Spread and persistence of a rugulosin-producing endophyte in Picea glauca seedlings

We have studied Picea glauca (white spruce) endophyte colonization and its affect on the growth of Choristoneura fumiferana (spruce budworm). Here we examine the spread and persistence of a rugulosin-producing endophyte and rugulosin in needles from trees maintained in the nursery, as well as in trees planted in a test field site. Additionally, we report toxicity of rugulosin against three P. glauca needle herbivores: C. fumiferana, Lambdina fiscellaria (hemlock looper) and Zeiraphera canadensis (spruce budmoth). Reduction in body weight for both the C. fumiferana and L. fiscellaria were observed at 25 and 50 μm, respectively, and head capsules were reduced at 100 and 150 μm. Z. canadensis larvae did not perform as well in tests due to an Aspergillus fumigatus infection, but were shown to be lighter when tested with 100 and 150 μm compared with controls. The endophyte and its toxin were shown to spread throughout the nursery-grown seedlings. After 3.5 and 4.5 y post-inoculation (one and two years in the test site), the inoculated endophyte and its toxin had remained present with an average rugulosin concentration of 1 μg g⁻¹.     

Inhibition of the M. tuberculosis 3β-hydroxysteroid dehydrogenase by azasteroids

The cholesterol metabolism pathway in Mycobacterium tuberculosis (M. tb) is a potential source of energy as well as secondary metabolite production that is important for survival of M. tb in the host macrophage. Oxidation and isomerization of 3β-hydroxysterols to 4-en-3-ones is requisite for sterol metabolism and the reaction is catalyzed by 3β-hydroxysteroid dehydrogenase (Rv1106c). Three series of 6-azasteroids and 4-azasteroids were employed to define the substrate preferences of M. tb 3β-hydroxysteroid dehydrogenase. 6-Azasteroids with large, hydrophobic side chains at the C17 position are the most effective inhibitors. Substitutions at C1, C2, C4 and N6 were poorly tolerated. Our structure-activity studies indicate that the 6-aza version of cholesterol is the best and tightest binding competitive inhibitor (K_i = 100 nM) of the steroid substrate and are consistent with cholesterol being the preferred substrate of M. tb 3β-hydroxysteroid dehydrogenase.     

Yeast two hybrid analyses reveal novel binary interactions between human cytomegalovirus-encoded virion proteins

Human cytomegalovirus (HCMV) is the largest human herpesvirus and its virion contains many viral encoded proteins found in the capsid, tegument, and envelope. In this study, we carried out a yeast two-hybrid (YTH) analysis to study potential binary interactions among 56 HCMV-encoded virion proteins. We have tested more than 3,500 pairwise combinations for binary interactions in the YTH analysis, and identified 79 potential interactions that involve 37 proteins. Forty five of the 79 interactions were also identified in human cells expressing the viral proteins by co-immunoprecipitation (co-IP) experiments. To our knowledge, 58 of the 79 interactions revealed by YTH analysis, including those 24 that were also identified in co-IP experiments, have not been reported before. Novel potential interactions were found between viral capsid proteins and tegument proteins, between tegument proteins, between tegument proteins and envelope proteins, and between envelope proteins. Furthermore, both the YTH and co-IP experiments have identified 9, 7, and 5 interactions that were involved with UL25, UL24, and UL89, respectively, suggesting that these "hub" proteins may function as the organizing centers for connecting multiple virion proteins in the mature virion and for recruiting other virion proteins during virion maturation and assembly. Our study provides a framework to study potential interactions between HCMV proteins and investigate the roles of protein-protein interactions in HCMV virion formation or maturation process.     

Photoresponse in the heterotrophic marine dinoflagellate Oxyrrhis marina

Expressed rhodopsins were detected by proteomic analysis in an investigation of potential signal receptors in the cell membrane of the marine heterotrophic dinoflagellate Oxyrrhis marina (CCMP604). We inferred these to be sensory rhodopsins, a type of G-protein-coupled receptor trans-membrane signaling molecule. Because phototactic behavior based on sensory rhodopsins has been reported in other protists, we investigated the photosensory response of O. marina. This dinoflagellate exhibited strongest positive phototaxis at low levels (2-3 μE/m(2)/s) of white light when the cells were previously light adapted and well fed. Positive phototaxis was also found for blue (450 nm), green (525 nm), and red (680 nm) wavelengths. In a further test, O. marina showed significantly greater phototaxis toward concentrated algal food illuminated by blue light to stimulate red chlorophyll-a autofluorescence in the prey, compared with using bleached algae as prey. Concentration of a cytoplasmic downstream messenger molecule, cyclic adenosine monophosphate, a component of the signaling pathway of G-protein-coupled receptor molecules, rapidly increased in O. marina cells after exposure to white light. In addition, treatment with hydroxylamine, a rhodopsin signaling inhibitor, significantly decreased their phototactic response. Our results demonstrate that a heterotrophic marine dinoflagellate can orient to light based on rhodopsins present in the outer cell membrane and may be able to use photosensory response to detect algal prey based on chlorophyll autofluorescence.     

