Intestinal hephaestin potentiates iron absorption in weanling, adult, and pregnant mice under physiological conditions

Adaptations in Gastrointestinal Nutrient Absorption and its Determinants During Pregnancy in Monogastric Mammals: A Scoping Review

CONTEXT

Pregnancy increases nutrient demand, but how nutrient uptake and its determinants adapt to facilitate this is unclear.

OBJECTIVE

This review aimed to identify and characterize evidence and evidence gaps regarding changes in gastrointestinal nutrient absorption and its determinants during pregnancy in monogastric mammals.

DATA SOURCES

A scoping review of peer-reviewed sources was conducted across PubMed, Scopus, Web of Science, Embase, and ProQuest (theses and dissertations) databases.

DATA EXTRACTION

Data extracted included species, pregnancy stages and outcomes. Where sufficient data for a given outcome was available, relative values were summarized graphically or in tables, to allow comparison across pregnancy stages and/or small intestine regions. Searches identified 26 855 sources, of which only 159 were eligible. Mechanistic studies were largely restricted to rodents, and most compared non- and late-pregnant groups, with fewer studies including early- or mid-pregnant groups.

DATA ANALYSIS

During pregnancy, there is some evidence for greater capacity for glucose uptake but unchanged amino acid uptake, and good evidence for increased uptake of calcium, iron, and zinc, and slower gastrointestinal passage of nutrients. The available evidence indicates that acute glucose uptake, gastric emptying, and the activities of sucrase, maltase, and lactase do not change during pregnancy. Gaps in the knowledge include the effects of pregnancy on uptake of specific amino acids, lipids, and most minerals and vitamins.

CONCLUSION

The results indicate that the gastrointestinal tract adapts during pregnancy to facilitate increased nutrient absorption. Additional data is required in order to assess the underlying mechanisms for and impacts on the absorption of many nutrients, as well as to determine the timing of these adaptations.

Mechanism and regulation of iron absorption throughout the life cycle

BACKGROUND

Iron plays a crucial role through various life stages of human. Iron homeostasis is primarily regulated by iron absorption which is mediated via divalent metal-ion transporter 1 (DMT1), and iron export protein ferroportin (FPN), as there is no active pathway for iron excretion from the body. Recent studies have shown that the magnitude of iron absorption changes through various life stages to meet changing iron requirements.

AIM OF REVIEW

This review aims to provide an overview of recent researches on the regulation of iron absorption throughout mammalian life cycle, with the potential to reveal novel molecules and pathways at special stage of life. Such insights may pave the way for new treatments for disorders associated with aberrant iron homeostasis in the future.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review first summarize the mechanism and regulation of iron absorption throughout various life stages, highlighting that regulatory mechanisms have developed to precisely align iron absorption to iron requirements. In adults, iron absorption is enhanced when body is deficient of iron, conversely, iron absorption is reduced when iron demand decreases via systemic regulator Hepcidin and cellular regulation. In the elderly, age-related inflammation, hormonal changes, and chronic diseases may affect the production of Hepcidin, affecting iron absorption. In infants, intestinal iron absorption and its regulatory mechanism are different from that in adults and there might be an alternative pathway independent of DMT1 and FPN due to high iron absorption. Unique to the fetus, iron is absorbed from maternal stores for its own use through the placenta and is regulated by maternal iron status. This review also proposes directions for further studies, offering promising avenues for developing new treatments for disorders associated with aberrant iron homeostasis.

In vivo silencing of intestinal DMT1 mitigates iron loading in β-thalassemia intermedia (Hbbth3/+) mice

ABSTRACT

β-thalassemia is an iron-loading anemia caused by homozygous mutation of the hemoglobin subunit β (HBB) gene. In β-thalassemia intermedia (βTI), a non-transfusion-dependent form of the disease, iron overload is caused by excessive absorption of dietary iron due to inappropriately low production of the iron-regulatory hormone hepcidin. Low hepcidin stabilizes the iron exporter ferroportin (FPN) on the basolateral membrane of enterocytes. High FPN activity may deplete intracellular iron and enhance expression of the predominant iron importer divalent metal-ion transporter 1 (DMT1). In mice, DMT1 mediates normal iron absorption under physiological conditions and excessive iron absorption in pathological iron overload (eg, hereditary hemochromatosis). Here, we hypothesized that DMT1 drives elevated iron absorption in βTI. Accordingly, we crossed Hbbth3/+ mice, a preclinical model of βTI, with intestine-specific DMT1-knockout mice. Ablation of intestinal DMT1 in Hbbth3/+ mice caused a pathophysiological shift from iron overload to an iron-deficiency phenotype with exacerbated anemia. DMT1 is thus required for iron absorption and iron loading in Hbbth3/+ mice. Based upon these outcomes, we further logically postulated that in vivo knockdown of intestinal DMT1 would mitigate iron loading in Hbbth3/+ mice. Ginger-derived, lipid nanoparticles carrying DMT1-specific (or control) small interfering RNAs (siRNAs) were administered by oral, intragastric gavage to 4-week-old Hbbth3/+ mice daily for 16 days. siRNA treatment reduced DMT1 expression by >80% and blunted iron loading, as indicated by significant reductions in liver iron and serum ferritin (which reflect body iron stores). These notable experimental outcomes establish intestinal DMT1 as a plausible therapeutic target to mitigate iron overload in βTI.

