Defining a stimuli-response relationship in compensatory lung growth following major resection

Depicting and predicting changes of lung after lobectomy for cancer by using CT images

Lobectomy is an effective and well-established therapy for localized lung cancer. This study aimed to assess the lung and lobe change after lobectomy and predict the postoperative lung volume. The study included 135 lung cancer patients from two hospitals who underwent lobectomy (32, right upper lobectomy (RUL); 31, right middle lobectomy (RML); 24, right lower lobectomy (RLL); 26, left upper lobectomy (LUL); 22, left lower lobectomy (LLL)). We initially employ a convolutional neural network model (nnU-Net) for automatically segmenting pulmonary lobes. Subsequently, we assess the volume, effective lung volume (ELV), and attenuation distribution for each lobe as well as the entire lung, before and after lobectomy. Ultimately, we formulate a machine learning model, incorporating linear regression (LR) and multi-layer perceptron (MLP) methods, to predict the postoperative lung volume. Due to the physiological compensation, the decreased TLV is about 10.73%, 8.12%, 13.46%, 11.47%, and 12.03% for the RUL, RML, RLL, LUL, and LLL, respectively. The attenuation distribution in each lobe changed little for all types of lobectomy. LR and MLP models achieved a mean absolute percentage error of 9.8% and 14.2%, respectively. Radiological findings and a predictive model of postoperative lung volume might help plan the lobectomy and improve the prognosis.

Lung organoids, useful tools for investigating epithelial repair after lung injury

Organoids are derived from stem cells or organ-specific progenitors. They display structures and functions consistent with organs in vivo. Multiple types of organoids, including lung organoids, can be generated. Organoids are applied widely in development, disease modelling, regenerative medicine, and other multiple aspects. Various human pulmonary diseases caused by several factors can be induced and lead to different degrees of lung epithelial injury. Epithelial repair involves the participation of multiple cells and signalling pathways. Lung organoids provide an excellent platform to model injury to and repair of lungs. Here, we review the recent methods of cultivating lung organoids, applications of lung organoids in epithelial repair after injury, and understanding the mechanisms of epithelial repair investigated using lung organoids. By using lung organoids, we can discover the regulatory mechanisms related to the repair of lung epithelia. This strategy could provide new insights for more effective management of lung diseases and the development of new drugs.

Inhalational delivery of induced pluripotent stem cell secretome improves postpneumonectomy lung structure and function

Cell-free secretory products (secretome) of human induced pluripotent stem cells (iPSCs) have been shown to attenuate tissue injury and facilitate repair and recovery. To examine whether iPSC secretome facilitates mechanically induced compensatory responses following unilateral pneumonectomy (PNX), litter-matched young adult female hounds underwent right PNX (removing 55%-58% of lung units), followed by inhalational delivery of either the nebulized-conditioned media containing induced pluripotent stem cell secretome (iPSC CM) or control cell-free media (CFM); inhalation was repeated every 5 days for 10 treatments. Lung function was measured under anesthesia pre-PNX and 10 days after the last treatment (8 wk post-PNX); detailed quantitative analysis of lung ultrastructure was performed postmortem. Pre-PNX lung function was similar between groups. Compared with CFM control, treatment with iPSC CM attenuated the post-PNX decline in lung diffusing capacity for carbon monoxide and membrane diffusing capacity, accompanied by a 24% larger postmortem lobar volume and distal air spaces. Alveolar double-capillary profiles were 39% more prevalent consistent with enhanced intussusceptive angiogenesis. Frequency distribution of the harmonic mean thickness of alveolar blood-gas barrier shifted toward the lowest values, whereas alveolar septal tissue volume and arithmetic septal thickness were similar, indicating septal remodeling and reduced diffusive resistance of the blood-gas barrier. Thus, repetitive inhalational delivery of iPSC secretome enhanced post-PNX alveolar angiogenesis and septal remodeling that are associated with improved gas exchange compensation. Results highlight the plasticity of the remaining lung units following major loss of lung mass that are responsive to broad-based modulation provided by the iPSC secretome.NEW & NOTEWORTHY To examine whether the secreted products of human induced pluripotent stem cells (iPSCs) facilitate innate adaptive responses following loss of lung tissue, adult dogs underwent surgical removal of one lung, then received repeated administration of iPSC secretory products via inhalational delivery compared with control treatment. Inhalation of iPSC secretory products enhanced capillary formation and beneficial structural remodeling in the remaining lung, leading to improved lung function.

