Defective forebrain development in mice lacking gp330/megalin

The cell biology of neurogenesis: toward an understanding of the development and evolution of the neocortex

Neural stem and progenitor cells have a central role in the development and evolution of the mammalian neocortex. In this review, we first provide a set of criteria to classify the various types of cortical stem and progenitor cells. We then discuss the issue of cell polarity, as well as specific subcellular features of these cells that are relevant for their modes of division and daughter cell fate. In addition, cortical stem and progenitor cell behavior is placed into a tissue context, with consideration of extracellular signals and cell-cell interactions. Finally, the differences across species regarding cortical stem and progenitor cells are dissected to gain insight into key developmental and evolutionary mechanisms underlying neocortex expansion.

Mechanisms of AT1a receptor-mediated uptake of angiotensin II by proximal tubule cells: a novel role of the multiligand endocytic receptor megalin

The present study tested the hypothesis that the multiligand endocytic receptor megalin is partially involved in the uptake of ANG II and downstream signaling responses in mouse proximal tubule cells (mPCT) by interacting with AT1a receptors. mPCT cells of wild-type (WT) and AT1a receptor-deficient (AT1a-KO) mice were treated with vehicle, the AT1 receptor blocker losartan (10 μM), or a selective megalin small interfering (si) RNA for 48 h. The uptake of fluorescein (FITC)-labeled ANG II (10 nM, 37°C) and downstream signaling responses were analyzed by fluorescence imaging and Western blotting. AT1a receptors and megalin were abundantly expressed in mPCT cells, whereas AT1a receptors were absent in AT1a-KO mPCT cells (P < 0.01). In WT mPCT cells, FITC-ANG II uptake was visualized at 30 min in the cytoplasm and in the nuclei 1 h after exposure. Losartan alone completely blocked the uptake of FITC-ANG II, whereas megalin siRNA inhibited only 30% of the response (P < 0.01). The remaining FITC-ANG II uptake in the presence of megalin siRNA was completely abolished by losartan. ANG II induced threefold increases in phosphorylated MAP kinases ERK1/2 and a onefold increase in phosphorylated sodium and hydrogen exchanger 3 (NHE3) proteins, which were also blocked by losartan and megalin-siRNA. By contrast, losartan and megalin siRNA had no effects on these signaling proteins in AT1a-KO mPCT cells. We conclude that the uptake of ANG II and downstream MAP kinases ERK1/2 and NHE3 signaling responses in mPCT cells are mediated primarily by AT1a receptors. However, megalin may also play a partial role in these responses to ANG II.

Sepp1(UF) forms are N-terminal selenoprotein P truncations that have peroxidase activity when coupled with thioredoxin reductase-1

Mouse selenoprotein P (Sepp1) consists of an N-terminal domain (residues 1-239) that contains one selenocysteine (U) as residue 40 in a proposed redox-active motif (-UYLC-) and a C-terminal domain (residues 240-361) that contains nine selenocysteines. Sepp1 transports selenium from the liver to other tissues by receptor-mediated endocytosis. It also reduces oxidative stress in vivo by an unknown mechanism. A previously uncharacterized plasma form of Sepp1 is filtered in the glomerulus and taken up by renal proximal convoluted tubule (PCT) cells via megalin-mediated endocytosis. We purified Sepp1 forms from the urine of megalin(-/-) mice using a monoclonal antibody to the N-terminal domain. Mass spectrometry revealed that the purified urinary Sepp1 consisted of N-terminal fragments terminating at 11 sites between residues 183 and 208. They were therefore designated Sepp1(UF). Because the N-terminal domain of Sepp1 has a thioredoxin fold, Sepp1(UF) were compared with full-length Sepp1, Sepp1(Δ240-361), and Sepp1(U40S) as a substrate of thioredoxin reductase-1 (TrxR1). All forms of Sepp1 except Sepp1(U40S), which contains serine in place of the selenocysteine, were TrxR1 substrates, catalyzing NADPH oxidation when coupled with H2O2 or tert-butylhydroperoxide as the terminal electron acceptor. These results are compatible with proteolytic cleavage freeing Sepp1(UF) from full-length Sepp1, the form that has the role of selenium transport, allowing Sepp1(UF) to function by itself as a peroxidase. Ultimately, plasma Sepp1(UF) and small selenium-containing proteins are filtered by the glomerulus and taken up by PCT cells via megalin-mediated endocytosis, preventing loss of selenium in the urine and providing selenium for the synthesis of glutathione peroxidase-3.

LRP2 mediates folate uptake in the developing neural tube

The low-density lipoprotein (LDL) receptor-related protein 2 (LRP2) is a multifunctional cell-surface receptor expressed in the embryonic neuroepithelium. Loss of LRP2 in the developing murine central nervous system (CNS) causes impaired closure of the rostral neural tube at embryonic stage (E) 9.0. Similar neural tube defects (NTDs) have previously been attributed to impaired folate metabolism in mice. We therefore asked whether LRP2 might be required for the delivery of folate to neuroepithelial cells during neurulation. Uptake assays in whole-embryo cultures showed that LRP2-deficient neuroepithelial cells are unable to mediate the uptake of folate bound to soluble folate receptor 1 (sFOLR1). Consequently, folate concentrations are significantly reduced in Lrp2(-/-) embryos compared with control littermates. Moreover, the folic-acid-dependent gene Alx3 is significantly downregulated in Lrp2 mutants. In conclusion, we show that LRP2 is essential for cellular folate uptake in the developing neural tube, a crucial step for proper neural tube closure.

Regulation of high-density lipoprotein metabolism

There is compelling evidence from human population studies that plasma levels of high-density lipoprotein (HDL) cholesterol correlate inversely with cardiovascular risk. Identification of this relationship has stimulated research designed to understand how HDL metabolism is regulated. The ultimate goal of these studies has been to develop HDL-raising therapies that have the potential to decrease the morbidity and mortality associated with atherosclerotic cardiovascular disease. However, the situation has turned out to be much more complex than originally envisaged. This is partly because the HDL fraction consists of multiple subpopulations of particles that vary in terms of shape, size, composition, and surface charge, as well as in their potential cardioprotective properties. This heterogeneity is a consequence of the continual remodeling and interconversion of HDL subpopulations by multiple plasma factors. Evidence that the remodeling of HDLs may impact on their cardioprotective properties is beginning to emerge. This serves to highlight the importance of understanding not only how the remodeling and interconversion of HDL subpopulations is regulated but also how these processes are affected by agents that increase HDL levels. This review provides an overview of what is currently understood about HDL metabolism and how the subpopulation distribution of these lipoproteins is regulated.

Clinical, genetic and imaging findings identify new causes for corpus callosum development syndromes

The corpus callosum is the largest fibre tract in the brain, connecting the two cerebral hemispheres, and thereby facilitating the integration of motor and sensory information from the two sides of the body as well as influencing higher cognition associated with executive function, social interaction and language. Agenesis of the corpus callosum is a common brain malformation that can occur either in isolation or in association with congenital syndromes. Understanding the causes of this condition will help improve our knowledge of the critical brain developmental mechanisms required for wiring the brain and provide potential avenues for therapies for callosal agenesis or related neurodevelopmental disorders. Improved genetic studies combined with mouse models and neuroimaging have rapidly expanded the diverse collection of copy number variations and single gene mutations associated with callosal agenesis. At the same time, advances in our understanding of the developmental mechanisms involved in corpus callosum formation have provided insights into the possible causes of these disorders. This review provides the first comprehensive classification of the clinical and genetic features of syndromes associated with callosal agenesis, and provides a genetic and developmental framework for the interpretation of future research that will guide the next advances in the field.

