Novel FMRP interaction networks linked to cellular stress

Silencing of the fragile X mental retardation 1 (FMR1) gene and consequently lack of synthesis of FMR protein (FMRP) are associated with fragile X syndrome, which is one of the most prevalent inherited intellectual disabilities, with additional roles in increased viral infection, liver disease, and reduced cancer risk. FMRP plays critical roles in chromatin dynamics, RNA binding, mRNA transport, and mRNA translation. However, the underlying molecular mechanisms, including the (sub)cellular FMRP protein networks, remain elusive. Here, we employed affinity pull-down and quantitative LC-MS/MS analyses with FMRP. We identified known and novel candidate FMRP-binding proteins as well as protein complexes. FMRP interacted with 180 proteins, 28 of which interacted with its N terminus. Interaction with the C terminus of FMRP was observed for 102 proteins, and 48 proteins interacted with both termini. This FMRP interactome comprises known FMRP-binding proteins, including the ribosomal proteins FXR1P, NUFIP2, Caprin-1, and numerous novel FMRP candidate interacting proteins that localize to different subcellular compartments, including CARF, LARP1, LEO1, NOG2, G3BP1, NONO, NPM1, SKIP, SND1, SQSTM1, and TRIM28. Our data considerably expand the protein and RNA interaction networks of FMRP, which thereby suggest that, in addition to its known functions, FMRP participates in transcription, RNA metabolism, ribonucleoprotein stress granule formation, translation, DNA damage response, chromatin dynamics, cell cycle regulation, ribosome biogenesis, miRNA biogenesis, and mitochondrial organization. Thus, FMRP seems associated with multiple cellular processes both under normal and cell stress conditions in neuronal as well as non-neuronal cell types, as exemplified by its role in the formation of stress granules.

Mechanisms of lymphatic system-specific viral replication and its potential role in autoimmune disease

Viral infections can be fatal because of the direct cytopathic effects of the virus or the induction of a strong, uncontrolled inflammatory response. Virus and host intrinsic characteristics strongly modulate the outcome of viral infections. Recently we determined the circumstances under which enhanced replication of virus within the lymphoid tissue is beneficial for the outcome of a disease. This enforced viral replication promotes anti-viral immune activation and, counterintuitively, accelerates virus control. In this review we summarize the mechanisms that contribute to enforced viral replication. Antigen-presenting cells and CD169⁺ macrophages exhibit enforced viral replication after infection with the model viruses lymphocytic choriomeningitis virus (LCMV) and vesicular stomatitis virus (VSV). Ubiquitin-specific peptidase 18 (Usp18), an endogenous type I interferon blocker in CD169⁺ macrophages, has been identified as a proviral gene, as are B cell activating factor (BAFF) and carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1). Lymphotoxins (LT) strongly enhance viral replication in the spleen and lymph nodes. All these factors modulate splenic architecture and thereby promote the development of CD169⁺ macrophages. Tumor necrosis factor alpha (TNF-α) and nuclear factor kappa-light-chain-enhancer of activated B cell signaling (NF-κB) have been found to promote the survival of infected CD169⁺ macrophages, thereby similarly promoting enforced viral replication. Association of autoimmune disease with infections is evident from (1) autoimmune phenomena described during a chronic virus infection; (2) onset of autoimmune disease simultaneous to viral infections; and (3) experimental evidence. Involvement of virus infection during onset of type I diabetes is strongly evident. Epstein-Bar virus (EBV) infection was discussed to be involved in the pathogenesis of systemic lupus erythematosus. In conclusion, several mechanisms promote viral replication in secondary lymphatic organs. Identifying such factors in humans is a challenge for future studies.

Balancing viral replication in spleen and liver determines the outcome of systemic virus infection

The innate immune system limits virus replication during systemic infection by producing type I interferons (IFN-I) but still has to allow viral replication to achieve maximal innate and adaptive immune activation. Some spleen and lymph node resident antigen presenting cells (APCs) show limited response to IFN-I due to expression of the endogenous inhibitor of IFN-I signaling, Usp18. Therefore, virus in this spleen niche replicates despite high levels of IFN-I. This enforced viral replication leads to an exorbitant propagation of viral antigens and viral RNA. Viral antigen leads to massive activation of the adaptive immune system, while viral RNA to activated innate immunity. In contrast to these APCs, liver resident Kupffer cells, take up most of the systemic virus and suppress its replication in response to IFN-I. In addition, virus specific CD8 + T cells which are primed in the spleen migrate to the liver and kill virus infected cells. In this review we discuss the different mechanisms, which influence immune activation in spleen and antiviral mechanisms in the liver and how they determine the outcome of virus infection.

