Sites of regulated phosphorylation that control K-Cl cotransporter activity.

Modulation of intracellular chloride concentration ([Cl(-)](i)) plays a fundamental role in cell volume regulation and neuronal response to GABA. Cl(-) exit via K-Cl cotransporters (KCCs) is a major determinant of [Cl(-)](I); however, mechanisms governing KCC activities are poorly understood. We identified two sites in KCC3 that are rapidly dephosphorylated in hypotonic conditions in cultured cells and human red blood cells in parallel with increased transport activity. Alanine substitutions at these sites result in constitutively active cotransport. These sites are highly phosphorylated in plasma membrane KCC3 in isotonic conditions, suggesting that dephosphorylation increases KCC3's intrinsic transport activity. Reduction of WNK1 expression via RNA interference reduces phosphorylation at these sites. Homologous sites are phosphorylated in all human KCCs. KCC2 is partially phosphorylated in neonatal mouse brain and dephosphorylated in parallel with KCC2 activation. These findings provide insight into regulation of [Cl(-)](i) and have implications for control of cell volume and neuronal function.

Novel mechanisms of PIEZO1 dysfunction in hereditary xerocytosis.

Mutations in PIEZO1 are the primary cause of hereditary xerocytosis, a clinically heterogeneous, dominantly inherited disorder of erythrocyte dehydration. We used next-generation sequencing-based techniques to identify PIEZO1 mutations in individuals from 9 kindreds referred with suspected hereditary xerocytosis (HX) and/or undiagnosed congenital hemolytic anemia. Mutations were primarily found in the highly conserved, COOH-terminal pore-region domain. Several mutations were novel and demonstrated ethnic specificity. We characterized these mutations using genomic-, bioinformatic-, cell biology-, and physiology-based functional assays. For these studies, we created a novel, cell-based in vivo system for study of wild-type and variant PIEZO1 membrane protein expression, trafficking, and electrophysiology in a rigorous manner. Previous reports have indicated HX-associated PIEZO1 variants exhibit a partial gain-of-function phenotype with generation of mechanically activated currents that inactivate more slowly than wild type, indicating that increased cation permeability may lead to dehydration of PIEZO1-mutant HX erythrocytes. In addition to delayed channel inactivation, we found additional alterations in mutant PIEZO1 channel kinetics, differences in response to osmotic stress, and altered membrane protein trafficking, predicting variant alleles that worsen or ameliorate erythrocyte hydration. These results extend the genetic heterogeneity observed in HX and indicate that various pathophysiologic mechanisms contribute to the HX phenotype.

Mutations in the Gardos channel (KCNN4) are associated with hereditary xerocytosis.

Hereditary xerocytosis (HX; MIM 194380) is an autosomal-dominant hemolytic anemia characterized by primary erythrocyte dehydration. In many patients, heterozygous mutations associated with delayed channel inactivation have been identified in PIEZO1. This report describes patients from 2 well-phenotyped HX kindreds, including from one of the first HX kindreds described, who lack predicted heterozygous PIEZO1-linked variants. Whole-exome sequencing identified novel, heterozygous mutations affecting the Gardos channel, encoded by the KCNN4 gene, in both kindreds. Segregation analyses confirmed transmission of the Gardos channel mutations with disease phenotype in affected individuals. The KCNN4 variants were different mutations in the same residue, which is highly conserved across species and within members of the small-intermediate family of calcium-activated potassium channel proteins. Both mutations were predicted to be deleterious by mutation effect algorithms. In sickle erythrocytes, the Gardos channel is activated under deoxy conditions, leading to cellular dehydration due to salt and water loss. The identification of KCNN4 mutations in HX patients supports recent studies that indicate it plays a critical role in normal erythrocyte deformation in the microcirculation and participates in maintenance of erythrocyte volume homeostasis.

Global transcriptome analysis and enhancer landscape of human primary T follicular helper and T effector lymphocytes.

T follicular helper (Tfh) cells are a subset of CD4(+) T helper cells that migrate into germinal centers and promote B-cell maturation into memory B and plasma cells. Tfh cells are necessary for promotion of protective humoral immunity following pathogen challenge, but when aberrantly regulated, drive pathogenic antibody formation in autoimmunity and undergo neoplastic transformation in angioimmunoblastic T-cell lymphoma and other primary cutaneous T-cell lymphomas. Limited information is available on the expression and regulation of genes in human Tfh cells. Using a fluorescence-activated cell sorting-based strategy, we obtained primary Tfh and non-Tfh T effector cells from tonsils and prepared genome-wide maps of active, intermediate, and poised enhancers determined by chromatin immunoprecipitation-sequencing, with parallel transcriptome analyses determined by RNA sequencing. Tfh cell enhancers were enriched near genes highly expressed in lymphoid cells or involved in lymphoid cell function, with many mapping to sites previously associated with autoimmune disease in genome-wide association studies. A group of active enhancers unique to Tfh cells associated with differentially expressed genes was identified. Fragments from these regions directed expression in reporter gene assays. These data provide a significant resource for studies of T lymphocyte development and differentiation and normal and perturbed Tfh cell function.

