Archived Research Summaries

The Myelin Repair Foundation is committed to advancing research as rapidly as possible through prompt publication of results.

June/July 2009

Myelin Repair Foundation Researchers identify signal that is important for the formation of the blood-brain barrier (BBB). Understanding these mechanisms may help develop therapeutics for patients with multiple sclerosis by rebuilding the BBB and limiting the damage by immune infiltration.

Proceedings of the National Academy of Sciences of the U.S.A. 2009 Jan 13;106(2):641-6.

Wnt/beta-catenin signaling is required for CNS, but not non-CNS, angiogenesis.

Authors: Richard Daneman, Dritan Agalliu, Lu Zhou, Frank Kuhnert, Calvin J. Kuo, and Ben A. Barres

Summary: The blood vessels in the central nervous system (CNS) form a specialized structure termed the blood-brain barrier (BBB) that limits the movement of molecules and ions from moving from the blood to the brain. This BBB is important for proper neuronal function as well as protecting the CNS from injury and disease. This is especially important in MS, where a breakdown of the BBB allows immune cells into the CNS which causes debilitating damage to the myelin. Therefore understanding the mechanisms of BBB formation may allow for repair of the BBB in patients suffering from multiple sclerosis, limiting the damage by immune infiltration.

In this paper we have identified that Wnt/beta-catenin signaling is important for the formation of the BBB. We demonstrate that Wnt signaling is specifically activated in CNS blood vessels during development, and that disruption of this leads to defects in CNS blood vessel formation, and the loss of specific BBB properties. This provides a new mechanistic understanding for BBB formation. Previous models suggest that blood vessel formation in all tissues was the same, and that BBB formation occurs after vessel formation. This study demonstrates that blood vessel formation in CNS uses a different molecular mechanism than in other tissues, and this is tied in with BBB formation. Understanding these mechanisms may help develop therapeutics for rebuilding the BBB in patients with multiple sclerosis.

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May 2009

Myelin Repair Foundation Researchers have developed a novel method to identify proteins that promote the survival of distinct populations of neurons in the brain. This approach may significantly enhance our understanding of the survival properties of neuronal populations at risk in neurological disorders.

Journal of Neuroscience (2008), August vol 28: pp 8294-305.

A Novel Purification Method for CNS Projection Neurons Leads to the Identification of Brain Vascular Cells As a Source of Trophic Support for Corticospinal Motor Neurons

Authors: Dugas JC, Mandemakers W, Rogers M, Ibrahim A, Daneman R, Barres BA.

Summary: Currently, one impediment to developing therapies targeting specific subpopulations of central nervous system (CNS) neurons is the inability to purify and examine distinct subpopulations of neurons in isolation. To address this problem, we have developed a novel purification scheme, called CTB immunopanning, in which the axons of a particular CNS neuron population are selectively labeled via localized injection of a tracer, cholera toxin beta (CTB). This tracer is transported back to neuronal cell bodies attached to labeled axons, at which point antibodies that recognize CTB can be used to gently purify the labeled subpopulation of neurons. As a proof of principle, we have utilized this novel method to purify two distinct populations of CNS neurons: retinal ganglion cells of the eye, and corticospinal motor neurons (CSMNs) of the cortex. We then utilized gene chip technology to perform a detailed characterization of gene expression in the purified population of CSMNs. Interestingly, we found that the CSMNs expressed receptors for several trophic (survival) factors that are normally produced at high levels by the cells that make up the blood vessels of the brain, called cortical endothelial cells and pericytes. In subsequent studies, we found that three of these trophic factors, CXCL12, IGF2, and pleiotrophin, were capable of promoting the survival and healthy maturation of isolated CSMNs in culture. Cumulatively, these findings demonstrate a useful method for the purification of several different types of CNS projection neurons, which in principle should work in many mammalian species, and provide evidence that endothelial-derived factors may represent an overlooked source of trophic support for neurons in the brain. The ability to isolate and study distinct subpopulations of neurons may facilitate the discovery of additional means of promoting neuronal survival in future investigations.

