Increased Thyroid Hormone Action Alleviates Hippocampal Damage by Downregulating Neuronal Type I Interferon Signaling/Necroptosis in Diabetes-Associated Cognitive Dysfunction

Background: Thyroid dysfunction plays an important role in the pathology of diabetes-associated cognitive dysfunction (DACD). However, thyroid hormone (TH) signaling and action changes in DACD brains remain unknown. This study evaluated the alternations in TH signaling and action in the brains of DACD mice and explored the beneficial effects of levothyroxine (L-T4) treatment. Methods: KK-Ay mice, serving as a spontaneous type 2 diabetes mellitus model, underwent intragastric administration of 10 ng/g and 20 ng/g of L-T4 solution or normal saline for 8 weeks. Age-matched C57BL/6J mice were used as normal controls. Cognitive and memory functions were examined through the open field and Morris water maze tests. Hippocampal TH signaling and pathogenic status were evaluated. The potential signaling pathways involved in the neuroprotective action of L-T4 were investigated through RNA sequencing and further verified through quantitative real-time PCR (qPCR), Western blotting (WB), immunofluorescence (IF), and fluorescent multiplex immunohistochemistry (mIHC) in vivo and vitro. Results: The expressions of hippocampal TH transporters (Mct8 and Oatp1c1), Dio2, and TH receptor were upregulated, whereas Dio3 as well as the TH-positive regulated genes MBP, Enpp2, and Klf9 were downregulated in DACD mice. Exogenous L-T4 partially alleviated cognitive and memory dysfunction and restored hippocampal neuronal activity by optimizing TH signaling. RNA sequencing provided insights into the role of type I interferon (IFN-I) signaling and necroptosis on the amelioration of hippocampal damage after L-T4 treatment. WB and qPCR further confirmed that the levels of key proteins for IFN-I signaling and necroptosis (p-STAT1, p-STAT2, IRF9, ZBP1, p-RIP3, and p-MLKL) were increased, but largely returned after L-T4 administration in vivo and T3 treatment in vitro. IF and mIHC revealed that IRF9 and p-MLKL colocalized in neurons, but not in astrocytes or microglia, of the hippocampus in DACD mice. The diabetes mellitus group had an increased number of IRF9⁺ p-MLKL⁺ NeuN⁺ cells, which decreased after L-T4 treatment. The elevated IFN-I signaling-mediated necroptosis in HT22 cells was also decreased by T3. Conclusion: We demonstrated abnormal hippocampal TH signaling and action in DACD. Promoting TH action with exogenous L-T4 ameliorated hippocampal impairment through inhibiting IFN-I signaling-induced necroptosis.