Unobstructed Multiscale Imaging of Tissue Sections for Ultrastructural Pathology Analysis by Backscattered Electron Scanning Microscopy

Work smart, not hard: How array tomography can help increase the ultrastructure data output

Transmission electron microscopy has been essential for understanding cell biology for over six decades. Volume electron microscopy tools, such as serial block face and focused ion beam scanning electron microscopy acquisition, brought a new era to ultrastructure analysis. 'Array Tomography' (AT) refers to sequential image acquisition of resin-embedded sample sections on a large support (coverslip, glass slide, silicon wafers) for immunolabelling with multiple fluorescent labels, occasionally combined with ultrastructure observation. Subsequently, the term was applied to generating and imaging a series of sections to acquire a 3D representation of a structure using scanning electron microscopy (SEM). Although this is a valuable application, the potential of AT is to facilitate many tasks that are difficult or even impossible to obtain by Transmission Electron Microscopy (TEM). Due to the straightforward nature and versatility of AT sample preparation and image acquisition, the technique can be applied practically to any biological sample for selected sections or volume electron microscopy analysis. Furthermore, in addition to the benefits described here, AT is compatible with morphological analysis, multiplex immunolabelling, immune-gold labelling, and correlative light and electron microscopy workflow applicable for single cells, tissue and small organisms. This versatility makes AT attractive not only for basic research but as a diagnostic tool with a simplified routine.

TREM2-dependent microglial function is essential for remyelination and subsequent neuroprotection

Disability in multiple sclerosis (MS) is driven in part by the failure of remyelination and progressive neurodegeneration. Microglia, and specifically triggering receptor expressed on myeloid cells 2 (TREM2), a factor highly expressed in microglia, have been shown to play an important role in remyelination. Here, using a focal demyelination model in the brain, we demonstrate that demyelination is persistent in TREM2 knockout mice, lasting more than 6 weeks after lysolecithin injection and resulting in substantial neurodegeneration. We also find that TREM2 knockout mice exhibit an altered glial response following demyelination. TREM2 knockout microglia demonstrate defects in migration and phagocytosis of myelin debris. In addition, human monocyte-derived macrophages from subjects with a TREM2 mutation prevalent in human disease also show a defect in myelin debris phagocytosis. Together, we highlight the central role of TREM2 signaling in remyelination and neuroprotection. These findings provide insights into how chronic demyelination might lead to axonal damage and could help identify novel neuroprotective therapeutic targets for MS.

Liver inter-organelle membrane contact sites revealed by serial section electron tomography

Inter-organelle membrane contact sites (MCSs) are defined as areas of close proximity between the membranes of two organelles (10-80nm). They have been implicated in many physiological processes such as Ca++, lipids or small molecules transfer, organelles biogenesis or dynamic and have an important role in many cellular processes such as apoptosis, autophagy, and signaling. Since the distance and the extent of these contacts are in the nanometer range, high resolution techniques are ideal for imaging these structures. It is for this reason that transmission electron microscopy (TEM) has been considered the gold standard for MCSs visualization and the first technique that described them. However, often TEM analysis is limited to 2D lacking information on the 3D association between the organelles involved in MCSs. To fully describe the complex architecture of MSCs and to unveil their role in cellular physiology a 3D analysis is required. This chapter provides a method for the analysis of MCSs using serial section electron tomography (ssET), a technique able to reconstruct in 3D at nanometer resolution cellular and subcellular structures. By applying this procedure, it was possible to elucidate the role of the contacts between Endoplasmic Reticulum (ER) and other organelles in liver lipid metabolism.

Imaging the Kidney with an Unconventional Scanning Electron Microscopy Technique: Analysis of the Subpodocyte Space in Diabetic Mice

Transmission electron microscopy (TEM) remains the gold standard for renal histopathological diagnoses, given its higher resolving power, compared with light microscopy. However, it imposes several limitations on pathologists, including longer sample preparation time and a small observation area. To overcome these, we introduced a scanning electron microscopy (SEM) technique for imaging resin-embedded semi-thin sections of renal tissue. We developed a rapid tissue preparation protocol for experimental models and human biopsies which, alongside SEM digital imaging acquisition of secondary electrons (SE–SEM), enables fast electron microscopy examination, with a resolution similar to that achieved by TEM. We used this unconventional SEM imaging approach to investigate the subpodocyte space (SPS) in BTBR ob/ob mice with type 2 diabetes. Analysis of semi-thin sections with secondary electrons revealed that the SPS had expanded in volume and covered large areas of the glomerular basement membrane, forming wide spaces between the podocyte body and the underlying filtering membrane. Our results show that SE–SEM is a valuable tool for imaging the kidney at the ultrastructural level, filling the magnification gap between light microscopy and TEM, and reveal that in diabetic mice, the SPS is larger than in normal controls, which is associated with podocyte damage and impaired kidney function.

