bims-muscge Biomed News
on Muscle stem cells and gene therapy
Issue of 2023–09–24
eightteen papers selected by
Chance Bowman, Dartmouth College
  1. Semaphorins: Missing Signals in Age-dependent Alteration of Neuromuscular Junctions and Skeletal Muscle Regeneration.
  2. Mimicking sarcolemmal damagein vitro: a contractile 3D model of skeletal muscle for drug testing in Duchenne muscular dystrophy.
  3. KLF4 regulates skeletal muscle development and regeneration by directly targeting P57 and Myomixer.
  4. Migration and differentiation of muscle stem cells are coupled by RhoA signalling during regeneration.
  5. Selenoprotein K knockdown induces apoptosis in skeletal muscle satellite cells via calcium dyshomeostasis-mediated endoplasmic reticulum stress.
  6. Advances in research on cell models for skeletal muscle atrophy.
  7. Actuated tissue engineered muscle grafts restore functional mobility after volumetric muscle loss.
  8. Absolute Quantification of mRNA Isoforms in Adult Stem Cells Using Microfluidic Digital PCR.
  9. Regulatory T cells require IL6 receptor alpha signaling to control skeletal muscle function and regeneration.
  10. Engineering extracellular vesicles to deliver CRISPR ribonucleoprotein for gene editing.
  11. Recent Advances in CRISPR/Cas9 Delivery Approaches for Therapeutic Gene Editing of Stem Cells.
  12. Unexpected partial RNA deletion by two different novel COL6A2 mutations leads to Ullrich congenital muscular dystrophy.
  13. Continuous directed evolution of a compact CjCas9 variant with broad PAM compatibility.
  14. Precision RNA base editing with engineered and endogenous effectors.
  15. Endogenous tenocyte activation underlies the regenerative capacity of the adult zebrafish tendon.
  16. Blocking insulin-like growth factor 1 receptor signaling pathway inhibits neuromuscular junction regeneration after botulinum toxin-A treatment.
  17. Loss of Trim28 in muscle alters mitochondrial signalling but not systemic metabolism.
  18. Muscle: an independent contributor to the neuromuscular spinal muscular atrophy disease phenotype.
  1. Aging Dis. 2023 Sep 03.
    Semaphorins: Missing Signals in Age-dependent Alteration of Neuromuscular Junctions and Skeletal Muscle Regeneration.
    Damon Fard, Alessandra Barbiera, Gabriella Dobrowolny, Luca Tamagnone, Bianca Maria Scicchitano.
      Skeletal muscle is characterized by a remarkable capacity to rearrange after physiological changes and efficiently regenerate. However, during aging, extensive injury, or pathological conditions, the complete regenerative program is severely affected, with a progressive loss of muscle mass and function, a condition known as sarcopenia. The compromised tissue repair program is attributable to the gradual depletion of stem cells and to altered regulatory signals. Defective muscle regeneration can severely affect re-innervation by motor axons, and neuromuscular junctions (NMJs) development, ultimately leading to skeletal muscle atrophy. Defects in NMJ formation and maintenance occur physiologically during aging and are responsible for the pathogenesis of several neuromuscular disorders. However, it is still largely unknown how neuromuscular connections are restored on regenerating fibers. It has been suggested that attractive and repelling signals used for axon guidance could be implicated in this process; in particular, guidance molecules called semaphorins play a key role. Semaphorins are a wide family of extracellular regulatory signals with a multifaceted role in cell-cell communication. Originally discovered as axon guidance factors, they have been implicated in cancer progression, embryonal organogenesis, skeletal muscle innervation, and other physiological and developmental functions in different tissues. In particular, in skeletal muscle, specific semaphorin molecules are involved in the restoration and remodeling of the nerve-muscle connections, thus emphasizing their plausible role to ensure the success of muscle regeneration. This review article aims to discuss the impact of aging on skeletal muscle regeneration and NMJs remodeling and will highlight the most recent insights about the role of semaphorins in this context.
    DOI:  https://doi.org/10.14336/AD.2023.0801
  2. Biofabrication. 2023 Sep 19.
    Mimicking sarcolemmal damagein vitro: a contractile 3D model of skeletal muscle for drug testing in Duchenne muscular dystrophy.
