Murine Epsins Play an Integral Role in Podocyte Function

BACKGROUND

Epsins, a family of evolutionarily conserved membrane proteins, play an essential role in endocytosis and signaling in podocytes.

METHODS

Podocyte-specific Epn1, Epn2, Epn3 triple-knockout mice were generated to examine downstream regulation of serum response factor (SRF) by cell division control protein 42 homolog (Cdc42).

RESULTS

Podocyte-specific loss of epsins resulted in increased albuminuria and foot process effacement. Primary podocytes isolated from these knockout mice exhibited abnormalities in cell adhesion and spreading, which may be attributed to reduced activation of cell division control protein Cdc42 and SRF, resulting in diminished β1 integrin expression. In addition, podocyte-specific loss of Srf resulted in severe albuminuria and foot process effacement, and defects in cell adhesion and spreading, along with decreased β1 integrin expression.

CONCLUSIONS

Epsins play an indispensable role in maintaining properly functioning podocytes through the regulation of Cdc42 and SRF-dependent β1 integrin expression.

A TLR4-TRIF-dependent signaling pathway is required for protective natural tumor-reactive IgM production by B1 cells

Metastatic cancer involving spread to the peritoneal cavity is referred to as peritoneal carcinomatosis and has a very poor prognosis. Our previous studies demonstrated a toll-like receptor 4 (TLR4) and C-type lectin receptor (CLR; Mincle/MCL) agonist pairing of monophosphoryl lipid A (MPL) and trehalose-6,6'-dicorynomycolate (TDCM) effectively inhibits peritoneal tumor growth and ascites development through a mechanism dependent upon B1a cell-produced natural IgM, complement, and phagocytes. In the current study, we investigated the requirement for TLR4 and Fc receptor common γ chain (FcRγ), required for Mincle/MCL signaling, in the MPL/TDCM-elicited response. MPL/TDCM significantly increased macrophages and Ly6Chi monocytes in the peritoneal cavity of both TLR4-/- and FcRγ-/- mice, suggesting redundancy in the signals required for monocyte/macrophage recruitment. However, B1 cell activation, antibody secreting cell differentiation, and tumor-reactive IgM production were defective in TLR4-/-, but not FcRγ-/- mice. TRIF was required for production of IgM reactive against tumor- and mucin-related antigens, but not phosphorylcholine, whereas TLR4 was required for production of both types of reactivities. Consistent with this, B1 cells lacking TLR4 or TRIF did not proliferate or differentiate into tumor-reactive IgM-producing cells in vitro and did not reconstitute MPL/TDCM-dependent protection against peritoneal carcinomatosis in CD19-/- mice. Our results indicate a TLR4/TRIF-dependent pathway is required by B1 cells for MPL/TDCM-elicited production of protective tumor-reactive natural IgM. The dependency on TRIF signaling for tumor-reactive, but not phosphorylcholine-reactive, IgM production reveals unexpected heterogeneity in TLR4-dependent regulation of natural IgM production, thereby highlighting important differences to consider when designing vaccines or therapies targeting these specificities.

Sensing and signaling of immunogenic extracellular RNAs restrain group 2 innate lymphoid cell-driven acute lung inflammation and airway hyperresponsiveness

Repeated exposures to environmental allergens in susceptible individuals drive the development of type 2 inflammatory conditions such as asthma, which have been traditionally considered to be mainly mediated by Th2 cells. However, emerging evidence suggest that a new innate cell type, group 2 innate lymphoid cells (ILC2), plays a central role in initiating and amplifying a type 2 response, even in the absence of adaptive immunity. At present, the regulatory mechanisms for controlling ILC2 activation remain poorly understood. Here we report that respiratory delivery of immunogenic extracellular RNA (exRNAs) derived from RNA- and DNA-virus infected cells, was able to activate a protective response against acute type 2 lung immunopathology and airway hyperresponsiveness (AHR) induced by IL-33 and a fungal allergen, A. flavus, in mice. Mechanistically, we found that the innate immune responses triggered by exRNAs had a potent suppressive effect in vivo on the proliferation and function of ILC2 without the involvement of adaptive immunity. We further provided the loss-of-function genetic evidence that the TLR3- and MAVS-mediated signaling axis is essential for the inhibitory effects of exRNAs in mouse lungs. Thus, our results indicate that the host detection of extracellular immunostimulatory RNAs generated during respiratory viral infections have an important function in the regulation of ILC2-driven acute lung inflammation.

FGFR1-mediated protocadherin-15 loading mediates cargo specificity during intraflagellar transport in inner ear hair-cell kinocilia

The mechanosensory hair cells of the inner ear are required for hearing and balance and have a distinctive apical structure, the hair bundle, that converts mechanical stimuli into electrical signals. This structure comprises a single cilium, the kinocilium, lying adjacent to an ensemble of actin-based projections known as stereocilia. Hair bundle polarity depends on kinociliary protocadherin-15 (Pcdh15) localization. Protocadherin-15 is found only in hair-cell kinocilia, and is not localized to the primary cilia of adjacent supporting cells. Thus, Pcdh15 must be specifically targeted and trafficked into the hair-cell kinocilium. Here we show that kinocilial Pcdh15 trafficking relies on cell type-specific coupling to the generic intraflagellar transport (IFT) transport mechanism. We uncover a role for fibroblast growth factor receptor 1 (FGFR1) in loading Pcdh15 onto kinociliary transport particles in hair cells. We find that on activation, FGFR1 binds and phosphorylates Pcdh15. Moreover, we find a previously uncharacterized role for clathrin in coupling this kinocilia-specific cargo with the anterograde IFT-B complex through the adaptor, DAB2. Our results identify a modified ciliary transport pathway used for Pcdh15 transport into the cilium of the inner ear hair cell and coordinated by FGFR1 activity.

More than just sugars: Conserved oligomeric Golgi complex deficiency causes glycosylation-independent cellular defects

The conserved oligomeric Golgi (COG) complex controls membrane trafficking and ensures Golgi homeostasis by orchestrating retrograde vesicle trafficking within the Golgi. Human COG defects lead to severe multisystemic diseases known as COG-congenital disorders of glycosylation (COG-CDG). To gain better understanding of COG-CDGs, we compared COG knockout cells with cells deficient to 2 key enzymes, Alpha-1,3-mannosyl-glycoprotein 2-beta-N-acetylglucosaminyltransferase and uridine diphosphate-glucose 4-epimerase (GALE), which contribute to proper N- and O-glycosylation. While all knockout cells share similar defects in glycosylation, these defects only account for a small fraction of observed COG knockout phenotypes. Glycosylation deficiencies were not associated with the fragmented Golgi, abnormal endolysosomes, defective sorting and secretion or delayed retrograde trafficking, indicating that these phenotypes are probably not due to hypoglycosylation, but to other specific interactions or roles of the COG complex. Importantly, these COG deficiency specific phenotypes were also apparent in COG7-CDG patient fibroblasts, proving the human disease relevance of our CRISPR knockout findings. The knowledge gained from this study has important implications, both for understanding the physiological role of COG complex in Golgi homeostasis in eukaryotic cells, and for better understanding human diseases associated with COG/Golgi impairment.

