Imaging cell biology in live animals: ready for prime time

Intravital Microscopy With an Airy Beam Light Sheet Microscope Improves Temporal Resolution and Reduces Surgical Trauma

Intravital microscopy has emerged as a powerful imaging tool, which allows the visualization and precise understanding of rapid physiological processes at sites of inflammation in vivo, such as vascular permeability and leukocyte migration. Leukocyte interactions with the vascular endothelium can be characterized in the living organism in the murine cremaster muscle. Here, we present a microscopy technique using an Airy Beam Light Sheet microscope that has significant advantages over our previously used confocal microscopy systems. In comparison, the light sheet microscope offers near isotropic optical resolution and faster acquisition speed, while imaging a larger field of view. With less invasive surgery we can significantly reduce side effects such as bleeding, muscle twitching, and surgical inflammation. However, the increased acquisition speed requires exceptional tissue stability to avoid imaging artefacts. Since respiratory motion is transmitted to the tissue under investigation, we have developed a relocation algorithm that removes motion artefacts from our intravital microscopy images. Using these techniques, we are now able to obtain more detailed 3D time-lapse images of the cremaster vascular microcirculation, which allow us to observe the process of leukocyte emigration into the surrounding tissue with increased temporal resolution in comparison to our previous confocal approach.

Coordination of force-generating actin-based modules stabilizes and remodels membranes in vivo

Membrane remodeling drives a broad spectrum of cellular functions, and it is regulated through mechanical forces exerted on the membrane by cytoplasmic complexes. Here, we investigate how actin filaments dynamically tune their structure to control the active transfer of membranes between cellular compartments with distinct compositions and biophysical properties. Using intravital subcellular microscopy in live rodents we show that a lattice composed of linear filaments stabilizes the granule membrane after fusion with the plasma membrane and a network of branched filaments linked to the membranes by Ezrin, a regulator of membrane tension, initiates and drives to completion the integration step. Our results highlight how the actin cytoskeleton tunes its structure to adapt to dynamic changes in the biophysical properties of membranes.

The extracellular matrix in tissue morphogenesis: No longer a backseat driver

The forces driving tissue morphogenesis are thought to originate from cellular activities. While it is appreciated that extracellular matrix (ECM) may also be involved, ECM function is assumed to be simply instructive in modulating the cellular behaviors that drive changes to tissue shape. However, there is increasing evidence that the ECM may not be the passive player portrayed in developmental biology textbooks. In this review we highlight examples of embryonic ECM dynamics that suggest cell-independent activity, along with developmental processes during which localized ECM alterations and ECM-autonomous forces are directing changes to tissue shape. Additionally, we discuss experimental approaches to unveil active ECM roles during tissue morphogenesis. We propose that it may be time to rethink our general definition of morphogenesis as a cellular-driven phenomenon and incorporate an underappreciated, and surprisingly dynamic ECM.

A spatial map of hepatic mitochondria uncovers functional heterogeneity shaped by nutrient-sensing signaling

In the liver, mitochondria are exposed to different concentrations of nutrients due to their spatial positioning across the periportal and pericentral axis. How the mitochondria sense and integrate these signals to respond and maintain homeostasis is not known. Here, we combine intravital microscopy, spatial proteomics, and functional assessment to investigate mitochondrial heterogeneity in the context of liver zonation. We find that periportal and pericentral mitochondria are morphologically and functionally distinct; beta-oxidation is elevated in periportal regions, while lipid synthesis is predominant in the pericentral mitochondria. In addition, comparative phosphoproteomics reveals spatially distinct patterns of mitochondrial composition and potential regulation via phosphorylation. Acute pharmacological modulation of nutrient sensing through AMPK and mTOR shifts mitochondrial phenotypes in the periportal and pericentral regions, linking nutrient gradients across the lobule and mitochondrial heterogeneity. This study highlights the role of protein phosphorylation in mitochondrial structure, function, and overall homeostasis in hepatic metabolic zonation. These findings have important implications for liver physiology and disease.

Advanced Imaging Techniques for the Characterization of Subcellular Organelle Structure in Pancreatic Islet β Cells

Type 2 diabetes (T2D) affects more than 32.3 million individuals in the United States, creating an economic burden of nearly $966 billion in 2021. T2D results from a combination of insulin resistance and inadequate insulin secretion from the pancreatic β cell. However, genetic and physiologic data indicate that defects in β cell function are the chief determinant of whether an individual with insulin resistance will progress to a diagnosis of T2D. The subcellular organelles of the insulin secretory pathway, including the endoplasmic reticulum, Golgi apparatus, and secretory granules, play a critical role in maintaining the heavy biosynthetic burden of insulin production, processing, and secretion. In addition, the mitochondria enable the process of insulin release by integrating the metabolism of nutrients into energy output. Advanced imaging techniques are needed to determine how changes in the structure and composition of these organelles contribute to the loss of insulin secretory capacity in the β cell during T2D. Several microscopy techniques, including electron microscopy, fluorescence microscopy, and soft X-ray tomography, have been utilized to investigate the structure-function relationship within the β cell. In this overview article, we will detail the methodology, strengths, and weaknesses of each approach. © 2024 American Physiological Society. Compr Physiol 14:5243-5267, 2024.

Spatial and Temporal Coordination of Force-generating Actin-based Modules Drives Membrane Remodeling In Vivo

Membrane remodeling drives a broad spectrum of cellular functions, and it is regulated through mechanical forces exerted on the membrane by cytoplasmic complexes. Here, we investigate how actin filaments dynamically tune their structure to control the active transfer of membranes between cellular compartments with distinct compositions and biophysical properties. Using intravital subcellular microscopy in live rodents we show that: a lattice composed of linear filaments stabilizes the granule membrane after fusion with the plasma membrane; and a network of branched filaments linked to the membranes by Ezrin, a regulator of membrane tension, initiates and drives to completion the integration step. Our results highlight how the actin cytoskeleton tunes its structure to adapt to dynamic changes in the biophysical properties of membranes.

