Focus on molecules: bestrophin (best-1)

Bestrophin3 Deficiency in Vascular Smooth Muscle Cells Activates MEKK2/3-MAPK Signaling to Trigger Spontaneous Aortic Dissection

BACKGROUND

Aortic dissection (AD) is a fatal cardiovascular disorder without effective medications due to unclear pathogenic mechanisms. Bestrophin3 (Best3), the predominant isoform of bestrophin family in vessels, has emerged as critical for vascular pathological processes. However, the contribution of Best3 to vascular diseases remains elusive.

METHODS

Smooth muscle cell-specific and endothelial cell-specific Best3 knockout mice (Best3SMKO and Best3ECKO, respectively) were engineered to investigate the role of Best3 in vascular pathophysiology. Functional studies, single-cell RNA sequencing, proteomics analysis, and coimmunoprecipitation coupled with mass spectrometry were performed to evaluate the function of Best3 in vessels.

RESULTS

Best3 expression in aortas of human AD samples and mouse AD models was decreased. Best3SMKO but not Best3ECKO mice spontaneously developed AD with age, and the incidence reached 48% at 72 weeks of age. Reanalysis of single-cell transcriptome data revealed that reduction of fibromyocytes, a fibroblast-like smooth muscle cell cluster, was a typical feature of human ascending AD and aneurysm. Consistently, Best3 deficiency in smooth muscle cells decreased the number of fibromyocytes. Mechanistically, Best3 interacted with both MEKK2 and MEKK3, and this interaction inhibited phosphorylation of MEKK2 at serine153 and MEKK3 at serine61. Best3 deficiency induced phosphorylation-dependent inhibition of ubiquitination and protein turnover of MEKK2/3, thereby activating the downstream mitogen-activated protein kinase signaling cascade. Furthermore, restoration of Best3 or inhibition of MEKK2/3 prevented AD progression in angiotensin II-infused Best3SMKO and ApoE-/- mice.

CONCLUSIONS

These findings unveil a critical role of Best3 in regulating smooth muscle cell phenotypic switch and aortic structural integrity through controlling MEKK2/3 degradation. Best3-MEKK2/3 signaling represents a novel therapeutic target for AD.

Differential Proximity of Perisynaptic Astrocytic Best1 at the Excitatory and Inhibitory Tripartite Synapses in APP/PS1 and MAOB-KO Mice Revealed by Lattice Structured Illumination Microscopy

Bestrophin-1 (Best1) is a GABA- and glutamate-permeable, Ca2+-activated Cl- channel, which is mainly expressed in astrocytes and localized at the microdomain or perisynaptic junction of the tripartite synapse. Distribution of Best1 is dramatically changed in pathological conditions such as Alzheimer's disease. However, it is still unknown whether Best1 is located at the glutamatergic or GABAergic tripartite synapses. Here, we utilized the Lattice structured illumination microscopy (Lattice SIM) to visualize Best1 expression at the perisynaptic junctions of the tripartite synapses in CA1 of mouse hippocampus. We performed co-labeling with antibodies against 1) Best1 and vesicular glutamate transporter-2 (vGLUT2) or 2) Best1 and vesicular GABA transporter (vGAT) to measure the proximity of Best1-containing perisynapse to glutamatergic or GABAergic presynapse, respectively. In addition, we examined two transgenic mouse lines of 1) APP/PS1 mouse showing high astrocytic MAOB activity and cytosolic GABA and 2) MAOB-KO mouse showing low astrocytic GABA. Lattice SIM images were further processed by Imaris, which allowed 3D-rendering and spot identification. We found that astrocytic Best1 was distributed closer to the glutamatergic synapses than GABAergic synapses in the wild-type mice. In APP/PS1 mice, Best1 distribution was significantly changed by moving away from the glutamatergic synapses while moving closer to the GABAergic synapses. On the contrary, in MAOB-KO mice, the Best1 distribution was dramatically changed by moving closer to the glutamatergic synapses and moving far away from the GABAergic synapses. Our findings propose that the proximity of Best1-containing perisynapses to presynapses dynamically changes according to the level of astrocytic cytosolic GABA.

Clinical Heterogeneity in Autosomal Recessive Bestrophinopathy with Biallelic Mutations in the BEST1 Gene

Autosomal recessive bestrophinopathy (ARB) has been reported as clinically heterogeneous. Eighteen patients (mean age: 22.5 years; 15 unrelated families) underwent ophthalmological examination, fundus photography, fundus autofluorescence, and optical coherence tomography (OCT). Molecular genetic testing of the BEST1 gene was conducted by the chain-terminating dideoxynucleotide Sanger methodology. Onset of symptoms (3 to 50 years of age) and best-corrected visual acuity (0.02–1.0) were highly variable. Ophthalmoscopic and retinal imaging defined five phenotypes. Phenotype I presented with single or confluent yellow lesions at the posterior pole and midperiphery, serous retinal detachment, and intraretinal cystoid spaces. In phenotype II fleck-like lesions were smaller and extended to the far periphery. Phenotype III showed a widespread continuous lesion with sharp peripheral demarcation. Single (phenotype IV) or multifocal (phenotype V) vitelliform macular dystrophy-like lesions were observed as well. Phenotypes varied within families and in two eyes of one patient. In addition, OCT detected hyperreflective foci (13/36 eyes) and choroidal excavation (11/36). Biallelic mutations were identified in each patient, six of which have not been reported so far [c.454C>T/p.(Pro152Ser), c.620T>A/p.(Leu207His), c.287_298del/p.(Gln96_Asn99del), c.199_200del/p.(Leu67Valfs*164), c.524del/p.(Ser175Thrfs*19), c.590_615del/p.(Leu197Profs*26)]. BEST1-associated ARB presents with a variable age of onset and clinical findings, that can be categorized in 5 clinical phenotypes. Hyperreflective foci and choroidal excavation frequently develop as secondary manifestations.

