Equilibrative nucleoside transporter 3 deficiency perturbs lysosome function and macrophage homeostasis

Identification of Structural and Molecular Features Involved in the Transport of 3'-Deoxy-Nucleoside Analogs by Human Equilibrative Nucleoside Transporter 3

Combination antiretroviral drug treatments depend on 3′-deoxy-nucleoside analogs such as 3′-azido-3′-deoxythymidine (AZT) and 2′3′-dideoxyinosine (DDI). Despite being effective in inhibiting human immunodeficiency virus replication, these drugs produce a range of toxicities, including myopathy, pancreatitis, neuropathy, and lactic acidosis, that are generally considered as sequelae to mitochondrial damage. Although cell surface–localized nucleoside transporters, such as human equilibrative nucleoside transporter 2 (hENT2) and human concentrative nucleoside transporter 1 (hCNT1), are known to increase the carrier-mediated uptake of 3′-deoxy-nucleoside analogs into cells, another ubiquitously expressed intracellular nucleoside transporter (namely, hENT3) has been implicated in the mitochondrial transport of 3′-deoxy-nucleoside analogs. Using site-directed mutagenesis, generation of chimeric hENTs, and ³H-permeant flux measurements in mutant/chimeric RNA–injected Xenopus oocytes, here we identified the molecular determinants of hENT3 that dictate membrane translocation of 3′-deoxy-nucleoside analogs. Our findings demonstrated that whereas hENT1 had no significant transport activity toward 3′-deoxy-nucleoside analogs, hENT3 was capable of transporting 3′-deoxy-nucleoside analogs similar to hENT2. Transport analyses of hENT3-hENT1 chimeric constructs demonstrated that the N-terminal half of hENT3 is primarily responsible for the hENT3–3′-deoxy-nucleoside analog interaction. In addition, mutagenic studies identified that 225D and 231L in the N-terminal half of hENT3 partially contribute to the ability of hENT3 to transport AZT and DDI. The identification of the transporter segment and amino acid residues that are important in hENT3 transport of 3′-deoxy-nucleoside analogs may present a possible mechanism for overcoming the adverse toxicities associated with 3′-deoxy-nucleoside analog treatment and may guide rational development of novel nucleoside analogs.

Tocilizumab for the Treatment of SLC29A3 Mutation Positive PHID Syndrome

Pigmentary hypertrichosis and non-autoimmune insulin-dependent diabetes mellitus (PHID) is associated with recessive mutations in SLC29A3, encoding the equilibrative nucleoside transporter hENT3 expressed in mitochondria, causing PHID and H syndromes, familial Rosai-Dorfman disease, and histiocytosis-lymphadenopathy-plus syndrome. Autoinflammation is increasingly recognized in these syndromes. We previously reported a 16-year-old girl with PHID syndrome associated with severe autoinflammation that was recalcitrant to interleukin-1 and tumor necrosis factor-α blockade. Tocilizumab is a humanized, monoclonal, anti-human interleukin-6 receptor antibody routinely used to treat arthritis in children and adults. Herein we report the first case of successful treatment of PHID syndrome using tocilizumab. Before commencing tocilizumab, there was evidence of significant systemic inflammation, and progressive sclerodermatous changes (physician global assessment [PGA] 7/10). Twelve weeks after starting tocilizumab (8 mg/kg every 2 weeks, intravenously) systemic inflammatory symptoms improved, and acute phase response markers normalized; serum amyloid A reduced from 178 to 8.4 mg/L. After a dose increase to 12 mg/kg every 2 weeks her energy levels, appetite, fevers, and night sweats further improved. Less skin tightness (PGA 5/10) was documented 12 months later. This excellent clinical and serological response was sustained over 48 months, and cutaneous sclerosis had improved further (PGA 3/10). Her height remained well below the 0.4th centile, and tocilizumab also had no impact on her diabetes or exocrine pancreatic insufficiency. Although the mechanism of autoinflammation of PHID remains uncertain, we suggest that tocilizumab should be the first choice when considering treatment of the autoinflammatory or cutaneous manifestations of this genetic disease.

H syndrome: 5 new cases from the United States with novel features and responses to therapy

BACKGROUND

H Syndrome is an autosomal recessive disorder characterized by cutaneous hyperpigmentation, hypertrichosis, and induration with numerous systemic manifestations. The syndrome is caused by mutations in SLC29A3, a gene located on chromosome 10q23, which encodes the human equilibrative transporter 3 (hENT3). Less than 100 patients with H syndrome have been described in the literature, with the majority being of Arab descent, and only a few from North America.

