Generation and characterisation of Rhd and Rhag null mice

Chloride Gradient Is Involved in Ammonium Influx in Human Erythrocytes

The ammonia/ammonium (NH3/NH4+, AM) concentration in human erythrocytes (RBCs) is significantly higher than in plasma. Two main possible mechanisms for AM transport, including simple and facilitated diffusion, are described; however, the driving force for AM transport is not yet fully characterized. Since the erythroid ammonium channel RhAG forms a structural unit with anion exchanger 1 (eAE1) within the ankyrin core complex, we hypothesized the involvement of eAE1 in AM transport. To evaluate the functional interaction between eAE1 and RhAG, we used a unique feature of RBCs to swell and lyse in isotonic NH4+ buffer. The kinetics of cell swelling and lysis were analyzed by flow cytometry and an original laser diffraction method, adapted for accurate volume sensing. The eAE1 role was revealed according to (i) the changes in cell swelling and lysis kinetics, and (ii) changes in intracellular pH, triggered by eAE1 inhibition or the modulation of eAE1 main ligand concentrations (Cl- and HCO3-). Additionally, the AM import kinetics was analyzed enzymatically and colorimetrically. In NH4+ buffer, RBCs concentration-dependently swelled and lysed when [NH4+] exceeded 100 mM. Cell swelling and hemolysis were tightly regulated by chloride concentration. The complete substitution of chloride with glutamate prevented NH4+-induced cell swelling and hemolysis, and the restoration of [Cl-] dose-dependently amplified the rates of RBC swelling and lysis and the percentage of hemolyzed cells. Similarly, eAE1 inhibition impeded cell swelling and completely prevented hemolysis. Accordingly, eAE1 inhibition, or a lack of chloride anions in the buffer, significantly decreased NH4+ import. Our data indicate that the eAE1-mediated chloride gradient is required for AM transport. Taken together, our data reveal a new player in AM transport in RBCs.

Reappraising the role of α5 integrin and the microenvironmental support in stress erythropoiesis

We previously studied the role of β1 integrin and some of its different α partners relevant to erythropoiesis. Although clear and consistent answers regarding the role of α4β1 (VLA-4) were evident, the role of its companion integrin α5β1 (VLA-5) was clouded by inconsistent outcomes in all prior publications. Furthermore, the functional consequences of integrin deficiencies only in microenvironmental (ME) cells supporting erythroid cell expansion and maturation post stress have never been explored. In the study described here, we created several additional mouse models in the aim of addressing unanswered questions regarding functional consequences of single or combined integrin deficiencies in erythroid cells or only in ME supporting cells. Our novel and expansive data solidified the intrinsic requirement of both α4 and α5 integrins in erythroid cells for their proliferative expansion and maturation in response to stress; α5 integrin alone, deleted either early in all hematopoietic cells or only in erythroid cell, has only a redundant role in proliferative expansion and is dispensable for erythroid maturation. By contrast, α4 integrin, on its own, exerts a dominant effect on timely and optimal erythroid maturation. Deficiency of both α4 and α5 integrins in ME cells, including macrophages, does not negatively influence stress response by normal erythroid cells, in great contrast to the effect of ME cells deficient in all β1 integrins. Collectively the present data offer deeper insight into the coordination of different β1 integrin functional activities in erythroid cells or in ME cells for optimal erythroid stress response.

Maximal Oxygen Consumption Is Reduced in Aquaporin-1 Knockout Mice

We have measured maximal oxygen consumption (V˙_O2,max) of mice lacking one or two of the established mouse red-cell CO₂ channels AQP1, AQP9, and Rhag. We intended to study whether these proteins, by acting as channels for O₂, determine O₂ exchange in the lung and in the periphery. We found that V˙_O2,max as determined by the Helox technique is reduced by ~16%, when AQP1 is knocked out, but not when AQP9 or Rhag are lacking. This figure holds for animals respiring normoxic as well as hypoxic gas mixtures. To see whether the reduction of V˙_O2,max is due to impaired O₂ uptake in the lung, we measured carotid arterial O₂ saturation (S_O2) by pulse oximetry. Neither under normoxic (inspiratory O₂ 21%) nor under hypoxic conditions (11% O₂) is there a difference in S_O2 between AQP1_null and WT mice, suggesting that AQP1 is not critical for O₂ uptake in the lung. The fact that the % reduction of V˙_O2,max is identical in normoxia and hypoxia indicates moreover that the limitation of V˙_O2,max is not due to an O₂ diffusion problem, neither in the lung nor in the periphery. Instead, it appears likely that AQP1_null animals exhibit a reduced V˙_O2,max due to the reduced wall thickness and muscle mass of the left ventricles of their hearts, as reported previously. We conclude that very likely the properties of the hearts of AQP1 knockout mice cause a reduced maximal cardiac output and thus cause a reduced V˙_O2,max, which constitutes a new phenotype of these mice.

