Insight into SLC9A3 deficiency-mediated micturition dysfunction caused by electrolyte imbalance

BACKGROUND

Solute carrier family nine isoform 3 (SLC9A3) is an Na⁺/H⁺ exchanger that regulates Ca₂⁺ homeostasis. SLC9A3 is largely involved in the transepithelial absorption of Na⁺/H⁺ and frequently functions in pair with a Cl^-/HCO3^- exchanger.

OBJECTIVE

To investigate the impact and pathophysiological mechanisms of long-term SLC9A3 deficiency on lower urinary tract symptoms (LUTS) in a mouse model MATERIALS AND METHODS: Slc9a3 knockout and wild-type mice (average >6 months) were used. The effects of SLC9A3 depletion on bladder and urethral functions and effectiveness of voiding were assessed using a cystometrogram (CMG). Histology, blood electrolytes, and gene expression were also analyzed.

RESULTS

The SLC9A3-deficient mice had smaller gross bladders than the wild-type mice. The CMG analysis revealed normal peak micturition pressure, higher threshold pressure, short intercontraction interval, less voided volume, and poor compliance in the SLC9A3-deficient mice, similar to clinical LUTS. Histological analysis revealed loose detrusor muscle and loss of transformability of the urothelium in the SLC9A3-deficient mice. Masson's trichrome analysis revealed severe collagen deposition in the detrusor muscle. Immunofluorescence staining also demonstrated a significant decrease in cytokeratins 5 and 20. Gene and protein expression analyses confirmed that SLC9A3 does not act directly on bladder tissue. Homeostasis was correlated with bladder dysfunction in the SLC9A3-deficient mice.

DISCUSSION

Fibrosis and collagen deposition in the bladder of the SLC9A3-deficient mice is due to bladder inflammation because of decreased blood flow and deregulated systemic homeostasis. Long-term SLC9A3 depletion causes progressive bladder dysfunction, similar to human LUTS.

CONCLUSION

Electrolyte imbalance causes SLC9A3 deficiency-mediated progressive micturition dysfunction.

Further delineation of SLC9A3-related congenital sodium diarrhea

BACKGROUND

Congenital sodium diarrhea (CSD) is a rare enteropathy displaying both broad variability in clinical severity and genetic locus and allelic heterogeneity. Eleven CSD patients were reported so far with SLC9A3 variants that impair the function of the encoded intestinal sodium-proton exchanger 3 (NHE3).

METHODS

We report a 4-year-old patient, born prematurely in the 35th week of gestation, with antenatal polyhydramnios and dilated intestinal loops, and with diarrhea of congenital onset, 2-6 times a day, and with polydipsia. She thrived age-appropriately under a normal family diet. Serum sodium levels were repeatedly normal but urinary sodium excretion was low. Exome sequencing revealed compound heterozygous variants in SLC9A3 as the likely cause of the congenital diarrhea.

RESULTS

While exome sequencing did not reveal pathogenic or likely pathogenic variants in other genes that cause syndromic or non-syndromic forms of congenital and intractable diarrheas, we identified novel compound heterozygous variants in SLC9A3, a complex allele with two missense changes, NP_004165.2:p.[Ser331Leu;Val449Ile] and in-trans the missense variant p.(Phe451Ser).

CONCLUSION

The clinical phenotype here appears to localize to the milder end of the known CSD spectrum, and the identified variants suggest that this is the twelfth patient reported to date with CSD due to mutations in SLC9A3.

