Maintaining live cells and tissue slices in the imaging setup

Live-cell single-molecule fluorescence microscopy for protruding organelles reveals regulatory mechanisms of MYO7A-driven cargo transport in stereocilia of inner ear hair cells

Stereocilia are unidirectional F-actin-based cylindrical protrusions on the apical surface of inner ear hair cells and function as biological mechanosensors of sound and acceleration. Development of functional stereocilia requires motor activities of unconventional myosins to transport proteins necessary for elongating the F-actin cores and to assemble the mechanoelectrical transduction (MET) channel complex. However, how each myosin localizes in stereocilia using the energy from ATP hydrolysis is only partially understood. In this study, we develop a methodology for live-cell single-molecule fluorescence microscopy of organelles protruding from the apical surface using a dual-view light-sheet microscope, diSPIM. We demonstrate that MYO7A, a component of the MET machinery, traffics as a dimer in stereocilia. Movements of MYO7A are restricted when scaffolded by the plasma membrane and F-actin as mediated by MYO7A's interacting partners. Here, we discuss the technical details of our methodology and its future applications including analyses of cargo transportation in various organelles.

Live-cell single-molecule fluorescence microscopy for protruding organelles reveals regulatory mechanisms of MYO7A-driven cargo transport in stereocilia of inner ear hair cells

Stereocilia are unidirectional F-actin-based cylindrical protrusions on the apical surface of inner ear hair cells and function as biological mechanosensors of sound and acceleration. Development of functional stereocilia requires motor activities of unconventional myosins to transport proteins necessary for elongating the F-actin cores and to assemble the mechanoelectrical transduction (MET) channel complex. However, how each myosin localizes in stereocilia using the energy from ATP hydrolysis is only partially understood. In this study, we develop a methodology for live-cell single-molecule fluorescence microscopy of organelles protruding from the apical surface using a dual-view light-sheet microscope, diSPIM. We demonstrate that MYO7A, a component of the MET machinery, traffics as a dimer in stereocilia. Movements of MYO7A are restricted when scaffolded by the plasma membrane and F-actin as mediated by MYO7A's interacting partners. Here, we discuss the technical details of our methodology and its future applications including analyses of cargo transportation in various organelles.

Using Ex Vivo Live Imaging to Investigate Cell Divisions and Movements During Mouse Dental Renewal

The continuously growing mouse incisor is emerging as a highly tractable model system to investigate the regulation of adult epithelial and mesenchymal stem cells and tooth regeneration. These progenitor populations actively divide, move, and differentiate to maintain tissue homeostasis and regenerate lost cells in a responsive manner. However, traditional analyses using fixed tissue sections could not capture the dynamic processes of cellular movements and interactions, limiting our ability to study their regulations. This paper describes a protocol to maintain whole mouse incisors in an explant culture system and live-track dental epithelial cells using multiphoton timelapse microscopy. This technique adds to our existing toolbox for dental research and allows investigators to acquire spatiotemporal information on cell behaviors and organizations in a living tissue. We anticipate that this methodology will help researchers further explore mechanisms that control the dynamic cellular processes taking place during both dental renewal and regeneration.

Advances in Confocal Microscopy and Selected Applications

Over the last 30 years, confocal microscopy has emerged as a primary tool for biological investigation across many disciplines. The simplicity of use and widespread accessibility of confocal microscopy ensure that it will have a prominent place in biological imaging for many years to come, even with the recent advances in light sheet and field synthesis microscopy. Since these more advanced technologies still require significant expertise to effectively implement and carry through to analysis, confocal microscopy-based approaches still remain the easiest way for biologists with minimal imaging experience to address fundamental questions about how their systems are arranged through space and time. In this review, we discuss a number of advanced applications of confocal microscopy for probing the spatiotemporal dynamics of biological systems.

