Cybrd1 (duodenal cytochrome b) is not necessary for dietary iron absorption in mice

Abstract

Mammalian nonheme iron absorption requires reduction of dietary iron for uptake by the divalent metal ion transport system in the intestine. This was thought to be mediated by duodenal cytochrome b (Cybrd1), a ferric reductase enzyme resident on the luminal surface of intestinal absorptive cells. To test its importance in vivo, we inactivated the murine Cybrd1 gene and assessed tissue iron stores in Cybrd1-null mice. We found that loss of Cybrd1 had little or no impact on body iron stores, even in the setting of iron deficiency. We conclude that other mechanisms must be available for the reduction of dietary iron.

Figure 1.

Targeted disruption of Cybrd1. (A) Cybrd1 wild-type locus and (B) targeted locus after introduction of an intronic floxed neomycin (Neo)/cytosine deaminase (CD) cassette and loxP sites (yellow triangles). Positions of 3′ and internal probes used for Southern blot analysis are shown as red bars A and B, respectively. (C) Cybrd1 locus after complete excision to inactivate the gene. Squiggled lines to left of diagrams in A-C indicate segments of DNA not included in the figure. Pink bars indicate the size of fragments digested by SacI using a 3′ probe by Southern analysis, and blue bars indicate the length of fragments digested by SphI using a 5′ probe by the same analysis. (D) Southern blot analysis of the 3′ end of the SphI digested locus in ES cells after Cre-mediated excision, using probe A. Clone 6 retains the floxed allele; clones 2, 4, and 5 have undergone complete excision, deleting exon 2. Clones 2 and 4 were used to obtain Cybrd1^-/- mice without residual Neo and CD cassettes. (E-F) Southern blot analysis of the 3′ end of the SacI digested locus in genomic DNA using probe A (E), and the internal probe B within exon 2 (F). Lanes 1, 3, 5, and 6 show wild-type mice, 2 and 8 show homozygous Cybrd1^-/- mice, and 4 and 7 show heterozygous Cybrd1^-/+ mice.

Figure 2.

Liver iron loading and duodenal protein expression in mice with Cybrd1 mutations. (A) There was slightly less nonheme liver iron content, on average, in Cybrd1^-/- animals compared with wild-type animals (12-week-old males: wild type [n = 3], Cybrd1^-/- [n = 3]; 4-week-old females: wild type [n = 9], Cybrd1^-/- [n = 7]; 12-week-old females: wild type [n = 10], Cybrd1^-/- [n = 9]). The differences did not reach statistical significance, but the trend was greatest in female mice maintained on an Fe(-) (iron deficient) diet. Values are shown as a percentage of the values seen in wild-type mice. (B) Liver nonheme iron content (calculated for whole liver and adjusted for body weight) in 12-week-old female mice fed an iron-deficient diet for 8 weeks (n = 9-10). Results in μg/g wet weight are expressed as mean ± SEM. Statistical analysis was performed using the unpaired Student t test, comparing mutant versus wild-type mice, P = .019. (C-D) Immunoblot analysis of duodenal lysates from Trf^hpx/hpx (lanes 1 and 7), iron-deficient wild-type (lanes 2-3), and iron-deficient Cybrd1^-/- (lanes 4-5) mice using anti-Cybrd1 antiserum (C) or anti-Slc11a2 antiserum (D). The single band shown in panel D ran between the 60-kDa and 85-kDa markers. No sample was loaded in lane 6. Protein amounts loaded per lane were 10 μg wild-type and Cybrd1^-/- lysate, and 2 μg (lane 1) and 4 μg (lane 7) Trf^hpx/hpx duodenal lysate. (E) Immunoblot analysis of duodenal lysates from Trf^hpx/hpx mice (lanes 1-2), a Cybrd1^-/- mouse maintained on an iron-deficient diet for 27 weeks (lane 3), a wild-type mouse maintained on an iron-deficient diet for 27 weeks (lane 4), and a wild-type mouse on a regular iron diet (lane 5) using anti-Slc11a2 antiserum, anti-Cybrd1 antibody, or anti-Slc40a1 antibody.

Figure 3.

Northern blot analysis in mice with Cybrd1 mutations. (A) Duodenal Slc40a1 mRNA expression and (B) liver Hamp mRNA expression. Samples were compared from Trf^hpx/hpx mice (lanes 1-3), iron-deficient Cybrd1^-/- mice (lanes 4-8), iron-deficient wild-type mice (lanes 9-13), and wild-type mice on normal diet (lanes 14-17). The identification numbers represent the same animals in panels A and B.