PRDM3/16 Regulate Chromatin Accessibility Required for NKX2-1 Mediated Alveolar Epithelial Differentiation and Function

Differential chromatin accessibility accompanies and mediates transcriptional control of diverse cell fates and their differentiation during embryogenesis. While the critical role of NKX2-1 and its transcriptional targets in lung morphogenesis and pulmonary epithelial cell differentiation is increasingly known, mechanisms by which chromatin accessibility alters the epigenetic landscape and how NKX2-1 interacts with other co-activators required for alveolar epithelial cell differentiation and function are not well understood. Here, we demonstrate that the paired domain zinc finger transcriptional regulators PRDM3 and PRDM16 regulate chromatin accessibility to mediate cell differentiation decisions during lung morphogenesis. Combined deletion of Prdm3 and Prdm16 in early lung endoderm caused perinatal lethality due to respiratory failure from loss of AT2 cell function. Prdm3/16 deletion led to the accumulation of partially differentiated AT1 cells and loss of AT2 cells. Combination of single cell RNA-seq, bulk ATAC-seq, and CUT&RUN demonstrated that PRDM3 and PRDM16 enhanced chromatin accessibility at NKX2-1 transcriptional targets in peripheral epithelial cells, all three factors binding together at a multitude of cell-type specific cis-active DNA elements. Network analysis demonstrated that PRDM3/16 regulated genes critical for perinatal AT2 cell differentiation, surfactant homeostasis, and innate host defense. Lineage specific deletion of PRDM3/16 in AT2 cells led to lineage infidelity, with PRDM3/16 null cells acquiring partial AT1 fate. Together, these data demonstrate that NKX2-1-dependent regulation of alveolar epithelial cell differentiation is mediated by epigenomic modulation via PRDM3/16.

Durable alveolar engraftment of PSC-derived lung epithelial cells into immunocompetent mice

Durable reconstitution of the distal lung epithelium with pluripotent stem cell (PSC) derivatives, if realized, would represent a promising therapy for diseases that result from alveolar damage. Here, we differentiate murine PSCs into self-renewing lung epithelial progenitors able to engraft into the injured distal lung epithelium of immunocompetent, syngeneic mouse recipients. After transplantation, these progenitors mature in the distal lung, assuming the molecular phenotypes of alveolar type 2 (AT2) and type 1 (AT1) cells. After months in vivo, donor-derived cells retain their mature phenotypes, as characterized by single-cell RNA sequencing (scRNA-seq), histologic profiling, and functional assessment that demonstrates continued capacity of the engrafted cells to proliferate and differentiate. These results indicate durable reconstitution of the distal lung's facultative progenitor and differentiated epithelial cell compartments with PSC-derived cells, thus establishing a novel model for pulmonary cell therapy that can be utilized to better understand the mechanisms and utility of engraftment.

Surfactant protein C mutation links postnatal type 2 cell dysfunction to adult disease

Mutations in the gene SFTPC, encoding surfactant protein C (SP-C), are associated with interstitial lung disease in children and adults. To assess the natural history of disease, we knocked in a familial, disease-associated SFTPC mutation, L188Q (L184Q [LQ] in mice), into the mouse Sftpc locus. Translation of the mutant proprotein, proSP-C^LQ, exceeded that of proSP-C^WT in neonatal alveolar type 2 epithelial cells (AT2 cells) and was associated with transient activation of oxidative stress and apoptosis, leading to impaired expansion of AT2 cells during postnatal alveolarization. Differentiation of AT2 to AT1 cells was also inhibited in ex vivo organoid culture of AT2 cells isolated from LQ mice; importantly, treatment with antioxidant promoted alveolar differentiation. Upon completion of alveolarization, Sftpc^LQ expression was downregulated, leading to resolution of chronic stress responses; however, the failure to restore AT2 cell numbers resulted in a permanent loss of AT2 cells that was linked to decreased regenerative capacity in the adult lung. Collectively, these data support the hypothesis that susceptibility to disease in adult LQ mice is established during postnatal lung development, and they provide a potential explanation for the delayed onset of disease in patients with familial pulmonary fibrosis.

IGF1R controls mechanosignaling in myofibroblasts required for pulmonary alveologenesis

Ventilation throughout life is dependent on the formation of pulmonary alveoli, which create an extensive surface area in which the close apposition of respiratory epithelium and endothelial cells of the pulmonary microvascular enables efficient gas exchange. Morphogenesis of the alveoli initiates at late gestation in humans and the early postnatal period in the mouse. Alveolar septation is directed by complex signaling interactions among multiple cell types. Here, we demonstrate that IGF1 receptor gene (Igf1r) expression by a subset of pulmonary fibroblasts is required for normal alveologenesis in mice. Postnatal deletion of Igf1r caused alveolar simplification, disrupting alveolar elastin networks and extracellular matrix without altering myofibroblast differentiation or proliferation. Moreover, loss of Igf1r impaired contractile properties of lung myofibroblasts and inhibited myosin light chain (MLC) phosphorylation and mechanotransductive nuclear YAP activity. Activation of p-AKT, p-MLC, and nuclear YAP in myofibroblasts was dependent on Igf1r. Pharmacologic activation of AKT enhanced MLC phosphorylation, increased YAP activation, and ameliorated alveolar simplification in vivo. IGF1R controls mechanosignaling in myofibroblasts required for lung alveologenesis.

