User:MRoidt3/Bestrophin1

Bestrophin 1 is a human protein encoded by the BEST1 gene (RPD ID - 5T5N/4RDQ).

File:Bestrophin-1 4RDQ Xray Cystallography.png

Calcium-activated chloride channel bestrophin-1 (BEST1), in complex with an Fab antibody fragment, chloride, and calcium. From RCSB PDB.

The bestrophins were first identified in humans by relating a BEST1 mutation with Best vitelliform macular dystrophy (BVMD). The bestrophin family is comprised of four ancestrally linked genes (BEST1, BEST2, BEST3, and BEST4) that code for integral membrane proteins. Mutations in the BEST1 gene have been determined as the primary cause for at least five different degenerative retinal diseases.

Background

The bestrophins are an ancient family of proteins that have been identified in nearly every organism studied. Over time, the overall structure of bestrophin has been conserved; bestrophins are always ion channels. Further studies have revealed at least four different types of bestrophins in every organism. In humans, all bestrophins function as calcium-activated anion channels, though different variations are localized to different areas of the body due to differing chromosomal locations and expression requirements. Specifically, the BEST1 gene on chromosome 11q13 encodes the Bestrophin 1 protein (Best1) in humans.

Structure in humans

Bestrophin 1 is an integral membrane protein found only in the retinal pigment epithelium (RPE) of the eye. Within the RPE layer, Best1 is primarily located on the basolateral plasma membrane. Protein crystallization structures indicate the primary ion channel function of Best1 as well as the calcium regulatory capabilities.Bestrophin 1 consists of 585 amino acids and both N- and the C-termini are located within the cell.

Calcium-activated chloride channel bestrophin-1 (BEST1), in complex with an Fab antibody fragment, chloride, and calcium. From RCSB PDB

The structure of Best1 consists of five identical subunits that each span the membrane four times and form a continuous, funnel-shaped pore via the second transmembrane domain. The pore is lined with various nonpolar, hydrophobic amino acids which help to act as a selectivity filter by forming at least 15 binding sites for each anion. Both the structure and the composition of the pore help to ensure that only small anions are able to move completely through the channel. The channel acts as two funnels working together in tandem. It begins with a semi-selective, narrow entryway for anions, and then opens to a larger, positively charged area which then leads to a narrower pathway that further limits the size of anions passing through the pore. A calcium clasp acts as a belting mechanism around the larger, middle section of the channel. Calcium ions control the opening and closing of the channel due to conformational changes caused by calcium binding at the C-terminus directly following the last transmembrane domain. The structure of the calcium-dependent chloride channel, which is bestrophin 1, contributes greatly to the overall function of the channel.

Localization in humans

The location of expression of the BEST1 gene is essential for protein functioning and mislocalization is often connected to a variety of retinal degenerative diseases. The BEST1 general is primarily expressed in the cytosol of the retinal pigment epithelium. The protein is typically contained in vesicles near the cellular membrane. There is also research to support that Best1 is localized and produced in the endoplasmic reticulum (intracellular organelle involved in protein and lipid synthesis). Best1 is typically expressed with other proteins also synthesized in the endoplasmic reticulum, such as calreticulin, calnexin and Stim-1. Calcium ion involvement in the countertransport of chloride ions also supports the idea that Best1 is involved in forming calcium stores within the cell. Best1 has also been located in the brain and spinal cord where it functions in glial cells and astrocytes to aid in the release of neurotransmitter GABA and in the transport of neurotransmitter glutamate. Although Best1 functions throughout the central nervous system, it is found in highest concentrations in retinal pigment epithelium within the retina of the eye. Though the mRNA that codes Best1 is located in other cell lines, only the retinal pigment epithelium, the brain, and the spinal cord have been identified to actually express the Best1 protein.

Function

Best1 primarily functions as an intracellular calcium-activated chloride channel on the cellular membrane that is not voltage dependent. Another function of Best1 is to control calcium ion release from the endoplasmic reticulum. By using chloride ions as counterions, Best1 is able to couple the movement of calcium to outside the cell with the movement of chloride ions. Therefore, the Best1 protein is also able to act as a regulator of calcium release and is an important component of intracellular calcium signaling within the human eye. Research has also shown that Best1 is permeable to select neurotransmitters such as GABA and glutamate. Thus, Best1 may be essential in synaptic inhibition of the central nervous system and for the release of glutamate by astrocytes.

Diseases

Best’s vitelliform macular dystrophy (BVMD)

Lipofuscin in a neuron

Best’s vitelliform macular dystrophy (BVMD) is one of the most common Best1-associated diseases. BVMD typically becomes noticeable in children and is represented by the buildup of lipofuscin (lipid residuals) lesions in the eye. Diagnosis normally follows an abnormal electrooculogram in which decreased activation of calcium channels in the basolateral membrane of the retinal pigment epithelium becomes apparent. A mutation in the BEST1 gene leads to a loss of channel function and eventually retinal degeneration. Although BVMD is an autosomal dominant form of macular dystrophy, expressivity varies within and between affected families although the overwhelming majority of affected families come from northern European descent. Typically, patients experience five progressively worsening stages, though timing and severity varies greatly. BVMD is often caused by the single missense mutations; however, amino acid deletions have also been identified. A loss of function of the Best1 chloride channel could likely explain some of the most common issues associated with BVMD: an inability to regulate intracellular ion concentrations and regulate overall cell volume. To date, over 100 disease-causing mutations have been related to BVMD as well as a number of other degenerative retinal diseases.

Adult-onset vitelliform macular dystrophy (AVMD)

Adult-onset vitelliform macular dystrophy (AVMD) consists of lesions similar to BVMD on the retina; however, the cause is not as definitive as BVMD. The inability to diagnosis AVMD via genetic testing makes differentiating between AVMD and pattern dystrophy difficult. It is also unknown whether there is truly a clinical difference between AVMD caused by BEST1 mutations and AVMD caused by PRPH2 mutations. AVMD usually involves less vision loss than BVMD and cases do not usually run in families.

Autosomal recessive bestrophinopathy (ARB)

Autosomal recessive bestrophinopathy (ARB) was first identified in 2008. Patients demonstrate a decrease in vision during the first ten years of life. Parents and family members typically show no abnormalities as the disease is autosomal recessive, indicating that both alleles of the BEST1 gene must be mutated. Vitelliform lesions are often present and some cases involve cystoid macular edema. In addition, other complications have been observed. Vision decreases slowly over time, although rates of decline vary among patients. Mutations causing ARB range from missense mutations to single base mutations in non-coding regions.

Autosomal dominant vitreoretinochoroidopathy

Autosomal dominant vitreoretinochoroidpathy was first identified in 1982 and presents itself in both eyes with decreases in peripheral vision due to excessive fluid and changes in eye retinal pigmentation. Early onset cataracts are also likely.

Retinitis pigmentosa (RP)

Fundus of patient with retinitis pigmentosa, mid stage

Retinitis pigmentosa was first described in 2009 and is thought to be a result of one of four different missense mutations in the BEST1 gene in patients of unrelated families. All affected individuals experience a diminished response to light within their retina and may have changes in pigmentation, pale optic discs, fluid accumulation and decreased visual acuity.

There are no known treatments or therapies for patients diagnosed with any of the above bestrophinopathies. However, researchers at the University of Arizona, the University of California - Santa Cruz, and University of Cambridge are exploring potential treatment options such as stem cell transplants of the retinal pigment epithelium.