Light and electron microscopic distribution of the AMPA receptor subunit, GluR2, in the spinal cord of control and G86R mutant superoxide dismutase transgenic mice

Journal of Comparative Neurology

Volumn 395, Issue 4, 1998, Pages 523-534

  Morrison, Brett M ^a   Janssen, William G M ^a   Gordon, Jon W ^a   Morrison, John H ^a  

^a MOUNT SINAI SCHOOL OF MEDICINE   (United States)

Author keywords

Amyotrophic lateral sclerosis; Excitotoxicity; Glutamate receptors; Motor neurons

Indexed keywords

AMPA RECEPTOR; GLUTAMATE RECEPTOR;

AMYOTROPHIC LATERAL SCLEROSIS; ARTICLE; IMMUNOREACTIVITY; MOTOR NEURON DISEASE; NERVE DEGENERATION; PATHOPHYSIOLOGY; PRIORITY JOURNAL; SPINAL CORD;

AMYOTROPHIC LATERAL SCLEROSIS; ANIMALS; CALCIUM; IMMUNOHISTOCHEMISTRY; MICE; MICE, TRANSGENIC; MICROSCOPY, IMMUNOELECTRON; MOTOR NEURONS; POINT MUTATION; RECEPTORS, METABOTROPIC GLUTAMATE; SPINAL CORD; SUPEROXIDE DISMUTASE;

EID: 0032527065     PISSN: 00219967     EISSN: None     Source Type: Journal    
DOI: 10.1002/(SICI)1096-9861(19980615)395:4<523::AID-CNE8>3.0.CO;2-3     Document Type: Article

References (95)

