Cholesterol-Dependent and -Independent CD40 Internalization and Signaling Activation in Cardiovascular Endothelial Cells

Potential role of soluble CD40 receptor in chronic inflammatory diseases

The CD40 receptor and its ligand CD154 are widely expressed in various immune-competent cells. Interaction of CD154 with CD40 is essential for B-cell growth, differentiation, and immunoglobulin class switching. Many other immune-competent cells involved in innate and adaptive immunity communicate through this co-stimulatory ligand-receptor dyad. CD40-CD154 interaction is involved in the pathogenesis of numerous inflammatory and autoimmune diseases. While CD40 and CD154 are membrane-bound proteins, their soluble counterparts are generated by proteolytic cleavage or alternative splicing. This review summarises current knowledge about the impact of single nucleotide polymorphisms in the human CD40 gene and compensatory changes in the plasma level of the soluble CD40 receptor (sCD40) isoform in related pro-inflammatory diseases. It discusses regulation patterns of the disintegrin metalloprotease ADAM17 function leading to ectodomain shedding of transmembrane proteins, such as pro-inflammatory adhesion molecules or CD40. The role of sCD40 as a potential biomarker for chronic inflammatory diseases will also be discussed.

Ectodomain Shedding by ADAM17 Increases the Release of Soluble CD40 from Human Endothelial Cells under Pro-Inflammatory Conditions

BACKGROUND

Homozygosity for the C allele of the -1T>C single nucleotide polymorphism (SNP) of the CD40 gene (rs1883832) is associated with susceptibility to coronary heart disease (CHD), enhanced CD40 expression, and shedding. The disintegrin metalloprotease ADAM17 can cleave various cell surface proteins. This study investigates an association between ADAM17-mediated CD40 shedding and inflammation in CC genotype human endothelial cells.

METHODS

Human umbilical vein endothelial cells (HUVEC) carrying the CC genotype were stimulated with soluble CD40 ligand (sCD40L) or tumor necrosis factor-α (TNFα). Messenger RNA and protein expression were determined with standard methods. Levels of high sensitive c-reactive protein (hs-CRP), interleukin-6 (IL-6), and sCD40 in plasma samples from patients with CHD were assessed using ELISA.

RESULTS

ADAM17 surface abundance was elevated following stimulation with CD40L and TNFα just as its regulator iRhom2. Inhibition of ADAM17 prevented TNFα-induced sCD40 and soluble vascular cell adhesion molecule-1 release into the conditioned medium and reinforced CD40 surface abundance. Secondary to inhibition of ADAM17, stimulation with CD40L or TNFα upregulated monocyte chemoattractant protein-1 mRNA and protein. Levels of sCD40 and the inflammatory biomarkers hs-CRP and IL-6 were positively correlated in the plasma of patients with CHD.

CONCLUSIONS

We provide a mechanism by which membrane-bound CD40 is shed from the endothelial cell surface by ADAM17, boosting sCD40 formation and limiting downstream CD40 signaling. Soluble CD40 may represent a robust biomarker for CHD, especially in conjunction with homozygosity for the C allele of the -1T>C SNP of the CD40 gene.

Functional association of a CD40 gene single-nucleotide polymorphism with the pathogenesis of coronary heart disease

AIMS

Endothelial dysfunction is a major contributor to the pathogenesis of atherosclerosis. CD40-CD40 ligand interactions confer a pro-inflammatory phenotype to endothelial cells (ECs). Recently, a thymine to cytosine transition (-1T>C) in the Kozak sequence of the CD40 gene (rs1883832) has been associated with coronary heart disease (CHD) in an Asian population. As there are no reports yet regarding its role in other ethnic groups, this study determines if the -1T>C single-nucleotide polymorphism (SNP) could be a risk factor for CHD in Caucasians by performing an association study and elucidates its functional consequence in cultured ECs.

METHODS AND RESULTS

Molecular and biochemical techniques, cell adhesion assays were used for genotype-stratified human EC characterization. SNP distribution in Caucasians was examined in a hospital-based case-control CHD study and serum levels of soluble CD40 (sCD40) were quantified by ELISA. The SNP in the CD40 gene affected baseline CD40 protein abundance on ECs. There was a genotype-dependent difference in CD40-mediated expression of pro-inflammatory genes. Monocyte adhesion was highest on the surface of cells homozygous for the C allele. Homozygosity for the C allele was associated with significant 2.32-fold higher odds of developing CHD as compared to TT genotype carriers. sCD40 plasma levels were genotype-dependently elevated in CHD patients, indicating a possible prognostic value.

