Expression and role of cubilin in the internalization of nutrients during the peri-implantation development of the rodent embryo

Toxicity Assessment of Biogenic Gold Nanoparticles on Crop Seeds and Zebrafish Embryos: Implications for Agricultural and Aquatic Ecosystems

The demand for food production has been growing exponentially due to the increase in the global population. Innovative approaches to enhance agricultural productivity have been explored, including the new applications of nanoparticles in agriculture. The nanoparticle application in agriculture can generate environmental and human health risks since nanoparticles can contaminate the soil and inevitably reach groundwater, potentially causing toxicity in aquatic organisms. In this study, we evaluated the benefits and toxicity of gold nanoparticles (GNPs), synthesized via green chemistry, on the growth of cultivated plants and in the zebrafish embryo model. GNPs were synthesized through an economical and environmentally friendly method using Brazilian red propolis (BRP) extract (BRP-GNPs). BRP-GNPs exhibited negative and positive effects on plant germination, depending on the concentration tested and the plant species involved. Moreover, BRP-GNPs induced developmental toxicity in fish embryos in a dose-dependent manner. Our results provide valuable insights for assessing the environmental risks of biogenic GNPs.

Preimplantation trophectoderm: A 'quick-fix' protector for embryo survival?

The trophectoderm (TE) epithelium forms the outer layer of the mammalian blastocyst and generates the blastocoel through vectorial transport. Its differentiation during cleavage, studied mainly in mouse, is integrated with blastocyst morphogenesis with key roles for cell polarisation, asymmetric cell divisions, cell signalling, regulatory transcription factors and cellular inheritance. The TE provides a physical and cellular protection to the emerging lineages of the embryo essential for the integrity of blastocyst development. Here, two examples of TE differentiation are considered in some detail where this immediate protective function for embryo survival is assessed: (i) cellular processes from TE at the polar-mural junctional zone in the early blastocyst that later form filopodia traversing the blastocoel, and (ii) the endocytic system which matures and polarises during differentiation. Understanding the broad role for TE in regulating early morphogenesis and environmental protection of the embryo, including these two examples, have clinical as well as biological relevance.

Research on the Mechanism of Metaldehyde on Pomacea canaliculata

Golden apple snail (Pomacea canaliculata), a major alien invasive organism in China, affects food production and poses a threat to human health. Metaldehyde is a highly effective, commonly used snail killer with low toxicity. Virulence determination, tissue section, iTRAQ and RNA interference were used to systematically study the toxicity of metaldehyde on P. canaliculata. The molluscicidal activity tests showed that metaldehyde exhibits strong toxicity against P. canaliculata. Physiological and biochemical data indicate that metaldehyde can cause damage to the gills, liver, pancreas, and kidneys of snails, also reduce the oxygen consumption rate and ammonia excretion rate of golden apple snails, and cause neurological diseases. The proteome of the gill region of the golden apple snail after exposure to metaldehyde was analyzed by using iTRAQ technology. A total of 360 differential proteins were identified, and four target proteins were screened, namely, alpha-protein kinase 1 (ALPK1), cubilin (CUBN), sodium- and chloride-dependent GABA transporter 2 (GAT2), and acetylcholinesterase (AChE). RNAi was used to target the four proteins. After the ALPK1 and CUBN protein genes were interfered with by metaldehyde treatment, it was found that the mortality rate of the golden apple snail significantly increased. However, interference of GAT2 and AChE protein genes by metaldehyde led to no significant change in the mortality rates of the snails. The histopathological observation of the gill showed that the rate of cilia shedding in the gill decreased after the interference of ALPK1 and CUBN protein genes.

Transcriptional network governing extraembryonic endoderm cell fate choice

The mechanism by which transcription factor (TF) network instructs cell-type-specific transcriptional programs to drive primitive endoderm (PrE) progenitors to commit to parietal endoderm (PE) versus visceral endoderm (VE) cell fates remains poorly understood. To address the question, we analyzed the single-cell transcriptional signatures defining PrE, PE, and VE cell states during the onset of the PE-VE lineage bifurcation. By coupling with the epigenomic comparison of active enhancers unique to PE and VE cells, we identified GATA6, SOX17, and FOXA2 as central regulators for the lineage divergence. Transcriptomic analysis of cXEN cells, an in vitro model for PE cells, after the acute depletion of GATA6 or SOX17 demonstrated that these factors induce Mycn, imparting the self-renewal properties of PE cells. Concurrently, they suppress the VE gene program, including key genes like Hnf4a and Ttr, among others. We proceeded with RNA-seq analysis on cXEN cells with FOXA2 knockout, in conjunction with GATA6 or SOX17 depletion. We found FOXA2 acts as a potent suppressor of Mycn while simultaneously activating the VE gene program. The antagonistic gene regulatory activities of GATA6/SOX17 and FOXA2 in promoting alternative cell fates, and their physical co-bindings at the enhancers provide molecular insights to the plasticity of the PrE lineage. Finally, we show that the external cue, BMP signaling, promotes the VE cell fate by activation of VE TFs and repression of PE TFs including GATA6 and SOX17. These data reveal a putative core gene regulatory module that underpins PE and VE cell fate choice.