Beyond water homeostasis: Diverse functional roles of mammalian aquaporins

BACKGROUND

Aquaporin (AQP) water channels are best known as passive transporters of water that are vital for water homeostasis.

SCOPE OF REVIEW

AQP knockout studies in whole animals and cultured cells, along with naturally occurring human mutations suggest that the transport of neutral solutes through AQPs has important physiological roles. Emerging biophysical evidence suggests that AQPs may also facilitate gas (CO2) and cation transport. AQPs may be involved in cell signalling for volume regulation and controlling the subcellular localization of other proteins by forming macromolecular complexes. This review examines the evidence for these diverse functions of AQPs as well their physiological relevance.

MAJOR CONCLUSIONS

As well as being crucial for water homeostasis, AQPs are involved in physiologically important transport of molecules other than water, regulation of surface expression of other membrane proteins, cell adhesion, and signalling in cell volume regulation.

GENERAL SIGNIFICANCE

Elucidating the full range of functional roles of AQPs beyond the passive conduction of water will improve our understanding of mammalian physiology in health and disease. The functional variety of AQPs makes them an exciting drug target and could provide routes to a range of novel therapies.

Genetic variation of the whole ICAM4 gene in Caucasians and African Americans

BACKGROUND

Landsteiner-Wiener (LW) is the human blood group system Number 16, which comprises two antithetical antigens, LW(a) and LW(b) and the high-prevalence antigen LW(ab) . LW is encoded by the intracellular adhesion molecule 4 (ICAM4) gene. The ICAM4 protein is part of the Rhesus complex in the red cell membrane and is involved in cell-cell adhesion.

STUDY DESIGN AND METHODS

We developed a method to sequence the whole 1.9-kb ICAM4 gene from genomic DNA in one amplicon. We determined the nucleotide sequence of Exons 1 to 3, the two introns, and 402-bp 5'-untranslated region (UTR) and 347-bp 3'-UTR in 97 Caucasian and 91 African American individuals.

RESULTS

Seven variant ICAM4 alleles were found, distinct from the wild-type ICAM4 allele (GenBank KF712272), known as LW*05 and encoding LW(a) . An effect of the LW(a) /LW(b) amino acid substitution on the protein structure was predicted by two of the three computational modeling programs used.

CONCLUSIONS

We describe a practical approach for sequencing and determining the ICAM4 alleles using genomic DNA. LW*05 is the ancestral allele, which had also been observed in a Neanderthal sample. All seven variant alleles are immediate derivatives of the prevalent LW*05 and caused by one single-nucleotide polymorphism (SNP) in each allele. Our data were consistent with the NHLBI GO Exome Sequencing Project (ESP) and the dbSNP databases, as all SNPs had been observed previously. Our study has the advantage over the other databases in that it adds haplotype (allele) information for the ICAM4 gene, clinically relevant in the field of transfusion medicine.

Mice expressing RHAG and RHD human blood group genes

Anti-RhD prophylaxis of haemolytic disease of the fetus and newborn (HDFN) is highly effective, but as the suppressive mechanism remains uncertain, a mouse model would be of interest. Here we have generated transgenic mice expressing human RhAG and RhD erythrocyte membrane proteins in the presence and, for human RhAG, in the absence, of mouse Rhag. Human RhAG associates with mouse Rh but not mouse Rhag on red blood cells. In Rhag knockout mice transgenic for human RHAG, the mouse Rh protein is “rescued” (re-expressed), and co-immunoprecipitates with human RhAG, indicating the presence of hetero-complexes which associate mouse and human proteins. RhD antigen was expressed from a human RHD gene on a BAC or from RHD cDNA under control of β-globin regulatory elements. RhD was never observed alone, strongly indicative that its expression absolutely depends on the presence of transgenic human RhAG. This first expression of RhD in mice is an important step in the creation of a mouse model of RhD allo-immunisation and HDFN, in conjunction with the Rh-Rhag knockout mice we have developed previously.

