Preservation of Fluorescence Signal and Imaging Optimization for Integrated Light and Electron Microscopy

Correlative light and electron microscopy imaging of proteinaceous deposits in cell cultures and brain tissues

Identifying protein deposits and associated components is crucial for understanding the pathogenesis of neurodegenerative disorders with intracellular or extracellular deposits. Correlative light and electron microscopy (CLEM) has emerged as a powerful tool to accurately study tissue and cellular pathology by examination of the same target at both microstructural and ultrastructural levels. However, the technical challenges with CLEM have limited its application to neuropathology. Here, we developed a simplified efficient CLEM method and applied it to a cell model that produces a high proportion of α-synuclein (αS) inclusions with immunopositivity to phosphorylated αS and the synaptic vesicle marker SV2A and synaptophysin. This approach incorporates modifications in sample processing and innovative fiducial marking techniques, which enhance antigen preservation and improve target registration, respectively. These advancements achieve an optimal balance in sensitivity, accuracy, efficiency, and cost-effectiveness compared to current CLEM methods employing different strategies. Using this method, we identified and analyzed αS inclusions in cell cultures, as well as various pathological protein deposits in postmortem brain tissues from individuals with a range of neurodegenerative disorders. Our findings replicate recently reported new features of αS pathology and also reveal unrecognized a variety forms of small αS inclusions in human brain, which provide valuable insights into mechanisms underlying Lewy-related pathology. Application of this enhanced CLEM method is a powerful tool in research on neurodegenerative disorders, including αS-opathies.

Fish gill chemosensing: knowledge gaps and inconsistencies

In this review, we explore the inconsistencies in the data and gaps in our knowledge that exist in what is currently known regarding gill chemosensors which drive the cardiorespiratory reflexes in fish. Although putative serotonergic neuroepithelial cells (NEC) dominate the literature, it is clear that other neurotransmitters are involved (adrenaline, noradrenaline, acetylcholine, purines, and dopamine). And although we assume that these agents act on neurons synapsing with the NECs or in the afferent or efferent limbs of the paths between chemosensors and central integration sites, this process remains elusive and may explain current discrepancies or species differences in the literature. To date it has been impossible to link the distribution of NECs to species sensitivity to different stimuli or fish lifestyles and while the gills have been shown to be the primary sensing site for respiratory gases, the location (gills, oro-branchial cavity or elsewhere) and orientation (external/water or internal/blood sensing) of the NECs are highly variable between species of water and air breathing fish. Much of what has been described so far comes from studies of hypoxic responses in fish, however, changes in CO2, ammonia and lactate have all been shown to elicit cardio-respiratory responses and all have been suggested to arise from stimulation of gill NECs. Our view of the role of NECs is broadening as we begin to understand the polymodal nature of these cells. We begin by presenting the fundamental picture of gill chemosensing that has developed, followed by some key unanswered questions about gill chemosensing in general.

Cdk5-dependent rapid formation and stabilization of dendritic spines by corticotropin-releasing factor

The neuropeptide corticotropin-releasing factor (CRF) exerts a pivotal role in modulating neuronal activity in the mammalian brain. The effects of CRF exhibit notable variations, depending on factors such as duration of exposure, concentration, and anatomical location. In the CA1 region of the hippocampus, the impact of CRF is dichotomous: chronic exposure to CRF impairs synapse formation and dendritic integrity, whereas brief exposure enhances synapse formation and plasticity. In the current study, we demonstrate long-term effects of acute CRF on the density and stability of mature mushroom spines ex vivo. We establish that both CRF receptors are present in this hippocampal region, and we pinpoint their precise subcellular localization within synapses by electron microscopy. Furthermore, both in vivo and ex vivo data collectively demonstrate that a transient surge of CRF in the CA1 activates the cyclin-dependent kinase 5 (Cdk5)-pathway. This activation leads to a notable augmentation in CRF-dependent spine formation. Overall, these data suggest that upon acute release of CRF in the CA1-SR synapse, both CRF-Rs can be activated and promote synaptic plasticity via activating different downstream signaling pathways, such as the Cdk5-pathway.

