Solute carrier SLC16A12 is critical for creatine and guanidinoacetate handling in the kidney

Creatine homeostasis and the kidney: comparison between kidney transplant recipients and healthy controls

Creatine is a natural nitrogenous organic acid that is integral to energy metabolism and crucial for proper cell functioning. The kidneys are involved in the first step of creatine production. With kidney transplantation being the gold-standard treatment for end-stage kidney disease, kidney transplant recipients (KTR) may be at risk of impaired creatine synthesis. We aimed to compare creatine homeostasis between KTR and controls. Plasma and urine concentrations of arginine, glycine, guanidinoacetate, creatine and creatinine were measured in 553 KTR and 168 healthy controls. Creatine intake was assessed using food frequency questionnaires. Iothalamate-measured GFR data were available in subsets of 157 KTR and 167 controls. KTR and controls had comparable body weight, height and creatine intake (all P > 0.05). However, the total creatine pool was 14% lower in KTR as compared to controls (651 ± 178 vs. 753 ± 239 mmol, P < 0.001). The endogenous creatine synthesis rate was 22% lower in KTR as compared to controls (7.8 ± 3.0 vs. 10.0 ± 4.1 mmol per day, P < 0.001). Despite lower GFR, the plasma guanidinoacetate and creatine concentrations were 21% and 41% lower in KTR as compared to controls (both P < 0.001). Urinary excretion of guanidinoacetate and creatine were 66% and 59% lower in KTR as compared to controls (both P < 0.001). In KTR, but not in controls, a higher measured GFR was associated with a higher endogenous creatine synthesis rate (std. beta: 0.21, 95% CI: 0.08; 0.33; P = 0.002), as well as a higher total creatine pool (std. beta: 0.22, 95% CI: 0.11; 0.33; P < 0.001). These associations were fully mediated (93% and 95%; P < 0.001) by urinary guanidinoacetate excretion which is consistent with production of the creatine precursor guanidinoacetate as rate-limiting factor. Our findings highlight that KTR have a disturbed creatine homeostasis as compared to controls. Given the direct relationship of measured GFR with endogenous creatine synthesis rate and the total creatine pool, creatine supplementation might be beneficial in KTR with low kidney function.Trial registration ID: NCT02811835.Trial registration URL: https://clinicaltrials.gov/ct2/show/NCT02811835 .

Extracellular sodium regulates fibroblast growth factor 23 (FGF23) formation

The bone-derived hormone fibroblast growth factor-23 (FGF23) has recently received much attention due to its association with chronic kidney disease and cardiovascular disease progression. Extracellular sodium concentration ([Na+]) plays a significant role in bone metabolism. Hyponatremia (lower serum [Na+]) has recently been shown to be independently associated with FGF23 levels in patients with chronic systolic heart failure. However, nothing is known about the direct impact of [Na+] on FGF23 production. Here, we show that an elevated [Na+] (+20 mM) suppressed FGF23 formation, whereas low [Na+] (-20 mM) increased FGF23 synthesis in the osteoblast-like cell lines UMR-106 and MC3T3-E1. Similar bidirectional changes in FGF23 abundance were observed when osmolality was altered by mannitol but not by urea, suggesting a role of tonicity in FGF23 formation. Moreover, these changes in FGF23 were inversely proportional to the expression of NFAT5 (nuclear factor of activated T cells-5), a transcription factor responsible for tonicity-mediated cellular adaptations. Furthermore, arginine vasopressin, which is often responsible for hyponatremia, did not affect FGF23 production. Next, we performed a comprehensive and unbiased RNA-seq analysis of UMR-106 cells exposed to low versus high [Na+], which revealed several novel genes involved in cellular adaptation to altered tonicity. Additional analysis of cells with Crisp-Cas9-mediated NFAT5 deletion indicated that NFAT5 controls numerous genes associated with FGF23 synthesis, thereby confirming its role in [Na+]-mediated FGF23 regulation. In line with these in vitro observations, we found that hyponatremia patients have higher FGF23 levels. Our results suggest that [Na+] is a critical regulator of FGF23 synthesis.

Implications of tRNA abundance on translation elongation across bovine tissues

Introduction: Translation is a crucial stage of gene expression. It may also act as an additional layer of regulation that plays an important role in gene expression and function. Highly expressed genes are believed to be codon-biased to support increased protein production, in which quickly translated codons correspond to highly abundant tRNAs. Synonymous SNPs, considered to be silent due to the degeneracy of the genetic code, may shift protein abundance and function through alterations in translational efficiency and suboptimal pairing to lowly abundant tRNAs. Methods: Here, we applied Quantitative Mature tRNA sequencing (QuantM-tRNAseq) and ribosome profiling across bovine tissues in order to investigate the relationship between tRNA expression and slowed translation. Results: Moreover, we have identified genes modulated at transcriptional and/or translational levels underlying tissue-specific biological processes. We have also successfully defined pausing sites that depict the regulatory information encoded within the open reading frame of transcripts, which could be related to translation rate and facilitate proper protein folding. This work offers an atlas of distinctive pausing sites across three bovine tissues, which provides an opportunity to predict codon optimality and understand tissue-specific mechanisms of regulating protein synthesis.

