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IMPACT STATEMENT - Successful clinical tissue engineering requires functional fidelity of the cultured cell to its counterpart, but this has been elusive in renal tissue engineering. Typically, renal proximal tubule cells in culture have a flattened morphology and do not express key transporters essential to their function. In this article, we show for the first time that substrate mechanical properties dictate differentiation of cultured renal proximal tubule cells. Remarkably, this effect was only discernable after 4 weeks in culture, longer than usually reported for this cell type. These results demonstrate a new tunable parameter to optimize cell differentiation in renal tissue engineering.
COX (cyclooxygenase)-derived prostaglandins regulate renal hemodynamics and salt and water homeostasis. Inhibition of COX activity causes blood pressure elevation. In addition, chronic analgesic abuse can induce renal injury, including papillary necrosis. COX-2 is highly expressed in the kidney papilla in renal medullary interstitial cells (RMICs). However, its role in blood pressure and papillary integrity in vivo has not been definitively studied. In mice with selective, inducible RMIC COX-2 deletion, a high-salt diet led to an increase in blood pressure that peaked at 4 to 5 weeks and was associated with increased papillary expression of AQP2 (aquaporin 2) and ENac (epithelial sodium channel) and decreased expression of cystic fibrosis transmembrane conductance regulator. With continued high-salt feeding, the mice with RMIC COX-2 deletion had progressive decreases in blood pressure from its peak. After return to a normal-salt diet for 3 weeks, blood pressure remained low and was associated with a persistent urinary concentrating defect. Within 2 weeks of institution of a high-salt diet, increased apoptotic RMICs and collecting duct cells could be detected in papillae with RMIC deletion of COX-2, and by 9 weeks of high salt, there was a striking loss of the papillae. Therefore, RMIC COX-2 expression plays a crucial role in renal handling water and sodium homeostasis, preventing salt-sensitive hypertension and maintaining structural integrity of papilla.
AIMS - We conducted a prospective study of emergency department (ED) patients with acute heart failure (AHF) to determine if worsening HF (WHF) could be predicted based on urinary electrolytes during the first 1-2 h of ED care. Loop diuretics are standard therapy for AHF patients. A subset of patients hospitalized for AHF will develop a blunted natriuretic response to loop diuretics, termed diuretic resistance, which often leads to WHF. Early detection of diuretic resistance could facilitate escalation of therapy and prevention of WHF.
METHODS AND RESULTS - Patients were eligible if they had an ED AHF diagnosis, had not yet received intravenous diuretics, had a systolic blood pressure > 90 mmHg, and were not on dialysis. Urine electrolytes and urine output were collected at 1, 2, 4, and 6 h after diuretic administration. Worsening HF was defined as clinically persistent or WHF requiring escalation of diuretics or administration of intravenous vasoactives after the ED stay. Of the 61 patients who qualified in this pilot study, there were 10 (16.3%) patients who fulfilled our definition of WHF. At 1 h after diuretic administration, patients who developed WHF were more likely to have low urinary sodium (9.5 vs. 43.0 mmol; P < 0.001) and decreased urine sodium concentration (48 vs. 80 mmol/L; P = 0.004) than patients without WHF. All patients with WHF had a total urine sodium of <35.4 mmol at 1 h (100% sensitivity and 60% specificity).
CONCLUSIONS - One hour after diuretic administration, a urine sodium excretion of <35.4 mmol was highly suggestive of the development of WHF. These relationships require further testing to determine if early intervention with alternative agents can prevent WHF.
© 2018 The Authors. ESC Heart Failure published by John Wiley & Sons Ltd on behalf of the European Society of Cardiology.
BACKGROUND - Fibroblast growth factor 23 (FGF23), a bone-derived hormone that regulates phosphorus and vitamin D metabolism, contributes to the pathogenesis of mineral and bone disorders in CKD and is an emerging cardiovascular risk factor. Central elements of FGF23 regulation remain incompletely understood; genetic variation may help explain interindividual differences.
METHODS - We performed a meta-analysis of genome-wide association studies of circulating FGF23 concentrations among 16,624 participants of European ancestry from seven cohort studies, excluding participants with eGFR<30 ml/min per 1.73 m to focus on FGF23 under normal conditions. We evaluated the association of single-nucleotide polymorphisms (SNPs) with natural log-transformed FGF23 concentration, adjusted for age, sex, study site, and principal components of ancestry. A second model additionally adjusted for BMI and eGFR.
