The citrate transporter SLC13A5 as a therapeutic target for kidney disease: evidence from Mendelian randomization to inform drug development.

BACKGROUND: Solute carrier family 13 member 5 (SLC13A5) is a Na+-coupled citrate co-transporter that mediates entry of extracellular citrate into the cytosol. SLC13A5 inhibition has been proposed as a target for reducing progression of kidney disease. The aim of this study was to leverage the Mendelian randomization paradigm to gain insight into the effects of SLC13A5 inhibition in humans, towards prioritizing and informing clinical development efforts. METHODS: The primary Mendelian randomization analyses investigated the effect of SLC13A5 inhibition on measures of kidney function, including creatinine and cystatin C-based measures of estimated glomerular filtration rate (creatinine-eGFR and cystatin C-eGFR), blood urea nitrogen (BUN), urine albumin-creatinine ratio (uACR), and risk of chronic kidney disease and microalbuminuria. Secondary analyses included a paired plasma and urine metabolome-wide association study, investigation of secondary traits related to SLC13A5 biology, a phenome-wide association study (PheWAS), and a proteome-wide association study. All analyses were compared to the effect of genetically predicted plasma citrate levels using variants selected from across the genome, and statistical sensitivity analyses robust to the inclusion of pleiotropic variants were also performed. Data were obtained from large-scale genetic consortia and biobanks, with sample sizes ranging from 5023 to 1,320,016 individuals. RESULTS: We found evidence of associations between genetically proxied SLC13A5 inhibition and higher creatinine-eGFR (p = 0.002), cystatin C-eGFR (p = 0.005), and lower BUN (p = 3 × 10-4). Statistical sensitivity analyses robust to the inclusion of pleiotropic variants suggested that these effects may be a consequence of higher plasma citrate levels. There was no strong evidence of associations of genetically proxied SLC13A5 inhibition with uACR or risk of CKD or microalbuminuria. Secondary analyses identified evidence of associations with higher plasma calcium levels (p = 6 × 10-13) and lower fasting glucose (p = 0.02). PheWAS did not identify any safety concerns. CONCLUSIONS: This Mendelian randomization analysis provides human-centric insight to guide clinical development of an SLC13A5 inhibitor. We identify plasma calcium and citrate as biologically plausible biomarkers of target engagement, and plasma citrate as a potential biomarker of mechanism of action. Our human genetic evidence corroborates evidence from various animal models to support effects of SLC13A5 inhibition on improving kidney function.

Extracellular citrate and metabolic adaptations of cancer cells.

It is well established that cancer cells acquire energy via the Warburg effect and oxidative phosphorylation. Citrate is considered to play a crucial role in cancer metabolism by virtue of its production in the reverse Krebs cycle from glutamine. Here, we review the evidence that extracellular citrate is one of the key metabolites of the metabolic pathways present in cancer cells. We review the different mechanisms by which pathways involved in keeping redox balance respond to the need of intracellular citrate synthesis under different extracellular metabolic conditions. In this context, we further discuss the hypothesis that extracellular citrate plays a role in switching between oxidative phosphorylation and the Warburg effect while citrate uptake enhances metastatic activities and therapy resistance. We also present the possibility that organs rich in citrate such as the liver, brain and bones might form a perfect niche for the secondary tumour growth and improve survival of colonising cancer cells. Consistently, metabolic support provided by cancer-associated and senescent cells is also discussed. Finally, we highlight evidence on the role of citrate on immune cells and its potential to modulate the biological functions of pro- and anti-tumour immune cells in the tumour microenvironment. Collectively, we review intriguing evidence supporting the potential role of extracellular citrate in the regulation of the overall cancer metabolism and metastatic activity.

The growing research toolbox for SLC13A5 citrate transporter disorder: a rare disease with animal models, cell lines, an ongoing Natural History Study and an engaged patient advocacy organization.

