Mechanosensing and mechanochemical transduction: how is mechanical energy sensed and converted into chemical energy in an extracellular matrix?

Gravity plays a central role in vertebrate development and evolution. Gravitational forces acting on mammalian tissues cause the net muscle forces required for locomotion to be higher on earth than on a body subjected to a microgravitational field. As body mass increases during development, the musculoskeleton must be able to adapt by increasing the size of its functional units. Thus mechanical forces required to do the work (mechanical energy) of locomotion must be sensed by cells and converted into chemical energy (synthesis of new tissue). Extracellular matrices (ECMs) are multicomponent tissues that transduce internal and external mechanical signals into changes in tissue structure and function through a process termed mechanochemical transduction. Under the influence of an external gravitational field, both mineralized and unmineralized vertebrate tissues exhibit internal tensile forces that serve to preserve a synthetic phenotype in the resident cell population. Application of additional external forces alters the balance between the external gravitational force and internal forces acting on resident cells leading to changes in the expression of genes and production of protein that ultimately may alter the exact structure and function of the extracellular matrix. Changes in the equilibrium between internal and external forces acting on ECMs and changes in mechanochemical transduction processes at the cellular level appear to be important mechanisms by which mammals adjust their needs to store, transmit, and dissipate energy that is required during development and for bodily movements. Mechanosensing is postulated to involve many different cellular and extracellular components. Mechanical forces cause direct stretching of protein-cell surface integrin binding sites that occur on all eukaryotic cells. Stress-induced conformational changes in the extracellular matrix may alter integrin structure and lead to activation of several secondary messenger pathways within the cell. Activation of these pathways leads to altered regulation of genes that synthesize and catabolize extracellular matrix proteins as well as to alterations in cell division. Another aspect by which mechanal signals are transduced involves deformation of gap junctions containing calcium-sensitive stretch receptors. Once activated, these channels trigger secondary messenger activation through pathways similar to those involved in integrin-dependent activation and allow cell-to-cell communications between cells with similar and different phenotypes. Another process by which mechanochemical transduction occurs is through the activation of ion channels in the cell membrane. Mechanical forces have been shown to alter cell membrane ion channel permeability associated with Ca(+2) and other ion fluxes. In addition, the application of mechanical forces to cells leads to the activation of growth factor and hormone receptors even in the absence of ligand binding. These are some of the mechanisms that have evolved in vertebrates by which cells respond to changes in external forces that lead to changes in tissue strcture and function.