Changes in the concentration of intracellular calcium as a result of synaptic activity control virtually all adaptive responses in the adult nervous system. Calcium activates mechanisms that affect synaptic connectivity, regulate learning and memory, promote survival, modulate pain, or cause cell death. Most activity-induced adaptations are initiated by synaptic NMDA receptors and require for their maintenance signal-induced changes in gene expression. For synapse to-nucleus communication neurons exploit the spatial and temporal diversity of calcium transients associated with electrical activation. Transcriptional responses depend on how calcium enters the neurons, the amplitude of the signal, how long it lasts and what subcellular compartment it invades. One means of conveying a signal to the nucleusinvolves ERK-MAP kinases that translocate to the nucleus and stimulate transcription factors upon being activated by a calcium micro-domain in the immediate vicinity of the site of calcium entry. However, the principal mediator in the dialogue between the synapse and the nucleus is calcium itself. Synaptic activity and NMDA receptor stimulation can initiate calcium transients that propagate towards the cellsoma and enter the cell nucleus.
Nuclear calcium: universal signal in neuronal survival and long-term memory
Nuclear calcium signaling controls the expression of a wide variety of target genes, primarily by stimulating CREB/CBP-mediated transcription. Our hypothesis is that nuclearcalcium acts as a universal signal for persistent adaptationsin the nervous system. To test this hypothesis, we have focused on two adaptive processes: activity-dependent neuronal survival (i.e. acquired neuroprotection) and memory formation. Using tools to interfere selectively with calcium signaling in the nucleus of hippocampal neurons, we have been able to demonstrate that the neuroprotection afforded by action potential bursting and synaptic NMDA receptor activation is indeed dependent upon nuclearcalcium signaling. Whole genome transcriptional profiling in hippocampal neurons revealed a nuclear calcium-regulated genomic survival program consisting of about a dozengenes. The neuroprotective activity of some of these genes has been demonstrated in vitro as well as in in vivo models of neurodegeneration. The role of nuclear calcium in learning and memory is being analyzed in the fruit fly Drosophilamelanogaster. The results obtained so far indicate that the formation of long-term memory following associative olfactory learning is impaired in transgenic flies expressing an inhibitor of nuclear calcium signaling.
Fig. 2: Organotypic hippocampal slice culture containing a neuron expression green fluorescent protein. The cell nuclei are stained with Hoechst. Image by Daniela Mauceri
Nuclear calcium imaging in vivo
A major challenge for the future is to develop tools and technologies to detect nuclear calcium signals in vivo. Given the central role of nuclear calcium in synaptic plasticity related gene expression, we are particularly interested in studying nuclear calcium transients during learning. We are using stereotaxic delivery of recombinant adeno-associated viruses to express recombinant calcium indicators targeted to the cell nucleus in the rodent brain. Techniques of imaging calcium signals in vivo are being developed. We have also generated transgenic Drosophila melanogaster expressing are combinant nuclear calcium indicator in the nervous system. These flies are being used to monitor nuclear calcium signals during associative olfactory learning.
|Fig 3: Differential signaling by synaptic and extrasynaptic NMDA receptors. Schematic illustration of calcium-regulated pathways controlling survival and memory. Illustration by Oliver Dick and Malte Wittmann.|
Extrasynaptic NMDA receptor signaling: CREB shut-off and cell death pathways
The transcription-promoting activities of synaptic NMDA receptor-induced nuclear calcium signals are antagonized by a calcium signaling pathway that is initiated by calcium flux through NMDA receptors located outside synaptic contacts. Extrasynaptic NMDA receptors couple to a CREB shut-off pathway and cause cell death. Thus, the decision whether a neuron survives (and perhaps undergoes plasticity) or dies after glutamate exposure is dependent on the location of the NMDA receptor activated. This concept of differential signaling by synaptic and extra synaptic NMDA receptors has wide-ranging implications, especially for the understanding and treatment of neuro-pathological conditions such as stroke in which brain damage may be caused by the stimulation of extrasynaptic NMDA receptors.
|Fig. 4: 3D image reconstruction of nuclei from hippocamal neurons. Picture by Gillian Queisser and Malte Wittmann|