Differential role of MyD88 and TRIF signaling in myeloid cells in the pathogenesis of autoimmune diabetes

Type 1 diabetes (T1D) is caused by the autoimmune destruction of the insulin-producing pancreatic beta cells. While the role of adaptive immunity has been extensively studied, the role of innate immune responses and particularly of Toll- like Receptor (TLR) signaling in T1D remains poorly understood. Here we show that myeloid cell-specific MyD88 deficiency considerably protected mice from the development of streptozotocin (STZ)-induced diabetes. The protective effect of MyD88 deficiency correlated with increased expression of the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO) in pancreatic lymph nodes from STZ-treated mice and in bone marrow-derived dendritic cells (BMDC) stimulated with apoptotic cells. Mice with myeloid cell specific TIR-domain-containing adapter-inducing interferon-β (TRIF) knockout showed a trend towards accelerated onset of STZ-induced diabetes, while TRIF deficiency resulted in reduced IDO expression in vivo and in vitro. Moreover, myeloid cell specific MyD88 deficiency delayed the onset of diabetes in Non-Obese Diabetic (NOD) mice, whereas TRIF deficiency had no effect. Taken together, these results identify MyD88 signaling in myeloid cells as a critical pathogenic factor in autoimmune diabetes, which is antagonized by TRIF-dependent responses. This differential function of MyD88 and TRIF depends at least in part on their opposite effects in regulating IDO expression in phagocytes exposed to apoptotic cells.

Quercetin protects against atherosclerosis by inhibiting dendritic cell activation

SCOPE

Quercetin is a typical flavonol with atheroprotective effects, but the effect of quercetin on dendritic cell (DC) maturation in relation to atherosclerosis has not yet been clearly defined. Thus, we investigated whether quercetin can inhibit DC maturation and evaluated its potential value in atherosclerosis progression in ApoE^-/- mice.

METHODS AND RESULTS

Quercetin consumption inhibited DC activation, inflammatory response and suppressed the progression of atherosclerosis in ApoE^-/- mice. Subsequently, quercetin treatment inhibited the phenotypic and functional maturation of DCs, as evidenced not only by downregulation of CD80, CD86, MHC-II, IL-6 and IL-12 but also by a reduction in the ability to stimulate T cell allogeneic proliferation. Finally, an in vitro study demonstrated that quercetin inhibited DC maturation via upregulation of Dabs, which then downregulated the Src/PI3K/Akt-NF-κB-inflammatory pathways.

CONCLUSIONS

Our data indicate that quercetin attenuates atherosclerosis progression by regulating DC activation via Dab2 protein expression.

Role of the myeloid differentiation primary response (MYD88) and TIR-domain-containing adapter-inducing interferon-β (TRIF) pathways in dengue

AIMS

Dengue disease courses with high viremia titers and high cytokine production suggesting viral replication and active immune response that could be related to viral evasion. One of the main targets of dengue virus (DENV) is monocyte/macrophage cells; however, little information regarding viral evasive mechanisms and pathway activation in monocytes infected by DENV is available. The aim of this study was to determine the role of myeloid differentiation primary response (MyD88), TIR-domain-containing adapter- inducing interferon-β (TRIF) and NF-kB pathways in viral replication and cytokine production in human monocyte cultures infected by DENV2.

MAIN METHODS

In this regard Pepinh- TRIF, Pepinh- MYD and pyrrolidine dithiocarbamate (PDTC) were used to inhibit TRIF, MYD88 and NF-kB pathways. Cytokine production was measured by ELISA.

KEY FINDINGS

Increased DENV replication and IFNα/β, TNF-α, IL-12 and IL-18 in infected cultures at 24h were found. All of these parameters were significantly decreased after TRIF, MYD88 or NF-kB inhibition. Association analysis between viral replication and cytokine production showed high significant positive correlation in TRIF and MYD88 treated cultures.

SIGNIFICANCE

This study shows that DENV2 induces activation of innate-immune response and transcription factors to drive viral expression and replication in the face of pro-inflammatory antiviral responses in vitro.

ABSENCE OF A THYROID PHENOTYPE IN SORTILIN-DEFICIENT MICE

OBJECTIVE

The Vps10p family member sortilin is expressed in thyroid epithelial cells where it contributes to recycling of the thyroid hormone precursor thyroglobulin (Tg), a process that is thought to render hormone release more effective. Here we investigated the functional impact of sortilin in the thyroid gland using sortilin-deficient mice.

METHODS

We measured free T4, thyroid-stimulating hormone (TSH) and Tg serum levels and studied thyroid morphology in 14 sortilin-deficient (Sort1)(-/-)and 12 wildtype (WT) mice.

RESULTS

Serum free T4 levels did not differ between Sort1(-/-)and WT females but were significantly lower in Sort1(-/-)males compared with WT (P = .0424). Neither serum TSH nor Tg levels differed between Sort1(-/-)and WT mice, regardless of sex. On the same line, no thyroid histology differences were observed.

CONCLUSION

Our findings seem to exclude a role of sortilin in thyroid hormone secretion, although it is possible that the absence of sortilin may result in a thyroid phenotype if combined with other molecular defects of thyroid hormone synthesis and secretion or under iodine deficiency.

The invadopodia scaffold protein Tks5 is required for the growth of human breast cancer cells in vitro and in vivo

The ability of cancer cells to invade underlies metastatic progression. One mechanism by which cancer cells can become invasive is through the formation of structures called invadopodia, which are dynamic, actin-rich membrane protrusions that are sites of focal extracellular matrix degradation. While there is a growing consensus that invadopodia are instrumental in tumor metastasis, less is known about whether they are involved in tumor growth, particularly in vivo. The adaptor protein Tks5 is an obligate component of invadopodia, and is linked molecularly to both actin-remodeling proteins and pericellular proteases. Tks5 appears to localize exclusively to invadopodia in cancer cells, and in vitro studies have demonstrated its critical requirement for the invasive nature of these cells, making it an ideal surrogate to investigate the role of invadopodia in vivo. In this study, we examined how Tks5 contributes to human breast cancer progression. We used immunohistochemistry and RNA sequencing data to evaluate Tks5 expression in clinical samples, and we characterized the role of Tks5 in breast cancer progression using RNA interference and orthotopic implantation in SCID-Beige mice. We found that Tks5 is expressed to high levels in approximately 50% of primary invasive breast cancers. Furthermore, high expression was correlated with poor outcome, particularly in those patients with late relapse of stage I/II disease. Knockdown of Tks5 expression in breast cancer cells resulted in decreased growth, both in 3D in vitro cultures and in vivo. Moreover, our data also suggest that Tks5 is important for the integrity and permeability of the tumor vasculature. Together, this work establishes an important role for Tks5 in tumor growth in vivo, and suggests that invadopodia may play broad roles in tumor progression.

Anti-inflammatory effects of extract from Haliotis discus hannai fermented with Cordyceps militaris mycelia in RAW264.7 macrophages through TRIF-dependent signaling pathway

In this study, Haliotis discus hannai (H. discus hannai) fermentation was attempted with Cordyceps militaris (C. militaris) mycelia using a solid culture. We tried to ferment H. discus hannai to determine the optimal conditions fermentation with regards to its anti-inflammatory effects. The extracts of H. discus hannai fermented with C. militaris mycelia (HFCM-5) showed higher nitric oxide inhibitory effects than H. discus hannai and C. militaris alone. HFCM-5 also decreased pro-inflammatory cytokines, TNF-α and IL-6 in a dose-dependent manner. HFCM-5 did not affect the MyD88-dependent pathway, but decreased phosphorylation of IRF3 and STAT1 which are involved in TRIF-dependent pathway. Taken together, our results suggest that HFCM-5 exerts its anti-inflammatory effects via TRIF signaling pathway and could potentially be used as a functional food in the regulation of inflammation.

