Herpesvirus capsids traverse the nuclear envelope by utilizing an unusual export pathway termed nuclear egress. In this process, the viral capsid is delivered into the perinuclear space, producing a vesicular intermediate after fission. After fusion with the outer nuclear membrane, the naked capsid is released into the cytosol. A recent study now suggests that this pathway might be an endogenous cellular pathway, co-opted by viruses, that serves to transport cellular cargo exceeding the size limit imposed by the nuclear pore complex. We propose that one function of this pathway is to transport nuclear protein aggregates to the cytosolic autophagy machinery. Our model has implications for our understanding of laminopathies and related diseases affecting proteins residing at the inner nuclear membrane and nuclear lamina.
Torsin A; LINC complex; chaperones; nuclear envelope; egress of nuclear aggregates (EGNA); premature aging
It is widely believed that calorie restriction (CR) can extend the lifespan of model organisms and protect against aging-related diseases. A potential CR mimetic is resveratrol, which may have beneficial effects against numerous diseases such as type 2 diabetes, cardiovascular diseases, and cancer in tissue culture and animal models. However, resveratrol in its current form is not ideal as therapy, because even at very high doses it has modest efficacy and many downstream effects. Identifying the cellular targets responsible for the effects of resveratrol and developing target-specific therapies will be helpful in increasing the efficacy of this drug without increasing its potential adverse effects. A recent discovery suggests that the metabolic effects of resveratrol may be mediated by inhibiting cAMP phosphodiesterases (PDEs), particularly PDE4. Here, we review the current literature on the metabolic and cardiovascular effects of resveratrol and attempt to shed light on the controversies surrounding its action.
resveratrol; PDE; AMPK; Sirt1; calorie restriction; diabetes
There is mounting evidence that the plasma membrane is highly dynamic and organized in a complex manner. The cortical cytoskeleton is proving to be a particularly important regulator of plasmalemmal organization, modulating the mobility of proteins and lipids in the membrane, facilitating their segregation and influencing their clustering. This organization plays a critical role in receptor-mediated signaling, especially in the case of immunoreceptors, which require lateral clustering for their activation. Based on recent developments, we discuss the structures and mechanisms whereby the cortical cytoskeleton regulates membrane dynamics and organization, and how the non-uniform distribution of immunoreceptors and their self-association may affect activation and signaling.
membrane domain; receptor clustering; immunoreceptor signaling; membrane skeleton; rafts; cytoskeleton
Mitochondria adopt a variety of different shapes in eukaryotic cells, ranging from multiple, small compartments to elaborate tubular networks. The establishment and maintenance of different mitochondrial morphologies depends, in part, on the equilibrium between opposing fission and fusion events. Recent studies in yeast, flies, worms and mammalian cells indicate that three high-molecular-weight GTPases control mitochondrial membrane dynamics. One of these is a dynamin-related GTPase that acts on the outer mitochondrial membrane to regulate fission. Recently, genetic approaches in budding yeast have identified additional components of the fission machinery. These and other new findings suggest a common mechanism for membrane fission events that has been conserved and adapted during eukaryotic evolution.
An increasing body of research on autophagy provides overwhelming evidence for its connection to various biological functions and human diseases. Beclin 1 is the first described mammalian autophagy protein and appears to act as a nexus point between autophagy, endosomal, and perhaps also cell death pathways. Beclin 1 performs these roles as part of a core complex that contains vacuolar sorting protein 34 (VPS34), a class III phosphatidylinositol-3 kinase. The precise mechanism of Beclin 1-mediated regulation of these cellular functions is unclear, but substantial progress has recently been made in identifying new players and their functions in Beclin 1-VSP34 complexes. Here, we review emerging studies, which are beginning to unveil physiological functions for Beclin 1-VPS34 in the central control of autophagic activity and other cellular trafficking events through the formation of distinct Beclin 1-VPS34 protein complexes.
One of the most ancient and highly conserved microRNAs (miRNAs), the let-7 family, has gained notoriety owing to its regulation of stem cell differentiation, essential role in normal development, as well as its tumor suppressor function. Mechanisms controlling let-7 expression have recently been uncovered, specifically the role of the RNA-binding protein Lin28 – a key developmental regulator - in blocking let-7 biogenesis. This review focuses on our current understanding of the Lin28-mediated control of let-7 maturation and highlights the central role of Lin28 in stem cell biology, development, control of glucose metabolism, and dysregulation in human disease. Manipulating the Lin28 pathway for the precise control of let-7 expression may therefore provide novel therapeutic opportunities for cancer and other diseases.
