Distinct ankyrin isoforms at neuron cell bodies and nodes of Ranvier resolved using erythrocyte ankyrin-deficient mice

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.

Isoforms of ankyrin-3 that lack the NH2-terminal repeats associate with mouse macrophage lysosomes

We have recently cloned and characterized ankyrin-3 (also called ankyrin(G)), a new ankyrin that is widely distributed, especially in epithelial tissues, muscle, and neuronal axons (Peters, L.L., K.M. John, F.M. Lu, E.M. Eicher, A. Higgins, M. Yialamas, L.C. Turtzo, A.J. Otsuka, and S.E. Lux. 1995. J. Cell Biol. 130: 313-330). Here we show that in mouse macrophages, ankyrin-3 is expressed exclusively as two small isoforms (120 and 100 kD) that lack the NH2-terminal repeats. Sequence analysis of isolated Ank3 cDNA clones, obtained by reverse transcription and amplification of mouse macrophage RNA (GenBank Nos. U89274 and U89275), reveals spectrin-binding and regulatory domains identical to those in kidney ankyrin-3 (GenBank No. L40631) preceded by a 29-amino acid segment of the membrane ("repeat") domain, beginning near the end of the last repeat. Antibodies specific for the regulatory and spectrin-binding domains of ankyrin-3 localize the protein to the surface of intracellular vesicles throughout the macrophage cytoplasm. It is not found on the plasma membrane. Also, epitope-tagged mouse macrophage ankyrin-3, transiently expressed in COS cells, associates with intracellular, not plasma, membranes. In contrast, ankyrin-1 (erythrocyte ankyrin, ankyrin(R)), which is also expressed in mouse macrophages, is located exclusively on the plasma membrane. The ankyrin-3-positive vesicles appear dark on phase-contrast microscopy. Two observations suggest that they are lysosomes. First, they are a late compartment in the endocytic pathway. They are only accessible to a fluorescent endocytic tracer (FITC-dextran) after a 24-h incubation, at which time all of the FITC-dextran-containing vesicles contain ankyrin-3 and vice versa. Second, the ankyrin-3-positive vesicles contain lysosomal-associated membrane glycoprotein (LAMP-1), a recognized lysosomal marker. This is the first evidence for the association of an ankyrin with lysosomes and is an example of two ankyrins present in the same cell that segregate to different locations.

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.

Hereditary spherocytosis: a review of the clinical and molecular aspects of the disease

Hereditary spherocytosis is a common and very heterogeneous hemolytic anemia caused by defects of the red cell membrane proteins. In recent years, major advances in our understanding of the red cell membrane skeleton and a better characterization of its individual components have allowed a brighter insight into the pathogenesis of the disease. In this article, we present an overview of the erythrocyte skeleton and its individual constituents. We also review the clinical aspects of the disease and describe the currently known molecular defects involving the membrane proteins which have been shown to play an essential role in the underlying mechanism of hereditary spherocytosis. Finally we examine several models that have been proposed in an attempt to clarify the mechanism leading from the initial molecular insult to the clinical phenotype.

Accumulation of synaptic vesicle proteins and cytoskeletal specializations at the peripheral node of Ranvier

Nodes of Ranvier of peripheral nerve fibres represent repetitive physiological axon constrictions. The nodal attenuation of the axon cylinder is expected to facilitate eliciting axon potentials. But as revealed by immunocytochemical analysis of synaptic vesicle proteins such as SV2 and synaptophysin, nodes are also sites of accumulation of the synaptic vesicle membrane compartment. Results from our studies and other laboratories suggest that the local nodal retardation of the axonally transported synaptic vesicle membrane compartment serves membrane processing and/or turnover. Nodes of Ranvier as well as incisures of Schmidt-Lanterman are rich in filamentous actin and can easily be depicted by fluoresceinated phalloidin. At the node and paranode phalloidin fluorescence appears to be mainly associated with the Schwann cell compartment. Immunofluorescence demonstrates that this compartment also contains myosin and spectrin. The nodal contents in actin and myosin may be effective in actively constricting the axon cylinder at both the node of Ranvier and the Schmidt-Lanterman incisures. This hypothesis is discussed in the light of the nodal cytoskeletal specializations of the axon cylinder and the ensheathing Schwann cell.

