The iron oxide mineral magnetite (Fe 3 O 4 ) is produced by various organisms to exploit magnetic and mechanical properties. Magnetotactic bacteria have become one of the best model organisms for studying magnetite biomineralization, as their genomes are sequenced and tools are available for their genetic manipulation. However, the chemical route by which magnetite is formed intracellularly within the so-called magnetosomes has remained a matter of debate. Here we used X-ray absorption spectroscopy at cryogenic temperatures and transmission electron microscopic imaging techniques to chemically characterize and spatially resolve the mechanism of biomineralization in those microorganisms. We show that magnetite forms through phase transformation from a highly disordered phosphate-rich ferric hydroxide phase, consistent with prokaryotic ferritins, via transient nanometric ferric (oxyhydr)oxide intermediates within the magnetosome organelle. This pathway remarkably resembles recent results on synthetic magnetite formation and bears a high similarity to suggested mineralization mechanisms in higher organisms.ferrihydrite | geomagnetism | hematite | nanoparticles | precursors

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are endocytosed by a clathrin- independent pathway into vesicles named GPI-AP–enriched early endosomal compartments (GEECs). We recently showed that the vacuolating toxin VacA secreted by Helicobacter pylori is endocytosed into the GEECs (Gauthier, N.C., P. Monzo, V. Kaddai, A. Doye, V. Ricci, and P. Boquet. 2005. Mol. Biol. Cell. 16:4852–4866). Unlike GPI-APs that are mostly recycled back to the plasma membrane, VacA reaches early endosomes (EEs) and then late endosomes (LEs), where vacuolation occurs.In this study, we used VacA to study the trafficking pathway between GEECs and LEs. We found that VacA routing from GEECs to LEs required polymerized actin. During this trafficking, VacA was transferred from GEECs to EEs associated with polymerized actin structures. The CD2-associated protein (CD2AP), a docking protein implicated in intracellular trafficking, bridged the filamentous actin (F-actin) structures with EEs containing VacA. CD2AP regulated those F-actin structures and was required to transfer VacA from GEECs to LEs. These results demonstrate that sorting from GEECs to LEs requires dynamic F-actin structures on EEs.

Activation of the p53 pathway in adipose tissue contributes to insulin resistance associated with obesity. However, the mechanisms of p53 activation and the effect on adipocyte functions are still elusive. Here we found a higher level of DNA oxidation and a reduction in telomere length in adipose tissue of mice fed a high-fat diet and an increase in DNA damage and activation of the p53 pathway in adipocytes. Interestingly, hallmarks of chronic DNA damage are visible at the onset of obesity. Furthermore, injection of lean mice with doxorubicin, a DNA damage-inducing drug, increased the expression of chemokines in adipose tissue and promoted its infiltration by proinflammatory macrophages and neutrophils together with adipocyte insulin resistance. In vitro, DNA damage in adipocytes increased the expression of chemokines and triggered the production of chemotactic factors for macrophages and neutrophils. Insulin signaling and effect on glucose uptake and Glut4 translocation were decreased, and lipolysis was increased. These events were prevented by p53 inhibition, whereas its activation by nutlin-3 reproduced the DNA damage-induced adverse effects. This study reveals that DNA damage in obese adipocytes could trigger p53-dependent signals involved in alteration of adipocyte metabolism and secretory function leading to adipose tissue inflammation, adipocyte dysfunction, and insulin resistance.

Insulin-induced IRS-1 serine phosphorylation is a complex process involving different sites and kinases. This complexity could be physiologically important to accurately regulate insulin signalling. Abnormal phosphorylation of these serine residues in hyperinsulinaemic state could participate in the down-regulation of insulin signalling.