Dysbalance of Astrocyte Calcium under Hyperammonemic Conditions

Abstract

Increased brain ammonium (NH4 +/NH3) plays a central role in the manifestation of hepatic encephalopathy (HE), a complex syndrome associated with neurological and psychiatric alterations, which is primarily a disorder of astrocytes. Here, we analysed the influence of NH4 +/NH3 on the calcium concentration of astrocytes in situ and studied the underlying mechanisms of NH4 +/NH3-evoked calcium changes, employing fluorescence imaging with Fura-2 in acute tissue slices derived from different regions of the mouse brain. In the hippocampal stratum radiatum, perfusion with 5 mM NH4 +/NH3 for 30 minutes caused a transient calcium increase in about 40% of astrocytes lasting about 10 minutes. Furthermore, the vast majority of astrocytes (∼90%) experienced a persistent calcium increase by ∼50 nM. This persistent increase was already evoked at concentrations of 1–2 mM NH4 +/NH3, developed within 10–20 minutes and was maintained as long as the NH4 +/NH3 was present. Qualitatively similar changes were observed in astrocytes of different neocortical regions as well as in cerebellar Bergmann glia. Inhibition of glutamine synthetase resulted in significantly larger calcium increases in response to NH4 +/NH3, indicating that glutamine accumulation was not a primary cause. Calcium increases were not mimicked by changes in intracellular pH. Pharmacological inhibition of voltage-gated sodium channels, sodium-potassium-chloride-cotransporters (NKCC), the reverse mode of sodium/calcium exchange (NCX), AMPA- or mGluR5-receptors did not dampen NH4 +/NH3-induced calcium increases. They were, however, significantly reduced by inhibition of NMDA receptors and depletion of intracellular calcium stores. Taken together, our measurements show that sustained exposure to NH4 +/NH3 causes a sustained increase in intracellular calcium in astrocytes in situ, which is partly dependent on NMDA receptor activation and on release of calcium from intracellular stores. Our study furthermore suggests that dysbalance of astrocyte calcium homeostasis under hyperammonemic conditions is a widespread phenomenon, which might contribute to the disturbance of neurotransmission during HE.

Activation of alpha‐2 adrenergic receptors stimulates GABA release by astrocytes

“…The effects of ammonium on [Ca 2+ ] i in astrocytes were demonstrated by other authors using brain slices. For instance, it was shown in work (Haack, Dublin, & Rose, ) that ammonium (5 mM) increased [Ca 2+ ] i in astrocytes by 50 nM. It was shown in other works using cell cultures that the addition of ATP increased [Ca 2+ ] i in astrocytes by over 600 nM (Nobile, Monaldi, Alloisio, Cugnoli, & Ferroni, ; Vermehren et al, ).…”

Astrogliopathology in neurological, neurodevelopmental and psychiatric disorders

“…Recent data, however, indicates much more complex molecular pathogenesis, i.e. exposure to increased ammonium decreases the expression of K ir 4.1 channels in astrocytes and compromises CNS K + homeostasis (Obara-Michlewska et al, 2014; Rangroo Thrane et al, 2013), affects the expression of Ca 2+ channels and deregulates Ca 2+ signaling and Ca 2+ homeostasis (Haack et al, 2014; Liang et al, 2014), causes pathological elevations in astroglial cytosolic Na + , impairs astroglial transport of H + hence affecting pH homeostasis (Kelly et al, 2009; Kelly and Rose, 2010) and triggers pathological glutamate release from astrocytes (Montana et al, 2014). Furthermore, the chronic exposure of astrocytes to ammonium impairs astroglial secretion of thrombospondin-1, which may negatively affect synaptogenesis (Jayakumar et al, 2014).…”

Translational potential of astrocytes in brain disorders

“…Recent observations demonstrated that in conditions of increased ammonium astrocytes rapidly undergo functional remodeling which severely compromises their homeostatic capabilities. This is represented by: (i) failure in K + buffering due to (at least in part) decreased expression of astroglial K ir 4.1 mediated through N-methyl D-aspartate (NMDA) type of glutamate receptors (Obara-Michlewska et al ., 2014; Rangroo Thrane et al ., 2013); (ii) aberrant astroglial Ca 2+ signaling and Ca 2+ homoeostasis due to an increase in expression of voltage-gated Ca 2+ channels and Ca 2+ -permeable transient receptor potential (TRP) channels as well as in abnormal Ca 2+ release from intracellular stores (Haack et al ., 2014; Liang et al ., 2014; Wang et al ., 2015); (iii) aberrant Ca 2+ signals trigger exocytotic astroglial glutamate release which may add to excitotoxic damage of the brain (Gorg et al ., 2010; Montana et al ., 2014); and (iv) massive pathological elevations in cytosolic Na + concentration and compromised H + transport which leads to abnormalities in pH regulation (Kelly et al ., 2009; Kelly and Rose, 2010). This astroglial remodeling may fundamentally contribute to the hyperammonemia damage.…”