Progesterone induces expression of Lrp2 in the murine uterus

Progesterone (P4) and progesterone receptor (PR) have important functions in uterine environment. In previous studies, using high density DNA microarray analysis, we identified low density lipoprotein receptor-related protein 2 (Lrp2) is one of the genes upregulated by P4 and PR. In present studies, we examined the expression of Lrp2 through real-time PCR, in situ hybridization and immunohistochemistry by P4-PR response. Lrp2 mRNA transcript was significantly increased after P4 treatment in the luminal and glandular epithelium of the wild-type mice. However, Lrp2 expression was not observed in the progesterone receptor knock out (PRKO) mice treated with P4. The expression of Lrp2 expression is not regulated by estrogen. During early pregnancy, the expression of Lrp2 was detected at 2.5 dpc and then significantly increased at 3.5 dpc in luminal and glandular epithelium. These results suggest that Lrp2 is a novel target gene by P4 and PR.

ARH directs megalin to the endocytic recycling compartment to regulate its proteolysis and gene expression

ARH is required for the trafficking of megalin from early endosomes to the endocytic recycling compartment, where megalin undergoes intramembrane proteolysis, releasing a tail fragment that represses megalin transcription.

Receptors internalized by endocytosis can return to the plasma membrane (PM) directly from early endosomes (EE; fast recycling) or they can traffic from EE to the endocytic recycling compartment (ERC) and recycle from there (slow recycling). How receptors are sorted for trafficking along these two pathways remains unclear. Here we show that autosomal recessive hypercholesterolemia (ARH) is required for trafficking of megalin, a member of the LDL receptor family, from EE to the ERC by coupling it to dynein; in the absence of ARH, megalin returns directly to the PM from EE via the connecdenn2/Rab35 fast recycling pathway. Binding of ARH to the endocytic adaptor AP-2 prevents fast recycling of megalin. ARH-mediated trafficking of megalin to the ERC is necessary for γ-secretase mediated cleavage of megalin and release of a tail fragment that mediates transcriptional repression. These results identify a novel mechanism for sorting receptors for trafficking to the ERC and link ERC trafficking to regulated intramembrane proteolysis (RIP) and expression of megalin.

Dietary vitamin E modulates differential gene expression in the rat hippocampus: Potential implications for its neuroprotective properties

A wide range of cell culture, animal and human epidemiological studies are suggestive of a role of vitamin E (VE) in brain function and in the prevention of neurodegeneration. However, the underlying molecular mechanisms remain largely unknown. In the current investigation Affymetrix gene chip technology was utilised to establish the impact of chronic VE deficiency on hippocampal genes expression. Male albino rats were fed either a VE deficient or standard diet (60 mg/kg feed) for a period of 9 months. Rats were sacrificed, the hippocampus removed and genes expression established in individual animals. VE deficiency showed to have a strong impact on genes expression in the hippocampus. An important number of genes found to be regulated by VE was associated with hormones and hormone metabolism, nerve growth factor, apoptosis, dopaminergic neurotransmission, and clearance of amyloid-beta and advanced glycated endproducts. In particular, VE strongly affected the expression of an array of genes encoding for proteins directly or indirectly involved in the clearance of amyloid beta, changes which are consistent with a protective effect of VE on Alzheimer's disease progression.

Renal uptake of the antiapoptotic protein survivin is mediated by megalin at the apical membrane of the proximal tubule

The inhibitor of apoptosis protein survivin is a bifunctional molecule that regulates cellular division and survival. We have previously shown that survivin protein can be found at high concentrations in the adult kidney, particularly in the proximal tubules. Here, survivin is localized primarily at the apical membrane, a pattern that may indicate absorption of the protein. Several proteins in primary urine are internalized by megalin, an endocytosis receptor, which is in principle found in the same localization as survivin. Immunolabeling for survivin in different species confirmed survivin signal localizing to the apical membrane of the proximal tubule. Immunoelectron microscopy also showed apical localization of survivin in human kidneys. Furthermore, in polarized human primary tubular cells endogenous as well as external recombinant survivin is stored in the apical region of the cells. Costaining of survivin and megalin by immunohistochemistry and immunoelectron microscopy confirmed colocalization. Finally, by surface plasmon resonance we were able to demonstrate that survivin binds megalin and cubilin and that megalin knockout mice lose survivin through the urine. Survivin accumulates at the apical membrane of the renal tubule by reuptake, which is achieved by the endocytic receptor megalin, collaborating with cubilin. For this to occur, survivin will have to circulate in the blood and be filtered into the primary urine. It is not known at this stage what the functional role of tubular survivin is. However, a small number of experimental and clinical reports implicate that renal survivin is important for functional integrity of the kidney.

Loss of β-carotene 15,15'-oxygenase in developing mouse tissues alters esterification of retinol, cholesterol and diacylglycerols

We provide novel insights into the function(s) of β-carotene-15,15'-oxygenase (CMOI) during embryogenesis. By performing in vivo and in vitro experiments, we showed that CMOI influences not only lecithin:retinol acyltransferase but also acyl CoA:retinol acyltransferase reaction in the developing tissues at mid-gestation. In addition, LC/MS lipidomics analysis of the CMOI-/- embryos showed reduced levels of four phosphatidylcholine and three phosphatidylethanolamine acyl chain species, and of eight triacylglycerol species with four or more unsaturations and fifty-two or more carbons in the acyl chains. Cholesteryl esters of arachidonate, palmitate, linoleate, and DHA were also reduced to less than 30% of control. Analysis of the fatty acyl CoA species ruled out a loss in fatty acyl CoA synthetase capability. Comparison of acyl species suggested significantly decreased 18:2, 18:3, 20:1, 20:4, or 22:6 acyl chains within the above lipids in CMOI-null embryos. Furthermore, LCAT, ACAT1 and DGAT2 mRNA levels were also downregulated in CMOI-/- embryos. These data strongly support the notion that, in addition to cleaving β-carotene to generate retinoids, CMOI serves an additional function(s) in retinoid and lipid metabolism and point to its role in the formation of specific lipids, possibly for use in nervous system tissue.

A forward genetic screen in mice identifies mutants with abnormal cortical patterning

Formation of a 6-layered cortical plate and axon tract patterning are key features of cerebral cortex development. Abnormalities of these processes may be the underlying cause for a range of functional disabilities seen in human neurodevelopmental disorders. To identify mouse mutants with defects in cortical lamination or corticofugal axon guidance, N-ethyl-N-nitrosourea (ENU) mutagenesis was performed using mice expressing LacZ reporter genes in layers II/III and V of the cortex (Rgs4-lacZ) or in corticofugal axons (TAG1-tau-lacZ). Four lines with abnormal cortical lamination have been identified. One of these was a splice site mutation in reelin (Reln) that results in a premature stop codon and the truncation of the C-terminal region (CTR) domain of reelin. Interestingly, this novel allele of Reln did not display cerebellar malformation or ataxia, and this is the first report of a Reln mutant without a cerebellar defect. Four lines with abnormal cortical axon development were also identified, one of which was found by whole-genome resequencing to carry a mutation in Lrp2. These findings demonstrated that the application of ENU mutagenesis to mice carrying transgenic reporters marking cortical anatomy is a sensitive and specific method to identify mutations that disrupt patterning of the developing brain.

