In vivo imaging of the mouse spinal cord using two-photon microscopy

In vivo imaging using two-photon microscopy in mice that have been genetically engineered to express fluorescent proteins in specific cell types has significantly broadened our knowledge of physiological and pathological processes in numerous tissues in vivo. In studies of the central nervous system (CNS), there has been a broad application of in vivo imaging in the brain, which has produced a plethora of novel and often unexpected findings about the behavior of cells such as neurons, astrocytes, microglia, under physiological or pathological conditions. However, mostly technical complications have limited the implementation of in vivo imaging in studies of the living mouse spinal cord. In particular, the anatomical proximity of the spinal cord to the lungs and heart generates significant movement artifact that makes imaging the living spinal cord a challenging task. We developed a novel method that overcomes the inherent limitations of spinal cord imaging by stabilizing the spinal column, reducing respiratory-induced movements and thereby facilitating the use of two-photon microscopy to image the mouse spinal cord in vivo. This is achieved by combining a customized spinal stabilization device with a method of deep anesthesia, resulting in a significant reduction of respiratory-induced movements. This video protocol shows how to expose a small area of the living spinal cord that can be maintained under stable physiological conditions over extended periods of time by keeping tissue injury and bleeding to a minimum. Representative raw images acquired in vivo detail in high resolution the close relationship between microglia and the vasculature. A timelapse sequence shows the dynamic behavior of microglial processes in the living mouse spinal cord. Moreover, a continuous scan of the same z-frame demonstrates the outstanding stability that this method can achieve to generate stacks of images and/or timelapse movies that do not require image alignment post-acquisition. Finally, we show how this method can be used to revisit and reimage the same area of the spinal cord at later timepoints, allowing for longitudinal studies of ongoing physiological or pathological processes in vivo.     

Generation of an OMgp allelic series in mice

The very limited ability to regenerate axons after injury in the mature mammalian central nervous system (CNS) has been partly attributed to the growth restrictive nature of CNS myelin. Oligodendrocyte myelin glycoprotein (OMgp) was identified as a major myelin‐derived inhibitor of axon growth. However, its role in axon regeneration in vivo is poorly understood. Here we describe the generation and molecular characterization of an OMgp allelic series. With a single gene targeting event and Cre/FLP mediated recombination, we generated an OMgp null allele with a LacZ reporter, one without a reporter gene, and an OMgp conditional allele. This allelic series will aid in the study of OMgp in adult CNS axon regeneration using mouse models of spinal cord injury. The conditional allele will overcome developmental compensation when employed with an inducible Cre, and allows for the study of temporal and tissue/cell type‐specific roles of OMgp in CNS injury‐induced axonal plasticity. genesis 47:751–756, 2009. © 2009 Wiley‐Liss, Inc.     

Biochemical Abnormalities in Spinal Cord Myelin and CNS Homogenates in Heterozygotes Affected by the Shiverer Mutation

Myelin was purified from the spinal cords of normal mice and mice heterozygous for the shiverer mutation, and measurements were made of the major myelin proteins and lipids and the specific activities of three myelin-associated enzymes. The myelin purified from the spinal cords of the heterozygotes (shi/+) was deficient by 30-40% in yield and had an apparently unique composition. In particular, when compared with normal mouse spinal cord myelin, there were more high-molecular-weight protein, less myelin basic protein, a higher protein-to-lipid ratio, and higher specific activities of 2',3'-cyclic nucleotide-3'-phosphohydrolase (EC 3.1.4.37) and carbonic anhydrase (EC 4.2.1.1) in the myelin purified from the shi/+ animals. These abnormalities were reflected in the composition of shi/+ whole spinal cord, where the protein-to-lipid ratio was intermediate between the respective values for +/+ and shi/shi spinal cords. Whole brains from shi/+ mice showed deficiencies in galactocerebroside and galactocerebroside sulfate and an increase in total phospholipid, and the lipid composition in the brains of the shi/shi mice was similar to that reported for another dysmyelinating mutant, quaking. The findings provide the first values for the lipids in normal mouse spinal cord myelin and show that heterozygotes are affected by the shiverer mutation. The observations imply that there can be considerable deviation from the normal CNS myelin content and composition without apparent qualitative morphological abnormalities or loss of function and that the amount of myelin basic protein available during myelination may influence the incorporation of other constituents into the myelin membranes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nogo and axon regeneration

Nogo-A is one of several neurite growth inhibitory components present in oligodendrocytes and CNS myelin membranes. Nogo has a crucial role in restricting axonal regeneration and compensatory fibre growth in the injured adult mammalian CNS. Recent studies have shown that in vivo applications of Nogo neutralizing antibodies, peptides blocking the Nogo receptor subunit NgR, or blockers of the postreceptor components Rho-A and ROCK induce long-distance axonal regeneration and compensatory sprouting, accompanied by an impressive enhancement of functional recovery, in the rat and mouse spinal cord.     

