Single-neuron NMDA receptor phenotype influences neuronal rewiring and reintegration following traumatic injury.

Alterations in the activity of neural circuits are a common consequence of traumatic brain injury (TBI), but the relationship between single-neuron properties and the aggregate network behavior is not well understood. We recently reported that the GluN2B-containing NMDA receptors (NMDARs) are key in mediating mechanical forces during TBI, and that TBI produces a complex change in the functional connectivity of neuronal networks. Here, we evaluated whether cell-to-cell heterogeneity in the connectivity and aggregate contribution of GluN2B receptors to [Ca(2+)]i before injury influenced the functional rewiring, spontaneous activity, and network plasticity following injury using primary rat cortical dissociated neurons. We found that the functional connectivity of a neuron to its neighbors, combined with the relative influx of calcium through distinct NMDAR subtypes, together contributed to the individual neuronal response to trauma. Specifically, individual neurons whose [Ca(2+)]i oscillations were largely due to GluN2B NMDAR activation lost many of their functional targets 1 h following injury. In comparison, neurons with large GluN2A contribution or neurons with high functional connectivity both independently protected against injury-induced loss in connectivity. Mechanistically, we found that traumatic injury resulted in increased uncorrelated network activity, an effect linked to reduction of the voltage-sensitive Mg(2+) block of GluN2B-containing NMDARs. This uncorrelated activation of GluN2B subtypes after injury significantly limited the potential for network remodeling in response to a plasticity stimulus. Together, our data suggest that two single-cell characteristics, the aggregate contribution of NMDAR subtypes and the number of functional connections, influence network structure following traumatic injury.

NR2A and NR2B subunits differentially mediate MAP kinase signaling and mitochondrial morphology following excitotoxic insult.

NMDA receptors are essential for neurotransmission and key mediators of synaptic signaling, but they can also trigger deleterious degenerative processes that lead to cell death. Growing evidence suggests that selective blockade of the heterogeneous subunits that comprise the NMDA receptor may enable better control of pharmacotherapies for treating neurological diseases and injuries. We investigated the relationship between NMDAR activation, MAPK signaling, and mitochondrial shape following an excitotoxic insult. NR2A- and NR2B-containing NMDARs differentially mediated acute changes in cytosolic calcium, alterations in mitochondrial morphology, and phosphorylation of the MAPKs ERK and JNK. Activation of NR2A-containing NMDARs was associated with JNK phosphorylation that was neuroprotective in neuronal cultures subjected to excitotoxicity. In contrast, activation of NR2B-containing NMDARs triggered calcium accumulation in mitochondria that was strongly associated with mitochondrial swelling and neuronal cell death. Indeed, while blockade of NR2B-containing receptors was neuroprotective, this protection was lost when NR2A-initiated JNK phosphorylation was inhibited. Given the modest selectivity of the NR2A inhibitor, NVP-AAM077, the results highlight the significance of the relative, rather than absolute, activation of these two NMDA subtypes in modulating cell death pathways. Therefore, the balance between concurrent activation of NR2B-containing and NR2A-containing NMDARs dictates neuronal fate following excitotoxicity.

N-methyl-D-aspartate receptor mechanosensitivity is governed by C terminus of NR2B subunit.

N-methyl-D-aspartate receptors (NMDARs), critical mediators of both physiologic and pathologic neurological signaling, have previously been shown to be sensitive to mechanical stretch through the loss of its native Mg(2+) block. However, the regulation of this mechanosensitivity has yet to be further explored. Furthermore, as it has become apparent that NMDAR-mediated signaling is dependent on specific NMDAR subtypes, as governed by the identity of the NR2 subunit, a crucial unanswered question is the role of subunit composition in observed NMDAR mechanosensitivity. Here, we used a recombinant system to assess the mechanosensitivity of specific subtypes and demonstrate that the mechanosensitive property is uniquely governed by the NR2B subunit. NR1/NR2B NMDARs displayed significant stretch sensitivity, whereas NR1/NR2A NMDARs did not respond to stretch. Furthermore, NR2B mechanosensitivity was regulated by PKC activity, because PKC inhibition reduced stretch responses in transfected HEK 293 cells and primary cortical neurons. Finally, using NR2B point mutations, we identified a PKC phosphorylation site, Ser-1323 on NR2B, as a unique critical regulator of stretch sensitivity. These data suggest that the selective mechanosensitivity of NR2B can significantly impact neuronal response to traumatic brain injury and illustrate that the mechanical tone of the neuron can be dynamically regulated by PKC activity.

