Increased CSF PKR activation in Alzheimer’s disease is associated with APOE4 genotype

Objective: Protein kinase R (PKR) is a stress-activated kinase implicated in translational control, neuroinflammation, and neurodegeneration. Although increased PKR activation was reported in Alzheimer’s disease (AD), prior studies relied on semi-quantitative methods. This study aimed to quantitatively assess cerebrospinal fluid (CSF) levels of total PKR and phosphorylated PKR (pPKR) in AD patients using the highly sensitive Single Molecule Array (SIMOA) platform and to evaluate their clinical and genetic associations. Methods: We developed and applied novel SIMOA assays to measure CSF concentrations of total PKR and pPKR in a cohort of 92 AD patients and neurological controls. Associations with established AD biomarkers, cognitive outcomes, and APOE ε4 (APOE4) genotype were examined. Results: CSF pPKR levels and the pPKR/PKR ratio were significantly elevated in AD patients compared with controls. CSF pPKR levels were positively correlated with phosphorylated tau concentrations and with a steeper longitudinal cognitive decline. Notably, among AD patients, higher CSF pPKR levels were associated with APOE4 carrier status, identifying a previously unreported association between this major genetic risk factor and PKR phosphorylation. In contrast, the pPKR/PKR ratio did not differ significantly according to APOE4 status among AD patients. Conclusions: Using ultrasensitive SIMOA assays, this study provides independent confirmation of increased CSF PKR phosphorylation in Alzheimer’s disease and identifies an association between APOE4 carrier status and pPKR concentration among patients with AD. These findings support further mechanistic and longitudinal studies to clarify the role of PKR in AD pathophysiology and to evaluate its potential as a biomarker candidate and therapeutic target.