Astrocyte-neuron signaling, a.k.a. gliotransmission, can modulate synaptic transmission/plasticity at tripartite synapses. Among the processes regulated by gliotransmission are sleep-regulation, respiration, and learning/memory. Despite these roles of gliotransmission in such fundamental life processes, its mechanism is not understood. Elucidating this mechanism should provide insights into basic brain processes, and suggest interventions when they go awry. Two early studies of astrocyte-neuron signaling explored the hypothesis that astrocytic glutamate release acts on neuronal glutamate receptors, but they led to different conclusions regarding the mechanism. One study concluded that glutamate is not the messenger but instead suggested that gap junctions might mediate astrocyte-neuron signaling. The other study concluded that the signaling is mediated by Ca2+-dependent glutamate release from astrocytes, subsequently shown to occur by regulated exocytosis of glutamate-containing vesicles. Virtually nothing is known about the subcellular distribution/localization of astrocyte release sites. It is not clear if they are localized uniquely to the tripartite synaptic regions of astrocytes or more broadly. There is much debate about the relative roles of exocytosis vs. gap junction-mediated communication as critical for astrocyte-neuron signals. Our preliminary data point to a novel, unifying hypothesis that these two mechanisms are, in fact, mechanistically linked.
The proposed investigation will gather valuable information on the novel role of connexin 43 in the distribution of stimulated exocytotic fusion/retrieval sites. Findings will be of general interest to any cell type expressing connexin 43 in the plasma membrane and containing synaptobrevin 2 positive secretory granules/vesicles; In the brain, astrocytes possess these attributes, while in bone osteoblasts fit these criteria. This new insight in connexin function has potential to change the way we think about brain and bone (dys)functions.
