Neural retina of a mouse aged 8 days is on top of a 4% gelatin block. After isolation of the photoreceptor layer (200 µm) by vibratome, the photoreceptors are seeded after mechanical and enzymatic dissociation for culture. The photoreceptor layer can be used for molecular, biochemical analyses or transplantation.
The retina is a part of the central nervous system that has organized architecture, with neurons in layers from the photoreceptors, both rods and cones in contact with the retinal pigmented epithelium in the most distant part on the retina considering the direction of light, and the ganglion cells in the most proximal distance. This architecture allows the isolation of the photoreceptor layer by vibratome sectioning. The dissected neural retina of a mouse aged 8 days is flat-embedded in 4% gelatin on top of a slice of 20% gelatin photoreceptor layer facing down. Using a vibratome and a double edged razor blade, the 100 µm thick inner retina is sectioned. This section contains the ganglion cells and the inner layer with notably the bipolar cells. An intermediary section of 15 µm is discarded before 200 µm of the outer retina containing the photoreceptors is recovered. The gelatin is removed by heating at 37 °C. Pieces of outer layer are incubated in 500 µl of Ringer's solution with 2 units of activated papain for 20 min at 37 °C. The reaction is stopped by adding 500 µl 10% fetal calf serum (FCS) in Dulbecco's Modified Eagle Medium (DMEM), then 25 units of DNAse I is added before centrifugation at RT, washed several times to remove serum and the cells are resuspended in 500 µl of DMEM and seeded at 1 x 105 cells/cm2. The cells are grown to 5 days in vitro and their viability scored using live/dead assay. The purity of the culture is first determined by microscopic observation during the experiment. The purity is then validated by seeding and fixing cells on a histological slide and analyzing using a rabbit polyclonal anti-SAG, a photoreceptor marker and mouse monoclonal anti-RHO, a rod photoreceptor specific marker. Alternatively, the photoreceptor layer (97% rods) can be used for gene or protein expression analysis and for transplantation.
The retina is an integral part of the central nervous system which has a conserved architecture among vertebrates. The neurons of the neural retina are organized in layers, with the most distant for the incident light, the photoreceptor layer in close contact with the retinal pigmented epithelium (RPE) at the back of the eye. Rod and cone photorecptors are light sensitive cells that rely on opsin sensitive molecules for photon capture. These molecules are enclosed on disk membranes of a cellular structure, located on the outer segment of the photoreceptor which points in the direction of the RPE1. This structure, which is most often affected very early in cases of photoreceptor degenerations, is renewed at the rate of 10% every day. The so called inner layer contains most of the other neurons that compute the signal received from the photoreceptors, the bipolar, amacrine and horizontal cells as well as the ganglion cells. These latter with their axons form a beam which is the optic nerve. This layering is so conserved that biologists have used the term displaced amacrine cells when the cells are found outside the inner plexiform layer2. Layers of neurons are distributed within an armature of radial Müller glial cells. Bipolar cells link photoreceptors to ganglion cells. They are located between the external plexiform layer and the inner plexiform layer. The ganglion cells form the inner plexiform layer in connection with the bipolar cells. The amacrin cells are named as association cells located in the inner plexiform layer between the bipolar cells and ganglion cells. The outer plexiform layer contains horizontal cells. This unique arrangement of neuronal layers of the central nervous system permits the isolation of the photoreceptor layer from the inner cell layer by slicing the flat-mounted retina using a vibratome.
Originally, this technique was used to isolate photoreceptors for transplantation in the eye of the rd1 mouse, a model of human retinitis pigmentosa (RP)3. The rd1 mouse carries a recessive mutation in the Pde6b gene which encodes for the rod-specific phosphodiesterase beta subunit. Recessive mutations of this gene result in RP in humans4. After rod photoreceptors have degenerated, the patient loses night vision, and surprisingly cone photoreceptors, which do not express the mutated gene, degenerate as a second step. Because the cones are required for color vision and visual acuity, the patients become progressively blind and an effective treatment for the disease has not yet developed. By grafting photoreceptor layer from a wild-type mouse the cone degeneration of the host mouse is delayed3,5. The rods lost in the rod-cone degenerative model could not be replaced by a transplant because the synaptic connection between rods and bipolar cells can only be obtained at a specific stage of retinal development, marked by the onset of Nrl expression6. The layer of photoreceptors is introduced by surgery in the sub-retinal space of the rod-less rd1 retina, between the RPE and the outer retina corresponding to only 3% of the remaining photoreceptors, the cones. Two weeks after surgery, 40% of the cones from the animal transplanted with a normal photoreceptor layer survived as compared to the animal transplanted with a normal layer of inner retinal cells, or to the sham-operated animal. The topography of cone survival, spread-out over the entire surface of the mutated retina located at the position of the grafted tissue indicates that the protective effect is due to a diffusible molecule7.
