11th International Neuroscience Winter Conference Symposium: Neuronal damage and repair in multiple sclerosis

The 11th International Neuroscience Winter Conference in Sölden took place from 31 March to 4 April 2009, and covered a broad spectrum of basic neuroscience topics as well as translational aspects in areas including fear, anxiety, Huntington’s disease and multiple sclerosis (MS). This article concentrates on a symposium that took place on 2 April and was organized by Mathias Bähr (University of Göttingen Medical Centre, Germany) and Frauke Zipp (Cecilie-Vogt-Klinik, Berlin, Germany).

In her lecture regarding neuronal damage and repair in the inflamed CNS, Zipp introduced novel data on neuronal pathology in MS and in its animal model, experimental autoimmune encephalomyelitis (EAE). Zipp’s group has experimental and clinical data for a marked loss of lower motor neurons in MS patients. They regularly found dying spinal motor neurons surrounded by CD3 ⁺ (as well as CD4 ⁺ and CD8 ⁺) T cells expressing TNF-related apoptosis-inducing ligand (TRAIL) in post-mortem tissue from MS patients. Based on these findings and on experimental data in EAE, their study indicates that damage to lower motor neurons and TRAIL-mediated inflammatory neurodegeneration in the spinal cord contribute to MS pathology [1].

Performing two-photon laser scanning microscopy in the brainstem of living mice suffering from EAE, the group furthermore visualized T cells directly attacking neurons and their processes. Prior to their effector function, the T cells exert distinct behavior at the BBB with CD4 ⁺ T-cell compartmentalization in the parenchyma along CNS vessels. This process is dependent on the chemokine CXCR4, while key adhesion molecules seem to have no major function in this process [2]. This is in contrast to their essential role in the transmigration step, in which integrin-mediated adhesion is necessary to overcome endothelial barriers. Within the perivascular compartment, the lymphocytes can interact with each other and/or other immune cells as a prerequisite for the sequential steps involved in the process of inflammation and immune regulation. Collectively, these findings support the idea that compartmentalization of activated CD4 ⁺ T lymphocytes in the target organ serves a similar purpose as in lymph nodes, that is to say, increasing the chance of T-cell receptor and coreceptor engagement, with the whole spectrum of possible outcomes between tolerance and immunity.

With regard to potential repair properties of the inflamed CNS, Zipp presented a study on the cell-fate decision of neural progenitor cells (NPCs) being affected by the redox state with oxidizing conditions favoring differentiation in astrocytes, and reducing conditions favoring neuron formation. To analyze the molecular mechanism underlying these effects, the group investigated a gene known as Hairy/enhancer of split 1 (Hes1), a transcriptional repressor of Mash1, which, in turn, is responsible for the activation of a neuron-specific transcription program. They found that under oxidizing conditions, the histone deacetylase Sirt1 and Hes1 form a complex that binds to and deacetylates histones at the Mash1 promoter. These events cause downregulation of Mash1 expression and block neuronal differentation. In a reducing environment, the Hes1–Sirt1 complex is not observed. Instead, Hes1 recruits transcription activators, such as CREB-binding protein to the Mash1 promoter, and this drives NPC towards a neuronal fate [3]. The influence of the redox state on NPC cell-fate decisions was eliminated by removal of Sirt1 activity either by RNAi or through the use of Sirt1 inhibitors. Thus, through its action at the Mash1 promoter, Sirt1 seems to act as a cell-fate decision switch in NPCs in vitro, with increased Sirt1 activity causing increased differentiation of NPCs into astrocytes at the expense of neurons. Using healthy mice and EAE, in vivo investigations followed. Injecting young mice with an oxidizing agent increased the number of newly differentiated Sirt1-positive astrocytes in the brain. Performing in utero electroporation of GFP-marked RNAi constructs against Sirt1 to deplete Sirt1, in collaboration with Robert Nitsch, an increased proportion of Mash1-positive cells was found in postnatal animals treated with an oxidizing agent. EAE induces an oxidizing environment and reactive astroglyosis. While Sirt1 expression was low in unaffected brain regions, inflamed areas with an abundance of invading leukocytes contained an increased number of cells positive for both GFAP and Sirt1.

