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Targeting the interrelation between microglia, neurons, and astrocytes to counteract tau-mediated synaptic impairments and neurodegeneration in AD.

Periodic Reporting for period 1 - SynapTau (Targeting the interrelation between microglia, neurons, and astrocytes to counteract tau-mediated synaptic impairments and neurodegeneration in AD.)

Reporting period: 2022-09-01 to 2024-08-31

The overall aim of SynapTau was to evaluate the contribution of microglia on tau-mediated synaptic loss in Alzheimer’s disease (AD), based on functional and structural synaptosomal changes, which may provide clues for developing original therapeutic strategies against a hitherto incurable disease. Determining the impact of tau accumulation on synaptic loss is critical, especially considering that synaptic dysfunction is an early key pathological event that correlates with cognitive decline in AD. Beyond tau and amyloid-β (Aβ) pathologies, neuroinflammation and gliosis are also hallmarks of the AD brain and are thought to play a pivotal role on synaptic loss. Accordingly, microglia depletion strongly attenuates tau-mediated neurodegeneration and synaptic loss . Despite the link between tau, synapses and inflammation in AD, we do not yet understand the specific interactions between synaptic proteins, glia and pathological tau that lead to synaptic loss. Therefore, my objective was to provide a better understanding of the role of microglia in tau-mediated synaptic loss based on my longstanding experience on the topic and on recent observations I made in the P301S mouse model of AD/tauopathy depleted for microglia. In a pilot study, I explored synaptosomes containing glial processes in close proximity to synapses to identify changes resulting from microglia depletion that account for synaptic protection against tau pathology. Two main original observations from this pilot study set the basis of SynapTau project: i) The upregulation of UCH-L1, an enzyme required for normal synaptic function; ii) Most of theupregulated proteins in the synaptosomes of microglia depleted-P301S mice are normally expressed by astrocytes.

Therefore, in SynapTau, I proposed to study potential protective mechanisms against tau-mediated synaptotoxicity set in motion by microglial depletion that target neurons (objective 1) and neuron/astrocyte interactions (objective 2). In objective 1, I hypothesize that enhancing UCH-L1 expression in neurons will protect against tau-mediated synaptic damages. In objective 2, I hypothesize that neuron-glia interactions are modified after microglia depletion, with changes in the formation and function of perisynaptic astrocytic processes that may support beneficial pathological outcomes. SynapTau is a highly significant project as it aims to discover new therapeutic targets to maintain synaptic integrity and function to slow down AD neurodegeneration and cognitive decline.

I was expecting to provide a better understanding of the interplay between tau pathology, gliosis and synaptic alterations in AD through in-depth proteomic, behavioral, electrophysiological, biochemical and brain imaging approaches. Moreover, this project will be an asset in obtaining a permanent position and becoming an independent researcher. Indeed, I obtained an ATIP-Avenir grant to start my own research lab, which began in January 2023 and was not compatible with my MSCA grant. For this reason, I had to terminate my MSCA grant on the 31st of December 2022.
I had to end the MSCA grant only 4 months after it started. During this time, I mostly focused my work on the WP4. The aim was to identify the proteome that is enriched at neuro-astrocyte junctions using an in vivo chemo-genetic approach that applies a cell-surface fragment complementation strategy . This innovative technique is based on the reconstitution of the enzymatic activity of a proximity-biotinylating enzyme, TurboID, at the neuron-astrocyte interface.
I will use the previously described strategy to direct N- and C-terminal TurboID fragments to the extracellular surface of neurons and astrocytes, respectively. To this end, we will use 2 different AAV-PHP.eB under the control of a neuronal or astrocytic promoter. Plasmid constructs has be purchased from Addgene: the AAV-hSynI-V5-TurboID(N)-GPI plasmid has be used to express the N-terminal half of TurboID on the neuronal surface, while the pZac2.1-GfaABC1D-TurboID(C)-HA-GPI plasmid drive the expression of the C-terminal half of TurboID on the surface of astrocytes. I redesigned the plasmid, produced it and then we produced the AAVs in collaboration with Genecust. AAV production has be done as previously described30, resulting in 2 AAVs: AAV-PHP.eB-hSynI-V5-N-TurboID (neuron-specific) and/or AAV-PHP.eB-pZac2.1-GfaABC1D-C-TurboID-HA (astrocyte-specific). Those AAVs have been tested in vitro for their ability to transfect the right cell types. They will be tested in the next months in neurons/astrocytes co-culture to confirm their ability to induce TurboID-associated biotynilation at the interface synapses/astrocytes.
While the grant has been ended quickly, the objective 2 is still an objective of my lab and we are currently working on it. This objective will help us dissect the dynamics of perisynaptic neuron-astrocyte changes after microglia depletion and provide a better understanding of synaptic changes after microglia depletion to find new therapeutical targets promoting synaptic welfare in AD. The most relevant targets will undergo the first stems for their validation using immunohistochemistry and western blot in post-mortem brain tissue from AD patients obtained at the Neuro-CEB biobank (La Salpetrière Hospital, Paris).