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1. Lipin1 plays complementary roles in myofibre stability and regeneration in dystrophic muscles

   https://doi.org/10.1113/JP284085  

   Jama, Abdulrahman ; Alshudukhi, Abdullah A. ; Burke, Steve ; Dong, Lixin ; Kamau, John Karanja ; Voss, Andrew Alvin ; Ren, Hongmei ( February 2023 , The Journal of Physiology)

   Duchenne muscular dystrophy (DMD) is a severe muscle wasting disorder caused by dystrophin mutations, leading to the loss of sarcolemmal integrity, and resulting in progressive myofibre necrosis and impaired muscle function. Our previous studies suggest that lipin1 is important for skeletal muscle regeneration and myofibre integrity. Additionally, we discovered that mRNA expression levels of lipin1 were significantly reduced in skeletal muscle of DMD patients and the mdx mouse model. To understand the role of lipin1 in dystrophic muscle, we generated dystrophin/lipin1 double knockout (DKO) mice, and compared the limb muscle pathology and function of wild‐type B10, muscle‐specific lipin1 deficient (lipin1^Myf5cKO), mdx and DKO mice. We found that further knockout of lipin1 in dystrophic muscle exhibited a more severe phenotype characterized by increased necroptosis, fibrosis and exacerbated membrane damage in DKO compared to mdx mice. In barium chloride‐induced muscle injury, both lipin1^Myf5cKOand DKO showed prolonged regeneration at day 14 post‐injection, suggesting that lipin1 is critical for muscle regeneration.In situcontractile function assays showed that lipin1 deficiency in dystrophic muscle led to reduced specific force production. Using a cell culture system, we found that lipin1 deficiency led to elevated expression levels of necroptotic markers and medium creatine kinase, which could be a result of sarcolemmal damage. Most importantly, restoration of lipin1 inhibited the elevation of necroptotic markers in differentiated primary lipin1‐deficient myoblasts. Overall, our data suggests that lipin1 plays complementary roles in myofibre stability and muscle function in dystrophic muscles, and overexpression of lipin1 may serve as a potential therapeutic strategy for dystrophic muscles.image

   We identified that lipin1 mRNA expression levels are significantly reduced in skeletal muscles of Duchenne muscular dystrophy patients and mdx mice.

   We found that further depletion of lipin1 in skeletal muscles of mdx mice induces more severe dystrophic phenotypes, including enhanced myofibre sarcolemma damage, muscle necroptosis, inflammation, fibrosis and reduced specific force production.

   Lipin1 deficiency leads to elevated expression levels of necroptotic markers, whereas restoration of lipin1 inhibits their expression.

   Our results suggest that lipin1 is functionally complementary to dystrophin in muscle membrane integrity and muscle regeneration.

    

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2. Skeletal Muscle Constructs Engineered from Human Embryonic Stem Cell Derived Myogenic Progenitors Exhibit Enhanced Contractile Forces When Differentiated in a Medium Containing EGM‐2 Supplements

   https://doi.org/10.1002/adbi.201900005  

   Xu, Bin ; Zhang, Mengen ; Perlingeiro, Rita C. R. ; Shen, Wei ( November 2019 , Advanced Biosystems)

   Three‐dimensional (3D) skeletal muscle constructs engineered from myogenic progenitors derived from human pluripotent stem cells (hPSCs) have a wide range of applications, but to date, such constructs generate lower specific tetanic force than adult human muscles. Methods enhancing functional muscle differentiation and force generation of these constructs are highly desirable. The finding of this study is that addition of the supplements in the endothelial cell growth medium‐2 (EGM‐2) to the myogenic differentiation medium can substantially enhance contractile force generation. For constructs differentiated for 4 weeks, addition of the EGM‐2 supplements in the first 2 weeks leads to tenfold and sevenfold increases in twitch and tetanic forces, respectively. The specific tetanic force generated by these constructs is 33 mN mm⁻², which is significantly higher than previously reported. These constructs show wider myotubes and higher gene expression levels for all skeletal muscle‐specific myosin heavy chain isoforms, suggesting that a more mature differentiation stage of the cells underlies the greater contractile force generation. The constructs exposed to these supplements for 4 weeks do not generate as high contractile forces, suggesting that prolonged treatment is not beneficial. These results suggest that temporal conditioning with the EGM‐2 supplements assists functional development of hPSC‐derived skeletal muscle constructs.

    

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3. Crosstalk between ERK and MRTF‐A signaling regulates TGFβ1‐induced epithelial‐mesenchymal transition

   https://doi.org/10.1002/jcp.30705  

   Nalluri, Sandeep M. ; Sankhe, Chinmay S. ; O'Connor, Joseph W. ; Blanchard, Paul L. ; Khouri, Joelle N. ; Phan, Steven H. ; Virgi, Gage ; Gomez, Esther W. ( February 2022 , Journal of Cellular Physiology)

