Defective forebrain development in mice lacking gp330/megalin

Cholesterol metabolism in the brain

The central nervous system accounts for only 2% of the whole body mass but contains almost a quarter of the unesterified cholesterol present in the whole individual. This sterol is largely present in two pools comprised of the cholesterol in the plasma membranes of glial cells and neurons and the cholesterol present in the specialized membranes of myelin. From 0.02% (human) to 0.4% (mouse) of the cholesterol in these pools turns over each day so that the absolute flux of sterol across the brain is only approximately 0.9% as rapid as the turnover of cholesterol in the whole body of these respective species. The input of cholesterol into the central nervous system comes almost entirely from in situ synthesis, and there is currently little evidence for the net transfer of sterol from the plasma into the brain of the fetus, newborn or adult. In the steady state in the adult, an equivalent amount of cholesterol must move out of the brain and this output is partly accounted for by the formation and excretion of 24S-hydroxycholesterol. This cholesterol turnover across the brain is increased in neurodegenerative disorders such as Alzheimer's disease and Niemann-Pick type C disease. Indirect evidence suggests that large amounts of cholesterol also turn over among the glial cells and neurons within the central nervous system during brain growth and neuron repair and remodelling. This internal recycling of sterol may involve ligands such as apolipoproteins E and AI, and one or more membrane transport proteins such as members of the low density lipoprotein receptor family. Changes in cholesterol balance across the whole body may, in some way, cause alterations in sterol recycling and apolipoprotein E expression within the central nervous system, which, in turn, may affect neuron and myelin integrity. Further elucidation of the processes controlling these events is very important to understand a variety of neurodegenerative disorders.

The membrane-associated guanylate kinase protein MAGI-1 binds megalin and is present in glomerular podocytes

The transmembrane endocytic receptor glycoprotein 330/megalin (hereafter referred to as megalin) is localized to the apical membrane domain of epithelial cells, where it is involved in the uptake of proteins from extracellular sources. The cytoplasmic domain of megalin contains amino acid motifs that have the potential to bind to other proteins, which may influence its localization or function. The yeast two-hybrid system was used to search for proteins that bind to the cytoplasmic tail of megalin, and a protein fragment from a mouse embryonic cDNA library that contained a single PDZ domain was identified. This protein, which was named glycoprotein 330-associated protein (GASP), appears to be a truncated mouse counterpart of the human and rat proteins atrophin-1-interacting protein-1 and synaptic scaffolding molecule, respectively. The interaction of GASP with megalin is mediated by the PDZ domain of GASP binding to the DSDV motif found at the carboxyl-terminus of megalin. A mutant version of megalin that lacks the terminal valine is unable to bind to GASP, illustrating the PDZ domain-dependent interaction between these two proteins. A close homolog of GASP, i.e., membrane-associated guanylate kinase with inverted orientation-1 (MAGI-1), is more ubiquitous in its tissue distribution (including kidney) and is also able to specifically bind to megalin via its fifth PDZ domain. Immunofluorescence studies of adult kidney revealed that MAGI-1 is expressed in the glomerulus of the kidney, in a manner that parallels the expression of the podocyte-specific protein glomerular epithelial protein 1. Western analysis of endogenous MAGI-1 from glomerular preparations suggests that it is associated with the cytoskeleton and seems to be expressed in a different form, compared with cell line-derived endogenous MAGI-1. The association of megalin with MAGI-1 may allow the assembly of a multiprotein complex, in which megalin may serve a nonendocytic function in glomerular podocytes.

Megalin and cubilin: synergistic endocytic receptors in renal proximal tubule

The multiligand, endocytic receptors megalin and cubilin are colocalized in the renal proximal tubule. They are heavily expressed in the apical endocytic apparatus. Megalin is a 600-kDa transmembrane protein belonging to the low-density lipoprotein-receptor family. The cytoplasmic tail contains three NPXY motifs that mediate the clustering in coated pits and are possibly involved in signaling functions. Cubilin, also known as the intestinal intrinsic factor-cobalamin receptor, is a 460-kDa receptor with no transmembrane domain and no known signal for endocytosis. Because the two receptors bind each other with high affinity and colocalize in several tissues, it is highly conceivable that megalin mediates internalization of cubilin and its ligands. Both receptors are important for normal tubular reabsorption of proteins, including albumin. Among the proteins normally filtered in the glomeruli, cubilin has been shown to bind albumin, immunoglobulin light chains, and apolipoprotein A-I. The variety of filtered ligands identified for megalin include vitamin-binding proteins, hormones, enzymes, apolipoprotein H, albumin, and beta(2)- and alpha(1)-microglobulin. Loss of these proteins and vitamins in the urine of megalin-deficient mice illustrates the physiological importance of this receptor.

Functional characterization of rat gp600/megalin promoter: combination of proximal Sp1 site and JCV repeat is important in rat gp600/megalin promoter activation

Gp600/megalin is an endocytic receptor belonging to the low-density lipoprotein receptor family. Up or down regulation of this protein were observed in certain disease states. To understand the mechanisms that control gp600/megalin gene expression, we cloned and functionally characterized a 738-bp fragment of the 5'-flanking region of rat gp600/megalin gene. A transcription start site was mapped to 33 bp downstream of TAGAAA sequence (TATA-like box). Multiple transcription factor binding sites were identified. Serial 5' deletions and transient transfection assays showed that the deletion fragment containing the Sp1 site proximal to the TATA-like box and a JCV repeat retained 80% of the promoter activity. Individual mutations of the proximal Sp1 site and JCV repeat reduced the promoter activity by 60 and 34% respectively. Double mutations of the proximal Sp1 site and JCV repeat produced a dramatic 80% reduction in the promoter activity. However, deletions and mutations or double mutations of other transcription factor binding sites in the promoter region had a minor effect on the promoter activity. These results indicate that the combination of proximal Sp1 site and the JCV repeat are necessary for activation of gp600/megalin expression. Moreover, Sp1 and Sp3 proteins interacted with the proximal and the distal Sp1 sites in the nuclear extracts of gp600/megalin expressing cell lines. TCF site seems to be involved in negative regulation of this promoter but no nuclear protein(s) were found to bind to this site. In addition, Ap2 site responsible for 28% promoter activity is able to bind two dominant unknown nuclear proteins. This functional characterization of the regulation of gp600/megalin gene is likely to advance the knowledge of the regulation of this gene in health and disease.

