Showing papers in "Journal of Cell Science in 2011"

Selective transfer of exosomes from oligodendrocytes to microglia by macropinocytosis

Abstract: The transfer of antigens from oligodendrocytes to immune cells has been implicated in the pathogenesis of autoimmune diseases. Here, we show that oligodendrocytes secrete small membrane vesicles called exosomes, which are specifically and efficiently taken up by microglia both in vitro and in vivo. Internalisation of exosomes occurs by a macropinocytotic mechanism without inducing a concomitant inflammatory response. After stimulation of microglia with interferon-γ, we observe an upregulation of MHC class II in a subpopulation of microglia. However, exosomes are preferentially internalised in microglia that do not seem to have antigen-presenting capacity. We propose that the constitutive macropinocytotic clearance of exosomes by a subset of microglia represents an important mechanism through which microglia participate in the degradation of oligodendroglial membrane in an immunologically ‘silent’ manner. By designating the capacity for macropinocytosis and antigen presentation to distinct cells, degradation and immune function might be assigned to different subtypes of microglia.

The regulation of autophagy - unanswered questions.

Abstract: Autophagy is an intracellular lysosomal (vacuolar) degradation process that is characterized by the formation of double-membrane vesicles, known as autophagosomes, which sequester cytoplasm. As autophagy is involved in cell growth, survival, development and death, the levels of autophagy must be properly regulated, as indicated by the fact that dysregulated autophagy has been linked to many human pathophysiologies, such as cancer, myopathies, neurodegeneration, heart and liver diseases, and gastrointestinal disorders. Substantial progress has recently been made in understanding the molecular mechanisms of the autophagy machinery, and in the regulation of autophagy. However, many unanswered questions remain, such as how the Atg1 complex is activated and the function of PtdIns3K is regulated, how the ubiquitin-like conjugation systems participate in autophagy and the mechanisms of phagophore expansion and autophagosome formation, how the network of TOR signaling pathways regulating autophagy are controlled, and what the underlying mechanisms are for the pro-cell survival and the pro-cell death effects of autophagy. As several recent reviews have comprehensively summarized the recent progress in the regulation of autophagy, we focus in this Commentary on the main unresolved questions in this field.

Lipid Map of the Mammalian Cell

Abstract: Technological developments, especially in mass spectrometry and bioinformatics, have revealed that living cells contain thousands rather than dozens of different lipids [for classification and nomenclature, see Fahy et al. ([Fahy et al., 2009][1])]. Now, the resulting questions are what is the

Fibrosis and adipogenesis originate from a common mesenchymal progenitor in skeletal muscle.

Abstract: Accumulation of adipocytes and collagen type-I-producing cells (fibrosis) is observed in muscular dystrophies. The origin of these cells had been largely unknown, but recently we identified mesenchymal progenitors positive for platelet-derived growth factor receptor alpha (PDGFRα) as the origin of adipocytes in skeletal muscle. However, the origin of muscle fibrosis remains largely unknown. In this study, clonal analyses show that PDGFRα + cells also differentiate into collagen type-I-producing cells. In fact, PDGFRα + cells accumulated in fibrotic areas of the diaphragm in the mdx mouse, a model of Duchenne muscular dystrophy. Furthermore, mRNA of fibrosis markers was expressed exclusively in the PDGFRα + cell fraction in the mdx diaphragm. Importantly, TGF-β isoforms, known as potent profibrotic cytokines, induced expression of markers of fibrosis in PDGFRα + cells but not in myogenic cells. Transplantation studies revealed that fibrogenic PDGFRα + cells mainly derived from pre-existing PDGFRα + cells and that the contribution of PDGFRα − cells and circulating cells was limited. These results indicate that mesenchymal progenitors are the main origin of not only fat accumulation but also fibrosis in skeletal muscle.

Increased ER-mitochondrial coupling promotes mitochondrial respiration and bioenergetics during early phases of ER stress

Abstract: Increasing evidence indicates that endoplasmic reticulum (ER) stress activates the adaptive unfolded protein response (UPR), but that beyond a certain degree of ER damage, this response triggers apoptotic pathways. The general mechanisms of the UPR and its apoptotic pathways are well characterized. However, the metabolic events that occur during the adaptive phase of ER stress, before the cell death response, remain unknown. Here, we show that, during the onset of ER stress, the reticular and mitochondrial networks are redistributed towards the perinuclear area and their points of connection are increased in a microtubule-dependent fashion. A localized increase in mitochondrial transmembrane potential is observed only in redistributed mitochondria, whereas mitochondria that remain in other subcellular zones display no significant changes. Spatial re-organization of these organelles correlates with an increase in ATP levels, oxygen consumption, reductive power and increased mitochondrial Ca2+ uptake. Accordingly, uncoupling of the organelles or blocking Ca2+ transfer impaired the metabolic response, rendering cells more vulnerable to ER stress. Overall, these data indicate that ER stress induces an early increase in mitochondrial metabolism that depends crucially upon organelle coupling and Ca2+ transfer, which, by enhancing cellular bioenergetics, establishes the metabolic basis for the adaptation to this response.

