Activator protein-1 contributes to high NaCl-induced increase in tonicity-responsive enhancer/osmotic response element-binding protein transactivating activity

Stress Factors as Possible Regulators of Pluripotent Stem Cell Survival and Differentiation

In recent years, extensive research efforts have been directed toward pluripotent stem cells, primarily due to their remarkable capacity for pluripotency. This unique attribute empowers these cells to undergo self-renewal and differentiate into various cell types originating from the ectoderm, mesoderm, and endoderm germ layers. The delicate balance and precise regulation of self-renewal and differentiation are essential for the survival and functionality of these cells. Notably, exposure to specific environmental stressors can activate numerous transcription factors, initiating a diverse array of stress response pathways. These pathways play pivotal roles in regulating gene expression and protein synthesis, ultimately aiming to preserve cell survival and maintain cellular functions. Reactive oxygen species, heat shock, hypoxia, osmotic stress, DNA damage, endoplasmic reticulum stress, and mechanical stress are among the examples of such stressors. In this review, we comprehensively discuss the impact of environmental stressors on the growth of embryonic cells. Furthermore, we provide a summary of the distinct stress response pathways triggered when pluripotent stem cells are exposed to different environmental stressors. Additionally, we highlight recent discoveries regarding the role of such stressors in the generation, differentiation, and self-renewal of induced pluripotent stem cells.

MAGED2 controls vasopressin-induced aquaporin-2 expression in collecting duct cells

Mutations in the Melanoma-Associated Antigen D2 (MAGED2) cause antenatal Bartter syndrome type 5 (BARTS5). This rare disease is characterized by perinatal loss of urinary concentration capability and large urine volumes. The underlying molecular mechanisms of this disease are largely unclear. Here, we study the effect of MAGED2 knockdown on kidney cell cultures using proteomic and phosphoproteomic analyses. In HEK293T cells, MAGED2 knockdown induces prominent changes in protein phosphorylation rather than changes in protein abundance. MAGED2 is expressed in mouse embryonic kidneys and its expression declines during development. MAGED2 interacts with G-protein alpha subunit (GNAS), suggesting a role in G-protein coupled receptors (GPCR) signalling. In kidney collecting duct cell lines, Maged2 knockdown subtly modulated vasopressin type 2 receptor (V2R)-induced cAMP-generation kinetics, rewired phosphorylation-dependent signalling, and phosphorylation of CREB. Maged2 knockdown resulted in a large increase in aquaporin-2 abundance during long-term V2R activation. The increase in aquaporin-2 protein was mediated transcriptionally. Taken together, we link MAGED2 function to cellular signalling as a desensitizer of V2R-induced aquaporin-2 expression. SIGNIFICANCE: In most forms of Bartter Syndrome, the underlying cause of the disease is well understood. In contrast, the role of MAGED2 mutations in a newly discovered form of Bartter Syndrome (BARTS5) is unknown. In our manuscript we could show that MAGED2 modulates vasopressin-induced protein and phosphorylation patterns in kidney cells, providing a broad basis for further studies of MAGED2 function in development and disease.

NFAT5-Mediated Signalling Pathways in Viral Infection and Cardiovascular Dysfunction

The nuclear factor of activated T cells 5 (NFAT5) is well known for its sensitivity to cellular osmolarity changes, such as in the kidney medulla. Accumulated evidence indicates that NFAT5 is also a sensitive factor to stress signals caused by non-hypertonic stimuli such as heat shock, biomechanical stretch stress, ischaemia, infection, etc. These osmolality-related and -unrelated stimuli can induce NFAT5 upregulation, activation and nuclear accumulation, leading to its protective role against various detrimental effects. However, dysregulation of NFAT5 expression may cause pathological conditions in different tissues, leading to a variety of diseases. These protective or pathogenic effects of NFAT5 are dictated by the regulation of its target gene expression and activation of its signalling pathways. Recent studies have found a number of kinases that participate in the phosphorylation/activation of NFAT5 and related signal proteins. Thus, this review will focus on the NFAT5-mediated signal transduction pathways. As for the stimuli that upregulate NFAT5, in addition to the stresses caused by hyperosmotic and non-hyperosmotic environments, other factors such as miRNA, long non-coding RNA, epigenetic modification and viral infection also play an important role in regulating NFAT5 expression; thus, the discussion in this regard is another focus of this review. As the heart, unlike the kidneys, is not normally exposed to hypertonic environments, studies on NFAT5-mediated cardiovascular diseases are just emerging and rapidly progressing. Therefore, we have also added a review on the progress made in this field of research.

Potential Role of Gene Regulator NFAT5 in the Pathogenesis of Diabetes Mellitus

Nuclear factor of activated T cells 5 (NFAT5), a Rel/nuclear factor- (NF-) κB family member, is the only known gene regulator of the mammalian adaptive response to osmotic stress. Exposure to elevated glucose increases the expression and nuclear translocation of NFAT5, as well as NFAT5-driven transcriptional activity in vivo and in vitro. Increased expression of NFAT5 is closely correlated with the progression of diabetes in patients. The distinct structure of NFAT5 governs its physiological and pathogenic roles, indicating its opposing functions. The ability of NFAT5 to maintain cell homeostasis and proliferation is impaired in patients with diabetes. NFAT5 promotes the formation of aldose reductase, pathogenesis of diabetic vascular complications, and insulin resistance. Additionally, NFAT5 activates inflammation at a very early stage of diabetes and induces persistent inflammation. Recent studies revealed that NFAT5 is an effective therapeutic target for diabetes. Here, we describe the current knowledge about NFAT5 and its relationship with diabetes, focusing on its diverse regulatory functions, and highlight the importance of this protein as a potential therapeutic target in patients with diabetes.

