Plasma Membrane-associated Annexin A6 Reduces Ca2+ Entry by Stabilizing the Cortical Actin Cytoskeleton

The annexins are a family of Ca²⁺- and phospholipid-binding proteins, which interact with membranes upon increase of [Ca²⁺]_i or during cytoplasmic acidification. The transient nature of the membrane binding of annexins complicates the study of their influence on intracellular processes. To address the function of annexins at the plasma membrane (PM), we fused fluorescent protein-tagged annexins A6, A1, and A2 with H- and K-Ras membrane anchors. Stable PM localization of membrane-anchored annexin A6 significantly decreased the store-operated Ca²⁺ entry (SOCE), but did not influence the rates of Ca²⁺ extrusion. This attenuation was specific for annexin A6 because PM-anchored annexins A1 and A2 did not alter SOCE. Membrane association of annexin A6 was necessary for a measurable decrease of SOCE, because cytoplasmic annexin A6 had no effect on Ca²⁺ entry as long as [Ca²⁺]_i was below the threshold of annexin A6-membrane translocation. However, when [Ca²⁺]_i reached the levels necessary for the Ca²⁺-dependent PM association of ectopically expressed wild-type annexin A6, SOCE was also inhibited. Conversely, knockdown of the endogenous annexin A6 in HEK293 cells resulted in an elevated Ca²⁺ entry. Constitutive PM localization of annexin A6 caused a rearrangement and accumulation of F-actin at the PM, indicating a stabilized cortical cytoskeleton. Consistent with these findings, disruption of the actin cytoskeleton using latrunculin A abolished the inhibitory effect of PM-anchored annexin A6 on SOCE. In agreement with the inhibitory effect of annexin A6 on SOCE, constitutive PM localization of annexin A6 inhibited cell proliferation. Taken together, our results implicate annexin A6 in the actin-dependent regulation of Ca²⁺ entry, with consequences for the rates of cell proliferation.Calcium entry into cells either through voltage- or receptor-operated channels, or following the depletion of intracellular stores is a major factor in maintaining intracellular Ca²⁺ homeostasis. Resting [Ca²⁺]_i is low (∼100 nm compared with extracellular [Ca²⁺]_ex of 1.2 mm) and can be rapidly increased by inositol triphosphate-mediated release from the intracellular Ca²⁺ stores (mostly endoplasmic reticulum (ER)³), or by channel-mediated influx across the plasma membrane (PM). Store-operated calcium entry (SOCE) has been proposed as the main process controlling Ca²⁺ entry in non-excitable cells (1), and the recent discovery of Orai1 and STIM provided the missing link between the Ca²⁺-release activated current (I_CRAC) and the ER Ca²⁺ sensor (2–4). Translocation of STIM within the ER, accumulation in punctae at the sites of contact with PM and activation of Ca²⁺ channels have been proposed as a model of its regulation of Orai1 activity (5, 6). However, many details of the functional STIM-Orai1 protein complex and its regulation remain to be elucidated. The actin cytoskeleton plays a major role in the regulation of SOCE, possibly by influencing the function of ion channels or by interfering with the interaction between STIM and Orai1 (7–9). However, the proteins connecting the actin cytoskeleton and SOCE activity at the PM have yet to be identified.The annexins are a multigene family of Ca²⁺- and phospholipid-binding proteins, which have been implicated in many Ca²⁺-regulated processes. Their C-terminal core is evolutionarily conserved and contains Ca²⁺-binding sites, their N-terminal tails are unique and enable the protein to interact with distinct cytoplasmic partners. At low [Ca²⁺]_i, annexins are diffusely distributed throughout the cytosol, however, after stimulation resulting in the increase of [Ca²⁺]_i, annexins are targeted to distinct subcellular membrane locations, such as the PM, endosomes, or secretory vesicles (10). Annexins are involved in the processes of vesicle trafficking, cell