SH3 domain of spectrin participates in the activation of Rac in specialized calpain-induced integrin signaling complexes

A Systematic Compilation of Human SH3 Domains: A Versatile Superfamily in Cellular Signaling

SRC homology 3 (SH3) domains are fundamental modules that enable the assembly of protein complexes through physical interactions with a pool of proline-rich/noncanonical motifs from partner proteins. They are widely studied modular building blocks across all five kingdoms of life and viruses, mediating various biological processes. The SH3 domains are also implicated in the development of human diseases, such as cancer, leukemia, osteoporosis, Alzheimer's disease, and various infections. A database search of the human proteome reveals the existence of 298 SH3 domains in 221 SH3 domain-containing proteins (SH3DCPs), ranging from 13 to 720 kilodaltons. A phylogenetic analysis of human SH3DCPs based on their multi-domain architecture seems to be the most practical way to classify them functionally, with regard to various physiological pathways. This review further summarizes the achievements made in the classification of SH3 domain functions, their binding specificity, and their significance for various diseases when exploiting SH3 protein modular interactions as drug targets.

The centrosomal recruitment of γ-tubulin and its microtubule nucleation activity is α-fodrin guided

The regulation and recruitment of γ-TuRCs, the prime nucleator of microtubules, to the centrosome are still thrust areas of research. The interaction of fodrin, a sub-plasmalemmal cytoskeletal protein, with γ-tubulin is a new area of interest. To understand the cellular significance of this interaction, we show that depletion of α-fodrin brings in a significant reduction of γ-tubulin in neural cell centrosomes making it functionally under-efficient. This causes a loss of nucleation ability that cannot efficiently form microtubules in interphase cells and astral microtubules in mitosis. Fluorescence Recovery after Photobleaching (FRAP) experiment implies that α-fodrin is important in the recruitment of γ-tubulin to the centrosome resulting in the aforementioned effects. Further, our experiments indicate that the interaction of α-fodrin with certain pericentriolar matrix proteins such as Pericentrin and CDK5RAP2 are crucial for the recruitment of γ-tubulin to the centrosome. Earlier we reported that α-fodrin limits the nucleation potential of γ-TuRC. In that context, this study suggests that α-fodrin is a γ-tubulin recruiting protein to the centrosome thus preventing cytoplasmic microtubule nucleation and thereby compartmentalizing the process to the centrosome for maximum efficiency. Summary statementα-fodrin is a γ-tubulin interacting protein that controls the process of γ-tubulin recruitment to the centrosome and thereby regulates the microtubule nucleation capacity spatially and temporally.

An alpha II spectrin mutant peptide with unstable scaffold structure and increased sensitivity to calpain cleavage

A spontaneous missense mutation in the alpha II spectrin (αII) gene, replacing a highly conserved arginine 1098 with the glutamine (R1098Q), causes progressive neurodegeneration in heterozygous mutant mice. The molecular mechanism underlying this phenotype is unknown but the accumulation of 150kD αII breakdown products in brains of homozygous mutant embryos suggests an imbalance in the substrate level control of αII cleavage by calpains. This is further supported by in silico simulation predicting unmasked calpain target site and increased spectrin scaffold bending and flexibility of R1098Q mutant peptide. Here, using spectroscopic and in situ enzymatic techniques, we aimed at obtaining direct experimental support for the impact of R1098Q mutation on the αII stability and its propensity for calpain-mediated degradation. Thermal circular dichroism analyses performed on recombinant wildtype and R1098Q mutant αII peptides, composed of spectrin repeat 9-10 revealed that although both had very similar secondary structure contents, thermal stability curve profiles varied and the observed midpoint of the unfolding transition (Tm) was 5.5 °C lower for the R1098Q peptide. Yet, the dynamic light scattering profiles of both peptides closely overlapped, implying the same thermal propensity to aggregate. Calpain digestion of plate-bound αII peptides with and without added calmodulin revealed an enhancement of the R1098Q peptide digestion rate relative to WT control. In summary, these results support the unstable scaffold structure of the R1098Q peptide as contributing to its enhanced intrinsic sensitivity to calpain and suggest physiologic relevance of a proper calpain/spectrin balance in preventing neurodegeneration.

