Modulation of epithelial sodium channel trafficking and function by sodium 4-phenylbutyrate in human nasal epithelial cells

HSP70-16 and VDAC3 jointly inhibit seed germination under cold stress in Arabidopsis

Abscisic acid (ABA) transport plays a crucial role in seed germination under unfavourable conditions such as cold stress. Both heat shock protein 70 (HSP70) and voltage-dependent anion channel (VDAC) protein are involved in cold stress responses in Arabidopsis. However, their roles in seed germination with regard to ABA signaling remain unknown. Here we demonstrated that Arabidopsis HSP70-16 and VDAC3 jointly suppress seed germination under cold stress conditions. At 4°C, both HSP70-16 and VDAC3 facilitated the efflux of ABA from the endosperm to the embryo and thus inhibited seed germination. HSP70-16 interacted with VDAC3 on the plasma membrane and in the nucleus, and the interplay between HSP70-16 and VDAC3 activated the opening of the VDAC3 ion channel. Our work established a novel function of HSP70-16 in seed germination under cold stress and a possible association of VDAC3 activity with ABA transportation from endosperm to embryo under cold stress conditions. This study reveals that HSP70-16 interacts with VDAC3 and facilitates the opening of the VDAC3 ion channel, which influences ABA efflux from endosperm to embryo, thus negatively regulates seed germination under cold stress.

Rescue by 4-phenylbutyrate of several misfolded creatine transporter-1 variants linked to the creatine transporter deficiency syndrome

Diseases arising from misfolding of SLC6 transporters have been reported over recent years, e.g. folding-deficient mutants of the dopamine transporter and of the glycine transporter-2 cause infantile/juvenile Parkinsonism dystonia and hyperekplexia, respectively. Mutations in the coding sequence of the human creatine transporter-1 (hCRT-1/SLC6A8) gene result in a creatine transporter deficiency syndrome, which varies in its clinical manifestation from epilepsy, mental retardation, autism, development delay and motor dysfunction to gastrointestinal symptoms. Some of the mutations in hCRT-1 occur at residues, which are highly conserved across the SLC6 family. Here, we examined 16 clinically relevant hCRT-1 variants to verify the conjecture that they were misfolded and that this folding defect was amenable to correction. Confocal microscopy imaging revealed that the heterologously expressed YFP-tagged mutant CRTs were trapped in the endoplasmic reticulum (ER), co-localised with the ER-resident chaperone calnexin. In contrast, the wild type hCRT-1 reached the plasma membrane. Preincubation of transiently transfected HEK293 cells with the chemical chaperone 4-phenylbutyrate (4-PBA) restored ER export and surface expression of as well as substrate uptake by several folding-deficient CRT-1 mutants. A representative mutant (hCRT-1-P544L) was expressed in rat primary hippocampal neurons to verify pharmacochaperoning in a target cell: 4-PBA promoted the delivery of hCRT-1-P544L to the neurite extensions. These observations show that several folding-deficient hCRT-1 mutants can be rescued. This proof-of-principle justifies the search for additional pharmacochaperones to restore folding of 4PBA-unresponsive hCRT-1 mutants. Finally, 4-PBA is an approved drug in paediatric use: this provides a rationale for translating the current insights into clinical trials. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Heat shock protein 70 and anti-heat shock protein 70 antibodies in nasal secretions of patients with chronic rhinosinusitis

BACKGROUND

The issue of heat shock protein (HSP) 70 and anti-HSP70 antibodies in chronic rhinosinusitis (CRS) has never been explored.

OBJECTIVE

To determine the nasal secretion (NS) levels of HSP70 and anti-HSP70 antibodies in patients with CRS with nasal polyps (CRSwNP) and patients with CRS without nasal polyps (CRSsNP), and to evaluate their associations with CRS clinical severity and correlation with NS interleukin (IL), IL-5 and interferon λ.

METHODS

CRS severity was determined by Lund-Mackay scores. Levels of immunoglobulin E (IgE), IL-4, IL-5, interferon λ, HSP70, and anti-HSP70 antibody levels in NS were measured by enzyme-linked immunosorbent assay.

