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

A modified density gradient proteomic-based method to analyze endolysosomal proteins in cardiac tissue

The importance of lysosomes in cardiac physiology and pathology is well established, and evidence for roles in calcium signaling is emerging. We describe a label-free proteomics method suitable for small cardiac tissue biopsies based on density-separated fractionation, which allows study of endolysosomal (EL) proteins. Density gradient fractions corresponding to tissue lysate; sarcoplasmic reticulum (SR), mitochondria (Mito) (1.3 g/mL); and EL with negligible contamination from SR or Mito (1.04 g/mL) were analyzed using Western blot, enzyme activity assay, and liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis (adapted discontinuous Percoll and sucrose differential density gradient). Kyoto Encyclopedia of Genes and Genomes, Reactome, Panther, and Gene Ontology pathway analysis showed good coverage of RAB proteins and lysosomal cathepsins (including cardiac-specific cathepsin D) in the purified EL fraction. Significant EL proteins recovered included catalytic activity proteins. We thus present a comprehensive protocol and data set of guinea pig atrial EL organelle proteomics using techniques also applicable for non-cardiac tissue.

Roles of palmitoylation in structural long-term synaptic plasticity

Long-term potentiation (LTP) and long-term depression (LTD) are important cellular mechanisms underlying learning and memory processes. N-Methyl-d-aspartate receptor (NMDAR)-dependent LTP and LTD play especially crucial roles in these functions, and their expression depends on changes in the number and single channel conductance of the major ionotropic glutamate receptor α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) located on the postsynaptic membrane. Structural changes in dendritic spines comprise the morphological platform and support for molecular changes in the execution of synaptic plasticity and memory storage. At the molecular level, spine morphology is directly determined by actin cytoskeleton organization within the spine and indirectly stabilized and consolidated by scaffold proteins at the spine head. Palmitoylation, as a uniquely reversible lipid modification with the ability to regulate protein membrane localization and trafficking, plays significant roles in the structural and functional regulation of LTP and LTD. Altered structural plasticity of dendritic spines is also considered a hallmark of neurodevelopmental disorders, while genetic evidence strongly links abnormal brain function to impaired palmitoylation. Numerous studies have indicated that palmitoylation contributes to morphological spine modifications. In this review, we have gathered data showing that the regulatory proteins that modulate the actin network and scaffold proteins related to AMPAR-mediated neurotransmission also undergo palmitoylation and play roles in modifying spine architecture during structural plasticity.

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.

Linkage of Metabolic Defects to Activated PIK3CA Alleles in Endothelial Cells Derived from Lymphatic Malformation

BACKGROUND

Lymphatic endothelial cells (LECs) derived from lymphatic malformations (LMs) bear activated PIK3CA alleles yet display an inflammatory gene expression profile. A basis for the inflammatory phenotype was sought by screening for coexisting somatic mutations.

METHODS AND RESULTS

Fourteen independent LEC populations bearing activated PIK3CA alleles were isolated from LM. These were characterized by the expression of growth and inflammatory genes (VEGFC, IL-6, COX-2, IL-8, HO-1, E-SEL) by qRT-PCR. Most commonly upregulated gene products were VEGFC, COX2, HO-1, and ANGPTL4. The specific inhibition of PI3K reduced VEGFC expression without resolving inflammation. Whole exome sequencing of six LM-LEC populations identified five novel somatically acquired alleles coexisting with activated PIK3CA alleles. Two affected genes regulate lipid droplet metabolism (FITM2 and ATG2A), two are gene regulators (MTA1 and TAF1L), and the fifth is an isoform of ANK3 (an endosomal/lysosomal protein). Inhibition of AMPK implicated its involvement in regulating COX-2 and HO-1 overexpression. ANGPTL4 expression was independent of AMPK and PI3K activity and reflected lipid stress demonstrated in normal LECs. AMPK activation with AICAR had a selective growth-limiting effect in a subset of LM-LEC isolates.

CONCLUSIONS

Inflammatory stress displayed by LM-LECs is consistent with errors in lipid metabolism that may be linked to acquired mutations. The acquisition of PIK3CA alleles may be a permissive event that antagonizes inflammation and metabolic defect.

Protein expression profiling in haemocytes and plasma of the Manila clam Ruditapes philippinarum in response to infection with Perkinsus olseni

The protein expression profiling in clam haemocytes and plasma in response to Perkinsus olseni was addressed. Adult Manila clams from a P. olseni-free bed were experimentally challenged with parasite zoospores to analyse immune response. In another experiment, the effects of longer term infection were assessed in adult clams collected from a P. olseni-affected bed, by comparing moderate to very heavily infected clams with non-infected ones. Haemocyte and plasma proteins were separated by two-dimensional electrophoresis; spot patterns were qualitatively compared between treatments within each experiment and the spots indicating differential protein expression associated with P. olseni challenge or with field infection were processed for protein identification. Fifteen clam proteins (four in haemocytes and eleven in plasma) of which expression was markedly affected by P. olseni were identified. Some of the identified proteins have a well-known role in clam immune response against the parasite, such as lysozyme and lectins. Rho GTPase-activating protein 6 could be a marker of resistance against P. olseni, which should be further studied.

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.

Isoforms of Spectrin and Ankyrin Reflect the Functional Topography of the Mouse Kidney

The kidney displays specialized regions devoted to filtration, selective reabsorption, and electrolyte and metabolite trafficking. The polarized membrane pumps, channels, and transporters responsible for these functions have been exhaustively studied. Less examined are the contributions of spectrin and its adapter ankyrin to this exquisite functional topography, despite their established contributions in other tissues to cellular organization. We have examined in the rodent kidney the expression and distribution of all spectrins and ankyrins by qPCR, Western blotting, immunofluorescent and immuno electron microscopy. Four of the seven spectrins (αΙΙ, βΙ, βΙΙ, and βΙΙΙ) are expressed in the kidney, as are two of the three ankyrins (G and B). The levels and distribution of these proteins vary widely over the nephron. αΙΙ/βΙΙ is the most abundant spectrin, found in glomerular endothelial cells; on the basolateral membrane and cytoplasmic vesicles in proximal tubule cells and in the thick ascending loop of Henle; and less so in the distal nephron. βΙΙΙ spectrin largely replaces βΙΙ spectrin in podocytes, Bowman’s capsule, and throughout the distal tubule and collecting ducts. βΙ spectrin is only marginally expressed; its low abundance hinders a reliable determination of its distribution. Ankyrin G is the most abundant ankyrin, found in capillary endothelial cells and all tubular segments. Ankyrin B populates Bowman’s capsule, podocytes, the ascending thick loop of Henle, and the distal convoluted tubule. Comparison to the distribution of renal protein 4.1 isoforms and various membrane proteins indicates a complex relationship between the spectrin scaffold, its adapters, and various membrane proteins. While some proteins (e.g. ankyrin B, βΙΙΙ spectrin, and aquaporin 2) tend to share a similar distribution, there is no simple mapping of different spectrins or ankyrins to most membrane proteins. The implications of this data are discussed.

