Assessment of Collagen in Translational Models of Lung Research

The extracellular matrix (ECM) plays an important role in lung health and disease. Collagen is the main component of the lung ECM, widely used for the establishment of in vitro and organotypic models of lung disease, and as scaffold material of general interest for the field of lung bioengineering. Collagen also is the main readout for fibrotic lung disease, where collagen composition and molecular properties are drastically changed and ultimately result in dysfunctional "scarred" tissue. Because of the central role of collagen in lung disease, quantification, determination of molecular properties, and three-dimensional visualization of collagen is important for both development and characterization of translational models of lung research. In this chapter, we provide a comprehensive overview on the various methodologies currently available for quantification and characterization of collagen including their detection principles, advantages, and disadvantages.

Self-assembly of mesoscale collagen architectures and applications in 3D cell migration

3D in vitro tumor models have recently been investigated as they can recapitulate key features in the tumor microenvironment. Reconstruction of a biomimetic scaffold is critical in these models. However, most current methods focus on modulating local properties, e.g. micro- and nano-scaled topographies, without capturing the global millimeter or intermediate mesoscale features. Here we introduced a method for modulating the collagen I-based extracellular matrix structure by disruption of fibrillogenesis and the gelation process through mechanical agitation. With this method, we generated collagen scaffolds that are thickened and wavy at a larger scale while featuring global softness. Thickened collagen patches were interconnected with loose collagen networks, highly resembling collagen architecture in the tumor stroma. This thickened collagen network promoted tumor cell dissemination. In addition, this novel modified scaffold triggered differences in morphology and migratory behaviors of tumor cells. Altogether, our method for altered collagen architecture paves new ways for studying in detail cell behavior in physiologically relevant biological processes. STATEMENT OF SIGNIFICANCE: Tumor progression usually involves chronic tissue damage and repair processes. Hallmarks of tumors are highly overlapped with those of wound healing. To mimic the tumor milieu, collagen-based scaffolds are widely used. These scaffolds focus on modulating microscale topographies and mechanics, lacking global architecture similarity compared with in vivo architecture. Here we introduced one type of thick collagen bundles that mimics ECM architecture in human skin scars. These thickened collagen bundles are long and wavy while featuring global softness. This collagen architecture imposes fewer steric restraints and promotes tumor cell dissemination. Our findings demonstrate a distinct picture of cell behaviors and intercellular interactions, highlighting the importance of collagen architecture and spatial heterogeneity of the tumor microenvironment.

Collagen I dysregulation is pivotal for ovarian cancer progression

As a principal matrisomal protein, collagen is involved in the regulation of the structural framework of extracellular matrix (ECM) and therefore is potentially crucial in determining the biophysical character of the ECM. It has been suggested that collagen architecture plays a role in ovarian cancer development, progression and therapeutic responses which led us to examine the collagen morphology in normal and cancerous ovarian tissue. Also, the behaviour of ovarian cancer cells cultured in four qualitatively different collagen gels was investigated. The results here provide evidence that collagen I morphology in the cancerous ovary is distinct from that in the normal ovary. Tumour-associated collagen I showed streams or channels of thick elongated collagen I fibrils. Moreover, fibril alignment was significantly more prevalent in endometrioid and clear cell cancers than other ovarian cancer subtypes. In this work, for the first-time collagen I architecture profiling (CAP) was introduced using histochemical staining, which distinguished between the collagen I morphologies of ovarian cancer subtypes. Immunohistochemical examination of ovarian normal and cancerous tissues also supported the notion that focal adhesion and Rho signalling are upregulated in ovarian cancers, especially in the high-grade serous tumours, as indicated by higher expression of p-FAK and p190RhoGEF. The results also support the concept that collagen I architecture, which might be collagen I concentration-dependent, influences proliferation in ovarian cancer cells. The study provides evidence that modification of collagen I architecture integrity is associated with ovarian cancer development and therapeutic responses.

