This protocol outlines the detailed steps of pre-embedding immunoelectron microscopy, with a focus on exploring synaptic circuits and protein localization in the retina.
The retina comprises numerous cells forming diverse neuronal circuits, which constitute the first stage of the visual pathway. Each circuit is characterized by unique features and distinct neurotransmitters, determining its role and functional significance. Given the intricate cell types within its structure, the complexity of neuronal circuits in the retina poses challenges for exploration. To better investigate retinal circuits and cross-talk, such as the link between cone and rod pathways, and precise molecular localization (neurotransmitters or neuropeptides), such as the presence of substance P-like immunoreactivity in the mouse retina, we employed a pre-embedding immunoelectron microscopy (immuno-EM) method to explore synaptic connections and organization. This approach enables us to pinpoint specific intercellular synaptic connections and precise molecular localization and could play a guiding role in exploring its function. This article describes the protocol, reagents used, and detailed steps, including (1) retina fixation preparation, (2) pre-embedding immunostaining, and (3) post-fixation and embedding.
The complexity of neuronal circuits in the retina presents challenges for exploration, considering the diverse cell types within its structure1,2. The initial step involves identifying synaptic connections between different cells and determining the cellular localization of specific neurotransmitters or neuropeptides. As molecular biology advances introduce new proteins, precise localization in the retina becomes crucial for understanding their functions and analyzing retinal circuits and synaptic connections3,4,5.
Due to the limited resolution of light microscopy, electron microscopy (EM) is commonly used to detect the subcellular structures of nerve cells. EM has various classifications, with conventional transmission electron microscopy (TEM) utilized for observing cell ultrastructures6,7,8,9. Immunoelectron microscopy (immuno-EM), which combines the spatial resolution of EM with the chemical identification ability of antibodies binding specifically to proteins10, stands out as the optimal and exclusive method for investigating synaptic connections and subcellular protein localization in the retina11,12.
Immuno-EM techniques can be divided into pre-embedding and post-embedding methods based on the order of embedding and antibody incubation. Compared with the post-embedding method, the pre-embedding approach is capable of large-scale and long-distance identification13,14,15, offering an optimal approach for studying cell processes like axons and dendrites. Additionally, this technique provides a strong signal and broad field of view, making it advantageous for comprehensive investigations of protein expression and molecular localization in the cytoplasm. This method proves particularly valuable in ensuring chemically identified structures that are visible throughout the entire cytoplasm, cells, or retina.
However, the post-embedding method, while having lower penetration or diffusion compared to the pre-embedding method, is not as sensitive16,17. In simple terms, if the goal is to explore the localization of specific neurotransmitters in the cytoplasm or synaptic terminals, the pre-embedding immuno-EM is the preferred method. Conversely, for identifying the localization of membrane receptors, it is more recommended to utilize post-embedding immunogold EM.
Given these considerations, we opt for the pre-embedding immuno-EM method to delve into retinal circuits, including the interaction between cone and rod pathways, and molecular localization, such as the distribution and synaptic organization of substance P-like immunoreactivity (SP-IR) in the mouse retina.
The care and handling of animals were approved by the Regulation of the Ethics Committee of Wenzhou Medical University in accordance with the ARVO guidelines. Adult mice (C57BL/6J, male and female, 8 to 12 weeks of age) were utilized in this research. The equipment and reagents needed for the study are listed in the Table of Materials.
1. Preparation for retina fixation
2. Pre-embedding immunostaining
3. Post-fixation and embedding
Figure 1 shows examples of control experiments without the incubation of primary antibodies against either protein kinase C alpha (PKCα) or SP, in which no immunoreactivity (IR) was found.
Figure 2 depicts the PKCα-IR in the mouse retina. PKCα serves as a marker for all rod bipolar cells (RBC) in the retina18. At the electron microscopy (EM) level, RBC can be identified through PKCα-IR, visualized by the diaminobenzidine (DAB) reaction product with high electron density. Figure 2A illustrates a PKCα-positive RBC dendrite forming a synapse with a rod terminal in the outer plexiform layer (OPL), while Figure 2B shows a cone-RBC synapse where a PKCα-positive RBC dendrite is post-synaptic to a cone terminal. Additionally, the DAB reaction product aids in identifying RBC terminals (Figure 2C) and axonal processes (Figure 2D) in the inner plexiform layer (IPL).
