Single-molecule live-cell imaging of clathrin-based endocytosis

Lipid Transport and Metabolism at the Blood-Brain Interface: Implications in Health and Disease

Many prospective studies have shown that a diet enriched in omega-3 polyunsaturated fatty acids (n-3 PUFAs) can improve cognitive function during normal aging and prevent the development of neurocognitive diseases. However, researchers have not elucidated how n-3 PUFAs are transferred from the blood to the brain or how they relate to cognitive scores. Transport into and out of the central nervous system depends on two main sets of barriers: the blood-brain barrier (BBB) between peripheral blood and brain tissue and the blood-cerebrospinal fluid (CSF) barrier (BCSFB) between the blood and the CSF. In this review, the current knowledge of how lipids cross these barriers to reach the CNS is presented and discussed. Implications of these processes in health and disease, particularly during aging and neurodegenerative diseases, are also addressed. An assessment provided here is that the current knowledge of how lipids cross these barriers in humans is limited, which hence potentially restrains our capacity to intervene in and prevent neurodegenerative diseases.

Combined Toxicity of Metal Nanoparticles: Comparison of Individual and Mixture Particles Effect

Toxicity of metal nanoparticles (NPs) are closely associated with increasing intracellular reactive oxygen species (ROS) and the levels of pro-inflammatory mediators. However, NP interactions and surface complexation reactions alter the original toxicity of individual NPs. To date, toxicity studies on NPs have mostly been focused on individual NPs instead of the combination of several species. It is expected that the amount of industrial and highway-acquired NPs released into the environment will further increase in the near future. This raises the possibility that various types of NPs could be found in the same medium, thereby, the adverse effects of each NP either could be potentiated, inhibited or remain unaffected by the presence of the other NPs. After uptake of NPs into the human body from various routes, protein kinases pathways mediate their toxicities. In this context, family of mitogen-activated protein kinases (MAPKs) is mostly efficient. Despite each NP activates almost the same metabolic pathways, the toxicity induced by a single type of NP is different than the case of co-exposure to the combined NPs. The scantiness of toxicological data on NPs combinations displays difficulties to determine, if there is any risk associated with exposure to combined nanomaterials. Currently, in addition to mathematical analysis (Response surface methodology; RSM), the quantitative-structure-activity relationship (QSAR) is used to estimate the toxicity of various metal oxide NPs based on their physicochemical properties and levels applied. In this chapter, it is discussed whether the coexistence of multiple metal NPs alter the original toxicity of individual NP. Additionally, in the part of "Toxicity of diesel emission/exhaust particles (DEP)", the known individual toxicity of metal NPs within the DEP is compared with the data regarding toxicity of total DEP mixture.

Uptake of rabies virus into epithelial cells by clathrin-mediated endocytosis depends upon actin

Rabies virus (RABV) causes a fatal zoonotic encephalitis. Disease symptoms require replication and spread of the virus within neuronal cells; however, in infected animals as well as in cell culture the virus replicates in a broad range of cell types. Here we use a single-cycle RABV and a recombinant vesicular stomatitis virus (rVSV) in which the glycoprotein (G) was replaced with that of RABV (rVSV RABV G) to examine RABV uptake into the African green monkey kidney cell line BS-C-1. Combining biochemical studies and real-time spinning-disk confocal fluorescence microscopy, we show that the predominant entry pathway of RABV particles into BS-C-1 cells is clathrin dependent. Viral particles enter cells in pits with elongated structures and incomplete clathrin coats which depend upon actin to complete the internalization process. By measuring the time of internalization and the abundance of the clathrin adaptor protein AP2, we further show that the pits that internalize RABV particles are similar to those that internalize VSV particles. Pharmacological perturbations of dynamin or of actin polymerization inhibit productive infection, linking our observations on particle uptake with viral infectivity. This work extends to RABV particles the finding that clathrin-mediated endocytosis of rhabdoviruses proceeds through incompletely coated pits which depend upon actin.

Alpha-synuclein and polyunsaturated fatty acids promote clathrin-mediated endocytosis and synaptic vesicle recycling

Alpha-synuclein (alphaS) is an abundant neuronal cytoplasmic protein implicated in Parkinson's disease (PD), but its physiological function remains unknown. Consistent with its having structural motifs shared with class A1 apolipoproteins, alphaS can reversibly associate with membranes and help regulate membrane fatty acid composition. We previously observed that variations in alphaS expression level in dopaminergic cultured cells or brains are associated with changes in polyunsaturated fatty acid (PUFA) levels and altered membrane fluidity. We now report that alphaS acts with PUFAs to enhance the internalization of the membrane-binding dye, FM 1-43. Specifically, alphaS expression coupled with exposure to physiological levels of certain PUFAs enhanced clathrin-mediated endocytosis in neuronal and non-neuronal cultured cells. Moreover, alphaS expression and PUFA-enhanced basal and -evoked synaptic vesicle (SV) endocytosis in primary hippocampal cultures of wild type (wt) and genetically depleted alphaS mouse brains. We suggest that alphaS and PUFAs normally function in endocytic mechanisms and are specifically involved in SV recycling upon neuronal stimulation.

