Re-expression of a developmentally restricted potassium channel in autoimmune demyelination: Kv1.4 is implicated in oligodendroglial proliferation

Altered Expression of Ion Channels in White Matter Lesions of Progressive Multiple Sclerosis: What Do We Know About Their Function?

Despite significant advances in our understanding of the pathophysiology of multiple sclerosis (MS), knowledge about contribution of individual ion channels to axonal impairment and remyelination failure in progressive MS remains incomplete. Ion channel families play a fundamental role in maintaining white matter (WM) integrity and in regulating WM activities in axons, interstitial neurons, glia, and vascular cells. Recently, transcriptomic studies have considerably increased insight into the gene expression changes that occur in diverse WM lesions and the gene expression fingerprint of specific WM cells associated with secondary progressive MS. Here, we review the ion channel genes encoding K⁺, Ca²⁺, Na⁺, and Cl^- channels; ryanodine receptors; TRP channels; and others that are significantly and uniquely dysregulated in active, chronic active, inactive, remyelinating WM lesions, and normal-appearing WM of secondary progressive MS brain, based on recently published bulk and single-nuclei RNA-sequencing datasets. We discuss the current state of knowledge about the corresponding ion channels and their implication in the MS brain or in experimental models of MS. This comprehensive review suggests that the intense upregulation of voltage-gated Na⁺ channel genes in WM lesions with ongoing tissue damage may reflect the imbalance of Na⁺ homeostasis that is observed in progressive MS brain, while the upregulation of a large number of voltage-gated K⁺ channel genes may be linked to a protective response to limit neuronal excitability. In addition, the altered chloride homeostasis, revealed by the significant downregulation of voltage-gated Cl^- channels in MS lesions, may contribute to an altered inhibitory neurotransmission and increased excitability.

Functional role of endogenous Kv1.4 in experimental demyelination

During neuroinflammation, the shaker type potassium channel Kv1.4 is re-expressed in oligodendrocytes (Ol), but not immune cells. Here, we analyze the role of endogenous Kv1.4 in two demyelinating animal models of multiple sclerosis. While Kv1.4 deficiency in primary murine Ol led to a decreased proliferation rate in vitro, it did not exert an effect on Ol proliferation or on the extent of de- or remyelination in the cuprizone model in vivo. However, in experimental autoimmune encephalomyelitis, Kv1.4^-/- mice exhibited a milder disease course and reduced Th1 responses. These data argue for an indirect effect of Kv1.4 on immune cells, possibly via glial cells.

Kir4.1 channels in NG2-glia play a role in development, potassium signaling, and ischemia-related myelin loss

The contribution of the inwardly rectifying K⁺ channel subtype Kir4.1 has been focused mainly on astrocytes, where they play important roles in the maintenance of resting membrane potential, extracellular K⁺ uptake, and facilitation of glutamate uptake in the central nervous system. Here, we report the role of Kir4.1 channels in NG2-glia during brain development, potassium signaling, and in an ischemic stroke disease model. Kir4.1 channels are widely expressed in NG2-glia during brain development. In the adult mouse hippocampus, Kir4.1 channels in NG2-glia constitute more than 80% of K⁺ channels inward currents. This large portion of Kir4.1 channel currents exhibits a deficit in NG2-glia as an initial response in a transient ischemic mouse model. Further evidence indicates that Kir4.1 deficits in NG2-glia potentially cause axonal myelin loss in ischemia through the association with oligodendrocyte-specific protein (OSP/Claudin-11), which unravels a potential therapeutic target in the treatment of ischemic stroke.

Feier Song and colleagues have examined Kir4.1 channels in the mouse brain, and found global expression of functional channels during development. They also show that depletion of Kir4.1 channels impacts demyelination in ischemic stroke

Fingolimod effects in neuroinflammation: Regulation of astroglial glutamate transporters?

