PKC-α contributes to high NaCl-induced activation of NFAT5 (TonEBP/OREBP) through MAPK ERK1/2

Impact of environmental salinity on the MAPK-NFAT5 pathway in Trachemys scripta elegans and its role in osmoregulaton

Globally, sea level rise (SLR) leads to salinization of coastal freshwater, in where organisms might be affected and in turn promote osmoregulation and adaptation in response to freshwater salinization. Trachemys scripta elegans a freshwater turtle species, exhibits remarkable tolerance to varying salinity environments, yet the underlying regulatory mechanisms remain poorly understood. This study aimed to elucidate the molecular mechanisms of osmoregulation in this species based on previous RNA-seq data. Our findings revealed that when exposed to 5 PSU (5 ‰) and 15 PSU (15 ‰) salinities, the turtles exhibited increased concentrations of ions (Na+, K+) and urea in urine, along with elevated osmotic pressures in both plasma and urine. Additionally, the protein levels of aquaporins (AQPs) and transporters of ions and organic osmolytes in the kidney were upregulated in saline water. Notably, the transcriptional level of the hypertonic regulator NFAT5 was significantly elevated, accompanied by an increase in phosphorylated NFAT5 levels in the nucleus of renal tubular epithelial cells. Furthermore, we observed upregulated phosphorylated levels of MAPKs in saline water. The use of MAPK inhibitors effectively blocked the transcription of NFAT5 and osmoregulatory target genes. Collectively, these results suggest that T. scripta elegans activates the MAPK-NFAT5 signaling pathway to modulate osmotic pressure in adaptation to saline water environments. Our study provides valuable insights into the osmoregulatory responses of aquatic organisms to saline environments and aids in understanding the adaptability of organisms inhabiting coastal areas facing rising sea levels.

E. hellem Ser/Thr protein phosphatase PP1 targets the DC MAPK pathway and impairs immune functions

Microsporidia are difficult to be completely eliminated once infected, and the persistence disrupts host cell functions. Here in this study, we aimed to elucidate the impairing effects and consequences of microsporidia on host DCs. Enterocytozoon hellem, one of the most commonly diagnosed zoonotic microsporidia species, was applied. In vivo models demonstrated that E. hellem-infected mice were more susceptible to further pathogenic challenges, and DCs were identified as the most affected groups of cells. In vitro assays revealed that E. hellem infection impaired DCs' immune functions, reflected by down-regulated cytokine expressions, lower extent of maturation, phagocytosis ability, and antigen presentations. E. hellem infection also detained DCs' potencies to prime and stimulate T cells; therefore, host immunities were disrupted. We found that E. hellem Ser/Thr protein phosphatase PP1 directly interacts with host p38α (MAPK14) to manipulate the p38α(MAPK14)/NFAT5 axis of the MAPK pathway. Our study is the first to elucidate the molecular mechanisms of the impairing effects of microsporidia on host DCs' immune functions. The emergence of microsporidiosis may be of great threat to public health.

Proteomic analyses of urinary exosomes identify novel potential biomarkers for early diagnosis of sickle cell nephropathy, a sex-based study

Sickle cell nephropathy (SCN) is a leading cause of morbidity and mortality in sickle cell disease (SCD). Early intervention is crucial for mitigating its effects. However, current diagnostic methods rely on generic tests and may not detect SCN until irreversible renal damage occurs. Therefore, specific biomarkers for early diagnosis of SCN are needed. Urinary exosomes, membrane-bound vesicles secreted by renal podocytes and epithelial cells, contain both common and cell type-specific membrane and cytosolic proteins, reflecting the physiologic and pathophysiologic states of the kidney. Using proteomics, we analyzed the proteomes of urinary exosomes from humanized SCD mice at 2 months (without albuminuria) and 4 months (with albuminuria) of age. Excretion of 164 proteins were significantly increased and 176 proteins was significantly decreased in the exosomes when mice developed albuminuria. Based on the relevance to SCD, chronic kidney disease and Western blot confirmation in mice, we analyzed protein abundance of heparanase, cathepsin C, α2-macroglobulin and sarcoplasmic endoplasmic Ca2+ ATPase-3 (SERCA3) in the urinary exosomes and urine of 18 SCD subjects without albuminuria and 12 subjects with albuminuria using Western blot analyses. Both male and female subjects increased or tended to increase the excretion of these proteins in their urinary exosomes upon developing albuminuria, but female subjects demonstrated stronger correlations between the excretion of these proteins and urine albumin creatinine ratio (UACR) compared to male subjects. In contrast, exosomal excretion of Tamm-Horsfall protein, β-actin and SHP-1 was independent of albuminuria. These findings provide a foundation for a time-course study to determine whether increases in the levels of these proteins precede the onset of albuminuria in patients, which will help determine the potential of these proteins as biomarkers for early detection of SCN.

