Expression and assembly of the erythroid membrane-skeletal proteins ankyrin (goblin) and spectrin in the morphogenesis of chicken neurons

The membrane-skeleton of adult chicken neurons in the cerebellum and optic system is composed of polypeptides structurally and functionally related to the erythroid proteins spectrin and ankyrin, respectively. Neuronal spectrin comprises two distinct complexes that share a common alpha subunit (Mr 240,000) but which have structurally distinct polymorphic subunits (beta' beta spectrin; Mr 220/225,000; gamma spectrin, Mr 235,000); the brain-specific form (alpha gamma spectrin or fodrin) and an erythrocyte-specific form (alpha beta' beta spectrin). Two structurally related isoforms of ankyrin have also been identified and are termed alpha (Mr 260,000) and beta (Mr 237,000) ankyrin. Immunofluorescence demonstrates that the variants of spectrin and ankyrin, respectively, have different distributions within neurons. On the one hand, alpha gamma spectrin and beta ankyrin are present throughout the neuron, in the perikaryon, dendrites, and axon, whereas alpha beta' spectrin and alpha ankyrin are localized exclusively in the perikaryon and dendrites where they are actively segregated from alpha gamma spectrin and other components of axonal transport. This asymmetric distribution of spectrin and ankyrin isoforms is established in distinct stages during neuronal morphogenesis. Early in cerebellar and retinal development, alpha gamma spectrin is expressed in mitotic cells. Subsequently beta ankyrin and alpha gamma spectrin are coexpressed in postmitotic cells and gradually accumulate on the plasma membrane in a uniform pattern throughout the neuron during the phase of cell growth. At the onset of synaptogenesis and the cessation of cell growth, their levels of synthesis decline sharply while the assembled proteins remained as stable membrane components. Concomitantly, there is a dramatic induction in the accumulation of alpha ankyrin and alpha beta' spectrin, whose assembly is limited to the plasma membrane of the perikarya and dendrites. These results demonstrate that two successive, developmentally regulated programs of ankyrin and spectrin expression and patterning on the plasma membrane are involved in the assembly of the spectrin-based asymmetry in the neuronal membrane-skeleton, and that their asymmetric distribution is actively maintained throughout the life of the neuron.     

From the spectrin gene to the assembly of the membrane skeleton

The complete nucleotide sequence coding for the chicken brain alpha-spectrin was determined. It comprises the entire coding frame, 5'- and 3'-untranslated sequences terminating in a poly(A)-tail. The deduced amino acid sequence shows that the alpha-chain contains 22 segments, 20 of which correspond to the typical 106 residue repeat of the human erythrocyte spectrin. Some segments non-homologous to the repeat structure reside in the middle and COOH-terminal regions. Sequence comparisons with other proteins show that these segments evidently harbour some structural and functional features such as: homology to alpha-actinin and dystrophin, two typical EF-hand structures (calcium-binding) and a putative calmodulin-binding site in the COOH-terminus and a sequence homologous to various src-tyrosine kinases and to phospholipase C in the middle of the molecule. Comparison of our sequence with other partial alpha-spectrin sequences shows that alpha-spectrin is well conserved in different species and that the human erythrocyte alpha-spectrin is divergent.     

Posttranslational control of membrane-skeleton (ankyrin and alpha beta- spectrin) assembly in early myogenesis

