NERVOUS SYSTEM DEVELOPMENT AND PLASTICITY
, Head, Section on Nervous System Development and Plasticity
Philip Lee, PhD, Research Fellow
Olena Bukalo, PhD, Postdoctoral Fellow
Jonathan Cohen, PhD, Postdoctoral Fellow
Yingchun Ni, PhD, Postdoctoral Fellow
Varsha Shukla, PhD, Postdoctoral Fellow
Eben Lichtman, BA, Predoctoral 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. The areas of study include interactions between neurons and glia, mechanisms of learning and memory, and effects of impulses on cell proliferation, differentiation, neurite outgrowth, synaptogenesis, synapse plasticity and remodeling, and myelination.
Activity-dependent neuron-glia interactions: myelin
The importance of neural impulse activity in regulating development of neurons is widely appreciated. Despite increasing recognition of the role of neural impulse activity in the development of non-neuronal cells in the brain (glia), few have considered the question of activity-dependent neuron-glia interactions outside the synapse. Our laboratory is exploring this question and working to identify the mechanisms and functional significance of activity-dependent communication between axons and glia.
Myelin is the spiral wrapping of membrane around axons that provides electrical insulation essential for rapid impulse conduction. Our findings show that myelinating glia can detect electrical activity in axons, thereby influencing glial development and myelination. Our 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 myelination 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; our results provide a cellular/molecular mechanism for this response.
We have shown that ATP is released from axons firing action potentials and 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 (CNS), we find that impulse activity increases myelination. Adenosine, derived from the breakdown of ATP released 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 mature, electrical activity can increase myelination in a different manner, which is dependent on ATP stimulating astrocytes, another type of glial cell, to release a factor (the cytokine LIF–leukemia inhibitory factor) promoting myelination by mature oligodendrocytes. Our research shows that, in the peripheral nervous system (PNS), ATP released from axons firing action potentials inhibits Schwann cell proliferation, arresting cellular differentiation at an immature stage and inhibiting myelination. Our cellular and electron microscopic studies on LIF knockout mice show that proliferation and differentiation of astrocytes and oligodendrocytes are impaired and that myelination is reduced.
Fields RD. Activity-dependent myelination. In: Fields RD, ed. Plasticity Beyond the Synapse. Cambridge University Press, in press.
Fields RD. Myelination: an overlooked mechanism of synaptic plasticity? The Neuroscientist 2005;11:528-31.
Fields RD. Schwann cell interactions with axons. In: Binder MD, Hirokawa N, Windhorst U, Hirsch MC, eds. Encyclopedia of Neuroscience, Springer, 2008; in press.
Ishibashi T, Dakin KA, Stevens B, Lee PR, Kozlov SV, Stewart CL, Fields RD. Astrocytes promote myelination in response to electrical impulses. Neuron 2006;49:823-32.
Neuron-glia signaling in astrocyte development and plasticity
In the early postnatal hippocampus, glial progenitor cells proliferate and differentiate into astrocytes. This developmental process is tightly correlated with the onset of spontaneous neural activity and may be relevant to how early experience can affect development of the brain. Our work shows that bidirectional signaling between neurons and glial cells contributes to the activity-dependent regulation of astrocyte development, which in turn affects neuronal development, synaptogenesis, and synaptic function. Purinergic signaling (via extracellular ATP) has broad effects on both proliferation and differentiation of glial progenitor cells. We found that ATP may be released from dorsal root ganglion (DRG) neurons following action potential firing and from astrocytes in response to neuronal activity, but the mechanisms of ATP release from neurons and astrocytes remain unclear. We are investigating the underlying mechanism of ATP release from DRG neurons and the contribution of SNARE-dependent ATP release from astrocytes. Expanding our findings on activity-dependent myelination (above), we found that interactions between purinergic signaling and cytokine signaling regulate hippocampal glial and neuronal development in an activity-dependent manner, with effects on long-term potentiation of synaptic strength in the hippocampus.
Bullock TH, Bennett MVL, Johnston D, Josephson R, Marder E, Fields RD. The neuron doctrine, redux. Science 2005;310:791-3.
Fields RD. Advances in understanding neuron-glia interactions. Neuron Glia Biol 2006;2:23-6.
Fields RD. Purinergic signalling in neuron-glia interactions. In: Fields RD, ed. Purinergic Signalling in Neuron-Glia Interactions; Novartis Symposium 276. Wiley Press;2006:148-74.
Fields RD, Burnstock G. Purinergic signalling in neuron-glia interactions. Nat Rev Neurosci 2006;7:423-36.
Jia M, Li MX, Fields RD, Nelson PG. Extracellular ATP in activity-dependent remodeling of the neuromuscular junction. Dev Neurobiol 2007;67:924-32.
