Human Umbilical Cord Mesenchymal Stem Cell-derived Exosomal miR-27b Attenuate Subretinal Fibrosis via Suppressing Epithelial-mesenchymal Transition by Targeting HOXC6

Background and aim Subretinal brosis resulting from neovascular age-related macular degeneration (nAMD) is one of the major causes of serious and irreversible vision loss worldwide, and no denite and effective treatment exists currently. Retinal pigmented epithelium (RPE) cells are crucial in maintaining the visual function of normal eyes and its epithelial–mesenchymal transition (EMT) is associated with the pathogenesis of subretinal brosis. Stem cells-derived exosomes have been reported to play a crucial part in tissue brosis by transferring their molecular contents. This study aimed to explore the effects of human umbilical cord (cid:0) derived mesenchymal stem cell exosomes (hucMSC-Exo) on subretinal brosis in vivo and in vitro and to investigate the anti-brotic mechanism of hucMSC-Exo. could suppress cells migration and promote the mesenchymal– epithelial transition by delivering miR-27b-3p. Analysis of the latent binding of miR-27b-3p to HOXC6 was made by bioinformatics prediction and luciferase reporter assays. subretinal brosis. HucMSCs-derived exosomal miR-27b could reverse the process of EMT induced by TGF-β2 via inhibiting HOXC6, which indicated that exosomal miR-27b/HOXC6 axis could play a vital role on ameliorating subretinal brosis. Our study put forward a promising therapeutic agent for the treatment of ocular brotic diseases, as well as comprehension into the mechanism of hucMSC-Exo under subretinal brosis.


Introduction
Subretinal brosis is a wound-healing response generated against CNV in nAMD responsible for severe blindness worldwide (1). In nAMD, CNV could stretch into Bruch's membrane or the retinal pigmented epithelium (RPE) layer, and subsequently form into a brovascular membrane, leading to retinal edema and subretinal hemorrhage, causing the destruction of photoreceptors and RPE cells (2). RPE cells are the main contributor to brotic scars at the end of nAMD. During subretinal brosis secondary to neovascular AMD, RPE cells lose their characteristic epithelial morphology and function and then transform into myo broblasts, which is called epithelial-mesenchymal transition (EMT). EMT is considered to be a key feature of pathological tissue that needs to be repaired in subretinal brosis (3).
Currently, the standard therapeutic modalities for nAMD is anti-vascular endothelial growth factor (VEGF) treatment. However, in the long run, frequent intravitreal administration of anti-VEGF drugs is not always able to successfully suppress the growth of CNV and its original e cacy will gradually decrease over time (4,5). Several studies explained that anti-VEGFA treatment promotes the formation of brotic scars in the subretinal space, and brotic changes could reduce the e cacy of anti-VEGFA pathway (6)(7)(8).
Therefore, the address of progressive brosis lesion including the reversal of EMT and the restoration of the epithelial phenotype will be potential molecular targets for neovascular AMD related subretinal brosis (9,10).
Mesenchymal stem cells (MSCs) are one of the most widely used stem cell types for immunomodulation, organ reconstruction, and tissue repair in clinical practice (11). It has been reported that their leading role in tissue repair is mediated by paracrine action (12); Among these paracrine factors, exosomes are considered to be a novel treatment tool for delivering functional proteins, mRNA, miRNA and lncRNA (13).They are small membrane-bound particles, and may serve as a strong candidate for cell-free therapies on account of overcoming the limitations of cell-based therapy while maintaining the advantages of their original cells (14). In addition, exosomes are also ideal carriers and act as a crucial role in intercellular communication for the reason that the exosomal membrane can protect their molecular content from degradation before reaching the target cells (15).
Increasing studies have identi ed that exosomes secreted from MSCs could elicit signi cant therapeutic effects by suppressing brosis and improving their function in multiple-organ brosis models, such as liver, kidney, myocardium, and several retinal injury models (16)(17)(18)(19)(20)(21)(22)(23). In addition, studies also reported that intravitreal injection of MSC-derived exosomes could ameliorate retinal laser injury by reducing damage, inhibiting apoptosis and in ammatory response (24). Treatment with MSC-derived exosomes retarded the growth of CNV and decreased the number of broblasts and collagen bers after laser photocoagulation, indicating that MSC-derived exosomes have great potential to reduce collagen formation in subretinal brosis (19). Moreover, Mathew et al. proved that exosomes derived from MSCs with a intravitreal injection could keep in the vitreous humor over 4 weeks, combining with the vitreous proteins in a dosedependent manner, indicating that administration of MSC-exosomes has a prolonged therapeutic effect and requires fewer injections (22).Based on the aforementioned nds, we believe that hucMSC-Exo could develop a cell-free regenerative therapy with great potential for treating subretinal brosis.