Emerging new role of NFAT5 in inducible nitric oxide synthase in response to hypoxia in mouse embryonic fibroblast cells

We previously described the protective role of the nuclear factor of activated T cells 5 (NFAT5) during hypoxia. Alternatively, inducible nitric oxide synthase (iNOS) is also induced by hypoxia. Some evidence indicates that NFAT5 is essential for the expression of iNOS in Toll-like receptor-stimulated macrophages and that iNOS inhibition increases NFAT5 expression in renal ischemia-reperfusion. Here we studied potential NFAT5 target genes stimulated by hypoxia in mouse embryonic fibroblast (MEF) cells. We used three types of MEF cells associated with NFAT5 gene: NFAT5 wild type (MEF-NFAT5+/+), NFAT5 knockout (MEF-NFAT5-/-), and NFAT5 dominant-negative (MEF-NFAT5Δ/Δ) cells. MEF cells were exposed to 21% or 1% O2 in a time course curve of 48 h. We found that, in MEF-NFAT5+/+ cells exposed to 1% O2, NFAT5 was upregulated and translocated into the nuclei, and its transactivation domain activity was induced, concomitant with iNOS, aquaporin 1 (AQP-1), and urea transporter 1 (UTA-1) upregulation. Interestingly, in MEF-NFAT5-/- or MEF-NFAT5Δ/Δ cells, the basal levels of iNOS and AQP-1 expression were strongly downregulated, but not for UTA-1. The upregulation of AQP-1, UTA-1, and iNOS by hypoxia was blocked in both NFAT5-mutated cells. The iNOS induction by hypoxia was recovered in MEF-NFAT5-/- MEF cells, when recombinant NFAT5 protein expression was reconstituted, but not in MEF-NFAT5Δ/Δ cells, confirming the dominant-negative effect of MEF-NFAT5Δ/Δ cells. We did not see the rescue effect on AQP-1 expression. This work provides novel and relevant information about the signaling pathway of NFAT5 during responses to oxygen depletion in mammalian cells and suggests that the expression of iNOS induced by hypoxia is dependent on NFAT5.

Regulation of Inflammatory Functions of Macrophages and T Lymphocytes by NFAT5

The transcription factor NFAT5, also known as TonEBP, belongs to the family of Rel homology domain-containing factors, which comprises the NF-κB proteins and the calcineurin-dependent NFAT1 to NFAT4. NFAT5 shares several structural and functional features with other Rel-family factors, for instance it recognizes DNA elements with the same core sequence as those bound by NFAT1 to 4, and like NF-κB it responds to Toll-like receptors (TLR) and activates macrophage responses to microbial products. On the other hand, NFAT5 is quite unique among Rel-family factors as it can be activated by hyperosmotic stress caused by elevated concentrations of extracellular sodium ions. NFAT5 regulates specific genes but also others that are inducible by NF-κB and NFAT1 to 4. The ability of NFAT5 to do so in response to hypertonicity, microbial products, and inflammatory stimuli may extend the capabilities of immune cells to mount effective anti-pathogen responses in diverse microenvironment and signaling conditions. Recent studies identifying osmostress-dependent and -independent functions of NFAT5 have broadened our understanding of how NFAT5 may modulate immune function. In this review we focus on the role of NFAT5 in macrophages and T cells in different contexts, discussing findings from in vivo mouse models of NFAT5 deficiency and reviewing current knowledge on its mechanisms of regulation. Finally, we propose several questions for future research.

COX-2 expression mediated by calcium-TonEBP signaling axis under hyperosmotic conditions serves osmoprotective function in nucleus pulposus cells

The nucleus pulposus (NP) of intervertebral discs experiences dynamic changes in tissue osmolarity because of diurnal loading of the spine. TonEBP/NFAT5 is a transcription factor that is critical in osmoregulation as well as survival of NP cells in the hyperosmotic milieu. The goal of this study was to investigate whether cyclooxygenase-2 (COX-2) expression is osmoresponsive and dependent on TonEBP, and whether it serves an osmoprotective role. NP cells up-regulated COX-2 expression in hyperosmotic media. The induction of COX-2 depended on elevation of intracellular calcium levels and p38 MAPK pathway, but independent of calcineurin signaling as well as MEK/ERK and JNK pathways. Under hyperosmotic conditions, both COX-2 mRNA stability and its proximal promoter activity were increased. The proximal COX-2 promoter (-1840/+123 bp) contained predicted binding sites for TonEBP, AP-1, NF-κB, and C/EBP-β. While COX-2 promoter activity was positively regulated by both AP-1 and NF-κB, AP-1 had no effect and NF-κB negatively regulated COX-2 protein levels under hyperosmotic conditions. On the other hand, TonEBP was necessary for both COX-2 promoter activity and protein up-regulation in response to hyperosmotic stimuli. Ex vivo disc organ culture studies using hypomorphic TonEBP^+/- mice confirmed that TonEBP is required for hyperosmotic induction of COX-2. Importantly, the inhibition of COX-2 activity under hyperosmotic conditions resulted in decreased cell viability, suggesting that COX-2 plays a cytoprotective and homeostatic role in NP cells for their adaptation to dynamically loaded hyperosmotic niches.

Dietary Salt Exacerbates Experimental Colitis

The Western diet is characterized by high protein, sugar, fat, and low fiber intake, and is widely believed to contribute to the incidence and pathogenesis of inflammatory bowel disease (IBD). However, high sodium chloride salt content, a defining feature of processed foods, has not been considered as a possible environmental factor that might drive IBD. We set out to bridge this gap. We examined murine models of colitis on either a high salt diet (HSD) or a low salt diet. We demonstrate that an HSD exacerbates inflammatory pathology in the IL-10-deficient murine model of colitis relative to mice fed a low salt diet. This was correlated with enhanced expression of numerous proinflammatory cytokines. Surprisingly, sodium accumulated in the colons of mice on an HSD, suggesting a direct effect of salt within the colon. Similar to the IL-10-deficient model, an HSD also enhanced cytokine expression during infection by Salmonella typhimurium This occurred in the first 3 d of infection, suggesting that an HSD potentiates an innate immune response. Indeed, in cultured dendritic cells we found that high salt media potentiates cytokine expression downstream of TLR4 activation via p38 MAPK and SGK1. A third common colitis model, administration of dextran sodium sulfate, was hopelessly confounded by the high sodium content of the dextran sodium sulfate. Our results raise the possibility that high dietary salt is an environmental factor that drives increased inflammation in IBD.