    Ainoa Tejedera-Villafranca, Marisol Montolio, Javier Ramón-Azcón, Juan M Fernández-Costa.
      Duchenne muscular dystrophy (DMD) is the most prevalent neuromuscular disease diagnosed in childhood. It is a progressive and wasting disease, characterized by a degeneration of skeletal and cardiac muscles caused by the lack of dystrophin protein. The absence of this crucial structural protein leads to sarcolemmal fragility, resulting in muscle fiber damage during contraction. Despite ongoing efforts, there is no cure available for DMD patients. One of the primary challenges is the limited efficacy of current preclinical tools, which fail in modeling the biological complexity of the disease. Human-based 3D cell culture methods appear as a novel approach to accelerate preclinical research by enhancing the reproduction of pathophysiological processes in skeletal muscle. In this work, we developed a patientderived functional 3D skeletal muscle model of DMD that reproduces the sarcolemmal damage found in the native DMD muscle. These bioengineered skeletal muscle tissues exhibit contractile functionality, as they responded to electrical pulse stimulation (EPS). Sustained contractile regimes induced the loss of myotube integrity, mirroring the pathological myotube breakdown inherent in DMD due to sarcolemmal instability. Moreover, damaged DMD tissues showed disease functional phenotypes, such as tetanic fatigue. We also evaluated the therapeutic effect of utrophin upregulator drug candidates on the functionality of the skeletal muscle tissues, thus providing deeper insight into the real impact of these treatments. Overall, our findings underscore the potential of bioengineered 3D skeletal muscle technology to advance DMD research and facilitate the development of novel therapies for DMD and related neuromuscular disorders.
    Keywords:  3D cell culture; Duchenne muscular dystrophy; disease modelling; drug testing; sarcolemmal damage; skeletal muscle; tissue engineering
    DOI:  https://doi.org/10.1088/1758-5090/acfb3d
  3. Cell Death Dis. 2023 09 18. 14(9): 612
    KLF4 regulates skeletal muscle development and regeneration by directly targeting P57 and Myomixer.
    Shufang Cai, Xiaoyu Wang, Rong Xu, Ziyun Liang, Qi Zhu, Meilin Chen, Zhuhu Lin, Chenggan Li, Tianqi Duo, Xian Tong, Enru Li, Zuyong He, Xiaohong Liu, Yaosheng Chen, Delin Mo.
      Krüppel-like factor 4 (KLF4) is an evolutionarily conserved zinc finger-containing transcription factor that regulates diverse cellular processes such as cell proliferation, apoptosis, and differentiation. Our previous study showed that KLF4 expression is upregulated in skeletal muscle ontogeny during embryonic development in pigs, suggesting its importance for skeletal muscle development and muscle function. We revealed here that KLF4 plays a critical role in skeletal muscle development and regeneration. Specific knockout of KLF4 in skeletal muscle impaired muscle formation further affecting physical activity and also defected skeletal muscle regeneration. In vitro, KLF4 was highly expressed in proliferating myoblasts and early differentiated cells. KLF4 knockdown promoted myoblast proliferation and inhibited myoblast fusion, while its overexpression showed opposite results. Mechanically, in proliferating myoblasts, KLF4 inhibits myoblast proliferation through regulating cell cycle arrest protein P57 by directly targeting its promoter; while in differentiated myoblasts, KLF4 promotes myoblast fusion by transcriptionally activating Myomixer. Our study provides mechanistic information for skeletal muscle development, reduced muscle strength and impaired regeneration after injury and unveiling the mechanism of KLF4 in myogenic regulation.
    DOI:  https://doi.org/10.1038/s41419-023-06136-w
  4. Open Biol. 2023 Sep;13(9): 230037
    Migration and differentiation of muscle stem cells are coupled by RhoA signalling during regeneration.
    Mirco Brondolin, Dylan Herzog, Sami Sultan, Fiona Warburton, Alessandra Vigilante, Robert D Knight.