Adult chicken skeletal muscle cells express polypeptides that are antigenically related to alpha-spectrin (Mr 240,000) and beta-spectrin (Mr 220,000-225,000), the major components of the erythrocyte membrane-skeleton, and to ankyrin (Mr 237,000; also termed goblin in chicken erythrocytes), which binds spectrin to the transmembrane anion transporter in erythrocytes. Comparative immunoblotting of SDS-solubilized extracts of presumptive myoblasts and fully differentiated myotubes cultured in vitro demonstrated that there is a dramatic accumulation of ankyrin and alpha- and beta-spectrin during myogenesis and a concomitant switch in the subunit composition of spectrin from alpha gamma to alpha beta. Analysis of early time points in myogenesis (12-96 h) revealed that these changes occur shortly after the main burst of cell fusion. To determine the temporal relationship between cell fusion and the accumulation of ankyrin and alpha- and beta-spectrin, we treated presumptive myoblasts with 2 mM EGTA, which resulted in the complete inhibition of cell fusion. The incorporation of [35S]methionine into total protein and, specifically, into alpha-, gamma-, and beta-spectrin remained the same in EGTA-treated and control cells. Analysis by immunoblotting of the amounts of ankyrin and alpha- and beta-spectrin in fusion-blocked cells revealed that there was no effect on accumulation for the first 19 h. However, there was then a dramatic cessation in their accumulation, and thereafter, the amount of each protein at steady state remained constant. Upon release from the EGTA block, the cells fused rapidly (less than 11 h), and the accumulation of ankyrin and alpha- and beta-spectrin was reinitiated after a lag period of 3-5 h at a rate similar to that in control cells. The inhibition in the accumulation of newly synthesized ankyrin, alpha-spectrin, and beta-spectrin in EGTA-treated myoblasts was not characteristic of all structural proteins, since the accumulation of the muscle-specific intermediate filament protein desmin was the same in control and fusion-blocked cells. These results show that in myogenesis, the synthesis of ankyrin and alpha- and beta-spectrin and their accumulation as a complex, although concurrent, are not coupled events. We hypothesize that the extent of assembly of these components of the membrane-skeleton in muscle cells is determined by a control mechanism(s) operative at the posttranslational level that is triggered near the time of cell fusion and the onset of terminal differentiation.     

Drosophila spectrin: the membrane skeleton during embryogenesis

The distribution of alpha-spectrin in Drosophila embryos was determined by immunofluorescence using affinity-purified polyclonal or monoclonal antibodies. During early development, spectrin is concentrated near the inner surface of the plasma membrane, in cytoplasmic islands around the syncytial nuclei, and, at lower concentrations, throughout the remainder of the cytoplasm of preblastoderm embryos. As embryogenesis proceeds, the distribution of spectrin shifts with the migrating nuclei toward the embryo surface so that, by nuclear cycle 9, a larger proportion of the spectrin is concentrated near the plasma membrane. During nuclear cycles 9 and 10, as the nuclei reach the cell surface, the plasma membrane-associated spectrin becomes concentrated into caps above the somatic nuclei. Concurrent with the mitotic events of the syncytial blastoderm period, the spectrin caps elongate at interphase and prophase, and divide as metaphase and anaphase progress. During cellularization, the regions of spectrin concentration appear to shift: spectrin increases near the growing furrow canal and concomitantly increases at the embryo surface. In the final phase of furrow growth, the shift in spectrin concentration is reversed: spectrin decreases near the furrow canal and concomitantly increases at the embryo surface. In gastrulae, spectrin accumulates near the embryo surface, especially at the forming amnioproctodeal invagination and cephalic furrow. During the germband elongation stage, the total amount of spectrin in the embryo increases significantly and becomes uniformly distributed at the plasma membrane of almost all cell types. The highest levels of spectrin are in the respiratory tract cells; the lowest levels are in parts of the forming gut. The spatial and temporal changes in spectrin localization suggest that this protein plays a role in stabilizing rather than initiating changes in structural organization in the embryo.     

Organizing the fluid membrane bilayer: diseases linked to spectrin and ankyrin

Ankyrin and spectrin were first discovered as binding partners in the membrane skeleton of human erythrocytes. Mutations in genes encoding these proteins cause hereditary spherocytosis. Recent advances reveal that ankyrin and spectrin are required for organization of a surprisingly diverse set of proteins, including ion channels and cell adhesion molecules that are localized in specialized membrane domains in many cell types. New insights into the cell biology of ankyrin and spectrin reveal that these proteins actively participate in assembly of specialized membrane domains in addition to their conventional maintenance role as scaffolding proteins. Recently described inherited human diseases due to defects in spectrin or ankyrin include spinocerebellar ataxia type 5 and a cardiac arrhythmia, termed sick sinus syndrome with bradycardia or ankyrin-B syndrome. Together, these studies identify an emerging paradigm for pathogenesis of human disease where failure in cellular localization of membrane-spanning proteins results in loss of physiological function.     

