Growth arrest of individual senile plaques in a model of Alzheimer's disease observed by in vivo multiphoton microscopy

VE-cadherin in arachnoid and pia mater cells serves as a suitable landmark for in vivo imaging of CNS immune surveillance and inflammation

Meninges cover the surface of the brain and spinal cord and contribute to protection and immune surveillance of the central nervous system (CNS). How the meningeal layers establish CNS compartments with different accessibility to immune cells and immune mediators is, however, not well understood. Here, using 2-photon imaging in female transgenic reporter mice, we describe VE-cadherin at intercellular junctions of arachnoid and pia mater cells that form the leptomeninges and border the subarachnoid space (SAS) filled with cerebrospinal fluid (CSF). VE-cadherin expression also marked a layer of Prox1+ cells located within the arachnoid beneath and separate from E-cadherin+ arachnoid barrier cells. In vivo imaging of the spinal cord and brain in female VE-cadherin-GFP reporter mice allowed for direct observation of accessibility of CSF derived tracers and T cells into the SAS bordered by the arachnoid and pia mater during health and neuroinflammation, and detection of volume changes of the SAS during CNS pathology. Together, the findings identified VE-cadherin as an informative landmark for in vivo imaging of the leptomeninges that can be used to visualize the borders of the SAS and thus potential barrier properties of the leptomeninges in controlling access of immune mediators and immune cells into the CNS during health and neuroinflammation.

Long-Term Repeatable In Vivo Monitoring of Amyloid-β Plaques and Vessels in Alzheimer's Disease Mouse Model with Combined TPEF/CARS Microscopy

Long-term, repeatable monitoring of the appearance and progress of Alzheimer's disease (AD) in real time can be extremely beneficial to acquire highly reliable diagnostic insights, which is crucial for devising apt strategies towards effective AD treatment. Herein, we present an optimized innovative cranial window imaging method for the long-term repeatable imaging of amyloid-β (Aβ) plaques and vessels in an AD mouse model. Basically, two-photon excitation fluorescence (TPEF) microscopy was used to monitor the fluorescently labeled Aβ plaques, whereas the label-free blood vessels were studied using coherent anti-Stokes Raman scattering (CARS) microscopy in the live in vivo AD mouse model. It was possible to clearly observe the Aβ deposition and vascular structure in the target cortex localization for 31 weeks in the AD mouse model using this method. The combined TPEF/CARS imaging studies were also instrumental in realizing the relationship between the tendency of Aβ deposition and ageing. Essentially, the progression of cerebral amyloid angiopathy (CAA) in the AD mouse model was quantitatively characterized, which revealed that the proportion Aβ deposition in the unit vessel can increase from 13.63% to 28.80% upon increasing the age of mice from 8 months old to 14 months old. The proposed imaging method provided an efficient, safe, repeatable platform with simple target localization aptitude towards monitoring the brain tissues, which is an integral part of studying any brain-related physiological or disease conditions to extract crucial structural and functional information.

A Model of Discovery: The Role of Imaging Established and Emerging Non-mammalian Models in Neuroscience

Rodents have been the dominant animal models in neurobiology and neurological disease research over the past 60 years. The prevalent use of rats and mice in neuroscience research has been driven by several key attributes including their organ physiology being more similar to humans, the availability of a broad variety of behavioral tests and genetic tools, and widely accessible reagents. However, despite the many advances in understanding neurobiology that have been achieved using rodent models, there remain key limitations in the questions that can be addressed in these and other mammalian models. In particular, in vivo imaging in mammals at the cell-resolution level remains technically difficult and demands large investments in time and cost. The simpler nervous systems of many non-mammalian models allow for precise mapping of circuits and even the whole brain with impressive subcellular resolution. The types of non-mammalian neuroscience models available spans vertebrates and non-vertebrates, so that an appropriate model for most cell biological questions in neurodegenerative disease likely exists. A push to diversify the models used in neuroscience research could help address current gaps in knowledge, complement existing rodent-based bodies of work, and bring new insight into our understanding of human disease. Moreover, there are inherent aspects of many non-mammalian models such as lifespan and tissue transparency that can make them specifically advantageous for neuroscience studies. Crispr/Cas9 gene editing and decreased cost of genome sequencing combined with advances in optical microscopy enhances the utility of new animal models to address specific questions. This review seeks to synthesize current knowledge of established and emerging non-mammalian model organisms with advances in cellular-resolution in vivo imaging techniques to suggest new approaches to understand neurodegeneration and neurobiological processes. We will summarize current tools and in vivo imaging approaches at the single cell scale that could help lead to increased consideration of non-mammalian models in neuroscience research.

Visualization of neurofibrillary tangle maturity in Alzheimer's disease: A clinicopathologic perspective for biomarker research

Neurofibrillary tangles, one of the neuropathologic hallmarks of Alzheimer's disease, have a dynamic lifespan of maturity that associates with progressive neuronal dysfunction and cognitive deficits. As neurofibrillary tangles mature, the biology of the neuron undergoes extensive changes that may impact biomarker recognition and therapeutic targeting. Neurofibrillary tangle maturity encompasses three levels: pretangles, mature tangles, and ghost tangles. In this review, we detail distinct and overlapping characteristics observed in the human brain regarding morphologic changes the neuron undergoes, conversion from intracellular to extracellular space, tau immunostaining patterns, and tau isoform expression changes across the lifespan of the neurofibrillary tangle. Post-translational modifications of tau such as phosphorylation, ubiquitination, conformational events, and truncations are discussed to contextualize tau immunostaining patterns. We summarize accumulated and emerging knowledge of neurofibrillary tangle maturity, discuss the current tools used to interpret the dynamic nature in the postmortem brain, and consider implications for cognitive dysfunction and tau biomarkers.

Optical Imaging of Beta-Amyloid Plaques in Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial, irreversible, and incurable neurodegenerative disease. The main pathological feature of AD is the deposition of misfolded β-amyloid protein (Aβ) plaques in the brain. The abnormal accumulation of Aβ plaques leads to the loss of some neuron functions, further causing the neuron entanglement and the corresponding functional damage, which has a great impact on memory and cognitive functions. Hence, studying the accumulation mechanism of Aβ in the brain and its effect on other tissues is of great significance for the early diagnosis of AD. The current clinical studies of Aβ accumulation mainly rely on medical imaging techniques, which have some deficiencies in sensitivity and specificity. Optical imaging has recently become a research hotspot in the medical field and clinical applications, manifesting noninvasiveness, high sensitivity, absence of ionizing radiation, high contrast, and spatial resolution. Moreover, it is now emerging as a promising tool for the diagnosis and study of Aβ buildup. This review focuses on the application of the optical imaging technique for the determination of Aβ plaques in AD research. In addition, recent advances and key operational applications are discussed.

Through the looking glass: A review of cranial window technology for optical access to the brain

Deciphering neurologic function is a daunting task, requiring understanding the neuronal networks and emergent properties that arise from the interactions among single neurons. Mechanistic insights into neuronal networks require tools that simultaneously assess both single neuron activity and the consequent mesoscale output. The development of cranial window technologies, in which the skull is thinned or replaced with a synthetic optical interface, has enabled monitoring neuronal activity from subcellular to mesoscale resolution in awake, behaving animals when coupled with advanced microscopy techniques. Here we review recent achievements in cranial window technologies, appraise the relative merits of each design and discuss the future research in cranial window design.

