Glial Cx43 hemichannels and neuronal Panx1 hemichannels and P2X7 receptors orchestrate presynaptic homeostatic plasticity

Abstract

The emerging role of glial cells in modulating neuronal excitability and synaptic strength is a growing field in neuroscience. In recent years, a pivotal role of gliotransmission in homeostatic presynaptic plasticity has been highlighted and glial-derived ATP arises as a key contributor. However, very little is known about the glial non-vesicular ATP-release pathway and how ATP participates in the modulation of synaptic strength. Here, we investigated the functional changes occurring in neurons upon chronic inactivity and the role of the purinergic signaling, connexin43 and pannexin1 hemichannels in this process. By using hippocampal dissociated cultures, we showed that blocking connexin43 and pannexin1 hemichannels decreases the amount of extracellular ATP. Moreover, Ca2+ imaging assays using Fluo-4/AM revealed that blocking connexin43, neuronal P2X7Rs and pannexin1 hemichannels decreases the amount of basal Ca2+ in neurons. A significant impairment in synaptic vesicle pool size was also evidenced under these conditions. Interestingly, rescue experiments where Panx1HCs are blocked showed that the compensatory adjustment of cytosolic Ca2+ was recovered after P2X7Rs activation, suggesting that Panx1 acts downstream P2X7Rs. These changes were accompanied by a modulation of neuronal permeability, as revealed by ethidium bromide uptake experiments. In particular, the permeability of neuronal P2X7Rs and pannexin1 hemichannels is increased upon 24 h of inactivity. Taken together, we have uncovered a role for connexin43-dependent ATP release and neuronal P2X7Rs and pannexin1 hemichannels in the adjustment of presynaptic strength by modulating neuronal permeability, the entrance of Ca2+ into neurons and the size of the recycling pool of synaptic vesicles.