Isolation and chromosomal localization of a novel nonerythroid ankyrin gene

Mechanisms underlying the role of ankyrin-B in cardiac and neurological health and disease

The ANK2 gene encodes for ankyrin-B (ANKB), one of 3 members of the ankyrin family of proteins, whose name is derived from the Greek word for anchor. ANKB was originally identified in the brain (B denotes “brain”) but has become most widely known for its role in cardiomyocytes as a scaffolding protein for ion channels and transporters, as well as an interacting protein for structural and signaling proteins. Certain loss-of-function ANK2 variants are associated with a primarily cardiac-presenting autosomal-dominant condition with incomplete penetrance and variable expressivity characterized by a predisposition to supraventricular and ventricular arrhythmias, arrhythmogenic cardiomyopathy, congenital and adult-onset structural heart disease, and sudden death. Another independent group of ANK2 variants are associated with increased risk for distinct neurological phenotypes, including epilepsy and autism spectrum disorders. The mechanisms underlying ANKB's roles in cells in health and disease are not fully understood; however, several clues from a range of molecular and cell biological studies have emerged. Notably, ANKB exhibits several isoforms that have different cell-type–, tissue–, and developmental stage– expression profiles. Given the conservation within ankyrins across evolution, model organism studies have enabled the discovery of several ankyrin roles that could shed important light on ANKB protein-protein interactions in heart and brain cells related to the regulation of cellular polarity, organization, calcium homeostasis, and glucose and fat metabolism. Along with this accumulation of evidence suggesting a diversity of important ANKB cellular functions, there is an on-going debate on the role of ANKB in disease. We currently have limited understanding of how these cellular functions link to disease risk. To this end, this review will examine evidence for the cellular roles of ANKB and the potential contribution of ANKB functional variants to disease risk and presentation. This contribution will highlight the impact of ANKB dysfunction on cardiac and neuronal cells and the significance of understanding the role of ANKB variants in disease.

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.

A Novel Mechanism for Human Cardiac Ankyrin-B Syndrome due to Reciprocal Chromosomal Translocation

BACKGROUND

Cardiac rhythm abnormalities are a leading cause of morbidity and mortality in developed countries. Loss-of-function variants in the ANK2 gene can cause a variety of cardiac rhythm abnormalities including sinus node dysfunction, atrial fibrillation and ventricular arrhythmias (called the "ankyrin-B syndrome"). ANK2 encodes ankyrin-B, a molecule critical for the membrane targeting of key cardiac ion channels, transporters, and signalling proteins.

METHODS AND RESULTS

Here, we describe a family with a reciprocal chromosomal translocation between chromosomes 4q25 and 9q26 that transects the ANK2 gene on chromosome 4 resulting in loss-of-function of ankyrin-B. Select family members with ankyrin-B haploinsufficiency due to the translocation displayed clinical features of ankyrin-B syndrome. Furthermore, evaluation of primary lymphoblasts from a carrier of the translocation showed altered levels of ankyrin-B as well as a reduced expression of downstream ankyrin-binding partners.

CONCLUSIONS

Thus, our data conclude that, similar to previously described ANK2 loss-of-function "point mutations", large chromosomal translocations resulting in ANK2 haploinsufficiency are sufficient to cause the human cardiac ankyrin-B syndrome. The unexpected ascertainment of ANK2 dysfunction via the discovery of a chromosomal translocation in this family, the determination of the familial phenotype, as well as the complexities in formulating screening and treatment strategies are discussed.

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'.

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.

A subtractive approach to characterize genes with regionalized expression in the gliogenic ventral neuroepithelium: identification of chick Sulfatase 1 as a new oligodendrocyte lineage gene

To address the question of the origin of glial cells and the mechanisms leading to their specification, we have sought to identify novel genes expressed in glial progenitors. We adopted suppression subtractive hybridization (SSH) to establish a chick cDNA library enriched for genes specifically expressed at 6 days of incubation (E6) in the ventral neuroepithelium, a tissue previously shown to contain glial progenitors. Screens were then undertaken to select differentially expressed cDNAs, and out of 82 unique SSH clones, 21 were confirmed to display a regionalized expression along the dorsoventral axis of the E6 ventral neuroepithelium. Among these, we identified a transcript coding for the chick orthologue of Sulf1, a recently identified cell surface sulfatase, as a new, early marker of oligodendrocyte (OL) precursors in the chick embryonic spinal cord. This study provides groundwork for the further identification of genes involved in glial specification.