Src-dependent Tks5 phosphorylation regulates invadopodia-associated invasion in prostate cancer cells

BACKGROUND

The Src tyrosine kinase substrate and adaptor protein Tks5 had previously been implicated in the invasive phenotype of normal and transformed cell types via regulation of cytoskeletal structures called podosomes/invadopodia. The role of Src-Tks5 signaling in invasive prostate cancer, however, had not been previously evaluated.

METHODS

We measured the relative expression of Tks5 in normal (n = 20) and cancerous (n = 184, from 92 patients) prostate tissue specimens by immunohistochemistry using a commercially available tumor microarray. We also manipulated the expression and activity of wild-type and mutant Src and Tks5 constructs in the LNCaP and PC-3 prostate cancer cell lines in order to ascertain the role of Src-Tks5 signaling in invadopodia development, matrix-remodeling activity, motility, and invasion.

RESULTS

Our studies demonstrated that Src was activated and Tks5 upregulated in high Gleason score prostate tumor specimens and in invasive prostate cancer cell lines. Remarkably, overexpression of Tks5 in LNCaP cells was sufficient to induce invadopodia formation and associated matrix degradation. This Tks5-dependent increase in invasive behavior further depended on Src tyrosine kinase activity and the phosphorylation of Tks5 at tyrosine residues 557 and 619. In PC-3 cells we demonstrated that Tks5 phosphorylation at these sites was necessary and sufficient for invadopodia-associated matrix degradation and invasion.

CONCLUSIONS

Our results suggest a general role for Src-Tks5 signaling in prostate tumor progression and the utility of Tks5 as a marker protein for the staging of this disease.

Receptor tyrosine kinase ubiquitylation involves the dynamic regulation of Cbl-Spry2 by intersectin 1 and the Shp2 tyrosine phosphatase

Ubiquitylation of receptor tyrosine kinases (RTKs) regulates their trafficking and lysosomal degradation. The multidomain scaffolding protein intersectin 1 (ITSN1) is an important regulator of this process. ITSN1 stimulates ubiquitylation of the epidermal growth factor receptor (EGFR) through enhancing the activity of the Cbl E3 ubiquitin ligase. However, the precise mechanism through which ITSN1 enhances Cbl activity is unclear. Here, we demonstrate that ITSN1 interacts with and recruits the Shp2 tyrosine phosphatase to Spry2 to enhance its dephosphorylation, thereby disrupting the inhibitory effect of Spry2 on Cbl and enhancing EGFR ubiquitylation. In contrast, expression of a catalytically inactive Shp2 mutant reversed the effect of ITSN1 on Spry2 dephosphorylation and decreased Cbl-mediated EGFR ubiquitylation. In addition, disruption of ITSN1 binding to Spry2 through point mutation of the Pro-rich ITSN1 binding site in Spry2 resulted in decreased Shp2-Spry2 interaction and enhanced Spry2 tyrosine phosphorylation. This study demonstrates that ITSN1 enhances Cbl activity, in part, by modulating the interaction of Cbl with Spry2 through recruitment of Shp2 phosphatase to the Cbl-Spry2 complex. These findings reveal a new level of complexity in the regulation of RTKs by Cbl through ITSN1 binding with Shp2 and Spry2.

The biological functions and signaling mechanisms of the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) regulates a wide range of cellular functions, including programmed cell death, axonal growth and degeneration, cell proliferation, myelination, and synaptic plasticity. The multiplicity of cellular functions governed by the receptor arises from the variety of ligands and co-receptors which associate with p75(NTR) and regulate its signaling. P75(NTR) promotes survival through interactions with Trk receptors, inhibits axonal regeneration via partnerships with Nogo receptor (Nogo-R) and Lingo-1, and promotes apoptosis through association with Sortilin. Signals downstream of these interactions are further modulated through regulated intramembrane proteolysis (RIP) of p75(NTR) and by interactions with numerous cytosolic partners. In this chapter, we discuss the intricate signaling mechanisms of p75(NTR), emphasizing how these signals are differentially regulated to mediate these diverse cellular functions.

NOX2 deficiency ameliorates cerebral injury through reduction of complexin II-mediated glutamate excitotoxicity in experimental stroke

Although NADPH oxidase (NOX)-mediated oxidative stress is considered one of the major mechanisms triggering the pathogenic actions of ischemic stroke and very recent studies have indicated that NADPH oxidase is a major source of reactive oxygen species (ROS) production controlling glutamate release, how neuronal NADPH oxidase activation is coupled to glutamate release is not well understood. Therefore, in this study, we used an in vivo transient middle cerebral artery occlusion model and in vitro primary cell cultures to test whether complexins, the regulators of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes necessary for vesicle fusion, are associated with NOX2-derived ROS and contribute to glutamate-mediated excitotoxicity in ischemic stroke. In this study, we first identified the upregulation of complexin II in the ischemic brain and evaluated its potential role in ischemic stroke showing that gene silencing of complexin II ameliorated cerebral injury as evidenced by reduced infarction volume, neurological deficit, and neuron necrosis accompanied by decreased glutamate levels, consistent with the results from NOX2(-/-) mice with ischemic stroke. We further demonstrated that complexin II expression was mediated by NOX2 in primary cultured neurons subjected to oxygen-glucose deprivation (OGD) and contributed to OGD-induced glutamate release and neuron necrosis via SNARE signaling. Taken together, these findings for the first time provide evidence that complexin II is a central target molecule that links NADPH oxidase-derived ROS to glutamate-mediated neuronal excitotoxicity in ischemic stroke.

Spinal toll-like receptor signaling and nociceptive processing: regulatory balance between TIRAP and TRIF cascades mediated by TNF and IFNβ

Toll-like receptors (TLRs) play a pivotal role in inflammatory processes, and individual TLRs have been investigated in nociception. We examined overlapping and diverging roles of spinal TLRs and their associated adaptor proteins in nociceptive processing. Intrathecal (IT) TLR2, TLR3, or TLR4 ligands (-L) evoked persistent (7-day) tactile allodynia (TA) that was abolished in respective TLR-deficient mice. Using Tnf(-/-) mice, we found that IT TLR2 and TLR4 TA was tumor necrosis factor (TNF) dependent, whereas TLR3 was TNF-independent. In Toll-interleukin 1 receptor (TIR) domain-containing adaptor protein (TIRAP; Tirap(-/-)) mice (downstream to TLR2 and TLR4), allodynia after IT TLR2-L and TLR4-L was abolished. Unexpectedly, in TIR-domain-containing adapter-inducing interferon-β (Trif(lps2)) mice (downstream of TLR3 and TLR4), TLR3-L allodynia was abrogated, but intrathecal TLR4-L produced a persistent increase (>21days) in TA. Consistent with a role for interferon (IFN) β (downstream to TIR-domain-containing adapter-inducing IFNβ [TRIF]) in regulating recovery after IT TLR4-L, prolonged allodynia was noted in Ifnar1(-/-) mice. Further, IT IFNβ given to Trif(lps2) mice reduced TLR4 allodynia. Hence, spinal TIR domain-containing adaptor protein (TIRAP) and TRIF cascades differentially lead to robust TA by TNF-dependent and independent pathways, whereas activation of TRIF modulated processing through type I IFN receptors. Based on these results, we believe that processes leading to the activation of these spinal TLRs initiate TNF-dependent and -independent cascades, which contribute to the associated persistent pain state. In addition, TRIF pathways are able to modulate the TNF-dependent pain state through IFNβ.

