Drosophila development requires spectrin network formation

αII-spectrin and βII-spectrin do not affect TGFβ1-induced myofibroblast differentiation

Mechanosensing of fibroblasts plays a key role in the development of fibrosis. So far, no effective treatments are available to treat this devastating disorder. Spectrins regulate cell morphology and are potential mechanosensors in a variety of non-erythroid cells, but little is known about the role of spectrins in fibroblasts. We investigate whether αII- and βII-spectrin are required for the phenotypic properties of adult human dermal (myo)fibroblasts. Knockdown of αII- or βII-spectrin in fibroblasts did not affect cell adhesion, cell size and YAP nuclear/cytosolic localization. We further investigated whether αII- and βII-spectrin play a role in the phenotypical switch from fibroblasts to myofibroblasts under the influence of the pro-fibrotic cytokine TGFβ1. Knockdown of spectrins did not affect myofibroblast formation, nor did we observe changes in the organization of αSMA stress fibers. Focal adhesion assembly was unaffected by spectrin deficiency, as was collagen type I mRNA expression and protein deposition. Wound closure was unaffected as well, showing that important functional properties of myofibroblasts are unchanged without αII- or βII-spectrin. In fact, fibroblasts stimulated with TGFβ1 demonstrated significantly lower endogenous mRNA levels of αII- and βII-spectrin. Taken together, despite the diverse roles of spectrins in a variety of other cells, αII- and βII-spectrin do not regulate cell adhesion, cell size and YAP localization in human dermal fibroblasts and are not required for the dermal myofibroblast phenotypical switch.

Spectrin tetramer formation is not required for viable development in Drosophila

The dominant paradigm for spectrin function is that (αβ)2-spectrin tetramers or higher order oligomers form membrane-associated two-dimensional networks in association with F-actin to reinforce the plasma membrane. Tetramerization is an essential event in such structures. We characterize the tetramerization interaction between α-spectrin and β-spectrins in Drosophila. Wild-type α-spectrin binds to both β- and βH-chains with high affinity, resembling other non-erythroid spectrins. However, α-spec(R22S), a tetramerization site mutant homologous to the pathological α-spec(R28S) allele in humans, eliminates detectable binding to β-spectrin and reduces binding to βH-spectrin ∼1000-fold. Even though spectrins are essential proteins, α-spectrin(R22S) rescues α-spectrin mutants to adulthood with only minor phenotypes indicating that tetramerization, and thus conventional network formation, is not the essential function of non-erythroid spectrin. Our data provide the first rigorous test for the general requirement for tetramer-based non-erythroid spectrin networks throughout an organism and find that they have very limited roles, in direct contrast to the current paradigm.

Genetic studies of spectrin in the larval fat body of Drosophila melanogaster: evidence for a novel lipid uptake apparatus

Spectrin cytoskeleton defects produce a host of phenotypes affecting the plasma membrane, cell polarity, and secretory membrane traffic. However, many of the underlying molecular mechanisms remain unexplained by prevailing models. Here we used the larval fat body of Drosophila melanogaster as a genetic model system to further elucidate mechanisms of αβ-spectrin function. The results provide unexpected new insights into spectrin function as well as mechanisms of dietary fat uptake and storage. We show that loss of α- or β-spectrin in the fat body eliminated a population of small cortical lipid droplets and altered plasma membrane architecture, but did not affect viability of the organism. We present a novel model in which αβ-spectrin directly couples lipid uptake at the plasma membrane to lipid droplet growth in the cytoplasm. In contrast, strong overexpression of β-spectrin caused fat body atrophy and larval lethality. Overexpression of β-spectrin also perturbed transport of dietary fat from the midgut to the fat body. This hypermorphic phenotype appears to be the result of blocking secretion of the lipid carrier lipophorin from fat cells. However, this midgut phenotype was never seen with spectrin loss of function, suggesting that spectrin is not normally required for lipophorin secretion or function. The β-spectrin hypermorphic phenotype was ameliorated by co-overexpression of α-spectrin. Based on the overexpression results here, we propose that β-spectrin family members may be prone to hypermorphic effects (including effects on secretion) if their activity is not properly regulated.

Orbit/CLASP is required for germline cyst formation through its developmental control of fusomes and ring canals in Drosophila males

Orbit, a Drosophila ortholog of microtubule plus-end enriched protein CLASP, plays an important role in many developmental processes involved in microtubule dynamics. Previous studies have shown that Orbit is required for asymmetric stem cell division and cystocyte divisions in germline cysts and for the development of microtubule networks that interconnect oocyte and nurse cells during oogenesis. Here, we examined the cellular localization of Orbit and its role in cyst formation during spermatogenesis. In male germline stem cells, distinct localization of Orbit was first observed on the spectrosome, which is a spherical precursor of the germline-specific cytoskeleton known as the fusome. In dividing stem cells and spermatogonia, Orbit was localized around centrosomes and on kinetochores and spindle microtubules. After cytokinesis, Orbit remained localized on ring canals, which are cytoplasmic bridges between the cells. Thereafter, it was found along fusomes, extending through the ring canal toward all spermatogonia in a cyst. Fusome localization of Orbit was not affected by microtubule depolymerization. Instead, our fluorescence resonance energy transfer experiments suggested that Orbit is closely associated with F-actin, which is abundantly found in fusomes. Surprisingly, F-actin depolymerization influenced neither fusome organization nor Orbit localization on the germline-specific cytoskeleton. We revealed that two conserved regions of Orbit are required for fusome localization. Using orbit hypomorphic mutants, we showed that the protein is required for ring canal formation and for fusome elongation mediated by the interaction of newly generated fusome plugs with the pre-existing fusome. The orbit mutation also disrupted ring canal clustering, which is essential for folding of the spermatogonia after cytokinesis. Orbit accumulates around centrosomes at the onset of spermatogonial mitosis and is required for the capture of one of the duplicated centrosomes onto the fusome. Moreover, Orbit is involved in the proper orientation of spindles towards fusomes during synchronous mitosis of spermatogonial cysts.