Mutation in a Highly Conserved COOH-Terminal Residue of Krüppel-Like Factor 1 Associated with Elevated Hb F in a Compound Heterozygous ß-Thalassemia Patient with a Nontransfusion-Dependent Thalassemia Phenotype.

We present a patient with a compound heterozygosity codon 39 (C > T) (ß0) [or ß39(C5)Gln→Stop (G39X); CAG > TAG; HBB: c.118C > T] and -87 (C > T) (ß+) (HBB: c.-137C > T) ß-globin mutations, a non transfusion-dependent thalassemia phenotype and 97.0% fetal hemoglobin. A novel heterozygous mutation was identified in a highly conserved residue in the COOH-terminus of the Krüppel-like factor 1, R360H, that likely altered DNA-binding and impaired transactivation.

Mutations in the mechanotransduction protein PIEZO1 are associated with hereditary xerocytosis.

Hereditary xerocytosis (HX, MIM 194380) is an autosomal dominant hemolytic anemia characterized by primary erythrocyte dehydration. Copy number analyses, linkage studies, and exome sequencing were used to identify novel mutations affecting PIEZO1, encoded by the FAM38A gene, in 2 multigenerational HX kindreds. Segregation analyses confirmed transmission of the PIEZO1 mutations and cosegregation with the disease phenotype in all affected persons in both kindreds. All patients were heterozygous for FAM38A mutations, except for 3 patients predicted to be homozygous by clinical and physiologic studies who were also homozygous at the DNA level. The FAM38A mutations were both in residues highly conserved across species and within members of the Piezo family of proteins. PIEZO proteins are the recently identified pore-forming subunits of channels that mediate mechanotransduction in mammalian cells. FAM38A transcripts were identified in human erythroid cell mRNA, and discovery proteomics identified PIEZO1 peptides in human erythrocyte membranes. These findings, the first report of mutation in a mammalian mechanosensory transduction channel-associated with genetic disease, suggest that PIEZO proteins play an important role in maintaining erythrocyte volume homeostasis.

Erythrocyte peripheral type benzodiazepine receptor/voltage-dependent anion channels are upregulated by Plasmodium falciparum.

Plasmodium falciparum relies on anion channels activated in the erythrocyte membrane to ensure the transport of nutrients and waste products necessary for its replication and survival after invasion. The molecular identity of these anion channels, termed "new permeability pathways" is unknown, but their currents correspond to up-regulation of endogenous channels displaying complex gating and kinetics similar to those of ligand-gated channels. This report demonstrates that a peripheral-type benzodiazepine receptor, including the voltage dependent anion channel, is present in the human erythrocyte membrane. This receptor mediates the maxi-anion currents previously described in the erythrocyte membrane. Ligands that block this peripheral-type benzodiazepine receptor reduce membrane transport and conductance in P falciparum-infected erythrocytes. These ligands also inhibit in vitro intraerythrocytic growth of P falciparum. These data support the hypothesis that dormant peripheral-type benzodiazepine receptors become the "new permeability pathways" in infected erythrocytes after up-regulation by P falciparum. These channels are obvious targets for selective inhibition in anti-malarial therapies, as well as potential routes for drug delivery in pharmacologic applications.

Patterns of histone H3 lysine 27 monomethylation and erythroid cell type-specific gene expression.

Post-translational histone modifications, acting alone or in a context-dependent manner, influence numerous cellular processes via their regulation of gene expression. Monomethylation of histone H3 lysine 27 (K27me1) is a poorly understood histone modification. Some reports describe depletion of K27Me1 at promoters and transcription start sites (TSS), implying its depletion at TSS is necessary for active transcription, while others have associated enrichment of H3K27me1 at TSS with increased levels of mRNA expression. Tissue- and gene-specific patterns of H3K27me1 enrichment and their correlation with gene expression were determined via chromatin immunoprecipitation on chip microarray (ChIP-chip) and human mRNA expression array analyses. Results from erythroid cells were compared with those in neural and muscle cells. H3K27me1 enrichment varied depending on levels of cell-type specific gene expression, with highest enrichment over transcriptionally active genes. Over individual genes, the highest levels of H3K27me1 enrichment were found over the gene bodies of highly expressed genes. In contrast to H3K4me3, which was highly enriched at the TSS of actively transcribing genes, H3K27me1 was selectively depleted at the TSS of actively transcribed genes. There was markedly decreased to no H3K27me1 enrichment in genes with low expression. At some locations, H3K27 monomethylation was also found to be associated with chromatin signatures of gene enhancers.

