Hereditary spherocytosis associated with deletion of human erythrocyte ankyrin gene on chromosome 8

Localized Leishmania major infection disrupts systemic iron homeostasis that can be controlled by oral iron supplementation

Leishmania parasites are heavily dependent on efficient iron acquisition from a tightly regulated host iron pool for survival and virulence. Prior studies uncovered multiple strategies adopted by the parasite to hijack the iron-regulatory network of macrophages. Despite these extensive studies with infected macrophages, there is limited knowledge of the effect of Leishmania infection on systemic iron homeostasis. This issue is particularly relevant for Leishmania major, which causes localized skin infection with minimal lymphatic spread. We show for the first time that L. major infection in the mouse footpad induced influx of iron at the site of infection through blood with simultaneous upregulation of transferrin receptor 1 and downregulation of phagolysosomal iron exporter Nramp1 expression in the footpad tissue. Interestingly, localized L. major infection had far-reaching effects beyond the infection site triggering anemia-like symptoms. This was evident from depleted physiological iron stores from the liver and bone marrow as well as reduced hemoglobin levels and deformed erythrocytes. The infected mice also developed splenomegaly with signs of splenic stress erythropoiesis as indicated by upregulation of several erythroid-related genes. These observations prompted us to provide oral iron supplementations to the L. major-infected mice, which resulted in a drastic reduction of the parasite load and restoration of iron homeostasis.

Pleiotropic Ankyrins: Scaffolds for Ion Channels and Transporters

The ankyrin proteins (Ankyrin-R, Ankyrin-B, and Ankyrin-G) are a family of scaffolding, or membrane adaptor proteins necessary for the regulation and targeting of several types of ion channels and membrane transporters throughout the body. These include voltage-gated sodium, potassium, and calcium channels in the nervous system, heart, lungs, and muscle. At these sites, ankyrins recruit ion channels, and other membrane proteins, to specific subcellular domains, which are then stabilized through ankyrin's interaction with the submembranous spectrin-based cytoskeleton. Several recent studies have expanded our understanding of both ankyrin expression and their ion channel binding partners. This review provides an updated overview of ankyrin proteins and their known channel and transporter interactions. We further discuss several potential avenues of future research that would expand our understanding of these important organizational proteins.

Prevalence and Phenotypic Effects of Copy Number Variants in Isolated Hypogonadotropic Hypogonadism

CONTEXT

The genetic architecture of isolated hypogonadotropic hypogonadism (IHH) has not been completely defined.

OBJECTIVE

To determine the role of copy number variants (CNVs) in IHH pathogenicity and define their phenotypic spectrum.

METHODS

Exome sequencing (ES) data in IHH probands (n = 1394) (Kallmann syndrome [IHH with anosmia; KS], n = 706; normosmic IHH [nIHH], n = 688) and family members (n = 1092) at the Reproductive Endocrine Unit and the Center for Genomic Medicine of Massachusetts General Hospital were analyzed for CNVs and single nucleotide variants (SNVs)/indels in 62 known IHH genes. IHH subjects without SNVs/indels in known genes were considered "unsolved." Phenotypes associated with CNVs were evaluated through review of patient medical records. A total of 29 CNVs in 13 genes were detected (overall IHH cohort prevalence: ~2%). Almost all (28/29) CNVs occurred in unsolved IHH cases. While some genes (eg, ANOS1 and FGFR1) frequently harbor both CNVs and SNVs/indels, the mutational spectrum of others (eg, CHD7) was restricted to SNVs/indels. Syndromic phenotypes were seen in 83% and 63% of IHH subjects with multigenic and single gene CNVs, respectively.

CONCLUSION

CNVs in known genes contribute to ~2% of IHH pathogenesis. Predictably, multigenic contiguous CNVs resulted in syndromic phenotypes. Syndromic phenotypes resulting from single gene CNVs validate pleiotropy of some IHH genes. Genome sequencing approaches are now needed to identify novel genes and/or other elusive variants (eg, noncoding/complex structural variants) that may explain the remaining missing etiology of IHH.

ANKRD37 inhibits trophoblast migration and invasion by regulating NF-κB pathway in preeclampsia

BACKGROUND

Inadequate trophoblast invasion is associated with preeclampsia (PE). Ankyrin repeat domain protein 37 (ANKRD37) has been reported to be abnormally expressed in PE placentas. However, the role of ANKRD37 in trophoblasts has not been investigated. We aimed to determine the functions of ANKRD37 in PE and to explore the molecular mechanisms.

METHODS

Here, fluorescence in situ hybridization, immunohistochemistry, Western blotting and quantitative real-time PCR were used to detect protein and mRNA expression levels. Cell Counting Kit-8 (CCK-8), EdU assay, flow cytometry, wound healing assay, Transwell assay and RNA sequencing were performed to study the role of ANKRD37 and the underlying mechanism in HTR8/SVneo and JEG-3 cells, and extravillous explant cultures were used to evaluate the migration and invasion abilities of extravillous cytotrophoblasts (EVTs).

RESULTS

We found that ANKRD37 expression was upregulated in PE placentas compared to normal pregnancy placentas. ANKRD37 knockdown enhanced trophoblast migration and invasion, promoted extravillous explant outgrowth, and regulated the expression of key invasion proteins, while ANKRD37 overexpression exerted the opposite effects. RNA sequencing indicated that NF-κB was the potential downstream pathway of ANKRD37, which was confirmed by the change in p-p65 and p-IκBα expression in JEG-3 and HTR8/SVneo cells.

CONCLUSIONS

Our findings suggest that high expression of ANKRD37 inhibits trophoblast cell migration and invasion possibly via NF-κB pathway, and may be related to the development of PE.