Vascular smooth muscle Notch signals regulate endothelial cell sensitivity to angiogenic stimulation

The evolutionarily conserved Notch signaling pathway is required for normal vascular development and function, and genetic associations link select Notch receptors and ligands to human clinical syndromes featuring blood vessel abnormalities and stroke susceptibility. A previously described mouse model engineered to suppress canonical Notch signaling in vascular smooth muscle cells (vSMCs) revealed surprising anatomical defects in arterial patterning and vessel maturation, suggesting that vSMCs have the functional capacity to influence blood vessel formation in a Notch signaling-dependent manner. In further analyses using this model system, we now show that explanted aortic ring tissue and Matrigel implants from the smooth muscle Notch signaling-deficient mice yield markedly diminished responses to angiogenic stimuli. Furthermore, cultured Notch signaling-deficient primary vSMCs have reduced proliferation and migration capacities and reveal diminished expression of PDGF receptor β and JAGGED1 ligand. These observations prompted a series of endothelial cell (EC)-vSMC co-culture experiments that revealed a requirement for intact vSMC Notch signals via JAGGED1 for efficient EC Notch1 receptor activation and EC proliferation. Taken together, these studies suggest a heterotypic model wherein Notch signaling in vSMCs provides early instructive cues to neighboring ECs important for optimal postnatal angiogenesis.     

Switches, excitable responses and oscillations in the Ring1B/Bmi1 ubiquitination system

In an active, self-ubiquitinated state, the Ring1B ligase monoubiquitinates histone H2A playing a critical role in Polycomb-mediated gene silencing. Following ubiquitination by external ligases, Ring1B is targeted for proteosomal degradation. Using biochemical data and computational modeling, we show that the Ring1B ligase can exhibit abrupt switches, overshoot transitions and self-perpetuating oscillations between its distinct ubiquitination and activity states. These different Ring1B states display canonical or multiply branched, atypical polyubiquitin chains and involve association with the Polycomb-group protein Bmi1. Bistable switches and oscillations may lead to all-or-none histone H2A monoubiquitination rates and result in discrete periods of gene (in)activity. Switches, overshoots and oscillations in Ring1B catalytic activity and proteosomal degradation are controlled by the abundances of Bmi1 and Ring1B, and the activities and abundances of external ligases and deubiquitinases, such as E6-AP and USP7.     

Direct observation of the iron binding sites in a ferritin

X-Ray analysis of the ferritin of Escherichia coli (Ec-FTN) and of Ec-FTN crystals soaked in (NH₄)₂Fe(SO₄)₂ has revealed the presence of three iron-binding sites per subunit. Two of these form a di-iron site in the centre of the subunit as has been proposed for the ‘ferroxidase centres’ of human ferritin H chains. This di-iron site, lying within the 4-alpha-helix bundle, resemble those of ribonucleotide reductase, methane monoxygenase and haemerythrin. The third iron is bound by ligands unique to Ec-FTN on the inner surface of the protein shell. It is speculated that this state may represent the nucleation centre of a novel type of Fe(III) cluster, recently observed in Ec-FTN.     

Progesterone Receptor Membrane Component 1 Is a Functional Part of the Glucagon-like Peptide-1 (GLP-1) Receptor Complex in Pancreatic β Cells

