Uptake and processing of 59Fe-labelled and 125I-labelled rat transferrin by early organogenesis rat conceptuses in vitro

The role of mammalian foetal membranes in early embryogenesis: Lessons from marsupials

Across mammals, early embryonic development is supported by uterine secretions taken up through the yolk sac and other foetal membranes (histotrophic nutrition). The marsupial conceptus is enclosed in a shell coat for the first two-thirds of gestation and nutrients pass to the embryo through the shell and the avascular bilaminar yolk sac. At around the time of shell rupture, part of the yolk sac is trilaminar and supplied with blood vessels. It attaches to the uterus and forms a choriovitelline placenta. Rapid growth of the embryo ensues, still supported by histotrophe as well as exchange of oxygen and nutrients between maternal and foetal blood vessels (haemotrophic nutrition). Few marsupials have a chorioallantoic placenta and the highly altricial newborn is delivered after a short gestation. Eutherian embryos pass through a similar sequence before there is a fully functional chorioallantoic placenta. In most orders, there is transient yolk sac placentation, but even before this, nutrients are transferred through an avascular yolk sac. Yolk sac placentation does not occur in rodents or catarrhine primates. Early embryonic development in the mouse is nonetheless dependent on histotrophic nutrition. In the first trimester of human pregnancy, uterine glands open to the intervillous space and secretion products are taken up by the trophoblast. Transfer of nutrients to the early human embryo also involves the yolk sac, which floats free in the exocoelom. Marsupials can therefore inform us about the role of foetal membranes and histotrophic nutrition in early embryogenesis, knowledge that can translate to eutherians.

Acute and Chronic Iron Overloading Differentially Modulates the Expression of Cellular Iron-homeostatic Molecules in Normal Rat Kidney

Little is known about the renal responses to acute iron overloading. This study measured the renal tubular expression of transferrin receptor-1 (TfR1), cubilin/megalin receptors, hepcidin, ferroportin, and ferritin chains following subacute intoxication of 40 male Wistar rats with a single oral dose of ferrous iron (300 mg/kg). The animals were randomly subdivided into 4 equal subgroups at the time of necropsy (1, 2, 4, and 8 hr). The results were compared with the controls ( n=15) and with the chronic group ( n=15), which received iron for 4 weeks (75 mg/kg/day; 5 days/week). Although both toxicity models inhibited TfR1, they upregulated the cubilin/megalin receptors and hepcidin, and triggered iron deposition in tubular cells. The ferritin heavy-chain and ferroportin were downregulated in the 2-hr and 4-hr acute subgroups, whereas chronic toxicity promoted their expression, compared with controls. Moreover, the 4-hr and 8-hr subgroups had higher intracellular Fe⁺² and marked cell apoptosis compared with the chronic group. In conclusion, the kidney appears to sustain iron reabsorption in both intoxication models. However, the cellular iron storage and exporter proteins were differentially expressed in both models, and their inhibition post-acute toxicity might contribute toward the intracellular accumulation of Fe⁺², oxidative stress, and ferroptosis.

Is low iron status a risk factor for neural tube defects?

BACKGROUND

Folic acid supplements can protect against neural tube defects (NTDs). Low folate and low vitamin B12 status may be maternal risk factors for having an NTD affected pregnancy. However, not all NTDs are preventable by having an adequate folate/ B12 status and other potentially modifiable factors may be involved. Folate and vitamin B12 status have important links to iron metabolism. Animal studies support an association between poor iron status and NTDs, but human data are scarce. We examined the relevance of low iron status in a nested NTD case-control study of women within a pregnant population-based cohort.

METHODS

Pregnant women were recruited between 1986 and 1990, when vitamin or iron supplementation in early pregnancy was rare. Blood samples, taken at an average of 14 weeks gestation, were used to measure ferritin and hemoglobin in 64 women during an NTD affected pregnancy and 207 women with unaffected pregnancies.

RESULTS

No significant differences in maternal ferritin or hemoglobin concentrations were observed between NTD affected and nonaffected pregnancies (case median ferritin 16.9 µg/L and hemoglobin 12.4 g/dl versus 15.4 µg/L and 12.3g/dl in controls). As reported previously, red cell folate and vitamin B12 concentrations were significantly lower in cases. Furthermore, there was no significant association of iron status with type of NTD lesion (anencephaly or spina bifida).

