Microcavity supported lipid membranes: versatile platforms for building asymmetric lipid bilayers and for protein recognition

Microcavity supported lipid bilayers (MSLB) are contact-free membranes suspended across aqueousfilled pores that maintain the lipid bilayer in a highly fluidic state and free from frictional interactions with substrate. Such platforms offer the prospect of liposome-like fluidity with the compositional versatility and addressability of supported lipid bilayers and thus offer significant opportunity for modelling membrane asymmetry, protein-membrane interactions and aggregation at the membrane interface. Herein, we evaluate their performance by studying the effect of transmembrane lipid asymmetry on lipid diffusivity, membrane viscosity and cholera toxin- ganglioside recognition across six symmetric and asymmetric membranes including binary compositions containing both fluid and gel phase, and ternary phase separated membrane compositions. Fluorescence lifetime correlation spectroscopy (FLCS) was used to determine the lateral mobility of lipid and protein, and electrochemical impedance spectroscopy (EIS) enabled detection of protein-membrane assembly over the nanomolar range. Transmembrane leaflet asymmetry was observed to have profound impact on membrane electrochemical resistance where the resistance of a ternary symmetric phase separated bilayer was found to be at least 2.6 times higher than the asymmetric bilayer with analogous composition at the distal leaflet but where the lower leaflet comprised only 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Similarly, the diffusion coefficient for MSLBs was observed to be 2.5 fold faster for asymmetric MSLBs where the lower leaflet is DOPC alone. Our results demonstrate that interplay of lipid packing across both membrane leaflets and concentration of GM1 both affect the extent of cholera toxin aggregation and consequent diffusion of the cholera-GM1 aggregates. Given that true biomembranes are both fluidic and asymmetric, MSLBs offer the opportunity for building greater biomimicry into biophysical models and the approach described demonstrates the value of MSLBs in studying aggregation and membrane associated multivalent interactions prevalent in many carbohydrates mediated processes

Basic fibroblast growth factor modifies the hypoxic response of human bone marrow stromal cells by ERK-mediated enhancement of HIF-1α activity

Human bone marrow stromal cells (hBMSCs, also known as bone marrow-derived mesenchymal stem cells) are promising tools for the cellular therapy of human pathologies related to various forms of hypoxia. Although the current concepts of their clinical use include the expansion of hBMSC in standard cell culture conditions, the effect of the mitogen-driven ex vivo expansion on the adaptation to the hypoxic environment is unknown. Here, we provide data that the basic fibroblast growth factor (FGF2) enhances the induction of a wide range of hypoxia-related adaptive genes in hypoxic hBMSCs. We identified that the FGF2 signal is transmitted by the ERK pathway similar to that of hypoxia that also utilises the distal elements of the same signalling machinery including the extracellular signal-regulated kinase 1/2 (ERK1/2) and mitogen-activated protein kinase kinases (MEK1/2) in hBMSCs. We found that the simultaneous activation of ERK1/2 by FGF2 and hypoxia transforms the activation dynamics from oscillatory into sustained one. Activated ERKs co-localise with stabilised hypoxia inducible factor-1α (HIF-1α) followed by the reduction of its nuclear mobility as well as increased DNA binding capacity leading to the up-regulation of hypoxia-adaptive genes. Our findings indicate that the status of the ERK pathway has significant impacts on the molecular adaptation of hBMSCs to the hypoxic milieu

Microcavity supported lipid bilayers; evaluation of drug- lipid membrane Interactions by electrochemical impedance and fluorescence correlation spectroscopy

Many drugs have intracellular or membrane-associated targets thus understanding their interaction with the cell membrane is of value in drug development. Cell-free tools used to predict membrane interactions should replicate the molecular organization of the membrane. Microcavity array supported lipid bilayer (MSLB) platform are versatile biophysical models of the cell membrane that combine liposome-like membrane fluidity with stability and addressability. We used an MSLB herein to interrogate drugmembrane interactions across seven drugs from different classes, including non-steroidal antiinflammatories; Ibuprofen (Ibu) and Diclofenac (Dic), antibiotics; Rifampicin (Rif), Levofloxacin (Levo) and Pefloxacin (Pef), and bisphosphonates; Alendronate (Ale) and Clodronate (Clo). Fluorescence lifetime correlation spectroscopy (FLCS) and electrochemical impedance spectroscopy (EIS) were used to evaluate the impact of drug on DOPC and binary bilayers over physiologically relevant drug concentrations. Whereas FLCS data revealed Ibu, Levo, Pef, Ale and Clo had no impact on lipid lateral mobility, EIS which is more sensitive to membrane structural change, indicated modest but significant decreases to membrane resistivity consistent with adsorption but weak penetration of drugs at the membrane. Ale and Clo, evaluated at pH 5.25, did not impact the impedance of the membrane except at concentrations exceeding 4mM. Conversely, Dic and Rif dramatically altered bilayer fluidity, suggesting their translocation through the bilayer and, EIS data, showed resistivity of the membrane decreased substantially with increasing drug concentration. Capacitance changes to the bilayer in most cases were insignificant. Using a Langmuir-Freundlich model to fit the EIS data, we propose Rsat as an empirical value that reflects permeation. Overall, the data indicate that Ibu, Levo, and Pef, adsorb at the interface of the lipid membrane but Dic and Rif interact strongly, permeating the membrane core modifying the water/ion permeability of the bilayer structure. These observations are discussed in the context of previously reported data on drug permeability and Log P