CASE PRESENTATION

Here we report five pediatric patients from three medical centers in the United States who were identified to have H syndrome by whole exome sequencing. These five patients, all of whom presented to pediatric rheumatologists prior to diagnosis, include two of Northern European descent, bringing the total number of Caucasian patients described to three. The patients share many of the characteristics previously reported with H syndrome, including hyperpigmentation, hypertrichosis, short stature, insulin-dependent diabetes, arthritis and systemic inflammation, as well as some novel features, including selective IgG subclass deficiency and autoimmune hepatitis. They share genetic mutations previously described in patients of the same ethnic background, as well as a novel mutation. In two patients, treatment with prednisone improved inflammation, however both patients flared once prednisone was tapered. In one of these patients, treatment with tocilizumab alone resulted in marked improvement in systemic inflammation and growth. The other had partial response to prednisone, azathioprine, and TNF inhibition; thus, his anti-TNF biologic was recently switched to tocilizumab due to persistent polyarthritis. Another patient improved on Methotrexate, with further improvement after the addition of tocilizumab.

CONCLUSION

H syndrome is a rare autoinflammatory syndrome with pleiotropic manifestations that affect multiple organ systems and is often mistaken for other conditions. Rheumatologists should be aware of this syndrome and its association with arthritis. It should be considered in patients with short stature and systemic inflammation, particularly with cutaneous findings. Some patients respond to treatment with biologics alone or in combination with other immune suppressants; in particular, treatment of systemic inflammation with IL-6 blockade appears to be promising. Overall, better identification and understanding of the pathophysiology may help devise earlier diagnosis and better treatment strategies.

Molecular determinants of acidic pH-dependent transport of human equilibrative nucleoside transporter 3

Equilibrative nucleoside transporters (ENTs) translocate hydrophilic nucleosides across cellular membranes and are essential for salvage nucleotide synthesis and purinergic signaling. Unlike the prototypic human ENT members hENT1 and hENT2, which mediate plasma membrane nucleoside transport at pH 7.4, hENT3 is an acidic pH-activated lysosomal transporter partially localized to mitochondria. Recent studies demonstrate that hENT3 is indispensable for lysosomal homeostasis, and that mutations in hENT3 can result in a spectrum of lysosomal storage-like disorders. However, despite hENT3's prominent role in lysosome pathophysiology, the molecular basis of hENT3-mediated transport is unknown. Therefore, we sought to examine the mechanistic basis of acidic pH-driven hENT3 nucleoside transport with site-directed mutagenesis, homology modeling, and [³H]adenosine flux measurements in mutant RNA-injected Xenopus oocytes. Scanning mutagenesis of putative residues responsible for pH-dependent transport via hENT3 revealed that the ionization states of Asp-219 and Glu-447, and not His, strongly determined the pH-dependent transport permissible-impermissible states of the transporter. Except for substitution with certain isosteric and polar residues, substitution of either Asp-219 or Glu-447 with any other residues resulted in robust activity that was pH-independent. Dual substitution of Asp-219 and Glu-447 to Ala sustained pH-independent activity over a broad range of physiological pH (pH 5.5-7.4), which also maintained stringent substrate selectivity toward endogenous nucleosides and clinically used nucleoside drugs. Our results suggest a putative pH-sensing role for Asp-219 and Glu-447 in hENT3 and that the size, ionization state, or electronegative polarity at these positions is crucial for obligate acidic pH-dependent activity.

The life cycle of phagosomes: formation, maturation, and resolution

Phagocytosis, the regulated uptake of large particles (>0.5 μm in diameter), is essential for tissue homeostasis and is also an early, critical component of the innate immune response. Phagocytosis can be conceptually divided into three stages: phagosome, formation, maturation, and resolution. Each of these involves multiple reactions that require exquisite spatial and temporal orchestration. The molecular events underlying these stages are being unraveled and the current state of knowledge is briefly summarized in this article.

Review of Rosai-Dorfman Disease: New Insights into the Pathogenesis of This Rare Disorder

Rosai-Dorfman disease (RDD) is a rare histiocytosis typically with bilateral painless cervical lymphadenopathy. Laboratory data are nonspecific, and the presence of emperipolesis in large foamy S-100+ CD1a- histiocytes is the prominent histologic feature. The pathogenesis of RDD still remains elusive. According to published studies, we propose that RDD cells might represent intermediate recruiting monocytes with differentiation blockade. Both disturbance of homoeostasis and inherent genomic alterations could contribute to initiation of the disorder through signal transduction. Several inflammatory molecules such as macrophage colony-stimulating factor, IL-1β, IL-6, and tumor necrosis factor-α also play a pivotal role in the development of this rare entity. Additional studies are needed to further elucidate the essence of the disease.