RESULTS

Forty-six patients with CRSsNP (25 women [54.3%] and 21 men [45.7%], mean [standard deviation {SD}]) age, 34.1 ± 12.3 years; 54 patients with CRSwNP (24 women [44.4%] and 30 men [55.6%], mean [SD] age, 37.9 ± 17.5 years). A group of 40 healthy subjects served as controls. Compared with the controls (with a mean [SD] NS HSP70 level of 0.05 ± 0.03 μg/mL), mean [SD] NS HSP70 levels in both the CRSsNP group (0.16 ± 0.07 μg/mL) and CRSwNP group (0.21 ± 0.10 μg/mL) were increased (p < 0.001). Similarly, the mean (SD) NS anti-HSP70 antibody levels were significantly higher in patients with CRSwNP (0.25 ± 0.09 optical density value [ODV]) compared with CRSsNP (0.13 ± 0.04 ODV) (p < 0.001) and healthy controls (0.14 ± 0.02 ODV) (p < 0.001). NS HSP70 in subjects with CRSwNP showed a significant positive correlation with the Lund-Mackay score (r = 0.31; p < 0.05). NS levels of either HSP70 or anti-HSP70 antibodies were strongly correlated with NS IL-4 in the CRSwNP group (r = 0.62, p < 0.001; and r = 0.69, p < 0.001, respectively).

CONCLUSION

NS concentrations of HSP70 and secretory IgA anti HSP70 antibodies are increased in CRSwNP (but not in CRSsNP) and correlate positively with the Lund-Mackay score, NS IL-4, and NS IL-5.

ERp29 regulates epithelial sodium channel functional expression by promoting channel cleavage

The epithelial Na(+) channel (ENaC) plays a key role in the regulation of blood pressure and airway surface liquid volume. ERp29 is a 29-kDa thioredoxin-homologous endoplasmic reticulum (ER) protein that has only a single cysteine instead of the usual thioredoxin CXXC motif. Our group previously demonstrated that ERp29 promotes biogenesis of the cystic fibrosis transmembrane conductance regulator (CFTR). On the basis of similarities of CFTR and ENaC trafficking, we hypothesized that ERp29 would also regulate ENaC biogenesis and functional expression. In epithelial cells, overexpression of wild-type (wt) ERp29 increased ENaC functional expression [amiloride-sensitive short-circuit current (Isc)] in Ussing chamber experiments, as well as the abundance of the cleaved form of γ-ENaC in whole cell lysates. In contrast, siRNA-mediated depletion of ERp29 or overexpression of a mutant ERp29 lacking its single cysteine (C157S ERp29) decreased ENaC functional expression. Cells in which wt ERp29 was overexpressed had a smaller fractional increase in amiloride-sensitive Isc when trypsin was applied to the apical surface to activate uncleaved ENaC, while cells in which C157S ERp29 was overexpressed or ERp29 was depleted had a significantly greater fractional increase in amiloride-sensitive Isc in response to trypsin. Interestingly, these observations were not associated with altered expression of β-ENaC at the apical surface. Instead, ERp29 appeared to promote the interaction of β-ENaC with the Sec24D cargo recognition component of the coat complex II ER exit machinery. Together, these data support the hypothesis that ERp29 directs ENaC toward the Golgi, where it undergoes cleavage during its biogenesis and trafficking to the apical membrane.

Conditioned media from mesenchymal stromal cells restore sodium transport and preserve epithelial permeability in an in vitro model of acute alveolar injury

Mesenchymal stromal cells (MSCs) or their media (MSC-M) were reported to reverse acute lung injury (ALI)-induced decrease of alveolar fluid clearance. To determine the mechanisms by which MSC-M exert their beneficial effects, an in vitro model of alveolar epithelial injury was created by exposing primary rat alveolar epithelial cells (AECs) to hypoxia (3% O2) plus cytomix, a combination of IL-1β, TNF-α, and IFN-γ. MSC-M were collected from human MSCs exposed for 12 h to either normoxia (MSC-M) or to hypoxia plus cytomix (HCYT-MSC-M). This latter condition was used to model the effect of alveolar inflammation and hypoxia on paracrine secretion of MSCs in the injured lung. Comparison of paracrine soluble factors in MSC media showed that the IL-1 receptor antagonist and prostaglandin E2 were markedly increased while keratinocyte growth factor (KGF) was twofold lower in HCYT-MSC-M compared with MSC-M. In AECs, hypoxia plus cytomix increased protein permeability, reduced amiloride-sensitive short-circuit current (AS-Isc), and also decreased the number of α-epithelial sodium channel (α-ENaC) subunits in the apical membrane. To test the effects of MSC media, MSC-M and HCYT-MSC-M were added for an additional 12 h to AECs exposed to hypoxia plus cytomix. MSC-M and HCYT-MSC-M completely restored epithelial permeability to normal. MSC-M, but not HCYT-MSC-M, significantly prevented the hypoxia plus cytomix-induced decrease of ENaC activity and restored apical α-ENaC channels. Interestingly, KGF-deprived MSC-M were unable to restore amiloride-sensitive sodium transport, indicating a possible role for KGF in the beneficial effect of MSC-M. These results indicate that MSC-M may be a preferable therapeutic option for ALI.

