Manipulating proteostasis to repair the F508del-CFTR defect in cystic fibrosis

Emerging Concepts in Defective Macrophage Phagocytosis in Cystic Fibrosis

Cystic fibrosis (CF) is caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Chronic inflammation and decline in lung function are major reasons for morbidity in CF. Mutant CFTR expressed in phagocytic cells such as macrophages contributes to persistent infection, inflammation, and lung disease in CF. Macrophages play a central role in innate immunity by eliminating pathogenic microbes by a process called phagocytosis. Phagocytosis is required for tissue homeostasis, balancing inflammation, and crosstalk with the adaptive immune system for antigen presentation. This review focused on (1) current understandings of the signaling underlying phagocytic mechanisms; (2) existing evidence for phagocytic dysregulation in CF; and (3) the emerging role of CFTR modulators in influencing CF phagocytic function. Alterations in CF macrophages from receptor initiation to phagosome formation are linked to disease progression in CF. A deeper understanding of macrophages in the context of CFTR and phagocytosis proteins at each step of phagosome formation might contribute to the new therapeutic development of dysregulated innate immunity in CF. Therefore, the review also indicates future areas of research in the context of CFTR and macrophages.

Cystic Fibrosis Lung Disease in the Aging Population

The demographics of the population with cystic fibrosis (CF) is continuously changing, with nowadays adults outnumbering children and a median predicted survival of over 40 years. This leads to the challenge of treating an aging CF population, while previous research has largely focused on pediatric and adolescent patients. Chronic inflammation is not only a hallmark of CF lung disease, but also of the aging process. However, very little is known about the effects of an accelerated aging pathology in CF lungs. Several chronic lung disease pathologies show signs of chronic inflammation with accelerated aging, also termed “inflammaging”; the most notable being chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). In these disease entities, accelerated aging has been implicated in the pathogenesis via interference with tissue repair mechanisms, alterations of the immune system leading to impaired defense against pulmonary infections and induction of a chronic pro-inflammatory state. In addition, CF lungs have been shown to exhibit increased expression of senescence markers. Sustained airway inflammation also leads to the degradation and increased turnover of cystic fibrosis transmembrane regulator (CFTR). This further reduces CFTR function and may prevent the novel CFTR modulator therapies from developing their full efficacy. Therefore, novel therapies targeting aging processes in CF lungs could be promising. This review summarizes the current research on CF in an aging population focusing on accelerated aging in the context of chronic airway inflammation and therapy implications.

Immunomodulatory Effects of Azithromycin Revisited: Potential Applications to COVID-19

The rapid advancement of the COVID-19 pandemic has prompted an accelerated pursuit to identify effective therapeutics. Stages of the disease course have been defined by viral burden, lung pathology, and progression through phases of the immune response. Immunological factors including inflammatory cell infiltration and cytokine storm have been associated with severe disease and death. Many immunomodulatory therapies for COVID-19 are currently being investigated, and preliminary results support the premise of targeting the immune response. However, because suppressing immune mechanisms could also impact the clearance of the virus in the early stages of infection, therapeutic success is likely to depend on timing with respect to the disease course. Azithromycin is an immunomodulatory drug that has been shown to have antiviral effects and potential benefit in patients with COVID-19. Multiple immunomodulatory effects have been defined for azithromycin which could provide efficacy during the late stages of the disease, including inhibition of pro-inflammatory cytokine production, inhibition of neutrophil influx, induction of regulatory functions of macrophages, and alterations in autophagy. Here we review the published evidence of these mechanisms along with the current clinical use of azithromycin as an immunomodulatory therapeutic. We then discuss the potential impact of azithromycin on the immune response to COVID-19, as well as caution against immunosuppressive and off-target effects including cardiotoxicity in these patients. While azithromycin has the potential to contribute efficacy, its impact on the COVID-19 immune response requires additional characterization so as to better define its role in individualized therapy.

