Astrocyte-derived ATP Promotes Synapse Elimination In The Thalamus

During early development, redundant synapses are eliminated to form elaborate functionally neural circuits, a process called synapse elimination. Although activity-dependent competition among different inputs has been suggested as the key mechanisms for synapse elimination, the underlying molecular and cellular mechanisms, especially the role of astrocyte in this process, remain unclear. Astrocyte releases a varieties of gliotransmitters in a Ca2+-dependent manner, and type 2 inositol 1,4,5-trisphosphate receptor (IP3R2) mediates Ca2+release from endoplasmic reticulum stores is the main source of intracellular Ca2+elevation. So we study the role of astrocyte in synapse elimination using mice deficient in the IP3R2 to specifically disturb intracellular Ca2+signaling in these cells.In mice, neurons in ventral posteromedial nucleus (VPm) of the thalamus receive projection from the principal trigeminal nucleus (Pr5) in the brainstem, relay whisker sensory information. Pr5-VPm connection in mice has proven to be an excellent model in which to investigate developmental synapse elimination in the central nervous system. By whole-cell patch recording in acute brain slices, we found most of VPm neurons receive one Pr5 input in WT mice at P16 but multiple Pr5 inputs in IP3R2 knockout mice (Itpr2-/-), indicating that knocking out astrocyte IP3R2 impaired synapse elimination. Consistent with the electrophysiological results, we observed more VGluT2 puncta numbers around the soma of VPm neuron in Itpr2-/-mice. Furthermore, a significant decrease in the basal ATP level in the brain was found in KO mice compared with that in WT mice. Intracerebalventricular injection of ATP, increased the basal ATP level in the brain, rescued the deficit in synapse elimination in Itpr2-/-mice, indicating that astrocytes mediate synapse elimination by releasing ATP. Next, we found that intracerebalventricular injection of ATPyS, the non-hydrolyzable ATP analog, but not adenosine, a degradation product of ATP, rescued the deficit in synapse elimination in Itpr2-/- mice. We also found that ATP activated presynaptic P2Y1 receptors to mediate long-term depression (LTD) and thus may serve as the mechanism for synapse elimination. These results were further supported by the finding that developmental synapse elimination was also impaired in P2Y1 knockout mice (P2ry1-/-) and was not rescued by ATP. Furthermore, intracerebroventricular injection of a selective P2Y1 agonist MRS-2365 also rescued the deficient of synapse elimination in Itpr2-/-mice.Taken together, our studies suggested that astrocytes play a crucial in developmental synapse elimination. Astrocytes release ATP in a Ca2+-dependent manner and activate presynaptic P2Y1 receptors to mediate LTD, making synapse activity weaken and eventually eliminated of this synapse.