[Show abstract][Hide abstract] ABSTRACT: 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 delivery of folate to neuroepithelial cells during neurulation. Uptake assays in whole embryo cultures showed that LRP2 deficient neuroepithelial cells are unable to mediate uptake of folate bound to soluble folate receptor 1 (sFOLR1). Consequently, folate concentrations are significantly reduced in Lrp2(-/-) embryos compared to control littermates. Moreover, the folic acid dependent gene Alx3 is significantly down regulated 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.
Journal of Cell Science 03/2014; · 5.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
Development 12/2012; 139(23):4311-9. · 6.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Distribution of selenium (Se) within the mammalian body is mediated by SePP (selenoprotein P), an Se-rich glycoprotein secreted by hepatocytes. Genetic and biochemical evidence indicate that the endocytic receptors ApoER2 (apolipoprotein E receptor 2) and megalin mediate tissue-specific SePP uptake. In the present study megalin-mutant mice were fed on diets containing adequate (0.15 p.p.m.) or low (0.08 p.p.m.) Se content and were analysed for tissue and plasma Se levels, cellular GPx (glutathione peroxidase) activities and protein expression patterns. Megalin-mutant mice displayed increased urinary Se loss, which correlated with SePP excretion in their urine. Accordingly, serum Se and SePP levels were significantly reduced in megalin-mutant mice, reaching marginal levels on the low-Se diet. Moreover, kidney Se content and expression of renal selenoproteins were accordingly reduced, as was SePP internalization along the proximal tubule epithelium. Although GPx4 expression was not altered in testis, Se and GPx activity in liver and brain were significantly reduced. When fed on a low-Se diet, megalin-mutant mice developed impaired movement co-ordination, but no astrogliosis. These findings suggest that megalin prevents urinary SePP loss and participates in brain Se/SePP uptake.
[Show abstract][Hide abstract] ABSTRACT: Megalin-mediated endocytic uptake constitutes the main pathway for clearance of plasma proteins from the glomerular filtrate in proximal tubules. Little is known, however, about mechanisms that control megalin expression and activity in the kidney. A widely discussed hypothesis states that upon ligand binding a regulated intramembrane proteolysis releases the cytosolic domain of megalin and this fragment subsequently modulates megalin gene transcription. Here, we tested this by generating a mouse model that co-expressed both the soluble intracellular domain and full-length megalin. Despite pronounced synthesis in the proximal tubules, the soluble intracellular domain failed to exert distinct effects on renal proximal tubular function, including megalin expression, endocytic retrieval of proteins, or global renal gene transcription. Hence, our study argues that the soluble intracellular domain does not have a role in regulating the activity of megalin in the kidney.
Kidney International 09/2010; 78(5):473-7. · 8.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The microenvironment of growth factors in the subependymal zone (SEZ) of the adult brain provides the instructive milieu for neurogenesis to proceed in this germinal niche. In particular, tight regulation of bone morphogenetic protein (BMP) signaling is essential to balance proliferative and non-proliferative cell fate specification. However, the regulatory pathways that control BMP signaling in the SEZ are still poorly defined. We demonstrate that LRP2, a clearance receptor for BMP4 is specifically expressed in ependymal cells of the lateral ventricles in the adult brain. Intriguingly, expression is restricted to the ependyma that faces the stem cell niche. Expression is not seen in ependyma elsewhere in the lateral ventricles or in the dentate gyrus, the second major neurogenic zone of the adult brain. We further show that lack of LRP2 expression in adult mice results in impaired proliferation of neural precursor cells in the SEZ resulting in decreased numbers of neuroblasts reaching the olfactory bulb. Reduced neurogenesis coincides with increased BMP4 expression and enhanced activation of downstream mediators phospho-SMAD1/5/8 and ID3 in the stem cell niche. Our findings suggest a novel mechanism whereby LRP2-mediated catabolism of BMP4 in the ependyma modulates the microenvironment of the SEZ and enables adult neurogenesis to proceed.