CFTR Correctors and Antioxidants Partially Normalize Lipid Imbalance but not Abnormal Basal Inflammatory Cytokine Profile in CF Bronchial Epithelial Cells

A deficiency in cystic fibrosis transmembrane conductance regulator (CFTR) function in CF leads to chronic lung disease. CF is associated with abnormalities in fatty acids, ceramides, and cholesterol, their relationship with CF lung pathology is not completely understood. Therefore, we examined the impact of CFTR deficiency on lipid metabolism and pro-inflammatory signaling in airway epithelium using mass spectrometric, protein array. We observed a striking imbalance in fatty acid and ceramide metabolism, associated with chronic oxidative stress under basal conditions in CF mouse lung and well-differentiated bronchial epithelial cell cultures of CFTR knock out pig and CF patients. Cell-autonomous features of all three CF models included high ratios of ω-6- to ω-3-polyunsaturated fatty acids and of long- to very long-chain ceramide species (LCC/VLCC), reduced levels of total ceramides and ceramide precursors. In addition to the retinoic acid analog fenretinide, the anti-oxidants glutathione (GSH) and deferoxamine partially corrected the lipid profile indicating that oxidative stress may promote the lipid abnormalities. CFTR-targeted modulators reduced the lipid imbalance and oxidative stress, confirming the CFTR dependence of lipid ratios. However, despite functional correction of CF cells up to 60% of non-CF in Ussing chamber experiments, a 72-h triple compound treatment (elexacaftor/tezacaftor/ivacaftor surrogate) did not completely normalize lipid imbalance or oxidative stress. Protein array analysis revealed differential expression and shedding of cytokines and growth factors from CF epithelial cells compared to non-CF cells, consistent with sterile inflammation and tissue remodeling under basal conditions, including enhanced secretion of the neutrophil activator CXCL5, and the T-cell activator CCL17. However, treatment with antioxidants or CFTR modulators that mimic the approved combination therapies, ivacaftor/lumacaftor and ivacaftor/tezacaftor/elexacaftor, did not effectively suppress the inflammatory phenotype. We propose that CFTR deficiency causes oxidative stress in CF airway epithelium, affecting multiple bioactive lipid metabolic pathways, which likely play a role in CF lung disease progression. A combination of anti-oxidant, anti-inflammatory and CFTR targeted therapeutics may be required for full correction of the CF phenotype.

Complex interplay between autophagy and oxidative stress in the development of pulmonary disease

The autophagic pathway involves the encapsulation of substrates in double-membraned vesicles, which are subsequently delivered to the lysosome for enzymatic degradation and recycling of metabolic precursors. Autophagy is a major cellular defense against oxidative stress, or related conditions that cause accumulation of damaged proteins or organelles. Selective forms of autophagy can maintain organelle populations or remove aggregated proteins. Dysregulation of redox homeostasis under pathological conditions results in excessive generation of reactive oxygen species (ROS), leading to oxidative stress and the associated oxidative damage of cellular components. Accumulating evidence indicates that autophagy is necessary to maintain redox homeostasis. ROS activates autophagy, which facilitates cellular adaptation and diminishes oxidative damage by degrading and recycling intracellular damaged macromolecules and dysfunctional organelles. The cellular responses triggered by oxidative stress include the altered regulation of signaling pathways that culminate in the regulation of autophagy. Current research suggests a central role for autophagy as a mammalian oxidative stress response and its interrelationship to other stress defense systems. Altered autophagy phenotypes have been observed in lung diseases such as chronic obstructive lung disease, acute lung injury, cystic fibrosis, idiopathic pulmonary fibrosis, and pulmonary arterial hypertension, and asthma. Understanding the mechanisms by which ROS regulate autophagy will provide novel therapeutic targets for lung diseases. This review highlights our current understanding on the interplay between ROS and autophagy in the development of pulmonary disease.

Drug efficacy and toxicity prediction: an innovative application of transcriptomic data

Drug toxicity and efficacy are difficult to predict partly because they are both poorly defined, which I aim to remedy here from a transcriptomic perspective. There are two major categories of drugs: (1) restorative drugs aiming to restore an abnormal cell, tissue, or organ to normal function (e.g., restoring normal membrane function of epithelial cells in cystic fibrosis), and (2) disruptive drugs aiming to kill pathogens or malignant cells. These two types of drugs require different definition of efficacy and toxicity. I outlined rationales for defining transcriptomic efficacy and toxicity and illustrated numerically their application with two sets of transcriptomic data, one for restorative drugs (treating cystic fibrosis with lumacaftor/ivacaftor aiming to restore the cellular function of epithelial cells) and the other for disruptive drugs (treating acute myeloid leukemia with prexasertib). The conceptual framework presented will help and sensitize researchers to collect data required for determining drug toxicity.