In the second lecture, Mathias Bähr further detailed the problem that with secondary progressive MS, many patients experience a continuous deterioration of their clinical symptoms and inflammatory and immunomodulatory treatment strategies become more and more ineffective. In order to study axonal destruction and neuronal cell death in relevant model systems, he explained the use of an optic neuritis model in brown Norway rats that has been established some time ago and allows monitoring of visual perimeters in pigmented rats such as electro-retinogram and visual-evoked potentials.

Unexpectedly, Bähr and his coworkers observed an early onset of axon degeneration and inflammation in the optic nerve and a loss of amplitudes in visual-evoked potentials before the onset of other clinical symptoms of EAE. This was also correlated with a massive loss of retinal ganglion cells (RGCs) in these animals. It could be demonstrated that axonal damage that took place in the optic nerve was mainly mediated by an upregulation of N-type calcium channels. A massive influx of calcium and initiation of secondary destructive cascades led to apoptotic cell death of the RGCs.

In order to test neuroprotective treatment strategies, several growth factors and antiapoptotic substances were tested, which were able to transiently rescue the ganglion cells but could not stop the cell death process, nor rescue optic nerve axons. Even the standard treatment of MS with high-dose cortisone, which leads to a significant reduction of the immune cells infiltrated (mainly macrophages in T cells) in the optic nerve, enhanced damage in the retina by downregulating protective signaling cascades in the RGCs. Only a combination of a high-dose cortisone treatment with a neuroprotective treatment (erythropoietin application) led to a significant recovery of visual functions.

Bähr demonstrated how these findings in the animal model system were recently translated into a clinical study with patients who experience a first ever episode of optic neuritis and now receive an add-on-regimen of high-dose cortisone and erythropoietin in a prospective, placebo-controlled, randomized multicenter trial.

In the third lecture given by Jaqueline Trotter (Johannes Gutenberg-Universität Mainz, Germany) the problem of remyelination of plaques and the recruitment of oligodendrocyte precursor cells was addressed. Trotter introduced the biological cell basis of intracellular trafficking of myelinproteins and relevant signaling cascades. Her group demonstrated that the src-family, tyrosine kinase Fyn, is one of the key regulators of the demyelination process and the signaling factor is trans-acting factor heterogeneous nuclear ribonucleoprotein (hnPNP), which is regulated by active Fyn.

Furthermore, Trotter could demonstrate that the neuronal adhesion molecule L1 binds to oligodendrocytes and leads to Fyn activation, thus opening a new avenue for influencing remyelination processes in demyelinated areas by activation of oligodendrocyte precursors in MS lesions.

In a final talk given by Sven Meuth (Würzburg, Germany), a new family of acid-sensitive potassium channels (TASK1, -2 and -3) that seem to be relevant for setting the resting potential and regulating neuronal excitability were introduced. These channels are expressed on T cells and neurons and might be key modulators of T-cell immunity and neurodegeneration. Pharmacological blockade of TASK channels or knockdown with siRNA resulted in reduced T-cell function, while overexpression of TASK had the opposite effect. The group demonstrated that mice lacking the TASK1 channel show significantly decreased clinical signs of EAE and markedly reduced axonal degeneration as compared with wild-type controls. The pathophysiological relevance of these findings for human disease is shown by a significant TASK channel upregulation in CD4 ⁺ and CD8 ⁺ T cells in relapsing-remitting MS (RRMS) patients during acute relapses as well as higher levels on CD8 ⁺ T cells of RRMS patients during clinically stable disease course. Importantly, putatively pathogenic T cells in cerebrospinal fluid exhibit elevated TASK channel levels, and TASK-positive T cells can be found in inflammatory lesions in MS tissue specimens, thus showing that modulation of these channels may allow the development of new immunomodulatory and protective treatment strategies in the future.

In summary, the symposium demonstrated that basic research with animal models of MS is opening up new insights into the complex network of autoimmune destruction, protection and repair in the nervous system and may in the future allow development of new treatment strategies for the new degenerative part of MS.