Miscibility of hBest1 and sphingomyelin in surface films - A prerequisite for interaction with membrane domains

Human bestrophin-1 (hBest1) is a transmembrane Ca²⁺- dependent anion channel, associated with the transport of Cl^-, HCO^3- ions, γ-aminobutiric acid (GABA), glutamate (Glu), and regulation of retinal homeostasis. Its mutant forms cause retinal degenerative diseases, defined as Bestrophinopathies. Using both physicochemical - surface pressure/mean molecular area (π/A) isotherms, hysteresis, compressibility moduli of hBest1/sphingomyelin (SM) monolayers, Brewster angle microscopy (BAM) studies, and biological approaches - detergent membrane fractionation, Laurdan (6-dodecanoyl-N,N-dimethyl-2-naphthylamine) and immunofluorescence staining of stably transfected MDCK-hBest1 and MDCK II cells, we report: 1) Ca²⁺, Glu and GABA interact with binary hBest1/SM monolayers at 35 °C, resulting in changes in hBest1 surface conformation, structure, self-organization and surface dynamics. The process of mixing in hBest1/SM monolayers is spontaneous and the effect of protein on binary films was defined as "fluidizing", hindering the phase-transition of monolayer from liquid-expanded to intermediate (LE-M) state; 2) in stably transfected MDCK-hBest1 cells, bestrophin-1 was distributed between detergent resistant (DRM) and detergent-soluble membranes (DSM) - up to 30 % and 70 %, respectively; in alive cells, hBest1 was visualized in both liquid-ordered (L_o) and liquid-disordered (L_d) fractions, quantifying protein association up to 35 % and 65 % with L_o and L_d. Our results indicate that the spontaneous miscibility of hBest1 and SM is a prerequisite to diverse protein interactions with membrane domains, different structural conformations and biological functions.

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.

Pathogenicity of new BEST1 variants identified in Italian patients with best vitelliform macular dystrophy assessed by computational structural biology

Background

Best vitelliform macular dystrophy (BVMD) is an autosomal dominant macular degeneration. The typical central yellowish yolk-like lesion usually appears in childhood and gradually worsens. Most cases are caused by variants in the BEST1 gene which encodes bestrophin-1, an integral membrane protein found primarily in the retinal pigment epithelium.

Methods

Here we describe the spectrum of BEST1 variants identified in a cohort of 57 Italian patients analyzed by Sanger sequencing. In 13 cases, the study also included segregation analysis in affected and unaffected relatives. We used molecular mechanics to calculate two quantitative parameters related to calcium-activated chloride channel (CaCC composed of 5 BEST1 subunits) stability and calcium-dependent activation and related them to the potential pathogenicity of individual missense variants detected in the probands.

Results

Thirty-six out of 57 probands (63% positivity) and 16 out of 18 relatives proved positive to genetic testing. Family study confirmed the variable penetrance and expressivity of the disease. Six of the 27 genetic variants discovered were novel: p.(Val9Gly), p.(Ser108Arg), p.(Asn179Asp), p.(Trp182Arg), p.(Glu292Gln) and p.(Asn296Lys). All BEST1 variants were assessed in silico for potential pathogenicity. Our computational structural biology approach based on 3D model structure of the CaCC showed that individual amino acid replacements may affect channel shape, stability, activation, gating, selectivity and throughput, and possibly also other features, depending on where the individual mutated amino acid residues are located in the tertiary structure of BEST1. Statistically significant correlations between mean logMAR best-corrected visual acuity (BCVA), age and modulus of computed BEST1 dimerization energies, which reflect variations in the in CaCC stability due to amino acid changes, permitted us to assess the pathogenicity of individual BEST1 variants.

Conclusions

Using this computational approach, we designed a method for estimating BCVA progression in patients with BEST1 variants.

Appropriately differentiated ARPE-19 cells regain phenotype and gene expression profiles similar to those of native RPE cells

PURPOSE

The RPE cell line ARPE-19 provides a dependable and widely used alternative to native RPE. However, replication of the native RPE phenotype becomes more difficult because these cells lose their specialized phenotype after multiple passages. Compounding this problem is the widespread use of ARPE-19 cells in an undifferentiated state to attempt to model RPE functions. We wished to determine whether suitable culture conditions and differentiation could restore the RPE-appropriate expression of genes and proteins to ARPE-19, along with a functional and morphological phenotype resembling native RPE. We compared the transcriptome of ARPE-19 cells kept in long-term culture with those of primary and other human RPE cells to assess the former's inherent plasticity relative to the latter.

METHODS

ARPE-19 cells at passages 9 to 12 grown in DMEM containing high glucose and pyruvate with 1% fetal bovine serum were differentiated for up to 4 months. Immunocytochemistry was performed on ARPE-19 cells grown on filters. Total RNA extracted from ARPE-19 cells cultured for either 4 days or 4 months was used for RNA sequencing (RNA-Seq) analysis using a 2 × 50 bp paired end protocol. The RNA-Seq data were analyzed to identify the affected pathways and recognize shared ontological classification among differentially expressed genes. RPE-specific mRNAs and miRNAs were assessed with quantitative real-time (RT)-PCR, and proteins with western blotting.

RESULTS

ARPE-19 cells grown for 4 months developed the classic native RPE phenotype with heavy pigmentation. RPE-expressed genes, including RPE65, RDH5, and RDH10, as well as miR-204/211, were greatly increased in the ARPE-19 cells maintained at confluence for 4 months. The RNA-Seq analysis provided a comprehensive view of the relative abundance and differential expression of the genes in the differentiated ARPE-19 cells. Of the 16,757 genes with detectable signals, nearly 1,681 genes were upregulated, and 1,629 genes were downregulated with a fold change of 2.5 or more differences between 4 months and 4 days of culture. Gene Ontology analysis showed that the upregulated genes were associated with visual cycle, phagocytosis, pigment synthesis, cell differentiation, and RPE-related transcription factors. The majority of the downregulated genes play a role in cell cycle and proliferation.

