Introduction
The loss of vision affects the quality of daily life and psychological state of patients, which places an enormous burden on not only the family but also the National Health Insurance budget. The retina, which consists of the neural retina and the retinal pigment epithelium (RPE), plays a pivotal role in light perception and signal transduction [
1]. Retinal degeneration (RD) is a group of diseases characterized by retinal cell degeneration, such as the cell loss of photoreceptors and/or the RPE in retinitis pigmentosa (RP), age-related macular degeneration (AMD) and Stargardt’s macular dystrophy, which are caused by inherited factors or acquired factors with an increasing incidence and prevalence in recent years [
2‐
7].
In inherited RD, thousands of mutations in hundreds of genes have been found [
8]. However, only one gene therapy has been approved by the United States Food and Drug Administration (FDA) for the treatment of patients with
RPE65-related inherited RD [
9]. There is no treatment for other inherited RD patients except for patients with RPE65 mutation. In general, it is difficult to obtain approval for gene therapies for clinical trials and routine clinical treatment. For acquired types of RD, such as AMD, only some patients, such as those with wet AMD, show slower progression of the disease after monthly treatment with anti-vascular endothelium growth factor (VEGF), which is extremely expensive [
10‐
12]. Given that only a very small number of patients with RD can be treated with the expensive or monthly treatment, for most of the remaining RD patients, including those with dry AMD, there are currently very few treatment options. Fortunately, with the emergence and advancements in stem cell research, human induced pluripotent stem cell (hiPSC)-derived RPE cells and/or hiPSC-derived photoreceptor cells seem to be promising treatments for RD patients [
13‐
25], since the RPE cells, which are located adjacent to the photoreceptor cells, play a vital role in maintaining the retinal homeostasis and normal vision.
Stem cell therapy for the treatment of RD has been investigated using several types of adult stem cells, such as bone marrow stem cells (BMSCs) [
26,
27], dental pulp stem cells (DPSCs) [
28‐
30], adipose-derived stem cells (ADSCs) [
31,
32] and neural stem cells (NSCs) [
33], as well as stem cell-derived cells, such as hiPSC-derived RPE cells and hiPSC-derived retinal stem and progenitor cells or retinal epithelium cells [
34,
35]. However, there has been no systematic study comparing the effects of different types of adult stem cells in RD treatment using an animal model of RD.
Furthermore, there have been very few studies that compared the treatment effects of human adult stem cells (hBMSCs, hDPSCs and hADSCs) or human fetal stem cells (human amniotic fluid stem cells, hAFSCs) and hiPSC-derived RPE cells in animals with RD (RCS rats) [
36‐
38], although clinical trials of the transplantation of specific stem cells or stem cell-derived cells, such as BMSCs (NCT03772938, NCT03011541, NCT02016508, NCT01920867, NCT01736059, and NCT01518127), ADSCs (NCT02024269), umbilical cord-derived mesenchymal stem cells (UC-MSCs; NCT05147701), human central nervous system stem cells (HuCNS-SCs; NCT02467634, NCT02137915, and NCT01632527), retinal stem and progenitor cells (NCT05187104), and stem cell-derived RPE cells (NCT04627428, NCT04339764, NCT03305029, NCT03178149, NCT03102138, NCT03046407, NCT02941991, NCT02903576, NCT02755428, NCT02749734, NCT02590692, NCT02563782, NCT02464956, NCT02463344, NCT02445612, NCT02286089, NCT02122159, NCT01691261, NCT01674829, NCT01625559, NCT01469832, NCT01345006, and NCT01344993) for the treatment of RD patients have been conducted [
39].
Among the many studies that have investigated stem cell therapy using animal models of RD, few studies have compared the protective effects or improvements in efficacy achieved in rats or mice with RD using only two or three different types of stem cells or stem cell-derived cells [
33,
40]. Mead et al. compared the abilities of three different types of human adult stem cells (DPSCs, BMSCs and ADSCs) to protect retinal ganglion cells in vitro [
40]. However, they did not compare the treatment effects of adult stem cells and hiPSC-derived RPE cells on retinal ganglion cells.
Sun and Takahashi et al. compared the neuroprotective efficacy of three cell types [hiPSC-RPE cells, BMSCs, and neural stem cells (NSCs)] in RD treatment in an immunocompromised mouse model,
rd1 mice [
33]. However, they did not evaluate the neuroprotective efficacy of several kinds of adult and/or fetal stem cells, such as ADSCs, DPSCs, AFSCs and BMSCs, but instead investigated this issue using only one cell line, BMSCs.
