Ligand-Independent Activation of Platelet-Derived Growth Factor Receptor B Promotes Contraction of Retinal Pigment Epithelial Cells

Research Article

Austin J Clin Ophthalmol. 2023; 10(2): 1144.

Ligand-Independent Activation of Platelet-Derived Growth Factor Receptor β Promotes Contraction of Retinal Pigment Epithelial Cells

Duan Y1,2#, Wu W3#, Cui J4, Matsubara JA4, Kazlauskas A5, Li X1* and Hetian Lei6*

1Tianjin Key Laboratory of Retinal Functions and Diseases, Tianjin Branch of National Clinical Research Center for Ocular Disease, Eye Institute and School of Optometry, Tianjin Medical University Eye Hospital, China

2Shanxi Bethune Hospital, Shanxi Academy of Medical Sciences, Tongji Shanxi Hospital, Third Hospital of Shanxi Medical University, China

3Department of Ophthalmology, Hunan Key Laboratory of Ophthalmology, National Clinical Research Center for Geriatric Disorders, Xiangya Hospital of Central South, China

4Department of Ophthalmology and Visual Sciences, The University of British Columbia, Canada

5Department of Ophthalmology, University of Illinois at Chicago, USA

6Shenzhen Eye Hospital, Jinan University, Shenzhen Eye Institute, China

*Corresponding author: Hetian LeiXiaorong Li, Department of Ophthalmology, University of Illinois at Chicago, 251 Fukang Road, Xiqing District, Tianjin 300384, China

Hetian Lei, Shenzhen Eye Hospital, Jinan University, Shenzhen Eye Institute, 18 Zetian Road, Futian District, Shenzhen, China

Received: January 30, 2023; Accepted: March 07, 2023; Published: March 14, 2023


Background: Epiretinal membranes in patients with Proliferative Vitreoretinopathy (PVR) consist of extracellular matrix and a number of cell types including Retinal Pigment Epithelial (RPE) cells and fibroblasts, whose contraction causes retinal detachment. In RPE cells depletion of Platelet-Derived Growth Factor (PDGF) receptor (PDGFR) β suppresses vitreous-induced Akt activation, where as in fibroblasts Akt activation through indirect activation of PDGFRa by growth factors outside the PDGF family (non-PDGFs) plays an essential role in experimental PVR. Whether non-PDGFs in the vitreous, however, were also able to activate PDGFRβ in RPE cells remained elusive.

Methods: We showed that expression of a truncated PDGFRβ lacking a PDGF-binding domain in the RPE cells whose PDGFRB gene had been silent using the CRISPR/Cas9 technology restored vitreous-induced Akt activation as well as cell proliferation, epithelial-mesenchymal transition, migration and contraction.

Results: We found that scavenging Reactive Oxygen Species (ROS) with N-acetyl-cysteine and inhibiting Src Family Kinases (SFKs) with their specific inhibitor SU6656 blunted the vitreous-induced activation of the truncated PDGFRβ and Akt as well as the cellular events related to the PVR pathogenesis.

Conclusions: These discoveries suggest that in RPE cells PDGFRβ can be activated indirectly by non-PDGFs in the vitreous via an intracellular pathway of ROS/SFKs to facilitate the development of PVR, thereby providing novel opportunities for PVR therapeutics.

Keywords: Vitreous Indirect Activation; PDGFRβ; Akt; Retinal Pigment Epithelial Cells; Proliferation; Epithelial-Mesenchymal Transition; Migration; Contraction


Platelet-Derived Growth Factor (PDGF) receptors (PDGFRs) were in 1994 firstly shown to be expressed in cells within Epi Retinal Membranes (ERMs) from patients with Proliferative Vitreoretinopathy (PVR) [1]. PVR is a fibrotic eye disease, which develops at a rate of 5-10% after surgery correction of a retinal detachment [2-4] and occurs at a rate of 40-60% after open ocular trauma [5,6]. In the PVR pathogenesis retinal cells including Retinal Pigment Epithelial (RPE) cells lodged in the vitreous after retinal repairing surgery or mechanistic retinal damage from Epi-or sub-Retinal Membranes (ERMs) after their proliferation, Epithelial-Mesenchymal Transition (EMT), migration and secretion of extracellular matrix [3,7]. The tracking force of the ERMs causes retinal detachment [8]. At present there is no approved medicine for this eye disease [9]; the only treatment option with the surgery leads to the poor sight recovery [10,11]. Therefore, it is urgent to develop a pharmacological approach for the therapy of PVR.

