To the Editor: Post-operative cognitive dysfunction (POCD) is described as a series of changes in the neurocognitive conditions and behaviors of patients, which occur within several weeks or even months after anesthesia and surgery. POCD occurs in 20% to 50% of patients, resulting in burden on patients, and exerting a significant social and economic impact.[1] Although an increased understanding of the effects of neuroinflammation and oxidative stress after surgery and anesthesia have clarified the underlying pathophysiology of POCD, many questions remain to be answered. At present, the treatment of POCD mainly focuses on prevention strategies, early identification, and perioperative risk factor management; and an effective treatment is still lacking. Dexmedetomidine (Dex) is a novel and highly selective α2 adrenergic receptor agonist. Perioperative Dex treatment can significantly reduce the incidence of POCD and inflammation, and it can improve post-operative neurocognitive function. Dex may inhibit the massive release of high mobility group protein B1 (HMGB1) by acting on the α2 adrenergic receptor to activate the PI3K/Akt signaling pathway, thus reducing the binding of HMGB1 with Toll-like receptor 4 and other membrane proteins, thus playing an anti-inflammatory role.[2] Dex can also enhance HMGB1-induced cognitive decline in inflammatory mice via vagal nerve stimulation. HMGB1 is released extracellularly after cell activation, stress, injury, or death and can promote an inflammatory response. High HMGB1 expression is associated with the occurrence of POCD. The inflammatory immune response may play an important role in the pathogenesis of POCD. Regulatory T (Treg) cells are key regulatory cells involved in inflammation and play an important role in immune tolerance and homeostasis. HMGB1 can downregulate the immune function of Treg cells by decreasing the expression of marker molecules on their surface and inhibiting cytokine secretion. Extracellular HMGB1 also intensifies the autoimmune process by impairing the stability of Treg cells.[3] In this study, we aimed to explore the effects and possible mechanisms of action of Dex in POCD.
Twenty-four male Sprague Dawley rats were randomly divided into three groups: sham, POCD, and Dex + POCD (n = 8 per group). This study was approved by the Ethics Committee of the Experimental Animal Center of Guangxi Medical University (No. 201909020). Rats in the POCD and Dex + POCD groups were intra-peritoneally injected with normal saline and 2% pentobarbital sodium (50 mg/kg) 30 min before undergoing splenectomy. The Dex + POCD group was simultaneously intraperitoneally injected with Dex (20 μg/kg). No splenectomy was performed in the sham group, but the rats were otherwise treated the same as the POCD group. The Morris water maze (MWM) test was performed to assess cognitive function. Blood was collected from the abdominal aorta of the rats, and the proportion of Treg cells in the peripheral blood was determined. Inflammation-related factors were also identified. The hippocampal and spleen tissues of rats were used for subsequent analysis.
The cognitive ability of the rats was measured using the MWM. Compared with the sham group, the escape latency time of rats was increased in the POCD group, while the swimming distance ratio in the original platform quadrant decreased. After Dex administration, the escape time was reduced and the ratio of swimming distance in the original platform quadrant increased [Figure 1A]. Furthermore, the serum tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and HMGB1 levels were significantly increased in the POCD group compared to those in the sham group, while the levels of TNF-α, IL-1β, and HMGB1 decreased after Dex administration, with the effects becoming more obvious with time [Figure 1B].
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Figure 1: Dex ameliorated POCD, reduces HMGB1, and modulates the activity of HMGB1-induced Treg cells in the sham, POCD, and Dex + POCD groups. (A) MWM was used to assay the cognitive ability of rats. (B) Serum TNF-α, IL-1β, and HMGB1 levels were detected at 1, 2, 3, 4, and 5 days. (C) Hematoxylin-eosin staining was applied to detect the morphological changes in the hippocampal tissue. (D) Immunochemistry was performed to detect HMGB1 expression in the hippocampus. (E) TUNEL was applied to detect the apoptosis rate of hippocampal cells. (F) Western blotting was performed to measure the expression of TNF-α, IL-1β, and HMGB1 in the hippocampus. (G) Proportion of Treg cells in the peripheral blood. (H) Western blotting was performed to detect HMGB1 and Foxp3 expression in the spleen tissue. (I) Western blotting was performed to detect IL-10, TGF-β, and Foxp3 expression in the hippocampus. ∗ P < 0.05 vs. sham group; †P < 0.05 vs. POCD group. Dex: Dexmedetomidine; HMGB1: High mobility group protein B1; MWM: Morris water maze; POCD: Postoperative cognitive dysfunction; Treg: Regulatory T cell.
