Microglia shape presynaptic properties at developing glutamatergic synapses

Abstract

Deficient neuron–microglia signaling during brain development is associated with abnormal synaptic maturation. However, the precise impact of deficient microglia function on synaptic maturation and the mechanisms involved remain poorly defined. Here we report that mice defective in neuron‐to‐microglia signaling via the fractalkine receptor (Cx3cr1 KO) show reduced microglial branching and altered motility and develop widespread deficits in glutamatergic neurotransmission. We characterized the functional properties of CA3–CA1 synapses in hippocampal slices from these mice and found that they display altered glutamatergic release probability, maintaining immature properties also at late developmental stages. In particular, CA1 synapses of Cx3cr1 KO show (i) immature AMPA/NMDA ratio across developmental time, displaying a normal NMDA component and a defective AMPA component of EPSC; (ii) defective functional connectivity, as demonstrated by reduced current amplitudes in the input/output curve; and (iii) greater facilitation in the paired pulse ratio (PPR), suggesting decreased release probability. In addition, minimal stimulation experiments revealed that excitatory synapses have normal potency, but an increased number of failures, confirming a deficit in presynaptic release. Consistently, KO mice were characterized by higher number of silent synapses in comparison to WT. The presynaptic deficits were corrected by performing experiments in conditions of high release probability (Ca2+/Mg2+ratio 8), where excitatory synapses showed normal synaptic multiplicity, AMPA/NMDA ratio, and proportion of silent synapses. These results establish that neuron–microglia interactions profoundly influence the functional maturation of excitatory presynaptic function.

WITHDRAWN: Complement C3-dependent glutamatergic synapse elimination in the developing hippocampus is region- and synapse-specific

“…While collectively, these results demonstrate that a complementdependent synapse elimination mechanism does not target synapses in the CA1 SR of the developing hippocampus, others have found that different microglial receptors mediate synapse elimination in this region. Deletion of the fractalkine receptor CX3CR1 was found to increase the number of dendritic spines in the CA1 SR (Paolicelli et al, 2011) as well as control pre-synaptic release probability (Basilico et al, 2019).…”

TREM2 is required for microglial instruction of astrocytic synaptic engulfment in neurodevelopment

“…Microglia have recently been shown to be required for postsynaptic maturation of CA1 neurons through a CX3CR1‐dependent mechanism (Basilico et al, ). To determine whether there were also TREM2‐dependent changes in postsynaptic function in this region, we evaluated mEPSC amplitude (Figure c), mEPSC rise and decay time and AMPA:NMDA ratios, but found no significant effect of Trem2 genotype on these measures of postsynaptic function (data not shown).…”

“…Complement is also involved in synaptic loss in response to acute brain injury (Alawieh, Langley, Weber, Adkins, & Tomlinson, 2018;Norris et al, 2018), chronic tau accumulation (Dejanovic et al, 2018), epilepsy (Wyatt, Witt, Barbaro, Cohen-Gadol, & Brewster, 2017), neuroinflammation (Watkins et al, 2016), and other disease contexts (Sekar et al, 2016). Other microglial signaling components also have important functions in both brain development and neurodegeneration, including CX3CR1 (Basilico et al, 2019;Paolicelli et al, 2011;Sheridan & Murphy, 2013) and progranulin (Baker et al, 2006;Lui et al, 2016), further illustrating that common mechanisms can mediate important microglial functions in brain health and disease (Tenner, Stevens, & Woodruff, 2018).…”

Local externalization of phosphatidylserine mediates developmental synaptic pruning by microglia

“…The involvement of microglia, the CNS resident immune cells and phagocytes, in synaptic refinement has been widely established. Microglial processes make direct and transient connections with subsets of neuronal synapses, with evidence for presynaptic and postsynaptic elements inside microglial lysosomes (Basilico et al, 2019;Filipello, Morini et al, 2018;Paolicelli et al, 2011;Schafer et al, 2012;Tremblay et al, 2010;Weinhard et al, 2018). Microglia-mediated synaptic pruning is a highly regulated process and occurs in several brain regions during developmental critical periods of synaptic refinement.…”