division, apoptosis, calcium signaling, and growth regulation (11), and frequent changes in expression levels of annexins are observed in disease (12, 13). Previously, using biochemical methods and imaging of fluorescent protein-tagged annexins in live cells, we demonstrated that annexins A1, A2, A4, and A6 interacted with the PM as well as with internal membrane systems in a highly coordinated manner (10, 14). In addition, there is evidence of Ca²⁺-independent membrane association of several annexins, including annexin A6 (15–19); some of which point to the existence of pH-dependent binding mechanisms (20–22). Given the fact that several annexins are present within any one cell, it is likely that they form a [Ca²⁺] and pH sensing system, with a regulatory influence on other signaling pathways.The role of annexins as regulators of ion channel activity has been addressed previously (23–25). In particular, annexin A6 has been implicated in regulation of the sarcoplasmic reticulum ryanodine-sensitive Ca²⁺ channel (25), the neuronal K⁺ and Ca²⁺ channels (26), and the cardiac Na⁺/Ca²⁺ exchanger (27). Cardiac-specific overexpression of annexin A6 resulted in lower basal [Ca²⁺], a depression of [Ca²⁺]_i transients and impaired cardiomyocyte contractility (28). In contrast, the cardiomyocytes from the annexin A6 null-mutant mice showed increased contractility and accelerated Ca²⁺ clearance (29). Consistent with its role in mediating the intracellular Ca²⁺ signals, especially Ca²⁺ influx, ectopic overexpression of annexin A6 in A431 cells, which lack endogenous annexin A6, resulted in inhibition of EGF-dependent Ca²⁺ entry (30).The difficulty of investigating the influence of annexins on signaling events occurring at the PM lies in the transient and reversible nature of their Ca²⁺ and pH-dependent lipid binding. Although the intracellular Ca²⁺ increase following receptor activation or Ca²⁺ influx promotes the association of the Ca²⁺-sensitive annexins A2 and A6 with the PM, the proteins quickly resume their cytoplasmic localization upon restoration of the basal [Ca²⁺]_i (14). Therefore, to investigate the effects of membrane-associated annexins on Ca²⁺ homeostasis and the cell signaling machinery, we aimed to develop a model system allowing for a constitutive membrane association of annexins. Here we used the PM-anchoring sequences of the H- and K-Ras proteins to target annexins A6 and A1 to the PM independently of [Ca²⁺]. The Ras GTPases are resident at the inner leaflet of the PM and function as molecular switches (31). The C-terminal 9 amino acids of H- and N-Ras and the C-terminal 14 amino acids of K-Ras comprise the signal sequences for membrane anchoring of Ras isoforms (32). Although the palmitoylation and farnesylation of the C terminus of H-Ras (tH) serves as a targeting signal for predominantly cholesterol-rich membrane microdomains at the PM (lipid rafts/caveolae) (33), the polybasic group and the lipid anchor of K-Ras (tK) ensures the association of K-Ras with cholesterol-poor PM membrane domains. Importantly, these minimal C-terminal amino acid sequences are sufficient to target heterologous proteins, for example GFP, to different microdomains at the PM and influence their trafficking (34).In the present study we fused annexins A6, A2, and A1 with fluorescent proteins and introduced the PM-anchoring sequences of either H-Ras (annexin-tH) or K-Ras (annexin-tK) at the C termini of the fusion constructs. We demonstrate that the constitutive PM localization of annexin A6 results in down-regulation of store-operated Ca²⁺ entry. Expression of membrane-anchored annexin A6 causes an accumulation of the cortical F-actin, and cytoskeletal destabilization with latrunculin A abolishes the inhibitory effect of PM-anchored annexin A6 on SOCE. Taken together, our results implicate annexin A6 in the maintenance of intracellular Ca²⁺ homeostasis via actin-dependent regulation of Ca²⁺ entry.     