Identification of a small cytoplasmic ankyrin (AnkG119) in the kidney and muscle that binds beta I sigma spectrin and associates with the Golgi apparatus

Ankyrins are a family of large, membrane-associated proteins that mediate the linkage of the cytoskeleton to a variety of membrane transport and receptor proteins. A repetitive 33-residue motif characteristic of domain I of ankyrin has also been identified in proteins involved with cell cycle control and development. We have cloned and characterized a novel ankyrin isoform, AnkG119 (GenBank accession No. U43965), from the human kidney which lacks part of this repetitive domain and associates in MDCK cells with beta I sigma spectrin and the Golgi apparatus, but not the plasma membrane. Sequence comparison reveals this ankyrin to be an alternative transcript of AnkG, a much larger ankyrin recently cloned from brain. AnkG119 has a predicted size of 119,201 D, and contains a 47-kD domain I consisting of 13 ankyrin repeat units, a 67-kD domain II with a highly conserved spectrin-binding motif, and a truncated 5-kD putative regulatory domain. An AnkG119 cDNA probe hybridized to a 6.0-kb message in human and rat kidney, placenta, and skeletal muscle. An antibody raised to AnkG119 recognized an apparent 116-kD peptide in rat kidney cortical tissue and MDCK cell lysates, and did not react with larger isoforms of ankyrin at 190 and 210 kD in these tissues, nor in bovine brain, nor with ankyrin from human erythrocytes. AnkG119 remains extractable in 0.5% Triton X-100, and assumes a punctuate cytoplasmic distribution in mature MDCK cells, in contrast to the Triton-stable plasma membrane localization of all previously described renal ankyrins. AnkG119 immunocreativity in subconfluent MDCK cells distributes with the Golgi complex in a pattern coincident with beta -COP and beta I sigma spectrin immunoreactivity. A fusion peptide containing residues 669-860 of AnkG119 interacts with beta I sigma 1 spectrin in vitro with a Kd = 4.2 +/- 4.0 ( +/- 2 SD) nM, and avidly binds the beta spectrin in MDCK cell lysates. Collectively, these data identify AnkG119 as a novel small ankyrin that binds and colocalizes with beta I sigma spectrin in the ER and Golgi apparatus, and possible on a subset of endosomes during the early stages of polarity development. We hypothesize that AnkG119 and beta I spectrin form a vesicular Golgi-associated membrane skeleton, promote the organization of protein microdomains within the Golgi and trans-Golgi networks, and contribute to polarized vesicle transport.

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.

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.

Possible involvement of the 440 kDa isoform of ankyrinB in neuritogenesis in human neuroblastoma NB-1 cells

Two isoforms of brain ankyrin, 440 kDa and 220 kDa ankyrinB, which are products of alternatively spliced pre-mRNA encoded by a single gene, are both expressed in human neuroblastoma NB-1 cells. Expression of the polypeptide and mRNA of the larger isoform increased upon induction of neurite outgrowth in NB-1 cells by dibutyryl cAMP, while those of the smaller isoform were unaffected. The expressed 440 kDa ankyrinB was concentrated at the tip of growing neurites and was partly co-localized with GAP-43. These results suggest that 440 kDa ankyrinB is one of the neuronal growth-associated proteins and provides an interesting example of gene regulation by alternative splicing in neuronal cells.

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.

Subcellular localization of the F5 protein to the neuronal membrane-associated cytoskeleton

F5 was identified originally as an interleukin-2-regulated gene in the murine helper T-lymphocyte clone L2. Subsequent studies demonstrated high levels of F5 mRNA and protein in mature neurons in adult mouse central and peripheral nervous systems. The F5 protein was present in dendrites and perikarya but not in axons. In the present studies, the intracellular localization of the F5 protein in adult mouse brain was determined by subcellular fractionation and Western blotting. Although the deduced F5 sequence predicts a soluble protein, virtually no F5 immunoreactivity was found in the cytosol. The F5 protein was restricted to the P2 crude mitochondrial and P3 crude microsomal particulate fractions. Within the P2 fraction, F5 protein was enriched in the P2B synaptosomal subfraction. The results of temperature-dependent phase separation with Triton X-114 and alkaline extraction with sodium carbonate of the P2 and P3 fractions were consistent with the F5 protein being an extrinsic membrane-associated protein. Although essentially all of the F5 protein in the P3 fraction was membrane-associated, a substantial proportion of P2-associated F5 protein and nearly all of the synaptosomal F5 protein was detergent-insoluble. Direct isolation and subfractionation of brain cytoskeleton confirmed colocalization of F5 immunoreactivity with the membrane-associated cytoskeleton and postsynaptic densities. These studies suggest that the F5 protein, which has a large number of potential phosphorylation sites, plays a role in membrane-cytoskeletal interactions and in dynamic aspects of synaptic structure or function.