Abnormal trafficking of endogenously expressed BMPR2 mutant allelic products in patients with heritable pulmonary arterial hypertension

More than 200 heterozygous mutations in the type 2 BMP receptor gene, BMPR2, have been identified in patients with Heritable Pulmonary Arterial Hypertension (HPAH). More severe clinical outcomes occur in patients with BMPR2 mutations by-passing nonsense-mediated mRNA decay (NMD negative mutations). These comprise 40% of HPAH mutations and are predicted to express BMPR2 mutant products. However expression of endogenous NMD negative BMPR2 mutant products and their effect on protein trafficking and signaling function have never been described. Here, we characterize the expression and trafficking of an HPAH-associated NMD negative BMPR2 mutation that results in an in-frame deletion of BMPR2 EXON2 (BMPR2ΔEx2) in HPAH patient-derived lymphocytes and in pulmonary endothelial cells (PECs) from mice carrying the same in-frame deletion of Exon 2 (Bmpr2 (ΔEx2/+) mice). The endogenous BMPR2ΔEx2 mutant product does not reach the cell surface and is retained in the endoplasmic reticulum. Moreover, chemical chaperones 4-PBA and TUDCA partially restore cell surface expression of Bmpr2ΔEx2 in PECs, suggesting that the mutant product is mis-folded. We also show that PECs from Bmpr2 (ΔEx2/+) mice have defects in the BMP-induced Smad1/5/8 and Id1 signaling axis, and that addition of chemical chaperones restores expression of the Smad1/5/8 target Id1. These data indicate that the endogenous NMD negative BMPRΔEx2 mutant product is expressed but has a folding defect resulting in ER retention. Partial correction of this folding defect and restoration of defective BMP signaling using chemical chaperones suggests that protein-folding agents could be used therapeutically in patients with these NMD negative BMPR2 mutations.

Hsc70 negatively regulates epithelial sodium channel trafficking at multiple sites in epithelial cells

The epithelial sodium channel (ENaC) plays an important role in homeostasis of blood pressure and of the airway surface liquid, and excess function of ENaC results in refractory hypertension (in Liddle's syndrome) and impaired mucociliary clearance (in cystic fibrosis). The regulation of ENaC by molecular chaperones, such as the 70-kDa heat shock protein Hsc70, is not completely understood. Our previously published data suggest that Hsc70 negatively affects ENaC activity and surface expression in Xenopus oocytes; here we investigate the mechanism by which Hsc70 acts on ENaC in epithelial cells. In Madin-Darby canine kidney cells stably expressing epitope-tagged αβγ-ENaC and with tetracycline-inducible overexpression of Hsc70, treatment with 5 μg/ml doxycycline increased total Hsc70 expression 20%. This increase in Hsc70 expression led to a decrease in ENaC activity and surface expression that corresponded to an increased rate of functional ENaC retrieval from the cell surface. In addition, Hsc70 overexpression decreased the association of newly synthesized ENaC subunits. These data support the hypothesis that Hsc70 inhibits ENaC functional expression at the apical surface of epithelia by regulating ENaC biogenesis and ENaC trafficking at the cell surface.

