Neuronal and astrocytic sodium-calcium exchanger differentially regulates calcium and sodium overload during ischemic stroke
Somayyeh Hamzei Taj, Pawan Kumar Thapaliya, Cordula Rakers, Niklas J. Gerkau, Christine R. Rose, Ghanim Ullah, Gabor C. Petzold
Somayyeh Hamzei Taj, Pawan Kumar Thapaliya, Cordula Rakers, Niklas J. Gerkau, Christine R. Rose, Ghanim Ullah, Gabor C. Petzold
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Research Article Neuroscience Vascular biology

Neuronal and astrocytic sodium-calcium exchanger differentially regulates calcium and sodium overload during ischemic stroke

Abstract

Spreading depolarizations (SDs) are propagating waves of near-complete breakdown of transmembrane ion gradients that occur during acute ischemic stroke and worsen outcome by driving calcium overload and glutamate release in neurons and astrocytes. The plasmalemmal sodium-calcium exchanger (NCX) plays a key role in such changes, in that the complex ionic disequilibrium during ischemia induces reverse-mode activity of NCX, leading to cellular calcium overload in exchange for sodium. However, the cell-type-specific roles of NCX in neurons and astrocytes during SDs remain unclear. Here, we used ion and glutamate reporters in an in vivo stroke model in mice carrying inducible, cell-specific deletions of NCX isoform 1. Neuronal NCX1 deletion reduced neuronal and astrocytic calcium transients, increased neuronal sodium transients, decreased extracellular glutamate levels, and raised SD initiation threshold. In contrast, astrocytic NCX1 deletion increased sodium transients in both neurons and astrocytes, and increased neuronal calcium as well as extracellular glutamate levels. A computational model of ischemia confirmed that these effects are consistent with reverse-mode NCX1 activity. Together, these findings indicate opposing roles of reverse-mode NCX1 during ischemia. Neuronal NCX1 promotes SD susceptibility, calcium overload, and glutamate release, whereas astrocytic NCX1 exerts protective effects by attenuating glutamate elevation and neuronal calcium accumulation.

Authors

Somayyeh Hamzei Taj, Pawan Kumar Thapaliya, Cordula Rakers, Niklas J. Gerkau, Christine R. Rose, Ghanim Ullah, Gabor C. Petzold

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Figure 1

Cell-specific NCX1 deletion and experimental paradigm.

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Cell-specific NCX1 deletion and experimental paradigm. (A) Immunohistoch...
(A) Immunohistochemistry using antibodies against NCX1, astrocytes (glial fibrillary acidic protein [GFAP]), and neurons (NeuN) showed that NCX1 is expressed by astrocytes (arrowheads) and neurons (arrows) in the cortex of Cre-negative control mice. (B) In astroglial NCX1-KO mice, most astrocytes appeared NCX1-negative, although punctuate signals remained in few astrocytes (arrowheads); neuronal NCX1 expression remained unchanged (arrows). (C) In neuronal NCX1-KO mice, neuronal NCX1 coverage was attenuated, although few neurons remained NCX1-positive (arrows); astroglial NCX1 coverage appeared unchanged (arrowheads). (D) Quantitative analysis of Cre-negative control mice, astroglial NCX1-KO mice, and neuronal NCX1-KO mice. NCX1-positive neurons or astrocytes, respectively, per field of view (FOV) were compared in the 3 lines. Astroglial coverage was significantly attenuated in astroglial NCX1-KO mice, and neuronal coverage was significantly attenuated in neuronal NCX1-KO mice (5 FOVs were averaged per animal; n = 5 animals per group; *P < 0.05, Kruskal-Wallis test followed by Dunn’s multiple-comparison test for all comparisons). (E) AAV5.GfaABC1D.cyto-GCaMP6f, and AAV1.syn1.jRGECO1a were co-injected into cortex to label astrocytes and neurons, respectively, with calcium indicators. One week later, tamoxifen was administered for Cre activation, followed by middle cerebral artery occlusion (MCAO) and 2-photon microscopy 4 weeks later. (F and G) During MCAO, ischemia-induced spreading depolarizations occurred, which were characterized by large, slowly progressing, transient calcium elevation in cortical neurons and astrocytes. Scale bars: 50 μm (A–C) and 100 μm (F and G).