Rho GTPases and their role in organizing the actin cytoskeleton.

Abstract: Cells receive extracellular stimuli in various ways: in the form of soluble molecules (growth factors, cytokines and hormones) that interact with cell-surface receptors; from adhesive interactions with the extracellular matrix; and from cell–cell adhesions. These stimuli act to generate changes in

Mechanical signaling through the cytoskeleton regulates cell proliferation by coordinated focal adhesion and Rho GTPase signaling.

Abstract: The notion that cell shape and spreading can regulate cell proliferation has evolved over several years, but only recently has this been linked to forces from within and upon the cell. This emerging area of mechanical signaling is proving to be wide-spread and important for all cell types. The microenvironment that surrounds cells provides a complex spectrum of different, simultaneously active, biochemical, structural and mechanical stimuli. In this milieu, cells probe the stiffness of their microenvironment by pulling on the extracellular matrix (ECM) and/or adjacent cells. This process is dependent on transcellular cell–ECM or cell–cell adhesions, as well as cell contractility mediated by Rho GTPases, to provide a functional linkage through which forces are transmitted through the cytoskeleton by intracellular force-generating proteins. This Commentary covers recent advances in the underlying mechanisms that control cell proliferation by mechanical signaling, with an emphasis on the role of 3D microenvironments and in vivo extracellular matrices. Moreover, as there is much recent interest in the tumor–stromal interaction, we will pay particular attention to exciting new data describing the role of mechanical signaling in the progression of breast cancer.

Bone remodelling at a glance

Abstract: The bone remodelling cycle (see Poster panel “The bone remodelling cycle”) maintains the integrity of the skeleton through the balanced activities of its constituent cell types. These are the bone-forming osteoblast, a cell that produces the organic bone matrix and aids its mineralisation ([

DNA-SCARS: distinct nuclear structures that sustain damage-induced senescence growth arrest and inflammatory cytokine secretion

Abstract: DNA damage can induce a tumor suppressive response termed cellular senescence. Damaged senescent cells permanently arrest growth, secrete inflammatory cytokines and other proteins and harbor persistent nuclear foci that contain DNA damage response (DDR) proteins. To understand how persistent damage foci differ from transient foci that mark repairable DNA lesions, we identify sequential events that differentiate transient foci from persistent foci, which we term ‘DNA segments with chromatin alterations reinforcing senescence’ (DNA-SCARS). Unlike transient foci, DNA-SCARS associate with PML nuclear bodies, lack the DNA repair proteins RPA and RAD51, lack single-stranded DNA and DNA synthesis and accumulate activated forms of the DDR mediators CHK2 and p53. DNA-SCARS form independently of p53, pRB and several other checkpoint and repair proteins but require p53 and pRb to trigger the senescence growth arrest. Importantly, depletion of the DNA-SCARS-stabilizing component histone H2AX did not deplete 53BP1 from DNA-SCARS but diminished the presence of MDC1 and activated CHK2. Furthermore, depletion of H2AX reduced both the p53-dependent senescence growth arrest and p53-independent cytokine secretion. DNA-SCARS were also observed following severe damage to multiple human cell types and mouse tissues, suggesting that they can be used in combination with other markers to identify senescent cells. Thus, DNA-SCARS are dynamically formed distinct structures that functionally regulate multiple aspects of the senescent phenotype.

Mechanisms of mechanical signaling in development and disease.

