Disease relevance of Lrp2

• Conformation and glycosylation of a megalin fragment correlate with nephritogenicity in Heymann nephritis [1].
• Rats immunized with the soluble, secreted protein encoded by a baculovirus construct elicited high titer anti-megalin autoantibodies and developed glomerular immune deposits and elevated proteinuria consistent with AHN [1].
• Recombinant proteins containing a 563-residue N-terminal sequence of megalin were obtained from Escherichia coli and baculovirus-insect cell expression systems [1].
• Serum antibodies against megalin (GP330) in patients with autoimmune thyroiditis [2].
• The nephritogenic antigen of Heymann's nephritis (HN), gp330, was previously demonstrated (4-9) to be a resident glycoprotein of coated pits in the glomerular and proximal tubule epithelium of rats, and anti-gp330 IgG given intravenously was found to form IDs in glomeruli (passive HN) [3].

High impact information on Lrp2

• We hypothesized that rats immunized with anti-Fx1A develop autoantibodies (auto-Abs) to Crry as well as to the megalin-containing HN antigenic complex, and that anti-Crry Abs promote the development of injury in HN by neutralizing the complement regulatory activity of Crry [4].
• In pHN, the formation of subepithelial immune deposits (ID) involves as antigenic targets the membrane glycoprotein gp330/megalin and the 44-kD receptor-associated protein (RAP) [5].
• Initial events in the formation of immune deposits in passive Heymann nephritis. gp330-anti-gp330 immune complexes form in epithelial coated pits and rapidly become attached to the glomerular basement membrane [3].
• When the IgG bound to glomeruli was eluted with acid citrate buffer 3 d after injection, it was found to specifically immunoprecipitate only gp330 from detergent-solubilized 125I-labeled kidney microvillar vesicles [3].
• The present study shows that the antifibrinolytic polypeptide, aprotinin, and the nephro- and ototoxic antibiotics, aminoglycosides, and polymyxin B compete for binding of 125I-urokinase-plasminogen activator inhibitor type-1 complexes to purified rabbit gp330 [6].

Chemical compound and disease context of Lrp2

• Active Heymann nephritis (AHN), a rat model of autoimmune glomerulonephritis, is induced by immunization with autologous megalin, a 600-kDa cell surface glycoprotein isolated from crude renal extracts [1].
• The antibody 14C1 recognized the previously described nephritogenic glycoprotein, gp 330, in microvillar brush border preparations, and reacted with material present on podocyte cell surfaces of normal rat kidneys, especially in coated pits, as well as with material in the glomerular deposits of rats with Heymann nephritis [7].
• A band of gp330, which was identified as a pathogenic antigen of Heymann nephritis by Kerjaschki D and Farquhar MG (Proc Natl Acad Sci USA 79:5557, 1982) was detected as another band by Coomassie blue staining and immunoblotting [8].
• Mycophenolate mofetil significantly reduced the production of autoantibodies against gp330 in rats with Heymann nephritis [9].

Biological context of Lrp2

• The amount of Tg internalized (measured by ELISA in the cell lysates) was reduced by RAP and 1H2, indicating that Tg endocytosis is partially mediated by megalin [10].
• Physiologically, Tg binding to megalin on thyroid cells may be facilitated by Tg interaction with heparin-like molecules (heparan sulfate proteoglycans) via adjacent binding sites [11].
• In contrast, antibodies to gp330 had a minimal effect on embryonic development and did not induce fetal malformations [12].
• These observations provide evidence that a megalin N-terminal domain includes B and T cell epitopes sufficient for a pathogenic autoimmune response and that a native-like conformation and glycosylation are essential for the induction of disease [1].
• Gene expression of vitamin D hydroxylase and megalin in the remnant kidney of nephrectomized rats [13].

Anatomical context of Lrp2

• Here we show that Fisher rat thyroid (FRTL-5) cells, a differentiated rat thyroid cell line, can bind and endocytose Tg via megalin [10].
• Megalin (gp330) is an endocytic receptor for thyroglobulin on cultured fisher rat thyroid cells [10].
• Tg transcytosis is a novel function of megalin, which usually transports ligands to lysosomes [14].
• Anti-megalin antisera, but not control sera, displace >90% bound MT from rat renal brush-border membranes [15].
• Megalin (gp330) is a large glycoprotein receptor found mainly on a group of absorptive epithelial cells, including renal proximal tubule, epididymal and thyroid cells [16].