CONCLUSIONS

The ARPE-19 cells cultured for 4 months developed a phenotype characteristic of native RPE and expressed proteins, mRNAs, and miRNAs characteristic of the RPE. Comparison of the ARPE-19 RNA-Seq data set with that of primary human fetal RPE, embryonic stem cell-derived RPE, and native RPE revealed an important overall similar expression ratio among all the models and native tissue. However, none of the cultured models reached the absolute values in the native tissue. The results of this study demonstrate that low-passage ARPE-19 cells can express genes specific to native human RPE cells when appropriately cultured and differentiated.

Effects of Ca2+ ions on bestrophin-1 surface films

Human bestrophin-1 (hBest1) is a transmembrane calcium-activated chloride channel protein - member of the bestrophin family of anion channels, predominantly expressed in the membrane of retinal pigment epithelium (RPE) cells. Mutations in the protein cause ocular diseases, named Bestrophinopathies. Here, we present the first Fourier transform infrared (FTIR) study of the secondary structure elements of hBest1, π/A isotherms and hysteresis, Brewster angle microscopy (BAM) and atomic force microscopy (AFM) visualization of the aggregation state of protein molecules dispersed as Langmuir and Langmuir-Blodgett films. The secondary structure of hBest1 consists predominantly of 3₁₀-helices (27.2%), α-helixes (16.3%), β-turns and loops (32.2%). AFM images of hBest1 suggest approximate lateral dimensions of 100×160Å and 75Å height. Binding of calcium ions (Ca²⁺) induces conformational changes in the protein secondary structure leading to assembly of protein molecules and changes in molecular and macro-organization of hBest1 in monolayers. These data provide basic information needed in pursuit of molecular mechanisms underlying retinal and other pathologies linked to this protein.

Unilateral BEST1-Associated Retinopathy

Purpose

To describe a series of patients with molecularly confirmed mutation in BEST1 causing Best disease but with unilateral clinical manifestation.

Design

Retrospective observational case series.

Methods

Setting: Moorfields Eye Hospital and Great Ormond Street Hospital, London (United Kingdom). Patients: Five patients (10 eyes) with uniocular manifestation of BEST1 mutation causing Best disease were ascertained retrospectively from the clinical and genetic databases. Main Outcome Measures: Patients had full ophthalmologic examination, color fundus photography, fundus autofluorescence imaging, spectral-domain optical coherence tomography, and detailed electrophysiological assessment. Genetic testing was performed.

Results

All cases had a clinical appearance typical of and consistent with Best disease at various stages, except that the presentation was unilateral. The reduced electrooculogram light rise was bilateral and in the context of normal electroretinograms therefore indicates generalized dysfunction at the level of the retinal pigment epithelium.

Conclusions

Mutation in BEST1 has variable penetrance and expressivity, and can be uniocular. The clinical and electrophysiological features described assist targeted mutational screening and alert to the potential diagnosis even when there is an atypical unilateral presentation.

BESTROPHIN1 mutations cause defective chloride conductance in patient stem cell-derived RPE

Bestrophin1 (BEST1) is expressed in human retinal pigment epithelium (RPE) and mutations in the BEST1 gene commonly cause retinal dysfunction and macular degeneration. BEST1 is presumed to assemble into a calcium-activated chloride channel and be involved in chloride transport but there is no direct evidence in live human RPE cells to support this idea. To test whether BEST1 functions as a chloride channel in living tissue, BEST1-mutant RPE (R218H, L234P, A243T) were generated from patient-derived induced pluripotent stem cells and compared with wild-type RPE in a retinal environment, using a biosensor that visualizes calcium-induced chloride ion flux in the cell. Calcium stimulation elicited chloride ion export in normal RPE but not in RPE derived from three patients with BEST1 mutations. These data, along with three-dimensional modeling, provide evidence that BEST1 assembles into a key calcium-sensing chloride channel in human RPE.

Contribution of Ion Channels in Calcium Signaling Regulating Phagocytosis: MaxiK, Cav1.3 and Bestrophin-1

Mutations in the BEST1 gene lead to a variety of retinal degenerations including Best's vitelliforme macular degeneration. The BEST1 gene product, bestrophin-1, is expressed in the retinal pigment epithelium (RPE). It is likely that mutant bestrophin-1 impairs functions of the RPE which support photoreceptor function and will thus lead to retinal degeneration. However, the RPE function which is influenced by bestrophin-1 is so far not identified. Previously we showed that bestrophin-1 interacts with L-type Ca²⁺ channels of the CaV1.3 subtype and that the endogenously expressed bestrophin-1 is required for intracellular Ca²⁺ regulation. A hallmark of Best's disease is the fast lipofuscin accumulation occurring already at young ages. Therefore, we addressed the hypothesis that bestrophin-1 might influence phagocytosis of photoreceptor outer segments (POS) by the RPE. Here, siRNA knock-down of bestrophin-1 expression as well as inhibition of L-type Ca²⁺ channel activity modulated the POS phagocytosis in vitro. In vivo CaV1.3 expression appeared to be diurnal regulated with a higher expression rate in the afternoon. Compared to wild-type littermates, Ca V 1.3 (-/-) mice showed a shift in the circadian POS phagocytosis with an increased activity in the afternoon. Thus we suggest that mutant bestrophin-1 leads to an impaired regulation of the POS phagocytosis by the RPE which would explain the fast lipofuscin accumulation in Best patients.

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.

