Mutations in LZTR1 drive human disease by dysregulating RAS ubiquitination

The leucine zipper-like transcriptional regulator 1 (LZTR1) protein, an adaptor for cullin 3 (CUL3) ubiquitin ligase complex, is implicated in human disease, yet its mechanism of action remains unknown. We found that Lztr1 haploinsufficiency in mice recapitulates Noonan syndrome phenotypes, whereas LZTR1 loss in Schwann cells drives dedifferentiation and proliferation. By trapping LZTR1 complexes from intact mammalian cells, we identified the guanosine triphosphatase RAS as a substrate for the LZTR1-CUL3 complex. Ubiquitome analysis showed that loss of Lztr1 abrogated Ras ubiquitination at lysine-170. LZTR1-mediated ubiquitination inhibited RAS signaling by attenuating its association with the membrane. Disease-associated LZTR1 mutations disrupted either LZTR1-CUL3 complex formation or its interaction with RAS proteins. RAS regulation by LZTR1-mediated ubiquitination provides an explanation for the role of LZTR1 in human disease.

Leptin receptor antagonism of iNKT cell function: a novel strategy to combat multiple myeloma

A hallmark of bone marrow changes with aging is the increase in adipocyte composition, but how this impacts development of multiple myeloma (MM) is unknown. Here, we report the role of the adipokine leptin as master regulator of anti-myeloma tumor immunity by modulating the invariant natural killer T (iNKT) cell function. A marked increase in serum leptin levels and leptin receptor (LR) expression on iNKT cells in MM patients and the 5T33 murine MM model was observed. MM cells and leptin synergistically counteracted anti-tumor functionality of both murine and human iNKT cells. In vivo blockade of LR signaling combined with iNKT stimulation resulted in superior anti-tumor protection. This was linked to persistent IFN-γ secretion upon repeated iNKT cell stimulation and a restoration of the dynamic antigen-induced motility arrest as observed by intravital microscopy, thereby showing alleviation of iNKT cell anergy. Overall our data reveal the LR axis as novel therapeutic target for checkpoint inhibition to treat MM.

Getting RIDD of RNA: IRE1 in cell fate regulation

Inositol-requiring enzyme 1 (IRE1) is the most conserved transducer of the unfolded protein response (UPR), a homeostatic response that preserves proteostasis. Intriguingly, via its endoribonuclease activity, IRE1 produces either adaptive or death signals. This occurs through both unconventional splicing of XBP1 mRNA and regulated IRE1-dependent decay of mRNA (RIDD). Whereas XBP1 mRNA splicing is cytoprotective in response to endoplasmic reticulum (ER) stress, RIDD has revealed many unexpected features. For instance, RIDD cleaves RNA at an XBP1-like consensus site but with an activity divergent from XBP1 mRNA splicing and can either preserve ER homeostasis or induce cell death. Here we review recent findings on RIDD and propose a model of how IRE1 RNase activity might control cell fate decisions.

Differential expression of the interleukin 5 receptor alpha isoforms in blood and tissue eosinophils of nasal polyp patients

BACKGROUND

Given the key role of interleukin-5 (IL-5) in eosinophil function, we investigated the regulated expression of the membrane-anchored (TM-IL-5Ralpha) isoform, or a secreted (SOL IL-5Ralpha) isoform, on both protein and transcript level in vitro and in vivo.

METHODS

A real-time PCR, FACS and ELISA were established to determine IL-5Ralpha isoform expression in peripheral blood and nasal tissue from control subjects and nasal polyp (NP) patients with or without asthma. Human peripheral blood eosinophils were incubated with IL-5 and were analyzed for SOL-IL-5Ralpha and TM-IL-5Ralpha mRNA and protein levels in comparison with CD-69 expression.

RESULTS

SOL-IL-5Ralpha and TM-IL-5Ralpha mRNA and protein expression was significantly increased in NP vs controls. In polyp tissue, SOL-IL-5Ralpha expression correlated to disease severity and eosinophils counts, whereas TM-IL-5Ralpha levels were inversely correlated to eosinophils counts and SOL-IL-5Ralpha expression. FACS analysis revealed increased CD-69 and decreased TM-IL-5Ralpha expression in NP tissue eosinophils vs blood eosinophils. Incubation of blood eosinophils with IL-5 caused up-regulation of CD-69 and down-regulation of TM-IL-5Ralpha after 2 and 24 h.

CONCLUSION

The expression of SOL-IL-5Ralpha and TM-IL-5Ralpha differs according to the eosinophil activation state and localization in the body (blood vs tissue) and may therefore be involved in the fine-tuning of the eosinophil homeostasis. Exposure of eosinophils to IL-5 reduces their responsiveness to IL-5 by regulated expression of the IL-5Ralpha isoforms. Since, TM-IL-5Ralpha is down-regulated and SOL-IL-5Ralpha (antagonistic) is upregulated in NP tissue, our findings are important to understand the clinical trials with anti-IL-5 in humans.

TLR-4, IL-1R and TNF-R signaling to NF-kappaB: variations on a common theme

Toll-like receptors (TLRs) as well as the receptors for tumor necrosis factor (TNF-R) and interleukin-1 (IL-1R) play an important role in innate immunity by regulating the activity of distinct transcription factors such as nuclear factor-kappaB (NF-kappaB). TLR, IL-1R and TNF-R signaling to NF-kappaB converge on a common IkappaB kinase complex that phosphorylates the NF-kappaB inhibitory protein IkappaBalpha. However, upstream signaling components are in large part receptor-specific. Nevertheless, the principles of signaling are similar, involving the recruitment of specific adaptor proteins and the activation of kinase cascades in which protein-protein interactions are controlled by poly-ubiquitination. In this review, we will discuss our current knowledge of NF-kappaB signaling in response to TLR-4, TNF-R and IL-1R stimulation, with a special focus on the similarities and dissimilarities among these pathways.

