Sorting of rat SPNT in renal epithelium is independent of N-glycosylation

Human Concentrative Nucleoside Transporter 3 (hCNT3, SLC28A3) Forms a Cyclic Homotrimer

Many anticancer and antiviral drugs are purine or pyrimidine analogues, which use membrane transporters to cross cellular membranes. Concentrative nucleoside transporters (CNTs) mediate the salvage of nucleosides and the transport of therapeutic nucleoside analogues across plasma membranes by coupling the transport of ligands to the sodium gradient. Of the three members of the human CNT family, CNT3 has the broadest selectivity and the widest expression profile. However, the molecular mechanisms of the transporter, including how it interacts with and translocates structurally diverse nucleosides and nucleoside analogues, are unclear. Recently, the crystal structure of vcCNT showed that the prokaryotic homologue of CNT3 forms a homotrimer. In this study, we successfully expressed and purified the wild type human homologue, hCNT3, demonstrating the homotrimer by size exclusion profiles and glutaraldehyde cross-linking. Further, by creating a series of cysteine mutants at highly conserved positions guided by comparative structure models, we cross-linked hCNT3 protomers in a cell-based assay, thus showing the existence of hCNT3 homotrimers in human cells. The presence and absence of cross-links at specific locations along TM9 informs us of important structural differences between vcCNT and hCNT3. Comparative modeling of the trimerization domain and sequence coevolution analysis both indicate that oligomerization is critical to the stability and function of hCNT3. In particular, trimerization appears to shorten the translocation path for nucleosides across the plasma membrane and may allow modulation of the transport function via allostery.

Glycosylation of solute carriers: mechanisms and functional consequences

Solute carriers (SLCs) are one of the largest groups of multi-spanning membrane proteins in mammals and include ubiquitously expressed proteins as well as proteins with highly restricted tissue expression. A vast number of studies have addressed the function and organization of SLCs as well as their posttranslational regulation, but only relatively little is known about the role of SLC glycosylation. Glycosylation is one of the most abundant posttranslational modifications of animal proteins and through recent advances in our understanding of protein-glycan interactions, the functional roles of SLC glycosylation are slowly emerging. The purpose of this review is to provide a concise overview of the aspects of glycobiology most relevant to SLCs, to discuss the roles of glycosylation in the regulation and function of SLCs, and to outline the major open questions in this field, which can now be addressed given major technical advances in this and related fields of study in recent years.

Human proteins with target sites of multiple post-translational modification types are more prone to be involved in disease

Many proteins can be modified by multiple types of post-translational modifications (Mtp-proteins). Although some post-translational modifications (PTMs) have recently been found to be associated with life-threatening diseases like cancers and neurodegenerative disorders, the underlying mechanisms remain enigmatic to date. In this study, we examined the relationship of human Mtp-proteins and disease and systematically characterized features of these proteins. Our results indicated that Mtp-proteins are significantly more inclined to participate in disease than proteins carrying no known PTM sites. Mtp-proteins were found significantly enriched in protein complexes, having more protein partners and preferred to act as hubs/superhubs in protein-protein interaction (PPI) networks. They possess a distinct functional focus, such as chromatin assembly or disassembly, and reside in biased, multiple subcellular localizations. Moreover, most Mtp-proteins harbor more intrinsically disordered regions than the others. Mtp-proteins carrying PTM types biased toward locating in the ordered regions were mainly related to protein-DNA complex assembly. Examination of the energetic effects of PTMs on the stability of PPI revealed that only a small fraction of single PTM events influence the binding energy of >2 kcal/mol, whereas the binding energy can change dramatically by combinations of multiple PTM types. Our work not only expands the understanding of Mtp-proteins but also discloses the potential ability of Mtp-proteins to act as key elements in disease development.