Structure and selectivity in bestrophin ion channels

Human bestrophin-1 (hBest1) is a calcium-activated chloride channel from the retinal pigment epithelium, where mutations are associated with vitelliform macular degeneration, or Best disease. We describe the structure of a bacterial homolog (KpBest) of hBest1 and functional characterizations of both channels. KpBest is a pentamer that forms a five-helix transmembrane pore, closed by three rings of conserved hydrophobic residues, and has a cytoplasmic cavern with a restricted exit. From electrophysiological analysis of structure-inspired mutations in KpBest and hBest1, we find a sensitive control of ion selectivity in the bestrophins, including reversal of anion/cation selectivity, and dramatic activation by mutations at the cytoplasmic exit. A homology model of hBest1 shows the locations of disease-causing mutations and suggests possible roles in regulation.

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.

Three-dimensional distribution of the vitelliform lesion, photoreceptors, and retinal pigment epithelium in the macula of patients with best vitelliform macular dystrophy

OBJECTIVE

To describe the anatomical phenotypes of Best vitelliform macular dystrophy (BVMD) with spectral-domain optical coherence tomography (SD-OCT) in a large series of patients with confirmed mutations in the BEST1 gene.

METHODS

In our retrospective observational case series, we assessed 15 patients (30 eyes) with a clinical diagnosis of vitelliform macular dystrophy who were found to have mutations in the BEST1 gene. Color fundus photographs and SD-OCT images were evaluated and compared with those of 15 age-matched controls (30 eyes). Using a validated 3-dimensional SD-OCT segmentation algorithm, we calculated the equivalent thickness of photoreceptors and the equivalent thickness of the retinal pigment epithelium for each patient. The photoreceptor equivalent thickness and the retinal pigment epithelium (RPE) equivalent thickness were compared in all patients, in a region of the macula outside the central lesion for patients with BVMD and outside the fovea in control patients. Paired t tests were used for statistical analysis.

RESULTS

The SD-OCT findings revealed that the vitelliform lesion consists of material above the RPE and below the outer segment tips. Additionally, drusen-like deposition of sub-RPE material was notable, and several patients exhibited a sub-RPE fibrotic nodule. Patients with BVMD had a mean photoreceptor equivalent thickness of 28.3 μm, and control patients had a mean photoreceptor equivalent thickness of 21.8 μm, a mean difference of 6.5 μm (P < .01), whereas the mean RPE equivalent thickness was not statistically different between patients with BVMD and control patients (P = .53).

CONCLUSIONS

The SD-OCT findings suggest that vitelliform material is located in the subretinal space and that BVMD is associated with diffuse photoreceptor outer segment abnormalities overlying a structurally normal RPE.

CLINICAL RELEVANCE

These findings provide new insight into the pathophysiology of BVMD and thus have implications for the development of therapeutic interventions.

Regulation of anterior chamber drainage by bicarbonate-sensitive soluble adenylyl cyclase in the ciliary body

Glaucoma is a leading cause of blindness affecting as many as 2.2 million Americans. All current glaucoma treatment strategies aim to reduce intraocular pressure (IOP). IOP results from the resistance to drainage of aqueous humor (AH) produced by the ciliary body in a process requiring bicarbonate. Once secreted into the anterior chamber, AH drains from the eye via two pathways: uveoscleral and pressure-dependent or conventional outflow (C(t)). Modulation of "inflow" and "outflow" pathways is thought to occur via distinct, local mechanisms. Mice deficient in the bicarbonate channel bestrophin-2 (Best2), however, exhibit a lower IOP despite an increase in AH production. Best2 is expressed uniquely in nonpigmented ciliary epithelial (NPE) cells providing evidence for a bicarbonate-dependent communicative pathway linking inflow and outflow. Here, we show that bicarbonate-sensitive soluble adenylyl cyclase (sAC) is highly expressed in the ciliary body in NPE cells, but appears to be absent from drainage tissues. Pharmacologic inhibition of sAC in mice causes a significant increase in IOP due to a decrease in C(t) with no effect on inflow. In mice deficient in sAC IOP is elevated, and C(t) is decreased relative to wild-type mice. Pharmacologic inhibition of sAC did not alter IOP or C(t) in sAC-deficient mice. Based on these data we propose that the ciliary body can regulate C(t) and that sAC serves as a critical sensor of bicarbonate in the ciliary body regulating the secretion of substances into the AH that govern outflow facility independent of pressure.

Autosomal recessive vitelliform macular dystrophy in a large cohort of vitelliform macular dystrophy patients

PURPOSE

To report 11 cases of autosomal recessive vitelliform macular dystrophy and to compare their molecular findings and phenotypic characteristics with those of patients with the more common and well-described dominant form of the disease.

METHODS

Blood samples were obtained from 435 unrelated individuals with a clinical diagnosis of vitelliform macular dystrophy and screened for mutations in the coding sequences of BEST1. Medical records and retinal photographs of selected patients were reviewed.

RESULTS

Nine of the 435 probands were found to have 2 plausible disease-causing variations in BEST1, while 198 individuals were found to have heterozygous variations compatible with autosomal dominant inheritance. Inheritance phase was determined in three of the recessive families. Six novel disease-causing mutations were identified among these recessive patients: Arg47Cys, IVS7-2A>G, IVS7+4G>A, Ile205del12ATCCTGCTCCAGAG, Pro274Arg, and Ile366delCAGGTGTGGC. Forty-four novel disease-causing mutations were identified among the patients with presumed autosomal dominant disease. The phenotype of patients with recessive alleles for BEST1 ranged from typical vitelliform lesions to extensive extramacular deposits.

CONCLUSION

The authors provide evidence that two abnormal BEST1 alleles, neither of which causes macular disease alone, can act in concert to cause early-onset vitelliform macular dystrophy.