Structure-function relationship of tumour necrosis factor and its mechanism of action

We have cloned the cDNAs of both human and mouse TNF and expressed them to high efficiency in Escherichia coli. Many transformed cell lines are sensitive to the cytotoxic action of TNF, especially in the presence of gamma-interferon, whereas normal cells either are unaffected or respond mitogenically. A number of human-mouse chimeric TNF genes have been constructed and expressed. All show biological activity but none of the chimeric proteins is neutralized by monoclonal antibodies to TNF. TNF has potent antitumour activity in nude mice carrying human xenografts or in mice bearing syngeneic tumours. In some systems direct effects can be demonstrated (in combination with species-specific gamma-interferon) but in others TNF acts indirectly. Combination of TNF with cytostatic drugs can also be effective in curing in vivo. The major limitation of the use of TNF is its toxicity. On many cell types TNF has an action similar to interleukin 1 (IL-1). At least some of the secondary, intracellular events may be identical for the two effectors. A possible mechanism of action of TNF is the release and metabolism of polyunsaturated fatty acids, which would explain the synthesis of prostaglandins and leukotrienes by many cell types after TNF treatment. The activation of the phospholipase can be blocked by corticoids. Some protease inhibitors protect cells from TNF-induced cytotoxicity but the target of these inhibitors has not been identified. Several genes are switched on by TNF (and by IL-1), including the gene for the 26 kDa protein recently identified as B cell stimulation factor 2. Events preceding death in rats include hypothermia, hypotension, acidosis and hypoglycaemia. All these effects can be largely eliminated by indomethacin pretreatment, with a resulting improvement in survival. As indomethacin does not inhibit the cytotoxic action of TNF on malignant cells it may form the basis for improved treatment protocols.

Novel role of WD40 and SOCS box protein-2 in steady-state distribution of granulocyte colony-stimulating factor receptor and G-CSF-controlled proliferation and differentiation signaling

Signals induced by granulocyte colony-stimulating factor (G-CSF), the major cytokine involved in neutrophil development, are tightly controlled by ligand-induced receptor internalization. Truncated G-CSF receptors (G-CSF-Rs) that fail to internalize show sustained proliferation and defective differentiation signaling. Steady-state forward routing also determines cell surface levels of cytokine receptors, but mechanisms controlling this are poorly understood. Here, we show that WD40 and suppressor of cytokine signaling (SOCS) box protein-2 (Wsb-2), an SOCS box-containing WD40 protein with currently unknown function, binds to the COOH-terminal region of G-CSF-R. Removal of this region did not affect internalization, yet resulted in increased membrane expression of G-CSF-R and enhanced proliferation signaling at the expense of differentiation induction. Conversely, Wsb-2 binding to the G-CSF-R reduced its cell surface expression and inhibited proliferation signaling. These effects depended on the SOCS box involved in ubiquitylation and on cytosolic lysines of G-CSF-R and imply a major role for ubiquitylation through the G-CSF-R C-terminus in forward routing of the receptor. Importantly, the Wsb-2 gene is commonly disrupted by virus integrations in mouse leukemia. We conclude that control of forward routing of G-CSF-R is essential for a balanced response of myeloid progenitors to G-CSF and suggest that disturbance of this balance may contribute to myeloid leukemia.

Analysis of leptin signalling in hematopoietic cells using an adapted MAPPIT strategy

The adipocyte-secreted hormone leptin participates in the regulation of hematopoiesis and enhances proliferation of hematopoietic cells. We used an adaptation of the MAPPIT mammalian two-hybrid method to study leptin signalling in a hematopoietic setting. We confirmed the known interactions of suppressor of cytokine signalling 3 (SOCS3) and STAT5 with the Y985 and Y1077 motifs of the leptin receptor, respectively. We also provide evidence for novel interactions at the Y1077 motif, including phospholipase C gamma and several members of the SOCS protein family, further underscoring the important role of the Y1077 motif in leptin signalling.

A role for leptin in the systemic inflammatory response syndrome (SIRS) and in immune response, an update

Leptin was originally identified as an adipocyte-derived cytokine with a key role in the regulation of the energy balance. Subsequent research revealed that leptin's biological action is not restricted to its effects on appetite and food intake, but instead has a much more pleiotropic character. There is now ample evidence that leptin has important functions in reproduction, hematopoiesis, HPA-axis endocrinology and angiogenesis. In this review we have focused on the effects of leptin in the antigen-specific immunity and in the inflammatory effector system.

Leptin, immune responses and autoimmune disease. Perspectives on the use of leptin antagonists

The pivotal role of leptin in regulating body weight and energy homeostasis is very well established. More recently, leptin also emerged as an important regulator of T-cell-dependent immunity. Reduced leptin levels, as observed during periods of starvation, correlate with an impaired cellular immune response, whereby especially the T(H)1 pro-inflammatory immune response appears to be affected. Physiologically, this could reflect the high energy demand of such processes, which are suppressed in animals or people with nutrient shortage. Several autoimmune diseases are T(H)1 T-cell dependent. In line with a pro-inflammatory role for leptin, animal models of leptin deficiency are markedly resistant to a variety of T-cell dependent autoimmune diseases. Here, we review the role of leptin in immune responses, with emphasis on autoimmune diseases. The design and potential use of leptin antagonists is also discussed.

Leptin: linking adipocyte metabolism with cardiovascular and autoimmune diseases

Leptin was originally discovered as an adipocyte-derived hormone involved in the central control of body weight and energy homeostasis. It is now clear that leptin is a pleiotropic cytokine, with activities on many peripheral cell types. These findings may help explain the surprising role of leptin in pathophysiological processes. Recent evidence suggests that leptin contributes to atherosclerosis and to the increased risk of cardiovascular disease in obese people. Leptin also appears to be involved in T-cell-dependent immunity and possibly in the development and maintenance of certain autoimmune diseases. Here, we review the role of leptin in cardiovascular and autoimmune diseases, and also briefly address the potential therapeutic use of leptin antagonists.

A role for leptin in the systemic inflammatory response syndrome (SIRS) and in immune response

Leptin was originally identified as an adipocyte-derived cytokine with a key role in the regulation of the energy balance. Subsequent research has, however, revealed that leptin's biological action is not restricted to its effects on appetite and food intake, but rather has a much more pleiotropic character. Evidence is now accumulating that it has important functions in reproduction, hematopoiesis, HPA-axis endocrinology and angiogenesis. In this review, we have focused on the effects of leptin in the immune system, which can be found in both the antigen-specific immunity and in the inflammatory effector system.

