Neurons and Astrocytes Respond Differentially to Ketogenic Diet and Ketone Body Exposure: Implications for the Treatment of Alzheimer’s Disease.

Abstract

Glucose hypometabolism and mitochondrial dysfunction are early deficits in Alzheimer’s disease (AD) brains that precede cognitive decline. Rescuing bioenergetic failure by providing an alternative fuel substrate is an attractive therapeutic strategy that may be beneficial in treating neurodegenerative disease. The ketogenic diet (KD) has been suggested as a treatment for AD. The KD is theorized to support the brain through the generation of ketone bodies that serve as an alternative fuel to glucose and possibly through other unknown mechanisms. KD and related neuroketotherapeutics have been shown to demonstrate molecular signaling effects including activating histone acetylation, increased BDNF signaling, reducing reactive oxygen species formation, and inhibiting the NLRP3 inflammasome. Historically many studies examine CNS function as though it is a homogenous bioenergetic compartment and under value how different CNS cell populations may respond to metabolic interventions or contribute to disease. In these collected works, we investigate the bioenergetic and molecular response of two primary CNS cell types, neurons and astrocytes, to ketogenic diet and ketone salt interventions both in vivo and in vitro. While neuronal response recapitulates many previously described effects in vitro by increasing mitochondrial respiration and histone acetylation, astrocytes exhibit no response to ketone body availability in these domains. Interestingly, neuronal response also was found to include reduced signaling through pro-growth PI3K-Akt-mTORC1 pathways indicating that ketone bodies concurrently support energy production while signaling nutrient scarcity and inducing cellular quiescence. We further expanded on these findings by performing cell type enrichment for neurons and astrocytes via magnetic assisted cell sorting and transcriptomic profiling of these cell populations following 90-day ketogenic diet intervention in v 16-week-old C57BL6/N mice. RNAseq and KEGG pathway analysis revealed that neuronal transcription was generally increased in response to ketogenic diet while astrocytic transcription was largely suppressed as compared to animals maintained on standard chow diet. Findings of KEGG pathway analysis indicated that the three most implicated pathways affected by KD in neurons by significance were Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease indicating KD may have important disease modifying properties regarding these neurodegenerative diseases. Finally, we sought to develop a new model to study the effects of persistent stable ketosis on brain health through the development of a new mouse model, the conditionally expressing malonyl-CoA insensitive carnitine-palmitoyl transferase 1A transgenic mouse. Early studies indicate that the CPT1AM593S mouse exhibits increased hepatic ketone body levels when crossed to Albumin-Cre strains, but increased ketosis is not observed in the blood. Acetyl-CoA generated from increased fatty acid β-oxidation may be increasingly exported as citrate in these animals. Taken together, these works have characterized that the bioenergetic and molecular responses of the CNS to ketone interventions may have important therapeutic effects for the treatment of brain aging and neurodegenerative disease, but the specific response is heavily modulated as a function of cell type. Understanding and appreciating the consequences of these effects, which are often antagonistic in nature, are critical for the development of an effective dietary mimetic of the ketogenic diet for the specific targeting of mechanisms of action not only by target, but also by target population.