CONCLUSION

The C allele of the CD40 SNP provokes a pro-inflammatory EC phenotype, compensated by an enhanced CD40 shedding to neutralize excess CD40 ligand. Homozygosity for the C allele is the cause for a genetic susceptibility to atherosclerosis and its sequelae.

The influx/efflux mechanisms of d-peptide ligand of nAChRs across the blood-brain barrier and its therapeutic value in treating glioma

A d-peptide ligand of the nicotine acetylcholine receptors (nAChRs), termed ^DCDX, enables drug delivery to the brain when incorporated into liposomes and has shown promise as a nanocarrier for treating brain diseases. However, few reports have described the mechanisms whereby ^DCDX-modified liposomes traverse the blood-brain barrier (BBB). Here, we studied the molecular mechanisms enabling ^DCDX (and its associated liposomes) to cross an in vitro BBB using a simulated cerebral endothelium monolayer formed by brain capillary endothelial cells (bEnd.3 cells). We also examined the mechanisms whereby ^DCDX-modified liposomes cross the BBB in vivo using the brain efflux-index method. Transport of ^DCDX and its modified liposomes was dominantly mediated via the lipid raft/caveolae endocytic pathway. Both the endoplasmic reticulum (ER) and Golgi complex participated in delivering ^DCDX-modified liposomes to the plasma membrane (PM). ^DCDX-modified liposomes also participated in the endosome/lysosome pathway (with high-efficiency BBB crossing observed in vitro), while competing for the ER/Golgi/PM pathway. In addition, nAChR α7 did not promote the transportation of ^DCDX-modified liposomes in vivo or in vitro, as assessed with α7-knockout mice and by performing α-bungarotoxin (α-Bgt) binding-competition experiments. P-glycoprotein (P-gp) was identified as the main efflux transporter across the BBB, in vivo and in vitro. Using a xenograft nude mouse model of human glioblastoma multiforme, blocking the efflux function of P-gp with verapamil enhanced the therapeutic efficiency of ^DCDX-modified liposomes that were formulated with doxorubicin against glioblastoma. The findings of this study reveal novel mechanisms underlying crossing of the BBB by ^DCDX-modified liposomes, suggesting that ^DCDX-modified liposomes can potentially serve as a powerful therapeutic tool for treating glioma.

Mitochondrial Double-Stranded RNA in Exosome Promotes Interleukin-17 Production Through Toll-Like Receptor 3 in Alcohol-associated Liver Injury

BACKGROUND AND AIMS

Mitochondrial double-stranded RNA (mtdsRNA) and its innate immune responses have been reported previously; however, mtdsRNA generation and its effects on alcohol-associated liver disease (ALD) remain unclear. Here, we report that hepatic mtdsRNA stimulates toll-like receptor 3 (TLR3) in Kupffer cells through the exosome (Exo) to enhance interleukin (IL)-17A (IL-17A) production in ALD.

APPROACH AND RESULTS

Following binge ethanol (EtOH) drinking, IL-17A production primarily increased in γδ T cells of wild-type (WT) mice, whereas the production of IL-17A was mainly facilitated by CD4+ T cells in acute-on-chronic EtOH consumption. These were not observed in TLR3 knockout (KO) or Kupffer cell-depleted WT mice. The expression of polynucleotide phosphorylase, an mtdsRNA-restricting enzyme, was significantly decreased in EtOH-exposed livers and hepatocytes of WT mice. Immunostaining revealed that mtdsRNA colocalized with the mitochondria in EtOH-treated hepatocytes from WT mice and healthy humans. Bioanalyzer analysis revealed that small-sized RNAs were enriched in EtOH-treated Exos (EtOH-Exos) rather than EtOH-treated microvesicles in hepatocytes of WT mice and humans. Quantitative real-time PCR and RNA sequencing analyses indicated that mRNA expression of mitochondrial genes encoded by heavy and light strands was robustly increased in EtOH-Exos from mice and humans. After direct treatment with EtOH-Exos, IL-1β expression was significantly increased in WT Kupffer cells but not in TLR3 KO Kupffer cells, augmenting IL-17A production of γδ T cells in mice and humans.

