Cells with a functioning molecular clock are better able to adapt to changes in glucose supply and recover more quickly from long-term malnutrition, as shown in a study published in eLife. This discovery helps explain why changes in the body’s circadian rhythm-such as night work and jet lag-can increase the risk of metabolic diseases such as diabetes.

Fine-tuning the Circadian Clock

The circadian clock is an endogenous time-keeping system with a period length of approximately 24 hours that organizes numerous physiological processes, particularly metabolism. At the molecular level, it is based on closely interlinked transcription-translation feedback loops. At the center of this regulation are the transcription factors CLOCK gene and BMAL1 gene. These proteins form a heterodimer complex that binds to specific DNA sequences and activates the transcription of various target genes. These target genes include the genes of the period and cryptochrome family, namely PER gene and CRY gene.

After transcription, the PER and CRY proteins are synthesized in the cytoplasm, where they initially accumulate. Over the course of several hours, they form protein complexes and then return to the cell nucleus. There, they inhibit the activity of the CLOCK-BMAL1 complex, thereby suppressing their own gene expression. This negative feedback causes the production of PER and CRY proteins to gradually decrease. Once the existing proteins have been degraded by cellular degradation processes, the inhibition of the CLOCK-BMAL1 complex is removed, and the cycle begins again. The time delay between activation, accumulation, and inhibition creates a self-sustaining rhythm of approximately 24 hours.

In addition to this central loop, there are additional regulatory mechanisms that contribute to the stabilization and fine-tuning of the circadian clock. The nuclear receptors REV-ERBα and RORα, which regulate the expression of BMAL1, play an important role in this process. While RORα activates the transcription of BMAL1, REV-ERBα has a repressive effect. This antagonistic interaction creates another feedback loop that increases the temporal precision and robustness of the circadian system.

Circadian Clock and Metabolism

The circadian clocks are closely linked to metabolism: on the one hand, the clock rhythmically modulates many metabolic pathways, and on the other hand, nutrients and metabolic stimuli influence the function of the clock. This is achieved by finely tuned feedback loops in which some positive components of the clock activate others, which in turn have a negative effect on the originally activating components. For example, glucose metabolism, lipid synthesis, and mitochondrial energy production are subject to circadian fluctuations. At the same time, metabolic signals act back on the clock and modulate its activity. One example of this is the enzyme SIRT1, whose activity depends on the cellular NAD⁺ level. Since this level is closely linked to the energy state of the cell, SIRT1 can influence the activity of circadian transcription factors and thus adapt the clock to the metabolic state. Another example is AMP-activated protein kinase (AMPK), which is activated when there is a lack of energy and can, among other things, influence the stability of CRY proteins.

This bidirectional coupling creates a finely tuned system in which the circadian clock coordinates metabolic processes, while nutrients, energy status, and metabolic signals modulate the function of the clock. This close interaction enables the organism to optimally adapt metabolic processes to the day-night rhythm and changing environmental conditions. “Since glucose influences so many signaling pathways, it is assumed that a glucose deficiency could impair the feedback loops of the circadian clock and hinder its ability to maintain a constant rhythm,” explains lead author Anita Szöke, a doctoral student at the Institute of Physiology at Semmelweis University in Budapest, Hungary. “We wanted to investigate how chronic glucose deficiency affects the molecular clock and what role the clock plays in adapting to hunger.”

Clock Components Have a Major Influence on the Balance of Energy Metabolism Within Cells

Using the fungus Neurospora crassa as a model, the team first investigated how a 40-hour glucose deficiency affected two core components of the clock, the so-called White Collar Complex (WCC), which consists of the two subunits WC-1 and 2, and Frequency (FRQ). They found that the concentrations of WC1 and 2 gradually decreased to about 15% and 20% of their pre-starvation levels, respectively, while FRQ concentrations remained unchanged but were altered by the addition of many phosphate groups (a process called hyperphosphorylation). Normally, hyperphosphorylation prevents FRQ from inhibiting WCC activity, so the authors speculated that the higher activity might accelerate the breakdown of WCC. When they examined the downstream actions of WCC, there were few differences between the starved cells and those still growing in glucose. Taken together, this suggests that the circadian clock continued to function robustly during glucose starvation, driving the rhythmic expression of cellular genes.

To further investigate the importance of the molecular clock in adapting to glucose deprivation, the team used a Neurospora strain lacking the WC-1 domain of WCC. They then compared gene expression levels after glucose deprivation with those of Neurospora containing an intact molecular clock. They found that long-term glucose deprivation affected more than 20% of coding genes and that 1,377 of these 9,758 coding genes (13%) showed strain-specific changes depending on whether the cells had a molecular clock or not. This suggests that the clock is an important mechanism for the cells’ response to glucose deprivation.

Next, the team investigated whether a functioning clock is important for cell recovery after glucose deprivation. They found that the growth of Neurospora cells without a functioning FRQ or WCC was significantly slower than that of normal cells after the addition of glucose, suggesting that a functioning clock supports cell regeneration. When they examined the glucose transport system of Neurospora, they also found that cells without a functioning clock were unable to boost the production of an important glucose transporter to transport more nutrients into the cell. “The clear differences in recovery behavior between fungal strains with and without functioning molecular clocks suggest that adaptation to changing nutrient availability is more efficient when a circadian clock is functioning in a cell,” explained lead author Krisztina Káldi, associate professor at Semmelweis University. This suggests that clock components have a major influence on balancing energy metabolism within cells and underscores the importance of the clock in regulating metabolism and health.