In the PDGFR family the products of two genes PDGFRA and PDGFRB can form three dimers: PDGFRaa, PDGFRaβ and PDGFRββ, which are Receptor Tyrosine Kinases (RTKs) [12]. There are activated PDGFRa and PDGFRβ in ERMs from PVR patients [13], and in the ERMs there are a variety of cell types including RPE cells, glial cells, fibroblasts and macrophages [8,14]. The PDGF family contains five protein members: PDGF-AA, -BB, -AB, -CC and -DD, which are produced by four genes: PDGFA, PDGFB, PDGFC and PDGFD. Notably, while PDGF-AA is a specific ligand for PDGFRaa, PDGF-BB can bind all the PDGFR dimers: PDGFRaa, -aβ and -β. In addition, PDGF-CC resembling PDGF-AB can bind to PDGFRaa and -aβ, whereas PDGF-DD can bind to PDGFRββ with a strong affinity, but to PDGFRa with a weak affinity [15,16].

Traditionally, binding of a ligand (e.g., PDGF) specifically to a RTK (e.g., PDGFR) induces the RTK’s dimerization and conformation change, leading to activation of its intracellular kinase domain that phosphorylates itself at a number of tyrosine sites. Subsequently, the phosphorylated tyrosine can be bound by intracellular enzymes with a Src Homology (SH)2 domain, for instance, Src, and adaptors including a p85 regulatory subunit of Phosphorinositide 3-Kinases (PI3K) [16-18]. As a consequence, the extracellular signal is transduced to a variety of intracellular enzymatic activation including the signaling pathway of PI3K/Akt. As a serine and threonine kinase, Akt plays an essential role in numerous cellular events including cell growth, survival, dedifferentiation, motility, and metabolism [19,20]. Up-regulation of Akt activity is strongly associated with a number of human diseases including cancer [17,21] and PVR [22-25].

In addition, PDGFRa could be activated indirectly via an intracellular route of Reactive Oxygen Species (ROS) and Src Family Kinases (SFKs) [24,26]. Here, ROS comes from Rac GTPases–regulated Nicotinamide Adenine Dinucleotide Phosphate (NADPH) oxidase [27] and mitochondria due to the decreased autophagy [25]. That is, growth factors outside the PDGF family (non-PDGFs) are able to activate the pathway of PDGFRa/PI3K/Akt/mTORC1 (mammalian target of rapamycin complex 1), resulting in a reduction in autophagy [25]. This module of action has been demonstrated using fibroblasts derived from genetically modified mouse embryos [25,28]; that is, indirect activation of PDGFRa via an intracellular route of ROS/SFKs contributes to the development of PVR [23,29].

RPE cells play a critical role in the PVR pathogenesis because these cells are the major component of ERMs from patients with PVR [8,14]. Thanks to the advent of the technology of the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-associated endonuclease (Cas)9 [6,30-32], we recent discovered that PDGFRβ is the predominant isoform among the PDGFRs in RPEM cells, which were the RPE cells derived from ERMs from patients with PVR [16,33,34], and that PDGFRβ plays an essential role in vitreous-induced activation of Akt in the RPEM cells and cellular events related to the PVR pathogenesis [16]. Thereby, we hypothesized that in RPE cells non-PDGFs in the vitreous were able to activate PDGFRβ indirectly via an intracellular pathway of ROS/SFKs, and a truncated PDGFRβ lacking a PDGF binding domain [35,36] was harnessed to test this hypothesis.

