Tschumperlin, Daniel J.

Pulmonary arterial stiffness is an independent risk factor for mortality in pulmonary hypertension (PH) and plays a critical role in PH pathophysiology. Our laboratory has recently demonstrated arterial stiffening early in experimental PH, along with evidence for a mechanobiological feedback loop by which arterial stiffening promotes further cellular remodeling behaviors (Liu F, Haeger CM, Dieffenbach PB, Sicard D, Chrobak I, Coronata AM, Suárez Velandia MM, Vitali S, Colas RA, Norris PC, Marinković A, Liu X, Ma J, Rose CD, Lee SJ, Comhair SA, Erzurum SC, McDonald JD, Serhan CN, Walsh SR, Tschumperlin DJ, Fredenburgh LE. JCI Insight 1: e86987, 2016). Cyclooxygenase-2 (COX-2) and prostaglandin signaling have been implicated in stiffness-mediated regulation, with prostaglandin activity inversely correlated to matrix stiffness and remodeling behaviors in vitro, as well as to disease progression in rodent PH models. The mechanism by which mechanical signaling translates to reduced COX-2 activity in pulmonary vascular cells is unknown. The present work investigated the transcriptional regulators Yes-associated protein (YAP) and WW domain-containing transcription regulator 1 (WWTR1, a.k.a., TAZ), which are known drivers of downstream mechanical signaling, in mediating stiffness-induced changes in COX-2 and prostaglandin activity in pulmonary artery smooth muscle cells (PASMCs). We found that YAP/TAZ activity is increased in PAH PASMCs and experimental PH and is necessary for the development of stiffness-dependent remodeling phenotypes. Knockdown of YAP and TAZ markedly induces COX-2 expression and downstream prostaglandin production by approximately threefold, whereas overexpression of YAP or TAZ reduces COX-2 expression and prostaglandin production to near undetectable levels. Together, our findings demonstrate a stiffness-dependent YAP/TAZ-mediated positive feedback loop that drives remodeling phenotypes in PASMCs via reduced COX-2 and prostaglandin activity. The ability to interrupt this critical mechanobiological feedback loop and enhance local prostaglandin activity via manipulation of YAP/TAZ signaling presents a highly attractive novel strategy for the treatment of PH.

Mechanical ventilation is necessary to support patients with acute lung injury, but also exacerbates injury through mechanical stress–activated signaling pathways. We show that stretch applied to cultured human cells, and to mouse lungs in vivo, induces robust expression of metallothionein, a potent antioxidant and cytoprotective molecule critical for cellular zinc homeostasis. Furthermore, genetic deficiency of murine metallothionein genes exacerbated lung injury caused by high tidal volume mechanical ventilation, identifying an adaptive role for these genes in limiting lung injury. Stretch induction of metallothionein required zinc and the zinc-binding transcription factor MTF1. We further show that mouse dietary zinc deficiency potentiates ventilator-induced lung injury, and that plasma zinc levels are significantly reduced in human patients who go on to develop acute respiratory distress syndrome (ARDS) compared with healthy and non-ARDS intensive care unit (ICU) controls, as well as with other ICU patients without ARDS. Taken together, our findings identify a potentially novel adaptive response of the lung to stretch and a critical role for zinc in defining the lung’s tolerance for mechanical ventilation. These results demonstrate that failure of stretch-adaptive responses play an important role in exacerbating mechanical ventilator–induced lung injury, and identify zinc and metallothionein as targets for lung-protective interventions in patients requiring mechanical ventilation.

Recent studies have implicated the Hippo pathway and its transcriptional effectors YAP and TAZ as necessary for fibroblast activation and tissue fibrosis. To test the specific and sufficient roles for TAZ in driving autonomous fibroblast activation, we cultured NIH3T3 fibroblasts expressing a doxycycline-inducible nuclear-localized mutant of TAZ (TAZ4SA) in scaffold-free 3D hanging drop spheroids, or on matrices of specified mechanical rigidity. Control NIH3T3 fibroblasts formed spheroids in hanging drop culture that remained stable and neither increased nor decreased in size significantly over 15 days. In contrast, TAZ4SA-transduced fibroblasts grew robustly in spheroid culture, and expressed enhanced levels of genes encoding profibrotic soluble factors connective tissue growth factor (CTGF), endothelin-1 (Et-1), and plasminogen activator inhibitor 1 (PAI-1). However, TAZ4SA expression was unable to enhance expression of extracellular matrix (ECM)-encoding genes Col1a1, Col1a2, Col3a1, or Fn1 in spheroid culture. Micromechanical testing indicated that spheroids composed of either control or TAZ4SA-expressing cells were highly compliant and indistinguishable in mechanical properties. In fibroblasts cultured on 2D matrices of compliance similar to spheroids, TAZ4SA expression was able to enhance contractile force generation, but was unable to enhance ECM gene expression. In contrast, culture on stiff hydrogels potentiated TAZ4SA enhancement of ECM expression. TAZ4SA enhancement of Col1a1 expression on soft matrices was potentiated by TGF-β1, while on stiff matrices it was abrogated by inhibition of myocardin-related transcription factor, demonstrating context-dependent crosstalk of TAZ with these pathways. These findings demonstrate sufficiency of TAZ activation for driving fibroblast proliferation, contraction, and soluble profibrotic factor expression, and mechanical context-dependent crosstalk of TAZ with other pathways in regulating Col1a1 expression.

