Corina M. Balut

Role of ubiquitylation and USP8-dependent deubiquitylation in the endocytosis and lysosomal targeting of plasma membrane KCa3.1

We recently demonstrated that plasma membrane KCa3.1 is rapidly endocytosed and targeted for lysosomal degradation via a Rab7‐ and ESCRT‐dependent pathway. Herein, we assess the role of ubiquitylation in this process. Using a biotin ligase acceptor peptide (BLAP)‐tagged KCa3.1, in combination with tandem ubiquitin binding entities (TUBEs), we demonstrate that KCa3.1 is polyubiquitylated following endocytosis. Hypertonic sucrose inhibited KCa3.1 endocytosis and resulted in a significant decrease in channel ubiquitylation. Inhibition of the ubiquitin‐activating enzyme (E1) with UBEI‐41 resulted in reduced KCa3.1 ubiquitylation and internalization. The general deubiquitylase (DUB) inhibitor, PR‐619 attenuated KCa3.1 degradation, indicative of deubiquitylation being required for lysosomal delivery. Using the DUB Chip, a protein microarray containing 35 DUBs, we demonstrate a time‐dependent association between KCa3.1 and USP8 following endocytosis, which was confirmed by coimmunoprecipitation. Further, overexpression of wild‐type USP8 accelerates channel deubiquitylation, while either a catalytically inactive mutant USP8 or siRNA‐mediated knockdown of USP8 enhanced accumulation of ubiquitylated KCa3.1, thereby inhibiting channel degradation. In summary, by combining BLAP‐tagged KCa3.1 with TUBEs and DUB Chip methodologies, we demonstrate that polyubiquitylation mediates the targeting of membrane KCa3.1 to the lysosomes and also that USP8 regulates the rate of KCa3.1 degradation by deubiquitylating KCa3.1 prior to lysosomal delivery.—Balut, C. M., Loch, C. M., Devor, D. C. Role of ubiquitylation and USP8‐dependent deubiquitylation in the endocytosis and lysosomal targeting of plasma membrane KCa3.1. FASEB J. 25, 3938–3948 (2011). www.fasebj.org

Recycling of the Ca2+-activated K+ Channel, KCa2.3, Is Dependent upon RME-1, Rab35/EPI64C, and an N-terminal Domain

Regulation of the number of Ca2+-activated K+ channels at the endothelial cell surface contributes to control of the endothelium-derived hyperpolarizing factor response, although this process is poorly understood. To address the fate of plasma membrane-localized KCa2.3, we utilized an extracellular epitope-tagged channel in combination with fluorescence and biotinylation techniques in both human embryonic kidney cells and the human microvascular endothelial cell line, HMEC-1. KCa2.3 was internalized from the plasma membrane and degraded with a time constant of 18 h. Cell surface biotinylation demonstrated that KCa2.3 was rapidly endocytosed and recycled back to the plasma membrane. Consistent with recycling, expression of a dominant negative (DN) RME-1 or Rab35 as well as wild type EPI64C, the Rab35 GTPase-activating protein, resulted in accumulation of KCa2.3 in an intracellular compartment. Expression of DN RME-1, DN Rab35, or wild type EPI64C resulted in a decrease in steady-state plasma membrane expression. Knockdown of EPI64C increased cell surface expression of KCa2.3. Furthermore, the effect of EPI64C was dependent upon its GTPase-activating proteins activity. Co-immunoprecipitation studies confirmed an association between KCa2.3 and both Rab35 and RME-1. In contrast to KCa2.3, KCa3.1 was rapidly endocytosed and degraded in an RME-1 and Rab35-independent manner. A series of N-terminal deletions identified a 12-amino acid region, Gly206–Pro217, as being required for the rapid recycling of KCa2.3. Deletion of Gly206–Pro217 had no effect on the association of KCa2.3 with Rab35 but significantly decreased the association with RME-1. These represent the first studies elucidating the mechanisms by which KCa2.3 is maintained at the plasma membrane.