Epidermal Growth Factor in the CNS: A Beguiling Journey from Integrated Cell Biology to Multiple Sclerosis. An Extensive Translational Overview

This article reviews the wealth of papers dealing with the different effects of epidermal growth factor (EGF) on oligodendrocytes, astrocytes, neurons, and neural stem cells (NSCs). EGF induces the in vitro and in vivo proliferation of NSCs, their migration, and their differentiation towards the neuroglial cell line. It interacts with extracellular matrix components. NSCs are distributed in different CNS areas, serve as a reservoir of multipotent cells, and may be increased during CNS demyelinating diseases. EGF has pleiotropic differentiative and proliferative effects on the main CNS cell types, particularly oligodendrocytes and their precursors, and astrocytes. EGF mediates the in vivo myelinotrophic effect of cobalamin on the CNS, and modulates the synthesis and levels of CNS normal prions (PrP^Cs), both of which are indispensable for myelinogenesis and myelin maintenance. EGF levels are significantly lower in the cerebrospinal fluid and spinal cord of patients with multiple sclerosis (MS), which probably explains remyelination failure, also because of the EGF marginal role in immunology. When repeatedly administered, EGF protects mouse spinal cord from demyelination in various experimental models of autoimmune encephalomyelitis. It would be worth further investigating the role of EGF in the pathogenesis of MS because of its multifarious effects.

     

The regional distribution of myelin oligodendrocyte glycoprotein (MOG) in the developing rat CNS: an in vivo immunohistochemical study

Using an immunohistochemical approach we have characterized the in vivo developmental distribution of myelin oligodendrocyte glycoprotein within the rat CNS. Myelin oligodendrocyte glycoprotein expression emerged in a non-uniform manner during the first 3 postnatal weeks. Although it was absent throughout the CNS of the newborn rat at postnatal day 0(P0), it had appeared in the spinal cord and brainstem by P7. The forebrain and cerebellum remained devoid of immunoreactivity until after P14. Myelin oligodendrocyte glycoprotein emerged at different times within the closely associated fasciculi of the dorsal funiculus. It appeared in the fasciculus cuneatus during the first postnatal week and in the fasciculus gracilis and corticospinal tracts during weeks 2 and 3 respectively. Myelin oligodendrocyte glycoprotein expression developed along a caudo-rostral gradient from spinal cord to forebrain and along an antero-posterior gradient within the CNS in general. The relationship between the onset of myelin oligodendrocyte glycoprotein expression and myelinogenesis was also investigated. In most regions, myelin oligodendrocyte glycoprotein expression lagged behind the initial appearance of myelin basic protein and Luxol Fast Blue-stained myelin by at least 1 week. These observations support the idea that myelin oligodendrocyte glycoprotein is the latest myelin protein to appear in development, only being expressed during the final stages of oligodendrocyte differentiation. Furthermore, the pattern of staggered expression within the dorsal columns indicates that localized, region-specific interactions may comprise a key element in the control of the terminal phases of oligodendrocyte differentiation.     

Live imaging of dorsal root axons after rhizotomy

The primary sensory axons injured by spinal root injuries fail to regenerate into the spinal cord, leading to chronic pain and permanent sensory loss. Regeneration of dorsal root (DR) axons into spinal cord is prevented at the dorsal root entry zone (DREZ), the interface between the CNS and PNS. Our understanding of the molecular and cellular events that prevent regeneration at DREZ is incomplete, in part because complex changes associated with nerve injury have been deduced from postmortem analyses. Dynamic cellular processes, such as axon regeneration, are best studied with techniques that capture real-time events with multiple observations of each living animal. Our ability to monitor neurons serially in vivo has increased dramatically owing to revolutionary innovations in optics and mouse transgenics. Several lines of thy1-GFP transgenic mice, in which subsets of neurons are genetically labeled in distinct fluorescent colors, permit individual neurons to be imaged in vivo(1). These mice have been used extensively for in vivo imaging of muscle(2-4) and brain(5-7), and have provided novel insights into physiological mechanisms that static analyses could not have resolved. Imaging studies of neurons in living spinal cord have only recently begun. Lichtman and his colleagues first demonstrated their feasibility by tracking injured dorsal column (DC) axons with wide-field microscopy(8,9). Multi-photon in vivo imaging of deeply positioned DC axons, microglia and blood vessels has also been accomplished(10). Over the last few years, we have pioneered in applying in vivo imaging to monitor regeneration of DR axons using wide-field microscopy and H line of thy1-YFP mice. These studies have led us to a novel hypothesis about why DR axons are prevented from regenerating within the spinal cord(11). In H line of thy1-YFP mice, distinct YFP+ axons are superficially positioned, which allows several axons to be monitored simultaneously. We have learned that DR axons arriving at DREZ are better imaged in lumbar than in cervical spinal cord. In the present report we describe several strategies that we have found useful to assure successful long-term and repeated imaging of regenerating DR axons. These include methods that eliminate repeated intubation and respiratory interruption, minimize surgery-associated stress and scar formation, and acquire stable images at high resolution without phototoxicity.     