Can iron chelators ameliorate viral infections?

The redox reactivity of iron is a double-edged sword for cell functions, being either essential or harmful depending on metal concentration and location. Deregulation of iron homeostasis is associated with several clinical conditions, including viral infections. Clinical studies as well as in silico, in vitro and in vivo models show direct effects of several viruses on iron levels. There is support for the strategy of iron chelation as an alternative therapy to inhibit infection and/or viral replication, on the rationale that iron is required for the synthesis of some viral proteins and genes. In addition, abnormal iron levels can affect signaling immune response. However, other studies report different effects of viral infections on iron homeostasis, depending on the class and genotype of the virus, therefore making it difficult to predict whether iron chelation would have any benefit. This review brings general aspects of the relationship between iron homeostasis and the nonspecific immune response to viral infections, along with its relevance to the progress or inhibition of the inflammatory process, in order to elucidate situations in which the use of iron chelators could be efficient as antivirals.

Fetal factors disrupt placental and maternal iron homeostasis in murine β-thalassemia

Pregnancy rates in β-thalassemia are increasing but the risk of complications is higher; thus, better understanding of maternal and fetal iron homeostasis in this disorder is needed. HbbTh3/+ (Th3/+) mice model human β-thalassemia. Both the murine and human diseases are characterized by low hepcidin, high iron absorption, and tissue iron overload, with concurrent anemia. We hypothesized that disordered iron metabolism in pregnant Th3/+ mice would negatively affect their unborn offspring. The experimental design included these groups: wild-type (WT) dams carrying WT fetuses (WT1); WT dams carrying WT and Th3/+ fetuses (WT2); Th3/+ dams carrying WT and Th3/+ fetuses (Th3/+); and age-matched, nonpregnant adult females. Serum hepcidin was low, and mobilization of splenic and hepatic storage iron was enhanced in all 3 groups of experimental dams. Intestinal 59Fe absorption was lower in Th3/+ dams (as compared with WT1/2 dams) but splenic 59Fe uptake was higher. Th3/+ dams had hyperferremia, which led to fetal and placenta iron loading, fetal growth restriction, and placentomegaly. Notably, Th3/+ dams loaded Th3/+ and WT fetuses, with the latter situation more closely mirroring human circumstances when mothers with thalassemia have relatively unaffected (thalassemia trait) offspring. Iron-related oxidative stress likely contributed to fetal growth impairment; enhanced placental erythropoiesis is a probable cause of placental enlargement. Moreover, high fetal liver iron transactivated Hamp; fetal hepcidin downregulated placental ferroportin expression, limiting placental iron flux and thus mitigating fetal iron loading. Whether gestational iron loading occurs in human thalassemic pregnancy, when blood transfusion can further elevate serum iron, is worth consideration.

The biology of mammalian multi-copper ferroxidases

The mammalian multicopper ferroxidases (MCFs) ceruloplasmin (CP), hephaestin (HEPH) and zyklopen (ZP) comprise a family of conserved enzymes that are essential for body iron homeostasis. Each of these enzymes contains six biosynthetically incorporated copper atoms which act as intermediate electron acceptors, and the oxidation of iron is associated with the four electron reduction of dioxygen to generate two water molecules. CP occurs in both a secreted and GPI-linked (membrane-bound) form, while HEPH and ZP each contain a single C-terminal transmembrane domain. These enzymes function to ensure the efficient oxidation of iron so that it can be effectively released from tissues via the iron export protein ferroportin and subsequently bound to the iron carrier protein transferrin in the blood. CP is particularly important in facilitating iron release from the liver and central nervous system, HEPH is the major MCF in the small intestine and is critical for dietary iron absorption, and ZP is important for normal hair development. CP and HEPH (and possibly ZP) function in multiple tissues. These proteins also play other (non-iron-related) physiological roles, but many of these are ill-defined. In addition to disrupting iron homeostasis, MCF dysfunction perturbs neurological and immune function, alters cancer susceptibility, and causes hair loss, but, despite their importance, how MCFs co-ordinately maintain body iron homeostasis and perform other functions remains incompletely understood.

