Tissue-specific mRNA Expression Profiles of Human Solute Carrier Transporter Superfamilies

Gene polymorphisms and thyroid hormone signaling: implication for the treatment of hypothyroidism

INTRODUCTION

Mutations and single nucleotide polymorphisms (SNPs) in the genes encoding the network of proteins involved in thyroid hormone signaling (TH) may have implications for the effectiveness of the treatment of hypothyroidism with LT4. It is conceivable that loss-of-function mutations or SNPs impair the ability of LT4 to be activated to T3, reach its targets, and ultimately resolve symptoms of hypothyroidism. Some of these patients do benefit from therapy containing LT4 and LT3.

METHODS

Here, we reviewed the PubMed and examined gene mutations and SNPs in the TH cellular transporters, deiodinases, and TH receptors, along with their impact on TH signaling, and potential clinical implications.

RESULTS

In some mechanisms, such as the Thr92Ala-DIO2 SNP, there is a compelling rationale for reduced T4 to T3 activation that limits the effectiveness of LT4 to restore euthyroidism. In other mechanisms, a potential case can be made but more studies with a larger number of individuals are needed.

DISCUSSION/CONCLUSION

Understanding the clinical impact of the genetic makeup of LT4-treated patients may help in the preemptive identification of those individuals that would benefit from therapy containing LT3.

CNS Drug Delivery in Stroke: Improving Therapeutic Translation From the Bench to the Bedside

Drug development for ischemic stroke is challenging as evidenced by the paucity of therapeutics that have advanced beyond a phase III trial. There are many reasons for this lack of clinical translation including factors related to the experimental design of preclinical studies. Often overlooked in therapeutic development for ischemic stroke is the requirement of effective drug delivery to the brain, which is critical for neuroprotective efficacy of several small and large molecule drugs. Advancing central nervous system drug delivery technologies implies a need for detailed comprehension of the blood-brain barrier (BBB) and neurovascular unit. Such knowledge will permit the innate biology of the BBB/neurovascular unit to be leveraged for improved bench-to-bedside translation of novel stroke therapeutics. In this review, we will highlight key aspects of BBB/neurovascular unit pathophysiology and describe state-of-the-art approaches for optimization of central nervous system drug delivery (ie, passive diffusion, mechanical opening of the BBB, liposomes/nanoparticles, transcytosis, intranasal drug administration). Additionally, we will discuss how endogenous BBB transporters represent the next frontier of drug delivery strategies for stroke. Overall, this review will provide cutting edge perspective on how central nervous system drug delivery must be considered for the advancement of new stroke drugs toward human trials.

Phosphate (Pi) Transporter PIT1 Induces Pi Starvation in Salmonella-Containing Vacuole in HeLa Cells

Salmonella enterica serovar Typhimurium (S. Typhimurium), an important foodborne pathogen, causes diarrheal illness and gastrointestinal diseases. S. Typhimurium survives and replicates in phagocytic and non-phagocytic cells for acute or chronic infections. In these cells, S. Typhimurium resides within Salmonella-containing vacuoles (SCVs), in which the phosphate (Pi) concentration is low. S. Typhimurium senses low Pi and expresses virulence factors to modify host cells. However, the mechanism by which host cells reduce the Pi concentration in SCVs is not clear. In this study, we show that through the TLR4-MyD88-NF-κB signaling pathway, S. Typhimurium upregulates PIT1, which in turn transports Pi from SCVs into the cytosol and results in Pi starvation in SCVs. Immunofluorescence and western blotting analysis reveal that after the internalization of S. Typhimurium, PIT1 is located on SCV membranes. Silencing or overexpressing PIT1 inhibits or promotes Pi starvation, Salmonella pathogenicity island-2 (SPI-2) gene expression, and replication in SCVs. The S. Typhimurium ΔmsbB mutant or silenced TLR4-MyD88-NF-κB pathway suppresses the expression of the SPI-2 genes and promotes the fusion of SCVs with lysosomes. Our results illustrate that S. Typhimurium exploits the host innate immune responses as signals to promote intracellular replication, and they provide new insights for the development of broad-spectrum therapeutics to combat bacterial infections.

Membrane transporters in cell physiology, cancer metabolism and drug response

By controlling the passage of small molecules across lipid bilayers, membrane transporters influence not only the uptake and efflux of nutrients, but also the metabolic state of the cell. With more than 450 members, the Solute Carriers (SLCs) are the largest transporter super-family, clustering into families with different substrate specificities and regulatory properties. Cells of different types are, therefore, able to tailor their transporter expression signatures depending on their metabolic requirements, and the physiological importance of these proteins is illustrated by their mis-regulation in a number of disease states. In cancer, transporter expression is heterogeneous, and the SLC family has been shown to facilitate the accumulation of biomass, influence redox homeostasis, and also mediate metabolic crosstalk with other cell types within the tumour microenvironment. This Review explores the roles of membrane transporters in physiological and malignant settings, and how these roles can affect drug response, through either indirect modulation of sensitivity or the direct transport of small-molecule therapeutic compounds into cells.

New insights in the genetic variant spectrum of SLC34A2 in pulmonary alveolar microlithiasis; a systematic review

Pulmonary alveolar microlithiasis (PAM) is a rare autosomal recessive lung disease caused by variants in the SLC34A2 gene encoding the sodium-dependent phosphate transport protein 2B, NaPi-2b. PAM is characterized by deposition of calcium phosphate crystals in the alveoli. Onset and clinical course vary considerably; some patients remain asymptomatic while others develop severe respiratory failure with a significant symptom burden and compromised survival. It is likely that PAM is under-reported due to lack of recognition, misdiagnosis, and mild clinical presentation. Most patients are genetically uncharacterized as the diagnostic confirmation of PAM has traditionally not included a genetic analysis. Genetic testing may in the future be the preferred tool for diagnostics instead of invasive methods. This systematic review aims to provide an overview of the growing knowledge of PAM genetics. Rare variants in SLC34A2 are found in almost all genetically tested patients. So far, 34 allelic variants have been identified in at least 68 patients. A majority of these are present in the homozygous state; however, a few are found in the compound heterozygous form. Most of the allelic variants involve only a single nucleotide. Half of the variants are either nonsense or frameshifts, resulting in premature termination of the protein or decay of the mRNA. There is currently no cure for PAM, and the only effective treatment is lung transplantation. Management is mainly symptomatic, but an improved understanding of the underlying pathophysiology will hopefully result in development of targeted treatment options. More standardized data on PAM patients, including a genetic diagnosis covering larger international populations, would support the design and implementation of clinical studies to the benefit of patients. Further genetic characterization and understanding of how the molecular changes influence disease phenotype will hopefully allow earlier diagnosis and treatment of the disease in the future.

Targeting NaPi2b in ovarian cancer

Novel biomarkers are needed to direct new treatments for ovarian cancer, a disease for which the standard of care remains heavily focused on platinum-based chemotherapy. Despite the success of PARP inhibitors, treatment options are limited, particularly in the platinum-resistant setting. NaPi2b is a cell surface sodium-dependent phosphate transporter that regulates phosphate homeostasis under normal physiological conditions and is a lineage marker that is expressed in select cancers, including ovarian, lung, thyroid, and breast cancers, with limited expression in normal tissues. Based on its increased expression in ovarian tumors, NaPi2b is a promising candidate to be studied as a biomarker for treatment and patient selection in ovarian cancer. In preclinical studies, the use of antibodies against NaPi2b showed that this protein can be exploited for tumor mapping and therapeutic targeting. Emerging data from phase 1 and 2 clinical trials in ovarian cancer have suggested that NaPi2b can be successfully detected in patient biopsy samples using immunohistochemistry, and the NaPi2b-targeting antibody-drug conjugate under evaluation appeared to elicit therapeutic responses. The aim of this review is to examine literature supporting NaPi2b as a novel biomarker for potential treatment and patient selection in ovarian cancer and to discuss the critical next steps and future analyses necessary to drive the study of this biomarker and therapeutic targeting forward.