CONCLUSIONS

EtOH-mediated generation of mtdsRNA contributes to TLR3 activation in Kupffer cells through exosomal delivery. Consequently, increased IL-1β expression in Kupffer cells triggers IL-17A production in γδ T cells at the early stage that may accelerate IL-17A expression in CD4+ T cells in the later stage of ALD. Therefore, mtdsRNA and TLR3 may function as therapeutic targets in ALD.

B Cell Endosomal RAB7 Promotes TRAF6 K63 Polyubiquitination and NF-κB Activation for Antibody Class-Switching

Upon activation by CD40 or TLR signaling, B lymphocytes activate NF-κB to induce activation-induced cytidine deaminase and, therefore, Ig class switch DNA recombination, as central to the maturation of the Ab and autoantibody responses. In this study, we show that NF-κB activation is boosted by colocalization of engaged immune receptors, such as CD40, with RAB7 small GTPase on mature endosomes, in addition to signals emanating from the receptors localized on the plasma membrane, in mouse B cells. In mature endosomes, RAB7 directly interacts with TRAF6 E3 ubiquitin ligase, which catalyzes K63 polyubiquitination for NF-κB activation. RAB7 overexpression in Cd19+/creRosa26fl-STOP-fl-Rab7 mouse B cells upregulates K63 polyubiquitination activity of TRAF6, enhances NF-κB activation and activation-induced cytidine deaminase induction, and boosts IgG Ab and autoantibody levels. This, together with the extensive intracellular localization of CD40 and the strong correlation of RAB7 expression with NF-κB activation in mouse lupus B cells, shows that RAB7 is an integral component of the B cell NF-κB activation machinery, likely through interaction with TRAF6 for the assembly of "intracellular membrane signalosomes."

Nigericin Promotes NLRP3-Independent Bacterial Killing in Macrophages

Altered microbiota has been associated with a number of diseases, including inflammatory bowel diseases, diabetes, and cancer. This dysregulation is thought to relate the host inflammatory response to enteric pathogens. Macrophages play a key role in host response to microbes and are involved in bacterial killing and clearance. This process is partially mediated through the potassium efflux-dependent, cytosolic, PYCARD-containing inflammasome protein complex. Surprisingly, we discovered an alternative mechanism for bacterial killing, independent of the NLRP3 inflammasome/PYCARD. Using the NLRP3 inflammasome-deficient Raw 264.7 and PYCARD-deficient J77 macrophages, which both lack PYCARD, we found that the potassium efflux activator nigericin enhances bacterial killing. Macrophage response to nigericin was examined by RT gene profiling and subsequent qPCR, which demonstrated altered expression of a series of genes involved in the IL-18 bacterial killing pathway. Based on our results we propose a model of bacterial killing, unrelated to NLRP3 inflammasome activation in macrophage cells. Improving understanding of the molecular pathways driving bacterial clearance within macrophage cells will aid in the development of novel immune-targeted therapeutics in a number of diseases.

Scaffold protein JLP is critical for CD40 signaling in B lymphocytes

CD40 expression on the surface of B lymphocytes is essential for their biological function and fate decision. The engagement of CD40 with its cognate ligand, CD154, leads to a sequence of cellular events in B lymphocytes, including CD40 cytoplasmic translocation, a temporal and spatial organization of effector molecules, and a cascade of CD40-induced signal transduction. The JLP scaffold protein was expressed in murine B lymphocytes. Using B lymphocytes from jlp-deficient mice, we observed that JLP deficiency resulted in defective CD40 internalization upon CD154/CD40 engagement. Examination of interactions and co-localization among CD40, JLP, dynein, and Rab5 in B lymphocytes suggested that CD40 internalization is a process of JLP-mediated vesicle transportation that depends on Rab5 and dynein. JLP deficiency also diminished CD40-dependent activation of MAPK and JNK, but not NF-κB. Inhibiting vesicle transportation from the direction of cell periphery to the cell center by a dynein inhibitor (ciliobrevin D) impaired both CD154-induced CD40 internalization and CD40-dependent MAPK activities in B lymphocytes. Collectively, our data demonstrate a novel role of the JLP scaffold protein in the bridging of CD154-triggered CD40 internalization and CD40-dependent signaling in splenic B lymphocytes.