Nephroprotective plant species used in traditional Mayan Medicine for renal-associated diseases

ETHNOPHARMACOLOGICAL RELEVANCE

The prevalence of kidney disease has increased rapidly in recent years and has emerged as one of the leading causes of mortality worldwide. Natural products have been suggested as valuable nephroprotective agents due to their multi-target and synergistic effects on modulating important proteins involved in kidney injury. There is a large number of plant species that have been used traditionally for kidney-related conditions in Mesoamerican medicine by different cultural groups that could provide a valuable source of nephroprotective therapeutic candidates and could lead to potential drug discovery.

AIM OF REVIEW

This review aims to provide an overview of the currently known efficacy of plant species used traditionally in Mesoamerica by Mayan groups to treat kidney-related conditions and to analyze the phytochemical, pharmacological, molecular, toxicological, and clinical evidence to contribute to public health efforts and for directing future research.

METHODS

Primary sources of plant use reports for traditional kidney-related disorders in Mesoamerica were searched systematically from library catalogs, theses, and scientific databases (PubMed, Google Scholar; and Science Direct), and were filtered according to usage frequency in Mayan groups and plant endemism. The database of traditional plants was further analyzed based on associations with published reports of the phytochemical, pharmacological, molecular, toxicological, and clinical evidence.

RESULTS

The most reported kidney-related conditions used traditionally in Mayan medicine involve reducing renal damage (a cultural interpretation that considers an inflammatory or infectious condition), cleaning or purifying the blood and kidney, reducing kidney pain, and eliminating kidney stones. A total of 208 plants used for kidney-related problems by 10 Mayan groups were found, representing 143 native species, where only 42 have reported pharmacological activity against kidney damage, mainly approached by in vitro and in vivo models of chemical- or drug-induced nephrotoxicity, diabetes nephropathy, and renal injury produced by hypertension. Nephroprotective effects are mainly mediated by reducing oxidative stress, inflammatory response, fibrosis mechanisms, and apoptosis in the kidney. The most common nephroprotective compounds associated with traditional Mayan medicine were flavonoids, terpenoids, and phenolic acids. The most widely studied traditional plants in terms of pharmacological evidence, bioactive compounds, and mechanisms of action, are Annona muricata L., Carica papaya L., Ipomoea batatas (L.) Lam., Lantana camara L., Sechium edule (Jacq.) Sw., Tagetes erecta L., and Zea mays L. Most of the plant species with reported pharmacological activity against kidney damage were considered safe in toxicological studies.

CONCLUSION

Available pharmacological reports suggest that several herbs used in traditional Mayan medicine for renal-associated diseases may have nephroprotective effects and consistent pharmacological evidence, nephroprotective compounds, and mechanisms of action in different models of kidney injury. However, more research is required to fully understand the potential of traditional Mayan medicine in drug discovery given the limited ethnobotanical studies and data available for most species with regards to identification on bioactive components, pharmacological mechanisms, and the scarce number of clinical studies.

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.

MiR-361-3p/Nfat5 Signaling Axis Controls Cementoblast Differentiation

The development of periodontal tissue is a complex process, including cementoblast proliferation and differentiation. Emerging reports suggest that microRNAs (miRNAs) play crucial roles in gene regulatory networks governing numerous biological processes. However, how miRNAs modulate cementoblast proliferation and differentiation remains largely unknown. In a previous study, we performed miRNA microarray profiling to fully reveal the expression patterns of miRNAs involved in cementoblast differentiation. We focused on miR-361-3p, which decreased during cementoblast differentiation. Overexpression of miR-361-3p resulted in decreased cementoblast differentiation, whereas the functional inhibition of miR-361-3p yielded the opposite effect. The bioinformatics approach identified nuclear factor of activated T-cell 5 (Nfat5) as a potential target of miR-361-3p, which was further verified by dual luciferase assay. Meanwhile, the expression pattern of Nfat5 was verified both in vitro and in vivo. Furthermore, knockdown of Nfat5 mimicked the inhibitory effect of overexpressing miR-361-3p in cementoblasts. Moreover, multiple signaling pathways, including the Erk1/2, JNK, p38, PI3K-Akt, and NF-κB pathways, were notably activated, and the Wnt/ß-catenin pathway was blocked by downregulation of Nfat5 or forced expression of miR-361-3p in cementoblast differentiation. Finally, the complementary approach demonstrated that miR-361-3p regulated cementoblast differentiation via or partially via Erk1/2 and PI3K-Akt. Overall, our study elucidated that the JNK, p38, NF-κB, and Wnt/ß-catenin pathways act as balancing players in the miR-361-3p/Nfat5 signaling axis during cementoblast differentiation.

