Quaking Is a Key Regulator of Endothelial Cell Differentiation, Neovascularization, and Angiogenesis

Abstract The capability to derive endothelial cell (ECs) from induced pluripotent stem cells (iPSCs) holds huge therapeutic potential for cardiovascular disease. This study elucidates the precise role of the RNA‐binding protein Quaking isoform 5 (QKI‐5) during EC differentiation from both mouse and human iPSCs (hiPSCs) and dissects how RNA‐binding proteins can improve differentiation efficiency toward cell therapy for important vascular diseases. iPSCs represent an attractive cellular approach for regenerative medicine today as they can be used to generate patient‐specific therapeutic cells toward autologous cell therapy. In this study, using the model of iPSCs differentiation toward ECs, the QKI‐5 was found to be an important regulator of STAT3 stabilization and vascular endothelial growth factor receptor 2 (VEGFR2) activation during the EC differentiation process. QKI‐5 was induced during EC differentiation, resulting in stabilization of STAT3 expression and modulation of VEGFR2 transcriptional activation as well as VEGF secretion through direct binding to the 3′ UTR of STAT3. Importantly, mouse iPS‐ECs overexpressing QKI‐5 significantly improved angiogenesis and neovascularization and blood flow recovery in experimental hind limb ischemia. Notably, hiPSCs overexpressing QKI‐5, induced angiogenesis on Matrigel plug assays in vivo only 7 days after subcutaneous injection in SCID mice. These results highlight a clear functional benefit of QKI‐5 in neovascularization, blood flow recovery, and angiogenesis. Thus, they provide support to the growing consensus that elucidation of the molecular mechanisms underlying EC differentiation will ultimately advance stem cell regenerative therapy and eventually make the treatment of cardiovascular disease a reality. The RNA binding protein QKI‐5 is induced during EC differentiation from iPSCs. RNA binding protein QKI‐5 was induced during EC differentiation in parallel with the EC marker CD144. Immunofluorescence staining showing that QKI‐5 is localized in the nucleus and stained in parallel with CD144 in differentiated ECs (scale bar = 50 µm). stem cells 2017 Stem Cells 2017;35:952–966


INTRODUCTION
Cardiovascular disease is a leading cause of mortality worldwide with pathology being significantly driven by progressive vascular endothelial cell (EC) dysfunction which regulates key pathogenic events such as infiltration of inflammatory cells, vascular smooth muscle proliferation, and platelet aggregation [1,2]. As cardiovascular disease progresses, ECs become depleted from the vascular lumen, and, without adequate replacement, nonperfusion and tissue ischaemia ensues. Repair and regeneration of ECs has been an important research focus for many years. As an important therapeutic avenue, the delivery of adult progenitor or stem cells to repair damaged vasculature has faced many limitations, such as identification and availability of appropriate, efficacious cell-types for therapy [3]. However, recently, the ability to derive ECs from induced pluripotent stem cells (iPSCs) has extended the scientific scope for regenerative medicine [4,5,7].
Cell reprogramming is offering new avenues for regenerative medicine with iPSCs proving able to differentiate into nearly all types of cells within the body [6][7][8]. This is a unique characteristic of iPSC technology, which offers a significant potential for cell-based therapies toward repairing tissues or organs destroyed by injury, degenerative diseases, aging, or cancer [4,5,8,9]. Generation of iPSCs also offers a promising strategy to create patient-specific cells [7,[10][11][12][13] with building evidence that these cells have therapeutic a The Wellcome-Wolfson Building, Centre for Experimental Medicine, Queen's University Belfast, United Kingdom; efficacy in animal models of disease [3,5,[14][15][16]. The concept of cell reprogramming is powerful [17][18][19][20] and has, so far, allowed the development of dynamic approaches to generate functional cells of interest and to switch cell fate [20]. We and others have recently demonstrated novel strategies of direct reprogramming toward functional ECs [21,22]. Both iPSCs and directly reprogrammed cells (partial iPSC-PiPSCs) can serve as useful tools not only for derivation of functional cells but also for establishing efficient protocols of differentiation and developing platforms to investigate the underlying mechanisms.