Cystic Fibrosis Defective Response to Infection Involves Autophagy and Lipid Metabolism

Cystic fibrosis (CF) is a hereditary disease, with 70% of patients developing a proteinopathy related to the deletion of phenylalanine 508. CF is associated with multiple organ dysfunction, chronic inflammation, and recurrent lung infections. CF is characterized by defective autophagy, lipid metabolism, and immune response. Intracellular lipid accumulation favors microbial infection, and autophagy deficiency impairs internalized pathogen clearance. Myriocin, an inhibitor of sphingolipid synthesis, significantly reduces inflammation, promotes microbial clearance in the lungs, and induces autophagy and lipid oxidation. RNA-seq was performed in Aspergillusfumigatus-infected and myriocin-treated CF patients’ derived monocytes and in a CF bronchial epithelial cell line. Fungal clearance was also evaluated in CF monocytes. Myriocin enhanced CF patients’ monocytes killing of A. fumigatus. CF patients’ monocytes and cell line responded to infection with a profound transcriptional change; myriocin regulates genes that are involved in inflammation, autophagy, lipid storage, and metabolism, including histones and heat shock proteins whose activity is related to the response to infection. We conclude that the regulation of sphingolipid synthesis induces a metabolism drift by promoting autophagy and lipid consumption. This process is driven by a transcriptional program that corrects part of the differences between CF and control samples, therefore ameliorating the infection response and pathogen clearance in the CF cell line and in CF peripheral blood monocytes.

Disorders of FZ-CRD; insights towards FZ-CRD folding and therapeutic landscape

The ER is hub for protein folding. Proteins that harbor a Frizzled cysteine-rich domain (FZ-CRD) possess 10 conserved cysteine motifs held by a unique disulfide bridge pattern which attains a correct fold in the ER. Little is known about implications of disease-causing missense mutations within FZ-CRD families. Mutations in FZ-CRD of Frizzled class receptor 4 (FZD4) and Muscle, skeletal, receptor tyrosine kinase (MuSK) and Receptor tyrosine kinase-like orphan receptor 2 (ROR2) cause Familial Exudative Vitreoretinopathy (FEVR), Congenital Myasthenic Syndrome (CMS), and Robinow Syndrome (RS) respectively. We highlight reported pathogenic inherited missense mutations in FZ-CRD of FZD4, MuSK and ROR2 which misfold, and traffic abnormally in the ER, with ER-associated degradation (ERAD) as a common pathogenic mechanism for disease. Our review shows that all studied FZ-CRD mutants of RS, FEVR and CMS result in misfolded proteins and/or partially misfolded proteins with an ERAD fate, thus we coin them as “disorders of FZ-CRD”. Abnormal trafficking was demonstrated in 17 of 29 mutants studied; 16 mutants were within and/or surrounding the FZ-CRD with two mutants distant from FZ-CRD. These ER-retained mutants were improperly N-glycosylated confirming ER-localization. FZD4 and MuSK mutants were tagged with polyubiquitin chains confirming targeting for proteasomal degradation. Investigating the cellular and molecular mechanisms of these mutations is important since misfolded protein and ER-targeted therapies are in development. The P344R-MuSK kinase mutant showed around 50% of its in-vitro autophosphorylation activity and P344R-MuSK increased two-fold on proteasome inhibition. M105T-FZD4, C204Y-FZD4, and P344R-MuSK mutants are thermosensitive and therefore, might benefit from extending the investigation to a larger number of chemical chaperones and/or proteasome inhibitors. Nonetheless, FZ-CRD ER-lipidation it less characterized in the literature and recent structural data sheds light on the importance of lipidation in protein glycosylation, proper folding, and ER trafficking. Current treatment strategies in-place for the conformational disease landscape is highlighted. From this review, we envision that disorders of FZ-CRD might be receptive to therapies that target FZ-CRD misfolding, regulation of fatty acids, and/or ER therapies; thus paving the way for a newly explored paradigm to treat different diseases with common defects.

Transglutaminase type 2 in the regulation of proteostasis

The maintenance of protein homeostasis (proteostasis) is a fundamental aspect of cell physiology that is essential for the survival of organisms under a variety of environmental and/or intracellular stress conditions. Acute and/or persistent stress exceeding the capacity of the intracellular homeostatic systems results in protein aggregation and/or damaged organelles that leads to pathological cellular states often resulting in cell death. These events are continuously suppressed by a complex macromolecular machinery that uses different intracellular pathways to maintain the proteome integrity in the various subcellular compartments ensuring a healthy cellular life span. Recent findings have highlighted the role of the multifunctional enzyme type 2 transglutaminase (TG2) as a key player in the regulation of intracellular pathways, such as autophagy/mitophagy, exosomes formation and chaperones function, which form the basis of proteostasis regulation under conditions of cellular stress. Here, we review the role of TG2 in these stress response pathways and how its various enzymatic activities might contributes to the proteostasis control.

