Phosphatidic acid phospholipase A1 mediates ER-Golgi transit of a family of G protein-coupled receptors.

The coat protein II (COPII)-coated vesicular system transports newly synthesized secretory and membrane proteins from the endoplasmic reticulum (ER) to the Golgi complex. Recruitment of cargo into COPII vesicles requires an interaction of COPII proteins either with the cargo molecules directly or with cargo receptors for anterograde trafficking. We show that cytosolic phosphatidic acid phospholipase A1 (PAPLA1) interacts with COPII protein family members and is required for the transport of Rh1 (rhodopsin 1), an N-glycosylated G protein-coupled receptor (GPCR), from the ER to the Golgi complex. In papla1 mutants, in the absence of transport to the Golgi, Rh1 is aberrantly glycosylated and is mislocalized. These defects lead to decreased levels of the protein and decreased sensitivity of the photoreceptors to light. Several GPCRs, including other rhodopsins and Bride of sevenless, are similarly affected. Our findings show that a cytosolic protein is necessary for transit of selective transmembrane receptor cargo by the COPII coat for anterograde trafficking.

In vivo reconstitution of gamma-secretase in Drosophila results in substrate specificity.

The intramembrane aspartyl protease gamma-secretase plays a fundamental role in several signaling pathways involved in cellular differentiation and has been linked with a variety of human diseases, including Alzheimer's disease. Here, we describe a transgenic Drosophila model for in vivo-reconstituted gamma-secretase, based on expression of epitope-tagged versions of the four core gamma-secretase components, Presenilin, Nicastrin, Aph-1, and Pen-2. In agreement with previous cell culture and yeast studies, coexpression of these four components promotes the efficient assembly of mature, proteolytically active gamma-secretase. We demonstrate that in vivo-reconstituted gamma-secretase has biochemical properties and a subcellular distribution resembling those of endogenous gamma-secretase. However, analysis of the cleavage of alternative substrates in transgenic-fly assays revealed unexpected functional differences in the activity of reconstituted gamma-secretase toward different substrates, including markedly reduced cleavage of some APP family members compared to cleavage of the Notch receptor. These findings indicate that in vivo under physiological conditions, additional factors differentially modulate the activity of gamma-secretase toward its substrates. Thus, our approach for the first time demonstrates the overall functionality of reconstituted gamma-secretase in a multicellular organism and the requirement for substrate-specific factors for efficient in vivo cleavage of certain substrates.

The big brain aquaporin is required for endosome maturation and notch receptor trafficking.

Activity of the big brain (bib) gene influences Notch signaling during Drosophila nervous system development. We demonstrate that Bib, which belongs to the aquaporin family of channel proteins, is required for endosome maturation in Drosophila epithelial cells. In the absence of Bib, early endosomes arrest and form abnormal clusters, and cells exhibit reduced acidification of endocytic trafficking organelles. Bib acts downstream of Hrs in early endosome morphogenesis and regulates biogenesis of endocytic compartments prior to the formation of Rab7-containing late endosomes. Abnormal endosome morphology caused by loss of Bib is accompanied by overaccumulation of Notch, Delta, and other signaling molecules as well as reduced intracellular trafficking of Notch to nuclei. Analysis of several endosomal trafficking mutants reveals a correlation between endosomal acidification and levels of Notch signaling. Our findings reveal an unprecedented role for an aquaporin in endosome maturation, trafficking, and acidification.

Endosomal entry regulates Notch receptor activation in Drosophila melanogaster.

Signaling through the transmembrane receptor Notch is widely used throughout animal development and is a major regulator of cell proliferation and differentiation. During canonical Notch signaling, internalization and recycling of Notch ligands controls signaling activity, but the involvement of endocytosis in activation of Notch itself is not well understood. To address this question, we systematically assessed Notch localization, processing, and signaling in a comprehensive set of Drosophila melanogaster mutants that block access of cargo to different endocytic compartments. We find that gamma-secretase cleavage and signaling of endogenous Notch is reduced in mutants that impair entry into the early endosome but is enhanced in mutants that increase endosomal retention. In mutants that block endosomal entry, we also uncover an alternative, low-efficiency Notch trafficking route that can contribute to signaling. Our data show that endosomal access of the Notch receptor is critical to achieve physiological levels of signaling and further suggest that altered residence in distinct endocytic compartments could underlie pathologies involving aberrant Notch pathway activation.

gamma-cleavage-independent functions of presenilin, nicastrin, and Aph-1 regulate cell-junction organization and prevent tau toxicity in vivo.

