Aquaporin-4-Deficient Mice Have Increased Extracellular Space without Tortuosity Change

Department of Neurosurgery and Brain and Spinal Injury Center, University of California, San Francisco, San Francisco, California 94110, USA.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 06/2008; 28(21):5460-4. DOI: 10.1523/JNEUROSCI.0257-08.2008
Source: PubMed


Aquaporin-4 (AQP4) is the major water channel expressed at fluid-tissue barriers throughout the brain and plays a crucial role in cerebral water balance. To assess whether these channels influence brain extracellular space (ECS) under resting physiological conditions, we used the established real-time iontophoresis method with tetramethylammonium (TMA(+)) to measure three diffusion parameters: ECS volume fraction (alpha), tortuosity (lambda), and TMA(+) loss (k'). In vivo measurements were performed in the somatosensory cortex of AQP4-deficient (AQP4(-/-)) mice and wild-type controls with matched age. Mice lacking AQP4 showed a 28% increase in alpha (0.23 +/- 0.007 vs 0.18 +/- 0.003) with no differences in lambda (1.62 +/- 0.04 vs 1.61 +/- 0.02) and k' (0.0045 +/- 0.0001 vs 0.0031 +/- 0.0009 s(-1)). Additional recordings in brain slices showed similarly elevated alpha in AQP4(-/-) mice, and no differences in lambda and k' between the two genotypes. This is the first direct comparison of ECS properties in adult mice lacking AQP4 water channels with wild-type animals and demonstrates a significant enlargement of the volume fraction but no difference in hindrance to TMA(+) diffusion, expressed as tortuosity. These findings provide direct evidence for involvement of AQP4 in modulation of the ECS volume fraction and provide a basis for future modeling of water and ion transport in the CNS.

