Spontaneous vasomotion propagates along pial arterioles in the awake mouse brain like stimulus-evoked vascular reactivity

Sensory stimulation evokes a local, vasodilation-mediated blood flow increase to the activated brain region, which is referred to as functional hyperemia. Spontaneous vasomotion is a change in arteriolar diameter that occurs without sensory stimulation, at low frequency (∼0.1 Hz). These vessel diameter changes are a driving force for perivascular soluble waste clearance, the failure of which has been implicated in neurodegenerative disease. Stimulus-evoked vascular reactivity is known to propagate along penetrating arterioles to pial arterioles, but it is unclear whether spontaneous vasomotion propagates similarly. We therefore imaged both stimulus-evoked and spontaneous changes in pial arteriole diameter in awake, head-fixed mice with 2-photon microscopy. By cross-correlating different regions of interest (ROIs) along the length of imaged arterioles, we assessed vasomotion propagation. We found that both during rest and during visual stimulation, one-third of the arterioles showed significant propagation (i.e., a wave), with a median (interquartile range) wave speed of 405 (323) µm/s at rest and 345 (177) µm/s during stimulation. In a second group of mice, with GCaMP expression in their vascular smooth muscle cells, we also found spontaneous propagation of calcium signaling along pial arterioles. In summary, we demonstrate that spontaneous vasomotion propagates along pial arterioles like stimulus-evoked vascular reactivity.

Calcium transients in nNOS neurons underlie distinct phases of the neurovascular response to barrel cortex activation in awake mice

Neuronal nitric oxide (NO) synthase (nNOS), a Ca2+ dependent enzyme, is expressed by distinct populations of neocortical neurons. Although neuronal NO is well known to contribute to the blood flow increase evoked by neural activity, the relationships between nNOS neurons activity and vascular responses in the awake state remain unclear. We imaged the barrel cortex in awake, head-fixed mice through a chronically implanted cranial window. The Ca2+ indicator GCaMP7f was expressed selectively in nNOS neurons using adenoviral gene transfer in nNOScre mice. Air-puffs directed at the contralateral whiskers or spontaneous motion induced Ca2+ transients in 30.2 ± 2.2% or 51.6 ± 3.3% of nNOS neurons, respectively, and evoked local arteriolar dilation. The greatest dilatation (14.8 ± 1.1%) occurred when whisking and motion occurred simultaneously. Ca2+ transients in individual nNOS neurons and local arteriolar dilation showed various degrees of correlation, which was strongest when the activity of whole nNOS neuron ensemble was examined. We also found that some nNOS neurons became active immediately prior to arteriolar dilation, while others were activated gradually after arteriolar dilatation. Discrete nNOS neuron subsets may contribute either to the initiation or to the maintenance of the vascular response, suggesting a previously unappreciated temporal specificity to the role of NO in neurovascular coupling.

NAD(P)H autofluorescence lifetime imaging enables single cell analyses of cellular metabolism of osteoblasts in vitro and in vivo via two-photon microscopy

Two-photon fluorescence lifetime microscopy (2P-FLIM) is a non-invasive optical technique that can obtain cellular metabolism information based on the intrinsic autofluorescence lifetimes of free and enzyme-bound NAD(P)H, which reflect the metabolic state of single cells within the native microenvironment of the living tissue. NAD(P)H 2P-FLIM was initially performed in bone marrow stromal cell (BMSC) cultures established from Col (I) 2.3GFP or OSX-mCherry mouse models, in which osteoblastic lineage cells were labelled with green or red fluorescence protein, respectively. Measurement of the mean NAD(P)H lifetime, τ_M, demonstrated that osteoblasts in osteogenic media had a progressively increased τ_M compared to cells in regular media, suggesting that osteoblasts undergoing mineralization had higher NAD⁺/NAD(P)H ratio and may utilize more oxidative phosphorylation (OxPhos). In vivo NAD(P)H 2P-FLIM was conducted in conjunction with two-photon phosphorescence lifetime microscopy (2P-PLIM) to evaluate cellular metabolism of GFP⁺ osteoblasts as well as bone tissue oxygen at different locations of the native cranial bone in Col (I) 2.3GFP mice. Our data showed that osteocytes dwelling within lacunae had higher τ_M than osteoblasts at the bone edge of suture and marrow space. Measurement of pO₂ showed poor correlation of pO₂ and τ_M in native bone. However, when NAD(P)H 2P-FLIM was used to examine osteoblast cellular metabolism at the leading edge of the cranial defects during repair in Col (I) 2.3GFP mouse model, a significantly lower τ_M was recorded, which was associated with lower pO₂ at an early stage of healing, indicating an impact of hypoxia on energy metabolism during bone tissue repair. Taken together, our current study demonstrates the feasibility of using non-invasive optical NAD(P)H 2P-FLIM technique to examine cellular energy metabolism at single cell resolution in living animals. Our data further support that both glycolysis and OxPhos are being used in the osteoblasts, with more mature osteoblasts exhibiting higher ratio of NAD⁺/NAD(P)H, indicating a potential change of energy mode during differentiation. Further experiments utilizing animals with genetic modification of cellular metabolism could enhance our understanding of energy metabolism in various cell types in living bone microenvironment.

Nitric oxide controls proliferation of Leishmania major by inhibiting the recruitment of permissive host cells

Nitric oxide (NO) is an important antimicrobial effector but also prevents unnecessary tissue damage by shutting down the recruitment of monocyte-derived phagocytes. Intracellular pathogens such as Leishmania major can hijack these cells as a niche for replication. Thus, NO might exert containment by restricting the availability of the cellular niche required for efficient pathogen proliferation. However, such indirect modes of action remain to be established. By combining mathematical modeling with intravital 2-photon biosensors of pathogen viability and proliferation, we show that low L. major proliferation results not from direct NO impact on the pathogen but from reduced availability of proliferation-permissive host cells. Although inhibiting NO production increases recruitment of these cells, and thus pathogen proliferation, blocking cell recruitment uncouples the NO effect from pathogen proliferation. Therefore, NO fulfills two distinct functions for L. major containment: permitting direct killing and restricting the supply of proliferation-permissive host cells.

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Direct killing of L. major by NO occurs only during the peak of the immune response

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Efficient L. major proliferation requires newly recruited monocyte-derived cells

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Loss of NO production increases both pathogen proliferation and monocyte recruitment

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NO dampens L. major proliferation indirectly, limiting the pathogen’s cellular niche

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

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

Neurovascular coupling and bilateral connectivity during NREM and REM sleep

To understand how arousal state impacts cerebral hemodynamics and neurovascular coupling, we monitored neural activity, behavior, and hemodynamic signals in un-anesthetized, head-fixed mice. Mice frequently fell asleep during imaging, and these sleep events were interspersed with periods of wake. During both NREM and REM sleep, mice showed large increases in cerebral blood volume ([HbT]) and arteriole diameter relative to the awake state, two to five times larger than those evoked by sensory stimulation. During NREM, the amplitude of bilateral low-frequency oscillations in [HbT] increased markedly, and coherency between neural activity and hemodynamic signals was higher than the awake resting and REM states. Bilateral correlations in neural activity and [HbT] were highest during NREM, and lowest in the awake state. Hemodynamic signals in the cortex are strongly modulated by arousal state, and changes during sleep are substantially larger than sensory-evoked responses.