Cystic Fibrosis: Proteostatic correctors of CFTR trafficking and alternative therapeutic targets

Introduction: Cystic fibrosis (CF) is the most frequent lethal orphan disease and is caused by mutations in the CFTR gene. The most frequent mutation F508del-CFTR affects multiple organs; infections and subsequent infections and complications in the lung lead to death. Areas covered: This review focuses on new targets and mechanisms that are attracting interest for the development of CF therapies. The F508del-CFTR protein is retained in the endoplasmic reticulum (ER) but has some function if it can traffic to the plasma membrane. Cell-based assays have been used to screen chemical libraries for small molecule correctors that restore its trafficking. Pharmacological chaperones are correctors that bind directly to the F508del-CFTR mutant and promote its folding and trafficking. Other correctors fall into a heterogeneous class of proteostasis modulators that act indirectly by altering cellular homeostasis. Expert opinion: Pharmacological chaperones have so far been the most successful correctors of F508del-CFTR trafficking, but their level of correction means that more than one corrector is required. Proteostasis modulators have low levels of correction but hold promise because some can correct several different CFTR mutations. Identification of their cellular targets and the potential for development may lead to new therapies for CF.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.

Defective proteostasis in celiac disease as a new therapeutic target

Cystic fibrosis (CF) is a disease caused by loss-of-function mutations affecting the CF transmembrane conductance regulator (CFTR), a chloride channel. Recent evidence indicates that CFTR is inhibited by a gluten/gliadin-derived peptide (P31-43), causing an acquired state of CFTR inhibition within the gut that contributes to the pathogenesis of celiac disease (CD). Of note, CFTR inhibition does not only cause intra- and extracellular ion imbalances but also affects proteostasis by activating transglutaminase-2 (TGM2) and by disabling autophagy. These three phenomena (CFTR inhibition, TGM2 activation, and autophagy impairment) engage in multiple self-amplifying circuitries, thus forming an "infernal trio". The trio hinders enterocytes from returning to homeostasis and instead locks them in an irreversible pro-inflammatory state that ultimately facilitates T lymphocyte-mediated immune responses against another gluten/gliadin-derived peptide (P57-68), which,upon deamidation by activated TGM2, becomes fully antigenic. Hence, the pathogenic protein gliadin exemplifies a food constituent the exceptional immunogenicity of which arises from a combination of antigenicity (conferred by deaminated P57-68) and adjuvanticity (conferred by P31-43). CF can be treated by agents targeting the "infernal trio" including CFTR potentiators, TGM2 inhibitors, and autophagy enhancers. We speculate that such agents may also be used for CD therapy and indeed could constitute close-to-etiological treatments of this enteropathy.

Adapting Proteostasis and Autophagy for Controlling the Pathogenesis of Cystic Fibrosis Lung Disease

Cystic fibrosis (CF), a fatal genetic disorder predominant in the Caucasian population, is caused by mutations in the cystic fibrosis transmembrane conductance regulator (Cftr) gene. The most common mutation is the deletion of phenylalanine from the position-508 (F508del-CFTR), resulting in a misfolded-CFTR protein, which is unable to fold, traffic and retain its plasma membrane (PM) localization. The resulting CFTR dysfunction, dysregulates variety of key cellular mechanisms such as chloride ion transport, airway surface liquid (ASL) homeostasis, mucociliary-clearance, inflammatory-oxidative signaling, and proteostasis that includes ubiquitin-proteasome system (UPS) and autophagy. A collective dysregulation of these key homoeostatic mechanisms contributes to the development of chronic obstructive cystic fibrosis lung disease, instead of the classical belief focused exclusively on ion-transport defect. Hence, therapeutic intervention(s) aimed at rescuing chronic CF lung disease needs to correct underlying defect that mediates homeostatic dysfunctions and not just chloride ion transport. Since targeting all the myriad defects individually could be quite challenging, it will be prudent to identify a process which controls almost all disease-promoting processes in the CF airways including underlying CFTR dysfunction. There is emerging experimental and clinical evidence that supports the notion that impaired cellular proteostasis and autophagy plays a central role in regulating pathogenesis of chronic CF lung disease. Thus, correcting the underlying proteostasis and autophagy defect in controlling CF pulmonary disease, primarily via correcting the protein processing defect of F508del-CFTR protein has emerged as a novel intervention strategy. Hence, we discuss here both the rationale and significant therapeutic utility of emerging proteostasis and autophagy modulating drugs/compounds in controlling chronic CF lung disease, where targeted delivery is a critical factor-influencing efficacy.

