Spinal apolipoprotein E is involved in inflammatory pain via regulating lipid metabolism and glial activation in the spinal dorsal horn

Introduction Inflammation and nerve injury promote astrocyte activation, which regulates the development and resolution of pain, in the spinal dorsal horn. APOE regulates lipid metabolism and is predominantly expressed in the astrocytes. However, the effect of astrocytic APOE and lipid metabolism on spinal cellular function is unclear. This study aimed to investigate the effect of spinal Apoe on spinal cellular functions using the complete Freund's adjuvant (CFA)-induced inflammatory pain mouse model. Methods After intraplantar injection of CFA, we assessed pain behaviors in C57BL6 and Apoe knockout (Apoe(-/-)) mice using von Frey and Hargreaves' tests and analyzed dorsal horn samples (L4-5) using western blotting, immunofluorescence, quantitative real-time polymerase chain reaction, and RNA sequencing. Results The Apoe levels were markedly upregulated at 2 h and on days 1 and 3 post-CFA treatment. Apoe was exclusively expressed in the astrocytes. Apoe(-/-) mice exhibited decreased pain on day 1, but not at 2 h, post-CFA treatment. Apoe-/- mice also showed decreased spinal neuron excitability and paw edema on day 1 post-CFA treatment. Global transcriptomic analysis of the dorsal horn on day 1 post-CFA treatment revealed that the differentially expressed mRNAs in Apoe(-/-) mice were associated with lipid metabolism and the immune system. Astrocyte activation was impaired in Apoe(-/-) mice on day 1 post-CFA treatment. The intrathecal injection of Apoe antisense oligonucleotide mitigated CFA-induced pain hypersensitivity. Conclusions Apoe deficiency altered lipid metabolism in astrocytes, exerting regulatory effects on immune response, astrocyte activation, and neuronal activity and consequently disrupting the maintenance of inflammatory pain after peripheral inflammation. Targeting APOE is a potential anti-nociception and anti-inflammatory strategy.