Functional assembly and purinergic activation of bestrophins

Evaluating BEST1 mutations in pluripotent stem cell-derived retinal pigment epithelial cells

Bestrophin-1 (BEST1) is a calcium-activated chloride channel (CaCC) predominantly expressed at the basolateral membrane of the retinal pigment epithelium (RPE). Over 250 mutations in the BEST1 gene have been documented to cause at least five retinal degenerative disorders, commonly termed bestrophinopathies, to which no treatment is currently available. Therefore, understanding the influences of BEST1 disease-causing mutations on the physiological function of BEST1 in RPE is critical for deciphering the pathology of bestrophinopathies and developing therapeutic strategies for patients. However, this task has been impeded by the rarity of BEST1 mutations and limited accessibility to native human RPE cells. Here, we describe a pluripotent stem cell (PSC)-based pipeline for reproducibly generating RPE cells expressing endogenous or exogenous mutant BEST1, which provides us with a powerful "disease-in-a-dish" approach for studying BEST1 mutations in physiological environments.

Restoration of mutant bestrophin-1 expression, localisation and function in a polarised epithelial cell model

Autosomal recessive bestrophinopathy (ARB) is a retinopathy caused by mutations in the bestrophin-1 protein, which is thought to function as a Ca²⁺-gated Cl⁻ channel in the basolateral surface of the retinal pigment epithelium (RPE). Using a stably transfected polarised epithelial cell model, we show that four ARB mutant bestrophin-1 proteins were mislocalised and subjected to proteasomal degradation. In contrast to the wild-type bestrophin-1, each of the four mutant proteins also failed to conduct Cl⁻ ions in transiently transfected cells as determined by whole-cell patch clamp. We demonstrate that a combination of two clinically approved drugs, bortezomib and 4-phenylbutyrate (4PBA), successfully restored the expression and localisation of all four ARB mutant bestrophin-1 proteins. Importantly, the Cl⁻ conductance function of each of the mutant bestrophin-1 proteins was fully restored to that of wild-type bestrophin-1 by treatment of cells with 4PBA alone. The functional rescue achieved with 4PBA is significant because it suggests that this drug, which is already approved for long-term use in infants and adults, might represent a promising therapy for the treatment of ARB and other bestrophinopathies resulting from missense mutations in BEST1.

Summary: Chemical chaperone 4PBA fully restores Cl⁻ conductance activity for mutant bestrophin-1 proteins associated with inherited retinal dystrophy, autosomal recessive bestrophinopathy.

Differential contribution of SLC26A9 to Cl(-) conductance in polarized and non-polarized epithelial cells

SLC26 proteins function as anion exchangers and Cl(-) channels. SLC26A9 has been proposed to be a constitutively active and CFTR-regulated anion conductance in human bronchial epithelia. This positive interaction between two Cl(-) channels has been questioned by others and evidence has been provided that CFTR rather inhibits the transport activity of SLC26A9. We therefore examined the functional interaction between CFTR and SLC26A9 in polarized airway epithelial cells and in non-polarized HEK293 cells expressing CFTR and SLC26A9. We found that SLC26A9 provides a constitutively active basal Cl(-) conductance in polarized grown CFTR-expressing CFBE airway epithelial cells, but not in cells expressing F508del-CFTR. In polarized CFTR-expressing cells, SLC26A9 also contributes to both Ca(2+) - and CFTR-activated Cl(-) secretion. In contrast in non-polarized HEK293 cells co-expressing CFTR/SLC26A9, the baseline Cl(-) conductance provided by SLC26A9 was inhibited during activation of CFTR. SLC26A9 and CFTR behave differentially in polarized and non-polarized cells, which may explain earlier conflicting data.

Disease-associated missense mutations in bestrophin-1 affect cellular trafficking and anion conductance

Bestrophin-1, an integral membrane protein encoded by the BEST1 gene, is localized predominantly to the basolateral membrane of the retinal pigment epithelium. Mutations in the BEST1 gene have been associated with Best vitelliforme macular dystrophy (BMD), a central retinopathy with autosomal dominant inheritance and variable penetrance. Over 120 disease-causing mutations are known, the majority of which result in amino acid substitutions within four mutational hot-spot regions in the highly conserved N-terminal half of the protein. Although initially thought to impair Cl⁻ channel function, the molecular pathology of BEST1 mutations is still controversial. We have analyzed the subcellular localization of 13 disease-associated BEST1 mutant proteins in polarized MDCK II cells, an established model of apical to basolateral protein sorting. Immunostaining demonstrated that nine of the 13 mutant proteins failed to integrate into the cell membrane. The defective proteins were predominantly retained in the cytoplasm, whereas wild-type bestrophin-1 revealed cell membrane localization. Functional analysis of I⁻ fluxes in HEK-293 cells showed that all mutants exhibited a significant reduction in anion conductance. Our data indicate that defective intracellular trafficking could be a common cause of BMD accompanied by impaired anion conductance, representing a loss of anion channel function that is probably due to mistargeting of mutant protein.