Peptide affinity analysis of proteins that bind to an unstructured region containing the transactivating domain of the osmoprotective transcription factor NFAT5

NFAT5 is a transcription factor originally identified because it is activated by hypertonicity and that activation increases expression of genes that protect against the adverse effects of the hypertonicity. However, its targets also include genes not obviously related to tonicity. The transactivating domain of NFAT5 is contained in its COOH-terminal region, which is predicted to be unstructured. Unstructured regions are common in transcription factors particularly in transactivating domains where they can bind co-regulatory proteins essential to their function. To identify potential binding partners of NFAT5 from either cytoplasmic or nuclear HEK293 cell extracts, we used peptide affinity chromatography followed by mass spectrometry. Peptide aptamer-baits consisted of overlapping 20 amino acid peptides within the predicted COOH-terminal unstructured region of NFAT5. We identify a total of 351 unique protein preys that associate with at least one COOH-terminal peptide bait from NFAT5 in either cytoplasmic or nuclear extracts from cells incubated at various tonicities (NaCl varied). In addition to finding many proteins already known to associate with NFAT5, we found many new ones whose function suggest novel aspects of NFAT5 regulation, interaction, and function. Relatively few of the proteins pulled down by peptide baits from NFAT5 are generally involved in transcription, and most, therefore, are likely to be specifically related to the regulation of NFAT5 or its function. The novel associated proteins are involved with cancer, effects of hypertonicity on chromatin, development, splicing of mRNA, transcription, and vesicle trafficking.

The Trafficking of the Water Channel Aquaporin-2 in Renal Principal Cells-a Potential Target for Pharmacological Intervention in Cardiovascular Diseases

Arginine-vasopressin (AVP) stimulates the redistribution of water channels, aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. By this AVP directs 10% of the water reabsorption from the 170 L of primary urine that the human kidneys produce each day. This review discusses molecular mechanisms underlying the AVP-induced redistribution of AQP2; in particular, it provides an overview over the proteins participating in the control of its localization. Defects preventing the insertion of AQP2 into the plasma membrane cause diabetes insipidus. The disease can be acquired or inherited, and is characterized by polyuria and polydipsia. Vice versa, up-regulation of the system causing a predominant localization of AQP2 in the plasma membrane leads to excessive water retention and hyponatremia as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure or liver cirrhosis. This article briefly summarizes the currently available pharmacotherapies for the treatment of such water balance disorders, and discusses the value of newly identified mechanisms controlling AQP2 for developing novel pharmacological strategies. Innovative concepts for the therapy of water balance disorders are required as there is a medical need due to the lack of causal treatments.

Peptide affinity analysis of proteins that bind to an unstructured NH2-terminal region of the osmoprotective transcription factor NFAT5

NFAT5 is an osmoregulated transcription factor that particularly increases expression of genes involved in protection against hypertonicity. Transcription factors often contain unstructured regions that bind co-regulatory proteins that are crucial for their function. The NH2-terminal region of NFAT5 contains regions predicted to be intrinsically disordered. We used peptide aptamer-based affinity chromatography coupled with mass spectrometry to identify protein preys pulled down by one or more overlapping 20 amino acid peptide baits within a predicted NH2-terminal unstructured region of NFAT5. We identify a total of 467 unique protein preys that associate with at least one NH2-terminal peptide bait from NFAT5 in either cytoplasmic or nuclear extracts from HEK293 cells treated with elevated, normal, or reduced NaCl concentrations. Different sets of proteins are pulled down from nuclear vs. cytoplasmic extracts. We used GeneCards to ascertain known functions of the protein preys. The protein preys include many that were previously known, but also many novel ones. Consideration of the novel ones suggests many aspects of NFAT5 regulation, interaction and function that were not previously appreciated, for example, hypertonicity inhibits NFAT5 by sumoylating it and the NFAT5 protein preys include components of the CHTOP complex that desumoylate proteins, an action that should contribute to activation of NFAT5.

SIRT1 contributes to aldose reductase expression through modulating NFAT5 under osmotic stress: In vitro and in silico insights

So far, a myriad of molecules were characterized to modulate NFAT5 and its downstream targets. Among these NFAT5 modifiers, SIRT1 was proposed to have a promising role in NFAT5 dependent events, yet the exact underlying mechanism still remains obscure. Hence, the link between SIRT1 and NFAT5-aldose reductase (AR) axis under osmotic stress, was aimed to be delineated in this study. A unique osmotic stress model was generated and its mechanistic components were deciphered in U937 monocytes. In this model, AR expression and nuclear NFAT5 stabilization were revealed to be positively regulated by SIRT1 through utilization of pharmacological modulators. Overexpression and co-transfection studies of NFAT5 and SIRT1 further validated the contribution of SIRT1 to AR and NFAT5. The involvement of SIRT1 activity in these events was mediated via modification of DNA binding of NFAT5 to AR ORE region. Besides, NFAT5 and SIRT1 were also shown to co-immunoprecipitate under isosmotic conditions and this interaction was disrupted by osmotic stress. Further in silico experiments were conducted to investigate if SIRT1 directly targets NFAT5. In this regard, certain lysine residues of NFAT5, when kept deacetylated, were found to contribute to its DNA binding and SIRT1 was shown to directly bind K282 of NFAT5. Based on these in vitro and in silico findings, SIRT1 was identified, for the first time, as a novel positive regulator of NFAT5 dependent AR expression under osmotic stress in U937 monocytes.

