Linking Microscopic Spatial Patterns of Tissue Destruction in Emphysema to Macroscopic Decline in Stiffness Using a 3D Computational Model

Pulmonary emphysema is a connective tissue disease characterized by the progressive destruction of alveolar walls leading to airspace enlargement and decreased elastic recoil of the lung. However, the relationship between microscopic tissue structure and decline in stiffness of the lung is not well understood. In this study, we developed a 3D computational model of lung tissue in which a pre-strained cuboidal block of tissue was represented by a tessellation of space filling polyhedra, with each polyhedral unit-cell representing an alveolus. Destruction of alveolar walls was mimicked by eliminating faces that separate two polyhedral either randomly or in a spatially correlated manner, in which the highest force bearing walls were removed at each step. Simulations were carried out to establish a link between the geometries that emerged and the rate of decline in bulk modulus of the tissue block. The spatially correlated process set up by the force-based destruction lead to a significantly faster rate of decline in bulk modulus accompanied by highly heterogeneous structures than the random destruction pattern. Using the Karhunen-Loève transformation, an estimator of the change in bulk modulus from the first four moments of airspace cell volumes was setup. Simulations were then obtained for tissue destruction with different idealized alveolar geometry, levels of pre-strain, linear and nonlinear elasticity assumptions for alveolar walls and also mixed destruction patterns where both random and force-based destruction occurs simultaneously. In all these cases, the change in bulk modulus from cell volumes was accurately estimated. We conclude that microscopic structural changes in emphysema and the associated decline in tissue stiffness are linked by the spatial pattern of the destruction process

Biomechanical determinants of emphysema progression in chronic obstructive pulmonary disease

Emphysema is a disease of the lung parenchyma associated with chronic obstructive pulmonary disease (COPD) and characterized by progressive, irreversible tissue destruction. While chronic inflammation due to repeated noxious particle exposure is the most common environmental risk factor, biomechanical stresses are also known to contribute. It is thought that inflammation-related enzymatic weakening predisposes tissue to mechanical failure, leading to self-propagating parenchymal destruction. However, essential questions regarding the underlying disease mechanisms and their link to overall lung decline remain unanswered. The overarching goals of this dissertation were to relate changes at the cell and tissue level to lung structure and function, and to determine how clinical interventions impact the mechanical balance of parenchymal tissue stresses. First, we use a computational network model of lung volume reduction, a palliative treatment for end-stage emphysema, to demonstrate how recent bronchoscopic, biomaterial-based treatments can achieve similar outcomes as traditional surgical procedures. Next, in a cohort of COPD patients with follow-up computed tomography (CT) imaging, we identify a previously unrecognized structural feature of emphysema that suggests a fundamentally new mechanism of disease progression and potential target for tissue engineering solutions. Finally, we describe the design and implementation of an ex vivo platform for cyclic stretching of precision-cut lung slices, demonstrating a stretch-dependent inflammatory response to acute cigarette smoke extract exposure. In summary, this work combines computational modeling, clinical imaging, and ex vivo measurements to characterize the biomechanical stresses driving emphysema progression and provide new insight that may inform more rational, patient-specific treatment strategies.2020-07-02T00:00:00

Informationist Supplement: A Collaboration to Improve Collaboration

Four librarians at Virginia Commonwealth University teamed up with faculty in mathematics and biomedical engineering to receive an NIH informationist administrative supplement. The supplement supports a research team developing a mathematical model of ventilator-induced lung inflammation. Our goals are to develop workflow strategies to enhance team communication, provide information and data management services, assist with dissemination of the research results, and assess the impact of the grant. We are currently examining the team’s workflow to find data management and communication gaps and are evaluating tools for group annotation and sharing of articles and references

