Genetic variation of basal iron status, ferritin and iron regulatory protein in mice: potential for modulation of oxidative stress

Comparison of the effects of high dietary iron levels on bone microarchitecture responses in the mouse strains 129/Sv and C57BL/6J

Iron is an essential nutrient for all living organisms. Both iron deficiency and excess can be harmful. Bone, a highly metabolic active organ, is particularly sensitive to fluctuations in iron levels. In this study, we investigated the effects of dietary iron overload on bone homeostasis with a specific focus on two frequently utilized mouse strains: 129/Sv and C57BL/6J. Our findings revealed that after 6 weeks on an iron-rich diet, 129/Sv mice exhibited a decrease in trabecular and cortical bone density in both vertebral and femoral bones, which was linked to reduced bone turnover. In contrast, there was no evidence of bone changes associated with iron overload in age-matched C57BL/6J mice. Interestingly, 129/Sv mice exposed to an iron-rich diet during their prenatal development were protected from iron-induced bone loss, suggesting the presence of potential adaptive mechanisms. Overall, our study underscores the critical role of genetic background in modulating the effects of iron overload on bone health. This should be considered when studying effects of iron on bone.

Adipose triglyceride lipase mediates lipolysis and lipid mobilization in response to iron-mediated negative energy balance

Maintenance of energy balance is essential for overall organismal health. Mammals have evolved complex regulatory mechanisms that control energy intake and expenditure. Traditionally, studies have focused on understanding the role of macronutrient physiology in energy balance. In the present study, we examined the role of the essential micronutrient iron in regulating energy balance. We found that a short course of dietary iron caused a negative energy balance resulting in a severe whole body wasting phenotype. This disruption in energy balance was because of impaired intestinal nutrient absorption. In response to dietary iron-induced negative energy balance, adipose triglyceride lipase (ATGL) was necessary for wasting of subcutaneous white adipose tissue and lipid mobilization. Fat-specific ATGL deficiency protected mice from fat wasting, but caused a severe cachectic response in mice when fed iron. Our work reveals a mechanism for micronutrient control of lipolysis that is necessary for regulating mammalian energy balance.

Supplementation with >Your< Iron Syrup Corrects Iron Status in a Mouse Model of Diet-Induced Iron Deficiency

The objective of this study was to compare the effects of >Your< Iron Syrup, a novel oral liquid iron-containing food supplement, with the commonly prescribed iron sulphate (Fe-sulphate) in a mouse model of diet-induced iron deficiency. Standard inbred BALB/cOlaHsd mice were fed low-iron diet for 11 weeks to induce significant decrease in blood haemoglobin and haematocrit and were then supplemented by gavage with either >Your< Iron Syrup or Fe-sulphate for two weeks. In >Your< Iron Syrup group, several markers of iron deficiency, such as serum iron concentration, transferrin saturation and ferritin level were significantly improved in both female and male mice. Fe-sulphate induced similar responses, except that it did not significantly increase iron serum in females and serum ferritin in both sexes. Fe-sulphate significantly increased liver-iron content which >Your< Iron Syrup did not. Transcription of Hamp and selected inflammatory genes in the liver was comparable between the two supplementation groups and with the Control diet group. Some sex-specific effects were noted, which were more pronounced and less variable in males. In conclusion, >Your< Iron Syrup was efficient, comparable and in some parameters superior to Fe-sulphate in improving iron-related parameters without inducing a response of selected liver inflammation markers in a mouse model of diet-induced iron deficiency.

Effect of mouse strain and diet on feasibility of MRI-based cell tracking in the liver

PURPOSE

MRI-based cell tracking identifies the location of magnetically labeled cells with hypointense voxels. Here we demonstrate a strain-dependent effect of liver MRI background on the feasibility of MRI-based cell tracking of transplanted cells in the mouse liver.

METHODS

FVB mice (GFP-LUC and NOG) and C57BL/6 mice (GFP+ and wild-type) were fed 3 different diets with varying iron content. In vivo T 2 ∗ -weighted images and T 2 ∗ maps of the liver were acquired at different ages. Magnetically labeled cancer cells were injected intrasplenically for hepatic migration; then, mice were imaged by in vivo MRI and bioluminescence imaging. Livers were also imaged ex vivo by magnetic particle imaging.

RESULTS

R 2 ∗ increased with age in FVB_NOG and FVB_GFP-LUC mice that were fed diets sufficient in iron. FVB_NOG mice developed a mottled appearance in their livers with age that did not occur in FVB_GFP-LUC mice. R 2 ∗ was unchanging with age in C57BL/6_GFP mice, and the liver remained bright and homogenous. Labeled cells were not detectable by MRI in some livers despite successful engraftment as shown by bioluminescence imaging and magnetic particle imaging.

