Calmodulin Regulates Fodrin Susceptibility to Cleavage by Calciumdependent Protease I

Membrane to cytosol redistribution of αII-spectrin drives extracellular vesicle biogenesis in malignant breast cells

Spectrin is a ubiquitous cytoskeletal protein that provides structural stability and supports membrane integrity. In erythrocytes, spectrin proteolysis leads to the biogenesis of plasma membrane extracellular vesicles (EVs). However, its role in non-erythroid or cancer-derived plasma membrane EVs biogenesis is unknown. This study aims to examine the role of αII-spectrin in malignant and non-malignant plasma membrane vesiculation. We developed a custom, automated cell segmentation plugin for the image processor, Fiji, that provides an unbiased assessment of high resolution confocal microscopy images of the subcellular distribution of αII-spectrin. We show that, in low vesiculating non-malignant MBE-F breast cells, prominent cortical spectrin localises to the cell periphery at rest. In comparison, cortical spectrin is diminished in high vesiculating malignant MCF-7 breast cells at rest. A cortical distribution of spectrin correlates with increased biomechanical stiffness as measured by Atomic Force Microscopy. Furthermore, cortical spectrin can be induced in malignant MCF-7 cells by treatment with known vesiculation modulators including the calcium chelator, BAPTA-AM or the calpain inhibitor II (ALLM). These results demonstrate that the subcellular localisation of spectrin is distinctly different in malignant and non-malignant cells at rest and shows that the redistribution of cortical αII-spectrin to the cytoplasm supports plasma membrane-derived EV biogenesis in malignant cells.

Age-dependent ataxia and neurodegeneration caused by an αII spectrin mutation with impaired regulation of its calpain sensitivity

The neuronal membrane-associated periodic spectrin skeleton (MPS) contributes to neuronal development, remodeling, and organization. Post-translational modifications impinge on spectrin, the major component of the MPS, but their role remains poorly understood. One modification targeting spectrin is cleavage by calpains, a family of calcium-activated proteases. Spectrin cleavage is regulated by activated calpain, but also by the calcium-dependent binding of calmodulin (CaM) to spectrin. The physiologic significance of this balance between calpain activation and substrate-level regulation of spectrin cleavage is unknown. We report a strain of C57BL/6J mice harboring a single αII spectrin point mutation (Sptan1 c.3293G > A:p.R1098Q) with reduced CaM affinity and intrinsically enhanced sensitivity to calpain proteolysis. Homozygotes are embryonic lethal. Newborn heterozygotes of either gender appear normal, but soon develop a progressive ataxia characterized biochemically by accelerated calpain-mediated spectrin cleavage and morphologically by disruption of axonal and dendritic integrity and global neurodegeneration. Molecular modeling predicts unconstrained exposure of the mutant spectrin's calpain-cleavage site. These results reveal the critical importance of substrate-level regulation of spectrin cleavage for the maintenance of neuronal integrity. Given that excessive activation of calpain proteases is a common feature of neurodegenerative disease and traumatic encephalopathy, we propose that damage to the spectrin MPS may contribute to the neuropathology of many disorders.

The functional importance of lamins, actin, myosin, spectrin and the LINC complex in DNA repair

Three major proteins in the nucleoskeleton, lamins, actin, and spectrin, play essential roles in maintenance of nuclear architecture and the integrity of the nuclear envelope, in mechanotransduction and mechanical coupling between the nucleoskeleton and cytoskeleton, and in nuclear functions such as regulation of gene expression, transcription and DNA replication. Less well known, but critically important, are the role these proteins play in DNA repair. The A-type and B-type lamins, nuclear actin and myosin, spectrin and the LINC (linker of nucleoskeleton and cytoskeleton) complex each function in repair of DNA damage utilizing various repair pathways. The lamins play a role in repair of DNA double-strand breaks (DSBs) by nonhomologous end joining (NHEJ) or homologous recombination (HR). Actin is involved in repair of DNA DSBs and interacts with myosin in facilitating relocalization of these DSBs in heterochromatin for HR repair. Nonerythroid alpha spectrin (αSpII) plays a critical role in repair of DNA interstrand cross-links (ICLs) where it acts as a scaffold in recruitment of repair proteins to sites of damage and is important in the initial damage recognition and incision steps of the repair process. The LINC complex contributes to the repair of DNA DSBs and ICLs. This review will address the important functions of these proteins in the DNA repair process, their mechanism of action, and the profound impact a defect or deficiency in these proteins has on cellular function. The critical roles of these proteins in DNA repair will be further emphasized by discussing the human disorders and the pathophysiological changes that result from or are related to deficiencies in these proteins. The demonstrated function for each of these proteins in the DNA repair process clearly indicates that there is another level of complexity that must be considered when mechanistically examining factors crucial for DNA repair.

The Spectrinome: The Interactome of a Scaffold Protein Creating Nuclear and Cytoplasmic Connectivity and Function

We provide a review of Spectrin isoform function in the cytoplasm, the nucleus, the cell surface, and in intracellular signaling. We then discuss the importance of Spectrin’s E2/E3 chimeric ubiquitin conjugating and ligating activity in maintaining cellular homeostasis. Finally we present spectrin isoform subunit specific human diseases. We have created the Spectrinome, from the Human Proteome, Human Reactome and Human Atlas data and demonstrated how it can be a useful tool in visualizing and understanding spectrins myriad of cellular functions.

Impact statement

Spectrin was for the first 12 years after its discovery thought to be found only in erythrocytes. In 1981, Goodman and colleagues1found that spectrin-like molecules were ubiquitously found in non-erythroid cells leading to a great multitude of publications over the next thirty eight years. The discovery of multiple spectrin isoforms found associated with every cellular compartment, and representing 2-3% of cellular protein, has brought us to today’s understanding that spectrin is a scaffolding protein, with its own E2/E3 chimeric ubiquitin conjugating ligating activity that is involved in virtually every cellular function. We cover the history, localized functions of spectrin isoforms, human diseases caused by mutations, and provide the spectrinome: a useful tool for understanding the myriad of functions for one of the most important proteins in all eukaryotic cells.

