Bestrophin-2 and glutamine synthetase form a complex for glutamate release.

Bestrophin-2 (BEST2) is a member of the bestrophin family of calcium-activated anion channels that has a critical role in ocular physiology1-4. Here we uncover a directional permeability of BEST2 to glutamate that heavily favours glutamate exit, identify glutamine synthetase (GS) as a binding partner of BEST2 in the ciliary body of the eye, and solve the structure of the BEST2-GS complex. BEST2 reduces cytosolic GS activity by tethering GS to the cell membrane. GS extends the ion conducting pathway of BEST2 through its central cavity and inhibits BEST2 channel function in the absence of intracellular glutamate, but sensitizes BEST2 to intracellular glutamate, which promotes the opening of BEST2 and thus relieves the inhibitory effect of GS. We demonstrate the physiological role of BEST2 in conducting chloride and glutamate and the influence of GS in non-pigmented ciliary epithelial cells. Together, our results reveal a novel mechanism of glutamate release through BEST2-GS.

Structures and gating mechanisms of human bestrophin anion channels.

Bestrophin-1 (Best1) and bestrophin-2 (Best2) are two members of the bestrophin family of calcium (Ca2+)-activated chloride (Cl-) channels with critical involvement in ocular physiology and direct pathological relevance. Here, we report cryo-EM structures of wild-type human Best1 and Best2 in various states at up to 1.8 Å resolution. Ca2+-bound Best1 structures illustrate partially open conformations at the two Ca2+-dependent gates of the channels, in contrast to the fully open conformations observed in Ca2+-bound Best2, which is in accord with the significantly smaller currents conducted by Best1 in electrophysiological recordings. Comparison of the closed and open states reveals a C-terminal auto-inhibitory segment (AS), which constricts the channel concentrically by wrapping around the channel periphery in an inter-protomer manner and must be released to allow channel opening. Our results demonstrate that removing the AS from Best1 and Best2 results in truncation mutants with similar activities, while swapping the AS between Best1 and Best2 results in chimeric mutants with swapped activities, underlying a key role of the AS in determining paralog specificity among bestrophins.

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.

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

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

Distinct expression requirements and rescue strategies for BEST1 loss- and gain-of-function mutations.

Genetic mutation of the human BEST1 gene, which encodes a Ca2+-activated Cl- channel (BEST1) predominantly expressed in retinal pigment epithelium (RPE), causes a spectrum of retinal degenerative disorders commonly known as bestrophinopathies. Previously, we showed that BEST1 plays an indispensable role in generating Ca2+-dependent Cl- currents in human RPE cells, and the deficiency of BEST1 function in patient-derived RPE is rescuable by gene augmentation (Li et al., 2017). Here, we report that BEST1 patient-derived loss-of-function and gain-of-function mutations require different mutant to wild-type (WT) molecule ratios for phenotypic manifestation, underlying their distinct epigenetic requirements in bestrophinopathy development, and suggesting that some of the previously classified autosomal dominant mutations actually behave in a dominant-negative manner. Importantly, the strong dominant effect of BEST1 gain-of-function mutations prohibits the restoration of BEST1-dependent Cl- currents in RPE cells by gene augmentation, in contrast to the efficient rescue of loss-of-function mutations via the same approach. Moreover, we demonstrate that gain-of-function mutations are rescuable by a combination of gene augmentation with CRISPR/Cas9-mediated knockdown of endogenous BEST1 expression, providing a universal treatment strategy for all bestrophinopathy patients regardless of their mutation types.

Structural and functional characterization of the bestrophin-2 anion channel.

The bestrophin family of calcium (Ca2+)-activated chloride (Cl-) channels, which mediate the influx and efflux of monovalent anions in response to the levels of intracellular Ca2+, comprises four members in mammals (bestrophin 1-4). Here we report cryo-EM structures of bovine bestrophin-2 (bBest2) bound and unbound by Ca2+ at 2.4- and 2.2-Å resolution, respectively. The bBest2 structure highlights four previously underappreciated pore-lining residues specifically conserved in Best2 but not in Best1, illustrating the differences between these paralogs. Structure-inspired electrophysiological analysis reveals that, although the channel is sensitive to Ca2+, it has substantial Ca2+-independent activity for Cl-, reflecting the opening at the cytoplasmic restriction of the ion conducting pathway even when Ca2+ is absent. Moreover, the ion selectivity of bBest2 is controlled by multiple residues, including those involved in gating.

Investigation and Restoration of BEST1 Activity in Patient-derived RPEs with Dominant Mutations.

BEST1 is a Ca2+-activated Cl- channel predominantly expressed in retinal pigment epithelium (RPE), and over 250 genetic mutations in the BEST1 gene have been identified to cause retinal degenerative disorders generally known as bestrophinopathies. As most BEST1 mutations are autosomal dominant, it is of great biomedical interest to determine their disease-causing mechanisms and the therapeutic potential of gene therapy. Here, we characterized six Best vitelliform macular dystrophy (BVMD)-associated BEST1 dominant mutations by documenting the patients' phenotypes, examining the subcellular localization of endogenous BEST1 and surface Ca2+-dependent Cl- currents in patient-derived RPEs, and analyzing the functional influences of these mutations on BEST1 in HEK293 cells. We found that all six mutations are loss-of-function with different levels and types of deficiencies, and further demonstrated the restoration of Ca2+-dependent Cl- currents in patient-derived RPE cells by WT BEST1 gene supplementation. Importantly, BEST1 dominant and recessive mutations are both rescuable at a similar efficacy by gene augmentation via adeno-associated virus (AAV), providing a proof-of-concept for curing the vast majority of bestrophinopathies.

