The Molecular Basis of Genetic Collagen Disorders and Its Clinical Relevance

A guide to the composition and functions of the extracellular matrix

Extracellular matrix (ECM) is a dynamic 3-dimensional network of macromolecules that provides structural support for the cells and tissues. Accumulated knowledge clearly demonstrated over the last decade that ECM plays key regulatory roles since it orchestrates cell signaling, functions, properties and morphology. Extracellularly secreted as well as cell-bound factors are among the major members of the ECM family. Proteins/glycoproteins, such as collagens, elastin, laminins and tenascins, proteoglycans and glycosaminoglycans, hyaluronan, and their cell receptors such as CD44 and integrins, responsible for cell adhesion, comprise a well-organized functional network with significant roles in health and disease. On the other hand, enzymes such as matrix metalloproteinases and specific glycosidases including heparanase and hyaluronidases contribute to matrix remodeling and affect human health. Several cell processes and functions, among them cell proliferation and survival, migration, differentiation, autophagy, angiogenesis, and immunity regulation are affected by certain matrix components. Structural alterations have been also well associated with disease progression. This guide on the composition and functions of the ECM gives a broad overview of the matrisome, the major ECM macromolecules, and their interaction networks within the ECM and with the cell surface, summarizes their main structural features and their roles in tissue organization and cell functions, and emphasizes the importance of specific ECM constituents in disease development and progression as well as the advances in molecular targeting of ECM to design new therapeutic strategies.

Contrasting Local and Macroscopic Effects of Collagen Hydroxylation

Collagen is heavily hydroxylated. Experiments show that proline hydroxylation is important to triple helix (monomer) stability, fibril assembly, and interaction of fibrils with other molecules. Nevertheless, experiments also show that even without hydroxylation, type I collagen does assemble into its native D-banded fibrillar structure. This raises two questions. Firstly, even though hydroxylation removal marginally affects macroscopic structure, how does such an extensive chemical change, which is expected to substantially reduce hydrogen bonding capacity, affect local structure? Secondly, how does such a chemical perturbation, which is expected to substantially decrease electrostatic attraction between monomers, affect collagen's mechanical properties? To address these issues, we conduct a benchmarked molecular dynamics study of rat type I fibrils in the presence and absence of hydroxylation. Our simulations reproduce the experimental observation that hydroxylation removal has a minimal effect on collagen's D-band length. We also find that the gap-overlap ratio, monomer width and monomer length are minimally affected. Surprisingly, we find that de-hydroxylation also has a minor effect on the fibril's Young's modulus, and elastic stress build up is also accompanied by tightening of triple-helix windings. In terms of local structure, de-hydroxylation does result in a substantial drop (23%) in inter-monomer hydrogen bonding. However, at the same time, the local structures and inter-monomer hydrogen bonding networks of non-hydroxylated amino acids are also affected. It seems that it is this intrinsic plasticity in inter-monomer interactions that preclude fibrils from undergoing any large changes in macroscopic properties. Nevertheless, changes in local structure can be expected to directly impact collagen's interaction with extra-cellular matrix proteins. In general, this study highlights a key challenge in tissue engineering and medicine related to mapping collagen chemistry to macroscopic properties but suggests a path forward to address it using molecular dynamics simulations.

Prolyl and lysyl hydroxylases in collagen synthesis

Collagens are the most abundant proteins in the extracellular matrix. They provide a framework to build organs and tissues and give structural support to make them resistant to mechanical load and forces. Several intra- and extracellular modifications are needed to make functional collagen molecules, intracellular post-translational modifications of proline and lysine residues having key roles in this. In this article, we provide a review on the enzymes responsible for the proline and lysine modifications, that is collagen prolyl 4-hydroxylases, 3-hydroxylases and lysyl hydroxylases, and discuss their biological functions and involvement in diseases.

Osteosynthesis of a Multifragment Femoral Shaft Fracture and Peri-Implant Refracture in an 83-Year-Old Patient with Osteogenesis Imperfecta

Introduction. Osteogenesis imperfecta (OI) is the term for a heterogenic group of conatal diseases that affect the bone formation. Eight different OI types are known. Patients with types III and IV frequently suffer from fractures without adequate trauma. The literature gives plenty advice for fracture treatment in pediatric OI patients, but there is less for adults, and no recommendations can be found for geriatric OI patients. Case Presentation. We report on an 83-year-old male who suffered from OI type IV. He was able to walk with an individually adapted gait orthosis. In an accident, the patient sustained a distal, multifragment, femoral shaft fracture. The fracture was openly reduced and fixated with a retrograde inserted elastic stable intramedullary nail (ESIN). Three months later, the patient was capable of walking without crutches. Due to another accident, he sustained a peri-implant refracture without failure of the ESIN. We immobilized the leg, and it achieved bony healing without reosteosynthesis. Eleven weeks later, he was again able to mobilize himself with full weight bearing. Discussion. We present a unique case of osteosynthesis in a distal, multifragment, femoral shaft fracture in a geriatric OI patient. No recommendations for the treatment of mature patients with OI can be found in the literature. We present our treatment concept and technique of osteosynthesis with an ESIN. Despite another accident with a peri-implant refracture, sufficient bony healing occurred, which allowed the patient to freely mobilize himself again.