Altered Astrocytic Swelling in the Cortex of α-Syntrophin-Negative GFAP/EGFP Mice

Abstract

Brain edema accompanying ischemic or traumatic brain injuries, originates from a disruption of ionic/neurotransmitter homeostasis that leads to accumulation of K+ and glutamate in the extracellular space. Their increased uptake, predominantly provided by astrocytes, is associated with water influx via aquaporin-4 (AQP4). As the removal of perivascular AQP4 via the deletion of α-syntrophin was shown to delay edema formation and K+ clearance, we aimed to elucidate the impact of α-syntrophin knockout on volume changes in individual astrocytes in situ evoked by pathological stimuli using three dimensional confocal morphometry and changes in the extracellular space volume fraction (α) in situ and in vivo in the mouse cortex employing the real-time iontophoretic method. RT-qPCR profiling was used to reveal possible differences in the expression of ion channels/transporters that participate in maintaining ionic/neurotransmitter homeostasis. To visualize individual astrocytes in mice lacking α-syntrophin we crossbred GFAP/EGFP mice, in which the astrocytes are labeled by the enhanced green fluorescent protein under the human glial fibrillary acidic protein promoter, with α-syntrophin knockout mice. Three-dimensional confocal morphometry revealed that α-syntrophin deletion results in significantly smaller astrocyte swelling when induced by severe hypoosmotic stress, oxygen glucose deprivation (OGD) or 50 mM K+. As for the mild stimuli, such as mild hypoosmotic or hyperosmotic stress or 10 mM K+, α-syntrophin deletion had no effect on astrocyte swelling. Similarly, evaluation of relative α changes showed a significantly smaller decrease in α-syntrophin knockout mice only during severe pathological conditions, but not during mild stimuli. In summary, the deletion of α-syntrophin markedly alters astrocyte swelling during severe hypoosmotic stress, OGD or high K+.

Turning down the volume: Astrocyte volume change in the generation and termination of epileptic seizures

“…Genetic deletion studies have provided evidence that AQP4 may be more than a simple, passive water channel, and is most likely not the sole mechanism for astrocyte water permeability. In one study, knockout of the anchoring protein α-syntrophin, which removes perivascular AQP4 (Amiry-Moghaddam et al 2004), significantly inhibited swelling in severe (~33%) hypoosmolar conditions, but had no effect on swelling in mild (~17%) hypoosmolar conditions (Anderova et al 2014). With a full knockout of AQP4 −/− , another study found nearly the opposite effect; little or no change in volume in 30% hACSF, but significant reduction in volume at 20% hACSF (Thrane et al 2011).…”

“…Disagreement exists as to whether astrocytic RVD occurs in intact tissue, as various groups have reported astrocytic swelling in brain slices without RVD (Andrew et al 1997; Hirrlinger et al 2008; Risher et al 2009). Others have observed swelling with an RVD, but only at certain hypoosmolar doses (Thrane et al 2011), and even swelling which does not recover following removal of the hypoosmolar stimulus (Anderova et al 2014). These data (with the possible exception of the latter study) may reflect the ability of intact tissue to maintain a relatively constant volume with more gradual changes in osmolarity, referred to as “isovolumetric” volume regulation or IVR (Franco et al 2000).…”

Morphological plasticity of astroglia: Understanding synaptic microenvironment

“…It appears that astrocytes should be able to regulate extracellular (volume) communications by swelling or shrinking (Mongin and Kimelberg, 2005;Reichenbach et al, 2010) and thus changing the distribution of the tunnels and sheets within the ECS (Kinney et al, 2013). This volume control by astrocytes can be modulated by aquaporins and Kir4.1 that are expressed in PAP membranes and control swelling (Anderova et al, 2014;Nagelhus et al, 2004;Thrane et al, 2011).…”

Brain edema: a valid endpoint for measuring hepatic encephalopathy?

“…Astrocytes are thought to be able to cope with brief exposures (30-90 min) to mild or moderate amounts of swelling by offloading osmolytes, including potentially excitotoxic neurotransmitters, a mechanism known as regulatory volume decrease (RVD) (Kimelberg 1987;Ordaz et al 2004;Thrane et al 2011;Anderova et al 2014). Conversely, hyperosmotic stress may be able to induce a regulatory volume increase (RVI) by uptake of osmolytes and water (Evanko et al 2004;Risher et al 2009).…”