Abstract
Background and aims Human colonoid cultures maintained under low-calcium (0.25 mM) conditions undergo differentiation spontaneously and, concomitantly, express a high level of tight junction proteins, but not desmosomal proteins. When calcium is included to a final concentration of 1.5 – 3.0 mM (provided either as a single agent or as a combination of calcium and minerals), there is little change in tight junction proteins but a strong up-regulation of desmosomal proteins and an increase in desmosome formation. The aim of this study was to assess functional consequences of the differences in calcium-mediated barrier protein expression.
Methods Human colonoid-derived epithelial monolayers were interrogated in transwell culture under low- or high-calcium conditions. Ion permeability and monolayer integrity were assessed by measuring trans-epithelial electrical resistance (TEER) across the confluent monolayer. Colonoid cohesiveness was assessed in parallel.
Results TEER values were high in the low-calcium environment and increased only modestly in response to calcium. In contrast, colonoid cohesiveness increased substantially with calcium supplementation. In both assays, the response to Aquamin was greater than the response to calcium alone. However, differences between interventions were small and only compared to the 0.25 mM calcium were they statistically significant. Consistent with these findings, occludin expression (a measure of tight junctions) was high at 0.25 mM calcium and did not increase with supplementation. Cadherin-17 and desmoglein-2 were weakly-expressed under low calcium conditions but increased with intervention.
Conclusions These findings indicate that low ambient calcium levels are sufficient to support formation of a permeability barrier in the colonic epithelium. Higher calcium levels are necessary to promote tissue cohesion and enhance barrier function. These findings may help explain how an adequate daily intake of calcium contributes to colonic health by improving barrier function, even though there is little change in colonic histological features over a wide range of calcium intake levels.
