Spectrin-based Membrane Skeleton Research Articles

β-actin function in platelets and red blood cells can be performed by γ-actin and is therefore independent of actin isoform protein sequence.

Actin is an essential component of the cytoskeleton in every eukaryotic cell. Cytoplasmic β-and γ-actin are over 99% identical to each other at the protein level, but are encoded by different genes and play distinct roles in vivo. Blood cells, especially red blood cells (RBC), contain almost exclusively β-actin, and it has been generally assumed that this bias is dictated by unique suitability of β-actin for RBC cytoskeleton function due to its specific amino acid sequence. Here we tested this assumption by analyzing the "β-coded γ-actin" (Actbcg) mouse model, in which β-actin gene is edited by five point mutations to produce γ-actin protein. Strikingly, despite lacking β-actin protein, Actbcg mice had no detectable phenotypes in RBCs, and no changes in the RBC shape, integrity, deformability, and molecular composition of their spectrin-based membrane skeleton. No actin-dependent changes are observed in platelets, another anucleate cell type enriched for β-actin. Our data show that, contrary to expectations, β-actin function in mature RBCs and platelets is independent of its protein sequence and its enrichment in hematopoiesis and mature blood cells is driven entirely by its nucleotide-dependent functions.

Structural basis of membrane skeleton organization in red blood cells

The spectrin-based membrane skeleton is a ubiquitous membrane-associated two-dimensional cytoskeleton underneath the lipid membrane of metazoan cells. Mutations of skeleton proteins impair the mechanical strength and functions of the membrane, leading to several different types of human diseases. Here, we report the cryo-EM structures of the native spectrin-actin junctional complex (from porcine erythrocytes), which is a specialized short F-actin acting as the central organizational unit of the membrane skeleton. While an α-/β-adducin hetero-tetramer binds to the barbed end of F-actin as a flexible cap, tropomodulin and SH3BGRL2 together create an absolute cap at the pointed end. The junctional complex is strengthened by ring-like structures of dematin in the middle actin layers and by patterned periodic interactions with tropomyosin over its entire length. This work serves as a structural framework for understanding the assembly and dynamics of membrane skeleton and offers insights into mechanisms of various ubiquitous F-actin-binding factors in other F-actin systems.

A hierarchical 3D finite element model of osteocyte: The spectrin membrane skeleton in mechanical transmission

Osteocytes in vivo are subjected to fluid shear stress and respond by initiating downstream mechano-transduction. Our previous studies demonstrated that the reticulate spectrin-based membrane skeleton (Spectrin-MS) beneath the cell membrane plays an important role in the mechanical property maintenance, as well as in the mechano-transduction of osteocytes. But how does the gridding extent of the spectrin-MS affect the mechanical transmission inside the cell remains unclear. To address this issue, a 3D finite element model (FEM) of osteocytes containing Spectrin-MS of varied gridding extent (representing polymerized/normal/depolymerized state) was established. The stress-strain responses of different components of the osteocyte to 15 dyn/cm2 fluid shear stress were obtained. Results showed that the depolymerized Spectrin-MS provided less support for the cell membrane and thus led more membrane deformation under shear stress. The depolymerization of Spectrin-MS also altered the mechanical transmission pathway inside the cell, and ultimately increased the stress transmitted to the nucleus. A compensative effect between the Spectrin-MS and cytoskeleton could be found in the mechanical transmission. These results indicate that the spectrin-MS is not only acting as a structural support, but also as a connecting linkage between the cell membrane and cytoskeleton. Future research of the mechanotransduction of osteocytes may benefit from considerations of the integration of the Spectrin-MS.

SHIP1 Controls Internal Platelet Contraction and αIIbβ3 Integrin Dynamics in Early Platelet Activation.