Vascular cell differentiation is achieved through the coordination of diverse molecular pathways [21,[23][24][25][26][27][28]. Due to this, there is a need to better understand the complex dynamics behind RNA regulation and how this, in turn, influences transcriptional control and protein expression. Elucidation of these complex molecular signals, which are evoked during iPSC and PiPSC differentiation toward ECs, may allow specific targeting of their activities to enhance cell differentiation and promote tissue regeneration. In an effort to elucidate the imperative mechanisms of RNA regulation during EC differentiation from iPSCs, this study focused on the RNAbinding protein Quaking (QKI). QKI is a member of the "STAR" (signal transduction and activation of RNA) family of proteins. These proteins are characterized by the presence of at least one RNA-binding motif (QKI containing a KH domain), SH2 and SH3 domains and potential phosphorylation sites. This suggests their involvement in signal transduction pathways [29,30], pre-mRNA splicing [31], exportation of mRNAs from the nucleus [32], protein translation, and mRNA stability. Functional studies of the mouse QKI gene have revealed that it has a key function in embryonic blood vessel formation and remodeling [33][34][35]37]. During vascular cell differentiation and reprogramming processes, QKI could play an important regulatory role in gene expression at the post-transcriptional level in either a miRNA-dependent or independent manner. Alternative splicing of genes, a process that RNA binding proteins are highly involved in, generates many forms of mature mRNA from the same gene is an essential process in development and a potent way to regulate gene expression at a posttranscriptional level. QKI has a number of alternative splicing isoforms, three of which have been associated with vascular development; QKI-5, QKI-6, and QKI-7. This further reinforces the importance of QKI in protein regulation and development. Each isoform contains identical RNA binding domains and differs only by its carboxy-terminal ends [36,37]. In this study, we focus on the QKI splicing isoform 5 (QKI-5). QKI-5 is most highly expressed during EC differentiation from iPSCs, it is an essential player during embryogenesis and notably, QKI-5 null mice are embryonic lethal [38,39,41].
We provide strong evidence that QKI-5 plays a critical role in the differentiation of ECs derived from iPSCs and acts as a key regulator of CD144 stabilization and vascular endothelial growth factor receptor 2 (VEGFR2) activation through STAT3 signaling. The RNA binding protein QKI-5 directly binds to 3 0 UTR of STAT3 and induced STAT3 mRNA stabilization. Remarkably, QKI-5 improved neovascularization and blood flow recovery in a hind limb ischemic model. Notably, human iPSCs (hiPSCs) overexpressing QKI-5 (OE-QKI-5) induced angiogenesis in Matrigel plug assays in vivo only 7 days after subcutaneous injection in SCID mice, highlighting, thus, a pivotal role of QKI-5 in neovascularization, blood flow recovery and angiogenesis.

Mouse iPSC and Embryonic Stem Cell Culture and Differentiation
Mouse iPSCs were generated in our laboratory using a similar approach as described previously [20,40]. Mouse iPSCs were cultured in gelatin-coated flasks (phosphate buffer saline [PBS] containing 0.02% gelatin from bovine skin; Sigma-Aldrich, SIGMA; England, UK http://www.sigmaaldrich.com/ united-kingdom.html) in Dulbecco's modified Eagle's medium (ATCC) supplemented with 10% fetal bovine serum (FBS), 100 IU/ml penicillin, and 100 lg/ml streptomycin (Thermo Fisher Scientific), 10 ng/ml recombinant human leukemia inhibitory factor (Millipore); and 0.1 mM 2-mercaptoethanol (Invitrogen) in a humidified incubator supplemented with 5% CO 2 . The cells were passaged every 2 days at a ratio of 1:6. Differentiation of iPSCs was induced by seeding the cells on type IV mouse collagen (5 lg/ml)-coated dishes in differentiation media (DM) containing a-modified Eagle's medium supplemented with 10% FBS (Invitrogen), 0.05 mM 2-mercaptoethanol, 100 units/ml penicillin, and 100 lg/ml streptomycin in the presence of 25 ng/ml VEGF for the time points indicated.
