Coulon A, Rabiller F, Takalo M, Roy A, Martiskainen H, Siedlecki-Wullich D, Mendes T, Lemeu C, Carvalho L-I, Ehrardt A, MelodeFerias AR, Hulsman M, Najdek C, Lannette-Weimann N, Freire-Regatillo A, Amouyel P, Charbonnier C, Dols-Icardo O, Jeskanen H, Kuulasmaa T, Kurki M, Hardy J, Wagner M, Heikkinen S, Holstege H, Makinen P, Nicolas G, Mead S, Wagner M, Ramirez A, Rauramaa T, Palotie A, Sims R, Soininen H, vanSwieten J, Williams J, Willman R-M, Bellenguez C, Grenier-Boley B, Gelle C, Lambert E, Ayral A-. Neuronal downregulation of PLCG2 impairs synaptic function and elicits Alzheimer disease hallmarks. 2024 May 01 10.1101/2024.04.29.591575 (version 1) bioRxiv.

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  1. This study by Coulon et al. is a tour de force, identifying and characterising an exciting neuronal function for the AD gene PLCγ2, which encodes a phospholipase enzyme involved in signal transduction.

    The multilaboratory team established a loss-of-function (LOF) screen for AD risk genes in primary neurons, measuring effects on synaptic morphology and function, based on assessment of synaptic density, using an shRNA lentiviral knockdown system in a rat neuronal culture. Short-hairpin RNAs (shRNAs) for 198 genes in more than 70 AD risk loci were tested. The screen identified genes whose knockdown resulted in high, or in low, synaptic density. These included PLCγ2, for which knockdown was associated with reduced synaptic density, with concomitant effects on electrophysiologic properties of the neurons in culture.

    Synaptic impairment may be a linking feature of several GWAS genes and synaptic failure is likely a key step early in mild cognitive impairment and AD.

    To translate their finding to a complete brain system, the authors induced PLCγ2 knockdown in mouse dentate gyrus cells, a key cell type for hippocampal function, which resulted in decreased neuronal complexity and synapse formation, confirming the effect of PLCγ2 in a mouse model.

    The study furthermore assessed LOF variants of PLCγ2 in genetic screens in AD patients from different clinical cohorts, identifying rare LOF variants of PLCγ2 that raise the risk of AD 10-fold. Interestingly, there are also PLCγ2 variants with lower risk for AD (Sims et al., 2017). The mechanisms of both LOF and protective variants may be reconciled by the synaptic function proposed by the authors.

    Interestingly, lower levels of PLCγ2, achieved by knockdown in human cultured neurons, that impaired synaptic function resulted in increased APP and Aβ. It also increased total tau and tau phosphorylation at multiple sites, which correlated with changes in phosphorylation status in kinases Akt (serine-473) and its substrate GSK3β (tyrosine-216), indicating a modulation of this pathway by PLCγ2 reduction—though, here the Akt serine-9 phosphorylation would be of interest as a readout of GSK3β regulation by Akt (Cross et al., 1995). Furthermore, inhibitor experiments may clarify the involvement of these kinases, or other kinases, downstream of PLCγ2, which may include PKC and CaMKinases, both known to be involved in synaptic function (Xia and Storm, 2005). Consistent with effects of PLCγ2 on tau, lower p-tau levels of the key epitope threonine-181 had been found in CSF of carriers of the protective P522R variant of PLCG2 (Kleineidam et al., 2020).

    Notably, restoring PLCγ2 expression mitigated phenotypes entirely, suggesting PLCG2 reduction is a persistent driver in the synaptic dysfunction and APP/tau aberrations.

    It will be interesting to see if PLCγ2 downregulation impairs synapses through Aβ and/or tau or through independent mechanisms. Notably, a recent study proposed that PLCGγ2 controls myelin-enriched lipids, suggesting a mechanism involving regulation of lipid metabolism (Hopp et al., 2023).

    Key advances of the study are:

    1. A neuronal function of PLCγ2 in AD pathology as compared with a largely microglial contribution suggested by GWAS in previous work (e.g., Andreone et al., 2020). PLCγ2 is expressed in neurons (Magno et al., 2019). The new paper provides strong evidence for a cell-autonomous function of PLCγ2 in neurons.
    2. PLCγ2 loss, or inhibition, may be proximally linked to causal events in AD. Though this must be established in additional models, it is a very exciting aspect of this work. The mechanism of APP and Ab increase is a key and exciting question to address: a starting point would be whether these happen at the transcriptional or post-transcriptional level for APP or whether there is an indirect effect through APP metabolism (i.e., cleaving or degrading enzymes). On this note, risk and protective variants of PLCγ2 were shown to modulate amyloidosis in mouse models (Tsai et al., 2023).
    3. AD risk gene function may be complex, combining effects in different cell types during disease processes and development. This may make targeting these factors challenging. However, PLCγ2 activators are being developed (Visvanathan et al., 2023) and we will certainly see more approaches to activating this pathway.

    It should, however, be kept in mind that the physiologic function of this pathway, both in neuronal synapse formation and maintenance, as well as in microglial cells, needs to be understood to move on with a therapeutic strategy targeting PLCγ2 activation. In this context, PLCγ2 is highly expressed in hematopoietic cells, which may be important when targeting this factor (e.g., Xu et al., 2023).

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