Aquaporin 4-Mediated Glutamate-Induced Astrocyte Swelling Is Partially Mediated through Metabotropic Glutamate Receptor 5 Activation

Abstract

Astrocytes are one of the most abundant cell types in the mammalian central nervous system (CNS), and astrocyte swelling is the primary event associated with brain edema. Glutamate, the principal excitatory amino acid neurotransmitter in the CNS, is released at high levels after brain injury including cerebral ischemia. This leads to astrocyte swelling, which we previously demonstrated is related to metabotropic glutamate receptor (mGluR) activation. Aquaporin 4 (AQP4), the predominant water channel in the brain, is expressed in astrocyte endfeet and plays an important role in brain edema following ischemia. Studies recently showed that mGluR5 is also expressed on astrocytes. Therefore, it is worth investigating whether AQP4 mediates the glutamate-induced swelling of astrocytes via mGluR5. In the present study, we found that 1 mM glutamate induced astrocyte swelling, quantified by the cell perimeter, but it had no effect on astrocyte viability measured by the cell counting kit-8 (CCK-8) and lactate dehydrogenase (LDH) assays. Quantitative reverse transcription polymerase chain reaction analyses revealed that AQP4, among AQP1, 4, 5, 9 and 11, was the main molecular expressed in cultured astrocytes. Glutamate-induced cell swelling was accompanied by a concentration-dependent change in AQP4 expression. Furthermore, RNAi technology revealed that AQP4 gene silencing inhibited glutamate-induced astrocyte swelling. Moreover, we found that mGluR5 expression was greatest among the mGluRs in cultured astrocytes and was co-expressed with AQP4. Activation of mGluR5 in cultured astrocytes using (S)-3,5-dihydroxyphenylglycine (DHPG), an mGluR5 agonist, mimicked the effect of glutamate. This effect was abolished by co-incubation with the mGluR5 antagonist fenobam but was not influenced by DL-threo-β-benzyloxyaspartic acid (DL-TBOA), a glutamate transporter inhibitor. Finally, experiments in a rat model of transient middle cerebral artery occlusion (tMCAO) revealed that co-expression of mGluR5 and AQP4 was increased in astrocyte endfeet around capillaries in the penumbra, and this was accompanied by brain edema. Collectively, these results suggest that glutamate induces cell swelling and alters AQP4 expression in astrocytes via mGluR5 activation, which may provide a novel approach for the treatment of edema following brain injury.

Connexin 43 hemichannels and pannexin‐1 channels contribute to the α‐synuclein‐induced dysfunction and death of astrocytes

“…In this study, we observed that astrocytes treated with α‐synuclein exhibited a significant decrease in cell survival, the latter response was strongly prevented by the blockade of Cx43 hemichannels rather than Panx1 channels. Relevantly, a recent study demonstrated that AQP4 water channel is crucial for the glutamate‐induced astrocyte swelling caused by mGluR 5 activation (Shi et al, ). Consistent with this, we observed that α‐synuclein‐evoked astrocyte death was strongly prevented by the blockade of both AQP4 or mGluR 5 , whereas inhibition of NDMA receptors resulted in similar weak protection than Panx1 channel blockers.…”

Involvement of p38 mitogen‐activated protein kinase in altered expressions of AQP1 and AQP4 after carbon monoxide poisoning in rat astrocytes

“…Cerebral oedema is a potentially lethal pathological state that occurs after brain injury, and it is also one of the most serious consequences of acute CO poisoning. Cerebral oedema is featured by accumulation of water molecules in brain cells and or in the extracellular space, which can be mainly classified into interstitial cerebral oedema, osmotic cerebral oedema, cytotoxic cerebral oedema and vasogenic cerebral oedema . Comprehensive molecular mechanisms in the acute CO poisoning remain to be elucidated.…”

Glial function (and dysfunction) in the normal & ischemic brain

“…Glutamate excitotoxicity, as noted above, is one of the major causes of ischemic injury, and glutamate transporters on oligodendrocytes, astrocytes, and microglia that under normal conditions mediate uptake of extracellular glutamate to maintain low levels in the extracellular space, in ischemia release glutamate into the extracellular space (Mifsud et al, 2014). Furthermore, brain injury-induced astrocyte swelling is likely mediated by glutamate transporters and metabotropic glutamate receptors (Shi et al, 2017), and it is well known that oligodendrocytes are susceptible to excitotoxicity mediated by AMPA/kainate ionotropic glutamate receptors (Mifsud et al, 2014). There is significant developmental upregulation of these non-NMDA receptors, and it has recently been shown that selective upregulation and downregulation of specific subunits of the NMDA receptors in subjects with periventricular leukomalacia may underlie NMDA-mediated vulnerability in the developing brain (Jantzie et al, 2015; Mifsud et al, 2014).…”

“…Astrocyte functions in the ‘healthy’ CNS include uptake of potassium, water (astrocytes contain abundant aquaporin-4, a water channel; Moftakhar et al, 2010; Shi et al, 2017), and neurotransmitters (glutamate, GABA, glycine), and release of energy substrates (e.g. lactate), neurotransmitter precursors (e.g.…”

“…BDNF, TNFalpha). Aqua-porin 4-mediated glutamate-induced astrocytic swelling may be partially mediated through the activation of metabotropic gluta-mate receptor 5 (Shi et al, 2017). The tight apposition of astrocytes to cerebral microvessels, neuronal perikarya, axons (in particular nodes of Ranvier) and synapses make them a crucial player in CNS function, metabolism and dysfunction.…”