The Role of Butyrylcholinesterase and Iron in the Regulation of Cholinergic Network and Cognitive Dysfunction in Alzheimer's Disease Pathogenesis

Alzheimer’s disease (AD), the most common form of dementia in elderly individuals, is marked by progressive neuron loss. Despite more than 100 years of research on AD, there is still no treatment to cure or prevent the disease. High levels of amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs) in the brain are neuropathological hallmarks of AD. However, based on postmortem analyses, up to 44% of individuals have been shown to have high Aβ deposits with no clinical signs, due to having a “cognitive reserve”. The biochemical mechanism explaining the prevention of cognitive impairment in the presence of Aβ plaques is still unknown. It seems that in addition to protein aggregation, neuroinflammatory changes associated with aging are present in AD brains that are correlated with a higher level of brain iron and oxidative stress. It has been shown that iron accumulates around amyloid plaques in AD mouse models and postmortem brain tissues of AD patients. Iron is required for essential brain functions, including oxidative metabolism, myelination, and neurotransmitter synthesis. However, an imbalance in brain iron homeostasis caused by aging underlies many neurodegenerative diseases. It has been proposed that high iron levels trigger an avalanche of events that push the progress of the disease, accelerating cognitive decline. Patients with increased amyloid plaques and iron are highly likely to develop dementia. Our observations indicate that the butyrylcholinesterase (BChE) level seems to be iron-dependent, and reports show that BChE produced by reactive astrocytes can make cognitive functions worse by accelerating the decay of acetylcholine in aging brains. Why, even when there is a genetic risk, do symptoms of the disease appear after many years? Here, we discuss the relationship between genetic factors, age-dependent iron tissue accumulation, and inflammation, focusing on AD.

Synaptic Loss in Alzheimer's Disease: Mechanistic Insights Provided by Two-Photon in vivo Imaging of Transgenic Mouse Models

Synapse loss is the strongest correlate for cognitive decline in Alzheimer's disease. The mechanisms underlying synapse loss have been extensively investigated using mouse models expressing genes with human familial Alzheimer's disease mutations. In this review, we summarize how multiphoton in vivo imaging has improved our understanding of synapse loss mechanisms associated with excessive amyloid in the living animal brain. We also discuss evidence obtained from these imaging studies for the role of cell-intrinsic calcium dyshomeostasis and cell-extrinsic activities of microglia, which are the immune cells of the brain, in mediating synapse loss.

Long term intravital single cell tracking under multiphoton microscopy

Visualizing and tracking cells over time in a living organism has been a much-coveted dream before the invention of intravital microscopy. The opaque nature of tissue was a major hurdle that was remedied by the multiphoton microscopy. With the advancement of optical imaging and fluorescent labeling tools, intravital high resolution imaging has become increasingly accessible over the past few years. Long-term intravital tracking of single cells (LIST) under multiphoton microscopy provides a unique opportunity to gain insight into the longitudinal changes in the morphology, migration, or function of cells or subcellular structures. It is particularly suitable for studying slow-evolving cellular and molecular events during normal development or disease progression, without losing the opportunity of catching fast events such as calcium signals. Here, we review the application of LIST under 2-photon microscopy in various fields of neurobiology and discuss challenges and new directions in labeling and imaging methods for LIST. Overall, this review provides an overview of current applications of LIST in mammals, which is an emerging field that will contribute to a better understanding of essential molecular and cellular events in health and disease.

Recent advances in nonlinear microscopy: Deep insights and polarized revelations

Nonlinear microscopy is a technique that utilizes nonlinear interactions between light and matter to image fluorescence and scattering phenomena in biological tissues. Very high peak intensities from focused short pulsed lasers are required for nonlinear excitation due to the extremely low probability of the simultaneous arrival of multiple photons of lower energy to excite fluorophores or interact with selective structures for harmonic generation. Combined with reduced scattering from the utilization of longer wavelengths, the inherent spatial confinement associated with achieving simultaneous arrival of photons within the focal volume enables deep imaging with low out-of-focus background for nonlinear imaging. This review provides an introduction to the different contrast mechanisms available with nonlinear imaging and instrumentation commonly used in nonlinear microscopy. Furthermore, we discuss some recent advances in nonlinear microscopy to extend the imaging penetration depth, conduct histopathological investigations on fresh tissues and examine the molecular order and orientation of molecules using polarization nonlinear microscopy.

Aβ Plaques

Aβ plaques are one of the two lesions in the brain that define the neuropathological diagnosis of Alzheimer's disease. Plaques are highly diverse structures; many of them include massed, fibrillar polymers of the Aβ protein referred to as Aβ-amyloid, but some lack the defining features of amyloid. Cellular elements in 'classical' plaques include abnormal neuronal processes and reactive glial cells, but these are not present in all plaques. Plaques have been given various names since their discovery in 1892, including senile plaques, amyloid plaques, and neuritic plaques. However, with the identification in the 1980s of Aβ as the obligatory and universal component of plaques, the term 'Aβ plaques' has become a unifying term for these heterogeneous formations. Tauopathy, the second essential lesion of the Alzheimer's disease diagnostic dyad, is downstream of Aβ-proteopathy, but it is critically important for the manifestation of dementia. The etiologic link between Aβ-proteopathy and tauopathy in Alzheimer's disease remains largely undefined. Aβ plaques develop and propagate via the misfolding, self-assembly and spread of Aβ by the prion-like mechanism of seeded protein aggregation. Partially overlapping sets of risk factors and sequelae, including inflammation, genetic variations, and various environmental triggers have been linked to plaque development and idiopathic Alzheimer's disease, but no single factor has emerged as a requisite cause. The value of Aβ plaques per se as therapeutic targets is uncertain; although some plaques are sites of focal gliosis and inflammation, the complexity of inflammatory biology presents challenges to glia-directed intervention. Small, soluble, oligomeric assemblies of Aβ are enriched in the vicinity of plaques, and these probably contribute to the toxic impact of Aβ aggregation on the brain. Measures designed to reduce the production or seeded self-assembly of Aβ can impede the formation of Aβ plaques and oligomers, along with their accompanying abnormalities; given the apparent long timecourse of the emergence, maturation and proliferation of Aβ plaques in humans, such therapies are likely to be most effective when begun early in the pathogenic process, before significant damage has been done to the brain. Since their discovery in the late 19th century, Aβ plaques have, time and again, illuminated fundamental mechanisms driving neurodegeneration, and they should remain at the forefront of efforts to understand, and therefore treat, Alzheimer's disease.

Microvessel occlusions alter amyloid-beta plaque morphology in a mouse model of Alzheimer's disease

Vascular dysfunction is correlated to the incidence and severity of Alzheimer's disease. In a mouse model of Alzheimer's disease (APP/PS1) using in vivo, time-lapse, multiphoton microscopy, we found that occlusions of the microvasculature alter amyloid-beta (Aβ) plaques. We used several models of vascular injury that varied in severity. Femtosecond laser-induced occlusions in single capillaries generated a transient increase in small, cell-sized, Aβ deposits visualized with methoxy-X04, a label of fibrillar Aβ. After occlusions of penetrating arterioles, some plaques changed morphology, while others disappeared, and some new plaques appeared within a week after the lesion. Antibody labeling of Aβ revealed a transient increase in non-fibrillar Aβ one day after the occlusion that coincided with the disappearance of methoxy-X04-labeled plaques. Four days after the lesion, anti-Aβ labeling decreased and only remained in patches unlabeled by methoxy-X04 near microglia. Histology in two additional models, sparse embolic occlusions from intracarotid injections of beads and infarction from photothrombosis, demonstrated increased labeling intensity in plaques after injury. These results suggest that microvascular lesions can alter the deposition and clearance of Aβ and confirm that Aβ plaques are dynamic structures, complicating the interpretation of plaque burden as a marker of Alzheimer's disease progression.