Spectrin and its interacting partners in nuclear structure and function

Nonerythroid αII-spectrin is a structural protein whose roles in the nucleus have just begun to be explored. αII-spectrin is an important component of the nucleoskelelton and has both structural and non-structural functions. Its best known role is in repair of DNA ICLs both in genomic and telomeric DNA. αII-spectrin aids in the recruitment of repair proteins to sites of damage and a proposed mechanism of action is presented. It interacts with a number of different groups of proteins in the nucleus, indicating it has roles in additional cellular functions. αII-spectrin, in its structural role, associates/co-purifies with proteins important in maintaining the architecture and mechanical properties of the nucleus such as lamin, emerin, actin, protein 4.1, nuclear myosin, and SUN proteins. It is important for the resilience and elasticity of the nucleus. Thus, αII-spectrin's role in cellular functions is complex due to its structural as well as non-structural roles and understanding the consequences of a loss or deficiency of αII-spectrin in the nucleus is a significant challenge. In the bone marrow failure disorder, Fanconi anemia, there is a deficiency in αII-spectrin and, among other characteristics, there is defective DNA repair, chromosome instability, and congenital abnormalities. One may speculate that a deficiency in αII-spectrin plays an important role not only in the DNA repair defect but also in the congenital anomalies observed in Fanconi anemia , particularly since αII-spectrin has been shown to be important in embryonic development in a mouse model. The dual roles of αII-spectrin in the nucleus in both structural and non-structural functions make this an extremely important protein which needs to be investigated further. Such investigations should help unravel the complexities of αII-spectrin's interactions with other nuclear proteins and enhance our understanding of the pathogenesis of disorders, such as Fanconi anemia , in which there is a deficiency in αII-spectrin. Impact statement The nucleoskeleton is critical for maintaining the architecture and functional integrity of the nucleus. Nonerythroid α-spectrin (αIISp) is an essential nucleoskeletal protein; however, its interactions with other structural and non-structural nuclear proteins and its functional importance in the nucleus have only begun to be explored. This review addresses these issues. It describes αIISp's association with DNA repair proteins and at least one proposed mechanism of action for its role in DNA repair. Specific interactions of αIISp with other nucleoskeletal proteins as well as its important role in the biomechanical properties of the nucleus are reviewed. The consequences of loss of αIISp, in disorders such as Fanconi anemia, are examined, providing insights into the profound impact of this loss on critical processes known to be abnormal in FA, such as development, carcinogenesis, cancer progression and cellular functions dependent upon αIISp's interactions with other nucleoskeletal proteins.

Nuclear alpha spectrin: Critical roles in DNA interstrand cross-link repair and genomic stability

Non-erythroid alpha spectrin (αIISp) is a structural protein which we have shown is present in the nucleus of human cells. It interacts with a number of nuclear proteins such as actin, lamin, emerin, chromatin remodeling factors, and DNA repair proteins. αIISp's interaction with DNA repair proteins has been extensively studied. We have demonstrated that nuclear αIISp is critical in DNA interstrand cross-link (ICL) repair in S phase, in both genomic (non-telomeric) and telomeric DNA, and in maintenance of genomic stability following ICL damage to DNA. We have proposed that αIISp acts as a scaffold aiding to recruit repair proteins to sites of damage. This involvement of αIISp in ICL repair and telomere maintenance after ICL damage represents new and critical functions for αIISp. These studies have led to development of a model for the role of αIISp in DNA ICL repair. They have been aided by examination of cells from patients with Fanconi anemia (FA), a repair-deficient genetic disorder in which a deficiency in αIISp leads to defective ICL repair in genomic and telomeric DNA, telomere dysfunction, and chromosome instability following DNA ICL damage. We have shown that loss of αIISp in FA cells is due to increased breakdown by the protease, µ-calpain. Importantly, we have demonstrated that this deficiency can be corrected by knockdown of µ-calpain and restoring αIISp levels to normal. This corrects a number of the phenotypic deficiencies in FA after ICL damage. These studies suggest a new and unexplored direction for therapeutically restoring genomic stability in FA cells and for correcting numerous phenotypic deficiencies occurring after ICL damage. Developing a more in-depth understanding of the importance of the interaction of αIISp with other nuclear proteins could significantly enhance our knowledge of the consequences of loss of αIISp on critical nuclear processes.

Functional Significance of Nuclear α Spectrin

Nonerythroid alpha spectrin (αIISp) interacts in the nucleus with an array of different proteins indicating its involvement in a number of diverse functions. However, the significance of these interactions and their functional importance has been a relatively unexplored area. The best documented role of nuclear αIISp is in DNA repair where it is critical for repair of DNA interstrand cross-links (ICLs), acting as a scaffold recruiting proteins to sites of damage in genomic and telomeric DNA. A deficiency in αIISp can importantly impact DNA ICL repair as is seen in cells from patients with the genetic disorder, Fanconi anemia (FA), where loss of αIISp leads to not only defects in repair of both genomic and telomeric DNA but also to telomere dysfunction and chromosome instability. This previously unexplored link between αIISp and telomere function is important in developing an understanding of maintenance of genomic stability after ICL damage. In FA cells, these defects in chromosome instability after ICL damage can be corrected when levels of αIISp are returned to normal by knocking down μ-calpain, a protease which cleaves αIISp. These studies suggest a new direction for correcting a number of the phenotypic defects in FA and could serve as a basis for therapeutic intervention. More in depth, examination of the interactions of αIISp with other proteins in the nucleus is of major importance in development of insights into the interacting key elements involved in the diverse processes occurring in the nucleus and the consequences loss of αIISp has on them.

Reactive protoplasmic and fibrous astrocytes contain high levels of calpain-cleaved alpha 2 spectrin

Calpain, a family of calcium-dependent neutral proteases, plays important roles in neurophysiology and pathology through the proteolytic modification of cytoskeletal proteins, receptors and kinases. Alpha 2 spectrin (αII spectrin) is a major substrate for this protease family, and the presence of the αII spectrin breakdown product (αΙΙ spectrin BDP) in a cell is evidence of calpain activity triggered by enhanced intracytoplasmic Ca(2+) concentrations. Astrocytes, the most dynamic CNS cells, respond to micro-environmental changes or noxious stimuli by elevating intracytoplasmic Ca(2+) concentration to become activated. As one measure of whether calpains are involved with reactive glial transformation, we examined paraffin sections of the human cerebral cortex and white matter by immunohistochemistry with an antibody specific for the calpain-mediated αΙΙ spectrin BDP. We also performed conventional double immunohistochemistry as well as immunofluorescent studies utilizing antibodies against αΙΙ spectrin BDP as well as glial fibrillary acidic protein (GFAP). We found strong immunopositivity in selected protoplasmic and fibrous astrocytes, and in transitional forms that raise the possibility of some of fibrous astrocytes emerging from protoplasmic astrocytes. Immunoreactive astrocytes were numerous in brain sections from cases with severe cardiac and/or respiratory diseases in the current study as opposed to our previous study of cases without significant clinical conditions that failed to reveal such remarkable immunohistochemical alterations. Our study suggests that astrocytes become αΙΙ spectrin BDP immunopositive in various stages of activation, and that spectrin cleavage product persists even in fully reactive astrocytes. Immunohistochemistry for αΙΙ spectrin BDP thus marks reactive astrocytes, and highlights the likelihood that calpains and their proteolytic processing of spectrin participate in the morphologic and physiologic transition from resting protoplasmic astrocytes to reactive fibrous astrocytes.

Calmodulin and CaMKII modulate ENaC activity by regulating the association of MARCKS and the cytoskeleton with the apical membrane

Phosphatidylinositol bisphosphate (PIP2) regulates epithelial sodium channel (ENaC) open probability. In turn, myristoylated alanine-rich C kinase substrate (MARCKS) protein or MARCKS-like protein 1 (MLP-1) at the plasma membrane regulates the delivery of PIP2 to ENaC. MARCKS and MLP-1 are regulated by changes in cytosolic calcium; increasing calcium promotes dissociation of MARCKS from the membrane, but the calcium-regulatory mechanisms are unclear. However, it is known that increased intracellular calcium can activate calmodulin and we show that inhibition of calmodulin with calmidazolium increases ENaC activity presumably by regulating MARCKS and MLP-1. Activated calmodulin can regulate MARCKS and MLP-1 in two ways. Calmodulin can bind to the effector domain of MARCKS or MLP-1, inactivating both proteins by causing their dissociation from the membrane. Mutations in MARCKS that prevent calmodulin association prevent dissociation of MARCKS from the membrane. Calmodulin also activates CaM kinase II (CaMKII). An inhibitor of CaMKII (KN93) increases ENaC activity, MARCKS association with ENaC, and promotes MARCKS movement to a membrane fraction. CaMKII phosphorylates filamin. Filamin is an essential component of the cytoskeleton and promotes association of ENaC, MARCKS, and MLP-1. Disruption of the cytoskeleton with cytochalasin E reduces ENaC activity. CaMKII phosphorylation of filamin disrupts the cytoskeleton and the association of MARCKS, MLP-1, and ENaC, thereby reducing ENaC open probability. Taken together, these findings suggest calmodulin and CaMKII modulate ENaC activity by destabilizing the association between the actin cytoskeleton, ENaC, and MARCKS, or MLP-1 at the apical membrane.