Multiphoton Microscopy for the Characterization of Cellular Behavior on Naturally Derived Polysaccharide Tissue Constructs With Irregular Surfaces for the Development of Platform Biomaterials

Over the past decade, the use of polymers as platform materials for biomedical applications including tissue engineering has been of rising interest. Recently, the use of naturally derived polysaccharides as 3-D scaffolds for tissue regeneration has shown promising material characteristics; however, due to complexities in composition, morphology, and optical properties, adequate spatial and temporal characterization of cellular behavior in these materials is lacking. Multiphoton microscopy has emerged as a viable tool for performing such quantification by permitting greater imaging depth while simultaneously minimizing un-favorable scattering and producing high-resolution optical cross sections for non-invasive analysis. Here we describe a method using endogenous contrast of cellulose nanofibers (CNF) using Second Harmonic Generation (SHG), combined with 2-photon fluorescence of Cell Tracker Orange for spatial and longitudinal imaging of cellular proliferation. Cell Tracker Orange is an ideal fluorophore to avoid the broad CNF autofluorescence allowing for segmentation of cells using a semi-automatic routine. Individual cells were identified using centroid locations for 3D cell proliferation. Overall, the methods presented are viable for investigation of cellular interactions with polysaccharide candidate biomaterials.

Dendritic Spines in the Spinal Cord: Live Action Pain

Dendritic spines are microscopic protrusions on neurons that house the postsynaptic machinery necessary for neurotransmission between neurons. As such, dendritic spine structure is intimately linked with synaptic function. In pathology, dendritic spine behavior and its contribution to disease are not firmly understood. It is well known that dendritic spines are highly dynamic in vivo. In our recent publication, we used an intravital imaging approach, which permitted us to repeatedly visualize the same neurons located in lamina II, a nociceptive processing region of the spinal cord. Using this imaging platform, we analyzed the intravital dynamics of dendritic spine structure before and after nerve injury-induced pain. This effort revealed a time-dependent relationship between the progressive increase in pain outcome, and a switch in the steady-state fluctuations of dendritic spine structure. Collectively, our in vivo study demonstrates how injury that leads to abnormal pain may also contribute to synapse-associated structural remodeling in nociceptive regions of the spinal cord dorsal horn. By combining our live-imaging approach with measures of neuronal activity, such as with the use of calcium or other voltage-sensitive dyes, we expect to gain a more complete picture of the relationship between dendritic spine structure and nociceptive physiology.

Input-Output Relationship of CA1 Pyramidal Neurons Reveals Intact Homeostatic Mechanisms in a Mouse Model of Fragile X Syndrome

Cellular hyperexcitability is a salient feature of fragile X syndrome animal models. The cellular basis of hyperexcitability and how it responds to changing activity states is not fully understood. Here, we show increased axon initial segment length in CA1 of the Fmr1^-/y mouse hippocampus, with increased cellular excitability. This change in length does not result from reduced AIS plasticity, as prolonged depolarization induces changes in AIS length independent of genotype. However, depolarization does reduce cellular excitability, the magnitude of which is greater in Fmr1^-/y neurons. Finally, we observe reduced functional inputs from the entorhinal cortex, with no genotypic difference in the firing rates of CA1 pyramidal neurons. This suggests that AIS-dependent hyperexcitability in Fmr1^-/y mice may result from adaptive or homeostatic regulation induced by reduced functional synaptic connectivity. Thus, while AIS length and intrinsic excitability contribute to cellular hyperexcitability, they may reflect a homeostatic mechanism for reduced synaptic input onto CA1 neurons.

In Vivo Imaging of Venous Thrombus and Pulmonary Embolism Using Novel Murine Venous Thromboembolism Model

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We established a novel clinically relevant murine DVT model at femoral/saphenous vein induced by flow restriction and light illumination.

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Our model newly succeeded in inducing DVT in a valve pocket and enabled spontaneous pulmonary embolism of fibrin-rich thrombus from lower extremity vein, reproducing the clinical VTE scenario.

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This model is suitable for motion-free in vivo high-resolution imaging of fibrin-rich DVT development and organization using 2-photon microscopy, enabling the real-time imaging of migration of platelets and leukocytes into the erythrocyte-rich DVT.

This work established a new murine venous thromboembolism (VTE) model. This model has multiple novel features representing clinical VTE that include the following: 1) deep venous thrombosis (DVT) was formed and extended in the long axis of femoral/saphenous vein; 2) thrombus was formed in a venous valve pocket; 3) deligation of suture-induced spontaneous pulmonary emboli of fibrin-rich DVT; and 4) cardiac motion-free femoral/saphenous vein allowed high-resolution intravital microscopic imaging of fibrin-rich DVT. This new model requires only commercially available epifluorescence microscopy. Therefore, this model has significant potential for better understanding of VTE pathophysiology.

Interstitial Migration of CD8αβ T Cells in the Small Intestine Is Dynamic and Is Dictated by Environmental Cues

The migratory capacity of adaptive CD8αβ T cells dictates their ability to locate target cells and exert cytotoxicity, which is the basis of immune surveillance for the containment of microbes and disease. The small intestine (SI) is the largest mucosal surface and is a primary site of pathogen entrance. Using two-photon laser scanning microscopy, we found that motility of antigen (Ag)-specific CD8αβ T cells in the SI is dynamic and varies with the environmental milieu. Pathogen-specific CD8αβ T cell movement differed throughout infection, becoming locally confined at memory. Motility was not dependent on CD103 but was influenced by micro-anatomical locations within the SI and by inflammation. CD8 T cells responding to self-protein were initially affected by the presence of self-Ag, but this was altered after complete tolerance induction. These studies identify multiple factors that affect CD8αβ T cell movement in the intestinal mucosa and show the adaptability of CD8αβ T cell motility.

Comparative Evaluation of Genetically Encoded Voltage Indicators

Imaging voltage using fluorescent-based sensors could be an ideal technique to probe neural circuits with high spatiotemporal resolution. However, due to insufficient signal-to-noise ratio (SNR), imaging membrane potential in mammalian preparations is still challenging. In recent years, many genetically encoded voltage indicators (GEVIs) have been developed. To compare them and guide decisions on which GEVI to use, we have characterized side by side the performance of eight GEVIs that represent different families of molecular constructs. We tested GEVIs in vitro with 1-photon imaging and in vivo with 1-photon wide-field imaging and 2-photon imaging. We find that QuasAr2 exhibited the best performance in vitro, whereas only ArcLight-MT could be used to reliably detect electrical activity in vivo with 2-photon excitation. No single GEVI was ideal for every experiment. These results provide a guide for choosing optimal GEVIs for specific applications.

The regional-dependent biaxial behavior of young and aged mouse skin: A detailed histomechanical characterization, residual strain analysis, and constitutive model

Skin fulfills several vital functions, many of which are dependent on its mechanical properties. Therefore, as mice have become an invaluable model for skin research, determining murine skin's mechanical properties is important. Specifically, skin's mechanical properties are important for functional tests as well as for prognostic and diagnostic purposes. Additionally, computational simulations of skin behavior are becoming commonplace, rendering accurate models of murine skin's constitutive behavior necessary. To date, our knowledge of mouse skin mechanics shows significant gaps. For example, there are no comprehensive reports correlating skin's mechanical properties with region, age, and direction. Moreover, mouse skin's residual strain behavior has not been reported on. In our current work, we set out to fill these gaps. Based on histology, 2-photon microscopy, and planar biaxial testing, while accurately tracking various reference configurations, we report on differences in gross structure, microstructural organization, and constitutive response of skin, and cast those properties into a versatile Fung-type hyperelastic constitutive law for three reference configurations. Our data is the most comprehensive report contrasting the mechanical properties of young (12 weeks) and aged (52 weeks) mouse skin and will, thus, be valuable to basic science as control data, and provide accurate constitutive laws for mouse skin modeling. STATEMENT OF SIGNIFICANCE: Our findings are significant as they fill several gaps in our understanding of mouse skin mechanics. This is particularly important as mouse skin is becoming a frequent and critical model of human skin for cosmetic and medical science. Specifically, we quantified how mechanical properties of mice skin vary with age, with location, and with direction. Additionally, we cast our findings into constitutive models that can be used by others for predictive computer simulations of skin behavior.

