Generation of Human Patient iPSC-derived Retinal Organoids to Model Retinitis Pigmentosa

Riepilogo

In this protocol, retinitis pigmentosa patient induced pluripotent stem cell (iPSC)-derived 3D retinal organoids were generated. Those organoids successfully recapitulated some clinical phenotypes of the retinitis pigmentosa disease.

Abstract

Retinitis pigmentosa (RP) is a rare and inherited retinal degenerative disease with a prevalence of approximately 1/4,000 people worldwide. The majority of RP patients have progressive photoreceptor degeneration leading to peripheral vision loss, night blindness, and finally, total blindness. To date, thousands of mutations in more than 90 genes have been reported to be associated with RP. Currently, there are few animal models available for all the affected genes and different types of mutations, which largely hampers the deciphering of the mechanisms underlying the gene/mutation pathology and limits treatment and drug development. Patient induced pluripotent stem cell (iPSC)-derived 3D retinal organoids (ROs) have provided a better system to model the human early-onset disease than cells and animals. In order to study RP, those patient-derived 3D retinal organoids were utilized to recapitulate the clinical phenotypes of RP. In the RP patient-derived ROs, Rhodopsin mislocalization was clearly displayed. Compared with other animal models, patient iPSC-derived retinal organoid models more closely recapitulated RP features and represent an ideal approach for investigating the disease pathogenesis and for drug development.

Introduzione

Human retinal diseases, such as retinitis pigmentosa and age-related macular degeneration, are poorly understood due to the lack of appropriate experimental models^1,2. Although the mouse retina is very similar to the human retina and is a powerful tool for studying the etiology of retinal degeneration, there are huge species differences between mice and humans^3,4. For instance, the nuclear architecture of the photoreceptor cells in mice and humans is different, and the mouse retina does not possess a macula^5,6. Induced pluripotent stem cell (iPSC) technology enables us to return the specialized cells of organisms to the initial pluripotent state through the "reprogramming" processes by combinations of transcription factors and/or compounds^7,8,9,10. Those iPSCs have nearly unlimited division and proliferation ability and could develop into various types of cells. Recently, iPSC-derived 3D retinal organoids have been developed to model the early events of human retinal development and to delineate the pathophysiology of human retinal diseases^{11,12,13,14,15}. Retinal organoids have many advantages: (1) they can be used to recapitulate in vivo retinal development and disease pathogenesis; (2) they can be used for high-throughput drug screening and preclinical trials of gene therapy; and (3) they can be used as preclinical evaluations of treatment options for retinal degenerative diseases^16,17.

One objective of this project was to study the pathogenesis of retinal pigmentosa (RP), a disease remaining incurable because of its extreme heterogeneity¹⁸. To date, over 90 genes have been identified to be associated with RP^19,20. The RPGR gene, which is considered one of the most prevalent causative genes of RP¹⁵, accounts for approximately 16% of all RP^4,21,22. iPSCs carrying a frameshift mutation in the RPGR gene have been successfully generated and differentiated into organized and stratified 3D retinal organoids¹⁴. By utilizing these organoids, abnormal photoreceptor layer morphology and the dislocation of opsins in photoreceptors were observed.

Altogether, a step-by-step and approachable protocol is described in detail here on how to generate patient-derived 3D retinal organoids^23,24. Those organoids successfully recapitulated some clinical phenotypes of the disease. This provides an encouraging model to study retinal development and disease mechanisms, for therapeutic screening, and to evaluate future preclinical gene therapy.

Protocollo

The protocol follows the guidelines of Capital Medical University's human research ethics committee.