Regulation of fibroblast growth factor-inducible 14 (Fn14) expression levels via ligand-independent lysosomal degradation

Fibroblast growth factor-inducible 14 (Fn14) is a highly inducible cytokine receptor that engages multiple intracellular signaling pathways, including nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK). Fn14 expression is regulated by several cytokines and growth factors, and Fn14 is transiently up-regulated after injury. In contrast, in states of chronic inflammatory disease and in some solid tumors, Fn14 is persistently up-regulated. However, the post-translational regulation of Fn14 expression has not been directly investigated. Thus, we examined Fn14 proteostasis in the presence and absence of the Fn14 ligand TNF-like weak inducer of apoptosis (TWEAK). Similar to other TNF receptor superfamily members, we found that TWEAK induces Fn14 internalization and degradation. Surprisingly, we also observed rapid, TWEAK-independent, constitutive Fn14 internalization and turnover. Fn14 levels are maintained in cell culture by ongoing synthesis and trafficking of the receptor, leading to subsequent down-regulation by lysosomal degradation. Unexpectedly, the extracellular domain of Fn14 is necessary and sufficient for constitutive turnover. Based on these findings, we propose a model in which constitutive down-regulation of Fn14 facilitates dynamic regulation of Fn14 protein levels and prevents spontaneous or inappropriate receptor signaling.

The impact of statins on FGF-2-stimulated human umbilical vein endothelial cells

AIM

To determine the effects of different types of statins on proliferative and migrative behaviors of basic fibroblastic growth factor (FGF)-2-stimulated endothelial cells.

MATERIALS AND METHODS

Human umbilical vein endothelial cells (HUVECs) were isolated and cultured. Groups were arranged in order to observe the impact of each individual substance alone, or under stimulation with statin on FGF-2-stimulated endothelial cells. Endothelial cells were stimulated with human growth factor (HGF), statins, methyl-β-cyclodextrin (β-MCD), and either farnesyl pyrophosphate (FPP) ammonium salt, or geranylgeranyl-pyrophosphate (GGPP), respectively. Cell proliferation analyses were performed 48 hours after stimulation and gaps between migration borders were used in migration analyses.

RESULTS

The statins showed significant antiproliferative and anti-migrative effects and inhibited the proliferative behavior of FGF-2. However, endothelial cell proliferation and migration were significantly increased after mevalonate co-incubation. Experiments with β-MCD indicated that the destruction of lipid rafts had a negative impact on the action of FGF-2. Stimulation of statin-incubated cells with FPP had no additional effect on proliferation or migration. Notably, although FGF-2 exerted a pro-migrative effect, the effect was not shown in the FPP + FGF-2 group. The anti-migrative actions of statins along with disruption of membrane integrity were reversed by the addition of GGPP.

CONCLUSION

The angiogenic effect of FGF-2 is suppressed through inhibition of the intracellular cholesterol biosynthesis via statins. Inhibitory effects of statins on FGF-2-stimulated HUVECs were observed to result from both the inhibition of isoprenylation and the destruction of lipid rafts on the cell membrane.

A novel role of the scaffolding protein JLP in tuning CD40-induced activation of dendritic cells

Receptor internalization is a common mechanism underlying surface receptor down-regulation (and thus receptor signaling) upon its engagement with the cognate ligand. Tight regulation of surface CD40 expression is critical in regulating different functional properties of dendritic cell (DC). Engagement of CD40 on mature DC and the cognate CD40 ligand on T cell activates c-Jun N-terminal MAPK, p38 and ERK1/2 MAPK pathways in mature DC. JNK-associated leucine zipper protein (JLP) is a scaffolding protein that interacted with p38 and JNK. The molecular mechanism underlying CD40 internalization and its physiological impact on DC functions remained unclear. Here we reported that the engagement of CD40 on the LPS-activated DC down-regulated the surface expression of CD40. We examined the role of the JLP protein in DC differentiation, and in the regulation of DC function(s) in vitro. In contrast to the abundant JLP expression observed in immortal cell lines, primary immature DC expressed low levels of the JLP proteins. The induction of the JLP protein expression was observed in the LPS-mature DC that were activated by CD40 ligation, and also in the poly I:C stimulated DC. JLP-silenced DC was impaired in regulating CD40 surface expression upon LPS stimulation and CD40 induced receptor internalization. Such aberrant change in the regulation of surface CD40 expression was associated with an augmented capacity of the JLP-silenced DC in IL-12 production. Collectively, our data identified a novel role of a scaffolding protein JLP in the regulation of surface CD40 expression and fine-tuning of DC function.

