Complement therapeutics in neurodegenerative diseases.

Neurodegenerative diseases (NDDs) such as Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis pose considerable therapeutic challenges, not only due to their complex pathophysiology, but also because any effective drug must be capable of penetrating the brain. Inflammation is a key feature of NDDs. Increasingly, the complement system, long studied in the context of host defence, has emerged as a central player in the brain, with roles extending far beyond its classical immune functions.

Knowing the enemy: strategic targeting of complement to treat Alzheimer disease.

The complement system protects against infection, positively responds to tissue damage, clears cell debris, directs and modulates the adaptive immune system, and functions in neuronal development, normal synapse elimination and intracellular metabolism. However, complement also has a role in aberrant synaptic pruning and neuroinflammation - processes that lead to a feedforward loop of inflammation, injury and neuronal death that can contribute to neurodegenerative and neurological disorders, including Alzheimer disease. This Review provides justification, largely from preclinical mouse models but also from correlates with human tissue and biomarkers, for targeting specific complement components for therapeutic intervention in Alzheimer disease.

APOE Christchurch enhances a disease-associated microglial response to plaque but suppresses response to tau pathology.

Background: Apolipoprotein E ε4 (APOE4) is the strongest genetic risk factor for late-onset Alzheimer's disease (LOAD). A recent case report identified a rare variant in APOE, APOE3-R136S (Christchurch), proposed to confer resistance to autosomal dominant Alzheimer's Disease (AD). However, it remains unclear whether and how this variant exerts its protective effects.

C5aR1 antagonism suppresses inflammatory glial responses and alters cellular signaling in an Alzheimer's disease mouse model.

Alzheimer's disease (AD) is the leading cause of dementia in older adults, and the need for effective, sustainable therapeutic targets is imperative. The complement pathway has been proposed as a therapeutic target. C5aR1 inhibition reduces plaque load, gliosis, and memory deficits in animal models, however, the cellular bases underlying this neuroprotection were unclear.

C5aR1 inhibition alleviates cranial radiation-induced cognitive decline.

Unlabelled: Cranial radiation therapy (RT) for brain cancers leads to an irreversible decline in cognitive function without an available remedy. Radiation-induced cognitive deficits (RICD) are a particularly pressing problem for the survivors of pediatric and low grade glioma (LGG) cancers who often live long post-RT lives. Radiation-induced elevated neuroinflammation and gliosis, triggered by the detrimental CNS complement cascade, lead to excessive synaptic and cognitive loss.

APOE Christchurch enhances a disease-associated microglial response to plaque but suppresses response to tau pathology.

Background: Apolipoprotein E ε4 (APOE4) is the strongest genetic risk factor for late-onset Alzheimer's disease (LOAD). A recent case report identified a rare variant in APOE, APOE3-R136S (Christchurch), proposed to confer resistance to autosomal dominant Alzheimer's Disease (AD). However, it remains unclear whether and how this variant exerts its protective effects.

Sex-specific associations between AD genotype and the microbiome of human amyloid beta knock-in (hAβ-KI) mice.

Introduction: Emerging evidence links changes in the gut microbiome to late-onset Alzheimer's disease (LOAD), necessitating examination of AD mouse models with consideration of the microbiome.

Methods: We used shotgun metagenomics and untargeted metabolomics to study the human amyloid beta knock-in (hAβ-KI) murine model for LOAD compared to both wild-type (WT) mice and a model for early-onset AD (3xTg-AD).

Results: Eighteen-month female (but not male) hAβ-KI microbiomes were distinct from WT microbiomes, with AD genotype accounting for 18% of the variance by permutational multivariate analysis of variance (PERMANOVA).

The Abca7 variant reduces Aβ generation, plaque load, and neuronal damage.

Background: Variants in ABCA7, a member of the ABC transporter superfamily, have been associated with increased risk for developing late onset Alzheimer's disease (LOAD).

Methods: CRISPR-Cas9 was used to generate an Abca7 variant in mice, modeling the homologous human ABCA7 variant, and extensive characterization was performed.

Results: Abca7 microglia show differential gene expression profiles upon lipopolysaccharide challenge and increased phagocytic capacity.

BIN1 suppresses glial response to plaques in mouse model of Alzheimer's disease.

Introduction: The BIN1 coding variant rs138047593 (K358R) is linked to Late-Onset Alzheimer's Disease (LOAD) via targeted exome sequencing.

Methods: To elucidate the functional consequences of this rare coding variant on brain amyloidosis and neuroinflammation, we generated BIN1 knock-in mice using CRISPR/Cas9 technology. These mice were subsequently bred with 5xFAD transgenic mice, which serve as a model for Alzheimer's pathology.

C5aR1 signaling promotes region- and age-dependent synaptic pruning in models of Alzheimer's disease.

