The Prion Protein and its Paralogue Doppel Affect Calcium Signalling in CHO Cells

The function of the prion protein (PrP), implicated in transmissible spongiform encephalopathies (TSEs), is largely unknown. We examined the possible influence of PrP on Ca homeostasis, by analysing local Ca fluctuations in cells transfected with PrP and Casensitive aequorin chimeras targeted to defined subcellular compartments. In agoniststimulated cells, the presence of PrP sharply increases the Ca concentration of sub-plasma membrane Ca domains, a feature that may explain the impairment of Ca-dependent neuronal excitability observed in TSEs. PrP also limits Ca release from the endoplasmic reticulum and Ca uptake by mitochondria, thus rendering unlikely the triggering of cell death pathways. Instead, cells expressing Doppel, a PrP paralogue, display opposite effects, which, however, are abolished by the co-expression of PrP. These findings are consistent with the functional interplay and antagonistic role attributed to the proteins, whereby PrP protects, and Doppel sensitises, cells towards stress conditions. Introduction The cellular prion protein (PrP) is a highly conserved cell surface glycoprotein, particularly expressed in the central nervous system (CNS), with a still unrecognised function. A conformationally modified isoform (PrP) of PrP is the major component of prions, the ethiological agent at the basis of fatal neurodegenerative disorders, called transmissible spongiform encephalopathies (TSEs). TSEs present as sporadic, genetic, and infectious illnesses, and include Creutzfeldt-Jacob disease (CJD) in humans, and bovine spongiform encephalopathy in cattle (Prusiner, 1998). The mechanism of PrP conversion into PrP is not yet elucidated, nor it is clear if the disease progression relates to PrP toxic effect, or to the deprivation of PrP functionality. The most prominent evidence in support of the former hypothesis is that most PrP-knockout mice remain viable, and do not develop spontaneous neurodegeneration (Bueler et al., 1992; Manson et al., 1994), even if the PrP gene is post-natal deleted (Mallucci et al., 2002). An essential role of PrP in cell survival comes, however, from the finding that wild type PrP transgenes abrogate the cerebellar degeneration and late-onset ataxia developed by some PrPknockout lines over expressing a protein, named Doppel (Dpl) (Sakaguchi et al., 1996; Moore et al., 1999, 2001; Li et al., 2000; Rossi et al., 2001). Dpl is normally absent in the CNS of adult animals and resembles truncated PrP; it lacks the copper binding N-terminus (Brown et al., 1997), while is structurally and biochemically similar to PrP carboxyl end (Silverman et