In Vivo Generation of Lung and Thyroid Tissues from Embryonic Stem Cells Using Blastocyst Complementation

Rationale: The regeneration and replacement of lung cells or tissues from induced pluripotent stem cell- or embryonic stem cell-derived cells represent future therapies for life-threatening pulmonary disorders but are limited by technical challenges to produce highly differentiated cells able to maintain lung function. Functional lung tissue-containing airways, alveoli, vasculature, and stroma have never been produced via directed differentiation of embryonic stem cells (ESCs) or induced pluripotent stem cells. We sought to produce all tissue components of the lung from bronchi to alveoli by embryo complementation.Objectives: To determine whether ESCs are capable of generating lung tissue in Nkx2-1^-/- mouse embryos with lung agenesis.Methods: Blastocyst complementation was used to produce chimeras from normal mouse ESCs and Nkx2-1^-/- embryos, which lack pulmonary tissues. Nkx2-1^-/- chimeras were examined using immunostaining, transmission electronic microscopy, fluorescence-activated cell sorter analysis, and single-cell RNA sequencing.Measurements and Main Results: Although peripheral pulmonary and thyroid tissues are entirely lacking in Nkx2-1 gene-deleted embryos, pulmonary and thyroid structures in Nkx2-1^-/- chimeras were restored after ESC complementation. Respiratory epithelial cell lineages in restored lungs of Nkx2-1^-/- chimeras were derived almost entirely from ESCs, whereas endothelial, immune, and stromal cells were mosaic. ESC-derived cells from multiple respiratory cell lineages were highly differentiated and indistinguishable from endogenous cells based on morphology, ultrastructure, gene expression signatures, and cell surface proteins used to identify cell types by fluorescence-activated cell sorter.Conclusions: Lung and thyroid tissues were generated in vivo from ESCs by blastocyst complementation. Nkx2-1^-/- chimeras can be used as "bioreactors" for in vivo differentiation and functional studies of ESC-derived progenitor cells.

EMC3 coordinates surfactant protein and lipid homeostasis required for respiration

Adaptation to respiration at birth depends upon the synthesis of pulmonary surfactant, a lipid-protein complex that reduces surface tension at the air-liquid interface in the alveoli and prevents lung collapse during the ventilatory cycle. Herein, we demonstrated that the gene encoding a subunit of the endoplasmic reticulum membrane complex, EMC3, also known as TMEM111 (Emc3/Tmem111), was required for murine pulmonary surfactant synthesis and lung function at birth. Conditional deletion of Emc3 in murine embryonic lung epithelial cells disrupted the synthesis and packaging of surfactant lipids and proteins, impaired the formation of lamellar bodies, and induced the unfolded protein response in alveolar type 2 (AT2) cells. EMC3 was essential for the processing and routing of surfactant proteins, SP-B and SP-C, and the biogenesis of the phospholipid transport protein ABCA3. Transcriptomic, lipidomic, and proteomic analyses demonstrated that EMC3 coordinates the assembly of lipids and proteins in AT2 cells that is necessary for surfactant synthesis and function at birth.

Differentiation of Human Pluripotent Stem Cells into Functional Lung Alveolar Epithelial Cells

Lung alveoli, which are unique to air-breathing organisms, have been challenging to generate from pluripotent stem cells (PSCs) in part because there are limited model systems available to provide the necessary developmental roadmaps for in vitro differentiation. Here we report the generation of alveolar epithelial type 2 cells (AEC2s), the facultative progenitors of lung alveoli, from human PSCs. Using multicolored fluorescent reporter lines, we track and purify human SFTPC+ alveolar progenitors as they emerge from endodermal precursors in response to stimulation of Wnt and FGF signaling. Purified PSC-derived SFTPC+ cells form monolayered epithelial "alveolospheres" in 3D cultures without the need for mesenchymal support, exhibit self-renewal capacity, and display additional AEC2 functional capacities. Footprint-free CRISPR-based gene correction of PSCs derived from patients carrying a homozygous surfactant mutation (SFTPB121ins2) restores surfactant processing in AEC2s. Thus, PSC-derived AEC2s provide a platform for disease modeling and future functional regeneration of the distal lung.

The Phosphatidylcholine Transfer Protein Stard7 is Required for Mitochondrial and Epithelial Cell Homeostasis

Mitochondria synthesize select phospholipids but lack the machinery for synthesis of the most abundant mitochondrial phospholipid, phosphatidylcholine (PC). Although the phospholipid transfer protein Stard7 promotes uptake of PC by mitochondria, the importance of this pathway for mitochondrial and cellular homeostasis represents a significant knowledge gap. Haploinsufficiency for Stard7 is associated with significant exacerbation of allergic airway disease in mice, including an increase in epithelial barrier permeability. To test the hypothesis that Stard7 deficiency leads to altered barrier structure/function downstream of mitochondrial dysfunction, Stard7 expression was knocked down in a bronchiolar epithelial cell line (BEAS-2B) and specifically deleted in lung epithelial cells of mice (Stard7^epi∆/∆). Stard7 deficiency was associated with altered mitochondrial size and membrane organization both in vitro and in vivo. Altered mitochondrial structure was accompanied by disruption of mitochondrial homeostasis, including decreased aerobic respiration, increased oxidant stress, and mitochondrial DNA damage that, in turn, was linked to altered barrier integrity and function. Both mitochondrial and barrier defects were largely corrected by targeting Stard7 to mitochondria or treating epithelial cells with a mitochondrial-targeted antioxidant. These studies suggest that Stard7-mediated transfer of PC is crucial for mitochondrial homeostasis and that mitochondrial dysfunction contributes to altered barrier permeability in Stard7-deficient mice.

