Signature molecular changes in the skeletal muscle of hindlimb unloaded mice

Hind-limb unloaded (HU) mouse is a well-recognized model of muscle atrophy; however the molecular changes in the skeletal muscle during unloading are poorly characterized. We have used Fourier transform infrared (FTIR) and Raman spectroscopy to evaluate the structure and behavior of signature molecules involved in regulating muscle structural and functional health. The FTIR and the Raman spectroscopic analysis of gastrocnemius muscles was compared between 16-18 weeks old HU c57Bl/6J mice and ground-based controls. The molecular components of the samples were identified by using the FTIR spectra from the control and the unloaded samples. The Raman spectra showed that the signals for asparagine and glutamine were reduced in HU mice, possibly indicating increased catabolism. The peaks for hydroxyproline and proline were split, pointing towards molecular breakdown and reduced tendon repair. We also report a consistently increased intensity in > 1300 cm-1 range in the Raman spectra along with a shift towards higher frequencies in the HU mice, indicating activation of sarcoplasmic reticulum (SR) stress during HU. SIGNIFICANCEMouse model of hindlimb unloading recapitulates many features of disuse muscle atrophy due to spaceflight and prolonged bed rest. However, a thorough understanding of molecular changes underlying muscle detriment in disuse partly remains elusive. We have applied the spectroscopic and density functional techniques conjointly to characterize the molecular changes in the skeletal muscle of hindlimb unloaded mice. A number of conformational changes and the breakdown of the molecular bonds in the skeletal muscle are observed, which can potentially dictate loss of muscle mass and strength during mechanical unloading. Our reporting of the signature spectral changes in the unloaded skeletal muscle can be a useful step towards the therapeutic interventions targeting specific molecules.