A Fresh Look at the Structure, Regulation, and Functions of Fodrin

Fodrin and its erythroid cell-specific isoform spectrin are actin-associated fibrous proteins that play crucial roles in the maintenance of structural integrity in mammalian cells, which is necessary for proper cell function. Normal cell morphology is altered in diseases such as various cancers and certain neuronal disorders. Fodrin and spectrin are two-chain (αβ) molecules that are encoded by paralogous genes and share many features but also demonstrate certain differences. Fodrin (in humans, typically a heterodimer of the products of the SPTAN1 and SPTBN1 genes) is expressed in nearly all cell types and is especially abundant in neuronal tissues, whereas spectrin (in humans, a heterodimer of the products of the SPTA1 and SPTB1 genes) is expressed almost exclusively in erythrocytes. To fulfill a role in such a variety of different cell types, it was anticipated that fodrin would need to be a more versatile scaffold than spectrin. Indeed, as summarized here, domains unique to fodrin and its regulation by Ca²⁺, calmodulin, and a variety of posttranslational modifications (PTMs) endow fodrin with additional specific functions. However, how fodrin structural variations and misregulated PTMs may contribute to the etiology of various cancers and neurodegenerative diseases needs to be further investigated.

Deficiency of αII-spectrin affects endothelial cell-matrix contact and migration leading to impairment of angiogenesis in vitro

Background

Precise coordination of cytoskeletal components and dynamic control of cell adhesion and migration are required for crucial cell processes such as differentiation and morphogenesis. We investigated the potential involvement of αII-spectrin, a ubiquitous scaffolding element of the membrane skeleton, in the adhesion and angiogenesis mechanism.

Methods

The cell models were primary human umbilical vein endothelial cells (HUVECs) and a human dermal microvascular endothelial cell line (HMEC-1). After siRNA- and shRNA-mediated knockdown of αII-spectrin, we assessed its expression and that of its partners and adhesion proteins using western blotting. The phenotypes of the control and spectrin-depleted cells were examined using immunofluorescence and video microscopy. Capillary tube formation was assessed using the thick gel Matrigel matrix-based method and a microscope equipped with a thermostatic chamber and a Nikon Biostation System camera.

Results

Knockdown of αII-spectrin leads to: modified cell shape; actin cytoskeleton organization with the presence of peripheral actin patches; and decreased formation of stress fibers. Spectrin deficiency affects cell adhesion on laminin and fibronectin and cell motility. This included modification of the localization of adhesion molecules, such as αVβ3- and α5-integrins, and organization of adhesion structures, such as focal points. Deficiency of αII-spectrin can also affect the complex mechanism of in vitro capillary tube formation, as demonstrated in a model of angiogenesis. Live imaging revealed that impairment of capillary tube assembly was mainly associated with a significant decrease in cell projection length and stability. αII-spectrin depletion is also associated with significantly decreased expression of three proteins involved in capillary tube formation and assembly: VE-cadherin, MCAM and β3-integrin.

Conclusion

Our data confirm the role of αII-spectrin in the control of cell adhesion and spreading. Moreover, our findings further support the participation of αII-spectrin in capillary tube formation in vitro through control of adhesion molecules, such as integrins. This indicates a new function of αII-spectrin in angiogenesis.

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 statement

This 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.

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.

αII-spectrin in T cells is involved in the regulation of cell-cell contact leading to immunological synapse formation?

T-lymphocyte activation after antigen presentation to the T-Cell Receptor (TCR) is a critical step in the development of proper immune responses to infection and inflammation. This dynamic process involves reorganization of the actin cytoskeleton and signaling molecules at the cell membrane, leading to the formation of the Immunological Synapse (IS). The mechanisms regulating the formation of the IS are not completely understood. Nonerythroid spectrin is a membrane skeletal protein involved in the regulation of many cellular processes, including cell adhesion, signaling and actin cytoskeleton remodeling. However, the role of spectrin in IS formation has not been explored. We used molecular, imaging and cellular approaches to show that nonerythroid αII-spectrin redistributes to the IS during T-cell activation. The redistribution of spectrin coincides with the relocation of CD45 and LFA-1, two components essential for IS formation and stability. We assessed the role of spectrin by shRNA-mediated depletion from Jurkat T cells and show that spectrin-depleted cells exhibit decreased adhesion and are defective in forming lamellipodia and filopodia. Importantly, IS formation is impaired in spectrin-depleted cells. Thus, spectrin may be engaged in regulation of distinct events necessary for the establishment and maturity of the IS: besides the involvement of spectrin in the control of CD45 and LFA-1 surface display, spectrin acts in the establishment of cell-cell contact and adhesion processes during the formation of the IS.

