Eric Hosy

ATP-gated purinergic P2X7 receptors are crucial ion channels involved in inflammation. They sense abnormal ATP release during stress or injury and are considered promising clinical targets for therapeutic intervention. However, despite their predominant expression in immune cells such as microglia, there is limited information on P2X7 membrane expression and regulation during inflammation at the single-molecule level, necessitating new labeling approaches to visualize P2X7 in native cells. Here, we present X7-uP , an unbiased, affinity-guided P2X7 chemical labeling reagent that selectively and covalently biotinylates endogenous P2X7 in BV2 cells, a murine microglial cell line, allowing subsequent labeling with streptavidin-Alexa 647 tailored for super-resolution imaging. We uncovered a nanoscale microglial P2X7 redistribution mechanism where evenly spaced individual receptors in quiescent cells undergo upregulation and clustering in response to the pro-inflammatory agent lipopolysaccharide and ATP, leading to synergistic interleukin-1β release. Our method thus offers a new approach to revealing endogenous P2X7 expression at the single-molecule level.

ATP-gated purinergic P2X7 receptors are crucial ion channels involved in inflammation. They sense abnormal ATP release during stress or injury and are considered promising clinical targets for therapeutic intervention. However, despite their predominant expression in immune cells such as microglia, there is limited information on P2X7 membrane expression and regulation during inflammation at the single-molecule level, necessitating new labeling approaches to visualize P2X7 in native cells. Here, we present X7-uP, an unbiased, affinity-guided P2X7 chemical labeling reagent that selectively and covalently biotinylates endogenous P2X7 in BV2 cells, a murine microglial cell line, allowing subsequent labeling with streptavidin-Alexa 647 tailored for super-resolution imaging. We uncovered a nanoscale microglial P2X7 redistribution mechanism where evenly spaced individual receptors in quiescent cells undergo upregulation and clustering in response to the pro-inflammatory agent lipopolysaccharide and ATP, leading to synergistic interleukin-1β release. Our method thus offers a new approach to revealing endogenous P2X7 expression at the single-molecule level.

ATP-gated purinergic P2X7 receptors are crucial ion channels involved in inflammation. They sense abnormal ATP release during stress or injury and are considered promising clinical targets for therapeutic intervention. However, despite their predominant expression in immune cells such as microglia, there is limited information on P2X7 membrane expression and regulation during inflammation at the single-molecule level, necessitating new labeling approaches to visualize P2X7 in native cells. Here, we present X7-uP , an unbiased, affinity-guided P2X7 chemical labeling reagent that selectively and covalently biotinylates endogenous P2X7 in BV2 cells, a murine microglial cell line, allowing subsequent labeling with streptavidin-Alexa 647 tailored for super-resolution imaging. We uncovered a nanoscale microglial P2X7 redistribution mechanism where evenly spaced individual receptors in quiescent cells undergo upregulation and clustering in response to the pro-inflammatory agent lipopolysaccharide and ATP, leading to synergistic interleukin-1β release. Our method thus offers a new approach to revealing endogenous P2X7 expression at the single-molecule level. Impact statement: An affinity-guided chemical strategy enabling highly specific biotinylation of P2X7 receptors reveals, by super-resolution microscopy, how the nanoscale organization of endogenous P2X7 in BV2 microglial cells dynamically changes upon activation.

A decade ago, in 2013, and over the course of 4 summer months, three separate observations were reported that each shed light independently on a new molecular organization that fundamentally reshaped our perception of excitatory synaptic transmission (Fukata et al., 2013; MacGillavry et al., 2013; Nair et al., 2013). This discovery unveiled an intricate arrangement of AMPA-type glutamate receptors and their principal scaffolding protein PSD-95, at synapses. This breakthrough was made possible, thanks to advanced super-resolution imaging techniques. It fundamentally changed our understanding of excitatory synaptic architecture and paved the way for a brand-new area of research. In this Progressions article, the primary investigators of the nanoscale organization of synapses have come together to chronicle the tale of their discovery. We recount the initial inquiry that prompted our research, the preceding studies that inspired our work, the technical obstacles that were encountered, and the breakthroughs that were made in the subsequent decade in the realm of nanoscale synaptic transmission. We review the new discoveries made possible by the democratization of super-resolution imaging techniques in the field of excitatory synaptic physiology and architecture, first by the extension to other glutamate receptors and to presynaptic proteins and then by the notion of trans-synaptic organization. After describing the organizational modifications occurring in various pathologies, we discuss briefly the latest technical developments made possible by super-resolution imaging and emerging concepts in synaptic physiology.