Next, we used a co-culture system as well as conditioned culture media to validate the fact that the protective effect relies on the secretion of a protein by rods8,9. We hypothesize that this protein will be expressed continuously and specifically by rods and that their death during the first phase of the disease will trigger secondary cone degeneration by the loss of a protective signal from rods in a non-cell autonomous manner10. Because of the importance of cone mediated central vision in primates this putative protein will be a highly relevant therapeutic tool for RP. Preserving the cones in RP would theoretically prevent a total of 1.5 million patients worldwide to become blind11. We have used a high content screening approach and a cone-enriched culture model to identify a cDNA encoding Rod-derived Cone Viability Factor (RdCVF) from a retinal cDNA library12. RdCVF is the spliced product of the NXNL1 gene which, interestingly is homologous to the gene encoding for thioredoxin proteins involved in redox homeostasis13. The second spliced product of the gene, RdCVFL is an enzyme that protects its target, the TAU protein against oxidative damage14. Administration of RdCVF prevents the secondary degeneration of cones and the loss of their visual function in a recessive, and dominant model of RP12,15. This demonstrates two important aspects of this innovative therapeutic strategy16. First, it can be applied in most of the RP cases in a gene-independent manner. Second, contrary to the competing factor CNTF, RdCVF survival is associated with the maintenance of visual function17. The absence of functional effect may explain the reason of the absence of clinical benefit of the administration of CNTF to RP patients18. RdCVF is most likely one of the important survival signal between rods and cones since cone rescue in vitro is inhibited by RdCVF immunodepletion12. In addition, the disruption of the rod-derived cone viability gene leads to photoreceptor dysfunction and susceptibility to oxidative stress19.
The use of photoreceptor layer is at the origin of identification of RdCVF and of a novel redox signal involved in neurodegenerative diseases20. This manuscript describes the protocol used to isolate and culture cells from the photoreceptor layer to characterize the activity of RdCVF. The photoreceptors can be maintained in culture for 5 to 7 days21. This technique can also be used to study the expression of photoreceptor specific genes.
NOTE: The procedure was approved by the ethics committee Darwin from University Pierre and Marie Curie (Ce5/2009/048)
1. Preparation of Gelatin Solution, Instruments, Vibratome, Culture Media and Culture Plate
2. Dissection of Entire Retina
3. Sealing of Flat-mounted Retina onto Gelatin Block
4. Sectioning the Retina
NOTE: This step is critical to obtain a layer of photoreceptor cells without the other retinal cells (Figure 1).
5. Dissociating Photoreceptor Cells
6 Culturing Photoreceptor Cells
Apart from transplantation, the photoreceptor layers have also been used to study cell signaling by seeding cells for making photoreceptor cultures12,22. Additionally, they are used to study gene expression and circadian rhythm23,24. We have used the cell photoreceptor layer from a wild-type mouse at post-natal day 8 to prepare photoreceptor cultures in the absence of FCS and maintained the cells for 5 days at 37 °C in an incubator with 5% CO2 (Figure 1E). The cells were then fixed using 4% paraformadehyde and then proceeded for immunocytochemical analysis using methods provided by the antibody suppliers either with mouse monoclonal anti-RHO (1:250, Millipore Mab5316) or rabbit polyclonal anti-SAG (1:200, a generous gift of Igal Gery and David Hicks). We used anti-RHO and anti-SAG (although anti-SAG label cones and rods) due to the fact that anti-SAG is an earlier marker, than anti-RHO. The proportion of cells positive for the anti-RHO antibody is lower than those positive for the anti-SAG antibodies (Figure 2). This can be explained by the fact that SAG, the rod arrestin is also expressed by cones which make 3% of photoreceptors in the isolated layer, or alternatively by the fact that during post-natal maturation of the retina, the expression of SAG precedes that of RHO25,26.
The photoreceptor layer was also used to monitor the specific expression of genes by photoreceptors of the wild-type retina and brain of animals aged 35 days RNA were prepared using CsCl ultracentrifugation 27 and hybridized to mouse DNA chip Array. The messengers for rhodopsin (Rho), S-arrestin (Sag) and cone-transducin (Gnat2) are expressed specifically in the retina as compared to the brain (Figure 3A). The expression is prominent in the photoreceptor layer (PR) than in the whole retina which encompasses the inner retinal layer showing that these genes are expressed indeed by photoreceptors. For Gnat2, the relative expression as compared to Rho and Sag shows that it is expressed by cones contained within the isolated photoreceptor layer. The expression of recoverin (Rcvrn) in the photoreceptor layer as compared to the whole retina is increased (Figure 3B).