A fused alpha-beta "mini-spectrin" mimics the intact erythrocyte spectrin head-to-head tetramer.

Head-to-head assembly of two spectrin heterodimers to form an actin-cross-linking tetramer is a physiologically dynamic interaction that contributes to red cell membrane integrity. Recombinant beta-spectrin C-terminal and alpha-spectrin N-terminal peptides can form tetramer-like univalent complexes, but they cannot evaluate effects of the open-closed dimer interactions or lateral associations of the two-spectrin strands on tetramer formation. In this study we produced and characterized a fused "mini-spectrin dimer" containing the beta-spectrin C-terminal region linked to the alpha-spectrin N-terminal region. This fused mini-spectrin mimics structural and functional properties of intact, full-length dimers and tetramers, including lateral association of the alpha and beta subunits in the dimer and formation of a closed dimer. High performance liquid chromatography gel filtration analyses of this mini-spectrin provide the first direct non-imaging experimental evidence for open and closed spectrin dimers and show that dimer-tetramer-oligomer interconversion is slow at low temperatures and accelerated at 30 degrees C, analogous to full-length spectrin. This protein exhibits wild type dimer-tetramer dissociation constants of approximately 1 mum at 30 degrees C, independent of initial oligomeric state. Conformational states of the mini-spectrin dimer were probed further using chemical cross-linking, which identified distinct groups of cross-links for "open" and "closed" dimers and confirmed the N-terminal region of alpha-spectrin remains highly flexible in the complex, exhibiting closely analogous structures to those observed for the isolated alpha-spectrin N-terminal using NMR (Park, S., Caffrey, M. S., Johnson, M. E., and Fung, L. W. (2003) J. Biol. Chem. 278, 21837-21844). This fusion protein should serve as a useful template for structural and functional studies of the divalent tetramer site.

A comprehensive model of the spectrin divalent tetramer binding region deduced using homology modeling and chemical cross-linking of a mini-spectrin.

Spectrin dimer-tetramer interconversion is a critical contributor to red cell membrane stability, but some properties of spectrin tetramer formation cannot be studied effectively using monomeric recombinant domains. To address these limitations, a fused αß mini-spectrin was produced that forms wild-type divalent tetramer complexes. Using this mini-spectrin, a medium-resolution structure of a seven-repeat bivalent tetramer was produced using homology modeling coupled with chemical cross-linking. Inter- and intramolecular cross-links provided critical distance constraints for evaluating and optimizing the best conformational model and appropriate docking interfaces. The two strands twist around each other to form a super-coiled, rope-like structure with the AB helix face of one strand associating with the opposing AC helix face. Interestingly, two tetramer site hereditary anemia mutations that exhibit wild-type binding in univalent head-to-head assays are located in the interstrand region. This suggests that perturbations of the interstrand region can destabilize spectrin tetramers and the membrane skeleton. The α subunit N-terminal cross-links to multiple sites on both strands, demonstrating that this non-homologous tail remains flexible and forms heterogeneous structures in the tetramer complex. Although no cross-links were observed involving the ß subunit non-homologous C-terminal tail, several cross-links were observed only when this domain was present, suggesting it induces subtle conformational changes to the tetramer site region. This medium-resolution model provides a basis for further studies of the bivalent spectrin tetramer site, including analysis of functional consequences of interstrand interactions and mutations located at substantial molecular distances from the tetramer site.

An insulator with barrier-element activity promotes alpha-spectrin gene expression in erythroid cells.

Understanding mechanisms controlling expression of the alpha-spectrin gene is important for understanding erythropoiesis, membrane biogenesis, and spectrin-linked hemolytic anemia. We showed previously that a minimal alpha-spectrin promoter directed low levels of expression only in early erythroid development, indicating elements outside the promoter are required for expression in adult erythrocytes. Addition of noncoding exon 1' and intron 1' conferred a 10-fold increase in activity in reporter gene assays. In this report, we used a transgenic mouse model to show that addition of exon 1' and intron 1' to the alpha-spectrin promoter conferred tissue-specific expression of a linked (A)gamma-globin gene in erythroid cells at all developmental stages. Expression was nearly position-independent, as 21 of 23 lines expressed the transgene, and gamma-globin protein was present in 100% of erythrocytes, indicating uniform expression. Additional in vivo studies revealed that exon 1' functions as an insulator with barrier-element activity. Chromatin immunoprecipitation assays demonstrated that this region was occupied by the upstream stimulatory factors 1/2 (USF1/USF2), similar to the well-characterized chicken HS4 insulator. These data identify the first barrier element described in an erythrocyte membrane protein gene and indicate that exon 1' and intron 1' are excellent candidate regions for mutations in patients with spectrin-linked hemolytic anemia.

