Sodium Transporters Are Involved in Lithium Influx in Brain Endothelial Cells

Variability in drug response to lithium (Li⁺) is poorly understood and significant, as only 40% of patients with bipolar disorder highly respond to Li⁺. Li⁺ can be transported by sodium (Na⁺) transporters in kidney tubules or red blood cells, but its transport has not been investigated at the blood-brain barrier (BBB). Inhibition and/or transcriptomic strategies for Na⁺ transporters such as NHE (SLC9), NBC (SLC4), and NKCC (SLC12) were used to assess their role on Li⁺ transport in human brain endothelial cells. Na⁺-free buffer was also used to examine Na⁺/Li⁺ countertransport (NLCT) activity. The BBB permeability of Li⁺ evaluated in the rat was 2% that of diazepam, a high passive diffusion lipophilic compound. Gene expression of several Na⁺ transporters was determined in hCMEC/D3 cells, human hematopoietic stem-cell-derived BBB models (HBLEC), and human primary brain microvascular endothelial cells (hPBMECs) and showed the following rank order with close expression profile: NHE1 > NKCC1 > NHE5 > NBCn1, while NHE2-4, NBCn2, and NBCe1-2 were barely detected. Li⁺ influx in hCMEC/D3 cells was increased in Na⁺-free buffer by 3.3-fold, while depletion of chloride or bicarbonate had no effect. DMA (NHE inhibitor), DIDS (anionic carriers inhibitor), and bumetanide (NKCC inhibitor) decreased Li⁺ uptake significantly in hCMEC/D3 by 52, 51, and 47%, respectively, while S0859 (NBC inhibitor) increased Li⁺ influx 2.3-fold. Zoniporide (NHE1 inhibitor) and siRNA against NHE1 had no effect on Li⁺ influx in hCMEC/D3 cells. Our study shows that NHE1 and/or NHE5, NBCn1, and NKCC1 may play a significant role in the transport of Li⁺ through the plasma membrane of brain endothelial cells.