Progress towards genetic and pharmacological therapies for keratin genodermatoses: current perspective and future promise

Hereditary keratin disorders of the skin and its appendages comprise a large group of clinically heterogeneous disfiguring blistering and ichthyotic diseases, primarily characterized by the loss of tissue integrity, blistering and hyperkeratosis in severely affected tissues. Pathogenic mutations in keratins cause these afflictions. Typically, these mutations in concert with characteristic features have formed the basis for improved disease diagnosis, prognosis and most recently therapy development. Examples include epidermolysis bullosa simplex, keratinopathic ichthyosis, pachyonychia congenita and several other tissue-specific hereditary keratinopathies. Understanding the molecular and genetic events underlying skin dysfunction has initiated alternative treatment approaches that may provide novel therapeutic opportunities for affected patients. Animal and in vitro disease modelling studies have shed more light on molecular pathogenesis, further defining the role of keratins in disease processes and promoting the translational development of new gene and pharmacological therapeutic strategies. Given that the molecular basis for these monogenic disorders is well established, gene therapy and drug discovery targeting pharmacological compounds with the ability to reinforce the compromised cytoskeleton may lead to promising new therapeutic strategies for treating hereditary keratinopathies. In this review, we will summarize and discuss recent advances in the preclinical and clinical modelling and development of gene, natural product, pharmacological and protein-based therapies for these disorders, highlighting the feasibility of new approaches for translational clinical therapy.

Hsp70 promotes epithelial sodium channel functional expression by increasing its association with coat complex II and its exit from endoplasmic reticulum

The epithelial sodium channel (ENaC) plays an important role in the homeostasis of blood pressure and of the airway surface liquid, and inappropriate regulation of ENaC results in refractory hypertension (in Liddle syndrome) and impaired mucociliary clearance (in cystic fibrosis). The regulation of ENaC by molecular chaperones, such as the 70-kDa heat shock protein Hsp70, is not completely understood. Building on the previous suggestion by our group that Hsp70 promotes ENaC functional and surface expression in Xenopus oocytes, we investigated the mechanism by which Hsp70 acts upon ENaC in epithelial cells. In Madin-Darby canine kidney cells stably expressing epitope-tagged αβγ-ENaC and with tetracycline-inducible overexpression of Hsp70, treatment with 1 or 2 μg/ml doxycycline increased total Hsp70 expression ~2-fold and ENaC functional expression ~1.4-fold. This increase in ENaC functional expression corresponded to an increase in ENaC expression at the apical surface of the cells and was not present when an ATPase-deficient Hsp70 was similarly overexpressed. The increase in functional expression was not due to a change in the rate at which ENaC was retrieved from the apical membrane. Instead, Hsp70 overexpression increased the association of ENaC with the Sec24D cargo recognition component of coat complex II, which carries protein cargo from the endoplasmic reticulum to the Golgi. These data support the hypothesis that Hsp70 promotes ENaC biogenesis and trafficking to the apical surface of epithelial cells.

Clinical and experimental applications of sodium phenylbutyrate

Histone acetyltransferase and histone deacetylase are enzymes responsible for histone acetylation and deacetylation, respectively, in which the histones are acetylated and deacetylated on lysine residues in the N-terminal tail and on the surface of the nucleosome core. These processes are considered the most important epigenetic mechanisms for remodeling the chromatin structure and controlling the gene expression. Histone acetylation is associated with gene activation. Sodium phenylbutyrate is a histone deacetylase inhibitor that has been approved for treatement of urea cycle disorders and is under investigation in cancer, hemoglobinopathies, motor neuron diseases, and cystic fibrosis clinical trials. Due to its characteristics, not only of histone deacetylase inhibitor, but also of ammonia sink and chemical chaperone, the interest towards this molecule is growing worldwide. This review aims to update the current literature, involving the use of sodium phenylbutyrate in experimental studies and clinical trials.

Effects of cellular, chemical, and pharmacological chaperones on the rescue of a trafficking-defective mutant of the ATP-binding cassette transporter proteins ABCB1/ABCB4

