The Impact of Alpha-Syntrophin Deletion on the Changes in Tissue Structure and Extracellular Diffusion Associated with Cell Swelling under Physiological and Pathological Conditions

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.

Activation of β‐adrenergic receptors in rat visual cortex expands astrocytic processes and reduces extracellular space volume

“…One of the strengths of this study is that both ECS analysis and EM analysis of the cellular components forming the ECS were conducted in the same preparation—acutely excised cortex that was maintained alive through oxygenated ACSF. The similarity of the ECS parameters α and λ with the in vivo values obtained previously (Yao et al, ; Dmytrenko et al, ) indicates that our acute slice preparation captured the in vivo state well.…”

“…Lower temperature has been shown not to impact ECS parameters. Previous studies demonstrated that λ and α in cortical slices incubated at 22–26°C and 33°C are similar to those measured in cortex in vivo at 37°C (Yao et al, ; Dmytrenko et al, ). The ACSF used for slice incubation and for diffusion measurements contained 0.5 mM TMA + and 200 μM ascorbate.…”

“…In this study, we used nine female Sprague‐Dawley rats (6–8 weeks, 127–233 g). Control ECS parameters between female (Sherpa et al, ) and male (Yao et al, ; Dmytrenko et al, ; Xie et al, ) rodents are similar. Nevertheless, female rats, 6–8 weeks old, were chosen in order to be able to compare ECS parameters obtained in this study with previous data from this lab that also used a similar age‐group of female rats (Hrabetova, ; Sherpa et al, ; Xiao et al, ).…”

Astrocytes and extracellular matrix in extrasynaptic volume transmission

“…Interestingly, at resting state, ECS volume in AQP4 or a-syn knockout mice (AQP4 2/2 or a-syn 2/2 , respectively) is larger than that found in control animals [42,53]. The higher initial value of a was proposed to be a protective mechanism against the fast increase in concentration of neurotoxic substances [54], and thus may contribute to a better outcome in AQP4 swelling, Dmytrenko et al [42] showed that AQP4-mediated water influx plays a role predominantly during pathological states (global ischaemia and strong hypotonic stress) or in states associated with increased potassium concentration, but not during milder stimuli modelling physiological neuronal activity (figure 2).…”

Inhibition of WNK3 Kinase Signaling Reduces Brain Damage and Accelerates Neurological Recovery After Stroke

“…Recent studies by Piala et al revealed WNK1 as a direct Cl - sensor, with Cl - binding at the active site and leading to Cl - -dependent inhibition of WNK1 autophosphorylation 13 . Increased tissue osmolality, or pathological changes in ionic contents, such as Cl - , may catalyze dysregulated WNK-SPAK/OSR1 activity by direct interaction with the WNK catalytic site 47-49 . Alternatively, inflammation may play a role in WNK-SPAK/OSR1 activation.…”