Tropomodulins: pointed-end capping proteins that regulate actin filament architecture in diverse cell types

Tropomodulins are a family of four proteins (Tmods 1-4) that cap the pointed ends of actin filaments in actin cytoskeletal structures in a developmentally regulated and tissue-specific manner. Unique among capping proteins, Tmods also bind tropomyosins (TMs), which greatly enhance the actin filament pointed-end capping activity of Tmods. Tmods are defined by a TM-regulated/Pointed-End Actin Capping (TM-Cap) domain in their unstructured N-terminal portion, followed by a compact, folded Leucine-Rich Repeat/Pointed-End Actin Capping (LRR-Cap) domain. By inhibiting actin monomer association and dissociation from pointed ends, Tmods regulate actin dynamics and turnover, stabilizing actin filament lengths and cytoskeletal architecture. In this review, we summarize the genes, structural features, molecular and biochemical properties, actin regulatory mechanisms, expression patterns, and cell and tissue functions of Tmods. By understanding Tmods' functions in the context of their molecular structure, actin regulation, binding partners, and related variants (leiomodins 1-3), we can draw broad conclusions that can explain the diverse morphological and functional phenotypes that arise from Tmod perturbation experiments in vitro and in vivo. Tmod-based stabilization and organization of intracellular actin filament networks provide key insights into how the emergent properties of the actin cytoskeleton drive tissue morphogenesis and physiology.

Transgene rescue identifies an essential function for Drosophila beta spectrin in the nervous system and a selective requirement for ankyrin-2-binding activity

The Gal4-UAS system was used to overexpress or knock down β spectrin with dsRNA in a variety of Drosophila tissues. Unexpectedly, overexpression in most tissues tested was lethal, whereas knockdown failed to produce a detectable phenotype in the same tissues. The lethality of a β spectrin mutation was rescued by expression of β spectrin in neurons.

The protein spectrin is ubiquitous in animal cells and is believed to play important roles in cell shape and membrane stability, cell polarity, and endomembrane traffic. Experiments here were undertaken to identify sites of essential β spectrin function in Drosophila and to determine whether spectrin and ankyrin function are strictly linked to one another. The Gal4-UAS system was used to drive tissue-specific overexpression of a β spectrin transgene or to knock down β spectrin expression with dsRNA. The results show that 1) overexpression of β spectrin in most of the cell types studied was lethal; 2) knockdown of β spectrin in most tissues had no detectable effect on growth or viability of the organism; and 3) nervous system-specific expression of a UAS-β spectrin transgene was sufficient to overcome the lethality of a loss-of-function β spectrin mutation. Thus β spectrin expression in other cells was not required for development of fertile adult males, although females lacking nonneuronal spectrin were sterile. Previous data indicated that binding of the DAnk1 isoform of ankyrin to spectrin was partially dispensable for viability. Domain swap experiments here uncovered a different requirement for neuronal DAnk2 binding to spectrin and establish that DAnk2-binding is critical for β spectrin function in vivo.

Tropomodulin1 is required for membrane skeleton organization and hexagonal geometry of fiber cells in the mouse lens

Hexagonal packing geometry is a hallmark of close-packed epithelial cells in metazoans. Here, we used fiber cells of the vertebrate eye lens as a model system to determine how the membrane skeleton controls hexagonal packing of post-mitotic cells. The membrane skeleton consists of spectrin tetramers linked to actin filaments (F-actin), which are capped by tropomodulin1 (Tmod1) and stabilized by tropomyosin (TM). In mouse lenses lacking Tmod1, initial fiber cell morphogenesis is normal, but fiber cell hexagonal shapes and packing geometry are not maintained as fiber cells mature. Absence of Tmod1 leads to decreased gammaTM levels, loss of F-actin from membranes, and disrupted distribution of beta2-spectrin along fiber cell membranes. Regular interlocking membrane protrusions on fiber cells are replaced by irregularly spaced and misshapen protrusions. We conclude that Tmod1 and gammaTM regulation of F-actin stability on fiber cell membranes is critical for the long-range connectivity of the spectrin-actin network, which functions to maintain regular fiber cell hexagonal morphology and packing geometry.

