From anemia to cerebellar dysfunction. A review of the ankyrin gene family

The spectrin cytoskeleton integrates endothelial mechanoresponses

Physiological blood flow induces the secretion of vasoactive compounds, notably nitric oxide, and promotes endothelial cell elongation and reorientation parallel to the direction of applied shear. How shear is sensed and relayed to intracellular effectors is incompletely understood. Here, we demonstrate that an apical spectrin network is essential to convey the force imposed by shear to endothelial mechanosensors. By anchoring CD44, spectrins modulate the cell surface density of hyaluronan and sense and translate shear into changes in plasma membrane tension. Spectrins also regulate the stability of apical caveolae, where the mechanosensitive PIEZO1 channels are thought to reside. Accordingly, shear-induced PIEZO1 activation and the associated calcium influx were absent in spectrin-deficient cells. As a result, cell realignment and flow-induced endothelial nitric oxide synthase stimulation were similarly dependent on spectrin. We conclude that the apical spectrin network is not only required for shear sensing but also transmits and distributes the resulting tensile forces to mechanosensors that elicit protective and vasoactive responses.

Ankyrins: Roles in synaptic biology and pathology

Ankyrins are broadly expressed adaptors that organize diverse membrane proteins into specialized domains and link them to the sub-membranous cytoskeleton. In neurons, ankyrins are known to have essential roles in organizing the axon initial segment and nodes of Ranvier. However, recent studies have revealed novel functions for ankyrins at synapses, where they organize and stabilize neurotransmitter receptors, modulate dendritic spine morphology and control adhesion to the presynaptic site. Ankyrin genes have also been highly associated with a range of neurodevelopmental and psychiatric diseases, including bipolar disorder, schizophrenia and autism, which all demonstrate overlap in their genetics, mechanisms and phenotypes. This review discusses the novel synaptic functions of ankyrin proteins in neurons, and places these exciting findings in the context of ANK genes as key neuropsychiatric disorder risk-factors.

Ankyrin-Like Protein AnkB Interacts with CatB, Affects Catalase Activity, and Enhances Resistance of Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola to Phenazine-1-Carboxylic Acid

Xanthomonas oryzae pv. oryzae, which causes rice bacterial leaf blight, and Xanthomonas oryzae pv. oryzicola, which causes rice bacterial leaf streak, are important plant-pathogenic bacteria. A member of the adaptor protein family, ankyrin protein, has been investigated largely in humans but rarely in plant-pathogenic bacteria. In this study, a novel ankyrin-like protein, AnkB, was identified in X. oryzae pv. oryzae and X. oryzae pv. oryzicola. The expression of ankB was significantly upregulated when these bacteria were treated with phenazine-1-carboxylic acid (PCA). ankB is located 58 bp downstream of the gene catB (which encodes a catalase) in both bacteria, and the gene expression of catB and catalase activity were reduced following ankB deletion in X. oryzae pv. oryzae and X. oryzae pv. oryzicola. Furthermore, we demonstrated that AnkB directly interacts with CatB by glutathione S-transferase (GST) pulldown assays. Deletion of ankB increased the sensitivity of X. oryzae pv. oryzae and X. oryzae pv. oryzicola to H₂O₂ and PCA, decreased bacterial biofilm formation, swimming ability, and exopolysaccharide (EPS) production, and also reduced virulence on rice. Together our results indicate that the ankyrin-like protein AnkB has important and conserved roles in antioxidant systems and pathogenicity in X. oryzae pv. oryzae and X. oryzae pv. oryzicola.IMPORTANCE This study demonstrates that the ankyrin protein AnkB directly interacts with catalase CatB in Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola. Ankyrin protein AnkB can affect the gene expression of catB, catalase activity, and sensitivity to H₂O₂ In Xanthomonas spp., the locations of genes ankB and catB and the amino acid sequence of AnkB are highly conserved. It is suggested that in prokaryotes, AnkB plays a conserved role in the defense against oxidative stress.

Association of ANK1 variants with new-onset type 2 diabetes in a Han Chinese population from northeast China

Previous studies have identified three loci (rs4737009, rs515071 and rs516946) in ankyrin 1 (ANK1) that are associated with type 2 diabetes mellitus (T2DM) in a number of ethnic groups. However, the impact of single nucleotide polymorphisms (SNPs) of ANK1 on T2DM in a Han Chinese population from northeast China has not yet been studied. The present study was undertaken to investigate the relationship between the ANK1 gene and new-onset T2DM in northeastern China. Three widely studied variants were genotyped and analyzed for T2DM susceptibility in 1,962 Chinese subjects (996 with T2DM and 966 healthy controls). Genotyping was performed using SNPscan™. The single-locus analysis, identified differences in the expression of rs515071 and rs516946 between cases and controls, with an odds ratio (OR) of 1.31 [95% confidence interval (CI), 1.10-1.55; P=0.002] and 1.32 (95% CI, 1.09-1.61; P=0.005) respectively, while there were no differences in the expression of rs4737009 between the groups. For the SNP of rs515071, the presence of AA or GA significantly reduced the risk of T2DM compared with GG (adjusted P=0.019, OR=0.78; 95% CI, 0.63-0.96). With respect to rs516946, individuals carrying TT or CT exhibited a decreased risk of T2DM compared with those with the CC allele (adjusted P=0.040, OR=0.79; 95% CI, 0.63-0.99). Furthermore, haplotype analysis indicated that the haplotype frequency of GC in T2DM cases was significantly higher than in controls (P=0.002, OR=1.31; 95% CI, 1.10-1.55). Furthermore, the rs516946-CC genotype was associated with a larger waist circumference (P=0.031). The present data indicated that ANK1 was a potential T2DM susceptibility gene in a Han Chinese population from northeastern China.

