Intrinsic voltage dependence of the epithelial Na+ channel is masked by a conserved transmembrane domain tryptophan

Evolution of the insect PPK gene family

Insect pickpocket (PPK) receptors mediate diverse functions, among them the detection of mechano- and chemo-sensory stimuli. Notwithstanding their relevance, studies on their evolution only focused on Drosophila. We have analyzed the genomes of 26 species of 8 orders including holometabolous and hemimetabolous insects (Blattodea, Orthoptera, Hemiptera, Phthiraptera, Hymenoptera, Lepidoptera, Coleoptera, and Diptera), to characterize the evolution of this gene family. PPKs were detected in all genomes analyzed, with 578 genes distributed in 7 subfamilies. According to our phylogeny ppk17 is the most divergent member, composing the new subfamily VII. PPKs evolved under a gene birth-and-death model that generated lineage-specific expansions usually located in clusters, while purifying selection affected several orthogroups. Subfamily V was the largest, including a mosquito-specific expansion that can be considered a new target for pest control. PPKs present a high gene turnover generating considerable variation. On one hand, Musca domestica (59), Aedes albopictus (51), Culex quinquefasciatus (48), and Blattella germanica (41) presented the largest PPK repertoires. On the other hand, Pediculus humanus (only ppk17), bees and ants (6-9) had the smallest PPK sets. A subset of prevalent PPKs was identified, indicating very conserved functions for these receptors. Finally, at least twenty percent of the sequences presented calmodulin-binding motifs, suggesting that these PPKs may amplify sensory responses similarly as proposed for D. melanogaster ppk25. Overall, this work characterized the evolutionary history of these receptors revealing relevant unknown gene sequence features and clade-specific expansions.

Deficient transient receptor potential vanilloid type 4 function contributes to compromised [Ca2+]i homeostasis in human autosomal-dominant polycystic kidney disease cells

Autosomal-dominant polycystic kidney disease (ADPKD) is a devastating disorder that is characterized by a progressive decline in renal function as a result of the development of fluid-filled cysts. Defective flow-mediated [Ca²⁺]_i responses and disrupted [Ca²⁺]_i homeostasis have been repeatedly associated with cyst progression in ADPKD. We have previously demonstrated that the transient receptor potential vanilloid type 4 (TRPV4) channel is imperative for flow-mediated [Ca²⁺]_i responses in murine distal renal tubule cells. To determine whether compromised TRPV4 function contributes to aberrant Ca²⁺ regulation in ADPKD, we assessed TRPV4 function in primary cells that were cultured from ADPKD and normal human kidneys (NHKs). Single-channel TRPV4 activity and TRPV4-dependent Ca²⁺ influxes were drastically reduced in ADPKD cells, which correlated with distorted [Ca²⁺]_i signaling. Whereas total TRPV4 protein levels were comparable in NHK and ADPKD cells, we detected a marked decrease in TRPV4 glycosylation in ADPKD cells. Tunicamycin-induced deglycosylation inhibited TRPV4 activity and compromised [Ca²⁺]_i signaling in NHK cells. Overall, we demonstrate that TRPV4 glycosylation and channel activity are diminished in human ADPKD cells compared with NHK cells, and that this contributes significantly to the distorted [Ca²⁺]_i dynamics. We propose that TRPV4 stimulation may be beneficial for restoring [Ca²⁺]_i homeostasis in cyst cells, thereby interfering with ADPKD progression.-Tomilin, V., Reif, G. A., Zaika, O., Wallace, D. P., Pochynyuk, O. Deficient transient receptor potential vanilloid type 4 function contributes to compromised [Ca²⁺]_i homeostasis in human autosomal-dominant polycystic kidney disease cells.

Dietary K+ and Cl- independently regulate basolateral conductance in principal and intercalated cells of the collecting duct

The renal collecting duct contains two distinct cell types, principal and intercalated cells, expressing potassium K_ir4.1/5.1 (KCNJ10/16) and chloride ClC-K2 (ClC-Kb in humans) channels on their basolateral membrane, respectively. Both channels are thought to play important roles in controlling systemic water-electrolyte balance and blood pressure. However, little is known about mechanisms regulating activity of K_ir4.1/5.1 and ClC-K2/b. Here, we employed patch clamp analysis at the single channel and whole cell levels in freshly isolated mouse collecting ducts to investigate regulation of K_ir4.1/5.1 and ClC-K2/b by dietary K⁺ and Cl^- intake. Treatment of mice with high K⁺ and high Cl^- diet (6% K⁺, 5% Cl^-) for 1 week significantly increased basolateral K⁺-selective current, single channel K_ir4.1/5.1 activity and induced hyperpolarization of basolateral membrane potential in principal cells when compared to values in mice on a regular diet (0.9% K⁺, 0.5% Cl^-). In contrast, basolateral Cl^--selective current and single channel ClC-K2/b activity was markedly decreased in intercalated cells under this condition. Substitution of dietary K⁺ to Na⁺ in the presence of high Cl^- exerted a similar inhibiting action of ClC-K2/b suggesting that the channel is sensitive to variations in dietary Cl^- per se. Cl^--sensitive with-no-lysine kinase (WNK) cascade has been recently proposed to orchestrate electrolyte transport in the distal tubule during variations of dietary K⁺. However, co-expression of WNK1 or its major downstream effector Ste20-related proline-alanine-rich kinase (SPAK) had no effect on ClC-Kb over-expressed in Chinese hamster ovary (CHO) cells. Treatment of mice with high K⁺ diet without concomitant elevations in dietary Cl^- (6% K⁺, 0.5% Cl^-) elicited a comparable increase in basolateral K⁺-selective current, single channel K_ir4.1/5.1 activity in principal cells, but had no significant effect on ClC-K2/b activity in intercalated cells. Furthermore, stimulation of aldosterone signaling by Deoxycorticosterone acetate (DOCA) recapitulated the stimulatory actions of high K⁺ intake on K_ir4.1/5.1 channels in principal cells but was ineffective to alter ClC-K2/b activity and basolateral Cl^- conductance in intercalated cells. In summary, we report that variations of dietary K⁺ and Cl^- independently regulate basolateral potassium and chloride conductance in principal and intercalated cells. We propose that such discrete mechanism might contribute to fine-tuning of urinary excretion of electrolytes depending on dietary intake.

