Serum D-asparagine concentration adjusted for eGFR could serve as a novel screening tool for urothelial carcinoma

The sensitivity of currently available screening tools for urothelial carcinoma (UC) remains unsatisfactory particularly at early stages. Hence, we aimed to establish a novel blood-based screening tool for urothelial carcinoma. We measured serum d-amino acid levels in 108 and 192 patients with and without UC individuals in the derivation cohort, and 15 and 25 patients with and without UC in the validation cohort. Serum d-asparagine levels were significantly higher in patients with UC than in those without UC (p < 0.0001). We developed a novel screening equation for the diagnosis of urothelial carcinoma using d-asparagine in serum and estimated the glomerular filtration rate (eGFR). Serum d-asparagine levels adjusted for eGFR exhibited high performance in the diagnosis of UC (AUC-ROC, 0.869; sensitivity, 80.6 %; specificity, 82.7 %), even in early-stage UC (AUC-ROC: 0.859, sensitivity: 83.3 %, specificity: 82.3 %), which were previously misdiagnosed via urinary occult blood or urine cytology. This established strategy combined with urinary occult blood, improves diagnostic ability (sensitivity: 93.7 %, specificity: 70.1 %).

<sc>A</sc> multi-hierarchical approach reveals <sc>d</sc>-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 -Alanine Affects the Circadian Clock to Regulate Glucose Metabolism in the Kidney

Key Points

d -Alanine affects the circadian clock to regulate gluconeogenesis in the kidney. d -Alanine itself has a clear intrinsic circadian rhythm, which is regulated by urinary excretion, and acts on the circadian rhythm. d -Alanine is a signal activator for circadian rhythm and gluconeogenesis through circadian transcriptional network.

Background

The aberrant glucose circadian rhythm is associated with the pathogenesis of diabetes. Similar to glucose metabolism in the kidney and liver, d -alanine, a rare enantiomer of alanine, shows circadian alteration, although the effect of d- alanine on glucose metabolism has not been explored. Here, we show that d- alanine acts on the circadian clock and affects glucose metabolism in the kidney.

Methods

The blood and urinary levels of d -alanine in mice were measured using two-dimensional high-performance liquid chromatography system. Metabolic effects of d -alanine were analyzed in mice and in primary culture of kidney proximal tubular cells from mice. Behavioral and gene expression analyses of circadian rhythm were performed using mice bred under constant darkness.

Results

d- Alanine levels in blood exhibited a clear intrinsic circadian rhythm. Since this rhythm was regulated by the kidney through urinary excretion, we examined the effect of d -alanine on the kidney. In the kidney, d -alanine induced the expressions of genes involved in gluconeogenesis and circadian rhythm. Treatment of d- alanine mediated glucose production in mice. Ex vivo glucose production assay demonstrated that the treatment of d -alanine induced glucose production in primary culture of kidney proximal tubular cells, where d -amino acids are known to be reabsorbed, but not in that of liver cells. Gluconeogenetic effect of d -alanine has an intraday variation, and this effect was in part mediated through circadian transcriptional network. Under constant darkness, treatment of d- alanine normalized the circadian cycle of behavior and kidney gene expressions.

Conclusions

d- Alanine induces gluconeogenesis in the kidney and adjusts the period of the circadian clock. Normalization of circadian cycle by d -alanine may provide the therapeutic options for life style–related diseases and shift workers.

Two-dimensional LC-MS/MS and three-dimensional LC analysis of chiral amino acids and related compounds in real-world matrices

Amino acids normally have a chiral carbon and d/l-enantiomers are present. Due to the homochirality features on the present Earth, l-enantiomers are predominant in the living beings and the d-enantiomers are rare. Along with the progress and development of cutting edge analytical methods, several d-amino acids were found even in the higher animals including humans, and their biological functions and diagnostic values have also been reported. However, the amounts of these d-amino acids are much lower than the l-forms, and development/utilization of highly sensitive and selective methods are practically essential to avoid the disturbance from uncountable intrinsic substances. In the present review, multi-dimensional HPLC methods for the determination of chiral amino acids, especially two-dimensional LC-MS/MS and three-dimensional LC methods, and their applications to a variety of real-world matrices are summarized.

