Regulating actin dynamics at membranes: a focus on dynamin

Dynamin, the large guanosine triphosphatase, is generally considered to have a key role in deforming membranes to create tubules or vesicles. Dynamin, particularly dynamin2 isoforms, also are localized with actin filaments, often at locations where cellular membranes undergo remodeling. Perturbing dynamin function interferes with endocytic traffic and actin function. Thus, dynamin may regulate actin filaments coordinately with its activities that remodel membranes. This review will highlight recent observations that provide clues to mechanisms whereby dynamin might coordinate membrane remodeling and actin filament dynamics during endocytic traffic, cell morphogenesis and cell migration.     

Regulating the reapers: activating metacaspases for programmed cell death

Research during the past two decades has revealed that specialized cysteine proteases act as conserved initiators or executioners for programmed cell death (PCD) in eukaryotes. Caspases were first identified as common regulators of PCD in metazoans, whereas the role of metacaspases (MCs) as regulators of cellular suicide in plants has only been shown genetically in the past several years. Together with recent biochemical and molecular characterizations of some of the representative MCs from different model systems, multiple mechanisms that can mediate the post-translational regulation of these proteases are beginning to emerge. Further elucidation of these regulatory pathways and definition of the downstream degradomes targeted by MCs should lead to a better understanding of cell death control in plants, protozoans, and fungi.     

Regulation of programmed cell death by basement membranes in embryonic development

The formation of the proamniotic cavity in the mammalian embryo is the earliest of many instances throughout development in which programmed cell death and the formation of epithelia play fundamental roles (Coucouvanis, E., and G.R. Martin. 1995. Cell. 83:279-287). To determine the role of the basement membrane (BM) in cavitation, we use embryoid bodies derived from mouse embryonic stem cells in which the LAMC1 genes have been inactivated to prevent BM deposition (Smyth, N., H.S. Vatansever, P. Murray, M. Meyer, C. Frie, M. Paulsson, and D. Edgar. 1999. J. Cell Biol. 144:151-610). We demonstrate here that LAMC1-/- embryoid bodies are unable to cavitate, and do not form an epiblast epithelium in the absence of a BM, although both embryonic ectodermal cells and extraembryonic endodermal cells do differentiate, as evidenced by the expression of cell-specific markers. Acceleration or rescue of BM deposition by exogenous laminin in wild-type or LAMC1-/- embryoid bodies, respectively, results in cavitation that is temporally and spatially associated with restoration of epiblast epithelial development. We conclude that the BM not only directly regulates development of epiblast epithelial cells, but also indirectly regulates the programmed cell death necessary for cavity formation.     

Supramolecular zippers elicit interbilayer adhesion of membranes producing cell death

Background

The fluorescent dye 10-N-nonyl acridine orange (NAO) is widely used as a mitochondrial marker. NAO was reported to have cytotoxic effects in cultured eukaryotic cells when incubated at high concentrations. Although the biochemical response of NAO-induced toxicity has been well identified, the underlying molecular mechanism has not yet been explored in detail.

Proapoptotic Bid binds to monolysocardiolipin,a new molecular connection between mitochondrial membranes and cell death

Recent evidence indicates that the mitochondrial lipid cardiolipin may be instrumental in the proapoptotic action of Bcl-2 family proteins on mitochondrial membranes, leading to the release of apoptogenic factors. However, contrasting evidence indicates that progressive loss of cardiolipin occurs during apoptosis. Here we show that Bid, a crucial proapoptotic protein that integrates the action of other Bcl-2 family members, exhibits discrete specificity for metabolites of cardiolipin, especially monolysocardiolipin (MCL). MCL, normally present in the remodelling of mitochondrial lipids, progressively increases in mitochondria during Fas-mediated apoptosis as a by-product of cardiolipin degradation, and also enhances Bid binding to membranes. MCL may thus play a crucial role in connecting lipid metabolism, relocation of Bid to mitochondria and integrated action of Bcl-2 proteins on mitochondrial membranes. We propose that Bid interaction with MCL 'primes' the mitochondrial outer membrane via segregation of lipid domains, facilitating membrane discontinuity and leakage of apoptogenic factors.     

Major cell death pathways at a glance

Cell death is a crucial process during development, homeostasis and immune regulation of multicellular organisms, and its dysregulation is associated with numerous pathologies. Cell death is often induced upon pathogen infection as part of the defense mechanism, and pathogens have evolved strategies to modulate host cell death. In this review, we will discuss the molecular mechanisms and physiological relevance of four major types of programmed cell death, namely apoptosis, necrosis, autophagic cell death and pyroptosis.     

Autophagy machinery mediates macroendocytic processing and entotic cell death by targeting single membranes

Autophagy normally involves the formation of double-membrane autophagosomes that mediate bulk cytoplasmic and organelle degradation. Here we report the modification of single-membrane vacuoles in cells by autophagy proteins. LC3 (Light chain 3) a component of autophagosomes, is recruited to single-membrane entotic vacuoles, macropinosomes and phagosomes harbouring apoptotic cells, in a manner dependent on the lipidation machinery including ATG5 and ATG7, and the class III phosphatidylinositol-3-kinase VPS34. These downstream components of the autophagy machinery, but not the upstream mammalian Tor (mTor)-regulated ULK-ATG13-FIP200 complex, facilitate lysosome fusion to single membranes and the degradation of internalized cargo. For entosis, a live-cell-engulfment program, the autophagy-protein-dependent fusion of lysosomes to vacuolar membranes leads to the death of internalized cells. As pathogen-containing phagosomes can be targeted in a similar manner, the death of epithelial cells by this mechanism mimics pathogen destruction. These data demonstrate that proteins of the autophagy pathway can target single-membrane vacuoles in cells in the absence of pathogenic organisms.     