Sequential degradation of alphaII and betaII spectrin by calpain in glutamate or maitotoxin-stimulated cells

Calpain-catalyzed proteolysis of II-spectrin is a regulated event associated with neuronal long-term potentiation, platelet and leukocyte activation, and other processes. Calpain proteolysis is also linked to apoptotic and nonapoptotic cell death following excessive glutamate exposure, hypoxia, HIV-gp120/160 exposure, or toxic injury. The molecular basis for these divergent consequences of calpain action, and their relationship to spectrin proteolysis, is unclear. Calpain preferentially cleaves II spectrin in vitro in repeat 11 between residues Y1176 and G1177. Unless stimulated by Ca++ and calmodulin (CaM), betaII spectrin proteolysis in vitro is much slower. We identify additional unrecognized sites in spectrin targeted by calpain in vitro and in vivo. Bound CaM induces a second II spectrin cleavage at G1230*S1231. BetaII spectrin is cleaved at four sites. One cleavage only occurs in the absence of CaM at high enzyme-to-substrate ratios near the betaII spectrin COOH-terminus. CaM promotes II spectrin cleavages at Q1440*S1441, S1447*Q1448, and L1482*A1483. These sites are also cleaved in the absence of CaM in recombinant II spectrin fusion peptides, indicating that they are probably shielded in the spectrin heterotetramer and become exposed only after CaM binds alphaII spectrin. Using epitope-specific antibodies prepared to the calpain cleavage sites in both alphaII and betaII spectrin, we find in cultured rat cortical neurons that brief glutamate exposure (a physiologic ligand) rapidly stimulates alphaII spectrin cleavage only at Y1176*G1177, while II spectrin remains intact. In cultured SH-SY5Y cells that lack an NMDA receptor, glutamate is without effect. Conversely, when stimulated by calcium influx (via maitotoxin), there is rapid and sequential cleavage of alphaII and then betaII spectrin, coinciding with the onset of nonapoptotic cell death. These results identify (i) novel calpain target sites in both alphaII and betaII spectrin; (ii) trans-regulation of proteolytic susceptibility between the spectrin subunits in vivo; and (iii) the preferential cleavage of alphaII spectrin vs betaII spectrin when responsive cells are stimulated by engagement of the NMDA receptor. We postulate that calpain proteolysis of spectrin can activate two physiologically distinct responses: one that enhances skeletal plasticity without destroying the spectrin-actin skeleton, characterized by preservation of betaII spectrin; or an alternative response closely correlated with nonapoptotic cell death and characterized by proteolysis of betaII spectrin and complete dissolution of the spectrin skeleton.

Distinct functions of alpha-Spectrin and beta-Spectrin during axonal pathfinding

Cell-shape changes during development require a precise coupling of the cytoskeleton with proteins situated in the plasma membrane. Important elements controlling the shape of cells are the Spectrin proteins that are expressed as a subcortical cytoskeletal meshwork linking specific membrane receptors with F-actin fibers. Here, we demonstrate that Drosophila karussell mutations affect beta-spectrin and lead to distinct axonal patterning defects in the embryonic CNS. karussell mutants display a slit-sensitive axonal phenotype characterized by axonal looping in stage-13 embryos. Further analyses of individual, labeled neuroblast lineages revealed abnormally structured growth cones in these animals. Cell-type-specific rescue experiments demonstrate that beta-Spectrin is required autonomously and non-autonomously in cortical neurons to allow normal axonal patterning. Within the cell, beta-Spectrin is associated with alpha-Spectrin. We show that expression of the two genes is tightly regulated by post-translational mechanisms. Loss of beta-Spectrin significantly reduces levels of neuronal alpha-Spectrin expression, whereas gain of beta-Spectrin leads to an increase in alpha-Spectrin protein expression. Because the loss of alpha-spectrin does not result in an embryonic nervous system phenotype, beta-Spectrin appears to act at least partially independent of alpha-Spectrin to control axonal patterning.

Spectrin functions upstream of ankyrin in a spectrin cytoskeleton assembly pathway

Prevailing models place spectrin downstream of ankyrin in a pathway of assembly and function in polarized cells. We used a transgene rescue strategy in Drosophila melanogaster to test contributions of four specific functional sites in beta spectrin to its assembly and function. (1) Removal of the pleckstrin homology domain blocked polarized spectrin assembly in midgut epithelial cells and was usually lethal. (2) A point mutation in the tetramer formation site, modeled after a hereditary elliptocytosis mutation in human erythrocyte spectrin, had no detectable effect on function. (3) Replacement of repetitive segments 4-11 of beta spectrin with repeats 2-9 of alpha spectrin abolished function but did not prevent polarized assembly. (4) Removal of the putative ankyrin-binding site had an unexpectedly mild phenotype with no detectable effect on spectrin targeting to the plasma membrane. The results suggest an alternate pathway in which spectrin directs ankyrin assembly and in which some important functions of spectrin are independent of ankyrin.

Continuous membrane-cytoskeleton adhesion requires continuous accommodation to lipid and cytoskeleton dynamics

The plasma membrane of most animal cells conforms to the cytoskeleton and only occasionally separates to form blebs. Previous studies indicated that many weak interactions between cytoskeleton and the lipid bilayer kept the surfaces together to counteract the normal outward pressure of cytoplasm. Either the loss of adhesion strength or the formation of gaps in the cytoskeleton enables the pressure to form blebs. Membrane-associated cytoskeleton proteins, such as spectrin and filamin, can control the movement and aggregation of membrane proteins and lipids, e.g., phosphoinositol phospholipids (PIPs), as well as blebbing. At the same time, lipids (particularly PIPs) and membrane proteins affect cytoskeleton and signaling dynamics. We consider here the roles of the major phosphatidylinositol-4,5-diphosphate (PIP2) binding protein, MARCKS, and PIP2 levels in controlling cytoskeleton dynamics. Further understanding of dynamics will provide important clues about how membrane-cytoskeleton adhesion rapidly adjusts to cytoskeleton and membrane dynamics.

