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Craniofacial and brain malformations in megalin-deficient newborn mice. (A) Head profiles of wild type (+/+) and megalin-deficient (-/-) newborn mouse. (B) Brains of +/+ and-/-mice after removal of the skin and bone. Arrows indicate olfactory bulbs. (C) Alizarin Red S/Alcian Blue staining of bone (red) and cartilage (blue) of a wild-type (specimen 3) and two variably affected megalin-/-mouse fetuses at day 18.5 (specimens 1 and 2). (D-F) Coronal paraffin sections (5 ,um) through the midbrain of a wild-type (D) and two megalin-/-newborn mice (E and F) stained with hematoxylin and eosin. cc, Corpus callosum; cp, protrusion of choroid plexus; lv, lateral ventricle; III, third ventricle; hc, holoprosencephalic cavity. (Bars = 1 mm.)
Source publication
gp330/megalin, a member of the low density lipoprotein (LDL) receptor gene family, is expressed on the apical surfaces of epithelial tissues, including the neuroepithelium, where it mediates the endocytic uptake of diverse macromolecules, such as cholesterol-carrying lipoproteins, proteases, and antiproteinases. Megalin knockout mice manifest abnor...
Citations
... During embryonic development, megalin plays a critical role in brain, cardiovascular, and lung development, as demonstrated by global megalin deficient mice. (1)(2)(3)(4) Megalin becomes most abundant in renal proximal tubule cells (PTCs) after birth, (5) and it functions primarily as an endocytic receptor in renal PTCs for many ligands including components of the reninangiotensin system. ...
Background
Pharmacological inhibition of megalin (also known as low-density lipoprotein receptor-related protein 2: LRP2) attenuates atherosclerosis in hypercholesterolemic mice. Since megalin is abundant in renal proximal tubule cells (PTCs), the purpose of this study was to determine whether PTC-specific deletion of megalin reduces hypercholesterolemia-induced atherosclerosis in mice.
Methods
Female Lrp2 f/f mice were bred with male Ndrg1 - Cre ERT2 +/0 mice to develop PTC-LRP2 +/+ and -/- littermates. To study atherosclerosis, all mice were to bred to an LDL receptor -/- background and fed a Western diet to induce atherosclerosis.
Results
PTC-specific megalin deletion did not attenuate atherosclerosis in LDL receptor -/- mice in either sex. Serendipitously, we discovered that PTC-specific megalin deletion led to interstitial infiltration of CD68+ cells and tubular atrophy. The pathology was only evident in male PTC-LRP2 -/- mice fed the Western diet, but not in mice fed a normal laboratory diet. Renal pathologies were also observed in male PTC-LRP2 -/- mice in an LDL receptor +/+ background fed the same Western diet, demonstrating that the renal pathologies were dependent on diet and not hypercholesterolemia. By contrast, female PTC-LRP2 -/- mice had no apparent renal pathologies. In vivo multiphoton microscopy demonstrated that PTC-specific megalin deletion dramatically diminished albumin accumulation in PTCs within 10 days of Western diet feeding. RNA sequencing analyses demonstrated the upregulation of inflammation-related pathways in kidney.
Conclusions
PTC-specific megalin deletion does not affect atherosclerosis, but leads to tubulointerstitial nephritis in mice fed Western diet, with severe pathologies in male mice.
Highlights
Deletion of megalin specifically in S1 and S2 of PTCs (proximal tubules) does not reduce atherosclerosis in hypercholesterolemic mice, irrespective of sex.
Deletion of megalin in S1 and S2 of PTCs induces TIN (tubulointerstitial nephritis) with severe renal pathological changes in male mice.
PTC-specific megalin deficiency-induced TIN occurs in both male LDL receptor -/- and LDL receptor +/+ mice fed a Wetsern diet.
The consumption of a Western diet exerts a crucial role in triggerring the observed TIN in male mice with PTC-specific megalin deletion.
