Defective Recycling and Exocytosis of Transferrin in Reticulocytes of hbd (Hemoglobin Deficit) Mice.

Mice with hemoglobin deficit (hbd) have a hypochromic, microcytic anemia inherited as an autosomal recessive. The mice also exhibit reticulocytosis, hyperferremia and elevated free protoporphyrin in red cells (Bannerman et al 1986 Proc Soc Exp Biol Med 182:52). The spontaneous mutation has been mapped to chromosome 19 (Bloom et al 1998 Mamm Genome 9:666), making it ripe for identification of the gene, given the power of mouse genetics. Until the actual locus is determined, however, some mice must be identified as +/? as dominant inheritance obscures the second allele. Prior studies established that hbd/hbd reticulocytes take up ¹²⁵I-labelled transferrin as well as +/? controls, but are less effective at incorporating ⁵⁹Fe from transferrin (Garrick et al 1987 Exp Hemato 15:671). Scatchard analysis showed the affinity of transferrin receptors for transferrin was similar in hbd/hbd versus +/? reticulocytes; slightly more receptors were present, however, on the surface of hbd/hbd reticulocytes.

We now report on aspects of transferrin-dependent iron metabolism in hbd/hbd reticulocytes to resolve the nature of the alterations in the transferrin cycle in these rodent mutants. Improved time resolution for ⁵⁹Fe uptake from transferrin by hbd/hbd versus +/? reticulocytes reveals that uptake in the first 2.5 min is indistinguishable; but that, by 5 min, incorporation by the mutant cells lags behind. Similarly, endocytosis of ¹²⁵I-labelled transferrin by mutant versus control reticulocytes occurs at essentially the same rate; while exocytosis is about 4-fold slower for hbd/hbd reticulocytes compared to +/? reticulocytes. We conclude that recycling and exocytosis of transferrin in reticulocytes of hbd/hbd mice is strikingly impaired. The mutation in these mice should be in a gene that causes this impairment.