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Sensitive factor attachment protein receptors (SNARE) proteins are important mediators of protein trafficking that regulate the membrane fusion of specific vesicle populations and their target organelles. The SNARE protein Ykt6 lacks a transmembrane domain and attaches to different organelle membranes. Mechanistically, Ykt6 activity is thought to be regulated by a conformational change from a closed cytosolic form to an open membrane-bound form, yet the mechanism that regulates this transition is unknown. We identified phosphorylation sites in the SNARE domain of Ykt6 that mediate Ykt6 membrane recruitment and are essential for cellular growth. Using proximity-dependent labeling and membrane fractionation, we found that phosphorylation regulates Ykt6 conversion from a closed to an open conformation. This conformational switch recruits Ykt6 to several organelle membranes, where it functionally regulates the trafficking of Wnt proteins and extracellular vesicle secretion in a concentration-dependent manner. We propose that phosphorylation of its SNARE domain leads to a conformational switch from a cytosolic, auto-inhibited Ykt6 to an active SNARE at different membranes.

Cryptochromes are a ubiquitous group of blue-light absorbing flavoproteins that in the mammalian retina have an important role in the circadian clock. In birds, cryptochrome 1a (Cry1a), localized in the UV/violet-sensitive S1 cone photoreceptors, is proposed to be the retinal receptor molecule of the light-dependent magnetic compass. The retinal localization of mammalian Cry1, homologue to avian Cry1a, is unknown, and it is open whether mammalian Cry1 is also involved in magnetic field sensing. To constrain the possible role of retinal Cry1, we immunohistochemically analysed 90 mammalian species across 48 families in 16 orders, using an antiserum against the Cry1 C-terminus that in birds labels only the photo-activated conformation. In the Carnivora families Canidae, Mustelidae and Ursidae, and in some Primates, Cry1 was consistently labeled in the outer segment of the shortwave-sensitive S1 cones. This finding would be compatible with a magnetoreceptive function of Cry1 in these taxa. In all other taxa, Cry1 was not detected by the antiserum that likely also in mammals labels the photo-activated conformation, although Western blots showed Cry1 in mouse retinal cell nuclei. We speculate that in the mouse and the other negative-tested mammals Cry1 is involved in circadian functions as a non-light-responsive protein.

The Wingless/Int1 (Wnt) signaling system plays multiple, essential roles in embryonic development, tissue homeostasis, and human diseases. Although many of the underlying signaling mechanisms are becoming clearer, the binding mode, kinetics, and selectivity of 19 mammalian WNTs to their receptors of the class Frizzled (FZD1–10) remain obscure. Attempts to investigate Wnt-FZD interactions are hampered by the difficulties in working with Wnt proteins and their recalcitrance to epitope tagging. Here, we used a fluorescently tagged version of mouse Wnt-3a for studying Wnt-FZD interactions. We observed that the enhanced GFP (eGFP)-tagged Wnt-3a maintains properties akin to wild-type (WT) Wnt-3a in several biologically relevant contexts. The eGFP-tagged Wnt-3a was secreted in an evenness interrupted (EVI)/Wntless-dependent manner, activated Wnt/β-catenin signaling in 2D and 3D cell culture experiments, promoted axis duplication in Xenopus embryos, stimulated low-density lipoprotein receptor-related protein 6 (LRP6) phosphorylation in cells, and associated with exosomes. Further, we used conditioned medium containing eGFP-Wnt-3a to visualize its binding to FZD and to quantify Wnt-FZD interactions in real time in live cells, utilizing a recently established NanoBRET-based ligand binding assay. In summary, the development of a biologically active, fluorescent Wnt-3a reported here opens up the technical possibilities to unravel the intricate biology of Wnt signaling and Wnt-receptor selectivity.