CONCLUSION

Strain, diet, and age are important considerations for MRI-based cell tracking in the liver. If a model with excessive liver iron must be used, alternative imaging methods such as magnetic particle imaging can be considered.

Hamp1 mRNA and plasma hepcidin levels are influenced by sex and strain but do not predict tissue iron levels in inbred mice

Iron homeostasis is tightly regulated, and the peptide hormone hepcidin is considered to be a principal regulator of iron metabolism. Previous studies in a limited number of mouse strains found equivocal sex- and strain-dependent differences in mRNA and serum levels of hepcidin and reported conflicting data on the relationship between hepcidin (Hamp1) mRNA levels and iron status. Our aim was to clarify the relationships between strain, sex, and hepcidin expression by examining multiple tissues and the effects of different dietary conditions in multiple inbred strains. Two studies were done: first, Hamp1 mRNA, liver iron, and plasma diferric transferrin levels were measured in 14 inbred strains on a control diet; and second, Hamp1 mRNA and plasma hepcidin levels in both sexes and iron levels in the heart, kidneys, liver, pancreas, and spleen in males were measured in nine inbred/recombinant inbred strains raised on an iron-sufficient or high-iron diet. Both sex and strain have a significant effect on both hepcidin mRNA (primarily a sex effect) and plasma hepcidin levels (primarily a strain effect). However, liver iron and diferric transferrin levels are not predictors of Hamp1 mRNA levels in mice fed iron-sufficient or high-iron diets, nor are the Hamp1 mRNA and plasma hepcidin levels good predictors of tissue iron levels, at least in males. We also measured plasma erythroferrone, performed RNA-sequencing analysis of liver samples from six inbred strains fed the iron-sufficient, low-iron, or high-iron diets, and explored differences in gene expression between the strains with the highest and lowest hepcidin levels.NEW & NOTEWORTHY Both sex and strain have a significant effect on both hepcidin mRNA (primarily a sex effect) and plasma hepcidin levels (primarily a strain effect). Liver iron and diferric transferrin levels are not predictors of Hamp1 mRNA levels in mice, nor are the Hamp1 mRNA and plasma hepcidin levels good predictors of tissue iron levels, at least in males.

Strain- and time-dependent alterations in hepatic iron metabolism in a murine model of nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis is a common liver disease that is often accompanied by dysregulated iron metabolism. The aim of the study was to test the hypothesis that aberrant iron metabolism in nonalcoholic steatohepatitis is modulated by genetic susceptibility to inflammation and oxidative stress. Hepatic histology and iron content were assessed in 3 inbred strains of mice (C57BL/6, BALB/c, and C3H/HeJ) fed an atherogenic diet (AD). Hepatic expression of genes relevant to iron metabolism, inflammation, and oxidative stress were quantitated by real-time reverse transcription-polymerase chain reaction. At 6 weeks on the AD, histologic injury and induction of inflammatory and oxidative stress-associated gene expression were most pronounced in C57BL/6. At 18 weeks on the AD, these parameters were similar in C57BL/6 and BALB/c. Atherogenic diet-fed C3H/HeJ showed milder responses at both time points. The AD was associated with decreased hepatic iron concentrations in all strains at 6 and 18 weeks. The decrease in hepatic iron concentrations did not correlate with changes in hepcidin expression and was not associated with altered expression of iron transporters. These findings are similar to those observed in models of obesity-induced steatosis and indicate that hepatic steatosis can be associated with depletion of iron stores that is not explained by upregulation of hepcidin expression by inflammation.

SIGNIFICANCE OF THE STUDY

Nonalcoholic steatohepatitis (NASH) is a common liver disease that often accompanies the metabolic syndrome. The latter condition has been linked to iron deficiency and diminished intestinal iron absorption, likely the result of hepcidin upregulation by chronic inflammation. Paradoxically, some NASH patients accumulate excess hepatic iron, which may increase fibrosis and cancer risk. Iron accumulation has been attributed to suppression of hepcidin by oxidative stress. The objective of this study was to investigate the contributions of inflammation and oxidative stress to altered hepatic iron metabolism in a murine model of NASH using inbred strains of mice with differing susceptibilities to injury.

Photoacoustic molecular imaging for in vivo liver iron quantitation

A recent study showed that ferritin is a suitable endogenous contrast agent for photoacoustic molecular imaging in cultured mammalian cells. We have therefore tested whether this imaging technique can be used for in vivo quantification of iron in mouse livers. To verify this hypothesis, we used multispectral optoacoustic tomography (MSOT) to image albino CD1 mice before and after experimental iron loading. Postmortem assays showed that the iron treatment caused a 15-fold increase in liver iron and a 40-fold increase in liver ferritin levels, while in vivo longitudinal analysis using MSOT revealed just a 1.6-fold increase in the ferritin/iron photoacoustic signal in the same animals. We conclude that MSOT can monitor changes in ferritin/iron levels in vivo, but its sensitivity is much lower than that of ex vivo iron assays.