Role of S3 and S4 Transmembrane Domain Charged Amino Acids in Channel Biogenesis and Gating of KCa2.3 and KCa3.1

The role of positively charged arginines in the fourth transmembrane domain (S4) and a single negatively charged amino acid in the third transmembrane domain (S3) on channel biogenesis and gating of voltage-gated K+ channels (Kv) has been well established. Both intermediate (KCa3.1) and small (KCa2.x) conductance, Ca2+-activated K+ channels have two conserved arginines in S4 and a single conserved glutamic acid in S3, although these channels are voltage-independent. We demonstrate that mutation of any of these charged amino acids in KCa3.1 or KCa2.3 to alanine, glutamine, or charge reversal mutations results in a rapid degradation (<30 min) of total protein, confirming the critical role of these amino acids in channel biogenesis. Mutation of the S4 arginine closest to the cytosolic side of KCa3.1 to histidine resulted in expression at the cell surface. Excised patch clamp experiments revealed that this Arg/His mutation had a dramatically reduced open probability (Po), relative to wild type channels. Additionally, we demonstrate, using a combination of short hairpin RNA, dominant negative, and co-immunoprecipitation studies, that both KCa3.1 and KCa2.3 are translocated out of the endoplasmic reticulum associated with Derlin-1. These misfolded channels are poly-ubiquitylated, recognized by p97, and targeted for proteasomal degradation. Our results suggest that S3 and S4 charged amino acids play an evolutionarily conserved role in the biogenesis and gating of KCa channels. Furthermore, these improperly folded K+ channels are translocated out of the endoplasmic reticulum in a Derlin-1- and p97-dependent fashion, poly-ubiquitylated, and targeted for proteasomal degradation.

Calcium-activated K + channels increase cell proliferation independent of K + conductance

The intermediate-conductance calcium-activated potassium channel (IK1) promotes cell proliferation of numerous cell types including endothelial cells, T lymphocytes, and several cancer cell lines. The mechanism underlying IK1-mediated cell proliferation was examined in human embryonic kidney 293 (HEK293) cells expressing recombinant human IK1 (hIK1) channels. Inhibition of hIK1 with TRAM-34 reduced cell proliferation, while expression of hIK1 in HEK293 cells increased proliferation. When HEK293 cells were transfected with a mutant (GYG/AAA) hIK1 channel, which neither conducts K(+) ions nor promotes Ca(2+) entry, proliferation was increased relative to mock-transfected cells. Furthermore, when HEK293 cells were transfected with a trafficking mutant (L18A/L25A) hIK1 channel, proliferation was also increased relative to control cells. The lack of functional activity of hIK1 mutants at the cell membrane was confirmed by a combination of whole cell patch-clamp electrophysiology and fura-2 imaging to assess store-operated Ca(2+) entry and cell surface immunoprecipitation assays. Moreover, in cells expressing hIK1, inhibition of ERK1/2 and JNK kinases, but not of p38 MAP kinase, reduced cell proliferation. We conclude that functional K(+) efflux at the plasma membrane and the consequent hyperpolarization and enhanced Ca(2+) entry are not necessary for hIK1-induced HEK293 cell proliferation. Rather, our data suggest that hIK1-induced proliferation occurs by a direct interaction with ERK1/2 and JNK signaling pathways.

Trafficking of intermediate (KCa3.1) and small (KCa2.x) conductance, Ca2+-activated K+ channels: A novel target for medicinal chemistry efforts?

