Molecular regulation of membrane resealing in 3T3 fibroblasts

An Overview of Cell Membrane Perforation and Resealing Mechanisms for Localized Drug Delivery

Localized and reversible plasma membrane disruption is a promising technique employed for the targeted deposition of exogenous therapeutic compounds for the treatment of disease. Indeed, the plasma membrane represents a significant barrier to successful delivery, and various physical methods using light, sound, and electrical energy have been developed to generate cell membrane perforations to circumvent this issue. To restore homeostasis and preserve viability, localized cellular repair mechanisms are subsequently triggered to initiate a rapid restoration of plasma membrane integrity. Here, we summarize the known emergency membrane repair responses, detailing the salient membrane sealing proteins as well as the underlying cytoskeletal remodeling that follows the physical induction of a localized plasma membrane pore, and we present an overview of potential modulation strategies that may improve targeted drug delivery approaches.

Membrane repair triggered by cholesterol-dependent cytolysins is activated by mixed lineage kinases and MEK

Repair of plasma membranes damaged by bacterial pore-forming toxins, such as streptolysin O or perfringolysin O, during septic cardiomyopathy or necrotizing soft tissue infections is mediated by several protein families. However, the activation of these proteins downstream of ion influx is poorly understood. Here, we demonstrate that following membrane perforation by bacterial cholesterol-dependent cytolysins, calcium influx activates mixed lineage kinase 3 independently of protein kinase C or ceramide generation. Mixed lineage kinase 3 uncouples mitogen-activated kinase kinase (MEK) and extracellular-regulated kinase (ERK) signaling. MEK signals via an ERK-independent pathway to promote rapid annexin A2 membrane recruitment and enhance microvesicle shedding. This pathway accounted for 70% of all calcium ion-dependent repair responses to streptolysin O and perfringolysin O, but only 50% of repair to intermedilysin. We conclude that mixed lineage kinase signaling via MEK coordinates microvesicle shedding, which is critical for cellular survival against cholesterol-dependent cytolysins.

Plasma membrane integrity: implications for health and disease

Plasma membrane integrity is essential for cellular homeostasis. In vivo, cells experience plasma membrane damage from a multitude of stressors in the extra- and intra-cellular environment. To avoid lethal consequences, cells are equipped with repair pathways to restore membrane integrity. Here, we assess plasma membrane damage and repair from a whole-body perspective. We highlight the role of tissue-specific stressors in health and disease and examine membrane repair pathways across diverse cell types. Furthermore, we outline the impact of genetic and environmental factors on plasma membrane integrity and how these contribute to disease pathogenesis in different tissues.

Mechanisms protecting host cells against bacterial pore-forming toxins

Pore-forming toxins (PFTs) are key virulence determinants produced and secreted by a variety of human bacterial pathogens. They disrupt the plasma membrane (PM) by generating stable protein pores, which allow uncontrolled exchanges between the extracellular and intracellular milieus, dramatically disturbing cellular homeostasis. In recent years, many advances were made regarding the characterization of conserved repair mechanisms that allow eukaryotic cells to recover from mechanical disruption of the PM membrane. However, the specificities of the cell recovery pathways that protect host cells against PFT-induced damage remain remarkably elusive. During bacterial infections, the coordinated action of such cell recovery processes defines the outcome of infected cells and is, thus, critical for our understanding of bacterial pathogenesis. Here, we review the cellular pathways reported to be involved in the response to bacterial PFTs and discuss their impact in single-cell recovery and infection.

Hypercapnia Alters Alveolar Epithelial Repair by a pH-Dependent and Adenylate Cyclase-Mediated Mechanism