In silico QTL mapping of basal liver iron levels in inbred mouse strains

Both iron deficiency and iron excess are detrimental in many organisms, and previous studies in both mice and humans suggest that genetic variation may influence iron status in mammals. However, these genetic factors are not well defined. To address this issue, we measured basal liver iron levels in 18 inbred strains of mice of both sexes on a defined iron diet and found ∼4-fold variation in liver iron in males (lowest 153 μg/g, highest 661 μg/g) and ∼3-fold variation in females (lowest 222 μg/g, highest 658 μg/g). We carried out a genome-wide association mapping to identify haplotypes underlying differences in liver iron and three other related traits (copper and zinc liver levels, and plasma diferric transferrin levels) in a subset of 14 inbred strains for which genotype information was available. We identified two putative quantitative trait loci (QTL) that contain genes with a known role in iron metabolism: Eif2ak1 and Igf2r. We also identified four putative QTL that reside in previously identified iron-related QTL and 22 novel putative QTL. The most promising putative QTL include a 0.22 Mb region on Chromosome 7 and a 0.32 Mb region on Chromosome 11 that both contain only one candidate gene, Adam12 and Gria1, respectively. Identified putative QTL are good candidates for further refinement and subsequent functional studies.

Repeated inhalations of diesel exhaust particles and oxidatively damaged DNA in young oxoguanine DNA glycosylase (OGG1) deficient mice

DNA repair may prevent increased levels of oxidatively damaged DNA from prolonged oxidative stress induced by, e.g. exposure to diesel exhaust particles (DEP). We studied oxidative damage to DNA in broncho-alveolar lavage cells, lungs, and liver after 4 x 1.5 h inhalations of DEP (20 mg/m3) in Ogg1-/- and wild type (WT) mice with similar extent of inflammation. DEP exposure increased lung levels of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in Ogg1-/- mice, whereas no effect on 8-oxodG or oxidized purines in terms of formamidopyrimidine DNA glycosylase (FPG) sites was observed in WT mice. In both unexposed and exposed Ogg1-/- mice the level of FPG sites in the lungs was 3-fold higher than in WT mice. The high basal level of FPG sites in Ogg1-/- mice probably saturated the assay and prevented detection of DEP-generated damage. In conclusion, Ogg1-/- mice have elevated pulmonary levels of FPG sites and accumulate genomic 8-oxodG after repeated inhalations of DEP.

Haptoglobin is degraded by iron in C57BL/6 mice: a possible link with endoplasmic reticulum stress

BACKGROUND

Haptoglobin is a glycoprotein produced mainly by the liver and secreted into the circulation. Haptoglobin, by virtue of its high affinity for hemoglobin, protects the tissues against hemoglobin-induced oxidative damage and allows heme iron recycling. Haptoglobin synthesis is controlled by various effectors, however, little is known concerning its regulation by iron. Haptoglobin regulation in C57BL/6 and 129sv mice fed on an iron-rich diet for 3 weeks was thus undertaken.

RESULTS

Iron induced a dramatic post-transcriptional decrease of liver and serum haptoglobin in C57BL/6 mice. In contrast, no alteration of haptoglobin expression was detected in 129sv mice. We assumed that the oxidative stress induced by iron in C57BL/6 mice altered the endoplasmic reticulum (ER) environment, leading to the incorrect folding of haptoglobin and its subsequent degradation. To test this hypothesis, the levels of the RE chaperone GRP78 were measured. This chaperone is known to assist protein folding in the RE during pathophysiological conditions. Interestingly, we found that the mRNA and protein levels of GRP78 were decreased in iron-fed C57BL/6 mice, while they were unchanged in iron-fed 129sv mice. These results suggest that the correct processing of haptoglobin (glycosylation, disulfide linkage, folding, and assembly) might be sensitive to ER stress and that, in the absence of GRP78-mediated assistance, Hp is degraded.

CONCLUSION

Our data demonstrate that iron regulates haptoglobin synthesis in C57BL/6 mice and suggest a possible link with iron-induced ER stress.

Genetic variation in Mon1a affects protein trafficking and modifies macrophage iron loading in mice

We undertook a quantitative trait locus (QTL) analysis in mice to identify modifier genes that might influence the severity of human iron disorders. We identified a strong QTL on mouse chromosome 9 that differentially affected macrophage iron burden in C57BL/10J and SWR/J mice. A C57BL/10J missense allele of an evolutionarily conserved gene, Mon1a, cosegregated with the QTL in congenic mouse lines. We present evidence that Mon1a is involved in trafficking of ferroportin, the major mammalian iron exporter, to the surface of iron-recycling macrophages. Differences in amounts of surface ferroportin correlate with differences in cellular iron content. Mon1a is also important for trafficking of cell-surface and secreted molecules unrelated to iron metabolism, suggesting that it has a fundamental role in the mammalian secretory apparatus.