RNA-Seq analysis of high NaCl-induced gene expression

High extracellular NaCl is known to change expression of numerous genes, many of which are regulated by the osmoprotective transcription factor nuclear factor of activated T cells-5 (NFAT5). In the present study we employed RNA-Seq to provide a comprehensive, unbiased account of genes regulated by high NaCl in mouse embryonic fibroblast cells (MEFs). To identify genes regulated by NFAT5 we compared wild-type MEFs (WT-MEFs) to MEFs in which mutation of the NFAT5 gene inhibits its transcriptional activity (Null-MEFs). In WT-MEFs adding NaCl to raise osmolality from 300 to 500 mosmol/kg for 24 h increases expression of 167 genes and reduces expression of 412. Raising osmolality through multiple passages (adapted cells) increases expression of 196 genes and reduces expression of 528. In Null-MEFs, after 24 h of high NaCl, expression of 217 genes increase and 428 decrease, while in adapted Null-MEFs 143 increase and 622 decrease. Fewer than 10% of genes are regulated in common between WT- and null-MEFs, indicating a profound difference in regulation of high-NaCl induced genes induced by NFAT5 compared with those induced in the absence of NFAT5. Based on our findings we suggest a mechanism for this phenomenon, which had previously been unexplained. The NFAT5 DNA-binding motif (osmotic response element) is overrepresented in the vicinity of genes that NFAT5 upregulates, but not genes that it downregulates. We used Gene Ontology and manual curation to determine the function of the genes targeted by NFAT5, revealing many novel consequences of NFAT5 transcriptional activity.

Global discovery of high-NaCl-induced changes of protein phosphorylation

High extracellular NaCl, such as in the renal medulla, can perturb and even kill cells, but cells mount protective responses that enable them to survive and function. Many high-NaCl-induced perturbations and protective responses are known, but the signaling pathways involved are less clear. Change in protein phosphorylation is a common mode of cell signaling, but there was no unbiased survey of protein phosphorylation in response to high NaCl. We used stable isotopic labeling of amino acids in cell culture coupled to mass spectrometry to identify changes in protein phosphorylation in human embryonic kidney (HEK 293) cells exposed to high NaCl. We reproducibly identify >8,000 unique phosphopeptides in 4 biological replicate samples with a 1% false discovery rate. High NaCl significantly changed phosphorylation of 253 proteins. Western analysis and targeted ion selection mass spectrometry confirm a representative sample of the phosphorylation events. We analyze the affected proteins by functional category to infer how altered protein phosphorylation might signal cellular responses to high NaCl, including alteration of cell cycle, cyto/nucleoskeletal organization, DNA double-strand breaks, transcription, proteostasis, metabolism of mRNA, and cell death.

Inhibitory phosphorylation of GSK-3β by AKT, PKA, and PI3K contributes to high NaCl-induced activation of the transcription factor NFAT5 (TonEBP/OREBP)

High NaCl activates the transcription factor nuclear factor of activated T cells 5 (NFAT5), leading to increased transcription of osmoprotective target genes. Kinases PKA, PI3K, AKT1, and p38α were known to contribute to the high NaCl-induced increase of NFAT5 activity. We now identify another kinase, GSK-3β. siRNA-mediated knock-down of GSK-3β increases NFAT5 transcriptional and transactivating activities without affecting high NaCl-induced nuclear localization of NFAT5 or NFAT5 protein expression. High NaCl increases phosphorylation of GSK-3β-S9, which inhibits GSK-3β. In GSK-3β-null mouse embryonic fibroblasts transfection of GSK-3β, in which serine 9 is mutated to alanine, so that it cannot be inhibited by phosphorylation at that site, inhibits high NaCl-induced NFAT5 transcriptional activity more than transfection of wild-type GSK-3β. High NaCl-induced phosphorylation of GSK-3β-S9 depends on PKA, PI3K, and AKT, but not p38α. Overexpression of PKA catalytic subunit α or of catalytically active AKT1 reduces inhibition of NFAT5 by GSK-3β, but overexpression of p38α together with its catalytically active upstream kinase, MKK6, does not. Thus, GSK-3β normally inhibits NFAT5 by suppressing its transactivating activity. When activated by high NaCl, PKA, PI3K, and AKT1, but not p38α, increase phosphorylation of GSK-3β-S9, which reduces the inhibitory effect of GSK-3β on NFAT5, and thus contributes to activation of NFAT5.

Mutations that reduce its specific DNA binding inhibit high NaCl-induced nuclear localization of the osmoprotective transcription factor NFAT5

The transcription factor nuclear factor of activated T cell 5 (NFAT5) is activated by the stress of hypertonicity (e.g., high NaCl). Increased expression of NFAT5 target genes causes accumulation of protective organic osmolytes and heat shock proteins. Under normotonic conditions (∼300 mosmol/kgH(2)O), NFAT5 is distributed between the nucleus and cytoplasm, hypertonicity causes it to translocate into the nucleus, and hypotonicity causes it to translocate into the cytoplasm. The mechanism of translocation is complex and not completely understood. NFAT5-T298 is a known contact site of NFAT5 with its specific DNA element [osmotic response element (ORE)]. In the present study, we find that mutation of NFAT5-T298 to alanine or aspartic acid not only reduces binding of NFAT5 to OREs (EMSA) but also proportionately reduces high NaCl-induced nuclear translocation of NFAT5. Combined mutation of other NFAT5 DNA contact sites (R293A/E299A/R302A) also greatly reduces both specific DNA binding and nuclear localization of NFAT5. NFAT5-T298 is a potential phosphorylation site, but, using protein mass spectrometry, we do not find phosphorylation at NFAT5-T298. Further, decreased high NaCl-induced nuclear localization of NFAT5 mutated at T298 does not involve previously known regulatory mechanisms, including hypotonicity-induced export of NFAT5, regulated by phosphorylation of NFAT5-S155, XPO1 (CRM1/exportin1)-mediated export of NFAT5 from the nucleus, or hypertonicity-induced elevation of NUP88, which enhances nuclear localization of NFAT5. We conclude that specific DNA binding of NFAT5 contributes to its nuclear localization, by mechanisms, as yet undetermined, but independent of ones previously described to regulate NFAT5 distribution.

