Osmotic regulation of the Na+/myo-inositol cotransporter and postinduction normalization

Targeted Urine Metabolomics for Monitoring Renal Allograft Injury and Immunosuppression in Pediatric Patients

Despite new advancements in surgical tools and therapies, exposure to immunosuppressive drugs related to non-immune and immune injuries can cause slow deterioration and premature failure of organ transplants. Diagnosis of these injuries by non-invasive urine monitoring would be a significant clinical advancement for patient management, especially in pediatric cohorts. We investigated the metabolomic profiles of biopsy matched urine samples from 310 unique kidney transplant recipients using gas chromatography-mass spectrometry (GC-MS). Focused metabolite panels were identified that could detect biopsy confirmed acute rejection with 92.9% sensitivity and 96.3% specificity (11 metabolites) and could differentiate BK viral nephritis (BKVN) from acute rejection with 88.9% sensitivity and 94.8% specificity (4 metabolites). Overall, targeted metabolomic analyses of biopsy-matched urine samples enabled the generation of refined metabolite panels that non-invasively detect graft injury phenotypes with high confidence. These urine biomarkers can be rapidly assessed for non-invasive diagnosis of specific transplant injuries, opening the window for precision transplant medicine.

Deficiency in the α1 subunit of Na+/K+-ATPase enhances the anti-proliferative effect of high osmolality in nucleus pulposus intervertebral disc cells

Intervertebral disc cells are constantly exposed to a hyperosmotic environment. Among cellular responses towards this stress is the inhibition of proliferation through the activation of p38 MAPK and p53. In an effort to further elucidate the biochemical pathways triggered by hyperosmotic stress, we assessed the high osmolality-induced transcriptional changes of bovine nucleus pulposus cells using whole-genome arrays. A 5- and a 24-h hyperosmotic treatment led to the differential expression of >100 and >200 genes, respectively, including nine genes encoding transporters (SLC4A11, SLC5A3, ATP1A1, SLC38A2, KCNK17, KCTD20, KCTD11, SLC7A5, and CLCA2). Differences in the transcriptional profile of these selected genes, as indicated by the microarrays experiments, were validated by qRT-PCR in 2D and 3D cell cultures, under hyperosmolar salt and sorbitol conditions, revealing the presence of a common triggering signal for osmotic adaptation. The key signaling molecules p38 MAPK and p53 were demonstrated to differently participate in the regulation of the aforementioned transporters. Finally, siRNA-mediated knocking-down of each one of the three transporters with the highest and sustained over-expression (i.e., SLC4A11, SLC5A3, and ATP1A1) had a distinct outcome on the transcriptional profile of the other transporters, on p38 MAPK and p53 phosphorylation and consequently on cell cycle progression. The inhibition of ATP1A1 had the most prominent effect on the transcription of the rest of the transporters and was found to enhance the anti-proliferative effect of hyperosmotic conditions through an increased G2/M cell cycle block, ascribing to this pump a central role in the osmoregulatory response of nucleus pulposus cells.

Role of N-glycosylation in renal betaine transport

The osmolyte and folding chaperone betaine is transported by the renal Na(+)-coupled GABA (γ-aminobutyric acid) symporter BGT-1 (betaine/GABA transporter 1), a member of the SLC6 (solute carrier 6) family. Under hypertonic conditions, the transcription, translation and plasma membrane (PM) insertion of BGT-1 in kidney cells are significantly increased, resulting in elevated betaine and GABA transport. Re-establishing isotonicity involves PM depletion of BGT-1. The molecular mechanism of the regulated PM insertion of BGT-1 during changes in osmotic stress is unknown. In the present study, we reveal a link between regulated PM insertion and N-glycosylation. Based on homology modelling, we identified two sites (Asn(171) and Asn(183)) in the extracellular loop 2 (EL2) of BGT-1, which were investigated with respect to trafficking, insertion and transport by immunogold-labelling, electron microscopy (EM), mutagenesis and two-electrode voltage clamp measurements in Xenopus laevis oocytes and uptake of radiolabelled substrate into MDCK (Madin-Darby canine kidney) and HEK293 (human embryonic kidney) cells. Trafficking and PM insertion of BGT-1 was clearly promoted by N-glycosylation in both oocytes and MDCK cells. Moreover, association with N-glycans at Asn(171) and Asn(183) contributed equally to protein activity and substrate affinity. Substitution of Asn(171) and Asn(183) by aspartate individually caused no loss of BGT-1 activity, whereas the double mutant was inactive, suggesting that N-glycosylation of at least one of the sites is required for function. Substitution by alanine or valine at either site caused a dramatic loss in transport activity. Furthermore, in MDCK cells PM insertion of N183D was no longer regulated by osmotic stress, highlighting the impact of N-glycosylation in regulation of this SLC6 transporter.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Polarized function of thick ascending limbs of Henle cells in osmoregulation

BACKGROUND

Organic osmolytes are necessary for osmoregulation in mammalian kidney. Since renal epithelial cells in many cases possess specific mechanisms both for uptake and osmotically regulated release, we investigated their localization in polarized cells.