Enhanced soluble interleukin-5 receptor alpha expression in nasal polyposis

BACKGROUND

Alternative splicing of the interleukin-5 receptor alpha (IL-5Ralpha)-subunit leads to the generation of a signalling, membrane-anchored (TM) isoform, or a secreted [soluble (SOL)], antagonistic variant. Given the key role of IL-5 in eosinophil function, we investigated SOL IL-5Ralpha expression pattern in an eosinophil-associated disease such as nasal polyposis (NP).

METHODS

An SOL IL-5Ralpha enzyme-linked immunosorbent assay and quantitative real-time polymerase chain reaction (PCR) were established and applied in serum, nasal secretion and nasal tissue of controls (n = 12), and NP patients (n = 42) with or without asthma.

RESULTS

Analysis of serum, nasal secretion, and nasal tissue samples revealed that SOL IL-5Ralpha protein concentrations were significantly increased in NP vs control tissue. Within the NP group, there was a significant up-regulation of SOL IL-5Ralpha in patients with systemic airway disease. These findings were confirmed at the mRNA level, using an optimized real-time reverse-transcriptase PCR procedure.

CONCLUSIONS

This report demonstrates SOL IL-5Ralpha transcript and protein up-regulation in NP. Soluble IL-5Ralpha differentiates nasal polyps with or without concomitant asthma. As SOL IL-5Ralpha is strongly up-regulated for disease and has antagonistic properties in vitro, our studies shed new light on the mechanisms of specific immunomodulatory therapies, such as anti-IL-5.

MAPPIT: a cytokine receptor-based two-hybrid method in mammalian cells

Identifying novel targets for therapy in allergic disease: protein interactions inside the cell Therapy of allergic disease currently relies on pharmacological manipulation of mediators or immunotherapy. Drugs have been developed to target specific mediators and their receptors: for example antihistamines blocking the H1 receptor have been refined to maximize antagonism and reduce central side-effects or adverse effects of activity on other receptors such as muscarinic cholinergic receptors. Traditional pharmacological approaches identify new surface receptors against which chemists will then design or screen compounds for activity: examples are H3 or H4 histamine receptors. With the advent of the sequenced human genome we are faced with a vast array of genes and proteins that interact to define normal physiology or indeed pathology. A major challenge to biotechnology is to evolve novel techniques to understand the function and interaction of these myriad proteins. One particular area of current interest is the signalling cascades downstream of surface receptors. For many years pathways have appeared overlapping and to offer little chance of specific intervention. However, greater understanding of the complexity and integration of signalling, together with the possibility of directing drugs to specific cells has aroused considerable interest in this area for novel therapeutics. Indeed, targeting events within the cell has been done for many years with steroids. Here, Jan Tavernier and colleagues describe some signalling pathways relevant to allergic disease and potential methods for understanding protein interactions that allow mapping of the cascades. In particular they describe an elegant new system of analysis of protein-protein interactions in a mammalian system, which they have developed, termed MAPPIT. The basis of the system is an engineered receptor with JAK kinase but which lacks STAT activation sites. To the cytoplasmic end of the receptor is added a bait protein of interest, and the cell line can then be transduced with plasmid containing 'prey' cDNA from a library of interest linked to an active STAT binding site. If this cDNA encodes a protein which, upon expression, is activated and recruited to the membrane complex, it will bind to the receptor via the bait, then STAT activation will occur and activate a reporter gene system such as luciferase or puromycin resistance. This novel system allows study of known protein-protein interactions by targeted mutagenesis, or screening for novel interactions. It has the advantage over existing systems such as yeast 2 hybrid that it uses mammalian cells and thus can reproduce the physiological conditions for protein processing or activation. As new genes and proteins are linked to the atopic phenotypes, systems such as this hold promise of rapidly defining their function and interacting proteins and may be important in linking genomics and proteomics with function and pharmacology in the future.

Down-modulation of type 1 interferon responses by receptor cross-competition for a shared Jak kinase

In contrast to the large number of class I and II cytokine receptors, only four Janus kinase (Jak) proteins are expressed in mammalian cells, implying the shared use of these kinases by many different receptor complexes. Consequently, if receptor numbers exceed the amount of available Jak, cross-interference patterns can be expected. We have engineered two model cellular systems expressing two different exogenous Tyk2-interacting receptors. A receptor chimera was generated wherein the extracellular part of the interferon type 1 receptor (Ifnar1) component of the interferon-alpha/beta receptor is replaced by the equivalent domain of the erythropoietin receptor. Despite Tyk2 activation, erythropoietin treatment of cells expressing this erythropoietin receptor/Ifnar1 chimera did not evoke any detectable IFN-type response. However, a dose-dependent interference with signal transduction via the endogenous Ifnar complex was found for STAT1, STAT2, STAT3, Tyk2, and Jak1 activation, for gene induction, and for antiviral activity. In a similar approach, cells expressing the beta1 chain of the interleukin-12 receptor showed a reduced transcriptional response to IFN-alpha as well as reduced STAT and kinase activation. In both model systems, titration of the Tyk2 kinase away from the Ifnar1 receptor chain accounts for the observed cross-interference.

A mediator role for metallothionein in tumor necrosis factor-induced lethal shock

Tumor necrosis factor (TNF) is a proinflammatory cytokine, which is centrally involved in several inflammatory disorders. Administration of TNF leads to a potentially lethal systemic inflammatory response syndrome (SIRS). We observed that (a) mice lacking functional genes for metallothionein 1 and 2 (MT-null) were protected compared with wild-type controls (P = 0.0078), and (b) mice overexpressing MT-1 (MT-TG) were more sensitized for the lethal effect of TNF than control mice (P = 0.0003), indicating a mediating role for MT in TNF induced SIRS. As MT is involved in the body zinc homeostasis, we tested whether zinc-deprivation or -supplementation alters the response to TNF. Although zinc-depletion strongly sensitized (P = 0.036), and pretreatment with zinc sulfate (ZnSO4) conferred protection against the deleterious effects of TNF (P < 0.0002), it was also found that the protection provided by zinc is independent of MT. Our observation that hsp70 is strongly induced in jejunum after ZnSO4 treatment, suggests a contribution of hsp70 in the protection against TNF. In addition, ZnSO4 cotreatment allowed complete regression of inoculated tumors with TNF and interferon gamma, leading to a significantly better survival (P = 0.0045).

