Deletion of calcineurin from GFAP-expressing astrocytes impairs excitability of cerebellar and hippocampal neurons through astroglial Na+ /K+ ATPase.

Astrocytes perform important housekeeping functions in the nervous system including maintenance of adequate neuronal excitability, although the regulatory mechanisms are currently poorly understood. The astrocytic Ca2+ /calmodulin-activated phosphatase calcineurin (CaN) is implicated in the development of reactive gliosis and neuroinflammation, but its roles, including the control of neuronal excitability, in healthy brain is unknown. We have generated a mouse line with conditional knockout (KO) of CaN B1 (CaNB1) in glial fibrillary acidic protein-expressing astrocytes (astroglial calcineurin KO [ACN-KO]). Here, we report that postnatal and astrocyte-specific ablation of CaNB1 did not alter normal growth and development as well as adult neurogenesis. Yet, we found that specific deletion of astrocytic CaN selectively impairs intrinsic neuronal excitability in hippocampal CA1 pyramidal neurons and cerebellar granule cells (CGCs). This impairment was associated with a decrease in after hyperpolarization in CGC, while passive properties were unchanged, suggesting impairment of K+ homeostasis. Indeed, blockade of Na+ /K+ -ATPase (NKA) with ouabain phenocopied the electrophysiological alterations observed in ACN-KO CGCs. In addition, NKA activity was significantly lower in cerebellar and hippocampal lysates and in pure astrocytic cultures from ACN-KO mice. While no changes were found in protein levels, NKA activity was inhibited by the specific CaN inhibitor FK506 in both cerebellar lysates and primary astroglia from control mice, suggesting that CaN directly modulates NKA activity and in this manner controls neuronal excitability. In summary, our data provide formal evidence for the notion that astroglia is fundamental for controlling basic neuronal functions and place CaN center-stage as an astrocytic Ca2+ -sensitive switch.

TGF-ß2 and TGF-ß3 from cultured ß-amyloid-treated or 3xTg-AD-derived astrocytes may mediate astrocyte-neuron communication.

Astrocytes participate in the development and resolution of neuroinflammation in numerous ways, including the release of cytokines and growth factors. Among many, astrocytes release transforming growth factors beta (TGF-ß) TGF-ß1, TGF-ß2 and TGF-ß3. TGF-ß1 is the most studied isoform, while production and release of TGF-ß2 and TGF-ß3 by astrocytes have been poorly characterized. Here, we report that purified cultures of hippocampal astrocytes produce mainly TGF-ß3 followed by TGF-ß2 and TGF-ß1. Furthermore, astrocytes release principally the active form of TGF-ß3 over the other two. Changes in release of TGF-ß were sensitive to the calcineurin (CaN) inhibitor FK506. Starvation had no effect on TGF-ß1 and TGF-ß3 while TGF-ß2 mRNA was significantly up-regulated in a CaN-dependent manner. We further investigated production and release of astroglial TGF-ß in Alzheimer's disease-related conditions. Oligomeric ß-amyloid (Aß) down-regulated TGF-ß1, while up-regulating TGF-ß2 and TGF-ß3, in a CaN-dependent manner. In cultured hippocampal astrocytes from 3xTg-AD mice, TGF-ß2 and TGF-ß3, but not TGF-ß1, were up-regulated, and this was CaN-independent. In hippocampal tissues from symptomatic 3xTg-AD mice, TGF-ß2 was up-regulated with respect to control mice. Finally, treatment with recombinant TGF-ßs showed that TGF-ß2 and TGF-ß3 significantly reduced PSD95 protein in cultured hippocampal neurons, and this effect was paralleled by conditioned media from Aß-treated astrocytes or from astrocytes from 3xTg-AD mice. Taken together, our data suggest that TGF-ß2 and TGF-ß3 are produced by astrocytes in a CaN-dependent manner and should be investigated further in the context of astrocyte-mediated neurodegeneration.