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.

Autophagy protects kidney from phosphate-induced mitochondrial injury.

Hyperphosphatemia is a common complication in patients with advanced chronic kidney disease (CKD) as well as an increased risk of cardiovascular mortality; however, the molecular mechanisms of phosphate-mediated kidney injury are largely unknown. Autophagy is a lysosomal degradation system, which plays protective roles against kidney diseases. Here, we studied the role of autophagy in kidney proximal tubular cells (PTECs) during phosphate overload. Temporal cessation of autophagy in drug-induced PTEC-specific autophagy-deficient mice that were fed high phosphate diet induced mild cytosolic swelling and an accumulation of SQSTM1/p62-and ubiquitin-positive protein aggregates in PTECs, indicating that phosphate overload requires enhanced autophagic activity for the degradation of increasing substrate. Morphological and biochemical analysis demonstrated that high phosphate activates mitophagy in PTECs in response to oxidative stress. PTEC-specific autophagy-deficient mice receiving heminephrectomy and autophagy-deficient cultured PTECs exhibited mitochondrial dysfunction, increased reactive oxygen species production, and reduced ATP production in response to phosphate overload, suggesting that high phosphate-induced autophagy counteracts mitochondrial injury and maintains cellular bioenergetics in PTECs. Thus, potentiating autophagic activity could be a therapeutic option for suppressing CKD progression during phosphate overload.

D-Amino acids and kidney diseases.

D-Amino acids are the recently detected enantiomers of L-amino acids. Accumulating evidence points their potential in solving the long-standing critical problems associated with the management of both chronic and acute kidney diseases. This includes estimating kidney function, early diagnosis and prognosis of chronic kidney disease, and disease monitoring. Among the D-amino acids, D-serine levels in the blood are strongly correlated with the glomerular filtration rate and are useful for estimating the function of the kidney. Urinary D-serine also reflects other conditions. The kidney proximal tubule reabsorbs serine with chiral-selectivity, with D-serine being reabsorbed much less efficiently than L-serine, and urinary excretion of D-serine is sensitive to the presence of kidney diseases. Therefore, assessing the intra-body dynamics of D-serine by measuring its level in blood and urinary excretion can be used to detect kidney diseases and assess pathophysiology. This new concept, the intra-body dynamics of D-serine, can be useful in the comprehensive management of kidney disease.

Proximal Tubule Autophagy Differs in Type 1 and 2 Diabetes.

BACKGROUND: Evidence of a protective role of autophagy in kidney diseases has sparked interest in autophagy as a potential therapeutic strategy. However, understanding how the autophagic process is altered in each disorder is critically important in working toward therapeutic applications. METHODS: Using cultured kidney proximal tubule epithelial cells (PTECs) and diabetic mouse models, we investigated how autophagic activity differs in type 1 versus type 2 diabetic nephropathy. We explored nutrient signals regulating starvation-induced autophagy in PTECs and used autophagy-monitoring mice and PTEC-specific autophagy-deficient knockout mice to examine differences in autophagy status and autophagy's role in PTECs in streptozotocin (STZ)-treated type 1 and db/db type 2 diabetic nephropathy. We also examined the effects of rapamycin (an inhibitor of mammalian target of rapamycin [mTOR]) on vulnerability to ischemia-reperfusion injury. RESULTS: Administering insulin or amino acids, but not glucose, suppressed autophagy by activating mTOR signaling. In db/db mice, autophagy induction was suppressed even under starvation; in STZ-treated mice, autophagy was enhanced even under fed conditions but stagnated under starvation due to lysosomal stress. Using knockout mice with diabetes, we found that, in STZ-treated mice, activated autophagy counteracts mitochondrial damage and fibrosis in the kidneys, whereas in db/db mice, autophagic suppression jeopardizes kidney even in the autophagy-competent state. Rapamycin-induced pharmacologic autophagy produced opposite effects on ischemia-reperfusion injury in STZ-treated and db/db mice. CONCLUSIONS: Autophagic activity in PTECs is mainly regulated by insulin. Consequently, autophagic activity differs in types 1 and 2 diabetic nephropathy, which should be considered when developing strategies to treat diabetic nephropathy by modulating autophagy.

Plasma pentraxin 3 (PTX3) level is associated with the disease activity of microscopic polyangiitis (MPA).

