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Mitogen-activated human mononuclear cells suppress calcium binding by sarcoplasmic reticulum
Abstract:
Weakness in inflammatory myopathy is commonly more prominent than is muscle fiber necrosis, suggesting additional explanations for its origin. Recent studies reveal that cell-mediated immune mechanisms are active in PMS-DMS. In experiments reported here, the effect of human mononuclear cell-derived mediator-rich supernatant upon calcium accumulation by SR membranes was examined. SR, by triggered binding and release of calcium in the vicinity of myofibrils, plays a key role in regulation of skeletal muscle contraction and relaxation. SR was derived from Lewis and Sprague-Dawley rat pelvic girdle muscles. Mediator-rich supernatants were generated by exposure of Hypaque-Ficoll-purified human mononuclear cells to PHA-P and Con-A during 72 hr of incubation in RPMl 1640 medium containing 10% autologous serum. Mitogens alone were without effect on SR. Suppression of ATP-dependent calcium accumulation was demonstrable within 40 min of exposure of SR to mediator-rich supernatants. The factor(s) producing suppression of calcium binding was nondialyzable, stable on storage at -70 degrees C, and active upon SR at physiologic calcium-loading levels. These findings may have important implications for understanding of the mechanism of weakness in inflammatory myopathies.
Insights
Mediator-rich supernatants from human immune cells impair muscle calcium regulation. This finding may explain weakness in inflammatory myopathies, where muscle fiber damage is less prominent than functional deficits.
Area of Science:
- Immunology
- Muscle Physiology
- Cell Biology
Background:
- Muscle weakness in inflammatory myopathies often exceeds observed muscle fiber necrosis.
- Cell-mediated immune mechanisms are implicated in polymyositis and dermatomyositis (PMS-DMS).
- Sarcoplasmic reticulum (SR) regulates skeletal muscle contraction via calcium binding and release.
Purpose of the Study:
- To investigate the effect of human mononuclear cell-derived mediators on calcium accumulation by SR membranes.
- To explore potential mechanisms underlying muscle weakness in inflammatory myopathies.
Main Methods:
- SR membranes were isolated from rat pelvic girdle muscles (Lewis and Sprague-Dawley).
- Mediator-rich supernatants were generated by incubating human mononuclear cells with mitogens (PHA-P, Con-A).
- The impact of these supernatants on ATP-dependent calcium accumulation by SR was assessed.
Main Results:
- Mediator-rich supernatants significantly suppressed ATP-dependent calcium accumulation by SR within 40 minutes.
- Mitogens alone did not affect SR calcium accumulation.
- The suppressive factor was nondialyzable, stable at -70°C, and effective at physiological calcium levels.
Conclusions:
- Human mononuclear cell mediators can impair sarcoplasmic reticulum calcium handling.
- This impairment of calcium regulation may contribute to muscle weakness observed in inflammatory myopathies.
- Further research into these mediators could reveal novel therapeutic targets.