RESULTS - We discovered 154 SNPs from five independent regions associated with FGF23 concentration. The SNP with the strongest association, rs17216707 (=3.0×10), lies upstream of , which encodes the primary catabolic enzyme for 1,25-dihydroxyvitamin D and 25-hydroxyvitamin D. Each additional copy of the T allele at this locus is associated with 5% higher FGF23 concentration. Another locus strongly associated with variations in FGF23 concentration is rs11741640, within and upstream of (a gene involved in renal phosphate transport). Additional adjustment for BMI and eGFR did not materially alter the magnitude of these associations. Another top locus (within , the ABO blood group transferase gene) was no longer statistically significant at the genome-wide level.
CONCLUSIONS - Common genetic variants located near genes involved in vitamin D metabolism and renal phosphate transport are associated with differences in circulating FGF23 concentrations.
Copyright © 2018 by the American Society of Nephrology.
BACKGROUND & AIMS - Inactivating mutations in MYO5B cause microvillus inclusion disease (MVID), but the physiological cause of the diarrhea associated with this disease is unclear. We investigated whether loss of MYO5B results in aberrant expression of apical enterocyte transporters.
METHODS - We studied alterations in apical membrane transporters in MYO5B-knockout mice, as well as mice with tamoxifen-inducible, intestine-specific disruption of Myo5b (VilCre;Myo5b mice) or those not given tamoxifen (controls). Intestinal tissues were collected from mice and analyzed by immunostaining, immunoelectron microscopy, or cultured enteroids were derived. Functions of brush border transporters in intestinal mucosa were measured in Ussing chambers. We obtained duodenal biopsy specimens from individuals with MVID and individuals without MVID (controls) and compared transporter distribution by immunocytochemistry.
RESULTS - Compared to intestinal tissues from littermate controls, intestinal tissues from MYO5B-knockout mice had decreased apical localization of SLC9A3 (also called NHE3), SLC5A1 (also called SGLT1), aquaporin (AQP) 7, and sucrase isomaltase, and subapical localization of intestinal alkaline phosphatase and CDC42. However, CFTR was present on apical membranes of enterocytes from MYO5B knockout and control mice. Intestinal biopsies from patients with MVID had subapical localization of NHE3, SGLT1, and AQP7, but maintained apical CFTR. After tamoxifen administration, VilCre;Myo5b mice lost apical NHE3, SGLT1, DRA, and AQP7, similar to germline MYO5B knockout mice. Intestinal tissues from VilCre;Myo5b mice had increased CFTR in crypts and CFTR localized to the apical membranes of enterocytes. Intestinal mucosa from VilCre;Myo5b mice given tamoxifen did not have an intestinal barrier defect, based on Ussing chamber analysis, but did have decreased SGLT1 activity and increased CFTR activity.
CONCLUSIONS - Although trafficking of many apical transporters is regulated by MYO5B, trafficking of CFTR is largely independent of MYO5B. Decreased apical localization of NHE3, SGLT1, DRA, and AQP7 might be responsible for dysfunctional water absorption in enterocytes of patients with MVID. Maintenance of apical CFTR might exacerbate water loss by active secretion of chloride into the intestinal lumen.
Copyright © 2018 AGA Institute. Published by Elsevier Inc. All rights reserved.
BACKGROUND - Accurately predicting the impact of rare nonsynonymous variants on disease risk is an important goal in precision medicine. Variants in the cardiac sodium channel (protein Na1.5; voltage-dependent cardiac Na+ channel) are associated with multiple arrhythmia disorders, including Brugada syndrome and long QT syndrome. Rare variants also occur in ≈1% of unaffected individuals. We hypothesized that in vitro electrophysiological functional parameters explain a statistically significant portion of the variability in disease penetrance.
METHODS - From a comprehensive literature review, we quantified the number of carriers presenting with and without disease for 1712 reported variants. For 356 variants, data were also available for 5 Na1.5 electrophysiological parameters: peak current, late/persistent current, steady-state V1/2 of activation and inactivation, and recovery from inactivation.
