Bestrophin interacts physically and functionally with protein phosphatase 2A

Therapeutic implications of nanodrug and tissue engineering for retinal pigment epithelium-related diseases

The retinal pigment epithelium (RPE), as a single layer of cells that performs multiple functions posteriorly in the eye, is a promising target site for the prevention and treatment of several clinical diseases, including proliferative diabetic retinopathy, age-related macular degeneration, chorionic neovascularization, and retinitis pigmentosa. In recent decades, several nanodrug delivery platforms and tissue-engineered RPE have been widely developed to treat RPE-related diseases. This work summarizes the recent advances in nanoplatforms and tissue engineering scaffolds developed in these fields. The diseases associated with pathological RPE and their common therapy strategies are first introduced. Then, the recent progress made with a variety of drug delivery systems is presented, with an emphasis on the modification strategies of nanomaterials for targeted delivery. Tissue engineering-mediated RPE transplantation for treating these diseases is subsequently described. Finally, the clinical translation challenges in these fields are discussed in depth. This article will offer readers a better understanding of emerging nanotechnology and tissue engineering related to the treatment of RPE-related diseases and could facilitate their widespread use in experiments in vivo and in clinical applications.

Self-organization and surface properties of hBest1 in models of biological membranes

The transmembrane Ca²⁺ - activated Cl^- channel - human bestrophin-1 (hBest1) is expressed in retinal pigment epithelium and mutations of BEST1 gene cause ocular degenerative diseases colectivelly referred to as "bestrophinopathies". A large number of genetical, biochemical, biophysical and molecular biological studies have been performed to understand the relationship between structure and function of the hBest1 protein and its pathophysiological significance. Here, we review the current understanding of hBest1 surface organization, interactions with membrane lipids in model membranes, and its association with microdomains of cellular membranes. These highlights are significant for modulation of channel activity in cells.

Structure and Function of the Bestrophin family of calcium-activated chloride channels

Bestrophins are a family of calcium-activated chloride channels (CaCCs) with relevance to human physiology and a myriad of eye diseases termed "bestrophinopathies". Since the identification of bestrophins as CaCCs nearly two decades ago, extensive studies from electrophysiological and structural biology perspectives have sought to define their key channel features including calcium sensing, gating, inactivation, and anion selectivity. The initial X-ray crystallography studies on the prokaryotic homolog of Best1, Klebsiella pneumoniae (KpBest), and the Best1 homolog from Gallus gallus (chicken Best1, cBest1), laid the foundational groundwork for establishing the architecture of Best1. Recent progress utilizing single-particle cryogenic electron microscopy has further elucidated the molecular mechanism of gating in cBest1 and, separately, the structure of Best2 from Bos taurus (bovine Best2, bBest2). Meanwhile, whole-cell patch clamp, planar lipid bilayer, and other electrophysiologic analyses using these models as well as the human Best1 (hBest1) have provided ample evidence describing the functional properties of the bestrophin channels. This review seeks to consolidate these structural and functional results to paint a broad picture of the underlying mechanisms comprising the bestrophin family's structure-function relationship.

NOVEL BEST1 MUTATIONS DETECTED BY NEXT-GENERATION SEQUENCING IN A CHINESE POPULATION WITH VITELLIFORM MACULAR DYSTROPHY

PURPOSE

To characterize novel BEST1 mutations and the phenotype-genotype correlations in vitelliform macular dystrophy in a Chinese population.

METHODS

Seventeen individuals affected by vitelliform macular dystrophy underwent detailed ophthalmic examinations, including a best-corrected visual acuity test, slit-lamp biomicroscopy, fundus photography, spectral-domain optical coherence tomography, fundus autofluorescence, fluorescein angiography, and optical coherence tomography angiography. Next-generation sequencing was used to screen 790 genes frequently involved in common inherited nonsyndromic eye diseases in all participants. Sanger sequencing was used to identify possible disease-causing variants.

RESULTS

The mean ± SD age of the patients was 44.20 ± 15.09 (range: 16-69) years. Seven novel BEST1 mutations were identified: six heterozygous missense (p.Thr307Asn, p.Ile295The, p.Leu75Pro, p.Thr2Ser, p.Ser79Tyr, and p.Val81Leu) and one frameshift (p.Glu115GlufsX120) mutation. Choroidal neovascularization was detected in two probands. One individual presented with subfoveal focal choroidal excavation. Arden ratios obtained by electrooculography were less than the 1.5 cutoff value in 7 patients. No mutations were identified in 2 patients, one of whom had a fundus appearance typical of vitelliform macular dystrophy and a decreased Arden ratio (1.2/1.2).

CONCLUSION

Patients with the same heterozygous BEST1 mutations exhibited varying phenotypes. Our results have expanded the BEST1 mutation spectrum in a Chinese population with vitelliform macular dystrophy.

Inhibition of Ca2+ channel surface expression by mutant bestrophin-1 in RPE cells

The BEST1 gene product bestrophin-1, a Ca²⁺ -dependent anion channel, interacts with Ca_V 1.3 Ca²⁺ channels in the retinal pigment epithelium (RPE). BEST1 mutations lead to Best vitelliform macular dystrophy. A common functional defect of these mutations is reduced trafficking of bestrophin-1 into the plasma membrane. We hypothesized that this defect affects the interaction partner Ca_V 1.3 channel affecting Ca²⁺ signaling and altered RPE function. Thus, we investigated the protein interaction between Ca_V 1.3 channels and bestrophin-1 by immunoprecipitation, Ca_V 1.3 activity in the presence of mutant bestrophin-1 and intracellular trafficking of the interaction partners in confluent RPE monolayers. We selected four BEST1 mutations, each representing one mutational hotspot of the disease: T6P, F80L, R218C, and F305S. Heterologously expressed L-type channels and mutant bestrophin-1 showed reduced interaction, reduced Ca_V 1.3 channel activity, and changes in surface expression. Transfection of polarized RPE (porcine primary cells, iPSC-RPE) that endogenously express Ca_V 1.3 and wild-type bestrophin-1, with mutant bestrophin-1 confirmed reduction of Ca_V 1.3 surface expression. For the four selected BEST1 mutations, presence of mutant bestrophin-1 led to reduced Ca_V 1.3 activity by modulating pore-function or decreasing surface expression. Reduced Ca_V 1.3 activity might open new ways to understand symptoms of Best vitelliform macular dystrophy such as reduced electro-oculogram, lipofuscin accumulation, and vision impairment.

