Emerging Pathophysiological Roles of Ketone Bodies

The discovery of insulin approximately a century ago greatly improved the management of diabetes, including many of its life-threatening acute complications like ketoacidosis. This breakthrough saved many lives and extended the healthy lifespan of many patients with diabetes. However, there is still a negative perception of ketone bodies stemming from ketoacidosis. Originally, ketone bodies were thought of as a vital source of energy during fasting and exercise. Furthermore, in recent years, research on calorie restriction and its potential impact on extending healthy lifespans, as well as studies on ketone bodies, have gradually led to a reevaluation of the significance of ketone bodies in promoting longevity. Thus, in this review, we discuss the emerging and hidden roles of ketone bodies in various organs, including the heart, kidneys, skeletal muscles, and brain, as well as their potential impact on malignancies and lifespan.

Muscle fibre transition and transcriptional changes of horse skeletal muscles during traditional Mongolian endurance training

BACKGROUND

Traditional Mongolian endurance training is an effective way to improve the athletic ability of the horse for endurance events and is widely used. This incorporates aerobic exercise and intermittent fasting and these altered physiologic conditions are associated with switches between muscle fibre types.

OBJECTIVES

To better understand the adaption of horse skeletal muscle to traditional Mongolian endurance training from muscle fibre characteristics and transcriptional levels and to explore possible molecular mechanisms associated with the endurance performance of horses.

STUDY DESIGN

Before-after study.

METHODS

Muscle fibre type switches and muscle transcriptome changes in six Mongolian horses were assessed during 4 weeks of training. Transcriptomic and histochemical analyses were performed. The activities of oxidative and glycolytic metabolic enzymes were analysed and we generated deep RNA-sequencing data relating to skeletal muscles.

RESULTS

A fast-to-slow muscle fibre transition occurred in horse skeletal muscles, with a concomitant increase of oxidative enzyme activity and decreased glycolytic enzyme activity. Numerous differentially expressed genes were involved in the control of muscle protein balance and degradation. Differential alternative splicing events were also found during training which included exon-skipping events in Ttn that were associated with muscle atrophy. Differentially expressed noncoding RNAs showed connections with muscle protein balance-related pathways and fibre type specification via the post-transcriptional regulation of miRNA.

MAIN LIMITATIONS

The study focuses on horse athletic ability only from the aspect of muscular adaptation.

CONCLUSION

Traditional Mongolian endurance training-induced muscle fibre transition and metabolic and transcriptional changes. Muscle-specific non-coding RNAs could contribute to these transcriptomic changes during training.

Acidosis attenuates CPT-I-supported bioenergetics as a potential mechanism limiting lipid oxidation

Fuel interactions in contracting muscle represent a complex interplay between enzymes regulating carbohydrate and fatty acid catabolism, converging in the mitochondrial matrix. While increasing exercise intensity promotes carbohydrate use at the expense of fatty acid oxidation, the mechanisms underlying this effect remain poorly elucidated. As a potential explanation, we investigated whether exercise-induced reductions in intramuscular pH (acidosis) attenuate carnitine palmitoyltransferase-I (CPT-I)-supported bioenergetics, the rate-limiting step for fatty acid oxidation within mitochondria. Specifically, we assessed the effect of a physiologically relevant reduction in pH (pH 7.2 versus 6.8) on single and mixed substrate respiratory responses in murine skeletal muscle isolated mitochondria and permeabilized fibers. While pH did not influence oxidative phosphorylation stoichiometry (ADP/O ratios), coupling efficiency, oxygen affinity, or ADP respiratory responses, acidosis impaired lipid bioenergetics by attenuating respiration with L-carnitine and palmitoyl-CoA, while enhancing the inhibitory effect of malonyl-CoA on CPT-I. These acidotic effects were largely retained following a single bout of intense exercise. At rest, pyruvate and succinate-supported respiration were also impaired by acidosis. However, providing more pyruvate and ADP at pH 6.8 to model increases in glycolytic flux and ATP turnover with intense exercise overcame the acidotic attenuation of carbohydrate-linked oxidative phosphorylation. Importantly, this situation is fundamentally different from lipids where CPT-I substrate sensitivity and availability is impaired at higher power outputs suggesting lipid metabolism may be more susceptible to the effects of acidosis, possibly contributing to fuel shifts with increasing exercise intensity.