Fingolimod is an oral sphingosine-1-phosphate-receptor modulator which reduces the recirculation of immune cells and may also directly target glial cells. Here we investigate effects of fingolimod on expression of astroglial glutamate transporters under pro-inflammatory conditions. In astrocyte cell culture, the addition of pro-inflammatory cytokines led to a significant downregulation of glutamate transporters glutamate transporter-1 (slc1a2/SLC1A2) and glutamate aspartate transporter (slc1a3/SLC1A3) expression on the mRNA or protein level. In this setting, the direct application of fingolimod-1 phosphate (F1P) on astrocytes did not change expression levels of slc1a2 and slc1a3 mRNA. The analysis of both transporters on the protein level by Western Blot and immunocytochemistry did also not reveal any effect of F1P. On a functional level, the addition of conditioned supernatants from F1P treated astrocytes to neuronal cell culture did not result in increased neurite growth. In experimental autoimmune encephalomyelitis as a model of multiple sclerosis, fingolimod treatment reduced T cell and macrophages/microglia mediated inflammation and also diminished astrocyte activation. At the same time, fingolimod restored the reduced expression of slc1a2 and slc1a3 in the inflamed spinal cord on the mRNA level and of SLC1A2 and SLC1A3 on the protein level, presumably via indirect, anti-inflammatory mechanisms. These findings provide further evidence for a predominantly peripheral effect of the compound in neuroinflammation.

Role of glial 14-3-3 gamma protein in autoimmune demyelination

Background

The family of 14-3-3 proteins plays an important role in the regulation of cell survival and death. Here, we investigate the role of the 14-3-3 gamma (14-3-3 γ) subunit for glial responses in autoimmune demyelination.

Methods

Expression of 14-3-3 γ in glial cell culture was investigated by reverse transcription polymerase chain reaction (RT-PCR) and immunocytochemistry. 14-3-3 γ knockout mice were subjected to murine myelin oligodendrocyte-induced experimental autoimmune encephalomyelitis (MOG-EAE), an animal model mimicking inflammatory features and neurodegenerative aspects of multiple sclerosis (MS).

Results

Expression studies in cell culture confined expression of 14-3-3 γ to both, oligodendrocytes (OL) and astrocytes. RT-PCR analysis revealed an increased expression of 14-3-3 γ mRNA in the spinal cord during the late chronic phase of MOG-EAE. At that stage, EAE was more severe in 14-3-3 γ knockout mice as compared to age- and gender-matched controls. Histopathological analyses on day 56 post immunization (p.i.) revealed significantly enhanced myelin damage as well as OL injury and secondary, an increase in axonal injury and gliosis in 14-3-3 γ −/− mice. At the same time, deficiency in 14-3-3 γ protein did not influence the immune response. Further histological studies revealed an increased susceptibility towards apoptosis in 14-3-3 γ-deficient OL in the inflamed spinal cord.

Conclusion

These data argue for a pivotal role of 14-3-3 γ-mediated signalling pathways for OL protection in neuroinflammation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0381-x) contains supplementary material, which is available to authorized users.

Targeting two-pore domain potassium channels - a promising strategy for treating T cell mediated autoimmunity

Two-pore domain potassium channels, such as TASK, are essential players in the regulation of the resting membrane potential. A recent publication in Experimental Neurology revealed that the pharmacological blockade of the two-pore domain potassium channel TASK1 improved clinical disease severity, the number of cellular infiltrates, as well as the degree of demyelination in experimental autoimmune encephalomyelitis, an animal model for multiple sclerosis. This effect was driven by a reduction in the generation of proinflammatory cytokines and proliferation of T lymphocytes. Thus, functional activity of T lymphocytes was identified to be TASK1 dependent suggesting that this class of potassium channels may represent an interesting target for treating T cell-driven autoimmunity directed against the central nervous system.

K+ channel alterations in the progression of experimental autoimmune encephalomyelitis

Voltage-gated K(+) (Kv) channels play critical roles not only in regulating synaptic transmission and intrinsic excitability of neurons, but also in controlling the function and proliferation of other cells in the central nervous system (CNS). The non-specific Kv channel blocker, 4-AminoPyridine (4-AP) (Dalfampridine, Ampyra®), is currently used to treat multiple sclerosis (MS), an inflammatory demyelinating disease. However, little is known how various types of Kv channels are altered in any inflammatory demyelinating diseases. By using established animal models for MS, experimental autoimmune encephalomyelitis (EAE), we report that expression and distribution patterns of Kv channels are altered in the CNS correlating with EAE severity. The juxtaparanodal (JXP) targeting of Kv1.2/Kvβ2 along myelinated axons is disrupted within demyelinated lesions in the white matter of spinal cord in EAE. Moreover, somatodendritic Kv2.1 channels in the motor neurons of lower spinal cord significantly decrease correlating with EAE severity. Interestingly, Kv1.4 expression surrounding lesions is markedly up-regulated in the initial acute phase of both EAE models. Its expression in glial fibrillary acidic protein (GFAP)-positive astrocytes further increases in the remitting phase of remitting-relapsing EAE (rrEAE), but decreases in late chronic EAE (chEAE) and the relapse of rrEAE, suggesting that Kv1.4-positive astrocytes may be neuroprotective. Taken together, our studies reveal myelin-dependent and -independent alterations of Kv channels in the progression of EAE and lay a solid foundation for future study in search of a better treatment for MS.

