Patching plasma membrane disruptions with cytoplasmic membrane

MG53 nucleates assembly of cell membrane repair machinery

Dynamic membrane repair and remodelling is an elemental process that maintains cell integrity and mediates efficient cellular function. Here we report that MG53, a muscle-specific tripartite motif family protein (TRIM72), is a component of the sarcolemmal membrane-repair machinery. MG53 interacts with phosphatidylserine to associate with intracellular vesicles that traffic to and fuse with sarcolemmal membranes. Mice null for MG53 show progressive myopathy and reduced exercise capability, associated with defective membrane-repair capacity. Injury of the sarcolemmal membrane leads to entry of the extracellular oxidative environment and MG53 oligomerization, resulting in recruitment of MG53-containing vesicles to the injury site. After vesicle translocation, entry of extracellular Ca(2+) facilitates vesicle fusion to reseal the membrane. Our data indicate that intracellular vesicle translocation and Ca(2+)-dependent membrane fusion are distinct steps involved in the repair of membrane damage and that MG53 may initiate the assembly of the membrane repair machinery in an oxidation-dependent manner.

Two-way traffic on the road to plasma membrane repair

Ca(2+) influx through plasma membrane wounds triggers a rapid-repair response that is essential for cell survival. Earlier studies showed that repair requires the exocytosis of intracellular vesicles. Exocytosis was thought to promote resealing by 'patching' the plasma membrane lesion or by facilitating bilayer restoration through reduction in membrane tension. However, cells also rapidly repair lesions created by pore-forming proteins, a form of injury that cannot be resealed solely by exocytosis. Recent studies indicate that, in cells injured by pores or mechanical abrasions, exocytosis is followed by lesion removal through endocytosis. Describing the relationship between wound-induced exocytosis and endocytosis has implications for the understanding of muscular degenerative diseases that are associated with defects in plasma membrane repair.

Modeling transmembrane transport through cell membrane wounds created by acoustic cavitation

Cells exposed to acoustic cavitation and other mechanical stresses can be transiently permeabilized to permit intracellular uptake of molecules, including drugs, proteins, and genes. Microscopic imaging and other studies suggest that intracellular loading occurs through plasma membrane wounds of submicrometer radius that reseal over time through the aggregation and fusion of lipid vesicles trafficked to the wound site. The goal of this study was to 1), determine the size of membrane wounds as a function of time after in vitro sonication of DU145 prostate cancer cells under conditions that caused extensive acoustic cavitation; and 2), theoretically model transport processes leading to intracellular loading. Our overall hypothesis was that intracellular loading is governed by passive diffusion through porous membrane wounds of up to 300-nm radius containing pores that permit entry of molecules up to at least 28-nm radius over a timescale of minutes. Experimental measurements showed intracellular loading of molecules with radii from 0.6 to 28 nm, where most loading occurred after sonication over a timescale up to minutes and where smaller molecules were taken up to a greater extent and over a longer timescale than larger molecules. Theoretical modeling predicted that membrane wounds would have a 300-nm radius initially and then would shrink, with a half life of 20 to 50 s. Uptake was shown to occur predominantly by diffusion and the increasing levels of uptake with decreasing molecular size was explained primarily by differences in molecular diffusivity and, for the largest molecule, geometrical hindrance within the wound. Mathematical modeling was simplified, because transport through porous wounds of possibly complex internal nanostructure was governed largely by transport at the edge of the wound, and depended only weakly on the size, number, and distribution of nanopores within the wound under the conditions relevant to this study. Overall, this study developed a theoretical framework for analysis of transmembrane transport through cell membrane wounds and thereby provided quantitative estimates of their size and lifetime.

Repair of injured plasma membrane by rapid Ca2+-dependent endocytosis

Ca²⁺ influx through plasma membrane lesions triggers a rapid repair process that was previously shown to require the exocytosis of lysosomal organelles (Reddy, A., E. Caler, and N. Andrews. 2001. Cell. 106:157–169). However, how exocytosis leads to membrane resealing has remained obscure, particularly for stable lesions caused by pore-forming proteins. In this study, we show that Ca²⁺-dependent resealing after permeabilization with the bacterial toxin streptolysin O (SLO) requires endocytosis via a novel pathway that removes SLO-containing pores from the plasma membrane. We also find that endocytosis is similarly required to repair lesions formed in mechanically wounded cells. Inhibition of lesion endocytosis (by sterol depletion) inhibits repair, whereas enhancement of endocytosis through disruption of the actin cytoskeleton facilitates resealing. Thus, endocytosis promotes wound resealing by removing lesions from the plasma membrane. These findings provide an important new insight into how cells protect themselves not only from mechanical injury but also from microbial toxins and pore-forming proteins produced by the immune system.

Biochemical analysis of the interaction of calcium with toposome: A major protein component of the sea urchin egg and embryo

We have investigated the biochemical and functional properties of toposome, a major protein component of sea urchin eggs and embryos. Atomic force microscopy was utilized to demonstrate that a Ca(2+)-driven change in secondary structure facilitated toposome binding to a lipid bilayer. Thermal denaturation studies showed that toposome was dependent upon calcium in a manner paralleling the effect of this cation on secondary and tertiary structure. The calcium-induced, secondary, and tertiary structural changes had no effect on the chymotryptic cleavage pattern. However, the digestion pattern of toposome bound to phosphatidyl serine liposomes did vary as a function of calcium concentration. We also investigated the interaction of this protein with various metal ions. Calcium, Mg(2+), Ba(2+), Cd(2+), Mn(2+), and Fe(3+) all bound to toposome. In addition, Cd(2+) and Mn(2+) displaced Ca(2+), prebound to toposome, while Mg(2+), Ba(2+), and Fe(3+) had no effect. Collectively, these results further enhance our understanding of the role of Ca(2+) in modulating the biological activity of toposome.

Acinar cell membrane disruption is an early event in experimental acute pancreatitis in rats

OBJECTIVE

To test the hypothesis that disruption of acinar cell membranes is the earliest event that takes place after the onset of acute pancreatitis.

