This manuscript describes a protocol for the in vivo imaging of the mouse retina with high-resolution spectral domain optical coherence tomography (SD-OCT). It focuses on retinal ganglion cells (RGC) in the peripapillary region, with several scanning and quantifying approaches described.
Structural changes in the retina are common manifestations of ophthalmic diseases. Optical coherence tomography (OCT) enables their identification in vivo—rapidly, repetitively, and at a high resolution. This protocol describes OCT imaging in the mouse retina as a powerful tool to study optic neuropathies (OPN). The OCT system is an interferometry-based, non-invasive alternative to common post mortem histological assays. It provides a fast and accurate assessment of retinal thickness, allowing the possibility to track changes, such as retinal thinning or thickening. We present the imaging process and analysis with the example of the Opa1delTTAG mouse line. Three types of scans are proposed, with two quantification methods: standard and homemade calipers. The latter is best for use on the peripapillary retina during radial scans; being more precise, is preferable for analyzing thinner structures. All approaches described here are designed for retinal ganglion cells (RGC) but are easily adaptable to other cell populations. In conclusion, OCT is efficient in mouse model phenotyping and has the potential to be used for the reliable evaluation of therapeutic interventions.
OCT is a diagnostic tool that facilitates the examination of retinal structures1, including the optic nerve head (ONH). Over the years it has become a dependable indicator of disease progression in humans2,3, as well as in rodents4,5. It uses interferometry to create cross-sectional images of retinal layers at a 2-µm axial resolution. The innermost layer is the retinal nerve fiber layer (RNFL), containing RGC axons, which is followed by the ganglion cell layer (GCL), containing mostly RGC bodies. Next is the inner plexiform layer (IPL), where RGC dendrites meet bipolar, horizontal, and amacrine cell axons. These, together with horizontal cells, form the inner nuclear layer (INL), and their protrusions connect with photoreceptor axons in the outer plexiform layer (OPL). This is followed by the outer nuclear layer (ONL), with photoreceptor cell bodies, and is separated from the photoreceptor layer by the outer limiting membrane (OLM), also called the inner segment/outer segment (IS/OS) layer. Finally, the last observable layers in the mouse retina are the retinal pigment epithelium (RPE) and the choroid (C). The RNFL alone is normally too thin to be measured in mice; thus, analyzing the RNFL/GCL instead is preferable4,5. Another possibility is the GC complex layer, which contains the latter in addition to the IPL, making it thicker and thus even easier to measure on OCT scans4. Consequently, OCT can provide insight into the pathological status of the retina, such as in OPNs.
Alternatively, the thickness of the mouse retina is often analyzed with post mortem histology. However, this technique faces limitations relating to tissue collection, fixation, cutting, staining, mounting, etc. Hence, some defects, such as subtle thickness changes, cannot be detected. Finally, because the same mouse cannot be tested at several time points, the number of animals per study greatly increases, unlike for OCT. All in all, the non-invasiveness, high-resolution, possibility for repetition, time monitoring in time, and ease of use of the OCT technology make it the method of choice in retinal disease studies.
Mouse models are used to identify gene defects and to elucidate molecular mechanisms underlying retinopathies6. OPN is a form of retinopathy with substantial damage to the optic nerve (ON), which is made up of approximately 1.2 million RGC axons. OPN can be focused on the ON or can be secondary to other disorders, inborn or not7, leading to visual field loss and later, blindness. Characteristic traits of OPN are RGC loss and ON damage, which can be observed in human OCT as RNFL and GCL thinning2,3. Meanwhile, the pathophysiology of OPN is still poorly understood, and hence the need to test mouse retinas remains.
This manuscript describes the imaging and quantification of retinal layer thickness, using the example of the Opa1delTTAG mouse line8,9, a model of dominant optic atrophy (DOA)10. To assess RGC pathophysiology, radial, rectangular, and annular scans were quantified. This was done either with standard calipers provided by the OCT software or with a homemade macro developed for an open-source image processing program. The standard calipers are difficult to manipulate and often thicker than the RNFL/GCL, while the homemade calipers are easy to use, reproducible, and more precise. The macro performs a measurement for an automatically detected layer, in 5 points and at fixed positions, on both sides of the ONH in the peripapillary region. The goal of the presented protocol is to describe OCT scan acquisition to specify retinal positioning, with a focus on RGCs.