1. Adams, R.D., M. Victor, and A.H. Ropper (1997) Principles of Neurology. New York: McGraw-Hill.
2. Alexianu, M.E., B.-K. Ho, A.H. Mohamed, V. La Bella, R.G. Smith, and S.H. Appel (1994) The role of calcium-binding proteins in selective motoneuron vulnerability in amyotrophic lateral sclerosis. Ann. Neurol. 36:846-858.
3. Antal, M., T.F. Freund, and E. Polgar (1990) Calcium-binding proteins, parvalbumin-and calbindin-D28k-immunoreactive neurons in the rat spinal cord and dorsal root ganglia: A light and electron microscopic study. J. Comp. Neurol. 295:467-484.
4. Beal, M.F., R.J. Ferrante, S.E. Browne, R.T. Matthews, N.W. Kowall, and R.H. Brown (1997) Increased 3-nitrotyrosine in both sporadic and familial amyotrophic lateral sclerosis. Ann. Neurol. 42:646-654.
5. Beckman, J.S., M. Carson, C.D. Smith, and W.H. Koppenol (1993) ALS, SOD and peroxynitrite. Nature 364:584.
6. Bonnot, A., M. Corio, G. Tramu, and D. Viala (1996) Immunocytochemical distribution of ionotropic glutamate receptor subunits in the spinal cord of the rabbit. J. Chem. Neuroanat. 11:267-278.
7. Boulter, J., M. Hollmann, A. O'Shea-Greenfield, M. Hartley, E. Deneris, C. Maron, and S. Heinemann (1990) Molecular cloning and functional expression of glutamate receptor subunit genes. Science 249:1033-1037.
8. Bristol, L.A. and J.D. Rothstein (1996) Glutamate transporter gene expression in amyotrophic lateral sclerosis motor cortex. Ann. Neurol. 39:676-679.
9. Brose, N., G.W. Huntley, Y. Stern-Bach, G. Sharma, J.H. Morrison, and S.F. Heinemann (1994) Differential assembly of coexpressed glutamate receptor subunits in neurons of rat cerebral cortex. J. Biol. Chem. 269:16780-16784.
10. Bruijn, L.I., M.F. Beal, M.W. Becher, J.B. Schulz, P.C. Wong, D.L. Price, and D.W. Cleveland (1997a) Elevated free nitrotyrosine levels, but not protein-bound nitrotyrosine or hydroxyl radicals, throughout amyotrophic lateral sclerosis (ALS)-like disease implicate tyrosine nitration as an aberrant in vivo property of one familial ALS-linked Superoxide dismutase 1 mutant. Proc. Natl. Acad. Sci. USA 94:7606-7611.
11. Bruijn, L.I., M.W. Becher, M.K. Lee, K.L. Anderson, N.A. Jenkins, N.G. Copeland, S.S. Sisodia, J.D. Rothstein, D.R. Borchelt, D.L. Price, and D.W. Cleveland (1997b) ALS-linked SOD1 mutant G85R mediates damage to astrocytes and promotes rapidly progressive disease with SOD1-containing inclusions. Neuron 18:327-338.
12. Burnashev, N., H. Monyer, P.H. Seeburg, and B. Sakmann (1992) Divalent ion permeability of AMPA receptor channels is dominated by the edited form of a single subunit. Neuron 8:189-198.
13. Burnashev, N., Z. Zhou, E. Neher, and B. Sakmann (1995) Fractional calcium currents through recombinant GluR channels of the NMDA, AMPA and kainate receptor subtypes. J. Physiol. 485:403-418.
14. 2+ dependent. Neuroreport 6:945-948.
15. Carriedo, S.G., H.Z. Yin, and J.H. Weiss (1996) Motor neurons are selectively vulnerable to AMPA/kainate receptor-mediated injury in vitro. J. Neurosci. 16:4069-4079.
16. Chard, P.S., D. Bleakman, S. Christakos, C.S. Fullmer, and R.J. Miller (1993) Calcium buffering properties of calbindin D-28k and parvalbumin in rat sensory neurones. J. Physiol. 472:341-357.
17. Choi, D.W. (1987) Ionic dependence of glutamate neurotoxicity. J. Neurosci. 7:369-379.
18. Crow, J.P., J.B. Sampson, Y. Zhuang, J.A. Thompson, and J.S. Beckman (1997) Decreased zinc affinity of amyotrophic lateral sclerosis-associated Superoxide dismutase mutants leads to enhanced catalysis of tyrosine nitration by peroxynitrite. J. Neurochem. 69:1936-1944.
19. DeFelipe, J., F. Conti, S.L. Van Eyck, and T. Manzoni (1988) Demonstration of glutamate-positive axon terminals forming asymmetric synapses in cat neocortex. Brain Res. 455:162-165.