Computational investigation of the changing patterns of subtype specific NMDA receptor activation during physiological glutamatergic neurotransmission.

NMDA receptors (NMDARs) are the major mediator of the postsynaptic response during synaptic neurotransmission. The diversity of roles for NMDARs in influencing synaptic plasticity and neuronal survival is often linked to selective activation of multiple NMDAR subtypes (NR1/NR2A-NMDARs, NR1/NR2B-NMDARs, and triheteromeric NR1/NR2A/NR2B-NMDARs). However, the lack of available pharmacological tools to block specific NMDAR populations leads to debates on the potential role for each NMDAR subtype in physiological signaling, including different models of synaptic plasticity. Here, we developed a computational model of glutamatergic signaling at a prototypical dendritic spine to examine the patterns of NMDAR subtype activation at temporal and spatial resolutions that are difficult to obtain experimentally. We demonstrate that NMDAR subtypes have different dynamic ranges of activation, with NR1/NR2A-NMDAR activation sensitive at univesicular glutamate release conditions, and NR2B containing NMDARs contributing at conditions of multivesicular release. We further show that NR1/NR2A-NMDAR signaling dominates in conditions simulating long-term depression (LTD), while the contribution of NR2B containing NMDAR significantly increases for stimulation frequencies that approximate long-term potentiation (LTP). Finally, we show that NR1/NR2A-NMDAR content significantly enhances response magnitude and fidelity at single synapses during chemical LTP and spike timed dependent plasticity induction, pointing out an important developmental switch in synaptic maturation. Together, our model suggests that NMDAR subtypes are differentially activated during different types of physiological glutamatergic signaling, enhancing the ability for individual spines to produce unique responses to these different inputs.

Calcium-permeable AMPA receptors appear in cortical neurons after traumatic mechanical injury and contribute to neuronal fate.

Traumatic brain injury (TBI) is one of the most disabling injuries in the population, with 1.5 million Americans new cases each year and 5.3 million Americans overall requiring long-term daily care as a result of their injuries. One critical aspect in developing effective treatments for TBI is determining if new, specific receptor populations emerge in the early phase after injury that can subsequently be targeted to reduce neuronal death after injury. One specific glutamate receptor subtype, the calcium-permeable AMPA receptor (CP-AMPAR), is becoming increasingly recognized for its role in physiological and pathophysiological processes. Although present in relatively low levels in the mature brain, recent studies show that CP-AMPARs can appear following ischemic brain injury or status epilepticus, and the mechanisms that regulate the appearance of these receptors include alterations in transcription, RNA editing, and receptor trafficking. In this report, we use an in vitro model of TBI to show a gradual appearance of CP-AMPARs four hours following injury to cortical neurons. Moreover, the appearance of these receptors is mediated by the phosphorylation of CaMKIIalpha following injury. Selectively blocking CP-AMPARs after mechanical injury leads to a significant reduction in the cell death that occurs 24 h following injury in untreated controls, and is similar in protection offered by broad-spectrum NMDA and AMPA receptor antagonists. These data point to a potentially new and more targeted therapeutic approach for treating TBI.

Linking impact to cellular and molecular sequelae of CNS injury: modeling in vivo complexity with in vitro simplicity.

Traumatic brain injury (TBI) represents one of most common disorders to the central nervous system (CNS). Despite significant efforts, though, an effective clinical treatment for TBI is not yet available. The complexity of human TBI is modeled with a broad group of experimental models, with each model matching some aspect of the human condition. In the past 15 years, these in vivo models were complemented with a group of in vitro models, with these in vitro models allowing investigators to more precisely identify the mechanism(s) of TBI, the different intracellular events that occur in acute period following injury, and the possible treatment of this injury in vitro. In this paper, we review the available in vitro models to study TBI, discuss their biomechanical basis for human TBI, and review the findings from these in vitro models. Finally, we synthesize the current knowledge and point out possible future directions for this group of models, especially in the effort toward developing new therapies for the traumatically brain injured patient.