Ezrin/Radixin/Moesin Are Required for the Purinergic P2X7 Receptor (P2X7R)-dependent Processing of the Amyloid Precursor Protein

The amyloid precursor protein (APP) can be cleaved by α-secretases in neural cells to produce the soluble APP ectodomain (sAPPα), which is neuroprotective. We have shown previously that activation of the purinergic P2X7 receptor (P2X7R) triggers sAPPα shedding from neural cells. Here, we demonstrate that the activation of ezrin, radixin, and moesin (ERM) proteins is required for the P2X7R-dependent proteolytic processing of APP leading to sAPPα release. Indeed, the down-regulation of ERM by siRNA blocked the P2X7R-dependent shedding of sAPPα. We also show that P2X7R stimulation triggered the phosphorylation of ERM. Thus, ezrin translocates to the plasma membrane to interact with P2X7R. Using specific pharmacological inhibitors, we established the order in which several enzymes trigger the P2X7R-dependent release of sAPPα. Thus, a Rho kinase and the MAPK modules ERK1/2 and JNK act upstream of ERM, whereas a PI3K activity is triggered downstream. For the first time, this work identifies ERM as major partners in the regulated non-amyloidogenic processing of APP.     

Soluble organic matrices of aragonitic skeletons of Merulinidae (Cnidaria, Anthozoa)

Our interpretation of the overall taxonomy and evolution of the Scleractinia, the most important reef builders in tropical areas, has long depended exclusively on morphology of the calcareous skeletons. The reported series of physical and biochemical characterizations of skeletons and the mineralizing matrices extracted from the skeletons allow, for the first time, the level of biochemical diversity among corallites of the same family to be estimated. Similarities and differences observed in the micro- and nanostructures of the skeletons reflect those of the soluble organic matrices. Sulphur is mainly associated with sulphated acidic sugars. The role of sulphated sugars on the biomineralization processes is still underestimated. The resulting data suggest that environmental conditions may act on the mineralization process through the detailed compositions of the mineralizing matrices.     

Rab6-interacting protein 1 links Rab6 and Rab11 function

Rab11 and Rab6 guanosine triphosphatases are associated with membranes of the recycling endosomes (REs) and Golgi complex, respectively. Evidence indicates that they sequentially regulate a retrograde transport pathway between these two compartments, suggesting the existence of proteins that must co-ordinate their functions. Here, we report the characterization of two isoforms of a protein, Rab6-interacting protein 1 (R6IP1), originally identified as a Rab6-binding protein. R6IP1 also binds to Rab11A in its GTP-bound conformation. In interphase cells, R6IP1 is targeted to the Golgi in a Rab6-dependent manner but can associate with Rab11-positive compartments when the level of Rab11A is increased within the cells. Fluorescence resonance energy transfer analysis using fluorescence lifetime imaging shows that the overexpression of R6IP1 promotes an interaction between Rab11A and Rab6 in living cells. Accordingly, the REs marked by Rab11 and transferrin receptor are depleted from the cell periphery and accumulate in the pericentriolar area. However, endosomal and Golgi membranes do not appear to fuse with each other. We also show that R6IP1 function is required during metaphase and cytokinesis, two mitotic steps in which a role of Rab6 and Rab11 has been previously documented. We propose that R6IP1 may couple Rab6 and Rab11 function throughout the cell cycle.     

Suppression of skeletal growth in scleractinian corals by decreasing ambient carbonate-ion concentration: a cross-family comparison

Biogenic calcification is influenced by the concentration of available carbonate ions. The recent confirmation of this for hermatypic corals has raised concern over the future of coral reefs because [CO(3)(2-)] is a decreasing function of increasing pCO(2) in the atmosphere. As one of the overriding features of coral reefs is their diversity, understanding the degree of variability between species in their ability to cope with a change in [CO(3)(2-)] is a priority. We cultured four phylogenetically and physiologically different species of hermatypic coral (Acropora verweyi, Galaxea fascicularis, Pavona cactus and Turbinaria reniformis) under 'normal' (280 micromol kg(-1)) and 'low' (140 micromol kg(-1)) carbonate-ion concentrations. The effect on skeletogenesis was investigated quantitatively (by calcification rate) and qualitatively (by microstructural appearance of growing crystalline fibres using scanning electron microscopy (SEM)). The 'low carbonate' treatment resulted in a significant suppression of calcification rate and a tendency for weaker crystallization at the distal tips of fibres. However, while the calcification rate was affected uniformly across species (13-18% reduction), the magnitude of the microstructural response was highly species specific: crystallization was most markedly affected in A. verweyi and least in T. reniformis. These results are discussed in relation to past records and future predictions of carbonate variability in the oceans.     

Microstructural and physico-chemical characterization of ‘centers of calcification’ in septa of some Recent scleractinian corals

In order to define the value of the concept of ‘center of calcification’, an attempt has been made to collect microstructural, physical, and chemical data from these particular structures. In each of the fifteen species studied, these data are compared with similar characteristics observed in the surrounding fibrous tissue. Results lead to a paradoxical conclusion. Although the existence of centers of calcification is sometimes denied, they have been evidenced by various techniques in septa of all the studied species, that belong to various families. Thus, ‘centers of calcification’ appear to be a basic component in the development of corallian septal architecture. But taking into account their microstructural and chemical peculiarities allows to introduce some changes in the current view concerning their role in skeletogenesis of Scleractinia.     

Shell layers of the black-lip pearl oyster Pinctada margaritifera: matching microstructure and composition

The nacre-prism transition of the mollusc shell Pinctada margaritifera was studied using scanning electron microscopy, electron probe micro-analysis (EPMA) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Mineralogical change is correlated with a change in organic matrix. Previous analyses had shown that sugars were involved in the transition layer (fibrous aragonite). The new Energy Dispersive Spectrometry (EDS) and TOF-SIMS maps show that the modifications at the layer boundary are complex, and that proteins and lipids are also involved. Detailed TOF-SIMS maps show that the thick organic envelopes surrounding the prisms, and between the prisms and the fibrous aragonitic layer, are not composed by regular layers, but are a patchwork of various molecules. The amino acid compositions of the nacreous and prismatic layer are compared thanks to the TOF-SIMS localized analyses.     

Multiscale structure of calcite fibres of the shell of the brachiopod Terebratulina retusa

The shells of rhynchonelliform brachiopods have an outer (primary) layer of acicular calcite and an inner (secondary) layer of calcite fibres which are parallel to the shell exterior. Atomic force microscopy (AFM) reveals that these fibres are composed of large triangular nanogranules of about 600-650 nm along their long axis. The nanogranules are composites of organic and inorganic components. As the shell grows, the fibres elongate with the calcite c-axis perpendicular to the fibre axis as demonstrated by electron backscatter diffraction (EBSD). Thus, despite being a composite structure comprising granules that are themselves composites, each fibre is effectively a single crystal. The combination of AFM and EBSD reveals the details of the structure and crystallography of these fibres. This knowledge serves to identify those aspects of biological control that must be understood to enable comprehension of the biological control exerted on the construction of these exquisite biomineral structures.