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.

Role of the Ca2+-activated Cl- channels bestrophin and anoctamin in epithelial cells

Two families of proteins, the bestrophins (Best) and the recently cloned TMEM16 proteins (anoctamin, Ano), recapitulate properties of Ca2+-activated Cl- currents. Best1 is strongly expressed in the retinal pigment epithelium and could have a function as a Ca2+-activated Cl- channel as well as a regulator of Ca2+ signaling. It is also present at much lower levels in other cell types including epithelial cells, where it regulates plasma membrane localized Cl- channels by controlling intracellular Ca2+ levels. Best1 interacts with important Ca2+-signaling proteins such as STIM1 and can interact directly with other Ca2+-activated Cl- channels such as TMEM16A. Best1 is detected in the endoplasmic reticulum (ER) where it shapes the dynamic ER structure and regulates cell proliferation, which could be important for renal cystogenesis. Ca2+-activated Cl- channels of the anoctamin family (TMEM16A) show biophysical and pharmacological properties that are typical for endogenous Ca2+-dependent Cl- channels. TMEM16 proteins are abundantly expressed and many reports demonstrate their physiological importance in epithelial as well as non-epithelial cells. These channels are also activated by cell swelling and can therefore control cell volume, proliferation and apoptosis. To fully understand the function and regulation of Ca2+-activated Cl- currents, it is necessary to appreciate that Best1 and TMEM16A are embedded in a protein network and that they probably operate in functional microdomains.

Morphological and functional changes in multifocal vitelliform retinopathy and biallelic mutations in BEST1

PURPOSE

To describe morphological and functional changes in a single patient with multifocal Best vitelliform macular dystrophy (BVMD) and to perform a genotype/phenotype correlation.

METHODS

The proband with multifocal BVMD and three of her family members were examined with electrooculography (EOG), full-field electroretinography (full-field ERG), multifocal electroretinography (mfERG) and optical coherence tomography (OCT). Genomic DNA was screened for mutation in the BEST1 gene by DNA sequencing analysis.

RESULTS

The proband was observed regularly during a follow-up period of 4 years. Full-field ERG demonstrated reduced and delayed responses of both rods and cones. OCT demonstrated intra- and subretinal fluid which seemed to fluctuate with periods of stress, similar to that seen in chronic central serous chorioretinopathy. Two distinct heterozygous BEST1 mutations were identified in the proband, the recurrent p.R141H mutation and the p.P233A mutation. Heterozygous p.R141H mutations were also identified in two family members, while p.P233A was a de novo mutation. Abnormal EOG findings were observed in both the proband and in the carriers of p.R141H. Heterozygous carriers showed delayed implicit times in a- and b-waves of combined total rod and cone full-field ERG responses.

CONCLUSIONS

The p.R141H mutation is frequently seen together with multifocal vitelliform retinopathy and biallelic mutations in BEST1. Our results show that carriers of the p.R141H mutation are clinically unaffected but present with abnormal EOG and full-field ERG findings. A patient with biallelic mutations of the BEST1 gene, causing multifocal BVMD with progressive, widespread functional disturbance of the retina, confirmed by full-field and mfERG is described.

Best's macular dystrophy in Australia: phenotypic profile and identification of novel BEST1 mutations

PURPOSE

(1) To evaluate the spectrum of BEST1 mutations within Australian Best Disease or vitelliform macular dystrophy (VMD) pedigrees, including any novel mutations; (2) to analyse the range of clinical presentations of this cohort; (3) to determine any possible genotype-phenotype correlations and (4) to compare clinical data of patients with phenotypic VMD, both with and without a BEST1 mutation.

PATIENTS AND METHODS

Patients with suspected VMD were referred to clinical centres for ophthalmological assessment and genetic screening. When a mutation was identified in a proband, further family members were invited for clinical and genetic screening.

RESULTS

We identified 42 patients with one of 13 BEST1 mutations. Seven mutations were novel. There were a further 14 probands in whom a BEST1 mutation was not identified. Median visual acuity in both VMD (mutation positive) and clinical VMD (no BEST1 mutation identified) groups reached driving standards (6/12 or better).

CONCLUSION

We did not identify any firm genotype-phenotype correlations in our Australian VMD pedigrees, in which there was a spectrum of BEST1 mutations and marked variation in clinical presentation. Genetic screening remains the gold standard for VMD diagnosis. Patients should be counselled that visual acuity might remain at or above driving standards in at least one eye even in the presence of a BEST1 mutation.

Multimodal fundus imaging in Best vitelliform macular dystrophy

BACKGROUND

Best vitelliform macular dystrophy (BVMD) is a rare autosomal dominant retinal disease of highly variable phenotypic expression. Interpretations of disease mechanisms based on histopathology, electrophysiology, genetic analysis, and retinal imaging are somewhat discordant in fundamental issues such as the location and extension of primary retinal changes. Herein we describe the morphological macular features in patients with BVMD undergoing simultaneous multimodal fundus imaging and compare to those of normal age-matched subjects.

METHODS

Comparative study including seven patients with BVMD (14 eyes) and seven age-matched healthy subjects (14 eyes). All participants were submitted to complete ophthalmological examination, fundus photography, and standardized multimodal fundus imaging protocol including Fourier-domain optical coherence tomography (Fd-OCT) combined with near-infrared reflectance and blue-light fundus autofluorescence (FAF).