Protective effects of glycerol and xylitol in keratinocytes exposed to hyperosmotic stress

Purpose: Our goal was to study whether glycerol and xylitol provide protection against osmotic stress in keratinocytes. Methods: The experiments were performed on HaCaT keratinocytes. Hyperosmotic stress was induced by the addition of sorbitol (450, 500 and 600 mOsm). Both polyols were applied at two different concentrations (glycerol: 0.027% and 0.27%, xylitol: 0.045% and 0.45%). Cellular viability and cytotoxicity were assessed, intracellular Ca²⁺ concentration was measured, and the RNA expression of inflammatory cytokines was determined by means of PCR. Differences among groups were analyzed with one-way ANOVA and Holm-Sidak post-hoc test. When the normality test failed, Kruskal-Wallis one-way analysis of variance on ranks, followed by Dunn's method for pairwise multiple comparison was performed. Results: The higher concentrations of the polyols were effective. Glycerol ameliorated the cellular viability while xylitol prevented the rapid Ca²⁺ signal. Both polyols suppressed the expression of IL-1α but only glycerol decreased the expression of IL-1β and NFAT5. Conclusions: Glycerol and xylitol protect keratinocytes against osmotic stress. Despite their similar chemical structure, the effect of these polyols displayed differences. Hence, joint application of glycerol and xylitol may be a useful therapeutic approach for different skin disorders.

Aberrant expression of alternative splicing variants in multiple sclerosis - A systematic review

OBJECTIVE

Alternative splicing is an important form of RNA processing that affects nearly all human genes. The differential expression of specific transcript and protein isoforms holds the potential of novel biomarkers for complex diseases. In this systematic review, we compiled the existing literature on aberrant alternative splicing events in multiple sclerosis (MS).

METHODS

A systematic literature search in the PubMed database was carried out and supplemented by screening the reference lists of the identified articles. We selected only MS-related original research studies which compared the levels of different isoforms of human protein-coding genes. A narrative synthesis of the research findings was conducted. Additionally, we performed a case-control analysis using high-density transcriptome microarray data to reevaluate the genes that were examined in the reviewed studies.

RESULTS

A total of 160 records were screened. Of those, 36 studies from the last two decades were included. Most commonly, peripheral blood samples were analyzed (32 studies), and PCR-based techniques were usually employed (27 studies) for measuring the expression of selected genes. Two studies used an exploratory genome-wide approach. Overall, 27 alternatively spliced genes were investigated. Nine of these genes appeared in at least two studies (CD40, CFLAR, FOXP3, IFNAR2, IL7R, MOG, PTPRC, SP140 and TNFRSF1A). The microarray data analysis confirmed differential alternative pre-mRNA splicing for 19 genes.

CONCLUSIONS

An altered RNA processing of genes mediating immune signaling pathways has been repeatedly implicated in MS. The analysis of individual exon-level expression patterns is stimulated by the advancement of transcriptome profiling technologies. In particular, the examination of genes encoded in MS-associated genetic regions may provide important insights into the pathogenesis of the disease and help to identify new biomarkers.

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.

