Light damage in rod outer segments: the effects of fixation on ultrastructural alterations

REG1A protects retinal photoreceptors from blue light damage

With the increased use of artificial light and the prolonged use of optoelectronic products, light damage (LD) to the human retina has been identified as a global vision-threatening problem. While there is evidence of a significant correlation between light-induced retinal damage and age-related vision impairment in age-related macular degeneration, it is unclear how light-induced retinal degeneration manifests itself and whether there are agents capable of preventing the development of LD in the retina. This study investigated a mechanism by which blue light leads to photoreceptor death. By observing blue light exposure in retinal organoids and photoreceptor cells, we concluded that there could be significant apoptosis of the photoreceptors. We demonstrate that regenerating islet-derived 1 alpha (REG1A) prevents photoreceptors from undergoing this LD-induced apoptosis by increasing expression of the anti-apoptotic gene Bcl2 and downregulating expression of the pro-apoptotic gene Bax, resulting in reduced mitochondrial damage and improved aerobic capacity in photoreceptor cells. For the first time, REG1A has been shown to restore mitochondrial function and cell apoptosis after LD-induced damage, suggesting its potential application in the prevention and treatment of retinal vision loss.

Current Progress in Expansion Microscopy: Chemical Strategies and Applications

Expansion microscopy (ExM) is a newly developed super-resolution technique, allowing visualization of biological targets at nanoscale resolution on conventional fluorescence microscopes. Since its introduction in 2015, many efforts have been dedicated to broaden its application range or increase the resolution that can be achieved. As a consequence, recent years have witnessed remarkable advances in ExM. This review summarizes recent progress in ExM, with the focus on the chemical aspects of the method, from chemistries for biomolecule grafting to polymer synthesis and the impact on biological analysis. The combination of ExM with other microscopy techniques, in search of additional resolution improvement, is also discussed. In addition, we compare pre- and postexpansion labeling strategies and discuss the impact of fixation methods on ultrastructure preservation. We conclude this review with a perspective on existing challenges and future directions. We believe that this review will provide a comprehensive understanding of ExM and facilitate its usage and further development.

Advanced Microscopy for Liver and Gut Ultrastructural Pathology in Patients with MVID and PFIC Caused by MYO5B Mutations

Mutations in the actin motor protein myosinVb (myo5b) cause aberrant apical cargo transport and the congenital enteropathy microvillus inclusion disease (MVID). Recently, missense mutations in myo5b were also associated with progressive familial intrahepatic cholestasis (MYO5B-PFIC). Here, we thoroughly characterized the ultrastructural and immuno-cytochemical phenotype of hepatocytes and duodenal enterocytes from a unique case of an adult MYO5B-PFIC patient who showed constant hepatopathy but only periodic enteric symptoms. Selected data from two other patients supported the findings. Advanced methods such as cryo-fixation, freeze-substitution, immuno-gold labeling, electron tomography and immuno-fluorescence microscopy complemented the standard procedures. Liver biopsies showed mislocalization of Rab11 and bile canalicular membrane proteins. Rab11-positive vesicles clustered around bile canaliculi and resembled subapical clusters of aberrant recycling endosomes in enterocytes from MVID patients. The adult patient studied in detail showed a severe, MVID-specific enterocyte phenotype, despite only a mild clinical intestinal presentation. This included mislocalization of numerous proteins essential for apical cargo transport and morphological alterations. We characterized the heterogeneous population of large catabolic organelles regarding their complex ultrastructure and differential distribution of autophagic and lysosomal marker proteins. Finally, we generated duodenal organoids/enteroids from biopsies that recapitulated all MVID hallmarks, demonstrating the potential of this disease model for personalized medicine.