METHODS

Cerulein and taurocholate pancreatitis were induced in rats. Furthermore, stimulation with different doses of bombesin, pilocarpine, and cerulein was performed. Five to 180 minutes after initiation of treatment, animals were killed. Disruption of cell membranes was detected by the penetration of the experimental animal's own albumin or immunoglobulin G (IgG) into acinar cells by immunocytological localization. Tissue was further analyzed by electron microscopy and electron microscopic immunostaining.

RESULTS

Animals with pancreatitis displayed significantly greater antialbumin and anti-IgG immunostaining in the cytoplasm of acinar cells and in vacuoles in comparison with controls, confirming membrane disruption. This was not detectable after stimulation with bombesin, pilocarpine, and nonsupramaximal doses of cerulein. The first changes were seen after 5 minutes of induction of pancreatitis. Results were verified by electron microscopy and electron microscopic immunohistochemistry.

CONCLUSIONS

The penetration of albumin and IgG into acinar cells indicates that wounding of their plasma membrane occurs at the onset of acute pancreatitis. Disruption of the membranes could be expected to allow the influx of calcium ions, causing massive intracellular alterations, and exit of molecules, such as enzymes from acinar cells.

Life and times of a cellular bleb

Blebs are spherical cellular protrusions that occur in many physiological situations. Two distinct phases make up the life of a bleb, each of which have their own biology and physics: expansion, which lasts approximately 30 s, and retraction, which lasts approximately 2 min. We investigate these phases using optical microscopy and simple theoretical concepts, seeking information on blebbing itself, and on cytomechanics in general. We show that bleb nucleation depends on pressure, membrane-cortex adhesion energy, and membrane tension, and test this experimentally. Bleb growth occurs through a combination of bulk flow of lipids and delamination from the cell cortex via the formation and propagation of tears. In extreme cases, this can give rise to a traveling wave around the cell periphery, known as "circus movement." When growth stalls, an actin cortex reforms under the bleb membrane, and retraction starts, driven by myosin-II. Using flicker spectroscopy, we find that retracting blebs are fivefold more rigid than expanding blebs, an increase entirely explained by the properties of the newly formed cortical actin mesh. Finally, using artificially nucleated blebs as pressure sensors, we show that cells rounded up in mitosis possess a substantial intracellular pressure.

Dysferlin and muscle membrane repair

The ability to repair membrane damage is conserved across eukaryotic cells and is necessary for the cells to survive a variety of physiological and pathological membrane disruptions. Membrane repair is mediated by rapid Ca(2+)-triggered exocytosis of various intracellular vesicles, such as lysosomes and enlargeosomes, which lead to the formation of a membrane patch that reseals the membrane lesion. Recent findings suggest a crucial role for dysferlin in this repair process in muscle, possibly as a Ca(2+) sensor that triggers vesicle fusion. The importance of membrane repair is highlighted by the genetic disease, dysferlinopathy, in which the primary defect is the loss of Ca(2+)-regulated membrane repair due to dysferlin deficiency. Future research on dysferlin and its interacting partners will enhance the understanding of this important process and provide novel avenues to potential therapies.

Lysosome repair enables host cell survival and bacterial persistence following Chlamydia trachomatis infection

Chlamydiae are obligate intracellular bacteria that replicate within the confines of a membrane-bound vacuole termed the inclusion. The final event in the infectious process is the disruption of the inclusion membrane and release of a multitude of infectious elementary bodies, each capable of eliciting a new infection. Strains of the trachoma biovar of Chlamydia trachomatis are released from the host cell without concomitant host cell death. In this study, analysis of events associated with chlamydial egress revealed that the integrity of the host cell plasma membrane was compromised prior to the inclusion membrane. This disruption was accompanied by the appearance of LAMP-1 at the infected cell surface, implicating lysosome repair of plasma membrane lesions in response to infection. Analysis of the effects of calcium chelators and actin stabilizing agents, indicated calcium-induced actin depolymerization as a requisite to lysosome-plasma membrane fusion and host cell survival. A consequence of this lysosome-mediated repair process, was the retention of residual bacteria within the surviving host cell, providing a unique mechanism for intracellular persistence of C. trachomatis.

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.

Calcium-regulated fusion of yolk granules is important for yolk degradation during early embryogenesis of Rhodnius prolixusStahl

This study examined the process of membrane fusion of yolk granules (YGs)during early embryogenesis of Rhodnius prolixus. We show that eggs collected at days 0 and 3 after oviposition contain different populations of YGs, for example day-3 eggs are enriched in large YGs (LYGs). Day-3 eggs also contain the highest free [Ca2+] during early embryogenesis of this insect. In vitro incubations of day-0 YGs with [Ca2+]similar to those found in day-3 eggs resulted in the formation of LYGs, as observed in vivo. Fractionation of LYGs and small YGs (SYGs) and their subsequent incubation with the fluorescent membrane marker PKH67 showed a calcium-dependent transference of fluorescence from SYGs to LYGs, possibly as the result of membrane fusion. Acid phosphatase and H+-PPase activities were remarkably increased in day-3 LYGs and in calcium-treated day-0 LYGs. Both fractions were found to contain vitellins as major components, and incubation of YGs with calcium induced yolk proteolysis in vitro. Altogether, our results suggest that calcium-induced membrane fusion events take part in yolk degradation, leading to the assembly of the yolk mobilization machinery.

Calcium signaling in neuronal motility

Neuronal motility is a fundamental feature that underlies the development, regeneration, and plasticity of the nervous system. Two major developmental events--directed migration of neuronal precursor cells to the proper positions and guided elongation of axons to their target cells--depend on large-scale neuronal motility. At a finer scale, motility is also manifested in many aspects of neuronal structures and functions, ranging from differentiation and refinement of axonal and dendritic morphology during development to synapse remodeling associated with learning and memory in the adult brain. As a primary second messenger that conveys the cytoplasmic actions of electrical activity and many neuroactive ligands, Ca(2+) plays a central role in the regulation of neuronal motility. Recent studies have revealed common Ca(2+)-dependent signaling pathways that are deployed for regulating cytoskeletal dynamics associated with neuronal migration, axon and dendrite development and regeneration, and synaptic plasticity.

