Degradation of Trafficking-defective Long QT Syndrome Type II Mutant Channels by the Ubiquitin-Proteasome Pathway

Reciprocal control of hERG stability by Hsp70 and Hsc70 with implication for restoration of LQT2 mutant stability

RATIONALE

The human ether-a-go-go-related gene (hERG) encodes the α subunit of the potassium current I(Kr). It is highly expressed in cardiomyocytes and its mutations cause long QT syndrome type 2. Heat shock protein (Hsp)70 is known to promote maturation of hERG. Hsp70 and heat shock cognate (Hsc70) 70 has been suggested to play a similar function. However, Hsc70 has recently been reported to counteract Hsp70.

OBJECTIVE

We investigated whether Hsc70 counteracts Hsp70 in the control of wild-type and mutant hERG stability.

METHODS AND RESULTS

Coexpression of Hsp70 with hERG in HEK293 cells suppressed hERG ubiquitination and increased the levels of both immature and mature forms of hERG. Immunocytochemistry revealed increased levels of hERG in the endoplasmic reticulum and on the cell surface. Electrophysiological studies showed increased I(Kr). All these effects of Hsp70 were abolished by Hsc70 coexpression. Heat shock treatment of HL-1 mouse cardiomyocytes induced endogenous Hsp70, switched mouse ERG associated with Hsc70 to Hsp70, increased I(Kr), and shortened action potential duration. Channels with disease-causing missense mutations in intracellular domains had a higher binding capacity to Hsc70 than wild-type channels and channels with mutations in the pore region. Knockdown of Hsc70 by small interfering RNA or heat shock prevented degradation of mutant hERG proteins with mutations in intracellular domains.

CONCLUSIONS

These results indicate reciprocal control of hERG stability by Hsp70 and Hsc70. Hsc70 is a potential target in the treatment of LQT2 resulting from missense hERG mutations.

Rescue of mutated cardiac ion channels in inherited arrhythmia syndromes

Inherited arrhythmia syndromes comprise an increasingly complex group of diseases involving mutations in multiple genes encoding ion channels, ion channel accessory subunits and channel interacting proteins, and various regulatory elements. These mutations serve to disrupt normal electrophysiology in the heart, leading to increased arrhythmogenic risk and death. These diseases have added impact as they often affect young people, sometimes without warning. Although originally thought to alter ion channel function, it is now increasingly recognized that mutations may alter ion channel protein and messenger RNA processing, to reduce the number of channels reaching the surface membrane. For many of these mutations, it is also known that several interventions may restore protein processing of mutant channels to increase their surface membrane expression toward normal. In this article, we reviewed inherited arrhythmia syndromes, focusing on long QT syndrome type 2, and discuss the complex biology of ion channel trafficking and pharmacological rescue of disease-causing mutant channels. Pharmacological rescue of misprocessed mutant channel proteins, or their transcripts providing appropriate small molecule drugs can be developed, has the potential for novel clinical therapies in some patients with inherited arrhythmia syndromes.

Mannose trimming is required for delivery of a glycoprotein from EDEM1 to XTP3-B and to late endoplasmic reticulum-associated degradation steps

Although the trimming of α1,2-mannose residues from precursor N-linked oligosaccharides is an essential step in the delivery of misfolded glycoproteins to endoplasmic reticulum (ER)-associated degradation (ERAD), the exact role of this trimming is unclear. EDEM1 was initially suggested to bind N-glycans after mannose trimming, a role presently ascribed to the lectins OS9 and XTP3-B, because of their in vitro affinities for trimmed oligosaccharides. We have shown before that ER mannosidase I (ERManI) is required for the trimming and concentrates together with the ERAD substrate and ERAD machinery in the pericentriolar ER-derived quality control compartment (ERQC). Inhibition of mannose trimming prevents substrate accumulation in the ERQC. Here, we show that the mannosidase inhibitor kifunensine or ERManI knockdown do not affect binding of an ERAD substrate glycoprotein to EDEM1. In contrast, substrate association with XTP3-B and with the E3 ubiquitin ligases HRD1 and SCF(Fbs2) was inhibited. Consistently, whereas the ERAD substrate partially colocalized upon proteasomal inhibition with EDEM1, HRD1, and Fbs2 at the ERQC, colocalization was repressed by mannosidase inhibition in the case of the E3 ligases but not for EDEM1. Interestingly, association and colocalization of the substrate with Derlin-1 was independent of mannose trimming. The HRD1 adaptor protein SEL1L had been suggested to play a role in N-glycan-dependent substrate delivery to OS9 and XTP3-B. However, substrate association with XTP3-B was still dependent on mannose trimming upon SEL1L knockdown. Our results suggest that mannose trimming enables delivery of a substrate glycoprotein from EDEM1 to late ERAD steps through association with XTP3-B.

Emerging concepts in the pharmacogenomics of arrhythmias: ion channel trafficking

Continuous, rhythmic beating of the heart requires exquisite control of expression, localization and function of cardiac ion channels - the foundations of the cardiac myocyte action potential. Disruption of any of these processes can alter the shape of the action potential, predisposing to cardiac arrhythmias. These arrhythmias can manifest in a variety of ways depending on both the channels involved and the type of disruption (i.e., gain or loss of function). As much as 1% of the population of developed countries is affected by cardiac arrhythmia each year, and a detailed understanding of the mechanism of each arrhythmia is crucial to developing and prescribing the proper therapies. Many of the antiarrhythmic drugs currently on the market were developed before the underlying cause of the arrhythmia was known, and as a result lack specificity, causing side effects. The majority of the available drugs target the conductance of cardiac ion channels, either by blocking or enhancing current through the channel. In recent years, however, it has become apparent that specific targeting of ion channel conductance may not be the most effective means for treatment. Here we review increasing evidence that suggests defects in ion channel trafficking play an important role in the etiology of arrhythmias, and small molecule approaches to correct trafficking defects will likely play an important role in the future of arrhythmia treatment.