Major quantitative trait locus for resting heart rate maps to a region on chromosome 4

Multiple studies have identified resting heart rate as a risk factor for cardiovascular disease independent of other cardiovascular disease risk factors (such as dyslipidemia and hypertension). Previous studies have examined heart rate in hypertensive individuals, but little is known about the genetic determination of resting heart rate in a normal population. Therefore, our objective was to perform a genome screen on a population containing normotensive and hypertensive individuals. We performed variance decomposition linkage analysis using maximum likelihood methods at approximately 10 cM intervals in 2209 individuals of predominantly North European ancestry. We estimated the heritability of resting heart rate to be 26% and obtained significant evidence of linkage (logarithm of the odds [LOD]=3.9) for resting heart rate on chromosome 4q. This signal is in the same region as a quantitative trait locus (QTL) for long QT syndrome 4 and a QTL for heart rate in rats. Within the 1-LOD unit support interval, there are 2 strong candidates: ankyrin-B and myozenin 2.

Truncating neurotrypsin mutation in autosomal recessive nonsyndromic mental retardation

A 4-base pair deletion in the neuronal serine protease neurotrypsin gene was associated with autosomal recessive nonsyndromic mental retardation (MR). In situ hybridization experiments on human fetal brains showed that neurotrypsin was highly expressed in brain structures involved in learning and memory. Immuno-electron microscopy on adult human brain sections revealed that neurotrypsin is located in presynaptic nerve endings, particularly over the presynaptic membrane lining the synaptic cleft. These findings suggest that neurotrypsin-mediated proteolysis is required for normal synaptic function and suggest potential insights into the pathophysiological bases of mental retardation.

Targeted ablation of NrCAM or ankyrin-B results in disorganized lens fibers leading to cataract formation

The NgCAM-related cell adhesion molecule (NrCAM) is an immunoglobulin superfamily member of the L1 subgroup that interacts intracellularly with ankyrins. We reveal that the absence of NrCAM causes the formation of mature cataracts in the mouse, whereas significant pathfinding errors of commissural axons at the midline of the spinal cord or of proprioceptive axon collaterals are not detected. Cataracts, the most common cause of visual impairment, are generated in NrCAM-deficient mice by a disorganization of lens fibers, followed by cellular disintegration and accumulation of cellular debris. The disorganization of fiber cells becomes histologically distinct during late embryonic development and includes abnormalities of the cytoskeleton and of connexin50-containing gap junctions. Furthermore, analysis of lenses of ankyrin-B mutant mice also reveals a disorganization of lens fibers at postnatal day 1, indistinguishable from that generated by the absence of NrCAM, indicating that NrCAM and ankyrin-B are required to maintain contact between lens fiber cells. Also, these studies provide genetic evidence of an interaction between NrCAM and ankyrin-B.

Ankyrin-Tiam1 interaction promotes Rac1 signaling and metastatic breast tumor cell invasion and migration

Tiam1 (T-lymphoma invasion and metastasis 1) is one of the known guanine nucleotide (GDP/GTP) exchange factors (GEFs) for Rho GTPases (e.g., Rac1) and is expressed in breast tumor cells (e.g., SP-1 cell line). Immunoprecipitation and immunoblot analyses indicate that Tiam1 and the cytoskeletal protein, ankyrin, are physically associated as a complex in vivo. In particular, the ankyrin repeat domain (ARD) of ankyrin is responsible for Tiam1 binding. Biochemical studies and deletion mutation analyses indicate that the 11-amino acid sequence between amino acids 717 and 727 of Tiam1 ((717)GEGTDAVKRS(727)L) is the ankyrin-binding domain. Most importantly, ankyrin binding to Tiam1 activates GDP/GTP exchange on Rho GTPases (e.g., Rac1). Using an Escherichia coli-derived calmodulin-binding peptide (CBP)-tagged recombinant Tiam1 (amino acids 393-728) fragment that contains the ankyrin-binding domain, we have detected a specific binding interaction between the Tiam1 (amino acids 393-738) fragment and ankyrin in vitro. This Tiam1 fragment also acts as a potent competitive inhibitor for Tiam1 binding to ankyrin. Transfection of SP-1 cell with Tiam1 cDNAs stimulates all of the following: (1) Tiam1-ankyrin association in the membrane projection; (2) Rac1 activation; and (3) breast tumor cell invasion and migration. Cotransfection of SP1 cells with green fluorescent protein (GFP)-tagged Tiam1 fragment cDNA and Tiam1 cDNA effectively blocks Tiam1-ankyrin colocalization in the cell membrane, and inhibits GDP/GTP exchange on Rac1 by ankyrin-associated Tiam1 and tumor-specific phenotypes. These findings suggest that ankyrin-Tiam1 interaction plays a pivotal role in regulating Rac1 signaling and cytoskeleton function required for oncogenic signaling and metastatic breast tumor cell progression.