Abstract: The responses of cells to chemical signals are relatively well characterized and understood. Cells also respond to mechanical signals in the form of externally applied force and forces generated by cell–matrix and cell–cell contacts. Many features of cell function that are generally considered to be under the control of chemical stimuli, such as motility, proliferation, differentiation and survival, can also be altered by changes in the stiffness of the substrate to which the cells are adhered, even when their chemical environment remains unchanged. Many examples from clinical and whole animal studies have shown that changes in tissue stiffness are related to specific disease characteristics and that efforts to restore normal tissue mechanics have the potential to reverse or prevent cell dysfunction and disease. How cells detect stiffness is largely unknown, but the cellular structures that measure stiffness and the general principles by which they work are beginning to be revealed. This Commentary highlights selected recent reports of mechanical signaling during disease development, discusses open questions regarding the physical mechanisms by which cells sense stiffness, and examines the relationship between studies in vitro on flat substrates and the more complex three-dimensional setting in vivo.

Overcoming anoikis – pathways to anchorage-independent growth in cancer

Abstract: Anoikis (or cell-detachment-induced apoptosis) is a self-defense strategy that organisms use to eliminate 'misplaced' cells, i.e. cells that are in an inappropriate location. Occasionally, detached or misplaced cells can overcome anoikis and survive for a certain period of time in the absence of the correct signals from the extracellular matrix (ECM). If cells are able to adapt to their new environment, then they have probably become anchorage-independent, which is one of the hallmarks of cancer cells. Anoikis resistance and anchorage-independency allow tumor cells to expand and invade adjacent tissues, and to disseminate through the body, giving rise to metastasis. Thus, overcoming anoikis is a crucial step in a series of changes that a tumor cell undergoes during malignant transformation. Tumor cells have developed a variety of strategies to bypass or overcome anoikis. Some strategies consist of adaptive cellular changes that allow the cells to behave as they would in the correct environment, so that induction of anoikis is aborted. Other strategies aim to counteract the negative effects of anoikis induction by hyperactivating survival and proliferative cascades. The recently discovered processes of autophagy and entosis also highlight the contribution of these mechanisms to rendering the cells in a dormant state until they receive a signal initiated at the ECM, thereby circumventing anoikis. In all situations, the final outcome is the ability of the tumor to grow and metastasize. A better understanding of the mechanisms underlying anoikis resistance could help to counteract tumor progression and prevent metastasis formation.

Lipid droplets are functionally connected to the endoplasmic reticulum in Saccharomyces cerevisiae.

Abstract: Cells store metabolic energy in the form of neutral lipids that are deposited within lipid droplets (LDs). In this study, we examine the biogenesis of LDs and the transport of integral membrane proteins from the endoplasmic reticulum (ER) to newly formed LDs. In cells that lack LDs, otherwise LD-localized membrane proteins are homogenously distributed in the ER membrane. Under these conditions, transcriptional induction of a diacylglycerol acyltransferase that catalyzes the formation of the storage lipid triacylglycerol (TAG), Lro1, is sufficient to drive LD formation. Newly formed LDs originate from the ER membrane where they become decorated by marker proteins. Induction of LDs by expression of the second TAG-synthesizing integral membrane protein, Dga1, reveals that Dga1 itself moves from the ER membrane to concentrate on LDs. Photobleaching experiments (FRAP) indicate that relocation of membrane proteins from the ER to LDs is independent of temperature and energy, and thus not mediated by classical vesicular transport routes. LD-localized membrane proteins are homogenously distributed at the perimeter of LDs, they are free to move over the LD surface and can even relocate back into the ER, indicating that they are not restricted to specialized sites on LDs. These observations indicate that LDs are functionally connected to the ER membrane and that this connection allows the efficient partitioning of membrane proteins between the two compartments.

Protein localization in disease and therapy

Abstract: The eukaryotic cell is organized into membrane-covered compartments that are characterized by specific sets of proteins and biochemically distinct cellular processes. The appropriate subcellular localization of proteins is crucial because it provides the physiological context for their function. In this Commentary, we give a brief overview of the different mechanisms that are involved in protein trafficking and describe how aberrant localization of proteins contributes to the pathogenesis of many human diseases, such as metabolic, cardiovascular and neurodegenerative diseases, as well as cancer. Accordingly, modifying the disease-related subcellular mislocalization of proteins might be an attractive means of therapeutic intervention. In particular, cellular processes that link protein folding and cell signaling, as well as nuclear import and export, to the subcellular localization of proteins have been proposed as targets for therapeutic intervention. We discuss the concepts involved in the therapeutic restoration of disrupted physiological protein localization and therapeutic mislocalization as a strategy to inactivate disease-causing proteins.

Hair follicle dermal papilla cells at a glance.