... LRP2 is a multi-ligand, endocytic receptor with a variety of significant physiological functions, but it mainly functions in the uptake of lipoprotein components, notably triglyceride/cholesterol-protein complexes (Christensen and Birn 2002). In mammals, LRP2 has been known as an endocytic receptor that mediates the cellular uptake of cholesterol into the developing visceral yolk sac (Assemat et al. 2005), and into rapidly dividing neuroepithelium before the establishment of a complete circulatory system in the embryo (Willnow et al. 1996). In C. elegans, LRP2 which is present on the apical surface of the hyp7 syncytium is thought to be involved in the endocytosis of dietary sterols such as cholesterol from the epidermis. ...
Low-density lipoprotein receptor-related protein 2 (LRP2) is a multifunctional endocytic receptor expressed in epithelial cells. In mammals, it acts as an endocytic receptor that mediates the cellular uptake of cholesterol-containing apolipoproteins to maintain lipid homeostasis. However, little is known about the role of LRP2 in lipid homeostasis in insects. In the present study, we investigated the function of LRP2 in the migratory locust Locusta migratoria (LmLRP2). The mRNA of LmLRP2 is widely distributed in various tissues, including integument, wing pads, foregut, midgut, hindgut, Malpighian tubules and fat body, and the amounts of LmLRP2 transcripts decreased gradually in the early stages and then increased in the late stages before ecdysis during the nymphal developmental stage. Fluorescence immunohistochemistry revealed that the LmLRP2 protein is mainly located in cellular membranes of the midgut and hindgut. Using RNAi to silence LmLRP2 caused molting defects in nymphs (more than 60%), and the neutral lipid was found to accumulate in the midgut and surface of the integument, but not in the fat body, of dsLmLRP2-treated nymphs. The results of a lipidomics analysis showed that the main components of lipids (diglyceride and triglyceride) were significantly increased in the midgut, but decreased in the fat body and hemolymph. Furthermore, the content of total triglyceride was significantly increased in the midgut, but markedly decreased in the fat body and hemolymph in dsLmLRP2-injected nymphs. Our results indicate that LmLRP2 is located in the cellular membranes of midgut cells, and is required for lipid export from the midgut to the hemolymph and fat body in locusts.
... These mice show loss of ventrally derived oligodendroglial cells as well as interneuronal population. Megalin-triggered signaling pathway directly connects the extracellular environment with changes at the gene expression level in these neuroepithelial cells ultimately affecting brain developmental pathways (Willnow et al., 1996;Spoelgen et al., 2005). ...
Neurons forming the human brain are generated during embryonic development by neural stem and progenitor cells via a process called neurogenesis. A crucial feature contributing to neural stem cell morphological and functional heterogeneity is cell polarity, defined as asymmetric distribution of cellular components. Cell polarity is built and maintained thanks to the interplay between polarity proteins and polarity-generating organelles, such as the endoplasmic reticulum (ER) and the Golgi apparatus (GA). ER and GA affect the distribution of membrane components and work as a hub where glycans are added to nascent proteins and lipids. In the last decades our knowledge on the role of polarity in neural stem and progenitor cells have increased tremendously. However, the role of traffic and associated glycosylation in neural stem and progenitor cells is still relatively underexplored. In this review, we discuss the link between cell polarity, architecture, identity and intracellular traffic, and highlight how studies on neurons have shaped our knowledge and conceptual framework on traffic and polarity. We will then conclude by discussing how a group of rare diseases, called congenital disorders of glycosylation (CDG) offers the unique opportunity to study the contribution of traffic and glycosylation in the context of neurodevelopment.
... Patients with various forms of Fanconi renotubular syndrome secrete low molecular weight proteins (such as vitamin D-binding protein (DBP), retinol-binding protein and β2-microglobulin) in the urine because of proximal tubular dysfunction (45,46). Megalin-null mice have high perinatal mortality rate due to impaired renal function, respiratory complications, and holoprosencephaly (47), suggesting that the protein also plays important roles in embryonic development. ...