Novel characteristics of a trafficking-defective G572R-hERG channel linked to hereditary long QT syndrome

BACKGROUND

The congenital long QT syndrome is a heterogeneous genetic disease associated with delayed cardiac repolarization, prolonged QT intervals, the development of ventricular arrhythmias and sudden death. Type 2 congenital long QT syndrome (LQT2) results from KCNH2 or hERG gene mutations. hERG encodes the K(v)11.1 alpha subunit of the rapidly activating delayed rectifier K(+) current in the heart. Studies of mutant hERG channels indicate that most LQT2 missense mutations generate trafficking-deficient K(v)11.1 channels.

OBJECTIVE

To identify the mechanism underlying G572R-hERG by using molecular and electrophysiological analyses.

METHODS AND RESULTS

To elucidate the electrophysiological properties of the G572R-hERG mutant channels, mutant hERG subunits were heterologously expressed in HEK293 cells alone or in combination with wild-type (WT)-hERG subunits. Patch-clamp techniques were used to record currents, and double immunofluorescence protein tagging and Western blotting were performed to examine the cellular trafficking of mutant subunits. When expressed alone, G572R-hERG subunits were not present in the cell membrane and did not produce detectable currents. When coexpressed with WT-hERG subunits, G572R-hERG decreased current density and altered gating properties of the WT-hERG channel.

CONCLUSION

The hERG-associated missense mutation G572R, like most LQT2 missense mutations, generates a trafficking-deficient phenotype. Furthermore, G572R-hERG causes a loss of function in hERG by a strong dominant negative effect on the WT-hERG channel.

Sorting things out through endoplasmic reticulum quality control

The endoplasmic reticulum (ER) is a highly organized and specialized organelle optimized for the production of proteins. It is comprised of a highly interconnected network of tubules that contain a large set of resident proteins dedicated to the maturation and processing of proteins that traverse the eukaryotic secretory pathway. As protein maturation is an imperfect process, frequently resulting in misfolding and/or the formation of aggregates, proteins are subjected to a series of evaluation processes within the ER. Proteins deemed native are sorted for anterograde trafficking, while immature or non-native proteins are initially retained in the ER in an attempt to rescue the aberrant products. Terminally misfolded substrates are eventually targeted for turnover through the ER-associated degradation or ERAD pathway to protect the cell from the release of a defective product. A clearer picture of the identity of the machinery involved in these quality control evaluation processes and their mechanisms of actions has emerged over the past decade.

Post-transcriptional control of human ether-a-go-go-related gene potassium channel protein by alpha-adrenergic receptor stimulation

Stimulation of alpha1-adrenoreceptors (alpha1-AR) acutely alters ion channel behavior via several signaling pathways [calcium and protein kinase C (PKC)]. Little is known about sustained alpha1-adrenergic/PKC signaling and channel regulation as may occur during cardiovascular disease states. Here we describe the effects of prolonged alpha1A-AR and PKC activity on human ether-a-go-go-related gene (HERG) K(+) channels (Kv11.1) expressed in a heterologous expression system. Stimulation of alpha1A-AR with phenylephrine or direct activation of PKC with phorbol ester increased HERG channel protein abundance and K(+) current density in a time- and dose-dependent manner. Channel augmentation reached a steady-state plateau within 24 h with a 2- to 6-fold induction. Phorbol ester and moderate alpha1A-AR stimulation enhanced HERG abundance in a PKC-dependent fashion but with stronger alpha1A-adrenergic stimulation; protein kinase A (PKA)-dependent activity also contributed. Comparable channel induction of other cardiac K(+) channels was not seen in this system. Comparison of wild-type HERG and channels with either mutated PKC phosphorylation sites (HERGDeltaPKC) or mutated PKA phosphorylation sites (HERGDeltaPKA) suggested that the mechanisms of augmentation of HERG by the two kinases were partially overlapping. The PKC-dependent effect was largely due to enhanced synthetic rates. Stimulation of alpha1-AR in cultured rat neonatal cardiac myocytes also enhanced the abundance of ERG channels. These findings show that alpha1A-AR stimulation is capable of influencing the balance of HERG channel synthesis and degradation via multiple signaling pathways, a process that may have relevance in cardiac diseases and treatment.

Sent to destroy: the ubiquitin proteasome system regulates cell signaling and protein quality control in cardiovascular development and disease

The ubiquitin proteasome system (UPS) plays a crucial role in biological processes integral to the development of the cardiovascular system and cardiovascular diseases. The UPS prototypically recognizes specific protein substrates and places polyubiquitin chains on them for subsequent destruction by the proteasome. This system is in place to degrade not only misfolded and damaged proteins, but is essential also in regulating a host of cell signaling pathways involved in proliferation, adaptation to stress, regulation of cell size, and cell death. During the development of the cardiovascular system, the UPS regulates cell signaling by modifying transcription factors, receptors, and structural proteins. Later, in the event of cardiovascular diseases as diverse as atherosclerosis, cardiac hypertrophy, and ischemia/reperfusion injury, ubiquitin ligases and the proteasome are implicated in protecting and exacerbating clinical outcomes. However, when misfolded and damaged proteins are ubiquitinated by the UPS, their destruction by the proteasome is not always possible because of their aggregated confirmations. Recent studies have discovered how these ubiquitinated misfolded proteins can be destroyed by alternative "specific" mechanisms. The cytosolic receptors p62, NBR, and histone deacetylase 6 recognize aggregated ubiquitinated proteins and target them for autophagy in the process of "selective autophagy." Even the ubiquitination of multiple proteins within whole organelles that drive the more general macro-autophagy may be due, in part, to similar ubiquitin-driven mechanisms. In summary, the crosstalk between the UPS and autophagy highlight the pivotal and diverse roles the UPS plays in maintaining protein quality control and regulating cardiovascular development and disease.

