Application of a membrane fusion assay for rapid drug screening

Application of High-Throughput Assays to Examine Phospho-Modulation of the Late Steps of Regulated Exocytosis

Abstract: Regulated exocytosis enables a range of physiological functions including neurotransmission, and the late steps (i.e., docking, priming and Ca²⁺-triggered membrane fusion) are modulated by a highly conserved set of proteins and lipids. Many of the molecular components and biochemical interactions required have been identified; the precise mechanistic steps they modulate and the biochemical interactions that need to occur across steps are still the subject of intense investigation. Particularly, although the involvement of phosphorylation in modulating exocytosis has been intensively investigated over the past three decades, it is unclear which phosphorylation events are a conserved part of the fundamental fusion mechanism and/or serve as part of the physiological fusion machine (e.g., to modulate Ca²⁺ sensitivity). Here, the homotypic fusion of cortical vesicles was monitored by utilizing new high-throughput, cost-effective assays to assess the influence of 17 small molecule phospho-modulators on docking/priming, Ca²⁺ sensitivity and membrane fusion. Specific phosphatases and casein kinase 2 are implicated in modulating the Ca²⁺ sensitivity of fusion, whereas sphingosine kinase is implicated in modulating the ability of vesicles to fuse. These results indicate the presence of multiple kinases and phosphatases on the vesicles and critical phosphorylation sites on vesicle membrane proteins and lipids that directly influence late steps of regulated exocytosis.

Critical role of cortical vesicles in dissecting regulated exocytosis: overview of insights into fundamental molecular mechanisms

Regulated exocytosis is one of the defining features of eukaryotic cells, underlying many conserved and essential functions. Definitively assigning specific roles to proteins and lipids in this fundamental mechanism is most effectively accomplished using a model system in which distinct stages of exocytosis can be effectively separated. Here we discuss the establishment of sea urchin cortical vesicle fusion as a model to study regulated exocytosis-a system in which the docked, release-ready, and late Ca(2+)-triggered steps of exocytosis are isolated and can be quantitatively assessed using the rigorous coupling of functional and molecular assays. We provide an overview of the insights this has provided into conserved molecular mechanisms and how these have led to and integrate with findings from other regulated exocytotic cells.

Reproductive and developmental effects of endocrine disrupters in invertebrates: in vitro and in vivo approaches

In order to gain basic understanding in the ecotoxicity of endocrine disrupting chemicals or EDCs (including natural chemicals and some pharmaceuticals), many international research groups are currently testing these chemicals using aquatic invertebrates. This paper discusses relevant examples to address key questions: which aquatic invertebrates are likely to be vulnerable to mammalian and non-mammalian EDCs; and which types of invertebrate chronic tests might be most sensitive and cost-effective to address potential environmental exposures? For a full review of invertebrate endocrine disrupter research see Endocrine Disruption in Invertebrates: Endocrinology, Testing and Assessment (1999). As an example, crustaceans are a particular focus of EDC research, reflecting their abundance in nature, commercial importance and their inclusion in the regulatory assessment schemes for active pharmaceutical ingredients (APIs). There is a diverse literature on the developmental and reproductive effects of mammalian EDCs in Crustacea, although there is growing evidence that such effects are probably not mediated via arthropod hormone systems. For example, recent studies in Europe using a marine copepod (Tisbe battagliai) life-cycle test have evaluated ecdysteroid agonists (e.g. 20-hydroxyecdysone), oestrogen agonists (e.g. diethylstilbestrol (DES), 17beta-oestradiol, oestrone and 17alpha-ethynylestradiol) and the pharmaceutical anti-oestrogen (ZM189, 154). While 20-hydroxyecdysone and DES were highly toxic, the other compounds tested show no significant toxicity to copepods. Furthermore, in vitro studies indicate that these environmental EDCs and several related APIs are not active against the ecdysteroid receptor. Therefore, other undefined modes of action appear to be responsible for crustacean toxicity in vivo and caution should be exercised before ascribing any apical effects to endogenous endocrine mechanisms, or before crustacean "EDC" data are extrapolated to other invertebrate taxa.