Ankyrin-R regulates fast-spiking interneuron excitability through perineuronal nets and Kv3.1b K+ channels

Neuronal ankyrins cluster and link membrane proteins to the actin and spectrin-based cytoskeleton. Among the three vertebrate ankyrins, little is known about neuronal Ankyrin-R (AnkR). We report AnkR is highly enriched in Pv⁺ fast-spiking interneurons in mouse and human. We identify AnkR-associated protein complexes including cytoskeletal proteins, cell adhesion molecules (CAMs), and perineuronal nets (PNNs). We show that loss of AnkR from forebrain interneurons reduces and disrupts PNNs, decreases anxiety-like behaviors, and changes the intrinsic excitability and firing properties of Pv⁺ fast-spiking interneurons. These changes are accompanied by a dramatic reduction in Kv3.1b K⁺ channels. We identify a novel AnkR-binding motif in Kv3.1b, and show that AnkR is both necessary and sufficient for Kv3.1b membrane localization in interneurons and at nodes of Ranvier. Thus, AnkR regulates Pv⁺ fast-spiking interneuron function by organizing ion channels, CAMs, and PNNs, and linking these to the underlying β1 spectrin-based cytoskeleton.

Global retardation and hereditary spherocytosis associated with a novel deletion of chromosome 8p11.21 encompassing KAT6A and ANK1

The loss of heterozygosity localized at chromosome segment 8p11.2 causes a contiguous gene syndrome, which mostly combined phenotype of Kallmann syndrome and hereditary spherocytosis. It has been documented that this combined phenotype is in association with both the deletion of the fibroblast growth factor receptor 1 (FGFR1) and ankyrin 1 (ANK1) genes. Here, we described a 6-year-old girl with microcephaly, global developmental delay, mental retardation, and hereditary spherocytosis, associated with a heterozygous pathogenic microdeletion of 1.9 Mb size at 8p11.21. Molecular analysis confirmed that the identified microdeletion contained two OMIM (Online Mendelian Inheritance in Man)genes, including ANK1 and lysine acetyltransferase 6 A (KAT6A), but not FGFR1. Therefore, the simultaneous occurrence of mild developmental delay and distinctive facial in this patient was associated with the pathogenic variation of the KAT6A.

Beyond antibodies: ankyrins and DARPins. From basic research to drug approval

This Pharmacological Perspective describes the pathway that, starting from the deep understanding of ankyrins - a family of proteins with high variability-binding and high specificity-binding characteristics - led to the development of a new class of recombinant-binding proteins, the DARPins (designed ankyrin repeat proteins). These are envisaged as alternatives to mAbs and related biologics, with the potential to overcome certain shortcomings of mAbs. DARPins have relatively low molecular weights (14-21kDas) and more favorable PK profiles than mAbs, are stable proteins that can be easily produced in Escherichia coli and can be used in their monovalent form or conjugated to other moieties, for example, polyethylene glycol (PEG) to enhance their half-life. DARPins can also be engineered to produce bi-specific or tri-specific compounds that bind different epitopes of the same target or two different targets. Abicipar, a first-in-class anti-VEGF-A DARPin had similar efficacy compared to anti-VEGF biologics (bevacizumab, ranibizumab) in preclinical studies and was not inferior to ranibizumab in the treatment of age-related macular degeneration (AMD) with a reduced number of intravitreal injections. Abicipar has recently been submitted for regulatory approval for use in AMD.

Autoinhibition of ankyrin-B/G membrane target bindings by intrinsically disordered segments from the tail regions

Ankyrins together with their spectrin partners are the master organizers of micron-scale membrane domains in diverse tissues. The 24 ankyrin (ANK) repeats of ankyrins bind to numerous membrane proteins, linking them to spectrin-based cytoskeletons at specific membrane microdomains. The accessibility of the target binding groove of ANK repeats must be regulated to achieve spatially defined functions of ankyrins/target complexes in different tissues, though little is known in this regard. Here we systemically investigated the autoinhibition mechanism of ankyrin-B/G by combined biochemical, biophysical and structural biology approaches. We discovered that the entire ANK repeats are inhibited by combinatorial and quasi-independent bindings of multiple disordered segments located in the ankyrin-B/G linkers and tails, suggesting a mechanistic basis for differential regulations of membrane target bindings by ankyrins. In addition to elucidating the autoinhibition mechanisms of ankyrins, our study may also shed light on regulations on target bindings by other long repeat-containing proteins.

Diffusion of glycophorin A in human erythrocytes

Several lines of evidence suggest that glycophorin A (GPA) interacts with band 3 in human erythrocyte membranes including: i) the existence of an epitope shared between band 3 and GPA in the Wright b blood group antigen, ii) the fact that antibodies to GPA inhibit the diffusion of band 3, iii) the observation that expression of GPA facilitates trafficking of band 3 from the endoplasmic reticulum to the plasma membrane, and iv) the observation that GPA is diminished in band 3 null erythrocytes. Surprisingly, there is also evidence that GPA does not interact with band 3, including data showing that: i) band 3 diffusion increases upon erythrocyte deoxygenation whereas GPA diffusion does not, ii) band 3 diffusion is greatly restricted in erythrocytes containing the Southeast Asian Ovalocytosis mutation whereas GPA diffusion is not, and iii) most anti-GPA or anti-band 3 antibodies do not co-immunoprecipitate both proteins. To try to resolve these apparently conflicting observations, we have selectively labeled band 3 and GPA with fluorescent quantum dots in intact erythrocytes and followed their diffusion by single particle tracking. We report here that band 3 and GPA display somewhat similar macroscopic and microscopic diffusion coefficients in unmodified cells, however perturbations of band 3 diffusion do not cause perturbations of GPA diffusion. Taken together the collective data to date suggest that while weak interactions between GPA and band 3 undoubtedly exist, GPA and band 3 must have separate interactions in the membrane that control their lateral mobility.

