MicroRNA-28 potentially regulates the photoreceptor lineage commitment of Müller glia-derived progenitors

Retinal degenerative diseases ultimately result into irreversible photoreceptor death or loss. At present, the most promising treatment for these diseases is cell replacement therapy. Müller glia are the major glia in the retina, displaying cardinal features of retinal progenitor cells, and can be candidate of seed cells for retinal degenerative diseases. Here, mouse retinal Müller glia dissociated and cultured in vitro amplified and were dedifferentiated into Müller glia-derived progenitors (MGDPs), demonstrating expression of stem/progenitor cell markers Nestin, Sox2 and self-renewal capacity. MicroRNAs (miRNAs) play unique roles in the retinogenesis, so we hypothesized miRNAs would contribute to photoreceptor lineage commitment of MGDPs. By TargetScan, Miranda, and Pictar bioinformatics, gain/loss-of-function models, dual luciferase assay, we identified and validated that miR-28 targeted the photoreceptor-specific CRX transcription factor. Anti-miR-28 could induce MGDPs to differentiate into neurons strongly expressing CRX and Rhodopsin, while miR-28 mimic suppressed CRX and Rhodopsin expression. Knockdown of CRX by siRNA blocked the expression of CRX and Rhodospin upregulated by anti-miR-28, indicating that anti-miR-28 potentially induced photoreceptor commitment of MGDPs by targeting CRX, but more experiments are necessary to confirm their role in differentiation.

Retinal degenerative diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP), despite very different etiologies, result into progressive visual impairment associated with photoreceptor damage or loss. These photoreceptors are terminally differentiated neurons and cannot regenerate. For therapy of photoreceptor degenerative diseases, various neuroprotective strategies could be beneficial in the early stages when most photoreceptors are still functional. However, photoreceptor transplantation may be the only effective therapy in the advanced stages when most photoreceptors are irreparably damaged or lost 1 . Photoreceptor transplantation probably is among the most feasible types of neural stem cell (NSC) repair because photoreceptors make short, single synaptic connections to inner retinal circuitry in contrast to most central neurons during integration into complex function circuitry which is a major impediment to replacement of lost neurons 2 .
During neurogenesis, radial glia constitute the majority of multipotent progenitors 3,4 . In the retina, Müller glia are the major radial cells 5,6 and display cardinal features of retinal progenitor cells. In fish, Müller glia can almost regenerate all types of damaged retinal neurons [7][8][9][10] . Unfortunately, this regenerative capacity has dwindled during vertebrate evolution to the point that a very limited number of Müller glia can re-enter the cell cycle in rodent retina [11][12][13] . However, Müller glia possess several advantages such as notably low risk of rejection, fewer ethical problems as therapeutic stem cells compared to embryonic stem cells. Therefore, methods to dedifferentiate Müller glia into MGDPs and direct them to the photoreceptor lineage may facilitate its use as seed cells for photoreceptor regeneration.
MiRNAs are short (21-24 nucleotides) noncoding RNAs that act as crucial regulators of gene transcription by binding to mRNA sequences with partial complementation and inhibiting subsequent translation. MiRNAs are thought to be ideal gene regulators to guide differentiation towards particular cell types due to their ease of transfection, resistance to nuclease action, and long half-life/bioactivity 14 . It is reported that miRNAs play unique roles in the fine-tuning of the proliferation/differentiation of retinal progenitor cells (RPCs) and contribute to the tightly controlled spatial and temporal sequences in retinal development 15 . Therefore, we hypothesized miRNAs might contribute to the differentiation of MGDPs into photoreceptors by binding the 3′ untranslated region (3′ UTR) of the cone-rod homeobox (CRX) gene, which encodes an important transcriptional factor involved in photoreceptor development and maturation 16,17 . By computer bioinformatics, gain/loss-of-function models and luciferase reporter assays, one identified miRNA, miR-28, was validated to target CRX. Furthermore, transfection of miR-28 inhibitor via lentivirus into MGDPs potentially facilitated commitment to the photoreceptor lineage as evidenced by increased expression of Rhodopsin and CRX. Knockdown of CRX by siRNA blocked the expression of CRX and Rhodospin upregulated by anti-miR-28, indicating that anti-miR-28 induced photoreceptor commitment of MGDPs by targeting CRX. Our results maybe suggest a new view of facilitating Müller glia as seed cells for photoreceptor regeneration in retinal degenerative diseases.