Lung cell injury and repair is a hallmark of the acute respiratory distress syndrome (ARDS). Lung protective mechanical ventilation strategies in these patients may lead to hypercapnia (HC). Although HC has been explored in the clinical context of ARDS, its effect upon alveolar epithelial cell (AEC) wounding and repair remains poorly understood. We have previously reported that HC alters the likelihood of AEC repair by a pH-sensitive but otherwise unknown mechanism. Adenylate cyclase (AC) is an attractive candidate as a putative AEC CO₂ sensor and effector as it is bicarbonate sensitive and controls key mediators of AEC repair. The effect of HC on AC activity and plasma membrane (PM) wound repair was measured in AEC type 1 exposed to normocapnia (NC, 40 Torr) or HC (80 Torr), ± tromethamine (THAM) or sodium bicarbonate (HCO3) ± AC probes in a micropuncture model of AEC injury relevant to ARDS. Intracellular pH and AC activity were measured and correlated with repair. HC decreased intracellular pH 0.56, cAMP by 37%, and absolute PM repair rate by 26%. Buffering or pharmacologic manipulation of AC reduced or reversed the effects of HC on AC activity (THAM 103%, HCO₃ 113% of NC cAMP, ns; Forskolin 168%, p < 0.05) and PM repair (THAM 87%, HCO₃ 108% of NC likelihood to repair, ns; Forskolin 160%, p < 0.01). These findings suggest AC to be a putative AEC CO₂ sensor and modulator of AEC repair, and may have implications for future pharmacologic targeting of downstream messengers of the AC-cAMP axis in experimental models of ARDS.

Frontiers of Complex Disease Mechanisms: Membrane Surface Tension May Link Genotype to Phenotype in Glaucoma

Although many monogenic diseases are understood based upon structural changes of gene products, less progress has been made concerning polygenic disease mechanisms. This article presents a new interdisciplinary approach to understand complex diseases, especially their genetic polymorphisms. I focus upon primary open angle glaucoma (POAG). Although elevated intraocular pressure (IOP) and oxidative stress are glaucoma hallmarks, the linkages between these factors and cell death are obscure. Reactive oxygen species (ROS) promote the formation of oxidatively truncated phosphoglycerides (OTP), free fatty acids, lysophosphoglycerides, oxysterols, and other chemical species that promote membrane disruption and decrease membrane surface tension. Several POAG-linked gene polymorphisms identify proteins that manage damaged lipids and/or influence membrane surface tension. POAG-related genes expected to participate in these processes include: ELOVL5, ABCA1, APOE4, GST, CYP46A1, MYOC, and CAV. POAG-related gene products are expected to influence membrane surface tension, strength, and repair. I propose that heightened IOP overcomes retinal ganglion cell (RGC) membrane compressive strength, weakened by damaged lipid accumulation, to form pores. The ensuing structural failure promotes apoptosis and blindness. The linkage between glaucoma genotype and phenotype is mediated by physical events. Force balancing between the IOP and compressive strength regulates pore nucleation; force balancing between pore line tension and membrane surface tension regulates pore growth. Similar events may contribute to traumatic brain injury, Alzheimer's disease, and macular degeneration.

Intrinsic repair protects cells from pore-forming toxins by microvesicle shedding

Pore-forming toxins (PFTs) are used by both the immune system and by pathogens to disrupt cell membranes. Cells attempt to repair this disruption in various ways, but the exact mechanism(s) that cells use are not fully understood, nor agreed upon. Current models for membrane repair include (1) patch formation (e.g., fusion of internal vesicles with plasma membrane defects), (2) endocytosis of the pores, and (3) shedding of the pores by blebbing from the cell membrane. In this study, we sought to determine the specific mechanism(s) that cells use to resist three different cholesterol-dependent PFTs: Streptolysin O, Perfringolysin O, and Intermedilysin. We found that all three toxins were shed from cells by blebbing from the cell membrane on extracellular microvesicles (MVs). Unique among the cells studied, we found that macrophages were 10 times more resistant to the toxins, yet they shed significantly smaller vesicles than the other cells. To examine the mechanism of shedding, we tested whether toxins with engineered defects in pore formation or oligomerization were shed. We found that oligomerization was necessary and sufficient for membrane shedding, suggesting that calcium influx and patch formation were not required for shedding. However, pore formation enhanced shedding, suggesting that calcium influx and patch formation enhance repair. In contrast, monomeric toxins were endocytosed. These data indicate that cells use two interrelated mechanisms of membrane repair: lipid-dependent MV shedding, which we term 'intrinsic repair', and patch formation by intracellular organelles. Endocytosis may act after membrane repair is complete by removing inactivated and monomeric toxins from the cell surface.

Why should cell biologists study microbial pathogens?