The Src homology 2 domain-containing inositol 5-phosphatase 1 (SHIP1) is known to dephosphorylate PtdIns(3,4,5)P3 into PtdIns(3,4)P2 and to interact with several signaling proteins though its docking functions. It has been shown to negatively regulate platelet adhesion and spreading on a fibrinogen surface and to positively regulate thrombus growth. In the present study, we have investigated its role during the early phase of platelet activation. Using confocal-based morphometric analysis, we found that SHIP1 is involved in the regulation of cytoskeletal organization and internal contractile activity in thrombin-activated platelets. The absence of SHIP1 has no significant impact on thrombin-induced Akt or Erk1/2 activation, but it selectively affects the RhoA/Rho-kinase pathway and myosin IIA relocalization to the cytoskeleton. SHIP1 interacts with the spectrin-based membrane skeleton, and its absence induces a loss of sustained association of integrins to this network together with a decrease in αIIbβ3 integrin clustering following thrombin stimulation. This αIIbβ3 integrin dynamics requires the contractile cytoskeleton under the control of SHIP1. RhoA activation, internal platelet contraction, and membrane skeleton integrin association were insensitive to the inhibition of PtdIns(3,4,5)P3 synthesis or SHIP1 phosphatase activity, indicating a role for the docking properties of SHIP1 in these processes. Altogether, our data reveal a lipid-independent function for SHIP1 in the regulation of the contractile cytoskeleton and integrin dynamics in platelets.

Differential dielectroscopic data on the relation of erythrocyte membrane skeleton to erythrocyte deformability and flicker.

Two dielectric relaxations, βsp (1.5MHz) and γ1sp (7MHz), have been detected on spectrin-based membrane skeleton (MS) of red blood cells (RBCs) using the plot of admittance changes at the spectrin denaturation temperature (Ivanov and Paarvanova in Bioelectrochemistry 110: 59-68, 2016, Electrochim Acta 317: 289-300, 2019a). In this study, we treated RBCs and RBC ghost membranes with agents that make membranes rigid and suppress membrane flicker, and studied the effect on βsp and γ1sp relaxations. Diamide (diazene dicarboxylic acid bis-(N,N-dimethylamide)) (up to 0.85mM), taurine mustard (tris(2-chloroethyl)amine) (up to 2mM), known to specifically cross-link and stiffen spectrin, and glutaraldehyde (up to 0.044%) all inhibited the relaxations in RBC ghost membranes. Similar inhibition was obtained resealing RBC ghost membranes with 2,3-diphosphoglicerate (up to 15mM), binding WGA (wheat germ agglutinin) (up to 0.025mg/ml) to exofacial aspect of RBCs, incubating RBCs in hypotonic (200mOsm) and hypertonic (600-900mOsm) media and depleting RBCs of ATP. By contrast, concanavalin A (1mg/ml) and DIDS (4,4'-diiso-thiocyanato stilbene-2,2'-disulfonic acid) (75μM, pH 8.2), both known to bind specifically band 3 integral protein of RBCs without effect on RBC membrane rigidity, did not affect the relaxations. We conclude there might be a relation between the strength of dielectric relaxations on MS spectrin and the deformability and flicker of RBC membrane.

An Axon-Pathfinding Mechanism Preserves Epithelial Tissue Integrity

Epithelial tissues form the boundaries of organs, where they perform a range of functions, including secretion, absorption, and protection. These tissues are commonly composed of discrete cell layers-sheets of cells that are one-cell thick. In multiple systems examined, epithelial cells round up and move in the apical direction before dividing, likely in response to neighbor-cell crowding [1-6]. Because of this movement, daughter cells may be born displaced from the tissue layer. Reintegration of these displaced cells supports tissue growth and maintains tissue architecture [4]. Two conserved IgCAMs (immunoglobulin superfamily cell adhesion molecules), neuroglian (Nrg) and fasciclin 2 (Fas2), participate in cell reintegration in the Drosophila follicular epithelium [4]. Like their vertebrate orthologs L1CAM and NCAM1/2, respectively, Nrg and Fas2 are cell adhesion molecules primarily studied in the context of nervous system development [7-10]. Consistent with this, we identify another neural IgCAM, Fasciclin 3 (Fas3), as a reintegration factor. Nrg, Fas2, and Fas3 are components of the insect septate junction, the functional equivalent of the vertebrate tight junction, but proliferating follicle cells do not have mature septate junctions, and we find that the septate junction protein neurexin IV does not participate in reintegration [11, 12]. Here, we show that epithelial reintegration works in the same way as IgCAM-mediated axon growth and pathfinding; it relies not only on extracellular adhesion but also mechanical coupling between IgCAMs and the lateral spectrin-based membrane skeleton. Our work indicates that reintegration is mediated by a distinct epithelial adhesion assembly that is compositionally and functionally equivalent to junctions made between axons.