Lin28; let-7; microRNA; TUTase; Stem Cells; Development; Cancer
In animal and plant cells, a wide range of key cellular processes that require the establishment of cell polarity are governed by Rho-GTPases. In contrast to animals and yeast, however, plants possess a single Rho-GTPase subfamily called ROP (Rho-like GTPases from plants). This raises the question of how plants achieve the high level of regulation required for polar cellular processes. It is becoming evident that plants have evolved specific regulators, including ROP-Guanine Exchange Factors (GEFs) and the Rop-interactive CRIB motif–containing proteins (RIC) effectors. Recent research shows that the spatiotemporal dynamics of ROPs, the cytoskeleton, endocytosis and exocytosis are intertwined. This review focuses on the proposed self-organizing nature of ROPs in plants and how ROP-mediated cellular mechanisms compare with those responsible for cell polarity in animals and yeast.
Cells use secreted signals (e.g. chemokines and growth factors) and sophisticated vehicles such as argosomes, cytonemes, tunneling nanotubes and exosomes to relay important information to other cells, often over large distances. Exosomes, 30–100-nm intraluminal vesicles of multivesicular bodies (MVB) released upon exocytic fusion of the MVB with the plasma membrane, are increasingly recognized as a novel mode of cell-independent communication. Exosomes have been shown to function in antigen presentation and tumor metastasis, and in transmitting infectious agents. However, little is known about the biogenesis and function of exosomes in polarized cells. In this review, we discuss new evidence suggesting that exosomes participate in the transport of morphogens and RNA, and thus influence cell polarity and developmental patterning of tissues.
All cellular proteins undergo continuous synthesis and degradation. This permanent renewal is necessary to maintain a functional proteome and to allow for rapid changes in levels of specific proteins with regulatory purposes. Although for a long time lysosomes were considered unable to contribute to the selective degradation of individual proteins, the discovery of chaperone-mediated autophagy (CMA) changed this notion. Here, we review the characteristics that set CMA apart from other types of lysosomal degradation and the subset of molecules that confer cells the capability to identify individual cytosolic proteins and direct them across the lysosomal membrane for degradation.
aging; cancer; chaperones; lysosomes; membrane proteins; neurodegeneration; protein degradation
+TIPs are a heterogeneous class of proteins that specifically bind to growing microtubule ends. Because dynamic microtubules are essential for many intracellular processes, +TIPs likely play important roles in regulating microtubule dynamics and microtubule interactions with other intracellular structures. End-binding proteins (EBs) recognize a structural cap at growing microtubule ends, and have emerged as central adaptors that mediate microtubule plus-end-tracking of potentially all other +TIPs. The majority of these +TIPs bind EBs through a short hydrophobic SxIP sequence motif and surrounding electrostatic interactions. These recent discoveries have resulted in a rapid expansion of the number of possible +TIPs. In this review, we outline our current understanding of the molecular mechanism of plus-end-tracking and provide an overview of SxIP-recruited +TIPs.
microtubule; microtubule dynamics; +TIPs; plus-end-tracking; EB1
Autophagy is a cell biological process ubiquitous to all eukaryotic cells, often referred to as a catabolic, lysosomal degradative pathway. However, current studies in mammalian systems suggest that autophagy plays an unexpectedly broad biogenesis role in protein trafficking and secretion. Autophagy supports alternative trafficking pathways for delivery of integral membrane proteins to the plasma membrane and affects secretion, including the constitutive, regulated and unconventional secretion pathways. Autophagy-based unconventional secretion, termed here ‘autosecretion’, is one of the pathways enabling leaderless cytosolic proteins to exit the cell without entering the ER-to-Golgi secretory pathway. In this review, we discuss the emerging underlying mechanisms of how autophagy affects different facets of secretion. We also describe the physiological roles of autosecretory cargos that are often associated with inflammatory processes and also play a role in the formation of specialized tissues and in tissue remodeling, expanding the immediate sphere of influence of autophagy from the intracellular to the extracellular space.
Autophagy; autosecretion; inflammasome; cystic fibrosis; IL-1; GRASP
Coordinated responses between the nucleus and mitochondria are essential for maintenance of homeostasis. For over 15 years, pools of nuclear transcription factors (TFs), such as p53 and nuclear hormone receptors, have been observed in the mitochondria. The contribution of the mitochondrial pool of these TFs to their well-defined biological actions is in some cases clear and in others not well understood. Recently, a small mitochondrial pool of the TF signal transducer and activator of transcription factor 3 (STAT3) was shown to modulate the activity of the electron transport chain. The mitochondrial function of STAT3 encompasses both its biological actions in the heart as well as its oncogenic effects. This review highlights advances in our understanding of how mitochondrial pools of nuclear TFs may influence the function of this organelle.
nuclear transcription factors; mitochondrial function; apoptosis; STAT3
Autophagy is a highly conserved process that allows cells, tissues and organs to survive onslaughts such as nutrient deprivation, inflammation, hypoxia and other stresses. The core component proteins that regulate autophagy are well known, and the formation of a double-membrane structure that encompasses cytosolic cargo, including protein aggregates and organelles, has been intensively studied. However, less is known about the inputs that specifically alter recruitment of these components and how post-translational modifications can influence autophagy flux, or the rate at which autophagy substrates are turned over. We propose that three types of post-translational modifications – phosphorylation, ubiquitylation and acetylation – are crucial for autophagy induction, regulation and fine-tuning, and are influenced by a variety of stimuli. Understanding these novel mechanisms of autophagy regulation will give us deeper insights into this process and potentially open up therapeutic avenues.
