Local externalization of phosphatidylserine mediates developmental synaptic pruning by microglia

Abstract

Neuronal circuit assembly requires the fine balance between synapse formation and elimination. Microglia, through the elimination of supernumerary synapses, have an established role in this process. While the microglial receptor TREM 2 and the soluble complement proteins C1q and C3 are recognized as key players, the neuronal molecular components that specify synapses to be eliminated are still undefined. Here, we show that exposed phosphatidylserine ( PS ) represents a neuronal “eat‐me” signal involved in microglial‐mediated pruning. In hippocampal neuron and microglia co‐cultures, synapse elimination can be partially prevented by blocking accessibility of exposed PS using Annexin V or through microglial loss of TREM 2. In vivo , PS exposure at both hippocampal and retinogeniculate synapses and engulfment of PS ‐labeled material by microglia occurs during established developmental periods of microglial‐mediated synapse elimination. Mice deficient in C1q, which fail to properly refine retinogeniculate connections, have elevated presynaptic PS exposure and reduced PS engulfment by microglia. These data provide mechanistic insight into microglial‐mediated synapse pruning and identify a novel role of developmentally regulated neuronal PS exposure that is common among developing brain structures.

An “Outside-In” and “Inside-Out” Consideration of Complement in the Multiple Sclerosis Brain: Lessons From Development and Neurodegenerative Diseases

“…Exposed PS have been shown in the cortex of adult mice in isolated synaptosomes that are also tagged by C1q (Györffy et al, 2018 ) and have also been observed in vivo at presynaptic inputs in the hippocampus and in the dLGN of developing mice (Scott-Hewitt et al, 2020 ). During development, deletion of C1q results in increased PS-labeled presynaptic inputs and decreased microglial engulfment of PS-labeled elements (Scott-Hewitt et al, 2020 ), implicating C1q in the elimination of PS-labeled synapses. It is likely that in this context, PS-labeled elements are engulfed by microglia via the phagocytic receptor TREM2 since, in hippocampal neuron and microglia co-cultures, synapse elimination can be partially prevented by deleting TREM2 on microglia (Scott-Hewitt et al, 2020 ).…”

“…Antibodies are the obvious candidate, but deposits of immunoglobulins were not detected in the MS hippocampus. An alternative target of C1q is an apoptotic signal on the surface of dead or dying cells, such as the previously identified ceramide transporter protein (Bode et al, 2014 ) or the externalization of phosphatidylserine (PS; Scott-Hewitt et al, 2020 ). Exposed PS have been shown in the cortex of adult mice in isolated synaptosomes that are also tagged by C1q (Györffy et al, 2018 ) and have also been observed in vivo at presynaptic inputs in the hippocampus and in the dLGN of developing mice (Scott-Hewitt et al, 2020 ).…”

“…An alternative target of C1q is an apoptotic signal on the surface of dead or dying cells, such as the previously identified ceramide transporter protein (Bode et al, 2014 ) or the externalization of phosphatidylserine (PS; Scott-Hewitt et al, 2020 ). Exposed PS have been shown in the cortex of adult mice in isolated synaptosomes that are also tagged by C1q (Györffy et al, 2018 ) and have also been observed in vivo at presynaptic inputs in the hippocampus and in the dLGN of developing mice (Scott-Hewitt et al, 2020 ). During development, deletion of C1q results in increased PS-labeled presynaptic inputs and decreased microglial engulfment of PS-labeled elements (Scott-Hewitt et al, 2020 ), implicating C1q in the elimination of PS-labeled synapses.…”

“…During development, deletion of C1q results in increased PS-labeled presynaptic inputs and decreased microglial engulfment of PS-labeled elements (Scott-Hewitt et al, 2020 ), implicating C1q in the elimination of PS-labeled synapses. It is likely that in this context, PS-labeled elements are engulfed by microglia via the phagocytic receptor TREM2 since, in hippocampal neuron and microglia co-cultures, synapse elimination can be partially prevented by deleting TREM2 on microglia (Scott-Hewitt et al, 2020 ). While this evidence point towards a potential role for an apoptotic signal in the C1q-mediated elimination of synapses which could be of significance in the MS brain, MS neurons are quite resistant to cell death (Dutta et al, 2011 ) which argues against C1q-driven opsonization of apoptotic neurons.…”

The effect of amyloid on microglia-neuron interactions before plaque onset occurs independently of TREM2 in a mouse model of Alzheimer's disease

“…Previous work in other pathological contexts suggest candidate factors, including chemokines such as CX3CL1 ( Lee et al, 2014 ) and CCL2/3 ( Welikovitch et al, 2020 ), factors released in response to oxidative stress ( Wilson et al, 2018 ), myelin debris, or extracellular ATP ( Cserép et al, 2020 ; Davalos et al, 2005 ; Neumann et al, 2009 ). Factors involved in microglia-neuron interactions during development, including complement or phosphatidylserine (PS) exposure ( Scott-Hewitt et al, 2020 ) could also play a role in the interactions we observe here in early AD. Recently, microglial interactions with neuronal soma have been linked to the level of neuronal activity, supported by TRPV1-expressing neurons activated by capsaicin exhibiting increased soma contacts by microglia ( Hughes and Appel, 2020 ).…”