Complementary mesoscale dynamics of spectrin and acto-myosin shape membrane territories during mechanoresponse

The spectrin-based membrane skeleton is a major component of the cell cortex. While expressed by all metazoans, its dynamic interactions with the other cortex components, including the plasma membrane or the acto-myosin cytoskeleton, are poorly understood. Here, we investigate how spectrin re-organizes spatially and dynamically under the membrane during changes in cell mechanics. We find spectrin and acto-myosin to be spatially distinct but cooperating during mechanical challenges, such as cell adhesion and contraction, or compression, stretch and osmolarity fluctuations, creating a cohesive cortex supporting the plasma membrane. Actin territories control protrusions and contractile structures while spectrin territories concentrate in retractile zones and low-actin density/inter-contractile regions, acting as a fence that organize membrane trafficking events. We unveil here the existence of a dynamic interplay between acto-myosin and spectrin necessary to support a mesoscale organization of the lipid bilayer into spatially-confined cortical territories during cell mechanoresponse.

The spectrin-based membrane skeleton is asymmetric and remodels during neural development in C. elegans.

Perturbation of spectrin-based membrane mechanics causes hereditary elliptocytosis and spinocerebellar ataxia, but the underlying cellular basis of pathogenesis remains unclear. Here, we introduced conserved disease-associated spectrin mutations into the Caenorhabditis elegans genome and studied the contribution of spectrin to neuronal migration and dendrite formation in developing larvae. The loss of spectrin resulted in ectopic actin polymerization outside of the existing front and secondary membrane protrusions, leading to defective neuronal positioning and dendrite morphology in adult animals. Spectrin accumulated in the lateral region and rear of migrating neuroblasts and redistributes from the soma into the newly formed dendrites, indicating that the spectrin-based membrane skeleton is asymmetric and remodels to regulate actin assembly and cell shape during development. We affinity-purified spectrin from C. elegans and showed that its binding partner ankyrin functions with spectrin. Asymmetry and remodeling of the membrane skeleton might enable spatiotemporal modulation of membrane mechanics for distinct developmental events.

Species-dependent variations in the dielectric activity of membrane skeleton of erythrocytes.

Previously detected βsp and γ1sp dielectric relaxations on the spectrin-based membrane skeleton (MS) of human red blood cells (RBCs) have been shown sensitive to the attachment of MS to the lipid-protein membrane. Such relaxations were now detected on the MS of mammal (rat, horse, bovine, sheep and goat) and "unstrained" chicken RBCs. To become "unstrained" chicken RBCs were subjected consecutively to cold (4°C, >20 h) and either colchicine (15 mM) or vinblastine (30 μM) (4°C, 1 h) that led to irreversible disassembly of their marginal band and an additional portion of their cytoskeleton. With the exception of bovine RBCs, the critical frequency (fc) of either relaxation increased, although at different rates, with the decrease in the volume of RBC species. The strong increase in fc of γ1sp relaxation from 2.5 MHz ("unstrained" chicken RBCs) to 13 MHz (goat RBCs) could indicate denser state of MS in smaller RBC species. The low values of fc of γ1sp relaxation in "unstrained" chicken RBCs (2.5 MHz) and bovine RBCs (4.5 instead of 9 MHz) could be related to their extraordinary thermal stability at the temperature of spectrin denaturation.

Spectrin-based membrane skeleton supports ciliogenesis.

