LRP2 explained

Low density lipoprotein receptor-related protein 2 also known as LRP-2 or megalin is a protein which in humans is encoded by the LRP2 gene.^{[1] [2] [3]}

Function

LRP2 was identified as the antigen of rat experimental membranous nephropathy (Heyman nephritis) and originally named gp330 and subsequently megalin^[4] and later LRP2. LRP2/megalin is a multiligand binding receptor found in the plasma membrane of many absorptive epithelial cells. LRP2 is an approximately 600kDa (4665 amino acids) transmembrane glycoprotein with structural similarities to the low density lipoprotein receptor (LDLR).^[5] LRP2 has a NPXY motif that is the binding site for Dab2 to initiate clathrin-mediated endocytosis.^[6] LRP2 forms a homodimer that changes conformation in response to pH.^[7] At pH 7.5 (extracellular pH), LRP2 is considered active, with the leucine loops in an open conformation to allow ligands to bind. At acidic endosomal pHs, the leucine loops collapse to prevent ligands binding.

LRP2 is expressed in epithelial cells of the thyroid (thyrocytes), where it can serve as a receptor for the protein thyroglobulin (Tg).^[8] LRP2 is also expressed on the apical surface of epithelial cells in the proximal tubule of the kidney. It is highly expressed in the first segment (S1) of the proximal tubule, with decreasing expression in the second (S2) and third segment (S3) of the proximal tubule. LRP2 is also expressed in podocytes, and antigenic response to LRP2 in podocytes is the primary cause of Heymann nephritis in rats.

LRP2/megalin functions to mediate endocytosis of ligands leading to degradation in lysosomes or transcytosis. LRP2/megalin can also form complexes with CUBAM, the cubilin and amnionless complex. Those complexes are able to reabsorb several molecules and can be inhibited by sodium maleate. LRP2 and CUBAM are responsible for the uptake of most of the filtered proteins that escape the glomerular filtration barrier in the proximal tubule of the kidney.^{[9] [10]} The endocytic capacity of the proximal tubule cells is dictated by the combined function of LRP2, CUBAM, and Dab2.

The epithelial cells of the proximal tubule are highly polarized and have a robust apical endocytic pathway, subapical compartmentalization, and large endocytic capacity. This pathway is mediated by LRP2 and CUBAM, where Dab2 binds to the cytoplasmic tails of both LRP2 and CUBAM to initiate clathrin-coated endocytosis. Once internalized, the endosomes release their clathrin coats and fuse with a dense subapical network of tubules to recycle receptors back to the apical surface. As the endosomes acidify, LRP2 release its cargo and undergoes a conformational change which collapses the binding pockets to inhibit ligands rebinding to LRP2 in the endosomes. Recycling of the LRP2 occurs from apical vacuoles with Rab11a positive endosomes, also referred to as dense apical tubules.^[11] The vesicles are directed back to the plasma membrane where LRP2 undergoes another conformational change due to the change in pH and becomes active again. According to LRP2/megalin kinetic modeling, the rate of megalin recycling and return to the apical surface from dense apical tubules has the largest impact on determining the overall endocytic capacity of proximal tubule cells and the endocytic rate of LRP2. The fraction of LRP2 at the apical surface is important for the continued ability of the protein to reabsorb filtered proteins in the proximal tubule to maintain the robust endocytic capacity of these cells.

Clinical significance

Disfunction in the LRP2-mediated endocytic trafficking and endocytic capacity in the proximal tubule can result in low molecular weight proteinuria, which is a hallmark of many diseases.

Mutations in the LRP2 gene are associated with Donnai-Barrow syndrome.^[12]

Dent's Disease (Dent 1) is associated with a drop in LRP2/megalin protein level in the proximal tubule with no detectable decrease in mRNA, suggesting that the loss of ClC-5, the gene mutated in Dent's Disease, shortens the half-life of the LRP2 receptor.^{[13] [14]} The loss of ClC-5 has been found to delay the early endosome maturation in the LRP2 trafficking in the proximal tubule cells.