In vivo imaging of canine lung deformation: effects of posture, pneumonectomy, and inhaled erythropoietin

Mechanical stresses on the lung impose the major stimuli for developmental and compensatory lung growth and remodeling. We used computed tomography (CT) to noninvasively characterize the factors influencing lobar mechanical deformation in relation to posture, pneumonectomy (PNX), and exogenous proangiogenic factor supplementation. Post-PNX adult canines received weekly inhalations of nebulized nanoparticles loaded with recombinant human erythropoietin (EPO) or control (empty nanoparticles) for 16 wk. Supine and prone CT were performed at two transpulmonary pressures pre- and post-PNX following treatment. Lobar air and tissue volumes, fractional tissue volume (FTV), specific compliance (Cs), mechanical strains, and shear distortion were quantified. From supine to prone, lobar volume and Cs increased while strain and shear magnitudes generally decreased. From pre- to post-PNX, air volume increased less and FTV and Cs increased more in the left caudal (LCa) than in other lobes. FTV increased most in the dependent subpleural regions, and the portion of LCa lobe that expanded laterally wrapping around the mediastinum. Supine deformation was nonuniform pre- and post-PNX; strains and shear were most pronounced in LCa lobe and declined when prone. Despite nonuniform regional expansion and deformation, post-PNX lobar mechanics were well preserved compared with pre-PNX because of robust lung growth and remodeling establishing a new mechanical equilibrium. EPO treatment eliminated posture-dependent changes in FTV, accentuated the post-PNX increase in FTV, and reduced FTV heterogeneity without altering absolute air or tissue volumes, consistent with improved microvascular blood volume distribution and modestly enhanced post-PNX alveolar microvascular reserves.NEW & NOTEWORTHY Mechanical stresses on the lung impose the major stimuli for lung growth. We used computed tomography to image deformation of the lung in relation to posture, loss of lung units, and inhalational delivery of the growth promoter erythropoietin. Following loss of one lung in adult large animals, the remaining lung expanded and grew while retaining near-normal mechanical properties. Inhalation of erythropoietin promoted more uniform distribution of blood volume within the remaining lung.

Erythropoietin inhalation enhances adult canine alveolar-capillary formation following pneumonectomy

Paracrine erythropoietin (EPO) signaling in the lung recruits endothelial progenitor cells, promotes cell maturation and angiogenesis, and is upregulated during canine postpneumonectomy (PNX) compensatory lung growth. To determine whether inhalational delivery of exogenous EPO augments endogenous post-PNX lung growth, adult canines underwent right PNX and received, via a permanent tracheal stoma, weekly nebulization of recombinant human EPO-containing nanoparticles or empty nanoparticles (control) for 16 wk. Lung function was assessed under anesthesia pre- and post-PNX. The remaining lobes were fixed for detailed morphometric analysis. Compared with control treatment, EPO delivery significantly increased serum EPO concentration without altering systemic hematocrit or hemoglobin concentration and abrogated post-PNX lipid oxidative stress damage. EPO delivery modestly increased post-PNX volume densities of the alveolar septum per unit of lung volume and type II epithelium and endothelium per unit of septal tissue volume in selected lobes. EPO delivery also augmented the post-PNX increase in alveolar double-capillary profiles, a marker of intussusceptive capillary formation, in all remaining lobes. EPO treatment did not significantly alter absolute resting lung volumes, lung and membrane diffusing capacities, alveolar-capillary blood volume, pulmonary blood flow, lung compliance, or extravascular alveolar tissue volumes or surface areas. Results established the feasibility of chronic inhalational delivery of growth-modifying biologics in a large animal model. Exogenous EPO selectively enhanced cytoprotection and alveolar angiogenesis in remaining lobes but not whole-lung extravascular tissue growth or resting function; the nonuniform response contributes to structure-function discrepancy, a major challenge for interventions aimed at amplifying the innate potential for compensatory lung growth.