Knockdown of mu-calpain in Fanconi anemia, FA-A, cells by siRNA restores alphaII spectrin levels and corrects chromosomal instability and defective DNA interstrand cross-link repair

We have previously shown that there is a deficiency in the structural protein, nonerythroid alpha spectrin (alphaIISp), in cells from patients with Fanconi anemia (FA). These studies indicate that this deficiency is due to the reduced stability of alphaIISp and correlates with a decreased level of repair of DNA interstrand cross-links and chromosomal instability in FA cells. An important factor in the stability of alphaIISp is its susceptibility to cleavage by the protease, mu-calpain. We hypothesized that an increased level of mu-calpain cleavage of alphaIISp in FA cells leads to an increased level of breakdown of alphaIISp and that knocking down expression of mu-calpain in FA cells should restore levels of alphaIISp and correct a number of the phenotypic defects observed. The results showed that there is increased mu-calpain activity in FA-A, FA-C, FA-D2, FA-F, and FA-G cells that could account for the deficiency in alphaIISp in these FA cells. Protein interaction studies indicated that FANCA and FANCG bind directly to mu-calpain. We hypothesize that this binding may lead to inhibition of mu-calpain activity in normal cells. Knocking down mu-calpain by siRNA in FA-A cells restored levels of alphaIISp to normal and reversed a number of the cellular deficiencies in these cells. It corrected the DNA repair defect and the chromosomal instability observed after exposure to a DNA interstrand cross-linking agent. These studies indicate that FA proteins may play an important role in maintaining the stability of alphaIISp in the cell by regulating its cleavage by mu-calpain. Thus, by reducing the level of breakdown of alphaIISp in FA cells, we may be able to reverse a number of the cellular deficiencies observed in this disorder.

Protein 4.2 binds to the carboxyl-terminal EF-hands of erythroid alpha-spectrin in a calcium- and calmodulin-dependent manner

Spectrin and protein 4.1 cross-link F-actin protofilaments into a network called the membrane skeleton. Actin and 4.1 bind to one end of beta-spectrin. The adjacent end of alpha-spectrin, called the EF-domain, is calmodulin-like, with calcium-dependent and calcium-independent EF-hands. It has no known function. However, the sph(1J)/sph(1J) mouse has very fragile red cells and lacks the last 13 amino acids in the EF-domain, suggesting the domain is critical for skeletal integrity. Using pulldown binding assays, we find the alpha-spectrin EF-domain either alone or incorporated into a mini-spectrin binds native and recombinant protein 4.2 at a previously identified region of 4.2 (G(3) peptide). Native 4.2 binds with an affinity comparable with other membrane skeletal interactions (K(d) = 0.30 microM). EF-domains bearing the sph(1J) mutation are inactive. Binding of protein 4.2 to band 3 (K(d) = 0.45 microM) does not interfere with the spectrin-4.2 interaction. Spectrin-4.2 binding is amplified by micromolar concentrations of Ca(2+) (but not Mg(2+)) by three to five times. Calmodulin also binds to the EF-domain (K(d) = 17 microM), and Ca(2+)-calmodulin blocks Ca(2+)-dependent binding of protein 4.2 but not Ca(2+)-independent binding. The data suggest that protein 4.2 is located near protein 4.1 at the spectrin-actin junctions. Because proteins 4.1 and 4.2 also bind to band 3, the erythrocyte anion channel, we suggest that one or both of these proteins cause a portion of band 3 to localize near the spectrin-actin junctions and provide another point of attachment between the membrane skeleton and the lipid bilayer.

The SH3 domain of alphaII spectrin is a target for the Fanconi anemia protein, FANCG

The structural protein nonerythroid alpha spectrin (alphaIISp) plays a role in the repair of DNA interstrand cross-links and is deficient in cells from patients with Fanconi anemia (FA), in which there is a defect in ability to repair such cross-links. We have proposed a model in which alphaIISp, whose stability is dependent on FA proteins, acts as a scaffold to aid in recruitment of repair proteins to sites of damage. In order to get a clearer understanding of the proposed role of FA proteins in maintaining stability of alphaIISp, yeast two-hybrid analysis was carried out to determine whether FA proteins directly interact with alphaIISp and, if so, to map the sites of interaction. Four overlapping regions of alphaIISp were constructed. FANCG interacted with one of these regions and specifically with the SH3 domain in this region of alphaIISp. The site of interaction in FANCG was mapped to a motif that binds to SH3 domains and contains a consensus sequence with preference for the SH3 domain of alphaIISp. This site of interaction was confirmed using site-directed mutagenesis. Two FA proteins that did not contain motifs that bind to SH3 domains, FANCC and FANCF, did not interact with the SH3 domain of alphaIISp. These results demonstrate that one of the FA proteins, FANCG, contains a motif that interacts directly with the SH3 domain of alphaIISp. We propose that this binding of FANCG to alphaIISp may be important for the stability of alphaIISp in cells and the role alphaIISp plays in the DNA repair process.

Prostaglandin-induced VASP phosphorylation controls alpha II-spectrin breakdown in apoptotic cells

In pathological conditions, the inflammatory mediator prostaglandin E2 (PGE2) has been shown to induce apoptosis through a cAMP-dependent pathway. However, underlying mechanisms have remained illusive. Irrespective whether apoptosis is induced by the intrinsic or extrinsic pathway, the cysteine protease caspase-3 becomes activated and cleaves many key proteins including spectrins. Cleavage of the plasma membrane-associated spectrins leads to cell shrinkage, membrane blebbing, the formation of apoptotic bodies, and irreversible cell death. Recently, we identified a novel interaction between alpha II-spectrin and vasodilator-stimulated phosphoprotein (VASP), which is abrogated by the cAMP-dependent protein kinase (PKA)-mediated phosphorylation of VASP. In the present study we investigated whether VASP binding to alpha II-spectrin affects spectrin breakdown in PGE2-induced apoptosis. PGE2 dose- and time-dependently triggered VASP phosphorylation. Following induction of apoptosis, caspase-3-mediated alpha II-spectrin breakdown and membrane blebbing were markedly delayed in wild-type as compared to VASP-deficient endothelial cells. This suggests that VASP binding to alpha II-spectrin attenuates alpha II-spectrin cleavage in apoptotic cells and that PGE2-induced VASP phosphorylation regulates this process. Our findings may therefore provide the molecular basis for PGE2-induced apoptosis in pathological events.

