Summary

Clearing óptico del sistema nervioso central del ratón Utilizando CLARITY pasiva

Published: June 30, 2016
doi:

Summary

Optical clearing techniques are revolutionizing the way tissues are visualized. In this report we describe modifications of the original Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging-compatible Tissue-hYdrogel (CLARITY) protocol that yields more consistent and less expensive results.

Abstract

Traditionally, tissue visualization has required that the tissue of interest be serially sectioned and imaged, subjecting each tissue section to unique non-linear deformations, dramatically hampering one’s ability to evaluate cellular morphology, distribution and connectivity in the central nervous system (CNS). However, optical clearing techniques are changing the way tissues are visualized. These approaches permit one to probe deeply into intact organ preparations, providing tremendous insight into the structural organization of tissues in health and disease. Techniques such as Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging-compatible Tissue-hYdrogel (CLARITY) achieve this goal by providing a matrix that binds important biomolecules while permitting light-scattering lipids to freely diffuse out. Lipid removal, followed by refractive index matching, renders the tissue transparent and readily imaged in 3 dimensions (3D). Nevertheless, the electrophoretic tissue clearing (ETC) used in the original CLARITY protocol can be challenging to implement successfully and the use of a proprietary refraction index matching solution makes it expensive to use the technique routinely. This report demonstrates the implementation of a simple and inexpensive optical clearing protocol that combines passive CLARITY for improved tissue integrity and 2,2′-thiodiethanol (TDE), a previously described refractive index matching solution.

Introduction

The ability to image complete neuroanatomical structures is immensely valuable for understanding the brain in health and disease. Traditionally, 3D imaging has required tissue sectioning to provide the axial resolution and to visualize deep anatomical structures. This approach can produce high-resolution data sets, but requires sophisticated image reconstruction techniques and is very labor intensive. As a result, it has been limited to the imaging of small volumes of tissue1-3. Optical sectioning, on the other hand, is well suited for the creation of high-resolution 3D images of fluorescently labeled tissues. Since optical sectioning is inherently three dimensional, it does not require extensive computation to produce a 3D image volume. However, light scattering and tissue opacity limit the depth at which tissues can be optically sectioned. The depth of imaging is limited to about 150 µm in laser scanning confocal microscopy and to less than 800 µm using two-photon excitation microscopy4-8.

In order to overcome these limitations, several optical clearing techniques have been recently developed and then further refined to permit the deep microscopic imaging of intact tissues. The use of benzyl alcohol and benzyl benzoate (BABB) to render fixed tissues transparent was among one of the earliest approaches9. However, this approach was limited by the quenching of fluorescence in the samples and incomplete clearing of highly myelinated structures10. Refinements of this technique, such as 3D Imaging of Solvent Cleared Organs (3DISCO), have led to very rapid and complete tissue clearing, but still suffer from rapid loss of fluorescent signals, especially yellow fluorescent protein (YFP)10. Water-based clearing solutions, such as Sca/e11 and SeeDB12, preserve fluorescent signals, but do not completely clear highly myelinated tissues. Clear, Unobstructed Brain Imaging Cocktails and Computational analysis (CUBIC) is a promising new optical clearing technology that appears to overcome the limitations of previously developed water-based clearing solutions13. In contrast with other optical clearing techniques, Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging-compatible Tissue-hYdrogel (CLARITY)14 embeds the brain in a porous matrix that provides structural integrity to proteins, nucleic acids and small molecules, while leaving lipids unbound. The lipids are then removed electrophoretically, culminating in an optically cleared brain that can be easily visualized and equally easily probed using commonly available techniques.

Electrophoretic tissue clearing (ETC) is used to remove lipids from hydrogel embedded tissues in CLARITY, however, ETC can be difficult to implement consistently and tissues subject to electrophoresis can exhibit tissue distortion, browning, loss of fluorescence and antibody reactivity. Passive lipid clearing avoids these limitations, but requires increased clearing times15-17. Passive clearing also permits the clearing of large numbers of samples in parallel, since it is not limited by the number of ETC apparatuses available. Decreasing the concentration of paraformaldehyde and excluding bis-acrylamide from the hydrogel have led to great increases in the speed of clearing at the expense of greater tissue expansion16. Less expensive alternatives to the refractive index matching solution used in the original CLARITY technique have been developed17-19. 2,2′-thiodiethanol (TDE) is an inexpensive and rapid brain clearing agent that reverses some of the tissue expansion that occurs during lipid removal18. This report incorporates passive clearing of hydrogel embedded tissue and the use of TDE as a refractive index matching solution to the original CLARITY technique to produce a highly reproducible and inexpensive protocol for the optical clearing of the mouse central nervous system.

