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This protocol describes methods for sectioning, staining, and imaging free-floating tissue sections of the mouse brain, followed by a detailed description of the analysis of astrocyte territory volume and astrocyte territory overlap or tiling.
Astrocytes possess an astounding degree of morphological complexity that enables them to interact with nearly every type of cell and structure within the brain. Through these interactions, astrocytes actively regulate many critical brain functions, including synapse formation, neurotransmission, and ion homeostasis. In the rodent brain, astrocytes grow in size and complexity during the first three postnatal weeks and establish distinct, non-overlapping territories to tile the brain. This protocol provides an established method for analyzing astrocyte territory volume and astrocyte tiling using free-floating tissue sections from the mouse brain. First, this protocol describes the steps for tissue collection, cryosectioning, and immunostaining of free-floating tissue sections. Second, this protocol describes image acquisition and analysis of astrocyte territory volume and territory overlap volume, using commercially available image analysis software. Lastly, this manuscript discusses the advantages, important considerations, common pitfalls, and limitations of these methods. This protocol requires brain tissue with sparse or mosaic fluorescent labeling of astrocytes, and is designed to be used with common lab equipment, confocal microscopy, and commercially available image analysis software.
Astrocytes are elaborately branched cells that perform many important functions in the brain1. In the mouse cortex, radial glial stem cells give rise to astrocytes during the late embryonic and early postnatal stages2. During the first three postnatal weeks, astrocytes grow in size and complexity, developing thousands of fine branches that directly interact with synapses1. Concurrently, astrocytes interact with neighboring astrocytes to establish discrete, non-overlapping territories to tile the brain3, while maintaining communication via gap junction channels4. Astrocyte morphology and organization are disrupted in many disease states following insult or injury5, indicating the importance of these processes for proper brain function. Analysis of astrocyte morphological properties during normal development, aging, and disease can provide valuable insights into astrocyte biology and physiology. Furthermore, analysis of astrocyte morphology following genetic manipulation is a valuable tool for discerning the cellular and molecular mechanisms that govern the establishment and maintenance of astrocyte morphological complexity.
Analysis of astrocyte morphology in the mouse brain is complicated by both astrocyte branching complexity and astrocyte tiling. Antibody staining using the intermediate filament glial fibrillary acidic protein (GFAP) as an astrocyte-specific marker captures only the major branches, and vastly underestimates astrocyte morphological complexity1. Other cell-specific markers such as glutamate transporter 1 (GLT-1; slc1a2), glutamine synthetase, or S100β do a better job of labeling astrocyte branches6, but introduce a new problem. Astrocyte territories are largely non-overlapping, but a small degree of overlap exists at the peripheral edges. Because of the complexity of branching, when neighboring astrocytes are labeled the same color, it is impossible to distinguish where one astrocyte ends and the other begins. Sparse or mosaic labeling of astrocytes with endogenous fluorescent proteins solves both problems: the fluorescent marker fills the cell to capture all of the branches and allows for imaging of individual astrocytes that can be distinguished from their neighbors. Several different strategies have been used to achieve sparse fluorescent labeling of astrocytes, with or without genetic manipulation, including viral injection, plasmid electroporation, or transgenic mouse lines. Details on the execution of these strategies are described in previously published studies and protocols1,7,8,9,10,11,12,13.
This article describes a method for measuring astrocyte territory volume from mouse brains with fluorescent labeling in a sparse population of astrocytes (Figure 1). Because the average diameter of an astrocyte in the mouse cortex is approximately 60 µm, 100 µm thick sections are used to improve the efficiency in capturing individual astrocytes in their entirety. Immunostaining is not required but is recommended to enhance the endogenous fluorescent signal for confocal imaging and analysis. Immunostaining may also enable better detection of fine astrocyte branches and reduce photobleaching of endogenous proteins during image acquisition. To improve antibody penetration into the thick sections, and to preserve tissue volume from sectioning through imaging, free-floating tissue sections are used. Analysis of astrocyte territory volume is performed using commercially available image analysis software. Additionally, this protocol describes a method for analysis of astrocyte tiling in tissue sections with mosaic labeling, where neighboring astrocytes express different fluorescent labels. This protocol has been used successfully in several recent studies1,8,9 to characterize astrocyte growth during normal brain development, as well as the impact of genetic manipulation on astrocyte development.