Calmodulin-binding proteins in the model organism Dictyostelium: a complete & critical review

Calmodulin is an essential protein in the model organism Dictyostelium discoideum. As in other organisms, this small, calcium-regulated protein mediates a diversity of cellular events including chemotaxis, spore germination, and fertilization. Calmodulin works in a calcium-dependent or -independent manner by binding to and regulating the activity of target proteins called calmodulin-binding proteins. Profiling suggests that Dictyostelium has 60 or more calmodulin-binding proteins with specific subcellular localizations. In spite of the central importance of calmodulin, the study of these target proteins is still in its infancy. Here we critically review the history and state of the art of research into all of the identified and presumptive calmodulin-binding proteins of Dictyostelium detailing what is known about each one with suggestions for future research. Two individual calmodulin-binding proteins, the classic enzyme calcineurin A (CNA; protein phosphatase 2B) and the nuclear protein nucleomorphin (NumA), which is a regulator of nuclear number, have been particularly well studied. Research on the role of calmodulin in the function and regulation of the various myosins of Dictyostelium, especially during motility and chemotaxis, suggests that this is an area in which future active study would be particularly valuable. A general, hypothetical model for the role of calmodulin in myosin regulation is proposed.

A mutant alphaII-spectrin designed to resist calpain and caspase cleavage questions the functional importance of this process in vivo

alpha- and beta-spectrins are components of molecular scaffolds located under the lipid bilayer and named membrane skeletons. Disruption of these scaffolds through mutations in spectrins demonstrated that they are involved in the membrane localization or the maintenance of proteins associated with them. The ubiquitous alphaII-spectrin chain bears in its central region a unique domain that is sensitive to several proteases such as calpains or caspases. The conservation of this region in vertebrates suggests that the proteolysis of alphaII-spectrin by these enzymes could be involved in important functions. To assess the role of alphaII-spectrin cleavage in vivo, we generated a murine model in which the exons encoding the region defining this cleavage sensitivity were disrupted by gene targeting. Surprisingly, homozygous mice expressing this mutant alphaII-spectrin appeared healthy, bred normally, and had no histological anomaly. Remarkably, the mutant alphaII-spectrin assembles correctly into the membrane skeleton, thus challenging the notion that this region is required for the stable biogenesis of the membrane skeleton in nonerythroid cells. Our finding also argues against a critical role of this particular alphaII-spectrin cleavage in either major cellular functions or in normal development.

Hypoxia-Induced Cytoskeleton Disruption in Alveolar Epithelial Cells

Alveolar hypoxia, a common feature of many respiratory disorders, has been previously reported to induce functional changes, particularly a decrease of transepithelial Na and fluid transport. In polarized epithelia, cytoskeleton plays a regulatory role in transcellular and paracellular transport of ions and fluid. We hypothesized that exposure to hypoxia could damage cytoskeleton organization, which in turn, may adversely affect ion and fluid transport. Primary rat alveolar epithelial cells (AEC) were exposed to either mild (3% O(2)) or severe (0.5% O(2)) hypoxia for 18 h or to normoxia (21% O(2)). First, mild and severe hypoxia induced a disorganization of actin, a major protein of the cytoskeleton, reflected by disruption of F-actin filaments. Second, alpha-spectrin, an apical cytoskeleton protein, which binds to actin cytoskeleton and Na transport proteins, was cleaved by hypoxia. Pretreatment of AEC by a caspase inhibitor (z-VAD-fmk; 90 microM) blunted hypoxia-induced spectrin cleavage as well as hypoxia-induced decrease in surface membrane alpha-ENaC and concomitantly induced a partial recovery of hypoxia-induced decrease of amiloride-sensitive Na transport at 3% O(2). Finally, tight junctions (TJs) proteins, which are linked to actin and are a determinant of paracellular permeability, were altered by mild and severe hypoxia: hypoxia induced a mislocalization of occludin from the TJ to cytoplasm and a decrease in zonula occludens-1 protein level. These modifications were associated with modest changes in paracellular permeability at 0.5% O(2,) as assessed by small 4-kD dextran flux and transepithelial resistance measurements. Together, these findings indicate that hypoxia disrupted cytoskeleton and TJ organization in AEC and may participate, at least in part, to hypoxia-induced decrease in Na transport.

Structure of the calmodulin alphaII-spectrin complex provides insight into the regulation of cell plasticity

AlphaII-spectrin is a major cortical cytoskeletal protein contributing to membrane organization and integrity. The Ca2+-activated binding of calmodulin to an unstructured insert in the 11th repeat unit of alphaII-spectrin enhances the susceptibility of spectrin to calpain cleavage but abolishes its sensitivity to several caspases and to at least one bacterially derived pathologic protease. Other regulatory inputs including phosphorylation by c-Src also modulate the proteolytic susceptibility of alphaII-spectrin. These pathways, acting through spectrin, appear to control membrane plasticity and integrity in several cell types. To provide a structural basis for understanding these crucial biological events, we have solved the crystal structure of a complex between bovine calmodulin and the calmodulin-binding domain of human alphaII-spectrin (Protein Data Bank ID code 2FOT). The structure revealed that the entire calmodulin-spectrin-binding interface is hydrophobic in nature. The spectrin domain is also unique in folding into an amphiphilic helix once positioned within the calmodulin-binding groove. The structure of this complex provides insight into the mechanisms by which calmodulin, calpain, caspase, and tyrosine phosphorylation act on spectrin to regulate essential cellular processes.

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

In this study, we used cultured cells spreading on beta3 integrin substrates to examine the possibility that spectrin is involved in signal transduction. Spectrin clustered with specialized calpain-induced beta3 integrin signaling complexes that mediate the initial attachment of cells and initiate Rac activation and lamellipodia extension. It was absent from focal complexes and focal adhesions, the integrin complexes that mediate adhesion in lamellipodia and fully spread cells. Spectrin contains a Src homology (SH3) domain of unknown function. Cells overexpressing this domain adhered and calpain-induced integrin signaling complexes formed. However, Rac activation, lamellipodia extension and cell spreading were inhibited. Spreading was restored by overexpression of constitutively active Rac. These studies point to a previously unrecognized role for spectrin and its SH3 domain in initiating Rac activation in the specialized integrin clusters that initiate cell adhesion and spreading. Thus, spectrin may have a pivotal role in initiating integrin-induced physiological and pathological events such as development, proliferation, cell survival, wound healing, metastasis and atherosclerosis.