LRP2 has been shown to play a role in the development of nephrotoxic acute kidney injury (AKI) by mediating the uptake of nephrotoxic agents.^[15] However, there have been no further studies to show the functional importance of LRP2 or CUBAM in the progression of AKI.

A decrease in LRP2 receptor expression has been reported in animal models of acute and chronic kidney diseases.

Interactions

LRP2 has been shown to associate with the following proteins in the plasma membrane/cytosol of cells:

• CUBAM,
• DAB2,^[16]
• DLG4,^[17]
• GIPC1,
• ITGB1BP1,
• LDL-receptor-related protein associated protein,^{[18] [19]}
• LDLRAP1,^[20]
• MAGI1,^[21]
• MAPK8IP1,
• MAPK8IP2,^{[22] [23]}
• NOS1AP,^[22] and
• SYNJ2BP.^[22]

LRP2 has been shown to bind to the following ligands:

• Albumin
• Vitamin D-binding protein
• Hemoglobin
• Myoglobin
• Angiotensin II
• Insulin
• Leptin
• Prolactin
• Epidermal Growth Factor
• Cathepsin B
• Immunoglobulin light chains
• Ca²⁺

Further reading

• Book: Longoni M, Kantarci S, Donnai D, Pober BR . Donnai-Barrow Syndrome . August 2008 . Adam MP, Mirzaa GM, Pagon RA, Wallace SE, Bean LJ, Gripp KW, Amemiya A . 6 . GeneReviews® [Internet] . Seattle (WA) . University of Washington, Seattle . 20301732 . https://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=donnai .
• Farquhar MG, Saito A, Kerjaschki D, Orlando RA . The Heymann nephritis antigenic complex: megalin (gp330) and RAP . Journal of the American Society of Nephrology . 6 . 1 . 35–47 . July 1995 . 7579068 . 10.1681/ASN.V6135 . free .
• Farquhar MG . The unfolding story of megalin (gp330): now recognized as a drug receptor . The Journal of Clinical Investigation . 96 . 3 . 1184 . September 1995 . 7657789 . 185736 . 10.1172/JCI118149 .
• Christensen EI, Birn H . Megalin and cubilin: multifunctional endocytic receptors . Nature Reviews. Molecular Cell Biology . 3 . 4 . 256–266 . April 2002 . 11994745 . 10.1038/nrm778 . 21893726 .
• Saito A, Takeda T, Hama H, Oyama Y, Hosaka K, Tanuma A, Kaseda R, Ueno M, Nishi S, Ogasawara S, Gondaira F, Suzuki Y, Gejyo F . 6 . Role of megalin, a proximal tubular endocytic receptor, in the pathogenesis of diabetic and metabolic syndrome-related nephropathies: protein metabolic overload hypothesis . Nephrology . 10 . Suppl . S26–S31 . October 2005 . 16174284 . 10.1111/j.1440-1797.2005.00453.x . 42737684 .
• Fisher CE, Howie SE . The role of megalin (LRP-2/Gp330) during development . Developmental Biology . 296 . 2 . 279–297 . August 2006 . 16828734 . 10.1016/j.ydbio.2006.06.007 .
• Christensen EI, Gliemann J, Moestrup SK . Renal tubule gp330 is a calcium binding receptor for endocytic uptake of protein . The Journal of Histochemistry and Cytochemistry . 40 . 10 . 1481–1490 . October 1992 . 1382088 . 10.1177/40.10.1382088 . 24323611 .
• Raychowdhury R, Niles JL, McCluskey RT, Smith JA . Autoimmune target in Heymann nephritis is a glycoprotein with homology to the LDL receptor . Science . 244 . 4909 . 1163–1165 . June 1989 . 2786251 . 10.1126/science.2786251 . 1989Sci...244.1163R .
• Orlando RA, Farquhar MG . Functional domains of the receptor-associated protein (RAP) . Proceedings of the National Academy of Sciences of the United States of America . 91 . 8 . 3161–3165 . April 1994 . 7512726 . 43535 . 10.1073/pnas.91.8.3161 . free . 1994PNAS...91.3161O .