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that regulates glucose homeostasis. Because of their direct stimulation of insulin secretion from pancreatic β cells, GLP-1 receptor (GLP-1R) agonists are now important therapeutic options for the treatment of type 2 diabetes. To better understand the mechanisms that control the insulinotropic actions of GLP-1, affinity purification and mass spectrometry (AP-MS) were employed to uncover potential proteins that functionally interact with the GLP-1R. AP-MS performed on Chinese hamster ovary cells or MIN6 β cells, both expressing the human GLP-1R, revealed 99 proteins potentially associated with the GLP-1R. Three novel GLP-1R interactors (PGRMC1, Rab5b, and Rab5c) were further validated through co-immunoprecipitation/immunoblotting, fluorescence resonance energy transfer, and immunofluorescence. Functional studies revealed that overexpression of PGRMC1, a novel cell surface receptor that associated with liganded GLP-1R, enhanced GLP-1-induced insulin secretion (GIIS) with the most robust effect. Knockdown of PGRMC1 in β cells decreased GIIS, indicative of positive interaction with GLP-1R. To gain insight mechanistically, we demonstrated that the cell surface PGRMC1 ligand P4-BSA increased GIIS, whereas its antagonist AG-205 decreased GIIS. It was then found that PGRMC1 increased GLP-1-induced cAMP accumulation. PGRMC1 activation and GIIS induced by P4-BSA could be blocked by inhibition of adenylyl cyclase/EPAC signaling or the EGF receptor–PI3K signal transduction pathway. These data reveal a dual mechanism for PGRMC1-increased GIIS mediated through cAMP and EGF receptor signaling. In conclusion, we identified several novel GLP-1R interacting proteins. PGRMC1 expressed on the cell surface of β cells was shown to interact with the activated GLP-1R to enhance the insulinotropic actions of GLP-1.Glucagon-like peptide-1 (GLP-1)¹ is a gastrointestinal hormone secreted by intestinal L cells upon food intake that is best known for its role in controlling glucose homeostasis. Acting through its cognate glucagon-like peptide-1 receptor (GLP-1R), GLP-1 has several important physiological and pharmacological functions. GLP-1 is best known for enhancing glucose-stimulated insulin secretion (GSIS) from the pancreatic β cells. Importantly, the insulinotropic properties of GLP-1 are maintained in patients with type 2 diabetes (1), which is characterized by insufficient insulin secretion from pancreatic β cells and an inability to maintain glucose homeostasis. Therefore, therapeutic strategies targeting GLP-1R have been developed to treat type 2 diabetes (2, 3). In addition to augmenting insulin secretion, GLP-1 has been known to improve glucose sensing, proinsulin biosynthesis, survival, and proliferation of β cells (3, 4) in a variety of experimental models. GLP-1 also has several extrapancreatic effects, including actions on the central nervous system to inhibit food intake (5), the stomach to decrease gastric emptying and gastric acid secretion (6), and the lungs to stimulate secretion of macromolecules from airways (7). Additionally, GLP-1 has an effect on the heart and possibly the kidney to modulate blood pressure and heart rate (8, 9).The GLP-1R is a member of the B1 family of G protein–coupled receptors (secretin receptor family). In mammals, GLP-1R is expressed in multiple tissues, including pancreatic β cells and δ cells (10), hypothalamus, lung, stomach, heart, kidney (11), and thyroid (12), which in part explains its diverse actions. Upon ligand binding, the GLP-1R is capable of coupling to diverse cell signal transduction pathways, but it is best known for its actions on G protein Gs α and adenylate cyclase activity to increase intracellular cAMP. It is known that other proteins can affect GLP-1R activity in addition to G proteins, including β-arrestin and caveolin, which affect receptor internalization and trafficking. β-Arrestin 1 is also required for proper GLP-1-stimulated cAMP production (13–15). More recently, it was shown that another B1 family member, gastric inhibitory polypeptide receptor heterodimerizes with GLP-1R, decreasing GLP-1-induced β-arrestin recruitment and mobilization (16). Very recently, our group identified several novel potential GLP-1R interactors using a membrane-based split-ubiquitin yeast two-hybrid (MYTH) assay (17). Three β cell–expressing membrane-bound interactors, solute carrier family 15 member 4 (SLC15A4), amyloid β A4 precursor-like protein 1 (APLP1), and adaptor-related protein complex 2 subunit mu (AP2M1), were further selected for individual knockdown in mouse insulinoma (MIN6) β cells using small interfering RNAs (siRNAs). GLP-1-induced insulin secretion was significantly enhanced when these genes were silenced, suggesting that these interactor proteins attenuate GLP-1R activity. These findings demonstrated that GLP-1R protein interactions are complex and the interactors can have measurable effects on receptor trafficking and downstream signaling. Such interactions may in part explain the diverse tissue-specific effects of GLP-1 and offer avenues for controlling GLP-1 actions in a tissue-selective manner.Although the MYTH system is well established (18) and has been applied to study G protein–coupled receptor interactomes (17), it is limited on two fronts. Firstly, it must be performed in yeast which is not an ideal representation of the mammalian system. Secondly, it is technically difficult to activate the receptor in MYTH, thus, effects of ligand stimulation on the receptor interactome cannot be assessed. Recently, affinity purification–mass spectrometry (AP-MS) has become a powerful tool for discovering and examining novel protein–protein interactions, including those between membrane-bound proteins in mammalian cells (19–21). In the current study, we applied AP-MS to discover novel GLP-1R interactors and employed a human GLP-1R harboring a FLAG^® epitope. GLP-1R-Flag was expressed in either Chinese hamster ovary (CHO) cells or MIN6 β cells, and interactors were studied in the presence or absence of GLP-1.