Fodrin CaM-binding domain cleavage by Pet from enteroaggregative Escherichia coli leads to actin cytoskeletal disruption

We have previously shown that the plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli produces cytotoxic and enterotoxic effects. Pet-intoxicated epithelial cells reveal contraction of the cytoskeleton and loss of actin stress fibres. Pet effects require its internalization into epithelial cells. We have also shown that Pet degrades erythroid spectrin. Pet delivery within the intestine suggests that Pet may degrade epithelial fodrin (non-erythroid spectrin). Here we demonstrate that Pet has affinity for alpha-fodrin (formally named alphaII spectrin) in vitro and in vivo and cleaves epithelial fodrin, causing its redistribution within the cells. When Pet has produced its cytoskeletal effects, fodrin is found in intracellular aggregates as membrane blebs. Pet cleaves recombinant GST-fodrin, generating two breakdown products of 37 and 72 kDa. Sequencing of the 37 kDa fragment demonstrated that the cleavage site occurred within fodrin's 11th repetitive unit between M1198 and V1199, in the calmodulin binding domain. Site-directed mutagenesis of these amino acids prevented fodrin degradation by Pet. Pet also cleaves epithelial fodrin from cultured Pet-treated cells. A mutant in the Pet serine protease motif was unable to cause fodrin redistribution or to cleave GST-fodrin. This is the first report showing cleavage of alpha-fodrin by a bacterial protease. Cleavage occurs in the middle of the calmodulin binding domain, which leads to cytoskeleton disruption.

c-Src binds alpha II spectrin's Src homology 3 (SH3) domain and blocks calpain susceptibility by phosphorylating Tyr1176

Spectrin is a ubiquitous heterodimeric scaffolding protein that stabilizes membranes and organizes protein and lipid microdomains on both the plasma membrane and intracellular organelles. Phosphorylation of beta-spectrin on Ser/Thr is well recognized. Less clear is whether alpha-spectrin is phosphorylated in vivo and whether spectrin is phosphorylated on tyrosine (pTyr). We affirmatively answer both questions. In cultured Madin-Darby canine kidney cells, alphaII spectrin undergoes in vivo tyrosine phosphorylation. Enhancement of the steady state level of pTyr-modified alphaII spectrin by vanadate, a phosphatase inhibitor, implies a dynamic balance between alphaII spectrin phosphorylation and dephosphorylation. Recombinant peptides containing the Src homology 3 domain of alphaII spectrin (but not the Src homology 3 domain of alphaI spectrin) bind specifically to phosphorylated c-Src in Madin-Darby canine kidney cell lysates, suggesting that this kinase is responsible for its in vivo phosphorylation. pTyr-modified alphaII spectrin is resistant to maitotoxin-induced cleavage by mu-calpain in vivo. In vitro studies of recombinant alphaII spectrin peptides representing repeats 9-12 identify two sites of pTyr modification. The first site is at Tyr(1073), a residue immediately adjacent to a region encoded by alternative exon usage (insert 1). The second site is at Tyr(1176). This residue flanks the major site of cleavage by the calcium-dependent protease calpain, and phosphorylation of Tyr(1176) by c-Src reduces the susceptibility of alphaII spectrin to cleavage by mu-calpain. Calpain cleavage of spectrin, activated by Ca(2+) and calmodulin, contributes to diverse cellular processes including synaptic remodeling, receptor-mediated endocytosis, apoptosis, and the response of the renal epithelial cell to ischemic injury. Tyrosine phosphorylation of alphaII spectrin now would appear to also mediate these events. The spectrin skeleton thus forms a point of convergence between kinase/phosphatase and Ca(2+)-mediated signaling cascades.

Participation of the conventional calpains in apoptosis

The conventional calpains, m- and micro-calpain, are suggested to be involved in apoptosis triggered by many different mechanisms. However, it has not been possible to definitively associate calpain function with apoptosis, largely because of the incomplete selectivity of the cell permeable calpain inhibitors used in previous studies. In the present study, Chinese hamster ovary (CHO) cell lines overexpressing micro-calpain or the highly specific calpain inhibitor protein, calpastatin, have been utilized to explore apoptosis signals that are influenced by calpain content. This approach allows unambiguous alteration of calpain activity in cells. Serum depletion, treatment with the endoplasmic reticulum (ER) calcium ATPase inhibitor thapsigargin, and treatment with calcium ionophore A23187 produced apoptosis in CHO cells, which was increased in calpain overexpressing cells and decreased by induced expression of calpastatin. Inhibition of calpain activity protected beta-spectrin, but not alpha-spectrin, from proteolysis. The calpains seemed not to be involved in apoptosis triggered by a number of other treatments. Calpain protected against TNF-alpha induced apoptosis. In contrast to previous studies, we found no evidence that calpains proteolyze I kappa B-alpha in TNF-alpha-stimulated cells. These studies indicate that the conventional calpains participate in some, but not all, apoptotic signaling mechanisms. In most cases, they contributed to apoptosis, but in at least one case, they were protective.

Evidence for the Involvement of annexin 6 in the trafficking between the endocytic compartment and lysosomes

Annexins are a family of calcium-dependent phospholipid-binding proteins, which have been implicated in a variety of biological processes including membrane trafficking. The annexin 6/lgp120 prelysosomal compartment of NRK cells was loaded with low-density lipoprotein (LDL) and then its transport from this endocytic compartment and its degradation in lysosomes were studied. NRK cells were microinjected with the mutated annexin 6 (anx6(1-175)), to assess the possible involvement of annexin 6 in the transport of LDL from the prelysosomal compartment. The results indicated that microinjection of mutated annexin 6, in NRK cells, showed the accumulation of LDL in larger endocytic structures, denoting retention of LDL in the prelysosomal compartment. To confirm the involvement of annexin 6 in the trafficking and the degradation of LDL we used CHO cells transfected with mutated annexin 6(1-175). Thus, in agreement with NRK cells the results obtained in CHO cells demonstrated a significant inhibition of LDL degradation in CHO cells expressing the mutated form of annexin 6 compared to controls overexpressing wild-type annexin 6. Therefore, we conclude that annexin 6 is involved in the trafficking events leading to LDL degradation.

Spectrin oligomerization is cooperatively coupled to membrane assembly: a linkage targeted by many hereditary hemolytic anemias?

In the erythrocyte, ankyrin is the major adapter protein linking tetramers of band 3 to the spectrin-actin cytoskeleton. This linkage involves a direct interaction between ankyrin and the 14th-15th repeat unit of beta-spectrin. The spectrin cytoskeleton itself is stabilized by the self-association of spectrin heterodimers into tetramers and larger oligomers, a process mediated by the 17th repeat unit of beta-spectrin and a short NH(2) -terminal sequence in alpha-spectrin. The self-association of spectrin and its ankyrin-mediated membrane binding have generally been considered independent events. We now demonstrate that spectrin self-association, the binding of spectrin to ankyrin, and the binding of ankyrin to the 43-kDa cytoplasmic domain of band 3 (cdb3) are coupled in a positively cooperative way. In solution, [(125)I]-labeled ankyrin was found by ND-PAGE3 to enhance the affinity of spectrin self-association by 10-fold. The reciprocal process was also true, in that spectrin tetramers and oligomers bound ankyrin with enhanced affinity relative to dimer spectrin. Saturation of the beta-spectrin self-association site by an NH(2) -terminal 80-kDa alpha-spectrin peptide enhanced the affinity of spectrin dimer for ankyrin, indicating a direct relationship between ankyrin binding and the occupancy of the beta-spectrin self-association site. cdb3 accentuated these cooperative interactions. Several inherited spectrin mutations that cause hemolytic disease but that do not directly destabilize the self-association or ankyrin-binding sites can be explained by these results. Three classes of mutations appear to disrupt cooperative coupling between self-association and ankyrin binding: (i) mutation of the linker sequences that join helices C and A in repeat units that intervene between the two functional sites, mutations that presumably block repeat-to-repeat transfer of conformational information; (ii) mutations in alpha-spectrin repeats 4 to 6 that disrupt the ability of this region to trans-regulate ankyrin binding by the adjacent beta-spectrin repeats 14-15; and (iii) exon-skipping mutations that shorten alpha-spectrin and force repeats 4 to 6 to fall out-of-register with the ankyrin-binding motif in beta-spectrin. Collectively, these results demonstrate a molecular mechanism whereby a membrane receptor can directly promote cytoskeletal assembly.

