Optical Imaging of the Nanoscale Structure and Dynamics of Biological Membranes.
Authors of this article are:
Wijesooriya C, Nyamekye C, Smith EA.
A summary of the article is shown below:
Biological membranes serve as the fundamental unit of life, allowing the compartmentalization of cellular contents into subunits with specific functions. The bilayer structure, consisting of lipids, proteins, small molecules and sugars, also serves many other complex functions in addition to maintaining the relative stability of the inner compartments. Signal transduction, regulation of solute exchange, active transport, and energy transduction through ion gradients all take place at biological membranes, primarily with the assistance of membrane proteins. For these functions, membrane structure is often critical. The fluid-mosaic model introduced by Singer and Nicolson in 1972 evokes the dynamic and fluid nature of biological membranes.1 According to this model, integral and peripheral proteins are oriented in a viscous phospholipid bilayer. Both proteins and lipids can diffuse laterally through the two-dimensional structure. Modern experimental evidence has shown, however, that the structure of the membrane is considerably more complex; various domains in the biological membranes, such as lipid rafts and confinement regions, form a more complicated molecular organization. The proper organization and dynamics of the membrane components are critical for the function of the entire cell. For example, cell signaling is often initiated at biological membranes and requires receptors to diffuse and assemble into complexes and clusters, and the resulting downstream events have consequences throughout the cell. Revealing the molecular level details of these signaling events is the foundation to understanding numerous unsolved questions regarding cellular life. Optical imaging methods have substantial utility in revealing information about biological materials. They offer simple sample preparation, the ability to non-invasively image samples in situ, the ability to simultaneously image several different properties, and compatibility with many other imaging techniques. The earliest applications of optical imaging to measure cellular membranes revealed their basic structure and their dynamic properties at the ensemble level. Many important molecular assemblies in biological membranes occur in the nanometer scale, thus diffraction-limited optical techniques are unable to resolve them. The development of super-resolution optical imaging techniques has accelerated the study of biological membranes, sometimes one molecule at a time. Within the past few years, multimodal and multicolor imaging approaches were developed to facilitate multivariable imaging of membranes, primarily with fluorescence contrast. Recent advances in Raman scattering techniques have paved the path to obtain chemical information at the nanoscale level. The introduction of novel and highly selective probes, advanced light sources, novel detectors with fast detection rates and high quantum yields, modifications to optics that provide optimized signals as well as recent big data efforts have all helped improve image quality and analysis, and have thus lead to a better understanding of membrane-related phenomena. This review summarizes the optical imaging instrumentation that has recently been developed or is being developed in order to measure membrane organization and dynamics as well as some of the key applications of these instruments for membrane studies. The developments and applications of fluorescence and Raman-based imaging methods are covered. Atomic force microscopy, mass spectroscopy, and electron microscopy methods are useful for revealing complementary details about membrane structure and dynamics, but will not be covered herein, nor will studies of model membranes, such as those using supported lipid bilayers.
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