Cubilin, the intrinsic factor-vitamin B12 receptor

Cubilin (CUBN), the intrinsic factor-vitamin B12 receptor is a large endocytic protein involved in various physiological functions: vitamin B12 uptake in the gut; reabsorption of albumin and maturation of vitamin D in the kidney; nutrient delivery during embryonic development. Cubilin is an atypical receptor, peripherally associated to the plasma membrane. The transmembrane proteins amnionless (AMN) and Lrp2/Megalin are the currently known molecular partners contributing to plasma membrane transport and internalization of Cubilin. The role of Cubilin/Amn complex in the handling of vitamin B12 in health and disease has extensively been studied and so is the role of the Cubilin-Lrp2 tandem in renal pathophysiology. Accumulating evidence strongly supports a role of Cubilin in some developmental defects including impaired closure of the neural tube. Are these defects primarily caused by the dysfunction of a specific Cubilin ligand or are they secondary to impaired vitamin B12 or protein uptake? We will present the established Cubilin functions, discuss the developmental data and provide an overview of the emerging implications of Cubilin in the field of cardiovascular disease and cancer pathogenesis.

Specification and role of extraembryonic endoderm lineages in the periimplantation mouse embryo

During mammalian embryo development, the correct formation of the first extraembryonic endoderm lineages is fundamental for successful development. In the periimplantation blastocyst, the primitive endoderm (PrE) is formed, which gives rise to the parietal endoderm (PE) and visceral endoderm (VE) during further developmental stages. These PrE-derived lineages show significant differences in both their formation and roles. Whereas differentiation of the PE as a migratory lineage has been suggested to represent the first epithelial-to-mesenchymal transition (EMT) in development, organisation of the epithelial VE is of utmost importance for the correct axis definition and patterning of the embryo. Despite sharing a common origin, the striking differences between the VE and PE are indicative of their distinct roles in early development. However, there is a significant disparity in the current knowledge of each lineage, which reflects the need for a deeper understanding of their respective specification processes. In this review, we will discuss the origin and maturation of the PrE, PE, and VE during the periimplantation period using the mouse model as an example. Additionally, we consider the latest findings regarding the role of the PrE-derived lineages and early embryo morphogenesis, as obtained from the most recent in vitro models.

Environmental Exposures around Conception: Developmental Pathways Leading to Lifetime Disease Risk

Environment around conception can influence the developmental programme with lasting effects on gestational and postnatal phenotype and with consequences for adult health and disease risk. Peri-conception exposure comprises a crucial part of the 'Developmental Origins of Health and Disease' (DOHaD) concept. In this review, we consider the effects of maternal undernutrition experienced during the peri-conception period in select human models and in a mouse experimental model of protein restriction. Human datasets indicate that macronutrient deprivation around conception affect the epigenome, with enduring effects on cardiometabolic and neurological health. The mouse model, comprising maternal low protein diet exclusively during the peri-conception period, has revealed a stepwise progression in altered developmental programming following induction through maternal metabolite deficiency. This progression includes differential effects in extra-embryonic and embryonic cell lineages and tissues, leading to maladaptation in the growth trajectory and increased chronic disease comorbidities. The timeline embraces an array of mechanisms across nutrient sensing and signalling, cellular, metabolic, epigenetic and physiological processes with a coordinating role for mTORC1 signalling proposed. Early embryos appear active participants in environmental sensing to optimise the developmental programme for survival but with the trade-off of later disease. Similar adverse health outcomes may derive from other peri-conception environmental experiences, including maternal overnutrition, micronutrient availability, pollutant exposure and assisted reproductive treatments (ART) and support the need for preconception health before pregnancy.

Loss of Cubilin, the intrinsic factor-vitamin B12 receptor, impairs visceral endoderm endocytosis and endodermal patterning in the mouse

The visceral endoderm is a polarized epithelial monolayer necessary for early embryonic development in rodents. A key feature of this epithelium is an active endocytosis and degradation of maternal nutrients, in addition to being the source of various signaling molecules or inhibitors required for the differentiation and patterning of adjacent embryonic tissues. Endocytosis across the visceral endoderm epithelium involves specific cell surface receptors and an extensive sub-membrane vesicular system with numerous apical vacuoles/lysosomes. We previously reported that Cubilin, the endocytic receptor for intrinsic factor-vitamin B12, albumin and apolipoproteinA-I/HDL allows maternal nutrient uptake by the visceral endoderm. In the present study, we show that the germline ablation of Cubilin impairs endodermal and mesodermal patterning, and results in developmental arrest at gastrulation. Notably, visceral endoderm dispersal is impeded in Cubilin null embryos. We further confirm the essential role of Cubilin in nutrient internalization by the early visceral endoderm and highlight its involvement in the formation of apical vacuoles. Our results reveal essential roles for Cubilin in early embryonic development, and suggest that in addition to its nutritive function, Cubilin sustains signaling pathways involved in embryonic differentiation and patterning.

Zebrafish as a Model for Toxicological Perturbation of Yolk and Nutrition in the Early Embryo

PURPOSE OF REVIEW

Developmental toxicity assessments often focus on structural outcomes and overlook subtle metabolic differences which occur during the early embryonic period. Deviant embryonic nutrition can result in later-life disease, including diabetes, obesity, and cardiovascular disease. Prior to placenta-mediated nutrient exchange, the human embryo requires maternally supplied nutritional substrates for growth, called yolk. Here, we compare the biology of the human and zebrafish yolk and review examples of toxicant-mediated perturbation of yolk defects, composition, and utilization.

RECENT FINDINGS

Zebrafish embryos, like human embryos, have a protruding yolk sac that serves as a nutritional cache. Aberrant yolk morphology is a common qualitative finding in fish embryotoxicity studies, but quantitative assessment and characterization provides an opportunity to uncover mechanistic targets of toxicant effects on embryonic nutrition. The zebrafish and the study of its yolk sac is an excellent model for uncovering toxicant disruptions to early embryonic nutrition and has potential to discover mechanistic insights into the developmental origins of health and disease.