Perfusion Window Chambers Enable Interventional Analyses of Tumor Microenvironments

Intravital microscopy (IVM) allows spatial and temporal imaging of different cell types in intact live tissue microenvironments. IVM has played a critical role in understanding cancer biology, invasion, metastases, and drug development. One considerable impediment to the field is the inability to interrogate the tumor microenvironment and its communication cascades during disease progression and therapeutic interventions. Here, a new implantable perfusion window chamber (PWC) is described that allows high-fidelity in vivo microscopy, local administration of stains and drugs, and longitudinal sampling of tumor interstitial fluid. This study shows that the new PWC design allows cyclic multiplexed imaging in vivo, imaging of drug action, and sampling of tumor-shed materials. The PWC will be broadly useful as a novel perturbable in vivo system for deciphering biology in complex microenvironments.

A spatial map of hepatic mitochondria uncovers functional heterogeneity shaped by nutrient-sensing signaling

In the liver, mitochondria are exposed to different concentrations of nutrients due to their spatial positioning across the periportal (PP) and pericentral (PC) axis. How these mitochondria sense and integrate these signals to respond and maintain homeostasis is not known. Here, we combined intravital microscopy, spatial proteomics, and functional assessment to investigate mitochondrial heterogeneity in the context of liver zonation. We found that PP and PC mitochondria are morphologically and functionally distinct; beta-oxidation was elevated in PP regions, while lipid synthesis was predominant in the PC mitochondria. In addition, comparative phosphoproteomics revealed spatially distinct patterns of mitochondrial composition and potential regulation via phosphorylation. Acute pharmacological modulation of nutrient sensing through AMPK and mTOR shifted mitochondrial phenotypes in the PP and PC regions, linking nutrient gradients across the lobule and mitochondrial heterogeneity. This study highlights the role of protein phosphorylation in mitochondrial structure, function, and overall homeostasis in hepatic metabolic zonation. These findings have important implications for liver physiology and disease.

Visualizing vasculature and its response to therapy in the tumor microenvironment

Tumor vasculature plays a critical role in the progression and metastasis of tumors, antitumor immunity, drug delivery, and resistance to therapies. The morphological and functional changes of tumor vasculature in response to therapy take place in a spatiotemporal-dependent manner, which can be predictive of treatment outcomes. Dynamic monitoring of intratumor vasculature contributes to an improved understanding of the mechanisms of action of specific therapies or reasons for treatment failure, leading to therapy optimization. There is a rich history of methods used to image the vasculature. This review describes recent advances in imaging technologies to visualize the tumor vasculature, with a focus on enhanced intravital imaging techniques and tumor window models. We summarize new insights on spatial-temporal vascular responses to various therapies, including changes in vascular perfusion and permeability and immune-vascular crosstalk, obtained from intravital imaging. Finally, we briefly discuss the clinical applications of intravital imaging techniques.

Bidirectional crosstalk between the peripheral nervous system and lymphoid tissues/organs

The central nervous system (CNS) influences the immune system generally by regulating the systemic concentration of humoral substances (e.g., cortisol and epinephrine), whereas the peripheral nervous system (PNS) communicates specifically with the immune system according to local interactions/connections. An imbalance between the components of the PNS might contribute to pathogenesis and the further development of certain diseases. In this review, we have explored the "thread" (hardwiring) of the connections between the immune system (e.g., primary/secondary/tertiary lymphoid tissues/organs) and PNS (e.g., sensory, sympathetic, parasympathetic, and enteric nervous systems (ENS)) in health and disease in vitro and in vivo. Neuroimmune cell units provide an anatomical and physiological basis for bidirectional crosstalk between the PNS and the immune system in peripheral tissues, including lymphoid tissues and organs. These neuroimmune interactions/modulation studies might greatly contribute to a better understanding of the mechanisms through which the PNS possibly affects cellular and humoral-mediated immune responses or vice versa in health and diseases. Physical, chemical, pharmacological, and other manipulations of these neuroimmune interactions should bring about the development of practical therapeutic applications for certain neurological, neuroimmunological, infectious, inflammatory, and immunological disorders/diseases.

High dynamic range 3D motion tracking using circular scans with optical coherence tomography

Motion artifacts, from such sources as heartbeats, respiration, or peristalsis, often degrade microscopic images or videos of live subjects. We have developed a method using circular optical coherence tomography (OCT) scans to track the transverse and axial motion of biological samples at speeds ranging from several micrometers per second to several centimeters per second. We achieve fast and high-precision measurements of the magnitude and direction of the sample's motion by adaptively controlling the circular scan pattern settings and applying interframe and intraframe analyses. These measurements are the basis of active motion compensation via feedback control for future in vivo microscopic and macroscopic imaging applications.

Improvement of imaging and image correction methods for the soft X-ray projection microscopy

BACKGROUND

The soft X-ray projection microscope has been developed for high resolution imaging of hydrated bio-specimens. Image blurring due to X-ray diffraction can be corrected by an iteration procedure. The correction is not efficient enough for all images, especially for low contrast chromosome images.

OBJECTIVE

The purpose of this study is to improve X-ray imaging techniques using a finer pinhole and reducing capture time, as well as to improve image correction methods. A method of specimen staining prior to the imaging was tested in order to capture images with high contrasts. The efficiency of the iteration procedure and its combined version with an image enhancement method was also assessed.

METHODS

In image correction, we used the iteration procedure and its combined version with an image enhancement technique. To capture higher contrast images, we stained chromosome specimens with the Platinum blue (Pt-blue) prior to the imaging.

RESULTS

The iteration procedure combined with image enhancement corrected the chromosome images with 329 or lower magnification effectively. Using the Pt-blue staining for the chromosome, images with high contrast have been captured and successfully corrected.

