The 37kDa/67kDa laminin receptor as a therapeutic target in prion diseases: potency of antisense LRP RNA, siRNAs specific for LRP mRNA and a LRP decoy mutant

Prion diseases are a group of rare, fatal neurodegenerative diseases, also known as transmissible spongiform encephalopathies (TSEs), that affect both animals and humans and include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep, chronic wasting disease in deer and elk and Creutzfeldt-Jakob disease (CJD) in humans. TSEs are usually rapidly progressive and clinical symptoms comprise dementia and loss of movement coordination. A common hallmark of TSEs is the accumulation of an abnormal isoform (PrPSc) of the host-encoded prion protein (PrPc) in the brains of affected animals and humans. PrPc is a highly conserved cell surface sialoglycoprotein that is expressed in several cell types, mainly neuronal cells, but its normal physiological function is still not known. However, PrPc is elementary for the acquisition and the replication of prion diseases. Several inhibitors of the PrPSc formation have been reported, but none of them showed great potency in an in vivo application. Thus, the identification of the 37kDa/67kDa laminin receptor (LRP/LR) as the cell surface receptor for prions opened a new direction for the development of a TSE therapy. Currently, no treatment to slow down or stop the disease process in humans with any form of CJD is established. However, several strategies have been investigated to find an anti-prion treatment including development of a vaccination therapy and screening for potent chemical compounds. In scrapie-infected neuronal cells, which represent a widely used and well characterized in vitro model for transmissible spongiform encephalopathies, the accumulation of PrPSc has been prevented by transfection of (i) antisense LRP RNA, (ii) small interfering RNAs targeting the LRP mRNA and (iii) incubation with the polyclonal anti-LRP antibody W3. Furthermore, the knock down of surface LRP/LR resulted in a reduction of the cellular PrP levels, suggesting an interference with the PrP internalization process. Thus, LRP/LR is required for the PrPSc propagation in vitro and involved in the PrPc metabolism.Due to the existence of several LR genes, a major step to investigate the role of the 37kDa/67kDa laminin receptor in scrapie pathogenesis in vivo is the generation of transgenic mice exhibiting a lower level of LRP/LR. Hemizygous transgenic mice that express LRP/LR antisense RNA under the control of the neuron-specific enolase (NSE) promoter were generated and showed a reduced LRP/LR protein level in the cerebellum and the hippocampus. Intracerebral inoculation of these transgenic mice with the scrapie agent will show, whether the accumulation of pathogenic PrPSc in the brain is delayed or prevented due to a reduced LRP/LR level. A further therapeutic anti-prion approach is given by LRP/LR deletion mutants that can be secreted to the cell culture medium and might act as decoys. Previously, it has been demonstrated that a transmembrane deletion mutant is able to prevent PrPc binding and internalization. In vitro studies using an N-terminally truncated LRP mutant, representing the extracellular domain of LRP/LR (LRP102-295::FLAG), revealed a reduced binding of (i) recombinant cellular PrP to mouse neuroblastoma cells, (ii) infectious moPrP 27-30 to BHK21 cells and (iii) interfered with the PrPSc propagation in chronically scrapie-infected mouse neuroblastoma cells. Furthermore, a cell free binding assay demonstrated the direct binding of the LRP102-295::FLAG mutant to both PrPc and PrPSc. These results together with the finding that that endogenous LRP levels remain unaffected by the expression of the mutant indicate that the secreted LRP102-295::FLAG mutant may act in a trans-dominant negative manner as a decoy by trapping PrP molecules. To investigate the therapeutic potential of the LRP102-295::FLAG decoy mutant in vivo transgenic mice were generated ectopically expressing LRP102-295::FLAG in the brain. Animals showed no phenotype and transgene expression was detected in cortical and cerebellar brain regions. An intracerebral prion inoculation of these mice will prove whether the expression of the LRP102-295::FLAG mutant can impair the PrPSc accumulation in the brain and can thus, act as a alternative therapeutic tool in prion diseases. The recent finding that experimental introduction of RNA can be used to interfere with the function of an endogenous gene (RNA interference) provided another tool for the development of gene-specific therapeutics. In order to evaluate a gene transfer therapeutic TSE strategy, human immunodeficiency virus (HIV)-derived vectors that express short hairpin RNA (shRNA) directed against the LRP mRNA were used. Following integration of LRP-shRNA-expressing lentiviral vectors into the genome of neuronal cells efficient LRP/LR downregulation was observed. In scrapie infected neuronal cells, downregulation of the LRP gene expression resulted in a diminishment of PrPSc propagation, providing a further therapeutic strategy in the development of a TSE treatment.