Construction of the metabolic reprogramming-associated gene signature for clear cell renal cell carcinoma prognosis prediction

BACKGROUND

Metabolism reprogramming is a hallmark that associates tumor growth, metastasis, progressive, and poor prognosis. However, the metabolism-related molecular patterns and mechanism in clear cell renal cell carcinoma (ccRCC) remain unclear. Herein, the purpose of this study was to identify metabolism-related molecular pattern and to investigate the characteristics and prognostic values of the metabolism-related clustering.

METHODS

We comprehensively analyzed the differentially expressed genes (DEGs), and metabolism-related genes (MAGs) in ccRCC based on the TCGA database. Consensus clustering was used to construct a metabolism-related molecular pattern. Then, the biological function, molecular characteristics, Estimate/immune/stomal scores, immune cell infiltration, response to immunotherapy, and chemotherapy were analyzed. We also identified the DEGs between subclusters and constructed a poor signature and risk model based on LASSO regression cox analysis and univariable and multivariable cox regression analyses. Then, a predictive nomogram was constructed and validated by calibration curves.

RESULTS

A total of 1942 DEGs (1004 upregulated and 838 downregulated) between ccRCC tumor and normal samples were identified, and 254 MRGs were screened out from those DEGs. Then, 526 ccRCC patients were divided into two subclusters. The 7 metabolism-related pathways enriched in cluster 2. And cluster 2 with high Estimate/immune/stomal scores and poor survival. While, cluster 1 with higher immune cell infiltrating, expression of the immune checkpoint, IFN, HLA, immune activation-related genes, response to anti-CTLA4 treatment, and chemotherapy. Moreover, we identified 295 DEGs between two metabolism-related subclusters and constructed a 15-gene signature and 9 risk factors. Then, a risk score was calculated and the patients into high- and low-risk groups in TCGA-KIRC and E-MTAB-1980 datasets. And the prediction viability of the risk score was validated by ROC curves. Finally, the clinicopathological characteristics (age and stage), risk score, and molecular clustering, were identified as independent prognostic variables, and were used to construct a nomogram for 1-, 3-, 5-year overall survival predicting. The calibration curves were used to verify the performance of the predicted ability of the nomogram.

CONCLUSION

Our finding identified two metabolism-related molecular subclusters for ccRCC, which facilitates the estimation of response to immunotherapy and chemotherapy, and prognosis after treatment.

SLC6A and SLC16A family of transporters: Contribution to transport of creatine and creatine precursors in creatine biosynthesis and distribution

Creatine (Cr) is needed to maintain high energy levels in cells. Since Cr plays reportedly a critical role in neurodevelopment and the immune system, Cr dynamics should be strictly regulated to control these physiological events. This review focuses on the role of transporters that recognize Cr and/or Cr precursors. Our previous studies revealed physiological roles of SLC6A and SLC16A family transporters in Cr dynamics. Creatine transporter (CRT/SLC6A8) contributes to the influx transport of Cr in Cr distribution. γ-Aminobutyric acid transporter 2 (GAT2/SLC6A13) mediates incorporation of guanidinoacetate (GAA), a Cr precursor, in the process of Cr biosynthesis. Monocarboxylate transporter 12 (MCT12/SLC16A12) functions as an efflux transporter for Cr and GAA, and contributes to the process of Cr biosynthesis. Accordingly, the SLC6A and SLC16A family of transporters play important roles in the process of Cr biosynthesis and distribution via permeation of Cr and Cr precursors across the plasma membrane.

Contribution of monocarboxylate transporter 12 to blood supply of creatine on the sinusoidal membrane of the hepatocytes

Creatine (Cr)/phosphocreatine has the ability to buffer the high-energy phosphate, thereby contributing to intracellular energy homeostasis. As Cr biosynthetic enzyme deficiency is reported to increase susceptibility to colitis under conditions of inflammatory stress, Cr is critical for maintaining intestinal homeostasis under inflammatory stress. Cr is mainly produced in the hepatocytes and then distributed to other organs of the body by the circulatory system. Since monocarboxylate transporter 9 (MCT9) and monocarboxylate transporter 12 (MCT12) have been reported to accept Cr as a substrate, these transporters are proposed as candidates for Cr efflux transporter in the liver. The aim of this study was to elucidate the transport mechanism on Cr supply from the hepatocytes. Immunohistochemical staining of the rat liver sections revealed that both MCT9 and MCT12 were localized on the sinusoidal membrane of the hepatocytes. In the transport studies using Xenopus laevis oocyte expression system, [¹⁴C]Cr efflux from MCT9- or MCT12-expressing oocytes was significantly greater than that from water-injected oocytes. [¹⁴C]Cr efflux from primary cultured hepatocytes was significantly decreased following MCT12 mRNA knockdown, whereas this efflux was not decreased after mRNA knockdown of MCT9. Based on the extent of MCT12 protein downregulation and Cr efflux after knockdown of MCT12 in primary cultured rat hepatocytes, the contribution ratio of MCT12 in Cr efflux was calculated as 76.4%. Our study suggests that MCT12 substantially contributes to the efflux of Cr at the sinusoidal membrane of the hepatocytes.NEW & NOTEWORTHY Our study is the first to identify the role of monocarboxylate transporter 12 (MCT12) as a transporter of creatine (Cr) in the liver. MCT12 was found to significantly contribute to the efflux of Cr on the sinusoidal membrane of the hepatocytes. Since hepatocytes are known to be involved in creatine biosynthesis, the present findings can be beneficial for the regulation of Cr biosynthesis and supply.