CONCLUSIONS

The image enhancement technique combining contrast enhancement and noise removal together was effective to obtain higher contrast images. As a result, the chromosome images with 329 or lower times magnification were corrected effectively. With Pt-blue staining, chromosome images with contrasts of 2.5 times higher than unstained case could be captured and corrected by the iteration procedure.

Intravital imaging to study cancer progression and metastasis

Navigation through the bulk tumour, entry into the blood vasculature, survival in the circulation, exit at distant sites and resumption of proliferation are all steps necessary for tumour cells to successfully metastasize. The ability of tumour cells to complete these steps is highly dependent on the timing and sequence of the interactions that these cells have with the tumour microenvironment (TME), including stromal cells, the extracellular matrix and soluble factors. The TME thus plays a major role in determining the overall metastatic phenotype of tumours. The complexity and cause-and-effect dynamics of the TME cannot currently be recapitulated in vitro or inferred from studies of fixed tissue, and are best studied in vivo, in real time and at single-cell resolution. Intravital imaging (IVI) offers these capabilities, and recent years have been a time of immense growth and innovation in the field. Here we review some of the recent advances in IVI of mammalian models of cancer and describe how IVI is being used to understand cancer progression and metastasis, and to develop novel treatments and therapies. We describe new techniques that allow access to a range of tissue and cancer types, novel fluorescent reporters and biosensors that allow fate mapping and the probing of functional and phenotypic states, and the clinical applications that have arisen from applying these techniques, reporters and biosensors to study cancer. We finish by presenting some of the challenges that remain in the field, how to address them and future perspectives.

Methodologies and Emerging Technologies for the Evaluation of the Hematopoietic System

Hematology and bone marrow analysis is central to our understanding of the hematopoietic system and how it responds to insults, and this session presented during the 2022 STP symposium provided a review of current and novel approaches for the evaluation of the hematopoietic system in the context of nonclinical investigations. This publication summarizes the information presented on novel approaches for evaluation of the hematopoietic system using automated hematology analyzers, including details around the quantitative assessment of bone marrow cell suspensions as well as introducing several newly available hematology parameters. It was followed by a discussion on intravital microscopy and live cell imaging and how these methods can assist with de-risking hematopoiesis-associated safety concerns, and a review of recent assays using artificial intelligence for the evaluation of bone marrow.

Intravital Imaging of Hepatic Blood Biliary Barrier in Live Mice

Understanding the kinetics and spatiotemporal interactions of living cells within the tissue environment requires real-time imaging. The introduction of two-photon microscopy has substantially boosted the power of live intravital imaging, making it possible to obtain information of individual cells in near-physiologic conditions within intact tissues nondestructively. Intravital imaging of the liver has proved useful in understanding its 3D structure, function, and dynamic cellular interactions. Recently we have shown that integrity of the blood-bile barrier in different physiologic and pathophysiologic conditions can be imaged in real time using intravital microscopy. Here we discuss the real-time intravital imaging method to visualize blood-bile barrier integrity in the murine liver. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Live imaging in the mouse liver Support Protocol: Monitoring vital signs of the mouse during live liver imaging Basic Protocol 2: Visualizing blood and bile transport using intravital microscopy.

Liver Zonation - Revisiting Old Questions With New Technologies

Despite the ever-increasing prevalence of non-alcoholic fatty liver disease (NAFLD), the etiology and pathogenesis remain poorly understood. This is due, in part, to the liver's complex physiology and architecture. The liver maintains glucose and lipid homeostasis by coordinating numerous metabolic processes with great efficiency. This is made possible by the spatial compartmentalization of metabolic pathways a phenomenon known as liver zonation. Despite the importance of zonation to normal liver function, it is unresolved if and how perturbations to liver zonation can drive hepatic pathophysiology and NAFLD development. While hepatocyte heterogeneity has been identified over a century ago, its examination had been severely hindered due to technological limitations. Recent advances in single cell analysis and imaging technologies now permit further characterization of cells across the liver lobule. This review summarizes the advances in examining liver zonation and elucidating its regulatory role in liver physiology and pathology. Understanding the spatial organization of metabolism is vital to further our knowledge of liver disease and to provide targeted therapeutic avenues.

Iterative tomography with digital adaptive optics permits hour-long intravital observation of 3D subcellular dynamics at millisecond scale

Long-term subcellular intravital imaging in mammals is vital to study diverse intercellular behaviors and organelle functions during native physiological processes. However, optical heterogeneity, tissue opacity, and phototoxicity pose great challenges. Here, we propose a computational imaging framework, termed digital adaptive optics scanning light-field mutual iterative tomography (DAOSLIMIT), featuring high-speed, high-resolution 3D imaging, tiled wavefront correction, and low phototoxicity with a compact system. By tomographic imaging of the entire volume simultaneously, we obtained volumetric imaging across 225 × 225 × 16 μm3, with a resolution of up to 220 nm laterally and 400 nm axially, at the millisecond scale, over hundreds of thousands of time points. To establish the capabilities, we investigated large-scale cell migration and neural activities in different species and observed various subcellular dynamics in mammals during neutrophil migration and tumor cell circulation.

Intravital Microscopy for the Study of Hepatic Glucose Uptake

The liver is central in maintaining glucose homeostasis. Indeed, impaired hepatic glucose uptake has been implicated in the development of hyperglycemia in type II diabetes (T2D) and non-alcoholic fatty liver disease (NAFLD). However, current approaches to evaluate glucose mobilization rely on indirect measurements that do not provide spatial and temporal information. Here, we describe confocal-based intravital microscopy (IVM) of the liver that allows the identification of hepatocyte spatial organization and glucose transport. Specifically, we describe a method to fluorescently label hepatic landmarks to identify different compartments within the liver. In addition, we outline an in vivo fluorescent glucose uptake assay to quantitatively measure glucose mobilization in space and time. These protocols allow direct investigation of hepatic glycemic control and can be further applied to murine models of liver disease. © Published 2021. This article is a U.S. Government work and is in the public domain in the USA. Basic Protocol 1: Mouse surgical procedure and positioning for liver intravital imaging Basic Protocol 2: Fluorescent labeling and intravital imaging of mouse hepatic compartments Basic Protocol 3: Mouse hepatic glucose uptake assay and intravital imaging analysis.

