Quantitative image mean squared displacement (iMSD) analysis of the dynamics of profilin 1 at the membrane of live cells

Comparison of MSD analysis from single particle tracking with MSD from images. Getting the best of both worlds

Fluorescence microscopy can provide valuable information about cell interior dynamics. Particularly, mean squared displacement (MSD) analysis is widely used to characterize proteins and sub-cellular structures' mobility providing the laws of molecular diffusion. The MSD curve is traditionally extracted from individual trajectories recorded by single-particle tracking-based techniques. More recently, image correlation methods like iMSD have been shown capable of providing averaged dynamic information directly from images, without the need for isolation and localization of individual particles. iMSD is a powerful technique that has been successfully applied to many different biological problems, over a wide spatial and temporal scales. The aim of this work is to review and compare these two well-established methodologies and their performance in different situations, to give an insight on how to make the most out of their unique characteristics. We show the analysis of the same datasets by the two methods. Regardless of the experimental differences in the input data for MSD or iMSD analysis, our results show that the two approaches can address equivalent questions for free diffusing systems. We focused on studying a range of diffusion coefficients between D = 0.001μm2s-1and D = 0.1μm2s-1, where we verified that the equivalence is maintained even for the case of isolated particles. This opens new opportunities for studying intracellular dynamics using equipment commonly available in any biophysical laboratory.

Angiotensin II Treatment Induces Reorganization and Changes in the Lateral Dynamics of Angiotensin II Type 1 Receptor in the Plasma Membrane Elucidated by Photoactivated Localization Microscopy Combined with Image Spatial Correlation Analysis

The mechanisms by which angiotensin II type 1 receptor is distributed and the diffusional pattern in the plasma membrane (PM) remain unclear, despite their crucial role in cardiovascular homeostasis. In this work, we obtained quantitative information of angiotensin II type 1 receptor (AT1R) lateral dynamics as well as changes in the diffusion properties after stimulation with ligands in living cells using photoactivated localization microscopy (PALM) combined with image spatial-temporal correlation analysis. To study the organization of the receptor at the nanoscale, expansion microscopy (ExM) combined with PALM was performed. This study revealed that AT1R lateral diffusion increased after binding to angiotensin II (Ang II) and the receptor diffusion was transiently confined in the PM. In addition, ExM revealed that AT1R formed nanoclusters at the PM and the cluster size significantly decreased after Ang II treatment. Taking these results together suggest that Ang II binding and activation cause reorganization and changes in the dynamics of AT1R at the PM.

Imaging Minimal Bacteria at the Nanoscale: a Reliable and Versatile Process to Perform Single-Molecule Localization Microscopy in Mycoplasmas

Mycoplasmas are the smallest free-living organisms. These bacteria are important models for both fundamental and synthetic biology, owing to their highly reduced genomes. They are also relevant in the medical and veterinary fields, as they are pathogenic to both humans and most livestock species. Mycoplasma cells have minute sizes, often in the 300- to 800-nm range. As these dimensions are close to the diffraction limit of visible light, fluorescence imaging in mycoplasmas is often poorly informative. Recently developed superresolution imaging techniques can break this diffraction limit, improving the imaging resolution by an order of magnitude and offering a new nanoscale vision of the organization of these bacteria. These techniques have, however, not been applied to mycoplasmas before. Here, we describe an efficient and reliable protocol to perform single-molecule localization microscopy (SMLM) imaging in mycoplasmas. We provide a polyvalent transposon-based system to express the photoconvertible fluorescent protein mEos3.2, enabling photo-activated localization microscopy (PALM) in most Mycoplasma species. We also describe the application of direct stochastic optical reconstruction microscopy (dSTORM). We showcase the potential of these techniques by studying the subcellular localization of two proteins of interest. Our work highlights the benefits of state-of-the-art microscopy techniques for mycoplasmology and provides an incentive to further the development of SMLM strategies to study these organisms in the future.

IMPORTANCE Mycoplasmas are important models in biology, as well as highly problematic pathogens in the medical and veterinary fields. The very small sizes of these bacteria, well below a micron, limits the usefulness of traditional fluorescence imaging methods, as their resolution limit is similar to the dimensions of the cells. Here, to bypass this issue, we established a set of state-of-the-art superresolution microscopy techniques in a wide range of Mycoplasma species. We describe two strategies: PALM, based on the expression of a specific photoconvertible fluorescent protein, and dSTORM, based on fluorophore-coupled antibody labeling. With these methods, we successfully performed single-molecule imaging of proteins of interest at the surface of the cells and in the cytoplasm, at lateral resolutions well below 50 nm. Our work paves the way toward a better understanding of mycoplasma biology through imaging of subcellular structures at the nanometer scale.

Heterogeneous Diffusion of Polystyrene Nanoparticles through an Alginate Matrix: The Role of Cross-linking and Particle Size

Most bacteria in natural and engineered environments grow and exist in biofilms. Recent investigations have shown that nanoparticles (NPs) interact with environmental biofilms, but these interactions are still not well characterized. Extracellular polymeric substances (EPS) are polymers secreted by bacteria to establish the functional and structural integrity of biofilms, and EPS porosity is a major contributor to NP access to and diffusion in biofilms. We used a synergistic combination of total internal reflection fluorescence microscopy and image correlation spectroscopy to monitor and map diffusion of fluorescent NPs in alginate yielding a detailed picture of the heterogeneous structure and connectivity of pores within a model EPS polymer. Using different sizes (20, 100, and 200 nm) of carboxylated polystyrene NPs, we examined how NP diffusive behaviors change as a result of calcium-induced cross-linking of the alginate matrix. This study reveals that cross-linking decreases NP diffusion coefficients and pore accessibility in an NP size-dependent manner and that NP movement through alginate matrices is anisotropic and heterogeneous. These results on heterogeneous and size-dependent movement within biofilms have important implications for future studies and simulations of NP-biofilm interactions.

Image mean square displacement to study the lateral mobility of Angiotensin II type 1 and Endothelin 1 type A receptors on living cells

The lateral mobility of membrane receptors provides insights into the molecular interactions of protein binding and the complex dynamic plasma membrane. The image mean square displacement (iMSD) analysis is a method used to extract qualitative and quantitative information of the protein diffusion law and infers how diffusion dynamic processes may change when the cellular environment is modified. The aim of the study was to describe the membrane diffusing properties of two G-protein-coupled receptors namely Angiotensin II type 1 (AT₁ ) and Endothelin 1 type A (ET_A ) receptors and their corresponding receptor-ligand complexes in living cells using total internal reflection fluorescent microscopy and iMSD analysis. This study showed that both AT₁ and ET_A receptors displayed a mix of three modes of diffusion: free, confined, and partially confined. The confined mode was the predominant at the plasma membrane of living cells and was not affected by ligand binding. However, the local diffusivity and the confinement zone of AT₁ receptors were reduced by the binding of its antagonist losartan, and the long-range diffusion with the local diffusivity coefficient of ET_A receptors was reduced upon exposure to its antagonist BQ123. To the best of our knowledge, this is the first study addressing the protein diffusion laws of these two receptors on living cells using total internal reflection fluorescence microscopy and iMSD.