PrP-knockout cerebellar granule neurons display altered Ca2+ homeostasis.

In spite we know much about the pathogenesis of prion disorders, our understanding of the molecular and cellular mechanisms that govern the onset of the disease, and the physiologic role of the cellular prion protein (PrPC), is still poor. A large body of evidence ascribes to the protein different biologic functions, such as the involvement in signal transduction events, but the extensive research devoted to PrPC physiology has not resulted in a proposition recapitulating the multiple, sometime contrasting, observations for its role in the cell. Interestingly, several lines of evidence suggest an intimate link between PrPC and the control of Ca2+ homeostasis. Ca2+, the most important second messenger of the cell, features in almost all signaling processes and governs key physiologic events. Yet, Ca2+ can lead to cell demise once its precisely tuned control is deranged. To explore the PrPC-Ca2+ connection, we have analyzed Ca2+ homeostasis in cerebellar granule cells (CGC) derived from wild-type, PrP-knockout, and PrP-overexpressing (3-fold the physiologic levels) congenic mice. Local Ca2+ fluxes were monitored by means of recombinant aequorins, Ca2+-sensitive photo-proteins, genetically targeted to different cell domains/organelles, and lentivirally delivered to the cells. Our results indicate that CGC deprived of PrPC have altered Ca2+ movements in the cytosolic domains beneath the plasma membrane and in the lumen of the endoplasmic reticulum. In particular, a persistently elevated Ca2+ concentration in the cytosolic domains of the plasma membrane, consequent to depletion of internal Ca2+ stores, was observed. Although the molecular aspects of this phenomenology are still unclear, the result is nonetheless stimulating, given that it confirms the involvement of PrPC in the control of Ca2+ homeostasis, thereby supporting the hypothesis that Ca2+ may act as common denominator for several roles attributed to PrPC. On the other hand, the persistence of high Ca2+ concentrations in cytosolic domains may justify further the higher excitability attributed to PrP-less neurons, and, concurrently, may support the notion that deregulation of Ca2+ metabolism may play a role in the pathogenesis of prion disease.