Interaction of toposome from sea-urchin yolk granules with dimyristoyl phosphatidylserine model membranes: a 2H-NMR study

The yolk granule is the most abundant membrane-bound organelle present in sea urchin eggs and embryos. The major protein component of this organelle, toposome, accounts for approximately 50% of the total yolk protein and has been shown to be localized to the embryonic cell surface. Extensive characterization in several laboratories has defined a role for toposome in mediating membrane-membrane interactions. The current study expands the analysis of toposome-membrane interaction by defining toposome-induced changes to the lipid bilayer. The effect of toposome on the biophysical properties of phosphatidyl serine (PS) multibilayers was investigated using deuterium nuclear magnetic resonance and perdeuterated dimyristoyl PS (DMPS-d(54)). Toposome was found to have little effect on DMPS-d(54) chain orientational order in both the gel and liquid-crystalline phases. Timescales for DMPS-d(54) reorientation were investigated using quadropole echo decay. Echo decay times were sensitive to toposome in the liquid-crystalline phase but not in the gel phase. Additional information about the perturbation of bilayer motions by toposome was obtained by analyzing its effect on the decay of Carr-Purcell-Meiboom-Gill echo trains. Collectively, these results suggest that toposome interacts peripherally with DMPS bilayers and that it increases the amplitude of lipid reorientation, possibly through local enhancement of bilayer curvature.

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).

Requirement for annexin A1 in plasma membrane repair

Ca2+ entering a cell through a torn or disrupted plasma membrane rapidly triggers a combination of homotypic and exocytotic membrane fusion events. These events serve to erect a reparative membrane patch and then anneal it to the defect site. Annexin A1 is a cytosolic protein that, when activated by micromolar Ca2+, binds to membrane phospholipids, promoting membrane aggregation and fusion. We demonstrate here that an annexin A1 function-blocking antibody, a small peptide competitor, and a dominant-negative annexin A1 mutant protein incapable of Ca2+ binding all inhibit resealing. Moreover, we show that, coincident with a resealing event, annexin A1 becomes concentrated at disruption sites. We propose that Ca2+ entering through a disruption locally induces annexin A1 binding to membranes, initiating emergency fusion events whenever and wherever required.

Biochemical analysis of a Ca2+-dependent membrane–membrane interaction mediated by the sea urchin yolk granule protein, toposome

Toposome, a high molecular mass protein, is an abundant component of the yolk granule in the sea urchin egg and embryo. Toposome is composed of a 160 kDa polypeptide that is proteolytically processed into smaller species of 120 and 90 kDa during embryonic development. The exact biological function of toposome during early development is unknown. In this study we have examined calcium binding to toposome and the effect of this binding on the secondary and tertiary structural characteristics of the purified protein. Initially, we used equilibrium dialysis to quantify calcium binding to toposome. Monophasic binding of up to 600 M of calcium per mole of protein was detected with an intrinsic dissociation constant (calcium) of 240 microm. Increasing concentrations of calcium resulted in an increase in alpha helical content from 3.0 to 22.0%, which occurred with an apparent dissociation constant (calcium) of 25 microm. In parallel experiments, toposome binding to liposomes required similar concentrations of calcium; an apparent dissociation constant (calcium) of 25 microm was recorded. Endogenous tryptophan fluorescence measurements, both in the presence and absence of liposomes, demonstrated that the tertiary structure is sensitive to increasing concentrations of calcium with an apparent dissociation constant (calcium) of 240 microm. Toposome-driven, liposome aggregation assays revealed a similar calcium requirement. Collectively, these results define a two-step model for calcium modulation of toposome structure and function.

Pharmacological characterization of NAADP-induced Ca2+ signals in starfish oocytes

The recently discovered second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) is central to the onset of intracellular Ca2+ signals induced by several stimuli, including fertilization. The nature of the Ca2+ pool mobilized by NAADP is still controversial. Depending on the cell type, NAADP may target either an acidic compartment with lysosomal properties or ryanodine receptors (RyRs) on endoplasmic reticulum. In addition, NAADP elicits a robust Ca2+ influx into starfish oocytes by activating a Ca2+-mediated current across the plasma membrane. In the present study, we employed the single-electrode intracellular recording technique to assess the involvement of either acidic organelles or RyRs in NAADP-elicited Ca2+ entry. We found that neither drugs which interfere with acidic compartments nor inhibitors of RyRs affected NAADP-induced depolarization. These data further support the hypothesis that a yet unidentified plasma membrane Ca2+ channel is the target of NAADP in starfish oocytes.

Raising the antioxidant levels within mouse muscle fibres does not affect contraction-induced injury

A protocol of 75 lengthening contractions (LCP) administered to skeletal muscles of mice causes an initial force deficit owing to the mechanical disruption of sarcomeres and a reduction in calcium release from the sarcoplasmic reticulum. During the 3 days following the LCP, a 'sealing off process' and inflammatory response occurs. The reactive oxygen species (ROS) released by invading inflammatory cells produce a secondary force deficit that is more severe than the initial deficit. The timing of the infiltration of inflammatory cells and increase in force deficit relative to the sealing off process is not well documented. We tested the null hypothesis that following a lifetime of overexpression of the genes for the intracellular antioxidants manganese superoxide dismutase, copper zinc superoxide dismutase or catalase in transgenic mice, the force deficits 3 days following the administration of a 75 LCP to in situ extensor digitorum longus muscles are not different from those of wild-type mice. Following the LCP, the force deficits ranged from 39 to 59% for the muscles of transgenic mice that overexpressed the genes for intracellular antioxidants and were not different from the force deficit of 44% observed for muscles of wild-type mice. The results provide evidence that the ROS damage does not occur within the cytosol of the injured fibres. Apparently, the hypercontraction of sarcomeres and accumulation of vesicles seal off and protect the intact portions of damaged fibres, such that the ROS damage and repair occurs in the milieu of the necrotic segments that are continuous with the extracellular matrix.

Wound-induced contractile ring: a model for cytokinesis

The actomyosin-based contractile ring is required for several biological processes, such as wound healing and cytokinesis of animal cells. Despite progress in defining the roles of this structure in both wound closure and cell division, we still do not fully understand how an actomyosin ring is spatially and temporally assembled, nor do we understand the molecular mechanism of its contraction. Recent results have demonstrated that microtubule-dependent local assembly of F-actin and myosin-II is present in wound closure and is similar to that in cytokinesis in animal cells. Furthermore, signalling factors such as small Rho GTPases have been shown to be involved in the regulation of actin dynamics during both processes. In this review we address recent findings in an attempt to better understand the dynamics of actomyosin contractile rings during wound healing as compared with the final step of animal cell division.