A shift in microvillus membrane fucosylation to sialylation by ethanol ingestion in rat intestine

The luminal surface of enterocytes is covered with glycocalyx which is rich in glycoproteins. Ethanol ingestion is shown to induce morphological and biochemical changes in the intestine. In this study, the effect of ethanol ingestion on membrane glycoproteins has been investigated. Chemical analysis of microvillus membranes revealed an increase in hexose and sialic acid contents, but a reduction in fucose levels in ethanol-fed rats compared with controls. The observed changes were apparent in animals fed with ethanol for 35–56 days compared with controls. Lectin-binding assay indicated an increase in Wheat germ agglutinin (affinity for GlcNAc/sialic acid) and a decrease in Aleuria aurantia (affinity for α-l-fucose) reactivity of brush borders in ethanol-fed animals for 4–8 weeks. Western blot analysis using biotin-labeled Wheat germ agglutinin revealed increased binding to proteins of M_r 66–205 kDa in ethanol-fed rats compared with controls. The binding of Aleuria aurantia to membrane proteins of M_r 97–185 kDa was reduced in ethanol-fed animals. These findings suggest that long-term ethanol feeding modulates the sialylation and fucosylation processes of microvillus membrane proteins in rat intestine. This could affect the intestinal digestive and absorptive functions in chronic alcoholism.     

Entomopathogenic nematodes induce components of systemic resistance in plants: Biochemical and molecular evidence

Antagonism between entomopathogenic nematodes (EPNs) and plant-parasitic nematodes (PPNs) has been documented over the past two decades but its mechanism and ecological significance remain elusive. We investigated the effects of Steinernema carpocapsae and its symbiotic bacterium, Xenorhabdus nematophila applied to the potting medium on pyrogallol peroxidase (P-peroxidase), guaiacol peroxidase (G-peroxidase) and catalase activities in Hosta sp. and Arabidopsis thaliana leaves as components of induced systemic resistance. We found that P-peroxidase activity was significantly higher in the leaves from hosta plants treated with S. carpocapsae infective juveniles (IJs) and S. carpocapsae infected insect cadavers than in the leaves from the control plants 2 weeks after treatment. The G-peroxidase activity was significantly higher in S. carpocapsae infected cadaver and X. nematophila treatments 10 and 15 days after treatment (DAT) and in S. carpocapsae IJs treatment 5 and 15 DAT. The catalase activity in hosta leaves was significantly higher in S. carpocapsae infected cadaver and X. nematophilus treatments compared with the control 5 and 15 DAT and in S. carpocapsae IJs treatment 5 and 10 DAT. Further, the catalase activity in A. thaliana leaves was significantly higher in S. carpocapsae IJs treatment than in the control 7 DAT. We also determined the effects of S. carpocapsae infected cadavers and S. carpocapsae IJs on PR1-gene expression in transgenic A. thaliana leaves through GUS (β-glucuronidase) activity assay and found that the PR1-gene was expressed in leaves from all treatments except the control. Thus, we conclude that the EPNs and their symbiotic bacteria can induce systemic resistance in plants which may explain the elusive antagonistic effect of EPNs on PPNs.     

K<Emphasis Type="Italic">i</Emphasis> DoQ: using docking based energy scores to develop ligand based model for predicting antibacterials

Background  

Identification of novel drug targets and their inhibitors is a major challenge in the field of drug designing and development. Diaminopimelic acid (DAP) pathway is a unique lysine biosynthetic pathway present in bacteria, however absent in mammals. This pathway is vital for bacteria due to its critical role in cell wall biosynthesis. One of the essential enzymes of this pathway is dihydrodipicolinate synthase (DHDPS), considered to be crucial for the bacterial survival. In view of its importance, the development and prediction of potent inhibitors against DHDPS may be valuable to design effective drugs against bacteria, in general.     