Intestinal iron absorption is appropriately modulated to match physiological demand for iron in wild-type and iron-loaded Hamp (hepcidin) knockout rats during acute colitis

Mucosal damage, barrier breach, inflammation, and iron-deficiency anemia (IDA) typify ulcerative colitis (UC) in humans. The anemia in UC appears to mainly relate to systemic inflammation. The pathogenesis of this ‘anemia of inflammation’ (AI) involves cytokine-mediated transactivation of hepatic Hamp (encoding the iron-regulatory hormone, hepcidin). In AI, high hepcidin represses iron absorption (and iron release from stores), thus lowering serum iron, and restricting iron for erythropoiesis (causing anemia). In less-severe disease states, inflammation may be limited to the intestine, but whether this perturbs iron homeostasis is uncertain. We hypothesized that localized gut inflammation will increase overall iron demand (to support the immune response and tissue repair), and that hepatic Hamp expression will decrease in response, thus derepressing (i.e., enhancing) iron absorption. Accordingly, we developed a rat model of mild, acute colitis, and studied iron absorption and homeostasis. Rats exposed (orally) to DSS (4%) for 7 days had intestinal (but not systemic) inflammation, and biomarker analyses demonstrated that iron utilization was elevated. Iron absorption was enhanced (by 2-3-fold) in DSS-treated, WT rats of both sexes, but unexpectedly, hepatic Hamp expression was not suppressed. Therefore, to gain a better understanding of regulation of iron absorption during acute colitis, Hamp KO rats were used for further experimentation. The severity of DSS-colitis was similar in Hamp KOs as in WT controls. In the KOs, increased iron requirements associated with the physiological response to colitis were satisfied by mobilizing hepatic storage iron, rather than by increasing absorption of enteral iron (as occurred in WT rats). In conclusion then, in both sexes and genotypes of rats, iron absorption was appropriately modulated to match physiological demand for dietary iron during acute intestinal inflammation, but regulatory mechanisms may not involve hepcidin.

Dietary Supplementation of EGF Ameliorates the Negatively Effects of LPS on Early-Weaning Piglets: From Views of Growth Performance, Nutrient Digestibility, Microelement Absorption and Possible Mechanisms

Simple Summary

This study aims to investigate how epidermal growth factor (EGF) attenuates the effect of lipopolysaccharide (LPS) on the growth performance, nutrient digestibility, microelement absorption of early-weaned pigs. A total of 48 early weaned piglets were randomly distributed to four groups consisting of a 2 × 2 factorial design. The main factors were the level of LPS (H_LPS = high LPS: 100 μg/kg body weight; Z_LPS = low LPS: 0 μg/kg body weight) and EGF (H_EGF = high EGF: 2 mg/kg diet; Z_EGF = low EGF: 0 mg/kg diet). Each group had four replicates and each replicate consisted of three piglets. The results showed that H_LPS level decreased the growth performance and the apparent digestibility of crude fat, while H_EGF level increased the average daily feed intake. The concentration of most microelements in the gastrointestinal tract chyme and feces were increased by H_LPS level and decreased by H_EGF level. The expression levels of most microelement transport-relative genes in the mucosa of gastrointestinal tissues were decreased by H_LPS level and increased by H_EGF level. In conclusion, dietary EGF could attenuate the negative effect of LPS exposure on the apparent digestibility of crude fat and microelement absorption through changing the expression levels of microelement transport-relative genes. EGF can be used as an additive to increase the essential trace elements absorption in the early weaning piglets.

