Hermansky-Pudlak syndrome interstitial pneumonia: it's the epithelium, stupid!

Modeling of Fibrotic Lung Disease Using 3D Organoids Derived from Human Pluripotent Stem Cells

The pathogenesis of idiopathic pulmonary fibrosis (IPF), an intractable interstitial lung disease, is unclear. Recessive mutations in some genes implicated in Hermansky-Pudlak syndrome (HPS) cause HPS-associated interstitial pneumonia (HPSIP), a clinical entity that is similar to IPF. We previously reported that HPS1^-/- embryonic stem cell-derived 3D lung organoids showed fibrotic changes. Here, we show that the introduction of all HPS mutations associated with HPSIP promotes fibrotic changes in lung organoids, while the deletion of HPS8, which is not associated with HPSIP, does not. Genome-wide expression analysis revealed the upregulation of interleukin-11 (IL-11) in epithelial cells from HPS mutant fibrotic organoids. IL-11 was detected predominantly in type 2 alveolar epithelial cells in end-stage IPF, but was expressed more broadly in HPSIP. Finally, IL-11 induced fibrosis in WT organoids, while its deletion prevented fibrosis in HPS4^-/- organoids, suggesting IL-11 as a therapeutic target. hPSC-derived 3D lung organoids are, therefore, a valuable resource to model fibrotic lung disease.

Proteostasis in pediatric pulmonary pathology

Protein homeostasis describes the tight supervision of protein synthesis, correct protein maturation and folding, as well as the timely disposal of unwanted and damaged proteins by the ubiquitin-proteasome pathway or the lysosome-autophagy route. The cellular processes involved in preservation of protein homeostasis are collectively called proteostasis. Dysregulation of proteostasis is an emerging common pathomechanism for chronic lung diseases in the adult and aged patient. There is also rising evidence that impairment of protein homeostasis contributes to early sporadic disease onset in pediatric lung diseases beyond the well-known hereditary proteostasis disorders such as cystic fibrosis and alpha-1 antitrypsin (AAT) deficiency. Identifying the pathways that contribute to impaired proteostasis will provide new avenues for therapeutic interference with the pathogenesis of chronic lung diseases in the young and adult. Here, we introduce the concept of proteostasis and summarize available evidence on dysregulation of proteostasis pathways in pediatric and adult chronic lung diseases.

Grainyhead-like 2 (GRHL2) distribution reveals novel pathophysiological differences between human idiopathic pulmonary fibrosis and mouse models of pulmonary fibrosis

Chronic injury of alveolar lung epithelium leads to epithelial disintegrity in idiopathic pulmonary fibrosis (IPF). We had reported earlier that Grhl2, a transcriptional factor, maintains alveolar epithelial cell integrity by directly regulating components of adherens and tight junctions and thus hypothesized an important role of GRHL2 in pathogenesis of IPF. Comparison of GRHL2 distribution at different stages of human lung development showed its abundance in developing lung epithelium and in adult lung epithelium. However, GRHL2 is detected in normal human lung mesenchyme only at early fetal stage (week 9). Similar mesenchymal reexpression of GRHL2 was also observed in IPF. Immunofluorescence analysis in serial sections from three IPF patients revealed at least two subsets of alveolar epithelial cells (AEC), based on differential GRHL2 expression and the converse fluorescence intensities for epithelial vs. mesenchymal markers. Grhl2 was not detected in mesenchyme in intraperitoneal bleomycin-induced injury as well as in spontaneously occurring fibrosis in double-mutant HPS1 and HPS2 mice, whereas in contrast in a radiation-induced fibrosis model, with forced Forkhead box M1 (Foxm1) expression, an overlap of Grhl2 with a mesenchymal marker was observed in fibrotic regions. Grhl2's role in alveolar epithelial cell plasticity was confirmed by altered Grhl2 gene expression analysis in IPF and further validated by in vitro manipulation of its expression in alveolar epithelial cell lines. Our findings reveal important pathophysiological differences between human IPF and specific mouse models of fibrosis and support a crucial role of GRHL2 in epithelial activation in lung fibrosis and perhaps also in epithelial plasticity.

Metabolic and storage lung diseases: spectrum of imaging appearances

Metabolic and storage diseases constitute a heterogeneous group of disorders that occur in the setting of altered biochemical homeostasis. Many of these disorders affect the lungs, either exclusively or as part of a systemic syndrome. For example, amyloidosis can be limited to the tracheobronchial tree or involve the kidneys, lungs and heart. The indolent course of some of these disorders and the non-specific clinical symptoms often result in a diagnostic challenge. Imaging, particularly high-resolution computed tomography (HRCT), is an invaluable asset in the diagnosis of these clinical conditions. Some metabolic and storage diseases have characteristic HRCT appearances, helping narrow the differential diagnosis. Correlation of the radiological and histopathological findings of this group of diseases has also helped improve understanding of these disorders. In addition, CT can offer guidance when tissue sampling is warranted and aid in histopathological diagnosis. This article describes the pertinent clinical features of the more common metabolic and storage diseases affecting the lungs, illustrates their respective HRCT findings and provides the relevant differential diagnosis.

TEACHING POINTS

• To recognise the various metabolic and storage lung diseases • To identify the characteristic imaging findings in various metabolic and storage lung diseases • To discuss the relevant differential diagnoses of each of these diseases.