Plasmalemmal dihydropyridine receptors modify the function of subplasmalemmal inositol 1,4,5-trisphosphate receptors: a hypothesis

Experimental observations on rat glomerulosa cells inspired a model which postulates that plasmalemmal dihydropyridine receptors are in juxtaposition and interaction with inositol 1,4,5-trisphosphate receptors in subplasmalemmal calciosomes. Activation of dihydropyridine receptors promotes the Ca2+ releasing effect of inositol 1,4,5-trisphosphate. The most important observations compatible with the model are the following: (1) angiotensin II does not influence Ca2+ influx during the peak phase of Ca2+ signal; (2) dihydropyridine drugs modify the initial peak of the Ca2+ signal induced by angiotensin II; (3) inhibitors of the dihydropyridine receptor reduce the initial Ca2+ signal also in the presence of 5 mM Ni2+, an inhibitor of voltage dependent Ca2+ influx; and (4) changes in extracellular K+ concentration within the physiological range also modify the cytoplasmic Ca2+ response to angiotensin II.

Axonal membrane-skeletal protein A60: association with a brain spectrin-binding activity and entry into cerebellar axons at a stage after the initiation of axonal growth

A60 is a 60-kDa component of the axonal cortical cytoskeleton in CNS neurones. It appears to be neurone specific and is tightly bound to brain membranes. In this study the cytoskeletal activities and developmental expression of A60 in rat cerebellum have been examined using the monoclonal antibody DR1. A60 in a partially purified soluble extract of brain membranes interacts selectively with brain but not erythrocyte spectrin. Because erythrocyte spectrin is more closely related to the dendritic form of spectrin than the axonal form, this raises the possibility that A60 localises in axons by interaction with the axonal form of spectrin only. A60 is not found in rat cerebellum before the day of birth. However, during postnatal development of the cerebellum (days 1-13) DR1 reactivity appears progressively. On postnatal day 1, a small population of cells in the mantle layer (presumptive Purkinje cells) is DR1 positive. There is no DR1 reactivity found in Purkinje cell axons during their initial phase of growth. By postnatal day 7, Purkinje cell bodies, initial dendritic segments, and the cerebellar white matter are all positive. This pattern of labelling is strengthened up until postnatal day 13. By contrast, in adult rat cerebellum, the location of A60 has changed so that it is most concentrated in axons, and dendritic staining is lost. These data indicate that A60 is a spectrin-binding component of the adult axonal membrane skeleton, the presence of which is only required in axons after the initial phase of growth.

Decrease of nerve Na+,K(+)-ATPase activity in the pathogenesis of human diabetic neuropathy

A decrease in Na+,K(+)-ATPase activity is claimed to play a central role in the pathogenesis of electrophysiological and morphological abnormalities that characterize the neuropathic complications in different animal models of diabetes mellitus. The peripheral nerves from 17 patients with either type I or type II diabetes mellitus were studied to assess the importance of changes in Na+,K(+)-ATPase activity in chronic human diabetic neuropathy. Sixteen nerves from age- and sex-matched normal individuals, and 12 nerves from non-diabetic neuropathic subjects undergoing vascular or orthopedic surgery served as negative and positive controls, respectively. All specimens were processed blind. Ouabain-sensitive ATPase activity was measured by a modified spectrophotometric coupled-enzyme assay. Standard histology, fiber teasing and electron microscopy were used to establish the normal or neuropathological patterns of surgical material. Morphometric analysis permitted calculation of fiber density in each nerve specimen and correlation of this figure with the relevant enzymatic activity. Na+,K(+)-ATPase activity was approximately 59% lower in nerves from diabetic patients than in normal controls (P < 0.01) and approximately 38% lower in nerves from non-diabetic patients with neuropathy (P < 0.01). Although nerves from both neuropathic conditions had significantly fewer fibers than those from normal individuals (diabetic -33%, and non-diabetic -22%), the decreases in Na+,K(+)-ATPase activity and fiber density were not correlated only in specimens from diabetic patients (r2 = 0.096; P = 0.22). Taken together with data from experimental animal models, these results suggest that the reduction in Na+,K(+)-ATPase activity in diabetic nerves is not an epiphenomenon secondary to fiber loss; rather, it may be an important factor in the pathogenesis and self-maintenance of human diabetic neuropathy.