20. Ferrante, R.J., L.A. Shinobu, J.B. Schulz, R.T. Matthews, C.E. Thomas, N.W. Kowall, M.E. Gurney, and M.F. Beal (1997) Increased 3-nitrotyrosine and oxidative damage in mice with a human copper/zinc Superoxide dismutase mutation. Ann. Neurol. 42:326-334.
21. Furuyama, T., H. Kiyama, K. Sato, H.T. Park, H. Maeno, H. Takagi, and M. Tohyama (1993) Region-specific expression of subunits of ionotropic glutamate receptors (AMPA-type, KA-type and NMDA receptors) in the rat spinal cord with special reference to nociception. Mol. Brain Res. 18:141-151.
22. Garcia-Segura, L.M., D. Baetens, J. Roth, A.W. Norman, and L. Orci (1984) Immunohistochemical mapping of calcium-binding protein immunore-activity in the rat central nervous system. Brain Res. 296:75-86.
23. 2+ permeability of AMPA receptors in principal neurons and interneurons in rat CNS. Neuron 15:193-204.
24. 2+ influx in hippocampal CA1 neurons of gerbil. J. Neurosci. 17:6179-6188.
25. 2+-permeable AMPA receptors. Nature 381:793-796.
26. Gurney, M.E., H. Pu, A.Y. Chiu, M.C. Dal Canto, C.Y. Polchow, D.D. Alexander, J. Caliendo, A. Hentati, Y.W. Kwon, H.-X. Deng, W. Chen, P. Zhai, R.L. Sufit, and T. Siddique (1994) Motor neuron degeneration in mice that express a human Cu,Zn Superoxide dismutase mutation. Science 264:1772-1775.
27. Hendrickson, A.E., S.P. Hunt, and J.Y. Wu (1981) Immunocytochemical localization of glutamic acid decarboxylase in monkey striate cortex. Nature 292:605-607.
28. Hendry, S.H.C., C.R. Houser, E.G. Jones, and J.E. Vaughn (1983) Synaptic organization of immunocytochemically identified GABA neurons in the monkey sensory-motor cortex. J. Neurocytol. 12:639-660.
29. Hirano, A. (1991) Cytopathology of amyotrophic lateral sclerosis. In L.P. Rowland (ed): Amyotrophic Lateral Sclerosis and Other Motor Neuron Diseases. New York: Raven Press, Ltd., pp. 91-101.
30. Hirano, A., L.T. Kurland, and G.P. Sayre (1967) Familial amyotrophic lateral sclerosis. Arch. Neurol. 16:232-243.
31. Hollmann, M. and S.F. Heinemann (1994) Cloned glutamate receptors. Annu. Rev. Neurosci. 17:31-108.
32. Hollmann, M., A. O'Shea-Greenfield, S.W. Rogers, and S. Heinemann (1989) Cloning by functional expression of a member of the glutamate receptor family. Nature 342:643-648.
33. Ikonomidou, C., Y. Qin Qin, J. Labruyere, and J.W. Olney (1996) Motor neuron degeneration induced by excitotoxin agonists has features in common with those seen in the SOD-1 transgenic mouse model of amyotrophic lateral sclerosis. J. Neuropathol. Exp. Neurol. 55:211-224.
34. Ince, P., N. Stout, P. Shaw, J. Slade, W. Hunziker, C.W. Heizmann, and K.G. Baimbridge (1993) Parvalbumin and calbindin D-28k in the human motor system and in motor neuron disease. Neuropathol. Appl. Neurobiol. 19:291-299.
35. Jakowec, M.W., A.J. Fox, L.J. Martin, and R.G. Kalb (1995a) Quantitative and qualitative changes in AMPA receptor expression during spinal cord development. Neuroscience 67:893-907.
36. Jakowec, M.W., L. Yen, and R.G. Kalb (1995b) In situ hybridization analysis of AMPA receptor subunit gene expression in the developing rat spinal cord. Neuroscience 67:909-920.
37. 2+ permeability of AMPA-type glutamate receptor channels in neocortical neurons caused by differential GluR-B subunit expression. Neuron 12:1281-1289.
38. Keinanen, K., W. Wisden, B. Sommer, P. Werner, A. Herb, T.A. Verdoorn, B. Sakmann, and P.H. Seeburg (1990) A family of AMPA-selective glutamate receptors. Science 249:556-560.
39. Kwak, S. and R. Nakamura (1995) Selective degeneration of inhibitory interneurons in the rat spinal cord induced by intrathecal infusion of acromelic acid. Brain Res. 702:61-71.
40. Kyrozis, A., P.A. Goldstein, M.J.S. Heath, and A.B. MacDermott (1995) Calcium entry through a subpopulation of AMPA receptors desensitized neighboring NMDA receptors in rat dorsal horn. J. Physiol. 485.2:373-381.
41. Leigh, P.N. and K. Ray-Chaudhuri (1994) Motor neuron disease. J. Neurol. Neurosurg. Psychol. 57:886-896.