RESULTS

In two eyes in the "subclinical" stage, Fd-OCT revealed thickening of the middle highly reflective layer (HRL) localized between the photoreceptors' inner/outer segments junction (inner-HRL) and RPE/Bruch's membrane reflective complex (outer-HRL) throughout the macula. In one eye in the "vitelliform" stage, a homogeneous hyper-reflective material on Fd-OCT was observed between the middle-HRL and outer-HRL; this material presented increased fluorescence on FAF. The outer nuclear layer (ONL) was thinned in the central macula and subretinal fluid was not identified in these earlier disease stages. In patients of "pseudohypopyon" (two eyes), "vitelliruptive" (eight eyes) and "atrophic" (one eye) stages, Fd-OCT revealed a variety of changes in the middle- and inner-HRLs and thinning of ONL. These changes were found to be associated with the level of visual acuity observed. Thickening of the middle-HRL was observed beyond the limits of the clinically evident macular lesion in all eyes.

CONCLUSIONS

Multimodal fundus imaging demonstrated thickening of the reflective layer corresponding to the photoreceptors' outer segments throughout the macula with no subretinal fluid accumulation as the earliest detectable feature in BVMD. Changes detected in the photoreceptors' reflective layers (middle- and inner- HRLs) and ONL thinning seemed to be progressive with direct implications for the level of visual acuity impairment observed among the different stages of the disease.

Chloride channels of intracellular membranes

Proteins implicated as intracellular chloride channels include the intracellular ClC proteins, the bestrophins, the cystic fibrosis transmembrane conductance regulator, the CLICs, and the recently described Golgi pH regulator. This paper examines current hypotheses regarding roles of intracellular chloride channels and reviews the evidence supporting a role in intracellular chloride transport for each of these proteins.

Functional and clinical data of Best vitelliform macular dystrophy patients with mutations in the BEST1 gene

PURPOSE

To analyze functional and clinical data of Best vitelliform macular dystrophy (VMD) patients with mutations in the BEST1 gene.

METHODS

Best VMD patients with BEST1 mutations were evaluated prospectively regarding age, age of onset, best-corrected visual acuity (BCVA), fundus autofluorescence, fluorescein angiography, optical coherence tomography, and electro-oculography. Mutations in BEST1 were established by direct sequencing.

RESULTS

Forty-six eyes of 23 patients (10 male, 13 female) were included in the study. We identified nine different BEST1 mutations (3/9 novel), in ten unrelated families. The age of patients ranged between 3 and 75 years; age of onset varied between 2 and 67 years. BCVA ranged between 20/20 and 20/200. On the basis of fundus biomicroscopy with direct illumination, using one widely accepted classification, the macular lesions could be counted as follows: 1. no lesion (normal fovea): eight eyes, five patients carrying a mutation on the BEST1 gene; 2. previtelliform lesions: six eyes, three affected patients; 3. vitelliform lesions: four eyes, two affected patients; 4. pseudohypopyon: three eyes, three affected patients; 5. vitelliruptive lesions (scrambled egg aspect with dispersion of the vitelliform material without sign of atrophy or fibrosis): ten eyes, six affected patients; 6. atrophic lesions (atrophy with or without residual dispersed material): seven eyes, five patients; 7. fibrotic lesions: eight eyes, five patients. Two patients presented unilateral Best VMD. Both eyes of two patients presented multifocal Best VMD features on fundus examination. Six eyes of four patients have been treated for choroidal neovascularization by thermic photocoagulation [one eye], photodynamic therapy [three eyes], and intravitreal ranibizumab injection [two eyes]. Comparison of interfamilial and intrafamilial clinical data between patients did not reveal differences in age, BCVA, and stage of the disease as evaluated by fundus autofluorescence, fluorescein angiography, and optical coherence tomography (p>0.05). Mean BCVA impairment showed a statistically significant correlation to a more advanced stage of the disease (p<0.001).

CONCLUSIONS

BEST1 mutations were not correlated with the severity of the functional and clinical data in the Best VMD patients examined.

Human disease-causing mutations disrupt an N-C-terminal interaction and channel function of bestrophin 1

Mutations in the human bestrophin 1 (hBest1) chloride channel cause Best vitelliform macular dystrophy. Although mutations in its transmembrane domains were found to alter biophysical properties of the channel, the mechanism for disease-causing mutations in its N and C termini remains elusive. We hypothesized that these mutations lead to channel dysfunction through disruption of an N-C-terminal interaction. Here, we present data demonstrating that hBest1 N and C termini indeed interact both in vivo and in vitro. In addition, using a spectrum-based fluorescence resonance energy transfer method, we showed that functional hBest1 channels in the plasma membrane were multimers. Disease-causing mutations in the N terminus (R19C, R25C, and K30C) and the C terminus (G299E, D301N, and D312N) caused channel dysfunction and disruption of the N-C interaction. Consistent with the functional and biochemical results, mutants D301N and D312N clearly reduced fluorescence resonance energy transfer signal, indicating that the N-C interaction was indeed perturbed. These results suggest that hBest1 functions as a multimer in the plasma membrane, and disruption of the N-C interaction by mutations leads to hBest1 channel dysfunction.

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.

Rescue of volume-regulated anion current by bestrophin mutants with altered charge selectivity

Mutations in human bestrophin-1 are linked to various kinds of retinal degeneration. Although it has been proposed that bestrophins are Ca(2+)-activated Cl(-) channels, definitive proof is lacking partly because mice with the bestrophin-1 gene deleted have normal Ca(2+)-activated Cl(-) currents. Here, we provide compelling evidence to support the idea that bestrophin-1 is the pore-forming subunit of a cell volume-regulated anion channel (VRAC) in Drosophila S2 cells. VRAC was abolished by treatment with RNAi to Drosophila bestrophin-1. VRAC was rescued by overexpressing bestrophin-1 mutants with altered biophysical properties and responsiveness to sulfhydryl reagents. In particular, the ionic selectivity of the F81C mutant changed from anionic to cationic when the channel was treated with the sulfhydryl reagent, sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES(-)) (P(Cs)/P(Cl) = 0.25 for native and 2.38 for F81C). The F81E mutant was 1.3 times more permeable to Cs(+) than Cl(-). The finding that VRAC was rescued by F81C and F81E mutants with different biophysical properties shows that bestrophin-1 is a VRAC in S2 cells and not simply a regulator or an auxiliary subunit. F81C overexpressed in HEK293 cells also exhibits a shift of ionic selectivity after MTSES(-) treatment, although the effect is quantitatively smaller than in S2 cells. To test whether bestrophins are VRACs in mammalian cells, we compared VRACs in peritoneal macrophages from wild-type mice and mice with both bestrophin-1 and bestrophin-2 disrupted (best1(-/-)/best2(-/-)). VRACs were identical in wild-type and best1(-/-)/best2(-/-) mice, showing that bestrophins are unlikely to be the classical VRAC in mammalian cells.

