Dynamic range and background filtering in raster image correlation spectroscopy

There is more to scanning than meets the eye: Raster Image Correlation Spectroscopy

Raster Image Correlation Spectroscopy (RICS) is a confocal image analysis method that can measure the diffusion and interactions of fluorescently labeled molecules in real time in solution and in living cells. RICS is easy to implement on commercial confocal microscopes and allows detailed investigations of complex biological systems and pathways. The method is especially robust for measurements in living cells using commonly used labels such as fluorescent proteins. Moreover, since its invention in 2005, the robustness and applicability of RICS has been significantly increased to allow, e.g., straightforward kinetic analyses, advanced image segmentation, parameter mapping, and multi-species analysis. In this review, we describe the methodological principles of RICS in a manner that is accessible to a broad readership, position RICS in relation to other fluorescence fluctuation techniques, highlight recent methodological advances and present exemplary applications of the method. With this review, we hope to facilitate the implementation of this powerful method into the everyday repertoire of confocal imaging approaches.

Fluorescence correlation spectroscopy for particle sizing: A notorious challenge

In many quantitative investigations of biological systems, including, e.g., the study of biomolecular interactions, assembly and disassembly, aggregation, micelle and vesicle formation, or drug encapsulation, accurate determination of particle sizes is of key interest. Fluorescence correlation spectroscopy (FCS), with its exceptional sensitivity for molecular diffusion properties, has long been proposed as a valuable method to size small, freely diffusible particles with superior precision. It is conceptually related to the more widespread particle sizing technique dynamic light scattering (DLS) but offers greater selectivity and sensitivity due to the use of fluorescence rather than scattered light. However, in spite of these apparent advantages, FCS has never become established as a biophysical routine for particle sizing. This is due to the fact that sensitivity can, under certain conditions, indeed be disadvantageous, as it renders the technique error prone and overly susceptible to signal disturbances. Here, we discuss the systematic challenges, as well as the advances made over the past decades, to employing FCS in polydisperse samples. The problematic role of large particles, a common issue in DLS and FCS, and the effect of fluorescent labeling are discussed in detail, along with strategies for respective error mitigation in experiments and data analysis. We expect this overview to guide future users in successfully applying FCS to their particle sizing problems in the hope of fostering a more widespread and routine use of FCS-based technology.

Hetero-pentamerization determines mobility and conductance of Glycine receptor α3 splice variants

Glycine receptors (GlyRs) are ligand-gated pentameric chloride channels in the central nervous system. GlyR-α3 is a possible target for chronic pain treatment and temporal lobe epilepsy. Alternative splicing into K or L variants determines the subcellular fate and function of GlyR-α3, yet it remains to be shown whether its different splice variants can functionally co-assemble, and what the properties of such heteropentamers would be. Here, we subjected GlyR-α3 to a combined fluorescence microscopy and electrophysiology analysis. We employ masked Pearson’s and dual-color spatiotemporal correlation analysis to prove that GlyR-α3 splice variants heteropentamerize, adopting the mobility of the K variant. Fluorescence-based single-subunit counting experiments revealed a variable and concentration ratio dependent hetero-stoichiometry. Via cell-attached single-channel electrophysiology we show that heteropentamers exhibit currents in between those of K and L variants. Our data are compatible with a model where α3 heteropentamerization fine-tunes mobility and activity of GlyR-α3 channels, which is important to understand and tackle α3 related diseases.