Functional stabilization of Kv1.5 protein by Hsp70 in mammalian cell lines

Physiological functions and pathological involvement of ion channel trafficking in the vasculature

A variety of ion channels regulate membrane potential and calcium influx in arterial smooth muscle and endothelial cells to modify vascular functions, including contractility. The current (I) generated by a population of ion channels is equally dependent upon their number (N), open probability (Po) and single channel current (i), such that I = N.PO .i. A conventional view had been that ion channels traffic to the plasma membrane in a passive manner, resulting in a static surface population. It was also considered that channels assemble with auxiliary subunits prior to anterograde trafficking of the multimeric complex to the plasma membrane. Recent studies have demonstrated that physiological stimuli can regulate the surface abundance (N) of several different ion channels in arterial smooth muscle and endothelial cells to control arterial contractility. Physiological stimuli can also regulate the number of auxiliary subunits present in the plasma membrane to modify the biophysical properties, regulatory mechanisms and physiological functions of some ion channels. Furthermore, ion channel trafficking becomes dysfunctional in the vasculature during hypertension, which negatively impacts the regulation of contractility. The temporal kinetics of ion channel and auxiliary subunit trafficking can also vary depending on the signalling mechanisms and proteins involved. This review will summarize recent work that has uncovered the mechanisms, functions and pathological modifications of ion channel trafficking in arterial smooth muscle and endothelial cells.

Kv1.5 channel mediates monosodium urate-induced activation of NLRP3 inflammasome in macrophages and arrhythmogenic effects of urate on cardiomyocytes

BACKGROUND

Gout is usually found in patients with atrial fibrillation (AF). K+ efflux is a common trigger of NLRP3 inflammasome activation which is involved in the pathogenesis of AF. We investigated the role of the K+ channel Kv1.5 in monosodium urate crystal (MSU)-induced activation of the NLRP3 inflammasome and electrical remodeling in mouse and human macrophages J774.1 and THP-1, and mouse atrial myocytes HL-1.

METHODS AND RESULTS

Macrophages, primed with lipopolysaccharide (LPS), were stimulated by MSU. HL-1 cells were incubated with the conditioned medium (CM) from MSU-stimulated macrophages. Western blot, ELISA and patch clamp were used. MSU induced caspase-1 expression in LPS-primed J774.1 cells and IL-1β secretion, suggesting NLRP3 inflammasome activation. A selective Kv1.5 inhibitor, diphenyl phosphine oxide-1 (DPO-1), and siRNAs against Kv1.5 suppressed the levels of caspase-1 and IL-1β. MSU reduced intracellular K+ concentration which was prevented by DPO-1 and siRNAs against Kv1.5. MSU increased expression of Hsp70, and Kv1.5 on the plasma membrane. siRNAs against Hsp70 were suppressed but heat shock increased the expression of Hsp70, caspase-1, IL-1β, and Kv1.5 in MSU-stimulated J774.1 cells. The CM from MSU-stimulated macrophages enhanced the expression of caspase-1, IL-1β and Kv1.5 with increased Kv1.5-mediated currents that shortened action potential duration in HL-1 cells. These responses were abolished by DPO-1 and a siRNA against Kv1.5.

CONCLUSIONS

Kv1.5 regulates MSU-induced activation of NLRP3 inflammasome in macrophages. MSUrelated activation of NLRP3 inflammasome and electrical remodeling in HL-1 cells are via macrophages. Kv1.5 may have therapeutic value for diseases related to gout-induced activation of the NLRP3 inflammsome, including AF.

Hsp70 promotes maturation of uromodulin mutants that cause familial juvenile hyperuricemic nephropathy and suppresses cellular damage

BACKGROUND

Familial juvenile hyperuricemic nephropathy (FJHN) is an autosomal dominant disorder caused by mutations in UMOD. Here we studied effects of genetic expression and pharmacological induction of Hsp70 on the UMOD mutants C112Y and C217G.

METHODS

We expressed wild type (WT), C112Y and C217G in HEK293 cells and studied their maturation and cellular damage using western blot and flow cytometry.