Ex-vivo Alzheimer's disease brain tissue investigation: a multiscale approach using 1060-nm swept source optical coherence tomography for a direct correlation to histology

Significance: Amyloid-beta ( A - β ) plaques are pathological protein deposits formed in the brain of Alzheimer's disease (AD) patients upon disease progression. Further research is needed to elucidate the complex underlying mechanisms involved in their formation using label-free, tissue preserving, and volumetric techniques. Aim: The aim is to achieve a one-to-one correlation of optical coherence tomography (OCT) data to histological micrographs of brain tissue using 1060-nm swept source OCT. Approach: A - β plaques were investigated in ex-vivo AD brain tissue using OCT with the capability of switching between two magnifications. For the exact correlation to histology, a 3D-printed tool was designed to generate samples with parallel flat surfaces. Large field-of-view (FoV) and sequentially high-resolution volumes at different locations were acquired. The large FoV served to align the OCT to histology images; the high-resolution images were used to visualize fine details. Results: The instrument and the presented method enabled an accurate correlation of histological micrographs with OCT data. A - β plaques were identified as hyperscattering features in both FoV OCT modalities. The plaques identified in volumetric OCT data were in good agreement with immunohistochemically derived micrographs. Conclusion: OCT combined with the 3D-printed tool is a promising approach for label-free, nondestructive, volumetric, and fast tissue analysis.

Intravital Imaging Techniques for Biomedical and Clinical Research

Intravital imaging, the direct visualization of cells and tissues within a living animal, is a technique that has been employed for the better part of a century. The advent of confocal and multiphoton microscopy has dramatically improved the power of intravital imaging, making it possible to obtain optical sections of tissues non-destructively. This review discusses the various techniques used for intravital imaging, describes how intravital imaging provides information about cellular and tissue dynamics not possible to be garnered by other techniques, and details several ways in which intravital imaging is making a direct impact on the clinical care of patients. © 2019 International Society for Advancement of Cytometry.

In Vivo Near-Infrared Two-Photon Imaging of Amyloid Plaques in Deep Brain of Alzheimer's Disease Mouse Model

Abnormal deposition of brain amyloid is a major hallmark of Alzheimer's disease (AD). The toxic extracellular amyloid plaques originating from the aberrant aggregation of beta-amyloid (Aβ) protein are considered to be the major cause of clinical deficits such as memory loss and cognitive impairment. Two-photon excited fluorescence (TPEF) microscopy provides high spatial resolution, minimal invasiveness, and long-term monitoring capability. TPEF imaging of amyloid plaques in AD transgenic mice models has greatly facilitated studies of the AD pathological mechanism. However, the imaging of deep cortical layers is still hampered by the conventional amyloid probes with short excitation/emission wavelength. In this work, we report that a near-infrared (NIR) probe, named CRANAD-3, is far superior for deep in vivo TPEF imaging of brain amyloid in comparison with the commonly used short-wavelength probe. Our findings show that the major interference for TPEF signal of the NIR probe is from the autofluorescence of lipofuscin, the "aging-pigment" in the brain. To eliminate the interference, we characterized the lipofuscin fluorescence in the aged brains of AD mice and found that it has unique broad emission and short lifetime. The lipofuscin signal can be clearly separated from the fluorescence of CRANAD-3 and fluorescent protein via a ratio-based unmixing method. Our results demonstrate the great advantages of NIR probes for in vivo deep-tissue imaging of amyloid plaques in AD.

In Vivo Imaging of Microglia With Multiphoton Microscopy

Neuroimaging has become an unparalleled tool to understand the central nervous system (CNS) anatomy, physiology and neurological diseases. While an altered immune function and microglia hyperactivation are common neuropathological features for many CNS disorders and neurodegenerative diseases, direct assessment of the role of microglial cells remains a challenging task. Non-invasive neuroimaging techniques, including magnetic resonance imaging (MRI), positron emission tomography (PET) and single positron emission computed tomography (SPECT) are widely used for human clinical applications, and a variety of ligands are available to detect neuroinflammation. In animal models, intravital imaging has been largely used, and minimally invasive multiphoton microcopy (MPM) provides high resolution detection of single microglia cells, longitudinally, in living brain. In this study, we review in vivo real-time MPM approaches to assess microglia in preclinical studies, including individual cell responses in surveillance, support, protection and restoration of brain tissue integrity, synapse formation, homeostasis, as well as in different pathological situations. We focus on in vivo studies that assess the role of microglia in mouse models of Alzheimer’s disease (AD), analyzing microglial motility and recruitment, as well as the role of microglia in anti-amyloid-β treatment, as a key therapeutic approach to treat AD. Altogether, MPM provides a high contrast and high spatial resolution approach to follow microglia chronically in vivo in complex models, supporting MPM as a powerful tool for deep intravital tissue imaging.

Amyloid-β Plaques in Clinical Alzheimer's Disease Brain Incorporate Stable Isotope Tracer In Vivo and Exhibit Nanoscale Heterogeneity

Alzheimer's disease (AD) is a neurodegenerative disorder with clinical manifestations of progressive memory decline and loss of executive function and language. AD affects an estimated 5.3 million Americans alone and is the most common form of age-related dementia with a rapidly growing prevalence among the aging population-those 65 years of age or older. AD is characterized by accumulation of aggregated amyloid-beta (Aβ) in the brain, which leads to one of the pathological hallmarks of AD-Aβ plaques. As a result, Aβ plaques have been extensively studied after being first described over a century ago. Advances in brain imaging and quantitative measures of Aβ in biological fluids have yielded insight into the time course of plaque development decades before and after AD symptom onset. However, despite the fundamental role of Aβ plaques in AD, in vivo measures of individual plaque growth, growth distribution, and dynamics are still lacking. To address this question, we combined stable isotope labeling kinetics (SILK) and nanoscale secondary ion mass spectrometry (NanoSIMS) imaging in an approach termed SILK-SIMS to resolve plaque dynamics in three human AD brains. In human AD brain, plaques exhibit incorporation of a stable isotope tracer. Tracer enrichment was highly variable between plaques and the spatial distribution asymmetric with both quiescent and active nanometer sub-regions of tracer incorporation. These data reveal that Aβ plaques are dynamic structures with deposition rates over days indicating a highly active process. Here, we report the first, direct quantitative measures of in vivo deposition into plaques in human AD brain. Our SILK-SIMS studies will provide invaluable information on plaque dynamics in the normal and diseased brain and offer many new avenues for investigation into pathological mechanisms of the disease, with implications for therapeutic development.