Hsp40 chaperones promote degradation of the HERG potassium channel

Loss of function mutations in the hERG (human ether-a-go-go related gene or KCNH2) potassium channel underlie the proarrhythmic cardiac long QT syndrome type 2. Most often this is a consequence of defective trafficking of hERG mutants to the cell surface, with channel retention and degradation at the endoplasmic reticulum. Here, we identify the Hsp40 type 1 chaperones DJA1 (DNAJA1/Hdj2) and DJA2 (DNAJA2) as key modulators of hERG degradation. Overexpression of the DJAs reduces hERG trafficking efficiency, an effect eliminated by the proteasomal inhibitor lactacystin or with DJA mutants lacking their J domains essential for Hsc70/Hsp70 activation. Both DJA1 and DJA2 cause a decrease in the amount of hERG complexed with Hsc70, indicating a preferential degradation of the complex. Similar effects were observed with the E3 ubiquitin ligase CHIP. Both the DJAs and CHIP reduce hERG stability and act differentially on folding intermediates of hERG and the disease-related trafficking mutant G601S. We propose a novel role for the DJA proteins in regulating degradation and suggest that they act at a critical point in secretory pathway quality control.

Extracellular K+ concentration controls cell surface density of IKr in rabbit hearts and of the HERG channel in human cell lines

Although the modulation of ion channel gating by hormones and drugs has been extensively studied, much less is known about how cell surface ion channel expression levels are regulated. Here, we demonstrate that the cell surface density of both the heterologously expressed K+ channel encoded by the human ether-a-go-go-related gene (HERG) and its native counterpart, the rapidly activating delayed rectifier K+ channel (IKr), in rabbit hearts in vivo is precisely controlled by extracellular K+ concentration ([K+]o) within a physiologically relevant range. Reduction of [K+]o led to accelerated internalization and degradation of HERG channels within hours. Confocal analysis revealed colocalization between HERG and ubiquitin during the process of HERG internalization, and overexpression of ubiquitin facilitated HERG degradation under low [K+]o. The HERG channels colocalized with a marker of multivesicular bodies during internalization, and the internalized HERG channels were targeted to lysosomes. Our results provide the first evidence to our knowledge that the cell surface density of a voltage-gated K+ channel, HERG, is regulated by a biological factor, extracellular K+. Because hypokalemia is known to exacerbate long QT syndrome (LQTS) and Torsades de pointes tachyarrhythmias, our findings provide a potential mechanistic link between hypokalemia and LQTS.

Lectin-like ERAD players in ER and cytosol

Protein quality control in the endoplasmic reticulum (ER) is an elaborate process conserved from yeast to mammals, ensuring that only newly synthesized proteins with correct conformations in the ER are sorted further into the secretory pathway. It is well known that high-mannose type N-glycans are involved in protein-folding events. In the quality control process, proteins that fail to achieve proper folding or proper assembly are degraded in a process known as ER-associated degradation (ERAD). The ERAD pathway comprises multiple steps including substrate recognition and targeting to the retro-translocation machinery, retrotranslocation from the ER into the cytosol, and proteasomal degradation through ubiquitination. Recent studies have documented the important roles of sugar-recognition (lectin-type) molecules for trimmed high-mannose type N-glycans and glycosidases in the ERAD pathways in both ER and cytosol. In this review, we discuss a fundamental system that monitors glycoprotein folding in the ER and the unique roles of the sugar-recognizing ubiquitin ligase and peptide:N-glycanase (PNGase) in the cytosolic ERAD pathway.

PKA phosphorylation of HERG protein regulates the rate of channel synthesis

Acute changes in cAMP and protein kinase A (PKA) signaling can regulate ion channel protein activities such as gating. Effects on channels due to chronic PKA signaling, as in stress or disease states, are less understood. We examined the effects of prolonged PKA activity on the human ether-a-go-go-related gene (HERG) K(+) channel in stably transfected human embryonic kidney (HEK)293 cells. Sustained elevation of cAMP by either chlorophenylthiol (CPT)-cAMP or forskolin increased the HERG channel protein abundance two- to fourfold within 24 h, with measurable difference as early as 4 h. The cAMP-induced augmentation was not due to changes in transcription and was specific for HERG compared with other cardiac K(+) channels (Kv1.4, Kv1.5, Kir2.1, and KvLQT1). PKA activity was necessary for the effect on HERG protein and did not involve other cAMP signaling pathways. Direct PKA phosphorylation of the HERG protein was responsible for the cAMP-induced augmentation. Enhanced abundance of HERG protein was detected in endoplasmic reticulum-enriched, Golgi, and plasma membrane without significant changes in trafficking rates or patterns. An increase in the K(+) current density carried by the HERG channel was also observed, but with a delay, suggesting that traffic to the surface is rate-limiting traffic. Acceleration of the HERG protein synthesis rate was the primary factor in the cAMP/PKA effect with lesser effects on protein stability. These results provide evidence for a novel mechanism whereby phosphorylation of a nascent protein dictates its rate of synthesis, resetting its steady-state abundance.