Genetic and teratogenic approaches to craniofacial development

Craniofacial malformations are the most common birth defects that occur in humans, with facial clefting representing the majority of these defects. Facial clefts can arise at any stage of development due to perturbations that alter the extracellular matrix as well as affect the patterning, migration, proliferation, and differentiation of cells. In this review, we focus on recent advances in the understanding of the developmental basis for facial clefting through the analysis of the effects of gene disruption experiments and treatments with teratogens in both chickens and mice. Specifically, we analyze the results of disruptions to genes such as Sonic hedgehog (Shh), epidermal growth factor receptor (EGFR), Distal-less (Dlx), and transforming growth factor beta 3 (TGFbeta3). We also describe the effects that teratogens such as retinoic acid, jervine, and cyclopamine have on facial clefting and discuss mechanisms for their action. In addition to providing insight into the bases for abnormal craniofacial growth, genetic and teratogenic techniques are powerful tools for understanding the normal developmental processes that generate and pattern the face.

The LDL receptor gene family: signaling functions during development

The traditional views regarding the biological functions of the low-density lipoprotein (LDL) receptor gene family have been revisited recently with new evidence that at least some of the members of this receptor family act as signal-transduction molecules. Known for their role in endocytosis, particularly of their namesake the LDLs, and for their role in the prevention of atherosclerosis, these receptors belong to an ancient family with numerous ligands, effector molecules and functions. Recent evidence implicates this family of receptors in diverse signaling pathways, long-term potentiation and neuronal degeneration.

Transcription factors Zic1 and Zic2 bind and transactivate the apolipoprotein E gene promoter

We have used the yeast one-hybrid system to identify transcription factors that bind to specific sequences in proximal regions of the apolipoprotein E gene promoter. The sequence between -163 and -124, that has been previously defined as a functional promoter element, was used as a bait to screen a human brain cDNA library. Ten cDNA clones that encoded portions of the human Zic1 (five clones) and Zic2 (five clones) transcription factors were isolated. Electrophoretic mobility shift assays confirmed the presence of a binding site for Zic1 and Zic2 in the -136/-125 region. Displacement of binding with oligonucleotides derived from adjacent sequences within the APOE promoter revealed the existence of two additional Zic-binding sequences in this promoter. These sequences were identified by electrophoretic mobility shift assays and mutational analysis in regions -65/-54 and -185/-174. Cotransfection of Zic1 and Zic2 expression vector and different APOE promoter-luciferase reporter constructs in U87 glioblastoma cell line showed that the three binding sites partially contributed to the trans-stimulation of the luciferase reporter. Ectopic expression of Zic1 and Zic2 in U87 cells also trans-stimulated the expression of the endogenous gene, increasing the amount of apolipoprotein E produced by glial cells. These data indicate that Zic proteins might contribute to the transcriptional activity of the apolipoprotein E gene and suggest that apolipoprotein E could mediate some of the developmental processes in which Zic proteins are involved.

The roles of cubilin and megalin, two multiligand receptors, in proximal tubule function: possible implication in the progression of renal disease

Proteins that have not been retained by the glomerulus are reabsorbed in the proximal tubule by endocytosis, a process that involves binding at the apical pole of the tubule cell, vesicular internalization and subsequent lysosomal degradation. Data presented in this review indicate that the initial recognition step involves two high molecular weight proteins, megalin and cubilin, which have multiligand properties and can therefore account for the wide variety of proteins reabsorbed. Given the potential importance of transepithelial protein traffic in the induction of interstitial fibrosis, the identification of these receptors may have implications in the progression of acute or chronic renal disease and may provide a target for therapeutic intervention.

Megalin in thyroid physiology and pathology

Megalin, a member of the low density lipoprotein endocytic receptor family, is expressed on the apical surface of thyroid epithelial cells, directly facing the follicle lumen, where colloid is stored in high concentrations. Studies in vivo and with cultured thyroid cells have provided evidence that megalin expression on thyroid cells is TSH-dependent. Thyroglobulin (Tg), the major protein component of the colloid and the precursor of thyroid hormones, binds to megalin with high affinity and megalin mediates in part its uptake by thyrocytes. Tg internalized by megalin avoids the lysosomal pathway and is delivered by transepithelial transport (transcytosis) to the basolateral membrane of thyrocytes, from which it is released into the bloodstream. This process competes with pathways leading to thyroid hormone release from Tg molecules, which occurs following internalization of Tg molecules from the colloid by other means of uptake (fluid phase endocytosis or endocytosis mediated by low affinity receptors) that result in proteolytic cleavage in the lyosomes. During transcytosis of Tg, a portion of megalin (secretory component) remains complexed with Tg and enters the circulation, where its detection may serve as a tool to identify the origin of serum Tg in patients with thyroid diseases. Tg endocytosis via megalin is facilitated by the interaction of Tg with cell surface heparan sulfate proteoglycans, which occurs via a carboxyl terminal heparin binding site of Tg functionally related with a major megalin binding site. Although autoantibodies against megalin can be found in the serum of approximately 50% of patients with autoimmune thyroiditis, a role of megalin in this and other thyroid diseases remains to be established.