Long-term in vivo microscopy of CAR T cell dynamics during eradication of CNS lymphoma in mice

Primary central nervous system lymphoma (PCNSL) is a highly malignant brain tumor with limited treatment options. Here, we show that genetically engineered T cells, expressing a chimeric antigen receptor, thoroughly infiltrate these tumors in mice. Combining intravital 2-photon microscopy with chronic cranial windows, we were able to visualize their intratumoral proliferation and intracerebral persistence for up to 159 d, leading to the eradication of large, established PCNSL and to long-term survival.

T cells expressing anti-CD19 chimeric antigen receptors (CARs) demonstrate impressive efficacy in the treatment of systemic B cell malignancies, including B cell lymphoma. However, their effect on primary central nervous system lymphoma (PCNSL) is unknown. Additionally, the detailed cellular dynamics of CAR T cells during their antitumor reaction remain unclear, including their intratumoral infiltration depth, mobility, and persistence. Studying these processes in detail requires repeated intravital imaging of precisely defined tumor regions during weeks of tumor growth and regression. Here, we have combined a model of PCNSL with in vivo intracerebral 2-photon microscopy. Thereby, we were able to visualize intracranial PCNSL growth and therapeutic effects of CAR T cells longitudinally in the same animal over several weeks. Intravenous (i.v.) injection resulted in poor tumor infiltration of anti-CD19 CAR T cells and could not sufficiently control tumor growth. After intracerebral injection, however, anti-CD19 CAR T cells invaded deeply into the solid tumor, reduced tumor growth, and induced regression of PCNSL, which was associated with long-term survival. Intracerebral anti-CD19 CAR T cells entered the circulation and infiltrated distant, nondraining lymph nodes more efficiently than mock CAR T cells. After complete regression of tumors, anti-CD19 CAR T cells remained detectable intracranially and intravascularly for up to 159 d. Collectively, these results demonstrate the great potential of anti-CD19 CAR T cells for the treatment of PCNSL.

Spontaneous Activity Patterns Are Altered in the Developing Visual Cortex of the Fmr1 Knockout Mouse

Fragile X syndrome (FXS) is the most prevalent inherited cause of autism and is accompanied by behavioral and sensory deficits. Errors in the wiring of the brain during early development likely contribute to these deficits, but the underlying mechanisms are unclear. Spontaneous activity patterns, which are required for fine-tuning neuronal networks before the senses become active, are perturbed in rodent models of FXS. Here, we investigated spontaneous network activity patterns in the developing visual cortex of the Fmr1 knockout mouse using in vivo calcium imaging during the second postnatal week, before eye opening. We found that while the frequency, mean amplitude and duration of spontaneous network events were unchanged in the knockout mouse, pair-wise correlations between neurons were increased compared to wild type littermate controls. Further analysis revealed that interneuronal correlations were not generally increased, rather that low-synchronization events occurred relatively less frequently than high-synchronization events. Low-, but not high-, synchronization events have been associated with retinal inputs previously. Since we found that spontaneous retinal waves were normal in the knockout, our results suggest that peripherally driven activity is underrepresented in the Fmr1 KO visual cortex. Therefore, we propose that central gating of retinal inputs may be affected in FXS and that peripherally and centrally driven activity patterns are already unbalanced before eye opening in this disorder.

Distinct Mechanisms for Visual and Motor-Related Astrocyte Responses in Mouse Visual Cortex

Astrocytes are a major cell type in the mammalian nervous system, are in close proximity to neurons, and show rich Ca²⁺ activity thought to mediate cellular outputs. Astrocytes show activity linked to sensory [1, 2] and motor [3, 4] events, reflecting local neural activity and brain-wide neuromodulatory inputs. Sensory responses are highly variable [5, 6, 7, 8, 9, 10], which may reflect interactions between distinct input types [6, 7, 9]. However, the diversity of inputs generating astrocyte activity, particularly during sensory stimulation and behavior, is not fully understood [11, 12]. Using a combination of Ca²⁺ imaging, a treadmill assay, and visual stimulation, we examined the properties of astrocyte activity in mouse visual cortex associated with motor or sensory events. Consistent with previous work, motor activity activated astrocytes across the cortex with little specificity, reflecting a diffuse neuromodulatory mechanism. In contrast, moving visual stimuli generated specific activity patterns that reflected the stimulus' trajectory within the visual field, precisely as one would predict if astrocytes reported local neural activity. Visual responses depended strongly on behavioral state, with astrocytes showing high amplitude Ca²⁺ transients during locomotion and little activity during stillness. Furthermore, the amplitudes of visual responses were highly correlated with pupil size, suggesting a role of arousal. Interestingly, while depletion of cortical noradrenaline abolished locomotor responses, visual responses were only reduced in amplitude and their spatiotemporal organization remained intact, suggesting two distinct types of inputs underlie visual responses. We conclude that cortical astrocytes integrate local sensory information and behavioral state, suggesting a role in information processing.

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Astrocytes of moving mice display robust retinotopic responses to visual stimuli

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Sensory responses are distinguishable from responses to locomotion

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Vision-driven responses are correlated to arousal

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Vision- and arousal-driven responses are differentially regulated by noradrenaline

Understanding the axonal response to injury by in vivo imaging in the mouse spinal cord: A tale of two branches

Understanding the basic properties of how axons respond to injury in the mammalian central nervous system (CNS) is of fundamental value for developing strategies to promote neural repair. Axons possess complex morphologies with stereotypical branching patterns. However, current knowledge of the axonal response to injury gives little consideration to axonal branches, nor do strategies to promote axon regeneration. This article reviews evidence from in vivo spinal cord imaging that axonal branches markedly impact the degenerative and regenerative responses to injury. At a major bifurcation point, depending on whether one or both axonal branches are injured, neurons may choose either a more self-preservative response or a more dynamic response. The stabilizing effect of the spared branch may underlie a well-known divergence in neuronal responses to injury, and illustrates an example where in vivo spinal cord imaging reveals insights that are difficult to elucidate with conventional histological methods.

Global analysis and Decay Associated Images (DAI) derived from Fluorescence Lifetime Imaging Microscopy (FLIM)

The extraction of fluorophore lifetimes in a biological sample provides useful information about the probe environment that is not readily available from fluorescence intensity alone. Cell membrane potential, pH, concentration of oxygen ([O₂]), calcium ([Ca²⁺]), NADH and other ions and metabolites are all regularly measured by lifetime-based techniques. These measurements provide invaluable knowledge about cell homeostasis, metabolism and communication with the cell environment. Fluorescence lifetime imaging microscopy (FLIM) produces spatial maps with time-correlated single-photon counting (TCSPC) histograms collected and analyzed at each pixel, but traditional TCSPC analysis is often hampered by the low number of photons that can reasonably be collected while maintaining high spatial resolution. More important, traditional analysis fails to employ the spatial linkages within the image. Here, we present a different approach, where we work under the assumption that mixtures of a global set of lifetimes (often only 2 or 3) can describe the entire image. We determine these lifetime components by globally fitting precise decays aggregated over large spatial regions of interest, and then we perform a pixel-by-pixel calculation of decay amplitudes (via simple linear algebra applied to coarser time-windows). This yields accurate amplitude images (Decay Associate Images, DAI) that contain stoichiometric information about the underlying mixtures while retaining single pixel resolution. We collected FLIM data of dye mixtures and bacteria expressing fluorescent proteins with a two-photon microscope system equipped with a commercial single-photon counting card, and we used these data to benchmark the gDAI program.