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.

An Adaptable Spectrin/Ankyrin-Based Mechanism for Long-Range Organization of Plasma Membranes in Vertebrate Tissues

Ankyrins are membrane-associated proteins that together with their spectrin partners are responsible for micron-scale organization of vertebrate plasma membranes, including those of erythrocytes, excitable membranes of neurons and heart, lateral membrane domains of columnar epithelial cells, and striated muscle. Ankyrins coordinate functionally related membrane transporters and cell adhesion proteins (15 protein families identified so far) within plasma membrane compartments through independently evolved interactions of intrinsically disordered sequences with a highly conserved peptide-binding groove formed by the ANK repeat solenoid. Ankyrins are coupled to spectrins, which are elongated organelle-sized proteins that form mechanically resilient arrays through cross-linking by specialized actin filaments. In addition to protein interactions, cellular targeting and assembly of spectrin/ankyrin domains also critically depend on palmitoylation of ankyrin-G by aspartate-histidine-histidine-cysteine 5/8 palmitoyltransferases, as well as interaction of beta-2 spectrin with phosphoinositide lipids. These lipid-dependent spectrin/ankyrin domains are not static but are locally dynamic and determine membrane identity through opposing endocytosis of bulk lipids as well as specific proteins. A partnership between spectrin, ankyrin, and cell adhesion molecules first emerged in bilaterians over 500 million years ago. Ankyrin and spectrin may have been recruited to plasma membranes from more ancient roles in organelle transport. The basic bilaterian spectrin-ankyrin toolkit markedly expanded in vertebrates through gene duplications combined with variation in unstructured intramolecular regulatory sequences as well as independent evolution of ankyrin-binding activity by ion transporters involved in action potentials and calcium homeostasis. In addition, giant vertebrate ankyrins with specialized roles in axons acquired new coding sequences by exon shuffling. We speculate that early axon initial segments and epithelial lateral membranes initially were based on spectrin-ankyrin-cell adhesion molecule assemblies and subsequently served as "incubators," where ion transporters independently acquired ankyrin-binding activity through positive selection.

Ankyrin-G palmitoylation and βII-spectrin binding to phosphoinositide lipids drive lateral membrane assembly

Ankyrin-G and βII-spectrin colocalize at sites of cell-cell contact in columnar epithelial cells and promote lateral membrane assembly. This study identifies two critical inputs from lipids that together provide a rationale for how ankyrin-G and βII-spectrin selectively localize to Madin-Darby canine kidney (MDCK) cell lateral membranes. We identify aspartate-histidine-histidine-cysteine 5/8 (DHHC5/8) as ankyrin-G palmitoyltransferases required for ankyrin-G lateral membrane localization and for assembly of lateral membranes. We also find that βII-spectrin functions as a coincidence detector that requires recognition of both ankyrin-G and phosphoinositide lipids for its lateral membrane localization. DHHC5/8 and βII-spectrin colocalize with ankyrin-G in micrometer-scale subdomains within the lateral membrane that are likely sites for palmitoylation of ankyrin-G. Loss of either DHHC5/8 or ankyrin-G-βII-spectrin interaction or βII-spectrin-phosphoinositide recognition through its pleckstrin homology domain all result in failure to build the lateral membrane. In summary, we identify a functional network connecting palmitoyltransferases DHHC5/8 with ankyrin-G, ankyrin-G with βII-spectrin, and βII-spectrin with phosphoinositides that is required for the columnar morphology of MDCK epithelial cells.

Proteomic analysis of clathrin interactions in trypanosomes reveals dynamic evolution of endocytosis

Endocytosis is a vital cellular process maintaining the cell surface, modulating signal transduction and facilitating nutrient acquisition. In metazoa, multiple endocytic modes are recognized, but for many unicellular organisms the process is likely dominated by the ancient clathrin-mediated pathway. The endocytic system of the highly divergent trypanosomatid Trypanosoma brucei exhibits many unusual features, including a restricted site of internalization, dominance of the plasma membrane by GPI-anchored proteins, absence of the AP2 complex and an exceptionally high rate. Here we asked if the proteins subtending clathrin trafficking in trypanosomes are exclusively related to those of higher eukaryotes or if novel, potentially taxon-specific proteins operate. Co-immunoprecipitation identified twelve T. brucei clathrin-associating proteins (TbCAPs), which partially colocalized with clathrin. Critically, eight TbCAPs are restricted to trypanosomatid genomes and all of these are required for robust cell proliferation. A subset, TbCAP100, TbCAP116, TbCAP161 and TbCAP334, were implicated in distinct endocytic steps by detailed analysis of knockdown cells. Coupled with the absence of orthologs for many metazoan and fungal endocytic factors, these data suggest that clathrin interactions in trypanosomes are highly lineage-specific, and indicate substantial evolutionary diversity within clathrin-mediated endocytosis mechanisms across the eukaryotes.

Genome-wide study identifies two loci associated with lung function decline in mild to moderate COPD

Accelerated lung function decline is a key COPD phenotype; however, its genetic control remains largely unknown. We performed a genome-wide association study using the Illumina Human660W-Quad v.1_A BeadChip. Generalized estimation equations were used to assess genetic contributions to lung function decline over a 5-year period in 4,048 European American Lung Health Study participants with largely mild COPD. Genotype imputation was performed using reference HapMap II data. To validate regions meeting genome-wide significance, replication of top SNPs was attempted in independent cohorts. Three genes (TMEM26, ANK3 and FOXA1) within the regions of interest were selected for tissue expression studies using immunohistochemistry. Two intergenic SNPs (rs10761570, rs7911302) on chromosome 10 and one SNP on chromosome 14 (rs177852) met genome-wide significance after Bonferroni. Further support for the chromosome 10 region was obtained by imputation, the most significantly associated imputed SNPs (rs10761571, rs7896712) being flanked by observed markers rs10761570 and rs7911302. Results were not replicated in four general population cohorts or a smaller cohort of subjects with moderate to severe COPD; however, we show novel expression of genes near regions of significantly associated SNPS, including TMEM26 and FOXA1 in airway epithelium and lung parenchyma, and ANK3 in alveolar macrophages. Levels of expression were associated with lung function and COPD status. We identified two novel regions associated with lung function decline in mild COPD. Genes within these regions were expressed in relevant lung cells and their expression related to airflow limitation suggesting they may represent novel candidate genes for COPD susceptibility.