Cross-talk between DNA damage response and the central carbon metabolic network underlies selective vulnerability of Purkinje neurons in ataxia-telangiectasia

Cerebellar ataxia is often the first and irreversible outcome in the disease of ataxia-telangiectasia (A-T), as a consequence of selective cerebellar Purkinje neuronal degeneration. A-T is an autosomal recessive disorder resulting from the loss-of-function mutations of the ataxia-telangiectasia-mutated ATM gene. Over years of research, it now becomes clear that functional ATM-a serine/threonine kinase protein product of the ATM gene-plays critical roles in regulating both cellular DNA damage response and central carbon metabolic network in multiple subcellular locations. The key question arises is how cerebellar Purkinje neurons become selectively vulnerable when all other cell types in the brain are suffering from the very same defects in ATM function. This review intended to comprehensively elaborate the unexpected linkages between these two seemingly independent cellular functions and the regulatory roles of ATM involved, their integrated impacts on both physical and functional properties, hence the introduction of selective vulnerability to Purkinje neurons in the disease will be addressed.

miR-183 and miR-96 orchestrate both glucose and fat utilization in skeletal muscle

Our knowledge of the coordination of fuel usage in skeletal muscle is incomplete. Whether and how microRNAs are involved in the substrate selection for oxidation is largely unknown. Here we show that mice lacking miR-183 and miR-96 have enhanced muscle oxidative phenotype and altered glucose/lipid homeostasis. Moreover, loss of miR-183 and miR-96 results in a shift in substrate utilization toward fat relative to carbohydrates in mice. Mechanistically, loss of miR-183 and miR-96 suppresses glucose utilization in skeletal muscle by increasing PDHA1 phosphorylation via targeting FoxO1 and PDK4. On the other hand, loss of miR-183 and miR-96 promotes fat usage in skeletal muscle by enhancing intramuscular lipolysis via targeting FoxO1 and ATGL. Thus, our study establishes miR-183 and miR-96 as master coordinators of fuel selection and metabolic homeostasis owing to their capability of modulating both glucose utilization and fat catabolism. Lastly, we show that loss of miR-183 and miR-96 can alleviate obesity and improve glucose metabolism in high-fat diet-induced mice, suggesting that miR-183 and miR-96 may serve as therapeutic targets for metabolic diseases.

Neither Postabsorptive Resting Nor Postprandial Fat Oxidation Are Related to Peak Fat Oxidation in Men With Chronic Paraplegia

The peak rate of fat oxidation (PFO) achieved during a graded exercise test is an important indicator of metabolic health. In healthy individuals, there is a significant positive association between PFO and total daily fat oxidation (FO). However, conditions resulting in metabolic dysfunction may cause a disconnect between PFO and non-exercise FO. Ten adult men with chronic thoracic spinal cord injury (SCI) completed a graded arm exercise test. On a separate day following an overnight fast (≥ 10 h), they rested for 60 min before ingesting a liquid mixed meal (600 kcal; 35% fat, 50% carbohydrate, 15% protein). Expired gases were collected and indirect calorimetry data used to determine FO at rest, before and after feeding, and during the graded exercise test. Participants had “good” cardiorespiratory fitness (VO_2peak: 19.2 ± 5.2 ml/kg/min) based on normative reference values for SCI. There was a strong positive correlation between PFO (0.30 ± 0.08 g/min) and VO_2peak (r = 0.86, p = 0.002). Additionally, postabsorptive FO at rest was significantly and positively correlated with postprandial peak FO (r = 0.77, p = 0.01). However, PFO was not significantly associated with postabsorptive FO at rest (0.08 ± 0.02 g/min; p = 0.97), postprandial peak FO (0.10 ± 0.03 g/min; p = 0.43), or incremental area under the curve postprandial FO (p = 0.22). It may be advantageous to assess both postabsorptive FO at rest and PFO in those with SCI to gain a more complete picture of their metabolic flexibility and long-term metabolic health.