The experimental protocol was approved by the Institut national de la santé et de la recherche médicale (Inserm; Montpellier, France), is consistent with the European directives, and complies with the ARVO Statement for the Use of Animals in Ophthalmic Research. It was carried out under the agreement of the Languedoc Roussillon Comity of Ethics in Animal Experimentation (CEEALR; nuCEEA-LR-12123).
1. Equipment Setup and Pre-imaging Preparation
NOTE: Here, OCT was performed on mouse retinas using the spectral domain (SD) ophthalmic imaging system (Figure 1A). The SD-OCT apparatus consists of a base and an animal imaging mount (AIM) with a rodent alignment stage (RAS) (Figure 1B). The base includes the computer, the OCT engine, the SD-OCT probe, and the mouse-specific lens. The probe is mounted onto the AIM, which includes the Z-translator. The RAS is used for mouse positioning thanks to the table with the X- and Y-translator, the cassette that can be rotated and swiveled, and the removable bite bar with the nose band. The software provided by the manufacturer allows for the acquisition and analysis of OCT files, although the latter may also be done with an open-source image processing program.
2. Mouse Preparation
3. Mouse Positioning
4. SD-OCT Imaging of the ONH and Retina
5. Acquisition Completion
6. Analysis
The SD-OCT technology enables retinal imagining and thickness analysis that is comparable to histology, but is faster and more detailed (Figure 3). As presented with wildtype C57Bl/6 mice, even though the quality of an SD-OCT scan (Figure 3A, right) is not as good as that of an image of a retinal cross-section (Figure 3A, left), it visualizes more layers (e.g., OLM). Moreover, it takes only about 40 min, including mouse preparation, versus days or weeks for histological analysis. Finally, it does not require processing and staining, such as haematoxylin, eosin, and saffron, which may damage the tissue and cause the collection of erroneous data. The retinal layers easily measurable in OCT encompass the RNFL/GCL, IPL, INL, OPL, ONL, IS/OS, RPE, and C (Figure 3B), therefore allowing for a complex study of the entire retina. As such, structural retinal changes reflect disease development. In the case of OPNs, this applies to RGCs and the ON, and thus the RNFL/GCL and IPL.
DOA is one of the most common OPNs and is characterized by RGC degeneration and the loss of the RNFL11. Due to mutations in the OPA1 gene12, it leads to visual impairment and blindness. Using the Opa1delTTAG mouse model, which carries the human recurrent c.2708delTTAG mutation, it was discovered that Opa1 haplo-insufficiency hinders vision in a sex-dependent manner8,9. This was determined on the basis of OCT measurements of retinal thickness, which showed a progressive thickening of the GC complex layer (Figure 4A) and the peripapillary RNFL (Figure 4B, 4C) in Opa1+/- females. In these experiments, the calculations were done with the standard calipers for rectangular scans and with an open-source image processing program for the annular scans. For radial scans, which often are of a lower quality and produce a minimum of 10 images per retina for analysis, a homemade macro was developed. A comparison of the standard and homemade calipers (Figure 4D) showed a significantly lower thickness of the RNFL/GCL and GC complex layers measured with the latter. This is because the standard calipers are much thicker and more difficult to place on the border of the layer. Therefore, it is best to avoid using the standard calipers for thin layers, especially on radial scans.
To summarize, the SD-OCT allows for mouse visual phenotyping that can be repeated across several time points. However, the OCT scan type and measuring method must be adapted to the investigated disease, and thus the retinal layer in question. Nevertheless, OCT provides sufficient information to identify defects in retinal structure. However, this must be further analyzed with another method to provide a complete understanding of the underlying mechanisms.
