Mutations in the murine fitness 1 gene result in defective hematopoiesis

Identification and characterization of mutations that disrupt normal hematopoiesis are essential for understanding the genetic pathways that control the development and regulation of the mammalian hematopoietic system. Previously, the fitness 1 gene was identified by five, independent mutations in N-ethyl-N-nitrosourea (ENU) saturation mutagenesis experiments within the albino (c) region of mouse chromosome 7 (MMU7). We report here that fit1 mutants are anemic, display numerous peripheral blood defects, and are deficient in early hematopoietic progenitor cell populations. The number of both erythroid and myeloid progenitors, as well as B cells, are reduced. These results implicate fit1 involvement in normal hematopoiesis and suggest that further characterization of the fit1 gene, and the five presumed point mutations of the gene, will lead to an improved understanding of normal hematopoiesis in the mouse.

A transgenic mouse model of hemoglobin S Antilles disease

Hemoglobin (Hb) S Antilles is a naturally occurring form of sickling human Hb but causes a more severe phenotype than Hb S. Two homozygous viable Hb S Antilles transgene insertions from Tg58Ru and Tg98Ru mice were bred into MHOAH mice that express high oxygen affinity (P50 approximately 24.5 mm Hg) rather than normal (P50 approximately 40 mm Hg) mouse Hbs. The rationale was that the high oxygen affinity MHOAH Hb, the lower oxygen affinity of Hb S Antilles than Hb S (P50 approximately 40 v 26.5 mm Hg), and the lower solubility of deoxygenated Hb S Antilles than Hb S (approximately 11 v 18 g/dL) would favor deoxygenation and polymerization of human Hb S Antilles in MHOAH mouse red blood cells (RBCs). The Tg58 x Tg98 mice produced have a high and balanced expression (approximately 50% each) of h alpha and h beta(S Antilles) globins, 25% to 35% of their RBCs are misshapen in vivo, and in vitro deoxygenation of their blood induces 30% to 50% of the RBCs to form classical looking, elongated sickle cells with pointed ends. Tg58 x Tg98 mice exhibit reticulocytosis, an elevated white blood cell count and lung and kidney pathology commonly found in sickle cell patients, which should make these mice useful for experimental studies on possible therapeutic intervention of sickle cell disease.

Functional analysis of the c-myb proto-oncogene

Targeted mutagenesis studies were initiated to determine the normal biological function of the c-myb proto-oncogene. While heterozygous mice are phenotypically indistinguishable from their wild-type littermates, homozygous mutant fetuses die at approximately 15.5 days of gestation apparently due to anemia, which results from an inability to switch from embryonic yolk sac to fetal liver erythropoiesis. Studies are currently being done to determine the extent of hematopoietic abnormalities in the homozygous mutant fetuses. In vitro assays for hematopoietic colony-forming cells have been used to determine the frequency of both erythroid and myeloid progenitors in the fetal livers of wild-type, heterozygous, and homozygous mutant c-myb fetuses. The reduced number of erythroid progenitors was not unexpected considering the mutant fetus's pale color and reduced hematocrit. The dramatically reduced number of colonies derived from myeloid progenitors in the mutant fetuses in comparison to the number detected in phenotypically normal littermates suggests that expression of the c-myb proto-oncogene is critical for the proliferation and/or differentiation of early hematopoietic progenitors and possibly hematopoietic stem cells. Other possible explanations would include a hematopoietic progenitor migration problem from the yolk sac to the fetal liver or a defect in the microenvironment of the liver. Whether the lymphoid lineage is also adversely affected by the lack of c-myb expression remains to be determined. RT-PCR and Northern blot analyses were used in an attempt to identify downstream genes which may be directly or indirectly regulated by the Myb gene product. While the levels of expression of several genes involved in erythropoiesis (GATA-1, NF-E2, SCL, and EpoR) were reduced in the livers of homozygous mutant fetuses in comparison to phenotypically normal littermates and one gene, Kit ligand (KL), was expressed at higher levels in the mutant livers, these results must be viewed with caution. The livers of the mutant fetuses have been shown to be hypocellular in comparison to those of phenotypically normal littermates (35). It is possible that the Myb gene product is directly or indirectly modulating the expression of these genes. Conversely, the alteration in expression may be due to the reduced number or absence of specific hematopoietic lineages in the livers of the mutant fetuses. Differential display has also been used to identify putative novel genes that are involved in hematopoiesis. Preliminary studies suggest that this may be a powerful methodology to compare the expression pattern of genes in the fetal liver of wild-type, heterozygous, and homozygous mutant littermates at 14.5 days of gestation. To date nearly 60% of the partial cDNAs subcloned analyzed have been shown to be differentially expressed. More importantly, 75% of the differentially expressed cDNAs that have been sequenced appear to encode novel genes. Whether any of these novel genes are involved in the c-myb transcriptional cascade remains to be determined. Overall, analysis of the c-myb mutant fetuses have provided valuable insight into the biological function of this interesting proto-oncogene. The continued analysis of this resource will undoubtedly provide additional information concerning the role of the c-myb gene in hematopoiesis.