Despite the constant advances in fluorescence imaging techniques, monitoring endogenous proteins still constitutes a major challenge in particular when considering dynamics studies or super-resolution imaging. We have recently evolved specific protein-based binders for PSD-95, the main postsynaptic scaffold proteins at excitatory synapses. Since the synthetic recombinant binders recognize epitopes not directly involved in the target protein activity, we consider them here as tools to develop endogenous PSD-95 imaging probes. After confirming their lack of impact on PSD-95 function, we validated their use as intrabody fluorescent probes. We further engineered the probes and demonstrated their usefulness in different super-resolution imaging modalities (STED, PALM, and DNA-PAINT) in both live and fixed neurons. Finally, we exploited the binders to enrich at the synapse genetically encoded calcium reporters. Overall, we demonstrate that these evolved binders constitute a robust and efficient platform to selectively target and monitor endogenous PSD-95 using various fluorescence imaging techniques.

Assembly of cell wall polysaccharides into specific patterns is required for plant growth. A complex of RAPID ALKALINIZATION FACTOR 4 (RALF4) and its cell wall–anchored LEUCINE-RICH REPEAT EXTENSIN 8 (LRX8)–interacting protein is crucial for cell wall integrity during pollen tube growth, but its molecular connection with the cell wall is unknown. Here, we show that LRX8-RALF4 complexes adopt a heterotetrametric configuration in vivo, displaying a dendritic distribution. The LRX8-RALF4 complex specifically interacts with demethylesterified pectins in a charge-dependent manner through RALF4’s polycationic surface. The LRX8-RALF4-pectin interaction exerts a condensing effect, patterning the cell wall’s polymers into a reticulated network essential for wall integrity and expansion. Our work uncovers a dual structural and signaling role for RALF4 in pollen tube growth and in the assembly of complex extracellular polymers.

Stomata optimize land plants’ photosynthetic requirements and limit water vapor loss. So far, all of the molecular and electrical components identified as regulating stomatal aperture are produced, and operate, directly within the guard cells. However, a completely autonomous function of guard cells is inconsistent with anatomical and biophysical observations hinting at mechanical contributions of epidermal origins. Here, potassium (K⁺) assays, membrane potential measurements, microindentation, and plasmolysis experiments provide evidence that disruption of the Arabidopsis thaliana K⁺ channel subunit gene AtKC1 reduces pavement cell turgor, due to decreased K⁺ accumulation, without affecting guard cell turgor. This results in an impaired back pressure of pavement cells onto guard cells, leading to larger stomatal apertures. Poorly rectifying membrane conductances to K⁺ were consistently observed in pavement cells. This plasmalemma property is likely to play an essential role in K⁺ shuttling within the epidermis. Functional complementation reveals that restoration of the wild-type stomatal functioning requires the expression of the transgenic AtKC1 at least in the pavement cells and trichomes. Altogether, the data suggest that AtKC1 activity contributes to the building of the back pressure that pavement cells exert onto guard cells by tuning K⁺ distribution throughout the leaf epidermis.

The pruning of dendritic spines during development requires autophagy. This process is facilitated by long-term depression (LTD)-like mechanisms, which has led to speculation that LTD, a fundamental form of synaptic plasticity, also requires autophagy. Here, we show that the induction of LTD via activation of NMDA receptors or metabotropic glutamate receptors initiates autophagy in the postsynaptic dendrites in mice. Dendritic autophagic vesicles (AVs) act in parallel with the endocytic machinery to remove AMPA receptor subunits from the membrane for degradation. During NMDAR-LTD, key postsynaptic proteins are sequestered for autophagic degradation, as revealed by quantitative proteomic profiling of purified AVs. Pharmacological inhibition of AV biogenesis, or conditional ablation of atg5 in pyramidal neurons abolishes LTD and triggers sustained potentiation in the hippocampus. These deficits in synaptic plasticity are recapitulated by knockdown of atg5 specifically in postsynaptic pyramidal neurons in the CA1 area. Conducive to the role of synaptic plasticity in behavioral flexibility, mice with autophagy deficiency in excitatory neurons exhibit altered response in reversal learning. Therefore, local assembly of the autophagic machinery in dendrites ensures the degradation of postsynaptic components and facilitates LTD expression.

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 of both humans and most livestock species. Mycoplasma cells have minute sizes, often in the 300-800 nanometers range. As these dimensions are close to the diffraction limit of visible light, fluorescence imaging in mycoplasmas is often poorly informative. Recently developed “Super-Resolution 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 photo-convertible 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 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 size of these bacteria, well below the 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 “Super-Resolution Microscopy” techniques in a wide range of Mycoplasma species. We describe two strategies: PALM, based on the expression of a specific photo-convertible 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 nanometers. Our work paves the way toward a better understanding of mycoplasma’s biology through imaging of subcellular structures at the nanometer scale.