Design and application of a cytokine-receptor-based interaction trap

Ligand-induced clustering of type I cytokine receptor subunits leads to trans-phosphorylation and activation of associated cytosolic janus kinases (JAKs). In turn, JAKs phosphorylate tyrosine residues in the receptor tails, leading to recruitment and activation of signalling molecules. Among these, signal transducers and activators of transcription (STATs) are important in the direct transmission of signals to the nucleus. Here, we show that incorporation of an interaction trap in a signalling-deficient receptor allows the identification of protein-protein interactions, using a STAT-dependent complementation assay. Mammalian protein-protein interaction trap (MAPPIT) adds to existing yeast two-hybrid procedures, as originally explored by Fields and Song, and permits the detection of both modification-independent and of phosphorylation-dependent interactions in intact human cells. We also demonstrate that MAPPIT can be used to screen complex complementary DNA libraries, and using this approach, we identify cytokine-inducible SH2-containing protein (CIS) and suppressor of cytokine signalling-2 (SOCS-2) as interaction partners of the phosphotyrosine 402 (Tyr 402)-binding motif in the erythropoietin receptor (EpoR). Importantly, this approach places protein-protein interactions in their normal physiological context, and is especially applicable to the in situ analysis of signal transduction pathways.

Morbillivirus in common seals stranded on the coasts of Belgium and northern France during summer 1998

Sixteen common seals (Phoca vitulina) were stranded on the Belgian and northern French coasts during the summer of 1998. Eleven (10 pups and one adult) were sampled for histopathological, immunohistochemical, serological, bacteriological, parasitological and virological investigations. The main gross findings were severe emaciation, acute haemorrhagic enteritis, acute pneumonia, interstitial pulmonary emphysema and oedema, and chronic ulcerative stomatitis. Microscopical lung findings were acute to subacute pneumonia with interstitial oedema and emphysema. Severe lymphocytic depletion was observed in lymph nodes. Severe acute to subacute meningoencephalitis was observed in one animal. Specific staining with two monoclonal antibodies directed against canine distemper virus (CDV) and phocine distemper virus was observed in a few lymphocytes in the spleen and lymph nodes of three seals. Anti-CDV neutralising antibodies were detected in sera from six animals. Seven of the seals were positive by reverse transcriptase-PCR for the morbillivirus phosphoprotein gene. The lesions observed were consistent with those in animals infected by a morbillivirus, and demonstrated that distemper has recently recurred in North Sea seals.

Neutralizing monoclonal antibodies can potentiate IL-5 signaling

IL-5 is a major determinant in the survival, differentiation and effector-functions of eosinophils. It mediates its effect upon binding and activation of a membrane bound receptor (R), composed of a ligand-specific alpha-chain and a beta-chain, shared with the receptors for IL-3 and granulocyte-macrophage colony-stimulating factor. We have generated and mapped the epitopes of three monoclonal antibodies (mAb) directed against this cytokine: the strong neutralizing mAb 5A5 and 1E1, and the very weak neutralizing mAb H30. We found that H30 as well as 5A5 can increase proliferation above the level induced by human (h)IL-5 alone, in a JAK-2-dependent manner, and at every sub-optimal hIL-5 concentration analyzed. This effect is dependent on mAb-mediated cross-linking of IL-5R complexes, and is only observed on cell lines expressing a hybrid human/mouse IL-5Ralpha-chain. We discuss these findings in view of the stoichiometric and topological requirements for an activated IL-5R. Since humanized anti-IL-5 mAb are currently in clinical testing, our findings imply that such mAb should be carefully evaluated for their potentiating effects.

Effect of mutations of N- and C-terminal charged residues on the activity of LCAT

On the basis of structural homology calculations, we previously showed that lecithin:cholesterol acyltransferase (LCAT), like lipases, belongs to the alpha/beta hydrolase fold family. As there is higher sequence conservation in the N-terminal region of LCAT, we investigated the contribution of the N- and C-terminal conserved basic residues to the catalytic activity of this enzyme. Most basic, and some acidic residues, conserved among LCAT proteins from different species, were mutated in the N-terminal (residues 1;-210) and C-terminal (residues 211;-416) regions of LCAT. Measurements of LCAT-specific activity on a monomeric substrate, on low density lipoprotein (LDL), and on reconstituted high density lipoprotein (rHDL) showed that mutations of N-terminal conserved basic residues affect LCAT activity more than those in the C-terminal region. This agrees with the highest conservation of the alpha/beta hydrolase fold and structural homology with pancreatic lipase observed for the N-terminal region, and with the location of most of the natural mutants reported for human LCAT. The structural homology between LCAT and pancreatic lipase further suggests that residues R80, R147, and D145 of LCAT might correspond to residues R37, K107, and D105 of pancreatic lipase, which form the salt bridges D105-K107 and D105-R37. Natural and engineered mutations at residues R80, D145, and R147 of LCAT are accompanied by a substantial decrease or loss of activity, suggesting that salt bridges between these residues might contribute to the structural stability of the enzyme.

A hydrophobic cluster at the surface of the human plasma phospholipid transfer protein is critical for activity on high density lipoproteins

The plasma phospholipid transfer protein (PLTP) belongs to the lipid transfer/lipopolysaccharide binding protein (LT/LBP) family, together with the cholesteryl ester transfer protein, the lipopolysaccharide binding protein (LBP) and the bactericidal permeability increasing protein (BPI). In the present study, we used the crystallographic data available for BPI to build a three-dimensional model for PLTP. Multiple sequence alignment suggested that, in PLTP, a cluster of hydrophobic residues substitutes for a cluster of positively charged residues found on the surface of LBP and BPI, which is critical for interaction with lipopolysaccharides. According to the PLTP model, these hydrophobic residues are situated on an exposed hydrophobic patch at the N-terminal tip of the molecule. To assess the role of this hydrophobic cluster for the functional activity of PLTP, single point alanine mutants were engineered. Phospholipid transfer from liposomes to high density lipoprotein (HDL) by the W91A, F92A, and F93A PLTP mutants was drastically reduced, whereas their transfer activity toward very low density lipoprotein and low density lipoprotein did not change. The HDL size conversion activity of the mutants was reduced to the same extent as the PLTP transfer activity toward HDL. Based on these results, we propose that a functional solvent-exposed hydrophobic cluster in the PLTP molecule specifically contributes to the PLTP transfer activity on HDL substrates.

