The ARF GAPs ELMOD1 and ELMOD3 act at the Golgi and Cilia to Regulate Ciliogenesis and Ciliary Protein Traffic

ELMODs are a family of three mammalian paralogs that display GTPase activating protein (GAP) activity towards a uniquely broad array of ADP-ribosylation factor (ARF) family GTPases that includes ARF-like (ARL) proteins. ELMODs are ubiquitously expressed in mammalian tissues, highly conserved across eukaryotes, and ancient in origin, being present in the last eukaryotic common ancestor. We described functions of ELMOD2 in immortalized mouse embryonic fibroblasts (MEFs) in the regulation of cell division, microtubules, ciliogenesis, and mitochondrial fusion. Here, using similar strategies with the paralogs ELMOD1 and ELMOD3, we identify novel functions and locations of these cell regulators and compare them to those of ELMOD2, allowing determination of functional redundancy among the family members. We found strong similarities in phenotypes resulting from deletion of either Elmod1 or Elmod3 and marked differences from those arising in Elmod2 deletion lines. Deletion of either Elmod1 or Elmod3 results in the decreased ability of cells to form primary cilia, loss of a subset of proteins from cilia, and accumulation of some ciliary proteins at the Golgi, predicted to result from compromised traffic from the Golgi to cilia. These phenotypes are reversed upon activating mutant expression of either ARL3 or ARL16, linking their roles to ELMOD1/3 actions.

Stimulation of immunity-linked genes by membrane disruption is linked to Golgi function and the ARF-1 GTPase

Immune-linked genes (ILGs) are activated in response to pathogens but can also be activated by lipid imbalance. Why pathogen attack and metabolic changes both impact ILG activation is unclear. Organelles in the secretory pathway have distinct protein and lipid components and genetically separable stress programs. These stress pathways activate restorative transcriptional programs when lipid ratios become unbalanced or during dysregulated protein folding and trafficking. We find that ILGs are specifically activated when membrane phosphatidylcholine ratios change in the secretory pathway. Consistent with this result, disruption of Golgi function in mutations targeting the ADP-ribosylation factor ARF-1 also activates ILG expression. Since increased protein secretion is altered by metabolic changes and pathogen responses, our data argue that ILG upregulation is a conserved, coordinated response to changes in trafficking resulting from intrinsic cues (lipid changes) or extrinsic stimulation (during the immune response). These findings uncover important and previously unexplored links between metabolism and the stress response.

The ARF GAPs ELMOD1 and ELMOD3 act at the Golgi and Cilia to Regulate Ciliogenesis and Ciliary Protein Traffic

ELMODs are a family of three mammalian paralogs that display GTPase activating protein (GAP) activity towards a uniquely broad array of ADP-ribosylation factor (ARF) family GTPases that includes ARF-like (ARL) proteins. ELMODs are ubiquitously expressed in mammalian tissues, highly conserved across eukaryotes, and ancient in origin, being present in the last eukaryotic common ancestor. We described functions of ELMOD2 in immortalized mouse embryonic fibroblasts (MEFs) in the regulation of cell division, microtubules, ciliogenesis, and mitochondrial fusion. Here, using similar strategies with the paralogs ELMOD1 and ELMOD3, we identify novel functions and locations of these cell regulators and compare them to those of ELMOD2, allowing determination of functional redundancy among the family members. We found strong similarities in phenotypes resulting from deletion of either Elmod1 or Elmod3 and marked differences from those arising in Elmod2 deletion lines. Deletion of either Elmod1 or Elmod3 results in the decreased ability of cells to form primary cilia, loss of a subset of proteins from cilia, and accumulation of some ciliary proteins at the Golgi, predicted to result from compromised traffic from the Golgi to cilia. These phenotypes are reversed upon expression of activating mutants of either ARL3 or ARL16, linking their roles to ELMOD1/3 actions. Thus, we believe that ELMOD1 and ELMOD3 perform multiple functions in cells, most prominently linked to ciliary biology and Golgi-ciliary traffic, and likely acting from more than one cellular location.

Role of ADP Ribosylation Factor Guanylate Kinase 1 in the Malignant Biological Behavior of Gastric Cancer