Resolving the gene expression maps of human first-trimester chorionic villi with spatial transcriptome

The placenta is important for fetal development in mammals, and spatial transcriptomic profiling of placenta helps to resolve its structure and function. In this study, we described the landscape of spatial transcriptome of human placental villi obtained from two pregnant women at the first trimester using the modified Stereo-seq method applied for paraformaldehyde (PFA) fixation samples. The PFA fixation of human placenta villi was better than fresh villi embedded in optimum cutting temperature (OCT) compound, since it greatly improved tissue morphology and the specificity of RNA signals. The main cell types in chorionic villi such as syncytiotrophoblasts (SCT), villous cytotrophoblasts (VCT), fibroblasts (FB), and extravillous trophoblasts (EVT) were identified with the spatial transcriptome data, whereas the minor cell types of Hofbauer cells (HB) and endothelial cells (Endo) were spatially located by deconvolution of scRNA-seq data. We demonstrated that the Stereo-seq data of human villi could be used for sophisticated analyses such as spatial cell-communication and regulatory activity. We found that the SCT and VCT exhibited the most ligand-receptor pairs that could increase differentiation of the SCT, and that the spatial localization of specific regulons in different cell types was associated with the pathways related to hormones transport and secretion, regulation of mitotic cell cycle, and nutrient transport pathway in SCT. In EVT, regulatory pathways such as the epithelial to mesenchyme transition, epithelial development and differentiation, and extracellular matrix organization were identified. Finally, viral receptors and drug transporters were identified in villi according to the pathway analysis, which could help to explain the vertical transmission of several infectious diseases and drug metabolism efficacy. Our study provides a valuable resource for further investigation of the placenta development, physiology and pathology in a spatial context.

Screening for drugs potentially interfering with MCT8-mediated T3 transport in vitro identifies dexamethasone and some commonly used drugs as inhibitors of MCT8 activity

BACKGROUND

Monocarboxylate transporter 8 (MCT8) is the first thyroid hormone transporter that has been linked to a human disease. Besides genetic alterations other factors might impair MCT8 activity.

AIM

This study aimed at investigating whether some common drugs having a structural similarity with TH and/or whose treatment is associated with thyroid function test abnormalities, or which behave as antagonists of TH action can inhibit MCT8-mediated T₃ transport.

METHODS

[¹²⁵I]T₃ uptake and efflux were measured in COS-7 cells transiently transfected with hMCT8 before and after exposure to increasing concentrations of hydrocortisone, dexamethasone, prednisone, prednisolone, amiodarone, desethylamiodarone, dronedarone, buspirone, carbamazepine, valproic acid, and L-carnitine. The mode of inhibition was also determined.

RESULTS

Dexamethasone significantly inhibited T₃ uptake at 10 μM; hydrocortisone reduced T₃ uptake only at high concentrations, i.e. at 500 and 1000 μM; prednisone and prednisolone were devoid of inhibitory potential. Amiodarone caused a reduction of T₃ uptake by MCT8 only at the highest concentrations used (44% at 50 μM and 68% at 100 μM), and this effect was weaker than that produced by desethylamiodarone and dronedarone; buspirone resulted a potent inhibitor, reducing T₃ uptake at 0.1-10 μM. L-Carnitine inhibited T₃ uptake only at 500 mM and 1 M. Kinetic experiments revealed a noncompetitive mode of inhibition for all compounds. All drugs inhibiting T₃ uptake did not affect T₃ release.

CONCLUSION

This study shows a novel effect of some common drugs, which is inhibition of T₃ transport mediated by MCT8. Specifically, dexamethasone, buspirone, desethylamiodarone, and dronedarone behave as potent inhibitors of MCT8.

Maternal exposure to bis(2-ethylhexyl) phthalate during the thyroid hormone-dependent stage induces persistent emotional and cognitive impairment in middle-aged offspring mice

Prenatal DEHP exposure can cause offspring neurodevelopmental toxicity, but the persistent effects of such exposure window are unclear. This study aimed to investigate the lasting neurobehavioral impact of DEHP on offspring following early exposure from GD9.5 (fetal neural tube closure) to GD16.5 (fetal thyroxin, TH, synthesis). Data showed maternal exposure to DEHP during the thyroid hormone-dependent stage induced a range of neurobehavioral phenotypic changes in adult and middle-aged mice, including anxiety, depression and cognitive impairment. Significant reductions in free TH, TH transporters, and TH metabolic enzyme deiodinase II (D2) were observed in the fetal brain, whereas D3 was elevated, indicating that TH signaling disruption was caused by in utero exposure. Gene expression analyses suggested the expression levels of the TH receptors Trα1, Trβ1 and their downstream target, brain-derived neurotrophic factor, were significantly attenuated, which may partially explain the mechanisms of neurodevelopmental impairment. This study provides new evidence of the persistent effects of sex-specific neurodevelopmental impairment due to in utero DEHP exposure, possibly through damage to the fetal brain TH signaling systems that causes lifelong brain damage. These results further suggest a profound neurobehavioral toxicity of DEHP that may be programmed during early developmental stage exposure and manifested later in life.

Mammalian monocarboxylate transporter 7 (MCT7/Slc16a6) is a novel facilitative taurine transporter

Monocarboxylate transporter 7 (MCT7) is an orphan transporter expressed in the liver, brain, and in several types of cancer cells. It has also been reported to be a survival factor in melanoma and breast cancers. However, this survival mechanism is not yet fully understood due to MCT7’s unidentified substrate(s). Therefore, here we sought to identify MCT7 substrate(s) and characterize the transport mechanisms by analyzing amino acid transport in HEK293T cells and polarized Caco-2 cells. Analysis of amino acids revealed significant rapid reduction in taurine from cells transfected with enhanced green fluorescent protein-tagged MCT7. We found that taurine uptake and efflux by MCT7 was pH-independent and that the uptake was not saturated in the presence of taurine excess of 200 mM. Furthermore, we found that monocarboxylates and acidic amino acids inhibited MCT7-mediated taurine uptake. These results imply that MCT7 may be a low-affinity facilitative taurine transporter. We also found that MCT7 was localized at the basolateral membrane in polarized Caco-2 cells and that the induction of MCT7 expression in polarized Caco-2 cells enhanced taurine permeation. Finally, we demonstrated that interactions of MCT7 with ancillary proteins basigin/CD147 and embigin/GP70 enhanced MCT7-mediated taurine transport. In summary, these findings reveal that taurine is a novel substrate of MCT7 and that MCT7-mediated taurine transport might contribute to the efflux of taurine from cells.

Oxalate Flux Across the Intestine: Contributions from Membrane Transporters

Epithelial oxalate transport is fundamental to the role occupied by the gastrointestinal (GI) tract in oxalate homeostasis. The absorption of dietary oxalate, together with its secretion into the intestine, and degradation by the gut microbiota, can all influence the excretion of this nonfunctional terminal metabolite in the urine. Knowledge of the transport mechanisms is relevant to understanding the pathophysiology of hyperoxaluria, a risk factor in kidney stone formation, for which the intestine also offers a potential means of treatment. The following discussion presents an expansive review of intestinal oxalate transport. We begin with an overview of the fate of oxalate, focusing on the sources, rates, and locations of absorption and secretion along the GI tract. We then consider the mechanisms and pathways of transport across the epithelial barrier, discussing the transcellular, and paracellular components. There is an emphasis on the membrane-bound anion transporters, in particular, those belonging to the large multifunctional Slc26 gene family, many of which are expressed throughout the GI tract, and we summarize what is currently known about their participation in oxalate transport. In the final section, we examine the physiological stimuli proposed to be involved in regulating some of these pathways, encompassing intestinal adaptations in response to chronic kidney disease, metabolic acid-base disorders, obesity, and following gastric bypass surgery. There is also an update on research into the probiotic, Oxalobacter formigenes, and the basis of its unique interaction with the gut epithelium. © 2021 American Physiological Society. Compr Physiol 11:1-41, 2021.

Drug delivery strategies for management of women's health issues in the upper genital tract

The female upper genital tract (UGT) hosts important reproductive organs including the cervix, uterus, fallopian tubes, and ovaries. Several pathologies affect these organ systems such as infections, reproductive issues, structural abnormalities, cancer, and inflammatory diseases that could have significant impact on women's overall health. Effective disease management is constrained by the multifaceted nature of the UGT, complex anatomy and a dynamic physiological environment. Development of drug delivery strategies that can overcome mucosal and safety barriers are needed for effective disease management. This review introduces the anatomy, physiology, and mucosal properties of the UGT and describes drug delivery barriers, advances in drug delivery technologies, and opportunities available for new technologies that target the UGT.