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April 2009

Myelin Repair Foundation sponsored research helps to develop new PET tags that make it possible to image myelin and see myelin repair for the first time, leading to earlier diagnosis and treatment of myelin-related diseases.

Journal of Medicinal Chemistry (2008), November 13:51 (21):6682-8.

Molecular probes for imaging myelinated white matter in CNS.

Authors: Wu C, Wei J, Tian D, Feng Y, Miller RH, Wang Y.

Summary: Myelination is one of the most fundamental biological processes of vertebrate nervous system development. Myelin sheaths provide a unique structure in the nervous system that fosters rapid and efficient conduction of impulses along axons. Abnormalities or changes in myelin occur in many acquired or inherited neurodegenerative diseases such as multiple sclerosis (MS). Recent studies showed that prevalence of multiple sclerosis has increased significantly, yet the reason why increasing number of people are afflicted by multiple sclerosis is unknown. In multiple sclerosis, demyelinated lesions are constantly formed that cannot be repaired through inherent remyelination process, causing axonal damage. Therefore, one promising therapy for multiple sclerosis is to restore or improve remyelination efficiency. To achieve this goal, efforts have been made to promote endogenous repair mechanisms and/or transplant an exogenous source of myelinating cells to the demyelinated regions.

To better assist in battling neurological disorders, physicians and researchers require an imaging tool that would allow direct examination of myelin sheaths in vivo. To date, magnetic resonance imaging (MRI) has been the primary tool for diagnosing and monitoring the evolution of white matter diseases. Unfortunately, any change in signal intensity of MRI typically reflects a change in tissue water content, which is a non-specific measure of overall changes in macroscopic tissue injury that can range from edema and inflammation to demyelination and axonal loss. As a result, the use of MRI as a primary measure of disease activity is often dissociated from the clinical outcome of disease-modifying therapies. A long-standing goal in multiple sclerosis research has been to develop a direct and quantitative measure of myelin content in vivo. However, the lack of molecular probes has limited the progress of myelin imaging and hindered efficacy evaluation of novel myelin repair therapies currently under development.

For this reason, we have developed a series of myelin-imaging agents for positron emission tomography (PET) that selectively localize to white matter in direct proportion to the extent of myelination. To date, we have identified several lead compounds that readily entered the brain and selectively localized to myelinated white matter. One type of these lead compounds has just been reported in the current issue of the Journal of Medicinal Chemistry (November, 2008, 51, 6682–6688). We found that these agents exhibit many biological properties suitable for in vivo myelin imaging. Indeed, one of the agents has been successfully radiolabelled with positron-emitting C-11 by Dr. Chunying Wu. Subsequent microPET studies in animal models from Dr. Robert Miller at Case Western Reserve University showed the brain can be directly imaged based on myelin contents. The main objective of these studies is to conduct full-scale pre-clinical studies to identify lead compounds in order to initiate clinical trials to validate these imaging agents as surrogate markers of demyelination and remyelination in patients afflicted by multiple sclerosis. Completion of these studies would also advance our understanding of the properties of myelinated white matter, and enhance our ability to visualize and monitor the neurological changes associated with demyelination.

This work has been supported by Myelin Repair Foundation. We wish to thank Rusty Bromley and Kumar Hari at Myelin Repair Foundation for their scientific inputs and inspiration. Additional support from Dana Foundation, National Multiple Sclerosis Society, and National Institute of Neurological Diseases and Stroke (R01 NS061837) are also acknowledged.

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March 2009

Myelin Repair Foundation Scientific Team Findings Have Important Implications for Understanding Why Remyelination Sometimes Fails to Occur in Multiple Sclerosis and How Remyelination May be Promoted

Neuron (2008), Nov 26;60(4):555-69.

Distinct stages of myelination regulated by gamma secretase and astrocytes in a rapidly myelinating CNS co-culture system.