TESS Research Foundation (TESS) is a patient-led nonprofit organization seeking to understand the basic biology and clinical impact of pathogenic variants in the SLC13A5 gene. TESS aims to improve the fundamental understanding of citrate's role in the brain, and ultimately identify treatments and cures for the associated disease. TESS identifies, organizes, and develops collaboration between researchers, patients, clinicians, and the pharmaceutical industry to improve the lives of those suffering from SLC13A5 citrate transport disorder. TESS and its partners have developed multiple molecular tools, cellular and animal models, and taken the first steps toward drug discovery and development for this disease. However, much remains to be done to improve our understanding of the disorder associated with SLC13A5 variants and identify effective treatments for this devastating disease. Here, we describe the available SLC13A5 resources from the community of experts, to foundational tools, to in vivo and in vitro tools, and discuss unanswered research questions needed to move closer to a cure.

Overview of research in SLC13A5 citrate transporter disorder SLC13A5 citrate transporter disorder is an ultra-rare, neurodevelopmental disorder that severely impacts cognition and motor control. It is characterized by frequent, intractable seizures that develop hours or days after birth, low tone, global developmental delay, a unique, varied, and difficult to categorize movement disorder, limited expressive verbal capabilities, tooth abnormalities, and increased citrate in both the CNS and serum. Seizures are frequently medically intractable, patients are often on multiple antiseizure medications and have frequent emergency room visits and hospitalizations for status epilepticus. SLC13A5 citrate transporter disorder is caused by mutations in the SLC13A5 gene which encodes a sodium-dependent citrate transporter, NaCT. NaCT is responsible for transporting citrate, a key molecule in cellular metabolism, from the extracellular space into cells, especially in the central nervous system and the liver. NaCT has been extensively studied in multiple animal models and affects lifespan and loss of some transporter activity actually improves metabolic syndrome in all animal species tested so far while causing mild neurological dysfunction in rodents. Although not definitively proven, it is presumed that loss of neuronal cell citrate transporter activity in the brain is the cause of seizures. Since the discovery of the disorder in 2014, there has been a rapid expansion in characterization of the disease. This has been aided by development of multiple models and molecular tools for studying wild type and mutant SLC13A5 making it a tractable candidate for therapeutic development. TESS Research Foundation is dedicated to driving SLC13A5 research and supporting children and families living with the disorder. Here, we describe the available SLC13A5 resources from the community of experts, to foundational tools, to in vivo and in vitro tools, and discuss unanswered research questions needed to move closer to a cure.

Selective, Temporary Postoperative Inhibition of Lymphangiogenesis by Integrin α5ß1 Blockade Improves Allograft Survival in a Murine Model of High-Risk Corneal Transplantation.

Background: Corneal inflammatory hem- and lymphangiogenesis significantly increase the risk for immune rejection after subsequent allogeneic corneal transplantation. The purpose of this study was to analyze the impact of temporary selective inhibition of lymphangiogenesis after transplantation on graft survival. Methods: Allogeneic transplantation from C57BL/6 mice to BalbC mice was performed as "high-risk" keratoplasty in a prevascularized corneal host bed (suture-induced inflammatory corneal neovascularization). The treatment group received integrin α5ß1-blocking small molecules (JSM6427) at the time of transplantation and for two weeks afterwards. Control mice received a vehicle solution. Grafts were evaluated weekly for graft rejection using an opacity score. At the end of the follow-up, immunohistochemical staining of corneal wholemounts for lymphatic vessels as well as CD11b+ immune cells was performed. Results: Temporary postoperative inhibition of lymphangiogenesis by JSM6427 improved the corneal graft survival significantly. At the end of the follow-up, no significant reduction in CD11b+ immunoreactive cells within the graft compared to controls was found. Conclusions: The significant improvement of corneal graft survival by the selective, temporary postoperative inhibition of lymphangiogenesis after keratoplasty using integrin antagonists shows the impact of lymphatic vessels in the early postoperative phase. Retarding lymphatic vessel ingrowth into the graft might be sufficient for the shift to immunological tolerance in the postoperative period, even after high-risk keratoplasty.

Expression of the plasma membrane citrate carrier (pmCiC) in human cancerous tissues-correlation with tumour aggressiveness.

