Astrocytic adenosine A₁ receptors: a new potential target for treating sepsis-associated encephalopathy

Article summary

In a recent article published in Nature Communications [1], Guo et al. [2] identified adenosine A₁ receptors (A₁ARs) in astrocytes as a key therapeutic target for treating sepsis-associated encephalopathy (SAE). Adenosine served as a signaling molecule, triggering early astrocyte reactivity, which in turn provoked microglial responses and mediated the development of SAE induced by systemic endotoxin lipopolysaccharide (LPS) challenge in mice. Inhibiting the effects mediated by A₁ARs in astrocytes prevented this early response, thereby alleviating a range of pathological manifestations of neuroinflammation triggered by increased peripheral and central adenosine levels. Enhancing Gi signaling in astrocytes with specific A₁AR deficiency could restore neuroinflammation following peripheral LPS challenge.

Commentary

Astrocytes have been recognized as a promising target for the treatment of neurological diseases [1, 3, 4], including SAE [5]. Interestingly, a recent publication presented evidence that astrocytic adenosine, acting via its A_2B receptor, plays a crucial role in the maintenance of physiological homeostasis brain function and so could be targeted for treatment of cognitive decline and sleep disorder [6, 7]. It also suggested that target astrocytic adenosine could be a novel strategy for the intervention of SAE. In the present study [1], the authors first investigated the role of adenosine in SAE during the onset and progression of systemic inflammation in mice following i.p. injection of LPS. They observed increased adenosine levels in the blood following systemic inflammation and found that adenosine increased blood–brain barrier (BBB) permeability, as measured by Evans blue (EB) extravasation. The team also found that BBB disruption peaked at 6 h post injection of LPS, coinciding with the elevated plasma adenosine levels. Additionally, LPS induced increased levels of inflammatory cytokines and reactive astrocytes and microglia, peaking at 2–6 h, indicating brain dysfunction induced by systemic inflammation.

Next, in vivo two-photon laser-scanning microscopy was used to monitor binding of endogenous adenosine to a novel sensor for adenosine (GRABAdo1.0) expressed in astrocytes within the somatosensory cortex. They found that systemic inflammation induced by LPS increased extracellular adenosine levels in the brain, reaching a plateau at 6 h, suggesting that peripheral adenosine could directly enhance central adenosine levels. When assessing the role of adenosine in neuroinflammatory responses, they saw upregulation of inflammation-related cytokines and markers of reactive astrocytes in the cerebral cortex, following adenosine i.v. injection in wild-type mice. Similar effects were observed with NECA (a non-selective adenosine analogue) and CPA (a selective A₁AR agonist) injections, while DPCPX (a selective A₁AR antagonist) blocked these effects.

To further explore the role of astrocyte-specific A₁R in adenosine-induced neuroinflammation, they generated astrocytic A₁AR conditional knockout mice (Adora1 cKO). NECA or CCPA injections in Adora1 cKO and control mice showed that the absence of astrocytic A₁ARs attenuated adenosine-induced neuroinflammation, suggesting that A₁AR signaling in astrocytes mediated neuroinflammatory responses triggered by adenosine. Subsequently, they continued to investigate the contribution of A₁AR-mediated early astrocyte reactivity to systemic inflammation. They conducted immunohistochemistry for c-Fos and phosphorylated STAT3 (P-STAT3), as well as high-throughput RNA sequencing, to found further evidence that the astrocyte response was enhanced via A₁AR signaling during the early stages of systemic inflammation triggered by LPS injection.

Astrocytes and microglia are essential glial cells in the central nervous system, and their interaction is crucial for regulating central inflammatory responses [8, 9]. Of note, in exploring the relationship between reactive astrocytes and microglia during the early stages of systemic inflammation, the researchers measured that the proportion of p65, which is a co-factor of NF-κB and an indicator of reactive microglia, that was in the nucleus. Nuclear translocation of p65 was reduced in Adora1 cKO mice compared to controls in the early stages of the systemic LPS model. In addition, they also analyzed the phagocytic ability of microglia and found that microglial phagocytosis was suppressed in the absence of astrocytic A₁ARs. Further analysis showed that microglial morphology and the expression of the P2ry12 gene, a microglial homeostasis marker, were consistent with these findings. Furthermore, they observed less disruption of the BBB and decreased infiltration of peripheral immune cells in Adora1 cKO mice compared to controls after LPS administration. These results suggested that the deficiency of astrocytic A₁AR signaling can inhibit microglial response and neuroinflammation, leading to improved BBB integrity and decreased peripheral immune cell infiltration.

It is well known that neurons are also involved in the regulation of systemic inflammatory responses. A recent study found that neurons participated in the body-brain circuit that regulated body inflammatory responses and kept a homeostatic balance between pro- and anti-inflammatory states [10]. In the present study, astrocytic A₁AR activation led to neuronal hyperactivity during systemic inflammation. The authors confirmed that elevated adenosine level impaired neuronal plasticity after LPS injection. In addition, in depression-like behavioral tests, Adora1 cKO mice exhibited ameliorated behavioral deficits induced by systemic inflammation, again suggesting that astrocytic A_1AR was involved in the occurrence of behavioral disorders associated with systemic inflammation.

A₁AR produced their effects via the Gi/o G proteins, so the authors next investigated the role of Gi in the effects discussed above using the chemogenetic tool, hM4Di, a designer receptors exclusively activated by designer drugs (DREADD), and the specific agonist of hM4Di, clozapine N-oxide (CNO). They found that chemogenetically enhancing Gi signaling with CNO in A₁AR-deficient astrocytes at the early phase post-peripheral LPS challenge could restore neuroinflammatory responses, further confirming that early reactive astrocytes enhance the microglial responses to systemic inflammation.

To sum up, this research provided novel insights at the molecular level into the role of adenosine in neuroinflammation induced by systemic inflammation and offered a rational for targeting astrocytic A₁ARs as an effective therapeutic strategy for treating neuroinflammation. Pharmacological intervention targeting adenosine A₁ receptors may attenuate symptoms of SAE. More to the point, this study provided the first proof that peripheral adenosine could pass the BBB and then directly enhance the central extracellular adenosine levels. While adenosine has been recognized as a beneficial signaling molecule in regulating brain metabolism and function in recent studies [6, 7], this study showed that excessive levels of adenosine have destructive effects on the central nervous system. Furthermore, the authors revealed an interaction between astrocytes and microglia that is regulated by adenosine and illustrated that early reactive astrocytes are an accelerant of reactive microglia in neuroinflammation. However, the interactions between astrocytes and other cells in the early stages of neuroinflammation remain to be explored. It is worth emphasizing that compelling evidence indicated that early reactive astrocytes are a driving factor of neuroinflammation in SAE.

Si-Le Liu

is a master student at the Chengdu University of Traditional Chinese Medicine. Her research interest is focusing on astrocytic purinergic signalling in depression.

Author contributions

SLL and YT wrote the main manuscript and all authors agreed with the final version of the manuscript.

Funding

This work is supported by the NSFC-RSF (82261138557), NSFC (82274668), and Sichuan Provincial Administration of Traditional Chinese Medicine (2023zd024).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval

This article does not contain any studies with human participants or animals performed by any of the authors.

Competing interests

The authors declare no competing interests.