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.

Immune and inflammatory pathways are involved in inherent bone marrow ossification

BACKGROUND

Bone marrow plays a key role in bone formation and healing. Although a subset of marrow explants ossifies in vitro without excipient osteoinductive factors, some explants do not undergo ossification. The disparity of outcome suggests a significant heterogeneity in marrow tissue in terms of its capacity to undergo osteogenesis.

QUESTIONS/PURPOSES

We sought to identify: (1) proteins and signaling pathways associated with osteogenesis by contrasting the proteomes of ossified and poorly ossified marrow explants; and (2) temporal changes in proteome and signaling pathways of marrow ossification in the early and late phases of bone formation.

METHODS

Explants of marrow were cultured. Media conditioned by ossified (n = 4) and poorly ossified (n = 4) subsets were collected and proteins unique to each group were identified by proteomic analysis. Proteomic data were processed to assess proteins specific to the early phase (Days 1-14) and late phase (Days 15-28) of the culture period. Pathways involved in bone marrow ossification were identified through bioinformatics.

RESULTS

Twenty-eight proteins were unique to ossified samples and eight were unique to poorly ossified ones. Twelve proteins were expressed during the early phase and 15 proteins were specific to the late phase. Several identified pathways corroborated those reported for bone formation in the literature. Immune and inflammatory pathways were specific to ossified samples.

CONCLUSIONS

The marrow explant model indicates the inflammatory and immune pathways to be an integral part of the osteogenesis process.

Inhibition of ankyrin-B expression reduces growth and invasion of human pancreatic ductal adenocarcinoma

BACKGROUND

In spite of the increasing knowledge of the molecular pathology of pancreatic ductal adenocarcinoma (PDAC), treatment of this tumor still remains an unresolved problem. Thus, the identification of 'novel' genes involved in pancreatic tumor progression is essential for early diagnosis and new treatment regimens of PDAC. Ankyrin-B (ANK2) was identified as being overexpressed in PDAC in a previous study by our group. ANK2 overexpression has been described in several tumors; however, the function of ANK2 in pancreatic carcinoma has not been elucidated.

MATERIALS AND METHODS

In the present study, we confirmed ANK2 overexpression in PDAC and analyzed the effects of ANK2 knockdown in the pancreatic tumor cell line PANC-1.

RESULTS

ANK2 silencing reduced the activity of FAK, ERK1/2 and p38. Decreased ANK2 expression restrained migration and invasive potential of PANC-1 cells. Moreover, silencing of ANK2 decreased the proliferation of the pancreatic tumor cells and reduced their tumorigenicity in vitro and in vivo.

CONCLUSION

Our results demonstrate that silencing of ANK2 expression reduced the malignant phenotype of pancreatic cancer cells, indicating that ANK2 represents a potential target for therapy of pancreatic cancer.

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.

Membrane domains based on ankyrin and spectrin associated with cell-cell interactions

Nodes of Ranvier and axon initial segments of myelinated nerves, sites of cell-cell contact in early embryos and epithelial cells, and neuromuscular junctions of skeletal muscle all perform physiological functions that depend on clustering of functionally related but structurally diverse ion transporters and cell adhesion molecules within microdomains of the plasma membrane. These specialized cell surface domains appeared at different times in metazoan evolution, involve a variety of cell types, and are populated by distinct membrane-spanning proteins. Nevertheless, recent work has shown that these domains all share on their cytoplasmic surfaces a membrane skeleton comprised of members of the ankyrin and spectrin families. This review will summarize basic features of ankyrins and spectrins, and will discuss emerging evidence that these proteins are key players in a conserved mechanism responsible for assembly and maintenance of physiologically important domains on the surfaces of diverse cells.

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

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

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.

Biosilica formation in spicules of the sponge Suberites domuncula: synchronous expression of a gene cluster

The formation of spicules is a complicated morphogenetic process in sponges (phylum Porifera). The primmorph system was used to demonstrate that in the demosponge Suberites domuncula the synthesis of the siliceous spicules starts intracellularly and is dependent on the concentration of silicic acid. To understand spicule formation, a cluster of genes was isolated. In the center of this cluster is the silicatein gene, which codes for the enzyme that synthesizes spicules. This gene is flanked by an ankyrin repeat gene at one side and by a tumor necrosis factor receptor-associated factor and a protein kinase gene at the other side. All genes are strongly expressed in primmorphs and intact animals after exposure to silicic acid, and this expression is restricted to those areas where the spicule formation starts or where spicules are maintained in the animals. Our observations suggest that in S. domuncula a coordinated expression of physically linked genes is essential for the synthesis of the major skeletal elements.

Molecular determinants of the structural and functional organization of the sarcoplasmic reticulum

The endo-sarcoplasmic reticulum of striated muscle cells consists of distinct functional domains that are extremely well organized both in terms of functional specialisation and of spatial organisation. Here we shall review recent evidence on the potential involvement of recently identified novel proteins and of cytoskeletal components in the structural and functional organization of the sarcoplasmic reticulum with respect to the surface membrane/T-tubule system and the contractile apparatus.

Obscurin is a ligand for small ankyrin 1 in skeletal muscle

The factors that organize the internal membranes of cells are still poorly understood. We have been addressing this question using striated muscle cells, which have regular arrays of membranes that associate with the contractile apparatus in stereotypic patterns. Here we examine links between contractile structures and the sarcoplasmic reticulum (SR) established by small ankyrin 1 (sAnk1), a approximately 17.5-kDa integral protein of network SR. We used yeast two-hybrid to identify obscurin, a giant Rho-GEF protein, as the major cytoplasmic ligand for sAnk1. The binding of obscurin to the cytoplasmic sequence of sAnk1 is mediated by a sequence of obscurin that is C-terminal to its last Ig-like domain. Binding was confirmed in two in vitro assays. In one, GST-obscurin, bound to glutathione-matrix, specifically adsorbed native sAnk1 from muscle homogenates. In the second, MBP-obscurin bound recombinant GST-sAnk1 in nitrocellulose blots. Kinetic studies using surface plasmon resonance yielded a K(D) = 130 nM. On subcellular fractionation, obscurin was concentrated in the myofibrillar fraction, consistent with its identification as sarcomeric protein. Nevertheless, obscurin, like sAnk1, concentrated around Z-disks and M-lines of striated muscle. Our findings suggest that obscurin binds sAnk1, and are the first to document a specific and direct interaction between proteins of the sarcomere and the SR.