Lateral mobility proteins and peptides in membranes

Biological membranes are composed of various lipids species of different acyl chain lengths and head groups. The lateral organisation and mobility of bilayer spanning proteins depends on lipid environment. Lateral mobility of proteins play a crucial role in cellular processes. Theoretical framework on lateral mobility of proteins in membrane was proposed by Saffman-Delbrück. This pioneering work comprises that lateral mobility is weakly dependent on the radius of the proteins but strongly dependent on thickness and viscosity of the membrane. In this thesis, integral proteins with different radius and transmembrane peptides are reconstituted in the membrane composed of lipids with different acyl chain length and head groups, and there lateral mobility are experimentally determined by fluorescence correlation spectroscopy

Optimizing fluorescent protein expression for quantitative fluorescence microscopy and spectroscopy using herpes simplex thymidine kinase promoter sequences

The modulation of expression levels of fluorescent fusion proteins (FFPs) is central for recombinant DNA technologies in modern biology as overexpression of proteins contributes to artifacts in biological experiments. In addition, some microscopy techniques such as fluorescence correlation spectroscopy (FCS) and single-molecule-based techniques are very sensitive to high expression levels of FFPs. To reduce the levels of recombinant protein expression in comparison with the commonly used, very strong CMV promoter, the herpes simplex virus thymidine kinase (TK) gene promoter, and mutants thereof were analyzed. Deletion mutants of the TK promoter were constructed and introduced into the Gateway (R) system for ectopic expression of enhanced green fluorescent protein (eGFP), monomeric cherry (mCherry), and FFPs containing these FPs. Two promoter constructs, TK2ST and TKTSC, were established, which have optimal low expression levels suitable for FCS studies in U2OS, HeLa CCL2, NIH 3T3, and BALB/c cells. Interestingly, when tested in these four cell lines, promoter constructs having a deletion within TK gene 5\u27-UTR showed significantly higher protein expression levels than the equivalent constructs lacking this deletion. This suggests that a negative regulatory element is localized within the TK gene 5\u27-UTR.Science Foundation Ireland. Grant Number: 06/CE/B1129 Else Kröner‐Fresenius‐Stiftung. Grant Number: 2013_A21

Lateral Diffusion of Membrane Proteins

We measured the lateral mobility of integral membrane proteins reconstituted in giant unilamellar vesicles (GUVs), using fluorescence correlation spectroscopy. Receptor, channel, and transporter proteins with 1-36 transmembrane segments (lateral radii ranging from 0.5 to 4 nm) and a α-helical peptide (radius of 0.5 nm) were fluorescently labeled and incorporated into GUVs. At low protein-to-lipid ratios (i.e., 10-100 proteins per μm2 of membrane surface), the diffusion coefficient D displayed a weak dependence on the hydrodynamic radius (R) of the proteins [D scaled with ln(1/R)], consistent with the Saffman-Delbrück model. At higher protein-to lipid ratios (up to 3000 μm-2), the lateral diffusion coefficient of the molecules decreased linearly with increasing the protein concentration in the membrane. The implications of our findings for protein mobility in biological membranes (protein crowding of ~25,000 μm-2) and use of diffusion measurements for protein geometry (size, oligomerization) determinations are discussed.

[Edicto sobre la contribución de frutos civiles, según el Real Decreto de 25 de junio de 1785]

Pie de imp. deducido del texto, fechado en Granada, 1785Inic. orladaEnc. Holandes

HeLa S3 cells stably expressing eGFP-Cdc45.

<p>Panel a, schematic diagram of eGFP-Cdc45 protein encoded by Cdc45L ORF cloned into pIC113gw vector. Panel b, total cell extract HeLaS3 cells (-ve) and Hela S3 cells stably expressing eGFP-Cdc45 (eGFP-Cdc45) normalized for protein content and analysed by western blotting using antibodies raised against Cdc45 and β-Actin, which serves as a loading control. Panel c, western blot analysis of immunoprecipitation of eGFP-Cdc45 using GFP-Trap IP from HeLa S3 cells transiently expressing eGFP-Cdc45. Verification of purification of eGFP-Cdc45 and co-immunoprecipitation of Mcm7 was carried out using antibodies raised against Mcm7 and Cdc45. Input (L), unbound (FT), mock IP (-ve) and IP from cells expressing eGFP-Cdc45 (eGFP) indicate yield of the IP and co-immunoprecipiation. Antibody light chain acts as a loading control.</p