The mechano-sensing role of the unique SH3 insertion in plakin domains revealed by Molecular Dynamics simulations

The plakin family of proteins, important actors in cross-linking force-bearing structures in the cell, contain a curious SH3 domain insertion in their chain of spectrin repeats (SRs). While SH3 domains are known to mediate protein-protein interactions, here, its canonical binding site is autoinhibited by the preceding SR. Under force, however, this SH3 domain could be released, and possibly launch a signaling cascade. We performed large-scale force-probe molecular dynamics simulations, across two orders of magnitude of loading rates, to test this hypothesis, on two prominent members of the plakin family: desmoplakin and plectin, obligate proteins at desmosomes and hemidesmosomes, respectively. Our simulations show that force unravels the SRs and abolishes the autoinhibition of the SH3 domain, an event well separated from the unfolding of this domain. The SH3 domain is free and fully functional for a significant portion of the unfolding trajectories. The rupture forces required for the two proteins significantly decrease when the SH3 domain is removed, which implies that the SH3 domain also stabilizes this junction. Our results persist across all simulations, and support a force-sensing as well as a stabilizing role of the unique SH3 insertion, putting forward this protein family as a new class of mechano-sensors.

α-Spectrin and integrins act together to regulate actomyosin and columnarization, and to maintain a monolayered follicular epithelium

The spectrin cytoskeleton crosslinks actin to the membrane, and although it has been greatly studied in erythrocytes, much is unknown about its function in epithelia. We have studied the role of spectrins during epithelia morphogenesis using the Drosophila follicular epithelium (FE). As previously described, we show that α-Spectrin and β-Spectrin are essential to maintain a monolayered FE, but, contrary to previous work, spectrins are not required to control proliferation. Furthermore, spectrin mutant cells show differentiation and polarity defects only in the ectopic layers of stratified epithelia, similar to integrin mutants. Our results identify α-Spectrin and integrins as novel regulators of apical constriction-independent cell elongation, as α-Spectrin and integrin mutant cells fail to columnarize. Finally, we show that increasing and reducing the activity of the Rho1-Myosin II pathway enhances and decreases multilayering of α-Spectrin cells, respectively. Similarly, higher Myosin II activity enhances the integrin multilayering phenotype. This work identifies a primary role for α-Spectrin in controlling cell shape, perhaps by modulating actomyosin. In summary, we suggest that a functional spectrin-integrin complex is essential to balance adequate forces, in order to maintain a monolayered epithelium.

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

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

αII-spectrin regulates invadosome stability and extracellular matrix degradation

Invadosomes are actin-rich adhesion structures involved in tissue invasion and extracellular matrix (ECM) remodelling. αII-Spectrin, an ubiquitous scaffolding component of the membrane skeleton and a partner of actin regulators (ABI1, VASP and WASL), accumulates highly and specifically in the invadosomes of multiple cell types, such as mouse embryonic fibroblasts (MEFs) expressing SrcY527F, the constitutively active form of Src or activated HMEC-1 endothelial cells. FRAP and live-imaging analysis revealed that αII-spectrin is a highly dynamic component of invadosomes as actin present in the structures core. Knockdown of αII-spectrin expression destabilizes invadosomes and reduces the ability of the remaining invadosomes to digest the ECM and to promote invasion. The ECM degradation defect observed in spectrin-depleted-cells is associated with highly dynamic and unstable invadosome rings. Moreover, FRAP measurement showed the specific involvement of αII-spectrin in the regulation of the mobile/immobile β3-integrin ratio in invadosomes. Our findings suggest that spectrin could regulate invadosome function and maturation by modulating integrin mobility in the membrane, allowing the normal processes of adhesion, invasion and matrix degradation. Altogether, these data highlight a new function for spectrins in the stability of invadosomes and the coupling between actin regulation and ECM degradation.