440-kD ankyrinB: structure of the major developmentally regulated domain and selective localization in unmyelinated axons

440-kD ankyrinB is an alternatively spliced variant of 220-kD ankyrinB, with a predicted 220-kD sequence inserted between the membrane/spectrin binding domains and COOH-terminal domain (Kunimoto, M., E. Otto, and V. Bennett. 1991. J. Cell Biol. 236:1372-1379). This paper presents the sequence of 2085 amino acids comprising the alternatively spliced portion of 440-kD ankyrinB, and provides evidence that much of the inserted sequence has the configuration of an extended random coil. Notable features of the inserted sequence include a hydrophilicity profile that contains few hydrophobic regions, and 220 predicted sites for phosphorylation by protein kinases (casein kinase 2, protein kinase C, and proline-directed protein kinase). Secondary structure and folding of the inserted amino acid residues were deduced from properties of recombinant polypeptides. Frictional ratios of 1.9-2.4 were calculated from Stokes radii and sedimentation coefficients, for polypeptides comprising 70% of the inserted sequence, indicating a highly asymmetric shape. Circular dichroism spectra of these polypeptides indicate a nonglobular structure with negligible alpha-helix or beta sheet folding. These results suggest a ball-and-chain model for 440-kD ankyrinB with a membrane-associated globular head domain and an extended filamentous tail domain encoded by the inserted sequence. Immunofluorescence and immunoblot studies of developing neonatal rat optic nerve indicate that 440-kD ankyrinB is selectively targeted to premyelinated axons, and that 440-kD ankyrinB disappears from these axons coincident with myelination. Hypomyelinated nerve tracts of the myelin-deficient Shiverer mice exhibit elevated levels of 440-kD ankyrinB. 440-kD ankyrinB thus is a specific component of unmyelinated axons and expression of 440-kD ankyrinB may be downregulated as a consequence of myelination.

Characterization of the neuroimmune protein F5: localization to the dendrites and perikarya of mature neurons and the basal aspect of choroid plexus epithelial cells

F5 was identified originally as an interleukin-2-regulated gene in L2 cells, a murine helper T-lymphocyte clone. In adult mouse, F5 mRNA was expressed at a modest level in lymphoid tissues, at a high level in mature neurons in the nervous system, but not in other tissues. Although the F5 sequence is highly conserved over evolution, the function of the F5 protein is unknown. In the present studies, the putative F5 protein-coding region was translated in vitro using a reticulocyte lysate system and in Escherichia coli, yielding a protein with the predicted molecular weight of 42 kDa. Polyclonal rabbit anti-F5 antibody was generated against a synthetic peptide corresponding to the C-terminus of the F5 protein. This antibody specifically recognized recombinant F5 protein. Western blot studies demonstrated a strongly-reactive 42-kDa band and a faint 39-kDa band in extracts of adult mouse brain regions, the levels of which paralleled F5 mRNA expression. Immunoperoxidase studies of adult mouse brain demonstrated F5 immunoreactivity in neuronal perikarya and dendrites but not axons. Neurons expressing the highest levels of F5 protein corresponded to those with the highest levels of F5 mRNA. Choroid plexus epithelial cells also exhibited strong reactivity localized to their basal aspect. These observations suggest that the F5 protein, expression of which appears to be regulated predominantly at the RNA level, may be involved in the maintenance of the functional or anatomic polarity of neurons and choroid plexus epithelial cells.

