Department of Physiology and Biophysics
University of Colorado School of Medicine
RC1 North Tower, P18-7113
Mail Stop 8307
Aurora, CO 80045
Molecular Structure, Function, and Regulation of Ion Channels
Ion channels are a diverse class of membrane transport systems that are essential for electrical signaling in the nervous system. Our group studies the molecular mechanisms that allow these ion channels to function, as well as the means used by the cell to regulate the number, distribution, and properties of ion channels. We focus on the voltage-gated sodium channel, having purified the molecule from the electric organ of the electric eel.
In characterizing the purified molecule, we have found that the sodium channel is comprised of a single large polypeptide that is highly glycosylated and modified by fatty acids. These extensive nonprotein domains are added to the channel structure posttranslationally, and there are strong indications that they may form part of the molecular mechanisms of voltage-sensing and ion conduction. In particular, we have found that removal of some of the large number of negatively charged sialic sugars greatly reduces the sensitivity of the channel to voltage changes. In addition, the complex opening/closing transitions of the channel are affected by the membrane lipid composition that surrounds the channel. Thus, we suggest that nonprotein domains must be considered as part of the functional apparatus of the sodium channel. We are also studying the biosynthesis of channels during development and pathological degeneration. For example, one project investigates changes in sodium channel distributions that occur during demyelination of nerves; the results obtained suggest that increased sodium channel synthesis may explain remissions experienced by patients suffering from multiple sclerosis. Other projects focus on the ways in which sodium channels are transported from nerve cell bodies to the remote domains on axons where they are needed for impulse conduction. An unusual aspect of this work concerns the role that glial cell-axon interactions play in controlling channel biosynthesis and localization at the nodes of Ranvier in myelinated nerves (as seen in orange bands in figure at right) . Lastly, we have found that different types of sodium channels ("isoforms") are differentially distributed in the nervous system. For example, unmyelinated pain fibers appear to have different channels than do myelinated fibers. These findings may be important in the development of drugs to control chronic pain.
In our studies we employ a variety of approaches, including protein biochemistry, recombinant DNA/molecular biology, immunology, and electrophysiology.
Wang, Z-J., Snell, L.D., Tabakoff, B., and Levinson, S.R. (2002), Inhibition of sodium channels by the novel antiepileptic compound DCUKA. Experimental Neurol. 178:129-138
Djouhri, L. Newton, R, Levinson, S.R., Carruthers, B., and Lawson, S.N. (2003) Sensory and electrophysiological properties of guinea-pig sensory neurones expressing Nav1.7 (PN1) Na+ channel subunit protein J. Physiol (Lond) 546: 565-576
Caldwell, J.H., and Levinson, S.R. (2004) The biology of voltage-gated sodium channels, in Molecular and cellular insights to ion channel biology, R.A. Maue, ed., Elsevier (Amsterdam).
Henry, M.A., Rzasa, R.S., Beeler, J.J.,1and Levinson, S.R. (2004) Caspr Reveals an Aggregation of Nodes and Flanking Node Free Zones at the Rat Trigeminal Sensory Root and Dorsal Root Entry Zones, Glia 49: 445-450
Yeomans, D.C., Levinson, S.R., Peters, M.C., Koszowski, A.G., Tzabazis, A.Z., Gilly, M.F., and Wilson, S.P (2005) Decrease in Inflammatory Hyperalgesia by Herpes Vector-Mediated Knockdown of Nav1.7 Sodium Channels in Primary Afferents. Human Gene Therapy 16:271-277
Leao, R.M., Kushmerick, C., Pinaud, R., Renden, R., Li, G-L., Taschenberger, H., Siprou, G., Levinson, S.R., and von Gersdorff, H. (2005) Presynaptic Na+ channels: Locus, development, and recovery from inactivation at a high fidelity synapse. J. Neurosci 25:3724-3738
Henry , M.A., Sorensen, H.J., Johnson, L.R., and Levinson, S.R. (2005) Localization of the Nav1.8 sodium channel isoform at nodes of Ranvier in normal human radicular tooth pulp. Neurosci. Lett. 380:32-36
Henry, M.A., Freking, A.R., Johnson, L.R., and Levinson, S.R. (2006) Increased sodium channel density at myelinated and demyelinated sites following an inflammatory and partial axotomy lesion of the rat infraorbital nerve. Pain 124:222-233
Henry, M.A., Freking, A.R., Johnson, L.R., and Levinson, S.R. (2007) Sodium channel Nav1.6 accumulates at the site of infraorbital nerve injury. BMC Neuroscience 8:56
O’Brien, B.J., Caldwell, J.H., Ehring, G.R., Bumstead O’Brien, K.M., Luo, S., and Levinson, S.R. (2008) Tetrodotoxin resistant voltage-gated sodium channels Nav1.8 and Nav1.9 are expressed in the retina. J. Comp. Neurol.508:940-951
Luo,S., Perry, G. M., Levinson, S.R. and Henry, M.A. (2008) Nav1.7 expression is increased in painful human dental pulp. Mol. Pain 4:16
Kao, C.Y. and Levinson, S.R. (1986), eds. "Tetrodotoxin, Saxitoxin, and the molecular biology of the sodium channel." Annals of the New York Academy of Sciences, volume 479.
