Triiodothyronine modulates LAT2 vesicle dynamics and potentiates β-adrenergic cAMP responses in astrocytes

Thyroid hormones (THs), triiodothyronine (T3) and thyroxine (T4), are essential for normal central nervous system development and function. In the brain, THs are taken up by neurons and glial cells, particularly astrocytes, through plasma membrane transporters, including the L-type amino acid transporter 2 (LAT2). Although elevated T3 levels in hyperthyroidism are associated with neurological symptoms, it remains unknown whether elevated T3 alters astrocytic LAT2 trafficking and surface availability, and whether such changes are accompanied by altered astrocyte signalling. Using primary astrocyte cultures and brain slice preparations, we examined how prolonged exposure to elevated T3 affects LAT2 localisation, vesicular dynamics, and noradrenaline (NA)-dependent signalling. Chronic T3 exposure reduced total and plasma membrane-associated LAT2, accompanied by decreased localisation to early endosomes but not lysosomes, indicating altered intracellular trafficking rather than lysosomal accumulation. In parallel, LAT2-containing vesicles were reduced in number by half but more mobile, and lysosomal vesicles also exhibited increased mobility. Given the known interactions between THs and the noradrenergic system, we next examined whether prolonged T3 exposure alters astrocytic responses to NA. We found that prolonged T3 exposure selectively potentiated NA-induced cyclic AMP (cAMP) signalling with only minor effects on Ca²⁺ response kinetics and no change in response amplitude. Together, these findings show that chronically elevated extracellular T3 reduces astrocytic LAT2 surface abundance, remodels astrocytic LAT2 trafficking and enhances noradrenergic cAMP signalling. While the reduced total LAT2 and lack of lysosomal accumulation suggest enhanced non-lysosomal degradation, the precise pathway, potentially involving the proteasome, remains to be established. These results identify astrocytes as potential mediators of TH-dependent changes in brain function and thus provide a cellular framework for understanding neurological manifestations of hyperthyroidism.