Kv1.1-dependent control of hippocampal neuron number as revealed by mosaic analysis with double markers

Megencephaly, or mceph, is a spontaneous frame-shift mutation of the mouse Kv1.1 gene. This mceph mutation results in a truncated Kv1.1 channel α-subunit without the channel pore domain or the voltage sensor. Interestingly, mceph/mceph mouse brains are enlarged and – unlike wild-type mouse brains – they keep growing throughout adulthood, especially in the hippocampus and ventral cortex. We used mosaic analysis with double markers (MADM) to identify the underlying mechanism. In mceph-MADM6 mice with only a small fraction of neurons homozygous for the mceph mutation, those homozygous mceph/mceph neurons in the hippocampus are more abundant than wild-type neurons produced by sister neural progenitors. In contrast, neither mceph/mceph astrocytes, nor neurons in the adjacent dorsal cortex (including the entorhinal and parietal cortex) exhibited overgrowth in the adult brain. The sizes of mceph/mceph hippocampal neurons were comparable to mceph/+ or wild-type neurons. Our mosaic analysis reveals that loss of Kv1.1 function causes an overproduction of hippocampal neurons, leading to an enlarged brain phenotype.

Dalfampridine in multiple sclerosis: from symptomatic treatment to immunomodulation

Multiple sclerosis (MS) is a neurodegenerative disease that is deemed to affect more than 2.1 million people worldwide, and for which there is no cure. Early symptoms of MS are believed to result from axonal demyelination leading to slowing or blockade of impulse conduction. The blockade of K+ channels has been proven to improve conduction deficiencies secondary to demyelination in patients with MS. Dalfampridine is a K+ channel blocker that was recently approved by FDA for the symptomatic treatment of ambulation hardship in MS. Understanding the mechanisms by which Dalfampridine exerts its therapeutic effects is a complex issue as it blocks a wide variety of K+ channels that are distributed across multiple cell types in the nervous system but also in the immune system, and because of their molecular identities remaining unknown. This review describes Dalfampridine potential roles at the cellular and molecular levels in MS pathogenesis.

The epigenetics of autoimmunity

The etiology of autoimmune diseases remains largely unknown. Concordance rates in monozygotic twins are lower than 50% while genome-wide association studies propose numerous significant associations representing only a minority of patients. These lines of evidence strongly support other complementary mechanisms involved in the regulation of genes expression ultimately causing overt autoimmunity. Alterations in the post-translational modification of histones and DNA methylation are the two major epigenetic mechanisms that may potentially cause a breakdown of immune tolerance and the perpetuation of autoimmune diseases. In recent years, several studies both in clinical settings and experimental models proposed that the epigenome may hold the key to a better understanding of autoimmunity initiation and perpetuation. More specifically, data support the impact of epigenetic changes in systemic lupus erythematosus, rheumatoid arthritis, multiple sclerosis and other autoimmune diseases, in some cases based on mechanistical observations. We herein discuss what we currently know and what we expect will come in the next future. Ultimately, epigenetic treatments already being used in oncology may soon prove beneficial also in autoimmune diseases.

Impact of global cerebral ischemia on K+ channel expression and membrane properties of glial cells in the rat hippocampus

Astrocytes and NG2 glia respond to CNS injury by the formation of a glial scar. Since the changes in K(+) currents in astrocytes and NG2 glia that accompany glial scar formation might influence tissue outcome by altering K(+) ion homeostasis, we aimed to characterize the changes in K(+) currents in hippocampal astrocytes and NG2 glia during an extended time window of reperfusion after ischemic injury. Global cerebral ischemia was induced in adult rats by bilateral, 15-min common carotid artery occlusion combined with low-pressure oxygen ventilation. Using the patch-clamp technique, we investigated the membrane properties of hippocampal astrocytes and NG2 glia in situ 2 hours, 6 hours, 1 day, 3 days, 7 days or 5 weeks after ischemia. Astrocytes in the CA1 region of the hippocampus progressively depolarized starting 3 days after ischemia, which coincided with decreased Kir4.1 protein expression in the gliotic tissue. Other K(+) channels described previously in astrocytes, such as Kir2.1, Kir5.1 and TREK1, did not show any changes in their protein content in the hippocampus after ischemia; however, their expression switched from neurons to reactive astrocytes, as visualized by immunohistochemistry. NG2 glia displayed increased input resistance, decreased membrane capacitance, increased delayed outwardly rectifying and A-type K(+) currents and decreased inward K(+) currents 3 days after ischemia, accompanied by their proliferation. Our results show that the membrane properties of astrocytes after ischemia undergo complex alterations, which might profoundly influence the maintenance of K(+) homeostasis in the damaged tissue, while NG2 glia display membrane currents typical of proliferating cells.

