The "C3aR Antagonist" SB290157 is a Partial C5aR2 Agonist

Complement C3/C3aR Signaling Pathway Inhibition Ameliorates Retinal Damage in Experimental Retinal Vein Occlusion

Purpose

Retinal vein occlusion (RVO) is a common retinal vascular disease that severely threatens visual function. This study aims to elucidate the role of the complement C3/C3aR signaling pathway in a laser-induced RVO mouse model and to explore its potential as a therapeutic target.

Methods

RVO was induced in C57BL/6J mice using laser photocoagulation combined with photosensitizer dye administration. Two days later, retinal tissues were collected for bulk RNA sequencing. The activation of the C3/C3aR signaling pathway was validated through RT-qPCR and Western blot. The C3aR antagonist SB290157 (C3aRA) was administered intravitreally and retinal morphological and functional changes were examined 1, 2, and 8 days later by optical coherence tomography (OCT), fundus photography (FP), and fluorescein angiography (FA), optomotor response (OKR) test, and electroretinogram (ERG).

Results

RVO mice exhibited marked increases in retinal thickness (P < 0.001) and fluorescence leakage (P < 0.01) compared to the sham-laser group. Bulk RNA-seq revealed significant upregulation of the complement pathway. Elevated expression of C3 and C3aR (P < 0.05) was confirmed by RT-qPCR and Western blot. Blocking C3aR with SB290157 significantly alleviated RVO-induced retinal edema, vascular leakage, and structural damage. Functional assessment showed that SB290157 treatment significantly improved contrast sensitivity (P < 0.05), increased b-wave (P < 0.001), and oscillatory potentials (Ops) amplitudes (P < 0.05) in RVO mice. RNA-seq analysis demonstrated that SB290157 significantly reduced the inflammatory mediator-related pathways and upregulated visual perception pathways (P < 0.05).

Conclusions

The complement C3/C3aR signaling pathway is critically involved in RVO-induced retinal damage and targeting this pathway may be a promising approach for RVO treatment.

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. We discuss promising strategies to slow the progression of cognitive loss with minimal undesired effects. The diverse interactions and functions of complement system components can influence biological processes in the healthy and diseased brain; here, these functions are described as a prerequisite to selecting appropriate, safe and effective therapeutic targets for translation to the clinic.

The role of the complement system in the response to cytotoxic therapy

The complement system is increasingly recognised as a key player in tumour progression and response to cancer treatment. Cytotoxic therapies, including chemo- and radiotherapy are standard-of-care for the majority of cancer patients. Cytotoxics have been found to alter the expression of complement system proteins and activation of components. Many recent reports highlight the role of local dysregulation of complement proteins in the tumour microenvironment and how targeting such dysregulation can have either anti- or pro-tumoricidal effects depending on several factors including treatment scheduling, the tumour type and its microenvironment characteristics. This review will explore the complex effects of cytotoxic therapy on complement regulation and what lessons can be learnt to identify the most effective way to therapeutically modulate complement system proteins for cancer therapy.

Functional profiling of the G protein-coupled receptor C3aR1 reveals ligand-mediated biased agonism

G protein-coupled receptors (GPCRs) are leading druggable targets for several medicines, but many GPCRs are still untapped for their therapeutic potential due to poor understanding of specific signaling properties. The complement C3a receptor 1 (C3aR1) has been extensively studied for its physiological role in C3a-mediated anaphylaxis/inflammation, and in TLQP-21-mediated lipolysis, but direct evidence for the functional relevance of the C3a and TLQP-21 ligands and signal transduction mechanisms are still limited. In addition, C3aR1 G protein coupling specificity is still unclear, and whether endogenous ligands, or drug-like compounds, show ligand-mediated biased agonism is unknown. Here, we demonstrate that C3aR1 couples preferentially to Gi/o/z proteins and can recruit β-arrestins to cause internalization. Furthermore, we showed that in comparison to C3a63-77, TLQP-21 exhibits a preference toward Gi/o-mediated signaling compared to β-arrestin recruitment and internalization. We also show that the purported antagonist SB290157 is a very potent C3aR1 agonist, where antagonism of ligand-stimulated C3aR1 calcium flux is caused by potent β-arrestin-mediated internalization. Finally, ligand-mediated signaling bias impacted cell function as demonstrated by the regulation of calcium influx, lipolysis in adipocytes, phagocytosis in microglia, and degranulation in mast cells. Overall, we characterize C3aR1 as a Gi/o/z-coupled receptor and demonstrate the functional relevance of ligand-mediated signaling bias in key cellular models. Due to C3aR1 and its endogenous ligands being implicated in inflammatory and metabolic diseases, these results are of relevance toward future C3aR1 drug discovery.