Ethics statement
All experiments on animals were approved by the Animal Care Committee of Shanghai General Hospital.
Principles of animal research abided by the guidelines of ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. The study was sanctioned by the Ethics Committee of the Shanghai General Hospital (Approval No.2020SQ120) and informed consents were acquired afore specimen collection.

Preparation of hucMSCs
Brie y, fresh umbilical cords were obtained and washed with phosphate-buffered saline (PBS) containing 1% penicillin and streptomycin. Then, the blood vessels were taken out and the remaining tissue was cut into 1-mm 3 pieces on culture plates. The medium was replaced every 3 days. Nonadherent fragments were discarded, and the adherent cells were subcultured with 0.25% trypsin. The hucMSCs in passages 3-7 were used for further experiments.
The immune phenotype of hucMSCs was characterized by flow cytometry using a Human MSC Analysis Kit (562245; BD Biosciences, USA) which contained four positive markers (FITC-CD90, PerCP™Cy5.5-CD105, APC-CD73, and PE-CD44), ve negative cocktails (PE-CD45, PE-CD34, PE-CD11b, PE-CD19, and PE-HLA-DR), and the respective isotype controls. The cells at a concentration of 5 10 6 cells/mL in PBS was stained following the BD's protocol. The analysis was carried out in a ow cytometer (Beckman Coulter, USA). Passage 3 hucMSCs were cultured in OriCell osteogenic and adipogenic differentiation media (Cyagen, Guangzhou, China) respectively as described by the manufacturer to identify the differentiation properties. After cultured for 23 days, hucMSCs were xed and dyed with Alizarin red for osteogenic cells and Oil red for adipose cells. The cells cultured in a normal medium were served as controls.
Extraction and identi cation of hucMSC-derived exosomes Exosome isolation was carried out following a previously published protocol(25) Brie y, hucMSCs were cultured and reached 50%-60% con uence. They were then placed in serum-free MSC medium (Nuwacell Biotechnology, RP02010-1 and RP02010-2, China). After 48 h of cultivation, the medium was obtained and centrifuged for 10 min at 300g and 30 min at 10000g. Next, the supernatant was ltered with a 0.22μm lter (Millipore Corp, USA) and was further ultracentrifuged at 100,000g for 70min in a SW32Ti rotor (Beckman Coulter, USA). After removing the supernatant, the pellet was resuspended in PBS and then centrifuged at 100,000g for 70min again. Finally, the pellet was stored at -80℃ after being collected in 200-300uL of PBS .

Exosome labeling
Puri ed exosomes were dyed with PKH67 (a green uorescent dye; Sigma-Aldrich, Germany) following the instructions. Brie y, exosomes were stained with 4 µL of PKH67 in 200 μL of Diluent C uid for 5 min. Next, an isovolumetric 1% BSA was appended to end the staining. PBS was used to wash the exosomes and re-puri ed via ultracentrifugation. Then, PKH67-labeled exosomes were incubated with ARPE19 cells for 12 h. Fluorescence microscopy (Leica Microsystems) was used to detect the green signals in cells.