Persistence of Mycobacterium avium subsp. paratuberculosis and other zoonotic pathogens during simulated composting, manure packing, and liquid storage of dairy manure

Livestock manures contain numerous microorganisms which can infect humans and/or animals, such as Escherichia coli O157:H7, Listeria monocytogenes, Salmonella spp., and Mycobacterium avium subsp. paratuberculosis (Mycobacterium paratuberculosis). The effects of commonly used manure treatments on the persistence of these pathogens have rarely been compared. The objective of this study was to compare the persistence of artificially inoculated M. paratuberculosis, as well as other naturally occurring pathogens, during the treatment of dairy manure under conditions that simulate three commonly used manure management methods: thermophilic composting at 55 degrees C, manure packing at 25 degrees C (or low-temperature composting), and liquid lagoon storage. Straw and sawdust amendments used for composting and packing were also compared. Manure was obtained from a large Ohio free-stall dairy herd and was inoculated with M. paratuberculosis at 10(6) CFU/g in the final mixes. For compost and pack treatments, this manure was amended with sawdust or straw to provide an optimal moisture content (60%) for composting for 56 days. To simulate liquid storage, water was added to the manure (to simulate liquid flushing and storage) and the slurry was placed in triplicate covered 4-liter Erlenmeyer flasks, incubated under ambient conditions for 175 days. The treatments were sampled on days 0, 3, 7, 14, 28, and 56 for the detection of pathogens. The persistence of M. paratuberculosis was also assessed by a PCR hybridization assay. After 56 days of composting, from 45 to 60% of the carbon in the compost treatments was converted to CO2, while no significant change in carbon content was observed in the liquid slurry. Escherichia coli, Salmonella, and Listeria were all detected in the manure and all of the treatments on day 0. After 3 days of composting at 55 degrees C, none of these organisms were detectable. In liquid manure and pack treatments, some of these microorganisms were detectable up to 28 days. M. paratuberculosis was detected by standard culture only on day 0 in all the treatments, but was undetectable in any treatment at 3 and 7 days. On days 14, 28, and 56, M. paratuberculosis was detected in the liquid storage treatment but remained undetectable in the compost and pack treatments. However, M. paratuberculosis DNA was detectable through day 56 in all treatments and up to day 175 in liquid storage treatments. Taken together, the results indicate that high-temperature composting is more effective than pack storage or liquid storage of manure in reducing these pathogens in dairy manure. Therefore, thermophilic composting is recommended for treatment of manures destined for pathogen-sensitive environments such as those for vegetable production, residential gardening, or application to rapidly draining fields.     

Genetic nature of eastern wolves: Past, present and future

Eastern North American wolves have long been recognized as morphologically distinct from both coyotes and gray wolves. This has led to questions regarding their origins and taxonomic status. Eastern wolves are mainly viewed as: (1) a smaller subspecies of gray wolf (Canis lupus lycaon), potentially the result of historical hybridization between gray wolves (C. lupus) and red wolves (C. rufus), (2) a hybrid, the result of gray wolf (C. lupus) and coyote (C. latrans) interbreeding, or (3) a distinct species, C. lycaon, closely related to the red wolf (C. rufus). Although debate persists, recent molecular studies suggest that the eastern wolf is not a gray wolf subspecies, nor the result of gray wolf/coyote hybridization. Eastern wolves were more likely a distinct species, C. lycaon, prior to the eastward spread of coyotes in the late 1800s. However, contemporary interbreeding exits between C. lycaon to both C. lupus and C. latrans over much of its present range complicating its present taxonomic characterization. While hybridization may be reducing the taxonomic distinctiveness of C. lycaon, it should not necessarily be viewed as negative influence. Hybridization may be enhancing the adaptive potential of eastern wolves, allowing them to more effectively exploit available resources in rapidly changing environments.     

5-azacytidine enhances the anti-leukemic activity of lintuzumab (SGN-33) in preclinical models of acute myeloid leukemia