Membrane skeleton of innervated and denervated fast- and slow-twitch muscle

We used confocal microscopy and immunoblotting to study membrane skeletal proteins of fast-twitch (extensor digitorum longus) and slow-twitch (soleus) muscles of the adult rat. In the extensor digitorum longus (EDL), beta-spectrin concentrates in costameres, whereas dystrophin is enriched at costameres but is also present in intercostameric regions. In the soleus, beta-spectrin and dystrophin underlie much of the sarcolemma, and intercostameric regions are difficult to detect. The EDL sarcolemma reorganizes following denervation to resemble soleus sarcolemma, but denervation does not significantly affect the latter. Consistent with these observations, soleus contains similar amounts of dystrophin but more beta-spectrin than EDL. Denervation increases beta-spectrin levels only in the EDL and dystrophin levels in both muscles. Denervation does not affect beta-fodrin, a beta-spectrin homolog expressed in embryonic myofibers. Thus, neuromuscular activity controls sarcolemmal organization and the levels of beta-spectrin and dystrophin, but not postnatal downregulation of beta-fodrin. The differences in organization of the sarcolemma may underlie the differential susceptibility of fast and slow myofibers to dystrophinopathies.

Synthesis, assembly, and turnover of alpha and beta-erythroid and nonerythroid spectrins in rat hippocampal neurons

The synthesis and turnover of alpha-erythroid, beta-erythroid, alpha-nonerythroid and beta-nonerythroid spectrins was investigated in cultured rat hippocampal neurons. [35S]methionine and subunit specific antibodies were used to label and immunoprecipitate newly synthesized spectrins in 12- to 14-day-old cultures. Synthesis experiments, performed under normal resting conditions, showed that the ratio of newly synthesized alpha-erythroid/beta-erythroid and alpha-nonerythroid/beta-nonerythroid spectrins is 1/1 (mol/mol) both in the soluble and insoluble fractions. Soluble and insoluble alpha and beta erythroid spectrin turn over rapidly (half-life=16-24 min). Soluble nonerythroid alpha-spectrin (half-life=80 min) and beta spectrin (half-life=53 min) turn over more slowly than their insoluble counterparts (30-34 min). The nonerythroid alpha spectrin turnover was significantly different (p<0.05) from the other measurements except for nonerythroid beta spectrin, indicating that these subunits are protected from rapid proteolytic degradation until they are assembled in the membrane skeleton.

Moderate posttraumatic hypothermia decreases early calpain-mediated proteolysis and concomitant cytoskeletal compromise in traumatic axonal injury

Traumatic brain injury (TBI) in animals and man generates widespread axonal injury characterized by focal axolemmal permeability changes, induction of calpain-mediated proteolysis, and neurofilament side-arm modification associated with neurofilament compaction (NFC) evolving to axonal disconnection. Recent observations have suggested that moderate hypothermia is neuroprotective in several models of TBI. Nevertheless, the pathway by which hypothermia prevents traumatic axonal injury (TAI) is still a matter of debate. The present study was conducted to evaluate the effects of moderate, early posttraumatic hypothermia on calpain-mediated spectrin proteolysis (CMSP), implicated in the pathogenesis of TAI. Using moderate (32 degrees C) hypothermia of 90 min duration without rewarming, the density of CMSP immunoreactive/damaged axons was quantified via LM analysis in vulnerable brain stem fiber tracts of hypothermic and normothermic rats subjected to impact acceleration TBI (90 min postinjury survival). To assess the influence of posthypothermic rewarming, a second group of animals was subjected to 90 min of hypothermia followed by 90 min of rewarming to normothermic levels when CMSP was analyzed to detect if any purported CMSP prevention persisted (180 min postinjury survival). Additionally, to determine if this protection translated into comparable cytoskeletal protection in the same foci showing decreased CMSP, antibodies targeting altered/compacted NF subunits were also employed. Moderate hypothermia applied in the acute postinjury period drastically reduced the number of damaged axons displaying CMSP at both time points and significantly reduced NFC immunoreactivity at 180 min postinjury. These results suggest that the neuroprotective effects of hypothermia in TBI are associated with the inhibition of axonal/cytoskeletal damage.

Calpain mediates integrin-induced signaling at a point upstream of Rho family members

Integrin-induced adhesion leads to cytoskeletal reorganizations, cell migration, spreading, proliferation, and differentiation. The details of the signaling events that induce these changes in cell behavior are not well understood but they appear to involve activation of Rho family members which activate signaling molecules such as tyrosine kinases, serine/threonine kinases, and lipid kinases. The result is the formation of focal complexes, focal adhesions, and bundles and networks of actin filaments that allow the cell to spread. The present study shows that mu-calpain is active in adherent cells, that it cleaves proteins known to be present in focal complexes and focal adhesions, and that overexpression of mu-calpain increased the cleavage of these proteins, induced an overspread morphology and induced an increased number of stress fibers and focal adhesions. Inhibition of calpain with membrane permeable inhibitors or by expression of a dominant negative form of mu-calpain resulted in an inability of cells to spread or to form focal adhesions, actin filament networks, or stress fibers. Cells expressing constitutively active Rac1 could still form focal complexes and actin filament networks (but not focal adhesions or stress fibers) in the presence of calpain inhibitors; cells expressing constitutively active RhoA could form focal adhesions and stress fibers. Taken together, these data indicate that calpain plays an important role in regulating the formation of focal adhesions and Rac- and Rho-induced cytoskeletal reorganizations and that it does so by acting at sites upstream of both Rac1 and RhoA.

Calpain-induced proteolysis of beta-spectrins

The calcium-activated neutral protease calpain is activated in several pathological conditions. Calpain usually hydrolyses one or only a few peptide bonds in its substrate. One prominent substrate for calpain is spectrin and it has been shown that alpha-spectrin is the preferred substrate. We now show that the beta-chain of spectrin is also a substrate for calpain proteolysis, and that the cleavage site in each beta-subunit is located at the very C-terminal part of the molecule. Surprisingly, beta1sigma-spectrin is cleaved at a different site than betaIsigma2- and betaIIsigma1-spectrins despite their high degree of sequence identity.

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.