[Function of intersectin in endocytosis and exocytosis]

Intersectin is an evolutionarily conserved multifunctional adaptor protein with multifunctional domains. These domains interact with components of the endocytic and exocytic pathways, such as the clathrin mediating synaptic vesicle recycling, the protein related to endocytosis via caveolae, the with-no-lysine kinases related to the regulation of renal outer medullar potassium, and the Cdc42 mediating exocytic pathway. Recently, the understanding of intersectin function in the pathogenesis of endocrine tumor and many neurodegenerative diseases such as Down syndrome, Alzheimer disease has been deepened. This article reviewed the structure and roles in endocytosis/exocytosis and diseases of intersectin.

Analysis of Gga null mice demonstrates a non-redundant role for mammalian GGA2 during development

Numerous studies using cultured mammalian cells have shown that the three GGAs (Golgi-localized, gamma-ear containing, ADP-ribosylation factor- binding proteins) function in the transport of cargo proteins between the trans- Golgi network and endosomes. However, the in vivo role(s) of these adaptor proteins and their possible functional redundancy has not been analyzed. In this study, the genes encoding GGAs1-3 were disrupted in mice by insertional mutagenesis. Loss of GGA1 or GGA3 alone was well tolerated whereas the absence of GGA2 resulted in embryonic or neonatal lethality, depending on the genetic background of the mice. Thus, GGA2 mediates a vital function that cannot be compensated for by GGA1and/or GGA3. The combined loss of GGA1 and GGA3 also resulted in a high incidence of neonatal mortality but in this case the expression level of GGA2 may be inadequate to compensate for the loss of the other two GGAs. We conclude that the three mammalian GGAs are essential proteins that are not fully redundant.

[The activation of TRIF-dependent signaling pathway in THP-1 cells induced by β₂ GPI/anti-β₂ GPI antibodies complex]

AIM

To observe whether the TIR-domain-containing adaptor inducing interferon-β (TRIF) is activated in THP-1 cells treated with β₂ GPI/anti-β₂ GPI complex and investigate the roles of TRIF-dependent signaling pathway of Toll-like receptor 4 (TLR4) in antiphospholipid syndrome (APS).

METHODS

The total RNA was extracted and the protein lysates were collected from THP-1 cells stimulated with β₂ GPI/anti-β₂ GPI complex. And the level of TRIF mRNA in THP-1 cells was detected by Real-time PCR (RT-PCR), TRIF protein expression was investigated by western blotting, respectively. Furthermore, whether TLR4 inhibitor, TAK-242, could interrupt the expression of TRIF as well as some inflammatory cytokines such as IL-6, IL-8 and TNF-α in THP-1 cells stimulated with β₂ GPI/anti-β₂ GPI complex was also investigated.

RESULTS

Both mRNA and protein levels of TRIF could be significantly increased in THP-1 cells with treatment of β₂ GPI/anti-β₂ GPI complex (100 mg/L). The expression of TRIF was shown in a manner of time-dependence, with the maximal levels at 1 hour (mRNA) and 2 hour (protein) stimulation respectively. The β₂ GPI/anti-β₂ GPI complex-induced TRIF and inflammatory cytokines including IL-6, IL-8 and TNF-α expression in THP-1 cells could be inhibited by TAK-242 (5 μmol/L).

CONCLUSION

TRIF-dependent signaling pathway of Toll-like receptor 4 is involved in the activation of THP-1 cells induced by β₂ GPI/anti-β₂GPI complex, suggesting that TRIF may play an important role in the pathogenesis of antiphospholipid syndrome.

The fifth adaptor protein complex

Adaptor protein (AP) complexes sort cargo into vesicles for transport from one membrane compartment of the cell to another. Four distinct AP complexes have been identified, which are present in most eukaryotes. We report the existence of a fifth AP complex, AP-5. Tagged AP-5 localises to a late endosomal compartment in HeLa cells. AP-5 does not associate with clathrin and is insensitive to brefeldin A. Knocking down AP-5 subunits interferes with the trafficking of the cation-independent mannose 6-phosphate receptor and causes the cell to form swollen endosomal structures with emanating tubules. AP-5 subunits can be found in all five eukaryotic supergroups, but they have been co-ordinately lost in many organisms. Concatenated phylogenetic analysis provides robust resolution, for the first time, into the evolutionary order of emergence of the adaptor subunit families, showing AP-3 as the basal complex, followed by AP-5, AP-4, and AP-1 and AP-2. Thus, AP-5 is an evolutionarily ancient complex, which is involved in endosomal sorting, and which has links with hereditary spastic paraplegia.

Adaptor protein (AP) complexes facilitate the trafficking of cargo from one membrane compartment of the cell to another by recruiting other proteins to particular types of vesicles. For over 10 years, it has been assumed that there are four, and only four, distinct AP complexes in eukaryotic cells. We report the existence of a fifth AP complex, AP-5. Immunolocalisation and RNAi knockdown experiments both indicate that AP-5 is involved in trafficking proteins from endosomes towards other membranous compartments. There are genetic links between AP-5 and hereditary spastic paraplegia, a group of human genetic disorders characterised by progressive spasticity in the lower limbs. Phylogenetic analyses indicate that AP-5 was already present in the last eukaryotic common ancestor over a billion years ago.

Basolateral sorting of syntaxin 4 is dependent on its N-terminal domain and the AP1B clathrin adaptor, and required for the epithelial cell polarity

Generation of epithelial cell polarity requires mechanisms to sort plasma membrane proteins to the apical and basolateral domains. Sorting involves incorporation into specific vesicular carriers and subsequent fusion to the correct target membranes mediated by specific SNARE proteins. In polarized epithelial cells, the SNARE protein syntaxin 4 localizes exclusively to the basolateral plasma membrane and plays an important role in basolateral trafficking pathways. However, the mechanism of basolateral targeting of syntaxin 4 itself has remained poorly understood. Here we show that newly synthesized syntaxin 4 is directly targeted to the basolateral plasma membrane in polarized Madin-Darby canine kidney (MDCK) cells. Basolateral targeting depends on a signal that is centered around residues 24–29 in the N-terminal domain of syntaxin 4. Furthermore, basolateral targeting of syntaxin 4 is dependent on the epithelial cell-specific clathrin adaptor AP1B. Disruption of the basolateral targeting signal of syntaxin 4 leads to non-polarized delivery to both the apical and basolateral surface, as well as partial intercellular retention in the trans-Golgi network. Importantly, disruption of the basolateral targeting signal of syntaxin 4 leads to the inability of MDCK cells to establish a polarized morphology which suggests that restriction of syntaxin 4 to the basolateral domain is required for epithelial cell polarity.

[Molecular physiology of receptor mediated endocytosis and its role in overcoming multidrug resistance]

Receptor-mediated endocytosis plays important role in the selective uptake of proteins at the plasma membrane of eukaryotic cells. Endocytosis regulates many processes of cell signalling by controlling the number of functional receptors on the cell surface. The article reviews the mechanism of clathrin-dependent endocytosis and the possibility of using this phenomenon for the targeted delivery of drugs. Use of certain proteins as targeting component of drug delivery systems can significantly improve the selectivity of this drug, as well as to overcome the multidrug resistance of cells resulting from the activity of the ABC-transporters.