Abstract: Mammalian skin is a highly tractable tissue in which to explore epithelial–mesenchymal interactions during development and in postnatal life ([Blanpain and Fuchs, 2009][1]; [Muller-Rover et al, 2001][2]; [Schmidt-Ullrich and Paus, 2005][3]; [Watt and Jensen, 2009][4]) One population of

Adipogenesis at a glance.

Abstract: The formation of adipocytes from precursor stem cells involves a complex and highly orchestrated programme of gene expression. Our understanding of the basic network of transcription factors that regulates adipogenesis has remained remarkably unchanged in recent years. However, this continues to be

Integrins and cadherins join forces to form adhesive networks.

Abstract: Cell-cell and cell-extracellular-matrix (cell-ECM) adhesions have much in common, including shared cytoskeletal linkages, signaling molecules and adaptor proteins that serve to regulate multiple cellular functions. The term 'adhesive crosstalk' is widely used to indicate the presumed functional communication between distinct adhesive specializations in the cell. However, this distinction is largely a simplification on the basis of the non-overlapping subcellular distribution of molecules that are involved in adhesion and adhesion-dependent signaling at points of cell-cell and cell-substrate contact. The purpose of this Commentary is to highlight data that demonstrate the coordination and interdependence of cadherin and integrin adhesions. We describe the convergence of adhesive inputs on cell signaling pathways and cytoskeletal assemblies involved in regulating cell polarity, migration, proliferation and survival, differentiation and morphogenesis. Cell-cell and cell-ECM adhesions represent highly integrated networks of protein interactions that are crucial for tissue homeostasis and the responses of individual cells to their adhesive environments. We argue that the machinery of adhesion in multicellular tissues comprises an interdependent network of cell-cell and cell-ECM interactions and signaling responses, and not merely crosstalk between spatially and functionally distinct adhesive specializations within cells.

Sirtuins at a Glance

Abstract: Sirtuins are NAD-dependent deacetylases that are highly conserved from bacteria to human and SIR2 was originally shown to extend lifespan in budding yeast ([Imai et al., 2000][1]; [Kaeberlein et al., 1999][2]). Since then, sirtuins have been shown to also regulate longevity in other lower organisms

Post-translational modifications of microtubules

Abstract: Microtubules--polymers of tubulin--perform essential functions, including regulation of cell shape, intracellular transport and cell motility. How microtubules are adapted to perform multiple diverse functions is not well understood. Post-translational modifications of tubulin subunits diversify the outer and luminal surfaces of microtubules and provide a potential mechanism for their functional specialization. Recent identification of a number of tubulin-modifying and -demodifying enzymes has revealed key roles of tubulin modifications in the regulation of motors and factors that affect the organization and dynamics of microtubules.

Fluorescent proteins at a glance

Abstract: The original green fluorescent protein (GFP) was discovered back in the early 1960s when researchers studying the bioluminescent properties of the Aequorea victoria jellyfish isolated a blue-light-emitting bioluminescent protein called aequorin together with another protein that was eventually named

Temporal changes in Hec1 phosphorylation control kinetochore–microtubule attachment stability during mitosis

Abstract: Precise control of the attachment strength between kinetochores and spindle microtubules is essential to preserve genomic stability. Aurora B kinase has been implicated in regulating the stability of kinetochore–microtubule attachments but its relevant kinetochore targets in cells remain unclear. Here, we identify multiple serine residues within the N-terminus of the kinetochore protein Hec1 that are phosphorylated in an Aurora-B-kinase-dependent manner during mitosis. On all identified target sites, Hec1 phosphorylation at kinetochores is high in early mitosis and decreases significantly as chromosomes bi-orient. Furthermore, once dephosphorylated, Hec1 is not highly rephosphorylated in response to loss of kinetochore–microtubule attachment or tension. We find that a subpopulation of Aurora B kinase remains localized at the outer kinetochore even upon Hec1 dephosphorylation, suggesting that Hec1 phosphorylation by Aurora B might not be regulated wholly by spatial positioning of the kinase. Our results define a role for Hec1 phosphorylation in kinetochore–microtubule destabilization and error correction in early mitosis and for Hec1 dephosphorylation in maintaining stable attachments in late mitosis.

The roles of flotillin microdomains - endocytosis and beyond

Abstract: Flotillins are membrane proteins that form microdomains in the plasma membrane of all mammalian cell types studied to date. They span the evolutionary spectrum, with proteins related to flotillins present in bacteria, fungi, plants and metazoans, which suggests that they perform important, and probably conserved, functions. Flotillins have been implicated in myriad processes that include endocytosis, signal transduction and regulation of the cortical cytoskeleton, yet the molecular mechanisms that underlie flotillin function in these different cases are still poorly understood. In this Commentary, we will provide an introduction to these intriguing proteins, summarise their proposed functions and discuss in greater detail some recent insights into the role of flotillin microdomains in endocytosis that have been provided by several independent studies. Finally, we will focus on the questions that are raised by these new experiments and their implications for future studies.

