Liver injury is a common pathophysiological basis of various liver diseases and is closely associated with inflammation. The nervous system modulates inflammatory responses by releasing neurotransmitters, neuropeptides, chemokines, and other regulatory molecules. It senses microbial invasion and tissue damage and reflexively regulates immune reactions. Although hepatic nerves are known to play a role in inflammation, the specific signals that trigger this response and the potential for neural-targeted therapies remain unclear. In Traditional Chinese Medicine, the principle that “sour enters the liver” suggests that sour stimulation can regulate hepatic physiological and pathological homeostasis. However, how the brain-liver axis functions and whether sour-responsive neurons influence liver inflammation remain unknown.
On March 7, 2025, a research team led by Peixiang Lan, Zhishui Chen, and Jing Liu from the Institute of Organ Transplantation, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, published an article in Journal of Hepatology titled “Sour neuronal signalling attenuates macrophage mediated liver injury.” The study demonstrates that sour stimulation alleviates hepatic ischemia-reperfusion injury (IRI) in both humans and mice. It further clarifies the key role of the brain-liver axis in liver IRI, showing that neuronally secreted TAFA2 (a member of the TAFA chemokine-like family) recruits CCR2+ macrophages to mediate this process. These findings offer new insights and potential therapeutic strategies for treating liver diseases.
To investigate the effects of sour taste on liver injury, the researchers established a mouse model of liver IRI and applied sour stimulation using citric acid. They found that sour stimulation reduced liver tissue damage and serum biomarkers. However, when nerve impulses were blocked by applying the local anesthetic dyclonine to the mouse tongue or by administering citric acid via gavage, the protective effects of sour stimulation disappeared. Furthermore, when the NMDAR antagonist was stereotactically injected into the VPM region of the mouse brain, sour stimulation no longer resulted in significant changes in serum biomarkers or liver injury after IRI. These results suggest that the nervous system plays a critical role in the sour-induced attenuation of liver IRI.
Figure 1. Sour stimulation reduces liver ischemia-reperfusion injury in mice via neural pathways.
To map the connections between the brain and liver, the researchers injected herpes simplex virus H129-hUbC-HBEGFP into the VPM region of the mouse brain and observed fluorescent protein expression in the brain, liver, and celiac ganglion (CG). After celiac ganglion removal (CGX), sour stimulation no longer alleviated liver tissue damage or reduced serum biomarkers in IRI mice. These findings further indicate that sour stimulation exerts its protective effect on the liver through a neural circuit involving the tongue, brain, and CG — forming a brain–CG–liver axis.
Figure 2. Distribution of H129-infected neurons in the brain and liver.
Sour Stimulation Reduces Liver IRI by Suppressing TAFA2 Expression
To investigate how sour stimulation affects gene expression in the liver and CG, the researchers performed single-nucleus RNA sequencing on liver and CG samples from three groups of mice: liver IRI, liver IRI with sour stimulation, and sham-operated controls. The results showed that liver IRI led to an expansion of immune cell clusters, which was reduced by sour stimulation. TAFA2 was specifically expressed in neurons and was upregulated during IRI; however, its expression was suppressed following sour stimulation. In the hippocampal neuronal cell line HT22, potassium ions induced TAFA2 expression, whereas other inflammatory factors did not. Inhibition of voltage-gated potassium channels reduced TAFA2 production and liver injury. These findings suggest that sour stimulation alleviates liver IRI by inhibiting potassium ion–induced TAFA2 expression in intrahepatic neurons, thereby mediating a neuroprotective effect.
Figure 3. Sour stimulation suppresses TAFA2 expression in endothelial cells during liver IRI.
To explore the functional role of neuronal TAFA2 in liver IRI, the researchers used TAFA2 knockout (KO) mice and TAFA2 knockdown mice generated via AAV vectors (AAV-hSyn-EGFP-sh-Tafa2). They found that TAFA2 deficiency or knockdown alleviated macrophage infiltration, liver tissue damage, cell death, and elevated serum biomarkers caused by IRI. These results further demonstrate that TAFA2 plays a critical pro-inflammatory role in liver IRI, and its inhibition can mitigate liver injury.
