Abstract. Doppel (Dpl) is a paralogue of the mammalian Prion (PrP) protein. It is abundant in testis and, unlike PrP, it is expressed at low levels in the adult central nervous system (CNS). Besides, Dpl overexpression correlates with some prion-disease pathological features, such as ataxia and death of cerebellar neurons. Recently, ectopic expression of doppel was found in two different tumor types, specifically in glial and haematological cancers. In this study the doppel gene (PRND) mRNA and protein expression in PRT-HU2 and IPDDC-A2 astrocytoma-derived cell lines was investigated. Northern blot analysis revealed two equally abundant PRND mRNA isoforms, while real-time PCR, on nuclear and cytoplasmic RNA fractions, and cRNA in situ hybridization, on astrocytoma cells and bioptical specimens, showed a nuclear retention of PRND transcripts. Western blot analysis showed that the amount of protein expressed is low compared to the level of mRNA. Moreover deglycosylation studies indicated that Dpl undergoes unusual glycosylation processes. Immunohistochemistry experiments demonstrated that Dpl was mainly localised in the cytoplasm of the astrocytic tumor cells, and that it failed to be GPI-anchored to the cell membrane. This unusual cellular localization was also confirmed through EGFP-Dpl expression in astrocytomas; on the contrary, HeLa cells exhibited the expected Dpl membrane localization. Our findings suggest an aberrant doppel gene expression pattern, characterized by a substantial nuclear retention of the transcript, an altered post-translational modification of the protein and an unusual cytoplasmic localization. IntroductionIn 1999, the first prion-gene paralogue, doppel (PRND), was described in rodents (1,2). Subsequently, the doppel gene was confirmed in humans (3), cattle, sheep (4), and goats (5). Comparative gene expression analysis was reported on different patterns of temporal and spatial expression among prion and doppel. Thus, whereas prion protein (PrP) is widely expressed, showing the highest expression profiles within the CNS (6), doppel shows barely detectable levels in most tissue (4,7). Doppel protein (Dpl) was found highly expressed only in adult and fetal testis (8), and different groups recently proposed an involvement in male gametogenesis (9-11). Based on the structural similarities between PrP (12) and Dpl (13), a role of doppel in the development of prion neurodegenerative diseases was hypothesised (14). Specifically, when ectopically expressed in some Prnp 0/0 transgenic lines, Dpl causes Purkinje cell death and ataxia (reviewed in ref. 15). However, Dpl-induced neurodegeneration can be rescued by the introduction of a Prnp transgene (16), suggesting the possibility that the protein interacts or competes with PrP in a sort of molecular antagonism-model (17). Further studies were performed to demonstrate a direct involvement of doppel in prion-diseases in humans (7) as well as in animals (18,19). From these analyses, doppel seems not to be associated with the diseases, neither if one ...

Doppel (PRND) is a paralogue of the mammalian prion (PRNP) gene. It is abundant in testis and, unlike PRNP, it is expressed at low levels in the adult central nervous system (CNS). Besides, doppel overexpression correlates with some prion-disease pathological features, such as ataxia and death of cerebellar neurons. Recently, ectopic expression of doppel was found in two different tumor types, specifically in glial and haematological cancers. In order to address clinical important issues, PRND mRNA expression was investigated in a panel of 111 astrocytoma tissue samples, histologically classified according to the World Health Organization (WHO) criteria (6 grade I pilocytic astrocytomas, 15 grade II low-grade astrocytomas, 26 grade III anaplastic astrocytomas and 64 grade IV glioblastoma multiforme). Real-time PRND gene expression profiling, after normalisation with GAPDH, revealed large differences between low (WHO I and II) and high grade (III and IV) of malignancy (P<0.001). Extensive differences in PRND gene expression were also found within each grade of malignancy, suggesting that PRND mRNA quantitation might be useful to distinguish astrocytoma subtypes, and important in disease stratification and in the assessment of specific treatment strategies.

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Doppel, a prion-like protein, is a GPI-membrane anchored protein generally not expressed in the Central Nervous System (CNS) of different mammalian species, including human. Nevertheless, in astrocytomas, a particular kind of glial tumors, the doppel encoding gene (PRND) is over-expressed and the corresponding protein product (Dpl) is ectopically localized in the cytoplasm of the tumor cells. In this study we have analysed the sub-cellular localization of Dpl using double-immunofluorescence staining and confocal microscopy examinations in two astrocytoma-derived human cell lines (IPDDC-A2 and D384-MG). Our results confirmed that Dpl is localized in the cytoplasm of the astrocytoma cells and indicated that it is mostly associated with Lamp-1 and Limp-2 positive lysosomal vesicles and, marginally, to the Golgi apparatus and other cellular organelles. Noticeably, none of the examined tumor cells showed a membrane-Dpl localization. The membrane-associated Dpl expression was restored after the transfection of the astrocytoma cells with mutated Dpl-expression vectors in its glycosylation sites. Additionally, Dpl showed altered expression and traffic using the acidotropic agent ammonium chloride, leading to the accumulation of Dpl in nascent exocytic vesicles. Altogether, these results indicated that in the astrocytic tumor cells Dpl has an altered biosynthetic trafficking, likely derived from abnormal post-translational processes: these modifications do not permit the localization of Dpl in correspondence of the plasma membrane and lead to its intracellular accumulation in the lysosomes. In these proteolytic compartments, the astrocytic tumor cells might provide to the degradation of the excess of a potentially cytotoxic Dpl product.