Structural determinants for rCNT2 sorting to the plasma membrane of polarized and non-polarized cells

rCNT2 (rat concentrative nucleoside transporter 2) (Slc28a2) is a purine-preferring concentrative nucleoside transporter. It is expressed in both non-polarized and polarized cells, where it is localized in the brush border membrane. Since no information about the domains implicated in the plasma membrane sorting of rCNT2 is available, the present study aimed to identify structural and functional requirements for rCNT2 trafficking. The comprehensive topological mapping of the intracellular N-terminal tail revealed two main features: (i) a glutamate-enriched region (NPGLELME) between residues 21 and 28 that seems to be implicated in the stabilization of rCNT2 in the cell surface, since mutagenesis of these conserved glutamates resulted in enhanced endocytosis; and (ii) mutation of a potential protein kinase CK2 domain that led to a loss of brush border-specific sorting. Although the shortest proteins assayed (rCNT2-74AA, -48AA and -37AA) accumulated intracellularly and lost their brush border membrane preference, they were still functional. A deeper analysis of CK2 implication in CNT2 trafficking, using a CK2-specific inhibitor [DMAT (2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole)] and other complementary mutations mimicking the negative charge provided by phosphorylation (S46D and S46E), demonstrated an effect of this kinase on rCNT2 activity. In summary, the N-terminal tail of rCNT2 contains dual sorting signals. An acidic region is responsible for its proper stabilization at the plasma membrane, whereas the putative CK2 domain (Ser46) is implicated in the apical sorting of the transporter.

Link between high-affinity adenosine concentrative nucleoside transporter-2 (CNT2) and energy metabolism in intestinal and liver parenchymal cells

Concentrative nucleoside transporter 2 (CNT2) is a high-affinity adenosine transporter that may play physiological roles beyond nucleoside salvage. Previous reports relate CNT2 function to modulation of purinergic signaling and energy metabolism in intestinal and liver parenchymal cells (Duflot et al., 2004, Mol Cell Biol 24:2710-2719; Aymerich et al., 2006, J Cell Sci 119:1612-1621). In the present study, to further examine the link between CNT2 and energy metabolism, CNT2 protein partners were identified using the bacterial two-hybrid and GST pull-down approaches. The N-terminal segment of CNT2 was used as bait, since proteins lacking this domain display impaired plasma membrane insertion and intracellular retention. Glucose-regulated protein 58 (GRP58) was identified as a potential rCNT2 partner in pull-down experiments. Two-hybrid screening performed against a liver human cDNA library led to the identification of aldolase B as another hCNT2 partner. Aldolase B-RFP and endogenous GRP58 separately co-localized with CNT2 in HeLa cells transfected with YFPrCNT2. CNT2 interaction with GRP58 was validated using co-immunoprecipitation experiments. In HeLa cells, fluorescence resonance energy transfer (FRET) efficiency increased upon fructose addition, consistent with a transient interaction between aldolase B and the transporter. The physiological basis for in vivo interactions was derived from experiments in which GRP58 was inhibited or overexpressed and aldolase B activity stimulated towards glycolysis. GRP58 appeared to be a negative effector of CNT2 function, whereas aldolase B flux modulated CNT2 activity via a mechanism involving acquisition of higher affinity for its substrates. These findings support the theory that CNT2 plays roles other than salvage and establishes links with energy metabolism.

SLC28 genes and concentrative nucleoside transporter (CNT) proteins

The human concentrative nucleoside transporter (hCNT) protein family has three members, hCNT1, 2, and 3, encoded by SLC28A1, A2, and A3 genes, respectively. hCNT1 and hCNT2 translocate pyrimidine- and purine-nucleosides, respectively, by a sodium-dependent mechanism, whereas hCNT3 shows broad substrate selectivity and the unique ability of translocating nucleosides both in a sodium- and a proton-coupled manner. hCNT proteins are also responsible for the uptake of most nucleoside-derived antiviral and anticancer drugs. Thus, hCNTs are key pharmacological targets. This review focuses on several crucial aspects of hCNT biology and pharmacology: protein structure-function, structural determinants for transportability, pharmacogenetics of hCNT-encoding genes, role of hCNT proteins in nucleoside-based therapeutics, and finally hCNT physiology.

