Sequential intravital imaging reveals in vivo dynamics of pancreatic tissue transplanted under the kidney capsule in mice

Recent Advances in Contrast-Enhanced Photoacoustic Imaging: Overcoming the Physical and Practical Challenges

For decades now, photoacoustic imaging (PAI) has been investigated to realize its potential as a niche biomedical imaging modality. Despite its highly desirable optical contrast and ultrasonic spatiotemporal resolution, PAI is challenged by such physical limitations as a low signal-to-noise ratio (SNR), diminished image contrast due to strong optical attenuation, and a lower-bound on spatial resolution in deep tissue. In addition, contrast-enhanced PAI has faced practical limitations such as insufficient cell-specific targeting due to low delivery efficiency and difficulties in developing clinically translatable agents. Identifying these limitations is essential to the continuing expansion of the field, and substantial advances in developing contrast-enhancing agents, complemented by high-performance image acquisition systems, have synergistically dealt with the challenges of conventional PAI. This review covers the past four years of research on pushing the physical and practical challenges of PAI in terms of SNR/contrast, spatial resolution, targeted delivery, and clinical application. Promising strategies for dealing with each challenge are reviewed in detail, and future research directions for next generation contrast-enhanced PAI are discussed.

Mouse models and human islet transplantation sites for intravital imaging

Human islet transplantations into rodent models are an essential tool to aid in the development and testing of islet and cellular-based therapies for diabetes prevention and treatment. Through the ability to evaluate human islets in an in vivo setting, these studies allow for experimental approaches to answer questions surrounding normal and disease pathophysiology that cannot be answered using other in vitro and in vivo techniques alone. Intravital microscopy enables imaging of tissues in living organisms with dynamic temporal resolution and can be employed to measure biological processes in transplanted human islets revealing how experimental variables can influence engraftment, and transplant survival and function. A key consideration in experimental design for transplant imaging is the surgical placement site, which is guided by the presence of vasculature to aid in functional engraftment of the islets and promote their survival. Here, we review transplantation sites and mouse models used to study beta cell biology in vivo using intravital microscopy and we highlight fundamental observations made possible using this methodology.

Optoacoustic Imaging of Glucagon-like Peptide-1 Receptor with a Near-Infrared Exendin-4 Analog

Limitations in current imaging tools have long challenged the imaging of small pancreatic islets in animal models. Here, we report the first development and in vivo validation testing of a broad-spectrum and high-absorbance near-infrared optoacoustic contrast agent, E4x12-Cy7. Our near-infrared tracer is based on the amino acid sequence of exendin-4 and targets the glucagon-like peptide-1 receptor (GLP-1R). Cell assays confirmed that E4x12-Cy7 has a high-binding affinity (dissociation constant, Kd, 4.6 ± 0.8 nM). Using the multispectral optoacoustic tomography, we imaged E4x12-Cy7 and optoacoustically visualized β-cell insulinoma xenografts in vivo for the first time. In the future, similar optoacoustic tracers that are specific for β-cells and combines optoacoustic and fluorescence imaging modalities could prove to be important tools for monitoring the pancreas for the progression of diabetes.

State-of-the-art microscopy to understand islets of Langerhans: what to expect next?

The discovery of Langerhans and microscopic description of islets in the pancreas were crucial steps in the discovery of insulin. Over the past 150 years, many discoveries in islet biology and type 1 diabetes have been made using powerful microscopic techniques. In the past decade, combination of new probes, animal and tissue models, application of new biosensors and automation of light and electron microscopic methods and other (sub)cellular imaging modalities have proven their potential in understanding the beta cell under (patho)physiological conditions. The imaging evolution, from fluorescent jellyfish to real‐time intravital functional imaging, the revolution in automation and data handling and the increased resolving power of analytical imaging techniques are now converging. Here, we review innovative approaches that address islet biology from new angles by studying cells and molecules at high spatiotemporal resolution and in live models. Broad implementation of these cellular imaging techniques will shed new light on cause/consequence of (mal)function in islets of Langerhans in the years to come.

In Vivo Assessment of Size-Selective Glomerular Sieving in Transplanted Human Induced Pluripotent Stem Cell-Derived Kidney Organoids

BACKGROUND

The utility of kidney organoids in regenerative medicine will rely on the functionality of the glomerular and tubular structures in these tissues. Recent studies have demonstrated the vascularization and subsequent maturation of human pluripotent stem cell-derived kidney organoids after renal subcapsular transplantation. This raises the question of whether the glomeruli also become functional upon transplantation.