VEGF-C attenuates renal damage in salt-sensitive hypertension

Salt-sensitive hypertension is a major risk factor for renal impairment leading to chronic kidney disease. High-salt diet leads to hypertonic skin interstitial volume retention enhancing the activation of the tonicity-responsive enhancer-binding protein (TonEBP) within macrophages leading to vascular endothelial growth factor C (VEGF-C) secretion and NOS3 modulation. This promotes skin lymphangiogenesis and blood pressure regulation. Whether VEGF-C administration enhances renal and skin lymphangiogenesis and attenuates renal damage in salt-sensitive hypertension remains to be elucidated. Hypertension was induced in BALB/c mice by a high-salt diet. VEGF-C was administered subcutaneously to high-salt-treated mice as well as control animals. Analyses of kidney injury, inflammation, fibrosis, and biochemical markers were performed in vivo. VEGF-C reduced plasma inflammatory markers in salt-treated mice. In addition, VEGF-C exhibited a renal anti-inflammatory effect with the induction of macrophage M2 phenotype, followed by reductions in interstitial fibrosis. Antioxidant enzymes within the kidney as well as urinary RNA/DNA damage markers were all revelatory of abolished oxidative stress under VEGF-C. Furthermore, VEGF-C decreased the urinary albumin/creatinine ratio and blood pressure as well as glomerular and tubular damages. These improvements were associated with enhanced TonEBP, NOS3, and lymphangiogenesis within the kidney and skin. Our data show that VEGF-C administration plays a major role in preserving renal histology and reducing blood pressure. VEGF-C might constitute an interesting potential therapeutic target for improving renal remodeling in salt-sensitive hypertension.

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.

Upregulation of DARS2 by HBV promotes hepatocarcinogenesis through the miR-30e-5p/MAPK/NFAT5 pathway

Background

Infection with the hepatitis B virus (HBV) is closely associated with the development of hepatocellular carcinoma (HCC). The osmoregulatory transcription factor nuclear factor of activated T-cells 5 (NFAT5) has been shown to play an important role in the development of many types of human cancers. The role of NFAT5 in HBV-associated HCC has never previously been investigated.

Methods

We compared expression profiles of NFAT5, DARS2 and miR-30e-5p in HCC samples, adjacent nontumor tissues and different hepatoma cell lines by quantitative real-time polymerase chain reaction and /or Western blot. Clinical data of HCC patients for up to 80 months were analyzed. The regulatory mechanisms upstream and convergent downstream pathways of NFAT5 in HBV-associated HCC were investigated by ChIP-seq, MSP, luciferase report assay and bioinformation anaylsis.

Results

We first found that higher levels of NFAT5 expression predict a good prognosis, suggesting that NFAT5 is a potential tumor-suppressing gene, and verified that NFAT5 promotes hepatoma cell apoptosis and inhibits cell growth in vitro. Second, our results showed that HBV could suppress NFAT5 expression by inducing hypermethylation of the AP1-binding site in the NFAT5 promoter in hepatoma cells. In addition, HBV also inhibited NFAT5 through miR-30e-5p targeted MAP4K4, and miR-30e-5p in turn inhibited HBV replication. Finally, we demonstrated that NFAT5 suppressed DARS2 by directly binding to its promoter. DARS2 was identified as an HCC oncogene that promotes HCC cell cycle progression and inhibits HCC cell apoptosis.

Conclusion

HBV suppresses NFAT5 through the miR-30e-5p/mitogen-activated protein kinase (MAPK) signaling pathway upstream of NFAT5 and inhibits the NFAT5 to enhance HCC tumorigenesis via the downstream target genes of DARS2.

Electronic supplementary material

The online version of this article (10.1186/s13046-017-0618-x) contains supplementary material, which is available to authorized users.

Progression of experimental autoimmune encephalomyelitis is associated with up-regulation of major sodium transporters in the mouse kidney cortex under a normal salt diet

Recent demonstrations of exacerbation of experimental autoimmune encephalomyelitis (EAE) by high salt diets prompted us to study whether EAE stimulated Na absorption by the renal cortex, a primary regulatory site for Na balance, even under a normal NaCl diet. We found that as EAE progressed from mild to severe symptoms, there were parallel increases in the protein abundance of NHE3 and αENaC and the Na,K-ATPase activity with an affiliated elevation of its β1-subunit protein. These effects are associated with increases in the protein levels of the well-known regulators SGK1 and scaffold NHERF2, and phosphorylation of ERK1/2. These effects of EAE could not be explained by reduction in water or food intake. We conclude that EAE progression is associated with up-regulation of major Na transporters, which is most likely driven by increased expression of SGK1 and NHERF2 and activation of ERK1/2. These data suggest that EAE progression increases Na absorption by the renal cortex.

Osmotic regulation of NFAT5 expression in RPE cells: The involvement of purinergic receptor signaling

PURPOSE

Systemic hypertension is a risk factor for age-related neovascular retinal diseases. The major condition that induces hypertension is the intake of dietary salt (NaCl) resulting in increased extracellular osmolarity. High extracellular NaCl was has been shown to induce angiogenic factor production in RPE cells, in part via the transcriptional activity of nuclear factor of activated T cell 5 (NFAT5). Here, we determined the signaling pathways that mediate the osmotic expression of the NFAT5 gene in RPE cells.

