Involvement of monocarboxylate transporter 7 in taurine efflux transport from rat retinal pericytes and capillary endothelial cells.

PURPOSE: Taurine exists abundantly in the retina and plays a vital role in retinal function. Monocarboxylate transporter 7 (MCT7) is found as a facilitative taurine transporter; however, its involvement in taurine dynamics in the retina is not yet fully understood. The purpose of the present study is to clarify the protein expression and function of MCT7 in retinal cells.

METHODS

Guinea pig antibodies were raised against the amino acid residues of rat MCT7, and immunostaining was performed to clarify protein expression in the rat retina. To characterize taurine influx into retinal pericytes, a [3H]taurine transport assay was conducted using TR-rPCT1 cells, immortalized rat retinal pericytes. A knockdown assay using MCT7 small interfering RNA (siRNA) was performed to examine the involvement of MCT7 in taurine efflux in TR-rPCT1 and immortalized rat retinal capillary endothelial (TR-iBRB2) cells.

RESULTS

The MCT7 protein expression was observed throughout the retinal layer. Immunostaining of isolated retinal capillaries revealed MCT7 expression in retinal pericytes and capillary endothelial cells. [3H]Taurine influx transport in TR-rPCT1 cells depends on temperature, concentration (Km = 11.2 μM), and extracellular Na+ and Cl-, and was inhibited by substrates for taurine transporter (TauT), suggesting the involvement of TauT in taurine influx in retinal pericytes. Moreover, MCT7 siRNA decreased MCT7 expression and [3H]taurine efflux in TR-rPCT1 and TR-iBRB2 cells, suggesting that the taurine efflux transport involves MCT7 at least partly.

CONCLUSIONS

The present study revealed that MCT7 functions as a taurine efflux transporter in both retinal pericytes and capillary endothelial cells.