Airway Epithelial KIF3A Regulates Th2 Responses to Aeroallergens

KIF3A, the gene encoding kinesin family member 3A, is a susceptibility gene locus associated with asthma; however, mechanisms by which KIF3A might influence the pathogenesis of the disorder are unknown. In this study, we deleted the mouse Kif3a gene in airway epithelial cells. Both homozygous and heterozygous Kif3a gene-deleted mice were highly susceptible to aeroallergens from Aspergillus fumigatus and the house dust mite, resulting in an asthma-like pathology characterized by increased goblet cell metaplasia, airway hyperresponsiveness, and Th2-mediated inflammation. Deletion of the Kif3a gene increased the severity of pulmonary eosinophilic inflammation and expression of cytokines (Il-4, Il-13, and Il-17a) and chemokine (Ccl11) RNAs following pulmonary exposure to Aspergillus extract. Inhibition of Kif3a disrupted the structure of motile cilia and impaired mucociliary clearance, barrier function, and epithelial repair, demonstrating additional mechanisms by which deficiency of KIF3A in respiratory epithelial cells contributes to pulmonary pathology. Airway epithelial KIF3A suppresses Th2 pulmonary inflammation and airway hyperresponsiveness following aeroallergen exposure, implicating epithelial microtubular functions in the pathogenesis of Th2-mediated lung pathology.

Reserve autophagic capacity in alveolar epithelia provides a replicative niche for influenza A virus

Autophagy contributes to cellular homeostasis through metabolite recycling and degradation of cytotoxic protein aggregates and damaged organelles. Although recent studies have established that the requirement for basal autophagy is largely tissue specific, the importance of autophagy for alveolar epithelial cell homeostasis remains an important knowledge gap. In the present study we generated two mouse models, with > 90% or > 50% recombination at the Atg5 locus in the distal respiratory epithelium, to assess the effect of dose-dependent decreases in autophagy on alveolar homeostasis. A 90% decrease in autophagy was well tolerated in young adult mice but resulted in alveolar septal thickening and altered lung mechanics in aged animals, consistent with accumulation of damage over time. By comparison, a 50% decrease in autophagy had no effect on alveolar structure or function throughout the murine life span, indicating that basal autophagy in this compartment exceeds that required for homeostasis. A 50% decrease in autophagy in the bronchoalveolar epithelium significantly attenuated influenza A/H3N2 viral replication, leading to improved lung structure and function and reduced morbidity and mortality after infection. The reserve of autophagic capacity in the alveolar epithelium may provide a niche for replication of influenza A virus.

Deficiency of the BiP cochaperone ERdj4 causes constitutive endoplasmic reticulum stress and metabolic defects

The BiP cochaperone ERdj4 removes misfolded proteins from the ER lumen by associating with ERAD machinery. Global deficiency of ERdj4 results in widespread constitutive ER stress, decreased survival, and metabolic derangements in mice. These findings indicate that the chaperone activity of ERdj4 is important for ER homeostasis in vivo.

Endoplasmic reticulum–localized DnaJ 4 (ERdj4) is an immunoglobulin-binding protein (BiP) cochaperone and component of the endoplasmic reticulum–associated degradation (ERAD) pathway that functions to remove unfolded/misfolded substrates from the ER lumen under conditions of ER stress. To elucidate the function of ERdj4 in vivo, we disrupted the ERdj4 locus using gene trap (GT) mutagenesis, leading to hypomorphic expression of ERdj4 in mice homozygous for the trapped allele (ERdj4^GT/GT). Approximately half of ERdj4^GT/GT mice died perinatally associated with fetal growth restriction, reduced hepatic glycogen stores, and hypoglycemia. Surviving adult mice exhibited evidence of constitutive ER stress in multiple cells/tissues, including fibroblasts, lung, kidney, salivary gland, and pancreas. Elevated ER stress in pancreatic β cells of ERdj4^GT/GT mice was associated with β cell loss, hypoinsulinemia, and glucose intolerance. Collectively these results suggest an important role for ERdj4 in maintaining ER homeostasis during normal fetal growth and postnatal adaptation to metabolic stress.