Results
Retinal progenitors were derived from mouse Müller glia. We isolated Müller glia from postnatal 5-7 day mice because there is a marked decrease in the proportion of cells re-entering the cell cycle in response to dedifferentiation culture conditions during the second postnatal week 18,19 . Passage 2 Müller glia cultured in vitro had uniform size and shape (Fig. 1A), and more than 90% were immunoreactive for the mature Müller cell markers vimentin and glutamine synthetase (GS) (Fig. 1B-E). Passage 2 Müller glia cultured in dedifferentiation medium for 3-7 days formed neurospheres with good refraction and well-defined boundaries ( Fig. 2A).  These neurospheres could be passaged more than 3 passages. 5-Ethynyl-2′-deoxyuridine (Edu) assay showed that many cells in passage 2 neurospheres had proliferative capacity ( Fig. 2B-D), meanwhile immunocytochemistry demonstrated strong expression of the stem/progenitor cell markers Nestin and SRY (sex determining region Y)-box 2 (Sox2) (Fig. 2E-H). These changes were consistent with Müller glia dedifferentiation into MGDPs.
To verify whether the 3′UTR of CRX was recognized by miR-28 or miR-7b, we generated luciferase reporter constructs containing a CRX 3′UTR with either wild type (wt) or mutated type of miR-28 (or miR-7b) binding sites. As shown in Fig. 4, there was no obvious change in luciferase activity in cells co-transfected with miR-7b mimic and dual-luciferase reporter plasmid (Fig. 4A). Co-transfection of miR-28 mimic and the dual-luciferase reporter plasmid containing wt binding sites reduced luciferase activity, while cells co-transfected with miR-28 mimic and reporter plasmid containing mutated binding sites exhibited no change in luciferase activity compared to controls (Fig. 4B). These results indicated that the putative binding site seed sequences of CRX were specifically recognized by miR-28, but not by miR-7b. Therefore, we selected miR-28 for further study on MGDP differentiation.

miR-28 potentially induced photoreceptor commitment of mice MGDPs.
To study the effects of miR-28-mediated CRX modulation on differentiation of MGDPs, MGDPs isolated from passage 2 neurospheres were infected with lentivirus containing miR-28 mimic or inhibitor. After 60 h, more than 80% of MGDPs were successfully infected as indicated by EGFP expression (Fig. 5A,B). For 7 days of differentiation, some MGDPs transfected with miR-28 inhibitor exhibited neuronal features, including condensed cytoplasm, one or more synaptic processes, and reduced cytoplasm:nucleus ratio (Fig. 5C), consistent with Jayaram H 20 . Furthermore, some cells demonstrated expression of the special photoreceptor marker Rhodopsin. Expression levels of mRNAs encoding the photoreceptor markers CRX (Fig. 6A) and Rhodopsin (Fig. 6B) were enhanced by miR-28 inhibition and suppressed by miR-28 mimic as assessed by real-time PCR. These mRNA expression changes were paralleled by protein expression changes measured by western blot (Fig. 6C,D). In contrast to Rhodopsin, these neuron-like cells did not express S-opsin by immunocytochemistry assay, and S-opsin protein was also not detected by western blot assay (not shown).
To investigate whether anti-miR-28 induced differentiation of MGDPs toward photoreceptor directly through CRX, MGDPs transfected with lenivirus mU6-MCS-Ubi-anti-miR-28-EGFP for 3 days were again transfected with siRNA harbouring no 3′UTR of CRX. Western blot demonstrated that si-CRX specifically knocked down the expression of CRX. The expression of CRX protein upregulated by anti-miR-28 was also partly blocked by CRX siRNA (Fig. S1) and Rhodopsin protein was not detected (data was not shown), indicating that anti-miR-28 could have a potential to drive MGDPs to photoreceptor differentiation by targeting CRX, but more experiments are necessary to confirm its role in differentiation.