Ankyrin G expression is associated with androgen receptor stability, invasiveness, and lethal outcome in prostate cancer patients

Ankyrin G (ANK3) is a member of the Ankyrin family, which functions to provide cellular stability by anchoring the cytoskeleton to the plasma membrane. Deregulation of ANK3 expression has been observed in multiple human cancers but its mechanism remains unknown. ANK3 expression in relation to disease progression and patients' outcome was investigated in two cohorts of prostate cancer (PCA). Mechanistic studies were carried out in vitro and in vivo using several PCA cell lines and the avian embryo model. Silencing ANK3 resulted in significant reduction of cell proliferation through an AR-independent mechanism. Decreased ANK3 expression delayed S phase to G2/M cell cycle transition and reduced the expression of cyclins A and B. However, cells with knocked-down ANK3 exhibited significant increase in cell invasion through an AR-dependent mechanism. Furthermore, we found that ANK3 is a regulator of AR protein stability. ANK3 knockdown also promoted cancer cell invasion and extravasations in vivo using the avian embryo model (p < 0.01). In human samples, ANK3 expression was dramatically upregulated in high grade intraepithelial neoplasia (HGPIN) and localized PCA (p < 0.0001). However, it was downregulated castration resistant stage (p < 0.0001) and showed inverse relation to Gleason score (p < 0.0001). In addition, increased expression of ANK3 in cancer tissues was correlated with better cancer-specific survival of PCA patients (p = 0.012).

KEY MESSAGE

Silencing ANK3 results in significant reduction of cell proliferation through an AR-independent mechanism. ANK3 knockdown results in significant increase in cell invasion through an AR-dependent mechanism. ANK3 is a regulator of AR protein stability. ANK3 knockdown also promotes cancer cell invasion and extravasation in vivo using the avian embryo model.

Bioinformatics approach to predict target genes for dysregulated microRNAs in hepatocellular carcinoma: study on a chemically-induced HCC mouse model

Background

Hepatocellular carcinoma (HCC) is an aggressive epithelial tumor which shows very poor prognosis and high rate of recurrence, representing an urgent problem for public healthcare. MicroRNAs (miRNAs/miRs) are a class of small, non-coding RNAs that attract great attention because of their role in regulation of processes such as cellular growth, proliferation, apoptosis. Because of the thousands of potential interactions between a single miR and target mRNAs, bioinformatics prediction tools are very useful to facilitate the task for individuating and selecting putative target genes. In this study, we present a chemically-induced HCC mouse model to identify differential expression of miRNAs during the progression of the hepatic injury up to HCC onset. In addition, we describe an established bioinformatics approach to highlight putative target genes and protein interaction networks where they are involved.

Results

We describe four miRs (miR-125a-5p, miR-27a, miR-182, miR-193b) which showed to be differentially expressed in the chemically-induced HCC mouse model. The miRs were subjected to four of the most used predictions tools and 15 predicted target genes were identified. The expression of one (ANK3) among the 15 predicted targets was further validated by immunoblotting. Then, enrichment annotation analysis was performed revealing significant clusters, including some playing a role in ion transporter activity, regulation of receptor protein serine/threonine kinase signaling pathway, protein import into nucleus, regulation of intracellular protein transport, regulation of cell adhesion, growth factor binding, and regulation of TGF-beta/SMAD signaling pathway. A network construction was created and links between the selected miRs, the predicted targets as well as the possible interactions among them and other proteins were built up.

Conclusions

In this study, we combined miRNA expression analysis, obtained by an in vivo HCC mouse model, with a bioinformatics-based workflow. New genes, pathways and protein interactions, putatively involved in HCC initiation and progression, were identified and explored.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-015-0836-1) contains supplementary material, which is available to authorized users.

A transcription blocker isolated from a designed repeat protein combinatorial library by in vivo functional screen

A highly diverse DNA library coding for ankyrin seven-repeat proteins (ANK-N5C) was designed and constructed by a PCR-based combinatorial assembly strategy. A bacterial melibiose fermentation assay was adapted for in vivo functional screen. We isolated a transcription blocker that completely inhibits the melibiose-dependent expression of α-galactosidase (MelA) and melibiose permease (MelB) of Escherichiacoli by specifically preventing activation of the melAB operon. High-resolution crystal structural determination reveals that the designed ANK-N5C protein has a typical ankyrin fold, and the specific transcription blocker, ANK-N5C-281, forms a domain-swapped dimer. Functional tests suggest that the activity of MelR, a DNA-binding transcription activator and a member of AraC family of transcription factors, is inhibited by ANK-N5C-281 protein. All ANK-N5C proteins are expected to have a concave binding area with negative surface potential, suggesting that the designed ANK-N5C library proteins may facilitate the discovery of binders recognizing structural motifs with positive surface potential, like in DNA-binding proteins. Overall, our results show that the established library is a useful tool for the discovery of novel bioactive reagents.

Ankyrin 3: genetic association with bipolar disorder and relevance to disease pathophysiology

Bipolar disorder (BD) is a multi-factorial disorder caused by genetic and environmental influences. It has a large genetic component, with heritability estimated between 59-93%. Recent genome-wide association studies (GWAS) using large BD patient populations have identified a number of genes with strong statistical evidence for association with susceptibility for BD. Among the most significant and replicated genes is ankyrin 3 (ANK3), a large gene that encodes multiple isoforms of the ankyrin G protein. This article reviews the current evidence for genetic association of ANK3 with BD, followed by a comprehensive overview of the known biology of the ankyrin G protein, focusing on its neural functions and their potential relevance to BD. Ankyrin G is a scaffold protein that is known to have many essential functions in the brain, although the mechanism by which it contributes to BD is unknown. These functions include organizational roles for subcellular domains in neurons including the axon initial segment and nodes of Ranvier, through which ankyrin G orchestrates the localization of key ion channels and GABAergic presynaptic terminals, as well as creating a diffusion barrier that limits transport into the axon and helps define axo-dendritic polarity. Ankyrin G is postulated to have similar structural and organizational roles at synaptic terminals. Finally, ankyrin G is implicated in both neurogenesis and neuroprotection. ANK3 and other BD risk genes participate in some of the same biological pathways and neural processes that highlight several mechanisms by which they may contribute to BD pathophysiology. Biological investigation in cellular and animal model systems will be critical for elucidating the mechanism through which ANK3 confers risk of BD. This knowledge is expected to lead to a better understanding of the brain abnormalities contributing to BD symptoms, and to potentially identify new targets for treatment and intervention approaches.