Pseudohypoaldosteronism type 1 and Liddle's syndrome mutations that affect the single-channel properties of the epithelial Na+ channel

These studies test whether three disease-causing mutations in genes (SCNN1A and SCNN1G) encoding subunits of the epithelial Na⁺ channel, ENaC, affect the biophysical and gating properties of this important renal ion channel. The S562P missense mutation in αENaC and the K106_S108delinsN mutation in γENaC are associated with pseudohypoaldosteronism type 1 (PHA1). The N530S missense mutation in γENaC causes Liddle’s syndrome. Incorporation of S562P into αENaC and K106_S108N into γENaC resulted in significant decreases in macroscopic ENaC currents. Conversely, incorporation of N530S into γENaC increased macroscopic ENaC current. The S562P substitution resulted in a nonfunctional channel. The K106_S108N mutation produced a functional channel having a normal macroscopic current–voltage relation, there was a slight but significant decrease in unitary conductance and a marked decrease in single-channel open probability. The N530S substitution increased single-channel open probability having no effect on the macroscopic current–voltage relation or unitary conductance of the channel. These findings are consistent with mutation of residues at 562 in αENaC and 530 in γENaC, and a 3′ splice site in SCNN1G (318-1 G→A; K106_108SdelinsN) resulting in aberrant ENaC activity due to changes in the biophysical and gating properties of the channel. Such changes likely contribute to the cellular mechanism underpinning the PHA1 and Liddle’s syndrome caused by these mutations in ENaC subunits.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

Structural mechanisms underlying the function of epithelial sodium channel/acid-sensing ion channel

PURPOSE OF REVIEW

The epithelial sodium channel/degenerin family encompasses a group of cation-selective ion channels that are activated or modulated by a variety of extracellular stimuli. This review describes findings that provide new insights into the molecular mechanisms that control the function of these channels.

RECENT FINDINGS

Epithelial sodium channels facilitate Na⁺ reabsorption in the distal nephron and hence have a role in fluid volume homeostasis and arterial blood pressure regulation. Acid-sensing ion channels are broadly distributed in the nervous system where they contribute to the sensory processes. The atomic structure of acid-sensing ion channel 1 illustrates the complex trimeric architecture of these proteins. Each subunit has two transmembrane spanning helices, a highly organized ectodomain and intracellular N-terminus and C-terminus. Recent findings have begun to elucidate the structural elements that allow these channels to sense and respond to extracellular factors. This review emphasizes the roles of the extracellular domain in sensing changes in the extracellular milieu and of the residues in the extracellular-transmembrane domains interface in coupling extracellular changes to the pore of the channel.

SUMMARY

Epithelial sodium channels and acid-sensing ion channels have evolved to sense extracellular cues. Future research should be directed toward elucidating how changes triggered by extracellular factors translate into pore opening and closing events.

Voltage-dependent gating underlies loss of ENaC function in Pseudohypoaldosteronism type 1

Here we explore the mechanism and associated structure-function implications of loss of function for epithelial Na(+) channel (ENaC) containing a pseudohypoaldosteronism type 1 (PHA-1)-causing missense point mutation. As expected, human ENaC that contained subunits harboring PHA-1-causing substitutions within an absolutely conserved, cytosolic Gly residue (e.g., βG37S) had significantly less activity. Unexpectedly, though, such substitution also results in voltage sensitivity with greater activity at hyperpolarizing potentials. This is a consequence of voltage-dependent changes in the single-channel open probability and is not species- or subunit-dependent. Voltage sensitivity in PHA-1 mutants stems from the disruption of critical structure, rather than the development of new properties resulting from the introduction of novel side chains. Residues near the conserved His-Gly sequence of G95 in α-mENaC are particularly important for voltage sensing. Although substitution of I93 in α-mENaC results in voltage sensing, it also slows the activation and deactivation kinetics enough to enable capture of the dynamic changes in single-channel open probability that account for changes in macroscopic activity. This provides definitive proof of the mechanism that underlies loss of function. In addition, the voltage dependence of ENaC with PHA-1 substitutions is akin to that which results from substitution of a critical, interfacial Trp residue conserved at the intracellular base of TM1 (e.g., W112 in α-mENaC). Dynamic interactions between similarly positioned His and Trp residues are essential for gating and the girdle-like structure that lines the intracellular mouth of the M2 proton channel. The similar residues in ENaC may serve a shared function, suggesting the possibility of an intracellular girdle just below the mouth of the ENaC pore.

ENaC structure and function in the wake of a resolved structure of a family member

Our understanding of epithelial Na(+) channel (ENaC) structure and function has been profoundly impacted by the resolved structure of the homologous acid-sensing ion channel 1 (ASIC1). The structure of the extracellular and pore regions provide insight into channel assembly, processing, and the ability of these channels to sense the external environment. The absence of intracellular structures precludes insight into important interactions with intracellular factors that regulate trafficking and function. The primary sequences of ASIC1 and ENaC subunits are well conserved within the regions that are within or in close proximity to the plasma membrane, but poorly conserved in peripheral domains that may functionally differentiate family members. This review examines functional data, including ion selectivity, gating, and amiloride block, in light of the resolved ASIC1 structure.