Figure 1: SD-OCT Ophthalmic Imaging System. (A) Overview of the base and AIM-RAS parts of the SD-OCT device. (B) Overview of the AIM-RAS components. A: computer, B: power supply, C: OCT engine reference arm, D: SD-OCT probe, E: mouse-specific lens, F: Z-translator, G: AIM-RAS table, H: Y-translator, I: cassette (rotator), J: bite bar, K – nose band, L: cassette swivel, M: X-translator, and N: aiming tip. This figute is color-coded as in Figure 2, with non-modulators of the retinal position marked in pink. Please click here to view a larger version of this figure.
Figure 2: Positioning the Retina. Horizontal (left) and vertical (right) view of the B-scan alignment. The arrows correspond to movements induced by the color-coded modulators. Please click here to view a larger version of this figure.
Figure 3: Retinal Layers. (A) Wildtype mouse retina histology after haematoxylin, eosin, and saffron staining (left) and SD-OCT (right); scale bar: 50 µm. (B) Retinal thickness measurements for 3 month-old wildtype mice; n = 14, mean ± SEM, scale bar: 50 µm. GC: ganglion cell, RNFL/GCL: retinal nerve fiber layer/ganglion cell layer, IPL: inner plexiform layer, INL: inner nuclear layer, OPL: outer plexiform layer, ONL: outer nuclear layer, IS/OS: photoreceptor inner segments/outer segments, RPE: retinal pigment epithelium, and C: choroid. Please click here to view a larger version of this figure.
Figure 4: Exemplary SD-OCT Measurements. (A) GC complex layer thickness in rectangular scans centered on the ONH, measured with the standard calipers; Opa1delTTAG mouse line, n = 4, mean ± SEM, ** p <0.01 assessed with Student's t-test. (B) Peripapillary RNFL in SD-OCT. (C) Peripapillary RNFL thickness in annular scans, determined as the RNFL area calculation per field; Opa1delTTAG female mice, n = 5-11, mean ± SEM, * p < 0.05 assessed with Student's t-test. (D) RNFL/GCL and GC complex layer thickness in radial scans, measured with the standard or homemade calipers for 3 or 10 scans, respectively; wildtype mice, n = 8, mean ± SEM, ** p <0.01, *** p <0.001 assessed with Student's t-test. RNFL/GCL – retinal nerve fiber layer/ganglion cell layer, GC: ganglion cell. Figures A-C adapted from Sarzi et al.9. Please click here to view a larger version of this figure.
The OCT system, a non-invasive in vivo imaging method, provides high-resolution retinal cross-section-like scans. Thus, its main advantage is its potential for detailed analysis, with the wonderful opportunity to transpose protocols routinely applied to humans to mouse models.
In the example of Opa1delTTAG mutant mice, SD-OCT results showed an increase of RNFL and GC complex layer thickness, which allowed for further exploration of DOA pathophysiology9. It would not have been possible solely with histological analysis. In comparison, histology does not provide the possibility to visualize the entire retina, unlike annular or radial OCT scans. Moreover, it is more time-consuming and costly, considering the increased number of animals in studies with several time points. In fact, SD-OCT paved the way to incriminate a new retinal cell type in DOA, the Müller cell9. This was done despite the fact that neither single-cell resolution nor specific cell identifications are possible with the system. On the contrary, the thickness-focused and/or general state-focused analysis of the peripapillary region is largely enough to detect cellular deterioration. Additional investigations with histology can then be conducted with a clear idea of what to look for. Therefore, the same method can also be applied to the evaluation of therapeutic interventions to prevent or brake retinal degeneration.