We have recently shown that cancer cells of various origins take up extracellular citrate through the plasma membrane citrate carrier (pmCiC), a specific plasma membrane citrate transporter. Extracellular citrate is required to support cancer cell metabolism, in particular fatty acid synthesis, mitochondrial activity, protein synthesis and histone acetylation. In addition, cancer cells tend to acquire a metastatic phenotype in the presence of extracellular citrate. Our recent study also showed that cancer-associated stromal cells synthesise and release citrate and that this process is controlled by cancer cells. In the present study, we evaluated the expression of pmCiC, fibroblast activation protein-α (FAP) and the angiogenesis marker cluster of differentiation 31 (CD31) in human cancer tissues of different origins. In the cohort studied, we found no correlation between disease stage and the expression of FAP or CD31. However, we have identified a clear correlation between pmCiC expression in cancer cells and cancer-associated stroma with tumour stage. It can be concluded that pmCiC is increased in cancer cells and in cancer-supporting cells in the tumour microenvironment at the later stages of cancer development, particularly at the metastatic sites. Therefore, pmCiC expression has the potential to serve as a prognostic marker, although further studies are needed.

The dual role of citrate in cancer.

Citrate is a key metabolite of the Krebs cycle that can also be exported in the cytosol, where it performs several functions. In normal cells, citrate sustains protein acetylation, lipid synthesis, gluconeogenesis, insulin secretion, bone tissues formation, spermatozoid mobility, and immune response. Dysregulation of citrate metabolism is implicated in several pathologies, including cancer. Here we discuss how cancer cells use citrate to sustain their proliferation, survival, and metastatic progression. Also, we propose two paradoxically opposite strategies to reduce tumour growth by targeting citrate metabolism in preclinical models. In the first strategy, we propose to administer in the tumor microenvironment a high amount of citrate, which can then act as a glycolysis inhibitor and apoptosis inducer, whereas the other strategy targets citrate transporters to starve cancer cells from citrate. These strategies, effective in several preclinical in vitro and in vivo cancer models, could be exploited in clinics, particularly to increase sensibility to current anti-cancer agents.

Targeting Longevity Gene SLC13A5: A Novel Approach to Prevent Age-Related Bone Fragility and Osteoporosis.

Reduced expression of the plasma membrane citrate transporter SLC13A5, also known as INDY, has been linked to increased longevity and mitigated age-related cardiovascular and metabolic diseases. Citrate, a vital component of the tricarboxylic acid cycle, constitutes 1-5% of bone weight, binding to mineral apatite surfaces. Our previous research highlighted osteoblasts' specialized metabolic pathway facilitated by SLC13A5 regulating citrate uptake, production, and deposition within bones. Disrupting this pathway impairs bone mineralization in young mice. New Mendelian randomization analysis using UK Biobank data indicated that SNPs linked to reduced SLC13A5 function lowered osteoporosis risk. Comparative studies of young (10 weeks) and middle-aged (52 weeks) osteocalcin-cre-driven osteoblast-specific Slc13a5 knockout mice (Slc13a5cKO) showed a sexual dimorphism: while middle-aged females exhibited improved elasticity, middle-aged males demonstrated enhanced bone strength due to reduced SLC13A5 function. These findings suggest reduced SLC13A5 function could attenuate age-related bone fragility, advocating for SLC13A5 inhibition as a potential osteoporosis treatment.

Cutting edge: lymphatic vessels, not blood vessels, primarily mediate immune rejections after transplantation.

The purpose of this study was to determine the relative importance of blood vessels (hemangiogenesis) versus lymphatic vessels (lymphangiogenesis) in mediating immunological responses after transplantation. Using the murine model of corneal transplantation, graft survival was compared in differentially prevascularized and avascular recipient beds. Donor corneas (C57BL/6) were transplanted into uninflamed or inflamed avascular, prehemvascularized only or prehemvascularized and prelymphvascularized recipient murine eyes (BALB/C). Selective inhibition of lymphangiogenesis was achieved using antivascular endothelial growth factor receptor 3 Abs and anti-integrin alpha5 small molecules. Grafts placed into only prehemvascularized recipient beds had a similarly good graft survival compared with grafts placed into completely avascular, normal recipients, whereas the pre-existence of lymphatic vessels significantly deteriorated corneal graft survival (p < 0.05). Lymphatic vessels seem to contribute significantly to graft rejection after (corneal) transplantation. That may allow for selective, temporary, perioperative antilymphangiogenic treatment to promote graft survival without affecting blood vessels, even after solid organ transplantation.

A Novel and Cross-Species Active Mammalian INDY (NaCT) Inhibitor Ameliorates Hepatic Steatosis in Mice with Diet-Induced Obesity.