      Skeletal muscle is highly regenerative and is mediated by a population of migratory adult muscle stem cells (muSCs). Effective muscle regeneration requires a spatio-temporally regulated response of the muSC population to generate sufficient muscle progenitor cells that then differentiate at the appropriate time. The relationship between muSC migration and cell fate is poorly understood and it is not clear how forces experienced by migrating cells affect cell behaviour. We have used zebrafish to understand the relationship between muSC cell adhesion, behaviour and fate in vivo. Imaging of pax7-expressing muSCs as they respond to focal injuries in trunk muscle reveals that they migrate by protrusive-based means. By carefully characterizing their behaviour in response to injury we find that they employ an adhesion-dependent mode of migration that is regulated by the RhoA kinase ROCK. Impaired ROCK activity results in reduced expression of cell cycle genes and increased differentiation in regenerating muscle. This correlates with changes to focal adhesion dynamics and migration, revealing that ROCK inhibition alters the interaction of muSCs to their local environment. We propose that muSC migration and differentiation are coupled processes that respond to changes in force from the environment mediated by RhoA signalling.
    Keywords:  cell motility; mechanotransduction; myogenesis; satellite cell; zebrafish
    DOI:  https://doi.org/10.1098/rsob.230037
  5. Poult Sci. 2023 Aug 22. pii: S0032-5791(23)00572-2. [Epub ahead of print]102(11): 103053
    Selenoprotein K knockdown induces apoptosis in skeletal muscle satellite cells via calcium dyshomeostasis-mediated endoplasmic reticulum stress.
    Rui-Feng Fan, Xue-Wei Chen, Han Cui, Hong-Yu Fu, Wan-Xue Xu, Jiu-Zhi Li, Hai Lin.
      Skeletal muscle satellite cells (SMSCs), known as muscle stem cells, play an important role in muscle embryonic development, post-birth growth, and regeneration after injury. Selenoprotein K (SELENOK), an endoplasmic reticulum (ER) resident selenoprotein, is known to regulate calcium ion (Ca2+) flux and ER stress (ERS). SELENOK deficiency is involved in dietary selenium deficiency-induced muscle injury, but the regulatory mechanisms of SELENOK in SMSCs development remain poorly explored in chicken. Here, we established a SELENOK deficient model to explore the role of SELENOK in SMSCs. SELENOK knockdown inhibited SMSCs proliferation and differentiation by regulating the protein levels of paired box 7 (Pax7), myogenic factor 5 (Myf5), CyclinD1, myogenic differentiation (MyoD), and Myf6. Further analysis exhibited that SELENOK knockdown markedly activated the ERS signaling pathways, which ultimately induced apoptosis in SMSCs. SELENOK knockdown-induced ERS is related with ER Ca2+ ([Ca2+]ER) overload via decreasing the protein levels of STIM2, Orai1, palmitoylation of inositol 1,4,5-trisphosphate receptor 1 (IP3R1), phospholamban (PLN), and plasma membrane Ca2+-ATPase (PMCA) while increasing the protein levels of sarco/endoplasmic Ca2+-ATPase 1 (SERCA1) and Na+/Ca2+ exchanger 1 (NCX1). Moreover, thimerosal, an activator of IP3R1, reversed the overload of [Ca2+]ER, ERS, and subsequent apoptosis caused by SELENOK knockdown. These findings indicated that SELENOK knockdown triggered ERS driven by intracellular Ca2+ dyshomeostasis and further induced apoptosis, which ultimately inhibited SMSCs proliferation and differentiation.
    Keywords:  apoptosis; calcium; endoplasmic reticulum stress; selenoprotein K; skeletal muscle satellite cell
    DOI:  https://doi.org/10.1016/j.psj.2023.103053
  6. Biomed Pharmacother. 2023 Sep 20. pii: S0753-3322(23)01315-X. [Epub ahead of print]167 115517
    Advances in research on cell models for skeletal muscle atrophy.
    Liwei Li, Chunman Huang, Jingqun Pang, Yongbin Huang, Xinxin Chen, Guanghua Chen.