All mice were used in accordance with the Institutional Animal Care and Use Committee (IACUC) at the University of North Carolina at Chapel Hill and the Division of Comparative Medicine (IACUC protocol number 21-116.0). Mice of both sexes at postnatal day 21 (P21) were used for these experiments. CD1 mice were obtained commercially (Table of Materials), and MADM9 WT:WT and MADM9 WT:KO mice were described previously9.
NOTE: This protocol requires brains with fluorescent protein expression in a sparse population of astrocytes. Fluorescent protein expression can be introduced genetically, virally, or by electroporation. Details of methods to sparsely label astrocytes are described in previously published studies and protocols1,7,8,9,10,11,12,13.
1. Tissue collection and preparation
CAUTION: Paraformaldehyde (PFA) is a hazardous chemical. Perform all steps with PFA in a chemical fume hood.
2. Cyrosectioning
NOTE: This sectioning method is intended to work with many different commercially available cryostats. With the cryostat used here (Table of Materials), the optimal specimen head cutting temperature is -23 °C, with an ambient chamber temperature between -23 °C and -25 °C.
CAUTION: The cryostat blade is extremely sharp. Use caution while manipulating the blade and operating the cryostat.
3. Immunostaining
NOTE: Perform all washes and incubations on an orbital platform shaker set to approximately 100 rpm. All steps are performed at room temperature, except the primary antibody incubation, which is performed at 4 °C. Prepare mounting media ahead of time by combining the ingredients in a 50 mL tube and mixing on a nutator overnight. Protect from light and store at 4 °C for up to 2 months. If the endogenous fluorescent signal is sufficient for imaging and analysis without the need for immunostaining, steps 3.1-3.9 can be skipped. If skipping immunostaining, perform three 10 min washes in TBS and proceed to step 3.10.
4. Confocal imaging
NOTE: This protocol gives general image acquisition guidelines that are widely applicable to different confocal microscopes, rather than specific details for a particular confocal and software interface.
5. Image analysis
NOTE: This protocol describes the steps for performing image analysis using commercially available image analysis software (i.e., Imaris; see Table of Materials). Other versions of this software may be used with minor modifications to the workflow. This protocol also requires MATLAB to run the Convex Hull XTension file (Supplemental File).
Figure 1 presents a schematic outline of the major steps and workflow for this protocol. Figure 2 shows screenshots of key steps using the image analysis software to generate a surface, generate spots close to the surface, and generate a convex hull. Figure 3 demonstrates the application of this technique to determine astrocyte territory overlap/tiling. In Figure 4, representative results from a pr...
This protocol describes an established method for analyzing astrocyte territory volume and astrocyte tiling in the mouse cortex, detailing all of the major steps beginning with perfusion and ending with image analysis. This protocol requires brains from mice that express fluorescent proteins in a sparse or mosaic population of astrocytes. Outside of this requirement, mice of any age may be used for this protocol, with only minor adjustments to perfusion settings and the volume of freezing media added to the embedding mol...
The authors have no conflicts of interest.
Microscopy was performed at the UNC Neuroscience Microscopy Core (RRID:SCR_019060), supported in part by funding from the NIH-NINDS Neuroscience Center Support Grant P30 NS045892 and the NIH-NICHD Intellectual and Developmental Disabilities Research Center Support Grant U54 HD079124. Figure 1 was created with BioRender.com. The images and data in Figure 4 are reprinted from a previous publication9 with permission from the publisher.