Calcium-regulated fusion of yolk granules during early embryogenesis ofPeriplaneta americana

This work reported membrane fusion of yolk granules (YGs) during early embryogenesis of the insect Periplaneta americana (P. americana). We showed that eggs from Day 5 of embryogenesis possess a greater amount of enlarged YGs in comparison with Day 1. Day 5 is also the period when the largest amount of free calcium is found (approximately 17 mM) within the oothecae from early embryogenesis. Treatment of Day 1-YGs fraction with 17 mM Ca2+ resulted in a YG size pattern very similar to the one observed in Day 5 eggs, where enlarged YGs were formed. YG membrane fusion was observed by fluorescent membrane dye transfer from previously labeled small YGs to larger ones and was also visualized by electron microscopy. We also showed that the small "in fusion" YGs seemed to be acidic, suggesting that acidification is correlated with YG membrane fusion. Hence, it was shown that YGs are capable of membrane fusion in a calcium-dependent manner and this process probably occurs in vivo during early embryogenesis of P. americana.

Multimodal Strategies for Resuscitating Injured Cells

Our cells and tissues are challenged constantly by exposure to extreme conditions that cause acute and chronic stress. Wounding at the cellular level is a common event, and results from cell exposure to supra-physiologic forces, or is the consequence of action by reactive chemical agents. An individual cellular wound results from either the alteration of protein or DNA structure, or the disruption of molecular assemblies, the most important of which is the cell's membranes. Tissue healing at the macroscopic level is a complex and coordinated process involving many different cell types while, in contrast, the wounds of individual cells heal primarily via biomolecular interactions. Like tissue wound healing, cellular wound healing involves the upregulation or acceleration of processes that are constitutively expressed in routine physiologic repair of cellular structures In addition, recent advances have been made in the identification of pharmaceutical strategies to aid the cellular repair response. Many of these strategies offer promise for augmenting the already present cellular repair mechanisms.

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.

Yolk granule tethering: a role in cell resealing and identification of several protein components

Homotypic fusion among echinoderm egg yolk granules has previously been reconstituted in vitro, and shown to be a rapid, Ca2+-triggered reaction that can produce extremely large (>10 μm diameter) fusion products. We here show that, prior to Ca2+-triggered fusion, yolk granules in vitro, if isolated in an appropriate buffer, became tethered to one another, forming large aggregates of more than 100 granules. Granule washing with mildly chaotropic salt abolished this tethering reaction, and prevented Ca2+-triggered formation of the large fusion products characteristic of tethered granules. Protein factors present in the wash restored tethering activity and these factors could be substantially enriched by anion exchange chromatography. The enriched fraction behaved under native conditions as a high molecular weight (∼670 kDa), multisubunit complex of at least seven proteins. Monoclonal antibodies directed against this complex of proteins were capable of immunodepleting tethering activity, confirming the role of the complex in granule tethering. These antibodies selectively stained the surface of yolk granules in the intact egg. We therefore propose a new role for tethering: it can promote the formation of large vesicular fusion products, such as those required for successful resealing. We have, moreover, identified several proteins that may be critical to this tethering mechanism.

Mechanisms of stretch-induced muscle damage in normal and dystrophic muscle: role of ionic changes

Muscle damage, characterized by prolonged weakness and delayed onset of stiffness and soreness, is common following contractions in which the muscles are stretched. Stretch-induced damage of this sort is more pronounced in the muscular dystrophies and the profound muscle damage observed in these conditions may involve similar pathways. It has been known for many years that damaged muscles accumulate calcium and that elevating calcium in normal muscles simulates many aspects of muscle damage. The changes in intracellular calcium, sodium and pH following stretched contractions are reviewed and the various pathways which have been proposed to allow ion entry are discussed. One possibility is that TRPC1 (transient receptor potential, canonical), a protein which seems to form both a stretch-activated channel and a store-operated channel, is the main source of Ca(2+) entry. The mechanisms by which the changes in intracellular ions contribute to reduced force production, to increased protein breakdown and to increased membrane permeability are considered. A hypothetical scheme for muscle damage which incorporates these ideas is presented.

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.

Cellular stress failure in ventilator-injured lungs

The clinical and experimental literature has unequivocally established that mechanical ventilation with large tidal volumes is injurious to the lung. However, uncertainty about the micromechanics of injured lungs and the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. In this review we focus on experimental evidence for lung cells as injury targets and the relevance of these studies for human ventilator-associated lung injury. In vitro, the stress-induced mechanical interactions between matrix and adherent cells are important for cellular remodeling as a means for preventing compromise of cell structure and ultimately cell injury or death. In vivo, these same principles apply. Large tidal volume mechanical ventilation results in physical breaks in alveolar epithelial and endothelial plasma membrane integrity and subsequent triggering of proinflammatory signaling cascades resulting in the cytokine milieu and pathologic and physiologic findings of ventilator-associated lung injury. Importantly, though, alveolar cells possess cellular repair and remodeling mechanisms that in addition to protecting the stressed cell provide potential molecular targets for the prevention and treatment of ventilator-associated lung injury in the future.

Hypercapnic acidosis impairs plasma membrane wound resealing in ventilator-injured lungs

The objective of this study was to assess the effects of hypercapnic acidosis on lung cell injury and repair by confocal microscopy in a model of ventilator-induced lung injury. Three groups of normocapnic, hypocapnic, and hypercapnic rat lungs were perfused ex vivo, either during or after injurious ventilation, with a solution containing the membrane-impermeant label propidium iodide. In lungs labeled during injurious ventilation, propidium iodide fluorescence identifies all cells with plasma membrane wounds, both permanent and transient, whereas in lungs labeled after injurious ventilation propidium iodide fluorescence identifies only cells with permanent plasma membrane wounds. Hypercapnia minimized the adverse effects of high-volume ventilation on vascular barrier function, whereas hypocapnia had the opposite effect. Despite CO2-dependent differences in lung mechanics and edema the number of injured subpleural cells per alveolus was similar in the three groups (0.48 +/- 0.34 versus 0.51 +/- 0.19 versus 0.43 +/- 0.20 for hypocapnia, normocapnia, and hypercapnia, respectively). However, compared with normocapnia the probability of wound repair was significantly reduced in hypercapnic lungs (63 versus 38%; p < 0.02). This finding was subsequently confirmed in alveolar epithelial cell scratch models. The potential relevance of these observations for lung inflammation and remodeling after mechanical injury is discussed.

