Abstract Recently, the amyloid-beta (Aβ) targeting antibody lecanemab has demonstrated modest therapeutic efficacy in slowing cognitive decline in people with Alzheimer’s disease (AD). Lecanemab clears amyloid plaques from the brain; however, plaque load does not correlate strongly with cognitive function. The strongest neuropathological correlate of cognitive decline in AD is synapse loss, which is exacerbated in the halo surrounding neuritic amyloid plaques where Aβ accumulates in remaining synapses. Here, we hypothesised that, through clearing plaques and the associated halo of soluble Aβ that can directly damage synapses, lecanemab could temper plaque-associated synapse loss. High-resolution imaging of temporal cortex tissue from people who died with AD (N=20) and age-matched controls (N=19) reveals lecanemab staining within individual pre and post-synaptic excitatory terminals in addition to plaque staining. The percentage of pre-synapses containing lecanemab-positive Aβ was over 200% higher in AD and the percentage of post-synapses was over 150% higher in AD than control tissue, with highest levels of synaptic lecanemab staining observed near plaques. These data demonstrate that lecanemab antibody recognises Aβ within synapses, warranting future work to determine whether lecanemab treatment slows cognitive decline, at least in part, through both clearing plaques and facilitating clearance or neutralisation of synaptic Aβ.

Microglia surrounding amyloid plaques in Alzheimer’s disease (AD) strongly upregulate HLA class II genes, but whether this transcriptional state reflects antigen presentation, and how it relates to genetic risk, remains unclear. Here we identify a broad repertoire of endogenous HLA-bound peptides in human brain extracts from individuals with and without AD. Using human microglia xenografts isolated after exposure to plaques, we show that amyloid pathology induces an HLA-expressing microglial state that presents a similarly diverse spectrum of peptide antigens. This response was accompanied by increased expression of costimulatory and coinhibitory molecules, supporting a transition towards an antigen presenting microglial state. Loss of PICALM, a major AD risk gene involved in endolysosomal trafficking, markedly amplified the microglial HLA response to amyloid pathology, and was associated with increased intracellular Aβ accumulation and modulation of immune regulatory proteins. These findings identify amyloid pathology as a driver of antigen presentation in human microglia and establish endolysosomal trafficking as a genetic regulator of this disease-associated immune state. Unexpectedly, the microglial immunopeptidome was not dominated by Aβ or tau, suggesting that amyloid plaques act less as a dominant antigenic source and more as a trigger of broader self-antigen presentation in the CNS by microglia.

While social and medical debate about the efficacy and safety of anti-Aβ immunotherapy is ongoing, one thing that emerged is that we have little understanding of the working mechanisms of these antibodies and this lack of knowledge complicates the interpretation of the clinical results. Here, we aimed to establish if microglia are required for the efficacy of Lecanemab, one of the most promising FDA-approved disease-modifying therapy for AD (Van Dyck et al. N Engl J Med 2023). To do so, we crossed App NL-G-F mice with Csf1r ΔFIRE/ΔFIRE mice (Rojo et al. Nat Commun 2019) to generate mice that show key features of Aβ pathology but genetically lack mouse microglia. We then assessed the effect of Lecanemab treatment on Aβ load and neuritic dystrophy. We demonstrate that Lecanemab lacks efficacy in the absence of microglia. On the other hand, when we xenotransplant human microglia into the brain of these mice (as described in Mancuso et al. Nat Neurosci 2019), we show that Lecanemab treatment significantly ameliorates both Aβ load and neuritic dystrophy. Furthermore, by employing scRNAseq on sorted human microglia, we demonstrate that Lecanemab treatment affects the transcriptome of the microglia by inducing a number of genes related phagocytosis, interferon response and immune activation. Functionally, we also established that Lecanemab-treated human microglia ingest more amyloid-β in vivo . Overall, we provide the first evidence that microglia are crucial for the efficacy of anti-Aβ immunotherapy and provide real insight into the working mechanisms of this first disease-modifying therapy for AD.