Functional Genomics of the Retina to Elucidate its Construction and Deconstruction

The retina is the light sensitive part of the eye and nervous tissue that have been used extensively to characterize the function of the central nervous system. The retina has a central position both in fundamental biology and in the physiopathology of neurodegenerative diseases. We address the contribution of functional genomics to the understanding of retinal biology by reviewing key events in their historical perspective as an introduction to major findings that were obtained through the study of the retina using genomics, transcriptomics and proteomics. We illustrate our purpose by showing that most of the genes of interest for retinal development and those involved in inherited retinal degenerations have a restricted expression to the retina and most particularly to photoreceptors cells. We show that the exponential growth of data generated by functional genomics is a future challenge not only in terms of storage but also in terms of accessibility to the scientific community of retinal biologists in the future. Finally, we emphasize on novel perspectives that emerge from the development of redox-proteomics, the new frontier in retinal biology.

Novel BEST1 gene mutations associated with two different forms of macular dystrophy in Tunisian families

BACKGROUND

Epidemiological studies of hereditary eye diseases allowed us to identify two Tunisian families suffering from macular dystrophies: Best vitelliform macular dystrophy (BVMD) and autosomal recessive bestrophinopathy (ARB). The purpose of the current study was to investigate the clinical characteristics and the underlying genetics of these two forms of macular dystrophy.

METHODS

Complete ophthalmic examination was performed including optical coherence tomography, electroretinography, electrooculography and autofluoresence imaging in all patients. Genomic DNA was extracted from peripheral blood collected from patients and family members.

RESULTS

Sanger sequencing of all exons of the BEST1 gene in both families identified two new mutations: a missense mutation c.C91A [p.L31 M] at the N-terminal transmembrane domain within the ARB family and a nonsense mutation C1550G (p.S517X) in the C-terminal domain segregating in the BVMD family.

CONCLUSIONS

Several mutations of the BEST1 gene have been reported which are responsible for numerous ocular pathologies. To the best of our knowledge, it is the first time we report mutations in this gene in Tunisian families presenting different forms of macular dystrophy. Our report also expands the list of pathogenic BEST1 genotypes and the associated clinical diagnosis.

Bestrophin-3 Expression in a Subpopulation of Astrocytes in the Neonatal Brain After Hypoxic-Ischemic Injury

Bestrophin-3, a potential candidate for a calcium-activated chloride channel, recently was suggested to have cell-protective functions. We studied the expression and alternative splicing of bestrophin-3 in neonatal mouse brain and after hypoxic-ischemic (HI) injury and in human neonatal brain samples. HI brain injury was induced in 9-day old mice by unilateral permanent common carotid artery occlusion in combination with exposure to 10% oxygen for 50 min. Endoplasmic reticulum stress was induced by thapsigargin treatment in primary culture of mouse brain astrocytes. We also investigated expression of bestrophin-3 protein in a sample of human neonatal brain tissue. Bestrophin-3 protein expression was detected with immunohistochemical methods and western blot; mRNA expression and splicing were analyzed by RT-PCR. HI induced a brain tissue infarct and a pronounced increase in the endoplasmic reticulum-associated marker CHOP. Three days after HI a population of astrocytes co-expressed bestrophin-3 and nestin in a penumbra-like area of the injured hemisphere. However, total levels of Bestrophin-3 protein in mouse cortex were reduced after injury. Mouse astrocytes in primary culture also expressed bestrophin-3 protein, the amount of which was reduced by endoplasmic reticulum stress. Bestrophin-3 protein was detected in astrocytes in the hippocampal region of the human neonatal brain which had patchy white matter gliosis and neuronal loss in the Sommer’s sector of the Ammon’s horn (CA1). Analysis of bestrophin-3 mRNA in mouse brain with and without injury showed the presence of two truncated spliced variants, but no full-length mRNA. Total amount of bestrophin-3 mRNA increased after HI, but showed only minor injury-related change. However, the splice variants of bestrophin-3 mRNA were differentially regulated after HI depending on the presence of tissue injury. Our results show that bestrophin-3 is expressed in neonatal mouse brain after injury and in the human neonatal brain with pathology. In mouse brain bestrophin-3 protein is upregulated in a specific astrocyte population after injury and is co-expressed with nestin. Splice variants of bestrophin-3 mRNA respond differently to HI, which might indicate their different roles in tissue injury.

An allosteric mechanism of inactivation in the calcium-dependent chloride channel BEST1

Bestrophin proteins are calcium (Ca²⁺)-activated chloride channels. Mutations in bestrophin 1 (BEST1) cause macular degenerative disorders. Whole-cell recordings show that ionic currents through BEST1 run down over time, but it is unclear whether this behavior is intrinsic to the channel or the result of cellular factors. Here, using planar lipid bilayer recordings of purified BEST1, we show that current rundown is an inherent property of the channel that can now be characterized as inactivation. Inactivation depends on the cytosolic concentration of Ca²⁺, such that higher concentrations stimulate inactivation. We identify a C-terminal inactivation peptide that is necessary for inactivation and dynamically interacts with a receptor site on the channel. Alterations of the peptide or its receptor dramatically reduce inactivation. Unlike inactivation peptides of voltage-gated channels that bind within the ion pore, the receptor for the inactivation peptide is on the cytosolic surface of the channel and separated from the pore. Biochemical, structural, and electrophysiological analyses indicate that binding of the peptide to its receptor promotes inactivation, whereas dissociation prevents it. Using additional mutational studies we find that the "neck" constriction of the pore, which we have previously shown to act as the Ca²⁺-dependent activation gate, also functions as the inactivation gate. Our results indicate that unlike a ball-and-chain inactivation mechanism involving physical occlusion of the pore, inactivation in BEST1 occurs through an allosteric mechanism wherein binding of a peptide to a surface-exposed receptor controls a structurally distant gate.

Protein Phosphatase 2A and Its Methylation Modulating Enzymes LCMT-1 and PME-1 Are Dysregulated in Tauopathies of Progressive Supranuclear Palsy and Alzheimer Disease

Hyperphosphorylated tau aggregates are characteristic of tauopathies including progressive supranuclear palsy (PSP) and Alzheimer disease (AD), but factors contributing to pathologic tau phosphorylation are not well understood. Here, we studied the regulation of the major tau phosphatase, the heterotrimeric AB55αC protein phosphatase 2 A (PP2A), in PSP and AD. The assembly and activity of this PP2A isoform are regulated by reversible carboxyl methylation of its catalytic C subunit, while the B subunit confers substrate specificity. We sought to address whether the decreases in PP2A methylation and its methylating enzyme, leucine carboxyl methyltransferase (LCMT-1), which are reported in AD, relate to tau pathology or to concomitant amyloid pathology by comparing them in the relatively pure tauopathy PSP. Immunohistochemical analysis of frontal cortices showed that methyl-PP2A is reduced while demethyl-PP2A is increased, with no changes in total PP2A or B55α subunit, resulting in a reduction in the methyl/demethyl PP2A ratio of 63% in PSP and 75% in AD compared to controls. Similarly, Western blot analyses showed a decrease of methyl-PP2A and an increase of demethyl-PP2A with a concomitant reduction in the methyl/demethyl PP2A ratio in both PSP (74%) and AD (76%) brains. This was associated with a decrease in LCMT-1 and an increase in the demethylating enzyme, protein phosphatase methylesterase (PME-1), in both diseases. These findings suggest that PP2A dysregulation in tauopathies may contribute to the accumulation of hyperphosphorylated tau and to neurodegeneration.