An ankryin-like protein in ascidian eggs and its role in the evolution of direct development

An erythrocyte anti-ankryin antibody was used to investigate the presence and distribution of ankryin in eggs and embryos of ascidian species with different modes of development. In eggs of the indirect developer Ascidia ceratodes, anti-ankryin reacted with a 210 kDa polypeptide which has an electrophoretic mobility similar to the vertebrate ankryins. Immunofluorescence microscopy showed that the ankryin-like protein is co-distributed with the myoplasm throughout development. It is restricted to a thin layer under the plasma membrane in unfertilised eggs, undergoes ooplasmic segregation to the posterior pole of the zygote after fertilisation, and is distributed to the tail muscle cells during cleavage and embryogenesis. After gastrulation and neurulation, lower levels of the ankryin-like protein, presumably of zygotic origin, were observed in brain cells and in the apical margin of epidermal cells. The ankryin-like protein was also localised in the myoplasm in eggs and embryos of another indirect developing species, Halocynthia roretzi. The ankryin-like protein may link the cytoskeleton with the plasma membrane in ascidian eggs, as it does in vertebrate erythrocytes. In contrast to A. ceratodes and H. roretzi, which are members of the families Ascidiidae and Pyuridae respectively, the pattern of ankryin-like protein expression was changed in five species in the family Molgulidae. These molgulid ascidians exhibit either indirect or direct development, and eggs of the direct developing species have lost or modified the myoplasm. The ankryin-like protein was present in young oocytes but failed to persist during oogenesis and disappeared in mature eggs and embryos of these molgulid species. The change in ankryin-like protein expression may be a preadaptation for loss of the myoplasm and the evolution of direct development.

Cl-/HCO3- exchange function differs in adult and fetal rat hippocampal neurons

We studied Cl-/HCO3- exchange function in acutely dissociated single hippocampal neurons from adult and fetal day 18 rat using a fluorescent intracellular pH (pHi) indicator dye. The presence of Cl-/HCO3- exchange activity was assayed by observing the elevation in pHi upon acute reversal of the Cl- gradient. Resting intracellular pH in acutely dissociated neurons of both adult and fetal tissue was significantly higher than that of cultured fetal hippocampal neurons (day 10-12 in culture). Acute removal of extracellular Cl- caused a rapid and reversible increase in pHi by 0.25 pH units in adult neurons but had virtually no effect in similarly dissociated fetal neurons. Cl-/HCO3- exchange activity was also undetectable in fetal cultured hippocampal neurons. The mRNA for the anion exchanger AE3 is expressed abundantly in adult rodent neurons. AE3 is a potential candidate molecule for the observed Cl-/HCO3- exchange activity. In situ hybridization was used to monitor expression of the AE3 gene in these two age groups. We found that both adult and fetal neurons express AE3 mRNA. These results indicate that AE3 may not function as a Cl-/HCO3- exchanger in fetal neurons, in contrast to its possible role in the adult brain.

Ankyrin immunoreactivity in adult rat cerebellum

An antibody recognizing all ankyrin isoforms was employed to localize ankyrin in the cerebellum by light and electron microscopy. White matter (myelinated fiber tracts) did not contain ankyrin. Granule cell bodies and axons contained ankyrin which extended into the parallel fibers and their synapses. Purkinje cells contained a nearly uniform plasma membrane undercoating of ankyrin in the cell body. However, in both of these cell types ankyrin was not detected in dendrites. Basket and stellate cells did not contain detectable ankyrin. Golgi neuroepithelial cell bodies and processes contained ankyrin while myelin and myelinated axons did not. These results indicate that the membrane skeletal protein ankyrin occurs in some, but not all, neuronal and glial cell types in the cerebellum. For the neurons in which it does occur its distribution is polarized, being limited to cell bodies and axons while not occurring in dendrites.

