LAMP, a new imaging assay of gap junctional communication unveils that Ca²⁺ influx inhibits cell coupling

Using a new class of photo-activatible fluorophores, we have developed a new imaging technique for measuring molecular transfer rates across gap junction connexin channels in intact living cells. This technique, named LAMP, involves local activation of a molecular fluorescent probe, NPE-HCCC2/AM, to optically label a cell. Subsequent dye transfer through gap junctions from labeled to unlabeled cells was quantified by fluorescence microscopy. Additional uncagings after prior dye transfers reached equilibrium enabled multiple measurements of dye transfer rates in the same coupled cell pair. Measurements in the same cell pair minimized variation due to differences in cell volume and number of gap junctions, allowing us to track acute changes in gap junction permeability. We applied the technique to study the regulation of gap junction coupling by intracellular Ca²⁺ ([Ca²⁺]_i). Although agonist or ionomycin exposure can raise bulk [Ca²⁺]_i to levels higher than those caused by capacitative Ca²⁺ influx, the LAMP assay revealed that only Ca²⁺ influx through the plasma membrane store-operated Ca²⁺ channels strongly reduced gap junction coupling. The noninvasive and quantitative nature of this imaging technique should facilitate future investigations of the dynamic regulation of gap junction communication.