RESULTS

Expression of C112Y or C217G increased pro-apoptotic proteins, decreased anti-apoptotic proteins, and induced cellular apoptosis as examined by annexin V staining and flow cytometry. Overexpression of Hsp70 or administration of an Hsp70 inducer geranylgeranylacetone (GGA) promoted maturation of the mutant proteins, increased their secreted forms, normalized the levels of pro- and anti-apoptotic proteins and suppressed apoptosis.

CONCLUSION

These findings indicated that Hsp70 enhanced maturation of C112Y and C217G and reduced cellular apoptosis, suggesting that Hsp70 induction might be of a therapeutic value for treatment of FJHN.

A Novel Treatment for Arrhythmias via the Control of the Degradation of Ion Channel Proteins

Although there are many reports on the regulation of ion channel expression in transcription and translation, few drugs have been studied to influence post-translational modification of ion channel proteins. The Kv1.5 channel is a potassium ion channel expressed in atrial muscle, belongs to the voltage-gated K⁺ channel superfamily, and forms an ultrarapid delayed rectifier potassium ion current. It is important to understand the fate of these channel proteins, as cardiac Kv1.5 mutations can cause arrhythmias. Disruption of quantitative and qualitative control mechanisms of channels leads to stagnation and degradation of intracellular channel proteins. As a result, ion channel proteins are not transported to the cell membrane and are involved in the development of atrial fibrillation. This review takes the Kv1.5 channel as an example and focuses on the degradation mechanism of ion channel proteins, and discusses its application to the treatment of arrhythmia by drugs that control the mechanism of ion channel protein degradation.

Evidence for Urate Uptake Through Monocarboxylate Transporter 9 Expressed in Mammalian Cells and Its Enhancement by Heat Shock

Background: Monocarboxylate transporter 9 (MCT9), an orphan transporter member of the solute carrier family 16 (SLC16), possibly reabsorbs uric acid in the renal tubule and has been suggested by genome-wide association studies to be involved in the development of hyperuricemia and gout. In this study we investigated the mechanisms regulating the expression of human (h) MCT9, its degradation, and physiological functions. Methods and Results: hMCT9-FLAG was stably expressed in HEK293 cells and its degradation, intracellular localization, and urate uptake activities were assessed by pulse-chase analysis, immunofluorescence, and [¹⁴C]-urate uptake experiments, respectively. hMCT9-FLAG was localized on the plasma membrane as well as in the endoplasmic reticulum and Golgi apparatus. The proteasome inhibitors MG132 and lactacystine increased levels of hMCT9-FLAG protein expression with enhanced ubiquitination, prolonged their half-life, and decreased [¹⁴C]-urate uptake. [¹⁴C]-urate uptake was increased by both heat shock (HS) and the HS protein inducer geranylgeranylacetone (GGA). Both HS and GGA restored the [¹⁴C]-urate uptake impaired by MG132. Conclusions: hMCT9 does transport urate and is degraded by a proteasome, inhibition of which reduces hMCT9 expression on the cell membrane and urate uptake. HS enhanced urate uptake through hMCT9.

Assistance for Folding of Disease-Causing Plasma Membrane Proteins

An extensive catalog of plasma membrane (PM) protein mutations related to phenotypic diseases is associated with incorrect protein folding and/or localization. These impairments, in addition to dysfunction, frequently promote protein aggregation, which can be detrimental to cells. Here, we review PM protein processing, from protein synthesis in the endoplasmic reticulum to delivery to the PM, stressing the main repercussions of processing failures and their physiological consequences in pathologies, and we summarize the recent proposed therapeutic strategies to rescue misassembled proteins through different types of chaperones and/or small molecule drugs that safeguard protein quality control and regulate proteostasis.