Quantitative Comparison of Dense-Core Amyloid Plaque Accumulation in Amyloid-β Protein Precursor Transgenic Mice

There exist several dozen lines of transgenic mice that express human amyloid-β protein precursor (AβPP) with Alzheimer's disease (AD)-linked mutations. AβPP transgenic mouse lines differ in the types and amounts of Aβ that they generate and in their spatiotemporal patterns of expression of Aβ assemblies, providing a toolkit to study Aβ amyloidosis and the influence of Aβ aggregation on brain function. More complete quantitative descriptions of the types of Aβ assemblies present in transgenic mice and in humans during disease progression should add to our understanding of how Aβ toxicity in mice relates to the pathogenesis of AD. Here, we provide a direct quantitative comparison of amyloid plaque burdens and plaque sizes in four lines of AβPP transgenic mice. We measured the fraction of cortex and hippocampus occupied by dense-core plaques, visualized by staining with Thioflavin S, in mice from young adulthood through advanced age. We found that the plaque burdens among the transgenic lines varied by an order of magnitude: at 15 months of age, the oldest age studied, the median cortical plaque burden in 5XFAD mice was already ∼4.5 times that of 21-month-old Tg2576 mice and ∼15 times that of 21-24-month-old rTg9191 mice. Plaque-size distributions changed across the lifespan in a line- and region-dependent manner. We also compared the dense-core plaque burdens in the mice to those measured in a set of pathologically-confirmed AD cases from the Nun Study. Cortical plaque burdens in Tg2576, APPSwePS1ΔE9, and 5XFAD mice eventually far exceeded those measured in the human cohort.

Amyloid Plaques Show Binding Capacity of Exogenous Injected Amyloid-β

Amyloid plaques, although inducing damage to the immediately surrounding neuropil, have been proposed to provide a relatively innocuous way to deposit toxic soluble amyloid-β (Aβ) species. Here we address this hypothesis by exploring spread and absorption of fluorescent Aβ to pre-existing amyloid plaques after local application in wild-type mice versus APP/PS1 transgenic mice with amyloid plaques. Local intracortical or intracerebroventricular injection of fluorescently-labeled Aβ in APP/PS1 mice with a high plaque density resulted in preferential accumulation of the peptide in amyloid plaques in both conventional postmortem histology and in live imaging using two-photon microscopy. These findings support the contention that amyloid plaques may act as buffers to protect neurons from the toxic effects of momentary high concentrations of soluble Aβ oligomers.

In Vivo Tau Imaging for a Diagnostic Platform of Tauopathy Using the rTg4510 Mouse Line

Association of tau deposition with neurodegeneration in Alzheimer's disease (AD) and related tau-positive neurological disorders collectively referred to as tauopathies indicates contribution of tau aggregates to neurotoxicity. The discovery of tau gene mutations in FTDP-17-tau kindreds has provided unequivocal evidence that tau abnormalities alone can induce neurodegenerative disorders. Therefore, visualization of tau accumulation would offer a reliable, objective index to aid in the diagnosis of tauopathy and to assess the disease progression. Positron emission tomography (PET) imaging of tau lesions is currently available using several tau PET ligands. Because most tau PET ligands have the property of an extrinsic fluorescent dye, these ligands are considered to be useful for both PET and fluorescence imaging. In addition, small-animal magnetic resonance imaging (MRI) is available for both structural and functional imaging. Using these advanced imaging techniques, in vivo studies on a mouse model of tauopathy will provide significant insight into the translational research of neurodegenerative diseases. In this review, we will discuss the utilities of PET, MRI, and fluorescence imaging for evaluating the disease progression of tauopathy.

Chip-on-the-tip compact flexible endoscopic epifluorescence video-microscope for in-vivo imaging in medicine and biomedical research

We demonstrate a 60 mg light video-endomicroscope with a cylindrical shape of the rigid tip of only 1.6 mm diameter and 6.7 mm length. A novel implementation method of the illumination unit in the endomicroscope is presented. It allows for the illumination of the biological sample with fiber-coupled LED light at 455 nm and the imaging of the red-shifted fluorescence light above 500 nm in epi-direction. A large numerical aperture of 0.7 leads to a sub-cellular resolution and yields to high-contrast images within a field of view of 160 μm. A miniaturized chip-on-the-tip CMOS image sensor with more than 150,000 pixels captures the multicolor images at 30 fps. Considering size, plug-and-play capability, optical performance, flexibility and weight, we hence present a probe which sets a new benchmark in the field of epifluorescence endomicroscopes. Several ex-vivo and in-vivo experiments in rodents and humans suggest future application in biomedical fields, especially in the neuroscience community, as well as in medical applications targeting optical biopsies or the detection of cellular anomalies.

Label-free imaging of amyloids using their intrinsic linear and nonlinear optical properties

The optical properties of amyloid fibers are often distinct from those of the source protein in its non-fibrillar form. These differences can be utilized for label-free imaging or characterization of such structures, which is particularly important for understanding amyloid fiber related diseases such as Alzheimer's and Parkinson's disease. We demonstrate that two amyloid forming proteins, insulin and β-lactoglobulin (β-LG), show intrinsic fluorescence with emission spectra that are dependent on the excitation wavelength. Additionally, a new fluorescence peak at about 430 nm emerges for β-LG in its amyloid state. The shift in emission wavelength is related to the red edge excitation shift (REES), whereas the additional fluorescence peak is likely associated with charge delocalization along the fiber backbone. Furthermore, the spherulitic amyloid plaque-like superstructures formed from the respective proteins were imaged label-free with confocal fluorescence, multiphoton excitation fluorescence (MPEF), and second-harmonic generation (SHG) microscopy. The latter two techniques in particular yield images with a high contrast between the amyloid fiber regions and the core of amorphously structured protein. Strong multiphoton absorption (MPA) for the amyloid fibers is a likely contributor to the observed contrast in the MPEF images. The crystalline fibrillar region provides even higher contrast in the SHG images, due to the inherently ordered non-centrosymmetric structure of the fibers together with their non-isotropic arrangement. Finally, we show that MPEF from the insulin spherulites exhibits a spectral dependence on the excitation wavelength. This behavior is consistent with the REES phenomenon, which we hypothesize is the origin of this observation. The presented results suggest that amyloid deposits can be identified and structurally characterized based on their intrinsic optical properties, which is important for probe-less and label-free identification and characterization of amyloid fibers in vitro and in complex biological samples.

Long-term in vivo single-cell tracking reveals the switch of migration patterns in adult-born juxtaglomerular cells of the mouse olfactory bulb

The behavior of adult-born cells can be easily monitored in cell culture or in lower model organisms, but longitudinal observation of individual mammalian adult-born cells in their native microenvironment still proves to be a challenge. Here we have established an approach named optical cell positioning system for long-term in vivo single-cell tracking, which integrates red-green-blue cell labeling with repeated angiography. By combining this approach with in vivo two-photon imaging technique, we characterized the in vivo migration patterns of adult-born neurons in the olfactory bulb. In contrast to the traditional view of mere radial migration of adult-born cells within the bulb, we found that juxtaglomerular cells switch from radial migration to long distance lateral migration upon arrival in their destination layer. This unique long-distance lateral migration has characteristic temporal (stop-and-go) and spatial (migratory, unidirectional or multidirectional) patterns, with a clear cell age-dependent decrease in the migration speed. The active migration of adult-born cells coincides with the time period of initial fate determination and is likely to impact on the integration sites of adult-born cells, their odor responsiveness, as well as their survival rate.