A novel neuron-enriched homolog of the erythrocyte membrane cytoskeletal protein 4.1

We report the molecular cloning and characterization of 4.1N, a novel neuronal homolog of the erythrocyte membrane cytoskeletal protein 4.1 (4.1R). The 879 amino acid protein shares 70, 36, and 46% identity with 4.1R in the defined membrane-binding, spectrin-actin-binding, and C-terminal domains, respectively. 4.1N is expressed in almost all central and peripheral neurons of the body and is detected in embryonic neurons at the earliest stage of postmitotic differentiation. Like 4.1R, 4.1N has multiple splice forms as evidenced by PCR and Western analysis. Whereas the predominant 4.1N isoform identified in brain is approximately 135 kDa, a smaller 100 kDa isoform is enriched in peripheral tissues. Immunohistochemical studies using a polyclonal 4.1N antibody revealed several patterns of neuronal staining, with localizations in the neuronal cell body, dendrites, and axons. In certain neuronal locations, including the granule cell layers of the cerebellum and dentate gyrus, a distinct punctate-staining pattern was observed consistent with a synaptic localization. In primary hippocampal cultures, mouse 4.1N is enriched at the discrete sites of synaptic contact, colocalizing with the postsynaptic density protein of 95 kDa (a postsynaptic marker) and glutamate receptor type 1 (an excitatory postsynaptic marker). By analogy with the roles of 4.1R in red blood cells, 4.1N may function to confer stability and plasticity to the neuronal membrane via interactions with multiple binding partners, including the spectrin-actin-based cytoskeleton, integral membrane channels and receptors, and membrane-associated guanylate kinases.

A spectrin membrane skeleton of the Golgi complex

The existence of a Golgi-localized membrane cytoskeleton has been revealed by the identification of two major components of the spectrin membrane skeleton, spectrin and ankyrin, that associate with the Golgi complex. Golgi spectrin was identified with an antibody specific for the beta-subunit of the erythroid isoform of spectrin (beta1Sigma1). This antibody recognizes a 220 kDa polypeptide that localizes to discrete regions of the Golgi complex and associates with Golgi membranes in a Brefeldin A sensitive manner. Two isoforms of Golgi ankyrin have been identified: a 119 kDa form (AnkG119) which represents a truncated, alternatively spliced isoform of a recently cloned novel ankyrin of the nervous system AnkG, and a larger 195 kDa ankyrin (Ank195) that cross-reacts with antibodies to erythrocyte ankyrin. A Golgi localized membrane skeleton composed of these unique membrane skeleton isoforms could serve a variety of important functions, including the maintenance of Golgi structural organization and the formation of discrete membrane domains within Golgi compartments.

Cloning and expression of cDNA encoding rat liver 60-kDa lysophospholipase containing an asparaginase-like region and ankyrin repeat

Mammalian tissues contain small form and large form lysophospholipases. Here we report the cloning, sequence, and expression of cDNA encoding the latter form of lysophospholipase using antibody raised against the enzyme purified from rat liver supernatant (Sugimoto, H., and Yamashita, S. (1994) J. Biol. Chem. 269, 6252-6258). The 2,539-base pair cDNA encoded 564 amino acid residues with a calculated Mr of 60,794. The amino-terminal two-thirds of the deduced amino acid sequence significantly resembled Escherichia coli asparaginase I with the putative asparaginase catalytic triad Thr-Asp-Lys and was followed by leucine zipper motif. The carboxyl-terminal region carried ankyrin repeat. When the cDNA was transfected into HEK293 cells, not only lysophospholipase activity but also asparaginase and platelet-activating factor acetylhydrolase activities were expressed. Reverse transcription-polymerase chain reaction revealed that the transcript occurred at high levels in liver and kidney but was hardly detectable in lung and heart from which large form lysophospholipases had been purified, suggesting the presence of multiple forms of large form lysophospholipase in mammalian tissues.