Xu et al. compared two subpopulations of rat BMSCs in terms of their ability to protect against RD progression in an animal model using Royal College of Surgeons (RCS) rats [
36] and later compared the protective effects of two subpopulations of human UC-MSCs [
38]. They did not investigate the protective effects of other types of stem cells, such as ADSCs, DPSCs and AFSCs, against RD progression.
Riera et al. compared the treatment effects of transplantation of two different RPE cell lines, which were differentiated from human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs), for RD in RCS rats [
37]. However, they did not compare the treatment effects of several types of adult and/or fetal stem cells with those of hESCs or hiPSC-derived RPE cells in the context of RD.
To our knowledge, there have been no studies investigating the protective effects and treatment efficacy of subretinal transplantation of several types of stem cells (hAFSCs, hDPSCs, hADSCs, hBMSCs, and hiPSCs) as well as hiPSC-derived RPE cells using an animal model of RD (RCS rats). RCS rats are a well-recognized and classical animal model of RD, which is caused by the mutation of the MER proto-oncogene, tyrosine kinase (
Mertk); in this model, RPE cells cannot phagocytose the outer segment of photoreceptor cells, which leads to progressive death of photoreceptor cells [
41‐
44]. Apoptosis of photoreceptor cells begins at approximately 21 days after birth in RCS rats, and the photoreceptor cells almost die when the rats are 2 to 3 months of age, which leads to severe loss of retinal structure and function [
37,
45,
46].
In this study, we investigated and compared the subretinal transplantation of several types of stem cells (hAFSCs, hDPSCs, hADSCs, hBMSCs, and hiPSCs) as well as hiPSC-derived RPE cells to determine the efficacy and protective effects of stem cells and hiPSC-derived RPE cells and to investigate their mechanisms in the treatment of RD in RCS rats. All the data manifested that stem cell based regenerative medicine, especially hiPSCs-derived RPE cells transplantation, showed excellent functional and structural recovery in RCS rats, which will be a promising treatment for clinical RD patients.
Discussion
Stem cell-based therapy is promising for RD patients. However, which stem cell types have better protective effects was unknown until this investigation. In this study, subretinal transplantation of hiPSC-RPE cells showed a better and longer effect than transplantation of hADSCs, hAFSCs, hBMSCs, hDPSCs, and hiPSCs in terms of protecting both visual function and retinal structure in an RD animal model in RCS rats. hADSCs, hAFSCs, hBMSCs, and hDPSCs yielded better preservation of the retinal structure and function than hiPSCs (HPS0077), although the effect was temporary, lasting until 4 weeks post-transplantation but not 8 weeks post-transplantation. Overall, hiPSC-RPE cells showed the best protective effect; hADSCs, hAFSCs, hBMSCs, and hDPSCs had the next best protective effect; and hiPSCs (HPS0077) had the weakest protective effect. Our study investigated the protective effect of each type of stem cell and performed a comparison in RCS retinas, and the results laid a foundation for future research aimed at optimizing stem cell-based therapies for RD and possibly other degenerative diseases.
Every cell line used in this study was validated using flow cytometry or immunostaining to confirm they are qualified cells with the expression of specific markers before subretinal transplantation. Our results confirmed the specific expression of the hMSC markers CD44, CD73, and CD105 in four types of hMSCs (hAFSCs, hADSCs, hDPSCs, and hBMSCs), as well as the lack of expression of the human hematopoietic progenitor marker CD34 [
48]. In addition, the human pluripotent markers Nanog, OCT4, and SSEA4 were detected in the hiPSC cell line HPS0077. The RPE cells used in this study were differentiated from hiPSCs (HPS0077), and they are referred to as hiPSC-RPE cells. These hiPSC-RPE cells exhibited the characteristic polygonal morphology and produced dark pigments, similar to native mature RPE cells [
55,
56]. In addition, the previously reported RPE-related markers MITF, PAX6, RPE65, and ZO-1 were all highly expressed in our hiPSC-RPE cells [
35,
37,
57,
58]. Although the validation of cells was carried out using specific markers expression, the cellular function and the purity of differentiated cells should be confirmed before real clinical treatments in future.
