γ-aminobutyric acid type B receptors are constitutively internalized via the clathrin-dependent pathway and targeted to lysosomes for degradation

Receptor internalization is recognized as an important mechanism for rapidly regulating cell surface numbers of receptors. However, there are conflicting results on the existence of rapid endocytosis of gamma- aminobutyric acid, type B ( GABAB) receptors. Therefore, we analyzed internalization of GABAB receptors expressed in HEK 293 cells qualitatively and quantitatively using immunocytochemical, cell surface enzyme- linked immunosorbent assay, and biotinylation methods. The data indicate the existence of rapid constitutive receptor internalization, with the first endocytosed receptors being observed in proximity of the plasma membrane after 10 min. After 120 min, a loss of about 40 - 50% of cell surface receptors was detected. Stimulation of GABA(B) receptors with GABA or baclofen did not enhance endocytosis of receptors, indicating the lack of agonist-induced internalization. The data suggest that GABA(B) receptors were endocytosed via the classical dynamin- and clathrin- dependent pathway and accumulated in an endosomal sorting compartment before being targeted to lysosomes for degradation. No evidence for recycling of receptors back to the cell surface was found. In conclusion, the results indicate the presence of constitutive internalization of GABAB receptors via clathrin-coated pits, which resulted in lysosomal degradation of the receptors.