Controversies over anti-amyloid immunotherapies underscore the need to elucidate their mechanisms of action. Here we demonstrate that Lecanemab, a leading anti-Aβ antibody, mediates amyloid clearance by triggering effector functions in the microglia. Using a human microglia xenograft model, we show that Lecanemab significantly reduces Aβ pathology and associated neuritic damage, while neither Fc-inactivated Lecanemab nor microglia deficiency elicit this effect despite intact plaque binding. Single-cell RNA sequencing and spatial transcriptomic analyses reveal that Lecanemab induces a focused transcriptional program that enhances phagocytosis, lysosomal degradation, metabolic reprogramming, interferon gamma genes, and antigen presentation. Finally, we identify SPP1/osteopontin as a major factor induced by Lecanemab treatment and demonstrate its role in promoting Aβ clearance. These findings highlight that effective amyloid removal depends on the engagement of microglia through Fc fragment, providing critical insights for optimizing anti-amyloid therapies in AD. ### Competing Interest Statement B.D.S. has been a consultant for Eli Lilly, Biogen, Janssen Pharmaceutica, Eisai, AbbVie and other companies and is now consultant to Muna Therapeutics. B.D.S is a scientific founder of Augustine Therapeutics and a scientific founder and stockholder of Muna Therapeutics. European Research Council, https://ror.org/0472cxd90, ERC-834682 CELLPHASE_AD VIB vzw KU Leuven and the Flemish Government, METH/21/05 Fonds voor Wetenschappelijk Onderzoek, G087523N KU Leuven, https://ror.org/05f950310 Queen Elisabeth Medical Foundation, https://ror.org/030x5z418 Stichting Alzheimer Onderzoek, SAO-FRA 20240017 Alzheimer's Association, AARF-22-968623

Controversies over anti-amyloid immunotherapies underscore the need to elucidate their mechanisms of action. Here we demonstrate that Lecanemab, a leading anti-β-amyloid (Aβ) antibody, mediates amyloid clearance by activating microglial effector functions. Using a human microglia xenograft mouse model, we show that Lecanemab significantly reduces Aβ pathology and associated neuritic damage, while neither fragment crystallizable (Fc)-silenced Lecanemab nor microglia deficiency elicits this effect despite intact plaque binding. Single-cell RNA sequencing and spatial transcriptomic analyses reveal that Lecanemab induces a focused transcriptional program that enhances phagocytosis, lysosomal degradation, metabolic reprogramming, interferon γ genes and antigen presentation. Finally, we identify SPP1/osteopontin as a major factor induced by Lecanemab treatment and demonstrate its role in promoting Aβ clearance. These findings highlight that effective amyloid removal depends on the engagement of microglia through the Fc fragment, providing critical insights for optimizing anti-amyloid therapies in Alzheimer's disease.

INTRODUCTION:The monoclonal antibodies Aducanumab, Lecanemab, Gantenerumab, and Donanemab were developed for the treatment of Alzheimer's disease (AD). METHODS:We used single-molecule detection and super-resolution imaging to characterize the binding of these antibodies to diffusible amyloid beta (Aβ) aggregates generated in-vitro and harvested from human brains. RESULTS:Lecanemab showed the best performance in terms of binding to the small-diffusible Aβ aggregates, affinity, aggregate coating, and the ability to bind to post-translationally modified species, providing an explanation for its therapeutic success. We observed a Braak stage-dependent increase in small-diffusible aggregate quantity and size, which was detectable with Aducanumab and Gantenerumab, but not Lecanemab, showing that the diffusible Aβ aggregates change with disease progression and the smaller aggregates to which Lecanemab preferably binds exist at higher quantities during earlier stages. DISCUSSION:These findings provide an explanation for the success of Lecanemab in clinical trials and suggests that Lecanemab will be more effective when used in early-stage AD. HIGHLIGHTS:Anti amyloid beta therapeutics are compared by their diffusible aggregate binding characteristics. In-vitro and brain-derived aggregates are tested using single-molecule detection. Lecanemab shows therapeutic success by binding to aggregates formed in early disease. Lecanemab binds to these aggregates with high affinity and coats them better.