The TRIF/TBK1/IRF-3 activation pathway is the primary inhibitory target of resveratrol, contributing to its broad-spectrum anti-inflammatory effects

Resveratrol, a stilbene type compound identified in wine and fruit juice, has been found to exhibit various pharmacological activities such as anti-oxidative, anti-cancerous, anti-inflammatory and anti-aging effects. Although numerous papers have explored the pharmacology of resveratrol in one particular cellular action, how this compound can have multiple effects simultaneously has not been fully addressed. In this study, therefore, we explored its broad-spectrum inhibitory mechanism using lipopolysaccharide (LPS)-mediated inflammatory responses and reporter gene assays involving overexpression of toll like receptor (TLR) adaptor molecules. Co-transfection of adaptor molecules such as (1) myeloid differentiation primary response gene 88 (MyD88), (2) Toll/4ll-1 Receptor-domain-containing adapter-inducing interferon-beta (TRIF), (3) TRIF-related adaptor molecule (TRAM), or (4) TANK-binding kinase (TBK) 1 strongly enhanced luciferase activity mediated by transcription factors including nuclear factor (NF)-KB, activator protein (AP)-1, and interferon regulatory factor (IRF)-3. Of the adaptor proteins, TRIF and TBK1 but not MyD88 and IKK enhanced luciferase activity mediated by these transcription factors. Resveratrol dose-dependently suppressed LPS-induced NO production in macrophages. It also blocked the increases in levels of mRNA for IFN-1, tumor necrosis factor (TNF)-alpha, and inducible nitric oxide synthase (iNOS) that were induced by LPS. Resveratrol diminished the translocation or activation of IRF-3 at 90min, c-Jun, a subunit of AP-1, and STAT-1 at 120 min, and p50, a subunit of NF-KB, at 60 and 90 min. Resveratrol strongly suppressed the up-regulation of luciferase activity induced by these adaptor molecules with IC50 values of 5 to 65 microM. In particular, higher inhibitory effects of resveratrol were when TRIF or TBK1 were overexpressed following cotransfection of luciferase constructs with IRF-3 binding sequences. Taken together, our data suggest that the suppression of TRIF and TBK1, which mediates transcriptional activation of NF-kappaB, AP-1, and IRF-3, contributes to resveratrol's broad-spectrum inhibitory activity, and that this compound can be further developed as a lead anti-inflammatory compound.

DIA1R is an X-linked gene related to Deleted In Autism-1

Background

Autism spectrum disorders (ASDs) are frequently occurring disorders diagnosed by deficits in three core functional areas: social skills, communication, and behaviours and/or interests. Mental retardation frequently accompanies the most severe forms of ASDs, while overall ASDs are more commonly diagnosed in males. Most ASDs have a genetic origin and one gene recently implicated in the etiology of autism is the Deleted-In-Autism-1 (DIA1) gene.

Methodology/Principal Findings

Using a bioinformatics-based approach, we have identified a human gene closely related to DIA1, we term DIA1R (DIA1-Related). While DIA1 is autosomal (chromosome 3, position 3q24), DIA1R localizes to the X chromosome at position Xp11.3 and is known to escape X-inactivation. The gene products are of similar size, with DIA1 encoding 430, and DIA1R 433, residues. At the amino acid level, DIA1 and DIA1R are 62% similar overall (28% identical), and both encode signal peptides for targeting to the secretory pathway. Both genes are ubiquitously expressed, including in fetal and adult brain tissue.

Conclusions/Significance

Examination of published literature revealed point mutations in DIA1R are associated with X-linked mental retardation (XLMR) and DIA1R deletion is associated with syndromes with ASD-like traits and/or XLMR. Together, these results support a model where the DIA1 and DIA1R gene products regulate molecular traffic through the cellular secretory pathway or affect the function of secreted factors, and functional deficits cause disorders with ASD-like symptoms and/or mental retardation.

Sortilin-mediated endocytosis determines levels of the frontotemporal dementia protein, progranulin

VIDEO ABSTRACT

The most common inherited form of Frontotemporal Lobar Degeneration (FTLD) known stems from Progranulin (GRN) mutation and exhibits TDP-43 plus ubiquitin aggregates. Despite the causative role of GRN haploinsufficiency in FTLD-TDP, the neurobiology of this secreted glycoprotein is unclear. Here, we examined PGRN binding to the cell surface. PGRN binds to cortical neurons via its C terminus, and unbiased expression cloning identifies Sortilin (Sort1) as a binding site. Sort1⁻/⁻ neurons exhibit reduced PGRN binding. In the CNS, Sortilin is expressed by neurons and PGRN is most strongly expressed by activated microglial cells after injury. Sortilin rapidly endocytoses and delivers PGRN to lysosomes. Mice lacking Sortilin have elevations in brain and serum PGRN levels of 2.5- to 5-fold. The 50% PGRN decrease causative in FTLD-TDP cases is mimicked in GRN+/⁻ mice, and is fully normalized by Sort1 ablation. Sortilin-mediated PGRN endocytosis is likely to play a central role in FTLD-TDP pathophysiology.

Integrin signalling adaptors: not only figurants in the cancer story

Current evidence highlights the ability of adaptor (or scaffold) proteins to create signalling platforms that drive cellular transformation upon integrin-dependent adhesion and growth factor receptor activation. The understanding of the biological effects that are regulated by these adaptors in tumours might be crucial for the identification of new targets and the development of innovative therapeutic strategies for human cancer. In this Review we discuss the relevance of adaptor proteins in signalling that originates from integrin-mediated cell-extracellular matrix (ECM) adhesion and growth factor stimulation in the context of cell transformation and tumour progression. We specifically underline the contribution of p130 Crk-associated substrate (p130CAS; also known as BCAR1), neural precursor cell expressed, developmentally down-regulated 9 (NEDD9; also known as HEF1), CRK and the integrin-linked kinase (ILK)-pinch-parvin (IPP) complex to cancer, along with the more recently identified p140 Cas-associated protein (p140CAP; also known as SRCIN1).

Immune evasion by Yersinia enterocolitica: differential targeting of dendritic cell subpopulations in vivo

CD4(+) T cells are essential for the control of Yersinia enterocolitica (Ye) infection in mice. Ye can inhibit dendritic cell (DC) antigen uptake and degradation, maturation and subsequently T-cell activation in vitro. Here we investigated the effects of Ye infection on splenic DCs and T-cell proliferation in an experimental mouse infection model. We found that OVA-specific CD4(+) T cells had a reduced potential to proliferate when stimulated with OVA after infection with Ye compared to control mice. Additionally, proliferation of OVA-specific CD4(+) T cells was markedly reduced when cultured with splenic CD8α(+) DCs from Ye infected mice in the presence of OVA. In contrast, T-cell proliferation was not impaired in cultures with CD4(+) or CD4(-)CD8α(-) DCs isolated from Ye infected mice. However, OVA uptake and degradation as well as cytokine production were impaired in CD8α(+) DCs, but not in CD4(+) and CD4(-)CD8α(-) DCs after Ye infection. Pathogenicity factors (Yops) from Ye were most frequently injected into CD8α(+) DCs, resulting in less MHC class II and CD86 expression than on non-injected CD8α(+) DCs. Three days post infection with Ye the number of splenic CD8α(+) and CD4(+) DCs was reduced by 50% and 90%, respectively. The decreased number of DC subsets, which was dependent on TLR4 and TRIF signaling, was the result of a faster proliferation and suppressed de novo DC generation. Together, we show that Ye infection negatively regulates the stimulatory capacity of some but not all splenic DC subpopulations in vivo. This leads to differential antigen uptake and degradation, cytokine production, cell loss, and cell death rates in various DC subpopulations. The data suggest that these effects might be caused directly by injection of Yops into DCs and indirectly by affecting the homeostasis of CD4(+) and CD8α(+) DCs. These events may contribute to reduced T-cell proliferation and immune evasion of Ye.