The photoreceptor layer was also used to monitor the expression of RHO and GNAT2 using western blotting following methods provided by the antibody suppliers and to compare to the inner retinal layer (Figure 4). Notice the absence of RHO and GNAT228, the markers of rods and cones respectively in the whole retina and in the retina of rd1 mouse at post-natal day 35.
Figure 1. Schematic view of the vibratome sectioning of mouse retina. (A) Installation of the flat-mounted retina with the photoreceptor facing down on the gelatin slice. (B) Section of the inner retina with the vibratome blade. (C) Section of the outer retina added of gelatin. (D) Control of the presence of photoreceptor at the edge of the fragment of retina after the isolation of photoreceptor layer. Scale bar used for green fluorescence is used of a white light microscope. The photoreceptors are located in the edge of the picture. (E) Cultured photoreceptor cells (post-five days). (F) Higher magnification of panel E.
Figure 2. Differential expression of RHO and SAG in mouse photoreceptor culture. (A) Staining of nuclei (blue), (B) SAG staining (green), (C) RHO staining (red). (D) Merged image. Because staining of SAG is expressed earlier than RHO staining, only two cells are double labeled SAG and RHO (see *). Scale bar: 23 µm. Please click here to view a larger version of this figure.
Figure 3. Comparison of the expression of three genes in brain and whole retina and photoreceptors of the wild-type animals aged thirty five days. (A) Rhodopsin (Rho), S-arrestin (Sag) and cone-transducin (Gnat2). (B) Calbindin (Calb1) and Recoverin (Rcvrn). The data are displayed as relative expression. The relative expression is an expression value obtained from microarray data after normaliszation with the software Robust Multi-array Average (RMA) available in the knowledge database KBaSS (http://kbass.institut-vision.org/KBaSS/).
Figure 4. Expression of RHO and GNAT2 in outer retina from a wild-type retina at post-natal day 35. Absence of their expression in the inner retina and in the retina of the rd1 mouse at post-natal day 35. ACTB, beta-actin. Please click here to view a larger version of this figure.
Name of Material / Equipment | Company | Catalog Number | Comments/Description |
Cytidine 5′-diphosphocholin* | Sigma-Aldrich | C0256 | 4.7 µM Cytidine 5′-diphosphocholin |
Cytidine 5′-diphosphoethanolamine* | Sigma-Aldrich | C0456 | 2.7 µM Cytidine 5′-diphosphoethanolamine |
Linoleïc acid/ bovine serum albumin (BSA)* | Sigma-Aldrich | L8384 | 100 µg/ml linoleïc acid/ bovine serum albumin (BSA) |
Triiodo-L-thyronine* | Sigma-Aldrich | T6397 | 0.03 µM Triiodo-L-thyronine |
96-wells plate | Greiner bio-one | 655-095 | |
Binocular microscope | Leica | MZ-75 | |
CO2 independent (CO2-i) | Life Technologies | 18045054 | |
DMEM | Life Technologies | 41966029 | |
Forceps n°5 Dumont | Bionic | 11254-20 | |
Gelatin from porcin skin type A | Sigma-Aldrich | G2500 | |
GeneChip | Affymetrix | U74v2 | |
Gentamicin solution | Life Technologies | 15710049 | |
Hydrocortison* | Sigma-Aldrich | H0888 | 0.55 µM hydrocortison |
Insulin* (I) | Sigma-Aldrich | I1884 (ITS) | 0.86 µM insulin (I) |
Papain | Worthington-biochem | WOLS03124 | |
Poly-D-Lysine | Sigma-Aldrich | P-6407 | |
Progesterone* | Sigma-Aldrich | P7556 | 2.0 µM progesterone |
Prostaglandin* | Sigma-Aldrich | P5172 | 0.28 µM prostaglandin |
Putrescine* | Sigma-Aldrich | P5780 | 182 µM putrescine |
Scalpel Albion | EMS | 72000 | |
Scissor | Moria | 15396-00 | |
Sodium Pyruvate* | Sigma-Aldrich | S8636 | 1 mM sodium pyruvate |
Sodium Selenite* (S) | Sigma-Aldrich | I1884 (ITS) | 0.29 µM Na2SeO3 (S) |
Taurine* | Sigma-Aldrich | T8691 | 3 mM taurine |
Transferrin* (T) | Sigma-Aldrich | I1884 (ITS) | 0.07 µM transferrin (T) |
Vibratome apparatus | Leica | VT1000-S | |
* Supplements |
Table 1: Specific material and equipment’s used.