Binding of an ankyrin-1 isoform to obscurin suggests a molecular link between the sarcoplasmic reticulum and myofibrils in striated muscles

Assembly of specialized membrane domains, both of the plasma membrane and of the ER, is necessary for the physiological activity of striated muscle cells. The mechanisms that mediate the structural organization of the sarcoplasmic reticulum with respect to the myofibrils are, however, not known. We report here that ank1.5, a small splice variant of the ank1 gene localized on the sarcoplasmic reticulum membrane, is capable of interacting with a sequence of 25 aa located at the COOH terminus of obscurin. Obscurin is a giant sarcomeric protein of approximately 800 kD that binds to titin and has been proposed to mediate interactions between myofibrils and other cellular structures. The binding sites and the critical aa required in the interaction between ank1.5 and obscurin were characterized using the yeast two-hybrid system, in in vitro pull-down assays and in experiments in heterologous cells. In differentiated skeletal muscle cells, a transfected myc-tagged ank1.5 was found to be selectively restricted near the M line region where it colocalized with endogenous obscurin. The M line localization of ank1.5 required a functional obscurin-binding site, because mutations of this domain resulted in a diffused distribution of the mutant ank1.5 protein in skeletal muscle cells. The interaction between ank1.5 and obscurin represents the first direct evidence of two proteins that may provide a direct link between the sarcoplasmic reticulum and myofibrils. In keeping with the proposed role of obscurin in mediating an interaction with ankyrins and sarcoplasmic reticulum, we have also found that a sequence with homology to the obscurin-binding site of ank1.5 is present in the ank2.2 isoform, which in striated muscles has been also shown to associate with the sarcoplasmic reticulum. Accordingly, a peptide containing the COOH terminus of ank2.2 fused with GST was found to bind to obscurin. Based on reported evidence showing that the COOH terminus of ank2.2 is necessary for the localization of ryanodine receptors and InsP3 receptors in the sarcoplasmic reticulum, we propose that obscurin, through multiple interactions with ank1.5 and ank2.2 isoforms, may assemble a large protein complex that, in addition to a structural function, may play a role in the organization of specific subdomains in the sarcoplasmic reticulum.

Identification of Ank(G107), a muscle-specific ankyrin-G isoform

We previously showed that alternatively spliced ankyrins-G, the Ank3 gene products, are expressed in skeletal muscle and localize to the postsynaptic folds and to the sarcoplasmic reticulum. Here we report the molecular cloning, tissue expression, and subcellular targeting of Ank(G107), a novel ankyrin-G from rat skeletal muscle. Ank(G107) lacks the entire ANK repeat domain and contains a 76-residue sequence near the COOH terminus. This sequence shares homology with COOH-terminal sequences of ankyrins-R and ankyrins-B, including the muscle-specific skAnk1. Despite widespread tissue expression of Ank3, the 76-residue sequence is predominantly detected in transcripts of skeletal muscle and heart, including both major 8- and 5.6-kb mRNAs of skeletal muscle. In 15-day-old rat skeletal muscle, antibodies against the 76-residue sequence localized to the sarcolemma and to the postsynaptic membrane and cross-reacted with three endogenous ankyrins-G, including one 130-kDa polypeptide that comigrated with in vitro translated Ank(G107). In adult muscle, these polypeptides appeared significantly decreased, and immunofluorescence labeling was no more detectable. Green fluorescent protein-tagged Ank(G107) transfected in primary cultures of rat myotubes was targeted to the plasma membrane. Deletion of the 76-residue insert resulted in additional cytoplasmic labeling suggestive of a reduced stability of Ank(G107) at the membrane. Recruitment of the COOH-terminal domain to the membrane was much less efficient but still possible only in the presence of the 76-residue insert. We conclude that the 76-residue sequence contributes to the localization and is essential to the stabilization of Ank(G107) at the membrane. These results suggest that tissue-dependent and developmentally regulated alternative processing of ankyrins generates isoforms with distinct sequences, potentially involved in specific protein-protein interactions during differentiation of the sarcolemma and, in particular, of the postsynaptic membrane.

beta III spectrin binds to the Arp1 subunit of dynactin

Cytoplasmic dynein is an intracellular motor responsible for endoplasmic reticulum-to-Golgi vesicle trafficking and retrograde axonal transport. The accessory protein dynactin has been proposed to mediate the association of dynein with vesicular cargo. Dynactin contains a 37-nm filament made up of the actin-related protein, Arp1, which may interact with a vesicle-associated spectrin network. Here, we demonstrate that Arp1 binds directly to the Golgi-associated betaIII spectrin isoform. We identify two Arp1-binding sites in betaIII spectrin, one of which overlaps with the actin-binding site conserved among spectrins. Although conventional actin binds weakly to betaIII spectrin, Arp1 binds robustly in the presence of excess F-actin. Dynein, dynactin, and betaIII spectrin co-purify on vesicles isolated from rat brain, and betaIII spectrin co-immunoprecipitates with dynactin from rat brain cytosol. In interphase cells, betaIII spectrin and dynactin both localize to cytoplasmic vesicles, co-localizing most significantly in the perinuclear region of the cell. In dividing cells, betaIII spectrin and dynactin co-localize to the developing cleavage furrow and mitotic spindle, a novel localization for betaIII spectrin. We hypothesize that the interaction between betaIII spectrin and Arp1 recruits dynein and dynactin to intracellular membranes and provides a direct link between the microtubule motor complex and its membrane-bounded cargo.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

A new spectrin, beta IV, has a major truncated isoform that associates with promyelocytic leukemia protein nuclear bodies and the nuclear matrix

We isolated cDNAs that encode a 77-kDa peptide similar to repeats 10-16 of beta-spectrins. Its gene localizes to human chromosome 19q13.13-q13.2 and mouse chromosome 7, at 7.5 centimorgans. A 289-kDa isoform, similar to full-length beta-spectrins, was partially assembled from sequences in the human genomic DNA data base and completely cloned and sequenced. RNA transcripts are seen predominantly in the brain, and Western analysis shows a major peptide that migrates as a 72-kDa band. This new gene, spectrin betaIV, thus encodes a full-length minor isoform (SpbetaIVSigma1) and a truncated major isoform (SpbetaIVSigma5). Immunostaining of cells shows a micropunctate pattern in the cytoplasm and nucleus. In mesenchymal stem cells, the staining concentrates at nuclear dots that stain positively for the promyelocytic leukemia protein (PML). Expression of SpbetaIVSigma5 fused to green fluorescence protein in cells produces nuclear dots that include all PML bodies, which double in number in transfected cells. Deletion analysis shows that partial repeats 10 and 16 of SpbetaIVSigma5 are necessary for nuclear dot formation. Immunostaining of whole-mount nuclear matrices reveals diffuse positivity with accentuation at PML bodies. Spectrin betaIV is the first beta-spectrin associated with a subnuclear structure and may be part of a nuclear scaffold to which gene regulatory machinery binds.