Abstract

Epidermal growth factor (EGF) plays an important role in nutrients absorption. However, whether it can be an effective additive to improve the growth performance and nutrients absorption in lipopolysaccharide (LPS) challenged early weaning piglets is still unknown. A 14-days trial was conducted to investigate how EGF attenuates the effect of LPS on the growth performance, nutrient digestibility, microelement absorption of early-weaned pigs, and study the underlying mechanism. A total of 48 early weaned piglets, aged 25 days, were randomly distributed to four groups (control, EGF, LPS and EGF + LPS groups) consisting of a 2 × 2 factorial design. The main factors were the level of LPS (H_LPS = high LPS: 100 μg/kg body weight; Z_LPS = low LPS: 0 μg/kg body weight) and EGF (H_EGF = high EGF: 2 mg/kg diet; Z_EGF = low EGF: 0 mg/kg diet). Each group had four replicates and each replicate consisted of three piglets. The results showed that piglets injected with H_LPS level significantly decreased the average daily gain (ADG), and significantly increased the feed conversion ratio (FCR) compared with the piglets injected with Z_LPS level, while piglets fed H_EGF level significantly increased the average daily feed intake (ADFI) compared with the piglets fed Z_EGF level (p < 0.05). Piglets injected with H_LPS level significantly decreased the apparent digestibility of crude fat compared with the piglets injected with Z_LPS level (p < 0.05). Piglets injected with H_LPS level significantly increased the concentration of most microelements in the gastrointestinal tract chyme and feces, and significantly decreased the expression levels of most microelement transport-relative genes in the mucosa of gastrointestinal tissues compared with the piglets injected with Z_LPS level (p < 0.05). Piglets fed H_EGF level significantly decreased the concentration of microelement in the gastrointestinal tract chyme and feces, and significantly increased the expression levels of the microelement transport-relative genes in the mucosa of gastrointestinal tissues compared with the piglets fed Z_EGF level (p < 0.05). In conclusion, dietary EGF could attenuate the negative effect of LPS exposure on the apparent digestibility of crude fat and microelement absorption of early-weaning piglets. EGF and LPS influenced the absorption of essential trace element through changing the expression levels of microelement transport-relative genes in the mucosa of gastrointestinal tissues. In the early weaning piglets, EGF can be used as an additive to increase the essential trace elements absorption.

Oral Administration of Ginger-Derived Lipid Nanoparticles and Dmt1 siRNA Potentiates the Effect of Dietary Iron Restriction and Mitigates Pre-Existing Iron Overload in Hamp KO Mice

Intestinal iron transport requires an iron importer (Dmt1) and an iron exporter (Fpn1). The hormone hepcidin regulates iron absorption by modulating Fpn1 protein levels on the basolateral surface of duodenal enterocytes. In the genetic, iron-loading disorder hereditary hemochromatosis (HH), hepcidin production is low and Fpn1 protein expression is elevated. High Fpn1-mediated iron export depletes intracellular iron, causing a paradoxical increase in Dmt1-mediated iron import. Increased activity of both transporters causes excessive iron absorption, thus initiating body iron loading. Logically then, silencing of intestinal Dmt1 or Fpn1 could be an effective therapeutic intervention in HH. It was previously established that Dmt1 knock down prevented iron-loading in weanling Hamp (encoding hepcidin) KO mice (modeling type 2B HH). Here, we tested the hypothesis that Dmt1 silencing combined with dietary iron restriction (which may be recommended for HH patients) will mitigate iron loading once already established. Accordingly, adult Hamp KO mice were switched to a low-iron (LFe) diet and (non-toxic) folic acid-coupled, ginger nanoparticle-derived lipid vectors (FA-GDLVs) were used to deliver negative-control (NC) or Dmt1 siRNA by oral, intragastric gavage daily for 21 days. The LFe diet reduced body iron burden, and experimental interventions potentiated iron losses. For example, Dmt1 siRNA treatment suppressed duodenal Dmt1 mRNA expression (by ~50%) and reduced serum and liver non-heme iron levels (by ~60% and >85%, respectively). Interestingly, some iron-related parameters were repressed similarly by FA-GDLVs carrying either siRNA, including 59Fe (as FeCl3) absorption (~20% lower), pancreatic non-heme iron (reduced by ~65%), and serum ferritin (decreased 40-50%). Ginger may thus contain bioactive lipids that also influence iron homeostasis. In conclusion, the combinatorial approach of FA-GDLV and Dmt1 siRNA treatment, with dietary iron restriction, mitigated pre-existing iron overload in a murine model of HH.