The ATP-binding cassette transporter ABCB4 is a phosphatidylcholine translocator specifically expressed at the bile canalicular membrane in hepatocytes, highly homologous to the multidrug transporter ABCB1. Variations in the ABCB4 gene sequence cause progressive familial intrahepatic cholestasis type 3. We have shown previously that the I541F mutation, when reproduced either in ABCB1 or in ABCB4, led to retention in the endoplasmic reticulum (ER)/Golgi. Here, Madin-Darby canine kidney cells expressing ABCB1-GFP were used as a model to investigate this mutant. We show that ABCB1-I541F is not properly folded and is more susceptible to in situ protease degradation. It colocalizes and coprecipitates with the ER chaperone calnexin and coprecipitates with the cytosolic chaperone Hsc/Hsp70. Silencing of calnexin or overexpression of Hsp70 have no effect on maturation of the mutant. We also tested potential rescue by chemical and pharmacological chaperones. Thapsigargin and sodium 4-phenyl butyrate were inefficient. Glycerol improved maturation and exit of the mutant from the ER. Cyclosporin A, a competitive substrate for ABCB1, restored maturation, plasma membrane expression, and activity of ABCB1-I541F. Cyclosporin A also improved maturation of ABCB4-I541F in Madin-Darby canine kidney cells. In HepG(2) cells transfected with ABCB4-I541F cDNA, cyclosporin A allowed a significant amount of the mutant protein to reach the membrane of bile canaliculi. These results show that the best strategy to rescue conformation-defective ABCB4 mutants is provided by pharmacological chaperones that specifically target the protein. They identify cyclosporin A as a potential novel therapeutic tool for progressive familial intrahepatic cholestasis type 3 patients.

4-Phenylbutyrate stimulates Hsp70 expression through the Elp2 component of elongator and STAT-3 in cystic fibrosis epithelial cells

Sodium 4-phenylbutyrate (4PBA) corrects trafficking of ΔF508-CFTR in Cystic Fibrosis (CF) epithelia, which is hypothesized to, at least in part, result from increased expression of Hsp70 (stress-induced 70 kDa heat shock protein). To identify other 4PBA-regulated proteins that may promote correction of ΔF508 trafficking, we performed differential display RT-PCR on mRNA from IB3-1 CF bronchiolar epithelial cells treated for 0-24 h with 1 mM 4PBA. In this screen, a STAT-3 (signal transducer and activator of transcription-3)-interacting protein, StIP-1 that regulates STAT-3 activation had transiently increased expression. StIP-1 is identical to Elongator protein 2 (Elp2), a component of the Elongator complex that regulates RNA polymerase II. Previous studies have suggested that Elongator regulates Hsp70 mRNA transcription, and that the Hsp70 promoter contains functional STAT-3-binding sites. We therefore tested the hypothesis that 4PBA increases Hsp70 expression by an Elongator- and STAT-3-dependent mechanism. 4PBA treatment of IB3-1 CF bronchiolar epithelial cells caused transiently increased expression of Hsp70 protein, as well as Elp2 protein and mRNA. Elp2 depletion by transfection of small interfering RNAs, reduced both Elp2 and Hsp70 protein expression. 4PBA also caused transient activation of STAT-3, and increased abundance of nuclear proteins that bind to the STAT-3-responsive element of the Hsp70 promoter. Luciferase reporter assays demonstrated that both Elp2 overexpression and 4PBA increase Hsp70 promoter activity, while Elp2 depletion blocked the ability of 4PBA to stimulate Hsp70 promoter activity. Together, these data suggest that Elp2 and STAT-3 mediate, at least in part, the stimulation of Hsp70 expression by 4PBA.

Pseudoxanthoma elasticum: progress in diagnostics and research towards treatment : Summary of the 2010 PXE International Research Meeting

Pseudoxanthoma elasticum (PXE), a prototypic heritable disorder with ectopic mineralization, manifests with characteristic skin findings, ocular involvement, and cardiovascular problems. The classic forms of PXE are due to loss-of-function mutations in the ABCC6 gene, which encodes ABCC6, a putative transmembrane efflux transporter expressed primarily in the liver. While considerable progress has recently been made in understanding the molecular genetics and pathomechanisms of PXE, no effective or specific treatment is currently available for this disorder. PXE International, the premiere patient advocacy organization, organized a workshop in November 2010 to assess the current state of diagnostics and research to develop an agenda towards treatment of PXE. This overview summarizes the progress in PXE research, with emphasis on molecular therapies for this, currently intractable, disorder.

Immortalized keratinocytes derived from patients with epidermolytic ichthyosis reproduce the disease phenotype: a useful in vitro model for testing new treatments

BACKGROUND

Epidermolytic ichthyosis (EI) is a skin fragility disorder caused by mutations in genes encoding suprabasal keratins 1 and 10. While the aetiology of EI is known, model systems are needed for pathophysiological studies and development of novel therapies.

OBJECTIVES

To generate immortalized keratinocyte lines from patients with EI for studies of EI cell pathology and the effects of chemical chaperones as putative therapies.