Spectrin: structure, function and disease

Spectrin is a large, cytoskeletal, and heterodimeric protein composed of modular structure of α and β subunits, it typically contains 106 contiguous amino acid sequence motifs called "spectrin repeats". Spectrin is crucial for maintaining the stability and structure of the cell membrane and the shape of a cell. Moreover, it contributes to diverse cell functions such as cell adhesion, cell spreading, and the cell cycle. Mutations of spectrin lead to various human diseases such as hereditary hemolytic anemia, type 5 spinocerebellar ataxia, cancer, as well as others. This review focuses on recent advances in determining the structure and function of spectrin as well as its role in disease.

Spectrins: a structural platform for stabilization and activation of membrane channels, receptors and transporters

This review focuses on structure and functions of spectrin as a major component of the membrane skeleton. Recent advances on spectrin function as an interface for signal transduction mediation and a number of data concerning interaction of spectrin with membrane channels, adhesion molecules, receptors and transporters draw a picture of multifaceted protein. Here, we attempted to show the current depiction of multitask role of spectrin in cell physiology. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Structural organization of the nine spectrin repeats of Kalirin

Sequence analysis suggests that KALRN, a Rho GDP/GTP exchange factor genetically linked to schizophrenia, could contain as many as nine tandem spectrin repeats (SRs). We expressed and purified fragments of Kalirin containing from one to five putative SRs to determine whether they formed nested structures that could endow Kalirin with the flexible rodlike properties characteristic of spectrin and dystrophin. Far-UV circular dichroism studies indicated that Kalirin contains nine SRs. On the basis of thermal denaturation, sensitivity to chemical denaturants, and the solubility of pairs of repeats, the nine SRs of Kalirin form nested structures. Modeling studies confirmed this conclusion and identified an exposed loop in SR5; consistent with the modeling, this loop was extremely labile to proteolytic cleavage. Analysis of a direpeat fragment (SR4:5) encompassing the region of Kalirin known to interact with NOS2, DISC-1, PAM, and Arf6 identified this as the least stable region. Analytical ultracentrifugation indicated that SR1:3, SR4:6, and SR7:9 were monomers and adopted an extended conformation. Gel filtration suggested that ΔKal7, a natural isoform that includes SR5:9, was monomeric and was not more extended than SR5:9. Similarly, the nine SRs of Kal7, which was also monomeric, were not more extended than SR5:9. The rigidity and flexibility of the nine SRs of Kal7, which separate its essential N-terminal Sec14p domain from its catalytic domain, play an essential role in its contribution to the formation and function of dendritic spines.

Cell organization, growth, and neural and cardiac development require αII-spectrin

Spectrin α2 (αII-spectrin) is a scaffolding protein encoded by the Spna2 gene and constitutively expressed in most tissues. Exon trapping of Spna2 in C57BL/6 mice allowed targeted disruption of αII-spectrin. Heterozygous animals displayed no phenotype by 2 years of age. Homozygous deletion of Spna2 was embryonic lethal at embryonic day 12.5 to 16.5 with retarded intrauterine growth, and craniofacial, neural tube and cardiac anomalies. The loss of αII-spectrin did not alter the levels of αI- or βI-spectrin, or the transcriptional levels of any β-spectrin or any ankyrin, but secondarily reduced by about 80% the steady state protein levels of βII- and βIII-spectrin. Residual βII- and βIII-spectrin and ankyrins B and G were concentrated at the apical membrane of bronchial and renal epithelial cells, without impacting cell morphology. Neuroepithelial cells in the developing brain were more concentrated and more proliferative in the ventricular zone than normal; axon formation was also impaired. Embryonic fibroblasts cultured on fibronectin from E14.5 (Spna2(-/-)) animals displayed impaired growth and spreading, a spiky morphology, and sparse lamellipodia without cortical actin. These data indicate that the spectrin-ankyrin scaffold is crucial in vertebrates for cell spreading, tissue patterning and organ development, particularly in the developing brain and heart, but is not required for cell viability.