Efferocytosis Signaling in the Regulation of Macrophage Inflammatory Responses

Since the pioneering work of Elie Metchnikoff and the discovery of cellular immunity, the phagocytic clearance of cellular debris has been considered an integral component of resolving inflammation and restoring function of damaged and infected tissues. We now know that the phagocytic clearance of dying cells (efferocytosis), particularly by macrophages and other immune phagocytes, has profound consequences on innate and adaptive immune responses in inflamed tissues. These immunomodulatory effects result from an array of molecular signaling events between macrophages, dying cells, and other tissue-resident cells. In recent years, many of these molecular pathways have been identified and studied in the context of tissue inflammation, helping us better understand the relationship between efferocytosis and inflammation. We review specific types of efferocytosis-related signals that can impact macrophage immune responses and discuss their relevance to inflammation-related diseases.

Inhibition of Transient Receptor Potential Channel Mucolipin-1 (TRPML1) by Lysosomal Adenosine Involved in Severe Combined Immunodeficiency Diseases

Impaired adenosine homeostasis has been associated with numerous human diseases. Lysosomes are referred to as the cellular recycling centers that generate adenosine by breaking down nucleic acids or ATP. Recent studies have suggested that lysosomal adenosine overload causes lysosome defects that phenocopy patients with mutations in transient receptor potential channel mucolipin-1 (TRPML1), a lysosomal Ca²⁺ channel, suggesting that lysosomal adenosine overload may impair TRPML1 and then lead to subsequent lysosomal dysfunction. In this study, we demonstrate that lysosomal adenosine is elevated by deleting adenosine deaminase (ADA), an enzyme responsible for adenosine degradation. We also show that lysosomal adenosine accumulation inhibits TRPML1, which is rescued by overexpressing ENT3, the adenosine transporter situated in the lysosome membrane. Moreover, ADA deficiency results in lysosome enlargement, alkalinization, and dysfunction. These are rescued by activating TRPML1. Importantly, ADA-deficient B-lymphocytes are more vulnerable to oxidative stress, and this was rescued by TRPML1 activation. Our data suggest that lysosomal adenosine accumulation impairs lysosome function by inhibiting TRPML1 and subsequently leads to cell death in B-lymphocytes. Activating TRPML1 could be a new therapeutic strategy for those diseases.

Newly Described Autoinflammatory Diseases in Pediatric Dermatology

Specific gene mutations leading to dysregulation of innate immune response produce the expanding spectrum of monogenic autoinflammatory diseases (AIDs). They are characterized by seemingly unprovoked, recurrent episodes of systemic inflammation in which a myriad of manifestations usually affect skin. Novel genetic technologies have led to the discovery of new AIDs and phenotypes that were not previously clinically described. Consequently the number of AIDs is continuously growing and their recognition and the disclosure of their pathophysiology will prompt early diagnosis and targeted treatment of affected patients. The objective of the present work is to review those newly described AIDs with prominent dermatologic manifestations that may constitute a major criterion for their diagnosis.

Equilibrative nucleoside transporters-A review

Equilibrative nucleoside transporters (ENTs) are polytopic integral membrane proteins that mediate the transport of nucleosides, nucleobases, and therapeutic analogs. The best-characterized ENTs are the human transporters hENT1 and hENT2. However, non-mammalian eukaryotic ENTs have also been studied (e.g., yeast, parasitic protozoa). ENTs are major pharmaceutical targets responsible for modulating the efficacy of more than 30 approved drugs. However, the molecular mechanisms and chemical determinants of ENT-mediated substrate recognition, binding, inhibition, and transport are poorly understood. This review highlights findings on the characterization of ENTs by surveying studies on genetics, permeant and inhibitor interactions, mutagenesis, and structural models of ENT function.

The plasma membrane monoamine transporter (PMAT): Structure, function, and role in organic cation disposition

Plasma membrane monoamine transporter (PMAT) is a new polyspecific organic cation transporter that transports a variety of biogenic amines and xenobiotic cations. Highly expressed in the brain, PMAT represents a major uptake₂ transporter for monoamine neurotransmitters. At the blood-cerebrospinal fluid (CSF) barrier, PMAT is the principal organic cation transporter for removing neurotoxins and drugs from the CSF. Here I summarize our latest understanding of PMAT and its roles in monoamine uptake and xenobiotic disposition.