METHODS

We derived keratinocytes from three patients with EI and one healthy control and established immortalized keratinocytes using human papillomavirus 16-E6/E7. Growth and differentiation characteristics, ability to regenerate organotypic epidermis, keratin expression, formation of cytoskeletal aggregates, and responses to heat shock and chemical chaperones were assessed.

RESULTS

The cell lines EH11 (K1_p.Val176_Lys197del), EH21 (K10_p.156Arg>Gly), EH31 (K10_p.Leu161_Asp162del) and NKc21 (wild-type) currently exceed 160 population doublings and differentiate when exposed to calcium. At resting state, keratin aggregates were detected in 9% of calcium-differentiated EH31 cells, but not in any other cell line. Heat stress further increased this proportion to 30% and also induced aggregates in 3% of EH11 cultures. Treatment with trimethylamine N-oxide and 4-phenylbutyrate (4-PBA) reduced the fraction of aggregate-containing cells and affected the mRNA expression of keratins 1 and 10 while 4-PBA also modified heat shock protein 70 (HSP70) expression. Furthermore, in situ proximity ligation assay suggested a colocalization between HSP70 and keratins 1 and 10. Reconstituted epidermis from EI cells cornified but EH21 and EH31 cells produced suprabasal cytolysis, closely resembling the in vivo phenotype.

CONCLUSIONS

These immortalized cell lines represent a useful model for studying EI biology and novel therapies.

Pseudoxanthoma elasticum, the paradigm of heritable ectopic mineralization disorders - can diet help?

Pseudoxanthoma elasticum (PXE) is a heritable multi-system disorder manifesting with characteristic cutaneous lesions, associated with ocular findings and cardiovascular involvement. The skin lesions, yellowish papules which coalesce into plaques of inelastic and leathery skin, demonstrate by histopathologic and ultrastructural examinations ectopic mineralization of dermal connective tissues, primarily the elastic structures. PXE is inherited in an autosomal recessive fashion due to mutations in the ABCC6 gene. Significant insights into the pathogenesis of PXE have been recently obtained from observations on the Abcc6(-/-) knockout mouse which mimics the genetic, histopathologic and ultrastructural features of PXE. This mouse model has provided a platform to test various treatment modalities to counteract the mineralization phenotypes. One of the intriguing findings emanating from these studies is that supplementation of the mouse diet with magnesium, at levels that are ∼5-fold higher than those in control diet, completely inhibits the development of tissue mineralization. These and related observations suggest that changes in the diet might counteract the progression of PXE and improve the quality of life of patients with this, currently intractable, disease.

Regulation of endogenous ENaC functional expression by CFTR and ΔF508-CFTR in airway epithelial cells

The functional expression of the epithelial sodium channel (ENaC) appears elevated in cystic fibrosis (CF) airway epithelia, but the mechanism by which this occurs is not clear. We tested the hypothesis that the cystic fibrosis transmembrane conductance regulator (CFTR) alters the trafficking of endogenously expressed human ENaC in the CFBE41o⁻ model of CF bronchial epithelia. Functional expression of ENaC, as defined by amiloride-inhibited short-circuit current (I(sc)) in Ussing chambers, was absent under control conditions but present in CFBE41o⁻ parental and ΔF508-CFTR-overexpressing cells after treatment with 1 μM dexamethasone (Dex) for 24 h. The effect of Dex was mimicked by incubation with the glucocorticoid hydrocortisone but not with the mineralocorticoid aldosterone. Application of trypsin to the apical surface to activate uncleaved, "near-silent" ENaC caused an additional increase in amiloride-sensitive I(sc) in the Dex-treated cells and was without effect in the control cells, suggesting that Dex increased ENaC cell surface expression. In contrast, Dex treatment did not stimulate amiloride-sensitive I(sc) in CFBE41o⁻ cells that stably express wild-type (wt) CFTR. CFBE41o⁻ wt cells also had reduced expression of α- and γ-ENaC compared with parental and ΔF508-CFTR-overexpressing cells. Furthermore, application of trypsin to the apical surface of Dex-treated CFBE41o⁻ wt cells did not stimulate amiloride-sensitive I(sc), suggesting that ENaC remained absent from the surface of these cells even after Dex treatment. We also tested the effect of trafficking-corrected ΔF508-CFTR on ENaC functional expression. Incubation with 1 mM 4-phenylbutyrate synergistically increased Dex-induced ENaC functional expression in ΔF508-CFTR-overexpressing cells. These data support the hypothesis that wt CFTR can regulate the whole cell, functional, and surface expression of endogenous ENaC in airway epithelial cells and that absence of this regulation may foster ENaC hyperactivity in CF airway epithelia.