METHODS

We transplanted kidney organoids under the renal capsule of the left kidney in immunodeficient mice followed by the implantation of a titanium imaging window on top of the kidney organoid. To assess glomerular function in the transplanted human pluripotent stem cell-derived kidney tissue 1, 2, and 3 weeks after transplantation, we applied high-resolution intravital multiphoton imaging through the imaging window during intravenous infusion of fluorescently labeled low and high molecular mass dextran molecules or albumin.

RESULTS

After vascularization, glomerular structures in the organoid displayed dextran and albumin size selectivity across their glomerular filtration barrier. We also observed evidence of proximal tubular dextran reuptake.

CONCLUSIONS

Our results demonstrate that human pluripotent stem cell-derived glomeruli can develop an appropriate barrier function and discriminate between molecules of varying size. These characteristics together with tubular presence of low molecular mass dextran provide clear evidence of functional filtration. This approach to visualizing glomerular filtration function will be instrumental for translation of organoid technology for clinical applications as well as for disease modeling.

Chemical Biology Toolbox for Studying Pancreatic Islet Function - A Perspective

The islets of Langerhans represent one of the many complex endocrine organs in mammals. Traditionally, islet function is studied by a mixture of physiological, cell biological, and molecular biological methods. Recently, novel techniques stemming from the ever-increasing toolbox provided by chemical laboratories have been added to the repertoire. Many emerging techniques will soon be available to manipulate and monitor islet function at the single-cell level and potentially in intact model animals, as well as in isolated human islets. Here, we review the most current small-molecule-based and genetically encoded molecular tool sets available to study islet function. We provide an outlook regarding future tool developments that will impact islet research, with a special focus on the interplay between different islet cell types.

Functional analysis of islet cells in vitro, in situ, and in vivo

The islet of Langerhans contains at least five types of endocrine cells producing distinct hormones. In response to nutrient or neuronal stimulation, islet endocrine cells release biochemicals including peptide hormones to regulate metabolism and to control glucose homeostasis. It is now recognized that malfunction of islet cells, notably insufficient insulin release of β-cells and hypersecretion of glucagon from α-cells, represents a causal event leading to hyperglycemia and frank diabetes, a disease that is increasing at an alarming rate to reach an epidemic level worldwide. Understanding the mechanisms regulating stimulus-secretion coupling and investigating how islet β-cells maintain a robust secretory activity are important topics in islet biology and diabetes research. To facilitate such studies, a number of biological systems and assay platforms have been developed for the functional analysis of islet cells. These technologies have enabled detailed analyses of individual islets at the cellular level, either in vitro, in situ, or in vivo.

A Novel Pancreatic Imaging Window for Stabilized Longitudinal In Vivo Observation of Pancreatic Islets in Murine Model

Longitudinal imaging of murine pancreas is technically challenging due to the mechanical softness of the tissue influenced by peristalsis. Here, we report a novel pancreatic imaging window for long-term stabilized cellular-level observation of the islets in the pancreas in vivo. By spatially separating the pancreas from the bowel movement and physiologic respiration with a metal plate integrated in the imaging window, we successfully tracked the pancreatic islets up to three weeks and visualized the dumbbell-shape transformation from the single islet. This window can be a useful tool for long-term cellular-level visualization of the microstructure in the pancreas.

Intravital imaging of vascular anomalies and extracellular matrix remodeling in orthotopic pancreatic tumors

Pancreatic cancers, both adenocarcinomas and endocrine tumors are characterized by varying levels of aberrant angiogenesis and fibrotic microenvironment. The difficulty to deliver drugs and treat the disease has been attributed in part to the vascular architecture and tissue/ECM density. Here we present longitudinal three-dimensional intravital imaging of vascular and tumor microenvironment remodeling in spontaneous transgenic tumors (RIP1-Tag2 insulinomas) and orthotopically injected tumors (KPC adenocarcinomas). Analysis of the data acquired in insulinomas revealed major differences in tumor blood vessel branching, fraction volume, number of branch points segments, vessel straightness and length compared to the normal tissue. The aggressive adenocarcinoma presented widespread peritumoral vascular remodeling and heterogeneous vascular distribution. Longitudinal imaging was used to acquire sequential vascular remodeling data during tumor progression. This work demonstrates the potential for using a pancreatic intravital imaging window for direct visualization of the tumor heterogenic microenvironments during tumor progression.