Spectrin interacts with EVL (Enabled/vasodilator-stimulated phosphoprotein-like protein), a protein involved in actin polymerization

BACKGROUND INFORMATION

The alpha- and beta-spectrin chains constitute the filaments of the spectrin-based skeleton, which was first identified in erythrocytes. The discovery of analogous structures at plasma membranes of eukaryotic cells has led to investigations of the role of this spectrin skeleton in many cellular processes. The alphaII-spectrin chain expressed in nucleated cells harbours in its central region several functional motifs, including an SH3 (Src homology 3) domain.

RESULTS

Using yeast two-hybrid screening, we have identified EVL [Enabled/VASP (vasodilator-stimulated phosphoprotein)-like protein] as a new potential partner of the alphaII-spectrin SH3 domain. In the present study, we investigated the interaction of the alphaII-spectrin SH3 domain with EVL and compared this with other proteins related to EVL [Mena (mammalian Enabled) and VASP]. We confirmed the in vitro interaction between EVL and the alphaII-spectrin SH3 domain by GST (glutathione S-transferase) pull-down assays, and showed that the co-expression of EVL with the alphaII-spectrin SH3 domain in COS-7 cells resulted in the partial delocalization of the SH3 domain from cytoplasm to filopodia and lamellipodia, where it was co-localized with EVL. In kidney epithelial and COS-7 cells, we demonstrated the co-immunoprecipitation of the alphaII-spectrin chain with over-expressed EVL. Immunofluorescence studies showed that the over-expression of EVL in COS-7 cells promoted the formation of filopodia and lamellipodia, and the expressed EVL was detected in filopodial tips and the leading edge of lamellipodia. In these cells over-expressing EVL, the alphaII-spectrin membrane labelling lagged behind EVL staining in lamellipodia and filopodia, with co-localization of these two stains in the contact area. In kidney epithelial cell lines, focused co-localization of spectrin with expressed EVL was observed in the membrane of the lateral domain, where the cell-cell contacts are reinforced.

CONCLUSIONS

The possible link between the spectrin-based skeleton and actin via the EVL protein suggests a new way of integrating the spectrin-based skeleton in areas of dynamic actin reorganization.

Mammalian alpha I-spectrin is a neofunctionalized polypeptide adapted to small highly deformable erythrocytes

Mammalian red blood cells, unlike those of other vertebrates, must withstand the rigors of circulation in the absence of new protein synthesis. Key to this is plasma membrane elasticity deriving from the protein spectrin, which forms a network on the cytoplasmic face. Spectrin is a tetramer (alphabeta)(2), made up of alphabeta dimers linked head to head. We show here that one component of erythrocyte spectrin, alphaI, is encoded by a gene unique to mammals. Phylogenetic analysis suggests that the other alpha-spectrin gene (alphaII) common to all vertebrates was duplicated after the emergence of amphibia, and that the resulting alphaI gene was preserved only in mammals. The activities of alphaI and alphaII spectrins differ in the context of the human red cell membrane. An alphaI-spectrin fragment containing the site of head-to-head interaction with the beta-chain binds more weakly than the corresponding alphaII fragment to this site. The latter competes so strongly with endogenous alphaI as to cause destabilization of membranes at 100-fold lower concentration than the alphaI fragment. The efficacies of alphaI/alphaII chimeras indicate that the partial structural repeat, which binds to the complementary beta-spectrin element, and the adjacent complete repeat together determine the strength of the dimer-dimer interaction on the membrane. Alignment of all available alpha-spectrin N-terminal sequences reveals three blocks of sequence unique to alphaI. Furthermore, human alphaII-spectrin is closer to fruitfly alpha-spectrin than to human alphaI-spectrin, consistent with adaptation of alphaI to new functions. We conclude that alphaI-spectrin represents a neofunctionalized spectrin adapted to the rapid make and break of tetramers.

The C-terminal domain ofDrosophilaβHeavy-spectrin exhibits autonomous membrane association and modulates membrane area

Current models of cell polarity invoke asymmetric cues that reorganize the secretory apparatus to induce polarized protein delivery. An important step in this process is the stabilization of the protein composition in each polarized membrane domain. The spectrin-based membrane skeleton is thought to contribute to such stabilization by increasing the half-life of many proteins at the cell surface. Genetic evidence is consistent with a negative role for Drosophila βHeavy-spectrin in endocytosis, but the inhibitory mechanism has not been elucidated. Here, we investigated the membrane binding properties of the C-terminal nonrepetitive domain of βHeavy-spectrin through its in vivo expression in transgenic flies. We found that this region is a membrane-association domain that requires a pleckstrin homology domain for full activity, and we showed for the first time that robust membrane binding by such a C-terminal domain requires additional contributions outside the pleckstrin homology. In addition, we showed that expression of the βHeavy-spectrin C-terminal domain has a potent effect on epithelial morphogenesis. This effect is associated with its ability to induce an expansion in plasma membrane surface area. The membrane expansions adopt a very specific bi-membrane structure that sequesters both the C-terminal domain and the endocytic protein dynamin. Our data provide supporting evidence for the inhibition of endocytosis by βHeavy-spectrin, and suggest that the C-terminal domain mediates this effect through interaction with the endocytic machinery. Spectrin may be an active partner in the stabilization of polarized membrane domains.