Epigenetic modulation of intestinal Na+/H+ exchanger-3 expression

Na+/H+ exchanger-3 (NHE3) is crucial for intestinal Na+ absorption, and its reduction has been implicated in infectious and inflammatory bowel diseases (IBD)-associated diarrhea. Epigenetic mechanisms such as DNA methylation are involved in the pathophysiology of IBD. Whether changes in DNA methylation are involved in modulating intestinal NHE3 gene expression is not known. Caco-2 and HuTu 80 cells were used as models of human intestinal epithelial cells. Normal C57/BL6, wild-type, or growth arrest and DNA damage-inducible 45b (GADD45b) knockout (KO) mice were used as in vivo models. NHE3 gene DNA methylation levels were assessed by MBDCap (MethyMiner) assays. Results demonstrated that in vitro methylation of NHE3 promoter construct (p-1509/+127) cloned into a cytosine guanine dinucleotide-free lucia vector decreased the promoter activity in Caco-2 cells. DNA methyltransferase inhibitor 5-azacytidine (10 μM, 24 h) caused a significant decrease in DNA methylation of the NHE3 gene and concomitantly increased NHE3 expression in Caco-2 cells. Similarly, 5-azacytidine treatment increased NHE3 mRNA levels in HuTu 80 cells. 5-Azacytidine treatment for 3 wk (10 mg/kg body wt ip, 3 times/wk) also resulted in an increase in NHE3 expression in the mouse ileum and colon. Small-interfering RNA knockdown of GADD45b (protein involved in DNA demethylation) in Caco-2 cells decreased NHE3 mRNA expression. Furthermore, there was a significant decrease in NHE3 mRNA and protein expression in the ileum and colon of GADD45b KO mice. Our findings demonstrate that NHE3 gene expression is regulated by changes in its DNA methylation. NEW & NOTEWORTHY Our studies for the first time demonstrate that Na+/H+ exchanger-3 gene expression is regulated by an epigenetic mechanism involving DNA methylation.

SLC9A3 Protein Is Critical for Acrosomal Formation in Postmeiotic Male Germ Cells

Solute carrier family 9 isoform 3 (SLC9A3), a Na⁺/H⁺ exchanger, regulates the transepithelial absorption of Na⁺ and water and is primarily expressed on the apical membranes of the intestinal epithelium, renal proximal tubule, epididymis, and vas deferens. Loss of the Slc9a3 allele in mice enhances intestinal fluid and causes diarrhoea as a consequence of diminished Na⁺ and HCO₃⁻ absorption. Hence, the loss also causes male infertility and reveals the abnormal dilated lumen of the rete testis and calcification in efferent ductules. However, whether loss of Slc9a3 alleles also disrupts mammalian spermatogenesis remains unknown. First, through immunoblotting, we determined that SLC9A3 is highly expressed in the murine testis compared with the small intestine, epididymis, and vas deferens. During murine spermatogenesis, SLC9A3 is specifically expressed in the acrosome region of round, elongating, and elongated spermatids through immunostaining. Furthermore, SLC9A3 signals are enriched in the acrosome of mature sperm isolated from the vas deferens. In Slc9a3 knockout (KO) mice, compared with the same-aged controls, the number of spermatids on the testicular section of the mice progressively worsened in mice aged 20, 35, and 60 days. Sperm isolated from the epididymis of Slc9a3 KO mice revealed severe acrosomal defects. Our data indicated that SLC9A3 has a vital role in acrosomal formation during spermiogenesis.

Intestinal brush-border Na+/H+ exchanger-3 drives H+-coupled iron absorption in the mouse

Divalent metal-ion transporter-1 (DMT1), the principal mechanism by which nonheme iron is taken up at the intestinal brush border, is energized by the H(+)-electrochemical potential gradient. The provenance of the H(+) gradient in vivo is unknown, so we have explored a role for brush-border Na(+)/H(+) exchanger (NHE) isoforms by examining iron homeostasis and intestinal iron handling in mice lacking NHE2 or NHE3. We observed modestly depleted liver iron stores in NHE2-null (NHE2(-/-)) mice stressed on a low-iron diet but no change in hematological or blood iron variables or the expression of genes associated with iron metabolism compared with wild-type mice. Ablation of NHE3 strongly depleted liver iron stores, regardless of diet. We observed decreases in blood iron variables but no overt anemia in NHE3-null (NHE3(-/-)) mice on a low-iron diet. Intestinal expression of DMT1, the apical surface ferrireductase cytochrome b reductase-1, and the basolateral iron exporter ferroportin was upregulated in NHE3(-/-) mice, and expression of liver Hamp1 (hepcidin) was suppressed compared with wild-type mice. Absorption of (59)Fe from an oral dose was substantially impaired in NHE3(-/-) compared with wild-type mice. Our data point to an important role for NHE3 in generating the H(+) gradient that drives DMT1-mediated iron uptake at the intestinal brush border.