Cholesterol modification of proteins

The demonstration over 30 years ago that inhibitors of cholesterol biosynthesis disrupt animal development suggested an intriguing connection between fundamental cellular metabolic processes and the more global processes of embryonic tissue patterning. Adding a new dimension to this relationship is the more recent finding that the Hedgehog family of tissue patterning factors are covalently modified by cholesterol. Here we review the mechanism of the Hedgehog autoprocessing reaction that results in this modification, and compare this reaction to that undergone by other autoprocessing proteins. We also discuss the biological consequences of cholesterol modification, in particular the use of cholesterol as a molecular handle in the spatial deployment of the protein signal in developing tissues. Finally, the developmental consequences of chemical and genetic disruption of cholesterol homeostasis are summarized, along with the potential importance of cholesterol-rich lipid rafts in production of and response to the Hh signal.

Evidence for the role of megalin in renal uptake of transthyretin

The kidney is a major organ for uptake of the thyroid hormone thyroxine (T(4)) and its conversion to the active form, triiodothyronine. In the plasma, one of the T(4) carriers is transthyretin (TTR). In the present study we observed that TTR, the transporter of both T(4) and retinol-binding protein, binds to megalin, the multiligand receptor expressed on the luminal surface of various epithelia including the renal proximal tubules. In the kidney, megalin plays an important role in tubular uptake of macromolecules filtered through the glomerulus. To evaluate the importance of megalin for renal uptake of TTR, we performed binding/uptake assays using immortalized rat yolk sac cells with high expression levels of megalin. Radiolabeled TTR, free as well as in complex with thyroxine or retinol-binding protein, was rapidly taken up by the cells, and the uptake was strongly inhibited by a polyclonal megalin antibody and by the receptor-associated protein, a chaperone-like protein inhibiting ligand binding to megalin. In cell culture, different TTR mutations presented different levels of cell association and degradation, suggesting that the structure of TTR is important for megalin recognition. Both the apo form and the T(4)-bound form were taken up by the cells. Analysis of urine from patients with Dent's disease, a renal tubular disorder that alters receptor-mediated endocytic reabsorption of proteins, identified TTR as an abundant excreted protein. Furthermore, analysis of kidney sections of megalin-deficient mice revealed no immunohistochemical TTR labeling in intracellular vesicles in the proximal tubule cells when compared with wild type control littermates. Taken together, the present data indicate that TTR represents a novel megalin ligand of importance in the thyroid hormone homeostasis.

Apolipoprotein E receptors: linking brain development and Alzheimer's disease

Alzheimer's disease is a debilitating neurodegenerative disorder that afflicts an increasing part of our ageing population. An isoform of apolipoprotein E, a protein that mediates the transport of lipids and cholesterol in the circulatory system, predisposes carriers of this allele to the common late-onset form of the disease. How this protein is related to a neurodegenerative disorder is an enigma. Mounting evidence indicates that apolipoprotein E receptors, which are abundantly expressed in most neurons in the central nervous system, also fulfill critical functions during brain development and may profoundly influence the pathogenesis of Alzheimer's disease.

Increased expression of cytoplasmic tail-containing form of gp600/megalin in active Heymann nephritis

Active Heymann nephritis of rat, an autoimmune glomerular disease, is a model of human membranous glomerulonephropathy. The major target autoantigen of this disease is a 600 kD renal glycoprotein, variously named gp600/gp330/megalin/LRP2. It has been shown previously that there is a marked increase in the gp600/megalin mRNA level in glomeruli of rats with this disease. A rabbit antibody was prepared against a peptide located at the very end of the cytoplasmic tail of rat gp600/megalin. Affinity purified anti-peptide antibody was monospecific and reacted only with gp600/megalin on the immunoblots of total kidney proteins. Immunofluorescence microscopy on frozen kidney sections probed with this antibody revealed the presence of the cytoplasmic tail-containing form of gp600/megalin in both proximal tubules and glomeruli. Expression in glomeruli of normal kidneys was less than in tubules but was clearly detectable. In active Heymann nephritis kidneys, expression of this form was increased most remarkably in glomeruli. These results, showing increased expression of the cytoplasmic tail-containing form in active Heymann nephritis glomeruli, contradict previous reports of its absence. This is the first study to show that there is increased expression of the cytoplasmic tail-containing form of gp600/megalin in the glomeruli of rats with this disease and its role in active Heymann nephritis is discussed.

RSH/Smith-Lemli-Opitz syndrome: a multiple congenital anomaly/mental retardation syndrome due to an inborn error of cholesterol biosynthesis

The RSH/Smith-Lemli-Opitz syndrome (RSH/SLOS) is an autosomal recessive multiple congenital anomaly/mental retardation syndrome caused by an inborn error of cholesterol biosynthesis. The RSH/SLOS phenotypic spectrum is broad; however, typical features include microcephaly, ptosis, a small upturned nose, micrognathia, postaxial polydactaly, second and third toe syndactaly, genital anomalies, growth failure, and mental retardation. RSH/SLOS is due to a deficiency of the 3beta-hydroxysterol Delta(7)-reductase, which catalyzes the reduction of 7-dehydrocholesterol (7-DHC) to cholesterol. This inborn error of cholesterol biosynthesis results in elevated serum and tissue 7-DHC levels. The 3beta-hydroxysterol Delta(7)-reductase gene (DHCR7) maps to chromosome 11q12-13, and to date 66 different mutations of this gene have been identified in RSH/SLOS patients. Identification of the biochemical basis of RSH/SLOS has led to development of therapeutic regimens based on dietary cholesterol supplementation and has increased our understanding of the role cholesterol plays during embryonic development.

Genetic approaches to understanding brain development: holoprosencephaly as a model

Holoprosencephaly (HPE) is the most common major developmental defect of the forebrain in humans. Clinical expression is variable, ranging from a small brain with a single cerebral ventricle and cyclopia to clinically unaffected carriers in familial HPE. Significant etiologic heterogeneity exists in HPE and includes both genetic and environmental causes. Defects in the cell signaling pathway involving the Sonic Hedgehog (SHH) gene, as well as defects in the cholesterol biosynthesis have been shown to cause HPE in humans. More recently, HPE genes from additional signaling pathways have been identified. These discoveries and current genetic approaches serve as a paradigm for studying normal and abnormal brain morphogenesis. MRDD Research Reviews 6:15-21, 2000.