hiPSCs Differentiation and Human Umbilical Vein ECs hiPSCs were pre-differentiated in low attachment plates using StemPro serum free media supplemented with Bone Morphogenetic Protein 4 (BMP4), Activin A, fibroblast growth factor (FGF), and VEGF for 5 days. The pre-differentiated cells were seeded on fibronectin (Sigma-Aldrich), while KDR endothelial precursor cells were magnetically sorted on day 6 using MicroBeads Kit (Miltenyi BIotec) and culturing in EGM-2 media (LONZA) for 3-9 days. QKI-5 was overexpressed or knockdown by lentiviral gene transfer on day 3 after KDR selection and the cells were harvested on day 3 and subjected to further analysis or labeled for the in vivo Matrigel plug assays. Human umbilical vein ECs (HUVECs) were seeded on collagen I coated plates and cultured in EGM-2 media. QKI-5 was overexpressed by lentiviral gene transfer and subjected to further analysis.

Experimental Hind Limb Ischemia
The mouse hind limb ischemia model was performed as described previously [21,42]. iPS-ECs were infected with FUW-QKI-5 or control empty vectors on day 4 of EC differentiation. After 48 hours, the cells were trypsinized and labeled with Molecular Probes Vybrant Cell Labeling (MP22885) before being injected intramuscularly into the adductors of C57BL/6 wild type mice.

Statistical Analysis
Data are expressed as the mean 6 SEM and were analyzed using GraphPad Prism 5 software with a two-tailed Student's t test for two groups or pairwise comparisons or analysis of variance (ANOVA). A value of *, p < .05; *, p < .05; **, p < .01; ***, p < .001 was considered significant.
Detailed Methods and Materials are in Supporting Information Appendix, Experimental Procedures

Pluripotent Stem Cells Differentiation toward ECs
Mouse iPSCs were differentiated toward ECs by seeding on collagen IV and DM supplemented with 25 ng/ml VEGF (DM 1 V) for 2-10 days. The differentiated ECs adopted a typical EC morphology (Fig. 1A, 1B). The efficiency of EC differentiation from mouse iPSCs was very high as FACs analysis is shown in Figure 1C (86.7% positive cells for CD144 [VE-Cadherin] and 87.2% positive cells for CD31 were obtained). These results were confirmed in both mRNA (Fig. 1D) and protein levels (Fig. 1E, 1F). During EC differentiation the pluripotent marker OCT4 was suppressed allowing the expression of the EC specific markers CD144 and CD31 to occur in a time point dependent manner (Fig. 1F). Indeed, immunofluorescence confocal microscope images revealed that ECs derived from iPSCs expressed EC markers (CD144, FLK-1, eNOS, and vWF) which are the characteristics of ECs (Fig. 1G). We also observed that the expression of the transcription factor STAT3 is a known essential player in VEGF signaling during vascular development. We are reporting that STAT3 was progressively induced during EC differentiation (Fig. 1H). Taken together, these data clearly demonstrate that mouse iPSCs have been differentiated toward ECs based on a highly efficient approach.