Small GTPase determinants for the Golgi processing and plasmalemmal expression of human ether-a-go-go related (hERG) K+ channels

The pro-arrhythmic Long QT syndrome (LQT) is linked to 10 different genes (LQT1-10). Approximately 40% of genotype-positive LQT patients have LQT2, which is characterized by mutations in the human ether-a-go-go related gene (hERG). hERG encodes the voltage-gated K(+) channel alpha-subunits that form the pore of the rapidly activating delayed rectifier K(+) current in the heart. The purpose of this study was to elucidate the mechanisms that regulate the intracellular transport or trafficking of hERG, because trafficking is impaired for about 90% of LQT2 missense mutations. Protein trafficking is regulated by small GTPases. To identify the small GTPases that are critical for hERG trafficking, we coexpressed hERG and dominant negative (DN) GTPase mutations in HEK293 cells. The GTPases Sar1 and ARF1 regulate the endoplasmic reticulum (ER) export of proteins in COPII and COPI vesicles, respectively. Expression of DN Sar1 inhibited the Golgi processing of hERG, decreased hERG current (I(hERG)) by 85% (n > or = 8 cells per group, *, p < 0.01), and reduced the plasmalemmal staining of hERG. The coexpression of DN ARF1 had relatively small effects on hERG trafficking. Surprisingly, the coexpression of DN Rab11B, which regulates the endosomal recycling, inhibited the Golgi processing of hERG, decreased I(hERG) by 79% (n > or = 8 cells per group; *, p < 0.01), and reduced the plasmalemmal staining of hERG. These data suggest that hERG undergoes ER export in COPII vesicles and endosomal recycling prior to being processed in the Golgi. We conclude that hERG trafficking involves a pathway between the ER and endosomal compartments that influences expression in the plasmalemma.

Stimulation of N-terminal truncated isoform of androgen receptor stabilizes human ether-á-go-go-related gene-encoded potassium channel protein via activation of extracellular signal regulated kinase 1/2

Proarrhythmic drugs induce long QT syndrome more frequently in women than men. The present study was designed to determine whether androgens regulate the function and expression of the human ether-á-go-go-related gene (HERG) encoded K+ channel, which is largely responsible for determining the QT interval. In a concentration-dependent manner (10(-9) to 10(-6) M for 24 h), 5alpha-dihydrotestosterone (5alpha-DHT) increased HERG protein abundance in HEK293 cells stably expressing HERG in the presence of coexpressed cardiac androgen receptor (AR) variant [N-terminal truncated isoform of AR (AR45)]. The elevation of HERG protein was seen in endoplasmic reticulum, Golgi, and plasma membrane without clear preferential colocalization. Coexpression of the more common form of the AR did not confer 5alpha-DHT augmentation of HERG protein. Proteasome inhibitors, N-acetyl-L-leucyl-L-leucyl-L-norleucinal and MG132 prevented the 5alpha-DHT- dependent enhancement of HERG, as did the lysosome inhibitor, bafilomycin A1. Consistently, the cycloheximide-based protein chase study showed that 5alpha-DHT prolonged HERG protein half-life. 5alpha-DHT/AR45 signaling induced phosphorylation of ERK1/2. Blockade of ERK1/2 with PD98059 and U0126 prevented the effect of androgen on HERG protein abundance. Functional studies showed that 5alpha-DHT treatment for 24 h increased HERG K+ current density in Chinese hamster ovary cells cotransfected with cDNAs of AR45 and HERG channels. Moreover, 5alpha-DHT also increased ether-á-go-go-related gene-encoded K+ channel protein abundance in isolated rabbit cardiac myocytes. In conclusion, these data provide evidence that stimulation of AR45 receptors by androgens up-regulates HERG K+ channel abundance and activity mainly through stabilizing HERG protein in an ERK1/2 dependent mechanism, and suggest a mechanism to explain the sex difference in the long QT syndrome.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Molecular chaperones and photoreceptor function

Molecular chaperones facilitate and regulate protein conformational change within cells. This encompasses many fundamental cellular processes: including the correct folding of nascent chains; protein transport and translocation; signal transduction and protein quality control. Chaperones are, therefore, important in several forms of human disease, including neurodegeneration. Within the retina, the highly specialized photoreceptor cell presents a fascinating paradigm to investigate the specialization of molecular chaperone function and reveals unique chaperone requirements essential to photoreceptor function. Mutations in several photoreceptor proteins lead to protein misfolding mediated neurodegeneration. The best characterized of these are mutations in the molecular light sensor, rhodopsin, which cause autosomal dominant retinitis pigmentosa. Rhodopsin biogenesis is likely to require chaperones, while rhodopsin misfolding involves molecular chaperones in quality control and the cellular response to protein aggregation. Furthermore, the specialization of components of the chaperone machinery to photoreceptor specific roles has been revealed by the identification of mutations in molecular chaperones that cause inherited retinal dysfunction and degeneration. These chaperones are involved in several important cellular pathways and further illuminate the essential and diverse roles of molecular chaperones.