Dynamin2, clathrin, and lipid rafts mediate endocytosis of the apical Na/K/2Cl cotransporter NKCC2 in thick ascending limbs

Steady-state surface levels of the apical Na/K/2Cl cotransporter NKCC2 regulate NaCl reabsorption by epithelial cells of the renal thick ascending limb (THAL). We reported that constitutive endocytosis of NKCC2 controls NaCl absorption in native THALs; however, the pathways involved in NKCC2 endocytosis are unknown. We hypothesized that NKCC2 endocytosis at the apical surface depends on dynamin-2 and clathrin. Measurements of steady-state surface NKCC2 and the rate of NKCC2 endocytosis in freshly isolated rat THALs showed that inhibition of endogenous dynamin-2 with dynasore blunted NKCC2 endocytosis by 56 ± 11% and increased steady-state surface NKCC2 by 67 ± 27% (p < 0.05). Expression of the dominant negative Dyn2K44A in THALs slowed the rate of NKCC2 endocytosis by 38 ± 8% and increased steady-state surface NKCC2 by 37 ± 8%, without changing total NKCC2 expression. Inhibition of clathrin-mediated endocytosis with chlorpromazine blunted NKCC2 endocytosis by 54 ± 6%, while preventing clathrin from interacting with synaptojanin also blunted NKCC2 endocytosis by 52 ± 5%. Disruption of lipid rafts blunted NKCC2 endocytosis by 39 ± 4% and silencing caveolin-1 by 29 ± 4%. Simultaneous inhibition of clathrin- and lipid raft-mediated endocytosis completely blocked NKCC2 internalization. We concluded that dynamin-2, clathrin, and lipid rafts mediate NKCC2 endocytosis and maintain steady-state apical surface NKCC2 in native THALs. These are the first data identifying the endocytic pathway for apical NKCC2 endocytosis.

Multivalent porous silicon nanoparticles enhance the immune activation potency of agonistic CD40 antibody

One of the fundamental paradigms in the use of nanoparticles to treat disease is to evade or suppress the immune system in order to minimize systemic side effects and deliver sufficient nanoparticle quantities to the intended tissues. However, the immune system is the body's most important and effective defense against diseases. It protects the host by identifying and eliminating foreign pathogens as well as self-malignancies. Here we report a nanoparticle engineered to work with the immune system, enhancing the intended activation of antigen presenting cells (APCs). We show that luminescent porous silicon nanoparticles (LPSiNPs), each containing multiple copies of an agonistic antibody (FGK45) to the APC receptor CD40, greatly enhance activation of B cells. The cellular response to the nanoparticle-based stimulators is equivalent to a 30-40 fold larger concentration of free FGK45. The intrinsic near-infrared photoluminescence of LPSiNPs is used to monitor degradation and track the nanoparticles inside APCs.

ARNO regulates VEGF-dependent tissue responses by stabilizing endothelial VEGFR-2 surface expression

AIMS

The vascular endothelial growth factor (VEGF) stimulates angiogenesis by induction of vessel permeability, proliferation, and migration of endothelial cells, an important process in ischaemic diseases. ADP-ribosylation factor (ARF) nucleotide-binding site opener (ARNO) (cytohesin-2) is a guanine exchange factor important for cellular signalling through ARF GTPases. However, a role for ARNO in VEGF-dependent endothelial processes has so far not been documented. Therefore, we investigated whether ARNO has a role in VEGF-dependent activation of endothelial cells and thus vessel permeability.

