Rapid translocation and insertion of the epithelial Na+ channel in response to RhoA signaling

Regulation of the epithelial sodium channel by phosphatidylinositides: experiments, implications, and speculations

Recent studies suggest that the activity of epithelial sodium channels (ENaC) is increased by phosphatidylinositides, especially phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)). Stimulation of phospholipase C by either adenosine triphosphate (ATP)-activation of purinergic P2Y receptors or epidermal growth factor (EGF)-activation of EGF receptors reduces membrane PI(4,5)P(2), and consequently decreases ENaC activity. Since ATP and EGF may be trapped in cysts formed by the distal tubule, it is possible that ENaC inhibition induced by ATP and EGF facilitates cyst formation in polycystic kidney diseases (PKD). However, some results suggest that ENaC activity is increased in PKD. In contrast to P2Y and EGF receptors, stimulation of insulin-like growth factor-1 (IGF-1) receptor by aldosterone or insulin produces PI(3,4,5)P(3), and consequently increases ENaC activity. The acute effect of aldosterone on ENaC activity through PI(3,4,5)P(3) possibly accounts for the initial feedback for blood volume recovery after hypovolemic hypotension. PI(4,5)P(2) and PI(3,4,5)P(3), respectively, interacts with the N terminus of beta-ENaC and the C terminus of gamma-ENaC. However, whether ENaC selectively binds to PI(4,5)P(2) and PI(3,4,5)P(3) over other anionic phospholipids remains unclear.

Acute regulation of the epithelial Na+ channel by phosphatidylinositide 3-OH kinase signaling in native collecting duct principal cells

Activity of the epithelial Na(+) channel (ENaC) is limiting for Na(+) reabsorption in the aldosterone-sensitive distal nephron. Hormones, including aldosterone and insulin, increase ENaC activity, in part by stimulating phosphatidylinositide 3-OH kinase (PI3-K) signaling. Recent studies in heterologous expression systems reveal a close spatiotemporal coupling between PI3-K signaling and ENaC activity with the phospholipid product of this kinase, PI(3,4,5)P(3), in some cases, directly binding the channel and increasing open probability (P(o)). This study tested whether this tight coupling plays a physiologic role in modulating ENaC activity in native tissue and polarized epithelial cells. IGF-I was found to increase Na(+) reabsorption across mpkCCD(c14) principal cell monolayers in a PI3-K-sensitive manner. Inhibition of PI3-K signaling, moreover, rapidly decreased Na(+) reabsorption and ENaC activity in mpkCCD(c14) cells that were treated with corticosteroids and IGF-I. These decreases paralleled changes in apical membrane PI(3,4,5)P(3) levels, demonstrating tight spatiotemporal coupling between ENaC activity and PI3-K/PI(3,4,5)P(3) signaling within this membrane. For further probing of the mechanism underpinning this coupling, cortical collecting ducts (CCD) were isolated from rat and split open to expose the apical membrane for patch-clamp analysis. Inhibition of PI3-K signaling with wortmannin and LY294002 but not its inactive analogue rapidly and markedly decreased the P(o) of ENaC. Moreover, IGF-I acutely increased P(o) of ENaC in CCD principal cells in a PI3-K-sensitive manner. Together, these observations stress the importance of tight spatiotemporal coupling between PI3-K signaling and ENaC within the apical membrane of principal cells to the physiologic control of this ion channel.

Quantifying RhoA facilitated trafficking of the epithelial Na+ channel toward the plasma membrane with total internal reflection fluorescence-fluorescence recovery after photobleaching

The epithelial Na(+) channel (ENaC) plays a central role in control of epithelial surface hydration and vascular volume. Similar to other ion channels, ENaC activity is set, in part, by its membrane levels. The small G protein RhoA increases ENaC activity by increasing the membrane levels of this channel. We hypothesize that RhoA increases ENaC activity by promoting channel trafficking to the plasma membrane. Few experimental methods are available to directly visualize trafficking of ion channels to the plasma membrane. Here we combine electrophysiology with two complementary imaging methods, total internal reflection fluorescence microscopy and fluorescence recovery after photobleaching, to study the mechanistic basis of RhoA actions on ENaC. Patch clamp results demonstrate that RhoA increases ENaC activity in an additive manner with dominant-negative dynamin. This is consistent with a mechanism of increased ENaC trafficking to the membrane. Direct visualization of ENaC movement near the plasma membrane with total internal reflection fluorescence-fluorescence recovery after photobleaching revealed that RhoA accelerates ENaC trafficking toward the membrane. RhoA-facilitated movement of the channel was sensitive to disrupting the endomembrane system. Moreover, facilitating retrieval decreased ENaC activity but not trafficking toward the membrane. ENaC at the plasma membrane clustered and was laterally immobile suggesting that the cytoskeleton tethers or corrals membrane resident channels or membrane-directed vesicles containing ENaC. Disrupting microtubules but not microfilaments led to reorganization of ENaC clusters and slowed trafficking toward the membrane. The cytoskeleton is an established target for RhoA signaling. We conclude that RhoA, likely through effects on the cytoskeleton, promotes ENaC trafficking to the plasma membrane to increase channel membrane levels and activity.