Sialidosis is an ultra-rare multisystemic lysosomal disease caused by mutations in the neuraminidase 1 (NEU1) gene. The severe type II form of the disease manifests with a prenatal/infantile or juvenile onset, bone abnormalities, severe neuropathology, and visceromegaly. A subset of these patients present with nephrosialidosis, characterized by abrupt onset of fulminant glomerular nephropathy. We studied the pathophysiological mechanism of the disease in 2 NEU1-deficient mouse models, a constitutive Neu1-knockout, Neu1ΔEx3, and a conditional phagocyte-specific knockout, Neu1Cx3cr1ΔEx3. Mice of both strains exhibited terminal urinary retention and severe kidney damage with elevated urinary albumin levels, loss of nephrons, renal fibrosis, presence of storage vacuoles, and dysmorphic mitochondria in the intraglomerular and tubular cells. Glycoprotein sialylation in glomeruli, proximal distal tubules, and distal tubules was drastically increased, including that of an endocytic reabsorption receptor megalin. The pool of megalin bearing O-linked glycans with terminal galactose residues, essential for protein targeting and activity, was reduced to below detection levels. Megalin levels were severely reduced, and the protein was directed to lysosomes instead of the apical membrane. Together, our results demonstrated that desialylation by NEU1 plays a crucial role in processing and cellular trafficking of megalin and that NEU1 deficiency in sialidosis impairs megalin-mediated protein reabsorption.
... Mice genetically disrupted for individual LCN2 receptors show severe phenotypes or die perinatally (44,(46)(47)(48), while mice lacking Lcn2 are viable (15). The fact that disruption of a potential LCN2 receptor provokes a more severe phenotype is not surprising, because the proposed receptors are multi-ligand binding receptors associated to many other pathways. ...
The human 25-kDa Lipocalin 2 (LCN2) was first identified and purified as a protein that in part is associated with gelatinase from neutrophils. This protein shows a high degree of sequence similarity with the deduced sequences of rat α2-microglobulin-related protein and the mouse protein 24p3. Based on its typical lipocalin fold, which consists of an eight-stranded, anti-parallel, symmetrical β-barrel fold structure it was initially thought that LCN2 is a circulating protein functioning as a transporter of small lipophilic molecules. However, studies in Lcn2 null mice have shown that LCN2 has bacteriostatic properties and plays a key role in innate immunity by sequestering bacterial iron siderophores. Numerous reports have further shown that LCN2 is involved in the control of cell differentiation, energy expenditure, cell death, chemotaxis, cell migration, and many other biological processes. In addition, important roles for LCN2 in health and disease have been identified in Lcn2 null mice and multiple molecular pathways required for regulation of Lcn2 expression have been identified. Nevertheless, although six putative receptors for LCN2 have been proposed, there is a fundamental lack in understanding of how these cell-surface receptors transmit and amplify LCN2 to the cell. In the present review we summarize the current knowledge on LCN2 receptors and discuss inconsistencies, misinterpretations and false assumptions in the understanding of these potential LCN2 receptors.
... In contrast to the restricted expression pattern of LRP2 in adults, LRP2 is more widely expressed during embryonic development [19,20]. LRP2 is required for proper development of numerous fetal tissues, including the brain, as is evident from the severe brain malformations observed in LRP2 knockout mice and in patients diagnosed with Donnai-Barrow syndrome [17,18,[21][22][23][24]. ...
Simple Summary
Epithelial tissues are the most common sites for the development of cancer. Loss of epithelial cell characteristics and dedifferentiation are hallmarks of cancer. A specialized and complex function in epithelial cells is receptor-mediated endocytosis. LRP2 (megalin) is the largest known endocytic membrane receptor of absorptive epithelia and mediates uptake of numerous ligands. However, its role and regulation in cancer has not been delineated. Therefore, we examined LRP2 expression across 33 cancer types in The Cancer Genome Atlas. As expected, we found the highest LRP2 expression in cancers arising from LRP2-expressing epithelia. However, in a subset of these tumors, we observed epigenetic silencing of LRP2. Interestingly, low LRP2 expression was associated with tumor cell dedifferentiation and poorer patient outcome, suggesting LRP2 is a potential cancer biomarker. Based on this, we warrant further studies on the functional role of LRP2 in tumors of epithelial origin and the implications of LRP2 downregulation.