METHODS

An immortalized epithelial cell line derived from the thick ascending limb of Henle's loop (TALH) was used to examine the transport characteristics of the apical and basolateral plasma membranes for osmotic regulation of organic osmolytes. Cells were cultured on filters in a two-compartment chamber.

RESULTS

In culture under hypertonic conditions the TALH cells accumulated in the following balance: sorbitoverline> betaine = myo-inositoverline> glycerophosphoryl choline (GPC). When extracellular osmolarity was decreased, then sorbitol was released on the apical side, whereas betaine and myo-inositol efflux occurred on the basolateral side. GPC release showed no preference of either side. Taurine did not seem to be necessary for osmoregulation under these conditions. Osmotically regulated myo-inositol and betaine uptake was located on the apical side, and choline uptake took place on both sides equally.

CONCLUSION

These results show that in renal epithelial cells, both osmotically induced release and the uptake of organic osmolytes are divided between the apical and the basolateral sides. This might be important for volume regulation.

Wortmannin and LY294002 inhibit myo-inositol accumulation by cultured bovine aorta endothelial cells and murine 3T3-L1 adipocytes

We have previously reported that myo-inositol uptake and metabolism is reduced in human fibroblasts derived from patients with ataxia telangiectasia (AT). Treating normal fibroblasts with 10-100 microM wortmannin duplicates some of the phenotypic properties of AT fibroblasts including the decrease in myo-inositol accumulation. In the present study we examined whether treatment of other types of mammalian cells with wortmannin or LY294002 altered myo-inositol uptake. Cultured bovine aorta endothelial cells or 3T3-L1 adipocytes were incubated with either wortmannin or LY294002, and afterwards, myo-inositol uptake and SMIT mRNA levels were determined. Incubating cultured bovine aorta endothelial cells and 3T3-L1 adipocytes with either wortmannin or LY294002 caused a time- and concentration-dependent decrease in myo-inositol accumulation that was independent of changes in SMIT mRNA levels. The effect of wortmannin and LY294002 on myo-inositol accumulation was not due to an increase in myo-inositol secretion. The effect of LY294002 on myo-inositol accumulation was reversible. Furthermore, the LY294002-induced decrease in myo-inositol accumulation was specific since the uptake of serine or choline by cultured bovine aorta endothelial cells and 3T3-L1 adipocytes treated with LY294002 was not significantly decreased. Co-incubation of cultured bovine aorta endothelial cells and 3T3-L1 adipocytes with either wortmannin or LY294002 and hyperosmotic medium caused a significant decrease in the induction of myo-inositol accumulation by hyperosmolarity without significantly affecting the hyperosmotic-induced increase in SMIT mRNA levels. These data suggest that myo-inositol accumulation is regulated post-translationally by wortmannin and LY294002.

Normalization of hyperosmotic-induced inositol uptake by renal and endothelial cells is regulated by NF-kappaB

Hyperosmolarity is a stress factor that has been shown to cause an increase in the transcription of the Na(+)-dependent myo-inositol cotransporter (SMIT). However, regulation of the reversion of SMIT mRNA levels and transporter activity following removal of hyperosmotic stress is less understood. Previously we have shown that postinduction normalization of SMIT mRNA levels and myo-inositol accumulation following removal of hyperosmotic stress is inhibited by actinomycin D and cycloheximide, suggesting that normalization requires RNA transcription and protein synthesis. We now demonstrate that removal of hyperosmotic stress causes an activation of the transcription factor NF-kappaB in renal and endothelial cells. Inhibiting NF-kappaB activation with pyrrolidine dithiocarbamate (PD) blocks the normalization of SMIT mRNA levels and myo-inositol accumulation on removal of the cells from hyperosmotic medium. These studies demonstrate that the downregulation of the myo-inositol transporter following reversal of hyperosmotic induction is regulated via the activation of NF-kappaB.

Normalizing mitochondrial superoxide production blocks three pathways of hyperglycaemic damage

Diabetic hyperglycaemia causes a variety of pathological changes in small vessels, arteries and peripheral nerves. Vascular endothelial cells are an important target of hyperglycaemic damage, but the mechanisms underlying this damage are not fully understood. Three seemingly independent biochemical pathways are involved in the pathogenesis: glucose-induced activation of protein kinase C isoforms; increased formation of glucose-derived advanced glycation end-products; and increased glucose flux through the aldose reductase pathway. The relevance of each of these pathways is supported by animal studies in which pathway-specific inhibitors prevent various hyperglycaemia-induced abnormalities. Hyperglycaemia increases the production of reactive oxygen species inside cultured bovine aortic endothelial cells. Here we show that this increase in reactive oxygen species is prevented by an inhibitor of electron transport chain complex II, by an uncoupler of oxidative phosphorylation, by uncoupling protein-1 and by manganese superoxide dismutase. Normalizing levels of mitochondrial reactive oxygen species with each of these agents prevents glucose-induced activation of protein kinase C, formation of advanced glycation end-products, sorbitol accumulation and NFkappaB activation.