Identification of adducin-binding residues on the cytoplasmic domain of erythrocyte membrane protein, band 3

Two major complexes form structural bridges that connect the erythrocyte membrane to its underlying spectrin-based cytoskeleton. Although the band 3-ankyrin bridge may account for most of the membrane-to-cytoskeleton interactions, the linkage between the cytoplasmic domain of band 3 (cdb3) and adducin has also been shown to be critical to membrane integrity. In the present paper, we demonstrate that adducin, a major component of the spectrin-actin junctional complex, binds primarily to residues 246-264 of cdb3, and mutation of two exposed glutamic acid residues within this sequence completely abrogates both α- and β-adducin binding. Because these residues are located next to the ankyrin-binding site on cdb3, it seems unlikely that band 3 can bind ankyrin and adducin concurrently, reducing the chances of an association between the ankyrin and junctional complexes that would significantly compromise erythrocyte membrane integrity. We also demonstrate that adducin binds the kidney isoform of cdb3, a spliceoform that lacks the first 65 amino acids of erythrocyte cdb3, including the central strand of a large β-pleated sheet. Because kidney cdb3 is not known to bind any of the common peripheral protein partners of erythrocyte cdb3, including ankyrin, protein 4.1, glyceraldehyde-3-phosphate dehydrogenase, aldolase, and phosphofructokinase, retention of this affinity for adducin was unexpected.

Modeling of band-3 protein diffusion in the normal and defective red blood cell membrane

We employ a two-component red blood cell (RBC) membrane model to simulate lateral diffusion of band-3 proteins in the normal RBC and in the RBC with defective membrane proteins. The defects reduce the connectivity between the lipid bilayer and the membrane skeleton (vertical connectivity), or the connectivity of the membrane skeleton itself (horizontal connectivity), and are associated with the blood disorders of hereditary spherocytosis (HS) and hereditary elliptocytosis (HE) respectively. Initially, we demonstrate that the cytoskeleton limits band-3 lateral mobility by measuring the band-3 macroscopic diffusion coefficients in the normal RBC membrane and in a lipid bilayer without the cytoskeleton. Then, we study band-3 diffusion in the defective RBC membrane and quantify the relation between band-3 diffusion coefficients and percentage of protein defects in HE RBCs. In addition, we illustrate that at low spectrin network connectivity (horizontal connectivity) band-3 subdiffusion can be approximated as anomalous diffusion, while at high horizontal connectivity band-3 diffusion is characterized as confined diffusion. Our simulations show that the band-3 anomalous diffusion exponent depends on the percentage of protein defects in the membrane cytoskeleton. We also confirm that the introduction of attraction between the lipid bilayer and the spectrin network reduces band-3 diffusion, but we show that this reduction is lower than predicted by the percolation theory. Furthermore, we predict that the attractive force between the spectrin filament and the lipid bilayer is at least 20 times smaller than the binding forces at band-3 and glycophorin C, the two major membrane binding sites. Finally, we explore diffusion of band-3 particles in the RBC membrane with defects related to vertical connectivity. We demonstrate that in this case band-3 diffusion can be approximated as confined diffusion for all attraction levels between the spectrin network and the lipid bilayer. By comparing the diffusion coefficients measured in horizontal vs. vertical defects, we conclude that band-3 mobility is primarily controlled by the horizontal connectivity.

A hierarchy of ankyrin-spectrin complexes clusters sodium channels at nodes of Ranvier

The scaffolding protein ankyrin-G is required for Na(+) channel clustering at axon initial segments. It is also considered essential for Na(+) channel clustering at nodes of Ranvier to facilitate fast and efficient action potential propagation. However, notwithstanding these widely accepted roles, we show here that ankyrin-G is dispensable for nodal Na(+) channel clustering in vivo. Unexpectedly, in the absence of ankyrin-G, erythrocyte ankyrin (ankyrin-R) and its binding partner βI spectrin substitute for and rescue nodal Na(+) channel clustering. In addition, channel clustering is also rescued after loss of nodal βIV spectrin by βI spectrin and ankyrin-R. In mice lacking both ankyrin-G and ankyrin-R, Na(+) channels fail to cluster at nodes. Thus, ankyrin R-βI spectrin protein complexes function as secondary reserve Na(+) channel clustering machinery, and two independent ankyrin-spectrin protein complexes exist in myelinated axons to cluster Na(+) channels at nodes of Ranvier.

Weighing in on molecular anchors: the role of ankyrin polypeptides in human arrhythmia

Loss-of-function gene variants which affect the biophysical properties of ion channel proteins have long been associated with the destabilization of cardiac electrical activity, leading to human arrhythmia and sudden cardiac death. However, recent studies have also demonstrated the importance of ion channel/transporter-anchoring molecules for normal cardiac function. Ankyrins are a family of membrane adaptor proteins whose role in metazoan physiology has been elucidated over the last quarter of a century, but with great strides taken in the last half decade with regard to cardiac cell physiology. The association of dysfunction in ankyrin-based cellular pathways with abnormal human cardiac function represents a surprising turn in the genetics of arrhythmias and sudden cardiac death, demonstrating an exciting new player in the field of 'channelopathies'.

A deep intronic mutation in the ankyrin-1 gene causes diminished protein expression resulting in hemolytic anemia in mice

Linkage between transmembrane proteins and the spectrin-based cytoskeleton is necessary for membrane elasticity of red blood cells. Mutations of the proteins that mediate this linkage result in various types of hemolytic anemia. Here we report a novel N-ethyl-N-nitrosourea-induced mutation of ankyrin-1, named hema6, which causes hereditary spherocytosis in mice through a mild reduction of protein expression. The causal mutation was traced to a single nucleotide transition located deep into intron 13 of gene Ank1. In vitro minigene splicing assay revealed two abnormally spliced transcripts containing cryptic exons from fragments of Ank1 intron 13. The inclusion of cryptic exons introduced a premature termination codon, which leads to nonsense-mediated decay of the mutant transcripts in vivo. Hence, in homozygous mice, only wild-type ankyrin-1 is expressed, albeit at 70% of the level in wild-type mice. Heterozygotes display a similar hereditary spherocytosis phenotype stemming from intermediate protein expression level, indicating the haploinsufficiency of the mutation. Weakened linkage between integral transmembrane protein, band 3, and underlying cytoskeleton was observed in mutant mice as the result of reduced high-affinity binding sites provided by ankyrin-1. Hema6 is the only known mouse mutant of Ank1 allelic series that expresses full-length canonical ankyrin-1 at a reduced level, a fact that makes it particularly useful to study the functional impact of ankyrin-1 quantitative deficiency.

