Hyperammonemia induces transport of taurine and creatine and suppresses claudin-12 gene expression in brain capillary endothelial cells in vitro

Ammonia is a key neurotoxin involved in the neurological complications of acute liver failure. The present study was undertaken to study the effects of exposure to pathophysiologically relevant concentrations of ammonium chloride on cultured brain capillary endothelial cells in order to identify mechanisms by which ammonia may alter blood-brain barrier function. Conditionally immortalized mouse brain capillary endothelial cells (TM-BBB) were used as an in vitro model of the blood-brain barrier. Gene expression of a series of blood-brain barrier transporters and tight junction proteins was assessed by quantitative real time PCR analysis. Exposure to ammonia (5 mM for 72 h) resulted in significant increases in mRNA levels of taurine transporter (TAUT; 2.0-fold increase) as well as creatine transporter (CRT; 1.9-fold increase) whereas claudin-12 mRNA expression was significantly reduced to 67.7% of control levels. Furthermore, [³H]taurine and [¹⁴C]creatine uptake were concomitantly increased following exposure to ammonia, suggesting that up-regulation of both TAUT and CRT under hyperammonemic conditions results in an increased function of these two transporters in TM-BBB cells. TAUT and CRT are respectively involved in osmoregulation and energy buffering in the brain, two systems that are thought to be affected in acute liver failure. Furthermore, claudin-12 down-regulation suggests that hyperammonemia may also affect tight junction integrity. Our results provide evidence that ammonia can alter brain capillary endothelial cell gene expression and transporter function. These findings may be relevant to pathological situations involving hyperammonemia, such as liver disease.