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

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 alpha-syntrophin (alpha-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 alpha-syn deletion on astrocyte morphology and changes in extracellular diffusion associated with cell swelling in vitro and in vivo. In mice lacking alpha-syn, we found higher resting values of the apparent diffusion coefficient of water (ADC(W)) and the extracellular volume fraction (alpha). No significant differences in tortuosity (lambda) or non-specific uptake (k'), were found between alpha-syn-negative (alpha-syn -/-) and alpha-syn-positive (alpha-syn +/+) mice. The deletion of alpha-syn resulted in a significantly smaller relative decrease in alpha 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 ADC(W) as well as of the ECS volume fraction alpha indicate milder cell swelling in alpha-syn -/- in comparison with alpha-syn +/+ mice. Shortly after terminal ischemia/anoxia induction, the onset of a steep rise in the extracellular potassium concentration and an increase in lambda was faster in alpha-syn -/- mice, but the final values did not differ between alpha-syn -/- and alpha-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.