Figure 3 displays the expression of substance P (SP) in both pre-and post-synaptic connections in the IPL of the mouse retina. SP-IR amacrine cells are presynaptic to SP-negative amacrine cells (Figure 3A) as well as SP-IR amacrine processes (data not shown). Furthermore, SP-IR amacrine cells are post-synaptic to bipolar terminals in the sublayer 3 (S3) and sublayer 5 (S5) levels of IPL, respectively (Figure 3B,C). Our previous research details the analysis of synapses where SP-IR amacrine cell processes form synaptic outputs onto other processes in the IPL19. Notably, the subcellular localization of SP is mostly found in synaptic vesicles in the presynaptic terminals (Figure 3).
Figure 1: Immuno-electron micrographs from control experiments. Both cone pedicle (CP) with two ribbons (arrowhead) (A) and rod spherule (RS) with one ribbon in the OPL (B) showed no staining. Scale bars: 500 nm (A), 200 nm (B). Please click here to view a larger version of this figure.
Figure 2: Immuno-electron micrographs labeled with PKCα antibody in the mouse retina. (A) RS connects with one PKCα-positive RBC (arrows indicate immunoreactivity) and two horizontal cells (H) in the OPL. (B) CP connects with PKCα-positive RBC in the OPL. (C) and (D) show the PKCα-positive RBC terminal and axonal process in the IPL, respectively. Scale bars: 200 nm (A), 500 nm (B), 500 nm (C), 500 nm (D). Please click here to view a larger version of this figure.
Figure 3: Immuno-electron micrographs labeled with SP antibody in the IPL of mouse retina. (A) SP-IR amacrine cell (A+) is presynaptic to an SP-negative amacrine cell (A-). SP-IR amacrine cells received inputs from bipolar terminals in the S3 (B) and S5 (C) levels of IPL, respectively. Scale bars: 200 nm (A), 200 nm (B), 200 nm (C). Please click here to view a larger version of this figure.
This article has described three critical steps for the successful observation of synaptic circuits and protein localization: (1) quick and weak fixation, (2) pre-embedding immunostaining, and (3) post-fixation and embedding.
We propose that fixation is the key step for a successful pre-embedding immuno-EM approach. Thus, the importance of fresh fixative and fast fixation is emphasized here, naming this principle the "4F principle," which is crucial in tissue preparation. However, achieving both enhanced antibody penetration and effective fixation poses a challenge20. To address this, glutaraldehyde was replaced with picric acid during the initial fixation to improve antibody penetration, as glutaraldehyde can disrupt antigenic determinants and affect immunostaining21. Picric acid, in contrast, better preserves membrane structure and tissue integrity22, albeit with a compromise in fixation efficacy. In the third step, 2% glutaraldehyde was used for post-fixation to optimize tissue fixation.
Key points of the protocol presented here include five steps: (1) Early fixation: Utilizing picric acid in the early stage for better antibody penetration while minimizing disruption to antigenic determinants. (2) Post-fixation: Using 2% glutaraldehyde to enhance tissue fixation after immunostaining. (3) Reduction treatment: Incubating tissue in 1% NaBH4 in 0.1 M PB (pH 7.4) for 30 min to restore much of the immunoreactivity and improve the immune reactivity of antigens and antibodies, intensifying positive signals23. (4) Tissue preparation: Removing vitreous and cutting retinal tissue into small strips to facilitate effective antibody incubation and ensure uniform signals. (5) Moreover, it is imperative to handle the retinal tissue with utmost care, avoiding any damage that could compromise subsequent observations throughout the entire procedure. These measures collectively contribute to the success of the pre-embedding immuno-EM method, allowing one to study diverse aspects such as cone-RBC synapses24, SP-IR molecular localization19, and the precise expression of α-Syn in the mouse retina25.
