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This protocol presents two techniques to isolate the subcellular compartments of murine rod photoreceptors for protein analysis. The first method utilizes live retinae and cellulose filter paper to separate rod outer segments, while the second employs lyophilized retinae and adhesive tape to peel away rod inner and outer segment layers.
Rod photoreceptors are highly polarized sensory neurons with distinct compartments. Mouse rods are long (~80 µm) and thin (~2 µm) and are laterally packed in the outermost layer of the retina, the photoreceptor layer, resulting in alignment of analogous subcellular compartments. Traditionally, tangential sectioning of the frozen flat-mounted retina has been used to study the movement and localization of proteins within different rod compartments. However, the high curvature of the rod-dominant mouse retina makes tangential sectioning challenging. Motivated by the study of protein transport between compartments, we developed two peeling methods that reliably isolate the rod outer segment (ROS) and other subcellular compartments for western blots. Our relatively quick and simple techniques deliver enriched and subcellular-specific fractions to quantitatively measure the distribution and redistribution of important photoreceptor proteins in normal rods. Moreover, these isolation techniques can also be easily adapted to isolate and quantitatively investigate the protein composition of other cellular layers within both healthy and degenerating retinae.
Rod photoreceptor cells, tightly packed in the outermost layer of the neural retina, are an integral part of dim light vision. To function as faithful photon counters, rods utilize a G-protein-based signaling pathway, termed phototransduction, to generate rapid, amplified, and reproducible responses to single photon capture. This response to light ultimately triggers a change in the current at the plasma membrane and is subsequently signaled to the rest of the visual system1. As their name implies, each rod cell has a distinct rod-like shape and exhibits a highly polarized cellular morphology, consisting of an outer segment (OS), inner segment (IS), cell body (CB), and synaptic terminal (ST). Each subcellular compartment has specific protein machinery (membrane-bound and soluble), biomolecular features, and protein complexes that play crucial roles such as visual phototransduction, general housekeeping and protein synthesis, and synaptic transmission2,3.
Over 30 years ago, the light-dependent reciprocal movement of subcellular proteins, specifically transducin (away from the OS) and arrestin (towards the OS), was first observed4,5,6,7. Early on, this observed phenomenon was received with skepticism, due in part to immunohistochemistry's vulnerability to epitope masking8. In the early 2000s, stimulus-dependent protein translocation was confirmed by using a rigorous and arduous physical sectioning technique9. Serial tangential sectioning of the frozen flat-mounted rodent retinae followed by immunoblotting revealed that transducin9,10, arrestin11,12, and recoverin13 all undergo subcellular redistribution in response to light. It is believed that light-driven translocation of these key signaling proteins not only regulates the sensitivity of the phototransduction cascade9,14,15, but may also be neuroprotective against light damage16,17,18. Because light-driven protein transport in rods appears to be very significant to rod cell biology and physiology, techniques that permit the isolation of different subcellular compartments to determine protein distribution are valuable research tools.
Currently, there are a few methods aimed at isolating the rod subcellular compartments. However, these methods can be lengthy and difficult to reproduce, or require a sizable amount of retinal isolate. Rod outer segment (ROS) preparations via density gradient centrifugation19, for example, is commonly used to separate the ROS from retinal homogenate. This method is widely used for western blot, but the procedure is very time consuming and requires a minimum of 8-12 murine retinae20. On the other hand, serial tangential sectioning of frozen murine and rat retinae has been successfully implemented in isolating the OS, IS, CB, and ST9,11,13. However, this method is technically challenging due to the necessity of fully flattening the small and highly curved murine retina to align the retinal layers prior to tangential sectioning. Since there are a plethora of mouse models and transgenic mice recapitulating diseases of the visual system, the creation of a technique that reliably, quickly, and easily separates individual rod compartments holds promise in revealing the physiologic processes that occur in each specialized compartment and the mechanisms that underlie visual processes in health and disease.
To facilitate these investigations, we describe two peeling methods that isolate rod subcellular compartments more easily than current protocols. The first peeling method, adapted from a technique to expose fluorescently labeled bipolar cells for patch clamp recording21, employs cellulose filter paper to sequentially remove the ROS from a live, isolated murine retina (Figure 1). The second method, adapted from a procedure that isolates the three primary retinal cell layers from a chick22 and frog23 retina, utilizes adhesive tape to remove the ROS and rod inner segment (RIS) from a lyophilized retina (Figure 2). Both procedures can be completed in 1 h and are considerably user-friendly. We provide validation of the effectiveness of these two separation protocols for western blot by utilizing dark-adapted and light-exposed retinae from C57BL/6J mice to demonstrate light-induced translocation of rod transducin (GNAT1) and arrestin (ARR1). Moreover, using the tape peeling method, we provide additional evidence that our technique can be used to examine and address inconsistencies between protein localization data acquired by immunocytochemistry (ICC) and western blots. Specifically, our technique showed that: 1) the protein kinase C-alpha (PKCα) isoform is present not only in bipolar cells, but also in murine ROS and RIS, albeit in low concentrations24,25, and 2) rhodopsin kinase (GRK1) is present predominantly in the isolated OS sample. These data demonstrate the effectiveness of our two peeling techniques for separating and quantifying specific rod and retinal proteins.