Actually Seeing What Is Going on - Intravital Microscopy in Tissue Engineering

Intravital microscopy (IVM) study approach offers several advantages over in vitro, ex vivo, and 3D models. IVM provides real-time imaging of cellular events, which provides us a comprehensive picture of dynamic processes. Rapid improvement in microscopy techniques has permitted deep tissue imaging at a higher resolution. Advances in fluorescence tagging methods enable tracking of specific cell types. Moreover, IVM can serve as an important tool to study different stages of tissue regeneration processes. Furthermore, the compatibility of different tissue engineered constructs can be analyzed. IVM is also a promising approach to investigate host reactions on implanted biomaterials. IVM can provide instant feedback for improvising tissue engineering strategies. In this review, we aim to provide an overview of the requirements and applications of different IVM approaches. First, we will discuss the history of IVM development, and then we will provide an overview of available optical modalities including the pros and cons. Later, we will summarize different fluorescence labeling methods. In the final section, we will discuss well-established chronic and acute IVM models for different organs.

Method for Acute Intravital Imaging of the Large Intestine in Live Mice

Intravital microscopy is an imaging technique aimed at the visualization of the dynamics of biological processes in live animals. In the last decade, the development of nonlinear optical microscopy has enormously increased the use of this technique, thus addressing key biological questions in different fields such as immunology, neurobiology and tumor biology. In addition, new upcoming strategies to minimize motion artifacts due to animal respiration and heartbeat have enabled the visualization in real time of biological processes at cellular and subcellular resolution. Recently, intravital microscopy has been applied to analyze different aspect of mucosal immunity in the gut. However, the majority of these studies have been performed on the small intestine. Although crucial aspects of the biology of this organ have been unveiled, the majority of intestinal pathologies in humans occur in the large intestine.Here, we describe a method to surgically expose and stabilize the large intestine in live mice and to perform short-term (up to 2 h) intravital microscopy.

The LTB4-BLT1 axis regulates actomyosin and β2-integrin dynamics during neutrophil extravasation

The eicosanoid leukotriene B4 (LTB4) relays chemotactic signals to direct neutrophil migration to inflamed sites through its receptor BLT1. However, the mechanisms by which the LTB4-BLT1 axis relays chemotactic signals during intravascular neutrophil response to inflammation remain unclear. Here, we report that LTB4 produced by neutrophils acts as an autocrine/paracrine signal to direct the vascular recruitment, arrest, and extravasation of neutrophils in a sterile inflammation model in the mouse footpad. Using intravital subcellular microscopy, we reveal that LTB4 elicits sustained cell polarization and adhesion responses during neutrophil arrest in vivo. Specifically, LTB4 signaling coordinates the dynamic redistribution of non-muscle myosin IIA and β2-integrin, which facilitate neutrophil arrest and extravasation. Notably, we also found that neutrophils shed extracellular vesicles in the vascular lumen and that inhibition of extracellular vesicle release blocks LTB4-mediated autocrine/paracrine signaling required for neutrophil arrest and extravasation. Overall, we uncover a novel complementary mechanism by which LTB4 relays extravasation signals in neutrophils during early inflammation response.

Next-generation fluorescent nucleic acids probes for microscopic analysis of intracellular nucleic acids

Fluorescence imaging of nucleic acids is a very important technique necessary to understand gene expression and the resulting changes in cell function. This mini-review focuses on sequence-specific fluorescence imaging of intracellular RNA and methylated DNA using fluorescent nucleic acid probes. A couple of functional fluorescent nucleic acid probes developed by our laboratory are introduced and the examples of their application to fluorescence imaging of intracellular nucleic acids are described.

Functional Fluorescence Microscopy Imaging: Quantitative Scanning-Free Confocal Fluorescence Microscopy for the Characterization of Fast Dynamic Processes in Live Cells

Functional fluorescence microscopy imaging (fFMI), a time-resolved (21 μs/frame) confocal fluorescence microscopy imaging technique without scanning, is developed for quantitative characterization of fast reaction-transport processes in solution and in live cells. The method is based on massively parallel fluorescence correlation spectroscopy (FCS). Simultaneous excitation of fluorescent molecules in multiple spots in the focal plane is achieved using a diffractive optical element (DOE). Fluorescence from the DOE-generated 1024 illuminated spots is detected in a confocal arrangement by a matching matrix detector comprising 32 × 32 single-photon avalanche photodiodes (SPADs). Software for data acquisition and fast auto- and cross-correlation analysis by parallel signal processing using a graphic processing unit (GPU) allows temporal autocorrelation across all pixels in the image frame in 4 s and cross-correlation between first- and second-order neighbor pixels in 45 s. We present here this quantitative, time-resolved imaging method with single-molecule sensitivity and demonstrate its usefulness for mapping in live cell location-specific differences in the concentration and translational diffusion of molecules in different subcellular compartments. In particular, we show that molecules without a specific biological function, e.g., the enhanced green fluorescent protein (eGFP), exhibit uniform diffusion. In contrast, molecules that perform specialized biological functions and bind specifically to their molecular targets show location-specific differences in their concentration and diffusion, exemplified here for two transcription factor molecules, the glucocorticoid receptor (GR) before and after nuclear translocation and the Sex combs reduced (Scr) transcription factor in the salivary gland of Drosophila ex vivo.