Before incubating the first antibody, it is essential to block retinal strips with 2% NGS to eliminate non-specific signal interference. The concentration of the first antibody must be determined based on the antibody itself, typically higher than that used for frozen section staining, and must be optimized beforehand. During the primary antibody incubation period, continuous rotation of the microcentrifuge tube is necessary to ensure full contact of the stacked retinal strips with the antibody, preventing incomplete penetration of stacked parts. To amplify the signal, DAB was pre-incubated before standard DAB staining, with the specific staining time typically ranging from 10-20 min. If the time is too short, the signal may be too weak for observation, and if it is too long, it can lead to false positive signals, affecting accurate judgment. Generally, the incubation time of dyes is directly dependent on the surface color of the tissue (brown to dark brown).
While experiments were performed with freezing tissue in liquid nitrogen to enhance the signal, as mentioned previously18,26,27, this method did amplify positive signals but resulted in poor membrane structure of retinal cells (data not shown). This structural degradation made it challenging to accurately identify the cell types. Considering this, the retinal tissue was not frozen repeatedly. To enhance the positive signal, the incubation time of antibodies was appropriately extended. The proposed protocol combines previous advantages, simplifying steps compared to agar embedding. This not only safeguards the antigenic determinants of proteins but also preserves the integrity of cell membranes. The overarching goal is to explore synaptic connections and cellular localization of neurotransmitters throughout the entire pathway. Therefore, the DAB reaction products in the protocol have not undergone silver intensification treatment, making it more suitable for investigating neurotransmitter-specific synaptic connections and pathways28.
Currently, there are limited methods for the chemical identification of pathways or neurotransmitters at the ultrastructural level. One such method is correlated light and electron microscopy (CLEM)29,30, which, however, is confined to a very small area and lacks the ability to identify connections throughout the entire pathway. The strengths of pre-embedding immuno-EM lie in its robust signal, wide range, and capability for long-distance analytical positioning and tracking13,14,15. However, this method also has its limitations, such as diffuse signals, making them less specific. For the precise localization of membrane receptors, especially those on the membrane, it may be more suitable to use post-embedding immunogold EM. This technique enables the direct counting of gold particles, facilitating the analysis of the number and variability of these membrane receptors16.
In the future, the integration of pre-embedding immuno-EM with volume EM holds promise as an effective method for the long-term exploration of identified neural circuits, representing the future perspectives for further development of this method. Volume EM focuses on structure reconstruction, relying on morphology for the identification of specific structures, lacking precise chemical identification31,32. By combining pre-embedding immuno-EM with volume EM, specific structures or substances can be identified during three-dimensional reconstruction, providing a more comprehensive and visually intuitive approach to the identification of neural circuits.
The authors have nothing to disclose.
This work was supported in part by Grants from the National Key Research and Development Program of China (2022YFA1105503), the State Key Laboratory of Neuroscience (SKLN-202103), Zhejiang Natural Science Foundation of China (Y21H120019).
1 mL syringe needle | kangdelai | ||
1% OsO4 | Electron Microscopy Science | 19100 | |
2,2,2-Tribromoethanol | Sigma-Aldrich | T48402 | |
8% Glutaraldehyde | Electron Microscopy Science | 16020 | |
8% Paraformaldehyde | Electron Microscopy Science | 157-8 | |
Acetone | Electron Microscopy Science | 10000 | |
Anti-rabbit PKC | Sigma-Aldrich | P4334 | |
Anti-Rabbit SP | Abcam | ab67006 | |
DAB Substrate kit | MXB Biotechnologies | KIT-9701/9702/9703 | |
Elbow scissors | Suzhou66 vision company | 54010 | |
Electron microscope | Phillips | CM120 | |
Epon resin | Electron Microscopy Science | 14910 | |
forcep | Suzhou66 vision company | S101A | |
Millipore filter paper | Merck Millipore | PR05538 | |
Na2HPO4· 12H2O | Sigma | 71650 | A component of phosphate buffer |
NaH2PO4· H2O | Sigma | 71507 | A component of phosphate buffer |
Picric acid | Electron Microscopy Science | 19550 | |
Sodium borohydride (NaBH4) | Sigma | 215511 | |
Tris | Solarbio | 917R071 | |
Ultramicrotome | Leica | ||
Uranyl acetate | Electron Microscopy Science | 22400 | |
VACTASTAIN ABC kit, Peroxidase (Rabbit IgG) | Vector Laboratories | PK-4001 |