Apolipoprotein E and apolipoprotein E receptors: normal biology and roles in Alzheimer disease

Apolipoprotein E (APOE) genotype is the major genetic risk factor for Alzheimer disease (AD); the ε4 allele increases risk and the ε2 allele is protective. In the central nervous system (CNS), apoE is produced by glial cells, is present in high-density-like lipoproteins, interacts with several receptors that are members of the low-density lipoprotein receptor (LDLR) family, and is a protein that binds to the amyloid-β (Aβ) peptide. There are a variety of mechanisms by which apoE isoform may influence risk for AD. There is substantial evidence that differential effects of apoE isoform on AD risk are influenced by the ability of apoE to affect Aβ aggregation and clearance in the brain. Other mechanisms are also likely to play a role in the ability of apoE to influence CNS function as well as AD, including effects on synaptic plasticity, cell signaling, lipid transport and metabolism, and neuroinflammation. ApoE receptors, including LDLRs, Apoer2, very low-density lipoprotein receptors (VLDLRs), and lipoprotein receptor-related protein 1 (LRP1) appear to influence both the CNS effects of apoE as well as Aβ metabolism and toxicity. Therapeutic strategies based on apoE and apoE receptors may include influencing apoE/Aβ interactions, apoE structure, apoE lipidation, LDLR receptor family member function, and signaling. Understanding the normal and disease-related biology connecting apoE, apoE receptors, and AD is likely to provide novel insights into AD pathogenesis and treatment.

The integrated landscape of driver genomic alterations in glioblastoma

Glioblastoma is one of the most challenging forms of cancer to treat. Here we describe a computational platform that integrates the analysis of copy number variations and somatic mutations and unravels the landscape of in-frame gene fusions in glioblastoma. We found mutations with loss of heterozygosity in LZTR1, encoding an adaptor of CUL3-containing E3 ligase complexes. Mutations and deletions disrupt LZTR1 function, which restrains the self renewal and growth of glioma spheres that retain stem cell features. Loss-of-function mutations in CTNND2 target a neural-specific gene and are associated with the transformation of glioma cells along the very aggressive mesenchymal phenotype. We also report recurrent translocations that fuse the coding sequence of EGFR to several partners, with EGFR-SEPT14 being the most frequent functional gene fusion in human glioblastoma. EGFR-SEPT14 fusions activate STAT3 signaling and confer mitogen independence and sensitivity to EGFR inhibition. These results provide insights into the pathogenesis of glioblastoma and highlight new targets for therapeutic intervention.

Identification of two maternal transmission ratio distortion loci in pedigrees of the Framingham heart study

Transmission ratio distortion (TRD) is indicated by the recovery of alleles in offspring in non-Mendelian proportions. An assumption of Mendelian proportion is central to many methods to identify disease-associated markers. This seems reasonable as, while TRD cases have been occasionally observed in various species few instances have been identified in humans. Here we search for evidence of paternal or maternal TRD with genome-wide SNP data of pedigrees from the Framingham Heart Study. After excluding many examples as better explained by genotyping errors we identified two maternal-specific TRD loci for autosomal SNPs rs6733122 and rs926716 (corrected P = 0.029 and P = 0.018) on LRP2 and ZNF133, respectively. The transmission ratios were as high as 1.7~1.8:1. Genotyping validation and further replication is still necessary to confirm the TRD. This study shows that there may be large-effect maternal-specific TRD loci of common SNPs in the human genome but that these are rare.

Maternal-fetal transfer of selenium in the mouse

Selenoprotein P (Sepp1) is taken up by receptor-mediated endocytosis for its selenium. The other extracellular selenoprotein, glutathione peroxidase-3 (Gpx3), has not been shown to transport selenium. Mice with genetic alterations of Sepp1, the Sepp1 receptors apolipoprotein E receptor-2 (apoER2) and megalin, and Gpx3 were used to investigate maternal-fetal selenium transfer. Immunocytochemistry (ICC) showed receptor-independent uptake of Sepp1 and Gpx3 in the same vesicles of d-13 visceral yolk sac cells, suggesting uptake by pinocytosis. ICC also showed apoER2-mediated uptake of maternal Sepp1 in the d-18 placenta. Thus, two selenoprotein-dependent maternal-fetal selenium transfer mechanisms were identified. Selenium was quantified in d-18 fetuses with the mechanisms disrupted. Maternal Sepp1 deletion, which lowers maternal whole-body selenium, decreased fetal selenium under selenium-adequate conditions but deletion of fetal apoER2 did not. Fetal apoER2 deletion did decrease fetal selenium, by 51%, under selenium-deficient conditions, verifying function of the placental Sepp1-apoER2 mechanism. Maternal Gpx3 deletion decreased fetal selenium, by 13%, but only under selenium-deficient conditions. These findings indicate that the selenoprotein uptake mechanisms ensure selenium transfer to the fetus under selenium-deficient conditions. The failure of their disruptions (apoER2 deletion, Gpx3 deletion) to affect fetal selenium under selenium-adequate conditions indicates the existence of an additional maternal-fetal selenium transfer mechanism.

Progression of microalbuminuria in SHR is associated with lower expression of critical components of the apical endocytic machinery in the renal proximal tubule

Cumulative epidemiological evidence indicates that the presence of microalbuminuria predicts a higher frequency of cardiovascular events, peripheral disease, and mortality in essential hypertension. Microalbuminuria may arise from increased glomerular permeability and/or reduced proximal tubular reabsorption of albumin by receptor-mediated endocytosis. This study aimed to evaluate the temporal pattern of urinary protein excretion and to test the hypothesis that progression of microalbuminuria is associated with decreased protein expression of critical components of the endocytic apparatus in the renal proximal tubule of spontaneously hypertensive rats (SHR). We found that urinary albumin excretion increased progressively with blood pressure in SHR from 6 to 21 wk of age. In addition, SDS-PAGE analysis of urinary proteins showed that microalbuminuric SHR virtually excreted proteins of the size of albumin or smaller (<70 kDa), typical of tubular proteinuria. Moreover, the protein abundance of the endocytic receptors megalin and cubilin as well as of the chloride channel ClC-5 progressively decreased in the renal cortex of SHR from 6 to 21 wk of age. Expression of the vacuolar H⁺-ATPase B2 subunit was also reduced in the renal cortex of 21-wk-old compared with both 6- and 14-wk-old SHR. Collectively, our study suggests that enhanced urinary protein excretion, especially of albumin, may be due, at least in part, to lower expression of key components of the apical endocytic apparatus in the renal proximal tubule. Finally, one may speculate that dysfunction of the apical endocytic pathway in the renal proximal tubule may contribute to the development of microalbuminuria in essential hypertension.