Abstract
More than 80% of human cancers originate in epithelial tissues. Loss of epithelial cell characteristics are hallmarks of tumor development. Receptor-mediated endocytosis is a key function of absorptive epithelial cells with importance for cellular and organismal homeostasis. LRP2 (megalin) is the largest known endocytic membrane receptor and is essential for endocytosis of various ligands in specialized epithelia, including the proximal tubules of the kidney, the thyroid gland, and breast glandular epithelium. However, the role and regulation of LRP2 in cancers that arise from these tissues has not been delineated. Here, we examined the expression of LRP2 across 33 cancer types in The Cancer Genome Atlas. As expected, the highest levels of LRP2 were found in cancer types that arise from LRP2-expressing absorptive epithelial cells. However, in a subset of tumors from these cancer types, we observed epigenetic silencing of LRP2. LRP2 expression showed a strong inverse correlation to methylation of a specific CpG site (cg02361027) in the first intron of the LRP2 gene. Interestingly, low expression of LRP2 was associated with poor patient outcome in clear cell renal cell carcinoma, papillary renal cell carcinoma, mesothelioma, papillary thyroid carcinoma, and invasive breast carcinoma. Furthermore, loss of LRP2 expression was associated with dedifferentiated histological and molecular subtypes of these cancers. These observations now motivate further studies on the functional role of LRP2 in tumors of epithelial origin and the potential use of LRP2 as a cancer biomarker.
... Molecular abnormalities in developing lungs can result in mortality and morbidity, including marginal lung changes that can promote subsequent chronic lung diseases [39][40][41][42][43]. This highlights the importance of understanding the transcriptomic effects of in utero ENDS aerosol exposures on developing lungs. ...
Unlabelled:
Currently, approximately 8 million adult Americans use electronic cigarettes (e-cigs) daily, including women of childbearing age. It is known that more than 10% of women smoke during their pregnancy, and recent surveys show that rates of maternal vaping are similar to rates of maternal cigarette smoking. However, the effects of inhaling e-cig aerosol on the health of fetuses remain unknown. The objective of the present study was to increase our understanding of the molecular effects caused by in utero exposures to e-cig aerosols on developing mouse lungs and, later in life, on the offspring's susceptibility to developing asthma.
Methods:
Pregnant mice were exposed throughout gestation to either filtered air or vanilla-flavored e-cig aerosols containing 18 mg/mL of nicotine. Male and female exposed mouse offspring were sacrificed at birth, and then the lung transcriptome was evaluated. Additionally, once sub-groups of male offspring mice reached 4 weeks of age, they were challenged with house dust mites (HDMs) for 3 weeks to assess asthmatic responses.
Results:
The lung transcriptomic responses of the mouse offspring at birth showed that in utero vanilla-flavored e-cig aerosol exposure significantly regulated 88 genes in males (62 genes were up-regulated and 26 genes were down-regulated), and 65 genes were significantly regulated in females (17 genes were up-regulated and 48 genes were down-regulated). Gene network analyses revealed that in utero e-cig aerosol exposure affected canonical pathways associated with CD28 signaling in T helper cells, the role of NFAT in the regulation of immune responses, and phospholipase C signaling in males, whereas the dysregulated genes in the female offspring were associated with NRF2-mediated oxidative stress responses. Moreover, we found that in utero exposures to vanilla-flavored e-cig aerosol exacerbated HDM-induced asthma in 7-week-old male mouse offspring compared to respective in utero air + HDM controls.
Conclusions:
Overall, these data demonstrate that in utero e-cig aerosol exposure alters the developing mouse lung transcriptome at birth in a sex-specific manner and provide evidence that the inhalation of e-cig aerosols is detrimental to the respiratory health of offspring by increasing the offspring' susceptibility to developing lung diseases later in life.
... LRP2 is expressed in many absorptive epithelia, particularly the apical membrane of the proximal tubules of the kidney, but also epithelia of the inner ear, thyroid, parathyroid, and alveoli [11][12][13][14] . LRP2 is also expressed in the central nervous system (CNS), including in neurons 15 , glia 16 , and the choroid plexus 17,18 , and plays signaling roles key to CNS development [19][20][21][22][23][24] . ...