Material and Methods

Major Reagents and Cell Culture

Antibodies against p-Akt (p-S473) (Catalog #: 9271), Akt (Catalog #: 9272), p-PDGFRβ (p-Y751, catalog #: 3166) and PDGFRβ (Catalog #: 3162) were purchased from Cell Signaling Technology (Danvers, MA), antibodies against Ki67 (Catolog #: ab243878) and a-smooth muscle act in (a-SMA) Catalog #: ab5694) were from Abcam (Danvers, MA), a heat shock protein (Hsp)90a (Catalog #: PA3-0137) was from ABR Affinity Bioreagents (Golden, CO) and a β-Actin antibody (Catalog #: sc-47778) was purchased from Santa Cruz Biotechnology (Santa Cruz, CA). HRP (Horseradish Peroxidase)-conjugated goat anti-rabbit IgG (Catalog #: sc-2004) and goat anti-mouse IgG (Catalog #: sc-2005) secondary antibodies were ordered from Santa Cruz Biotechnology. Enhanced chemiluminescent substrate for detection of horseradish peroxidase was from Thermo Fisher Scientific (Waltham, MA). N-Acetyl-Cysteine (NAC, a scavenger of ROS) and SU6656 (a selective inhibitor of Src family kinases) [23] were purchased from Sigma (St. Luis, MO) and Calbiochem (San Diego, CA), respectively.

Rabbit Vitreous (RV) was prepared by dissection from the rabbit eyeball when it was still frozen, and the thawed vitreous was centrifuged at 4°C for 5 minutes at 10,000×g. The resulting supernatant was used for all analyses. There is hardly detectable PDGF in RV as demonstrated previously [23].

RPEM cells were derived from an epiretinal membrane from a patient with grade C PVR and expressed RPE-cells’ markers including keratin as described previously [33; Xin, 2020 #1365]. These RPEM cells were gifts from Dr. Joanne Matsubara at the University of British Columbia, Canada, and grown in Dulbecco’s Modified Eagle’s Medium/nutrient mixture (DMEM/F12, Thermo Fisher Scientific) supplemented with 10% Fetal Bovine Serum (FBS) and antibiotics of streptomycin (50ug/ml) and penicillin (50 units/ml). When these cells were sub-cultured, 1:2 split was performed in their 90% confluence.

Human Embryonic Kidney (HEK) 293GPG cells were gifts from the Kazlauskas lab at the Schepens Eye Research Institute (Boston, MA) and were grown in high-glucose (4.5g/L) DMEM supplemented with 10% FBS, G418 (0.3mg/ml), tetracycline (1ug/ml) and puromycin (2ug/ml). These 293GPG cells were stably transfected with genes of vesicular stomatitis virus including the gag, pol, and the VSV-Ggene. All mammalian cells were cultured at 37°C in a humidified incubator with 5% CO2 [37].

Construction of PDGFRβΔx

Construction of PDGFRβΔx was completed in two steps. First, the PDGFRβΔx lacking amino acids 38 to 442 of the human PDGFRβwas cloned into the PVZ-ApaI-NotI-EcoRI-XbaI-SalI-PstI-HindII vector [35] using EcoRI/Xbal. Then the digested PDGFRβΔxDNA fragment with EcoRI/SalI was subcloned into pLXSHD-EcoRI-HpaI-XhoI-BamHI vector digested withEcoRI/Xhol. The resultant construct was termed pLXSHD-PDGFRβΔx and verified by Sanger DNA sequencing at the MGH DNA core facility (Cambridge, MA).