Genetic analysis of familial Alzheimer's disease has revealed that mutations in the gamma-secretase enzyme presenilin promote toxic Abeta secretion; however, presenilin mutations might also influence tau hyperphosphorylation and neurodegeneration through gamma-secretase-independent mechanisms. To address this possibility and determine whether other components of the gamma-secretase complex possess similar regulatory functions, we analyzed the roles of presenilin, nicastrin, and aph-1 in a Drosophila model for tau-induced neurodegeneration. Here, we show that presenilin and nicastrin prevent tau toxicity by modulating the PI3K/Akt/GSK3beta phosphorylation pathway, whereas aph-1 regulates aPKC/PAR-1 activities. Moreover, we found that these transmembrane proteins differentially regulate the intracellular localization of GSK3beta and aPKC at cell junctions. Inhibition of gamma-secretase activity neither interfered with these kinase pathways nor induced aberrant tau phosphorylation. These results establish new in vivo molecular functions for the three components of the gamma-secretase complex and reveal a different mechanism that might contribute to neuronal degeneration in Alzheimer's disease.

Notch signaling: a different sort makes the cut.

Notch signaling is regulated by ubiquitination of the receptor and its extracellular ligands. New studies reveal distinct ubiquitination-dependent endosomal sorting pathways in which ligand-bound Notch is activated while unliganded Notch is recycled or degraded, facilitating signaling while preventing inappropriate activation of unstimulated receptors.

Molecular pathology of dityrosine cross-links in proteins: structural and functional analysis of four proteins.

The dityrosine bond (DT) is an oxidative covalent cross-link between two tyrosines. DT cross-linking is increasingly identified as a marker of oxidative stress, aging and disease, and has been detected in diverse pathologies. While DT cross- linked proteins have been documented, the consequences of the DT link on the structure and function of the so modified proteins are yet to be understood. With this in view, we have studied the properties of intermolecular DT-dimers of four proteins of diverse functions, namely the enzyme ribonuclease A, the signal protein calmodulin, and the eye lens proteins alpha- and gamma B-crystallins. We find that DT is formed through radical reactions and type I photosensitization (including .OH, O2- and OONO-), but not by 1O2 and NO, (which modify his, trp and met more readily). Tyr residues on the surface of the protein make DT bonds (intra- and intermolecular) most readily and preferentially. The conformation of each of these DT-dimers, monitored by spectroscopy, is seen not to be significantly altered in comparison to that of the parent monomer, but the structural stability of the DT cross-linked molecule is lower than that of the parent native monomer. The DT-dimer is denatured at a lower temperature, and at lower concentrations of urea or guanidinium chloride. The effect of DT-cross-linking on the biological activities of these proteins was next studied. The enzymatic activity of the DT-dimer of ribonuclease A is not lost but lowered. DT-dimerization of lens alpha-crystallin did not significantly affect the chaperone-like ability; it inhibits the self-aggregation and precipitation of target proteins just as well as the parent, unmodified alpha-crystallin does. DT-dimerization of gamma B-crystallin is however seen to lead to more ready aggregation and precipitation, a point of interest in cataract. In the case of calmodulin, we could generate both intermolecular and intramolecular DT cross-linking, and study both the DT-dimer and DT-monomer. The DT-dimer binds smooth muscle light chain kinase and also Ca2+, but less efficiently and over a broad concentration range than the native monomer. The intramolecular DT-monomer is weaker in all these respects, presumably since it is structurally more constrained. These results suggest that DT cross-linking of globular proteins weakens their structural stability and compromises (though does not abolish) their biological activity, both of which are pathologically relevant. The intramolecular DT cross-link would appear to lead to more severe structural and functional consequences.