Preparation of biologically active subcellular fractions using the Balch homogenizer

RNF43 induces the turnover of protease-activated receptor 2 in colon cancer

Post-translational modification of G-protein coupled receptors (GPCRs) plays a central role in tissue hemostasis and cancer. The molecular mechanism of post-translational regulation of protease-activated receptors (PARs), a subgroup of GPCRs is yet understudied. Here we show that the cell-surface transmembrane E3 ubiquitin ligase ring finger 43 (RNF43) is a negative feedback regulator of PAR₂ , impacting PAR₂ -induced signaling and colon cancer growth. RNF43 co-associates with PAR₂ , promoting its membrane elimination and degradation as shown by reduced cell surface biotinylated PAR₂ levels and polyubiquitination. PAR₂ degradation is rescued by R-spondin2 in the presence of leucine-rich repeat-containing G-protein-coupled receptor5 (LGR5). In fact, PAR₂ acts jointly with LGR5, as recapitulated by increased β-catenin levels, transcriptional activity, phospho-LRP6, and anchorage-independent colony growth in agar. Animal models of the chemically induced AOM/DSS colon cancer of wt versus Par2/f2rl1 KO mice as also the 'spleen-liver' colon cancer metastasis, allocated a central role for PAR₂ in colon cancer growth and development. RNF43 is abundantly expressed in the Par2/f2rl1 KO-treated AOM/DSS colon tissues while its level is very low to nearly null in colon cancer adenocarcinomas of the wt mice. The same result is obtained in the 'spleen-liver' model of spleen-inoculated cells, metastasized to the liver. High RNF43 expression is observed in the liver upon shRNA -Par2 silencing. "Limited-dilution-assay" performed in mice in-vivo, assigned PAR₂ as a member of the cancer stem cell niche compartment. Collectively, we elucidate an original regulation of PAR₂ oncogene, a member of cancer stem cells, by RNF43 ubiquitin ligase. It impacts β-catenin signaling and colon cancer growth.

Simplet/Fam53b is required for Wnt signal transduction by regulating β-catenin nuclear localization

Canonical β-catenin-dependent Wnt signal transduction is important for several biological phenomena, such as cell fate determination, cell proliferation, stem cell maintenance and anterior-posterior axis formation. The hallmark of canonical Wnt signaling is the translocation of β-catenin into the nucleus where it activates gene transcription. However, the mechanisms regulating β-catenin nuclear localization are poorly understood. We show that Simplet/Fam53B (Smp) is required for Wnt signaling by positively regulating β-catenin nuclear localization. In the zebrafish embryo, the loss of smp blocks the activity of two β-catenin-dependent reporters and the expression of Wnt target genes, and prevents nuclear accumulation of β-catenin. Conversely, overexpression of smp increases β-catenin nuclear localization and transcriptional activity in vitro and in vivo. Expression of mutant Smp proteins lacking either the nuclear localization signal or the β-catenin interaction domain reveal that the translocation of Smp into the nucleus is essential for β-catenin nuclear localization and Wnt signaling in vivo. We also provide evidence that mammalian Smp is involved in regulating β-catenin nuclear localization: the protein colocalizes with β-catenin-dependent gene expression in mouse intestinal crypts; siRNA knockdown of Smp reduces β-catenin nuclear localization and transcriptional activity; human SMP mediates β-catenin transcriptional activity in a dose-dependent manner; and the human SMP protein interacts with human β-catenin primarily in the nucleus. Thus, our findings identify the evolutionary conserved SMP protein as a regulator of β-catenin-dependent Wnt signal transduction.

Isolation of a specialized, antigen-loaded early endosomal subpopulation by flow cytometry

Isolation and characterization of antigen-containing endosomes remains difficult utilizing standard purification techniques. Here, we describe a method, which allows isolation of antigen-loaded endosomes, that is based on flow cytometrical analysis and sorting. We specifically isolated antigen-containing endosomes from cells that had taken up fluorochrome-labeled ovalbumin via mannose receptor-mediated endocytosis. The protocol described here allows for the isolation of pure fractions of ovalbumin-containing endosomes and the extraction of proteins from these endosomes for analysis by western blot. Importantly, this protocol can be easily extended to other fluorochrome-labeled antigens and to primary antigen-presenting cells like bone marrow-derived dendritic cells.

