Differential impact of high-salt levels in vitro and in vivo on macrophage core functions

The consumption of processed food is on the rise leading to huge intake of excess dietary salt, which strongly correlates with development of hypertension, often leading to cardiovascular diseases such as stroke and heart attack, as well as activation of the immune system. The effect of salt on macrophages is especially interesting as they are able to sense high sodium levels in tissues leading to transcriptional changes. In the skin, macrophages were shown to influence lymphatic vessel growth which, in turn, enables the transport of excess salt and thereby prevents the development of high blood pressure. Furthermore, salt storage in the skin has been linked to the onset of pro-inflammatory effector functions of macrophages in pathogen defence. However, there is only little known about the mechanisms which are involved in changing macrophage function to salt exposure. Here, we characterize the response of macrophages to excess salt both in vitro and in vivo. Our results validate and strengthen the notion that macrophages exhibit chemotactic migration in response to salt gradients in vitro. Furthermore, we demonstrate a reduction in phagocytosis and efferocytosis following acute salt challenge in vitro. While acute exposure to a high-salt diet in vivo has a less pronounced impact on macrophage core functions such as phagocytosis, our data indicate that prolonged salt challenge may exert a distinct effect on the function of macrophages. These findings suggest a potential role for excessive salt sensing by macrophages in the manifestation of diseases related to high-salt diets and explicitly highlight the need for in vivo work to decipher the physiologically relevant impact of excess salt on tissue and cell function.

TRIM71 reactivation enhances the mitotic and hair cell-forming potential of cochlear supporting cells

Cochlear hair cell loss is a leading cause of deafness in humans. Neighboring supporting cells have some capacity to regenerate hair cells. However, their regenerative potential sharply declines as supporting cells undergo maturation (postnatal day 5 in mice). We recently reported that reactivation of the RNA-binding protein LIN28B restores the hair cell-regenerative potential of P5 cochlear supporting cells. Here, we identify the LIN28B target Trim71 as a novel and equally potent enhancer of supporting cell plasticity. TRIM71 is a critical regulator of stem cell behavior and cell reprogramming; however, its role in cell regeneration is poorly understood. Employing an organoid-based assay, we show that TRIM71 re-expression increases the mitotic and hair cell-forming potential of P5 cochlear supporting cells by facilitating their de-differentiation into progenitor-like cells. Our mechanistic work indicates that TRIM71's RNA-binding activity is essential for such ability, and our transcriptomic analysis identifies gene modules that are linked to TRIM71 and LIN28B-mediated supporting cell reprogramming. Furthermore, our study uncovers that the TRIM71-LIN28B target Hmga2 is essential for supporting cell self-renewal and hair cell formation.

Reactivation of the progenitor gene Trim71 enhances the mitotic and hair cell-forming potential of cochlear supporting cells

Cochlear hair cell loss is a leading cause of deafness in humans. Neighboring supporting cells have some capacity to regenerate hair cells. However, their regenerative potential sharply declines as supporting cells undergo maturation (postnatal day 5 in mice). We recently reported that reactivation of the RNA-binding protein LIN28B restores the hair cell-regenerative potential of P5 cochlear supporting cells. Here, we identify the LIN28B target Trim71 as a novel and equally potent enhancer of supporting cell plasticity. TRIM71 is a critical regulator of stem cell behavior and cell reprogramming, however, its role in cell regeneration is poorly understood. Employing an organoid-based assay, we show that TRIM71 reactivation increases the mitotic and hair cell-forming potential of P5 cochlear supporting cells by facilitating their de-differentiation into progenitor-like cells. Our mechanistic work indicates that TRIM71’s RNA-binding activity is essential for such ability, and our transcriptomic analysis identifies gene modules that are linked to TRIM71 and LIN28B-mediated supporting cell reprogramming. Furthermore, our study uncovers that the TRIM71-LIN28B target Hmga2 is essential for supporting cell self-renewal and hair cell formation.

Multiple centrosomes enhance migration and immune cell effector functions of mature dendritic cells

Centrosomes play a crucial role during immune cell interactions and initiation of the immune response. In proliferating cells, centrosome numbers are tightly controlled and generally limited to one in G1 and two prior to mitosis. Defects in regulating centrosome numbers have been associated with cell transformation and tumorigenesis. Here, we report the emergence of extra centrosomes in leukocytes during immune activation. Upon antigen encounter, dendritic cells pass through incomplete mitosis and arrest in the subsequent G1 phase leading to tetraploid cells with accumulated centrosomes. In addition, cell stimulation increases expression of polo-like kinase 2, resulting in diploid cells with two centrosomes in G1-arrested cells. During cell migration, centrosomes tightly cluster and act as functional microtubule-organizing centers allowing for increased persistent locomotion along gradients of chemotactic cues. Moreover, dendritic cells with extra centrosomes display enhanced secretion of inflammatory cytokines and optimized T cell responses. Together, these results demonstrate a previously unappreciated role of extra centrosomes for regular cell and tissue homeostasis.

A Stable Chemokine Gradient Controls Directional Persistence of Migrating Dendritic Cells

Navigation of dendritic cells (DCs) from the site of infection to lymphoid organs is guided by concentration gradients of CCR7 ligands. How cells interpret chemokine gradients and how they couple directional sensing to polarization and persistent chemotaxis has remained largely elusive. Previous experimental systems were limited in the ability to control fast de novo formation of the final gradient slope, long-lasting stability of the gradient and to expose cells to dynamic stimulation. Here, we used a combination of microfluidics and quantitative in vitro live cell imaging to elucidate the chemotactic sensing strategy of DCs. The microfluidic approach allows us to generate soluble gradients with high spatio-temporal precision and to analyze actin dynamics, cell polarization, and persistent directional migration in both static and dynamic environments. We demonstrate that directional persistence of DC migration requires steady-state characteristics of the soluble gradient instead of temporally rising CCL19 concentration, implying that spatial sensing mechanisms control chemotaxis of DCs. Kymograph analysis of actin dynamics revealed that the presence of the CCL19 gradient is essential to stabilize leading edge protrusions in DCs and to determine directionality, since both cytoskeletal polarization and persistent chemotaxis are abrogated in the range of seconds when steady-state gradients are perturbed. In contrast to Dictyostelium amoeba, DCs are unable to decode oscillatory stimulation of soluble chemokine traveling waves into a directional response toward the wave source. These findings are consistent with the notion that DCs do not employ adaptive temporal sensing strategies that discriminate temporally increasing and decreasing chemoattractant concentrations in our setting. Taken together, in our experimental system DCs do not depend on increasing absolute chemokine concentration over time to induce persistent migration and do not integrate oscillatory stimulation. The observed capability of DCs to migrate with high directional persistence in stable gradients but not when subjected to periodic temporal cues, identifies spatial sensing as a key requirement for persistent chemotaxis of DCs.

