Samuel T. Hess

Bio: Samuel T. Hess is an academic researcher from University of Maine. The author has contributed to research in topics: Microscopy & Photoactivated localization microscopy. The author has an hindex of 28, co-authored 68 publications receiving 10278 citations. Previous affiliations of Samuel T. Hess include Yale University & National Institutes of Health.

Imaging coexisting fluid domains in biomembrane models coupling curvature and line tension

Abstract: Lipid bilayer membranes--ubiquitous in biological systems and closely associated with cell function--exhibit rich shape-transition behaviour, including bud formation and vesicle fission. Membranes formed from multiple lipid components can laterally separate into coexisting liquid phases, or domains, with distinct compositions. This process, which may resemble raft formation in cell membranes, has been directly observed in giant unilamellar vesicles. Detailed theoretical frameworks link the elasticity of domains and their boundary properties to the shape adopted by membranes and the formation of particular domain patterns, but it has been difficult to experimentally probe and validate these theories. Here we show that high-resolution fluorescence imaging using two dyes preferentially labelling different fluid phases directly provides a correlation between domain composition and local membrane curvature. Using freely suspended membranes of giant unilamellar vesicles, we are able to optically resolve curvature and line tension interactions of circular, stripe and ring domains. We observe long-range domain ordering in the form of locally parallel stripes and hexagonal arrays of circular domains, curvature-dependent domain sorting, and membrane fission into separate vesicles at domain boundaries. By analysing our observations using available membrane theory, we are able to provide experimental estimates of boundary tension between fluid bilayer domains.

Three-dimensional sub-100 nm resolution fluorescence microscopy of thick samples.

Abstract: Imaging volumes as thick as whole cells at three-dimensional (3D) super-resolution is required to reveal unknown features of cellular organization. We report a light microscope that generates images with translationally invariant 30 x 30 x 75 nm resolution over a depth of several micrometers. This method, named biplane (BP) FPALM, combines a double-plane detection scheme with fluorescence photoactivation localization microscopy (FPALM) enabling 3D sub-diffraction resolution without compromising speed or sensitivity.

Large-scale fluid/fluid phase separation of proteins and lipids in giant plasma membrane vesicles

Abstract: The membrane raft hypothesis postulates the existence of lipid bilayer membrane heterogeneities, or domains, supposed to be important for cellular function, including lateral sorting, signaling, and trafficking. Characterization of membrane lipid heterogeneities in live cells has been challenging in part because inhomogeneity has not usually been definable by optical microscopy. Model membrane systems, including giant unilamellar vesicles, allow optical fluorescence discrimination of coexisting lipid phase types, but thus far have focused on coexisting optically resolvable fluid phases in simple lipid mixtures. Here we demonstrate that giant plasma membrane vesicles (GPMVs) or blebs formed from the plasma membranes of cultured mammalian cells can also segregate into micrometer-scale fluid phase domains. Phase segregation temperatures are widely spread, with the vast majority of GPMVs found to form optically resolvable domains only at temperatures below ≈25°C. At 37°C, these GPMV membranes are almost exclusively optically homogenous. At room temperature, we find diagnostic lipid phase fluorophore partitioning preferences in GPMVs analogous to the partitioning behavior now established in model membrane systems with liquid-ordered and liquid-disordered fluid phase coexistence. We image these GPMVs for direct visual characterization of protein partitioning between coexisting liquid-ordered-like and liquid-disordered-like membrane phases in the absence of detergent perturbation. For example, we find that the transmembrane IgE receptor FceRI preferentially segregates into liquid-disordered-like phases, and we report the partitioning of additional well known membrane associated proteins. Thus, GPMVs now provide an effective approach to characterize biological membrane heterogeneities.

Biological and Chemical Applications of Fluorescence Correlation Spectroscopy: A Review†

