Twenty-four hour rhythmicity in mitochondrial network connectivity and mitochondrial respiration; a study in human skeletal muscle biopsies of young lean and older individuals with obesity

OBJECTIVE

Mitochondrial network dynamics may play role in metabolic homeostasis. Whether mitochondrial network dynamics are involved in adaptations to day-night fluctuations in energy supply and demand is unclear. Here we visualized and quantified the mitochondrial network morphology in human skeletal muscle of young healthy lean and older individuals with obesity over the course of 24 hours METHODS: Muscle biopsies taken at 5 timepoints over a 24-hour period obtained from young healthy lean and older metabolically impaired obese males were analyzed for mitochondrial network integrity with confocal laser scanning microscopy. Variation of level of fragmentation over the course of the day were aligned with variation of mitochondrial respiration over the day RESULTS: Young healthy lean individuals displayed a day-night rhythmicity in mitochondrial network morphology, which aligned with the day-night rhythmicity of mitochondrial respiratory capacity, with a more fused network coinciding with higher mitochondrial respiratory capacity. In the older individuals with obesity, the mitochondrial network was more fragmented overall compared to young healthy lean individuals and completely lacked 24 hour rhythmicity, which was also true for the mitochondrial respiratory capacity CONCLUSIONS: Our data shows a paralleled rhythmicity between mitochondrial network morphology and mitochondrial oxidative capacity, which oscillates over the course of a mimicked real-life day in human skeletal muscle of young, healthy lean individuals. In older individuals with obesity, the lack of a 24-hour rhythmicity in mitochondrial network connectivity was also aligned with a lack in respiratory capacity. This suggests that 24-hour rhythmicity in mitochondrial network connectivity is a determinant of rhythmicity in mitochondrial respiratory capacity. Thus, restoring mitochondrial network integrity may promote mitochondrial respiratory capacity and hence contribute to blunting the metabolic aberrations in individuals with a disturbed 24-hour rhythmicity in metabolism, like older individuals with obesity.

Divergent remodeling of the skeletal muscle metabolome over 24 h between young, healthy men and older, metabolically compromised men

24 h whole-body substrate metabolism and the circadian clock within skeletal muscle are both compromised upon metabolic disease in humans. Here, we assessed the 24 h muscle metabolome by serial muscle sampling performed under 24 h real-life conditions in young, healthy (YH) men versus older, metabolically compromised (OMC) men. We find that metabolites associated with the initial steps of glycolysis and hexosamine biosynthesis are higher in OMC men around the clock, whereas metabolites associated with glutamine-alpha-ketoglutarate, ketone, and redox metabolism are lower in OMC men. The night period shows the largest number of differently expressed metabolites. Both groups demonstrate 24 h rhythmicity in half of the metabolome, but rhythmic metabolites only partially overlap. Specific metabolites are only rhythmic in YH men (adenosine), phase shifted in OMC men (cis-aconitate, flavin adenine dinucleotide [FAD], and uridine diphosphate [UDP]), or have a reduced 24 h amplitude in OMC men (hydroxybutyrate and hippuric acid). Our data highlight the plasticity of the skeletal muscle metabolome over 24 h and large divergence across the metabolic health spectrum.

The influence of bright and dim light on substrate metabolism, energy expenditure and thermoregulation in insulin-resistant individuals depends on time of day

AIMS/HYPOTHESIS

In our modern society, artificial light is available around the clock and most people expose themselves to electrical light and light-emissive screens during the dark period of the natural light/dark cycle. Such suboptimal lighting conditions have been associated with adverse metabolic effects, and redesigning indoor lighting conditions to mimic the natural light/dark cycle more closely holds promise to improve metabolic health. Our objective was to compare metabolic responses to lighting conditions that resemble the natural light/dark cycle in contrast to suboptimal lighting in individuals at risk of developing metabolic diseases.

METHODS

Therefore, we here performed a non-blinded, randomised, controlled, crossover trial in which overweight insulin-resistant volunteers (n = 14) were exposed to two 40 h laboratory sessions with different 24 h lighting protocols while staying in a metabolic chamber under real-life conditions. In the Bright day-Dim evening condition, volunteers were exposed to electric bright light (~1250 lx) during the daytime (08:00-18:00 h) and to dim light (~5 lx) during the evening (18:00-23:00 h). Vice versa, in the Dim day-Bright evening condition, volunteers were exposed to dim light during the daytime and bright light during the evening. Randomisation and allocation to light conditions were carried out by sequential numbering. During both lighting protocols, we performed 24 h indirect calorimetry, and continuous core body and skin temperature measurements, and took frequent blood samples. The primary outcome was plasma glucose focusing on the pre- and postprandial periods of the intervention.