Despite therapeutic advances, the poor prognoses for acute myeloid leukemia (AML) and intermediate and high-risk myelodysplastic syndromes (MDS) point to the need for better treatment options. AML and MDS cells express the myeloid marker CD33, making it amenable to CD33-targeted therapy. Lintuzumab (SGN-33), a humanized monoclonal anti-CD33 antibody undergoing clinical evaluation, induced meaningful responses in a Phase 1 clinical trial and demonstrated anti-leukemic activity in preclinical models. Recently, it was reported that 5-azacytidine (Vidaza™) prolonged the overall survival of a group of high risk MDS and AML patients. To determine whether the combination of lintuzumab and 5-azacytidine would be beneficial, a mouse xenograft model of disseminated AML was used to evaluate the combination. There was a significant reduction in tumor burden and an increase in overall survival in mice treated with lintuzumab and 5-azacytidine. The effects were greater than that obtained with either agent alone. As the in vivo anti-leukemic activity of lintuzumab was dependent upon the presence of mouse effector cells including macrophages and neutrophils, in vitro effector function assays were used to assess the impact of 5-azacytidine on lintuzumab activity. The results show that 5-azacytidine significantly enhanced the ability of lintuzumab to promote tumor cell killing through antibody-dependent cellular cytotoxicity (ADCC) and phagocytic (ADCP) activities. These results suggest that lintuzumab and 5-azacytidine act in concert to promote tumor cell killing. Additionally, these findings provide the rationale to evaluate this combination in the clinic.Key words: CD33, monoclonal antibody, immunotherapy, myeloid malignancies, 5-azacytidine, epigenetic therapies, hypermethylation, effector function     

Annexin A6—Linking Ca signaling with cholesterol transport

Annexin A6 (AnxA6) belongs to a conserved family of Ca²⁺-dependent membrane-binding proteins. Like other annexins, the function of AnxA6 is linked to its ability to bind phospholipids in cellular membranes in a dynamic and reversible fashion, in particular during the regulation of endocytic and exocytic pathways. High amounts of AnxA6 sequester cholesterol in late endosomes, thereby lowering the levels of cholesterol in the Golgi and the plasma membrane. These AnxA6-dependent redistributions of cellular cholesterol pools give rise to reduced cytoplasmic phospholipase A2 (cPLA₂) activity, retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. In addition to regulating cholesterol and caveolin distribution, AnxA6 acts as a scaffold/targeting protein for several signaling proteins, the best characterized being the Ca²⁺-dependent membrane targeting of p120GAP to downregulate Ras activity. AnxA6 also stimulates the Ca²⁺-inducible involvement of PKC in the regulation of HRas and possibly EGFR signal transduction pathways. The ability of AnxA6 to recruit regulators of the EGFR/Ras pathway is likely potentiated by AnxA6-induced actin remodeling. Accordingly, AnxA6 may function as an organizer of membrane domains (i) to modulate intracellular cholesterol homeostasis, (ii) to create a scaffold for the formation of multifactorial signaling complexes, and (iii) to regulate transient membrane-actin interactions during endocytic and exocytic transport. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.     

Ras/mitogen-activated protein kinase (MAPK) signaling modulates protein stability and cell surface expression of scavenger receptor SR-BI

The mitogen-activated protein kinase (MAPK) Erk1/2 has been implicated to modulate the activity of nuclear receptors, including peroxisome proliferator activator receptors (PPARs) and liver X receptor, to alter the ability of cells to export cholesterol. Here, we investigated if the Ras-Raf-Mek-Erk1/2 signaling cascade could affect reverse cholesterol transport via modulation of scavenger receptor class BI (SR-BI) levels. We demonstrate that in Chinese hamster ovary (CHO) and human embryonic kidney (HEK293) cells, Mek1/2 inhibition reduces PPARα-inducible SR-BI protein expression and activity, as judged by reduced efflux onto high density lipoprotein (HDL). Ectopic expression of constitutively active H-Ras and Mek1 increases SR-BI protein levels, which correlates with elevated PPARα Ser-21 phosphorylation and increased cholesterol efflux. In contrast, SR-BI levels are insensitive to Mek1/2 inhibitors in PPARα-depleted cells. Most strikingly, Mek1/2 inhibition promotes SR-BI degradation in SR-BI-overexpressing CHO cells and human HuH7 hepatocytes, which is associated with reduced uptake of radiolabeled and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyane-labeled HDL. Loss of Mek1/2 kinase activity reduces SR-BI expression in the presence of bafilomycin, an inhibitor of lysosomal degradation, indicating down-regulation of SR-BI via proteasomal pathways. In conclusion, Mek1/2 inhibition enhances the PPARα-dependent degradation of SR-BI in hepatocytes.