One quarter of all deaths worldwide each year result from infectious diseases caused by microbial pathogens. Pathogens infect and cause disease by producing virulence factors that target host cell molecules. Studying how virulence factors target host cells has revealed fundamental principles of cell biology. These include important advances in our understanding of the cytoskeleton, organelles and membrane-trafficking intermediates, signal transduction pathways, cell cycle regulators, the organelle/protein recycling machinery, and cell-death pathways. Such studies have also revealed cellular pathways crucial for the immune response. Discoveries from basic research on the cell biology of pathogenesis are actively being translated into the development of host-targeted therapies to treat infectious diseases. Thus there are many reasons for cell biologists to incorporate the study of microbial pathogens into their research programs.

Plasma Membrane Repair: A Central Process for Maintaining Cellular Homeostasis

Plasma membrane repair is a conserved cellular response mediating active resealing of membrane disruptions to maintain homeostasis and prevent cell death and progression of multiple diseases. Cell membrane repair repurposes mechanisms from various cellular functions, including vesicle trafficking, exocytosis, and endocytosis, to mend the broken membrane. Recent studies increased our understanding of membrane repair by establishing the molecular machinery contributing to membrane resealing. Here, we review some of the key proteins linked to cell membrane repair.

Membrane Repair: Mechanisms and Pathophysiology

Eukaryotic cells have been confronted throughout their evolution with potentially lethal plasma membrane injuries, including those caused by osmotic stress, by infection from bacterial toxins and parasites, and by mechanical and ischemic stress. The wounded cell can survive if a rapid repair response is mounted that restores boundary integrity. Calcium has been identified as the key trigger to activate an effective membrane repair response that utilizes exocytosis and endocytosis to repair a membrane tear, or remove a membrane pore. We here review what is known about the cellular and molecular mechanisms of membrane repair, with particular emphasis on the relevance of repair as it relates to disease pathologies. Collective evidence reveals membrane repair employs primitive yet robust molecular machinery, such as vesicle fusion and contractile rings, processes evolutionarily honed for simplicity and success. Yet to be fully understood is whether core membrane repair machinery exists in all cells, or whether evolutionary adaptation has resulted in multiple compensatory repair pathways that specialize in different tissues and cells within our body.

Zinc Binding to MG53 Protein Facilitates Repair of Injury to Cell Membranes

Zinc is an essential trace element that participates in a wide range of biological functions, including wound healing. Although Zn(2+) deficiency has been linked to compromised wound healing and tissue repair in human diseases, the molecular mechanisms underlying Zn(2+)-mediated tissue repair remain unknown. Our previous studies established that MG53, a TRIM (tripartite motif) family protein, is an essential component of the cell membrane repair machinery. Domain homology analysis revealed that MG53 contains two Zn(2+)-binding motifs. Here, we show that Zn(2+) binding to MG53 is indispensable to assembly of the cell membrane repair machinery. Live cell imaging illustrated that Zn(2+) entry from extracellular space is essential for translocation of MG53-containing vesicles to the acute membrane injury sites for formation of a repair patch. The effect of Zn(2+) on membrane repair is abolished in mg53(-/-) muscle fibers, suggesting that MG53 functions as a potential target for Zn(2+) during membrane repair. Mutagenesis studies suggested that both RING and B-box motifs of MG53 constitute Zn(2+)-binding domains that contribute to MG53-mediated membrane repair. Overall, this study establishes a base for Zn(2+) interaction with MG53 in protection against injury to the cell membrane.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Linking exocytosis and endocytosis during phagocytosis

Phagocytosis is used by macrophages, dendritic cells and neutrophils to capture and destroy pathogens and particulate antigens. Although localized assembly of actin filaments is the driving force for particle internalization, exocytosis of intracellular compartments, and in particular endocytic compartments, has been shown recently to be required for the early steps of phagosome formation. Here we report on the different compartments undergoing exocytosis during phagocytosis, with a special focus on late endosomes. We then compare this process with secretion from lysosomes or lysosome-related organelles in specialized cells. Finally, we discuss how some of the molecular mechanisms responsible for lysosome-related organelle secretion could also be implicated in phagosome formation.