RESULTS

Spending the day in bright light resulted in a greater increase in postprandial triacylglycerol levels following breakfast, but lower glucose levels preceding the dinner meal at 18:00 h, compared with dim light (5.0 ± 0.2 vs 5.2 ± 0.2 mmol/l, n = 13, p=0.02). Dim day-Bright evening reduced the increase in postprandial glucose after dinner compared with Bright day-Dim evening (incremental AUC: 307 ± 55 vs 394 ± 66 mmol/l × min, n = 13, p=0.009). After the Bright day-Dim evening condition the sleeping metabolic rate was identical compared with the baseline night, whereas it dropped after Dim day-Bright evening. Melatonin secretion in the evening was strongly suppressed for Dim day-Bright evening but not for Bright day-Dim evening. Distal skin temperature for Bright day-Dim evening was lower at 18:00 h (28.8 ± 0.3°C vs 29.9 ± 0.4°C, n = 13, p=0.039) and higher at 23:00 h compared with Dim day-Bright evening (30.1 ± 0.3°C vs 28.8 ± 0.3°C, n = 13, p=0.006). Fasting and postprandial plasma insulin levels and the respiratory exchange ratio were not different between the two lighting protocols at any time.

CONCLUSIONS/INTERPRETATION

Together, these findings suggest that the indoor light environment modulates postprandial substrate handling, energy expenditure and thermoregulation of insulin-resistant volunteers in a time-of-day-dependent manner.

TRIAL REGISTRATION

ClinicalTrials.gov NCT03829982.

FUNDING

We acknowledge the financial support from the Netherlands Cardiovascular Research Initiative: an initiative with support from the Dutch Heart Foundation (CVON2014-02 ENERGISE).

Circadian misalignment disturbs the skeletal muscle lipidome in healthy young men

Circadian misalignment, as seen in shift work, is associated with an increased risk to develop type 2 diabetes. In an experimental setting, we recently showed that a rapid day-night shift for 3 consecutive nights leads to misalignment of the core molecular clock, induction of the PPAR pathway, and insulin resistance in skeletal muscle of young, healthy men. Here, we investigated if circadian misalignment affects the skeletal muscle lipidome and intramyocellular lipid droplet characteristics, explaining the misalignment-induced insulin resistance. Fourteen healthy men underwent one aligned and one circadian misalignment period, both consisting of ~3.5 days. In the misaligned condition, day and night were rapidly shifted by 12 hours leading to opposite eating, sleep, and activity times compared with the aligned condition. For each condition, two muscle biopsies were taken from the m. vastus lateralis in the morning and evening and subjected to semi-targeted lipidomics and confocal microscopy analysis. We found that only 2% of detected lipids were different between morning and evening in the aligned condition, whereas 12% displayed a morning-evening difference upon misalignment. Triacylglycerols, in particular species of a carbon length ≥55, were the most abundant lipid species changed upon misalignment. Cardiolipins were decreased upon misalignment, whereas phosphatidylcholines consistently followed the same morning-evening pattern, suggesting regulation by the circadian clock. Cholesteryl esters adjusted to the shifted behavior. Lipid droplet characteristics remained unaltered upon misalignment. Together, these findings show that simulated shift work disturbs the skeletal muscle lipidome, which may contribute to misalignment-induced insulin resistance.

Day-night rhythm of skeletal muscle metabolism is disturbed in older, metabolically compromised individuals

OBJECTIVE

Skeletal muscle mitochondrial function and energy metabolism displays day-night rhythmicity in healthy, young individuals. Twenty-four-hour rhythmicity of metabolism has been implicated in the etiology of age-related metabolic disorders. Whether day-night rhythmicity in skeletal muscle mitochondrial function and energy metabolism is altered in older, metabolically comprised humans remains unknown.

METHODS

Twelve male overweight volunteers with impaired glucose tolerance and insulin sensitivity stayed in a metabolic research unit for 2 days under free living conditions with regular meals. Indirect calorimetry was performed at 5 time points (8 AM, 1 PM, 6 PM, 11 PM, 4 AM), followed by a muscle biopsy. Mitochondrial oxidative capacity was measured in permeabilized muscle fibers using high-resolution respirometry.

RESULTS

Mitochondrial oxidative capacity did not display rhythmicity. The expression of circadian core clock genes BMAL1 and REV-ERBα showed a clear day-night rhythm (p < 0.001), peaking at the end of the waking period. Remarkably, the repressor clock gene PER2 did not show rhythmicity, whereas PER1 and PER3 were strongly rhythmic (p < 0.001). On the whole-body level, resting energy expenditure was highest in the late evening (p < 0.001). Respiratory exchange ratio did not decrease during the night, indicating metabolic inflexibility.