Topology, tissue distribution, and transcriptional level of SLC34s in response to Pi and pH in grass carp Ctenopharyngodon idella

In the present study, two new SLC34 family members, named slc34a1b and slc34a2a, were isolated and characterized from grass carp Ctenopharyngodon idella. Topology, tissue distribution, and transcriptional response to phosphorus (Pi) and pH were compared among three members of SLC34 family (slc34a1b, slc34a2a, and slc34a2b) in grass carp. The length of validated cDNAs of grass carp slc34a1b and slc34a2a was 1494 bp and 1902 bp, and these two cDNAs encoded 497 and 633 amino acid residues, respectively. The domain analysis showed that three SLC34 members of grass carp contain architecture similar to that in mammals. Moreover, the mRNA of three slc34s was widely expressed in nine tissues (heart, brain, intestine, kidney, liver, muscle, gill, spleen, and skin), but at various levels. Our results revealed that 6 mM and 9 mM Pi incubation significantly reduced the mRNA expression of three slc34s in both CIK and L8824 cell lines from grass carp. The expression of slc34a1b was decreased in the CIK cells, but not in the L8824 cells after 3 mM Pi incubation. In CIK cells, 3 mM Pi incubation downregulated the expression of slc34a1b and slc34a2a, but not slc34a2b. In addition, the expression of three slc34s was significantly reduced at acidic pH in the CIK cells. Taken together, we characterized three SLC34 family members, revealed their specific distribution among different tissues, and elucidated their transcriptional responses to Pi and pH in two cell lines from grass carp. Our findings provide an insight into the physiological function of three SLC34s in fish.

Clonal osteoblastic cell lines with CRISPR/Cas9-mediated ablation of Pit1 or Pit2 show enhanced mineralization despite reduced osteogenic gene expression

Multiple actions of extracellular Pi on the skeletal cells are likely to be partly mediated by type III sodium/phosphate (Na⁺/Pi) cotransporters Pit1 and Pit2, although the details are not fully understood. In the current study, to determine the roles of Pit1 and Pit2 in osteoblasts, we generated Pit1-knockout (KO) and Pit2-KO osteoblastic cells by applying CRISPR/Cas9 genome editing to an osteoblastic cell line MC3T3-E1 subclone 4. The extracellular Pi level was increased in the Pit1-KO and Pit2-KO clones due to the reduced Pi uptake. Interestingly, in vitro mineralization was accelerated in the Pit1-KO and Pit2-KO clones, although the induction of the expression of osteogenic marker genes was suppressed. In the cells before mineralization, extracellular levels of pyrophosphate (PPi) and adenosine triphosphate (ATP) were increased in the Pit1-KO and Pit2-KO clones, which might be attributable to the reduced expression and activity of tissue-nonspecific alkaline phosphatase (TNSALP). A 24-h treatment with high Pi reduced the expression and activity of TNSALP, suggesting that the suppression of TNSALP in the Pit1-KO and Pit2-KO clones was caused by the increased availability of extracellular Pi. Lentiviral gene transfer of Pit1 and Pit2 restored the changes observed in Pit1-KO and Pit2-KO clones, respectively. The expressions of P2Y2 and P2X7 which encode receptors for extracellular ATP were altered in the Pit1-KO and Pit2-KO clones, suggesting an influence on purinergic signaling. In mineralized cells after long-term culture, intracellular levels of PPi and ATP were higher in the Pit1-KO and Pit2-KO clones. Taken together, ablation of Pit1 or Pit2 in this osteoblastic cell model led to accelerated mineralization, suppressed TNSALP and altered the levels of extracellular and intracellular PPi and ATP, which might be partly mediated by changes in the availability of extracellular Pi.

Targeting OCT3 attenuates doxorubicin-induced cardiac injury

Doxorubicin is a commonly used anticancer agent that can cause debilitating and irreversible cardiac injury. The initiating mechanisms contributing to this side effect remain unknown, and current preventative strategies offer only modest protection. Using stem-cell-derived cardiomyocytes from patients receiving doxorubicin, we probed the transcriptomic landscape of solute carriers and identified organic cation transporter 3 (OCT3) (SLC22A3) as a critical transporter regulating the cardiac accumulation of doxorubicin. Functional validation studies in heterologous overexpression models confirmed that doxorubicin is transported into cardiomyocytes by OCT3 and that deficiency of OCT3 protected mice from acute and chronic doxorubicin-related changes in cardiovascular function and genetic pathways associated with cardiac damage. To provide proof-of-principle and demonstrate translational relevance of this transport mechanism, we identified several pharmacological inhibitors of OCT3, including nilotinib, and found that pharmacological targeting of OCT3 can also preserve cardiovascular function following treatment with doxorubicin without affecting its plasma levels or antitumor effects in multiple models of leukemia and breast cancer. Finally, we identified a previously unrecognized, OCT3-dependent pathway of doxorubicin-induced cardiotoxicity that results in a downstream signaling cascade involving the calcium-binding proteins S100A8 and S100A9. These collective findings not only shed light on the etiology of doxorubicin-induced cardiotoxicity, but also are of potential translational relevance and provide a rationale for the implementation of a targeted intervention strategy to prevent this debilitating side effect.

The Complexities of Organ Crosstalk in Phosphate Homeostasis: Time to Put Phosphate Sensing Back in the Limelight

Phosphate homeostasis is essential for health and is achieved via interaction between the bone, kidney, small intestine, and parathyroid glands and via intricate processes involving phosphate transporters, phosphate sensors, and circulating hormones. Numerous genetic and acquired disorders are associated with disruption in these processes and can lead to significant morbidity and mortality. The role of the kidney in phosphate homeostasis is well known, although it is recognized that the cellular mechanisms in murine models and humans are different. Intestinal phosphate transport also appears to differ in humans and rodents, with recent studies demonstrating a dominant role for the paracellular pathway. The existence of phosphate sensing has been acknowledged for decades; however, the underlying molecular mechanisms are poorly understood. At least three phosphate sensors have emerged. PiT2 and FGFR1c both act as phosphate sensors controlling Fibroblast Growth Factor 23 secretion in bone, whereas the calcium-sensing receptor controls parathyroid hormone secretion in response to extracellular phosphate. All three of the proposed sensors are expressed in the kidney and intestine but their exact function in these organs is unknown. Understanding organ interactions and the mechanisms involved in phosphate sensing requires significant research to develop novel approaches for the treatment of phosphate homeostasis disorders.

Next generation risk assessment (NGRA): Bridging in vitro points-of-departure to human safety assessment using physiologically-based kinetic (PBK) modelling - A case study of doxorubicin with dose metrics considerations

Using the chemical doxorubicin (DOX), the objective of the present study was to evaluate the impact of dose metrics selection in the new approach method of integrating physiologically-based kinetic (PBK) modelling and relevant human cell-based assays to inform a priori the point of departure for human health risk. We reviewed the literature on the clinical consequences of DOX treatment to identify dosing scenarios with no or mild cardiotoxicity observed. Key concentrations of DOX that induced cardiomyocyte toxicity in vitro were derived from studies of our own and others. A human population-based PBK model of DOX was developed and verified against pharmacokinetic data. The model was then used to predict plasma and extracellular and intracellular heart concentrations of DOX under selected clinical settings and compared with in vitro outcomes, based on several dose metrics: C_max (maximum concentration) or AUC (area under concentration-time curve) in free or total form of DOX. We found when using in vitro assays to predict cardiotoxicity for DOX, AUC is a better indicator. Our study illustrates that when appropriate dose metrics are used, it is possible to combine PBK modelling with in vitro-derived toxicity information to define margins of safety and predict low-risk human exposure levels.

Constitutive depletion of Slc34a2/NaPi-IIb in rats causes perinatal mortality

Absorption of dietary phosphate (Pi) across intestinal epithelia is a regulated process mediated by transcellular and paracellular pathways. Although hyperphosphatemia is a risk factor for the development of cardiovascular disease, the amount of ingested Pi in a typical Western diet is above physiological needs. While blocking intestinal absorption has been suggested as a therapeutic approach to prevent hyperphosphatemia, a complete picture regarding the identity and regulation of the mechanism(s) responsible for intestinal absorption of Pi is missing. The Na+/Pi cotransporter NaPi-IIb is a secondary active transporter encoded by the Slc34a2 gene. This transporter has a wide tissue distribution and within the intestinal tract is located at the apical membrane of epithelial cells. Based on mouse models deficient in NaPi-IIb, this cotransporter is assumed to mediate the bulk of active intestinal absorption of Pi. However, whether or not this is also applicable to humans is unknown, since human patients with inactivating mutations in SLC34A2 have not been reported to suffer from Pi depletion. Thus, mice may not be the most appropriate experimental model for the translation of intestinal Pi handling to humans. Here, we describe the generation of a rat model with Crispr/Cas-driven constitutive depletion of Slc34a2. Slc34a2 heterozygous rats were indistinguishable from wild type animals under standard dietary conditions as well as upon 3 days feeding on low Pi. However, unlike in humans, homozygosity resulted in perinatal lethality.