Association of spectrin-like proteins with the actin-organized aggregate of endoplasmic reticulum in the Spitzenkörper of gravitropically tip-growing plant cells

Spectrin-like epitopes were immunochemically detected and immunofluorescently localized in gravitropically tip-growing rhizoids and protonemata of characean algae. Antiserum against spectrin from chicken erythrocytes showed cross-reactivity with rhizoid proteins at molecular masses of about 170 and 195 kD. Confocal microscopy revealed a distinct spherical labeling of spectrin-like proteins in the apices of both cell types tightly associated with an apical actin array and a specific subdomain of endoplasmic reticulum (ER), the ER aggregate. The presence of spectrin-like epitopes, the ER aggregate, and the actin cytoskeleton are strictly correlated with active tip growth. Application of cytochalasin D and A23187 has shown that interfering with actin or with the calcium gradient, which cause the disintegration of the ER aggregate and abolish tip growth, inhibits labeling of spectrin-like proteins. At the beginning of the graviresponse in rhizoids the labeling of spectrin-like proteins remained in its symmetrical position at the cell tip, but was clearly displaced to the upper flank in gravistimulated protonemata. These findings support the hypothesis that a displacement of the Spitzenkörper is required for the negative gravitropic response in protonemata, but not for the positive gravitropic response in rhizoids. It is evident that the actin/spectrin system plays a role in maintaining the organization of the ER aggregate and represents an essential part in the mechanism of gravitropic tip growth.

Ankyrins and cellular targeting of diverse membrane proteins to physiological sites

Ankyrins are spectrin-binding proteins that associate via ANK repeats with a variety of ion channels/pumps, calcium release channels and cell adhesion molecules. Recent studies in mice indicate that ankyrins have a physiological role in restricting voltage-gated sodium channels and members of the L1 CAM family of cell adhesion molecules to excitable membranes in the central nervous system and in targeting calcium-release channels to the calcium homeostasis compartment of striated muscle.

Skin-specific expression of ank-3(93), a novel ankyrin-3 splice variant

Ankyrins represent a protein family whose members are associated with membrane proteins and the actin cytoskeleton. The principal ankyrin domain structure comprises an amino-terminal membrane-binding, a spectrin-binding, and a regulatory domain, and can be modulated by alternative splicing. In order to investigate the role of ankyrin-3 in skin, we have isolated three complete ankyrin-3 cDNA clones of 5.8 kb, 5.2 kb, and 2.5 kb by reverse transcription-polymerase chain reaction of mouse skin RNA. DNA sequencing confirmed the isolated clones to be splice variants of ankyrin-3. Of these, the smallest cDNA represents a novel ankyrin named ankyrin-3(93). Surprisingly, this novel ankyrin subtype lacks not only all ankyrin repeats, but also the first 75 amino acids of the spectrin-binding domain. Immuno-fluorescence analysis of mouse skin showed that ankyrin-3 is expressed in all living layers of mouse epidermis. Here, it predominates along the basal and lateral membranes of the basal layer in addition to an even cytoplasmic distribution. In primary mouse keratinocytes grown at elevated Ca2+ levels, ankyrin-3(93) was localized along the plasma membrane and throughout the cell in a Golgi-like fashion. Depending on fixation conditions, nuclear staining became apparent in many cells. In agreement with previous data, northern blotting revealed a widespread distribution of the two larger ankyrin splice variants. In contrast, the mRNA coding for ankyrin-3(93)was restricted to mouse skin. Reverse transcription-polymerase chain reaction of mouse skin RNA strongly suggested additional ankyrin isoforms in skin. Our data on ankyrin-3(93), which lacks a part of the spectrin-binding domain that regulates the affinity to spectrin, suggests a new function for this member of the ankyrin family.

betaIV spectrin, a new spectrin localized at axon initial segments and nodes of ranvier in the central and peripheral nervous system

We report the identification of betaIV spectrin, a novel spectrin isolated as an interactor of the receptor tyrosine phosphatase-like protein ICA512. The betaIV spectrin gene is located on human and mouse chromosomes 19q13.13 and 7b2, respectively. Alternative splicing of betaIV spectrin generates at least four distinct isoforms, numbered betaIVSigma1-betaIVSigma4 spectrin. The longest isoform (betaIVSigma1 spectrin) includes an actin-binding domain, followed by 17 spectrin repeats, a specific domain in which the amino acid sequence ERQES is repeated four times, several putative SH3-binding sites and a pleckstrin homology domain. betaIVSigma2 and betaIVSigma3 spectrin encompass the NH(2)- and COOH-terminal halves of betaIVSigma1 spectrin, respectively, while betaIVSigma4 spectrin lacks the ERQES and the pleckstrin homology domain. Northern blots revealed an abundant expression of betaIV spectrin transcripts in brain and pancreatic islets. By immunoblotting, betaIVSigma1 spectrin is recognized as a protein of 250 kD. Anti-betaIV spectrin antibodies also react with two additional isoforms of 160 and 140 kD. These isoforms differ from betaIVSigma1 spectrin in terms of their distribution on subcellular fractionation, detergent extractability, and phosphorylation. In islets, the immunoreactivity for betaIV spectrin is more prominent in alpha than in beta cells. In brain, betaIV spectrin is enriched in myelinated neurons, where it colocalizes with ankyrin(G) 480/270-kD at axon initial segments and nodes of Ranvier. Likewise, betaIV spectrin is concentrated at the nodes of Ranvier in the rat sciatic nerve. In the rat hippocampus, betaIVSigma1 spectrin is detectable from embryonic day 19, concomitantly with the appearance of immunoreactivity at the initial segments. Thus, we suggest that betaIVSigma1 spectrin interacts with ankyrin(G) 480/270-kD and participates in the clustering of voltage-gated Na(+) channels and cell-adhesion molecules at initial segments and nodes of Ranvier.