Autophagy augmentation alleviates cigarette smoke-induced CFTR-dysfunction, ceramide-accumulation and COPD-emphysema pathogenesis

In this study, we aimed to investigate precise mechanism(s) of sphingolipid-imbalance and resulting ceramide-accumulation in COPD-emphysema. Where, human and murine emphysema lung tissues or human bronchial epithelial cells (Beas2b) were used for experimental analysis. We found that lungs of smokers and COPD-subjects with increasing emphysema severity demonstrate sphingolipid-imbalance, resulting in significant ceramide-accumulation and increased ceramide/sphingosine ratio, as compared to non-emphysema/non-smoker controls. Next, we found a substantial increase in emphysema chronicity-related ceramide-accumulation in murine (C57BL/6) lungs, while sphingosine levels only slightly increased. In accordance, the expression of the acid ceramidase decreased after CS-exposure. Moreover, CS-induced (sub-chronic) ceramide-accumulation was significantly (p < 0.05) reduced by treatment with TFEB/autophagy-inducing drug, gemfibrozil (GEM), suggesting that autophagy regulates CS-induced ceramide-accumulation. Next, we validated experimentally that autophagy/lipophagy-induction using an anti-oxidant, cysteamine, significantly (p < 0.05) reduces CS-extract (CSE)-mediated intracellular-ceramide-accumulation in p62 + aggresome-bodies. In addition to intracellular-accumulation, we found that CSE also induces membrane-ceramide-accumulation by ROS-dependent acid-sphingomyelinase (ASM) activation and plasma-membrane translocation, which was significantly controlled (p < 0.05) by cysteamine (an anti-oxidant) and amitriptyline (AMT, an inhibitor of ASM). Cysteamine-mediated and CSE-induced membrane-ceramide regulation was nullified by CFTR-inhibitor-172, demonstrating that CFTR controls redox impaired-autophagy dependent membrane-ceramide accumulation. In summary, our data shows that CS-mediated autophagy/lipophagy-dysfunction results in intracellular-ceramide-accumulation, while acquired CFTR-dysfunction-induced ASM causes membrane ceramide-accumulation. Thus, CS-exposure alters the sphingolipid-rheostat leading to the increased membrane- and intracellular- ceramide-accumulation inducing COPD-emphysema pathogenesis that is alleviated by treatment with cysteamine, a potent anti-oxidant with CFTR/autophagy-augmenting properties.

Oxidative stress, autophagy and airway ion transport

Mucociliary clearance is critically important in protecting the airways from infection and from the harmful effects of smoke and various inspired substances known to induce oxidative stress and persistent inflammation. An essential feature of the clearance mechanism involves regulation of the periciliary liquid layer on the surface of the airway epithelium, which is necessary for normal ciliary beating and maintenance of mucus hydration. The underlying ion transport processes associated with airway surface hydration include epithelial Na⁺ channel-dependent Na⁺ absorption occurring in parallel with CFTR and Ca²⁺-activated Cl^- channel-dependent anion secretion, which are coordinately regulated to control the depth of the periciliary liquid layer. Oxidative stress is known to cause both acute and chronic effects on airway ion transport function, and an increasing number of studies in the past few years have identified an important role for autophagy as part of the physiological response to the damaging effects of oxidation. In this review, recent studies addressing the influence of oxidative stress and autophagy on airway ion transport pathways, along with results showing the potential of autophagy modulators in restoring the function of ion channels involved in transepithelial electrolyte transport necessary for effective mucociliary clearance, are presented.

Silencing of the Hsp70-specific nucleotide-exchange factor BAG3 corrects the F508del-CFTR variant by restoring autophagy

The protein chaperones heat shock protein 70 (Hsp70) and Hsp90 are required for de novo folding of proteins and protect against misfolding-related cellular stresses by directing misfolded or slowly folding proteins to the ubiquitin/proteasome system (UPS) or autophagy/lysosomal degradation pathways. Here, we examined the role of the Bcl2-associated athanogene (BAG) family of Hsp70-specific nucleotide-exchange factors in the biogenesis and functional correction of genetic variants of the cystic fibrosis transmembrane conductance regulator (CFTR) whose mutations cause cystic fibrosis (CF). We show that siRNA-mediated silencing of BAG1 and -3, two BAG members linked to the clearance of misfolded proteins via the UPS and autophagy pathways, respectively, leads to functional correction of F508del-CFTR and other disease-associated CFTR variants. BAG3 silencing was the most effective, leading to improved F508del-CFTR stability, trafficking, and restoration of cell-surface function, both alone and in combination with the FDA-approved CFTR corrector, VX-809. We also found that the BAG3 silencing-mediated correction of F508del-CFTR restores the autophagy pathway, which is defective in F508del-CFTR-expressing cells, likely because of the maladaptive stress response in CF pathophysiology. These results highlight the potential therapeutic benefits of targeting the cellular chaperone system to improve the functional folding of CFTR variants contributing to CF and possibly other protein-misfolding-associated diseases.

