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Progress in Biophysics and Molecular... May 2005The original papers of Hodgkin and Huxley (J. Physiol. 116 (1952a) 449, J. Physiol. 116 (1952b) 473, J. Physiol. 116 (1952c) 497, J. Physiol. 117 (1952d) 500) have... (Review)
Review
The original papers of Hodgkin and Huxley (J. Physiol. 116 (1952a) 449, J. Physiol. 116 (1952b) 473, J. Physiol. 116 (1952c) 497, J. Physiol. 117 (1952d) 500) have provided a benchmark in our understanding of cellular excitability. Not surprisingly, their model of the membrane action potential (AP) requires revisions even for the squid giant axon, the preparation for which it was originally formulated. The mechanisms they proposed for the voltage-gated potassium and sodium ion currents, IK, and INa, respectively, have been superceded by more recent formulations that more accurately describe voltage-clamp measurements of these components. Moreover, the current-voltage relation for IK has a non-linear dependence upon driving force that is well described by the Goldman-Hodgkin-Katz (GHK) relation, rather than the linear dependence on driving force found by Hodgkin and Huxley. Furthermore, accumulation of potassium ions in the extracellular space adjacent to the axolemma appears to be significant even during a single AP. This paper describes the influence of these various modifications in their model on the mathematically reconstructed AP. The GHK and K+ accumulation results alter the shape of the AP, whereas the modifications in IK and INa gating have surprisingly little effect. Perhaps the most significant change in their model concerns the amplitude of INa, which they appear to have overestimated by a factor of two. This modification together with the GHK and the K+ accumulation results largely remove the discrepancies between membrane excitability of the squid giant axon and the Hodgkin and Huxley (J. Physiol. 117 (1952d) 500) model previously described (Clay, J. Neurophysiol. 80 (1998) 903).
Topics: Action Potentials; Animals; Axons; Decapodiformes; Ion Channel Gating; Models, Neurological; Potassium; Potassium Channels; Sodium
PubMed: 15561301
DOI: 10.1016/j.pbiomolbio.2003.12.004 -
Journal of Neurochemistry Nov 2003Myelin-axolemmal interactions regulate many cellular and molecular events, including gene expression, oligodendrocyte survival and ion channel clustering. Here we report...
Myelin-axolemmal interactions regulate many cellular and molecular events, including gene expression, oligodendrocyte survival and ion channel clustering. Here we report the biochemical fractionation and enrichment of distinct subcellular domains from myelinated nerve fibers. Using antibodies against proteins found in compact myelin, non-compact myelin and axolemma, we show that a rigorous procedure designed to purify myelin also results in the isolation of the myelin-axolemmal complex, a high-affinity protein complex consisting of axonal and oligodendroglial components. Further, the isolation of distinct subcellular domains from galactolipid-deficient mice with disrupted axoglial junctions is altered in a manner consistent with the delocalization of axolemmal proteins observed in these animals. These results suggest a paradigm for identification of proteins involved in neuroglial signaling.
Topics: Animals; Brain Chemistry; Centrifugation, Density Gradient; Electrophoresis, Polyacrylamide Gel; Galactose; Glycosphingolipids; Immunoblotting; Immunohistochemistry; Macromolecular Substances; Mice; Myelin Sheath; Nerve Fibers, Myelinated; Optic Nerve; Subcellular Fractions
PubMed: 14622129
DOI: 10.1046/j.1471-4159.2003.02075.x -
Neuron Jun 2003Dystroglycan is a central component of the dystrophin-glycoprotein complex implicated in the pathogenesis of several neuromuscular diseases. Although dystroglycan is...
Dystroglycan is a central component of the dystrophin-glycoprotein complex implicated in the pathogenesis of several neuromuscular diseases. Although dystroglycan is expressed by Schwann cells, its normal peripheral nerve functions are unknown. Here we show that selective deletion of Schwann cell dystroglycan results in slowed nerve conduction and nodal changes including reduced sodium channel density and disorganized microvilli. Additional features of mutant mice include deficits in rotorod performance, aberrant pain responses, and abnormal myelin sheath folding. These data indicate that dystroglycan is crucial for both myelination and nodal architecture. Dystroglycan may be required for the normal maintenance of voltage-gated sodium channels at nodes of Ranvier, possibly by mediating trans interactions between Schwann cell microvilli and the nodal axolemma.
