Light sheet microscopy with acoustic sample confinement

Contactless sample confinement would enable a whole host of new studies in developmental biology and neuroscience, in particular, when combined with long-term, wide-field optical imaging. To achieve this goal, we demonstrate a contactless acoustic gradient force trap for sample confinement in light sheet microscopy. Our approach allows the integration of real-time environmentally controlled experiments with wide-field low photo-toxic imaging, which we demonstrate on a variety of marine animal embryos and larvae. To illustrate the key advantages of our approach, we provide quantitative data for the dynamic response of the heartbeat of zebrafish larvae to verapamil and norepinephrine, which are known to affect cardiovascular function. Optical flow analysis allows us to explore the cardiac cycle of the zebrafish and determine the changes in contractile volume within the heart. Overcoming the restrictions of sample immobilisation and mounting can open up a broad range of studies, with real-time drug-based assays and biomechanical analyses.

Myelination of Neuronal Cell Bodies when Myelin Supply Exceeds Axonal Demand

The correct targeting of myelin is essential for nervous system formation and function. Oligodendrocytes in the CNS myelinate some axons, but not others, and do not myelinate structures including cell bodies and dendrites [1]. Recent studies indicate that extrinsic signals, such as neuronal activity [2, 3] and cell adhesion molecules [4], can bias myelination toward some axons and away from cell bodies and dendrites, indicating that, in vivo, neuronal and axonal cues regulate myelin targeting. In vitro, however, oligodendrocytes have an intrinsic propensity to myelinate [5, 6, 7] and can promiscuously wrap inert synthetic structures resembling neuronal processes [8, 9] or cell bodies [4]. A current therapeutic goal for the treatment of demyelinating diseases is to greatly promote oligodendrogenesis [10, 11, 12, 13]; thus, it is important to test how accurately extrinsic signals regulate the oligodendrocyte’s intrinsic program of myelination in vivo. Here, we test the hypothesis that neurons regulate myelination with sufficient stringency to always ensure correct targeting. Surprisingly, however, we find that myelin targeting in vivo is not very stringent and that mistargeting occurs readily when oligodendrocyte and myelin supply exceed axonal demand. We find that myelin is mistargeted to neuronal cell bodies in zebrafish mutants with fewer axons and independently in drug-treated zebrafish with increased oligodendrogenesis. Additionally, by increasing myelin production of oligodendrocytes in zebrafish and mice, we find that excess myelin is also inappropriately targeted to cell bodies. Our results suggest that balancing oligodendrocyte-intrinsic programs of myelin supply with axonal demand is essential for correct myelin targeting in vivo and highlight potential liabilities of strongly promoting oligodendrogenesis.

•

Balance between axons and myelin production regulates its targeting in vivo

•

Excess myelin is mistargeted to cell bodies

•

Low, but not zero, level of mistargeting during normal development

•

Potential implications for myelin-promoting therapies

Drug discovery for remyelination and treatment of MS

Glia constitute the majority of the cells in our nervous system, yet there are currently no drugs that target glia for the treatment of disease. Given ongoing discoveries of the many roles of glia in numerous diseases of the nervous system, this is likely to change in years to come. Here we focus on the possibility that targeting the oligodendrocyte lineage to promote regeneration of myelin (remyelination) represents a therapeutic strategy for the treatment of the demyelinating disease multiple sclerosis, MS. We discuss how hypothesis driven studies have identified multiple targets and pathways that can be manipulated to promote remyelination in vivo, and how this work has led to the first ever remyelination clinical trials. We also highlight how recent chemical discovery screens have identified a host of small molecule compounds that promote oligodendrocyte differentiation in vitro. Some of these compounds have also been shown to promote myelin regeneration in vivo, with one already being trialled in humans. Promoting oligodendrocyte differentiation and remyelination represents just one potential strategy for the treatment of MS. The pathology of MS is complex, and its complete amelioration may require targeting multiple biological processes in parallel. Therefore, we present an overview of new technologies and models for phenotypic analyses and screening that can be exploited to study complex cell-cell interactions in in vitro and in vivo systems. Such technological platforms will provide insight into fundamental mechanisms and increase capacities for drug-discovery of relevance to glia and currently intractable disorders of the CNS.

Production and titering of recombinant adeno-associated viral vectors

In recent years recombinant adeno-associated viral vectors (AAV) have become increasingly valuable for in vivo studies in animals, and are also currently being tested in human clinical trials. Wild-type AAV is a non-pathogenic member of the parvoviridae family and inherently replication-deficient. The broad transduction profile, low immune response as well as the strong and persistent transgene expression achieved with these vectors has made them a popular and versatile tool for in vitro and in vivo gene delivery. rAAVs can be easily and cheaply produced in the laboratory and, based on their favourable safety profile, are generally given a low safety classification. Here, we describe a method for the production and titering of chimeric rAAVs containing the capsid proteins of both AAV1 and AAV2. The use of these so-called chimeric vectors combines the benefits of both parental serotypes such as high titres stocks (AAV1) and purification by affinity chromatography (AAV2). These AAV serotypes are the best studied of all AAV serotypes, and individually have a broad infectivity pattern. The chimeric vectors described here should have the infectious properties of AAV1 and AAV2 and can thus be expected to infect a large range of tissues, including neurons, skeletal muscle, pancreas, kidney among others. The method described here uses heparin column purification, a method believed to give a higher viral titer and cleaner viral preparation than other purification methods, such as centrifugation through a caesium chloride gradient. Additionally, we describe how these vectors can be quickly and easily titered to give accurate reading of the number of infectious particles produced.