Khaled Zemoura

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

Background: The amount of cell surface GABAB receptors determines the strength of GABAB-mediated inhibition of neuronal excitability. Results: GABAB receptors are Lys48-linked polyubiquitinated and degraded by proteasomes via ERAD. Conclusion: ERAD constitutively degrades GABAB 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 GABAB receptors. Metabotropic GABAB 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 GABAB receptor trafficking are studied to some extent, it is currently unclear whether receptor degradation actively controls the number of GABAB receptors available for signaling. Here we tested the hypothesis that proteasomal degradation contributes to the regulation of GABAB receptor expression levels. Blocking proteasomal activity in cultured cortical neurons considerably enhanced total and cell surface expression of GABAB receptors, indicating the constitutive degradation of the receptors by proteasomes. Proteasomal degradation required Lys48-linked polyubiquitination of lysines 767/771 in the C-terminal domain of the GABAB2 subunit. Inactivation of these ubiquitination sites increased receptor levels and GABAB 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 GABAB receptors with the essential ERAD components Hrd1 and p97. In conclusion, the data support a model in which the fraction of GABAB 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.

Modulation of cell surface GABA(B) receptors by desensitization, trafficking and regulated degradation.

Inhibitory neurotransmission ensures normal brain function by counteracting and integrating excitatory activity. γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mammalian central nervous system, and mediates its effects via two classes of receptors: the GABA(A) and GABA(B) receptors. GABA(A) receptors are heteropentameric GABA-gated chloride channels and responsible for fast inhibitory neurotransmission. GABA(B) receptors are heterodimeric G protein coupled receptors (GPCR) that mediate slow and prolonged inhibitory transmission. The extent of inhibitory neurotransmission is determined by a variety of factors, such as the degree of transmitter release and changes in receptor activity by posttranslational modifications (e.g., phosphorylation), as well as by the number of receptors present in the plasma membrane available for signal transduction. The level of GABA(B) receptors at the cell surface critically depends on the residence time at the cell surface and finally the rates of endocytosis and degradation. In this review we focus primarily on recent advances in the understanding of trafficking mechanisms that determine the expression level of GABA(B) receptors in the plasma membrane, and thereby signaling strength.

Ischemia-like Oxygen and Glucose Deprivation Mediates Down-regulation of Cell Surface γ-Aminobutyric AcidB Receptors via the Endoplasmic Reticulum (ER) Stress-induced Transcription Factor CCAAT/Enhancer-binding Protein (C/EBP)-homologous Protein (CHOP)

Background: ER stress associated with cerebral ischemia induces the expression of the transcription factor CHOP. Results: Interaction with CHOP down-regulates cell surface GABAB receptors and, thus, GABAB receptor-mediated neuronal inhibition. Conclusion: Interaction of CHOP with GABAB receptors in the ER prevents forward trafficking of the receptors. Significance: This mechanism is expected to contribute to excitotoxicity in cerebral ischemia. Cerebral ischemia frequently leads to long-term disability and death. Excitotoxicity is believed to be the main cause for ischemia-induced neuronal death. Although a role of glutamate receptors in this process has been firmly established, the contribution of metabotropic GABAB receptors, which control excitatory neurotransmission, is less clear. A prominent characteristic of ischemic insults is endoplasmic reticulum (ER) stress associated with the up-regulation of the transcription factor CCAAT/enhancer-binding protein-homologous protein (CHOP). After inducing ER stress in cultured cortical neurons by sustained Ca2+ release from intracellular stores or by a brief episode of oxygen and glucose deprivation (in vitro model of cerebral ischemia), we observed an increased expression of CHOP accompanied by a strong reduction of cell surface GABAB receptors. Our results indicate that down-regulation of cell surface GABAB receptors is caused by the interaction of the receptors with CHOP in the ER. Binding of CHOP prevented heterodimerization of the receptor subunits GABAB1 and GABAB2 and subsequent forward trafficking of the receptors to the cell surface. The reduced level of cell surface receptors diminished GABAB receptor signaling and, thus, neuronal inhibition. These findings indicate that ischemia-mediated up-regulation of CHOP down-regulates cell surface GABAB receptors by preventing their trafficking from the ER to the plasma membrane. This mechanism leads to diminished neuronal inhibition and may contribute to excitotoxicity in cerebral ischemia.

Proteasomal degradation of γ-aminobutyric acidB receptors is mediated by the interaction of the GABAB2 C terminus with the proteasomal ATPase Rtp6 and regulated by neuronal activity.

