An an Vitro and in Silico Investigation of the Role of Nmda Receptor Subtypes Following Mechanical injury

The N-methyl D-aspartate receptor (NMDAR), a common glutamate receptor found throughout the brain, has long been implicated as the major mediator of the pathology seen after traumatic brain injury (TBI). However, given their critical role in physiologic function of neural networks, complete inhibition of these receptors is an unsuitable therapeutic strategy. Thus, further investigation into how these receptors respond to injury is required to identify more directed therapeutic targets. Here, we aimed to use two unique experimental models to further investigate the role of NMDARs in the neuronal response to TBI, with specific emphasis on the contribution of different NMDAR subtypes. TBI produces a unique disease paradigm containing mechanical and biochemical components, which can both affect NMDAR activity. We sought to isolate the effects of both these components and then to examine how they combine to create a unique injury response. We utilized a recombinant system expressing known NMDAR subtypes to first examine the action of mechanical stretch on specific subtypes. We demonstrated that mechanosensitivity of the NMDAR is indeed dependent on its subunit composition, with the NR2B subunit conferring stretch sensitivity. Further, we were able to investigate the regulation of NR2B mechanosensitivity and found that a single PKC phosphorylation site on the NR2B C-terminal tail can critically control stretch sensitivity. We next developed a computational model of a single dendritic spine to evaluate the patterns of activation among NMDAR subtypes in both physiologic and pathologic glutamatergic signaling. We demonstrate that the presence of multiple NMDAR subtypes on the dendritic spine enables the ability for a single synapse to produce unique responses to different glutamate inputs. Importantly, we discovered that injury induced release of synaptic glutamate vesicles results in enhanced contribution of NR2B containing receptors. Finally, we have shown that the collective effects of TBI can drastically enhance the calcium influx from synaptic and extrasynaptic NR1/NR2B-NMDARs, an NMDAR subtype known to mediate pro-death signaling. Together, our data demonstrates that the NR2B subunit represents a unique pathologic sensor for TBI, and could represent an intriguing target of manipulation in the development of improved TBI therapeutics.