To grow artificial muscles or mend natural tissues it is necessary to stimulate muscle cell contraction and differentiation. In vitro, this is typically achieved by electrical stimulation, mechanical stretching or light pulses. Each method, however, presents challenges in terms of generation of toxic by-products, invasiveness or limited applicability. A. Marino et al. now show that mild photothermal stimulation can also activate myotube contraction (ACS Nano http://doi.org/bznk; 2017).

To do so they use silica–gold core–shell nanoparticles that absorb in the near-infrared (the ability to penetrate through deep layers of tissues makes the near-infrared the spectral region of choice for nanomedical applications). When they incubate the myotubes with the nanoparticles and irradiate with near-infrared light pulses, the authors measure a 5 °C increase in the local temperature. Concomitantly, they observe a large increase in myotube contraction compared with control cells subjected to irradiation but with no nanoparticles. In line with previous reports, they demonstrate that the enhanced contraction is related to an increase in the interaction between the two major protein filaments in the myotubes, actin and myosin. This is a potential advantage over alternative treatments such as electro-stimulation, which works by modulating the cytosolic calcium levels and thus is associated with an increased risk of cell death due to an elevated intracellular concentration of this ion.

Credit: American Chemical Society

Additionally, under an extended heat treatment, during which cells are repeatedly irradiated with near-infrared pulses for five days, Marino et al. also observe the overexpression of heat shock proteins, which have been shown to protect tissues from apoptotic damage and to promote mitochondria biogenesis. The authors suggest that these observations could be used to design a wireless system for switching on muscle contraction in vivo while boosting the defences against apoptosis and skeletal muscle loss.

The confocal fluorescence image depicts the cytoplasmic localization of the core–shell nanoparticles (white) in C2C12 muscle cells; cell nuclei are shown in blue, and the actin and myosin filaments in red and green, respectively.