Background: The expression level of GABAB receptors is controlled by proteasomal degradation. Results: Proteasomal degradation of GABAB receptors is mediated by interaction with Rpt6 and modulated by neuronal activity. Conclusion: The level of neuronal activity regulates via proteasomal degradation the ER pool of GABAB receptor competent for forward trafficking. Significance: This mechanism might contribute to homeostatic neuronal plasticity. Regulation of cell surface expression of neurotransmitter receptors is crucial for determining synaptic strength and plasticity, but the underlying mechanisms are not well understood. We previously showed that proteasomal degradation of GABAB receptors via the endoplasmic reticulum (ER)-associated protein degradation (ERAD) machinery determines the number of cell surface GABAB receptors and thereby GABAB receptor-mediated neuronal inhibition. Here, we show that proteasomal degradation of GABAB receptors requires the interaction of the GABAB2 C terminus with the proteasomal AAA-ATPase Rpt6. A mutant of Rpt6 lacking ATPase activity prevented degradation of GABAB receptors but not the removal of Lys48-linked ubiquitin from GABAB2. Blocking ERAD activity diminished the interaction of Rtp6 with GABAB receptors resulting in increased total as well as cell surface expression of GABAB receptors. Modulating neuronal activity affected proteasomal activity and correspondingly the interaction level of Rpt6 with GABAB2. This resulted in altered cell surface expression of the receptors. Thus, neuronal activity-dependent proteasomal degradation of GABAB receptors by the ERAD machinery is a potent mechanism regulating the number of GABAB receptors available for signaling and is expected to contribute to homeostatic neuronal plasticity.

Several posttranslational modifications act in concert to regulate gephyrin scaffolding and GABAergic transmission.

GABAA receptors (GABAARs) mediate the majority of fast inhibitory neurotransmission in the brain via synergistic association with the postsynaptic scaffolding protein gephyrin and its interaction partners. However, unlike their counterparts at glutamatergic synapses, gephyrin and its binding partners lack canonical protein interaction motifs; hence, the molecular basis for gephyrin scaffolding has remained unclear. In this study, we identify and characterize two new posttranslational modifications of gephyrin, SUMOylation and acetylation. We demonstrate that crosstalk between SUMOylation, acetylation and phosphorylation pathways regulates gephyrin scaffolding. Pharmacological intervention of SUMO pathway or transgenic expression of SUMOylation-deficient gephyrin variants rescued gephyrin clustering in CA1 or neocortical neurons of Gabra2-null mice, which otherwise lack gephyrin clusters, indicating that gephyrin SUMO modification is an essential determinant for scaffolding at GABAergic synapses. Together, our results demonstrate that concerted modifications on a protein scaffold by evolutionarily conserved yet functionally diverse signalling pathways facilitate GABAergic transmission.

The positive allosteric GABAB receptor modulator rac-BHFF enhances baclofen-mediated analgesia in neuropathic mice

Neuropathic pain is associated with impaired inhibitory control of spinal dorsal horn neurons, which are involved in processing pain signals. The metabotropic GABAB receptor is an important component of the inhibitory system and is highly expressed in primary nociceptors and intrinsic dorsal horn neurons to control their excitability. Activation of GABAB receptors with the orthosteric agonist baclofen effectively reliefs neuropathic pain but is associated with severe side effects that prevent its widespread application. The recently developed positive allosteric GABAB receptor modulators lack most of these side effects and are therefore promising drugs for the treatment of pain. Here we tested the high affinity positive allosteric modulator rac-BHFF for its ability to relief neuropathic pain induced by chronic constriction of the sciatic nerve in mice. rac-BHFF significantly increased the paw withdrawal threshold to mechanical stimulation in healthy mice, indicating an endogenous GABABergic tone regulating the sensitivity to mechanical stimuli. Surprisingly, rac-BHFF displayed no analgesic activity in neuropathic mice although GABAB receptor expression was not affected in the dorsal horn as shown by quantitative receptor autoradiography. However, activation of spinal GABAB receptors by intrathecal injection of baclofen reduced hyperalgesia and its analgesic effect was considerably potentiated by co-application of rac-BHFF. These results indicate that under conditions of neuropathic pain the GABAergic tone is too low to provide a basis for allosteric modulation of GABAB receptors. However, allosteric modulators would be well suited as an add-on to reduce the dose of baclofen required to achieve analgesia.

Lys-63-linked Ubiquitination of γ-Aminobutyric Acid (GABA), Type B1, at Multiple Sites by the E3 Ligase Mind Bomb-2 Targets GABAB Receptors to Lysosomal Degradation

GABAB receptors are heterodimeric G protein-coupled receptors, which control neuronal excitability by mediating prolonged inhibition. The magnitude of GABAB receptor-mediated inhibition essentially depends on the amount of receptors in the plasma membrane. However, the factors regulating cell surface expression of GABAB receptors are poorly characterized. Cell surface GABAB receptors are constitutively internalized and either recycled to the plasma membrane or degraded in lysosomes. The signal that sorts GABAB receptors to lysosomes is currently unknown. Here we show that Mind bomb-2 (MIB2)-mediated Lys-63-linked ubiquitination of the GABAB1 subunit at multiple sites is the lysosomal sorting signal for GABAB receptors. We found that inhibition of lysosomal activity in cultured rat cortical neurons increased the fraction of Lys-63-linked ubiquitinated GABAB receptors and enhanced the expression of total as well as cell surface GABAB receptors. Mutational inactivation of four putative ubiquitination sites in the GABAB1 subunit significantly diminished Lys-63-linked ubiquitination of GABAB receptors and prevented their lysosomal degradation. We identified MIB2 as the E3 ligase triggering Lys-63-linked ubiquitination and lysosomal degradation of GABAB receptors. Finally, we show that sustained activation of glutamate receptors, a condition occurring in brain ischemia that down-regulates GABAB receptors, considerably increased the expression of MIB2 and Lys-63-linked ubiquitination of GABAB receptors. Interfering with Lys-63-linked ubiquitination by overexpressing ubiquitin mutants or GABAB1 mutants deficient in Lys-63-linked ubiquitination prevented glutamate-induced down-regulation of the receptors. These findings indicate that Lys-63-linked ubiquitination of GABAB1 at multiple sites by MIB2 controls sorting of GABAB receptors to lysosomes for degradation under physiological and pathological conditions.