1. Cell culture and generation of iPSCs

1. Choose RPGR patients for this study. Here, three patients, one familial carrier and three healthy controls, were used. Patient 1 possessed a mutation c.1685_1686delAT in exon 14 of the RPGR gene, patient 2 harbored a mutation c.2234_2235delGA in exon 15 of the RPGR gene, and patient 3 had a mutation c.2403_2404delAG in exon 15 of the RPGR gene. Choose three healthy volunteers as controls¹⁴. Collect peripheral whole blood samples from the patients and healthy controls with venipuncture, and put them in blood collection tubes (Table of Materials).
2. Isolate peripheral blood mononuclear cells (PBMCs) by density gradient centrifugation with a density gradient medium²⁵. Expand CD34⁺ PBMCs activated with cytokine in 8 mL of growth enhancement medium (Table 1) in a Petri dish.
3. After 1 week, electroporate the CD34-positive peripheral blood mononuclear cells with a transfection system (Table of Materials) and choose CD34-positive cells and the inbuilt T-01 Program without any modification. Transfect a cocktail of reprogramming plasmids (2 µg) that could produce human OCT4, SOX2, NANOG, LIN28, c-MYC, and KLF4 proteins.
4. Seed the transfected cells in vitronectin human recombinant protein-coated six-well plates in 2 mL of DMEM/ F12 supplemented with B-27 (1x), N-2 (1x), PD0325901 (0.5 µM), bFGF (100 ng/mL), CHIR99021 (3 µM), HA-100 (10 µM) , A-83-01 (0.5 µM), and hLIF (1000 U/mL) to maintain the growth of the iPSCs.
5. 3 weeks after transduction, pick the colonies of iPSCs manually, subsequently transfer them into six-well plates, and maintain them in reagent B (Table of Materials) for expansion.
   NOTE: The six-well plates should be coated with vitronectin human recombinant protein. Dilute vitronectin solution in DPBS to reach a final concentration of 10 µg/mL. Add the diluted vitronectin to tissue culture-treated six-well plates. Swirl the cultureware to spread the vitronectin solution evenly across the surface of the six-well plates. Incubate the plates at 37 °C for at least 1 h.
6. Ensure that cells are disaggregated with 0.5 mM EDTA when the iPSCs reach around 80%-90% confluence and that the splitting ratio is 1:8 or 1:10. On the day of passage, add blebbistatin (2.5 µM) to reagent B (Table 1).
   NOTE: Blebbistatin was used to promote cell adhesion and stem cell viability and prevent stem cell apoptosis. 18 h later, the medium should be refreshed with reagent B.

2. Generation of human ROs

NOTE: The iPSCs must be dissociated when the iPSCs reach around 80%-90% confluence.

1. Dissociate the human iPSC (hiPSC) colonies using 0.5 mM EDTA at 37 ◦C for 5 min.
2. Segregate the hiPSC clumps into single cells by gentle pipetting. Count the cells for maintenance and grow them in reagent B (Table 1).
3. On day 0, seed 12,000 cells/well in 96 V-bottomed conical wells for cell reaggregation in 100 µL of differentiation medium (Table 1).
4. On day 6, carefully aspirate the cell culture medium and add 100 µL of the differentiation medium containing 20 μM Y 27632 and 1.5 nM (55 ng/mL) hBMP4.
5. On day 9, replace half of the differentiation medium to maintain the hBMP4 concentration at around 0.75 nM.
6. Aspire 60 µL of the differentiation medium from each well and add 60 µL of the fresh differentiation medium containing 20 μM Y-27632 to each well.
7. On day 12, replace half of the differentiation medium with fresh differentiation medium to achieve 0.375 nM hBMP4.
8. On day 18, transfer the aggregates into a new Petri dish with 10 mL tips and cut each aggregate into 2-4 pieces with a microsurgical knife under an inverted microscope to generate more organoids.
9. Transfer the aggregates from the same group into one 15 mL centrifuge tube. When the aggregates settle down to the bottom of the tubes, aspirate the supernatant.
10. Resuspend the aggregates with 1 mL of neural retina medium (DMEM/F12, 10% fetal bovine serum, 1% N-2 supplement, 0.5 µM retinoic acid, 0.1 mM taurine, 1% penicillin-streptomycin), and transfer them into non-stick Petri dishes with 15 mL of neural retina medium in it. Put 10-15 retinal organoids in one Petri dish.
11. Refresh the culture medium every 5 days. Keep the culture in the dark since retinoic acid is sensitive to light (Figure 1) and check the formation of the self-organizing human retinal tissue with an inverted microscope^26,27,28,29.