Field-Emission Scanning Electron Microscope as a Tool for Large-Area and Large-Volume Ultrastructural Studies

Simple Summary

Ultrastructural studies of cells and tissues are usually performed using transmission electron microscopy (TEM), which enables imaging at the highest possible resolution. The weak point of TEM is the limited ability to analyze the ultrastructure of large areas and volumes of biological samples. This limitation can be overcome by using modern field-emission scanning electron microscopy (FE-SEM) with high-sensitivity detection, which enables the creation of TEM-like images from the flat surfaces of resin-embedded biological specimens. Several FE-SEM-based techniques for two- and three-dimensional ultrastructural studies of cells, tissues, organs, and organisms have been developed in the 21st century. These techniques have created a new era in structural biology and have changed the role of the scanning electron microscope (SEM) in biological and medical laboratories. Since the premiere of the first commercially available SEM in 1965, these instruments were used almost exclusively to obtain topographical information over a large range of magnifications. Currently, FE-SEM offers many attractive possibilities in the studies of cell and tissue ultrastructure, and they are presented in this review.

Abstract

The development of field-emission scanning electron microscopes for high-resolution imaging at very low acceleration voltages and equipped with highly sensitive detectors of backscattered electrons (BSE) has enabled transmission electron microscopy (TEM)-like imaging of the cut surfaces of tissue blocks, which are impermeable to the electron beam, or tissue sections mounted on the solid substrates. This has resulted in the development of methods that simplify and accelerate ultrastructural studies of large areas and volumes of biological samples. This article provides an overview of these methods, including their advantages and disadvantages. The imaging of large sample areas can be performed using two methods based on the detection of transmitted electrons or BSE. Effective imaging using BSE requires special fixation and en bloc contrasting of samples. BSE imaging has resulted in the development of volume imaging techniques, including array tomography (AT) and serial block-face imaging (SBF-SEM). In AT, serial ultrathin sections are collected manually on a solid substrate such as a glass and silicon wafer or automatically on a tape using a special ultramicrotome. The imaging of serial sections is used to obtain three-dimensional (3D) information. SBF-SEM is based on removing the top layer of a resin-embedded sample using an ultramicrotome inside the SEM specimen chamber and then imaging the exposed surface with a BSE detector. The steps of cutting and imaging the resin block are repeated hundreds or thousands of times to obtain a z-stack for 3D analyses.

EMbedding and Backscattered Scanning Electron Microscopy: A Detailed Protocol for the Whole-Specimen, High-Resolution Analysis of Cardiovascular Tissues

Currently, an ultrastructural analysis of cardiovascular tissues is significantly complicated. Routine histopathological examinations and immunohistochemical staining suffer from a relatively low resolution of light microscopy, whereas the fluorescence imaging of plaques and bioprosthetic heart valves yields considerable background noise from the convoluted extracellular matrix that often results in a low signal-to-noise ratio. Besides, the sectioning of calcified or stent-expanded blood vessels or mineralised heart valves leads to a critical loss of their integrity, demanding other methods to be developed. Here, we designed a conceptually novel approach that combines conventional formalin fixation, sequential incubation in heavy metal solutions (osmium tetroxide, uranyl acetate or lanthanides, and lead citrate), and the embedding of the whole specimen into epoxy resin to retain its integrity while accessing the region of interest by grinding and polishing. Upon carbon sputtering, the sample is visualised by means of backscattered scanning electron microscopy. The technique fully preserves calcified and stent-expanded tissues, permits a detailed analysis of vascular and valvular composition and architecture, enables discrimination between multiple cell types (including endothelial cells, vascular smooth muscle cells, fibroblasts, adipocytes, mast cells, foam cells, foreign-body giant cells, canonical macrophages, neutrophils, and lymphocytes) and microvascular identities (arterioles, venules, and capillaries), and gives a technical possibility for quantitating the number, area, and density of the blood vessels. Hence, we suggest that our approach is capable of providing a pathophysiological insight into cardiovascular disease development. The protocol does not require specific expertise and can be employed in virtually any laboratory that has a scanning electron microscope.

Preparation of Samples for Large-Scale Automated Electron Microscopy of Tissue and Cell Ultrastructure

Manual selection of targets in experimental or diagnostic samples by transmission electron microscopy (TEM), based on single overview and detail micrographs, has been time-consuming and susceptible to bias. Substantial information and throughput gain may now be achieved by the automated acquisition of virtually all structures in a given EM section. Resulting datasets allow the convenient pan-and-zoom examination of tissue ultrastructure with preserved microanatomical orientation. The technique is, however, critically sensitive to artifacts in sample preparation. We, therefore, established a methodology to prepare large-scale digitization samples (LDS) designed to acquire entire sections free of obscuring flaws. For evaluation, we highlight the supreme performance of scanning EM in transmission mode compared with other EM technology. The use of LDS will substantially facilitate access to EM data for a broad range of applications.