Wound-healing assay
A total of 2 × 10 5 cells/well were plated and serum-starved overnight. Then, scratches on ARPE-19 cell monolayers were made with a sterilized 200-μL pipette tip. Images were recorded after 0, 24, and 48 h, and the wound recovery was analyzed using ImageJ. The migration capacity was determined by a percentage of wound closure.
Transwell assay ARPE-19 cells (3 × 10 4 ) resuspended in 200 μL of serum-free medium were placed in the upper chamber with a polycarbonate membrane (8-μm pore size, Corning, USA). Then, 600 μL of DMEM/F12 supplemented with 10% FBS was added to the lower chamber. After incubation for 12 h, cells were stained with 0.1% crystal violet. For visualization, images of cultured cells were collected and counted in random different ve elds.

Cells transfection with miRNA
Lipofectamine 2000 (Invitrogen, USA) was used for transfection. The cells were cultivated in six-well plates upon cell fusion of 60%. The instructions from GenePharma (Shanghai) were followed for the agomir and antagomir (100 pmol) transfection. The cells were harvested for the following e ciency and function assay 48 h after transfection.

RNA isolation and quantitation
We extracted total RNA from hucMSC-Exo using TRIzol Reagent (Takara, Japan). Before isopropanol precipitation, Dr.GenTLE Precipitation Carrier (#9094, TaKaRa,Japan) was added as a co-precipitant to increase the yield of exosomes RNA, which was reverse transcribed into cDNA with a Mir-X miRNA First-Strand Synthesis Kit (Cat. No. 638313, TaKaRa). qRT-PCR was performed using the TB Green Premix Ex Taq II (Tli RNaseH Plus) (RR820A, TaKaRa) and then detected with an ABI 7500 qPCR instrument (Thermo Fisher Scienti c). The relative expression of miRNAs or mRNAs was analyzed using the 2 −ΔΔCt method. U6 or GAPDH were deemed as an internal control. The primer sequences were acquired from GenePharma (Shanghai) ( Table 1). Laser-induced CNV model and drug administration C57BL/6J mice aged 6-8 weeks were used in this study. All animals were housed at the Laboratory Animal Center of Shanghai General Hospital. The laser-induced subretinal brosis model was established as previously described and observed for 35 days to generate subretinal brosis(1). In brief, four to six laser spots (532 nm, 180 mW, 100 ms; Novus Spectra, Japan) were selected at each fundus around the optic disc. The disruption of Bruch's membrane was con rmed by subretinal bubble formation immediately after laser application. HucMSC-Exo or PBS in 2 µL was injected into the vitreous cavity immediately after laser injury. After injection, the mice were randomly sacri ced (n = 15) on day 7, day 21, and day 35 for further quanti cation of CNV and subretinal brosis.
Choroidal at-mount and immuno uorescence staining The areas of CNV and collagen bers were determined on choroidal at mounts on day 7, day 21, and day 35. Mouse eyecups were xed in 4% PFA, and anterior segments were removed before cutting four to six radial incisions to be attened. Then, the RPE-choroid complexes were washed, blocked with 5% goat serum albumin and 0.3% Triton X-100, and then incubated with FITC-labeled isolectin-B4 (IB4) (Vector Laboratories, 1:100) (for evaluation of CNV) and collagen type I antibody (ab34710, Abcam, dilution 1:100) (for evaluating subretinal brosis) at 4°C overnight. The secondary antibody against collagen type I was Cy3-conjugated donkey anti-rabbit IgG (Jackson ImmunoResearch Labs, 1:200).

Hematoxylin and eosin and Masson staining
The eyes were removed after 7, 21, and 35 days and xed for 2 h at 4°C in 4% PFA, then dehydrated and embedded in para n. Hematoxylin and eosin (H&E) staining was conducted following speci c protocols. Masson's trichrome staining was carried out with a trichrome staining kit (ab150686, Abcam). The rate was automatically averaged using ImageJ (MD, USA).