4-Phenylbutyric acid treatment rescues trafficking and processing of a mutant surfactant protein-C

Mutations in the SFTPC gene, encoding surfactant protein-C (SP-C), are associated with interstitial lung disease (ILD). Knowledge of the intracellular fate of mutant SP-C is essential in the design of therapies to correct trafficking/processing of the proprotein, and to prevent the formation of cytotoxic aggregates. We assessed the potential of a chemical chaperone to correct the trafficking and processing of three disease-associated mutant SP-C proteins. HEK293 cells were stably transfected with wild-type (SP-C(WT)) or mutant (SP-C(L188Q), SP-C(Δexon4), or SP-C(I73T)) SP-C, and cell lines with a similar expression of SP-C mRNA were identified. The effects of the chemical chaperone 4-phenylbutyric acid (PBA) and lysosomotropic drugs on intracellular trafficking to the endolysosomal pathway and the subsequent conversion of SP-C proprotein to mature peptide were assessed. Despite comparable SP-C mRNA expression, proprotein concentrations varied greatly: SP-C(I73T) was more abundant than SP-C(WT) and was localized to the cell surface, whereas SP-C(Δexon4) was barely detectable. In contrast, SP-C(L188Q) and SP-C(WT) proprotein concentrations were comparable, and a small amount of SP-C(L188Q) was localized to the endolysosomal pathway. PBA treatment restored the trafficking and processing of SP-C(L188Q) to SP-C(WT) concentrations, but did not correct the mistrafficking of SP-C(I73T) or rescue SP-C(Δexon4). PBA treatment also promoted the aggregation of SP-C proproteins, including SP-C(L188Q). This study provides proof of the principle that a chemical chaperone can correct the mistrafficking and processing of a disease-associated mutant SP-C proprotein.

Comparative proteomic analysis of lung lamellar bodies and lysosome-related organelles

Pulmonary surfactant is a complex mixture of lipids and proteins that is essential for postnatal function. Surfactant is synthesized in alveolar type II cells and stored as multi-bilayer membranes in a specialized secretory lysosome-related organelle (LRO), known as the lamellar body (LB), prior to secretion into the alveolar airspaces. Few LB proteins have been identified and the mechanisms regulating formation and trafficking of this organelle are poorly understood. Lamellar bodies were isolated from rat lungs, separated into limiting membrane and core populations, fractionated by SDS-PAGE and proteins identified by nanoLC-tandem mass spectrometry. In total 562 proteins were identified, significantly extending a previous study that identified 44 proteins in rat lung LB. The lung LB proteome reflects the dynamic interaction of this organelle with the biosynthetic, secretory and endocytic pathways of the type II epithelial cell. Comparison with other LRO proteomes indicated that 60% of LB proteins were detected in one or more of 8 other proteomes, confirming classification of the LB as a LRO. Remarkably the LB shared 37.8% of its proteins with the melanosome but only 9.9% with lamellar bodies from the skin. Of the 229 proteins not detected in other LRO proteomes, a subset of 34 proteins was enriched in lung relative to other tissues. Proteins with lipid-related functions comprised a significant proportion of the LB unique subset, consistent with the major function of this organelle in the organization, storage and secretion of surfactant lipid. The lung LB proteome will facilitate identification of molecular pathways involved in LB biogenesis, surfactant homeostasis and disease pathogenesis.

Meckel-Gruber syndrome protein MKS3 is required for endoplasmic reticulum-associated degradation of surfactant protein C

Autosomal dominant mutations in the SFTPC gene are associated with idiopathic pulmonary fibrosis, a progressive lethal interstitial lung disease. Mutations that cause misfolding of the encoded proprotein surfactant protein C (SP-C) trigger endoplasmic reticulum (ER)-associated degradation, a pathway that segregates terminally misfolded substrate for retrotranslocation to the cytosol and degradation by proteasome. Microarray screens for genes involved in SP-C ER-associated degradation identified MKS3/TMEM67, a locus previously linked to the ciliopathy Meckel-Gruber syndrome. In this study, MKS3 was identified as a membrane glycoprotein predominantly localized to the ER. Expression of MKS3 was up-regulated by genetic or pharmacological inducers of ER stress. The ER lumenal domain of MKS3 interacted with a complex that included mutant SP-C and associated chaperones, whereas the region predicted to encode the transmembrane domains of MKS3 interacted with cytosolic p97. Deletion of the transmembrane and cytosolic domains abrogated interaction of MKS3 with p97 and resulted in accumulation of mutant SP-C proprotein; knockdown of MKS3 also inhibited degradation of mutant SP-C. These results support a model in which MKS3 links the ER lumenal quality control machinery with the cytosolic degradation apparatus.

Nedd4-2-mediated ubiquitination facilitates processing of surfactant protein-C

We previously proposed a model of surfactant protein (SP)-C biosynthesis in which internalization of the proprotein from the limiting membrane of the multivesicular body to internal vesicles represents a key step in the processing and secretion of SP-C. To test this hypothesis, alanine mutagenesis of the N-terminal propeptide of SP-C was performed. Adenoviruses encoding mutant proproteins were infected into type II cells isolated from Sftpc(-/-) mice, and media analyzed for secreted SP-C 24 hours after infection. Mutation of S(12)PPDYS(17) completely blocked secretion of SP-C. PPDY (PY motif) has previously been shown to bind WW domains of neural precursor cell-expressed developmentally down-regulated (Nedd) 4-like E3 ubiquitin ligases. Purified recombinant glutathione S-transferase-SP-C propeptide (residues 1-35) bound recombinant Nedd4-2 strongly, and Nedd4 weakly; the S(12)PPDYS(17)mutation abrogated binding of SP-C to Nedd4-2. Immobilized recombinant Nedd4-2 WW domain captured SP-C proprotein from mouse type II cell lysates; in the reverse pulldown, endogenous SP-C in type II cells was captured by recombinant Nedd4-2. To determine if the interaction of Nedd4-2 and SP-C resulted in ubiquitination, the SP-C proprotein was immunoprecipitated from transiently transfected human embryonic kidney 293 cells, and analyzed by SDS-PAGE/Western blotting with ubiquitin antibody. Two ubiquitinated forms of SP-C were detected; ubiquitination was blocked by mutation of K6, but not K34, in the SP-C propeptide. Mutation of K6 also inhibited processing of SP-C proprotein to the mature peptide in human embryonic kidney 293 cells. Nedd4-2-mediated ubiquitination regulates lumenal relocation of SP-C, leading to processing and, ultimately, secretion of SP-C.