Effects of guanidinoacetic acid supplementation on nitrogen retention and methionine flux in cattle

Creatine stores high-energy phosphate bonds in muscle and is synthesized in the liver through methylation of guanidinoacetic acid (GAA). Supplementation of GAA may therefore increase methyl group requirements, and this may affect methyl group utilization. Our experiment evaluated the metabolic responses of growing cattle to postruminal supplementation of GAA, in a model where methionine (Met) was deficient, with and without Met supplementation. Seven ruminally cannulated Holstein steers (161 kg initial body weight [BW]) were limit-fed a soybean hull-based diet (2.7 kg/d dry matter) and received continuous abomasal infusions of an essential amino acid (AA) mixture devoid of Met to ensure that no AA besides Met limited animal performance. To provide energy without increasing the microbial protein supply, all steers received ruminal infusions of 200 g/d acetic acid, 200 g/d propionic acid, and 50 g/d butyric acid, as well as abomasal infusions of 300 g/d glucose. Treatments, provided abomasally, were arranged as a 2 × 3 factorial in a split-plot design, and included 0 or 6 g/d of l-Met and 0, 7.5, and 15 g/d of GAA. The experiment included six 10-d periods. Whole body Met flux was measured using continuous jugular infusion of 1-13C-l-Met and methyl-2H3-l-Met. Nitrogen retention was elevated by Met supplementation (P < 0.01). Supplementation with GAA tended to increase N retention when it was supplemented along with Met, but not when it was supplemented without Met. Supplementing GAA linearly increased plasma concentrations of GAA and creatine (P < 0.001), but treatments did not affect urinary excretion of GAA, creatine, or creatinine. Supplementation with Met decreased plasma homocysteine (P < 0.01). Supplementation of GAA tended (P = 0.10) to increase plasma homocysteine when no Met was supplemented, but not when 6 g/d Met was provided. Protein synthesis and protein degradation were both increased by GAA supplementation when no Met was supplemented, but decreased by GAA supplementation when 6 g/d Met were provided. Loss of Met through transsulfuration was increased by Met supplementation, whereas synthesis of Met from remethylation of homocysteine was decreased by Met supplementation. No differences in transmethylation, transsulfuration, or remethylation reactions were observed in response to GAA supplementation. The administration of GAA, when methyl groups are not limiting, has the potential to improve lean tissue deposition and cattle growth.

Metabolic Basis of Creatine in Health and Disease: A Bioinformatics-Assisted Review

Creatine (Cr) is a ubiquitous molecule that is synthesized mainly in the liver, kidneys, and pancreas. Most of the Cr pool is found in tissues with high-energy demands. Cr enters target cells through a specific symporter called Na⁺/Cl⁻-dependent Cr transporter (CRT). Once within cells, creatine kinase (CK) catalyzes the reversible transphosphorylation reaction between [Mg²⁺:ATP⁴⁻]²⁻ and Cr to produce phosphocreatine (PCr) and [Mg²⁺:ADP³⁻]⁻. We aimed to perform a comprehensive and bioinformatics-assisted review of the most recent research findings regarding Cr metabolism. Specifically, several public databases, repositories, and bioinformatics tools were utilized for this endeavor. Topics of biological complexity ranging from structural biology to cellular dynamics were addressed herein. In this sense, we sought to address certain pre-specified questions including: (i) What happens when creatine is transported into cells? (ii) How is the CK/PCr system involved in cellular bioenergetics? (iii) How is the CK/PCr system compartmentalized throughout the cell? (iv) What is the role of creatine amongst different tissues? and (v) What is the basis of creatine transport? Under the cellular allostasis paradigm, the CK/PCr system is physiologically essential for life (cell survival, growth, proliferation, differentiation, and migration/motility) by providing an evolutionary advantage for rapid, local, and temporal support of energy- and mechanical-dependent processes. Thus, we suggest the CK/PCr system acts as a dynamic biosensor based on chemo-mechanical energy transduction, which might explain why dysregulation in Cr metabolism contributes to a wide range of diseases besides the mitigating effect that Cr supplementation may have in some of these disease states.