Cilia are remarkable cellular devices that power cell motility and transduce extracellular signals. To assemble a cilium, a cylindrical array of 9 doublet microtubules push out an extension of the plasma membrane. Membrane tension regulates cilium formation; however, molecular pathways that link mechanical stimuli to ciliogenesis are unclear. Using genome editing, we introduced hereditary elliptocytosis (HE)- and spinocerebellar ataxia (SCA)-associated mutations into the Caenorhabditis elegans membrane skeletal protein spectrin. We show that these mutations impair mechanical support for the plasma membrane and change cell shape. RNA sequencing (RNA-seq) analyses of spectrin-mutant animals uncovered a global down-regulation of ciliary gene expression, prompting us to investigate whether spectrin participates in ciliogenesis. Spectrin mutations affect intraflagellar transport (IFT), disrupt axonemal microtubules, and inhibit cilium formation, and the endogenous spectrin periodically distributes along cilia. Mammalian spectrin also localizes in cilia and regulates ciliogenesis. These results define a previously unrecognized yet conserved role of spectrin-based mechanical support for cilium biogenesis.

The role of spectrin in cell adhesion and cell–cell contact

Spectrins are proteins that are responsible for many aspects of cell function and adaptation to changing environments. Primarily the spectrin-based membrane skeleton maintains cell membrane integrity and its mechanical properties, together with the cytoskeletal network a support cell shape. The occurrence of a variety of spectrin isoforms in diverse cellular environments indicates that it is a multifunctional protein involved in numerous physiological pathways. Participation of spectrin in cell–cell and cell–extracellular matrix adhesion and formation of dynamic plasma membrane protrusions and associated signaling events is a subject of interest for researchers in the fields of cell biology and molecular medicine. In this mini-review, we focus on data concerning the role of spectrins in cell surface activities such as adhesion, cell–cell contact, and invadosome formation. We discuss data on different adhesion proteins that directly or indirectly interact with spectrin repeats. New findings support the involvement of spectrin in cell adhesion and spreading, formation of lamellipodia, and also the participation in morphogenetic processes, such as eye development, oogenesis, and angiogenesis. Here, we review the role of spectrin in cell adhesion and cell–cell contact.Impact statementThis article reviews properties of spectrins as a group of proteins involved in cell surface activities such as, adhesion and cell–cell contact, and their contribution to morphogenesis. We show a new area of research and discuss the involvement of spectrin in regulation of cell–cell contact leading to immunological synapse formation and in shaping synapse architecture during myoblast fusion. Data indicate involvement of spectrins in adhesion and cell–cell or cell–extracellular matrix interactions and therefore in signaling pathways. There is evidence of spectrin’s contribution to the processes of morphogenesis which are connected to its interactions with adhesion molecules, membrane proteins (and perhaps lipids), and actin. Our aim was to highlight the essential role of spectrin in cell–cell contact and cell adhesion.

Tenectin recruits integrin to stabilize bouton architecture and regulate vesicle release at the Drosophila neuromuscular junction.

Assembly, maintenance and function of synaptic junctions depend on extracellular matrix (ECM) proteins and their receptors. Here we report that Tenectin (Tnc), a Mucin-type protein with RGD motifs, is an ECM component required for the structural and functional integrity of synaptic specializations at the neuromuscular junction (NMJ) in Drosophila. Using genetics, biochemistry, electrophysiology, histology and electron microscopy, we show that Tnc is secreted from motor neurons and striated muscles and accumulates in the synaptic cleft. Tnc selectively recruits αPS2/βPS integrin at synaptic terminals, but only the cis Tnc/integrin complexes appear to be biologically active. These complexes have distinct pre- and postsynaptic functions, mediated at least in part through the local engagement of the spectrin-based membrane skeleton: the presynaptic complexes control neurotransmitter release, while postsynaptic complexes ensure the size and architectural integrity of synaptic boutons. Our study reveals an unprecedented role for integrin in the synaptic recruitment of spectrin-based membrane skeleton.

Vector Analysis of Cytoskeletal Structural Tension and the Mechanisms that Underpin Spectrin-Related Forces in Pyroptosis.