Ankyrin-binding proteins related to nervous system cell adhesion molecules: candidates to provide transmembrane and intercellular connections in adult brain

A major class of ankyrin-binding glycoproteins have been identified in adult rat brain of 186, 155, and 140 kD that are alternatively spliced products of the same pre-mRNA. Characterization of cDNAs demonstrated that ankyrin-binding glycoproteins (ABGPs) share 72% amino acid sequence identity with chicken neurofascin, a membrane-spanning neural cell adhesion molecule in the Ig super-family expressed in embryonic brain. ABGP polypeptides have the following features consistent with a role as ankyrin-binding proteins in vitro and in vivo: (a) ABGPs and ankyrin associate as pure proteins in a 1:1 molar stoichiometry; (b) the ankyrin-binding site is located in the COOH-terminal 21 kD of ABGP186 which contains the predicted cytoplasmic domain; (c) ABGP186 is expressed at approximately the same levels as ankyrin (15 pmoles/milligram of membrane protein); and (d) ABGP polypeptides are co-expressed with the adult form of ankyrinB late in postnatal development and are colocalized with ankyrinB by immunofluorescence. Similarity in amino acid sequence and conservation of sites of alternative splicing indicate that genes encoding ABGPs and neurofascin share a common ancestor. However, the major differences in developmental expression reported for neurofascin in embryos versus the late postnatal expression of ABGPs suggest that ABGPs and neurofascin represent products of gene duplication events that have subsequently evolved in parallel with distinct roles. The predicted cytoplasmic domains of rat ABGPs and chicken neurofascin are nearly identical to each other and closely related to a group of nervous system cell adhesion molecules with variable extracellular domains, which includes L1, Nr-CAM, and Ng-CAM of vertebrates, and neuroglian of Drosophila. The ankyrin-binding site of rat ABGPs is localized to the C-terminal 200 residues which encompass the cytoplasmic domain, suggesting the hypothesis that ability to associate with ankyrin may be a shared feature of neurofascin and related nervous system cell adhesion molecules.

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.

An isoform-specific mutation in the protein 4.1 gene results in hereditary elliptocytosis and complete deficiency of protein 4.1 in erythrocytes but not in nonerythroid cells

Multiple protein 4.1 isoforms are expressed in a variety of tissues through complex alternative pre-mRNA splicing events, one function of which is to regulate use of two alternative translation initiation signals. Late erythroid cells express mainly the downstream initiation site for synthesis of prototypical 80-kD isoforms; nonerythroid cells in addition use an upstream site to encode higher molecular mass isoform(s). In this study, we examined the effects of a 5' gene rearrangement in a family with hereditary elliptocytosis and complete deficiency of erythrocyte 4.1 protein on 4.1 isoform expression in erythroid vs. nonerythroid cells. Patient 4.1 mRNAs from reticulocytes, fibroblasts, and B lymphocytes were amplified by reverse transcriptase/polymerase chain reaction techniques and shown to exhibit a 318-nucleotide deletion that encompasses the downstream AUG, but leaves intact the upstream AUG. Immunoblot analysis revealed a total deficiency of 4.1 in patient red cells and a selective deficiency of 80-kD isoform(s) but not high molecular weight 4.1 in patient nonerythroid cells. Thus, the 4.1 gene mutation in this family produces an isoform-specific deficiency that is manifested clinically in tissue-specific fashion, such that red cells are affected but other cell types are unaffected because of tissue-specific differences in RNA splicing and translation initiation.

Neuronal polarity

The axonal and somatodendritic domains of neurons differ in their cytoskeletal and membrane composition, complement of organelles, and capacity for macromolecular synthesis. Recently there has been progress in elucidating the cellular mechanisms that underlie the establishment and maintenance of neuronal polarity, including microtubule organization and the sorting, transport, and anchoring of membrane proteins.

Isoforms of the Na,K-ATPase are present in both axons and dendrites of hippocampal neurons in culture

The distributions of isoforms of the Na,K-ATPase alpha subunit were determined in mature cultured hippocampal neurons and in a polarized epithelial cell line. We find that hippocampal neurons express the alpha 1 and alpha 3 isoforms in the membranes of both axons and dendrites. In contrast the alpha 1 and alpha 3 proteins are exclusively basolateral when expressed endogenously or by stable transfection in renal epithelial cells. These data suggest that epithelial cells and hippocampal neurons localize these proteins by different mechanisms. These observations contrast with those made for the vesicular stomatitis virus and the influenza glycoproteins, which are polarized in both epithelial and neuronal cells.

