Acute connexin43 temporal and spatial expression in response to ischemic stroke

Unravelling Contributions of Astrocytic Connexin 43 to the Functional Activity of Brain Neuron-Glial Networks under Hypoxic State In Vitro

Brain hypoxia remains an Achilles' heel for public health that must be urgently addressed. Hypoxic damage affects both neurons and glial cells, particularly astrocytes, which are in close dynamic bi-directional communication, and are organized in plastic and tightly regulated networks. However, astroglial networks have received limited attention regarding their influence on the adaptive functional rearrangements of neural networks to oxygen deficiency. Herein, against the background of astrocytic Cx43 gap junction blockade by the selective blocker Gap19, we evaluated the features of spontaneous calcium activity and network characteristics of cells in primary cultures of the cerebral cortex, as well as the expression levels of metabotropic glutamate receptors 2 (mGluR2) and 5 (mGluR5) in the early and late periods after simulated hypoxia in vitro. We showed that, under normoxic conditions, blockade of Cx43 leads to an increase in the expression of metabotropic glutamate receptors mGluR2 and mGluR5 and long-term modulation of spontaneous calcium activity in primary cortical cultures, primarily expressed in the restructuring of the functional architectonics of neuron-glial networks through reducing the level of correlation between cells in the network and the percentage of existing correlated connections between cells. Blocking Cx43 during hypoxic injury has a pronounced neuroprotective effect. Together with the increased expression of mGluR5 receptors, a decrease in mGluR2 expression to the physiological level was found, which suggests the triggering of alternative molecular mechanisms of cell adaptation to hypoxia. Importantly, the blockade of Cx43 in hypoxic damage contributed to the maintenance of both the main parameters of the spontaneous calcium activity of primary cortical cultures and the functional architectonics of neuron-glial networks while maintaining the profile of calcium oscillations and calcium signal communications between cells at a highly correlated level. Our results demonstrate the crucial importance of astrocytic networks in functional brain adaptation to hypoxic damage and could be a promising target for the development of rational anti-hypoxic therapy.

First Responders to Hyperosmotic Stress in Murine Astrocytes: Connexin 43 Gap Junctions Are Subject to an Immediate Ultrastructural Reorganization

Simple Summary

Gap junctions are intercellular channels that provide the means for direct transport of small molecules, ions, and water between connected cells. With these functions, gap junctions are essential for the maintenance of astrocytic homeostasis and of particular importance in the context of pathophysiological disbalances. These include the hyperosmolar hyperglycemic syndrome or the pathology after brain trauma. We demonstrate that short-term hyperosmolarity reduces intercellular communication via gap junctions. These functional changes coincide with the transformation of gap junction ultrastructure as evidenced by freeze-fracture replica immunolabeling and transmission electron microscopy. The hyperosmolarity-induced immediate changes in the ultrastructural assembly of connexons, the protein constituents of gap junction channels, have not been described in astrocytes before and are revealing the coherence of structure and function in gap junctions. Phosphorylation of Connexin 43, the main gap junction protein in astrocytes, at amino acid 368 (Serine) might link the two.

Abstract

In a short-term model of hyperosmotic stress, primary murine astrocytes were stimulated with a hyperosmolar sucrose solution for five minutes. Astrocytic gap junctions, which are mainly composed of Connexin (Cx) 43, displayed immediate ultrastructural changes, demonstrated by freeze–fracture replica immunogold labeling: their area, perimeter, and distance of intramembrane particles increased, whereas particle numbers per area decreased. Ultrastructural changes were, however, not accompanied by changes in Cx43 mRNA expression. In contrast, transcription of the gap junction regulator zonula occludens (ZO) protein 1 significantly increased, whereas its protein expression was unaffected. Phosphorylation of Serine (S) 368 of the Cx43 C–terminus has previously been associated with gap junction disassembly and reduction in gap junction communication. Hyperosmolar sucrose treatment led to enhanced phosphorylation of Cx43S368 and was accompanied by inhibition of gap junctional intercellular communication, demonstrated by a scrape loading-dye transfer assay. Taken together, Cx43 gap junctions are fast reacting elements in response to hyperosmolar challenges and can therefore be considered as one of the first responders to hyperosmolarity. In this process, phosphorylation of Cx43S368 was associated with disassembly of gap junctions and inhibition of their function. Thus, modulation of the gap junction assembly might represent a target in the treatment of brain edema or trauma.