Combination of a pesticide exposure and a bacterial challenge: in vivo effects on immune response of Pacific oyster, Crassostrea gigas (Thunberg)

To assess the impact of pollution induced by pesticides on Pacific oyster, Crassostrea gigas, health in France, in vivo effects of combined pesticide exposure and bacterial challenge on cell activities and gene expression in hemocytes were tested using flow cytometry and real-time PCR. As a first step, an in vivo model of experimental contamination was developed. Pacific oysters were exposed to a mixture of eight pesticides (atrazine, glyphosate, alachlor, metolachlor, fosetyl-alumimium, terbuthylazine, diuron and carbaryl) at environmentally relevant concentrations over a 7-day period. Hemocyte parameters (cell mortality, enzyme activities and phagocytosis) were monitored using flow cytometry and gene expression was evaluated by real-time PCR (RT-PCR). The expression of 19 genes involved in C. gigas hemocyte functions was characterized using RT-PCR. After 7 days of exposure, phagocytosis was significantly reduced and the 19 selected genes were down-regulated in treated animals. As a second step, the experimental contamination method previously developed was used to study interactions between pesticide exposure and bacterial challenge by intramuscular injection of two Vibrio splendidus-related pathogenic strains. Oyster mortality and expression of 10 of the 19 selected genes were followed 4 and 24h post-injection. Oyster mortality was higher in pesticide-treated oysters compared to untreated oysters after the bacterial challenge. Gene expression was up-regulated in pesticide-treated oysters compared to untreated oysters after the bacterial challenge. We hypothesize that gene over-expression due to an interaction between pesticides and bacteria could lead to an injury of host tissues, resulting in higher mortality rates. In conclusion, this study is the first to show effects of pesticides at environmentally relevant concentrations on C. gigas hemocytes and to hypothesize that pesticides modulate the immune response to a bacterial challenge in oysters.

The Smaller Isoforms of Ankyrin 3 Bind to the p85 Subunit of Phosphatidylinositol 3′-Kinase and Enhance Platelet-derived Growth Factor Receptor Down-regulation

The Src homology 2 (SH2) domains of the p85 subunit of phosphatidylinositol 3'-kinase have been shown to bind to the tyrosine-phosphorylated platelet-derived growth factor receptor (PDGFR). Previously, we have demonstrated that p85 SH2 domains can also bind to the serine/threonine kinase A-Raf via a unique phosphorylation-independent interaction. In this report, we describe a new phosphotyrosine-independent p85 SH2-binding protein, ankyrin 3 (Ank3). In general, ankyrins serve a structural role by binding to both integral membrane proteins at the plasma membrane and spectrin/fodrin proteins of the cytoskeleton. However, smaller isoforms of Ank3 lack the membrane domain and are localized to late endosomes and lysosomes. We found that p85 binds directly to these smaller 120- and 105-kDa Ank3 isoforms. Both the spectrin domain and the regulatory domain of Ank3 are involved in binding to p85. At least two domains of p85 can bind to Ank3, and the interaction involving the p85 C-SH2 domain was found to be phosphotyrosine-independent. Overexpression of the 120- or 105-kDa Ank3 proteins resulted in significantly enhanced PDGFR degradation and a reduced ability to proliferate in response to PDGF. Ank3 overexpression also differentially regulated signaling pathways downstream from the PDGFR. Chloroquine, an inhibitor of lysosomal-mediated degradation pathways, blocked the ability of Ank3 to enhance PDGFR degradation. Immunofluorescence experiments demonstrated that both small Ank3 isoforms colocalized with the lysosomal-associated membrane protein and with p85 and the PDGFR. These results suggest that Ank3 plays an important role in lysosomal-mediated receptor down-regulation, likely through a p85-Ank3 interaction.

Cell death in Porifera: molecular players in the game of apoptotic cell death in living fossils

Apoptosis represents the morphological manifestation of programmed cell death and, paradoxically at first sight, it is a prerequisite for metazoan life. Thus, apoptosis is responsible for the demise of cells during many physiological processes. It is also accountable for the death of cells following exposure to countless stimuli. Therefore, it is obvious that apoptosis must be regulated by a complex network of various molecular signaling pathways. Research during the past 20 years has led to the identification of major functional groups of molecules involved in apoptotic pathways. These include members of the Bcl-2 superfamily, members of the TNF family, caspases, and their activators. Yet, the evolutionary conservation of those elements of the apoptotic machinery was only established from nematode to man. Sponges (phylum Porifera) are characterized by a remarkable regeneration capacity and longevity. Furthermore, they represent the phylogenetically oldest still extant metazoan taxon. Thus, research on these living fossils opens a window to the past, to the dawn of metazoan life. It allows us to trace the evolution of programmed cell death and its core components. This review summarizes the key findings and concepts which have emerged from studies of apoptosis in Porifera.

Novel UNC-44 AO13 ankyrin is required for axonal guidance in C. elegans, contains six highly repetitive STEP blocks separated by seven potential transmembrane domains, and is localized to neuronal processes and the periphery of neural cell bodies

Conventional ankyrins are cortical cytoskeletal proteins that form an ankyrin-spectrin meshwork underlying the plasma membrane. We report here the unusual structure of a novel ankyrin (AO13 ankyrin, 775,369 Da, 6994 aa, pI = 4.45) that is required for proper axonal guidance in Caenorhabditis elegans. AO13 ankyrin contains the ANK repeat and spectrin-binding domains found in other ankyrins, but differs from all others in that the acidic carboxyl region contains six blocks of serine/threonine/glutamic acid/proline rich (STEP) repeats separated by seven hydrophobic domains. The STEP repeat blocks are composed primarily of sequences related to ETTTTTTVTREHFEPED(E/D)X(n)VVESEEYSASGSPVPSE (E/K)DVE(H/R)VI, and the hydrophobic domains contain sequences related to PESGEESDGEGFGSKVLGFAKK[AGMVAGGVVAAPVALAAVGA]KAAYDALKKDDDEE, which includes a potential transmembrane domain (in brackets). Recombinant protein fragments of AO13 ankyrin were used to prepare polyclonal antisera against the spectrin-binding domain (AO271 Ab), the conventional ankyrin regulatory domain (AO280 Ab), the AO13 ankyrin STEP domain (AO346 Ab), the AO13 ankyrin STEP + hydrophobic domain (AO289 Ab), and against two carboxyl terminal domain fragments (AO263 Ab and AO327 Ab). Western blot analysis with these Ab probes demonstrated multiple protein isoforms. By immunofluorescence microscopy, the antispectrin-binding and regulatory domain (AO271 and AO280) antibodies recognized many cell types, including neurons, and stained the junctions between cells. The AO13 ankyrin-specific (AO289 and AO346) antibodies showed a neurally restricted pattern, staining nerve processes and the periphery of neural cell bodies. These results are consistent with a role for AO13 ankyrin in neural development.

