Actin-membrane interaction in fibroblasts: what proteins are involved in this association?

In this review we discuss some of the proteins for which a role in linking actin to the fibroblast plasma membrane has been suggested. We focus on the family of proteins related to erythrocyte spectrin, proteins that have generally been viewed as having an organization and a function in actin-membrane attachment similar to those of erythrocyte spectrin. Experiments in which we precipitated the nonerythrocyte spectrin within living fibroblasts have led us to question this supposed similarity of organization and function of the nonerythrocyte and erythrocyte spectrins. Intracellular precipitation of fibroblast spectrin does not affect the integrity of the major actin-containing structures, the stress fiber microfilament bundles. Unexpectedly, however, we found that the precipitation of spectrin results in a condensation and altered distribution of the vimentin class of intermediate filaments in most cells examined. Although fibroblast spectrin may have a role in the attachment of some of the cortical, submembranous actin, it is surprising how little the intracellular immunoprecipitation of the spectrin affects the cells. Several proteins have been found concentrated at the ends of stress fibers, where the actin filaments terminate at focal contacts. Two of these proteins, alpha-actinin and fimbrin, have properties that suggest that they are not involved in the attachment of the ends of the bundles to the membrane but are more probably involved in the organization and cross-linking of the filaments within the bundles. On the other hand, vinculin and talin are two proteins that interact with each other and may form part of a chain of attachments between the ends of the microfilament bundles and the focal contact membrane. Their role in this attachment, however, has not been established and further work is needed to examine their interaction with actin and to identify any other components with which they may interact, particularly in the plasma membrane

Aggregation reroutes molecules from a recycling to a vesicle-mediated secretion pathway during reticulocyte maturation

Endocytosis of the Tf/TfR complex is essentially the only pathway active in maturing reticulocytes, while exosomes, formed by invagination of the endosomal membrane, pro,ide a mechanism to eliminate seemingly obsolescent proteins, including the TfR, when their function is completed, In this study, we examined molecular trafficking in the recycling and exosome-directed pathways during endocytosis in maturing reticulocytes. To this end, the dow of two exogenously inserted fluorescent lipid analogs, N-(N-[6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]caproyl])sphingomyelin (C6-NBD-SM) and N-(lissamine rhodamine B sulfonyl) phosphatidyl ethanolamine (N-Rh-PE) was monitored and compared to that of the transferrin (Tf)/Tf receptor (TfR) complex, Prior to elimination via exosomes, the TfR actively recycles with a half-time of approx. 2 minutes, The recycling kinetics of C6-NBD-SM, as bulk plasma membrane marker, are identical to those of the apoTf/TfR complex, as shown by fluorescence microscopy and biochemical analysis. By contrast, although efficiently internalized along the same pathway, N-Rh-PE does not return to the cell surface, More specifically, sucrose gradient analysis and immunoisolation experiments demonstrated that N-Rh-PE accumulates in exosomes, which are eventually released into the extracellular medium, Fluorometric measurements showed that exogenously inserted N-Rh-PE is present in the reticulocyte plasma membrane as small molecular clusters, Moreover, a close correlation was observed between the fate of crosslinked proteins, including the TfR and acetylcholinesterase (AChE), and the fate of the clustered lipid N-Rh-PE, Thus antibody-induced aggregation of specific proteins like the TfR and AChE, which are normally sorted into exosomes during reticulocyte maturation, enhances their shedding by the exosomal pathway. Taken together, the results support the hypothesis that aggregation of either proteins or lipids act as a general sorting signal for exosomal processing, thereby inhibiting reentry in a recycling pathway and providing an effective means for clearing molecules from the cell surface and their eventual elimination from the cells

APOBEC3G/3F mediates intrinsic resistance of monocyte-derived dendritic cells to HIV-1 infection