Production of Retrovirus

In the production of retrovirus there were three steps: transfection, collection of retrovirus, and concentration. Transfection: Gently mix lipofectamine 2000 (156μl, Thermo Fisher Scientific) pLXSHDor pLXSHD-PDGFRβΔx (25μg) in an OPTIMEM medium (1.8ml, Thermo Fisher Scientific), incubate these mixtures at room temperature for 30 minutes so as to form liposomesentraping the DNA, and then transfer these liposomes drop wise into the 293GPG cells, which were in about 70% confluence in a 15-cm cell culture dish. Notably, during transfection, the growth medium was changed to 10ml OPTIMEM, and after transfection for 7-10 hours, a 12ml virus-producing medium (high glucose DMEM with 10% FBS) was added. In the following morning, a 20ml fresh virus-producing medium was used to replace the old medium. Collection of retrovirus: harvest the culture media containing retroviruses after transfection at 48,72,96,120 hours, and spin at 1500rpm for 10 minutes to remove cells and debris. Concentration: Spin the supernatant containing the virus at 25,000×g, 4°C for 90 minutes, dissolve the white pellet on the bottom of the centrifuge tube in 300μl of sterile TNE buffer (50mM Tris pH7.8, 130mM NaCl, 1mM EDTA), and then gently rotate the solution overnight at 4°C to obtain the retrovirus [38].

Expression of PDGFRβΔx in RPEM cells

Depletion of PDGFRβ in RPEM cells was achieved using the CRISPR/Cas9 technology with a PB3 sgRNA (GCCTGGTCGTCACACCCCC) guiding SpCas9 to cleave human genomic PDGFRB at exon 3 as described previously [16]. These PDGFRβ-depleted RPEM cells were infected by the concentrated retrovirus in DMEM supplemented with 10% FBS and 8μg/mL polybrene (hexadimethrine bromide; Sigma, St. Louis, MO). Cells expressing PDGFRβΔx were selected in a histidine-free DMEM supplemented with 2mM L-histidinol dihydrochoride (Sigma), and the levels of the PDGFRβΔx in these cells were determined by western blot with an anti–PDGFRβ antibody recognizing the intracellular domain of PDGFRβ [16].

Western Blot

As described previously [16], proteins in cell lysates after centrifugal clarification at 13,000×g for 10 minutes were mixed with a sample buffer and denatured by boiling for 5 minutes. Subsequently the soluble proteins in the sample buffer were separated by 10% SDS-polyacrylamide gel electrophoresis. The proteins in the gel were then transferred to polyvinylidene difluoride membranes for western blot analysis with desired antibodies [16].

Cell Proliferation Assay

RPEM cells after trypsin detachment were counted and seeded into wells of a 24-well plate in DMEM/F12 with 10% FBS at a density of 3×104 cells/well. After attaching the plate, the RPEM cells were treated with DMEM/F12 only or RV (1:3 dilution in DMEM/F12) with additional NAC (5mM), SU6656 (1μM) or their solvent. The cells were trypsin detached after treatment for 48 hours for cell counting. Each experimental condition was treated in duplicate, and data from three independent experiments were subjected to statistic analysis [26,29].

In addition, after reaching 80% confluence, cells were cultured in a serum-free medium overnight, and then the cells were treated with RV plus Ki67 for additional 48 hours. Next, a Ki67 primary antibody was incubated with the cells overnight at 4°C, and the fluorescent secondary antibody was incubated for 1 hour in the dark at room temperature. Finally, the cells were stained with 4’,6-Diamidino-2-Phenylindole (DAPI) for 10 minutes, and the slides were mounted for photographs. The data were analyzed in Image J software for counting the Ki67-stained cell numbers [39].

An Epithelial-Mesenchymal Transition Assay

When RPEM cells grew to 90% confluence in 24-well culture plates, they were serum-starved overnight and then treated with RV or RV in the presence of NAC, SU6656 for additional 48 hours. The resultant lysates were subjected to western blot using desired antibodies, among which there was an antibody against a-SMA, a protein marker of EMT [40,41].

A Scratch Wound Assay

When RPEM cells grew to near confluence in wells of a 24-well plate, the wells were scratched with a 200μl pipet tip. After washing with Phosphate-Buffered Solution (PBS), the cells were treated with a medium of DMEM/F12 only or RV (1:3 dilution in DMEM/F12) with or without addition of NAC (5mM) or SU6656 (1μM). 16 hours later the wound areas were photographed and analyzed with Adobe Photoshop CS6 software. Data from three independent experiments were subjected to statistic analysis [16,42].

