Quantification of small amounts of hemoglobin in polyacrylamide gels with benzidine

Notch2 is required for formation of the placental circulatory system, but not for cell-type specification in the developing mouse placenta

We have previously reported that a mutation in the ankyrin repeats of mouse Notch2 results in embryonic lethality by embryonic day 11.5 (E11.5), showing developmental retardation at E10.5. This indicated that Notch2 plays an essential role in postimplantation development in mice. Here, we demonstrate that whole embryo culture can circumvent developmental retardation of Notch2 mutant embryos for up to 1 day, suggesting that the lethality was primarily caused by extraembryonic defects. Histological examinations revealed delayed entry of maternal blood into the mutant placenta and poor blood sinus formation at later stages. Notch2-expressing cells appeared around maternal blood sinuses. Specification of trophoblast subtypes appeared not to be drastically disturbed and expression of presumptive downstream genes of Notch2 signaling was not altered by the Notch2 mutation. Thus, in the developing mouse placenta, Notch2 is unlikely to be involved in cell fate decisions, but rather participates in formation of maternal blood sinuses. In aggregation chimeras with wild-type tetraploid embryos, the mutant embryos developed normally until E12.5, but died before E13.5. The chimeric placentas showed a restored maternal blood sinus formation when compared with the mutant placentas, but not at the level of wild-type diploid placentas. Therefore, it was concluded that the mutant suffers from defects in maternal blood sinus formation. Thus, Notch2 is not cell autonomously required for the early cell fate determination of subtypes of trophoblast cells, but plays an indispensable role in the formation of maternal blood sinuses in the developing mouse placenta.

Binding of acellular, native and cross-linked human hemoglobins to haptoglobin: enhanced distribution and clearance in the rat

It is well established that hemoglobin resulting from red cell lysis binds to haptoglobin in plasma to form a complex. The increased molecular size precludes its filtration by the kidneys, redirecting it toward hepatocellular entry. Chemically cross-linked hemoglobins are designed to be resistant to renal excretion, even in the absence of haptoglobin. The manner in which binding to haptoglobin influences the pharmacokinetics of acellular cross-linked and native hemoglobins was investigated after intravenous injection of radiolabeled native human hemoglobin and trimesyl-(Lys82)beta-(Lys82)beta cross-linked human hemoglobin, at trace doses, into rats. Under these conditions, there is sufficient plasma haptoglobin for binding with hemoglobin. In vitro binding assayed by size-exclusion chromatography for bound and free hemoglobin revealed that, at <8 muM hemoglobin, native human hemoglobin was completely bound to rat haptoglobin, whereas only approximately 30% of trimesyl-(Lys82)beta-(Lys82)beta cross-linked hemoglobin was bound. Plasma disappearance of low doses (0.31 mumol/kg) of native and cross-linked hemoglobins was monoexponential (half-life = 23 and 33 min, respectively). The volume of distribution (40 vs. 19 ml/kg) and plasma clearance (1.22 vs. 0.4 ml.min(-1).kg(-1)) were higher for native than for cross-linked hemoglobin. Native and cross-linked human hemoglobins were found primarily in the liver, and not in the kidney, heart, lung, or spleen, mostly as degradation products. These pharmacokinetic findings suggest that the binding of hemoglobin to haptoglobin enhances its hepatocellular entry, clearance, and distribution.

Hemoglobin switching in Rana/Xenopus erythroid heterokaryons: factors mediating the metamorphic hemoglobin switch are conserved

Hemoglobin switching, which occurs in all classes of vertebrates as well as in certain invertebrates, is due to developmental regulation of different globin genes which are typically arranged in clustered families. By fusing erythroid cells of different developmental programs, trans-acting factors that regulate this switch in gene expression have been detected [Ramseyer et al. (1989): Dev Biol 133:262-271]. Adult erythroid cells of one anuran species, Xenopus laevis, were fused with tadpole erythroid cells of another frog, Rana catesbeiana, creating developmental erythroid heterokaryons that synthesize adult Rana globin mRNA and hemoglobins. The results show that factors from adult Xenopus erythroid cells are capable of inducing adult Rana globin gene expression in the Rana tadpole erythroid cell nucleus. We have used the cross-induction of adult Rana hemoglobin synthesis in these adult Xenopus/Rana tadpole erythroid heterokaryons to address two practical questions, answers to which may be helpful in isolating developmental stage-specific globin gene regulatory proteins: 1) Are erythroblasts which are actively expressing globin mRNAs and hemoglobins richer in specific globin-inducing activities than other stages of erythroid cellular differentiation? 2) Do mature, circulating erythrocytes still have the activities necessary to mediate the cross-induction of Hb synthesis? The results reported here show that the answers to both questions are affirmative and show that quiescent, fully differentiated adult erythroid cells are still capable of expressing the trans-activator(s). These findings show that factors which mediate the metamorphic hemoglobin switch are conserved between these two genera of frogs.