A new 440-kD isoform is the major ankyrin in neonatal rat brain

This report describes initial characterization of a 440-kD isoform of brain ankyrin (ankyrinB) representing an alternatively spliced mRNA product of the gene encoding the major isoform of ankyrin in adult human brain (Otto, E., M. Kunimoto, T. McLaughlin, V. Bennett, J. Cell Biology. 114:241-253). Northern and immunoblot analyses indicate that 440-kD ankyrinB includes the spectrin and membrane-binding domains as well as a regulatory domain of the major 220-kD isoform. 440-kD ankyrinB contains, in addition, a sequence of a predicted size of 220 kD which is inserted between the regulatory domain and spectrin/membrane-binding domains. 440-kD ankyrinB has properties expected of a peripherally associated membrane-skeletal protein: it is exclusively present in the particulate fraction of brain homogenates, is extracted with NaOH, and remains associated with Triton-X-100-resistant structures. Expression of 440-kD ankyrinB in rat brain began at birth before other ankyrins could be detected, peaked 10 d after birth, and then decreased progressively to 30% of the maximum in adults. Expression of the 220-kD ankyrinB and ankyrinR (erythroid ankyrin) began approximately 10 d after the 440-kD isoform, increased rapidly between 10 and 15 d after birth, and finally achieved their maximal levels in adults. 440-kD ankyrinB is present in approximately equivalent amounts in all regions of neonatal brain while in adult brain it is present in highest levels in cerebellum and lowest in brain stem. 440-kD ankyrinB was localized by immunofluorescence in regions of neonatal and adult brain containing primarily dendrites and unmyelinated axons. 440-kD ankyrinB thus may play a specialized role in neuronal processes.

Primary structure and domain organization of human alpha and beta adducin

Adducin is a membrane-skeletal protein which is a candidate to promote assembly of a spectrin-actin network in erythrocytes and at sites of cell-cell contact in epithelial tissues. The complete sequence of both subunits of human adducin, alpha (737 amino acids), and beta (726 amino acids) has been deduced by analysis of the cDNAs. The two subunits have strikingly conserved amino acid sequences with 49% identity and 66% similarity, suggesting evolution by gene duplication. Each adducin subunit has three distinct domains: a 39-kD NH2-terminal globular protease-resistant domain, connected by a 9-kD domain to a 33-kD COOH-terminal protease-sensitive tail comprised almost entirely of hydrophilic amino acids. The tail is responsible for the high frictional ratio of adducin noted previously, and was visualized by EM. The head domains of both adducin subunits exhibit a limited sequence similarity with the NH2-terminal actin-binding motif present in members of the spectrin superfamily and actin gelation proteins. The COOH-termini of both subunits contain an identical, highly basic stretch of 22 amino acids with sequence similarity to the MARCKS protein. Predicted sites of phosphorylation by protein kinase C include the COOH-terminus and sites at the junction of the head and tail. Northern blot analysis of mRNA from rat tissues, K562 erythroleukemia cells and reticulocytes has shown that alpha adducin is expressed in all the tissues tested as a single message size of 4 kb. In contrast, beta adducin shows tissue specific variability in size of mRNA and level of expression. A striking divergence between alpha and beta mRNAs was noted in reticulocytes, where alpha adducin mRNA is present in at least 20-fold higher levels than that of beta adducin. The beta subunit thus is a candidate to perform a limiting role in assembly of functional adducin molecules.

Purkinje cell degeneration associated with erythroid ankyrin deficiency in nb/nb mice

Mice homozygous for the nb mutation (Chromosome 8) have a severe hemolytic anemia and develop a psychomotor disorder at 6 mo of age. The nb/nb mice are deficient in erythroid ankyrin (Ank-1) but, until the present study, the role of Ank-1 and of Ank-2 (brain ankyrin) in disease genesis was unknown. In normal erythroid tissues, we show that two major transcripts are expressed from Ank-1, and one of these is also present at high levels in the cerebellum. By in situ hybridization and immunocytochemistry, Ank-1 localizes to the cerebellar Purkinje cells and, to a lesser extent, the granule cells. In nb/nb mice, Ank-1 transcripts are markedly reduced in both erythroid and neural tissue, and nb/nb Purkinje cells and granule cells are nearly devoid of Ank-1. The neurological syndrome appears concurrently with a dramatic loss of Purkinje cells. Ank-2 maps to Chromosome 3 and its expression is unaffected by the nb mutation. We conclude that Ank-1 is specifically required for Purkinje cell stability and, in its absence, Purkinje cell loss and neurological symptoms appear.