Single-Cell Electroporation for Measuring In Vivo Calcium Dynamics in Microglia

Similar to many nonexcitable cells, microglia utilize intracellular Ca²⁺ signaling for the communication with each other as well as neurons and astrocytes and for triggering a magnitude of their executive functions. However, in vivo measurements of the intracellular Ca²⁺ dynamics in microglia have been challenging due to technical reasons. Here, we describe an approach utilizing a single-cell electroporation technique to facilitate the study of microglial Ca²⁺ signaling in the living brain.

Morphology-Based Distinction Between Healthy and Pathological Cells Utilizing Fourier Transforms and Self-Organizing Maps

The appearance and the movements of immune cells are driven by their environment. As a reaction to a pathogen invasion, the immune cells are recruited to the site of inflammation and are activated to prevent a further spreading of the invasion. This is also reflected by changes in the behavior and the morphological appearance of the immune cells. In cancerous tissue, similar morphokinetic changes have been observed in the behavior of microglial cells: intra-tumoral microglia have less complex 3-dimensional shapes, having less-branched cellular processes, and move more rapidly than those in healthy tissue. The examination of such morphokinetic properties requires complex 3D microscopy techniques, which can be extremely challenging when executed longitudinally. Therefore, the recording of a static 3D shape of a cell is much simpler, because this does not require intravital measurements and can be performed on excised tissue as well. However, it is essential to possess analysis tools that allow the fast and precise description of the 3D shapes and allows the diagnostic classification of healthy and pathogenic tissue samples based solely on static, shape-related information. Here, we present a toolkit that analyzes the discrete Fourier components of the outline of a set of 2D projections of the 3D cell surfaces via Self-Organizing Maps. The application of artificial intelligence methods allows our framework to learn about various cell shapes as it is applied to more and more tissue samples, whilst the workflow remains simple.

Comparison between reflectance confocal microscopy and 2-photon microscopy in early detection of cutaneous radiation injury in a mouse model in vivo

Cutaneous radiation injury (CRI) is a skin injury caused by high-dose exposure of ionizing radiation (IR). For proper treatment, early detection of CRI before clinical symptoms is important. Optical microscopic techniques such as reflectance confocal microscopy (RCM) and 2-photon microscopy (TPM) have been tested as the early diagnosis method by detecting cellular changes. In this study, RCM and TPM were compared in the detection of cellular changes caused by CRI in an in vivo mouse model. CRI was induced on the mouse hindlimb skin with various IR doses and the injured skin regions were imaged longitudinally by both modalities until the onset of clinical symptoms. Both RCM and TPM detected the changes of epidermal cells and sebaceous glands before clinical symptoms in different optical contrasts. RCM detected changes of cell morphology and scattering property based on light reflection. TPM detected detail changes of cellular structures based on autofluorescence of cells. Since both RCM and TPM were sensitive to the early stage CRI by using different contrasts, the optimal method for clinical CRI diagnosis could be either individual methods or their combination.

Long-term In Vivo Calcium Imaging of Astrocytes Reveals Distinct Cellular Compartment Responses to Sensory Stimulation

Localized, heterogeneous calcium transients occur throughout astrocytes, but the characteristics and long-term stability of these signals, particularly in response to sensory stimulation, remain unknown. Here, we used a genetically encoded calcium indicator and an activity-based image analysis scheme to monitor astrocyte calcium activity in vivo. We found that different subcellular compartments (processes, somata, and endfeet) displayed distinct signaling characteristics. Closer examination of individual signals showed that sensory stimulation elevated the number of specific types of calcium peaks within astrocyte processes and somata, in a cortical layer-dependent manner, and that the signals became more synchronous upon sensory stimulation. Although mice genetically lacking astrocytic IP3R-dependent calcium signaling (Ip3r2-/-) had fewer signal peaks, the response to sensory stimulation was sustained, suggesting other calcium pathways are also involved. Long-term imaging of astrocyte populations revealed that all compartments reliably responded to stimulation over several months, but that the location of the response within processes may vary. These previously unknown characteristics of subcellular astrocyte calcium signals provide new insights into how astrocytes may encode local neuronal circuit activity.

Multiphoton Intravital Calcium Imaging

Multiphoton intravital calcium imaging is a powerful technique that enables high-resolution longitudinal monitoring of cellular and subcellular activity hundreds of microns deep in the living organism. This unit addresses the application of 2-photon microscopy to imaging of genetically encoded calcium indicators (GECIs) in the mouse brain. The protocols in this unit enable real-time intravital imaging of intracellular calcium concentration simultaneously in hundreds of neurons, or at the resolution of single synapses, as mice respond to sensory stimuli or perform behavioral tasks. Protocols are presented for implantation of a cranial imaging window to provide optical access to the brain and for 2-photon image acquisition. Protocols for implantation of both open skull and thinned skull windows for single or multi-session imaging are described. © 2018 by John Wiley & Sons, Inc.

Identification of polystyrene nanoparticle penetration across intact skin barrier as rare event at sites of focal particle aggregations

The question whether nanoparticles can cross the skin barrier is highly debated. Even in intact skin rare events of deeper penetration have been reported, but technical limitations and possible artifacts require careful interpretation. In this study, horizontal scanning by 2-photon microscopy (2 PM) of full-thickness human skin samples placed in a lateral position yielded highly informative images for skin penetration studies of fluorescently tagged nanoparticles. Scanning of large fields of view allowed for detailed information on interfollicular and follicular penetration in tissue blocks without damaging the sample. Images in histomorphological correlation showed that 2P-excited fluorescence signals of fluorescently tagged 20 and 200 nm polystyrene nanoparticles preferentially accumulated in the stratum corneum (SC) and in the upper part of vellus hair follicles (HFs). Rare events of deeper penetration in the SC and in the infundibulum of vellus HFs were observed at sites of high focal particle aggregations. Wide-field 2 PM allows for imaging of nanoparticle penetration in large tissue blocks, whereas total internal reflection microscopy (TIRFM) enables selective detection of individual nanoparticles as well as clusters of nanoparticles in the SC and within the epidermal layer directly beneath the SC, thus confirming barrier crossing with high sensitivity.

