muscle across 27 developmental stages and detected 52 918 high-confidence circRNAs. A total of 2916 of these circRNAs are conserved across human, mouse, and pig, including four circRNAs (circFgfr2, circQrich1, circMettl9, and circCamta1) that were differentially expressed (|log2 fold change| > 1 and adjusted P value < 0.05) in various myogenesis systems. We further focused on a conserved circRNA produced from the fibroblast growth factor receptor 2 (Fgfr2) gene, termed circFgfr2, which was found to inhibit myoblast proliferation and promote differentiation and skeletal muscle regeneration. Mechanistically, circFgfr2 acted as a sponge for miR-133 to regulate the mitogen-activated protein kinase kinase kinase 20 (Map3k20) gene and JNK/MAPK pathway. Importantly, transcription factor Kruppel like factor 4 (Klf4), the downstream target of the JNK/MAPK pathway, directly bound to the promoter of circFgfr2 and affected its expression via an miR-133/Map3k20/JNK/Klf4 auto-regulatory feedback loop. RNA binding protein G3BP stress granule assembly factor 1 (G3bp1) inhibited the biogenesis of circFgfr2. CONCLUSIONS: The present study provides a comprehensive circRNA resource for skeletal muscle study. The functional and mechanistic analysis of circFgfr2 uncovered a circRNA-mediated auto-regulatory feedback loop regulating myogenesis and muscle regeneration, which provides new insight to further understand the regulatory mechanism of circRNAs.

Sp1-Mediated circRNA circHipk2 Regulates Myogenesis by Targeting Ribosomal Protein Rpl7.

Circular RNAs (circRNAs) represent a class of covalently closed single-stranded RNA molecules that are emerging as essential regulators of various biological processes. The circRNA circHipk2 originates from exon 2 of the Hipk2 gene in mice and was reported to be involved in acute promyelocytic leukemia and myocardial injury. However, the functions and mechanisms of circHipk2 in myogenesis are largely unknown. Here, to deepen our knowledge about the role of circHipk2, we studied the expression and function of circHipk2 during skeletal myogenesis. We found that circHipk2 was mostly distributed in the cytoplasm, and dynamically and differentially expressed in various myogenesis systems in vitro and in vivo. Functionally, overexpression of circHipk2 inhibited myoblast proliferation and promoted myotube formation in C2C12 cells, whereas the opposite effects were observed after circHipk2 knockdown. Mechanistically, circHipk2 could directly bind to ribosomal protein Rpl7, an essential 60S preribosomal assembly factor, to inhibit ribosome translation. In addition, we verified that transcription factor Sp1 directly bound to the promoter of circHipk2 and affected the expression of Hipk2 and circHipk2 in C2C12 myoblasts. Collectively, these findings identify circHipk2 as a candidate circRNA regulating ribosome biogenesis and myogenesis proliferation and differentiation.