James Young (PhD, University of Edinburgh)
7-25A Medical Sciences Building
University of Alberta
Edmonton, Alberta Canada T6G 2H7
Killam Annual Professorship, University of Alberta 2010-2011
AHFMR (AI-HS) Senior Investigator, University of Alberta 2010-2012
AHFMR Scientist, University of Alberta 1990-2010
Director, Membrane Protein Research Group*, University of Alberta 2005-2007
Mechanistic, physiologic and therapeutic studies of nucleoside transport proteins
Accounting for an average of 10% of genes across species, membrane transport proteins fulfill essential biological functions and are intimately involved in the development, progression and treatment of human disease. In his research, Dr Young has discovered and characterized the proteins responsible for the cellular uptake and release of nucleosides and their derivatives. As precursors of nucleotides, nucleosides are central cellular metabolites and several (e.g., adenosine) also have diverse physiological functions through interactions with cell-surface purinergic receptors. Nucleoside analogs are used to treat viral infections and many types of cancer, including leukemias, lymphomas and various solid tumors. Nucleosides, including those with antineoplastic activities, are hydrophilic molecules, and transportability across cell membranes via specialized nucleoside transport proteins is a critical determinant of their metabolism and pharmacologic actions. Two mechanisms of nucleoside transport have been identified in human and other mammalian cells and tissues: bidirectional equilibrative processes driven by chemical gradients, and concentrative processes driven by electrochemical gradients.
Dr Young has been an international leader in nucleoside transport for more than 30 years. Below are described some of his major contributions to this field of research.
Contribution 1. A breakthrough series of papers describing the molecular identification of the membrane proteins responsible for the three main concentrative (Na+-dependent) nucleoside transport processes of human (h) and other mammalian cells. The proteins belong to a new family which we designated as CNT (SLC28 in humans). hCNT1-3 transport both physiologic nucleosides and anticancer and antiviral nucleoside drugs. CNTs are evolutionarily old and widely distributed in both eukaryotes and prokaryotes [see e.g. Cloning and functional expression of a complementary DNA encoding a mammalian nucleoside transport protein. J. Biol. Chem. 269:17757-17760 (1994) for the first paper in this series].
Contribution 2. A breakthrough series of papers describing the molecular identification of the corresponding membrane proteins responsible for the two main equilibrative (Na+-independent) nucleoside transport processes of human (h) and other mammalian cells. The proteins belong to a new family which we designated as ENT (SLC29 in humans). hENT1 and hENT2, like their CNT counterparts, transport both physiologic nucleosides and anticancer and antiviral nucleoside drugs. ENTs are widely distributed in eukaryotes [see e.g. Cloning of a human nucleoside transporter implicated in the cellular uptake of adenosine and chemotherapeutic drugs. Nature Medicine 3:89-93 (1997) for the first paper in this series]. Most recently, we have identified two further human ENT isoforms (hENT3, hENT4), one of which (hENT3) is intracellular in localization. Both are activated at acidic pH, and may be proton-coupled [listed in Selected Recent Publications].
Contribution 3. An on-going series of chimeric, mutagenesis and heterologous expression studies to understand structure/function relationships in CNT and ENT nucleoside transport proteins [see e.g. Identification of amino acid residues responsible for the pyrimidine and purine nucleoside specificities of human Na+-nucleoside cotransporters hCNT1 and hCNT2. J. Biol. Chem. 274:24475-24484 (1999) for the first paper in this series]. Other more recent molecular studies of CNTs and ENTs can be found in Selected Recent Publications.
Contribution 4. Use of steady-state and presteady-state two microelectrode voltage-clamp electrophysiology in Xenopus oocytes to study the cation-selectivity, coupling ratios, turnover number, partial reactions of the transport cycle and other properties of wild-type and mutant human and other CNTs not accessible by radioisotope flux other techniques [see e.g. Electrophysiological characterization of a recombinant human Na+-coupled nucleoside transporter (hCNT1) produced in Xenopus oocytes. J. Physiol. (Lond.) 558:807-823 (2004)]. Other papers in this series can be found in Selected Recent Publications.