High phosphate impairs arterial endothelial function through AMPK-related pathways in mouse resistance arteries

AIMS

In patients with renal disease, high serum phosphate shows a relationship with cardiovascular risk. We speculate that high phosphate (HP) impairs arterial vasodilation via the endothelium and explore potential underlying mechanisms.

METHODS

Isolated vessel relaxation, endothelial function, glomerular filtration rate (GFR), oxidative stress status and protein expression were assessed in HP diet mice. Mitochondrial function and protein expression were assessed in HP-treated human umbilical vein endothelial cells (HUVECs).

RESULTS

High phosphate (1.3%) diet for 12 weeks impaired endothelium-dependent relaxation in mesenteric arteries, kidney interlobar arteries and afferent arterioles; reduced GFR and the blood pressure responses to acute administration of acetylcholine. The PPARα/LKB1/AMPK/eNOS pathway was attenuated in the endothelium of mesenteric arteries from HP diet mice. The observed vasodilatory impairment of mesenteric arteries was ameliorated by PPARα agonist WY-14643. The phosphate transporter PiT-1 knockdown prevented HP-mediated suppression of eNOS activity by impeding phosphorus influx in HUVECs. Endothelium cytoplasmic and mitochondrial reactive oxygen species (ROS) were increased in HP diet mice. Moreover HP decreased the expression of mitochondrial-related antioxidant genes. Finally, mitochondrial membrane potential and PGC-1α expression were reduced by HP treatment in HUVECs, which was partly restored by AMPKα agonist.

CONCLUSIONS

HP impairs endothelial function by reducing NO bioavailability via decreasing eNOS activity and increasing mitochondrial ROS, in which the AMPK-related signalling pathways may play a key role.

The impact of type III sodium-dependent phosphate transporters (Pit 1 and Pit 2) on podocyte and kidney function

The sodium-dependent phosphate transporters Pit 1 and Pit 2 belong to the solute carrier 20 (SLC20) family of membrane proteins. They are ubiquitously distributed in the human body. Their crucial function is the intracellular transport of inorganic phosphate (Pi) in the form of H₂ PO₄ ^- . They are one of the main elements in maintaining physiological phosphate homeostasis. Recent data have emerged that indicate novel roles of Pit 1 and Pit 2 proteins besides the well-known function of Pi transporters. These membrane proteins are believed to be precise phosphate sensors that mediate Pi-dependent intracellular signaling. They are also involved in insulin signaling and influence cellular insulin sensitivity. In diseases that are associated with hyperphosphatemia, such as diabetes and chronic kidney disease (CKD), disturbances in the function of Pit 1 and Pit 2 are observed. Phosphate transporters from the SLC20 family participate in the calcification of soft tissues, mainly blood vessels, during the course of CKD. The glomerulus and podocytes therein can also be a target of pathological calcification that damages these structures. A few studies have demonstrated the development of Pi-dependent podocyte injury that is mediated by Pit 1 and Pit 2. This paper discusses the role of Pit 1 and Pit 2 proteins in podocyte function, mainly in the context of the development of pathological calcification that disrupts permeability of the renal filtration barrier. We also describe the mechanisms that may contribute to podocyte damage by Pit 1 and Pit 2.

Novel mineral regulatory pathways in ovine pregnancy: I. phosphate, klotho signaling, and sodium-dependent phosphate transporters

Appropriate mineralization of the fetal skeleton requires an excess of phosphate in the fetus compared to the mother. However, mechanisms for placental phosphate transport are poorly understood. This study aimed to identify phosphate regulatory pathways in ovine endometria and placentae throughout gestation. Suffolk ewes were bred with fertile rams upon visual detection of estrus (Day 0). On Days 9, 12, 17, 30, 70, 90, 110, and 125 of pregnancy (n = 3-14/Day), ewes were euthanized and hysterectomized. Phosphate abundance varied across gestational days in uterine flushings, allantoic fluid, and homogenized endometria and placentae (P < 0.05). The expression of mRNAs for sodium-dependent phosphate transporters (SLC20A1 and SLC20A2) and klotho signaling mediators (FGF7, FGF21, FGF23, FGFR1-4, KL, KLB, ADAM10, and ADAM17) were quantified by qPCR. Day 17 conceptus tissue expressed SLC20A1, SLC20A2, KLB, FGF7, FGF21, FGF23, FGFR1, and FGFR2 mRNAs. Both sodium-dependent phosphate transporters and klotho signaling mediators were expressed in endometria and placentae throughout gestation. Gestational day influenced the expression of SLC20A1, ADAM10, ADAM17, FGF21, FGFR1, and FGFR3 mRNAs in both endometria and placentae (P < 0.05). Gestational day influenced endometrial expression of FGF7 (P < 0.001), and placental expression of FGF23 (P < 0.05). Immunohistochemistry confirmed that both FGF23 and KL proteins were expressed in endometria and placentae throughout gestation. The observed spatiotemporal profile of KL-FGF signaling suggests a potential role in the establishment of pregnancy and regulation of fetal growth. This study provides a platform for further mechanistic investigation into the role for KL-FGF signaling in the regulation of phosphate transport at the ovine maternal-conceptus interface.

A review of species differences in the control of, and response to, chemical-induced thyroid hormone perturbations leading to thyroid cancer

This review summarises the current state of knowledge regarding the physiology and control of production of thyroid hormones, the effects of chemicals in perturbing their synthesis and release that result in thyroid cancer. It does not consider the potential neurodevelopmental consequences of low thyroid hormones. There are a number of known molecular initiating events (MIEs) that affect thyroid hormone synthesis in mammals and many chemicals are able to activate multiple MIEs simultaneously. AOP analysis of chemical-induced thyroid cancer in rodents has defined the key events that predispose to the development of rodent cancer and many of these will operate in humans under appropriate conditions, if they were exposed to high enough concentrations of the affecting chemicals. There are conditions however that, at the very least, would indicate significant quantitative differences in the sensitivity of humans to these effects, with rodents being considerably more sensitive to thyroid effects by virtue of differences in the biology, transport and control of thyroid hormones in these species as opposed to humans where turnover is appreciably lower and where serum transport of T4/T3 is different to that operating in rodents. There is heated debate around claimed qualitative differences between the rodent and human thyroid physiology, and significant reservations, both scientific and regulatory, still exist in terms of the potential neurodevelopmental consequences of low thyroid hormone levels at critical windows of time. In contrast, the situation for the chemical induction of thyroid cancer, through effects on thyroid hormone production and release, is less ambiguous with both theoretical, and actual data, showing clear dose-related thresholds for the key events predisposing to chemically induced thyroid cancer in rodents. In addition, qualitative differences in transport, and quantitative differences in half life, catabolism and turnover of thyroid hormones, exist that would not operate under normal situations in humans.

Monocarboxylate Transporter 8 Deficiency: From Pathophysiological Understanding to Therapy Development

Genetic defects in the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) result in MCT8 deficiency. This disorder is characterized by a combination of severe intellectual and motor disability, caused by decreased cerebral thyroid hormone signalling, and a chronic thyrotoxic state in peripheral tissues, caused by exposure to elevated serum T3 concentrations. In particular, MCT8 plays a crucial role in the transport of thyroid hormone across the blood-brain-barrier. The life expectancy of patients with MCT8 deficiency is strongly impaired. Absence of head control and being underweight at a young age, which are considered proxies of the severity of the neurocognitive and peripheral phenotype, respectively, are associated with higher mortality rate. The thyroid hormone analogue triiodothyroacetic acid is able to effectively and safely ameliorate the peripheral thyrotoxicosis; its effect on the neurocognitive phenotype is currently under investigation. Other possible therapies are at a pre-clinical stage. This review provides an overview of the current understanding of the physiological role of MCT8 and the pathophysiology, key clinical characteristics and developing treatment options for MCT8 deficiency.