Targeting transglutaminase 2 partially restores extracellular matrix structure but not alveolar architecture in experimental bronchopulmonary dysplasia

The generation, maturation and remodelling of the extracellular matrix (ECM) are essential for the formation of alveoli during lung development. Alveoli formation is disturbed in preterm infants that develop bronchopulmonary dysplasia (BPD), where collagen fibres are malformed, and perturbations to lung ECM structures may underlie BPD pathogenesis. Malformed ECM structures might result from abnormal protein cross-linking, in part attributable to the increased expression and activity of transglutaminase 2 (TGM2) that have been noted in affected patient lungs, as well as in hyperoxia-based BPD animal models. The objective of the present study was to assess whether TGM2 plays a causal role in normal and aberrant lung alveolarization. Targeted deletion of Tgm2 in C57BL/6J mice increased septal thickness and reduced gas-exchange surface area in otherwise normally developing lungs. During aberrant lung alveolarization that occurred under hyperoxic conditions, collagen structures in Tgm2^-/- mice were partially protected from the impact of hyperoxia, where normal dihydroxylysinonorleucine and hydroxylysylpiridinoline collagen cross-link abundance was restored; however, the lung alveolar architecture remained abnormal. Inhibition of transglutaminases (including TGM2) with cysteamine appreciably reduced transglutaminase activity in vivo, as assessed by N^ε -(γ-l-glutamyl)-l-lysine abundance and TGM catalytic activity, and restored normal dihydroxylysinonorleucine and hydroxylysylpiridinoline collagen cross-link abundance under pathological conditions. Furthermore, a moderate improvement in alveoli size and gas-exchange surface density was noted in cysteamine-treated mouse lungs in which BPD was modelled. These data indicate that TGM2 plays a role in normal lung alveolarization, and contributes to the formation of aberrant ECM structures during disordered lung alveolarization.

CFTR Deletion Confers Mitochondrial Dysfunction and Disrupts Lipid Homeostasis in Intestinal Epithelial Cells

Background: Cystic Fibrosis (CF) is a genetic disease in which the intestine exhibits oxidative and inflammatory markers. As mitochondria are the central source and the main target of reactive oxygen species, we hypothesized that cystic fibrosis transmembrane conductance regulator (CFTR) defect leads to the disruption of cellular lipid homeostasis, which contributes to mitochondrial dysfunction. Methods. Mitochondrial functions and lipid metabolism were investigated in Caco-2/15 cells with CFTR knockout (CFTR^-/-) engineered by the zinc finger nuclease technique. Experiments were performed under basal conditions and after the addition of the pro-oxidant iron-ascorbate (Fe/Asc) complex. Results. Mitochondria of intestinal cells with CFTR^-/-, spontaneously showed an altered redox homeostasis characterised by a significant decrease in the expression of PPARα and nuclear factor like 2. Consistent with these observations, 8-oxoguanine-DNA glycosylase, responsible for repair of ROS-induced DNA lesion, was weakly expressed in CFTR^-/- cells. Moreover, disturbed fatty acid β-oxidation process was evidenced by the reduced expression of CPT1 and acyl-CoA dehydrogenase long-chain in CFTR^-/- cells. The decline of mitochondrial cytochrome c and B-cell lymphoma 2 expression pointing to magnified apoptosis. Mitochondrial respiration was also affected as demonstrated by the low expression of respiratory oxidative phosphorylation (OXPHOS) complexes and a high adenosine diphosphate/adenosine triphosphate ratio. In contrast, the FAS and ACC enzymes were markedly increased, thereby indicating lipogenesis stimulation. This was associated with an augmented secretion of lipids, lipoproteins and apolipoproteins in CFTR^-/- cells. The addition of Fe/Asc worsened while butylated hydroxy toluene partially improved these processes. Conclusions: CFTR silencing results in lipid homeostasis disruption and mitochondrial dysfunction in intestinal epithelial cells. Further investigation is needed to elucidate the mechanisms underlying the marked abnormalities in response to CFTR deletion.