Statistical analysis
GraphPad Prism Version 8.2 was used for statistical analysis. All data were expressed as mean ± standard deviation (SD). Statistical analysis between two sets of data was performed with the Student t test. Comparisons between multiple groups were analyzed by one-way analysis of variance. A P value less than 0.05 indicated a statistically signi cant difference.

Isolation and identi cation of hucMSCs
HucMSCs were successfully isolated from Wharton's jelly region of umbilical cords. After culturing for two weeks, the cells around tissue blocks displayed a broblast-like morphology and attached to the plastic surface (Fig. 1a). After passage, they grew rapidly and arranged in a spiral pattern (Fig. 1b). MSCs could undergo differentiation to osteocytes and adipocytes and their multipotency property was detected by Oil red O staining (Fig. 1c, d) and Alizarin red respectively (Fig.1e, f). The expression of surface markers of hucMSCs was determined by ow cytometry. The positive markers of CD73, CD90, and CD105 along with negative cocktail markers (CD19, CD34, CD45, CD14, and HLA-DR) were shown in Figure.1g. Mouse isotypic IgG was used as a control. These results showed that hucMSCs were successfully isolated and identi ed.
Exosome puri cation, characterization, andinternalization into cells HucMSC-Exo were purified from the third to seventh generation medium by serial centrifugation as described in Figure. 2a. As shown in Figure. 2b, the round or oval membranous vesicle morphology of exosomes was evaluated under a transmission electron microscope. Nanoparticle tracking analysis showed that the distribution of the particle size mainly focused on 30 to 160 nm particle size (Fig. 2c).
Western blot analysis showed that traditional exosomal markers (CD9, TSG101, CD63, and HSP70) were expressed obviously in isolated hucMSC-Exo (Fig. 2d). Therefore, it was con rmed that hucMSC-Exo were isolated successfully. Furthermore, PKH67-labeled exosomes were observed under a confocal microscope. The result revealed that exosomes were internalized by ARPE19 cells and intensively distributed around the nucleus (Fig. 2e).

hucMSC-Exo alleviated laser-induced CNV and subretinal brosis in mice
To explore the anti-brotic in uence of hucMSC-Exo on subretinal brosis in vivo, laser-induced CNV and subretinal brosis model was established and intravitreal injection was performed immediately (Fig. 3a). Masson staining performed on choroidal at mounts revealed that obvious disruption change could observed in the choroidal layer and outer nuclear layers within the center of laser burn 7days after laser photocoagulation. Subsequently, newly formed vessels and retinal edema were discovered to be extended into the subretinal space 7days and 21 days after laser photocoagulation. The collagen area was obviously decreased in hucMSC-Exo group compared with control group after 21 and 35 days (Fig. 3b, P < 0.05). Furthermore, immuno uorescence staining of choroidal at-mount indicated that the area of CNV was up to a maximum at day 7 and declined gradually between day 21 and day 35. Importantly, fewer vascular channels (isolectin-B4) and area of brosis (type 1 collagen) were observed in the subretinal space after 21 days in the hucMSC-Exo group (Fig. 3c, P < 0.05). These ndings indicated that hucMSC-Exo strongly reduced laser-induced subretinal brosis in mice.

hucMSC-Exo inhibited cell migration
TGF-β2-induced EMT has been experimented in numerous cell lines as brosis models, and ARPE19 cells are one of the leading effector cells in subretinal brosis (26,27). Hence, different concentrations of TGF-β2 were added to ARPE19 cell mediums to establish a successful RPE EMT model and further to verify the roles of hucMSC-Exo under the subretinal brosis environment. When ARPE-19 cells were supplied with diverse concentrations of TGF-β2 for 48 h, the epithelial markers expression was downregulated, whereas expression of mesenchymal markers, such as N-cadherin, Vimentin, and α-SMA, were upregulated in protein level (Fig. 4a, b). Stimulation of ARPE-19 cells with TGF-β2 at 10ng/ml resulted in a signi cant morphological alteration, with a transition from typical cobblestone-like cells to an elongated spindle-shaped mesenchymal-like cells (Fig. 4c). Compared with control group, the stimulated cells displayed a loss of epithelial characteristics and the acquisition of mesenchymal characteristics under the immuno uorescence microscope (Fig. 4d). These results suggested that ARPE-19 cells undergone a process of EMT after treatment with 10 ng/mL TGF-β2.
Additionally, Scratch assays showed that cells treated with TGF-β2 exhibited an enhanced migratory ability, while cells treated with MSC-derived exosomes exerted an opposite effect after 24 and 48 h (P < 0.05) (Fig. 4e). Similar results were observed in transwell assay, indicating that fewer cells migrated through the membrane in the hucMSC-Exo group than in the TGF-β2-treated groups after 12 h of incubation (Fig. 4f). These assays consistently demonstrated that exosomes derived from MSC treatment significantly inhibited cell migration.

