Trafficking of prion proteins through a caveolae-mediated endosomal pathway

Abstract

To understand the posttranslational conversion of the cellular prion protein (PrPC) to its pathologic conformation, it is important to define the intracellular trafficking pathway of PrPC within the endomembrane system. We studied the localization and internalization of PrPC in CHO cells using cryoimmunogold electron microscopy. At steady state, PrPC was enriched in caveolae both at the TGN and plasma membrane and in interconnecting chains of endocytic caveolae. Protein A–gold particles bound specifically to PrPC on live cells. These complexes were delivered via caveolae to the pericentriolar region and via nonclassical, caveolae-containing early endocytic structures to late endosomes/lysosomes, thereby bypassing the internalization pathway mediated by clathrin-coated vesicles. Endocytosed PrPC-containing caveolae were not directed to the ER and Golgi complex. Uptake of caveolae and degradation of PrPC was slow and sensitive to filipin. This caveolae-dependent endocytic pathway was not observed for several other glycosylphosphatidyl inositol (GPI)-anchored proteins. We propose that this nonclassical endocytic pathway is likely to determine the subcellular location of PrPC conversion.

Altered Glycosylated PrP Proteins Can Have Different Neuronal Trafficking in Brain but Do Not Acquire Scrapie-like Properties

“…7A). This observation is in accord with several results previously obtained both in vitro and in vivo with different detection techniques (41)(42)(43)(44)(45)(46)(47)(48).…”

Arf6-independent GPI-anchored Protein-enriched Early Endosomal Compartments Fuse with Sorting Endosomes via a Rab5/Phosphatidylinositol-3′-Kinase–dependent Machinery

“…Although there are reports of internalization of GPI-anchored proteins via caveolae (Nichols, 2002;Peters et al, 2003), our results show that the caveolar pathway does not seem to be the major pathway for endocytosis of a majority of GPIanchored proteins in CHO and MEF cells, although the virus particle SV40 is internalized via a clathrin-, dynamin-, Arf6-independent pathway in cells that do not express caveolae (Damm et al, 2005). However, the virus does not colocalize with EEA1, or dextran and resides in a pH-neutral compartment.…”

“…In recent studies, it has been shown that GPI-anchored prion protein (PrP C ) is enriched in caveolae at the plasma membrane, and a caveolar uptake mechanism for PrP C and its conversion to PrP sc was proposed. However, the study demonstrated that PrP C followed a different endocytic route than other GPIanchored proteins, such as CD59, which showed no enrichment in caveolae (Peters et al, 2003). Other instances of such protein-specific modulation are binding of a complex of the urokinase-type plasminogen activator (uPA) and its GPIanchored uPA receptor to the ␣-2 macroglobulin receptor/ low-density lipoprotein receptor-related protein (Nykjaer et al, 1992;Conese et al, 1995); and GPI-anchored CD14, which interacts with bacterial lipopolysaccharide (LPS) bound to the plasma-LPS-binding protein (Poussin et al, 1998;Triantafilou et al, 2001).…”

PrPc and reggies/flotillins are contained in and released via lipid-rich vesicles in Jurkat T cells

“…Release is further supported by the absence of F-actin labeling around the electron-lucent, reggie-and PrP c -positive vesicles at the EM level that appear as plasma membrane invaginations devoid of surrounding F-actin [26,27]. Release of lipid vesicle contents containing reggie and PrP c proteins by unstimulated exocytosis is therefore highly suggestive, especially as proteins of the Rab and SNARE family with functions in membrane transport and fusion have recently been detected in lipid droplet-containing fractions [18] where we found synaptobrevin-Ab binding in immunoblots. This opens the interesting possibility that propagation of prion infectivity may, in addition to cell contact-mediated pathways, also occur via release of prion-loaded lipid vesicles, and so participate in the neuroimmune transfer between follicular dendritic cells and sympathetic nerves that has recently been shown [28].…”

“…The situation for PrP c is quite similar, as it has always been detected in the non-cytosolic surface of membranes, suggesting that transport of PrP c and the reggies to the lipid-rich vesicle core may occur via a similar mechanism. The lipid-rich vesicles are most likely generated by fusion of smaller vesicles [18,23], so that the surface-to-volume relation becomes increasingly smaller during the course of maturation by ongoing fusion. Therefore, this transfer process may be enhanced due to lack of surface space in the course of lipid body maturation.…”