Aims: Pyroptotic cells are characterized by plasma swelling, membrane blebbing, and disintegration of the cell membrane mediated by spectrin-based membrane skeleton and intercellular competitive tension activities. The spectrin-based membrane skeleton is involved in membrane organization through the regulation of intercellular tension. Using genetically encoded tension sensors to attain noninvasive force measurements in structural proteins, we investigated how cytoskeletal structural tension influences changes in plasma morphology during pyroptosis and the regulatory mechanism of cytoskeletal structural tension that underpins pyroptosis. Results: The results indicate that increasing spectrin tension is caused by osmotic swelling. Hightened tension of spectrin was closely associated with the shrink tension transmitted synergistically by microfilaments (MFs) and microtubules (MTs). However, the increment of spectrin tension in pyroptotic cells was controlled antagonistically by MF and MT forces. Different from MF tension, outward MT forces participated in the formation of membrane blebs. Spectrin tension caused by inward MF forces resisted pyroptosis swelling. Stabilization of MF and MT structure had little influence on intracellular tension and pyroptosis deformation. Pyroptosis-induced cytoskeletal structural tension was highly dependent on calcium signaling and reactive oxygen species generation. Blocking of membrane pores, nonselective ion flux, or elimination of caspase-1 cleavage resulted in the remission of structural forces associated with pyroptosis failure. Innovation and Conclusions: The data suggest that subcellular tension, in terms of magnitude and vector, is integral to pyroptosis through the mediation of swelling and blebbing and the elimination of structural tension, especially MT forces, may result in pyroptosis inhibition.

βIII Spectrin Is Necessary for Formation of the Constricted Neck of Dendritic Spines and Regulation of Synaptic Activity in Neurons.

Dendritic spines are postsynaptic structures in neurons often having a mushroom-like shape. Physiological significance and cytoskeletal mechanisms that maintain this shape are poorly understood. The spectrin-based membrane skeleton maintains the biconcave shape of erythrocytes, but whether spectrins also determine the shape of nonerythroid cells is less clear. We show that βIII spectrin in hippocampal and cortical neurons from rodent embryos of both sexes is distributed throughout the somatodendritic compartment but is particularly enriched in the neck and base of dendritic spines and largely absent from spine heads. Electron microscopy revealed that βIII spectrin forms a detergent-resistant cytoskeletal network at these sites. Knockdown of βIII spectrin results in a significant decrease in the density of dendritic spines. Surprisingly, the density of presynaptic terminals is not affected by βIII spectrin knockdown. However, instead of making normal spiny synapses, the presynaptic structures in βIII spectrin-depleted neurons make shaft synapses that exhibit increased amplitudes of miniature EPSCs indicative of excessive postsynaptic excitation. Thus, βIII spectrin is necessary for formation of the constricted shape of the spine neck, which in turn controls communication between the synapse and the parent dendrite to prevent excessive excitation. Notably, mutations of SPTNB2 encoding βIII spectrin are associated with neurodegenerative syndromes, spinocerebellar ataxia Type 5, and spectrin-associated autosomal recessive cerebellar ataxia Type 1, but molecular mechanisms linking βIII spectrin functions to neuronal pathologies remain unresolved. Our data suggest that spinocerebellar ataxia Type 5 and spectrin-associated autosomal recessive cerebellar ataxia Type 1 pathology likely arises from poorly controlled synaptic activity that leads to excitotoxicity and neurodegeneration.SIGNIFICANCE STATEMENT Dendritic spines are small protrusions from neuronal dendrites that make synapses with axons of other neurons in the brain. Dendritic spines usually have a mushroom-like shape, which is essential for brain functions, because aberrant spine morphology is associated with many neuropsychiatric disorders. The bulbous head of a mushroom-shaped spine makes the synapse, whereas the narrow neck transmits the incoming signals to the dendrite and supposedly controls the signal propagation. We show that a cytoskeletal protein βIII spectrin plays a key role for the formation of narrow spine necks. In the absence of βIII spectrin, dendritic spines collapse onto dendrites. As a result, synaptic strength exceeds acceptable levels and damages neurons, explaining pathology of human syndromes caused by βIII spectrin mutations.

Red cell membrane disorders.