Topics: Animals; Animals, Newborn; Cell Membrane; Cells, Cultured; Cytoskeletal Proteins; Dystroglycans; Laminin; Macromolecular Substances; Membrane Glycoproteins; Mice; Mice, Knockout; Movement Disorders; Mutation; Myelin Sheath; Neural Conduction; Peripheral Nerves; Protein Binding; Ranvier's Nodes; Schwann Cells; Sodium Channels; Wallerian Degeneration
PubMed: 12797959
DOI: 10.1016/s0896-6273(03)00301-5 -
The Journal of Cell Biology Jun 2002An axonal complex of cell adhesion molecules consisting of Caspr and contactin has been found to be essential for the generation of the paranodal axo-glial junctions...
An axonal complex of cell adhesion molecules consisting of Caspr and contactin has been found to be essential for the generation of the paranodal axo-glial junctions flanking the nodes of Ranvier. Here we report that although the extracellular region of Caspr was sufficient for directing it to the paranodes in transgenic mice, retention of the Caspr-contactin complex at the junction depended on the presence of an intact cytoplasmic domain of Caspr. Using immunoelectron microscopy, we found that a Caspr mutant lacking its intracellular domain was often found within the axon instead of the junctional axolemma. We further show that a short sequence in the cytoplasmic domain of Caspr mediated its binding to the cytoskeleton-associated protein 4.1B. Clustering of contactin on the cell surface induced coclustering of Caspr and immobilized protein 4.1B at the plasma membrane. Furthermore, deletion of the protein 4.1B binding site accelerated the internalization of a Caspr-contactin chimera from the cell surface. These results suggest that Caspr serves as a "transmembrane scaffold" that stabilizes the Caspr/contactin adhesion complex at the paranodal junction by connecting it to cytoskeletal components within the axon.
Topics: Amino Acid Sequence; Animals; Brain Chemistry; Cell Adhesion Molecules; Cell Adhesion Molecules, Neuronal; Cell Line; Cells, Cultured; Contactins; Cytoplasm; Cytoskeletal Proteins; Humans; Intercellular Junctions; Membrane Proteins; Mice; Mice, Knockout; Models, Biological; Neurons; Neuropeptides; Optic Nerve; Ranvier's Nodes; Receptors, Cell Surface; Sciatic Nerve; Sequence Deletion; Transgenes
PubMed: 12082082
DOI: 10.1083/jcb.200203050 -
Biochemistry. Biokhimiia Apr 2002Functionally active Na+,K+-ATPase isozymes containing three types of the catalytic subunits (alpha1, alpha2, and alpha3) were obtained from calf brain by two methods:...
Functionally active Na+,K+-ATPase isozymes containing three types of the catalytic subunits (alpha1, alpha2, and alpha3) were obtained from calf brain by two methods: selective removal of contaminating proteins according to Jorgensen (1974) and selective solubilization of the enzyme with subsequent reformation of the membrane structure according to Esmann (1988). All preparations were characterized with respect to ouabain-inhibition constants. The presence of the cytoskeleton protein tubulin (beta3 isoform) in the high-molecular-weight complex of Na+,K+-ATPase alpha3beta1 isozyme from brain stem axolemma and the junction between Na+,K+-ATPase alpha3 subunit and tubulin beta3 subunit are shown for the first time.
Topics: Amino Acid Sequence; Animals; Brain Stem; Cattle; Isoenzymes; Molecular Sequence Data; Neurons; Sodium-Potassium-Exchanging ATPase; Tubulin
PubMed: 11996666
DOI: 10.1023/a:1015202610797 -
Current Biology : CB Feb 2002In myelinated fibers of the vertebrate nervous system, glial-ensheathing cells interact with axons at specialized adhesive junctions, the paranodal septate-like...
In myelinated fibers of the vertebrate nervous system, glial-ensheathing cells interact with axons at specialized adhesive junctions, the paranodal septate-like junctions. The axonal proteins paranodin/Caspr and contactin form a cis complex in the axolemma at the axoglial adhesion zone, and both are required to stabilize the junction. There has been intense speculation that an oligodendroglial isoform of the cell adhesion molecule neurofascin, NF155, expressed at the paranodal loop might be the glial receptor for the paranodin/Caspr-contactin complex, particularly since paranodin/Caspr and NF155 colocalize to ectopic sites in the CNS of the dysmyelinated mouse Shiverer mutant. We report that the extracellular domain of NF155 binds specifically to transfected cells expressing the paranodin/Caspr-contactin complex at the cell surface. This region of NF155 also binds the paranodin/Caspr-contactin complex from brain lysates in vitro. In support of the functional significance of this interaction, NF155 antibodies and the extracellular domain of NF155 inhibit myelination in myelinating cocultures, presumably by blocking the adhesive relationship between the axon and glial cell. These results demonstrate that the paranodin/Caspr-contactin complex interacts biochemically with NF155 and that this interaction is likely to be biologically relevant at the axoglial junction.
