A miniature Couette to generate shear for flow cytometry: studying real-time modulation of intracellular calcium in monocytic cells.

Extracellular hydrodynamic forces may be transmitted to the interior of cells through the alteration of integrin conformation and affinity. Integrin activation regulates leukocyte recruitment, cell activation, and transmigration. The cellular and molecular mechanisms for integrin activation are not precisely known, although intracellular calcium signaling is involved. Flow cytometry offers a versatile way to study intracellular calcium signaling in real-time. We report a novel method to generate defined shear by using a miniature Couette. Testing involved measuring shear-induced intracellular calcium signals of human monoblastoid U937 cells in suspension. The Couette was connected externally to a flow cytometer and pressurized at 6 PSI (4.1 N/m(2) ). Cells were subjected to a well-defined shear between 0 and 1,000 s(-1) and delivered continuously within 10 s to a FACScan at 1 µl/s. Intracellular calcium levels and the percentage of cells activated increased as shear increased in duration and intensity.

Mechanical properties of primary cilia regulate the response to fluid flow.

The primary cilium is a ubiquitous organelle present on most mammalian cells. Malfunction of the organelle has been associated with various pathological disorders, many of which lead to cystic disorders in liver, pancreas, and kidney. Primary cilia have in kidney epithelial cells been observed to generate intracellular calcium in response to fluid flow, and disruption of proteins involved in this calcium signaling lead to autosomal dominant polycystic kidney disease, implying a direct connection between calcium signaling and cyst formation. It has also been shown that there is a significant lag between the onset of flow and initiation of the calcium signal. The present study focuses on the mechanics of cilium bending and the resulting calcium signal. Visualization of real-time cilium movements in response to different types of applied flow showed that the bending is fast compared with the initiation of calcium increase. Mathematical modeling of cilium and surrounding membrane was performed to deduce the relation between bending and membrane stress. The results showed a delay in stress buildup that was similar to the delay in calcium signal. Our results thus indicate that the delay in calcium response upon cilia bending is caused by mechanical properties of the cell membrane.

A quantitative approach for studying IgE-FcepsilonRI aggregation.

Aggregation of cell surface receptors is a ubiquitous means of initiating signal transduction in many cellular systems. In this manuscript, we describe a combined theoretical and experimental approach based on multiparameter flow cytometry for measuring the time course of ligand induced aggregation of IgE-FcepsilonRI on RBL cells. By fluorescently labeling both the ligand and surface IgE (sIgE), we have developed an assay that permits us to simultaneously measure both occupancy of sIgE combining sites and association of antigen with the cell surface. This allows for a direct calculation of the degree of receptor aggregation present on the cell. By employing new mixing technologies developed for flow cytometry, we are able to look at aggregation in the sub second time domain. To extend our work, we have synthesized a new set of chemically well defined ligands (of valences 1-3) to use as probes in our studies. We show that the magnitude of the cellular response is dramatically increased as the valence of our ligand is raised from two to three.

Regulation of cell adhesion by affinity and conformational unbending of alpha4beta1 integrin.

Rapid activation of integrins in response to chemokine-induced signaling serves as a basis for leukocyte arrest on inflamed endothelium. Current models of integrin activation include increased affinity for ligand, molecular extension, and others. In this study, using real-time fluorescence resonance energy transfer to assess alpha(4)beta(1) integrin conformational unbending and fluorescent ligand binding to assess affinity, we report at least four receptor states with independent regulation of affinity and unbending. Moreover, kinetic analysis of chemokine-induced integrin conformational unbending and ligand-binding affinity revealed conditions under which the affinity change was transient whereas the unbending was sustained. In a VLA-4/VCAM-1-specific myeloid cell adhesion model system, changes in the affinity of the VLA-4-binding pocket were reflected in rapid cell aggregation and disaggregation. However, the initial rate of cell aggregation increased 9-fold upon activation, of which only 2.5-fold was attributable to the increased affinity of the binding pocket. These data show that independent regulation of affinity and conformational unbending represents a novel and fundamental mechanism for regulation of integrin-dependent adhesion in which the increased affinity appears to account primarily for the increasing lifetime of the alpha(4)beta(1) integrin/VCAM-1 bond, whereas the unbending accounts for the increased capture efficiency.