The transcobalamin receptor knockout mouse: a model for vitamin B12 deficiency in the central nervous system

The membrane receptor (TCblR/CD320) for transcobalamin (TC)-bound cobalamin (Cbl) facilitates the cellular uptake of Cbl. A genetically modified mouse model involving ablation of the CD320 gene was generated to study the effects on cobalamin homeostasis. The nonlethal nature of this knockout and the lack of systemic cobalamin deficiency point to other mechanisms for cellular Cbl uptake in the mouse. However, severe cobalamin depletion in the central nervous system (CNS) after birth (P<0.01) indicates that TCblR is the only receptor responsible for Cbl uptake in the CNS. Metabolic Cbl deficiency in the brain was evident from the increased methylmalonic acid (P<0.01-0.04), homocysteine (P<0.01), cystathionine (P<0.01), and the decreased S-adenosylmethionine/S-adenosyl homocysteine ratio (P<0.01). The CNS pathology of Cbl deficiency seen in humans may not manifest in this mouse model; however, it does provide a model with which to evaluate metabolic pathways and genes affected.

Endocytic receptor-mediated control of morphogen signaling

Receptor-mediated endocytosis provides a mechanism by which cells take up signaling molecules from the extracellular space. Recent studies have shown that one class of endocytic receptors, the low-density lipoprotein receptor-related proteins (LRPs), is of particular relevance for embryonic development. In this Primer, we describe how LRPs constitute central pathways that modulate morphogen presentation to target tissues and cellular signal reception, and how LRP dysfunction leads to developmental disturbances in many species.

The developing chicken yolk sac acquires nutrient transport competence by an orchestrated differentiation process of its endodermal epithelial cells

During chicken yolk sac (YS) growth, mesodermal cells in the area vasculosa follow the migrating endodermal epithelial cell (EEC) layer in the area vitellina. Ultimately, these cells form the vascularized YS that functions in nutrient transfer to the embryo. How and when EECs, with their apical aspect directly contacting the oocytic yolk, acquire the ability to take up yolk macromolecules during the vitellina-to-vasculosa transition has not been investigated. In addressing these questions, we found that with progressive vascularization, the expression level in EECs of the nutrient receptor triad, LRP2-cubilin-amnionless, changes significantly. The receptor complex, competent for uptake of yolk proteins, is produced by EECs in the area vasculosa but not in the area vitellina. Yolk components endocytosed by LRP2-cubilin-amnionless, preformed and newly formed lipid droplets, and yolk-derived very low density lipoprotein, shown to be efficiently endocytosed and lysosomally processed by EECs, probably provide substrates for resynthesis and secretion of nutrients, such as lipoproteins. In fact, as directly demonstrated by pulse-chase experiments, EECs in the vascularized, but not in the avascular, region efficiently produce and secrete lipoproteins containing apolipoprotein A-I (apoA-I), apoB, and/or apoA-V. In contrast, perilipin 2, a lipid droplet-stabilizing protein, is produced exclusively by the EECs of the area vitellina. These data suggest a differentiation process that orchestrates the vascularization of the developing YS with the induction of yolk uptake and lipoprotein secretion by EECs to ensure embryo nutrition.

Cholesterol: its regulation and role in central nervous system disorders

Cholesterol is a major constituent of the human brain, and the brain is the most cholesterol-rich organ. Numerous lipoprotein receptors and apolipoproteins are expressed in the brain. Cholesterol is tightly regulated between the major brain cells and is essential for normal brain development. The metabolism of brain cholesterol differs markedly from that of other tissues. Brain cholesterol is primarily derived by de novo synthesis and the blood brain barrier prevents the uptake of lipoprotein cholesterol from the circulation. Defects in cholesterol metabolism lead to structural and functional central nervous system diseases such as Smith-Lemli-Opitz syndrome, Niemann-Pick type C disease, and Alzheimer's disease. These diseases affect different metabolic pathways (cholesterol biosynthesis, lipid transport and lipoprotein assembly, apolipoproteins, lipoprotein receptors, and signaling molecules). We review the metabolic pathways of cholesterol in the CNS and its cell-specific and microdomain-specific interaction with other pathways such as the amyloid precursor protein and discuss potential treatment strategies as well as the effects of the widespread use of LDL cholesterol-lowering drugs on brain functions.

SeP, ApoER2 and megalin as necessary factors to maintain Se homeostasis in mammals

Selenoprotein P (SeP) is an extracellular protein containing ten selenium atoms in the form of selenocysteine, secreted mainly from the liver. About 60% of the whole plasma selenium level is present in SeP, which makes it a useful biomarker of selenium nutritional status. The main functions of SeP are transport and storage of selenium in plasma. It is especially an important protein for the brain, testes and kidneys where the supplementation of the proper amount of Se ensures the synthesis of selenoenzymes with antioxidant properties.Recently, it has been found that SeP uptake in kidneys, testes and brain depends on the apolipoprotein receptor 2 (ApoER2) and lipoprotein megalin receptor (Lrp2). Megalin receptor represents a physiological SeP receptor in kidneys, mediating the re-uptake of secreted SeP from the primary urine. The absence of a functional megalin receptor causes a significant reduction of plasma selenium and the SeP levels as a result of Se excretion. ApoER2 is a SeP receptor in the brain and testes which uptakes Se from the extracellular fluid. Deletion of ApoER2 in mice leads to a lowered selenium level in the brain and testes, neurological dysfunction, production of abnormal spermatozoa, infertility and even death when the subjects are fed a low-selenium diet.

Renal LRP2 expression in man and chicken is estrogen-responsive

In mammals, low-density lipoprotein receptor-related protein-2 (LRP2) is an endocytic receptor that binds multiple ligands and is essential for a wide range of physiological processes. To gain new insights into the biology of this complex protein, we have initiated the molecular characterization of the LRP2 homolog from an oviparous species, the chicken (Gallus gallus). The galline LRP2 cDNA encodes a membrane protein of 4658 residues. Overall, the galline and human proteins are 73% identical, indicating that the avian gene has been well conserved over 300 million years. Unexpectedly, LRP2 transcript and protein levels in the kidney of females and estrogen-treated roosters were significantly higher than those in untreated males. The estrogen-responsiveness of avian LRP2 may be related to the dramatic differences in lipoprotein metabolism between mature roosters and laying hens. Newly identified potential estrogen-responsive elements (ERE) in the human and galline LRP2 gene, and additional Sp1 sites present in the promoter of the chicken gene, are compatible with both direct estrogen induction via the classical ligand-induced ERE pathway and the indirect transcription factor crosstalk pathway engaging the Sp1 sites. In agreement with this assumption, estrogen induction of LRP2 was observed not only in primary cultured chicken kidney cells, but also human kidney cell lines. These findings point to novel regulatory features of the LRP2 gene resulting in sex-specific receptor expression.

Megalin mediates transepithelial albumin clearance from the alveolar space of intact rabbit lungs

The alveolo-capillary barrier is effectively impermeable to large solutes such as proteins. A hallmark of acute lung injury/acute respiratory distress syndrome is the accumulation of protein-rich oedema fluid in the distal airspaces. Excess protein must be cleared from the alveolar space for recovery; however, the mechanisms of protein clearance remain incompletely understood. In intact rabbit lungs 29.8 ± 2.2% of the radio-labelled alveolar albumin was transported to the vascular compartment at 37°C within 120 min, as assessed by real-time measurement of 125I-albumin clearance from the alveolar space. At 4°C or 22°C significantly lower albumin clearance (3.7 ± 0.4 or 16.2 ± 1.1%, respectively) was observed. Deposition of a 1000-fold molar excess of unlabelled albumin into the alveolar space or inhibition of cytoskeletal rearrangement or clathrin-dependent endocytosis largely inhibited the transport of 125I-albumin to the vasculature, while administration of unlabelled albumin to the vascular space had no effect on albumin clearance. Furthermore, albumin uptake capacity was measured as about 0.37 mg ml−1 in cultured rat lung epithelial monolayers, further highlighting the (patho)physiological relevance of active alveolar epithelial protein transport. Moreover, gene silencing and pharmacological inhibition of the multi-ligand receptor megalin resulted in significantly decreased albumin binding and uptake in monolayers of primary alveolar type II and type I-like and cultured lung epithelial cells. Our data indicate that clearance of albumin from the distal air spaces is facilitated by an active, high-capacity, megalin-mediated transport process across the alveolar epithelium. Further understanding of this mechanism is of clinical importance, since an inability to clear excess protein from the alveolar space is associated with poor outcome in patients with acute lung injury/acute respiratory distress syndrome.

