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Endocytic receptor LRP2/megalin-of holoprosencephaly and renal Fanconi syndrome

August 2017
Pflügers Archiv - European Journal of Physiology 469(Pt 3)

August 2017
469(Pt 3)

DOI:10.1007/s00424-017-1992-0

Authors:

Megalin (or LRP2) is an endocytic receptor that plays a central role in embryonic development and adult tissue homeostasis. Loss of this receptor in congenital or acquired diseases results in multiple organ dysfunctions, including forebrain malformation (holoprosencephaly) and renal reabsorption defects (renal Fanconi syndrome). Here, we describe current concepts of the mode of receptor action that include co-receptors and a repertoire of different ligands, and we discuss how these interactions govern functional integrity of the kidney and the brain, and cause disease when defective.

LRP2/megalin is a member of the LDL receptor gene family. The structural organization of members of the family of low-density lipoprotein receptor-related proteins (LRPs) is depicted. Only receptors expressed in mammalian cell types are shown. Unifying structural elements in the extracellular domain of these endocytic receptors are clusters of complement-type repeats. They consist of basic modules of approximately 40 amino acid lengths each that fold into a octohedral cage around a central calcium ion and serve as calcium-dependent ligand-binding sites [27]. In addition, their extracellular domains harbor β-propellers, structural elements that function as an alternate substrate for the ligand-binding domain, and enable pH-dependent release of bound ligands in endosomes [78]. The localization of interaction motifs in the cytoplasmic tail of LRP2/megalin is highlighted in the inset to the right [33]. LDLR low-density lipoprotein receptor, VLDLR very low-density lipoprotein receptor

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Cellular fate of ligands internalized by LRP2/megalin. Endocytosis starts with a ligand binding to LRP2/megalin at the target cell surface. Receptor and cargo complexes internalize from the plasma membrane and move to early endosomes. A drop in luminal pH in endosomes disrupts binding of most ligands to LRP2/megalin, enabling the unliganded receptor to recycle back to the cell surface while the ligand traffics through late endosomal compartments to lysosomes for degradation. Some ligand/receptor complexes resist the low pH in early endosomes and recycle back to the cell surface, resulting in re-secretion of the ligand. A third pathway involves trafficking of receptor cargo from early endosomes to the basolateral membrane for transcytosis. Cytosolic adaptors binding to the cytoplasmic tail of LRP2/megalin to promote (+) or inhibit (−) distinct steps in receptor sorting are indicated. AP2 adaptor related protein complex 2, ARH autosomal recessive hypercholesterolemia, DAB2 Disabled homolog 2, GIPC PDZ domain containing family member 1

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LRP2/megalin-mediated endocytosis in epithelial cells of the renal proximal tubule. a Renal uptake and activation of 25-OH vitamin D3. Complexes of vitamin D-binding protein (DBP) and 25-OH vitamin D3 are cleared from the glomerular filtrate by association of DBP with LRP2/megalin on the apical surface of epithelial cells in the proximal tubule [70]. Internalized 25-OH vitamin D3/DBP complexes are delivered to early endosomes for separation from the receptor. From there, they move to lysosomes where DBP is degraded to enable transport of 25-(OH) vitamin D3 metabolites to mitochondria for hydroxylation [21, 35]. Produced 1,25-(OH)2 vitamin D3 is released into the interstitial fluid where it associates with circulating DBP. Intracellular transport of vitamin D metabolites is facilitated by intracellular vitamin D-binding proteins (IDBP)-1 and IDBP-2 [100]. b Genetic disorders of the endo-lysosomal system in the proximal convoluted tubule. LRP2/megalin and cubilin consitute the main endocytic receptors in the renal proximal tubule for clearance of low-molecular weight plasma proteins from the glomerular filtrate. Endocytic activity also requires the actions of chloride voltage-gated channel 5 (CLCN5) and Lowe oculocerebrorenal syndrome protein (OCRL). CLCN5 encodes ClC-5, a Cl⁻/H⁺ exchanger that facilitates acidification and trafficking of endosomal vesicles. OCRL encodes inositol polyphosphate-5-phosphatase, an enzyme required for proper vesicular trafficking between the intracellular compartments and the plasma membrane. Cystinosin is a cystine transporter in lysosomes. Several inherited disorders target components of the endo-lysosomal system in the proximal convoluted tubules including LRP2 (Donnai-Barrow syndrome; Mendelian inheritance in man (MIM) no. 222448), Cubilin (Imerslund-Graesbeck syndrome; MIM no. 261100), CLCN5 (Dent disease 1; MIM no. 300009), OCRL (Lowe syndrome, Dent disease 2; MIM no. 309000), and CNTS (nephropathic cystinosis; MIM no. 219800). Typically, these disorders lead to urinary loss of filtered solutes, and may progress to chronic kidney disease and renal failure

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LRP2/megalin-mediated endocytosis controls SHH trafficking and signaling. a LRP2/megalin promotes SHH signaling during forebrain patterning. During neurulation, LRP2/megalin is expressed on the apical surface of neuroepithelial cells where it forms a co-receptor complex with Patched 1 (PTCH1). SHH binding induces internalization of LRP2/megalin and PTCH1 complexes, alleviating PTCH1-dependent suppression of SMO, the effector of cellular SHH signal transduction. Internalized PTCH1 is subject to lysosomal degradation while SHH is recycled by LRP2/megalin to increase local morphogen concentrations in the neural tube. b LRP2/megalin antagonizes SHH signals in the eye. In the developing retina, LRP2 is expressed in non-pigmented ciliary epithelial cells in the ciliary marginal zone (CMZ). Binding of SHH to LRP2 results in cellular uptake and lysosomal degradation of the morphogen. As a consequence of absent SHH, PTCH1 continues to inhibit SMO in the CMZ and to impair morphogen action.