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 members of the low density lipoprotein (LDL) receptor gene family bind a broad spectrum of extracellular ligands. Traditionally, they had been regarded as mere cargo receptors that promote the endocytosis and lysosomal delivery of these ligands. However, recent genetic experiments in mice have revealed critical functions for two LDL receptor family members, the very low density lipoprotein receptor and the apoE receptor-2, in the transmission of extracellular signals and the activation of intracellular tyrosine kinases. This process regulates neuronal migration and is crucial for brain development. Signaling through these receptors requires the interaction of their cytoplasmic tails with the intracellular adaptor protein Disabled-1 (DAB1). Here, we identify an extended set of cytoplasmic proteins that might also participate in signal transmission by the LDL receptor gene family. Most of these novel proteins are adaptor or scaffold proteins that contain PID or PDZ domains and function in the regulation of mitogen-activated protein kinases, cell adhesion, vesicle trafficking, or neurotransmission. We show that binding of DAB1 interferes with receptor internalization suggesting a mechanism by which signaling through this class of receptors might be regulated. Taken together, these findings imply much broader physiological functions for the LDL receptor family than had previously been appreciated. They form the basis for the elucidation of the molecular pathways by which cells respond to the diversity of ligands that bind to these multifunctional receptors on the cell surface.

Cubilin is an albumin binding protein important for renal tubular albumin reabsorption

Using affinity chromatography and surface plasmon resonance analysis, we have identified cubilin, a 460-kDa receptor heavily expressed in kidney proximal tubule epithelial cells, as an albumin binding protein. Dogs with a functional defect in cubilin excrete large amounts of albumin in combination with virtually abolished proximal tubule reabsorption, showing the critical role for cubilin in the uptake of albumin by the proximal tubule. Also, by immunoblotting and immunocytochemistry we show that previously identified low-molecular-weight renal albumin binding proteins are fragments of cubilin. In addition, we find that mice lacking the endocytic receptor megalin show altered urinary excretion, and reduced tubular reabsorption, of albumin. Because cubilin has been shown to colocalize and interact with megalin, we propose a mechanism of albumin reabsorption mediated by both of these proteins. This process may prove important for understanding interstitial renal inflammation and fibrosis caused by proximal tubule uptake of an increased load of filtered albumin.

Cytosolic adaptor protein Dab2 is an intracellular ligand of endocytic receptor gp600/megalin

gp600/megalin, an endocytic receptor, belongs to the low-density lipoprotein receptor family. It is most abundant in the renal proximal tubular cells, where it is implicated in the reabsorption of a number of molecules filtered through the glomerulus. The cytoplasmic tail (CT) of gp600/megalin contains a number of sequence similarities, which indicate that gp600/megalin might be involved in signal transduction. To find intracellular proteins that would interact with the gp600/megalin CT, a human kidney cDNA library was screened by using the yeast two-hybrid system. The phosphotyrosine interaction domain (PID) of the Disabled protein 2 (Dab2), a mammalian structural analogue of Drosophila Disabled, was found to bind to the gp600/megalin CT in this system. The interaction between these two proteins was confirmed by a binding assay in vitro and by the co-immunoprecipitation of both proteins from renal cell lysates. The gp600/megalin CT contains three PsiXNPXY motifs (in which Psi represents a hydrophobic residue) that are potentially able to interact with PID. Analysis of the CT deletion and point-mutation variants of gp600/megalin by the two-hybrid system revealed that the third PsiXNPXY motif is most probably involved in this interaction. Dab2 is a mitogen-responsive phosphoprotein thought to be an adaptor molecule involved in signal transduction, and a suggested negative regulator of cell growth. Dab2 is the first intracellular ligand identified for gp600/megalin; gp600/megalin is the first known transmembrane receptor that interacts with the cytosolic protein Dab2. We speculate that their interaction might involve gp600/megalin in signal transduction pathways or might mediate the intracellular trafficking of this receptor.

The Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is one of the archetypical multiple congenital malformation syndromes. The recent discovery of the biochemical cause of SLOS and the subsequent redefinition of SLOS as an inborn error of cholesterol metabolism have led to important new treatment possibilities for affected patients. Moreover, the recent recognition of the important role of cholesterol in vertebrate embryogenesis, especially with regard to the hedgehog embryonic signalling pathway and its effects on the expression of homeobox genes, has provided an explanation for the abnormal morphogenesis in the syndrome. The well known role of cholesterol in the formation of steroid hormones has also provided a possible explanation for the abnormal behavioural characteristics of SLOS.

Role of cholesterol in embryonic development

We showed previously that 3 distal inhibitors of cholesterol synthesis are highly teratogenic in rats. AY 9944 and BM 15766 inhibit 7-dehydrocholesterol reductase, which catalyzes the last step of cholesterol synthesis, and triparanol inhibits Delta(24)-dehydrocholesterol reductase, which catalyzes the last step in another pathway. These molecules cause holoprosencephalic brain anomalies. Under certain experimental conditions, other anomalies (of the limbs and male genitalia) are also observed. Assays performed by gas chromatography-mass spectrometry (GC-MS) show hypocholesterolemia and an accumulation of precursors. These data indicate that this animal model can be considered a model of Smith-Lemli-Opitz syndrome. Smith-Lemli-Opitz syndrome is a recessive autosomal genetic disease characterized by malformations (microcephaly, corpus callosum agenesis, holoprosencephaly, and mental retardation), male pseudohermaphroditism, finger anomalies, and failure to thrive. The syndrome has been attributed to a deficit in 7-dehydrocholesterol reductase. As assayed by GC-MS, the sterol status of these patients indicates severe hypocholesterolemia and an accumulation of precursors: 7-dehydrocholesterol, 8-dehydrocholesterol, and oxidized derivatives. The presence of 7-dehydrocholesterol in the serum of patients is pathognomonic of the disease. The developmental gene Shh (sonic hedgehog) plays a key role in brain, limb, and genital development; it was shown recently that the Shh protein has to be covalently linked to cholesterol to be active. This is the first time that a posttranslational function has been attributed to cholesterol. There is an obvious relation between Shh dysfunction and the malformations observed in our experiments and in patients with Smith-Lemli-Opitz syndrome. However, the exact relation remains to be clarified. It is clear, however, that the role of cholesterol in embryonic development must be taken into account.