Interaction of bestrophin-1 and Ca2+ channel β-subunits: identification of new binding domains on the bestrophin-1 C-terminus

Bestrophin-1 modulates currents through voltage-dependent L-type Ca²⁺ channels by physically interacting with the β-subunits of Ca²⁺ channels. The main function of β-subunits is to regulate the number of pore-forming Ca_V-subunits in the cell membrane and modulate Ca²⁺ channel currents. To understand the influence of full-length bestrophin-1 on β-subunit function, we studied binding and localization of bestrophin-1 and Ca²⁺ channel subunits, together with modulation of Ca_V1.3 Ca²⁺ channels currents. In heterologeous expression, bestrophin-1 showed co-immunoprecipitation with either, β3-, or β4-subunits. We identified a new highly conserved cluster of proline-rich motifs on the bestrophin-1 C-terminus between amino acid position 468 and 486, which enables possible binding to SH3-domains of β-subunits. A bestrophin-1 that lacks these proline-rich motifs (ΔCT-PxxP bestrophin-1) showed reduced efficiency to co-immunoprecipitate with β3 and β4-subunits. In the presence of ΔCT-PxxP bestrophin-1, β4-subunits and Ca_V1.3 subunits partly lost membrane localization. Currents from Ca_V1.3 subunits were modified in the presence of β4-subunit and wild-type bestrophin-1: accelerated time-dependent activation and reduced current density. With ΔCTPxxP bestrophin-1, currents showed the same time-dependent activation as with wild-type bestrophin-1, but the current density was further reduced due to decreased number of Ca²⁺ channels proteins in the cell membrane. In summary, we described new proline-rich motifs on bestrophin-1 C-terminus, which help to maintain the ability of β-subunits to regulate surface expression of pore-forming Ca_V Ca²⁺-channel subunits.

Bestrophin is important for the rhythmic but not the tonic contraction in rat mesenteric small arteries

AIMS

We have previously characterized a cGMP-dependent Ca(2+)-activated Cl(-) current in vascular smooth muscle cells (SMCs) and have shown its dependence on bestrophin-3 expression. We hypothesize that this current is important for synchronization of SMCs in the vascular wall. In the present study, we aimed to test this hypothesis by transfecting rat mesenteric small arteries in vivo with siRNA specifically targeting bestrophin-3.

METHODS AND RESULTS

The arteries were tested 3 days after transfection in vitro for isometric force development and for intracellular Ca(2+) in SMCs. Bestrophin-3 expression was significantly reduced compared with arteries transfected with mutated siRNA. mRNA levels for bestrophin-1 and -2 were also significantly reduced by bestrophin-3 down-regulation. This is suggested to be secondary to specific bestrophin-3 down-regulation since siRNAs targeting different exons of the bestrophin-3 gene had identical effects on bestrophin-1 and -2 expression. The transfection affected neither the maximal contractile response nor the sensitivity to norepinephrine and arginine-vasopressin. The amplitude of agonist-induced vasomotion was significantly reduced in arteries down-regulated for bestrophins compared with controls, and asynchronous Ca(2+) waves appeared in the SMCs. The average frequency of vasomotion was not different. 8Br-cGMP restored vasomotion in arteries where the endothelium was removed, but oscillation amplitude was still significantly less in bestrophin-down-regulated arteries. Thus, vasomotion properties were consistent with those previously characterized for rat mesenteric small arteries. Data from our mathematical model are consistent with the experimental results.

CONCLUSION

This study demonstrates the importance of bestrophins for synchronization of SMCs and strongly supports our hypothesis for generation of vasomotion.