Spectrin-based skeleton as an actor in cell signaling

This review focuses on the recent advances in functions of spectrins in non-erythroid cells. We discuss new data concerning the commonly known role of the spectrin-based skeleton in control of membrane organization, stability and shape, and tethering protein mosaics to the cellular motors and to all major filament systems. Particular effort has been undertaken to highlight recent advances linking spectrin to cell signaling phenomena and its participation in signal transduction pathways in many cell types.

Novel role for the Lu/BCAM-spectrin interaction in actin cytoskeleton reorganization

Lu/BCAM (Lutheran/basal cell-adhesion molecule) is a laminin 511/521 receptor expressed in erythroid and endothelial cells, and in epithelial tissues. The RK573-574 (Arg573-Lys574) motif of the Lu/BCAM cytoplasmic domain interacts with αI-spectrin, the main component of the membrane skeleton in red blood cells. In the present paper we report that Lu/BCAM binds to the non-erythroid αII-spectrin via the RK573-574 motif. Alanine substitution of this motif abolished the Lu/BCAM-spectrin interaction, enhanced the half-life of Lu/BCAM at the MDCK (Madin-Darby canine kidney) cell surface, and increased Lu/BCAM-mediated cell adhesion and spreading on laminin 511/521. We have shown that the Lu/BCAM-spectrin interaction mediated actin reorganization during cell adhesion and spreading on laminin 511/521. This interaction was involved in a laminin 511/521-to-actin signalling pathway leading to stress fibre formation. This skeletal rearrangement was associated with an activation of the small GTP-binding protein RhoA, which depended on the integrity of the Lu/BCAM laminin 511/521-binding site. It also required a Lu/BCAM-αII-spectrin interaction, since its disruption decreased stress fibre formation and RhoA activation. We conclude that the Lu/BCAM-spectrin interaction is required for stress fibre formation during cell spreading on laminin 511/521, and that spectrin acts as a signal relay between laminin 511/521 and actin that is involved in actin dynamics.

Crystal structure of a rigid four-spectrin-repeat fragment of the human desmoplakin plakin domain

The plakin protein family serves to connect cell-cell and cell-matrix adhesion molecules to the intermediate filament cytoskeleton. Desmoplakin (DP) is an integral part of desmosomes, where it links desmosomal cadherins to the intermediate filaments. The 1056-amino-acid N-terminal region of DP contains a plakin domain common to members of the plakin family. Plakin domains contain multiple copies of spectrin repeats (SRs). We determined the crystal structure of a fragment of DP, residues 175-630, consisting of four SRs and an inserted SH3 domain. The four repeats form an elongated, rigid structure. The SH3 domain is present in a loop between two helices of an SR and interacts extensively with the preceding SR in a manner that appears to limit inter-repeat flexibility. The intimate intramolecular association of the SH3 domain with the preceding SR is also observed in plectin, another plakin protein, but not in α-spectrin, suggesting that the SH3 domain of plakins contributes to the stability and rigidity of this subfamily of SR-containing proteins.

The growth cone cytoskeleton in axon outgrowth and guidance

Axon outgrowth and guidance to the proper target requires the coordination of filamentous (F)-actin and microtubules (MTs), the dynamic cytoskeletal polymers that promote shape change and locomotion. Over the past two decades, our knowledge of the many guidance cues, receptors, and downstream signaling cascades involved in neuronal outgrowth and guidance has increased dramatically. Less is known, however, about how those cascades of information converge and direct appropriate remodeling and interaction of cytoskeletal polymers, the ultimate effectors of movement and guidance. During development, much of the communication that occurs between environmental guidance cues and the cytoskeleton takes place at the growing tip of the axon, the neuronal growth cone. Several articles on this topic focus on the "input" to the growth cone, the myriad of receptor types, and their corresponding cognate ligands. Others investigate the signaling cascades initiated by receptors and propagated by second messenger pathways (i.e., kinases, phosphatases, GTPases). Ultimately, this plethora of information converges on proteins that associate directly with the actin and microtubule cytoskeletons. The role of these cytoskeletal-associated proteins, as well as the cytoskeleton itself in axon outgrowth and guidance, is the subject of this article.