Estimating the sensitivity of mechanosensitive ion channels to membrane strain and tension

Bone adapts to its environment by a process in which osteoblasts and osteocytes sense applied mechanical strain. One possible pathway for the detection of strain involves mechanosensitive channels and we sought to determine their sensitivity to membrane strain and tension. We used a combination of experimental and computational modeling techniques to gain new insights into cell mechanics and the regulation of mechanosensitive channels. Using patch-clamp electrophysiology combined with video microscopy, we recorded simultaneously the evolution of membrane extensions into the micropipette, applied pressure, and membrane currents. Nonselective mechanosensitive cation channels with a conductance of 15 pS were observed. Bleb aspiration into the micropipette was simulated using finite element models incorporating the cytoplasm, the actin cortex, the plasma membrane, cellular stiffening in response to strain, and adhesion between the membrane and the micropipette. Using this model, we examine the relative importance of the different cellular components in resisting suction into the pipette and estimate the membrane strains and tensions needed to open mechanosensitive channels. Radial membrane strains of 800% and tensions of 5 10(-4) N.m(-1) were needed to open 50% of mechanosensitive channels. We discuss the relevance of these results in the understanding of cellular reactions to mechanical strain and bone physiology.

Dysferlin and the plasma membrane repair in muscular dystrophy

Muscular dystrophy covers a group of genetically determined disorders that cause progressive weakness and wasting of the skeletal muscles. Dysferlin was identified as a gene mutated in limb-girdle muscular dystrophy (type 2B) and Miyoshi myopathy. The discovery of dysferlin revealed a new family of proteins, known as the ferlin family, which includes four different genes. Recent work suggests the function of dysferlin in membrane repair and demonstrates that defective membrane repair is a novel mechanism of muscle degeneration. These findings reveal the importance of a basic cellular function in skeletal muscle and a new class of muscular dystrophy where the defect lies in the maintenance, not the structure, of the plasma membrane. Here, we discuss the current knowledge of dysferlin function in the repair of the plasma membrane of the skeletal muscle cells.

Functional role of a high mol mass protein complex in the sea urchin yolk granule

We have investigated the biochemical and functional characteristics of the major protein constituents of the yolk granule organelle present in sea urchin eggs and embryos. Compositional analysis, using sodium dodecyl sulfate polyacrylamide gel electrophoresis, revealed distinctly different polypeptide patterns under reducing and non-reducing conditions. In the presence of reducing agent, a 240 kDa species dissociated into polypeptides of apparent mol mass 160, 120 and 90 k. The relatedness of these polypeptides to the 240 kDa species was demonstrated in protein gel blot and peptide mapping analyses. The profile of yolk granule polypeptides was dynamic during embryonic development with the disappearance of the 160 kDa species and the coincidental appearance of lower mol mass polypeptides. However, the 240 kDa complex was detected even after the disappearance of the 160 kDa polypeptide. The 240 kDa complex was released from yolk granules in the absence of calcium and the purified species was shown to bind liposomes in a calcium-dependent manner. In addition, the 240 kDa complex possessed a calcium-dependent, liposome aggregating activity. The 240 kDa species could also induce the aggregation of yolk granules, previously denuded of the complex following treatment with either ethylenediaminetetraacetic acid or trypsin. Collectively, these results demonstrate the dynamic characteristics of the yolk granule 240 kDa protein complex and offer insights into a possible functional role.

Uptake and trafficking of DNA in keratinocytes: evidence for DNA-binding proteins

The skin is an interesting organ for human gene therapy due to accessibility, immunologic potential and synthesis capabilities. In this study, we attempted to visualize and measure the uptake of naked FITC-labeled plasmid by FACS analysis detecting up to 15% internalization in a dose- and time-dependent manner. Cycloheximide treatment inhibited the uptake by >90%, suggesting a protein-mediated uptake. The inhibition of different internalization pathways demonstrated that blocking macropinocytosis (by amiloride and N,N-dimethylamylorid) reduced DNA uptake by >85%, while the inhibition of clathrin-coated pits (by chlorpromazine) and caveolae (by nystatin/filipin III) did not limit the uptake. Colocalization studies using confocal laser microscopy revealed a time-dependent accumulation of plasmid DNA in endosomes and lysosomes. When a green fluorescent protein (GFP) expression vector was used, specific GFP-RNA became detectable by reverse transcriptase-PCR, whereas measurable amounts of protein could not be identified in FACS experiments. To detect the potential DNA receptors on the keratinocyte surface, membrane proteins were extracted and subjected to South-Western blotting using digoxigenin-labeled calf thymus and lambda-phage DNA. Two DNA-binding proteins, ezrin and moesin, known as plasma membrane-actin linkers, were identified by one- and two-dimensional-South-Western blots and matrix-assisted laser desorption and ionization-mass spectrometry. Ezrin and moesin are functionally associated with a number of transmembrane receptors such as the EGF, CD44 or ICAM-1 receptor. Taken together, naked plasmid DNA seems to enter human keratinocytes through different pathways, mainly by macropinocytosis. Two DNA-binding proteins were identified that seemed to be involved in binding/trafficking of internalized DNA.

Plasma Membrane Disruption: Repair, Prevention, Adaptation

Many metazoan cells inhabit mechanically stressful environments and, consequently, their plasma membranes are frequently disrupted. Survival requires that the cell rapidly repair or reseal the disruption. Rapid resealing is an active and complex structural modification that employs endomembrane as its primary building block, and cytoskeletal and membrane fusion proteins as its catalysts. Endomembrane is delivered to the damaged plasma membrane through exocytosis, a ubiquitous Ca2+-triggered response to disruption. Tissue and cell level architecture prevent disruptions from occurring, either by shielding cells from damaging levels of force, or, when this is not possible, by promoting safe force transmission through the plasma membrane via protein-based cables and linkages. Prevention of disruption also can be a dynamic cell or tissue level adaptation triggered when a damaging level of mechanical stress is imposed. Disease results from failure of either the preventive or resealing mechanisms.