RNA-Binding Protein QKI Is Induced during EC Differentiation from iPSCs QKI-5 was significantly induced during EC differentiation from iPSCs and this occurred in parallel with the EC markers CD144 and CD31 ( Fig. 2A). It is important to note that only QKI-5 mRNA splicing isoform was induced during EC differentiation. QKI-6 and QKI-7 expression levels were also tested showing no change or decrease, respectively ( Fig. 2A). Time-course experiments also demonstrated that QKI-5 was progressively induced after 4 days post-differentiation from iPSCs (Fig. 2B). There was a comparable induction of CD144 and STAT3 (Fig.  2B). Immunofluorescence images demonstrated that the alternative splicing isoform QKI-5 was localized to the EC nucleus in both mouse iPSC-derived ECs and in HUVECs (Fig. 2C-2E). Also in both EC-types, CD144 and CD31 were expressed in the plasma membrane (Fig. 2D, 2E). These results demonstrated that QKI-5 expression is induced during EC differentiation from mouse iPSCs, and its expression is maintained in mature ECs.
A number of additional experiments were performed to determine whether QKI-5 plays a regulatory role in iPSC differentiation toward ECs. Initially, a pure population (100%) of differentiated ECs was obtained based on CD144 selection, as it is a robust EC-specific marker. Based on this pure population of ECs derived from iPSCs, a high expression of QKI-5 was observed and this was comparable to the EC markers CD144, VEGFA, and the transcription factor STAT3 (Supporting Information Fig. S1A-S1F).
Further experiments performed on mouse embryonic stem cells (ESCs) differentiated to ECs, using a similar approach as the iPSCs, verified that QKI-5 expression was induced in parallel with CD144 expression (Supporting Information Fig. S2A-S2D). These results indicated that QKI-5 may be implicated in differentiation of ECs derived from mouse iPSC and ESCs. To elucidate the underlying mechanisms regulated by QKI-5 in EC differentiation, QKI-5 was overexpressed by lentiviral gene transfer at day 4 of EC differentiation using ECs derived from iPSCs. The cells were harvested on day 6 and subjected to further analysis. QKI-5 overexpression significantly enhanced expression of the endothelial markers CD144, CD31, eNOS, and VEGFA and the transcription factor STAT3 (Fig. 2F, 2G). Surprisingly, QKI-5 overexpression also induced the in activation of the CD144 (VE-Cadherin) promoter as revealed by luciferase assays (Fig. 2H).
Importantly, QKI-5 also induced the expression of the arterial specific EC marker Ephrin B2, but it did not affect the expression of venous (CoupTFII) and lymphatic (Lyve1) markers. Moreover, QKI-5 induced EC differentiation in an ECspecific manner, while markers of other cells lineages such as neuronal (Nestin), cardiac (Mef2c), or smooth muscle cell (SMA) were not altered by QKI-5 overexpression (Fig. 2I). These data reveal that QKI-5 induced EC differentiation toward an arterial lineage.

QKI-5 Is Capable of Initiating EC Differentiation from iPSCs
Further experiments have revealed that QKI-5 leads to the stabilization of VE-Cadherin adhesion molecule during the EC differentiation process. When differentiated ECs were treated with Actinomycin D for 6 hours or cyclohexamide for 24 hours, CD144 (VE-Cadherin) expression was stabilized in the presence of QKI-5 (Supporting Information Fig. S3A-S3D), respectively. Moreover, when differentiated ECs were treated with Actinomycin D in a time point experiment from 0 to 24 hours, CD144 expression was stabilized as a decay curve as is shown in Supporting Information Figure S3E. These results demonstrate that QKI-5 induced the expression and stabilization of CD144 during EC differentiation. Additional experiments have been conducted to address the important question whether QKI-5 is able to initiate EC differentiation from iPSCs. When QKI-5 was overexpressed in undifferentiated iPSCs, the cells started expressing the arterial marker Ephrin B2 3 days later, while venous, lymphatic, or markers of other cell lineages, such as Nestin and SMA, were not expressed (Supporting Information Fig. S4A). Importantly, EC markers were highly induced upon QKI-5 overexpression in   S4B). Finally, to shed light on the upstream signaling, which activates QKI-5 expression in early stages of EC differentiation from iPSCs, transcription factor binding analysis on the QKI-5 promoter has predicted a potential binding site for the transcription factor ETS1. ETS factors, and in particular ETS1 and Etv2, have been reported as key factors that regulate endothelial development [43]. Interestingly, ETS1 is induced during EC differentiation from iPSCs in a time-dependent manner (Supporting Information Fig. S4C), while overexpression of ETS1 in early stages of EC differentiation has led to induction of QKI-5 (Supporting Information Fig. S4D). These results demonstrate that QKI-5 is able to initiate EC differentiation from iPSCs, and ETS1 may be implicated in the activation of QKI-5 in the early stages of EC differentiation.

