Bestrophin-2 mediates bicarbonate transport by goblet cells in mouse colon

Abstract

Anion transport by the colonic mucosa maintains the hydration and pH of the colonic lumen, and its disruption causes a variety of diarrheal diseases. Cholinergic agonists raise cytosolic Ca 2+ levels and stimulate anion secretion, but the mechanisms underlying this effect remain unclear. Cholinergic stimulation of anion secretion may occur via activation of Ca 2+ -activated Cl -channels (CaCCs) or an increase in the Cl -driving force through CFTR after activation of Ca 2+ -dependent K + channels. Here we investigated the role of a candidate CaCC protein, bestrophin-2 (Best2), using Best2 -/-mice. Cholinergic stimulation of anion current was greatly reduced in Best2 -/-mice, consistent with our proposed role for Best2 as a CaCC. However, immunostaining revealed Best2 localized to the basolateral membrane of mucin-secreting colonic goblet cells, not the apical membrane of Cl --secreting enterocytes. In addition, in the absence of HCO 3 -, cholinergic-activated current was identical in control and Best2 -/-tissue preparations, which suggests that most of the Best2 current was carried by HCO 3 -. These data delineate an alternative model of cholinergic regulation of colonic anion secretion in which goblet cells play a critical role in HCO 3 -homeostasis. We therefore propose that Best2 is a HCO 3 -channel that works in concert with a Cl:HCO 3 -exchanger in the apical membrane to affect transcellular HCO 3 -transport. Furthermore, previous models implicating CFTR in cholinergic Cl -secretion may be explained by substantial downregulation of Best2 in Cftr -/-mice.

Role of epithelial cells in the pathogenesis and treatment of inflammatory bowel disease

“…Recent studies have shown that a newly identified Clchannel, bestrophin2 (BEST2), is expressed specifically by colonic goblet cells [46,47]. Mice lacking BEST2 develops spontaneous colitis, and are more susceptible to DSS challenge, suggesting that this channel may play a distinct role to maintain goblet cell-dependent barrier functions [46]. We have further identified that BEST2 expression is down-regulated in the inflamed colonic mucosa of UC patients [47].…”

“…Besides mucin production, goblet cells secretes cytokines such as IL-7 [44] and thus may exert immunoregulatory functions, and can also deliver luminal antigens to the underlying DCs [45]. Recent studies have shown that a newly identified Clchannel, bestrophin2 (BEST2), is expressed specifically by colonic goblet cells [46,47]. Mice lacking BEST2 develops spontaneous colitis, and are more susceptible to DSS challenge, suggesting that this channel may play a distinct role to maintain goblet cell-dependent barrier functions [46].…”

Regulation of Anterior Chamber Drainage by Bicarbonate-sensitive Soluble Adenylyl Cyclase in the Ciliary Body

“…Topical application of brinzolamide led to a greater decrease in IOP than was observed in Best2 ϩ/ϩ mice, suggesting a role for bicarbonate in these effects. Indeed, we have since shown that Best2 functions as a critical bicarbonate channel in colon goblet cells (10), and it is likely that it functions similarly in NPE cells. This led us to examine the role of sAC in regulation of IOP.…”

“…Recently, we demonstrated that bestrophin 2 (Best2) functions as a HCO 3 Ϫ channel in colon goblet cells (10). In the eye, Best2 is uniquely localized to the basal membrane of NPE cells (11,12).…”

Stoichiometry and specific assembly of Best ion channels

“…Also, the possibility of bestrophins forming heteromeric complexes in native tissues is limited due to their restricted distribution. The hBest1 mainly expresses in retina and RPE cells (1,11,44), whereas hBest2 and hBest4 express mostly in different cell types in colon (goblet cells and small intestine, respectively) (45,46).…”