      Skeletal muscle, the largest organ in the human body, plays a crucial role in supporting and defending the body and is essential for movement. It also participates in regulating the processes of protein synthesis and degradation. Inhibition of protein synthesis and activation of degradation metabolism can both lead to the development of skeletal muscle atrophy, a pathological condition characterized by a decrease in muscle mass and fiber size. Many physiological and pathological conditions can cause a decline in muscle mass, but the underlying mechanisms of its pathogenesis remain incompletely understood, and the selection of treatment strategies and efficacy evaluations vary. Moreover, the early symptoms of this condition are often not apparent, making it easily overlooked in clinical practice. Therefore, it is necessary to develop and use cell models to understand the etiology and influencing factors of skeletal muscle atrophy. In this review, we summarize the methods used to construct skeletal muscle cell models, including hormone, inflammation, cachexia, genetic engineering, drug, and physicochemical models. We also analyze, compare, and evaluate the various construction and assessment methods.
    Keywords:  Cell models; Myotubes; Skeletal muscle atrophy
    DOI:  https://doi.org/10.1016/j.biopha.2023.115517
  7. Biomaterials. 2023 Sep 08. pii: S0142-9612(23)00325-3. [Epub ahead of print]302 122317
    Actuated tissue engineered muscle grafts restore functional mobility after volumetric muscle loss.
    Erin Rousseau, Ritu Raman, Tigist Tamir, Angel Bu, Shriya Srinivasan, Naomi Lynch, Robert Langer, Forest M White, Michael J Cima.
      Damage that affects large volumes of skeletal muscle tissue can severely impact health, mobility, and quality-of-life. Efforts to restore muscle function by implanting tissue engineered muscle grafts at the site of damage have demonstrated limited restoration of force production. Various forms of mechanical and biochemical stimulation have been shown to have a potentially beneficial impact on graft maturation, vascularization, and innervation. However, these approaches yield unpredictable and incomplete recovery of functional mobility. Here we show that targeted actuation of implanted grafts, via non-invasive transcutaneous light stimulation of optogenetic engineered muscle, restores motor function to levels similar to healthy mice 2 weeks post-injury. Furthermore, we conduct phosphoproteomic analysis of actuated engineered muscle in vivo and in vitro to show that repeated muscle contraction alters signaling pathways that play key roles in skeletal muscle contractility, adaptation to injury, neurite growth, neuromuscular synapse formation, angiogenesis, and cytoskeletal remodeling. Our study uncovers changes in phosphorylation of several proteins previously unreported in the context of muscle contraction, revealing promising mechanisms for leveraging actuated muscle grafts to restore mobility after volumetric muscle loss.
    Keywords:  Neuromuscular; Phosphoproteomics; Regenerative medicine; Skeletal muscle; Tissue engineering; Volumetric muscle loss
    DOI:  https://doi.org/10.1016/j.biomaterials.2023.122317
  8. Bio Protoc. 2023 Sep 05. 13(17): e4811
    Absolute Quantification of mRNA Isoforms in Adult Stem Cells Using Microfluidic Digital PCR.
    Shubhangi Das Barman, Zofija Frimand, Antoine De Morree.
      Adult stem cells play key roles in homeostasis and tissue repair. These cells are regulated by a tight control of transcriptional programs. For example, muscle stem cells (MuSCs), located beneath the basal lamina, exist in the quiescent state but can transition to an activated, proliferative state upon injury. The control of MuSC state depends on the expression levels of myogenic transcription factors. Recent studies revealed the presence of different mRNA isoforms, with distinct biological regulation. Quantifying the exact expression levels of the mRNA isoforms encoding these myogenic transcription factors is therefore key to understanding how MuSCs switch between cell states. Previously, quantitative real-time polymerase chain reaction (qRT-PCR) has been used to quantify RNA expression levels. However, qRT-PCR depends on large amounts of RNA input and only measures relative abundance. Here, we present a protocol for the absolute quantification of mRNA isoforms using microfluidic digital PCR (mdPCR). Primary MuSCs isolated from individual skeletal muscles (gastrocnemius and masseter) are lysed, and their RNA is reverse-transcribed into cDNA and copied into double-stranded DNA. Following exonuclease I digestion to remove remaining single-stranded DNA, the samples are loaded onto a mdPCR chip with TaqMan probes targeting the mRNA isoforms of interest, whereupon target molecules are amplified in nanoliter chambers. We demonstrate that mdPCR can give exact molecule counts per cell for mRNA isoforms encoding the myogenic transcription factor Pax3. This protocol enables the absolute quantification of low abundant mRNA isoforms in a fast, precise, and reliable way.