CONCLUSION

We conclude that low maternal iron status during early pregnancy is not an independent risk factor for NTDs. Adding iron to folic acid for periconceptional use may improve iron status but is not likely to prevent NTDs.

Maternal iron deficiency identifies critical windows for growth and cardiovascular development in the rat postimplantation embryo

Imbalances in nutrition during pregnancy can lead to long-, as well as short-term consequences, a phenomenon known as fetal programming. However, there is little information about when the fetus is most sensitive to its environment during gestation. We hypothesize that different fetal systems are most vulnerable to nutritional stress during periods of maximal growth and differentiation. We used iron (Fe) deficiency, which causes hypertension in the offspring, to test this hypothesis. We examined development between embryonic day (E) 10.5 and 12.5, when cardiovascular development is maximal, using whole embryo culture. Female rats were fed Fe-deficient or control diet for 4 wk before mating and up to E10.5. The embryos were cultured for 48 h in 95% rat serum collected from males fed either a control or Fe-deficient diet. Growth was impaired and heart size increased in embryos taken from Fe-deficient mothers and cultured in deficient serum compared with control embryos cultured in control serum. To test whether restoring normal Fe levels could reverse these effects, we cultured embryos from control and deficient dams in either control or deficient medium. The yolk sac circulation of embryos from dams fed either diet cultured in deficient medium was less developed, with a thinner and less branched network than that in all embryos cultured in control serum. The heart was enlarged in embryos of deficient dams cultured in deficient serum compared with the heart size of those cultured in control serum. Culturing embryos in control serum reversed these changes. We conclude, therefore, that this period of cardiovascular organogenesis is one of the sensitive windows during which optimal Fe status is critical for normal development.

Megalin and cubilin: multifunctional endocytic receptors

The ability to take up substances from the surrounding environment not only provides cells with vital nutrients, but also enables the selective transport of substances from one compartment to another. Megalin and cubilin are two structurally different endocytic receptors that interact to serve such functions. Evidence has accumulated in recent years to indicate that these receptors have important functions in both normal physiology and pathology.

Megalin-dependent cubilin-mediated endocytosis is a major pathway for the apical uptake of transferrin in polarized epithelia

Cubilin is a 460-kDa protein functioning as an endocytic receptor for intrinsic factor vitamin B(12) complex in the intestine and as a receptor for apolipoprotein A1 and albumin reabsorption in the kidney proximal tubules and the yolk sac. In the present study, we report the identification of cubilin as a novel transferrin (Tf) receptor involved in catabolism of Tf. Consistent with a cubilin-mediated endocytosis of Tf in the kidney, lysosomes of human, dog, and mouse renal proximal tubules strongly accumulate Tf, whereas no Tf is detectable in the endocytic apparatus of the renal tubule epithelium of dogs with deficient surface expression of cubilin. As a consequence, these dogs excrete increased amounts of Tf in the urine. Mice with deficient synthesis of megalin, the putative coreceptor colocalizing with cubilin, also excrete high amounts of Tf and fail to internalize Tf in their proximal tubules. However, in contrast to the dogs with the defective cubilin expression, the megalin-deficient mice accumulate Tf on the luminal cubilin-expressing surface of the proximal tubule epithelium. This observation indicates that megalin deficiency causes failure in internalization of the cubilin-ligand complex. The megalin-dependent, cubilin-mediated endocytosis of Tf and the potential of the receptors thereby to facilitate iron uptake were further confirmed by analyzing the uptake of (125)I- and (59)Fe-labeled Tf in cultured yolk sac cells.