Lentivirus-GFP was reported to label RPE cells for detection in vivo [
34]. Therefore, we tried to label stem cells such as hAFSCs and hDPSCs using lentivirus-GFP. We observed green fluorescence from hAFSCs and hDPSCs in vitro under the microscope after transduction of lentivirus-GFP into the cells, but the green fluorescence was found to be extremely weak in the eyes of RCS rats by fundus photography in vivo after subretinal transplantation of the cells into RCS rats. In addition, a potential risk of changing cellular processes or gene expression by the use of transgenic markers GFP still exists. CellTracker was also previously reported for the detection of RPE cells in vivo by several researchers [
37,
42] and is considered safer than lentivirus-GFP transduction. Our results demonstrated that CellTracker staining of the cells was suitable for labeling and observation of all six cell lines, namely, hiPSC-RPE, hAFSCs, hADSCs, hBMSCs, hDPSCs, and hiPSCs, in vivo. The green fluorescence in fundus photographs was evidence of successful transplantation of the cells into the subretinal space. Although we have proofed transplanted cells were successful transplanted into the subretinal space by the green fluorescence of CellTraker, a long-lasting cell tracker which can trace cells for months or years would be beneficial for the evaluation of cell integration and survival in vivo in future.
In RCS rats, a tendency to follow rotating stripes (qOMR) or a preference for the dark (LDB) can partially reflect visual function [
59,
60], although the reliability of the results is influenced by involuntary movements of the rats, such as chewing their digits or licking their fur. Our results showed an increase in qOMR index values at most of the spatial frequencies in groups subjected to transplantation of all six cell lines compared with the RCS rat group subjected to PBS injection or the age-matched noninjection group from week 1 to week 4 post-injection. In addition, most of the higher qOMR values appeared at a spatial frequency of approximately 0.2, which was consistent with previous studies in which animals showed the best performance following the stripes at the same spatial frequency of 0.2 [
51]. Although the qOMR index values were improved in the RCS rats subjected to transplantation of cells, no significant difference in the duration of time spent in the light and dark chambers was found in LDB testing under natural light conditions. Because the RCS rats had RD and impaired vision, light of a strong intensity should be used for irradiation of the light chamber rather than natural light conditions to induce more significant differences between the light and dark chambers. Moreover, other behavioral assays assisting in visual function evaluation, such as a watermaze assay, should also take into consideration in the further study.
ERG is an objective visual electrophysiological examination that is considered the gold standard for evaluating the visual function of the retina [
61,
62]. The ERG performance of RCS rats decreased dramatically with the progression of RD [
42]. In our results, we observed the preservation of ERG performance to varying degrees in the groups subjected to transplantation of all six cell lines (the hiPSC-RPE group had slightly higher performance than the four hMSC groups (the hAFSC, hADSC, hBMSC, and hDPSC groups) and much higher performance than the hiPSC group, the PBS injection group or the age-matched noninjection group at 4 weeks post-injection. However, only the hiPSC-RPE group maintained partial ERG performance at 8 weeks post-injection, which indicated that hiPSC-RPE cells induced better and longer protection of visual function than other cells, whereas hAFSCs, hADSCs, hBMSCs, hDPSCs, and hiPSCs induced temporary protection that lasted only until 4 weeks.
The photoreceptor cells in the retina of RCS rats began to undergo apoptosis from postnatal week 3 and were nearly completely lost when the rats were 2 to 3 months of age [
45,
46]. The cell body of photoreceptor cells lies in the ONL of the retina, and a thicker ONL or a greater number of nuclei per column in the ONL indicates the existence of more photoreceptor cells. In our results, hiPSC-RPE cells led to the preservation of approximately 4 nuclei per column in the ONL and slowed the progression of retinal degeneration even at 8 weeks post-injection; at this time, the RCS rats were 2 to 3 months old, and the most severe loss of photoreceptor cells was expected in nontreated RCS rats. Other types of cells, such as hAFSCs, hADSCs, hBMSCs, hDPSCs, and hiPSCs, preserved the thickness of the ONL only at week 4 post-injection but not at week 8 post-injection.
The results of retinal histological analysis were also consistent with the above visual function results obtained by ERG. It is known that the well-organized retinal structure is the foundation of the normal visual function. Photoreceptor cells lies in the ONL are responsible for the photoreception and phototransduction, which are crucial for vision. Stem cell-based transplantation preserved the visual function detected by ERG with a result of maintaining the thickness of ONL, which was evaluated by the histological analysis.
hAFSCs are fetal stem cells and have been reported to have stronger differentiation abilities than other hMSCs, such as hBMSCs and hADSCs [
63]. hDPSCs have also been reported to exhibit higher proliferation than other hMSCs, such as hBMSCs and hADSCs [
64]. However, we did not observe differences among different types of hMSCs in the protection of visual function or ONL thickness subretinal transplantation into RCS rats.
