Ketogenic diet

The ketogenic diet (KD) is based on a ratio of high fat, low carbohydrates and sufficient protein, to reprogram the body to metabolize fats for energy requirements instead of carbohydrates. Ketone bodies are comprised by three compounds: β-hydroxybutyrate, acetoacetate and acetone, with the latter being less important as energy source. Ketone bodies are produced by fatty acid oxidation in the liver. They can enter the CNS via monocarboxylate transporters (MCT) and be integrated in local metabolism (Figure 4.4).

Figure 4.4I Model of metabolic support of ketone bodies. Fatty acids in the blood stream enter the liver and are converted to ketone bodies by fatty acid oxidation. The conversion of fatty acids into acetoacetate requires the enzyme 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2).

Ketone bodies can be transported by monocarboxylate transporters (MCT1,2 or 7). The three enzymes involved in ketolysis are 3-hydroxybutyrate dehydrogenase 1 (BDH1), 3-oxoacid CoA-transferase 1 (OXCT1) and acetyl-coA acetylCoA-transferase 1 (ACAT1) leading finally to acetyl-CoA production. Acetyl-CoA can be used in the tricarboxylic acid (TCA) cycle or to build-up fatty acids and cholesterol involving the following selected enzymes: 3-hydroxy-3-methylglutaryl-CoA synthase 1 (HMGCS1), 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), farnesyl-diphosphate farnesyltransferase also referred to as squalene synthase (FDFT1), acetyl-CoA-carboxylase (ACC1), fatty acid synthases (FASN β-hydroxybutyrate dehydrogenase 1.

MCT1 is mainly expressed by astrocytes and endothelial cells. Whereas MCT2 was predominantly expressed by neurons, but also oligodendrocytes and microglia revealed MCT2 expression (Pellerin et al., 2005; Zhang et al., 2014). Once ketone bodies were imported into cells of the CNS, BDH1 interconverts β-hydroxybutyrate to acetoacetate. OXCT1, the rate-limiting enzyme of ketolysis, converts acetoacetate to aceto-acetyl-CoA. ACAT1 facilitated the reaction of aceto-acetyl-CoA to two molecules of acetyl-CoA. Acetyl-CoA can be used for example as energy source in the TCA cycle or utilized to synthesize fatty acids or cholesterol, but further many other metabolic pathways of acetyl-CoA are possible. In newborn rodents, which rely on the maternal, ketone body rich milk as food source, ketone bodies provide ~ 30%

of the brains energy requirements (Cremer, 1982). This proportion decreases drastically after weaning, which might reflect reduced activity of the main enzymes for ketone body lysis, BDH1 (β-hydroxybutyrate dehydrogenase 1), OXCT1 (3-oxoacid CoA-transferase 1) and ACAT1 (acetyl-CoA acetyltransferase 1). (Krebs et al., 1971). It was shown that in young rodents, ketone bodies are used to synthesize lipids and especially cholesterol, which is predominantly integrated in the myelin sheath (Koper et al., 1981). Furthermore, ketone bodies as metabolites were proven to be the main supplier of lipid and cholesterol synthesis, even in the presence of sufficient glucose (Webber and Edmond, 1977). A benefit for lipid synthesis by ketone bodies compared to glucose might be that aceto-acetyl-CoA, which is produced during ketolysis can be directly integrated in fatty acid synthesis. In contrast glycolytic derived acetyl-CoA must first be converted to aceto-acetyl-CoA (Morris, 2005).

4.1.6.1 Treatment potential of the ketogenic diet

In the early 1920s the ketogenic diet was developed and first applied in the treatment of epilepsy (Liu et al., 2018). Since then, researchers in a broad field of neurological related diseases discovered the potential of the ketogenic diet, but so far little is known about the function of KD. Hereafter, I summarize interesting findings providing insight in the neuroprotective role of ketone bodies

Treatment of KD in epilepsy provided anticonvulsant and anti-epileptogenic properties (Liu et al., 2018). In a clinical treatment trial of childhood epilepsy with KD it was shown that two-third of the children treated with KD had a seizure reduction of 50-90% (Neal et al., 2008). Direct mechanistic insight of KD action in epilepsy and seizure control is unknown, but it has been speculated that a combination of enhanced ketosis, reduced glycolysis and increased amounts of fatty acids leads to a metabolic switch which induces neuroprotection. Mechanisms that might be involved in KD neuroprotection include an increase in oxidative phosphorylation and enhanced mitochondrial biogenesis which can causes reduction of reactive oxygen species (ROS) (Bough and Rho, 2007). Furthermore, impact of KD function on the reduction of mitochondrial oxidative stress release might be provided by the fact that elevated amounts of fatty acids induce the expression of mitochondrial uncoupling proteins (UCP) via induction of PPARα (Masino and Rho, 2012). UCPs function via

upregulation of mitochondria biogenesis, control of calcium flux and reduce free radical formation (Andrews et al., 2005). The results are supported by KD fed mice, which showed increased abundance of cerebral UCPs associated with reduced ROS levels (Sullivan et al., 2004). Moreover, KD treatment has been postulated to ameliorate mitochondrial dysfunction. In a therapeutic approach, mice treated with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), which blocks mitochondrial complex I of the electron transport chain, were supplied with D-β-hydroxybutyrate infusion (Abou-Sleiman et al., 2006). Interestingly, neurons of D-β-hydroxybutyrate treated mice were protected from the neurotoxin. Further in vivo studies in cultured neurons provided evidence that D-β-hydroxybutyrate functions by elevating oxidative phosphorylation of mitochondria in a complex II dependent manner (Tieu et al., 2003). Besides the neuroprotective function of KD, another study provided insight in the anti-inflammatory role of ketone bodies by decreasing pro-inflammatory cytokines (Yang and Cheng, 2010).

In summary, KD provided anticonvulsant and anti-inflammatory properties, increased oxidative phosphorylation and reduced oxidative stress in mitochondria.

Furthermore, the metabolic switch induced by KD treatment might enhance myelination due to fact that ketone bodies are predominantly used for the synthesis of fatty acids and especially cholesterol (Webber and Edmond, 1977). These findings could be of great interest for the treatment of neurodegenerative diseases associated with hypomyelination and mitochondrial malfunctions such as PMD.