Microglial functions rely on their morphodynamic versatility and inflammatory response, yet the molecular determinants, particularly ion channels and receptors, remain poorly understood. Here, we identify chloride intracellular channel 1 (CLIC1), a protein known to exist in both soluble and membrane-associated forms, as highly enriched in human and murine microglia, with minimal expression in other brain cells. Acute blockade or genetic deletion of CLIC1 markedly attenuates microglial surveillance by reducing ramification and motility, without affecting chemotaxis. This phenotype is recapitulated in xenografted human microglia and human brain tissue. Mechanistically, CLIC1 effects involve interactions with actin-binding ezrin, radixin, and moesin (ERM) proteins, suggesting a role in linking the plasma membrane to the cytoskeleton. Contrary to its name, CLIC1 functions are chloride-independent and thus unlikely to reflect ion channel activity. This is supported by patch-clamp electrophysiology revealing lack of chloride conductance in surveillant microglia. Following ATP-evoked activation, CLIC1 blockade strongly suppresses NLRP3-dependent interleukin-1β release, suggesting therapeutic potential against neuroinflammation.

AbstractThe role of microglia in the amyloid cascade of Alzheimer’s disease (AD) is debated due to conflicting findings. Using a genetic and a pharmacological approach we demonstrate that depletion of microglia before amyloid-β (Aβ) plaque deposition, leads to a reduction in plaque numbers and neuritic dystrophy, confirming their role in plaque initiation. Transplanting human microglia restores Aβ plaque formation. While microglia depletion reduces insoluble Aβ levels, soluble Aβ concentrations stay consistent, challenging the view that microglia clear Aβ. In later stages, microglial depletion decreases plaque compaction and increases neuritic dystrophy, suggesting a protective role. Human microglia with the TREM2R47H/R47H mutation exacerbate plaque pathology, emphasizing the importance of non-reactive microglia in the initiation of the amyloid cascade. Adaptive immune depletion (Rag2-/-) does not affect microglia’s impact on plaque formation. These findings clarify conflicting reports, identifying microglia as key drivers of amyloid pathology, and raise questions about optimal therapeutic strategies for AD.

Monoclonal antibodies Aducanumab, Lecanemab, Gantenerumab, and Donanemab have been developed for treatment of Alzheimer's disease. Here, we have used single-molecule detection techniques and super-resolution imaging to characterise the binding of these antibodies to beta-amyloid aggregates including human post-mortem brain samples. Lecanemab is the best antibody in terms of binding to the small-soluble beta-amyloid aggregates, affinity, aggregate coating, and the ability to bind to post-translationally modified species, explaining its therapeutic success. ### Competing Interest Statement B.D.S. has been a consultant for Eli Lilly, Biogen, Janssen Pharmaceutica, Eisai, AbbVie and other companies, but not on their antibody programs. He is consultant to Muna Therapeutics. B.D.S is a scientific founder of Augustine Therapeutics and a scientific founder and stockholder of Muna Therapeutics.

Extracellular beta-amyloid aggregation and inflammation are in a complex and not fully understood interplay during hyperphosphorylated tau aggregation and pathogenesis of Alzheimer's disease. Our group has previously shown that an immune challenge with tumour necrosis factor alpha can alter extracellular beta-sheet containing aggregates in human-induced pluripotent stem cell-derived cortical neurons carrying familial Alzheimer's disease-related presenilin 1 mutations. Here, using single-molecule detection and super-resolution imaging techniques, we quantified and characterized the intra- and extracellular beta-amyloid and AT8-positive tau aggregates. Our results indicate a pre-existing Alzheimer's disease-like pathology caused by the presenilin 1 mutation, with increased beta-amyloid aggregates in both the cell lysate and conditioned media compared to isogenic controls and also increased intracellular tau aggregates. The main effect of tumour necrosis factor alpha treatment on presenilin 1 neurons was the formation of larger intracellular beta-amyloid aggregates. In contrast, isogenic controls showed more significant changes with tumour necrosis factor alpha treatment with an increase in beta-amyloid aggregates in the media but not intracellularly and an increase in tau aggregates in both the media and cell lysate, suggesting a chronic inflammation-driven mechanism for the development of sporadic Alzheimer's disease. Remarkably, we also found significant morphological differences between intra- and extracellular beta-amyloid and tau aggregates in human-induced pluripotent stem cell-derived cortical neurons, suggesting these neurons can only clear aggregates when small, and that larger aggregates stay inside the neurons. While majority of the beta-amyloid aggregates were cleared into the media, a greater portion of the tau aggregates remained intracellular. This size-dependent aggregate clearance was also shown to be conserved in vivo, using soaked and homogenized mouse and human post-mortem Alzheimer's disease brain samples. As such, our results are proposing a previously unknown, size-dependent aggregate clearance mechanism, which can possibly explain the intracellular aggregation of tau and extracellular aggregation of beta-amyloid.