Authors: Watkins T, Emery B, Mulinyawe S, and Barres B

Summary: Myelination is a highly orchestrated, multi-step process. Precursor cells generate oligodendrocytes which extend processes that recognize, adhere to, ensheath, and finally wrap axons. The molecular mechanism of each of these steps is poorly understood. Mechanistic studies of central nervous system (CNS) myelination have been hindered by the lack of a rapidly myelinating culture system. In this paper, the authors describe a versatile new culture method in which reaggregated CNS neurons, which generate a dense beds of axons, are cultured together with purified oligodendrocyte precursor cells (OPCs) in a specially defined culture medium that enables myelination to occur. The OPCs migrate along these axons and then differente into oligodendrocytes that ensheath axons in as few as three days, with wrapping and the generation of compact myelin by the sixth day. Using this technique the authors were able to timelapse CNS myelination for the first time. The authors found unexpectedly that oligodendrocytes myelinate all of the axons they will ever myelinate concurrently during a brief window early in their differentiation. They found that the first stage of myelination, axon ensheathment, was powerfully promoted by application of drug inhibitors to an enzyme in oligodendrocytes called γ-secretase, which was not previously known to control myelination. Minimal myelin wrapping occurred unless astrocytes, or medium conditioned by astrocytes, was added to the culture system. These findings demonstrate that each stage of the myelination process is differentially regulated. By use of this new culture system, the identity of the relevant oligodendrocyte γ-secretase and astrocyte derived signals can be investigated. Moreover, taken together, these findings imply the existence of an as yet unidentified nuclear program that enables myelination during a limited time period early during oligodendrocyte differentiation. These findings therefore have important implications for understanding why remyelination sometimes fails to occur in Multiple Sclerosis and how remyelination may be promoted.

These findings therefore have important implications for understanding why remyelination sometimes fails to occur in Multiple Sclerosis and how remyelination may be promoted.

Complete text of this article can be found at www.pubmed.com.

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February 2009

Myelin Repair Foundation Scientists gain new understanding of demyelinating disease development and the ability to test therapeutic strategies as a result of their discoveries surrounding the role that gene mutation, Nur7, plays in degeneration of the CNS

Journal of Neuroscience (2008): 28(45):11537-11549.

Nur7 is a nonsense mutation in the mouse aspartoacylase gene that causes spongy degeneration of the CNS.

Authors: Traka M, Wollmann RL, Cerda SR, Dugas J, Barres BA, Popko B.

Summary: Canavan disease (CD) is a rare recessive autosomal white matter (myelin) leukodystrophy that is caused by mutations of the aspartoacylase gene (ASPA). The disease has a high prevalence in people of Askenazi Jewish origin, and it is characterized by the presence of widespread spongy degeneration of the brain and spinal cord. Affected individuals suffer from mental retardation, weakness, blindness and functional disability, and most die by the age of five. ASPA is highly expressed in mature oligodendrocytes, where it catalyzes the hydrolysis of the most abundant amino acid in the brain, N-acetylaspartate (NAA) to acetate and aspartic acid. Since CD was linked to ASPA gene mutations a considerable effort has been made towards understanding the mechanism of disease pathology. Despite these efforts, there is not yet a clear link between deficient NAA hydrolysis and the myelin degeneration observed in CD. This manuscript describes the identification of a nonsense mutation, Q193X, in the Aspa gene of the previously described ENU-induced nur7 mouse mutant (Aspanur7) that closely resembles the ASPA gene mutations described in CD patients. The Aspanur7 mutation results in the absence of detectable Aspa protein expression in Aspanur7 homozygous mutant mice, which display severe spongy degeneration and oligodendrocyte loss throughout the CNS, strikingly resembling CD. The progression of the CD pathology in older Aspanur7/nur7 mice was examined and it was found that despite the extensive vacuolation, recruitment of new oligodendrocytes occurs in the lesion sites, followed by appearance of normally myelinated axons in the affected brain areas. This finding suggests that the increased numbers of oligodendrocytes found in the lesion sites of human CD patients might be evidence of an effort to remyelinate. Moreover, this study demonstrates significant axonal loss in the cerebellum of older Aspanur7 mutants, which correlates with the severe ataxia that develops at later disease stages. These data suggest that myelin degeneration in CD leads to axonal damage and loss, which might be the underlying cause of motor and sensory disabilities that patients develop in advanced-stage disease. Thus, the Aspanur7/nur7 mouse provides a new authentic model for the characterization of CD pathogenesis and the testing of therapeutic strategies.