The GPA-dependent, spherostomatocytosis mutant AE1 E758K induces GPA-independent, endogenous cation transport in amphibian oocytes.

The previously undescribed heterozygous missense mutation E758K was discovered in the human AE1/SLC4A1/band 3 gene in two unrelated patients with well-compensated hereditary spherostomatocytic anemia (HSt). Oocyte surface expression of AE1 E758K, in contrast to that of wild-type AE1, required coexpressed glycophorin A (GPA). The mutant polypeptide exhibited, in parallel, strong GPA dependence of DIDS-sensitive (36)Cl(-) influx, trans-anion-dependent (36)Cl(-) efflux, and Cl(-)/HCO(3)(-) exchange activities at near wild-type levels. AE1 E758K expression was also associated with GPA-dependent increases of DIDS-sensitive pH-independent SO(4)(2-) uptake and oxalate uptake with altered pH dependence. In marked contrast, the bumetanide- and ouabain-insensitive (86)Rb(+) influx associated with AE1 E758K expression was largely GPA-independent in Xenopus oocytes and completely GPA-independent in Ambystoma oocytes. AE1 E758K-associated currents in Xenopus oocytes also exhibited little or no GPA dependence. (86)Rb(+) influx was higher but inward cation current was lower in oocytes expressing AE1 E758K than previously reported in oocytes expressing the AE1 HSt mutants S731P and H734R. The pharmacological inhibition profile of AE1 E758K-associated (36)Cl(-) influx differed from that of AE1 E758K-associated (86)Rb(+) influx, as well as from that of wild-type AE1-mediated Cl(-) transport. Thus AE1 E758K-expressing oocytes displayed GPA-dependent surface polypeptide expression and anion transport, accompanied by substantially GPA-independent, pharmacologically distinct Rb(+) flux and by small, GPA-independent currents. The data strongly suggest that most of the increased cation transport associated with the novel HSt mutant AE1 E758K reflects activation of endogenous oocyte cation permeability pathways, rather than cation translocation through the mutant polypeptide.

Aberrant splicing contributes to severe α-spectrin-linked congenital hemolytic anemia.

The etiology of severe hemolytic anemia in most patients with recessive hereditary spherocytosis (rHS) and the related disorder hereditary pyropoikilocytosis (HPP) is unknown. Whole exome sequencing of DNA from probands of 24 rHS or HPP kindreds identified numerous mutations in erythrocyte membrane α-spectrin (SPTA1). Twenty-eight mutations were novel, with null alleles frequently found in trans to missense mutations. No mutations were identified in a third of SPTA1 alleles (17/48). Whole genome sequencing revealed linkage disequilibrium between the common rHS-linked α-spectrinBug Hill polymorphism and a rare intron 30 variant in all 17 mutation-negative alleles. In vitro minigene studies and in vivo splicing analyses revealed the intron 30 variant changes a weak alternate branch point (BP) to a strong BP. This change leads to increased utilization of an alternate 3' splice acceptor site, perturbing normal α-spectrin mRNA splicing and creating an elongated mRNA transcript. In vivo mRNA stability studies revealed the newly created termination codon in the elongated transcript activates nonsense mediated decay leading to spectrin deficiency. These results demonstrate a unique mechanism of human genetic disease contributes to the etiology of a third of cases of rHS, facilitating diagnosis and treatment of severe anemia, and identifying a new target for therapeutic manipulation.

Ankyrin-linked hereditary spherocytosis in an African-American kindred.