NFAT5 is activated by hypoxia: role in ischemia and reperfusion in the rat kidney

The current hypothesis postulates that NFAT5 activation in the kidney's inner medulla is due to hypertonicity, resulting in cell protection. Additionally, the renal medulla is hypoxic (10–18 mmHg); however there is no information about the effect of hypoxia on NFAT5. Using in vivo and in vitro models, we evaluated the effect of reducing the partial pressure of oxygen (PO₂) on NFAT5 activity. We found that 1) Anoxia increased NFAT5 expression and nuclear translocation in primary cultures of IMCD cells from rat kidney. 2) Anoxia increased transcriptional activity and nuclear translocation of NFAT5 in HEK293 cells. 3) The dose-response curve demonstrated that HIF-1α peaked at 2.5% and NFAT5 at 1% of O₂. 4) At 2.5% of O₂, the time-course curve of hypoxia demonstrated earlier induction of HIF-1α gene expression than NFAT5. 5) siRNA knockdown of NFAT5 increased the hypoxia-induced cell death. 6) siRNA knockdown of HIF-1α did not affect the NFAT5 induction by hypoxia. Additionally, HIF-1α was still induced by hypoxia even when NFAT5 was knocked down. 7) NFAT5 and HIF-1α expression were increased in kidney (cortex and medulla) from rats subjected to an experimental model of ischemia and reperfusion (I/R). 7) Experimental I/R increased the NFAT5-target gene aldose reductase (AR). 8) NFAT5 activators (ATM and PI3K) were induced in vitro (HEK293 cells) and in vivo (I/R kidneys) with the same timing of NFAT5. 8) Wortmannin, which inhibits ATM and PI3K, reduces hypoxia-induced NFAT5 transcriptional activation in HEK293 cells. These results demonstrate for the first time that NFAT5 is induced by hypoxia and could be a protective factor against ischemic damage.

Innovative therapy for Classic Galactosemia - tale of two HTS

Classic Galactosemia is an autosomal recessive disorder caused by the deficiency of galactose-1-phosphate uridylyltransferase (GALT), one of the key enzymes in the Leloir pathway of galactose metabolism. While the neonatal morbidity and mortality of the disease are now mostly prevented by newborn screening and galactose restriction, long-term outcome for older children and adults with this disorder remains unsatisfactory. The pathophysiology of Classic Galactosemia is complex, but there is convincing evidence that galactose-1-phosphate (gal-1P) accumulation is a major, if not the sole pathogenic factor. Galactokinase (GALK) inhibition will eliminate the accumulation of gal-1P from both dietary sources and endogenous production, and efforts toward identification of therapeutic small molecule GALK inhibitors are reviewed in detail. Experimental and computational high-throughput screenings of compound libraries to identify GALK inhibitors have been conducted, and subsequent studies aimed to characterize, prioritize, as well as to optimize the identified positives have been implemented to improve the potency of promising compounds. Although none of the identified GALK inhibitors inhibits glucokinase and hexokinase, some of them cross-inhibit other related enzymes in the GHMP small molecule kinase superfamily. While this finding may render the on-going hit-to-lead process more challenging, there is growing evidence that such cross-inhibition could also lead to advances in antimicrobial and anti-cancer therapies.

Hyperosmotic stress strongly potentiates serum response factor (SRF)-dependent transcriptional activity in Ehrlich Lettré Ascites cells through a mechanism involving p38 mitogen-activated protein kinase

Long-term osmotic stress results in altered gene transcription, however, with the exception of the TonE/TonEBP system, the underlying mechanisms are poorly understood. We previously showed that upon osmotic shrinkage of Ehrlich Lettré Ascites (ELA) fibroblasts, the MEK1-ERK1/2 pathway is transiently inhibited while p38 MAPK is activated, in turn impacting on cell survival (Pedersen et al., 2007, Cell Physiol Biochem 20: 735-750). Here, we show that downstream of these kinases, two transcription factors with major roles in control of cell proliferation and death, serum response factor (SRF) and cAMP response element-binding protein (CREB) are differentially regulated in ELA cells. SRF Ser(103) phosphorylation and SRF-dependent transcriptional activity were strongly augmented 5-30 min and 24 h, respectively, after hyperosmotic stress (50% increase in extracellular ionic strength), in a p38 MAPK-dependent manner. In contrast, CREB Ser(133) was transiently dephosphorylated upon osmotic shrinkage. The ERK1/2 effector ribosomal S kinase (RSK) and the ERK1/2- and p38 MAPK effector mitogen- stress-activated protein kinase 1 (MSK1) both phosphorylate CREB at Ser(133) . RSK and MSK1 were dephosphorylated within 5 min of shrinkage. MSK1 phosphorylation recovered within 30 min in a p38-MAPK-dependent manner. CREB was transiently dephosphorylated after shrinkage in a manner exacerbated by p38 MAPK inhibition or MSK1 knockdown, but unaffected by inhibition of RSK. In conclusion, in ELA cells, hyperosmotic stress activates SRF in a p38 MAPK-dependent manner and transiently inactivates CREB, likely due to MSK1 inactivation. We suggest that these events contribute to shrinkage-induced changes in gene transcription and death/survival balance.

