A multi-hierarchical approach reveals d-serine as a hidden substrate of sodium-coupled monocarboxylate transporters.

Transporter research primarily relies on the canonical substrates of well-established transporters. This approach has limitations when studying transporters for the low-abundant micromolecules, such as micronutrients, and may not reveal physiological functions of the transporters. While d-serine, a trace enantiomer of serine in the circulation, was discovered as an emerging biomarker of kidney function, its transport mechanisms in the periphery remain unknown. Here, using a multi-hierarchical approach from body fluids to molecules, combining multi-omics, cell-free synthetic biochemistry, and ex vivo transport analyses, we have identified two types of renal d-serine transport systems. We revealed that the small amino acid transporter ASCT2 serves as a d-serine transporter previously uncharacterized in the kidney and discovered d-serine as a non-canonical substrate of the sodium-coupled monocarboxylate transporters (SMCTs). These two systems are physiologically complementary, but ASCT2 dominates the role in the pathological condition. Our findings not only shed light on renal d-serine transport, but also clarify the importance of non-canonical substrate transport. This study provides a framework for investigating multiple transport systems of various trace micromolecules under physiological conditions and in multifactorial diseases.

D-amino Acids Ameliorate Experimental Colitis and Cholangitis by Inhibiting Growth of Proteobacteria: Potential Therapeutic Role in Inflammatory Bowel Disease.

BACKGROUND & AIMS: D-amino acids, the chiral counterparts of protein L-amino acids, were primarily produced and utilized by microbes, including those in the human gut. However, little was known about how orally administered or microbe-derived D-amino acids affected the gut microbial community or gut disease progression. METHODS: The ratio of D- to L-amino acids was analyzed in feces and blood from patients with ulcerative colitis (UC) and healthy controls. Also, composition of microbe was analyzed from patients with UC. Mice were treated with D-amino acid in dextran sulfate sodium colitis model and liver cholangitis model. RESULTS: The ratio of D- to L-amino acids was lower in the feces of patients with UC than that of healthy controls. Supplementation of D-amino acids ameliorated UC-related experimental colitis and liver cholangitis by inhibiting growth of Proteobacteria. Addition of D-alanine, a major building block for bacterial cell wall formation, to culture medium inhibited expression of the ftsZ gene required for cell fission in the Proteobacteria Escherichia coli and Klebsiella pneumoniae, thereby inhibiting growth. Overexpression of ftsZ restored growth of E. coli even when D-alanine was present. We found that D-alanine not only inhibited invasion of pathological K. pneumoniae into the host via pore formation in intestinal epithelial cells but also inhibited growth of E. coli and generation of antibiotic-resistant strains. CONCLUSIONS: D-amino acids might have potential for use in novel therapeutic approaches targeting Proteobacteria-associated dysbiosis and antibiotic-resistant bacterial diseases by means of their effects on the intestinal microbiota community.

Implanted cannula-mediated repetitive administration of Abeta25-35 into the mouse cerebral ventricle effectively impairs spatial working memory.

Amyloid beta (Abeta) is closely related to the onset of Alzheimer's disease (AD). To construct AD animal models, a bolus administration of a large dose of toxic Abeta into the cerebral ventricles of rodents has been performed in earlier studies. In parallel, a continuous infusion system via an osmotic pump into the cerebral ventricle has been developed to make a rat AD model. In this study, we developed a mouse AD model by repetitive administration of Abeta25-35 via a cannula implanted into the cerebral ventricle. Using this administration system, we reproducibly constructed a mouse with impaired spatial working memory. In accordance with the occurrence of the abnormal mouse behavior, we found that the number of choline acetyltransferase (ChAT)-positive neurons was reduced in paraventricular regions of brains of Abeta25-35-administered mice in a dose-dependent manner. Considering that the repetitive administration of a small dose of toxic Abeta via an implanted cannula leads to a brain status more resembling that of the AD patients than a bolus injection of a large dose of Abeta, and therapeutic as well as toxic agents are able to be repeatedly and reliably administered via an implanted cannula, we concluded that the implanted cannula-bearing AD mouse model is useful for development of new AD therapy.