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 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 mechanism for Aβ-induced neuronal Ca2+ dyshomeostasis.

The cellular prion protein (PrP^C) is an ubiquitous cell surface protein mostly expressed in neurons, where it localizes to both pre- and post-synaptic membranes. PrP^C aberrant conformers are the major components of mammalian prions, the infectious agents responsible for incurable neurodegenerative disorders. PrP^C was also proposed to bind aggregated misfolded proteins/peptides, and to mediate their neurotoxic signal. In spite of long-lasting research, a general consensus on the precise pathophysiologic mechanisms of PrP^C has not yet been reached. Here we review our recent data, obtained by comparing primary neurons from PrP-expressing and PrP-knockout mice, indicating a central role of PrP^C in synaptic transmission and Ca²⁺ homeostasis. Indeed, by controlling gene expression and signaling cascades, PrP^C is able to optimize glutamate secretion and regulate Ca²⁺ entry via store-operated channels and ionotropic glutamate receptors, thereby protecting neurons from threatening Ca²⁺ overloads and excitotoxicity. We will also illustrate and discuss past and unpublished results demonstrating that Aβ oligomers perturb Ca²⁺ homeostasis and cause abnormal mitochondrial accumulation of reactive oxygen species by possibly affecting the PrP-dependent downregulation of Fyn kinase activity.

The function of the prion protein (PrP^c), implicated in transmissible spongiform encephalopathies (TSEs), is largely unknown. We examined the possible influence of PrP^con Ca²⁺homeostasis, by analyzing local Ca²⁺fluctuations in cells transfected with PrP^cand Ca²⁺-sensitive aequorin chimeras targeted to defined subcellular compartments. In agonist-stimulated cells, the presence of PrP^csharply increases the Ca²⁺concentration of subplasma membrane Ca²⁺domains, a feature that may explain the impairment of Ca²⁺-dependent neuronal excitability observed in TSEs. PrP^calso limits Ca²⁺release from the endoplasmic reticulum and Ca²⁺uptake by mitochondria, thus rendering unlikely the triggering of cell death pathways. Instead, cells expressing Doppel, a PrP^cparalogue, display opposite effects, which, however, are abolished by the coexpression of PrP^c. These findings are consistent with the functional interplay and antagonistic role attributed to the proteins, whereby PrP^cprotects, and Doppel sensitizes, cells toward stress conditions.

According to the `protein only' hypothesis, modification of the 3‐dimensional fold of the constituent cellular protein, PrP^C, into the disease‐associated isoform, PrP^Sc, is the cause of neurodegenerative diseases in animals and humans. Here we describe the high‐level synthesis in Escherichia coli, and purification in the monomeric form, of a histidine‐tagged full‐length mature PrP (25–249) of bovine brain, termed His‐PrP. Based on biochemical and spectroscopic data, His‐PrP displays characteristics expected for the PrP^C isoform. The reported expression system should allow the production of quantities of bovine PrP^C sufficient to permit 3‐dimensional structure determinations.