Victor Forge
In muscle physiology, the term PCSA refers to the physiological cross-sectional area, which is the area of the cross-section of a muscle perpendicular to its fibres, usually at its largest point. It is used to describe the contraction properties of pennate muscles and is calculated using the formula developed in 1975 by Alexander and Vernon. PCSA is not the same as ACSA (anatomical cross-sectional area), which is the area of the cross-section of a muscle perpendicular to its longitudinal axis.
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What You'll Learn
- Sca-1 influences the innate immune response during skeletal muscle regeneration
- Transplantation of skeletal muscle-derived Sca-1+/PW1+/Pax7- interstitial cells (PICs) improves cardiac function
- Sca-1 negatively regulates proliferation and differentiation of muscle cells
- The role of supporting cell populations in satellite cell-mediated muscle repair
- Sca-1 is required for the self-renewal of hematopoietic and mesenchymal stem cells
Sca-1 influences the innate immune response during skeletal muscle regeneration
Skeletal muscle regeneration is a complex process that involves the interplay of various biological mechanisms. One critical aspect of this process is the role played by the innate immune response, which acts to clear necrotic and damaged tissue from the injured muscle. This initial clearance sets the stage for subsequent regenerative processes to occur effectively.
Sca-1, or Stem Cell Antigen-1, is a molecule that has been identified as having a significant influence on the innate immune response during skeletal muscle regeneration. Sca-1 is highly expressed in multiple populations of the immune system and plays a crucial role in immune cell function. When Sca-1 is deficient or absent (as in Sca-1−/− mice models), the innate immune response is impaired, leading to inefficient muscle regeneration.
One of the key ways in which Sca-1 influences the innate immune response during muscle regeneration is by affecting the recruitment of IgM and complement C3 to the injured muscle. IgM is a natural antibody that plays a critical role in the clearance of necrotic and apoptotic cells. In the context of muscle regeneration, IgM binds to autoantigens presented upon tissue damage, facilitating the removal of damaged tissue and promoting the regeneration process. However, in Sca-1−/− mice, there is a defect in the recruitment of IgM to regenerating muscle, leading to decreased phagocytosis by macrophages and contributing to a fibrotic phenotype.
Additionally, Sca-1−/− mice display significantly decreased peritoneal B-1a cell numbers. B-1a cells are a subset of self-renewing B cells that produce IgM. The reduction in B-1a cell numbers further contributes to the impaired innate immune response in Sca-1 deficient mice. Furthermore, these mice fail to mount a normal response to ischemia-reperfusion injury, which is consistent with a defect in the innate immune-mediated recognition of necrotic tissue damage.
The collective evidence suggests that Sca-1 plays a novel and important role in modulating the innate immune response during skeletal muscle regeneration. By influencing the recruitment of IgM, complement C3, and B-1a cells, Sca-1 contributes to the efficient clearance of damaged tissue and subsequent regeneration. A better understanding of the immuno-myogenic processes involving Sca-1 will help promote and enhance muscle regeneration, particularly in the context of muscle injuries and diseases.
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Transplantation of skeletal muscle-derived Sca-1+/PW1+/Pax7- interstitial cells (PICs) improves cardiac function
Sca-1 is a glycosyl phosphatidylinositol-anchored cell surface protein that has been widely used as a marker to isolate hematopoietic stem cells. It is highly expressed in multiple populations of the immune system and plays a critical role in the innate immune response during skeletal muscle regeneration. Sca-1 is required for the self-renewal of hematopoietic and mesenchymal stem cells, as well as for T cell development and function.
In a study by Ruchaya et al., the transplantation of skeletal muscle-derived Sca-1+/PW1+/Pax7- interstitial cells (PICs) was found to improve cardiac function and attenuate remodeling in mice subjected to myocardial infarction. Myocardial infarction (MI) is characterised by a lack of blood flow to the heart, resulting in significant cell death and a reduction in cardiomyocytes. While advances in early diagnosis and intervention have increased the survival rate of MI sufferers, therapeutic treatments only prevent further damage to the myocardium rather than encouraging cardiac repair and regeneration.
The study found that PICs, which can be easily sourced from skeletal muscle, have significant reparative potential. When transplanted into the myocardially infarcted mouse heart, PICs engrafted and significantly improved cardiac function and remodelling. This improvement was observed as early as one week post-transplantation, with treated mice showing a higher EF compared to untreated MI mice.
The findings suggest that PIC transplantation acts through an indirect paracrine mechanism, stimulating endogenous repair processes, including angiogenesis. PIC transplantation also increased the number of proliferating BrdU+ cells, of which a proportion were positive for cardiomyocyte, smooth muscle, and endothelial cell markers. These results support the hypothesis that skeletal muscle-derived Sca-1+ PICs have beneficial effects in improving cardiac function and reducing cardiac remodelling after intramyocardial transplantation in mice post-MI.
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Sca-1 negatively regulates proliferation and differentiation of muscle cells
Sca-1, or stem cell antigen-1, is a protein that is a member of the Ly-6 family and is involved in the regulation of cell proliferation, differentiation, and self-renewal in multiple tissues. It is highly expressed in various immune system populations, and the innate immune response it elicits plays a critical role in normal muscle regeneration. Sca-1 is necessary for the self-renewal of hematopoietic and mesenchymal stem cells and T cell development and function.
