Microglia, the immune cells of the brain, can have a beneficial effect in Alzheimer’s disease by phagocytosing amyloid-β. Two-photon in vivo imaging of neuron loss in the intact brain of living Alzheimer’s disease mice revealed an involvement of microglia in neuron elimination, indicated by locally increased number and migration velocity of microglia around lost neurons. Knockout of the microglial chemokine receptor Cx3cr1, which is critical in neuron-microglia communication, prevented neuron loss.

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Alzheimer's disease (AD) represents the most common age-related neurodegenerative disorder. It is characterized by the invariant accumulation of the ␤-amyloid peptide (A␤), which mediates synapse loss and cognitive impairment in AD. Current therapeutic approaches concentrate on reducing A␤ levels and amyloid plaque load via modifying or inhibiting the generation of A␤. Based on in vivo two-photon imaging, we present evidence that side effects on the level of dendritic spines may counteract the beneficial potential of these approaches. Two potent ␥-secretase inhibitors (GSIs), DAPT (N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester) and LY450139 (hydroxylvaleryl monobenzocaprolactam), were found to reduce the density of dendritic spines in wild-type mice. In mice deficient for the amyloid precursor protein (APP), both GSIs had no effect on dendritic spine density, demonstrating that ␥-secretase inhibition decreases dendritic spine density via APP. Independent of the effects of ␥-secretase inhibition, we observed a twofold higher density of dendritic spines in the cerebral cortex of adult APP-deficient mice. This observation further supports the notion that APP is involved in the modulation of dendritic spine density-shown here for the first time in vivo.

Protein transport into the mammalian endoplasmic reticulum depends on nucleoside triphosphates. Photoaffinity labelling of microsomes with azido‐ATP prevents protein transport at the level of association of precursor proteins with the components of the transport machinery, Sec61alpha and TRAM proteins. The same phenotype of inactivation was observed after depleting a microsomal detergent extract of ATP‐binding proteins by passage through ATP‐agarose and subsequent reconstitution of the pass‐through into proteoliposomes. Transport was restored by co‐reconstitution of the ATP eluate. This eluate showed eight distinct bands in SDS gels. We identified five lumenal proteins (Grp170, Grp94, BiP/Grp78, calreticulin and protein disulfide isomerase), one membrane protein (ribophorin I) and two ribosomal proteins (L4 and L5). In addition to BiP (Grp78), Grp170 was most efficiently retained on ATP‐agarose. Purified BiP did not stimulate transport activity. Sequence analysis revealed a striking similarity of Grp170 and the yeast microsomal protein Lhs1p which was recently shown to be involved in protein transport into yeast microsomes. We suggest that Grp170 mediates efficient insertion of polypeptides into the microsomal membrane at the expense of nucleoside triphosphates.

Caveolae and caveolin-containing detergent-insoluble glycolipid-enriched rafts (DIG) have been implicated to function as plasma membrane microcompartments or domains for the preassembly of signaling complexes, keeping them in the basal inactive state. So far, only limited in vivo evidence is available for the regulation of the interaction between caveolae-DIG and signaling components in response to extracellular stimuli. Here, we demonstrate that in isolated rat adipocytes, synthetic intracellular caveolin binding domain (CBD) peptide derived from caveolin-associated pp59Lyn (10 to 100 M) or exogenous phosphoinositolglycan derived from glycosyl-phosphatidylinositol (GPI) membrane protein anchor (PIG; 1 to 10 M) triggers the concentrationdependent release of caveolar components and the GPI-anchored protein Gce1, as well as the nonreceptor tyrosine kinases pp59Lyn and pp125 Fak, from interaction with caveolin (up to 45 to 85%). This dissociation, which parallels redistribution of the components from DIG to non-DIG areas of the adipocyte plasma membrane (up to 30 to 75%), is accompanied by tyrosine phosphorylation and activation of pp59Lyn and pp125Fak (up to 8-and 11-fold) but not of the insulin receptor. This correlates well to increased tyrosine phosphorylation of caveolin and the insulin receptor substrate protein 1 (up to 6-and 15-fold), as well as elevated phosphatidylinositol-3 kinase activity and glucose transport (to up to 7-and 13-fold). Insulin-mimetic signaling by both CBD peptide and PIG as well as redistribution induced by CBD peptide, but not by PIG, was blocked by synthetic intracellular caveolin scaffolding domain (CSD) peptide. These data suggest that in adipocytes a subset of signaling components is concentrated at caveolae-DIG via the interaction between their CBD and the CSD of caveolin. These inhibitory interactions are relieved by PIG. Thus, caveolae-DIG may operate as signalosomes for insulin-independent positive cross talk to metabolic insulin signaling downstream of the insulin receptor based on redistribution and accompanying activation of nonreceptor tyrosine kinases.