The complement C3-complement factor D-C3a receptor signalling axis regulates cardiac remodelling in right ventricular failure

Failure of the right ventricle plays a critical role in any type of heart failure. However, the mechanism remains unclear, and there is no specific therapy. Here, we show that the right ventricle predominantly expresses alternative complement pathway-related genes, including Cfd and C3aR1. Complement 3 (C3)-knockout attenuates right ventricular dysfunction and fibrosis in a mouse model of right ventricular failure. C3a is produced from C3 by the C3 convertase complex, which includes the essential component complement factor D (Cfd). Cfd-knockout mice also show attenuation of right ventricular failure. Moreover, the plasma concentration of CFD correlates with the severity of right ventricular failure in patients with chronic right ventricular failure. A C3a receptor (C3aR) antagonist dramatically improves right ventricular dysfunction in mice. In summary, we demonstrate the crucial role of the C3-Cfd-C3aR axis in right ventricular failure and highlight potential therapeutic targets for right ventricular failure.

Right ventricular (RV) failure is clinically crucial, but there is no specific therapy. Here, the authors show that the complement alternative pathway is activated in RV failure and that blockade of the pathway ameliorates RV failure in mice.

Four Chemotherapeutic Compounds That Limit Blood-Brain-Barrier Invasion by Toxoplasma gondii

Background: Toxoplasma gondii, an intracellular protozoan parasite, exists in the host brain as cysts, which can result in Toxoplasmic Encephalitis (TE) and neurological diseases. However, few studies have been conducted on TE, particularly on how to prevent it. Previous proteomics studies have showed that the expression of C3 in rat brains was up-regulated after T. gondii infection. Methods: In this study, we used T. gondii to infect mice and bEnd 3 cells to confirm the relation between T. gondii and the expression of C3. BEnd3 cells membrane proteins which directly interacted with C3a were screened by pull down. Finally, animal behavior experiments were conducted to compare the differences in the inhibitory ability of TE by four chemotherapeutic compounds (SB290157, CVF, NSC23766, and Anxa1). Results: All chemotherapeutic compounds in this study can inhibit TE and cognitive behavior in the host. However, Anxa 1 is the most suitable material to inhibit mice TE. Conclusion: T. gondii infection promotes TE by promoting host C3 production. Anxa1 was selected as the most appropriate material to prevent TE among four chemotherapeutic compounds closely related to C3.

Unexpected Off-Target Activities for Recombinant C5a in Human Macrophages

The anaphylatoxin C5a is core effector of complement activation. C5a exerts potent proinflammatory and immunomodulatory actions through interacting with its C5a receptors, C5aR1 and C5aR2, modulating multiple signaling and functional activities of immune cells. Native C5a contains a large N-linked glycosylation site at Asn⁶⁴, which accounts for up to 25% of its m.w. To date, the vast majority of published studies examining C5a are performed using Escherichia coli-generated recombinant C5a, which is readily available from numerous commercial suppliers, but lacks this glycosylation moiety. However, a plasma-purified "native" form of C5a is also commercially available. The different size and glycosylation of these two C5a versions could have functional implications. Therefore, the current study aimed to compare recombinant human C5a to purified plasma-derived human C5a in driving the signaling and functional activities of human primary macrophages. We found that both versions of C5a displayed similar potencies at triggering C5aR1- and C5aR2-mediated cell signaling, but elicited distinct functional responses in primary human monocyte-derived macrophages. Multiple commercial sources of recombinant C5a, but not the plasma-purified or a synthetic C5a version, induced human monocyte-derived macrophages to produce IL-6 and IL-10 in a C5a receptor-independent manner, which was driven through Syk and NF-κB signaling and apparently not due to endotoxin contamination. Our results, therefore, offer caution against the sole use of recombinant human C5a, particularly in functional/cytokine assays conducted in human primary immune cells, and suggest studies using recombinant human C5a should be paired with C5aR1 inhibitors or purified/synthetic human C5a to confirm relevant findings.