All experiments were performed according to the local institutional guidelines of the committee on research animal care from the University of Southern California (USC).
1. Live cell retinal peeling method
2. Lyophilized retina peeling method
3. Western blot sample preparation for peeling isolations
The present strategies were developed to provide relatively rapid and simple methods to isolate and analyze proteins among specific rod subcellular compartments for western blot analysis. We applied two sequential peeling techniques (Figure 1 and Figure 2) followed by immunoblotting to demonstrate that these methods could reliably be used to detect the known distribution of rod transducin (GNAT1) and arrestin (ARR1) in both dark- and light-adapted animals. To va...
Many retinal diseases affect rod photoreceptor cells, leading to rod death and, ultimately, complete vision loss37. A significant portion of the genetic and mechanistic origins of human retinal degeneration have been successfully recapitulated in numerous mouse models over the years. In that context, the ability to easily and selectively separate individual rod subcellular compartments from the small mouse retina would greatly enhance our understanding of the localized biochemical and molecular un...
The authors declare that they have no competing interests.
This work was supported by NIH Grant EY12155, EY027193, and EY027387 to JC. We are thankful to Dr. Spyridon Michalakis (Caltech, Pasadena, USA) and Natalie Chen (USC, Los Angeles, USA) for proofreading the manuscript. We would also like to thank Dr. Seth Ruffins (USC, Los Angeles, USA) and Dr. Janos Peti-Peterdi (USC, Los Angeles, USA) for providing the necessary equipment to collect the author provided footage. Material from: Kasey Rose et al, Separation of photoreceptor cell compartments in mouse retina for protein analysis, Molecular Neurodegeneration, published [2017], [Springer Nature].
Name | Company | Catalog Number | Comments |
100 mL laboratory or media bottle equipped with a tubing cap adapter | N/A | N/A | |
100% O2 tank | N/A | N/A | |
1000mL Bottle Top Filter, PES Filter Material, 0.22 μm | Genesee Scientific | 25-235 | |
4X SDS Sample Buffer | Millipore Sigma | 70607-3 | |
50 mL Falcon tube | Fisher Scientific | 14-432-22 | |
95% O2 and 5% CO2 tank | N/A | N/A | |
Ames’ Medium with L-glutamine, without bicarbonate | Sigma-Aldrich | A1420 | |
CaCl2 (99%, dihydrate) | Sigma | C-3881 | |
Drierite (Anhydrous calcium sulfate, >98% CaSO4, >2% CoCl2) | WA Hammond Drierite Co LTD | 21005 | |
Falcon Easy-Grip Petri Dish (polystyrene, 35 x 10 mm) | Falcon-Corning | 08-757-100A | |
Falcon Easy-Grip Tissue Culture Dish (60 x 15 mm) | Falcon-Corning | 08-772F | |
Feather Scalpel (No. 10, 40 mm) | VWR | 100499-578 | |
Feather Scalpel (No. 11, 40 mm) | VWT | 100499-580 | |
KCl (99%) | Sigma | P-4504 | |
Kimble Kontes pellet pestle | Sigma | z359971 | |
Labconco Fast-Freeze Flasks | Labconco | N/A | |
LN2 (liquid nitrogen) + Dewar flask or similar vacuum flask | N/A | N/A | |
MgCl2 | Sigma | M-9272 | |
Milli-Q/de-ionized water | EMD Millipore | N/A | |
Na2HPO4 (powder) | J.T. Baker | 4062-01 | |
NaCl (crystal) | EMD Millipore | Sx0420-3 | |
NaHCO3 | Amresco | 0865 | |
OmniPur EDTA | EMD | 4005 | |
OmniPur HEPES, Free Acid | EMD | 5320 | |
Parafilm M | Sigma-Aldrich | P7793 | |
Reynolds Wrap Aluminum Foil | Reynolds Brands | N/A | |
Scotch Magic Tape (12.7 mm x 32.9 m) | Scotch-3M | N/A | |
Sodium deoxycholate | Sigma-Aldrich | D67501 | |
Spectrafuge mini centrifuge | Labnet International, Inc | C1301 | |
Tissue incubation chamber (purchased or custom made) | N/A | N/A | |
Tris-HCl | J.T.Baker | 4103-02 | |
Triton X-100 | Signma-Aldrich | T8787 | |
VirTis Benchtop 2K Lyophilizer or equivalent machine | SP Scientific | N/A | |
VWR Grade 413 Filer Paper (diameter 5.5 cm, pore size 5 μm) | VWR | 28310-015 | |
Whatman Grade 1 Qualitative Filter Paper (diameter 9 cm, pore size 11 μm) | Whatman/GE Healthcare | 1001-090 | |
Wide bore transfer pipet, Global Scientific | VWR | 76285-362 |
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