3. Analysis of retinal organoids

1. Immunofluorescence staining of hiPSC-derived 3D retinae
   1. Fix the 3D retinae in 1 mL of fresh 4% paraformaldehyde in DPBS for 20 min at 4 °C. Then, rinse the organoids with 1 mL of DPBS buffer.
   2. Measure 0.5 g of agarose (Table of Materials), and mix the agarose powder with 100 mL of DPBS in a microwavable flask. Use a microwave to dissolve the agarose for 1-3 min until it is completely dissolved. Let the agarose solution cool down to about 40 °C.
   3. Embed them in 2% in DPBS.
   4. Slice the samples into sections with a thickness of 50 µm using a vibratome. Place the sliced sections on adhesion microscope slides (Table of Materials) and store them at −80 °C.
   5. Thaw the slices for 15 min at room temperature (RT).
   6. Wash the slices with DPBS for 5 min, and repeat 3x.
   7. Use 0.5% Triton X-100 in DPBS solution to permeabilize the slices for 15 min at RT.
   8. Remove the permeabilization solution, wash the slices with DPBS for 5 min, and repeat 3x.
   9. Incubate the slices with 1% BSA and 0.5% Triton X-100 in DPBS for 30 min at RT.
   10. Incubate the slices with anti-Rhodopsin antibody (1:1000) and anti-L/M opsin antibody (1:1000) diluted in 1% BSA and 0.1% Triton X-100 for 16 h at 4 °C.
   11. Wash the slices with DPBS for 5 min, and repeat 3x.
   12. Incubate the slices with donkey anti-mouse and rabbit secondary antibody (1:500) diluted in 1% BSA and 0.1% Triton X-100 for 1 h at RT.
   13. Add 30 nM 4',6-diamidino-2-phenylindole (DAPI) in DPBS for 15 min at RT for nuclear staining.
   14. Wash the slices with DPBS for 5 min, and repeat 5x.
   15. Mount the slices with embedding medium (Table of Materials) and cover the slides (Table of Materials). Keep in a dark box and store at 4 °C for up to 4 weeks.
2. RNA extraction and RNA sequencing
   1. Collect 1-3 (according to sizes) 3D retinae derived from each control and patient at the same differentiation time points (day 0, day 47, day 91, day 121, and day 151) in 1 mL of RNA extraction reagent (Table of Materials) on ice.
   2. Use the homogenizer (Table of Materials) to disrupt the samples, 1 min ON, 30 s OFF, for three cycles, and check the sample for disruption.
   3. Stop the homogenization when the samples are totally homogenized, and vortex the tube vigorously for 30 s.
   4. Incubate the samples on ice for 15 min.
   5. Centrifuge the samples at 13,800 x g for 15 min at 4 °C and transfer the supernatant to a new 2 mL RNase-free tube.
   6. Add 0.4 mL of chloroform to the tube, vortex, and centrifuge at 13,800 x g for 10 min at 4 °C.
   7. Transfer a portion (about 2/3) of the colorless upper phase to a new RNase-free tube carefully. Do not aspirate or disturb the white interface containing DNA.
   8. Add 0.5 mL of cold isopropanol, vortex for 30 s, and centrifuge at 13,800 x g for 15 min at 4 °C. A pellet forms on the side of the tube bottom.
   9. Carefully aspirate the supernatant with a pipette tip and keep the pellet.
      NOTE: The RNA pellet is jelly-like and transparent when the purity is very high.
   10. Add 1 mL of cold 70% ethanol, vortex the samples vigorously for 30 s, and centrifuge at 13,800 x g for 10 min at 4 °C.
   11. Carefully aspirate the supernatant with a pipette tip and air-dry the pellet in the hood at RT for 5-10 min. Do not overdry the pellet.
   12. Add 20-50 µL of RNase-free water and resuspend the pellet carefully by pipetting up and down.
   13. Apply 1 µL of RNA on a spectrophotometer (Table of Materials). A total of around 10 µg of RNA from controls and patients respectively were used for Illumina library preparation³⁰.
   14. Store the RNA at −80 °C.