Spectrin alpha II and beta II isoforms interact with high affinity at the tetramerization site

Spectrin tetramers form by the interaction of two alpha-beta dimers through two helices close to the C-terminus of a beta subunit and a single helix at the N-terminus of an alpha subunit. Early work on spectrin from solid tissues (typified by alphaII and betaII polypeptides) indicated that it forms a more stable tetramer than erythroid spectrin (alphaI-betaI). In the present study, we have probed the molecular basis of this phenomenon. We have quantified the interactions of N-terminal regions of two human alpha polypeptides (alphaI and alphaII) with the C-terminal regions of three beta isoforms (betaISigma1, betaIISigma1 and betaIISigma2). alphaII binds either betaII form with a much higher affinity than alphaI binds betaISigma1 ( K (d) values of 5-9 nM and 840 nM respectively at 25 degrees C). betaIISigma1 and betaIISigma2 are splice variants with different C-terminal extensions outside the tetramerization site: these extensions affect the rate rather than the affinity of alpha subunit interaction. alphaII spectrin interacts with each beta subunit with higher affinity than alphaI, and the betaII polypeptides have higher affinities for both alpha chains than betaISigma1. The first full repeat of the alpha subunit has a major role in determining affinity. Enthalpy changes in the alphaII-betaIISigma2 interaction are large, but the entropy change is comparatively small. The interaction is substantially reduced, but not eliminated, by concentrated salt solutions. The high affinity and slow overall kinetics of association and dissociation of alphaII-betaII spectrin may suit it well to a role in strengthening cell junctions and providing stable anchor points for transmembrane proteins at points specified by cell-adhesion molecules.

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.

Spectrin: the ghost in the machine

It has long been speculated that spectrin, the actin crosslinking and molecular scaffold protein, is involved in the development of apicobasal polarity in epithelia. While spectrins can undoubtedly influence the protein content of specific membrane domains, recent genetic evidence indicates that this activity is not necessary for the establishment or maintenance of this axis. Instead, these studies point to critical roles in tissue stability and morphogenesis. A possible role in cellular contractility is highlighted in this review.

Hereditary spherocytosis in zebrafish riesling illustrates evolution of erythroid beta-spectrin structure, and function in red cell morphogenesis and membrane stability

Spectrins are key cytoskeleton proteins with roles in membrane integrity, cell morphology, organelle transport and cell polarity of varied cell types during development. Defects in erythroid spectrins in humans result in congenital hemolytic anemias with altered red cell morphology. Although well characterized in mammals and invertebrates, analysis of the structure and function of non-mammalian vertebrate spectrins has been lacking. The zebrafish riesling (ris) suffers from profound anemia, where the developing red cells fail to assume terminally differentiated erythroid morphology. Using comparative genomics, erythroid beta-spectrin (sptb) was identified as the gene mutated in ris. Zebrafish Sptb shares 62.3% overall identity with the human ortholog and phylogenetic comparisons suggest intragenic duplication and divergence during evolution. Unlike the human and murine orthologs, the pleckstrin homology domain of zebrafish Sptb is not removed in red cells by alternative splicing. In addition, apoptosis and abnormal microtubule marginal band aggregation contribute to hemolysis of mutant erythrocytes, which are features not present in mammalian red cells with sptb defects. This study presents the first genetic characterization of a non-mammalian vertebrate sptb and demonstrates novel features of red cell hemolysis in non-mammalian red cells. Further, we propose that the distinct mammalian erythroid morphology may have evolved from specific modifications of Sptb structure and function.

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.

Control of epithelial cell shape and polarity

The polarised character of a cell is often obvious from its shape and is largely dependent on the actin cytoskeleton and the membrane-associated cell cortex---a dense network comprising spectrin and other related proteins. Spatially and functionally distinct protein scaffolds, assembled from transmembrane and cytoplasmic proteins, provide the cues for polarisation. Recent data have provided new insights into the molecular nature of these cues and the mechanisms by which they may be translated into a polarised phenotype.

Genetic control of epithelial cell polarity: lessons from Drosophila

The cell biology of polarized epithelial cells is a field of major interest to cell and developmental biologists. In addition to the study of epithelial cells in tissue culture, genetically tractable systems have been employed to examine the functional importance of individual molecules. Here I review recent progress that has been made using Drosophila as a model system to study apical-basal epithelial cell polarity. In this system, a large number of genes have been identified that are essential for the development and maintenance of the apical-basal polarity of epithelial cells in different developmental contexts. In this article, the recent progress in three models, i.e., establishment of the ectoderm and midgut epithelia during embryogenesis, epithelial polarity of imaginal discs, and epithelial development of the follicle cells during oogenesis will be discussed.

Apical spectrin is essential for epithelial morphogenesis but not apicobasal polarity in Drosophila

Changes in cell shape and position drive morphogenesis in epithelia and depend on the polarized nature of its constituent cells. The spectrin-based membrane skeleton is thought to be a key player in the establishment and/or maintenance of cell shape and polarity. We report that apical beta(Heavy)-spectrin (beta(H)), a terminal web protein that is also associated with the zonula adherens, is essential for normal epithelial morphogenesis of the Drosophila follicle cell epithelium during oogenesis. Elimination of beta(H) by the karst mutation prevents apical constriction of the follicle cells during mid-oogenesis, and is accompanied by a gross breakup of the zonula adherens. We also report that the integrity of the migratory border cell cluster, a group of anterior follicle cells that delaminates from the follicle epithelium, is disrupted. Elimination of beta(H) prevents the stable recruitment of alpha-spectrin to the apical domain, but does not result in a loss of apicobasal polarity, as would be predicted from current models describing the role of spectrin in the establishment of cell polarity. These results demonstrate a direct role for apical (alphabeta(H))(2)-spectrin in epithelial morphogenesis driven by apical contraction, and suggest that apical and basolateral spectrin do not play identical roles in the generation of apicobasal polarity.