TG2 regulates the heat-shock response by the post-translational modification of HSF1

Heat-shock factor 1 (HSF1) is the master transcription factor that regulates the response to proteotoxic stress by controlling the transcription of many stress-responsive genes including the heat-shock proteins. Here, we show a novel molecular mechanism controlling the activation of HSF1. We demonstrate that transglutaminase type 2 (TG2), dependent on its protein disulphide isomerase activity, triggers the trimerization and activation of HSF1 regulating adaptation to stress and proteostasis impairment. In particular, we find that TG2 loss of function correlates with a defect in the nuclear translocation of HSF1 and in its DNA-binding ability to the HSP70 promoter. We show that the inhibition of TG2 restores the unbalance in HSF1-HSP70 pathway in cystic fibrosis (CF), a human disorder characterized by deregulation of proteostasis. The absence of TG2 leads to an increase of about 40% in CFTR function in a new experimental CF mouse model lacking TG2. Altogether, these results indicate that TG2 plays a key role in the regulation of cellular proteostasis under stressful cellular conditions through the modulation of the heat-shock response.

DPAGT1-CDG: Functional analysis of disease-causing pathogenic mutations and role of endoplasmic reticulum stress

Pathogenic mutations in DPAGT1 are manifested as two possible phenotypes: congenital disorder of glycosylation DPAGT1-CDG (also known as CDG-Ij), and limb-girdle congenital myasthenic syndrome (CMS) with tubular aggregates. UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosamine phosphotransferase (GPT), the protein encoded by DPAGT1, is an endoplasmic reticulum (ER)-resident protein involved in an initial step in the N-glycosylation pathway. The aim of the present study was to examine the effect of six variants in DPAGT1 detected in patients with DPAGT1-CDG, and the role of endoplasmic reticulum stress, as part of the search for therapeutic strategies to use against DPAGT1-CDG. The effect of the six mutations, i.e., c.358C>A (p.Leu120Met), c.791T>G (p.Val264Gly), c.901C>T (p.Arg301Cys), c.902G>A (p.Arg301His), c.1154T>G (p.Leu385Arg), and of the novel mutation c.329T>C (p.Phe110Ser), were examined via the analysis of DPAGT1 transcriptional profiles and GTP levels in patient-derived fibroblasts. In addition, the transient expression of different mutations was analysed in COS-7 cells. The results obtained, together with those of bioinformatic studies, revealed these mutations to affect the splicing process, the stability of GTP, or the ability of this protein to correctly localise in the ER membrane. The unfolded protein response (UPR; the response to ER stress) was found not to be active in patient-derived fibroblasts, unlike that seen in cells from patients with PMM2-CDG or DPM1-CDG. Even so, the fibroblasts of patients with DPAGT1-CDG seemed to be more sensitive to the stressor tunicamycin. The present work improves our knowledge of DPAGT1-CDG and provides bases for developing tailored splicing and folding therapies.

Role of second-hand smoke (SHS)-induced proteostasis/autophagy impairment in pediatric lung diseases

Background

Exposure to second-hand tobacco smoke (SHS) is one of the prime risk factors for chronic lung disease development. Smoking during pregnancy may lead to birth defects in the newborn that include pulmonary dysfunction, increased susceptibility to opportunistic pathogens, or initiation of childhood respiratory manifestations such as bronchopulmonary dysplasia (BPD). Moreover, exposure to SHS in early childhood can have negative impact on lung health, although the exact mechanisms are unclear. Autophagy is a crucial proteostatic mechanism modulated by cigarette smoke (CS) in adult lungs. Here, we sought to investigate whether SHS exposure impairs autophagy in pediatric lungs.

Methods

Pregnant C57BL/6 mice were exposed to room air or SHS for 14 days. The newborn pups were subsequently exposed to room air or SHS (5 h/day) for 1 or 14 days, and lungs were harvested. Soluble and insoluble protein fractions isolated from pediatric mice lungs were subjected to immunoblotting for ubiquitin (Ub), p62, VCP, HIF-1α, and β-actin.

Results

Our data shows that short-term exposure to SHS (1 or 14 days) leads to proteostasis and autophagy-impairment as evident by significant increase in accumulation of ubiquitinated proteins (Ub), p62 (impaired-autophagy marker) and valosin-containing protein (VCP) in the insoluble protein fractions of pediatric mice lungs. Moreover, increased HIF-1α levels in SHS-exposed mice lungs points towards a novel mechanism for SHS-induced lung disease initiation in the pediatric population. Validating the in vivo studies, we demonstrate that treatment of human bronchial epithelial cells (Beas2b cells) with the proteasome inhibitor (MG-132) induces HIF-1α expression that is controlled by co-treatment with autophagy-inducing drug, cysteamine.