Adaptation and increased susceptibility to infection associated with constitutive expression of misfolded SP-C

Mutations in the gene encoding SP-C (surfactant protein C; SFTPC) have been linked to interstitial lung disease (ILD) in children and adults. Expression of the index mutation, SP-C(Deltaexon4), in transiently transfected cells and type II cells of transgenic mice resulted in misfolding of the proprotein, activation of endoplasmic reticulum (ER) stress pathways, and cytotoxicity. In this study, we show that stably transfected cells adapted to chronic ER stress imposed by the constitutive expression of SP-C(Deltaexon4) via an NF-kappaB-dependent pathway. However, the infection of cells expressing SP-C(Deltaexon4) with respiratory syncytial virus resulted in significantly enhanced cytotoxicity associated with accumulation of the mutant proprotein, pronounced activation of the unfolded protein response, and cell death. Adaptation to chronic ER stress imposed by misfolded SP-C was associated with increased susceptibility to viral-induced cell death. The wide variability in the age of onset of ILD in patients with SFTPC mutations may be related to environmental insults that ultimately overwhelm the homeostatic cytoprotective response.

Partial SP-B deficiency perturbs lung function and causes air space abnormalities

Surfactant protein B (SP-B) is required for function of newborn and adult lung, and partial deficiency has been associated with susceptibility to lung injury. In the present study, transgenic mice were produced in which expression of SP-B in type II epithelial cells was conditionally regulated. Concentrations of SP-B were maintained at 60–70% of that normally present in control. Immunostaining for SP-B demonstrated cellular heterogeneity in expression of the protein. In subsets of type II cells in which SP-B staining was decreased, immunostaining for pro-SP-C was increased and lamellar body ultrastructure was disrupted, consistent with focal SP-B deficiency. Fluorescence-activated cell sorting analyses of freshly isolated type II cells identified a population of cells with low SP-B content and a smaller population with increased SP-B content, confirming nonuniform expression of the SP-B transgene. Focal air space enlargement, without cellular infiltration or inflammation, was observed. Pressure-volume curves indicated that maximal tidal volume was unchanged; however, hysteresis was modestly altered and residual volumes were significantly decreased in the SP-B-deficient mice. Chronic, nonuniform SP-B deficiency perturbed pulmonary function and caused air space enlargement.

Surfactant protein D influences surfactant ultrastructure and uptake by alveolar type II cells

Surfactant protein D (SP-D) is a member of the collectin family of the innate host defense proteins. In the lung, SP-D is expressed primarily by type II cells. Gene-targeted SP-D-deficient [SP-D(-/-)] mice have three- to fivefold higher surfactant lipid pool sizes. However, surfactant synthesis and secretion by type II cells and catabolism by alveolar macrophages are normal in SP-D(-/-) mice. Therefore, we hypothesized that SP-D might regulate surfactant homeostasis by influencing surfactant structure, thereby altering its uptake by type II cells. Large (LA) and small aggregate (SA) surfactant were isolated from bronchoalveolar lavage fluid (BALF) from SP-D(-/-), wild-type [SP-D(+/+)], and transgenic mice in which SP-D was expressed under conditional control of doxycycline in alveolar type II cells. Uptake of both LA and SA isolated from SP-D(-/-) mice by normal type II cells was decreased. Abnormally dense lipid forms were observed by electron microscopy of LA from SP-D(-/-) mice. SA from SP-D(-/-) mice consisted of atypical multilamellated small vesicles. Abnormalities in surfactant uptake by type II cells and in surfactant ultrastructure were corrected by conditional expression of SP-D in vivo. Preincubation of BALF from SP-D(-/-) mice with SP-D changed surfactant ultrastructure to be similar to that of SP-D(+/+) mice in vitro. The rapid changes in surfactant structure, increased uptake by type II cells, and decreased pool sizes normally occurring in the postnatal period were not seen in SP-D(-/-) mice. SP-D regulates uptake and catabolism by type II cells and influences the ultrastructure of surfactant in the alveolus.