Quantitative Electron Microscopy to Study HCMV Morphogenesis

The generation and release of mature virions from human cytomegalovirus (HCMV) infected cells is a multistep process, involving a profound reorganization of cellular structures and various stages of virus particle morphogenesis in different cellular compartments. Although the general steps of HCMV morphogenesis are known, it has become clear that the detailed molecular mechanisms are complex and dependent on various viral factors and cellular pathways. The lack of a full understanding of HCMV virion morphogenesis emphasizes the need of imaging techniques to visualize the different stages of virion assembly, such as electron microscopy. Here, we describe various electron microscopy techniques and the methodology of high-pressure freezing and freeze substitution for sample preparation to visualize HCMV morphogenesis. These methods are used in our laboratory in combination with a thorough quantification to characterize phenotypic alterations and to identify the function of viral and cellular proteins for the various morphogenesis stages.

An optimized approach using cryofixation for high-resolution 3D analysis by FIB-SEM

Compared with conventional two-dimensional transmission electron microscopy (TEM), focused ion beam scanning electron microscopy (FIB-SEM) can provide more comprehensive 3D information on cell substructures at the nanometer scale. Biological samples prepared by cryofixation using high-pressure freezing demonstrate optimal preservation of the morphology of cellular structures, as these are arrested instantly in their near-native states. However, samples from cryofixation often show a weak back-scatter electron signal and bad image contrast in FIB-SEM imaging. In addition, it is impossible to do large amounts of heavy metal staining. This is commonly achieved via established osmium impregnation (OTO) en bloc staining protocols. Here, we compared the FIB-SEM image quality of brain tissues prepared using several common freeze-substitution media, and we developed an approach that overcomes these limitations through a combination of osmium tetroxide, uranyl acetate, tannic acid, and potassium permanganate at proper concentrations, respectively. Using this optimized sample preparation protocol for high-pressure freezing and freeze-substitution, perfect smooth membrane morphology, even of the lipid bilayers of the cell membrane, was readily obtained using FIB-SEM. In addition, our protocol is broadly applicable and we demonstrated successful application to brain tissues, plant tissues, Caenorhabditis elegans, Candida albicans, and chlorella. This approach combines the potential of cryofixation for 3D large volume analysis of subcellular structures with the high-resolution capabilities of FIB-SEM.

High-quality ultrastructural preservation using cryofixation for 3D electron microscopy of genetically labeled tissues

Electron microscopy (EM) offers unparalleled power to study cell substructures at the nanoscale. Cryofixation by high-pressure freezing offers optimal morphological preservation, as it captures cellular structures instantaneously in their near-native state. However, the applicability of cryofixation is limited by its incompatibility with diaminobenzidine labeling using genetic EM tags and the high-contrast en bloc staining required for serial block-face scanning electron microscopy (SBEM). In addition, it is challenging to perform correlated light and electron microscopy (CLEM) with cryofixed samples. Consequently, these powerful methods cannot be applied to address questions requiring optimal morphological preservation. Here, we developed an approach that overcomes these limitations; it enables genetically labeled, cryofixed samples to be characterized with SBEM and 3D CLEM. Our approach is broadly applicable, as demonstrated in cultured cells, Drosophila olfactory organ and mouse brain. This optimization exploits the potential of cryofixation, allowing for quality ultrastructural preservation for diverse EM applications.

Ultrastructural Characterization of Phagophores Using Electron Tomography on Cryoimmobilized and Freeze Substituted Samples

Electron tomography has significantly contributed to recent findings regarding the biogenesis of the phagophore, an organelle which initiates autophagic sequestration. The information obtained from 1.9nm slices through the tomograms have revealed that during biogenesis the phagophore is in contact with the membranes of apposing organelles to form tubular connections and membrane contact sites (MCSs). The most reported and established tubular connections occur between the phagophore and the endoplasmic reticulum. However, as the phagophore continues to grow and expand, connections and MCSs have also been reported to occur between the phagophore and several other organelles in a possible attempt to utilize lipids for membrane expansion from alternative sources. Since the lifespan of the phagophore is only a few minutes and membrane connections and MCSs are very dynamic, capturing these two events requires precision during fixation. Up to date there is no quicker alternative for sample preservation in transmission electron microscopy than cryoimmobilization. In this report, we describe our protocol for cryoimmobilization using high-pressure freezing and freeze substitution, and report our first findings on phagophore morphology using this approach.

