The ketone body metabolite β‐hydroxybutyrate induces an antidepression‐associated ramification of microglia via HDACs inhibition‐triggered Akt‐small RhoGTPase activation

Abstract

Direct induction of macrophage ramification has been shown to promote an alternative (M2) polarization, suggesting that the ramified morphology may determine the function of immune cells. The ketone body metabolite β-hydroxybutyrate (BHB) elevated in conditions including fasting and low-carbohydrate ketogenic diet (KD) can reduce neuroinflammation. However, how exactly BHB impacts microglia remains unclear. We report that BHB as well as its producing stimuli fasting and KD induced obvious ramifications of murine microglia in basal and inflammatory conditions in a reversible manner, and these ramifications were accompanied with microglial profile toward M2 polarization and phagocytosis. The protein kinase B (Akt)-small RhoGTPase axis was found to mediate the effect of BHB on microglial shape change, as (i) BHB activated the microglial small RhoGTPase (Rac1, Cdc42) and Akt; (ii) Akt and Rac1-Cdc42 inhibition abolished the pro-ramification effect of BHB; (iii) Akt inhibition prevented the activation of Rac1-Cdc42 induced by BHB treatment. Incubation of microglia with other classical histone deacetylases (HDACs) inhibitors, but not G protein-coupled receptor 109a (GPR109a) activators, also induced microglial ramification and Akt activation, suggesting that the BHB-induced ramification of microglia may be triggered by HDACs inhibition. Functionally, Akt inhibition was found to abrogate the effects of BHB on microglial polarization and phagocytosis. In neuroinflammatory models induced by lipopolysaccharide (LPS) or chronic unpredictable stress (CUS), BHB prevented the microglial process retraction and depressive-like behaviors, and these effects were abolished by Akt inhibition. Our findings for the first time showed that BHB exerts anti-inflammatory actions via promotion of microglial ramification.

Metabolic Reprograming of Microglia in the Regulation of the Innate Inflammatory Response

“…Accordingly, since a given microglia subpopulation can plastically modify its phenotype and function in response to signals from the microenvironment (10), the targeting of specific microglial phenotypes in a proper time window could represent a more selective and efficacious approach and represent the current challenge of this field of research. One recent experimental approach proposes to induce a ketogenic state in microglia, suppressing glucose utilization to reduce inflammation, tissue loss and functional impairment after brain injury (41)(42)(43). The activation of the G-proteincoupled receptor 109A (GPR109A) with b-hydroxybutyrate (41,43,96) on microglial cells attenuates the NF-kB signaling and the production of pro-inflammatory cytokines, promoting a microglial neuroprotective phenotype in a mouse model of PD (42).…”

“…One recent experimental approach proposes to induce a ketogenic state in microglia, suppressing glucose utilization to reduce inflammation, tissue loss and functional impairment after brain injury (41)(42)(43). The activation of the G-proteincoupled receptor 109A (GPR109A) with b-hydroxybutyrate (41,43,96) on microglial cells attenuates the NF-kB signaling and the production of pro-inflammatory cytokines, promoting a microglial neuroprotective phenotype in a mouse model of PD (42). Also, a metabolic switch toward oxidative metabolism might contribute to promote a protective microglia in some pathophysiological conditions, resulting in the production of metabolites beneficial for neurons (18,97).…”

“…Glucose metabolism controls NOX activation by the NADH-dependent transcriptional co-repressor C-terminal binding protein (CtBP) that affects nuclear factor kappalight-chain-enhancer of activated B cell (NF-κB) signaling and the expression of inducible nitric oxide synthase (iNOS) (35,36). Interestingly, microglia also express the monocarboxylic transporter (MCT) 1 and 2 and absorb lactate and ketons (37) and it has been demonstrated that a ketogenic diet is correlated with a suppression of microglia activation (38)(39)(40) likely due to the inhibition of histone deacetylases (HDACs) by ketonic bodies, which decreases NK-kB signaling (41)(42)(43). Moreover, silencing HDAC activity affects microglia during development and in adulthood, as a function of the activation state, suggesting that epigenetic changes affect cellular metabolism in activated microglia, modulating microglia function (44).…”

β-hydroxybutyrate and its metabolic effects on age-associated pathology

“…Additionally, β-HB prevented microglial process retraction and depressive-like behaviors by HDAC inhibition. β-HB-induced ramification and Akt activation in microglia abrogated HDAC activity, resulting in a further reduction in neuroinflammation in microglia 66 . Butyrate is a short-chain carboxylic acid produced in the gut.…”

The ketogenic diet in disease and development

“…However, βHB also triggered increased expression of glucose transporters in the murine brain following fasting via epigenetic mechanisms, indicating a balanced interplay between metabolic regulation and levels of glucose and βHB (Tanegashima et al, 2017). Ketone bodies also affect microglial function; βHB increased microglial ramification due to its function as an inhibitor of histone deacetylation (Huang et al, 2018), inhibited inflammasome activation (Deora et al, 2017), and decreased production of pro‐inflammatory cytokines following lipopolysaccharide exposure (Fu et al, 2015).…”