Imaging of Leukocyte Trafficking in Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is characterized by a progressive decline of cognitive functions. The neuropathological features of AD include amyloid beta (Aβ) deposition, intracellular neurofibrillary tangles derived from the cytoskeletal hyperphosphorylated tau protein, amyloid angiopathy, the loss of synapses, and neuronal degeneration. In the last decade, inflammation has emerged as a key feature of AD, but most studies have focused on the role of microglia-driven neuroinflammation mechanisms. A dysfunctional blood-brain barrier has also been implicated in the pathogenesis of AD, and several studies have demonstrated that the vascular deposition of Aβ induces the expression of adhesion molecules and alters the expression of tight junction proteins, potentially facilitating the transmigration of circulating leukocytes. Two-photon laser scanning microscopy (TPLSM) has become an indispensable tool to dissect the molecular mechanisms controlling leukocyte trafficking in the central nervous system (CNS). Recent TPLSM studies have shown that vascular deposition of Aβ in the CNS promotes intraluminal neutrophil adhesion and crawling on the brain endothelium and also that neutrophils extravasate in the parenchyma preferentially in areas with Aβ deposits. These studies have also highlighted a role for LFA-1 integrin in neutrophil accumulation in the CNS of AD-like disease models, revealing that LFA-1 inhibition reduces the corresponding cognitive deficit and AD neuropathology. In this article, we consider how current imaging techniques can help to unravel new inflammation mechanisms in the pathogenesis of AD and identify novel therapeutic strategies to treat the disease by interfering with leukocyte trafficking mechanisms.

Quantitative multimodal multiparametric imaging in Alzheimer's disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder, causing changes in memory, thinking, and other dysfunction of brain functions. More and more people are suffering from the disease. Early neuroimaging techniques of AD are needed to develop. This review provides a preliminary summary of the various neuroimaging techniques that have been explored for in vivo imaging of AD. Recent advances in magnetic resonance (MR) techniques, such as functional MR imaging (fMRI) and diffusion MRI, give opportunities to display not only anatomy and atrophy of the medial temporal lobe, but also at microstructural alterations or perfusion disturbance within the AD lesions. Positron emission tomography (PET) imaging has become the subject of intense research for the diagnosis and facilitation of drug development of AD in both animal models and human trials due to its non-invasive and translational characteristic. Fluorodeoxyglucose (FDG) PET and amyloid PET are applied in clinics and research departments. Amyloid beta (Aβ) imaging using PET has been recognized as one of the most important methods for the early diagnosis of AD, and numerous candidate compounds have been tested for Aβ imaging. Besides in vivo imaging method, a lot of ex vivo modalities are being used in the AD researches. Multiphoton laser scanning microscopy, neuroimaging of metals, and several metal bioimaging methods are also mentioned here. More and more multimodality and multiparametric neuroimaging techniques should improve our understanding of brain function and open new insights into the pathophysiology of AD. We expect exciting results will emerge from new neuroimaging applications that will provide scientific and medical benefits.

Imaging Multimodalities for Dissecting Alzheimer's Disease: Advanced Technologies of Positron Emission Tomography and Fluorescence Imaging

The rapid progress in advanced imaging technologies has expanded our toolbox for monitoring a variety of biological aspects in living subjects including human. In vivo radiological imaging using small chemical tracers, such as with positron emission tomography, represents an especially vital breakthrough in the efforts to improve our understanding of the complicated cascade of neurodegenerative disorders including Alzheimer's disease (AD), and it has provided the most reliable visible biomarkers for enabling clinical diagnosis. At the same time, in combination with genetically modified animal model systems, the most recent innovation of fluorescence imaging is helping establish diverse applications in basic neuroscience research, from single-molecule analysis to animal behavior manipulation, suggesting the potential utility of fluorescence technology for dissecting the detailed molecular-based consequence of AD pathophysiology. In this review, our primary focus is on a current update of PET radiotracers and fluorescence indicators beneficial for understanding the AD cascade, and discussion of the utility and pitfalls of those imaging modalities for future translational research applications. We will also highlight current cutting-edge genetic approaches and discuss how to integrate individual technologies for further potential innovations.

Evaluation of Injured Axons Using Two-Photon Excited Fluorescence Microscopy after Spinal Cord Contusion Injury in YFP-H Line Mice

Elucidation of the process of degeneration of injured axons is important for the development of therapeutic modules for the treatment of spinal cord injuries. The aim of this study was to establish a method for time-lapse observation of injured axons in living animals after spinal cord contusion injury. YFP (yellow fluorescent protein)-H transgenic mice, which we used in this study, express fluorescence in their nerve fibers. Contusion damage to the spinal cord at the 11th vertebra was performed by IH (Infinite Horizon) impactor, which applied a pressure of 50 kdyn. The damaged spinal cords were re-exposed during the observation period under anesthesia, and then observed by two-photon excited fluorescence microscopy, which can observe deep regions of tissues including spinal cord axons. No significant morphological change of injured axons was observed immediately after injury. Three days after injury, the number of axons decreased, and residual axons were fragmented. Seven days after injury, only fragments were present in the damaged tissue. No hind-limb movement was observed during the observation period after injury. Despite the immediate paresis of hind-limbs following the contusion injury, the morphological degeneration of injured axons was delayed. This method may help clarification of pathophysiology of axon degeneration and development of therapeutic modules for the treatment of spinal cord injury.

BACE1 (β-secretase) inhibitors for the treatment of Alzheimer's disease

BACE1 (β-secretase, memapsin 2, Asp2) has emerged as a promising target for the treatment of Alzheimer's disease. BACE1 is an aspartic protease which functions in the first step of the pathway leading to the production and deposition of amyloid-β peptide (Aβ). Its gene deletion showed only mild phenotypes. BACE1 inhibition has direct implications in the Alzheimer's disease pathology without largely affecting viability. However, inhibiting BACE1 selectively in vivo has presented many challenges to medicinal chemists. Since its identification in 2000, inhibitors covering many different structural classes have been designed and developed. These inhibitors can be largely classified as either peptidomimetic or non-peptidic inhibitors. Progress in these fields resulted in inhibitors that contain many targeted drug-like characteristics. In this review, we describe structure-based design strategies and evolution of a wide range of BACE1 inhibitors including compounds that have been shown to reduce brain Aβ, rescue the cognitive decline in transgenic AD mice and inhibitor drug candidates that are currently in clinical trials.

A method for longitudinal, transcranial imaging of blood flow and remodeling of the cerebral vasculature in postnatal mice

In the weeks following birth, both the brain and the vascular network that supplies it undergo dramatic alteration. While studies of the postnatal evolution of the pial vasculature and blood flow through its vessels have been previously done histologically or acutely, here we describe a neonatal reinforced thin‐skull preparation for longitudinally imaging the development of the pial vasculature in mice using two‐photon laser scanning microscopy. Starting with mice as young as postnatal day 2 (P2), we are able to chronically image cortical areas >1 mm², repeatedly for several consecutive days, allowing us to observe the remodeling of the pial arterial and venous networks. We used this method to measure blood velocity in individual vessels over multiple days, and show that blood flow through individual pial venules was correlated with subsequent diameter changes. This preparation allows the longitudinal imaging of the developing mammalian cerebral vascular network and its physiology.

We developed a technique to longitudinally image blood vessels in the neonatal mouse cortex transcranially using two‐photon microscopy. The blood vessels on the surface of the brain undergo substantial pruning after birth. Blood flow through a vessel was correlated with the subsequent diameter change of the vessel.