Blood levels of d-amino acids reflect the clinical course of COVID-19

d-Amino acids, rare enantiomers of amino acids, have been identified as biomarkers and therapeutic options for COVID-19. Methods for monitoring recovery are necessary for managing COVID-19. On the other hand, the presence of SARS-CoV2 virus in the blood is associated with worse outcomes. We investigated the potential of d-amino acids for assessing recovery from severe COVID-19. In patients with severe COVID-19 requiring artificial ventilation, the blood levels of d-amino acids, including d-alanine, d-proline, d-serine, and d-asparagine, which were lower than the normal range before treatment, quickly and transiently increased and surpassed the upper limit of the normal range. This increase preceded the recovery of respiratory function, as indicated by ventilation weaning. The increase in blood d-amino acid levels was associated with the disappearance of the virus in the blood, but not with inflammatory manifestations or blood cytokine levels. d-Amino acids are sensitive biomarkers that reflect the recovery of the clinical course and blood viral load. Dynamic changes in blood d-amino acid levels are key indicators of clinical course.

d-Alanine as a biomarker and a therapeutic option for severe influenza virus infection and COVID-19

Since the outbreak of coronavirus disease 2019 (COVID-19), biomarkers for evaluating severity, as well as supportive care to improve clinical course, remain insufficient. We explored the potential of d-amino acids, rare enantiomers of amino acids, as biomarkers for assessing disease severity and as protective nutrients against severe viral infections. In mice infected with influenza A virus (IAV) and in patients with severe COVID-19 requiring artificial ventilation or extracorporeal membrane oxygenation, blood levels of d-amino acids, including d-alanine, were reduced significantly compared with those of uninfected mice or healthy controls. In mice models of IAV infection or COVID-19, supplementation with d-alanine alleviated severity of clinical course, and mice with sustained blood levels of d-alanine showed favorable prognoses. In severe viral infections, blood levels of d-amino acids, including d-alanine, decrease, and supplementation with d-alanine improves prognosis. d-Alanine has great potentials as a biomarker and a therapeutic option for severe viral infections.

Measurement of glomerular filtration rate using endogenous d-serine clearance in living kidney transplant donors and recipients

BACKGROUND

Endogenous molecules that provide an unbiased and a precise evaluation of kidney function are still necessary. We explored the potential of clearance of d-serine, a rare enantiomer of serine and a biomarker of kidney function, as a measure of glomerular filtration rate (GFR).

METHODS

This was a cross-sectional observational study of 200 living kidney transplant donors and recipients enrolled between July 2019 and December 2020 in a single Japanese center, for whom GFR was measured by clearance of inulin (C-in). Clearance of d-serine (C-dSer) was calculated based on blood and urine levels of d-serine, as measured by two-dimensional high-performance liquid chromatography. Analytical performance was assessed by calculating biases. Utilizing data from 129 participants, we developed equations for C-in based on C-dSer and C-cre using a linear regression model, and the performance was validated in 68 participants.

FINDINGS

The means of C-in and C-dSer were 66.7 and 55.7 mL/min/1.73 m² of body surface area, respectively, in the entire cohort. C-dSer underestimated C-in with a proportional bias of 22.0% (95% confidence interval, 14.2-29.8%) and a constant bias of -1.24 (-5.78-3.31), whereas the proportional bias was minor to that of C-cre (34.6% [31.1-38.2%] and 2.47 (-1.18-6.13) for proportional and constant bias, respectively). Combination of C-dSer and C-cre measured C-in with an equation of 0.391 × C-dSer + 0.418 × C-cre + 3.852, which reduced the proportional bias (6.5% [-0.2-13.1%] and -4.30 [-8.87-0.28] for proportional and constant bias, respectively). In the validation dataset, this equation performed well with median absolute residual of 3.5 [2.3-4.8], and high ratio of agreement (ratios of 30% and 15% different from C-in [P₃₀ and P₁₅] of 98.5 [91.4-100] and 89.7 [80.0-95.2], respectively).

INTERPRETATION

The smaller proportional bias compared to that of C-cre is an advantage of C-dSer as a measure of C-in. Combinational measurement of d-serine and creatinine, two endogenous molecules, has the potential to serve as a measure of GFR with precision and minor biases and can support important clinical decisions.