Arginine-based cationic liposomes accelerate T cell activation and differentiation in vitro

Cationic liposomes are versatile lipid nanocarriers to improve the pharmacological properties of drug payloads. Recent advantages include the application of their intrinsic immunostimulatory effects to enhance immune activation. Herein, we report for the first time the structural effect of cationic lipids in promoting T cell activation and differentiation in vitro. Two types of cationic liposomes R3C14 and R5C14 were prepared from single type of lipids Arg-C3-Clu2C14 or Arg-C5-Clu2C14, which bear arginine head group and ditetradecyl tails but vary in the carbon number of the spacer in between. Murine CD8 or CD4 T cells were pretreated with 50 μM of each type of liposomes for 2 h, followed by stimulation with anti-CD3/CD28 antibodies for 24 h. In comparison to liposome-untreated T cells, R5C14-pretreatment induced a robust T cell activation (IL-2, CD25⁺) and differentiation into effector cells (CD44^high, CD62L^low), whereas R3C14 did not show comparable effect. Furthermore, a weak activation of nuclear factor of activated T cells (NFAT) was detected in Jurkat-Lucia NFAT cells (InvivoGen), suggesting a potential signaling pathway for the liposomal effect. Although R5C14 liposomes did not activate T cells without subsequent CD3/CD28 stimulation, this study implied a recessive effect of some cationic adjuvant in priming T cells to enhance their responsiveness to antigens.

Impaired neurogenesis alters brain biomechanics in a neuroprogenitor-based genetic subtype of congenital hydrocephalus

Hydrocephalus, characterized by cerebral ventricular dilatation, is routinely attributed to primary defects in cerebrospinal fluid (CSF) homeostasis. This fosters CSF shunting as the leading reason for brain surgery in children despite considerable disease heterogeneity. In this study, by integrating human brain transcriptomics with whole-exome sequencing of 483 patients with congenital hydrocephalus (CH), we found convergence of CH risk genes in embryonic neuroepithelial stem cells. Of all CH risk genes, TRIM71/lin-41 harbors the most de novo mutations and is most specifically expressed in neuroepithelial cells. Mice harboring neuroepithelial cell-specific Trim71 deletion or CH-specific Trim71 mutation exhibit prenatal hydrocephalus. CH mutations disrupt TRIM71 binding to its RNA targets, causing premature neuroepithelial cell differentiation and reduced neurogenesis. Cortical hypoplasia leads to a hypercompliant cortex and secondary ventricular enlargement without primary defects in CSF circulation. These data highlight the importance of precisely regulated neuroepithelial cell fate for normal brain-CSF biomechanics and support a clinically relevant neuroprogenitor-based paradigm of CH.

Maintaining proteostasis under mechanical stress

Cell survival, tissue integrity and organismal health depend on the ability to maintain functional protein networks even under conditions that threaten protein integrity. Protection against such stress conditions involves the adaptation of folding and degradation machineries, which help to preserve the protein network by facilitating the refolding or disposal of damaged proteins. In multicellular organisms, cells are permanently exposed to stress resulting from mechanical forces. Yet, for long time mechanical stress was not recognized as a primary stressor that perturbs protein structure and threatens proteome integrity. The identification and characterization of protein folding and degradation systems, which handle force-unfolded proteins, marks a turning point in this regard. It has become apparent that mechanical stress protection operates during cell differentiation, adhesion and migration and is essential for maintaining tissues such as skeletal muscle, heart and kidney as well as the immune system. Here, we provide an overview of recent advances in our understanding of mechanical stress protection.

Ultra-Thin Porous PDLLA Films Promote Generation, Maintenance, and Viability of Stem Cell Spheroids

Three-dimensional (3D) culture bridges and minimizes the gap between in vitro and in vivo states of cells and various 3D culture systems have been developed according to different approaches. However, most of these approaches are either complicated to operate, or costive to scale up. Therefore, a simple method for stem cell spheroid formation and preservation was proposed using poly(D,L-lactic acid) porous thin film (porous nanosheet), which were fabricated by a roll-to-roll gravure coating method combining a solvent etching process. The obtained porous nanosheet was less than 200 nm in thickness and had an average pore area of 6.6 μm² with a porosity of 0.887. It offered a semi-adhesive surface for stem cells to form spheroids and maintained the average spheroid diameter below 100 μm for 5 days. In comparison to the spheroids formed in suspension culture, the porous nanosheets improved cell viability and cell division rate, suggesting the better feasibility to be applied as 3D culture scaffolds.

TRIM71 Deficiency Causes Germ Cell Loss During Mouse Embryogenesis and Is Associated With Human Male Infertility

Mutations affecting the germline can result in infertility or the generation of germ cell tumors (GCT), highlighting the need to identify and characterize the genes controlling germ cell development. The RNA-binding protein and E3 ubiquitin ligase TRIM71 is essential for embryogenesis, and its expression has been reported in GCT and adult mouse testes. To investigate the role of TRIM71 in mammalian germ cell embryonic development, we generated a germline-specific conditional Trim71 knockout mouse (cKO) using the early primordial germ cell (PGC) marker Nanos3 as a Cre-recombinase driver. cKO mice are infertile, with male mice displaying a Sertoli cell-only (SCO) phenotype which in humans is defined as a specific subtype of non-obstructive azoospermia characterized by the absence of germ cells in the seminiferous tubules. Infertility in male Trim71 cKO mice originates during embryogenesis, as the SCO phenotype was already apparent in neonatal mice. The in vitro differentiation of mouse embryonic stem cells (ESCs) into PGC-like cells (PGCLCs) revealed reduced numbers of PGCLCs in Trim71-deficient cells. Furthermore, TCam-2 cells, a human GCT-derived seminoma cell line which was used as an in vitro model for PGCs, showed proliferation defects upon TRIM71 knockdown. Additionally, in vitro growth competition assays, as well as proliferation assays with wild type and CRISPR/Cas9-generated TRIM71 mutant NCCIT cells showed that TRIM71 also promotes proliferation in this malignant GCT-derived non-seminoma cell line. Importantly, the PGC-specific markers BLIMP1 and NANOS3 were consistently downregulated in Trim71 KO PGCLCs, TRIM71 knockdown TCam-2 cells and TRIM71 mutant NCCIT cells. These data collectively support a role for TRIM71 in PGC development. Last, via exome sequencing analysis, we identified several TRIM71 variants in a cohort of infertile men, including a loss-of-function variant in a patient with an SCO phenotype. Altogether, our work reveals for the first time an association of TRIM71 deficiency with human male infertility, and uncovers further developmental roles for TRIM71 in the germline during mouse embryogenesis.