Identification of the Y985 and Y1077 motifs as SOCS3 recruitment sites in the murine leptin receptor

The leptin system provides a link between adipose mass and the central nervous system. The appetite suppressing effects of leptin are impaired in most obese patients and some mutant mice strains. Herein we describe how suppressor of cytokine signalling 3 (SOCS3), a potential mediator of this leptin resistance is recruited into the activated murine leptin receptor complex. Using a functional assay based on inhibition of leptin mediated reporter induction, and using phosphopeptide affinity chromatography we show binding of SOCS3 to the highly conserved phosphorylated Tyr-985 and Tyr-1077 motifs within the mouse leptin receptor.

Interleukin-9 enhances interleukin-5 receptor expression, differentiation, and survival of human eosinophils

Interleukin-9 (IL-9) has been implicated in the pathogenesis of allergic disorders. To examine the interaction between IL-9 and eosinophils, we evaluated mature peripheral blood eosinophils for their expression of the specific alpha-subunit of the IL-9 receptor (IL-9R-alpha). The expression of IL-9R-alpha by human eosinophils was detected at the messenger RNA (mRNA) and protein levels by reverse transcriptase-polymerase chain reaction (RT-PCR), flow cytometry, and immunocytochemical analysis, respectively. Functional analyses demonstrated that recombinant human (rh)IL-9 inhibited in vitro peripheral blood human eosinophil apoptosis in a concentration-dependent manner. We then examined the role of IL-9 in eosinophil differentiation using the human cord blood CD34(+) cells and human promyelocytic leukemia cells (HL-60). The addition of IL-9 to CD34(+) cells cultured in IL-3 and IL-5 enhanced eosinophil development, and IL-9 alone induced the expression of IL-5R-alpha. IL-9 also up-regulated the IL-5R-alpha chain cell surface expression during terminal eosinophil differentiation of the HL-60 cell line. Our findings suggest that IL-9 may potentiate in vivo eosinophil function by increasing their survival and IL-5-mediated differentiation and maturation. Taken together, these results suggest a mechanism by which IL-9 potentiates airway and tissue eosinophilia.

Pathological findings in two fin whales (Balaenoptera physalus) with evidence of morbillivirus infection

Two immature female fin whales stranded on the Belgian and French coastlines, were examined post mortem. The main gross findings were massive parasitic infestation, associated with a large thrombus in one whale, and severe emaciation. Microscopical investigations revealed multinucleated syncytia with large intranuclear inclusion bodies in various tissues, and positive immunolabelling for morbillivirus antigens. Other evidence of morbillivirus infection was provided by the demonstration of specific viral structures in syncytia and in cell cultures, and the detection of neutralizing antibodies to canine distemper virus. To the authors>> knowledge, this is the first firm report of morbillivirus infection in baleen whales.

Headgroup specificity of lecithin cholesterol acyltransferase for monomeric and vesicular phospholipids

In this study, we investigated how the nature of the phospholipid head group and the macromolecular structure of the phospholipid, either as a monomer or incorporated into a lipid matrix, influence the activity of lecithin cholesterol acyltransferase (LCAT). As substrates we used 1,2-bis-(1-pyrenebutanoyl)-phosphatidylcholine, 1, 2-bis-(1-pyrenebutanoyl)-phosphatidylethanolamine and 1, 2-bis-(1-pyrenebutanoyl)-phosphatidyl-alcohols, either as monomers or incorporated into small unilamellar vesicles consisting of dipalmitoylphosphatidylcholine ether. The rate of hydrolysis of the pyrene-labeled phospholipids was determined both by fluorescence and by high performance liquid chromatography. V(max) and K(m) were calculated for the different substrates. The data show that V(max) is 10- to 30-fold higher for the hydrolysis of monomeric phosphatidylcholine (PC) compared to phosphatidylethanolamine (PE) and the phosphatidylalcohols, while K(m) values are comparable. When the fluorescent substrates were incorporated into dipalmitoylphosphatidylcholine ether vesicles, we observed a 4- to 10-fold increase of V(max) for PE and the phosphatidylalcohols, and no significant change for K(m). V(max) for PC remained the same. Natural LCAT mutants causing Fish-Eye Disease (FED) and analogues of these mutants expressed in Cos-1 cells, had similar activity on monomeric PC and PE. These data suggest that the activity of LCAT is determined both by the molecular structure of the phospholipid and by its macromolecular properties. The LCAT activity on monomeric substrates decreases as: phosphatidylcholine&z. Gt;phosphatidylethanolamine congruent withphosphatidylpropanol congruent withphosphatidylethanol congruent withphosphatidylethyleneglycol. The incorporation of PE and the phosphatidylalcohols into a matrix of dipalmitoylphosphatidylcholine decreases the specificity of the phospholipid head group.

Identification and expression analysis of leptin-regulated immediate early response and late target genes

Using PC12 cells as an in vitro model system, we have identified a series of transcripts induced through activation of the leptin receptor. On the basis of kinetic studies, two distinct gene sets could be discerned: signal transducer and activator of transciption-3 (STAT-3), suppressor of cytokine signalling-3 (SOCS-3), MT-II (metallothionein-II), the serine/threonine kinase fibroblast-growth-factor-inducible kinase (Fnk) and modulator recognition factor (MRF-1), which are immediate early response genes, and pancreatitis-associated protein I (PAP I), squalene epoxidase, uridine diphosphate glucuronosyltransferase and annexin VIII, which are late induced target genes. At late time points a strong co-stimulation with beta-nerve growth factor or with the adenylate cyclase activator forskolin was observed. To assess the validity of the PC12-cell model system, we examined the effect of leptin administration on the gene transcription of STAT-3, MT-II, Fnk and PAP I in vivo. Leptin treatment of leptin-deficient ob/ob mice increased the STAT-3, SOCS-3, MT-II and Fnk mRNA, and MT-I protein levels in liver, whereas, in jejunum, expression of PAP I mRNA was down-regulated. Furthermore, administration of leptin to starved wild-type mice enhanced the expression of MT-II and Fnk mRNA in liver, but decreased MT-II and PAP I mRNA expression in jejunum. These findings may help to explain the obese phenotype observed in some colonies of MT-I- and MT-II-null mice and/or the observation that leptin protects against tumour-necrosis-factor toxicity in vivo.