Discussion
We demonstrated that anti-miR-28 potentially induced photoreceptor lineage commitment of MGDPs as evidenced by neuron-like morphological changes and upregulation of the photoreceptor markers CRX and rhodopsin at both the mRNA and protein levels. The expression of CRX and Rhodopsin protein were blocked by CRX siRNA, suggesting anti-miR-28 induced MGDPs to differentiate into photoreceptors commitment by targeting CRX. The CRX gene is the earliest known photoreceptor markers, regulating development and maturation of rods and cones. Besides postnatal rod, the embryonic-stage CRX-positive donor cells can also integrate within the outer nuclear layer of retina and differentiate into new rods and cones, which are critical to visual function. The ratio of rods to cones is similar to that of the host retina (20:1) 21,22 . In this study, we did not detect the expression of S-opsin, the marker of cones, in contrast to rhodospin. Maybe too few cones were generated for finding. Alternatively, it is unlikely that our methodology was inappropriate as we tested primers from 3 different companies (unpublished observations).
Although orthodenticle homeobox 2 (OTX2) rather than CRX is the key regulatory factor of photoreceptor fate, overexpression of CRX is also capable to induce the retinal progenitors and retinal endogenous stem cells such as the iris or ciliary-derived cells to differentiate to light-sensitive photoreceptor phenotypes in vivo and vitro. While cornea-derived stem cells by misexpression of CRX only express photoreceptor-specific protein not to be functional, and none of neural stem cells from hippocampus acquires Rhodopsin immunoreactivity [23][24][25][26][27][28] . Retinal Müller glia are also knows as one of endogenous stem cells and could be induced to photoreceptor phenotypes by upregulation of CRX regulated by anti-miR-28 as demonstrated in our study. Our results further proved that retinal Müller glia has capacity to facilitate photoreceptor regeneration, consistent with the literatures 5, 20 , but we did not go a step further to detect photoreceptor function, which was one of limits of our study, and more experiments are needed to confirm the role of anti-miR-28 in differentiation.
In the retinogenesis, miRNAs play unique roles in the fine-tuning of the proliferation/differentiation of RPCs and contribute to the tightly controlled spatial and temporal sequences in development 15 . Sole miRNA even can induce the commitment of neuronal differentiation of stem/progenitor cells, for example, anti-miR-410 can induce retinal pigment epithelium differentiation in stem cells 29,30 . Ectopic miR-124 expression is sufficient to induce MGDPs differentiate to early neuronal lineages 31 . We similarly demonstrated that anti-miR-28 potentially induced photoreceptor lineage commitment of MGDPs, which further proved the viewpoint that distinct miR-NAs must be down-regulated to generate the latest neuron types 32 .
In conclusion, anti-miR-28 potentially induce MGDPs to differentiate to photoreceptor commitment by targeting CRX, suggesting a new view of facilitating Müller glia as seed cells for photoreceptor regeneration in retinal degenerative diseases. But more study was needed to further assess cone differentiation of MGDPs and whether these differentiated photoreceptor-like cells can integrate in the host retina and develop mature. Cell Culture. Müller glia were isolated and dedifferentiated according to previous methods [33][34][35] . Briefly, C57BL/6J mice (5-7 days old) were sacrificed by cervical dislocation under isoflurane anesthesia. Eyes were enucleated and neuroretina dissected without disrupting the underlying retinal pigmented epithelium starting 0.5 mm from the ora serrata to avoid contamination by retinal stem cells at the ciliary margin. Neuroretinas were dissociated into small aggregates and digested by trypsin-EDTA (0.25% trypsin, 2% EDTA, Gibco, USA) for 5 minutes. Digestion was stopped by addition of Dulbecco's modified Eagle's medium F12 (DMEM/F12, HyClone, USA) supplemented with 15% fetal bovine serum (FBS, Gibco). Cell suspensions were filtrated through a 0.75 μm stainless steel sieve and centrifuged at 1000 rpm for 5 min. The cell pellets were resuspended in DMEM/F-12 containing 15% FBS, seeded on 25 cm 2 flasks, and grown at 37 °C under a 5% CO 2 atmosphere. The cells were passaged via trypsinization every 4-5 days. Passage 2 Müller cells were used in the experiments.

