Myosin - still a good reference for skeletal muscle fibre classification?

Abstract

Myosin heavy chain (MHC) isoforms have been taken for the molecular signature of skeletal muscle fibres based on their relationship to mechanical properties – maximum shortening velocity, power – and ATP consumption. This relation between MHCs, fibre mechanics and energy suggests that they direct a concerted myofibre molecular and functional program. Switching MHC isoforms had been reported to change ‘myofibre type’, but finding two MHC isoforms expressed in a single myofibre made interpreting their influence on fibre contractile properties more complex and raised an intriguing question about their relation to intracellular Ca2+ homeostasis. Most studies of skeletal muscle fibre physiology focus on either sarcolemmal excitation/sarcoplasmic reticulum Ca2+ release–uptake or myofibrillar protein function. They overlook more general aspects of the increasingly complex cascade that leads to muscle contraction, probably due to technical difficulties in exploring both components simultaneously in the same myofibre. Work by Calderon et al. (2010), in this issue of The Journal of Physiology, fills a gap in our knowledge in this regard. They examined cytosolic Ca2+ transient kinetics in a significant number of muscle fibres expressing pure and hybrid MHCs. Loading myofibres with MagFluo-4, a relatively low-affinity indicator, allowed them to track changes in cytosolic Ca2+ concentration in response to field electrical stimulation, a technique that reproduces physiological stimulation through the nerve. MHC isoforms were characterized in individual fibres with SDS-PAGE gels. The authors succeeded in isolating viable, intact, long fibres from the extensor digitorum longus (EDL) and soleus muscles to examine intracellular Ca2+ kinetics in a larger variety of MHC isoforms than a single mouse muscle can provide. Their work tested the previously proposed hypothesis correlating MHC isoforms and Ca2+ transient kinetics. EDL and soleus muscle fibres exhibited a continuum in cytosolic Ca2+ transient kinetics from slow to fast that parallels the MHC continuum from pure type I (slow-twitch oxidative) to pure IIB (fast-twitch glycolytic), with IIA (fast-twitch oxidative), IIX (fast-twitch glycolytic) and various combinations in between. Although presumed, this proposal was never explored in such detail or in myofibres exhibiting the whole spectrum of MHC composition. This work prompts important questions about discrepancies in the relationship between MHC composition and Ca2+ kinetics. Fibres IIX and IIB extensively overlap in all Ca2+ kinetic parameters, while IIA fibres behave more like fibres IIX and IIB in the cytosolic Ca2+ rising phase and type I fibres in Ca2+ removal. To some extent, Ca2+ kinetics is related to MHC composition, but given its great variability within MHC isoforms and similarity among different fibre types, regulation of fibre functional phenotype may be a consequence of a set of genes rather than a single master-switch gene (Spangenburg & Booth, 2003). We cannot exclude the possibility that undiscovered MHCs explain the overlap. This interpretation would be consistent with previous studies suggesting that with ageing and disuse, properties of a muscle fibre type can change while MHC isoform content remains the same (Canepari et al. 2009). Calderon et al. provide support for the concept that the widely used classification based on MHC isoforms partially accounts for the functional and molecular diversity of myofibres. The number of SR release units correlates with the rate of SR Ca2+ release. Higher density has been reported in fast- compared to slow-twitch muscles (Delbono & Meissner, 1996). Parvalbumin, an endogenous Ca2+ chelator that rapidly buffers cytosolic Ca2+ concentration, correlates with fibre relaxation rate. These data indicate that aspects of muscle contraction depend on different molecular substrates and that orchestrated programs shape contraction kinetics. A study exploring graded relaxation, rate of cytosolic Ca2+ decline, MHC isoforms and parvalbumin is needed. We also need a study of Ca2+ transient kinetics in superfast and embryonic MHC using an experimental approach similar to that of Calderon et al. to better understand the molecular basis of muscle contraction. For more than a decade, myofibre classification based on MHC isoforms proved useful in the study of human and animal skeletal muscle. However, a new nomenclature that prioritizes the cell's functional, proteomic, genomic and epigenetic hierarchical organization would be desirable. Recently discovered miRNAs encoded within myosin genes that regulate muscle gene expression and performance (Van Rooij et al. 2009) support this need and also the leading role of myosin in muscle phenotype. Therefore, growing understanding of the complexity of skeletal muscle signalling and organization will probably make formulating a new, simple, unifying classification a difficult task.