42. Levitan, I.B. (1994) Modulation of ion channels by protein phosphorylation and dephosphorylation. Annu. Rev. Physiol. 56:193-212.
43. Lledo, P.-M., B. Somasundaram, A.J. Morton, P.C. Emson, and W.T. Mason (1992) Stable transfection of calbindin-D28k into the GH3 cell line alters calcium currents and intracellular calcium homeostasis. Neuron 9:943-954.
44. Lukas, W. and K.A. Jones (1994) Cortical neurons containing calretinin are selectively resistant to calcium overload and excitotoxicity in vitro. Neuroscience 61:307-316.
45. Luque, J.M., Z. Bleuel, P. Malherbe, and J.G. Richards (1994) Alternatively spliced isoforms of the N-methyl-D-aspartate receptor subunit 1 are differentially distributed within the rat spinal cord. Neuroscience 63:629-635.
46. Maas, S., T. Melcher, and P.H. Seeburg (1997) Mammalian RNA-dependent deaminases and edited mRNAs. Curr. Opin. Cell Biol. 9:343-349.
47. Margulies, J.E., R.W. Cohen, M.S. Levine, and J.B. Watson (1993) Decreased GluR2(B) receptor subunit mRNA expression in cerebellar neurons at risk for degeneration. Dev. Neurosci. 15:110-120.
48. Martin, L.J., C.D. Blackstone, A.I. Levey, R.L. Huganir, and D.L. Price (1993) AMPA glutamate receptor subunits are differentially distributed in rat brain. Neuroscience 53:327-358.
49. Mattson, M.P., B. Rychlik, C. Chu, and S. Christakos (1991) Evidence for calcium-reducing and excitoprotective roles for the calcium-binding protein calbindin-D28k in cultured hippocampal neurons. Neuron 6:41-51.
50. Meldrum, B. and J. Garthwaite (1990) Excitatory amino acid neurotoxicity and neurodegenerative disease. Trends Pharmacol. Sci. 11:379-387.
51. Morrison, B.M., J.W. Gordon, M.E. Ripps, and J.H. Morrison (1996) Quantitative immunocytochemical analysis of the spinal cord in G86R Superoxide dismutase transgenic mice: Neurochemical correlates of selective vulnerability. J. Comp. Neurol. 373:619-631.
52. Morrison, B.M., W.G. Janssen, J.W. Gordon, and J.H. Morrison (1998) Time course of neuropathology in the spinal cord of G86R Superoxide dismutase transgenic mice. J. Comp. Neurol. 391:64-77.
53. Nakanishi, S. (1992) Molecular diversity of glutamate receptors and implications for brain function. Science 258:597-603.
54. Orrenius, S., M.J. Burkitt, G.E.N. Kass, J.M. Dypbukt, and P. Nicotera (1992) Calcium ions and oxidative cell injury. Ann. Neurol. 32:S33-S42.
55. Paschen, W., J. Schmitt, C. Gissel, and E. Dux (1997) Developmental changes of RNA editing of glutamate receptor subunits GluR5 and GluR6: In vivo versus in vitro. Dev. Brain Res. 98:271-280.
56. Pellegrini-Giampietro, D.E., R.S. Zukin, M.V.L. Bennett, S. Cho, and W.A. Pulsinelli (1992) Switch in glutamate receptor subunit gene expression in CA1 subfield of hippocampus following global ischemia in rats. Proc. Natl. Acad. Sci. USA 89:10499-10503.
57. Perry, T.L., S. Hansen, and K. Jones (1987) Brain glutamate deficiency in amyotrophic lateral sclerosis. Neurology 37:1845-1848.
58. Peters, A. and K.M. Harriman (1992) Different kinds of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells. III. Origins and frequency of occurrence of the terminals. J. Neurocytol. 21:679-692.
59. Petralia, R.S., Y.-X. Wang, and R.J. Wenthold (1994a) Histological and ultrastructural localization of the kainate receptor subunits, KA2 and GluR6/7, in the rat nervous system using selective antipeptide antibodies. J. Comp. Neurol. 349:85-110.
60. Petralia, R.S., N. Yokotani, and R.J. Wenthold (1994b) Light and electron microscope distribution of the NMDA receptor subunit NMDAR1 in the rat nervous system using a selective anti-peptide antibody. J. Neurosci. 14:667-696.
61. Petralia, R.S., Y.-X. Wang, E. Mayat, and R.J. Wenthold (1997) Glutamate receptor subunit 2-selective antibody shows a differential distribution of calcium-impermeable AMPA receptors among populations of neurons. J. Comp. Neurol. 385:456-476.