Hydroxytamoxifen interaction with human erythrocyte membrane and induction of permeabilization and subsequent hemolysis

4-Hydroxytamoxifen (OHTAM) is the most active metabolite of the widely prescribed anticancer drug tamoxifen (TAM) used in breast cancer therapy. This work describes the effects of OHTAM on isolated human erythrocytes, using standardized test conditions, to check for a putative contribution to the TAM-induced hemolysis and to study basic mechanisms involved in the interaction of OHTAM with cell membranes. Incubation of isolated human erythrocytes with relatively high concentrations of OHTAM results in a concentration-dependent hemolysis, its hemolytic effect being about one-third of that induced by TAM. OHTAM-induced hemolysis is prevented by either alpha-tocopherol (alpha-T) or alpha-tocopherol acetate (alpha-TAc) and it occurs in the absence of oxygen consumption and hemoglobin oxidation, ruling out the oxidative damage of erythrocytes. However, OHTAM remarkably increases the osmotic fragility of erythrocytes, increasing the susceptibility of erythrocytes to hypotonic lysis. Additionally, the hemoglobin release induced by OHTAM is preceded by a rapid efflux of intracellular K(+). Therefore, our data suggest that OHTAM-induced hemolysis does not contribute to TAM-induced hemolytic anemia and it is a much weaker toxic drug as compared with TAM. Moreover, at variance with the membrane disrupting effects of TAM, OHTAM promotes perturbation of the membrane's backbone region due to its strong binding to proteins with consequent formation of membrane paths of permeability to small solutes and retention of large solutes like hemoglobin, followed by osmotic swelling and cell lysis. The prevention of OHTAM-induced hemolysis by alpha-T and alpha-TAc is probably committed to the permeability sealing resulting from structural stabilization of membrane.

Hemolysis of human erythrocytes induced by tamoxifen is related to disruption of membrane structure

Tamoxifen (TAM), the antiestrogenic drug most widely prescribed in the chemotherapy of breast cancer, induces changes in normal discoid shape of erythrocytes and hemolytic anemia. This work evaluates the effects of TAM on isolated human erythrocytes, attempting to identify the underlying mechanisms on TAM-induced hemolytic anemia and the involvement of biomembranes in its cytostatic action mechanisms. TAM induces hemolysis of erythrocytes as a function of concentration. The extension of hemolysis is variable with erythrocyte samples, but 12.5 microM TAM induces total hemolysis of all tested suspensions. Despite inducing extensive erythrocyte lysis, TAM does not shift the osmotic fragility curves of erythrocytes. The hemolytic effect of TAM is prevented by low concentrations of alpha-tocopherol (alpha-T) and alpha-tocopherol acetate (alpha-TAc) (inactivated functional hydroxyl) indicating that TAM-induced hemolysis is not related to oxidative membrane damage. This was further evidenced by absence of oxygen consumption and hemoglobin oxidation both determined in parallel with TAM-induced hemolysis. Furthermore, it was observed that TAM inhibits the peroxidation of human erythrocytes induced by AAPH, thus ruling out TAM-induced cell oxidative stress. Hemolysis caused by TAM was not preceded by the leakage of K(+) from the cells, also excluding a colloid-osmotic type mechanism of hemolysis, according to the effects on osmotic fragility curves. However, TAM induces release of peripheral proteins of membrane-cytoskeleton and cytosol proteins essentially bound to band 3. Either alpha-T or alpha-TAc increases membrane packing and prevents TAM partition into model membranes. These effects suggest that the protection from hemolysis by tocopherols is related to a decreased TAM incorporation in condensed membranes and the structural damage of the erythrocyte membrane is consequently avoided. Therefore, TAM-induced hemolysis results from a structural perturbation of red cell membrane, leading to changes in the framework of the erythrocyte membrane and its cytoskeleton caused by its high partition in the membrane. These defects explain the abnormal erythrocyte shape and decreased mechanical stability promoted by TAM, resulting in hemolytic anemia. Additionally, since membrane leakage is a final stage of cytotoxicity, the disruption of the structural characteristics of biomembranes by TAM may contribute to the multiple mechanisms of its anticancer action.

Poisson-distributed active fusion complexes underlie the control of the rate and extent of exocytosis by calcium

We have investigated the consequences of having multiple fusion complexes on exocytotic granules, and have identified a new principle for interpreting the calcium dependence of calcium-triggered exocytosis. Strikingly different physiological responses to calcium are expected when active fusion complexes are distributed between granules in a deterministic or probabilistic manner. We have modeled these differences, and compared them with the calcium dependence of sea urchin egg cortical granule exocytosis. From the calcium dependence of cortical granule exocytosis, and from the exposure time and concentration dependence of N-ethylmaleimide inhibition, we determined that cortical granules do have spare active fusion complexes that are randomly distributed as a Poisson process among the population of granules. At high calcium concentrations, docking sites have on average nine active fusion complexes.