QKI-5 Induced VEGFR2 Activation and VEGF Secretion through Direct Binding of the 3 0 UTR Region of STAT3
Formation of vascular networks is highly dependent on VEGF as a critical regulator of EC differentiation and vasculogenesis during development. Overexpression of QKI-5 by lentiviral gene transfer induced the secretion of VEGF on day 6 during the EC differentiation process (Fig. 3A). Similarly, QKI-5 overexpression led to activation of VEGFR2 during EC differentiation from iPSCs (Fig. 3B). To shed light on the signaling cascade initiated by the VEGFR transcriptional activation, the expression of the transcription factors STAT3, JAK-1, and AP-1 was assessed. Over-expression of QKI-5 leads to enhanced transcriptional activation of the VEGFR2 signaling pathway, which seems to be related to increased expression of VEGFA and autocrine stimulation of its cognate receptor (Fig. 3C). Notably, overexpression of QKI-5 induced the expression of EC markers, CD144 and CD31, in parallel with the induction of JAK-1, STAT3, and phosphorylation of STAT3 (Fig. 3D, and quantification in Fig. 3E). These results revealed that QKI-5 is implicated in the activation of the VEGFR2-regulatory binding sites AP1, STAT3, as well as STAT3 phosphorylation. Additional experiments were performed to investigate the nature of QKI-5-mediated activation of VEGFR. When STAT3 was knocked down by shRNA during EC differentiation, the previously observed upregulation of EC markers (CD144, CD31, and eNOS) and VEGFA, mediated by QKI-5 was ablated (Fig. 3F). Furthermore, luciferase assays revealed that knockdown of STAT3 by shRNA ablated the QKI-5-mediated activation of VEGFR2 (Supporting Information Fig. S5A). In agreement with the above results, when the differentiated ECs were treated with inhibitors to block the JAK-1 and STAT3 pathways, QKI-5 did not activate the EC markers CD31, CD144, eNOS, FLK-1 (VEGFR2), and VEGFA (Supporting Information Fig. S5B). Taken together, these data demonstrate that QKI-5 modulates the activation of VEGFR and EC differentiation through the transcription factor STAT3. Interestingly, the KH DOMAIN-mutant QKI-5 constructs did not induce the expression of the EC markers and STAT3 signaling when it was overexpressed in the iPSCs (Supporting Information Fig. S6A-S6D). These results suggest that QKI-5 binds candidate RNAs in the JAK1-STAT3 signaling pathway and regulates EC differentiation. Importantly, luciferase assays have shown that QKI-5 activated the 3 0 UTR of STAT3. Remarkably, QKI-5 was unable to activate the 3 0 UTR of STAT3 when the QKI motif was deleted (Fig. 3G). Moreover, RNA binding assays have confirmed that QKI binds directly to the 3 0 UTR of STAT3 (Fig. 3G, lower panel). Moreover, when differentiated ECs were treated with Actinomycin D in a time point experiment from 0 to 24 hours, STAT3 expression was stabilized as a decay curve is shown in Figure  3H. These results clearly demonstrate that the RNA binding protein QKI-5 induced VEGFR2 activation and VEGF secretion through direct binding of the 3 0 UTR of STAT3.

