Anne E.West, Wen G.Chen, Matthew B.Dalva, Ricardo E.Dolmetsch, Jon M.Kornhauser, Adam J.Shaywitz, Mari A.Takasu, Xu Tao, and Michael E.Greenberg*

Division of Neuroscience, Children’s Hospital, 300 Longwood Avenue, Boston, MA 02115

Plasticity is a remarkable feature of the brain, allowing neuronal structure and function to accommodate to patterns of electrical activity. One component of these long-term changes is the activity-driven induction of new gene expression, which is required for both the long-lasting long-term potentiation of synaptic transmission associated with learning and memory, and the activity-dependent survival events that help to shape and wire the brain during development. We have characterized molecular mechanisms by which neuronal membrane depolarization and subsequent calcium influx into the cytoplasm lead to the induction of new gene transcription. We have identified three points within this cascade of events where the specificity of genes induced by different types of stimuli can be regulated. By using the induction of the gene that encodes brain-derived neurotrophic factor (BDNF) as a model, we have found that the ability of a calcium influx to induce transcription of this gene is regulated by the route of calcium entry into the cell, by the pattern of phosphorylation induced on the transcription factor cAMP-response element (CRE) binding protein (CREB), and by the complement of active transcription factors recruited to the BDNF promoter. These results refine and expand the working model of activity-induced gene induction in the brain, and help to explain how different types of neuronal stimuli can activate distinct transcriptional responses.

Electrical activity within the brain rapidly encodes information about the world, but for these fleeting perceptions to have a lasting impact, long-term changes in the structure and function of neurons must follow. At the synapse, neurotransmitter reception initiates a number of biochemical signaling cascades in the postsynaptic cell, one of the most important of which is the elevation of intracellular calcium (1). This calcium rise is a critical component of signaling pathways whose functional consequences include activity-dependent survival (2) and the synaptic plasticity of long-term potentiation (LTP; ref. 3).

Early events induced by the rise of calcium in dendrites are likely to be local, resulting from posttranslational modifications of the synaptic machinery. However, for long-term structural and functional changes in the neuron, the calcium signal must regulate the expression of new gene products. There are several potential calcium-dependent steps in the process of new gene expression: elements of mRNA transcription, elongation, splicing, stability, and translation have all been suggested to be regulated by calcium in neurons. Especially exciting evidence has been accumulating in favor of the idea that some calcium-regulated mRNA translation occurs locally at postsynaptic sites, providing a means for rapid and accurate expression of activity-induced gene products at activated synapses (4, 5). The relative importance of dendritic mRNA translation in calcium-dependent biological processes will become clearer when the specific mRNAs that are regulated in this manner are identified.

Our laboratory has focused on the mechanisms by which calcium regulates new gene transcription. This line of investigation began with the observation that application of acetylcholine receptor agonists to the PC12 pheochromocytoma cell line induces rapid transcriptional initiation of the c-fos protooncogene in a manner that depends on calcium influx (6). c-Fos is a transcription factor whose mRNA expression had been shown to be rapidly induced when quiescent fibroblasts are stimulated by growth factors to reenter the cell cycle (7). That c-fos could also be induced in neurons suggested that an equally intricate pattern of regulated transcription might be responsible for cellular responses to neuronal activity. Indeed it has subsequently been shown that transcription of a large number of genes, encoding both transcription factors and molecules that function at synapses, is induced by synaptic activity and subsequent calcium influx (8, 9). These activity-dependent changes in gene transcription have been shown to be required for both neuronal survival (10) and for the maintenance of the late phase of LTP (11).

One of the best studied of the activity-induced genes is brain-derived neurotrophic factor (BDNF). BDNF is a small secreted protein that acts by binding to its receptors, the tyrosine kinase TrkB and the low-affinity neurotrophin receptor p75 (12). Ligation of TrkB receptors by BDNF promotes the activation of signaling pathways that both rapidly modify the function of local synaptic targets and also have long-term effects on gene transcription. BDNF was originally identified as an important neuronal survival factor in the central nervous system (13, 14). BDNF promotes survival through inactivation of components of the cell death machinery and also through activation of the transcription factor cAMP-response element binding protein (CREB), which drives expression of the prosurvival gene Bcl-2 (15, 16). In addition to its role in neuronal survival, BDNF also modulates synaptic activity (17). Mice lacking the BDNF gene show impaired hippocampal LTP, and overexpression of BDNF rescues this defect (18, 19). Both presynaptic and postsynaptic mechanisms may underlie this effect on synaptic plasticity. BDNF application to presynaptic terminals of Xenopus spinal neurons enhances both spontaneous and evoked release of neurotransmitter via a mechanism that may involve the phosphorylation and functional regulation of synaptic vesicle proteins (17, 20). At the postsynaptic membrane, BDNF may act as a neurotransmitter itself, as rapid pulsatile application of BDNF to neurons can cause TrkB-dependent membrane depolarization within milliseconds (21).

Perhaps because BDNF plays so many important roles in neuronal development and function, the expression and action of this protein is tightly regulated in time and space. In particular, expression of BDNF mRNA is highly responsive to electrical

Front Matter (R1-R5)

1 Introduction (10994-10995)

2 Neural Roles for Heme Oxygenase: Contrasts to Nitric Oxide Synthase (10996-11002)

3 Presynaptic Kainate Receptors at Hippocampal Mossy Fiber Synapses (11003-11008)

4 Retrograde Signaling at Central Synapses (11009-11015)

5 Controlling Potassium Channel Activities: Interplay Between the Membrane and Intracellular Factors (11016-11023)

6 Calcium Regulation of Neuronal Gene Expression (11024-11031)

7 Sensory Experience and Sensory Activity Regulate Chemosensory Receptor Gene Expression in Caenorhabditis Elegans (11032-11038)

8 Presenilin, Notch, and the Genesis and Treatment of Alzheimer's Disease (11039-11041)

9 FosB: A Sustained Molecular Switch for Addiction (11042-11046)

10 Glutabatergic Modulation of Hyperactivity in Mice Lacking the Dopamine Transporter (11047-11054)

11 Zinc Induces a Src Family Kinase-Medicated Up-Regulation of NMDA Receptor Activity and Excitotoxicity (11055-11061)

12 Regulation of Cyclin-Dependent Kinase 5 and Casein Kinase 1 by Metabotropic Glutamate Receptors (11062-11068)