Direct interaction with 14-3-3γ promotes surface expression of Best1 channel in astrocyte

Background

Bestrophin-1 (Best1) is a calcium-activated anion channel (CAAC) that is expressed broadly in mammalian tissues including the brain. We have previously reported that Best1 is expressed in hippocampal astrocytes at the distal peri-synaptic regions, called microdomains, right next to synaptic junctions, and that it disappears from the microdomains in Alzheimer’s disease mouse model. Although Best1 appears to be dynamically regulated, the mechanism of its regulation and modulation is poorly understood. It has been reported that a regulatory protein, 14-3-3 affects the surface expression of numerous membrane proteins in mammalian cells.

Methods

The protein-protein interaction between Best1 and 14-3-3γ was confirmed by yeast-two hybrid assay and BiFC method. The effect of 14-3-3γ on Best1-mediated current was measured by whole-cell patch clamp technique.

Results

We identified 14-3-3γ as novel binding partner of Best1 in astrocytes: among 7 isoforms of 14-3-3 protein, only 14-3-3γ was found to bind specifically. We determined a binding domain on the C-terminus of Best1 which is critical for an interaction with 14-3-3γ. We also revealed that interaction between Best1 and 14-3-3γ was mediated by phosphorylation of S358 in the C-terminus of Best1. We confirmed that surface expression of Best1 and Best1-mediated whole-cell current were significantly decreased after a gene-silencingof 14-3-3γ without a significant change in total Best1 expression in cultured astrocytes. Furthermore, we discovered that 14-3-3γ-shRNA reduced Best1-mediated glutamate release from hippocampal astrocyte by recording a PAR1 receptor-induced NMDA receptor-mediated current from CA1 pyramidal neurons in hippocampal slices injected with adenovirus carrying 14-3-3γ-shRNA. Finally, through a structural modeling, we found critical amino acid residues containing S358 of Best1 exhibiting binding affinities to 14-3-3γ.

Conclusions

14-3-3γ promotes surface expression of Best1 channel in astrocytes through direct interaction.

Effects and mechanism of GA-13315 on the proliferation and apoptosis of KB cells in oral cancer

The present study describes the effects and mechanism of GA-13315 on the proliferation and apoptosis of KB cells in oral cancer. Oral cancer is twice as common in men than women. More than 90% of oral cancers in men and 85% in women are linked to lifestyle and environmental factors. PPP2R2B methylation may be associated with survival and prognosis in patients with gliomas. In tumor cell proliferation and apoptosis, the mechanism of PPP2R2B remains unclear. In the present study, we found that PPP2R2B expression of H1299 cells is significantly decreased after being treated by GA-13315. KB cells were isolated from patients with oral cancer and treated with GA-13315 (5 µM). Cells without GA-13315 treatment served as the control group. An MTT experiment was performed to detect the post-treatment cell growth between the groups. A flow cytometry was used to detect cell apoptosis. Western blot analysis and quantitative polymerase chain reaction methods were used for detecting the expression of PPP2R2B. Compared with the control group, the cell proliferation of the treatment group slowed after being treated with GA-13315. The difference was statistically significant (P<0.05). Western blotting showed that the PPP2R2B expression of cells was reduced after being treated with GA-13315. Compared with the control group, the difference was statistically significant (P<0.05). According to results from the Transwell migration assay, the invasiveness of the KB cells of oral cancer were weakened after being treated by GA-13315. GA-13315 can accelerate the apoptosis of oral cancer cells and presents a dose correlation. The biological effect is exerted through the decrease of PPP2R2B.

Bestrophin 1 and retinal disease

Mutations in the gene BEST1 are causally associated with as many as five clinically distinct retinal degenerative diseases, which are collectively referred to as the "bestrophinopathies". These five associated diseases are: Best vitelliform macular dystrophy, autosomal recessive bestrophinopathy, adult-onset vitelliform macular dystrophy, autosomal dominant vitreoretinochoroidopathy, and retinitis pigmentosa. The most common of these is Best vitelliform macular dystrophy. Bestrophin 1 (Best1), the protein encoded by the gene BEST1, has been the subject of a great deal of research since it was first identified nearly two decades ago. Today we know that Best1 functions as both a pentameric anion channel and a regulator of intracellular Ca²⁺ signaling. Best1 is an integral membrane protein which, within the eye, is uniquely expressed in the retinal pigment epithelium where it predominantly localizes to the basolateral plasma membrane. Within the brain, Best1 expression has been documented in both glial cells and astrocytes where it functions in both tonic GABA release and glutamate transport. The crystal structure of Best1 has revealed critical information about how Best1 functions as an ion channel and how Ca²⁺ regulates that function. Studies using animal models have led to critical insights into the physiological roles of Best1 and advances in stem cell technology have allowed for the development of patient-derived, "disease in a dish" models. In this article we review our knowledge of Best1 and discuss prospects for near-term clinical trials to test therapies for the bestrophinopathies, a currently incurable and untreatable set of diseases.

PP2A as a master regulator of the cell cycle

Protein phosphatase 2A (PP2A) plays a critical multi-faceted role in the regulation of the cell cycle. It is known to dephosphorylate over 300 substrates involved in the cell cycle, regulating almost all major pathways and cell cycle checkpoints. PP2A is involved in such diverse processes by the formation of structurally distinct families of holoenzymes, which are regulated spatially and temporally by specific regulators. Here, we review the involvement of PP2A in the regulation of three cell signaling pathways: wnt, mTOR and MAP kinase, as well as the G1→S transition, DNA synthesis and mitotic initiation. These processes are all crucial for proper cell survival and proliferation and are often deregulated in cancer and other diseases.

What do we know about gliotransmitter release from astrocytes?

Astrocytes participate in information processing by actively modulating synaptic properties via gliotransmitter release. Various mechanisms of astrocytic release have been reported, including release from storage organelles via exocytosis and release from the cytosol via plasma membrane ion channels and pumps. It is still not fully clear which mechanisms operate under which conditions, but some of them, being Ca²⁺-regulated, may be physiologically relevant. The properties of Ca²⁺-dependent transmitter release via exocytosis or via ion channels are different and expected to produce different extracellular transmitter concentrations over time and to have distinct functional consequences. The molecular aspects of these two release pathways are still under active investigation. Here, we discuss the existing morphological and functional evidence in support of either of them. Transgenic mouse models, specific antagonists and localization studies have provided insight into regulated exocytosis, albeit not in a systematic fashion. Even more remains to be uncovered about the details of channel-mediated release. Better functional tools and improved ultrastructural approaches are needed in order fully to define specific modalities and effects of astrocytic gliotransmitter release pathways.