QKI-5 Has a Critical Role during EC Differentiation and Vascular Tube Formation
When QKI-5 expression was suppressed by shRNA during EC differentiation, this resulted in a significant suppression of the EC markers CD144 and CD31 (Fig. 4A, 4B) and the transcription factor STAT3 (Fig. 4B). Moreover, knockdown of QKI-5 caused inhibition in the transcriptional activation of VEGFR (Fig. 4C). Furthermore, knockdown of QKI-5 by shRNA resulted in inhibition of angiogenesis in Matrigel plugs in vivo (Fig.  4D), which further reinforced the notion that QKI-5 plays a critical role during EC differentiation and subsequent angiogenic function of the derived cells.

QKI-5 Induced Angiogenesis in Both in Vitro and in Vivo
QKI-5 appears to play an important role in iPSC-derived ECs during the formation of vascular networks in vitro and in vivo. These processes are greatly dependent on the presence of VEGF, although these experiments were conducted in the absence of exogenous VEGF. Notably, overexpression of QKI-5 by lentiviral gene transfer induced vascular tube formation in vitro (Fig. 4E, quantified in the right panel as total tube length/field [mM]) and vessel formation in vivo (Fig. 4F, quantified in the right panel as capillary density) when compared with controls. CD144 immunofluorescence staining verified the presence of differentiated cells in in vivo vascular tubes and demonstrated that QKI-5 over-expressing iPSCs displayed well-formed vascular structures and enhanced engraftment ability when compared with the control cells (Fig. 4G). Moreover, the nascent vessels in vivo (matrigel plugs) were stabilized by pericytes/vascular smooth muscle cells in the presence of QKI-5 as smooth muscle alpha-actin (SMA) staining is shown in Supporting Information Figure S7. These results demonstrate that QKI-5 induced the angiogenesis in both in vitro and in vivo Matrigel plug assays.

QKI-5 Significantly Improved Neovascularization and Blood Flow Recovery in the Hind Limb Ischemic Model
ECs derived from mouse iPSCs (iPS-ECs CTL) or iPSC ECs OE-QKI-5 (iPS-ECs OE-QKI-5) were labeled with Vybrant and injected intramuscularly immediately after induction of hind limb ischemia in mice. PBS was also used as an additional control. After 7 and 14 days, delivery of iPS-ECs OE-QKI-5 improved neovascularization and promoted significantly higher blood flow in the ischemic limbs compared to the nonmodified, iPS-ECs controls or PBS controls (Fig. 5A-5D). Limbs receiving iPS-ECs OE-QKI-5 also displayed significantly higher capillary numbers in the musculature in comparison with the controls (Fig. 5E, 5F) as shown by staining of adductor muscle sections with CD144 (Fig. 5G) (Fig. 5E,  5F).
Finally, when adductor muscle sections from iPS-ECs CTL or iPS-ECs OE-QKI-5 injected animals were stained and quantified with Vybrant and EC markers, iPS-ECs OE-QKI-5 were found to display an enhanced engraftment ability compared with controls ( Fig. 5H; Supporting Information Fig. S8B). These results indicate that iPS-ECs OE-QKI-5 display characteristic endothelial functions when tested in vivo. Importantly, no tumors have been detected in any animals during the duration of these experiments highlighting that these pluripotent stem cells were fully differentiated into ECs.

