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EMERGING RESEARCH FRONTS

Bruce D. Trapp talks with ScienceWatch.com and answers a few questions about this month's Emerging Research Front in the field of Neuroscience & Behavior.
Trapp Article: Premyelinating oligodendrocytes in chronic lesions of multiple sclerosis
Authors: Chang, A;Tourtellotte, WW;Rudick, R;Trapp , BD
Journal: N ENGL J MED, 346 (3): 165-173 JAN 17 2002
Addresses:
Cleveland Clin Fdn, Lerner Res Inst, Dept Neurosci, NC30, 9500 Euclid Ave, Cleveland, OH 44195 USA.
Cleveland Clin Fdn, Lerner Res Inst, Dept Neurosci, NC30, Cleveland, OH 44195 USA.
Cleveland Clin Fdn, Mellen Ctr Multiple Sclerosis, Cleveland, OH 44195 USA.
(addresses may have been truncated.)

Why do you think your paper is highly cited?

The paper is highly cited because it is of interest to developmental neurobiologists and clinical multiple sclerosis researchers. From the basic science perspective, there is considerable interest in how oligodendrocytes are generated and what regulates their differentiation into myelin-forming cells during development and following demyelination.

The best example of repair of the adult human brain is the generation of new oligodendrocytes that remyelinate demyelinated axons in the lesions of multiple sclerosis. Unfortunately, this repair process is not always successful. Because the Multiple Sclerosis (MS) brain can endogenously replace myelin, it is likely that the repair process can be therapeutically manipulated and enhanced in those lesions where repair fails. Remyelination requires generation of new oligodendrocytes from an oligodendrocyte progenitor cell.

Our studies investigated whether oligodendrocyte progenitor cells, production of new oligodendrocytes, or differentiation of oligodendrocytes limits remyelination in chronic MS lesions. We demonstrate that the replacement of oligodendrocytes in MS lesions recapitulates many of the steps described in development. Specifically, we detected oligodendrocyte progenitor cells and pre-myelinating oligodendrocytes in chronic demyelinated MS lesions and show positive correlation between the distributions of these two cell types.

These observations support the contention that neither oligodendrocyte progenitor cells nor oligodendrocyte production necessarily limit the remyelination process. The observation that has received the most attention is the detection of the pre-myelinating oligodendrocytes. These cells express myelin proteins and extend processes that follow individual axons, but they fail to form myelin around these axons. Thus, in addition to increased oligodendrocyte production, we identified enhancement of oligodendrocyte differentiation as a therapeutic target for remyelinating therapies.

Because many of the demyelinated axons in chronic lesions appeared dystrophic, failure of remyelination by pre-myelinating oligodendrocytes may be due to the lack of appropriate signals from axons. It is well established that axons regulate the myelination process and how this is accomplished is a major interest of basic scientists. Thus, our data is cited in the context of the need to better understand the molecular mechanisms of this axonal regulation, which could lead to therapeutic approaches to enhance repair of chronic MS lesions.

Our data is also of interest to the clinical MS researcher because it impacts design of future clinical trials. Transplantation of oligodendrocyte-producing cells is a major focus of pre-clinical research. Much of this research appropriately focuses on the type and source of the cells to be transplanted. Equally important is the transplant recipient and transplant site.

Chronic MS lesions have advantages as transplant sites and were the target for a Phase I trial testing the safety of Schwann cell transplantation. Our paper, however, raises the possibility that chronic MS lesions may not be an appropriate transplantation site. Chronic lesions may support production of premyelinating oligodendrocytes from transplanted cells (as occurs endogenously).

However, if the environment of chronic MS lesions does not provide the appropriate signals for pre-myelinating oligodendrocytes to successfully remyelinate axons, increasing oligodendrocyte numbers will not result in remyelination, nor benefit the patient.

Does it describe a new discovery, methodology, or synthesis of knowledge?

As is often the case, new methodology and synthesis of knowledge led to new information. From the methodological point of view, the challenge was to apply techniques that we developed for basic science studies of myelination in rodents to human tissue. The first challenge was to develop a rapid autopsy protocol for MS brains.

The Cleveland Clinic has one of the largest clinical MS programs in the world. In collaboration with Dr. Richard Rudick and other neurologists at our Mellen Center for Multiple Sclerosis Treatment and Research, we established a protocol to rapidly access MS brains. The success of this program reflects the dedicated MS patients who prospectively signed-up to participate in this endeavor.

Our post-mortem protocol includes an in-situ MRI. Half the brain is processed for MRI-pathology correlations and half for morphological and biochemical analysis. We have processed over 50 brains and spinal cords with post-mortem intervals averaging around five hours. Thus, we have a significant collection of well-fixed brains for our studies.

The next methodological challenge was to get our immunocytochemical techniques to work on human tissue, which was not an easy task. We do not utilize routine histological stains or paraffin embedded tissue. Our approach is to use immunocytochemistry to decipher the three dimensional relationship of cells in 30um-thick sections using confocal microscopy. Because my laboratory collects and processes the tissue in the autopsy suite, we can prospectively modify tissue collection to our needs. For example, our antibodies that detect oligodendrocyte progenitor cells do not work on tissue slices that have been fixed for extended periods of time. We limited fixation of brain slices to two days for these studies. Dr. Ansi Chang, the lead author of the paper, spent an enormous amount of time in optimizing the staining protocols for human tissue.