Post-transcriptional control of circadian rhythms.

Abstract: Circadian rhythms exist in most living organisms. The general molecular mechanisms that are used to generate 24-hour rhythms are conserved among organisms, although the details vary. These core clocks consist of multiple regulatory feedback loops, and must be coordinated and orchestrated appropriately for the fine-tuning of the 24-hour period. Many levels of regulation are important for the proper functioning of the circadian clock, including transcriptional, post-transcriptional and post-translational mechanisms. In recent years, new information about post-transcriptional regulation in the circadian system has been discovered. Such regulation has been shown to alter the phase and amplitude of rhythmic mRNA and protein expression in many organisms. Therefore, this Commentary will provide an overview of current knowledge of post-transcriptional regulation of the clock genes and clock-controlled genes in dinoflagellates, plants, fungi and animals. This article will also highlight how circadian gene expression is modulated by post-transcriptional mechanisms and how this is crucial for robust circadian rhythmicity.

Integrin α5β1 facilitates cancer cell invasion through enhanced contractile forces

Abstract: Cell migration through connective tissue, or cell invasion, is a fundamental biomechanical process during metastasis formation. Cell invasion usually requires cell adhesion to the extracellular matrix through integrins. In some tumors, increased integrin expression is associated with increased malignancy and metastasis formation. Here, we have studied the invasion of cancer cells with different α5β1 integrin expression levels into loose and dense 3D collagen fiber matrices. Using a cell sorter, we isolated from parental MDA-MB-231 breast cancer cells two subcell lines expressing either high or low amounts of α5β1 integrins (α5β1high or α5β1low cells, respectively). α5β1high cells showed threefold increased cell invasiveness compared to α5β1low cells. Similar results were obtained for 786-O kidney and T24 bladder carcinoma cells, and cells in which the α5 integrin subunit was knocked down using specific siRNA. Knockdown of the collagen receptor integrin subunit α2 also reduced invasiveness, but to a lesser degree than knockdown of integrin subunit α5. Fourier transform traction microscopy revealed that the α5β1high cells generated sevenfold greater contractile forces than α5β1low cells. Cell invasiveness was reduced after addition of the myosin light chain kinase inhibitor ML-7 in α5β1high cells, but not in α5β1low cells, suggesting that α5β1 integrins enhance cell invasion through enhanced transmission and generation of contractile forces.

Super-resolution microscopy at a glance.

Abstract: Advances in microscopy and cell biology are intimately intertwined, with new visualization possibilities often leading to dramatic leaps in our understanding of how cells function. The recent unprecedented technical innovation of super-resolution microscopy has changed the limits of optical

Mesenchymal stem cell migration is regulated by fibronectin through α5β1-integrin-mediated activation of PDGFR-β and potentiation of growth factor signals

Abstract: Cell migration during vascular remodelling is regulated by crosstalk between growth factor receptors and integrin receptors, which together coordinate cytoskeletal and motogenic changes. Here, we report extracellular matrix (ECM)-directed crosstalk between platelet-derived growth factor receptor (PDGFR)-β and α5β1-integrin, which controls the migration of mesenchymal stem (stromal) cells (MSCs). Cell adhesion to fibronectin induced α5β1-integrin-dependent phosphorylation of PDGFR-β in the absence of growth factor stimulation. Phosphorylated PDGFR-β co-immunoprecipitated with α5-integrin and colocalised with α5β1-integrin in the transient tidemarks of focal adhesions. Adhesion to fibronectin also strongly potentiated PDGF-BB-induced PDGFR-β phosphorylation and focal adhesion kinase (FAK) activity, in an α5β1-integrin-dependent manner. PDGFR-β-induced phosphoinositide 3-kinase (PI3K) and Akt activity, actin reorganisation and cell migration were all regulated by fibronectin and α5β1-integrin. This synergistic relationship between α5β1-integrin and PDGFR-β is a fundamental determinant of cell migration. Thus, fibronectin-rich matrices can prime PDGFR-β to recruit mesenchymal cells at sites of vascular remodelling.

Aquaporins at a glance.