Protocol

Todos los experimentos se realizaron de acuerdo con el Cuidado de Animales institucional y el empleo directrices del Comité (IACUC). ratones Thy1-YFP, PLP-EGFP y PV-tdTomato se utilizaron para estos experimentos, pero cualquier ratones que expresan las proteínas fluorescentes se pueden borrar y la imagen con éxito. 1. Preparación del tejido Precaución: El paraformaldehído (PFA) y acrilamida son tóxicos e irritantes. Realizar experimentos que utilizan estos r…

Representative Results

La visualización de la organización estructural del cerebro es fundamental para la comprensión de cómo la morfología y la conectividad afectan la función cerebral en la salud y la enfermedad. técnicas de compensación ópticas hacen posible poblaciones celulares imagen en 3D en los tejidos intactos, que nos permite estudiar la morfología y la conectividad de manera cohesiva. Los ratones que expresan proteínas fluoresc…

Discussion

compensación pasiva de los tejidos de hidrogel incrustado (claridad pasiva) es un método sencillo y barato para la limpieza de piezas grandes de tejido. Este enfoque no requiere un equipo dedicado y se puede realizar fácilmente en un agitador de temperatura controlada. En el lapso de unas pocas semanas, los tejidos, incluso grandes, altamente mielinizados, como todo un cerebro o la médula espinal, llegarán a ser transparentes y adecuados para la microscopía. A pesar de que este informe se ha centrado en la limpiez…

Disclosures

The authors have nothing to disclose.

Acknowledgements

Este trabajo fue apoyado generosamente por los Institutos Nacionales de / Instituto Nacional de Trastornos Neurológicos y Accidentes Cerebrovasculares (NIH / NINDS) y la subvención 1R01NS086981 Conrad N. Hilton Foundation Salud. Agradecemos al Dr. Laurent Bentolila y el Dr. Mateo Schibler por su valiosa asistencia con microscopía confocal. Agradecemos a todos los que han contribuido al foro CLARITY (http://forum.claritytechniques.org). Agradecemos especialmente el Dr. Karl Deisseroth para la apertura de su laboratorio para enseñar esta técnica fascinante. Los autores están muy agradecidos por el generoso apoyo de la Organización de Investigación Médica Brain Mapping, Mapeo Cerebral Fundación textuales, Pierson-Lovelace Fundación, la Fundación Ahmanson, Fundación de Caridad Capital Group Companies, William M. y R. Linda Fondo Filantrópico Dietel, y el Fondo de Northstar . Las investigaciones realizadas en esta publicación también fue apoyado en parte por el Centro Nacional de Recursos para la Investigación y la Oficina del Director de la NaciónAl Institutos de Salud bajo los números de adjudicación C06RR012169, C06RR015431, y S10OD011939. El contenido es responsabilidad exclusiva de los autores y no representa necesariamente la opinión oficial de los Institutos Nacionales de Salud.

Materials

10X phosphate buffered saline (PBS) Fisher BP399-1 buffers
32% paraformaldehyde (PFA) Electron Microscopy Sciences 15714-5 perfusion and hydrogel
2,2'-thiodiethanol (TDE) Sigma-Aldrich 166782-500G refractive index matching solution
40% acrylamide Bio-Rad 161-0140 hydrogel
2% bis-acrylamide Bio-Rad 161-0142 hydrogel
VA-044 initiator Wako Pure Chemical Industries, Ltd. VA044 hydrogel
boric acid Sigma-Aldrich B7901 clearing buffer
sodium dodecyl sulfate (SDS) Fisher BP166-5 clearing buffer
sodium hydroxide Fisher 55255-1 buffers
sodium azide Sigma-Aldrich 52002-100G preservative
triton x-100 Sigma-Aldrich X100-500G buffers
heparin Sigma-Aldrich H3393-50KU perfusion
sodium nitrate Fisher BP360-500 perfusion
DAPI Molecular Probes D1306 nuclear stain
99.9% isoflurane Phoenix 57319-559-06 anesthetic
hydrocholoric acid Fisher A1445-500 buffers
glass bottom dish Willco HBSB-5040 Willco dishes
reusable adhesive Bostick 371351 Blu-Tack
silicon elastomer World Precision Instruments KWIK-CAST Kwik-Cast
lint-free wipe Kimberly-Clark 34120 KimWipe
mouse brain matrix Roboz SA-2175 sectioning tissue
matrix blades Roboz RS-9887 sectioning tissue
peristaltic pump Cole-Parmer 77122-24 pefusion
laser scanning confocal microscope Leica SP5 microscopy
imaging software Leica LAS AF microscopy

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Cite This Article
Roberts, D. G., Johnsonbaugh, H. B., Spence, R. D., MacKenzie-Graham, A. Optical Clearing of the Mouse Central Nervous System Using Passive CLARITY. J. Vis. Exp. (112), e54025, doi:10.3791/54025 (2016).

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