Modulation of C5a–C5aR1 signaling alters the dynamics of AD progression

Abstract

Background The complement system is part of the innate immune system that clears pathogens and cellular debris. In the healthy brain, complement influences neurodevelopment and neurogenesis, synaptic pruning, clearance of neuronal blebs, recruitment of phagocytes, and protects from pathogens. However, excessive downstream complement activation that leads to generation of C5a, and C5a engagement with its receptor C5aR1, instigates a feed-forward loop of inflammation, injury, and neuronal death, making C5aR1 a potential therapeutic target for neuroinflammatory disorders. C5aR1 ablation in the Arctic (Arc) model of Alzheimer’s disease protects against cognitive decline and neuronal injury without altering amyloid plaque accumulation. Methods To elucidate the effects of C5a–C5aR1 signaling on AD pathology, we crossed Arc mice with a C5a-overexpressing mouse (ArcC5a+) and tested hippocampal memory. RNA-seq was performed on hippocampus and cortex from Arc, ArcC5aR1KO, and ArcC5a+ mice at 2.7–10 months and age-matched controls to assess mechanisms involved in each system. Immunohistochemistry was used to probe for protein markers of microglia and astrocytes activation states. Results ArcC5a+ mice had accelerated cognitive decline compared to Arc. Deletion of C5ar1 delayed or prevented the expression of some, but not all, AD-associated genes in the hippocampus and a subset of pan-reactive and A1 reactive astrocyte genes, indicating a separation between genes induced by amyloid plaques alone and those influenced by C5a–C5aR1 signaling. Biological processes associated with AD and AD mouse models, including inflammatory signaling, microglial cell activation, and astrocyte migration, were delayed in the ArcC5aR1KO hippocampus. Interestingly, C5a overexpression also delayed the increase of some AD-, complement-, and astrocyte-associated genes, suggesting the possible involvement of neuroprotective C5aR2. However, these pathways were enhanced in older ArcC5a+ mice compared to Arc. Immunohistochemistry confirmed that C5a–C5aR1 modulation in Arc mice delayed the increase in CD11c-positive microglia, while not affecting other pan-reactive microglial or astrocyte markers. Conclusion C5a–C5aR1 signaling in AD largely exerts its effects by enhancing microglial activation pathways that accelerate disease progression. While C5a may have neuroprotective effects via C5aR2, engagement of C5a with C5aR1 is detrimental in AD models. These data support specific pharmacological inhibition of C5aR1 as a potential therapeutic strategy to treat AD.

C5aR1 signaling promotes region and age dependent synaptic pruning in models of Alzheimer’s Disease

“…C3 has been shown to be involved in synapse pruning (19)(20)(21)44) C3 staining, using an antibody that recognizes whole C3 as well as its cleavage products, was observed mainly on astrocytes and deposited on the neuropil. Our results showed a high amount of C3 positive astrocytes, mainly in the vicinity of amyloid plaques (data not shown), which parallels previous bulk RNAseq and IHC results from our lab showing a strong astroglial response (characterized by an increased in GFAP and C3) in the hippocampus of Arctic and Arc-C5aR1KO mice at 7 and 10 months of age (35). Here, our results at 10 months in the CA3-SL region showed a significant increase of C3 puncta (excluding astroglia-associated C3) in the Arc mice when compared to WT littermates, which was further increased in the Arc-C5aR1KO mice…”

“…Previous studies in our lab and others, have shown that the genetic or pharmacological inhibition of C5a-C5aR1 signaling reduced pathology, and rescued synaptic and cognitive loss in several mouse models of AD (31)(32)(33)(34)(35), suggesting that C5a-C5aR1 signaling might be a better therapeutic target than blocking the upstream components of the complement cascade. Moreover, two different C5aR1 antagonist have already been proven safe in humans in different clinical trials for autoimmune disorders, further emphasizing the potential for C5aR1 as a therapeutic target (36,37).…”

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

“…2-photon in vivo studies can be used to overcome this limitation in future studies. Additionally, since only female mice were used for the present study, possible microglial sex-specific changes in response to PMX205 should be assessed, although genetic ablation of C5aR1 in another mouse model of amyloidosis (Arctic) did not show sex-related effects in behavior, gliosis or gene expression in either the AD characteristics or the effect of a deletion of C5aR1 [ 17 , 21 ]. Finally, peripheral inflammation certainly can affect the progression of AD-pathology ([ 81 , 82 ] and reviewed in [ 83 ]), and it remains to be seen what the contribution of PMX205 mediated peripheral effects may be.…”

“…We have previously reported a neuroprotective role of C1q in the absence of triggering downstream activation [ 12 , 13 ], but we and others have also demonstrate a beneficial effect of a genetic ablation of C1q, C3 and CR3 in AD mouse models [ 14 – 16 ]. In addition, C3a is generated upon complement activation and binds to C3aR, whose expression is induced in microglia in mouse models of amyloidosis including very clear tissue and temporal induction recently published by this lab [ 17 ]. Both in the brain and in the periphery, C3a-C3aR signaling can be either pro- or anti-inflammatory depending on the developmental or disease state [ 18 ].…”

Mechanisms of abnormal adult hippocampal neurogenesis in Alzheimer’s disease

“…However, in the process of AD disease, excessive activation of downstream complement will lead to the production of C5a, which can bind with the receptor and cause a series of negative reactions such as inflammation, injury and neuronal death ( Ager et al, 2010 ). It has been shown that in AD, the C5a-C5aR1 signaling pathway can accelerate disease progression by enhancing the activation of microglia and exacerbating neuroinflammation ( Carvalho et al, 2022 ). In the process of finding an effective treatment for AD, people found that sigma1 receptors (S1Rs) may regulate AHN, prevent Aβ-induced neurotoxicity and modulate the pathophysiology of AD, and have great potential as new targets for treatment ( Penke et al, 2018 ; Ma et al, 2021 ).…”