Nogo-A expressed in Schwann cells impairs axonal regeneration after peripheral nerve injury

Injured axons in mammalian peripheral nerves often regenerate successfully over long distances, in contrast to axons in the brain and spinal cord (CNS). Neurite growth-inhibitory proteins, including the recently cloned membrane protein Nogo-A, are enriched in the CNS, in particular in myelin. Nogo-A is not detectable in peripheral nerve myelin. Using regulated transgenic expression of Nogo-A in peripheral nerve Schwann cells, we show that axonal regeneration and functional recovery are impaired after a sciatic nerve crush. Nogo-A thus overrides the growth-permissive and -promoting effects of the lesioned peripheral nerve, demonstrating its in vivo potency as an inhibitor of axonal regeneration.     

Heterodyne detected nonlinear optical imaging in a lock‐in free manner

We report a compact, cost‐effective tuned amplifier for frequency‐selective amplification of the modulated signal in heterodyne detected nonlinear optical microscopy. Our method improved the signal to noise ratio by an order of magnitude compared to conventional lock‐in detection, as demonstrated through stimulated Raman scattering imaging of live cells and tissues at the speed of 2 μsec/pixel. Application of the tuned amplifier to transient absorption microscopy is also demonstrated. The increased signal to noise ratio allowed epi‐detected in vivo imaging of myelin and blood in rat spinal cord with high spatial resolution. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Correlation Between 2'',3''-Cyclic Nucleotide 3''-Phosphohydrolase Activity and Demyelination In Vitro Using a Syngeneic System

Cultures of myelinated SJL/J fetal mouse spinal cord were incubated with serum and lymphoid cells from syngeneic animals with experimental allergic encephalomyelitis (EAE) induced by syngeneic spinal cord homogenate (SSCH) in complete Freund's adjuvant or others injected with complete Freund's adjuvant alone. After 24 or 48 h of exposure, demyelination was determined by light microscopic examination and quantification of 2',3'-cyclic nucleotide 3'-phosphohydrolase activity. Cultures exposed to spleen or lymph node cells from SSCH-sensitized animals showed the greatest alterations in myelin and decreases in 2',3'-cyclic nucleotide 3'-phosphohydrolase activity whereas serum from these animals had less effect. Cells and serum from complete Freund's adjuvant-injected control animals also induced structural changes in myelin that were significantly less than changes induced by cells and serum from animals with EAE. These experiments show that lymphoid cells and serum obtained from SJL/J mice with acute EAE affected myelin biochemistry and morphology in syngeneic CNS cultures.     

Three-dimensional imaging of the unsectioned adult spinal cord to assess axon regeneration and glial responses after injury

Studying regeneration in the central nervous system (CNS) is hampered by current histological and imaging techniques because they provide only partial information about axonal and glial reactions. Here we developed a tetrahydrofuran-based clearing procedure that renders fixed and unsectioned adult CNS tissue transparent and fully penetrable for optical imaging. In large spinal cord segments, we imaged fluorescently labeled cells by 'ultramicroscopy' and two-photon microscopy without the need for histological sectioning. We found that more than a year after injury growth-competent axons regenerated abundantly through the injury site. A few growth-incompetent axons could also regenerate when they bypassed the lesion. Moreover, we accurately determined quantitative changes of glial cells after spinal cord injury. Thus, clearing CNS tissue enables an unambiguous evaluation of axon regeneration and glial reactions. Our clearing procedure also renders other organs transparent, which makes this approach useful for a large number of preclinical paradigms.     