HIV-1 infects immature dendritic cells (iDCs), but infection is inefficient compared with activated CD4+ T cells and only involves a small subset of iDCs. We analyzed whether this could be attributed to specific cellular restrictions during the viral life cycle. To study env-independent restriction to HIV-1 infection, we used a single-round infection assay with HIV-1 pseudotyped with vesicular stomatitis virus G protein (HIV-VSVG). Small interfering RNA-mediated depletion of APOBEC3G/3F (A3G/3F), but not TRIM5α, enhanced HIV-1 infection of iDCs, indicating that A3G/3F controls the sensitivity of iDCs to HIV-1 infection. Furthermore, sequences of HIV reverse transcripts revealed G-to-A hypermutation of HIV genomes during iDC infection, demonstrating A3G/3F cytidine deaminase activity in iDCs. When we separated the fraction of iDCs that was susceptible to HIV, we found the cells to be deficient in A3G messenger RNA and protein. We also noted that during DC maturation, which further reduces susceptibility to infection, A3G levels increased. These findings high-light a role for A3G/3F in explaining the resistance of most DCs to HIV-1 infection, as well as the susceptibility of a fraction of iDCs. An increase in the A3G/3F-mediated intrinsic resistance of iDCs could result in a block of HIV infection at its mucosal point of entry

Flocking and reorientation transition in the 4-state active Potts model

We study the active 4-state Potts model (APM) on the square lattice in which active particles have four internal states corresponding to the four directions of motion. A local alignment rule inspired by the ferromagnetic 4-state Potts model and self-propulsion via biased diffusion according to the internal particle states leads to flocking at high densities and low noise. We compute the phase diagram of the APM and explore the flocking dynamics in the region, in which the high-density (liquid) phase coexists with the low-density (gas) phase and forms a fluctuating band of coherently moving particles. As a function of the particle self-propulsion velocity, a novel reorientation transition of the phase-separated profiles from transversal to longitudinal band motion is revealed, which is absent in the Vicsek model and the active Ising model. We further construct a coarse-grained hydrodynamic description of the model which validates the results for the microscopic model

Study of Human Cortical Microstructure Using Magnetization Transfer and T2* Mapping with Application in Multiple Sclerosis