Human spectrin Src homology 3 domain binding protein 1 regulates macropinocytosis in NIH 3T3 cells

Macropinocytosis is an endocytic process that occurs through non-clathrin coated vesicles larger than 0.2 microm in diameter. Although macropinocytic vesicles are readily visualized in cultured cells by the introduction of fluorescent, water-soluble dyes into the culture medium, protein markers associated with this type of vesicles have not yet been well defined. Here, we report that human spectrin SH3 domain binding protein 1, or Hssh3bp1, associates with macropinosomes in NIH 3T3 fibroblasts. Hssh3bp1 macropinosomes are heterogeneous in morphology and size, do not endocytose transferrin and are resistant to brefeldin A treatment. Cytochalasin D, and wortmannin block endocytosis of fluorescent dyes into the Hssh3bp1 macropinosomes and dramatically affect their morphology. Overexpression of Hssh3bp1-green fluorescent protein abolished fusion of vesicles resulting in a decreased endocytosis of fluorescence dyes, thus suggesting a potential regulatory role of Hssh3bp1 in macropinocytosis. In the macropinosomes of NIH 3T3 cells, Hssh3bp1 associates with a 200-kDa protein that crossreacts with a monoclonal antibody to the erythroid alpha-spectrin SH3 domain. Thus macropinosomes in cells may contain a spectrin-like protein.

Ankyrins

This review is focused on ankyrin which is a protein linker between the integral membrane proteins and spectrin-based cytoskeleton. Structure and distribution of different ankyrin isoforms that are products of alternative-spliced genes are described. Interaction of ankyrins with various membranes is considered. Special attention is paid to ankyrin participation in signal transduction and in assembly of integral membrane proteins in specialized membrane domains.

ankA: an Ehrlichia phagocytophila group gene encoding a cytoplasmic protein antigen with ankyrin repeats

Human granulocytic ehrlichiosis (HGE) is a potentially fatal, tick-borne disease caused by a bacterium related or identical to Ehrlichia phagocytophila. To identify and characterize E. phagocytophila group-specific protein antigen genes, we prepared and screened HGE agent and Ehrlichia equi genomic DNA expression libraries using polyclonal equine E. equi antibodies. Two clones, one each from HGE agent and E. equi, that were recognized specifically by antibodies to the E. phagocytophila group ehrlichiae had complete open reading frames of 3,693 and 3,615 nucleotides, respectively. The two clones were 96.6% identical and predicted a protein with at least 11 tandemly repeated ankyrin motifs. Thus, the gene was named ank (for ankyrin). When the encoded protein, named AnkA, was expressed in Escherichia coli, it was recognized by antibodies from rabbits and mice immunized with the HGE agent, sera from humans convalescent from HGE, and sera from horses convalescent from HGE and E. equi infection. Monospecific AnkA antibodies reacted with proteins in HGE agent immunoblots, and AnkA monoclonal antibodies detected cytoplasmic antigen in E. phagocytophila group bacteria and also detected antigen associated with chromatin in infected but not uninfected HL-60 cell cultures. These results suggest that this Ehrlichia protein may influence host cell gene expression.

Spectrin tethers and mesh in the biosynthetic pathway

The paradox of how the Golgi and other organelles can sort a continuous flux of protein and lipid but maintain temporal and morphological stability remains unresolved. Recent discoveries highlight a role for the cytoskeleton in guiding the structure and dynamics of organelles. Perhaps one of the more striking, albeit less expected, of these discoveries is the recognition that a spectrin skeleton associates with many organelles and contributes to the maintenance of Golgi structure and the efficiency of protein trafficking in the early secretory pathway. Spectrin interacts directly with phosphoinositides and with membrane proteins. The small GTPase ARF, a key player in Golgi dynamics, regulates the assembly of the Golgi spectrin skeleton through its ability to control phosphoinositide levels in Golgi membranes, whereas adapter molecules such as ankyrin link spectrin to other membrane proteins. Direct interactions of spectrin with actin and centractin (ARP1) provide a link to dynein, myosin and presumably other motors involved with intracellular transport. Building on the recognized ability of spectrin to organize macromolecular complexes of membrane and cytosolic proteins into a multifaceted scaffold linked to filamentous structural elements (termed linked mosaics), recent evidence supports a similar role for spectrin in organelle function and the secretory pathway. Two working models accommodate much of the available data: the Golgi mesh hypothesis and the spectrin ankyrin adapter protein tethering system (SAATS) hypothesis.

Association of spectrin with a subcompartment of the endoplasmic reticulum in honeybee photoreceptor cells

The endoplasmic reticulum (ER) in honeybee photoreceptors is organized into structurally distinct subregions. The most prominent of these, the submicrovillar network of ER cisternae, is tightly associated with actin filaments. Electron microscopic techniques have demonstrated that the ER-associated actin filaments are regularly spaced at 60-80 nm and cross-bridged by filamentous structures. A polyclonal antibody against Drosophila alpha-spectrin has been used to examine the distribution of spectrin in the photoreceptors. On Western blots of bee retina, the antibody identifies a 260-kDa protein that exhibits biochemical and immunological properties characteristic of alpha-spectrin. Immunofluorescence microscopy has shown that alpha-spectrin codistributes with the submicrovillar ER but not with other ER subdomains. After cytochalasin-B-induced depolymerization of the ER-associated F-actin system, alpha-spectrin remains colocalized with the ER, indicating that alpha-spectrin is bound to the ER membrane. The F-actin/spectrin system associated with the submicrovillar ER may stabilize the shape of this ER subcompartment and may play a role in maintaining functional ER subregions.

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.

New insights into functions of erythroid proteins in nonerythroid cells

This article presents new insights into potential roles that three erythrocyte cytoskeletal proteins, protein 4.1, ankyrin, and spectrin, may play in nonerythroid nucleated cells. Each of these proteins is encoded by several closely related genes characterized by complex alternative splicing of their pre-mRNA, thus resulting in the cellular expression of a broad repertoire of isoforms that can adopt tissue- and cell-specific distribution. This could account for the presence of skeletal networks in intracellular organelles such as lysosomes, the Golgi apparatus, or the nucleus. In addition to providing structural support to cell membranes, these cytoskeletal proteins regulate the functions of various transmembrane proteins they interact with, in particular ion channels, as well as the activity of membrane-bound enzymes. Thus, they appear to be key players in major unsuspected cell functions such as protein sorting, dynamics of nuclear architecture during mitosis, or regulation of signal transduction pathways.