Bovine Pericardium of High Fibre Dispersion Has High Fatigue Life and Increased Collagen Content; Potentially an Untapped Source of Heart Valve Leaflet Tissue

Bioprosthetic heart valves (BHVs) are implanted in aortic valve stenosis patients to replace the native, dysfunctional valve. Yet, the long-term performance of the glutaraldehyde-fixed bovine pericardium (GLBP) leaflets is known to reduce device durability. The aim of this study was to investigate a type of commercial-grade GLBP which has been over-looked in the literature to date; that of high collagen fibre dispersion (HD). Under uniaxial cyclic loading conditions, it was observed that the fatigue behaviour of HD GLBP was substantially equivalent to GLBP in which the fibres are highly aligned along the loading direction. It was also found that HD GLBP had a statistically significant 9.5% higher collagen content when compared to GLBP with highly aligned collagen fibres. The variability in diseased BHV delivery sites results in unpredictable and complex loading patterns across leaflets in vivo. This study presents the possibility of a shift from the traditional choice of circumferentially aligned GLBP leaflets, to that of high fibre dispersion arrangements. Characterised by its high fatigue life and increased collagen content, in addition to multiple fibre orientations, GLBP of high fibre dispersion may provide better patient outcomes under the multi-directional loading to which BHV leaflets are subjected in vivo.

Frequency dependent inelastic response of collagen architecture of pig dermis under cyclic tensile loading: An experimental study

The evaluation of collagen architecture of the dermis in response to mechanical stimulation is important as it affects the macroscopic mechanical properties of the dermis. A detailed understanding of the processes involved in the alteration of the collagen structure is required to correlate the mechanical stimulation with tissue remodeling. This study investigated the effect of cyclic frequencies i.e. low (0.1 Hz), medium (2.0 Hz), and high (5.0 Hz) (physiological range) in the alteration of pig dermis collagen structure and its correlation with the macroscopic mechanical response of the dermis. The assessment of the collagen structure of virgin and mechanical tested specimens at tropocollagen, collagen fibril, and fiber level was performed using Fourier-transform infrared-attenuated total reflection (FTIR-ATR), atomic force microscopy (AFM), and scanning electron microscopy (SEM) respectively. After 10³ cycles, a significantly higher alteration in collagen structure with discrete plastic-type damage was found for low frequency. This frequency dependent alteration of the collagen structure was found in correlation with the dermis macroscopic response. The value of inelastic strain, stress softening, damage parameter (reduction in elastic modulus), and reduction in energy dissipation were observed significantly large for slow frequency. A power-law based empirical relations, as a function of frequency and number of cycles, were proposed to predict the value of inelastic strain and damage parameter. This study also suggests that hierarchical structural response against the mechanical stimulation is time-dependent rather than cycle-dependent, may affect the tissue remodeling.

Peripapillary sclera architecture revisited: A tangential fiber model and its biomechanical implications

The collagen fiber architecture of the peripapillary sclera (PPS), which surrounds the scleral canal, is a critical factor in determining the mechanical response of the optic nerve head (ONH) to variations in intraocular pressure (IOP). Experimental and clinical evidence point to IOP-induced deformations within the scleral canal as important contributing factors of glaucomatous neural tissue damage and consequent vision loss. Hence, it is imperative to understand PPS architecture and biomechanics. Current consensus is that the fibers of the PPS form a closed ring around the canal to support the delicate neural tissues within. We propose an alternative fiber architecture for the PPS, in which the scleral canal is supported primarily by long-running fibers oriented tangentially to the canal. We present evidence that this tangential model is consistent with histological observations in multiple species, and with quantitative measurements of fiber orientation obtained from small angle light scattering and wide-angle X-ray scattering. Using finite element models, we investigated the biomechanical implications of a tangential fiber PPS architecture. We found that the tangential arrangement of fibers afforded better mechanical support to the tissues within the scleral canal as compared to a simple circumferential ring of fibers or a combination of fibers oriented radially and circumferentially. We also found that subtle variations from a tangential orientation could reproduce clinically observed ONH behavior which has yet to be explained using current theories of PPS architecture and simulation, namely, the contraction of the scleral canal under elevated IOP.

STATEMENT OF SIGNIFICANCE

It is hypothesized that vision loss in glaucoma is due to excessive mechanical deformation within the neural tissue inside the scleral canal. This study proposes a new model for how the collagen of the peripapillary sclera surrounding the canal is organized to support the delicate neural tissue inside. Previous low-resolution studies of the peripapillary sclera suggested that the collagen fibers are arranged in a ring around the canal. Instead, we provide microscopic evidence suggesting that the canal is also supported by long-running interwoven fibers oriented tangentially to the canal. We demonstrate that this arrangement has multiple biomechanical advantages over a circular collagen arrangement and can explain previously unexplained experimental findings including contraction of the scleral canal under elevated intraocular pressure.

