NEUROBIOLOGY: PROGENITOR CELLS

Co-Chairs: Robert H. Miller, Ph.D., and Scott Pomeroy, M.D., Ph.D.

Participants: Keith L. Black Lester Drewes Howard A. Fine Joseph C. Gloriosa James E. Goldman

Eric C. Holland David Kaplan Marla B. Luskin Maiken Needergaard Larry Pizzi Greg Riggins

David H. Rowitch Evan Snyder Harald Sontheimer Susan L. Weiner

STATEMENT OF THE PROBLEM

The vertebrate central nervous system (CNS) is a unique tissue in terms of cell number and diversity. During development, the major classes of neural cells are derived from cells of the neuroepithelium. At present it is not known how these divergent cell types are specified and how they relate to highly heterogeneous brain tumors. There is also a current lack of understanding of the cells of origin and cell lineage associations for distinct tumor types. To design specific therapeutic approaches requires a detailed understanding of the signal transduction pathways utilized by different tumor cells in regulating cell fate, including cell proliferation and cell death and differentiation, as well as a comparison of these pathways with those utilized by normal neural progenitors.

An understanding of tumor cell biology depends on defining interactions between tumor cells and their immediate neural environment to elucidate how the environment influences tumor cell behavior and how tumor cells influence local neural function.

CHALLENGES AND QUESTIONS

• What are the cells of origin that give rise to distinct brain tumor types? How do distinct brain tumor types correlate with neural progenitors in the developing CNS? Are distinct tumor types derived from:

-- Multipotent stem cells?

-- Specified progenitor cells in the developing CNS?

-- Specified progenitor cells in the adult CNS?

-- Differentiated cells in the adult CNS?

• What approaches will yield a comprehensive molecular characterization of tumor and normal neural progenitor cells? What model systems are best suited to define tumor cell origins? Can human neural stem cells and their derivatives be used in defining tumor cell origins?

• What extracellular and intracellular signaling systems regulate the fate of brain tumor cells? How are the proliferation, differentiation, and survival of distinct tumor subtypes regulated? Are signaling pathways in tumor cells similar to those utilized by normal neural cells during development? Can specific molecular targets be identified in tumor signal transduction pathways for therapeutic treatments? What are the best cellular models with which to address signaling issues?

• What are the functionally significant cellular interactions between the founder cells of neural tumors and the local neural environment? What are the physiological properties of distinct tumor cell types? What is the influence of the local neural environment on tumor cells? How do influences of the immune system impinge on neural tumor cells? What interactions between the endothelium and tumor cells contribute to tumor expansion? What approaches are required to effectively address interactions between tumor cells and their environment?

BARRIERS

• There is extensive cellular diversity among neural cell types and brain tumor subtypes.

• There is a lack of understanding of the basic biology of neural cell fate determination, particularly within glial lineages.

• Insufficient research effort is being brought to bear on the cellular neurobiology of brain tumors.

• The lack of interaction between neurobiologists and neurooncologists has precluded the effective translation of progress in basic research to brain tumor research.

• There is a lack of appropriate molecular screening systems such as microchip arrays for further advancement of molecular classification and identification of specific signaling systems.

• There is a lack of appropriate models in which to study brain tumor biology

• There is a lack of representative cell lines in which to study the extrinsic and intrinsic signal systems that control normal and tumor progenitor cell fate.

• Current tissue banks are limited, and their existence or mode of access is not obvious to non-brain tumor neurobiologists.

RESEARCH AND SCIENTIFIC PRIORITIES

Priority 1: Create a detailed characterization of the cell of origin of different brain tumor types.

This requires a clear definition of the cellular and molecular characteristics of neural cells during development. Basic biologic studies suggest that tumors might arise from multipotential stem cells, specified progenitor cells in the developing or adult central nervous system (CNS), or dedifferentiation of mature neural cells. To address this issue requires the following:

• Identification of new cell-surface markers and specific receptors for extracellular ligands

• Identification of lineage- and stage-specific transcription factors and other intracellular signaling molecules

• Application of these markers for molecular classification of brain tumors such that relationships between cells at distinct developmental stages of normal development and tumors become apparent

• Studies of tumor cellular biology provide a potentially important approach to defining molecular targets that could lead to further understanding of normal neural development

• Analyses of glial development and definition of glial progenitors in the vertebrate CNS. This reflects a high incidence of glial tumors and the relatively low emphasis on glial progenitor research by the neurobiology community.