Cholesterol and its derivatives in Sonic Hedgehog signaling and cancer

The connection between the Hedgehog (HH) pathway and cholesterol has been recognized since the early days that shaped our current understanding of this unique pathway. Cholesterol and related lipids are intricately linked to HH signaling: from the role of cholesterol in HH biosynthesis to the modulation of HH signal reception and transduction by other sterols, passing by the phylogenetic relationships among many components of the HH pathway that resemble or contain lipid-binding domains. Here I review the connections between HH signaling, cholesterol and its derivatives and analyze the potential implications for HH-dependent cancers.

Mutation mapping and identification by whole-genome sequencing

Genetic mapping of mutations in model systems has facilitated the identification of genes contributing to fundamental biological processes including human diseases. However, this approach has historically required the prior characterization of informative markers. Here we report a fast and cost-effective method for genetic mapping using next-generation sequencing that combines single nucleotide polymorphism discovery, mutation localization, and potential identification of causal sequence variants. In contrast to prior approaches, we have developed a hidden Markov model to narrowly define the mutation area by inferring recombination breakpoints of chromosomes in the mutant pool. In addition, we created an interactive online software resource to facilitate automated analysis of sequencing data and demonstrate its utility in the zebrafish and mouse models. Our novel methodology and online tools will make next-generation sequencing an easily applicable resource for mutation mapping in all model systems.

LRP2 is an auxiliary SHH receptor required to condition the forebrain ventral midline for inductive signals

Sonic hedgehog (SHH) is a regulator of forebrain development that acts through its receptor, patched 1. However, little is known about cellular mechanisms at neurulation, whereby SHH from the prechordal plate governs specification of the rostral diencephalon ventral midline (RDVM), a major forebrain organizer. We identified LRP2, a member of the LDL receptor gene family, as a component of the SHH signaling machinery in the RDVM. LRP2 acts as an apical SHH-binding protein that sequesters SHH in its target field and controls internalization and cellular trafficking of SHH/patched 1 complexes. Lack of LRP2 in mice and in cephalic explants results in failure to respond to SHH, despite functional expression of patched 1 and smoothened, whereas overexpression of LRP2 variants in cells increases SHH signaling capacity. Our data identify a critical role for LRP2 in SHH signaling and reveal the molecular mechanism underlying forebrain anomalies in mice and patients with Lrp2 defects.

Low-density lipoprotein receptor (LDLR) family orchestrates cholesterol homeostasis

The LDLR family of proteins is involved in lipoproteins trafficking. While the role of LDLR in cardiovascular disease has been widely studied, only recently the role of other members of the LDLR proteins in lipoprotein homeostasis and atherosclerosis has emerged. LDLR, VLDLR, and LRPs bind and internalize apoE- and apoB-containing lipoprotein, including LDL and VLDL, and regulate their cellular uptake. LRP6 is a unique member of this family for its function as a co-receptor for Wnt signal transduction. The work in our laboratory has shown that LRP6 also plays a key role in lipoprotein and TG clearance, glucose homoeostasis, and atherosclerosis. The role of these receptor proteins in pathogenesis of diverse metabolic risk factors is emerging, rendering them targets of novel therapeutics for metabolic syndrome and atherosclerosis. This manuscript reviews the physiological role of the LDLR family of proteins and describes its involvement in pathogenesis of hyperlipidemia and atherosclerosis.

Megalin mediates the influence of sonic hedgehog on oligodendrocyte precursor cell migration and proliferation during development

Oligodendrocyte precursor cells (OPCs) of the optic nerve are generated in the preoptic area, from where they migrate to colonize it entirely. Sonic hedgehog (Shh) induces the proliferation of these cells as well as influencing their migration, acting through its canonical receptor (Ptc-1). However, the multiligand receptor megalin (or LRP-2) is also involved in Shh-induced OPC proliferation and migration, and thus, we have evaluated the relevance of this interaction. During the stages at which Shh influences OPC development, we found megalin to be selectively expressed by optic nerve astrocytes, whereas Ptc-1 and Gli1 were found in OPCs. Indeed, this pattern of expression paralleled the rostral-caudal expression of the three Shh-related molecules during the time course of plp-dm20(+) -OPC colonization. The blockage of megalin partially abolished OPC chemoattraction and fully impaired Shh-induced proliferation. Using in vitro co-cultures of dissociated optic nerve cells, we demonstrated that Shh was internalized by astrocytes via megalin, and sufficient Shh was subsequently released to produce the biological effects on OPCs observed in the nerve. Together, these data indicate that at least part of the influence of Shh on OPCs is mediated by megalin during optic nerve development, and that astrocytes expressing megalin transiently capture Shh to present it to OPCs and/or to control the gradient of this molecule during development.

Lipid metabolism and glial lipoproteins in the central nervous system

Lipoproteins in the central nervous system (CNS) are not incorporated from the blood but are formed mainly by glial cells within the CNS. In addition, cholesterol in the CNS is synthesized endogenously because the blood-brain barrier segregates the CNS from the peripheral circulation. Apolipoprotein (apo) E is a major apo in the CNS. In normal condition, apo E is secreted from glia, mainly from astrocytes, and forms cholesterol-rich lipoproteins by ATP-binding cassette transporters. Subsequently, apo E-containing glial lipoproteins supply cholesterol and other components to neurons via a receptor-mediated process. Recent findings demonstrated that receptors of the low density lipoprotein (LDL) receptor family not only internalize lipoproteins into the cells but also, like signaling receptors, transduce signals upon binding the ligands. In this review, the regulation of lipid homeostasis will be discussed as well as roles of lipoproteins and functions of receptors of LDL receptor family in the CNS. Furthermore, the relation between lipid metabolism and Alzheimer's disease (AD) is discussed.

Retinal proliferation response in the buphthalmic zebrafish, bugeye

The zebrafish retina regenerates in response to acute retinal lesions, replacing damaged neurons with new neurons. In this study we test the hypothesis that chronic stress to inner retinal neurons also triggers a retinal regeneration response in the bugeye zebrafish. Mutations in the lrp2 gene in zebrafish are associated with a progressive eye phenotype (bugeye) that models several risk factors for human glaucoma including buphthalmos (enlarged eyes), elevated intraocular pressure (IOP), and upregulation of genes related to retinal ganglion cell pathology. The retinas of adult bugeye zebrafish showed high rates of ongoing proliferation which resulted in the production of a small number of new retinal neurons, particularly photoreceptors. A marker of mechanical cell stress, Hsp27, was strongly expressed in inner retinal neurons and glia of bugeye retinas. The more enlarged eyes of individual bugeye zebrafish showed disrupted retinal lamination, and a persistent reduced density of neurons in the ganglion cell layer (GCL), although total numbers of GCL neurons were higher than in control eyes. Despite the presence of a proliferative response to damage, the adult bugeye zebrafish remained behaviorally blind. These findings suggest the existence of an unsuccessful regenerative response to a persistent pathological condition in the bugeye zebrafish.