Complete text of this article can be found at www.pubmed.com.

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January 2009

Cutting Edge: Myelin Repair Foundation Scientists find that a specialized subset of dendritic cells could play a regulatory role in suppressing CNS inflammation and perhaps limit damage to myelin

Journal of Immunology (2008): 180:6457-6461.

Cutting Edge: CNS plasmacytoid dendritic cells regulate the severity of relapsing experimental autoimmune encephalomyelitis.

Authors: Bailey-Bucktrout, S. L., S. Caulkins, G. Goings, J. A. Fischer, A. Dzionek, and S. D. Miller.

Scientific Summary: Plasmacytoid dendritic cells (pDC) have both stimulatory and regulatory affects on T cell responses, and are found in multiple sclerosis tissue. pDCs are a major CNS-infiltrating DC population during experimental autoimmune encephalomyelitis (EAE), but unlike myeloid DCs (mDC), appeared to have a minor role in T cell activation and epitope spreading. We show that depletion of pDCs during either the acute or relapse phases of EAE resulted in exacerbation of disease severity. pDC depletion did not affect the frequency of myelin specific CD4+ T cells in peripheral lymphoid organs, but significantly enhanced CNS CD4+ T cell activation, as well as IL-17 and IFN-γ production. Moreover, CNS pDCs suppressed CNS mDC-driven production of IL-17, IFN-γ and IL-10 in an IDO-independent manner. The data demonstrate that pDCs play a critical regulatory role in negatively regulating pathogenic CNS CD4+ T cell responses highlighting a new role for pDCs in inflammatory autoimmune disease.

Lay Summary: This paper shows that a specialized subset of dendritic cells, called plasmacytoid DCs are present in the central nervous system of both multiple sclerosis patients and mice with the animal model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). Interestingly, specifically deleting these cells led to a more severe course of EAE suggesting that they play of regulatory role attempting to suppress CNS inflammation and perhaps limit damage to myelin. Experiments are continuing to determine if injecting these cells into mice with ongoing EAE could lead to an effective disease therapy.

Complete text of this article can be found at www.pubmed.com.

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November 2008

Myelin Repair Foundation Scientists Develop a New Resource for Understanding Brain Development and Function

Journal of Neuroscience (2008) Volume 28, pages 264-78.

A transcriptome database for astrocytes, neurons, and oligodendrocytes: a new resource for understanding brain development and function.

Authors: Cahoy JD, Emery B, Kaushal A, Foo LC, Zamanian JL, Christopherson KS, Xing Y, Lubischer JL, Krieg PA, Krupenko SA, Thompson WJ, Barres BA.

The genetic programs that control how oligodendrocytes are generated and how they myelinate axons are poorly understood. In order to better understand what genes are expressed by oligodendrocytes and the other brain cell types they interact with, neurons and astrocytes, in this paper the authors, developed novel methods to enable these main brain cell types to be isolated and highly purified. These new purification procedures then allow extraction and analysis of the genes expressed by astrocytes, neurons, and oligodendrocytes for the first time. Using a new technology called gene profiling, the authors successfully created a “transcriptome” database which is a detailed and quantitative description of expression levels of over 20,000 genes. These transcriptomes were generated from cells purified from developing and mature brain tissue. By comparing the pattern of gene expression between these cell types, the authors identified large numbers of genes that are uniquely expressed by each of these 3 cell types including oligodendrocytes. These results are important because they have (a) revealed a large number of novel candidate myelin genes that may be critical in allowing the formation of myelin sheathes, (b) revealed highly expressed oligodendrocyte specific transcription factors that are novel candidates for specifying oligodendrocyte cell generation, and (c) identified new astrocyte and neuronal signaling molecules that may help to coordinate and direct the process of myelination. These findings provide a catalog of great value for identifying new therapeutic targets for promoting remyelination in M.S.

Complete text of this article can be found at www.pubmed.com.

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MRF-008

Read about the clinical trial of a potential MS drug to protect and repair myelin.