Hepatitis A and hepatitis C viruses: divergent infection outcomes marked by similarities in induction and evasion of interferon responses

Hepatitis A and hepatitis C viruses (HAV and HCV) are both positive-strand ribonucleic acid (RNA) viruses with hepatotropic lifestyles. Despite several important differences, they share many biological and molecular features and similar genome replication schemes. Despite this, HAV infections are usually effectively controlled by the host with elimination of the virus, whereas HCV most often is able to establish lifelong persistent infection. The mechanisms underlying this difference are unknown. The cellular helicases RIG-I and MDA5, and Toll-like receptor 3, are pattern recognition receptors that sense virus-derived RNAs within hepatocytes in the liver. Activation of these receptors leads to their interaction with specific adaptor proteins, mitochondrial antiviral signaling protein (MAVS) and TIR-domain-containing adapter-inducing interferon-β (TRIF), respectively, which engage downstream kinases to activate two crucial transcription factors, nuclear factor kappa B (NF-κB) and interferon regulatory factor 3 (IRF3). This results in the induction of interferons (IFNs) and IFN-stimulated genes that ultimately establish an antiviral state. These signaling pathways are central to host antiviral defense and thus frequent targets for viral interference. Both HAV and HCV express proteases that target signal transduction through these pathways and that block the induction of IFNs upon sensing of viral RNA by these receptors. An understanding of the differences and similarities in the early innate immune responses to these infections is likely to provide important insights into the mechanism underlying the long-term persistence of HCV.

Inhibition of TIR domain signaling by TcpC: MyD88-dependent and independent effects on Escherichia coli virulence

Toll-like receptor signaling requires functional Toll/interleukin-1 (IL-1) receptor (TIR) domains to activate innate immunity. By producing TIR homologous proteins, microbes inhibit host response induction and improve their own survival. The TIR homologous protein TcpC was recently identified as a virulence factor in uropathogenic Escherichia coli (E. coli), suppressing innate immunity by binding to MyD88. This study examined how the host MyD88 genotype modifies the in vivo effects of TcpC and whether additional, TIR-domain containing proteins might be targeted by TcpC. In wild type mice (wt), TcpC enhanced bacterial virulence, increased acute mortality, bacterial persistence and tissue damage after infection with E. coli CFT073 (TcpC+), compared to a ΔTcpC deletion mutant. These effects were attenuated in Myd88(-/-) and Tlr4(-/-) mice. Transcriptomic analysis confirmed that TcpC inhibits MYD88 dependent gene expression in CFT073 infected human uroepithelial cells but in addition the inhibitory effect included targets in the TRIF and IL-6/IL-1 signaling pathways, where MYD88 dependent and independent signaling may converge. The effects of TcpC on bacterial persistence were attenuated in Trif (-/-) or Il-1β (-/-) mice and innate immune responses to ΔTcpC were increased, confirming that Trif and Il-1β dependent targets might be involved in vivo, in addition to Myd88. Furthermore, soluble TcpC inhibited Myd88 and Trif dependent TLR signaling in murine macrophages. Our results suggest that TcpC may promote UTI-associated pathology broadly, through inhibition of TIR domain signaling and downstream pathways. Dysregulation of the host response by microbial TcpC thus appears to impair the protective effects of innate immunity, while promoting inflammation and tissue damage.

Reduction of intersectin1-s induced apoptosis of human glioblastoma cells

Malignant gliomas have a high proliferation ability and high tendency to invade diffusely into surrounding healthy brain tissues, thereby precluding their successful surgical removal. Intersectin1 (also called ITSN1) as a molecular linker in the central nervous system is well known as an important regulator of endocytosis and exocytosis. ITSN1 has two isoforms: ITSN1-l and ITSN1-s. In this study, we show that siRNA-mediated down regulation of ITSN1-s induced glioma cells apoptosis. In addition, we demonstrate the possible mechanisms by which ITSN1-s functions in glioma cells apoptosis. Our data demonstrate that several key proteins, including FAK, Akt, Bcl-2, BAD which are critical for cells apoptosis were probably involved in ITSN1-s signaling pathways. Our results indicate that ITSN1-s is an effecter in regulation of gliomas cells apoptosis, and identify that ITSN1-s may be a new potentially anti-apoptosis target for therapeutic of gliomas.

Role of the TLR signaling molecule TRIF in β-cell function and glucose homeostasis

Type 2 diabetes is a metabolic and inflammatory disease characterized by deteriorating islet function and increased levels of inflammatory cytokines. The inflammatory milieu induced in type 2 diabetes exacerbates islet dysfunction and insulin resistance, and therapies that target inflammation can improve glycemic control in patients with type 2 diabetes. Inflammation in type 2 diabetes may be the result of the stimulation of Toll-like receptors (TLRs), one of the many mediators of inflammation. TLRs can be activated by both exogenous and endogenous ligands, and are responsible for activating NFκB and interferon- inducible inflammatory gene expression. We examined the role of the TIR-domain containing adaptor-inducing interferon-β (TRIF or TICAM-1), a major signaling molecule for TLR3 and TLR4, in b-cell function and glucose homeostasis by examining mice lacking TRIF (Trif⁻(/)⁻), TLR3 (Tlr3⁻(/)⁻) or TLR4 (Tlr4⁻(/)⁻). Male, 10-week old Trif⁻(/)⁻ mice exhibit a moderate but significant increase in fasting blood glucose compared to C57BL/6 controls (12.0 ± 0.9 vs. 9.7 ± 0.4 mM; p < 0.05) as well as impaired glucose tolerance revealed by IPGTT (AUC: 2850 ± 236 vs. 2050 ± 108; p < 0.005) whereas Tlr3⁻(/)⁻ and Tlr4⁻(/)⁻ mice have normal glucose tolerance. Interestingly, Trif⁻(/)⁻ mice have normal insulin sensitivity yet have increased plasma insulin levels (180 ± 22 vs. 89 ± 24 pM; p < 0.05). Islets isolated from Trif⁻(/)⁻ mice have impaired glucose-stimulated insulin secretion, with a diminished first-phase insulin response to glucose. Immunohistological analysis revealed that age-matched Trif⁻(/)⁻ and control mice have normal islet morphology, although Trif⁻(/)⁻ mice have increased b-cell mass (3.5 ± 0.9 vs. 1.7 ± 0.2 mg; p < 0.05). In summary, mice lacking TRIF have hyperglycemia associated with b-cell dysfunction that may be partly compensated for by increased b-cell mass. These studies suggest a role for TLR signaling in glucose homeostasis, and raise the possibility that TRIF signaling is required for normal b-cell function.