Contribution of sponge genes to unravel the genome of the hypothetical ancestor of Metazoa (Urmetazoa)

Recently the term Urmetazoa, as the hypothetical metazoan ancestor, was introduced to highlight the finding that all metazoan phyla including the Porifera (sponges) are derived from one common ancestor. Sponges as the evolutionarily oldest, still extant phylum, are provided with a complex network of structural and functional molecules. Analyses of sponge genomes from Demospongiae (Suberites domuncula and Geodia cydonium), Calcarea (Sycon raphanus) and Hexactinellida (Aphrocallistes vastus) have contributed also to the reconstruction of the evolutionary position of Metazoa with respect to Fungi. Furthermore, these analyses have provided evidence that the characteristic evolutionary novelties of Metazoa, such as the extracellular matrix molecules, the cell surface receptors, the nervous signal transduction molecules as well as the immune molecule existing in Porifera, share high sequence and in some aspects also functional similarities to related polypeptides found in other metazoan phyla. During the transition to Metazoa new domains occurred; as one example, the formation of the death domain from the ankyrin is outlined. In parallel, domanial proteins have been formed, such as the receptor tyrosine kinases. The metazoan essentials have been defined by analyzing and comparing the sponge sequences with the related sequences from the metazoans Homo sapiens, Caenorhabditis elegans and Drosophila melanogaster, the fungus Saccharomyces cerevisiae and the plant Arabidopsis thaliana. The data revealed that those sponge molecules grouped to cell adhesion cell recognition proteins are predominantly found in Protostomia and Deuterostomia while they are missing in Fungi and Viridiplantae. Moreover, evidence is presented allowing the conclusion that the sponge molecules are more closely related to the corresponding molecules from H. sapiens than to those of C. elegans or D. melanogaster. Especially surprising was the finding that the Demospongiae are provided with elements of adaptive immunity.

Caspase remodeling of the spectrin membrane skeleton during lens development and aging

Terminal differentiation of lens fiber cells resembles the apoptotic process in that organelles are lost, DNA is fragmented, and changes in membrane morphology occur. However, unlike classically apoptotic cells, which are disintegrated by membrane blebbing and vesiculation, aging lens fiber cells are compressed into the center of the lens, where they undergo cell-cell fusion and the formation of specialized membrane interdigitations. In classically apoptotic cells, caspase cleavage of the cytoskeletal protein alpha-spectrin to approximately 150-kDa fragments is believed to be important for membrane blebbing. We report that caspase(s) cleave alpha-spectrin to approximately 150-kDa fragments and beta-spectrin to approximately 120- and approximately 80-kDa fragments during late embryonic chick lens development. These fragments continue to accumulate with age so that in the oldest fiber cells of the adult lens, most, if not all, of the spectrin is cleaved to discrete fragments. Thus, unlike classical apoptosis, where caspase-cleaved spectrin is short lived, lens fiber cells contain spectrin fragments that appear to be stable for the lifetime of the organism. Moreover, fragmentation of spectrin results in reduced membrane association and thus may lead to permanent remodeling of the membrane skeleton. Partial and specific proteolysis of membrane skeleton components by caspases may be important for age-related membrane changes in the lens.

Ankyrin gene mutations in japanese patients with hereditary spherocytosis

We studied mutations of the ankyrin-1 (ANK-1) gene of genomic DNA from Japanese patients with hereditary spherocytosis (HS). Forty-nine patients from 46 unrelated families were included in this study. Of these patients, 19 cases from 16 unrelated families had HS of autosomal-dominant inheritance, and 30 patients had non-autosomal-dominant HS. Fifteen mutations of the ANK-1 gene pathognomonic for HS were identified: 4 nonsense mutations, 7 frameshift mutations, and 4 abnormal splicing mutations. These 15 mutations have not been previously reported. The frameshift mutations were found from exon 1 to exon 26, corresponding particularly to the band 3-binding domain of ankyrin. The nonsense mutations, on the contrary, were present mostly at the 3'-terminal side, especially in the spectrin-binding domain and the regulatory domain. The patients with ankyrin gene mutations tended to be more anemic with a higher level of reticulocytosis than those without these mutations. Fifteen silent mutations of the ANK-1 gene, most of which have previously been detected in HS patients in Western populations, were also found. The allele frequency of these silent mutations in the HS patients was nearly identical to that in normal subjects. There was no difference between the Japanese and Western populations in the allele frequency of these gene polymorphisms in healthy subjects or HS patients.

A minimal ankyrin promoter linked to a human gamma-globin gene demonstrates erythroid specific copy number dependent expression with minimal position or enhancer dependence in transgenic mice

In red blood cells ankyrin (ANK-1) provides the primary linkage between the erythrocyte membrane skeleton and the plasma membrane. We have previously demonstrated that a 271-bp 5'-flanking region of the ANK-1 gene has promoter activity in erythroid, but not non-erythroid, cell lines. To determine whether the ankyrin promoter could direct erythroid-specific expression in vivo, we analyzed transgenic mice containing the ankyrin promoter fused to the human (A)gamma-globin gene. Sixteen of 17 lines expressed the transgene in erythroid cells indicating nearly position-independent expression. We also observed a significant correlation between the level of Ank/(A)gamma-globin mRNA and transgene copy number. The level of Ank/(A)gamma mRNA averaged 11% of mouse alpha-globin mRNA per gene copy at all developmental stages. The addition of the HS2 enhancer from the beta-globin locus control region to the Ank/(A)gamma-globin transgene resulted in Ank/(A)gamma-globin mRNA expression in embryonic and fetal erythroid cells in six of eight lines but resulted in absent or dramatically reduced levels of Ank/(A)gamma-globin mRNA in adult erythroid cells in eight of eight transgenic lines. These data indicate that the minimal ankyrin promoter contains all sequences necessary and sufficient for erythroid-specific, copy number-dependent, position-independent expression of the human (A)gamma-globin gene.