We demonstrate the dual role of microglia in Alzheimer’s disease (AD), initially harmful, by seeding amyloid plaques, and protective in later stages by compacting amyloid plaques. Early microglial depletion using pharmacological or genetic blockage of CSF1R reduces plaque load and associated neuritic dystrophy, while human microglia transplantation restores plaque formation, confirming their seeding role. Transplanted TREM2R47H/R47H microglia exacerbate plaque pathology, highlighting microglia as key initiators of the amyloid pathology cascade. ### Competing Interest Statement B.D.S. has been a consultant for Eli Lilly, Biogen, Janssen Pharmaceutica, Eisai, AbbVie and other companies and is now consultant to Muna Therapeutics. B.D.S is a scientific founder of Augustine Therapeutics and a scientific founder and stockholder of Muna Therapeutics.

The proper maturation of emotional and sensory circuits requires fine-tuning of serotonin (5-HT) level during early postnatal development. Consistently, dysfunctions of the serotonergic system have been associated with neurodevelopmental psychiatric diseases, including autism spectrum disorders (ASD). However, the mechanisms underlying the developmental effects of 5-HT remain partially unknown, one obstacle being the action of 5-HT on different cell types. Here, we focused on microglia, which play a role in brain wiring refinement, and we investigated whether the control of these cells by 5-HT is relevant for neurodevelopment and spontaneous behaviors in mice. Since the main 5-HT sensor in microglia is the 5-HT2B receptor subtype, we prevented 5-HT signaling specifically in microglia by conditional invalidation of the Htr2b gene in these cells. We observed that abrogating the serotonergic control of microglia during early postnatal development affects the phagolysosomal compartment of these cells and their proximity to dendritic spines and perturbs neuronal circuits maturation. Furthermore, this early ablation of microglial 5-HT2B receptors leads to adult hyperactivity in a novel environment and behavioral defects in sociability and flexibility. Importantly, we show that these behavioral alterations result from a developmental effect, since they are not observed when microglial Htr2b invalidation is induced later, at P30 onward. Thus, a primary alteration of 5-HT sensing in microglia, during a critical time window between birth and P30, is sufficient to impair social and flexibility skills. This link between 5-HT and microglia may explain the association between serotonergic dysfunctions and behavioral traits like impaired sociability and inadaptability to novelty, which are prominent in psychiatric disorders such as ASD.

The astrocytic cystine/glutamate antiporter system xc– (with xCT as the specific subunit) imports cystine in exchange for glutamate and has been shown to interact with multiple pathways in the brain that are dysregulated in age-related neurological disorders, including glutamate homeostasis, redox balance, and neuroinflammation. In the current study, we investigated the effect of genetic xCT deletion on lactacystin (LAC)- and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced degeneration of the nigrostriatal pathway, as models for Parkinson’s disease (PD). Dopaminergic neurons of adult xCT knock-out mice (xCT–/–) demonstrated an equal susceptibility to intranigral injection of the proteasome inhibitor LAC, as their wild-type (xCT+/+) littermates. Contrary to adult mice, aged xCT–/– mice showed a significant decrease in LAC-induced degeneration of nigral dopaminergic neurons, depletion of striatal dopamine (DA) and neuroinflammatory reaction, compared to age-matched xCT+/+ littermates. Given this age-related protection, we further investigated the sensitivity of aged xCT–/– mice to chronic and progressive MPTP treatment. However, in accordance with our previous observations in adult mice (Bentea et al., 2015a), xCT deletion did not confer protection against MPTP-induced nigrostriatal degeneration in aged mice. We observed an increased loss of nigral dopaminergic neurons, but equal striatal DA denervation, in MPTP-treated aged xCT–/– mice when compared to age-matched xCT+/+ littermates. To conclude, we reveal age-related protection against proteasome inhibition-induced nigrostriatal degeneration in xCT–/– mice, while xCT deletion failed to protect nigral dopaminergic neurons of aged mice against MPTP-induced toxicity. Our findings thereby provide new insights into the role of system xc– in mechanisms of dopaminergic cell loss and its interaction with aging.