Rare copy number variants disrupt genes regulating vascular smooth muscle cell adhesion and contractility in sporadic thoracic aortic aneurysms and dissections

Thoracic aortic aneurysms and dissections (TAAD) cause significant morbidity and mortality, but the genetic origins of TAAD remain largely unknown. In a genome-wide analysis of 418 sporadic TAAD cases, we identified 47 copy number variant (CNV) regions that were enriched in or unique to TAAD patients compared to population controls. Gene ontology, expression profiling, and network analysis showed that genes within TAAD CNVs regulate smooth muscle cell adhesion or contractility and interact with the smooth muscle-specific isoforms of α-actin and β-myosin, which are known to cause familial TAAD when altered. Enrichment of these gene functions in rare CNVs was replicated in independent cohorts with sporadic TAAD (STAAD, n = 387) and inherited TAAD (FTAAD, n = 88). The overall prevalence of rare CNVs (23%) was significantly increased in FTAAD compared with STAAD patients (Fisher's exact test, p = 0.03). Our findings suggest that rare CNVs disrupting smooth muscle adhesion or contraction contribute to both sporadic and familial disease.

The effect of the lipid-binding site of the ankyrin-binding domain of erythroid beta-spectrin on the properties of natural membranes and skeletal structures

It was previously shown that the beta-spectrin ankyrin-binding domain binds lipid domains rich in PE in an ankyrin-dependent manner, and that its N-terminal sequence is crucial in interactions with phospholipids. In this study, the effect of the full-length ankyrin-binding domain of β-spectrin on natural erythrocyte and HeLa cell membranes was tested. It was found that, when encapsulated in resealed erythrocyte ghosts, the protein representing the full-length ankyrin-binding domain strongly affected the shape and barrier properties of the erythrocyte membrane, and induced partial spectrin release from the membrane, while truncated mutants had no effect. As found previously (Bok et al. Cell Biol. Int. 31 (2007) 1482–94), overexpression of the full-length GFP-tagged ankyrin-binding domain aggregated and induced aggregation of endogenous spectrin, but this was not the case with overexpression of proteins truncated at their N-terminus. Here, we show that the aggregation of spectrin was accompanied by the aggregation of integral membrane proteins that are known to be connected to spectrin via ankyrin, i.e. Na⁺K⁺ATP-ase, IP3 receptor protein and L1 CAM. By contrast, the morphology of the actin cytoskeleton remained unchanged and aggregation of cadherin E or N did not occur upon the overexpression of either full-length or truncated ankyrin-binding domain proteins. The obtained results indicate a substantial role of the lipid-binding part of the β-spectrin ankyrin-binding domain in the determination of the membrane and spectrin-based skeleton functional properties.

Increased aortic calpain-1 activity mediates age-associated angiotensin II signaling of vascular smooth muscle cells

BACKGROUND

Angiotensin II (Ang II) signaling, including matrix metalloproteinase type II (MMP2) activation, has been linked to an age-associated increase in migration capacity of vascular smooth muscle cells (VSMC), and to other proinflammatory features of arterial aging. Calpain-1 activation is required for MMP2 expression in fibroblasts and is induced in cardiomyocytes by Ang II. The consequences of engagement of calpain-1 with its substrates, however, in governing the age-associated proinflammatory status within the arterial wall, remains unknown.

METHODOLOGY/PRINCIPAL FINDINGS

The present findings demonstrate that transcription, translation, and activity of calpain-1 are significantly up-regulated in rat aortae or early-passage aortic VSMC from old (30-mo) rats compared to young (8-mo). Dual immunolabeling of the arterial wall indicates that colocalization of calpain-1 and Ang II increases within the aged arterial wall. To further explore the relationship of calpain-1 to Ang II, we chronically infused Ang II into young rats, and treated cultured aortic rings or VSMC with Ang II. We also constructed adenoviruses harboring calpain-1 (CANP1) or its endogenous inhibitor calpastatin (CAST) and infected these into VSMC. Ang II induces calpain-1 expression in the aortic walls in vivo and ex vivo and VSMC in vitro. The Ang II mediated, age-associated increased MMP2 activity and migration in VSMC are both blocked by calpain inhibitor 1 or CAST. Over-expression of calpain-1 in young VSMC results in cleavage of intact vimentin, and an increased migratory capacity mimicking that of old VSMC, which is blocked by the MMP inhibitor, GM6001.

