As a consequence of the increasing prevalence of obesity, type 2 diabetes (T2D) and its associated cardiovascular complications have emerged as one of the leading causes of death in Western countries. Although more common lifestyle interventions such as exercise training programs and dietary regimes are effective, novel intervention options and treatment targets to improve insulin sensitivity, a major hallmark of T2D, are warranted to counter this epidemic.
Cold exposure is a novel lifestyle intervention with a great therapeutic potential which is increasingly gaining attention among the scientific community. Our observations thus far indicate that skeletal muscle activation during cold exposure, i.e. shivering, is a key pre-requisite for cold-induced improvements in glucose homeostasis in patients with T2D and overweight/obese individuals. In this regard, we have observed that prolonged (10-day), intermittent (6h/day), mild cold acclimation resulted in a marked ~40% increase in peripheral insulin sensitivity in type 2 diabetes patients (1). When, however, in a follow-up study we took measures to minimize/eliminate shivering, we failed to observe changes in insulin sensitivity (2). Based on these observations, our current research within DMRG exclusively focuses on the effects of shivering thermogenesis on metabolic health parameters in patients with T2D and individuals at risk of developing the disease. Important questions we aim to address through our research include ‘Can we use cold exposure as a lifestyle method to prevent and treat diabetes and co-morbidities linked to the disease?’, ‘How much cold exposure is enough to see clinically relevant results?’, ‘Do all individuals respond optimally to cold therapy?’ and ‘What are the underlying mechanisms of cold-induced improvements in metabolic health?’. Ultimately, we aim to unravel the therapeutic potential of cold in the management of type 2 diabetes and its co-morbidities, as well as to set a firm ground for further research into this topic.
- Hanssen MJ, Hoeks J, Brans B, van der Lans AA, Schaart G, van den Driessche JJ, et al. Short-term cold acclimation improves insulin sensitivity in patients with type 2 diabetes mellitus. Nat Med. 2015;21(8):863-5.
- Remie CME, Moonen MPB, Roumans KHM, Nascimento EBM, Gemmink A, Havekes B, et al. Metabolic responses to mild cold acclimation in type 2 diabetes patients. Nat Commun. 2021;12(1):1516.
Non-alcoholic fatty liver (NAFL), the excessive accumulation of fat in the liver in the absence of excessive alcohol consumption, is the most common liver disorder in western society and is strongly associated with insulin resistance and increases the risk for the development of type 2 diabetes mellitus. NAFL is thought to be the result of an imbalance between lipid storage (due to increased delivery and synthesis), and disposal. Little is known about how hepatic fat causes insulin resistance and knowledge on the importance of delivery, synthesis and disposal pathways in causing NAFL in humans is sparse. Gaining a better understanding of the mechanisms underlying hepatic fat accumulation and its relation to insulin resistance is crucial in the development of effective treatment and prevention strategies for type 2 diabetes and NAFL.
To this end, we develop and apply state-of-the art Magnetic Resonance Spectroscopy (MRS) techniques, including 1H-MRS for quantification of hepatic fat content and composition (fatty acid saturation) and determination of hepatic acetylcarnitine, 13C-MRS for quantification of hepatic glycogen levels, and 31P-MRS for determination of hepatic energy metabolites, such as ATP and inorganic phosphate. In our research, we combine this MR-methodology with other state-of-the art techniques, such as stable isotope methods, including the use of D2O for measurements of de novo lipogenesis (DNL) and gluconeogenesis, hyperinsulinemic-euglycemic clamp methodology to assess insulin sensitivity and PET-methodology to measure organ-specific glucose uptake.
In this research line we use lifestyle interventions and pharmaceutical interventions as tools to study the etiology of insulin resistance and type 2 diabetes focusing on the role of the liver. Our current research goals on liver metabolism are:
- Investigate the relation between hepatic fat storage and insulin-stimulated hepatic glucose uptake
- Investigate the metabolic effects of ketohexokinase inhibition in individuals with NAFL
- Investigate the importance of hepatic saturated fatty acid content and DNL in determining hepatic insulin resistance
- Investigate the role of overnight hepatic gluconeogenesis in prediabetes
Circadian rhythmicity of metabolism and glycemic control
Type 2 diabetes (T2D) is one of the most prevalent metabolic diseases worldwide, characterized by inadequate pancreatic insulin secretion and insulin resistance. Recent insights have linked our intrinsic circadian system to metabolism regulation. The master pacemaker is generated in the hypothalamic suprachiasmatic nucleus (SCN), which establishes phase coherence in the body by synchronizing peripheral oscillators. This molecular clock consists of a transcriptional-translational feedback loop that involves the core clock genes BMAL1 and CLOCK, which induce the expression of their own repressors CRY and PER, generating ~24-hour oscillations. We previously discovered 24-hour rhythmicity in whole body lipid and glucose metabolism, whole body resting energy expenditure, substrate selection and mitochondrial respiration in healthy young men. Interestingly, a follow-up study indicated that these diurnal oscillations were compromised in older, insulin-resistant man. Particularly, older insulin-resistant men were less capable to switch to lipid oxidation during the night upon fasting, also known as metabolic inflexibility. In addition, we found genes associated with the clock to be altered in their expression pattern over 24-hours. This suggests that metabolic inflexibility in insulin resistance may be related to the dysregulation of the circadian system. As a result, alternative approaches aimed at re-synchronizing “broken clocks” in metabolically compromised individuals provide novel opportunities for the prevention and treatment of T2D.
