Distinct Roles of N-Terminal Fatty Acid Acylation of the Salinity-Sensor Protein SOS3

The Salt-Overly-Sensitive (SOS) pathway controls the net uptake of sodium by roots and the xylematic transfer to shoots in vascular plants. SOS3/CBL4 is a core component of the SOS pathway that senses calcium signaling of salinity stress to activate and recruit the protein kinase SOS2/CIPK24 to the plasma membrane to trigger sodium efflux by the Na/H exchanger SOS1/NHX7. However, despite the well-established function of SOS3 at the plasma membrane, SOS3 displays a nucleo-cytoplasmic distribution whose physiological meaning is not understood. Here, we show that the N-terminal part of SOS3 encodes structural information for dual acylation with myristic and palmitic fatty acids, each of which commands a different location and function of SOS3. N-myristoylation at glycine-2 is essential for plasma membrane association and recruiting SOS2 to activate SOS1, whereas S-acylation at cysteine-3 redirects SOS3 toward the nucleus. Moreover, a poly-lysine track in positions 7–11 that is unique to SOS3 among other Arabidopsis CBLs appears to be essential for the correct positioning of the SOS2-SOS3 complex at the plasma membrane for the activation of SOS1. The nuclear-localized SOS3 protein had limited bearing on the salt tolerance of Arabidopsis. These results are evidence of a novel S-acylation dependent nuclear trafficking mechanism that contrasts with alternative subcellular targeting of other CBLs by S-acylation.

SLC8 family of sodium/calcium exchangers in GtoPdb v.2021.3

The sodium/calcium exchangers (NCX) use the extracellular sodium concentration to facilitate the extrusion of calcium out of the cell. Alongside the plasma membrane Ca2+-ATPase (PMCA) and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA), as well as the sodium/potassium/calcium exchangers (NKCX, SLC24 family), NCX allow recovery of intracellular calcium back to basal levels after cellular stimulation. When intracellular sodium ion levels rise, for example, following depolarisation, these transporters can operate in the reverse direction to allow calcium influx and sodium efflux, as an electrogenic mechanism. Structural modelling suggests the presence of 9 TM segments, with a large intracellular loop between the fifth and sixth TM segments [1].

SLC8 family of sodium/calcium exchangers (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database

The sodium/calcium exchangers (NCX) use the extracellular sodium concentration to facilitate the extrusion of calcium out of the cell. Alongside the plasma membrane Ca2+-ATPase (PMCA) and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA), as well as the sodium/potassium/calcium exchangers (NKCX, SLC24 family), NCX allow recovery of intracellular calcium back to basal levels after cellular stimulation. When intracellular sodium ion levels rise, for example, following depolarisation, these transporters can operate in the reverse direction to allow calcium influx and sodium efflux, as an electrogenic mechanism. Structural modelling suggests the presence of 9 TM segments, with a large intracellular loop between the fifth and sixth TM segments.

Abstract 087: Human Stomach Cell Gastrin Inhibits Renal NHE3 and NaKATPase in Concert With the Renal D1R

