ADH-induced recycling of fluid-phase marker from endosomes to the mucosal surface in toad bladder

Regulation of the epithelial sodium channel by membrane trafficking

The epithelial Na(+) channel (ENaC) is a major regulator of salt and water reabsorption in a number of epithelial tissues. Abnormalities in ENaC function have been directly linked to several human disease states including Liddle's syndrome, psuedohypoaldosteronism, and cystic fibrosis and may be implicated in states as diverse as salt-sensitive hypertension, nephrosis, and pulmonary edema. ENaC activity in epithelial cells is highly regulated both by open probability and number of channels. Open probability is regulated by a number of factors, including proteolytic processing, while ENaC number is regulated by cellular trafficking. This review discusses current understanding of apical membrane delivery, cell surface stability, endocytosis, retrieval, and recycling of ENaC and the molecular partners that have so far been shown to participate in these processes. We review known sites and mechanisms of hormonal regulation of trafficking by aldosterone, vasopressin, and insulin. While many details of the regulation of ENaC trafficking remain to be elucidated, knowledge of these mechanisms may provide further insights into ENaC activity in normal and disease states.

Acute ENaC stimulation by cAMP in a kidney cell line is mediated by exocytic insertion from a recycling channel pool

Acute hormonal regulation of the epithelial sodium channel (ENaC) in tight epithelia increases transcellular Na⁺ transport via trafficking of intracellular channels to the apical surface. The fate of the channels removed from the apical surface following agonist washout is less clear. By repetitively stimulating polarized mouse cortical collecting duct (mCCD, _MPKCCD₁₄) epithelia, we evaluated the hypothesis that ENaC recycles through an intracellular pool to be available for reinsertion into the apical membrane. Short circuit current (I_SC), membrane capacitance (C_T), and conductance (G_T) were recorded from mCCD epithelia mounted in modified Ussing chambers. Surface biotinylation of ENaC demonstrated an increase in channel number in the apical membrane following cAMP stimulation. This increase was accompanied by a 83 ± 6% (n = 31) increase in I_SC and a 15.3 ± 1.5% (n = 15) increase in C_T. Selective membrane permeabilization demonstrated that the C_T increase was due to an increase in apical membrane capacitance. I_SC and C_T declined to basal levels on stimulus washout. Repetitive cAMP stimulation and washout (∼1 h each cycle) resulted in response fatigue; ΔI_SC decreased ∼10% per stimulation–recovery cycle. When channel production was blocked by cycloheximide, ΔI_SC decreased ∼15% per stimulation cycle, indicating that newly synthesized ENaC contributed a relatively small fraction of the channels mobilized to the apical membrane. Selective block of surface ENaC by benzamil demonstrated that channels inserted from a subapical pool made up >90% of the stimulated I_SC, and that on restimulation a large proportion of channels retrieved from the apical surface were reinserted into the apical membrane. Channel recycling was disrupted by brefeldin A, which inhibited ENaC exocytosis, by chloroquine, which inhibited ENaC endocytosis and recycling, and by latrunculin A, which blocked ENaC exocytosis. A compartment model featuring channel populations in the apical membrane and intracellular recycling pool provided an adequate kinetic description of the I_SC responses to repetitive stimulation. The model supports the concept of ENaC recycling in response to repetitive cAMP stimulation.

Coupling of vasopressin-induced intracellular Ca2+ mobilization and apical exocytosis in perfused rat kidney collecting duct