Figure 4. Both TAFA2 KO and knockdown reduce liver IRI in mice.
TAFA2 Interacts with Macrophages
To characterize immune cell changes during liver IRI, single-cell RNA sequencing revealed that the proportion of macrophage clusters increased during IRI, accompanied by upregulation of inflammation-related genes such as Il1β, Il6, Tnf, and S100. Sour stimulation reduced the expression of these genes. Using FLAG-tagged TAFA2 binding assays and flow cytometry, the study confirmed that TAFA2 specifically binds to macrophages. Furthermore, TAFA2 knockout or knockdown inhibited macrophage infiltration into the liver during IRI. In vitro experiments also showed that TAFA2 can activate bone marrow–derived macrophages and induce the expression of pro-inflammatory cytokines, confirming that macrophages are activated by TAFA2 and mediate liver IRI.
Figure 5. TAFA2 increases macrophage proportion during IRI and promotes the production of inflammatory cytokines.
TAFA2 Interacts with Macrophages via CCR2
To investigate the mechanism by which TAFA2 interacts with macrophages and to understand the role of CCR2, the researchers conducted immunoprecipitation, mass spectrometry, and flow cytometry experiments, which confirmed that CCR2 is a specific receptor for TAFA2 on macrophages. RNA-seq analysis revealed that the transcriptional profiles of bone marrow–derived macrophages (BMDMs) stimulated by TAFA2 or CCL2 were nearly identical. Sixty-six genes related to adhesion, metabolism, and Ras signaling pathways were upregulated, while 195 genes associated with metabolism and ribosomal pathways were downregulated. TAFA2 also induced higher expression levels of inflammatory genes such as Il1α, Il1β, Il6, and Tnfα, as well as interferon-stimulated genes, thereby enhancing macrophage-mediated inflammatory responses.
Further in vivo experiments showed that the pro-inflammatory effect of CCL2 on macrophages is CCR2-dependent, and TAFA2 primarily promotes macrophage-driven inflammation via CCR2. However, other potential receptors for TAFA2 may also exist on macrophages.
Figure 6. TAFA2 interacts with CCR2 receptors on the surface of macrophages.
Sour Stimulation Alleviates Liver IRI During Human Hepatectomy
To validate the effect of sour stimulation on liver IRI in humans, an open-label, randomized, blank-controlled clinical trial was conducted in Wuhan. A total of 66 patients undergoing hepatectomy were enrolled and divided into a sour group (administered 25 mM citric acid via oral rinsing for 5 minutes every 8 hours, starting 24 hours before surgery) and a control group. Six patients were excluded due to intraoperative liver ischemia lasting more than 30 minutes, and four were excluded due to poor compliance (failure to follow the oral rinse instructions). The results showed that postoperative serum ALT and AST levels in the sour group were significantly lower than those in the control group. Notably, the incidence of severe liver injury (ALT > 500 U/L) was 0% in the sour group, compared to 40% in the control group. Moreover, in mouse models, sour stimulation similarly alleviated liver IRI. These findings confirm that sour stimulation can effectively reduce the risk of severe postoperative liver injury, mitigating liver IRI in both humans and mice during hepatectomy.
Figure 7. Sour stimulation reduces liver IRI during human hepatectomy.
Conclusion
This study is the first to reveal the regulatory role of the brain–liver axis in liver injury and to elucidate the mechanism by which sour stimulation alleviates hepatic damage via neural signaling pathways. These findings provide a novel intervention strategy for reducing ischemia-reperfusion injury during clinical liver surgery, with significant potential for clinical application.
All viral vectors used in this study are available from Brain Case Biotech( bd@ebraincase.com)
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