Contribution of TIR domain-containing adapter inducing IFN-beta-mediated IL-18 release to LPS-induced liver injury in mice

BACKGROUND/AIMS

After treatment with heat-killed Propionibacterium acnes mice show dense hepatic granuloma formation. Such mice develop liver injury in an interleukin (IL)-18-dependent manner after challenge with a sublethal dose LPS. As previously shown, LPS-stimulated Kupffer cells secrete IL-18 depending on caspase-1 and Toll-like receptor (TLR)-4 but independently of its signal adaptor myeloid differentiation factor 88 (MyD88), suggesting importance of another signal adaptor TIR domain-containing adapter inducing IFN-beta (TRIF). Nalp3 inflammasome reportedly controls caspase-1 activation. Here we investigated the roles of MyD88 and TRIF in P. acnes-induced hepatic granuloma formation and LPS-induced caspase-1 activation for IL-18 release.

METHODS

Mice were sequentially treated with P. acnes and LPS, and their serum IL-18 levels and liver injuries were determined by ELISA and ALT/AST measurement, respectively. Active caspase-1 in LPS-stimulated Kupffer cells was determined by Western blotting.

RESULTS

Macrophage-ablated mice lacked P. acnes-induced hepatic granuloma formation and LPS-induced serum IL-18 elevation and liver injury. Myd88(-/-) Kupffer cells, but not Trif(-/-) cells, exhibited normal caspase-1 activation upon TLR4 engagement in vitro. Myd88(-/-) mice failed to develop hepatic granulomas after P. acnes treatment and liver injury induced by LPS challenge. In contrast, Trif(-/-) mice normally formed the hepatic granulomas, but could not release IL-18 or develop the liver injury. Nalp3(-/-) mice showed the same phenotypes of Trif(-/-) mice.

CONCLUSIONS

Propionibacterium acnes treatment MyD88-dependently induced hepatic granuloma formation. Subsequent LPS TRIF-dependently activated caspase-1 via Nalp3 inflammasome and induced IL-18 release, eventually leading to the liver injury.

MyD88 and TRIF mediate the cyclic adenosine monophosphate (cAMP) induced corticotropin releasing hormone (CRH) expression in JEG3 choriocarcinoma cell line

BACKGROUND

Classically protein kinase A (PKA) and transcription factor activator protein 1 (AP-1) mediate the cyclic AMP (cAMP) induced-corticotrophin releasing hormone (CRH) expression in the placenta. However enteric Gram (-) bacterial cell wall component lipopolysaccharide (LPS) may also induce-CRH expression via Toll like receptor (TLR)4 and its adaptor molecule Myd88. Here we investigated the role of MyD88, TRIF and IRAK2 on cAMP-induced CRH promoter activation in JEG3 cells in the absence of LPS/TLR4 stimulation.

METHODS

JEG3 cells were transfected with CRH-luciferase and Beta-galactosidase expression vectors and either empty or dominant-negative (DN)-MyD88, DN-TRIF or DN-IRAK2 vectors using Fugene6 (Roche). cAMP-induced CRH promoter activation was examined by using a luminometer and luciferase assay. Calorimetric Beta-galactosidase assays were performed to correct for transfection efficiency. Luciferase expression vectors of cAMP-downstream molecules, CRE and AP-1, were used to further examine the signaling cascades.

RESULTS

cAMP stimulation induced AP-1 and CRE promoter expression and led to dose-dependent CRH promoter activation in JEG3 cells. Inhibition of MyD88 signaling blocked cAMP-induced CRE and CRH promoter activation. Inhibition of TRIF signaling blocked cAMP-induced CRH but not CRE expression, while inhibition of IRAK2 did not have an effect on cAMP-induced CRH expression.

CONCLUSION

MyD88 and TRIF exert direct regulatory effect on cAMP-induced CRH promoter activation in JEG3 cells in the absence of infection. MyD88 most likely interacts with molecules upstream of IRAK2 to regulate cAMP-induced CRH expression.

TRAF6 is required for generation of the B-1a B cell compartment as well as T cell-dependent and -independent humoral immune responses

TNF receptor superfamily members, such as CD40 and the Toll-like receptors (TLRs), regulate many aspects of B cell differentiation and activation. TRAF6 is an intracellular signaling adaptor molecule for these receptors, but its role in B cells has not been clarified by previous genetic approaches, as the systemic deletion of the TRAF6 gene results in perinatal lethality. Here we show that B cell-specific TRAF6 deficiency results in a reduced number of mature B cells in the bone marrow and spleen. Optimal T cell-dependent (TD) antigen responses, as characterized by isotype switching and long-lived plasma cell generation, are also impaired in B cell-specific TRAF6-deficient mice. B cell-specific TRAF6-deficient mice also exhibit lower levels of serum IgM and IgG2b and defective antigen-specific IgM production in response to T cell-independent (TI) antigens. Unexpectedly, TRAF6-deficient B cell progenitors are unable to generate CD5(+) B-1 cells. These results reveal critical roles for TRAF6 in TD and TI humoral immune responses and in inductive fate decisions necessary to generate the B-1 B cell compartment.

PLIC proteins or ubiquilins regulate autophagy-dependent cell survival during nutrient starvation

Ubiquilins (UBQLNs) are adaptor proteins thought to deliver ubiquitinated substrates to proteasomes. Here, we show a role for UBQLN in autophagy: enforced expression of UBQLN protects cells from starvation-induced death, whereas depletion of UBQLN renders cells more susceptible. The UBQLN protective effect requires the autophagy-related genes ATG5 and ATG7, two essential components of autophagy. The ubiquitin-associated domain of UBQLN mediates both its association with autophagosomes and its protective effect against starvation. Depletion of UBQLN delays the delivery of autophagosomes to lysosomes. This study identifies a new role for UBQLN in regulating the maturation of autophagy, expanding the involvement of ubiquitin-related proteins in this process.

Sortilin is a putative postendocytic receptor of thyroglobulin

The Vps10p family member sortilin is involved in various cell processes, including protein trafficking. Here we found that sortilin is expressed in thyroid epithelial cells (thyrocytes) in a TSH-dependent manner, that the hormone precursor thyroglobulin (Tg) is a high-affinity sortilin ligand, and that binding to sortilin occurs after Tg endocytosis, resulting in Tg recycling. Sortilin was found to be expressed intracellularly in thyrocytes, as observed in mouse, human, and rat thyroid as well as in FRTL-5 cells. Sortilin expression was demonstrated to be TSH dependent, both in FRTL-5 cells and in mice treated with methimazole and perchlorate. Plasmon resonance binding assays showed that Tg binds to sortilin in a concentration-dependent manner and with high affinity, with Kd values that paralleled the hormone content of Tg. In addition, we found that Tg and sortilin interact in vivo and in cultured cells, as observed by immunoprecipitation, in mouse thyroid extracts and in COS-7 cells transiently cotransfected with sortilin and Tg. After incubation of FRTL-5 cells with exogenous, labeled Tg, sortilin and Tg interacted intracellularly, presumably within the endocytic pathway, as observed by immunofluorescence and immunoelectron microscopy, the latter technique showing some degree of Tg recycling. This was confirmed in FRTL-5 cells in which Tg recycling was reduced by silencing of the sortilin gene and in CHO cells transfected with sortilin in which recycling was increased. Our findings provide a novel pathway of Tg trafficking and a novel function of sortilin in the thyroid gland, the functional impact of which remains to be established.