QKI-5 Has a Key Role in EC Differentiation Derived from hiPSCs
Generation of high-fidelity and stable EC populations, derived from human pluripotent cells, is a major limitation that the EC differentiation and cell reprogramming field are facing at the moment. Thus, we investigated whether QKI-5 has a role in EC differentiation derived from hiPSCs. hiPSCs were generated and differentiated in low attachment plates using Stem-Pro serum free media supplemented with BMP4, Activin A, FGF, and VEGF for 5 days. Then, a KDR positive population was selected and cultured in Fibronectin coated plates supplemented with EGM-2 media for 3-9 days. The derived hiPSC-ECs displayed a typical pattern of EC specific markers such as CD144, CD31, eNOS, and vWF (Fig. 6A). The efficiency of EC differentiation from hiPSCs was 62% as shown by FACs analysis for the positive cells for the EC marker CD144 (Fig. 6B). QKI-5 was expressed during EC differentiation from hiPSCs in parallel with VE-cadherin expression (Fig. 6C, 6D). QKI-5 overexpression by lentiviral gene transfer significantly induced the expression of the EC markers CD144, CD31, and eNOS (Fig. 6E, 6F and signaling of VEGFR [KDR], VEGFA, and STAT3 during EC differentiation (Fig. 6G). Further data from luciferase assays also confirmed that QKI-5 induced the transcriptional activation of CD144 and VEGFR in human cells (Fig. 6H, 6I). In contrast, knockdown of QKI-5 by shRNA suppressed the transcriptional activation of both CD144 and VEGFR (Fig. 6H, 6I). Importantly, QKI-5 only induced the expression of arterial specific EC marker and not the expression of venous and lymphatic markers (Fig. 6J). These data reveal that QKI-5 is implicated in EC differentiation from hiPSCs toward an arterial EC specification.

hiPSC-ECs OE-QKI-5 Induced Angiogenesis in Vivo
As iPS-ECs in the presence of QKI-5 maintained a stable phenotype for many passages without drifting to various nonendothelial phenotypes (as it has already been shown in Fig.  2), we further investigated whether QKI-5 had a similar role in mature ECs. For these experiments, QKI-5 was overexpressed in HUVECs by lentiviral gene transfer. These lentiviral constructs expressed mCherry, and the efficiency of infection was monitored by in vivo imaging using a fluorescence microscope (Fig. 7A). Indeed, QKI-5 was highly overexpressed in these mature ECs (Fig. 7B) and was able to induce further the expression of CD144 and VEGFA in both protein and mRNA levels (Fig. 7C, 7D). Importantly, QKI-5 overexpression did not induce the expression of non-EC markers such as SMA and SM22 (Fig. 7C). These results highlight that QKI-5 induces EC differentiation from hiPSCs and, in parallel, has an important role in the maintenance of the EC phenotype in both differentiated and mature ECs. Additional experiments with Matrigel plus assays in vivo provided further support to the above findings. hiPSC-ECs OE-QKI-5 significantly induced angiogenesis only 7 days after subcutaneous injection of ECs OE-QKI-5-iPSCs labeled with Vybrant cell surface marker in SCID mice, in comparison with the iPS-ECs CTL expressing an empty lentiviral vector (HE staining in Fig. 7E). CD31 immunofluorescence staining confirmed the presence of differentiated cells in in vivo vascular tubes and demonstrated that QKI-5 overexpressing iPSCs displayed well-formed vascular structures (Fig. 7G) and enhanced engraftment ability when co-stained with CD144 and Vybrant compared with the control cells (Fig.  7F, 7H). In addition, a human specific antibody for CD31 was used to stain exclusively the human cells in the Matrigel plugs and co-stained with the Vybrant to confirm the presence of the human cells and the specificity of the Vybrant labeling (Fig. 7I). These results clearly demonstrate that QKI-5 induced angiogenesis in vivo Matrigel plug assays highlighting its pivotal role in EC differentiation derived from hiPSCs.