Contribution 5. Systematic kinetic analysis of transport of anticancer and antiviral nucleoside drug by different recombinant hCNTs and hENTs produced in the Xenopus oocyte heterologous expression system. In addition to radioisotope flux measurements, these studies use electrophysiology to demonstrate hCNT-mediated drug transport [see e.g. Gemcitabine transport mediated by recombinant plasma membrane mammalian nucleoside transporters expressed in Xenopus oocytes. J. Natl. Cancer Inst. 91:1876-1881 (1999) for the first paper in this series]. Other more recent papers describing recombinant CNT and ENT transport of anticancer and antiviral nucleoside drugs and other nucleoside analogs can be found in Selected Recent Publications.
Contribution 6. A landmark series of collaborative studies with clinical colleagues that establish hENT1 and hCNT3 as strong predictive markers of patient outcome following anticancer nucleoside drug chemotherapy. In pancreatic cancer, for example, dual high expression of hCNT3 and hENT1 correlate with a dramatic 8-fold increase in patient median overall survival time from 12 to 95 months [see e.g. The absence of human equilibrative nucleoside transporter 1 (hENT1) is associated with reduced survival in patients with gemcitabine-treated pancreas adenocarcinoma. Clin. Cancer Res. 10:6956-6961 (2004) for the first paper in this series]. Other recent papers in this series can be found in Selected Recent Publications.
Damaraju, V, Surapaneni, S, Kumar, G, Ng, AM, Yao, SYM, Young, JD, Cass, CE and Tong, Z. (2012). Role of human nucleoside transporters in the uptake and cytotoxicity of azacitidine and decitabine. Nucleosides, Nucleotides & Nucleic Acids 31:236-255.
Huysmans, GH, Chan, N, Baldwin JM, Postis, VL, Tzokov, SB, Deacon, SE, Yao, SYM, Young, JD, McPherson, MJ, Bullough, PA, Baldwin, SA. (2012). A urea channel from Bacillus cereus reveals a novel hexameric structure. Biochem. J. 445:157-166.
Witkowska K, Smith, KM, Yao, SYM, Ng, AML, O.Neill, D, Karpinski, E, Young, JD, Cheeseman, CI. (2012). Human SLC2A9a and SLC2A9b isoforms mediate electrogenic transport of urate with different characteristics in the presence of hexoses. Am. J. Physiol. Renal Physiol. 303:F527-539.
Vaziri, H, Baldwin, SA, Baldwin JM, Adams DG, Young, JD, Postis, VL. (2012). Use of molecular modelling to probe the mechanism of the nucleoside transporter NupG. Mol. Membr. Biol. [Epub ahead of print, Dec. 21] .
Damaraju, V.L., Smith, K.M., Mowles, D., Nowak, I., Karpinski, E., Young, J.D., Robins, M. and Cass C.E. Interaction of fused-pyrimidine nucleoside analogs with human concentrative nucleoside transporter 1. Mol. Pharmacol. 81:82-90 (2011). PubMed Accession Number: 20854794. [Journal Impact factor 4.53]
Graham, K., Yao, S.Y.M., Johnson, L., Mowles, D., Ng, A.M.L., Wilkinson, J., Young, J.D. and Cass, C.E. Nucleoside transporter gene expression in wild-type and mENT1 knockout mice. Biochem. Cell Biol. 89:236-2145 (2011). PubMed Accession Number: 21455274. [Journal Impact factor 2.61]
Kost, S., Sun, C., Graham, K., Cass, C.E., Young, J.D., Albensi, B.C. and Parkinson, F.E. Behavioral effects of elevated expression of human equilibrative nucleoside transporter 1 in mice. Behav. Brain Res. 224:44-49 (2011). PubMed Accession Number: 21645551. [Journal Impact factor 3.22]
Yao, S.Y.M., Ng, A.M.L., Cass, C.E., Baldwin, S.A. and Young, J.D. Nucleobase transport by human equilibrative nucleoside transporter 1 (hENT1). J. Biol. Chem. 286:32552-32562 (2011). PubMed Accession Number: 21795683. [Journal Impact factor 5.52]
Damaraju, V.J., Mowles, D., Smith, K.M., Yao, S.Y.M., Young, J.D., Marquez, V.E. and Cass, C.E. Influence of sugar ring conformation on transportability of nucleosides by human nucleoside transporters. Chem. Bio. Chem. [Epub ahead of print, Nov. 4, 2011]. PubMed Accession Number: 22052809. [Journal Impact factor 3.82]