Assessment of Histiotrophic Nutrition Using Fluorescent Probes

Histiotrophic nutrition is a process whereby the rodent visceral yolk sac (VYS) internalizes exogenous macromolecules, degrades them, and sends the degradation products to the embryo. Quantification and visualization of histiotrophic nutrition can be accomplished using fluorescent tracer molecules such as fluorescein isothiocyanate-conjugated albumin (FITC-albumin). The methods are simple and can provide complimentary functional and structural information in studies of the effects of embryotoxicants on visceral yolk sac function.

Cubilin, the Intrinsic Factor-Vitamin B12 Receptor in Development and Disease

Gp280/Intrinsic factor-vitamin B12 receptor/Cubilin (CUBN) is a large endocytic receptor serving multiple functions in vitamin B12 homeostasis, renal reabsorption of protein or toxic substances including albumin, vitamin D-binding protein or cadmium. Cubilin is a peripheral membrane protein consisting of 8 Epidermal Growth Factor (EGF)-like repeats and 27 CUB (defined as Complement C1r/C1s, Uegf, BMP1) domains. This structurally unique protein interacts with at least two molecular partners, Amnionless (AMN) and Lrp2/Megalin. AMN is involved in appropriate plasma membrane transport of Cubilin whereas Lrp2 is essential for efficient internalization of Cubilin and its ligands. Observations gleaned from animal models with Cubn deficiency or human diseases demonstrate the importance of this protein. In this review addressed to basic research and medical scientists, we summarize currently available data on Cubilin and its implication in renal and intestinal biology. We also discuss the role of Cubilin as a modulator of Fgf8 signaling during embryonic development and propose that the Cubilin-Fgf8 interaction may be relevant in human pathology, including in cancer progression, heart or neural tube defects. We finally provide experimental elements suggesting that some aspects of Cubilin physiology might be relevant in drug design.

Transcriptional profiling of embryos lacking the lipoprotein receptor SR-B1 reveals a regulatory circuit governing a neurodevelopmental or metabolic decision during neural tube closure

Background

The high-density lipoprotein receptor SR-B1 mediates cellular uptake of several lipid species, including cholesterol and vitamin E. During early mouse development, SR-B1 is located in the maternal-fetal interface, where it facilitates vitamin E transport towards the embryo. Consequently, mouse embryos lacking SR-B1 are vitamin E-deficient, and around half of them fail to close the neural tube and show cephalic neural tube defects (NTD). Here, we used transcriptomic profiling to identify the molecular determinants of this phenotypic difference between SR-B1 deficient embryos with normal morphology or with NTD.

Results

We used RNA-Seq to compare the transcriptomic profile of three groups of embryos retrieved from SR-B1 heterozygous intercrosses: wild-type E9.5 embryos (WT), embryos lacking SR-B1 that are morphologically normal, without NTD (KO-N) and SR-B1 deficient embryos with this defect (KO-NTD). We identified over 1000 differentially expressed genes: down-regulated genes in KO-NTD embryos were enriched for functions associated to neural development, while up-regulated genes in KO-NTD embryos were enriched for functions related to lipid metabolism. Feeding pregnant dams a vitamin E-enriched diet, which prevents NTD in SR-B1 KO embryos, resulted in mRNA levels for those differentially expressed genes that were more similar to KO-N than to KO-NTD embryos. We used gene regulatory network analysis to identify putative transcriptional regulators driving the different embryonic expression profiles, and identified a regulatory circuit controlled by the androgen receptor that may contribute to this dichotomous expression profile in SR-B1 embryos. Supporting this possibility, the expression level of the androgen receptor correlated strongly with the expression of several genes involved in neural development and lipid metabolism.

Conclusions

Our analysis shows that normal and defective embryos lacking SR-B1 have divergent expression profiles, explained by a defined set of transcription factors that may explain their divergent phenotype. We propose that distinct expression profiles may be relevant during early development to support embryonic nutrition and neural tube closure.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5110-2) contains supplementary material, which is available to authorized users.

Placental Origins of Chronic Disease

Epidemiological evidence links an individual's susceptibility to chronic disease in adult life to events during their intrauterine phase of development. Biologically this should not be unexpected, for organ systems are at their most plastic when progenitor cells are proliferating and differentiating. Influences operating at this time can permanently affect their structure and functional capacity, and the activity of enzyme systems and endocrine axes. It is now appreciated that such effects lay the foundations for a diverse array of diseases that become manifest many years later, often in response to secondary environmental stressors. Fetal development is underpinned by the placenta, the organ that forms the interface between the fetus and its mother. All nutrients and oxygen reaching the fetus must pass through this organ. The placenta also has major endocrine functions, orchestrating maternal adaptations to pregnancy and mobilizing resources for fetal use. In addition, it acts as a selective barrier, creating a protective milieu by minimizing exposure of the fetus to maternal hormones, such as glucocorticoids, xenobiotics, pathogens, and parasites. The placenta shows a remarkable capacity to adapt to adverse environmental cues and lessen their impact on the fetus. However, if placental function is impaired, or its capacity to adapt is exceeded, then fetal development may be compromised. Here, we explore the complex relationships between the placental phenotype and developmental programming of chronic disease in the offspring. Ensuring optimal placentation offers a new approach to the prevention of disorders such as cardiovascular disease, diabetes, and obesity, which are reaching epidemic proportions.