RÉSUMÉ Description du problème: La sclérose en plaques (SEP) est une maladie dévastatrice touchant plus de 100.000 personnes au Canada (MS Society of Canada). Les déficits fonctionnels engendrés par la maladie peuvent se traduire en troubles moteurs, cognitifs et sensoriels ayant un grand impact sur les activités sociales et professionnelles des patients. Le coût socio-économique de la SEP est colossal. Premièrement, la qualité de vie des patients ainsi que celle de leur familles peut se voir considérablement altérée. Deuxièmement, les traitements diminuant les effets handicapant de la SEP sont extrêmement dispendieux, leur coût annuel est estimé à plusieurs milliards de dollars au Canada (Karampampa et al. 2012) ainsi qu’aux USA (Hartung et al. 2015). De nos jours, la SEP ne se soigne pas et les détails de sa pathophysiologie restent obscures. La SEP est une maladie du système nerveux central, chronique, inflammatoire et démyélinisante. Elle est caractérisé par la formation de lésions inflammatoires et démyélinisantes prenant place dans la moelle épinière et dans les matières blanche et grise du cerveau. Bien que l’Imagerie par Résonance Magnétique (IRM) soit resté l’outil principal de diagnostic de SEP, les lésions observées dans la matière blanche ne corrélaient que très peu avec les déficits fonctionnels observés. Récemment, il à été montré que la démyélinisation de la matière grise est un meilleur indice de l’aggravement fonctionnel (Mainero et al. 2015). Cependant, les techniques d’IRM classiques sont difficilement utilisables pour l’imagerie du cortex, en effet, son épaisseur est seulement 2 à 4 mm et la résolution spatiale d'une IRM standard est de l’ordre de grandeur de 1 mm, ce qui n’est pas suffisant pour examiner précisément la pathologies corticales. L’IRM à ultra-haut champ (7 Tesla) à été montré capable d’imager des détails microstructurels du cortex, grâce à un gain en résolution et en signal sur bruit. Récamment, il a été montré que la relaxation transverse (apellée T2*) acquise à 7 Tesla est un marqueur sensible de la progression de la pathologie corticale des patients polyscléreux, notamment, de la démyélinisation corticale (Pitt et al. 2010; Mainero et al. 2015; Cohen-Adad et al. 2011). Cependant, des effets confondants réduisent la spécificité qu’a le contraste T2* à quantifier la myéline (notamment, le contenu en fer, ou les vaisseaux sanguins) (Hwang et al. 2010; Lee et al. 2012). Une seconde mesure indépendante serait bénéfique pour augmenter la spécificité d’une potentielle estimation de quantitée de myéline. Le Ratio de Transfert de Magnétisation (MTR) à aussi été démontré sensible à la myéline dans le cortex (Derakhshan et al. 2014; Chen et al. 2013) et serait une excellent mesure----------ABSTRACT Problem description: Multiple Sclerosis (MS) is a devastating disease affecting around 100,000 people in Canada (MS Society of Canada). The functional deficits resulting from the disease include motor, cognitive and somatic troubles, affecting the social and professional activities of MS patients. The socio-economic cost of MS is colossal. Firstly, life quality of MS patients and those of their family members can be drastically hampered. Secondly, existing treatments that reduce handicapping effects of MS are expensive, with an annual cost estimated in billions of dollars in Canada (Karampampa et al. 2012) and in the USA (Hartung et al. 2015). To date, MS is not curable and its pathophysiological mechanisms are still obscure. MS is known to be a chronic, inflammatory, demyelinating disease of the central nervous system. It is characterized by the formation of inflammatory and demyelinating lesions in the spinal cord and in the brain’s white and gray matters. While Magnetic Resonance Imaging (MRI) has been the main tool for diagnosing MS, correlations of white matter lesions with functional deficits remain poor. Recently, it was shown that grey matter demyelination provides a more specific assessment of functional worsening (Mainero et al. 2015). However, it is difficult to image the grey matter with standard MRI methods because the cortex is only 2-4 mm thick and the spatial resolution of standard MRI system is on the order of 1 mm, which is not sufficient for proper examination of cortical pathology. Ultra-high field MRI (7 Tesla) was shown to reveal microstructural features thanks to an increase in signal to noise ratio and spatial resolution. Recently, transverse relaxation (characterized by a time constant: T2*) at 7 Tesla was shown to be a sensitive marker of pathology and disease progression associated with demyelination in the cortex of MS patients (Pitt et al. 2010; Mainero et al. 2015; Cohen-Adad et al. 2011). However, several confounds hamper the specificity of T2* measures (iron content, blood vessels) (Hwang et al. 2010; Lee et al. 2012). An independent measure would increase the specificity to the myelin content. Magnetization Transfer Ratio (MTR) imaging has been shown to be sensitive to myelin content (Derakhshan et al. 2014; Chen et al. 2013) and thus would be an excellent complementary measure because its underlying contrast mechanisms are different than that from T2*. However, mapping MTR and T2* in the cortex is challenging because the cortical ribbon is thin, highly convoluted and its geometry varies across individuals

How stickiness can speed up diffusion in confined systems

The paradigmatic model for heterogeneous media used in diffusion studies is built from reflecting obstacles and surfaces. It is well known that the crowding effect produced by these reflecting surfaces slows the dispersion of Brownian tracers. Here, using a general adsorption desorption model with surface diffusion, we show analytically that making surfaces or obstacles attractive can accelerate dispersion. In particular, we show that this enhancement of diffusion can exist even when the surface diffusion constant is smaller than that in the bulk. Even more remarkably, this enhancement effect occurs when the effective diffusion constant, when restricted to surfaces only, is lower than the effective diffusivity with purely reflecting boundaries. We give analytical formulas for this intriguing effect in periodic arrays of spheres as well as undulating micro-channels. Our results are confirmed by numerical calculations and Monte Carlo simulations