Longitudinal in vivo two-photon fluorescence imaging

Fluorescence microscopy is an essential technique for the basic sciences, especially biomedical research. Since the invention of laser scanning confocal microscopy in the 1980s, which enabled imaging both fixed and living biological tissue with 3D precision, high-resolution fluorescence imaging has revolutionized biological research. Confocal microscopy, by its very nature, has one fundamental limitation. Due to the confocal pinhole, deep tissue fluorescence imaging is not practical. In contrast (no pun intended), two-photon fluorescence microscopy allows, in principle, the collection of all emitted photons from fluorophores in the imaged voxel, dramatically extending our ability to see deep into living tissue. Since the development of transgenic mice with genetically encoded fluorescent protein in neocortical cells in 2000, two-photon imaging has enabled the dynamics of individual synapses to be followed for up to 2 years. Since the initial landmark contributions to this field in 2002, the technique has been used to understand how neuronal structure are changed by experience, learning, and memory and various diseases. Here we provide a basic summary of the crucial elements that are required for such studies, and discuss many applications of longitudinal two-photon fluorescence microscopy that have appeared since 2002.

Longitudinal PET-MRI reveals β-amyloid deposition and rCBF dynamics and connects vascular amyloidosis to quantitative loss of perfusion

The dynamics of β-amyloid deposition and related second-order physiological effects, such as regional cerebral blood flow (rCBF), are key factors for a deeper understanding of Alzheimer's disease (AD). We present longitudinal in vivo data on the dynamics of β-amyloid deposition and the decline of rCBF in two different amyloid precursor protein (APP) transgenic mouse models of AD. Using a multiparametric positron emission tomography and magnetic resonance imaging approach, we demonstrate that in the presence of cerebral β-amyloid angiopathy (CAA), β-amyloid deposition is accompanied by a decline of rCBF. Loss of perfusion correlates with the growth of β-amyloid plaque burden but is not related to the number of CAA-induced microhemorrhages. However, in a mouse model of parenchymal β-amyloidosis and negligible CAA, rCBF is unchanged. Because synaptically driven spontaneous network activity is similar in both transgenic mouse strains, we conclude that the disease-related decline of rCBF is caused by CAA.

Immunotherapy alleviates amyloid-associated synaptic pathology in an Alzheimer's disease mouse model

Accumulation of amyloid-beta leads to loss of functional synapses in Alzheimer’s disease. Dorostkar et al. report that immunotherapy against oligomeric amyloid-beta in the Tg2576 mouse model attenuates synapse loss near plaques, and abolishes it elsewhere. Sequestering oligomeric amyloid-beta may counteract synaptic pathology, even while fibrillar amyloid load remains unchanged.

Cognitive decline in Alzheimer’s disease is attributed to loss of functional synapses, most likely caused by synaptotoxic, oligomeric forms of amyloid-β. Many treatment options aim at reducing amyloid-β levels in the brain, either by decreasing its production or by increasing its clearance. We quantified the effects of immunotherapy directed against oligomeric amyloid-β in Tg2576 mice, a mouse model of familial Alzheimer’s disease. Treatment of 12-month-old mice with oligomer-specific (A-887755) or conformation-unspecific (6G1) antibodies for 8 weeks did not affect fibrillar plaque density or growth. We also quantified densities of DLG4 (previously known as PSD95) expressing post-synapses and synapsin expressing presynapses immunohistochemically. We found that both pre- and post-synapses were strongly reduced in the vicinity of plaques, whereas distant from plaques, in the cortex and hippocampal CA1 field, only post-synapses were reduced. Immunotherapy alleviated this synapse loss. Synapse loss was completely abolished distant from plaques, whereas it was only attenuated in the vicinity of plaques. These results suggest that fibrillar plaques may act as reservoirs for synaptotoxic, oligomeric amyloid-β and that sequestering oligomers suffices to counteract synaptic pathology. Therefore, cognitive function may be improved by immunotherapy even when the load of fibrillar amyloid remains unchanged.

In vivo imaging reveals sigmoidal growth kinetic of β-amyloid plaques

A major neuropathological hallmark of Alzheimer’s disease is the deposition of amyloid plaques in the brains of affected individuals. Amyloid plaques mainly consist of fibrillar β-amyloid, which is a cleavage product of the amyloid precursor protein. The amyloid-cascade-hypothesis postulates Aβ accumulation as the central event in initiating a toxic cascade leading to Alzheimer’s disease pathology and, ultimately, loss of cognitive function. We studied the kinetics of β-amyloid deposition in Tg2576 mice, which overexpress human amyloid precursor protein with the Swedish mutation. Utilizing long-term two-photon imaging we were able to observe the entire kinetics of plaque growth in vivo. Essentially, we observed that plaque growth follows a sigmoid-shaped curve comprising a cubic growth phase, followed by saturation. In contrast, plaque density kinetics exhibited an asymptotic progression. Taking into account the fact that a critical concentration of Aβ is required to seed new plaques, we can propose the following kinetic model of β-amyloid deposition in vivo. In the early cubic phase, plaque growth is not limited by Aβ concentration and plaque density increases very fast. During the transition phase, plaque density stabilizes whereas plaque volume increases strongly reflecting a robust growth of the plaques. In the late asymptotic phase, Aβ peptide production becomes rate-limiting for plaque growth. In conclusion, the present study offers a direct link between in vitro and in vivo studies facilitating the translation of Aβ-lowering strategies from laboratory models to patients.

Rapid β-amyloid deposition and cognitive impairment after cholinergic denervation in APP/PS1 mice

Although extensive evidence supports the role of β-amyloid (Aβ) in Alzheimer disease (AD), the neurotoxic mechanisms underlying AD pathogenesis are not understood. On the other hand, neuronal loss is the pathologic feature that best correlates with cognitive impairment. We hypothesized that cholinergic neurodegeneration may lead to Aβ deposition and tested this by inducing selective cholinergic lesions in APPswe/PS1dE9 mice with murine p75 saporin (mu p75-SAP). Intracerebroventricular lesions that removed approximately 50% of cholinergic innervation to the cortex and hippocampus were induced in animals with incipient (∼3 months) and marked (∼7 months of age) Aβ deposition. Cranial windows were implanted, and Aβ deposition was monitored in vivo using multiphoton microscopy. Deposition of Aβ was increased as soon as 7 days after the lesion, and this effect was maintained up to 3 months later. Postmortem studies using immunohistochemistry with an anti-Aβ antibody corroborated these findings in both cerebral cortex and hippocampus. Tau phosphorylation was also significantly increased after the lesions. Cholinergic denervation resulted in early memory impairment at 3 months of age that worsened with age (∼7 months); there was a synergistic effect between cholinergic denervation and the presence of APP/PS1 transgenes. Altogether, our data suggest that cholinergic denervation may trigger Aβ deposition and synergistically contribute to cognitive impairment in AD patients.

Amyloid PET in clinical practice: Its place in the multidimensional space of Alzheimer's disease

Amyloid imaging is currently introduced to the market for clinical use. We will review the evidence demonstrating that the different amyloid PET ligands that are currently available are valid biomarkers for Alzheimer-related β amyloidosis. Based on recent findings from cross-sectional and longitudinal imaging studies using different modalities, we will incorporate amyloid imaging into a multidimensional model of Alzheimer's disease. Aside from the critical role in improving clinical trial design for amyloid-lowering drugs, we will also propose a tentative algorithm for when it may be useful in a memory clinic environment. Gaps in our evidence-based knowledge of the added value of amyloid imaging in a clinical context will be identified and will need to be addressed by dedicated studies of clinical utility.