METHODS AND RESULTS

ARNO expression was observed in endothelial cells in vitro by RT-PCR, western blotting, and immunofluorescence as well as ex vivo by immunohistochemical staining of mouse aorta. Treatment with the cytohesin inhibitor SecinH3 or with an ARNO siRNA prevented VEGF-dependent Akt activation, assessed by detection of phosphorylated Akt, and proliferation of endothelial cells in vitro, measured by methylthiazoletetrazolium (MTT) reduction. In addition, ARNO suppression reduced VEGF-induced permeability in vessels of the mouse (C57BL/6) cremaster muscle in vivo, as measured by extravasation of fluorescein isothiocyanate (FITC)-dextran. Moreover, ARNO knock-down accelerated ligand-induced reduction in vascular endothelial growth factor receptor-2 (VEGFR-2) surface expression, internalization, and degradation, as assessed by flow cytometry and western blotting, respectively.

CONCLUSION

Our findings indicate an important and novel role for endothelial ARNO in VEGF-dependent initiation of angiogenesis by regulation of VEGFR-2 internalization in endothelial cells, resulting in the activation of the Akt pathway, vessel permeability, and ultimately endothelial proliferation. Thus, ARNO may be a new essential player in endothelial signalling and angiogenesis.

Molecular regulation of NKCC2 in the thick ascending limb

The kidney plays an essential role in blood pressure regulation by controlling short-term and long-term NaCl and water balance. The thick ascending limb of the loop of Henle (TAL) reabsorbs 25-30% of the NaCl filtered by the glomeruli in a process mediated by the apical Na(+)-K(+)-2Cl(-) cotransporter NKCC2, which allows Na(+) and Cl(-) entry from the tubule lumen into TAL cells. In humans, mutations in the gene coding for NKCC2 result in decreased or absent activity characterized by severe salt and volume loss and decreased blood pressure (Bartter syndrome type 1). Opposite to Bartter's syndrome, enhanced NaCl absorption by the TAL is associated with human hypertension and animal models of salt-sensitive hypertension. TAL NaCl reabsorption is subject to exquisite control by hormones like vasopressin, parathyroid, glucagon, and adrenergic agonists (epinephrine and norepinephrine) that stimulate NaCl reabsorption. Atrial natriuretic peptides or autacoids like nitric oxide and prostaglandins inhibit NaCl reabsorption, promoting salt excretion. In general, the mechanism by which hormones control NaCl reabsorption is mediated directly or indirectly by altering the activity of NKCC2 in the TAL. Despite the importance of NKCC2 in renal physiology, the molecular mechanisms by which hormones, autacoids, physical factors, and intracellular ions regulate NKCC2 activity are largely unknown. During the last 5 years, it has become apparent that at least three molecular mechanisms determine NKCC2 activity. As such, membrane trafficking, phosphorylation, and protein-protein interactions have recently been described in TALs and heterologous expression systems as mechanisms that modulate NKCC2 activity. The focus of this review is to summarize recent data regarding NKCC2 regulation and discuss their potential implications in physiological control of TAL function, renal physiology, and blood pressure regulation.

Tyrosine nitration limits stretch-induced CD40 expression and disconnects CD40 signaling in human endothelial cells

Hemodynamic forces are important effectors of endothelial cell phenotype and function. Because CD40-CD154 interactions between endothelial cells and mononuclear leukocytes or activated platelets play an important role in vascular dysfunction, we investigated the effects of cyclic stretch on CD40 expression in human cultured endothelial cells. Short-term stretch transiently up-regulated CD40 expression while long-term stretch resulted in a distinct decline in CD40 protein which was prevented by inhibition of the 20S proteasome or scavenging of peroxynitrite. Tyrosine nitration of CD40 also occurred under static conditions on addition of authentic peroxynitrite, and according to mass spectrometry analysis Tyr-82 but not Tyr-31 was its target in the native protein. Immunofluorescence analysis of endothelial cells transduced with a control or Tyr-82 to Ala mutated AAV9-CD40-eGFP expression construct confirmed a peroxynitrite-dependent redistribution of the protein from the cell membrane to the cytoplasm, which was prevented by methyl-β-cyclodextrin. Moreover, CD154-stimulated IL-12p40 and E-selectin expression markedly decreased after exposure to authentic peroxynitrite or cyclic stretch, respectively. Coimmunoprecipitation demonstrated a decreased binding of TRAF2 and TRAF6 to the CD40 protein after tyrosine nitration. Through this posttranslational oxidative modification of an important costimulatory molecule, endothelial cells are able to quickly adapt to unfavorable hemodynamics and maintain their anti-inflammatory phenotype.