Unrestricted diffusion of exogenous and endogenous PIP(2 )in baby hamster kidney and Chinese hamster ovary cell plasmalemma

We used two approaches to characterize the lateral mobility of phosphatidylinositol 4,5-bisphosphate (PIP(2)) in the plasmalemma of baby hamster kidney and Chinese hamster ovary fibroblasts. First, nitrobenzoxadiazole-labeled C6-phosphatidylcholine and C16-PIP(2) were incorporated into plasma membrane "lawns" ( approximately 20 x 30 microm) from these cells and into the outer monolayer of intact cells. Diffusion coefficients determined by fluorescence recovery after photobleaching were similar for the two lipids and were higher in lawns, approximately 0.3 microm(2)/s, than on the cell surface, approximately 0.1 microm(2)/s. For membrane lawns, the fractional recoveries (75-90%) were close to those expected from the fraction of total membrane bleached, and labeling by the probes was several times greater than for intact cells. Second, we analyzed cells expressing M1 muscarinic receptors and green fluorescent protein fused with PIP(2)-binding pleckstrin-homology domains, Tubby domains or diacylglycerol (DAG)-binding C1 domains. On-cell gigaseal patches were formed with pipette tips >5 microm in diameter. When the agonist carbachol (0.3 mM: ) was applied either within or outside of the pipette, lipid signals crossed the pipette barrier rapidly in both directions and membrane blebbing occurred on both membrane sides. Accurate simulations of lipid gradients required diffusion coefficients >1 microm(2)/s. Exogenous DAG also crossed the pipette barrier rapidly. In summary, we found no evidence for restricted diffusion of signaling lipids in these cells. The lower mobility and incorporation of phospholipid at the extracellular leaflet may reflect a more ordered and condensed extracellular monolayer, as expected from previous studies.

Bluetongue virus entry into cells

Bluetongue virus (BTV) is a member of the Orbivirus genus within the Reoviridae family. Like those of other members of the family, BTV particles are nonenveloped and contain two distinct capsids, namely, an outer capsid and an inner capsid or core. The two outer capsid proteins, VP2 and VP5, are involved in BTV entry into cells and in the delivery of the transcriptionally active core to the target cell cytoplasm. However, very little is known about the precise mechanism of BTV entry. In this report, using RNA interference, we demonstrate that inhibition of the clathrin-dependent endocytic pathway correlates with reduced BTV internalization and subsequent replication. Furthermore, by using the ATPase inhibitor bafilomycin A1, we show that exposure of the virus to acidic pH is required for productive infection. Moreover, microscopic analysis of cells incubated with BTV indicated that the virus is internalized into early endosomes, where separation of the outer capsid and inner core occurs. Together, our data indicate that BTV undergoes low-pH-induced penetration in early endosomes following clathrin-mediated endocytosis from the plasma membrane, supporting a stepwise model for BTV entry and penetration.

Membrane curvature and mechanisms of dynamic cell membrane remodelling

Membrane curvature is no longer seen as a passive consequence of cellular activity but an active means to create membrane domains and to organize centres for membrane trafficking. Curvature can be dynamically modulated by changes in lipid composition, the oligomerization of curvature scaffolding proteins and the reversible insertion of protein regions that act like wedges in membranes. There is an interplay between curvature-generating and curvature-sensing proteins during vesicle budding. This is seen during vesicle budding and in the formation of microenvironments. On a larger scale, membrane curvature is a prime player in growth, division and movement.

Phosphoinositide regulation of clathrin-mediated endocytosis

Endocytosis of transmembrane receptors largely occurs via clathrin-coated vesicles that bud from the plasma membrane and deliver their cargo to the endosomal system for recycling or degradation. PIs (phosphoinositides) control the timing and localization of endocytic membrane trafficking by recruiting adaptors and other components of the transport machinery, thereby being part of a coincidence detection system in adaptor-mediated vesicle transport. Activation of organelle- and substrate-specific PI kinases by small GTPases such as Arf (ADP-ribosylation factor) and other factors may result in local changes of PI content, thereby regulating activity-dependent endocytic events including the recycling of synaptic vesicle membranes at nerve terminals. One such example is the PtdIns(4)P 5-kinase-mediated formation of PI(4,5)P2 [PtdIns(4,5)P2], which is required for the exo- and endo-cytic cycling of presynaptic vesicles and secretory granules. Over the last few years, protein X-ray crystallography in combination with biochemical and cell biological assays has been used to investigate the structure and function of many PI-binding proteins, including protein components of the endocytic machinery. These studies have provided molecular insights into the mechanisms by which PI(4,5)P2 recruits and activates adaptor proteins and their binding partners. In this mini-review, I will discuss the pathways of PI(4,5)P2 formation and its interactions with endocytic trafficking adaptors.