Intravital microscopy in mammalian multicellular organisms

Imaging subcellular processes in live animals is no longer a dream. The development of Intravital Subcellular Microscopy (ISMic) combined with the astounding repertoire of available mouse models makes it possible to investigate processes such as membrane trafficking in mammalian living tissues under native conditions. This has provided the unique opportunity to answer questions that cannot be otherwise addressed in reductionist model systems and to link cell biology to tissue pathophysiology.

Revealing the morphology and function of the cochlea and middle ear with optical coherence tomography

Optical coherence tomography (OCT) has revolutionized physiological studies of the hearing organ, the vibration and morphology of which can now be measured without opening the surrounding bone. In this review, we provide an overview of OCT as used in the otological research, describing advances and different techniques in vibrometry, angiography, and structural imaging.

Synthesis of Dye-Modified Oligonucleotides via Copper(I)-Catalyzed Alkyne Azide Cycloaddition Using On- and Off-Bead Approaches

Fluorescence molecular imaging is widely used to visualize and observe different biomolecules, in particular DNA and RNA, in vivo and in real time. Typically, DNA strands are tagged with only one fluorophore, and, in the case of molecular beacons, an additional quencher is conjugated, which bears the risk of false-positive or false-negative results because only fluorescence intensities at one fluorescence wavelength (color) are compared. To address this drawback, the concept of "DNA/RNA traffic lights," which is characterized by a fluorescence color change due to energy transfer between two dyes, was developed by our working group. For these DNA and RNA systems, the oligonucleotides are post-synthetically labeled, specifically after solid-phase synthesis by chemical means, with a fluorescent dye using copper(I)-catalyzed cycloaddition at the 2' position of single uridines. In order to functionalize oligonucleotides with several different labels, an on-resin method is required to ensure the necessary selectivity. This unit describes two different CuAAC ("click") approaches-in solution (post-synthetic) and on solid phase (during synthesis)-for the attachment of fluorophores to the 2' position of DNA. © 2018 by John Wiley & Sons, Inc.

Unraveling the host's immune response to infection: Seeing is believing

It has long been appreciated that understanding the interactions between the host and the pathogens that make us sick is critical for the prevention and treatment of disease. As antibiotics become increasingly ineffective, targeting the host and specific bacterial evasion mechanisms are becoming novel therapeutic approaches. The technology used to understand host‐pathogen interactions has dramatically advanced over the last century. We have moved away from using simple in vitro assays focused on single‐cell events to technologies that allow us to observe complex multicellular interactions in real time in live animals. Specifically, intravital microscopy (IVM) has improved our understanding of infection, from viral to bacterial to parasitic, and how the host immune system responds to these infections. Yet, at the same time it has allowed us to appreciate just how complex these interactions are and that current experimental models still have a number of limitations. In this review, we will discuss the advances in vivo IVM has brought to the study of host‐pathogen interactions, focusing primarily on bacterial infections and innate immunity.

Long-Term Cell Fate Tracking of Individual Renal Cells Using Serial Intravital Microscopy

Intravital multiphoton microscopy of the kidney is a powerful technique to study alterations in tissue morphology and function simultaneously in the living animal and represents a dynamic and developing research tool in the field. Recent technological advances include serial intravital multiphoton microscopy of the same kidney regions over several weeks and combined with ex vivo histology for cellular biomarker expression of the same cells, which had been subject to serial imaging before. Thus, serial intravital multiphoton microscopy followed by ex vivo histology provides unique tools to perform long-term cell fate tracing of the same renal cells during physiological and pathophysiological conditions, thereby allowing the detection of structural changes of the same renal cells over time. Examples include renal cell migration and proliferation while linking these events to local functional alterations and eventually to the expression of distinct cellular biomarkers. Here, we provide a detailed step-by-step protocol to facilitate serial intravital multiphoton microscopy for long-term in vivo tracking of renal cells and subsequent ex vivo histology for immunohistological staining of the same cells in the fixed tissue.

Cdc42 negatively regulates endocytosis during apical membrane maintenance in live animals

Lumen establishment and maintenance are fundamental for tubular organs physiological functions. Most of the studies investigating the mechanisms regulating this process have been carried out in cell cultures or in smaller organisms, whereas little has been done in mammalian model systems in vivo. Here we used the salivary glands of live mice to examine the role of the small GTPase Cdc42 in the regulation of the homeostasis of the intercellular canaliculi, a specialized apical domain of the acinar cells, where protein and fluid secretion occur. Depletion of Cdc42 in adult mice induced a significant expansion of the apical canaliculi, whereas depletion at late embryonic stages resulted in a complete inhibition of their postnatal formation. In addition, intravital subcellular microscopy revealed that reduced levels of Cdc42 affected membrane trafficking from and toward the plasma membrane, highlighting a novel role for Cdc42 in membrane remodeling through the negative regulation of selected endocytic pathways.

Performance of convolutional neural networks for identification of bacteria in 3D microscopy datasets

Three-dimensional microscopy is increasingly prevalent in biology due to the development of techniques such as multiphoton, spinning disk confocal, and light sheet fluorescence microscopies. These methods enable unprecedented studies of life at the microscale, but bring with them larger and more complex datasets. New image processing techniques are therefore called for to analyze the resulting images in an accurate and efficient manner. Convolutional neural networks are becoming the standard for classification of objects within images due to their accuracy and generalizability compared to traditional techniques. Their application to data derived from 3D imaging, however, is relatively new and has mostly been in areas of magnetic resonance imaging and computer tomography. It remains unclear, for images of discrete cells in variable backgrounds as are commonly encountered in fluorescence microscopy, whether convolutional neural networks provide sufficient performance to warrant their adoption, especially given the challenges of human comprehension of their classification criteria and their requirements of large training datasets. We therefore applied a 3D convolutional neural network to distinguish bacteria and non-bacterial objects in 3D light sheet fluorescence microscopy images of larval zebrafish intestines. We find that the neural network is as accurate as human experts, outperforms random forest and support vector machine classifiers, and generalizes well to a different bacterial species through the use of transfer learning. We also discuss network design considerations, and describe the dependence of accuracy on dataset size and data augmentation. We provide source code, labeled data, and descriptions of our analysis pipeline to facilitate adoption of convolutional neural network analysis for three-dimensional microscopy data.