The stem cell-specific protein TRIM71 inhibits maturation and activity of the pro-differentiation miRNA let-7 via two independent molecular mechanisms

The stem cell-specific RNA-binding protein TRIM71/LIN-41 was the first identified target of the pro-differentiation and tumor suppressor miRNA let-7. TRIM71 has essential functions in embryonic development and a proposed oncogenic role in several cancer types, such as hepatocellular carcinoma. Here, we show that TRIM71 regulates let-7 expression and activity via two independent mechanisms. On the one hand, TRIM71 enhances pre-let-7 degradation through its direct interaction with LIN28 and TUT4, thereby inhibiting let-7 maturation and indirectly promoting the stabilization of let-7 targets. On the other hand, TRIM71 represses the activity of mature let-7 via its RNA-dependent interaction with the RNA-Induced Silencing Complex (RISC) effector protein AGO2. We found that TRIM71 directly binds and stabilizes let-7 targets, suggesting that let-7 activity inhibition occurs on active RISCs. MiRNA enrichment analysis of several transcriptomic datasets from mouse embryonic stem cells and human hepatocellular carcinoma cells suggests that these let-7 regulatory mechanisms shape transcriptomic changes during developmental and oncogenic processes. Altogether, our work reveals a novel role for TRIM71 as a miRNA repressor and sheds light on a dual mechanism of let-7 regulation.

NCX1 represents an ionic Na+ sensing mechanism in macrophages

Inflammation and infection can trigger local tissue Na+ accumulation. This Na+-rich environment boosts proinflammatory activation of monocyte/macrophage-like cells (MΦs) and their antimicrobial activity. Enhanced Na+-driven MΦ function requires the osmoprotective transcription factor nuclear factor of activated T cells 5 (NFAT5), which augments nitric oxide (NO) production and contributes to increased autophagy. However, the mechanism of Na+ sensing in MΦs remained unclear. High extracellular Na+ levels (high salt [HS]) trigger a substantial Na+ influx and Ca2+ loss. Here, we show that the Na+/Ca2+ exchanger 1 (NCX1, also known as solute carrier family 8 member A1 [SLC8A1]) plays a critical role in HS-triggered Na+ influx, concomitant Ca2+ efflux, and subsequent augmented NFAT5 accumulation. Moreover, interfering with NCX1 activity impairs HS-boosted inflammatory signaling, infection-triggered autolysosome formation, and subsequent antibacterial activity. Taken together, this demonstrates that NCX1 is able to sense Na+ and is required for amplifying inflammatory and antimicrobial MΦ responses upon HS exposure. Manipulating NCX1 offers a new strategy to regulate MΦ function.

The mRNA repressor TRIM71 cooperates with Nonsense-Mediated Decay factors to destabilize the mRNA of CDKN1A/p21

Nonsense-mediated decay (NMD) plays a fundamental role in the degradation of premature termination codon (PTC)-containing transcripts, but also regulates the expression of functional transcripts lacking PTCs, although such 'non-canonical' functions remain ill-defined and require the identification of factors targeting specific mRNAs to the NMD machinery. Our work identifies the stem cell-specific mRNA repressor protein TRIM71 as one of these factors. TRIM71 plays an essential role in embryonic development and is linked to carcinogenesis. For instance, TRIM71 has been correlated with advanced stages and poor prognosis in hepatocellular carcinoma. Our data shows that TRIM71 represses the mRNA of the cell cycle inhibitor and tumor suppressor CDKN1A/p21 and promotes the proliferation of HepG2 tumor cells. CDKN1A specific recognition involves the direct interaction of TRIM71 NHL domain with a structural RNA stem-loop motif within the CDKN1A 3'UTR. Importantly, CDKN1A repression occurs independently of miRNA-mediated silencing. Instead, the NMD factors SMG1, UPF1 and SMG7 assist TRIM71-mediated degradation of CDKN1A mRNA, among other targets. Our data sheds light on TRIM71-mediated target recognition and repression mechanisms and uncovers a role for this stem cell-specific factor and oncogene in non-canonical NMD, revealing the existence of a novel mRNA surveillance mechanism which we have termed the TRIM71/NMD axis.

Cytohesin-3 is required for full insulin receptor signaling and controls body weight via lipid excretion

Insulin plays a central role in regulating metabolic homeostasis and guanine-nucleotide exchange factors of the cytohesin family have been suggested to be involved in insulin signal transduction. The Drosophila homolog of cytohesin-3, steppke, has been shown to be essential for insulin signaling during larval development. However, genetic evidence for the functional importance of cytohesin-3 in mammals is missing. We therefore analyzed the consequences of genetic cytohesin-3-deficiency on insulin signaling and function in young and aged mice, using normal chow or high-fat diet (HFD). Insulin-receptor dependent signaling events are significantly reduced in liver and adipose tissue of young cytohesin-3-deficient mice after insulin-injection, although blood glucose levels and other metabolic parameters remain normal in these animals. Interestingly, however, cytohesin-3-deficient mice showed a reduced age- and HFD-induced weight gain with a significant reduction of body fat compared to wild-type littermates. Furthermore, cytohesin-3-deficient mice on HFD displayed no alterations in energy expenditure, but had an increased lipid excretion instead, as well as a reduced expression of genes essential for bile acid synthesis. Our findings show for the first time that an intact cyth3 locus is required for full insulin signaling in mammals and might constitute a novel therapeutic target for weight reduction.

SOCS1 and SOCS3 Target IRF7 Degradation To Suppress TLR7-Mediated Type I IFN Production of Human Plasmacytoid Dendritic Cells

Type I IFN production of plasmacytoid dendritic cells (pDCs) triggered by TLR-signaling is an essential part of antiviral responses and autoimmune reactions. Although it was well-documented that members of the cytokine signaling (SOCS) family regulate TLR-signaling, the mechanism of how SOCS proteins regulate TLR7-mediated type I IFN production has not been elucidated yet. In this article, we show that TLR7 activation in human pDCs induced the expression of SOCS1 and SOCS3. SOCS1 and SOCS3 strongly suppressed TLR7-mediated type I IFN production. Furthermore, we demonstrated that SOCS1- and SOCS3-bound IFN regulatory factor 7, a pivotal transcription factor of the TLR7 pathway, through the SH2 domain to promote its proteasomal degradation by lysine 48-linked polyubiquitination. Together, our results demonstrate that SOCS1/3-mediated degradation of IFN regulatory factor 7 directly regulates TLR7 signaling and type I IFN production in pDCs. This mechanism might be targeted by therapeutic approaches to either enhance type I IFN production in antiviral treatment or decrease type I IFN production in the treatment of autoimmune diseases.

N-glycosylation converts non-glycoproteins into mannose receptor ligands and reveals antigen-specific T cell responses in vivo

N-glycosylation is generally accepted to enhance the immunogenicity of antigens because of two main reasons. First, the attachment of glycans enables recognition by endocytic receptors like the mannose receptor (MR) and hence increased uptake by dendritic cells (DCs). Second, foreign glycans are postulated to be immunostimulatory and their recognition could induce DC activation. However, a direct comparison between the immunogenicity of N-glycosylated vs. de-glycosylated proteins in vivo and a direct effect of N-glycosylated antigens on the intrinsic capacity of DCs to activate T cells have not been assessed so far.To analyze whether enforced N-glycosylation is a suited strategy to enhance the immunogenicity of non-glycosylated antigens for vaccination studies, we targeted non-glycoproteins towards the MR by introduction of artificial N-glycosylation using the methylotrophic yeast Komagataella phaffii (previously termed Pichia pastoris). We could demonstrate that the introduction of a single N-X-S/T motif was sufficient for efficient MR-binding and internalization. However, addition of N-glycosylated proteins neither influenced DC maturation nor their general capacity to activate T cells, pointing out that enforced N-glycosylation does not increase the immunogenicity of the antigen per se. Additionally, increased antigen-specific cytotoxic T cell responses in vivo after injection of N-glycosylated compared to de-glycosylated proteins were observed but this effect strongly depended on the epitope tested. A beneficial effect of N-glycosylation on antibody production could not be detected, which might be due to MR-cross-linking on DCs and to concomitant differences in IL-6 production by CD4+ T cells.These observations point out that the effect of N-glycosylation on antigen immunogenicity can vary between different antigens and therefore might have important implications for the development of vaccines using K. phaffii.

