Regulatory Roles of Fibril-Forming Collagens in Cartilaginous Tissue Biomechanics and Mechanobiology
Authors:   , Wang Chao , Han Lin 
Published: July 2023

In the knee joint, articular cartilage and meniscus work synergistically to enable our daily activities such as walking, running and jumping. Although the importance of maintaining the health of both tissues cannot be emphasized more and the injury prevalence of both tissues are common, the state of William Hunter in 1743 that "once it destroyed, it never repairs" still holds true after three centuries. This can be attributed to both tissues' poor capability of intrinsic repair due to their characteristics of non/low vascularity, innervation and very low cellular mitotic activity. Furthermore, despite decades of progresses in tissue engineering, successful regeneration of both tissues remains elusive. This is, at least partly, due to lacking a comprehensive understanding of the molecular activities that govern the assembly of the complex hierarchical matrix architecture, in turn, the reciprocal interactions between the matrix and residing cells. To address these limitations, a far-reaching knowledge of the roles of fibril-forming collagen, a key component in the extracellular matrix of both articular cartilage and meniscus, could serve as an indispensable foundation to better understand the tissue function. Therefore, first part of this dissertation defense is to elucidate the role of type III collagen in regulating the morphology, nanostructure and biomechanical properties of both articular cartilage and meniscus by studying the phenotypic changes in type III collagen haploinsufficiency mice. Second portion is to elucidate the nanomechanics and micromechanobiological role of the pericellular matrix (PCM), the immediate microniche of residing cells, in fibrocartilage. We queried the roles of proteoglycan and type V collagen in the regulating the structural integrity, matrix assembly and intracellular calcium signaling of murine meniscus. Our findings, for the first time, uncovered the structural and mechanical role of two regulatory collagens, type III and V, in cartilaginous tissues matrices, providing better clinical comprehension to Ehlers-Danlos syndrome (EDS), a rare human genetic disease caused by collagen gene mutations. We also defined the unique molecular and functional properties of the PCM in fibrocartilage, providing a path for enhancing fibrocartilage regeneration by modulating the PCM-mediated cell mechanotransduction. The following chapter provides an up-to-date summary of the current understanding of cartilage ECM functions and its applications in cartilage regeneration. It begins with the discussion of the activities of regulatory collagens and proteoglycans in the function, establishment and maintenance of cartilage ECM, followed by a special focus on the PCM, then, a summary on direct applications of ECM and PCM in cartilage regeneration, finally, concludes with a summary and future outlook. And the last chapter provides a brief summary of Dr. Alan J. Grodzinsky's transformative studies on understanding the aggrecan molecular mechanics. By developing and applying a series of atomic force microscopy (AFM)-based nanomechanical tools, the "bottle-brush"-like ultrastructure of aggrecan was directly visualized for the first time. In addition, molecular mechanics of aggrecan was studied using a physiological-like biomimetic assembly of aggrecan on multiple fronts, including compression, dynamic loading, shear and adhesion. These studies not only generated new insights into the development, aging and disease of cartilage degeneration, but established a foundation for developing and evaluating novel cartilage regeneration strategies.