Cholesterol in signal transduction

Membrane cholesterol impinges on signal transduction in several ways, which is highlighted in particular by the Hedgehog signaling pathway. In Hedgehog signaling, cholesterol is important for ligand biogenesis, as well as for signal transduction in receiving cells. Hedgehog ligands are post-translationally modified by cholesterol, and the Hedgehog receptor, Patched, is structurally similar to the Niemann-Pick C1 protein, which functions in intracellular lipid transport. Although the exact role of cholesterol in Hedgehog signal transduction remains elusive and is probably multifaceted, studies over the past year have implicated raft membrane subdomains, cholesterol transport and a link between protein and lipid trafficking in endocytic compartments.

Cubilin, a high-density lipoprotein receptor

The metabolism of HDL particles is a complex biological process involving various regulating factors in plasma and different cellular receptors. In addition to the well-established scavenger receptor BI-mediated selective HDL-cholesteryl ester uptake in liver and steroidogenic tissues, evidence has been provided that HDL also undergoes holoparticle endocytosis in different tissues. Recently, a novel receptor expressed in various absorptive epithelia was disclosed as a high affinity receptor for endocytosis of HDL and lipid-poor apolipoprotein AI. This receptor, designated cubilin, may play an important role in the renal clearance of filterable apolipoprotein AI/HDL and in the maternal-fetal transport of cholesterol.

Receptor-associated protein is important for normal processing of megalin in kidney proximal tubules

The receptor-associated protein (RAP) has been identified as a chaperone regulating the expression and processing of the LDL receptor-related protein. RAP also binds to the related 600-kD multiligand endocytic receptor megalin expressed in many absorptive epithelia including renal proximal tubule. The present study examines the effect of RAP gene disruption on megalin expression and subcellular distribution in the proximal tubule as well as the effect on tubular protein reabsorption. It is shown that RAP is important for the normal expression and function of megalin. Megalin expression was reduced to approximately 23% estimated by immunoblotting and supported by immunocytochemistry and by the amount of megalin recovered by RAP affinity chromatography. Light- and electron microscope immunocytochemistry as well as analyses on separated membrane fractions showed significant changes in the subcellular distribution of megalin. A significant reduction in the normal brush border labeling was observed in association with increased labeling of rough endoplasmic reticulum and the smooth paramembranous endoplasmic reticulum along the basolateral membranes. RAP deficiency was associated with changes in urinary protein composition, enabling the identification of alpha-amylase as a new ligand for megalin. In addition, an increased excretion of vitamin D-binding protein, a recently identified ligand to megalin, was observed supporting changes in tubular protein reabsorption. The present data show that RAP is of crucial importance for normal processing and function of megalin, suggesting a chaperone-like function of this protein in the kidney proximal tubule.

Towards a greater understanding of the pathogenesis of holoprosencephaly

Holoprosencephaly is a malformation of the cerebral hemispheres resulting in the absence of the inter-hemispheric fissure along with other defects of brain development. Frequently midline defects of the craniofacial structures are also present. This malformation sequence has been of interest for many years because of the well recognized genetic and environmental pathogeneses, although the molecular pathogenesis remained elusive. Recent studies have begun clarifying the molecular pathogenesis of holoprosencephaly. Herein is reviewed the syndromes associated with holoprosencephaly, the pathology of this disorder, genetic and environment factors, and a current understanding of the molecular pathogenesis of this disorder.

Embryonic lethality and defective neural tube closure in mice lacking squalene synthase

Squalene synthase (SS) catalyzes the reductive head-to-head condensation of two molecules of farnesyl diphosphate to form squalene, the first specific intermediate in the cholesterol biosynthetic pathway. We used gene targeting to knock out the mouse SS gene. The mice heterozygous for the mutation (SS+/-) were apparently normal. SS+/- mice showed 60% reduction in the hepatic mRNA levels of SS compared with SS+/+ mice. Consistently, the SS enzymatic activities were reduced by 50% in the liver and testis. Nevertheless, the hepatic cholesterol synthesis was not different between SS+/- and SS+/+ mice, and plasma lipoprotein profiles were not different irrespective of the presence of the low density lipoprotein receptor, indicating that SS is not a rate-limiting enzyme in the cholesterol biosynthetic pathway. The mice homozygous for the disrupted SS gene (SS-/-) were embryonic lethal around midgestation. E9.5-10.5 SS-/- embryos exhibited severe growth retardation and defective neural tube closure. The lethal phenotype was not rescued by supplementing the dams either with dietary squalene or cholesterol. We speculate that cholesterol is required for the development, particularly of the nervous system, and that the chorioallantoic circulatory system is not mature enough to supply the rapidly growing embryos with maternal cholesterol at this developmental stage.