Arginine vasopressin (AVP) regulates the osmotic water permeability of the kidney collecting duct by inducing exocytotic insertion of aquaporin-2 into apical membrane. The coupling between AVP-induced intracellular Ca2+ mobilization and apical exocytosis was investigated in isolated perfused rat inner medullary collecting duct (IMCD) segments using confocal fluorescence microscopy. Changes of [Ca2+]i in IMCD cells were measured with fluo-4. A novel confocal imaging technique using a styryl dye, FM1-43, was developed to monitor real-time exocytosis induced by arginine vasopressin. AVP (0.1 nM) triggered a rapid increase of [Ca2+]i in IMCD cells, followed by sustained oscillations. Ratiometric measurement of [Ca2+]i confirmed that the observed [Ca2+]i oscillation was a primary event and was not secondary to changes in cell volume. The frequencies of [Ca2+]i oscillations in each IMCD cell were independent and time variant. 1-Deamino-8-D-arginine vasopressin (a V2 receptor agonist, 0.1 nM) simulated the effects of AVP by triggering [Ca2+]i oscillations. In the absence of extracellular Ca2+, ryanodine (0.1 mM) inhibited AVP-induced Ca2+ mobilization. AVP (0.1 nM) triggered accumulative apical exocytosis in IMCD cells within 20 s after application. Pre-incubating the IMCD with an intracellular Ca2+ chelator, BAPTA, prevented AVP-induced intracellular Ca2+ mobilization, apical exocytosis, and increase of osmotic water permeability. These results indicate that AVP, via the V2 receptor, triggers a calcium signalling cascade observed as [Ca2+]i oscillations in the IMCD and that intracellular Ca2+ mobilization is required for exocytotic insertion of aquaporin-2.

Luminal L-alanine stimulates exocytosis at the K+-conductive apical membrane of Aplysia enterocytes

In Aplysia intestine, stimulation of Na+ absorption with luminal alanine increases apical membrane K+ conductance (GK,a), which presumably regulates enterocyte volume during stimulated Na+ absorption. However, the mechanism responsible for the sustained increase in plasma membrane K+ conductance is not known for any nutrient-absorbing epithelium. In the present study, we have begun to test the hypothesis that the alanine-induced increase in GK,a in Aplysia enterocytes results from exocytic insertion of K+ channels into the apical membrane. We used the fluid-phase marker horseradish peroxidase to assess the effect of alanine on apical membrane exocytosis and conventional microelectrode techniques to assess the effect of alanine on fractional capacitance of the apical membrane (fCa). Luminal alanine significantly increased apical membrane exocytosis from 1.04 +/- 0.30 to 1.39 +/- 0.38 ng. min-1. cm-2. To measure fCa, we modeled the Aplysia enterocyte as a double resistance-capacitance (RC) electric circuit arranged in series. Several criteria were tested to confirm application of the model to the enterocytes, and all satisfied the model. When added to the luminal surface, alanine significantly increased fCa from 0.27 +/- 0. 02 to 0.33 +/- 0.04 (n = 10) after 4 min. There are two possible explanations for our findings: 1) the increase in exocytosis, which adds membrane to the apical plasma membrane, prevents plasma membrane fracture, and 2) the increase in exocytosis delivers K+ channels to the apical membrane by exocytic insertion. After the alanine-induced depolarization of apical membrane potential (Va), there is a strong correlation (r = 0.96) between repolarization of Va, which reflects the increase in GK,a, and increase in fCa. This correlation supports the exocytic insertion hypothesis for activation of GK,a.

Antidiuretic hormone and exocytosis: lessons from neurosecretion

Many cells, both single and epithelial, are programmed for exocytosis. In most cases, the contents of cytoplasmic vesicles are delivered rapidly and directly to the extracellular fluid. The process has been intensively studied in the chromaffin cell and the nerve terminal, where, as in other cells, exocytosis is under a complex type of cytoskeletal control. An array of vesicle-associated proteins mediates attachment of the vesicles to the cytoskeleton, their release, and their fusion with the plasma membrane. Two functional pools of vesicles, the releasable and reserve pool, carry out immediate and long-term secretory activity. Some of the mediators of neurotransmitter vesicle fusion, originally thought to be restricted to neurosecretory cells, have now been found in nonneuronal cells. The mammalian collecting duct and the amphibian bladder are also engaged in exocytosis. In both epithelia, antidiuretic hormone (ADH) induces the transfer of water channels from cytoplasmic vesicles to the apical cell membrane. The process is slower than in the nerve terminal and ends with channel placement rather than the extrusion of vesicular contents. Nevertheless, there are several respects in which cytoskeletal control, vesicle positioning in the cell, docking, and fusion may prove to resemble the events in neurosecretion. This review begins with a survey of cytoskeletal structure and function in the erythrocyte, the chromaffin cell, and the nerve terminal and then presents current studies of ADH-induced exocytosis, emphasizing common themes in cytoskeletal control.