Mouse model of proximal tubule endocytic dysfunction

BACKGROUND

Several studies have indicated the central role of the megalin/cubilin multiligand endocytic receptor complex in protein reabsorption in the kidney proximal tubule. However, the poor viability of the existing megalin-deficient mice precludes further studies and comparison of homogeneous groups of mice.

METHODS

Megalin- and/or cubilin-deficient mice were generated using a conditional Cre-loxP system, where the Cre gene is driven by the Wnt4 promoter. Kidney tissues from the mice were analysed for megalin and cubilin expression by quantitative reverse transcription-polymerase chain reaction, western blotting and immunohistochemistry. Renal albumin uptake was visualized by immunohistochemistry. Twenty-four-hour urine samples were collected in metabolic cages and analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and western blotting. Urinary albumin/creatinine ratios were measured by ELISA and the alkaline picrate method.

RESULTS

The Meg(lox/lox);Cre(+), Cubn(lox/lox);Cre(+) and Meg(lox/lox), Cubn(lox/lox);Cre(+) mice were all viable, fertile and developed normal kidneys. Megalin and/or cubilin expression, assessed by immunohistology and western blotting, was reduced by >89%. Consistent with this observation, the mice excreted megalin and cubilin ligands such as transferrin and albumin in addition to low-molecular weight proteins. We further show that megalin/cubilin double-deficient mice excrete albumin with an average of 1.45 ± 0.54 mg/day, suggesting a very low albumin concentration in the glomerular ultrafiltrate.

CONCLUSIONS

We report here the efficient genetic ablation of megalin, cubilin or both, using a Cre transgene driven by the Wnt4 promoter. The viable megalin/cubilin double-deficient mice now allow for detailed large-scale group analysis, and we anticipate that the mice will be of great value as an animal model for proximal tubulopathies with disrupted endocytosis.

Dietary fat impacts fetal growth and metabolism: uptake of chylomicron remnant core lipids by the placenta

The fetus requires significant energy for growth and development. Although glucose is a major source of energy for the fetus, other maternal nutrients also appear to promote growth. Thus, the goal of these studies was to determine whether triglyceride-rich remnants are taken up by the placenta and whether maternal dietary lipids, independently of adiposity, can impact fetal growth. To accomplish our first goal, chylomicron particles were duallly labeled with cholesteryl ester and triglycerides. The placenta took up remnant particles/core lipids at rates greater than adipose tissue and skeletal muscle but less than the liver. Although the placenta expresses apoE receptors, uptake of chylomicron remnants and/or core lipids can occur independently of apoE. To determine the impact of dietary lipid on fetal growth, independent of maternal adiposity, females were fed high-fat diets (HFD) for 1 mo; there was no change in adiposity or leptin levels prior to or during pregnancy of dams fed HFD. Fetal masses were greater in dams fed HFD, and mRNA levels of proteins involved in fatty acid oxidation (CPT I, PPARα), but not glucose oxidation (pyruvate kinase) or other regulatory processes (HNF-4α, LXR), were increased with maternal dietary fat. There was also no change in mRNA levels of proteins involved in placental glucose and fatty acid transport, and GLUT1 protein levels in microvillous membranes were similar in placentas of dams fed either diet. Thus, the ability of the placenta to take up chylomicron remnant core lipids likely contributes to accelerated fetal growth in females fed high fat diets.

MicroRNA gene expression signatures in the developing neural tube

BACKGROUND

Neurulation requires precise, spatio-temporal expression of numerous genes and coordinated interaction of signal transduction and gene regulatory networks, disruption of which may contribute to the etiology of neural tube defects (NTDs). MicroRNAs (miRNAs) are key modulators of cell and tissue differentiation. To define potential roles of miRNAs in development of the murine neural tube (NT), miRNA microarray analysis was conducted to establish expression profiles, and identify miRNA target genes and functional gene networks.

METHODS

The miRNA expression profiles in murine embryonic NTs derived from gestational days 8.5, 9.0, and 9.5 were defined and compared utilizing miRXplore microarrays from Miltenyi Biotec GmbH, Bergisch Gladbach, Germany. Gene expression changes were verified by TaqMan quantitative Real-Time PCR. The clValid R package and the UPGMA (hierarchical) clustering method were utilized for cluster analysis of the microarray data. Functional associations among selected miRNAs were examined via Ingenuity Pathway Analysis.

RESULTS

The miRXplore chips enabled examination of 609 murine miRNAs. Expression of approximately 12% of these was detected in murine embryonic NTs. Clustering analysis revealed several developmentally regulated expression clusters among these expressed genes. Target analysis of differentially expressed miRNAs enabled identification of numerous target genes associated with cellular processes essential for normal NT development. Utilization of Ingenuity Pathway Analysis revealed interactive biologic networks which connected differentially expressed miRNAs with their target genes, and highlighted functional relationships.

CONCLUSIONS

The present study defined unique gene expression signatures of a range of miRNAs in the developing NT during the critical period of NT morphogenesis. Analysis of miRNA target genes and gene interaction pathways revealed that specific miRNAs might direct expression of numerous genes encoding proteins, which have been shown to be indispensable for normal neurulation. This study is the first to identify miRNA expression profiles and their potential regulatory networks in the developing mammalian NT.

Retinal proliferation response in the buphthalmic zebrafish, bugeye

The zebrafish retina regenerates in response to acute retinal lesions, replacing damaged neurons with new neurons. In this study we test the hypothesis that chronic stress to inner retinal neurons also triggers a retinal regeneration response in the bugeye zebrafish. Mutations in the lrp2 gene in zebrafish are associated with a progressive eye phenotype (bugeye) that models several risk factors for human glaucoma including buphthalmos (enlarged eyes), elevated intraocular pressure (IOP), and upregulation of genes related to retinal ganglion cell pathology. The retinas of adult bugeye zebrafish showed high rates of ongoing proliferation which resulted in the production of a small number of new retinal neurons, particularly photoreceptors. A marker of mechanical cell stress, Hsp27, was strongly expressed in inner retinal neurons and glia of bugeye retinas. The more enlarged eyes of individual bugeye zebrafish showed disrupted retinal lamination, and a persistent reduced density of neurons in the ganglion cell layer (GCL), although total numbers of GCL neurons were higher than in control eyes. Despite the presence of a proliferative response to damage, the adult bugeye zebrafish remained behaviorally blind. These findings suggest the existence of an unsuccessful regenerative response to a persistent pathological condition in the bugeye zebrafish.

Loss of Lrp2 in zebrafish disrupts pronephric tubular clearance but not forebrain development

Low-density lipoprotein receptor-related protein 2 (LRP2) is a multifunctional cell surface receptor conserved from nematodes to humans. In mammals, it acts as regulator of sonic hedgehog and bone morphogenetic protein pathways in patterning of the embryonic forebrain and as a clearance receptor in the adult kidney. Little is known about activities of this LRP in other phyla. Here, we extend the functional elucidation of LRP2 to zebrafish as a model organism of receptor (dys)function. We demonstrate that expression of Lrp2 in embryonic and larval fish recapitulates the patterns seen in mammalian brain and kidney. Furthermore, we studied the consequence of receptor deficiencies in lrp2 and in lrp2b, a homologue unique to fish, using ENU mutagenesis or morpholino knockdown. While receptor-deficient zebrafish suffer from overt renal resorption deficiency, their brain development proceeds normally, suggesting evolutionary conservation of receptor functions in pronephric duct clearance but not in patterning of the teleost forebrain.