Annuality, perenniality and cell death

This essay considers annuality and perenniality as quantitative traits and discusses the application of established and new genetic tools to the analysis of plant life histories. Annual/perennial status is a function of meristem determinacy in combination with the processes of cell death and disposal employed by plants to generate well-adapted anatomies and morphologies. Creeping perennials, like clover or bracken, seem to move around in the environment. They do this by extending into unoccupied space while the oldest tissues behind the growing and mature regions senesce, die and decompose. Trees do essentially the same thing, except that they develop vertically and the old dead tissue does not disappear but instead persists as wood. A root system is a kind of upended vertical perennial. The balance between exploratory growth and the wave of tissue death that succeeds it is a major determinant of perenniality. So although perenniality and annuality may appear to be dramatically different traits, extremes of behaviour can arise by a relatively minor change in the relationship between growth and death. This conclusion is supported by evidence from genome dosage studies, from the practical experiences of breeding perennial-type traits into annual backgrounds and from molecular cladistics. Applications of methods for the genetic analysis of quantitative characters are described, including the exploitation of introgression mapping in Lolium-Festuca and quantitative trait locus mapping in cereals and other species.     

Autophagy and cell death: no longer at odds

Autophagy has been associated with both cell survival and cell death, but the role of autophagy in cell death has been controversial. In this issue, Berry and Baehrecke (2007) report that autophagy is involved in physiological cell death during Drosophila development and is controlled by similar mechanisms as those that control its function in cell survival.     

NLRs at the intersection of cell death and immunity

Inflammation is a crucial element of the host response to cellular insult. Pathogen-induced inflammation includes a molecular pathway which proceeds through activation of the protease caspase-1 to the release of the inflammatory cytokines interleukin-1 (IL-1) and IL-18. Importantly, pathogens may also induce forms of cell death that have inherently pro-inflammatory features. Here, we review recent evidence demonstrating that NLR (nucleotide-binding domain, leucine-rich repeat containing) family proteins serve as a common component of both caspase-1-activated apoptotic pathways and caspase-independent necrotic pathways. Parallels are drawn between NLR protein function and the activity of structurally similar proteins involved in cell death: the apoptotic mediator APAF1 (apoptotic-protease-activating factor 1) and the plant disease resistance NBS-LRR (nucleotide-binding site leucine-rich repeats) proteins.     

Mitochondria in cell life, death and disease

In animal cell, mitochondria are the main sites of the synthesis of ATP required for cell functioning and survival. On the other hand, mitochondria play a key role in initiating cell programmed death (apoptosis). In addition, defects in the mitochondrial genome and in the nuclear genome encoding mitochondrial proteins may result in malfunctioning of these organelles and, as result, in diseases of the whole organism. This article contains basic information on the functioning of oxidative phosphorylation and on mitochondrial production of reactive oxygen species. It also describes initiation of apoptosis at the mitochondrial level. Finally, it briefly presents some most common genetic defects responsible for "mitochondrial diseases".     

Transport of L-proline,L-proline-containing peptides and related drugs at mammalian epithelial cell membranes

Summary. Membrane transport of L-proline has received considerable attention in basic and pharmaceutical research recently. Of the most recently cloned members of the solute carrier family, two are “proline transporters”. The amino acid transporter PAT1, expressed in intestine, kidney, brain and other organs, mediates the uptake of proline and derivatives in a pH gradient-dependent manner. The Na⁺-dependent proline transporter SIT1, cloned in 2005, exhibits the properties of the long-sought classical IMINO system. Proline-containing peptides are of interest for several reasons. Many biologically important peptide sequences contain highly conserved proline residues. Xaa-Pro peptides are very often resistant to enzymatic hydrolysis and display, in contrast to Pro-Xaa peptides, a high affinity to the H⁺/peptide cotransporter PEPT1 which is expressed in intestinal, renal, lung and biliary duct epithelial cells. Furthermore, several orally available drugs are recognized by PEPT1 as Xaa-Pro analogues due to their sterical resemblance to small peptides.     

Conformational diseases and ER stress-mediated cell death: apoptotic cell death and autophagic cell death

The expanded polyglutamine (polyQ) tracts observed in autosomal dominant neurodegenerative disorders have the tendency to form intracellular aggregates, thus enhancing apoptotic cell death and the formation of autophagic vesicles. PolyQ accumulation inhibits the ER-associated degradation system (ERAD) resulting in reduced retrotranslocation from the ER and increased accumulation of misfolded proteins in the lumen of ER. Autophagy is an early cellular defense mechanism associated with ER stress, but prolonged ER stress may induce autophagic cell death, with destruction of cellular components and apoptotic cell death. Endoplasmic reticulum (ER) stress may be the key signal for both of these events.     

Mitochondria,oxidative stress and cell death

In addition to the well-established role of the mitochondria in energy metabolism, regulation of cell death has recently emerged as a second major function of these organelles. This, in turn, seems to be intimately linked to their role as the major intracellular source of reactive oxygen species (ROS), which are mainly generated at Complex I and III of the respiratory chain. Excessive ROS production can lead to oxidation of macromolecules and has been implicated in mtDNA mutations, ageing, and cell death. Mitochondria-generated ROS play an important role in the release of cytochrome c and other pro-apoptotic proteins, which can trigger caspase activation and apoptosis. Cytochrome c release occurs by a two-step process that is initiated by the dissociation of the hemoprotein from its binding to cardiolipin, which anchors it to the inner mitochondrial membrane. Oxidation of cardiolipin reduces cytochrome c binding and results in an increased level of “free” cytochrome c in the intermembrane space. Conversely, mitochondrial antioxidant enzymes protect from apoptosis. Hence, there is accumulating evidence supporting a direct link between mitochondria, oxidative stress and cell death.