Targeting of the dual oxidase 2 N-terminal region to the plasma membrane

A comprehensive molecular analysis of bovine coronavirus strains isolated from Brazil and comparison of a wild-type and cell culture-adapted strain associated with respiratory disease

Bovine coronavirus (BCoV) has dual tropisms that can trigger enteric and respiratory diseases in cattle. Despite its global distribution, BCoV field strains from Brazil remain underexplored in studies investigating the virus's worldwide circulation. Another research gap involves the comparative analysis of S protein sequences in BCoV isolates from passages in cell lines versus direct sequencing from clinical samples. Therefore, one of the objectives of our study was to conduct a comprehensive phylogenetic analysis of BCoV strains identified from Brazil, including a respiratory strain obtained during this study, comparing them with global and ancestral BCoV strains. Additionally, we performed a comparative analysis between wild-type BCoV directly sequenced from the clinical sample (nasal secretion) and the cell culture-adapted strain, utilizing the Sanger method. The field strain and multiple cell passage in cell culture (HRT-18) adapted BCoV strain (BOV19 NS) detected in this study were characterized through molecular and phylogenetic analyses based on partial fragments of 1,448 nt covering the hypervariable region of the S gene. The analyses have demonstrated that different BCoV strains circulating in Brazil, and possibly Brazilian variants, constitute a new genotype (putative G15 genotype). Compared with the ancestral prototype (Mebus strain) of BCoV, 33 nt substitutions were identified of which 15 resulted in non-synonymous mutations (nine transitions and six transversions). Now, compared with the wild-type strain was identified only one nt substitution in nt 2,428 from the seventh passage onwards, which resulted in transversion, neutral-neutral charge, and one substitution of asparagine for tyrosine at aa residue 810 (N810Y).

CMPK2 restricts Zika virus replication by inhibiting viral translation

Flaviviruses continue to emerge as global health threats. There are currently no Food and Drug Administration (FDA) approved antiviral treatments for flaviviral infections. Therefore, there is a pressing need to identify host and viral factors that can be targeted for effective therapeutic intervention. Type I interferon (IFN-I) production in response to microbial products is one of the host's first line of defense against invading pathogens. Cytidine/uridine monophosphate kinase 2 (CMPK2) is a type I interferon-stimulated gene (ISG) that exerts antiviral effects. However, the molecular mechanism by which CMPK2 inhibits viral replication is unclear. Here, we report that CMPK2 expression restricts Zika virus (ZIKV) replication by specifically inhibiting viral translation and that IFN-I- induced CMPK2 contributes significantly to the overall antiviral response against ZIKV. We demonstrate that expression of CMPK2 results in a significant decrease in the replication of other pathogenic flaviviruses including dengue virus (DENV-2), Kunjin virus (KUNV) and yellow fever virus (YFV). Importantly, we determine that the N-terminal domain (NTD) of CMPK2, which lacks kinase activity, is sufficient to restrict viral translation. Thus, its kinase function is not required for CMPK2's antiviral activity. Furthermore, we identify seven conserved cysteine residues within the NTD as critical for CMPK2 antiviral activity. Thus, these residues may form an unknown functional site in the NTD of CMPK2 contributing to its antiviral function. Finally, we show that mitochondrial localization of CMPK2 is required for its antiviral effects. Given its broad antiviral activity against flaviviruses, CMPK2 is a promising potential pan-flavivirus inhibitor.

The Intestinal Redox System and Its Significance in Chemotherapy-Induced Intestinal Mucositis

Chemotherapy-induced intestinal mucositis (CIM) is a significant dose-limiting adverse reaction brought on by the cancer treatment. Multiple studies reported that reactive oxygen species (ROS) is rapidly produced during the initial stages of chemotherapy, when the drugs elicit direct damage to intestinal mucosal cells, which, in turn, results in necrosis, mitochondrial dysfunction, and ROS production. However, the mechanism behind the intestinal redox system-based induction of intestinal mucosal injury and necrosis of CIM is still undetermined. In this article, we summarized relevant information regarding the intestinal redox system, including the composition and regulation of redox enzymes, ROS generation, and its regulation in the intestine. We innovatively proposed the intestinal redox “Tai Chi” theory and revealed its significance in the pathogenesis of CIM. We also conducted an extensive review of the English language-based literatures involving oxidative stress (OS) and its involvement in the pathological mechanisms of CIM. From the date of inception till July 31, 2021, 51 related articles were selected. Based on our analysis of these articles, only five chemotherapeutic drugs, namely, MTX, 5-FU, cisplatin, CPT-11, and oxaliplatin were shown to trigger the ROS-based pathological mechanisms of CIM. We also discussed the redox system-mediated modulation of CIM pathogenesis via elaboration of the relationship between chemotherapeutic drugs and the redox system. It is our belief that this overview of the intestinal redox system and its role in CIM pathogenesis will greatly enhance research direction and improve CIM management in the future.

DUOX1 in mammalian disease pathophysiology

Dual oxidase 1 (DUOX1) is a member of the protein family of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases. DUOX1 has several normal physiological, immunological, and biochemical functions in different parts of the body. Dysregulated oxidative metabolism interferes with various disease pathologies and numerous therapeutic options are based on targeting cellular redox pathways. DUOX1 forms an important enzymatic source of biological oxidants, and DUOX1 expression is frequently dysregulated in various diseases. While this review shortly addresses the biochemical and cellular properties and proposed physiological roles of DUOX1, its main purpose is to summarize the current knowledge with respect to the potential role of DUOX1 enzyme in disease pathology, especially in mammalian organisms. Although DUOX1 is normally prominently expressed in epithelial lineages, it is frequently silenced in epithelial-derived cancers by epigenetic mechanisms. While an abundance of information is available on DUOX1 transcription in different diseases, an increasing number of mechanistic studies indicate a causative relationship between DUOX1 function and disease pathophysiology. Additionally, specific functions of the DUOX1 maturation factor, DUOXA1, will also be addressed. Lastly, urgent and outstanding questions on the field of DUOX1 will be discussed that could provide valuable new diagnostic tools and novel therapeutic options.