Megalin knockout mice as an animal model of low molecular weight proteinuria

Megalin is an endocytic receptor expressed on the luminal surface of the renal proximal tubules. The receptor is believed to play an important role in the tubular uptake of macromolecules filtered through the glomerulus. To elucidate the role of megalin in vivo and to identify its endogenous ligands, we analyzed the proximal tubular function in mice genetically deficient for the receptor. We demonstrate that megalin-deficient mice exhibit a tubular resorption deficiency and excrete low molecular weight plasma proteins in the urine (low molecular weight proteinuria). Proteins excreted include small plasma proteins that carry lipophilic compounds including vitamin D-binding protein, retinol-binding protein, alpha(1)-microglobulin and odorant-binding protein. Megalin binds these proteins and mediates their cellular uptake. Urinary loss of carrier proteins in megalin-deficient mice results in concomitant loss of lipophilic vitamins bound to the carriers. Similar to megalin knockout mice, patients with low molecular weight proteinuria as in Fanconi syndrome are also shown to excrete vitamin/carrier complexes. Thus, these results identify a crucial role of the proximal tubule in retrieval of filtered vitamin/carrier complexes and the central role played by megalin in this process.

Molecular mechanisms of holoprosencephaly

Holoprosencephaly (HPE) is the most common developmental defect of the forebrain in humans. Several distinct human genes for holoprosencephaly have now been identified. They include Sonic hedgehog (SHH), ZIC2, and SIX3. Many additional genes involved in forebrain development are rapidly being cloned and characterized in model vertebrate organisms. These include Patched (Ptc), Smoothened (Smo), cubitus interuptus (ci)/Gli, wingless (wg/Wnt, decapentaplegic (dpp)/BMP, Hedgehog interacting protein (Hip), nodal, Smads, One-eyed pinhead (Oep), and TG-Interacting Factor (TGIF). However, further analysis is needed before their roles in HPE can be established. Here we present an overview of the presently known genes causing human holoprosencephaly and describe candidate genes involved in forebrain development identified in other systems. A model is discussed for how these genes may interact within and between several different signaling pathways to direct the formation of the forebrain.

A head-activator binding protein is present in hydra in a soluble and a membrane-anchored form

The neuropeptide head activator plays an important role for proliferation and determination of stem cells in hydra. By affinity chromatography a 200 kDa head-activator binding protein, HAB, was isolated from the multiheaded mutant of Chlorohydra viridissima. Partial amino acid sequences were used to clone the HAB cDNA which coded for a receptor with a unique alignment of extracellular modules, a transmembrane domain, and a short carboxy-terminal cytoplasmic tail. A mammalian HAB homologue with identical alignment of these modules is expressed early in brain development. Specific antibodies revealed the presence of HAB in hydra as a transmembrane receptor, but also as secreted protein, both capable of binding head activator. Secretion of HAB during regeneration and expression in regions of high determination potential hint at a role for HAB in regulating the concentration and range of action of head activator.

The mammalian low-density lipoprotein receptor family

The low-density lipoprotein (LDL) receptor (LDL-R) family consists of cell-surface receptors that recognize extracellular ligands and internalize them for degradation by lysosomes. The LDL-R is the prototype of this family, which also contains very-low-density lipoprotein receptors (VLDL-R), apolipoprotein E receptor 2, LRP, and megalin. The family members contain four major structural modules: the cysteine-rich complement-type repeats, epidermal growth factor precursor-like repeats, a transmembrane domain, and a cytoplasmic domain. Each structural module serves distinct and important functions. These receptors bind several structurally dissimilar ligands. It is proposed that instead of a primary sequence, positive electrostatic potential in different ligands constitutes a receptor binding domain. This family of receptors plays crucial roles in various physiologic functions. LDL-R plays an important role in cholesterol homeostasis. Mutations cause familial hypercholesterolemia and premature coronary artery disease. LDL-R-related protein plays an important role in the clearance of plasma-activated alpha 2-macroglobulin and apolipoprotein E-enriched lipoproteins. It is essential for fetal development and has been associated with Alzheimer's disease. Megalin is the major receptor in absorptive epithelial cells of the proximal tubules and an antigenic determinant for Heymann nephritis in rats. Mutations in a chicken homolog of VLDL-R cause female sterility and premature atherosclerosis. This receptor is not expressed in liver tissue; however, transgenic expression of VLDL-R in liver corrects hypercholesterolemia in experiment animals, which suggests that it can be a candidate for gene therapy for various hyperlipidemias. The functional importance of individual receptors may lie in their differential tissue expression. The regulation of expression of these receptors occurs at the transcriptional level. Expression of the LDL-R is regulated by intracellular sterol levels involving novel membrane-bound transcription factors. Other members of the family are not regulated by sterols. All the members are, however, regulated by hormones and growth factors, but the mechanisms of regulation by hormones have not been elucidated. Studies of these receptors have provided important insights into receptor structure-function and mechanisms of ligand removal and catabolism. It is anticipated that increased knowledge about the LDL-R family members will open new avenues for the treatment of many disorders.

Steroid hormones: Interactions with membrane-bound receptors

Steroid hormones are generally thought to pass easily across a plasma membrane into a cell, interacting once inside with soluble nuclear receptors, but recent experiments have demonstrated the importance of membrane-bound receptors in mediating the activity and the metabolism of steroid hormones.

Expression and regulation of LRP-2/megalin in epithelial cells lining the efferent ducts and epididymis during postnatal development