INTRODUCTION:Besides proteasome dysfunction, neuroinflammation is a common feature in the pathogenesis of Parkinson's disease (PD). Accordingly, peripheral inflammation has been shown to increase the susceptibility of the brain for nigrostriatal degeneration by inducing activation of glial cells and release of pro-inflammatory cytokines in the brain. Given that current animal models of PD fail to recapitulate the pathophysiology occurring in idiopathic PD, the aim of this study was to combine two pathogenic mechanisms (i.e. neuroinflammation and proteasome inhibition) to create a dual-hit mouse model of PD. METHODS:We repeatedly injected mice with a low dose of LPS (250 μg/kg/day i. p. for four days) to induce neuroinflammation, followed by a unilateral intranigral injection of lactacystin (LAC; 3 μg). Seven days later, mice were evaluated behaviorally to assess locomotion, anxiety- and depressive-like behavior. Nigrostriatal degeneration was analyzed by measuring striatal dopamine loss as well as loss of nigral dopaminergic neurons. Neuroinflammation was confirmed by quantifying microglial cells in the substantia nigra (SN) and cytokine expression in the striatum. RESULTS:Repeated systemic LPS injections increase the number of microglial cells in the SN and induce a mixed profile of pro- and anti-inflammatory cytokines in the striatum without affecting the integrity of the nigrostriatal pathway. Systemic LPS-induced neuroinflammation, however, increases the susceptibility of the nigrostriatal pathway for LAC-induced degeneration. CONCLUSION:Recapitulating two relevant etiopathogenic mechanisms of PD - neuroinflammation and proteasome inhibition-, we propose this dual-hit model as a relevant mouse model for PD that could be used to investigate potential therapeutic targets.

The contribution of glial transporters to glutamate movement across the membrane has been identified as a potential target for anti-seizure therapies. Two such glutamate transporters, GLT-1 and system x(c)(-), are expressed on glial cells, and modulation of their expression and function have been identified as a means by which seizures, neuronal injury, and gliosis can be reduced in models of brain injury. While GLT-1 is responsible for the majority of glutamate uptake in the brain, system x(c)(-), releases glutamate in the extracellular cleft in exchange for cystine and represents as such the major source of hippocampal extracellular glutamate. Using the Theiler's Murine Encephalomyelitis Virus (TMEV) model of viral-induced epilepsy, we have taken two well-studied approaches, one pharmacological, one genetic, to investigate the potential role(s) of GLT-1 and system x(c)(-) in TMEV-induced pathology. Our findings suggest that the methods we utilized to modulate these glial transporters, while effective in other models, are not sufficient to reduce the number or severity of behavioral seizures in TMEV-infected mice. However, genetic knockout of xCT, the specific subunit of system x(c)(-) may have cellular effects, as we observed a slight decrease in neuronal injury caused by TMEV and an increase in astrogliosis in the CA1 region of the hippocampus. Furthermore, xCT knockout caused an increase in GLT-1 expression selectively in the cortex. These findings have significant implications for both the characterization of the TMEV model as well as for future efforts to discover novel and effective anti-seizure drugs.