Enhanced inflow and outflow rates despite lower IOP in bestrophin-2-deficient mice

PURPOSE

Bestrophin-2 (Best2), a putative Cl(-) channel is expressed in the nonpigmented epithelium (NPE). Disruption of Best2 in mice results in a diminished intraocular pressure (IOP). Aqueous humor dynamics were compared in Best2(+/+) and Best2(-/-) mice, to better understand the contribution of Best2 to IOP.

METHODS

Measurements of IOP, episcleral venous pressure (EVP), conventional outflow facility (C(t)), aqueous humor production (F(a)), and anterior chamber volume (V(a)) were made using anterior chamber cannulation. Conventional (F(c)) and uveoscleral outflow (F(u)), and rate of aqueous humor turnover, were calculated from measured data. The anterior chamber was examined in live mice by optical coherence tomography (OCT) and postmortem by light microscopy.

RESULTS

IOP in Best2(-/-) mice was lower compared with Best2(+/+) littermates. EVP was unchanged. Since Best2 is expressed in NPE cells, the hypothesis was that Best2 is involved in generating aqueous flow. However, F(a) in Best2(-/-) mice was increased by approximately 73% compared with Best2(+/+) mice. This was accompanied by increases in F(c) and F(u). Aqueous humor turnover was enhanced more than twofold in Best2(-/-) mice. No evidence of developmental structural changes was noted.

CONCLUSIONS

Best2 appears to antagonize the formation of aqueous humor and cause an inhibition of both F(c) and F(u), despite being expressed only in NPE cells. These data support the hypothesis that the inflow and outflow pathways communicate via soluble agents present in the aqueous humor and implicate Best2 as a critical mediator of that communication.

Epithelial phenotype and the RPE: is the answer blowing in the Wnt?

Cells of the human retinal pigment epithelium (RPE) have a regular epithelial cell shape within the tissue in situ, but for reasons that remain elusive the RPE shows an incomplete and variable ability to re-develop an epithelial phenotype after propagation in vitro. In other epithelial cell cultures, formation of an adherens junction (AJ) composed of E-cadherin plays an important early inductive role in epithelial morphogenesis, but E-cadherin is largely absent from the RPE. In this review, the contribution of cadherins, both minor (E-cadherin) and major (N-cadherin), to RPE phenotype development is discussed. Emphasis is placed on the importance for future studies of actin cytoskeletal remodeling during assembly of the AJ, which in epithelial cells results in an actin organization that is characteristically zonular. Other markers of RPE phenotype that are used to gauge the maturation state of RPE cultures including tissue-specific protein expression, protein polarity, and pigmentation are described. An argument is made that RPE epithelial phenotype, cadherin-based cell-cell adhesion and melanization are linked by a common signaling pathway: the Wnt/beta-catenin pathway. Analyzing this pathway and its intersecting signaling networks is suggested as a useful framework for dissecting the steps in RPE morphogenesis. Also discussed is the effect of aging on RPE phenotype. Preliminary evidence is provided to suggest that light-induced sub-lethal oxidative stress to cultured ARPE-19 cells impairs organelle motility. Organelle translocation, which is mediated by stress-susceptible cytoskeletal scaffolds, is an essential process in cell phenotype development and retention. The observation of impaired organelle motility therefore raises the possibility that low levels of stress, which are believed to accompany RPE aging, may produce subtle disruptions of cell phenotype. Over time these would be expected to diminish the support functions performed by the RPE on behalf of photoreceptors, theoretically contributing to aging retinal disease such as age-related macular degeneration (AMD). Analyzing sub-lethal stress that produces declines in RPE functional efficiency rather than overt cell death is suggested as a useful future direction for understanding the effects of age on RPE organization and physiology. As for phenotype and pigmentation, a role for the Wnt/beta-catenin pathway is also suggested in regulating the RPE response to oxidative stress. Exploration of this pathway in the RPE therefore may provide a unifying strategy for advancing our understanding of both RPE phenotype and the consequences of mild oxidative stress on RPE structure and function.

The best disease-linked Cl- channel hBest1 regulates Ca V 1 (L-type) Ca2+ channels via src-homology-binding domains

Mutations in the bestrophin-1 (Best1) gene are linked to several kinds of macular degeneration in both humans and dogs. Although bestrophins have been shown clearly to be Cl(-) ion channels, it is controversial whether Cl(-) channel dysfunction can explain the diseases. It has been suggested that bestrophins are multifunctional proteins: they may regulate voltage-gated Ca(2+) channels in addition to functioning as Cl(-) channels. Here, we show that human Best1 gene (hBest1) differentially modulates Ca(V)1.3 (L-type) voltage-gated Ca(2+) channels through association with the Ca(V)beta subunit. In transfected human embryonic kidney 293 cells, hBest1 inhibited Ca(V)1.3. Inhibition of Ca(V)1.3 was not observed in the absence of the beta subunit. Also, the hBest1 C terminus binds to Ca(V)beta subunits, suggesting that the effect of hBest1 was mediated by the Ca(V)beta subunit. The region of hBest1 responsible for the effect was localized to a region (amino acids 330-370) in the cytoplasmic C terminus that contains a predicted src-homology-binding domain that is not present in other bestrophin subtypes. Mutation of Pro(330) and Pro(334) abolished the effects of hBest1 on Ca(V)1.3. The effect was specific to hBest1; it was not observed with mouse Best1 (mBest1), mBest2, or mBest3. Wild-type hBest1 and the disease-causing mutants R92S, G299R, and D312N inhibited Ca(V) currents the same amount, whereas the A146K and G222E mutants were less effective. We propose that hBest1 regulates Ca(V) channels by interacting with the Ca(V)beta subunit and altering channel availability. Our findings reveal a novel function of bestrophin in regulation of Ca(V) channels and suggest a possible mechanism for the role of hBest1 in macular degeneration.