METHODS

Cultured human RPE cells were stimulated with high (+100 mM) NaCl. Alterations in gene and protein expression were determined with real-time reverse transcriptase (RT)-PCR and western blot analysis, respectively.

RESULTS

NaCl-induced NFAT5 gene expression was fully inhibited by calcium chelation and blockers of inositol triphosphate (IP₃) receptors and phospholipases C and A₂. Blockers of phospholipases C and A₂ also prevented the NaCl-induced increase of the cellular NFAT5 protein level. Inhibitors of multiple intracellular signaling transduction pathways and kinases, including p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun NH₂-terminal kinase (JNK), phosphatidylinositol-3 kinase (PI3K), protein kinases A and C, Src tyrosine kinases, and calpains, as well as cyclooxygenase inhibitors, decreased the NaCl-induced expression of the NFAT5 gene. In addition, autocrine purinergic signaling mediated by a release of ATP and a nucleoside transporter-mediated release of adenosine, activation of P2X₇, P2Y₁, P2Y₂, and adenosine A₁ receptors, but not adenosine A_2A receptors, is required for the full expression of the NFAT5 gene under hyperosmotic conditions. NaCl-induced NFAT5 gene expression is in part dependent on the activity of nuclear factor κB (NF-κB). The NaCl-induced expression of NFAT5 protein was prevented by inhibitors of phospholipases C and A₂ and an inhibitor of NF-κB, but it was not prevented by a P2Y₁ inhibitor.

CONCLUSIONS

The data suggest that in addition to calcium signaling and activation of inflammatory enzymes, autocrine/paracrine purinergic signaling contributes to the stimulatory effect of hyperosmotic stress on the expression of the NFAT5 gene in RPE cells. It is suggested that high intake of dietary salt induces RPE cell responses, which may contribute to age-related retinal diseases.

Mitogen-activated protein kinases as key players in osmotic stress signaling

BACKGROUND

Osmotic stress arises from the difference between intracellular and extracellular osmolality. It induces cell swelling or shrinkage as a consequence of water influx or efflux, which threatens cellular activities. Mitogen-activated protein kinases (MAPKs) play central roles in signaling pathways in osmotic stress responses, including the regulation of intracellular levels of inorganic ions and organic osmolytes.

SCOPE OF REVIEW

The present review summarizes the cellular osmotic stress response and the function and regulation of the vertebrate MAPK signaling pathways involved. We also describe recent findings regarding apoptosis signal-regulating kinase 3 (ASK3), a MAP3K member, to demonstrate its regulatory effects on signaling molecules beyond MAPKs.

MAJOR CONCLUSIONS

MAPKs are rapidly activated by osmotic stress and have diverse roles, such as cell volume regulation, gene expression, and cell survival/death. There is significant cell type specificity in the function and regulation of MAPKs. Based on its activity change during osmotic stress and its regulation of the WNK1-SPAK/OSR1 pathway, ASK3 is expected to play important roles in osmosensing mechanisms and cellular functions related to osmoregulation.

GENERAL SIGNIFICANCE

MAPKs are essential for various cellular responses to osmotic stress; thus, the identification of the upstream regulators of MAPK pathways will provide valuable clues regarding the cellular osmosensing mechanism, which remains elusive in mammals. The elucidation of in vivo MAPK functions is also important because osmotic stress in physiological and pathophysiological conditions often results from changes in the intracellular osmolality. These studies potentially contribute to the establishment of therapeutic strategies against diseases that accompany osmotic perturbation.

Osmotic Induction of Angiogenic Growth Factor Expression in Human Retinal Pigment Epithelial Cells

BACKGROUND

Although systemic hypertension is a risk factor of age-related macular degeneration, antihypertensive medications do not affect the risk of the disease. One condition that induces hypertension is high intake of dietary salt resulting in increased blood osmolarity. In order to prove the assumption that, in addition to hypertension, high osmolarity may aggravate neovascular retinal diseases, we determined the effect of extracellular hyperosmolarity on the expression of angiogenic cytokines in cultured human retinal pigment epithelial (RPE) cells.