Significant advances have been made in our understanding of the structural basis for altered cell function in various inherited red cell membrane disorders with reduced red cell survival and resulting hemolytic anemia. The current review summarizes these advances as they relate to defining the molecular and structural basis for disorders involving altered membrane structural organization (hereditary spherocytosis [HS] and hereditary elliptocytosis [HE]) and altered membrane transport function (hereditary overhydrated stomatocytosis and hereditary xerocytosis). Mutations in genes encoding membrane proteins that account for these distinct red cell phenotypes have been identified. These molecular insights have led to improved understanding of the structural basis for altered membrane function in these disorders. Weakening of vertical linkage between the lipid bilayer and spectrin-based membrane skeleton leads to membrane loss in HS. In contrast, weakening of lateral linkages among different skeletal proteins leads to membrane fragmentation and decreased surface area in HE. The degrees of membrane loss and resultant increases in cell sphericity determine the severity of anemia in these two disorders. Splenectomy leads to amelioration of anemia by increasing the circulatory red cell life span of spherocytic red cells that are normally sequestered by the spleen. Disordered membrane cation permeability and resultant increase or decrease in red cell volume account for altered cellular deformability of hereditary overhydrated stomatocytosis and hereditary xerocytosis, respectively. Importantly, splenectomy is not beneficial in these two membrane transport disorders and in fact contraindicated due to severe postsplenectomy thrombotic complications.

Prevalent presence of periodic actin–spectrin-based membrane skeleton in a broad range of neuronal cell types and animal species

Actin, spectrin, and associated molecules form a periodic, submembrane cytoskeleton in the axons of neurons. For a better understanding of this membrane-associated periodic skeleton (MPS), it is important to address how prevalent this structure is in different neuronal types, different subcellular compartments, and across different animal species. Here, we investigated the organization of spectrin in a variety of neuronal- and glial-cell types. We observed the presence of MPS in all of the tested neuronal types cultured from mouse central and peripheral nervous systems, including excitatory and inhibitory neurons from several brain regions, as well as sensory and motor neurons. Quantitative analyses show that MPS is preferentially formed in axons in all neuronal types tested here: Spectrin shows a long-range, periodic distribution throughout all axons but appears periodic only in a small fraction of dendrites, typically in the form of isolated patches in subregions of these dendrites. As in dendrites, we also observed patches of periodic spectrin structures in a small fraction of glial-cell processes in four types of glial cells cultured from rodent tissues. Interestingly, despite its strong presence in the axonal shaft, MPS is disrupted in most presynaptic boutons but is present in an appreciable fraction of dendritic spine necks, including some projecting from dendrites where such a periodic structure is not observed in the shaft. Finally, we found that spectrin is capable of adopting a similar periodic organization in neurons of a variety of animal species, including Caenorhabditis elegans, Drosophila, Gallus gallus, Mus musculus, and Homo sapiens.

Quantitation of Compositional Changes in the Non-Erythroid Membrane Skeleton due to External Forces

The spectrin based membrane skeleton plays important roles in the mechanical and biological functions of living cells. Unlike the membrane skeleton of red blood cells, its non-erythroid counterpart has seen very little attention. Yet, it is assumed that also in non-erythrocytes the membrane skeleton is mostly composed of heterotetramers of α-II and β-II spectrin. Because of α-spectrins’ potential role as mechanotransducers, we investigate the effect of mechanical stimulation on the spectrin-ankyrin B membrane skeleton of NIH 3T3 fibroblasts and murine C3H 10T½ cells. Using quantitative fluorescence microscopy and biochemical techniques, the absolute changes in cellular spectrin and ankyrin B content were determined. Before stimulation the spectrins and ankyrin B make up more than 10% of cytosolic and membrane proteins. Thus, they constitute a much more significant part of the non-erythroid cytoskeleton in these cells than previously assumed. Interestingly, both cell types contain all four major isoforms of spectrin. However, while α-II spectrin is the dominant α spectrin in 3T3, in 10T½ cells it is the ‘erythroid’ α-I spectrin. Also, at least half of the α spectrins do not have β-spectrin counterparts and thus cannot be part of the classical heterotetramere structure. Mechanical stimulation decreases the overall spectrin amount by up to 60% with α-spectrins exhibiting the largest loss. In contrast, the number of ankyrin B copies increases by almost 30%. Furthermore, the fraction of polyubiquitinated spectrin actually increases, which in part could account for the reduction in α-spectrins and is in line with their proposed mechanically enhanced self-ubiquitination activity.