Defects in protein folding in congenital hypothyroidism

Primary congenital hypothyroidism (CH) is the most common endocrine disease in children and one of the most common preventable causes of both cognitive and motor deficits. CH is a heterogeneous group of thyroid disorders in which inadequate production of thyroid hormone occurs due to defects in proteins involved in the gland organogenesis (dysembryogenesis) or in multiple steps of thyroid hormone biosynthesis (dyshormonogenesis). Dysembryogenesis is associated with genes responsible for the development or growth of thyroid cells: such as NKX2-1, FOXE1, PAX8, NKX2-5, TSHR, TBX1, CDCA8, HOXD3 and HOXB3 resulting in agenesis, hypoplasia or ectopia of thyroid gland. Nevertheless, the etiology of the dysembryogenesis remains unknown for most cases. In contrast, the majority of patients with dyshormonogenesis has been linked to mutations in the SLC5A5, SLC26A4, SLC26A7, TPO, DUOX1, DUOX2, DUOXA1, DUOXA2, IYD or TG genes, which usually originate goiter. About 800 genetic mutations have been reported to cause CH in patients so far, including missense, nonsense, in-frame deletion and splice-site variations. Many of these mutations are implicated in specific domains, cysteine residues or glycosylation sites, affecting the maturation of nascent proteins that go through the secretory pathway. Consequently, misfolded proteins are permanently entrapped in the endoplasmic reticulum (ER) and are translocated to the cytosol for proteasomal degradation by the ER-associated degradation (ERAD) machinery. Despite of all these remarkable advances in the field of the CH pathogenesis, several points on the development of this disease remain to be elucidated. The continuous study of thyroid gene mutations with the application of new technologies will be useful for the understanding of the intrinsic mechanisms related to CH. In this review we summarize the present status of knowledge on the disorders in the protein folding caused by thyroid genes mutations.

Antimicrobial actions of dual oxidases and lactoperoxidase

The NOX/DUOX family of NADPH oxidases are transmembrane proteins generating reactive oxygen species as their primary enzymatic products. NADPH oxidase (NOX) 1-5 and Dual oxidase (DUOX) 1 and 2 are members of this family. These enzymes have several biological functions including immune defense, hormone biosynthesis, fertilization, cell proliferation and differentiation, extracellular matrix formation and vascular regulation. They are found in a variety of tissues such as the airways, salivary glands, colon, thyroid gland and lymphoid organs. The discovery of NADPH oxidases has drastically transformed our view of the biology of reactive oxygen species and oxidative stress. Roles of several isoforms including DUOX1 and DUOX2 in host innate immune defense have been implicated and are still being uncovered. DUOX enzymes highly expressed in the respiratory and salivary gland epithelium have been proposed as the major sources of hydrogen peroxide supporting mucosal oxidative antimicrobial defenses. In this review, we shortly present data on DUOX discovery, structure and function, and provide a detailed, up-to-date summary of discoveries regarding antibacterial, antiviral, antifungal, and antiparasitic functions of DUOX enzymes. We also present all the literature describing the immune functions of lactoperoxidase, an enzyme working in partnership with DUOX to produce antimicrobial substances.

Dual oxidase: a novel therapeutic target in allergic disease

NADPH oxidases (NOXs) represent a family of enzymes that mediate regulated cellular production of reactive oxygen species (ROS) and play various functional roles in physiology. Among the NOX family, the dual oxidases DUOX1 and DUOX2 are prominently expressed in epithelial cell types at mucosal surfaces and have therefore been considered to have important roles in innate host defence pathways. Recent studies have revealed important insights into the host defence mechanisms of DUOX enzymes, which control innate immune response pathways in response to either microbial or allergic triggers. In this review, we discuss the current level of understanding with respect to the biological role(s) of DUOX enzymes and the unique role of DUOX1 in mediating innate immune responses to epithelial injury and allergens and in the development of allergic disease. These novel findings highlight DUOX1 as an attractive therapeutic target, and opportunities for the development of selective inhibitor strategies will be discussed.

Compound heterozygous DUOX2 gene mutations (c.2335-1G>C/c.3264_3267delCAGC) associated with congenital hypothyroidism. Characterization of complex cryptic splice sites by minigene analysis

Iodide Organification defects (IOD) represent 10% of cases of congenital hypothyroidism (CH) being the main genes affected that of TPO (thyroid peroxidase) and DUOX2 (dual oxidasa 2). From a patient with clinical and biochemical criteria suggestive with CH associated with IOD, TPO and DUOX2 genes were analyzed by means of PCR-Single Strand Conformation Polymorphism analysis and sequencing. A novel heterozygous compound to the mutations c.2335-1G>C (paternal mutation, intron 17) and c.3264_3267delCAGC (maternal mutation, exon 24) was identified in the DUOX2 gene. Ex-vivo splicing assays and subsequent RT-PCR and sequencing analyses were performed on mRNA isolated from the HeLa cells transfected with wild-type and mutant pSPL3 expression vectors. The wild-type and c.2335-1G>C mutant alleles result in the complete inclusion or exclusion of exon 18, or in the activation of an exonic cryptic 5' ss with the consequent deletion of 169 bp at the end of this exon. However, we observed only a band of the expected size in normal thyroid tissue by RT-PCR. Additionally, the c.2335-1G>C mutation activates an unusual cryptic donor splice site in intron 17, located at position -14 of the authentic intron 17/exon 18 junction site, with an insertion of the last 14 nucleotides of the intron 17 in mutant transcripts with complete and partial inclusion of exon 18. The theoretical consequences of splice site mutation, predicted with the bioinformatics NNSplice, Fsplice, SPL, SPLM and MaxEntScan programs were investigated and evaluated in relation with the experimental evidence. These analyses confirm that c.2335-1G>C mutant allele would result in the abolition of the authentic splice acceptor site. The results suggest the coexistence in our patient of four putative truncated proteins of 786, 805, 806 and 1105 amino acids, with conservation of peroxidase-like domain and loss of gp91(phox)/NOX2-like domain. In conclusion a novel heterozygous compound was identified being responsible of IOD. Cryptic splicing sites have been characterized in DUOX2 gene for the first time. The use of molecular biology techniques is a valuable tool for understanding the molecular pathophysiology of this type of thyroid defects.