Is prenatal myo-inositol deficiency a mechanism of CNS injury in galactosemia?

Classic Galactosemia due to galactose-1-phosphate uridyltransferase (GALT) deficiency is associated with apparent diet-independent complications including cognitive impairment, learning problems and speech defects. As both galactose-1-phosphate and galactitol may be elevated in cord blood erythrocytes and amniotic fluid despite a maternal lactose-free diet, endogenous production of galactose may be responsible for the elevated fetal galactose metabolites, as well as postnatal CNS complications. A prenatal deficiency of myo-inositol due to an accumulation of both galactose-1- phosphate and galactitol may play a role in the production of the postnatal CNS dysfunction. Two independent mechanisms may result in fetal myo-inositol deficiency: competitive inhibition of the inositol monophosphatase1 (IMPA1)-mediated hydrolysis of inositol monophosphate by high galactose-1- phosphate levels leading to a sequestration of cellular myo-inositol as inositol monophosphate and galactitol-induced reduction in SMIT1-mediated myo-inositol transport. The subsequent reduction of myo-inositol within fetal brain cells could lead to inositide deficiencies with resultant perturbations in calcium and protein kinase C signaling, the AKT/mTOR/ cell growth and development pathway, cell migration, insulin sensitivity, vescular trafficking, endocytosis and exocytosis, actin cytoskeletal remodeling, nuclear metabolism, mRNA export and nuclear pore complex regulation, phosphatidylinositol-anchored proteins, protein phosphorylation and/or endogenous iron "chelation". Using a knockout animal model we have shown that a marked deficiency of myo-inositol in utero is lethal but the phenotype can be rescued by supplementing the drinking water of the pregnant mouse. If myo-inositol deficiency is found to exist in the GALT-deficient fetal brain, then the use of myo-inositol to treat the fetus via oral supplementation of the pregnant female may warrant consideration.

Mediator of DNA damage checkpoint 1 (MDC1) contributes to high NaCl-induced activation of the osmoprotective transcription factor TonEBP/OREBP

Background

Hypertonicity, such as induced by high NaCl, increases the activity of the transcription factor TonEBP/OREBP whose target genes increase osmoprotective organic osmolytes and heat shock proteins.

Methodology

We used mass spectrometry to analyze proteins that coimmunoprecipitate with TonEBP/OREBP in order to identify ones that might contribute to its high NaCl-induced activation.

Principal Findings

We identified 20 unique peptides from Mediator of DNA Damage Checkpoint 1 (MDC1) with high probability. The identification was confirmed by Western analysis. We used small interfering RNA knockdown of MDC1 to characterize its osmotic function. Knocking down MDC1 reduces high NaCl-induced increases in TonEBP/OREBP transcriptional and transactivating activity, but has no significant effect on its nuclear localization. We confirm six previously known phosphorylation sites in MDC1, but do not find evidence that high NaCl increases phosphorylation of MDC1. It is suggestive that MDC1 acts as a DNA damage response protein since hypertonicity reversibly increases DNA breaks, and other DNA damage response proteins, like ATM, also associate with TonEBP/OREBP and contribute to its activation by hypertonicity.

Conclusions/Significance

MDC1 associates with TonEBP/OREBP and contributes to high NaCl-induced increase of that factor's transcriptional activity.

Osmoprotective transcription factor NFAT5/TonEBP modulates nuclear factor-kappaB activity

Tonicity responsive binding protein (TonEBP) is a transcription factor that plays a key role in osmoprotection. Here, we demonstrate enhanced activity of prosurvival NF-κB—at the onset of hypertonic challenge that depends on p38 kinase—and Akt-dependent formation of p65-TonEBP complexes that bind to elements of NF-κB-responsive genes.

Tonicity-responsive binding-protein (TonEBP or NFAT5) is a widely expressed transcription factor whose activity is regulated by extracellular tonicity. TonEBP plays a key role in osmoprotection by binding to osmotic response element/TonE elements of genes that counteract the deleterious effects of cell shrinkage. Here, we show that in addition to this “classical” stimulation, TonEBP protects cells against hypertonicity by enhancing nuclear factor-κB (NF-κB) activity. We show that hypertonicity enhances NF-κB stimulation by lipopolysaccharide but not tumor necrosis factor-α, and we demonstrate overlapping protein kinase B (Akt)-dependent signal transduction pathways elicited by hypertonicity and transforming growth factor-α. Activation of p38 kinase by hypertonicity and downstream activation of Akt play key roles in TonEBP activity, IκBα degradation, and p65 nuclear translocation. TonEBP affects neither of these latter events and is itself insensitive to NF-κB signaling. Rather, we reveal a tonicity-dependent interaction between TonEBP and p65 and show that NF-κB activity is considerably enhanced after binding of NF-κB-TonEBP complexes to κB elements of NF-κB–responsive genes. We demonstrate the key roles of TonEBP and Akt in renal collecting duct epithelial cells and in macrophages. These findings reveal a novel role for TonEBP and Akt in NF-κB activation on the onset of hypertonic challenge.

c-Abl mediates high NaCl-induced phosphorylation and activation of the transcription factor TonEBP/OREBP

The transcription factor TonEBP/OREBP promotes cell survival during osmotic stress. High NaCl-induced phosphorylation of TonEBP/OREBP at tyrosine-143 was known to be an important factor in increasing its activity in cell culture. We now find that TonEBP/OREBP also is phosphorylated at tyrosine-143 in rat renal inner medulla, dependent on the interstitial osmolality. c-Abl seemed likely to be the kinase that phosphorylates TonEBP/OREBP because Y143 is in a consensus c-Abl phosphorylation site. We now confirm that, as follows. High NaCl increases c-Abl activity. Specific inhibition of c-Abl by imatinib, siRNA, or c-Abl kinase dead drastically reduces high NaCl-induced TonEBP/OREBP activity by reducing its nuclear location and transactivating activity. c-Abl associates with TonEBP/OREBP (coimmunoprecipitation) and phosphorylates TonEBP/OREBP-Y143 both in cell and in vitro. High NaCl-induced activation of ataxia telangiectasia mutated, previously known to contribute to activation of TonEBP/OREBP, depends on c-Abl activity. Thus, c-Abl is the kinase responsible for high NaCl-induced phosphorylation of TonEBP/OREBP-Y143, which contributes to its increased activity.

