Astrocytes release d-serine by a large vesicle

Abstract

Long-term potentiation (LTP) of synaptic transmission in the CA1 region of the hippocampus depends on activation of N-methyl-D-aspartate receptors (NMDARs), which can be regulated by Ca2+-dependent release of D-serine from astrocytes. The detailed mechanism underlying astrocytic D-serine release is still unknown. In this study, we found that clamping astrocytic [Ca2+] at 100-150 nM or puffing artificial cerebrospinal fluid (ACSF) into the extracellular space (weak mechanical stimulation) enhanced synaptic activation of NMDARs. The enhancement was blocked by the NMDAR glycine site antagonist DCKA, glycine saturation, and infusion of astrocytes with D-Amino Acid Oxidase (DAAO) and the serine racemase inhibitor L-erythro-3-hydroxyaspartate (HoAsp), suggesting the involvement of astrocytic D-serine release. Intracellular 100-150 nM [Ca2+] or puffing ACSF stimulated astrocytes to generate D-serine-containing large vesicles (1-3 μm), exocytotic fusion of which released D-serine. The formation of astrocytic large vesicles involved intracellular fusion of small vesicles and/or other organelles. Spontaneous fusion of large vesicles occurred occasionally in astrocytes at rest, contributing to baseline D-serine levels, which increased the rising slope of NMDAR post-burst potentiation (PBP) without altering the PBP peak amplitude. Thus, under physiological conditions, astrocytic D-serine release by large vesicles facilitated weak theta-burst (TBS consisting of 5 bursts), but not strong TBS (TBS consisting of 10 bursts) stimulation-induced LTP.

Dysfunctional synapse in Alzheimer's disease – A focus on NMDA receptors

“…Functionally, synaptic and extrasynaptic NMDARs are gated by different co-agonists (Papouin et al, 2012), respectively, D-serine released by astrocytes (Kang et al, 2013) and glycine released by both astrocytes and neurons (Holopainen and Kontro, 1989), as observed in both the hippocampus and in cerebellar granule cell cultures (Figure 1). Papouin and colleagues demonstrated that the availability of the co-agonists matches the preferential affinity of each subunit for its own co-agonist and that glycine and D-serine inhibit NMDAR surface trafficking in a subunit-dependent manner, influencing also…”

Two Distinct Modes of Exocytotic Fusion Pore Expansion in Large Astrocytic Vesicles

“…These large vesicles could also be detected by DIC microscopy (supplemental Fig. S2, A-C), by patch clamp loading of astrocytes with Alexa Fluor-594 and Fluo-4 potassium, or by Fluo-4 AM loading of astrocytes (25,32,33). Repeated puffs increased FM 1-43 fluorescence around large vesicles, probably due to increased endocytosis of FM 1-43 (39).…”

“…Large astrocytic vesicles were also induced by puffing ACSF (weak mechanical stimulation) in the absence of tetrodotoxin. The fusion of puffing ACSF-induced large vesicles resulted in D-serine-mediated enhancement of NMDA receptor activation without SICs (33).…”

“…Exocytotic Fusion of Large Astrocytic Vesicles-Our previous studies have demonstrated that astrocytes release glutamate and D-serine through fusion of large vesicles (25,32,33). To monitor the fusion process of large astrocytic vesicles, we puffed glutamate (50 mM) together with FM 1-43 (20 M) in the presence of tetrodotoxin (1 M) and imaged FM 1-43 fluorescence using two-photon microscopy.…”

“…One type of astrocytic glutamate release that activates a slowly decaying transient inward current (SIC) 3 in neurons (28 -31) is through exocytotic fusion of a large astrocytic vesicle in a SNARE-dependent manner (25,32). Large astrocytic vesicles also contain D-serine (33), and co-release of glutamate and D-serine induces NMDA receptor-mediated neuronal SICs. The size of these large astrocytic vesicles (ϳ3 m) is similar to vacuoles in yeast (34) and about 100 times larger than synaptic vesicles (ϳ30 nm).…”