In skeletal muscle, Sca-1 inhibits both proliferation and differentiation of myogenic cells, and its expression is dynamically regulated during muscle regeneration. Mice lacking Sca-1 exhibit increased fibrosis following muscle injury. Studies have shown that Sca-1 expression is negatively regulated by TGF-β1, and this inhibition is Smad3-dependent. TGF-β1 represses Sca-1 expression in T cells and immune cell populations derived from the spleen.
The role of Sca-1 in muscle regeneration is further supported by evidence from Sca-1−/− mice, which display inefficient muscle regeneration and increased fibrotic indices post-injury. Additionally, loss of Sca-1 in a mouse model of muscular dystrophy (mdx) exacerbates fibrosis. These findings demonstrate that Sca-1 plays a regulatory role in fibrosis during acute and chronic muscle injuries.
Furthermore, Sca-1 influences the innate immune response during skeletal muscle regeneration. Loss of Sca-1 affects the recruitment of IgM and complement C3 to injured muscle, and Sca-1−/− mice exhibit decreased peritoneal B-1a cell numbers. These observations suggest a novel role for Sca-1 in modulating B-1a cell numbers and IgM and complement deposition in regenerating tissue.
In summary, Sca-1 negatively regulates proliferation and differentiation in muscle cells, particularly myogenic cells, through mechanisms involving TGF-β1 signalling and modulation of the innate immune response during muscle regeneration.
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The role of supporting cell populations in satellite cell-mediated muscle repair
Skeletal muscle is a flexible organ capable of repair and remodelling in response to damage. It relies on local stem cell populations for support, with satellite cells being the primary stem cell population that contributes myonuclei. These cells reside between the basal lamina and sarcolemma of myofibres in a state of dormancy or quiescence. Upon activation, satellite cells can enter the myogenic program, proliferate, and donate myonuclei to a damaged myofibre or replenish the pool of satellite cells.
The efficient repair and regeneration of skeletal muscle is dependent on the continuous crosstalk between various cell populations. For instance, muscle-resident mast cells are involved in the initial inflammatory response by mediating neutrophil recruitment to the site of injury. Mast cells respond to injury in the first few hours, secreting TNF-α at low concentrations that stimulate satellite cell proliferation.
Macrophages are another important supporting cell population. Following the phagocytosis of cellular debris, macrophages transition from a pro- to an anti-inflammatory phenotype. Anti-inflammatory macrophages limit the inflammatory reaction to muscle damage and trigger the latter half of the muscle repair process by secreting specific cytokines. Pro-inflammatory macrophages are a source of IGF-1, which promotes repair by stimulating satellite cell activation, proliferation, and differentiation.
Fibro/adipogenic progenitors (FAPs) are also involved in the muscle repair process. Transplanting FAPs from young muscles into old muscles restored histone deacetylase inhibitor (HDACi) function, promoting differentiation and fusion of myoblasts. Follistatin, which is upregulated within satellite cells, mediates the interaction between FAPs and satellite cells and is known to promote satellite cell fusion.
Additionally, endothelial cells aid in muscle repair by promoting immune-cell-mediated angiogenesis. Angiotensin II, a pro-angiogenic factor released by myoblasts, increases myotube length and branching in vitro. It is also a regulator of satellite cell dynamics.
In summary, the repair and regeneration of skeletal muscle involve intricate interactions between satellite cells and various supporting cell populations, including mast cells, macrophages, FAPs, and endothelial cells. These cells communicate with each other to regulate complex processes such as myogenesis, inflammation, angiogenesis, and fibrosis, ultimately contributing to the effective repair and regeneration of skeletal muscle.
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Sca-1 is required for the self-renewal of hematopoietic and mesenchymal stem cells
Sca-1, or stem cell antigen 1, is a glycosyl phosphatidylinositol-anchored cell surface protein. It is expressed extensively in immune cells and plays a critical role in the innate immune response during skeletal muscle regeneration. Sca-1 has been detected in multiple tissues, including cells of hematopoietic lineages, the mammary gland, liver, prostate, heart, and skeletal muscle.
Sca-1 is essential for the self-renewal of hematopoietic stem cells and mesenchymal stem cells. Hematopoietic stem cells (HSCs) are multi-potent and self-renewing, giving rise to hematopoietic progenitor cell (HPC) populations with gradually narrowing differentiation and self-renewal potential. Sca-1 is also expressed on a subset of bone marrow stromal cells, which include mesenchymal stem cells. Longitudinal studies have shown that Sca-1 deficiency leads to normal bone development but with age, these mice exhibit dramatically decreased bone mass, resulting in brittle bones. This suggests that Sca-1 is necessary for the self-renewal of mesenchymal progenitors.
The role of Sca-1 in hematopoietic stem cell self-renewal has been studied by transplanting Sca-1−/− bone marrow cells into wild-type recipient mice. While self-renewal of Sca-1−/− HSC appeared to be normal, lineage skewing was observed in B cells, NK cells, and granulocytes/macrophages derived from Sca-1−/− HSC. Overexpression of Sca-1 in mouse and human stem/progenitor cells resulted in decreased in vitro myeloid activity.
The loss of Sca-1 in mouse models has been shown to affect the recruitment of IgM and complement C3 to injured muscle, leading to inefficient muscle regeneration and increased fibrosis. Sca-1−/− mice also display significantly decreased peritoneal B-1a cell numbers and fail to mount a normal response to ischemia-reperfusion injury. These observations reveal a novel role for Sca-1 in modulating B-1a cell number and B-1a-driven deposition of IgM and complement in regenerating tissue.
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