Calcineurin proteolysis in astrocytes: Implications for impaired synaptic function

https://doi.org/10.1016/j.bbadis.2016.05.007Get rights and content
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Highlights

  • Study used a novel antibody for identifying proteolyzed, active calcineurin.

  • Calcineurin proteolysis appeared in astrocytes of human Alzheimer's disease brain.

  • Antibody labeling was very intense around amyloid plaques and microinfarcts.

  • AAV expression of active calcineurin in astrocytes caused synaptic deficits in rats.

  • Calcineurin proteolysis is a key pathogenic mechanism resulting in synaptic alterations.

Abstract

Mounting evidence suggests that astrocyte activation, found in most forms of neural injury and disease, is linked to the hyperactivation of the protein phosphatase calcineurin. In many tissues and cell types, calcineurin hyperactivity is the direct result of limited proteolysis. However, little is known about the proteolytic status of calcineurin in activated astrocytes. Here, we developed a polyclonal antibody to a high activity calcineurin proteolytic fragment in the 45–48 kDa range (ΔCN) for use in immunohistochemical applications. When applied to postmortem human brain sections, the ΔCN antibody intensely labeled cell clusters in close juxtaposition to amyloid deposits and microinfarcts. Many of these cells exhibited clear activated astrocyte morphology. The expression of ΔCN in astrocytes near areas of pathology was further confirmed using confocal microscopy. Multiple NeuN-positive cells, particularly those within microinfarct core regions, also labeled positively for ΔCN. This observation suggests that calcineurin proteolysis can also occur within damaged or dying neurons, as reported in other studies. When a similar ΔCN fragment was selectively expressed in hippocampal astrocytes of intact rats (using adeno-associated virus), we observed a significant reduction in the strength of CA3-CA1 excitatory synapses, indicating that the hyperactivation of astrocytic calcineurin is sufficient for disrupting synaptic function. Together, these results suggest that proteolytic activation of calcineurin in activated astrocytes may be a central mechanism for driving and/or exacerbating neural dysfunction during neurodegenerative disease and injury.

Abbreviations

ΔCN
calcineurin proteolytic fragment
AD
Alzheimer's disease
AID
autoinhibitory domain
IHC
immunohistochemistry
beta-amyloid
SMTG
superior and middle temporal gyri
NeuN
neuronal-specific nuclear protein
DAB
3,3′-diaminobenzidine
CaM
calmodulin
N-terminus
amino-terminus
C-terminus
carboxy-terminus
FL-CN
full length calcineurin
GFAP
glial fibrillary acidic protein
CA1
cornus ammonis 1
H&E
hematoxylin & eosin
CNS
central nervous system
VCID
vascular cognitive impairment and dementia

Keywords

Calcineurin
Proteolysis
Astrocytes
Alzheimer's disease
Microinfarct

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