Animals (Ethics statement). All animal experiments in this study conformed to the Guidelines for Animal
The neurosphere assay is frequently used to isolate and propagate neural stem cells. We generated neurospheres by standard techniques. Passage 2 Müller cells were dissociated using Accutase (Invitrogen, USA) and cultured on ultra low attachment 6-well plates(Corning, USA) in a serum-free dedifferentiation medium containing DMEM/F12 (Gibco), 1 × N2 supplement (Gibco), 2 × B27 supplement without Vitamin A (Gibco), 20 ng/ml murine epidermal growth factor (EGF, Peprotech, USA), 10 ng/ml murine basic fibroblast growth factor (bFGF2, Peprotech), and 2 mM L-glutamine (HyClone) at 1 × 10 5 cells/cm 2 . Half of the dedifferentiation medium was changed every other day. Immunocytochemical analysis. Immunocytochemical analysis was performed as previously described.

Edu proliferation assay.
For the Edu proliferation assay, neurospheres were seeded on poly-L-lysine-coated slides. After 24 h, the slides were processed according to the manufacture's instructions (Ribobio, China). Fluorescent images were recorded by confocal or fluorescent microscopy.
MicroRNA prediction. The bioinformatics tools TargetScan (http://www.targetscan.org), Miranda (http:// www.mircrona.org/microrna/home.do), and Pictar (http://pictar.mdc-berlin.de) were used to search for miRNAs that target the CRX 3′ UTR. These algorithms were selected because they provided complementary information. MiRNAs were predicted by these bioinformatics individually, and the overlapping miRNAs were chosen for subsequent experiments. The selected miRNAs mimics or inhibitions (Genecopoeia, USA) were transiently transfected to MGDPs, those that were not negatively related to CRX were excluded from further study.

MicroRNA-28 lentivirus vector construction and transfection of MGDPs. The mouse
pre-miR-28 sequence was amplified from genomic DNA of mouse fibroblasts and cloned into the lentivirus expression vector mU6-MCS-Ubi-EGFP (provided by Gene, China). Primers used for amplification were as follows: Mmu-miR-28-5p-inhibitor, forward 5′-AAGGAGCTCACAGTCTATTGAG-3′ and reverse 5′-CTCAATAGACTGTGAGCTCCTT-3′; miR-28, 5′-ATTGAGTTGCCTTTCTGATT CTCCCACTAGATTGTGAGCTGCTGGAGGGCAGGCACT-3′ and reverse 5′-AGTGCCTGCCCTCCAGC A G C T C A C A AT C TA G T G G G A G A AT C A G A A A G G C A A C T C A ATA G A C T G T G A G C T C C TTGAAGGTAGGGACC -3′.
293T cells were transfected with lentiviral plasmid (empty EGFP control, miR-28, or miR-28 inhibitor), as well as lentiviral packaging plasmids including pLP1 and pLP2 using the FuGENE ® HD transfection reagent (Roche). Viral -bearing supernatant was collected and cellular debris removed by syringe filtering (0.45 μm pore size; Millipore). To improve the transfection efficiency, passage 2 neurospheres were digested by Accutase into single cell suspensions and cultured as monolayer at 4 × 10 5 MGDPs per well in 6-well plates (Beaver, China) with serum-free NSC medium. MGDPs were divided into four groups: A (control group with no lentivirus transfection), B (empty vector group transfected with lentivirus mU6-MCS-Ubi-EGFP), C (miR-28-overexpression group