Nanostructure and mechanics of collagen fibrils from osteogenesis imperfecta mice and chronic asthma assessed with atomic force microscopy

Abstract

A number of pathologies are characterized by long term functional impairment of tissues and organs in the human body. Some case pathologies are directly associated with gene mutations that alter the biosynthesis of collagen; the basic structural element and most vital protein of biological tissues in vertebrates.

An example is osteogenesis imperfecta, a heritable bone disorder characterized by abnormally increased number of bone fractures. Other pathologies, however, develop over time as a result of tissue remodelling. In the case of asthma, the mechanical performance of the bronchial airways are now thought to be affliated with alterations in the quality and quantity of collagen fibrils. This thesis is based on the hypothesis that both the structure and biochemistry is altered in collagen{related pathologies, and as a result of these alterations the mechanical properties of collagen fibrils are impaired.

The mechanical assessment of individual collagen fibrils (of larger than 100 nm in diameter) has been accomplished with a number of techniques, but atomic force microscopy (AFM) has been most widely used between several research groups and is the most promising technique at hand for this task. The main aim of this thesis was to assess the structure-mechanical function of collagen fibrils in osteogenesis imperfecta and asthma by employing AFM cantilever based-nanoindentation.

Prior to conducting scientific studies, the AFM cantilever based-nanoindentation was successfully validated with conventional nanoindentation, a well established technique in thin film nanometrology. Collagen fibrils from the oim mouse model of osteogenesis imperfecta were mechanically characterized. This, as well as an in vitro study, using ribose, to artificially cross-link collagen fibrils, demonstrated that the indentation modulus was much dependent on the amount of non{enzymatic cross-links present in collagen fibrils. Further, it became clear that alterations in the structure can have an effect on the type of cross-link predominantly formed, as well as the hydration behaviour of collagen fibrils, and hence also on elasticity and in further instance likely on the ductility. Collagen fibrils from asthmatic donors showed close a trend towards a lower indentation modulus compared to collagen fibrils from healthy donors. An unparalleled biochemistry study could in part complement on this difference but further experimentation is required to draw conclusions with certainty.

To date there are several avenues that could explain the nanomechanical changes seen in asthmatic collagen fibrils, yet the biochemical assays necessary to answer to these are, unfortunately, beyond the scope of this thesis. One possible change may be the amount of collagen types I and III, the abundant collagen types found in the submucosa. It is suggested that the fibril diameter of collagen type I is decreased with increasing the amount of collagen type III. The biochemistry study suggested a trend towards lower amount of type III collagen in asthmatics. In consequence of a possible decreased fibril diameter in asthmatics the density of the subepithelial layer in the airways could change and hence the mechanical properties at the tissue level. Beyond structural changes, there could also be changes in the immature to mature cross-links which would affect the fibril mechanics. This is supported by evidence of decreasing indentation modulus with increase of the immature to mature cross-linking ratio. Moreover, a strong effect of non-enzymatic glycation end products on collagen fibril mechanics was observed. The indentation modulus of collagen fibrils with artificially induced AGEs (advanced glycation end products) was significantly increased.

This finding suggests that relative amount of glycosylated collagen fibrils in asthma could also be a contributing factor to the development of mechanical impairment of asthmatic airways. Nevertheless, findings within this thesis suggest that the biochemistry of collagen is directly associated with its mechanical performance. It has become clear during this thesis that the AFM serves as an important tool to characterize mechanically basic biological elements at the submicrometre scale. The technique used was highly effective towards associating the mechanics of individual collagen fibrils with pathology and biochemistry in model systems. Paralleled data are still needed to provide a holistic picture of a clear pathology state. Importantly, the technique is able to characterize the mechanics of individual collagen fibrils and the studies presented within this thesis have brought interesting results as well as further questions to be answered. With wider use of the technique it is anticipated that some of the issues raised within this thesis can also be discussed and answered by a larger research community.

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Identifiers

ORCID for Philipp J. Thurner: orcid.org/0000-0001-7588-9041

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