Prolonged activation of NMDA receptors promotes dephosphorylation and alters postendocytic sorting of GABA _B receptors

Abstract

Slow and persistent synaptic inhibition is mediated by metabotropic GABA B receptors (GABA B Rs). GABA B Rs are responsible for the modulation of neurotransmitter release from presynaptic terminals and for hyperpolarization at postsynaptic sites. Postsynaptic GABA B Rs are predominantly found on dendritic spines, adjacent to excitatory synapses, but the control of their plasma membrane availability is still controversial. Here, we explore the role of glutamate receptor activation in regulating the function and surface availability of GABA B Rs in central neurons. We demonstrate that prolonged activation of NMDA receptors (NMDA-Rs) leads to endocytosis, a diversion from a recycling route, and subsequent lysosomal degradation of GABA B Rs. These sorting events are paralleled by a reduction in GABA B R-dependent activation of inwardly rectifying K + channel currents. Postendocytic sorting is critically dependent on phosphorylation of serine 783 (S783) within the GABA B R2 subunit, an established substrate of AMP-dependent protein kinase (AMPK). NMDA-R activation leads to a rapid increase in phosphorylation of S783, followed by a slower dephosphorylation, which results from the activity of AMPK and protein phosphatase 2A, respectively. Agonist activation of GABA B Rs counters the effects of NMDA. Thus, NMDA-R activation alters the phosphorylation state of S783 and acts as a molecular switch to decrease the abundance of GABA B Rs at the neuronal plasma membrane. Such a mechanism may be of significance during synaptic plasticity or pathological conditions, such as ischemia or epilepsy, which lead to prolonged activation of glutamate receptors.he availability of neurotransmitter receptors, a major determinant of synaptic efficacy, is regulated by coordinated mechanisms of intracellular trafficking that deliver newly synthesized receptors to the plasma membrane and remove them for storage, recycling, or degradation (1). The molecular mechanisms controlling the availability of GABA B receptors (GABA B Rs), which are central players in the modulation of excitatory and inhibitory synaptic activity, are unclear.GABA B Rs mediate slow and prolonged inhibitory synaptic signals (2, 3). Consistent with these roles, modifications in the function of GABA B Rs are implicated in epilepsy, anxiety, stress, sleep disorders, nociception, depression, cognition, and addictive mechanisms to drugs of abuse (3-7). GABA B Rs are members of the G protein-coupled receptor (GPCR) superfamily and are obligatory heteromers composed of two related subunits, namely GABA B R1 and GABA B R2 (3, 8). GABA B R1 binds agonist with high affinity, whereas GABA B R2 mediates coupling to Gαi (9, 10). GABA B Rs are located in GABA-ergic and glutamatergic pre-and postsynaptic terminals, but their distribution does not coincide with the active zone, postsynaptic density, or inhibitory postsynaptic specializations. Rather, they are perisynaptic receptors activated by GABA spillover (3, 11). Stimulation of GABA B Rs decreases the levels of cAMP, inhibits neurotr...

-Aminobutyric Acid Type B (GABAB) Receptor Internalization Is Regulated by the R2 Subunit

“…2C). Therefore, after being internalized, GABA B receptors are recruited to early endosomes, from where they are either recycled via recycling endosomes to be reinserted into the plasma membrane or are degraded in the lysosomes via the late endosomes, consistent with previous findings (14,15,27,28). The process of recycling would account for the plateau phase in the time profiles of surface fluorescence (Fig.…”

Activation of AMP-activated Protein Kinase Regulates Hippocampal Neuronal pH by Recruiting Na+/H+ Exchanger NHE5 to the Cell Surface

“…How activation of NMDA receptors stimulated this translocation process was not examined. However, it is tempting to speculate that this may involve activity-dependent activation of AMPK, as has recently been described in hippocampal neurons (94). Activity-driven increases in intracellular Ca 2ϩ can activate AMPK indirectly by stimulating its upstream regulator CaMKK␤, which is highly abundant in brain (95).…”

Heterodimeric coiled-coil interactions of human GABA B receptor

“…As a result, the coiled-coil interactions between GBR1b and GBR2 directly mask the di-leucine motif of GBR1b. Second, the phosphorylation state of GABA B receptor affects its cell surface stability (29)(30)(31). Specifically, dephosphorylation of Ser784′ in GBR2 is associated with decreased expression of the receptor at the neuronal plasma membrane (31).…”