Mutations of ankyrin-1 are the most frequent cause of the inherited hemolytic anemia, hereditary spherocytosis (HS), in people of European ancestry. Ankyrin-1, which provides the primary linkage between the erythrocyte membrane skeleton and the plasma membrane, has numerous isoforms generated by alternative splicing, alternate polyadenylation, use of tissue-specific promoters, and alternate NH(2) or COOH-termini. Mutation detection in erythrocyte membrane protein genes, including ankyrin, has been a challenge, primarily due to the large size of these genes, and the apparent frequent occurrence of HS-associated null alleles. Using denaturing high-performance liquid chromatography (DHPLC), we screened the ankyrin gene of the proband of a large, three generation African-American kindred with ankyrin-deficient HS. DHPLC yielded an abnormal chromatogram for exon 1. Examination of the corresponding exon 1 sequence in genomic DNA from the proband revealed heterozygosity for a mutation of the initiator methionine (ATG to ATA Met 1 Ile). Coupled in vitrotranscription/translation studies with rabbit reticulocyte lysates demonstrated that the wild-type ankyrin erythroid cDNA initiates only from the known initiator methionine, indicating that the use of alternate initiator methionine is not a mechanism of isoform diversity in erythroid cells. The mutant ankyrin allele, unlike some initiator methionine mutations that utilize downstream codons for translation initiation, was associated with a null allele. This is the first report describing ankyrin-linked HS in an African-American kindred.

Chromatin architecture and transcription factor binding regulate expression of erythrocyte membrane protein genes.

Erythrocyte membrane protein genes serve as excellent models of complex gene locus structure and function, but their study has been complicated by both their large size and their complexity. To begin to understand the intricate interplay of transcription, dynamic chromatin architecture, transcription factor binding, and genomic organization in regulation of erythrocyte membrane protein genes, we performed chromatin immunoprecipitation (ChIP) coupled with microarray analysis and ChIP coupled with massively parallel DNA sequencing in both erythroid and nonerythroid cells. Unexpectedly, most regions of GATA-1 and NF-E2 binding were remote from gene promoters and transcriptional start sites, located primarily in introns. Cooccupancy with FOG-1, SCL, and MTA-2 was found at all regions of GATA-1 binding, with cooccupancy of SCL and MTA-2 also found at regions of NF-E2 binding. Cooccupancy of GATA-1 and NF-E2 was found frequently. A common signature of histone H3 trimethylation at lysine 4, GATA-1, NF-E2, FOG-1, SCL, and MTA-2 binding and consensus GATA-1-E-box binding motifs located 34 to 90 bp away from NF-E2 binding motifs was found frequently in erythroid cell-expressed genes. These results provide insights into our understanding of membrane protein gene regulation in erythropoiesis and the regulation of complex genetic loci in erythroid and nonerythroid cells and identify numerous candidate regions for mutations associated with membrane-linked hemolytic anemia.

Failure of terminal erythroid differentiation in EKLF-deficient mice is associated with cell cycle perturbation and reduced expression of E2F2.

Erythroid Krüppel-like factor (EKLF) is a Krüppel-like transcription factor identified as a transcriptional activator and chromatin modifier in erythroid cells. EKLF-deficient (Eklf(-/-)) mice die at day 14.5 of gestation from severe anemia. In this study, we demonstrate that early progenitor cells fail to undergo terminal erythroid differentiation in Eklf(-/-) embryos. To discover potential EKLF target genes responsible for the failure of erythropoiesis, transcriptional profiling was performed with RNA from wild-type and Eklf(-/-) early erythroid progenitor cells. These analyses identified significant perturbation of a network of genes involved in cell cycle regulation, with the critical regulator of the cell cycle, E2f2, at a hub. E2f2 mRNA and protein levels were markedly decreased in Eklf(-/-) early erythroid progenitor cells, which showed a delay in the G(1)-to-S-phase transition. Chromatin immunoprecipitation analysis demonstrated EKLF occupancy at the proximal E2f2 promoter in vivo. Consistent with the role of EKLF as a chromatin modifier, EKLF binding sites in the E2f2 promoter were located in a region of EKLF-dependent DNase I sensitivity in early erythroid progenitor cells. We propose a model in which EKLF-dependent activation and modification of the E2f2 locus is required for cell cycle progression preceding terminal erythroid differentiation.

A complex splicing defect associated with homozygous ankyrin-deficient hereditary spherocytosis.

Defects in erythrocyte ankyrin are the most common cause of typical, dominant hereditary spherocytosis (HS). Detection of ankyrin gene mutations has been complicated by allelic heterogeneity, large gene size, frequent de novo mutations, and associated mRNA instability. Using denaturing high-performance liquid chromatography (DHPLC)-based mutation detection, a mutation in the splice acceptor of exon 17 was discovered in a Turkish family. Reticulocyte RNA and functional minigene splicing assays in heterologous cells revealed that this mutation was associated with a complex pattern of aberrant splicing, suggesting that removal of intron 16 is important for ordered ankyrin mRNA splicing. As predicted by clinical, laboratory, and biochemical studies, the parents were heterozygous and the proband was homozygous for this mutation. These data indicate that DHPLC offers a highly sensitive, economic, and rapid method for mutation detection and, unlike previously suggested, homozygosity for a mutation associated with dominant ankyrin-linked HS may be compatible with life.