Role of N-glycosylation in trafficking of apical membrane proteins in epithelia

Polarized distribution of plasma membrane transporters and receptors in epithelia is essential for vectorial functions of epithelia. This polarity is maintained by sorting of membrane proteins into apical or basolateral transport containers in the trans-Golgi network and/or endosomes followed by their delivery to the appropriate plasma membrane domains. Sorting depends on the recognition of sorting signals in proteins by specific sorting machinery. In the present review, we summarize experimental evidence for and against the hypothesis that N-glycans attached to the membrane proteins can act as apical sorting signals. Furthermore, we discuss the roles of N-glycans in the apical sorting event per se and their contribution to folding and quality control of glycoproteins in the endoplasmic reticulum or retention of glycoproteins in the plasma membrane. Finally, we review existing hypotheses on the mechanism of apical sorting and discuss the potential roles of the lectins, VIP36 and galectin-3, as putative apical sorting receptors.

The role of nucleoside transporters in cancer chemotherapy with nucleoside drugs

Nucleoside analogs are important components of treatment regimens for various malignancies. Nucleoside-specific membrane transporters mediate plasma membrane permeation of physiologic nucleosides and most nucleoside analogs, for which the initial event is cellular conversion of nucleosides to active agents. Understanding of the roles of nucleoside transporters in nucleoside drug toxicity and resistance will provide opportunities for potentiating anticancer efficacy and avoiding resistance. Because transportability is a possible determinant of toxicity and resistance of many nucleoside analogs, nucleoside transporter abundance might be a prognostic marker to assess drug resistance. Elucidation of the structural determinants of nucleoside analogs for interaction with transporter proteins as well as the structural features of transporter proteins required for permeant interaction and translocation will lead to "transportability guidelines" for the rational design and therapeutic application of nucleoside analogs as anticancer drugs. It should eventually be possible to develop clinical assays that predict sensitivity and/or resistance to nucleoside anti-cancer drugs and thus to identify those patient populations that will most likely benefit from optimal nucleoside analog treatments. This review discusses recent results from structure/function studies of human nucleoside transporters, the role of nucleoside transport processes in the cytotoxicity and resistance of several anticancer nucleoside analogs and strategies to improve the nucleoside transporter-related anticancer effects of nucleoside analogs.

Renal nucleoside transporters: physiological and clinical implicationsThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

Renal handling of physiological and pharmacological nucleosides is a major determinant of their plasma levels and tissue availabilities. Additionally, the pharmacokinetics and normal tissue toxicities of nucleoside drugs are influenced by their handling in the kidney. Renal reabsorption or secretion of nucleosides is selective and dependent on integral membrane proteins, termed nucleoside transporters (NTs) present in renal epithelia. The 7 known human NTs (hNTs) exhibit varying permeant selectivities and are divided into 2 protein families: the solute carrier (SLC) 29 (SLC29A1, SLC29A2, SLC29A3, SLC29A4) and SLC28 (SLC28A1, SLC28A2, SLC28A3) proteins, otherwise known, respectively, as the human equilibrative NTs (hENTs, hENT1, hENT2, hENT3, hENT4) and human concentrative NTs (hCNTs, hCNT1, hCNT2, hCNT3). The well characterized hENTs (hENT1 and hENT2) are bidirectional facilitative diffusion transporters in plasma membranes; hENT3 and hENT4 are much less well known, although hENT3, found in lysosomal membranes, transports nucleosides and is pH dependent, whereas hENT4–PMAT is a H+-adenosine cotransporter as well as a monoamine–organic cation transporter. The 3 hCNTs are unidirectional secondary active Na+-nucleoside cotransporters. In renal epithelial cells, hCNT1, hCNT2, and hCNT3 at apical membranes, and hENT1 and hENT2 at basolateral membranes, apparently work in concert to mediate reabsorption of nucleosides from lumen to blood, driven by Na+ gradients. Secretion of some physiological nucleosides, therapeutic nucleoside analog drugs, and nucleotide metabolites of therapeutic nucleoside and nucleobase drugs likely occurs through various xenobiotic transporters in renal epithelia, including organic cation transporters, organic anion transporters, multidrug resistance related proteins, and multidrug resistance proteins. Mounting evidence suggests that hENT1 may have a presence at both apical and basolateral membranes of renal epithelia, and thus may participate in both selective secretory and reabsorptive fluxes of nucleosides. In this review, the renal handling of nucleosides is examined with respect to physiological and clinical implications for the regulation of human kidney NTs and adenosine signaling, intracellular nucleoside transport, and nephrotoxicities associated with some nucleoside drugs.