Plasmalemmal sealing of transected mammalian neurites is a gradual process mediated by Ca2+-regulated proteins

Cultured mammalian PC12 or B104 cells do not instantaneously restore a plasmalemmal barrier (seal) after neurite transection, as measured using fluorescent dye probes of various sizes and saline solutions with different [Ca(2+)](o). Rather, transected cells gradually (from 15 to 60 min postseverance) exclude probes (dye molecules) of progressively smaller size. Furthermore, an inhibitor (calpeptin) of a Ca(2+)-activated cysteine protease (calpain) and antibodies or toxins to a Ca(2+)-regulated protein (synaptotagmin) and other membrane fusion proteins (syntaxin and synaptobrevin) inhibit plasmalemmal sealing. These data obtained using molecular probes on mammalian cell lines are consistent with previous data on invertebrate giant axons indicating that Ca(2+) plays many roles in the formation, accumulation, and fusion/interaction of vesicles gradually forming a seal at a site of plasmalemmal damage.

Mass spectrometric approach for identifying putative plasma membrane proteins of Arabidopsis leaves associated with cold acclimation

Although enhancement of freezing tolerance in plants during cold acclimation is closely associated with an increase in the cryostability of plasma membrane, the molecular mechanism for the increased cryostability of plasma membrane is still to be elucidated. In Arabidopsis, enhanced freezing tolerance was detectable after cold acclimation at 2 degrees C for as short as 1 day, and maximum freezing tolerance was attained after 1 week. To identify the plasma membrane proteins that change in quantity in response to cold acclimation, a highly purified plasma membrane fraction was isolated from leaves before and during cold acclimation, and the proteins in the fraction were separated with gel electrophoresis. We found that there were substantial changes in the protein profiles after as short as 1 day of cold acclimation. Subsequently, using matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS), we identified 38 proteins that changed in quantity during cold acclimation. The proteins that changed in quantity during the first day of cold acclimation include those that are associated with membrane repair by membrane fusion, protection of the membrane against osmotic stress, enhancement of CO2 fixation, and proteolysis.

Mechanical injury and repair of cells

OBJECTIVE

To concisely review the field of cell plasma membrane disruption (torn cell surface) and repair.

MAIN POINTS

Plasma membrane disruption is a common form of cell injury under physiologic conditions, after trauma, in certain muscular dystrophies, and during certain forms of clinical intervention. Rapid repair of a disruption is essential to cell survival and involves a complex and active cell response that includes membrane fusion and cytoskeletal activation. Tissues, such as cardiac and skeletal muscle, adapt to a disruption injury by hypertrophying. Cells adapt by increasing the efficiency of their resealing response.

CONCLUSION

Plasma membrane disruption is an important cellular event in both health and disease. The disruption repair mechanism is now well understood at the cellular level, but much remains to be learned at the molecular level. Cell and tissue level adaptational responses to the disruption either prevent its further occurrence or facilitate future repairs. Therapeutically useful drugs might result if, using this accumulating knowledge, chemical agents can be developed that can enhance repair or adaptive responses.

Defective membrane repair in dysferlin-deficient muscular dystrophy

Muscular dystrophy includes a diverse group of inherited muscle diseases characterized by wasting and weakness of skeletal muscle. Mutations in dysferlin are linked to two clinically distinct muscle diseases, limb-girdle muscular dystrophy type 2B and Miyoshi myopathy, but the mechanism that leads to muscle degeneration is unknown. Dysferlin is a homologue of the Caenorhabditis elegans fer-1 gene, which mediates vesicle fusion to the plasma membrane in spermatids. Here we show that dysferlin-null mice maintain a functional dystrophin-glycoprotein complex but nevertheless develop a progressive muscular dystrophy. In normal muscle, membrane patches enriched in dysferlin can be detected in response to sarcolemma injuries. In contrast, there are sub-sarcolemmal accumulations of vesicles in dysferlin-null muscle. Membrane repair assays with a two-photon laser-scanning microscope demonstrated that wild-type muscle fibres efficiently reseal their sarcolemma in the presence of Ca2+. Interestingly, dysferlin-deficient muscle fibres are defective in Ca2+-dependent sarcolemma resealing. Membrane repair is therefore an active process in skeletal muscle fibres, and dysferlin has an essential role in this process. Our findings show that disruption of the muscle membrane repair machinery is responsible for dysferlin-deficient muscle degeneration, and highlight the importance of this basic cellular mechanism of membrane resealing in human disease.

Controlled damage in thick specimens by multiphoton excitation

Controlled damage by light energy has been a valuable tool in studies of cell function. Here, we show that the Ti:Sapphire laser in a multiphoton microscope can be used to cause localized damage within unlabeled cells or tissues at greater depths than previously possible. We show that the damage is due to a multiphoton process and made wounds as small as 1 microm in diameter 20 microm from the surface. A characteristic fluorescent scar allows monitoring of the damage and identifies the wound site in later observations. We were able to lesion a single axon within a bundle of nerves, locally interrupt organelle transport within one axon, cut dendrites in a zebrafish embryo, ablate a mitotic pole in a sea urchin egg, and wound the plasma membrane and nuclear envelope in starfish oocytes. The starfish nucleus collapsed approximately 1 h after wounding, indicating that loss of compartmentation barrier makes the structure unstable; surprisingly, the oocyte still completed meiotic divisions when exposed to maturation hormone, indicating that the compartmentalization and translocation of cdk1 and its regulators is not required for this process. Multiphoton excitation provides a new means for producing controlled damage deep within tissues or living organisms.

The endomembrane requirement for cell surface repair

The capacity to reseal a plasma membrane disruption rapidly is required for cell survival in many physiological environments. Intracellular membrane (endomembrane) is thought to play a central role in the rapid resealing response. We here directly compare the resealing response of a cell that lacks endomembrane, the red blood cell, with that of several nucleated cells possessing an abundant endomembrane compartment. RBC membrane disruptions inflicted by a mode-locked Ti:sapphire laser, even those initially smaller than hemoglobin, failed to reseal rapidly. By contrast, much larger laser-induced disruptions made in sea urchin eggs, fibroblasts, and neurons exhibited rapid, Ca(2+)-dependent resealing. We conclude that rapid resealing is not mediated by simple physiochemical mechanisms; endomembrane is required.