Cellular Differentiation of Human Monocytes Is Regulated by Time-Dependent Interleukin-4 Signaling and the Transcriptional Regulator NCOR2

Human in vitro generated monocyte-derived dendritic cells (moDCs) and macrophages are used clinically, e.g., to induce immunity against cancer. However, their physiological counterparts, ontogeny, transcriptional regulation, and heterogeneity remains largely unknown, hampering their clinical use. High-dimensional techniques were used to elucidate transcriptional, phenotypic, and functional differences between human in vivo and in vitro generated mononuclear phagocytes to facilitate their full potential in the clinic. We demonstrate that monocytes differentiated by macrophage colony-stimulating factor (M-CSF) or granulocyte macrophage colony-stimulating factor (GM-CSF) resembled in vivo inflammatory macrophages, while moDCs resembled in vivo inflammatory DCs. Moreover, differentiated monocytes presented with profound transcriptomic, phenotypic, and functional differences. Monocytes integrated GM-CSF and IL-4 stimulation combinatorically and temporally, resulting in a mode- and time-dependent differentiation relying on NCOR2. Finally, moDCs are phenotypically heterogeneous and therefore necessitate the use of high-dimensional phenotyping to open new possibilities for better clinical tailoring of these cellular therapies.

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In vitro monocyte cultures model in vivo inflammatory dendritic cells and macrophages

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Monocyte-derived dendritic cells integrate interleukin-4 signaling time dependently

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NCOR2 controls differentiation of in vitro generated monocyte-derived dendritic cells

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In vitro generated monocyte-derived cells are phenotypically heterogeneous

MAPK Signaling and Inflammation Link Melanoma Phenotype Switching to Induction of CD73 during Immunotherapy

Evolution of tumor cell phenotypes promotes heterogeneity and therapy resistance. Here we found that induction of CD73, the enzyme that generates immunosuppressive adenosine, is linked to melanoma phenotype switching. Activating MAPK mutations and growth factors drove CD73 expression, which marked both nascent and full activation of a mesenchymal-like melanoma cell state program. Proinflammatory cytokines like TNFα cooperated with MAPK signaling through the c-Jun/AP-1 transcription factor complex to activate CD73 transcription by binding to an intronic enhancer. In a mouse model of T-cell immunotherapy, CD73 was induced in relapse melanomas, which acquired a mesenchymal-like phenotype. We also detected CD73 upregulation in melanoma patients progressing under adoptive T-cell transfer or immune checkpoint blockade, arguing for an adaptive resistance mechanism. Our work substantiates CD73 as a target to combine with current immunotherapies, but its dynamic regulation suggests limited value of CD73 pretreatment expression as a biomarker to stratify melanoma patients. Cancer Res; 77(17); 4697-709. ©2017 AACR.

Amplification of N-Myc is associated with a T-cell-poor microenvironment in metastatic neuroblastoma restraining interferon pathway activity and chemokine expression

Immune checkpoint inhibitors have significantly improved the treatment of several cancers. T-cell infiltration and the number of neoantigens caused by tumor-specific mutations are correlated to favorable responses in cancers with a high mutation load. Accordingly, checkpoint immunotherapy is thought to be less effective in tumors with low mutation frequencies such as neuroblastoma, a neuroendocrine tumor of early childhood with poor outcome of the high-risk disease group. However, spontaneous regressions and paraneoplastic syndromes seen in neuroblastoma patients suggest substantial immunogenicity. Using an integrative transcriptomic approach, we investigated the molecular characteristics of T-cell infiltration in primary neuroblastomas as an indicator of pre-existing immune responses and potential responsiveness to checkpoint inhibition. Here, we report that a T-cell-poor microenvironment in primary metastatic neuroblastomas is associated with genomic amplification of the MYCN (N-Myc) proto-oncogene. These tumors exhibited lower interferon pathway activity and chemokine expression in line with reduced immune cell infiltration. Importantly, we identified a global role for N-Myc in the suppression of interferon and pro-inflammatory pathways in human and murine neuroblastoma cell lines. N-Myc depletion potently enhanced targeted interferon pathway activation by a small molecule agonist of the cGAS-STING innate immune pathway. This promoted chemokine expression including Cxcl10 and T-cell recruitment in microfluidics migration assays. Hence, our data suggest N-Myc inhibition plus targeted IFN activation as adjuvant strategy to enforce cytotoxic T-cell recruitment in MYCN-amplified neuroblastomas.

The ubiquitin ligase LIN41/TRIM71 targets p53 to antagonize cell death and differentiation pathways during stem cell differentiation

Rapidity and specificity are characteristic features of proteolysis mediated by the ubiquitin-proteasome system. Therefore, the UPS is ideally suited for the remodeling of the embryonic stem cell proteome during the transition from pluripotent to differentiated states and its inverse, the generation of inducible pluripotent stem cells. The Trim-NHL family member LIN41 is among the first E3 ubiquitin ligases to be linked to stem cell pluripotency and reprogramming. Initially discovered in C. elegans as a downstream target of the let-7 miRNA, LIN41 is now recognized as a critical regulator of stem cell fates as well as the timing of neurogenesis. Despite being indispensable for embryonic development and neural tube closure in mice, the underlying mechanisms for LIN41 function in these processes are poorly understood. To better understand the specific contributions of the E3 ligase activity for the stem cell functions of LIN41, we characterized global changes in ubiquitin or ubiquitin-like modifications using Lin41-inducible mouse embryonic stem cells. The tumor suppressor protein p53 was among the five most strongly affected proteins in cells undergoing neural differentiation in response to LIN41 induction. We show that LIN41 interacts with p53, controls its abundance by ubiquitination and antagonizes p53-dependent pro-apoptotic and pro-differentiation responses. In vivo, the lack of LIN41 is associated with upregulation of Grhl3 and widespread caspase-3 activation, two downstream effectors of p53 with essential roles in neural tube closure. As Lin41-deficient mice display neural tube closure defects, we conclude that LIN41 is critical for the regulation of p53 functions in cell fate specification and survival during early brain development.