Megalin and cubilin in proximal tubule protein reabsorption: from experimental models to human disease

Proximal tubule protein uptake is mediated by 2 receptors, megalin and cubilin. These receptors rescue a variety of filtered ligands, including biomarkers, essential vitamins, and hormones. Receptor gene knockout animal models have identified important functions of the receptors and have established their essential role in modulating urinary protein excretion. Rare genetic syndromes associated with dysfunction of these receptors have been identified and characterized, providing additional information on the importance of these receptors in humans. Using various disease models in combination with receptor gene knockout, the implications of receptor dysfunction in acute and chronic kidney injury have been explored and have pointed to potential new roles of these receptors. Based on data from animal models, this paper will review current knowledge on proximal tubule endocytic receptor function and regulation, and their role in renal development, protein reabsorption, albumin uptake, and normal renal physiology. These findings have implications for the pathophysiology and diagnosis of proteinuric renal diseases. We will examine the limitations of the different models and compare the findings to phenotypic observations in inherited human disorders associated with receptor dysfunction. Furthermore, evidence from receptor knockout mouse models as well as human observations suggesting a role of protein receptors for renal disease will be discussed in light of conditions such as chronic kidney disease, diabetes, and hypertension.

Lipid transport to avian oocytes and to the developing embryo

Studies of receptor-mediated lipoprotein metabolic pathways in avian species have revealed that physiological intricacies of specific cell types are highly analogous to those in mammals. A prime example for the power of comparative studies across different animal kingdoms, elucidated in the chicken, is that the expression of different lipoprotein receptors in somatic cells and oocytes are the key to oocyte growth. In avian species, yolk precursor transport from the hen's liver to rapidly growing oocytes and the subsequent transfer of yolk nutrients via the yolk sac to the developing embryo are highly efficient processes. Oocytes grow from a diameter of 5 mm to 2.5-3 cm in only 7 days, and the yolk sac transfers nutrients from the yolk stored in the mature oocyte to the embryo within just 2 weeks. The underlying key transport mechanism is receptor-mediated endocytosis of macromolecules, i.e., of hepatically synthesized yolk precursors for oocyte growth, and of mature yolk components for embryo nutrition, respectively. Recently, the receptors involved, as well as the role of lipoprotein synthesis in the yolk sac have been identified. As outlined here, lipoprotein degradation/resynthesis cycles and the expression of lipoprotein receptors are not only coordinated with the establishment of the follicular architecture embedding the oocyte, but also with the generation of the yolk sac vasculature essential for nutrient transfer to the embryo.

Amino acid starvation induced by protease inhibition produces differential alterations in redox status and the thiol proteome in organogenesis-stage rat embryos and visceral yolk sacs

The process of embryonic nutrition in rodent conceptuses during organogenesis has been shown to involve a dominant histiotrophic mechanism where essential developmental substrates and micronutrients are supplied as whole maternal proteins or cargoes associated with proteins. The histiotrophic nutrition pathways (HNP) responsible for uptake and initial processing of proteins across maternal-conceptal interfaces involve uptake via receptor mediated endocytosis and protein degradation via lysosomal proteolysis. Chemical inhibition of either process can lead to growth deficits and malformation in the embryo (EMB), but selective inhibition of either HNP component will elicit a different subset of developmental perturbations. In vitro, whole embryo culture exposure of GD10 or GD11 rat conceptuses to the natural protease inhibitor, leupeptin, leads to significant reductions in all measured embryonic growth parameters as well as a myriad of other effects. Leupeptin doses of 10 μM or 20 μM over a 26-h period (GD10-GD11) and 50 μM over a 3 h pulse period produced significant decreases in the clearance of FITC-albumin from culture media. The near complete loss of acid soluble fluorescence and increased total visceral yolk sac (VYS) protein content confirmed the selective inhibition of proteolysis. Inhibition of lysosomal proteolysis thus deprives the developing EMB of essential nutrient amino acids producing conditions akin to amino acid starvation, but may also cause direct effects on pathways critical for normal growth and differentiation. Following leupeptin exposure for 26 or 6 h, total glutathione (GSH) concentrations dropped significantly in the VYS, but only slightly in yolk sac (YSF) and amniotic (AF) fluids. Cys concentrations increased in VYS and EMB, but dropped in YSF and AF fluids. Redox potentials (Eh) for the glutathione disulfide (GSSG)/glutathione (GSH) redox couple trended significantly toward the positive, confirming the net oxidation of conceptual tissues following leupeptin treatment. Analysis of the thiol proteome showed few alterations to specific pathways mapped to the Kyoto Encyclopedia of Genes and Genomes Pathway database, but did reveal significant increases in concentrations of proteins associated with glycolysis/gluconeogenesis in the VYS and decreased concentrations proteins associated with ribosome biogenesis and function in the EMB. A subset of proteins elevated by >2-23-fold in the VYS were identified as serum (blood) proteins and represent the maternal-side proteins captured by the VYS and which are not degraded in the lysosomes as a result of leupeptin's inhibitory action. The observed constellation of proteins decreased in the EMB by leupeptin represent proteins from several adaptive pathways that are commonly altered in responses to amino acid starvation. These studies show clear differential responses to protease inhibition in VYS and EMB during organogenesis and suggest the possibility for additional roles of redox regulation, cellular adaptations and metabolic insufficiency caused by protease inhibition.