Four-dimensional microglia response to anti-Aβ treatment in APP/PS1xCX3CR1/GFP mice

Senile plaques, mainly composed of amyloid-β (Aβ), are a major hallmark of Alzheimer disease (AD), and immunotherapy is a leading therapeutic approach for Aβ clearance. Although the ultimate mechanisms for Aβ clearance are not well known, characteristic microglia clusters are observed in the surround of senile plaques, and are implicated both in the elimination of Aβ as well as the deleterious inflammatory effects observed in AD patients after active immunization. Therefore, analyzing the direct effect of immunotherapy on microglia, using longitudinal in vivo multiphoton microscopy can provide important information regarding the role of microglia in immunotherapy. While microglia were observed to surround senile plaques, topical anti-Aβ antibody administration, which led to a reduction in plaque size, directed microglia toward senile plaques, and the overall size of microglia and number of processes were increased. In some cases, we observed clusters of microglia in areas of the brain that did not have detectable amyloid aggregates, but this did not predict the deposition of new plaques in the area within a week of imaging, indicating that microglia react to but do not precipitate amyloid aggregation. The long-term presence of large microglial clusters in the surrounding area of senile plaques suggests that microglia cannot effectively remove Aβ unless anti-Aβ antibody is administered. All together, these data suggest that although there is a role for microglia in Aβ clearance, it requires an intervention like immunotherapy to be effective.

Reducing available soluble β-amyloid prevents progression of cerebral amyloid angiopathy in transgenic mice

Cerebral amyloid angiopathy (CAA), the accumulation of β-amyloid (Aβ) in the walls of leptomeningeal and cortical blood vessels of the brain, is a major cause of intracerebral hemorrhage and cognitive impairment and is commonly associated with Alzheimer disease. The progression of CAA, as measured in transgenic mice by longitudinal imaging with multiphoton microscopy, occurs in a predictable linear manner. The dynamics of Aβ deposition in and clearance from vascular walls and their relationship to the concentration of Aβ in the brain are poorly understood. We manipulated Aβ levels in the brain using 2 approaches: peripheral clearance via administration of the amyloid binding "peripheral sink" protein gelsolin and direct inhibition of its formation via administration of LY-411575, a small-molecule γ-secretase inhibitor. We found that gelsolin and LY-411575 both reduced the rate of CAA progression in Tg2576 mice from untreated rates of 0.58% ± 0.15% and 0.52% ± 0.09% to 0.11% ± 0.18% (p = 0.04) and -0.17% ± 0.09% (p < 0.001) of affected vessel per day, respectively, in the absence of an immune response. The progression of CAA was also halted when gelsolin was combined with LY-411575 (-0.004% ± 0.10%, p < 0.003). These data suggest that CAA progression can be prevented with non-immune approaches that may reduce the availability of soluble Aβ but without evidence of substantial amyloid clearance from vessels.

Amyloid plaque formation precedes dendritic spine loss

Amyloid-beta plaque deposition represents a major neuropathological hallmark of Alzheimer's disease. While numerous studies have described dendritic spine loss in proximity to plaques, much less is known about the kinetics of these processes. In particular, the question as to whether synapse loss precedes or follows plaque formation remains unanswered. To address this question, and to learn more about the underlying kinetics, we simultaneously imaged amyloid plaque deposition and dendritic spine loss by applying two-photon in vivo microscopy through a cranial window in double transgenic APPPS1 mice. As a result, we first observed that the rate of dendritic spine loss in proximity to plaques is the same in both young and aged animals. However, plaque size only increased significantly in the young cohort, indicating that spine loss persists even many months after initial plaque appearance. Tracking the fate of individual spines revealed that net spine loss is caused by increased spine elimination, with the rate of spine formation remaining constant. Imaging of dendritic spines before and during plaque formation demonstrated that spine loss around plaques commences at least 4 weeks after initial plaque formation. In conclusion, spine loss occurs, shortly but with a significant time delay, after the birth of new plaques, and persists in the vicinity of amyloid plaques over many months. These findings hence give further hope to the possibility that there is a therapeutic window between initial amyloid plaque deposition and the onset of structural damage at spines.

Stable size distribution of amyloid plaques over the course of Alzheimer disease

Amyloid β plaques are a key pathologic feature of Alzheimer disease (AD), but whether plaque sizes increase or stabilize over the course of AD is unknown. We measured the size distribution of total immunoreactive (10D5-positive) and dense-core (Thioflavin S-positive) plaques in the temporal neocortex of a large group of subjects with AD and age-matched plaque-bearing subjects without dementia to test the hypothesis that amyloid plaques continue to grow along with the progression of the disease. The size of amyloid β (10D5)-positive plaques did not differ between groups, whereas dense-core plaques from the group with AD were slightly larger than those from the group without dementia (∼25%-30%, p = 0.01). Within the group with AD, dense-core plaque size did not independently correlate with duration of clinical disease (from 4 to 21 years, p = 0.68), whereas 10D5-positive plaque size correlated negatively with disease duration (p = 0.01). By contrast, an earlier age of symptom onset strongly predicted a larger postmortem plaque size; this effect was independent of disease duration and the presence of the APOE[Latin Small Letter Open E]4 allele (p = 0.0001). We conclude that plaques vary in size among patients, with larger size distributions correlating with an earlier age of onset, but plaques do not substantially increase in size over the clinical course of the disease.

Imaging neocortical neurons through a chronic cranial window

The rich structural dynamics of axonal arbors and neuronal circuitry can only be revealed through direct and repeated observations of the same neuron(s) over time, preferably in vivo. This protocol describes a long-term, high-resolution method for imaging neocortical neurons in vivo, using a combination of two-photon laser scanning microscopy (2PLSM) and a surgically implanted chronic cranial window. The window is used because the skull of most mammals is too opaque to allow high-resolution imaging of cortical neurons. Using this method, it is feasible to image the smallest neuronal structures in the superficial layers of the neocortex, such as dendritic spines and axonal boutons. Because the surface area of the craniotomy is relatively large, this technique is even suitable for use when labeled neurons are relatively uncommon. The surgery and imaging procedures are illustrated with examples from our studies of structural plasticity in the developing or adult mouse brain. The protocol is optimized for adult mice; we have used mice up to postnatal day 511 (P511). With minor modifications, it is possible to image neurons in rats and mice from P2. Most of our studies have used the Thy1 promoter to drive expression of fluorophores in subsets of cortical neurons.

Optical window preparation for two-photon imaging of microglia in mice

Microglia are the primary immune effector cells of the brain parenchyma. They are distributed throughout the brain at various densities. Two-photon fluorescence microscopy, together with expression of fluorescent proteins in microglia, has enabled the study of these fascinating cells in vivo. Imaging studies have shown, for example, that microglia continually survey their cellular environment and immediately respond to injury. However, we still know very little about their roles in various parts of the developing and adult brain or their diverse effector functions in aging and different disease states. Experimental procedures have been developed for minimally invasive short- and long-term two-photon imaging of microglial cells in cortical regions of the intact mouse brain. This protocol presents two methods for the preparation of the optical window that is needed for two-photon imaging of microglia. The thinned skull method should be used whenever possible. Skull thinning enables transcranial two-photon imaging while minimizing external influences that might disturb normal microglia physiology and brain homeostasis. The sealed craniotomy preparation is useful for short-term investigation of microglia.

