The inter-connection between fibrosis and microvascular remodeling in idiopathic pulmonary fibrosis: Reality or just a phenomenon

The sclerotic component of metabolic syndrome: Fibroblast activities may be the central common denominator driving organ function loss and death

Fibrosis is a common feature of more than 50 different diseases and the cause of more than 35% of deaths worldwide, of which liver, kidney, skin, heart and, recently, lungs are receiving the most attention. Tissue changes, resulting in loss of organ function, are both a cause and consequence of disease and outcome. Fibrosis is caused by an excess deposition of extracellular matrix proteins, which over time results in impaired organ function and organ failure, and the pathways leading to increased fibroblast activation are many. This narrative review investigated the common denominator of fibrosis, fibroblasts, and the activation of fibroblasts, in response to excess energy consumption in liver, kidney, heart, skin and lung fibrosis. Fibroblasts are the main drivers of organ function loss in lung, liver, skin, heart and kidney disease. Fibroblast activation in response to excess energy consumption results in the overproduction of a range of collagens, of which types I, III and VI seem to be the essential drivers of disease progression. Fibroblast activation may be quantified in serum, enabling profiling and selection of patients. Activation of fibroblasts results in the overproduction of collagens, which deteriorates organ function. Patient profiling of fibroblast activities in serum, quantified as collagen production, may identify an organ death trajectory, better enabling identification of the right treatment for use in different metabolic interventions. As metabolically activated patients have highly elevated risk of kidney, liver and heart failure, it is essential to identify which organ to treat first and monitor organ status to correct treatment regimes. In direct alignment with this, it is essential to identify the right patients with the right organ deterioration trajectory for enrolment in clinical studies.

Lung injury-induced activated endothelial cell states persist in aging-associated progressive fibrosis

Progressive lung fibrosis is associated with poorly understood aging-related endothelial cell dysfunction. To gain insight into endothelial cell alterations in lung fibrosis we performed single cell RNA-sequencing of bleomycin-injured lungs from young and aged mice. Analysis reveals activated cell states enriched for hypoxia, glycolysis and YAP/TAZ activity in ACKR1+ venous and TrkB+ capillary endothelial cells. Endothelial cell activation is prevalent in lungs of aged mice and can also be detected in human fibrotic lungs. Longitudinal single cell RNA-sequencing combined with lineage tracing demonstrate that endothelial activation resolves in young mouse lungs but persists in aged ones, indicating a failure of the aged vasculature to return to quiescence. Genes associated with activated lung endothelial cells states in vivo can be induced in vitro by activating YAP/TAZ. YAP/TAZ also cooperate with BDNF, a TrkB ligand that is reduced in fibrotic lungs, to promote capillary morphogenesis. These findings offer insights into aging-related lung endothelial cell dysfunction that may contribute to defective lung injury repair and persistent fibrosis.

Vascular Contribution to Lung Repair and Fibrosis

Lungs are constantly exposed to environmental perturbations and therefore have remarkable capacity to regenerate in response to injury. Sustained lung injuries, aging, and increased genomic instability, however, make lungs particularly susceptible to disrepair and fibrosis. Pulmonary fibrosis constitutes a major cause of morbidity and is often relentlessly progressive, leading to death from respiratory failure. The pulmonary vasculature, which is critical for gas exchanges and plays a key role during lung development, repair, and regeneration, becomes aberrantly remodeled in patients with progressive pulmonary fibrosis. Although capillary rarefaction and increased vascular permeability are recognized as distinctive features of fibrotic lungs, the role of vasculature dysfunction in the pathogenesis of pulmonary fibrosis has only recently emerged as an important contributor to the progression of this disease. This review summarizes current findings related to lung vascular repair and regeneration and provides recent insights into the vascular abnormalities associated with the development of persistent lung fibrosis.

Microcomputed tomography visualization and quantitation of the pulmonary arterial microvascular tree in mouse models of chronic lung disease

Pulmonary vascular dysfunction is characterized by remodeling and loss of microvessels in the lung and is a major manifestation of chronic lung diseases (CLD). In murine models of CLD, the small arterioles and capillaries are the first and most prevalent vessels that are affected by pruning and remodeling. Thus, visualization of the pulmonary arterial vasculature in three dimensions is essential to define pruning and remodeling both temporally and spatially and its role in the pathogenesis of CLD, aging, and tissue repair. To this end, we have developed a novel method to visualize and quantitate the murine pulmonary arterial circulation using microcomputed tomography (µCT) imaging. Using this perfusion technique, we can quantitate microvessels to approximately 6 µM in diameter. We hypothesize that bleomycin-induced injury would have a significant impact on the arterial vascular structure. As proof of principle, we demonstrated that as a result of bleomycin-induced injury at peak fibrosis, significant alterations in arterial vessel structure were visible in the three-dimensional models as well as quantification. Thus, we have successfully developed a perfusion methodology and complementary analysis techniques, which allows for the reconstruction, visualization, and quantitation of the mouse pulmonary arterial microvasculature in three-dimensions. This tool will further support the examination and understanding of angiogenesis during the development of CLD as well as repair following injury.

Dysfunctional ERG signaling drives pulmonary vascular aging and persistent fibrosis

Vascular dysfunction is a hallmark of chronic diseases in elderly. The contribution of the vasculature to lung repair and fibrosis is not fully understood. Here, we performed an epigenetic and transcriptional analysis of lung endothelial cells (ECs) from young and aged mice during the resolution or progression of bleomycin-induced lung fibrosis. We identified the transcription factor ETS-related gene (ERG) as putative orchestrator of lung capillary homeostasis and repair, and whose function is dysregulated in aging. ERG dysregulation is associated with reduced chromatin accessibility and maladaptive transcriptional responses to injury. Loss of endothelial ERG enhances paracrine fibroblast activation in vitro, and impairs lung fibrosis resolution in young mice in vivo. scRNA-seq of ERG deficient mouse lungs reveales transcriptional and fibrogenic abnormalities resembling those associated with aging and human lung fibrosis, including reduced number of general capillary (gCap) ECs. Our findings demonstrate that lung endothelial chromatin remodeling deteriorates with aging leading to abnormal transcription, vascular dysrepair, and persistent fibrosis following injury.

