Tight Coupling of Astrocyte pH Dynamics to Epileptiform Activity Revealed by Genetically Encoded pH Sensors

Abstract

Astrocytes can both sense and shape the evolution of neuronal network activity and are known to possess unique ion regulatory mechanisms. Here we explore the relationship between astrocytic intracellular pH dynamics and the synchronous network activity that occurs during seizure-like activity. By combining confocal and two-photon imaging of genetically encoded pH reporters with simultaneous electrophysiological recordings, we perform pH measurements in defined cell populations and relate these to ongoing network activity. This approach reveals marked differences in the intracellular pH dynamics between hippocampal astrocytes and neighboring pyramidal neurons in rodent in vitro models of epilepsy. With three different genetically encoded pH reporters, astrocytes are observed to alkalinize during epileptiform activity, whereas neurons are observed to acidify. In addition to the direction of pH change, the kinetics of epileptiform-associated intracellular pH transients are found to differ between the two cell types, with astrocytes displaying significantly more rapid changes in pH. The astrocytic alkalinization is shown to be highly correlated with astrocytic membrane potential changes during seizure-like events and mediated by an electrogenic Na ϩ /HCO 3 Ϫ cotransporter. Finally, comparisons across different cell-pair combinations reveal that astrocytic pH dynamics are more closely related to network activity than are neuronal pH dynamics. This work demonstrates that astrocytes exhibit distinct pH dynamics during periods of epileptiform activity, which has relevance to multiple processes including neurometabolic coupling and the control of network excitability.

Ubiquitin S65 phosphorylation engenders a pH-sensitive conformational switch

“…Thus, phosphorylated polyUb can be either recruited or generated onsite. Upon oxidative stress, neurodegeneration, and aging, Ub phosphorylation level drastically increases (4,8,31), whereas mitophagy, apoptosis, ischemia, inflammation, aging, and many pathophysiological conditions are all associated with changes in cellular pH (32)(33)(34)(35). Altogether, it is likely that certain functions of pUb are only turned on in response to changes in both Ub phosphorylation level and environmental pH.…”

Heterogeneity of Activity-Induced Sodium Transients between Astrocytes of the Mouse Hippocampus and Neocortex: Mechanisms and Consequences

“…Brief afferent stimulation of glutamatergic fibers and pressure application of glutamate induced transient sodium elevations in somata of neurons and astrocytes in slices obtained from juvenile mice. While the sodium indicator SBFI shows a slight pH sensitivity Ransom, 1996, 1997), expected pH changes are rather small (Raimondo et al, 2016) and are thus not likely to significantly distort SBFI signals. As expected from earlier work (Callaway and Ross, 1997;Bennay et al, 2008;Langer and Rose, 2009;Mondragão et al, 2016;Miyazaki and Ross, 2017), neuronal sodium elevations were largely blocked upon inhibition of ionotropic glutamate receptors, confirming that these represent the main sodium influx pathways at postsynaptic sites.…”

Excitatory GABAergic signalling is associated with acquired benzodiazepine resistance in status epilepticus

“…The use of animals was approved by either the University of Cape Town (mouse) or the University of Oxford (rat) animal ethics committees. Organotypic brain slices were prepared using 7 day old animals and followed the protocol originally described by (Stoppini et al, 1991) (for details see Raimondo et al (2016)).…”