Do little embryos make big decisions? How maternal dietary protein restriction can permanently change an embryo’s potential, affecting adult health

Periconceptional environment may influence embryo development, ultimately affecting adult health. Here, we review the rodent model of maternal low-protein diet specifically during the preimplantation period (Emb-LPD) with normal nutrition during subsequent gestation and postnatally. This model, studied mainly in the mouse, leads to cardiovascular, metabolic and behavioural disease in adult offspring, with females more susceptible. We evaluate the sequence of events from diet administration that may lead to adult disease. Emb-LPD changes maternal serum and/or uterine fluid metabolite composition, notably with reduced insulin and branched-chain amino acids. This is sensed by blastocysts through reduced mammalian target of rapamycin complex 1 signalling. Embryos respond by permanently changing the pattern of development of their extra-embryonic lineages, trophectoderm and primitive endoderm, to enhance maternal nutrient retrieval during subsequent gestation. These compensatory changes include stimulation in proliferation, endocytosis and cellular motility, and epigenetic mechanisms underlying them are being identified. Collectively, these responses act to protect fetal growth and likely contribute to offspring competitive fitness. However, the resulting growth adversely affects long-term health because perinatal weight positively correlates with adult disease risk. We argue that periconception environmental responses reflect developmental plasticity and ‘decisions’ made by embryos to optimise their own development, but with lasting consequences.

Stress-induced enzyme activation primes murine embryonic stem cells to differentiate toward the first extraembryonic lineage

Extracellular stresses influence transcription factor (TF) expression and therefore lineage identity in the peri-implantation mouse embryo and its stem cells. This potentially affects pregnancy outcome. To understand the effects of stress signaling during this critical period of pregnancy, we exposed cultured murine embryonic stem cells (mESCs) to hyperosmotic stress. We then measured stress-enzyme-dependent regulation of key pluripotency and lineage TFs. Hyperosmotic stress slowed mESC accumulation due to slowing of the cell cycle over 72 h, after a small apoptotic response within 12 h. Phosphoinositide 3-kinase (PI3K) enzymatic signaling was responsible for stem cell survival under stressed conditions. Stress initially triggered mESC differentiation after 4 h through MEK1, c-Jun N-terminal kinase (JNK), and PI3K enzymatic signaling, which led to proteasomal degradation of Oct4, Nanog, Sox2, and Rex1 TF proteins. Concurrent with this post-transcriptional effect was the decreased accumulation of potency TF mRNA transcripts. After 12-24 h of stress, cells adapted, cell cycle resumed, and Oct4 and Nanog mRNA and protein expression returned to approximately normal levels. The TF protein recovery was mediated by p38MAPK and PI3K signaling, as well as by MEK2 and/or MEK1. However, due to JNK signaling, Rex1 expression did not recover. Probing for downstream lineages revealed that although mESCs did not differentiate morphologically during 24 h of stress, they were primed to differentiate by upregulating markers of the first lineage differentiating from mESCs, extraembryonic endoderm. Thus, although two to three TFs that mark pluripotency recover expression by 24 h of stress, there is nonetheless sustained Rex1 suppression and a priming of mESCs for differentiation to the earliest lineage.

Deficits in receptor-mediated endocytosis and recycling in cells from mice with Gpr107 locus disruption

GPR107 is a type III integral membrane protein that was initially predicted to be a member of the family of G-protein-coupled receptors. This report shows that deletion of Gpr107 leads to an embryonic lethal phenotype that is characterized by a reduction in cubilin transcript abundance and a decrease in the representation of multiple genes implicated in the cubilin-megalin endocytic receptor complex (megalin is also known as LRP2). Gpr107-null fibroblast cells exhibit reduced transferrin internalization, decreased uptake of low-density lipoprotein (LDL) receptor-related protein-1 (LRP1) cargo and resistance to toxins. Colocalization studies and proteomic analyses suggest that GPR107 associates with clathrin and the retromer protein VPS35 and that GPR107 might be responsible for the return of receptors to the plasma membrane from endocytic compartments. The highly selective deficits observed in Gpr107-null cells indicate that GPR107 interacts directly or indirectly with a limited subset of surface receptors.

Mouse early extra-embryonic lineages activate compensatory endocytosis in response to poor maternal nutrition

Mammalian extra-embryonic lineages perform the crucial role of nutrient provision during gestation to support embryonic and fetal growth. These lineages derive from outer trophectoderm (TE) and internal primitive endoderm (PE) in the blastocyst and subsequently give rise to chorio-allantoic and visceral yolk sac placentae, respectively. We have shown maternal low protein diet exclusively during mouse preimplantation development (Emb-LPD) is sufficient to cause a compensatory increase in fetal and perinatal growth that correlates positively with increased adult-onset cardiovascular, metabolic and behavioural disease. Here, to investigate early mechanisms of compensatory nutrient provision, we assessed the influence of maternal Emb-LPD on endocytosis within extra-embryonic lineages using quantitative imaging and expression of markers and proteins involved. Blastocysts collected from Emb-LPD mothers within standard culture medium displayed enhanced TE endocytosis compared with embryos from control mothers with respect to the number and collective volume per cell of vesicles with endocytosed ligand and fluid and lysosomes, plus protein expression of megalin (Lrp2) LDL-family receptor. Endocytosis was also stimulated using similar criteria in the outer PE-like lineage of embryoid bodies formed from embryonic stem cell lines generated from Emb-LPD blastocysts. Using an in vitro model replicating the depleted amino acid (AA) composition found within the Emb-LPD uterine luminal fluid, we show TE endocytosis response is activated through reduced branched-chain AAs (leucine, isoleucine, valine). Moreover, activation appears mediated through RhoA GTPase signalling. Our data indicate early embryos regulate and stabilise endocytosis as a mechanism to compensate for poor maternal nutrient provision.