Microcirculation research in community hospitals - challenges and chances

Community hospitals provide ideal conditions for large clinical studies because of the high volume of unselected patients admitted every year. With regard to microcirculatory studies, there are still some feasibility problems which are not solved yet. First of all, the lack of reliable automated software to analyze microcirculatory images represents the most important issue. Secondly, hardware aspects still need improvements regarding portability and miniaturization. Finally, to conduct studies of the microcirculation in a community hospital is also always a funding issue. The cost of the measurement device is hereby only one factor. Main cost factor is the personnel.

Discovering Macrophage Functions Using In Vivo Optical Imaging Techniques

Macrophages are an important component of host defense and inflammation and play a pivotal role in immune regulation, tissue remodeling, and metabolic regulation. Since macrophages are ubiquitous in human bodies and have versatile physiological functions, they are involved in virtually every disease, including cancer, diabetes, multiple sclerosis, and atherosclerosis. Molecular biological and histological methods have provided critical information on macrophage biology. However, many in vivo dynamic behaviors of macrophages are poorly understood and yet to be discovered. A better understanding of macrophage functions and dynamics in pathogenesis will open new opportunities for better diagnosis, prognostic assessment, and therapeutic intervention. In this article, we will review the advances in macrophage tracking and analysis with in vivo optical imaging in the context of different diseases. Moreover, this review will cover the challenges and solutions for optical imaging techniques during macrophage intravital imaging.

Site Specific Modification of Adeno-Associated Virus Enables Both Fluorescent Imaging of Viral Particles and Characterization of the Capsid Interactome

Adeno-associated viruses (AAVs) are attractive gene therapy vectors due to their low toxicity, high stability, and rare integration into the host genome. Expressing ligands on the viral capsid can re-target AAVs to new cell types, but limited sites have been identified on the capsid that tolerate a peptide insertion. Here, we incorporated a site-specific tetracysteine sequence into the AAV serotype 9 (AAV9) capsid, to permit labelling of viral particles with either a fluorescent dye or biotin. We demonstrate that fluorescently labelled particles are detectable in vitro, and explore the utility of the method in vivo in mice with time-lapse imaging. We exploit the biotinylated viral particles to generate two distinct AAV interactomes, and identify several functional classes of proteins that are highly represented: actin/cytoskeletal protein binding, RNA binding, RNA splicing/processing, chromatin modifying, intracellular trafficking and RNA transport proteins. To examine the biological relevance of the capsid interactome, we modulated the expression of two proteins from the interactomes prior to AAV transduction. Blocking integrin αVβ6 receptor function reduced AAV9 transduction, while reducing histone deacetylase 4 (HDAC4) expression enhanced AAV transduction. Our method demonstrates a strategy for inserting motifs into the AAV capsid without compromising viral titer or infectivity.

A cerebellar window for intravital imaging of normal and disease states in mice

The cerebellum is a prominent part of the vertebrate hindbrain that is critically involved in the regulation of important body functions such as movement coordination, maintenance of balance and posture, and motor control. Here, we describe a cerebellar window that provides access to the mouse cerebellum for intravital imaging, thereby allowing for a detailed characterization of the dynamic processes in this region of the brain. First, the skull overlying the cerebellum is removed, and then the window is applied to the region of interest. Windows may be exchanged depending on the desired imaging modality. This technique has a variety of applications. In the setting of medulloblastoma, spontaneous or orthotopically implanted lesions can be imaged, and tumor morphology and size can be monitored using ultrasonography. Multiphoton laser-scanning microscopy (MPLSM) or optical-frequency-domain imaging (OFDI) can be applied for in vivo visualization and analysis of cellular and vascular structures in a variety of disease states, including malignancies and ataxia telangiectasia. This protocol describes a novel and rapid method for cerebellar window construction that can be set up in under an hour.

Quantitative, Label-Free Evaluation of Tissue-Engineered Skeletal Muscle Through Multiphoton Microscopy

The lack of tools for assessing engineered tissue viability and function in a noninvasive manner is a major regulatory and translational challenge facing tissue engineers. Label-free, nonlinear optical molecular imaging (OMI) has utilized endogenous nicotinamide adenine dinucleotide and flavin adenine dinucleotide fluorescence to indicate metabolic activity. Similarly, second harmonic generation (SHG) signals from myosin and collagen can measure overall muscle structural integrity and function. The purpose of this study was to demonstrate these OMI techniques for the first time in engineered skeletal muscle and to develop a novel method for evaluating our engineered skeletal muscle units (SMUs) before implantation. Three experimental groups were studied: Control, Steroid Supplemented, and Metabolically Stressed SMUs. After imaging and analysis in ImageJ, a redox ratio (RR) metric was calculated to indicate metabolic activity, and a structure ratio metric was calculated to reflect structural composition. In addition, function was evaluated as tetanic force production in response to electrical stimulation. In living tissues, the RRs successfully distinguished control and metabolically stressed SMUs in both monolayer and 3D form. OMI of myosin and collagen SHG similarly differentiated control SMUs from the steroid supplemented group. With respect to function, steroid supplementation significantly increased active force generation. When comparing functional and OMI measures, a significant correlation was present between overall myosin density and active force generation. This work demonstrates the potential for using label-free OMI to evaluate tissue-engineered skeletal muscle constructs. The positive correlation between structural OMI measures and force production suggests that OMI could potentially serve as an accurate predictor of functional behaviors, such as integration and tissue regeneration, after implantation. This noninvasive OMI methodology, demonstrated for the first time in engineered skeletal muscle, could prove invaluable for assessing our tissue engineering technology and confirming release criteria for validation.