The role of purinergic signaling on deformation induced injury and repair responses of alveolar epithelial cells

Cell wounding is an important driver of the innate immune response of ventilator-injured lungs. We had previously shown that the majority of wounded alveolus resident cells repair and survive deformation induced insults. This is important insofar as wounded and repaired cells may contribute to injurious deformation responses commonly referred to as biotrauma. The central hypothesis of this communication states that extracellular adenosine-5′ triphosphate (ATP) promotes the repair of wounded alveolus resident cells by a P2Y2-Receptor dependent mechanism. Using primary type 1 alveolar epithelial rat cell models subjected to micropuncture injury and/or deforming stress we show that 1) stretch causes a dose dependent increase in cell injury and ATP media concentrations; 2) enzymatic depletion of extracellular ATP reduces the probability of stretch induced wound repair; 3) enriching extracellular ATP concentrations facilitates wound repair; 4) purinergic effects on cell repair are mediated by ATP and not by one of its metabolites; and 5) ATP mediated cell salvage depends at least in part on P2Y2-R activation. While rescuing cells from wounding induced death may seem appealing, it is possible that survivors of membrane wounding become governors of a sustained pro-inflammatory state and thereby perpetuate and worsen organ function in the early stages of lung injury syndromes. Means to uncouple P2Y2-R mediated cytoprotection from P2Y2-R mediated inflammation and to test the preclinical efficacy of such an undertaking deserve to be explored.

Wound repair: toward understanding and integration of single-cell and multicellular wound responses

The importance of wound healing to medicine and biology has long been evident, and consequently, wound healing has been the subject of intense investigation for many years. However, several relatively recent developments have added new impetus to wound repair research: the increasing application of model systems; the growing recognition that single cells have a robust, complex, and medically relevant wound healing response; and the emerging recognition that different modes of wound repair bear an uncanny resemblance to other basic biological processes such as morphogenesis and cytokinesis. In this review, each of these developments is described, and their significance for wound healing research is considered. In addition, overlapping mechanisms of single-cell and multicellular wound healing are highlighted, and it is argued that they are more similar than is often recognized. Based on this and other information, a simple model to explain the evolutionary relationships of cytokinesis, single-cell wound repair, multicellular wound repair, and developmental morphogenesis is proposed. Finally, a series of important, but as yet unanswered, questions is posed.

Muscle membrane repair and inflammatory attack in dysferlinopathy

Repair of plasma membrane tears is an important normal physiological process that enables the cells to survive a variety of physiological and pathological membrane lesions. Dysferlin was the first protein reported to play a crucial role in this repair process in muscle, and recently, several other proteins including Mitsugumin 53 (MG53), annexin and calpain were also found to participate. These findings have now established the framework of the membrane repair mechanism. Defective membrane repair in dysferlin-deficient muscle leads to the development of muscular dystrophy associated with remarkable muscle inflammation. Recent studies have demonstrated a crosstalk between defective membrane repair and immunological attack, thus unveiling a new pathophysiological mechanism of dysferlinopathy. Here I summarize and discuss the latest progress in the molecular mechanisms of membrane repair and the pathogenesis of dysferlinopathy. Discussion about potential therapeutic applications of these findings is also provided.

A plasma membrane wound proteome: reversible externalization of intracellular proteins following reparable mechanical damage

Cells in mechanically active tissues undergo constant plasma membrane damage that must be repaired to allow survival. To identify wound-associated proteins, a cell-impermeant, thiol-reactive biotinylation reagent was used to label and subsequently isolate intracellular proteins that become exposed on the surface of cultured cells after plasma membrane damage induced by scraping from substratum or crushing with glass beads. Scrape-damaged cells survived injury and were capable of forming viable colonies. Proteins that were exposed to the cell surface were degraded or internalized a few seconds to several minutes after damage, except for vimentin, which was detectable on the cell surface for at least an hour after injury. Seven major biotinylated protein bands were identified on SDS-PAGE gels. Mass spectrometric studies identified cytoskeletal proteins (caldesmon-1 and vimentin), endoplasmic reticulum proteins (ERp57, ERp5, and HSP47), and nuclear proteins (lamin C, heterogeneous nuclear ribonucleoprotein F, and nucleophosmin-1) as major proteins exposed after injury. Although caldesmon was a major wound-associated protein in calpain small subunit knock-out fibroblasts, it was rapidly degraded in wild-type cells, probably by calpains. Lamin C exposure after wounding was most likely the consequence of nuclear envelope damage. These studies document major intracellular proteins associated with the cell surface of reversibly damaged somatic cells. The studies also show that externalization of some proteins reported to have physiologic or pathologic roles on the cell surface can occur in cells undergoing plasma membrane damage and subsequent repair.