Ankyrin-B is required for intracellular sorting of structurally diverse Ca2+ homeostasis proteins

This report describes a congenital myopathy and major loss of thymic lymphocytes in ankyrin-B (-/-) mice as well as dramatic alterations in intracellular localization of key components of the Ca(2+) homeostasis machinery in ankyrin-B (-/-) striated muscle and thymus. The sarcoplasmic reticulum (SR) and SR/T-tubule junctions are apparently preserved in a normal distribution in ankyrin-B (-/-) skeletal muscle based on electron microscopy and the presence of a normal pattern of triadin and dihydropyridine receptor. Therefore, the abnormal localization of SR/ER Ca ATPase (SERCA) and ryanodine receptors represents a defect in intracellular sorting of these proteins in skeletal muscle. Extrapolation of these observations suggests defective targeting as the basis for abnormal localization of ryanodine receptors, IP3 receptors and SERCA in heart, and of IP3 receptors in the thymus of ankyrin-B (-/-) mice. Mis-sorting of SERCA 2 and ryanodine receptor 2 in ankyrin-B (-/-) cardiomyocytes is rescued by expression of 220-kD ankyrin-B, demonstrating that lack of the 220-kD ankyrin-B polypeptide is the primary defect in these cells. Ankyrin-B is associated with intracellular vesicles, but is not colocalized with the bulk of SERCA 1 or ryanodine receptor type 1 in skeletal muscle. These data provide the first evidence of a physiological requirement for ankyrin-B in intracellular targeting of the calcium homeostasis machinery of striated muscle and immune system, and moreover, support a catalytic role that does not involve permanent stoichiometric complexes between ankyrin-B and targeted proteins. Ankyrin-B is a member of a family of adapter proteins implicated in restriction of diverse proteins to specialized plasma membrane domains. Similar mechanisms involving ankyrins may be essential for segregation of functionally defined proteins within specialized regions of the plasma membrane and within the Ca(2+) homeostasis compartment of the ER.

Functional domains of the very low density lipoprotein receptor: molecular analysis of ligand binding and acid-dependent ligand dissociation mechanisms

The very low density lipoprotein (VLDL) receptor is closely related in structure to the low density lipoprotein receptor. The ectodomain of these endocytic receptors is composed of modules which include clusters of cysteine-rich class A repeats, epidermal growth factor (EGF)-like repeats, tyrosine-tryptophan-threonine-aspartic acid (YWTD) repeats and an O-linked sugar domain. To identify important functional regions within the ectodomain of the VLDL receptor, we produced a mutant receptor in which the EGF, YWTD and O-linked sugar domains were deleted. Cells transfected with the mutant receptor were able to bind and internalize (125)I-labeled receptor associated protein (RAP). In contrast to the wild-type receptor, however, RAP did not dissociate from the mutant receptor and consequently was not degraded. Immunofluoresence data indicated that once bound to the mutant receptor, fluorescent-labeled RAP co-localized with markers of the endosomal pathway, whereas, in cells expressing the wild-type receptor, RAP fluorescence co-localized with lysosomal markers. Thus this deleted region is responsible for ligand uncoupling within the endosomes. To identify regions responsible for ligand recognition, soluble receptor fragments containing the eight cysteine-rich class A repeats were produced. (125)I-RAP and (125)I-labeled urokinase-type plasminogen activator:plasminogen activator inhibitor type I (uPA:PAI-1) complexes bound to the soluble fragment with K(D, app) values of 0.3 and 14 nM, respectively. Deletion analysis demonstrate that high affinity RAP binding requires the first four cysteine-rich class A repeats (L1-4) in the VLDL receptor while the second repeat (L2) appears responsible for binding uPA:PAI-1 complexes. Together, these results confirm that ligand uncoupling occurs via an allosteric-type mechanism in which pH induced changes in the EGF and/or YWTD repeats alter the ligand binding properties at the amino-terminal portion of the molecule.

Rearrangements of Actin Cytoskeleton during Infection with Escherichia coli O157 in Macrophages

The lamina propria of the large intestine is rich in macrophages, and they might be one of the first lines of the host defense in enterohemorrhagic Escherichia coli (EHEC) O157:H7 infection. Although macrophages were infected with them, they can survive the EHEC O157 infection. We examined the structural rearrangements of the actin cytoskeleton during the microbial infection process. Macrophage actin filaments were rearranged in the following sequence; 1) disappearance of the actin filament bundles in the cytoplasm, 2) accumulation of actin filaments under the cell surface, and 3) construction of actin networks underlying the endosome membrane. Before infection, actin filaments were distributed under the cell surface and in bundles located in the macrophage cytoplasm. Within 2 min, infection caused a rapid and marked loss of the actin filament bundles that had run parallel to the long axis of the cell. Concomitant with the loss, actin filaments became more markedly distributed under the cell surface. In the formation of the endosome, new networks of actin filaments were constructed below the phagosome membrane. The networks contained a large amount of actin as well as a fodrin-like immunoreactivity. The thickness of the networks reached about 400 nm under the phagosome membrane. The actin networks disappeared again after the bacterial digestion. The results of this study showed that actin filaments undergo three major rearrangements of the actin filaments during the infection in macrophages, and suggested that the third rearrangement is mediated by actin-binding proteins, such as a fodrin-like molecules. These morphological changes in macrophages were not clear after infection with other strains of Escherichia coli.

The localization of myosin VI at the golgi complex and leading edge of fibroblasts and its phosphorylation and recruitment into membrane ruffles of A431 cells after growth factor stimulation

Myosin VI is an unconventional myosin that may play a role in vesicular membrane traffic through actin rich regions of the cytoplasm in eukaryotic cells. In this study we have cloned and sequenced a cDNA encoding a chicken intestinal brush border myosin VI. Polyclonal antisera were raised to bacterially expressed fragments of this myosin VI. The affinity purified antibodies were highly specific for myosin VI by immunoblotting and immunoprecipitation and were used to study the localization of the protein by immunofluorescence and immunoelectron microscopy. It was found that in NRK and A431 cells, myosin VI was associated with both the Golgi complex and the leading, ruffling edge of the cell as well as being present in a cytosolic pool. In A431 cells in which cell surface ruffling was stimulated by EGF, myosin VI was phosphorylated and recruited into the newly formed ruffles along with ezrin and myosin V. In vitro experiments suggested that a p21-activated kinase (PAK) might be the kinase responsible for phosphorylation in the motor domain. These results strongly support a role for myosin VI in membrane traffic on secretory and endocytic pathways.