L-T4 Therapy in the Presence of Pharmacological Interferents

Pharmacological interference on L-thyroxine (L-T4) therapy can be exerted at several levels, namely from the hypothalamus/pituitary through the intestine, where the absorption of exogenous L-T4 takes place. A number of medications interfere with L-T4 therapy, some of them also being the cause of hypothyroidism. The clinician should be aware that some medications simply affect thyroid function tests with no need of modifying the dose of L-T4 that the patient was taking prior to their prescription. Usually, the topic of pharmacological interference on L-T4 therapy addresses the patient with primary hypothyroidism, in whom periodic measurement of serum thyrotropin (TSH) is the biochemical target. However, this minireview also addresses the patient with central hypothyroidism, in whom the biochemical target is serum free thyroxine (FT4). This minireview also addresses two additional topics. One is the costs associated with frequent monitoring of the biochemical target when L-T4 is taken simultaneously with the interfering drug. The second topic is the issue of metabolic/cardiovascular complications associated with undertreated hypothyroidism.

Pulmonary alveolar microlithiasis

Pulmonary alveolar microlithiasis (PAM) is a fascinating rare lung disease that is associated with the accumulation of hydroxyapatite microliths within the lumen of the alveolar spaces. In most patients, PAM is discovered incidentally on radiographs performed for other purposes, and the typical disease course is characterised by slowly progressive respiratory insufficiency over decades. Recent genetic analyses that have revealed that the deficiency of the sodium-phosphate cotransporter NPT2B is the cause of PAM have enabled the development of powerful animal models that inform our approach to disease management and treatment. Here we review the epidemiology and molecular pathophysiology of PAM, as well as the diagnostic approach, clinical manifestations, radiographic and pathologic features, and clinical management of the disease. Although there are no proven treatments for PAM, progress in our understanding of disease pathogenesis is providing insights that suggest strategies for trials.

A review of the epidemiology and molecular pathophysiology of pulmonary alveolar microlithiasishttps://bit.ly/3lBgM7p

Isolation and Characterization of Three Sodium-Phosphate Cotransporter Genes and Their Transcriptional Regulation in the Grass Carp Ctenopharyngodon idella

It is important to explore the regulatory mechanism of phosphorus homeostasis in fish, which help avoid the risk of P toxicity and prevent P pollution in aquatic environment. The present study obtained the full-length cDNA sequences and the promoters of three SLC20 members (slc20a1a, slc20a1b and slc20a2) from grass carp Ctenopharyngodon idella, and explored their responses to inorganic phosphorus (Pi). Grass carp SLC20s proteins possessed conservative domains and amino acid sites relevant with phosphorus transport. The mRNAs of three slc20s appeared in the nine tissues, but their expression levels were tissue-dependent. The binding sites of three transcription factors (SREBP1, NRF2 and VDR) were predicted on the slc20s promoters. The mutation and EMSA analysis indicated that: (1) SREBP1 binding site (−783/−771 bp) negatively but VDR (−260/−253 bp) binding site positively regulated the activities of slc20a1a promoter; (2) SREBP1 (−1187/−1178 bp), NRF2 (−572/−561 bp) and VDR(615/−609 bp) binding sites positively regulated the activities of slc20a1b promoter; (3) SREBP1 (−987/−977 bp), NRF2 (−1469/−1459 bp) and VDR (−1124/−1117 bp) binding sites positively regulated the activities of the slc20a2 promoter. Moreover, Pi incubation significantly reduced the activities of three slc20s promoters, and Pi-induced transcriptional inactivation of slc20s promoters abolished after the mutation of the VDR element but not SREBP1 and NRF2 elements. Pi incubation down-regulated the mRNA levels of three slc20s. For the first time, our study elucidated the transcriptional regulatory mechanisms of SLC20s and their responses to Pi, which offered new insights into the Pi homeostatic regulation and provided the basis for reducing phosphorus discharge into the waters.

Intestinal epithelial ablation of Pit-2/Slc20a2 in mice leads to sustained elevation of vitamin D3 upon dietary restriction of phosphate

AIM

Several Na⁺ -dependent phosphate cotransporters, namely NaPi-IIb/SLC34A2, Pit-1/SLC20A1 and Pit-2/SLC20A2, are expressed at the apical membrane of enterocytes but their contribution to active absorption of phosphate is unclear. The aim of this study was to compare their pattern of mRNA expression along the small and large intestine and to analyse the effect of intestinal depletion of Pit-2 on phosphate homeostasis.

METHODS

Intestinal epithelial Pit-2-deficient mice were generated by crossing floxed Pit-2 with villin-Cre mice. Mice were fed 2 weeks standard or low phosphate diets. Stool, urine, plasma and intestinal and renal tissue were collected. Concentration of electrolytes and hormones, expression of mRNAs and proteins and intestinal transport of tracers were analysed.

RESULTS

Intestinal mRNA expression of NaPi-IIb and Pit-1 is segment-specific, whereas the abundance of Pit-2 mRNA is more homogeneous. In ileum, NaPi-IIb mRNA expression is restricted to enterocytes, whereas Pit-2 mRNA is found in epithelial and non-epithelial cells. Overall, their mRNA expression is not regulated by dietary phosphate. The absence of Pit-2 from intestinal epithelial cells does not affect systemic phosphate homeostasis under normal dietary conditions. However, in response to dietary phosphate restriction, Pit-2-deficient mice showed exacerbated hypercalciuria and sustained elevation of 1,25(OH)₂ vitamin D₃ .

CONCLUSIONS

In mice, the intestinal Na⁺ /phosphate cotransporters are not coexpressed in all segments. NaPi-IIb but not Pit-2 mRNA is restricted to epithelial cells. Intestinal epithelial Pit-2 does not contribute significantly to absorption of phosphate under normal dietary conditions. However, it may play a more significant role upon dietary phosphate restriction.

Monocarboxylate Transporters (SLC16): Function, Regulation, and Role in Health and Disease

The solute carrier family 16 (SLC16) is comprised of 14 members of the monocarboxylate transporter (MCT) family that play an essential role in the transport of important cell nutrients and for cellular metabolism and pH regulation. MCTs 1-4 have been extensively studied and are involved in the proton-dependent transport of L-lactate, pyruvate, short-chain fatty acids, and monocarboxylate drugs in a wide variety of tissues. MCTs 1 and 4 are overexpressed in a number of cancers, and current investigations have focused on transporter inhibition as a novel therapeutic strategy in cancers. MCT1 has also been used in strategies aimed at enhancing drug absorption due to its high expression in the intestine. Other MCT isoforms are less well characterized, but ongoing studies indicate that MCT6 transports xenobiotics such as bumetanide, nateglinide, and probenecid, whereas MCT7 has been characterized as a transporter of ketone bodies. MCT8 and MCT10 transport thyroid hormones, and recently, MCT9 has been characterized as a carnitine efflux transporter and MCT12 as a creatine transporter. Expressed at the blood brain barrier, MCT8 mutations have been associated with an X-linked intellectual disability, known as Allan-Herndon-Dudley syndrome. Many MCT isoforms are associated with hormone, lipid, and glucose homeostasis, and recent research has focused on their potential roles in disease, with MCTs representing promising novel therapeutic targets. This review will provide a summary of the current literature focusing on the characterization, function, and regulation of the MCT family isoforms and on their roles in drug disposition and in health and disease. SIGNIFICANCE STATEMENT: The 14-member solute carrier family 16 of monocarboxylate transporters (MCTs) plays a fundamental role in maintaining intracellular concentrations of a broad range of important endogenous molecules in health and disease. MCTs 1, 2, and 4 (L-lactate transporters) are overexpressed in cancers and represent a novel therapeutic target in cancer. Recent studies have highlighted the importance of MCTs in glucose, lipid, and hormone homeostasis, including MCT8 in thyroid hormone brain uptake, MCT12 in carnitine transport, and MCT11 in type 2 diabetes.

Sodium hydrogen exchanger 9 NHE9 (SLC9A9) and its emerging roles in neuropsychiatric comorbidity

Variations in SLC9A9 gene expression and protein function are associated with multiple human diseases, which range from Attention-deficit/hyperactivity disorder (ADHD) to glioblastoma multiforme. In an effort to determine the full spectrum of human disease associations with SLC9A9, we performed a systematic review of the literature. We also review SLC9A9's biochemistry, protein structure, and function, as well as its interacting partners with the goal of identifying mechanisms of disease and druggable targets. We report gaps in the literature regarding the genes function along with consistent trends in disease associations that can be used to further research into treating the respective diseases. We report that SLC9A9 has strong associations with neuropsychiatric diseases and various cancers. Interestingly, we find strong overlap in SLC9A9 disease associations and propose a novel role for SLC9A9 in neuropsychiatric comorbidity. In conclusion, SLC9A9 is a multifunctional protein that, through both its endosome regulatory function and its protein-protein interaction network, has the ability to modulate signaling axes, such as the PI3K pathway, among others.