Dual Ca2+-dependent gates in human Bestrophin1 underlie disease-causing mechanisms of gain-of-function mutations.

Mutations of human BEST1, encoding a Ca2+-activated Cl- channel (hBest1), cause macular degenerative disorders. Best1 homolog structures reveal an evolutionarily conserved channel architecture highlighted by two landmark restrictions (named the "neck" and "aperture", respectively) in the ion conducting pathway, suggesting a unique dual-switch gating mechanism, which, however, has not been characterized well. Using patch clamp and crystallography, we demonstrate that both the neck and aperture in hBest1 are Ca2+-dependent gates essential for preventing channel leakage resulting from Ca2+-independent, spontaneous gate opening. Importantly, three patient-derived mutations (D203A, I205T and Y236C) lead to Ca2+-independent leakage and elevated Ca2+-dependent anion currents due to enhanced opening of the gates. Moreover, we identify a network of residues critically involved in gate operation. Together, our results suggest an indispensable role of the neck and aperture of hBest1 for channel gating, and uncover disease-causing mechanisms of hBest1 gain-of-function mutations.

Differentiation, Maintenance, and Analysis of Human Retinal Pigment Epithelium Cells: A Disease-in-a-dish Model for BEST1 Mutations.

Although over 200 genetic mutations in the human BEST1 gene have been identified and linked to retinal degenerative diseases, the pathological mechanisms remain elusive mainly due to the lack of a good in vivo model for studying BEST1 and its mutations under physiological conditions. BEST1 encodes an ion channel, namely BESTROPHIN1 (BEST1), which functions in retinal pigment epithelium (RPE); however, the extremely limited accessibility to native human RPE cells represents a major challenge for scientific research. This protocol describes how to generate human RPEs bearing BEST1 disease-causing mutations by induced differentiation from human pluripotent stem cells (hPSCs). As hPSCs are self-renewable, this approach allows researchers to have a steady source of hPSC-RPEs for various experimental analyses, such as immunoblotting, immunofluorescence, and patch clamp, and thus provides a very powerful disease-in-a-dish model for BEST1-associated retinal conditions. Notably, this strategy can be applied to study RPE (patho)physiology and other genes of interest natively expressed in RPE.

Expression and Purification of Mammalian Bestrophin Ion Channels.

The human genome encodes four bestrophin paralogs, namely BEST1, BEST2, BEST3, and BEST4. BEST1, encoded by the BEST1 gene, is a Ca2+-activated Cl- channel (CaCC) predominantly expressed in retinal pigment epithelium (RPE). The physiological and pathological significance of BEST1 is highlighted by the fact that over 200 distinct mutations in the BEST1 gene have been genetically linked to a spectrum of at least five retinal degenerative disorders, such as Best vitelliform macular dystrophy (Best disease). Therefore, understanding the biophysics of bestrophin channels at the single-molecule level holds tremendous significance. However, obtaining purified mammalian ion channels is often a challenging task. Here, we report a protocol for the expression of mammalian bestrophin proteins with the BacMam baculovirus gene transfer system and their purification by affinity and size-exclusion chromatography. The purified proteins have the potential to be utilized in subsequent functional and structural analyses, such as electrophysiological recording in lipid bilayers and crystallography. Importantly, this pipeline can be adapted to study the functions and structures of other ion channels.

ATP activates bestrophin ion channels through direct interaction.

Human Bestrophin1 (hBest1) is a Ca2+-activated Cl- channel in retinal pigment epithelium (RPE) essential for retina physiology, and its mutation results in retinal degenerative diseases that have no available treatments. Here, we discover that hBest1's channel activity in human RPE is significantly enhanced by adenosine triphosphate (ATP) in a dose-dependent manner. We further demonstrate a direct interaction between ATP and bestrophins, and map the ATP-binding motif on hBest1 to an intracellular loop adjacent to the channel activation gate. Importantly, a disease-causing mutation of hBest1 located within the ATP-binding motif, p.I201T, diminishes ATP-dependent activation of the channel in patient-derived RPE, while the corresponding mutants in bestrophin homologs display defective ATP binding and a conformational change in the ATP-binding motif. Taken together, our results identify ATP as a critical activator of bestrophins, and reveal the molecular mechanism of an hBest1 patient-specific mutation.

Patient-specific mutations impair BESTROPHIN1's essential role in mediating Ca2+-dependent Cl- currents in human RPE.

Mutations in the human BEST1 gene lead to retinal degenerative diseases displaying progressive vision loss and even blindness. BESTROPHIN1, encoded by BEST1, is predominantly expressed in retinal pigment epithelium (RPE), but its physiological role has been a mystery for the last two decades. Using a patient-specific iPSC-based disease model and interdisciplinary approaches, we comprehensively analyzed two distinct BEST1 patient mutations, and discovered mechanistic correlations between patient clinical phenotypes, electrophysiology in their RPEs, and the structure and function of BESTROPHIN1 mutant channels. Our results revealed that the disease-causing mechanism of BEST1 mutations is centered on the indispensable role of BESTROPHIN1 in mediating the long speculated Ca2+-dependent Cl- current in RPE, and demonstrate that the pathological potential of BEST1 mutations can be evaluated and predicted with our iPSC-based 'disease-in-a-dish' approach. Moreover, we demonstrated that patient RPE is rescuable with viral gene supplementation, providing a proof-of-concept for curing BEST1-associated diseases.