Nitrate reductase of Neurospora crassa: the functional role of individual amino acids in the heme domain as examined by site-directed mutagenesis

The enzyme nitrate reductase, which catalyzes the reduction of nitrate to nitrite, is a multi-redox center homodimeric protein. Each polypeptide subunit is approximately 100 kDa in size and contains three separate domains, one each for a flavin, a heme-iron, and a molybdopterin cofactor. The heme-iron domain of nitrate reductase has homology with the simple redox protein, cytochrome b5, whose crystal structure was used to predict a three-dimensional structure for the heme domain. Two histidine residues have been identified that appear to coordinate the iron of the heme moiety, while other residues may be important in the folding or the function of the heme pocket. Site-directed mutagenesis was employed to obtain mutants that encode nitrate reductase derivatives with eight different single amino acid substitutions within the heme domain, including the two central histidine residues. Replacement of one of these histidines by alanine resulted in a completely nonfunctional enzyme whereas replacement of the other histidine resulted in a stable and functional enzyme with a lower affinity for heme. Certain amino acid substitutions appeared to cause a rapid turnover of the heme domain, whereas other substitutions were tolerated and yielded a stable and fully active enzyme. Three different single amino acid replacements within the heme domain led to a dramatic change in regulation of nitrate reductase synthesis, with significant expression of the enzyme even in the absence of nitrate induction.

Intracellular signals for developmental hemoglobin switching

We have detected trans-acting factors that regulate developmental hemoglobin switching by fusing erythroid cells of different developmental programs. Adult erythroid cells of one anuran species, Xenopus laevis, were fused with tadpole erythroid cells of another frog, Rana catesbeiana. In a second set of experiments, dimethyl sulfoxide-induced murine erythroleukemia cells, which express only adult mouse globins, were fused with Rana tadpole erythroid cells, which express only embryonic and fetal-like globins. Adult Rana globin gene expression was detected in both sets of transient heterokaryons at 6 hr after fusion. Dot blots and Northern blots of total RNA from the heterokaryons contained material that reacted with an adult Rana alpha-globin probe; newly synthesized adult Rana hemoglobin tetramers were detected with native polyacrylamide gel electrophoresis. These results show that developmental stage-specific transacting factors for globin genes can function across vertebrate classes (mammalia to amphibia) and suggest that the mechanisms that regulate developmental hemoglobin switching are highly conserved.

Formation of transient polykaryons by fusion of erythrocytes of different developmental programs

We report here two methods of fusing erythroid cells from bullfrogs (Rana catesbeiana), using polyethylene glycol or calcium phosphate, which yield masses of polykaryons in which the cytoplasms and nuclei of tadpole and adult frog erythroid cells are intermixed. The masses of fused cells carry out protein synthesis in culture, including the assembly of normal hemoglobin (Hb) tetramers. In these polykaryons there is reactivation of the expression of specific Hbs that have previously been "turned off" in vivo as the result of either a developmental Hb switch or normal cellular differentiation and RBC maturation. For example, the products of fusion of tadpole erythroblasts with adult frog mature RBCs synthesize adult Hb, whereas neither cell population alone does so. Recent experiments have taken advantage of a Hb-expression polymorphism that we discovered in this species, such that some tadpoles have greatly reduced expression of one of the larval Hbs (Hb Td-4). Fusion of erythroblasts from such tadpoles with RBC from frogs that had expressed Hb Td-4 when they were tadpoles produces polykaryons that synthesize Hb Td-4, indicating there is a trans factor that stimulates Td-4 expression. Heterospecific erythroid cell polykaryons can be constructed in an analogous manner, facilitating the study of trans-acting factors that regulate specific globin gene expression during development.

Determination of hemoglobin expression patterns in erythroid cells of Rana catesbeiana tadpoles

1. Rana catesbeiana (bullfrog) tadpoles are heterogeneous in the relative amounts of four major tadpole hemoglobins (Hbs), as well as in the relative amounts of two tadpole red blood cell types in the peripheral blood. 2. Previous work has shown that this heterogeneity is present at all stages of larval development and growth. 3. Although some tadpoles lack one of the Hbs in their peripheral blood (i.e. the electrophoretically slowest form, Td-4), the missing Hb can be found in the erythropoietic organ from which it emanates (the kidneys), indicating that the heterogeneity results from quantitative differences in gene expression. 4. We wished to know whether this in vivo regulation is subject to external environmental perturbation and report that tadpoles of known Hb phenotypes regenerate precisely the pre-anemia Hb profile during early as well as late stages of recovery from phenylhydrazine-induced anemia. 5. These and other results indicate that the in vivo mechanism for regulating the pattern of Hb expression has become firmly determined in the erythropoietic system by the earliest larval stage of development.

In vivo regulation of hemoglobin phenotypes of developing Rana catesbeiana

We have examined the effects of phenylhydrazine-induced anemia on the in vivo synthesis of specific hemoglobins at larval, metamorphic, and post-metamorphic stages of the bullfrog Rana catesbeiana, and have found that at all stages the animals qualitatively and quantitatively regenerate their pre-anemia hemoglobin profiles, with one exception: Animals approaching or undergoing the metamorphic hemoglobin switch synthesize only adult hemoglobin during recovery from anemia. We conclude that the ontogenetic progression of hemoglobins in R. catesbeiana is regulated at the level of differentiation of distinct erythroid cell lines, each committed to expressing a particular hemoglobin phenotype; this regulation is unperturbed by anemia.