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INVITED REVIEW

Endocytic receptor LRP2/megalin—of holoprosencephaly

and renal Fanconi syndrome

Thomas E. Willnow

&Annabel Christ

Received: 24 April 2017 /Revised: 2 May 2017 /Accepted: 3 May 2017 /Published online: 11 May 2017

#Springer-Verlag Berlin Heidelberg 2017

Abstract Megalin (or LRP2) is an endocytic receptor that

plays a central role in embryonic development and adult tissue

homeostasis. Loss of this receptor in congenital or acquired

diseases results in multiple organ dysfunctions, including

forebrain malformation (holoprosencephaly) and renal reab-

sorption defects (renal Fanconi syndrome). Here, we describe

current concepts of the mode of receptor action that include

co-receptors and a repertoire of different ligands, and we dis-

cuss how these interactions govern functional integrity of the

kidney and the brain, and cause disease when defective.

Keywords Endocytosis .Forebrain development .

Proteinuria .Proximal convoluted tubules .Sonic hedgehog .

Vitami n D

Introduction

Endocytosis is the main mechanism that enables selective up-

take of macromolecules by cells. It is facilitated by endocytic

receptors on the cell surface that bind specific ligands and

mediate their internalization in endocytic vesicles that pinch

off the plasma membrane to deliver their cargo to intracellular

compartments. Receptor-mediated endocytosis serves to

control the extracellular concentration of metabolites, and it

supplements cells with bioactive molecules required for cellu-

lar metabolism or signal reception.

Likely, the cell type with the highest endocytic capacity in

the human body is the epithelial cell in the proximal convo-

luted tubule (PCT) of the kidney. PCT cells clear massive

amounts of proteins from the glomerular filtrate (approximate-

ly 7 g per day and kidney) producing an end-proximal tubular

fluid almost completely devoid of protein. The endocytic re-

ceptor in PCT cells responsible for achieving this formidable

task is a protein known as megalin or LRP2. In recent years,

studies in animal models and in patients have elucidated the

molecular details of the LRP2/megalin clearance pathway in

the kidney, as well as its relevance for functional integrity and

for inborn or acquired defects of the renal proximal tubule.

Here, we describe the role of LRP2/megalin as endocytic

receptor in the embryonic and the adult mammalian organism.

We mainly focus on its relevance for the resorptive capacity of

the healthy and the diseased kidney, but we also discuss what

studies in the kidney have told us about equally important

functions of this receptor in other organs.

LRP2/megalin is a receptor of the LDL receptor gene

family

LRP2/megalin was initially identified as autoantigen in an ex-

perimental model of membranous nephropathy, called Heymann

nephritis [40]. In Heymann nephritis, immunization of rats with

crude renal membrane preparations resulted in extensive im-

mune deposits in the glomerular basement membrane, mimick-

ing glomerulonephritis, an inflammatory process damaging the

human kidney. Subsequent efforts localized the major

autoantigen in Heymann nephritis to the luminal surface of the

PCT cells and resulted in the cloning of a giant 600 kDa

protein-termed megalin (or gp330) [79].Basedonitsstructural

This article is part of the special issue on Functional Anatomy of the

Kidney in Health and Disease in Pflügers Archiv –European Journal of

Physiology

*Thomas E. Willnow

willnow@mdc-berlin.de

*Annabel Christ

annabel.christ@mdc-berlin.de

Max-Delbrueck Center for Molecular Medicine, Robert-Roessle-Str.

10, 13125 Berlin, Germany

Pflugers Arch - Eur J Physiol (2017) 469:907–916

DOI 10.1007/s00424-017-1992-0

Content courtesy of Springer Nature, terms of use apply. Rights reserved.

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Article

Dec 2014

Background/aims: Experimental and clinical evidence suggested that monocyte chemoattractant protein-1 (MCP-1/CCL2) has a role in the development of interstitial inflammation and renal failure in polycystic kidney disease (PKD). We investigated whether bindarit, an inhibitor of MCP-1/CCL2 synthesis, could influence the evolution of PKD in PCK rats. Methods: PCK rats were treated from 5 to 15 weeks of age with vehicle or bindarit. Sprague-Dawley rats served as control. For in vitro studies, murine podocytes were exposed to albumin with or without bindarit. Results: MCP-1 mRNA was upregulated in the kidney of PCK rats and reduced by bindarit. Treatment limited overexpression of MCP-1 protein by epithelial cells of dilated tubules and cysts, and interstitial inflammatory cells. Excessive renal accumulation of monocytes/macrophages was lowered by bindarit by 41%. Serum creatinine slightly increased in PCK rats on vehicle and was similar to controls after bindarit. Kidney and liver cysts were not affected by treatment. Bindarit significantly reduced progressive proteinuria of PCK rats. The antiproteinuric effect was associated with the restoration of the defective nephrin expression in podocytes of PCK rats. Bindarit limited podocyte foot process effacement and ameliorated slit diaphragm frequency. In cultured podocytes, bindarit reduced MCP-1 production in response to albumin and inhibited albumin-induced cytoskeletal remodeling and cell migration. Conclusion: This study showed that although bindarit did not prevent renal cyst growth, it limited interstitial inflammation and renal dysfunction and reduced proteinuria in PKD. Thus, bindarit could be considered a therapeutic intervention complementary to therapies specifically acting to block renal cyst growth.

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