Mammalian INDY (mINDY, NaCT, gene symbol SLC13A5) is a potential target for the treatment of metabolically associated fatty liver disease (MAFLD). This study evaluated the effects of a selective, cross-species active, non-competitive, non-substrate-like inhibitor of NaCT. First, the small molecule inhibitor ETG-5773 was evaluated for citrate and succinate uptake and fatty acid synthesis in cell lines expressing both human NaCT and mouse Nact. Once its suitability was established, the inhibitor was evaluated in a diet-induced obesity (DIO) mouse model. DIO mice treated with 15 mg/kg compound ETG-5773 twice daily for 28 days had reduced body weight, fasting blood glucose, and insulin, and improved glucose tolerance. Liver triglycerides were significantly reduced, and body composition was improved by reducing fat mass, supported by a significant reduction in the expression of genes for lipogenesis such as SREBF1 and SCD1. Most of these effects were also evident after a seven-day treatment with the same dose. Further mechanistic investigation in the seven-day study showed increased plasma ß-hydroxybutyrate and activated hepatic adenosine monophosphate-activated protein kinase (AMPK), reflecting findings from Indy (-/-) knockout mice. These results suggest that the inhibitor ETG-5773 blocked citrate uptake mediated by mouse and human NaCT to reduce liver steatosis and body fat and improve glucose regulation, proving the concept of NaCT inhibition as a future liver treatment for MAFLD.

alpha5beta1 Integrin blockade inhibits lymphangiogenesis in airway inflammation.

The integrin alpha5beta1 has been previously implicated in tumor angiogenesis, but its role in the remodeling of both blood vessels and lymphatics during inflammation is at an early stage of understanding. We examined this issue using a selective, small-molecule inhibitor of alpha5beta1 integrin, 2-aroylamino-3-{4-[(pyridin-2-ylaminomethyl)heterocyclyl]phenyl}propionic acid (JSM8757), in a model of sustained airway inflammation in mice with Mycoplasma pulmonis infection, which is known to be accompanied by robust blood vessel remodeling and lymphangiogenesis. The inhibitor significantly decreased the proliferation of lymphatic endothelial cells in culture and the number of lymphatic sprouts and new lymphatics in airways of mice infected for 2 weeks but did not reduce remodeling of blood vessels in the same airways. In inflamed airways, alpha5 integrin immunoreactivity was present on lymphatic sprouts, but not on collecting lymphatics or blood vessels, and was not found on any lymphatics of normal airways. Macrophages, potential targets of the inhibitor, did not have alpha5 integrin immunoreactivity in inflamed airways. In addition, macrophage recruitment, assessed in infected airways by quantitative reverse transcription-polymerase chain reaction measurements of expression of the marker protein ionized calcium-binding adapter molecule 1 (Iba1), was not reduced by JSM8757. We conclude that inhibition of the alpha5beta1 integrin reduces lymphangiogenesis in inflamed airways after M. pulmonis infection because expression of the integrin is selectively increased on lymphatic sprouts and plays an essential role in lymphatic growth.

SAR of N-phenyl piperidine based oral integrin alpha5beta1 antagonists.

Recently, a new class of selective integrin alpha5beta1inhibitors consisting of a heterocyclic based scaffold was published. Herein the SAR and pharmacokinetic profiles of N-phenyl piperidine derivatives are described.

Discovery of orally available integrin alpha5beta1 antagonists.

Previous research within our laboratories identified the 3-hydroxypyrrolidine scaffold 1 as a new and selective integrin alpha5beta1 inhibitor class which was designed for local administration. Herein the discovery of new orally available integrin alpha5beta1 inhibitor scaffolds for potential systemic treatment is described.

Mapping the ligand-binding pocket of integrin alpha5beta1 using a gain-of-function approach.

