Enhanced epithelial Na(+) channel (ENaC) activity in mouse endometrial epithelium by upregulation of gammaENaC subunit

Visualization of preimplantation uterine fluid absorption in mice using Alexa Fluor™ 488 Hydrazide†

Uterine fluid plays important roles in supporting early pregnancy events and its timely absorption is critical for embryo implantation. In mice, its volume is maximum on day 0.5 post-coitum (D0.5) and approaches minimum upon embryo attachment ~D4.0. Its secretion and absorption in ovariectomized rodents were shown to be promoted by estrogen and progesterone (P4), respectively. The temporal mechanisms in preimplantation uterine fluid absorption remain to be elucidated. We have established an approach using intraluminally injected Alexa Fluor™ 488 Hydrazide (AH) in preimplantation control (RhoAf/f) and P4-deficient RhoAf/fPgrCre/+ mice. In control mice, bulk entry (seen as smeared cellular staining) via uterine luminal epithelium (LE) decreases from D0.5 to D3.5. In P4-deficient RhoAf/fPgrCre/+ mice, bulk entry on D0.5 and D3.5 is impaired. Exogenous P4 treatment on D1.5 and D2.5 increases bulk entry in D3.5 P4-deficient RhoAf/fPgrCre/+ LE, while progesterone receptor (PR) antagonist RU486 treatment on D1.5 and D2.5 diminishes bulk entry in D3.5 control LE. The abundance of autofluorescent apical fine dots, presumptively endocytic vesicles to reflect endocytosis, in the LE cells is generally increased from D0.5 to D3.5 but its regulation by exogenous P4 or RU486 is not obvious under our experimental setting. In the glandular epithelium (GE), bulk entry is rarely observed and green cellular dots do not show any consistent differences among all the investigated conditions. This study demonstrates the dominant role of LE but not GE, the temporal mechanisms of bulk entry and endocytosis in the LE, and the inhibitory effects of P4-deficiency and RU486 on bulk entry in the LE in preimplantation uterine fluid absorption.

Timeline of Changes in Biomarkers Associated with Spinal Cord Injury-Induced Polyuria

Deficits in upper and lower urinary tract function, which include detrusor overactivity, urinary incontinence, detrusor-sphincter dyssynergia, and polyuria, are among the leading issues that arise after spinal cord injury (SCI) affecting quality of life. Given that overproduction of urine (polyuria) has been shown to be associated with an imbalance in key regulators of body fluid homeostasis, the current study examined the timing of changes in levels of various relevant hormones, peptides, receptors, and channels post-contusion injury in adult male Wistar rats. The results show significant up- or downregulation at various time points, beginning at 7 days post-injury, in levels of urinary atrial natriuretic peptide, serum arginine vasopressin (AVP), kidney natriuretic peptide receptor-A, kidney vasopressin-2 receptor, kidney aquaporin-2 channels, and kidney epithelial sodium channels (β- and γ-, but not α-, subunits). The number of AVP-labeled neurons in the hypothalamus (supraoptic and -chiasmatic, but not paraventricular, nuclei) was also significantly altered at one or more time points. These data show significant fluctuations in key biomarkers involved in body fluid homeostasis during the post-SCI secondary injury phase, suggesting that therapeutic interventions (e.g., desmopressin, a synthetic analogue of AVP) should be considered early post-SCI.

Perinatal administration of bisphenol A alters the expression of tight junction proteins in the uterus and reduces the implantation rate

We studied the effect of bisphenol-A (BPA) administration to rats, during the perinatal period, on the fertility of F1 generation and on the expression of tight junction (TJ) proteins in the uterus during early pregnancy. Pregnant Wistar dams (F0) received: BPA-L (0.05mg/kg/day), BPA-H (20mg/kg/day) or vehicle, from gestational day (GD) 6 to lactation day 21. F1 female pups were mated at 3 months of age and sacrificed at GD 1, 3, 6, and 7. Serum hormonal levels, ovulation rate, number of implantation sites and expression of TJ proteins in the uterus of F1 females were evaluated. BPA treatment induced no change in ovulation rate, but induced alterations in progesterone (P₄) and estradiol (E₂) serum levels, and in implantation rate. With regards to TJ proteins, BPA-H increased claudin-1 during all GDs; eliminated the peaks of claudins -3 and -4 at GD 3 and 6, respectively; and decreased claudin-7 at GD 6, ZO-1 from GD 1-6, and claudin-3 at GD 7 in stromal cells. BPA-L instead, eliminated claudin-3 peak at GD 3, increased claudin-4 and decreased claudin-7 from GD 1-6, decreased claudin-1 at GD 3 and 7 and claudin-4 at GD 7 in stromal cells. BPA-L also decreased ZO-1 at GDs 1 and 3 and increased ZO-1 at GD 6. Thus, BPA treatment during perinatal period perturbed, when the animals reached adulthood and became pregnant, the particular expression of TJ proteins in the uterine epithelium and reduced in consequence the number of implantation sites.