Relationship between structure and biochemical phenotype of lecithin:cholesterol acyltransferase (LCAT) mutants causing fish-eye disease

In order to test the hypothesis that fish-eye disease (FED) is due to a deficient activation of lecithin:cholesterol acyltransferase (LCAT) by its co-factor apolipoprotein (apo) A-I, we overexpressed the natural mutants T123I, N131D, N391S, and other engineered mutants in Cos-1 cells. Esterase activity was measured on a monomeric phospholipid enelogue, phospholipase A(2) activity was measured on reconstituted high density lipoprotein (HDL), and acyltransferase activity was measured both on rHDL and on low density lipoprotein (LDL). The natural FED mutants have decreased phospholipase A(2) activity on rHDL, which accounts for the decreased acyltransferase activity previously reported. All mutants engineered at positions 131 and 391 had decreased esterase activity on a monomeric substrate and decreased acyltransferase activity on LDL. In contrast, mutations at position 123 preserved these activities and specifically decreased phospholipase A(2) and acyltransferase activites on rHDL. Mutations of hydrophilic residues in amphipathic helices alpha 3;-4 and alpha His to an alanine did not affect the mutants' activity on rHDL. Based upon the 3D model built for human LCAT, we designed a new mutant F382A, which had a biochemical phenotype similar to the natural T123I FED mutant. These data suggest that residues T123 and F382, located N-terminal of helices alpha 3-4 and alpha His, contribute specifically to the interaction of LCAT with HDL and possibly with its co-factor apoA-I. Residues N131 and N391 seem critical for the optimal orientation of the two amphipathic helices necessary for the recognition of a lipoprotein substrate by the enzyme.

Interleukin 5 regulates the isoform expression of its own receptor alpha-subunit

The receptor for interleukin 5 (IL-5) consists of a cytokine-specific alpha chain (IL-5Ralpha) and a signaling beta chain, which is shared with interleukin 3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF). These 3 cytokines can act in eosinophil development and activation in vitro, but gene deletion or antibody blocking of IL-5 largely ablates eosinophilic responses in models of allergic disease or helminth infection. We investigated factors acting in differential IL-5Ralpha gene splicing to generate either the membrane-anchored isoform (TM-IL-5Ralpha) which associates with the common beta chain to allow IL-5 responsiveness, or a secreted, antagonist variant (SOL-IL-5Ralpha). In a murine myeloid cell line (FDC-P1), transfected with minigenes allowing expression of either IL-5Ralpha variant, IL-5 itself, but not IL-3 or GM-CSF, stimulated a reversible switch toward expression of TM-IL-5Ralpha. A switch from predominantly soluble isoform to TM-IL-5Ralpha messenger RNA (mRNA) expression was also seen during IL-5-driven eosinophil development from human umbilical cord blood-derived CD34(+) cells; this was accompanied by surface expression of IL-5Ralpha and acquisition of functional responses to IL-5. IL-3 and GM-CSF also supported eosinophil development and up-regulation of TM-IL-5Ralpha mRNA in this system, but this was preceded by expression of IL-5 mRNA and was inhibited by monoclonal antibody to IL-5. These data suggest IL-5-specific signaling, not shared by IL-3 and GM-CSF, leading to a switch toward up-regulation of functional IL-5Ralpha and, furthermore, that IL-3 and GM-CSF-driven eosinophil development is dependent on IL-5, providing an explanation for the selective requirement of IL-5 for expansion of the eosinophil lineage. (Blood. 2000;95:1600-1607)

Evidence for local eosinophil differentiation within allergic nasal mucosa: inhibition with soluble IL-5 receptor

Eosinophil differentiation occurs within the bone marrow in response to eosinopoietic cytokines, particularly IL-5. Recently, however, eosinophil precursors (CD34/IL-5Ralpha+ cells) and IL-5 mRNA+ cells have been identified within the lungs of asthmatics, indicating that a population of eosinophils may differentiate in situ. In this report, we examined the presence of eosinophil precursors within allergic nasal mucosa and examined whether they undergo local differentiation following ex vivo stimulation. We cultured human nasal mucosa obtained from individuals with seasonal allergic rhinitis with either specific allergen, recombinant human IL-5 (rhIL-5), or allergen + soluble IL-5Ralpha (sIL-5Ralpha), shown to antagonize IL-5 function. Simultaneous immunocytochemistry and in situ hybridization demonstrated that there were fewer cells coexpressing CD34 immunoreactivity and IL-5Ralpha mRNA following culture with allergen or rhIL-5, compared with medium alone. Immunostaining revealed that the number of major basic protein (MBP) immunoreactive cells (eosinophils) was higher within tissue stimulated with allergen or rhIL-5, compared with unstimulated tissue. In situ hybridization detected an increase in IL-5 mRNA+ cells in sections from tissue cultured with allergen, compared with medium alone. These effects were not observed in tissue cultured with a combination of allergen and sIL-5Ralpha. Colocalization analysis indicated this expression to be mainly, but not exclusively, T cell (44%) and eosinophil (10%) derived. Our findings suggest that a subset of eosinophils may differentiate locally within allergic nasal mucosa, in what appears to be a highly IL-5-dependent fashion, and imply that this process might be regulated in vivo by endogenous production of sIL-5Ralpha.

A sensitive and versatile bioassay for ligands that signal through receptor clustering

The induced expression of xanthine-guanine phosphoribosyl transferase (XGPRT) by low concentrations (-2 pg/ml) of interferon-alpha (IFN-alpha) or IFN-beta, in the 2fTPGH cell line caused a 50% cytotoxicity when these cells were grown in medium containing 6-thioguanine. We extended the application of this sensitive, reliable, and easy bioassay to other members of the cytokine family. To activate the IFN signaling pathway, we made receptor chimeras, consisting of the IFN type I receptor intracellular and transmembrane domains, fused to either the interleukin-5 (IL-5) receptors or erythropoietin (Epo) receptor extracellular domains as model systems. 2fTGH cells, stably transfected with these receptor chimeras, responded to very low concentrations of IL-5 or Epo (IC50 values of approximately 15 pg and 3 pg/ml, respectively) and thus can be used as a very sensitive bioassay for both ligands. Background activity of IL-5, Epo, tumor necrosis factor (TNF), IL-6, or leptin on cells that did not carry the receptor chimeras was very low. This methodology can in principle be extended to any ligand that acts via clustering of its receptors.