Reconstitution assay system for ceramide transport with semi-intact cells

The intracellular transport of lipids from the sites of their synthesis to their appropriate destination is a critical step for lipid metabolism. One well-defined inter-organelle lipid movement is the transport of ceramide by ceramide transport protein (CERT). Ceramide, a key intermediate for both sphingomyelin and glycosphingolipids, is synthesized at the endoplasmic reticulum and delivered to the Golgi apparatus to be converted to sphingomyelin. CERT delivers ceramide from the ER to the Golgi apparatus in a non-vesicular and ATP-dependent manner. This chapter describes a reconstitution assay system for ceramide transport with semi-intact cells, which is useful for the study of the CERT-mediated inter-organelle transport of ceramide.

The STAT3 beacon: IL-6 recurrently activates STAT 3 from endosomal structures

Endocytic trafficking plays an important role in signal transduction. Signal transducer and activator of transcription 3 (STAT3) and mitogen-activate protein kinase (MAPK) have both been localized to endosomal structures and are dependent upon endocytosis for downstream function. While the dependence of MAPK signaling upon endosomes has been well characterized, the involvement of endosomes in regulating STAT3 signaling has not been defined. Consequently, this study evaluated the role of endosomes in the initiation, modulation, amplification and persistence of interleukin-6(IL-6)-induced STAT3 signal transduction and transcription, and utilized IL-6-induced MAPK signaling as a comparator. Using pharmacologic treatment and temperature control of endocytic trafficking, pulse-chase treatments and in vitro kinase assays, STAT3 was found to interact with endosomes in a markedly different fashion than MAPK. STAT3 was activated by direct interaction with internal structures upstream of the late endosome following IL-6 exposure and persistent STAT3 signaling depended upon recurrent activation from endocytic structures. Further, STAT3 subcellular localization was not dependent upon endocytic trafficking. Instead, STAT3 transiently interacted with endosomes and relocated to the nucleus by an endosome-independent mechanism. Finally, endocytic trafficking played a central role in regulating STAT3 serine 727 phosphorylation through crosstalk with the MAPK signaling system. Together, these data reveal endosomes as central to the genesis, course and outcome of STAT3 signal transduction and transcription.

Evidence for abnormal forward trafficking of AMPA receptors in frontal cortex of elderly patients with schizophrenia

Several lines of evidence point to alterations of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor trafficking in schizophrenia. Multiple proteins, including synapse-associated protein 97 (SAP97), glutamate receptor-interacting protein 1 (GRIP1), and N-ethylmaleimide sensitive factor (NSF), facilitate the forward trafficking of AMPA receptors toward the synapse. Once localized to the synapse, AMPA receptors are trafficked in a complex endosomal system. We hypothesized that alterations in the expression of these proteins and alterations in the subcellular localization of AMPA receptors in endosomes may contribute to the pathophysiology of schizophrenia. Accordingly, we measured protein expression of SAP97, GRIP1, and NSF in the dorsolateral prefrontal cortex and found an increase in the expression of SAP97 and GRIP1 in schizophrenia. To determine the subcellular localization of AMPA receptor subunits, we developed a technique to isolate early endosomes from post-mortem tissue. We found increased GluR1 receptor subunit protein in early endosomes in subjects with schizophrenia. Together, these data suggest that there is an alteration of forward trafficking of AMPA receptors as well as changes in the subcellular localization of an AMPA receptor subunit in schizophrenia.

A spatio-temporal analysis of matrix protein and nucleocapsid trafficking during vesicular stomatitis virus uncoating

To study VSV entry and the fate of incoming matrix (M) protein during virus uncoating we used recombinant viruses encoding M proteins with a C-terminal tetracysteine tag that could be fluorescently labeled using biarsenical (Lumio) compounds. We found that uncoating occurs early in the endocytic pathway and is inhibited by expression of dominant-negative (DN) Rab5, but is not inhibited by DN-Rab7 or DN-Rab11. Uncoating, as defined by the separation of nucleocapsids from M protein, occurred between 15 and 20 minutes post-entry and did not require microtubules or an intact actin cytoskeleton. Unexpectedly, the bulk of M protein remained associated with endosomal membranes after uncoating and was eventually trafficked to recycling endosomes. Another small, but significant fraction of M distributed to nuclear pore complexes, which was also not dependent on microtubules or polymerized actin. Quantification of fluorescence from high-resolution confocal micrographs indicated that after membrane fusion, M protein diffuses across the endosomal membrane with a concomitant increase in fluorescence from the Lumio label which occurred soon after the release of RNPs into the cytoplasm. These data support a new model for VSV uncoating in which RNPs are released from M which remains bound to the endosomal membrane rather than the dissociation of M protein from RNPs after release of the complex into the cytoplasm following membrane fusion.