To further improve the utility of OCT, homemade calipers were developed and had a much higher precision than the standard ones. Even though the standard is thicker than the RNFL/GCL, some teams use it anyway, but for larger layers13. Here, we focused the comparative analysis on RGCs in 10 radial scans per retina, all in the peripapillary region. The RNFL alone was not measurable on the radial scans either way. This layer was too thin and vague; therefore, the RNFL/GCL and the GC complex layer were measured instead. At the same time, we succeeded in measuring the RNFL using annular scans, which proved beneficial to mouse phenotyping. However, the reliability may be controversial. In all these approaches, the critical step was to center the scans on the ONH and to visualize the retina without shadows and opacities. The former can be easily adjusted by following the steps of the protocol in terms of the positioning of the retina. The latter depends upon the transmittance of the cornea and the crystalline. For example, if the ophthalmic gel is spread unevenly, the scan is blurry and/or the retina appears bent. To fix this, it would be enough to properly re-apply the gel. If the crystalline is opaque, the scan is dark or incomplete. The solution here would be to repeat the scan another day, if the transparency of the crystalline returns. Another possible reason for a bad-quality scan is the presence of obstacles, such as whiskers or eyelashes. These can be easily removed by setting aside and applying a little of the ophthalmic gel to hold them in place. Other analytic approaches that differ in terms of equipment type, scan type, angle, and other parameters exist as well and have varying numbers of analyzed images. This must be considered if the result quality is still not satisfactory. For instance, Liu et al. took radial scans at several angles13, in comparison to our radial scans at only one, reporting slightly thicker layers. Nevertheless, the OCT acquisition and analytic approaches proposed in this manuscript are suited for analyzing RGCs in the peripapillary mouse retina.
In conclusion, OCT is a technology with great potential. It enables the detection of subtle changes in the retinal structure—including the RGCs, especially in regards to the OPNs—and proves indispensable to vision science. Therefore, the presented protocol is practical for OPN mouse model phenotyping, as well as for the evaluation of novel therapies.
The authors have nothing to disclose.
This work was supported by Inserm, Université Montpellier, Retina France, Union National des Aveugles et Déficients Visuels (UNADEV), Association Syndrome de Wolfram, Fondation pour la Recherche Médicale, Fondation de France, and the Laboratory of Excellence EpiGenMed program.
Mice | |||
Opa1delTTAG mouse | Institute for Neurosciences in Montpellier, INSERM UMR 1051, France | – | Opa1 knock-in mice carrying OPA1 c.2708_2711delTTAG mutation on C57Bl6/J background |
Name | Company | Catalog Number | コメント |
Equipment | |||
EnVisu R2200 SD-OCT Imaging System | Bioptigen, Leica Microsystems, Germany | – | Spectral-Domain Optic Coherence Tomography system |
EnVisu R2200 SD-OCT Imaging System Software | Bioptigen, Leica Microsystems, Germany | – | Software for OCT acquisition and analysis |
ImageJ 1.48v | Wayne Rasband, National Institutes of Health, USA | – | Software for analysis, requires downloading and installing two hommade macros: http://dev.mri.cnrs.fr/projects/imagej-macros/wiki/Retina_Tool |
Self-regulating heating plate | Bioseb, France | BIO-062 | Protection against hypothermia |
Name | Company | Catalog Number | コメント |
Supplies | |||
Nose Band | – | – | Elastic band |
Gauze pads 3"x3" | Curad, USA | CUR20434ERB | Protection against hypothermia |
Dual Ended Cotton tip applicator | Essence of Beauty, CVS Health Corporation, USA | – | Gel application |
Cotton Twists | CentraVet, France | T.7979C.CS | Mouse positioning |
Name | Company | Catalog Number | コメント |
Reagents and Drugs | |||
Néosynéphrine Faure 10% | Laboratoires Europhtha, Monaco | – | Eye dilatation |
Mydriaticum 0.5% | Laboratoires Théa, France | 3397908 | Eye dilatation |
Cebesine 0.4% | Laboratoire Chauvin, Bausch&Lomb, France | 3192342 | Local anesthesia |
Imalgene 1000 | Merial, France/CentraVet, France | IMA004 | General anesthesia |
Rompun | Bayer Healthcare, Germany/CentraVet, France | ROM001 | General anesthesia, analgesia, muscle relaxation |
NaCl 0,9% | Laboratoire Osalia, France | 103697114 | Physiological serum |
Systene Ultra | Alcon, Novartis, USA | – | Hydration of eyes |
GenTeal' | Alcon, Novartis, USA | – | Ophtalmic gel to minimize light refraction and opacities |
Aniospray Surf 29 | Laboratoires Anios, France | 59844 | Desinfectant |