Major acute-phase alpha(1)protein in the rat: structure, molecular cloning, and regulation of mRNA levels

Rat major acute-phase alpha(1)protein (MAP) was characterized by determining its secondary structure, ligand binding and partial amino acid sequence. A cDNA clone expressing MAP and coding for the entire mature protein was isolated from a cDNA library in E. coli prepared from rat liver mRNA. By hybridization to nick translated cDNA, mRNA for MAP was found only in liver, where it increased 17-fold during acute inflammation. Constant proportions of rates of leucine incorporation into MAP over mRNA levels in liver indicated that the regulation of the synthesis of MAP is due to a change in the rate of synthesis and/or the stability of mRNA for MAP, but not the rate of its translation.

Monooxygenase activity of human hemoglobin: role of quaternary structure in the preponderant activity of the beta subunits within the tetramer

Catalysis of para hydroxylation of aniline was measured for human ferrihemoglobin and various derivatives in a reconstituted system consisting of the appropriate hemoprotein (at 4 microM heme), reduced nicotinamide adenine dinucleotide phosphate (NADPH), cytochrome P-450 reductase, and aniline under atmospheric O2. The isolated subunits of hemoglobin (alpha 3+ and beta 3+4) were prepared by treatment with p-(hydroxymercuri)benzoate. Semihemoglobin (alpha heme2 beta 02) was prepared from ferrihemoglobin and apohemoglobin. Converse valency hybrids alpha 3+2(beta 2+-CO)2 and (alpha 2+-CO)2 beta 3+2 were prepared from appropriately ligated alpha and beta subunits. After chromatography, the hemoglobin derivatives were characterized by visible and 1H NMR spectroscopy and electrophoresis. At the same concentration of aniline, the alpha and beta subunits were much less active than the normal tetramer. alpha-Semihemoglobin and the alpha 3+2(beta 2+-CO)2 hybrid also displayed lower hydroxylase activity. The (alpha 2+-CO)2 beta 3+2 hybrid was about as active as normal alpha 3+2 beta 3+2. This result suggests that the activity of tetrameric hemoglobin primarily involves the beta subunits. Also transfer of the beta subunits from the beta 4 molecular environment to the alpha 2 beta 2 state enhances their monooxygenase activity approximately 15-fold. The hemoglobin derivatives were differently susceptible to substrate inhibition, the beta 4 species being most sensitive. Estimates of Vmax from the linear portions of the corresponding Lineweaver-Burk plots showed agreement within a factor of 2.5 for all of the hemoglobin derivatives, suggesting that the intrinsic O2-activating capacities of the derivatives are similar.(ABSTRACT TRUNCATED AT 250 WORDS)

Embryonic and larval hemoglobins during the early development of the bullfrog, Rana catesbeiana

The main hemoglobin (Hb) found in Shumway (embryonic) stage 25 bullfrogs is that which we have designated Td-4. The other major tadpole Hbs (Td-1, 2, and 3) predominate during Taylor and Kollros (larval) stages I-XVIII. We propose that Td-4 is an embryonic Hb, whereas Td-1, 2, and 3 are larval (fetal-like) Hbs. Embryonic Hb Td-4 continues to be synthesized during the larval stages. During the larval period, the average peripheral blood Hb profile changes very little with morphological stage or general growth. However, there is great heterogeneity in the embryonic:larval Hb ratio among individual tadpoles of a given stage or weight, apparently due to differential Hb and red cell production by the two active erythropoietic sites, mesonephric kidneys (Td-4), and liver (Td-1, 2, 3).

Erythrocyte differentiation during the metamorphic hemoglobin switch of Rana catesbeiana

Anurans (frogs and toads) switch from tadpole to adult hemoglobin synthesis during metamorphosis. A number of workers have attempted to determine whether tadpole and adult Hb types are expressed in the same or different erythroid cells during the switch. If the different Hb types are found in different cells during the transition, the switch in globin gene expression occurs at an early stage of cellular differentiation. Previous studies, in which immunocytochemical techniques were used to approach this question, are in conflict in regard to the metamorphic Hb switch of the North American bullfrog Rana catesbeiana. We have purified newly differentiating erythroid cells from the blood of metamorphosing tadpoles by using Percoll gradients. These new cells have an immature morphology, are very active in the synthesis of adult Hb, and contain no detectable tadpole Hb. The tadpole cells have no detectable adult Hb, are synthetically inactive, increase in density during the switch, and are then cleared from the circulation. Thus, only adult Hb expression is detected in newly differentiating erythroid cells during metamorphosis.

Use of 3,3',5,5'-tetramethylbenzidine (TMB) in the identification of erythroid colonies

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