The digestive track secretes gastrin in response to sodium ingestion stimulating increased renal sodium excretion. We previously showed that gastrin secreted by human colon cancer cells (SW626) can bind to cell surface cholecystokinin receptors on renal proximal tubule cells (RPTC), modulating the natriuretic dopaminergic system. We tested the hypothesis that a similar gastro/renal axis exists in humans. Novel human stomach antrum G-cell lines from 6 separate individuals were isolated and each shown to express gastrin mRNA and protein, and bind Phaseolus vulgaris Leucoagglutinin (PHA-l) by fluorescence lectin affinity (marker for gastrin secreting cells). It was determined that the D1 receptor (D1R) is also found on G-Cells. In human stomach tissue, gastrin expression was increased by Fenofibrate treatment, (PPARα agonist, p= 0.07, in 3 live ex-vivo cultured human stomachs). Moreover, we tested the effect of gastrin on CCKB2 receptors in human RPTC. Gastrin (100 nM 15 min.) increased RPTC phospholipace-C (PLC) activity by 1.07± 0.01 fold, N=35, p<0.0001, using a PLC-FRET biosensor, CYPHR, but did not increase cAMP levels using the specific cAMP-FRET biosensor, ICUE-YR. Both gastrin and SKF83822 (100 nM, a cAMP specific D1R agonist, 15 min) alone reduced sodium influx into RPTC via NHE3 (from VEH 100± 4.7% to 73.5± 6.2% or 74.1± 4.3% respectively N=6, p<0.05), but gastrin along with SKF83822 decreased sodium influx more than either alone (57.2 ± 5.8% N=6, p<0.05). Sodium efflux via NaKATPase was reduced by SKF83822 (from VEH 100± 3.9% to 84.2± 3.3%, N=6, p<0.05), but not gastrin alone, however SKF83822 along with gastrin reduced sodium efflux more than SKF83822 alone (72.3±5.1% vs N=6, p<0.05). Additionally we found that Angiotensin II (AngII, 10 nM, 15 min.) increased NHE3 activity (12.3± 3.6% N=6, p<0.01) and this increase was completely blocked by gastrin (N=6, p<0.01). The PLC inhibitor U73122 reversed the inhibitory effect of gastrin on NHE3 and NaKATPase. Gastrin was also found to decrease the amount of fluorescent AngII binding to RPTCs (27.4± 6.1% N=6, p<0.01) and this decrease was completely blocked by U73122. Thus, both stomach gastrin and the renal D1R inhibit NHE3 and NaKATPase, to increase sodium elimination from the body after salt is ingested.

Defective organellar acidification as a cause of cystic fibrosis lung disease: reexamination of a recurring hypothesis

The cellular mechanisms by which loss-of-function mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel produce cystic fibrosis (CF) lung disease remain uncertain. Defective organellar function has been proposed as an important determinant in the pathogenesis of CF lung disease. According to one hypothesis, reduced CFTR chloride conductance in organelles in CF impairs their acidification by preventing chloride entry into the organelle lumen, which is needed to balance the positive charge produced by proton entry. According to a different hypothesis, CFTR mutation hyperacidifies organelles by an indirect mechanism involving unregulated sodium efflux through epithelial sodium channels. There are reports of defective Golgi, endosomal and lysosomal acidification in CF epithelial cells, defective phagolysosomal acidification in CF alveolar macrophages, and organellar hyperacidification in CF respiratory epithelial cells. The common theme relating too high or low organellar pH to cellular dysfunction and CF pathogenesis is impaired functioning of organellar enzymes, such as those involved in ceramide metabolism and protein processing in epithelial cells and antimicrobial activity in alveolar macrophages. We review here the evidence for defective organellar acidification in CF. Significant technical and conceptual concerns are discussed regarding the validity of data showing too high/low organellar pH in CF cells, and rigorous measurements of organellar pH in CF cells are reviewed that fail to support defective organellar acidification in CF. Indeed, there is an expanding body of evidence supporting the involvement of non-CFTR chloride channels in organellar acidification. We conclude that biologically significant involvement of CFTR in organellar acidification is unlikely.

Diminished social status affects ionoregulation at the gills and kidney in rainbow trout (Oncorhynchus mykiss)

Competition for social status can result in physiological differences between individuals, including differences in ionoregulatory ability. Subordinate rainbow trout (Oncorhynchus mykiss) had two-fold higher uptake rates of sodium across the gill and two-fold higher whole-body sodium efflux rates than the dominant fish with which they were paired. Sodium efflux was then divided into branchial and renal components, both of which were higher in subordinates. Branchial sodium efflux accounted for 95%–98% of sodium loss. Plasma sodium concentrations were more variable, although not significantly different, in subordinate fish, suggesting that the increased loss of sodium in these trout is compensated for by an increase in uptake rates. Urine flow rates and plasma cortisol concentrations were higher in subordinate fish, but there was no difference in glomerular filtration rate between dominants and subordinates. Renal sodium reabsorption was significantly reduced in subordinates. In summary, the ionoregulation of subordinate individuals was altered, most likely occurring as a result of stress-induced changes in gill permeability, resulting in a higher throughput of water and increased branchial sodium efflux. These changes in ionoregulatory ability have many physiological implications, including the increased susceptibility of subordinates to ionoregulatory challenges and an increased metabolic cost of ionoregulation.