• Kounnas MZ, Chappell DA, Strickland DK, Argraves WS . Glycoprotein 330, a member of the low density lipoprotein receptor family, binds lipoprotein lipase in vitro . The Journal of Biological Chemistry . 268 . 19 . 14176–14181 . July 1993 . 7686151 . 10.1016/S0021-9258(19)85224-9 . free .
• Kounnas MZ, Loukinova EB, Stefansson S, Harmony JA, Brewer BH, Strickland DK, Argraves WS . Identification of glycoprotein 330 as an endocytic receptor for apolipoprotein J/clusterin . The Journal of Biological Chemistry . 270 . 22 . 13070–13075 . June 1995 . 7768901 . 10.1074/jbc.270.22.13070 . free .
• Korenberg JR, Argraves KM, Chen XN, Tran H, Strickland DK, Argraves WS . Chromosomal localization of human genes for the LDL receptor family member glycoprotein 330 (LRP2) and its associated protein RAP (LRPAP1) . Genomics . 22 . 1 . 88–93 . July 1994 . 7959795 . 10.1006/geno.1994.1348 .
• Lundgren S, Hjälm G, Hellman P, Ek B, Juhlin C, Rastad J, Klareskog L, Akerström G, Rask L . 6 . A protein involved in calcium sensing of the human parathyroid and placental cytotrophoblast cells belongs to the LDL-receptor protein superfamily . Experimental Cell Research . 212 . 2 . 344–350 . June 1994 . 8187828 . 10.1006/excr.1994.1153 .
• Moestrup SK, Nielsen S, Andreasen P, Jørgensen KE, Nykjaer A, Røigaard H, Gliemann J, Christensen EI . 6 . Epithelial glycoprotein-330 mediates endocytosis of plasminogen activator-plasminogen activator inhibitor type-1 complexes . The Journal of Biological Chemistry . 268 . 22 . 16564–16570 . August 1993 . 8344937 . 10.1016/S0021-9258(19)85456-X . free .
• Hjälm G, Murray E, Crumley G, Harazim W, Lundgren S, Onyango I, Ek B, Larsson M, Juhlin C, Hellman P, Davis H, Akerström G, Rask L, Morse B . 6 . Cloning and sequencing of human gp330, a Ca(2+)-binding receptor with potential intracellular signaling properties . European Journal of Biochemistry . 239 . 1 . 132–137 . July 1996 . 8706697 . 10.1111/j.1432-1033.1996.0132u.x . free .
• Willnow TE, Hilpert J, Armstrong SA, Rohlmann A, Hammer RE, Burns DK, Herz J . Defective forebrain development in mice lacking gp330/megalin . Proceedings of the National Academy of Sciences of the United States of America . 93 . 16 . 8460–8464 . August 1996 . 8710893 . 38693 . 10.1073/pnas.93.16.8460 . free . 1996PNAS...93.8460W .
• Cui S, Verroust PJ, Moestrup SK, Christensen EI . Megalin/gp330 mediates uptake of albumin in renal proximal tubule . The American Journal of Physiology . 271 . 4 Pt 2 . F900–F907 . October 1996 . 8898021 . 10.1152/ajprenal.1996.271.4.F900 .
• Lundgren S, Carling T, Hjälm G, Juhlin C, Rastad J, Pihlgren U, Rask L, Akerström G, Hellman P . 6 . Tissue distribution of human gp330/megalin, a putative Ca(2+)-sensing protein . The Journal of Histochemistry and Cytochemistry . 45 . 3 . 383–392 . March 1997 . 9071320 . 10.1177/002215549704500306 . free .
• Birn H, Verroust PJ, Nexo E, Hager H, Jacobsen C, Christensen EI, Moestrup SK . Characterization of an epithelial approximately 460-kDa protein that facilitates endocytosis of intrinsic factor-vitamin B12 and binds receptor-associated protein . The Journal of Biological Chemistry . 272 . 42 . 26497–26504 . October 1997 . 9334227 . 10.1074/jbc.272.42.26497 . free .