Hereditary spherocytosis, elliptocytosis, and other red cell membrane disorders

Hereditary spherocytosis and elliptocytosis are the two most common inherited red cell membrane disorders resulting from mutations in genes encoding various red cell membrane and skeletal proteins. Red cell membrane, a composite structure composed of lipid bilayer linked to spectrin-based membrane skeleton is responsible for the unique features of flexibility and mechanical stability of the cell. Defects in various proteins involved in linking the lipid bilayer to membrane skeleton result in loss in membrane cohesion leading to surface area loss and hereditary spherocytosis while defects in proteins involved in lateral interactions of the spectrin-based skeleton lead to decreased mechanical stability, membrane fragmentation and hereditary elliptocytosis. The disease severity is primarily dependent on the extent of membrane surface area loss. Both these diseases can be readily diagnosed by various laboratory approaches that include red blood cell cytology, flow cytometry, ektacytometry, electrophoresis of the red cell membrane proteins, and mutational analysis of gene encoding red cell membrane proteins.

Identification of contact sites between ankyrin and band 3 in the human erythrocyte membrane

The red cell membrane is stabilized by a spectrin/actin-based cortical cytoskeleton connected to the phospholipid bilayer via multiple protein bridges. By virtue of its interaction with ankyrin and adducin, the anion transporter, band 3 (AE1), contributes prominently to these bridges. In a previous study, we demonstrated that an exposed loop comprising residues 175-185 of the cytoplasmic domain of band 3 (cdB3) constitutes a critical docking site for ankyrin on band 3. In this paper, we demonstrate that an adjacent loop, comprising residues 63-73 of cdB3, is also essential for ankyrin binding. Data that support this hypothesis include the following. (1) Deletion or mutation of residues within the latter loop abrogates ankyrin binding without affecting cdB3 structure or its other functions. (2) Association of cdB3 with ankyrin is inhibited by competition with the loop peptide. (3) Resealing of the loop peptide into erythrocyte ghosts alters membrane morphology and stability. To characterize cdB3-ankyrin interaction further, we identified their interfacial contact sites using molecular docking software and the crystal structures of D(3)D(4)-ankyrin and cdB3. The best fit for the interaction reveals multiple salt bridges and hydrophobic contacts between the two proteins. The most important ion pair interactions are (i) cdB3 K69-ankyrin E645, (ii) cdB3 E72-ankyrin K611, and (iii) cdB3 D183-ankyrin N601 and Q634. Mutation of these four residues on ankyrin yielded an ankyrin with a native CD spectrum but little or no affinity for cdB3. These data define the docking interface between cdB3 and ankyrin in greater detail.

A de novo interstitial deletion of 8p11.2 including ANK1 identified in a patient with spherocytosis, psychomotor developmental delay, and distinctive facial features

The contiguous gene syndrome involving 8p11.2 is recognized as a combined phenotype of both Kallmann syndrome and hereditary spherocytosis, because the genes responsible for these 2 clinical entities, the fibroblast growth factor receptor 1 (FGFR1) and ankyrin 1 (ANK1) genes, respectively, are located in this region within a distance of 3.2Mb. We identified a 3.7Mb deletion of 8p11.2 in a 19-month-old female patient with hereditary spherocytosis. The identified deletion included ANK1, but not FGFR1, which is consistent with the absence of any phenotype or laboratory findings of Kallmann syndrome. Compared with the previous studies, the deletion identified in this study was located on the proximal end of 8p, indicating a pure interstitial deletion of 8p11.21. This patient exhibited mild developmental delay and distinctive facial findings in addition to hereditary spherocytosis. Thus, some of the genes included in the deleted region would be related to these symptoms.

Sequence-specific recognition of a PxLPxI/L motif by an ankyrin repeat tumbler lock

Ankyrin repeat family A protein 2 (ANKRA2) interacts with the plasma membrane receptor megalin and the class IIa histone deacetylases HDAC4 and HDAC5. We report that the ankyrin repeat domains of ANKRA2 and its close paralog regulatory factor X-associated ankyrin-containing protein (RFXANK) recognize a PxLPxI/L motif found in diverse binding proteins, including HDAC4, HDAC5, HDAC9, megalin, and regulatory factor X, 5 (RFX5). Crystal structures of the ankyrin repeat domain of ANKRA2 in complex with its binding peptides revealed that each of the middle three ankyrin repeats of ANKRA2 recognizes a residue from the PxLPxI/L motif in a tumbler-lock binding mode, with ANKRA2 acting as the lock and the linear binding motif serving as the key. Structural analysis showed that three disease-causing mutations in RFXANK affect residues that are critical for binding to RFX5. These results suggest a fundamental principle of longitudinal recognition of linear sequences by a repeat-type domain. In addition, phosphorylation of serine 350, a residue embedded within the PxLPxI/L motif of HDAC4, impaired the binding of ANKRA2 but generated a high-affinity docking site for 14-3-3 proteins, which may help sequester this HDAC in the cytoplasm. Thus, the binding preference of the PxLPxI/L motif is signal-dependent. Furthermore, proteome-wide screening suggested that a similar phosphorylation-dependent switch may operate in other pathways. Together, our findings uncover a previously uncharacterized sequence- and signal-dependent peptide recognition mode for a repeat-type protein domain.

Disruption of spectrin-like cytoskeleton in differentiating keratinocytes by PKCδ activation is associated with phosphorylated adducin

Spectrin is a central component of the cytoskeletal protein network in a variety of erythroid and non-erythroid cells. In keratinocytes, this protein has been shown to be pericytoplasmic and plasma membrane associated, but its characteristics and function have not been established in these cells. Here we demonstrate that spectrin increases dramatically in amount and is assembled into the cytoskeleton during differentiation in mouse and human keratinocytes. The spectrin-like cytoskeleton was predominantly organized in the granular and cornified layers of the epidermis and disrupted by actin filament inhibitors, but not by anti-mitotic drugs. When the cytoskeleton was disrupted PKCδ was activated by phosphorylation on Thr505. Specific inhibition of PKCδ^(Thr505) activation with rottlerin prevented disruption of the spectrin-like cytoskeleton and the associated morphological changes that accompany differentiation. Rottlerin also inhibited specific phosphorylation of the PKCδ substrate adducin, a cytoskeletal protein. Furthermore, knock-down of endogenous adducin affected not only expression of adducin, but also spectrin and PKCδ, and severely disrupted organization of the spectrin-like cytoskeleton and cytoskeletal distribution of both adducin and PKCδ. These results demonstrate that organization of a spectrin-like cytoskeleton is associated with keratinocytes differentiation, and disruption of this cytoskeleton is mediated by either PKCδ^(Thr505) phosphorylation associated with phosphorylated adducin or due to reduction of endogenous adducin, which normally connects and stabilizes the spectrin-actin complex.