    Keywords:  Alternative polyadenylation; Digital PCR; Microfluidics; Muscle stem cells; Pax3; Skeletal muscle; mRNA isoforms
    DOI:  https://doi.org/10.21769/BioProtoc.4811
  9. Cell Metab. 2023 Sep 15. pii: S1550-4131(23)00331-5. [Epub ahead of print]
    Regulatory T cells require IL6 receptor alpha signaling to control skeletal muscle function and regeneration.
    Maike Becker, Sini S Joseph, Francisco Garcia-Carrizo, Robby Z Tom, Daria Opaleva, Isabelle Serr, Matthias H Tschöp, Tim J Schulz, Susanna M Hofmann, Carolin Daniel.
      Muscle-residing regulatory T cells (Tregs) control local tissue integrity and function. However, the molecular interface connecting Treg-based regulation with muscle function and regeneration remains largely unexplored. Here, we show that exercise fosters a stable induction of highly functional muscle-residing Tregs with increased expression of amphiregulin (Areg), EGFR, and ST2. Mechanistically, we find that mice lacking IL6Rα on T cells (TKO) harbor significant reductions in muscle Treg functionality and satellite and fibro-adipogenic progenitor cells, which are required for muscle regeneration. Using exercise and sarcopenia models, IL6Rα TKO mice demonstrate deficits in Tregs, their functional maturation, and a more pronounced decline in muscle mass. Muscle injury models indicate that IL6Rα TKO mice have significant disabilities in muscle regeneration. Treg gain of function restores impaired muscle repair in IL6Rα TKO mice. Of note, pharmacological IL6R blockade in WT mice phenocopies deficits in muscle function identified in IL6Rα TKO mice, thereby highlighting the clinical implications of the findings.
    Keywords:  IL6Ra signaling; exercise; immune tissue crosstalk; immune-metabolic crosstalk; immunometabolism; injury; muscle function; niche-specific Tregs; tissue Tregs; voluntary wheel running
    DOI:  https://doi.org/10.1016/j.cmet.2023.08.010
  10. J Extracell Vesicles. 2023 09;12(9): e12343
    Engineering extracellular vesicles to deliver CRISPR ribonucleoprotein for gene editing.
    Joseph Andrew Whitley, Houjian Cai.
      Clustered regularly interspaced palindromic repeats (CRISPR) is a gene editing tool with tremendous therapeutic potential. Recently, ribonucleoprotein (RNP) complex-based CRISPR systems have gained momentum due to their reduction of off-target editing. This has coincided with the emergence of extracellular vesicles (EVs) as a therapeutic delivery vehicle due to its low immunogenicity and high capacity for manipulation. EVs are cell-derived membranous nanoparticles which mediate the intercellular transfer of molecular components. Current technologies achieve CRISPR RNP encapsulation into EVs through EVs biogenesis, thereby avoiding unnecessary physical, chemical or biological manipulations to the vesicles directly. Herein, we identify sixteen EVs-based CRISPR RNP encapsulation strategies, each with distinct genetic features to encapsulate CRISPR RNP. According to the molecular mechanism facilitating the encapsulation process, there are six strategies of encapsulating Cas9 RNP into virus-like particles based on genetic fusion, seven into EVs based on protein tethering, and three based on sgRNA-coupled encapsulation. Additionally, the incorporation of a targeting moiety to the EVs membrane surface through EVs biogenesis confers tropism and increases delivery efficiency to specific cell types. The targeting moieties include viral envelope proteins, recombinant proteins containing a ligand peptide, single-chain fragment variable (scFv) antibodies, and integrins. However, current strategies still have a number of limitations which prevent their use in clinical trials. Among those, the incorporation of viral proteins for encapsulation of Cas9 RNP have raised issues of biocompatibility due to host immune response. Future studies should focus on genetically engineering the EVs without viral proteins, enhancing EVs delivery specificity, and promoting EVs-based homology directed repair. Nevertheless, the integration of CRISPR RNP encapsulation and tropism technologies will provide strategies for the EVs-based delivery of CRISPR RNP in gene therapy and disease treatment.
