Calcineurin Controls Cellular Prion Protein Expression in Mouse Astrocytes

Abstract

Prion diseases arise from the conformational conversion of the cellular prion protein (PrPC) into a self-replicating prion isoform (PrPSc). Although this process has been studied mostly in neurons, a growing body of evidence suggests that astrocytes express PrPC and are able to replicate and accumulate PrPSc. Currently, prion diseases remain incurable, while downregulation of PrPC represents the most promising therapy due to the reduction of the substrate for prion conversion. Here we show that the astrocyte-specific genetic ablation or pharmacological inhibition of the calcium-activated phosphatase calcineurin (CaN) reduces PrPC expression in astrocytes. Immunocytochemical analysis of cultured CaN-KO astrocytes and isolation of synaptosomal compartments from the hippocampi of astrocyte-specific CaN-KO (ACN-KO) mice suggest that PrPC is downregulated both in vitro and in vivo. The downregulation occurs without affecting the glycosylation of PrPC and without alteration of its proteasomal or lysosomal degradation. Direct assessment of the protein synthesis rate and shotgun mass spectrometry proteomics analysis suggest that the reduction of PrPC is related to the impairment of global protein synthesis in CaN-KO astrocytes. When WT-PrP and PrP-D177N, a mouse homologue of a human mutation associated with the inherited prion disease fatal familial insomnia, were expressed in astrocytes, CaN-KO astrocytes showed an aberrant localization of both WT-PrP and PrP-D177N variants with predominant localization to the Golgi apparatus, suggesting that ablation of CaN affects both WT and mutant PrP proteins. These results provide new mechanistic details in relation to the regulation of PrP expression in astrocytes, suggesting the therapeutic potential of astroglial cells.

Calcineurin Signalling in Astrocytes: From Pathology to Physiology and Control of Neuronal Functions

“…In astrocytes, the canonical view on CaN signaling considers the activation of pro-inflammatory CaN → NFAT/NF-kB axes governing astrocytic transcriptional remodeling in pathological conditions [ 19 , 21 , 95 ]. Recent reports from our lab challenged this paradigm, suggesting that in healthy astrocytes transcription-independent CaN activity is required for maintenance of basal protein synthesis possibly through dephosphorylation of eIF2α, thereby regulating astrocytic and neuronal functions such as neuronal excitability and memory formation [ 22 , 55 , 117 , 151 ]. To provide a framework for further investigation of the role and mechanisms of activation of CaN in astrocytes in physiology and pathology, we propose a model of the transition of astrocytic CaN from the transcription-independent functions in healthy CNS to the activation of CaN → NFAT/NF-kB-mediated transcriptional reprograming in diseases accompanied by neuroinflammatory reaction.…”

Protein synthesis inhibition and loss of homeostatic functions in astrocytes from an Alzheimer’s disease mouse model: a role for ER-mitochondria interaction

“…1a) and Western blot (WB) analysis (Fig. 1b) of puromycin incorporation in neo-synthetized peptides (SUrface SEnsing of Translation (SUnSET) method [22,23]) (Fig. 1).…”

Genetic deletion of astrocytic calcineurin B1 prevents cognitive impairment and neuropathology development in acute and chronic mouse models of Alzheimer's disease

“…Recently, physiological functions of ACN have been linked to a number of homeostatic and signaling functions including metabolism and clearance of Aβ and tau, that could be traced back to CaN control over astrocytic proteostasis (Dematteis et al, 2022; Dematteis, Restelli, et al, 2020; Furman & Norris, 2014; Lim et al, 2022; Tapella et al, 2020; Tapella et al, 2021). It could be, thus, speculated that the CaN‐dependent alterations of astrocytic proteostasis might turn into a reduced homeostatic support, for example, through an alteration in the expression and release of neurotrophic, neuroprotective, and signaling factors (Lim et al, 2022; Verkhratsky et al, 2019).…”

“…In the healthy brain, CaN is mostly expressed in neurons where it is essential for synaptic plasticity and memory formation (Baumgärtel & Mansuy, 2012), but much less in astrocytes (Tapella et al, 2020) where it regulates neuronal excitability and protein expression at the post‐transcriptional level (Dematteis et al, 2022; Dematteis, Restelli, et al, 2020; Tapella et al, 2020; Tapella et al, 2021). During AD, CaN is over activated mainly in astrocytes and by interacting with NFAT induces synaptic/cognitive dysfunction, glutamate dysregulation, and neuroinflammation (Sompol & Norris, 2018).…”