Functional mimicry of the LDL receptor-associated protein through multimerization of a minimized third domain.

The LDL receptor-associated protein (RAP) is a ligand for the LDL receptor-related protein (LRP1). The first and third domains of RAP can each bind to one of many sequence-related pairs of complement-type repeats (CR) found within the LRP1 ectodomain. Multiple sites of interaction between the multivalent RAP ligand and the multivalent LRP1 receptor yield strong binding avidity for the complex. The third domain of RAP can be significantly truncated, with material retention of monovalent CR pair-binding affinity, provided that the minimized sequence is stabilized with an intramolecular disulfide bond. We demonstrate that the avidity of full-length RAP for LRP1 in vitro can be partially reconstituted by assembly of truncated, disulfide-linked RAP peptides on tetravalent streptavidin or bivalent immunoglobulin scaffolds. The peptide complex with streptavidin shows pronounced hepatotropism in vivo, replicating the biodistribution of full-length RAP.

Minimization of the third domain of the LDL receptor-associated protein (RAP).

The third domain of the low-density lipoprotein receptor-associated protein (RAP d3) binds with high-affinity to pairs of complement-type repeats (CR) within the LDLR family of receptors. Structural analyses have defined the contact surface between RAP d3 and a CR pair from the low-density lipoprotein receptor (LDLR). Much of the sequence of RAP d3 has been proposed to stabilize the receptor-binding region without participating directly in formation of the contact surface. We have developed a truncated version of RAP d3 in which these scaffolding regions are excised and replaced with a single, intramolecular disulfide bond. This substitution allows for deletion of as much as a third of the RAP d3 sequence with substantial retention of receptor-binding ability.

Efficient transfer of receptor-associated protein (RAP) across the blood-brain barrier.

We have sought to identify a high-capacity transport system that mediates transcytosis of proteins from the blood to the brain. The 39 kDa receptor-associated protein (RAP) functions as a specialized endoplasmic reticulum chaperone assisting in the folding and trafficking of members of the low-density lipoprotein (LDL) receptor family. RAP efficiently binds to these receptors and antagonizes binding of other ligands. Previous studies have shown that two large members of the LDL receptor family, LDL receptor-related protein 1 (LRP1) and LDL receptor-related protein 2 (LRP2 or megalin), possess the ability to mediate transcytosis of ligands across the brain capillary endothelium. Here, we tested whether blood-borne RAP crosses the blood-brain barrier (BBB) by LRP1- or megalin-mediated transport by studying the pharmacokinetics of [125I]-RAP transport into the brain in intact mice and across cell monolayers in vitro. Our results show that [125I]-RAP is relatively stable in blood for 30 minutes and has a mean influx constant of 0.62+/-0.08 microl/g-minute from blood to brain. In situ brain perfusion in blood-free buffer shows that transport of [125I]-RAP across the BBB is a saturable process. Capillary depletion of brain homogenates indicates that 70% of [125I]-RAP is localized in the parenchyma rather than in the vasculature of the brain. Results of transport in stably transfected MDCK cells are consistent with the hypothesis that megalin mediates most of the apical-to-basolateral transport across polarized epithelial cells. The inhibition of [125I]-RAP influx by excess RAP and the involvement of megalin indicate the presence of a saturable transport system at the BBB. The higher permeability of RAP compared with that of melanotransferrin and transferrin show that the LRP receptor is a high capacity transport system. These studies suggest that RAP may provide a novel means of protein-based drug delivery to the brain.

Overexpression of inactive arylsulphatase mutants and in vitro activation by light-dependent oxidation with vanadate.

Arylsulphatases B (ASB) and A (ASA) are subject to a unique post-translational modification that is required for their function. The modification reaction, conversion of an active-site cysteine into a formylglycine, becomes saturated when these enzymes are overexpressed. We have removed the possibility of in vivo modification by expressing mutants of ASB and ASA in which the active-site cysteine is substituted with a serine. These mutants are expressed much more efficiently when compared with the native enzymes under identical conditions. The purified ASB mutant can then be converted into catalytically active ASB in vitro using vanadate and light.

Lipoprotein receptor binding, cellular uptake, and lysosomal delivery of fusions between the receptor-associated protein (RAP) and alpha-L-iduronidase or acid alpha-glucosidase.

Enzyme replacement therapy for lysosomal storage disorders depends on efficient uptake of recombinant enzyme into the tissues of patients. This uptake is mediated by oligosaccharide receptors including the cation-independent mannose 6-phosphate receptor and the mannose receptor. We have sought to exploit alternative receptor systems that are independent of glycosylation but allow for efficient delivery to the lysosome. Fusions of the human lysosomal enzymes alpha-l-iduronidase or acid alpha-glucosidase with the receptor-associated protein were efficiently endocytosed by lysosomal storage disorder patient fibroblasts, rat C6 glioma cells, mouse C2C12 myoblasts, and recombinant Chinese hamster ovary cells expressing individual members of the low-density lipoprotein receptor family. Uptake of the fusions exceeded that of phosphorylated enzyme in all cases, often by an order of magnitude or greater. Uptake was specifically mediated by members of the low-density lipoprotein receptor protein family and was followed by delivery of the fusions to the lysosome. The advantages of the lipoprotein receptor system over oligosaccharide receptor systems include more efficient cellular delivery and the potential for transcytosis of ligands across tight endothelia, including the blood-brain barrier.