Genetic disorders coupled to ROS deficiency

Maintaining the redox balance between generation and elimination of reactive oxygen species (ROS) is critical for health. Disturbances such as continuously elevated ROS levels will result in oxidative stress and development of disease, but likewise, insufficient ROS production will be detrimental to health. Reduced or even complete loss of ROS generation originates mainly from inactivating variants in genes encoding for NADPH oxidase complexes. In particular, deficiency in phagocyte Nox2 oxidase function due to genetic variants (CYBB, CYBA, NCF1, NCF2, NCF4) has been recognized as a direct cause of chronic granulomatous disease (CGD), an inherited immune disorder. More recently, additional diseases have been linked to functionally altered variants in genes encoding for other NADPH oxidases, such as for DUOX2/DUOXA2 in congenital hypothyroidism, or for the Nox2 complex, NOX1 and DUOX2 as risk factors for inflammatory bowel disease. A comprehensive overview of novel developments in terms of Nox/Duox-deficiency disorders is presented, combined with insights gained from structure-function studies that will aid in predicting functional defects of clinical variants.

When an Intramolecular Disulfide Bridge Governs the Interaction of DUOX2 with Its Partner DUOXA2

AIMS

The dual oxidase 2 (DUOX2) protein belongs to the NADPH oxidase (NOX) family. As H2O2 generator, it plays a key role in both thyroid hormone biosynthesis and innate immunity. DUOX2 forms with its maturation factor, DUOX activator 2 (DUOXA2), a stable complex at the cell surface that is crucial for the H2O2-generating activity, but the nature of their interaction is unknown. The contribution of some cysteine residues located in the N-terminal ectodomain of DUOX2 in a surface protein-protein interaction is suggested. We have investigated the involvement of different cysteine residues in the formation of covalent bonds that could be of critical importance for the function of the complex.

RESULTS

We report the identification and the characterization of an intramolecular disulfide bond between cys-124 of the N-terminal ectodomain and cys-1162 of an extracellular loop of DUOX2, which has important functional implications in both export and activity of DUOX2. This intramolecular bridge provides structural support for the formation of interdisulfide bridges between the N-terminal domain of DUOX2 and the two extracellular loops of its partner, DUOXA2.

INNOVATION

Both stability and function of the maturation factor, DUOXA2, are dependent on the oxidative folding of DUOX2, indicating that DUOX2 displays a chaperone-like function with respect to its partner.

CONCLUSIONS

The oxidative folding of DUOX2 that takes place in the endoplasmic reticulum (ER) appears to be a key event in the trafficking of the DUOX2/DUOXA2 complex as it promotes an appropriate conformation of the N-terminal region, which is propitious to subsequent covalent interactions with the maturation factor, DUOXA2.

Hypothyroidism-associated missense mutation impairs NADPH oxidase activity and intracellular trafficking of Duox2

In the thyroid gland Duox2-derived H2O2 is essential for thyroid hormone biosynthesis. Several patients were identified with partial or severe iodide organification defects caused by mutation in the gene for Duox2 or its maturation factor, DuoxA2. A Duox2-deficient (Duox2(thyd)) mouse model enabled in vivo investigation of its critical function in thyroid tissues, but its roles proposed in host defense or other innate responses in nonthyroid tissues remain less certain. These mice carry a spontaneous DUOX2 missense mutation, a T→G transversion, in exon 16 that changes the highly conserved valine 674 to glycine and results in severe congenital hypothyroidism. The exact mechanism underlying the effects of the V674G mutation has not been elucidated at the molecular or cellular level. To determine how the V674G mutation leads to congenital hypothyroidism, we introduced the same mutation into human Duox2 or Duox1 cDNAs and expressed them in HEK-293 cells stably expressing the corresponding DuoxA proteins. We found that the valine→glycine mutant Duox proteins fail to produce H2O2, lose their plasma membrane localization pattern, and are retained within the endoplasmic reticulum. The Duox2 mutant binds to DuoxA2, but appears to be unstable owing to this retention. Immunohistochemical staining of Duox2 in murine salivary gland ducts showed that Duox2 in mutant mice loses its condensed apical plasma membrane localization pattern characteristic of wild-type Duox2 and accumulates in punctate vesicular structures within cells. Our findings demonstrate that changing the highly conserved valine 674 in Duox2 leads to impaired subcellular targeting and reactive oxygen species release required for hormonogenesis, resulting in congenital hypothyroidism.

Nox NADPH oxidases and the endoplasmic reticulum

SIGNIFICANCE

Understanding isoform- and context-specific subcellular Nox reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase compartmentalization allows relevant functional inferences. This review addresses the interplay between Nox NADPH oxidases and the endoplasmic reticulum (ER), an increasingly evident player in redox pathophysiology given its role in redox protein folding and stress responses.

