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C5aR1 antagonism suppresses microglial polarization and mitigates disease progression in a mouse model of Alzheimer’s disease

ABSTRACT: Despite the discovery of PMX205 more than 20 years ago and its reported beneficial effects in Alzheimer’s disease, the specific mechanism that might be driving the improvement in cognition are still not know. Here, we used the Tg2576 mouse model of Alzheimer’s disease and treated them with PMX205 at the onset of the amyloid pathology to further determine the effects of this C5aR1 antagonist on microglial cells. The results presented in this study demonstrated a neuroprotective effect of PMX205, which rescues the excessive synaptic pruning and synapse loss associated with Alzheimer’s disease. This finding seems to be linked to the reduction of a unique microglial subpopulation associated to synaptic pruning in the PMX205 treated mice. Interestingly, we also show here that blocking C5a-C5aR1 signaling in the Tg2576 mouse model of AD results in the increase of the DAM2 microglial subpopulation, suggesting that PMX205 might be inducing a disease mitigating phenotype on microglial cells. Our data further supports the use of C5aR1 antagonists as potential therapeutic targets to treat or slow the progression of Alzheimer’s disease.

ORGANISM(S): Mus musculus

PROVIDER: GSE200942 | GEO | 2022/05/02

REPOSITORIES: GEO

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Project description:The complement system is part of the innate immune system that works to clear pathogens and cellular debris. In the central nervous system (CNS) complement activation can promote synaptic pruning clearance of neuronal blebs recruitment of phagocytes and protection from pathogens. However in a neuropathologic environment complement activation may contribute to inflammatory pathways neuronal dysfunction and in the Alzheimer’s disease (AD) brain cognitive decline. If complement activation proceeds to the cleavage of C5 and thus generation of C5a engagement of C5a with the receptor C5aR1 can instigate a feed-forward loop of inflammation injury and neuronal death thus making this molecule a potential target for modulation in AD therapeutics. The Arctic (Arc) AD mouse model known to rapidly accumulate fibrillar amyloid plaques was crossed to a model that lacks the receptor for C5a (ArcC5aR1KO) or to a transgenic mouse that generates C5a under the GFAP promoter (ArcC5a+). ArcticC5a+ mice showed accelerated loss of spatial memory compared to Arc mice. While eliminating C5aR1 did not alter amyloid plaque accumulation in this AD model C5aR1KO delayed or prevented the expression of important AD-associated genes in the hippocampus indicating a separation between those genes induced by amyloid plaques and those influenced by C5a-C5aR1 signaling. C5ar1 deletion also reduced/delayed the expression of select pan-reactive and A1 reactive astrocyte genes. ArcC5aR1KO showed delayed expression of genes enriched for biological processes that are significant in the AD context such as regulation of inflammatory signaling microglial cell activation astrocyte migration and lysosome pathway. Interestingly overexpression of C5a also delayed the increase of some AD- complement and astrocyte-associated genes perhaps mediated by C5aR2 and emphasizing the importance of selectively suppressing C5aR1. Immunohistochemical investigation further confirmed that modulation of C5a-C5aR1 either delayed or reduced some reactive microglial markers in the Arc hippocampus including CD11b and CD11c. These results suggest that C5a-C5aR1 signaling in the context of AD largely exerts its effects by suppressing those microglial activation pathways that accelerate disease enhancing pathways. Given the highly focused modulation of a common driver of neurotoxicity pharmacological inhibition of this C5aR1 signaling pathway is a promising therapeutic strategy to treat AD.

Project description:Many forms of synaptic plasticity are critically dependent upon production of cGMP to trigger activity-dependent increases in synaptic size and strength. Phosphodiesterase 9A (PDE9A) is a high affinity, cGMP-specific phosphodiesterase with widespread distribution in the central nervous system. Inhibition of PDE9A results in significant accumulation of cGMP in brain tissue and cerebrospinal fluid (CSF) of rodents, and increases CSF cGMP in human volunteers. We hypothesized that chronic exposure to a PDE9A inhibitor, and the associated elevations in brain cGMP could provide a therapeutic benefit to vulnerable synapses chronically exposed to Aβ in transgenic mice over-expressing human mutant amyloid precursor protein (Tg2576 mice). A total of N=20 animals per group of 4 month old Tg2576 mice and non-transgenic littermates (WT) were implanted with Alzet osmotic minipumps to deliver vehicle or the PDE9A inhibitor PF-04447943. Neurobehavioral outcomes were measured as conditioned fear response after 28 days of treatment and subsequently brains were harvested for measurement of Aβ, gene expression profiling or synaptic density as assessed by Golgi staining of dendrites. Dendritic spine density on apical dendrites of CA1 neurons exhibited a small but significant deficit in the density of dendritic spines in vehicle treated Tg2576 mice as compared to WT mice. This deficit was ameliorated by 30 days of exposure to PF-04447943. No significant drug effect was observed in WT mice. No significant effects of drug treatment were observed on Aβ levels in Tg2576 mice. Behavioral analysis of Tg2576 mice showed deficits in fear conditioning as early as 2 months old, and therefore were considered unlikely to be due to the accumulation of oligomeric Aβ. These deficits were not affected by drug treatment. Transcriptional profiles of Tg2576 mice treated with drug compared to vehicle showed evidence of regulation of pathways related to synaptic plasticity and remodeling of the dendritic cytoskeleton, consistent with stabilization of vulnerable spine structure. These data supports the hypothesis that PDE9A inhibition can stabilize vulnerable synapses early in the Alzheimer’s disease process. 4-month old Tg2576 mice and non-transgenic (WT) littermates were implanted with minipumps to deliver vehicle or the PDE9A-inhibitor, PF-04447943, for 28 days. At the end of the study, hippocampus and frontal cortex were dissected from n=8 mice per group, RNA was isolated, and hybridized to Affymetrix 430 2.0 mouse whole genome microarray.

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C5aR1 antagonism suppresses microglial polarization and mitigates disease progression in a mouse model of Alzheimer’s disease

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