Lysosomal solute carrier transporters gain momentum in research

Emerging evidence indicates that lysosome function extends beyond macromolecular degradation. Genetic and functional defects in components of the lysosomal transport machinery cause lysosomal storage disorders implicating the lysosomal solute carrier (SLC) transporters as essential to vital cell processes. The pathophysiology and therapeutic potential of lysosomal SLC transporters are highlighted here, focusing on recent discoveries in autophagic amino acid sensing (SLC38A9), phagocytic regulation in macrophages (SLC29A3, SLC15A3/A4), adenosine triphosphate (ATP) exocytosis in neurotransmission (SLC17A9), and lysosomal transport of maytansine catabolites into the cytoplasm (SLC46A3).

Lysosomal Disorders Drive Susceptibility to Tuberculosis by Compromising Macrophage Migration

A zebrafish genetic screen for determinants of susceptibility to Mycobacterium marinum identified a hypersusceptible mutant deficient in lysosomal cysteine cathepsins that manifests hallmarks of human lysosomal storage diseases. Under homeostatic conditions, mutant macrophages accumulate undigested lysosomal material, which disrupts endocytic recycling and impairs their migration to, and thus engulfment of, dying cells. This causes a buildup of unengulfed cell debris. During mycobacterial infection, macrophages with lysosomal storage cannot migrate toward infected macrophages undergoing apoptosis in the tuberculous granuloma. The unengulfed apoptotic macrophages undergo secondary necrosis, causing granuloma breakdown and increased mycobacterial growth. Macrophage lysosomal storage similarly impairs migration to newly infecting mycobacteria. This phenotype is recapitulated in human smokers, who are at increased risk for tuberculosis. A majority of their alveolar macrophages exhibit lysosomal accumulations of tobacco smoke particulates and do not migrate to Mycobacterium tuberculosis. The incapacitation of highly microbicidal first-responding macrophages may contribute to smokers’ susceptibility to tuberculosis.

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Lysosomal storage diseases reduce macrophage endocytic recycling and migration

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Reduced macrophage migration increases tuberculosis severity via granuloma breakdown

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Tobacco smoke particles accumulate in lysosomes of smokers’ alveolar macrophages

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Lysosomal particles reduce smokers’ macrophage migration to infecting mycobacteria

A Novel and Fast Purification Method for Nucleoside Transporters

Nucleoside transporters (NTs) play critical biological roles in humans, and to understand the molecular mechanism of nucleoside transport requires high-resolution structural information. However, the main bottleneck for structural analysis of NTs is the production of pure, stable, and high quality native protein for crystallization trials. Here we report a novel membrane protein expression and purification strategy, including construction of a high-yield membrane protein expression vector, and a new and fast purification protocol for NTs. The advantages of this strategy are the improved time efficiency, leading to high quality, active, stable membrane proteins, and the efficient use of reagents and consumables. Our strategy might serve as a useful point of reference for investigating NTs and other membrane proteins by clarifying the technical points of vector construction and improvements of membrane protein expression and purification.

Identification of a novel mutation in solute carrier family 29, member 3 in a Chinese patient with H syndrome

Background:

H syndrome (OMIM 612391) is a recently described autosomal recessive genodermatosis characterized by indurated hyperpigmented and hypertrichotic skin, as well as other systemic manifestations. Most of the cases occurred in the Middle East areas or nearby countries such as Spain or India. The syndrome is caused by mutations in solute carrier family 29, member 3 (SLC29A3), the gene encoding equilibrative nucleoside transporter 3. The aim of this study was to identify pathogenic SLC29A3 mutations in a Chinese patient clinically diagnosed with H syndrome.

Methods:

Peripheral blood samples were collected from the patient and his parents. Genomic DNA was isolated by the standard method. All six SLC29A3 exons and their flanking intronic sequences were polymerase chain reaction (PCR)-amplified and the PCR products were subjected to direct sequencing.

Results:

The patient, an 18-year-old man born to a nonconsanguineous Chinese couple, had more extensive cutaneous lesions, involving both buttocks and knee. In his genomic DNA, we identified a novel homozygous insertion-deletion, c. 1269_1270delinsA, in SLC29A3. Both of his parents were carriers of the mutation.

Conclusions:

We have identified a pathogenic mutation in a Chinese patient with H syndrome.

High-density P300 enhancers control cell state transitions

Background

Transcriptional enhancers are frequently bound by a set of transcription factors that collaborate to activate lineage-specific gene expression. Recently, it was appreciated that a subset of enhancers comprise extended clusters dubbed stretch- or super-enhancers (SEs). These SEs are located near key cell identity genes, and enriched for non-coding genetic variations associated with disease. Previously, SEs have been defined as having the highest density of Med1, Brd4 or H3K27ac by ChIP-seq. The histone acetyltransferase P300 has been used as a marker of enhancers, but little is known about its binding to SEs.