RECENT ADVANCES

Catalytic/regulatory transmembrane subunits are synthesized in the ER and their processing includes folding, N-glycosylation, heme insertion, p22phox heterodimerization, as shown for phagocyte Nox2. Dual oxidase (Duox) maturation also involves the regulation by ER-resident Duoxa2. The ER is the activation site for some isoforms, typically Nox4, but potentially other isoforms. Such location influences redox/Nox-mediated calcium signaling regulation via ER targets, such as sarcoendoplasmic reticulum calcium ATPase (SERCA). Growing evidence suggests that Noxes are integral signaling elements of the unfolded protein response during ER stress, with Nox4 playing a dual prosurvival/proapoptotic role in this setting, whereas Nox2 enhances proapoptotic signaling. ER chaperones such as protein disulfide isomerase (PDI) closely interact with Noxes. PDI supports growth factor-dependent Nox1 activation and mRNA expression, as well as migration in smooth muscle cells, and PDI overexpression induces acute spontaneous Nox activation.

CRITICAL ISSUES

Mechanisms of PDI effects include possible support of complex formation and RhoGTPase activation. In phagocytes, PDI supports phagocytosis, Nox activation, and redox-dependent interactions with p47phox. Together, the results implicate PDI as possible Nox organizer.

FUTURE DIRECTIONS

We propose that convergence between Noxes and ER may have evolutive roots given ER-related functional contexts, which paved Nox evolution, namely calcium signaling and pathogen killing. Overall, the interplay between Noxes and the ER may provide relevant insights in Nox-related (patho)physiology.

A comparative in silico characterization of functional and physicochemical properties of 3FTx (three finger toxin) proteins from four venomous snakes

Snake venom is an abundant resource of diverse pharmacologically bioactive proteins and peptides and a good natural source of drug lead compounds and used as important research tools in the field of toxicology, pharmacology and neuroscience. Three finger toxins (3FTx) is an important super-family of snake venom proteins which has a conserved three finger like appearance in three dimensional structures. Members of 3FTx family show a wide array of pharmacological effects by targeting different receptors and ion channels with high specificity and many of them are being investigated as potential drug target. Therefore, with a vision to verdict a new edge and attempt we determined the amino acid compositional (%) profile, physiochemical properties, secondary structural and functional analysis and phylogenetic relationship of three finger toxins present in four different elapid snake species namely, Naja naja, Astrotia stokesii, Hydrophis cyanocintus and Pelamis platura using different bioinformatics tools. From the outcome of the current studies, it will be possible to know about a range of biological functions which are responsible mainly for the glowing amino acid composition profile of these proteins. Amino acid composition (%) profile although represents differential amount of different amino acid residues which encompasses a family precise model but all the protein sequence have a conserved amount of cysteine. The analysis of physicochemical properties can be used as a basic approach to contribute in developing rational drug through protein engineering and understanding different physiological function which will be beneficial for the welfare of human being.

Conserved cysteine residues provide a protein-protein interaction surface in dual oxidase (DUOX) proteins

Intramolecular disulfide bond formation is promoted in oxidizing extracellular and endoplasmic reticulum compartments and often contributes to protein stability and function. DUOX1 and DUOX2 are distinguished from other members of the NOX protein family by the presence of a unique extracellular N-terminal region. These peroxidase-like domains lack the conserved cysteines that confer structural stability to mammalian peroxidases. Sequence-based structure predictions suggest that the thiol groups present are solvent-exposed on a single protein surface and are too distant to support intramolecular disulfide bond formation. To investigate the role of these thiol residues, we introduced four individual cysteine to glycine mutations in the peroxidase-like domains of both human DUOXs and purified the recombinant proteins. The mutations caused little change in the stabilities of the monomeric proteins, supporting the hypothesis that the thiol residues are solvent-exposed and not involved in disulfide bonds that are critical for structural integrity. However, the ability of the isolated hDUOX1 peroxidase-like domain to dimerize was altered, suggesting a role for these cysteines in protein-protein interactions that could facilitate homodimerization of the peroxidase-like domain or, in the full-length protein, heterodimeric interactions with a maturation protein. When full-length hDUOX1 was expressed in HEK293 cells, the mutations resulted in decreased H2O2 production that correlated with a decreased amount of the enzyme localized to the membrane surface rather than with a loss of activity or with a failure to synthesize the mutant proteins. These results support a role for the cysteine residues in intermolecular disulfide bond formation with the DUOX maturation factor DUOXA1.

Tetraspanin is required for generation of reactive oxygen species by the dual oxidase system in Caenorhabditis elegans

Reactive oxygen species (ROS) are toxic but essential molecules responsible for host defense and cellular signaling. Conserved NADPH oxidase (NOX) family enzymes direct the regulated production of ROS. Hydrogen peroxide (H₂O₂) generated by dual oxidases (DUOXs), a member of the NOX family, is crucial for innate mucosal immunity. In addition, H₂O₂ is required for cellular signaling mediated by protein modifications, such as the thyroid hormone biosynthetic pathway in mammals. In contrast to other NOX isozymes, the regulatory mechanisms of DUOX activity are less understood. Using Caenorhabditis elegans as a model, we demonstrate that the tetraspanin protein is required for induction of the DUOX signaling pathway in conjunction with the dual oxidase maturation factor (DUOXA). In the current study, we show that genetic mutation of DUOX (bli-3), DUOXA (doxa-1), and peroxidase (mlt-7) in C. elegans causes the same defects as a tetraspanin tsp-15 mutant, represented by exoskeletal deficiencies due to the failure of tyrosine cross-linking of collagen. The deficiency in the tsp-15 mutant was restored by co-expression of bli-3 and doxa-1, indicating the involvement of tsp-15 in the generation of ROS. H₂O₂ generation by BLI-3 was completely dependent on TSP-15 when reconstituted in mammalian cells. We also demonstrated that TSP-15, BLI-3, and DOXA-1 form complexes in vitro and in vivo. Cell-fusion-based analysis suggested that association with TSP-15 at the cell surface is crucial for BLI-3 activation to release H₂O₂. This study provides the first evidence for an essential role of tetraspanin in ROS generation.