Reactive oxygen species delay control of lymphocytic choriomeningitis virus

Cluster of differentiation (CD)8(+) T cells are like a double edged sword during chronic viral infections because they not only promote virus elimination but also induce virus-mediated immunopathology. Elevated levels of reactive oxygen species (ROS) have been reported during virus infections. However, the role of ROS in T-cell-mediated immunopathology remains unclear. Here we used the murine lymphocytic choriomeningitis virus to explore the role of ROS during the processes of virus elimination and induction of immunopathology. We found that virus infection led to elevated levels of ROS producing granulocytes and macrophages in virus-infected liver and spleen tissues that were triggered by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Lack of the regulatory subunit p47phox of the NADPH oxidase diminished ROS production in these cells. While CD8(+) T cells exhibited ROS production that was independent of NADPH oxidase expression, survival and T-cell function was elevated in p47phox-deficient (Ncf1(-/-)) mice. In the absence of p47phox, enhanced T-cell immunity promoted virus elimination and blunted corresponding immunopathology. In conclusion, we find that NADPH-mediated production of ROS critically impairs the immune response, impacting elimination of virus and outcome of liver cell damage.

Osmotic shock-induced suicidal death of erythrocytes

Osmotic shock triggers eryptosis, a suicidal death of erythrocytes characterized by cell shrinkage, cell membrane blebbing and phosphatidylserine exposure at the cell surface. Phosphatidylserine-exposing erythrocytes are recognized by macrophages, engulfed, degraded and thus cleared from circulating blood. Eryptosis following osmotic shock is mediated by two distinct signalling pathways. On the one hand, osmotic shock stimulates a cyclooxygenase leading to formation of prostaglandin E2 and subsequent activation of Ca2+-permeable cation channels. On the other hand, osmotic shock activates a phospholipase A2 leading to release of platelet activating factor, which in turn activates a sphingomyelinase and thus stimulates the formation of ceramide. The increased cytosolic Ca2+ concentrations on the one hand and ceramide on the other trigger phospholipid scrambling of the cell membrane with the subsequent shift of phosphatidylserine from the inner to the outer cell membrane leaflet. Ca2+ further activates Ca2+-sensitive K+ channels leading to cellular KCl loss and further cell shrinkage. The cation channels are inhibited by Cl- anions, erythropoietin and dopamine. The sphingomyelinase is inhibited by high concentrations of urea. Thus, the high Cl- and urea concentrations in renal medulla presumably prevent the triggering of eryptosis despite hyperosmolarity. The mechanisms involved in eryptosis may not only affect the survival of erythrocytes but may be similarly operative in nucleated cells exposed to osmotic shock.

Ion channels in cell proliferation and apoptotic cell death

Cell proliferation and apoptosis are paralleled by altered regulation of ion channels that play an active part in the signaling of those fundamental cellular mechanisms. Cell proliferation must--at some time point--increase cell volume and apoptosis is typically paralleled by cell shrinkage. Cell volume changes require the participation of ion transport across the cell membrane, including appropriate activity of Cl- and K+ channels. Besides regulating cytosolic Cl- activity, osmolyte flux and, thus, cell volume, most Cl- channels allow HCO3- exit and cytosolic acidification, which inhibits cell proliferation and favors apoptosis. K+ exit through K+ channels may decrease intracellular K+ concentration, which in turn favors apoptotic cell death. K+ channel activity further maintains the cell membrane potential, a critical determinant of Ca2+ entry through Ca2+ channels. Cytosolic Ca2+ may trigger mechanisms required for cell proliferation and stimulate enzymes executing apoptosis. The switch between cell proliferation and apoptosis apparently depends on the magnitude and temporal organization of Ca2+ entry and on the functional state of the cell. Due to complex interaction with other signaling pathways, a given ion channel may play a dual role in both cell proliferation and apoptosis. Thus, specific ion channel blockers may abrogate both fundamental cellular mechanisms, depending on cell type, regulatory environment and condition of the cell. Clearly, considerable further experimental effort is required to fully understand the complex interplay between ion channels, cell proliferation and apoptosis.

