Accumulation of CK-MM is impaired in innervated and contracting cultured muscle fibers of Duchenne muscular dystrophy patients

Generation of human muscle fibers and satellite-like cells from human pluripotent stem cells in vitro

Progress toward finding a cure for muscle diseases has been slow because of the absence of relevant cellular models and the lack of a reliable source of muscle progenitors for biomedical investigation. Here we report an optimized serum-free differentiation protocol to efficiently produce striated, millimeter-long muscle fibers together with satellite-like cells from human pluripotent stem cells (hPSCs) in vitro. By mimicking key signaling events leading to muscle formation in the embryo, in particular the dual modulation of Wnt and bone morphogenetic protein (BMP) pathway signaling, this directed differentiation protocol avoids the requirement for genetic modifications or cell sorting. Robust myogenesis can be achieved in vitro within 1 month by personnel experienced in hPSC culture. The differentiating culture can be subcultured to produce large amounts of myogenic progenitors amenable to numerous downstream applications. Beyond the study of myogenesis, this differentiation method offers an attractive platform for the development of relevant in vitro models of muscle dystrophies and drug screening strategies, as well as providing a source of cells for tissue engineering and cell therapy approaches.

Ultrastructural distribution of the M form of creatine phosphokinase in human muscle by immunogold labeling

Creatine phosphokinase regenerates ATP from ADP using creatine phosphate. Isoenzymes of creatine phosphokinase are bound to certain cellular structures or are compartmentalized in areas of the cell, and this has been used as a basis for defining the role of these isoenzymes in energy metabolism. The M isoenzyme of creatine phosphokinase has been morphologically associated with the M-line of striated muscle in many species. In this present study the ultrastructural distribution and the relative concentration of the M form of creatine phosphokinase in human muscle tissue was determined using immunogold and electron microscopy. The M-line of the sarcomere, comprising only 3-4% of the sarcomere area, was found to contain over 20% of the total M isoenzyme signal of the entire sarcomere. This technique represents a quantitative, ultrastructural method to study the subcellular distribution of this isoenzyme. These data suggest that localized concentrations of M-CPK may be important for normal energy metabolism, and may also serve as a foundation for a better understanding of the relationship between abnormal creatine metabolism and the pathogenesis of neuromuscular disease.

Effect of various agents on the cytoplasmic calcium concentration in cultured human muscle cells

1. We determined the cytoplasmic Ca2+ concentration ([Ca2+]i) in cultured human muscle cells using the fluorescent indicator Quin-2. 2. The [Ca2+]i was dependent on the external Ca2+ concentration. Acetylcholine in the presence of external Ca2+ caused a transient increase in [Ca2+]i. Inhibition by nifedipine indicated that this response was mediated through activated voltage-operated channels. In nominally Ca2(+)-free buffer acetylcholine did not markedly increase [Ca2+]i. Therefore, the increase in [Ca2+]i as a response to depolarization is mainly due to influx of external Ca2+. 3. Various concentrations of caffeine did not influence the [Ca2+]i. Dantrolene decreased [Ca2+]i, both in the presence and absence of external Ca2+. The reduction probably resulted from an action of dantrolene on the intracellular Ca2+ stores, since dantrolene did not influence 45Ca2+ influx or efflux and caffeine partially counteracted the reduction.

Paralysis of innervated cultured human muscle fibers affects enzymes differentially

Increased accumulation of muscle-specific isozyme (MSI) of creatine kinase (CK), lactate dehydrogenase (LDH), glycogen phosphorylase (GP), and phosphoglycerate mutase (PGAM) occurs with development and indicates muscle fiber maturation. The expression of MSIs of those four enzymes is greatly enhanced in innervated-contracting as compared to noninnervated and noncontracting cultured human muscle fibers. We have now studied the effect of contractile activity on developmental accumulation of MSIs in innervated-contracting, innervated-paralyzed (2 microM tetrodotoxin for 30 days), and noninnervated-noncontracting cultured human muscle fibers. Muscle acetylcholinesterase (AChE) and total enzyme activities were also studied under the same conditions. We observed a different dependency on contractile activity between total enzymatic activities of CK, LDH, and AChE, which were substantially reduced after paralysis, and GP and PGAM, which were unchanged. The expression of MSIs of CK, GP, PGAM, and LDH was always significantly increased in innervated as compared to noninnervated fibers. While the expression of MSIs of GP and PGAM was the same in contracting-innervated and paralyzed-innervated muscle fibers, the expression of MSIs of CK and LDH in paralyzed-innervated muscle fibers was very slightly decreased as compared to their contracting-innervated controls. Our studies demonstrate that in human muscle: (1) total enzymatic activities and the expression of MSIs of GP and PGAM are regulated by neuronal effect(s); (2) total enzymatic activities of CK, LDH, and AChE depend mainly on muscle contractile activity; and (3) MSIs of CK and LDH are regulated predominantly by neuronal factors and to a much lesser degree by muscle contractile activity.