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Available from: Xiaoming Yao, Jan 14, 2015
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    • "There are controversial reports in the literature about the tortuosity changes in AQP4-deficient mice observed using different methods. While the study of Yao and colleagues [26] is in agreement with our findings, diffusion studies with dextran polymers using fluorescence recovery after photobleaching (FRAP) showed, in contrast, a decrease of about 10–20% in tortuosity in the neocortex of AQP4−/− mice [30], [31]. Since the recording intervals are significantly shorter and the molecular weight of the used extracellular marker is higher in the FRAP method compared to RTI-TMA method, the diffusion parameters measured by these methods may differ. "
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    ABSTRACT: Aquaporin-4 (AQP4) is the primary cellular water channel in the brain and is abundantly expressed by astrocytes along the blood-brain barrier and brain-cerebrospinal fluid interfaces. Water transport via AQP4 contributes to the activity-dependent volume changes of the extracellular space (ECS), which affect extracellular solute concentrations and neuronal excitability. AQP4 is anchored by α-syntrophin (α-syn), the deletion of which leads to reduced AQP4 levels in perivascular and subpial membranes. We used the real-time iontophoretic method and/or diffusion-weighted magnetic resonance imaging to clarify the impact of α-syn deletion on astrocyte morphology and changes in extracellular diffusion associated with cell swelling in vitro and in vivo. In mice lacking α-syn, we found higher resting values of the apparent diffusion coefficient of water (ADCW) and the extracellular volume fraction (α). No significant differences in tortuosity (λ) or non-specific uptake (k'), were found between α-syn-negative (α-syn -/-) and α-syn-positive (α-syn +/+) mice. The deletion of α-syn resulted in a significantly smaller relative decrease in α observed during elevated K(+) (10 mM) and severe hypotonic stress (-100 mOsmol/l), but not during mild hypotonic stress (-50 mOsmol/l). After the induction of terminal ischemia/anoxia, the final values of ADCW as well as of the ECS volume fraction α indicate milder cell swelling in α-syn -/- in comparison with α-syn +/+ mice. Shortly after terminal ischemia/anoxia induction, the onset of a steep rise in the extracellular potassium concentration and an increase in λ was faster in α-syn -/- mice, but the final values did not differ between α-syn -/- and α-syn +/+ mice. This study reveals that water transport through AQP4 channels enhances and accelerates astrocyte swelling. The substantially altered ECS diffusion parameters will likely affect the movement of neuroactive substances and/or trophic factors, which in turn may modulate the extent of tissue damage and/or drug distribution.
    PLoS ONE 07/2013; 8(7):e68044. DOI:10.1371/journal.pone.0068044 · 3.23 Impact Factor
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    • "In contrast to the slowed diffusion produced by brain edema and seizure activity, ECS diffusion was faster in Aqp4 2/2 mice, indicating ECS expansion in AQP4 deficiency (Binder et al., 2004b). Similar results were obtained with follow-up studies using the TMA 1 method (Yao et al., 2008). "
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    ABSTRACT: Recent studies have implicated glial cells in modulation of synaptic transmission, so it is plausible that glial cells may have a functional role in the hyperexcitability characteristic of epilepsy. Indeed, alterations in distinct astrocyte membrane channels, receptors, and transporters have all been associated with the epileptic state. This review focuses on the potential roles of the glial water channel aquaporin-4 (AQP4) in modulation of brain excitability and in epilepsy. We will review studies of mice lacking AQP4 (Aqp4(-/-) mice) or α-syntrophin (an AQP4 anchoring protein) and discuss the available human studies demonstrating alterations of AQP4 in human epilepsy tissue specimens. We will conclude with new studies of AQP4 regulation and discuss the potential role of AQP4 in the development of epilepsy (epileptogenesis). While many questions remain unanswered, the available data indicate that AQP4 and its molecular partners may represent important new therapeutic targets.
    Glia 08/2012; 60(8):1203-14. DOI:10.1002/glia.22317 · 6.03 Impact Factor
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    • "However, alveus stimulation produced smaller [K 1 ] o increases in the CA1 pyramidal layer of AQP4-de- ficient mice. Given that wt astrocytes rapidly and reversibly swell in response to elevation in [K 1 ] o (Risher et al., 2009) with an up to 30% decrease in extracellular space (Dietzel et al., 1980), these findings indicate concurrent movement of water and K 1 through the membrane: In AQP4-deficient mice, water flux is reduced leading to enhanced extracellular space (Binder et al., 2004; Yao et al., 2008) and smaller [K 1 ] o elevations. Fig. 3. Deletion of AQP4 improves spatial redistribution of K 1 in the CA1 region. "
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    ABSTRACT: Aquaporin-4 (AQP4) is the main water channel in the brain and primarily localized to astrocytes where the channels are thought to contribute to water and K(+) homeostasis. The close apposition of AQP4 and inward rectifier K(+) channels (Kir4.1) led to the hypothesis of direct functional interactions between both channels. We investigated the impact of AQP4 on stimulus-induced alterations of the extracellular K(+) concentration ([K(+)](o)) in murine hippocampal slices. Recordings with K(+)-selective microelectrodes combined with field potential analyses were compared in wild type (wt) and AQP4 knockout (AQP4(-/-)) mice. Astrocyte gap junction coupling was assessed with tracer filling during patch clamp recording. Antidromic fiber stimulation in the alveus evoked smaller increases and slower recovery of [K(+)](o) in the stratum pyramidale of AQP4(-/-) mice indicating reduced glial swelling and a larger extracellular space when compared with control tissue. Moreover, the data hint at an impairment of the glial Na(+)/K(+) ATPase in AQP4-deficient astrocytes. In a next step, we investigated the laminar profile of [K(+)](o) by moving the recording electrode from the stratum pyramidale toward the hippocampal fissure. At distances beyond 300 μm from the pyramidal layer, the stimulation-induced, normalized increases of [K(+)](o) in AQP4(-/-) mice exceeded the corresponding values of wt mice, indicating facilitated spatial buffering. Astrocytes in AQP4(-/-) mice also displayed enhanced tracer coupling, which might underlie the improved spatial re- distribution of [K(+)](o) in the hippocampus. These findings highlight the role of AQP4 channels in the regulation of K(+) homeostasis.
    Glia 06/2011; 59(6):973-80. DOI:10.1002/glia.21169 · 6.03 Impact Factor
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