Radiation-induced eCIRP Causes Alzheimer's Disease-like Neuronal Tau Phosphorylation.

Cranial radiotherapy is associated with neuroinflammation, cognitive dysfunction, and dementia. Tau pathology plays a key role in Alzheimer's disease (AD) but has not been studied in the context of radiation injury. Extracellular cold-inducible RNA-binding protein (eCIRP) is a novel neuroinflammatory mediator that activates neuronal IL6Rα/p25/Cdk5, leading to tau hyperphosphorylation. We hypothesized that radiation promotes pathological tau phosphorylation via the eCIRP/IL6Rα/p25/Cdk5 pathway, which can be mitigated by the eCIRP inhibitor C23. Adult C57BL/6 and CIRP-/- mice were exposed to a single dose of 10-Gy X-rays at a dose-rate of 1 Gy/min. Hippocampal tissue lysates were analyzed for p25, phosphorylated tau, and CIRP using Western blotting. Cdk5 activity was assessed utilizing a modified Cdk5/p35 kinase enzyme system. Serum and cerebrospinal fluid (CSF) eCIRP was quantified using ELISA. eCIRP (1 µg) was injected intracerebroventricularly (icv) and C23 (8 mg/kg BW) retro-orbitally. N2a and primary neurons were pre-treated with 3 μg/ml IL-6Rα neutralizing Abs or IgG control, or 25 µg/mL C23 and stimulated with up to 2.5 μg/mL eCIRP for 48 h. Irradiation significantly increased hippocampal p25 expression, Cdk5 activity, and tau phosphorylation [AT-8 (Ser202/Thr205/Ser208), and combined Ser199/Ser202 and Ser396 p-tau antibodies] at 48 h post-irradiation. eCIRP increased by 1.2-fold in the serum and 2.6-fold in the CSF, and CIRP increased by 65% in hippocampal lysates after irradiation. Irradiated CIRP-/- mice had a 44% reduction in hippocampal p25 and a 59% reduction in hippocampal phosphorylated tau. Mice icv-injected with eCIRP presented p25 and p-tau protein levels as high as irradiated mice. eCIRP increased p-tau in N2a and primary neurons and was inhibited by IL-6Rα Abs and by C23. C23 markedly attenuated the radiation-induced increase in hippocampal p25 and tau phosphorylation. Radiation causes AD-like tau phosphorylation via the eCIRP/IL-6Rα/p25/Cdk5 pathway, which is effectively mitigated by targeting eCIRP with C23. eCIRP is a novel neuroinflammatory mediator with consequential effects for tau pathology and cognitive dysfunction. These novel findings are directly relevant to the pathogenesis and mitigation of radiation-induced cognitive dysfunction, as well as to the etiopathogenesis of AD.