Expression and immunolocalisation of the endocytic receptors megalin and cubilin in the human yolk sac and placenta across gestation

Megalin and cubilin are multifunctional endocytic receptors associated with many transporting epithelia. They play an essential role in transport of nutrients through the visceral yolk sac of rodents during embryogenesis. Here, we immunolocalise them to the endodermal layer of the human yolk sac, and to the syncytiotrophoblast and cytotrophoblast cells of placental villi. In villi, the protein level of both receptors increased with gestation. The mRNA for megalin remained constant, while that encoding cubilin increased with gestation. These results suggest megalin and cubilin may be important in human maternal–fetal transfer, and that they increase across gestation to facilitate this function.

Cubilin, a high affinity receptor for fibroblast growth factor 8, is required for cell survival in the developing vertebrate head

Cubilin (Cubn) is a multiligand endocytic receptor critical for the intestinal absorption of vitamin B12 and renal protein reabsorption. During mouse development, Cubn is expressed in both embryonic and extra-embryonic tissues, and Cubn gene inactivation results in early embryo lethality most likely due to the impairment of the function of extra-embryonic Cubn. Here, we focus on the developmental role of Cubn expressed in the embryonic head. We report that Cubn is a novel, interspecies-conserved Fgf receptor. Epiblast-specific inactivation of Cubn in the mouse embryo as well as Cubn silencing in the anterior head of frog or the cephalic neural crest of chick embryos show that Cubn is required during early somite stages to convey survival signals in the developing vertebrate head. Surface plasmon resonance analysis reveals that fibroblast growth factor 8 (Fgf8), a key mediator of cell survival, migration, proliferation, and patterning in the developing head, is a high affinity ligand for Cubn. Cell uptake studies show that binding to Cubn is necessary for the phosphorylation of the Fgf signaling mediators MAPK and Smad1. Although Cubn may not form stable ternary complexes with Fgf receptors (FgfRs), it acts together with and/or is necessary for optimal FgfR activity. We propose that plasma membrane binding of Fgf8, and most likely of the Fgf8 family members Fgf17 and Fgf18, to Cubn improves Fgf ligand endocytosis and availability to FgfRs, thus modulating Fgf signaling activity.

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.

Characterization of endocytic compartments after holo-high density lipoprotein particle uptake in HepG2 cells

Holo-high density lipoprotein (HDL) particle uptake, besides selective lipid uptake, constitutes an alternative pathway to regulate cellular cholesterol homeostasis. In the current study, the cellular path of holo-HDL particles was investigated in human liver carcinoma cells (HepG2) using combined light and electron microscopical methods. The apolipoprotein moiety of HDL was visualized with different markers: horseradish peroxidase, colloidal gold and the fluorochrome Alexa(568), used in fluorescence microscopy and after photooxidation correlatively at the ultrastructural level. Time course experiments showed a rapid uptake of holo-HDL particles, an accumulation in endosomal compartments, with a plateau after 1-2 h of continuous uptake, and a clearance 1-2 h upon replacement by unlabeled HDL. Correlative microscopy, using HDL-Alexa(568)-driven diaminobenzidine (DAB) photooxidation, identified the fluorescent organelles as DAB-positive multivesicular bodies (MVBs) in the electron microscope; their luminal contents but not the internal vesicles were stained. Labeled MVBs increased in numbers and changed shapes along with the duration of uptake, from polymorphic organelles with multiple surface domains and differently shaped protrusions dominating at early times of uptake to compact bodies with mainly tubular appendices and densely packed vesicles after later times. Differently shaped and labeled surface domains and appendices, as revealed by three dimensional reconstructions, as well as images of homotypic fusions indicate the dynamics of the HDL-positive MVBs. Double staining visualized by confocal microscopy, along with the electron microscopic data, shows that holo-HDL particles after temporal storage in MVBs are only to a minor degree transported to lysosomes, which suggests that different mechanisms are involved in cellular HDL clearance, including resecretion.

Advances in the understanding of cobalamin assimilation and metabolism

The haematological and neurological consequences of cobalamin deficiency define the essential role of this vitamin in key metabolic reactions. The identification of cubilin-amnionless as the receptors for intestinal absorption of intrinsic factor-bound cobalamin and the plasma membrane receptor for cellular uptake of transcobalamin bound cobalamin have provided a clearer understanding of the absorption and cellular uptake of this vitamin. As the genes involved in the intracellular processing of cobalamins and genetic defects of these pathways are identified, the metabolic disposition of cobalamins and the proteins involved are being recognized. The synthesis of methylcobalamin and 5'-deoxyadenosylcobalamin, their utilization in conjunction with methionine synthase and methylmalonylCoA mutase, respectively, and the metabolic consequences of defects in these pathways could provide insights into the clinical presentation of cobalamin deficiency.

Receptor-mediated endocytosis in renal proximal tubule

Proteins filtered in renal glomeruli are removed from the tubular fluid by endocytosis in the proximal tubule mediated by the two receptors megalin and cubilin. After endocytic uptake, the proteins are transferred to lysosomes for degradation, while the receptors are returned to the apical cell membrane by receptor recycling in dense apical tubules. In the renal proximal tubule, there is no significant transcellular transport of protein. The reabsorptive process is extremely efficient as evidenced by the virtual protein free urine in humans. The two receptors bind a variety of ligands. The process serves not only to remove the proteins from the ultrafiltrate but also to conserve a variety of essential substances such as vitamins and trace elements carried by plasma proteins. The endocytic apparatus is highly developed in the proximal tubule demonstrating the high capacity of the cells; however, under certain circumstances like diseases affecting the glomeruli, the system is overloaded resulting in proteinuria.