Recio-Pinto, E., Duch, D.S., Levinson, S.R. and Urban, B.W. (1987). Purified and unpurified sodium channels from eel electroplax in planar lipid bilayers. J. Gen. Physiol. 90:375-395.
Thornhill, W.B. and Levinson, S.R. (1987). Biosynthesis of electroplax sodium channels in eel electrocytes and Xenopus Oocytes. Biochemistry 26:4381-4388.
Recio-Pinto, E., Duch, D.S., Urban, B.W., Thornhill, W.B. and Levinson, S.R. (1990). Neuraminidase treatment modifies the function of eel sodium channels reconstituted in planar lipid bilayers. Neuron 5:675-684.
Levinson, S.R., Thornhill, W.B., Duch, D.S., Recio-Pinto, E., and Urban, B.W. (1990) The role of nonprotein domains in the function and synthesis of voltage-gated sodium channels.Ion Channels, Vol II. T. Narahashi, ed., Plenum Publishing, pp 33-64
England, J.D., Gamboni, F., Levinson, S.R., and Finger, T.E. (1990) Formation of new distributions of sodium channels along demyelinated axons. Proc. Natl. Acad. Sci. (USA) 87 6777-6780
Ivey, S., Thornhill, W.B., and Levinson, S.R. (1991) Monoclonal antibodies raised against posttranslational domains of the electroplax sodium channel. J.Membr. Biol. 121:215-222
England, J.D., Gamboni, F., and Levinson, S.R. (1991) Increased numbers of sodium channels form along demyelinated axons. Brain Res. 548: 334-337
Levinson, S.R., and Thornhill, W.B.(1992), Use of cell-free translation systems and pulse labeling methods to study ion channel biosynthesis and trafficking", Methods in Enzymology 207: 659-670
England, J.D., Gamboni, F., Ferguson, M.A., and Levinson, S.R.(1994) Sodium channels accumulate at the tips of injured axons. Muscle & Nerve 17: 593-598
Levinson, S.R., (1995) The structure and mechanism of voltage-gated ion ion channels, Invited chapter, Principles of Physiology and Biophysics, N. Sperelakis, ed., Academic Press
Turner, R.W., Maler, L., Deerinck, Levinson, S.R., and Ellisman, M.H., TTX-sensitive sodium channels underlie oscillatory discharge in a vertebrate sensory neuron. (1994) J. Neuroscience 14: 6453-6471
Dugandzija-Novakovic, S., Koszowski, A. G., Levinson, S.R., and Shrager, P. (1995) Clustering of Na+ channels and node of Ranvier formation in remyelinating axons. J. Neuroscience 15:492-503
Tzoumaka, E-E., Novakovic, S.D., Levinson, S.R., and Shrager, P. (1995) Na+ channel aggregation in remyelinating mouse sciatic axons following transection. Glia,15:188-194
England, J.D., Shrager, P., and Levinson, S.R. (1996) Immunocytochemical investigations of the sodium channels along nodal and paranodal portions of demyelinated axons (invited review). Microscopy and Research Technique 34:445-451
Novakovic, S.D., Deerinck, T.J., Levinson, S.R., Shrager, P., and Ellisman, M.H. (1996) Clusters of axonal Na+ channels adjacent to remyelinating Schwann cells. J. Neurocytol.25:403-412
England, J.D., Happel, L.T., Kline, D.G., Gamboni, F., Thouron, C.L., Liu, Z.P., and Levinson, S.R. (1996) Sodium channel accumulation in painful human neuromas. Neurology 47:272-276
Vabnick, I., Novakovic, S.D., Levinson, S.R., Schachner, M., and Shrager, P., (1996) The clustering of axonal sodium channels during development of the peripheral nervous system. J. Neurosci. 16:4914-4922
Bennett, E., Urcan, M.S., Tinkle, S.S., Koszowski, A.G., and Levinson, S.R. (1997) Contribution of sialic acid to the voltage-depndence of sodium channel gating: A possible electrostatic mechanism. J. Gen. Physiol. 109:327-343
Toledo-Aral, J.J., Moss, B.L., He, Z-J., Koszowski, A.G., Whisenand, W., Levinson, S.R., Wolf, J.J., Silos Santiago, I., Halegoua, S., and Mandel, G. (1997) Identification of PN1, a predominant voltage-dependent sodium channel expressed principally in peripheral neurons. Proc. Natl. Acad. Sci. (USA) 94:1527-1532
