Abnormal Sodium Channel Distribution In Optic Nerve Axons In A Model Of Inflammatory DeMyelination
Matthew J. Craner, Albert C. Lo, Joel A. Black and Stephen G. Waxman
Brain, Vol. 126, No. 7, 1552-1561, July 2003
Yale University School of Medicine, Department of Neurology and PVA/EPVA Center for NeuroScience Research, New Haven and Rehabilitation Research Center, VA Connecticut Healthcare System, West Haven, CT, USA
Myelinated fibres are characterized by the aggregation of Nav1.6 Sodium Channels within the Axon membrane at Nodes of Ranvier, where their presence supports Saltatory Conduction.
In this study, we used ImmunoCytoChemical methods to study the organization of Sodium Channels along Axons in Experimental Allergic Encephalomyelitis (EAE), a model of Multiple Sclerosis.
We studied Axons within the Optic Nerve, a CNS tract commonly affected in Multiple Sclerosis, and their cell bodies of origin (Retinal Ganglion Cells), using subtype-specific AntiBodies.
Generated against Sodium Channel subtypes Nav1.1, Nav1.2, Nav1.3 and Nav1.6, which previously have been shown to be expressed by Retinal Ganglion Cells.
We demonstrate a significant switch from Nav1.6 to Nav1.2 expression in the Optic Nerve in EAE.
There was a reduction in frequency of Nav1.6-positive Nodes (84.5% Nav1.6-ImmunoPositive Nodes in control versus 32.9% in EAE) and increased frequency of Nav1.2-positive Nodes (11.8% Nav1.2 ImmunoPositive Nodes in control versus 74.9% in EAE).
Moreover, we observed a significant increase in the number of linear (presumably DeMyelinated) Axonal profiles demonstrating extended diffuse ImmunoStaining for Nav1.2 in EAE versus control Optic Nerves.
These changes within the Optic Nerve are paralleled by decreased levels of Nav1.6 and increased Nav1.2 protein, together with increased levels of Nav1.2 mRNA, within Retinal Ganglion Cells in EAE.
Our findings of a loss of Nav1.6 and increased expression of Nav1.2 suggest that electrogenesis in EAE may revert to a stage similar to that observed in immature Retinal Ganglion Cells in which Nav1.2 Channels support Conduction of Action Potentials along Axons.
Expression Of Na(v)1.8 Sodium Channels Perturbs The Firing Patterns Of Cerebellar Purkinje Cells
Renganathan M, Gelderblom M, Black JA, Waxman SG
Brain Res 2003 Jan 10;959(2):235-42
Yale Medical School, Department of Neurology LCI-707 and PVA/EPVA NeuroScience Research Center, P.O. Box 208018, 06510, New Haven, CT, USA
The Sensory Neuron specific Sodium Channel Na(v)1.8/SNS exhibits depolarized Voltage-dependence of inactivation, slow inactivation and rapid repriming, which differentiate it from other Voltage-gated Sodium Channels.
Na(v)1.8 is normally selectively expressed at high levels in Sensory Ganglion Neurons, but not within the CNS. However, expression of Na(v)1.8 mRNA and protein are upregulated within Cerebellar Purkinje Cells in animal models of Multiple Sclerosis (MS), and in human MS.
To examine the effect of expression of Na(v)1.8 on the activity pattern of Purkinje Cells, we biolistically introduced Na(v)1.8 cDNA into these cells in vitro.
We report here that Na(v)1.8 can be functionally expressed at physiological levels (similar to the levels in DRG Neurons where Na(v)1.8 is normally expressed) within Purkinje Cells.
And, that its expression alters the activity of these Neurons in three ways:
- Increasing the amplitude and duration of Action Potentials
- Decreasing the proportion of Action Potentials that are conglomerate and
- The number of spikes per conglomerate Action Potential
- Contributing to the production of sustained, pacemaker-like Impulse trains in response to depolarization
These results provide support for the hypothesis that the expression of Na(v)1.8 Channels within Purkinje Cells, which occurs in MS, may perturb their function.
Axotomy Does Not Up-Regulate Expression Of Sodium Channel Na(v)1.8 In Purkinje Cells
Black JA, Dusart I, Sotelo C, Waxman SG
Brain Res Mol Brain Res 2002 May 30;101(1-2):126-31
Yale University School of Medicine, Department of Neurology and PVA/EPVA Center for NeuroScience Research, New Haven, CT 06510, USA
Aberrant expression of the Sensory Neuron Specific (SNS) Sodium Channel Na(v)1.8 has been demonstrated in Cerebellar Purkinje Cells in experimental models of Multiple Sclerosis (MS) and in human MS.
The aberrant expression of Na(v)1.8, which is normally present in primary Sensory Neurons but not in the CNS, may perturb Cerebellar function, but the mechanisms that trigger it are not understood.
Because Axotomy can provoke changes in Na(v)1.8 expression in Dorsal Root Ganglion (DRG) Neurons, we tested the hypothesis that Axotomy can provoke an up-regulation of Na(v)1.8 expression in Purkinje Cells, using a surgical model that transects Axons of Purkinje Cells in Lobules IIIb-VII in the rat.
In Situ Hybridization and ImmunoCytoChemistry did not reveal an up-regulation of Na(v)1.8 mRNA or protein in Axotomized Purkinje Cells. Hybridization and ImmunoStaining signals for the Sodium Channel Na(v)1.6 were clearly present.
Demonstrating that Sodium Channel transcripts and protein were present in experimental Cerebella. These results demonstrate that Axotomy does not trigger the expression of Na(v)1.8 in Purkinje Cells.
Factors Directly Affecting Impulse Transmission In Inflammatory DeMyelinating Disease: Recent Advances In Our Understanding
Smith KJ, Hall SM
Curr Opin Neurol 2001 Jun;14(3):289-98
Guy's, King's and St Thomas' School of Medicine, Dentistry and BioMedical Sciences, Department of NeuroImmunology, NeuroInflammation Research Group, Guy's Campus, London SE1 9RT, UK
DeMyelination and inflammation both contribute to the Neurological deficits characteristic of Multiple Sclerosis and Guillain-Barre Syndrome.
Conduction deficits attributable to DeMyelination are well known, but it is becoming clear that factors such as Nitric Oxide, Endocaine, Cytokines, and AntiGanglioside AntiBodies also play significant roles.
DeMyelination directly affects Conduction and also causes changes in both the distribution and repertoire of expressed Axolemmal Ion Channels, which in turn affect Impulse propagation and can promote HyperExcitability.
In conducting Axons, sustained trains of Impulses can produce intermittent Conduction Failure, and, in the presence of Nitric Oxide exposure, can also cause Axonal Degeneration.
Other factors impairing Impulse transmission include Nodal widening, Glutamate toxicity, and disturbances of both the Blood-Brain Barrier and Synaptic Transmission.