Risultati

The schematic illustration describes the differentiation procedures to generate healthy and patient iPSC-derived retinal organoids (Figure 1). From iPSC to ROs, variations can be produced owing to several factors. The status of the iPSC is the determinant step of the RO generation. In addition, it is highly recommended that researchers should record every step, catalog, and lot number of all media so that the entire experiments are trackable. In Figure 2A, the i...

Discussione

Retinal organoids are 3D, laminated structures derived from hiPSCs or embryonic stem cells (ESCs) and feature as a very promising model to mimic the spatial and temporal patterns of human retinal development^31,32. The ROs consist of various types of retinal cells, including photoreceptors, bipolar cells, ganglion cells, amacrine cells, horizontal cells, and Müller glia³³. 2D culture cannot precisely mimic the orientation and developme...

Materiali

Name	Company	Catalog Number	Comments
96 V-bottomed conical wells	Sumitomo Bakelite	MS-9096VZ
A-83–01	R&D Systems	2939/10
Adhesion microscope slides	CITOtest	188105
Agarose	Gene Tech	111760
Amaxa Nucleofector 2b Device	Lonza	AAB-1001	Transfection system
B-27	Thermo Fisher Scientific	17504044
bFGF	R&D Systems	3718-FB
Blebbistatin	Nuwacell Biotechnologies	RP01008
Blood collection tube	BD Vacutainer EDTA	366643
CHIR99021	TOCRIS	4423/10
Cover slides	CITOGLAS	10212440C
cTarget hPSC Medium	Nuwacell Biotechnologies	RP01020
DAPI	Invitrogen	D-1306
DMEM/Ham’s F12	Gibco	10565-042
Donkey anti-mouse 488	Invitrogen	A-21202
Donkey anti-rabbit 594	Invitrogen	A-21207
EDTA	Nuwacell Biotechnologies	RP01007
Embedding medium	FluorSaveTM Reagent	345789
EX-CYTE growth enhancement medium	Sigma	811292	Growth enhancement medium
Fetal bovine serum	Gibco	04-002-1A
Ficoll	Sigma-Aldrich	26873-85-8	Density gradient medium
FLT3L	Peprotech	300-19
GlutaMAX	Life Technologies	35050-061	L-glutamine supplement
HA-100	STEMCELL Technologies	72482
Ham’s F12	Gibco	11765-054
hLIF	Thermo Fisher Scientific	AF-250-NA
Homogenizer	EDEN lab	D-130
IL-3	Peprotech	213-13
IL-6	Peprotech	200-06
Iscove’s Modified Dulbecco Medium	Gibco	12440053
KnockOut Serum Replacement - Multi-Species	Gibco	A3181502	Serum replacement media
L/M-opsin	Millipore	ab5405
Monothioglycerol	Sigma	M6145
N-2 supplement	Thermo Fisher Scientific	17502048
Nanodrop Spectrophotometer	Thermo Fisher Scientific	ND2000	Spectrophotometer
ncEpic 125x Supplement	Nuwacell Biotechnologies	RP01001-02	125x Supplement
ncEpic Basal Medium	Nuwacell Biotechnologies	RP01001-01	Basal hpsc medium
ncLaminin511 human recombinant protein	Nuwacell Biotechnologies	RP01025
PD0325901	STEMCELL Technologies	72182
Penicillin-streptomycin	Gibco	15140-122
Recombinant human BMP4	R&D Systems	314-BP
Retinoic acid	Sigma	R2625
Rhodopsin	Sigma	O4886
RNeasy Mini Kit	Qiagen	74104
RNeasy Mini Kit	Qiagen	74104
sIL6-R	Thermo Fisher Scientific	RP-75602
StemSpan SFEM medium	STEMCELL Technologies	09600
Taurine	Sigma	T8691
Trizol reagent	Invitrogen	15596026
Vitronectin	Nuwacell Biotechnologies	RP01002
V-Lance knife	Alcon Surgical	8065912001