Abnormal red cell features associated with hereditary neurodegenerative disorders

This review discusses the mechanisms involved in the generation of thorny red blood cells (RBCs), known as acanthocytes, in patients with neuroacanthocytosis, a heterogenous group of neurodegenerative hereditary disorders that include chorea-acanthocytosis (ChAc) and McLeod syndrome (MLS). Although molecular defects associated with neuroacanthocytosis have been identified recently, their pathophysiology and the related RBC abnormalities are largely unknown. Studies in ChAc RBCs have shown an altered association between the cytoskeleton and the integral membrane protein compartment in the absence of major changes in RBC membrane composition. In ChAc RBCs, abnormal Lyn kinase activation in a Syk-independent fashion has been reported recently, resulting in increased band 3 tyrosine phosphorylation and perturbation of the stability of the multiprotein band 3-based complexes bridging the membrane to the spectrin-based membrane skeleton. Similarly, in MLS, the absence of XK-protein, which is associated with the spectrin-actin-4.1 junctional complex, is associated with an abnormal membrane protein phosphorylation state, with destabilization of the membrane skeletal network resulting in generation of acanthocytes. A novel mechanism in generation of acanthocytes involving abnormal Lyn activation, identified in ChAc, expands the acanthocytosis phenomenon toward protein-protein interactions, controlled by phosphorylation-related abnormal signaling.

The Cellular Content of Non-Erythroid Spectrins and Ankyrins is Modulated by External Forces

Cells are exposed to mechanical forces in the form of stretch, shear stress, and hydro-static pressure and they transduce those forces into physical as well as biochemical responses. An important, mechanical cell-component that also takes part in this process of mechano-transduction is the spectrin based membrane skeleton. In the context of non-erythroid cells this structure has seen very little attention so far, while its counterpart in erythroid cells, which lack any other significant mechanical structure, has been extensively studied. For example, it was investigated that the erythroid α-spectrin has an E2/E3 ubiquitination site that allows it to either ubiquitinate itself or spectrin associated Ankyrin. Interestingly, the corresponding domain in non-erythroid α-Spectrins is both conserved and is located at a mechano-sensitive site within a tri-helical spectrin repeat. Thus, we looked at the change in non-erythroid Spectrins' and Ankyrins', suspected targets' contents in NIH-3T3 Fibroblasts and multipotent-10T1/2 cells as they are exposed to stretch magnitudes of 20% at a frequency of 1Hz. Using a combination of fluorescence microscopy, quantitative gel analysis and immunoprecipitation, we find that spectrin content drastically decreases by up to 80%, while ankyrin content increases by up to 30% in the two cell types when they are exposed to bi-axial stretch. Technically, we were also able to establish that quantitative fluorescence microscopy on immunostained fixed cells is as suitable to measure relative changes in protein content as immunoprecipitated Coomassie blue stained protein gels. Finally, our results from co-immunoprecipitation and Western blotting indeed suggest that the cause of these changes in protein content is the poly-ubiqutination activity of spectrin on itself as well as ankyrin. Since both proteins act also as scaffolds for membrane associated proteins, these force induced changes should also impact membrane organization and structure.

Spectrin and phospholipids — the current picture of their fascinating interplay

The spectrin-based membrane skeleton is crucial for the mechanical stability and resilience of erythrocytes. It mainly contributes to membrane integrity, protein organization and trafficking. Two transmembrane protein macro-complexes that are linked together by spectrin tetramers play a crucial role in attaching the membrane skeleton to the cell membrane, but they are not exclusive. Considerable experimental data have shown that direct interactions between spectrin and membrane lipids are important for cell membrane cohesion. Spectrin is a multidomain, multifunctional protein with several distinctive structural regions, including lipid-binding sites within CH tandem domains, a PH domain, and triple helical segments, which are excellent examples of ligand specificity hidden in a regular repetitive structure, as recently shown for the ankyrin-sensitive lipid-binding domain of beta spectrin. In this review, we summarize the state of knowledge about interactions between spectrin and membrane lipids.