Neuronal Gap Junction Coupling Is Regulated by Glutamate and Plays Critical Role in Cell Death during Neuronal Injury

Abstract

In the mammalian CNS, excessive release of glutamate and overactivation of glutamate receptors are responsible for the secondary (delayed) neuronal death following neuronal injury, including ischemia, traumatic brain injury (TBI) and epilepsy. The coupling of neurons by gap junctions (electrical synapses) increases during neuronal injury. We report here that the ischemic increase in neuronal gap junction coupling is regulated by glutamate via group II metabotropic glutamate receptors (mGluR). Specifically, using electrotonic coupling, western blots and siRNA in the mouse somatosensory cortex in vivo and in vitro, we demonstrate that activation of group II mGluRs increases background levels of neuronal gap junction coupling and expression of connexin 36 (Cx36; neuronal gap junction protein) and inactivation of group II mGluRs prevents the ischemia-mediated increases in the coupling and Cx36 expression. We also show that the regulation is via cAMP/PKA-dependent signaling and post-transcriptional control of Cx36 expression and that other glutamate receptors are not involved in these regulatory mechanisms. Further, using the analysis of neuronal death, we show that inactivation of group II mGluRs or genetic elimination of Cx36 both dramatically reduce ischemia-mediated neuronal death in vitro and in vivo. Similar results are obtained using in vitro models of TBI and epilepsy. Our results indicate that neuronal gap junction coupling is a critical component of glutamate-dependent neuronal death. They also suggest that causal link among group II mGluR function, neuronal gap junction coupling and neuronal death has a universal character and operates in different types of neuronal injuries.

Correlation between connexin and traumatic brain injury in patients

“…On the other hand, changes in connexin levels may exacerbate brain dysfunctions (Fonseca, Green, & Nicholson, 2002). It was previously reported by in vitro and in vivo models of TBI and epilepsy, neuronal gap junction coupling through CX36 can rise along with ischemia‐stimulated neuron death that is regulated by glutamate‐mediated mechanisms (Wang et al., 2012). CX43 mRNAs, which are primarily seen in astrocytes in the rat, were significantly enhanced following spinal cord injury (Lee, Lindqvist, Kiehn, Widenfalk, & Olson, 2005).…”

The regulation and role of neuronal gap junctions during neuronal injury

“…However, clinical trials for the majority of NMDAR antagonists have failed. 23 Based on our previous results and results obtained in the most recent study, we have proposed a novel model for the mechanisms of glutamate-dependent excitotoxicity 13 (Fig. 1B).…”

“…13 We used oxygen-glucose deprivation (OGD) in mouse mature neuronal somatosensory cortical cultures as a model of cortical ischemia in vitro. We also used photothrombotic focal ischemia in the somatosensory cortex of adult mice as a model of ischemia in vivo.…”

“…We also established that the regulation by group II www.landesbioscience.com Channels 391 ARTICLE ADDENDUM ARTICLE ADDENDUM neuronal gap junction coupling and, via regulation of neuronal gap junctions, they also control death/survival mechanisms in injured neurons. 13 It has been suggested previously [20][21][22] that one of the most critical factors, responsible for the secondary (delayed) neuronal death is excessive release of glutamate from injured cells that causes glutamate-dependent excitotoxicity. The excitotoxic mechanisms of glutamate are well-characterized and include hyperactivation of glutamate receptors (primarily NMDARs), massive influx of Ca 2+ ions and overactivation of Ca 2+ -dependent signaling pathways that eventually causes death of neurons [20][21][22] ( Fig.…”

“…15 Therefore, in our most recent study, we tested whether mechanisms regulating ischemic increase in neuronal gap junction coupling also regulate ischemic neuronal death. 13 The amount of neuronal death was measured using methyl thiazolyl tetrazolium assay in neuronal cultures and Fluoro-Jade B staining in brain sections. We found that inactivation of group II mGluRs, that prevents the ischemic increase in neuronal gap junction coupling and Cx36 expression, also dramatically reduces ischemia-mediated neuronal death in vivo and in vitro.…”