Time-lapsed, large-volume, high-resolution intravital imaging for tissue-wide analysis of single cell dynamics

Pathologists rely on microscopy to diagnose disease states in tissues and organs. They utilize both high-resolution, high-magnification images to interpret the staining and morphology of individual cells, as well as low-magnification overviews to give context and location to these cells. Intravital imaging is a powerful technique for studying cells and tissues in their native, live environment and can yield sub-cellular resolution images similar to those used by pathologists. However, technical limitations prevent the straightforward acquisition of low-magnification images during intravital imaging, and they are hence not typically captured. The serial acquisition, mosaicking, and stitching together of many high-resolution, high-magnification fields of view is a technique that overcomes these limitations in fixed and ex vivo tissues. The technique however, has not to date been widely applied to intravital imaging as movements caused by the living animal induce image distortions that are difficult to compensate for computationally. To address this, we have developed techniques for the stabilization of numerous tissues, including extremely compliant tissues, that have traditionally been extremely difficult to image. We present a novel combination of these stabilization techniques with mosaicked and stitched intravital imaging, resulting in a process we call Large-Volume High-Resolution Intravital Imaging (LVHR-IVI). The techniques we present are validated and make large volume intravital imaging accessible to any lab with a multiphoton microscope.

In vivo functional and morphological characterization of bone and striated muscle microcirculation in NSG mice

Organ-specific microcirculation plays a central role in tumor growth, tumor cell homing, tissue engineering, and wound healing. Mouse models are widely used to study these processes; however, these mouse strains often possess unique microhemodynamic parameters, making it difficult to directly compare experiments. The full functional characterization of bone and striated muscle microcirculatory parameters in non-obese diabetic-severe combined immunodeficiency/y-chain; NOD-Prkds IL2rg (NSG) mice has not yet been reported. Here, we established either a dorsal skinfold chamber or femur window in NSG mice (n = 23), allowing direct analysis of microcirculatory parameters in vivo by intravital fluorescence microscopy at 7, 14, 21, and 28 days after chamber preparation. Organ-specific differences were observed. Bone had a significantly lower vessel density but a higher vessel diameter than striated muscle. Bone also showed higher effective vascular permeability than striated muscle. The centerline velocity values were similar in the femur window and dorsal skinfold chamber, with a higher volumetric blood flow in bone. Interestingly, bone and striated muscle showed similar tissue perfusion rates. Knowledge of physiological microhemodynamic values of bone and striated muscle in NSG mice makes it possible to analyze pathophysiological processes at these anatomic sites, such as tumor growth, tumor metastasis, and tumor microcirculation, as well as the response to therapeutic agents.

Non-invasive intravital imaging of head and neck squamous cell carcinomas in live mice

Head and neck squamous cell carcinoma is one of the most common cancers with a 50% 5-year survival rate. Understanding the mechanisms that control development, progression, and spreading of the tumor to distal sites is of paramount importance to develop effective therapies. Here, we describe a minimally invasive procedure, which enables performing intravital microscopy of the mouse tongue in models for oral cancer and provides structural and dynamic information of the tumors at cellular and subcellular level.

Concerted actions of distinct nonmuscle myosin II isoforms drive intracellular membrane remodeling in live animals

Membrane remodeling plays a fundamental role during a variety of biological events. However, the dynamics and the molecular mechanisms regulating this process within cells in mammalian tissues in situ remain largely unknown. In this study, we use intravital subcellular microscopy in live mice to study the role of the actomyosin cytoskeleton in driving the remodeling of membranes of large secretory granules, which are integrated into the plasma membrane during regulated exocytosis. We show that two isoforms of nonmuscle myosin II, NMIIA and NMIIB, control distinct steps of the integration process. Furthermore, we find that F-actin is not essential for the recruitment of NMII to the secretory granules but plays a key role in the assembly and activation of NMII into contractile filaments. Our data support a dual role for the actomyosin cytoskeleton in providing the mechanical forces required to remodel the lipid bilayer and serving as a scaffold to recruit key regulatory molecules.

ROS in Cancer: The Burning Question

An unanswered question in human health is whether antioxidation prevents or promotes cancer. Antioxidation has historically been viewed as chemopreventive, but emerging evidence suggests that antioxidants may be supportive of neoplasia. We posit this contention to be rooted in the fact that ROS do not operate as one single biochemical entity, but as diverse secondary messengers in cancer cells. This cautions against therapeutic strategies to increase ROS at a global level. To leverage redox alterations towards the development of effective therapies necessitates the application of biophysical and biochemical approaches to define redox dynamics and to functionally elucidate specific oxidative modifications in cancer versus normal cells. An improved understanding of the sophisticated workings of redox biology is imperative to defeating cancer.

Experimental cannabidiol treatment reduces early pancreatic inflammation in type 1 diabetes

BACKGROUND

Destruction of the insulin-producing beta cells in type 1 diabetes (T1D) is induced by invasion of immune cells causing pancreatic inflammation. Cannabidiol (CBD), a phytocannabinoid, derived from the plant, Cannabis sativa, was shown to lower the incidence of diabetes in non-obese diabetic (NOD) mice, an animal model of spontaneous T1D development.

OBJECTIVE

The goal of this study was to investigate the impact of experimental CBD treatment on early pancreatic inflammation in T1D by intravital microscopy (IVM) in NOD mice.

METHODS

Seven-week-old female NOD mice were prophylactically administered daily 5 mg/kg CBD or control vehicle i.p. five times weekly for ten weeks. Animals underwent IVM following confirmation of T1D diagnosis by blood glucose testing. Leukocyte activation and functional capillary density (FCD) were quantified via IVM.