Primary Traumatic Axonopathy in Mice Subjected to Impact Acceleration: A Reappraisal of Pathology and Mechanisms with High-Resolution Anatomical Methods

Traumatic axonal injury (TAI) is a common neuropathology in traumatic brain injury and is featured by primary injury to axons. Here, we generated TAI with impact acceleration of the head in male Thy1-eYFP-H transgenic mice in which specific populations of neurons and their axons are labeled with yellow fluorescent protein. This model results in axonal lesions in multiple axonal tracts along with blood-brain barrier disruption and neuroinflammation. The corticospinal tract, a prototypical long tract, is severely affected and is the focus of this study. Using optimized CLARITY at single-axon resolution, we visualized the entire corticospinal tract volume from the pons to the cervical spinal cord in 3D and counted the total number of axonal lesions and their progression over time. Our results divulged the presence of progressive traumatic axonopathy that was maximal at the pyramidal decussation. The perikarya of injured corticospinal neurons atrophied, but there was no evidence of neuronal cell death. We also used CLARITY at single-axon resolution to explore the role of the NMNAT2-SARM1 axonal self-destruction pathway in traumatic axonopathy. When we interfered with this pathway by genetically ablating SARM1 or by pharmacological strategies designed to increase levels of Nicotinamide (Nam), a feedback inhibitor of SARM1, we found a significant reduction in the number of axonal lesions early after injury. Our findings show that high-resolution neuroanatomical strategies reveal important features of TAI with biological implications, especially the progressive axonopathic nature of TAI and the role of the NMNAT2-SARM1 pathway in the early stages of axonopathy.SIGNIFICANCE STATEMENT In the first systematic application of novel high-resolution neuroanatomical tools in neuropathology, we combined CLARITY with 2-photon microscopy, optimized for detection of single axonal lesions, to reconstruct the injured mouse brainstem in a model of traumatic axonal injury (TAI) that is a common pathology associated with traumatic brain injury. The 3D reconstruction of the corticospinal tract at single-axon resolution allowed for a more advanced level of qualitative and quantitative understanding of TAI. Using this model, we showed that TAI is an axonopathy with a prominent role of the NMNAT2-SARM1 molecular pathway, that is also implicated in peripheral neuropathy. Our results indicate that high-resolution anatomical models of TAI afford a level of detail and sensitivity that is ideal for testing novel molecular and biomechanical hypotheses.

Microglia modulation through external vagus nerve stimulation in a murine model of Alzheimer's disease

Chronically activated microglia contribute to the development of neurodegenerative diseases such as Alzheimer's disease (AD) by the release of pro-inflammatory mediators that compromise neuronal function and structure. Modulating microglia functions could be instrumental to interfere with disease pathogenesis. Previous studies have shown anti-inflammatory effects of acetylcholine (ACh) or norepinephrine (NE), which mainly activates the β-receptors on microglial cells. Non-invasive vagus nerve stimulation (nVNS) is used in treatment of drug-resistant depression, which is a risk factor for developing AD. The vagus nerve projects to the brainstem's locus coeruleus from which noradrenergic fibers reach to the Nucleus Basalis of Meynert (NBM) and widely throughout the brain. Pilot studies showed first signs of cognitive-enhancing effects of nVNS in AD patients. In this study, the effects of nVNS on mouse microglia cell morphology were analyzed over a period of 280 min by 2-photon laser scanning in vivo microscopy. Total branch length, average branch order and number of branches, which are commonly used indicators for the microglial activation state were determined and compared between young and old wild-type and amyloid precursor protein/presenilin-1 (APP/PS1) transgenic mice. Overall, these experiments show strong morphological changes in microglia, from a neurodestructive to a neuroprotective phenotype, following a brief nVNS in aged animals, especially in APP/PS1 animals, whereas microglia from young animals were morphologically unaffected.

Altered Synapse Stability in the Early Stages of Tauopathy

Synapse loss is a key feature of dementia, but it is unclear whether synaptic dysfunction precedes degenerative phases of the disease. Here, we show that even before any decrease in synapse density, there is abnormal turnover of cortical axonal boutons and dendritic spines in a mouse model of tauopathy-associated dementia. Strikingly, tauopathy drives a mismatch in synapse turnover; postsynaptic spines turn over more rapidly, whereas presynaptic boutons are stabilized. This imbalance between pre- and post-synaptic stability coincides with reduced synaptically driven neuronal activity in pre-degenerative stages of the disease.

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Density of cortical axonal boutons and dendritic spines is reduced early in tauopathy

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Abnormalities in synaptic stability and size exist before decreases in synapse density

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Turnover of dendritic spines is elevated, whereas presynaptic boutons are stabilized

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Neuronal activity is reduced at stages associated with mismatched synaptic turnover

The SNARE Protein Syntaxin-1a Plays an Essential Role in Biphasic Exocytosis of the Incretin Hormone Glucagon-Like Peptide 1

Exocytosis of the hormone glucagon-like peptide 1 (GLP-1) by the intestinal L cell is essential for the incretin effect after nutrient ingestion and is critical for the actions of dipeptidyl peptidase 4 inhibitors that enhance GLP-1 levels in patients with type 2 diabetes. Two-photon microscopy revealed that exocytosis of GLP-1 is biphasic, with a first peak at 1-6 min and a second peak at 7-12 min after stimulation with forskolin. Approximately 75% of the exocytotic events were represented by compound granule fusion, and the remainder were accounted for by full fusion of single granules under basal and stimulated conditions. The core SNARE protein syntaxin-1a (syn1a) was expressed by murine ileal L cells. At the single L-cell level, first-phase forskolin-induced exocytosis was reduced to basal (P < 0.05) and second-phase exocytosis abolished (P < 0.05) by syn1a knockout. L cells from intestinal-epithelial syn1a-deficient mice demonstrated a 63% reduction in forskolin-induced GLP-1 release in vitro (P < 0.001) and a 23% reduction in oral glucose-stimulated GLP-1 secretion (P < 0.05) in association with impairments in glucose-stimulated insulin release (by 60%; P < 0.01) and glucose tolerance (by 20%; P < 0.01). The findings identify an exquisite mechanism of metered secretory output that precisely regulates release of the incretin hormone GLP-1 and hence insulin secretion after a meal.

Dynamic intravital imaging of cell-cell interactions in the lymph node

In the last decade, the application of 2-photon intravital microscopy as a tool to study cell interactions in different areas of the immune system has offered an unprecedented opportunity to understand the complexity of cell behavior in relation to immune functions. In this review we describe the latest advances in the field of live imaging in the lymph nodes, grouping the different cell populations in 2 compartments according to their motility: the sessile compartment, which is formed by resident cells of stromal origin, macrophages, and resident dendritic cells, and the motile compartment, which is mainly formed by T and B lymphocytes. Here we review how the use of in vivo imaging has contributed to our understanding of the role of these cells in the initiation of the immune response in the draining lymph nodes.

Cell type specificity of neurovascular coupling in cerebral cortex

Identification of the cellular players and molecular messengers that communicate neuronal activity to the vasculature driving cerebral hemodynamics is important for (1) the basic understanding of cerebrovascular regulation and (2) interpretation of functional Magnetic Resonance Imaging (fMRI) signals. Using a combination of optogenetic stimulation and 2-photon imaging in mice, we demonstrate that selective activation of cortical excitation and inhibition elicits distinct vascular responses and identify the vasoconstrictive mechanism as Neuropeptide Y (NPY) acting on Y1 receptors. The latter implies that task-related negative Blood Oxygenation Level Dependent (BOLD) fMRI signals in the cerebral cortex under normal physiological conditions may be mainly driven by the NPY-positive inhibitory neurons. Further, the NPY-Y1 pathway may offer a potential therapeutic target in cerebrovascular disease.

