Annexin A6 is a scaffold for PKCα to promote EGFR inactivation

Protein kinase Cα (PKCα) can phosphorylate the epidermal growth factor receptor (EGFR) at threonine 654 (T654) to inhibit EGFR tyrosine phosphorylation (pY-EGFR) and the associated activation of downstream effectors. However, upregulation of PKCα in a large variety of cancers is not associated with EGFR inactivation, and factors determining the potential of PKCα to downregulate EGFR are yet unknown. Here, we show that ectopic expression of annexin A6 (AnxA6), a member of the Ca(2+) and phospholipid-binding annexins, strongly reduces pY-EGFR levels while augmenting EGFR T654 phosphorylation in EGFR overexpressing A431, head and neck and breast cancer cell lines. Reduced EGFR activation in AnxA6 expressing A431 cells is associated with reduced EGFR internalization and degradation. RNA interference (RNAi)-mediated PKCα knockdown in AnxA6 expressing A431 cells reduces T654-EGFR phosphorylation, but restores EGFR tyrosine phosphorylation, clonogenic growth and EGFR degradation. These findings correlate with AnxA6 interacting with EGFR, and elevated AnxA6 levels promoting PKCα membrane association and interaction with EGFR. Stable expression of the cytosolic N-terminal mutant AnxA6(1-175), which cannot promote PKCα membrane recruitment, does not increase T654-EGFR phosphorylation or the association of PKCα with EGFR. AnxA6 overexpression does not inhibit tyrosine phosphorylation of the T654A EGFR mutant, which cannot be phosphorylated by PKCα. Most strikingly, stable plasma membrane anchoring of AnxA6 is sufficient to recruit PKCα even in the absence of EGF or Ca(2+). In summary, AnxA6 is a new PKCα scaffold to promote PKCα-mediated EGFR inactivation through increased membrane targeting of PKCα and EGFR/PKCα complex formation.

Blebbing confers resistance against cell lysis

The plasma membrane constitutes a barrier that maintains the essential differences between the cytosol and the extracellular environment. Plasmalemmal injury is a common event during the life of many cells that often leads to their premature, necrotic death. Blebbing - a display of plasmalemmal protrusions - is a characteristic feature of injured cells. In this study, we disclose a previously unknown role for blebbing in furnishing resistance to plasmalemmal injury. Blebs serve as precursors for injury-induced intracellular compartments that trap damaged segments of the plasma membrane. Hence, loss of cytosol and the detrimental influx of extracellular constituents are confined to blebs that are sealed off from the cell body by plugs of annexin A1 - a Ca(2+)- and membrane-binding protein. Our findings shed light on a fundamental process that contributes to the survival of injured cells. By targeting annexin A1/blebbing, new therapeutic approaches could be developed to avert the necrotic loss of cells in a variety of human pathologies.

Intracellular Ca(2+) operates a switch between repair and lysis of streptolysin O-perforated cells

Pore-forming (poly)peptides originating from invading pathogens cause plasma membrane damage in target cells, with consequences as diverse as proliferation or cell death. However, the factors that define the outcome remain unknown. We show that in cells maintaining an intracellular Ca(2+) concentration [Ca(2+)](i) below a critical threshold of 10 microM, repair mechanisms seal off 'hot spots' of Ca(2+) entry and shed them in the form of microparticles, leading to [Ca(2+)](i) reduction and cell recovery. Cells that are capable of preventing an elevation of [Ca(2+)](i) above the critical concentration, yet are unable to complete plasma membrane repair, enter a prolonged phase of [Ca(2+)](i) oscillations, accompanied by a continuous shedding of microparticles. When [Ca(2+)](i) exceeds the critical concentration, an irreversible formation of ceramide platforms within the plasma membrane and their internalisation drives the dying cells beyond the 'point of no return'. These findings show that the extent of [Ca(2+)](i) elevation determines the fate of targeted cells and establishes how different Ca(2+)-dependent mechanisms facilitate either cell survival or death.

Statin therapy induces ultrastructural damage in skeletal muscle in patients without myalgia

Muscle pain and weakness are frequent complaints in patients receiving 3-hydroxymethylglutaryl coenzymeA (HMG CoA) reductase inhibitors (statins). Many patients with myalgia have creatine kinase levels that are either normal or only marginally elevated, and no obvious structural defects have been reported in patients with myalgia only. To investigate further the mechanism that mediates statin-induced skeletal muscle damage, skeletal muscle biopsies from statin-treated and non-statin-treated patients were examined using both electron microscopy and biochemical approaches. The present paper reports clear evidence of skeletal muscle damage in statin-treated patients, despite their being asymptomatic. Though the degree of overall damage is slight, it has a characteristic pattern that includes breakdown of the T-tubular system and subsarcolemmal rupture. These characteristic structural abnormalities observed in the statin-treated patients were reproduced by extraction of cholesterol from skeletal muscle fibres in vitro. These findings support the hypothesis that statin-induced cholesterol lowering per se contributes to myocyte damage and suggest further that it is the specific lipid/protein organization of the skeletal muscle cell itself that renders it particularly vulnerable.

