A cluster of familial Creutzfeldt-Jakob disease mutations recapitulate conserved residues in Doppel: a case of molecular mimicry?

The prion protein family: Diversity, rivalry, and dysfunction

The prion gene family currently consists of three members: Prnp which encodes PrP(C), the precursor to prion disease associated isoforms such as PrP(Sc); Prnd which encodes Doppel, a testis-specific protein involved in the male reproductive system; and Sprn which encodes the newest PrP-like protein, Shadoo, which is expressed in the CNS. Although the identification of numerous candidate binding partners for PrP(C) has hinted at possible cellular roles, molecular interpretations of PrP(C) activity remain obscure and no widely-accepted view as to PrP(C) function has emerged. Nonetheless, studies into the functional interrelationships of prion proteins have revealed an interesting phenomenon: Doppel is neurotoxic to cerebellar cells in a manner which can be blocked by either PrP(C) or Shadoo. Further examination of this paradigm may help to shed light on two prominent unanswered questions in prion biology: the functional role of PrP(C) and the neurotoxic pathways initiated by PrP(Sc) in prion disease.

Selective PrP-like protein, doppel immunoreactivity in dystrophic neurites of senile plaques in Alzheimer's disease

Doppel (Dpl) is a prion-like protein encoded by the gene PRND, which has been found downstream of the prion gene PRNP in several species. The present study examines by immunohistochemistry Dpl expression in brain samples from 10 patients with Alzheimer's disease (AD), three patients with Pick's disease, four patients with Parkinson's disease, eight patients with diffuse Lewy body disease (DLBD), six patients with sporadic Creutzfeldt-Jakob disease (CJD) methionine/methionine at the codon 129, two patients with sporadic CJD methionine/valine at the codon 129 and numerous kuru plaques in the cerebellum, one patient with fatal familial insomnia (FFI), and 10 age-matched controls. In the adult human brain, Dpl immunoreactivity was restricted to scattered granule cells of the cerebellum and scattered small granules in the cerebral cortex. Dpl immunoreactivity was seen around betaA4 amyloid deposits in neuritic plaques, but not in diffuse plaques, AD and the common form of DLBD. Neurofibrillary tangles, Pick bodies and Lewy bodies were not stained with anti-Dpl antibodies. No modifications in Dpl immunoreactivity were observed in CJD excepting those associated with accompanying senile plaques. No Dpl-positive deposits were seen in FFI. Whether Dpl in neuritic plaques may attenuate amyloid-induced oxidative stress and participate in the glial response around amyloid cores is discussed in light of the few available data on Dpl functions.

Stability and conformational properties of doppel, a prion-like protein, and its single-disulphide mutant

Both prion protein and the structurally homologous protein doppel are associated with neurodegenerative disease by mechanisms which remain elusive. We have prepared murine doppel, and a mutant with one of the two disulphide bonds removed, in the expectation of increasing the similarity of doppel to prion protein in terms of conformation and stability. Unfolding studies of doppel and the mutant have been performed using far-UV CD over a range of solution conditions known to favour the alpha-->beta transformation of recombinant prion protein. Only partial unfolding of doppel or the mutant occurs at elevated temperature, but both exhibit full and reversible unfolding in chemical denaturation with urea. Doppel is significantly less stable than prion protein, and this stability is further reduced by removal of the disulphide bond between residues 95-148. Both doppel and the mutant are observed to unfold by a two-state mechanism, even under the mildly acidic conditions where prion protein forms an equilibrium intermediate with enhanced beta-structure, potentially analogous to the conversion of the cellular form of the prion protein into the infectious form (PrP(C)-->PrP(Sc)). Furthermore, no direct interaction of either doppel protein with prion protein, either in the alpha-form or the beta-rich conformation, was detectable spectroscopically. These studies indicate that, in spite of the similarity in secondary structure between the doppel and prion protein, there are significant differences in their solution properties. The fact that neither doppel nor its mutant exhibited the alpha-->beta transformation of the prion protein suggests that this conversion property may be dependent on unique sequences specific to the prion protein.

The PrP-like protein Doppel binds copper

Doppel (Dpl) is a glycosylphosphatidylinositol-anchored protein expressed in the testis. It exhibits 26% sequence identity with the prion protein (PrP) but lacks the octarepeat region implicated as the major copper-binding domain. Contrary to expectations, Cu(II) induced a 26% reduction in the intrinsic fluorescence of Dpl(27-154) and a calculated K(d) for a single-site model of 0.16 +/- 0.08 microm. Other metals had minimal effects on fluorescence quenching. Matrix-assisted laser desorption ionization mass spectrometry of a Dpl peptide revealed binding of copper (but not other metals) to the helical alphaB/B'-loop-alphaC subregion of Dpl. Fluorescence quenching and equilibrium dialysis analyses of this Dpl(101-145) peptide were compatible with a binding site of K(d) = 0.4 microm. Diethylpyrocarbonate footprinting (Qin, K., Yang, Y., Mastrangelo, P., and Westaway, D. (2002) J. Biol. Chem. 277, 1981-1990) of Dpl(27-154) defined one residue/molecule was protected by copper from diethylpyrocarbonate adduct formation, and reiteration of this analysis with Dpl(101-145) suggested that His(131) may contribute to Cu(II) binding. Taken together, our data indicate that the alpha-helical region of mouse Dpl possesses a selective copper-binding site with a submicromolar K(d) and perhaps one or more lower affinity sites. Although metallated forms of Dpl might exist in vivo, analyses of Tg(Dpl)10329 mice were inconsistent with reports that Dpl expression is associated with increased carbonylation and nitrosylation of brain proteins. Thus, rather than comprising an important source of free radical damage, copper binding may serve to modulate the activity, stability, or localization of the Dpl protein.