CD44 expression on murine tissues

CD44 is a cell surface glycoprotein found on lymphoid and epithelial cells. Its primary function on lymphocytes and macrophages is to mediate interaction with endothelium, while its function on epithelial cells is not known. The protein has many different forms, generated by alternative mRNA splicing and by post-translational modification, which may mediate different functions. During previous work on murine lung tumor cells, mAb 133–13A was isolated and shown to recognize a surface glycoprotein, P100, of 90–100 × 10(3) M(r). Amino acid sequence analysis of purified P100 indicates that it is CD44. Since few data exist to indicate which forms of CD44 are present in different normal tissues, mAb 133–13A was used to analyze CD44 expression in mouse tissue. Quantitative data on the distribution of CD44(P100) in mice show that spleen, thymus, liver, intestine, uterus and choroid of the eye are major sites of expression. In addition, epithelia of adrenals, esophagus and trachea are CD44(P100) positive. Previous work on human cell lines has implicated a high molecular mass (130-160 × 10(3) M(r)) form of the glycoprotein as the form expressed in epithelial cells and carcinomas. Isolation of CD44 proteins from lymphoid tissues in the mouse indicate that, as in human lymphoid tissue, the low molecular mass form (80-90 × 10(3) M(r)) is predominately expressed. These data show that both small (approximately 81 × 10(3) M(r)) and large forms of the glycoprotein are expressed in basal epithelia of esophagus and trachea and in salivary gland, while only the small form is expressed in epithelium of the adrenal cortex and in the murine lung and mammary carcinomas studied. While these data cannot distinguish between specific splice variants, they show that the large forms of CD44 are minor components in normal tissue and seem to be found only in basal epithelium. The CD44 of low M(r) found in epithelial tissues is probably associated with lymphoid cell types in the tissues.

Immunologic and hematologic consequences in mice of exposure to ozone

Immunologic and hematologic effects of continuous, chronic exposure of mice to ozone are reported. A consistent decrease was found in ability of spleen cells from exposed mice to engage in primary antibody formation. Results from an investigation of the possible presence of suppressor cells were inconclusive Separation of spleen cells into adherent and nonadherent populations revealed suggestive differences between normal and ozone-exposed mice. Among myelopoietic progenitors examined, CFU-S, CFU-GM, CFU-E, CFU-ME, and BFU-E, only the last two named were found to be altered after ozone exposure. The possible bases and implications of these findings are discussed.

A comparison of stem cell populations and hemoglobin switching in normal versus beta-thalassemic mice

The hematopoietic stem cell concentrations in tissues of homozygous beta-thalassemic and non-thalassemic fetuses and neonates were compared by using the spleen colony-forming units (CFU-S) assay. The relative quantities of embryonic and adult hemoglobins were also determined for fetuses. Beta-thalassemic fetuses had a reduced incidence of CFU-S in the liver throughout gestation, but after birth the beta-thalassemic neonates maintained a greater number of CFU-S in the liver for an extended period. The incidence of CFU-S in the bone marrow was not different for the two groups. The beta-thalassemic mice exhibited a significant expansion of CFU-S in the spleen beyond 11 days after birth. The switch from the synthesis of primarily embryonic to primarily adult hemoglobins in circulating erythrocytes in beta-thalassemic fetuses appeared later than the switch in normal fetuses. These observations establish that the developmental timing and expansion of hematopoiesis are perturbed in beta-thalassemic mice.