Connexin43 promotes angiogenesis through activating the HIF-1α/VEGF signaling pathway under chronic cerebral hypoperfusion

Chronic cerebral hypoperfusion, a major vascular contributor to vascular cognitive impairment and dementia, can exacerbate small vessel pathology. Connexin43, the most abundant gap junction protein in brain tissue, has been found to be critically involved in the pathological changes of vascular cognitive impairment and dementia caused by chronic cerebral hypoperfusion. However, the precise mechanisms underpinning its role are unclear. We established a mouse model via bilateral common carotid arteries stenosis on connexin43 heterozygous male mice and demonstrated that connexin43 improves brain blood flow recovery by mediating reparative angiogenesis under chronic cerebral hypoperfusion, which subsequently reduces the characteristic pathologies of vascular cognitive impairment and dementia including white matter lesions and irreversible neuronal injury. We additionally found that connexin43 mediates hypoxia inducible factor-1α expression and then activates the PKA signaling pathway to regulate vascular endothelial growth factor-induced angiogenesis. All the above findings were replicated in bEnd.3 cells treated with 375 µM CoCl₂in vitro. These results suggest that connexin 43 could be instrumental in developing potential therapies for vascular cognitive impairment and dementia caused by chronic cerebral hypoperfusion.

Exogenous Adenosine Antagonizes Excitatory Amino Acid Toxicity in Primary Astrocytes

Excitatory toxicity is still a hot topic in the study of ischemic stroke, and related research has focused mainly on neurons. Adenosine is an important neuromodulator that is known as a "biosignature" in the central nervous system (CNS). The protective effect of exogenous adenosine on neurons has been confirmed, but its mechanism remains elusive. In this study, astrocytes were pretreated with adenosine, and the effects of an A2a receptor (A2aR) inhibitor (SCH58261) and A2b receptor (A2bR) inhibitor (PSB1115) on excitatory glutamate were investigated. An oxygen glucose deprivation/reoxygenation (OGD/R) and glutamate model was generated in vitro. Post-model assessment included expression levels of glutamate transporters (glt-1), gap junction protein (Cx43) and glutamate receptor (AMPAR), Na+-K+-ATPase activity, and diffusion distance of dyes. Glutamate and glutamine contents were determined at different time points. The results showed that (1) adenosine could improve the function of Na+-K+-ATPase, upregulate the expression of glt-1, and enhance the synthesis of glutamine in astrocytes. This effect was associated with A2aR activation but not with A2bR activation. (2) Adenosine could inhibit the expression of gap junction protein (Cx43) and reduce glutamate diffusion. Inhibition of A2aR attenuated adenosine inhibition of gap junction intercellular communication (GJIC) in the OGD/R model, while it enhanced adenosine inhibition of GJIC in the glutamate model, depending on the glutamate concentration. (3) Adenosine could cause AMPAR gradually entered the nucleus from the cytoplasm, thereby reducing the expression of AMPAR on the cell membrane. Taken together, the results indicate that adenosine plays a role of anti-excitatory toxicity effect in protection against neuronal death and the functional recovery of ischemic stroke mainly by targeting astrocytes, which are closely related to A2aR. The present study provided a scientific basis for adenosine prevention and ischemic stroke treatment, thereby providing a new approach for alleviating ischemic stroke.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

Danegaptide Enhances Astrocyte Gap Junctional Coupling and Reduces Ischemic Reperfusion Brain Injury in Mice

Ischemic stroke is a complex and devastating event characterized by cell death resulting from a transient or permanent arterial occlusion. Astrocytic connexin43 (Cx43) gap junction (GJ) proteins have been reported to impact neuronal survival in ischemic conditions. Consequently, Cx43 could be a potential target for therapeutic approaches to stroke. We examined the effect of danegaptide (ZP1609), an antiarrhythmic dipeptide that specifically enhances GJ conductance, in two different rodent stroke models. In this study, danegaptide increased astrocytic Cx43 coupling with no significant effects on Cx43 hemichannel activity, in vitro. Using matrix-assisted laser desorption ionization imaging mass spectrometry (MALDI IMS) the presence of danegaptide within brain tissue sections were detected one hour after reperfusion indicating successful transport of the dipeptide across the blood brain barrier. Furthermore, administration of danegaptide in a novel mouse brain ischemia/reperfusion model showed significant decrease in infarct volume. Taken together, this study provides evidence for the therapeutic potential of danegaptide in ischemia/reperfusion stroke.

Herbal Compatibility of Ginseng and Rhubarb Exerts Synergistic Neuroprotection in Cerebral Ischemia/Reperfusion Injury of Rats

Objective

Ischemic stroke is a complex multifactorial disease caused by interactions among polygenetic, environmental, and lifestyle factors with limited effective treatments. Multi-herbal formulae have long been used for stroke through herbal compatibility in traditional Chinese medicine (TCM); however, there is still a lack of evidence due to their unimaginable complexity. Herbal pairs represent the simplest and basic features of multi-herbal formulae, which are of great significance in clarifying herbal compatibility. Here, we aim to investigate the neuroprotective effects of the herbal compatibility of Ginseng and Rhubarb on a cerebral ischemia/reperfusion (I/R) injury model of rats.