3D Fiber Orientation in Atherosclerotic Carotid Plaques

Atherosclerotic plaque rupture is the primary trigger of fatal cardiovascular events. Fibrillar collagen in atherosclerotic plaques and their directionality are anticipated to play a crucial role in plaque rupture. This study aimed assessing 3D fiber orientations and architecture in atherosclerotic plaques for the first time. Seven carotid plaques were imaged ex-vivo with a state-of-the-art Diffusion Tensor Imaging (DTI) technique, using a high magnetic field (9.4Tesla) MRI scanner. A 3D spin-echo sequence with uni-polar diffusion sensitizing pulsed field gradients was utilized for DTI and fiber directions were assessed from diffusion tensor measurements. The distribution of the 3D fiber orientations in atherosclerotic plaques were quantified and the principal fiber orientations (circumferential, longitudinal or radial) were determined. Overall, 52% of the fiber orientations in the carotid plaque specimens were closest to the circumferential direction, 34% to the longitudinal direction, and 14% to the radial direction. Statistically no significant difference was measured in the amount of the fiber orientations between the concentric and eccentric plaque sites. However, concentric plaque sites showed a distinct structural organization, where the principally longitudinally oriented fibers were closer to the luminal side and the principally circumferentially oriented fibers were located more abluminally. The acquired unique information on 3D plaque fiber direction will help understanding pathobiological mechanisms of atherosclerotic plaque progression and pave the road to more realistic biomechanical plaque modeling for rupture assessment.

MRI magic-angle effect in femorotibial cartilages of the red kangaroo

OBJECTIVE

Kangaroo knee cartilages are robust tissues that can support knee flexion and endure high levels of compressive stress. This study aimed to develop a detailed understanding of the collagen architecture in kangaroo knee cartilages and thus obtain insights into the biophysical basis of their function.

DESIGN

Cylindrical/square plugs from femoral and tibial hyaline cartilage and tibial fibrocartilage were excised from the knees of three adult red kangaroos. Multi-slice, multi-echo MR images were acquired at the sample orientations 0° and 55° ("magic angle") with respect to the static magnetic field. Maps of the transverse relaxation rate constant (R₂) and depth profiles of R₂ and its anisotropic component (R₂^A) were constructed from the data.

RESULTS

The R₂^A profiles confirmed the classic three-zone organisation of all cartilage samples. Femoral hyaline cartilage possessed a well-developed, thick superficial zone. Tibial hyaline cartilage possessed a very thick radial zone (80% relative thickness) that exhibited large R₂^A values consistent with highly ordered collagen. The R₂^A profile of tibial fibrocartilage exhibited a unique region near the bone (bottom 5-10%) consistent with elevated proteoglycan content ("attachment sub-zone").

CONCLUSIONS

Our observations suggest that the well-developed superficial zone of femoral hyaline cartilage is suitable for supporting knee flexion; the thick and well-aligned radial zone of tibial hyaline cartilage is adapted to endure high compressive stress; while the innermost part of the radial zone of tibial fibrocartilage may facilitate anchoring of the collagen fibres to withstand high shear deformation. These findings may inspire new designs for cartilage tissue engineering.

Regional Mapping of Flow and Wall Characteristics of Intracranial Aneurysms

The evolution of intracranial aneurysms (IAs) is thought to be driven by progressive wall degradation in response to abnormal hemodynamics. Previous studies focused on the relationship between global hemodynamics and wall properties. However, hemodynamics, wall structure and mechanical properties of cerebral aneurysms can be non-uniform across the aneurysm wall. Therefore, the aim of this work is to introduce a methodology for mapping local hemodynamics to local wall structure in resected aneurysm specimens. This methodology combines image-based computational fluid dynamics, tissue resection, micro-CT imaging of resected specimens mounted on 3D-printed aneurysm models, alignment to 3D vascular models, multi-photon microscopy of the wall, and regional mapping of hemodynamics and wall properties. This approach employs a new 3D virtual marking tool for surgeons to delineate the location of the resected specimen directly on the 3D model, while in the surgical suite. The case of a middle cerebral artery aneurysm is used to illustrate the application of this methodology to the assessment of the relationship between local wall shear stress and local wall properties including collagen fiber organization and wall geometry. This methodology can similarly be used to study the relationship between local intramural stresses and local wall structure.