Synaptic activity can boost neuroprotection through a mechanism that requires synapse-to-nucleus communication and calcium signals in the cell nucleus. Here we show that in hippocampal neurons nuclear calcium is one of the most potent signals in neuronal gene expression. The induction or repression of 185 neuronal activity-regulated genes is dependent upon nuclear calcium signaling. The nuclear calcium-regulated gene pool contains a genomic program that mediates synaptic activity-induced, acquired neuroprotection. The core set of neuroprotective genes consists of 9 principal components, termed Activity-regulated Inhibitor of Death (AID) genes, and includes Atf3, Btg2, GADD45β, GADD45γ, Inhibin β-A, Interferon activated gene 202B, Npas4, Nr4a1, and Serpinb2, which strongly promote survival of cultured hippocampal neurons. Several AID genes provide neuroprotection through a common process that renders mitochondria more resistant to cellular stress and toxic insults. Stereotaxic delivery of AID gene-expressing recombinant adeno-associated viruses to the hippocampus confers protection in vivo against seizure-induced brain damage. Thus, treatments that enhance nuclear calcium signaling or supplement AID genes represent novel therapies to combat neurodegenerative conditions and neuronal cell loss caused by synaptic dysfunction, which may be accompanied by a deregulation of calcium signal initiation and/or propagation to the cell nucleus. The dialogue between the synapse and the nucleus plays an important role in the physiology of neurons because it links brief changes in the membrane potential to the transcriptional regulation of genes critical for neuronal survival and long-term memory. The propagation of activity-induced calcium signals to the cell nucleus represents a major route for synapse-to-nucleus communication. Here we identified nuclear calcium-regulated genes that are responsible for a neuroprotective shield that neurons build up upon synaptic activity. We found that among the 185 genes controlled by nuclear calcium signaling, a set of 9 genes had strong survival promoting activity both in cell culture and in an animal model of neurodegeneration. The mechanism through which several genes prevent cell death involves the strengthening of mitochondria against cellular stress and toxic insults. The discovery of an activity-induced neuroprotective gene program suggest that impairments of synaptic activity and synapse-to-nucleus signaling, for example due to expression of Alzheimer's disease protein or in aging, may comprise the cells' own neuroprotective system eventually leading to cell death. Thus, malfunctioning of nuclear calcium signaling could be a key etiological factor common to many neuropathological conditions, providing a simple and unifying concept to explain disease- and aging-related cell loss.

The recent identification of the mitochondrial Ca²⁺ uniporter gene (Mcu/Ccdc109a) has enabled us to address its role, and that of mitochondrial Ca²⁺ uptake, in neuronal excitotoxicity. Here we show that exogenously expressed Mcu is mitochondrially localized and increases mitochondrial Ca²⁺ levels following NMDA receptor activation, leading to increased mitochondrial membrane depolarization and excitotoxic cell death. Knockdown of endogenous Mcu expression reduces NMDA-induced increases in mitochondrial Ca²⁺, resulting in lower levels of mitochondrial depolarization and resistance to excitotoxicity. Mcu is subject to dynamic regulation as part of an activity-dependent adaptive mechanism that limits mitochondrial Ca²⁺ overload when cytoplasmic Ca²⁺ levels are high. Specifically, synaptic activity transcriptionally represses Mcu, via a mechanism involving the nuclear Ca²⁺ and CaM kinase-mediated induction of Npas4, resulting in the inhibition of NMDA receptor-induced mitochondrial Ca²⁺ uptake and preventing excitotoxic death. This establishes Mcu and the pathways regulating its expression as important determinants of excitotoxicity, which may represent therapeutic targets for excitotoxic disorders.

Activation of extrasynaptic N-methyl-d-aspartate (NMDA) receptors causes neurodegeneration and cell death. The disease mechanism involves a pathological triad consisting of mitochondrial dysfunction, loss of integrity of neuronal structures and connectivity, and disruption of excitation–transcription coupling caused by CREB (cyclic adenosine monophosphate–responsive element-binding protein) shut-off and nuclear accumulation of class IIa histone deacetylases. Interdependency within the triad fuels an accelerating disease progression that culminates in failure of mitochondrial energy production and cell loss. Both acute and slowly progressive neurodegenerative conditions, including stroke, Alzheimer’s disease, amyotrophic lateral sclerosis, and Huntington’s disease, share increased death signaling by extrasynaptic NMDA receptors caused by elevated extracellular glutamate concentrations or relocalization of NMDA receptors to extrasynaptic sites. Six areas of therapeutic objectives are defined, based on which a broadly applicable combination therapy is proposed to combat the pathological triad of extrasynaptic NMDA receptor signaling that is common to many neurodegenerative diseases.

Interface targeting skirts excitotoxicity: Nearly all attempts to use traditional N -methyl- d -aspartate receptor (NMDAR) antagonists to treat neurodegenerative diseases have failed. This is because NMDARs are not only promoters of neuronal death but also have essential physiological roles in synaptic plasticity and cognitive functions such as learning and memory. Yan et al. explored the structural basis of NMDAR coupling to neuronal cell death (see the Perspective by Jones). The death-promoting activity, but not the essential physiological function, was mediated by the physical interaction of NMDARs with TRPM4, a calcium-impermeable ion channel activated by intracellular calcium, depolarization, and temperature. A subsequent structure-based computational drug screen led to the discovery of neuroprotective small molecules that block the NMDAR-TRPM4 interaction interface but spare the critical healthy NMDAR function. Science , this issue p. eaay3302 ; see also p. 168

Synapse-to-nucleus signaling triggered by synaptic NMDA receptors can lead to the buildup of a neuroprotective shield. Nuclear calcium activating the cAMP response element binding protein (CREB) plays a key role in neuroprotection acquired by synaptic activity. Here we show that in mouse hippocampal neurons, the transcription factorAtf3(activating transcription factor 3) is a direct target of CREB. Induction of ATF3 expression by CREB in hippocampal neurons was initiated by calcium entry through synaptic NMDA receptors and required nuclear calcium transients and calcium/calmodulin-dependent protein kinase IV activity. Acting as a transcriptional repressor, ATF3 protects cultured hippocampal neurons from apoptosis and extrasynaptic NMDA receptor-induced cell death triggered by bath application of NMDA or oxygen–glucose deprivation. Expression of ATF3in vivousing stereotaxic delivery of recombinant adeno-associated virus reduces brain damage following a cerebral ischemic insult in mice. Conversion of ATF3 to a transcriptional activator transforms ATF3 into a potent prodeath protein that kills neurons in cell culture and, when expressedin vivoin the hippocampus, ablates the neuronal cell layer. These results link nuclear calcium-CREB signaling to an ATF3-mediated neuroprotective gene repression program, indicating that activity-dependent shutoff of genes is an important process for survival. ATF3 supplementation may counteract age- and disease-related neuronal cell loss caused by a reduction in synaptic activity, malfunctioning of calcium signaling toward and within the nucleus (“nuclear calciopathy”), or increases in death signaling by extrasynaptic NMDA receptors.