Conclusions

SHS-exposure induced proteostasis/autophagy impairment can mediate the initiation of chronic lung disease in pediatric subjects. Hence, our data warrants the evaluation of proteostasis/autophagy-inducing drugs, such as cysteamine, as a potential therapeutic intervention strategy for SHS-induced pediatric lung diseases.

Cysteamine re-establishes the clearance of Pseudomonas aeruginosa by macrophages bearing the cystic fibrosis-relevant F508del-CFTR mutation

Cystic fibrosis (CF), the most common lethal monogenic disease in Caucasians, is characterized by recurrent bacterial infections and colonization, mainly by Pseudomonas aeruginosa, resulting in unresolved airway inflammation. CF is caused by mutations in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) protein, which functions as a chloride channel in epithelial cells, macrophages, and other cell types. Impaired bacterial handling by macrophages is a feature of CF airways, although it is still debated how defective CFTR impairs bacterial killing. Recent evidence indicates that a defective autophagy in CF macrophages leads to alterations of bacterial clearance upon infection. Here we use bone marrow-derived macrophages from transgenic mice to provide the genetic proof that defective CFTR compromises both uptake and clearance of internalized Pseudomonas aeruginosa. We demonstrate that the proteostasis regulator cysteamine, which rescues the function of the most common F508del-CFTR mutant and hence reduces lung inflammation in CF patients, can also repair the defects of CF macrophages, thus restoring both bacterial internalization and clearance through a process that involves upregulation of the pro-autophagic protein Beclin 1 and re-establishment of the autophagic pathway. Altogether these results indicate that cysteamine restores the function of several distinct cell types, including that of macrophages, which might contribute to its beneficial effects on CF.

Cystic fibrosis lung environment and Pseudomonas aeruginosa infection

Background

The airways of patients with cystic fibrosis (CF) are highly complex, subject to various environmental conditions as well as a distinct microbiota. Pseudomonas aeruginosa is recognized as one of the most important pulmonary pathogens and the predominant cause of morbidity and mortality in CF. A multifarious interplay between the host, pathogens, microbiota, and the environment shapes the course of the disease. There have been several excellent reviews detailing CF pathology, Pseudomonas and the role of environment in CF but only a few reviews connect these entities with regards to influence on the overall course of the disease. A holistic understanding of contributing factors is pertinent to inform new research and therapeutics.

Discussion

In this article, we discuss the deterministic alterations in lung physiology as a result of CF. We also revisit the impact of those changes on the microbiota, with special emphasis on P. aeruginosa and the influence of other non-genetic factors on CF. Substantial past and current research on various genetic and non-genetic aspects of cystic fibrosis has been reviewed to assess the effect of different factors on CF pulmonary infection. A thorough review of contributing factors in CF and the alterations in lung physiology indicate that CF lung infection is multi-factorial with no isolated cause that should be solely targeted to control disease progression. A combinatorial approach may be required to ensure better disease outcomes.

Conclusion

CF lung infection is a complex disease and requires a broad multidisciplinary approach to improve CF disease outcomes. A holistic understanding of the underlying mechanisms and non-genetic contributing factors in CF is central to development of new and targeted therapeutic strategies.

SYVN1, NEDD8, and FBXO2 Proteins Regulate ΔF508 Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Ubiquitin-mediated Proteasomal Degradation

We previously reported that delivery of a microRNA-138 mimic or siRNA against SIN3A to cultured cystic fibrosis (ΔF508/ΔF508) airway epithelia partially restored ΔF508-cystic fibrosis transmembrane conductance regulator (CFTR)-mediated cAMP-stimulated Cl^- conductance. We hypothesized that dissecting this microRNA-138/SIN3A-regulated gene network would identify individual proteins contributing to the rescue of ΔF508-CFTR function. Among the genes in the network, we rigorously validated candidates using functional CFTR maturation and electrolyte transport assays in polarized airway epithelia. We found that depletion of the ubiquitin ligase SYVN1, the ubiquitin/proteasome system regulator NEDD8, or the F-box protein FBXO2 partially restored ΔF508-CFTR-mediated Cl^- transport in primary cultures of human cystic fibrosis airway epithelia. Moreover, knockdown of SYVN1, NEDD8, or FBXO2 in combination with corrector compound 18 further potentiated rescue of ΔF508-CFTR-mediated Cl^- conductance. This study provides new knowledge of the CFTR biosynthetic pathway. It suggests that SYVN1 and FBXO2 represent two distinct multiprotein complexes that may degrade ΔF508-CFTR in airway epithelia and identifies a new role for NEDD8 in regulating ΔF508-CFTR ubiquitination.