Topics: Animals; Axons; Brain; CHO Cells; Cell Adhesion; Cell Adhesion Molecules; Cell Adhesion Molecules, Neuronal; Coculture Techniques; Contactins; Cricetinae; Macromolecular Substances; Membrane Glycoproteins; Models, Biological; Nerve Fibers, Myelinated; Nerve Growth Factors; Neuroglia; Neuropeptides; Protein Binding; Protein Isoforms; Protein Structure, Tertiary; Rats; Receptors, Cell Surface; Transfection
PubMed: 11839274
DOI: 10.1016/s0960-9822(01)00680-7 -
Journal of Neurophysiology Feb 2002Human nerve fibers exhibit a distinct pattern of threshold fluctuation following a single action potential known as the recovery cycle. We developed geometrically and...
Human nerve fibers exhibit a distinct pattern of threshold fluctuation following a single action potential known as the recovery cycle. We developed geometrically and electrically accurate models of mammalian motor nerve fibers to gain insight into the biophysical mechanisms that underlie the changes in axonal excitability and regulate the recovery cycle. The models developed in this study incorporated a double cable structure, with explicit representation of the nodes of Ranvier, paranodal, and internodal sections of the axon as well as a finite impedance myelin sheath. These models were able to reproduce a wide range of experimental data on the excitation properties of mammalian myelinated nerve fibers. The combination of an accurate representation of the ion channels at the node (based on experimental studies of human, cat, and rat) and matching the geometry of the paranode, internode, and myelin to measured morphology (necessitating the double cable representation) were needed to match the model behavior to the experimental data. Following an action potential, the models generated both depolarizing (DAP) and hyperpolarizing (AHP) afterpotentials. The model results support the hypothesis that both active (persistent Na(+) channel activation) and passive (discharging of the internodal axolemma through the paranodal seal) mechanisms contributed to the DAP, while the AHP was generated solely through active (slow K(+) channel activation) mechanisms. The recovery cycle of the fiber was dependent on the DAP and AHP, as well as the time constant of activation and inactivation of the fast Na(+) conductance. We propose that experimentally documented differences in the action potential shape, strength-duration relationship, and the recovery cycle of motor and sensory nerve fibers can be attributed to kinetic differences in their nodal Na(+) conductances.
Topics: Action Potentials; Animals; Computer Simulation; Mammals; Models, Neurological; Motor Neurons; Nerve Fibers, Myelinated; Potassium Channels; Ranvier's Nodes; Refractory Period, Electrophysiological; Sodium Channels
PubMed: 11826063
DOI: 10.1152/jn.00353.2001 -
The Journal of Cell Biology Dec 2001Myelin-associated glycoprotein (MAG) is expressed in periaxonal membranes of myelinating glia where it is believed to function in glia-axon interactions by binding to a...
Myelin-associated glycoprotein (MAG) is expressed in periaxonal membranes of myelinating glia where it is believed to function in glia-axon interactions by binding to a component of the axolemma. Experiments involving Western blot overlay and coimmunoprecipitation demonstrated that MAG binds to a phosphorylated neuronal isoform of microtubule-associated protein 1B (MAP1B) expressed in dorsal root ganglion neurons (DRGNs) and axolemma-enriched fractions from myelinated axons of brain, but not to the isoform of MAP1B expressed by glial cells. The expression of some MAP1B as a neuronal plasma membrane glycoprotein (Tanner, S.L., R. Franzen, H. Jaffe, and R.H. Quarles. 2000. J. Neurochem. 75:553-562.), further documented here by its immunostaining without cell permeabilization, is consistent with it being a binding partner for MAG on the axonal surface. Binding sites for a MAG-Fc chimera on DRGNs colocalized with MAP1B on neuronal varicosities, and MAG and MAP1B also colocalized in the periaxonal region of myelinated axons. In addition, expression of the phosphorylated isoform of MAP1B was increased significantly when DRGNs were cocultured with MAG-transfected COS cells. The interaction of MAG with MAP1B is relevant to the known role of MAG in affecting the cytoskeletal structure and stability of myelinated axons.