Irreversibility of Pulmonary Fibrosis

Pulmonary fibrosis, a kind of terminal pathological changes in the lung, is caused by aberrant wound healing, deposition of extracellular matrix (ECM), and eventually replacement of lung parenchyma by ECM. Pulmonary fibrosis induced by acute lung injury and some diseases is reversible under treatment. While idiopathic pulmonary fibrosis is persistent and irreversible even after treatment. Currently, the pathogenesis of irreversible pulmonary fibrosis is not fully elucidated. The known factors associated with the development of irreversible fibrosis include apoptosis resistance of (myo)fibroblasts, dysfunction of pulmonary vessel, cell mitochondria and autophagy, aberrant epithelia hyperplasia and lipid metabolism disorder. In this review, other than a brief introduction of reversible pulmonary fibrosis, we focus on the underlying pathogenesis of irreversible pulmonary fibrosis from the above aspects as well as preclinical disease models, and also suggest directions for future studies.

Vascular dysfunction in aged mice contributes to persistent lung fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive disease thought to result from impaired lung repair following injury and is strongly associated with aging. While vascular alterations have been associated with IPF previously, the contribution of lung vasculature during injury resolution and fibrosis is not well understood. To compare the role of endothelial cells (ECs) in resolving and non-resolving models of lung fibrosis, we applied bleomycin intratracheally to young and aged mice. We found that injury in aged mice elicited capillary rarefaction, while injury in young mice resulted in increased capillary density. ECs from the lungs of injured aged mice relative to young mice demonstrated elevated pro-fibrotic and reduced vascular homeostasis gene expression. Among the latter, Nos3 (encoding the enzyme endothelial nitric oxide synthase, eNOS) was transiently upregulated in lung ECs from young but not aged mice following injury. Young mice deficient in eNOS recapitulated the non-resolving lung fibrosis observed in aged animals following injury, suggesting that eNOS directly participates in lung fibrosis resolution. Activation of the NO receptor soluble guanylate cyclase in human lung fibroblasts reduced TGFβ-induced pro-fibrotic gene and protein expression. Additionally, loss of eNOS in human lung ECs reduced the suppression of TGFβ-induced lung fibroblast activation in 2D and 3D co-cultures. Altogether, our results demonstrate that persistent lung fibrosis in aged mice is accompanied by capillary rarefaction, loss of EC identity, and impaired eNOS expression. Targeting vascular function may thus be critical to promote lung repair and fibrosis resolution in aging and IPF.

Histological hallmarks and role of Slug/PIP axis in pulmonary hypertension secondary to pulmonary fibrosis

Pulmonary hypertension secondary to pulmonary fibrosis (PF-PH) is one of the most common causes of PH, and there is no approved therapy. The molecular signature of PF-PH and underlying mechanism of why pulmonary hypertension (PH) develops in PF patients remains understudied and poorly understood. We observed significantly increased vascular wall thickness in both fibrotic and non-fibrotic areas of PF-PH patient lungs compared to PF patients. The increased vascular wall thickness in PF-PH patients is concomitant with a significantly increased expression of the transcription factor Slug within the macrophages and its target prolactin-induced protein (PIP), an extracellular matrix protein that induces pulmonary arterial smooth muscle cell proliferation. We developed a novel translational rat model of combined PF-PH that is reproducible and shares similar histological features (fibrosis, pulmonary vascular remodeling) and molecular features (Slug and PIP upregulation) with human PF-PH. We found Slug inhibition decreases PH severity in our animal model of PF-PH. Our study highlights the role of Slug/PIP axis in PF-PH.

Deregulated angiogenesis in chronic lung diseases: a possible role for lung mesenchymal progenitor cells (2017 Grover Conference Series)

Chronic lung disease (CLD), including pulmonary fibrosis (PF) and chronic obstructive pulmonary disease (COPD), is the fourth leading cause of mortality worldwide. Both are debilitating pathologies that impede overall tissue function. A common co-morbidity in CLD is vasculopathy, characterized by deregulated angiogenesis, remodeling, and loss of microvessels. This substantially worsens prognosis and limits survival, with most current therapeutic strategies being largely palliative. The relevance of angiogenesis, both capillary and lymph, to the pathophysiology of CLD has not been resolved as conflicting evidence depicts angiogenesis as both reparative or pathologic. Therefore, we must begin to understand and model the underlying pathobiology of pulmonary vascular deregulation, alone and in response to injury induced disease, to define cell interactions necessary to maintain normal function and promote repair. Capillary and lymphangiogenesis are deregulated in both PF and COPD, although the mechanisms by which they co-regulate and underlie early pathogenesis of disease are unknown. The cell-specific mechanisms that regulate lung vascular homeostasis, repair, and remodeling represent a significant gap in knowledge, which presents an opportunity to develop targeted therapies. We have shown that that ABCG2^pos multipotent adult mesenchymal stem or progenitor cells (MPC) influence the function of the capillary microvasculature as well as lymphangiogenesis. A balance of both is required for normal tissue homeostasis and repair. Our current models suggest that when lymph and capillary angiogenesis are out of balance, the non-equivalence appears to support the progression of disease and tissue remodeling. The angiogenic regulatory mechanisms underlying CLD likely impact other interstitial lung diseases, tuberous sclerosis, and lymphangioleiomyomatosis.