// Kerstin Menck 1,* , Florian Klemm 1,* , Julia Christina Gross 2 , Tobias Pukrop 1 , Dirk Wenzel 3 , Claudia Binder 1 1 Dept of Hematology/Oncology, University Medicine, Gottingen, Germany 2 Div. Signaling and Functional Genomics, German Cancer Research Center, Heidelberg, Germany 3 Max Planck Institute for Biophysical Chemistry, Gottingen, Germany * These authors contributed equally to this work. Correspondence: Claudia Binder, email: // Keywords : Breast cancer, exosomes, Evi, microvesicles, macrophages, Wnt 5a Received : August 27, 2013 Accepted : October 19, 2013 Published : October 21, 2013 Abstract Recently, we have shown that macrophage (MΦ)-induced invasion of breast cancer cells requires upregulation of Wnt 5a in MΦ leading to activation of β-Catenin-independent Wnt signaling in the tumor cells. However, it remained unclear, how malignant cells induce Wnt 5a in MΦ and how it is transferred back to the cancer cells. Here we identify two types of extracellular particles as essential for this intercellular interaction in both directions. Plasma membrane-derived microvesicles (MV) as well as exosomes from breast cancer cells, although biologically distinct populations, both induce Wnt 5a in MΦ. In contrast, the particle-free supernatant and vesicles from benign cells, such as platelets, have no such effect. Induction is antagonized by the Wnt inhibitor Dickkopf-1. Subsequently, Wnt 5a is shuttled via responding MΦ-MV and exosomes to the tumor cells enhancing their invasion. Wnt 5a export on both vesicle fractions depends at least partially on the cargo protein Evenness interrupted (Evi). Its knockdown leads to Wnt 5a depletion of both particle populations and reduced vesicle-mediated invasion. In conclusion, MV and exosomes are critical for MΦ-induced invasion of cancer cells since they are responsible for upregulation of MΦ-Wnt 5a as well as for its delivery to the recipient cells via a reciprocal loop. Although of different biogenesis, both populations share common features regarding function and Evi-dependent secretion of non-canonical Wnts.

Cryptochromes are a ubiquitous group of blue-light absorbing flavoproteins that in the mammalian retina have an important role in the circadian clock. In birds, cryptochrome 1a (Cry1a), localized in the UV/violet-sensitive S1 cone photoreceptors, is proposed to be the retinal receptor molecule of the light-dependent magnetic compass. The retinal localization of mammalian Cry1, homologue to avian Cry1a, is unknown and it is open whether mammalian Cry1 is also involved in magnetic field sensing. To constrain the possible role of retinal Cry1, we immunohistochemically analysed 90 mammalian species across 48 families in 16 orders, using an antiserum against the Cry1 C-terminus that in birds labels only the photo-activated conformation. In the Carnivora families Canidae, Mustelidae and Ursidae and in some Primates, Cry1 was consistently labeled in the outer segment of the shortwave-sensitive S1 cones. This finding would be compatible with a magnetoreceptive function of Cry1 in these taxa. In all other taxa, Cry1 was not detected by the antiserum that likely also in mammals labels the photo-activated conformation, although Western blots showed Cry1 in mouse retinal cell nuclei. We speculate that in the mouse and the other negative-tested mammals Cry1 is involved in circadian functions as a non-light-responsive protein.

Cryptochromes, blue-light absorbing proteins involved in the circadian clock, have been proposed to be the receptor molecules of the avian magnetic compass. In birds, several cryptochromes occur: Cryptochrome 2, Cryptochrome 4 and two splice products of Cryptochrome 1, Cry1a and Cry1b. With an antibody not distinguishing between the two splice products, Cryptochrome 1 had been detected in the retinal ganglion cells of garden warblers during migration. A recent study located Cry1a in the outer segments of UV/V-cones in the retina of domestic chickens and European robins, another migratory species. Here we report the presence of cryptochrome 1b (eCry1b) in retinal ganglion cells and displaced ganglion cells of European Robins, Erithacus rubecula. Immuno-histochemistry at the light microscopic and electron microscopic level showed eCry1b in the cell plasma, free in the cytosol as well as bound to membranes. This is supported by immuno-blotting. However, this applies only to robins in the migratory state. After the end of the migratory phase, the amount of eCry1b was markedly reduced and hardly detectable. In robins, the amount of eCry1b in the retinal ganglion cells varies with season: it appears to be strongly expressed only during the migratory period when the birds show nocturnal migratory restlessness. Since the avian magnetic compass does not seem to be restricted to the migratory phase, this seasonal variation makes a role of eCry1b in magnetoreception rather unlikely. Rather, it could be involved in physiological processes controlling migratory restlessness and thus enabling birds to perform their nocturnal flights. peerReviewed