The Formation of Microthrombi in Parenchymal Microvessels after Traumatic Brain Injury Is Independent of Coagulation Factor XI

Microthrombus formation and bleeding worsen the outcome after traumatic brain injury (TBI). The aim of the current study was to characterize these processes in the brain parenchyma after experimental TBI and to determine the involvement of coagulation factor XI (FXI). C57BL/6 mice (n = 101) and FXI-deficient mice (n = 15) were subjected to controlled cortical impact (CCI). Wild-type mice received an inhibitory antibody against FXI (14E11) or control immunoglobulin G 24 h before or 30 or 120 min after CCI. Cerebral microcirculation was visualized in vivo by 2-photon microscopy 2-3 h post-trauma and histopathological outcome was assessed after 24 h. TBI induced hemorrhage and microthrombus formation in the brain parenchyma (p < 0.001). Inhibition of FXI activation or FXI deficiency did not reduce cerebral thrombogenesis, lesion volume, or hemispheric swelling. However, it also did not increase intracranial hemorrhage. Formation of microthrombosis in the brain parenchyma after TBI is independent of the intrinsic coagulation cascade since it was not reduced by inhibition of FXI. However, since targeting FXI has well-established antithrombotic effects in humans and experimental animals, inhibition of FXI could represent a reasonable strategy for the prevention of deep venous thrombosis in immobilized patients with TBI.

Laser-Mediated Microlesions in Mouse Neocortex to Investigate Neuronal Degeneration and Regeneration

In vivo two-photon (2P) imaging enables neural circuitry to be repeatedly visualized in both normal conditions and following trauma. This protocol describes how laser-mediated neuronal microlesions can be created in the cerebral cortex using an ultrafast laser without causing a significant inflammatory reaction or compromising the blood-brain barrier. Furthermore, directives are provided for the acute and chronic in vivo imaging of the lesion site, as well as for post-hoc analysis of the lesion site in fixed tissue, which can be correlated with the live imaging phase.

Angiopoietin-4 increases permeability of blood vessels and promotes lymphatic dilation

The angiopoietin (Ang) ligands are potential therapeutic targets for lymphatic related diseases, which include lymphedema and cancer. Ang-1 and Ang-2 functions are established, but those of Ang-4 are poorly understood. We used intravital fluorescence microscopy to characterize Ang-4 actions on T241 murine fibrosarcoma-associated vessels in mice. The diameters of lymphatic vessels draining Ang-4- or VEGF-C (positive control)-expressing tumors increased to 123 and 135 μm, respectively, and parental, mock-transduced (negative controls) and tumors expressing Ang-1 or Ang-2 remained at baseline (∼60 μm). Ang-4 decreased human dermal lymphatic endothelial cell (LEC) monolayer permeability by 27% while increasing human dermal blood endothelial cell (BEC) monolayer permeability by 200%. In vivo, Ang-4 stimulated a 4.5-fold increase in tumor-associated blood vessel permeability compared with control when measured using intravital quantitative multiphoton microscopy. Ang-4 activated receptor signaling in both LECs and BECs, evidenced by tyrosine kinase with Ig and endothelial growth factor homology domains-2 (TIE2) receptor, protein kinase B, and Erk1,2 phosphorylation detectable by immunoblotting. These data suggest that Ang-4 actions are mediated through cell-type-specific networks and that lymphatic vessel dilation occurs secondarily to increased vascular leakage. Ang-4 also promoted survival of LECs. Thus, blocking Ang-4 may prune the draining lymphatic vasculature and decrease interstitial fluid pressure (IFP) by reducing vascular permeability.

Identification of the Vascular Source of Vasogenic Brain Edema following Traumatic Brain Injury Using In Vivo 2-Photon Microscopy in Mice

Vasogenic brain edema due to vascular leakage is one of the most important factors determining the clinical outcome of patients following acute brain injury. To date, performing a detailed in vivo quantification of vascular leakage has not been possible. Here, we used in vivo 2-photon microscopy (2-PM) to determine the spatial (3D) and temporal development of vasogenic brain edema following traumatic brain injury (TBI) in mice; in addition, we identified the vessel types involved in vascular leakage. Thirteen male Tie2-GFP mice (6-8 weeks old) were subjected to controlled cortical impact (CCI) or a sham operation; subsequently, a cranial window was prepared adjacent to the injury site, and tetramethylrhodamine-dextran (TMRM, 40 mg/kg, MW 40,000) was injected intravenously to visualize blood plasma leakage. Parenchymal fluorescence intensity was monitored in three regions for 2-4 h post-CCI, reaching from the surface of the brain to a depth of 300 μm, and TMRM leakage was measured as an increase in TMRM fluorescence intensity outside the vessel lumen and in the parenchyma. In the CCI group, vascular leakage was detected in all investigated regions as early as 2.5 h post-injury. This leakage increased over time and was more pronounced proximal to the primary contusion. Both arterioles and venules contributed similarly to brain edema formation and their contribution was independent of vessel size; however, capillaries were the major contributor to leakage. In summary, using 2-PM to perform in vivo 3D deep-brain imaging, we found that TBI induces vascular leakage from capillaries, venules, and arterioles. Thus, all three vessel types are involved in trauma-induced brain edema and should be considered when developing novel therapies for preventing vasogenic brain edema.

Imaging CD8+ T cells during diverse viral infections

CD8+ T cells play a critical role in host defense against pathogens and tumors. Much of our current knowledge of the activation and subsequent effector activities of CD8+ T cells has been gained using ex vivo approaches examining the T cell population en masse for surface phenotype, activation status and the production of effector molecules. Thus, the precise behaviors and diversity of individual CD8+ T cells responding to virus infection in vivo have not been extensively explored, leaving many unanswered questions relevant to the rational design of antiviral vaccines and therapeutics. Recently, intravital multiphoton microscopy (MPM) has been used to image CD8+ T cell priming after infection with disparate viral pathogens ranging from small RNA viruses encoding few proteins to DNA viruses producing hundreds of viral proteins (many immunomodulatory). After priming, effector CD8+ T cells have been visualized in virus-infected tissue, both during primary infection and after transitioning to tissue resident memory cells (TRM). Here, I highlight recent advances in our understanding of antiviral CD8+ T cell responses revealed through intravital MPM.

Bitter tasting compounds dilate airways by inhibiting airway smooth muscle calcium oscillations and calcium sensitivity

BACKGROUND AND PURPOSE

While selective, bitter tasting, TAS2R agonists can relax agonist-contracted airway smooth muscle (ASM), their mechanism of action is unclear. However, ASM contraction is regulated by Ca²⁺ signalling and Ca²⁺ sensitivity. We have therefore investigated how the TAS2R10 agonists chloroquine, quinine and denotonium regulate contractile agonist-induced Ca²⁺ signalling and sensitivity.

EXPERIMENTAL APPROACH

Airways in mouse lung slices were contracted with either methacholine (MCh) or 5HT and bronchodilation assessed using phase-contrast microscopy. Ca²⁺ signalling was measured with 2-photon fluorescence microscopy of ASM cells loaded with Oregon Green, a Ca²⁺-sensitive indicator (with or without caged-IP₃). Effects on Ca²⁺ sensitivity were assessed on lung slices treated with caffeine and ryanodine to permeabilize ASM cells to Ca²⁺ .

KEY RESULTS

The TAS2R10 agonists dilated airways constricted by either MCh or 5HT, accompanied by inhibition of agonist-induced Ca²⁺ oscillations. However, in non-contracted airways, TAS2R10 agonists, at concentrations that maximally dilated constricted airways, did not evoke Ca²⁺ signals in ASM cells. Ca²⁺ increases mediated by the photolysis of caged-IP₃ were also attenuated by chloroquine, quinine and denotonium. In Ca²⁺-permeabilized ASM cells, the TAS2R10 agonists dilated MCh- and 5HT-constricted airways.