Low density lipoprotein receptor-related protein-2/megalin (LRP-2) is a receptor belonging to the low density lipoprotein receptor family that mediates endocytosis and lysosomal degradation of a variety of ligands including apolipoprotein J (Apo J)/clusterin/SGP-2. LRP-2 has been shown to be expressed regionally in the adult rat epididymis. In this study, we describe the pattern of expression of LRP-2 in the efferent ducts and epididymis during postnatal development of the rat and examine the role of testicular luminally derived substances on its expression. The expression of LRP-2 was analyzed immunocytochemically in tissues of normal animals ranging in age from postnatal day 7-90 and in 15-day-old efferent-duct-ligated animals sacrificed at later ages. In the efferent ducts, LRP-2 expression, appearing as a dense band on the apical surface of the nonciliated epithelial cells, was noted as early as day 7, well before the entry of sperm, Sertoli-cell-derived secretory products, and high levels of androgens. Efferent duct ligation studies further revealed that expression under this condition was comparable to controls at all later ages examined, suggesting that the factor regulating its expression was not a luminally derived testicular substance. In normal untreated animals, LRP-2 expression was not apparent at any of the ages examined in the proximal initial segment of the epididymis. By comparison, the distal initial segment, although having no LRP-2 expression from 7-15 days, showed expression in principal cells by day 21 which intensified at days 29 and 39. However, by day 49 and at later ages (56 and 90), LRP-2 immunoreactivity over principal cells became spotty or with weak or moderate reactivity in some cells and none in others. LRP-2 expression in the intermediate zone, proximal caput, corpus, and cauda regions also appeared in principal cells by day 21, intensified at days 29 and 39 and persisted as such at all later ages examined, correlating with high levels of androgens shown to occur by day 39. Although LRP-2 expression in the distal caput region was evident in principal cells at days 21 and 29, it became spotty with weak, moderate, or absent reactivity over principal cells at all later ages. These data suggest that LRP-2 expression is under the influence of both stimulatory and region-specific inhibitory factors. Analysis of 15-day-old efferent-duct-ligated animals at all later ages examined revealed that there was no change in LRP-2 expression along the entire epididymis, suggesting that both the stimulatory and inhibitory factors are not luminally derived testicular substances. The observed pattern of LRP-2 expression in all regions of the epididymis, except the distal caput region, was similar to that described for Apo J internalization by principal cells during postnatal development, showing a correlation between LRP-2 expression and its ligand, Apo J. In summary, LRP-2 expression in the epididymis undergoes region-specific changes during postnatal development and appears to be influenced by both stimulatory and inhibitory factors.

The intrinsic factor-vitamin B12 receptor, cubilin, is a high-affinity apolipoprotein A-I receptor facilitating endocytosis of high-density lipoprotein

Cubilin is the intestinal receptor for the endocytosis of intrinsic factor-vitamin B12. However, several lines of evidence, including a high expression in kidney and yolk sac, indicate it may have additional functions. We isolated apolipoprotein A-I (apoA-I), the main protein of high-density lipoprotein (HDL), using cubilin affinity chromatography. Surface plasmon resonance analysis demonstrated a high-affinity binding of apoA-I and HDL to cubilin, and cubilin-expressing yolk sac cells showed efficient 125I-HDL endocytosis that could be inhibited by IgG antibodies against apoA-I and cubilin. The physiological relevance of the cubilin-apoA-I interaction was further emphasized by urinary apoA-I loss in some known cases of functional cubilin deficiency. Therefore, cubilin is a receptor in epithelial apoA-I/HDL metabolism.

Apolipoprotein E immunoreactivity in human and mouse olfactory bulb

We evaluated the distribution of apolipoprotein E (apoE) immunoreactivity in mouse and human olfactory bulbs. ApoE immunoreactivity was present in the olfactory nerve and around the glomeruli. Immunoreactivity was seen in somata that appeared to be glial. No neuronal staining was seen. The apoE immunoreactivity was also present in the mouse olfactory bulb subependymal layer. The specificity of immunoreactivity was confirmed with apoE-deficient mice (apoE gene knock-out mice, apoE KO) which did not display any immunoreactivity. The presence of apoE around glomeruli and nerve suggest that apoE is associated with the continuous degeneration and regeneration processes that occur in the olfactory nerve. Experimental manipulation of the olfactory nerve may be a useful technique to determine the functions of apoE in a well-defined neural system.

Evidence for an essential role of megalin in transepithelial transport of retinol

Transepithelial transport of retinol is linked to retinol-binding protein (RBP), which is taken up and also synthesized in a number of epithelia. By immunocytochemistry of human, rat, and mouse renal proximal tubules, a strong staining in apical endocytic vacuoles, lysosomes, endoplasmic reticulum, Golgi, and basal vesicles was observed, in accordance with luminal endocytic uptake as well as a constitutive synthesis and basal secretion of RBP. Analysis of mice with target disruption of the gene for the major endocytic receptor of proximal tubules, megalin, revealed no RBP in proximal tubules of these mice. Western blotting and HPLC of the urine of the megalin-deficient mice instead revealed a highly increased urinary excretion of RBP and retinol, demonstrating that glomerular filtered RBP-retinol of megalin-deficient mice escapes uptake by proximal tubules. A direct megalin-mediated uptake of purified RBP-retinol was indicated by surface plasmon resonance analysis and uptake in immortalized rat yolk sac cells. Uptake was partially inhibited by a polyclonal megalin antibody and the receptor-associated protein. The present data show that the absence of RBP-binding megalin causes a significantly increased loss of RBP and retinol in the urine, demonstrating a crucial role of megalin in vitamin A homeostasis.

Molecular genetics of the Smith-Lemli-Opitz syndrome and postsqualene sterol metabolism

The Smith-Lemli-Opitz syndrome is a disorder of morphogenesis resulting from an enzymatic defect in the last step of cholesterol metabolism (reduction of 7-dehydrocholesterol). Analysis of the defective gene and identification of mutations therein have paved the way for the study of the molecular genetics of the disorder which is caused by numerous different mutations. Future efforts should identify a postulated intracellular signalling activity of sterol intermediates, isolate proteins that govern the sterol traffic between intracellular compartments, structurally characterize the enzyme delta 7-sterol reductase defective in the Smith-Lemli-Opitz syndrome and investigate the pathomechanism of sterol depletion-induced dysmorphogenesis.