Small-volume rapid-mix device for subsecond kinetic analysis in flow cytometry.

BACKGROUND: Rapid-mix flow cytometry has emerged as a powerful tool for mechanistic analysis of ligand binding, cell response, and molecular assembly. Although progress has come from improving sample delivery capabilities, little attention has been paid to the volumetric requirements associated with precious biological reagents. METHODS: By using programmable syringes, valves, and other fluidic components, we created a modular, precisely regulated rapid-mix device for the delivery of small-volume samples to the flow cytometer. The device was tested using a bead-based assay in which the binding kinetics between native biotin and fluorescein biotin-bearing beads were characterized. RESULTS: Bead suspensions and reagents paired in 35- to 45-microl aliquots were efficiently mixed by the device and delivered to the flow cytometer. Kinetic data associated with the fluorescein biotin beads were analyzed and used to calibrate the performance characteristics of the device in terms of sample delivery and mixing efficiency. CONCLUSION: The rapid-mix device is capable of detecting subsecond kinetics of biological reactions using microliter volume of samples. Dimensions of the device have been minimized, and the quantitative aspects of sample delivery and analysis have been optimized. Further, the modular design has been optimized for adaptation to a variety of experimental protocols.

Relationship between molecular and cellular dissociation rates for VLA-4/VCAM-1 interaction in the absence of shear stress.

The rate of leukocyte recruitment to and detachment from the vasculature contributes to cellular tethering, rolling, firm adherence, and migration across an endothelium layer. The molecular rates depend on the type and number of bound integrin or selectin adhesion molecules, shear force acting on the bound adhesion molecules, and affinity state of integrins. Although little is known of the effect that the number of adhesion molecules has on leukocyte recruitment, it has been shown that firm adhesion for cells in suspension may be mediated by small numbers of bound adhesion molecules. We studied the disaggregation of aggregates composed of B78H1 cells transfected with human vascular cell adhesion molecule-1 (VCAM-1) and human monoblastoid U937 cells expressing Very Late Antigen-4 (VLA-4). Aggregate disaggregation rates were obtained and compared to dissociation rates for soluble rhVCAM-1 ligand and monoblastoid U937 cells. Under conditions without shear stress, it was found that average cellular disaggregation rates were a factor of 1.3 +/- 0.4 times slower than molecular dissociation rates for the 1 mM Mn(2+) and 1 mM Mn(2+) + 1 mM Ca(2+) conditions. A simple mathematical model was used to predict how much smaller the dissociation constant would be if the number of bonds holding an aggregate varied from one bond to N bonds under conditions without shear stress. The average number of adhesion bonds holding the cell aggregates together was found to be 1.5 +/- 0.7. This suggests that a few bonds were needed to form cellular aggregates and that increased aggregation was related to integrin affinity changes and not due to clustering or increased bond numbers.

Conformational regulation of alpha 4 beta 1-integrin affinity by reducing agents. "Inside-out" signaling is independent of and additive to reduction-regulated integrin activation.