Calcium/calmodulin-dependent protein kinase IV is cleaved by caspase-3 and calpain in SH-SY5Y human neuroblastoma cells undergoing apoptosis

We have previously demonstrated cleavage of alpha-spectrin by caspase-3 and calpain during apoptosis in SH-SY5Y neuroblastoma cells (Nath, R., Raser, K. J., Stafford, D., Hajimohammadreza, I., Posner, A., Allen, H., Talanian, R. V., Yuen, P., Gilbertsen, R. B., and Wang, K. K. (1996) Biochem. J. 319, 683-690). We demonstrate here that calcium/calmodulin-dependent protein kinase IV (CaMK IV) is cleaved during apoptosis by caspase-3 and calpain. We challenged SH-SY5Y cells with the pro-apoptotic agent thapsigargin. Western blot analysis revealed major CaMK IV breakdown products of 40, 38, and 33 kDa. Digestion of control SH-SY5Y lysate with purified caspase-3 produced a 38-kDa CaMK IV fragment; digestion with purified calpain produced a major fragment of 40 kDa. Pretreatment with carbobenzoxy-Asp-CH2OC(O)-2,6-dichlorobenzene or Z-Val-Ala-Asp-fluoromethylketone was able to block the caspase-3-mediated production of the 38-kDa fragment both in situ and in vitro. Calpain inhibitor II similarly blocked formation of the calpain-mediated 40-kDa fragment both in situ and in vitro. Digestion of recombinant CaMK IV by other caspase family members revealed that only caspase-3 produces a fragmentation pattern consistent to that seen in situ. The major caspase-3 and calpain cleavage sites are respectively identified as PAPD176*A and CG201*A, both within the CaMK IV catalytic domain. Furthermore, calmodulin-stimulated protein kinase activity decreases within 6 h in thapsigargin-treated SH-SY5Y. The loss of activity precedes cell death.

Inhibition of phospholipase D activity by fodrin. An active role for the cytoskeleton

Phospholipase D (PLD) is a major enzyme implicated in important cellular processes such as secretion and proliferation. The knowledge of its regulation is essential to understand the control of these phenomena. Several proteins activating PLD have been described in the last years. In this report, we chromatographed bovine brain cytosolic proteins to identify fodrin, the non-erythroid spectrin, as the first described inhibitor of PLD. A cytosolic fraction with an inhibitory effect on PLD activity loses its capacity after immunoprecipitation of fodrin. Moreover, at 1 nM, purified fodrin blocks fully and quickly PLD activity, whatever the stimuli used. In contrast, fodrin has no effect on adenylate cyclase activity. Fodrin-analogous proteins like dimeric or tetrameric erythroid spectrin have the same inhibitory effect on PLD, at higher concentrations. Other cytoskeletal proteins, actin and vimentin, are inefficient on PLD inhibition. The mechanisms implicated in PLD modulation such as post-translational modifications of fodrin and the role of small G-proteins on the cytoskeleton regulation are discussed. In conclusion, this study reveals that fodrin is involved in the control of PLD activity, suggesting that the cytoskeleton could have an active role in control of secretion and proliferation.

Redistribution of fodrin in an in vitro wound healing model of the corneal epithelium

We previously observed the redistribution of a membrane skeletal protein, fodrin, after wounding in the corneal epithelium in vivo. In this study, we made an in vitro wound healing model using cultured corneal epithelial cells to investigate the redistribution mechanism of fodrin in the corneal epithelial cells. The distributional change of fodrin from the plasmalemma to the cytoplasm was observed soon after wounding by indirect immunofluorescence microscopy and laser scanning confocal microscopy. A similar change was caused by treating intact cells with phorbol-12-myristate-13-acetate (PMA), but not with calcium ionophore, A23187. The redistribution occurred even in cells pretreated with 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetomethyl ester (BAPTA-AM) before wounding. The redistribution caused by wounding or by PMA was inhibited by pretreating the cells with protein kinase C inhibitors, H-7 or calphostin C. Moreover, the reagents were found to slow down the migration of corneal epithelial cells after wounding. These results suggest that the redistribution of fodrin in the wounded corneal epithelium is caused through the activation of protein kinase C and might be related to the ensuing cell migration.

Domain structure of calpain: mapping the binding site for calpastatin

The peptide EKLGERDDTIPPEYRELLEKKTGV was synthesized to mimic the central consensus sequence of calpastatin, the specific, endogenous inhibitor of the calpains (EC 3.4.22.17). The peptide competitively inhibits hydrolysis of casein by either micro- or milli-calpain but does not affect the activity of other proteases. This inhibitory peptide was preferentially cross-linked to milli-calpain in the presence of calcium using the heterobifunctional cross-linking reagent m-maleimidobenzoyl-N-hydroxysuccinimide ester. Cross-linking of the peptide was blocked by calpastatin. The site of cross-linking for the peptide within milli-calpain was localized using random chemical cleavage of the enzyme-peptide complex at cysteine residues. Calpain fragments were identified as amino-terminal fragments through reactivity with a peptide-specific antiserum or as non-amino-terminal fragments through incorporation of 14C from 14CN. Analysis of the control and cross-linked fragments, from experiments using both milli-calpain and micro-calpain, maps the chemical cross-linking site to cysteine-497 and localizes the binding site for the calpastatin-like peptide to this highly conserved region of domain III of calpains catalytic subunit.

No evidence for calpain I involvement in fodrin rearrangements linked to regulated secretion

Stimulation of secretion in chromaffin and parotid acinar cells is associated with dramatic rearrangements of the subplasmalemmal cytoskeleton, notably of fodrin and F-actin. It has been proposed that a proteolytic cleavage of fodrin resulting from an activation of the neutral calcium activated protease (calpain) could be responsible for these changes. Using an affinity-purified anti-alpha-fodrin antibody, several cleavage products of fodrin could clearly be detected following incubation of total cell homogenates from chromaffin and parotid acinar cells with purified calpain I. On the other hand, maximum stimulation of secretion of chromaffin cells by nicotine, and of parotid acinar cells by carbachol plus isoproterenol, was not associated with an increased appearance of cleavage products of fodrin. This result is not compatible with the 'proteolytic cleavage' hypothesis.

Characterization of calcium binding to spectrins

Calcium binding to brain and erythrocyte spectrins was studied at physiological ionic strength by a calcium overlay assay and aqueous two-phase partitioning. When the spectrins were immobilized on nylon membranes by slot blotting, the overlay assay showed that even though both spectrins bound 45Ca2+, the brain protein displayed much greater affinity for calcium ions than erythrocyte spectrin did. Since the observed binding was weaker than that displayed by calmodulin under similar conditions, the overlay assay results indicated that the binding must be weaker than 1 microM. The phase partition experiments showed that there are at least two sites for calcium on brain spectrin and that calcium binding to one of these sites is reduced significantly by magnesium ions. From the partition isotherm, the dissociation constants were estimated as 50 microM for the Mg(2+)-independent site and 150 microM for the Mg(2+)-dependent site. The phase partition results also showed that erythrocyte spectrin bound calcium ions at least 1 order of magnitude weaker. By examining calcium binding to slot-blotted synthetic peptides, we identified two binding sites in brain spectrin. One mapped to the second putative calcium binding site (EF-hand) in alpha-spectrin and the other to the 36 amino acid residue long insert in domain 11. In addition, a tryptic fragment derived from the C-terminal of erythrocyte alpha-spectrin, which contained the two postulated EF-hands, also bound calcium. These findings suggest that the calcium signal system may also involve direct binding of calcium to spectrin beside known calcium modulators such as calmodulin and calpain.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro digestion of dystrophin by calcium-dependent proteases, calpains I and II

Dystrophin is a cytoskeletal protein which is thought to play an important role in membrane physiology since its absence (due to gene deficiency) leads to the symptoms of Duchenne muscular dystrophy (DMD). Some disruption in the regulation of intracellular free Ca2+ levels could lead to DMD-like symptoms. In this study, calpains, which are very active calcium-dependent proteases, were examined for their capacity to hydrolyse dystrophin in vitro. The results show that calpains are able to split dystrophin and produce breakdown products of different sizes (the degree of cleavage being dependent on the incubation time with proteases). The time-course of protease degradation was examined by Western immunoblot using three polyclonal sera which were characterized as being specific to the central (residues 1173-1728) and two distal parts of the molecule ie specific to the N-terminal (residues 43-760) or the C-terminal (residues 3357-3660) extremities of the dystrophin molecule. The cleavage patterns of dystrophin showed an accumulation of some major protease-resistant fragments of high relative molecular mass (250-370 kDa). These observations demonstrate that calpains digest dystrophin very rapidly when the calcium concentration is compatible with their activation. For instance, it is clear that calpains first give rise to large dystrophin products in which the C-terminal region is lacking. These observations suggest that dystrophin antibodies specific to the central domain of the molecule should be used to detect dystrophin for diagnostic purposes and before any conclusion as to the presence or absence of dystrophin can be deduced from results obtained using immunoanalyses of muscle biopsies.