Hyperpolarized 3He Magnetic Resonance Imaging Phenotypes of Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is the third leading cause of death in the world. Identifying clinically relevant COPD phenotypes has the potential to reduce the global burden of COPD by helping to alleviate symptoms, slow disease progression and prevent exacerbation by stratifying patient cohorts and forming targeted treatment plans. In this regard, quantitative pulmonary imaging with hyperpolarized 3He magnetic resonance imaging (MRI) and thoracic computed tomography (CT) have emerged as ways to identify and measure biomarkers of lung structure and function. 3He MRI may be used as a tool to probe both functional and structural properties of the lung whereby static-ventilation maps allow the direct visualization of ventilated lung regions and 3He apparent diffusion coefficient maps show the lung microstructure at alveolar scales. At the same time, thoracic CT provides quantitative measurements of lung density and airway wall and lumen dimensions. Together, MRI and CT may be used to characterize the relative contributions of airways disease and emphysema on overall lung function, providing a way to phenotype underlying disease processes in a way that conventional measurements of airflow, taken at the mouth, cannot. Importantly, structure-function measurements obtained from 3He MRI and CT can be extracted from the whole-lung or from individual lung lobes, providing direct information on specific lung regions. In this thesis, my goal was to identify pulmonary imaging phenotypes to provide a better understanding of COPD pathophysiology in ex-smokers with and without airflow limitation. This thesis showed: 1) ex-smokers without airflow limitation had imaging evidence of subclinical lung and vascular disease, 2) pulmonary abnormalities in ex- smokers without airflow limitation were spatially related to airways disease and very mild emphysema, and, 3) in ex-smokers with COPD, there were distinct apical-basal lung phenotypes associated with disease severity. Collectively, these findings provide strong evidence that quantitative pulmonary imaging phenotypes may be used to characterize the underlying pathophysiology of very mild or early COPD and in patients with severe disease

Predicting changes in lung structure and function during emphysema progression through network modeling methods

Emphysema is a type of Chronic Obstructive Pulmonary Disease (COPD) characterized by breathing difficulties due to airflow obstruction, and results in structural and functional changes of the lungs. Structural changes include alveolar wall destruction and the formation of enlarged alveoli, or bullae, which appear as low attenuation areas in the CT image of emphysematous lungs. Functional changes include increased lung compliance and decreased bulk modulus in emphysematous lungs. Previous mathematical and computational models have attempted to explain either general lung structure or function, but have not linked the two to explore patient-specific lung mechanics. We propose that we can link the structure and function by creating CT-based spring network models of the lung parenchyma and manipulating these networks to predict the regional tissue stiffness and global pressure-volume relationship of the lung during disease progression. The goal of this thesis is to predict these patient-specific changes during emphysema progression by approximating the lung tissue stiffness distribution from CT densities and predicting parenchymal destruction over time from high-strain regions of a non-linear elastic spring network representing lung tissue. First, we used simple spring network models to determine the appropriate non-linear spring force-extension equation to implement into the full lung network. We then mapped a spring network onto a CT image to create a lung network, applied the non-linear force-extension equation to the network springs, and developed a lung deflation model to capture the quasi-static pressure-volume curve of the lung. Finally, we reduced the stiffness of high-strain regions of the lung network and deflated the model to predict the loss of tissue elastance and the reduced bulk modulus over time. Our method shows evidence of a reduced bulk modulus and similar tissue destruction between predicted and actual lung networks, but further development and testing are necessary to create more accurate prediction network models

The role of nicotine, a7 nicotinic acetylcholine receptors and extracellular matrix remodeling in pulmonary fibrosis.

The median survival for idiopathic pulmonary fibrosis (IPF) patients from diagnosis is a dismal 3 years. This condition is characterized by pulmonary fibroproliferation and excess production and disordered deposition of extracellular matrix (ECM) proteins resulting in obliteration of the original tissue architecture, loss of lung function and eventual death due to respiratory failure. The main hindrance to the development of effective treatments against pulmonary fibrosis is the late detection of its progression and is often of unknown cause. Tobacco smoke represents the most important environmental factor linked to the development of pulmonary fibrosis, with over 60% of IPF patients current or ex-smokers, yet exactly how tobacco influences lung injury and repair is unknown. Research in this area has been hampered by the fact that tobacco is a very complex substance, containing thousands of chemicals. Due to this complexity, I have pursued a different approach and focused on factors, specifically nicotine, which might render the lung susceptible to fibrosis and contribute to the early pathophysiology of IPF. In this dissertation, I extend the work of Dr. Jesse Roman’s lab to investigate additional extracellular matrix modifications via nicotine exposure, including collagen type I. Investigating the cellular receptors and molecular mechanisms mediating the effects of nicotine on fibroblast collagen production/deposition and the potential role of nicotine-induced remodeling in rendering the host susceptible to pulmonary fibrosis are explored through 5 chapters: 1) The effects of nicotine on lung fibroblast proliferation and collagen expression/deposition in vitro and in vivo, and the cholinergic receptors responsible for these effects. 2) The effects of chronic nicotine exposure on injury-induced fibrosis. 3) The impact of chronic nicotine exposure on survival after bleomycin-lung injury. 4) A new diagnostic physiological formula for earlier detection of pulmonary fibrosis progression in IPF patients. 5) A clinical review on Hermansky-Pudlak syndrome, an orphan disease characterized by the natural formation of pulmonary fibrosis. This work provides a detailed understanding of the mechanisms by which tobacco promotes lung remodeling, leading to the development of better tools for diagnostic tracking, care and treatment of these patients