The autophagy effector Beclin 1: a novel BH3-only protein

BH3 domains were originally discovered in the context of apoptosis regulators and they mediate binding of proapoptotic Bcl-2 family members to antiapoptotic Bcl-2 family members. Yet, recent studies indicate that BH3 domains do not function uniquely in apoptosis regulation; they also function in the regulation of another critical pathway involved in cellular and tissue homeostasis called autophagy. Antiapoptotic Bcl-2 homologs downregulate autophagy through interactions with the essential autophagy effector and haploinsufficient tumor suppressor, Beclin 1. Beclin 1 contains a BH3 domain, similar to that of Bcl-2 proteins, which is necessary and sufficient for binding to antiapoptotic Bcl-2 homologs and required for Bcl-2-mediated inhibition of autophagy. This review will summarize the evidence that the BH3 domain of Beclin 1 serves as a key structural motif that enables Bcl-2 to function not only as an antiapoptotic protein, but also as an antiautophagy protein.

Toll-like receptor-4 coordinates the innate immune response of the kidney to renal ischemia/reperfusion injury

Toll-like receptors (TLRs) can detect endogenous danger molecules released upon tissue injury resulting in the induction of a proinflammatory response. One of the TLR family members, TLR4, is constitutively expressed at RNA level on renal epithelium and this expression is enhanced upon renal ischemia/reperfusion (I/R) injury. The functional relevance of this organ-specific upregulation remains however unknown. We therefore investigated the specific role of TLR4 and the relative contribution of its two downstream signaling cascades, the MyD88-dependent and TRIF-dependent cascades in renal damage by using TLR4−/−, MyD88−/− and TRIF-mutant mice that were subjected to renal ischemia/reperfusion injury. Our results show that TLR4 initiates an exaggerated proinflammatory response upon I/R injury, as reflected by lower levels of chemokines and infiltrating granulocytes, less renal damage and a more preserved renal function in TLR4−/− mice as compared to wild type mice. In vitro studies demonstrate that renal tubular epithelial cells can coordinate an immune response to ischemic injury in a TLR4-dependent manner. In vivo we found that epithelial- and leukocyte-associated functional TLR4 contribute in a similar proportion to renal dysfunction and injury as assessed by bone marrow chimeric mice. Surprisingly, no significant differences were found in renal function and inflammation in MyD88−/− and TRIF-mutant mice compared with their wild types, suggesting that selective targeting of TLR4 directly may be more effective for the development of therapeutic tools to prevent I/R injury than targeting the intracellular pathways used by TLR4. In conclusion, we identified TLR4 as a cellular sentinel for acute renal damage that subsequently controls the induction of an innate immune response.

The IFN-inducible GTPase LRG47 (Irgm1) negatively regulates TLR4-triggered proinflammatory cytokine production and prevents endotoxemia

LRG47/Irgm1, a 47-kDa IFN-inducible GTPase, plays a major role in regulating host resistance as well as the hemopoietic response to intracellular pathogens. LRG47 expression in macrophages has been shown previously to be stimulated in vitro by bacterial LPS, a TLR4 ligand. In this study, we demonstrate that induction of LRG47 by LPS is not dependent on MyD88 signaling, but rather, requires STAT-1 and IFN-beta. In addition, LRG47-deficient mice are highly susceptible to LPS, but not TLR2 ligand-induced shock, an outcome that correlates with enhanced proinflammatory cytokine production in vitro and in vivo. Further analysis revealed that LPS-stimulated LRG47-deficient macrophages display enhanced phosphorylation of p38, a downstream response associated with TLR4/MyD88 rather than IFN-beta/STAT-1 signaling. In contrast, LPS-induced phosphorylation of IFN regulatory factor-3 and expression of IFN-beta or the type I IFN-regulated genes, CCL5 and CCL10, were unaltered in LRG47(-/-) cells. Together, these observations indicate that in LPS-stimulated murine macrophages LRG47 is induced by IFN-beta and negatively regulates TLR4 signaling to prevent excess proinflammatory cytokine production and shock. Thus, our findings reveal a new host-protective function for this GTPase in the response to pathogenic encounter.

Role for MyD88-independent, TRIF pathway in lipid A/TLR4-induced endotoxin tolerance

Repeated exposure to low doses of endotoxin results in progressive hyporesponsiveness to subsequent endotoxin challenge, a phenomenon known as endotoxin tolerance. In spite of its clinical significance in sepsis and characterization of the TLR4 signaling pathway as the principal endotoxin detection mechanism, the molecular determinants that induce tolerance remain obscure. We investigated the role of the TRIF/IFN-beta pathway in TLR4-induced endotoxin tolerance. Lipid A-induced homotolerance was characterized by the down-regulation of MyD88-dependent proinflammatory cytokines TNF-alpha and CCL3, but up-regulation of TRIF-dependent cytokine IFN-beta. This correlated with a molecular phenotype of defective NF-kappaB activation but a functional TRIF-dependent STAT1 signaling. Tolerance-induced suppression of TNF-alpha and CCL3 expression was significantly relieved by TRIF and IFN regulatory factor 3 deficiency, suggesting the involvement of the TRIF pathway in tolerance. Alternatively, selective activation of TRIF by poly(I:C)-induced tolerance to lipid A. Furthermore, pretreatment with rIFN-beta also induced tolerance, whereas addition of IFN-beta-neutralizing Ab during the tolerization partially alleviated tolerance to lipid A but not TLR2-induced endotoxin homo- or heterotolerance. Furthermore, IFNAR1-/- murine embryonal fibroblast and bone-marrow derived macrophages failed to induce tolerance. Together, these observations constitute evidence for a role of the TRIF/IFN-beta pathway in the regulation of lipid A/TLR4-mediated endotoxin homotolerance.

Dialogue between LKB1 and AMPK: a hot topic at the cellular pole

Disruption of cell architecture and change of energy metabolism are two traits of malignant cells. Yet, there was scant evidence that these two cancer hallmarks involved perturbations of a common signaling pathway. Enter LKB1, a kinase that is a tumor suppressor and that is an upstream activator of the adenosine monophosphate (AMP)-activated protein kinase (AMPK), a key sensor of cellular energy status. Four studies now reveal that LKB1 signals through AMPK to facilitate the formation of tight junctions and to maintain epithelial polarity. Thus, LKB1 appears to be a novel class of tumor suppressor that acts as an energy-sensing and polarity checkpoint.

Cell autonomous sorting and surface positioning in the formation of primitive endoderm in embryoid bodies

The differentiation and formation of the primitive endoderm in early embryos can be mimicked in vitro by the aggregation of embryonic stem cells to form embryoid bodies. We present morphological evidence that primitive endoderm cells often first locate in the interior of embryoid bodies and subsequently migrate to the surface. Cell mixing experiments indicate that surface positioning is an intrinsic property of endoderm epithelial cells. Moreover, Disabled-2 (Dab2) is required for surface sorting and positioning of the endoderm cells: when Dab2 expression was eliminated, the differentiated endoderm epithelial cells distributed throughout the interior of the embryoid bodies. Surprisingly, E-cadherin is dispensable for primitive endoderm differentiation and surface sorting in embryoid bodies. These results support the model that primitive endoderm cells first emerge in the interior of the inner cell mass and are subsequently sorted to the surface to form the primitive endoderm.