A splice site mutation in hERG leads to cryptic splicing in human long QT syndrome

Mutations in the human ether-a-go-go-related gene (hERG) cause type 2 long QT syndrome. In this study, we investigated the pathogenic mechanism of the hERG splice site mutation 2398+1G>C and the genotype-phenotype relationship of mutation carriers in three unrelated kindreds with long QT syndrome. The effect of 2398+1G>C on mRNA splicing was studied by analysis of RNA isolated from lymphocytes of index patients and using minigenes expressed in HEK293 cells and neonatal rat ventricular myocytes. RT-PCR analysis revealed that the 2398+1G>C mutation disrupted the normal splicing and activated a cryptic splice donor site in intron 9, leading to the inclusion of 54 nt of the intron 9 sequence in hERG mRNA. The cryptic splicing resulted in an in-frame insertion of 18 amino acids in the middle of the cyclic nucleotide binding domain. In patch clamp experiments the splice mutant did not generate hERG current. Western blot and immunostaining studies showed that the mutant expressed an immature form of hERG protein that failed to reach the plasma membrane. Coexpression of the mutant and wild-type channels led to a dominant negative suppression of wild-type channel function by intracellular retention of heteromeric channels. Our results demonstrate that 2398+1G>C activates a cryptic site and generates a full-length hERG protein with an insertion of 18 amino acids, which leads to a trafficking defect of the mutant channel.

Improved functional expression of recombinant human ether-a-go-go (hERG) K+ channels by cultivation at reduced temperature

Background

HERG potassium channel blockade is the major cause for drug-induced long QT syndrome, which sometimes cause cardiac disrhythmias and sudden death. There is a strong interest in the pharmaceutical industry to develop high quality medium to high-throughput assays for detecting compounds with potential cardiac liability at the earliest stages of drug development. Cultivation of cells at lower temperature has been used to improve the folding and membrane localization of trafficking defective hERG mutant proteins. The objective of this study was to investigate the effect of lower temperature maintenance on wild type hERG expression and assay performance.

Results

Wild type hERG was stably expressed in CHO-K1 cells, with the majority of channel protein being located in the cytoplasm, but relatively little on the cell surface. Expression at both locations was increased several-fold by cultivation at lower growth temperatures. Intracellular hERG protein levels were highest at 27°C and this correlated with maximal ³H-dofetilide binding activity. In contrast, the expression of functionally active cell surface-associated hERG measured by patch clamp electrophysiology was optimal at 30°C. The majority of the cytoplasmic hERG protein was associated with the membranes of cytoplasmic vesicles, which markedly increased in quantity and size at lower temperatures or in the presence of the Ca²⁺-ATPase inhibitor, thapsigargin. Incubation with the endocytic trafficking blocker, nocodazole, led to an increase in hERG activity at 37°C, but not at 30°C.

Conclusion

Our results are consistent with the concept that maintenance of cells at reduced temperature can be used to boost the functional expression of difficult-to-express membrane proteins and improve the quality of assays for medium to high-throughput compound screening. In addition, these results shed some light on the trafficking of hERG protein under these growth conditions.

Endoplasmic reticulum retention and rescue by heteromeric assembly regulate human ERG 1a/1b surface channel composition

Defects in the trafficking of subunits encoded by the human ether-à-go-go-related gene (hERG1) can lead to catastrophic arrhythmias and sudden cardiac death due to a reduction in I(Kr)-mediated repolarization. Native I(Kr) channels are composed of two alpha subunits, hERG 1a and 1b. In heterologous expression systems, hERG 1b subunits efficiently produce current only in heteromeric combination with hERG 1a. We used Western blot analysis and electrophysiological recordings in HEK-293 cells and Xenopus oocytes to monitor hERG 1b maturation in the secretory pathway and to determine the factors regulating surface expression of hERG 1b subunits. We found that 1b subunits expressed alone were largely retained in the endoplasmic reticulum (ER), thus accounting for the poor functional expression of homomeric 1b currents. Association with hERG 1a facilitated 1b ER export and surface expression. We show that hERG 1b subunits fail to mature because of an "RXR" ER retention signal specific to the 1b N terminus of the human sequence and not conserved in other species. Mutating the RXR facilitated maturation and functional expression of homomeric hERG 1b channels in a charge-dependent manner. Co-expression of the 1b RXR mutants with hERG 1a did not further enhance 1b maturation, suggesting that hERG 1a promotes 1b trafficking by overcoming the RXR-mediated retention. Thus, selective trafficking mechanisms regulate subunit composition of surface hERG channels.

In and out of the ER: protein folding, quality control, degradation, and related human diseases

A substantial fraction of eukaryotic gene products are synthesized by ribosomes attached at the cytosolic face of the endoplasmic reticulum (ER) membrane. These polypeptides enter cotranslationally in the ER lumen, which contains resident molecular chaperones and folding factors that assist their maturation. Native proteins are released from the ER lumen and are transported through the secretory pathway to their final intra- or extracellular destination. Folding-defective polypeptides are exported across the ER membrane into the cytosol and destroyed. Cellular and organismal homeostasis relies on a balanced activity of the ER folding, quality control, and degradation machineries as shown by the dozens of human diseases related to defective maturation or disposal of individual polypeptides generated in the ER.

Nonsense mutations in hERG cause a decrease in mutant mRNA transcripts by nonsense-mediated mRNA decay in human long-QT syndrome

BACKGROUND

Long-QT syndrome type 2 (LQT2) is caused by mutations in the human ether-a-go-go-related gene (hERG). More than 30% of the LQT2 mutations result in premature termination codons. Degradation of premature termination codon-containing mRNA transcripts by nonsense-mediated mRNA decay is increasingly recognized as a mechanism for reducing mRNA levels in a variety of human diseases. However, the role of nonsense-mediated mRNA decay in LQT2 mutations has not been explored.

