Beta-Hydroxybutyrate (BHB) Elicits Concentration-Dependent Anti-Inflammatory Effects on Microglial Cells which are Reversible by Blocking its Monocarboxylate (MCT) Importer

Chase Garcia^*Ariana BanerjeeClaire MontgomeryLauren AdcockIzumi MaezawaJon RamseyAna Cristina GrodzkiKyoungmi KimGino Cortopassi^*

• University of California, Davis, Davis, United States

The Ketogenic diet (KD) increases mouse lifespan and healthspan, and improves late-life memory. This raises the question as to the mechanism behind this effect. In mice on a KD blood beta-hydroxybutyrate (BHB) levels uniquely rise higher compared to those on a control diet (CD). BHB is therefore considered a key signaling and metabolic mediator of KD's effects and benefits. BHB crosses the blood-brain barrier and rescued memory, improved cognitive function, and increased neuronal plasticity in 2 different mouse models of Alzheimer's disease (PS1/APP and 5XFAD). At the cellular level, microglia are thought to play a critical role in the physiologic basis of memory due to their important role in synaptic development, plasticity, and connectivity. Conversely, microglial dysfunction and inflammation are connected to cognitive decline and neurodegenerative diseases. Because of this, one explanatory hypothesis for these positive therapeutic observations in mice is that the KD and BHB drive memory and longevity benefits through their anti-inflammatory actions on microglia. We investigated the concentration-dependence of BHB's anti-inflammatory effects in BV2 microglial cells. We focused on 1.5mM BHB, which reflects blood levels in mice and humans on a KD. At this concentration, BHB significantly and concentration-dependently decreased 1) inflammatory cytokine expression (IL-6, TNF-α, IL-1β) 2) inflammatory morphological changes, and 3) activation of p-ERK and p-p38MAPK, which are key pathways involved in microglial inflammation. We show for the first time that the expression of Alzheimer's risk gene TREM2 is modified by dietarily-achievable 1.5mM BHB. BHB's anti-inflammatory, morphological, biochemical, and TREM2 effects were blocked by a Monocarboxylate Transporter (MCT) inhibitor, supporting the idea that BHB must enter microglia to elicit some of its anti-inflammatory effects. These results support a hypothesis that blood BHB levels achievable on a KD elicit significant concentration-dependent antiinflammatory effects in microglia. Increasing BHB concentration through sustained KD, or BHB supplements, may lower microglial inflammatory tone and provide benefit in age-related memory loss.