The role of collagen in energy storage and dissipation in extracellular matrix

Gravity plays a central role in vertebrate development and evolution. Gravitational forces present on earth increase the muscle forces required to support locomotion and other bodily movements. Increased muscle forces lead to increased energy stored in tendons and other extracellular matrices (ECMs) during movement that is eventually dissipated as heat by muscle cells. The role of collagen molecular and fibrillar stretching and slippage in energy storage and dissipation in ECMs has been studied in tendon, articular cartilage, and blood vessel wall. The results of these studies suggest that collagen is a block copolymer of alternating flexible and rigid regions with energy storage occurring primarily in the flexible regions devoid of the aromatic amino acids, proline, and hydroxyproline. Energy storage in collagen involves increase in steric energy associated with stretching of pairs of charged amino acids that are present in flexible regions of both the molecule and fibril; these regions coincide with the positively stained bands seen in the D period when collagen fibrils are stained with heavy metals and observed in the electron microscope. Energy storage is also associated with changes in steric energy of backbone carbonyl oxygen groups and stretching of the triple helical backbone. Energy stored in collagen fibrils of the ECM is transduced by the attached cells found in the ECM into changes in cell division and protein synthesis. The purpose of this chapter is to relate the structure of collagen fibrils in extracellular matrix (ECM) to the molecular and fibrillar bases of energy storage and dissipation. Besides this the relationship between energy storage and mechanochemical transduction by cells found in theECMis explored and a model of how energy storage may lead to changes in cell division, gene expression, and protein synthesis in the ECM is presented.