Type I alveolar epithelial phenotype in primary culture

Type I alveolar epithelial cells (ATIs) are very large, thin cells, which extend across several air sacs and cover more than 95% of the alveolar surface area. ATIs are the target of many insults, including ventilator-induced lung injury, and are generally considered terminally differentiated cells arising from type II cell (ATII) lineage. ATIs have proven difficult to harvest and maintain in primary culture, which is why much of ATI biology has been inferred from studies on ex vivo, ATII-derived, so-called ATI-like cells. We report on a modified approach to rat ATI harvest and primary culture, which yielded the following observations: (1) rat ATI can be harvested and maintained with a high degree of purity in primary culture; (2) in vitro growth characteristics of primary ATIs differ from those of ATII-derived ATI-like cells; ATIs, but not ex vivo, ATII-derived ATI-like cells, are capable of cell division; (3) ATIs readily repair plasma membrane wounds without the subsequent loss of their ability to divide; (4) ATI monolayers heal scratch wounds primarily by cell spreading and migration. Although the ability of ATIs to divide may be limited to the in vitro environment, we do believe that their role in alveolar wound repair deserves to be revisited, and the molecular control of ATI-ATII plasticity further explored.

Calcium-sensing beyond neurotransmitters: functions of synaptotagmins in neuroendocrine and endocrine secretion

Neurotransmitters, neuropeptides and hormones are released through the regulated exocytosis of SVs (synaptic vesicles) and LDCVs (large dense-core vesicles), a process that is controlled by calcium. Synaptotagmins are a family of type 1 membrane proteins that share a common domain structure. Most synaptotagmins are located in brain and endocrine cells, and some of these synaptotagmins bind to phospholipids and calcium at levels that trigger regulated exocytosis of SVs and LDCVs. This led to the proposed synaptotagmin-calcium-sensor paradigm, that is, members of the synaptotagmin family function as calcium sensors for the regulated exocytosis of neurotransmitters, neuropeptides and hormones. Here, we provide an overview of the synaptotagmin family, and review the recent mouse genetic studies aimed at understanding the functions of synaptotagmins in neurotransmission and endocrine-hormone secretion. Also, we discuss potential roles of synaptotagmins in non-traditional endocrine systems.

Synaptotagmin-7 is a principal Ca2+ sensor for Ca2+ -induced glucagon exocytosis in pancreas

Hormones such as glucagon are secreted by Ca(2+)-induced exocytosis of large dense-core vesicles, but the mechanisms involved have only been partially elucidated. Studies of pancreatic beta-cells secreting insulin revealed that synaptotagmin-7 alone is not sufficient to mediate Ca(2+)-dependent insulin granule exocytosis, and studies of chromaffin cells secreting neuropeptides and catecholamines showed that synaptotagmin-1 and -7 collaborate as Ca(2+) sensors for exocytosis, and that both are equally involved. As no other peptide secretion was analysed, it remains unclear whether synaptotagmins generally act as Ca(2+) sensors in large dense-core vesicle exocytosis in endocrine cells, and if so, whether synaptotagmin-7 always functions with a partner in that role. In particular, far less is known about the mechanisms underlying Ca(2+)-triggered glucagon release from alpha-cells than insulin secretion from beta-cells, even though insulin and glucagon together regulate blood glucose levels. To address these issues, we analysed the role of synaptotagmins in Ca(2+)-triggered glucagon exocytosis. Surprisingly, we find that deletion of a single synaptotagmin isoform, synaptotagmin-7, nearly abolished Ca(2+)-triggered glucagon secretion. Moreover, single-cell capacitance measurements confirmed that pancreatic alpha-cells lacking synaptotagmin-7 exhibited little Ca(2+)-induced exocytosis, whereas all other physiological and morphological parameters of the alpha-cells were normal. Our data thus identify synaptotagmin-7 as a principal Ca(2+) sensor for glucagon secretion, and support the notion that synaptotagmins perform a universal but selective function as individually acting Ca(2+) sensors in neurotransmitter, neuropeptide, and hormone secretion.