Structure and Function of Asthma Evaluated Using Pulmonary Imaging

Asthma has been understood to affect the airways in a spatially heterogeneous manner for over six decades. Computational models of the asthmatic lung have suggested that airway abnormalities are diffusely and randomly distributed throughout the lung, however these mechanisms have been challenging to measure in vivo using current clinical tools. Pulmonary structure and function are still clinically characterized by the forced expiratory volume in one-second (FEV1) – a global measurement of airflow obstruction that is unable to capture the underlying regional heterogeneity that may be responsible for symptoms and disease worsening. In contrast, pulmonary magnetic resonance imaging (MRI) provides a way to visualize and quantify regional heterogeneity in vivo, and preliminary MRI studies in patients suggest that airway abnormalities in asthma are spatially persistent and not random. Despite these disruptive results, imaging has played a limited clinical role because the etiology of ventilation heterogeneity in asthma and its long-term pattern remain poorly understood. Accordingly, the objective of this thesis was to develop a deeper understanding of the pulmonary structure and function of asthma using functional MRI in conjunction with structural computed tomography (CT) and oscillometry, to provide a foundation for imaging to guide disease phenotyping, personalized treatment and prediction of disease worsening. We first evaluated the biomechanics of ventilation heterogeneity and showed that MRI and oscillometry explained biomechanical differences between asthma and other forms of airways disease. We then evaluated the long-term spatial and temporal nature of airway and ventilation abnormalities in patients with asthma. In nonidentical twins, we observed a spatially-matched CT airway and MRI ventilation abnormality that persisted for seven-years; we estimated the probability of an identical defect occurring in time and space to be 1 in 130,000. In unrelated asthmatics, ventilation defects were spatially-persistent over 6.5-years and uniquely predicted longitudinal bronchodilator reversibility. Finally, we investigated the entire CT airway tree and showed that airways were truncated in severe asthma related to thickened airway walls and worse MRI ventilation heterogeneity. Together, these results advance our understanding of asthma as a non-random disease and support the use of MRI ventilation to guide clinical phenotyping and treatment decisions

A novel scaffold for cell-based lung tissue engineering

Chronic obstructive pulmonary disease (COPD) is the fourth leading cause of death worldwide and numbers are rising. Tissue engineering approaches have the potential to improve lung function and treat diseases such as COPD. Our aim is to investigate a highly porous and elastic gelatine scaffold, Surgispon®, for potential uses in cell-based lung tissue engineering by generating an alveolar-like structure. Surgispon scaffolds were crosslinked for stability, and their pore size, pore connectivity, and cytotoxicity were investigated. Human lung epithelial (A549) and fibroblast (35FLH) cell lines and primary porcine lung cells were cultured on Surgispon scaffolds, both separately and in co cultures. Surgispon was used to create an airliquid interface (ALI) to differentiate primary lung cells, and migration studies were performed in combination with decellularised lung tissue. Uncrosslinked Surgispon dissolved rapidly in solution, crosslinking promoted stability beyond 60 days in cell culture conditions. Pore size and interconnectivity were determined via imaging and μCT analysis, establishing similarity to alveolar diameter. Surgispon scaffolds supported cell attachment and growth, with no requirement for chemical coating. Primary lung epithelial cells differentiated into ciliated cells and self-organised into bronchospheres/organoids when cultured on Surgispon with an ALI. Cell migration occurred both from scaffold to tissue and vice versa, demonstrating the potential use of Surgispon scaffolds in clinical COPD research. In summary, Surgispon® scaffolds are an effective alveolar mimic due to being suitable for 3D cell culture, differentiation, and featuring interconnective pores of a size approximate to alveolar diameter. These features promote the prospect of Surgispon scaffolds as a potential scaffold and/or cell delivery system for use in lung tissue engineering and to help combat respiratory disease

