Recently, it was discovered that amino acids, the building blocks of proteins, exist in two different forms: L- and D-form. While all natural proteins consist exclusively of L-amino acids, the function of D-amino acids has been poorly understood until now, even though they are present in the food that we consume daily.
Now, a multi-institutional research team led by Osaka University has discovered a function of a D-form amino acid: D-Alanine. So, what does it do, and how did they discover its function? To understand this, we need some background information.
The circadian clock, a natural oscillation of our bodies that follows the 24-hour cycle of day and night, influences many biological processes. One of these is gluconeogenesis, where new glucose can be produced instead of carbohydrate intake to maintain energy levels. Although it was known that gluconeogenesis varies with the circadian rhythm, the reason for this was unknown.
There were, however, some clues. D-Alanine is found in tissues that metabolize glucose, and it has been reported that trace levels of D-Alanine in blood and urine vary with the circadian rhythm. Using special devices and advanced analytical techniques, the researchers were able to detect and quantify traces of the rare amino acid. The team was then able to confirm that the D-Alanine level reliably changes with the circadian clock; the fluctuations are caused by the removal of D-Alanine through urine, a process controlled by the kidney. They also showed that sleep plays a key role in regulating the D-Alanine level.
„We decided to investigate which genes are expressed when the kidney is exposed to D-Alanine. We used a deep-learning analysis with an iterative random-forest algorithm to identify the target genes. We found that D-Alanine upregulated genes associated with gluconeogenesis and genes that are known to be related to the circadian rhythm.“
Shinsuke Sakai, lead author of the study
An analysis of transcription factors, which are proteins that regulate gene expression, revealed that the changes induced by D-Alanine were mediated by a protein called Cry2, known as an important circadian regulator. When the circadian rhythm was disrupted, treatment with D-Alanine improved the circadian rhythm.
„Through these experiments, we were able to demonstrate that D-Alanine represents a connection between gluconeogenesis in the kidney and the circadian clock,“ said senior author Tomonori Kimura, „and that D-Alanine activates gluconeogenesis through the circadian transcriptional network.“
The uncovering of the connection between D-Alanine and the circadian clock represents a major advance in our understanding of these rare D-amino acids. An exciting possibility is new treatments for diseases related to glucose, such as diabetes, and the circadian clock, such as sleep disorders.
Sakai, S., et al. (2023) d-Alanine influences the circadian clock to regulate glucose metabolism in the kidney. Nephron 360. doi.org/10.34067/KID.0000000000000345.