RESULTS

CBD-treated NOD mice developed T1D later and showed significantly reduced leukocyte activation and increased FCD in the pancreatic microcirculation.

CONCLUSIONS

Experimental CBD treatment reduced markers of inflammation in the microcirculation of the pancreas studied by intravital microscopy.

Evaluation of noise limits to improve image processing in soft X-ray projection microscopy

Soft X-ray microscopy has been developed for high resolution imaging of hydrated biological specimens due to the availability of water window region. In particular, a projection type microscopy has advantages in wide viewing area, easy zooming function and easy extensibility to computed tomography (CT). The blur of projection image due to the Fresnel diffraction of X-rays, which eventually reduces spatial resolution, could be corrected by an iteration procedure, i.e., repetition of Fresnel and inverse Fresnel transformations. However, it was found that the correction is not enough to be effective for all images, especially for images with low contrast. In order to improve the effectiveness of image correction by computer processing, we in this study evaluated the influence of background noise in the iteration procedure through a simulation study. In the study, images of model specimen with known morphology were used as a substitute for the chromosome images, one of the targets of our microscope. Under the condition that artificial noise was distributed on the images randomly, we introduced two different parameters to evaluate noise effects according to each situation where the iteration procedure was not successful, and proposed an upper limit of the noise within which the effective iteration procedure for the chromosome images was possible. The study indicated that applying the new simulation and noise evaluation method was useful for image processing where background noises cannot be ignored compared with specimen images.

Kinetics of milk lipid droplet transport, growth, and secretion revealed by intravital imaging: lipid droplet release is intermittently stimulated by oxytocin

The lipid droplet (LD) fraction of milk has attracted special attention because it supplies preformed lipids for neonatal development, and the assembled LDs are secreted by a unique apocrine mechanism. Because many aspects of this key process remain uncharacterized, we developed a facile method for the intravital imaging of mammary cells in transgenic mice that express fluorescently tagged marker proteins. Using these techniques, we describe the first kinetic analysis of LD growth and secretion at peak lactation in real time. LD transit from basal to apical regions was slow (0-2 μm/min) and frequently intermittent. Droplets grew by the fusion of preexisting droplets, with no restriction on the size of fusogenic partners. Most droplet expansion took several hours and occurred in apical nucleation centers, either close to or in association with the apical surface. Droplets even continued to expand as they were emerging from the cell. Contrary to expectations, LDs attached to the apical plasma membrane but still associated with the cytoplasm were released after oxytocin-mediated contraction of the myoepithelium. Thus milk LD secretion is an intermittently regulated process. This novel procedure will have broad application for investigating trafficking events within the mammary epithelium in real time.

Divide or Conquer: Cell Cycle Regulation of Invasive Behavior

Cell invasion through the basement membrane (BM) occurs during normal embryonic development and is a fundamental feature of cancer metastasis. The underlying cellular and genetic machinery required for invasion has been difficult to identify, due to a lack of adequate in vivo models to accurately examine invasion in single cells at subcellular resolution. Recent evidence has documented a functional link between cell cycle arrest and invasive activity. While cancer progression is traditionally thought of as a disease of uncontrolled cell proliferation, cancer cell dissemination, a critical aspect of metastasis, may require a switch from a proliferative to an invasive state. In this work, we review evidence that BM invasion requires cell cycle arrest and discuss the implications of this concept with regard to limiting the lethality associated with cancer metastasis.

Introduction to Modern Methods in Light Microscopy

For centuries, light microscopy has been a key method in biological research, from the early work of Robert Hooke describing biological organisms as cells, to the latest in live-cell and single-molecule systems. Here, we introduce some of the key concepts related to the development and implementation of modern microscopy techniques. We briefly discuss the basics of optics in the microscope, super-resolution imaging, quantitative image analysis, live-cell imaging, and provide an outlook on active research areas pertaining to light microscopy.

Liver kinase B1 regulates hepatocellular tight junction distribution and function in vivo

Liver kinase B1 (LKB1) and its downstream effector AMP-activated protein kinase (AMPK) play critical roles in polarity establishment by regulating membrane trafficking and energy metabolism. In collagen sandwich-cultured hepatocytes, loss of LKB1 or AMPK impaired apical ABCB11 (Bsep) trafficking and bile canalicular formation. In the present study, we used liver-specific (albumin-Cre) LKB1 knockout mice (LKB1(-/-) ) to investigate the role of LKB1 in the maintenance of functional tight junction (TJ) in vivo. Transmission electron microscopy examination revealed that hepatocyte apical membrane with microvilli substantially extended into the basolateral domain of LKB1(-/-) livers. Immunofluorescence studies revealed that loss of LKB1 led to longer and wider canalicular structures correlating with mislocalization of the junctional protein, cingulin. To test junctional function, we used intravital microscopy to quantify the transport kinetics of 6-carboxyfluorescein diacetate (6-CFDA), which is processed in hepatocytes into its fluorescent derivative 6-carboxyfluorescein (6-CF) and secreted into the canaliculi. In LKB1(-/-) mice, 6-CF remained largely in hepatocytes, canalicular secretion was delayed, and 6-CF appeared in the blood. To test whether 6-CF was transported through permeable TJ, we intravenously injected low molecular weight (3 kDa) dextran in combination with 6-CFDA. In wild-type mice, 3 kDa dextran remained in the vasculature, whereas it rapidly appeared in the abnormal bile canaliculi in LKB1(-/-) mice, confirming that junctional disruption resulted in paracellular exchange between the blood stream and the bile canaliculus.

CONCLUSION

LKB1 plays a critical role in regulating the maintenance of TJ and paracellular permeability, which may explain how various drugs, chemicals, and metabolic states that inhibit the LKB1/AMPK pathway result in cholestasis. (Hepatology 2016;64:1317-1329).