The role of the myosin ATPase activity in adaptive thermogenesis by skeletal muscle

Abstract

Resting skeletal muscle is a major contributor to adaptive thermogenesis, i.e., the thermogenesis that changes in response to exposure to cold or to overfeeding. The identification of the "furnace" that is responsible for increased heat generation in resting muscle has been the subject of a number of investigations. A new state of myosin, the super relaxed state (SRX), with a very slow ATP turnover rate has recently been observed in skeletal muscle (Stewart et al. in Proc Natl Acad Sci USA 107:430-435, 2010). Inhibition of the myosin ATPase activity in the SRX was suggested to be caused by binding of the myosin head to the core of the thick filament in a structural motif identified earlier by electron microscopy. To be compatible with the basal metabolic rate observed in vivo for resting muscle, most myosin heads would have to be in the SRX. Modulation of the population of this state, relative to the normal relaxed state, was proposed to be a major contributor to adaptive thermogenesis in resting muscle. Transfer of only 20% of myosin heads from the SRX into the normal relaxed state would cause muscle thermogenesis to double. Phosphorylation of the myosin regulatory light chain was shown to transfer myosin heads from the SRX into the relaxed state, which would increase thermogenesis. In particular, thermogenesis by myosin has been proposed to play a role in the dissipation of calories during overfeeding. Up-regulation of muscle thermogenesis by pharmaceuticals that target the SRX would provide new approaches to the treatment of obesity or high blood sugar levels.

Fig. 1

Fluorescence intensities are shown as a function of time during the chase phase of two single-nucleotide turnover experiments in permeable rabbit fast-twitch muscle fibers. The fluorescence decay occurring during a chase with ATP following an incubation with mantATP [2′-/3′-O-(N′-methylanthraniloyl)-ATP] is shown (red, open circles). The rise in fluorescence intensity occurring during the inverse experiment in which the fiber was first incubated with ATP followed by a chase with mantATP is also shown (blue, open squares). The composition of the incubation and chase phases are shown above, color-coded with the data. The fluorescence changes in two phases, a rapid phase which has a time constant of approximately 20 s followed by a slow phase with a time constant of 230 s. The slower phase is attributed to the slow release of nucleotides from a fraction of myosin heads, which are in a “super relaxed” state. Reproduced with permission from (Stewart et al. 2010)

Fig. 2

The three-state model proposed for myosin with approximate nucleotide turnover times. In active muscle, myosin heads are primarily in the active state with rapid nucleotide turnover. In relaxed muscle, myosin can be in one of two states: (1) the normally relaxed state, with a turnover time approximately equal to that of purified myosin, which is 6 s for rabbit myosin at in vivo temperature or (2) the super relaxed state (SRX), with a much slower turnover time, 230 s. Two factors have been shown to alter rates between the relaxed state and the SRX. Myosin heads in active cycles increase the rate from the SRX to the relaxed state (Fig. 4). Phosphorylation (∼P) of the regulatory light chain (RLC) also favors the relaxed state, although it is not known whether this is due to increased rates from the SRX to the relaxed state, or to a decreased rate in the reverse direction

Fig. 3

Three-dimensional reconstruction of the frozen–hydrated tarantula thick filament, filtered to 2-nm resolution. The three-dimensional map segment shows four 14.5 crowns of interacting heads. Scale bar: 14.5 nm. Right High-resolution structural models of two relaxed smooth heavy meromyosins in the conserved “J” structural motif (Wendt et al. 1999), which could be fitted, with minor modifications, into the myosin density on the tarantula thick filament. Reproduced with permission from Alamo et al. (2008)

Fig. 4

Fluorescence intensities are shown as a function of time during a chase with a relaxing solution (open circles, red) or with an activating solution (closed squares, blue). In the relaxing solution, there is a slow release of nucleotides from the SRX, while in the activating solution all nucleotides are released in the rapid phase. The fibers were initially relaxed in 250 uM mantATP. The chase solution was either a relaxing solution, containing 4 mM ATP, pCa approx. 9, or an activating solution containing 4 mM ATP and calcium, pCa = 5.7. The fibers in the activating solution generated 60% of maximal tension

Fig. 5

Components of energy use in the human body. Energy intake in the diet for an average adult human in our modern society (i.e. not engaged in intense physical activity) is approximately 8 MJ day⁻¹. Approximately two-thirds of this energy is required for obligatory cellular functions. A variable amount is used to power physical activity. Adaptive thermogenesis, responding to exposure to cold and to nutritional state, is highly variable and typically amounts to around 25% of the total. The difference between energy intake and energy used by the factors listed above is either stored to—or retrieved from—lipids, proteins and glycogen.

Fig. 6

Changes in non-exercise activity thermogenesis (NEAT) with overfeeding in healthy human subjects who were overfed by 4.2 MJ day⁻¹ for 8 weeks. Fat gain is plotted on the X-axis. Change in NEAT was calculated from NEAT values measured before and after overfeeding: NEAT = total daily energy expenditure − (basic metabolic rate + thermic effect of food). The energy expended in NEAT decreases as the weight gain increases, showing that excess energy intake is approximately balanced between either NEAT or fat gain. Reproduced by permission from Levine (2004)