Contribution of SHP-1 protein tyrosine phosphatase to osmotic regulation of the transcription factor TonEBP/OREBP

Hypertonicity activates the transcription factor TonEBP/OREBP, resulting in increased expression of osmoprotective genes, including those responsible for accumulation of organic osmolytes and heat-shock proteins. Phosphorylation of TonEBP/OREBP contributes to its activation. Several of the kinases that are involved were previously identified, but the phosphatases were not. In the present studies we screened a genomewide human phosphatase siRNA library in human embryonic kidney (HEK)293 cells for effects on TonEBP/OREBP transcriptional activity. We found that siRNAs against 57 phosphatases significantly alter TonEBP/OREBP transcriptional activity during normotonicity (290 mosmol/kg) or hypertonicity (500 mosmol/kg, NaCl added) or both. Most siRNAs increase TonEBP/OREBP activity, implying that the targeted phosphatases normally reduce that activity. We further studied in detail SHP-1, whose knockdown by its specific siRNA increases TonEBP/OREBP transcriptional activity at 500 mosmol/kg. We confirmed that SHP-1 is inhibitory by overexpressing it, which reduces TonEBP/OREBP transcriptional activity at 500 mosmol/kg. SHP-1 dephosphorylates TonEBP/OREBP at a known regulatory site, Y143, both in vivo and in vitro. It inhibits TonEBP/OREBP by both reducing TonEBP/OREBP nuclear localization, which is Y143 dependent, and by lowering high NaCl-induced TonEBP/OREBP transactivating activity. SHP-1 coimmunoprecipitates with TonEBP/OREBP and vice versa, suggesting that they are physically associated in the cell. High NaCl inhibits the effect of SHP-1 on TonEBP/OREBP by increasing phosphorylation of SHP-1 on Ser591, which reduces its phosphatase activity and localization to the nucleus. Thus, TonEBP/OREBP is extensively regulated by phosphatases, including SHP-1, whose inhibition by high NaCl increases phosphorylation of TonEBP/OREBP at Y143, contributing to the nuclear localization and activation of TonEBP/OREBP.

TonEBP regulates hyperosmolality-induced arginine vasotocin gene expression in the chick (Gallus domesticus)

Arginine vasotocin (AVT) is expressed mainly in the paraventircular and supraoptic nuclei of the hypothalamus in chicken. This peptide is known to act as an antidiuretic hormone and its gene expression is stimulated by hyperosmolality. However, the transcription factors that regulate the AVT gene expression induced by hyperosmolality are still unknown. In this study, we examined the role of hyper-tonicity enhancer binding protein (TonEBP) in the transcriptional regulation of AVT gene in chicken. TonEBP mRNA expression levels increased at 1h after salt-loading treatment in the hypothalamus. This increase preceded that in AVT and c-fos mRNA expression. Intracerebroventricular injections of TonEBP antisense oligonucleotides, before the salt-loading treatment, prevented the increase in AVT gene expression. These results, all together, suggest that the transcription factor TonEBP may be involved in the regulation of AVT genes expression in response to a hyperosmotic environment in chicken.

Hyperosmolar sodium chloride, p38 mitogen activated protein and cytokine-mediated inflammation

Although there is increasing clinical evidence that high salt intake contributes to cardiovascular events and deaths seemingly independent of hypertension, the molecular mechanism for increased atherogenesis remains unclear. Vessel wall inflammation secondary to proinflammatory cytokines is one mechanism for atherogenesis. The role of mitogen activated protein kinase (MAPK) p38 in cytokine production such as IL-1, TNF-alpha, and IL-8 are well established. The link between inflammation and salt intake likely includes p38 MAPK as hyperosmolar sodium chloride triggers phosphorylation of p38 MAPK and stimulates gene expression and synthesis of proinflammatory cytokines. Hence a possible link of high salt intake, inflammation, and atherogenesis may be one molecular mechanism for the association of high salt intake and cardiovascular events.

Cellular stress response pathway system as a sentinel ensemble in toxicological screening

High costs, long test times, and societal concerns related to animal use have required the development of in vitro assays for the rapid and cost-effective toxicological evaluation and characterization of compounds in both the pharmaceutical and environmental arenas. Although the pharmaceutical industry has developed very effective, high-throughput in vitro assays for determining the therapeutic potential of compounds, the application of this approach to toxicological screening has been limited. A primary reason for this is that while drug candidate screens are directed to a specific target/mechanism, xenobiotics can cause toxicity through any of a myriad of undefined interactions with cellular components and processes. Given that it is not practical to design assays that can interrogate each potential toxicological target, an integrative approach is required if there is to be a rapid and low-cost toxicological evaluation of chemicals. Cellular stress response pathways offer a viable solution to the creation of a set of integrative assays as there is a limited and hence manageable set (a small ensemble of 10 or less) of major cellular stress response pathways through which cells mount a homoeostatic response to toxicants and which also participate in cell fate/death decisions. Further, over the past decades, these pathways have been well characterized at a molecular level thereby enabling the development of high-throughput cell-based assays using the components of the pathways. Utilization of the set of cellular stress response pathway-based assays as indicators of toxic interactions of chemicals with basic cellular machinery will potentially permit the clustering of chemicals based on biological response profiles of common mode of action (MOA) and also the inference of the specific MOA of a toxicant. This article reviews the biochemical characteristics of the stress response pathways, their common architecture that enables rapid activation during stress, their participation in cell fate decisions, the essential nature of these pathways to the organism, and the biochemical basis of their cross-talk that permits an assay ensemble screening approach. Subsequent sections describe how the stress pathway ensemble assay approach could be applied to screening potentially toxic compounds and discuss how this approach may be used to derive toxicant MOA from the biological activity profiles that the ensemble strategy provides. The article concludes with a review of the application of the stress assay concept to noninvasive in vivo assessments of chemical toxicants.