Deprivation of pantothenic acid elicits a movement disorder and azoospermia in a mouse model of pantothenate kinase-associated neurodegeneration

We asked whether a movement disorder could be elicited by deprivation of pantothenic acid (PA; vitamin B5), the substrate for the enzyme pantothenate kinase 2 (PANK2), which is deficient in the inherited neurological disorder PKAN (pantothenate kinase-associated neurodegeneration formerly called Hallervorden-Spatz syndrome). This study was undertaken because mice made null for Pank2 failed to show the neurological manifestations of the human disease. Wild-type and Pank2 mutant mice were fed pantothenic acid-deficient diets and were monitored for general health, fertility and movement compared with animals on control diets over time. Mice of both genotypes on PA-deficient diets exhibited poor grooming, greying of fur and decreased body weight. With PA deprivation, wild-type mice manifested azoospermia (a phenotype also seen in Pank2 mice) as well as a movement disorder with a low-lying pelvis and slow steps. Rear limbs appeared to drag and occasionally extended into unnatural postures for 16-17 s duration, possibly indicative of dystonia. Movement disruption probably also occurs in PA-deprived Pank2 mutant mice, but they died precipitously before undergoing detailed analysis. Remarkably, restoration of dietary PA led to recovery of general health and grooming, weight gain, reversal of the movement disorder, and reappearance of mature sperm within 4 weeks. This study confirms the primacy of PA metabolism in the mechanism of disease in PKAN. PA deprivation provides a useful phenocopy for PKAN and allows us to test pharmacological and other interventional strategies in the treatment of this devastating disease.

Mapping genes responsible for strain-specific iron phenotypes in murine chromosome substitution strains

The highly variable clinical phenotype observed in patients homozygous for the C282Y mutation of the hereditary hemochromatosis gene (HFE) is likely due to the influence of non-HFE modifier genes. The primary functional abnormality causing iron overload in hemochromatosis is hyper-absorption of dietary iron. We found that iron absorption in inbred mice varies in a strain-specific manner, as does the pattern of iron distribution to the liver and spleen. A/J mice absorbed approximately twice the amount of 59Fe delivered by gavage compared to the C57BL/6 strain. Genetic comparisons between A/J and C57BL/6 were facilitated by the availability of consomic chromosome substitution strains (CSS). Each CSS has an individual chromosome pair from A/J on an otherwise C57BL/6J background. We found that iron absorption and iron content in liver and in spleen were continuous variables suggesting that each trait is under multigenic control. No trait co-segregated among the CSS. Chromosome 5 from A/J, however, imparted the highest iron absorption phenotype and multiple CSS had absorption levels equivalent to A/J. Chromosomes 9 and X were associated with high spleen iron content. These data suggest that multiple genes contribute to the regulation of iron absorption and that individual organ iron phenotypes are independently regulated.

Disparate patterns of age-related changes in lipid peroxidation in long-lived naked mole-rats and shorter-lived mice

A key tenet of the oxidative stress theory of aging is that levels of accrued oxidative damage increase with age. Differences in damage generation and accumulation therefore may underlie the natural variation in species longevity. We compared age-related profiles of whole-organism lipid peroxidation (urinary isoprostanes) and liver lipid damage (malondialdehyde) in long living naked mole-rats [maximum lifespan (MLS) > 28.3 years] and shorter-living CB6F1 hybrid mice (MLS approximately 3.5 years). In addition, we compared age-associated changes in liver non-heme iron to assess how intracellular conditions, which may modulate oxidative processes, are affected by aging. Surprisingly, even at a young age, concentrations of both markers of lipid peroxidation, as well as of iron, were at least twofold (P < 0.005) greater in naked mole tats than in mice. This refutes the hypothesis that prolonged naked mole-rat longevity is due to superior protection against oxidative stress. The age-related profiles of all three parameters were distinctly species specific. Rates of lipid damage generation in mice were maintained throughout adulthood, while accrued damage in old animals was twice that of young mice. In naked mole-rats, urinary isoprostane excretion declined by half with age (P < 0.001), despite increases in tissue iron (P < 0.05). Contrary to the predictions of the oxidative stress theory, lipid damage levels did not change with age in mole-rats. These data suggest that the patterns of age-related changes in levels of markers of oxidative stress are species specific, and that the pronounced longevity of naked mole-rats is independent of oxidative stress parameters.