The spectrin-based skeleton at the postsynaptic membrane of the neuromuscular junction

Membrane skeletons, in particular the spectrin-based skeleton, are thought to participate in the organization of specialized membrane domains by restricting integral proteins to specific membrane sites. In the neuromuscular junction, discrete isoforms of spectrin and ankyrin, the peripheral protein that links spectrin to the membrane, colocalize with voltage-dependent sodium channels and N-CAM at the troughs of the postsynaptic membrane folds. Moreover, beta-spectrin, N-CAM, and sodium channels become clustered at the endplate during a period of time coincident with postsynaptic fold formation and synapse maturation. These observations suggest a role of the spectrin skeleton in directing and maintaining postsynaptic accumulations of sodium channels and N-CAM. In addition, the coexistence of spectrin and dystrophin at the troughs of the junctional folds raises the question of their respective functions in this membrane domain, where both cytoskeletal proteins have the potential to associate with sodium channels via ankyrin and syntrophin, respectively. Possible scenarios are discussed here with respect to accumulating evidence from studies of assembly of similar membrane domains in neurons.

The MSN1 and NHP6A genes suppress SWI6 defects in Saccharomyces cerevisiae

Ankyrin (ANK) repeats were first found in the Swi6 transcription factor of Saccharomyces cerevisiae and since then were identified in many proteins of eukaryotes and prokaryotes. These repeats are thought to serve as protein association domains. In Swi6, ANK repeats affect DNA binding of both the Swi4/Swi6 and Mbp1/Swi6 complexes. We have previously described generation of random mutations within the ANK repeats of Swi6 that render the protein temperature sensitive in its ability to activate HO transcription. Two of these SWI6 mutants were used in a screen for high copy suppressors of this phenotype. We found that MSN1, which encodes a transcriptional activator, and NHP6A, which encodes an HMG-like protein, are able to suppress defective Swi6 function. Both of these gene products are involved in HO transcription, and Nhp6A may also be involved in CLN1 transcription. Moreover, because overexpression of NHP6A can suppress caffeine sensitivity of one of the SWI6 ANK mutants, swi6-405, other SWI6-dependent genes may also be affected by Nhp6A. We hypothesize that Nhp6A and Msn1 modulate Swi6-dependent gene transcription indirectly, through effects on chromatin structure or other transcription factors, because we have not been able to demonstrate that either Msn1 or Nhp6A interact with the Swi4/Swi6 complex.

Polarization of the Na+, K(+)-ATPase in epithelia derived from the neuroepithelium

The neuroepithelium generates a fascinating group of epithelia. One of their intriguing properties is how they polarize the distribution of the Na+, K(+)-ATPase. Typically, this ion pump is concentrated in the basolateral membrane, but it is concentrated in the apical membranes of the retinal pigment epithelium and the epithelium of the choroid plexus. A comparison of their development with that of systemic epithelia yields insights into how cells polarize the distribution of this and other membrane proteins. The polarization of the Na+, K(+)-ATPase depends upon the interplay between different sorting signals and different types of polarity mechanisms. These include intracellular targeting signals that direct the delivery of newly synthesized proteins, and maintenance signals that stabilize proteins in the proper membrane domain. Conflicting signals appear to be arranged in a hierarchy that can be rearranged as cells respond to certain environmental stimuli. Part of this response is mediated by changes in the distribution and composition of the cortical cytoskeleton.

Cloning and characterization of 4.1G (EPB41L2), a new member of the skeletal protein 4.1 (EPB41) gene family

The prototypical erythrocyte membrane skeletal protein 4.1 (HGMW-approved symbol EPB41), here designated 4.1R, is encoded by a large, complexly spliced gene located on human chromosome 1p32-p33. In this paper we report evidence for a second 4.1 gene, 4.1G (HGMW-approved symbol EPB41L2), which maps to human chromosome 6q23 and is widely expressed among human tissues. The complete nucleotide sequence of 4.1G cDNA predicts a 113-kDa protein that exhibits three regions of high homology to 4.1R, including the membrane binding domain, the spectrinactin binding domain, and the C-terminal domain. Interspersed among the shared domains are unique sequences that may define functional differences between 4.1R and 4.1G. Specific isoforms of 4.1R and 4.1G exhibit differential subcellular localizations. These results expand the 4.1 gene superfamily and demonstrate that the diverse cellular complement of 4.1 isoforms results from both alternative splicing and expression of distinct genes.

beta-Spectrin is colocalized with both voltage-gated sodium channels and ankyrinG at the adult rat neuromuscular junction

Voltage-gated sodium channels (VGSCs) are concentrated in the depths of the postsynaptic folds at mammalian neuromuscular junctions (NMJs) where they facilitate action potential generation during neuromuscular transmission. At the nodes of Ranvier and the axon hillocks of central neurons, VGSCs are associated with the cytoskeletal proteins, beta-spectrin and ankyrin, which may help to maintain the high local density of VGSCs. Here we show in skeletal muscle, using immunofluorescence, that beta-spectrin is precisely colocalized with both VGSCs and ankyrinG, the nodal isoform of ankyrin. In en face views of rat NMJs, acetylcholine receptors (AChRs), and utrophin immunolabeling are organized in distinctive linear arrays corresponding to the crests of the postsynaptic folds. In contrast, beta-spectrin, VGSCs, and ankyrinG have a punctate distribution that extends laterally beyond the AChRs, consistent with a localization in the depths of the folds. Double antibody labeling shows that beta-spectrin is precisely colocalized with both VGSCs and ankyrinG at the NMJ. Furthermore, quantification of immunofluorescence in labeled transverse sections reveals that beta-spectrin is also concentrated in perijunctional regions, in parallel with an increase in labeling of VGSCs and ankyrinG, but not of dystrophin. These observations suggest that interactions with beta-spectrin and ankyrinG help to maintain the concentration of VGSCs at the NMJ and that a common mechanism exists throughout the nervous system for clustering VGSCs at a high density.