Histological analysis of hippocampal tissue showed that the hippocampal neuron cells in the POCD group were severely damaged, with an irregular arrangement and unclear cell structure. HMGB1 significantly decreased cell density, which was significantly rescued by Dex treatment [Figure 1C]. HMGB1 levels were significantly increased in the POCD group compared to the sham group, while the level of HMGB1 was decreased after Dex administration [Figure 1D and Supplementary Figure 1A, https://links.lww.com/CM9/B241]. The POCD group had an increased rate of apoptosis in the hippocampus compared to the sham group, whereas the Dex + POCD group had a decreased rate of apoptosis [Figure 1E and Supplementary Figure 1B, https://links.lww.com/CM9/B241]. Moreover, TNF-α, IL-1β, and HMGB1 expression in the hippocampus in the POCD group were significantly increased compared with that in the sham group, while the expression of TNF-α, IL-1β, and HMGB1 was decreased after Dex administration [Figure 1F].
Analysis of the proportion of Treg cells in the peripheral blood showed that, compared with the sham group, the proportion was decreased in the POCD group, and was increased after Dex administration [Figure 1G]. Foxp3 is a key transcription factor controlling the development and function of Treg cells. Foxp3 expression was decreased and HMGB1 expression was increased in the spleen tissue of the POCD group compared to the sham group. After Dex treatment, HMGB1 expression decreased, and Foxp3 expression increased [Figure 1H]. The hippocampal expression of IL-10, TGF-β, and Foxp3 was lower in the POCD group than in the sham group. After Dex treatment, IL-10, TGF-β, and Foxp3 expression levels increased [Figure 1I].
The pathological process of POCD is related to both neuroinflammation and microglial proliferation. Immune inflammation plays an important role in POCD progression. In rodents, POCD is associated with inflammatory activation of hippocampal microglia. HMGB1 is an abundant nuclear and cytoplasmic protein in mammalian cells; it is released from activated innate immune cells or dead cells, and its role in inflammatory diseases has garnered significant interest. Interestingly, Dex exhibits protective properties under inflammatory conditions, and can thus play a protective role in HMGB1-induced cell damage. Dex may exert a protective effect against traumatic brain injury-induced acute lung injury by acting on the HMGB1-RAGE pathway.[4] Dex has also been shown to inhibit spinal cord microglial activation in mice with spinal cord ischemia-reperfusion injury via the LeT-7a-1/2-3P/HMGB1 pathway.[5] Therefore, in this study, we constructed in vivo POCD animal models and found that Dex improved POCD. Furthermore, the levels of TNF-α, IL-1β, and HMGB1 decreased after Dex treatment.
Tregs play a crucial role in maintaining immune homeostasis and preventing autoimmunity. Treg cells are defined by the expression of the transcription factor Foxp3, which exerts a strong regulatory effect on the function and plasticity of Treg cells. Dex has been shown to effectively improve the function of Treg cells and establishes a new helper T (Th)1/Th2 balance in patients with Graves’ disease.[6] HMGB1 aggravates lipopolysaccharide-induced acute lung injury by inhibiting the activity and function of Tregs. However, the knockout of HMGB1 in tumor cells weakens their ability to induce Treg cells. The protective effect of Dex against cystic echinococcosis may be related to the upregulation of IL-10 and TGF-β1 levels by Treg cells, and the inhibition of Th cells. In the absence of Tregs, the ability to inhibit inflammation is completely lost after treatment with Dex.[7] In this study, we found that IL-10 and TGF-β levels increased after Dex treatment and that Dex affected the activity of Treg cells. Furthermore, Foxp3 expression increased and HMGB1 expression decreased after Dex administration.
Overall, our study demonstrated the effects and possible mechanisms of action of Dex on POCD. Dex ameliorates POCD, reduces HMGB1 release, and modulates HMGB1-induced Treg cell activity. Our study provides a reference and basis for the clinical treatment and prognosis of POCD in the future and will help to enrich new treatment strategies for POCD.
Acknowledgements
We are grateful for the technical support provided by Guangxi Medical University and Ningbo Medical Center Lihuili Hospital.
Funding
The present study was supported by grants from the Ningbo Nature Science Fundation (No. 2019A610267).
Conflicts of interest
None.
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