Constitutive endocytosis and recycling of NKCC2 in rat thick ascending limbs

The Na-K-2Cl cotransporter (NKCC2) mediates NaCl absorption by the thick ascending limb of Henle's loop (THAL). Exocytosis and endocytosis regulates surface expression of most transporters. However, little is known about the mechanism of NKCC2 trafficking in the absence of stimulating hormones and whether this mechanism contributes to regulation of steady-state surface expression of apical NKCC2 in the THAL. We tested whether NKCC2 undergoes constitutive endocytosis that regulates steady-state surface NKCC2 and NaCl reabsorption in THALs. We measured steady-state surface NKCC2 levels and the rate of NKCC2 endocytosis by surface biotinylation and Western blot and confocal microscopy of isolated perfused rat THALs. We observed constitutive NKCC2 endocytosis over 30 min that averaged 21.5 ± 2.7% of the surface pool. We then tested whether methyl-β-cyclodextrin (MβCD), a compound that inhibits endocytosis by chelating membrane cholesterol, blocked NKCC2 endocytic retrieval. We found that 30-min treatment with MβCD (5 mM) blocked NKCC2 endocytosis by 81% (P < 0.01). Blockade of endocytosis by MβCD induced accumulation of NKCC2 at the apical membrane as demonstrated by a 60 ± 16% (P < 0.05) increase in steady-state surface expression and enhanced apical surface NKCC2 immunostaining in isolated, perfused THALs. Acute treatment with MβCD did not change the total pool of NKCC2. MβCD did not affect NKCC2 trafficking when it was complexed with cholesterol before treatment. Inhibition endocytosis with MβCD enhanced NKCC2-dependent NaCl entry by 57 ± 16% (P < 0.05). Finally, we observed that a fraction of retrieved NKCC2 recycles back to the plasma membrane (36 ± 7%) over 30 min. We concluded that constitutive NKCC2 trafficking maintains steady-state surface NKCC2 and regulates NaCl reabsorption in THALs. These are the first data showing an increase in apical membrane NKCC2 in THALs by altering the rates of constitutive NKCC2 trafficking, rather than by stimulation of hormone-dependent signaling.

Role of lipid rafts in virus infection

Rafts are domains of the plasma membrane, enriched in cholesterol and sphingolipids; they form a platform for signaling proteins and receptors. The lipid rafts are utilized in the replication cycle of numerous viruses. Internalization receptors of many viruses localize to rafts or are recruited there after virus binding. Arrays of signal transduction proteins found in rafts contribute to efficient trafficking and productive infection. Some viruses are dependent on raft domains for the biogenesis of their membranous replication structures. Finally, rafts are often important in virus assembly and budding. Subsequently, raft components in the viral envelope may be vital for the entry to a new host cell. Here, we summarize the current knowledge of the involvement of rafts in virus infection.

Troglitazone inhibits cell proliferation by attenuation of epidermal growth factor receptor signaling independent of peroxisome proliferator-activated receptor gamma

Peroxisome proliferator-activated receptors (PPAR) belong to the nuclear hormone receptor superfamily of ligand-dependent transcription factors. Recent results have shown that agonists of PPARgamma, such as troglitazone (TGZ), can inhibit cell proliferation and promote cell differentiation independent of PPARgamma. In the present study, we provide evidence that TGZ may bind directly to EGFR and trigger its signaling and internalization independent of PPARgamma. Detailed studies revealed that prolonged incubation with TGZ effectively attenuated EGFR signaling by targeting the receptor to the endo-lysosomal degradation machinery. Although the extracellular signal-regulated kinase-signaling pathway was transiently activated by TGZ in EGFR overexpressing cancer cells, inhibition of EGF-induced Akt phosphorylation most likely accounted for the growth arrest of tumor cells caused by TGZ at pharmacologically achievable concentrations. Therefore, we have provided a new line of evidence indicating that TGZ inhibits cell proliferation by promoting EGFR degradation and attenuating Akt phosphorylation.