An alternate promoter directs expression of a truncated, muscle-specific isoform of the human ankyrin 1 gene

Ankyrin 1, an erythrocyte membrane protein that links the underlying cytoskeleton to the plasma membrane, is also expressed in brain and muscle. We cloned a truncated, muscle-specific ankyrin 1 cDNA composed of novel 5' sequences and 3' sequences previously identified in the last 3 exons of the human ankyrin 1 erythroid gene. Northern blot analysis revealed expression restricted to cardiac and skeletal muscle tissues. Deduced amino acid sequence of this muscle cDNA predicted a peptide of 155 amino acids in length with a hydrophobic NH2 terminus. Cloning of the corresponding chromosomal gene revealed that the ankyrin 1 muscle transcript is composed of four exons spread over approximately 10 kilobase pairs of DNA. Reverse transcriptase-polymerase chain reaction of skeletal muscle cDNA identified multiple cDNA isoforms created by alternative splicing. The ankyrin 1 muscle promoter was identified as a (G + C)-rich promoter located > 200 kilobase pairs from the ankyrin 1 erythroid promoter. An ankyrin 1 muscle promoter fragment directed high level expression of a reporter gene in cultured C2C12 muscle cells, but not in HeLa or K562 (erythroid) cells. DNA-protein interactions were identified in vitro at a single Sp1 and two E box consensus binding sites contained within the promoter. A MyoD cDNA expression plasmid transactivated an ankyrin 1 muscle promoter fragment/reporter gene plasmid in a dose-dependent fashion in both HeLa and K562 cells. A polyclonal antibody raised to human ankyrin 1 muscle-specific sequences reacted with peptides of 28 and 30 kDa on immunoblots of human skeletal muscle.

Morphogenesis of the node of Ranvier: co-clusters of ankyrin and ankyrin-binding integral proteins define early developmental intermediates

AnkyrinG 480/270 kDa and three ankyrin-binding integral membrane proteins (neurofascin, NrCAM, and the voltage-dependent sodium channel) colocalize within a specialized domain of the spectrin-actin network found at axonal segments of nodes of Ranvier in myelinated axons. Before myelination in embryonic nerves, ankyrinG 480/270 kDa and the related ankyrin isoform ankyrinB 440 kDa are co-expressed along with NrCAM in an abundant, continuous distribution along the length of axons. This study has resolved intermediate stages in the developmental transition from a continuous distribution of ankyrinG 480/270 kDa in all axons to a highly polarized localization at the node of Ranvier in the developing rat sciatic nerve. The first detected event is formation of clusters containing the cell adhesion molecules neurofascin and NrCAM at sites independent of myelin-associated glycoprotein (MAG)-staining Schwann cell processes. Subsequent steps involve recruitment of ankyrinG 480/270 kDa and the voltage-dependent sodium channel to cluster sites containing cell adhesion molecules, and elaboration of MAG-staining Schwann cell processes adjacent to these cluster sites. Formation of the mature node of Ranvier results from the fusion of asynchronously formed pairs of clusters associated with MAG-positive Schwann cells flanking the site of presumed node formation. Studies with the hypomyelinating mutant mouse trembler demonstrate that the elaboration of compact myelin is not required for the formation of these clustered nodal intermediates. Clustering of neurofascin and NrCAM precedes redistribution of ankyrinG 480/270 kDa and the voltage-dependent sodium channel, suggesting that the adhesion molecules define the initial site for subsequent assembly of ankyrin and the voltage-dependent sodium channel.

Structure and organization of the human ankyrin-1 gene. Basis for complexity of pre-mRNA processing

Ankyrin-1 (ANK-1) is an erythrocyte membrane protein that is defective in many patients with hereditary spherocytosis, a common hemolytic anemia. In the red cell, ankyrin-1 provides the primary linkage between the membrane skeleton and the plasma membrane. To gain additional insight into the structure and function of this protein and to provide the necessary tools for further genetic studies of hereditary spherocytosis patients, we cloned the human ANK-1 chromosomal gene. Characterization of the ANK-1 gene genomic structure revealed that the erythroid transcript is composed of 42 exons distributed over approximately 160 kilobase pairs of DNA. Comparison of the genomic structure with the protein domains reveals a near-absolute correlation between the tandem repeats encoding the membrane-binding domain of ankyrin with the location of the intron/exon boundaries in the corresponding part of the gene. Erythroid stage-specific, complex patterns of alternative splicing were identified in the region encoding the regulatory domain of ankyrin-1. Novel brain-specific transcripts were also identified in this region, as well as in the "hinge" region between the membrane-binding and spectrin-binding domains. Utilization of alternative polyadenylation signals was found to be the basis for the previously described, stage-specific 9.0- and 7.2-kilobase pair transcripts of the ANK-1 gene.

Identification of O-linked N-acetylglucosamine modification of ankyrinG isoforms targeted to nodes of Ranvier

AnkyrinGs of 270 and 480 kDa are localized at nodes of Ranvier and are candidates to couple the voltage-dependent sodium channel and neurofascin to the spectrin/actin network. This study presents evidence that these ankyrins contain O-linked GlcNAc residues and identifies as the site of glycosylation a serine-rich domain that distinguishes them from other ankyrin isoforms. The 480-kDa ankyrinG, extracted from brain membranes associated with wheat germ agglutinin-affinity columns, was [3H]galactose-labeled with UDP-[3H] galactose and galactosyltransferase, and cross-reacted with an antibody against O-GlcNAc monosaccharides. AnkyrinG-associated sugars are O-linked monosaccharides based on resistance to peptide-N-glycosidase F and analysis of saccharides released by beta-elimination. The serine-rich domain is the site of glycosylation based on wheat germ agglutinin binding activity of polypeptides produced by in vitro translation in reticulocyte lysates. Immunofluorescence revealed co-localization of ankyrinG and O-GlcNAc immunoreactivity at nodes of Ranvier. These observations suggest that ankyrin at the node of Ranvier is O-GlcNAc-glycosylated and are the first demonstration of a post-translational modification that is concentrated at the node of Ranvier and not in adjacent areas of myelinated axons.