ROS are highly reactive molecules, which can be inappropriately produced during aerobic metabolism or by exogenous stresses such as exposure to UV light and radiation. ROS interact with cellular components including nucleic acids, lipids, and proteins and irreversibly inhibit their functions. However, ROS are essential for innate host defense and multiple physiological processes and are generated by conserved NADPH oxidase (NOX) family enzymes. The release of ROS by ROS generator enzymes must be properly controlled, as chronic oxidative stress can cause an imbalance of the redox state and is often associated with disease and aging. Using C. elegans as a model, we identified a tetraspanin (TSP-15) protein as a new key component of the ROS generation system controlled by dual oxidase (BLI-3), a unique NOX isozyme in C. elegans. Mutants of both bli-3 and tsp-15 developed the same defects in extracellular matrix cross-linking. Using a combination of genetics and reconstitution experiments in mammalian cells, we have demonstrated a novel requirement of tetraspanin for dual oxidase-dependent ROS generation via complex formation at the cell surface.

Structural stability and heme binding potential of the truncated human dual oxidase 2 (DUOX2) peroxidase domain

The essential role of human dual oxidase 2 (hDUOX2) in thyroid hormone biosynthesis defines this member of the NOX/DUOX family, whose absence due to mutation has been directly related to disease, specifically hypothyroidism. Both human DUOX isoforms, hDUOX1 and hDUOX2, are expressed in thyroid tissue; however, hDUOX1 cannot compensate for inactivation of hDUOX2, suggesting that each enzyme is differentially regulated and/or functions in a unique manner. In efforts to uncover relevant structural and functional differences we have expressed and purified the peroxidase domain of hDUOX2(1-599) for direct comparison with the previously studied hDUOX1(1-593). As was shown for hDUOX1, the truncated hDUOX2 domain purifies without a bound heme co-factor and displays no peroxidase activity. However, hDUOX2(1-599) displays greater stability than hDUOX1(1-593). Surprisingly, upon titration with heme, both isoforms bind heme with a low micromolar affinity, demonstrating that they retain a heme binding site. A conformational difference in the full-length protein and/or a protein-protein interaction may be required to increase the heme binding affinity.

Receptor activation of NADPH oxidases

Reactive oxygen species (ROS) have been implicated in many intra- and intercellular processes. High levels of ROS are generated as part of the innate immunity in the respiratory burst of phagocytic cells. Low levels of ROS, however, are generated in a highly controlled manner by various cell types to act as second messengers in redox-sensitive pathways. A NADPH oxidase has been initially described as the respiratory burst enzyme in neutrophils. Stimulation of this complex enzyme system requires specific signaling cascades linking it to membrane-receptor activation. Subsequently, a family of NADPH oxidases has been identified in various nonphagocytic cells. They mainly differ in containing one out of seven homologous catalytic core proteins termed NOX1 to NOX5 and DUOX1 or 2. NADPH oxidase activity is controlled by regulatory subunits, including the NOX regulators p47phox and p67phox, their homologs NOXO1 and NOXA1, or the DUOX1 or 2 regulators DUOXA1 and 2. In addition, the GTPase Rac modulates activity of several of these enzymes. Recently, additional proteins have been identified that seem to have a regulatory function on NADPH oxidase activity under certain conditions. We will thus summarize molecular pathways linking activation of different membrane-bound receptors with increased ROS production of NADPH oxidases.

Targeting and regulation of reactive oxygen species generation by Nox family NADPH oxidases

Nox family NADPH oxidases serve a variety of functions requiring reactive oxygen species (ROS) generation, including antimicrobial defense, biosynthetic processes, oxygen sensing, and redox-based cellular signaling. We explored targeting, assembly, and activation of several Nox family oxidases, since ROS production appears to be regulated both spatially and temporally. Nox1 and Nox3 are similar to the phagocytic (Nox2-based) oxidase, functioning as multicomponent superoxide-generating enzymes. Factors regulating their activities include cytosolic activator and organizer proteins and GTP-Rac. Their regulation varies, with the following rank order: Nox2 > Nox1 > Nox3. Determinants of subcellular targeting include: (a) formation of Nox-p22(phox) heterodimeric complexes allowing plasma membrane translocation, (b) phospholipids-binding specificities of PX domain-containing organizer proteins (p47(phox) or Nox organizer 1 (Noxo1 and p40(phox)), and (c) variably splicing of Noxo1 PX domains directing them to nuclear or plasma membranes. Dual oxidases (Duox1 and Duox2) are targeted by different mechanisms. Plasma membrane targeting results in H(2)O(2) release, not superoxide, to support extracellular peroxidases. Human Duox1 and Duox2 have no demonstrable peroxidase activity, despite their extensive homology with heme peroxidases. The dual oxidases were reconstituted by Duox activator 2 (Duoxa2) or two Duoxa1 variants, which dictate maturation, subcellular localization, and the type of ROS generated by forming stable complexes with Duox.