CONCLUSIONS

Mitochondrial oxidative capacity does not show a day-night rhythm in older, overweight participants with impaired glucose tolerance and insulin sensitivity. In addition, gene expression of PER2 in skeletal muscle indicates that rhythmicity of the negative feedback loop of the molecular clock is disturbed. CLINICALTRIALS.

GOV ID

NCT03733743.

Skeletal muscle in healthy humans exhibits a day-night rhythm in lipid metabolism

OBJECTIVE

Human energy metabolism is under the regulation of the molecular circadian clock; we recently reported that mitochondrial respiration displays a day-night rhythm under study conditions that are similar to real life. Mitochondria are interconnected with lipid droplets, which are of importance in fuel utilization and play a role in muscle insulin sensitivity. Here, we investigated if skeletal muscle lipid content and composition also display day-night rhythmicity in healthy, lean volunteers.

METHODS

Skeletal muscle biopsies were obtained from 12 healthy lean male volunteers every 5 h over a 24 h period. Volunteers were provided with standardized meals, and biopsies were taken 4.5 h after each last meal. Lipid droplet size and number were investigated by confocal microscopy. Additionally, the muscle lipidome was assessed using UPLC/HRMS-based semi-targeted lipidomics.

RESULTS

Confocal microscopy revealed diurnal differences in intramyocellular lipid content (P < 0.05) and lipid droplet size in oxidative type 1 muscle fibers (P < 0.01). Lipidomics analysis revealed that 13% of all detected lipids displayed significant day-night rhythmicity. The most rhythmic lipid species were glycerophospholipids and diacylglycerols (DAG), with the latter being the largest fraction (>50% of all rhythmic species). DAG levels showed a day-night pattern with a trough at 1 PM and a peak at 4 AM.

CONCLUSIONS

Using two distinct methods, our findings show that myocellular lipid content and whole muscle lipid composition vary across the day-night cycle under normal living conditions. In particular, day-night rhythmicity was present in over half of the DAG lipid species. Future studies are needed to investigate whether rhythmicity in DAG is functionally related to insulin sensitivity and how this might be altered in prediabetes.

Circadian misalignment induces fatty acid metabolism gene profiles and compromises insulin sensitivity in human skeletal muscle

Circadian misalignment, such as in shift work, has been associated with obesity and type 2 diabetes. However, direct effects of circadian misalignment on skeletal muscle insulin sensitivity and the muscle molecular circadian clock have never been studied in humans. Here, we investigated insulin sensitivity and muscle metabolism in 14 healthy young lean men [age 22.4 ± 2.8 years; body mass index (BMI) 22.3 ± 2.1 kg/m2 (mean ± SD)] after a 3-d control protocol and a 3.5-d misalignment protocol induced by a 12-h rapid shift of the behavioral cycle. We show that short-term circadian misalignment results in a significant decrease in muscle insulin sensitivity due to a reduced skeletal muscle nonoxidative glucose disposal (rate of disappearance: 23.7 ± 2.4 vs. 18.4 ± 1.4 mg/kg per minute; control vs. misalignment; P = 0.024). Fasting glucose and free fatty acid levels as well as sleeping metabolic rate were higher during circadian misalignment. Molecular analysis of skeletal muscle biopsies revealed that the molecular circadian clock was not aligned to the inverted behavioral cycle, and transcriptome analysis revealed the human PPAR pathway as a key player in the disturbed energy metabolism upon circadian misalignment. Our findings may provide a mechanism underlying the increased risk of type 2 diabetes among shift workers.

BclI Glucocorticoid Receptor Polymorphism in Relation to Arterial Stiffening and Cardiac Structure and Function: The Hoorn and CODAM Studies

BACKGROUND

Chronic glucocorticoid excess is associated with arterial stiffening and cardiac dysfunction. The BclI glucocorticoid receptor (GR) polymorphism increases GR sensitivity and is associated with a higher body mass index (BMI) and some proxies for cardiovascular disease (CVD). Whether BclI influences arterial stiffening and cardiac dysfunction is currently unknown. Therefore, the aim of the present study was to investigate the association of the BclI polymorphism with arterial stiffening and cardiac structure and function.

METHODS

We conducted an observational cohort study, combining 2 cohort studies designed to investigate genetic and metabolic determinants of CVD. We genotyped 1,124 individuals (age: 64.7 ± 8.5 years) from the Hoorn study and Cohort on Diabetes and Atherosclerosis Maastricht (CODAM) study for BclI. Several arterial stiffening indices of the carotid (Hoorn and CODAM study), brachial and femoral artery and the carotid-femoral pulse wave velocity (Hoorn study only) were determined. In addition, in the Hoorn study, we determined several variables of cardiac structure and function.