The Bordetella type III secretion system effector BteA contains a conserved N-terminal motif that guides bacterial virulence factors to lipid rafts

The Bordetella type III secretion system (T3SS) effector protein BteA is necessary and sufficient for rapid cytotoxicity in a wide range of mammalian cells. We show that BteA is highly conserved and functionally interchangeable between Bordetella bronchiseptica, Bordetella pertussis and Bordetella parapertussis. The identification of BteA sequences required for cytotoxicity allowed the construction of non-cytotoxic mutants for localization studies. BteA derivatives were targeted to lipid rafts and showed clear colocalization with cortical actin, ezrin and the lipid raft marker GM1. We hypothesized that BteA associates with the cytoplasmic face of lipid rafts to locally modulate host cell responses to Bordetella attachment. B. bronchiseptica adhered to host cells almost exclusively to GM1-enriched lipid raft microdomains and BteA colocalized to these same sites following T3SS-mediated translocation. Disruption of lipid rafts with methyl-beta-cyclodextrin protected cells from T3SS-induced cytotoxicity. Localization to lipid rafts was mediated by a 130-amino-acid lipid raft targeting domain at the N-terminus of BteA, and homologous domains were identified in virulence factors from other bacterial species. Lipid raft targeting sequences from a T3SS effector (Plu4750) and an RTX-type toxin (Plu3217) from Photorhabdus luminescens directed fusion proteins to lipid rafts in a manner identical to the N-terminus of BteA.

Caveolae-mediated internalization of occludin and claudin-5 during CCL2-induced tight junction remodeling in brain endothelial cells

Disturbance of the tight junction (TJ) complexes between brain endothelial cells leads to increased paracellular permeability, allowing leukocyte entry into inflamed brain tissue and also contributing to edema formation. The current study dissects the mechanisms by which a chemokine, CCL2, induces TJ disassembly. It investigates the potential role of selective internalization of TJ transmembrane proteins (occludin and claudin-5) in increased permeability of the brain endothelial barrier in vitro. To map the internalization and intracellular fate of occludin and claudin-5, green fluorescent protein fusion proteins of these TJ proteins were generated and imaged by fluorescent microscopy with simultaneous measurement of transendothelial electrical resistance. During CCL2-induced reductions in transendothelial electrical resistance, claudin-5 and occludin became internalized via caveolae and further processed to early (EEA1+) and recycling (Rab4+) endosomes but not to late endosomes. Western blot analysis of fractions collected from a sucrose gradient showed the presence of claudin-5 and occludin in the same fractions that contained caveolin-1. For the first time, these results suggest an underlying molecular mechanism by which the pro-inflammatory chemokine CCL2 mediates brain endothelial barrier disruption during CNS inflammation.

CD40/CD40L interaction induces Abeta production and increases gamma-secretase activity independently of tumor necrosis factor receptor associated factor (TRAF) signaling

CD40, a member of tumor necrosis factor receptor superfamily, and its cognate ligand CD40L are both elevated in the brain of Alzheimer's disease (AD) patients compared to controls. We have shown that pharmacological or genetic interruption of CD40/CD40L interaction results in mitigation of AD-like pathology in vivo in transgenic AD mouse models, and in vitro. Recently, we showed that CD40L stimulation could increase Abeta levels via NFkappaB signaling, presumably through TRAFs. In the present work, using CD40 mutants, we show that CD40L can increase levels of Abeta(1-40), Abeta(1-42), sAPPbeta, sAPPalpha and CTFbeta independently of TRAF signaling. We report an increase in mature/immature APP ratio after CD40L treatment of CD40wt and CD40-mutant cells, reflecting alterations in APP trafficking. In addition, results from CD40L treatment of a neuroblastoma cell line over-expressing the C-99 APP fragment suggest that CD40L has an effect on gamma-secretase. Furthermore, inhibition of gamma-secretase activity significantly reduces sAPPbeta levels in the CD40L treated HEK/APPsw CD40wt and the CD40-mutant cells. The latter suggests CD40/CD40L interaction primarily acts on gamma-secretase and affects beta-secretase via a positive feedback mechanism. Taken together, our data suggest that CD40/CD40L interaction modulates APP processing independently of TRAF signaling.