The alpha(4)beta(1)-integrin (very late antigen-4 (VLA-4), CD49d/CD29) is an adhesion receptor involved in the interaction of lymphocytes, dendritic cells, and stem cells with the extracellular matrix and endothelial cells. This and other integrins have the ability to regulate their affinity for ligands through a process termed "inside-out" signaling that affects cell adhesion avidity. Several mechanisms are known to regulate integrin affinity and conformation: conformational changes induced by separation of the C-terminal tails, divalent ions, and reducing agents. Recently, we described a fluorescent LDV-containing small molecule that was used to monitor VLA-4 affinity changes in live cells (Chigaev, A., Blenc, A. M., Braaten, J. V., Kumaraswamy, N., Kepley, C. L., Andrews, R. P., Oliver, J. M., Edwards, B. S., Prossnitz, E. R., Larson, R. S., and Sklar, L. A. (2001) J. Biol. Chem. 276, 48670-48678). Using the same molecule, we also developed a fluorescence resonance energy transfer-based assay to probe the "switchblade-like" opening of VLA-4 upon activation. Here, we investigated the effect of reducing agents on the affinity and conformational state of the VLA-4 integrin simultaneously with cell activation initiated by inside-out signaling through G protein-coupled receptors or Mn(2+) in live cells in real time. We found that reducing agents (dithiothreitol and 2,3-dimercapto-1-propanesulfonic acid) induced multiple states of high affinity of VLA-4, where the affinity change was accompanied by an extension of the integrin molecule. Bacitracin, an inhibitor of the reductive function of the plasma membrane, diminished the effect of dithiothreitol, but had no effect on inside-out signaling. Based on this result and differences in the kinetics of integrin activation, we conclude that conformational activation of VLA-4 by inside-out signaling is independent of and additive to reduction-regulated integrin activation.

Real-time analysis of very late antigen-4 affinity modulation by shear.

Shear promotes endothelial recruitment of leukocytes, cell activation, and transmigration. Mechanical stress on cells caused by shear can induce a rapid integrin conformational change and activation, followed by an increase in binding to the extracellular matrix. The molecular mechanism of increased avidity is unknown. We have shown previously that the affinity of the alpha(4)beta(1) integrin, very late antigen-4 (VLA-4), measured with an LDV-containing small molecule, varies with cellular avidity, measured from cell disaggregation rates. In this study, we measured in real time affinity changes of VLA-4 in response to shear. The resulting affinity was comparable with the state mediated by receptor signaling and corresponded in time with intracellular Ca(2+) responses. Ca(2+) ionophores and N,N'-[1,2-ethanediyl-bis(oxy-2,1-phenylene)]bis[N-[2-[(acetyloxy)methoxy]-2-oxoethyl]]-, bis[(acetyloxy)methyl]ester demonstrate that the affinity regulation of VLA-4 in the presence of shear was related to Ca(2+) signaling. Pertussis toxin treatment implicates G(i) in an unknown pathway that connects shear, Ca(2+) elevation, VLA-4 affinity, and cell avidity.

Alpha4beta1 integrin affinity changes govern cell adhesion.

Integrin alpha4beta1 is a receptor for vascular cell adhesion molecule-1 and fibronectin. It is important in lymphopoiesis, inflammatory recruitment of leukocytes, and other situations that require cell adhesion to the vascular endothelium. The avidity of the cells expressing alpha4beta1 integrin can be rapidly changed by chemokines and chemoattractants. Different mechanisms, including changes in the number of interacting molecules due to the alteration of the receptor topology or changes in the affinity of the individual bonds, have been proposed to explain the nature of these fast changes in avidity. Recently, we described a fluorescent LDV-containing small molecule, which we used to monitor the affinity changes on live cells in real time (Chigaev, A., Blenc, A. M., Braaten, J. V., Kumaraswamy, N., Kepley, C. L., Andrews, R. P., Oliver, J. M., Edwards, B. S., Prossnitz, E. R., Larson, R. S. et al. (2001) J. Biol. Chem. 276, 48670-48678). Here we show that the affinity of the small molecule probe as well as the native ligand vascular cell adhesion molecule-1 varies in parallel when the integrin is modulated with divalent cations and that the affinity modulation leads to the changes in cell avidity. Using formyl peptide receptor-transfected U937 cells, we further show that the time course of avidity changes in response to the receptor activation coincides with the time course of the affinity changes. Taken together, these data are consistent with the idea that affinity regulation is a major factor that governs the avidity of cell adhesion mediated by the alpha4 integrin.