The role of bestrophin has been difficult to fully explain until a recent review by Strauss and Rosenthal.
24 Bestrophin is now seen as a regulator of intracellular calcium stores rather than a CaCC.
25 Earlier findings in rat models where mutant bestrophin was overexpressed did not reduce the light-rise as expected and led to doubts over bestrophin being the generator of the light rise.
26 Further confusion arose with increases in the amplitude of the light-rise demonstrated in mouse models of Best's disease.
27 A calcium-dependent chloride channel has been demonstrated in cultured canine RPE cells
28 and is also expressed in chick RPE.
29 The CaCC TMEM16A (ANO1) of the anoctamin family is widely expressed in epithelia where they regulate cell volume, apoptosis, and proliferation.
30 Knockout mice demonstrate decreased chloride secretion in in multiple secretory epithelia
31 and TMEM16A is expressed in mouse and human ocular epithelia.
32 Best1 and TMEM16A function as a micro domain in renal and lung epithelia and it is plausible that TMEM16A is the CaCC in the RPE that regulates cell volume, while bestrophin regulates intracellular calcium stores. The main recent findings about the nature of bestrophin-1 are that the protein is not expressed in the basolateral membrane as previously thought,
20 but is associated with the endoplasmic reticulum where it regulates store-operated calcium entry.
33 The key recent findings are that bestrophin-1 colocalizes with Stim-1, a protein found in the endoplasmic reticulum and whose role is to sense the levels of calcium stores. When stores are low, Stim-1 may increase cytoplasmic concentrations of calcium for re-uptake into the endoplasmic reticulum through plasma membrane calcium channels, such as Orai,
34,35 via a physical interaction.
36 The finding that bestrophin-1 colocalizes with Stim-1 and regulates store-operated calcium entry, provides an elegant resolution to the confusion surrounding the role of bestrophin-1 in the RPE, and the findings of normal light-rises in some individuals with Best's disease. Gomez et al.
33 were able to demonstrate that in the RPE bestrophin-1 regulates the majority of calcium entry to the cytosol following depletion of endoplasmic reticulum stores. The increase in intracellular calcium is through a direct interaction between the C-terminus of the L-type calcium channels in the plasma membranes and bestrophin-1 in the endoplasmic reticulum. In addition bestrophin-1 acts as a chloride channel by conducting chloride ions as the counter-ion into the endoplasmic reticulum to facilitate the re-uptake of calcium through the endoplasmic reticulum Ca-ATPase pump. The RPE cells also express the Stim-1/Orai channels that contribute less to the overall replenishment of cytosolic calcium following depletion of the stores.
30,31