[Early embryogenesis in mammals: stem cells and first commitment steps]

In mammals, embryonic and extraembryonic cell lineages segregate during the first steps of cell differentiation in the preimplantation embryo. Indeed, mammal embryos contain very low energy stocks and thus get ready for implantation very early to be able to absorb nutrients from the mother, first through the yolk sac and then through the placenta. These first steps involve classical genetic and morphogenetic processes as well as specific mechanisms of early embryo development such as epigenetic reprogramming and maintenance of pluripotent cells. Embryo analysis led to the isolation of embryonic stem (ES) cells, granted by the 2007 Nobel prize of Medicine (to M. Evans, M. Capecchi and O. Smithies) and which offer strong hopes for cell therapy.

Microarray analysis of E9.5 reduced folate carrier (RFC1; Slc19a1) knockout embryos reveals altered expression of genes in the cubilin-megalin multiligand endocytic receptor complex

BACKGROUND

The reduced folate carrier (RFC1) is an integral membrane protein and facilitative anion exchanger that mediates delivery of 5-methyltetrahydrofolate into mammalian cells. Adequate maternal-fetal transport of folate is necessary for normal embryogenesis. Targeted inactivation of the murine RFC1 gene results in post-implantation embryolethality, but daily folic acid supplementation of pregnant dams prolongs survival of homozygous embryos until mid-gestation. At E10.5 RFC1-/- embryos are developmentally delayed relative to wildtype littermates, have multiple malformations, including neural tube defects, and die due to failure of chorioallantoic fusion. The mesoderm is sparse and disorganized, and there is a marked absence of erythrocytes in yolk sac blood islands. The identification of alterations in gene expression and signaling pathways involved in the observed dysmorphology following inactivation of RFC1-mediated folate transport are the focus of this investigation.

RESULTS

Affymetrix microarray analysis of the relative gene expression profiles in whole E9.5 RFC1-/- vs. RFC1+/+ embryos identified 200 known genes that were differentially expressed. Major ontology groups included transcription factors (13.04%), and genes involved in transport functions (ion, lipid, carbohydrate) (11.37%). Genes that code for receptors, ligands and interacting proteins in the cubilin-megalin multiligand endocytic receptor complex accounted for 9.36% of the total, followed closely by several genes involved in hematopoiesis (8.03%). The most highly significant gene network identified by Ingenuitytrade mark Pathway analysis included 12 genes in the cubilin-megalin multiligand endocytic receptor complex. Altered expression of these genes was validated by quantitative RT-PCR, and immunohistochemical analysis demonstrated that megalin protein expression disappeared from the visceral yolk sac of RFC1-/- embryos, while cubilin protein was widely misexpressed.

CONCLUSION

Inactivation of RFC1 impacts the expression of several ligands and interacting proteins in the cubilin-amnionless-megalin complex that are involved in the maternal-fetal transport of folate and other nutrients, lipids and morphogens such as sonic hedgehog (Shh) and retinoids that play critical roles in normal embryogenesis.

A Chinese hamster ovarian cell line imports cholesterol by high density lipoprotein degradation

Plasma high density lipoprotein (HDL) is inversely associated with the development of atherosclerosis. HDL exerts its atheroprotective role through involvement in reverse cholesterol transport in which HDL is loaded with cholesterol at the periphery and transports its lipid load back to the liver for disposal. In this pathway, HDL is not completely dismantled but only transfers its lipids to the cell. Here we present evidence that a Chinese hamster ovarian cell line (CHO7) adapted to grow in lipoprotein-deficient media degrades HDL and concomitantly internalizes HDL-derived cholesterol. Delivery of HDL cholesterol to the cell was demonstrated by a down-regulation of cholesterol biosynthesis, an increase in total cellular cholesterol content and by stimulation of cholesterol esterification after HDL treatment. This HDL degradation pathway is distinct from the low density lipoprotein (LDL) receptor pathway but also degrades LDL. 25-Hydroxycholesterol, a potent inhibitor of the LDL receptor pathway, down-regulated LDL degradation in CHO7 cells only in part and did not down-regulate HDL degradation. Dextran sulfate released HDL bound to the cell surface of CHO7 cells, and heparin treatment released protein(s) contributing to HDL degradation. The involvement of heparan sulfate proteoglycans and lipases in this HDL degradation was further tested by two inhibitors genistein and tetrahydrolipstatin. Both blocked HDL degradation significantly. Thus, we demonstrate that CHO7 cells degrade HDL and LDL to supply themselves with cholesterol via a novel degradation pathway. Interestingly, HDL degradation with similar properties was also observed in a human placental cell line.

The role of megalin (LRP-2/Gp330) during development

Megalin (LRP-2/GP330), a member of the LDL receptor family, is an endocytic receptor expressed mainly in polarised epithelial cells. Identified as the pathogenic autoantigen of Heymann nephritis in rats, its functions have been studied in greatest detail in adult mammalian kidney, but there is increasing recognition of its involvement in embryonic development. The megalin homologue LRP-1 is essential for growth and development in Caenorhabditis elegans and megalin plays a role in CNS development in zebrafish. There is now also evidence for a homologue in Drosophila. However, most research concerns mammalian embryogenesis; it is widely accepted to be important during forebrain development and the developing renal proximal tubule. Megalin is also expressed in lung, eye, intestine, uterus, oviduct, and male reproductive tract. It is found in yolk sacs and the outer cells of pre-implantation mouse embryos, where interactions with cubilin result in nutrient endocytosis, and it may be important during implantation. Models for megalin interaction(s) with Sonic Hedgehog (Shh) have been proposed. The importance of Shh signalling during embryogenesis is well established; how and when megalin interacts with Shh is becoming a pertinent question in developmental biology.