Mathematical modeling analysis of hepatic uric acid disposition using human sandwich-cultured hepatocytes

Uric acid is biosynthesized from purine by xanthine oxidase (XO) mainly in the liver and is excreted into urine and feces. Although several transporters responsible for renal and intestinal handling of uric acid have been reported, information on hepatic transporters is limited. In the present study, we studied quantitative contribution of transporters for hepatic handling of uric acid by mathematical modeling analysis in human sandwich-cultured hepatocytes (hSCH). Stable isotope-labeled hypoxanthine, hypoxanthine-¹³C2,¹⁵N (HX), was incubated with hSCH and formed ¹³C2,¹⁵N-labeled xanthine (XA) and uric acid (UA) were measured by LC-MS/MS time dependently. Rate constants for metabolism and efflux and uptake transport across sinusoidal and bile canalicular membranes of HX, XA and UA were estimated in the presence of inhibitors of XO and uric acid transporters. An XO inhibitor allopurinol significantly decreased metabolisms of HX and XA. Efflux into bile canalicular lumen was negligible and sinusoidal efflux was considered main efflux pathway of formed UA. Transporter inhibition study highlighted that GLUT9 strongly and MRP4 intermediately contribute to the sinusoidal efflux of UA with minor contribution of NPT1/4. Modeling analysis developed in the present study should be useful for quantitative prediction of uric acid disposition in liver.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

Expression of NaPi-IIb in rodent and human kidney and upregulation in a model of chronic kidney disease

Na⁺-coupled phosphate cotransporters from the SLC34 and SLC20 families of solute carriers mediate transepithelial transport of inorganic phosphate (Pi). NaPi-IIa/Slc34a1, NaPi-IIc/Slc34a3, and Pit-2/Slc20a2 are all expressed at the apical membrane of renal proximal tubules and therefore contribute to renal Pi reabsorption. Unlike NaPi-IIa and NaPi-IIc, which are rather kidney-specific, NaPi-IIb/Slc34a2 is expressed in several epithelial tissues, including the intestine, lung, testis, and mammary glands. Recently, the expression of NaPi-IIb was also reported in kidneys from rats fed on high Pi. Here, we systematically quantified the mRNA expression of SLC34 and SLC20 cotransporters in kidneys from mice, rats, and humans. In all three species, NaPi-IIa mRNA was by far the most abundant renal transcript. Low and comparable mRNA levels of the other four transporters, including NaPi-IIb, were detected in kidneys from rodents and humans. In mice, the renal expression of NaPi-IIa transcripts was restricted to the cortex, whereas NaPi-IIb mRNA was observed in medullary segments. Consistently, NaPi-IIb protein colocalized with uromodulin at the luminal membrane of thick ascending limbs of the loop of Henle segments. The abundance of NaPi-IIb transcripts in kidneys from mice was neither affected by dietary Pi, the absence of renal NaPi-IIc, nor the depletion of intestinal NaPi-IIb. In contrast, it was highly upregulated in a model of oxalate-induced kidney disease where all other SLC34 phosphate transporters were downregulated. Thus, NaPi-IIb may contribute to renal phosphate reabsorption, and its upregulation in kidney disease might promote hyperphosphatemia.

Role of sodium-dependent Pi transporter/Npt2c on Pi homeostasis in klotho knockout mice different properties between juvenile and adult stages

SLC34A3/NPT2c/NaPi-2c/Npt2c is a growth-related NaPi cotransporter that mediates the uptake of renal sodium-dependent phosphate (Pi). Mutation of human NPT2c causes hereditary hypophosphatemic rickets with hypercalciuria. Mice with Npt2c knockout, however, exhibit normal Pi metabolism. To investigate the role of Npt2c in Pi homeostasis, we generated α-klotho-/- /Npt2c-/- (KL2cDKO) mice and analyzed Pi homeostasis. α-Klotho-/- (KLKO) mice exhibit hyperphosphatemia and markedly increased kidney Npt2c protein levels. Genetic disruption of Npt2c extended the lifespan of KLKO mice similar to that of α-Klotho-/- /Npt2a-/- mice. Adult KL2cDKO mice had hyperphosphatemia, but analysis of Pi metabolism revealed significantly decreased intestinal and renal Pi (re)absorption compared with KLKO mice. The 1,25-dihydroxy vitamin D3 concentration was not reduced in KL2cDKO mice compared with that in KLKO mice. The KL2cDKO mice had less severe soft tissue and vascular calcification compared with KLKO mice. Juvenile KL2cDKO mice had significantly reduced plasma Pi levels, but Pi metabolism was not changed. In Npt2cKO mice, plasma Pi levels began to decrease around the age of 15 days and significant hypophosphatemia developed within 21 days. The findings of the present study suggest that Npt2c contributes to regulating plasma Pi levels in the juvenile stage and affects Pi retention in the soft and vascular tissues in KLKO mice.

Solute Carrier Transportome in Chemotherapy-Induced Adverse Drug Reactions

Members of the solute carrier (SLC) family of transporters are responsible for the cellular influx of a broad range of endogenous compounds and xenobiotics. These proteins are highly expressed in the gastrointestinal tract and eliminating organs such as the liver and kidney, and are considered to be of particular importance in governing drug absorption and elimination. Many of the same transporters are also expressed in a wide variety of organs targeted by clinically important anticancer drugs, directly affect cellular sensitivity to these agents, and indirectly influence treatment-related side effects. Furthermore, targeted intervention strategies involving the use of transport inhibitors have been recently developed, and have provided promising lead candidates for combinatorial therapies associated with decreased toxicity. Gaining a better understanding of the complex interplay between transporter-mediated on-target and off-target drug disposition will help guide the further development of these novel treatment strategies to prevent drug accumulation in toxicity-associated organs, and improve the safety of currently available treatment modalities. In this report, we provide an update on this rapidly emerging field with particular emphasis on anticancer drugs belonging to the classes of taxanes, platinum derivatives, nucleoside analogs, and anthracyclines.

Variants in MCT10 protein do not affect FT3 levels in athyreotic patients

PURPOSE

Several single-nucleotide polymorphisms in genes encoding for transporters have been associated with serum thyroid hormone concentrations with inconsistent results. The aim of this study was to assess the clinical significance of the rs17606253 in SLC16A10 gene alone and in combination with the DIO2 Thr92Ala variation in athyreotic patients.

METHODS

One-hundred patients submitted to total thyroidectomy and treated with levothyroxine were included. Pre- and post surgical serum TSH levels did not differ by more than ± 0.5 mIU/l.

RESULTS

Both patients carrying the wild-type allele or heterozygous for rs17606253 in SLC16A10 gene had a significant reduction in FT3 post surgical levels (p = 0.01 and p < 0.0001, respectively) while Thr92Ala in DIO2 gene was associated with reduced FT3 levels for heterozygous and rare homozygous patients (p < 0.0001 and p = 0.01, respectively). We identified two groups ("FT3 unchanged" and "FT3 reduced") using a cutoff of at least 0.5 pg/ml as a significant variation between pre- and post surgical FT3 values. In this case, the rs17606253 was not statistically associated with reduced FT3 levels at genotype and allele levels. On the contrary, the Thr92Ala in DIO2 gene was confirmed statistically associated with reduced FT3 levels after surgery with a p = 0.035 at genotype level and p = 0.014 at allele level.

CONCLUSIONS

We confirmed the role of DIO2 Thr92Ala polymorphism on T3 levels. On the contrary, SLC16A1 rs17606253 polymorphism did not impair hormone levels in athyreotic patients treated with levothyroxine therapy.

Differential expression and immunoreactivity of thyroid hormone transporters MCT8 and OATP1C1 in rat ovary

Thyroid hormones (THs) regulate several physiological processes in female mammals, many of which are related to reproduction such as steroidogenesis in the ovary, oocyte and granulosa cells maturation, follicular development and differentiation, and ovulation. THs actions require the presence of THs transporters to facilitate their cellular uptake and efflux. MCT8 and OATP1C1 are the principal THs transporters. The aim of the present study was to determine the gene expression and cellular localization of MCT8 and OATP1C1 in the rat ovary during the diestrus-II cycle phase. Ovaries of virgin adult rats were histologically processed. Reverse Transcription-PCR and immunohistochemistry analyses for MCT8 and OATP1C1 were done. MCT8 gene expression level was significantly higher (P ≤ 0.01) than that of OATP1C1 in the rat ovary. MCT8 and OATP1C1 were found in all types of ovarian cells but with different immunoreactivity. MCT8 showed stronger immunoreactivity in tertiary and Graafian follicles, corpus luteum and blood vessels, whereas OATP1C1's immunoreactivity was stronger in stroma cells, tunica albuginea, and blood vessels. Our results provide evidence that THs and their transporters are both necessary for ovarian function and that any alteration in these transporters could interfere with reproductive processes such as ovulation and steroidogenesis, compromising fertility.