Molecular composition of the node of Ranvier: identification of ankyrin-binding cell adhesion molecules neurofascin (mucin+/third FNIII domain-) and NrCAM at nodal axon segments

Neurofascin, NrCAM, L1, and NgCAM are a family of Ig/FNIII cell adhesion molecules that share ankyrin-binding activity in their cytoplasmic domains, and are candidates to form membrane-spanning complexes with members of the ankyrin family of spectrin-binding proteins in a variety of cellular contexts in the nervous system. Specialized forms of ankyrin, 270 kD and/or 480 kD ankyrinG are components of the membrane undercoat of axons at the node of Ranvier. This paper focuses on definition of the isoforms of ankyrin-binding cell adhesion molecules localized with ankyrinG at the nodal axon segment. The exon usage of two major forms of neurofascin was determined by isolation of full-length cDNAs and used to prepare isoform-specific antibodies. An isoform of neurofascin containing a mucin-like domain and lacking the third FNIII domain was concentrated at axon initial segments and colocalized at nodes of Ranvier with ankyrinG and the voltage-dependent sodium channel. An alternative form of neurofascin lacking the mucin-like domain and containing the third FNIII domain was present in unmyelinated axons. The antibody initially raised against neurofascin was used to screen a rat brain cDNA expression library. In addition to neurofascin, this screen yielded a clone with 80% sequence identity to NrCAM from chicken. The sequences of two full-length cDNAs are presented. NrCAM is most closely related to neurofascin among the other members of the L1/neurofascin/NgCAM family, with over 70% identity between cytoplasmic domains. NrCAM, visualized with antibodies specific for the ecto-domain, also was found to be coexpressed with neurofascin at nodes of Ranvier and at axon initial segments. This is the first characterization of defined neuronal cell adhesion molecules localized to axonal membranes at the node of Ranvier of myelinated axons.

Mechanism for binding site diversity on ankyrin. Comparison of binding sites on ankyrin for neurofascin and the Cl-/HCO3- anion exchanger

Ankyrins are a family of spectrin-binding proteins that associate with at least seven distinct membrane proteins, including ion transporters and cell adhesion molecules. The membrane-binding domain of ankyrin is comprised of a tandem array of 24 ANK repeats organized into four 6-repeat folding domains. Tandem arrays of ANK repeats have been proposed to mediate protein interactions in a variety of proteins including factors involved in the regulation of transcription and the cell cycle. This report provides several new insights into the versatility of ANK repeats of ankyrin in protein recognition, using neurofascin and the Cl-/HCO3- anion exchanger as model ligands and ankyrinR as the prototypic ankyrin. Different combinations of ANK repeat domains from this ankyrin form two distinct, high affinity binding sites for neurofascin. One site requires both repeat domains 3 and 4. The other site involves both repeat domains 2 and 3, although domain 2 has significant activity alone. The sites appear to be independent with Kd values of 3 and 14 nM, respectively. Both the Cl-/HCO3- anion exchanger and neurofascin can interact simultaneously with repeat domains 3 and 4, because neurofascin is unable to displace binding of the anion exchanger cytoplasmic domain to domains 3 and 4, despite having a 3-5-fold higher affinity. These results demonstrate two levels of diversity in the binding sites on ankyrin: one resulting from different combinations of ANK repeat domains and another from different determinants within the same combination of repeat domains. One consequence of this diversity is that ankyrin can accommodate two neurofascin molecules as well as the anion exchanger through interactions mediated by ANK repeats. The ability of ankyrin to simultaneously associate with multiple types of membrane proteins is an unanticipated finding with implications for the assembly of integral membrane proteins into specialized regions of the plasma membrane.

An ankyrin-related gene (unc-44) is necessary for proper axonal guidance in Caenorhabditis elegans

Caenorhabditis elegans unc-44 mutations result in aberrant axon guidance and fasciculation with inappropriate partners. The unc-44 gene was cloned by transposon tagging, and verified by genetic and molecular analyses of six transposon-induced alleles and their revertants. Nucleotide sequence analyses demonstrated that unc-44 encodes a series of putative ankyrin-related proteins, including AO49 ankyrin (1815 aa, 198.8 kD), AO66 ankyrin (1867 aa, 204 kD), and AO13 ankyrin (< or = 4700 aa, < or = 517 kD). In addition to the major set of approximately 6 kb alternatively spliced transcripts, minor transcripts were observed at approximately 3, 5, 7, and 14 kb. Evidence is provided that mutations in the approximately 14-kb AO13 ankyrin transcript are responsible for the neuronal defects. These molecular studies provide the first evidence that ankyrin-related molecules are required for axonal guidance.

A novel protein that interacts with the death domain of Fas/APO1 contains a sequence motif related to the death domain

Signaling for cell death by Fas/APO1 occurs via a distinct region in its intracellular domain. This region contains a conserved sequence motif, the death domain motif, that is also found in the intracellular domains of the p55 tumor necrosis factor receptor and the low affinity nerve growth factor receptor, as well as in the regulatory domain of the ankyrins. A novel protein that specifically binds to the death domain of Fas/APO1 but not to Fas/APO1 molecules with a loss of function point mutation occurring in lprcg mice was cloned by a two-hybrid screen of a HeLa cells' cDNA library. The cloned protein itself contains a death domain motif, and this region binds to the death domain of Fas/APO1, while the region upstream to the death domain prompts self-association of the protein. Induced expression of the protein results in ligand-independent triggering of cytotoxicity, suggesting that it is involved in cell death induction by Fas/APO1.

Identification of an M(r) 60,000 polypeptide unique to the meiotic spindle of the mouse oocyte

The mouse oocyte expresses an M(r) 60,000 (p60) polypeptide that is associated with the first and second meiotic spindles. Immunoreactive p60 was not detectable in the meiotic spindles of male germ cells or in mitotic spindles. P60 was identified with a polyclonal antibody whose predominant activity is directed against ankyrin. However, immunoadsorption experiments demonstrated that p60 is not an ankyrin isoform and represents a secondary activity of the polyclonal antibody. Circumstantial evidence suggest that p60 may be a microtubule-associated protein. Since the most obvious difference between the female meiotic spindle and other spindles is the long half-life of the former, we hypothesize that p60 may function in the maintenance of the long-lived female meiotic apparatus.