Hyperuricemia as a Risk Factor for Atrial Fibrillation Due to Soluble and Crystalized Uric Acid

Among the several independent risk factors for atrial fibrillation (AF), hyperuricemia has been widely accepted as associated with the incidence of paroxysmal or persistent AF, as well as with the risk of AF in patients undergoing cardiovascular surgery. The electrophysiological mechanism of AF involves electrical remodeling of the arrhythmogenic substrate and abnormal automaticity as trigger. Both electrical and structural remodeling mediated by oxidative stress derived from either xanthine oxidoreductase (XOR), soluble uric acid (UA) or monosodium urate (MSU) crystals might be plausible explanations for the association of AF with hyperuricemia. XOR generates reactive oxygen species (ROS) that lead to atrial structural remodeling via inflammation. Soluble UA accumulates intracellularly through UA transporters (UAT), shortening the atrial action potential via enhanced expression and activity of Kv1.5 channel proteins. Intracellular accumulation of soluble UA generates ROS in atrial myocytes via nicotinamide adenine dinucleotide phosphate oxidase, which phosphorylates ERK/Akt and heat shock factor 1 (HSF1), thereby increasing transcription and translation of Hsp70, which stabilizes Kv1.5. In macrophages, MSU activates the NLRP3 inflammasome and proteolytic processing mediated by caspase-1 with enhanced interleukin (IL)-1β and IL-18 secretion. Use of an XOR inhibitor, antioxidants, a UAT inhibitor such as a uricosuric agent, and an NLRP3 inflammasome inhibitor, might become a potential strategy to reduce the risk of hyperuricemia-induced AF, and control serum UA level.

The endosomal trafficking factors CORVET and ESCRT suppress plasma membrane residence of the renal outer medullary potassium channel (ROMK)

Protein trafficking can act as the primary regulatory mechanism for ion channels with high open probabilities, such as the renal outer medullary (ROMK) channel. ROMK, also known as Kir1.1 (KCNJ1), is the major route for potassium secretion into the pro-urine and plays an indispensable role in regulating serum potassium and urinary concentrations. However, the cellular machinery that regulates ROMK trafficking has not been fully defined. To identify regulators of the cell-surface population of ROMK, we expressed a pH-insensitive version of the channel in the budding yeast Saccharomyces cerevisiae We determined that ROMK primarily resides in the endoplasmic reticulum (ER), as it does in mammalian cells, and is subject to ER-associated degradation (ERAD). However, sufficient ROMK levels on the plasma membrane rescued growth on low-potassium medium of yeast cells lacking endogenous potassium channels. Next, we aimed to identify the biological pathways most important for ROMK regulation. Therefore, we used a synthetic genetic array to identify non-essential genes that reduce the plasma membrane pool of ROMK in potassium-sensitive yeast cells. Genes identified in this screen included several members of the endosomal complexes required for transport (ESCRT) and the class-C core vacuole/endosome tethering (CORVET) complexes. Mass spectroscopy analysis confirmed that yeast cells lacking an ESCRT component accumulate higher potassium concentrations. Moreover, silencing of ESCRT and CORVET components increased ROMK levels at the plasma membrane in HEK293 cells. Our results indicate that components of the post-endocytic pathway influence the cell-surface density of ROMK and establish that components in this pathway modulate channel activity.

E3 ligase CHIP and Hsc70 regulate Kv1.5 protein expression and function in mammalian cells