Deerinck, T.J., Levinson, S.R., Bennett, G.V., and Ellisman, M.H. (1997) Clustering of voltage-sensitive sodium channels on axons is independent of direct Schwann cell contact in the dystrophic mouse. J. Neurosci. 17:5080-5088
Kaplan, M.R., Meyer-Franke, A., Lambert, S., Bennett, V., Duncan, I.D., Levinson, S.R., and Barres, B.A. (1997) A new role for oligodendrocytes: induction of axonal sodium channel clustering. Nature 386:724-728
Koszowski, A.G., Owens, G.C., and Levinson, S.R. (1998) The effect of the mouse mutation claw paw on myelination and nodal frequency in sciatic nerves. J. Neurosci. 18:5859-5868
Levinson, S.R. (1998) The structure and mechanism of voltage-gated ion channels. In Cell Physiology Source Book, 2nd edition, N. Sperelakis, ed., Academic Press, NY
Vabnick, I., Messing, A., Chiu, S.Y., Levinson, S.R., Schachner, M., Roder, J., Li, C., Novakovic, S., and Shrager, P. (1997) Sodium channel distribution in axons of hypomyelinated and MAG nul mutant mice. J. Neurosci. Res. 50:321-336.
Rasband, M.N., Trimmer, J.S., Schwarz, T.L., Levinson, S.R., Ellisman, M.H., Schachner, M., and Shrager, P. (1998) Potassium channel distribution, clustering, and function in remyelinating rat axons. J. Neurosci. 18:36-47.
Gee, S.H., Madhaven, R., Levinson, S.R., Caldwell, J.H., Sealock, R., and Froehner, S.C. (1998) Interaction of muscle and brain sodium channels with multiple members of the syntrophin family of dystrophin-associated proteins. J. Neurosci. 18:128-137.
Vabnick, I., Trimmer, J.S., Schwarz, T.L., Levinson, S.R., Risal, D., and Shrager, P. (1999) Dynamic potassium channel distributions during axonal development prevent aberrant firing patterns. J. Neurosci. 19:747-758
Gould, H.J. III, Gould, T.N., Paul, D., England, J.D., Liu, Z.P., Reeb, S.C., and Levinson, S.R. (1999) Development of inflammatory hypersensitivity and augmentation of sodium channels in rat dorsal root ganglia. Brain Research 824:296-299
Ching, W., Zanazzi, G., Levinson, S.R., and Salzer, J.L., (1999) Clustering of neuronal sodium channels requires contact with myelinating Schwann cells. J. Neurocytol. 28:295-301
Weber, P. Bartsch, U., Rasband, M.N., Czaniera, R., Lang, Y., Bluethmann, H., Margolis, R.U., Levinson, S.R., Shrager, P., Montag, D., and Schachner, M. (1999) Mice deficient for tenascin-R display alterations of the extracellular matrix and decreased axonal conduction velocities in the CNS. J. Neurosci. 19:4245-4262
Rasband, M.N., Peles, E., Trimmer, J.S., Levinson, S.R., Lux, S.E., and Shrager, P. (1999) Dependence of nodal sodium channel clustering on paranodal axoglial contact in the developing CNS. J. Neurosci. 19:7516-7528
Caldwell, J.H., Schaller, K.L., Lasher, R.S., Peles, E., and Levinson, S.R. (2000) Sodium channel Nav1.6 is localized at nodes of Ranvier, dendrites, and synapses. Proc. Natl. Acad. Sci. USA 97:5616-5620
Levinson, S.R. and W.A. Sather, (2001) The structure and mechanism of voltage-gated ion ion channels, in Cell Physiology Source Book, 3nd edition, pp 455-477, N. Sperelakis, ed., Academic Press, NY
Boiko T., Rasband, M.N., Levinson, S.R., Caldwell, J.H., Mandel, G.Trimmer, J.S., and Matthews, G. (2001) Compact myelin dictates the differential targeting of two sodium channel isoforms in the same axon. Neuron 30: 91–104
Kaplan, M.R., Cho, M-H., Ullian, E.M., Isom, L.L., Levinson, S.R., and Barres, B.A., (2001) Differential control of clustering of the sodium channels Nav1.2 and Nav1.6 at developing CNS nodes of Ranvier. Neuron 30: 105-119