Correlative Light Electron Microscopy: Connecting Synaptic Structure and Function

Many core paradigms of contemporary neuroscience are based on information obtained by electron or light microscopy. Intriguingly, these two imaging techniques are often viewed as complementary, yet separate entities. Recent technological advancements in microscopy techniques, labeling tools, and fixation or preparation procedures have fueled the development of a series of hybrid approaches that allow correlating functional fluorescence microscopy data and ultrastructural information from electron micrographs from a singular biological event. As correlative light electron microscopy (CLEM) approaches become increasingly accessible, long-standing neurobiological questions regarding structure-function relation are being revisited. In this review, we will survey what developments in electron and light microscopy have spurred the advent of correlative approaches, highlight the most relevant CLEM techniques that are currently available, and discuss its potential and limitations with respect to neuronal and synapse-specific applications.

Studying Lysosomal Membrane Permeabilization by Analyzing the Release of Preloaded BSA-Gold Particles into the Cytosol

In addition to techniques involving assaying the release of endogenous lysosomal molecules into the cytosol, the endocytic system can be preloaded with exogenous fluorescent or electron-dense tracers. These tracers will translocate into the cytosol upon lysosomal membrane permeabilization and have the advantage of being detectable directly without additional labeling. Another benefit is that the tracers can be made more abundant than most endogenous lysosomal molecules, which facilitates their detection. Tracers that can be analyzed with fluorescence microscopy include low-molecular-mass molecules such as sulforhodamine B and also fluorescent polymers of dextran that are available in a wide range of molecular masses. This protocol shows how, for electron-microscopic analysis, cells can be fed with colloidal gold or ferrofluid particles complexed to bovine serum albumin. Although electron microscopy entails a high-resolution analysis, which can be advantageous, we caution how it is important to note that particulate tracers are larger than many endogenous lysosomal molecules and might be released only upon extensive membrane permeabilization.

Correlative 3D imaging: CLSM and FIB-SEM tomography using high-pressure frozen, freeze-substituted biological samples

Correlative light and electron microscopy aims at combining data from different imaging modalities, ideally from the same area of the one sample, in order to achieve a more holistic view of the hierarchical structural organization of cells and tissues. Modern 3D imaging techniques opened up new possibilities to expand morphological studies into the third dimension at the nanometer scale. Here we present an approach to correlate 3D light microscopy data with volume data from focused ion beam-scanning electron microscopy. An adapted sample preparation method based on high-pressure freezing for structure preservation, followed by freeze-substitution for multimodal en bloc imaging, is described. It is based on including fluorescent labeling during freeze-substitution, which enables histological context description of the structure of interest by confocal laser scanning microscopy prior to high-resolution electron microscopy. This information can be employed to relocate the respective structure in the electron microscope. This approach is most suitable for targeted small 3D volume correlation and has the potential to extract statistically relevant data of structural details for systems biology.

Bridging microscopes: 3D correlative light and scanning electron microscopy of complex biological structures

The rationale of correlative light and electron microscopy (CLEM) is to collect data on different information levels--ideally from an identical area on the same sample--with the aim of combining datasets at different levels of resolution to achieve a more holistic view of the hierarchical structural organization of cells and tissues. Modern three-dimensional (3D) imaging techniques in light and electron microscopy opened up new possibilities to expand morphological studies into the third dimension at the nanometer scale and over various volume dimensions. Here, we present two alternative approaches to correlate 3D light microscopy (LM) data with scanning electron microscopy (SEM) volume data. An adapted sample preparation method based on high-pressure freezing for structure preservation, followed by freeze-substitution for multimodal en-bloc imaging or serial-section imaging is described. The advantages and potential applications are exemplarily shown on various biological samples, such as cells, individual organisms, human tissue, as well as plant tissue. The two CLEM approaches presented here are per se not mutually exclusive, but have their distinct advantages. Confocal laser scanning microscopy (CLSM) and focused ion beam-SEM (FIB-SEM) is most suitable for targeted 3D correlation of small volumes, whereas serial-section LM and SEM imaging has its strength in large-area or -volume screening and correlation. The second method can be combined with immunocytochemical methods. Both methods, however, have the potential to extract statistically relevant data of structural details for systems biology.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.