Kv1.5 confers ultra-rapid delayed-rectifier potassium channel current (IKur) which contributes to repolarization of the atrial action potential. Kv1.5 proteins, degraded via the ubiquitin-proteasome pathway, decreased in some atrial fibrillation patients. Carboxyl-terminus heat shock cognate 70-interacting protein (CHIP), an E3 ubiquitin ligase, is known to ubiquitinate short-lived proteins. Here, we investigated the roles of CHIP in Kv1.5 degradation to provide insights into the mechanisms of Kv1.5 decreases and treatments targeting Kv1.5 for atrial fibrillation. Coexpression of CHIP with Kv1.5 in HEK293 cells increased Kv1.5 protein ubiquitination and decreased the protein level. Immunofluorescence revealed decreases of Kv1.5 proteins in the endoplasmic reticulum and on the cell membrane. A siRNA against CHIP suppressed Kv1.5 protein ubiquitination and increased its protein level. CHIP mutants, lacking either the N-terminal tetratricopeptide region domain or the C-terminal U-box domain, failed to exert these effects on Kv1.5 proteins. Immunoprecipitation showed that CHIP formed complexes with Kv1.5 proteins and heat shock cognate protein 70 (Hsc70). Effects of Hsc70 on Kv1.5 were similar to CHIP by altering interaction of CHIP with Kv1.5 protein. Coexpression of CHIP and Hsc70 with Kv1.5 additionally enhanced Kv1.5 ubiquitination. Kv1.5 currents were decreased by overexpression of CHIP or Hsc70 but were increased by knockdown of CHIP or Hsc70 in HEK 293 cells stably expressing Kv1.5. These effects of CHIP and Hsc70 were also observed on endogenous Kv1.5 in HL-1 mouse cardiomyocytes, decreasing IKur and prolonging action potential duration. These results indicate that CHIP decreases the Kv1.5 protein level and functional channel by facilitating its degradation in concert with chaperone Hsc70.

The role of the cytosolic HSP70 chaperone system in diseases caused by misfolding and aberrant trafficking of ion channels

Protein-folding diseases are an ongoing medical challenge. Many diseases within this group are genetically determined, and have no known cure. Among the examples in which the underlying cellular and molecular mechanisms are well understood are diseases driven by misfolding of transmembrane proteins that normally function as cell-surface ion channels. Wild-type forms are synthesized and integrated into the endoplasmic reticulum (ER) membrane system and, upon correct folding, are trafficked by the secretory pathway to the cell surface. Misfolded mutant forms traffic poorly, if at all, and are instead degraded by the ER-associated proteasomal degradation (ERAD) system. Molecular chaperones can assist the folding of the cytosolic domains of these transmembrane proteins; however, these chaperones are also involved in selecting misfolded forms for ERAD. Given this dual role of chaperones, diseases caused by the misfolding and aberrant trafficking of ion channels (referred to here as ion-channel-misfolding diseases) can be regarded as a consequence of insufficiency of the pro-folding chaperone activity and/or overefficiency of the chaperone ERAD role. An attractive idea is that manipulation of the chaperones might allow increased folding and trafficking of the mutant proteins, and thereby partial restoration of function. This Review outlines the roles of the cytosolic HSP70 chaperone system in the best-studied paradigms of ion-channel-misfolding disease--the CFTR chloride channel in cystic fibrosis and the hERG potassium channel in cardiac long QT syndrome type 2. In addition, other ion channels implicated in ion-channel-misfolding diseases are discussed.

Hsp90 prevents interaction between CHIP and HERG proteins to facilitate maturation of wild-type and mutant HERG proteins

AIMS

We examined the role of Hsp90 in expression and maturation of wild-type (WT) and mutant ether-a-go-go related gene (HERG) proteins by using Hsp90 inhibitors, geldanamycin (GA) and radicicol, and Hsp90 overexpression.

METHODS AND RESULTS

The proteins were expressed in HEK293 cells or collected from HL-1 mouse cardiomyocytes, and analysed by western blotting, immunoprecipitation, immunofluorescence, and whole-cell patch-clamp techniques. GA and radicicol suppressed maturation of HERG-FLAG proteins and increased their immature forms. Co-expression of Hsp90 counteracted the effects of Hsp90 inhibitors and suppressed ubiquitination of HERG proteins. Overexpressed Hsp90 also inhibited the binding of endogenous C-terminus of Hsp70-interacting protein (CHIP) to HERG-FLAG proteins. Hsp90-induced increase of functional HERG proteins was verified by their increased expression on the cell surface and enhanced HERG channel currents. CHIP overexpression decreased both mature and immature forms of HERG-FLAG proteins in cells treated with GA. Hsp90 facilitated maturation of endogenous ERG proteins, whereas CHIP decreased both forms of ERG proteins in HL-1 cells. Mutant HERG proteins harbouring disease-causing missense mutations were mainly in the immature form and had a higher binding capacity to CHIP than the WT; Hsp90 overexpression suppressed this association. Overexpressed Hsp90 increased the mature form of HERG(1122fs/147) proteins, reduced its ubiquitinated form, increased its immunoreactivity in the endoplasmic reticulum and on the plasma membrane, and increased the mutant-mediated membrane current. CHIP overexpression decreased the immature form of HERG(1122fs/147) proteins.