Noninvasive intravital high-resolution imaging of pancreatic neuroendocrine tumours

Preclinical trials of cancer drugs in animal models are important for drug development. The Rip1Tag2 (RT2) transgenic mouse, a model of pancreatic neuroendocrine tumours (PNET), has provided immense knowledge about PNET biology, although tumour progression occurs in a location inaccessible for real-time monitoring. To overcome this hurdle we have developed a novel platform for intravital 3D imaging of RT2 tumours to facilitate real-time studies of cancer progression. Pre-oncogenic islets retrieved from RT2 mice were implanted into the anterior chamber of the eye (ACE) of host mice, where they engrafted on the iris, recruited blood vessels and showed continuous growth. Noninvasive confocal and two-photon laser-scanning microscopy through the transparent cornea facilitated high-resolution imaging of tumour growth and angiogenesis. RT2 tumours in the ACE expanded up to 8-fold in size and shared hallmarks with tumours developing in situ in the pancreas. Genetically encoded fluorescent reporters enabled high-resolution imaging of stromal cells and tumour cell migration. Sunitinib treatment impaired RT2 tumour angiogenesis and growth, while overexpression of the vascular endothelial growth factor (VEGF)-B increased tumour angiogenesis though tumour growth was impaired. In conclusion, we present a novel platform for intravital high-resolution and 3D imaging of PNET biology and cancer drug assessment.

A Versatile, Portable Intravital Microscopy Platform for Studying Beta-cell Biology In Vivo

The pancreatic islet is a complex micro-organ containing numerous cell types, including endocrine, immune, and endothelial cells. The communication of these systems is lost upon isolation of the islets, and therefore the pathogenesis of diabetes can only be fully understood by studying this organized, multicellular environment in vivo. We have developed several adaptable tools to create a versatile platform to interrogate β-cell function in vivo. Specifically, we developed β-cell-selective virally-encoded fluorescent protein biosensors that can be rapidly and easily introduced into any mouse. We then coupled the use of these biosensors with intravital microscopy, a powerful tool that can be used to collect cellular and subcellular data from living tissues. Together, these approaches allowed the observation of in vivo β-cell-specific ROS dynamics using the Grx1-roGFP2 biosensor and calcium signaling using the GcAMP6s biosensor. Next, we utilized abdominal imaging windows (AIW) to extend our in vivo observations beyond single-point terminal measurements to collect longitudinal physiological and biosensor data through repeated imaging of the same mice over time. This platform represents a significant advancement in our ability to study β-cell structure and signaling in vivo, and its portability for use in virtually any mouse model will enable meaningful studies of β-cell physiology in the endogenous islet niche.

Intravital visualization of the primary cilium, tubule flow, and innate immune cells in the kidney utilizing an abdominal window imaging approach

The renal primary cilium is a small microtubule-based appendage thought to have mechano/chemosensory roles detecting changes in the fluid passing through the nephron. Mutations affecting cilium structure or function of ciliary-localized proteins result in a spectrum of diseases termed ciliopathies, with prevalent phenotypes such as the formation of renal cysts and fibrosis. While many studies have been conducted using fixed kidney sections or live imaging of cells in culture to investigate the cilium, examination in the context of a living murine kidney remains to be conducted. Previously, our lab generated the SSTR3GFP mouse to study cilium dynamics in vivo and found novel cilium behaviors that occurred following alteration of heart rate, blood pressure, and tubule flow. In this manuscript, we utilize multiple transgenic mouse models and an abdominal window imaging approach to observe primary cilia and tubule flow dynamics, immune cell movement, and renal Ca2+ signaling as it occurs in real time within a live mouse kidney. We present this window method as an approach that can be used in combination with various fluorescently labeled transgenic mice to investigate renal physiology, pathology, and function in vivo in longitudinal studies for as long as 5weeks.

A Bright Future for Fluorescence Imaging of Fungi in Living Hosts

Traditional in vivo investigation of fungal infection and new antifungal therapies in mouse models is usually carried out using post mortem methodologies. However, biomedical imaging techniques focusing on non-invasive techniques using bioluminescent and fluorescent proteins have become valuable tools. These new techniques address ethical concerns as they allow reduction in the number of animals required to evaluate new antifungal therapies. They also allow better understanding of the growth and spread of the pathogen during infection. In this review, we concentrate on imaging technologies using different fungal reporter proteins. We discuss the advantages and limitations of these different reporters and compare the efficacy of bioluminescent and fluorescent proteins for fungal research.