Biological and chemical approaches to diseases of proteostasis deficiency

Many diseases appear to be caused by the misregulation of protein maintenance. Such diseases of protein homeostasis, or "proteostasis," include loss-of-function diseases (cystic fibrosis) and gain-of-toxic-function diseases (Alzheimer's, Parkinson's, and Huntington's disease). Proteostasis is maintained by the proteostasis network, which comprises pathways that control protein synthesis, folding, trafficking, aggregation, disaggregation, and degradation. The decreased ability of the proteostasis network to cope with inherited misfolding-prone proteins, aging, and/or metabolic/environmental stress appears to trigger or exacerbate proteostasis diseases. Herein, we review recent evidence supporting the principle that proteostasis is influenced both by an adjustable proteostasis network capacity and protein folding energetics, which together determine the balance between folding efficiency, misfolding, protein degradation, and aggregation. We review how small molecules can enhance proteostasis by binding to and stabilizing specific proteins (pharmacologic chaperones) or by increasing the proteostasis network capacity (proteostasis regulators). We propose that such therapeutic strategies, including combination therapies, represent a new approach for treating a range of diverse human maladies.

Mechanisms of ENaC regulation and clinical implications

The epithelial Na+ channel (ENaC) transports Na+ across tight epithelia, including the distal nephron. Different paradigms of ENaC regulation include extrinsic and intrinsic factors that affect the expression, single-channel properties, and intracellular trafficking of the channel. In particular, recent discoveries highlight new findings regarding proteolytic processing, ubiquitination, and recycling of the channel. Understanding the regulation of this channel is critical to the understanding of various clinical phenomena, including normal physiology and several diseases of kidney and lung epithelia, such as blood pressure (BP) control, edema, and airway fluid clearance. Significant progress has been achieved in this active field of research. Although ENaC is classically thought to be a mediator of BP and volume status through Na+ reabsorption in the distal nephron, several studies in animal models highlight important roles for ENaC in lung pathophysiology, including in cystic fibrosis. The purpose of this review is to highlight the various modes and mechanisms of ENaC regulation, with a focus on more recent studies and their clinical implications.

Hsc70 regulates cell surface ASIC2 expression and vascular smooth muscle cell migration

Recent studies suggest members of the degenerin (DEG)/epithelial Na(+) channel (ENaC)/acid-sensing ion channel (ASIC) protein family play an important role in vascular smooth muscle cell (VSMC) migration. In a previous investigation, we found suppression of a certain DEG/ENaC/ASIC member, ASIC2, increased VSMC chemotactic migration, raising the possibility that ASIC2 may play an inhibitory role. Because ASIC2 protein was retained in the cytoplasm, we reasoned increasing surface expression of ASIC2 might unmask the inhibitory role of ASIC2 in VSMC migration so we could test the hypothesis that ASIC2 inhibits VSMC migration. Therefore, we used the chemical chaperone glycerol to enhance ASIC2 expression. Glycerol 1) increased cytoplasm ASIC2 expression, 2) permitted detection of ASIC2 at the cell surface, and 3) inhibited platelet-derived growth factor (PDGF)-bb mediated VSMC migration. Furthermore, ASIC2 silencing completely abolished the inhibitory effect of glycerol on migration, suggesting upregulation of ASIC2 is responsible for glycerol-induced inhibition of VSMC migration. Because other investigators have shown that glycerol regulates ENaC/ASIC via interactions with a certain heat shock protein, heat shock protein 70 (Hsc70), we wanted to determine the importance of Hsc70 on ASIC2 expression in VSMCs. We found that Hsc70 silencing increases ASIC2 cell surface expression and inhibits VSMC migration, which is abolished by cosilencing ASIC2. These data demonstrate that Hsc70 inhibits ASIC2 expression, and, when the inhibitory effect of Hsc70 is removed, ASIC2 expression increases, resulting in reduced VSMC migration. Because VSMC migration contributes to vasculogenesis and remodeling following vascular injury, our findings raise the possibility that ASIC2-Hsc70 interactions may play a role in these processes.