Abstract: The mathematical concept of fluorescence correlation spectroscopy (FCS) 1 (1) emerged from quasi-elastic light scattering (QELS) spectroscopy ( 2) in the early 1970s. Compared to light scattering, the enhanced sensitivity of fluorescence to changes in molecular structure, chemistry, and local environment makes FCS a superior analytical tool for chemical kinetics studies ( 1, 3-5). The primary motivation for the invention of FCS was the study of chemical kinetics at very dilute concentrations in biological systems, such as the reversible binding reaction between ethidium bromide, a fluorescent nucleic acid synthesis inhibitor, and DNA ( 1). Theoretical and experimental studies ( 3, 4, 6) soon established that FCS could measure not only diffusion coefficients but also chemical rate constants, concentration, aggregation, and rotational dynamics ( 3-9). Building on this foundation, significant advances in the understanding of lipid diffusion in membranes were made soon after the birth of FCS ( 10) using a confocal microscope geometry ( 11) introduced into FCS by Koppel et al. ( 7) and still used today. Recently, technological advances in detectors, autocorrelation electronics, and confocal microscopy were incorporated into FCS, mainly in the laboratories of R. Rigler and M. Eigen (12, 13). A detailed theoretical framework on the effects of translational and rotational motion of a fluorescent molecule undergoing chemical reactions, in a three-dimensional (3D) Gaussian observation volume, has been introduced ( 14). Statistical analysis provided the basis for optimizing the signal-to-noise ratio (S/N) in FCS ( 15). Furthermore, analytic functions describing molecular translation (16), shot noise effects on higher-order fluorescence fluctuation moments ( 17), and the effect of the observation volume (18) on the S/N were explored. Finally, the ability of FCS to resolve multiple species with equivalent diffusion properties was extended by probability analysis of fluorescence intensity distributions ( 19-22). These advances extend the horizon of FCS in biological and chemical studies (for a recent review, see ref 23).

Lipid Rafts As a Membrane-Organizing Principle

Abstract: Cell membranes display a tremendous complexity of lipids and proteins designed to perform the functions cells require. To coordinate these functions, the membrane is able to laterally segregate its constituents. This capability is based on dynamic liquid-liquid immiscibility and underlies the raft concept of membrane subcompartmentalization. Lipid rafts are fluctuating nanoscale assemblies of sphingolipid, cholesterol, and proteins that can be stabilized to coalesce, forming platforms that function in membrane signaling and trafficking. Here we review the evidence for how this principle combines the potential for sphingolipid-cholesterol self-assembly with protein specificity to selectively focus membrane bioactivity.

Nonlinear magic: multiphoton microscopy in the biosciences

Abstract: Multiphoton microscopy (MPM) has found a niche in the world of biological imaging as the best noninvasive means of fluorescence microscopy in tissue explants and living animals. Coupled with transgenic mouse models of disease and 'smart' genetically encoded fluorescent indicators, its use is now increasing exponentially. Properly applied, it is capable of measuring calcium transients 500 microm deep in a mouse brain, or quantifying blood flow by imaging shadows of blood cells as they race through capillaries. With the multitude of possibilities afforded by variations of nonlinear optics and localized photochemistry, it is possible to image collagen fibrils directly within tissue through nonlinear scattering, or release caged compounds in sub-femtoliter volumes.

A variant of yellow fluorescent protein with fast and efficient maturation for cell-biological applications

Abstract: The green fluorescent protein (GFP) from the jellyfish Aequorea victoria has provided a myriad of applications for biological systems Over the last several years, mutagenesis studies have improved folding properties of GFP (refs 1,2) However, slow maturation is still a big obstacle to the use of GFP variants for visualization These problems are exacerbated when GFP variants are expressed at 37 degrees C and/or targeted to certain organelles Thus, obtaining GFP variants that mature more efficiently is crucial for the development of expanded research applications Among Aequorea GFP variants, yellow fluorescent proteins (YFPs) are relatively acid-sensitive, and uniquely quenched by chloride ion (Cl-) For YFP to be fully and stably fluorescent, mutations that decrease the sensitivity to both pH and Cl- are desired Here we describe the development of an improved version of YFP named "Venus" Venus contains a novel mutation, F46L, which at 37 degrees C greatly accelerates oxidation of the chromophore, the rate-limiting step of maturation As a result of other mutations, F64L/M153T/V163A/S175G, Venus folds well and is relatively tolerant of exposure to acidosis and Cl- We succeeded in efficiently targeting a neuropeptide Y-Venus fusion protein to the dense-core granules of PC12 cells Its secretion was readily monitored by measuring release of fluorescence into the medium The use of Venus as an acceptor allowed early detection of reliable signals of fluorescence resonance energy transfer (FRET) for Ca2+ measurements in brain slices With the improved speed and efficiency of maturation and the increased resistance to environment, Venus will enable fluorescent labelings that were not possible before

Far-Field Optical Nanoscopy

Abstract: In 1873, Ernst Abbe discovered what was to become a well-known paradigm: the inability of a lens-based optical microscope to discern details that are closer together than half of the wavelength of light. However, for its most popular imaging mode, fluorescence microscopy, the diffraction barrier is crumbling. Here, I discuss the physical concepts that have pushed fluorescence microscopy to the nanoscale, once the prerogative of electron and scanning probe microscopes. Initial applications indicate that emergent far-field optical nanoscopy will have a strong impact in the life sciences and in other areas benefiting from nanoscale visualization.