Endoplasmic Reticulum-associated Degradation Controls Cell Surface Expression of γ-Aminobutyric Acid, Type B Receptors

Background: The amount of cell surface GABA(B) receptors determines the strength of GABA(B)-mediated inhibition of neuronal excitability. Results: GABA(B) receptors are Lys(48)-linked polyubiquitinated and degraded by proteasomes via ERAD. Conclusion: ERAD constitutively degrades GABA(B) receptors and thereby determines the number of functional receptors available for signaling. Significance: Modulation of ERAD activity may be a mechanism to adjust the level of functional GABA(B) receptors. Metabotropic GABA(B) receptors are crucial for controlling the excitability of neurons by mediating slow inhibition in the CNS. The strength of receptor signaling depends on the number of cell surface receptors, which is thought to be regulated by trafficking and degradation mechanisms. Although the mechanisms of GABA(B) receptor trafficking are studied to some extent, it is currently unclear whether receptor degradation actively controls the number of GABA(B) receptors available for signaling. Here we tested the hypothesis that proteasomal degradation contributes to the regulation of GABA(B) receptor expression levels. Blocking proteasomal activity in cultured cortical neurons considerably enhanced total and cell surface expression of GABA(B) receptors, indicating the constitutive degradation of the receptors by proteasomes. Proteasomal degradation required Lys(48)-linked polyubiquitination of lysines 767/771 in the C-terminal domain of the GABA(B2) subunit. Inactivation of these ubiquitination sites increased receptor levels and GABA(B) receptor signaling in neurons. Proteasomal degradation was mediated by endoplasmic reticulum-associated degradation (ERAD) as shown by the accumulation of receptors in the endoplasmic reticulum upon inhibition of proteasomes, by the increase of receptor levels, as well as receptor signaling upon blocking ERAD function, and by the interaction of GABA(B) receptors with the essential ERAD components Hrd1 and p97. In conclusion, the data support a model in which the fraction of GABA(B) receptors available for plasma membrane trafficking is regulated by degradation via the ERAD machinery. Thus, modulation of ERAD activity by changes in physiological conditions may represent a mechanism to adjust receptor numbers and thereby signaling strength.