Hippocampal synaptic plasticity
It is widely appreciated that there are two types of memory: short-term and long-term. 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 the conversion process or what 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; both 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. The objective of our research is to (1) understand how the appropriate patterns of impulses control regulatory networks by leading to different forms of synaptic plasticity and (2) identify new molecular mechanisms regulating synaptic strength.
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. One finding of particular interest concerns the gene encoding BDNF, which is a growth factor. Our research shows that mRNA for BDNF is expressed at different levels depending on whether firing of post-synaptic CA1 neurons is coincident with excitatory synaptic firing from presynaptic neurons.
Bukalo O, Fields RD. Signaling to the nucleus in long-term memory. In: Fields RD, ed. Plasticity Beyond the Synapse. Cambridge University Press, in press.
Fields RD. Making memories stick. Sci Am 2005;292:75-81.
Fields RD, ed. Plasticity Beyond the Synapse. Cambridge University Press, in press.
Lee PR, Cohen JE, Becker K, Fields RD. Gene expression in the conversion of early-phase to late-phase long-term potentiation. Ann NY Acad Sci 2005;1048:259-71.
Regulation of gene expression by action potential firing patterns
All information in our nervous system is coded in the pattern of neural impulse activity. Given that experience regulates nervous system structure and function, it follows that gene activity in neurons must be regulated by the pattern of neural impulse activity. We have tested such a 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. After stimulation, we measured mRNA and protein expression by gene arrays, quantitative RT-PCR, Western blot, and immunocytochemistry. The results confirm our hypothesis that precise patterns of impulse activity can turn specific genes on or off. 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. The role of post-transcriptional gene regulation mediated by mRNA stability and transport is also under investigation in hippocampal and DRG neurons. Our findings provide an improved 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
NF-1 is an autosomal dominant condition leading to nerve sheath tumors that can become malignant. 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 the malignant Schwann cells. Among the genes are those associated with all aspects of cellular function involved in malignancy, including genes that regulate cell proliferation, motility, growth factor, and immune system responses. We have observed widespread suppression of many genes involved in immune responses in these Schwann tumor cells, perhaps helping the cells escape detection by the immune system.
Lee PR, Cohen JE, Fields RD. Immune system evasion by peripheral nerve sheath tumor. Neurosci Lett 2006;397:126-9.
Stevens B. Cross-talk between growth factor and purinergic signalling regulates Schwann cell proliferation. In: Fields RD, ed. Purinergic Signalling in Neuron-Glia Interactions;Novartis Symposium 276. Wiley Press;2006:148-74.
Ampullae of Lorenzini
Ampullae of Lorenzini are sense organs of sharks, rays, and chimaeras that detect weak electric fields generated by many physiological and biological processes in the aquatic environment. These sense organs can detect voltage gradients as weak as 5 billionths of a volt/cm via a mechanism of sensory transduction that is only partly understood. The organs are distantly related to the sense cells in the human inner ear, but the anatomic features of the electrosensory organs provide an excellent preparation for anatomic and physiologic properties of sensory transduction and synaptic plasticity.
Fields RD. The shark’s electric sense. Sci Am 2007;297:74-81.
Fields RD, Fields KD, Fields MC. Semiconductor gel in shark sense organs? Neurosci Lett 2007;426:166-70.
Publications Related to Other Work
Cohen JE, Fields RD. CaMKII activation by extracellular Ca2+ depletion in dorsal root ganglion neurons. Cell Calcium 2006;39:445-54.
Fields RD, Lee PR, Cohen JE. Temporal integration of intracellular calcium signaling networks in regulating gene expression by action potentials. Cell Calcium 2005;37:433-42.
Publications Related to Work as Adjunct Professor, University of Maryland, NACS Program
Fields, RD. Beyond the neuron doctrine. Sci Am Mind 2006;17:20-7.
Fields, RD. Case of the loud eyeballs. Sci Am Mind 2007;18:12-3.
Fields, RD. Erasing memories. Sci Am Mind 2006;16:28-33.
Fields, RD. Meditations on the brain. Sci Am Mind 2006;17:42-3.
Fields, RD. New brain cells go to work. Sci Am Mind 2007;18:30-5.
Fields, RD. Sex and the secret nerve. Sci Am Mind 2007;18:20-7.
Fields, RD. Tears, blushing, and goosebumps. Odyssey 2006;November:18-23.
Fields, RD. Unforgettable. Odyssey 2006;November:38-40.
1 Tomoko Ishibashi, PhD, former Postdoctoral Fellow
2 Kelly Dakin, BS, former Predoctoral Fellow
3 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
Ken McCarthy, PhD, University of North Carolina, Chapel Hill, NC
Philip Nelson, PhD, Scientist Emeritus, NICHD, Bethesda, MD
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.
