Human Doppel and prion protein share common membrane microdomains and internalization pathways

Doppel is the first identified homologue of the prion protein (PrPc) implicated in prion disease. Doppel is considered an N-truncated form of PrPc, and shares with PrPc several structural and biochemical features. When over expressed in the brain of some PrP knockout animals, it provokes cerebellar ataxia. As this phenotype is rescued by reintroducing the PrP gene, it has been suggested that Doppel and PrPc have antagonistic functions and may compete for a common ligand. However, a direct interaction between the two proteins has recently been observed. To investigate whether the neuronal environment is suitable for such possibility, human Doppel and PrPc were expressed separately, or together, in neuroblastoma cells, and then studied by biochemical and immunomicroscopic tools, as well as in intact cells expressing fluorescent fusion constructs. The results demonstrate that Doppel and PrPc co-patch extensively at the plasma membrane, and get internalized together after ganglioside cross-linking by cholera toxin or addition of an antibody against only one of the proteins. These processes no longer occur if the integrity of rafts is disrupted. We also show that, whereas each protein expressed alone occupies Triton X-100-insoluble membrane microdomains, co-transfected Doppel and PrPc redistribute together into a less ordered lipidic environment. All these features are consistent with interactions occurring between Doppel and PrPc in our neuronal cell model.