The cellular consequences of N-Methyl-D-Aspartate receptor (NMDAR) stimulation depend on the receptors' subcellular localization. Synaptic NMDARs promote plasticity and survival whereas extrasynaptic NMDARs mediate excitotoxicity and contribute to cell death in neurodegenerative diseases. The mechanisms that couple activation of extrasynaptic NMDARs to cell death remain incompletely understood. We here show that activation of extrasynaptic NMDARs by bath application of NMDA or L-glutamate leads to the upregulation of a group of 19 microRNAs in cultured mouse hippocampal neurons. In contrast, none of these microRNAs is induced upon stimulation of synaptic activity. Increased microRNA expression depends on the pri-miRNA processing enzyme Drosha, but not on de novo gene transcription. These findings suggest that toxic NMDAR signaling involves changes in the expression levels of particular microRNAs.
Synaptic NMDA receptors activating nuclear calcium-driven adaptogenomics control a potent body-own neuroprotective mechanism, referred to as acquired neuroprotection. Viral vector-mediated gene transfer in conjunction with stereotactic surgery has previously demonstrated the proficiency of several nuclear calcium-regulated genes to protect in vivo against brain damage caused by toxic extrasynaptic NMDA receptor signaling following seizures or stroke. Here we used noninvasive nose-to-brain administration of Activin A and SerpinB2, two secreted nuclear calcium-regulated neuroprotectants, for post-injury treatment of brain damage following middle cerebral artery occlusion (MCAO) in C57BL/6N mice. The observed reduction of the infarct volume was comparable to the protection obtained by intracerebroventricular injection of recombinant Activin A or SerpinB2 or by stereotactic delivery 3 weeks prior to the injury of a recombinant adeno-associated virus containing an expression cassette for the potent neuroprotective transcription factor Npas4. These results establish post-injury, nose-to-brain delivery of Activin A and SerpinB2 as effective and possibly clinically applicable treatments of acute and chronic neurodegenerative conditions. Synaptic NMDA receptors activating nuclear calcium-driven adaptogenomics control a potent body-own neuroprotective mechanism, referred to as acquired neuroprotection. Viral vector-mediated gene transfer in conjunction with stereotactic surgery has previously demonstrated the proficiency of several nuclear calcium-regulated genes to protect in vivo against brain damage caused by toxic extrasynaptic NMDA receptor signaling following seizures or stroke. Here we used noninvasive nose-to-brain administration of Activin A and SerpinB2, two secreted nuclear calcium-regulated neuroprotectants, for post-injury treatment of brain damage following middle cerebral artery occlusion (MCAO) in C57BL/6N mice. The observed reduction of the infarct volume was comparable to the protection obtained by intracerebroventricular injection of recombinant Activin A or SerpinB2 or by stereotactic delivery 3 weeks prior to the injury of a recombinant adeno-associated virus containing an expression cassette for the potent neuroprotective transcription factor Npas4. These results establish post-injury, nose-to-brain delivery of Activin A and SerpinB2 as effective and possibly clinically applicable treatments of acute and chronic neurodegenerative conditions.
The health of neurons is critically dependent on the relative signaling intensities of survival-promoting synaptic and death-inducing extrasynaptic NMDA receptors. Here, we show that BDNF is a regulator of this balance and promotes neuroprotection by reducing toxic NMDA receptor signaling. BDNF acts by initiating synaptic NMDA-receptor/nuclear-calcium-driven adaptogenomics, leading to increased expression of inhibin β-A (inhba). Inhibin β-A (its homodimer is known as activin A) in turn reduces neurotoxic extrasynaptic NMDA-receptor-mediated calcium influx, thereby shielding neurons against mitochondrial dysfunction, a major cause of excitotoxicity. Thus, BDNF induces acquired neuroprotection by enhancing synaptic activity and lowering extrasynaptic NMDA receptor death signaling through a nuclear calcium-inhibin β-A pathway. This process, which confers protection against ischemic brain damage in a mouse stroke model, may be compromised in Huntington's disease, Alzheimer's disease, or aging-related neurodegenerative conditions that are associated with reduced BDNF levels and/or enhanced extrasynaptic NMDA receptor signaling.
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 factor Atf3 (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 ATF3 in vivo using 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 expressed in vivo in 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.
The function of MeCP2, a methylated DNA-interacting protein that may act as a global chromatin modifier, is controlled by its phosphorylation on serine 421. Here we show that in hippocampal neurons, nuclear calcium signaling controls synaptic activity-induced phosphorylation of MeCP2 on serine 421. Pharmacological inhibition of calcium/calmodulin-dependent protein (CaM)kinases blocked activity-induced MeCP2 serine 421 phosphorylation. CaM kinase II (CaMKII) but not CaMKIV, the major nuclear CaM kinase in hippocampal neurons, appeared to mediate this phosphorylation event. Biochemical subcellular fractionations and immunolocalization studies revealed that several isoforms of CaMKII (i.e. CaMKIIα, -β, -γ, and -δ) are expressed in the cytosol but are also detectable in the cell nucleus of hippocampal neurons, suggesting that nuclear CaMKII catalyzes MeCP2 serine 421 phosphorylation. Thus, in addition to the classical nuclear calcium-CaMKIV-CREB/CBP (cAMP-response element-binding protein/CREB-binding protein) pathway that regulates transcription of specific target genes, nuclear calcium may also modulate genome-wide the chromatin state in response to synaptic activity via nuclear CaMKII-MeCP2 signaling.
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
In the adult brain, vascular endothelial growth factor D (VEGFD) is required for structural integrity of dendrites and cognitive abilities. Alterations of dendritic architectures are hallmarks of many neurologic disorders, including stroke-induced damage caused by toxic extrasynaptic NMDA receptor (eNMDAR) signaling. Here we show that stimulation of eNMDARs causes a rapid shutoff of VEGFD expression, leading to a dramatic loss of dendritic structures. Using the mouse middle cerebral artery occlusion (MCAO) stroke model, we have established the therapeutic potential of recombinant mouse VEGFD delivered intraventricularly to preserve dendritic architecture, reduce stroke-induced brain damage, and facilitate functional recovery. An easy-to-use therapeutic intervention for stroke was developed that uses a new class of VEGFD-derived peptide mimetics and postinjury nose-to-brain delivery.
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