Abstract
Aquaporins (AQPs) facilitate water diffusion through the plasma membrane. Brain aquaporin-4 (AQP4) is present in astrocytes and has critical roles in normal and disease physiology. We previously showed that a 24.9% decrease in AQP4 expression after in vivo silencing resulted in a 45.8% decrease in tissue water mobility as interpreted from magnetic resonance imaging apparent diffusion coefficients (ADC). Similar to previous in vitro studies we show decreased expression of the gap junction protein connexin 43 (Cx43) in vivo after intracortical injection of siAQP4 in the rat. Moreover, siAQP4 induced a loss of dye-coupling between astrocytes in vitro, further demonstrating its effect on gap junctions. In contrast, silencing of Cx43 did not alter the level of AQP4 or water mobility (ADC) in the brain. We hypothesized that siAQP4 has off-target effects on Cx43 expression via modification of miRNA expression. The decreased expression of Cx43 in siAQP4-treated animals was associated with up-regulation of miR224, which is known to target AQP4 and Cx43 expression. This could be one potential molecular mechanism responsible for the effect of siAQP4 on Cx43 expression, and the resultant decrease in astrocyte connectivity and dramatic effects on ADC values and water mobility.
Highlights
Water movements in the brain are critical for cellular function by regulating cell volume and homeostasis between extracellular and intracellular compartments
The temporal pattern of decrease was dissimilar between both proteins with a more rapid decrease in AQP4 expression compared to connexin 43 (Cx43) (Fig. 2B)
Aquaporin 4 (AQP4) is the most abundant water channel in the brain and its location on astrocytic endfeet that are in contact with blood vessels and synapses demonstrate that it is a key player in water diffusion, homeostasis and edema formation in numerous brain disorders[2]
Summary
Water movements in the brain are critical for cellular function by regulating cell volume and homeostasis between extracellular and intracellular compartments. In our previous studies using siAQP4, we made the interesting observation that a 27% decrease in AQP4 expression elicited a 50% decrease in water mobility, as interpreted from magnetic resonance imaging (MRI) derived apparent diffusion coefficients (ADC)[3]. Such robust effects on water diffusion led us to hypothesize that siAQP4 may have other sites of action (off-target) in addition to decreasing the levels of AQP4 expression (see Fig. 1A). In primary astrocyte cultures it has been previously reported that siAQP4 results in decreased expression of the gap junction protein connexin 43 (Cx43)[5]. The changes in miRNA expression is one molecular mechanism that could explain how siAQP4 modulates Cx43 expression in the rodent brain
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