Abstract Malignant esophageal pathology typically requires resection of the esophagus and reconstruction to restore foregut continuity. Reconstruction options are limited and morbid. The esophagus represents a useful target for tissue engineering strategies based on relative simplicity in comparison to other organs. The ideal tissue engineered conduit would have sufficient and ideally matched mechanical tolerances to native esophageal tissue. Current methods for mechanical testing of esophageal tissues both in vivo and ex vivo are typically destructive, alter tissue conformation, ignore anisotropy, or are not able to be performed in fluid media. The aim of this study was to investigate biomechanical properties of swine esophageal tissues through nondestructive testing utilizing sonometry ex vivo . This method allows for biomechanical determination of tissue properties, particularly longitudinal and circumferential moduli and strain energy functions. The relative contribution of mucosal–submucosal layers and muscular layers are compared to composite esophagi. Swine thoracic esophageal tissues ( n = 15) were tested by pressure loading using a continuous pressure pump system to generate stress. Preconditioning of tissue was performed by pressure loading with the pump system and pre-straining the tissue to in vivo length before data was recorded. Sonometry using piezocrystals was utilized to determine longitudinal and circumferential strain on five composite esophagi. Similarly, five mucosa–submucosal and five muscular layers from thoracic esophagi were tested independently. This work on esophageal tissues is consistent with reported uniaxial and biaxial mechanical testing and reported results using strain energy theory and also provides high resolution displacements, preserves native architectural structure and allows assessment of biomechanical properties in fluid media. This method may be of use to characterize mechanical properties of tissue engineered esophageal constructs.

Pulmonary arterial (PA) stiffness is associated with increased mortality in patients with pulmonary hypertension (PH); however, the role of PA stiffening in the pathogenesis of PH remains elusive. Here, we show that distal vascular matrix stiffening is an early mechanobiological regulator of experimental PH. We identify cyclooxygenase-2 (COX-2) suppression and corresponding reduction in prostaglandin production as pivotal regulators of stiffness-dependent vascular cell activation. Atomic force microscopy microindentation demonstrated early PA stiffening in experimental PH and human lung tissue. Pulmonary artery smooth muscle cells (PASMC) grown on substrates with the stiffness of remodeled PAs showed increased proliferation, decreased apoptosis, exaggerated contraction, enhanced matrix deposition, and reduced COX-2–derived prostanoid production compared with cells grown on substrates approximating normal PA stiffness. Treatment with a prostaglandin I 2 analog abrogated monocrotaline-induced PA stiffening and attenuated stiffness-dependent increases in proliferation, matrix deposition, and contraction in PASMC. Our results suggest a pivotal role for early PA stiffening in PH and demonstrate the therapeutic potential of interrupting mechanobiological feedback amplification of vascular remodeling in experimental PH.

Lung diseases such as emphysema, interstitial fibrosis, and pulmonary vascular diseases cause significant morbidity and mortality, but despite substantial mechanistic understanding, clinical management options for them are limited, with lung transplantation being implemented at end stages. However, limited donor lung availability, graft rejection, and long-term problems after transplantation are major hurdles to lung transplantation being a panacea. Bioengineering the lung is an exciting and emerging solution that has the ultimate aim of generating lung tissues and organs for transplantation. In this article we capture and review the current state of the art in lung bioengineering, from the multimodal approaches, to creating anatomically appropriate lung scaffolds that can be recellularized to eventually yield functioning, transplant-ready lungs. Strategies for decellularizing mammalian lungs to create scaffolds with native extracellular matrix components vs. de novo generation of scaffolds using biocompatible materials are discussed. Strengths vs. limitations of recellularization using different cell types of various pluripotency such as embryonic, mesenchymal, and induced pluripotent stem cells are highlighted. Current hurdles to guide future research toward achieving the clinical goal of transplantation of a bioengineered lung are discussed.

Abstract The extracellular matrix (ECM) of the lung serves as both a scaffold for resident cells and a mechanical support for respiratory function. The ECM is deposited during development and undergoes continuous turnover and maintenance during organ growth and homeostasis. Cells of the mesenchyme, including the tissue resident fibroblast, take a leading role in depositing and organizing the matrix and do so in an anatomically distinct fashion, with differing composition, organization, and mechanical properties within the airways, vessels, and alveoli of the lung. Recent technological advancements have allowed the lung’s ECM biochemical composition and mechanical properties to be studied with improved resolution, thereby identifying novel disease-related changes in ECM characteristics. In parallel, efforts to study cells seeded on normal and disease-derived matrices have illustrated the powerful role the ECM can play in altering key functions of lung resident cells. The mechanical properties of the matrix have been identified as an important modifier of cell–matrix adhesions, with matrices of pathologic stiffness promoting profibrotic signaling and cell function. Ongoing work is identifying both mechanically activated pathways in mesenchymal cells and disease-related ECM molecules that biochemically regulate cell function. Uncovering the control systems by which cells respond to and regulate the matrix, and the failures in these systems that underlie aberrant repair, remains a major challenge. Progress in this area will be an essential element in efforts to engineer functional lung tissue for regenerative approaches and will be key to identifying new therapeutic strategies for lung diseases characterized by disturbed matrix architecture.