Linking brain innate immunity, energy metabolism, and HFpEF: Hypothalamic complement C3/C3a/C3aR pathway drives cardiometabolic syndrome and HFpEF via central SOCS3 activation (DGK-Forschungsstipendium)

Obesity, a major risk factor for diabetes, hypertension, and hypercholesterolemia, promotes heart failure with preserved ejection fraction (HFpEF). While lipid-driven hypothalamic inflammation via NFkB has been linked to obesity, the role of the hypothalamic complement system in energy imbalance and cardiovascular disease is still unknown. As complement inhibitors progress through clinical trials, this study aims to clarify the role of hypothalamic complement activation in cardiometabolic syndrome and HFpEF, supporting future translational efforts in using complement antagonists in cardiovascular disease.

Methods

C57BL/6J (WT) mice received the C3aR1 antagonists SB290157, JR14a (effective on murine and human receptors), or vehicle, and were fed a high-fat diet (HFD). C3 knockout (C3 KO) and hypothalamus-specific C3aR1 KO mice (C3aR flox x LepR cre) were also fed HFD. In a therapeutic setup, WT mice were made obese with 18 weeks of HFD before JR14a or vehicle was initiated. C3b expression and co-localization of leptin receptor (LepR) with C3aR1 in the hypothalamic arcuate nucleus (ARC) were examined via IHC. Intracellular C3 in microglia and astrocytes were measured by flow cytometry.  Hypothalamic leptin resistance, glucose tolerance, and diastolic function were assessed in vivo. In vitro, mHypoE-46 cell line was used to study C3a’s effects on leptin-driven AgRP release and SOCS3 expression.

Key Results

·      Diet-induced obesity (DIO) impairs insulin sensitivity and central leptin signaling, elevating hypothalamic C3a and C3b expression (Fig 1 A–H).

·      Murine C3aR1 antagonist SB290157 protects against DIO, preserving glucose tolerance and leptin sensitivity (Fig 1I–K).

·      Confocal microscopy showed C3aR co-localizes with hypothalamic, appetite-regulating neurons expressing LepR (Fig 1L). Indeed, both C3 knockout (KO) and hypothalamus-specific C3aR1 KO mice were protected from DIO, showing reduced hypothalamic SOCS3 activation and preserved leptin sensitivity (Fig 2 A–H), underscoring the critical role of hypothalamic C3aR axis in metabolic syndrome.

·      Echocardiography showed improved diastolic function in mice with hypothalamic C3aR knockout (Fig 2 I).

·      Novel C3aR antagonist JR14a, also active on human receptors, protects against DIO as well as insulin resistance (Fig 2 J-O) and reduces weight in already obese mice, improving metabolic state (Fig 2 P-S, W). This is associated with improved diastolic function, reduced LV wall thickness, and lower NT-proBNP levels, suggesting therapeutic potential for HFpEF (Fig 2 T-V).

·      Flow cytometry identified astrocytes as the main source of hypothalamic C3 in obese mice, not microglia (Fig 2 X-Y). In vitro, C3a blocked leptin-mediated suppression of orexigenic AgRP secretion by mHypoE-46 cells. Mechanistically, C3aR1 activation enhanced SOCS3 expression, negatively regulating leptin receptor signaling (Fig 2 Z-AA).

Conclusion and translational perspective

For the first time, we demonstrate that diet-induced obesity activates the hypothalamic complement C3a/C3aR axis. Hypothalamus-specific C3aR knockout protects against central leptin resistance, obesity and HFpEF. The novel, brain permeable, C3aR antagonist JR14a emerges as a promising therapeutic agent for targeting cardiometabolic syndrome and HFpEF, warranting progression to early clinical trials.

This project was supported by the DZHK Clinician Scientist Programme and by a DGK Research Fellowship to ER.