Retinal light toxicity

The ability of light to enact damage on the neurosensory retina and underlying structures has been well understood for hundreds of years. While the eye has adapted several mechanisms to protect itself from such damage, certain exposures to light can still result in temporal or permanent damage. Both clinical observations and laboratory studies have enabled us to understand the various ways by which the eye can protect itself from such damage. Light or electromagnetic radiation can result in damage through photothermal, photomechanical, and photochemical mechanisms. The following review seeks to describe these various processes of injury and many of the variables, which can mitigate these modes of injury.

The shape of caveolae is omega-like after glutaraldehyde fixation and cup-like after cryofixation

Caveolae were defined as flask- or omega-shaped plasma membrane invaginations, abundant in adipocytes, fibroblasts, endothelial and smooth muscle cells. The major protein component of caveolar membranes is an integral membrane protein named caveolin. We compared the freeze-fracture behavior of caveolae in glutaraldehyde-fixed and cryofixed mouse fibroblast cells and found distinct differences. In glutaraldehyde-fixed cells almost all caveolae were cross-fractured through their pore and only very few caveolar membranes were membrane-fractured. We found the reverse situation in rapid frozen cells without any chemical fixation where most of the caveolae were membrane-fractured, showing different degrees of invagination from nearly flat to deeply invaginated. In ultrathin sections of glutaraldehyde-fixed heart endothelial cells, caveolae exhibit the well known omega-like shape. In high-pressure frozen, freeze-substituted and low temperature embedded heart endothelial cells, the caveolae frequently exhibit a cup-like shape without any constriction or pore. The cup-like caveolar shape could also be shown by tilt series analysis of freeze-fracture replicas obtained from cryofixed cells. Freeze-fracture immunolabeling of caveolin-1 revealed a lateral belt-like caveolin alignment. These findings point out that the constricted "neck" region of caveolae in most cases is an effect that is caused and intensified by the glutaraldehyde fixation. Our data indicate that caveolae in vivo show all degrees of invagination from nearly flat via cup-like depressed to in a few cases omega-like.

Electron microscopy of high pressure frozen samples: bridging the gap between cellular ultrastructure and atomic resolution

Transmission electron microscopy has provided most of what is known about the ultrastructural organization of tissues, cells, and organelles. Due to tremendous advances in crystallography and magnetic resonance imaging, almost any protein can now be modeled at atomic resolution. To fully understand the workings of biological "nanomachines" it is necessary to obtain images of intact macromolecular assemblies in situ. Although the resolution power of electron microscopes is on the atomic scale, in biological samples artifacts introduced by aldehyde fixation, dehydration and staining, but also section thickness reduces it to some nanometers. Cryofixation by high pressure freezing circumvents many of the artifacts since it allows vitrifying biological samples of about 200 mum in thickness and immobilizes complex macromolecular assemblies in their native state in situ. To exploit the perfect structural preservation of frozen hydrated sections, sophisticated instruments are needed, e.g., high voltage electron microscopes equipped with precise goniometers that work at low temperature and digital cameras of high sensitivity and pixel number. With them, it is possible to generate high resolution tomograms, i.e., 3D views of subcellular structures. This review describes theory and applications of the high pressure cryofixation methodology and compares its results with those of conventional procedures. Moreover, recent findings will be discussed showing that molecular models of proteins can be fitted into depicted organellar ultrastructure of images of frozen hydrated sections. High pressure freezing of tissue is the base which may lead to precise models of macromolecular assemblies in situ, and thus to a better understanding of the function of complex cellular structures.