Multiple polymorphic loci determine basal hepatic and splenic iron status in mice

Polymorphisms of genes linked to iron metabolism may account for individual variability in hemochromatosis and iron status connected with liver and cardiovascular diseases, cancers, toxicity, and infection. Mouse strains exhibit marked differences in levels of non-heme iron, with C57BL/6J and SWR showing low and high levels, respectively. The genetic basis for this variability was examined using quantitative trait loci (QTL) analysis together with expression profiling and chromosomal positions of known iron-related genes. Non-heme iron levels in liver and spleen of C57BL/6J x SWR F2 mice were poorly correlated, indicating independent regulation. Highly significant (P < .01) polymorphic loci were found on chromosomes 2 and 16 for liver and on chromosomes 8 and 9 for spleen. With sex as a covariate, additional significant or suggestive (P < 0.1) QTL were detected on chromosomes 7, 8, 11, and 19 for liver and on chromosome 2 for spleen. A gene array showed no clear association between most loci and differential iron-related gene expression. The gene for transferrin and a transferrin-like gene map close to the QTL on chromosome 9. Transferrin saturation was significantly lower in C57BL/6J mice than in SWR mice, but there was no significant difference in the serum level of transferrin, hepatic expression, or functional change in cDNA sequence. beta2-Microglobulin, which, unlike other loci, was associated with C57BL/6J alleles, is a candidate for the chromosome 2 QTL for higher iron. In conclusion, the findings show the location of polymorphic genes that determine basal iron status in wild-type mice. Human equivalents may be pertinent in predisposition to hepatic and other disorders.

Relationship between body iron stores and diquat toxicity in male Fischer-344 rats

The effects of body iron stores on diquat (DQ)-induced toxicity were examined in male Fischer-344 rats, which are sensitive to this herbicide. The rats (5 weeks old) were fed diets containing 40 (lower iron storage [LIS] group) or 320 ppm iron (higher iron storage [HIS] group) for 5 weeks. The concentrations of nonheme iron and ferritin in the liver and kidney were significantly higher in the HIS group than in the LIS group (P<0.0001), although there was no significant differences between the HIS and LIS groups in hematological parameters, including red blood cell count, hemoglobin concentration, and mean corpuscular volume. Three hours after administration of 0.1 mmol DQ/kg, serum alanine aminotransferase and urea nitrogen were significantly higher than in controls (saline injection) for both the LIS and HIS groups (P<0.01), and, after DQ injection, these parameters were significantly higher in the HIS group than in the LIS group (P<0.01). When the rats were injected with 0.075 or 0.1 mmol DQ/kg, the survival time was significantly shorter in the HIS group than in the LIS group (P<0.05). These findings suggest that higher body iron stores result in more severe DQ toxicity in Fischer-344 rats.

Animal models with enhanced erythropoiesis and iron absorption

The regulation of iron absorption is of considerable interest in mammals since excretion is minimal. Recent advances in iron metabolism have expounded the molecular mechanisms by which iron absorption is attuned to the physiological demands of the body. The pinnacle was the discovery and identification of hepcidin, a hepatic antimicrobial peptide that regulates absorption to maintain iron homeostasis. While the intricacies of its expression and regulation by HFE, transferrin receptor 2 and hemojuvelin are still speculative, hepcidin responsiveness has correlated negatively with iron absorption in different models and disorders of iron metabolism. Consequently, hepcidin expression is repressed to enhance iron absorption during stimulated erythropoiesis even in situations of elevated iron stores. Animal models have been crucial to the advances in understanding iron metabolism and the present review focuses on phenylhydrazine treated and hypotransferrinaemic rodents. These, respectively, experimental and genetic models of enhanced erythropoiesis highlight the shifting focus of iron absorption regulation from the marrow to the liver.

Proinflammatory effects of iron sucrose in chronic kidney disease

Inflammation is a central component of progressive chronic kidney disease (CKD). Iron promotes oxidative stress and inflammatory response in animals and promotes progressive CKD. Parenteral iron provokes oxidative stress in patients with CKD; however, its potential to provoke an inflammatory response is unknown. In 20 veterans with CKD, 100 mg iron sucrose was administered intravenously over 5 min and urinary excretion rate and plasma concentration of monocyte chemoattractant protein-1 (MCP-1) were measured at timed intervals over 24 h. Patients were then randomized to placebo or N-acetyl cysteine (NAC) 600 mg b.i.d. and the experiment was repeated at 1 week. Iron sucrose markedly increased plasma concentration and urinary excretion rate of MCP-1 at baseline and at 1 week visits (P < 0.0001 for time effect). Urinary excretion peaked at 30 min and plasma concentration at 15 min. Plasma MCP-1 concentration fell from 164 +/- 17.7 to 135 +/- 17.7 pg/ml with NAC, whereas it remained unchanged from 133 +/- 12.5 to 132 +/- 17.7 pg/ml with placebo (P=0.001 for visit x antioxidant drug interaction). There was a reduction in MCP-1 urinary excretion rate from visit 1 to 2. At the baseline visit, the urinary excretion rate averaged 305 +/- 66 pg/min and at the second visit 245 +/- 67 pg/min (mean difference 60 +/- 28 pg/min, P = 0.030). There was no improvement in urinary MCP-1 excretion with NAC. In conclusion, iron sucrose causes rapid and transient generation and/or release of MCP-1 plasma concentration and increases urinary excretion rate, and systemic MCP-1 level but the urinary excretion rate is not abrogated with the antioxidant NAC. These results may have implications for the progression of CKD with parenteral iron.

