Abstract
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.
Highlights
The Ca2+/calmodulin (CaM)-activated serine-threonine phosphatase, calcineurin (CaN), is abundantly expressed in neurons (Klee et al, 1979) and in other brain cellular types, including astrocytes (Lim et al, 2016)
To formally demonstrate astrocyte-specific CaN B1 (CaNB1) deletion, we made use of fluorescence-activated cell sorting (FACS) and isolated Draq5+/glutamate/aspartate transporter (GLAST)+/CD90.2- astrocytes from the CB of postnatal day 20 (P20) ACN-KO and ACN-Ctr mice
We found that significantly lower expression of CaNB1 protein in ACN-KO compared with ACN-Ctr mice
Summary
The Ca2+/calmodulin (CaM)-activated serine-threonine phosphatase, calcineurin (CaN), is abundantly expressed in neurons (Klee et al, 1979) and in other brain cellular types, including astrocytes (Lim et al, 2016). Two catalytic (CaNAα and CaNAβ) and one regulatory (CaNB1) subunits are expressed in the brain and the selective suppression of CaN activity in mouse models is routinely achieved by deletion of the regulatory CaNB1 subunit (Zeng et al, 2001; Miyakawa et al, 2003). Neuronal CaN plays a crucial role in the control of neurotransmission, synaptic plasticity and memory formation (Zeng et al, 2001; Baumgärtel et al, 2008). Astroglial CaN regulates neuronal excitability in a murine model of Alzheimer’s Disease (Sompol et al, 2017) and modulates synaptic strength and long-term plasticity in a rat model of traumatic brain injury (Furman et al, 2016). There is no evidence about the role of astroglial CaN in neuronal excitability in the healthy brain
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