Name | Company | Catalog Number | Comments |
#5 forceps | Roboz | RS-5045 | |
1 mL TB Syringe | Becton Dickinson (BD) | 309623 | |
10x TBS (tris-buffered saline) | 30 g Tris, 80 g NaCl, 2 g KCl, HCl to pH 7.4, dH2O to 1 L; store at room temperature (RT) | ||
12-well plate | Genesee Scientific | 25-106MP | |
1x TBS | 100 mL 10x TBS + 900 mL dH2O; store at RT | ||
1x TBS + Heparin | 28.2 mg Heparin + 250 mL 1x TBS; store at 4 °C | ||
24-well plate | Genesee Scientific | 25-107MP | |
30% Sucrose in TBS | 15 g sucrose, 1x TBS to 50 mL; store at 4 °C | ||
4% PFA (paraformaldehyde) in TBS | 40 g PFA, 4-6 NaOH pellets, 100 mL 10x TBS, dH2O to 1 L; store at 4 °C | ||
Avertin | 0.3125 g tri-bromoethanol, 0.625 mL methylbutanol, dH2O to 25 mL; store at 4 °C; discard 2 weeks after making | ||
Blocking and antibody buffer | 10% goat serum in TBST; store at 4 °C | ||
CD1 mice | Charles River | 022 | |
Collection vial for brains | Fisher Scientific | 03-337-20 | |
Confocal acquisition software | Olympous | FV31S-SW | |
Confocal microscope | Olympus | FV3000RS | |
Coverslips | Fisher Scientific | 12544E | |
Cryostat | Thermo Scientific | CryoStar NX50 | |
Cryostat blade | Thermo Scientific | 3052835 | |
DAPI | Invitrogen | D1306 | |
Embedding mold | Polysciences | 18646A-1 | |
Freezing Medium | 2:1 30% sucrose:OCT; store at RT | ||
GFP antibody | Aves Labs | GFP1010 | |
Glycerol | Thermo Scientific | 158920010 | |
Goat anti-chicken 488 | Invitrogen | A-11039 | |
Goat anti-rabbit 594 | Invitrogen | A11037 | |
Goat Serum | Gibco | 16210064 | |
Heparin | Sigma-Aldrich | H3149 | |
Hydrochloric acid | Sigma-Aldrich | 258148 | |
Imaris | Bitplane | N/A | Version 9.8.0 |
MATLAB | MathWorks | N/A | |
Metal lunch tin | AQUARIUS | N/A | From Amazon, "DIY Large Fun Box" |
Methylbutanol | Sigma-Aldrich | 152463 | |
Micro Dissecting Scissors | Roboz | RS-5921 | |
Mouting medium | 20mM Tris pH8.0, 90% Glycerol, 0.5% N-propyl gallate ; store at 4 °C; good for up to 2 months | ||
Nailpolish | VWR | 100491-940 | |
N-propyl gallate | Sigma-Aldrich | 02370-100G | |
O.C.T. | Fisher Scientific | 23-730-571 | |
Oil | Olympus | IMMOIL-F30CC | Specific to microscope/objective |
Operating Scissors 6" | Roboz | RS-6820 | |
Orbital platform shaker | Fisher Scientific | 88861043 | Minimum speed needed: 25 rpm |
Paintbrush | Bogrinuo | N/A | From Amazon, "Detail Paint Brushes - Miniature Brushes" |
Paraformaldehyde | Sigma-Aldrich | P6148 | |
Pasteur pipet (5.75") | VWR | 14672-608 | |
Pasteur pipet (9") | VWR | 14672-380 | |
Potassium chloride | Sigma-Aldrich | P9541-500G | |
Razor blade | Fisher Scientific | 12-640 | |
RFP antibody | Rockland | 600-401-379 | |
Sectioning medium | 1:1 glycerol:1x TBS; store at RT | ||
Slides | VWR | 48311-703 | |
Sodium chrloide | Fisher Scientific | BP358-212 | |
Sodium hydroxide | Sigma-Aldrich | S5881 | |
Sucrose | Sigma-Aldrich | S0389 | |
TBST (TBS + Triton X-100) | 0.2% Triton in 1x TBS; store at RT | ||
Transfer Pipet | VWR | 414004-002 | |
Tri-bromoethanol | Sigma-Aldrich | T48402 | |
Tris(hydroxymethyl)aminomethane | Thermo Scientific | 424570025 | |
Triton X-100 | Sigma-Aldrich | 93443 | |
Triton X-100 (high-quality) | Fisher Scientific | 50-489-120 | |
XTSpotsConvexHull | N/A | N/A | custom XTension provide as supplementary material |
Buffers and Solutions | |||
10x TBS | xx mM Tris, xx mM NaCl, xx mM KCl, pH 7.4 | ||
1x TBS | |||
1x TBS + Heparin | add xx mg Heparin to xx mL of 1x TBS | ||
4% PFA | |||
30% Sucrose in TBS | |||
Freezing Medium | |||
Sectioning medium | |||
TBST | 0.2% Triton in 1x TBS | ||
Blocking and antibody buffer | 10% goat serum in 1x TBST | ||
Mouting medium |
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