Immobilisation induces sizeable and sustained reductions in forearm glucose uptake in just 24 h but does not change lipid uptake in healthy men

KEY POINTS

The trajectory, magnitude and localisation of metabolic perturbations caused by immobilisation (IMM) are unresolved. Forearm glucose uptake (FGU) in response to glucose feeding was determined in healthy men before and during 72 h of forearm IMM, and the same measurements were made in the non-IMM contralateral limb at baseline and 72 h. In a similar study design, FGU and forearm lipid uptake were determined after a high fat mixed-meal (HFMM) in IMM and non-IMM limbs. FGU was reduced by 38%, 57% and 46% following 24, 48 and 72 h IMM, respectively, but was unchanged in the non-IMM limb. A similar FGU response to IMM was observed after a HFMM, and forearm lipid uptake was unchanged. A sizeable reduction in FGU occurs in just 24 h of IMM, which is sustained thereafter and specific to the IMM limb, making unloading per se the likely rapid driver of dysregulation.

ABSTRACT

The trajectory and magnitude of metabolic perturbations caused by muscle disuse are unknown yet central to understanding the mechanistic basis of immobilisation-associated metabolic dysregulation. To address this gap, forearm glucose uptake (FGU) was determined in 10 healthy men (age 24.9 ± 0.6 years, weight 71.9 ± 2.6 kg, BMI 22.6 ± 0.6 kg/m² ) during a 180 min oral glucose challenge before (0) and after 24, 48 and 72 h of arm immobilisation, and before and after 72 h in the contralateral non-immobilised arm (Study A). FGU was decreased from baseline at 24 h (38%, P = 0.04), 48 h (57%, P = 0.01) and 72 h (46%, P = 0.06) of immobilisation, and was also 63% less than the non-immobilised limb at 72 h (P = 0.002). In a second study, FGU and forearm lipid uptake were determined in nine healthy men (age 22.4 ± 1.3 years, weight 71.4 ± 2.8 kg, BMI 22.6 ± 0.8 kg/m² ) during a 420 min mixed-meal challenge before (0) and after 24 and 48 h of arm immobilisation and before and after 72 h in the contralateral non-immobilised arm (Study B). FGU responses were similar to Study A, and forearm lipid uptake was unchanged from pre-immobilisation in both arms over the study. A sizeable decrement in FGU in response to glucose feeding occurred within 24 h of immobilisation that was sustained and specific to the immobilised limb. Increasing lipid availability had no additional impact on the rate or magnitude of these responses or on lipid uptake. These findings highlight a lack of muscle contraction per se as a fast-acting physiological insult to FGU.

Transcriptional Changes Involved in Atrophying Muscles during Prolonged Fasting in Rats

Food deprivation resulting in muscle atrophy may be detrimental to health. To better understand how muscle mass is regulated during such a nutritional challenge, the current study deciphered muscle responses during phase 2 (P2, protein sparing) and phase 3 (P3, protein mobilization) of prolonged fasting in rats. This was done using transcriptomics analysis and a series of biochemistry measurements. The main findings highlight changes for plasma catabolic and anabolic stimuli, as well as for muscle transcriptome, energy metabolism, and oxidative stress. Changes were generally consistent with the intense use of lipids as fuels during P2. They also reflected increased muscle protein degradation and repressed synthesis, in a more marked manner during P3 than P2 compared to the fed state. Nevertheless, several unexpected changes appeared to be in favor of muscle protein synthesis during fasting, notably at the level of the phosphatidylinositol-3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway, transcription and translation processes, and the response to oxidative stress. Such mechanisms might promote protein sparing during P2 and prepare the restoration of the protein compartment during P3 in anticipation of food intake for optimizing the effects of an upcoming refeeding, thereby promoting body maintenance and survival. Future studies should examine relevance of such targets for improving nitrogen balance during catabolic diseases.

