Rustan 2017 MiP2017
Rustan AC (2017)
With respect to energy metabolism cultured myotubes are glycolytic, show a low mitochondrial oxidative activity and fuel preference for carbohydrate over lipids. The glucose transporter GLUT1:GLUT4 ratio is higher in cultured myotubes compared to adult skeletal muscle resulting in lower insulin responsiveness on glucose transport. However, despite the reduced insulin-responsiveness, the mechanisms involved in glucose uptake in vivo are conserved in vitro.
With respect to fiber type comparison myotubes express mostly fast myosin heavy chain (MyHC). However, studies have shown that human satellite cells isolated from either fast or slow muscle fibers form myotubes in vitro which co-express both fast and slow fibers independently of the fiber type from which they originated. Slow MyHC can be increased by remodeling of myotubes i.e. electrical pulse stimulation (EPS) also showing a plasticity potential of these cells.
Limitations with the myotube cell model are a lack of the in vivo microenvironment, but this can be improved using co-cultures (e.g. adipocytes and motor neurons). There are also differences in gene expression compared to in vivo (mostly decreased). There are low expressions of many genes important in lipid metabolism and mitochondrial function. Compared to skeletal muscles in vivo cultured myotubes show a lack of fiber maturation, are more quiescent (but may contract spontaneously) and undergo senescence. It should be noted that the ability of the myoblast to fuse and differentiate into myotubes and metabolic processes can gradually become impaired with increasing passage number.
One important feature is that the diabetic phenotype is conserved in myotubes established from type 2 diabetic (T2D) subjects with respect to glucose metabolism. A decreased insulin-stimulated glucose transport and glycogen synthesis, and glycogen synthase activity has been observed compared to cells from healthy donors. Moreover, T2D myotubes also have impaired fatty acid handling demonstrated as a decreased complete fatty acid oxidation and a lower adaptability to enhance lipid oxidation with increased fatty acid availability. Recently, it was shown reduced capacity for lipid storage and defects in lipolysis in myotubes from T2D subjects. The ability of skeletal muscles to switch between lipid and glucose oxidation (metabolic flexibility) also appears to be an intrinsic characteristic, as it was retained in vitro. The precise mechanisms by which myotubes are able to retain the in vivo characteristics are not known. However, a combination of genetic and epigenetic mechanisms are probably involved. Finally, it has been demonstrated correlations between different in vivo metabolic parameters, such as respiratory quotient dynamics, insulin sensitivity, O2 consumption, body fat mass, plasma free fatty acids, and fatty acid oxidation measured in primary cells from the respective donor.
Taken together, human myotubes seem to be a valuable model for the study of skeletal muscle glucose and lipid metabolism in vitro under normal as well as disease conditions. Some limitations have been noted in the differentiation status (fiber type expression) of the cells and energy metabolism, but these can be improved by proper treatment, such as chronic low-frequency EPS to mimic endurance exercise. In vitro vs. in vivo translatability should be further studied, since epigenetic mechanisms are possibly translated to cultured myotubes.
• Bioblast editor: Kandolf G
Labels: Pathology: Diabetes
Organism: Human Tissue;cell: Skeletal muscle
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