Introduction: Synaptic loss is a hallmark of Alzheimer's disease (AD) that correlates with cognitive decline in AD patients. Complement-mediated synaptic pruning has been associated with this excessive loss of synapses in AD. Here, we investigated the effect of C5aR1 inhibition on microglial and astroglial synaptic pruning in two mouse models of AD.

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

Introduction: Synaptic loss is a hallmark of Alzheimer's disease (AD) that correlates with cognitive decline in AD patients. Complement-mediated synaptic pruning has been associated with this excessive loss of synapses in AD. Here, we investigated the effect of C5aR1 inhibition on microglial and astroglial synaptic pruning in two mouse models of AD.

C5aR1 antagonism suppresses inflammatory glial gene expression and alters cellular signaling in an aggressive Alzheimer's model.

Alzheimer's disease (AD) is the leading cause of dementia in older adults, and the need for effective, sustainable therapeutic targets is imperative. Pharmacologic inhibition of C5aR1 reduces plaque load, gliosis and memory deficits in animal models. However, the cellular basis underlying this neuroprotection and which processes were the consequence of amyloid reduction vs alteration of the response to amyloid were unclear.

A Trem2 mouse model without cryptic splicing drives age- and disease-dependent tissue damage and synaptic loss in response to plaques.

Background: The TREM2 R47H variant is one of the strongest genetic risk factors for late-onset Alzheimer's Disease (AD). Unfortunately, many current Trem2 mouse models are associated with cryptic mRNA splicing of the mutant allele that produces a confounding reduction in protein product. To overcome this issue, we developed the Trem2 (Normal Splice Site) mouse model in which the Trem2 allele is expressed at a similar level to the wild-type Trem2 allele without evidence of cryptic splicing products.

Complement in the Brain: Contributions to Neuroprotection, Neuronal Plasticity, and Neuroinflammation.

The complement system is an ancient collection of proteolytic cascades with well-described roles in regulation of innate and adaptive immunity. With the convergence of a revolution in complement-directed clinical therapeutics, the discovery of specific complement-associated targetable pathways in the central nervous system, and the development of integrated multi-omic technologies that have all emerged over the last 15 years, precision therapeutic targeting in Alzheimer disease and other neurodegenerative diseases and processes appears to be within reach. As a sensor of tissue distress, the complement system protects the brain from microbial challenge as well as the accumulation of dead and/or damaged molecules and cells.

Degenerate mapping of environmental location presages deficits in object-location encoding and memory in the 5xFAD mouse model for Alzheimer's disease.

A key challenge in developing diagnosis and treatments for Alzheimer's disease (AD) is to detect abnormal network activity at as early a stage as possible. To date, behavioral and neurophysiological investigations in AD model mice have yet to conduct a longitudinal assessment of cellular pathology, memory deficits, and neurophysiological correlates of neuronal activity. We therefore examined the temporal relationships between pathology, neuronal activities and spatial representation of environments, as well as object location memory deficits across multiple stages of development in the 5xFAD mice model and compared these results to those observed in wild-type mice.

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

Multiple studies have recognized the involvement of the complement cascade during Alzheimer's disease pathogenesis. However, the specific role of C5a-C5aR1 signaling in the progression of this neurodegenerative disease is still not clear. Furthermore, its potential as a therapeutic target to treat AD still remains to be elucidated.

Modulation of C5a-C5aR1 signaling alters the dynamics of AD progression.

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.

Impact of COVID-19 on the Onset and Progression of Alzheimer's Disease and Related Dementias: A Roadmap for Future Research.

COVID-19 causes lasting neurological symptoms in some survivors. Like other infections, COVID-19 may increase risk of cognitive impairment. This perspective highlights four knowledge gaps about COVID-19 that need to be filled to avoid this possible health issue.

Complement as a powerful "influencer" in the brain during development, adulthood and neurological disorders.

The complement system was long considered as only a powerful effector arm of the immune system that, while critically protective, could lead to inflammation and cell death if overactivated, even in the central nervous system (CNS). However, in the past decade it has been recognized as playing critical roles in key physiological processes in the CNS, including neurogenesis and synaptic remodeling in the developing and adult brain. Inherent in these processes are the interactions with cells in the brain, and the cascade of interactions and functional consequences that ensue.

C1q and SRPX2 regulate microglia mediated synapse elimination during early development in the visual thalamus but not the visual cortex.

The classical complement cascade mediates synapse elimination in the visual thalamus during early brain development. However, whether the primary visual cortex also undergoes complement-mediated synapse elimination during early visual system development remains unknown. Here, we examined microglia-mediated synapse elimination in the visual thalamus and the primary visual cortex of early postnatal C1q and SRPX2 knockout mice.