Mutations in zebrafish lrp2 result in adult-onset ocular pathogenesis that models myopia and other risk factors for glaucoma

The glaucomas comprise a genetically complex group of retinal neuropathies that typically occur late in life and are characterized by progressive pathology of the optic nerve head and degeneration of retinal ganglion cells. In addition to age and family history, other significant risk factors for glaucoma include elevated intraocular pressure (IOP) and myopia. The complexity of glaucoma has made it difficult to model in animals, but also challenging to identify responsible genes. We have used zebrafish to identify a genetically complex, recessive mutant that shows risk factors for glaucoma including adult onset severe myopia, elevated IOP, and progressive retinal ganglion cell pathology. Positional cloning and analysis of a non-complementing allele indicated that non-sense mutations in low density lipoprotein receptor-related protein 2 (lrp2) underlie the mutant phenotype. Lrp2, previously named Megalin, functions as an endocytic receptor for a wide-variety of bioactive molecules including Sonic hedgehog, Bone morphogenic protein 4, retinol-binding protein, vitamin D-binding protein, and apolipoprotein E, among others. Detailed phenotype analyses indicated that as lrp2 mutant fish age, many individuals—but not all—develop high IOP and severe myopia with obviously enlarged eye globes. This results in retinal stretch and prolonged stress to retinal ganglion cells, which ultimately show signs of pathogenesis. Our studies implicate altered Lrp2-mediated homeostasis as important for myopia and other risk factors for glaucoma in humans and establish a new genetic model for further study of phenotypes associated with this disease.

Complex genetic inheritance, including variable penetrance and severity, underlies many common eye diseases. In this study, we present analysis of a zebrafish mutant, bugeye, which shows complex inheritance of multiple ocular phenotypes that are known risk factors for glaucoma, including high myopia, elevated intraocular pressure, and up-regulation of stress-response genes in retinal ganglion cells. Molecular genetic analysis revealed that mutations in low density lipoprotein receptor-related protein 2 (lrp2) underlie the mutant phenotypes. Lrp2 is a large transmembrane protein expressed in epithelia of the eye. It facilitates transport and clearance of multiple secreted bioactive factors through receptor-mediated endocytosis. Glaucoma, a progressive blinding disorder, usually presents in adulthood and is characterized by optic nerve damage followed by ganglion cell death. In bugeye/lrp2 mutants, ganglion cell death was significantly elevated, but surprisingly moderate, and therefore they do not model this endpoint of glaucoma. As such, bugeye/lrp2 mutants should be considered valuable as a genetic model (A) for buphthalmia, myopia, and regulated eye growth; (B) for identifying genes and pathways that modify the observed ocular phenotypes; and (C) for studying the initiation of retinal ganglion cell pathology in the context of high myopia and elevated intraocular pressure.

Megalin/LRP2 expression is induced by peroxisome proliferator-activated receptor -alpha and -gamma: implications for PPARs' roles in renal function

Background

Megalin is a large endocytic receptor with relevant functions during development and adult life. It is expressed at the apical surface of several epithelial cell types, including proximal tubule cells (PTCs) in the kidney, where it internalizes apolipoproteins, vitamins and hormones with their corresponding carrier proteins and signaling molecules. Despite the important physiological roles of megalin little is known about the regulation of its expression. By analyzing the human megalin promoter, we found three response elements for the peroxisomal proliferator-activated receptor (PPAR). The objective of this study was to test whether megalin expression is regulated by the PPARs.

Methodology/Principal Findings

Treatment of epithelial cell lines with PPARα or PPARγ ligands increased megalin mRNA and protein expression. The stimulation of megalin mRNA expression was blocked by the addition of specific PPARα or PPARγ antagonists. Furthermore, PPAR bound to three PPAR response elements located in the megalin promoter, as shown by EMSA, and PPARα and its agonist activated a luciferase construct containing a portion of the megalin promoter and the first response element. Accordingly, the activation of PPARα and PPARγ enhanced megalin expression in mouse kidney. As previously observed, high concentrations of bovine serum albumin (BSA) decreased megalin in PTCs in vitro; however, PTCs pretreated with PPARα and PPARγ agonists avoided this BSA-mediated reduction of megalin expression. Finally, we found that megalin expression was significantly inhibited in the PTCs of rats that were injected with BSA to induce tubulointerstitial damage and proteinuria. Treatment of these rats with PPARγ agonists counteracted the reduction in megalin expression and the proteinuria induced by BSA.

Conclusions

PPARα/γ and their agonists positively control megalin expression. This regulation could have an important impact on several megalin-mediated physiological processes and on pathophysiologies such as chronic kidney disease associated with diabetes and hypertension, in which megalin expression is impaired.

β-Carotene and its cleavage enzyme β-carotene-15,15'-oxygenase (CMOI) affect retinoid metabolism in developing tissues

The mammalian embryo relies on maternal circulating retinoids (vitamin A derivatives) for development. β-Carotene is the major human dietary provitamin A. β-Carotene-15,15'-oxygenase (CMOI) has been proposed as the main enzyme generating retinoid from β-carotene in vivo. CMOI is expressed in embryonic tissues, suggesting that β-carotene provides retinoids locally during development. We performed loss of CMOI function studies in mice lacking retinol-binding protein (RBP), an established model of embryonic vitamin A deficiency (VAD). We show that, unexpectedly, lack of CMOI in the developing tissues further exacerbates the severity of VAD and thus the embryonic malformations of RBP(-/-) mice. Since β-carotene was not present in any of the mouse diets, we unveiled a novel action of CMOI independent from its β-carotene cleavage activity. We also show for the first time that CMOI exerts an additional function on retinoid metabolism by influencing retinyl ester formation via modulation of lecithin:retinol acyltransferase (LRAT) activity, at least in developing tissues. Finally, we demonstrate unequivocally that β-carotene can serve as an alternative vitamin A source for the in situ synthesis of retinoids in developing tissues by the action of CMOI.

Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family

The evolutionarily ancient low-density lipoprotein (LDL) receptor gene family represents a class of widely expressed cell surface receptors. Since the dawn of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors have evolved. In accordance with the now obsolete 'one-gene-one-function' hypothesis, these cell surface receptors were originally perceived as mere transporters of lipoproteins, lipids, and nutrients or as scavenger receptors, which remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular environment and the cell surface. This picture has since undergone a fundamental change. Experimental evidence has replaced the perception that these receptors serve merely as cargo transporters. Instead it is now clear that the transport of macromolecules is inseparably intertwined with the molecular machinery by which cells communicate with each other. Lipoprotein receptors are essentially sensors of the extracellular environment that participate in a wide range of physiological processes by physically interacting and coevolving with primary signal transducers as co-regulators. Furthermore, lipoprotein receptors modulate cellular trafficking and localization of the amyloid precursor protein (APP) and the β-amyloid peptide (Aβ), suggesting a role in the pathogenesis of Alzheimer's disease. Moreover, compelling evidence shows that LDL receptor family members are involved in tumor development and progression.