The presence of bestrophin-1 modulates the Ca2+ recruitment from Ca2+ stores in the ER

Bestrophin-1, mainly analyzed in overexpression experiments, functions as Ca(2+)-dependent Cl(-) channel. Analysis of endogenously expressed bestrophin-1 suggested an influence on intracellular Ca(2+). The aim of the study is to analyze the influence of endogenously expressed bestrophin-1 on Ca(2+) homeostasis. Primary cultures of retinal pigment epithelial (RPE) cells were established from wild-type and bestrophin-1-deficient mice. Intracellular free Ca(2+) ([Ca(2+)](i)) was recorded by Ca(2+) imaging; through immunocytochemistry and differential centrifugation, subcellular localization of bestrophin-1 was analyzed. RPE cells of bestrophin-1-deficient mice showed higher levels of resting [Ca(2+)](i) than cells from wild-type mice. In cells from knockout mice and wild-type mice, ATP led to increases in [Ca(2+)](i) subsequent to phospholipase C activation. ATP-induced Ca(2+) in bestrophin-1-deficient mice rose faster and decayed slower. In cells from wild-type mice, ATP led to [Ca(2+)](i) increase via depletion of Ca(2+) from thapsigargin-sensitive stores. In cells from bestrophin-1-deficient mice, ATP-dependent increase in [Ca(2+)](i) resulted in 40% of cells from depletion of bafilomycin-sensitive and in 60% from thapsigargin-sensitive Ca(2+) stores. After differential centrifugation, bestrophin-1 was found in fractions enriched of ClC-3 Cl channel and myosin-7A. Co-localization analysis of bestrophin-1, with beta-catenin or pan-cadherin, in fresh sections of porcine retina, revealed bestrophin-1 in the basolateral membrane. A portion of endogenously expressed bestrophin-1,localized in the endoplasmic reticulum, influenced uptake of Ca(2+) into Ca(2+) stores. Therefore, bestrophin-1 possibly conducts Cl(-) as counter ion for Ca(2+) uptake into cytosolic Ca(2+) stores.

Course of visual decline in relation to the Best1 genotype in vitelliform macular dystrophy

PURPOSE

To describe the disease course in patients with vitelliform macular dystrophy (VMD) with a Best1 mutation and to determine the association between Best1 genotype and visual prognosis.

DESIGN

Consecutive case series.

PARTICIPANTS

Fifty-three patients with VMD with Best1 mutations from 27 Dutch families, aged 11 to 87 years.

METHODS

Best-corrected visual acuity (VA), fundus appearance, and Arden ratio on the electro-oculogram (EOG) during clinical follow-up were assessed from medical records. Mutation analysis of the Best1 gene was performed on DNA samples using denaturing high-pressure liquid chromatography and direct sequencing.

MAIN OUTCOME MEASURES

Cumulative lifetime risk of visual decline below 0.5, 0.3, and 0.1 for the entire group and stratified for genotype.

RESULTS

Median age of onset of visual symptoms was 33 years (range: 2-78). The cumulative risk of VA below 0.5 (20/40) was 50% at 55 years and 75% at 66 years. The cumulative risk of decline less than 0.3 (20/63) was 50% by age 66 years and 75% by age 74 years. Two patients progressed to VA less than 0.1 (20/200). Fourteen different mutations were found. Most patients (96%) had missense mutations; the Thr6Pro, Ala10Val, and Tyr227Asn mutations were most common. Visual decline was significantly faster in patients with an Ala10Val mutation than either the Thr6Pro or the Tyr227Asn mutation (P=0.001).

CONCLUSIONS

Age of onset of visual symptoms varies greatly among patients with VMD. All patients show a gradual decrease in VA, and most progress to visual impairment at a relatively late age. Our data suggest a phenotype-genotype correlation, because the Ala10Val mutation has a more rapid disease progression than other common mutations.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

ER-localized bestrophin 1 activates Ca2+-dependent ion channels TMEM16A and SK4 possibly by acting as a counterion channel

Bestrophins form Ca(2+)-activated Cl(-) channels and regulate intracellular Ca(2+) signaling. We demonstrate that bestrophin 1 is localized in the endoplasmic reticulum (ER), where it interacts with stromal interacting molecule 1, the ER-Ca(2+) sensor. Intracellular Ca(2+) transients elicited by stimulation of purinergic P2Y(2) receptors in HEK293 cells were augmented by hBest1. The p21-activated protein kinase Pak2 was found to phosphorylate hBest1, thereby enhancing Ca(2+) signaling and activation of Ca(2+)-dependent Cl(-) (TMEM16A) and K(+) (SK4) channels. Lack of bestrophin 1 expression in respiratory epithelial cells of mBest1 knockout mice caused expansion of ER cisterns and induced Ca(2+) deposits. hBest1 is, therefore, important for Ca(2+) handling of the ER store and may resemble the long-suspected counterion channel to balance transient membrane potentials occurring through inositol triphosphate (IP(3))-induced Ca(2+) release and store refill. Thus, bestrophin 1 regulates compartmentalized Ca(2+) signaling that plays an essential role in Best macular dystrophy, inflammatory diseases such as cystic fibrosis, as well as proliferation.