Long-term potentiation of exocytosis and cell membrane repair in fibroblasts

We previously found that a microdisruption of the plasma membrane evokes Ca(2+)-regulated exocytosis near the wound site, which is essential for membrane resealing. We demonstrate herein that repeated membrane disruption reveals long-term potentiation of Ca(2+)-regulated exocytosis in 3T3 fibroblasts, which is closely correlated with faster membrane resealing rates. This potentiation of exocytosis is cAMP-dependent protein kinase A dependent in the early stages (minutes), in the intermediate term (hours) requires protein synthesis, and for long term (24 h) depends on the activation of cAMP response element-binding protein (CREB). We were able to demonstrate that wounding cells activated CREB within 3.5 h. In all three phases, the increase in the amount of exocytosis was correlated with an increase in the rate of membrane resealing. However, a brief treatment with forskolin, which is effective for short-term potentiation and which could also activate CREB, was not sufficient to induce long-term potentiation of resealing. These results imply that long-term potentiation by CREB required activation by another, cAMP-independent pathway.

NAADP mobilizes Ca(2+) from reserve granules, lysosome-related organelles, in sea urchin eggs

Nicotinic acid adenine dinucleotide phosphate (NAADP) mobilizes Ca(2+) in many cells and species. Unlike other Ca(2+)-mobilizing messengers, NAADP mobilizes Ca(2+) from an unknown store that is not the endoplasmic reticulum, the store traditionally associated with messenger-mediated Ca(2+) signaling. Here, we demonstrate the presence of a Ca(2+) store in sea urchin eggs mobilized by NAADP that is dependent on a proton gradient maintained by an ATP-dependent vacuolar-type proton pump. Moreover, we provide pharmacological and biochemical evidence that this Ca(2+) store is the reserve granule, the functional equivalent of a lysosome in the sea urchin egg. These findings represent an unsuspected mechanism for messenger-mediated Ca(2+) release from lysosome-related organelles.

Interactions between calcium release pathways: multiple messengers and multiple stores

The discovery of cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) as Ca(2+) releasing messengers has provided additional insight into how complex Ca(2+) signalling patterns are generated. There is mounting evidence that these molecules along with the more established messenger, myo-inositol 1,4,5-trisphosphate (IP(3)), have a widespread messenger role in shaping Ca(2+) signals in many cell types. These molecules have distinct structures and act on specific Ca(2+) release mechanisms. Emerging principles are that cADPR enhances the Ca(2+) sensitivity of ryanodine receptors (RYRs) to produce prolonged Ca(2+) signals through Ca(2+)-induced Ca(2+) release (CICR), while NAADP acts on a novel Ca(2+) release mechanism to produce a local trigger Ca(2+) signal which can be amplified by CICR by recruiting other Ca(2+) release mechanisms. Whilst IP(3) and cADPR mobilise Ca(2+) from the endoplasmic reticulum (ER), recent evidence from the sea urchin egg suggests that the major NAADP-sensitive Ca(2+) stores are reserve granules, acidic lysosomal-related organelles. In this review we summarise the role of multiple Ca(2+) mobilising messengers, Ca(2+) release channels and Ca(2+) stores, and the interplay between them, in the generation of specific Ca(2+) signals. Focusing upon cADPR and NAADP, we discuss how cellular stimuli may draw upon different combinations of these messengers to produce distinct Ca(2+) signalling signatures.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

Repairing a torn cell surface: make way, lysosomes to the rescue

Biological membranes are often described as `self-sealing' structures. If indeed membranes do have an inherent capacity for repair, does this explain how a cell can rapidly reseal a very large (1-1000 μm2)disruption in its plasma membrane? It is becoming increasingly clear that, in nucleated animal cells, the cytoplasm plays an active and essential role in resealing. A rapid and apparently chaotic membrane fusion response is initiated locally in the cytoplasm by the Ca2+ that floods in through a disruption: cytoplasmic vesicles are thereby joined with one another(homotypically) and with the surrounding plasma membrane (exocytotically). As a consequence, internal membrane is added to cell surface membrane at the disruption site. In the case of large disruptions, this addition is hypothesized to function as a `patch'. In sea urchin eggs, the internal compartment used is the yolk granule. Several recent studies have significantly advanced our understanding of how cells survive disruption-inducing injuries. In fibroblasts, the lysosome has been identified as a key organelle in resealing. Protein markers of the lysosome membrane appear on the surface of fibroblasts at sites of disruption. Antibodies against lysosome-specific proteins, introduced into the living fibroblast,inhibit its resealing response. In gastric eptithelial cells, local depolymerization of filamentous actin has been identified as a crucial step in resealing: it may function to remove a barrier to lysosome-plasma membrane contact leading to exocytotic fusion. Plasma membrane disruption in epithelial cells induces depolymerization of cortical filamentous actin and, if this depolymerization response is inhibited, resealing is blocked. In the Xenopus egg, the cortical cytoskeleton has been identified as an active participant in post-resealing repair of disruption-related damage to underlying cell cortex. A striking, highly localized actin polymerization response is observable around the margin of cortical defects. A myosin powered contraction occurring within this newly formed zone of F-actin then drives closure of the defect in a purse-string fashion.