Calcium-dependent freezing tolerance in Arabidopsis involves membrane resealing via synaptotagmin SYT1

Plant freezing tolerance involves the prevention of lethal freeze-induced damage to the plasma membrane. We hypothesized that plant freezing tolerance involves membrane resealing, which, in animal cells, is accomplished by calcium-dependent exocytosis following mechanical disruption of the plasma membrane. In Arabidopsis thaliana protoplasts, extracellular calcium enhanced not only freezing tolerance but also tolerance to electroporation, which typically punctures the plasma membrane. However, calcium did not enhance survival when protoplasts were exposed to osmotic stress that mimicked freeze-induced dehydration. Calcium-dependent freezing tolerance was also detected with leaf sections in which ice crystals intruded into tissues. Interestingly, calcium-dependent freezing tolerance was inhibited by extracellular addition of an antibody against the cytosolic region of SYT1, a homolog of synaptotagmin known to be a calcium sensor that initiates exocytosis. This inhibition indicates that the puncture allowing the antibody to flow into the cytoplasm occurs during freeze/thawing. Thus, we propose that calcium-dependent freezing tolerance results from resealing of the punctured site. Protoplasts or leaf sections isolated from Arabidopsis SYT1-RNA interference (RNAi) plants lost calcium-dependent freezing tolerance, and intact SYT1-RNAi plants had lower freezing tolerance than control plants. Taken together, these findings suggest that calcium-dependent freezing tolerance results from membrane resealing and that this mechanism involves SYT1 function.

Neuraminidase 1 is a negative regulator of lysosomal exocytosis

Lysosomal exocytosis is a Ca2+-regulated mechanism that involves proteins responsible for cytoskeletal attachment and fusion of lysosomes with the plasma membrane. However, whether luminal lysosomal enzymes contribute to this process remains unknown. Here we show that neuraminidase NEU1 negatively regulates lysosomal exocytosis in hematopoietic cells by processing the sialic acids on the lysosomal membrane protein LAMP-1. In macrophages from NEU1-deficient mice, a model of the disease sialidosis, and in patients' fibroblasts, oversialylated LAMP-1 enhances lysosomal exocytosis. Silencing of LAMP-1 reverts this phenotype by interfering with the docking of lysosomes at the plasma membrane. In neu1-/- mice the excessive exocytosis of serine proteases in the bone niche leads to inactivation of extracellular serpins, premature degradation of VCAM-1, and loss of bone marrow retention. Our findings uncover an unexpected mechanism influencing lysosomal exocytosis and argue that exacerbations of this process form the basis for certain genetic diseases.

Cell wounding and repair in ventilator injured lungs

Acute lung injury (ALI) is a common, frequently hospital-acquired condition with a high morbidity and mortality. The stress associated with invasive mechanical ventilation represents a potentially harmful exposure, and attempts to minimize deforming stress through low tidal ventilation have proven efficacious. Lung cells are both sensors and transducers of deforming stress, and are frequently wounded in the setting of mechanical ventilation. Cell wounding may be one of the drivers of the innate immunologic and systemic inflammatory response associated with mechanical ventilation. These downstream effects of mechanotransduction have been referred to collectively as "Biotrauma". Our review will focus on cellular stress failure, that is cell wounding, and the mechanisms mediating subsequent plasma membrane repair, we hold that a better mechanistic understanding of cell plasticity, deformation associated remodeling and repair will reveal candidate approaches for lung protective interventions in mechanically ventilated patients. We will detail one such intervention, lung conditioning with hypertonic solutions as an example of ongoing research in this arena.

Ibuprofen reduces Abeta, hyperphosphorylated tau and memory deficits in Alzheimer mice