   Epithelial‐mesenchymal transition (EMT) is a physiological process that is essential during embryogenesis and wound healing and also contributes to pathologies including fibrosis and cancer. EMT is characterized by marked gene expression changes, loss of cell–cell contacts, remodeling of the cytoskeleton, and acquisition of enhanced motility. In the late stages of EMT, cells can exhibit myofibroblast‐like properties with enhanced expression of the mesenchymal protein marker α‐smooth muscle actin and contractile activity. Transforming growth factor (TGF)‐β1 is a well‐known inducer of EMT and it activates a plethora of signaling cascades including extracellular signal‐regulated kinase (ERK). Previous reports have demonstrated a role for ERK signaling in the early stages of EMT, but the molecular impacts of ERK signaling on the late stages of EMT are still unknown. Here, we found that inhibition of the phosphorylation of ERK enhances focal adhesions, stress fiber formation, cell contractility, and gene expression changes associated with TGFβ1‐induced EMT in mammary epithelial cells. These effects are mediated in part by the phosphorylation state and subcellular localization of myocardin‐related transcription factor‐A. These findings indicate that the intricate crosstalk between signaling cascades plays an important role in regulating the progression of EMT and suggests new approaches to control EMT processes.

    

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4. Direct intraoperative measurement of isometric contractile properties in living human muscle

   https://doi.org/10.1113/JP284092  

   Binder‐Markey, Benjamin I. ; Persad, Lomas S. ; Shin, Alexander Y. ; Litchy, William J. ; Kaufman, Kenton R. ; Lieber, Richard L. ( April 2023 , The Journal of Physiology)

   Skeletal muscle's isometric contractile properties are one of the classic structure–function relationships in all of biology allowing for extrapolation of single fibre mechanical properties to whole muscle properties based on the muscle's optimal fibre length and physiological cross‐sectional area (PCSA). However, this relationship has only been validated in small animals and then extrapolated to human muscles, which are much larger in terms of length and PCSA. The present study aimed to measure directly thein situproperties and function of the human gracilis muscle to validate this relationship. We leveraged a unique surgical technique in which a human gracilis muscle is transferred from the thigh to the arm, restoring elbow flexion after brachial plexus injury. During this surgery, we directly measured subject specific gracilis muscle force–length relationshipin situand propertiesex vivo. Each subject's optimal fibre length was calculated from their muscle's length‐tension properties. Each subject's PCSA was calculated from their muscle volume and optimal fibre length. From these experimental data, we established a human muscle fibre‐specific tension of 171 kPa. We also determined that average gracilis optimal fibre length is 12.9 cm. Using this subject‐specific fibre length, we observed an excellent fit between experimental and theorical active length‐tension curves. However, these fibre lengths were about half of the previously reported optimal fascicle lengths of 23 cm. Thus, the long gracilis muscle appears to be composed of relatively short fibres acting in parallel that may not have been appreciated based on traditional anatomical methods.image

   Skeletal muscle's isometric contractile properties represent one of the classic structure–function relationships in all of biology and allow scaling single fibre mechanical properties to whole muscle properties based on the muscle's architecture.

   This physiological relationship has only been validated in small animals but is often extrapolated to human muscles, which are orders of magnitude larger.

   We leverage a unique surgical technique in which a human gracilis muscle is transplanted from the thigh to the arm to restore elbow flexion after brachial plexus injury, aiming to directly measure muscles propertiesin situand test directly the architectural scaling predictions.

   Using these direct measurements, we establish human muscle fibre‐specific tension of ∼170 kPa.

   Furthermore, we show that the gracilis muscle actually functions as a muscle with relatively short fibres acting in parallelvs. long fibres as previously assumed based on traditional anatomical models.

    

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5. Skeletal muscle: A review of molecular structure and function, in health and disease

   https://doi.org/10.1002/wsbm.1462  

   Mukund, Kavitha ; Subramaniam, Shankar ( August 2019 , WIREs Systems Biology and Medicine)

   Decades of research in skeletal muscle physiology have provided multiscale insights into the structural and functional complexity of this important anatomical tissue, designed to accomplish the task of generating contraction, force and movement. Skeletal muscle can be viewed as a biomechanical device with various interacting components including the autonomic nerves for impulse transmission, vasculature for efficient oxygenation, and embedded regulatory and metabolic machinery for maintaining cellular homeostasis. The “omics” revolution has propelled a new era in muscle research, allowing us to discern minute details of molecular cross‐talk required for effective coordination between the myriad interacting components for efficient muscle function. The objective of this review is to provide a systems‐level, comprehensive mapping the molecular mechanisms underlying skeletal muscle structure and function, in health and disease. We begin this review with a focus on molecular mechanisms underlying muscle tissue development (myogenesis), with an emphasis on satellite cells and muscle regeneration. We next review the molecular structure and mechanisms underlying the many structural components of the muscle: neuromuscular junction, sarcomere, cytoskeleton, extracellular matrix, and vasculature surrounding muscle. We highlight aberrant molecular mechanisms and their possible clinical or pathophysiological relevance. We particularly emphasize the impact of environmental stressors (inflammation and oxidative stress) in contributing to muscle pathophysiology including atrophy, hypertrophy, and fibrosis.

   This article is categorized under:

   Physiology > Mammalian Physiology in Health and Disease

   Developmental Biology > Developmental Processes in Health and Disease

   Models of Systems Properties and Processes > Cellular Models

    

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