Comparison of the microbicidal and muramidase activities of mouse lysozyme M and P

Lysozyme is one of the most abundant antimicrobial proteins in the airspaces of the lung. Mice express two lysozyme genes, lysozyme M and P, but only the M enzyme is detected in abundance in lung tissues. Disruption of the lysozyme M locus significantly increased bacterial burden and mortality following intratracheal infection with a Gram-negative bacterium. Unexpectedly, significant lysozyme enzyme activity (muramidase activity) was detected in the airspaces of uninfected lysozyme M-/- mice, amounting to 25% of the activity in wild-type mice. Muramidase activity in lysozyme M-/- mice was associated with increased lysozyme P mRNA and protein in lung tissue and bronchoalveolar lavage fluid respectively. The muramidase activity of recombinant lysozyme P was less than that of recombinant M lysozyme. Recombinant P lysozyme was also less effective in killing selected Gram-negative bacteria, requiring higher concentrations than lysozyme M to achieve the same level of killing. The lower antimicrobial activity of P lysozyme, coupled with incomplete compensation by P lysozyme in lysozyme M-/- mice, probably accounts for the increased susceptibility of null mice to infection. Recombinant lysozyme M and P were equally effective in killing selected Gram-positive organisms. This outcome suggests that disruption of both M and P loci would significantly increase susceptibility to airway infections, particularly those associated with colonization by Gram-positive organisms.

N-terminally extended surfactant protein (SP) C isolated from SP-B-deficient children has reduced surface activity and inhibited lipopolysaccharide binding

In both humans and mice, a deficiency of surfactant protein B (SP-B) is associated with a decreased concentration of mature SP-C and accumulation of a larger SP-C peptide, denoted SP-C(i), which is not observed under normal conditions. Isolation of hydrophobic polypeptides from the lungs of children who died with two different SP-B mutations yielded pure SP-C(i) and showed only trace amounts of mature SP-C. Determination of the SP-C(i) covalent structure revealed a 12-residue N-terminal peptide segment, followed by a 35-residue segment that is identical to mature SP-C. The SP-C(i) structure determined herein is similar to that of a proposed late intermediate in the processing of proSP-C, suggesting that SP-C(i) is the immediate precursor of SP-C. In bronchoalveolar lavage fluid from transgenic mice with a focal deficiency of SP-B, SP-C(i) was detected in the biophysically active, large aggregate fraction and was associated with membrane structures that are typical for a large aggregate surfactant. However, unlike SP-C, SP-C(i) exhibited a very poor ability to promote phospholipid adsorption, gave high surface tension during cyclic film compression, and did not bind lipopolysaccharide in vitro. SP-C(i) is thus capable of associating with surfactant lipids, but its N-terminal dodecapeptide segment must be proteolytically removed to generate a biologically functional peptide. The results of this study indicate that the early postnatal fatal respiratory distress seen in SP-B-deficient children is combined with the near absence of active variants of SP-C.

Processing of Pulmonary Surfactant Protein B by Napsin and Cathepsin H

Surfactant protein B (SP-B) is an essential constituent of pulmonary surfactant. SP-B is synthesized in alveolar type II cells as a preproprotein and processed to the mature peptide by the cleavage of NH2- and COOH-terminal peptides. An aspartyl protease has been suggested to cleave the NH2-terminal propeptide resulting in a 25-kDa intermediate. Napsin, an aspartyl protease expressed in alveolar type II cells, was detected in fetal lung homogenates as early as day 16 of gestation, 1 day before the onset of SP-B expression and processing. Napsin was localized to multivesicular bodies, the site of SP-B proprotein processing in type II cells. Incubation of SP-B proprotein from type II cells with a crude membrane extract from napsin-transfected cells resulted in enhanced levels of a 25-kDa intermediate. Purified napsin cleaved a recombinant SP-B/EGFP fusion protein within the NH2-terminal propeptide between Leu178 and Pro179, 22 amino acids upstream of the NH2 terminus of mature SP-B. Cathepsin H, a cysteine protease also implicated in pro-SP-B processing, cleaved SP-B/EGFP fusion protein 13 amino acids upstream of the NH2 terminus of mature SP-B. Napsin did not cleave the COOH-terminal peptide, whereas cathepsin H cleaved the boundary between mature SP-B and the COOH-terminal peptide and at several other sites within the COOH-terminal peptide. Knockdown of napsin by small interfering RNA resulted in decreased levels of mature SP-B and mature SP-C in type II cells. These results suggest that napsin, cathepsin H, and at least one other enzyme are involved in maturation of the biologically active SP-B peptide.

Pneumonitis and emphysema in sp-C gene targeted mice

SP-C-deficient (SP-C -/-) mice developed a severe pulmonary disorder associated with emphysema, monocytic infiltrates, epithelial cell dysplasia, and atypical accumulations of intracellular lipids in type II epithelial cells and alveolar macrophages. Whereas alveolar and tissue surfactant phospholipid pools were increased, levels of other surfactant proteins were not altered (SP-B) or were modestly increased (SP-A and SP-D). Analysis of pressure-volume curves and forced oscillatory dynamics demonstrated abnormal respiratory mechanics typical of emphysema. Lung disease was progressive, causing weight loss and cardiomegaly. Extensive alveolar remodeling was accompanied by type II cell hyperplasia, obliteration of pulmonary capillaries, and widespread expression of alpha-smooth muscle actin, indicating myofibroblast transformation in the lung parenchyma. Dysplastic epithelial cells lining conducting airways stained intensely for the mucin, MUC5A/C. Tissue concentrations of proinflammatory cytokines were not substantially altered in the SP-C (-/-) mice. Production of matrix metalloproteinases (MMP-2 and MMP-9) was increased in alveolar macrophages from SP-C (-/-) mice. Absence of SP-C caused a severe progressive pulmonary disorder with histologic features consistent with interstitial pneumonitis.