Deletion of the Glutaredoxin-2 Gene Protects Mice from Diet-Induced Weight Gain, Which Correlates with Increased Mitochondrial Respiration and Proton Leaks in Skeletal Muscle

Aims: The aim of this study was to determine whether deleting the gene encoding glutaredoxin-2 (GRX2) could protect mice from diet-induced weight gain. Results: Subjecting wild-type littermates to a high fat diet (HFD) induced a significant increase in overall body mass, white adipose tissue hypertrophy, lipid droplet accumulation in hepatocytes, and higher circulating insulin and triglyceride levels. In contrast, GRX2 heterozygotes (GRX2^+/-) fed an HFD had a body mass, white adipose tissue weight, and hepatic and circulating lipid and insulin levels similar to littermates fed a control diet. Examination of the bioenergetics of muscle mitochondria revealed that this protective effect was associated with an increase in respiration and proton leaks. Muscle mitochondria from GRX2^+/- mice had a ∼2- to 3-fold increase in state 3 (phosphorylating) respiration when pyruvate/malate or succinate served as substrates and a ∼4-fold increase when palmitoyl-carnitine was being oxidized. Proton leaks were ∼2- to 3-fold higher in GRX2^+/- muscle mitochondria. Treatment of mitochondria with either guanosine diphosphate, genipin, or octanoyl-carnitine revealed that the higher rate of O₂ consumption under state 4 conditions was associated with increased leaks through uncoupling protein-3 and adenine nucleotide translocase. GRX2^+/- mitochondria also had better protection from oxidative distress. Innovation: For the first time, we demonstrate that deleting the Grx2 gene can protect from diet-induced weight gain and the development of obesity-related disorders. Conclusions: Deleting the Grx2 gene protects mice from diet-induced weight gain. This effect was related to an increase in muscle fuel combustion, mitochondrial respiration, proton leaks, and reactive oxygen species handling. Antioxid. Redox Signal. 31, 1272-1288.

Multi-dimensional Roles of Ketone Bodies in Fuel Metabolism, Signaling, and Therapeutics

Ketone body metabolism is a central node in physiological homeostasis. In this review, we discuss how ketones serve discrete fine-tuning metabolic roles that optimize organ and organism performance in varying nutrient states and protect from inflammation and injury in multiple organ systems. Traditionally viewed as metabolic substrates enlisted only in carbohydrate restriction, observations underscore the importance of ketone bodies as vital metabolic and signaling mediators when carbohydrates are abundant. Complementing a repertoire of known therapeutic options for diseases of the nervous system, prospective roles for ketone bodies in cancer have arisen, as have intriguing protective roles in heart and liver, opening therapeutic options in obesity-related and cardiovascular disease. Controversies in ketone metabolism and signaling are discussed to reconcile classical dogma with contemporary observations.

Determinants of resting lipid oxidation in response to a prior bout of endurance exercise

A single bout of exercise can alter subsequent resting metabolism for many hours and into the next day. However, differences between men and women, effects of nutritional state, and relative effects of resting metabolic rate (RMR) and respiratory exchange ratio (RER) in controlling the increase in lipid oxidation (Lox) after exercise are not yet clear. Effects of aerobic capacity (Vo2 peak) and exercise bout parameters (intensity and volume) also remain to be clearly elucidated as does the time course of changes after exercise. We performed a meta-analysis to assess these potential moderators of the impact of endurance exercise [effect sizes (ESs)] on subsequent Lox at rest (ES = 0.91; 95% CI: 0.69-1.12), on the day of exercise (ES = 1.22; 95% CI: 0.89-1.55), and on the following day (ES = 0.60; 95% CI: 0.35-0.85). ES for the exercise-related increase in resting Lox was significantly greater in men than women in the postabsorptive state but similar in the postprandial state. The ES for depression of RER after exercise was similar between men and women, while the ES for RMR in the postabsorptive state tended to be higher in men than women. Finally, Vo2 peak and exercise energy expenditure (EEE), but not intensity, were predictive of postexercise Lox. The findings indicate importance of EEE and fitness for ability to achieve robust enhancement of Lox after exercise. The results additionally indicate a gender difference in postexercise Lox that is dependent on nutritional state, as the ES for Lox was lower in women only in the postabsorptive state.