Associations of Lrp2 with chemical compounds

• Megalin-bound Tg was releasable by heparin [10].
• Megalin-mediated transcytosis may regulate the extent of thyroid hormone release [14].
• CONCLUSION: Coordinated and reciprocal alterations of the increase in CYP27B1 mRNA and the decrease in CYP24 mRNA may play a pivotal role in maintaining the plasma level of 1 alpha,25(OH)2D3 in the face of reduced nephron mass and/or megalin expression [13].
• Thyroxine treatment of TDX rats resulted in reversal of all of these effects, including an increase to nearly 24% of cubilin associated with megalin [17].
• Phosphoinositide 3-kinase inhibits megalin-mediated transcytosis of thyroglobulin across thyroid epithelial cells at a post-sorting level [18].

Physical interactions of Lrp2

• First, MT binds megalin, but not cubilin, in direct surface plasmon resonance studies [15].
• Moreover, Sp1 and Sp3 proteins interacted with the proximal and the distal Sp1 sites in the nuclear extracts of gp600/megalin expressing cell lines [19].
• Plasminogen binding to gp330 could be completely inhibited by the addition of exogenous gp330 [20].
• Megalin bound leptin in the presence of Ca(2+) and mediated its cellular internalization and degradation [21].
• Quartz-crystal microbalance analysis revealed that L-FABP bound to megalin, a multiligand endocytotic receptor on PTC, in a Ca2+-dependent manner [22].

Enzymatic interactions of Lrp2

• These results indicate that autoantibody obtained from the immune deposits in the glomeruli of rats with active Heymann nephritis does not inhibit the binding of plasminogen to gp330 but does significantly alter the urokinase catalyzed activation of plasminogen to plasmin [23].

Regulatory relationships of Lrp2

• Furthermore, the 210-kDa Tg polypeptide inhibited megalin binding to intact Tg by approximately 70% [11].
• Megalin expressed in the lower intestine appears not to have a significant role in the absorption of lysozyme [24].

Other interactions of Lrp2

• Third, megalin and fluorescently labeled MT colocalize in BN-16 cells, as shown by fluorescent microscopic techniques [15].
• These inhibitory effects and those of a monoclonal antimegalin antibody were not additive, indicating that calmodulin acts on the megalin-mediated pathway [25].
• Previous studies have identified two high-molecular weight (280 and 330 kd) glycoproteins expressed by coated pits of the proximal renal tubule and yolk sac and have further established that, in vivo, antibodies to gp280 but not to gp330 induce fetal malformations [12].
• Using nephrectomized rats, we examined changes in mRNA levels of CYP27B1 (25-hydroxyvitamin D3-1 alpha-hydroxylase), CYP24 (25-hydroxyvitamin D3-24-hydroxylase), and vitamin D receptor in relation to megalin, recently found to participate in renal vitamin D metabolism [13].
• In the thyroid, megalin (a low-density lipoprotein receptor family member) serves as an endocytic receptor for thyroglobulin [26].

Analytical, diagnostic and therapeutic context of Lrp2

• All three recombinant proteins were reactive in Western blots with rabbit anti-megalin antiserum, whereas the insect cell-derived proteins reacted preferentially in Western blot and ELISA with anti-megalin autoantibodies from rats with AHN induced by native megalin [1].
• Furthermore, megalin affinity chromatography and surface plasmon resonance analysis revealed a calcium-dependent high affinity binding of the 460-kDa receptor to megalin, which thereby may mediate its vesicular trafficking [27].
• We developed an assay, based on flow cytometry, to measure binding of serum IgGs to L2 cells, a rat carcinoma cell line that expresses abundant megalin [2].
• To identify megalin's apical sorting signal, we generated deletion mutants and chimeric minireceptors composed of complementary regions of megalin and LDL receptor-related protein (LRP) and assessed the distribution of the mutants in Madin-Darby canine kidney (MDCK) cells by immunofluorescence and cell surface biotinylation [28].
• Cubilin and megalin expression and their interaction in the rat intestine: effect of thyroidectomy [17].

References