Analysis of the mobilities of band 3 populations associated with ankyrin protein and junctional complexes in intact murine erythrocytes

Current models of the erythrocyte membrane depict three populations of band 3: (i) a population tethered to spectrin via ankyrin, (ii) a fraction attached to the spectrin-actin junctional complex via adducin, and (iii) a freely diffusing population. Because many studies of band 3 diffusion also distinguish three populations of the polypeptide, it has been speculated that the three populations envisioned in membrane models correspond to the three fractions observed in diffusion analyses. To test this hypothesis, we characterized band 3 diffusion by single-particle tracking in wild-type and ankyrin- and adducin-deficient erythrocytes. We report that ∼40% of total band 3 in wild-type murine erythrocytes is attached to ankyrin, whereas ∼33% is immobilized by adducin, and ∼27% is not attached to any cytoskeletal anchor. More detailed analyses reveal that mobilities of individual ankyrin- and adducin-tethered band 3 molecules are heterogeneous, varying by nearly 2 orders of magnitude and that there is considerable overlap in diffusion coefficients for adducin and ankyrin-tethered populations. Taken together, the data suggest that although the ankyrin- and adducin-immobilized band 3 can be monitored separately, significant heterogeneity still exists within each population, suggesting that structural and compositional properties likely vary considerably within each band 3 complex.

Single-cell electrical lysis of erythrocytes detects deficiencies in the cytoskeletal protein network

The network of erythrocyte cytoskeletal proteins significantly influences erythrocyte physical and biological properties. Here we show that the kinetics of erythrocyte lysis during exposure to an electric field is sensitively correlated with defects in the cytoskeletal network. Histograms compiled from single-cell electrical lysis data show characteristics of erythrocyte populations that are deficient in a specific cytoskeletal protein, revealing the presence of cell subpopulations.

Mouse Fem1b interacts with and induces ubiquitin-mediated degradation of Ankrd37

Ankyrin repeat domain 37 (Ankrd37), a protein containing ankyrin repeats (ARs) and a putative nuclear localization signal (NLS), is highly conserved from zebrafish to humans. In mouse testes, Ankrd37 protein was initially present in the cytoplasm of elongating spermatids, and finally restricted to the nuclei of spermatozoa during spermatogenesis. Ankrd37 bound to feminization 1 homolog b (Fem1b) as indicated by yeast two-hybrid screening and co-immunoprecipitation assays. Ankrd37 facilitated the transport of Fem1b protein from cytoplasm to nuclei in co-transfected CHO cells. In addition, the protein level of Ankrd37 was decreased in a Fem1b dose-dependent manner as shown by the transfection experiments, and Ankrd37 was ubiquitinated in the presence of Fem1b. As the nematode Fem-1 has been shown to target its downstream effector TRA-1 for ubiquitin-mediated degradation, we report in the present study that mouse Fem1b targets Ankrd37 for degradation in the same manner.

Protein 4.2 binds to the carboxyl-terminal EF-hands of erythroid alpha-spectrin in a calcium- and calmodulin-dependent manner

Spectrin and protein 4.1 cross-link F-actin protofilaments into a network called the membrane skeleton. Actin and 4.1 bind to one end of beta-spectrin. The adjacent end of alpha-spectrin, called the EF-domain, is calmodulin-like, with calcium-dependent and calcium-independent EF-hands. It has no known function. However, the sph(1J)/sph(1J) mouse has very fragile red cells and lacks the last 13 amino acids in the EF-domain, suggesting the domain is critical for skeletal integrity. Using pulldown binding assays, we find the alpha-spectrin EF-domain either alone or incorporated into a mini-spectrin binds native and recombinant protein 4.2 at a previously identified region of 4.2 (G(3) peptide). Native 4.2 binds with an affinity comparable with other membrane skeletal interactions (K(d) = 0.30 microM). EF-domains bearing the sph(1J) mutation are inactive. Binding of protein 4.2 to band 3 (K(d) = 0.45 microM) does not interfere with the spectrin-4.2 interaction. Spectrin-4.2 binding is amplified by micromolar concentrations of Ca(2+) (but not Mg(2+)) by three to five times. Calmodulin also binds to the EF-domain (K(d) = 17 microM), and Ca(2+)-calmodulin blocks Ca(2+)-dependent binding of protein 4.2 but not Ca(2+)-independent binding. The data suggest that protein 4.2 is located near protein 4.1 at the spectrin-actin junctions. Because proteins 4.1 and 4.2 also bind to band 3, the erythrocyte anion channel, we suggest that one or both of these proteins cause a portion of band 3 to localize near the spectrin-actin junctions and provide another point of attachment between the membrane skeleton and the lipid bilayer.

Ankyrin-based patterning of membrane microdomains: new insights into a novel class of cardiovascular diseases

The organization of membrane-spanning proteins within discrete microdomains is critical for their physiologic function. This is especially important in the heart, where ion transporter and force-transducing microdomains are responsible for excitation-contraction coupling, anisotropic depolarization, and mechanotransduction. The following review will discuss recent advances in our understanding of the patterning of ion channel and force-transmitting membrane microdomains in cardiomyocytes, focusing on the T-tubule and intercalated disc.