hucMSC-Exo inhibited the expression of EMT-associated proteins
The alteration of representative proteins is an important feature of RPE cells under the subretinal brosis environment. The expression of EMT-related indicators and cell adhesion markers were examined to evaluate the effect of hucMSC-Exo in ARPE19 cells. Immuno uorescence results revealed that the expression of intercellular tight junction ZO-1 was abrogated by TGF-β2, which was alleviated by hucMSC-Exo. The mesenchymal proteins α-SMA and vimentin were distributed around the cells because of TGF-β2 induction. However, their expression markedly decreased upon exposure to hucMSC-Exo in combination with TGFβ2 compared with the TGF-β2 alone group (Fig. 5a). Besides, hucMSC-Exo signi cantly upregulated the expression of occludin while decreasing the expression of mesenchymal markers (Vimentin, N-cadherin, and α-SMA) compared with the TGF-β2 group (Fig. 5b). The aforementioned results indicated that hucMSC-Exo reduced subretinal brosis through suppressing EMT.

Detection of miR-27b expression in hucMSC-Exo
Studies have shown that miRNAs are enriched in MSC-derived exosomes and play a pivotal role in cellular communication. The present study aimed to identify the miRNAs that contribute to the therapeutic effects of hucMSC-Exo on subretinal brosis. We found that eight miRNAs were widely reported to be a strong enrichment in hucMSC-Exo and closely correlated with brosis (Fig. 6a). Next, the expression of these miRNAs was detected using qRT-PCR. The results indicated that miR-27b-3p expression was the highest in hucMSC-Exo relative to HSF-Exo (Fig. 6b). The average miR-27b levels relative to U6 in hucMSC-Exo after treatment with RNase (0.5 µg/µL) and 0.1% Triton X-100 for 30 min were measured by qRT-PCR, which veri ed the presence of miR-27b in hucMSC-Exo. As shown in Figure  6C, exosomes treated with RNase did not show a signi cantly low level of miR-27b unless they were cotreated with Triton X-100, which degraded the exosomes membrane (Fig. 6c). Hence, miR-27b was protected within the intact exosomes from RNase degradation, which was considered to be a real EV-miRNA. Furthermore, the elevated levels of miR-27 were con rmed in hucMSC-Exo. Therefore, miR-27b-3p was selected as a potential candidate to mediate the therapeutic effects of hucMSC-Exo in the model.
hucMSC-Exo transferred a high level of miR-27b to ARPE19 cells ARPE19 cells treated with exosomes were transfected with an miR-27b-3p inhibitor (antagomiR-27b-3p) to explore the effect of miR-27b-3p in hucMSC-Exo on TGFβ2-induced ARPE-19 cells. The results showed a marked decline in miR-27b-3p expression following antagomiR-27b-3p transfection (Fig. 7a). Therefore, the study con rmed that transferring miR-27b-3p was one of the mechanisms for hucMSC-Exo to suppress TGF-β2-meditated EMT in ARPE19 cells. The transfection with antagomiR-27b-3p led to a downregulation of epithelial marker occludin, while increasing the