Notes and References

1. Web site: Entrez Gene: LRP2 low density lipoprotein-related protein 2.
2. Korenberg JR, Argraves KM, Chen XN, Tran H, Strickland DK, Argraves WS . Chromosomal localization of human genes for the LDL receptor family member glycoprotein 330 (LRP2) and its associated protein RAP (LRPAP1) . Genomics . 22 . 1 . 88–93 . July 1994 . 7959795 . 10.1006/geno.1994.1348 .
3. Farquhar MG . The unfolding story of megalin (gp330): now recognized as a drug receptor . The Journal of Clinical Investigation . 96 . 3 . 1184 . September 1995 . 7657789 . 185736 . 10.1172/JCI118149 .
4. Farquhar MG, Saito A, Kerjaschki D, Orlando RA . The Heymann nephritis antigenic complex: megalin (gp330) and RAP . Journal of the American Society of Nephrology . 6 . 1 . 35–47 . July 1995 . 7579068 . 10.1681/ASN.V6135 . free .
5. Eshbach ML, Weisz OA . Receptor-Mediated Endocytosis in the Proximal Tubule . Annual Review of Physiology . 79 . 1 . 425–448 . February 2017 . 27813828 . 5512543 . 10.1146/annurev-physiol-022516-034234 .
6. Gallagher H, Oleinikov AV, Fenske C, Newman DJ . The adaptor disabled-2 binds to the third psi xNPxY sequence on the cytoplasmic tail of megalin . Biochimie . 86 . 3 . 179–182 . March 2004 . 15134832 . 10.1016/j.biochi.2004.03.001 .
7. Beenken A, Cerutti G, Brasch J, Guo Y, Sheng Z, Erdjument-Bromage H, Aziz Z, Robbins-Juarez SY, Chavez EY, Ahlsen G, Katsamba PS, Neubert TA, Fitzpatrick AW, Barasch J, Shapiro L . 6 . Structures of LRP2 reveal a molecular machine for endocytosis . Cell . 186 . 4 . 821–836.e13 . February 2023 . 36750096 . 9993842 . 10.1016/j.cell.2023.01.016 .
8. Zheng G, Marino' M, Zhao J, McCluskey RT . Megalin (gp330): a putative endocytic receptor for thyroglobulin (Tg) . Endocrinology . 139 . 3 . 1462–1465 . March 1998 . 9492085 . 10.1210/endo.139.3.5978 . free .
9. Weisz OA . Endocytic adaptation to functional demand by the kidney proximal tubule . The Journal of Physiology . 599 . 14 . 3437–3446 . July 2021 . 34036593 . 8715547 . 10.1113/JP281599 .
10. Long KR, Rbaibi Y, Bondi CD, Ford BR, Poholek AC, Boyd-Shiwarski CR, Tan RJ, Locker JD, Weisz OA . 6 . Cubilin-, megalin-, and Dab2-dependent transcription revealed by CRISPR/Cas9 knockout in kidney proximal tubule cells . American Journal of Physiology. Renal Physiology . 322 . 1 . F14–F26 . January 2022 . 34747197 . 8698540 . 10.1152/ajprenal.00259.2021 .
11. Shipman KE, Long KR, Cowan IA, Rbaibi Y, Baty CJ, Weisz OA . An Adaptable Physiological Model of Endocytic Megalin Trafficking in Opossum Kidney Cells and Mouse Kidney Proximal Tubule . Function . 3 . 6 . zqac046 . 2022-10-28 . 36325513 . 9614980 . 10.1093/function/zqac046 .
12. Kantarci S, Al-Gazali L, Hill RS, Donnai D, Black GC, Bieth E, Chassaing N, Lacombe D, Devriendt K, Teebi A, Loscertales M, Robson C, Liu T, MacLaughlin DT, Noonan KM, Russell MK, Walsh CA, Donahoe PK, Pober BR . 6 . Mutations in LRP2, which encodes the multiligand receptor megalin, cause Donnai-Barrow and facio-oculo-acoustico-renal syndromes . Nature Genetics . 39 . 8 . 957–959 . August 2007 . 17632512 . 2891728 . 10.1038/ng2063 .
