Activation of skeletal muscle calpain-3 by eccentric exercise in humans does not result in its translocation to the nucleus or cytosol

Murphy RM, Vissing K, Latchman H, Lamboley C, McKenna MJ, Overgaard K, Lamb GD. Activation of skeletal muscle calpain-3 by eccentric exercise in humans does not result in its translocation to the nucleus or cytosol. J Appl Physiol 111: 1448-1458, 2011. First published August 11, 2011; doi: 10.1152/japplphysiol.00441.2011.-The skeletal muscle-specific calpain-3 protease is likely involved in muscle repair, although the mechanism is not known. Physiological activation of calpain-3 occurs 24 h following eccentric exercise in humans. Functional consequences of calpain-3 activation are not known; however, calpain-3 has been suggested to be involved in nuclear signaling via NF-kappa B. To test this and help identify how/where calpain-3 acts, we investigated whether calpain-3 autolysis (hence, activation) following eccentric exercise results in translocation from its normal myofibrillar location to the nucleus or the cytosol. In resting human skeletal muscle, the majority (87%) of calpain-3 was present in myofibrillar fractions, with only a small proportion (<10%) in an autolyzed state. Enriched nuclear fractions contained similar to 8% of the total calpain-3, which was present in a predominantly (>80%) autolyzed state. Using freshly dissected human muscle fibers to identify freely diffusible proteins, we showed that only similar to 5% of the total calpain-3 pool was cytosolic. At 3 and 24 h following eccentric step exercise, there was an similar to 70% increase in autolysis in whole muscle samples (n = 11, P < 0.05, by 1-way ANOVA with repeated measures and Newman-Keuls post hoc analysis). This exercise-induced autolysis was attributed to myofibrillar-bound calpain-3, since neither the amount of calpain-3 nor the proportion autolyzed was significantly changed in enriched nuclear or cytosolic fractions following the exercise intervention. We present a model for calpain-3 localization at rest and following activation in human skeletal muscle and suggest that the functional importance of calpain-3 remains predominantly tightly associated with its localization within the myofibrillar compartment.