Optical prediction of single muscle fiber force production using a combined biomechatronics and second harmonic generation imaging approach

Skeletal muscle is an archetypal organ whose structure is tuned to match function. The magnitude of order in muscle fibers and myofibrils containing motor protein polymers determines the directed force output of the summed force vectors and, therefore, the muscle’s power performance on the structural level. Structure and function can change dramatically during disease states involving chronic remodeling. Cellular remodeling of the cytoarchitecture has been pursued using noninvasive and label-free multiphoton second harmonic generation (SHG) microscopy. Hereby, structure parameters can be extracted as a measure of myofibrillar order and thus are suggestive of the force output that a remodeled structure can still achieve. However, to date, the parameters have only been an indirect measure, and a precise calibration of optical SHG assessment for an exerted force has been elusive as no technology in existence correlates these factors.  We engineered a novel, automated, high-precision biomechatronics system into a multiphoton microscope allows simultaneous isometric Ca2+-graded force or passive viscoelasticity measurements and SHG recordings. Using this MechaMorph system, we studied force and SHG in single EDL muscle fibers from wt and mdx mice; the latter serves as a model for compromised force and abnormal myofibrillar structure. We present Ca2+-graded isometric force, pCa-force curves, passive viscoelastic parameters and 3D structure in the same fiber for the first time. Furthermore, we provide a direct calibration of isometric force to morphology, which allows noninvasive prediction of the force output of single fibers from only multiphoton images, suggesting a potential application in the diagnosis of myopathies.New Biomechatronics-Multiphoton Microscopy predicts the degree of muscle weakness from the cellular structure in 3DA laser-scanning multiphoton microscope that diagnoses cellular-level structural defects in skeletal muscle in 3D has been combined with a miniaturized biomechatronics diagnostic machine developed by German researchers. Structural changes to muscle cells, such as the branching and splitting of individual fibers, or subcellular remodeling of cellular skeleton, have been associated with degenerative ailments in patients. Now, Oliver Friedrich at the Friedrich-Alexander-University in Erlangen and colleagues have developed a technique that links the force a muscle cell can apply with its cellular architecture. The team used a special version of multiphoton microscopy, Second Harmonic Generation, to examine single muscle fibers extracted from wild type mice or muscular dystrophy ones carrying a mutation in the dystrophin gene. Simultaneously, an automated system measures the mechanical properties of contractions induced by defined concentrations of calcium ions, or how fibers react to passive stress. Computer analysis yielded linear correlations between optical images and muscle function that may be useful in predicting biomechanical disease in patients from optical assessment of patient biopsies using multiphoton imaging.