We examined the effects of ibuprofen on cognitive deficits, Abeta and tau accumulation in young triple transgenic (3xTg-AD) mice. 3xTg-AD mice were fed ibuprofen-supplemented chow between 1 and 6 months. Untreated 3xTg-AD mice showed significant impairment in the ability to learn the Morris water maze (MWM) task compared to age-matched wild-type (WT) mice. The performance of 3xTg-AD mice was significantly improved with ibuprofen treatment compared to untreated 3xTg-AD mice. Ibuprofen-treated transgenic mice showed a significant decrease in intraneuronal oligomeric Abeta and hyperphosphorylated tau (AT8) immunoreactivity in the hippocampus. Confocal microscopy demonstrated co-localization of conformationally altered (MC1) and early phosphorylated tau (CP-13) with oligomeric Abeta, and less co-localization of oligomeric Abeta and later forms of phosphorylated tau (AT8 and PHF-1) in untreated 3xTg-AD mice. Our findings show that prophylactic treatment of young 3xTg-AD mice with ibuprofen reduces intraneuronal oligomeric Abeta, reduces cognitive deficits, and prevents hyperphosphorylated tau immunoreactivity. These findings provide further support for intraneuronal Abeta as a cause of cognitive impairment, and suggest that pathological alterations of tau are associated with intraneuronal oligomeric Abeta accumulation.

Identification of Secretory Granule Phosphatidylinositol 4,5-Bisphosphate-interacting Proteins Using an Affinity Pulldown Strategy

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) synthesis is required for calcium-dependent exocytosis in neurosecretory cells. We developed a PtdIns(4,5)P2 bead pulldown strategy combined with subcellular fractionation to identify endogenous chromaffin granule proteins that interact with PtdIns(4,5)P2. We identified two synaptotagmin isoforms, synaptotagmins 1 and 7; spectrin; alpha-adaptin; and synaptotagmin-like protein 4 (granuphilin) by mass spectrometry and Western blotting. The interaction between synaptotagmin 7 and PtdIns(4,5)P2 and its functional relevance was investigated. The 45-kDa isoform of synaptotagmin 7 was found to be highly expressed in adrenal chromaffin cells compared with PC12 cells and to mainly localize to secretory granules by subcellular fractionation, immunoisolation, and immunocytochemistry. We demonstrated that synaptotagmin 7 binds PtdIns(4,5)P2 via the C2B domain in the absence of calcium and via both the C2A and C2B domains in the presence of calcium. We mutated the polylysine stretch in synaptotagmin 7 C2B and demonstrated that this mutant domain lacks the calcium-independent PtdIns(4,5)P2 binding. Synaptotagmin 7 C2B domain inhibited catecholamine release from digitonin-permeabilized chromaffin cells, and this inhibition was abrogated with the C2B polylysine mutant. These data indicate that synaptotagmin 7 C2B-effector interactions, which occur via the polylysine stretch, including calcium-independent PtdIns(4,5)P2 binding, are important for chromaffin granule exocytosis.

Rehabilitation and the single cell

Cellular damage triggers rapid resealing of the plasma membrane and repair of the cortical cytoskeleton. Plasma membrane resealing results from calcium-dependent fusion of membranous organelles and the plasma membrane at the site of the damage. Cortical cytoskeletal repair results from local assembly of actin filaments (F-actin), myosin-2 and microtubules into an array that closes around the original wound site. Control of the cytoskeletal response is exerted by local activation of the small GTPases, Rho and Cdc42. Recent work has given insight into both the membrane fusion and cytoskeletal responses to plasma membrane damage and we propose that Rho GTPase activation results at least in part from the events that drive membrane repair.

Patients with a non-dysferlin Miyoshi myopathy have a novel membrane repair defect

Two autosomal recessive muscle diseases, limb girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy (MM), are caused by mutations in the dysferlin gene. These mutations result in poor ability to repair cell membrane damage, which is suggested to be the cause for this disease. However, many patients who share clinical features with MM-type muscular dystrophy do not carry mutations in dysferlin gene. To understand the basis of MM that is not due to mutations in dysferlin gene, we analyzed cells from patients in one such family. In these patients, we found no defects in several potential candidates - annexin A2, caveolin-3, myoferlin and the MMD2 locus on chromosome 10p. Similar to dysferlinopathy, these cells also exhibit membrane repair defects and the severity of the defect correlated with severity of their disease. However, unlike dysferlinopathy, none of the conventional membrane repair pathways are defective in these patient cells. These results add to the existing evidence that cell membrane repair defect may be responsible for MM-type muscular dystrophy and indicate that a previously unsuspected genetic lesion that affects cell membrane repair pathway is responsible for the disease in the non-dysferlin MM patients.

