Gram-positive bacterial pathogens that secrete cytotoxic pore-forming toxins, such as Staphylococcus aureus and Streptococcus pneumoniae, cause a substantial burden of disease. Inspired by the principles that govern natural toxin-host interactions, we have engineered artificial liposomes that are tailored to effectively compete with host cells for toxin binding. Liposome-bound toxins are unable to lyse mammalian cells in vitro. We use these artificial liposomes as decoy targets to sequester bacterial toxins that are produced during active infection in vivo. Administration of artificial liposomes within 10 h after infection rescues mice from septicemia caused by S. aureus and S. pneumoniae, whereas untreated mice die within 24-33 h. Furthermore, liposomes protect mice against invasive pneumococcal pneumonia. Composed exclusively of naturally occurring lipids, tailored liposomes are not bactericidal and could be used therapeutically either alone or in conjunction with antibiotics to combat bacterial infections and to minimize toxin-induced tissue damage that occurs during bacterial clearance.

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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 þ concentration [Ca 2 þ ] i below a critical threshold of 10 lM, repair mechanisms seal off 'hot spots' of Ca 2 þ entry and shed them in the form of microparticles, leading to [ Plasma membrane pores formed by cytotoxic proteins and peptides disrupt the permeability barrier in a target cell. Pathogens gain access and kill host cells by secreting pore-forming toxins, whereas the blood complement system utilises the pore-forming proteins of membrane attack complexes to eliminate both pathogens and the pathogen-invaded cells. 1,2 As in particular, cells of the blood and the vascular systems are permanently exposed to potential deadly attacks by a variety of pore-forming (poly)peptides, it is not surprising that mechanisms repairing the damaged plasma membrane have evolved. [3][4][5][6] Biological effects occurring in the wake of membrane permeabilisation and its subsequent repair are multifaceted. Apart from the two obvious end points, complete recovery or death, recovering cells can newly acquire numerous (patho)physiological functions. 3,7,8 A rise in intracellular Ca 2 þ concentration [Ca 2 þ ] i is critical for successful plasma membrane repair and cell recovery, 9,10 whereas an intracellular Ca 2 þ overload is held responsible for the death of pore-bearing cells. 11 In addition, Ca 2 þ influx, followed by transcriptional activation, is thought to induce a variety of biological responses associated with sublytic effects of pore-forming toxins. 3,4,7,8 Thus, it appears that the extent of [Ca 2 þ ] i elevation following pore formation determines the fate of a targeted cell. Consequently, Morgan et al. 11 suggested that in nucleated cells, an initial increase in [Ca 2 þ ] i stimulates the recovery processes, allowing the cell to withstand a limited complement attack. The recovery might be associated with cellular activation and the production of inflammatory modulators, which, in turn, amplify an ongoing inflammatory response. 3,11 The authors further hypothesised that a more severe membrane damage causes a sharp rise in [Ca 2 þ ] i , which overwhelms all recovery processes. 11 However, how [Ca 2 þ ] i determines cell fate, how the acquisition of novel functions is initiated and how the 'point of no return' is defined remain unknown.In this study, we have undertaken a simultaneous, real-time characterisation of [Ca 2 þ ] i and plasma membrane dynamics in living cells permeabilised with the bacterial pore-forming toxin streptolysin O (SLO). Our data show that the fate of SLOperforated cells is dependent on their ability to control the extent of a pore-induced elevation in [Ca 2 þ ] i . We detail Ca 2 þ -dependent mechanisms that elicit either repair or irreversible structural changes in the plasma membrane and show how intracellula...

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 injuryinduced 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 þ -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.

We have achieved a comprehensive view of a general plasma membrane repair mechanism after injury by a major bacterial toxin.

Uterine quiescence is essential for successful pregnancy. Cholesterol and triglycerides are markedly increased in pregnancy. Cholesterol is enriched in microdomains of the plasma membrane known as rafts and caveolae. Both lipid rafts and caveolae have been implicated in cellular signaling cascades. The purpose of this work was to investigate whether manipulation of cholesterol content alters uterine contractility. Late pregnancy (19-21 days) rats were humanely euthanized and strips of longitudinal myometrium were then dissected. Force and Ca(2+) measurements were simultaneously recorded and cholesterol increased by the addition of 5 mg/ml cholesterol or 0.25 mg/ml low-density lipoproteins (LDLs) or reduced by 2% methyl-beta-cyclodextrin (MCD) or 2 U/ml cholesterol oxidase addition to the perfusate. Both LDLs and cholesterol profoundly inhibited spontaneous uterine force production and associated Ca(2+) transients; frequency, amplitude, and duration of contraction were all significantly reduced compared with preceding control contractions. Force and Ca(2+) were also reduced by cholesterol when 1 nM oxytocin was used to stimulate the myometrium. Uterine activity was significantly increased by cholesterol extraction with MCD or cholesterol oxidase treatment. Electron microscopy confirmed the lipid raft disrupting effect of MCD, as formerly electron microscopy-visible caveolae in the myometrial cell membrane all but disappeared after MCD treatment. These data show that uterine smooth muscle cell cholesterol content is critically important for functional activity. A novel finding of our study is that cholesterol is inhibitory for force generation. It may be one of the mechanisms operating to maintain uterine quiescence throughout gestation and may also contribute to difficulties in labor suffered by obese women.