The very low density lipoprotein receptor regulates urokinase receptor catabolism and breast cancer cell motility in vitro

The very low density lipoprotein receptor (VLDLr) binds diverse ligands, including urokinase-type plasminogen activator (uPA) and uPA-plasminogen activator inhibitor-1 (PAI-1) complex. In this study, we characterized the effects of the VLDLr on the internalization, catabolism, and function of the uPA receptor (uPAR) in MCF-7 and MDA-MB-435 breast cancer cells. When challenged with uPA.PAI-1 complex, MDA-MB-435 cells internalized uPAR; this process was inhibited by 80% when the activity of the VLDLr was neutralized with receptor-associated protein (RAP). To determine whether internalized uPAR is degraded, we studied the catabolism of [35S]methionine-labeled uPAR. In the absence of exogenous agents, the uPAR catabolism t(1)/(2) was 8.2 h. uPA.PAI-1 complex accelerated uPAR catabolism (t(1)/(2) to 1.8 h), while RAP inhibited uPAR catabolism in the presence (t(1)/(2) of 7.8 h) and absence (t(1)/(2) of 16.9 h) of uPA.PAI-1 complex, demonstrating a critical role for the VLDLr. When MCF-7 cells were cultured in RAP, cell surface uPAR levels increased gradually, reaching a new steady-state in 3 days. The amount of uPA which accumulated in the medium also increased. Culturing in RAP for 3 days increased MCF-7 cell motility by 2.2 +/- 0.1-fold and by 4.4 +/- 0.3-fold when 1.0 nM uPA was added. The effects of RAP on MCF-7 cell motility were entirely abrogated by an antibody which binds uPA and prevents uPA binding to uPAR. MCF-7 cells that were cultured in RAP demonstrated increased levels of activated mitogen-activated protein kinases. Furthermore, the MEK inhibitor, PD098059, decreased the motility of RAP-treated cells without affecting control cultures. These studies suggest a model in which the VLDLr regulates autocrine uPAR-initiated signaling and thereby regulates cellular motility.

Megalin in normal tissues and carcinoma cells carries oligo/poly alpha2,8 deaminoneuraminic acid as a unique posttranslational modification

In rat kidney, megalin, a member of the low density lipoprotein receptor gene family, is the sole glycoprotein which carries oligo/poly alpha2,8 deaminoneuraminic acid (KDN) as a posttranslational modification. We have investigated immunoprecipitated megalin from rat brain, lung and placenta, mouse yolk sac carcinoma and megalin synthesizing carcinoma cell lines, for presence of this unique glycan structure. Our immunoblot analysis revealed the presence of oligo/poly alpha2,8 KDN on megalin in all the studied normal tissues and carcinoma cells. Furthermore, it is demonstrated to be part of oligosaccharides O-glycosidically linked to megalin.

Identification of megalin/gp330 as a receptor for lipoprotein(a) in vitro

Lipoprotein(a) [Lp(a)] is an atherogenic lipoprotein of unknown physiological function. The mechanism of Lp(a) atherogenicity as well as its catabolic pathways are only incompletely understood at present. In this report, we show that the low density lipoprotein receptor (LDLR) gene family member megalin/glycoprotein (gp) 330 is capable of binding and mediating the cellular uptake and degradation of Lp(a) in vitro. A mouse embryonic yolk sac cell line with native expression of megalin/gp330 but genetically deficient in LDLR-related protein (LRP) and a control cell line carrying a double knockout for both LRP and megalin/gp330 were compared with regard to their ability to bind, internalize, and degrade dioctadecyltetramethylindocarbocyanine perchlorate (DiI)-fluorescence-labeled Lp(a) as well as equimolar amounts of 125I-labeled Lp(a) and LDL. Uptake and degradation of radiolabeled Lp(a) by the megalin/gp330-expressing cells were, on average, 2-fold higher than that of control cells. This difference could be completely abolished by addition of the receptor-associated protein, an inhibitor of ligand binding to megalin/gp330. Mutual suppression of the uptake of 125I-Lp(a) and of 125I-LDL by both unlabeled Lp(a) and LDL suggested that Lp(a) uptake is mediated at least partially by apolipoprotein B100. Binding and uptake of DiI-Lp(a) resulted in strong signals on megalin/gp330-expressing cells versus background only on control cells. In addition, we show that purified megalin/gp330, immobilized on a sensor chip, directly binds Lp(a) in a Ca2+-dependent manner with an affinity similar to that for LDL. We conclude that megalin/gp330 binds Lp(a) in vitro and is capable of mediating its cellular uptake and degradation.

Megalin antagonizes activation of the parathyroid hormone receptor

Parathyroid hormone (PTH) is predominantly cleared from the circulation by glomerular filtration and degradation in the renal proximal tubules. Here, we demonstrate that megalin, a multifunctional endocytic receptor in the proximal tubular epithelium, mediates the uptake and degradation of PTH. Megalin was purified from kidney membranes as the major PTH-binding protein and shown in BIAcore analysis to specifically bind full-length PTH and amino-terminal PTH fragments (Kd 0.5 microM). Absence of the receptor in megalin knockout mice resulted in 4-fold increased levels of amino-terminal PTH fragments in the urine. In F9 cells expressing both megalin and the PTH/PTH-related peptide receptor (PTH/PTHrP receptor), uptake and lysosomal degradation of the hormone was mediated through megalin. Blocking megalin-mediated clearance of PTH resulted in 3-fold increased stimulation of the PTH/PTHrP receptor. These data provide evidence that megalin is involved in the renal catabolism of PTH and potentially antagonizes PTH/PTHrP receptor activity in the proximal tubular epithelium.

An endocytic pathway essential for renal uptake and activation of the steroid 25-(OH) vitamin D3

Steroid hormones may enter cells by diffusion through the plasma membrane. However, we demonstrate here that some steroid hormones are taken up by receptor-mediated endocytosis of steroid-carrier complexes. We show that 25-(OH) vitamin D3 in complex with its plasma carrier, the vitamin D-binding protein, is filtered through the glomerulus and reabsorbed in the proximal tubules by the endocytic receptor megalin. Endocytosis is required to preserve 25-(OH) vitamin D3 and to deliver to the cells the precursor for generation of 1,25-(OH)2 vitamin D3, a regulator of the calcium metabolism. Megalin-/- mice are unable to retrieve the steroid from the glomerular filtrate and develop vitamin D deficiency and bone disease.