CONCLUSIONS/SIGNIFICANCE

Calpain-1 activation is a pivotal molecular event in the age-associated arterial Ang II/MMP2 signaling cascade that is linked to cytoskeleton protein restructuring, and VSMC migration. Therefore, targeting calpain-1 has the potential to delay or reverse the arterial remodeling that underlies age-associated diseases i.e. atherosclerosis.

Calpain activation is involved in early caspase-independent neurodegeneration in the hippocampus following status epilepticus

Evidence for increased calpain activity has been described in the hippocampus of rodent models of temporal lobe epilepsy. However, it is not known whether calpains are involved in the cell death that accompanies seizures. In this work, we characterized calpain activation by examining the proteolysis of calpain substrates and in parallel we followed cell death in the hippocampus of epileptic rats. Male Wistar rats were injected with kainic acid (10 mg/kg) intraperitoneally and killed 24 h later, after development of grade 5 seizures. We observed a strong Fluoro-Jade labeling in the CA1 and CA3 areas of the hippocampus in the rats that received kainic acid, when compared with saline-treated rats. Immunohistochemistry and western blot analysis for the calpain-derived breakdown products of spectrin showed evidence of increased calpain activity in the same regions of the hippocampus where cell death is observed. No evidence was found for caspase activation, in the same conditions. Treatment with the calpain inhibitor MDL 28170 significantly prevented the neurodegeneration observed in CA1. Taken together, our data suggest that early calpain activation, but not caspase activation, is involved in neurotoxicity in the hippocampus after status epilepticus.

Scaffold mediated regulation of MAPK signaling and cytoskeletal dynamics: a perspective

Cell migration is critical for many physiological processes and is often misregulated in developmental disorders and pathological conditions including cancer and neurodegeneration. MAPK signaling and the Rho family of proteins are known regulators of cell migration that exert their influence on cellular cytoskeleton during cell adhesion and migration. Here we review data supporting the view that localized ERK signaling mediated through recently identified scaffold proteins may regulate cell migration.

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.

Structural analysis of the plakin domain of bullous pemphigoid antigen1 (BPAG1) suggests that plakins are members of the spectrin superfamily

Bullous pemphigoid antigen 1 (BPAG1) is a member of the plakin family of proteins. The plakins are multi-domain proteins that have been shown to interact with microtubules, actin filaments and intermediate filaments, as well as proteins found in cellular junctions. These interactions are mediated through different domains on the plakins. The interactions between plakins and components of specialized cell junctions such as desmosomes and hemidesmosomes are mediated through the so-called plakin domain, which is a common feature of the plakins. We report the crystal structure of a stable fragment from BPAG1, residues 226-448, defined by limited proteolysis of the whole plakin domain. The structure, determined by single-wavelength anomalous diffraction phasing from a selenomethionine-substituted crystal at 3.0 A resolution, reveals a tandem pair of triple helical bundles closely related to spectrin repeats. Based on this structure and analysis of sequence conservation, we propose that the architecture of plakin domains is defined by two pairs of spectrin repeats interrupted by a putative Src-Homology 3 (SH3) domain.

A role for the intracellular protease calpain in the activation of store-operated calcium entry in human platelets

Here, we report a novel role for the cysteine protease calpain in store-operated calcium entry. Several structurally and mechanistically unrelated inhibitors of calpain inhibited Ca2+ entry activated in human platelets by thapsigargin-evoked Ca2+ store depletion or the physiological agonist thrombin, whereas inhibitors of other cysteine proteases were without effect. The use of the cell-permeable fluorogenic calpain substrate 7-amino-4-chloromethylcoumarin, t-BOC-l-leucyl-l-methionine amide revealed rapid activation of calpain which was closely temporally correlated with Ca2+ store depletion even in the absence of a rise in cytosolic [Ca2+]. Calpain inhibition prevented the tyrosine phosphorylation of several proteins upon Ca2+ store depletion, suggesting that calpain may lie upstream of protein tyrosine phosphorylation that is known to be required for the activation of store-operated Ca2+ entry in human platelets. Earlier studies using calpain inhibitors may need reinterpretation in the light of this finding that calpain plays a role in the activation of physiological Ca2+ entry pathways.