Autosomal Recessive Bestrophinopathy Is Not Associated With the Loss of Bestrophin-1 Anion Channel Function in a Patient With a Novel BEST1 Mutation

PURPOSE

Mutations in BEST1, encoding bestrophin-1 (Best1), cause autosomal recessive bestrophinopathy (ARB). Encoding bestrophin-1 is a pentameric anion channel localized to the basolateral plasma membrane of the RPE. Here, we characterize the effects of the mutations R141H (CGC > CAC) and I366fsX18 (c.1098_1100+7del), identified in a patient in our practice, on Best1 trafficking, oligomerization, and channel activity.

METHODS

Currents of Cl- were assessed in transfected HEK293 cells using whole-cell patch clamp. Best1 localization was assessed by confocal microscopy in differentiated, human-induced pluripotent stem cell-derived RPE (iPSC-RPE) cells following expression of mutants via adenovirus-mediated gene transfer. Oligomerization was evaluated by coimmunoprecipitation in iPSC-RPE and MDCK cells.

RESULTS

Compared to Best1, Best1 I366fsX18 currents were increased while Best1 R141H Cl- currents were diminished. Coexpression of Best1 R141H with Best1 or Best1 I366fsX18 resulted in rescued channel activity. Overexpressed Best1, Best1 R141H, and Best1 I366fsX18 were all properly localized in iPSC-RPE cells; Best1 R141H and Best1 I366fsX18 coimmunoprecipitated with endogenous Best1 in iPSC-RPE cells and with each other in MDCK cells.

CONCLUSIONS

The first 366 amino acids of Best1 are sufficient to mediate channel activity and homo-oligomerization. The combination of Best1 and Best1 R141H does not cause disease, while Best1 R141H together with Best1 I366fsX18 causes ARB. Since both combinations generate comparable Cl- currents, this indicates that ARB in this patient is not caused by a loss of channel activity. Moreover, Best1 I366fsX18 differs from Best1 in that it lacks most of the cytosolic C-terminal domain, suggesting that the loss of this region contributes significantly to the pathogenesis of ARB in this patient.

The bestrophin- and TMEM16A-associated Ca(2+)- activated Cl(–) channels in vascular smooth muscles

The presence of Ca²⁺-activated Cl^– currents (I_Cl(Ca)) in vascular smooth muscle cells (VSMCs) is well established. I_Cl(Ca) are supposedly important for arterial contraction by linking changes in [Ca²⁺]_i and membrane depolarization. Bestrophins and some members of the TMEM16 protein family were recently associated with I_Cl(Ca). Two distinct I_Cl(Ca) are characterized in VSMCs; the cGMP-dependent I_Cl(Ca) dependent upon bestrophin expression and the ‘classical’ Ca²⁺-activated Cl^– current, which is bestrophin-independent. Interestingly, TMEM16A is essential for both the cGMP-dependent and the classical I_Cl(Ca). Furthermore, TMEM16A has a role in arterial contraction while bestrophins do not. TMEM16A’s role in the contractile response cannot be explained however only by a simple suppression of the depolarization by Cl^– channels. It is suggested that TMEM16A expression modulates voltage-gated Ca²⁺ influx in a voltage-independent manner and recent studies also demonstrate a complex role of TMEM16A in modulating other membrane proteins.

Interaction of Bestrophin-1 with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in surface films

Human bestrophin-1 (hBest1) is a transmembrane channel protein, predominantly expressed in the membrane of retinal pigment epithelium (RPE) cells. Although it is clear that hBest1's interactions with lipids are crucial for its function such studies were not performed as the protein was not purified. Here we describe an effective purification of hBest1 from Madin-Darby Canine Kidney (MDCK) cells via simple gel-filtration and affinity chromatographic steps, which makes possible to probe the protein interplay with lipids. The interaction of the purified hBest1 with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was studied in Langmuir monolayers. The surface pressure (π)-area (A) isotherms and compression/expansion isocycles of POPC monolayer were recorded in absence and presence of hBest1 in the subphase. The π(A) isotherms were analyzed in terms of surface compressional modulus and via two-dimensional virial equation of state. The dilatational rheological properties of the surface films and their surface potential were also measured. The morphology of the films was observed by Brewster angle microscopy. The inclusion of the protein in the film subphase does not lead to in-depth penetration of hBest1 but interaction takes place in the headgroup region of the monolayer. The hBest1/POPC interaction resulted in formation of more condensed films, which rheological properties and lateral structure differed significantly from the pure POPC monolayers. Our study sheds light on the still unclear question how hBest1 gets in touch with biomembrane phospholipids of eukaryotic cells that might be of key importance for the proper structure and function of RPE biomembranes.

Disease-causing mutations associated with four bestrophinopathies exhibit disparate effects on the localization, but not the oligomerization, of Bestrophin-1

BEST1 encodes Bestrophin-1 (Best1), a homo-oligomeric, integral membrane protein localized to the basolateral plasma membrane of the retinal pigment epithelium. Mutations in BEST1 cause five distinct retinal degenerative diseases, including adult vitelliform macular dystrophy (AVMD), autosomal recessive bestrophinopathy (ARB), autosomal dominant vitreoretinochoroidopathy (ADVIRC), and retinitis pigmentosa (RP). The mechanisms underlying these diseases and why mutations cause one disease over another are, for the most part, unknown. To gain insights into these four diseases, we expressed 28 Best1 mutants fused to YFP in polarized MDCK monolayers and, via confocal microscopy and immunofluorescence, live-cell FRET, and reciprocal co-immunoprecipitation experiments, screened these mutants for defects in localization and oligomerization. All 28 mutants exhibited comparable FRET efficiencies to and co-immunoprecipitated with WT Best1, indicating unimpaired oligomerization. RP- and ADVIRC-associated mutants were properly localized to the basolateral plasma membrane of cells, while two AVMD and most ARB mutants were mislocalized. When co-expressed, all mislocalized mutants caused mislocalization of WT Best1 to intracellular compartments. Our current and past results indicate that mislocalization of Best1 is not an absolute feature of any individual bestrophinopathy, occurring in AVMD, BVMD, and ARB. Furthermore, some ARB mutants that do not also cause dominant disease cause mislocalization of Best1, indicating that mislocalization is not a cause of disease, and that absence of Best1 activity from the plasma membrane is tolerated. Lastly, we find that the ARB truncation mutants L174Qfs*57 and R200X can form oligomers with WT Best1, indicating that the first ∼174 amino acids of Best1 are sufficient for oligomerization to occur.