62. Phend, K.D., R.J. Weinberg, and A. Rustioni (1992) Techniques to optimize post-embedding single and double staining for amino acid neurotransmitters. J. Histochem. Cytochem. 40:1011-1020.
63. Phend, K.D., A. Rustioni, and R.J. Weinberg (1995) An osmium free method of epon embedment that preserves both ultrastructure and antigenicity for post-embedding immunohistochemistry. J. Histochem. Cytochem. 43:283-292.
64. Plaitakis, A. and J.T. Caroscio (1987) Abnormal glutamate metabolism in amyotrophic lateral sclerosis. Ann. Neurol. 22:575-579.
65. Plaitakis, A., E. Constantakakis, and J. Smith (1988) The neuroexcitotoxic amino acids glutamate and aspartate are altered in the spinal cord and brain in amyotrophic lateral sclerosis. Ann. Neurol. 24:446-449.
66. 3H-glutamate transport in synaptasomes. Synapse 18:21-26.
67. Puchalski, R.B., J.-C. Louis, N. Brose, S.F. Traynelis, J. Egebjerg, V. Kukekov, R.J. Wenthold, S.W. Rogers, F. Lin, T. Moran, J.H. Morrison, and S.F. Heinemann (1994) Selective RNA editing and subunit assembly of native glutamate receptors. Neuron 13:131-147.
68. Reaume, A.G., J.L. Elliott, E.K. Hoffman, N.W. Kowall, R.J. Ferrante, D.F. Siwek, H.M. Wilcox, D.G. Flood, M.F. Beal, R.H. Brown, R.W. Scott, and W.D. Snider (1996) Motor neurons in Cu/Zn Superoxide dismutase-deficient mice develop normally but exhibit enhanced cell death after axonal injury. Nature Genet. 13:43-47.
69. Ren, K. and M.A. Ruda (1994) A comparative study of the calcium-binding proteins calbindin-D28k, calretinin, calmodulin and parvalbumin in the rat spinal cord. Brain Res. Rev. 19:163-179.
70. Ripps, M.E., G.W. Huntley, P.R. Hof, J.H. Morrison, and J.W. Gordon (1995) Transgenic mice expressing an altered murine Superoxide dismutase gene provide an animal model of amyotrophic lateral sclerosis. Proc. Natl. Acad. Sci. USA 92:689-693.
71. Rosen, D.R., T. Siddique, D. Patterson, D.A. Figlewicz, P. Sapp, A. Hentati, D. Donaldson, J. Goto, J.P. O'Regan, H.-X. Deng, Z. Rahmani, A. Krizus, D. McKenna-Yasek, A. Cayabyab, S.M. Gaston, R. Berger, R.E. Tanzi, J.J. Halperin, B. Herzfeldt, R. Van den Bergh, W.-Y. Huang, T. Bird, G. Deng, D.W. Mulder, C. Smyth, N.G. Laing, E. Soriano, M.A. Pericak-Vance, J. Haines, G.A. Rouleau, J.S. Gusella, H.R. Horvitz, and R.H. Brown, Jr. (1993) Mutations in Cu/Zn Superoxide dismutase are associated with familial amyotrophic lateral sclerosis. Nature 362:59-62.
72. Rothstein, J.D., G. Tsai, R.W. Kuncl, L. Clawson, D.R. Cornblath, D.B. Drachman, A. Pestronk, B.L. Stauch, and J.T. Coyle (1990) Abnormal excitatory amino acid metabolism in amyotrophic lateral sclerosis. Ann. Neurol. 28:18-25.
73. Rothstein, J.D., L.J. Martin, and R.W. Kuncl (1992) Decreased glutamate transport by the brain and spinal cord in amyotrophic lateral sclerosis. N. Engl. J. Med. 326:1464-1468.
74. Rothstein, J.D., L. Jin, M. Dykes-Hoberg, and R.W. Kuncl (1993) Chronic inhibition of glutamate uptake produces a model of slow neurotoxicity. Proc. Natl. Acad. Sci. USA 90:6591-6595.
75. Rothstein, J.D., M. Van Kammen, A.I. Levey, L.J. Martin, and R.W. Kuncl (1995) Selective loss of glial glutamate transporter GLT-1 in amyotrophic lateral sclerosis. Ann. Neurol. 38:73-84.
76. Rothstein, J.D., M. Dykes-Hoberg, C.A. Pardo, L.A. Bristol, L. Jin, R.W. Kuncl, Y. Kanai, M.A. Hediger, Y. Wang, J.P. Schielke, and D.F. Welty (1996) Knockout of glutamate transporters reveals a major role for astroglial transport in excitotoxicity and clearance of glutamate. Neuron 16:675-686.
77. Sato, K., H. Kiyama, and M. Tohyama (1993) The differential expression patterns of messenger RNAs encoding non-N-methyl-D-aspartate glutamate receptor subunits (GluR1-4) in the rat brain. Neuroscience 52:515-539.