The rat visceral yolk sac internalizes maternal transferrin and secretes hydrolyzed products towards the fetus

The uptake of transferrin by the rat visceral yolk sac membranes, and the fate of this protein, were measured in a two-chambered system which allowed access to both surfaces of these membranes, i.e. that facing the maternal compartment and that facing the fetal compartment. 125I-labeled transferrin was internalized by the maternal surface of the visceral yolk sac but not by the fetal surface. Following internalization, this transferrin was degraded and the amino acids were secreted exclusively towards the fetal compartment. Transcytosis of intact transferrin was not detected in either direction. These results suggest that transport across the rat visceral yolk sac bound to maternally derived transferrin is not a major mechanism of iron transport in vivo. These results support a role for the visceral yolk sac in fetal metabolism, or supplying the fetus with amino acids derived from degradation of specific maternal plasma proteins, in this case, transferrin.

The presence of transcription factors in chicken albumin, yolk and blastoderm

Embryonic development is determined by preset intrinsic programs and extrinsic signals. To explore the possibility that transcription factors are present at the onset of development, preparations of yolk, albumin, and blastoderm from unfertilized and fertilized white Leghorn chicken eggs were screened by a panel of 16 transcription factor antibodies with Western blot techniques. Yolk was positive for 13 transcription factors, whereas blastoderm was positive for 10, and albumin was positive for 5. In yolk, several transcription factors, GATA-2, E2F-1, MyoD, and TFIID, were developmentally regulated. These results indicate that intracellular yolk and extracellular albumin contain transcription factors which presumably influence early chick embryonic development from prefertilization to the late blastoderm stage. Thus, the utility of preset maternal transcription factors within yolk and albumin complement maternally derived mRNA to determine the early development of the zygote.

Mouse mutant embryos lacking huntingtin are rescued from lethality by wild-type extraembryonic tissues

Mouse embryos nullizygous for a targeted disruption of the Huntington's disease gene homologue (Hdh), which encodes a protein (huntingtin) of unknown biochemical function, become developmentally retarded and disorganized, and die early in development. Using chimeric analysis, we demonstrate that extensively chimeric embryos derived by injection of Hdh null ES cells into wild-type host blastocysts are rescued from lethality. In contrast, when wild-type ES cells are injected into Hdh null blastocysts, the chimeric embryos are morphologically indistinguishable from Hdh null mutants derived from natural matings, and die shortly after gastrulation. Therefore, the primary defect in the absence of huntingtin lies in extraembryonic tissues, whereas the epiblast and its derivatives are affected secondarily. It is likely that the mutation results in impairment of the nutritive functions of the visceral endoderm, which otherwise appears to differentiate normally, as evidenced by the expression of several specific marker genes. Consistent with preliminary histochemical analysis indicating that at least the transport of ferric ions is defective in Hdh mutants and in conjunction with the known localization of huntingtin in the membranes of vesicles associated with microtubules, we hypothesize that this protein is involved in the intracellular trafficking of nutrients in early embryos.

Uptake of microparticles by rat visceral yolk sac

The visceral yolk sac (VYS) is responsible for a major part of the amino acid nutrition of the early post-implantation rat embryo and possibly also at the fetal stage of gestation. The mechanism involves endocytic uptake of proteins by the tissue's epithelial cells followed by intralysosomal digestion to amino acids. The amino acid so generated are used for protein synthesis in both the embryo and the VYS. Previous reports had indicated that the endocytic capacity of the VYS might be limited to exclude larger macromolecules. This study demonstrates that Percoll, which comprises 30-nm silica particles coated with polyvinylpyrrolidone (PVP), is as effectively captured by the 17.5-day rat VYS cultured in vitro as PVP itself. Uptake of 125I-labelled Percoll was progressive with time over 5 h and was inhibited by a low incubation temperature, 2,4-dinitrophenol (50 micrograms/ml), EGTA (5 mM), colchicine (10 micrograms/ml) or cytochalasin B (10 micrograms/ml). After uptake of 125I-labelled Percoll, VYSs released only 20 per cent of their radioactivity when re-incubated in fresh medium for 3 h. These data, and electron micrographs showing Percoll in intracellular vacuoles, are all consistent with uptake by endocytosis. Percoll's rate of uptake by the VYS indicates that, like 125I-labelled PVP, it enters the cell chiefly by fluid-phase pinocytosis. It is concluded that endocytosis by the VYS will efficiently capture even the largest globular proteins, and that previous indications of a relatively low size exclusion reflected the loosely coiled configuration of the synthetic polymers used in the earlier studies.