Cell Biology

The blood-brain barrier (BBB), a crucial protection mechanism in the central nervous system (CNS), is a selective barrier comprised of endothelial cells. It hampers the development of therapeutic and diagnostic tools for neurological diseases due to the poor penetration of most of these agents. Rationally engineered nanoparticles (NP) can facilitate the transport of therapeutic and diagnostic agents across the BBB. However, evaluating BBB penetration by NP majorly relies on the use of expensive and time-consuming animal experiments with low throughput. In vitro BBB models composed of brain endothelial cells can be a useful tool to rapidly screen multiple NP formulations to compare their BBB penetration ability and identify optimal formulations for in vivo validation. In this protocol, we present an in vitro model of BBB developed using murine cerebral cortex endothelial cells (bEnd.3). bEnd.3 is a commercially available, easy to manipulate cell line that forms tight junctions with potent paracellular barrier property. The protocol includes culturing of bEnd.3 cells, establishment of the in vitro model, and assessing NP permeability. We believe that, due to its simplicity and consistency, this step-by-step protocol can be easily used by researchers to screen NP-based drug delivery systems for BBB penetration.

Graphic abstract:

ATP13A2/PARK9 is a late endo-/lysosomal P5B transport ATPase that is associated with several neurodegenerative disorders. We recently characterized ATP13A2 as a lysosomal polyamine exporter, which sheds light on the molecular identity of the unknown mammalian polyamine transport system. Here, we describe step by step a protocol to measure radiolabeled polyamine transport in reconstituted vesicles from yeast cells overexpressing human ATP13A2. This protocol was developed as part of our recent publication (van Veen et al., 2020) and will be useful for characterizing the transport function of other putative polyamine transporters, such as isoforms of the P5B transport ATPases.

The Xenopus oocyte is a powerful system for the exogenous expression and functional characterization of plant membrane transport proteins. Until now, a number of potassium transporters and channels have been identified in oocytes expression system by the two-electrode voltage clamp technology. It is difficult to characterize K⁺/H⁺ anti-transporters, especially, electroneutral transporter. The K⁺ efflux assay system enables easy, fast, large-scale measurement of the transporters activity without two-electrode voltage clamp technology. This protocol describes a technique to measure the efflux activity of potassium transporter in oocytes expressing system.

Stopped-Flow Light Scattering (SFLS) is a method devised to analyze the kinetics of fast chemical reactions that result in a significant change of the average molecular weight and/or in the shape of the reaction substrates. Several modifications of the original stopped-flow system have been made leading to a significant extension of its technical applications. One of these modifications allows the biophysical characterization of the water and solute permeability of biological and artificial membranes.

Here, we describe a protocol of SFLS to measure the glycerol permeability of isolated human red blood cells (RBCs) and evaluate the pharmacokinetics properties (selectivity and potency) of isoform-specific inhibitors of AQP3, AQP7 and AQP9, three mammalian aquaglyceroporins allowing transport of glycerol across membranes. Suspensions of RBCs (1% hematocrit) are exposed to an inwardly directed gradient of 100 mM glycerol in a SFLS apparatus at 20 °C and the resulting changes in scattered light intensity are recorded at a monochromatic wavelength of 530 nm for 120 s. The SFLS apparatus is set up to have a dead time of 1.6-ms and 99% mixing efficiency in less than 1 ms. Data are fitted to a single exponential function and the related time constant (, seconds) of the cell-swelling phase of light scattering corresponding to the osmotic movement of water that accompanies the entry of glycerol into erythrocytes is measured. The coefficient of glycerol permeability (P_gly, cm/s) of RBCs is calculated with the following equation:



where (s) is the fitted exponential time constant and S/V is the surface-to-volume ratio (cm^-1) of the analyzed RBC specimen. Pharmacokinetics of the isoform-specific inhibitors of AQP3, AQP7 and AQP9 are assessed by evaluating the extent of RBC P_gly values resulting after the exposure to serial concentrations of the blockers.

The spatiotemporal localization of different intracellular factors in real-time and their detection in live cells are important parameters to understand dynamic protein-based processes. Therefore, there is a demand to perform live-cell imaging and to measure endogenous protein dynamics in single cells. However, fluorescent labeling of endogenous protein in living cells without overexpression of fusion proteins or genetic tagging has not been routinely possible. Here we describe a versatile antibody-based imaging approach (VANIMA) to be able to precisely locate and track endogenous proteins in living cells. The labeling is achieved by the efficient and harmless delivery of fluorescent dye-conjugated antibodies or antibody fragments (Fabs) into living cells and the specific binding of these antibodies to the target protein inside of the cell. Our protocol describes step by step the procedure from testing of the suitability of the desired antibody, over the digestion of the antibody to Fabs until the labeling and the delivery by electroporation of the antibody or Fab into the cells. VANIMA can be adapted to any monoclonal antibody, self-produced or commercial, and many different metazoan cell lines. Additionally, our method is simple to implement and can be used not only to visualize and track endogenous factors, but also to specifically label posttranslational modifications, which cannot be achieved by any other labeling technique so far.

