Background: Obesity represents a significant risk factor for the development of diabetes, hypertension, hypercholesterolemia, and thus, atherosclerosis. Although both peripheral complement activation and lipid-mediated hypothalamic inflammation have been implicated in the development of obesity and subsequent atherosclerosis, the role of hypothalamic complement system in dysregulated energy homeostasis during obesity has not yet been investigated. As various complement inhibitors are currently being evaluated in clinical trials, and additional agents are in preclinical development, we here aim to elucidate the functional impact of hypothalamic complement activation in metabolic syndrome and subsequent cardiovascular disease, to pave the way for future translational approaches by using complement receptor antagonists in patients.

Methods: C57BL/6J mice were treated with 30 mg/kg/d complement 3a receptor (C3aR1) antagonist SB290157 or vehicle and fed with high fat diet (HFD, 60% fat) or chow. After 6 or 12 weeks of treatment mice were sacrificed. Hypothalamic sphingolipid accumulation was determined by liquid chromatography–mass spectrometry (LC–MS). Hypothalamic glial fibrillary acidic protein (GFAP) expression, indicating chronic inflammation, was measured by immunohistochemistry. TNF-α, leptin and C3 expression were determined by enzyme-linked immunosorbent assay. Signal transducer and activator of transcription-3 (STAT3) phosphorylation was analyzed via immunoblotting. Leptin resistance and glucose tolerance test we performed according to standardized protocols. 

Results: Obesity due to chronic HFD leads to hypothalamic accumulation of toxic lipid species (Fig. B). Subsequent inflammation, indicated by increased astroglioses (Fig. C) and TNF-α expression (Fig. D), is accompanied by impaired insulin sensitivity and central leptin resistance, fueling hyperphagia (Figs. E, F, G, H), thus maintaining excessive weight gain (Figs. A, I). For the first time, we were able to show that diet-induced obesity leads to increased C3 expression in the hypothalamus (Fig. L). Stinkingly, 6-week treatment with C3aR1 antagonist SB290157 is not only able to reduce weight gain of mice under HFD (Fig. J), but also to protect against hypothalamic leptin resistance, as shown by enhanced STAT3 phosphorylation after leptin injection (Fig. M) and maintain insulin sensitivity as shown by intraperitoneal glucose tolerance test (Fig. K).

Conclusion and perspectives:

Diet-induced obesity is accompanied by hypothalamic complement activation whereby complement receptor inhibition protects against the development of central leptin resistance and obesity. In order to further underlie our hypothesis, we are currently breeding a hypothalamus-specific C3aR1 knock-out mice model using the Cre/loxP-system. Regarding the recently described importance neuroimmune cardiovascular interfaces in the context of atherosclerosis, we will furthermore apply a PCSK9 adeno-associated virus vector (AAV) to render an atherosclerotic disposition in the conditional C3aR1 knock-out model, to elucidate the importance of hypothalamic complement system in atherosclerosis.