Correlative Imaging to Detect Rare HIV Reservoirs and Associated Damage in Tissues

Correlative light-electron microscopy (CLEM) has evolved in the last decades, especially after significant developments in sample preparation, imaging acquisition, software, spatial resolution, and equipment, including confocal, live-cell, super-resolution, and electron microscopy (scanning, transmission, focused ion beam, and cryo-electron microscopy). However, the recent evolution of different laser-related techniques, such as mass spectrometry imaging (MSI) and laser capture microdissection, could further expand spatial imaging capabilities into high-resolution OMIC approaches such as proteomic, lipidomics, small molecule, and drug discovery. Here, we will describe a protocol to integrate the detection of rare viral reservoirs with imaging mass spectrometry.

One for All, All for One: A Close Look at In-Resin Fluorescence Protocols for CLEM

Sample preparation is the novel bottleneck for high throughput correlative light and electron microscopy (CLEM). Protocols suitable for both imaging methods must therefore balance the requirements of each technique. For fluorescence light microscopy, a structure of interest can be targeted using: 1) staining, which is often structure or tissue specific rather than protein specific, 2) dye-coupled proteins or antibodies, or 3) genetically encoded fluorescent proteins. Each of these three methods has its own advantages. For ultrastructural investigation by electron microscopy (EM) resin embedding remains a significant sample preparation approach, as it stabilizes the sample such that it withstands the vacuum conditions of the EM, and enables long-term storage. Traditionally, samples are treated with heavy metal salts prior to resin embedding, in order to increase imaging contrast for EM. This is particularly important for volume EM (vEM) techniques. Yet, commonly used contrasting agents (e.g., osmium tetroxide, uranyl acetate) tend to impair fluorescence. The discovery that fluorescence can be preserved in resin-embedded specimens after mild heavy metal staining was a game changer for CLEM. These so-called in-resin fluorescence protocols present a significant leap forward for CLEM approaches towards high precision localization of a fluorescent signal in (volume) EM data. Integrated microscopy approaches, combining LM and EM detection into a single instrument certainly require such an “all in one” sample preparation. Preserving, or adding, dedicated fluorescence prior to resin embedding requires a compromise, which often comes at the expense of EM imaging contrast and membrane visibility. Especially vEM can be strongly hampered by a lack of heavy metal contrasting. This review critically reflects upon the fundamental aspects of resin embedding with regard to 1) specimen fixation and the physics and chemistry underlying the preservation of protein structure with respect to fluorescence and antigenicity, 2) optimization of EM contrast for transmission or scanning EM, and 3) the choice of embedding resin. On this basis, various existing workflows employing in-resin fluorescence are described, highlighting their common features, discussing advantages and disadvantages of the respective approach, and finally concluding with promising future developments for in-resin CLEM.

Fluorescence CLEM in biology: historic developments and current super-resolution applications

Correlative light and electron microscopy (CLEM) is a powerful imaging approach that allows the direct correlation of information obtained on a light and an electron microscope. There is a growing interest in the application of CLEM in biology, mainly attributable to technical advances in field of fluorescence microscopy in the past two decades. In this review, we summarize the important developments in CLEM for biological applications, focusing on the combination of fluorescence microscopy and electron microscopy. We first provide a brief overview of the early days of fluorescence CLEM usage starting with the initial rise in the late 1970s and the subsequent optimization of CLEM workflows during the following two decades. Next, we describe how the engineering of fluorescent proteins and the development of super-resolution fluorescence microscopy have significantly renewed the interest in CLEM resulting in the present application of fluorescence CLEM in many different areas of cellular and molecular biology. Lastly, we present the promises and challenges for the future of fluorescence CLEM discussing novel workflows, probe development and quantification possibilities.