Our research makes use of various non-invasive/invasive measurements including:
- Indirect calorimetry: to measure energy expenditure, substrate utilization and respiratory exchange ratio (RER) around the clock
- Skeletal muscle biopsies: to determine skeletal muscle mitochondrial function (e.g., high resolution respirometry), protein content of markers involved in glucose and lipid metabolism and/or genes and protein levels of proteins involved in the molecular clock (e.g., CLOCK and BMAL1), cultivation of myotubes to study cell metabolism in vitro, confocal microscopy to study lipid droplet morphology and mitochondrial dynamics
- Blood withdrawals: to determine circulating metabolites (e.g., glucose, triglycerides, FFA’s) and hormones (e.g., insulin and melatonin)
- Stimulation of different light regimes over 24h within our highly controlled metabolic chambers
We investigate whether timing of behavioural interventions (timing) can improve 24-hour rhythmicity of the intrinsic circadian system, potentially leading to improved glucose homeostasis and metabolic health and serve as a novel strategy in the prevention and treatment of T2D.
To explore this, our current research goals are:
- To investigate the potential benefit of scheduled natural daylight exposure vs. artificial light exposure in improving glucose control in T2D individuals
- To investigate if regular exercise training can improve 24h rhythmicity in substrate metabolism in individuals with prediabetes
- To investigate whether an acute, high intensity exercise bout performed either in the morning or late afternoon differentially affects substrate oxidation at night in individuals with prediabetes
- To investigate whether prolonged exercise training performed either in the morning or late afternoon differentially affects insulin sensitivity and 24 substrate metabolism in individuals with prediabetes
Non-invasive tissue metabolism
Within our research group, we focus on the development and application of novel non-invasive imaging strategies to study tissue metabolism, including sophisticated magnetic resonance spectroscopy (MRS) sequences and positron emission tomography (PET) imaging. Examples of our developed MRS sequences are the non-invasive detection and quantification of acetylcarnitine, NAD+/NADH, and the assessment of hepatic fatty acid composition. We also established measurements of mitochondrial function through phosphocreatine resynthesis rate in skeletal muscle and energy state (PCr/ATP) in cardiac muscle (by 31P-MRS), glycogen, and ectopic lipid deposition (by 13C and 1H-MRS respectively). The non-invasive nature of these measurements offers the opportunity to measure repeatedly, allowing to adequately study e.g. metabolite dynamics or treatment responses over time.
The current studies in this research line not only focus on the development and improvement of novel imaging sequences, but also on the validation and application of these sequences in clinical studies. Examples of our current research goals in such studies are:
- To develop a 31P-MRS sequence that can suppress α-ATP resonances to uncover NAD+/NADH metabolites in the skeletal muscle
- To develop a single-shot 1H-MRS sequence with dual water/lipid suppression for intrahapetic acetylcarnitine detection
- To investigate the combined effect of Nicotinamide Riboside (NR) supplementation and exercise on mitochondrial function and NAD metabolites in the skeletal muscle of older overweight humans.
- To determine the association between insulin-stimulated hepatic glucose uptake measured by dynamic PET and liver fat content and fatty acid composition, measured by 1H-MRS in overweight/obese volunteers.
Mitochondrial network and lipid droplet dynamics
Insulin resistance, an important hallmark of type 2 diabetes, is associated with mitochondrial dysfunction and increased accumulation of lipid droplets in the skeletal muscle. In the early 2000s it was published that lipid droplet accumulation in skeletal muscle perse is not detrimental. In healthy skeletal muscle lipid droplets are dynamic, i.e. storing and releasing fatty acids depending on energy demand, and interact with mitochondria. Mitochondria form a reticular network which undergoes a continuous cycle of fusion and fission events in order to maintain mitochondrial quality and to deal with fluctuations in energy demand and supply. Diminished dynamics in lipid droplets and mitochondrial networks are thought to play a role in the development of insulin resistance. In addition, mitochondrial network and lipid droplet dynamics are potential targets for insulin sensitizing interventions, such as exercise training and cold exposure.
To this end we have developed and applied advanced microscopy techniques such as STimulated Emission Depletion (STED), confocal laser scanning, and Correlative Light and Electron Microscopy (CLEM) on muscle biopsies obtained in our cross-sectional and intervention studies. In addition, to gain more mechanistical insights we applied live-cell spinning disk confocal microscopy on cultured human primary myotubes obtained from well-phenotyped donors. In combination with custom written scripts mitochondrial network morphology, lipid droplet morphology, subcellular lipid droplet location, protein content in/on individual organelles, mitochondria-lipid droplet interactions, and live cell imaging of mitochondrial network and lipid droplet dynamics can be quantitatively studied.
With these techniques we have shown that mitochondrial networks have a more fragmented character in individuals with type 2 diabetes. In addition, lipid droplet characteristics, such as morphology and protein coating, correspond to static lipid droplets in these individuals. Improved insulin sensitivity upon exercise training is associated with a shift in lipid droplet morphology towards an athlete-like phenotype. Our current research in this research line focuses on:
- Lipid droplet and mitochondrial network dynamics as a target in interventions improving insulin sensitivity and metabolic health
- How mitochondrial networks and lipid droplets deal with fluctuations in energy demand and supply