Contributions of leukemia inhibitory factor receptor and oncostatin M receptor to signal transduction in heterodimeric complexes with glycoprotein 130

Leukemia inhibitory factor (LIF), cardiotrophin-1, ciliary neurotrophic factor, and oncostatin M (OSM) lead to heterodimerization of LIF receptor (LIFR) or the OSM-specific receptor (OSMR) with glycoprotein (gp) 130, the common receptor subunit for IL-6-type cytokines. Thereby intracellular signaling via Janus kinases (Jaks) and STAT transcription factors is initiated. We investigated the contributions of LIFR and OSMR to signal transduction in the context of heterodimers with gp130. Chimeric receptors based on the extracellular parts of the IL-5R alpha- and beta-chains were generated, allowing the induced heterodimerization of two different cytoplasmic tails. Our studies demonstrate that upon heterodimerization with the gp130 cytoplasmic region, the cytoplasmic parts of both LIFR and OSMR were critical for activation of an acute phase protein promoter in HepG2 hepatoma cells. The membrane-proximal region of LIFR or OSMR was crucial for the ability of such receptor complexes to induce DNA binding of STAT1 and STAT3 in COS-7 cells. Membrane-distal regions of LIFR and OSMR contributed to STAT activation even in the absence of gp130 STAT recruitment sites. We further show that the Janus kinases Jak1 and Jak2 constitutively associated with receptor constructs containing the cytoplasmic part of LIFR, OSMR, or gp130, respectively. Homodimers of the LIFR or OSMR cytoplasmic regions did not elicit responses in COS-7 cells but did in HepG2 cells and in MCF-7 breast carcinoma cells. Thus, in spite of extensive functional similarities, differential signaling abilities of gp130, LIFR, and OSMR may become evident in a cell-type-specific manner.

Dimerization of the interferon type I receptor IFNaR2-2 is sufficient for induction of interferon effector genes but not for full antiviral activity

We constructed chimeric receptors wherein the extracellular domain of the erythropoietin receptor (EpoR) was fused to the transmembrane and intracellular domains of the interferon (IFN) type I receptor subunits, IFNaR1 or IFNaR2-2. Transfection into 2fTGH and Tyk2-deficient 11,1 cells showed that EpoR/IFNaR2-2 alone was able to transduce a signal upon stimulation with erythropoietin (Epo), as judged by induction of the interferon type I-inducible 6-16 promoter. In contrast, protection against infection with encephalomyocarditis virus or vesicular stomatitis virus was reduced or absent, respectively. To further investigate the role of IFNaR1 in the induction of an antiviral state, we analyzed the Epo- versus IFNalpha-induced transcription of a set of genes, involved in antiviral protection. Up to 24 h after stimulation with Epo or IFNalpha, comparable transcription of the p56, dsRNA-dependent protein kinase, 2'-5'A synthetase, and MxA genes was seen. However, at later time points, only in the case of Epo induction, a sharp decrease of mRNA levels was observed. Western blotting analysis of dsRNA-dependent protein kinase showed a similar pattern at the protein level. Taken together, our results imply a role for IFNaR1 in the induction of sustained mRNA and protein levels that are likely required for optimal antiviral activity.

Analysis of Tyr to Phe and fa/fa leptin receptor mutations in the PC12 cell line

Weight regulation through body-fat content and energy homeostasis, is regulated mainly through the actions of leptin. Herein, we analyse the effect of mutations in the mouse leptin receptor using the PC12 pheochromocytoma cell line as a model system. Both the induction of pancreatitis associated protein 1 and metallothionein-II, two leptin regulated genes in PC12, was evaluated. Tyr to Phe mutations in the cytoplasmic tail of the mouse leptin receptor confirmed the critical role of Tyr1138 (a YxxQ motif) and STAT-3 activation for induction of leptin-induced genes in PC12. In addition, the Tyr985Phe mutation showed enhanced responsiveness to leptin, which was even more pronounced in combination with Tyr1077Phe. The short isoform of the leptin receptor showed complete loss of stimulation of both genes. In contrast, a leptin receptor devoid of all Tyr residues in its cytoplasmic tail was still capable of a limited induction of the PAP 1 gene. A mutant mouse leptin receptor containing the fa/fa mutation showed constitutive signalling and impaired responsiveness to leptin. Treatment with the adenylate cyclase activator forskolin alone, in the absence of leptin was sufficient to obtain full induction of both genes.

Identification of a Leu-lle internalization motif within the cytoplasmic domain of the leukaemia inhibitory factor receptor

Leukaemia inhibitory factor (LIF) signals via a heterodimeric receptor complex comprised of the LIF receptor (LIFR) and the interleukin (IL)-6 signal transducer gp130. Upon binding to its cognate receptor LIF is internalized. In this study, we show that the LIFR is endocytosed independently of gp130. By using a heterochimaeric receptor system we identified a dileucine-based internalization motif within the cytoplasmic domain of the LIFR. Our findings suggest that a heterodimeric LIFR/gp130 complex and homodimeric gp130/gp130 complex are endocytosed via distinct internalization signals.

Eosinophil development and bone marrow and tissue eosinophils in atopic asthma

BACKGROUND

Eosinophils develop from bone marrow (BM) progenitors, and interleukin-5 (IL-5) and eotaxin may act in expansion and mobilisation of BM eosinophils in asthma.

METHODS

We have examined phenotypic changes as CD34+ cells develop to the eosinophil lineage in vitro, and have evaluated BM eosinophils from asthmatic and control subjects for expression of the eotaxin receptor, CCR3.

RESULTS

Acquisition of receptors for IL-5 and CCR3 was an early event in eosinophil development. There were increased CD34+ cells, and mature and immature CCR3+ eosinophils in BM from asthmatics.

CONCLUSION

These data suggest that IL-5 may act early in eosinophil development, and that eotaxin has the capacity to mobilise a BM eosinophil pool in asthma.