CONCLUSIONS AND IMPLICATIONS

TAS2R10 agonists reversed bronchoconstriction by inhibiting agonist-induced Ca²⁺ oscillations while simultaneously reducing the Ca²⁺ sensitivity of ASM cells. Reduction of Ca²⁺ oscillations may be due to inhibition of Ca²⁺ release through IP₃ receptors. Further characterization of bronchodilatory TAS2R agonists may lead to the development of novel therapies for the treatment of bronchoconstrictive conditions.

An evolving autoimmune microenvironment regulates the quality of effector T cell restimulation and function

Defining the processes of autoimmune attack of tissues is important for inhibiting continued tissue destruction. In type 1 diabetes, it is not known how cytotoxic effector T cell responses evolve over time in the pancreatic islets targeted for destruction. We used two-photon microscopy of live, intact, individual islets to investigate how progression of islet infiltration altered the behavior of infiltrating islet-specific CD8(+) T cells. During early-islet infiltration, T-cell interactions with CD11c(+) antigen-presenting cells (APCs) were stable and real-time imaging of T cell receptor (TCR) clustering provided evidence of TCR recognition in these stable contacts. Early T cell-APC encounters supported production of IFN-γ by T effectors, and T cells at this stage also killed islet APCs. At later stages of infiltration, T-cell motility accelerated, and cytokine production was lost despite the presence of higher numbers of infiltrating APCs that were able to trigger T-cell signaling in vitro. Using timed introduction of effector T cells, we demonstrate that elements of the autoimmune-tissue microenvironment control the dynamics of autoantigen recognition by T cells and their resulting pathogenic effector functions.

Neurovascular Network Explorer 1.0: a database of 2-photon single-vessel diameter measurements with MATLAB(®) graphical user interface

We present a database client software—Neurovascular Network Explorer 1.0 (NNE 1.0)—that uses MATLAB^® based Graphical User Interface (GUI) for interaction with a database of 2-photon single-vessel diameter measurements from our previous publication (Tian et al., 2010). These data are of particular interest for modeling the hemodynamic response. NNE 1.0 is downloaded by the user and then runs either as a MATLAB script or as a standalone program on a Windows platform. The GUI allows browsing the database according to parameters specified by the user, simple manipulation and visualization of the retrieved records (such as averaging and peak-normalization), and export of the results. Further, we provide NNE 1.0 source code. With this source code, the user can database their own experimental results, given the appropriate data structure and naming conventions, and thus share their data in a user-friendly format with other investigators. NNE 1.0 provides an example of seamless and low-cost solution for sharing of experimental data by a regular size neuroscience laboratory and may serve as a general template, facilitating dissemination of biological results and accelerating data-driven modeling approaches.

Functional improvement and maturation of rat and human engineered heart tissue by chronic electrical stimulation

Spontaneously beating engineered heart tissue (EHT) represents an advanced in vitro model for drug testing and disease modeling, but cardiomyocytes in EHTs are less mature and generate lower forces than in the adult heart. We devised a novel pacing system integrated in a setup for videooptical recording of EHT contractile function over time and investigated whether sustained electrical field stimulation improved EHT properties. EHTs were generated from neonatal rat heart cells (rEHT, n=96) or human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (hEHT, n=19). Pacing with biphasic pulses was initiated on day 4 of culture. REHT continuously paced for 16-18 days at 0.5Hz developed 2.2× higher forces than nonstimulated rEHT. This was reflected by higher cardiomyocyte density in the center of EHTs, increased connexin-43 abundance as investigated by two-photon microscopy and remarkably improved sarcomere ultrastructure including regular M-bands. Further signs of tissue maturation include a rightward shift (to more physiological values) of the Ca(2+)-response curve, increased force response to isoprenaline and decreased spontaneous beating activity. Human EHTs stimulated at 2Hz in the first week and 1.5Hz thereafter developed 1.5× higher forces than nonstimulated hEHT on day 14, an ameliorated muscular network of longitudinally oriented cardiomyocytes and a higher cytoplasm-to-nucleus ratio. Taken together, continuous pacing improved structural and functional properties of rEHTs and hEHTs to an unprecedented level. Electrical stimulation appears to be an important step toward the generation of fully mature EHT.

Super-resolution 2-photon microscopy reveals that the morphology of each dendritic spine correlates with diffusive but not synaptic properties

The structure of dendritic spines suggests a specialized function in compartmentalizing synaptic signals near active synapses. Indeed, theoretical and experimental analyses indicate that the diffusive resistance of the spine neck is sufficient to effectively compartmentalize some signaling molecules in a spine for the duration of their activated lifetime. Here we describe the application of 2-photon microscopy combined with stimulated emission depletion (STED-2P) to the biophysical study of the relationship between synaptic signals and spine morphology, demonstrating the utility of combining STED-2P with modern optical and electrophysiological techniques. Morphological determinants of fluorescence recovery time were identified and evaluated within the context of a simple compartmental model describing diffusive transfer between spine and dendrite. Correlations between the neck geometry and the amplitude of synaptic potentials and calcium transients evoked by 2-photon glutamate uncaging were also investigated.

Fluorescence microscopy gets faster and clearer: roles of photochemistry and selective illumination

Significant advances in fluorescence microscopy tend be a balance between two competing qualities wherein improvements in resolution and low light detection are typically accompanied by losses in acquisition rate and signal-to-noise, respectively. These trade-offs are becoming less of a barrier to biomedical research as recent advances in optoelectronic microscopy and developments in fluorophore chemistry have enabled scientists to see beyond the diffraction barrier, image deeper into live specimens, and acquire images at unprecedented speed. Selective plane illumination microscopy has provided significant gains in the spatial and temporal acquisition of fluorescence specimens several mm in thickness. With commercial systems now available, this method promises to expand on recent advances in 2-photon deep-tissue imaging with improved speed and reduced photobleaching compared to laser scanning confocal microscopy. Superresolution microscopes are also available in several modalities and can be coupled with selective plane illumination techniques. The combination of methods to increase resolution, acquisition speed, and depth of collection are now being married to common microscope systems, enabling scientists to make significant advances in live cell and in situ imaging in real time. We show that light sheet microscopy provides significant advantages for imaging live zebrafish embryos compared to laser scanning confocal microscopy.

The Bee as a Model to Investigate Brain and Behavioural Asymmetries

The honeybee Apis mellifera, with a brain of only 960,000 neurons and the ability to perform sophisticated cognitive tasks, has become an excellent model in life sciences and in particular in cognitive neurosciences. It has been used in our laboratories to investigate brain and behavioural asymmetries, i.e., the different functional specializations of the right and the left sides of the brain. It is well known that bees can learn to associate an odour stimulus with a sugar reward, as demonstrated by extension of the proboscis when presented with the trained odour in the so-called Proboscis Extension Reflex (PER) paradigm. Bees recall this association better when trained using their right antenna than they do when using their left antenna. They also retrieve short-term memory of this task better when using the right antenna. On the other hand, when tested for long-term memory recall, bees respond better when using their left antenna. Here we review a series of behavioural studies investigating bees’ lateralization, integrated with electrophysiological measurements to study asymmetries of olfactory sensitivity, and discuss the possible evolutionary origins of these asymmetries. We also present morphological data obtained by scanning electron microscopy and two-photon microscopy. Finally, a behavioural study conducted in a social context is summarised, showing that honeybees control context-appropriate social interactions using their right antenna, rather than the left, thus suggesting that lateral biases in behaviour might be associated with requirements of social life.

An experimental protocol for in vivo imaging of neuronal structural plasticity with 2-photon microscopy in mice

INTRODUCTION

Structural plasticity with synapse formation and elimination is a key component of memory capacity and may be critical for functional recovery after brain injury. Here we describe in detail two surgical techniques to create a cranial window in mice and show crucial points in the procedure for long-term repeated in vivo imaging of synaptic structural plasticity in the mouse neocortex.