Synthesis and assembly of spectrin during avian erythropoiesis: Stoichiometric assembly but unequal synthesis of α and β spectrin

The synthesis and assembly of spectrin was investigated in erythroid cells during chicken embryo development. Immunoprecipitation of Triton X-100-soluble and -insoluble cytoskeletal fractions with α- and β-spectrin antisera show that, at steady state, α and β spectrin are present in stoichiometric amounts, and exclusively, in the cytoskeleton. However, pulse labeling of cells and in vitro translation of total erythroid cell RNA reveal that α spectrin is synthesized in a two to three fold excess over β spectrin. Pulse-chase experiments show that newly synthesized α and β spectrin are present in both the cytoskeletal and soluble fractions, and that stoichiometric amounts are stably assembled in the cytoskeleton. On the other hand, there is a severalfold excess of α relative to β spectrin in the soluble fraction, both of which turn over with a half-life of 50 min. In cells from 4 day old embryos, more than 80% of the newly synthesized β spectrin, but only 10% of the α spectrin, are present in the cytoskeleton. Thus, early in development, the association of α and β spectrin with the membrane-cytoskeleton may be rate-limited by the amount of β spectrin synthesized. Later on in erythroid development, progressively lesser proportions of newly synthesized β spectrin are present in the cytoskeleton, suggesting that during development, the rate of association of β spectrin with the membrane-cytoskeleton becomes limited by some other membrane-cytoskeletal component.     

Reaction of Se with SH groups in spectrin is involved in the stabilization of erythrocyte membrane skeleton

Na2SeO3 supplementation in the dialysis medium could obviously prevent the dissociation of spectrin from the erythrocyte membranes. Such Se effect could be eliminated by pretreatment of erythrocyte membranes with a SH-blocking reagent, iodoacetamide(IAA) or addition of a SH-compound, dithio-threitol. The fluorescence intensity of erythrocyte membranes labelled with the fluorescent probe N-(3-pyrenyl)-maleimide decreased with increasing Na2SeO3 concentration used for pretreatment of ghosts. 31P-NMR spectra of erythrocyte membrane dialyzed in the presence or absence of Na2SeO3 concentration showed a difference in chemical shift anisotropy (delta sigma) between these two samples. These data suggest that the stabilization effect is based on changes in lipid-protein interaction and conformation of membrane skeletal components induced by reaction of their SH groups with Na2SeO3.     

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

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

Phosphatidylserine binding sites in erythroid spectrin: location and implications for membrane stability

The erythrocyte membrane is a composite structure consisting of a lipid bilayer tethered to the spectrin-based membrane skeleton. Two complexes of spectrin with other proteins are known to participate in the attachment. Spectrin has also been shown to interact with phosphatidylserine (PS), a component of the lipid bilayer, which is confined to its inner leaflet. That there may be multiple sites of interaction with PS in the spectrin sequence has been inferred, but they have not hitherto been identified. Here we have explored the interaction of PS-containing liposomes with native alpha- and beta-spectrin chains and with recombinant spectrin fragments encompassing the entire sequences of both chains. We show that both alpha-spectrin and beta-spectrin bind PS and that sites of high affinity are located within 8 of the 38 triple-helical structural repeats which make up the bulk of both chains; these are alpha8, alpha9-10, beta2, beta3, beta4, beta12, beta13, and beta14, and PS affinity was also found in the nonhomologous N-terminal domain of the beta-chain. No other fragments of either chain showed appreciable binding. Binding of spectrin and its constituent chains to mixed liposomes of PS and phosphatidylcholine (PC) depended on the proportion of PS. Binding of spectrin dimers to PS liposomes was inhibited by single repeats containing PS binding sites. It is noteworthy that the PS binding sites in beta-spectrin are grouped in close proximity to the sites of attachment both of ankyrin and of 4.1R, the proteins engaged in attachment of spectrin to the membrane. We conjecture that direct interaction of spectrin with PS in the membrane may modulate its interactions with the proteins and that (considering also the known affinity of 4.1R for PS) the formation of PS-rich lipid domains, which have been observed in the red cell membrane, may be a result.     