An alternative first exon in the distal end of the erythroid ankyrin gene leads to production of a small isoform containing an NH2-terminal membrane anchor

Mouse erythroid ankyrin is encoded by the Ank1 gene on Chromosome 8. The best studied isoform is 210 kDa and contains three large functional domains. We have recently reported a small Ank1 isoform (relative mobility 25 kDa) that localizes to the M and Z lines in skeletal muscle. Analyses of cDNA and genomic clones show that three transcripts of 3.5, 2.0, and 1.6 kb code for this protein. The different transcript sizes are due to their 3'-untranslated regions. They are encoded by a new first exon located in intron 39 of the Ank1 gene and three previously described Ank1 exons (40, 41, and 42). The 5'-flanking region contains a putative muscle-specific promoter. The sequence of the first 72 amino acids is novel and is predicted to form a transmembrane helix at the NH2-terminus. Functional testing of the putative transmembrane segment indicates that it acts as a membrane anchor, suggesting that the new Ank1 isoform may play an important role in organizing the contractile apparatus within the cell.

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.

Ank3 (epithelial ankyrin), a widely distributed new member of the ankyrin gene family and the major ankyrin in kidney, is expressed in alternatively spliced forms, including forms that lack the repeat domain

We cloned a novel ankyrin, Ank3, from mouse kidney cDNA. The full-length transcript is predicted to encode a 214-kD protein containing an 89 kD, NH2 terminal "repeat" domain; a 65 kD, central "spectrin-binding" domain; and a 56 kD, COOH-terminal "regulatory" domain. The Ank3 gene maps to mouse Chromosome 10, approximately 36 cM from the centromere, a locus distinct from Ank1 and Ank2. Ank3 is the major kidney ankyrin. Multiple transcripts of approximately 7.5, 6.9, 6.3, 5.7, 5.1, and 4.6 kb are highly expressed in kidney where Ank1 and Ank2 mRNAs are barely detectable. The smaller mRNAs (< or = 6.3 kb) lack the entire repeat domain. These transcripts have a unique 5'untranslated region and NH2-terminal sequence and encode a predicted protein of 121 kD. Two small sequences of 21 and 18 amino acids are alternatively spliced at the junction of the repeat and spectrin-binding domains in the larger (> or = 6.9 kb) RNAs. Alternative splicing of a 588 bp sequence (corresponding to a 21.5-kD acidic amino acid sequence) within the regulatory domain also occurs. Ank3 is much more widely expressed than previously described ankyrins. By Northern hybridization or immunocytochemistry, it is present in most epithelial cells, in neuronal axons, in muscle cells, and in megakaryocytes/platelets, macrophages, and the interstitial cells of Leydig (testis). On immunoblots, an antibody raised to a unique regions of the regulatory domain detects multiple Ank3 isoforms in the kidney (215, 200, 170, 120, 105 kD) and in other tissues. The 215/200 kD and 120/105-kD kidney proteins are close to the sizes predicted for the 7.5/6.9- and 6.3/5.7-kb RNAs (with/without the 588-bp acidic insert). Interestingly, it appears that Ank3 exhibits a polarized distribution only in tissues that express the approximately 7.0-kb isoforms, the only isoforms in the kidney that contain the repeat domain. In tissues where smaller transcripts (< or = 6.3 kb) are expressed. Ank3 is diffusely distributed in some or all cells and may be associated with cytoplasmic structures. We conclude that Ank3 is a broadly distributed epithelial ankyrin and is the major ankyrin in the kidney and other tissues, where it plays an important role in the polarized distribution of many integral membrane proteins.

Brain spectrin: of mice and men

This article reviews our current knowledge of the structure of alpha spectrins and beta spectrins in the brain, as well as their location and expression within neural tissue. We discuss the known protein interactions of brain spectrin isoforms, and then describe results that suggest an important role for spectrin (alpha SpII sigma 1/beta SpII sigma 1) in the Ca(2+)-regulated release of neurotransmitters. Evidence that supports a role for spectrin in the docking of synaptic vesicles to the presynaptic plasma membrane and as a Ca2+ sensor protein that unclamps the fusion machinery is described, along with the Casting the Line model, which summarizes the information. We finish with a discussion of the value of spectrin and ankyrin-deficient mouse models in deciphering spectrin function in neural tissue.

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

The focus of this review is on the ankyrin gene family, key elements in the interaction of the spectrin-based membrane skeleton with the plasma membrane in a variety of tissues and multicellular organisms. The structure/function relationships of ankyrin molecules are reviewed, illustrating how these proteins are uniquely suited to serve as adaptors between the membrane skeleton and a number of integral membrane proteins. Advances in the understanding of ankyrin biology in the brain are discussed and used to show how ankyrins may be involved in the establishment and/or maintenance of specialized plasma membrane domains. Finally, recent research in hematological and neurological disorders are reviewed, suggesting that ankyrins have a role in the development of human disease.