Simultaneous degradation of alphaII- and betaII-spectrin by caspase 3 (CPP32) in apoptotic cells

The degradation of alphaII- and betaII-spectrin during apoptosis in cultured human neuroblastoma SH-SY5Y cells was investigated. Immunofluorescent staining showed that the collapse of the cortical spectrin cytoskeleton is an early event following staurosporine challenge. This collapse correlated with the generation of a series of prominent spectrin breakdown products (BDPs) derived from both alphaII- and betaII-subunits. Major C-terminal alphaII-spectrin BDPs were detected at approximately 150, 145, and 120 kDa (alphaII-BDP150, alphaII-BDP145, and alphaII-BDP120, respectively); major C-terminal betaII-spectrin BDPs were at approximately 110 and 85 kDa (betaII-BDP110 and betaII-BDP85, respectively). N-terminal sequencing of the major fragments produced in vitro by caspase 3 revealed that alphaII-BDP150 and alphaII-BDP120 were generated by cleavages at DETD1185*S1186 and DSLD1478*S1479, respectively. For betaII-spectrin, a major caspase site was detected at DEVD1457*S1458, and both betaII-BDP110 and betaII-BDP85 shared a common N-terminal sequence starting with Ser1458. An additional cleavage site near the C terminus, at ETVD2146*S2147, was found to account for betaII-BDP85. Studies using specific caspase or calpain inhibitors indicate that the pattern of spectrin breakdown during apoptosis differs from that during non-apoptotic cell death. We postulate that in concert with calpain, caspase rapidly targets critical sites in both alphaII- and betaII-spectrin and thereby initiates a rapid dissolution of the spectrin-actin cortical cytoskeleton with apoptosis.

Drosophila betaHeavy-spectrin is essential for development and contributes to specific cell fates in the eye

The spectrin membrane skeleton is a ubiquitous cytoskeletal structure with several cellular roles, including the maintenance of cell integrity, determination of cell shape and as a contributor to cell polarity. We have isolated mutations in the gene encoding βHeavy-spectrin in Drosophila, and have named this essential locus karst. karst mutant individuals have a pleiotropic phenotype characterized by extensive larval lethality and, in adult escapers, rough eyes, bent wings, tracheal defects and infertility. Within karst mutant eyes, a significant number of ommatidia specifically lack photoreceptor R7 alongside more complex morphological defects. Immunolocalization of betaHeavy-spectrin in wild-type eye-antennal and wing imaginal discs reveals that betaHeavy-spectrin is present in a restricted subdomain of the membrane skeleton that colocalizes with DE-cadherin. We propose a model where normal levels of Sevenless signaling are dependent on tight cell-cell adhesion facilitated by the betaHeavy-spectrin membrane skeleton. Immunolocalization of betaHeavy-spectrin in the adult and larval midgut indicates that it is a terminal web protein, but we see no gross morphological defects in the adult apical brush border in karst mutant flies. Rhodamine phalloidin staining of karst mutant ovaries similarly reveals no conspicuous defect in the actin cytoskeleton or cellular morphology in egg chambers. This is in contrast to mutations in alpha-spectrin, the molecular partner of betaHeavy-spectrin, which affect cellular structure in both the larval gut and adult ovaries. Our results emphasize the fundamental contribution of the spectrin membrane skeleton to normal development and reveals a critical interplay between the integrity of a cell's membrane skeleton, the structure of cell-cell contacts and cell signaling.

Motifs involved in interchain binding at the tail-end of spectrin

Segments 20-22 of alpha-spectrin and 1-3 of beta-spectrin are required for high avidity interchain binding at the tail-end of the molecule. Here, sequence analysis guided by the crystal structure of spectrin's repeating segments was used to redefine the boundaries of a repetitive beta segment that is critical for interchain binding and demonstrate the contribution of non-repetitive spectrin segments in high avidity interchain binding. Our results show that several motifs together are required for high avidity binding, indicating that interchain binding at the tail-end of the spectrin molecule depends on the long distance coordination of several different elements. We also explored the role of unusual motifs contained in beta segments involved in interchain binding. A row of basic residues and a row of small hydrophobic residues were found not to be required for interchain binding, suggesting that their conservation among species reflects functions unrelated to interchain binding. The octamer between segments beta 2 and beta 3 that maintains a specific register between true binding sites was found to have an indirect role in interchain binding by stabilizing neighboring segments. A 5-residue domain in segment beta 2 (EKPPK) was required for interchain binding because it sustains normal helix-helix interactions within segments beta 2.

Spectrin self-association site: characterization and study of beta-spectrin mutations associated with hereditary elliptocytosis

Most of hereditary elliptocytosis (HE) cases are related to a spectrin dimer (SpD) self-association defect. The severity of haemolysis is correlated with the extent of the SpD self-association defect, which itself depends on the location of the mutation regarding the tetramerization site. This site is presumed to involve the first C helix of the alpha chain and the last two helices, A and B, of the beta chain to reconstitute a triple helical structure (A, B and C), as observed along spectrin. Using recombinant peptides, we demonstrated that the first C helix of the alpha chain and the last two helices of the beta chain alone are not sufficient to establish interactions, which only occurred when a complete triple-helical repeat was added to each partner. One adjacent repeat is necessary to stabilize the conformation of both N- and C-terminal structures directly involved in the interaction site and is sufficient to generate a binding affinity similar to that observed in the native molecule. Producing peptides carrying a betaHE mutation, we reproduced the tetramerization defect as observed in patients. Therefore, the betaW2024R and betaW2061R mutations, which replace the invariant tryptophan and a residue located in the hydrophobic core, respectively, affect alpha-beta interactions considerably. In contrast, the betaA2013V mutation, which modifies a residue located outside any presumed interacting regions, has a minor effect on the interaction.