Models of autoimmune demyelination in the central nervous system: on the way to translational medicine

Multiple sclerosis (MS) is the most common neurologic disease of young adults. In the recent years, our understanding on disease pathomechanisms has considerably improved and new therapies have emerged. Yet a cure for this devastating disorder is still a far cry away and human resources on ex vivo specimens are limited. More than 70 years after its first description, experimental autoimmune encephalomyelitis (EAE) remains an important tool to understand concepts of T cell mediated autoimmunity as well as the roles of the innate and the humoral immune systems. Some EAE models also well reflect mechanisms of tissue damage including demyelination, axonal injury and also cortical changes. A limitation of the classical EAE model is a neglect of CD8 T cell mediated immune mechanisms. Moreover, well characterized models for primary progressive MS or demyelination patterns involving primary oligodendrocyte dystrophy are still not available. Yet many current therapeutic concepts including glatiramer acetate or natalizumab stem from their successful first application in EAE models. New strategies include the widespread use of conditional knockout mice to understand the cell-type specific function of single genes, innovative approaches to establish models on the roles of B cells and CD8 T cells as well as on the relation of inflammation to primary degeneration. In summary, EAE models continue to play an important role in neuroimmunology thereby also stimulating research in other fields of the neurosciences and immunobiology.

Evaluating epigenetic landmarks in the brain of multiple sclerosis patients: a contribution to the current debate on disease pathogenesis

The evidence suggesting a role of epigenetics in the definition of complex trait diseases is rapidly increasing. The gender prevalence of multiple sclerosis, the low level concordance in homozygous twins and the linkage to several genetic loci, suggest an epigenetic component to the definition of this demyelinating disorder. While the immune etio-pathogenetic mechanism of disease progression has been well characterized, still relatively little is known about the initial events contributing to onset and progression of the demyelinating lesion. This article addresses the challenging question of whether loss of the mechanisms of epigenetic regulation of gene expression in the myelinating cells may contribute to the pathogenesis of multiple sclerosis, by affecting the repair process and by modulating the levels of enzymes involved in neo-epitope formation. The role of altered post-translational modifications of nucleosomal histones and DNA methylation in white matter oligodendroglial cells are presented in terms of pathogenetic concepts and the relevance to therapeutic intervention is then discussed.

Pattern of axonal injury in murine myelin oligodendrocyte glycoprotein induced experimental autoimmune encephalomyelitis: implications for multiple sclerosis

Axonal damage is a correlate for increasing disability in multiple sclerosis. Animal models such as experimental autoimmune encephalomyelitis (EAE) may help to develop better therapeutical neuroprotective strategies for the human disease. Here we investigate the pattern of axonal injury in murine myelin oligodendrocyte glycoprotein peptide 35-55 (MOG) induced EAE. Inflammatory infiltration, axonal densities and expression of amyloid precursor protein (APP), neurofilaments (SMI31 and 32) as well as expression of sodium channels were quantified in lesions, the perilesional area and normal appearing white matter (NAWM). Quantification of T cells and macrophages revealed a significant reduction of inflammatory infiltration at later disease stages despite an increase of demyelinated areas and persistent clinical disability. In lesions, axonal density was already significantly reduced early and throughout all investigated disease stages. A significant axonal loss was also seen in the grey matter and at later time points in the perilesion as well as NAWM. Numbers of axons characterized by non-phosphorylated neurofilaments and re-distribution of sodium channels 1.2 and 1.6 increased over the course of MOG-EAE whilst APP positive axons peaked at the maximum of disease. Finally, double-labeling experiments revealed a strong colocalization of sodium channels with APP, neurofilaments and the axonal nodal protein Caspr, but not glial and myelin markers in actively demyelinating lesions. In summary, progressive axonal loss distant from lesions is mainly associated with changes in neurofilament phosphorylation, re-distribution of sodium channels and demyelination. This axonal loss is dissociated from acute inflammatory infiltration and markedly correlates with clinical impairment. Consequently, therapeutic intervention may be promising at early stages of EAE focusing on inflammation, or later in disease targeting degenerative mechanisms.