Mitigating Phototoxicity during Multiphoton Microscopy of Live Drosophila Embryos in the 1.0–1.2 μm Wavelength Range

Light-induced toxicity is a fundamental bottleneck in microscopic imaging of live embryos. In this article, after a review of photodamage mechanisms in cells and tissues, we assess photo-perturbation under illumination conditions relevant for point-scanning multiphoton imaging of live Drosophila embryos. We use third-harmonic generation (THG) imaging of developmental processes in embryos excited by pulsed near-infrared light in the 1.0–1.2 µm range. We study the influence of imaging rate, wavelength, and pulse duration on the short-term and long-term perturbation of development and define criteria for safe imaging. We show that under illumination conditions typical for multiphoton imaging, photodamage in this system arises through 2- and/or 3-photon absorption processes and in a cumulative manner. Based on this analysis, we derive general guidelines for improving the signal-to-damage ratio in two-photon (2PEF/SHG) or THG imaging by adjusting the pulse duration and/or the imaging rate. Finally, we report label-free time-lapse 3D THG imaging of gastrulating Drosophila embryos with sampling appropriate for the visualisation of morphogenetic movements in wild-type and mutant embryos, and long-term multiharmonic (THG-SHG) imaging of development until hatching.     

Coherent anti-stokes Raman scattering imaging of axonal myelin in live spinal tissues

We present a vibrational imaging study of axonal myelin under physiological conditions by laser-scanning coherent anti-Stokes Raman scattering (CARS) microscopy. We use spinal cord white matter strips that are isolated from guinea pigs and kept alive in oxygen bubbled Krebs' solution. Both forward- and epi-detected CARS are used to probe the parallel axons in the spinal tissue with a high vibrational contrast. With the CARS signal from CH₂ vibration, we have measured the ordering degree and the spectral profile of myelin lipids. Via comparison with the ordering degrees of lipids in myelin figures formed of controlled lipid composition, we show that the majority of the myelin membrane is in the liquid ordered phase. By measuring the myelin thickness and axon diameter, the value of g ratio is determined to be 0.68 with forward- and 0.63 with epi-detected CARS. Detailed structures of the node of Ranvier and Schmidt-Lanterman incisure are resolved. We have also visualized the ordering of water molecules between adjacent bilayers inside the myelin. Our observations provide new insights into myelin organization, complementary to the knowledge from light and electron microscopy studies of fixed and dehydrated tissues. In addition, we have demonstrated simultaneous CARS imaging of myelin and two-photon excitation fluorescence imaging of intra- and extraaxonal Ca²⁺. The current work opens up a new approach to the study of spinal cord injury and demyelinating diseases.     

Physiological Notch signaling promotes gliogenesis in the developing peripheral and central nervous systems

Constitutive activation of the Notch pathway can promote gliogenesis by peripheral (PNS) and central (CNS) nervous system progenitors. This raises the question of whether physiological Notch signaling regulates gliogenesis in vivo. To test this, we conditionally deleted Rbpsuh (Rbpj) from mouse PNS or CNS progenitors using Wnt1-Cre or Nestin-Cre. Rbpsuh encodes a DNA-binding protein (RBP/J) that is required for canonical signaling by all Notch receptors. In most regions of the developing PNS and spinal cord, Rbpsuh deletion caused only mild defects in neurogenesis, but severe defects in gliogenesis. These resulted from defects in glial specification or differentiation, not premature depletion of neural progenitors, because we were able to culture undifferentiated progenitors from the PNS and spinal cord despite their failure to form glia in vivo. In spinal cord progenitors, Rbpsuh was required to maintain Sox9 expression during gliogenesis, demonstrating that Notch signaling promotes the expression of a glial-specification gene. These results demonstrate that physiological Notch signaling is required for gliogenesis in vivo, independent of the role of Notch in the maintenance of undifferentiated neural progenitors.     

In Situ Fluorescence Imaging of Myelination

We describe a novel fluorescent dye, 3-(4-aminophenyl)-2H-chromen-2-one (termed case myelin compound or CMC), that can be used for in situ fluorescent imaging of myelin in the vertebrate nervous system. When administered via intravenous injection into the tail vein, CMC selectively stained large bundles of myelinated fibers in both the central nervous system (CNS) and the peripheral nervous system (PNS). In the CNS, CMC readily entered the brain and selectively localized in myelinated regions such as the corpus callosum and cerebellum. CMC also selectively stained myelinated nerves in the PNS. The staining patterns of CMC in a hypermyelinated mouse model were consistent with immunohistochemical staining. Similar to immunohistochemical staining, CMC selectively bound to myelin sheaths present in the white matter tracts. Unlike CMC, conventional antibody staining for myelin basic protein also stained oligodendrocyte cytoplasm in the striatum as well as granule layers in the cerebellum. In vivo application of CMC was also demonstrated by fluorescence imaging of myelinated nerves in the PNS. (J Histochem Cytochem 58:611–621, 2010)     

