3-Hydroxybutyrate, a metabolite in sustaining neuronal cell vitality: a mendelian randomization and in vitro experimentation.

Background: Recent research has implicated mitochondrial DNA copy number (mtDNA-CN) and Tau protein levels in the blood as potential biomarkers for early Alzheimer's disease (AD) risk assessment, correlating with metabolite profiles. However, intermediary metabolites mediating these associations remain elusive.

Methods: Employing a two-sample and a mediation Mendelian randomization (MR) analysis of the IEU OpenGWAS database, involving 383,476 participants from a genome-wide association study (GWAS) and an exome-wide association study (ExWAS), we identified intermediary metabolites linking mtDNA-CN and Tau.Meanwhile, the effects of mediating metabolites on HT22 cell viability and its mitochondrial morphology were also assessed in conjunction with in vitro experiments.

Results: Our study revealed an association of mtDNA-CN on Tau (OR = 3.102, 95% CI: 1.016-9.472, P = 0.047), as well as on other 31 metabolites such as 3-Hydroxybutyrate (3HB), Docosahexaenoic acid (DHA), Acetate, Albumin, Apolipoprotein A-I (APOA1), and so on. Notably, 3HB was further implicated in a relationship with Tau (OR = 6.030, 95% CI: 1.054-34.491, P = 0.043), acting as a mediator between mtDNA-CN and Tau. In vitro experiments demonstrated that 3HB positively sustained HT22 cell viability by MTT assay and mitigated mitochondrial swelling under low glucose conditions, as observed via HIS-SIM. In Western blot (WB) and quantitative real-time PCR (qPCR) assays, phosphorylation levels of Tau at serine 262 (p-Tau262) and serine 396 (p-Tau396) were tended to decline following 3HB intervention. Additionally, a positive correlation was identified between 3HB concentration and mtDNA-CN.

Conclusions: These findings underscore the potential of 3HB as a biomarker and mediator in early AD risk assessment. Moreover, 3HB's ability to enhance cell viability, maintain mitochondrial morphology, decrease phosphorylated Tau protein expression and increase mtDNA-CN under stressful conditions, suggesting its therapeutic potential in improving the imbalance of energy metabolism in the AD brain.