Complex lung physiology and airway inflammation in adults with asthma and fixed airflow obstruction

Background: Fixed airflow obstruction (FAO) occurs in asthma despite adequate treatment and no or minimal smoking history. FAO in asthma is more common in older people or those with long-standing disease and associated with poor outcomes. Airflow obstruction occurs in the small airways and is thought to be due to airway remodelling and driven by inflammation. Changes to the lung tissue, which may result in alteration of the lungs elastic properties such as loss of lung elastic recoil, may also contribute. The underlying mechanisms leading to FAO in asthma are poorly understood. Aim: To explore the physiological properties of both the airways and lung tissue and airway inflammation in older non-smokers with asthma, and to assess how they may contribute to FAO. Method: Non-smoking adults >40 years old with asthma were treated with two months of high dose inhaled corticosteroid/long-acting beta-agonist (ICS/LABA). Subsequently standard lung function and small airway function was measured using the forced oscillation technique (FOT) and the multiple breath nitrogen washout test (MBNW). Lung elastic recoil was measured using the oesophageal balloon technique. Airway inflammation was measured using bronchoalveolar lavage fluid obtained during bronchoscopy. Results: Non-smoking adults with asthma (n=19) demonstrated moderate FAO; small airway dysfunction, as measured by FOT and MBNW; increased lung compliance and loss of elastic recoil and variable airway inflammation. Worse airflow obstruction was associated with increased lung compliance. Increased airway resistance and small airway dysfunction in the acinar airways was associated with a loss of lung elastic recoil. Cross-sectional assessment of airway inflammation was not associated with lung function impairment. Conclusion: Changes to the lungs elastic properties results in increased compliance or reduced lung elastic recoil and make a significant contribution to FAO and small airway dysfunction in older non-smokers with asthma. ‘Lung remodelling’ is a potential pathological process leading to lung tissue changes and may be an alternate asthma paradigm. Underlying cellular mechanisms need further investigation

Analysis of matrix metalloproteinases in cancer cell signaling and extracellular behavior

Despite the fact that over the past two decades the total death rate has declined up to twenty percent, cancer remains the second leading cause of death in the United States and accounts for nearly one in every four deaths. It is therefore of paramount importance that new strategies continue to develop in an effort to curb both incidence and treatment of disease. The current research landscape is focused on developing strategies to disrupt molecular signatures of cancer cell types, commonly known as targeted therapy. Of particular importance in the advancement of targeted therapies are matrix metalloproteinases (MMPs), a family of endopeptidases whose primary function lies in cleaving extracellular matrix (ECM) proteins and are frequently dysregulated in cancer. While research regarding MMPs is decades old, their significance in the signal transduction of several oncogenic pathways is yet to be fully explored. In addition, a dearth of quantitative data exists describing the action of MMPs in three dimensional (3D) networks, a configuration that causes cells to express vastly different behaviors compared to traditional two-dimensional (2D) in vitro culture methods. This dissertation aims to further elucidate the intimate relationships between MMPs, the ECM, cancer pathway signaling, and cell migration. First, the behavioral crosstalk between MMPs and the ECM is studied using quantitative methods in 3D matrices. Next, the role of MMPs in both Ras oncogenic and HER2 positive breast cancer is probed via extensive protein expression analysis. Finally, the behavioral aspects of MMPs in 3D are assessed marrying both in vitro data with a computational model to predict migration response. The results reveal that MMPs exhibit a bidirectional relationship with respect to matrix architecture, and the ability to regulate and be regulated by the ECM. In addition, it is concluded that MMPs play a significant role in both active Ras and HER2 upregulated cancer signaling. Finally, the data demonstrates the robustness and accuracy of our methods in manufacturing a model to predict migration in 3D matrices. The work described here promises to further enhance the knowledge of MMPs in cancer and potentially inform future drug development endeavors