Results

We establish that P300 marks a similar SE repertoire in embryonic stem cells as previously reported using Med1 and H3K27ac. We also exemplify a role for SEs in mouse T helper cell fate decision. Similarly, upon activation of macrophages by bacterial endotoxin, we found that many SE-associated genes encode inflammatory proteins that are strongly up-regulated. These SEs arise from small, low-density enhancers in unstimulated macrophages. We also identified expression quantitative trait loci (eQTL) in human monocytes that lie within such SEs. In macrophages and Th17 cells, inflammatory SEs can be perturbed either genetically or pharmacologically thus revealing new avenues to target inflammation.

Conclusions

Our findings support the notion that P300-marked SEs can help identify key nodes of transcriptional control during cell fate decisions. The SE landscape changes drastically during cell differentiation and cell activation. As these processes are crucial in immune responses, SEs may be useful in revealing novel targets for treating inflammatory diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1905-6) contains supplementary material, which is available to authorized users.

Equilibrative nucleoside transporter 3 depletion in β-cells impairs mitochondrial function and promotes apoptosis: Relationship to pigmented hypertrichotic dermatosis with insulin-dependent diabetes

Loss of function recessive mutations in the SLC29A3 gene that encodes human equilibrative nucleoside transporter 3 (ENT3) have been identified in patients with pigmented hypertrichotic dermatosis with insulin-dependent diabetes (PHID). ENT3 is a member of the equilibrative nucleoside transporter (ENT) family whose primary function is mediating transport of nucleosides and nucleobases. The aims of this study were to characterise ENT3 expression in islet β-cells and identify the effects of its depletion on β-cell mitochondrial activity and apoptosis. RT-PCR amplification identified ENT3 expression in human and mouse islets and exocrine pancreas, and in MIN6 β-cells. Immunohistochemistry using human and mouse pancreas sections exhibited extensive ENT3 immunostaining of β-cells, which was confirmed by co-staining with an anti-insulin antibody. In addition, exposure of dispersed human islet cells and MIN6 β-cells to MitoTracker and an ENT3 antibody showed co-localisation of ENT3 to β-cell mitochondria. Consistent with this, Western blot analysis confirmed enhanced ENT3 immunoreactivity in β-cell mitochondria-enriched fractions. Furthermore, ENT3 depletion in β-cells increased mitochondrial DNA content and promoted an energy crisis characterised by enhanced ATP-linked respiration and proton leak. Finally, inhibition of ENT3 activity by dypridamole and depletion of ENT3 by siRNA-induced knockdown resulted in increased caspase 3/7 activities in β-cells. These observations demonstrate that ENT3 is predominantly expressed by islet β-cells where it co-localises with mitochondria. Depletion of ENT3 causes mitochondrial dysfunction which is associated with enhanced β-cell apoptosis. Thus, apoptotic loss of islet β-cells may contribute to the occurrence of autoantibody-negative insulin-dependent diabetes in individuals with non-functional ENT3 mutations.

High yield expression and purification of equilibrative nucleoside transporter 7 (ENT7) from Arabidopsis thaliana

BACKGROUND

Equilibrative nucleoside transporters (ENTs) facilitate the import of nucleosides and their analogs into cells in a bidirectional, non-concentrative manner. However, in contrast to their name, most characterized plant ENTs act in a concentrative manner. A direct characterization of any ENT protein has been hindered due to difficulties in overexpression and obtaining pure recombinant protein.

METHODS

The equilibrative nucleoside transporter 7 from Arabidopsis thaliana (AtENT7) was expressed in Xenopus laevis oocytes to assess mechanism of substrate uptake. Recombinant protein fused to enhanced green fluorescent protein (eGFP) was expressed in Pichia pastoris to characterize its oligomeric state by gel filtration and substrate binding by microscale thermophoresis (MST).

RESULTS

AtENT7 expressed in X. laevis oocytes works as a classic equilibrative transporter. The expression of AtENT7-eGFP in the P. pastoris system yielded milligram amounts of pure protein that exists as stable homodimers. The concentration dependent binding of purine and pyrimidine nucleosides to the purified recombinant protein, assessed by MST, confirmed that AtENT7-eGFP is properly folded. For the first time the binding of nucleobases was observed for AtENT7.

SIGNIFICANCE

The availability of pure recombinant AtENT7 will permit detailed kinetic and structural studies of this unique member of the ENT family and, given the functional similarity to mammalian ENTs, will serve as a good model for understanding the structural basis of translocation mechanism for the family.