Consistent with the previous studies [
33,
37,
52,
65], RCS rats or other animal models of RD, which underwent a progressive retinal degeneration, can be halted by stem cell-based therapy. Our study confirmed the protective effect of different stem cells and hiPSC-RPE cells on RCS rats with various degrees, and figured out the most effective stem cell-based therapy for RD that is the subretinal transplantation of hiPSC-RPE cells. It is promising to treat patients with RD or other degenerative diseases by transplantation of hiPSC-RPE cells or other regenerative cells to recover and maintain cell function in future. However, there are still some challenges should be considered seriously. For instance, the purity of stem cell derived cells and the long-term safety of transplanting those cells should be assessed carefully [
17]; a world recognized and standard protocol to prepare and transplant cells should be established; a potential cellular rejection after cell transplantation also should take into consideration; the ethical and regulatory aspects related to stem cell-based therapy should be treated seriously [
66].
We speculated that the protective effect of subretinal transplantation of these cells against RD in RCS rats partially relied on the trophic factors that the cells secreted [
67] and the cell fusion and materials transfer between the transplanted stem cells and the native cells. Growth factors or pathways related to GDNF [
68,
69], BDNF [
70,
71], PEDF [
72,
73], VEGF [
74,
75], and TGF-β [
76] were reported to have neuroprotective effects. We observed the secretion of three growth factors, GDNF, TGF-β, and BDNF, by ELISA in all six cell lines we transplanted. However, the secretion of PEDF and VEGF was observed only in hiPSC-RPE cells. A more detailed and comprehensive analysis of the trophic factors of the transplanted cells secreted in vivo, using microarray or RNA expression assay may contribute to a better understanding of the molecular mechanisms in future. In addition, we speculated that another reason why hiPSC-RPE cells had the strongest retinal protection ability in RCS rats was that hiPSC-RPE cells may replace partially dysfunctional native RPE cells, which needs to be confirmed in further studies. For instance, the integration of transplanted cells with native cells should be confirmed by using an electron microscopy, and the function of transplanted cells should be evaluated in vivo.
Although we confirmed that hiPSC-RPE cell transplantation showed the best and longest protective effect in the retinas of RCS rats compared to the transplantation of other cells investigated in this study, such as hAFSCs, hADSCs, hBMSCs, hDPSCs, and hiPSCs, which also rescued the visual function of RCS rats, but the effect was temporary and lasted only 4 weeks. Besides, the potential of hESCs, hESCs-RPE cells, and hiPSC-MSC cells in RD treatment still needs to be investigated and compared in future. In addition, we confirmed the protective effect of subretinal transplantation of these cells against RD in RCS rats partially relied on the secretion of growth factors by the cells. hMSCs and hiPSCs as well as hiPSC-RPE cells can secrete GDNF, BDNF, and TGF-β. hiPSC-RPE cells can secrete the growth factors PEDF and VEGF, which cannot be secreted by hMSCs or hiPSCs. However, there are still some limitations need to be further investigated. First, it should be considered how long each cell can survive in the eyes of RCS rats after subretinal transplantation by long-term follow-up experiments using cell tracker on the retinal section. Second, it should be more specifically explored and compared how is the protective mechanism of hESCs, hESCs-RPE cells, or hiPSC-MSC cells on RD progress of RCS rats by combining the microarray assays and cell integration detection using electron microscope. Third, it needs to be explored whether the number of transplanted cells or numbers of injection time give influence on the treatment outcome by setting up a series of gradient cell concentrations or injection frequency for transplantation. Furthermore, the methods chosen to deliver cells may also influence the outcomes. For instance, cell sheet transplantation kept the integrity and well-aligned structure of cells, but showed large surgical trauma without obvious vision improvement in clinical patients [
77,
78]; transplantation of cell suspension showed some effect, but with the risk of backflow. Therefore, injectable hydrogels loaded cell transplantation should be an alternative delivery method for patients in the future by loading cells on injectable hydrogels, which can be solidified in situ at physiological condition. In addition, immunosuppression drugs were used to avoid cell rejection in this study. How to avoid the usage of immunosuppression drugs will benefit patients who will receive cell transplantation, which may include the personalized therapy using self-origin cells from patients or generate regenerative cells from hypo-immunogenic stem cells.
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