FUNDING

Japan Society for the Promotion of Science (JSPS, grant number 17H04188), Japan Agency of Medical Research and Development (AMED, JP20gm5010001), Osaka Kidney Bank (OKF19-0010), Shiseido Co., Ltd and KAGAMI Inc.

Identification of Diabetic Nephropathy in Patients Undergoing Kidney Biopsy through Blood and Urinary Profiles of d-Serine

Background

The diagnosis of diabetic nephropathy (DN), the major cause of ESKD, requires kidney biopsy. d-Serine, present only in trace amounts in humans, is a biomarker for kidney diseases and shows potential to distinguish the origin of kidney diseases, whose diagnoses usually require kidney biopsy. We extended this concept and examined the potential of d-serine in the diagnosis of DN.

Methods

We enrolled patients with biopsy sample-proven DN and primary GN (minimal change disease and IgA nephropathy) and participants without kidney disease. A total of 388 participants were included in this study, and d-serine levels in blood and urine were measured using two-dimensional high-performance liquid chromatography, and urinary fractional excretion (FE) of d-serine was calculated. Using data from 259 participants, we developed prediction models for detecting DN by logistic regression analyses, and the models were validated in 129 participants.

Results

A d-serine blood level of >2.34 μM demonstrated a high specificity of 83% (95% CI, 70% to 93%) for excluding participants without kidney diseases. In participants with a d-serine blood level >2.34 μM, the threshold of 47% in FE of d-serine provided an optimal threshold for the detection of DN (AUC, 0.85 [95% CI, 0.76 to 0.95]; sensitivity, 79% [95% CI, 61% to 91%]; specificity, 83% [95% CI, 67% to 94%]). This plasma-high and FE-high profile of d-serine in combination with clinical factors (age, sex, eGFR, and albuminuria) correctly predicted DN with a sensitivity of 91% (95% CI, 72% to 99%) and a specificity of 79% (95% CI, 63% to 80%), and outperformed the model based on clinical factors alone in the validation dataset (P<0.02).

Conclusions

Analysis of d-serine in blood and urinary excretion is useful in identifying DN in patients undergoing kidney biopsy. Profiling of d-serine in patients with kidney diseases supports the suitable treatment through the auxial diagnosis of the origins of kidney diseases.

d-Serine Mediates Cellular Proliferation for Kidney Remodeling

Background

d-serine, a long-term undetected enantiomer of serine, is a biomarker that reflects kidney function and disease activity. The physiologic functions of d-serine are unclear.

Methods

The dynamics of d-serine were assessed by measuring d-serine in human samples of living kidney donors using two-dimensional high-performance liquid chromatography, and by autoradiographic studies in mice. The effects of d-serine on the kidney were examined by gene expression profiling and metabolic studies using unilateral nephrectomy mice, and genetically modified cells.

Results

Unilateral nephrectomy in human living kidney donors decreases urinary excretion and thus increases the blood level of d-serine. d-serine is quickly and dominantly distributed to the kidney on injection in mice, suggesting the kidney is a main target organ. Treatment of d-serine at a low dose promotes the enlargement of remnant kidney in mouse model. Mechanistically, d-serine activates the cell cycle for tissue remodeling through an mTOR-related pathway.

Conclusions

d-serine is a physiologic molecule that promotes kidney remodeling. Besides its function as a biomarker, d-serine has a physiologic activity that influences kidney function.

Intra-body dynamics of D-serine reflects the origin of kidney diseases

Introduction

d-Serine, present only in trace amounts in humans, is now recognized as a biomarker of chronic kidney disease (CKD). CKD is heterogeneous in its original kidney diseases, whose diagnoses require kidney biopsy. In this study, we examined whether the intra-body dynamics of d-serine, indexed by its blood and urinary levels, reflects the origin of kidney diseases.

Methods

Patients with six kinds of kidney disease undergoing kidney biopsy were enrolled in a single center. Levels of d- and l-serine were measured using two-dimensional high-performance liquid chromatography. The associations between the origin of kidney diseases and the intra-body dynamics of d-serine were examined using multivariate cluster analyses.

Results

Unlike the non-CKD profile, patients with CKD showed broadly-distributed profiles of intra-body dynamics of d-serine. The plasma level of d-serine plays a key role in the detection of kidney diseases, whereas a combination of plasma and urinary levels of d-serine distinguished the origin of CKD, especially lupus nephritis.

