Astrocytic IP3 Rs: Contribution to Ca2+ signalling and hippocampal LTP.

Astrocytes regulate hippocampal synaptic plasticity by the Ca2+ dependent release of the N-methyl d-aspartate receptor (NMDAR) co-agonist d-serine. Previous evidence indicated that d-serine release would be regulated by the intracellular Ca2+ release channel IP3 receptor (IP3 R), however, genetic deletion of IP3 R2, the putative astrocytic IP3 R subtype, had no impact on synaptic plasticity or transmission. Although IP3 R2 is widely believed to be the only functional IP3 R in astrocytes, three IP3 R subtypes (1, 2, and 3) have been identified in vertebrates. Therefore, to better understand gliotransmission, we investigated the functionality of IP3 R and the contribution of the three IP3 R subtypes to Ca2+ signalling. As a proxy for gliotransmission, we found that long-term potentiation (LTP) was impaired by dialyzing astrocytes with the broad IP3 R blocker heparin, and rescued by exogenous d-serine, indicating that astrocytic IP3 Rs regulate d-serine release. To explore which IP3 R subtypes are functional in astrocytes, we used pharmacology and two-photon Ca2+ imaging of hippocampal slices from transgenic mice (IP3 R2-/- and IP3 R2-/- ;3-/- ). This approach revealed that underneath IP3 R2-mediated global Ca2+ events are an overlooked class of IP3 R-mediated local events, occurring in astroglial processes. Notably, multiple IP3 Rs were recruited by high frequency stimulation of the Schaffer collaterals, a classical LTP induction protocol. Together, these findings show the dependence of LTP and gliotransmission on Ca2+ release by astrocytic IP3 Rs. GLIA 2017;65:502-513.

Co-agonists differentially tune GluN2B-NMDA receptor trafficking at hippocampal synapses.

The subunit composition of synaptic NMDA receptors (NMDAR), such as the relative content of GluN2A- and GluN2B-containing receptors, greatly influences the glutamate synaptic transmission. Receptor co-agonists, glycine and D-serine, have intriguingly emerged as potential regulators of the receptor trafficking in addition to their requirement for its activation. Using a combination of single-molecule imaging, biochemistry and electrophysiology, we show that glycine and D-serine relative availability at rat hippocampal glutamatergic synapses regulate the trafficking and synaptic content of NMDAR subtypes. Acute manipulations of co-agonist levels, both ex vivo and in vitro, unveil that D-serine alter the membrane dynamics and content of GluN2B-NMDAR, but not GluN2A-NMDAR, at synapses through a process requiring PDZ binding scaffold partners. In addition, using FRET-based FLIM approach, we demonstrate that D-serine rapidly induces a conformational change of the GluN1 subunit intracellular C-terminus domain. Together our data fuels the view that the extracellular microenvironment regulates synaptic NMDAR signaling.