13. Shipman KE, Weisz OA . Making a Dent in Dent Disease . Function . 1 . 2 . zqaa017 . 2020-09-14 . 33015630 . 7519470 . 10.1093/function/zqaa017 .
14. Shipman KE, Baty CJ, Long KR, Rbaibi Y, Cowan IA, Gerges M, Marciszyn AL, Kashlan OB, Tan RJ, Edwards A, Weisz OA . 6 . Impaired Endosome Maturation Mediates Tubular Proteinuria in Dent Disease Cell Culture and Mouse Models . Journal of the American Society of Nephrology . 34 . 4 . 619–640 . April 2023 . 36758125 . 10.1681/ASN.0000000000000084 . 10103310 . 256737627 .
15. Nielsen R, Christensen EI, Birn H . Megalin and cubilin in proximal tubule protein reabsorption: from experimental models to human disease . Kidney International . 89 . 1 . 58–67 . January 2016 . 26759048 . 10.1016/j.kint.2015.11.007 . free .
16. Oleinikov AV, Zhao J, Makker SP . Cytosolic adaptor protein Dab2 is an intracellular ligand of endocytic receptor gp600/megalin . The Biochemical Journal . 347 . Pt 3 . 613–621 . May 2000 . 10769163 . 1220996 . 10.1042/0264-6021:3470613 .
17. Larsson M, Hjälm G, Sakwe AM, Engström A, Höglund AS, Larsson E, Robinson RC, Sundberg C, Rask L . 6 . Selective interaction of megalin with postsynaptic density-95 (PSD-95)-like membrane-associated guanylate kinase (MAGUK) proteins . The Biochemical Journal . 373 . Pt 2 . 381–391 . July 2003 . 12713445 . 1223512 . 10.1042/BJ20021958 .
18. Lou X, McQuistan T, Orlando RA, Farquhar MG . GAIP, GIPC and Galphai3 are concentrated in endocytic compartments of proximal tubule cells: putative role in regulating megalin's function . Journal of the American Society of Nephrology . 13 . 4 . 918–927 . April 2002 . 11912251 . 10.1681/ASN.V134918 . free .
19. Orlando RA, Farquhar MG . Functional domains of the receptor-associated protein (RAP) . Proceedings of the National Academy of Sciences of the United States of America . 91 . 8 . 3161–3165 . April 1994 . 7512726 . 43535 . 10.1073/pnas.91.8.3161 . free . 1994PNAS...91.3161O .
20. Nagai M, Meerloo T, Takeda T, Farquhar MG . The adaptor protein ARH escorts megalin to and through endosomes . Molecular Biology of the Cell . 14 . 12 . 4984–4996 . December 2003 . 14528014 . 284800 . 10.1091/mbc.E03-06-0385 .
21. Patrie KM, Drescher AJ, Goyal M, Wiggins RC, Margolis B . The membrane-associated guanylate kinase protein MAGI-1 binds megalin and is present in glomerular podocytes . Journal of the American Society of Nephrology . 12 . 4 . 667–677 . April 2001 . 11274227 . 10.1681/ASN.V124667 . free .
22. Gotthardt M, Trommsdorff M, Nevitt MF, Shelton J, Richardson JA, Stockinger W, Nimpf J, Herz J . 6 . Interactions of the low density lipoprotein receptor gene family with cytosolic adaptor and scaffold proteins suggest diverse biological functions in cellular communication and signal transduction . The Journal of Biological Chemistry . 275 . 33 . 25616–25624 . August 2000 . 10827173 . 10.1074/jbc.M000955200 . free .
23. Petersen HH, Hilpert J, Militz D, Zandler V, Jacobsen C, Roebroek AJ, Willnow TE . Functional interaction of megalin with the megalinbinding protein (MegBP), a novel tetratrico peptide repeat-containing adaptor molecule . Journal of Cell Science . 116 . Pt 3 . 453–461 . February 2003 . 12508107 . 10.1242/jcs.00243 . free .