expression of mesenchymal markers (Vimentin, N-cadherin, and α-SMA) when compared with the hucMSC-Exo group (Fig. 7b). Consistently, scratch and transwell assays also con rmed that the migration of cells treated with hucMSC-Exo was enhanced if they were also transfected with antagomiR-27b-3p (Fig. 7d, e). The study similarly con rmed that hucMSC-Exo enhanced the expression of tight junction ZO-1 destroyed by TGF-β2, and this effect was eliminated by additional antagomiR-27b-3p treatment. In addition, the expression of mesenchymal proteins α-SMA and Vimentin signi cantly was increased upon exposure to antagomiR-27b-3p compared with the control group, as detected by immuno uorescence (Fig. 7c). Together, these results revealed that hucMSC-Exo was enriched in miR-27b-3p, which could regulate the functionality of TGFβ2-induced ARPE-19 cells.
Enhancing the expression of miR-27b inhibited TGFβ2-induced EMT To gain insights into whether miR-27b-3p could exert an impact on TGF-β2-induced EMT, series of experiments were conducted. First, the transfection e ciency of agomiR-27b-3p was detected by qRT-PCR (Fig. 8a). In addition, the addition of miR-27b-3p markedly rescued the loss of epithelial phenotype ZO-1 and inhibited the EMT of ARPE19 cells compared with the TGF-β2 group, as demonstrated by immuno uorescence and western blot analysis (Fig.8b, c). Furthermore, the activation of TGF-β2induced migration was obviously suppressed by miR-27b-3p overexpression in comparison with that in RPE cells without miR-27b-3p pretreatment (Fig. 8d, e). Together, our results con rmed that miR-27b-3p could directly inhibit the EMT of TGFβ2-induced ARPE19 cells in vitro.
Identi cation of candidate genes regulated by miR-27b As miR-27b markedly suppressed EMT in vitro, the present study concentrated on explicating the potential target of miR-27b in RPE cells. Bioinformatics analysis was performed with ve online analysis instruments (miRanda, miRTarbase, TargetScan, miRDB, and miRsystem). As a result, 7 genes were predicted to be direct targets of miR-27b-3p, including HOXC6 (Figure 9a). Among the predicted target genes, the previous literatures have reported that HOXC6 could act as a vital role in EMT (28)(29)(30), therefore we chose it as the candidate. In addition, the dual-luciferase reporter assay was applied to con rm whether HOXC6 was a target for miR-27b-3p. The results illustrated that miR-27b-3p overexpression inhibited the luciferase activity of psiCHECK-2-HOXC6-WT, but it did not affect the luciferase activity of psiCHECK-2-HOXC6-MT (Fig. 9b). These data indicated that HOXC6 was a binding target of miR-27b-3p. Consistently, western blot analysis indicated that the expression of HOXC6 was signi cantly downregulated in ARPE19 cells following transfection with miR-27b-3p mimics while exerted the opposite effect in ARPE19 cells transfected with miR-27b-3p inhibitor (Fig. 9c), which illustrated that HOXC6 was a direct target of miR-27b-3p.