Bestrophin-2 is involved in the generation of intraocular pressure

PURPOSE

The bestrophin family of proteins has been demonstrated to generate or regulate Ca2+-activated Cl(-) conductances. Mutations in bestrophin-1 (Best1) cause several blinding eye diseases, but little is known about other bestrophin family members. This study involved disruption of the Best2 gene in mice.

METHODS

The mouse Best2 gene was disrupted by replacing exons 1, 2, and part of exon 3 with a Lac Z. The expression profile of Bestrophin-2 (Best2) was examined using RT-PCR, X-gal staining, and immunohistochemistry. Intraocular pressure (IOP) was measured by anterior chamber cannulation.

RESULTS

RT-PCR of mouse tissues revealed Best2 mRNA in eye, colon, nasal epithelia, trachea, brain, lung, and kidney. X-gal staining, confirmed expression in colon epithelia and in the eye, in the nonpigmented epithelia (NPE). Best2 was not expressed in RPE cells. Best2 protein was observed only in NPE and colon epithelia. The absence of Best2 had no obvious deleterious effect on the mice. However, the Best2-/- mice were found to have significantly (P < 0.02) diminished IOP with respect to the Best2+/+ and Best2+/- littermates. The Best2-/- and Best2+/- mice responded better to the carbonic anhydrase inhibitor brinzolamide than did their Best2+/+ littermates, although the beta-blocker timolol brought IOP to the same level, regardless of genotype.

CONCLUSIONS

Best2 plays a role in the generation of IOP by regulating formation of aqueous humor, and inhibition of Best2 function represents an attractive new avenue for regulating IOP in individuals with glaucoma.

Bestrophin gene mutations cause canine multifocal retinopathy: a novel animal model for best disease

PURPOSE

Canine multifocal retinopathy (cmr) is an autosomal recessive disorder of multiple dog breeds. The disease shares a number of clinical and pathologic similarities with Best macular dystrophy (BMD), and cmr is proposed as a new large animal model for Best disease.

METHODS

cmr was characterized by ophthalmoscopy and histopathology and compared with BMD-affected patients. BEST1 (alias VMD2), the bestrophin gene causally associated with BMD, was evaluated in the dog. Canine ortholog cDNA sequence was cloned and verified using RPE/choroid 5'- and 3'-RACE. Expression of the canine gene transcripts and protein was analyzed by Northern and Western blotting and immunocytochemistry. All exons and the flanking splice junctions were screened by direct sequencing.

RESULTS

The clinical phenotype and pathology of cmr closely resemble lesions of BMD. Canine VMD2 spans 13.7 kb of genomic DNA on CFA18 and shows a high level of conservation among eukaryotes. The transcript is predominantly expressed in RPE/choroid and encodes bestrophin, a 580-amino acid protein of 66 kDa. Immunocytochemistry of normal canine retina demonstrated specific localization of protein to the RPE basolateral plasma membranes. Two disease-specific sequence alterations were identified in the canine VMD2 gene: a C(73)T stop mutation in cmr1 and a G(482)A missense mutation in cmr2.

CONCLUSIONS

The authors propose these two spontaneous mutations in the canine VMD2 gene, which cause cmr, as the first naturally occurring animal model of BMD. Further development of the cmr models will permit elucidation of the complex molecular mechanism of these retinopathies and the development of potential therapies.

Macular degeneration

Macular degeneration is a term that describes a large group of conditions that are collectively the most common cause of irreversible vision loss in the developed world. Approximately one in three people will be affected to some degree by the time they reach 75 years of age. Many forms of macular degeneration have a significant genetic component, and a large amount of progress has recently been made in understanding the genes involved. In the future, genetic testing may allow specific preventive treatments to be delivered to individuals at risk, decades before the disease would ordinarily become manifest.

Light and alcohol evoked electro-oculograms in cystic fibrosis

Cystic fibrosis (CF) is caused by a defect in the cystic fibrosis transmembrane conductance regulator (CFTR) which is a chloride channel. CFTR is expressed in the retinal pigment epithelium (RPE) where it is believed to be important in generating the fast oscillations (FOs) and potentially contributing to the light-electro-oculogram (EOG). The role of CFTR in the alcohol-EOG is unknown. We recruited six individuals with CF (three homozygotes for Delta508 and three heterozygous for Delta508) and recorded the light- and alcohol-EOGs as well as the FOs and compared them to a control group. The results showed that in the CF group the amplitude of the alcohol- and light-EOGs were normal. However, the time to peak of the light- and alcohol-rises were significantly faster than in the control group. We conclude that CFTR is not primarily responsible for the alcohol- or light-rises but is involved in altering the timing of these responses. The FOs showed differences between the homozygotes, heterozygotes and the controls. The amplitudes were significantly higher and the time to the dark troughs were significantly slower in the heterozygote group compared to both controls and the homozygotes. In contrast, the homozygotes did not differ in either amplitude or the timing of the FOs compared to the controls.