Profiling solute-carrier transporters in key metabolic tissues during the postpartum evolution of mammary epithelial cells from nonsecretory to secretory

Modifications in the abundance of solute-carrier (SLC) transcripts in tandem with adjustments in genes-associated with energy homeostasis during the postpartum transition of the mammary epithelial cells (MEC) from nonsecretory to secretory is pivotal for supporting milk synthesis. The goal of this study was to identify differentially expressed SLC genes across key metabolic tissues between late pregnancy and onset of lactation. Total RNA was isolated from the mammary, liver, and adipose tissues collected from rat dams on day 20 of pregnancy (P20) and day 1 of lactation (L1) and gene expression was measured with Rat 230 2.0 Affymetrix GeneChips. LIMMA was utilized to identify the differential gene expression patterns between P20 and L1 tissues. Transcripts engaged in conveying anions, cations, carboxylates, sugars, amino acids, metals, nucleosides, vitamins, and fatty acids were significantly increased (P < 0.05) in MEC during the P20 to L1 shift. Downregulated (P < 0.05) genes in the mammary during the physiological transition included GLUT8 and SLC45a3. In the liver, SLC genes encoding for anion, carbonyl, and nucleotide sugar transporters were upregulated (P < 0.05) at L1. while genes facilitating transportation of anions and hexose were increased (P < 0.05), from P20 to L1 in the adipose tissue. GLUT1 and GLUT4 in the liver, along with GLUT4 and SGLT2 in the adipose tissue, were repressed (P < 0.05) at L1. Our results illustrate that MEC exhibit dynamic molecular plasticity during the nonsecretory to secretory transition and increase biosynthetic capacity through a coordinated tissue specific SLC transcriptome modification to facilitate substrate transfer.

Renal phosphate handling and inherited disorders of phosphate reabsorption: an update

Renal phosphate handling critically determines plasma phosphate and whole body phosphate levels. Filtered phosphate is mostly reabsorbed by Na⁺-dependent phosphate transporters located in the brush border membrane of the proximal tubule: NaPi-IIa (SLC34A1), NaPi-IIc (SLC34A3), and Pit-2 (SLC20A2). Here we review new evidence for the role and relevance of these transporters in inherited disorders of renal phosphate handling. The importance of NaPi-IIa and NaPi-IIc for renal phosphate reabsorption and mineral homeostasis has been highlighted by the identification of mutations in these transporters in a subset of patients with infantile idiopathic hypercalcemia and patients with hereditary hypophosphatemic rickets with hypercalciuria. Both diseases are characterized by disturbed calcium homeostasis secondary to elevated 1,25-(OH)₂ vitamin D₃ as a consequence of hypophosphatemia. In vitro analysis of mutated NaPi-IIa or NaPi-IIc transporters suggests defective trafficking underlying disease in most cases. Monoallelic pathogenic mutations in both SLC34A1 and SLC34A3 appear to be very frequent in the general population and have been associated with kidney stones. Consistent with these findings, results from genome-wide association studies indicate that variants in SLC34A1 are associated with a higher risk to develop kidney stones and chronic kidney disease, but underlying mechanisms have not been addressed to date.

Genome-wide scans of myopia in Pennsylvania Amish families reveal significant linkage to 12q15, 8q21.3 and 5p15.33

Myopia is one of the most common ocular disorders in the world, yet the genetic etiology of the disease remains poorly understood. Specialized founder populations, such as the Pennsylvania Amish, provide the opportunity to utilize exclusive genomic architecture, like unique haplotypes, to better understand the genetic causes of myopia. We perform genetic linkage analysis on Pennsylvania Amish families that have a strong familial history of myopia to map any potential causal variants and genes for the disease. 293 individuals from 25 extended families were genotyped on the Illumina ExomePlus array and merged with previous microsatellite data. We coded myopia affection as a binary phenotype; myopia was defined as having a mean spherical equivalent (MSE) of less than or equal to - 1 D (diopters). Two-point and multipoint parametric linkage analyses were performed under an autosomal dominant model. When allowing for locus heterogeneity, we identified two novel genome-wide significantly linked variants at 12q15 (heterogeneity LOD, HLOD = 3.77) in PTPRB and at 8q21.3 (HLOD = 3.35) in CNGB3. We identified further three genome-wide significant variants within a single family. These three variants were located in exons of SLC6A18 at 5p15.33 (LODs ranged from 3.51 to 3.37). Multipoint analysis confirmed the significant signal at 5p15.33 with six genome-wide significant variants (LODs ranged from 3.6 to 3.3). Further suggestive evidence of linkage was observed in several other regions of the genome. All three novel linked regions contain strong candidate genes, especially CNGB3 on 8q21.3, which has been shown to affect photoreceptors and cause complete color blindness. Whole genome sequencing on these regions is planned to conclusively elucidate the causal variants.

Gastrointestinal Malabsorption of Thyroxine

Levothyroxine, a largely prescribed drug with a narrow therapeutic index, is often a lifelong treatment. The therapeutic efficacy of T4 may be marred by behavioral, pharmacologic, and pathologic issues acting as interfering factors. Despite a continuous search for an optimal T4 treatment, a significant number of patients fail to show a complete chemical and/or clinical response to this reference dose of T4. Gastrointestinal malabsorption of oral T4 represents an emerging cause of refractory hypothyroidism and may be more frequent than previously reputed. In this review, we examine the pharmacologic features of T4 preparations and their linkage with the intestinal absorption of the hormone. We have stressed the major biochemical and pharmacologic characteristics of T4 and its interaction with the putative transporter at the intestinal level. We have examined the interfering role of nutrients, foods, and drugs on T4 absorption at the gastric and intestinal levels. The impact of gastrointestinal disorders on T4 treatment efficacy has been also analyzed, in keeping with the site of action and the interfering mechanisms. Based on the evidence obtained from the literature, we also propose a schematic diagnostic workup for the most frequent and often hidden gastrointestinal diseases impairing T4 absorption.

Physiological regulation of phosphate by vitamin D, parathyroid hormone (PTH) and phosphate (Pi)

Inorganic phosphate (Pi) is an abundant element in the body and is essential for a wide variety of key biological processes. It plays an essential role in cellular energy metabolism and cell signalling, e.g. adenosine and guanosine triphosphates (ATP, GTP), and in the composition of phospholipid membranes and bone, and is an integral part of DNA and RNA. It is an important buffer in blood and urine and contributes to normal acid-base balance. Given its widespread role in almost every molecular and cellular function, changes in serum Pi levels and balance can have important and untoward effects. Pi homoeostasis is maintained by a counterbalance between dietary Pi absorption by the gut, mobilisation from bone and renal excretion. Approximately 85% of total body Pi is present in bone and only 1% is present as free Pi in extracellular fluids. In humans, extracellular concentrations of inorganic Pi vary between 0.8 and 1.2 mM, and in plasma or serum Pi exists in both its monovalent and divalent forms (H2PO4- and HPO42-). In the intestine, approximately 30% of Pi absorption is vitamin D regulated and dependent. To help maintain Pi balance, reabsorption of filtered Pi along the renal proximal tubule (PT) is via the NaPi-IIa and NaPi-IIc Na+-coupled Pi cotransporters, with a smaller contribution from the PiT-2 transporters. Endocrine factors, including, vitamin D and parathyroid hormone (PTH), as well as newer factors such as fibroblast growth factor (FGF)-23 and its coreceptor α-klotho, are intimately involved in the control of Pi homeostasis. A tight regulation of Pi is critical, since hyperphosphataemia is associated with increased cardiovascular morbidity in chronic kidney disease (CKD) and hypophosphataemia with rickets and growth retardation. This short review considers the control of Pi balance by vitamin D, PTH and Pi itself, with an emphasis on the insights gained from human genetic disorders and genetically modified mouse models.

