AFM/TIRF force clamp measurements of neurosecretory vesicle tethers reveal characteristic unfolding steps

Exploring the structural dynamics of the vesicle priming machinery

Various cell types release neurotransmitters, hormones and many other compounds that are stored in secretory vesicles by exocytosis via the formation of a fusion pore traversing the vesicular membrane and the plasma membrane. This process of membrane fusion is mediated by the Soluble N-ethylmaleimide-Sensitive Factor Attachment Proteins REceptor (SNARE) protein complex, which in neurons and neuroendocrine cells is composed of the vesicular SNARE protein Synaptobrevin and the plasma membrane proteins Syntaxin and SNAP25 (Synaptosomal-Associated Protein of 25 kDa). Before a vesicle can undergo fusion and release of its contents, it must dock at the plasma membrane and undergo a process named 'priming', which makes it ready for release. The primed vesicles form the readily releasable pool, from which they can be rapidly released in response to stimulation. The stimulus is an increase in Ca2+ concentration near the fusion site, which is sensed primarily by the vesicular Ca2+ sensor Synaptotagmin. Vesicle priming involves at least the SNARE proteins as well as Synaptotagmin and the accessory proteins Munc18, Munc13, and Complexin but additional proteins may also participate in this process. This review discusses the current views of the interactions and the structural changes that occur among the proteins of the vesicle priming machinery.

Automated Bio-AFM Generation of Large Mechanome Data Set and Their Analysis by Machine Learning to Classify Cancerous Cell Lines

Mechanobiological measurements have the potential to discriminate healthy cells from pathological cells. However, a technology frequently used to measure these properties, i.e., atomic force microscopy (AFM), suffers from its low output and lack of standardization. In this work, we have optimized AFM mechanical measurement on cell populations and developed a technology combining cell patterning and AFM automation that has the potential to record data on hundreds of cells (956 cells measured for publication). On each cell, 16 force curves (FCs) and seven features/FC, constituting the mechanome, were calculated. All of the FCs were then classified using machine learning tools with a statistical approach based on a fuzzy logic algorithm, trained to discriminate between nonmalignant and cancerous cells (training base, up to 120 cells/cell line). The proof of concept was first made on prostate nonmalignant (RWPE-1) and cancerous cell lines (PC3-GFP), then on nonmalignant (Hs 895.Sk) and cancerous (Hs 895.T) skin fibroblast cell lines, and demonstrated the ability of our method to classify correctly 73% of the cells (194 cells in the database/cell line) despite the very high degree of similarity of the whole set of measurements (79-100% similarity).

An atomic force microscopy and total internal reflection fluorescence microscopy correlated system (AFM-TIRF) for fluorescence imaging and spectroscopy of a single particle

Combining atomic force microscopy (AFM) with other optical microscopic techniques is pivotal in nanoscale investigations, particularly leveraging the surface-sensitive properties of total internal reflection fluorescence microscopy (TIRF). A novel design that integrates AFM with a multi-wavelength TIRF is displayed, providing simultaneous fluorescence imaging and spectral acquisition capabilities. We elaborate on the considerations in the instrument design process and demonstrate the performance and potential applications of the instrument through fluorescence imaging and spectroscopy testing of individual nanoparticles. This AFM and TIRF correlated system (AFM-TIRF) emerges as a promising option for single-molecule fluorescence studies, enabling simultaneous manipulation and detection of fluorescence from individual molecules.

Advanced Imaging Approaches to Reveal Molecular Mechanisms Governing Neuroendocrine Secretion

Identification of the molecular mechanisms governing neuroendocrine secretion and resulting intercellular communication is one of the great challenges of cell biology to better understand organism physiology and neurosecretion disruption-related pathologies such as hypertension, neurodegenerative, or metabolic diseases. To visualize molecule distribution and dynamics at the nanoscale, many imaging approaches have been developed and are still emerging. In this review, we provide an overview of the pioneering studies using transmission electron microscopy, atomic force microscopy, total internal reflection microscopy, and super-resolution microscopy in neuroendocrine cells to visualize molecular mechanisms driving neurosecretion processes, including exocytosis and associated fusion pores, endocytosis and associated recycling vesicles, and protein-protein or protein-lipid interactions. Furthermore, the potential and the challenges of these different advanced imaging approaches for application in the study of neuroendocrine cell biology are discussed, aiming to guide researchers to select the best approach for their specific purpose around the crucial but not yet fully understood neurosecretion process.

How did correlative atomic force microscopy and super-resolution microscopy evolve in the quest for unravelling enigmas in biology?

With the invention of the Atomic Force Microscope (AFM) in 1986 and the subsequent developments in liquid imaging and cellular imaging it became possible to study the topography of cellular specimens under nearly physiological conditions with nanometric resolution. The application of AFM to biological research was further expanded with the technological advances in imaging modes where topographical data can be combined with nanomechanical measurements, offering the possibility to retrieve the biophysical properties of tissues, cells, fibrous components and biomolecules. Meanwhile, the quest for breaking the Abbe diffraction limit restricting microscopic resolution led to the development of super-resolution fluorescence microscopy techniques that brought the resolution of the light microscope comparable to the resolution obtained by AFM. The instrumental combination of AFM and optical microscopy techniques has evolved over the last decades from integration of AFM with bright-field and phase-contrast imaging techniques at first to correlative AFM and wide-field fluorescence systems and then further to the combination of AFM and fluorescence based super-resolution microscopy modalities. Motivated by the many developments made over the last decade, we provide here a review on AFM combined with super-resolution fluorescence microscopy techniques and how they can be applied for expanding our understanding of biological processes.