Interleukin 3 promotes erythroid burst formation in "serum-free" cultures without detectable erythropoietin

Erythroid burst-forming units (BFU-E) from mouse bone marrow were grown for 7 days in agar or serum-free methylcellulose cultures in the presence or absence of erythropoietin (Ep) and/or interleukin 3 (IL-3). It was found that IL-3, even in the absence of serum and detectable Ep, was able to stimulate the full development of many erythroid bursts. This IL-3 effect was cell-dose dependent and did not appear to correlate with Ep dose. Spontaneous bursts and those stimulated by Ep only were rare and when seen were very small relative to those produced by IL-3 or IL-3 plus Ep. When addition of IL-3 or Ep to 7-day cultures was delayed, IL-3 but not Ep was shown to maintain BFU-E. No evidence was found by radioimmunoassay that Ep was produced or released in 7-day, "serum-free" cultures of bone marrow nor was Ep activity detected in culture media except those to which it had been added deliberately.

In vitro studies of erythropoietic progenitors (CFU-E) in marrow from neonatal and young mice

Reports that cells from neonatal animals produce CFU-E in response to erythropoietin (Ep) at doses lower than are required by adults indicated that young mice would provide a good system to study the hormone response of erythropoietic progenitors. Studies presented here confirm the increased Ep responsiveness until 3-4 weeks of age reported previously by others. Along with the enhancement in their Ep responsiveness, bone marrow cells from young animals produce many more CFU-E per 10(5) cells plated than do adult animals; i.e., neonates have a higher plating efficiency for CFU-E in the presence of erythropoietin. Neonatal animals, in contrast to adults, contain in addition erythropoietic progenitors in their bone marrow capable of growth in vitro without added hormone. Because of the large numbers of "endogenous" colonies, a study was done to explore the possibility that a small amount of Ep was present in the culture medium. Antiserum to erythropoietin was added directly to cultures in another study to remove any undetected hormone. The results indicate that CFU-E from neonatal mouse bone marrow can develop in culture in the absence of detectable Ep.

Thymic involvement in control of bone marrow growth. Use of T-cell-depleted hybrid mice

Because of mounting evidence of involvement of thymus-derived cells in blood formation we have studied the growth of transplanted bone marrow in mice extensively depleted of T-lymphocytes (TCD). Poor growth of parental marrow was found not to be appreciably altered in TCD hybrid recipients. However, parental thymic lymphocytes even in massive doses were not able to augment hemopoiesis in TCD hosts, in contrast to findings from sham-thymectomized or age control mice. This indication that a third (host) cell takes part in the thymocyte-marrow stem cell interaction was reinforced by the finding that isogenic (hybrid) thymocytes administered to TCD mice 5 weeks before the final irradiation restored their ability to support thymocyte-induced augmentation of parental marrow growth. Data were obtained from theta-poor sham-thymectomized irradiated controls which are interpreted as evidence for a suppressor T cell. Thymocytes administered with marrow produced a shift toward granulopoiesis in TCD mice as well as in controls. From this finding we infer that although thymus-derived cells are intimately involved in regulation of myelopoiesis, the effect of administered thymic lymphocytes on the differentiative pathway does not depend on host T cells.

Ability of thymic lymphocytes to alter CFU kinetics in radiation chimeras

It is known that the poor colony-forming ability of B6 bone marrow transplanted into B6D2F1 hybrids can be improved if B6 lymphocytes are given in addition. It was recently reported that the augmenting lymphocytes decrease the doubling time of differentiating hemopoietic cells. To determine whether thymus cells alter the self-renewal of CFUs in this parent leads to F1 combination, retransplantation and 3H-thymidine 'suicide' were employed as methods to determine the cell-division rate. We have observed that in the presence of thymocytes, parental bone marrow cells are seeded more efficiently in the spleen, and the lag phase of the CFUs growth curve is shortened. However, thymic lymphocytes do not increase the slope of the exponential growth phase of CFUs.

Augmentation of marrow growth by thymocytes separated by discontinuous albumin density-gradient centrifugation

Mouse thymocytes were separated by discontinuous albumin density-gradient centrifugation. The ability of B6 thymocytes from the gradient fractions to increase the number of spleen nodules formed by B6 marrow cells in B6D2F1 recipients was compared with that of similar numbers of B6 thymocytes from an unfractionated cell suspension. Thymocytes from any of the gradient fractions were not more effective than thymocytes from an unfractionated suspension in augmenting marrow-cell growth, indicating that the presumptive 'effective cell type' in this system cannot be separated from the total thymocyte population by its buoyant density properties.