    Keywords:  CRISPR; Cas9/sgRNA ribonucleoprotein; delivery; encapsulation; extracellular vesicles; tropism
    DOI:  https://doi.org/10.1002/jev2.12343
  11. Stem Cell Rev Rep. 2023 Sep 18.
    Recent Advances in CRISPR/Cas9 Delivery Approaches for Therapeutic Gene Editing of Stem Cells.
    Malihe Lotfi, Dorsa Morshedi Rad, Samaneh Sharif Mashhadi, Atefeh Ashouri, Majid Mojarrad, Sina Mozaffari-Jovin, Shima Farrokhi, Maryam Hashemi, Marzieh Lotfi, Majid Ebrahimi Warkiani, Mohammad Reza Abbaszadegan.
      Rapid advancement in genome editing technologies has provided new promises for treating neoplasia, cardiovascular, neurodegenerative, and monogenic disorders. Recently, the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system has emerged as a powerful gene editing tool offering advantages, including high editing efficiency and low cost over the conventional approaches. Human pluripotent stem cells (hPSCs), with their great proliferation and differentiation potential into different cell types, have been exploited in stem cell-based therapy. The potential of hPSCs and the capabilities of CRISPR/Cas9 genome editing has been paradigm-shifting in medical genetics for over two decades. Since hPSCs are categorized as hard-to-transfect cells, there is a critical demand to develop an appropriate and effective approach for CRISPR/Cas9 delivery into these cells. This review focuses on various strategies for CRISPR/Cas9 delivery in stem cells.
    Keywords:  CRISPR/Cas9 system; Delivery technologies; Gene editing; Stem cell therapy; Stem cells
    DOI:  https://doi.org/10.1007/s12015-023-10585-3
  12. QJM. 2023 Sep 21. pii: hcad209. [Epub ahead of print]
    Unexpected partial RNA deletion by two different novel COL6A2 mutations leads to Ullrich congenital muscular dystrophy.
    Songchao Xu, Liangshan Li, Lu Shen, Xiaoyu Wang, Wei Feng, Shiguo Liu.
      Congenital muscular dystrophies (CMDs) are a group of rare genetic diseases that primarily affect the muscle and are characterized by progressive degeneration and weakness(1, 2). Ullrich congenital muscular dystrophy (UCMD) is a rare type of autosomal dominant or recessive CMDs, mainly caused by mutations in the related genes leading to loss of collagen VI with an earlier onset time and progressive clinical symptoms(1, 3). We describe a case which presented UCMD caused by novel COL6A2 mutations.
    DOI:  https://doi.org/10.1093/qjmed/hcad209
  13. Nat Chem Biol. 2023 Sep 21.
    Continuous directed evolution of a compact CjCas9 variant with broad PAM compatibility.
    Lukas Schmidheini, Nicolas Mathis, Kim Fabiano Marquart, Tanja Rothgangl, Lucas Kissling, Desirée Böck, Christelle Chanez, Jingrui Priscilla Wang, Martin Jinek, Gerald Schwank.
      CRISPR-Cas9 genome engineering is a powerful technology for correcting genetic diseases. However, the targeting range of Cas9 proteins is limited by their requirement for a protospacer adjacent motif (PAM), and in vivo delivery is challenging due to their large size. Here, we use phage-assisted continuous directed evolution to broaden the PAM compatibility of Campylobacter jejuni Cas9 (CjCas9), the smallest Cas9 ortholog characterized to date. The identified variant, termed evoCjCas9, primarily recognizes N4AH and N5HA PAM sequences, which occur tenfold more frequently in the genome than the canonical N3VRYAC PAM site. Moreover, evoCjCas9 exhibits higher nuclease activity than wild-type CjCas9 on canonical PAMs, with editing rates comparable to commonly used PAM-relaxed SpCas9 variants. Combined with deaminases or reverse transcriptases, evoCjCas9 enables robust base and prime editing, with the small size of evoCjCas9 base editors allowing for tissue-specific installation of A-to-G or C-to-T transition mutations from single adeno-associated virus vector systems.
    DOI:  https://doi.org/10.1038/s41589-023-01427-x
  14. Nat Biotechnol. 2023 Sep 21.
    Precision RNA base editing with engineered and endogenous effectors.
