Local externalization of phosphatidylserine mediates developmental synaptic pruning by microglia

Abstract

Neuronal circuits assembly requires the fine equilibrium between synapse formation and elimination. Microglia, through the elimination of supernumerary synapses, have an established role in this process. While the microglial receptor TREM2 and the soluble complement proteins C1q and C3 are recognized key players in this process, the neuronal molecular components that tag synapses to be eliminated are still undefined. Here we show that exposed phosphatidylserine (PS) represents a neuronal 'eat-me' signal enabling microglial-mediated synapse pruning. In hippocampal neuron and microglia co-cultures, synapse elimination can be prevented by blocking accessibility of exposed PS using Annexin V or through microglial loss of TREM2. In vivo, exposed PS is detectable at both hippocampal and retinogeniculate synapses, where exposure coincides with the onset of synapse elimination and increased PS engulfment by microglia. Mice deficient in C1q, which fail to properly refine retinogeniculate connections, display elevated exposed PS and reduced PS engulfment by microglia. These data provide mechanistic insight into microglial-mediated synapse pruning and identify a novel role of developmentally regulated PS exposure that is common among developing brain structures.

WITHDRAWN: Complement C3-dependent glutamatergic synapse elimination in the developing hippocampus is region- and synapse-specific

“…The amount of signal above threshold was quantified in the 3-D stack, yielding the volume of signal. For co-localization analysis, the binarized channels were combined using the Boolean 'AND' function, similar to previous studies (Cheadle et al, 2018;Filipello, Morini et al, 2018;Li et al, 2020;Litvinchuk et al, 2018;Lui et al, 2016;Scott-Hewitt et al, 2020;Vasek et al, 2016Werneburg et al, 2020. Each measure was averaged across FOVs and slides for each animal and normalized to the WT mean.…”

Identification of Neuronal Pentraxins as Synaptic Binding Partners of C1q and the Involvement of NP1 in Synaptic Pruning in Adult Mice

“…Very little is currently known about the interaction partners of the initial complement component C1q in the CNS, especially on the synaptic surface that might drive synapse recognition. Earlier, we have shown in adult mice that C1q-tagging of synapses is linked to local apoptotic-like processes such as phosphatidylserine externalization ( 18 ), which has been recently pointed out as a key factor in developmental synaptic pruning by microglia ( 19 ). Nevertheless, members of the family of neuronal pentraxins (NPs) are suggested to be potential binding partners of C1q ( 20 ) because they share substantial structural homology with acute-phase immune proteins and well-known C1q binding partners: pentraxin 3 (PTX3) from the long-pentraxin family ( 21 ), the short-pentraxin C-reactive protein and serum amyloid P component ( 22 ).…”

Deleterious Alteration of Glia in the Brain of Alzheimer’s Disease

“…Some current studies show that microglia could selectively engulf supernumerary synapses in response to phosphatidylserine (PS), so-called “eat me” signals released from neurons. On the contrary, it bypasses engulfment when transmembrane immunoglobulin CD47 is expressed in neurons as a “don’t eat me‘ signal [ 20 , 21 ].…”