Critical role of aquaporin-4 (AQP4) in astrocytic Ca2+ signaling events elicited by cerebral edema.

Aquaporin-4 (AQP4) is a primary influx route for water during brain edema formation. Here, we provide evidence that brain swelling triggers Ca(2+) signaling in astrocytes and that deletion of the Aqp4 gene markedly interferes with these events. Using in vivo two-photon imaging, we show that hypoosmotic stress (20% reduction in osmolarity) initiates astrocytic Ca(2+) spikes and that deletion of Aqp4 reduces these signals. The Ca(2+) signals are partly dependent on activation of P2 purinergic receptors, which was judged from the effects of appropriate antagonists applied to cortical slices. Supporting the involvement of purinergic signaling, osmotic stress was found to induce ATP release from cultured astrocytes in an AQP4-dependent manner. Our results suggest that AQP4 not only serves as an influx route for water but also is critical for initiating downstream signaling events that may affect and potentially exacerbate the pathological outcome in clinical conditions associated with brain edema.

Somatic Accumulation of GluA1-AMPA Receptors Leads to Selective Cognitive Impairments in Mice.

The GluA1 subunit of the L-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) plays a crucial, but highly selective, role in cognitive function. Here we analyzed AMPAR expression, AMPAR distribution and spatial learning in mice (Gria1R/R ), expressing the "trafficking compromised" GluA1(Q600R) point mutation. Our analysis revealed somatic accumulation and reduction of GluA1(Q600R) and GluA2, but only slightly reduced CA1 synaptic localization in hippocampi of adult Gria1R/R mice. These immunohistological changes were accompanied by a strong reduction of somatic AMPAR currents in CA1, and a reduction of plasticity (short-term and long-term potentiation, STP and LTP, respectively) in the CA1 subfield following tetanic and theta-burst stimulation. Nevertheless, spatial reference memory acquisition in the Morris water-maze and on an appetitive Y-maze task was unaffected in Gria1R/R mice. In contrast, spatial working/short-term memory during both spontaneous and rewarded alternation tasks was dramatically impaired. These findings identify the GluA1(Q600R) mutation as a loss of function mutation that provides independent evidence for the selective role of GluA1 in the expression of short-term memory.