Contraction Assay

RPEM cells were trypsin detached, counted and mixed with collagen I (INAMED, Fremont, CA) on ice. The final collagen I concentration was 1.5mg/ml, cell density was at 1×106cells/ml, and the pH value was adjusted to 7.2 as described previously [23,29]. 300μl cell-gel mixture was transferred into wells of a 24-well plate, which had been preincubated with 5mg/ml bovine serum albumin/PBS for at least 8 hours. 90 minutes later the collagen gel was polymerized at 37°C, and 0.5ml DMEM/F12 or RV (1:3 dilution in DMEM/F12) with or without additional NAC (5mM) or SU6656 (1μM). The gel diameter was measured and photographed on day 2 or 3 for further analysis. Data from three independent experiments were subjected to statistic analysis [16,29,43].


As described previously [8,16], data were collected from three independent experiments for analysis using ordinary one-way analysis of variance (ANOVA) followed by Tukey’s Honest Significant Difference (HSD) post hoc test. A significant difference between groups was determined by a P value less than 0.05.

Institutional Review Board Statement

Collection of epiretinal membranes from patients with proliferative vitreoretinopathy for this study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board of the Vancouver Hospital and University of British Columbia Clinical Research Ethics Board.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Animal Use

The protocol for the use of animals was approved by the Schepens Eye Research Institute Animal Care and Use Committee (Boston, MA), and all animal surgeries adhered to the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research and followed the ARRIVE guidelines (


PDGFRβ in the RPE cells plays a critical role in vitreous-induced activation of Akt and cellular responses intrinsic to the pathogenesis of PVR [16], but whether non-PDGFs in the vitreous play a part in these signaling and cellular events was unknown. Our goal herein was to answer this intriguing question, and our hypothesis was that non-PDGFs were able to indirectly activate PDGFRβ via an intracellular pathway of ROS/SFKs based previous findings [26]. To test this hypothesis, we employed a truncated PDGFRβ lacking a PDGF binding domain [35,36] and specific inhibitors of ROS and SFKs. The experiments showed that non-PDGFs in the vitreous could activate PDGFRβ and Akt via the intracellular domain of PDGFR, leading to cellular responses of proliferation, EMT, migration and contraction related to the development of PVR.

Expression of a Truncated PDGFRβ Lacking a PDGF Binding Domain in the PDGFRB-Silent RPE Cells Restores Vitreous-Induced Akt Activation

To seek an answer to the question whether non-PDGFs in the vitreous activated PDGFRβ via an intracellular pathway in the RPE cells, a truncated PDGFRβ that did not have a PDGF-binding domain that spans from 38th to 442th amino acids of PDGFRβ [35] was expressed in the RPE cells whose PDGFRB had been silent using the CRISPR/Cas9 technology [16]. The single guide (sg) RNA denoted as PB3 guiding SpCas9 to specifically edit genomic PDGFRB could not recognize the mutant PDGFRB encoding the truncated PDGFRβ (Figure 1). In this CRISPR/Cas9 system, the PB3-sgRNA was designed to target human PDGFRB at exon 3, and the three nucleotides (GGG) of the Protospacer Adjacent Motif (PAM) for the PB3-sgRNA were absent from the mutant PDGFRB (Figure 1A). Thereby we constructed a retroviral vector with the PDGFRB mutant to express the truncated PDGFRβ. This truncated PDGFRβ was named as PDGFRβx (Figure 1B). The retrovirus containing DNA sequence coding PDGFRβ△x was used to infect the PDGFRB-silent RPEM cells (Figure 2). As expected, PDGFRβ△x was successfully expressed in the PDGFRβ-depleted RPEM cells, and PDGF-BB activated PDGFRβ and Akt in the control RPEM cells, but neither in the PDGFRB-silent or in the PDGFRβ△x-expressed RPEM cells (Figure 2). However, RV, in which there are no detectable PDGFs as determined previously [26], activated not only PDGFRβ but also PDGFRβ△x in the PDGFRβ or PDGFRβ△x-expressed RPEM cells (Figure 2). Notably, expression of PDGFRβ△x in the PDGFRβ-depleted RPEM cells restored vitreous-induced activation of Akt (Figure 2).