The contributions of normal variation and genetic background to mammalian gene expression

BACKGROUND

Qualitative and quantitative variability in gene expression represents the substrate for external conditions to exert selective pressures for natural selection. Current technologies allow for some forms of genetic variation, such as DNA mutations and polymorphisms, to be determined accurately on a comprehensive scale. Other components of variability, such as stochastic events in cellular transcriptional and translational processes, are less well characterized. Although potentially important, the relative contributions of genomic versus epigenetic and stochastic factors to variation in gene expression have not been quantified in mammalian species.

RESULTS

In this study we compared microarray-based measures of hepatic transcript abundance levels within and between five different strains of Mus musculus. Within each strain 23% to 44% of all genes exhibited statistically significant differences in expression between genetically identical individuals (positive false discovery rate of 10%). Genes functionally associated with cell growth, cytokine activity, amine metabolism, and ubiquitination were enriched in this group. Genetic divergence between individuals of different strains also contributed to transcript abundance level differences, but to a lesser extent than intra-strain variation, with approximately 3% of all genes exhibiting inter-strain expression differences.

CONCLUSION

These results indicate that although DNA sequence fixes boundaries for gene expression variability, there remain considerable latitudes of expression within these genome-defined limits that have the potential to influence phenotypes. The extent of normal or expected natural variability in gene expression may provide an additional level of phenotypic opportunity for natural selection.

High-throughput non-heme iron assay for animal tissues

Iron has been widely studied in nearly every realm of biology. However, current methodologies, such as genetic mapping or mutation screening, have been difficult to apply due to the lack of robust high-throughput methods for quantifying iron levels from cells or tissues. The measurement of total iron levels in tissues, usually done with atomic absorption spectroscopy, is impractical for large numbers of samples and includes the contribution of heme iron from hemoglobin contained in red blood cells. The measurement of non-heme iron by reaction with a bathophenanthroline reagent, a commonly used assay reported more than 30 years ago, is also not feasible for large-scale analyses because it is cuvette-based. We therefore have modified this method to a microplate format that will facilitate large-scale analysis. The microplate assay is highly sensitive and specific, and is a simple and effective method for the measurement of non-heme iron for animal tissues that will enable the application of high-throughput of genetic methodologies.

Different expression pattern of hepcidin genes in the liver and pancreas of C57BL/6N and DBA/2N mice

BACKGROUND/AIMS

Male C57BL/6 and DBA/2 mice differ in their liver iron content. The aim of this study was to examine possible differences in the expression of hepcidin genes (Hamp and Hamp2) between the two strains.

METHODS

Hepatic mRNAs were quantified by real-time PCR.

RESULTS

Ferroportin1, transferrin receptor 2 and HAMP mRNA levels displayed no significant strain differences. However, HAMP2 mRNA levels were higher in DBA/2N mice. In both strains, HAMP2 mRNA content was sex-dependent, with higher values in female animals. Both hepatic HAMP and HAMP2 mRNA levels were elevated by iron overload, but treatment with lipopolysaccharide increased only HAMP mRNA. Lipopolysaccharide also elevated the amount of HAMP mRNA in the pancreas, while pancreatic HAMP2 mRNA levels were decreased. Sequence analysis of hepcidin amplicons from DBA/2N mice predicted an Asn-->Lys substitution at position 73 of the HAMP peptide and a Ser-->Phe substitution at position 76 of the HAMP2 peptide.

CONCLUSIONS

Hepatic Hamp2 expression displays considerable strain- and sex-dependent variation. Lipopolysaccharide increases expression of Hamp both in the liver and pancreas, but Hamp2 does not respond to lipopolysaccharide treatment. The significance of the amino acid substitutions in hepcidin peptides in DBA/2N mice is at present unknown.