METHODS AND RESULTS

We examined the expression of hERG mRNA in lymphocytes from patients carrying the R1014X mutation using a technique of allele-specific transcript quantification. The R1014X mutation led to a reduced level of mutant mRNA compared with that of the wild-type allele. The decrease in mutant mRNA also was observed in the LQT2 nonsense mutations W1001X and R1014X using hERG minigenes expressed in HEK293 cells or neonatal rat ventricular myocytes. Treatment with the protein synthesis inhibitor cycloheximide or RNA interference-mediated knockdown of the Upf1 protein resulted in the restoration of mutant mRNA to levels comparable to that of the wild-type minigene, suggesting that hERG nonsense mutations are subject to nonsense-mediated mRNA decay.

CONCLUSIONS

These results indicate that LQT2 nonsense mutations cause a decrease in mutant mRNA levels by nonsense-mediated mRNA decay rather than production of truncated proteins. Our findings suggest that the degradation of hERG mutant mRNA by nonsense-mediated mRNA decay is an important mechanism in LQT2 patients with nonsense or frameshift mutations.

N-glycan structure dictates extension of protein folding or onset of disposal

The endoplasmic reticulum (ER) is the site of folding for proteins that are resident in the ER or that are destined for the Golgi, endosomes, lysosomes, the plasma membrane, or secretion. Cotranslational addition of preassembled glucose(3)-mannose(9)-N-acetylglucosamine(2) core oligosaccharides (N-glycosylation) is a common event for polypeptides synthesized in this compartment. Protein-bound oligosaccharides are exposed to several ER glycanases that sequentially remove terminal glucose or mannose residues. Their activity must be tightly regulated because the N-glycan composition determines whether the associated protein is subjected to folding attempts in the ER lumen or whether it is retrotranslocated into the cytosol and degraded.

Glycoprotein folding and the role of EDEM1, EDEM2 and EDEM3 in degradation of folding-defective glycoproteins

Proteins synthesized in the endoplasmic reticulum (ER) lumen are exposed to several dedicated chaperones and folding factors that ensure efficient maturation. Nevertheless, protein folding remains error-prone and mutations in the polypeptide sequence may significantly reduce folding-efficiency. Folding-incompetent proteins carrying N-glycans are extracted from futile folding cycles in the calnexin chaperone system upon intervention of EDEM1, EDEM2 and EDEM3, three ER-stress-induced members of the glycosyl hydrolase 47 family. This review describes current knowledge about mechanisms regulating folding and disposal of glycoproteins.

Mechanisms of cardiac potassium channel trafficking

The regulation of ion channels involves more than just modulation of their synthesis and kinetics, as controls on their trafficking and localization are also important. Although the body of knowledge is fairly large, the entire trafficking pathway is not known for any one channel. This review summarizes current knowledge on the trafficking of potassium channels that are expressed in the heart. Our knowledge of channel assembly, trafficking through the Golgi apparatus and on to the surface is covered, as are controls on channel surface retention and endocytosis.

Heteromeric assembly of human ether-à-go-go-related gene (hERG) 1a/1b channels occurs cotranslationally via N-terminal interactions

Alternate transcripts of the human ether-à-go-go-related gene (hERG1) encode two subunits, hERG 1a and 1b, which form potassium channels regulating cardiac repolarization, neuronal firing frequency, and neoplastic cell growth. The 1a and 1b subunits are identical except for their unique, cytoplasmic N termini, and they readily co-assemble in heterologous and native systems. We tested the hypothesis that interactions of nascent N termini promote heteromeric assembly of 1a and 1b subunits. We found that 1a and 1b N-terminal fragments bind in a direct and dose-dependent manner. hERG1 hetero-oligomerization occurs in the endoplasmic reticulum where co-expression of N-terminal fragments with hERG1 subunits disrupted oligomerization and core glycosylation. The disruption of core glycosylation, a cotranslational event, allows us to pinpoint these N-terminal interactions to the earliest steps in biogenesis. Thus, N-terminal interactions mediate hERG 1a/1b assembly, a process whose perturbation may represent a new mechanism for disease.

N-glycan processing in ER quality control

Glycosylation of asparagine residues in Asn-x-Ser/Thr motifs is a common covalent modification of proteins in the lumen of the endoplasmic reticulum (ER). By substantially contributing to the overall hydrophilicity of the polypeptide, pre-assembled core glycans inhibit possible aggregation caused by the inevitable exposure of hydrophobic patches on the as yet unstructured chains. Thereafter, N-glycans are modified by ER-resident enzymes glucosidase I (GI), glucosidase II (GII), UDP-glucose:glycoprotein glucosyltransferase (UGT) and mannosidase(s) and become functional appendices that determine the fate of the associated polypeptide. Recent work has improved our understanding of how the removal of terminal glucose residues from N-glycans allows newly synthesized proteins to access the calnexin chaperone system; how substrate retention in this specialized chaperone system is regulated by de-/re-glucosylation cycles catalyzed by GII and UGT1; and how acceleration of N-glycan dismantling upon induction of EDEM variants promotes ER-associated degradation (ERAD) under conditions of ER stress. In particular, characterization of cells lacking certain ER chaperones has revealed important new information on the mechanisms regulating protein folding and quality control. Tight regulation of N-glycan modifications is crucial to maintain protein quality control, to ensure the synthesis of functional polypeptides and to avoid constipation of the ER with folding-defective polypeptides.

