Aberrant Ferroportin 1 Regulation and Iron Homeostasis Interferes with Development of the Spleen Stroma during Murine Embryogenesis.

We have previously identified a regulatory mutation in the ferroportin 1 (Fpn1) locus in polycythaemia (Pcm) mutant mice, leading to transient iron overload and polycythemia during postnatal development. A 58-base pair deletion within the Fpn1 promoter region caused aberrant transcription initiation and resulted in the absence of the iron-responsive element in the 5′ untranslated region of the vast majority of Fpn1 transcripts, thereby disrupting translational regulation of Fpn1 expression. Interestingly, at birth Pcm mutant pups were remarkable for severe iron deficiency and a hypochromic, microcytic anemia. Here, we demonstrate that decreased placental syncytiotrophoblast expression of Fpn1 protein governs decreased embryonic iron levels during late gestation, implicating Fpn1 in maternal-to-fetal iron transport in the placenta. The decreased placental expression of Fpn1 protein appeared to be regulated at the transcriptional level. In striking contrast, increased Fpn1 protein was observed in the embryonic spleen during late gestation, mediated by a post-transcriptional mechanism. This correlated with a significant increase in apoptotic cell death of spleen stromal cells, culminating in a progressive regression of the spleen during late embryogenesis, more severe in Pcm homozygotes. Importantly, Pcm describes a phase in spleen development subsequent to that affected in mutant alleles of the transcription factors Wt1, Hox11, Bapx1, and capsulin. During postnatal development, whereas Pcm heterozygotes retain a significant amount of spleen tissue, Pcm homozygotes display only a spleen rudiment, and are likely to be functionally asplenic throughout postnatal life. Phenylhydrazine treatment of Pcm heterozygotes during early postnatal development demonstrated a reduced functional capacity for erythroid hyperplasia in the spleen. Despite the significantly reduced organ size, splenic white pulp follicles appear to form normally in both Pcm heterozygotes and homozygotes. However, severe abnormalities of the red pulp sinusoidal endothelial are observed in mutant spleens using markers specific for endothelial cell subpopulations. Therefore, the Pcm mutation represents a novel murine model for disrupted splenogenesis that is due to aberrant tissue-specific regulation of the iron transporter Fpn1 during embryonic development.