Combinatorial and distinct roles of α₅ and α₄ integrins in stress erythropoiesis in mice

To delineate the role of specific members of β₁ integrins in stress erythropoiesis in the adult, we compared the response to phenylhydrazine stress in 3 genetically deficient models. The survival of β₁-conditionally deficient mice after phenylhydrazine is severely compromised because of their inability to mount a successful life saving splenic erythroid response, a phenotype reproduced in β₁(Δ/Δ) reconstituted animals. The response of bone marrow to phenylhydrazine-induced stress was, unlike that of spleen, appropriate in terms of progenitor cell expansion and mobilization to peripheral blood although late differentiation defects qualitatively similar to those in spleen were present in bone marrow. In contrast to β₁-deficient mice, α₄(Δ/Δ) mice showed only a kinetic delay in recovery and similar to β₁(Δ/Δ), terminal maturation defects in both bone marrow and spleen, which were not present in VCAM-1(Δ/Δ) mice. Convergence of information from these comparative studies lends new insight to the distinct in vivo roles of α₄ and α₅ integrins in erythroid stress, suggesting that the presence of mainly α₅β₁ integrin in all hematopoietic progenitor cells interacting with splenic microenvironmental ligands/cells is instrumental for their survival and accumulation during hemolytic stress, whereas presence of α₄ or of both α₅ and α₄, is important for completion of terminal maturation steps.

Claudin 13, a member of the claudin family regulated in mouse stress induced erythropoiesis

Mammals are able to rapidly produce red blood cells in response to stress. The molecular pathways used in this process are important in understanding responses to anaemia in multiple biological settings. Here we characterise the novel gene Claudin 13 (Cldn13), a member of the Claudin family of tight junction proteins using RNA expression, microarray and phylogenetic analysis. We present evidence that Cldn13 appears to be co-ordinately regulated as part of a stress induced erythropoiesis pathway and is a mouse-specific gene mainly expressed in tissues associated with haematopoietic function. CLDN13 phylogenetically groups with its genomic neighbour CLDN4, a conserved tight junction protein with a putative role in epithelial to mesenchymal transition, suggesting a recent duplication event. Mechanisms of mammalian stress erythropoiesis are of importance in anaemic responses and expression microarray analyses demonstrate that Cldn13 is the most abundant Claudin in spleen from mice infected with Trypanosoma congolense. In mice prone to anaemia (C57BL/6), its expression is reduced compared to strains which display a less severe anaemic response (A/J and BALB/c) and is differentially regulated in spleen during disease progression. Genes clustering with Cldn13 on microarrays are key regulators of erythropoiesis (Tal1, Trim10, E2f2), erythrocyte membrane proteins (Rhd and Gypa), associated with red cell volume (Tmcc2) and indirectly associated with erythropoietic pathways (Cdca8, Cdkn2d, Cenpk). Relationships between genes appearing co-ordinately regulated with Cldn13 post-infection suggest new insights into the molecular regulation and pathways involved in stress induced erythropoiesis and suggest a novel, previously unreported role for claudins in correct cell polarisation and protein partitioning prior to erythroblast enucleation.

Genome-wide identification of TAL1's functional targets: insights into its mechanisms of action in primary erythroid cells

Coordination of cellular processes through the establishment of tissue-specific gene expression programs is essential for lineage maturation. The basic helix-loop-helix hemopoietic transcriptional regulator TAL1 (formerly SCL) is required for terminal differentiation of red blood cells. To gain insight into TAL1 function and mechanisms of action in erythropoiesis, we performed ChIP-sequencing and gene expression analyses from primary fetal liver erythroid cells. We show that TAL1 coordinates expression of genes in most known red cell-specific processes. The majority of TAL1's genomic targets require direct DNA-binding activity. However, one-fifth of TAL1's target sequences, mainly among those showing high affinity for TAL1, can recruit the factor independently of its DNA binding activity. An unbiased DNA motif search of sequences bound by TAL1 identified CAGNTG as TAL1-preferred E-box motif in erythroid cells. Novel motifs were also characterized that may help distinguish activated from repressed genes and suggest a new mechanism by which TAL1 may be recruited to DNA. Finally, analysis of recruitment of GATA1, a protein partner of TAL1, to sequences occupied by TAL1 suggests that TAL1's binding is necessary prior or simultaneous to that of GATA1. This work provides the framework to study regulatory networks leading to erythroid terminal maturation and to model mechanisms of action of tissue-specific transcription factors.