METHODS

Transgenic Thy1-YFP(H) mice expressing yellow-fluorescent protein (YFP) in layer-5 pyramidal neurons were prepared under anesthesia for in vivo imaging of dendritic spines in the parietal cortex either with an open-skull glass or thinned skull window. After a recovery period of 14 days, imaging sessions of 45-60 min in duration were started under fluothane anesthesia. To reduce respiration-induced movement artifacts, the skull was glued to a stainless steel plate fixed to metal base. The animals were set under a two-photon microscope with multifocal scanhead splitter (TriMScope, LaVision BioTec) and the Ti-sapphire laser was tuned to the optimal excitation wavelength for YFP (890 nm). Images were acquired by using a 20×, 0.95 NA, water-immersion objective (Olympus) in imaging depth of 100-200 μm from the pial surface. Two-dimensional projections of three-dimensional image stacks containing dendritic segments of interest were saved for further analysis. At the end of the last imaging session, the mice were decapitated and the brains removed for histological analysis.

RESULTS

Repeated in vivo imaging of dendritic spines of the layer-5 pyramidal neurons was successful using both open-skull glass and thinned skull windows. Both window techniques were associated with low phototoxicity after repeated sessions of imaging.

CONCLUSIONS

Repeated imaging of dendritic spines in vivo allows monitoring of long-term structural dynamics of synapses. When carefully controlled for influence of repeated anesthesia and phototoxicity, the method will be suitable to study changes in synaptic structural plasticity after brain injury.

Neurovascular coupling: in vivo optical techniques for functional brain imaging

Optical imaging techniques reflect different biochemical processes in the brain, which is closely related with neural activity. Scientists and clinicians employ a variety of optical imaging technologies to visualize and study the relationship between neurons, glial cells and blood vessels. In this paper, we present an overview of the current optical approaches used for the in vivo imaging of neurovascular coupling events in small animal models. These techniques include 2-photon microscopy, laser speckle contrast imaging (LSCI), voltage-sensitive dye imaging (VSDi), functional photoacoustic microscopy (fPAM), functional near-infrared spectroscopy imaging (fNIRS) and multimodal imaging techniques. The basic principles of each technique are described in detail, followed by examples of current applications from cutting-edge studies of cerebral neurovascular coupling functions and metabolic. Moreover, we provide a glimpse of the possible ways in which these techniques might be translated to human studies for clinical investigations of pathophysiology and disease. In vivo optical imaging techniques continue to expand and evolve, allowing us to discover fundamental basis of neurovascular coupling roles in cerebral physiology and pathophysiology.

Ionotropic receptors at hippocampal mossy fibers: roles in axonal excitability, synaptic transmission, and plasticity

Dentate granule cells process information from the enthorinal cortex en route to the hippocampus proper. These neurons have a very negative resting membrane potential and are relatively silent in the slice preparation. They are also subject to strong feed-forward inhibition. Their unmyelinated axon or mossy fiber ramifies extensively in the hilus and projects to stratum lucidum where it makes giant en-passant boutons with CA3 pyramidal neurons. There is compelling evidence that mossy fiber boutons express presynaptic GABA_A receptors, which are commonly found in granule cell dendrites. There is also suggestive evidence for the presence of other ionotropic receptors, including glycine, NMDA, and kainate receptors, in mossy fiber boutons. These presynaptic receptors have been proposed to lead to mossy fiber membrane depolarization. How this phenomenon alters the excitability of synaptic boutons, the shape of presynaptic action potentials, Ca²⁺ influx and neurotransmitter release has remained elusive, but high-resolution live imaging of individual varicosities and direct patch-clamp recordings have begun to shed light on these phenomena. Presynaptic GABA_A and kainate receptors have also been reported to facilitate the induction of long-term potentiation at mossy fiber—CA3 synapses. Although mossy fibers are highly specialized, some of the principles emerging at this connection may apply elsewhere in the CNS.

Diffusion properties of molecules at the blood-brain interface: potential contributions of astrocyte endfeet to diffusion barrier functions

Molecular diffusion in the extracellular space (ECS) plays a key role in determining tissue physiology and pharmacology. The blood-brain barrier regulates the exchange of substances between the brain and the blood, but the diffusion properties of molecules at this blood-brain interface, particularly around the astrocyte endfeet, are poorly characterized. In this study, we used 2-photon microscopy and acute brain slices of mouse neocortex and directly assessed the diffusion patterns of fluorescent molecules. By observing the diffusion of unconjugated and 10-kDa dextran-conjugated Alexa Fluor 488 from the ECS of the brain parenchyma to the blood vessels, we find various degrees of diffusion barriers at the endfeet: Some allow the invasion of dye inside the endfoot network while others completely block it. Detailed analyses of the time course for dye clearance support the existence of a tight endfoot network capable of acting as a diffusion barrier. Finally, we show that this diffusion pattern collapses under pathological conditions. These data demonstrate the heterogeneous nature of molecular diffusion dynamics around the endfeet and suggest that these structures can serve as the diffusion barrier. Therefore, astrocyte endfeet may add another layer of regulation to the exchange of molecules between blood vessels and brain parenchyma.

Review of advanced imaging techniques

Pathology informatics encompasses digital imaging and related applications. Several specialized microscopy techniques have emerged which permit the acquisition of digital images (“optical biopsies”) at high resolution. Coupled with fiber-optic and micro-optic components, some of these imaging techniques (e.g., optical coherence tomography) are now integrated with a wide range of imaging devices such as endoscopes, laparoscopes, catheters, and needles that enable imaging inside the body. These advanced imaging modalities have exciting diagnostic potential and introduce new opportunities in pathology. Therefore, it is important that pathology informaticists understand these advanced imaging techniques and the impact they have on pathology. This paper reviews several recently developed microscopic techniques, including diffraction-limited methods (e.g., confocal microscopy, 2-photon microscopy, 4Pi microscopy, and spatially modulated illumination microscopy) and subdiffraction techniques (e.g., photoactivated localization microscopy, stochastic optical reconstruction microscopy, and stimulated emission depletion microscopy). This article serves as a primer for pathology informaticists, highlighting the fundamentals and applications of advanced optical imaging techniques.

Whole isolated neocortical and hippocampal preparations and their use in imaging studies

This study shows that two whole isolated preparations from the young mouse, the neocortical 'slab' and the hippocampal formation, are useful for imaging studies requiring both global monitoring using light transmittance (LT) imaging and high resolution cellular monitoring using 2-photon laser scanning microscopy (2PLSM). These preparations share advantages with brain slices such as maintaining intrinsic neuronal properties and avoiding cardiac or respiratory movement. Important additional advantages include the maintenance of all local input and output pathways, the absence of surfaces injured by slicing and the preservation of three-dimensional tissue structure. Using evoked extracellular field recording, we demonstrate long-term (hours) viability of both whole preparations. We then show that propagating cortical events such as anoxic depolarization (AD) and spreading depression (SD) can be imaged in both preparations, yielding results comparable to those in brain slices but retaining the tissue's three-dimensional structure. Using transgenic mice expressing green fluorescent protein (GFP) in pyramidal and granule cell neurons, 2PLSM confirms that these preparations are free of the surface damage observed in sliced brain tissue. Moreover the neurons undergo swelling with accompanying dendritic beading following AD induced by simulated ischemia, similar to cortical damage described in vivo.