No compensatory lung growth after resection in a one-year follow-up cohort of patients with lung cancer

Background

As compensatory lung growth after lung resection has been studied in animals of various ages and in one case report in a young adult, it has not been studied in a cohort of adults operated for lung cancer.

Methods

A prospective study including patients with lung cancer was conducted over two years. Parenchymal mass was calculated using computed tomography before (M0) and at 3 and 12 months (M3 and M12) after surgery. Respiratory function was estimated by plethysmography and CO/NO lung transfer (D_LCO and D_LNO). Pulmonary capillary blood volume (Vc) and membrane conductance for CO (Dm_CO) were calculated. Insulin-like growth factor-1 (IGF-1) and insulin-like growth factor binding protein-3 (IGFBP-3) plasma concentrations were measured simultaneously.

Results

Forty-nine patients underwent a pneumonectomy (N=12) or a lobectomy (N=37) thirty two completed the protocol. Among all patients, from M3 to M12 the masses of the operated lungs (239±58 to 238±72 g in the lobectomy group) and of the non-operated lungs (393±84 to 377±68 g) did not change. Adjusted by the alveolar volume (V_A), D_LNO/V_A decreased transiently by 7% at M3, returning towards the M0 value at M12. Both Vc and Dm_CO increased slightly between M3 and M12. IGF-1 and IGFBP-3 concentrations did not change at M3, IGF-1 decreased significantly from M3 to M12.

Conclusions

Compensatory lung growth did not occur over one year after lung surgery. The lung function data could suggest a slight recruitment or distension of capillaries owing to the likely hemodynamic alterations. An angiogenesis process is unlikely.

Platelet-derived growth factor receptor-α and Ras-related C3 botulinum toxin substrate-1 regulate mechano-responsiveness of lung fibroblasts

Platelet-derived growth factor (PDGF)-A, which only signals through PDGF-receptor-α (PDGFR-α), is required for secondary alveolar septal formation. Although PDGFR-α distinguishes mesenchymal progenitor cells during the saccular stage, PDGFR-α-expressing alveolar cells persist through adulthood. PDGF-A sustains proliferation, limits apoptosis, and maintains α-smooth muscle actin (α-SMA)-containing alveolar cells, which congregate at the alveolar entry ring at postnatal day (P)12. PDGFR-α-expressing, α-SMA-containing alveolar cells redistribute in the elongating septum, suggesting that they migrate to the alveolar entry rings, where mechanical tension is higher. We hypothesized that PDGFR-α and Ras-related C3 botulinum toxin substrate 1(Rac1) are required for mechanosensitive myofibroblast migration. Spreading of PDGFR-α-deficient lung fibroblasts was insensitive to increased rigidity, and their migration was not reduced by Rac1-guanine exchange factor (GEF)-inhibition. PDGFR-α-expressing fibroblasts migrated toward stiffer regions within two-dimensional substrates by increasing migrational persistence (durotaxis). Using a Förster resonance energy transfer (FRET) biosensor for Rac1-GTP, we observed that PDGFR-α was required for fibroblast Rac1 responsiveness to stiffness within a three-dimensional collagen substrate, which by itself increased Rac1-FRET. Rho-GTPase stabilized, whereas Rac1-GTPase increased the turnover of focal adhesions. Under conditions that increased Rac1-GTP, PDGFR-α signaled through both phosphoinositide-3-kinase (PIK) or Src to engage the Rac1 GEF dedicator of cytokinesis-1 (Dock180) and p21-activated-kinase interacting exchange factor-β (βPIX). In cooperation with collagen fibers, these signaling pathways may guide fibroblasts toward the more rigid alveolar entry ring during secondary septation. Because emphysema and interstitial fibrosis disrupt the parenchymal mechanical continuum, understanding how mechanical factors regulate fibroblast migration could elicit strategies for alveolar repair and regeneration.