Mutations of alpha spectrin and labial block cuprophilic cell differentiation and acid secretion in the middle midgut of Drosophila larvae

Mutations in Drosophila alpha spectrin cause larval lethality and defects in cell shape and adhesion (J. Lee et al., 1993, J. Cell Biol. 123, 1797-1809). Here we examined the effects of two lethal alpha spectrin alleles (alpha-specrg41 and alpha-specrg35) on development and function of the larval midgut. Homozygous null alpha-specrg41-mutant larvae exhibited a striking defect in middle midgut acidification. In contrast, many homozygous alpha-specrg35 mutants were capable of acidification, indicating partial function of the truncated alpha-specrg35 product. Acidification was also blocked by a mutation in the labial gene, which is required for differentiation of cuprophilic cells in the midgut, suggesting that these cells secrete acid. We found that two isoforms of spectrin (alphabeta and alphabetaH) are segregated within the basolateral and apical domains of cuprophilic cells, respectively. The most conspicuous defect in cuprophilic cells from labial and alpha spectrin mutants was in morphogenesis of the invaginated apical domain, although basolateral defects may also contribute to the acidification phenotype. Acid secretion in vertebrate systems is thought to involve the polarized activities of apical proton pumps and basolateral anion exchangers, both of which interact with spectrin. We propose that the alpha-specrg41 mutation in Drosophila interferes with the polarized activities of homologous molecules that drive acid secretion in cuprophilic cells.

alpha-Spectrin is required for ovarian follicle monolayer integrity in Drosophila melanogaster

To understand the role of the spectrin-based membrane skeleton in generating epithelial polarity, we characterized the distribution of membrane skeletal components in Drosophila ovarian follicle cells and in somatic clones of mutant cells that lack alpha-spectrin. Immunolocalization data reveal that wild-type follicle cells contain two populations of spectrin heterodimers: a network of alphabeta heterodimers concentrated on the lateral plasma membrane and an alphabetaH population targeted to the apical surface. Induction of somatic clones lacking alpha-spectrin leads to follicle cell hyperplasia. Surprisingly, elimination of alpha-spectrin from follicle cells does not appear to prevent the assembly of conventional beta-spectrin and ankyrin at the lateral domain of the follicle cell plasma membrane. However, the alpha-subunit is essential for the correct localization of betaH-spectrin to the apical surface. As a consequence of disrupting the apical membrane skeleton a distinct sub population of follicle cells undergoes unregulated proliferation which leads to the loss of monolayer organization and disruption of the anterior-posterior axis of the oocyte. These results suggest that the spectrin-based membrane skeleton is required in a developmental pathway that controls follicle cell monolayer integrity and proliferation.

The neurogenic genes egghead and brainiac define a novel signaling pathway essential for epithelial morphogenesis during Drosophila oogenesis

Notch (N) and other neurogenic genes have been implicated in two fundamental processes, lateral specification of cell fates, and epithelial development. Previous studies have suggested that the neurogenic gene brainiac (brn) is specifically required for epithelial development (Goode, S., Morgan, M., Liang, Y-P. and Mahowald, A. P. (1996). Dev. Biol. 178, 35–50). In this report we show that egghead (egh), a gene with phenotypes identical to brn, encodes for a novel, putative secreted or transmembrane protein. We describe the role of egh and brn germline function in the morphogenesis of the follicular epithelium from the time it is born through the time that it migrates towards the oocyte late in oogenesis. By comparing the function of germline egh and brn to N during oogenesis, we have obtained direct evidence for the involvement of Notch in maintenance of the follicle cell epithelium, and the specificity of brn and egh in epithelial development during oogenesis. The most striking phenotype observed for all three genes is a loss of apical-basal polarity and accumulation of follicular epithelial cells in multiple layers around the oocyte. The spatiotemporal onset of this adenoma-like phenotype correlates with the differential accumulation of egh transcripts in the oocyte at stage 4 of oogenesis. In contrast to N, we find that brn and egh are essential for the organization, but not specification, of stalk and polar cells. The expression patterns and functional requirements of brn, egh, and N lead us to propose that these genes mediate follicular morphogenesis by regulating germline-follicle cell adhesion. This proposal offers explanations for (1) the involvement of egh and brn in N-mediated epithelial development, but not lateral specification, (2) why brn and egh embryonic neurogenic phenotypes are not as severe as N phenotypes, and (3) how egh and brn influence Egfr-mediated processes. The correlation between the differential expression of egh in the oocyte and the differential requirement for brn, egh, and N in maintaining the follicular epithelium around the oocyte, suggests that Egghead is a critical component of a differential oocyte-follicle cell adhesive system.