    Laura S Pfeiffer, Thorsten Stafforst.
      RNA base editing refers to the rewriting of genetic information within an intact RNA molecule and serves various functions, such as evasion of the endogenous immune system and regulation of protein function. To achieve this, certain enzymes have been discovered in human cells that catalyze the conversion of one nucleobase into another. This natural process could be exploited to manipulate and recode any base in a target transcript. In contrast to DNA base editing, analogous changes introduced in RNA are not permanent or inheritable but rather allow reversible and doseable effects that appeal to various therapeutic applications. The current practice of RNA base editing involves the deamination of adenosines and cytidines, which are converted to inosines and uridines, respectively. In this Review, we summarize current site-directed RNA base-editing strategies and highlight recent achievements to improve editing efficiency, precision, codon-targeting scope and in vivo delivery into disease-relevant tissues. Besides engineered editing effectors, we focus on strategies to harness endogenous adenosine deaminases acting on RNA (ADAR) enzymes and discuss limitations and future perspectives to apply the tools in basic research and as a therapeutic modality. We expect the field to realize the first RNA base-editing drug soon, likely on a well-defined genetic disease. However, the long-term challenge will be to carve out the sweet spot of the technology where its unique ability is exploited to modulate signaling cues, metabolism or other clinically relevant processes in a safe and doseable manner.
    DOI:  https://doi.org/10.1038/s41587-023-01927-0
  15. NPJ Regen Med. 2023 Sep 19. 8(1): 52
    Endogenous tenocyte activation underlies the regenerative capacity of the adult zebrafish tendon.
    Stephanie L Tsai, Steffany Villaseñor, Rishita R Shah, Jenna L Galloway.
      Tendons are essential, frequently injured connective tissues that transmit forces from muscle to bone. Their unique highly ordered, matrix-rich structure is critical for proper function. While adult mammalian tendons heal after acute injuries, endogenous tendon cells, or tenocytes, fail to respond appropriately, resulting in the formation of disorganized fibrovascular scar tissue with impaired function and increased propensity for re-injury. Here, we show that, unlike mammals, adult zebrafish tenocytes activate upon injury and fully regenerate the tendon. Using a full tear injury model in the adult zebrafish craniofacial tendon, we defined the hallmark stages and cellular basis of tendon regeneration through multiphoton imaging, lineage tracing, and transmission electron microscopy approaches. Remarkably, we observe that zebrafish tendons regenerate and restore normal collagen matrix ultrastructure by 6 months post-injury (mpi). Tendon regeneration progresses in three main phases: inflammation within 24 h post-injury (hpi), cellular proliferation and formation of a cellular bridge between the severed tendon ends at 3-5 days post-injury (dpi), and re-differentiation and matrix remodeling beginning from 5 dpi to 6 mpi. Importantly, we demonstrate that pre-existing tenocytes are the main cellular source of regeneration, proliferating and migrating upon injury to ultimately bridge the tendon ends. Finally, we show that TGF-β signaling is required for tenocyte recruitment and bridge formation. Collectively, our work debuts and aptly positions the adult zebrafish tendon as an invaluable comparative system to elucidate regenerative mechanisms that may inspire new therapeutic strategies.
    DOI:  https://doi.org/10.1038/s41536-023-00328-w
  16. Cell Death Dis. 2023 09 16. 14(9): 609
    Blocking insulin-like growth factor 1 receptor signaling pathway inhibits neuromuscular junction regeneration after botulinum toxin-A treatment.
    Hiroki Ishihara, Yoshinori Otani, Kazuki Tanaka, Hisao Miyajima, Huy Xuan Ngo, Masashi Fujitani.
