La rentabilité de l'investissement dans l'éducation au Sénégal

Depuis les années 60, la rentabilité du système éducatif suscite un intérêt croissant auprès des décideurs lorsqu'il s'agit de faire un choix d'investissement. Toutefois, la plupart des travaux consacrés à cette problématique portent sur les rendements internes de l'éducation. Cette étude se propose de mesurer la rentabilité externe de l'investissement relativement important, consenti dans l'éducation par les ménages (rendement privé) et la collectivité (rendement social). Il est reconnu que l'État et les ménages sénégalais consacrent d'importantes ressources à l'éducation sans avoir cependant une idée précise du rendement de leur investissement. Cette étude devrait combler cette lacune. La méthode utilisée est celle du taux de rendement interne appliquée aux données de l'Enquête Sénégalaise Auprès des Ménages (ESAM) de 1995. Les revenus nets bruts proviennent de ceux de l'année 1995. L'étude montre qu'au Sénégal l'éducation est en général un secteur d'investissement rentable où le rendement privé est plus élevé que le rendement social. De plus, une analyse par sous-secteur fait ressortir que l'investissement social dans le secondaire et le supérieur n'est pas rentable contrairement à l'enseignement primaire et moyen. Enfin, le chômage et le redoublement ont un impact négatif sur les rentabilités sociale et privée.Rentabilité interne de l'éducation, méthode avantages-coût, rendements privés et sociaux

Mechanisms behind the polarized distribution of lipids in epithelial cells

Epithelial cells are polarized cells and typically display distinct plasma membrane domains: basal plasma membrane domains face the underlying tissue, lateral domains contact adjacent cells and apical domains face the exterior lumen. Each membrane domain is endowed with a specific macromolecular composition that constitutes the functional identity of that domain. Defects in apical-basal plasma membrane polarity altogether or more subtle defects in the composition of either apical or basal plasma membrane domain can give rise to severe diseases. Lipids are the main component of cellular membranes and mechanisms that control their polarized distribution in epithelial cells are emerging. In particular sphingolipids and phosphatidylinositol lipids have taken center stage in the organization of the apical and basolateral plasma membrane domain. This short review article discusses mechanisms that contribute to the polarized distribution of lipids in epithelial cells

Pseudomonas aeruginosa exploits a PIP3-dependent pathway to transform apical into basolateral membrane

Pseudomonas aeruginosa, an important human pathogen, preferentially binds and enters injured cells from the basolateral (BL) surface. We previously demonstrated that activation of phosphatidylinositol 3-kinase (PI3K) and Akt are necessary and sufficient for P. aeruginosa entry from the apical (AP) surface and that AP addition of phosphatidylinositol 3,4,5-trisphosphate (PIP3) is sufficient to convert AP into BL membrane (Kierbel, A., A. Gassama-Diagne, K. Mostov, and J.N. Engel. 2005. Mol. Biol. Cell. 16:2577–2585; Gassama-Diagne, A., W. Yu, M. ter Beest, F. Martin-Belmonte, A. Kierbel, J. Engel, and K. Mostov. 2006. Nat. Cell Biol. 8:963–970). We now show that P. aeruginosa subverts this pathway to gain entry from the AP surface. In polarized monolayers, P. aeruginosa binds near cell–cell junctions without compromising them where it activates and recruits PI3K to the AP surface. Membrane protrusions enriched for PIP3 and actin accumulate at the AP surface at the site of bacterial binding. These protrusions lack AP membrane markers and are comprised of BL membrane constituents, which are trafficked there by transcytosis. The end result is that this bacterium transforms AP into BL membrane, creating a local microenvironment that facilitates its colonization and entry into the mucosal barrier

Phosphoinositide 3-kinase p110δ promotes lumen formation through the enhancement of apico-basal polarity and basal membrane organization

Signalling triggered by adhesion to the extracellular matrix plays a key role in the spatial orientation of epithelial polarity and formation of lumens in glandular tissues. Phosphoinositide 3-kinase signalling in particular is known to influence the polarization process during epithelial cell morphogenesis. Here, using Madin–Darby canine kidney epithelial cells grown in 3D culture, we show that the p110d isoform of phosphoinositide 3-kinase co-localizes with focal adhesion proteins at the basal surface of polarized cells. Pharmacological, siRNA- or kinase-dead-mediated inhibition of p110d impair the early stages of lumen formation, resulting in inverted polarized cysts, with no laminin or type IV collagen assembly at cell/extracellular matrix contacts. p110d also regulates the organization of focal adhesions and membrane localization of dystroglycan. Thus, we uncover a previously unrecognized role for p110d in epithelial cells in the orientation of the apico-basal axis and lumen formation

Ca2+ influx and phosphoinositide signalling are essential for the establishment and maintenance of cell polarity in monospores from the red alga Porphyra yezoensis