The cell surface expression level of the human interleukin-5 receptor alpha subunit determines the agonistic/antagonistic balance of the human interleukin-5 E13Q mutein

The human interleukin-5 (IL-5) receptor consists of an alpha-chain that specifically binds the ligand with intermediate affinity, and a beta c-chain, that associates with the IL-5/IL-5R alpha complex, leading to a high-affinity, signal transducing receptor complex. Structure-function studies showed that modification of the putative beta c-chain binding site in IL-5 (E13Q mutein) converted the molecule into an antagonist. However, analysis of the effect of this mutant IL-5 on COS-1 cells transfected with both receptor subunits, did not show reduced interaction with the beta c subunit [Tavernier, J., Tuypens, T., Verhee, A., Plaetinck, G., Devos, R., Van der Heyden, J., Guisez, Y. & Oefner, C. (1995) Proc. Natl Acad. Sci. USA 89, 7041-7045]. To gain more insight into the mechanism of IL-5 antagonism by E13Q, we tested its biological activity on two FDC-P1 subclones that express clearly different numbers of alpha-subunits yet an almost constant number of murine beta c-subunits. Here we show that E13Q has a biological activity comparable to wild-type IL-5 only when a high number of alpha-chains is present on the cells. Confirming the critical role of the IL5R alpha cell-surface expression level, treatment with suboptimal doses of a neutralising anti-IL-5R alpha antibody results in reduced activity of the mutant but not of wild-type IL-5.

Characterization of functional residues in the interfacial recognition domain of lecithin cholesterol acyltransferase (LCAT)

Lecithin cholesterol acyltransferase (LCAT) is an interfacial enzyme active on both high-density (HDL) and low-density lipoproteins (LDL). Threading alignments of LCAT with lipases suggest that residues 50-74 form an interfacial recognition site and this hypothesis was tested by site-directed mutagenesis. The (delta56-68) deletion mutant had no activity on any substrate. Substitution of W61 with F, Y, L or G suggested that an aromatic residue is required for full enzymatic activity. The activity of the W61F and W61Y mutants was retained on HDL but decreased on LDL, possibly owing to impaired accessibility to the LDL lipid substrate. The decreased activity of the single R52A and K53A mutants on HDL and LDL and the severer effect of the double mutation suggested that these conserved residues contribute to the folding of the LCAT lid. The membrane-destabilizing properties of the LCAT 56-68 helical segment were demonstrated using the corresponding synthetic peptide. An M65N-N66M substitution decreased both the fusogenic properties of the peptide and the activity of the mutant enzyme on all substrates. These results suggest that the putative interfacial recognition domain of LCAT plays an important role in regulating the interaction of the enzyme with its organized lipoprotein substrates.

Effects of natural mutations in lecithin:cholesterol acyltransferase on the enzyme structure and activity

A molecular model was built for human lecithin:cholesterol acyltransferase (LCAT) based upon the structural homology between this enzyme and lipases (Peelman et al. 1998. Prot. Sci. 7: 585-597). We proposed that LCAT belongs to the alpha/beta hydrolase fold family, and that the central domain of LCAT consists of a mixed seven-stranded beta-pleated sheet with four alpha-helices and loops linking the beta-strands. The catalytic triad of LCAT was identified as Asp345 and His377, as well as Ser181. This model is used here for the interpretation of the structural defects linked to the point mutations identified in LCAT, which cause either familial LCAT deficiency (FLD) or fish-eye disease (FED). We show that these mutations occur in separate domains of the 3D structure of the enzyme. Most mutations causing familial LCAT deficiency are either clustered in the vicinity of the catalytic triad or affect conserved structural elements in LCAT. Most mutations causing fish-eye disease are localized on the outer hydrophilic surface of the amphipathic helical segments. These mutations affect only minimally the overall structure of the enzyme, but are likely to impair the interaction of the enzyme with its co-factor and/or substrate.

Interleukin-4, interleukin-5, and granulocyte-macrophage colony-stimulating factor receptor expression in chronic sinusitis and response to topical steroids

Chronic sinusitis and its associated eosinophilic infiltrate are believed to be mediated, at least in part, by the upregulation of Th-2 cytokines, including interleukin-4, interleukin-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF). Interleukin-4 is involved in IgE production and in eosinophil recruitment through upregulation of vascular cell adhesion molecule-1. Interleukin-5 and GM-CSF are involved in eosinophil growth and survival. The aim of this study was to investigate the expression of receptors for these cytokines in the sinus mucosa of subjects with chronic sinusitis. Using the technique of in situ hybridization to detect specific cytokine receptor messenger RNA, we studied the sinus mucosa of subjects with nonallergic chronic sinusitis, subjects with allergic chronic sinusitis, subjects with allergic chronic sinusitis treated with topical steroids, and normal controls. Our data demonstrate higher expression of interleukin-4 receptor in subjects with allergic chronic sinusitis than in controls (p < 0.001) and higher expression of interleukin-5 receptor in both subjects with nonallergic chronic sinusitis and subjects with allergic chronic sinusitis than in controls (p < 0.001, p < 0.001). The expression of interleukin-4 receptor and interleukin-5 receptor was higher in subjects with allergic chronic sinusitis than in subjects with nonallergic chronic sinusitis (p < 0.001). GM-CSF receptor expression was also found to be higher in subjects with allergic chronic sinusitis and subjects with nonallergic chronic sinusitis than in controls (p < 0.001, p < 0.001). In contrast to interleukin-4 receptor and interleukin-5 receptor, however, expression of GM-CSF receptor was higher in subjects with nonallergic chronic sinusitis than in subjects with allergic chronic sinusitis (p < 0.001). In subjects with allergic chronic sinusitis treated with topical corticosteroids, the expression of interleukin-4 receptor and interleukin-5 receptor messenger RNA levels was significantly lower than levels in patients with allergic chronic sinusitis who were not taking topical steroids (p < 0.001, p < 0.001). Steroid treatment had no effect on GM-CSF receptor messenger RNA expression. In conclusion, our data support a role for Th-2 cytokine receptors in the pathophysiology of chronic sinusitis. Further, our data lend support to the theory that differential activation of distinct cytokine pathways mediates inflammation in chronic sinusitis depending on whether there is associated allergy. Finally, treatment with topical corticosteroids has been demonstrated in chronic sinusitis to downregulate receptors for interleukin-4 and interleukin-5.