Disease-causing mutations in BEST1 gene are associated with altered sorting of bestrophin-1 protein

Mutations in BEST1 gene, encoding the bestrophin-1 (Best1) protein are associated with macular dystrophies. Best1 is predominantly expressed in the retinal pigment epithelium (RPE), and is inserted in its basolateral membrane. We investigated the cellular localization in polarized MDCKII cells of disease-associated Best1 mutant proteins to study specific sorting motifs of Best1. Real-time PCR and western blots for endogenous expression of BEST1 in MDCK cells were performed. Best1 mutant constructs were generated using site-directed mutagenesis and transfected in MDCK cells. For protein sorting, confocal microscopy studies, biotinylation assays and statistical methods for quantification of mislocalization were used. Analysis of endogenous expression of BEST1 in MDCK cells revealed the presence of BEST1 transcript but no protein. Confocal microscopy and quantitative analyses indicate that transfected normal human Best1 displays a basolateral localization in MDCK cells, while cell sorting of several Best1 mutants (Y85H, Q96R, L100R, Y227N, Y227E) was altered. In contrast to constitutively active Y227E, constitutively inactive Y227F Best1 mutant localized basolaterally similar to the normal Best1 protein. Our data suggest that at least three basolateral sorting motifs might be implicated in proper Best1 basolateral localization. In addition, non-phosphorylated tyrosine 227 could play a role for basolateral delivery.

Drosophila bestrophin-1 currents are regulated by phosphorylation via a CaMKII dependent mechanism

Cell swelling induced by hypo-osmotic stress results in activation of volume-regulated anion channels (VRAC) that drive a compensatory regulatory volume decrease. We have previously shown that the Best1 gene in Drosophila encodes a VRAC that is also activated by increases in intracellular Ca²⁺. The role of Best1 as a VRAC has recently been independently confirmed by the Clapham lab in an unbiased RNAi screen. Although dBest1 is clearly a volume-regulated channel, its mechanisms of regulation remain unknown. Here we investigate Drosophila Best1 (dBest1) regulation using the Drosophila S2 cell model system. Because dBest1 activates slowly after establishing whole-cell recording, we tested the hypothesis that the channel is activated by phosphorylation. Two experiments indicate that phosphorylation is required for dBest1 activation: nonspecific protein kinase inhibitors or intracellular perfusion with the non-hydrolyzable ATP analog AMP-PNP dramatically reduce the amplitude of dBest1 currents. Furthermore, intracellular perfusion with ATP-γ-S augments channel activation. The kinase responsible for dBest1 activation is likely Ca²⁺/calmodulin dependent kinase II (CaMKII), because specific inhibitors of this kinase dramatically inhibit dBest1 current activation. Neither specific PKA inhibitors nor inactive control inhibitors have effects on dBest1currents. Our results demonstrate that dBest1 currents are regulated by phosphorylation via a CaMKII dependent mechanism.

Novel and homozygous BEST1 mutations in Chinese patients with Best vitelliform macular dystrophy

PURPOSE

The purpose of this study was to investigate the BEST1 gene mutations in Chinese patients with Best vitelliform macular dystrophy (BVMD).

METHODS

Twenty-six subjects from 7 Chinese families with BVMD and 100 unrelated healthy Chinese subjects without a family history of BVMD were screened for mutations in the BEST1 gene by direct sequencing. The subjects underwent complete ophthalmologic examination and BEST1 gene screening.

RESULTS

Six novel missense mutations (Thr2Asn, Leu75Phe, Ser144Asn, Arg255Trp, Pro297Thr, and Asp301Gly) and 1 previously reported mutation (Arg218Cys) were identified. Each family was found to have a unique BEST1 mutation that segregated with the disease. Two of the six novel mutations are located within the four previously reported common mutation clusters within the BEST1 gene. One family with patients having homozygous Leu75Phe mutations did not have the more severe BVMD phenotype. None of the patients with mutations was identified among the 100 healthy control subjects.

CONCLUSION

A large number of unique novel missense mutations was found in Chinese patients with BVMD, suggesting considerable interethnic differences between the mutation sites in the BEST1 gene in different populations. The few truncating BEST1 mutations and the lack of a more severe phenotype in homozygous patients suggest that the missense BEST1 mutation may produce a dominant negative effect on wild-type BEST1 gene.

Chloride channels: often enigmatic, rarely predictable

Until recently, anion (Cl(-)) channels have received considerably less attention than cation channels. One reason for this may be that many Cl(-) channels perform functions that might be considered cell-biological, like fluid secretion and cell volume regulation, whereas cation channels have historically been associated with cellular excitability, which typically happens more rapidly. In this review, we discuss the recent explosion of interest in Cl(-) channels, with special emphasis on new and often surprising developments over the past five years. This is exemplified by the findings that more than half of the ClC family members are antiporters, and not channels, as was previously thought, and that bestrophins, previously prime candidates for Ca(2+)-activated Cl(-) channels, have been supplanted by the newly discovered anoctamins and now hold a tenuous position in the Cl(-) channel world.

Bestrophins and retinopathies

Best vitelliform macular dystrophy (BVMD, also called Best's disease) is a dominantly inherited, juvenile-onset form of macular degeneration, which is characterized by abnormal accumulation of yellow pigment in the outer retina and a depressed electro-oculogram light peak (LP). Over 100 disease-causing mutations in human bestrophin-1 (hBest1) are closely linked to BVMD and several other retinopathies. However, the physiological role of hBest1 and the mechanisms of retinal pathology remain obscure partly because hBest1 has been described as a protein with multiple functions including a Ca2+-activated Cl- channel, a Ca2+ channel regulator, a volume-regulated Cl- channel, and a HCO3- channel. This review focuses on how dysfunction of hBest1 is related to the accumulation of yellow pigment and a decreased LP. The dysfunction of hBest1 as a HCO3- channel or a volume-regulated Cl- channel may be associated with defective regulation of the subretinal fluid or phagocytosis of photoreceptor outer segments by retinal pigment epithelium cells, which may lead to fluid and pigment accumulation.

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.