[Show abstract][Hide abstract] ABSTRACT: Cerebral ischemic small vessel disease (SVD) is the leading cause of vascular dementia and a major contributor to stroke in humans. Dominant mutations in NOTCH3 cause cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), a genetic archetype of cerebral ischemic SVD. Progress toward understanding the pathogenesis of this disease and developing effective therapies has been hampered by the lack of a good animal model. Here, we report the development of a mouse model for CADASIL via the introduction of a CADASIL-causing Notch3 point mutation into a large P1-derived artificial chromosome (PAC). In vivo expression of the mutated PAC transgene in the mouse reproduced the endogenous Notch3 expression pattern and main pathological features of CADASIL, including Notch3 extracellular domain aggregates and granular osmiophilic material (GOM) deposits in brain vessels, progressive white matter damage, and reduced cerebral blood flow. Mutant mice displayed attenuated myogenic responses and reduced caliber of brain arteries as well as impaired cerebrovascular autoregulation and functional hyperemia. Further, we identified a substantial reduction of white matter capillary density. These neuropathological changes occurred in the absence of either histologically detectable alterations in cerebral artery structure or blood-brain barrier breakdown. These studies provide in vivo evidence for cerebrovascular dysfunction and microcirculatory failure as key contributors to hypoperfusion and white matter damage in this genetic model of ischemic SVD.
The Journal of clinical investigation 02/2010; 120(2):433-45. · 15.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Previously, the relevance of lipoproteins and their receptors has mainly been discussed in terms of cholesterol clearance in the adult organism. Now, findings from nematodes to fruit flies to mammals all point towards novel and unexpected roles for lipoprotein metabolism in the control of key regulatory pathways in the developing embryo, including signaling through steroid hormones and throughout the hedgehog and Wnt signaling pathways. Here, we discuss the emerging view of how lipoproteins and their receptors regulate embryogenesis.
Development 10/2007; 134(18):3239-49. · 6.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Androgens and estrogens are transported bound to the sex hormone binding globulin (SHBG). SHBG is believed to keep sex steroids inactive and to control the amount of free hormones that enter cells by passive diffusion. Contrary to the free hormone hypothesis, we demonstrate that megalin, an endocytic receptor in reproductive tissues, acts as a pathway for cellular uptake of biologically active androgens and estrogens bound to SHBG. In line with this function, lack of receptor expression in megalin knockout mice results in impaired descent of the testes into the scrotum in males and blockade of vagina opening in females. Both processes are critically dependent on sex-steroid signaling, and similar defects are seen in animals treated with androgen- or estrogen-receptor antagonists. Thus, our findings uncover the existence of endocytic pathways for protein bound androgens and estrogens and their crucial role in development of the reproductive organs.
[Show abstract][Hide abstract] ABSTRACT: Neuronal lineage formation in the developing olfactory epithelium has been extensively studied at the cellular level, but little is known about the genes that control proliferation and differentiation of neuronal progenitor cells. Here, we report that the Wilms' tumour zinc-finger protein, Wt1, is required for normal formation of the olfactory epithelium. Wt1 was detected by immunohistochemistry in the developing olfactory epithelium of wild-type embryos between gestational days E9.5 and E18.5. Embryos with complete lack of Wt1 and embryos with selective ablation of the alternatively spliced Wt1(+KTS) isoform both had thinner olfactory epithelia and fewer neuronal progenitor cells than do normal animals. Mash1 and neurogenin 1, two basic helix-loop-helix transcription factors with critical functions during olfactory neuron development, were reduced in the Wt1(+KTS)-/- mutants compared with the wild-type mice. Stable expression of the Wt1(+KTS) isoform, but not of the Wt1(-KTS) variant, upregulated Mash1 mRNA and protein in vitro. The olfactory epithelia of mouse embryos, which lacked the Wt1(-KTS) protein, appeared normal. However, formation of the neural retina was severely impaired in the Wt1(-KTS)-/- mutants. These findings demonstrate that the Wt1(+KTS) protein, which has been proposed to play a role in mRNA processing, acts upstream of Mash1 to promote the development of the olfactory epithelium. Furthermore, neuron formation depends on distinct functions of alternatively spliced Wt1 products in the embryonic retina and the olfactory epithelium.
Development 04/2005; 132(6):1327-36. · 6.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Megalin is a low-density lipoprotein receptor-related protein (LRP2) expressed in the neuroepithelium and the yolk sac of the early embryo. Absence of megalin expression in knockout mice results in holoprosencephaly, indicating an essential yet unidentified function in forebrain development. We used mice with complete or conditional megalin gene inactivation in the embryo to demonstrate that expression of megalin in the neuroepithelium but not in the yolk sac is crucial for brain development. During early forebrain development, megalin deficiency leads to an increase in bone morphogenic protein (Bmp) 4 expression and signaling in the rostral dorsal neuroepithelium, and a subsequent loss of sonic hedgehog (Shh) expression in the ventral forebrain. As a consequence of absent SHH activity, ventrally derived oligodendroglial and interneuronal cell populations are lost in the forebrain of megalin-/- embryos. Similar defects are seen in models with enhanced signaling through BMPs, central regulators of neural tube patterning. Because megalin mediates endocytic uptake and degradation of BMP4, these findings indicate a role for megalin in neural tube specification, possibly by acting as BMP4 clearance receptor in the neuroepithelium.