RESULTS

We identified 155 homozygous carriers (GG), 498 heterozygous carriers (CG), and 471 noncarriers (CC) of the BclI polymorphism. BclI genotypes did not display significant differences in variables of arterial stiffening (e.g., carotid distensibility coefficient (DC): 12.41 ± 5.37 vs. 12.87 ± 5.55 10-3/kPa [mean ± SD]; P = 0.38; homozygous vs. noncarriers). In addition, no clear differences in estimates of cardiac structure and function were found.

CONCLUSIONS

Even though BclI is associated with a higher BMI and some proxies of CVD, our results do not support the concept that BclI carrier status is associated with greater arterial stiffening or cardiac dysfunction.

Demonstration of a day-night rhythm in human skeletal muscle oxidative capacity

OBJECTIVE

A disturbed day-night rhythm is associated with metabolic perturbations that can lead to obesity and type 2 diabetes mellitus (T2DM). In skeletal muscle, a reduced oxidative capacity is also associated with the development of T2DM. However, whether oxidative capacity in skeletal muscle displays a day-night rhythm in humans has so far not been investigated.

METHODS

Lean, healthy subjects were enrolled in a standardized living protocol with regular meals, physical activity and sleep to reflect our everyday lifestyle. Mitochondrial oxidative capacity was examined in skeletal muscle biopsies taken at five time points within a 24-hour period.

RESULTS

Core-body temperature was lower during the early night, confirming a normal day-night rhythm. Skeletal muscle oxidative capacity demonstrated a robust day-night rhythm, with a significant time effect in ADP-stimulated respiration (state 3 MO, state 3 MOG and state 3 MOGS, p < 0.05). Respiration was lowest at 1 PM and highest at 11 PM (state 3 MOGS: 80.6 ± 4.0 vs. 95.8 ± 4.7 pmol/mg/s). Interestingly, the fluctuation in mitochondrial function was also observed in whole-body energy expenditure, with peak energy expenditure at 11 PM and lowest energy expenditure at 4 AM (p < 0.001). In addition, we demonstrate rhythmicity in mRNA expression of molecular clock genes in human skeletal muscle.

CONCLUSIONS

Our results suggest that the biological clock drives robust rhythms in human skeletal muscle oxidative metabolism. It is tempting to speculate that disruption of these rhythms contribute to the deterioration of metabolic health associated with circadian misalignment.

BclI glucocorticoid receptor polymorphism in relation to cardiovascular variables: the Hoorn and CODAM studies

OBJECTIVE

Excess glucocorticoids are known to cause hypertension and cardiovascular disease (CVD). The BclI glucocorticoid receptor (GR) polymorphism increases glucocorticoid sensitivity and is associated with adverse metabolic effects. Previous studies investigating cardiovascular implications have shown inconsistent results. Therefore, the aim of the present study was to investigate the association of the BclI polymorphism with blood pressure, atherosclerosis, low-grade inflammation, endothelial dysfunction, and prevalent CVD.

DESIGN

Observational cohort study, combining two cohort studies designed to investigate genetic and metabolic determinants of CVD.

METHODS

We genotyped 1228 individuals (aged 64.7 years±8.5) from the Cohort on Diabetes and Atherosclerosis Maastricht (CODAM) study and Hoorn study for the BclI polymorphism. We measured blood pressure, ankle-brachial index (ABI), and carotid intima-media thickness (cIMT). Low-grade inflammation and endothelial dysfunction scores were computed by averaging Z-scores of six low-grade inflammation markers and four endothelial dysfunction markers respectively. Prevalent CVD was assessed with questionnaires, hospital records, ECG, and ABI.

RESULTS

Homozygous carriers (GG) had higher mean arterial pressure (103.8±12.4  mmHg vs 101.6±12.2  mmHg (mean±S.D.); P<0.05) compared with non-carriers (CC). Homozygous carriers had lower ABI compared with heterozygous carriers (CG) (1.08±0.13 vs 1.11±0.14; P<0.05). After adjustment for all covariates in the full model, the association with ABI was no longer significant. BclI was not associated with systolic blood pressure, cIMT, low-grade inflammation, endothelial dysfunction, and prevalent CVD.

CONCLUSIONS

The BclI polymorphism of the GR gene may contribute to an unfavorable cardiovascular profile; however, the effects on cardiovascular variables appear to be limited and partly mediated by the metabolic phenotype exerted by BclI.