R. Douglas Fields, PhD, Head, Section on Nervous System Development and Plasticity
Philip Lee, PhD, Research Fellow
Jonathan Cohen, PhD, Postdoctoral Fellow
Tomoko Ishibashi, PhD, Postdoctoral Fellow
Varsha Shukla, PhD, Postdoctoral Fellow
Peter Wadeson, BS, Laboratory Technician

Our research is concerned with understanding the molecular mechanisms by which functional activity in the brain regulates development of the nervous system during late stages of fetal development and early postnatal life. The two main objectives of our research program are (1) to understand how the expression of genes controlling the developing structure and function of the nervous system are regulated by patterned neural impulse activity and (2) to determine the functional consequences of neural impulse activity on major developmental processes. Areas of study include interactions between neurons and glia, the mechanisms of learning and memory, and the effects of impulses on cell proliferation, differentiation, neurite outgrowth, synaptogenesis, synapse remodeling, and myelination.

Activity-dependent neuron-glia interactions

Ishibashi, Dakin, ¹ Stevens, ² Lee, Fields; in collaboration with Chen, Schwarzschild, Stewart

The importance of neural impulse activity in regulating the development of neurons is widely appreciated. Until recently, however, researchers did not recognize the role of such activity in the development of non-neuronal cells in the brain (glia). Accordingly, few have considered the question of activity-dependent neuron-glia interactions outside the synapse. Our laboratory is exploring this issue and is working to identify the mechanisms and functional significance of activity-dependent communication between axons and glia.

Myelin, which is the membrane wrapping around axons in a spiral, provides the electrical insulation essential for rapid impulse conduction. Our findings show that myelinating glia can detect electrical activity in axons and that electrical activity influences glial development and myelination. These findings are relevant to early childhood development and open new avenues for therapeutic approaches to treating demyelinating disease, but they are also expanding our current concepts of activity-dependent plasticity in the brain.

Myelination in humans continues through childhood and into adult life and may represent a largely unexplored mechanism for activity-dependent plasticity. By regulating the speed of conduction, myelination can increase synaptic strength by coordinating temporal summation of convergent inputs. Indeed, recent research from other laboratories indicates that learning new skills or raising animals in an enriched environment increases myelination of fiber tracts in the brain, and our results provide a cellular/molecular mechanism for this response.

We have shown that ATP is released from axons firing action potentials and that it that this release activates purinergic receptors on myelinating glia, causing an increase in intracellular calcium, activation of transcription factors, regulation of gene expression, and control of glial development, cell proliferation, and myelination. In the central nervous system, we find that impulse activity increases myelination. Adenosine, derived from the breakdown of ATP from electrically active axons, stimulates differentiation of oligodendrocyte progenitor cells (OPCs) to a promyelinating stage and increases myelination by acting on P1 receptors. After OPCs are mature, electrical activity can increase myelination in a different manner that depends on ATP's stimulating astrocytes, another type of glial cell, to release the cytokine LIF (leukemia inhibitory factor), promoting myelination by mature oligodendrocytes. Our research shows that, in the peripheral nervous system, ATP released from axons firing action potentials inhibits Schwann cell proliferation, arresting cellular differentiation at an immature stage and inhibiting myelination.

Hippocampal synaptic plasticity

Cohen, Lee, Fields; in collaboration with Becker

It is widely appreciated that there are two types of memory: short-term and long-term memory. It has been well known for decades that gene expression is necessary for converting short-term into long-term memory, but it is not known how signals reach the nucleus to initiate this process or which genes make memories permanent. Long-term potentiation (LTP) and long-term depression (LTD) are two widely studied forms of synaptic plasticity that can be recorded electrophysiologically in the hippocampus and that are believed to represent a cellular basis for memory. We are using custom cDNA microarrays to investigate the signaling pathways, genes, and proteins involved in these forms of synaptic plasticity. Our research has identified sets of transcription factors, structural genes, and signaling pathways that are regulated by activity patterns leading selectively to different types of synaptic plasticity. The objective of our research is to understand how gene-regulatory networks are controlled by the appropriate patterns of impulses leading to different forms of synaptic plasticity and to identify new molecular mechanisms regulating synaptic strength. One finding of particular interest concerns the gene that encodes BDNF, a growth factor. Our research shows that mRNA for BDNF is expressed at different levels depending on whether or not firing of post-synaptic CA1 neurons is coincident with excitatory synaptic firing from pre-synaptic neurons.

Regulation of gene expression by action potential firing patterns

Lee, Cohen, Fields; in collaboration with Becker

All information in our nervous system is coded in the pattern of neural impulse activity. Given that nervous system structure and function are regulated by experience, it follows that gene activity in neurons must be regulated by the pattern of neural impulse activity. We have tested this hypothesis by using custom cDNA arrays for gene expression profiling after stimulating nerve cells to fire in different patterns by delivering electrical stimulation through platinum electrodes in specially designed cell culture dishes. Using gene arrays, quantitative RT-PCR, Western blot, and immunocytochemistry, we measured mRNA and protein expression after stimulation. The results confirm our hypothesis that precise patterns of impulse activity can turn on or turn off specific genes. The experiments are revealing signaling pathways and gene-regulatory networks that respond selectively to appropriate temporal patterns of action potential firing in neurons. Temporal aspects of intracellular calcium signaling are particularly important in regulating gene expression according to neural impulse firing patterns in normal and pathological conditions. Our findings provide a better understanding of how nervous system development and plasticity may be regulated by information coded in the temporal pattern of impulse firing in the brain.

Nerve sheath tumor

Lee, Cohen; in collaboration with Becker, De Vries

NF-1 is an autosomal dominant condition leading to nerve sheath tumors that can develop into malignant cancer. We are using gene expression profiling in combination with quantitative RT-PCR and biochemical methods to investigate the processes involved in the transition from a benign to a malignant peripheral nerve sheath tumor. Our studies identify hundreds of genes that are dysregulated in malignant Schwann cells, including those associated with all aspects of cellular function involved in malignancy, such as, for example, those regulating cell proliferation, motility, and growth factor and immune system responses. We have observed widespread suppression of many genes involved in immune responses in these tumor cells, which may help the cells escape detection by the immune system.

Publications Related to Other Work as Adjunct Professor, University of Maryland

¹ Kelly Dakin, BS, former Predoctoral Fellow
² Beth Stevens, PhD, former Biologist

COLLABORATORS

Kevin Becker, PhD, Research Resources Branch, NIA, Baltimore, MD
Jiang-Fan Chen, MD, PhD, Boston University School of Medicine, Boston, MA
George De Vries, PhD, Loyola University, Chicago, IL
Michael Schwarzschild, MD, PhD, Massachusetts General Hospital, Charlestown, MA
Colin L. Stewart, DPhil, Laboratory of Cancer and Developmental Biology, NCI, Frederick, MD

For further information, contact fieldsd@mail.nih.gov.