PGE(2) in the regulation of programmed erythrocyte death

Hyperosmotic shock, energy depletion, or removal of extracellular Cl(-) activates Ca(2+)-permeable cation channels in erythrocyte membranes. Subsequent Ca(2+) entry induces erythrocyte shrinkage and exposure of phosphatidylserine (PS) at the erythrocyte surface. PS-exposing cells are engulfed by macrophages. The present study explored the signalling involved. Hyperosmotic shock and Cl(-) removal triggered the release of prostaglandin E(2) (PGE(2)). In whole-cell recording, activation of the cation channels by Cl(-) removal was abolished by the cyclooxygenase inhibitor diclophenac. In FACS analysis, phospholipase-A(2) inhibitors quinacrine and palmitoyltrifluoromethyl-ketone, and cyclooxygenase inhibitors acetylsalicylic acid and diclophenac, blunted the increase of PS exposure following Cl(-) removal. PGE(2) (but not thromboxane) induced cation channel activation, increase in cytosolic Ca(2+) concentration, cell shrinkage, PS exposure, calpain activation, and ankyrin-R degradation. The latter was attenuated by calpain inhibitors-I/II, while PGE(2)-induced PS exposure was not. In conclusion, hyperosmotic shock or Cl(-) removal stimulates erythrocyte PS exposure through PGE(2) formation and subsequent activation of Ca(2+)-permeable cation channels.

Involvement of ceramide in hyperosmotic shock-induced death of erythrocytes

Erythrocytes lack nuclei and mitochondria, the organelles important for apoptosis of nucleated cells. However, following increase of cytosolic Ca(2+) activity, erythrocytes undergo cell shrinkage, cell membrane blebbing and breakdown of phosphatidylserine asymmetry, all features typical for apoptosis in nucleated cells. The same events are observed following osmotic shock, an effect mediated in part by activation of Ca(2+)-permeable cation channels. However, erythrocyte death following osmotic shock is blunted but not prevented in the absence of extracellular Ca(2+) pointing to additional mechanisms. As shown in this study, osmotic shock (950 mOsm) triggers sphingomyelin breakdown and formation of ceramide. The stimulation of annexin binding following osmotic shock is mimicked by addition of ceramide or purified sphingomyelinase and significantly blunted by genetic (aSM-deficient mice) or pharmacologic (50 microM 3,4-dichloroisocoumarin) knockout of sphingomyelinase. The effect of ceramide is blunted but not abolished in the absence of Ca(2+). Conversely, osmotic shock-induced annexin binding is potentiated in the presence of sublethal concentrations of ceramide. In conclusion, ceramide and Ca(2+) entry through cation channels concert to trigger erythrocyte death during osmotic shock.

K+ channel activation by all three isoforms of serum- and glucocorticoid-dependent protein kinase SGK

The serum- and glucocorticoid-dependent kinase SGK1 was originally identified as a glucocorticoid-sensitive gene. Subsequently, the two homologous kinases SGK2 and SGK3 have been cloned, being products of distinct genes, which are differentially expressed and share 80% identity in amino acid sequence in their catalytic domains. While SGK1 has been shown to activate ion channels, including K(+) channels, the functions of SGK2 and SGK3 have not been examined. The present study was therefore performed to elucidate the effect of SGK1, SGK2, and SGK3 on electrical properties of renal epithelial cells. To this end human embryonic kidney (HEK293) cells were transfected with the kinases and ion-channel activity determined using the patch-clamp technique. In non-transfected cells and in cells transfected with the empty GFP construct a voltage-gated K(+) current was observed amounting to 303+/-19 pA ( n=13) and 299+/-29 pA ( n=23), respectively. Transfection with SGK1, SGK2 or SGK3 increased the voltage-gated K(+) current to 1056+/-152 pA ( n=17), 555+/-47 pA ( n=17), and 775+/-98 pA ( n=16), respectively. The K(+) current was fully blocked by 3 mM tetraethylammonium chloride and inhibited 45% by the Kv1 channel blocker margatoxin (10 nM). In dual electrode voltage-clamp experiments SGK isoforms up-regulated Kv1 voltage-gated K(+)channels expressed in Xenopus laevis oocytes. The present observations thus reveal a powerful stimulating effect of all three isoforms of SGK on K(+) channels. Those effects may participate in regulation of epithelial transport, cell proliferation, and neuromuscular excitability.

Cerebral localization and regulation of the cell volume-sensitive serum- and glucocorticoid-dependent kinase SGK1