The oligomeric state of spectrin in the rat erythrocyte membrane skeleton

The oligomeric state of spectrin in the erythrocyte membrane skeleton of the rat was investigated following extraction in a low ionic strength buffer for 24 and 96 h. All analyses were quantitatively compared with preparations from human erythrocyte membranes. After nondenaturing agarose-polyacrylamide gel electrophoresis, the human samples revealed their characteristic spectrin oligomer pattern; there were high molecular weight complexes near the origin of the gel, followed by several high order oligomers, tetramers, and dimers. The pattern in the rat membrane skeleton also included tetramers and a high molecular weight complex band, but had only one oligomer and no dimers. With time the high molecular weight complex diminished and oligomers accumulated in both the rat and human, while dimers accumulated only in the human and tetramers accumulated only in the rat. Tetramers decreased with time in the human. Extraction of spectrin increased with time and was greater from rat than the human red cell membrane at both time points. The percentage of spectrin and actin in the low ionic strength extract was similar between species, as analyzed by SDS-polyacrylamide electrophoresis, staining, and densitometry. Proteins 4.1 and 4.9 were present in greater percentages in the human. The only temporal effect on monomeric protein composition was an increase of protein A in the rat. There was no species difference in protein A percentage at 24 h, but at 96 h the rat was greater than the human. The results suggest that there are significant differences in the structural arrangement of the rat and human erythrocyte membrane skeleton.     

Essential control of an endothelial cell ISOC by the spectrin membrane skeleton

Mechanism(s) underlying activation of store-operated Ca2+ entry currents, ISOC, remain incompletely understood. F-actin configuration is an important determinant of channel function, although the nature of interaction between the cytoskeleton and ISOC channels is unknown. We examined whether the spectrin membrane skeleton couples Ca2+ store depletion to Ca2+ entry. Thapsigargin activated an endothelial cell ISOC (-45 pA at -80 mV) that reversed at +40 mV, was inwardly rectifying when Ca2+ was the charge carrier, and was inhibited by La3+ (50 microM). Disruption of the spectrin-protein 4.1 interaction at residues A207-V445 of betaSpIISigma1 decreased the thapsigargin-induced global cytosolic Ca2+ response by 50% and selectively abolished the endothelial cell ISOC, without altering activation of a nonselective current through cyclic nucleotide-gated channels. In contrast, disruption of the spectrin-actin interaction at residues A47-K186 of betaSpIISigma1 did not decrease the thapsigargin-induced global cytosolic Ca2+ response or inhibit ISOC. Results indicate that the spectrin-protein 4.1 interaction selectively controls ISOC, indicating that physical coupling between calcium release and calcium entry is reliant upon the spectrin membrane skeleton.     

A model of spectrin as a concertina in the erythrocyte membrane skeleton

To maintain its distinctive biconcave shape, the erythrocyte has a skeleton composed largely of the protein spectrin, which associates closely and exclusively with the cell membrane. Although the membrane skeleton forms through specific protein-protein interactions of defined stoichiometry, it has a flexible structure and organization due to the unusual molecular properties of spectrin. Here we describe these properties and propose a model to account for the extensibility of spectrin and for its organization in the skeleton.     

Interaction of calmodulin with the red cell and its membrane skeleton and with spectrin

The binding of calmodulin to red cell membrane cytoskeletons and to purified spectrin from red cells and bovine brain spectrin (fodrin) has been examined. Under physiological solvent conditions binding can be measured by ultracentrifugal pelleting assays. The membrane cytoskeletons contained a single class of binding sites, with a concentration similar to that of spectrin dimers and an association constant of 1.5 X 10(5) M-1. Binding is calcium dependent and is suppressed by the calmodulin inhibitor trifluoperazine. The binding showed a marked dependence on ionic strength, with a maximum at 0.05 M, and a steep dependence on pH, with a maximum at pH 6.5. It was unaffected by 5 mM magnesium. An azidocalmodulin derivative, under the conditions of our experiments, did not label the spectrin-containing complex, although it could be used to demonstrate binding to fodrin. Binding of calmodulin to spectrin tetramers and fodrin in solution could be demonstrated by a pelleting assay after addition of F-actin. Calculations (which are necessarily rough) suggest that at the free calcium concentration prevailing in a normal red cell about 1 in 20 of the calmodulin binding sites in spectrin will be occupied; this proportion will rise rapidly with increasing intracellular calcium. To determine whether inhibition of calmodulin binding to red cell proteins disturbs the control of cell shape, as has been suggested, calcium ions were removed from the cell by addition of an ionophore and of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid to the external medium. This did not affect the discoid shape. Trifluoperazine still induced stomatocytosis, exactly as in untreated cells.(ABSTRACT TRUNCATED AT 250 WORDS)