Comparative proximity biotinylation implicates the small GTPase RAB18 in sterol mobilization and biosynthesis.

Loss of functional RAB18 causes the autosomal recessive condition Warburg Micro syndrome. To better understand this disease, we used proximity biotinylation to generate an inventory of potential RAB18 effectors. A restricted set of 28 RAB18 interactions were dependent on the binary RAB3GAP1-RAB3GAP2 RAB18-guanine nucleotide exchange factor complex. Twelve of these 28 interactions are supported by prior reports, and we have directly validated novel interactions with SEC22A, TMCO4, and INPP5B. Consistent with a role for RAB18 in regulating membrane contact sites, interactors included groups of microtubule/membrane-remodeling proteins, membrane-tethering and docking proteins, and lipid-modifying/transporting proteins. Two of the putative interactors, EBP and OSBPL2/ORP2, have sterol substrates. EBP is a Δ8-Δ7 sterol isomerase, and ORP2 is a lipid transport protein. This prompted us to investigate a role for RAB18 in cholesterol biosynthesis. We found that the cholesterol precursor and EBP-product lathosterol accumulates in both RAB18-null HeLa cells and RAB3GAP1-null fibroblasts derived from an affected individual. Furthermore, de novo cholesterol biosynthesis is impaired in cells in which RAB18 is absent or dysregulated or in which ORP2 expression is disrupted. Our data demonstrate that guanine nucleotide exchange factor-dependent Rab interactions are highly amenable to interrogation by proximity biotinylation and may suggest that Micro syndrome is a cholesterol biosynthesis disorder.

Next generation sequencing of progressive colorectal liver metastases after portal vein embolization.

Portal vein embolization (PVE) can be required to stimulate liver regeneration before hepatectomy for colorectal liver metastasis (CRCLM), however PVE may also trigger CRCLM progression in patients initially exhibiting chemotherapy response. Using RNA-seq, we aimed to determine the molecular networks involved in metastatic progression in this context. A prospective study including all CRCLM patients undergoing PVE prior to hepatectomy was conducted. Paired biopsies of metastatic lesions were obtained prior to and after PVE and total RNA was isolated and used to prepare Illumina rRNA-depleted TruSeq stranded cDNA libraries for HiSeq 100 bp paired-end sequencing. Patients were classified with progression of disease (PDPVE) or stable disease (SDPVE) post-PVE using 3D-CT tumor volumetric analysis. RESULTS: Twenty patients were included, 13 (65.0%) in the PDPVE group (median 58.0% (18.6-234.3) increase in tumor volume) and 7 (35.0%) in the SDPVE group exhibiting continuous chemotherapy response (median -14.3% (-40.8 to -2.8) decrease in tumor volume) (p < 0.0001). Our results showed that progressive CRCLM after PVE undergo gene expression changes that indicate activation of core cancer pathways (IL-17 (p = 5.94 × 10-03), PI3K (p = 8.71 × 10-03), IL6 and IGF-1 signaling pathways), consistent with changes driven by cytokines and growth factors. Differential expression analysis in a paired model of progression (EdgeR, DeSeq) identified significantly dysregulated genes in the PDPVE group (FOS, FOSB, RAB20, IRS2). CONCLUSION: Differentially expressed genes and pathways with known links to cancer and metastasis were identified post-PVE in patients with disease progression. Highlighting these molecular changes is a crucial first step towards development of targeted therapeutic strategies that may mitigate the effects of PVE on tumor growth.

Bicaudal-C associates with a Trailer Hitch/Me31B complex and is required for efficient Gurken secretion.

Bicaudal-C (Bic-C) is a multiple KH-domain RNA-binding protein required for Drosophila oogenesis and, maternally, for embryonic patterning. In early oogenesis, Bic-C negatively regulates target mRNAs, including Bic-C, by recruiting the CCR4 deadenylase through a direct association with its NOT3 subunit. Here, we identify a novel function for Bic-C in secretion of the TGF-alpha homolog Gurken (Grk). In Bic-C mutant egg chambers, Grk is sequestered within actin-coated structures during mid-oogenesis. As a consequence, Egfr signalling is not efficiently activated in the dorsal-anterior follicle cells. This phenotype is strikingly similar to that of trailer hitch (tral) mutants. Consistent with the idea that Bic-C and Tral act together in Grk secretion, Bic-C co-localizes with Tral within cytoplasmic granules, and can be co-purified with multiple protein components of a Tral mRNP complex. Taken together, our results implicate translational regulation by Bic-C and Tral in the secretory pathway.

Bicaudal-C recruits CCR4-NOT deadenylase to target mRNAs and regulates oogenesis, cytoskeletal organization, and its own expression.

Bicaudal-C (Bic-C) encodes an RNA-binding protein required maternally for patterning the Drosophila embryo. We identified a set of mRNAs that associate with Bic-C in ovarian ribonucleoprotein complexes. These mRNAs are enriched for mRNAs that function in oogenesis and in cytoskeletal regulation, and include Bic-C RNA itself. Bic-C binds specific segments of the Bic-C 5' untranslated region and negatively regulates its own expression by binding directly to NOT3/5, a component of the CCR4 core deadenylase complex, thereby promoting deadenylation. Bic-C overexpression induces premature cytoplasmic-streaming, a posterior-group phenotype, defects in Oskar and Kinesin heavy chain:betaGal localization as well as dorsal-appendage defects. These phenotypes are largely reciprocal to those of Bic-C mutants, and they affect cellular processes that Bic-C-associated mRNAs are known, or predicted, to regulate. We conclude that Bic-C regulates expression of specific germline mRNAs by controlling their poly(A)-tail length.

A new paradigm for translational control: inhibition via 5'-3' mRNA tethering by Bicoid and the eIF4E cognate 4EHP.

Translational control is a key genetic regulatory mechanism implicated in regulation of cell and organismal growth and early embryonic development. Initiation at the mRNA 5' cap structure recognition step is frequently targeted by translational control mechanisms. In the Drosophila embryo, cap-dependent translation of the uniformly distributed caudal (cad) mRNA is inhibited in the anterior by Bicoid (Bcd) to create an asymmetric distribution of Cad protein. Here, we show that d4EHP, an eIF4E-related cap binding protein, specifically interacts with Bcd to suppress cad translation. Translational inhibition depends on the Bcd binding region (BBR) present in the cad 3' untranslated region. Thus, simultaneous interactions of d4EHP with the cap structure and of Bcd with BBR renders cad mRNA translationally inactive. This example of cap-dependent translational control that is not mediated by canonical eIF4E defines a new paradigm for translational inhibition involving tethering of the mRNA 5' and 3' ends.