Microbiome-derived metabolites are important for the microbiome-gut-brain axis and the discovery of new disease treatments. d-Alanine (d-Ala) is found in many animals as a potential co-agonist of the N-methyl-d-aspartate receptors (NMDAR), receptors widely used in the nervous and endocrine systems. The gut microbiome, diet and putative endogenous synthesis are the potential sources of d-Ala in animals, although there is no direct evidence to show the distribution and racemization of gut-absorbed l-/d-Ala with regards to host-microbe interactions in mammals. In this work, we utilized germ-free mice to control the interference from microbiota and isotopically labeled l-/d-Ala to track their biodistribution and racemization in vivo. Results showed time-dependent biodistribution of gut-absorbed d-Ala, particularly accumulation of gut-absorbed d-Ala in pancreatic tissues, brain, and pituitary. No endogenous synthesis of d-Ala via racemization was observed in germ-free mice. The sources of d-Ala in mice were revealed as microbiota and diet, but not endogenous racemization. This work indicates the importance of further investigating the in vivo biological functions of gut-microbiome derived d-Ala, particularly on NMDAR-related activities, for d-Ala as a potential signaling molecules in the microbiome-gut-brain axis.

D ‐alanine ( d ‐Ala) and several other d ‐amino acids ( d ‐AAs) act as hormones and neuromodulators in nervous and endocrine systems. Unlike the endogenously synthesized d ‐serine in animals, d ‐Ala may be from exogenous sources, e.g., diet and intestinal microorganisms. However, it is unclear if the capability to produce d ‐Ala and other d ‐AAs varies among different microbial strains in the gut. We isolated individual microorganisms of rat gut microbiota and profiled their d ‐AA production in vitro, focusing on d ‐Ala. Serial dilutions of intestinal contents from adult male rats were plated on agar to obtain clonal cultures. Using MALDI‐TOF MS for rapid strain typing, we identified 38 unique isolates, grouped into 11 species based on 16S rRNA gene sequences. We then used two‐tier screening to profile bacterial d ‐AA production, combining a d ‐amino acid oxidase‐based enzymatic assay for rapid assessment of non‐acidic d ‐AA amount and chiral LC–MS/MS to quantify individual d ‐AAs, revealing 19 out of the 38 isolated strains as d ‐AA producers. LC–MS/MS analysis of the eight top d ‐AA producers showed high levels of d ‐Ala in all strains tested, with substantial inter‐ and intra‐species variations. Though results from the enzymatic assay and LC–MS/MS analysis aligned well, LC–MS/MS further revealed the existence of d ‐glutamate and d ‐aspartate, which are poor substrates for this enzymatic assay. We observed large inter‐ and intra‐species variation of d ‐AA production profiles from rat gut microbiome species, demonstrating the importance of chemical profiling of gut microbiota in addition to sequencing, furthering the idea that microbial metabolites modulate host physiology.

The pancreatic islets of Langerhans are clusters of cells that function as endocrine units synthesizing and releasing insulin and a range of additional peptide hormones. The structural and chemical characteristics of islets change during type 2 diabetes development. Although a range of metabolites including neurotransmitters has been reported in rodent islets, the involvement of these cell-to-cell signaling molecules within human pancreatic islets in the pathophysiology of type 2 diabetes is not well known, despite studies suggesting that these molecules impact intra- and inter-islet signaling pathways. We characterize the enigmatic cell-to-cell signaling molecules, d-serine (d-Ser) and d-aspartate (d-Asp), along with multiple classical neurotransmitters and related molecules, in healthy versus type 2 diabetes-affected human islets using capillary electrophoresis separations. Significantly reduced d-Ser percentage and gamma-aminobutyric acid (GABA) levels were found in type 2 diabetes-affected islets compared to healthy islets. In addition, the negative correlations of many of the signaling molecules, such as d-Ser percentage (r = −0.35), d-Asp (r = −0.32), serotonin (r = −0.42), and GABA (r = −0.39) levels, with hemoglobin A1c (HbA1c) levels and thus with the progression of type 2 diabetes further demonstrate the disruption in intra- or inter-islet signaling pathways and suggest that these cell-to-cell signaling molecules may be potential therapeutic targets.

The cell-to-cell signaling role of d-amino acids (d-AAs) in the mammalian endocrine system, particularly in the islets of Langerhans, has drawn growing interest for their potential involvement in modulating glucose metabolism. Previous studies found colocalization of serine racemase [produces d-serine (d-Ser)] and d-alanine (d-Ala) within insulin-secreting beta cells and d-aspartate (d-Asp) within glucagon-secreting alpha cells. Expressed in the islets, functional N-methyl-d-aspartate receptors are involved in the modulation of glucose-stimulated insulin secretion and have binding sites for several d-AAs. However, knowledge of the regulation of d-AA levels in the islets during glucose stimulation as well as the response of islets to different levels of extracellular d-AAs is limited. In this study, we determined the intracellular and extracellular levels of d-Ser, d-Ala, and d-Asp in cultures of isolated rodent islets exposed to different levels of extracellular glucose. We found that the intracellular levels of the enantiomers demonstrated large variability and, in general, were not affected by extracellular glucose levels. However, significantly lower levels of extracellular d-Ser and d-Ala were observed in the islet media supplemented with 20 mM concentration of glucose compared to the control condition utilizing 3 mM glucose. Glucose-induced oscillations of intracellular free calcium concentration ([Ca²⁺]_i), a proxy for insulin secretion, were modulated by the exogenous application of d-Ser and d-Ala but not by their l-stereoisomers. Our results provide new insights into the roles of d-AAs in the biochemistry and function of pancreatic islets.