Complex phosphatase regulation of Ca2+-activated Cl- currents in pulmonary arterial smooth muscle cells

The present study was undertaken to determine whether the two ubiquitously expressed Ca(2+)-independent phosphatases PP1 and PP2A regulate Ca(2+)-activated Cl(-) currents (I(Cl(Ca))) elicited by 500 nM [Ca(2+)](i) in rabbit pulmonary artery (PA) myocytes dialyzed with or without 3 mM ATP. Reverse transcription-PCR experiments revealed the expression of PP1alpha, PP1beta/delta, PP1gamma, PP2Aalpha, PP2Abeta, PP2Balpha (calcineurin (CaN) Aalpha), and PP2Bbeta (CaN Abeta) but not PP2Bgamma (CaN Agamma) in rabbit PA. Western blot and immunofluorescence experiments confirmed the presence of all three PP1 isoforms and PP2A. Intracellular dialysis with a peptide inhibitor of calcineurin (CaN-AIP); the non-selective PP1/PP2A inhibitors okadaic acid (0.5, 10, or 30 nM), calyculin A (10 nM), or cantharidin (100 nM); and the selective PP1 inhibitor NIPP-1 (100 pM) potently antagonized the recovery of I(Cl(Ca)) in cells dialyzed with no ATP, whereas the PP2A-selective antagonist fostriecin (30 or 150 nM) was ineffective. The combined application of okadaic acid (10 nM) and CaN-autoinhibitory peptide (50 microM) did not potentiate the response of I(Cl(Ca)) in 0 ATP produced by maximally inhibiting CaN or PP1/PP2A alone. Consistent with the non-additive effects of either classes of phosphatases, the PP1 inhibitor NIPP-1 (100 pM) antagonized the recovery of I(Cl(Ca)) induced by exogenous CaN Aalpha (0.5 microM). These results demonstrate that I(Cl(Ca)) in PA myocytes is regulated by CaN and PP1 and/or PP2A. Our data also suggest the existence of a functional link between these two classes of phosphatases.

Dysregulation of human bestrophin-1 by ceramide-induced dephosphorylation

Best vitelliform macular dystrophy is an inherited autosomal dominant, juvenile onset form of macular degeneration caused by mutations in a chloride ion channel, human bestrophin-1 (hBest1). Mutations in Best1 have also been linked to several other forms of retinopathy. In addition to mutations, hBest1 dysfunction might come about by disruption of other processes that regulate Best1 function. Here we show that hBest1 chloride channel activity is regulated by ceramide and phosphorylation. We have identified a protein kinase C (PKC) phosphorylation site (serine 358) in hBest1 that is important for sustained channel function. Channel activity is maintained by PKC activators, protein phosphatase inhibitors, or pseudo-phosphorylation by substitution of glutamic acid for serine 358. When ceramide levels are elevated by exogenous addition of ceramide to the bath, by addition of bacterial sphingomyelinase, or by hypertonic stress, S358 is rapidly dephosphorylated. The dephosphorylation is mediated by protein phosphatase 2A. Hypertonic stress-induced dephosphorylation is blocked by a dihydroceramide, an inactive form of ceramide, and manumycin, an inhibitor of neutral sphingomyelinase. Our results support a model in which ceramide accumulation during early stages of retinopathy inhibits hBest1 function, leading to abnormal fluid transport across the retina, and enhanced inflammation.

Functional roles of bestrophins in ocular epithelia

There are four members of the bestrophin family of proteins in the human genome, of which two are known to be expressed in the eye. The gene BEST1 (formerly VMD2) which encodes the protein bestrophin-1 (Best1) was first identified in 1998. Mutations in this gene have now been associated with four clinically distinguishable human eye diseases, collectively referred to as "bestrophinopathies". Over the last decade, laboratories have sought to understand how Best1 mutations could result in eye diseases that range in presentation from macular degeneration to nanophthalmos. The majority of our knowledge comes from studies that have sought to understand how Best1 mutations or dysfunction could induce the classical symptoms of the most common of these diseases: Best vitelliform macular dystrophy (BVMD). BVMD is a dominant trait that is characterized electrophysiologically by a diminished electrooculogram light peak with a normal clinical electroretinogram. This together with the localization of Best1 to the retinal pigment epithelium (RPE) basolateral plasma membrane and data from heterologous expression studies, have led to the proposal that Best1 generates the light peak, and that bestrophins are a family of Ca(2+) activated Cl(-) channels (CaCCs). However, data from Best1 knock-out and knock-in mice, coupled with the recent discovery of a recessive bestrophinopathy suggest that Best1 does not generate the light peak. Recently Best2 was found to be expressed in non-pigmented epithelia in the ciliary body. However, aqueous dynamics in Best2 knock-out mice do not support a role for Best2 as a Cl(-) channel. Thus, the purported CaCC function of the bestrophins and how loss of this function relates to clinical disease needs to be reassessed. In this article, we examine data obtained from tissue-type and animal models and discuss the current state of bestrophin research, what roles Best1 and Best2 may play in ocular epithelia and ocular electrophysiology, and how perturbation of these functions may result in disease.

Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies

This article reviews the current state of knowledge about the bestrophins, a newly identified family of proteins that can function both as Cl(-) channels and as regulators of voltage-gated Ca(2+) channels. The founding member, human bestrophin-1 (hBest1), was identified as the gene responsible for a dominantly inherited, juvenile-onset form of macular degeneration called Best vitelliform macular dystrophy. Mutations in hBest1 have also been associated with a small fraction of adult-onset macular dystrophies. It is proposed that dysfunction of bestrophin results in abnormal fluid and ion transport by the retinal pigment epithelium, resulting in a weakened interface between the retinal pigment epithelium and photoreceptors. There is compelling evidence that bestrophins are Cl(-) channels, but bestrophins remain enigmatic because it is not clear that the Cl(-) channel function can explain Best disease. In addition to functioning as a Cl(-) channel, hBest1 also is able to regulate voltage-gated Ca(2+) channels. Some bestrophins are activated by increases in intracellular Ca(2+) concentration, but whether bestrophins are the molecular counterpart of Ca(2+)-activated Cl(-) channels remains in doubt. Bestrophins are also regulated by cell volume and may be a member of the volume-regulated anion channel family.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.

Biallelic mutation of BEST1 causes a distinct retinopathy in humans

We describe a distinct retinal disorder, autosomal-recessive bestrophinopathy (ARB), that is consequent upon biallelic mutation in BEST1 and is associated with central visual loss, a characteristic retinopathy, an absent electro-oculogram light rise, and a reduced electroretinogram. Heterozygous mutations in BEST1 have previously been found to cause the two dominantly inherited disorders, Best macular dystrophy and autosomal-dominant vitreoretinochoroidopathy. The transmembrane protein bestrophin-1, encoded by BEST1, is located at the basolateral membrane of the retinal pigment epithelium in which it probably functions as a Cl(-) channel. We sequenced BEST1 in five families, identifying DNA variants in each of ten alleles. These encoded six different missense variants and one nonsense variant. The alleles segregated appropriately for a recessive disorder in each family. No clinical or electrophysiological abnormalities were identified in any heterozygotes. We conducted whole-cell patch-clamping of HEK293 cells transfected with bestrophin-1 to measure the Cl(-) current. Two ARB missense isoforms severely reduced channel activity. However, unlike two other alleles previously associated with Best disease, cotransfection with wild-type bestrophin-1 did not impair the formation of active wild-type bestrophin-1 channels, consistent with the recessive nature of the condition. We propose that ARB is the null phenotype of bestrophin-1 in humans.