A Drosophila dystrophin-related protein, MSP-300, is required for embryonic muscle morphogenesis

Proteins from the spectrin superfamily contribute to cell polarity and shape during the morphogenetic that accompany embryogenesis. Drosophila MSP-300, a member of the spectrin superfamily, is expressed in somatic, visceral and heart embryonic muscles. Cloning and sequence analysis of various spliced forms of MSP-300 reveals functional and structural similarities between MSP-300 and vertebrate Dystrophin, the product of the Duchenne Muscular Dystrophy gene. The identification of a strain mutant for the MSP-300 gene is described. Analysis of the somatic muscle phenotype in MSP-300 mutant embryos suggests that the protein contributes to the integrity of the somatic and visceral muscle during periods of significant morphogenetic change. Functional synergism between MSP-300 and laminin is demonstrated by the analysis of the phenotype of embryos mutant for both genes. The enhancement of aberrant muscle phenotype in the double mutants suggests a link between MSP-300 and laminin function in mediating proper extension of the myotube towards the epidermal muscle attachment site. In addition, both genes function to establish gut integrity. In view of the functional and structural similarities between MSP-300 and Dystrophin, it is postulated that Dystrophin is not only required for proper muscle function in adult life but also contributes to muscle morphogenesis during the development of the vertebrate embryo.

The tumor suppressor gene, lethal(2)giant larvae (1(2)g1), is required for cell shape change of epithelial cells during Drosophila development

Inactivation of the lethal(2)giant larvae (l(2)gl) gene results in malignant transformation of imaginal disc cells and neuroblasts of the larval brain in Drosophila. Subcellular localization of the l(2)gl gene product, P127, and its biochemical characterization have indicated that it participates in the formation of the cytoskeletal network. In this paper, genetic and phenotypic analyses of a temperature-sensitive mutation (l(2)glts3) that behaves as a hypomorphic allele at restrictive temperature are presented. In experimentally overaged larvae obtained by using mutants in the production of ecdysone, the l(2)glts3 mutation displays a tumorous potential. This temperature-sensitive allele of the l(2)gl gene has been used to describe the primary function of the gene before tumor progression. A reduced contribution of both maternal and zygotic activities in l(2)glts3 homozygous mutant embryos blocks embryogenesis at the end of germ-band retraction. The mutant embryos are consequently affected in dorsal closure and head involution and show a hypertrophy of the midgut. These phenotypes are accompanied by an arrest of the cell shape changes normally occurring in lateral epidermis and in epithelial midgut cells. l(2)gl activity is also necessary for larval fife and the critical period falls within the third instar larval stage. Finally, l(2)gl activity is required during oogenesis and mutations in the gene disorganize egg chambers and cause abnormalities in the shape of follicle cells, which are eventually internalized within the egg chamber. These results together with the tumoral phenotype of epithelial imaginal disc cells strongly suggest that the l(2)gl product is required in vivo in different types of epithelial cells to control their shape during development.

Neuroglian-mediated cell adhesion induces assembly of the membrane skeleton at cell contact sites

The protein ankyrin links integral membrane proteins to the spectrin-based membrane skeleton. Ankyrin is often concentrated within restricted membrane domains of polarized epithelia and neurons, but the mechanisms responsible for membrane targeting and its segregation within a continuous lipid bilayer remain unexplained. We provide evidence that neuroglian, a cell adhesion molecule related to L1 and neurofascin, can transmit positional information directly to ankyrin and thereby polarize its distribution in Drosophila S2 tissue culture cells. Ankyrin was not normally associated with the plasma membrane of these cells. Upon expression of an inducible neuroglian minigene, however, cells aggregated into large clusters and ankyrin became concentrated at sites of cell-cell contact. Spectrin was also recruited to sites of cell contact in response to neuroglian expression. The accumulation of ankyrin at cell contacts required the presence of the cytoplasmic domain of neuroglian since a glycosyl phosphatidylinositol-linked form of neuroglian failed to recruit ankyrin to sites of cell-cell contact. Double-labeling experiments revealed that, whereas ankyrin was strictly associated with sites of cell-cell contact, neuroglian was more broadly distributed over the cell surface. A direct interaction between neuroglian and ankyrin was demonstrated using yeast two-hybrid analysis. Thus, neuroglian appears to be activated by extracellular adhesion so that ankyrin and the membrane skeleton selectively associate with sites of cell contact and not with other regions of the plasma membrane.

Phylogenetic occurrence of coiled coil proteins: implications for tissue structure in metazoa via a coiled coil tissue matrix

We examined GenBank sequence files with a heptad repeat analysis program to assess the phylogenetic occurrence of coiled coil proteins, how heptad repeat domains are organized within them, and what structural/functional categories they comprise. Of 102,007 proteins analyzed, 5.95% (6,074) contained coiled coil domains; 1.26% (1,289) contained "extended" (> 75 amino acid) domains. While the frequency of proteins containing coiled coils was surprisingly constant among all biota, extended coiled coil proteins were fourfold more frequent in the animal kingdom and may reflect early events in the divergence of plants and animals. Structure/function categories of extended coils also revealed phylogenetic differences. In pathogens and parasites, many extended coiled coil proteins are external and bind host proteins. In animals, the majority of extended coiled coil proteins were identified as constituents of two protein categories: 1) myosins and motors; or 2) components of the nuclear matrix-intermediate filament scaffold. This scaffold, produced by sequential extraction of epithelial monolayers in situ, contains only 1-2% of the cell mass while accurately retaining morphological features of living epithelium and is greatly enriched in proteins with extensive, interrupted coiled coil forming domains. The increased occurrence of this type of protein in metazoa compared with plants or protists leads us to hypothesize a tissue-wide matrix of coiled coil interactions underlying metazoan differentiated cell and tissue structure.

Spectrin: on the path from structure to function

New structural analyses of the spectrin family of actin cross-linking proteins are providing molecular explanations for both the interchain binding between the alpha and beta chains of spectrin and the intermolecular associations between spectrin and other proteins. Additionally, the analyses bring into focus a conformation which may explain aspects of spectrin's interaction with lipids.