Stem cell activity fluctuates throughout an organism’s lifetime to maintain homeostatic conditions in all tissues. As animals develop and age, their organs must remodel and regenerate themselves in response to environmental and physiological demands. Recently, the highly conserved Hippo signaling pathway, discovered in Drosophila melanogaster, has been implicated as a key regulator of organ size control across species. Deregulation is associated with substantial overgrowth phenotypes and eventual onset of cancer in various tissues. Importantly, emerging evidence suggests that the Hippo pathway can modulate its effects on tissue size by the direct regulation of stem cell proliferation and maintenance. These findings provide an attractive model for how this pathway might communicate physiological needs for growth to tissue-specific stem cell pools. In this review, we summarize the current and emerging data linking Hippo signaling to stem cell function
stem cells; Hippo signaling; Yap; size regulation; regeneration; tumorigenesis
To date, most studies of Rho GTPase regulation have focused on the classic GTPase cycle – GTP binding and hydrolysis – controlled by guanine nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs) and GDP-dissociation inhibitors (GDIs). Recent investigations have unveiled important additional regulatory mechanisms: microRNA (miRNA) regulating post-transcriptional processing of Rho GTPase-encoding mRNAs; palmitoylation and nuclear targeting affecting intracellular distribution; post-translational phosphorylation, transglutamination and AMPylation impacting Rho GTPase signaling; and ubiquitination controlling Rho GTPase protein stability and turnover. These modes of regulation add to the complexity of the Rho GTPase signaling network and allow precise spatiotemporal control of individual Rho GTPases. This review will discuss these ‘unconventional’ modes of regulation and their contribution to cellular function, focusing on post-transcriptional and post-translational events beyond the classic GTPase cycle regulatory model.
small GTPases; microRNAs; ubiquitination; phosphorylation; transglutamination
Eukaryotic cells must constantly degrade both intracellular and extracellular material in order to maintain cellular and organismal homeostasis. Two engulfment pathways, autophagy and phagocytosis, mediate the turnover of intracellular and extracellular substrates by delivering material to the lysosome. Historically these were thought to be separate pathways, but recent studies have revealed the direct participation of autophagy proteins in phagocytosis. Autophagy proteins lipidate LC3 onto phagosomes and other macroendocytic vacuole membranes, and are required for lysosomal degradation of engulfed cargo, demonstrating an autophagosome-independent role for autophagy proteins in mediating the turnover of extracellular substrates. This review discusses the biological systems where autophagy proteins have been found to regulate lysosome fusion to non-autophagic membranes.
autophagy; phagocytosis; entosis; engulfment; lysosome
Neurodegenerative diseases are typically late-onset progressive disorders that affect neural function and integrity. Although most attention has been focused on the genetic underpinnings of familial disease, mechanisms are likely shared with more predominant sporadic forms, which can be influenced by age, environment and genetic inputs. Previous work has largely addressed the roles of select protein-coding genes; however, disease pathogenesis is complicated and can be modulated through not just protein-coding genes, but also regulatory mechanisms mediated by the exploding world of small non-coding RNAs. Here, we focus on emerging roles of miRNAs in age-associated events impacting long-term brain integrity and neurodegenerative disease.
microRNA; neurodegenerative disease; aging; miRNA biogenesis; dicer; small RNAs
Inter-organellar communication and interactions are necessary and accepted consequences of the segregation of biochemical functions in subcellular organelles. Recently, Heidi McBride and her collaborators found a novel link between mitochondria and peroxisomes in their discovery of mitochondria-derived-vesicles (MDVs) that appear to fuse with a fraction of pre-existing peroxisomes in mammalian cells. We discuss the potential role of this vesicle population in the context of pathways for the exchange of metabolites and/or macromolecules between these compartments.
Cytokinesis is an event common to all organisms that involves the precise coordination of independent pathways involved in cell-cycle regulation and microtubule, membrane, actin and organelle dynamics. In animal cells, the spindle midzone/midbody with associated endo-membrane system are required for late cytokinesis events, including furrow ingression and scission. In plants, cytokinesis is mediated by the phragmoplast, an array of microtubules, actin filaments and associated molecules that act as a framework for the future cell wall. In this article (which is part of the Cytokinesis series), we discuss recent studies that highlight the increasing number of similarities in the components and function of the spindle midzone/midbody in animals and the phragmoplast in plants, suggesting that they might be analogous structures.
