Collagen Organization Within the Cartilage of Trpv4 −/− Mice Studied with Two‐Photon Microscopy and Polarized Second Harmonic Generation

The Diverse Physiological Functions of Mechanically Activated Ion Channels in Mammals

Many aspects of mammalian physiology are mechanically regulated. One set of molecules that can mediate mechanotransduction are the mechanically activated ion channels. These ionotropic force sensors are directly activated by mechanical inputs, resulting in ionic flux across the plasma membrane. While there has been much research focus on the role of mechanically activated ion channels in touch sensation and hearing, recent data have highlighted the broad expression pattern of these molecules in mammalian cells. Disruption of mechanically activated channels has been shown to impact (a) the development of mechanoresponsive structures, (b) acute mechanical sensing, and (c) mechanically driven homeostatic maintenance in multiple tissue types. The diversity of processes impacted by these molecules highlights the importance of mechanically activated ion channels in mammalian physiology. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Imaging of muscle activity-induced morphometric changes in fibril network of myofascia by two-photon microscopy

Myofascia, deep fascia enveloping skeletal muscles, consists of abundant collagen and elastin fibres that play a key role in the transmission of muscular forces. However, understanding of biomechanical dynamics in myofascia remains very limited due to less quantitative and relevant approaches for in vivo examination. The purpose of this study was to evaluate the myofascial fibril structure by means of a quantitative approach using two-photon microscopy (TPM) imaging in combination with intravital staining of Evans blue dye (EBD), a far-red fluorescence dye, which potentially labels elastin. With focus on myofascia of the tibial anterior (TA) muscle, the fibril structure intravitally stained with EBD was observed at the depth level of collagen fibrous membrane above the muscle belly. The EBD-labelled fibril structure and orientation in myofascia indicated biomechanical responses to muscle activity and ageing. The orientation histograms of EBD-labelled fibrils were significantly modified depending upon the intensity of muscle activity and ageing. Moreover, the density of EBD-labelled fibrils in myofascia decreased with habitual exercise but increased with muscle immobilization or ageing. In particular, the diameter of EBD-labelled fibrils in aged mice was significantly higher. The orientation histograms of EBD-labelled fibrils after habitual exercise, muscle immobilization and ageing showed significant differences compared to control. Indeed, the histograms in bilateral TA myofascia of exercise mice made simple waveforms without multiple sharp peaks, whilst muscular immobilization or ageing significantly shifted a histogram with sustaining multiple sharp peaks. Therefore, the dynamics of fibre network with EBD fluorescence in response to the biomechanical environment possibly indicate functional tissue adaptation in myofascia. Furthermore, on the basis of the knowledge that neutrophil recruitment occurs locally in working muscles, we suggested the unique reconstruction mechanism involving neutrophilic elastase in the myofascial fibril structure. In addition to the elastolytic susceptibility of EBD-labelled fibrils, distinct immunoreactivities and activities of neutrophil elastase in the myofascia were observed after electric pulse stimulation-induced muscle contraction for 15 min. Our findings of EBD-labelled fibril dynamics in myofascia through quantitative approach using TPM imaging and intravital fluorescence labelling potentially brings new insights to examine muscle physiology and pathology.