Tropomodulin3-Null Mice Are Embryonic Lethal with Anemia Due to Defects in Erythroblast Enucleation and Erythroblast-Macrophage Islands in the Fetal Liver

Abstract 84

Tropomodulin1 (Tmod1) binds tropomyosin and caps the pointed ends of the short actin filaments in the spectrin-actin network of red blood cells (RBCs). Tmod1-null mice display a mild sphero-elliptocytic anemia due to mis-regulation of actin filament lengths and a disrupted membrane skeleton. The mild phenotype may be explained by the compensation of Tmod3, which is not found in wild-type RBCs but exists in Tmod1-null RBCs (one-fifth level of Tmod1). Our experiments with human erythroblasts show that the expression of Tmod1 increases during terminal differentiation while the level of Tmod3 is relatively constant, only decreasing at a very late stage. To investigate the role of Tmod3 in RBCs, we created a Tmod3 knockout mouse from ES cells (#RRF004, BayGenomics) with a gene-trap vector insertion into intron 1 of Tmod3. Both RT-PCR and western-blot results show that the expression of Tmod3 is abolished in Tmod3−/− mice. Tmod3+/− mice are viable and fertile, while Tmod3−/− animals are embryonic lethal, with most nulls dying between E16.5-E17.5. Tmod3−/− embryos are pale and anemic with a smaller fetal liver, suggesting that the lethality might be due to defective definitive erythropoiesis. This is supported by analysis of peripheral blood, which shows fewer definitive enucleated erythroblasts in Tmod3-null embryos. Flow-cytometry of fetal liver erythroblasts labeled with Ter119 and CD71 indicates that the late stage R3 population is reduced by about one-third in absence of Tmod3, while R1-R2 populations are somewhat increased. In addition, Annexin V staining shows a two-fold increase in apoptotic cells in the fetal liver, most of which are in the R1 population. Measurement of enucleation frequency in R populations shows a marked reduction of enucleated cells as the erythroblasts mature through the R3-R5 populations. These data indicate that definitive erythropoiesis is defective due to impaired erythroblast terminal differentiation in absence of Tmod3. To determine the underlying mechanisms, we used histology and confocal fluorescence microscopy to investigate the morphology and actin cytoskeleton of erythroblasts in process of enucleation. These experiments show abnormal nuclear morphology in orthochromatic Tmod3-null fetal liver erythroblasts, as well as defective F-actin contractile ring assembly in Tmod3−/− erythroblasts in process of nuclear expulsion, suggesting a role for Tmod3 in enucleation. Since macrophages are required for production of definitive erythroblasts and enucleation in vivo, we explored the role of macrophages in the Tmod3−/− phenotype. Immunofluorescence staining of fetal liver cryosections with F4/80, Ter119 and Hoechst reveals that macrophages display strikingly less dendritic morphologies in the Tmod3−/− mice, with macrophages sometimes containing Ter119-positive nucleated erythroblasts. Isolation of native erythroblast-macrophage islands from fetal liver demonstrates that islands isolated from Tmod3−/− fetal livers contain fewer erythroblasts compared with those from wild-type fetal liver. Further, reconstitution experiments indicate that erythroblasts from Tmod3−/− fetal liver are unable to form normal islands, indicating that Tmod3 function is required in erythroblasts. In conclusion, our study shows that knockout of Tmod3 leads to defective definitive erythropoiesis and embryonic lethality in mice, due to defects in island formation and abnormal enucleation. These data suggest that Tmod3-mediated actin remodeling may be required for erythroblast-macrophage adhesion as well as contractile ring assembly during erythroblast enucleation.