Absorption and secretion of peptide and protein cargoes across single-cell thick mucosal and endothelial barriers occurs by active endocytic and vesicular trafficking that connects one side of the epithelial or endothelial cell (the lumen) with the other (the serosa or blood). Assays that assess this pathway must robustly control for non-specific and passive solute flux through weak or damaged intercellular junctions that seal the epithelial or endothelial cells together. Here we describe an in vitro cell culture Transwell assay for transcytosis of therapeutic peptides linked covalently to various species of the glycosphingolipid GM1. We recently used this assay to develop technology that harnesses endogenous mechanism of lipid sorting across epithelial cell barriers to enable oral delivery of peptide and protein therapeutics.

Boron (B) is essential for plant growth and taken up by plant roots as boric acid. Under B limitation, B uptake and translocation in plants are dependent on the boric acid channels located in the plasma membrane. Xenopus leavis oocyte is a reliable heterologous expression system to characterize transport activities of boric acid channels and related major intrinsic proteins (aquaporins). Here, we outline the protocols for expression of boric acid channels and boric acid uptake assay in Xenopus leavis oocytes.

Microvesicle (MVs) are submicron-sized membranous vesicles that are either actively released from cells via secretory compartments or shed from cell surface membranes. MVs are generated by many cell types and serve as vehicles that transfer biological information (e.g., protein, mRNA, and miRNA) to distant cells, thereby affecting their gene expression, proliferation, differentiation, and function. Although their physiological functions are not clearly defined, recent studies have shown their therapeutic potential for tissue repair and regeneration. While MVs can be isolated readily from mesenchymal stem cells (MSCs) and other cell types from various sources, the yield of MVs under conventional culture condition in vitro is one of the limiting factors for both the in vivo functional study as well as in vitro molecular analysis. Here, we provide a protocol to increase the yield of microvesicles by preconditioning MSCs with rat brain extract.

Mesenchymal stem/stromal cells (MSC) are adult stem cells which have been shown to improve survival, enhance bacterial clearance and alleviate inflammation in pre-clinical models of acute respiratory distress syndrome (ARDS) and sepsis. These diseases are characterised by uncontrolled inflammation often underpinned by bacterial infection. The mechanisms of MSC immunomodulatory effects are not fully understood yet. We sought to investigate MSC cell contact-dependent communication with alveolar macrophages (AM), professional phagocytes which play an important role in the lung inflammatory responses and anti-bacterial defence. With the use of a basic direct co-culture system, confocal microscopy and flow cytometry we visualised and effectively quantified MSC mitochondrial transfer to AM through tunnelling nanotubes (TNT). To model the human AM, primary monocytes were isolated from human donor blood and differentiated into macrophages (monocyte derived macrophages, MDM) in the presence of granulocyte macrophage colony-stimulating factor (GM-CSF), thus allowing adaptation of an AM-like phenotype (de Almeida et al., 2000; Guilliams et al., 2013). Human bone-marrow derived MSC, were labelled with mitochondria-specific fluorescent stain, washed extensively, seeded into the tissue culture plate with MDMs at the ratio of 1:20 (MSC/MDM) and co-cultured for 24 h. TNT formation and mitochondrial transfer were visualised by confocal microscopy and semi-quantified by flow cytometry. By using the method we described here we established that MSC use TNTs as the means to transfer mitochondria to macrophages. Further studies demonstrated that mitochondrial transfer enhances macrophage oxidative phosphorylation and phagocytosis. When TNT formation was blocked by cytochalasin B, MSC effect on macrophage phagocytosis was completely abrogated. This is the first study to demonstrate TNT-mediated mitochondrial transfer from MSC to innate immune cells.

Vascular leakage is an important feature in several diseases, such as septic shock, viral hemorrhagic fever, cancer metastasis and ischemia-reperfusion injuries. Thus establishing assays for measuring endothelial permeability will provide insight into the establishment or progression of such diseases. Here, we provide transwell permeability assay and electrical impedance sensing assay for studying endothelial permeability in vitro. With these methods, the effect of a molecule on endothelial permeability could be defined.