The Role of N-Linked Glycosylation in Protein Folding, Membrane Targeting, and Substrate Binding of Human Organic Anion Transporter hOAT4

We used a novel approach to evaluate how the addition/acquisition and processing/modification of N-linked oligosaccharides play a role in the functional maturation of human organic anion transporter hOAT4. Inhibition of acquisition of oligosaccharides in hOAT4 by mutating asparagine to glutamine and by tunicamycin treatment was combined with the expression of wild-type hOAT4 in a series of mutant Chinese hamster ovary (CHO)-Lec cells defective in the different steps of glycosylation processing. We showed that both the disruption of the glycosylation sites by mutagenesis and the inhibition of glycosylation by tunicamycin treatment resulted in a nonglycosylated hOAT4, which was unable to target to the cell surface. In contrast, hOAT4 synthesized in mutant CHO-Lec cells, carrying different structural forms of sugar moieties (mannose-rich in Lec1 cells, sialic acid-deficient in Lec2 cells, and sialic acid/galactose-deficient in Lec8 cells) were able to traffic to the cell surface. However, hOAT4 expressed in CHO-Lec1 cells had significantly lower binding affinity for its substrates compared with that expressed in parental CHO cells. This study provided novel information that addition/acquisition of oligosaccharides but not the processing of the added oligosaccharides participates in the membrane insertion of hOAT4. Processing of added oligosaccharides from mannose-rich type to complex type is important for enhancing the binding affinity of hOAT4 for its substrates. Glycosylation could therefore serve as a means to specifically regulate hOAT4 function in vivo.

A mouse model of sitosterolemia: absence of Abcg8/sterolin-2 results in failure to secrete biliary cholesterol

BACKGROUND

Mutations in either of two genes comprising the STSL locus, ATP-binding cassette (ABC)-transporters ABCG5 (encoding sterolin-1) and ABCG8 (encoding sterolin-2), result in sitosterolemia, a rare autosomal recessive disorder of sterol trafficking characterized by increased plasma plant sterol levels. Based upon the genetics of sitosterolemia, ABCG5/sterolin-1 and ABCG8/sterolin-2 are hypothesized to function as obligate heterodimers. No phenotypic difference has yet been described in humans with complete defects in either ABCG5 or ABCG8. These proteins, based upon the defects in humans, are responsible for regulating dietary sterol entry and biliary sterol secretion.

METHODS

In order to mimic the human disease, we created, by a targeted disruption, a mouse model of sitosterolemia resulting in Abcg8/sterolin-2 deficiency alone. Homozygous knockout mice are viable and exhibit sitosterolemia.

RESULTS

Mice deficient in Abcg8 have significantly increased plasma and tissue plant sterol levels (sitosterol and campesterol) consistent with sitosterolemia. Interestingly, Abcg5/sterolin-1 was expressed in both liver and intestine in Abcg8/sterolin-2 deficient mice and continued to show an apical expression. Remarkably, Abcg8 deficient mice had an impaired ability to secrete cholesterol into bile, but still maintained the ability to secrete sitosterol. We also report an intermediate phenotype in the heterozygous Abcg8+/- mice that are not sitosterolemic, but have a decreased level of biliary sterol secretion relative to wild-type mice.

CONCLUSION

These data indicate that Abcg8/sterolin-2 is necessary for biliary sterol secretion and that loss of Abcg8/sterolin-2 has a more profound effect upon biliary cholesterol secretion than sitosterol. Since biliary sitosterol secretion is preserved, although not elevated in the sitosterolemic mice, this observation suggests that mechanisms other than by Abcg8/sterolin-2 may be responsible for its secretion into bile.