The regulatory role of calmodulin in the proteolysis of individual neurofilament proteins by calpain

The in vitro degradation of individual neurofilament proteins by calpain and the effects of calmodulin on this proteolysis were studied. Two major results are reported. First, in the presence of calcium, calmodulin binds to the 200-kD neurofilament protein, but only weakly associates with the 150-kD neurofilament protein. The 70-kD neurofilament protein shows no specific calmodulin-binding. Second, calmodulin inhibits the calpain-mediated degradation of the 200-kD neurofilament protein, but does not alter the hydrolysis of the 150-kD and 70-kD neurofilament proteins. In addition, calmodulin is able to bind to the 200-kD neurofilament protein in the presence of other neurofilament subunits, indicating that calmodulin may play a role in the regulation of the metabolism of the 200-kD neurofilament protein in vivo.

Degradation of microtubule-associated protein 2 and brain spectrin by calpain: a comparative study

The in vitro degradation of microtubule-associated protein 2 (MAP-2) and spectrin by the calcium-dependent neutral protease calpain was studied. Five major results are reported. First, MAP-2 isolated from twice-cycled microtubules (2 X MT MAP-2) was extremely sensitive to calpain-induced hydrolysis. Even at an enzyme-to-substrate ratio (wt/wt) of 1:200, 2 X MT MAP-2 was significantly degraded by calpain. Second, MAP-2 purified from the total brain heat-stable fraction (total MAP-2) was significantly more resistant to calpain-induced hydrolysis compared with 2 X MT MAP-2. Third, MAP-2a and MAP-2b were proteolyzed similarly by calpain, although some relative resistance of MAP-2b was observed. Fourth, the presence of calmodulin significantly increased the extent of calpain-induced hydrolysis of the alpha-subunit of spectrin. Fifth, the two neuronal isoforms of brain spectrin (240/235 and 240/235E, referred to as alpha/beta N and alpha/beta E, respectively) showed different sensitivities to calpain. alpha N-spectrin was significantly more sensitive to calpain-induced degradation compared to alpha E-spectrin. Among other things, these results suggest a role for the calpain-induced degradation of MAP-2, as well as spectrin, in such physiological processes as alterations in synaptic efficacy, dendritic remodeling, and in pathological processes associated with neurodegeneration.

Neuronal fodrin proteolysis occurs independently of excitatory amino acid-induced neurotoxicity

In cultured cerebellar granule cells, the total amount of fodrin alpha subunit increased 3-fold between 0 and 10 days in vitro and fodrin mRNA increased 5-fold. The exposure of cerebellar neurons to NMDA induced the accumulation of a 150 kd proteolytic fragment of fodrin. The NMDA-induced breakdown of fodrin was time-, concentration-, and Ca2(+)-dependent and was inhibited by APV, Mg2+, or the calpain I inhibitor N-acetyl-Leu-Leu-norleucinal. Kainate caused fodrin proteolysis through indirect activation of NMDA receptors. Quisqualate was ineffective. The NMDA-induced degradation of fodrin occurred under conditions that did not cause degeneration of cultured cerebellar neurons. These results show that Ca2+/calpain I-dependent proteolysis of fodrin is selectively associated with NMDA receptor activation; however, fodrin proteolysis per se does not play a causal role in NMDA-induced toxicity in cerebellar granule cells.

Spectrin: a structural mediator between diverse plasma membrane proteins and the cytoplasm

The spectrin skeleton of non-erythroid cells is likely to interact with a variety of integral membrane proteins and participate both in stable linkages as well as dynamic structures capable of rapid disassembly and assembly. The basis for diversity of roles for spectrin includes multiple, functionally distinct isoforms of spectrin, ankyrin and other associated proteins, regulation of protein interactions through phosphorylation and calcium/calmodulin, as well as differential expression of accessory proteins that determine the organization and localization of spectrin in cells. Spectrin is highly conserved from Drosophila to man and is likely to be involved in fundamental aspects of membrane structure requiring long range order and organization. Spectrin is a candidate to interact with many integral membrane proteins in roles basic to metazoan cells which must associate into tissues. Organization of cells into tissues requires loss of cell motility, formation of specialized membrane domains and assembly of cell junctions, which are all activities potentially involving spectrin. Future challenges lie in devising direct experiments to evaluate the functions of spectrin in cells and tissues.

In vitro digestion of spectrin, protein 4.1 and ankyrin by erythrocyte calcium dependent neutral protease (calpain I)

1. In whole ghosts, ankyrin, protein 4.1, protein band 3 and spectrin are lysed by purified calpain I in the presence of calcium. 2. Limited calpain lysis of purified ankyrin results in several peptides, including a 85 kD peptide bearing the ankyrin interaction site for the protein band 3 internal fragment (43 kD), and a 55 kD peptide carrying the ankyrin-spectrin interaction site. 3. These peptides are differently phosphorylated: the 85 kD by cytosol casein kinase, and the 55 kD by membrane casein kinase. 4. Protein 4.1 lysis mainly produces a 30 kD peptide resistant to proteolysis. 5. The spectrin beta-chain is more sensitive to calpain cleavage than the alpha chain; both chains seem to be cleaved in a similar sequential manner. 6. Limited proteolysis of spectrin dimer does not impede tetramerization in vitro.

Calpains (intracellular calcium-activated cysteine proteinases): structure-activity relationships and involvement in normal and abnormal cellular metabolism

1. Calpains (calcium-activated cysteine proteinases) have evolved by gene fusion events involving calmodulin-like genes, cysteine proteinase genes and other sequences of unknown origin. 2. The enzymes are composed of two non-identical subunits, each of which contains functional calcium-binding sequences. 3. Calpains are inhibited by the endogenous protein inhibitor, calpastatin and some calmodulin antagonists are also inhibitors of calpain. A number of synthetic proteinase inhibitors also inhibit calpains. 4. Calpains can be activated by phospholipids, an endogenous protein activator and some amino acid derivatives. 5. Various protein substrates for calpains have been recognized in vitro, but the identity of in situ substrates remains unclear. 6. Proposals have been made for calpain function, including involvement in signal transduction, platelet activation, cell fusion, mitosis and cytoskeleton and contractile protein turnover. 7. Calpain and calpastatin expression is altered in a number of abnormal states including muscular dystrophy, muscle denervation and tenotomy, hypertension and platelet abnormalities.