Air Bubbles Activate Complement and Trigger Hemostasis and C3-Dependent Cytokine Release Ex Vivo in Human Whole Blood

Air bubbles trigger a C3-driven thromboinflammation in human whole blood.

Blocking C3, but not C5, attenuates the air-induced inflammation.

Avoiding ambient air in test tubes attenuates thromboinflammation.

Venous air embolism, which may complicate medical and surgical procedures, activates complement and triggers thromboinflammation. In lepirudin-anticoagulated human whole blood, we examined the effect of air bubbles on complement and its role in thromboinflammation. Whole blood from 16 donors was incubated with air bubbles without or with inhibitors of C3, C5, C5aR1, or CD14. Complement activation, hemostasis, and cytokine release were measured using ELISA and quantitative PCR. Compared with no air, incubating blood with air bubbles increased, on average, C3a 6.5-fold, C3bc 6-fold, C3bBbP 3.7-fold, C5a 4.6-fold, terminal complement complex sC5b9 3.6-fold, prothrombin fragments 1+2 (PTF1+2) 25-fold, tissue factor mRNA (TF-mRNA) 26-fold, microparticle tissue factor 6.1-fold, β-thromboglobulin 26-fold (all p < 0.05), and 25 cytokines 11-fold (range, 1.5–78-fold; all p < 0.0001). C3 inhibition attenuated complement and reduced PTF1+2 2-fold, TF-mRNA 5.4-fold, microparticle tissue factor 2-fold, and the 25 cytokines 2.7-fold (range, 1.4–4.9-fold; all p < 0.05). C5 inhibition reduced PTF1+2 2-fold and TF-mRNA 12-fold (all p < 0.05). C5 or CD14 inhibition alone reduced three cytokines, including IL-1β (p = 0.02 and p = 0.03). Combined C3 and CD14 inhibition reduced all cytokines 3.9-fold (range, 1.3–9.5-fold; p < 0.003) and was most pronounced for IL-1β (3.2- versus 6.4-fold), IL-6 (2.5- versus 9.3-fold), IL-8 (4.9- versus 8.6-fold), and IFN-γ (5- versus 9.5-fold). Antifoam activated complement and was avoided. PTF1+2 was generated in whole blood but not in plasma. In summary, air bubbles activated complement and triggered a C3-driven thromboinflammation. C3 inhibition reduced all mediators, whereas C5 inhibition reduced only TF-mRNA. Combined C5 and CD14 inhibition reduced IL-1β release. These data have implications for future mechanistic studies and possible pharmacological interventions in patients with air embolism.

The molecular identity of the TLQP-21 peptide receptor

The TLQP-21 neuropeptide has been implicated in functions as diverse as lipolysis, neurodegeneration and metabolism, thus suggesting an important role in several human diseases. Three binding targets have been proposed for TLQP-21: C3aR1, gC1qR and HSPA8. The aim of this review is to critically evaluate the molecular identity of the TLQP-21 receptor and the proposed multi-receptor mechanism of action. Several studies confirm a critical role for C3aR1 in TLQP-21 biological activity and a largely conserved mode of binding, receptor activation and signaling with C3a, its first-identified endogenous ligand. Conversely, data supporting a role of gC1qR and HSPA8 in TLQP-21 activity remain limited, with no signal transduction pathways being described. Overall, C3aR1 is the only receptor for which a necessary and sufficient role in TLQP-21 activity has been confirmed thus far. This conclusion calls into question the validity of a multi-receptor mechanism of action for TLQP-21 and should inform future studies.

Development of Potent and Selective Agonists for Complement C5a Receptor 1 with In Vivo Activity

The anaphylatoxin C5a is a complement peptide associated with immune-related disorders. C5a binds with equal potency to two GPCRs, C5aR1 and C5aR2. Multiple C5a peptide agonists have been developed to interrogate the C5a receptor function but none show selectivity for C5aR1. To address these limitations, we developed potent and stable peptide C5aR1 agonists that display no C5aR2 activity and over 1000-fold selectivity for C5aR1 over C3aR. This includes BM213, which induces C5aR1-mediated calcium mobilization and pERK1/2 signaling but not β-arrestin recruitment, and BM221, which exhibits no signaling bias. Both ligands are functionally similar to C5a in human macrophage cytokine release assays and in a murine in vivo neutrophil mobilization assay. BM213 showed antitumor activity in a mouse model of mammary carcinoma. We anticipate that these C5aR1-selective agonists will be useful research tools to investigate C5aR1 function.