Integrin alpha5beta1 is a key receptor for the extracellular matrix protein fibronectin. Antagonists of human integrin alpha5beta1 have therapeutic potential as anti-angiogenic agents in cancer and diseases of the eye. However, the structure of the integrin is unsolved and the atomic basis of fibronectin and antagonist binding by integrin alpha5beta1 is poorly understood. In the present study, we demonstrate that zebrafish alpha5beta1 integrins do not interact with human fibronectin or the human alpha5beta1 antagonists JSM6427 and cyclic peptide CRRETAWAC. Zebrafish alpha5beta1 integrins do bind zebrafish fibronectin-1, and mutagenesis of residues on the upper surface and side of the zebrafish alpha5 subunit beta-propeller domain shows that these residues are important for the recognition of the Arg-Gly-Asp (RGD) motif and the synergy sequence [Pro-His-Ser-Arg-Asn (PHSRN)] in fibronectin. Using a gain-of-function analysis involving swapping regions of the zebrafish integrin alpha5 subunit with the corresponding regions of human alpha5 we show that blades 1-4 of the beta-propeller are required for human fibronectin recognition, suggesting that fibronectin binding involves a broad interface on the side and upper face of the beta-propeller domain. We find that the loop connecting blades 2 and 3 of the beta-propeller, the D3-A3 loop, contains residues critical for antagonist recognition, with a minor role played by residues in neighbouring loops. A new homology model of human integrin alpha5beta1 supports an important function for D3-A3 loop residues Trp157 and Ala158 in the binding of antagonists. These results will aid the development of reagents that block integrin alpha5beta1 functions in vivo.

An alpha5beta1 integrin inhibitor attenuates glioma growth.

Integrins are heterodimeric transmembrane proteins, which mediate cell-cell and cell-extracellular matrix (ECM) interaction. We show, that an inhibitor of alpha5 beta1 integrin (alpha5beta1), JSM6427, attenuated glioma growth and decreased the density of microglia at the tumor border. 21 days after glioma cell injection into an experimental mouse model, the tumor volume was significantly smaller after treating animals for 14 days with JSM6427 as compared to controls. We could demonstrate the expression of integrin alpha5beta1 on both microglia and glioma cells using flow cytometry. In a slice culture we could compare glioma growth in the presence and absence of microglia. Slices injected with glioma cells were treated with the integrin inhibitor JSM6427 and showed a significant reduction in tumor size as compared to control. Depleting microglial cells from the slice culture by treatment with clodronate liposomes abrogated the effect of JSM6427 on glioma invasion indicating that the presence of microglia is required. We show further, that microglial migration, and proliferation was attenuated dose-dependently by JSM6427.

Breaking the dogma of the metal-coordinating carboxylate group in integrin ligands: introducing hydroxamic acids to the MIDAS to tune potency and selectivity.

A suitable substitute: All integrin receptors bind their ligands, which contain an aspartate residue, in the metal-ion- dependent adhesion site (MIDAS). So far all attempts to replace the carboxyl group of aspartate with other, pharmacologically favorable isosteric groups have failed. Now it has been shown that a hydroxamic acid group can replace the carboxyl group; the resulting ligand retains its high binding activity. The picture shows one such ligand in the binding site of alphavbeta3.

Rational design of highly active and selective ligands for the alpha5beta1 integrin receptor.

The inhibition of integrin function is a major challenge in medicinal chemistry. Potent ligands are currently in different stages of clinical trials for the antiangiogenic therapy of cancer and age-related macula degeneration (AMD). The subtype alpha5beta1 has recently been drawn into the focus of research because of its genuine role in angiogenesis. In our previous work we could demonstrate that the lack of structural information about the receptor could be overcome by a homology model based on the X-ray structure of the alphavbeta3 integrin subtype and the sequence similarities between both receptors. In this work, we describe the rational design and synthesis of high affinity alpha5beta1 binders, and the optimisation of selectivity against alphavbeta3 by means of extensive SAR studies and docking experiments. A first series of compounds based on the tyrosine scaffold resulted in affinities in the low and even subnanomolar range and selectivities of 400-fold against alphavbeta3. The insights about the structure-activity relationship gained from tyrosine-based ligands could be successfully transferred to ligands that bear an aza-glycine scaffold to yield alpha5beta1 ligands with affinities of approximately 1 nm and selectivities that exceed 10(4)-fold. The ligands have already been successfully employed as selective alpha5beta1 ligands in biological research and might serve as lead structures for antiangiogenic cancer therapy.

The longevity gene INDY (I'm Not Dead Yet) in metabolic control: Potential as pharmacological target.