Ca2+‐activated K+ (KCa) channels play a pivotal role in the physiology of a wide variety of tissues and disease states, including vascular endothelia, secretory epithelia, certain cancers, red blood cells (RBC), neurons, and immune cells. Such widespread involvement has generated an intense interest in elucidating the function and regulation of these channels, with the goal of developing pharmacological strategies aimed at selective modulation of KCa channels in various disease states. Herein we give an overview of the molecular and functional properties of these channels and their therapeutic importance. We discuss the achievements made in designing pharmacological tools that control the function of KCa channels by modulating their gating properties. Moreover, this review discusses the recent advances in our understanding of KCa channel assembly and anterograde trafficking toward the plasma membrane, the micro‐domains in which these channels are expressed within the cell, and finally the retrograde trafficking routes these channels take following endocytosis. As the regulation of intracellular trafficking by agonists as well as the protein–protein interactions that modify these events continue to be explored, we anticipate this will open new therapeutic avenues for the targeting of these channels based on the pharmacological modulation of KCa channel density at the plasma membrane.

ESCRT-dependent targeting of plasma membrane localized KCa3.1 to the lysosomes

The number of intermediate-conductance, Ca(2+)-activated K(+) channels (KCa3.1) present at the plasma membrane is deterministic in any physiological response. However, the mechanisms by which KCa3.1 channels are removed from the plasma membrane and targeted for degradation are poorly understood. Recently, we demonstrated that KCa3.1 is rapidly internalized from the plasma membrane, having a short half-life in both human embryonic kidney cells (HEK293) and human microvascular endothelial cells (HMEC-1). In this study, we investigate the molecular mechanisms controlling the degradation of KCa3.1 heterologously expressed in HEK and HMEC-1 cells. Using immunofluorescence and electron microscopy, as well as quantitative biochemical analysis, we demonstrate that membrane KCa3.1 is targeted to the lysosomes for degradation. Furthermore, we demonstrate that either overexpressing a dominant negative Rab7 or short interfering RNA-mediated knockdown of Rab7 results in a significant inhibition of channel degradation rate. Coimmunoprecipitation confirmed a close association between Rab7 and KCa3.1. On the basis of these findings, we assessed the role of the ESCRT machinery in the degradation of heterologously expressed KCa3.1, including TSG101 [endosomal sorting complex required for transport (ESCRT)-I] and CHMP4 (ESCRT-III) as well as VPS4, a protein involved in the disassembly of the ESCRT machinery. We demonstrate that TSG101 is closely associated with KCa3.1 via coimmunoprecipitation and that a dominant negative TSG101 inhibits KCa3.1 degradation. In addition, both dominant negative CHMP4 and VPS4 significantly decrease the rate of membrane KCa3.1 degradation, compared with wild-type controls. These results are the first to demonstrate that plasma membrane-associated KCa3.1 is targeted for lysosomal degradation via a Rab7 and ESCRT-dependent pathway.

Immunofluorescence-based assay to identify modulators of the number of plasma membrane KCa3.1 channels

BACKGROUND Intermediate conductance Ca2+-dependent K+ channels (KCa3.1) have been proposed as therapeutic targets for numerous diseases. We recently characterized the endocytic fate of these channels; leading to the possibility that this can be pharmacologically manipulated, thereby altering the number of channels (N) at the plasma membrane. RESULTS & DISCUSSION We demonstrate that plasma membrane-localized KCa3.1 can be rapidly(10 min) tagged with a fluorophore using a combination of a biotin ligase (BirA) acceptor peptide-tagged channel and an ER-localized BirA. Endocytosis of KCa3.1 was quantified using a 96-well plate format, demonstrating that the ubiquitin-activating enzyme E1 inhibitor UBEI-41, blocks the endocytosis of KCa3.1. CONCLUSION We describe a novel method for identifying modulators of KCa endocytosis and demonstrate this can be used to modulate Nat the plasma membrane. It is anticipated that altering N will provide novel therapeutic strategies for targeting these channels in disease.

