Heterologous Expression of Aedes aegypti Cation Chloride Cotransporter 2 (aeCCC2) in Xenopus laevis Oocytes Induces an Enigmatic Na(+)/Li(+) Conductance

The yellow fever mosquito Aedes aegypti possesses three genes encoding putative Na(+)-coupled cation chloride cotransporters (CCCs): aeNKCC1, aeCCC2, and aeCCC3. To date, none of the aeCCCs have been functionally characterized. Here we expressed aeCCC2 heterologously in Xenopus oocytes and measured the uptake of Li(+) (a tracer for Na(+)) and Rb(+) (a tracer for K(+)). Compared to control (H(2)O-injected) oocytes, the aeCCC2-expressing oocytes exhibited significantly greater uptake of Li(+), but not Rb(+). However, the uptake of Li(+) was neither Cl(−)-dependent nor inhibited by thiazide, loop diuretics, or amiloride, suggesting unconventional CCC activity. To determine if the Li(+)-uptake was mediated by a conductive pathway, we performed two-electrode voltage clamping (TEVC) on the oocytes. The aeCCC2 oocytes were characterized by an enhanced conductance for Li(+) and Na(+), but not K(+), compared to control oocytes. It remains to be determined whether aeCCC2 directly mediates the Na(+)/Li(+) conductance or whether heterologous expression of aeCCC2 stimulates an endogenous cation channel in the oocyte plasma membrane.