Enhanced excretion of vitamin D binding protein in type 1 diabetes: a role in vitamin D deficiency?

CONTEXT

Vitamin D deficiency is an increasingly recognized comorbidity in patients with both type 1 (T1D) and type 2 diabetes, particularly associated with the presence of diabetic nephropathy.

OBJECTIVE

Because we have previously reported enhanced excretion of megalin in the urine of T1D patients with microalbuminuria, we hypothesized that concurrent urinary loss of the megalin ligand, vitamin D binding protein, might contribute mechanistically to vitamin D deficiency.

DESIGN AND PARTICIPANTS

Examining a study cohort of 115 subjects with T1D, aged 14-40 yr, along with 55 age-matched healthy control subjects, we measured plasma and urine concentrations of vitamin D binding protein (VDBP) along with serum concentrations of total calcium, parathyroid hormone, 25-hydroxyvitamin D, and 1, 25-dihydroxyvitamin D; these results were compared between groups and investigated for relationships with metabolic control status or with albuminuria.

MAIN OUTCOME MEASURE

Between-group differences in urinary VDBP concentration were the main outcome measures.

RESULTS

A marked increase in the urinary excretion of VDBP was apparent in subjects with T1D, compared with control subjects. Using multivariate regression modeling, significant correlates of urinary VDBP excretion included microalbuminuria (P = 0.004), glycosylated hemoglobin (P = 0.010), continuous glucose monitoring system average capillary glucose (P = 0.047), and serum 1,25(OH)(2)D concentrations (P = 0.037). Vitamin D deficiency or insufficiency was slightly more prevalent in diabetic subjects with albuminuria, coincident with the increase in urine VDBP excretion.

CONCLUSIONS

These findings suggest that, theoretically, exaggerated urinary loss of VDBP in T1D, particularly in persons with albuminuria, could contribute mechanistically to vitamin D deficiency in this disease.

Tubular toxicity of proteinuria

Proteinuria is a prognostic indicator of progressive kidney disease and poor cardiovascular outcomes. Abnormally filtered bioactive macromolecules interact with proximal tubular epithelial cells (PTECs), which results in the development of proteinuric nephropathy. This condition is characterized by alterations in PTEC growth, apoptosis, gene transcription and inflammatory cytokine production as a consequence of dysregulated signaling pathways that are stimulated by proteinuric tubular fluid. The megalin-cubilin complex mediates the uptake of several proteins, including albumin, into PTECs. Megalin might also possess intrinsic signaling properties and the ability to regulate cell signaling pathways and gene transcription after processing regulated intramembrane proteolysis. Megalin could, therefore, link abnormal PTEC albumin exposure with altered growth factor receptor activation, proinflammatory and profibrotic signaling, and gene transcription. Evidence now suggests that other PTEC pathways for protein reabsorption of (patho)physiological importance might be mediated by the neonatal Fc receptor and CD36.

Analysis of the expression pattern of the carrier protein transthyretin and its receptor megalin in the human scalp skin and hair follicles: hair cycle-associated changes

Transthyretin is a serum and cerebrospinal fluid protein synthesized early in development by the liver, choroid plexus and several other tissues. It is a carrier protein for the antioxidant vitamins, retinol, and thyroid hormones. Transthyretin helps internalize thyroxine and retinol-binding protein into cells by binding to megalin, which is a multi-ligand receptor expressed on the luminal surface of various epithelia. We investigated the expression of transthyretin and its receptor megalin in the human skin; however, their expression pattern in the hair follicle is still to be elucidated. This study addresses this issue and tests the hypothesis that "the expression of transthyretin and megalin undergoes hair follicle cycle-dependent changes." A total of 50 normal human scalp skin biopsies were examined (healthy females, 53-62 years) using immunofluorescence staining methods and real-time PCR. In each case, 50 hair follicles were analyzed (35, 10, and 5 follicles in anagen, catagen, and telogen, respectively). Transthyretin and megalin were prominently expressed in the human scalp skin and hair follicles, on both gene and protein levels. The concentrations of transthyretin and megalin were 0.12 and 0.03 Ul/ml, respectively, as indicated by PCR. The expression showed hair follicle cycle-associated changes i.e., strong expression during early and mature anagen, very weak expression during catagen and moderate expression during telogen. The expression values of these proteins in the anagen were statistically significantly higher than those of either catagen or telogen hair follicles (P ≤ 0.001). This study provides the first morphologic indication that transthyretin and megalin are variably expressed in the human scalp skin and hair follicles. It also reports variations in the expression of these proteins during hair follicle cycling. The clinical ramifications of these findings are open for further investigations.

Hedgehog signaling update

In vertebrate hedgehog signaling, hedgehog ligands are processed to become bilipidated and then multimerize, which allows them to leave the signaling cell via Dispatched 1 and become transported via glypicans and megalin to the responding cells. Hedgehog then interacts with a complex of Patched 1 and Cdo/Boc, which activates endocytic Smoothened to the cilium. Patched 1 regulates the activity of Smoothened (1) via Vitamin D3, which inhibits Smoothened in the absence of hedgehog ligand or (2) via oxysterols, which activate Smoothened in the presence of hedgehog ligand. Hedgehog ligands also interact with Hip1, Patched 2, and Gas1, which regulate the range as well as the level of hedgehog signaling. In vertebrates, Smoothened is shortened at its C-terminal end and lacks most of the phosphorylation sites of importance in Drosophila. Cos2, also of importance in Drosophila, plays no role in mammalian transduction, nor do its homologs Kif7 and Kif27. The cilium may provide a function analogous to that of Cos2 by linking Smoothened to the modulation of Gli transcription factors. Disorders associated with the hedgehog signaling network follow, including nevoid basal cell carcinoma syndrome, holoprosencephaly, Smith-Lemli-Opitz syndrome, Greig cephalopolysyndactyly syndrome, Pallister-Hall syndrome, Carpenter syndrome, and Rubinstein-Taybi syndrome.

Hedgehog signaling update

In vertebrate hedgehog signaling, hedgehog ligands are processed to become bilipidated and then multimerize, which allows them to leave the signaling cell via Dispatched 1 and become transported via glypicans and megalin to the responding cells. Hedgehog then interacts with a complex of Patched 1 and Cdo/Boc, which activates endocytic Smoothened to the cilium. Patched 1 regulates the activity of Smoothened (1) via Vitamin D3, which inhibits Smoothened in the absence of hedgehog ligand or (2) via oxysterols, which activate Smoothened in the presence of hedgehog ligand. Hedgehog ligands also interact with Hip1, Patched 2, and Gas1, which regulate the range as well as the level of hedgehog signaling. In vertebrates, Smoothened is shortened at its C-terminal end and lacks most of the phosphorylation sites of importance in Drosophila. Cos2, also of importance in Drosophila, plays no role in mammalian transduction, nor do its homologs Kif7 and Kif27. The cilium may provide a function analogous to that of Cos2 by linking Smoothened to the modulation of Gli transcription factors. Disorders associated with the hedgehog signaling network follow, including nevoid basal cell carcinoma syndrome, holoprosencephaly, Smith-Lemli-Opitz syndrome, Greig cephalopolysyndactyly syndrome, Pallister-Hall syndrome, Carpenter syndrome, and Rubinstein-Taybi syndrome.