Hyperosmolarity impedes the cross-priming competence of dendritic cells in a TRIF-dependent manner

Tissue osmolarity varies among different organs and can be considerably increased under pathologic conditions. Hyperosmolarity has been associated with altered stimulatory properties of immune cells, especially macrophages and dendritic cells. We have recently reported that dendritic cells upon exposure to hypertonic stimuli shift their profile towards a macrophage-M2-like phenotype, resulting in attenuated local alloreactivity during acute kidney graft rejection. Here, we examined how hyperosmotic microenvironment affects the cross-priming capacity of dendritic cells. Using ovalbumin as model antigen, we showed that exposure of dendritic cells to hyperosmolarity strongly inhibits activation of antigen-specific T cells despite enhancement of antigen uptake, processing and presentation. We identified TRIF as key mediator of this phenomenon. Moreover, we detected a hyperosmolarity-triggered, TRIF-dependent clustering of MHCI loaded with the ovalbumin-derived epitope, but not of overall MHCI molecules, providing a possible explanation for a reduced T cell activation. Our findings identify dendritic cells as important players in hyperosmolarity-mediated immune imbalance and provide evidence for a novel pathway of inhibition of antigen specific CD8⁺ T cell response in a hypertonic micromilieu.

Dynamin2 controls Rap1 activation and integrin clustering in human T lymphocyte adhesion

Leukocyte trafficking is crucial to facilitate efficient immune responses. Here, we report that the large GTPase dynamin2, which is generally considered to have a key role in endocytosis and membrane remodeling, is an essential regulator of integrin-dependent human T lymphocyte adhesion and migration. Chemical inhibition or knockdown of dynamin2 expression significantly reduced integrin-dependent T cell adhesion in vitro. This phenotype was not observed when T cells were treated with various chemical inhibitors which abrogate endocytosis or actin polymerization. We furthermore detected dynamin2 in signaling complexes and propose that it controls T cell adhesion via FAK/Pyk2- and RapGEF1-mediated Rap1 activation. In addition, the dynamin2 inhibitor-induced reduction of lymphocyte adhesion can be rescued by Rap1a overexpression. We demonstrate that the dynamin2 effect on T cell adhesion does not involve integrin affinity regulation but instead relies on its ability to modulate integrin valency. Taken together, we suggest a previously unidentified role of dynamin2 in the regulation of integrin-mediated lymphocyte adhesion via a Rap1 signaling pathway.

Ceramide synthase 2 facilitates S1P-dependent egress of thymocytes into the circulation in mice

Well-defined gradients of the lipid mediator sphingosine-1-phosphate (S1P) direct chemotactic egress of mature thymocytes from the thymus into the circulation. Although it is known that these gradients result from low S1P levels in the thymic parenchyma and high S1P concentrations at the exit sites and in the plasma, the biochemical mechanisms that regulate these differential S1P levels remain unclear. Several studies demonstrated that ceramide synthase 2 (Cers2) regulates the levels of the S1P precursor sphingosine. We, therefore, investigated whether Cers2 is involved in the regulation of S1P gradients and S1P-dependent egress into the circulation. By analyzing Cers2-deficient mice, we demonstrate that Cers2 limits the levels of S1P in thymus and blood to maintain functional S1P gradients that mediate thymocyte emigration into the circulation. This function is specific for Cers2, as we also show that Cers4 is not involved in the regulation of thymic egress. Our study identified Cers2 as an important regulator of S1P-dependent thymic egress, and thus contributes to the understanding of how S1P gradients are maintained in vivo.

Syk phosphorylation - a gravisensitive step in macrophage signalling

BACKGROUND

The recognition of pathogen patterns followed by the production of reactive oxygen species (ROS) during the oxidative burst is one of the major functions of macrophages. This process is the first line of defence and is crucial for the prevention of pathogen-associated diseases. There are indications that the immune system of astronauts is impaired during spaceflight, which could result in an increased susceptibility to infections. Several studies have indicated that the oxidative burst of macrophages is highly impaired after spaceflight, but the underlying mechanism remained to be elucidated. Here, we investigated the characteristics of reactive oxygen species production during the oxidative burst after pathogen pattern recognition in simulated microgravity by using a fast-rotating Clinostat to mimic the condition of microgravity. Furthermore, spleen tyrosine kinase (Syk) phosphorylation, which is required for ROS production, and the translocation of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) to the nucleus were monitored to elucidate the influence of altered gravity on macrophage signalling.

RESULTS

Simulated microgravity leads to significantly diminished ROS production in macrophages upon zymosan, curdlan and lipopolysaccharide stimulation. To address the signalling mechanisms involved, Syk phosphorylation was examined, revealing significantly reduced phosphorylation in simulated microgravity compared to normal gravity (1 g) conditions. In contrast, a later signalling step, the translocation of NF-κB to the nucleus, demonstrated no gravity-dependent alterations.

CONCLUSIONS

The results obtained in simulated microgravity show that ROS production in macrophages is a highly gravisensitive process, caused by a diminished Syk phosphorylation. In contrast, NF-κB signalling remains consistent in simulated microgravity. This difference reveals that early signalling steps, such as Syk phosphorylation, are affected by microgravity, whereas the lack of effects in later steps might indicate adaptation processes. Taken together, this study clearly demonstrates that macrophages display impaired signalling upon pattern recognition when exposed to simulated microgravity conditions, which if verified in real microgravity this may be one reason why astronauts display higher susceptibility to infections.

Liver sinusoidal endothelial cell-mediated CD8 T cell priming depends on co-inhibitory signal integration over time

The initiation of adaptive immunity requires cell-to-cell contact between T cells and antigen-presenting cells. Together with immediate TCR signal transduction, the formation of an immune synapse (IS) is one of the earliest events detected during T cell activation. Here, we show that interaction of liver sinusoidal endothelial cells (LSEC) with naive CD8 T cells, which induces CD8 T cells without immediate effector function, is characterized by a multi-focal type IS. The co-inhibitory molecule B7H1, which is pivotal for the development of non-responsive LSEC-primed T cells, did not alter IS structure or TCRβ/CD11a cluster size or density, indicating that IS form does not determine the outcome of LSEC-mediated T cell activation. Instead, PD-1 signaling during CD8 T cell priming by LSEC repressed IL-2 production as well as sustained CD25 expression. When acting during the first 24 h of LSEC/CD8 T cell interaction, CD28 co-stimulation inhibited the induction of non-responsive LSEC-primed T cells. However, after more than 36 h of PD-1 signaling, CD28 co-stimulation failed to rescue effector function in LSEC-primed T cells. Together, these data show that during LSEC-mediated T cell priming, integration of co-inhibitory PD-1 signaling over time turns on a program for CD8 T cell development, that cannot be overturned by co-stimulatory signals.