Expression and function of NFAT5 in medullary thick ascending limb (mTAL) cells

The contribution of nuclear factor of activated T cells 5 (NFAT5) to the regulation of tumor necrosis factor-alpha (TNF) production in medullary thick ascending limb (mTAL) cells is unclear. RT-PCR analysis was performed on primary cultures of mouse mTAL cells and freshly isolated mTAL tubules to determine which NFAT isoforms are present in this nephron segment. Primer pairs were designed, based on published sequences for mouse NFAT1-5, to produce fragments of approximately 200 bp. Analysis of PCR products by gel electrophoresis and subsequent DNA sequencing indicated that cells and tubules contained mRNA for all five NFAT isoforms. The relative expression of NFAT isoforms was then determined using quantitative real-time RT-PCR. The data indicate that NFAT isoforms 5 >/= 1 are the predominant isoforms present in mTAL cells and tubules. Western blot analysis demonstrated constitutive expression of NFAT5 in nuclear extracts from mTAL tubules and primary culture cells; expression in mTAL cells also was detected by immunofluorescence. Expression of NFAT5 was increased in mTAL cells transiently transfected with an NFAT5 overexpression vector (pcDNA3.1-NFAT5), resulting in increased basal and calcium-sensing receptor (CaR)-mediated TNF production. Transient transfection of mTAL cells with a small hairpin RNA vector that targeted exon 8 of NFAT5 (U6-N5 ex8) significantly inhibited TNF promoter activity. Transient transfection with U6-N5 ex8 also reduced nuclear expression of NFAT5, TNF mRNA accumulation, and attenuated CaR-mediated activation of Cl(-) entry into polarized mTAL cells. Collectively, these data suggest that activation of NFAT5 is part of a TNF-dependent pathway that inhibits apical Cl(-) influx in the mTAL after activation of CaR.

EGF receptor signaling is involved in expression of osmoprotective TonEBP target gene aldose reductase under hypertonic conditions

Renal medullary cells adapt to their hyperosmotic environment by enhanced expression of various osmoprotective genes. Although it is clearly established that TonEBP contributes to the expression of these genes, neither the precise signaling mechanism by which hypertonicity activates TonEBP is completely understood, nor is it known whether a membrane-bound osmosenser, corresponding to yeast and bacteria, is present in mammalian cells. We found evidence that metalloproteinase (MMP)-dependent activation of the epidermal growth factor receptor (EGFR) signals to TonEBP and stimulates the expression of the TonEBP target gene aldose reductase (AR) under hypertonic conditions. Phosphorylation of EGFR and the downstream MAP kinases ERK1/2 and p38 was significantly enhanced by high NaCl in Madin-Darby canine kidney (MDCK) cells. Conversely, the broad-spectrum MMP inhibitor GM6001 or the EGFR inhibitor AG1478 diminished phosphorylation of EGFR, p38, and ERK1/2, the induction of AR mRNA and protein, and AR promoter reporter activity in response to hypertonicity. Accordingly, neutralizing antibodies against the putative EGFR ligand transforming growth factor-alpha (TGF-alpha) abolished AR induction during osmotic stress. Furthermore, tonicity-induced phosphorylation of p38 and ERK1/2 and expression of AR were reduced significantly in MDCK cells transfected with a dominant-negative Ras construct. These effects were not caused by reduced nuclear abundance of TonEBP during osmotic stress; however, inhibition of EGFR or p38 diminished TonEBP transactivation activity under hypertonic conditions. The contribution of MMP/EGFR signaling in vivo was confirmed in C57BL/6 mice, in which treatment with GM6001 was associated with reduced AR induction following dehydration. Taken together, these results indicate that osmotic stress induces MMP-dependent activation of EGFR, likely via shedding of TGF-alpha, and downstream activation of Ras and the MAP kinases p38 and ERK1/2, which stimulate TonEBP transactivation activity. This EGFR-Ras-MAPK pathway contributes to TonEBP transcriptional activation and targets gene expression during osmotic stress, thus establishing a membrane-associated signal input that contributes to the regulation of TonEBP activity.

Computational modeling and in silico analysis of differential regulation of myo-inositol catabolic enzymes in Cryptococcus neoformans

Background

Inositol is a key cellular metabolite for many organisms. Cryptococcus neoformans is an opportunistic pathogen which primarily infects the central nervous system, a region of high inositol concentration, of immunocompromised individuals. Through the use of myo-inositol oxygenase C. neoformans can catabolize inositol as a sole carbon source to support growth and viability.

Results

Three myo-inositol oxygenase gene sequences were identified in the C. neoformans genome. Differential regulation was suggested by computational analyses of the three gene sequences. This included examination of the upstream regulatory regions, identifying ORE/TonE and UAS_INO sequences, conserved introns/exons, and in frame termination sequences. Homology modeling of the proteins encoded by these genes revealed key differences in the myo-inositol active site.

Conclusion

The results suggest there are two functional copies of the myo-inositol oxygenase gene in the C. neoformans genome. The functional genes are differentially expressed in response to environmental inositol concentrations. Both the upstream regulatory regions of the genes and the structure of the specific proteins suggest that MIOX1 would function when inositol concentrations are low, whereas MIOX2 would function when inositol concentrations are high.