Towards a better understanding of neuron-NG2 glia signaling

Abstract

Grey matter NG2 glia constitutes a heterogeneous glial population whose functions remain incompletely understood. Their ability to make synapses with neurons is unique among glial cells, but the purpose of these connections is still elusive. NG2 glia can sense both glutamatergic and GABAergic inputs thanks to the expression of AMPA receptors (AMPARs) and GABA receptors (GABARs). In the hippocampus, Schaffer collaterals (SCs) activate AMPARs in NG2 glia, giving rise to small post-synaptic currents (PSCs). Cerebellar climbing fibers (CFs) also form synapses with NG2 glia, producing much larger PSCs. This work aims to assess mechanisms underlying these regional differences and to better understand the role of NG2 glia AMPARs in influencing the activity of neuronal networks and mouse behaviour. Patch-clamp analyses also allowed for determining the properties of AMPARs and GABARs expressed by hippocampal NG2 glia at different developmental stages. Taking advantage of a mouse line with inducible deletion of AMPARs subunits GluA1-4 in NG2 glia (GluAko) we performed field potential recordings (fEPSPs) to assess possible impairments in long-term plasticity. Memory and motor abilities of GluAko mice were also evaluated with a series of behavioural tests. Comparing evoked AMPAR-mediated PSCs in hippocampal and cerebellar NG2 glia while applying intra- and extracellular calcium-permeable AMPAR (CP-AMPAR) antagonists revealed a higher expression of these receptors in the cerebellum. Moreover, application of a competitive AMPAR antagonist during ePSC recordings demonstrated that a larger glutamate transient is sensed by cerebellar NG2 glia, due to multivesicular release from CFs. This data shows that NG2 glia in the cerebellum can make more efficient synapses compared to the hippocampal counterpart, thanks to a higher expression of CP-AMPARs and a larger glutamate concentration in the synaptic cleft. Field potential recordings in slices from GluAko mice unraveled impaired long-term potentiation in the hippocampus and cerebellum, which was rescued pharmacologically in both regions by applying a TrkB receptor agonist, hinting at a crucial role of NG2 glia in regulating BDNF levels. Notably, GluAko mice displayed reduced locomotor activity in most of the tasks, which indicates the presence of motor deficits or anxious behaviour. The data presented in this work addresses unresolved questions concerning the role and properties of neuron-NG2 glia synapses within the grey matter and their possible impact on neuronal communication, suggesting that proper signaling to NG2 glia is essential for a correct functioning of the neuronal network.