Resources needed:

To define cellular origins of different brain tumor subtypes, the following resources are required:

• Model development: There is a shortage of effective cell-based models. Present emphasis is on mouse and rat models, but the use of other animals, including zebrafish, dogs, and others should not be excluded. New in vitro models also need to be developed. Emphasis could be placed on human stem cells/progenitor cells in this system, since species differences may exist. Progress in this area would be enhanced through the development and analyses of multiple model systems rather than a focus on a single or restricted number of models or cell lines.

• Development of microarrays to generate genetic, molecular, and biochemical information. Specifically, development of custom-designed neurodevelopment arrays are important.

• Tissue banking: Wide access to such tissue resources is critically important.

• Development of tissue arrays for further advancement of cellular classification

• Development of infrastructure to encourage interaction between scientists in basic neuroscience and neurooncology: These should include the development of new funding mechanisms that directly allow interdisciplinary teams of scientist to address cellular issues of brain tumor biology.

Priority 2: Develop an understanding of the regulation of mitogenic and anti-mitogenic control of tumor and normal neural progenitor cells.

A thorough understanding of extracellular signaling systems and signal transduction pathways that control progression through the cell cycle and specific inhibitors of progression through the cell cycle is required. Studies should be directed at dissecting and identifying targets of cell growth, cell death and survival, and differentiation in normal and tumor progenitors. It seems likely that each distinct tumor type will utilize a different set of regulatory signaling pathways and share some common effector mechanisms. Comparison of regulatory signaling pathways will lead to enhanced understanding of neural tumors and the development of potential targets for specific intervention strategies. In addition, such studies are likely to provide molecular definitions of progenitor cells at critical developmental junctions and may characterize their derivative tumors.

Resources needed:

Defining the signaling systems involved in control of normal and tumor progenitor cell fate will require the following:

• Additional support for basic biological research in defining signal transduction pathways in neural progenitor cells, with particular emphasis on glial progenitors

• Facilitation of substantive interactions among scientists focused on normal developmental issues and those focused on glial tumor biology. Organization of specific workshops and the development of joint funding programs between basic and clinical research teams may accomplish this.

• Development of microarrays to identify novel signaling pathways in distinct cell types.

• Development of new in vivo and in vitro model systems in which to explore functional requirements for discrete signaling pathways. Specifically, given the limitation of cellular material from distinct tumor types, a need was recognized to develop new cell lines that more closely represent native tumor cells by using recently developed molecular techniques.

Priority 3: Understand the interactions of brain tumors with their immediate neural environment. Studies directed at dissecting tumor-brain interactions should focus on the following areas:

• Further characterization of the physiological properties of tumor cells and the composition of the neural environment are needed. The ionic environment and activation of distinct channels in tumor cells may regulate cell morphology, proliferation, and motility. A broader understanding of interactions of tumor cells with microglial cells and cells of the immune system is warranted. Developmental studies have begun to provide evidence that neural cells respond to immune cells and their influences in a number of ways; these include control of cell morphology, migration, and proliferation. Defining tumor cell responses to immunological influences appears timely.

• Interactions between tumor cells and the local endothelium are poorly understood. Regulation of the blood-brain barrier, control of vessel formation, and other aspects of this interaction require additional analyses.

Resources Needed

The resources required to address this research priority include:

• Basic research in non-neuronal cell ion channel expression and function

• Models of neovascularization in vitro

• Identification of the distinct molecular properties of brain endothelium, perivascular astrocytes, and microglia

• Facilitation of interactions between neurobiologists, brain tumor biologists, and immunologists through the development of novel interdisciplinary funding programs and focused meetings or workshops