Dynamin- and Rab5-Dependent Endocytosis of a Ca2+-Activated K+ Channel, KCa2.3

Regulation of the number of ion channels at the plasma membrane is a critical component of the physiological response. We recently demonstrated that the Ca2+-activated K+ channel, KCa2.3 is rapidly endocytosed and enters a Rab35- and EPI64C-dependent recycling compartment. Herein, we addressed the early endocytic steps of KCa2.3 using a combination of fluorescence and biotinylation techniques. We demonstrate that KCa2.3 is localized to caveolin-rich domains of the plasma membrane using fluorescence co-localization, transmission electron microscopy and co-immunoprecipitation (co-IP). Further, in cells lacking caveolin-1, we observed an accumulation of KCa2.3 at the plasma membrane as well as a decreased rate of endocytosis, as assessed by biotinylation. We also demonstrate that KCa2.3 and dynamin II are co-localized following endocytosis as well as demonstrating they are associated by co-IP. Further, expression of K44A dynamin II resulted in a 2-fold increase in plasma membrane KCa2.3 as well as a 3-fold inhibition of endocytosis. Finally, we evaluated the role of Rab5 in the endocytosis of KCa2.3. We demonstrate that expression of a dominant active Rab5 (Q79L) results in the accumulation of newly endocytosed KCa2.3 on to the membrane of the Rab5-induced vacuoles. We confirmed this co-localization by co-IP; demonstrating that KCa2.3 and Rab5 are associated. As expected, if Rab5 is required for the endocytosis of KCa2.3, expression of a dominant negative Rab5 (S34N) resulted in an approximate 2-fold accumulation of KCa2.3 at the plasma membrane. This was confirmed by siRNA-mediated knockdown of Rab5. Expression of the dominant negative Rab5 also resulted in a decreased rate of KCa2.3 endocytosis. These results demonstrate that KCa2.3 is localized to a caveolin-rich domain within the plasma membrane and is endocytosed in a dynamin- and Rab5-dependent manner prior to entering the Rab35/EPI64C recycling compartment and returning to the plasma membrane.

Investigation of the Ba2+-Sensitive NH4+ Transport Pathways in the Apical Cell Membrane of Primary Cultured Rabbit MTAL Cells

Background: Several apical ammonium (NH₄⁺/NH₃) transport pathways have been described in medullary thick ascending limb (MTAL) cells. The exact nature and importance of some of these pathways remain controversial. Methods: Ammonium transport in primary cultured rabbit MTAL cells was investigated by measuring intracellular pH (pH_i). Results: To create physiological conditions, experiments were performed in the symmetrical presence of NH₄Cl, which acidified the cells to pH_i 6.89. When blockers of apical NH₄⁺ transport were used, the cells alkalinized due to a decreased NH₄⁺ loading. The following values (pH units) were observed: bumetanide, +0.05; verapamil, +0.04; Ba²⁺ and Cs⁺, +0.19; tertiapin, +0.09. Tetraethylammonium had no effect. Depolarizing the cells by increasing the K⁺ concentration alkalinized the cells by 0.16 pH units. Because NH₄⁺ might enter through nonspecific channels, ammonium pulse experiments were performed: an NH₄Cl pulse acidified controls as well as depolarized cells. In contrast, when Ba²⁺, Cs⁺ or tertiapin were present, an NH₄Cl pulse alkalinized the cells. The pharmacological profile of this apical NH₄⁺ transport pathway correlates with the renal outer medullary K⁺ (ROMK) channel. Indirect immunofluorescence showed the presence of the ROMK protein. Conclusion: In these MTAL cells the Ba²⁺-sensitive component of NH₄⁺ transport is predominant and consists of permeation of NH₄⁺ through an apical ROMK-related channel.

Analysis of mitochondrial pH and ion concentrations.

Detailed practical information is provided with emphasis on mapping cytosolic and mitochondrial pH, mitochondrial Na(+), and briefly also aspects related to mitochondrial Ca(2+) measurements in living cells, as grown on (un)coated glass coverslips. This chapter lists (laser scanning confocal) microscope instrumentation and setup requirements for proper imaging conditions, cell holders, and an easy-to-use incubator stage. For the daily routine of preparing buffer and calibration solutions, extensive annotated protocols are provided. In addition, detailed measurement and image analysis protocols are given to routinely obtain optimum results with confidence, while avoiding a number of typical pitfalls.