Two-photon imaging of microglia in the mouse cortex in vivo

Microglia are the primary immune effector cells of the brain parenchyma. They are distributed throughout the brain at various densities. Two-photon fluorescence microscopy, together with expression of fluorescent proteins in microglia, has enabled study of these fascinating cells in vivo. Imaging studies have shown, for example, that microglia continually survey their cellular environment and immediately respond to injury. However, we still know very little about their roles in various parts of the developing and adult brain or their diverse effector functions in aging and different disease states. Experimental procedures have been developed for minimally invasive short- and long-term two-photon imaging of microglial cells in cortical regions of the intact mouse brain. This protocol describes two-photon imaging of microglia in the mouse cortex in vivo, using mice which have had a head plate implanted and have been prepared with either a thinned skull or optical window. Technical pitfalls, limitations, and alternative approaches are also discussed.

Longitudinal imaging of Alzheimer pathology using [11C]PIB, [18F]FDDNP and [18F]FDG PET

PURPOSE

[(11)C]PIB and [(18)F]FDDNP are PET tracers for in vivo detection of the neuropathology underlying Alzheimer's disease (AD). [(18)F]FDG is a glucose analogue and its uptake reflects metabolic activity. The purpose of this study was to examine longitudinal changes in these tracers in patients with AD or mild cognitive impairment (MCI) and in healthy controls.

METHODS

Longitudinal, paired, dynamic [(11)C]PIB and [(18)F]FDDNP (90 min each) and static [(18)F]FDG (15 min) PET scans were obtained in 11 controls, 12 MCI patients and 8 AD patients. The mean interval between baseline and follow-up was 2.5 years (range 2.0-4.0 years). Parametric [(11)C]PIB and [(18)F]FDDNP images of binding potential (BP(ND)) and [(18)F]FDG standardized uptake value ratio (SUVr) images were generated.

RESULTS

A significant increase in global cortical [(11)C]PIB BP(ND) was found in MCI patients, but no changes were observed in AD patients or controls. Subsequent regional analysis revealed that this increase in [(11)C]PIB BP(ND) in MCI patients was most prominent in the lateral temporal lobe (p < 0.05). For [(18)F]FDDNP, no changes in global BP(ND) were found. [(18)F]FDG uptake was reduced at follow-up in the AD group only, especially in frontal, parietal and lateral temporal lobes (all p < 0.01). Changes in global [(11)C]PIB binding (ρ = -0.42, p < 0.05) and posterior cingulate [(18)F]FDG uptake (ρ = 0.54, p < 0.01) were correlated with changes in Mini-Mental-State Examination score over time across groups, whilst changes in [(18)F]FDDNP binding (ρ = -0.18, p = 0.35) were not.

CONCLUSION

[(11)C]PIB and [(18)F]FDG track molecular changes in different stages of AD. We found increased amyloid load in MCI patients and progressive metabolic impairment in AD patients. [(18)F]FDDNP seems to be less useful for examining disease progression.

Chronic calcium imaging in neuronal development and disease

Neuronal circuits develop, adjust to experience and degenerate in response to injury or disease in the course of weeks and months. Available recording techniques, however, typically sample physiological properties of identified neurons on the time scale of minutes and hours. Thus, in order to obtain a full understanding of a long term physiological process data need to be extrapolated from numerous experimental sessions and animals, often collected blindly and under variable conditions. The generation and ongoing engineering of genetically encoded calcium indicators creates an opportunity to repeatedly record activity from the same individual neurons in vivo over weeks, months and potentially the entire lifetime of a model organism. Chronic calcium imaging with genetically encoded indicators thus may allow to establish functional biographies of identified neuronal cell types in the brain and to reveal the physiological relevance of structural changes as they occur under natural and pathological conditions.

Chronic in vivo imaging in the mouse spinal cord using an implanted chamber

Understanding and treatment of spinal cord pathology is limited in part by a lack of longitudinal in vivo imaging strategies at the cellular level. We developed a chronically implanted spinal chamber and surgical procedure suitable for time-lapse in vivo multiphoton microscopy of mouse spinal cord without the need for repeat surgical procedures. Repeated imaging was routinely achieved for more than five weeks post-operatively with up to ten separate imaging sessions. We observed neither motor function deficit nor neuropathology in the spinal cord as a result of chamber implantation. Using this chamber we quantified microglia and afferent axon dynamics following a laser-induced spinal cord lesion and observed massive microglia infiltration within one day along with a heterogeneous dieback of axon stumps. By enabling chronic imaging studies over timescales ranging from minutes to months, our method offers an ideal platform for understanding cellular dynamics in response to injury and therapeutic interventions.

Spines, plasticity, and cognition in Alzheimer's model mice

The pathological hallmarks of Alzheimer's disease (AD)--widespread synaptic and neuronal loss and the pathological accumulation of amyloid-beta peptide (Aβ) in senile plaques, as well as hyperphosphorylated tau in neurofibrillary tangles--have been known for many decades, but the links between AD pathology and dementia and effective therapeutic strategies remain elusive. Transgenic mice have been developed based on rare familial forms of AD and frontotemporal dementia, allowing investigators to test in detail the structural, functional, and behavioral consequences of AD-associated pathology. Here, we review work on transgenic AD models that investigate the degeneration of dendritic spine structure, synaptic function, and cognition. Together, these data support a model of AD pathogenesis in which soluble Aβ initiates synaptic dysfunction and loss, as well as pathological changes in tau, which contribute to both synaptic and neuronal loss. These changes in synapse structure and function as well as frank synapse and neuronal loss contribute to the neural system dysfunction which causes cognitive deficits. Understanding the underpinnings of dementia in AD will be essential to develop and evaluate therapeutic approaches for this widespread and devastating disease.

Transcranial two-photon imaging of the living mouse brain

This protocol describes imaging of the living mouse brain through a thinned skull using two-photon microscopy. This transcranial two-photon laser-scanning microscope (TPLSM) imaging method allows high-resolution imaging of fluorescently labeled neurons, microglia, astrocytes, and blood vessels, as well as subcellular structures such as dendritic spines and axonal varicosities. The surgical procedure that is required to allow imaging thins the cranium so that it becomes optically transparent. Once learned, the surgery can be performed in ∼30 min, and imaging can follow immediately. The procedure can be repeated multiple times, allowing brain cells and tissues to be studied in the same animals over short or long time intervals, depending on the design of the experiment. Two-photon imaging through a thinned and intact skull avoids side effects caused by skull removal and is a minimally invasive method for studying the living mouse brain under physiological and pathological conditions.

Increase in the density of resting microglia precedes neuritic plaque formation and microglial activation in a transgenic model of Alzheimer's disease

The formation of cerebral senile plaques composed of amyloid β peptide (Aβ) is a fundamental feature of Alzheimer's disease (AD). Glial cells and more specifically microglia become reactive in the presence of Aβ. In a triple transgenic model of AD (3 × Tg-AD), we found a significant increase in activated microglia at 12 (by 111%) and 18 (by 88%) months of age when compared with non-transgenic (non-Tg) controls. This microglial activation correlated with Aβ plaque formation, and the activation in microglia was closely associated with Aβ plaques and smaller Aβ deposits. We also found a significant increase in the area density of resting microglia in 3 × Tg-AD animals both at plaque-free stage (at 9 months by 105%) and after the development of A plaques (at 12 months by 54% and at 18 months by 131%). Our results show for the first time that the increase in the density of resting microglia precedes both plaque formation and activation of microglia by extracellular Aβ accumulation. We suggest that AD pathology triggers a complex microglial reaction: at the initial stages of the disease the number of resting microglia increases, as if in preparation for the ensuing activation in an attempt to fight the extracellular Aβ load that is characteristic of the terminal stages of the disease.