Biogenesis of lamellar bodies, lysosome-related organelles involved in storage and secretion of pulmonary surfactant

Lamellar bodies are members of a subclass of lysosome-related organelles referred to as secretory lysosomes. The principal constituents of the lamellar body, surfactant phospholipids, are organized into tightly packed, bilayer membranes in a process that is strongly influenced by the lung-specific, hydrophobic peptide SP-B. Newly synthesized SP-B is transported from the Golgi to the lamellar body via multivesicular bodies; in contrast, preliminary evidence suggests that newly synthesized surfactant phospholipids are transported from the ER and incorporated into the internal membranes of the lamellar body via a distinct pathway.

Maintenance of the mouse type II cell phenotype in vitro

The purpose of this study was to identify culture conditions for maintenance of isolated mouse type II cells with intact surfactant protein (SP) and phospholipid production. Type II cells were isolated from 6-wk-old mice and cultured on Matrigel matrix-rat tail collagen (70:30 vol/vol) in bronchial epithelial cell growth medium minus hydrocortisone plus 5% charcoal-stripped FBS and 10 ng/ml keratinocyte growth factor. Under these conditions, type II cells actively produced surfactant phospholipids and proteins for at least 7 days. Synthesis and secretion of surfactant phospholipids and SP-A, -B, -C, and -D declined on day 1 of culture but recovered by day 3, reaching levels comparable to or exceeding freshly isolated cells by day 5. Abundant lamellar bodies were readily apparent in cells examined on days 5 and 7, and a surfactant pellet was recovered by centrifugation of media harvested on each day of culture. Secretion of SP-B, SP-C, and phosphatidylcholine was stimulated by phorbol 12-myristate 13-acetate and was inhibited by compound 48/80. When tested with a bubble surfactometer, surfactant secreted by type II cells on day 5 of culture lowered surface tension to 5.2 +/- 2.3 mN/m. This is the first description of the synthesis and secretion of a functional surfactant complex by mouse type II cells after 7 days in primary culture.

Overexpression of surfactant protein-C mature peptide causes neonatal lethality in transgenic mice

Surfactant replacement preparations containing either surfactant protein (SP)-B or SP-C significantly improve lung function in surfactant-deficient infants, suggesting that these peptides may be functionally redundant. SP-B is absent and SP-C is greatly diminished in the airspaces of SP-B (-/-) mice, which die of respiratory distress syndrome (RDS) shortly after birth. The goal of this study was to determine if elevated expression of SP-C mature peptide could reverse the neonatal lethality in SP-B (-/-) mice. SP-C peptide (residues 24-57 of mouse SP-C proprotein) with a hemagglutinin epitope (SP-C(24-57)HA) was expressed in type II cells of transgenic mice, with the goal of crossing these animals into the SP-B (-/-) background. Unexpectedly, expression of the SP-C(24-57)HA transgene resulted in delayed/arrested lung development and lethal, neonatal RDS of all transgenic progeny in two independent transgenic lines. In transgenic mice, SP-C(24-57)HA was localized predominantly to the endoplasmic reticulum and Golgi; in contrast, SP-B and SP-C were very difficult to detect in the endoplasmic reticulum of wild-type mice. These results suggest that elevated expression of SP-C(24-57)HA in type II cells resulted in aggregation of SP-C in the early secretory pathway, leading to cytotoxicity and, ultimately, altered lung development.

Altered stability of pulmonary surfactant in SP-C-deficient mice

The surfactant protein C (SP-C) gene encodes an extremely hydrophobic, 4-kDa peptide produced by alveolar epithelial cells in the lung. To discern the role of SP-C in lung function, SP-C-deficient (-/-) mice were produced. The SP-C (-/-) mice were viable at birth and grew normally to adulthood without apparent pulmonary abnormalities. SP-C mRNA was not detected in the lungs of SP-C (-/-) mice, nor was mature SP-C protein detected by Western blot of alveolar lavage from SP-C (-/-) mice. The levels of the other surfactant proteins (A, B, D) in alveolar lavage were comparable to those in wild-type mice. Surfactant pool sizes, surfactant synthesis, and lung morphology were similar in SP-C (-/-) and SP-C (+/+) mice. Lamellar bodies were present in SP-C (-/-) type II cells, and tubular myelin was present in the alveolar lumen. Lung mechanics studies demonstrated abnormalities in lung hysteresivity (a term used to reflect the mechanical coupling between energy dissipative forces and tissue-elastic properties) at low, positive-end, expiratory pressures. The stability of captive bubbles with surfactant from the SP-C (-/-) mice was decreased significantly, indicating that SP-C plays a role in the stabilization of surfactant at low lung volumes, a condition that may accompany respiratory distress syndrome in infants and adults.