Objectives: To study the effects of ADP ribosylation factor guanylate kinase 1 gene (ASAP1) on the biological behavior of malignant gastric cancer (GC), and explore its possible molecular mechanisms in tumorigenesis and tumor progression.Methods: Quantitative PCR and western blotting (WB) were performed to measure ASAP1 mRNA and protein expression in the GES-1 epithelial cell line, which is derived from normal human mucosa, and three GC cell lines. Molecular biology techniques such as lentivirus packaging, infection, and screening were used to obtain BGC823 GC cells overexpressing ASAP1 and BGC823 and MKN45 cells with ASAP1 knocked down. The Cell Counting Kit-8 assay, colony formation assay, flow cytometry using Annexin V/propidium iodide, Transwell migration and invasion assays, and scratch assay were used to assess the malignant biological behavior of GC cells with ASAP1 overexpression and knockdown. WB was conducted to evaluate the effects of ASAP1 expression on angiogenesis, as well as on the expression of matrix metalloproteinases (MMPs), apoptotic proteins, and epithelial-mesenchymal transition (EMT)-related proteins. Nude mice bearing transplanted tumors were evaluated to determine the effect of ASAP1 knockdown on BGC823 GC cells.Results: ASAP1 expression in GC cells was greater than that in GES-1 normal gastric mucosal epithelial cells. ASAP1 overexpression significantly enhanced the proliferation, invasion, and migration of GC cells and reduced apoptosis; whereas ASAP1 knockdown significantly reduced the proliferation, invasion, and migration of GC cells and promoted apoptosis. In the ASAP1-knockdown group, expression of cleaved-caspase 3, cleaved-poly-ADP-ribose polymerase (PARP), and the epithelial marker E-cadherin increased significantly, whereas the expression of MMP2, MMP9, vascular endothelial growth factor A (VEGFA), and the mesenchymal markers N-cadherin, and vimentin decreased significantly (P<0.01). Knockdown of ASAP1 inhibited the growth of subcutaneously implanted tumors in nude mice.Conclusions: ASAP1 overexpression strongly promotes—whereas knockdown of ASAP1 effectively weakens—the malignant biological behavior of GC cells, possibly by reducing VEGFA expression and thus reducing angiogenesis, upregulating the expression of cleaved-caspase 3 and cleaved-PARP, and reducing the activity of MMPs and EMT.

ARF GTPases and Their Ubiquitous Role in Intracellular Trafficking Beyond the Golgi

Molecular switches of the ADP-ribosylation factor (ARF) GTPase family coordinate intracellular trafficking at all sorting stations along the secretory pathway, from the ER-Golgi-intermediate compartment (ERGIC) to the plasma membrane (PM). Their GDP-GTP switch is essential to trigger numerous processes, including membrane deformation, cargo sorting and recruitment of downstream coat proteins and effectors, such as lipid modifying enzymes. While ARFs (in particular ARF1) had mainly been studied in the context of coat protein recruitment at the Golgi, COPI/clathrin-independent roles have emerged in the last decade. Here we review the roles of human ARF1-5 GTPases in cellular trafficking with a particular emphasis on their roles in post-Golgi secretory trafficking and in sorting in the endo-lysosomal system.

Characterization of lipoprotein lipase storage vesicles in 3T3-L1 adipocytes

Lipoprotein lipase (LPL) is a secreted triglyceride lipase involved in the clearance of very-low-density lipoproteins and chylomicrons from circulation. LPL is expressed primarily in adipose and muscle tissues and transported to the capillary lumen. LPL secretion is regulated by insulin in adipose tissue, however few studies have examined the regulatory and trafficking steps involved in secretion. Here we describe the intracellular localization and insulin-dependent trafficking of LPL in 3T3-L1 adipocytes. We compared LPL trafficking to the better characterized trafficking pathways taken by leptin and GLUT4. We show that LPL trafficking shares some characteristics of these other pathways, but that LPL subcellular localization and trafficking are distinct from GLUT4 and leptin. LPL secretion occurs slowly in response to insulin and rapidly in response to the calcium ionophore ionomycin. This regulated trafficking is dependent on Golgi protein kinase D and the ADP-ribosylation factor GTPase ARF1 localized to caveolar membrane domains. Together, these data give support to a new trafficking pathway for soluble cargo active in adipocytes.

ARL15 modulates magnesium homeostasis through N-glycosylation of CNNMs

Cyclin M (CNNM1-4) proteins maintain cellular and body magnesium (Mg2+) homeostasis. Using various biochemical approaches, we have identified members of the CNNM family as direct interacting partners of ADP-ribosylation factor-like GTPase 15 (ARL15), a small GTP-binding protein. ARL15 interacts with CNNMs at their carboxyl-terminal conserved cystathionine-β-synthase (CBS) domains. In silico modeling of the interaction between CNNM2 and ARL15 supports that the small GTPase specifically binds the CBS1 and CNBH domains. Immunocytochemical experiments demonstrate that CNNM2 and ARL15 co-localize in the kidney, with both proteins showing subcellular localization in the endoplasmic reticulum, Golgi apparatus and the plasma membrane. Most importantly, we found that ARL15 is required for forming complex N-glycosylation of CNNMs. Overexpression of ARL15 promotes complex N-glycosylation of CNNM3. Mg2+ uptake experiments with a stable isotope demonstrate that there is a significant increase of 25Mg2+ uptake upon knockdown of ARL15 in multiple kidney cancer cell lines. Altogether, our results establish ARL15 as a novel negative regulator of Mg2+ transport by promoting the complex N-glycosylation of CNNMs.