Comparative analysis of the mechanical signals in lung development and compensatory growth

This review compares the manner in which physical stress imposed on the parenchyma, vasculature and thorax and the thoraco-pulmonary interactions, drive both developmental and compensatory lung growth. Re-initiation of anatomical lung growth in the mature lung is possible when the loss of functioning lung units renders the existing physiologic-structural reserves insufficient for maintaining adequate function and physical stress on the remaining units exceeds a critical threshold. The appropriate spatial and temporal mechanical interrelationships and the availability of intra-thoracic space, are crucial to growth initiation, follow-on remodeling and physiological outcome. While the endogenous potential for compensatory lung growth is retained and may be pharmacologically augmented, supra-optimal mechanical stimulation, unbalanced structural growth, or inadequate remodeling may limit functional gain. Finding ways to optimize the signal-response relationships and resolve structure-function discrepancies are major challenges that must be overcome before the innate compensatory ability could be fully realized. Partial pneumonectomy reproducibly removes a known fraction of functioning lung units and remains the most robust model for examining the adaptive mechanisms, structure-function consequences and plasticity of the remaining functioning lung units capable of regeneration. Fundamental mechanical stimulus-response relationships established in the pneumonectomy model directly inform the exploration of effective approaches to maximize compensatory growth and function in chronic destructive lung diseases, transplantation and bioengineered lungs.

Perfusion-related stimuli for compensatory lung growth following pneumonectomy

Following pneumonectomy (PNX), two separate mechanical forces act on the remaining lung: parenchymal stress caused by lung expansion, and microvascular distension and shear caused by increased perfusion. We previously showed that parenchymal stress and strain explain approximately one-half of overall compensation; the remainder was presumptively attributed to perfusion-related factors. In this study, we directly tested the hypothesis that perturbation of regional pulmonary perfusion modulates post-PNX lung growth. Adult canines underwent banding of the pulmonary artery (PAB) to the left caudal (LCa) lobe, which caused a reduction in basal perfusion to LCa lobe without preventing the subsequent increase in its perfusion following right PNX while simultaneously exaggerating the post-PNX increase in perfusion to the unbanded lobes, thereby creating differential perfusion changes between banded and unbanded lobes. Control animals underwent sham pulmonary artery banding followed by right PNX. Pulmonary function, regional pulmonary perfusion, and high-resolution computed tomography of the chest were analyzed pre-PNX and 3-mo post-PNX. Terminally, the remaining lobes were fixed for detailed morphometric analysis. Results were compared with corresponding lobes in two control (Sham banding and normal unoperated) groups. PAB impaired the indices of post-PNX extravascular alveolar tissue growth by up to 50% in all remaining lobes. PAB enhanced the expected post-PNX increase in alveolar capillary formation, measured by the prevalence of double-capillary profiles, in both unbanded and banded lobes. We conclude that perfusion distribution provides major stimuli for post-PNX compensatory lung growth independent of the stimuli provided by lung expansion and parenchymal stress and strain.

Lung Structure and the Intrinsic Challenges of Gas Exchange

Structural and functional complexities of the mammalian lung evolved to meet a unique set of challenges, namely, the provision of efficient delivery of inspired air to all lung units within a confined thoracic space, to build a large gas exchange surface associated with minimal barrier thickness and a microvascular network to accommodate the entire right ventricular cardiac output while withstanding cyclic mechanical stresses that increase several folds from rest to exercise. Intricate regulatory mechanisms at every level ensure that the dynamic capacities of ventilation, perfusion, diffusion, and chemical binding to hemoglobin are commensurate with usual metabolic demands and periodic extreme needs for activity and survival. This article reviews the structural design of mammalian and human lung, its functional challenges, limitations, and potential for adaptation. We discuss (i) the evolutionary origin of alveolar lungs and its advantages and compromises, (ii) structural determinants of alveolar gas exchange, including architecture of conducting bronchovascular trees that converge in gas exchange units, (iii) the challenges of matching ventilation, perfusion, and diffusion and tissue-erythrocyte and thoracopulmonary interactions. The notion of erythrocytes as an integral component of the gas exchanger is emphasized. We further discuss the signals, sources, and limits of structural plasticity of the lung in alveolar hypoxia and following a loss of lung units, and the promise and caveats of interventions aimed at augmenting endogenous adaptive responses. Our objective is to understand how individual components are matched at multiple levels to optimize organ function in the face of physiological demands or pathological constraints.