Conclusion

Intra-body dynamics of d-serine have the potential to predict the origin of kidney diseases. Monitoring of d-serine may guide specific treatments for the origin of kidney diseases.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10157-021-02052-5.

Three approaches to improving performance of liquid chromatography using contour maps with pressure, time, and number of theoretical plates

Attempts to improve HPLC performance often focus on increasing the speed or separation performance. In this article, both the flow rate and column length are optimized as separation conditions, while observing the number of theoretical plates and hold-up time with isocratic elutions. In addition, the upper pressure limit must be simultaneously considered as the boundary condition. Approaches based on the optimal velocity (Opt.) are often adopted; but the kinetic performance limit (KPL) in Desmet's method can also be utilized for three-dimensional graphing with axes of pressure, time, and number of theoretical plates. Here, two approaches involving pressure increase are introduced, beginning with the condition of optimal linear velocity: one aimed at greater speed and the other at higher resolution. Coefficients of pressure-application are derived to measure the effectiveness of the intermediate conditions between the Opt. and KPL methods. In the third approach, the hold-up time is extended while maintaining a fixed pressure. Coefficients of time-extension are also derived, to determine the effectiveness to improve the separation performance.

Determination of phenylalanine enantiomers in the plasma and urine of mammals and ᴅ-amino acid oxidase deficient rodents using two-dimensional high-performance liquid chromatography

A two-dimensional (2D) HPLC system focusing on the determination of phenylalanine (Phe) enantiomers in mammalian physiological fluids has been developed. ᴅ-Phe is indicated to have potential values as a disease biomarker and therapeutic molecule in several neuronal and metabolic disorders, thus the regulation of ᴅ-Phe in mammals is a matter of interest. However, the precise determination of amino acid enantiomers is difficult in complex biological samples, and the development of an analytical method with practically acceptable sensitivity, selectivity and throughput is expected. In the present study, a 2D-HPLC system equipped with a reversed-phase column in the 1st dimension and an enantioselective column in the 2nd dimension has been designed, following the fluorescence derivatization of the target amino acid enantiomers with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F). The analytical method was validated using both plasma and urine samples, and successfully applied to human, rat and mouse fluids. Trace levels of ᴅ-Phe were determined in the plasma, and the %ᴅ values were around 0.1% for all species. In the urine, relatively large amounts of ᴅ-Phe were observed, and the %ᴅ values for humans, rats and mice were 3.99, 1.76 and 5.25%, respectively. The relationships between the enzymatic activity of ᴅ-amino acid oxidase (DAO) and the amounts of intrinsic ᴅ-Phe have also been clarified, and high ᴅ-Phe amounts were observed (around 0.3% in the plasma and around 50% in the urine) in the DAO deficient rats and mice.

d-Amino acid oxidase deficiency is caused by a large deletion in the Dao gene in LEA rats

d-Amino acids, enantiomers of l-amino acids, are increasingly recognized as physiologically active molecules as well as potential biomarkers for diseases. d-Amino acid oxidase (DAO) catalyzes the oxidative deamination of d-amino acids and is present in a wide variety of organisms from yeasts to humans. Previous studies indicated that LEA rats lacked DAO activity, and levels of d-Ser and d-Ala were markedly increased in their tissues, suggesting a mutated locus responsible for the lack of Dao activity (ldao) existed in the LEA genome. Sequence analysis identified deletion breakpoints located in intron 4-5 of the Dao gene and intron 1-2 of the Svop gene, resulting in a 54.1-kb deletion which encompassed exons 5-12 of the Dao gene and exons 2-16 of the Svop gene. We developed a novel congenic rat strain, F344-Dao^ldao, harboring the Dao^ldao mutation from LEA rats delivered onto the F344 genetic background. Compared to the parental F344 strain, in F344-Dao^ldao rats d-Ala was markedly increased in both cerebrum and cerebellum, while d-Ser content was increased in cerebellum but not cerebrum. d-Ala, d-Ser, d-Pro and d-Leu levels were also elevated in F344-Dao^ldao plasma. F344-Dao^ldao rats represent a novel model system that will aid in elucidating the physiological functions of d-amino acids in vivo. (203 words).