Immunohistochemical analysis of transferrin receptor: regional and cellular distribution in the hypotransferrinemic (hpx) mouse brain.

The hypotransferrinemic (hpx) mouse mutant produces <1% of the normal circulating level of transferrin (Tf). Heterozygote animals of this strain (hpx/+) have approximately 50% of normal plasma Tf levels. In this study we examine the cellular and regional distribution of Tf receptor (Tf-R) in the brain of wild type, hpx/+ and mutant (hpx/hpx) mice. Also, using slot-blot (immunoblot) analysis, we describe the relative amount of Tf-R in brain microvessels of hpx/+ animals compared with wild type. Tf-R was seen primarily in neurons throughout the brains of wild type, hpx/+ and hpx/hpx animals. Gray matter areas immunoreacted more robustly than white matter areas. Oligodendrocytes and third ventricle tanycytes, both of which we have previously described as iron-positive, did not immunoreact for Tf-R. Tf-R immunohistochemical reaction in wild type, hpx/+ and hpx/hpx brains appeared similar. Immunoblot analysis of isolated cortical microvessels from wild type and hpx/+ animals revealed no upregulation of Tf-R expression in hpx/+ (relative to normal) despite a 50% decrease in circulating Tf levels. These results indicate that Tf-R is primarily expressed by neurons and that half normal levels of Tf (hpx/+) or transferrin supplementation (hpx/hpx) are apparently sufficient for normal expression and distribution of Tf-R. Because of the lack of circulating Tf, but unaltered Tf-R expression, hpx mice could serve as a model for delivery of therapeutic agents via the Tf/Tf-R system.

Cellular distribution of iron, transferrin, and ferritin in the hypotransferrinemic (Hp) mouse brain.

Hypotransferrinemic (Hp) mice have a point mutation or small deletion in the transferrin (Tf) gene, resulting in defective splicing of precursor Tf mRNA. Hp animals produce < 1% of normal Tf levels and require supplemental serum or purified Tf for survival. Because of the lack of endogenous brain Tf, we examined regional and cellular distributions of iron and iron regulatory proteins (Tf and ferritin) in selected brain regions of Hp mice. The regional distribution of iron, Tf, and ferritin in Hp brain was similar to normal except for the pattern of iron staining in hippocampus. The cellular distribution of iron, ferritin, and Tf was similar between Hp and normal animals. The predominant cell type staining for Tf and iron was oligodendrocytes. Qualitative observations suggest that the number of cells staining for iron was similar between Hp and normal mice, whereas the number of Hp Tf-positive cells was reduced. Ferritin immunostaining was similar in both cases. However, ferritin-positive cells were predominantly astrocytes, an observation unique to mice among species studied previously. Western blot analysis revealed that Tf present in Hp brain was of exogenous origin (from supplemental injections). Presumably, Tf transports the iron found in Hp oligodendrocytes. These data demonstrate that, despite reduced endogenous Hp brain Tf, iron and plasma Tf migrate or are transported to the appropriate cells (oligodendrocytes), bringing into question the role of endogenous brain Tf in extracellular iron transport.