Improving the Accuracy of Registration-Based Biomechanical Analysis: A Finite Element Approach to Lung Regional Strain Quantification

Tissue deformation plays an important role in lung physiology, as lung parenchyma largely deforms during spontaneous ventilation. However, excessive regional deformation may lead to lung injury, as observed in patients undergoing mechanical ventilation. Thus, the accurate estimation of regional strain has recently received great attention in the intensive care community. In this work, we assess the accuracy of regional strain maps computed from direct differentiation of B-Spline (BS) interpolations, a popular technique employed in non-rigid registration of lung computed tomography (CT) images. We show that, while BS-based registration methods give excellent results for the deformation transformation, the strain field directly computed from BS derivatives results in predictions that largely oscillate, thus introducing important errors that can even revert the sign of strain. To alleviate such spurious behavior, we present a novel finite-element (FE) method for the regional strain analysis of lung CT images. The method follows from a variational strain recovery formulation, and delivers a continuous approximation to the strain field in arbitrary domains. From analytical benchmarks, we show that the FE method results in errors that are a fraction of those found for the BS method, both in an average and pointwise sense. The application of the proposed FE method to human lung CT images results in 3D strain maps are heterogeneous and smooth, showing high consistency with specific ventilation maps reported in the literature. We envision that the proposed FE method will considerably improve the accuracy of image-based biomechanical analysis, making it reliable enough for routine medical applications.

Lung Regeneration: Endogenous and Exogenous Stem Cell Mediated Therapeutic Approaches

The tissue turnover of unperturbed adult lung is remarkably slow. However, after injury or insult, a specialised group of facultative lung progenitors become activated to replenish damaged tissue through a reparative process called regeneration. Disruption in this process results in healing by fibrosis causing aberrant lung remodelling and organ dysfunction. Post-insult failure of regeneration leads to various incurable lung diseases including chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis. Therefore, identification of true endogenous lung progenitors/stem cells, and their regenerative pathway are crucial for next-generation therapeutic development. Recent studies provide exciting and novel insights into postnatal lung development and post-injury lung regeneration by native lung progenitors. Furthermore, exogenous application of bone marrow stem cells, embryonic stem cells and inducible pluripotent stem cells (iPSC) show evidences of their regenerative capacity in the repair of injured and diseased lungs. With the advent of modern tissue engineering techniques, whole lung regeneration in the lab using de-cellularised tissue scaffold and stem cells is now becoming reality. In this review, we will highlight the advancement of our understanding in lung regeneration and development of stem cell mediated therapeutic strategies in combating incurable lung diseases.

A review of recent developments and applications of morphometry/stereology in lung research

Design-based stereology is the gold standard of morphometry in lung research. Here, we analyze the current use of morphometric and stereological methods in lung research and provide an overview on recent methodological developments and biological observations made by the use of stereology. Based on this analysis we hope to provide useful recommendations for a good stereological practice to further the use of advanced and unbiased stereological methods.

Repair and regeneration of the respiratory system: complexity, plasticity, and mechanisms of lung stem cell function

Respiratory disease is the third leading cause of death in the industrialized world. Consequently, the trachea, lungs, and cardiopulmonary vasculature have been the focus of extensive investigations. Recent studies have provided new information about the mechanisms driving lung development and differentiation. However, there is still much to learn about the ability of the adult respiratory system to undergo repair and to replace cells lost in response to injury and disease. This Review highlights the multiple stem/progenitor populations in different regions of the adult lung, the plasticity of their behavior in injury models, and molecular pathways that support homeostasis and repair.