Ankyrin protein networks in membrane formation and stabilization

In eukaryotic cells, ankyrins serve as adaptor proteins that link membrane proteins to the underlying cytoskeleton. These adaptor proteins form protein complexes consisting of integral membrane proteins, signalling molecules and cytoskeletal components. With their modular architecture and ability to interact with many proteins, ankyrins organize and stabilize these protein networks, thereby establishing the infrastructure of membrane domains with specialized functions. To this end, ankyrin collaborates with a number of proteins including cytoskeletal proteins, cell adhesion molecules and large structural proteins. This review addresses the targeting and stabilization of protein networks related to ankyrin interactions with the cytoskeletal protein β-spectrin, L1-cell adhesion molecules and the large myofibrillar protein obscurin. The significance of these interactions for differential targeting of cardiac proteins and neuronal membrane formation is also presented. Finally, this review concludes with a discussion about ankyrin dysfunction in human diseases such as haemolytic anaemia, cardiac arrhythmia and neurological disorders.

Adducin forms a bridge between the erythrocyte membrane and its cytoskeleton and regulates membrane cohesion

The erythrocyte membrane skeleton is the best understood cytoskeleton. Because its protein components have homologs in virtually all other cells, the membrane serves as a fundamental model of biologic membranes. Modern textbooks portray the membrane as a 2-dimensional spectrin-based membrane skeleton attached to a lipid bilayer through 2 linkages: band 3-ankyrin-beta-spectrin and glycophorin C-protein 4.1-beta-spectrin.(1-7) Although evidence supports an essential role for the first bridge in regulating membrane cohesion, rupture of the glycophorin C-protein 4.1 interaction has little effect on membrane stability.(8) We demonstrate the existence of a novel band 3-adducin-spectrin bridge that connects the spectrin/actin/protein 4.1 junctional complex to the bilayer. As rupture of this bridge leads to spontaneous membrane fragmentation, we conclude that the band 3-adducin-spectrin bridge is important to membrane stability. The required relocation of part of the band 3 population to the spectrin/actin junctional complex and its formation of a new bridge with adducin necessitates a significant revision of accepted models of the erythrocyte membrane.

A novel 8 Mb interstitial deletion of chromosome 8p12-p21.2

We report on a girl with delayed mental and motor development, ophthalmological abnormalities, and peripheral neuropathy. Chromosome analysis suggested a deletion within chromosome 8p. Further investigation by array-based comparative genomic hybridization (array-CGH) delineated an 8 Mb interstitial deletion on the short arm of chromosome 8. The breakpoints are located at chromosome bands 8p12 and 8p21.2. Forty-two known genes including gonadotropin-releasing hormone 1 (GNRH1), transcription factor EBF2, exostosin-like 3 (EXTL3), glutathione reductase (GSR), and neuregulin 1 (NRG1), are located within the deleted region on chromosome 8p. A comparison of our patient with the cases described in the literature is presented, and we discuss the genotype-phenotype correlation in our patient. This is the first report of array-CGH analysis of an interstitial deletion at chromosome 8p.

Imaging of the diffusion of single band 3 molecules on normal and mutant erythrocytes

Membrane-spanning proteins may interact with a variety of other integral and peripheral membrane proteins via a diversity of protein-protein interactions. Not surprisingly, defects or mutations in any one of these interacting components can impact the physical and biological properties on the entire complex. Here we use quantum dots to image the diffusion of individual band 3 molecules in the plasma membranes of intact human erythrocytes from healthy volunteers and patients with defects in one of their membrane components, leading to well-known red cell pathologies (hereditary spherocytosis, hereditary elliptocytosis, hereditary hydrocytosis, Southeast Asian ovalocytosis, and hereditary pyropoikilocytosis). After characterizing the motile properties of the major subpopulations of band 3 in intact normal erythrocytes, we demonstrate that the properties of these subpopulations of band 3 change significantly in diseased cells, as evidenced by changes in the microscopic and macroscopic diffusion coefficients of band 3 and in the compartment sizes in which the different band 3 populations can diffuse. Because the above membrane abnormalities largely arise from defects in other membrane components (eg, spectrin, ankyrin), these data suggest that single particle tracking of band 3 might constitute a useful tool for characterizing the general structural integrity of the human erythrocyte membrane.

Clinical and molecular characterization of two patients with a 6.75 Mb overlapping deletion in 8p12p21 with two candidate loci for congenital heart defects

Clinical and molecular characteristics of two patients with a 6.75Mb overlapping interstitial deletion in the 8p12p21 region are described and compared with previously reported cases with an overlapping deletion. The most common characteristics of interstitial deletions of proximal 8p are developmental delay, postnatal microcephaly and growth retardation. Other frequently reported findings are hypogonadism associated with haploinsufficiency of GNRH1 and ocular problems. Congenital heart anomalies are also common and might at least to some extent be due to haploinsufficiency of NKX2-6 or NRG1. The aforementioned clinical characteristics should be considered in the care of patients with a proximal interstitial 8p12p21 deletion.

Ankrd7, a novel gene specifically expressed in Sertoli cells and its potential roles in Sertoli cell maturation

The somatic Sertoli cells play an essential role in testis determination and spermatogenesis by providing nutrition and structural support. In the current study, we report on the novel Ankrd7 gene that contains five ankyrin repeat domains. This gene was specifically expressed in Sertoli cells and was regulated in a maturation-dependent manner. Its expression was restricted to testicular tissue, and its mRNA could be detected in testes at as early as 14 dpp (days post partum) using RT-PCR analysis. In both testicular tissue sections and in vitro cultured Sertoli cells, the Ankrd7 protein was localized to the nucleus of the Sertoli cell. Immuno-histochemistry and immunocytochemistry investigations showed that the protein was detectable in testicular tissues at 20 dpp, at which time Sertoli cells were gradually differentiating into their mature cellular form. These results suggest that Ankrd7 is probably involved in the process of Sertoli cell maturation and in spermatogenesis.