EDEM1 regulates ER-associated degradation by accelerating de-mannosylation of folding-defective polypeptides and by inhibiting their covalent aggregation

Proteins expressed in the endoplasmic reticulum (ER) are covalently modified by co-translational addition of pre-assembled core glycans (glucose(3)-mannose(9)-N-acetylglucosamine(2)) to asparagines in Asn-X-Ser/Thr motifs. N-Glycan processing is essential for protein quality control in the ER. Cleavages and re-additions of the innermost glucose residue prolong folding attempts in the calnexin cycle. Progressive loss of mannoses is a symptom of long retention in the ER and elicits preparation of terminally misfolded polypeptides for dislocation into the cytosol and proteasome-mediated degradation. The ER stress-induced protein EDEM1 regulates disposal of folding-defective glycoproteins and has been described as a mannose-binding lectin. Here we show that elevation of the intralumenal concentration of EDEM1 accelerates ER-associated degradation (ERAD) by accelerating de-mannosylation of terminally misfolded glycoproteins and by inhibiting formation of covalent aggregates upon release of terminally misfolded ERAD candidates from calnexin. Acceleration of Man(9) or Man(5)N-glycans dismantling upon overexpression was fully blocked by substitution in EDEM1 of one catalytic residue conserved amongst alpha1,2-mannosidases, thus suggesting that EDEM1 is an active mannosidase. This mutation did not affect the chaperone function of EDEM1.

Functional analysis of the extracellular cysteine residues in the human organic anion transporting polypeptide, OATP2B1

Organic anion transporting polypeptide (OATP) superfamily member 2B1 (OATP2B1) mediates the uptake of steroid hormone precursors and selected drugs in the placenta, liver, mammary gland, brain, and intestine. This action is modulated by sulfhydryl reagents. Common to all OATPs is a large extracellular loop between transmembrane domains IX and X with 10 conserved cysteines. To elucidate the structure-function relationship of this cysteine rich ectodomain, a truncated OATP2B1 lacking 10 extracellular cysteines (OATP2B1(Delta489-557)) and 10 OATP2B1 mutants containing individual Cys-to-Ala substitutions were generated and expressed in CHO-K1 cells. The immunolocalization, cell-surface expression, transport activity, and free cysteine labeling with N-biotinoylaminoethylmethane-thiosulfonate of mutant proteins and wild-type OATP2B1 were compared. OATP2B1(Delta489-557) accumulated intracellularly. Nine Cys-to-Ala substitutions, C489A, C495A, C504A, C516A, C520A, C539A, C541A, C553A, and C557A, were misprocessed, appearing predominantly as core-glycosylated, 60-kDa proteins and as 180-kDa complexes. Only C493A was a fully glycosylated 75-kDa protein expressed at the cell surface. Thapsigargin partially corrected the misprocessing of mutants. Compared with OATP2B1, C493A and C557A transported estrone-3-sulfate and dehydroepiandrosterone sulfate less efficiently, whereas all other mutants were functionally impaired. MTSEA labeled free cysteines in all Cys-to-Ala mutants but not in OATP2B1, suggesting that all 10 extracellular cysteines are normally disulfide-bonded. Our findings show that the trafficking and function of OATP2B1 is vulnerable to changes in the cysteine residues of extracellular loop IX-X.

hERG potassium channels and cardiac arrhythmia

hERG potassium channels are essential for normal electrical activity in the heart. Inherited mutations in the HERG gene cause long QT syndrome, a disorder that predisposes individuals to life-threatening arrhythmias. Arrhythmia can also be induced by a blockage of hERG channels by a surprisingly diverse group of drugs. This side effect is a common reason for drug failure in preclinical safety trials. Insights gained from the crystal structures of other potassium channels have helped our understanding of the block of hERG channels and the mechanisms of gating.

Most LQT2 mutations reduce Kv11.1 (hERG) current by a class 2 (trafficking-deficient) mechanism

BACKGROUND

The KCNH2 or human ether-a-go-go related gene (hERG) encodes the Kv11.1 alpha-subunit of the rapidly activating delayed rectifier K+ current (IKr) in the heart. Type 2 congenital long-QT syndrome (LQT2) results from KCNH2 mutations that cause loss of Kv11.1 channel function. Several mechanisms have been identified, including disruption of Kv11.1 channel synthesis (class 1), protein trafficking (class 2), gating (class 3), or permeation (class 4). For a few class 2 LQT2-Kv11.1 channels, it is possible to increase surface membrane expression of Kv11.1 current (IKv11.1). We tested the hypotheses that (1) most LQT2 missense mutations generate trafficking-deficient Kv11.1 channels, and (2) their trafficking-deficient phenotype can be corrected.

METHODS AND RESULTS

Wild-type (WT)-Kv11.1 channels and 34 missense LQT2-Kv11.1 channels were expressed in HEK293 cells. With Western blot analyses, 28 LQT2-Kv11.1 channels had a trafficking-deficient (class 2) phenotype. For the majority of these mutations, the class 2 phenotype could be corrected when cells were incubated for 24 hours at reduced temperature (27 degrees C) or in the drugs E4031 or thapsigargin. Four of the 6 LQT2-Kv11.1 channels that had a wild-type-like trafficking phenotype did not cause loss of Kv11.1 function, which suggests that these channels are uncommon sequence variants.

CONCLUSIONS

This is the first study to identify a dominant mechanism, class 2, for the loss of Kv11.1 channel function in LQT2 and to report that the class 2 phenotype for many of these mutant channels can be corrected. This suggests that if therapeutic strategies to correct protein trafficking abnormalities can be developed, it may offer clinical benefits for LQT2 patients.

A cytoplasmic peptide: N-glycanase

A cytoplasmic peptide:N-glycanase (PNGase) has been implicated in the proteasomal degradation of aberrant glycoproteins synthesized in the endoplasmic reticulum. The reaction is believed to be important for subsequent proteolysis by the proteasome since bulky N-glycan chains on misfolded glycoproteins may impair their efficient entry into the interior of the cylinder-shaped 20S proteasome, where the active sites of the proteases reside. The deglycosylation reaction by PNGase brings about two major changes on substrate proteins; one is a removal of N-glycan chains, and the other is the introduction of negative charge(s) into the core peptide by converting glycosylated asparagine residue(s) into aspartic acid residue(s). Therefore, PNGase action can be accurately monitored by detecting both changes using two different methods; that is, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) for deglycosylation and isoelectric focusing for detection of introduction of negative charge(s) into core proteins. This chapter will describe the simple in vivo as well as in vitro assay method to detect PNGase activity.

