Researchers from Belgium and Luxembourg have uncovered data suggesting that a lack of sleep sharply affects the level of proteins associated with processes that require energy, supporting the scientific view that sleep serves a crucial role in restoring energy and molecular stocks.
In a study published May 22 in the online journal Proteome Science, the researchers looked into the effects of sleep deprivation on a proteomic level and found that in the hippocampus of rats, the levels of proteins associated with cell metabolism, energy pathways, and protein processing all increased. At the same time, they observed a lower abundance of proteins in the adrenals that are associated with cell metabolism, protein assembly, and transcription regulation.
Taken together, the findings support the view that sleep serves to help the body replenish energy and certain molecules, the authors write in the article. While the overall conclusion may not be unexpected, the study is one of a small handful that have looked at how the body responds to sleep deprivation on a protein level and identifies specific proteins that may be affected the most from sleep, or, conversely, suffer the most from a lack of sleep.
A provisional version of the study is available here.
The authors say that all known vertebrate species require sleep suggesting it “might underpin one or several vital functions,” including thermoregulation, energy conservation, immune defense, tissue restoration, and brain plasticity.
While several studies have looked into gene transcription of the wake-sleep cycle, fewer have explored what effect sleep deprivation has on the proteomic level. However, “the characterization of protein changes that contribute to the cellular phenotype is an indispensable complement to genomic studies in understanding the link between cellular activity and behavior,” the authors said.
Referencing earlier research by Radhika Basheer and colleagues, the Belgian and Luxembourger team used a rat model and compared proteins in the hippocampus and adrenals in rats that were deprived of sleep for four hours with those that were allowed to sleep for the same period.
The authors chose to study changes in the hippocampus because it “plays an important role in spatial memory” for both humans and rodents, and because it is a target of stress hormones. Adrenals were chosen because they are physiologically affected by sleep deprivation.
For their research, they used 2D DIGE technology for protein separation then identified them with mass spectrometers from Bruker and Applied Biosystems. Several search engines and databases were also used.
The authors hypothesize that proteins whose abundance increased in sleep-deprived rats “could indicate a large number of cellular mechanisms involved in the maintenance of wakefulness, energy metabolism, and maybe some cognitive function.”
In total, 31 proteins were identified in the hippocampus or adrenal to have been modified after sleep deprivation. They note, however, that a rise in protein levels may not be reflective of just an increase in translation rate, “but also a reduced degradation of proteins, a post-translational modification of proteins, or a combination of these events,” and 2D DIGE does not allow for the distinction between these possibilities.
In the hippocampus, proteins with higher abundance after sleep deprivation serve five main functions: cell metabolism, energy pathways, transport and vesicle trafficking, cytoskeleton processing, and protein processing, which all require energy.
“An increase in the expression of genes related to energy metabolism has already been described in the literature. Our results support the hypothesis of a fast adaptation of neurons and/or glial cells to the increased metabolic demand of wakefulness relative to sleep,” the researchers wrote. They note also that no proteins related to stress response were identified in the hippocampus.
The protein that had the highest ratio in sleep-deprived rats, compared to rats that were allowed to sleep during the same period, was dihydrophrimidinase-related protein 2, which plays an important role in axon specification and elongation. In the adult brain, expression of the protein is down-regulated but remains in structures “that retain their capacity for differentiation and plasticity as well as in a subpopulation of oligodendrocytes,” the authors said. “One hypothesis could be that this protein may be involved in neuron plasticity in the hippocampus during sleep deprivation.”
They also saw an increased abundance of Rab GDI-alpha, vesicle fusing ATPase (NSF), and the alpha-soluble NSF attachment protein (alpha-SNAP) in the hippocampus, which they said suggests increased regulation of transport and vesicle trafficking “especially the docking and dissociation of vesicles to their target organelles.”
An increase regulation of vesicle traffic, they add, could underlie a “general increase in synaptic activity reflecting the induced motor activity and alertness as well as a reaction to a stressful situation.”
In the sleeping rats, the authors found higher abundance in the hippocampus of protein disulfide isomerase A3 and YPEL4.
The first protein has two main roles: to catalyze disulfide bond formation and isomerization in proteins, and to inhibit their aggression. Other researchers have suggested increased levels of protein disulfide isomerase A3 may be involved in controlling proper folding of newly synthesized proteins. The authors, however, note that because the sleep episodes in their experiment were short, higher abundance of the protein in this instance may be associated with the control of the release of previously inactivated proteins “a process that could be faster than an activation and translation of genes related to sleep.”
The function of YPEL4 is not yet fully known, but according to the authors, it is a protein whose gene is highly conserved and expressed in various eukaryotic organisms, suggesting it plays an important role in the maintenance of life. Its subcellular location suggests a function in cell division, they add.
In contrast to the hippocampus, not many proteins were found to be in higher abundance in the adrenals of rats that had been sleep-deprived, though one was of particular note, glucose-6-phosphate dehydorgenase, an enzyme that catalyzes the first step of the oxidative phase of the pentose phosphate pathway, and so is involved in the synthesis of NAPDH which may act to counter oxidative stress resulting from sleep-deprivation, the authors said.
In the adrenals of undisturbed rats, proteins were found to be abundant in several proteins associated with cell metabolism. These include aldehyde dehydrogenase precursor, carboxylesterase, and serine.
One protein that was observed, NADH deydrogenase 1-alpha, is specifically involved in ATP synthesis, and the potential increase in ATP could be advantageous for increased activity in metabolic pathways.
Other high-abundance proteins that were observed include steroidogenic acute regulator protein and heat shock 70kiloDalton protein 5, both of whom are involved in protein assembly and transcription regulation.
Based on their findings, the authors hypothesize that proteins whose abundance increased in sleep-deprived rats “could indicate a large number of cellular mechanisms involved in the maintenance of wakefulness, energy metabolism, and maybe some cognitive function.”
In addition, they said that the increased abundance of proteins in the adrenals of rats that were not sleep-deprived contributes to the “view of sleep as a mechanism of energy and restoration of molecular stocks.”
For future work, they said that attention could be focused on how sleep deprivation affects cognitive processes on a molecular level since sleep deprivation “extends the effects of synaptic potentiation on subsequent homeostatic processes.”
Further they suggest research on the differences in protein abundances at different points in sleep-deprivation.