Lack of fibulin-3 causes early aging and herniation, but not macular degeneration in mice

A mutation in the EFEMP1 gene causes Malattia Leventinese, an inherited macular degenerative disease with strong similarities to age-related macular degeneration. EFEMP1 encodes fibulin-3, an extracellular matrix protein of unknown function. To investigate its biological role, the murine Efemp1 gene was inactivated through targeted disruption. Efemp1(-/-) mice exhibited reduced reproductivity, and displayed an early onset of aging-associated phenotypes including reduced lifespan, decreased body mass, lordokyphosis, reduced hair growth, and generalized fat, muscle and organ atrophy. However, these mice appeared to have normal wound healing ability. Efemp1(-/-) mice on a C57BL/6 genetic background developed multiple large hernias including inguinal hernias, pelvic prolapse and protrusions of the xiphoid process. In contrast, Efemp1(-/-) mice on a BALB/c background rarely had any forms of hernias, indicating the presence of modifiers for fibulin-3's function in different mouse strains. Histological analysis revealed a marked reduction of elastic fibers in fascia, a thin layer of connective tissue maintaining and protecting structures throughout the body. No apparent macular degeneration associated defects were found in Efemp1(-/-) mice, suggesting that loss of fibulin-3 function is not the mechanism by which the mutation in EFEMP1 causes macular degeneration. These data demonstrate that fibulin-3 plays an important role in maintaining the integrity of fascia connective tissues and regulates aging.

Calcium-dependent chloride conductance in epithelia: is there a contribution by Bestrophin?

Although known for more than 20 years, the molecular identity of epithelial Ca(2+)-activated Cl(-) channels remains obscure. Previous candidate proteins did not hold initial promises, and thus, new hope is put into the recently identified family of bestrophin proteins, as they reflect many of the properties found for native channels. Mutations in the bestrophin gene cause an autosomal form of macular dystrophy of the retina. Bestrophin 1 is assumed to form the basolateral Ca(2+)-activated Cl(-) channel in the retinal pigment epithelium of the eye. Other data suggest that bestrophin is a regulator of voltage gated Ca(2+) channels. Structural information on bestrophins is available and a Cl(-) selective filter has been localized to the second transmembrane domain of bestrophin. It is possible that bestrophins function as physiologically regulated Cl(-) channels only in association with additional subunits and auxiliary proteins. Little is known about expression of bestrophin in gland acinar cells, which show a pronounced Ca(2+)-activated Cl(-) secretion. In airways and intestinal epithelia, bestrophins could be particularly important in diseases such as cystic fibrosis and secretory diarrhea.

Na,K‐ATPase α1‐subunit dephosphorylation by protein phosphatase 2A is necessary for its recruitment to the plasma membrane

In alveolar epithelial cells, G-protein coupled-receptors agonists (GPCR) induce the recruitment of the Na,K-ATPase to the plasma membrane. Here we report that for the recruitment of the Na,K-ATPase to occur, dephosphorylation of its alpha1-subunit at serine 18 is necessary, as demonstrated by in vitro phosphorylation, mutation of the serine 18 to alanine, and use of a specific phospho-antibody. Several approaches strongly suggest dephosphorylation to be mediated by protein phosphatase 2A (PP2A): 1) Na,K-ATPase dephosphorylation and recruitment were prevented by okadaic acid (OA); 2) the Na,K-ATPase alpha1-subunit is an in vitro substrate for PP2A; and 3) glutathione S-transferase (GST)-fusion proteins binding assays demonstrate a direct interaction between the catalytic subunit of PP2A and the first 90 amino acids of the Na,K-ATPase alpha1-subunit. Finally, GPCR agonists induced a rapid translocation of PP2A from the cytosol to the membrane fraction, which corresponded with increased coimmunoprecipitation and colocalization of PP2A and the Na,K-ATPase. Accordingly, we provide evidence that GPCR agonists promote PP2A translocation to the membrane fraction, leading to the dephosphorylation of the Na,K-ATPase alpha1-subunit at the serine 18 residue and its recruitment to the cell plasma membrane, which is of biological and physiological importance.

The bestrophin family of anion channels: identification of prokaryotic homologues

The human disease protein, Bestrophin-1, associated with vitelliform macular dystrophy, has recently been shown to be an integral membrane anion channel-forming protein. In this study we have recovered all bestrophin homologues from the NCBI database and analyzed their sequences using bioinformatic approaches. Eukaryotic homologues were found in animals and fungi but not in plants or protozoans, and prokaryotic homologues distantly related to the eukaryotic proteins, were identified in certain Gram-negative bacterial kingdoms but not in Gram-positive bacteria or archaea. Our analyses suggest a uniform 4 TMS topology for most of these homologues with regions of conservation overlapping and preceding the odd numbered TMSs and overlapping and following the even numbered TMSs. Well-conserved motifs were identified in both the eukaryotic and the prokaryotic homologues, and these proved to overlap, suggesting common structural and functional properties. Phylogenetic analyses revealed that the eukaryotic proteins cluster according to organismal type, and that the prokaryotic proteins sometimes (but not always) do so. This suggests that eukaryotic paralogues arose exclusively by recent gene duplication events although both early and late gene duplication events occurred in prokaryotes.

A Novel Mutation of the VMD2 Gene in a Chinese Family with Best Vitelliform Macular Dystrophy

Introduction: In this paper, we report a novel VMD2 gene mutation in a Chinese family with Best vitelliform macular dystrophy. Materials and Methods: Ophthalmologic examination and optical coherence tomography (OCT) were performed in 2 members of this family. Mutational screening was performed by single-strand conformation polymorphism (SSCP) and direct sequencing of PCR-amplified DNA fragments, corresponding to the 11 exons of the gene. Results: Sequence analysis identified a previously unreported C to G change, predicting a Phe-113-Leu substitution. Both the proband and his sister harboured this novel mutation. Each had bilateral vitelliform lesions. Conclusions: A novel mutation in the VMD2 gene (C427G) was found in Chinese patients with Best vitelliform macular dystrophy. Key words: Best macular dystrophy, Bestrophin, Retinal pigment epithelium

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Targeted disruption of fibulin-4 abolishes elastogenesis and causes perinatal lethality in mice