Bestrophin and TMEM16-Ca(2+) activated Cl(-) channels with different functions

In the past, a number of candidates have been proposed to form Ca(2+) activated Cl(-) currents, but it is only recently that two families of proteins, the bestrophins and the TMEM16-proteins, recapitulate reliably the properties of Ca(2+) activated Cl(-) currents. Bestrophin 1 is strongly expressed in the retinal pigment epithelium, but also at lower levels in other cell types. Bestrophin 1 may form Ca(2+) activated chloride channels and, at the same time, affect intracellular Ca(2+) signaling. In epithelial cells, bestrophin 1 probably controls receptor mediated Ca(2+) signaling. It may do so by facilitating Ca(2+) release from the endoplasmic reticulum, thereby indirectly activating membrane localized Ca(2+)-dependent Cl(-) channels. In contrast to bestrophin 1, the Ca(2+) activated Cl(-) channel TMEM16A (anoctamin 1, ANO1) shows most of the biophysical and pharmacological properties that have been attributed to Ca(2+)-dependent Cl(-) channels in various tissues. TMEM16A is broadly expressed in both mouse and human tissues and is of particular importance in epithelial cells. Thus exocrine gland secretion as well as electrolyte transport by both respiratory and intestinal epithelia requires TMEM16A. Because of its role for Ca(2+)-dependent Cl(-) secretion in human airways, it is likely to become a prime target for the therapy of cystic fibrosis lung disease, caused by defective cAMP-dependent Cl(-) secretion. It will be very exciting to learn, how TMEM16A and other TMEM16-proteins are activated upon increase in intracellular Ca(2+), and whether the other nine members of the TMEM16 family also form Cl(-) channels with properties similar to TMEM16A.

Loss of TMEM16A causes a defect in epithelial Ca2+-dependent chloride transport

Molecular identification of the Ca(2+)-dependent chloride channel TMEM16A (ANO1) provided a fundamental step in understanding Ca(2+)-dependent Cl(-) secretion in epithelia. TMEM16A is an intrinsic constituent of Ca(2+)-dependent Cl(-) channels in cultured epithelia and may control salivary output, but its physiological role in native epithelial tissues remains largely obscure. Here, we demonstrate that Cl(-) secretion in native epithelia activated by Ca(2+)-dependent agonists is missing in mice lacking expression of TMEM16A. Ca(2+)-dependent Cl(-) transport was missing or largely reduced in isolated tracheal and colonic epithelia, as well as hepatocytes and acinar cells from pancreatic and submandibular glands of TMEM16A(-/-) animals. Measurement of particle transport on the surface of tracheas ex vivo indicated largely reduced mucociliary clearance in TMEM16A(-/-) mice. These results clearly demonstrate the broad physiological role of TMEM16A(-/-) for Ca(2+)-dependent Cl(-) secretion and provide the basis for novel treatments in cystic fibrosis, infectious diarrhea, and Sjöegren syndrome.

Bestrophin 1 promotes epithelial-to-mesenchymal transition of renal collecting duct cells

Bestrophin 1 (Best1) controls intracellular Ca(2+) concentration, induces Ca(2+)-activated Cl(-) conductance, and increases proliferation of colon carcinoma cells. Here, we show that expression of Best1 in mouse renal collecting duct (CD) cells causes i) an increase in cell proliferation, ii) a loss of amiloride-sensitive Na(+) absorption, iii) induction of Ca(2+)-dependent Cl(-) conductance (CaCC), and iv) epithelial-to-mesenchymal transition. During conditions of high proliferation or when we exposed CD cells to serum or TGF-beta1, we observed upregulation of Best1, increased CaCC, redistribution of the epithelial-to-mesenchymal transition marker beta-catenin, and upregulation of vimentin. In contrast, suppression of Best1 by RNAi inhibited proliferation, reduced CaCC, and downregulated markers of EMT. CaCC and expression of Best1 were independent of the cell cycle but clearly correlated to cell proliferation and cell density. During renal inflammation in LPS-treated mice or after unilateral ureteral obstruction, we observed transient upregulation of Best1. These data indicate that repression of cell proliferation, CaCC, and expression of Best1 occurs during mesenchymal-to-epithelial transition once CD cells polarize and terminally differentiate. These results may suggest a role for Best1 in renal fibrosis and tissue repair.