Immunogold Labeling of Cryosections from High-Pressure Frozen Cells

Immunogold labeling of cryosections according to Tokuyasu (Tokuyasu KT. A technique for ultracyotomy of cell suspensions and tissues. J Cell Biol 1973;57:551-565), is an important and widely used method for immunoelectron microscopy. These sections are cut from material that is chemically fixed at room temperature (room temperature fixation, RTF). Lately in many morphological studies fast freezing followed by cryosubstitution fixation (CSF) is used instead of RTF. We have explored some new methods for applying immunogold labeling on cryosections from high-pressure frozen cells (HepG2 cells, primary chondrocytes) and tissues (cartilage and exocrine pancreas). As immunolabeling has to be carried out on thawed and stable sections, we explored two ways to achieve this: (1) The section fixation method, as briefly reported before (Liou W et al. Histochem Cell Biol 1996;106:41-58 and Möbius W et al. J Histochem Cytochem 2002;50:43-55.) in which cryosections from freshly frozen cells were stabilized in mixtures of sucrose and methyl cellulose and varying concentrations of glutaraldehyde, formaldehyde and uranyl acetate (UA). Only occasionally does this method reveal section areas with excellent cell preservation and negatively stained membranes like Tokuyasu sections of RTF material. (Liou et al.) (2) The rehydration method, a novel approach, in which CSF with glutaraldehyde and/or osmium tetroxide (OsO4) was followed by rehydration and cryosectioning as in the Tokuyasu method. Especially, the addition of UA and low concentrations of water to the CSF medium favored superb membrane contrast. Immunogold labeling was as efficient as with the Tokuyasu method.

Cytomegalovirus primary envelopment occurs at large infoldings of the inner nuclear membrane

We have investigated the morphogenesis of human and murine cytomegalovirus by transmission electron microscopy after high-pressure freezing, freeze substitution, and plastic embedding. We observed large tubular infoldings of the inner nuclear membrane that were free of lamina and active in primary envelopment and subsequent transport of capsids to the nuclear periphery. Semiquantitative determinations of the enlarged inner nuclear membrane area and the location of the primary envelopment of nucleocapsids demonstrated that this structure represents a virus-induced specialized membrane domain at which the particles are preferentially enveloped. This is a previously undescribed structural element relevant in cytomegalovirus morphogenesis.

New comprehension of the apicoplast of Sarcocystis by transmission electron tomography

BACKGROUND INFORMATION

Apicomplexan parasites (like Plasmodium, Toxoplasma, Eimeria and Sarcocystis) contain a distinctive organelle, the apicoplast, acquired by a secondary endosymbiotic process analogous to chloroplasts and mitochondria. The apicoplast is essential for long-term survival of the parasite. This prokaryotic origin implies that molecular and metabolic processes in the apicoplast differ from those of the eukaryotic host cells and therefore offer options for specific chemotherapeutic treatment. We studied the apicoplast in high-pressure frozen and freeze-substituted cysts of Sarcocystis sp. from roe deer (Capreolus capreolus) to get better insight in apicoplast morphology.

RESULTS AND CONCLUSIONS

We observed that the apicoplast contains four continuous membranes. The two inner membranes have a circular shape with a constant distance from each other and large-sized protein complexes are located between them. The two outer membranes have irregular shapes. The periplastid membrane also contains large-sized protein complexes, while the outer membrane displays protuberances into the parasite cytoplasm. In addition, it is closely associated with the endoplasmic reticulum by 'contact sites'.