Development 02/2005; 132(2):405-14. · 6.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To study the mechanisms by which missense mutations in alpha-tropomyosin cause familial hypertrophic cardiomyopathy, we generated transgenic rats overexpressing alpha-tropomyosin with one of two disease-causing mutations, Asp(175)Asn or Glu(180)Gly, and analyzed phenotypic changes at molecular, morphological, and physiological levels. The transgenic proteins were stably integrated into the sarcomere, as shown by immunohistochemistry using a human-specific anti-alpha-tropomyosin antibody, ARG1. In transgenic rats with either alpha-tropomyosin mutation, molecular markers of cardiac hypertrophy were induced. Ca(2+) sensitivity of cardiac skinned-fiber preparations from animals with mutation Asp(175)Asn, but not Glu(180)Gly, was decreased. Furthermore, elevated frequency and amplitude of spontaneous Ca(2+) waves were detected only in cardiomyocytes from animals with mutation Asp(175)Asn, suggesting an increase in intracellular Ca(2+) concentration compensating for the reduced Ca(2+) sensitivity of isometric force generation. Accordingly, in Langendorff-perfused heart preparations, myocardial contraction and relaxation were accelerated in animals with mutation Asp(175)Asn. The results allow us to propose a hypothesis of the pathogenetic changes caused by alpha-tropomyosin mutation Asp(175)Asn in familial hypertrophic cardiomyopathy on the basis of changes in Ca(2+) handling as a sensitive mechanism to compensate for alterations in sarcomeric structure.
[Show abstract][Hide abstract] ABSTRACT: Slowed relaxation in diabetic cardiomyopathy (CM) is partially related to diminished expression of the sarcoplasmic reticulum (SR) Ca2+-ATPase SERCA2a. To evaluate the impact of SERCA2a overexpression on SR Ca2+ handling in diabetic CM, we 1) generated transgenic rats harboring a human cytomegalovirus enhancer/chicken beta-actin promotor-controlled rat SERCA2 transgene (SERCA2-TGR), 2) characterized their SR phenotype, and 3) examined whether transgene expression may rescue SR Ca2+ transport in streptozotocin-induced diabetes. The transgene was expressed in all heart chambers. Compared to wild-type (WT) rats, a heterozygous line exhibited increased SERCA2 mRNA (1.5-fold), SERCA2 protein (+26%) and SR Ca2+ uptake (+37%). Phospholamban expression was not altered. In SERCA2-TGR, contraction amplitude (+48%) and rates of contraction (+34%) and relaxation (+35%) of isolated papillary muscles (PM) were increased (P2+ uptake and SERCA2 protein of SERCA2-TGR were 1.3-fold higher (P2+ uptake, accelerates relaxation and compensates, in part, for depressed Ca2+ uptake in diabetic CM. Therefore, SERCA2 expression might constitute an important therapeutic target to rescue cardiac SR Ca2+ handling in diabetes.
The FASEB Journal 11/2002; 16(12):1657-9. · 5.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Glomerular disease is one of the most common causes of end-stage renal failure. Increasing evidence suggests that these glomerulopathies are frequently caused by primary lesions in the renal podocytes. One of the major consequences of podocyte lesions is the accumulation of mesangial matrix in the glomerular basement membrane, a process called glomerulosclerosis. Mesangial sclerosis is one of the most consistent findings in Denys-Drash patients and can be caused by dominant mutations in the Wilms' tumor 1 gene (WT1). The underlying mechanism, however, is poorly understood. WT1 is expressed in the podocytes throughout life, but its function in this cell type is unknown. Combining Wt1-knockout and inducible yeast artificial chromosome transgenic mouse models, we demonstrate that reduced expression levels of WT1 result in either crescentic glomerulonephritis or mesangial sclerosis depending on the gene dosage. Strikingly, the two podocyte-specific genes nphs1 and podocalyxin are dramatically downregulated in mice with decreased levels of Wt1, suggesting that these two genes act downstream of Wt1. Taken together, our data provide genetic evidence that reduced levels of Wt1 are responsible for the pathogenesis of two distinct renal diseases and offer a molecular explanation for the increased occurrence of glomerulosclerosis in patients with WAGR syndrome.
Human Molecular Genetics 04/2002; 11(6):651-9. · 7.69 Impact Factor