Two-way communication between ex vivo tissues on a microfluidic chip: application to tumor-lymph node interaction

Experimentally accessible tools to replicate the complex biological events of in vivo organs offer the potential to reveal mechanisms of disease and potential routes to therapy. In particular, models of inter-organ communication are emerging as the next essential step towards creating a body-on-a-chip, and may be particularly useful for poorly understood processes such as tumor immunity. In this paper, we report the first multi-compartment microfluidic chip that continuously recirculates a small volume of media through two ex vivo tissue samples to support inter-organ cross-talk via secreted factors. To test on-chip communication, protein release and capture were quantified using well-defined artificial tissue samples and model proteins. Proteins released by one sample were transferred to the downstream reservoir and detectable in the downstream sample. Next, the chip was applied to model the communication between a tumor and a lymph node, to test whether on-chip dual-organ culture could recreate key features of tumor-induced immune suppression. Slices of murine lymph node were co-cultured with tumor or healthy tissue on-chip with recirculating media, then tested for their ability to respond to T cell stimulation. Interestingly, lymph node slices co-cultured with tumor slices appeared more immunosuppressed than those co-cultured with healthy tissue, suggesting that the chip may successfully model some features of tumor-immune interaction. In conclusion, this new microfluidic system provides on-chip co-culture of pairs of tissue slices under continuous recirculating flow, and has the potential to model complex inter-organ communication ex vivo with full experimental accessibility of the tissues and their media.

Preparation of organotypic brain slice cultures for the study of Alzheimer's disease

Alzheimer's disease, the most common cause of dementia, is a progressive neurodegenerative disorder characterised by amyloid-beta deposits in extracellular plaques, intracellular neurofibrillary tangles of aggregated tau, synaptic dysfunction and neuronal death.

Transgenic rodent models to study Alzheimer’s mimic features of human disease such as age-dependent accumulation of abnormal beta-amyloid and tau, synaptic dysfunction, cognitive deficits and neurodegeneration. These models have proven vital for improving our understanding of the molecular mechanisms underlying AD and for identifying promising therapeutic approaches. However, modelling neurodegenerative disease in animals commonly involves aging animals until they develop harmful phenotypes, often coupled with invasive procedures.

We have developed a novel organotypic brain slice culture model to study Alzheimer’s disease using 3xTg-AD mice which brings the potential of substantially reducing the number of rodents used in dementia research from an estimated 20,000 per year. Using a McIllwain tissue chopper, we obtain 36 x 350 micron slices from each P8-P9 mouse pup for culture between 2 weeks and 6 months on semi-permeable 0.4 micron pore membranes, considerably reducing the numbers of animals required to investigate multiple stages of disease. This tractable model also allows the opportunity to modulate multiple pathways in tissues from a single animal. We believe that this model will most benefit dementia researchers in the academic and drug discovery sectors.

We validated the slice culture model against aged mice, showing that the molecular phenotype closely mimics that displayed in vivo, albeit in an accelerated timescale. We showed beneficial outcomes following treatment of slices with agents previously shown to have therapeutic effects in vivo, and we also identified new mechanisms of action of other compounds. Thus, organotypic brain slice cultures from transgenic mouse models expressing Alzheimer’s disease-related genes may provide a valid and sensitive replacement for in vivo studies that do not involve behavioural analysis.

Imaging morphogenesis

The hostile environment of the microscope stage poses numerous challenges to successful imaging of morphogenesis in live tissues. This review aims to highlight some of the main practical considerations to take into account when embarking on a project to image cell behaviour in the context of cells' normal surroundings. Scrutiny of these activities is likely to be the most informative approach to understanding mechanical morphogenesis but is often confounded by the substantial technical difficulties involved in imaging samples over extended periods of time. Repeated observation of cells in live tissue requires that strategies be adopted to prioritize the stability of the sample, ensuring that it remains viable and develops normally while being held in a manner accessible to microscopic examination. Key considerations when creating reliable protocols for time-lapse imaging may be broken down into three main criteria; labelling, mounting and image acquisition. Choices and compromises made here, however, will directly influence image quality, and even small refinements can substantially improve what information may be extracted from images. Live imaging of tissue is difficult but paying close attention to the basics along with a little innovation is likely to be well rewarded.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

Developmental changes in microglial mobilization are independent of apoptosis in the neonatal mouse hippocampus