Methods

Male adult SD rats were randomly divided into a sham group, a normal saline (NS) group, a Ginseng group, a Rhubarb group, and a Ginseng + Rhubarb (GR) group, a Carbenoxolone [CBX, gap junction (GJ) specific inhibitor] group, and a GR + CBX group. Each group was further assigned into four subgroups according to ischemic time (6 h, 1 day, 3 days, and 7 days). The cerebral I/R injury model was established according to the modified Zea Longa method. The Neurological Deficiency Score (NDS) was assessed by the Zea-Longa scale; the cerebral infarction area was detected by TTC (2,3,5-triphenyltetrazolium chloride) staining; and the expression of connexin-43 (Cx43) and aquaporin-4 (AQP4) were detected based on an immunofluorescence technique and quantitative real-time-PCR.

Results

Compared to the I/R group, both the independent and combined use of Ginseng and Rhubarb can significantly improve NDS (P < 0.05), decrease the percentage of the cerebral infarction area around the infarction penumbra (P < 0.05) and down-regulate the expression of Cx43 and AQP4 after I/R injury (P < 0.05). The GR had more significant effects than that of Ginseng and Rhubarb (P < 0.05). Compared with the GR group, the GR + CBX group significantly improved in NDS (P < 0.05), and decreased the percentage of the cerebral infarction area (P < 0.05) and expression of Cx43 and AQP4 protein (P < 0.05).

Conclusion

The herbal compatibility of Ginseng and Rhubarb synergistically exerts neuroprotective function during acute cerebral I/R injury, mainly through reducing the expression of Cx43 and AQP4.

Expression and role of CaMKII and Cx43 in a rat model of post-stroke depression

Expression of Ca²⁺/CaM-dependent protein kinase II (CaMKII) and connexin 43 (Cx43) in a rat model of post-stroke depression (PSD) was investigated. Rats were separated into control group (10 rats underwent a sham operation and were not ligated after incision), PSD group (13 PSD rats) and KN93 group (12 rats were treated with KN93, an inhibitor of CaMKII, on the basis of the PSD group). After PSD modeling, Longa scoring was performed, and an open field test as well as a step-through test were carried out to observe rat behavior. RT-qPCR and western blot analysis were used to detect the expression of CaMKII and CX43 in the hippocampus tissue. On the 14th day, the Longa scores in the PSD and KN93 groups were higher than that in the control group (P<0.05), while on the 18th day, Longa score was higher in the PSD group than that in the control and KN93 groups, and higher in the KN93 group than that in the control group (both P<0.05). In the PSD group, the Longa score on the 18th day was significantly higher than that on the 14th day, whereas in the KN93 group, the Longa score on the 18th day was significantly lower than that on the 14th day (both P<0.05). Compared with the PSD group on the 18th day, the passive avoidance defects in the KN93 group were improved, and the frequency of activity in the open field test was significantly increased. On the 18th day, the expression levels of the mRNA and protein of CaMKII were higher in the PSD group than in the control group, whereas those of Cx43 were lower in the PSD group than those in the control group (P<0.05). The mRNA and protein expression levels of CaMKII in the KN93 group were lower than those in the PSD group, but higher than those in the control group. In PSD rats, CaMKII expression is upregulated, but Cx43 expression is downregulated, and both CaMKII and Cx43 may participate in PSD. The inhibitor of CaMKII, KN93, can improve the depression in PSD rats.

Oxygen-Glucose Deprivation in Mouse Astrocytes is Associated with Ultrastructural Changes in Connexin 43 Gap Junctions

In the intact brain, astrocytes play an important role in a number of physiological functions like spatial buffering of potassium, maintenance of calcium homeostasis, neurotransmitter release, regulation of the cerebral blood flow, and many more. As pathophysiological events upon hypoxic-ischemic brain injury include excitotoxicity by glutamate release as well as oxidative stress, astrocytes and their gap junction-based syncytium are of major relevance for regulating the extent of resulting brain damage. The gap junction protein Connexin (Cx) 43 contributes mainly to the astrocytic intercellular communication. As little is known about the ultrastructural assemblage of Cx43 and its changes in response to hypoxic events, we chose temporary oxygen and glucose deprivation with subsequent reoxygenation (OGD-R) as a metabolic inhibition model of hypoxia in primary murine astrocytes. Gap junction morphology and assembly/disintegration were analyzed at the ultrastructural level using freeze-fracture replica immunolabeling. The exposure of cultured astrocytes to short-term OGD-R resulted in the activation of ERK1/2 (p44/p42), downregulation of Cx43 protein expression, and the rearrangement of Cx43 particles within the cell membrane and within gap junctions. These changes in gap junction morphology were associated with phosphorylation of Cx43 at Serine 368. Analysis of the nearest-neighbor distance within gap junction plaques revealed the loosening of Cx43 particle clusters. Together with the observation of additional connexons being present in the vicinity of gap junction plaques after OGD-R treatment, our study indicates that changes in gap junction assembly are associated with the early phase of hypoxic cell damage.