The expression levels of three raft-associated molecules in cultivated vascular cells are dependent on culture conditions

Relaying a signal across the plasma membrane requires functional connections between the partner molecules. Membrane microdomains or lipid rafts provide an environment in which such specific interactions can take place. The integrity of these sites is often taken for granted when signalling pathways are investigated in cell culture. However, it is well known that smooth muscle and endothelial cells undergo cytoskeletal rearrangements during monolayer culturing. Likewise affected--and with potentially important consequences for signalling events--is the organization of the plasma membrane. The expression levels of three raft markers were massively upregulated, and raft-associated 5'-nucleotidase activity increased in conventional monolayer cultures as compared with a spheroidal coculture model, shown to promote the differentiation of endothelial cells. Our data point to a shift of raft components in monolayer cultures and demonstrate potential advantages of the spheroid coculture system for investigation of raft-mediated signalling events in endothelial cells.

Membrane segregation and downregulation of raft markers during sarcolemmal differentiation in skeletal muscle cells

Muscle contraction implies flexibility in combination with force resistance and requires a high degree of sarcolemmal organization. Smooth muscle cells differentiate largely from mesenchymal precursor cells and gradually assume a highly periodic sarcolemmal organization. Skeletal muscle undergoes an even more striking differentiation programme, leading to cell fusion and alignment into myofibrils. The lipid bilayer of each cell type is further segregated into raft and non-raft microdomains of distinct lipid composition. Considering the extent of developmental rearrangement in skeletal muscle, we investigated sarcolemmal microdomain organization in skeletal and smooth muscle cells. The rafts in both muscle types are characterized by marker proteins belonging to the annexin family which localize to the inner membrane leaflet, as well as glycosyl-phosphatidyl-inositol (GPI)-anchored enzymes attached to the outer leaflet. We demonstrate that the profound structural rearrangements that occur during skeletal muscle maturation coincide with a striking decrease in membrane lipid segregation, downregulation of annexins 2 and 6, and a significant decrease in raft-associated 5'-nucleotidase activity. The relative paucity of lipid rafts in mature skeletal in contrast to smooth muscle suggests that the organization of sarcolemmal microdomains contributes to the muscle-specific differences in stimulatory responses and contractile properties.

Functional dissection of the major structural protein of bluetongue virus: identification of key residues within VP7 essential for capsid assembly

A lattice of VP7 trimers forms the surface of the icosahedral bluetongue virus (BTV) core. To investigate the role of VP7 oligomerization in core assembly, a series of residues for substitution were predicted based on crystal structures of BTV type 10 VP7 molecule targeting the monomer-monomer contacts within the trimer. Seven site-specific substitution mutations of VP7 have been created using cDNA clones and were employed to produce seven recombinant baculoviruses. The effects of these mutations on VP7 solubility, ability to trimerize and formation of core-like particles (CLPs) in the presence of the scaffolding VP3 protein, were investigated. Of the seven VP7 mutants examined, three severely affected the stability of CLP, while two other mutants had lesser effect on CLP stability. Only one mutant had no apparent effect on the formation of the stable capsid. One mutant in which the conserved tyrosine at residue 271 (lower domain helix 6) was replaced by arginine formed insoluble aggregates, implying an effect in the folding of the molecule despite the prediction that such a change would be accommodated. All six soluble VP7 mutants were purified, and their ability to trimerize was examined. All mutants, including those that did not form stable CLPs, assembled into stable trimers, implying that single substitution may not be sufficient to perturb the complex monomer-monomer contacts, although subtle changes within the VP7 trimer could destabilize the core. The study highlights some of the key residues that are crucial for BTV core assembly and illustrates how the structure of VP7 in isolation underrepresents the dynamic nature of the assembly process at the biological level.

Effects of domain-switching and site-directed mutagenesis on the properties and functions of the VP7 proteins of two orbiviruses

Based on the crystal structure of the VP7 major core protein of bluetongue virus serotype 10 (BTV-10) and that of the top domain of the VP7 protein of African horsesickness virus serotype 4 (AHSV-4), chimeras and site-directed mutants of the proteins were constructed and the products analyzed with respect to their properties and functions. Chimeras with the central upper domain of BTV-10 VP7 replaced by that of AHSV-4 VP7 (construct BAB) formed trimers, as did the converse construct (ABA). Further, both proteins exhibited the expected conformational epitopes of the constituent sequences. Using BAB VP7 it was demonstrated that residues of the upper domain of AHSV-4 VP7 contribute to the observed insolubility of the protein. By contrast, ABA VP7 protein was as soluble as wild-type BTV-10 VP7. Replacement of selected amino acid residues in the top domain (e.g., A167 by R; F209 by T) improved the solubility of BAB VP7. Since the trimeric BAB and ABA VP7 proteins did not form core-like particles (CLPs) when coexpressed with BTV VP3, it was concluded that trimerization of chimeric VP7 is not sufficient for CLP formation. When the N-terminal region of the ABA protein (aa 1-120) was replaced by the respective sequences of BTV VP7 (construct BBA), the protein aggregated and did not form CLPs with coexpressed BTV VP3, most likely due to disruption of the required contacts between the N- and C-terminal regions of the bottom domain, leading to incorrect folding of the chimera.