Crosstalk between sentinel and helper macrophages permits neutrophil migration into infected uroepithelium

The phagocytes of the innate immune system, macrophages and neutrophils, contribute to antibacterial defense, but their functional specialization and cooperation is unclear. Here, we report that three distinct phagocyte subsets play highly coordinated roles in bacterial urinary tract infection. Ly6C(-) macrophages acted as tissue-resident sentinels that attracted circulating neutrophils and Ly6C(+) macrophages. Such Ly6C(+) macrophages played a previously undescribed helper role: once recruited to the site of infection, they produced the cytokine TNF, which caused Ly6C(-) macrophages to secrete CXCL2. This chemokine activated matrix metalloproteinase-9 in neutrophils, allowing their entry into the uroepithelium to combat the bacteria. In summary, the sentinel macrophages elicit the powerful antibacterial functions of neutrophils only after confirmation by the helper macrophages, reminiscent of the licensing role of helper T cells in antiviral adaptive immunity. These findings identify helper macrophages and TNF as critical regulators in innate immunity against bacterial infections in epithelia.

The oxidative burst reaction in mammalian cells depends on gravity

Gravity has been a constant force throughout the Earth’s evolutionary history. Thus, one of the fundamental biological questions is if and how complex cellular and molecular functions of life on Earth require gravity. In this study, we investigated the influence of gravity on the oxidative burst reaction in macrophages, one of the key elements in innate immune response and cellular signaling. An important step is the production of superoxide by the NADPH oxidase, which is rapidly converted to H₂O₂ by spontaneous and enzymatic dismutation. The phagozytosis-mediated oxidative burst under altered gravity conditions was studied in NR8383 rat alveolar macrophages by means of a luminol assay. Ground-based experiments in “functional weightlessness” were performed using a 2 D clinostat combined with a photomultiplier (PMT clinostat). The same technical set-up was used during the 13th DLR and 51st ESA parabolic flight campaign. Furthermore, hypergravity conditions were provided by using the Multi-Sample Incubation Centrifuge (MuSIC) and the Short Arm Human Centrifuge (SAHC). The results demonstrate that release of reactive oxygen species (ROS) during the oxidative burst reaction depends greatly on gravity conditions. ROS release is 1.) reduced in microgravity, 2.) enhanced in hypergravity and 3.) responds rapidly and reversible to altered gravity within seconds. We substantiated the effect of altered gravity on oxidative burst reaction in two independent experimental systems, parabolic flights and 2D clinostat / centrifuge experiments. Furthermore, the results obtained in simulated microgravity (2D clinorotation experiments) were proven by experiments in real microgravity as in both cases a pronounced reduction in ROS was observed. Our experiments indicate that gravity-sensitive steps are located both in the initial activation pathways and in the final oxidative burst reaction itself, which could be explained by the role of cytoskeletal dynamics in the assembly and function of the NADPH oxidase complex.

Salt-dependent chemotaxis of macrophages

Besides their role in immune system host defense, there is growing evidence that macrophages may also be important regulators of salt homeostasis and blood pressure by a TonEBP-VEGF-C dependent buffering mechanism. As macrophages are known to accumulate in the skin of rats fed under high salt diet conditions and are pivotal for removal of high salt storage, the question arose how macrophages sense sites of high sodium storage. Interestingly, we observed that macrophage-like RAW264.7 cells, murine bone marrow-derived macrophages and peritoneal macrophages recognize NaCl hypertonicity as a chemotactic stimulus and migrate in the direction of excess salt concentration by using an in vitro transwell migration assay. While RAW264.7 cells migrated toward NaCl in a dose-dependent fashion, no migratory response toward isotonic or hypotonic media controls, or other osmo-active agents, e.g. urea or mannitol, could be detected. Interestingly, we could not establish a specific role of the osmoprotective transcription factor TonEBP in regulating salt-dependent chemotaxis, since the specific migration of bone marrow-derived macrophages following RNAi of TonEBP toward NaCl was not altered. Although the underlying mechanism remains unidentified, these data point to a thus far unappreciated role for NaCl-dependent chemotaxis of macrophages in the clearance of excess salt, and suggest the existence of novel NaCl sensor/effector circuits, which are independent of the TonEBP system.

The cytohesin paralog Sec7 of Dictyostelium discoideum is required for phagocytosis and cell motility

Background

Dictyostelium harbors several paralogous Sec7 genes that encode members of three subfamilies of the Sec7 superfamily of guanine nucleotide exchange factors. One of them is the cytohesin family represented by three members in D. discoideum, SecG, Sec7 and a further protein distinguished by several transmembrane domains. Cytohesins are characterized by a Sec7-PH tandem domain and have roles in cell adhesion and migration.

Results

We study here Sec7. In vitro its PH domain bound preferentially to phosphatidylinositol 3,4-bisphosphate (PI(3,4)P₂), phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃). When following the distribution of GFP-Sec7 in vivo we observed the protein in the cytosol and at the plasma membrane. Strikingly, when cells formed pseudopods, macropinosomes or phagosomes, GFP-Sec7 was conspicuously absent from areas of the plasma membrane which were involved in these processes. Mutant cells lacking Sec7 exhibited an impaired phagocytosis and showed significantly reduced speed and less persistence during migration. Cellular properties associated with mammalian cytohesins like cell-cell and cell-substratum adhesion were not altered. Proteins with roles in membrane trafficking and signal transduction have been identified as putative interaction partners consistent with the data obtained from mutant analysis.

Conclusions

Sec7 is a cytosolic component and is associated with the plasma membrane in a pattern distinctly different from the accumulation of PI(3,4,5)P₃. Mutant analysis reveals that loss of the protein affects cellular processes that involve membrane flow and the actin cytoskeleton.

Impact of altered gravity on the ROS-signalling in macrophages (P1293)

Astronauts suffer from a higher susceptibility to infections during spaceflights. Different stressors could lead to a diminished immune defense. Macrophages present the first line of defense of the innate immune system. Their main challenge is the recognition, engulfment and destruction of pathogenic bacteria, viruses and dead cells. The killing of bacteria is accomplished by the production of Reactive Oxygen Species (ROS) during oxidative burst. We investigated the effects of altered gravity on the pattern recognition and ROS production of macrophages. Our results show that real microgravity (parabolic flight), simulated microgravity (2D fast-rotating clinostat) as well as hypergravity (centrifuge) alter the production of ROS after stimulation of cell surface receptors Dectin, TLR2/6, complement and Fcγ. We also find that phosphorylation of the tyrosine kinase Syk, an essential link between pattern recognition, cytoskeleton and ROS production, is sensitive to microgravity. We can conclude that both microgravity and hypergravity cause very fast changes in ROS production, characterising the oxidative burst as a gravisensitive processes. Altered gravity might have impact on the interaction of different pattern recognition signalling pathways, which are crucial for the bacteria clearance in terms of ROS production. Therefore, the study of ROS signalling under altered gravity is not only of interest for humans in space, but also for human health on Earth.

Salt-dependent chemotaxis of macrophages (P5122)

Recent studies support the hypothesis that macrophages are not only essential for efficient immune responses, but are also regulators of local tissue electrolyte composition and hence arterial hypertension. Blockade of this macrophage-dependent regulatory axis resulted in skin electrolyte accumulation and blood pressure increase. Based on the observation that macrophages accumulate in skin tissue of rats fed under high salt diet conditions, the question arose whether macrophages actively migrate to sites of high sodium storage. We therefore investigated the migratory behavior of RAW264.7 macrophages, murine bone marrow-derived macrophages and murine peritoneal macrophages toward different NaCl gradients, showing that NaCl may act as a chemotactic stimulus per se. We also assessed the reorganization of the actin cytoskeleton, a potential NaCl-induced chemokine CCL2 production and the role of the osmoprotective transcription factor TonEBP in salt-dependent chemotaxis. Although the underlying mechanism remains unidentified, we propose that salt-dependent chemotaxis might be an important functional feature of macrophages within the context of excess salt clearance in vivo.