A widely expressed betaIII spectrin associated with Golgi and cytoplasmic vesicles

Spectrin is an important structural component of the plasma membrane skeleton. Heretofore-unidentified isoforms of spectrin also associate with Golgi and other organelles. We have discovered another member of the beta-spectrin gene family by homology searches of the GenBank databases and by 5' rapid amplification of cDNA ends of human brain cDNAs. Collectively, 7,938 nucleotides of contiguous clones are predicted to encode a 271,294-Da protein, called betaIII spectrin, with conserved actin-, protein 4.1-, and ankyrin-binding domains, membrane association domains 1 and 2, a spectrin dimer self-association site, and a pleckstrin-homology domain. betaIII spectrin transcripts are concentrated in the brain and present in the kidneys, liver, and testes and the prostate, pituitary, adrenal, and salivary glands. All of the tested tissues contain major 9.0-kb and minor 11.3-kb transcripts. The human betaIII spectrin gene (SPTBN2) maps to chromosome 11q13 and the mouse gene (Spnb3) maps to a syntenic region close to the centromere on chromosome 19. Indirect immunofluorescence studies of cultured cells using antisera specific to human betaIII spectrin reveal a Golgi-associated and punctate cytoplasmic vesicle-like distribution, suggesting that betaIII spectrin associates with intracellular organelles. This distribution overlaps that of several Golgi and vesicle markers, including mannosidase II, p58, trans-Golgi network (TGN)38, and beta-COP and is distinct from the endoplasmic reticulum markers calnexin and Bip. Liver Golgi membranes and other vesicular compartment markers cosediment in vitro with betaIII spectrin. betaIII spectrin thus constitutes a major component of the Golgi and vesicular membrane skeletons.

Biogenesis of polarized epithelial cells during kidney development in situ: roles of E-cadherin-mediated cell-cell adhesion and membrane cytoskeleton organization

Organization of proteins into structurally and functionally distinct plasma membrane domains is an essential characteristic of polarized epithelial cells. Based on studies with cultured kidney cells, we have hypothesized that a mechanism for restricting Na/K-ATPase to the basal-lateral membrane involves E-cadherin-mediated cell-cell adhesion and integration of Na/K-ATPase into the Triton X-100-insoluble ankyrin- and spectrin-based membrane cytoskeleton. In this study, we examined the relevance of these in vitro observations to the generation of epithelial cell polarity in vivo during mouse kidney development. Using differential detergent extraction, immunoblotting, and immunofluorescence histochemistry, we demonstrate the following. First, expression of the 220-kDa splice variant of ankyrin-3 correlates with the development of resistance to Triton X-100 extraction for Na/K-ATPase, E-cadherin, and catenins and precedes maximal accumulation of Na/K-ATPase. Second, expression of the 190-kDa slice variant of ankyrin-3 correlates with maximal accumulation of Na/K-ATPase. Third, Na/K-ATPase, ankyrin-3, and fodrin specifically colocalize at the basal-lateral plasma membrane of all epithelial cells in which they are expressed and during all stages of nephrogenesis. Fourth, the relative immunofluorescence staining intensities of Na/K-ATPase, ankyrin-3, and fodrin become more similar during development until they are essentially identical in adult kidney. Thus, renal epithelial cells in vivo regulate the accumulation of E-cadherin-mediated adherens junctions, the membrane cytoskeleton, and Na/K-ATPase through sequential protein expression and assembly on the basal-lateral membrane. These results are consistent with a mechanism in which generation and maintenance of polarized distributions of these proteins in vivo and in vitro involve cell-cell adhesion, assembly of the membrane cytoskeleton complex, and concomitant integration and retention of Na/K-ATPase in this complex.

Identification of a novel ankyrin isoform (AnkG190) in kidney and lung that associates with the plasma membrane and binds alpha-Na, K-ATPase

Ankyrins are a family of adapter molecules that mediate linkages between integral membrane and cytoskeletal proteins. Such interactions are crucial to the polarized distribution of membrane proteins in transporting epithelia. We have cloned and characterized a novel 190-kDa member of this family from a rat kidney cDNA library, which we term AnkG190 based on the predicted size and homology with the larger neuronal AnkG isoform. AnkG190 displays a unique 31-residue amino terminus, a repeats domain consisting of 24 repetitive 33-residue motifs, a spectrin binding domain, and a truncated regulatory domain. Probes derived from the unique amino terminus hybridize to an 8-kilobase message exclusively in kidney and lung and specifically to the kidney outer medullary collecting ducts by in situ hybridization. Transfections of Madin-Darby canine kidney and COS-7 epithelial cell lines with a full-length AnkG190 construct result in (a) expression at the lateral plasma membrane, (b) functional assembly with the cytoskeleton, and (c) interaction with at least one membrane protein, the Na,K-ATPase. Two independent Na,K-ATPase binding domains on AnkG190 are demonstrated as follows: one within the distal 12 ankyrin repeats, and a second site within the spectrin binding domain. Thus, ankyrins may interact with integral membrane proteins in a pleiotropic manner that may involve complex tertiary structural determinants.

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.

AnkyrinG is associated with the postsynaptic membrane and the sarcoplasmic reticulum in the skeletal muscle fiber

Ankyrins are a multi-gene family of peripheral proteins that link ion channels and cell adhesion molecules to the spectrin-based skeleton in specialized membrane domains. In the mammalian skeletal myofiber, ankyrins were immunolocalized in several membrane domains, namely the costameres, the postsynaptic membrane and the triads. Ank1 and Ank3 transcripts were previously detected in skeletal muscle by northern blot analysis. However, the ankyrin isoforms associated with these domains were not identified, with the exception of an unconventional Ank1 gene product that was recently localized at discrete sites of the sarcoplasmic reticulum. Here we study the expression and subcellular distribution of the Ank3 gene products, the ankyrinsG, in the rat skeletal muscle fiber. Northern blot analysis of rat skeletal muscle mRNAs using domain-specific Ank3 cDNA probes revealed two transcripts of 8.0 kb and 5.6 kb containing the spectrin-binding and C-terminal, but not the serine-rich, domains. Reverse transcriptase PCR analysis of rat skeletal muscle total RNA confirmed the presence of Ank3 transcripts that lacked the serine-rich and tail domains, a major insert of 7813 bp at the junction of the spectrin-binding and C-terminal domains that was previously identified in brain Ank3 transcripts. Immunoblot analysis of total skeletal muscle homogenates using ankyrinG-specific antibodies revealed one major 100 kDa ankyrinG polypeptide. Immunofluorescence labeling of rat diaphragm cryosections showed that ankyrin(s)G are selectively associated with (1) the depths of the postsynaptic membrane folds, where the voltage-dependent sodium channel and N-CAM accumulate, and (2) the sarcoplasmic reticulum, as confirmed by codistribution with the sarcoplasmic reticulum Ca2+-ATPase (SERCA 1). At variance with ankyrin(s)G, ankyrin(s)R (ank1 gene products) accumulate at the sarcolemma and at sarcoplasmic structures, in register with A-bands. Both ankyrin isoforms codistributed over Z-lines and at the postsynaptic membrane. These data extend the notion that ankyrins are differentially localized within myofibers, and point to a role of the ankyrinG family in the organization of the sarcoplasmic reticulum and the postsynaptic membrane.