The annexins, a family of Ca 2؉ -and lipid-binding proteins, are involved in a range of intracellular processes. Recent findings have implicated annexin A1 in the resealing of plasmalemmal injuries. Here, we demonstrate that another member of the annexin protein family, annexin A6, is also involved in the repair of plasmalemmal lesions induced by a bacterial pore-forming toxin, streptolysin O. An injury-induced elevation in the intra- 2؉ -sensitivities provide a cell with the means to react promptly to a limited injury in its early stages and, at the same time, to withstand a sustained injury accompanied by the continuous formation of plasmalemmal lesions.The annexins are a family of Ca 2ϩ -binding proteins expressed in most phyla and species (1-4). Twelve annexins are present in vertebrates (A1-A11 and A13) with different splice variants (1). Annexins share a common folding motif, the "annexin core," which harbors the Ca 2ϩ -and membrane-binding sites (2-4). In their Ca 2ϩ -bound form, the annexins translocate from the cytoplasm to the plasma membrane and associate with negatively charged phospholipids (2-4). The N-terminal region precedes the conserved core and is unique for a given member of the annexin family. It mediates interactions with protein ligands and regulates the annexin-membrane association (2-4). Different annexins have been shown to orchestrate a variety of intracellular processes, ranging from the regulation of membrane dynamics to cell migration, proliferation, and apoptosis (2-12). However, the intriguing question why the majority of cells express several annexins, which differ only slightly in their biochemical properties, remains unanswered.Recent findings have implicated annexin A1 in the resealing of plasmalemmal lesions following cell injury (13,14). An injury-induced rise in the local concentration of intracellular Ca 2ϩ (15) is sensed by annexin A1 and triggers its binding to the plasma membrane at the site of the injury (13,14). Subsequently, annexin A1 promotes fusion of the damaged membrane around the pore, forming sealed, lesion-containing structures: large, cytosol-containing blebs (14) or smaller, cytosol-free microvesicles (16). The microvesicles subsequently can be shed by the cell (16, 17).Here, we show that, similar to annexin A1, annexin A6 is directly involved in the repair of plasmalemmal lesions induced by streptolysin O (SLO).2 The shedding of microvesicles appears to be predominant in the elimination of pores by annexin A6-dependent repair. Annexin A6 requires lower [Ca 2ϩ ] i for its plasmalemmal binding and, thus, responds faster to an injury than annexin A1. Correspondingly, a plasmalemmal lesion can be repaired by annexin A6 even without involvement of annexin A1; however, the concerted action of both annexins is instrumental for the efficient repair of multiple, simultaneously occurring plasmalemmal lesions. EXPERIMENTAL PROCEDURESReagents-Monoclonal anti-annexin A6 and anti-annexin A1 antibodies were from BD Biosciences; an antiserum against SLO was from Bioacad...

The plasmalemma of smooth muscle cells is periodically banded. This arrangement ensures efficient transmission of contractile activity, via the firm, actin-anchoring regions, while the more elastic caveolaecontaining "hinge" regions facilitate rapid cellular adaptation to changes in cell length. Since cellular mechanics are undoubtedly regulated by components of the membrane and cytoskeleton, we have investigated the potential role played by annexins (a family of phospholipid-and actin-binding, Ca 2؉ -regulated proteins) in regulating sarcolemmal organization. Stimulation of smooth muscle cells elicited a relocation of annexin VI from the cytoplasm to the plasmalemma. In smooth, but not in striated muscle extracts, annexins II and VI coprecipitated with actomyosin and the caveolar fraction of the sarcolemma at elevated Ca 2؉ concentrations. Recombination of actomyosin, annexins, and caveolar lipids in the presence of Ca 2؉ led to formation of a structured precipitate. Participation of all 3 components was required, indicating that a Ca 2؉ -dependent, cytoskeleton-membrane complex had been generated. This association, which occurred at physiological Ca 2؉ concentrations, corroborates our biochemical fractionation and immunohistochemical findings and suggests that annexins play a role in regulating sarcolemmal organization during smooth muscle contraction.