Topics: Animals; Axons; COS Cells; Coculture Techniques; Ganglia, Spinal; Microtubule-Associated Proteins; Myelin-Associated Glycoprotein; Neuroglia; Neurons; Phosphorylation; Protein Binding; Rats; Transfection
PubMed: 11733546
DOI: 10.1083/jcb.200108137 -
Neuron May 2001Rapid nerve impulse conduction depends on specialized membrane domains in myelinated nerve, the node of Ranvier, the paranode, and the myelinated internodal region. We...
Rapid nerve impulse conduction depends on specialized membrane domains in myelinated nerve, the node of Ranvier, the paranode, and the myelinated internodal region. We report that GPI-linked contactin enables the formation of the paranodal septate-like axo-glial junctions in myelinated peripheral nerve. Contactin clusters at the paranodal axolemma during Schwann cell myelination. Ablation of contactin in mutant mice disrupts junctional attachment at the paranode and reduces nerve conduction velocity 3-fold. The mutation impedes intracellular transport and surface expression of Caspr and leaves NF155 on apposing paranodal myelin disengaged. The contactin mutation does not affect sodium channel clustering at the nodes of Ranvier but alters the location of the Shaker-type Kv1.1 and Kv1.2 potassium channels. Thus, contactin is a crucial part in the machinery that controls junctional attachment at the paranode and ultimately the physiology of myelinated nerve.
Topics: Aging; Animals; Axons; Cell Adhesion Molecules, Neuronal; Contactins; Crosses, Genetic; Gene Expression Regulation, Developmental; Kv1.1 Potassium Channel; Kv1.2 Potassium Channel; Mice; Mice, Inbred C57BL; Mice, Knockout; Microscopy, Electron; Models, Neurological; Nerve Fibers, Myelinated; Neuroglia; Potassium Channels; Potassium Channels, Voltage-Gated; Ranvier's Nodes; Receptors, Cell Surface; Schwann Cells; Sciatic Nerve
PubMed: 11395001
DOI: 10.1016/s0896-6273(01)00296-3 -
The Journal of Neuroscience : the... Jun 2001Netrins are a family of secreted proteins that function as chemotropic axon guidance cues during neural development. Here we demonstrate that netrin-1 continues to be...
Netrins are a family of secreted proteins that function as chemotropic axon guidance cues during neural development. Here we demonstrate that netrin-1 continues to be expressed in the adult rat spinal cord at a level similar to that in the embryonic CNS. In contrast, netrin-3, which is also expressed in the embryonic spinal cord, was not detected in the adult. In situ hybridization analysis demonstrated that cells in the white matter and the gray matter of the adult spinal cord express netrin-1. Colocalization studies using the neuronal marker NeuN revealed that netrin-1 is expressed by multiple classes of spinal interneurons and motoneurons. Markers identifying glial cell types indicated that netrin-1 is expressed by most, if not all, oligodendrocytes but not by astrocytes. During neural development, netrin-1 has been proposed to function as a diffusible long-range cue for growing axons. We show that in the adult spinal cord the majority of netrin-1 protein is not freely soluble but is associated with membranes or the extracellular matrix. Fractionation of adult spinal cord white matter demonstrated that netrin-1 was absent from fractions enriched for compact myelin but was enriched in fractions containing periaxonal myelin and axolemma, indicating that netrin-1 protein may be localized to the periaxonal space. These findings suggest that in addition to its role as a long-range guidance cue for developing axons, netrin may have a short-range function associated with the cell surface that contributes to the maintenance of appropriate neuronal and axon-oligodendroglial interactions in the mature nervous system.
Topics: Aging; Animals; Antigens, Differentiation; Astrocytes; Axons; Cell Lineage; Cell Membrane; Cloning, Molecular; Extracellular Matrix; Gene Expression Regulation, Developmental; Genes, Reporter; Interneurons; Male; Mice; Mice, Transgenic; Molecular Sequence Data; Motor Neurons; Myelin Sheath; Nerve Growth Factors; Nerve Tissue Proteins; Netrin-1; Netrins; Neurons; Oligodendroglia; Rats; Rats, Sprague-Dawley; Spinal Cord; Subcellular Fractions; Tumor Suppressor Proteins
PubMed: 11356879
DOI: 10.1523/JNEUROSCI.21-11-03911.2001