Novel Monoclonal Antibodies Against Mouse C1q: Characterisation and Development of a Quantitative ELISA for Mouse C1q

Microglial-derived C1q integrates into neuronal ribonucleoprotein complexes and impacts protein homeostasis in the aging brain

Neuroimmune interactions mediate intercellular communication and underlie critical brain functions. Microglia, CNS-resident macrophages, modulate the brain through direct physical interactions and the secretion of molecules. One such secreted factor, the complement protein C1q, contributes to complement-mediated synapse elimination in both developmental and disease models, yet brain C1q protein levels increase significantly throughout aging. Here, we report that C1q interacts with neuronal ribonucleoprotein (RNP) complexes in an age-dependent manner. Purified C1q protein undergoes RNA-dependent liquid-liquid phase separation (LLPS) in vitro, and the interaction of C1q with neuronal RNP complexes in vivo is dependent on RNA and endocytosis. Mice lacking C1q have age-specific alterations in neuronal protein synthesis in vivo and impaired fear memory extinction. Together, our findings reveal a biophysical property of C1q that underlies RNA- and age-dependent neuronal interactions and demonstrate a role of C1q in critical intracellular neuronal processes.

C1q as a target molecule to treat human disease: What do mouse studies teach us?

The complement system is a field of growing interest for pharmacological intervention. Complement protein C1q, the pattern recognition molecule at the start of the classical pathway of the complement cascade, is a versatile molecule with additional non-canonical actions affecting numerous cellular processes. Based on observations made in patients with hereditary C1q deficiency, C1q is protective against systemic autoimmunity and bacterial infections. Accordingly, C1q deficient mice reproduce this phenotype with susceptibility to autoimmunity and infections. At the same time, beneficial effects of C1q deficiency on disease entities such as neurodegenerative diseases have also been described in murine disease models. This systematic review provides an overview of all currently available literature on the C1q knockout mouse in disease models to identify potential target diseases for treatment strategies focusing on C1q, and discusses potential side-effects when depleting and/or inhibiting C1q.

Terminal complement pathway activation drives synaptic loss in Alzheimer's disease models

Complement is involved in developmental synaptic pruning and pathological synapse loss in Alzheimer's disease. It is posited that C1 binding initiates complement activation on synapses; C3 fragments then tag them for microglial phagocytosis. However, the precise mechanisms of complement-mediated synaptic loss remain unclear, and the role of the lytic membrane attack complex (MAC) is unexplored. We here address several knowledge gaps: (i) is complement activated through to MAC at the synapse? (ii) does MAC contribute to synaptic loss? (iii) can MAC inhibition prevent synaptic loss? Novel methods were developed and optimised to quantify C1q, C3 fragments and MAC in total and regional brain homogenates and synaptoneurosomes from WT and AppNL-G-F Alzheimer's disease model mouse brains at 3, 6, 9 and 12 months of age. The impact on synapse loss of systemic treatment with a MAC blocking antibody and gene knockout of a MAC component was assessed in Alzheimer's disease model mice. A significant increase in C1q, C3 fragments and MAC was observed in AppNL-G-F mice compared to controls, increasing with age and severity. Administration of anti-C7 antibody to AppNL-G-F mice modulated synapse loss, reflected by the density of dendritic spines in the vicinity of plaques. Constitutive knockout of C6 significantly reduced synapse loss in 3xTg-AD mice. We demonstrate that complement dysregulation occurs in Alzheimer's disease mice involving the activation (C1q; C3b/iC3b) and terminal (MAC) pathways in brain areas associated with pathology. Inhibition or ablation of MAC formation reduced synapse loss in two Alzheimer's disease mouse models, demonstrating that MAC formation is a driver of synapse loss. We suggest that MAC directly damages synapses, analogous to neuromuscular junction destruction in myasthenia gravis.

Paeoniflorin prevents aberrant proliferation and differentiation of intestinal stem cells by controlling C1q release from macrophages in chronic colitis

The pathological features of inflammatory bowel disease necessitate therapeutic strategies aimed at restoring intestinal mucosal barrier function in addition to controlling inflammation. Paeoniflorin, a bioactive herbal constituent isolated from the root of Paeonia albiflora Pall, has been reported to protect against acute colitis in mice. However, the direct molecular target of paeoniflorin in preventing colitis remains elusive. Here, we evaluated the therapeutical effects of Paeoniflorin using IL-10^-/- chronic colitis model, and explored the precise mechanism of action involved. Our results demonstrated that intragastric administration of Paeoniflorin significantly ameliorated inflammatory response and restored the aberrant intestinal proliferation and differentiation in IL-10^-/-colitis mice. By utilizing a chemical biology approach, we identified C1qa, a crucial component of C1q, is the direct target of Paeoniflorin. Binding of Paeoniflorin to C1qa prevented the cleavage of C1q on macrophages, resulting in the aggregation of surface membrane-anchored C1q and the diminished C1q secretion. The excessive surface membrane-anchored C1q significantly enhanced the phagocytic capability of macrophages and promoted the elimination of infiltrated bacteria and inflammatory cells in mouse colon. The reduced C1q secretion conferred by Paeoniflorin dampened Wnt/β-catenin signaling activation, thereby rectifying the aberrant proliferation and differentiation of intestinal stem cells (ISCs). In summary, our study demonstrates that Paeoniflorin can orchestrate mucosal healing and intestinal inflammation elimination through C1q-bridged macrophage-ISCs crosstalk, highlighting a novel strategy to treat chronic colitis by restoring mucosal homeostasis via targeting C1q.