Cellular mechanotransduction relies on tension-induced and chaperone-assisted autophagy

Mechanical tension is an ever-present physiological stimulus essential for the development and homeostasis of locomotory, cardiovascular, respiratory, and urogenital systems. Tension sensing contributes to stem cell differentiation, immune cell recruitment, and tumorigenesis. Yet, how mechanical signals are transduced inside cells remains poorly understood. Here, we identify chaperone-assisted selective autophagy (CASA) as a tension-induced autophagy pathway essential for mechanotransduction in muscle and immune cells. The CASA complex, comprised of the molecular chaperones Hsc70 and HspB8 and the cochaperone BAG3, senses the mechanical unfolding of the actin-crosslinking protein filamin. Together with the chaperone-associated ubiquitin ligase CHIP, the complex initiates the ubiquitin-dependent autophagic sorting of damaged filamin to lysosomes for degradation. Autophagosome formation during CASA depends on an interaction of BAG3 with synaptopodin-2 (SYNPO2). This interaction is mediated by the BAG3 WW domain and facilitates cooperation with an autophagosome membrane fusion complex. BAG3 also utilizes its WW domain to engage in YAP/TAZ signaling. Via this pathway, BAG3 stimulates filamin transcription to maintain actin anchoring and crosslinking under mechanical tension. By integrating tension sensing, autophagosome formation, and transcription regulation during mechanotransduction, the CASA machinery ensures tissue homeostasis and regulates fundamental cellular processes such as adhesion, migration, and proliferation.

Vav1 regulates MHCII expression in murine resting and activated B cells

Vav1 is a guanine nucleotide exchange factor (GEF) for Rho GTPases, which is exclusively expressed in cells of the hematopoietic system. In addition to its well-documented GEF activity, it was suggested to have other functions due to the presence of multiple domains and nuclear localization signals in its protein structure. Although GEF-dependent and GEF-independent functions of vav have been implicated in T-cell development and T-cell receptor signaling, the role of vav1 in antigen-presenting cells is poorly understood. We found that vav1 is an important regulator of MHCII expression and transport. Microarray analysis of unstimulated bone marrow-derived macrophages revealed a novel role of vav1 in transcriptional regulation of the MHCII locus, possibly by indirect means. Primary immune cells from vav1-deficient mice had a significantly lower constitutive surface expression of MHCII with the strongest impact observed on splenic and peritoneal B cells. Impaired MHCII expression resulted in a diminished capacity for T-cell activation. Using 6-thio-GTP, a specific inhibitor of the GEF function of vav1, we were able to show that the GEF activity is required for MHCII upregulation in B cells after stimulation with LPS. Furthermore, our data show that vav1 not only affects transcription of the MHCII locus but also is an important regulator of MHCII protein transport to the cell surface.

Atad3 function is essential for early post-implantation development in the mouse

The mitochondrial AAA+-ATPase ATAD3 is implicated in the regulation of mitochondrial and ER dynamics and was shown to be necessary for larval development in Caenorhabditis elegans. In order to elucidate the relevance of ATAD3 for mammalian development, the phenotype of an Atad3 deficient mouse line was analyzed. Atad3 deficient embryos die around embryonic day E7.5 due to growth retardation and a defective development of the trophoblast lineage immediately after implantation into the uterus. This indicates an essential function of Atad3 for the progression of the first steps of post-implantation development at a time point when mitochondrial biogenesis and ATP production by oxidative phosphorylation are required. Therefore, murine Atad3 plays an important role in the biogenesis of mitochondria in trophoblast stem cells and in differentiating trophoblasts. At the biochemical level, we report here that ATAD3 is present in five native mitochondrial protein complexes of different sizes, indicating complex roles of the protein in mitochondrial architecture and function.

Rapid hierarchical assembly of medium-size DNA cassettes

Synthetic biology applications call for efficient methods to generate large gene cassettes that encode complex gene circuits in order to avoid simultaneous delivery of multiple plasmids encoding individual genes. Multiple methods have been proposed to achieve this goal. Here, we describe a novel protocol that allows one-step cloning of up to four gene-size DNA fragments, followed by a second assembly of these concatenated sequences into large circular DNA. The protocols described here comprise a simple, cheap and fast solution for routine construction of cassettes with up to 10 gene-size components.

Repression of the genome organizer SATB1 in regulatory T cells is required for suppressive function and inhibition of effector differentiation

Regulatory T cells (T(reg) cells) are essential for self-tolerance and immune homeostasis. Lack of effector T cell (T(eff) cell) function and gain of suppressive activity by T(reg) cells are dependent on the transcriptional program induced by Foxp3. Here we report that repression of SATB1, a genome organizer that regulates chromatin structure and gene expression, was crucial for the phenotype and function of T(reg) cells. Foxp3, acting as a transcriptional repressor, directly suppressed the SATB1 locus and indirectly suppressed it through the induction of microRNAs that bound the SATB1 3' untranslated region. Release of SATB1 from the control of Foxp3 in T(reg) cells caused loss of suppressive function, establishment of transcriptional T(eff) cell programs and induction of T(eff) cell cytokines. Our data support the proposal that inhibition of SATB1-mediated modulation of global chromatin remodeling is pivotal for maintaining T(reg) cell functionality.

Tetraspanins CD9 and CD81 are molecular partners of trimeric FcɛRI on human antigen-presenting cells

BACKGROUND

Most functions of tetraspanins are not related to cell-surface receptor ligand binding, but are mediated by direct interactions with their partner proteins. Functions of trimeric FcɛRI, expressed by antigen-presenting cells (APCs), range from amplification of allergic inflammatory reactions to their active suppression. Cell-type-specific protein-protein interactions might play a role in the regulation of these bidirectional tasks. Therefore, we intended to study the interactions of trimeric FcɛRI with tetraspanins.

METHODS

The expression levels of tetraspanins CD9, CD37, CD53, CD63, CD81, CD82, and CD151 on skin dendritic cells of atopic dermatitis (AD) patients or healthy individuals were detected by flow cytometry. Tetraspanin expression on FcɛRI(pos) and FcɛRI(neg) monocyte subpopulations was evaluated. Flow cytometry, confocal microscopy, immunoprecipitation, and immunoblotting experiments were performed to observe the relationship between tetraspanins CD9 and CD81 and FcɛRI. Furthermore, plate stimulation experiments were performed, and cytokines in the supernatants were detected.

RESULTS

We found that human FcɛRI(pos) APCs expressed high amounts of tetraspanins and that the tetraspanins CD9 and CD81 were associated with FcɛRI. Concomitant activation of FcɛRI and CD9 on human monocytes increased FcɛRI-mediated cytokine release.