Abstract: With a concentration of 55,000 mM, water is by far the most prevalent molecule in biological systems. For many years it was assumed that biological membranes are freely water permeable, so that the existence of a family of water channels would not have been predicted. However, biophysical studies in

Stress-specific composition, assembly and kinetics of stress granules in Saccharomyces cerevisiae

Abstract: Eukaryotic cells respond to cellular stresses by the inhibition of translation and the accumulation of mRNAs in cytoplasmic RNA–protein (ribonucleoprotein) granules termed stress granules and P-bodies. An unresolved issue is how different stresses affect formation of messenger RNP (mRNP) granules. In the present study, we examine how sodium azide (NaN3), which inhibits mitochondrial respiration, affects formation of mRNP granules as compared with glucose deprivation in budding yeast. We observed that NaN3 treatment inhibits translation and triggers formation of P-bodies and stress granules. The composition of stress granules induced by NaN3 differs from that of glucose-deprived cells by containing eukaryotic initiation factor (eIF)3, eIF4A/B, eIF5B and eIF1A proteins, and by lacking the heterogeneous nuclear RNP (hnRNP) protein Hrp1. Moreover, in contrast with glucose-deprived stress granules, NaN3-triggered stress granules show different assembly rules, form faster and independently from P-bodies and dock or merge with P-bodies over time. Strikingly, addition of NaN3 and glucose deprivation in combination, regardless of the order, always results in stress granules of a glucose deprivation nature, suggesting that both granules share an mRNP remodeling pathway. These results indicate that stress granule assembly, kinetics and composition in yeast can vary in a stress-specific manner, which we suggest reflects different rate-limiting steps in a common mRNP remodeling pathway.

LSR defines cell corners for tricellular tight junction formation in epithelial cells

Abstract: Epithelial cell contacts consist of not only bicellular contacts but also tricellular contacts, where the corners of three cells meet. At tricellular contacts, tight junctions (TJs) generate specialized structures termed tricellular TJs (tTJs) to seal the intercellular space. Tricellulin is the only known molecular component of tTJs and is involved in the formation of tTJs, as well as in the normal epithelial barrier function. However, the detailed molecular mechanism of how tTJs are formed and maintained remains elusive. Using a localization-based expression cloning method, we identified a novel tTJ-associated protein known as lipolysis-stimulated lipoprotein receptor (LSR). Upon LSR knockdown in epithelial cells, tTJ formation was affected and the epithelial barrier function was diminished. Tricellulin accumulation at the tricellular contacts was also diminished in these cells. By contrast, LSR still accumulated at the tricellular contacts upon tricellulin knockdown. Analyses of deletion mutants revealed that the cytoplasmic domain of LSR was responsible for the recruitment of tricellulin. On the basis of these observations, we propose that LSR defines tricellular contacts in epithelial cellular sheets by acting as a landmark to recruit tricellulin for tTJ formation.

BMP2, but not BMP4, is crucial for chondrocyte proliferation and maturation during endochondral bone development

Abstract: The BMP signaling pathway has a crucial role in chondrocyte proliferation and maturation during endochondral bone development. To investigate the specific function of the Bmp2 and Bmp4 genes in growth plate chondrocytes during cartilage development, we generated chondrocyte-specific Bmp2 and Bmp4 conditional knockout (cKO) mice and Bmp2,Bmp4 double knockout (dKO) mice. We found that deletion of Bmp2 and Bmp4 genes or the Bmp2 gene alone results in a severe chondrodysplasia phenotype, whereas deletion of the Bmp4 gene alone produces a minor cartilage phenotype. Both dKO and Bmp2 cKO mice exhibit severe disorganization of chondrocytes within the growth plate region and display profound defects in chondrocyte proliferation, differentiation and apoptosis. To understand the mechanism by which BMP2 regulates these processes, we explored the specific relationship between BMP2 and Runx2, a key regulator of chondrocyte differentiation. We found that BMP2 induces Runx2 expression at both the transcriptional and post-transcriptional levels. BMP2 enhances Runx2 protein levels through inhibition of CDK4 and subsequent prevention of Runx2 ubiquitylation and proteasomal degradation. Our studies provide novel insights into the genetic control and molecular mechanism of BMP signaling during cartilage development.

COPII-mediated vesicle formation at a glance.

Abstract: Eukaryotic cells contain a number of different membrane compartments that have specialized roles. Each compartment depends on a specific mixture of proteins for its identity and function. For many compartments, proteins arrive by way of small membrane vesicles that travel through the cell from an