Secretion of surfactant protein C, an integral membrane protein, requires the N-terminal propeptide

Proteolytic processing of surfactant protein C (SP-C) proprotein in multivesicular bodies of alveolar type II cells results in a 35-residue mature peptide, consisting of a transmembrane domain and a 10-residue extramembrane domain. SP-C mature peptide is stored in lamellar bodies (a lysosomal-like organelle) and secreted with surfactant phospholipids into the alveolar space. This study was designed to identify the peptide domain of SP-C required for sorting and secretion of this integral membrane peptide. Deletion analyses in transiently transfected PC12 cells and isolated mouse type II cells suggested the extramembrane domain of mature SP-C was cytosolic and sufficient for sorting to the regulated secretory pathway. Intratracheal injection of adenovirus encoding SP-C mature peptide resulted in secretion into the alveolar space of wild type mice but not SP-C (-/-) mice. SP-C secretion in null mice was restored by the addition of the N-terminal propeptide. The cytosolic domain, consisting of the N- terminal propeptide and extramembrane domain of mature SP-C peptide, supported secretion of the transmembrane domain of platelet-derived growth factor receptor. Collectively, these studies indicate that the N-terminal propeptide of SP-C is required for intracellular sorting and secretion of SP-C.

The role of homodimers in surfactant protein B function in vivo

Surfactant protein B (SP-B) is detected in the airways as a sulfhydryl-dependent dimer (M(r) approximately 16,000). To test the hypothesis that formation of homodimers is critical for SP-B function, the cysteine residue reported to be involved in SP-B dimerization was mutated to serine (Cys(248) --> Ser) and the mutated protein was targeted to the distal respiratory epithelium of transgenic mice. Transgenic lines which demonstrated appropriate processing, sorting, and secretion of human SP-B monomer were crossed with SP-B +/- mice to achieve expression of human monomer in the absence of endogenous SP-B dimer (hSP-B(mon), mSP-B-/-). In two of three transgenic lines, hSP-B(mon), mSP-B-/- mice had normal lung structure, complete processing of SP-C proprotein, well formed lamellar bodies, and normal longevity. Pulmonary function studies revealed an altered hysteresis curve for hSP-B(mon), mSP-B-/- mice relative to wild type mice. Large aggregate surfactant fractions from hSP-B(mon), mSP-B-/- mice resulted in higher minimum surface tension in vitro compared with surfactant from wild type mice. Surfactant lipids supplemented with 2% hSP-B monomer resulted in slower adsorption and higher surface tension than surfactant with 2% hSP-B dimer. Taken together, these data indicate a role for SP-B dimer in surface tension reduction in the alveolus.

Ablation of a critical surfactant protein B intramolecular disulfide bond in transgenic mice

The 79-amino acid, mature SP-B peptide contains three intramolecular disulfide bonds shared by all saposin-like proteins. This study tested the hypothesis that the disulfide bond formed between cysteine residues 35 and 46 (residues 235 and 246 of the SP-B proprotein) is essential for proper function of SP-B. To test the role of this bridge in SP-B function in vivo, a construct was generated in which cysteine residues 235 and 246 of the human SP-B proprotein were mutated to serine and cloned under the control of the 3.7-kilobase hSP-C promoter (hSP-B(C235S/C246S)). In two transgenic mouse lines, expression of the mutant peptide in the wild-type murine SP-B background was invariably lethal in the neonatal period. In four additional lines, survival was inversely related to the level of transgene expression. To test the ability of the mutant peptide to functionally replace the wild-type protein, transgenic mice were crossed into the SP-B null background. No animals that expressed hSP-B(C235S/C246S) in the murine SP-B-/- background survived the neonatal period. hSP-B(C235S/C246S) proprotein accumulated in the endoplasmic reticulum and was not processed to the mature, biologically active peptide. The results of these studies demonstrate that the intramolecular bridge between residues 235 and 246 is critical for intracellular trafficking of SP-B and suggest that overexpression of mutant SP-B in the wild-type background may be lethal.

Surfactant protein B (SP-B) -/- mice are rescued by restoration of SP-B expression in alveolar type II cells but not Clara cells

Surfactant protein B (SP-B) mRNA and protein are restricted to alveolar Type II and Clara cells in the respiratory epithelium. In order to investigate the function of SP-B in these distinct cell types, transgenic mice were generated in which SP-B expression was selectively restored in Type II cells or Clara cells of SP-B -/- mice. The 4.8-kilobase murine SP-C promoter was used to generate 3 transgenic lines which expressed human SP-B in Type II cells (mSP-C/hSP-B). Likewise, the 2.3-kilobase murine CCSP promoter was used to generate two transgenic lines which expressed human SP-B in Clara cells (mCCSP/hSP-B). mSP-C/hSP-B and mCCSP/hSP-B transgenic mice were subsequently bred to SP-B +/- mice in order to selectively express SP-B in Type II cells or Clara cells of SP-B -/- mice. Selective restoration of SP-B expression in Type II cells completely rescued the neonatal lethal phenotype in SP-B -/- mice. Expression of SP-B in some, but not all Type II cells of SP-B -/- mice, allowed postnatal survival, but resulted in significantly altered lung architecture and function. Selective restoration of SP-B expression in Clara cells of SP-B -/- mice resulted in respiratory dysfunction and invariable neonatal death, related to the complete absence of mature SP-B peptide in these mice. These results indicate that expression and processing of the SP-B proprotein to the mature peptide in Type II cells is absolutely required for lung function in vivo and that SP-B expression in Clara cells cannot substitute for this function.