No abstract

Injury to the glomerular podocyte is a key mechanism in human glomerular disease and podocyte repair is an important therapeutic target. In Fabry disease, podocyte injury is caused by the intracellular accumulation of globotriaosylceramide. This study identifies in the human podocyte three endocytic receptors, mannose 6-phosphate/insulin-like growth II receptor, megalin, and sortilin and demonstrates their drug delivery capabilities for enzyme replacement therapy. Sortilin, a novel α-galactosidase A binding protein, reveals a predominant intracellular expression but also surface expression in the podocyte. The present study provides the rationale for the renal effect of treatment with α-galactosidase A and identifies potential pathways for future non-carbohydrate based drug delivery to the kidney podocyte and other potential affected organs.

Sortilin is a member of the Vps10p domain family of neuropeptide and neurotrophin binding neuronal receptors. The family members interact with and partly share a variety of ligands and partake in intracellular sorting and protein transport as well as in transmembrane signal transduction. Thus, sortilin mediates the transport of both neurotensin and nerve growth factor and interacts with their respective receptors to facilitate ligand-induced signaling. Here we report that ciliary neurotrophic factor (CNTF), and related ligands targeting the established CNTF receptor ␣, binds to sortilin with high affinity. We find that sortilin may have at least two functions: one is to provide rapid endocytosis and the removal of CNTF, something which is not provided by CNTF receptor ␣, and the other is to facilitate CNTF signaling through the gp130/leukemia inhibitory factor (LIF) receptor ␤ heterodimeric complex. Interestingly, the latter function is independent of both the CNTF receptor ␣ and ligand binding to sortilin but appears to implicate a direct interaction with LIF receptor ␤. Thus, sortilin facilitates the signaling of all helical type 1 cytokines, which engage the gp130/LIF receptor ␤ complex.

The low-density lipoprotein receptor (LDLR) family of endocytosis receptors has been implicated in binding and endocytosis of a large number of structurally unrelated proteins, including apolipoproteins, proteaseinhibitor complexes, extracellular matrix proteins, and hormone carriers. In mammals, this receptor family includes LDLR itself, low-density lipoprotein receptorrelated protein-1A (LRP-1A), LRP-1B, megalin or LRP-2, very-low-density lipoprotein receptor (VLDLR), and apolipoprotein E receptor-2. These Keywords low-density lipoprotein receptor-related protein; plasminogen activator inhibitor 1; sorting protein-related receptor; urokinase plasminogen activator; very-low-density lipoprotein receptor Some endocytosis receptors related to the low-density lipoprotein receptor, including low-density lipoprotein receptor-related protein-1A, very-lowdensity lipoprotein receptor, and sorting protein-related receptor, bind protease-inhibitor complexes, including urokinase-type plasminogen activator (uPA), plasminogen activator inhibitor-1 (PAI-1), and the uPA-PAI-1 complex. The unique capacity of these receptors for high-affinity binding of many structurally unrelated ligands renders mapping of receptor-binding surfaces of serpin and serine protease ligands a special challenge. We have mapped the receptor-binding area of the uPA-PAI-1 complex by site-directed mutagenesis. Substitution of a cluster of basic residues near the 37-loop and 60-loop of uPA reduced the receptor-binding affinity of the uPA-PAI-1 complex approximately twofold. Deletion of the N-terminal growth factor domain of uPA reduced the affinity 2-4-fold, depending on the receptor, and deletion of both the growth factor domain and the kringle reduced the affinity sevenfold. The binding affinity of the uPA-PAI-1 complex to the receptors was greatly reduced by substitution of basic and hydrophobic residues in a-helix D and a-helix E of PAI-1. The localization of the implicated residues in the 3D structures of uPA and PAI-1 shows that they form a continuous receptor-binding area spanning the serpin as well as the A-chain and the serine protease domain of uPA. Our results suggest that the 10-100-fold higher affinity of the uPA-PAI-1 complex compared with the free components depends on the bonus effect of bringing the binding areas on uPA and PAI-1 together on the same binding entity.Abbreviations a 1 -PI, a 1 -antiproteinase inhibitor; CTR, complement type repeat; HEK293T, human embryonic kidney cell line 293T; LDLR, low-density lipoprotein receptor; LRP, low-density lipoprotein receptor-related protein; PAI-1, plasminogen activator inhibitor 1; RAP, receptor-associated protein; RCL, reactive centre loop; sorLA, sorting protein-related receptor; SPD, serine protease domain; tPA, tissue-type plasminogen activator; uPA, urokinase-type plasminogen activator; uPAR, uPA receptor; VLDLR, very-low-density lipoprotein receptor.

Cardiotrophin-like cytokine:cytokine-like factor-1 (CLC:CLF-1) is a heterodimeric neurotropic cytokine that plays a crucial role during neuronal development. Mice lacking CLC:CLF-1 die soon after birth due to a suckling defect and show reduced numbers of motor neurons. Humans carrying mutations in CLC:CLF-1 develop similar disorders, known as Sohar-Crisponi or cold-induced sweating syndrome, and have a high risk of early death. It is well known that CLC binds the ciliary neurotrophic factor receptor α (CNTFRα) and is a prerequisite for signaling through the gp130/leukemia inhibitory factor receptor β (LIFRβ) heterodimer, whereas CLF-1 serves to promote the cellular release of CLC. However, the precise role of CLF-1 is unclear. Here, we report that CLF-1, based on its binding site for CLC and on two additional and independent sites for CNTFRα and sorLA, is a key player in CLC and CNTFRα signaling and turnover. The site for CNTFRα enables CLF-1 to promote CLC:CNTFRα complex formation and signaling. The second site establishes a link between the endocytic receptor sorLA and the tripartite CLC:CLF-1:CNTFRα complex and allows sorLA to downregulate the CNTFRα pool in stimulated cells. Finally, sorLA may bind and concentrate the tripartite soluble CLC:CLF-1:CNTFRα complex on cell membranes and thus facilitate its signaling through gp130/LIFRβ.