Elastic fibers provide tissues with elasticity which is critical to the function of arteries, lungs, skin, and other dynamic organs. Loss of elasticity is a major contributing factor in aging and diseases. However, the mechanism of elastic fiber development and assembly is poorly understood. Here, we show that lack of fibulin-4, an extracellular matrix molecule, abolishes elastogenesis. fibulin-4-/- mice generated by gene targeting exhibited severe lung and vascular defects including emphysema, artery tortuosity, irregularity, aneurysm, rupture, and resulting hemorrhages. All the homozygous mice died perinatally. The earliest abnormality noted was a uniformly narrowing of the descending aorta in fibulin-4-/- embryos at embryonic day 12.5 (E12.5). Aorta tortuosity and irregularity became noticeable at E15.5. Histological analysis demonstrated that fibulin-4-/- mice do not develop intact elastic fibers but contain irregular elastin aggregates. Electron microscopy revealed that the elastin aggregates are highly unusual in that they contain evenly distributed rod-like filaments, in contrast to the amorphous appearance of normal elastic fibers. Desmosine analysis indicated that elastin cross-links in fibulin-4-/- tissues were largely diminished. However, expression of tropoelastin or lysyl oxidase mRNA was unaffected in fibulin-4-/- mice. In addition, fibulin-4 strongly interacts with tropoelastin and colocalizes with elastic fibers in culture. These results demonstrate that fibulin-4 plays an irreplaceable role in elastogenesis.

A short motif in the C-terminus of mouse bestrophin 3 [corrected] inhibits its activation as a Cl channel

Bestrophins are a new family of anion channels. Here, we examined the Cl channel activity of mBest4. Surprisingly, wild type mouse bestrophin-4 (mBest4) did not induce functional Cl channels when over-expressed in HEK293 cells. However, deletion of part of the C-terminus (residues 353-669) produced large Cl currents, suggesting the presence of a C-terminal motif that inhibited Cl channel function. Deletion of a short motif (356-364) or substitution of certain residues in this motif with alanines also resulted in expression of robust Cl currents. The channel activity of the mBest4 protein lacking the C-terminus (residues 353-669) was specifically inhibited by co-expression of C-terminal fragments of mBest4 having the inhibitory motif, suggesting that the C-terminal motif blocked mBest4 channel activity probably by interacting with the channel pore.

Hydrodynamic properties of porcine bestrophin-1 in Triton X-100

Bestrophin-1 (Best-1) is an integral membrane protein, defects in which cause Best vitelliform macular dystrophy. Best-1 is proposed to function as a Cl- channel and/or a regulator of Ca++ channels. A tetrameric (or pentameric) stoichiometry has been reported for recombinant best-1. Using a combination of gel exclusion chromatography and velocity sedimentation we examined the quaternary structure of native best-1 and found that it migrates as a single species with a Stokes radius of 7.3 nm, sedimentation coefficient (S20,w) of 4.9, and partial specific volume (nu) of 0.80 ml/g. The mass of the protein-detergent complex is calculated to be 206 kDa, with the protein component estimated to be approximately 138 kDa. Given a monomeric mass of 68 kDa, we conclude that native best-1 solubilized with Triton X-100 is a homodimer. The differences between this observation and a prior report were examined by comparing recombinant best-1 with tissue derived best-1 using gel exclusion chromatography. Much of the recombinant best-1 eluted in the column void (Vo) fraction, unlike that extracted from RPE cells. We conclude that the minimal functional unit of best-1 is dimeric. This stoichiometry differs from that previously measured for recombinant best-1, suggesting that further studies are necessary to determine the stoichiometry of functional best-1 in RPE membranes.

Diversity of Cl(-) channels

Cl(-) channels are widely found anion pores that are regulated by a variety of signals and that play various roles. On the basis of molecular biologic findings, ligand-gated Cl(-) channels in synapses, cystic fibrosis transmembrane conductors (CFTRs) and ClC channel types have been established, followed by bestrophin and possibly by tweety, which encode Ca(2+)-activated Cl(-) channels. The ClC family has been shown to possess a variety of functions, including stabilization of membrane potential, excitation, cell-volume regulation, fluid transport, protein degradation in endosomal vesicles and possibly cell growth. The molecular structure of Cl(-) channel types varies from 1 to 12 transmembrane segments. By means of computer-based prediction, functional Cl(-) channels have been synthesized artificially, revealing that many possible ion pores are hidden in channel, transporter or unidentified hydrophobic membrane proteins. Thus, novel Cl(-)-conducting pores may be occasionally discovered, and evidence from molecular biologic studies will clarify their physiologic and pathophysiologic roles.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

Structure and function of CLCA proteins

CLCA proteins were discovered in bovine trachea and named for a calcium-dependent chloride conductance found in trachea and in other secretory epithelial tissues. At least four closely located gene loci in the mouse and the human code for independent isoforms of CLCA proteins. Full-length CLCA proteins have an unprocessed mass ratio of approximately 100 kDa. Three of the four human loci code for the synthesis of membrane-associated proteins. CLCA proteins affect chloride conductance, epithelial secretion, cell-cell adhesion, apoptosis, cell cycle control, mucus production in asthma, and blood pressure. There is a structural and probable functional divergence between CLCA isoforms containing or not containing beta4-integrin binding domains. Cell cycle control and tumor metastasis are affected by isoforms with the binding domains. These isoforms are expressed prominently in smooth muscle, in some endothelial cells, in the central nervous system, and also in secretory epithelial cells. The isoform with disrupted beta4-integrin binding (hCLCA1, pCLCA1, mCLCA3) alters epithelial mucus secretion and ion transport processes. It is preferentially expressed in secretory epithelial tissues including trachea and small intestine. Chloride conductance is affected by the expression of several CLCA proteins. However, the dependence of the resulting electrical signature on the expression system rather than the CLCA protein suggests that these proteins are not independent Ca2+-dependent chloride channels, but may contribute to the activity of chloride channels formed by, or in conjunction with, other proteins.

[Function of bestrophin]

Clarification of the function of bestrophin, the gene product of VMD2, establishes a basis for the understanding of the pathomechanisms leading to Best's vitelliform macular degeneration. Studies of heterologously expressed bestrophin showed that bestrophin can function as a Cl(-) channel. All four known bestrophins were found to display Cl(-) channel activity. A loss in Cl(-) channel function would elegantly explain the development of the leading symptom for Best's disease, the reduction of the light peak amplitude in the patient's electro-oculogram. However, there are still gaps in the chain of evidence demonstrating that bestrophin is a Cl(-) channel, and this hypothesis is inconsistent with newly published follow-up observations. In an alternative hypothesis bestrophin appears as a regulator of voltage-dependent Ca(2+) channels assuming an indirect involvement of bestrophin in the generation of the light peak. Further studies on either bestrophin-deficient mice or transgenic mice will show that either one of the hypotheses is right or maybe both will be proven correct, showing bestrophin as a Cl(-) channel and Ca(2+) channel regulator.