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.