Mechanisms of pharmacological rescue of trafficking-defective hERG mutant channels in human long QT syndrome

Long QT syndrome type 2 is caused by mutations in the human ether-a-go-go-related gene (hERG). We previously reported that the N470D mutation is retained in the endoplasmic reticulum (ER) but can be rescued to the plasma membrane by hERG channel blocker E-4031. The mechanisms of ER retention and how E-4031 rescues the N470D mutant are poorly understood. In this study, we investigated the interaction of hERG channels with the ER chaperone protein calnexin. Using coimmunoprecipitation, we showed that the immature forms of both wild type hERG and N470D associated with calnexin. The association required N-linked glycosylation of hERG channels. Pulse-chase analysis revealed that N470D had a prolonged association with calnexin compared with wild type hERG and E-4031 shortened the time course of calnexin association with N470D. To test whether the prolonged association of N470D with calnexin is due to defective folding of mutant channels, we studied hERG channel folding using the trypsin digestion method. We found that N470D and the immature form of wild type hERG were more sensitive to trypsin digestion than the mature form of wild type hERG. In the presence of E-4031, N470D became more resistant to trypsin even when its ER-to-Golgi transport was blocked by brefeldin A. These results suggest that defective folding of N470D contributes to its prolonged association with calnexin and ER retention and that E-4031 may restore proper folding of the N470D channel leading to its cell surface expression.

Ubiquitylation of Ion Channels

Ubiquitylation (i.e., covalent attachment of ubiquitin moieties to proteins) of ion channels allows regulation of their activity and fate. Nedd4/Nedd4-like ubiquitin-protein ligases bind to, ubiquitylate, and modulate the internalization of several channels bearing PY motifs, whereas endoplasmic reticulum-associated degradation (involving ubiquitylation) plays an important role in the biogenesis of normal and defective channels.

Evidence for proteasomal degradation of Kv1.5 channel protein

BACKGROUND

The voltage-gated potassium channel Kv1.5 plays a critical role in the maintenance of the membrane potential. While protein degradation is one of the major mechanisms for the regulation of channel functions, little is known on the degradation mechanism of Kv1.5.

METHODS AND RESULTS

Kv1.5 was expressed in COS cells and its degradation, intracellular localization, and channel activities were assessed by pulse-chase analysis, immunofluorescence, and patch clamp techniques, respectively. Expressed Kv1.5 had a half-life time of approximately 6.7 h, which was prolonged by the proteasome inhibitors of MG132, ALLN, proteasomal inhibitor 1, or lactacystine, but not by a lysosomal inhibitor chloroquine. MG132 increased the protein level of Kv1.5, as well as the level of its ubiquitinated form in a dose-dependent manner. Similar effects of MG132 on endogenous Kv1.5 were seen in cultured rat atrial cells. Within a cell, Kv1.5 was mainly localized in both the endoplasmic reticulum and Golgi apparatus. MG132 increased the immunoreactivity of Kv1.5 in these compartments and also increased Ik(ur) currents through the cell-surface Kv1.5. Pretreatment with either brefeldin A or colchicine abolished MG132-induced increase in Ik(ur) currents.

CONCLUSION

Kv1.5 is degraded by the proteasome. The inhibition of the proteasome increased Ik(ur) currents secondary to stabilization of the channel protein in the endoplasmic reticulum/Golgi apparatus.

Specific serine proteases selectively damage KCNH2 (hERG1) potassium channels and I(Kr)

KCNH2 (hERG1) encodes the alpha-subunit proteins for the rapidly activating delayed rectifier K+ current (I(Kr)), a major K+ current for cardiac myocyte repolarization. In isolated myocytes I(Kr) frequently is small in amplitude or absent, yet KCNH2 channels and I(Kr) are targets for drug block or mutations to cause long QT syndrome. We hypothesized that KCNH2 channels and I(Kr) are uniquely sensitive to enzymatic damage. To test this hypothesis, we studied heterologously expressed K+, Na+, and L-type Ca2+ channels, and in ventricular myocytes I(Kr), slowly activating delayed rectifier K+ current (I(Ks)), and inward rectifier K+ current (I(K1)), by using electrophysiological and biochemical methods. 1) Specific exogenous serine proteases (protease XIV, XXIV, or proteinase K) selectively degraded KCNH2 current (I(KCNH2)) and its mature channel protein without damaging cell integrity and with minimal effects on the other channel currents; 2) immature KCNH2 channel protein remained intact; 3) smaller molecular mass KCNH2 degradation products appeared; 4) protease XXIV selectively abolished I(Kr); and 5) reculturing HEK-293 cells after protease exposure resulted in the gradual recovery of I(KCNH2) and its mature channel protein over several hours. Thus the channel protein for I(KCNH2) and I(Kr) is uniquely sensitive to proteolysis. Analysis of the degradation products suggests selective proteolysis within the S5-pore extracellular linker, which is structurally unique among Kv channels. These data provide 1) a new mechanism to account for low I(Kr) density in some isolated myocytes, 2) evidence that most complexly glycosylated KCNH2 channel protein is in the plasma membrane, and 3) new insight into the rate of biogenesis of KCNH2 channel protein within cells.