A novel homozygous missense mutation of the dual oxidase 2 (DUOX2) gene in an adult patient with large goiter

OBJECTIVE

To describe the first adult case of large goiter associated with a novel R1110Q mutation in the dual oxidase 2 (DUOX2) gene. She was initially euthyroid, and developed hypothyroidism later in her forties. DUOX2 is an essential enzyme in iodine organification of thyroid hormone biosynthesis. Only infant cases of congenital hypothyroidism due to mutations of the DUOX2 gene have been reported. Biallelic mutation of DUOX2 is thought to lead to total iodine organification defect. PATIENTS AND MEASUREMENT: This 57-year-old woman became first aware of goiter around the age of 20 years. Since the goiter had enlarged gradually, she consulted us at the age of 32 years. Goiter was soft, and thyroid function was normal. Antithyroid antibodies were negative. Both physical and mental development was normal. Three of her nine siblings and her mother had large goiters. At the age of 44 years, thyroid function demonstrated subclinical hypothyroidism. She started to take levo-thyroxine at a dose of 100 mug/day to reduce goiter. At the age of 56 years, goiter size remained the same. The perchlorate discharge rate was 72.8%, suggesting partial iodine organification defect. Thus, thyroid peroxidase (TPO) gene and DUOX2 gene were analyzed.

RESULTS

There was no mutation in the TPO gene, but a novel homozygous mutation (R1110Q) in the DUOX2 gene was identified. The same heterozygous mutation was detected in her two sons and two grandchildren. This mutation was not detected in 104 control alleles and was located at a site differing from any other reported mutations in the DUOX2 gene.

CONCLUSIONS

This homozygous missense mutation can be associated with thyroid dysfunction and goiter formation of an enlarged thyroid gland.

NADPH oxidases in lung biology and pathology: host defense enzymes, and more

The deliberate production of reactive oxygen species (ROS) by phagocyte NADPH oxidase is widely appreciated as a critical component of antimicrobial host defense. Recently, additional homologs of NADPH oxidase (NOX) have been discovered throughout the animal and plant kingdoms, which appear to possess diverse functions in addition to host defense, in cell proliferation, differentiation, and in regulation of gene expression. Several of these NOX homologs are also expressed within the respiratory tract, where they participate in innate host defense as well as in epithelial and inflammatory cell signaling and gene expression, and fibroblast and smooth muscle cell proliferation, in response to bacterial or viral infection and environmental stress. Inappropriate expression or activation of NOX/DUOX during various lung pathologies suggests their specific involvement in respiratory disease. This review summarizes the current state of knowledge regarding the general functional properties of mammalian NOX enzymes, and their specific importance in respiratory tract physiology and pathology.

Congenital hypothyroidism, dwarfism, and hearing impairment caused by a missense mutation in the mouse dual oxidase 2 gene, Duox2

Dual oxidases generate the hydrogen peroxide needed by thyroid peroxidase for the incorporation of iodine into thyroglobulin, an essential step in thyroid hormone synthesis. Mutations in the human dual oxidase 2 gene, DUOX2, have been shown to underlie several cases of congenital hypothyroidism. We report here the first mouse Duox2 mutation, which provides a new genetic model for studying the specific function of DUOX2 in the thyroid gland and in other organ systems where it is hypothesized to play a role. We mapped the new spontaneous mouse mutation to chromosome 2 and identified it as a T>G base pair change in exon 16 of Duox2. The mutation changes a highly conserved valine to glycine at amino acid position 674 (V674G) and was named "thyroid dyshormonogenesis" (symbol thyd) to signify a defect in thyroid hormone synthesis. Thyroid glands of mutant mice are goitrous and contain few normal follicles, and anterior pituitaries are dysplastic. Serum T(4) in homozygotes is about one-tenth the level of controls and is accompanied by a more than 100-fold increase in TSH. The weight of adult mutant mice is approximately half that of littermate controls, and serum IGF-I is reduced. The cochleae of mutant mice exhibit abnormalities characteristic of hypothyroidism, including a delayed formation of the inner sulcus and tunnel of Corti and an abnormally thickened tectorial membrane. Hearing thresholds of adult mutant mice are on average 50-60 decibels (dB) above those of controls.

Missense mutations of dual oxidase 2 (DUOX2) implicated in congenital hypothyroidism have impaired trafficking in cells reconstituted with DUOX2 maturation factor

Dual oxidase 2 (DUOX2), a reduced NAD phosphate:O2 oxidoreductase flavoprotein, is a component of the thyrocyte H2O2 generator required for hormone synthesis at the apical plasma membrane. We recently identified a specific DUOX2 maturation factor (DUOXA2) that is necessary and sufficient for expression of functional DUOX2 in mammalian cell lines. We have now used a DUOXA2 reconstituted system to provide the first characterization of natural DUOX2 missense variants (Q36H, R376W, D506N) at the molecular level, analyzing their impact on H2O2 generation, trafficking, stability, folding, and DUOXA2 interaction. The Q36H and R376W mutations completely prevent routing of DUOX2 to the cell surface. The mutant proteins are predominantly present as core N-glycosylated, thiol-reduced folding intermediates, which are retained by the quality control system within the endoplasmic reticulum (ER) as indicated by increased complexation with the lectin calnexin. D506N displays a partial deficiency phenotype with reduced surface expression of a mutant protein with normal intrinsic activity in generating H2O2. D506N N-glycan moieties are not subject to normal modification in the Golgi apparatus, suggesting that nonnative protein can escape the quality control in the ER. Oxidative folding of DUOX2 in the ER appears to be the rate-limiting step in the maturation of DUOX2, but is not facilitated by DUOXA2. Rather, DUOXA2 allows rapid ER exit of folded DUOX2 or enhanced degradation of mutant DUOX2 proteins not competent for ER exit. DUOXA2 may thus be part of a secondary quality control system specific for DUOX2.

