Protocol to measure the membrane tension and bending modulus of cells using optical tweezers and scanning electron microscopy.

The elastic properties of cell membranes, particularly the membrane tension and bending modulus, are known to be key regulators of cellular functions. Here, we present a correlative and integrated tool based on optical tweezers and scanning electron microscopy to accurately determine these properties in a variety of cell types. Although there are intrinsic difficulties associated with correlative experiments, we believe that the methods presented can be considered a suitable protocol for determining the elastic properties of cell membranes.

Membrane Elastic Properties During Neural Precursor Cell Differentiation.

Neural precursor cells differentiate into several cell types that display distinct functions. However, little is known about how cell surface mechanics vary during the differentiation process. Here, by precisely measuring membrane tension and bending modulus, we map their variations and correlate them with changes in neural precursor cell morphology along their distinct differentiation fates.

Are cell membrane nanotubes the ancestors of the nervous system?

Cell membrane nanotubes, variously referred to as tunneling nanotubes and cytonemes, are currently the focus of much interest. They are of ancient origin, as indicated by their opportunistic use for cell invasion by pathogens, including bacteria and virus, and by their employment in bacterial networking. They play a significant role in cancer invasion and in the explanation of glioblastoma resistance to treatment.

Negative optical torque on a microsphere in optical tweezers.

We show that the optical force field in optical tweezers with elliptically polarized beams has the opposite handedness for a wide range of particle sizes and for the most common configurations. Our method is based on the direct observation of the particle equilibrium position under the effect of a transverse Stokes drag force, and its rotation around the optical axis by the mechanical effect of the optical torque. We find overall agreement with theory, with no fitting, provided that astigmatism, which is characterized separately, is included in the theoretical description.

Cell membrane biophysics with optical tweezers.

Membrane elastic properties play important roles in regulating cell shape, motility, division and differentiation. Here I review optical tweezer (OT) investigations of membrane surface tension and bending modulus, emphasizing didactic aspects and insights provided for cell biology. OT measurements employ membrane-attached microspheres to extract long cylindrical nanotubes named tethers.

Effects of cytoskeletal drugs on actin cortex elasticity.

Mechanical properties of cells are known to be influenced by the actin cytoskeleton. In this article, the action of drugs that interact with the actin cortex is investigated by tether extraction and rheology experiments using optical tweezers. The influences of Blebbistatin, Cytochalasin D and Jasplakinolide on the cell mechanical properties are evaluated.

Exact Theory of Optical Tweezers and Its Application to Absolute Calibration.

Optical tweezers have become a powerful tool for basic and applied research in cell biology. Here, we describe an experimentally verified theory for the trapping forces generated by optical tweezers based on first principles that allows absolute calibration. For pedagogical reasons, the steps that led to the development of the theory over the past 15 years are outlined.

Rheological properties of cells measured by optical tweezers.

Background: The viscoelastic properties of cells have been investigated by a variety of techniques. However, the experimental data reported in literature for viscoelastic moduli differ by up to three orders of magnitude. This has been attributed to differences in techniques and models for cell response as well as to the natural variability of cells.

Membrane elastic properties and cell function.

Recent studies indicate that the cell membrane, interacting with its attached cytoskeleton, is an important regulator of cell function, exerting and responding to forces. We investigate this relationship by looking for connections between cell membrane elastic properties, especially surface tension and bending modulus, and cell function. Those properties are measured by pulling tethers from the cell membrane with optical tweezers.

Cell cytoskeleton and tether extraction.

We perform a detailed investigation of the force × deformation curve in tether extraction from 3T3 cells by optical tweezers. Contrary to conventional wisdom about tethers extracted from cells, we find that actin filaments are present within them, so that a revised theory of tether pulling from cells is called for. We also measure steady and maximum tether force values significantly higher than previously published ones for 3T3 cells.

Does the glory have a simple explanation?

The explanation for the meteorological glory provided by the complex angular momentum theory is revisited in response to comments that a simpler physical picture would be desirable. New results that confirm the tunneling origin of the glory and the roles of resonances and surface waves in this phenomenon are presented, and expressions for averaged angular distribution and polarization features are given.

Structure and elastic properties of tunneling nanotubes.

We investigate properties of a reported new mechanism for cell-cell interactions, tunneling nanotubes (TNT's). TNT's mediate actin-based transfer of vesicles and organelles and they allow signal transmission between cells. The effects of lateral pulling with polystyrene beads trapped by optical tweezers on TNT's linking separate U-87 MG human glioblastoma cells in culture are described.

Towards absolute calibration of optical tweezers.

Aiming at absolute force calibration of optical tweezers, following a critical review of proposed theoretical models, we present and test the results of Mie-Debye-spherical aberration (MDSA) theory, an extension of a previous (MD) model, taking account of spherical aberration at the glass-water interface. This first-principles theory is formulated entirely in terms of experimentally accessible parameters (none adjustable). Careful experimental tests of the MDSA theory, undertaken at two laboratories, with very different setups, are described.

Light tunneling in clouds.

Solar radiation, traveling outside cloud water droplets, excites sharp resonances and surface waves by tunneling into the droplets. This effect contributes substantially to the total absorption (typically, of the order of 20%) and yields the major contribution to backscattering, producing the meteorological glory. Usual computational practices in atmospheric science misrepresent resonance contributions and cannot be relied on in the assessment of possible anomalies in cloud absorption.

Forward optical glory.

Forward optical glory effects in Mie scattering are displayed here for the first time to our knowledge. These effects include regular oscillations in Mie efficiency factors and characteristic deviations from zero polarization in near-forward scattering, which are observable for real refractive indices near radical2 and 2. Complex angular momentum theory predicts the period of oscillation correctly and shows the important role played by surface waves with shortcuts through the sphere.