Impact of non-neuronal mechanisms on synaptic transmission

Abstract

The brain receives, encodes and shares information between neurons via synaptic transmission. This kind of communication is generally exclusive to neurons, and was soon found to be activity-dependent and therefore plastic. Numerous mechanisms by which synaptic activity can be modulated by the signaling partners themselves such as changes to the firing rate or release of retrograde messengers have been described. However, an increasing number of studies revealing the many ways in which other players in the brain, primarily glial cells but also the extracellular matrix and the neurovascular unit can influence synaptic transmission, shift this neuron-centered perspective towards a more complex system. To shed further light on this topic we used high-resolution microscopy, 2P glutamate uncaging, electrophysiological methods and RNA sequencing in mouse brain slices to investigate the role of oligodendrocyte precursor cells (OPCs), astrocytes and the blood-brain barrier (BBB) in modulating synaptic activity. <br/> OPCs are the single source for oligodendrocytes in the CNS, constitute the only glial cell type that receives synaptic input from neurons and remain present throughout the entire lifespan. Neuronal activity is believed to play a key role in adult oligodendrogenesis and myelination, which renders neuron-OPC crosstalk central to a field of intense research for therapeutic approaches. We found that embryonic OPCs exhibit a unique transcriptional profile different from both, neuronal precursors and postnatal OPCs, and do not require synaptic innervation from neurons to develop. <br/> Astroglial perisynaptic sheaths in close contact with the synapse make up the so-called tripartite synapse and modulate synaptic activity via several different functions like transmitter secretion as well as generation of a local extracellular microenvironment. Astrocytic coverage varies between brain regions and individual synapses and can change. We demonstrated that astrocytic coverage inversely correlates with spine size and that smaller spines were better shielded from invading glutamate than bigger spines. Furthermore, we experimentally assessed the glutamate spread in the neuropil. Glutamate diffused further than previously thought and could regularly lead to synaptic cross-talk. Our experiments identified astrocytic glutamate transporters as an essential regulator for the spread of glutamate, which together with differential coverage of individual synapses could control synaptic signaling. <br/> The BBB is part of the neurovascular unit permitting the controlled entry of substances from the blood stream into the brain and ensuring brain homeostasis for proper functioning of synaptic mechanisms. We developed a new approach to study the BBB by combining guided micropipette perfusion of brain vesicles with multiphoton imaging, which unlike previously existing models of the BBB allows spatially and temporally controlled experiments. This offers new ways to deepen our understanding of how changes to the neurovascular unit modify neuronal function and synaptic transmission.