Embryonic stem (ES) cells, like all cell types, are defined by their unique transcriptional signatures. The ability of ES cells to self-renew or exit the pluripotent state and enter differentiation requires extensive changes in their transcriptome and epigenome. Remarkably, transcriptional programs governing each cell fate must remain sufficiently malleable so that expression of only a handful of transcriptional activators can override the pre-existing state by collaborating with an unexpectedly elaborate collection of coactivators to specify, restrict and stabilize the new state. Here, we discuss recent advances in our understanding of how the same coactivator can interpret multiple lines of information encoded by different activators and integrate signals from diverse regulators into stem cell-specific transcriptional outputs.
self-renewal; pluripotency; transcription activation; Mediator; TFIID; DNA repair
Cadherins embody a superfamily of cell-surface glycoproteins whose ectodomains contain multiple repeats of β-sandwich EC (extracellular cadherin) domains that adopt a similar fold to immunoglobulin domains. The best characterized cadherins are the vertebrate “classical” cadherins, which mediate adhesion via trans homodimerization between their membrane-distal EC1 domains that extend from apposed cells, and assemble intercellular adherens junctions through cis clustering. To form mature trans adhesive dimers, cadherin domains from apposed cells dimerize in a “strand-swapped” conformation. This occurs in a two-step binding process involving a fast-binding intermediate called the “X-dimer”. Trans dimers are less flexible than cadherin monomers, a factor which drives junction assembly following cell-cell contact by reducing the entropic cost associated with the formation of lateral cis oligomers. Cadherins outside of the classical subfamily appear to have evolved distinct adhesive mechanisms which are just now beginning to be understood.
Classical apoptotic cell death is now sufficiently well understood to be interrogated with mathematical modeling and to be skillfully manipulated with targeted drugs for clinical benefit. However, a biological black hole has emerged with the realization that apoptosis regulators are functionally multipolar. BCL-2 family proteins appear to have much greater effects on cells than can be explained by their known roles in apoptosis. While these effects may be observable simply because the cell is not dead, the general assumption is that BCL-2 proteins have yet undiscovered biochemical activities. Conversely, these yet uncharacterized day-jobs may underlie their profound effects on cell survival, challenging current assumptions about classical apoptosis. Even their sub-mitochondrial localizations remain controversial. Here we attempt to integrate seemingly conflicting information with the prospect that BCL-2 proteins themselves may be the critical crosstalk between life and death.
apoptosis; Bcl-2; Bcl-xL; mitochondria; inner mitochondrial membrane; energetics
Photoactivatable fluorescent proteins (PA-FPs) are molecules that switch to a new fluorescent state in response to activation to generate a high level of contrast. Over the past eight years, several types of PA-FPs have been developed. The PA-FPs fluoresce green or red, or convert from green to red in response to activating light. Others reversibly switch between ‘off’ and ‘on’ in response to light. The optical “highlighting” capability of PA-FPs has led to the rise of novel imaging techniques providing important new biological insights. These range from in cellulo pulse-chase labeling for tracking subpopulations of cells, organelles or proteins under physiological settings, to super-resolution imaging of single molecules for determining intracellular protein distributions at nanometer precision. This review surveys the expanding array of PA-FPs, including their advantages and disadvantages, and highlights their use in novel imaging methodologies.
The nervous and vascular systems are both exquisitely branched and complicated systems and their proper development requires careful guidance of nerves and vessels. The recent realization that common ligand-receptor pairs are used in guiding the patterning of both systems has prompted the question of whether similar signaling pathways are used in both systems. This review highlights recent progress in our understanding of the similarities and differences in the intracellular signaling mechanisms downstream of semaphorins, ephrins, and VEGF in neurons and endothelial cells during neural and vascular development. We present evidence that similar intracellular signaling principles underlying cytoskeletal regulation are used to control neural and vascular guidance, while the specific molecules used in neurons and endothelial cells are often different.
The ability to reproduce relies in most eukaryotes on specialized cells called gametes. Gametes are formed by the process of meiosis in which, after a single round of replication, two successive cell divisions reduce the ploidy of the genome. Fusion of gametes at fertilization reconstitutes diploidy. In most animal species, chromosome segregation during female meiosis occurs on spindles assembled in the absence of the major microtubule-organizing center, the centrosome. In mammals, oocyte meiosis is error-prone and underlies the majority of birth aneuploidies. Here, we review recent work on acentrosomal spindle formation and chromosome alignment/separation during oocyte meiosis in different animal models.