The low-density lipoprotein (LDL) receptor-related protein 2 (LRP2 or megalin) is representative of the phylogenetically conserved subfamily of giant LDL receptor-related proteins, which function in endocytosis and are implicated in diseases of the kidney and brain. Here, we report high-resolution cryoelectron microscopy structures of LRP2 isolated from mouse kidney, at extracellular and endosomal pH. The structures reveal LRP2 to be a molecular machine that adopts a conformation for ligand binding at the cell surface and for ligand shedding in the endosome. LRP2 forms a homodimer, the conformational transformation of which is governed by pH-sensitive sites at both homodimer and intra-protomer interfaces. A subset of LRP2 deleterious missense variants in humans appears to impair homodimer assembly. These observations lay the foundation for further understanding the function and mechanism of LDL receptors and implicate homodimerization as a conserved feature of the LRP receptor subfamily.
... Megalin, also known as LRP-2, is the largest sized protein in the family and can bind a wide variety of ligands. While it can play a role in reabsorption of various molecules in the proximal renal tubule, including insulin [81], it can also act as a cell signaling transducer within the CNS [80,82]. LRP-8, also referred to as apolipoprotein E receptor 2 (apoER2), has been recognized as a signal transducer critical in brain development [83]. ...
Accumulating evidence suggests there is an alternative insulin transporter besides the insulin receptor at the blood-brain barrier (BBB), responsible for shuttling insulin from the circulation into the brain. In this review, we summarize key features of the BBB and what makes it unique compared to other capillary beds; summarize what we know about insulin BBB transport; provide an extensive list of diseases, physiological states, and serum factors tested in modifying insulin BBB transport; and lastly, highlight potential alternative transport systems that may be involved in or have already been tested in mediating insulin BBB transport. Identifying the transport system for insulin at the BBB would aide in controlling central nervous system (CNS) insulin levels in multiple diseases and conditions including Alzheimer’s disease (AD) and obesity, where availability of insulin to the CNS is limited. Keywords: Insulin, transport, blood-brain barrier
... Megalin-deficient mice have abnormal neuroepithelial proliferation, an ordinary ventricular system, and an incomplete olfactory system (Fisher and Howie, 2006). Generally, megalin forms a megalin/cubilin complex to fulfill its role by combining cubilin, a 460 kDa intrinsic factor vitamin B12 receptor protein (Willnow et al., 1996). Other ligands for megalin include insulin, insulin-like growth factor 1 (IGF-1), albumin, MMP9, hemoglobin, vitamin D-binding protein (DBP), retinol-binding protein (RBP), and β2-microglobulin (Cui et al., 1996;Orlando et al., 1998;Christensen et al., 1999;Nykjaer et al., 1999;Gburek et al., 2002). ...
Ischemic and hemorrhagic stroke (including intracerebral hemorrhage, intraventricular hemorrhage, and subarachnoid hemorrhage) is the dominating cause of disability and death worldwide. Neuroinflammation, blood–brain barrier (BBB) disruption, neuronal death are the main pathological progress, which eventually causes brain injury. Increasing evidence indicated that lipocalin 2 (LCN2), a 25k-Da acute phase protein from the lipocalin superfamily, significantly increased immediately after the stroke and played a vital role in these events. Meanwhile, there exists a close relationship between LCN2 levels and the worse clinical outcome of patients with stroke. Further research revealed that LCN2 elimination is associated with reduced immune infiltrates, infarct volume, brain edema, BBB leakage, neuronal death, and neurological deficits. However, some studies revealed that LCN2 might also act as a beneficial factor in ischemic stroke. Nevertheless, the specific mechanism of LCN2 and its primary receptors (24p3R and megalin) involving in brain injury remains unclear. Therefore, it is necessary to investigate the mechanism of LCN2 induced brain damage after stroke. This review focuses on the role of LCN2 and its receptors in brain injury and aiming to find out possible therapeutic targets to reduce brain damage following stroke.