Immunoglobulins and complement in demyelination induced in mice by Theiler's virus

Intracerebral inoculation of Theiler's murine encephalomyelitis virus (TMEV) produces chronic demyelination and persistent infection in the central nervous system (CNS) of susceptible SJL mice. This series of experiments examined the contribution of humoral immunity and C to myelin destruction. As in multiple sclerosis, mice persistently infected with TMEV had elevated levels of IgG and oligoclonal bands in the cerebrospinal fluid (CSF). Immunoblot studies revealed that even in animals exhibiting profound demyelination, IgG in the serum and CSF was directed primarily at virus antigen rather than at normal myelin components. Inflammatory cells positive for Ig were distributed mainly around blood vessels, but occasionally they infiltrated the spinal cord parenchyma. Rare examples of myelin sheaths positive for IgG were found by immunoelectron microscopy in spinal cord sections from infected mice; the third component of complement (C3) was commonly found in the walls of CNS blood vessels but not on myelin. Neither serum nor CSF IgG from infected mice bound to myelin sheaths or other CNS components in sections of normal syngeneic spinal cord. There were significantly more demyelinating lesions in infected mice depleted of C components with cobra venom factor. These data do not support a humoral autoimmune basis for the CNS demyelination that occurs in association with persistent TMEV infection. However, the humoral immune response directed at TMEV antigens may either limit virus spread or promote virus persistence.     

Up-Regulation of Carbonic Anhydrase Isozyme IV in CNS Myelin of Mice Genetically Deficient in Carbonic Anhydrase II

Abstract: Carbonic anhydrase (CA) II is the major CA isozyme in the brain, where it participates in acid-base homeostasis, fluid transport, and myelin synthesis. The CA II deficiency [CA(II)D] mutation in the mouse results in structural changes in the glial cells in the CNS and in decreased susceptibility to seizures, but no detectable changes in myelin yield and ultrastructure. We compared the CA isozymes in brain and spinal cord fractions, as well as in purified myelin, between CA(II)D and control mice. CA(II)D resulted in a much lower total CA specific activity in all tissues examined but in higher CA IV specific activities in soluble and membrane-associated fractions and pure myelin. Western blots of purified myelin showed a band corresponding to CA IV in CA(II)D mice. This band was weak or undetectable in myelin samples from normal mice. Immunocytochemical staining demonstrated CA IV in oligodendrocytes and myelinated tracts in normal mouse brains and stronger staining of the same structures in brains of CA(II)D mutants. We conclude that CA(II)D mutation in the mouse up-regulates CNS CA IV. We speculate that this up-regulation could mitigate the effect of CA(II)D on myelin formation and maintenance.     

Visualizing the “sandwich” structure of osteocytes in their native environment deep in bone in vivo

Osteocytes are the most abundant cells in bone and always the focus of bone research. They are embedded in the highly scattering mineralized bone matrix. Consequently, visualizing osteocytes deep in bone with subcellular resolution poses a major challenge for in vivo bone research. Here we overcome this challenge by demonstrating 3‐photon imaging of osteocytes through the intact mouse skull in vivo. Through broadband transmittance characterization, we establish that the excitation at the 1700‐nm window enables the highest optical transmittance through the skull. Using label‐free third‐harmonic generation (THG) imaging excited at this window, we visualize osteocytes through the whole 140‐μm mouse skull and 155 μm into the brain in vivo. By developing selective labeling technique for the interstitial space, we visualize the “sandwich” structure of osteocytes in their native environment. Our work provides novel imaging methodology for bone research in vivo.      

Regeneration of sensory axons within the injured spinal cord induced by intraganglionic cAMP elevation

The peripheral branch of primary sensory neurons regenerates after injury, but there is no regeneration when their central branch is severed by spinal cord injury. Here we show that microinjection of a membrane-permeable analog of cAMP in lumbar dorsal root ganglia markedly increases the regeneration of injured central sensory branches. The injured axons regrow into the spinal cord lesion, often traversing the injury site. This result mimics the effect of a conditioning peripheral nerve lesion. We also demonstrate that sensory neurons exposed to cAMP in vivo, when subsequently cultured in vitro, show enhanced growth of neurites and an ability to overcome inhibition by CNS myelin. Thus, stimulating cAMP signaling increases the intrinsic growth capacity of injured sensory axons. This approach may be useful in promoting regeneration after spinal cord injury.