Calpain involvement in the remodeling of cytoskeletal anchorage complexes

Cells offer different types of cytoskeletal anchorages: transitory structures such as focal contacts and perennial ones such as the sarcomeric cytoskeleton of muscle cells. The turnover of these structures is controlled with different timing by a family of cysteine proteases activated by calcium, the calpains. The large number of potential substrates present in each of these structures imposes fine tuning of the activity of the proteases to avoid excessive action. This phenomenon is thus guaranteed by various types of regulation, ranging from a relatively high calcium concentration necessary for activation, phosphorylation of substrates or the proteases themselves with either a favorable or inhibitory effect, possible intervention of phospholipids, and the presence of a specific inhibitor and its possible degradation before activation. Finally, formation of multiprotein complexes containing calpains offers a new method of regulation.

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.

Caspase-8 promotes cell motility and calpain activity under nonapoptotic conditions

Significant caspase-8 activity has been found in normal and certain tumor cells, suggesting that caspase-8 possesses an alternative, nonapoptotic function that may contribute to tumor progression. In this article, we report that caspase-8 promotes cell motility. In particular, caspase-8 is required for the optimal activation of calpains, Rac, and lamellipodial assembly. This represents a novel nonapoptotic function of caspase-8 acting at the intersection of the caspase-8 and calpain proteolytic pathways to coordinate cell death versus cell motility signaling.

Stimulation of Gαq-coupled M1 muscarinic receptor causes reversible spectrin redistribution mediated by PLC, PKC and ROCK

Spectrin is a cytoskeletal protein that plays a role in formation of the specialized plasma membrane domains. However, little is known of the molecular mechanism that regulates responses of spectrin to extracellular stimuli, such as activation of G-protein-coupled receptor (GPCR). We have found that αII spectrin is a component of the Gαq/11-associated protein complex in CHO cells stably expressing the M1 muscarinic receptor, and investigated the effect of activation of GPCR on the cellular localization of yellow-fluorescent-protein-tagged αII spectrin. Stimulation of Gαq/11-coupled M1 muscarinic receptor triggered reversible redistribution of αII spectrin following a rise in intracellular Ca2+ concentration. This redistribution, accompanied by non-apoptotic membrane blebbing, required an intact actin cytoskeleton and was dependent on activation of phospholipase C, protein kinase C, and Rho-associated kinase ROCK. Muscarinic-agonist-induced spectrin remodeling appeared particularly active at localized domains, which is clear contrast to that caused by constitutive activation of ROCK and to global rearrangement of the spectrin lattice caused by changes in osmotic pressure. These results suggest a role for spectrin in providing a dynamic and reversible signaling platform to the specific domains of the plasma membrane in response to stimulation of GPCR.

Regulating cell migration: calpains make the cut

The calpain family of proteases has been implicated in cellular processes such as apoptosis, proliferation and cell migration. Calpains are involved in several key aspects of migration, including: adhesion and spreading; detachment of the rear; integrin- and growth-factor-mediated signaling; and membrane protrusion. Our understanding of how calpains are activated and regulated during cell migration has increased as studies have identified roles for calcium and phospholipid binding, autolysis, phosphorylation and inhibition by calpastatin in the modulation of calpain activity. Knockout and knockdown approaches have also contributed significantly to our knowledge of calpain biology, particularly with respect to the specific functions of different calpain isoforms. The mechanisms by which calpain-mediated proteolysis of individual substrates contributes to cell motility have begun to be addressed, and these efforts have revealed roles for proteolysis of specific substrates in integrin activation, adhesion complex turnover and membrane protrusion dynamics. Understanding these mechanisms should provide avenues for novel therapeutic strategies to treat pathological processes such as tumor metastasis and chronic inflammatory disease.