Discussion
Subretinal brosis de nes the end stage of nAMD, leading to irreversible vision loss; Currently there is no available treatment to either prevent it or treat it. (1). Thus, novel therapeutic methods need to be urgently developed to prevent vision loss and reduce subretinal scar formation in nAMD. Intravitreal administration of MSC has been reported to have an encouraging therapeutic effect on multiple retinal disease models, including retinal damage, in ammation, and nerve injury (23). Despite the bene cial effects of MSCs in repairing retinal injury and restoring the visual system, their application is con ned to ethical issues, poor diffusion through biological barriers and possible complications such as epiretinal membranes, in ammatory response and malignant transformation (31). It is now clear that they exert most of their biological effects through the paracrine secretion of soluble factors and exosomes (21). Exosomes derived from MSCs have been shown to own all the superiority of MSCs in limiting the extent of damage, reducing apoptosis and restricting in ammation (16,20,32). Importantly, exosomes have a reliable stability, a better ability to penetrate deep tissues and avoid immune attack, a lower chance of immunological rejection, and no risk of malignant transformation or vitreoretinal proliferation and uncertain differentiation in MSCs (33,34).
MicroRNA, a class of small noncoding RNAs, serves as a regulator of gene expression at the posttranscriptional level, causing a translational arrest or mRNA degradation (35). They are involved in regulating basic cellular processes such as proliferation and differentiation (36). In addition, many studies analyzed the miRNA disorders in patients with AMD which suggested that the miRNA-based treatment may put forward a promising novel strategy to ameliorate AMD-related tissue damage (37).Our preliminary literature screening and PCR results indicated that miR-27b-3p from hucMSC-Exo might be one of the critical miRNAs in subretinal brosis. MiR-27b was discovered to be observably upregulated in nAMD plasma, and upregulation of miR-27b markedly blocked the cell proliferation and migration in PDGF-BB-induced ARPE19 cells (38,39). Although various growth factors, including platelet-derived growth factor (PDGF), hepatocyte growth factor(HGF), connective tissue growth factor (CTGF) and transforming growth factor-β (TGF-β) are involved in the pathogenesis, our present study focused on TGF-β2 because it is predominant in aqueous humor and its concentration in the vitreous humor has a correlation with the progression of retinal brosis. Moreover, PDGF, TGF-β1, and CTGF are known to be targets of TGF-β2 signaling, implying that TGF-β2 can coordinate the secondary effects of these other factors on EMT and brosis (40).Importantly, previous studies have demonstrated that suppression of TGF-β2-induced EMT could reduce the development of collagen bers and brotic membrane and thereby against brosis-relative retinal diseases (41),and there is no relevant report showing that miR-27b is involved in the EMT of RPE cells induced by TGF-β2. Therefore, we aimed to explore the effects of hucMSC-Exo and its containing miR-27b on subretinal brosis in vivo and in vitro and further investigated its anti-brotic mechanism.
This study was novel in demonstrating that hucMSC-Exo had the ability to alleviate subretinal brosis, including the decrease in both CNV and subretinal brosis, the reduction of migration, and the reversal of EMT markers in TGF-β2-induced RPE cells. Moreover, we found that hucMSC-Exo inhibited subretinal brosis by the delivery of miR-27b because when silencing the expression of miR-27b in hucMSC-Exo, the inhibitory effects were weakened, but it was rescued after miR-27b overexpression. Bioinformatics analysis and luciferase reporter assay revealed that miR-27b-3p directly targeted the 3′-UTR of HOXC6.
Consistently, upregulation of miR-27b-3p markedly downregulated HOXC6 expression at the protein level(P<0.05). HOXC6, a member of the homeobox superfamily, plays pivotal roles in numerous cellular processes, including cell apoptosis, proliferation and differentiation (42). Studies have shown that silencing of HOXC6 suppressed EMT via the inhibition of the TGF-β/Smad signaling pathway in cervical carcinoma cells (28). Therefore, miR-27b-3p is likely to block the activation of the TGF-β/Smad signaling pathway in RPE cells partly by targeting HOXC6 gene.
This study had several limitations. Firstly, miR-27b-3p is just one of the signi cantly increased miRNAs in hucMSC-Exo compared with HSF-exosomal miRNAs. Although hucMSC-Exo strongly inhibited the activation of EMT in RPE cells and reduced subretinal brosis, the roles of other miRNAs also deserve further investigation. Secondly, although the experiments strongly demonstrated that miR-27b-3p had an obvious inhibitory effect on the EMT of ARPE19 cells, the same effect in animals needs to be further veri ed. Finally, a large amount of studies have demonstrated that miRNAs packaged in MSC-Exos were functional molecules mediated bene cial effects (43), but the therapeutic effect of proteins and other bene cial components also deserves further research. In brief, we believe that exosomes derived from MSCs have a great potential to be superior candidates in the therapy of retina diseases in various clinical stages and an in-depth study about its whole effect and mechanism are needed in the future.