The prion protein constitutively controls neuronal store-operated Ca2+ entry through Fyn kinase

Abstract

The prion protein (PrPC) is a cell surface glycoprotein mainly expressed in neurons, whose misfolded isoforms generate the prion responsible for incurable neurodegenerative disorders. Whereas PrPC involvement in prion propagation is well established, PrPC physiological function is still enigmatic despite suggestions that it could act in cell signal transduction by modulating phosphorylation cascades and Ca2+ homeostasis. Because PrPC binds neurotoxic protein aggregates with high-affinity, it has also been proposed that PrPC acts as receptor for amyloid-β (Aβ) oligomers associated with Alzheimer’s disease (AD), and that PrPC-Aβ binding mediates AD-related synaptic dysfunctions following activation of the tyrosine kinase Fyn. Here, use of gene-encoded Ca2+ probes targeting different cell domains in primary cerebellar granule neurons (CGN) expressing, or not, PrPC, allowed us to investigate whether PrPC regulates store-operated Ca2+ entry (SOCE) and the implication of Fyn in this control. Our findings show that PrPC attenuates SOCE, and Ca2+ accumulation in the cytosol and mitochondria, by constitutively restraining Fyn activation and tyrosine phosphorylation of STIM1, a key molecular component of SOCE. This data establishes the existence of a PrPC-Fyn-SOCE triad in neurons. We also demonstrate that treating cerebellar granule and cortical neurons with soluble Aβ(1–42) oligomers abrogates the control of PrPC over Fyn and SOCE, suggesting a PrPC-dependent mechanizm for Aβ-induced neuronal Ca2+ dyshomeostasis.

Neuronal pathophysiology featuring PrPC and its control over Ca2+ metabolism

“…On the one hand, PrP C prevents glutamate potential excitotoxicity because the protein downregulates the channel activity of the NMDAR by interacting directly with one of its subunits [16], and by reducing the affinity for the co-agonist glycine in a copper-dependent manner [33]. On the other hand, in line with suggestions that PrP C affects phosphorylation-based signaling pathways [2,34], we have also shown that PrP C downregulates both SOCE and AMPAR by controlling specific phosphorylation events on the respective molecular machineries (see above) [21,24]. Yet, if the phenomenology of the PrP C -Ca 2+ connection in the above instances is quite clear, mechanistic clues of these links still warrant a better definition.…”

“…It was also reported that PrP C -mediated Ab effects imply the engagement of Fyn [12]. On this, we added another important piece of information by showing that Ab oligomers subvert the tripartite connection between PrP C , Fyn and SOCE in both CGN and cortical neurons [21]. This enabled us to suggest that disruption of SOCE-mediated Ca 2+ signaling could contribute to PrP C -dependent effects in AD.…”

“…Becoming activated upon a decrease of ER Ca 2+ content, SOCE serves to refill intracellular Ca 2+ reservoirs and to regulate key cell parameters, from gene expression to tissue development and function [19]. Our work demonstrated that, also in the case of SOCE, PrP C served to oppose dangerous Ca 2+ overloads because Ca 2+ accumulation in PM subdomains was attenuated compared to PrP-KO CGN, and this reflected in lower cytosolic and mitochondrial Ca 2+ fluxes [20,21]. In addition, combined biochemical approaches allowed us to substantiating the alleged involvement of PrP C in signal transduction related to the modulation of phosphorylation cascades, in particular that governed by the Src Tyr-kinase Fyn [2].…”

“…In addition, combined biochemical approaches allowed us to substantiating the alleged involvement of PrP C in signal transduction related to the modulation of phosphorylation cascades, in particular that governed by the Src Tyr-kinase Fyn [2]. In fact, PrP C was found to constitutively downregulate Fyn activation and the Tyr-phosphorylation of the stromal interaction molecule 1 [21], an ER transmembrane Ca 2+ sensor which is key to the mechanism linking ER Ca 2+ depletion to stimulation of the channel-forming proteins of SOCE [19].…”

“…Preliminary data collected in this study indicate that such a Ca 2+ -mobilizing mechanism regulates the differentiation of CGN cultured under basal conditions (Peggion et al, unpublished observations). In light of this information, and the alleged role of PrP C in (neuronal) cell differentiation [2,38], and in SOCE modulation [20,21], it seems plausible that PrP C regulates neurogenesis via SOCE. Although we did not observe any apparent alteration of PrP-KO CGN differentiation compared to the PrP-expressing counterpart in our experimental constraints, still an involvement of the PrP C -SOCE connection in neuronal development under different in vitro and/or in vivo settings cannot yet be excluded.…”