Glycosylphosphatidylinositols: More than just an anchor?

There is increasing interest in the role of glycosylphosphatidylinositol (GPI) anchors that attach some proteins to cell membranes. Far from being biologically inert, GPIs influence the targeting, intracellular trafficking and function of the attached protein. Our recent paper demonstrated the role of sialic acid on the GPI of the cellular prion protein (PrP(C)). The "prion diseases" arise following the conversion of PrP(C) to a disease-associated isoform called PrP(Sc) or "prion". Our paper showed that desialylated PrP(C) inhibited PrP(Sc) formation. Aggregated PrP(Sc) creates a signaling platform in the cell membrane incorporating and activating cytoplasmic phospholipase A2 (cPLA2), an enzyme that regulates PrP(C) trafficking and hence PrP(Sc) formation. The presence of desialylated PrP(C) caused the dissociation of cPLA2 from PrP-containing platforms, reduced the activation of cPLA2 and inhibited PrP(Sc) production. We concluded that sialic acid contained within the GPI attached to PrP(C) modifies local membrane microenvironments that are important in PrP-mediated cell signaling and PrP(Sc) formation.

Prion diseases occur following the conversion of a normal host protein (the cellular prion protein (PrPC)) into disease-associated isoforms (PrPSc) or “prions” which accumulate within the brain causing neurodegeneration. Our recent paper examined the role of the glycosylphosphatidylinositol (GPI) anchor that links PrPC to cell membranes upon the properties of PrPC and consequently whether PrPC was converted to PrPSc. As efficient PrPSc formation occurs only when PrPC is targeted to specific membrane micro-domains called lipid rafts, the factors affecting the cellular targeting and intracellular trafficking of PrPC are critical in regulating PrPSc formation. The GPI anchor targets PrPC to lipid rafts that are required for efficient PrPSc formation. Our recent paper showed that the targeting of PrPC to those lipid rafts involved in PrPSc formation was dependent upon the composition of the GPI anchor, specifically the presence of sialic acid. We reported 3 major observations:

That desialylated PrPC behaved differently from PrPC with regards to protein targeting, intracellular trafficking, its effects on membrane composition, cell signaling and critically, it was not converted to PrPSc.

That desialylated PrPC inhibited the conversion of PrPC to PrPSc.

That desialylated PrPC disrupted cell signaling mediated by PrPSc.

Although GPI-anchored proteins are targeted to lipid rafts, there exist many different, heterogeneous lipid rafts and PrPSc formation probably occurs in only a subset of these. The composition of lipid rafts surrounding GPI-anchored proteins is dependent upon multiple interactions between the protein, glycans and membrane lipids, and consequently PrPC and desialylated PrPC were found within different lipid rafts. We hypothesized that sialic acid in the GPI has a direct effect upon the composition of the surrounding membrane. Immunoprecipitation and analysis of lipid rafts surrounding PrP proteins demonstrated higher concentrations of gangliosides and cholesterol associated with lipid rafts containing desialylated PrPC than with lipid rafts containing PrPC. The functional consequences of these changes were 2-fold; desialylated PrPC remained within lipid rafts after cholesterol depletion, whereas PrPC redistributed to the normal cell membrane, and that desialylated PrPC had a demonstrably longer half-life than PrPC in neurons. We speculated that if sialic acid contained within the GPI competes with gangliosides for sialic acid-binding proteins, then the removal of sialic acid would allow the incorporation of more gangliosides into PrPC-containing rafts. Gangliosides help sequester cholesterol that increases membrane rigidity and stabilize lipid rafts. Thus the increased concentrations of gangliosides surrounding desialylated PrPC would explain the observed increased cholesterol concentration in lipid rafts surrounding desialylated PrPC. This hypothesis is compatible with reports that the concentrations of gangliosides in lipid rafts affects the expression of PrPC.

When neurons from transgenic mice in which the PrP protein had been deleted (Prnp(0(0) neurons) were pulsed with PrPSc, we found that PrPC was converted to PrPSc, but desialylated PrPC was not. Perhaps of greater interest were observations that in wildtype neurons and neuronal cell lines the presence of desialylated PrPC significantly reduced the conversion of PrPC to PrPSc. While most potential therapeutics for prion diseases are targeted at the protein component of PrP, our work highlights the importance of the underlying cell membrane. Since the composition and hence the function of lipid rafts is controlled by an “induced fit” model we hypothesized that the binding of desialylated PrPC to PrPSc modified the lipid rafts involved in PrPSc formation. Because the composition of lipid rafts is affected by the glycan composition of GPIs we expected that the lipid rafts surrounding PrPSc:PrPC complexes would differ from that of lipid rafts surrounding complexes of PrPSc:desialylated PrPC. We proposed that the binding of desialylated PrPC to PrPSc changes the composition of the lipid rafts so that it inhibits the conversion of PrPC to PrPSc.

The coalescence of outer membrane lipid raft proteins affects the composition of the cytoplasmic leaflet and its association with signaling molecules. The clustering of sialic acid-containing GPIs attached to PrP proteins activates cPLA2, an enzyme that promotes PrPSc formation. This occurs naturally as a consequence of PrPSc self-aggregation, and cPLA2 is concentrated within PrPSc-containing lipid rafts. The binding of desialylated PrPC to PrPSc changed the underlying membrane so that it no longer captured and activated cPLA2. This reduced the activation of cPLA2 by existing PrPSc and hindered the conversion of PrPC to PrPSc. It is noteworthy that desialylated PrPC is surrounded by more gangliosides than PrPC, which is consistent with reports that gangliosides inhibit the activation of cPLA2.

We concluded that sialic acid in the GPI anchors affects the properties of PrPC, altering the surrounding membrane; PrP-induced cell signaling and the trafficking of PrPC. Critically desialylated PrPC reduced the activation of cPLA2 and PrPSc formation in prion-infected cells. We propose that sialic acid on the GPI anchor attached to PrPC affects its precise membrane targeting and the subsequent cell signaling that is conducive to its conversion to PrPSc.