Estrogen, progesterone, and genistein differentially regulate levels of expression of α-, β-, and γ-epithelial sodium channel (ENaC) and α-sodium potassium pump (Na⁺/K⁺-ATPase) in the uteri of sex steroid-deficient rats

Estrogen, progesterone, and genistein could induce changes in uterine fluid volume and Na(+) concentration. Progesterone upregulates expression of epithelial sodium channel (ENaC) and Na(+)/K(+)-ATPase which contributed toward these changes. However, effects of estrogen and genistein were unknown. This study therefore investigated changes in expression of these proteins in the uterus under estrogen, progesterone, and genistein influences to further understand mechanisms underlying sex steroids and phytoestrogen effects on uterine fluid Na(+) regulation. In this study, uteri of ovariectomized female rats receiving 7-day treatment with genistein (25, 50, and 100 mg/kg/day), estrogen (0.8 × 10(-4) mg/kg/day), or progesterone (4 mg/kg/day) were harvested, and expression levels of α-, β-, and γ-ENaC proteins and messenger RNAs (mRNAs) and α-Na(+)/K(+)-ATPase protein were determined by Western blotting (proteins) and real-time polymerase chain reaction (mRNA). Meanwhile, distribution of α-, β-, and γ-ENaC and α-Na(+)/K(+)-ATPase proteins in the uterus was identified by immunohistochemistry. Our findings indicated that expression of α-, β-, and γ-ENaC proteins and mRNAs and α-Na(+)/K(+)-ATPase protein were enhanced under progesterone influence. Lower expressions were noted under estrogen and genistein influences compared to progesterone. Under estrogen, progesterone, and genistein influences, α- and β-ENaC were distributed at apical membrane and γ-ENaC was distributed at apical and basolateral membranes of uterine luminal epithelia. Under progesterone influence, α-Na(+)/K(+)-ATPase was highly expressed at basolateral membrane. In conclusion, high expression of α-, β-, and γ-ENaC and α-Na(+)/K(+)-ATPase under progesterone influence would contribute toward increased uterine fluid Na(+) reabsorption, whereas lesser expression of these proteins under estrogen and genistein influences would contribute toward lower reabsorption of uterine fluid Na(+).

Regulation of miR-101/miR-199a-3p by the epithelial sodium channel during embryo implantation: involvement of CREB phosphorylation

In our previous study, we have demonstrated that the epithelial sodium channel (ENaC) mediates the embryo-derived signals leading to the activation of CREB and upregulation of cyclooxygenase type 2 (COX2) required for embryo implantation. This study aims to investigate whether microRNAs (miRNAs) are involved in the ENaC-induced upregulation of COX2 during embryo implantation. The results show that the levels of miR-101 and miR-199a-3p, two COX2 targeting miRNAs, are reduced by ENaC activation, and increased by ENaC inhibition or knock-down of ENaC subunit (ENaCα) in human endometrial surface epithelial (HES) cells or in mouse uteri during implantation. Phosphorylation of CREB is induced by the activation of ENaC, and blocked by ENaC inhibition or knockdown in HES cells. Knockdown of ENaCα or CREB in HES cells or in mouse uterus in vivo results in increases in miR-101 and miR-199a-3p, accompanied with decreases in COX2 protein levels and reduction in implantation rate. The downregulation of COX2 caused by knockdown of ENaC or CREB can be recovered by the inhibitors of miR-101 or miR-199a-3p in HES cells. These results reveal a novel molecular mechanism modulating COX2 expression during embryo implantation via ENaC-dependent CREB activation and COX2-targeting miRNAs.