Age-dependent loss of PTP and LTP in the hippocampus of PrP-null mice

We have investigated synaptic function in the hippocampus in mice of different ages carrying a null mutation in the PrP gene. Experiments carried out in vivo and in vitro in two laboratories revealed no differences in the ability of juvenile and young adult control and PrP-null mice to express long-term potentiation, paired-pulse facilitation, or posttetanic potentiation in either the dentate gyrus or in the CA1 region. However, we found a significant reduction in the level of posttetanic potentiation and long-term potentiation in the CA1 region of aged PrP-null mice. These results are discussed in relationship to reported increased levels of oxidative stress in older PrP-null mice.

Iron status in mice carrying a targeted disruption of lactoferrin

Lactoferrin is a member of the transferrin family of iron-binding glycoproteins present in milk, mucosal secretions, and the secondary granules of neutrophils. While several physiological functions have been proposed for lactoferrin, including the regulation of intestinal iron uptake, the exact function of this protein in vivo remains to be established. To directly assess the physiological functions of lactoferrin, we have generated lactoferrin knockout (LFKO(-/-)) mice by homologous gene targeting. LFKO(-/-) mice are viable and fertile, develop normally, and display no overt abnormalities. A comparison of the iron status of suckling offspring from LFKO(-/-) intercrosses and from wild-type (WT) intercrosses showed that lactoferrin is not essential for iron delivery during the postnatal period. Further, analysis of adult mice on a basal or a high-iron diet revealed no differences in transferrin saturation or tissue iron stores between WT and LFKO(-/-) mice on either diet, although the serum iron levels were slightly elevated in LFKO-/- mice on the basal diet. Consistent with the relatively normal iron status, in situ hybridization analysis demonstrated that lactoferrin is not expressed in the postnatal or adult intestine. Collectively, these results support the conclusion that lactoferrin does not play a major role in the regulation of iron homeostasis.

A genetic view of iron homeostasis

Inherited disorders of iron metabolism are invariably disorders of iron balance or distribution. This review describes the proteins known to be involved in establishing and maintaining iron balance, and discusses regulation of iron homeostasis in the context of three cell types: intestinal enterocytes, reticuloendothelial macrophages, and hepatocytes. It emphasizes information gleaned from the use of genetic analyses, particularly in mice, and poses new questions to help advance our understanding of iron balance.

Expression of the two mRNA isoforms of the iron transporter Nramp2/DMTI in mice and function of the iron responsive element

Nramp2/DMT1 is a transmembrane proton-coupled Fe(2+) transporter. Two different mRNAs are generated by alternative splicing; isoform I contains an iron responsive element (IRE), whereas isoform II does not. They encode two proteins differing at their C-terminal end and by their subcellular localization. IRE-mediated stabilization of isoform I mRNA is thought to stimulate DMT1 expression in response to iron deficiency. We have measured the two mRNAs by real-time quantitative PCR in several mouse tissues, in normal conditions or following injection of phenylhydrazine, a potent haemolytic agent. Isoform I mRNA is expressed in the duodenum and is induced by stimulation of erythropoiesis, whereas the non-IRE isoform is mostly induced in erythropoietic spleen. Surprisingly, both isoforms are highly expressed in the kidney and are not regulated by erythropoiesis. To evaluate the role of the IRE in regulating isoform I mRNA stability, in response to variations in cell iron status, several constructs were made in pCDNA3 with either a normal or a mutated IRE placed at the 3' end of a stable mRNA. These constructs were transfected into HT29 cells and mRNAs were analysed after growing cells in the presence or absence of exogenous iron. There was no difference in the level of expression of the different messages, suggesting that the IRE does not regulate stability of isoform I mRNA. The half-life of the endogenous IRE-mRNA was also measured following actinomycin D addition in iron- or desferrioxamine-treated cells. Decay of the mRNA was very similar in both conditions. These results suggest that additional transcriptional regulations at the promoter level, or iron-dependent regulation of alternative splicing are likely to participate in the induction of isoform I mRNA by iron deficiency.