Bluetongue virus VP6 protein binds ATP and exhibits an RNA-dependent ATPase function and a helicase activity that catalyze the unwinding of double-stranded RNA substrates

RNA-dependent ATPase and helicase activities have been identified associated with the purified VP6 protein of bluetongue virus, a member of the Orbivirus genus of double-stranded RNA (dsRNA; Reoviridae family) viruses. In addition, the protein has an ATP binding activity. RNA unwinding of duplexes occurred with both 3' and 5' overhang templates, as well as with blunt-ended dsRNA, an activity not previously identified in other viral helicases. Although little sequence similarity to other helicases was detected, certain similarities to motifs commonly attributed to such proteins were identified.

A new form of particulate single and multiple immunogen delivery system based on recombinant bluetongue virus-derived tubules

The development of particular vector systems for the presentation of immunogenic epitopes provides a powerful approach for the delivery of antigens. These include the core-like particles formed by recombinant bluetongue virus (BTV) capsid proteins VP3 and VP7 synthesized in insect cells by recombinant baculoviruses. Previously we have reported localization of an antigenic site on the surface of tubular structures formed by the nonstructural protein NS1 of BTV, and their potential use for epitope presentation. In this study foreign sequences ranging form 44 to 116 aa in length and representing 44 aa sequence from Clostridium difficile toxin A, 48 aa of the hepatitis B virus preS2 region, and the whole of bovine leukemia virus p15 protein were inserted at the C-terminus of BTV-10 NS1. The chimeric NS1 genes were expressed using recombinant baculoviruses and the ability of the mutated NS1 proteins to form tubules was investigated. All chimeric constructs formed tubular structures which carried the foreign antigenic sequences exposed on the surface of the tubules and were highly immunogenic. When Sf cells were coinfected with three recombinant baculoviruses expressing chimeric NS1 proteins with different epitopes, their simultaneous assembly into the same tubule was demonstrated. This observation opens up the possibility of using recombinant NS1 tubules as carriers for the delivery of multiple epitopes.

Characterization and modification of the carboxy-terminal sequences of bluetongue virus type 10 NS1 protein in relation to tubule formation and location of an antigenic epitope in the vicinity of the carboxy terminus of the protein

Bluetongue virus produces large numbers of tubules during infection. The tubules are formed from a 552-amino-acid, 64-kDa NS1 protein encoded by the viral double-stranded RNA segment M6. A series of deletion and extension mutants of bluetongue virus serotype 10 NS1 has been generated and expressed in insect cells in order to identify the carboxy-terminal components of the protein which are important for tubule formation. The mutants AcCT5 and AcCT10, lacking 5 and 10 of the carboxy-terminal residues, respectively, were prepared. By analyzing their abilities to form tubules, it was shown that AcCT5 was capable of this function whereas AcCT10 was not, indicating that the last five amino acids are not strongly involved in NS1 tubule formation. Extension mutants including foreign antigenic sequences involving up to 16 amino acids added to the C terminus of NS1 were shown to form tubules, although an extension of 19 amino acids inhibited tubule formation. Analysis of a panel of monoclonal antibodies has established that an NS1 antigenic site is located near the carboxy terminus of the protein. It appears to be exposed on the surface of tubules. The opportunities to develop new vaccines using recombinant NS1 to deliver foreign epitopes are discussed.

Mutation of either of two cysteine residues or deletion of the amino or carboxy terminus of nonstructural protein NS1 of bluetongue virus abrogates virus-specified tubule formation in insect cells

Virus-specific tubules are characteristic of orbivirus infections and are likely to play an important role in virus morphogenesis. It has been shown that for bluetongue virus (BTV), the prototype orbivirus in the family Reoviridae, the virus-encoded NS1 protein forms tubules in insect cells when the BTV segment M6 gene is expressed by using a baculovirus vector. To understand the function of NS1 tubules and to identify the sequences involved in their polymerization, a series of mutant NS1 genes was generated and expressed in insect cell cultures by using baculovirus vectors. Three of the mutants were deletion mutants. One (AcNS1.dNT10) lacked 10 of the amino-terminal amino acids, and the other two mutants (AcNS1.dCT20 and AcNS1.dCT43) lacked 20 or 43 of the carboxy-terminal amino acids. In addition, site-directed mutants were constructed in which various single cysteines or pairs of cysteines were changed to serines. The ability of each mutant protein to form tubules was investigated. None of the deletion mutants formed tubules. The constructs in which the cysteines at amino acid positions 337 and/or 340 were replaced by serines (e.g., AcNS1.C337S,C340S) also did not form tubules. Instead, the NS1 protein of these and the deletion mutants made ribbon-like structures which formed large aggregates. Mutations involving six other cysteines (i.e., AcNS1.C37S,C43S,AcNS1.C462S,C465S, AcNS1.C104S, and AcNS1.C364S) produced tubules. The results show that both the amino and carboxy termini of the NS1 protein molecule and the cysteines at residues 337 and 340 are essential for tubule formation.