Novel homozygous SLC29A3 mutations among two unrelated Egyptian families with spectral features of H-syndrome

OBJECTIVES

H syndrome and pigmented hypertrichosis with insulin-dependent diabetes mellitus (PHID) had been described as two autosomal recessive disorders. We aim to screen for pathogenic SLC29A3 mutations in two unrelated Egyptian families with affected siblings of these overlapping syndromes.

METHODS

Clinical, laboratory, histopathological, and radiological characteristics of individuals probably diagnosed as H and/or PHID syndrome were reported. Mutation analysis of SLC29A3 gene was performed for all members of the two Egyptian families.

RESULTS

All affected individuals were females; proband of family-I (A1961) displayed overlapping features of H syndrome and PHID, while her younger brother (A1962) was asymptomatic. A1961 presented with previously undescribed features; absent pectoralis major muscle and a supracondylar bony spur in left humerus. In family-II, probands (A1965 and A1966) had clinical features consistent with classical H syndrome with unique early onset of cutaneous phenomena at birth. Mutation analysis of SLC29A3 revealed homozygous mutation previously reported in literature c.1279G>A [p.G427S] in A1961 and unexpectedly in the asymptomatic A1962 of family-I. Probands of family-II were homozygous for a novel mutation c.401G>A [p.R134H], in the same codon that was published in an Indian boy [p.R134C].

CONCLUSIONS

We emphasize the inter- and intra-familial genetic heterogeneity among Egyptian patients with overlapping features of SLC29A3 disorders. This suggests the presence of other factors like regulatory genes or epigenetic factors that may explain variable disease manifestations and severity.

Nucleoside transporter proteins as biomarkers of drug responsiveness and drug targets

Nucleoside and nucleobase analogs are currently used in the treatment of solid tumors, lymphoproliferative diseases, viral infections such as hepatitis and AIDS, and some inflammatory diseases such as Crohn. Two gene families are implicated in the uptake of nucleosides and nucleoside analogs into cells, SCL28 and SLC29. The former encodes hCNT1, hCNT2, and hCNT3 proteins. They translocate nucleosides in a Na⁺ coupled manner with high affinity and some substrate selectivity, being hCNT1 and hCNT2 pyrimidine- and purine-preferring, respectively, and hCNT3 a broad selectivity transporter. SLC29 genes encode four members, being hENT1 and hENT2 the only two which are unequivocally implicated in the translocation of nucleosides and nucleobases (the latter mostly via hENT2) at the cell plasma membrane. Some nucleoside-derived drugs can also interact with and be translocated by members of the SLC22 gene family, particularly hOCT and hOAT proteins. Inter-individual differences in transporter function and perhaps, more importantly, altered expression associated with the disease itself might modulate the transporter profile of target cells, thereby determining drug bioavailability and action. Drug transporter pharmacology has been periodically reviewed. Thus, with this contribution we aim at providing a state-of-the-art overview of the clinical evidence generated so far supporting the concept that these membrane proteins can indeed be biomarkers suitable for diagnosis and/or prognosis. Last but not least, some of these transporter proteins can also be envisaged as drug targets, as long as they can show “transceptor” functions, in some cases related to their role as modulators of extracellular adenosine levels, thereby providing a functional link between P1 receptors and transporters.

Naked-eye and fluorescence detection of basic pH and F−with a 1,8-naphthalimide-based multifunctional probe

A novel multifunctional colorimetric and fluorescent 1,8-naphthalimide-based probe toward basic pH and F⁻has been developed.

An aptamer intrinsically comprising 5-fluoro-2'-deoxyuridine for targeted chemotherapy

An aptamer specifically binding the interleukin-6 receptor and intrinsically comprising multiple units of the nucleoside analogue 5-fluoro-2'-deoxyuridine can exert a cytostatic effect direcly on certain cells presenting the receptor. Thus the modified aptamer fulfils the requirements for active drug targeting in an unprecedented manner. It can easily be synthesized in a single enzymatic step and it binds to a cell surface receptor that is conveyed into the lysosome. Upon degradation of the aptamer by intracellular nucleases the active drug is released within the targeted cells exclusively. In this way the aptamer acts as a prodrug meeting two major prerequisites of a drug delivery system: specific cell targeting and the controlled release of the drug triggered by an endogenous stimulus.