Renal Subcapsular Transplantation of PSC-Derived Kidney Organoids Induces Neo-vasculogenesis and Significant Glomerular and Tubular Maturation In Vivo

Human pluripotent stem cell (hPSC)-derived kidney organoids may facilitate disease modeling and the generation of tissue for renal replacement. Long-term application, however, will require transferability between hPSC lines and significant improvements in organ maturation. A key question is whether time or a patent vasculature is required for ongoing morphogenesis. Here, we show that hPSC-derived kidney organoids, derived in fully defined medium conditions and in the absence of any exogenous vascular endothelial growth factor, develop host-derived vascularization. In vivo imaging of organoids under the kidney capsule confirms functional glomerular perfusion as well as connection to pre-existing vascular networks in the organoids. Wide-field electron microscopy demonstrates that transplantation results in formation of a glomerular basement membrane, fenestrated endothelial cells, and podocyte foot processes. Furthermore, compared with non-transplanted organoids, polarization and segmental specialization of tubular epithelium are observed. These data demonstrate that functional vascularization is required for progressive morphogenesis of human kidney organoids.

The allosteric AKT inhibitor, MK2206, decreases tumor growth and invasion in patient derived xenografts of endometrial cancer

The purpose of this study was to test the effect of MK2206, an allosteric inhibitor of AKT, on the growth and invasion of patient-derived xenografts (PDX) of endometrial cancer. Three PDX lines, USC1 (uterine serous), EEC2 (endometrioid grade 2) and EEC4 (endometrioid grade 3) of endometrial cancer were grafted under the renal capsule of NSG mice. After 2 weeks of tumor growth the mice were treated with vehicle or 120mg/kg MK2206 twice a week for 3 weeks. Growth of all 3 PDX lines of different type and grade was significantly inhibited in response to MK2206 compared with vehicle control. Histological analysis revealed invasion and spread of EEC2 and EEC4 tumors were significantly decreased with MK2206 treatment. Immunohistochemical analysis showed a decrease in Ki67 in EEC2 upon MK2206 treatment, while USC1 and EEC4 tumors did not show differences in Ki67 levels. PR levels were evident in EEC2 which dramatically increased upon MK2206 treatment. In vitro analysis of EEC4 and AN3CA cells showed a dose-dependent decrease in p(Ser473)-AKT and p(Thr308)-AKT with MK2206. Invasion of EEC4 and AN3CA cells also significantly decreased after 36h and 72h of MK2206 treatment. PDX tumors provide an appropriate model for the testing of compounds that incorporates the heterogeneous nature of endometrial cancer. Further studies to determine efficacy of MK2206 alone or in combination with other compounds can also identify predictors of response to these pathway inhibitors.

Surgical preparation of rats and mice for intravital microscopic imaging of abdominal organs

Intravital microscopy is a powerful research tool that can provide insight into cellular and subcellular events that take place in organs in the body. However, meaningful results can only be obtained from animals whose physiology is preserved during the process of microscopy. Here I discuss the importance of preserving the overall state of health of the animal, methods of anesthesia, surgical techniques for intravital microscopy of various abdominal organs, methods to maintain and monitor the physiology of the animal during microscopy and associated peri- and post-operative recovery considerations.

Procedures and applications of long-term intravital microscopy

Intravital microscopy (IVM) is increasingly used in biomedical research to study dynamic processes at cellular and subcellular resolution in their natural environment. Long-term IVM especially can be applied to visualize migration and proliferation over days to months within the same animal without recurrent surgeries. Skin can be repetitively imaged without surgery. To intermittently visualize cells in other organs, such as liver, mammary gland and brain, different imaging windows including the abdominal imaging window (AIW), dermal imaging window (DIW) and cranial imaging window (CIW) have been developed. In this review, we describe the procedure of window implantation and pros and cons of each technique as well as methods to retrace a position of interest over time. In addition, different fluorescent biosensors to facilitate the tracking of cells for different purposes, such as monitoring cell migration and proliferation, are discussed. Finally, we consider new techniques and possibilities of how long-term IVM can be even further improved in the future.