METHODOLOGY/PRINCIPAL FINDINGS

Hyperosmolarity was induced by the addition of 100 mM NaCl or sucrose to the culture medium. Hypoxia and oxidative stress were induced by the addition of the hypoxia mimetic CoCl2 and H2O2, respectively. Alterations in gene expression were determined with real-time RT-PCR. Secretion of bFGF was evaluated by ELISA. Cell viability was determined by trypan blue exclusion. Nuclear factor of activated T cell 5 (NFAT5) expression was knocked down with siRNA. Hyperosmolarity induced transcriptional activation of bFGF, HB-EGF, and VEGF genes, while the expression of other cytokines such as EGF, PDGF-A, TGF-β1, HGF, and PEDF was not or moderately altered. Hypoxia induced increased expression of the HB-EGF, EGF, PDGF-A, TGF-β1, and VEGF genes, but not of the bFGF gene. Oxidative stress induced gene expression of HB-EGF, but not of bFGF. The hyperosmotic expression of the bFGF gene was dependent on the activation of p38α/β MAPK, JNK, PI3K, and the transcriptional activity of NFAT5. The hyperosmotic expression of the HB-EGF gene was dependent on the activation of p38α/β MAPK, ERK1/2, and JNK. The hyperosmotic expression of bFGF, HB-EGF, and VEGF genes was reduced by inhibitors of TGF-β1 superfamily activin receptor-like kinase receptors and the FGF receptor kinase, respectively. Hyperosmolarity induced secretion of bFGF that was reduced by inhibition of autocrine/paracrine TGF-β1 signaling and by NFAT5 siRNA, respectively. Hyperosmolarity decreased the viability of the cells; this effect was not altered by exogenous bFGF and HB-EGF. Various vegetable polyphenols (luteolin, quercetin, apigenin) inhibited the hyperosmotic expression of bFGF, HB-EGF, and NFAT5 genes.

CONCLUSION

Hyperosmolarity induces transcription of bFGF and HB-EGF genes, and secretion of bFGF from RPE cells. This is in part mediated by autocrine/paracrine TGF-β1 and FGF signaling. It is suggested that high intake of dietary salt resulting in osmotic stress may aggravate neovascular retinal diseases via stimulation of the production of angiogenic factors in RPE cells, independent of hypertension.

TonEBP modulates the protective effect of taurine in ischemia-induced cytotoxicity in cardiomyocytes

Taurine, which is found at high concentration in the heart, exerts several protective actions on myocardium. Physically, the high level of taurine in heart is maintained by a taurine transporter (TauT), the expression of which is suppressed under ischemic insult. Although taurine supplementation upregulates TauT expression, elevates the intracellular taurine content and ameliorates the ischemic injury of cardiomyocytes (CMs), little is known about the regulatory mechanisms of taurine governing TauT expression under ischemia. In this study, we describe the TonE (tonicity-responsive element)/TonEBP (TonE-binding protein) pathway involved in the taurine-regulated TauT expression in ischemic CMs. Taurine inhibited the ubiquitin-dependent proteasomal degradation of TonEBP, promoted the translocation of TonEBP into the nucleus, enhanced TauT promoter activity and finally upregulated TauT expression in CMs. In addition, we observed that TonEBP had an anti-apoptotic and anti-oxidative role in CMs under ischemia. Moreover, the protective effects of taurine on myocardial ischemia were TonEBP dependent. Collectively, our findings suggest that TonEBP is a core molecule in the protective mechanism of taurine in CMs under ischemic insult.

Meta-Analysis of Multiple Sclerosis Microarray Data Reveals Dysregulation in RNA Splicing Regulatory Genes

Abnormalities in RNA metabolism and alternative splicing (AS) are emerging as important players in complex disease phenotypes. In particular, accumulating evidence suggests the existence of pathogenic links between multiple sclerosis (MS) and altered AS, including functional studies showing that an imbalance in alternatively-spliced isoforms may contribute to disease etiology. Here, we tested whether the altered expression of AS-related genes represents a MS-specific signature. A comprehensive comparative analysis of gene expression profiles of publicly-available microarray datasets (190 MS cases, 182 controls), followed by gene-ontology enrichment analysis, highlighted a significant enrichment for differentially-expressed genes involved in RNA metabolism/AS. In detail, a total of 17 genes were found to be differentially expressed in MS in multiple datasets, with CELF1 being dysregulated in five out of seven studies. We confirmed CELF1 downregulation in MS (p = 0.0015) by real-time RT-PCRs on RNA extracted from blood cells of 30 cases and 30 controls. As a proof of concept, we experimentally verified the unbalance in alternatively-spliced isoforms in MS of the NFAT5 gene, a putative CELF1 target. In conclusion, for the first time we provide evidence of a consistent dysregulation of splicing-related genes in MS and we discuss its possible implications in modulating specific AS events in MS susceptibility genes.