Copper nutrition and biochemistry and human (patho)physiology

The essential trace mineral copper plays important roles in human physiology and pathophysiology. Disruption of copper homeostasis may underlie the development of ischemic heart disease, and connective tissue and neurodegenerative disorders. Copper also likely participates in the host response to bacterial infection and is further implicated more broadly in regulating immunity. Recent studies further associate copper with disruption of lipid homeostasis, as is frequently seen in, for example, non-alcoholic fatty liver disease (NAFLD). Moreover, continuing investigation of copper chaperones has revealed new roles for these intracellular copper-binding proteins. Despite these (and many other) significant advances, many questions related to copper biology remain unanswered. For example, what are the most sensitive and specific biomarkers of copper status, and which ones are useful in marginal (or "sub-clinical" copper deficiency)? Further research on this topic is required to inform future investigations of copper metabolism in humans (so the copper status of study participants can be fully appreciated). Also, are current recommendations for copper intake adequate? Recent studies suggest that overt copper deficiency is more common than once thought, and further, some have suggested that the copper RDAs for adults may be too low. Additional human balance and interventional studies are necessary and could provide the impetus for reconsidering the copper RDAs in the future. These and myriad other unresolved aspects of copper nutrition will undoubtedly be the focus of future investigation.

Expression of soluble, active, fluorescently tagged hephaestin in COS and CHO cell lines

Hephaestin (Hp) is a trans-membrane protein, which plays a critical role in intestinal iron absorption. Hp was originally identified as the gene responsible for the phenotype of sex-linked anaemia in the sla mouse. The mutation in the sla protein causes accumulation of dietary iron in duodenal cells, causing severe microcytic hypochromic anaemia. Although mucosal uptake of dietary iron is normal, export from the duodenum is inhibited. Hp is homologous to ceruloplasmin (Cp), a member of the family of multi copper ferroxidases (MCFs) and possesses ferroxidase activity that facilitates iron release from the duodenum and load onto the serum iron transport protein transferrin. In the present study, attempts were made to produce biologically active recombinant mouse hephaestin as a secretory form tagged with green fluorescent protein (GFP), Hpsec-GFP. Plasmid expressing Hpsec-GFP was constructed and transfected into COS and CHO cells. The GFP aided the monitoring expression in real time to select the best conditions to maximise expression and provided a tag for purifying and analysing Hpsec-GFP. The protein had detectable oxidase activity as shown by in-gel and solution-based assays. The methods described here can provide the basis for further work to probe the interaction of hephaestin with other proteins using complementary fluorescent tags on target proteins that would facilitate the fluorescence resonance energy transfer measurements, for example with transferrin or colocalisation studies, and help to discover more about hephaestin works at the molecular level.

Iron and manganese transport in mammalian systems

Studies in recent years have significantly expanded, refined, and redefined the repertoire of transporters and other proteins involved in iron and manganese (Mn) transport and homeostasis. In this review, we discuss highlights of the recent literature on iron and Mn transport, focusing on the roles of membrane transporters and related proteins. Studies are considered from the vantage point of main organs, tissues, and cell types that actively control whole-body iron or Mn homeostasis, with emphasis on studies in which in vivo metal transport was measured directly or implicated by using knockout mouse models. Overviews of whole-body and cellular iron and Mn homeostasis are also provided to give physiological context for key transporters and to highlight how they participate in the uptake, intracellular trafficking, and efflux of each metal. Important similarities and differences in iron and Mn transport are noted, and future research opportunities and challenges are identified.

Animal Models of Normal and Disturbed Iron and Copper Metabolism

Research on the interplay between iron and copper metabolism in humans began to flourish in the mid-20th century, and diseases associated with dysregulated homeostasis of these essential trace minerals are common even today. Iron deficiency is the most frequent cause of anemia worldwide, leading to significant morbidity, particularly in developing countries. Iron overload is also quite common, usually being the result of genetic mutations which lead to inappropriate expression of the iron-regulatory hormone hepcidin. Perturbations of copper homeostasis in humans have also been described, including rare genetic conditions which lead to severe copper deficiency (Menkes disease) or copper overload (Wilson disease). Historically, the common laboratory rat (Rattus norvegicus) was the most frequently utilized species to model human physiology and pathophysiology. Recently, however, the development of genetic-engineering technology combined with the worldwide availability of numerous genetically homogenous (i.e., inbred) mouse strains shifted most research on iron and copper metabolism to laboratory mice. This created new opportunities to understand the function of individual genes in the context of a living animal, but thoughtful consideration of whether mice are the most appropriate models of human pathophysiology was not necessarily involved. Given this background, this review is intended to provide a guide for future research on iron- and copper-related disorders in humans. Generation of complementary experimental models in rats, swine, and other mammals is now facile given the advent of newer genetic technologies, thus providing the opportunity to accelerate the identification of pathogenic mechanisms and expedite the development of new treatments to mitigate these important human disorders.

Effects of egg phosvitin on mucosal transcriptional profiles and luminal microbiota composition in murine colon

The effects of egg phosvitin on epithelial gene expression, abundance of bacteria in colon and their correlation have been explored.