Wound closure in the lamellipodia of single cells: mediation by actin polymerization in the absence of an actomyosin purse string

The actomyosin purse string is an evolutionarily conserved contractile structure that is involved in cytokinesis, morphogenesis, and wound healing. Recent studies suggested that an actomyosin purse string is crucial for the closure of wounds in single cells. In the present study, morphological and pharmacological methods were used to investigate the role of this structure in the closure of wounds in the peripheral cytoplasm of sea urchin coelomocytes. These discoidal shaped cells underwent a dramatic form of actin-based centripetal/retrograde flow and occasionally opened and closed spontaneous wounds in their lamellipodia. Fluorescent phalloidin staining indicated that a well defined fringe of actin filaments assembles from the margin of these holes, and drug studies with cytochalasin D and latrunculin A indicated that actin polymerization is required for wound closure. Additional evidence that actin polymerization is involved in wound closure was provided by the localization of components of the Arp2/3 complex to the wound margin. Significantly, myosin II immunolocalization demonstrated that it is not associated with wound margins despite being present in the perinuclear region. Pharmacological evidence for the lack of myosin II involvement in wound closure comes from experiments in which a microneedle was used to produce wounds in cells in which actomyosin contraction was inhibited by treatment with kinase inhibitors. Wounds produced in kinase inhibitor-treated cells closed in a manner similar to that seen with control cells. Taken together, our results suggest that an actomyosin purse string mechanism is not responsible for the closure of lamellar wounds in coelomocytes. We hypothesize that the wounds heal by means of a combination of the force produced by actin polymerization alone and centripetal flow. Interestingly, these cells did assemble an actomyosin structure around the margin of phagosome-like membrane invaginations, indicating that myosin is not simply excluded from the periphery by some general mechanism. The results indicate that the actomyosin purse string is not the only mechanism that can mediate wound closure in single cells.

An actin barrier to resealing

Plasma membrane disruption is a common form of cell injury in many normal biological environments, including many mammalian tissues. Survival depends on the initiation of a rapid resealing response that is mounted only in the presence of physiological levels of extracellular Ca2+. Vesicle-vesicle and vesicle-plasma membrane fusion events occurring in cortical cytoplasm surrounding the defect are thought to be a crucial element of the resealing mechanism. However, in mammalian cells, the vesicles used in this fusion reaction (endosomes/lysosomes) are not present in a ‘pre-docked’ configuration and so must be brought into physical contact with one another and with the plasma membrane. We propose that a requisite prelude to fusion is the disassembly in local cell cortex of the physical barrier constituted by filamentous actin. Consistent with this hypothesis, we found that rat gastric epithelial (RGM1) cell cortical staining with phalloidin was apparently reduced at presumptive disruption sites. Moreover, flow cytofluorometric analysis of wounded RGM1 populations revealed a small, but significant, Ca2+-dependent reduction in whole cell phalloidin staining. The functional significance of this disruption-induced depolymerization response was confirmed in several independent tests. Introduction into RGM1 cells of the filamentous actin-depolymerizing agent, DNase1, enhanced resealing, although cytochalasin treatment, by itself, had no effect. By contrast, when the filamentous actin cytoskeleton was stabilized experimentally, using phalloidin or jasplakinolide, resealing was strongly inhibited. Cells in wounded cultures displayed an enhanced cortical array of filamentous actin, and resealing by such cells was enhanced strongly by both cytochalasin and DNase 1, demonstrating the specific reversibility of a biologically mediated, polymerization-induced inhibition of resealing. We conclude that localized filamentous actin disassembly removes a cortical barrier standing in the way of membrane-membrane contacts leading to resealing-requisite homotypic and exocytotic fusion events.

Plasma membrane repair is mediated by Ca(2+)-regulated exocytosis of lysosomes

Plasma membrane wounds are repaired by a mechanism involving Ca(2+)-regulated exocytosis. Elevation in intracellular [Ca(2+)] triggers fusion of lysosomes with the plasma membrane, a process regulated by the lysosomal synaptotagmin isoform Syt VII. Here, we show that Ca(2+)-regulated exocytosis of lysosomes is required for the repair of plasma membrane disruptions. Lysosomal exocytosis and membrane resealing are inhibited by the recombinant Syt VII C(2)A domain or anti-Syt VII C(2)A antibodies, or by antibodies against the cytosolic domain of Lamp-1, which specifically aggregate lysosomes. We further demonstrate that lysosomal exocytosis mediates the resealing of primary skin fibroblasts wounded during the contraction of collagen matrices. These findings reveal a fundamental, novel role for lysosomes: as Ca(2+)-regulated exocytic compartments responsible for plasma membrane repair.

Life without a cell membrane: regeneration of protoplasts from disintegrated cells of the marine green alga Bryopsis plumosa

When the multi-nucleate giant cells of the green alga Bryopsis plumosa (Huds.) Ag. are injured, the protoplasm is extruded from the cells and can generate spontaneously numerous new cells. The cell organelles aggregate rapidly in seawater and become covered with a gelatinous envelope within 15 minutes. A lipid cell membrane is formed inside the envelope within 9 to 12 hours and about 15% of the original cell membrane is recycled to make the membrane of new protoplasts. Cytochemical studies using Nile Red and various enzymes revealed that the primary envelope is initially composed of polysaccharides, and then transformed into a polysaccharide-lipid complex. Fluorescein diacetate staining showed that the primary envelope has some characteristics of a cell membrane including semi-permeability and selective transport of materials. The aggregation of cell organelles appears to be mediated by two kinds of materials, one present in vacuolar sap and the other on the surface of the cell organelles. About a thousand new cells were generated from a single disintegrated branch and 40% of them eventually developed into mature plants.

Coping with the inevitable: how cells repair a torn surface membrane

Disruption of the cell plasma membrane is a commonplace occurrence in many mechanically challenging, biological environments. 'Resealing' is the emergency response required for cell survival. Resealing is triggered by Ca2+ entering through the disruption; this causes vesicles present in cytoplasm underlying the disruption site to fuse rapidly with one another (homotypically) and also with the adjacent plasma membrane (heterotypically/exocytotically). The large vesicular products of homotypic fusion are added as a reparative 'patch' across the disruption, when its resealing requires membrane replacement. The simultaneous activation of the local cytoskeleton supports these membrane fusion events. Resealing is clearly a complex and dynamic cell adaptation, and, as we emphasize here, may be an evolutionarily primitive one that arose shortly after the ancestral eukaryote lost its protective cell wall.

Conserved mechanisms of repair: from damaged single cells to wounds in multicellular tissues

The capacities to repair minor membrane holes in damaged single cells, and the more major damage sustained when a multicellular tissue is wounded, both involve a series of ancient and highly conserved processes. In this review, we discuss what is known about how the plasma membrane of a single cell and its underlying cortical cytoplasm are repaired following cell damage, and how multicellular wounds to the embryonic and adult skin are also able to heal. Pivotal for all these processes is the actin cytoskeleton and we draw analogies between the actin machineries that drive repair and those that appear to underlie several genetically tractable morphogenetic processes that occur during Drosophila and Caenorhabditis elegans embryogenesis.