Because of the colocalization of AQP4 and inwardly rectifying K+ channels in glial endfeet, the hypothesis arose that AQP4 was indirectly involved in K+ reuptake (Nagelhus et al.,1999; Nielsen et al.,1997). Impaired K+ clearance from the ECS following the intense neuronal activity accompanying the seizure would lead to prolonged depolarization of neurons and inhibit seizure termination (Rutecki et al.,1985; Steinhäuser and Seifert,2002; Traynelis and Dingledine,1988; Yaari et al.,1986). Indeed, in addition to modulation of brain water transport, AQP4 and its known molecular partners have been hypothesized to modulate ion homeostasis (Manley et al.,2004; Simard and Nedergaard,2004). During rapid neuronal firing, extracellular [K+] increases from ∼3 mM to a maximum of 10-12 mM; and K+ released by active neurons is thought to be primarily taken up by glial cells (Heinemann and Lux,1977; Somjen,2002; Sykova,1997; Xiong and Stringer,1999). Such K+ reuptake into glial cells could be AQP4-dependent, as water influx coupled to K+ influx is thought to underlie activity-induced glial cell swelling (Walz,1987,1992). In support of this possibility was the known subcellular colocalization of AQP4 with the inwardly rectifying K+ channel Kir4.1 in the retina (Connors et al.,2004; Nagelhus et al.,1999; Nagelhus et al.,2004). Kir4.1−/− mice, like Aqp4−/− mice (Li et al.,2002; Li and Verkman,2001), demonstrate abnormal retinal and cochlear physiology presumably due to altered K+ homeostasis (Kofuji et al.,2000; Marcus et al.,2002; Neusch et al.,2001; Rozengurt et al.,2003). Kir4.1 is thought to contribute to K+ reuptake and spatial K+ buffering by glial cells (Newman,1986,1993; Newman et al.,1984; Newman and Karwoski,1989), and pharmacological or genetic inactivation of Kir4.1 leads to impairment of extracellular K+ regulation (Ballanyi et al.,1987; Djukic et al.,2007; Haj-Yasein et al.,2011a; Kofuji et al.,2000; Kofuji and Newman,2004; Neusch et al.,2006; Seifert et al.,2009).
To address the possibility that AQP4 deficiency was associated with a deficit in K+ homeostasis, K+ dynamics were examined in vivo in Aqp4−/− mice (Binder et al.,2006). Neither baseline [K+]o nor the “Lux-Heinemann ceiling” level of activity-induced [K+]o elevation (∼12 mM) (Heinemann and Lux,1977; Somjen,2002) were altered in AQP4 deficiency, indicating that basic K+ homeostasis was intact. Based on K+ measurements made from cortex with double-barreled K+-sensitive microelectrodes, stimulation-induced rises in [K+]o were quite different in Aqp4−/− compared to wild-type mice; in particular, there was a markedly slower rise and decay time for post-stimulus changes in [K+]o in Aqp4−/− mice (Binder et al.,2006). A similar delay in K+ kinetics was observed following cortical spreading depression in Aqp4−/− mice using a fluorescent K+ sensor (Padmawar et al.,2005).
Slowed [K+]o rise time is consistent with increased ECS volume fraction in Aqp4−/− mice (Binder et al.,2004b). Slowed [K+]o decay is possibly due to impaired K+ reuptake into Aqp4−/− astrocytes. Interestingly, there is no difference in expression of Kir4.1 protein (Binder et al.,2006) or Kir4.1 immunoreactivity (Hsu et al.,2011) in Aqp4−/− mice nor AQP4 immunoreactivity in Kir4.1−/− mice (Hsu et al.,2011). In addition, no alterations were observed in membrane potential, barium-sensitive Kir4.1 K+ current or current-voltage curves in Aqp4−/− retinal Müller cells (Ruiz-Ederra et al.,2007) or brain astrocytes (Zhang and Verkman,2008). Lack of alteration of Kir channels in Aqp4−/− mice suggests the interesting possibility that the slowed [K+]o decay may be a secondary effect of slowed water extrusion (“deswelling”) following stimulation, but this has not been directly demonstrated.
A recent study investigated the impact of AQP4 on stimulus-induced alterations of [K+]o in hippocampal slices. Antidromic stimulation evoked smaller increases and slower recovery of [K+]o in the stratum pyramidale of Aqp4−/− mice, consistent with the previous in vivo studies in cortex. Interestingly, astrocyte gap junction coupling as assessed with tracer filling during patch clamp recording demonstrated enhanced tracer coupling in Aqp4−/− mice, and laminar profiles indicated enhanced spatial redistribution of K+ (Strohschein et al.,2011). The functional consequences of alterations in gap junctional coupling are not completely clear; however the complete absence of astrocytic gap junctions impairs K+ homeostasis (Wallraff et al.,2006). To further assess the direct link between AQP4 and K+ homeostasis, it would be interesting to perform similar studies in the new glial-conditional Aqp4 knockout mouse line (Haj-Yasein et al.,2011b).