Uroporphyria in mice: thresholds for hepatic CYP1A2 and iron

In mice treated with 5-aminolevulinic acid (ALA) and polyhalogenated aromatic compounds, the levels of both hepatic cytochrome P450 (CYP)1A2 and iron-which can be quite different among inbred strains-are critical in causing experimental uroporphyria. Here we investigate the development of uroporphyria as a function of CYP1A2 and iron levels in the liver of mice having a common C57BL/6 genetic background. We compared Cyp1a2(-/-) knockout mice, Cyp1a2(+/-) heterozygotes, Cyp1a2(+/+) wild type, and Cyp1a2(+/+) mice pretreated with a low dose of 3,3',4,4',5-pentachlorobiphenyl (PCB126) (4 microg/kg). Cyp1a2(+/-) mice contain about 60% of the hepatic CYP1A2 content of Cyp1a2(+/+) mice, and the PCB126-pretreated Cyp1a2(+/+) mice have about twice the wild-type levels of CYP1A2. ALA- and iron-treated Cyp1a2(+/+) mice are known to accumulate hepatic uroporphyrin; this accumulation was increased 7-fold by pretreatment with the low dose of PCB126. ALA- and iron-treated Cyp1a2(+/-) heterozygote mice accumulated no uroporphyrin in 4 weeks, but by 8 weeks accumulated significant amounts of uroporphyrin. As previously reported, the ALA- and iron-treated Cyp1a2(-/-) knockout mouse has no CYP1A2 and exhibits no detectable uroporphyrin accumulation. Iron dose-response curves in ALA- and PCB126-treated Cyp1a2(+/+) mice showed that hepatic iron levels greater than 850 microg/g liver were required to produce significant uroporphyrin accumulation in the liver. Other measures of hepatic effects of iron (iron-response element-binding protein [IRP]-iron response element [IRE] binding activity and accumulation of protoporphyrin from ALA) decreased when the level of iron was considerably lower than 850 microg/g liver. At low iron doses, accumulation of iron was principally in Kupffer cells, whereas at the higher doses (required to stimulate uroporphyrin accumulation), more iron was found in parenchymal cells. We conclude that small changes in hepatic CYP1A2 levels can dramatically affect uroporphyria in C57BL/6 mice, providing the animals have been sufficiently loaded with iron; these data might be clinically relevant to acquired (sporadic) porphyria cutanea tarda, because humans show greater than 60-fold genetic differences in hepatic basal CYP1A2.

Genetic differences in hepatic lipid peroxidation potential and iron levels in mice

Oxidative damage to macromolecules, including lipids, has been hypothesized as a mechanism of aging. One end product of lipid peroxidation, malondialdehyde (MDA), is often quantified as a measure of oxidative damage to lipids. We used a commercial colorimetric assay for MDA (Bioxytech LPO-586, Oxis International, Portland, OR) to measure lipid peroxidation potential in liver tissue from young (2 month) male mice from recombinant inbred (RI) mouse strains from the C57BL/6J (B6)xDBA/2J (D2) series (BXD). The LPO-586 assay (LPO) reliably detected significant differences (P<0.0001) in lipid peroxidation potential between the B6 and D2 parental strains, and yielded a more than two-fold variation across the BXD RI strains. In both B6 and D2 mice, LPO results were greater in old (23 month) mice, with a larger age-related increase in the D2 strain. As the level of iron can influence lipid peroxidation, we also measured hepatic non-heme iron levels in the same strains. Although iron level exhibited a slightly negative overall correlation (r(2)=0.119) with LPO results among the entire group of BXD RI strains, a sub-group with lower LPO values were highly correlated (r(2)=0.704). LPO results were also positively correlated with iron levels from a group of 8 other inbred mouse strains (r(2)=0.563). The BXD RI LPO data were statistically analyzed to nominate quantitaive trait loci (QTL). A single marker, Zfp4, which maps to 55.2 cM on chromosome 8, achieved a significance level of P<0.0006. At least two potentially relevant candidate genes reside close to this chromosomal position. Hepatic lipid peroxidation potential appears to be a strain related trait in mice that is amenable to QTL analysis.

Mouse strain differences determine severity of iron accumulation in Hfe knockout model of hereditary hemochromatosis

Hereditary hemochromatosis (HH) is a common disorder of iron metabolism caused by mutation in HFE, a gene encoding an MHC class I-like protein. Clinical studies demonstrate that the severity of iron loading is highly variable among individuals with identical HFE genotypes. To determine whether genetic factors other than Hfe genotype influence the severity of iron loading in the murine model of HH, we bred the disrupted murine Hfe allele onto three different genetically defined mouse strains (AKR, C57BL/6, and C3H), which differ in basal iron status and sensitivity to dietary iron loading. Serum transferrin saturations (percent saturation of serum transferrin with iron), hepatic and splenic iron concentrations, and hepatocellular iron distribution patterns were compared for wild-type (Hfe +/+), heterozygote (Hfe +/-), and knockout (Hfe -/-) mice from each strain. Although the Hfe -/- mice from all three strains demonstrated increased transferrin saturations and liver iron concentrations compared with Hfe +/+ mice, strain differences in severity of iron accumulation were striking. Targeted disruption of the Hfe gene led to hepatic iron levels in Hfe -/- AKR mice that were 2.5 or 3.6 times higher than those of Hfe -/- C3H or Hfe -/- C57BL/6 mice, respectively. The Hfe -/- mice also demonstrated strain-dependent differences in transferrin saturation, with the highest values in AKR mice and the lowest values in C3H mice. These observations demonstrate that heritable factors markedly influence iron homeostasis in response to Hfe disruption. Analysis of mice from crosses between C57BL/6 and AKR mice should allow the mapping and subsequent identification of genes modifying the severity of iron loading in this murine model of HH.