Most new discoveries require a synthesis of knowledge. My long-standing interest in the basic science of myelination was a big factor in design of the hypothesis on which our paper was based. We set out to ask two basic questions. First, do oligodendrocyte progenitor cells (OPCs) limit remyelination in chronic MS lesions? We partly addressed this question in a paper published in 1999, by detecting cells that expressed phenotypic marker of OPCs.

Of course, this observation did not prove that these cells had the ability to generate oligodendrocytes. However, detection of OPCs and pre-myelinating oligodendrocytes in the same lesions established the presence of functional OPC in chronic lesions and addressed the second hypothesis by demonstrating that oligodendrocyte production does not always limit remyelination of chronic lesions. The significantly new discovery was the failure of premyelinating oligodendrocytes to form myelin.

Would you summarize the significance of your paper in layman’s terms?

The significance of our finding is a positive one for the MS patient. The damaged brain of multiple sclerosis patients never gives up trying to repair itself. This is important and encouraging because we have identified the stage of the repair process that is blocked. This provides targets for the development of new therapeutics that will optimize MS brain repair.

How did you become involved in this research and were any particular problems encountered along the way?

My entire research career has focused on the cellular and molecular biology of myelination. My Ph.D. dissertation investigated the effects of essential fatty acid deficiency on myelination. My Ph.D. advisor, Joseph Bernsohn, had hypothesized that essential fatty acids may lower susceptibility to MS, based upon to the high intake of raw fish (a source of essential fatty acids) and low MS incidence in Japan.

While there was a theoretical focus on MS, my studies were restricted to rodents. Following my Ph.D., I joined the laboratory of Dr. Henry deF. Webster at NIH as a postdoctoral fellow where two major influences shaped my career. First, Dr. Webster was interested in both diseases of myelin and the basic sciences of myelination. I learned to extend basic science observations to relevant questions of disease mechanisms. Second, Dr. Nancy Sternberger was also a fellow in Dr. Webster’s laboratory and her husband, Ludwig, was developing the peroxidase-anti-peroxidase method for localizing antigens in tissue sections.

We had access to these reagents well before they became commercially available. This catalyzed a shift in my research to proteins and immunocytochemistry, a focus I maintain today. I purified several peripheral nerve myelin proteins in the laboratory of Dr. Richard H. Quarles at NIH, generated antibodies and investigated their expression during myelination. I published my first papers on human disease, including a description of Schwann cell remyelination of demyelinated axons in MS spinal cord and several papers on the pathogenesis of human peripheral neuropathies.

Following my postdoctoral fellowship, I joined the Department of Neurology at Johns Hopkins University. This was a tremendous environment. I had outstanding mentorship from Drs. Jack Griffin, Guy McKhann, Richard Johnson, and many others who reinforced my interest in applying basic science observations to human disease mechanisms. I learned the importance and value of collaborating with clinicians who understood the diseases and who could help put my observations in the context of disease pathogenesis and progression. It was there that I had my first exposure to rapid autopsy protocols that were developed to investigate the pathogenesis of HIV-related CNS diseases.

I became interested in MS pathogenesis and was fortunate enough to obtain several postmortem MS brains. It was a very difficult decision to leave this environment for the Cleveland Clinic in 1994, because I was generating some exciting data regarding axonal degeneration in MS. Following my move to Cleveland, I turned much of my clinical focus to MS and established the rapid autopsy program.

As part of my basic science research program, we identified reliable markers for oligodendrocyte progenitor cells and premyelinating oligodendrocytes. It was an obvious extension to determine if these cells limit remyelination in chronic MS lesions. While I do not recall any particular problems encountered along the way, an obstacle to our research of MS tissue is the lack of brains from patients during early stages of the disease.

Many lesions in early stages of the disease remyelinate and it would be informative to characterize the environments that promote successful remyelination. For the bigger picture, however, this obstacle is good because the neurologists have greatly improved the quality of life and lifespan of MS patients. Since that’s what our research is really about, we will have to find other ways to characterize environments that promote remyelination.

Where do you see your research leading in the future?

We are looking for mechanisms that enhance myelin repair and limit neurodegeneration in MS brains. Degeneration of chronically demyelinated axons is a major cause of the continuous and irreversible neurological disability that most MS patients eventually endure. Remyelination is the best documented neuroprotective therapy. Transplantation of progenitor cells and manipulation of the endogenous repair process represent viable repair approaches and both should be pursued.

We have a special interest in small-molecule therapies that may be combined with transplantation. We also need a better understanding of how myelin keeps the axon happy and functional. If one could find a way to make the demyelinated axon think it is myelinated we could avoid some of the complication of transplant therapies and provide an effective mechanism for delaying disease progression.

Do you foresee any social or political implications for your research?

Social and political implications will increase the closer we get to a transplantation therapy. The issue will be the source of the transplanted cells. We are very interested in stem cells in the adult brain. One possible source for generating the adult CNS stem cells is human embryonic stem cells, a topic with both social and political implications. Fetal human brain is a proven source of oligodendrocyte progenitor cells and a political issue in the state of Ohio, where state law forbids research on any fetal tissue derived from elective procedures.

Bruce D. Trapp, Ph.D.
Chairman
Department of Neurosciences
Lerner Research Institute
The Cleveland Clinic
Cleveland, OH, USA

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2008 : February 2008 : Bruce D. Trapp