Ovarian hyperstimulation affects fluid transporters in the uterus: a potential mechanism in uterine receptivity

Controlled ovarian hyperstimulation is commonly used in fertility treatment. Evidence suggests that this could alter the endometrial environment and influence implantation rate. However, the mechanisms underlying this disruption are unknown. A recently developed rat ovarian hyperstimulation (OH) model found alterations in the localisation and expression of several molecules associated with implantation, as well as an increase in luminal fluid at the time of implantation. The present study investigated the effects of OH in rats on the expression of fluid-transporting molecules aquaporin 5 (AQP5) and claudin 4. The expression of these proteins was investigated in uterine luminal epithelial cells of rats undergoing OH and compared with normal pregnancy. There was a significant increase in AQP5 protein in OH rats at the time of implantation, along with a loss of the mesometrial staining gradient, which is thought to contribute to implantation position. At the same time, there was a significant decrease in claudin 4 protein. These results suggest that OH in rats causes a dysregulation in uterine fluid dynamics through modifications to fluid-transporting molecules, resulting in an unfavourable implantation environment for the blastocyst.

The hormonal control of uterine luminal fluid secretion and absorption

The secretion of uterine luminal fluid initially provides a transport and support medium for spermatozoa and unimplanted embryos, while the absorption of uterine luminal fluid in early pregnancy results in the closure of the lumen and allows blastocysts to establish intimate contact with the uterine epithelium. We have established an in vivo perfusion technique of the lumen to study the hormonal control of the events in the peri-implantation period. Fluorescein-labelled dextran was included in the perfusion medium to monitor fluid movements and the concentrations of Na(+) and CI(-) ions in the effluent were monitored. Using an established regimen of steroid treatment of ovariectomized rats mimicking early pregnancy, oestradiol caused fluid secretion, while progesterone resulted in an amiloride-sensitive fluid absorption. Fluid absorption peaked at about the expected time of implantation. The effect of progesterone could be inhibited by treatment with a high dose of oestradiol, by the anti-progestin RU486, and by the presence of an intra-uterine contraceptive device. Studies of expression of Na(+) and CI(-) channels (ENaC, CFTR) indicated that these channels were subject to tissue-specific regulation within the uterus, but more work is required to determine their role and the factors controlling their abundance and localization in early pregnancy.

Noncoordinate regulation of ENaC: paradigm lost?

The epithelial sodium channel (ENaC) is composed of the three homologous subunits α, β, and γ. The basic oligomerization process inferred from all studies in heterologous systems is preferential assembly of the three subunits into a single oligomeric form. However, there is also considerable evidence that channels composed of only α-, αβ-, or αγ-subunits can form under some circumstances and that individual subunits expressed in heterologous systems can traffic to the cell membrane. In cells that express endogenous ENaC, the three subunits are often synthesized in a differential fashion, with one or two subunits expressed constitutively while the other(s) are induced by different physiological stimuli in parallel with increased ENaC activity. This phenomenon, which we term noncoordinate regulation, has been observed for both whole cell and apical membrane ENaC subunit expression. Several other heteromeric membrane proteins have also been observed to have differential rates of either turnover or trafficking of individual subunits after biosynthesis and membrane localization. Here, we examine the possibility that noncoordinate regulation of ENaC subunits may represent another mechanism in the arsenal of physiological responses to diverse stimuli.

The gamma-subunit of ENaC is more important for channel surface expression than the beta-subunit

The amiloride-sensitive epithelial sodium channel (ENaC) plays a critical role in fluid and electrolyte homeostasis and is composed of three homologous subunits: alpha, beta, and gamma. Only heteromultimeric channels made of alphabetagammaENaC are efficiently expressed at the cell surface, resulting in maximally amiloride-sensitive currents. To study the relative importance of various regions of the beta- and gamma-subunits for the expression of functional ENaC channels at the cell surface, we constructed hemagglutinin (HA)-tagged beta-gamma-chimeric subunits composed of beta- and gamma-subunit regions and coexpressed them with HA-tagged alphabeta- and alphagamma-subunits in Xenopus laevis oocytes. The whole cell amiloride-sensitive sodium current (DeltaI(ami)) and surface expression of channels were assessed in parallel using the two-electrode voltage-clamp technique and a chemiluminescence assay. Because coexpression of alphagammaENaC resulted in larger DeltaI(ami) and surface expression compared with coexpression of alphabetaENaC, we hypothesized that the gamma-subunit is more important for ENaC trafficking than the beta-subunit. Using chimeras, we demonstrated that channel activity is largely preserved when the highly conserved second cysteine rich domains (CRD2) of the beta- and gamma-subunits are exchanged. In contrast, exchanging the whole extracellular loops of the beta- and the gamma-subunits largely reduced ENaC currents and ENaC expression in the membrane. This indicates that there is limited interchangeability between molecular regions of the two subunits. Interestingly, our chimera studies demonstrated that the intracellular termini and the two transmembrane domains of gammaENaC are more important for the expression of functional channels at the cell surface than the corresponding regions of betaENaC.