activity in the brain, being induced by a wide range of stimuli including seizures (22), electrical stimulation that produces LTP (23), and patterned visual input (24). BDNF is categorized as an immediate-early gene, meaning that the stimulus-induced transcriptional initiation of new BDNF mRNA occurs rapidly and without the need for new protein synthesis (25); stimuli induce the expression of new BDNF mRNA through the posttranslational modification of preexisting transcription factors. The function of BDNF protein expressed as a result of this activity-induced transcription can be assessed by acutely blocking the action of BDNF, either through the addition of function-blocking anti-BDNF Abs or soluble TrkB receptor extracellular domains that sequester newly synthesized and secreted BDNF. This approach has been used to study the role of BDNF in calcium-dependent neuronal survival. Depolarization and subsequent calcium influx promote the survival of cortical neurons in culture and also drive an increase in the transcription of BDNF mRNA (26). To determine whether stimulus-induced expression of BDNF is required for calcium-dependent neuronal survival, function-blocking BDNF Abs were added to the culture medium when the cells were depolarized, leading to a blockade of the ability of BDNF to activate TrkB receptors. Under these conditions, neuronal survival is reduced, suggesting that activity-induced BDNF expression is required for calcium-dependent neuronal survival. Similar experiments have implicated acutely synthesized BDNF in LTP. Sequestration of BDNF up to 1 h after the induction of hippocampal LTP causes previously potentiated synaptic transmission to return to baseline, suggesting that BDNF expression driven by stimuli that produce LTP may be required for the maintenance of synaptic potentiation (27).

By using the induction of BDNF mRNA as an assay, we have explored in detail the mechanism of calcium-induced neuronal gene expression. Our studies, in combination with the work of numerous other labs, have revealed pathways that lead from the outer membrane of the cell, where calcium first enters the cell to the nucleus, where transcription is initiated. A summary of these pathways is presented in Fig. 1. Neurotransmitter reception and membrane depolarization open ligand- and voltage-gated calcium channels in the cell membrane, allowing the influx of extracellular calcium into the cell. This calcium is quickly bound by proteins that sit at the top of calcium-activated signaling cascades. A large number of pathways can respond to the elevation of intracellular calcium, and activation of these signal-transduction cascades amplifies the calcium signal while carrying it to the nucleus. Within the nucleus, the transcription factor CREB seems to be prebound to the BDNF promoter in an inactive form. Phosphorylation of CREB by calcium-regulated kinase cascades stimulates the recruitment of components of the basal transcription machinery to the BDNF promoter, and then new BDNF mRNA is synthesized.

Despite the elucidation of this general mechanism by which calcium elevation induces neuronal gene expression, a number of questions remain to be addressed. For example, there are many routes of calcium entry in neurons, and it has been widely observed that not all paths of entry are equally efficient at inducing the expression of activity-induced genes including BDNF. This observation raises the question, how is it possible that some calcium channels are tightly coupled to gene expression whereas others are not? Downstream of calcium entry, calcium-activated signaling pathways couple to transcription through the phosphorylation of CREB, but certain phosphorylation events render CREB competent to drive transcription whereas others do not. We have studied the molecular mechanisms of calcium-activated CREB-dependent transcription and investigated how CREB phosphorylation affects the formation of active transcriptional complexes. Finally, although CREB is one calcium-responsive transcription factor that contributes to

BDNF transcription, mutational analysis of the BDNF promoter has revealed that there are additional non-CREB binding elements required for calcium-dependent BDNF transcription. We have used these elements to identify other calcium-regulated transcription factors that drive BDNF transcription and have explored how this complex of transcriptional activators gives specificity to transcriptional initiation at the BDNF promoter. In this article, we present our past and current work addressing these issues and describe how these findings have contributed to our working model of neuronal calcium-induced gene expression.

The first step in calcium regulation of neuronal gene expression is the influx of calcium into the cytoplasm. There are four primary routes of calcium entry into the cytoplasm of the postsynaptic neuron: extracellular calcium can enter through the ligand-gated ion channels of the N-methyl-D-aspartate-type (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type (AMPA) (28) glutamate receptors, through voltage-

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)