Tau-dependent microtubule disassembly initiated by prefibrillar beta-amyloid

Alzheimer's Disease (AD) is defined histopathologically by extracellular β-amyloid (Aβ) fibrils plus intraneuronal tau filaments. Studies of transgenic mice and cultured cells indicate that AD is caused by a pathological cascade in which Aβ lies upstream of tau, but the steps that connect Aβ to tau have remained undefined. We demonstrate that tau confers acute hypersensitivity of microtubules to prefibrillar, extracellular Aβ in nonneuronal cells that express transfected tau and in cultured neurons that express endogenous tau. Prefibrillar Aβ42 was active at submicromolar concentrations, several-fold below those required for equivalent effects of prefibrillar Aβ40, and microtubules were insensitive to fibrillar Aβ. The active region of tau was localized to an N-terminal domain that does not bind microtubules and is not part of the region of tau that assembles into filaments. These results suggest that a seminal cell biological event in AD pathogenesis is acute, tau-dependent loss of microtubule integrity caused by exposure of neurons to readily diffusible Aβ.

The mechanisms of cell membrane repair: A tutorial guide to key experiments

The best way to approach a new area is to study closely a sample of the key papers, and spread out from there. In this tutorial paper I present my personal selection of papers introducing concepts in the study of the mechanisms of cell membrane repair. For a more comprehensive review up to 2003, I refer the student to McNeil and Steinhardt (2003).

Rapid increase in plasma membrane chloride permeability during wound resealing in starfish oocytes

Plasma membrane wound repair is an important but poorly understood process. We used femtosecond pulses from a Ti-Sapphire laser to make multiphoton excitation–induced disruptions of the plasma membrane while monitoring the membrane potential and resistance. We observed two types of wounds that depolarized the plasma membrane. At threshold light levels, the membrane potential and resistance returned to prewound values within seconds; these wounds were not easily observed by light microscopy and resealed in the absence of extracellular Ca²⁺. Higher light intensities create wounds that are easily visible by light microscopy and require extracellular Ca²⁺ to reseal. Within a few seconds the membrane resistance is ∼100-fold lower, while the membrane potential has depolarized from −80 to −30 mV and is now sensitive to the Cl⁻ concentration but not to that of Na⁺, K⁺, or H⁺. We suggest that the chloride sensitivity of the membrane potential, after wound resealing, is due to the fusion of chloride-permeable intracellular membranes with the plasma membrane.

There's more to life than neurotransmission: the regulation of exocytosis by synaptotagmin VII

Among the 16 known vertebrate synaptotagmins, only Syt I, IV and VII are also present in C. elegans and Drosophila, suggesting that these isoforms play especially important roles in vivo. Extensive evidence indicates that Syt I is a synaptic vesicle Ca(2+) sensor essential for rapid neurotransmitter release. It has been suggested that the ubiquitously expressed Syt VII also regulates synaptic vesicle exocytosis, despite its presence in several tissues in addition to the brain. Here, we discuss recent genetic and biochemical evidence that does not support this view. Syt VII null mutants do not have a neurological phenotype, and the protein is found on the membrane of lysosomes and some non-synaptic secretory granules, where it regulates Ca(2+)-triggered exocytosis and plasma membrane repair.

An emergency response team for membrane repair

On demand, rapid Ca(2+)-triggered homotypic and exocytic membrane-fusion events are required to repair a torn plasma membrane, and we propose that this emergency-based fusion differs fundamentally from other rapid, triggered fusion reactions. Emergency fusion might use a specialized protein and organelle emergency response team that can simultaneously promote impromptu homotypic fusion events between organelles and exocytic fusion events along the vertices between these fusion products and the plasma membrane.

Membrane resealing: synaptotagmin VII keeps running the show

Ca2+ influx, an immediate consequence of plasma membrane disruption, triggers a resealing mechanism involving exocytosis. Although this has been known for about a decade, a better understanding of the organelles involved and of the molecular machinery controlling membrane repair has been slower to emerge. Recent studies have changed this picture, by identifying lysosomes as exocytotic vesicles involved in membrane resealing and the Ca2+-binding protein synaptotagmin VII as a regulator of this process. New evidence reinforces the role of the C2A and C2B domains of synaptotagmin VII in plasma membrane repair, highlighting the importance of this molecule as a powerful tool for future studies.