–>The retina is a model organ in biology. Study of the retina led to 6 major discoveries in biology. It is at the origin of the first tumor suppressor gene RB1. It reveals the molecular link between the receptor tyrosine kinases and the MAP kinases through the interaction with Son of sevenless. It was involved in the discovery of PAX6, the first master control gene for organ morphogenesis. It is at the center of the genetic association of Complement factor H (CFH) with age-related macular degeneration (AMD), the first disease susceptibility gene identified by genome wide association screen (GWAS). Finally, it led to the first successful gene therapy for Leber congenital amaurosis, the first corrective gene therapy trial in human of any inherited disease. The structure of this organ is conserved in most vertebrate species. Its accessibility for manipulation in vivo has encouraged early on functional genomics investigations on this integral part of the central nervous system.
We show here how to separate the photoreceptor layer from the inner layer of the retina by vibratome sectioning. This step is critical to obtain pure photoreceptor cultures. Our dissection protocol makes the contamination by RPE cells very unlikely.
One of the major challenges is the flattening of the retina that is necessary to section properly the inner and the outer retina. The sectioning of the retina is best on retina with small diameter as those from rodents and this diameter is a limitation of the technique with current available material.
It is advisable before starting a biologically meaningful experiment, to practice on a sample of the retina. We illustrate the method by representative results obtained from cultured photoreceptor cells, using the material to perform expression study of both mRNAs and proteins. Expression studies can also be performed alternatively on sections obtained by laser capture microdissection, but the cultures are best performed using vibratome sectioning. We could have used the technique of microdissection laser with a complete different strategy. But, to collect the photoreceptor layer with the microdissection apparatus for culture, it would be necessary to avoid fixative and this would complicate the current methodology significantly.
We are developing a protocol aimed at studying the kinetics of gene and protein expression in vibratome sections during the degeneration of photoreceptors in models of retinitis pigmentosa. We believe that the detailed description of the protocol will be useful to researchers in the field of retinal biology, and most particularly for proteomic and metabolomic studies.
The authors have nothing to disclose.
Igal Gery and David Hicks for anti-SAG antibodies. Ram Fridlich for reading the manuscript.
Name of Material / Equipment | Company | Catalog Number | Comments/Description |
Cytidine 5′-diphosphocholin* | Sigma-Aldrich | C0256 | 4.7 µM Cytidine 5′-diphosphocholin |
Cytidine 5′-diphosphoethanolamine* | Sigma-Aldrich | C0456 | 2.7 µM Cytidine 5′-diphosphoethanolamine |
Linoleïc acid/ bovine serum albumin (BSA)* | Sigma-Aldrich | L8384 | 100 µg/ml linoleïc acid/ bovine serum albumin (BSA) |
Triiodo-L-thyronine* | Sigma-Aldrich | T6397 | 0.03 µM Triiodo-L-thyronine |
96-wells plate | Greiner bio-one | 655-095 | |
Binocular microscope | Leica | MZ-75 | |
CO2 independent (CO2-i) | Life Technologies | 18045054 | |
DMEM | Life Technologies | 41966029 | |
Forceps n°5 Dumont | Bionic | 11254-20 | |
Gelatin from porcin skin type A | Sigma-Aldrich | G2500 | |
GeneChip | Affymetrix | U74v2 | |
Gentamicin solution | Life Technologies | 15710049 | |
Hydrocortison* | Sigma-Aldrich | H0888 | 0.55 µM hydrocortison |
Insulin* (I) | Sigma-Aldrich | I1884 (ITS) | 0.86 µM insulin (I) |
Papain | Worthington-biochem | WOLS03124 | |
Poly-D-Lysine | Sigma-Aldrich | P-6407 | |
Progesterone* | Sigma-Aldrich | P7556 | 2.0 µM progesterone |
Prostaglandin* | Sigma-Aldrich | P5172 | 0.28 µM prostaglandin |
Putrescine* | Sigma-Aldrich | P5780 | 182 µM putrescine |
Scalpel Albion | EMS | 72000 | |
Scissor | Moria | 15396-00 | |
Sodium Pyruvate* | Sigma-Aldrich | S8636 | 1 mM sodium pyruvate |
Sodium Selenite* (S) | Sigma-Aldrich | I1884 (ITS) | 0.29 µM Na2SeO3 (S) |
Taurine* | Sigma-Aldrich | T8691 | 3 mM taurine |
Transferrin* (T) | Sigma-Aldrich | I1884 (ITS) | 0.07 µM transferrin (T) |
Vibratome apparatus | Leica | VT1000-S | |
* Supplements |