Ion Channels In Multiple Sclerosis

The presence of intact ParaNodal junctions on Myelinated Axons in the CNS and PNS is crucial for both Myelin Sheath attachment and Saltatory Impulse Conduction.

The Axonal GlycoProtein Contactin-associated protein (Caspr) is expressed in the ParaNodal region and plays an important role in the creation and maintenance of these Adhesive Junctions.

In the present study, AntiBodies to Caspr were used to assess the integrity of ParaNodal Junctions on Myelinated Axons in Brain and Spinal Cord tissue from subjects with longstanding Multiple Sclerosis, a Neurological Disorder that affects both Myelin and Axons.

Triple ImmunoFluorescence combined with Confocal Laser Scanning Microscopy showed that Axons in the DeMyelinated center of the 36 Brain and 16 Spinal Cord Multiple Sclerosis lesions studied were devoid of Caspr ImmunoReactivity.

Suggesting that Axons down regulate the expression of Caspr following DeMyelination. Additional data indicated that Caspr reappears in the ParaNodal region with the formation of new Myelin Sheaths.

Immuno labelling further revealed that Caspr on Myelinated Axons in border regions was often no longer concentrated in the ParaNodal region.

But, was also present in the InterNodal region a phenomenon particularly common in the borders of the more chronic lesions in the collection.

Myelinated Axons with long Caspr-positive stretches were often present at a considerable distance from the lesion edges.

These findings raise the possibility that the aberrant location of Caspr is an early sign of impending Myelin loss.

This would imply that DeMyelination continues at a slow rate in established lesions. The diameters of Caspr-positive structures on some Myelinated Axons near the lesion edges were also increased.

Moreover, the gap between individual Myelin Sheaths on these apparently swollen Axons was widened occasionally and a very small Myelin Sheath plus additional Caspr-positive structures had sometimes formed in the enlarged space.

This finding thus suggests that the formation of new Myelin in Multiple Sclerosis is not only induced following the loss of complete InterNodes but also in response to broadening of the Nodal region.

Interestingly, alterations in the expression and localization of Caspr were observed in tissue from both subjects with the Primary and Secondary/Progressive form of Multiple Sclerosis.

In summary, the present study provides ImmunoHistoChemical evidence that ParaNodal Junctions on some Myelinated Axons in the borders of lesions of patients with Chronic Progressive Multiple Sclerosis are no longer intact.

This may impair Saltatory Impulse Conduction and lead to further Myelin loss, thereby contributing to disease progression in Multiple Sclerosis.

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.

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:

1. Increasing the amplitude and duration of Action Potentials
2. Decreasing the proportion of Action Potentials that are conglomerate and
   • The number of spikes per conglomerate Action Potential
3. 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.

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.

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.

Clinical abnormalities in Multiple Sclerosis (MS) have classically been considered to be caused by DeMyelination and/or Axonal degeneration.

The possibility of molecular changes in Neurons, such as the deployment of abnormal repertoires of Ion Channels that would alter Neuronal ElectroGenic properties, has not been considered.

Sensory Neuron-Specific Sodium Channel SNS displays a depolarized Voltage dependence, slower activation and inactivation kinetics, and more rapid recovery from inactivation than classical "fast" Sodium Channels.

SNS is selectively expressed in Spinal Sensory and Trigeminal Ganglion Neurons within the Peripheral Nervous System and is not expressed within the normal Brain.

Here we show that Sodium Channel SNS mRNA and protein, which are not present within the Cerebellum of control mice, are expressed within Cerebellar Purkinje Cells in a mouse model of MS, Chronic Relapsing Experimental Allergic Encephalomyelitis.

We also demonstrate SNS mRNA and protein expression within Purkinje Cells from tissue obtained postmortem from patients with MS, but not in control subjects with no Neurological Disease.

These results demonstrate a change in Sodium Channel expression in Neurons within the Brain in an animal model of MS and in humans with MS.

And suggest, that abnormal patterns of Neuronal Ion Channel expression may contribute to clinical abnormalities such as Ataxia in these disorders.

Voltage-and Ligand-gated Ion Channels are key players in Synaptic transmission and Neuron-Glia communication in the Nervous System.

Expression of these proteins can be regulated at several levels (Transcriptional, Translational, or PostTranslational) and by multiple ExtraCellular Factors in the developing and mature Nervous System.

A wide variety of Hormones and Growth Factors have been identified as important in Neural Cell differentiation, which is a complex process involving the acquisition of cell-type-specific Ion Channel phenotypes.

Much literature has already accumulated describing the structural and functional characteristics of Ion Channels, but relatively little is known about the factors.

That influence their synthesis and cell surface expression, although this area has generated considerable interest in the context of Neural Cell development.

This article reviews several examples of regulated expression of these Channels by Cellular Factors, namely Peptide Growth Factors and Steroid Hormones.

And discusses, where applicable, current understanding of molecular mechanisms underlying such regulation of Voltage-and NeuroTransmitter-gated Ion Channels.

The ElectroPhysiologist's view of Brain Astrocytes has changed markedly in recent years. In the past Astrocytes were viewed as passive, K⁺ selective cells.

But, it is now evident that they are capable of expressing Voltage- and Ligand-activated Channels previously thought to be restricted to Neurons.

The functional importance of most of these Ion Channels is not understood at present. However, from studies of Astrocytes cultured from different species and Brain regions.

We learned that like their Neuronal counterparts, Astrocytes are a heterogeneous group of Brain Cells showing similar heterogeneity in their Ion-Channel expression.

Not only are subpopulations of Astrocytes within areas of the Brain equipped with specific sets of Ion Channels but, furthermore, regional heterogeneity is apparent.

In addition, Astrocyte Ion Channel expression is dynamic and changes during development. Some Ion Channels are only expressed postnatally, yet others appear to be expressed only during certain stages of development.

Interestingly, the expression of some Astrocyte Channels, including Na⁺, Ca2⁺, and some K⁺ Channels, appears to be controlled by Neurons via mechanisms that are presently unknown.

Some studies suggest roles for Astrocyte Channels in basic cell processes such as cell proliferation. Thus, although the role of some Astrocyte Channels remains unclear.

Our understanding of Astrocyte physiology is starting to take shape and points towards roles of Ion Channels not involved in ElectroGenesis.