The asymmetrical distribution of F-actin directed by cell polarity has been observed during the migration of monospores from the red alga Porphyra yezoensis. The significance of Ca2+ influx and phosphoinositide signalling during the formation of cell polarity in migrating monospores was analysed pharmacologically. The results indicate that the inhibition of the establishment of cell polarity, as judged by the ability of F-actin to localize asymmetrically, cell wall synthesis, and development into germlings, occurred when monospores were treated with inhibitors of the Ca2+ permeable channel, phospholipase C (PLC), diacylglycerol kinase, and inositol-1,4,5-trisphosphate receptor. Moreover, it was also found that light triggered the establishment of cell polarity via photosynthetic activity but not its direction, indicating that the Ca2+ influx and PLC activation required for the establishment of cell polarity are light dependent. By contrast, inhibition of phospholipase D (PLD) prevented the migration of monospores but not the asymmetrical localization of F-actin. Taken together, these findings suggest that there is functional diversity between the PLC and PLD signalling systems in terms of the formation of cell polarity; the former being critical for the light-dependent establishment of cell polarity and the latter playing a role in the maintenance of established cell polarity

Reversible binding and rapid diffusion of proteins in complex with inositol lipids serves to coordinate free movement with spatial information

Polyphosphoinositol lipids convey spatial information partly by their interactions with cellular proteins within defined domains. However, these interactions are prevented when the lipids' head groups are masked by the recruitment of cytosolic effector proteins, whereas these effectors must also have sufficient mobility to maximize functional interactions. To investigate quantitatively how these conflicting functional needs are optimized, we used different fluorescence recovery after photobleaching techniques to investigate inositol lipid–effector protein kinetics in terms of the real-time dissociation from, and diffusion within, the plasma membrane. We find that the protein–lipid complexes retain a relatively rapid (∼0.1–1 µm2/s) diffusion coefficient in the membrane, likely dominated by protein–protein interactions, but the limited time scale (seconds) of these complexes, dictated principally by lipid–protein interactions, limits their range of action to a few microns. Moreover, our data reveal that GAP1IP4BP, a protein that binds PtdIns(4,5)P2 and PtdIns(3,4,5)P3 in vitro with similar affinity, is able to “read” PtdIns(3,4,5)P3 signals in terms of an elongated residence time at the membrane

Complex polarity: building multicellular tissues through apical membrane traffic

The formation of distinct subdomains of the cell surface is crucial for multicellular organism development. The most striking example of this is apical-basal polarization. What is much less appreciated is that underpinning an asymmetric cell surface is an equally dramatic intracellular endosome rearrangement. Here, we review the interplay between classical cell polarity proteins and membrane trafficking pathways, and discuss how this marriage gives rise to cell polarization. We focus on those mechanisms that regulate apical polarization, as this is providing a number of insights into how membrane traffic and polarity are regulated at the tissue level

PIP5KIβ Selectively Modulates Apical Endocytosis in Polarized Renal Epithelial Cells

Localized synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] at clathrin coated pits (CCPs) is crucial for the recruitment of adaptors and other components of the internalization machinery, as well as for regulating actin dynamics during endocytosis. PtdIns(4,5)P2 is synthesized from phosphatidylinositol 4-phosphate by any of three phosphatidylinositol 5-kinase type I (PIP5KI) isoforms (α, β or γ). PIP5KIβ localizes almost exclusively to the apical surface in polarized mouse cortical collecting duct cells, whereas the other isoforms have a less polarized membrane distribution. We therefore investigated the role of PIP5KI isoforms in endocytosis at the apical and basolateral domains. Endocytosis at the apical surface is known to occur more slowly than at the basolateral surface. Apical endocytosis was selectively stimulated by overexpression of PIP5KIβ whereas the other isoforms had no effect on either apical or basolateral internalization. We found no difference in the affinity for PtdIns(4,5)P2-containing liposomes of the PtdIns(4,5)P2 binding domains of epsin and Dab2, consistent with a generic effect of elevated PtdIns(4,5)P2 on apical endocytosis. Additionally, using apical total internal reflection fluorescence imaging and electron microscopy we found that cells overexpressing PIP5KIβ have fewer apical CCPs but more internalized coated structures than control cells, consistent with enhanced maturation of apical CCPs. Together, our results suggest that synthesis of PtdIns(4,5)P2 mediated by PIP5KIβ is rate limiting for apical but not basolateral endocytosis in polarized kidney cells. PtdIns(4,5)P2 may be required to overcome specific structural constraints that limit the efficiency of apical endocytosis. © 2013 Szalinski et al

Arf6 regulates AP-1B–dependent sorting in polarized epithelial cells

Activated Arf6 directs membrane recruitment of the clathrin adaptor AP-1B to maintain membrane polarization in epithelial cells

The Distribution of Phosphatidylinositol 4,5-Bisphosphate in Acinar Cells of Rat Pancreas Revealed with the Freeze-Fracture Replica Labeling Method

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] is a phospholipid that has been implicated in multiple cellular activities. The distribution of PI(4,5)P2 has been analyzed extensively using live imaging of the GFP-coupled phospholipase C-δ1 pleckstrin homology domain in cultured cell lines. However, technical difficulties have prevented the study of PI(4,5)P2 in cells of in vivo tissues. We recently developed a method to analyze the nanoscale distribution of PI(4,5)P2 in cultured cells by using the quick-freezing and freeze-fracture replica labeling method. In principle, this method can be applied to any cell because it does not require the expression of artificial probes. In the present study, we modified the method to study cells of in vivo tissues and applied it to pancreatic exocrine acinar cells of the rat. We found that PI(4,5)P2 in the plasma membrane is distributed in an equivalent density in the apical and basolateral domains, but exists in a significantly higher concentration in the gap junction. The intracellular organelles did not show labeling for PI(4,5)P2. The results are novel or different from the reported distribution patterns in cell lines and highlight the importance of studying cells differentiated in vivo