Nucleoside transporters in the purinome

The purinome is a rich complex of proteins and cofactors that are involved in fundamental aspects of cellular homeostasis and cellular responses. The purinome is evolutionarily ancient and is made up of thousands of members. Our understanding of the mechanisms linking some parts of this complex network and the physiological relevance of the various connections is well advanced. However, our understanding of other parts of the purinome is less well developed. Our research focuses on the adenosine or nucleoside transporters (NTs), which are members of the membrane purinome. Nucleoside transporters are integral membrane proteins that are responsible for the flux of nucleosides, such as adenosine, and nucleoside analog drugs, used in a variety of anti-cancer, anti-viral and anti-parasite therapies, across cell membranes. Nucleoside transporters form the SLC28 and SLC29 families of solute carriers and the protein members of these families are widely distributed in human tissues including the central nervous system (CNS). NTs modulate purinergic signaling in the CNS primarily through their effects on modulating prevailing adenosine levels inside and outside the cell. By clearing the extracellular milieu of adenosine, NTs can terminate adenosine receptor-dependent signaling and this raises the possibility of regulatory feedback loops that tie together receptor signaling with transporter function. Despite the important role of NTs as modulators of purinergic signaling in the human body, very little is known about the nature or underlying mechanisms of regulation of either the SLC28 or SLC29 families, particularly within the context of the CNS purinome. Here we provide a brief overview of our current understanding of the regulation of members of the SLC29 family and highlight some interesting avenues for future research.

Ophthalmologic Findings in H Syndrome: A Unique Diagnostic Clue

BACKGROUND

H syndrome is an autosomal recessive histiocytosis with multisystemic involvement caused by mutations in the SLC29A3 gene. The term H syndrome was coined to denote the major clinical findings which include hyperpigmentation, hypertrichosis, hearing loss, hepatosplenomegaly, hypogonadism, hyperglycemia/diabetes mellitus and hallux valgus/flexion contractures. Almost 100 individuals affected with this disorder have been reported, however, a thorough evaluation of the ophthalmologic features of H syndrome has not yet been performed.

MATERIALS AND METHODS

Ophthalmic examination of a 50-year-old male with H syndrome. Mutation analysis of SLC29A3 was also performed in this patient.

RESULTS

Ophthalmic findings included; shallow orbits with exorbitism, bilateral pterygium, limbal thickening, corneal arcus and cortical cataract. We also review ophthalmologic findings in previously reported H syndrome patients.

CONCLUSIONS

The presence of dilated lateral scleral vessels, corneal arcus and shallow orbits should raise the suspicion of H syndrome, especially when seen in young age.

The Concise Guide to PHARMACOLOGY 2013/14: transporters

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Transporters are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

The human concentrative and equilibrative nucleoside transporter families, SLC28 and SLC29

Nucleoside transport in humans is mediated by members of two unrelated protein families, the SLC28 family of cation-linked concentrative nucleoside transporters (CNTs) and the SLC29 family of energy-independent, equilibrative nucleoside transporters (ENTs). These families contain three and four members, respectively, which differ both in the stoichiometry of cation coupling and in permeant selectivity. Together, they play key roles in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis. Moreover, they facilitate cellular uptake of several nucleoside and nucleobase drugs used in cancer chemotherapy and treatment of viral infections. Thus, the transporter content of target cells can represent a key determinant of the response to treatment. In addition, by regulating the concentration of adenosine available to cell surface receptors, nucleoside transporters modulate many physiological processes ranging from neurotransmission to cardiovascular activity. This review describes the molecular and functional properties of the two transporter families, with a particular focus on their physiological roles in humans and relevance to disease treatment.

Agenesis of the inferior vena cava in H syndrome due to a novel SLC29A3 mutation

We present a 10-year-old girl with typical clinical features of H syndrome. Complete agenesis of the inferior vena cava was found on echocardiography and radiologic studies. Mutation analysis of the SLC29A3 gene revealed a novel nonsense mutation. This unique case extends the clinical and mutation spectrum associated with H syndrome and underlines the importance of routine cardiac screening in this disorder.

Regulation of lysosome biogenesis and functions in osteoclasts

In order to resorb the mineralized bone extracellular matrix, the osteoclast relies on the generation of a resorption lacuna characterized by the presence of specific proteases and a low pH. Hence, bone resorption by osteoclasts is highly dependent on lysosomes, the organelles specialized in intra- and extracellular material degradation. This is best illustrated by the fact that multiple forms of human osteopetrosis are caused by mutations in genes encoding for lysosomal proteins. Yet, until recently, the molecular mechanisms regulating lysosomal biogenesis and function in osteoclasts were poorly understood. Here we review the latest developments in the study of lysosomal biogenesis and function in osteoclasts with an emphasis on the transcriptional control of these processes.

The Thr224Asn mutation in the VPS45 gene is associated with the congenital neutropenia and primary myelofibrosis of infancy

VPS45 is a new gene associated with severe infections and bone marrow failure in infancy that can be treated by bone marrow transplantation. The mutation affects intracellular storage and transport and results in increased programmed cell death in neutrophils and bone marrow.