Etidronate attenuates tactile allodynia by spinal ATP release inhibition in mice with partial sciatic nerve ligation

Etidronate is widely used as a therapeutic agent for osteoporosis. We have recently shown that intrathecal administration of etidronate into mice produces an analgesic effect against the capsaicin-induced nociceptive behavior. However, the effect of etidronate on neuropathic pain at the spinal level remains unknown. Therefore, we examined whether etidronate attenuates pain after partial sciatic nerve ligation (PSNL). We evaluated tactile allodynia 7 days after PSNL by measuring paw withdrawal with the von Frey filament test. The mRNA and protein levels of SLC17A9 in the ipsilateral lumbar dorsal spinal cord of PSNL-operated mice were determined using real-time PCR and western blotting, respectively. PSNL-induced tactile allodynia was attenuated by oral and intrathecal administration of etidronate, with maximum efficiency at 90 and 60 min after injection, respectively. The anti-allodynic effect of intrathecally administered etidronate was completely inhibited by an intrathecal administration of adenosine triphosphate (ATP). The solute carrier family, SLC17, mediates the transport of pain transmitters, like ATP and glutamate. Indeed, we detected several members of the SLC17 family in the mouse dorsal lumbar spinal cord. Among the detected mRNAs, only Slc17a9, encoding for neuronal vesicular ATP transporter, was significantly increased upon PSNL. SLC17A9 protein levels were also significantly increased. In mice subjected to PSNL, SLC17A9 was present in neurons and microglia, but not in astrocytes of the lumbar superficial dorsal horn. Collectively, our results suggest that etidronate produces its anti-allodynic effects by inhibiting SLC17A9-dependent exocytotic ATP release from the dorsal horn in mice subjected to PSNL.

The role of SLC34A2 in intestinal phosphate absorption and phosphate homeostasis

There has recently been significant interest in the concept of directly targeting intestinal phosphate transport to control hyperphosphatemia in patients with chronic kidney disease. However, we do not have a complete understanding of the cellular mechanisms that govern dietary phosphate absorption. Studies in the 1970s documented both active and passive pathways for intestinal phosphate absorption. However, following the cloning of the intestinal SLC34 cotransporter, NaPi-IIb, much of the research focused on the role of this protein in active transcellular phosphate absorption and the factors involved in its regulation. Generation of a conditional NaPi-IIb knockout mouse has demonstrated that this protein is critical for the maintenance of skeletal integrity during periods of phosphate restriction and that under normal physiological conditions, the passive sodium-independent pathway is likely be the more dominant pathway for intestinal phosphate absorption. The review aims to summarise the most recent developments in our understanding of the role of the intestine in phosphate homeostasis, including the acute and chronic renal adaptations that occur in response to dietary phosphate intake. Evidence regarding the overall contribution of the transcellular and paracellular pathways for phosphate absorption will be discussed, together with the clinical benefit of inhibiting these pathways for the treatment of hyperphosphatemia in chronic kidney disease.

Analysis of opossum kidney NaPi-IIc sodium-dependent phosphate transporter to understand Pi handling in human kidney

BACKGROUND

The role of Na⁺-dependent inorganic phosphate (Pi) transporters in the human kidney is not fully clarified. Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is caused by loss-of-function mutations in the IIc Na⁺-dependent Pi transporter (NPT2c/Npt2c/NaPi-IIc) gene. Another Na⁺-dependent type II transporter, (NPT2A/Npt2a/NaPi-IIa), is also important for renal Pi reabsorption in humans. In mice, Npt2c deletion does not lead to hypophosphatemia and rickets because Npt2a compensates for the impaired Pi reabsorption. To clarify the differences between mouse and human, we investigated the relation between NaPi-IIa and NaPi-IIc functions in opossum kidney (OK) cells.

METHODS

We cloned NaPi-IIc from OK cells and created opossum NaPi-IIc (oNaPi-IIc) antibodies. We used oNaPi-IIc small interference (si)RNA and investigated the role of NaPi-IIc in Pi transport in OK cells.

RESULTS

We cloned opossum kidney NaPi-IIc cDNAs encoding 622 amino acid proteins (variant1) and examined their pH- and sodium-dependency. The antibodies reacted specifically with 75-kDa and 150-kDa protein bands, and the siRNA of NaPi-IIc markedly suppressed endogenous oNaPi-IIc in OK cells. Treatment with siRNA significantly suppressed the expression of NaPi-4 (NaPi-IIa) protein and mRNA. oNaPi-IIc siRNA also suppressed Na⁺/H⁺ exchanger regulatory factor 1 expression in OK cells.

CONCLUSION

These findings suggest that NaPi-IIc is important for the expression of NaPi-IIa (NaPi-4) protein in OK cells. Suppression of Npt2c may downregulate Npt2a function in HHRH patients.

Phosphate as a Signaling Molecule and Its Sensing Mechanism

In mammals, phosphate balance is maintained by influx and efflux via the intestines, kidneys, bone, and soft tissue, which involves multiple sodium/phosphate (Na+/Pi) cotransporters, as well as regulation by several hormones. Alterations in the levels of extracellular phosphate exert effects on both skeletal and extra-skeletal tissues, and accumulating evidence has suggested that phosphate itself evokes signal transduction to regulate gene expression and cell behavior. Several in vitro studies have demonstrated that an elevation in extracellular Piactivates fibroblast growth factor receptor, Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular signal-regulated kinase) pathway and Akt pathway, which might involve the type III Na+/Picotransporter PiT-1. Excessive phosphate loading can lead to various harmful effects by accelerating ectopic calcification, enhancing oxidative stress, and dysregulating signal transduction. The responsiveness of mammalian cells to altered extracellular phosphate levels suggests that they may sense and adapt to phosphate availability, although the precise mechanism for phosphate sensing in mammals remains unclear. Unicellular organisms, such as bacteria and yeast, use some types of Pitransporters and other molecules, such as kinases, to sense the environmental Piavailability. Multicellular animals may need to integrate signals from various organs to sense the phosphate levels as a whole organism, similarly to higher plants. Clarification of the phosphate-sensing mechanism in humans may lead to the development of new therapeutic strategies to prevent and treat diseases caused by phosphate imbalance.

Hereditary hypophosphatemic rickets with hypercalciuria: pathophysiology, clinical presentation, diagnosis and therapy

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH; OMIM: 241530) is a rare autosomal recessive disorder with an estimated prevalence of 1:250,000 that was originally described by Tieder et al. Individuals with HHRH carry compound-heterozygous or homozygous (comp/hom) loss-of-function mutations in the sodium-phosphate co-transporter NPT2c. These mutations result in the development of urinary phosphate (Pi) wasting and hypophosphatemic rickets, bowing, and short stature, as well as appropriately elevated 1,25(OH)₂D levels, which sets this fibroblast growth factor 23 (FGF23)-independent disorder apart from the more common X-linked hypophosphatemia. The elevated 1,25(OH)₂D levels in turn result in hypercalciuria due to enhanced intestinal calcium absorption and reduced parathyroid hormone (PTH)-dependent calcium-reabsorption in the distal renal tubules, leading to the development of kidney stones and/or nephrocalcinosis in approximately half of the individuals with HHRH. Even heterozygous NPT2c mutations are frequently associated with isolated hypercalciuria (IH), which increases the risk of kidney stones or nephrocalcinosis threefold in affected individuals compared with the general population. Bone disease is generally absent in individuals with IH, in contrast to those with HHRH. Treatment of HHRH and IH consists of monotherapy with oral Pi supplements, while active vitamin D analogs are contraindicated, mainly because the endogenous 1,25(OH)₂D levels are already elevated but also to prevent further worsening of the hypercalciuria. Long-term studies to determine whether oral Pi supplementation alone is sufficient to prevent renal calcifications and bone loss, however, are lacking. It is also unknown how therapy should be monitored, whether secondary hyperparathyroidism can develop, and whether Pi requirements decrease with age, as observed in some FGF23-dependent hypophosphatemic disorders, or whether this can lead to osteoporosis.

Drug transporters and anthracycline-induced cardiotoxicity

The solute carrier superfamily comprises of uptake transporters that can contribute to the absorption and elimination of a broad array of clinically important drugs. Recent studies have suggested that the tissue-specific expression of these transporters may have important consequences for an individual's susceptibility to drug-induced organ damage or to drug–drug interactions. Polymorphic variants have been identified in genes encoded by this family, and some of these have been associated with functional changes in transport function and response to anthracycline-induced toxicity and efficacy. Here, we review recent advances in the role solute carrier transporters play in anthracycline-induced cardiotoxicity, highlight potential implications of genetic variants that may contribute to drug response and discuss novel technologies to study mechanisms of anthracycline transport.