Temporal sequence of changes in rat retina after UV-A and blue light exposure

Two spectral types of retinal light damage were induced in pigmented rats by irradiating small retinal patches at either 380 or 470 nm. The temporal sequence of changes in the retina was followed for up to 2 months by funduscopy and histology. For both damage types, fundus changes were best visible after 3 days. Histology showed that 380 nm specifically damaged photoreceptor cells, particularly the rods. All cell compartments of the rods, including the nucleus were affected already after 3 h. In the next days, damaged rods degenerated. At high doses (2.5 x the funduscopic threshold dose) all rods in the irradiated area were lost, resulting in a local photoreceptor lesion, which was still present at 2 months after the irradiation. At 470 nm, damage occurred both in the photoreceptor layer and in the pigment epithelium. Acute changes, at 1 h after irradiation, consisted mainly of damaged mitochondria in these layers. Next, the pigment epithelium showed swelling, an altered melanin distribution and, at high doses (2.5 x threshold), interruptions of the monolayer. Degeneration of photoreceptor cells was initially limited to a few scattered cells, but 3 days after high doses focal areas of massive degeneration were seen. At late stages, the cells of the pigment epithelium recovered and the photoreceptor layer showed a loss of cells. The results show that the spectral damage types are distinct in the early phases, indicating that different mechanisms are involved. Yet, the end effect of both damage types after exposure at doses up to 2.5 x the funduscopic threshold is remarkably similar and consists of local photoreceptor lesions.

Retinal damage by light in the golden hamster: an ultrastructural study in the retinal pigment epithelium and Bruch's membrane

The mechanism of the toxicity of light on the retina remains unclear despite a large number of investigations. The purpose of this study is to identify and localize the ultrastructural changes and the site of the earliest damage after intense light exposure. Nine adult Syrian golden hamsters (Mesocricetus auratus) have been maintained under constant illumination with a high-pressure mercury lamp (HQJ R 80 W Deluxe, Osram, Berlin, light intensity 1000 lx) for 12 h, followed by an additional 3 h in the dark. Light damage is assessed by light and electron microscopy. Morphological evaluation reveals focal damage to the retinal pigment epithelial (RPE) cells in close proximity to less-affected RPE cells and normal photoreceptors. Collagen fibers in Bruch's membrane lose their parallel orientation. Occasionally, fusion of cell membranes of neighboring rod outer segments (ROS) is also observed. Continuous, 12 h exposure of hamsters to intense light results in initial focal damage to some RPE cells, such that severely damaged RPE cells are found adjacent to intact RPE cells. Only slight damage to the photoreceptors is evident, suggesting that the sequence of the pathological changes resulting from light begins with damage to the RPE cells and associated Bruch's membrane.

Critical role of TrkB and brain-derived neurotrophic factor in the differentiation and survival of retinal pigment epithelium

In the vertebrate eye, the retinal pigment epithelium (RPE) and the neural retina arise from a single layer of neuroectoderm. Factors influencing the differentiation of retinal neurons have been identified; however, little is known about molecules directing the differentiation of the RPE. Here we have found that the neurotrophin brain-derived neurotrophic factor (BDNF) plays an autocrine role in the differentiation and survival of Xenopus laevis RPE. Fluorescent in situ hybridization studies showed a precise co-expression of BDNF and its receptor trkB in the retinal neuroepithelium and actively differentiating RPE; in vitro studies demonstrated survival- and differentiation-promoting effects in serum-free explants and dissociated cultures. When a dominant negative mutant of the trkB receptor was expressed in developing embryos, severe arrest of RPE differentiation was seen with persistence of nestin- and Notch-positive neuroblasts.

Double-layer coating for field-emission cryo-scanning electron microscopy--present state and applications

Imaging of fast-frozen samples is the most direct approach for electron microscopy of organic material. It prevents chemical fixation and drying artifacts. Frozen samples can be replicated and imaged in the transmission electron microscope (TEM), or they can be directly visualized in the cryo-scanning electron microscope (cryo-SEM). Double-layer coating combines these two techniques and many of their advantages. With this method, the frozen bulk sample is coated similar to the TEM-replica technique with, for example, a shadow of platinum (at an angle of 45 degrees) and an additional layer of carbon. Then, the sample is cryo-transferred to an SEM equipped with a cold stage and imaged with the material-dependent backscattered electron signal that shows the platinum distribution. With this method, charging artifacts and the effects of beam damage are significantly reduced. Although currently the resolution of the replica technique cannot be surpassed, the method greatly facilitates the processing of brittle, rapidly frozen samples because no replica cleaning is necessary. This makes the method especially suitable for high-pressure frozen samples.