The regulation of metabolic processes by the Indy (I'm Not Dead Yet) (SLC13A5/NaCT) gene was revealed through studies in Drosophila melanogaster and Caenorhabditis elegans. Reducing the expression of Indy in these species extended their life span by a mechanism resembling caloric restriction, without reducing food intake. In D. melanogaster, mutating the Indy gene reduced body fat content, insulin-like proteins and reactive oxygen species production. Subsequent studies indicated that Indy encodes a citrate transporter located on the cell plasma membrane. The transporter is highly expressed in the mammalian liver. We generated a mammalian knock out model deleting the mammalian homolog mIndy (SLC13A5). The knock out animals were protected from HFD induced obesity, fatty liver and insulin resistance. Moreover, we have shown that inducible and liver selective knock down of mIndy protects against the development of fatty liver and insulin resistance and that obese humans with type 2 diabetes and non-alcoholic fatty liver disease have increased levels of mIndy. Therefore, the transporter mINDY (NaCT) has been proposed to be an 'ideal target for the treatment of metabolic disease'. A small molecule inhibitor of the mINDY transporter has been generated, normalizing glucose levels and reducing fatty liver in a model of diet induced obese mice. Taken together, studies from lower organisms, mammals and humans suggest that mINDY (NaCT) is an attractive target for the treatment of metabolic disease.

Multiple N-methylation by a designed approach enhances receptor selectivity.

An unselective cyclic peptide integrin ligand was sequentially N-methylated by a designed approach, where only the externally oriented (solvent exposed) amide bonds were N-methylated. The N-methylation resulted in tremendous enhancement in selectivity among the different integrin receptor subtypes (alpha5beta1, alphavbeta3, and alphaIIbbeta3). Conformational and docking studies were performed, which suggested that the receptor selectivity is principally caused by reduced backbone flexibility due to N-methylation.

Design and synthesis of a new class of selective integrin alpha5beta1 antagonists.

Starting from the structure of integrin alphavbeta3 in a complex with a peptidic ligand plus SAR data on nonpeptidic ligands, we derived a new class of integrin alpha5beta1 antagonists (1). Several synthesis strategies were applied to evaluate the chemical space around the essential pharmacophore groups R1 to R3 to obtain highly active and selective pyrrolidine derivatives as integrin alpha5beta1 antagonists. Integrin selectivity was controlled by switching from a sulfonamide moiety to a mesitylene amide moiety for R3. This finding represents a general feature for modulating selectivity toward other related integrin receptors. On the basis of the encouraging results from various in vitro studies, the most active compounds were selected for further in vivo studies in animal models and preclinical development.

Bevacizumab as a potent inhibitor of inflammatory corneal angiogenesis and lymphangiogenesis.

PURPOSE: To analyze whether bevacizumab can inhibit inflammatory angiogenesis and lymphangiogenesis in the cornea. Bevacizumab (Avastin; Roche, Welwyn Garden City, UK) is a recombinant, humanized, monoclonal antibody against VEGF-A that has been approved by the U.S. Food and Drug Administration for the treatment of colon carcinomas. METHODS: The mouse model of suture-induced corneal neovascularization was used to assess the antihemangiogenic and antilymphangiogenic effect of bevacizumab by systemic and topical application. Corneal flatmounts were stained with LYVE-1 as a specific lymphatic vascular endothelial marker and CD31 as a pan-endothelial marker, and blood and lymph vascularized areas were analyzed morphometrically. The inhibitory effect of bevacizumab on lymphatic endothelial cells (LECs) was analyzed with a colorimetric (BrdU) proliferation ELISA. The binding ability of bevacizumab to murine VEGF-A was analyzed by Western blot, ELISA, and surface plasmon resonance. RESULTS: The systemic and topical applications of bevacizumab significantly inhibited the outgrowth of blood (P < 0.006 and P < 0.0001, respectively) and lymphatic (P < 0.002 and P < 0.0001, respectively) vessels. Inhibition of the proliferation of LECs was also significant (P < 0.0001). Western blot analysis, ELISA, and the surface plasmon resonance assay showed that bevacizumab binds murine VEGF-A. CONCLUSIONS: Topical or systemic application of bevacizumab inhibits both inflammation-induced angiogenesis and lymphangiogenesis in the cornea. This finding suggests an important role of VEGF-A in corneal lymphangiogenesis. Bevacizumab may be useful in preventing immune rejections after penetrating keratoplasty or tumor metastasis via lymphatic vessels.