The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology

For a long time, superoxide generation by an NADPH oxidase was considered as an oddity only found in professional phagocytes. Over the last years, six homologs of the cytochrome subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the phagocyte NADPH oxidase itself (NOX2/gp91(phox)), the homologs are now referred to as the NOX family of NADPH oxidases. These enzymes share the capacity to transport electrons across the plasma membrane and to generate superoxide and other downstream reactive oxygen species (ROS). Activation mechanisms and tissue distribution of the different members of the family are markedly different. The physiological functions of NOX family enzymes include host defense, posttranlational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. NOX enzymes also contribute to a wide range of pathological processes. NOX deficiency may lead to immunosuppresion, lack of otoconogenesis, or hypothyroidism. Increased NOX activity also contributes to a large number or pathologies, in particular cardiovascular diseases and neurodegeneration. This review summarizes the current state of knowledge of the functions of NOX enzymes in physiology and pathology.

Duox2 exhibits potent heme peroxidase activity in human respiratory tract epithelium

The dual oxidase isozymes Duox1 and Duox2 exhibit functional NADPH:O(2) oxidoreductase activity in thyroid and respiratory tract cells and are thought to be essential for H(2)O(2) generation in these tissues. However, it is not universally accepted that the heme peroxidase domains of the Duox isozymes are functional. To address this question, we modulated Duox2 expression in human tracheobronchial epithelial (TBE) cell culture systems and quantified peroxidase activity. We discovered that interferon-gamma (IFN-gamma) induced robust peroxidase activity in TBE cells that paralleled Duox2 expression. IFN-gamma-induced peroxidase activity was abolished in the presence of sodium azide, which implicated the activation of a heme peroxidase. IFN-gamma-induced peroxidase activity was abolished in TBE cell lines expressing anti-Duox2 short hairpin RNA transcripts. Together, these data unequivocally demonstrated that Duox2 contains a functional heme peroxidase in intact respiratory tract epithelium.

Role of Nox family NADPH oxidases in host defense

The phagocytic NADPH oxidase is recognized as a critical component of innate immunity, responsible for generation of microbicidal reactive oxygen species (ROS). This enzyme is one representative of the Nox family of oxidases (Nox1-Nox5, Duox1, and Duox2) that exhibit diverse expression patterns and appear to serve a variety of functions related to ROS generation. Mounting evidence now suggests that several of these novel oxidases also serve in host defense, particularly those showing high expression along epithelial surfaces exposed to the external environment. Within these sites, Nox enzymes tend to be located on apical cell surfaces and release ROS into extracellular environments, where they can be used by known antimicrobial peroxidases. Moreover, microbial factors were shown in several cases to cause higher ROS production, either by direct oxidase activation or by inducing higher oxidase expression. Several oxidases are also induced by immune cytokines, including interferon-gamma, interleukin (IL)-4, and IL-13. Although most of the evidence supporting host defense roles for mammalian nonphagocytic oxidases remains circumstantial, recent evidence indicates that Drosophila Duox plays a role in host resistance to infection. Finally, oxidative defense against invading pathogens appears to be an ancient protective mechanism, because related oxidases are known to participate in disease resistance in plants.

Physiology and pathophysiology of the DUOXes

The dual oxidases (DUOXes) 1 and 2 are named based on their having both a domain homologous to the NADPH-oxidase of the phagocyte NADPH-oxidase gp91( phox )/NOX2 and a domain homologous to thyroid peroxidase. The DUOX1 and DUOX2 mRNAs were originally cloned from thyroid tissue, and the corresponding proteins were recognized as intricate components of the thyroid hormone synthesis process, providing hydrogen peroxide essential for the organification of iodide. The function of DUOX2 in thyroid hormonogenesis has been firmly established by linking the congenital hypothyroid phenotype "total iodide organification defect" to biallelic inactivating DUOX2 mutations. Based on the expression of both DUOXes in combination with a peroxidase in a range of different tissues and functional studies; the concept evolves that DUOX is important not only for thyroid hormonogenesis but also as an integral part of the host defense system of mucosal surfaces, participates in the control of epithelial infection, augments surface B-cell receptor signaling in lymphocytes, and is involved in generating a respiratory burst at fertilization.

Dual oxidases

Reactive oxygen species (ROS) have an important role in various physiological processes including host defence, mitogenesis, hormone biosynthesis, apoptosis and fertilization. Currently, the most characterized ROS-producing system operates in phagocytic cells, where ROS generated during phagocytosis act in host defence. Recently, several novel homologues of the phagocytic oxidase have been discovered and this protein family is now designated as the NOX/DUOX family of NADPH oxidases. NOX/DUOX enzymes function in a variety of tissues, including colon, kidney, thyroid gland, testis, salivary glands, airways and lymphoid organs. Importantly, members of the enzyme family are also found in non-mammalian species, including Caenorhabditis elegans and sea urchin. The physiological functions of novel NADPH oxidase enzymes are currently largely unknown. This review focuses on our current knowledge about dual oxidases.

Identification of the maturation factor for dual oxidase. Evolution of an eukaryotic operon equivalent

Dual oxidase 2 (DUOX2), an NADPH:O(2) oxidoreductase flavoprotein, is a component of the thyroid H(2)O(2) generator crucial for hormone synthesis at the apical membrane. Mutations in DUOX2 produce congenital hypothyroidism in humans. However, no functional DUOX-based NADPH oxidase has ever been reconstituted at the plasma membrane of transfected cells. It has been proposed that DUOX retention in the endoplasmatic reticulum (ER) of heterologous systems is due to the lack of an unidentified component required for functional maturation of the enzyme. By data mining of a massively parallel signature sequencing tissue expression data base, we identified an uncharacterized gene named DUOX maturation factor (DUOXA2) arranged head-to-head to and co-expressed with DUOX2. A paralog (DUOXA1) was similarly linked to DUOX1. The genomic rearrangement leading to linkage of ancient DUOX and DUOXA genes could be traced back before the divergence of echinoderms. We demonstrate that co-expression of DUOXA2, an ER-resident transmembrane protein, allows ER-to-Golgi transition, maturation, and translocation to the plasma membrane of functional DUOX2 in a heterologous system. The identification of DUOXA genes has important implications for studies of the molecular mechanisms controlling DUOX expression and the molecular genetics of congenital hypothyroidism.