Objective: The cystine/glutamate antiporter system x(c)(-) could represent a new target for antiepileptogenic treatments due to its crucial roles in glutamate homeostasis and neuroinflammation. To demonstrate this, we compared epilepsy development and seizure susceptibility in xCT knockout mice (xCT(-/-)) and in littermate controls (xCT(+/+)) in different chronic models of epilepsy. Methods: Mice were surgically implanted with electrodes in the basolateral amygdala and chronically stimulated to develop self-sustained status epilepticus (SSSE); continuous video-electroencephalography monitoring was performed for 28 days after SE and hippocampal histopathology was assessed. Corneal kindling was induced by twice daily electrical stimulation at 6 Hz and maintenance of the fully kindled state was evaluated. Next, messenger RNA (mRNA) and protein levels of xCT and of the proteins involved in the phosphoinositide 3-kinase (PI3K)/Akt/glycogen synthase kinase 3 beta (GSK-3 beta)/eukaryotic initiation factor 2 alpha (eIF2 alpha)/activating transcription factor 4 (ATF4) signaling pathway were measured at different time points during epileptogenesis in NMRI mice treated with pilocarpine. Finally, the anticonvulsant effect of sulfasalazine (SAS), a nonselective system x(c)(-) inhibitor, was assessed against 6 Hz-evoked seizures in pilocarpine-treated mice. Results: In the SSSE model, xCT(-/-) mice displayed a significant delayed epileptogenesis, a reduced number of spontaneous recurrent seizures, and less pronounced astrocytic and microglial activation. Moreover, xCT(-/-) mice showed reduced seizure severity during 6 Hz kindling development and a lower incidence of generalized seizures during the maintenance of the fully kindled state. In pilocarpine-treated mice, protein levels of the PI3K/Akt/GSK-3 beta/eIF2 alpha/ATF4 pathway were increased during the chronic phase of the model, consistent with previous findings in the hippocampus of patients with epilepsy. Finally, repeated administration of SAS protected pilocarpine-treated mice against acute 6 Hz seizure induction, in contrast to sham controls, in which system x(c)(-) is not activated. Significance: Inhibition of system x(c)(-) could be an attractive target for the development of new therapies with a potential for disease modification in epilepsy.

Objectives: The cystine/glutamate antiporter (system xc-) is believed to contribute to nonvesicular glutamate release from glial cells in various brain areas. Although recent investigations implicate system xc- in mood disorders, unambiguous evidence has not yet been established. Therefore, we evaluated the possible role of system xc- in the depressive state. Methods: We conducted a protein expression analysis of the specific subunit of system xc- (xCT) in brain regions of the corticosterone mouse model, Flinders Sensitive Line rat model and post-mortem tissue of depressed patients. We next subjected system xc- deficient mice to the corticosterone model and analysed their behaviour in several tests. Lastly, we subjected additional cohorts of xCT-deficient and wild-type mice to N-acetylcysteine treatment to unveil whether the previously reported antidepressant-like effects are dependent upon system xc-. Results: We did not detect any changes in xCT expression levels in the animal models or patients compared to proper controls. Furthermore, loss of system xc- had no effect on depression- and anxiety-like behaviour. Finally, the antidepressant-like effects of N-acetylcysteine are not mediated via system xc-. Conclusions: xCT protein expression is not altered in the depressed brain and system xc- deficiency does not affect depression-associated behaviour in the corticosterone mouse model.

Accumulating evidence shows a key function for astrocytic connexin43 (Cx43) signaling in epilepsy. However, the lack of experimental distinction between Cx43 gap junction channels (GJCs) and hemichannels (HCs) has impeded the identification of the exact contribution of either channel configurations to epilepsy. We therefore investigated whether TAT-Gap19, a Cx mimetic peptide that inhibits Cx43 HCs but not the corresponding Cx43 GJCs, influences experimentally induced seizures in rodents. Dye uptake experiments in acute hippocampal slices of mice demonstrated that astroglial Cx43 HCs open in response to the chemoconvulsant pilocarpine and this was inhibited by TAT-Gap19. In vivo, pilocarpine-induced seizures as well as the accompanying increase in D-serine microdialysate levels were suppressed by Cx43 HC inhibition. Moreover, the anticonvulsant action of TAT-Gap19 was reversed by exogenous D-serine administration, suggesting that Cx43 HC inhibition protects against seizures by lowering extracellular D-serine levels. The anticonvulsive properties of Cx43 HC inhibition were further confirmed in electrical seizure mouse models, i.e. an acute 6 Hertz (Hz) model of refractory seizures and a chronic 6 Hz corneal kindling model. Collectively, these results indicate that Cx43 HCs play a role in seizures and underscore their potential as a novel and druggable target in epilepsy treatment.