      Botulinum toxin-A (BTX) administration into muscle is an established treatment for conditions with excessive muscle contraction. However, botulinum therapy has short-term effectiveness, and high-dose injection of BTX could induce neutralizing antibodies against BTX. Therefore, prolonging its effects could be beneficial in a clinical situation. Insulin-like growth factor-1 receptor (IGF1R) and its ligands, insulin-like growth factor (IGF) -I and II, regulate the physiological and pathological processes of the nervous system. It has been suggested that IGF1R is involved in the process after BTX administration, but the specific regeneration mechanism remains unclear. Therefore, this study aimed to determine how inhibition of IGF1R signaling pathway affects BTX-induced muscle paralysis. The results showed that anti-IGF1R antibody administration inhibited the recovery from BTX-induced neurogenic paralysis, and the synaptic components at the neuromuscular junction (NMJ), mainly post-synaptic components, were significantly affected by the antibody. In addition, the wet weight or frequency distribution of the cross-sectional area of the muscle fibers was regulated by IGF1R, and sequential antibody administration following BTX treatment increased the number of Pax7+-satellite cells in the gastrocnemius (GC) muscle, independent of NMJ recovery. Moreover, BTX treatment upregulated mammalian target of rapamycin (mTOR)/S6 kinase signaling pathway, HDAC4, Myog, Fbxo32/MAFbx/Atrogin-1 pathway, and transcription of synaptic components, but not autophagy. Finally, IGF1R inhibition affected only mTOR/S6 kinase translational signaling in the GC muscle. In conclusion, the IGF1R signaling pathway is critical for NMJ regeneration via specific translational signals. IGF1R inhibition could be highly beneficial in clinical practice by decreasing the number of injections and total dose of BTX due to the prolonged duration of the effect.
    DOI:  https://doi.org/10.1038/s41419-023-06128-w
  17. J Endocrinol. 2023 Sep 01. pii: JOE-23-0210. [Epub ahead of print]
    Loss of Trim28 in muscle alters mitochondrial signalling but not systemic metabolism.
    Emily J King, Simon T Bond, Christine Yang, Yingying Liu, Anna C Calkin, Darren C Henstridge, Brian G Drew.
      Type 2 diabetes mellitus (T2DM), a condition characterised by insulin resistance (IR) and skeletal muscle mitochondrial abnormalities, is a leading cause of death in developed societies. Much work has postulated that improving pathways linked to mitochondrial health, including autophagy, may be a potential avenue to prevent or treat T2DM. Given recent data indicating a role for Tripartite motif-containing 28 (TRIM28) in autophagy and mitochondrial pathways, we investigated whether muscle specific deletion of TRIM28 might impact on obesity, glucose tolerance and IR in mice. We studied two different muscle-specific (MCK-cre and ACTA1-cre-ERT2) TRIM28 knock out (KO) models, which were phenotyped during and after being fed a chow or high fat diet (HFD). Whilst muscle specific deletion of TRIM28 in both models demonstrated alterations in markers of mitochondrial activity and autophagy in skeletal muscle, we did not observe major impacts on the majority of metabolic measures in these mice. Specifically, we demonstrate that deletion of TRIM28 in skeletal muscle of mice during (MCK-cre) or post-development (ACTA1-cre-ERT2) does not prevent HFD-induced obesity or glucose intolerance. These findings are consistent with those reported previously in relation to autophagy and mitochondria in other cell types, and thus warrant further study into the biological role Trim28 has in relation to mitochondrial function.
    DOI:  https://doi.org/10.1530/JOE-23-0210
  18. JCI Insight. 2023 09 22. pii: e171878. [Epub ahead of print]8(18):
    Muscle: an independent contributor to the neuromuscular spinal muscular atrophy disease phenotype.
    Narendra N Jha, Jeong-Ki Kim, Yoon-Ra Her, Umrao R Monani.
      Spinal muscular atrophy (SMA) is a pediatric-onset neuromuscular disorder caused by insufficient survival motor neuron (SMN) protein. SMN restorative therapies are now approved for the treatment of SMA; however, they are not curative, likely due to a combination of imperfect treatment timing, inadequate SMN augmentation, and failure to optimally target relevant organs. Here, we consider the implications of imperfect treatment administration, focusing specifically on outcomes for skeletal muscle. We examine the evidence that muscle plays a contributing role in driving neuromuscular dysfunction in SMA. Next, we discuss how SMN might regulate the health of myofibers and their progenitors. Finally, we speculate on therapeutic outcomes of failing to raise muscle SMN to healthful levels and present strategies to restore function to this tissue to ensure better treatment results.
    DOI:  https://doi.org/10.1172/jci.insight.171878
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