A nonsense mutation 1669Glu-->Ter within the regulatory domain of human erythroid ankyrin leads to a selective deficiency of the major ankyrin isoform (band 2.1) and a phenotype of autosomal dominant hereditary spherocytosis

We describe a nonsense mutation in the regulatory domain of erythroid ankyrin associated with autosomal dominant hereditary spherocytosis with a selective deficiency of the ankyrin isoform 2.1 (55% of normal), a deficiency of spectrin (58% of normal) proportional to the decrease in ankyrin 2.1, and a normal content of the other main ankyrin isoform, protein 2.2. PCR amplification of cDNA encoding the regulatory domain of ankyrin revealed a marked decreased in the ratio of ankyrin 2.1 mRNA to the ankyrin 2.2 mRNA. Sequencing of ankyrin gene in the region where the 2.1 and 2.2 mRNA differ detected a nonsense mutation 1669Glu-->Ter (GAA-->TAA) in one ankyrin allele. Only normal ankyrin 2.1 mRNA was detected in the reticulocyte RNA. Since the alternative splicing within the regulatory domain of ankyrin retains codon 1669 in ankyrin 2.1 mRNA and removes it from ankyrin 2.2 mRNA, we propose that the 1669Glu-->Ter mutation decreases the stability of the abnormal ankyrin 2.1 mRNA allele leading to a decreased synthesis of ankyrin 2.1 and a secondary deficiency of spectrin.

Expression of four immunoglobulin superfamily adhesion molecules (L1, Nr-CAM/Bravo, neurofascin/ABGP, and N-CAM) in the developing mouse spinal cord

To identify cell adhesion molecules (CAMs) expressed by mammalian motoneurons, we applied the polymerase chain reaction to a murine motor neuron-like cell line, NSC-34. Using primers derived from a group of L1-related CAMs, we cloned two alternatively spliced forms of mouse L1, which differ by a 12-base-pair insert, plus putative murine orthologs of the chicken cell adhesion molecules Nr-CAM/Bravo and neurofascin. All four mRNAs are expressed in NSC-34 cells, but only neurofascin and the insert-minus form of L1 are expressed in its neuroblastoma parent, N18TG2. Analysis of RNA in neonatal tissues reveals expression largely restricted to the brain and spinal cord. In situ hybridization histochemistry of spinal cord shows that motoneurons express L1, Nr-CAM, and neurofascin as well as N-CAM. L1 and N-CAM RNAs are detected throughout the period studied (from embryonic day [E]11 to postnatal day [P]28), whereas Nr-CAM is expressed only at early ages (< E15) and neurofascin is predominantly expressed postnatally. Moreover, each CAM is expressed by distinct subsets of neighboring cells and at distinct times. For example, Nr-CAM mRNA is present in floor plate cells of embryonic spinal cord, whereas neurofascin is expressed by a subset of glia postnatally. Finally, we show that each CAM has a distinct spatiotemporal pattern of expression in dorsal root ganglia.

AnkyrinG. A new ankyrin gene with neural-specific isoforms localized at the axonal initial segment and node of Ranvier

We have characterized a new ankyrin gene, expressed in brain and other tissues, that is subject to extensive tissue-specific alternative mRNA processing. The full-length polypeptide has a molecular mass of 480 kDa and includes a predicted globular head domain, with membrane- and spectrin-binding activities, as well as an extended "tail" domain. We term this gene ankyrinG based on its giant size and general expression. Two brain-specific isoforms of 480 kDa and 270 kDa were identified that contain a unique stretch of sequence highly enriched in serine and threonine residues immediately following the globular head domain. Antibodies against the serine-rich domain and spectrin-binding domain revealed labeling of nodes of Ranvier and axonal initial segments. Ankyrin-binding proteins also known to be localized in these specialized membrane domains include the voltage-dependent sodium channel, the sodium/potassium ATPase, sodium/calcium exchanger, and members of the neurofascin/L1 family of cell adhesion molecules. The neural-specific ankyrinG polypeptides are candidates to participate in maintenance/targeting of ion channels and cell adhesion molecules to nodes of Ranvier and axonal initial segments.

Cytoskeletal reorganization underlying growth cone motility

Recent studies have implicated cytoskeletal dynamics as an important component in directing neuronal outgrowth. By using modern imaging techniques to observe the kinetics of individual cytoskeletal elements in living cells, these results have converged upon a common theme: functional coupling between the intracellular cytoskeleton and extracellular substrates, and regulation thereof, appears to be crucial in controlling neuronal migration.

The fodrin-ankyrin cytoskeleton of choroid plexus preferentially colocalizes with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2

A unique feature of the choroid plexus as a single-layer epithelium is its localization of Na+K(+)-ATPase at its apical (lumenal) surface. In contrast, a band 3 (AE1)-related anion exchanger protein has been localized to the basolateral surface of the choroid plexus. Both Na+K(+)-ATPase and AE1 in other tissues have been shown to bind via ankyrin to the spectrin-actin-based membrane cytoskeleton. Since linkage of integral membrane proteins to the membrane cytoskeleton is important for their restriction to specialized domains of the cell surface, we investigated the polarity of the choroid plexus membrane cytoskeleton. We developed isoform-specific antibodies to confirm the identity of choroid plexus band 3-related polypeptide as AE2. We demonstrated that ankyrin, fodrin/spectrin, actin, myosin, and alpha-actinin are predominantly apical in choroid plexus and preferentially colocalize with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2. Colchicine administration did not alter the polarity of apical cytoskeletal and transport proteins or basolateral AE2 in choroid plexus, suggesting that biosynthetic targeting of these proteins is not microtubule dependent. In choroid plexus papilloma, Na+K(+)-ATPase and AE2 were decreased in amount and failed to preserve their polarized distributions.

Membrane interactions with the actin cytoskeleton

Recent advances have been made in our understanding of the direct binding of actin to integral membrane proteins. New information has been obtained about indirect actin-membrane associations through spectrin superfamily members and through proteins at the cytoplasmic surfaces of focal contacts and adherens junctions.