CONCLUSION

Taken together, we show for the first time that CD9 and CD81 act as molecular partners of trimeric FcɛRI on human APC, which might be of importance in allergic diseases such as AD.

Targeting multifunctional proteins by virtual screening: structurally diverse cytohesin inhibitors with differentiated biological functions

Virtual screening (VS) of chemical libraries formatted in silico provides an alternative to experimental high-throughput screening (HTS) for the identification of small molecule modulators of protein function. We have tailored a VS approach combining fingerprint similarity searching and support vector machine modeling toward the identification of small molecular probes for the study of cytohesins, a family of cytoplasmic regulator proteins with multiple cellular functions. A total of 40 new structurally diverse inhibitors were identified, and 26 of these compounds were more active than the primary VS template, a single known inhibitory chemotype, in at least one of three different assays (guanine nucleotide exchange, Drosophila insulin signaling, and human leukocyte cell adhesion). Moreover, these inhibitors displayed differential inhibitory profiles. Our findings demonstrate that, at least for the cytohesins, computational extrapolation from known active compounds was capable of identifying small molecular probes with highly diversified functional profiles.

Myeloid cell-restricted insulin receptor deficiency protects against obesity-induced inflammation and systemic insulin resistance

A major component of obesity-related insulin resistance is the establishment of a chronic inflammatory state with invasion of white adipose tissue by mononuclear cells. This results in the release of pro-inflammatory cytokines, which in turn leads to insulin resistance in target tissues such as skeletal muscle and liver. To determine the role of insulin action in macrophages and monocytes in obesity-associated insulin resistance, we conditionally inactivated the insulin receptor (IR) gene in myeloid lineage cells in mice (IR(Deltamyel)-mice). While these animals exhibit unaltered glucose metabolism on a normal diet, they are protected from the development of obesity-associated insulin resistance upon high fat feeding. Euglycemic, hyperinsulinemic clamp studies demonstrate that this results from decreased basal hepatic glucose production and from increased insulin-stimulated glucose disposal in skeletal muscle. Furthermore, IR(Deltamyel)-mice exhibit decreased concentrations of circulating tumor necrosis factor (TNF) alpha and thus reduced c-Jun N-terminal kinase (JNK) activity in skeletal muscle upon high fat feeding, reflecting a dramatic reduction of the chronic and systemic low-grade inflammatory state associated with obesity. This is paralleled by a reduced accumulation of macrophages in white adipose tissue due to a pronounced impairment of matrix metalloproteinase (MMP) 9 expression and activity in these cells. These data indicate that insulin action in myeloid cells plays an unexpected, critical role in the regulation of macrophage invasion into white adipose tissue and in the development of obesity-associated insulin resistance.

A cytohesin homolog in Dictyostelium amoebae

BACKGROUND

Dictyostelium, an amoeboid motile cell, harbors several paralogous Sec7 genes that encode members of three distinct subfamilies of the Sec7 superfamily of Guanine nucleotide exchange factors. Among them are proteins of the GBF/BIG family present in all eukaryotes. The third subfamily represented with three members in D. discoideum is the cytohesin family that has been thought to be metazoan specific. Cytohesins are characterized by a Sec7 PH tandem domain and have roles in cell adhesion and migration.

PRINCIPAL FINDINGS

Dictyostelium SecG exhibits highest homologies to the cytohesins. It harbors at its amino terminus several ankyrin repeats that are followed by the Sec7 PH tandem domain. Mutants lacking SecG show reduced cell-substratum adhesion whereas cell-cell adhesion that is important for development is not affected. Accordingly, multicellular development proceeds normally in the mutant. During chemotaxis secG(-) cells elongate and migrate in a directed fashion towards cAMP, however speed is moderately reduced.

SIGNIFICANCE

The data indicate that SecG is a relevant factor for cell-substrate adhesion and reveal the basic function of a cytohesin in a lower eukaryote.

The nuclear receptor PPAR gamma selectively inhibits Th17 differentiation in a T cell-intrinsic fashion and suppresses CNS autoimmunity

T helper cells secreting interleukin (IL)-17 (Th17 cells) play a crucial role in autoimmune diseases like multiple sclerosis (MS). Th17 differentiation, which is induced by a combination of transforming growth factor (TGF)-beta/IL-6 or IL-21, requires expression of the transcription factor retinoic acid receptor-related orphan receptor gamma t (ROR gamma t). We identify the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR gamma) as a key negative regulator of human and mouse Th17 differentiation. PPAR gamma activation in CD4(+) T cells selectively suppressed Th17 differentiation, but not differentiation into Th1, Th2, or regulatory T cells. Control of Th17 differentiation by PPAR gamma involved inhibition of TGF-beta/IL-6-induced expression of ROR gamma t in T cells. Pharmacologic activation of PPAR gamma prevented removal of the silencing mediator for retinoid and thyroid hormone receptors corepressor from the ROR gamma t promoter in T cells, thus interfering with ROR gamma t transcription. Both T cell-specific PPAR gamma knockout and endogenous ligand activation revealed the physiological role of PPAR gamma for continuous T cell-intrinsic control of Th17 differentiation and development of autoimmunity. Importantly, human CD4(+) T cells from healthy controls and MS patients were strongly susceptible to PPAR gamma-mediated suppression of Th17 differentiation. In summary, we report a PPAR gamma-mediated T cell-intrinsic molecular mechanism that selectively controls Th17 differentiation in mice and in humans and that is amenable to pharmacologic modulation. We therefore propose that PPAR gamma represents a promising molecular target for specific immunointervention in Th17-mediated autoimmune diseases such as MS.

Systemic antigen cross-presented by liver sinusoidal endothelial cells induces liver-specific CD8 T-cell retention and tolerization

Peripheral CD8 T-cell tolerance can be generated outside lymphatic tissue in the liver, but the course of events leading to tolerogenic interaction of hepatic cell populations with circulating T-cells remain largely undefined. Here we demonstrate that preferential uptake of systemically circulating antigen by murine liver sinusoidal endothelial cells (LSECs), and not by other antigen-presenting cells in the liver or spleen, leads to cross-presentation on major histocompatibility complex (MHC) I molecules, which causes rapid antigen-specific naïve CD8 T-cell retention in the liver but not in other organs. Using bone-marrow chimeras and a novel transgenic mouse model (Tie2-H-2K(b) mice) with endothelial cell-specific MHC I expression, we provide evidence that cross-presentation by organ-resident and radiation-resistant LSECs in vivo was both essential and sufficient to cause antigen-specific retention of naïve CD8 T-cells under noninflammatory conditions. This was followed by sustained CD8 T-cell proliferation and expansion in vivo, but ultimately led to the development of T-cell tolerance.

CONCLUSION

Our results show that cross-presentation of circulating antigens by LSECs caused antigen-specific retention of naïve CD8 T-cells and identify antigen-specific T-cell adhesion as the first step in the induction of T-cell tolerance.