Red cell membrane: past, present, and future

As a result of natural selection driven by severe forms of malaria, 1 in 6 humans in the world, more than 1 billion people, are affected by red cell abnormalities, making them the most common of the inherited disorders. The non-nucleated red cell is unique among human cell type in that the plasma membrane, its only structural component, accounts for all of its diverse antigenic, transport, and mechanical characteristics. Our current concept of the red cell membrane envisions it as a composite structure in which a membrane envelope composed of cholesterol and phospholipids is secured to an elastic network of skeletal proteins via transmembrane proteins. Structural and functional characterization of the many constituents of the red cell membrane, in conjunction with biophysical and physiologic studies, has led to detailed description of the way in which the remarkable mechanical properties and other important characteristics of the red cells arise, and of the manner in which they fail in disease states. Current studies in this very active and exciting field are continuing to produce new and unexpected revelations on the function of the red cell membrane and thus of the cell in health and disease, and shed new light on membrane function in other diverse cell types.

Hereditary spherocytosis

Hereditary spherocytosis is a common inherited disorder that is characterised by anaemia, jaundice, and splenomegaly. It is reported worldwide and is the most common inherited anaemia in individuals of northern European ancestry. Clinical severity is variable with most patients having a well-compensated haemolytic anaemia. Some individuals are asymptomatic, whereas others have severe haemolytic anaemia requiring erythrocyte transfusion. The primary lesion in hereditary spherocytosis is loss of membrane surface area, leading to reduced deformability due to defects in the membrane proteins ankyrin, band 3, beta spectrin, alpha spectrin, or protein 4.2. Many isolated mutations have been identified in the genes encoding these membrane proteins; common hereditary spherocytosis-associated mutations have not been identified. Abnormal spherocytes are trapped and destroyed in the spleen and this is the main cause of haemolysis in this disorder. Common complications are cholelithiasis, haemolytic episodes, and aplastic crises. Splenectomy is curative but should be undertaken only after careful assessment of the risks and benefits.

Dysfunction in ankyrin-B-dependent ion channel and transporter targeting causes human sinus node disease

The identification of nearly a dozen ion channel genes involved in the genesis of human atrial and ventricular arrhythmias has been critical for the diagnosis and treatment of fatal cardiovascular diseases. In contrast, very little is known about the genetic and molecular mechanisms underlying human sinus node dysfunction (SND). Here, we report a genetic and molecular mechanism for human SND. We mapped two families with highly penetrant and severe SND to the human ANK2 (ankyrin-B/AnkB) locus. Mice heterozygous for AnkB phenocopy human SND displayed severe bradycardia and rate variability. AnkB is essential for normal membrane organization of sinoatrial node cell channels and transporters, and AnkB is required for physiological cardiac pacing. Finally, dysfunction in AnkB-based trafficking pathways causes abnormal sinoatrial node (SAN) electrical activity and SND. Together, our findings associate abnormal channel targeting with human SND and highlight the critical role of local membrane organization for sinoatrial node excitability.

Exon organization and novel alternative splicing of the human ANK2 gene: implications for cardiac function and human cardiac disease

Recent findings illustrate a critical role for ankyrin-B function in normal cardiovascular physiology. Specifically, decreased expression of ankyrin-B in mice or human mutations in the ankyrin-B gene (ANK2) results in potentially fatal cardiac arrhythmias. Despite the clear role of ankyrin-B in heart, the mechanisms underlying transcriptional regulation of ANK2 are unknown. In fact, to date there is no description of ANK2 genomic organization. The aims of this study were to provide a comprehensive description of the ANK2 gene and to evaluate the relative expression of alternative splicing events associated with ANK2 transcription in heart. Using reverse-transcriptase PCR on mRNA isolated from human hearts, we identify seven new exons associated with the ANK2 gene including an alternative first exon located approximately 145 kb upstream of the previously-identified first exon. In addition, we identify over thirty alternative splicing events associated with ANK2 mRNA transcripts. Using real-time PCR and exon boundary-spanning primers to selectively amplify these splice variants, we demonstrate that these variants are expressed at varying levels in human heart. Finally, ankyrin-B immunoblot analysis demonstrates the expression of a heterogeneous population of ankyrin-B polypeptides in heart. ANK2 consists of 53 exons that span approximately 560 kb on human chromosome 4. Additionally, our data demonstrates that ANK2 is subject to complex transcriptional regulation that likely results in differential ankyrin-B polypeptide function.

In situ hybridization to metaphase chromosomes and interphase nuclei

In situ hybridization is used to determine the chromosomal map location and the relative order of genes and DNA sequences within a chromosomal band. It can also be used to detect aneuploidy, gene amplification, and subtle chromosomal rearrangements. Fluorescence in situ hybridization (FISH), probably the most widely used method, is described in the first basic protocol. Two support protocols are provided to amplify weak fluorescent signals obtained in FISH. Nonisotopic probes can also be detected by enzymatic reactions using horseradish peroxidase or alkaline phosphatase, as described in alternate protocols. Nonisotopic labeling of DNA probes by nick translation is described in a support protocol. The order of closely spaced FISH probes along chromosomes in interphase nuclei can be determined. A basic protocol for isotopic in situ hybridization (IISH) with (3)H is provided followed by a support protocol for preparation of autoradiographic emulsion.

An 11-amino acid beta-hairpin loop in the cytoplasmic domain of band 3 is responsible for ankyrin binding in mouse erythrocytes

The best-studied cytoskeletal system is the inner surface of the erythrocyte membrane, which provides an erythrocyte with the structural support needed to be stable yet flexible as it passes through the circulation. Current structural models predict that the spectrin-actin-based cytoskeletal network is attached to the plasma membrane through interactions of the protein ankyrin, which binds to both spectrin and the cytoplasmic domain of the transmembrane protein band 3. The crystal structure of the cytoplasmic domain of band 3 predicted that the ankyrin binding site was located on a beta-hairpin loop in the cytoplasmic domain. In vitro, deletion of this loop eliminated ankyrin affinity for band 3 without affecting any other protein-band 3 interaction. To evaluate the importance of the ankyrin-band 3 linkage to membrane properties in vivo, we generated mice with the nucleotides encoding the 11-aa beta-hairpin loop in the mouse Slc4a1 gene replaced with sequence encoding a diglycine bridge. Mice homozygous for the loop deletion were viable with mildly spherocytic and osmotically fragile erythrocytes. In vitro, homozygous ld/ld erythrocytes were incapable of binding ankyrin, but contrary to all previous predictions, abolishing the ankyrin-band 3 linkage destabilized the erythrocyte membrane to a lesser degree than complete deficiencies of either band 3 or ankyrin. Our data indicate that as yet uncharacterized interactions between other membrane proteins must significantly contribute to linkage of the spectrin-actin-based membrane cytoskeleton to the plasma membrane.

