Role of PrPC glycosylation in health and disease

Glycosylation is the most abundant post-translational modification of proteins and has the ability to change the physical properties of the protein and its cell biology. The cellular prion protein (PrPC) is a membrane bound host glycoprotein present in a number of isoforms in vivo due to variable occupancy of the two N-linked glycosylation sites. The function of PrPC is still unclear but it is essential for disease in transmissible spongiform encephalopathies (TSEs). The significance of the PrP glycoforms in the physiological function is unknown. Gene targeted mice have been created with point mutations that selectively abolish the glycosylation sites of PrPC. These GlycoD mutants have been used to study the effect of glycosylation at the different sites on the cell biology of PrPC. This study showed that both glycosylation sites played a role in the cell biology of PrPC. Removal of a single or both glycosylation sites significantly reduced total PrPC protein. The relative amount of the truncated protein produced through proteolytic cleaving was slightly reduced in the GlycoD mutants; however the proportion of truncated to full length PrP was increased, further reducing full length protein. The maintenance of truncated protein levels indicates a potential importance of the fragment in PrPC function. Wild type PrP is predominantly diglycosylated and localised to the cell surface. In this study it was shown that all GlycoD mutants had reduced amounts of cell surface PrPC and an increased proportion of PrPC associated with the secretory pathway. Removal of either the first or the second glycosylation site produced changes in cell biology that were almost indistinguishable from each other whilst disruption of both glycosylation sites produces a more extreme phenotype than removal of a single site. Previous studies have shown an altered susceptibility for TSE disease GlycoD mice. An in vitro conversion assay was used to investigate the ability of the glycoforms to initiate conversion from PrPC to the disease associated PrPSc. Mice which had only the second site abolished were much more efficient at seeding conversion than all other glycoforms. This may reflect the difference in susceptibility between the two monoglycosylated PrPs but does not explain the increased resistance compared to wild type mice. All other GlycoD mutants had similar seeding times to wild type mice despite having increased TSE resistance. The differences observed in the cell biology of PrPC of the GlycoD mutants may go some way to explaining the differences in TSE susceptibilities seen with these mice

The Glycosylation Status of PrPC Is a Key Factor in Determining Transmissible Spongiform Encephalopathy Transmission between Species

The risk of transmission of transmissible spongiform encephalopathies (TSE) between different species has been notoriously unpredictable because the mechanisms of transmission are not fully understood. A transmission barrier between species often prevents infection of a new host with a TSE agent. Nonetheless, some TSE agents are able to cross this barrier and infect new species, with devastating consequences. The host PrP(C) misfolds during disease pathogenesis and has a major role in controlling the transmission of agents between species, but sequence compatibility between host and agent PrP(C) does not fully explain host susceptibility. PrP(C) is posttranslationally modified by the addition of glycan moieties which have an important role in the infectious process. Here, we show in vivo that glycosylation of the host PrP(C) has a significant impact on the transmission of TSE between different host species. We infected mice carrying different glycosylated forms of PrP(C) with two human agents (sCJDMM2 and vCJD) and one hamster strain (263K). The absence of glycosylation at both or the first PrP(C) glycosylation site in the host results in almost complete resistance to disease. The absence of the second site of N-glycan has a dramatic effect on the barrier to transmission between host species, facilitating the transmission of sCJDMM2 to a host normally resistant to this agent. These results highlight glycosylation of PrP(C) as a key factor in determining the transmission efficiency of TSEs between different species. IMPORTANCE The risks of transmission of TSE between different species are difficult to predict due to a lack of knowledge over the mechanisms of disease transmission; some strains of TSE are able to cross a species barrier, while others do not. The host protein, PrP(C), plays a major role in disease transmission. PrP(C) undergoes posttranslational glycosylation, and the addition of these glycans may play a role in disease transmission. We infected mice that express different forms of glycosylated PrP(C) with three different TSE agents. We demonstrate that changing the glycosylation status of the host can have profound effects on disease transmission, changing host susceptibility and incubation times. Our results show that PrP(C) glycosylation is a key factor in determining risks of TSE transmission between species

Gene Therapy: A Promising Approach to Treating Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a severe autosomal recessive disease caused by a genetic defect in the survival motor neuron 1 (SMN1) gene, which encodes SMN, a protein widely expressed in all eukaryotic cells. Depletion of the SMN protein causes muscle weakness and progressive loss of movement in SMA patients. The field of gene therapy has made major advances over the past decade, and gene delivery to the central nervous system (CNS) by in vivo or ex vivo techniques is a rapidly emerging field in neuroscience. Despite Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis being among the most common neurodegenerative diseases in humans and attractive targets for treatment development, their multifactorial origin and complicated genetics make them less amenable to gene therapy. Monogenic disorders resulting from modifications in a single gene, such as SMA, prove more favorable and have been at the fore of this evolution of potential gene therapies, and results to date have been promising at least. With the estimated number of monogenic diseases standing in the thousands, elucidating a therapeutic target for one could have major implications for many more. Recent progress has brought about the commercialization of the first gene therapies for diseases, such as pancreatitis in the form of Glybera, with the potential for other monogenic disease therapies to follow suit. While much research has been carried out, there are many limiting factors that can halt or impede translation of therapies from the bench to the clinic. This review will look at both recent advances and encountered impediments in terms of SMA and endeavor to highlight the promising results that may be applicable to various associated diseases and also discuss the potential to overcome present limitations.Depto. de Bioquímica y Biología MolecularFac. de MedicinaTRUEpu