Alterations in gene expression in T1 alpha null lung: a model of deficient alveolar sac development

Background

Development of lung alveolar sacs of normal structure and size at late gestation is necessary for the gas exchange process that sustains respiration at birth. Mice lacking the lung differentiation gene T1α [T1α(-/-)] fail to form expanded alveolar sacs, resulting in respiratory failure at birth. Since little is known about the molecular pathways driving alveolar sacculation, we used expression microarrays to identify genes altered in the abnormal lungs and, by inference, may play roles in normal lung morphogenesis.

Results

Altered expression of genes related to cell-cell interaction, such as ephrinA3, are observed in T1α(-/-) at E18.5. At term, FosB, Egr1, MPK-1 and Nur77, which can function as negative regulators of the cell-cycle, are down-regulated. This is consistent with the hyperproliferation of peripheral lung cells in term T1α (-/-) lungs reported earlier. Biochemical assays show that neither PCNA nor p21 are altered at E18.5. At term in contrast, PCNA is increased, and p21 is decreased.

Conclusion

This global analysis has identified a number of candidate genes that are significantly altered in lungs in which sacculation is abnormal. Many genes identified were not previously associated with lung development and may participate in formation of alveolar sacs prenatally.

Targeted disruption of cubilin reveals essential developmental roles in the structure and function of endoderm and in somite formation

Background

Cubilin is a peripheral membrane protein that interacts with the integral membrane proteins megalin and amnionless to mediate ligand endocytosis by absorptive epithelia such as the extraembryonic visceral endoderm (VE).

Results

Here we report the effects of the genetic deletion of cubilin on mouse embryonic development. Cubilin gene deletion is homozygous embryonic lethal with death occurring between 7.5–13.5 days post coitum (dpc). Cubilin-deficient embryos display developmental retardation and do not advance morphologically beyond the gross appearance of wild-type 8–8.5 dpc embryos. While mesodermal structures such as the allantois and the heart are formed in cubilin mutants, other mesoderm-derived tissues are anomalous or absent. Yolk sac blood islands are formed in cubilin mutants but are unusually large, and the yolk sac blood vessels fail to undergo remodeling. Furthermore, somite formation does not occur in cubilin mutants. Morphological abnormalities of endoderm occur in cubilin mutants and include a stratified epithelium in place of the normally simple columnar VE epithelium and a stratified cuboidal epithelium in place of the normally simple squamous epithelium of the definitive endoderm. Cubilin-deficient VE is also functionally defective, unable to mediate uptake of maternally derived high-density lipoprotein (HDL).

Conclusion

In summary, cubilin is required for embryonic development and is essential for the formation of somites, definitive endoderm and VE and for the absorptive function of VE including the process of maternal-embryo transport of HDL.

An improved single-cell cDNA amplification method for efficient high-density oligonucleotide microarray analysis

A systems-level understanding of a small but essential population of cells in development or adulthood (e.g. somatic stem cells) requires accurate quantitative monitoring of genome-wide gene expression, ideally from single cells. We report here a strategy to globally amplify mRNAs from single cells for highly quantitative high-density oligonucleotide microarray analysis that combines a small number of directional PCR cycles with subsequent linear amplification. Using this strategy, both the representation of gene expression profiles and reproducibility between individual experiments are unambiguously improved from the original method, along with high coverage and accuracy. The immediate application of this method to single cells in the undifferentiated inner cell masses of mouse blastocysts at embryonic day (E) 3.5 revealed the presence of two populations of cells, one with primitive endoderm (PE) expression and the other with pluripotent epiblast-like gene expression. The genes expressed differentially between these two populations were well preserved in morphologically differentiated PE and epiblast in the embryos one day later (E4.5), demonstrating that the method successfully detects subtle but essential differences in gene expression at the single-cell level among seemingly homogeneous cell populations. This study provides a strategy to analyze biophysical events in medicine as well as in neural, stem cell and developmental biology, where small numbers of distinctive or diseased cells play critical roles.

Transfer of maternal cholesterol to embryo and fetus in pregnant mice

Cholesterol is essential for antenatal development. However, the transport of maternal cholesterol to the embryo has not been sufficiently studied, and that to the fetus is still controversial. To this end, a 1 mg dose of [3,4-(13)C(2)]cholesterol was injected daily into pregnant mice and the labeled cholesterol was measured by gas chromatography-mass spectrometry. After venous injections from days 10 to 17 of gestation, [(13)C]cholesterol levels in total ((12)C and (13)C) cholesterol were increased to 5.1% and 2.8% in maternal and fetal plasma, respectively. Labeled cholesterol was identified in the liver, kidneys, and intestines, but not in the brain, of the fetus. After injections from days 1 to 8, [(13)C]cholesterol levels were increased to 12.4% and 8.0% of total cholesterol in maternal plasma and the embryo, respectively. The level of 11.5% in the yolk sac was higher than that in the embryo. Intrauterine transfer of maternal cholesterol to the embryo as well as the fetus was evident in mice, and both the placenta and the yolk sac appear to be sites of intermediate passage in murine pregnancy.