Regulation of Cell Surface GABAB Receptors: Contribution to Synaptic Plasticity in Neurological Diseases

γ-Amino butyric acid (GABA(B)) receptors are heterodimeric G protein-coupled receptors expressed throughout the central nervous system in virtually all neurons. They control the excitability of neurons via activation of different downstream effector systems in pre- and postsynaptic neurons and as such regulate all major brain functions including synaptic plasticity, neuronal network activity, and neuronal development. Accordingly, GABA(B) receptors have been implicated in a variety of neurological disorders and thus are regarded as promising drug targets. A key factor determining the extent of GABA(B) receptor-mediated inhibition is the level of receptors at the cell surface available for signaling. There is increasing evidence that cell surface expression of functional GABA(B) receptors is affected in neurological diseases. This diminishes inhibitory control of neuronal excitation and may contribute to the disease state. Here, we discuss recent findings on mechanisms involved in regulating cell surface expression of GABA(B) receptors in addiction, neuropathic pain, and brain ischemia.

Chapter Three – Regulation of Cell Surface GABAB Receptors: Contribution to Synaptic Plasticity in Neurological Diseases

γ-Amino butyric acid (GABA(B)) receptors are heterodimeric G protein-coupled receptors expressed throughout the central nervous system in virtually all neurons. They control the excitability of neurons via activation of different downstream effector systems in pre- and postsynaptic neurons and as such regulate all major brain functions including synaptic plasticity, neuronal network activity, and neuronal development. Accordingly, GABA(B) receptors have been implicated in a variety of neurological disorders and thus are regarded as promising drug targets. A key factor determining the extent of GABA(B) receptor-mediated inhibition is the level of receptors at the cell surface available for signaling. There is increasing evidence that cell surface expression of functional GABA(B) receptors is affected in neurological diseases. This diminishes inhibitory control of neuronal excitation and may contribute to the disease state. Here, we discuss recent findings on mechanisms involved in regulating cell surface expression of GABA(B) receptors in addiction, neuropathic pain, and brain ischemia.

Ca2+/Calmodulin-Dependent Protein Kinase II (CaMKII) β-Dependent Phosphorylation of GABAB1 Triggers Lysosomal Degradation of GABAB Receptors via Mind Bomb-2 (MIB2)-Mediated Lys-63-Linked Ubiquitination

The G protein-coupled GABAB receptors, constituted from GABAB1 and GABAB2 subunits, are important regulators of neuronal excitability by mediating long-lasting inhibition. One factor that determines receptor availability and thereby the strength of inhibition is regulated protein degradation. GABAB receptors are constitutively internalized from the plasma membrane and are either recycled to the cell surface or degraded in lysosomes. Lys-63-linked ubiquitination mediated by the E3 ligase Mind bomb-2 (MIB2) is the signal that sorts GABAB receptors to lysosomes. However, it is unknown how Lys-63-linked ubiquitination and thereby lysosomal degradation of the receptors is regulated. Here, we show that Ca2+/calmodulin-dependent protein kinase II (CaMKII) promotes MIB2-mediated Lys-63-linked ubiquitination of GABAB receptors. We found that inhibition of CaMKII in cultured rat cortical neurons increased cell surface GABAB receptors, whereas overexpression of CaMKIIβ, but not CaMKIIα, decreased receptor levels. This effect was conveyed by Lys-63-linked ubiquitination of GABAB1 at multiple sites mediated by the E3 ligase MIB2. Inactivation of the CaMKII phosphorylation site on GABAB1(Ser-867) strongly reduced Lys-63-linked ubiquitination of GABAB receptors and increased their cell surface expression, whereas the phosphomimetic mutant GABAB1(S867D) exhibited strongly increased Lys-63-linked ubiquitination and reduced cell surface expression. Finally, triggering lysosomal degradation of GABAB receptors by sustained activation of glutamate receptors, a condition occurring in brain ischemia, was accompanied with a massive increase of GABAB1(Ser-867) phosphorylation-dependent Lys-63-linked ubiquitination of GABAB receptors. These findings indicate that CaMKIIβ-dependent Lys-63-linked ubiquitination of GABAB1 at multiple sites controls sorting of GABAB receptors to lysosomes for degradation under physiological and pathological condition.