During CNS development, microglia transform from highly mobile amoeboid-like cells to primitive ramified forms and, finally, to highly branched but relatively stationary cells in maturity. The factors that control developmental changes in microglia are largely unknown. Because microglia detect and clear apoptotic cells, developmental changes in microglia may be controlled by neuronal apoptosis. Here, we assessed the extent to which microglial cell density, morphology, motility, and migration are regulated by developmental apoptosis, focusing on the first postnatal week in the mouse hippocampus when the density of apoptotic bodies peaks at postnatal day 4 and declines sharply thereafter. Analysis of microglial form and distribution in situ over the first postnatal week showed that, although there was little change in the number of primary microglial branches, microglial cell density increased significantly, and microglia were often seen near or engulfing apoptotic bodies. Time-lapse imaging in hippocampal slices harvested at different times over the first postnatal week showed differences in microglial motility and migration that correlated with the density of apoptotic bodies. The extent to which these changes in microglia are driven by developmental neuronal apoptosis was assessed in tissues from BAX null mice lacking apoptosis. We found that apoptosis can lead to local microglial accumulation near apoptotic neurons in the pyramidal cell body layer but, unexpectedly, loss of apoptosis did not alter overall microglial cell density in vivo or microglial motility and migration in ex vivo tissue slices. These results demonstrate that developmental changes in microglial form, distribution, motility, and migration occur essentially normally in the absence of developmental apoptosis, indicating that factors other than neuronal apoptosis regulate these features of microglial development.

Immunohistochemical and calcium imaging methods in wholemount rat retina

In this paper we describe the tools, reagents, and the practical steps that are needed for: 1) successful preparation of wholemount retinas for immunohistochemistry and, 2) calcium imaging for the study of voltage gated calcium channel (VGCC) mediated calcium signaling in retinal ganglion cells. The calcium imaging method we describe circumvents issues concerning non-specific loading of displaced amacrine cells in the ganglion cell layer.

A method to investigate radial glia cell behavior using two-photon time-lapse microscopy in an ex vivo model of spinal cord development

The mammalian central nervous system (CNS) develops from multipotent progenitor cells, which proliferate and differentiate into the various cell types of the brain and spinal cord. Despite the wealth of knowledge from progenitor cell culture studies, there is a significant lack of understanding regarding dynamic progenitor cell behavior over the course of development. This is in part due to shortcomings in the techniques available to study these processes in living tissues as they are occurring. In order to investigate cell behavior under physiologically relevant conditions we established an ex vivo model of the developing rat spinal cord. This method allows us to directly observe specific populations of cells ex vivo in real time and over extended developmental periods as they undergo proliferation, migration, and differentiation in the CNS. Previous investigations of progenitor cell behavior have been limited in either spatial or temporal resolution (or both) due to the necessity of preserving tissue viability and avoiding phototoxic effects of fluorescent imaging. The method described here overcomes these obstacles. Using two-photon and confocal microscopy and transfected organotypic spinal cord slice cultures we have undertaken detailed imaging of a unique population of neural progenitors, radial glial cells. This method uniquely enables analysis of large populations as well as individual cells; ultimately resulting in a 4D dataset of progenitor cell behavior for up to 7 days during embryonic development. This approach can be adapted to study a variety of cell populations at different stages of development using appropriate promoter driven fluorescent protein expression. The ability to control the tissue micro-environment makes this ex vivo method a powerful tool to elucidate the underlying molecular mechanisms regulating cell behavior during embryonic development.

Imaging microglia in brain slices and slice cultures

Here we describe a method for imaging fluorescently labeled parenchymal microglia (MG) in excised neonatal or adult rodent brain tissue slices. Using multichannel confocal or two-photon time-lapse imaging, the approach affords real-time analyses of MG behaviors, including motility, migration, chemotaxis, proliferation, and phagocytosis in live brain tissues. The method is applicable to acutely prepared tissue slices from developing and adult rodents and to slice cultures derived from neonatal rodents, including transgenic and green fluorescent protein reporter mice. A variety of fluorescent tags can be used to study the structure and physiology of MG in these preparations. Moreover, bath application of reagents (such as ATP) can establish spatial and temporal gradients that induce chemokinesis- and chemotaxis-like MG migration in tissue slices. Thus, the approach is useful for dissecting the molecular basis of MG behaviors and testing whether candidate reagents alter MG behavior and function in semi-intact central nervous system tissue preparations.