Just getting into cells is not enough: mechanisms underlying 4-(N)-stearoyl gemcitabine solid lipid nanoparticle's ability to overcome gemcitabine resistance caused by RRM1 overexpression

Gemcitabine is a deoxycytidine analog that is widely used in the chemotherapy of many solid tumors. However, acquired tumor cell resistance often limits its use. Previously, we discovered that 4-(N)-stearoyl gemcitabine solid lipid nanoparticles (4-(N)-GemC18-SLNs) can overcome multiple acquired gemcitabine resistance mechanisms, including RRM1 overexpression. The present study was designed to elucidate the mechanisms underlying the 4-(N)-GemC18-SLNs' ability to overcome gemcitabine resistance. The 4-(N)-GemC18 in the 4-(N)-GemC18-SLNs entered tumor cells due to clathrin-mediated endocytosis of the 4-(N)-GemC18-SLNs into the lysosomes of the cells, whereas the 4-(N)-GemC18 alone in solution entered cells by diffusion. We substantiated that it is the way the 4-(N)-GemC18-SLNs deliver the 4-(N)-GemC18 into tumor cells that allows the gemcitabine hydrolyzed from the 4-(N)-GemC18 to be more efficiently converted into its active metabolite, gemcitabine triphosphate (dFdCTP), and thus more potent against gemcitabine-resistant tumor cells than 4-(N)-GemC18 or gemcitabine alone. Moreover, we also showed that the RRM1-overexpressing tumor cells were also cross-resistant to cytarabine, another nucleoside analog commonly used in cancer therapy, and 4-(N)-stearoyl cytarabine carried by solid lipid nanoparticles can also overcome the resistance. Therefore, formulating the long-chain fatty acid amide derivatives of nucleoside analogs into solid lipid nanoparticles may represent a platform technology to increase the antitumor activity of the nucleoside analogs and to overcome tumor cell resistance to them.

Mutation in the SLC29A3 gene: a new cause of a monogenic, autoinflammatory condition

Germline mutations in the SLC29A3 gene result in a range of recessive, clinically related syndromes: H syndrome, pigmented hypertrichosis with insulin-dependent diabetes mellitus syndrome, Faisalabad histiocytosis, and sinus histiocytosis with massive lymphadenopathy. The main symptoms of these diseases are hyperpigmentation with hypertrichosis, sensorineural deafness, diabetes, short stature, uveitis, and Rosai-Dorfman like histiocytosis. Here, we report the case of an 11-month-old boy with early-onset, recurrent episodes of unprovoked fever lasting 7 to 10 days and associated with pericardial effusion, abdominal pain, diarrhea, and inflammation. Physical examination revealed hyperpigmentation with hypertrichosis, dysmorphic features, and spleen and liver enlargement. Failure to thrive, sensorineural deafness, retarded psychomotor development, and a Rosai-Dorfman like cheek lesion developed subsequently. The febrile episodes did not respond to tumor necrosis factor α antagonists and interleukin-1. Sequencing of the SLC29A3 gene revealed a homozygous missense mutation c.1088G>A (p.Arg363Gln). These observations suggest that a newly identified mutation in the SLC29A3 gene may be associated with an autoinflammatory disorder. Genetic defects in SLC29A3 should be considered in patients with autoinflammatory manifestations, recurrent febrile attacks, and 1 or more of the symptoms found in the broad spectrum of SLC29A3-related disorders (especially hyperpigmentation with hypertrichosis).

Whole-exome sequencing identifies mutations in the nucleoside transporter gene SLC29A3 in dysosteosclerosis, a form of osteopetrosis

Dysosteosclerosis (DSS) is the form of osteopetrosis distinguished by the presence of skin findings such as red-violet macular atrophy, platyspondyly and metaphyseal osteosclerosis with relative radiolucency of widened diaphyses. At the histopathological level, there is a paucity of osteoclasts when the disease presents. In two patients with DSS, we identified homozygous or compound heterozygous missense mutations in SLC29A3 by whole-exome sequencing. This gene encodes a nucleoside transporter, mutations in which cause histiocytosis-lymphadenopathy plus syndrome, a group of conditions with little or no skeletal involvement. This transporter is essential for lysosomal function in mice. We demonstrate the expression of Slc29a3 in mouse osteoclasts in vivo. In monocytes from patients with DSS, we observed reduced osteoclast differentiation and function (demineralization of calcium surface). Our report highlights the pleomorphic consequences of dysfunction of this nucleoside transporter, and importantly suggests a new mechanism for the control of osteoclast differentiation and function.