Three Mutations (p.Q36H, p.G418fsX482, and g.IVS19-2A>C) in the Dual Oxidase 2 Gene Responsible for Congenital Goiter and Iodide Organification Defect

Background: Iodide organification defects are associated with mutations in the dual oxidase 2 (DUOX2) gene and are characterized by a positive perchlorate discharge test. These mutations produce a congenital goitrous hypothyroidism, usually transmitted in an autosomal recessive mode.

Methods: We studied the complete coding sequence of the human DUOX2 gene by single-strand conformational polymorphism (SSCP) analysis of DNA from 17 unrelated patients with iodide organification defects. Samples showing an aberrant pattern were directly sequenced. All mutations were validated by SSCP analysis. Finally, the effect of a splicing mutation was studied by construction of minigenes.

Results: Genomic DNA sequencing revealed 3 novel mutations [c.108G>C (p.Q36H), c.1253delG (p.G418fsX482), and g.IVS19-2A>C] and 1 previously reported mutation [c.2895-2898delGTTC (p.S965fsX994)] in 2 families with 1 (family 1) and 2 (family 2) affected members. This implies the inheritance of 2 compound heterozygous mutations, p.Q36H and p.S965fsX994 in family 1 and p.G418fsX482 and g.IVS19-2A>C in family 2. The c.1253delG mutation was associated with a c.1254C>A transversion. In vitro transcription analysis showed that exon 20 is skipped entirely when the g.IVS19-2A>C mutation is present. The wild-type glutamine residue at position 36 is strictly conserved.

Conclusions: Two previously unknown compound heterozygous mutations in the DUOX2 gene, p.Q36H/p.S965fsX994 and p.G418fsX482/g.IVS19-2A>C, are responsible for iodide organification defects in 2 unrelated families. Identification of the molecular basis of this disorder might be helpful for understanding the pathophysiology of this congenital hypothyroidism.

Dual oxidase-2 has an intrinsic Ca2+-dependent H2O2-generating activity

Duox2 (and probably Duox1) is a glycoflavoprotein involved in thyroid hormone biosynthesis, as the thyroid H2O2 generator functionally associated with Tpo (thyroperoxidase). So far, because of the impairment of maturation and of the targeting process, transfecting DUOX into nonthyroid cell lines has not led to the expression of a functional H2O2-generating system at the plasma membrane. For the first time, we investigated the H2O2-generating activity in the particulate fractions from DUOX2- and DUOX1-transfected HEK293 and Chinese hamster ovary cells. The particulate fractions of these cells stably or transiently transfected with human or porcine DUOX cDNA demonstrate a functional NADPH/Ca2+-dependent H2O2-generating activity. The immature Duox proteins had less activity than pig thyrocyte particulate fractions, and their activity depended on their primary structures. Human Duox2 seemed to be more active than human Duox1 but only half as active as its porcine counterpart. TPO co-transfection produced a slight increase in the enzymatic activity, whereas p22(phox), the 22-kDa subunit of the leukocyte NADPH oxidase, had no effect. In previous studies on the mechanism of H2O2 formation, it was shown that mature thyroid NADPH oxidase does not release O2*- but H2O2. Using a spin-trapping technique combined with electron paramagnetic resonance spectroscopy, we confirmed this result but also demonstrated that the partially glycosylated form of Duox2, located in the endoplasmic reticulum, generates superoxide in a calcium-dependent manner. These results suggest that post-translational modifications during the maturation process of Duox2 could be implicated in the mechanism of H2O2 formation by favoring intramolecular superoxide dismutation.

Identification of a novel partner of duox: EFP1, a thioredoxin-related protein

H(2)O(2) is a crucial substrate of thyroproxidase (TPO) to iodinate thyroglobulin and synthesize thyroid hormones in thyroid. ThOX proteins (thyroid oxidase) also called Duox are believed to be responsible for H(2)O(2) generation. Duoxs expressed in transfected cells do not generate an active system, nor permit their membrane localization suggesting that other proteins are required to fulfill these functions. In this study, we demonstrate interactions of Duoxs with TPO and with p22(phox) without any effect on Duox activity. By yeast two-hybrid method using EF-hand fragment of dog Duox1 as the bait we have isolated EFP1 (EF-hand binding protein 1), one partner of Duoxs that belongs to the thioredoxin-related protein family. EFP1 shares moderate similarities with other members of thioredoxin-related proteins, but the characteristic active site and the folding structures are well conserved. EFP1 can be co-immunoprecipitated with Duoxs in transfected COS cells as well as in primary cultured human thyrocytes. It interacts also with TPO but not thyroglobulin. Immunofluorescence studies show that EFP1 and Duox proteins are co-localized inside the transfected cells, suggesting that EFP1 is not sufficient to induce either the expression of Duox at the plasma membrane or to permit H(2)O(2) production. EFP1 and Duox mRNA share similar distribution in nine different tissues. These results suggest that EFP1 could be one of the partners in the assembly of the multiprotein complex constituting the thyroid H(2)O(2) generating system but is certainly not sufficient to permit H(2)O(2) generation.