78. Schmitt, J., E. Dux, C. Gissel, and W. Paschen (1996) Regional analysis of developmental changes in the extent of GluR6 mRNA editing in rat brain. Dev. Brain Res. 91:153-157.
79. Seeburg, P.H. (1993) The molecular biology of mammalian glutamate receptor channels. Trends Neurosci. 16:359-365.
80. Sommer, B., M. Kohler, R. Sprengel, and P.H. Seeburg (1991) RNA editing in brain controls a determinant of ion flow in glutamate-gated channels. Cell 67:11-19.
81. Tachibana, M., R.J. Wenthold, H. Morioka, and R.S. Petralia (1994) Light and electron microscopic immunocytochemical localization of AMPA-selective glutamate receptors in the rat spinal cord. J. Comp. Neurol. 344:431-454.
82. Tandan, R. (1994) Clinical features and differential diagnosis of classical motor neuron disease. In A.C. Williams (ed): Motor Neuron Disease. London: Chapman and Hall, pp. 3-28.
83. Temkin, R., D. Lowe, P. Jensen, H. Hatt, and D.O. Smith (1997) Expression of glutamate receptor subunits in alpha-motoneurons. Mol. Brain. Res. 52:38-45.
84. Tolle, T.R., A. Berthele, W. Zieglgansberger, P.H. Seeberg, and W. Wisden (1993) The differential expression of 16 NMDA and non-NMDA receptor subunits in the rat spinal cord and in periaqueductal gray. J. Neurosci 13:5009-5028.
85. Tolle, T.R., A. Berthele, W. Zieglgansberger, P.H. Seeburg, and W. Wisden (1995) Flip and flop variants of AMPA receptors in the rat lumbar spinal cord. Eur. J. Neurosci. 7:1414-1419.
86. Tomiyama, M., R. Rodriguez-Puertas, R. Cortes, A. Christnacher, B. Sommer, A. Pazos, J.M. Palacios, and G. Mengod (1996) Differential regional distribution of AMPA receptor subunit messenger RNAs in the human spinal cord as visualized by in situ hybridization. Neuroscience 75:901-915.
87. 2+ chelators reduce early excitotoxic and ischemic neuronal injury in vitro and in vivo. Neuron 3 11:221-235.
88. Virgo, L., S. Samarasinghe, and J. de Belleroche (1996) Analysis of AMPA receptor subunit mRNA expression in control and ALS spinal cord. Neuroreport 7:2507-2511.
89. Vissavajjhala, P., W.G.M. Janssen, Y. Hu, A.H. Gazzaley, T. Moran, P.R. Hof, and J.H. Morrison (1996) Synaptic distribution of the AMPA-GluR2 subunit and its colocalization with calcium-binding proteins in rat cerebral cortex: An immunohistochemical study using a GluR2-specific monoclonal antibody. Exp. Neurol. 142:296-312.
90. Volterra, A., D. Trotti, C. Tromba, S. Floridi, and G. Racagni (1994) Glutamate uptake inhibition by oxygen free radicals in rat cortical astrocytes. J. Neurosci. 14:2924-2932.
91. Wiedau-Pazos, M., J.J. Goto, S. Rabizadeh, E.B. Gralla, J.A. Roe, M.K. Lee, J.S. Valentine, and D.E. Bredesen (1996) Altered reactivity of superoxide dismutase in familial amyotrophic lateral sclerosis. Science 271:515-518.
92. Williams, T.L., P.G. Ince, A.E. Oakley, and P.J. Shaw (1996) An immunocytochemical study of the distribution of AMPA selective glutamate receptor subunits in the normal human motor system. Neuroscience 74:185-198.
93. Williams, T.L., N.C. Day, P.G. Ince, M.R.C. Path, R.K. Kamboj, and P.J. Shaw (1997) Calcium-permeable amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors: A molecular determinant of selective vulnerability in amyotrophic lateral sclerosis. Ann. Neurol. 42:200-207.
94. Wong, P.C., C.A. Pardo, D.R. Borchelt, M.K. Lee, N.G. Copeland, N.A. Jenkins, S.S. Sisodia, D.W. Cleveland, and D.L. Price (1995) An adverse property of a familial ALS-linked SOD1 mutation causes motor neuron disease characterized by vacuolar degeneration of mitochondria. Neuron 14:1105-1116.
95. Yim, M.B., J.-H. Kang, H.-S. Yim, H.-S. Kwak, P.B. Chock, and E.R. Stadtman (1996) A gain-of-function of an amyotrophic lateral sclerosis-associated Cu,Zn-superoxide dismutase mutant: An enhancement of free radical formation due to a decrease in Km for hydrogen peroxide. Proc. Natl. Acad. Sci. USA 93:5709-5714.