Ocular phenotypes in a mouse model of impaired glucocerebrosidase activity

Mutations in the GBA1 gene encoding glucocerebrosidase (GCase) are linked to Gaucher (GD) and Parkinson's Disease (PD). Since some GD and PD patients develop ocular phenotypes, we determined whether ocular phenotypes might result from impaired GCase activity and the corresponding accumulation of glucosylceramide (GluCer) and glucosylsphingosine (GluSph) in the Gba1^D409V/D409V knock-in (Gba KI/KI; "KI") mouse. Gba KI mice developed age-dependent pupil dilation deficits to an anti-muscarinic agent; histologically, the iris covered the anterior part of the lens with adhesions between the iris and the anterior surface of the lens (posterior synechia). This may prevent pupil dilation in general, beyond an un-responsiveness of the iris to anti-muscarinics. Gba KI mice displayed atrophy and pigment dispersion of the iris, and occlusion of the iridocorneal angle by pigment-laden cells, reminiscent of secondary open angle glaucoma. Gba KI mice showed progressive thinning of the retina consistent with retinal degeneration. GluSph levels were increased in the anterior and posterior segments of the eye, suggesting that accumulation of lipids in the eye may contribute to degeneration in this compartment. We conclude that the Gba KI model provides robust and reproducible eye phenotypes which may be used to test for efficacy and establish biomarkers for GBA1-related therapies.

Neutrophil serine protease 4 is required for mast cell-dependent vascular leakage

Vascular leakage, or edema, is a serious complication of acute allergic reactions. Vascular leakage is triggered by the release of histamine and serotonin from granules within tissue-resident mast cells. Here, we show that expression of Neutrophil Serine Protease 4 (NSP4) during the early stages of mast cell development regulates mast cell-mediated vascular leakage. In myeloid precursors, the granulocyte-macrophage progenitors (GMPs), loss of NSP4 results in the decrease of cellular levels of histamine, serotonin and heparin/heparan sulfate. Mast cells that are derived from NSP4-deficient GMPs have abnormal secretory granule morphology and a sustained reduction in histamine and serotonin levels. Consequently, in passive cutaneous anaphylaxis and acute arthritis models, mast cell-mediated vascular leakage in the skin and joints is substantially reduced in NSP4-deficient mice. Our findings reveal that NSP4 is required for the proper storage of vasoactive amines in mast cell granules, which impacts mast cell-dependent vascular leakage in mouse models of immune complex-mediated diseases.

IRE1α Disruption in Triple-Negative Breast Cancer Cooperates with Antiangiogenic Therapy by Reversing ER Stress Adaptation and Remodeling the Tumor Microenvironment

Cancer cells exploit the unfolded protein response (UPR) to mitigate endoplasmic reticulum (ER) stress caused by cellular oncogene activation and a hostile tumor microenvironment (TME). The key UPR sensor IRE1α resides in the ER and deploys a cytoplasmic kinase-endoribonuclease module to activate the transcription factor XBP1s, which facilitates ER-mediated protein folding. Studies of triple-negative breast cancer (TNBC)-a highly aggressive malignancy with a dismal posttreatment prognosis-implicate XBP1s in promoting tumor vascularization and progression. However, it remains unknown whether IRE1α adapts the ER in TNBC cells and modulates their TME, and whether IRE1α inhibition can enhance antiangiogenic therapy-previously found to be ineffective in patients with TNBC. To gauge IRE1α function, we defined an XBP1s-dependent gene signature, which revealed significant IRE1α pathway activation in multiple solid cancers, including TNBC. IRE1α knockout in TNBC cells markedly reversed substantial ultrastructural expansion of their ER upon growth in vivo. IRE1α disruption also led to significant remodeling of the cellular TME, increasing pericyte numbers while decreasing cancer-associated fibroblasts and myeloid-derived suppressor cells. Pharmacologic IRE1α kinase inhibition strongly attenuated growth of cell line-based and patient-derived TNBC xenografts in mice and synergized with anti-VEGFA treatment to cause tumor stasis or regression. Thus, TNBC cells critically rely on IRE1α to adapt their ER to in vivo stress and to adjust the TME to facilitate malignant growth. TNBC reliance on IRE1α is an important vulnerability that can be uniquely exploited in combination with antiangiogenic therapy as a promising new biologic approach to combat this lethal disease. SIGNIFICANCE: Pharmacologic IRE1α kinase inhibition reverses ultrastructural distension of the ER, normalizes the tumor vasculature, and remodels the cellular TME, attenuating TNBC growth in mice.