A 20-year-old woman, who was suffering from appetite loss, weight loss and livedo reticularis for one and half months, was referred to our hospital. On admission, laboratory studies demonstrated proteinuria (1.0 g/g Cr), hematuria (erythrocytes': 50 - 99/HPF), ,.enal dysfunction (Cr : 2.09 mg/dL), elevated C reactive protein (CRP: 10.82 mg/dL), elevated MPO-ANCA titer (11.6 U/mL) and elevated pentraxin3 (PTX3: 24.05 ng/mL). Her kidney and skin biopsy revealed massive crescentic necrotizing glomerulonephritis and leukocytoclastic vasculitis, respectively. She was diagnosed with microscopic polyangiitis (MPA), and treated with 500 mg/day of intravenous methyl-prednisolone (mPSL) for 3 days followed by 40 mg/day of oral PSL, rituximab (375 mg/m² once a week for a month) and plasma exchange. When PSL tapered to 30 mg/day in 4 weeks, her renal function was only partially recovered, while the CRP level had been normalized and the MPO-ANCA titer was almost negative (3.6 IU/mL). To evaluate histological activity, a second renal biopsy was conducted, which showed fibrocellular crescents in 32% of her glomeruli. The PTX3 level remained high (14.82 ng/mL) at that point. Taken together, the vasculitis was considered to be active still. Steroid pulse therapy for 3 days was administered again, followed by oral PSL 30 mg/day. Her renal function completely recovered in 70 days. The PTX3 level also normalized in 161 days. PTX3 is one of the short pentraxins, produced by a variety of cell types in response to pro-inflammatory signals such as IL-1 and TNF-α. It was reported that PTX3 reflects activity of vasculitis independently from CRP. In the presenting case, when the second renal biopsy revealed a histologically active lesion of the vasculitis, PTX3 was elevated independently from CRP and MPO-ANCA, suggesting that PTX3 may be a more sensitive marker of the disease activity than other tests.

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.

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.

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.

Utility of d-serine monitoring in kidney disease.

d-Serine, a long-term undetected enantiomer of serine, is now showing its potential as a biomarker for kidney diseases. The intra-body dynamics of d-serine, currently defined by blood levels and urinary excretion dynamics, are useful for a comprehensive assessment of kidney function and disease activity. Thus, widespread adoption of d-serine as a biomarker can resolve the long-standing clinical challenge of the early detection and prognostic prediction of kidney diseases. Accuracy and reliability of the measurements are particularly important because these measurements will affect treatment decisions and thus impact the patient's emotional state and quality of life. Accordingly, this review focuses on current clinical challenges in kidney diseases and the potential for monitoring of d-serine to overcome these issues, and discuss the requirements of accurate quantification.

D-Serine reflects kidney function and diseases.

D-Amino acids, long-term undetected enantiomers of L-amino acids, are now emerging as potential biomarkers, especially for kidney diseases. Management of chronic kidney disease (CKD), a global problem with its high prevalence and poor prognosis, is currently unsatisfactory due to the difficulty in estimating kidney function and in early detection of diseases. We now show that intra-body dynamics of D-serine reflect kidney function and diseases. The blood level of D-serine correlated well with the actual glomerular filtration ratio, a key kidney function. This correlation was compatible with those of conventional kidney markers, and blood level of D-serine was relatively unaffected by such clinical factors as body size. The balance between excretion and reabsorption of amino acids by the kidney was controlled with chiral selectivity, and the reabsorption of D-serine was sensitive to the presence of CKD. The combination of blood level and urinary dynamics of D-serine effectively distinguished CKD from non-CKD. These lines of evidence provide new insights into the enantioselective amino acid dynamics in the human body that reflect disease pathophysiology. D-Serine may serve as a vital biomarker that suppress CKD onset through the precise assessment of kidney function and the diagnosis of CKD.

Dynamics of D-serine reflected the recovery course of a patient with rapidly progressive glomerulonephritis.

We experienced a case of a 36-year-old female with rapidly progressive glomerulonephritis (RPGN) due to anti-neutrophil cytoplasmic antibody (ANCA)-associated nephritis and systemic lupus erythematosus (SLE) nephritis. Chiral amino acid metabolomics revealed a prominent profile of D-serine in this patient. At the fulminant period of RPGN, the level of plasma D-serine, a potential biomarker in CKD that reflects actual glomerular filtration ratio (GFR), was extremely high. On the other hand, urinary fractional excretion (FE) of D-serine, which was usually much higher than that of L-isoform, was 0% in this patient. These abnormal D-serine profiles normalized in response to the intensive treatment. Normalizations of blood D-serine levels were in parallel with those of blood creatinine levels and potentially reflect the recovery of GFR. FE of D-serine increased transiently before the normalization of D-serine profile, suggesting that kidney promotes urinary excretion of D-serine for the normalization of plasma D-serine level. These unexplored clinical features of D-serine well reflected the clinical course of this patient. Blood D-serine level can also serve as a biomarker in acute kidney injury (AKI) or RPGN, and, in combination with FE of D-serine, may render the clinical practitioners to judge the efficacy of intensive treatments.