The serum- and glucocorticoid-dependent kinase SGK1 is regulated by alterations of cell volume, whereby cell shrinkage increases and cell swelling decreases the transcription, expression and activity of SGK1. The kinase is expressed in all human tissues studied including the brain. The present study was performed to localize the sites of SGK1 transcription in the brain, to elucidate the influence of the hydration status on SGK1 transcription and to explore the functional significance of altered SGK1 expression. Northern blot analysis of human brain showed SGK1 to be expressed in all cerebral structures examined: amygdala, caudate nucleus, corpus callosum, hippocampus, substantia nigra, subthalamic nucleus and thalamus. In situ hybridization and immunohistochemistry in the rat revealed increased expression of SGK1 in neurons of the hippocampal area CA3 after dehydration, compared with similar slices from brains of euvolaemic rats. Additionally, several oligodendrocytes, a few microglial cells, but no astrocytes, were positive for SGK1. The abundance of SGK1 mRNA in the temporal lobe, including hippocampus, was increased by dehydration and SGK1 transcription in neuroblastoma cells was stimulated by an increase of extracellular osmolarity. Co-expression studies in Xenopus laevis oocytes revealed that SGK1 markedly increased the activity of the neuronal K+ channel Kv1.3. As activation of K+ channels modifies excitation of neuronal cells, SGK1 may participate in the regulation of neuronal excitability.

Na+ transport by the neural glutamine transporter ATA1

Transfer of glutamine between astrocytes and neurons is an essential part of the glutamate-glutamine cycle in the brain. Here we have investigated how the neural glutamine transporter (rATA1/GlnT) works. Rat ATA1 was expressed in Xenopus laevis oocytes and examined using two-electrode voltage-clamp recordings, ion-sensitive microelectrodes and tracer flux experiments. Glutamine transport via rATA1 was electrogenic and caused inward currents that did not reverse at positive holding potentials. Currents were induced by a variety of neutral amino acids in the following relative order Ala>Ser/Gln/Asn/His/Cys/Met >MeAIB/Gly>Thr/Pro/Tyr/Val, where MeAIB is the amino acid analogue N-methylaminoisobutyric acid. The uptake of glutamine and the corresponding currents depended on Na+ and pH. Hill-coefficient and flux studies with 22NaCl indicated a cotransport stoichiometry 1 Na+ per transport cycle. The transporter also showed uncoupled Na+ transport, particularly when alanine was used as the substrate. Although substrate uptake increased strongly with increasing pH, no change of intracellular pH was observed during transport. A decrease of the intracellular pH similarly inhibited glutamine transport via ATA1, suggesting that the pH dependence was an allosteric effect on the transporter.

NOD/LtSz-Rag1null mice: an immunodeficient and radioresistant model for engraftment of human hematolymphoid cells, HIV infection, and adoptive transfer of NOD mouse diabetogenic T cells

Development of a small animal model for the in vivo study of human immunity and infectious disease remains an important goal, particularly for investigations of HIV vaccine development. NOD/Lt mice homozygous for the severe combined immunodeficiency (Prkdcscid) mutation readily support engraftment with high levels of human hematolymphoid cells. However, NOD/LtSz-scid mice are highly radiosensitive, have short life spans, and a small number develop functional lymphocytes with age. To overcome these limitations, we have backcrossed the null allele of the recombination-activating gene (Rag1) for 10 generations onto the NOD/LtSz strain background. Mice deficient in RAG1 activity are unable to initiate V(D)J recombination in Ig and TCR genes and lack functional T and B lymphocytes. NOD/LtSz-Rag1null mice have an increased mean life span compared with NOD/LtSz-scid mice due to a later onset of lymphoma development, are radioresistant, and lack serum Ig throughout life. NOD/LtSz-Rag1null mice were devoid of mature T or B cells. Cytotoxic assays demonstrated low NK cell activity. NOD/LtSz-Rag1null mice supported high levels of engraftment with human lymphoid cells and human hemopoietic stem cells. The engrafted human T cells were readily infected with HIV. Finally, NOD/LtSz-Rag1null recipients of adoptively transferred spleen cells from diabetic NOD/Lt+/+ mice rapidly developed diabetes. These data demonstrate the advantages of NOD/LtSz-Rag1null mice as a radiation and lymphoma-resistant model for long-term analyses of engrafted human hematolymphoid cells or diabetogenic NOD lymphoid cells.

Supportive and concrete services for teenage oncology patients

The social worker involved with the Adolescent Unit at Roswell Park Memorial Institute offers two types of services--concrete and supportive. The concrete services include referrals to community agencies and assistance with transportation, discharge plans, and financial aid. The supportive services include counseling with patients, parents, spouses, and significant others. Helping the young patients deal with diagnoses, treatments, and possible death are major focal points of the social workers' involvement. Leading the patient and the family to an understanding of the changes they are experiencing and assisting them in their adjustment are tasks the social worker frequently faces. At Roswell Park Memorial Institute, the composition the staff assigned to the Adolescent Unit, the unit's physical layout, the family suppers, the availability of peer support for both patients and parents, and the concentrated team approach all contribute to the accomplishment of social work goals for teenage oncology patients and their families.