RESULTS - We found that peak and late current significantly associate with Brugada syndrome (<0.001; ρ=-0.44; Spearman rank test) and long QT syndrome disease penetrance (<0.001; ρ=0.37). Steady-state V1/2 activation and recovery from inactivation associate significantly with Brugada syndrome and long QT syndrome penetrance, respectively. Continuous estimates of disease penetrance align with the current American College of Medical Genetics classification paradigm.
CONCLUSIONS - Na1.5 in vitro electrophysiological parameters are correlated with Brugada syndrome and long QT syndrome disease risk. Our data emphasize the value of in vitro electrophysiological characterization and incorporating counts of affected and unaffected carriers to aid variant classification. This quantitative analysis of the electrophysiological literature should aid the interpretation of Na1.5 variant electrophysiological abnormalities and help improve Na1.5 variant classification.
© 2018 American Heart Association, Inc.
We recently reported the case of a young patient with multisystem failure carrying a de novo mutation in SLC12A2, the gene encoding the Na-K-2Cl cotransporter-1 (NKCC1). Heterologous expression studies in nonepithelial cells failed to demonstrate dominant-negative effects. In this study, we examined expression of the mutant cotransporter in epithelial cells. Using Madin-Darby canine kidney (MDCK) cells grown on glass coverslips, permeabilized support, and Matrigel, we show that the fluorescently tagged mutant cotransporter is expressed in cytoplasm and at the apical membrane and affects epithelium integrity. Expression of the mutant transporter at the apical membrane also results in the mislocalization of some of the wild-type transporter to the apical membrane. This mistargeting is specific to NKCC1 as the Na-K-ATPase remains localized on the basolateral membrane. To assess transporter localization in vivo, we created a mouse model using CRISPR/cas9 that reproduces the 11 bp deletion in exon 22 of Slc12a2. Although the mice do not display an overt phenotype, we show that the colon and salivary gland expresses wild-type NKCC1 abundantly at the apical pole, confirming the data obtained in cultured epithelial cells. Enough cotransporter must remain, however, on the basolateral membrane to participate in saliva secretion, as no significant decrease in saliva production was observed in the mutant mice.
UNC-8 and MEC-4 are two members of the degenerin/epithelial Na channel (DEG/ENaC) family of voltage-independent Na channels that share a high degree of sequence homology and functional similarity. For example, both can be hyperactivated by genetic mutations [UNC-8(d) and MEC-4(d)] that induce neuronal death by necrosis. Both depend in vivo on chaperone protein MEC-6 for function, as demonstrated by the finding that neuronal death induced by hyperactive UNC-8 and MEC-4 channels is prevented by null mutations in mec-6. UNC-8 and MEC-4 differ functionally in three major ways: 1) MEC-4 is calcium permeable, whereas UNC-8 is not; 2) UNC-8, but not MEC-4, is blocked by extracellular calcium and magnesium in the micromolar range; and 3) MEC-6 increases the number of MEC-4 channels at the cell surface in oocytes but does not have this effect on UNC-8. We previously reported that Capermeability of MEC-4 is conferred by the second transmembrane domain. We show here that the extracellular "finger" domain of UNC-8 is sufficient to mediate inhibition by divalent cations and that regulation by MEC-6 also depends on this region. Thus, our work confirms that the finger domain houses residues involved in gating of this channel class and shows for the first time that the finger domain also mediates regulation by chaperone protein MEC-6. Given that the finger domain is the most divergent region across the DEG/ENaC family, we speculate that it influences channel trafficking and function in a unique manner depending on the channel subunit.
Two genes encode the Na -K -2Cl cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na , 1K , and 2Cl across the plasma membrane of cells. Na -K -2Cl cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na /K pump. In many cells, NKCC1 accumulates Cl above its electrochemical potential equilibrium, thereby facilitating Cl channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca channels, leading to Ca influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl . In combination with the Na /K pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K -driven fluid secretion in inner ear, and possibly in Na -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na /K pump participates in reabsorbing 30% of the glomerular-filtered Na . Overall, many critical physiological functions are maintained by the activity of the two Na -K -2Cl cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.
Copyright © 2018 American Physiological Society. All rights reserved.