C1q classical complement pathway inhibition: A novel strategy to dampen complement activation and inflammation in alzheimer’s disease

Abstract

Alzheimer’s disease (AD) is a neurological disorder characterised by memory loss and which affects millions over the age of 65. Due to its multifactorial causality, it has proved difficult to find a cure. The primary pathological hallmark of the disorder is the deposition and impaired clearance of extracellular Amyloid-beta plaques. This in turn triggers a cascade of events such as neuronal loss, chronic neuroinflammation and brain atrophy, which manifest behaviorally as memory loss, and will eventually culminate in death. One promising avenue for future therapeutic benefit is the study of neuro-inflammatory effectors; in particular, the complement system, and its primary recognition subcomponent C1q (composed of the globular heads ghA, ghB and ghC). The complement system is a part of the innate immune system that enhances the body’s ability to remove pathogens and aberrant proteins. Once activated, the complement system acts to maintain homeostatic balance within the neuronal milieu by pathogen recognition, opsonisation, inflammatory stimulation, and direct apoptosis through the membrane attack complex of the classical complement pathway. An activated complement system has been implicated in the perpetuation of the chronic inflammatory state observed in AD. C1q the main recognition component binds to Aβ and triggers a whole cascade of events including the activation of the classical complement pathway. which results in the recruitment of microglia and the secretion of inflammatory cytokines. An exaggerated or insufficient activation of the complement system contributes towards the perpetuation of neurotoxicity and neuroinflammation. In this study we cloned mutant homotrimeric forms of the globular heads which are capable of binding to Αβ, but incapable of activating the CCP. We assessed their ability to bind to Αβ via ELISA, and their ability to inhibit complement mediated haemolysis. The homotrimers ghA3, ghB3 and ghC3 bound to Aβ and protected against CCP induced haemolysis in a dose dependent manner. We further demonstrated a downregulation in the mRNA and protein expression of inflammatory mediators via qPCR and western blot post globular head treatment. Finally, we assessed the BV-2 transcriptome post ghB3 treatment and observed an initial upregulation in the expression of inflammatory genes which was severely downregulated 12 hrs post treatment. Indicating that inhibition of the CCP leads to a faster post inflammatory recovery and therefore may be useful towards the prevention of chronic inflammation.