Erythrocyte disorders in the perinatal period

Anemia is a commonly encountered problem in the fetal and neonatal period, and can lead to significant morbidity and mortality. Intrinsic disorders of the erythrocyte, such as the hemoglobinopathies, enzyme deficiencies, and membrane defects are common causes of neonatal anemia. Genetic diseases that lead to decreased erythrocyte production, such as Diamond-Blackfan anemia, Schwachman-Diamond syndrome, and Congential Dyserythropoietic Anemia, are rare causes of perinatal anemia, but are important to recognize as they are often associated with other congenital abnormalities and require specialized treatment. This review focuses on the perinatal presentation and management of intrinsic erythrocyte disorders, as well as on the diagnosis and management of genetic conditions leading to erythrocyte underproduction.

Vision impairment with an interstitial deletion of the short arm of chromosome 8

We describe the phenotype of a male infant with an interstitial deletion of the short arm of chromosome 8 (p. 11.2-p. 21). Visual impairment is a major feature in this case. The clinical, radiographic and electrodiagnostic findings are presented. Only four other cases have been reported in which visual problems are associated with a deletion of 8 p.

Genomic organization, differential expression, and interaction of SQUAMOSA promoter-binding-like transcription factors and microRNA156 in rice

Transcription factors play essential roles in the developmental processes of plants. Many such factors are regulated by microRNAs (miRNAs). SQUAMOSA (SQUA) promoter-binding-like (SPL) genes encode plant-specific transcription factors, some of which contain complementary sequences of miRNA156. In this study, 19 rice (Oryza sativa) SPL (OsSPL) genes and 12 rice miRNA156 (OsmiR156) precursors were identified in the rice genome. Sequence and experimental analysis suggested that 11 OsSPL genes were putative targets of OsmiR156. Plant SPL proteins were classified into six subgroups based on the phylogenetic analysis of SQUA promoter-binding protein domain. Diverse exon-intron structures and distinct organizations of putative motifs beyond the SQUA promoter-binding protein domains were identified in the OsSPL gene family. Transcript level analysis of OsSPL genes in various rice tissues and organs revealed different tempospatial expression patterns. More than half of the OsSPL genes including most OsmiR156-targeted genes are predominantly expressed in the young panicles, whereas OsmiR156 genes are predominantly expressed in the young shoots and leaves of rice. Overexpression of two OsmiR156 genes (OsmiR156b and OsmiR156h) in rice resulted in severe dwarfism, strongly reduced panicle size, and delayed flowering, suggesting that OsmiR156 and OsSPL target genes are involved in various developmental processes, especially the flower development of rice. Different patterns of transcript changes (decreased or unchanged) of different target genes in same tissue and of same target gene in different tissues detected in the OsmiR156-overexpressing plants suggested diverse interactions between OsmiR156 and OsSPL target genes in a tissue-specific manner.

Molecular mechanisms of cellular mechanics

Mechanical forces play an essential role in cellular processes as input, output, and signals. Various protein complexes in the cell are designed to handle, transform and use such forces. For instance, proteins of muscle and the extracellular matrix can withstand considerable stretching forces, hearing-related and mechanosensory proteins can transform weak mechanical stimuli into electrical signals, and regulatory proteins are suited to forcing DNA into loops to control gene expression. Here we review the structure-function relationship of four protein complexes with well defined and representative mechanical functions. The first example is titin, a protein that confers passive elasticity on muscle. The second system is the elastic extracellular matrix protein, fibronectin, and its cellular receptor integrin. The third protein system is the transduction apparatus in hearing and other mechanical senses, likely containing cadherin and ankyrin repeats. The last system is the lac repressor protein, which regulates gene expression by looping DNA. This review focuses on atomic level descriptions of the physical mechanisms underlying the various mechanical functions of the stated proteins.

In Search of the Hair-Cell Gating Spring

Mechanotransduction in vertebrate hair cells involves a biophysically defined elastic element (the "gating spring") that pulls on the transduction channels. The tip link, a fine filament made of cadherin 23 linking adjacent stereocilia in hair-cell bundles, has been suggested to be the gating spring. However, TRP channels that mediate mechanotransduction in Drosophila, zebrafish, and mice often have cytoplasmic domains containing a large number of ankyrin repeats that are also candidates for the gating spring. We have explored the elastic properties of cadherin and ankyrin repeats through molecular dynamics simulations using crystallographic structures of proteins with one cadherin repeat or 4 and 12 ankyrin repeats, and using models of 17 and 24 ankyrin repeats. The extension and stiffness of large ankyrin-repeat structures were found to match those predicted by the gating-spring model. Our results suggest that ankyrin repeats of TRPA1 and TRPN1 channels serve as the gating spring for mechanotransduction.

Spectrin, alpha-actinin, and dystrophin

Spectrin family proteins represent an important group of actin-bundling and membrane-anchoring proteins found in diverse structures from yeast to man. Arising from a common ancestral alpha-actinin gene through duplications and rearrangements, the family has increased to include the spectrins and dystrophin/utrophin. The spectrin family is characterized by the presence of spectrin repeats, actin binding domains, and EF hands. With increasing divergence, new domains and functions have been added such that spectrin and dystrophin also contain specialized protein-protein interaction motifs and regions for interaction with membranes and phospholipids. The acquisition of new domains also increased the functional complexity of the family such that the proteins perform a range of tasks way beyond the simple bundling of actin filaments by alpha-actinin in S. pombe. We discuss the evolutionary, structural, functional, and regulatory roles of the spectrin family of proteins and describe some of the disease traits associated with loss of spectrin family protein function.