CONCLUSION

Enhancement of HERG protein expression through Hsp90 inhibition of CHIP binding might be a novel therapeutic strategy for long QT syndrome 2 caused by trafficking abnormalities of HERG proteins.

Stabilization of Kv1.5 channel protein by bepridil through its action as a chemical chaperone

While bepridil has been reported to alter the stability of ion channel proteins, the precise mechanism of action remains unclear. We examined the effect of bepridil on the stability of Kv1.5 channel proteins expressed in COS7 cells. Bepridil at 0.3-30 μM increased the protein level of Kv1.5 channels in a concentration-dependent manner. Chase experiments showed that bepridil delayed the degradation process of Kv1.5 channel proteins in the same manner as a proteasomal inhibitor, MG132, did. Bepridil increased the immunofluorescent signal of Kv1.5 channel proteins in the endoplasmic reticulum (ER) and Golgi apparatus and on the cell surface. The cell fraction experiment also showed bepridil-induced increases in Kv1.5 in the ER, Golgi apparatus, and the cell membrane. Bepridil at a lower concentration of 1 μM had no effect on the proteasome activity in vitro. A blocker of the ultrarapid delayed-rectifier K(+) channel current, 4-aminopyridine (4AP), abolished bepridil-induced increases in Kv1.5. Kv1.5-medicated membrane currents measured as 4AP-sensitive currents were increased by bepridil. Taken together, we conclude that bepridil stabilizes Kv1.5 proteins at the ER through an action as a chemical chaperone, thereby increasing the density of Kv1.5 channels in the cell membrane.

Transcriptional activation of the anchoring protein SAP97 by heat shock factor (HSF)-1 stabilizes K(v) 1.5 channels in HL-1 cells

BACKGROUND AND PURPOSE

The expression of voltage-dependent K⁺ channels (K_v) 1.5 is regulated by members of the heat shock protein (Hsp) family. We examined whether the heat shock transcription factor 1 (HSF-1) and its inducer geranylgeranylacetone (GGA) could affect the expression of K_v1.5 channels and its anchoring protein, synapse associated protein 97 (SAP97).

EXPERIMENTAL APPROACH

Transfected mouse atrial cardiomyocytes (HL-1 cells) and COS7 cells were subjected to luciferase reporter gene assay and whole-cell patch clamp. Protein and mRNA extracts were subjected to Western blot and quantitative real-time polymerase chain reaction.

KEY RESULTS

Heat shock of HL-1 cells induced expression of Hsp70, HSF-1, SAP97 and K_v1.5 proteins. These effects were reproduced by wild-type HSF-1. Both heat shock and expression of HSF-1, but not the R71G mutant, increased the SAP97 mRNA level. Small interfering RNA (siRNA) against SAP97 abolished HSF-1-induced increase of K_v1.5 and SAP97 proteins. A luciferase reporter gene assay revealed that the SAP97 promoter region (from −919 to −740) that contains heat shock elements (HSEs) was required for this induction. Suppression of SIRT1 function either by nicotinamide or siRNA decreased the level of SAP97 mRNA. SIRT1 activation by resveratrol had opposing effects. A treatment of the cells with GGA increased the level of SAP97 mRNA, K_v1.5 proteins and I_Kur current, which could be modified with either resveratrol or nicotinamide.

CONCLUSIONS AND IMPLICATIONS

HSF-1 induced transcription of SAP97 through SIRT1-dependent interaction with HSEs; the increase in SAP97 resulted in stabilization of K_v1.5 channels. These effects were mimicked by GGA.

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.