DISCUSSION
iPSCs represent an attractive cellular approach for regenerative medicine today as they can be used to generate therapeutic cells of almost any type and, importantly, can be harnessed as patient-specific cells toward autologous cell therapy. In this study, using the model of iPSCs differentiation toward ECs, the RNA-binding protein QKI-5 was found to be an important regulator of VE-cadherin stabilization and VEGFR2 transcriptional activation during the EC differentiation process. Notably, QKI-5 overexpression induced the activation of EC markers, while knockdown by shRNA suppressed EC differentiation. In this study, the role of QKI-5 has been elucidated in EC differentiation from iPSCs. iPSCs have been generated based on a highly efficient approach, fully characterized and forced to differentiate toward ECs. QKI-5 was found to be induced during EC differentiation from iPSCs, and its expression was shown to be maintained at high levels in mature ECs. It has been demonstrated that QKI-5 plays a role in the induction and stabilization of CD144 and activation of VEGFR-regulatory binding sites AP1 and STAT3 and induction of STAT3 phosphorylation. Importantly, QKI-5 modulated the activation of VEGFR through direct binding of the 3 0 UTR region of STAT3. The notion that QKI-5 indeed played an important role during EC differentiation was further supported from additional data which clearly demonstrated that knockdown of QKI-5 resulted in inhibition of angiogenesis in vivo. Remarkably, ECs derived from iPS OE-QKI-5 improved neovascularization and blood flow recovery (almost 100%) in a hind limb ischemia model by showing an enhanced engraftment capacity when compared with non-modified iPS-ECs or PBS control groups. Notably, hiPSCs OE-QKI-5 induced angiogenesis in Matrigel plug assays in vivo only 7 days after subcutaneously injection in SCID mice, highlighting a clear functional benefit of QKI-5 in neovascularization, blood flow recovery, and angiogenesis.
RNA-binding proteins add an additional layer of complexity to a series of events, which include processing and splicing of pre-mRNA, export to the cytoplasm, quality control     [44]. All of these events, from initiation of transcription by key transcription factors, to the stability and effective translation of the message, are regulated by the presence of specific nucleotide sequences which are bound by specific RNAbinding proteins [45].
CD144 is a strictly endothelial specific adhesion molecule and is of vital importance in maintaining and controlling the endothelial phenotype through appropriate cell-cell contacts [16]. CD144 is essential during embryonic angiogenesis and, thus, the elucidation of the underlying mechanisms that regulate CD144 are important in understanding the functions permeating vascular permeability, cell proliferation, apoptosis, and modulation of VEGFR. A recent study has shown that QKI directly binds to the 3 0 UTR of CD144 [46]. In our study, we provide further support to the above notion that QKI-5 induced the mRNA stabilization of CD144.
VEGFA is one of the earliest markers for the endothelial lineage during development [47,48], and this growth factor is also critical for vascular repair and neovascularization [49]. The data obtained in this study indicate that QKI-5 regulates EC expression of VEGFA and autocrine stimulation of VEGFR and this pathway is controlled, at least in part, through STAT3 activation and phosphorylation. Interestingly, knockdown of STAT3 by shRNA ablated QKI-5-mediated activation of the VEGFR and subsequent expression of EC markers. It has been reported that constitutive STAT3 activity up-regulates VEGFR [50], while STAT3 protein binds to the VEGFR promoter inducing VEGFR promoter activity [50]. Interestingly, QKI-5 regulates the expression of STAT3 through direct binding in the 3 0 UTR leading to mRNA stabilization. Notably, the KH DOMAIN-mutant QKI-5 construct did not induce the expression of the EC markers, STAT3 signaling when it was overexpressed in iPSCs. Importantly, QKI-5 was unable to activate the 3 0 UTR of STAT3 when the QKI motif was deleted. These results demonstrate that QKI-5 binds to the 3 0 UTR of STAT3 in the JAK1-STAT3 signaling pathway and regulates EC differentiation toward an arterial lineage.
It has previously been reported that QKI-5 knockout mice display a range of blood vessel defects during development which result in embryonic lethality [39]. Additional studies of the extraembryonic yolk sac have also reported that QKI regulates visceral endoderm differentiated function at the cellular level, including the local synthesis of retinoic acid [29], which then exerts paracrine control of ECs within the adjacent mesoderm. QKI is also highly expressed during normal cardiac development, particularly in the outflow tract, suggesting potentially unique functions in the developing heart [51] and vascular smooth muscle cell development [38]. Taken together, this is strong evidence that QKI is highly conserved in the early embryo throughout the evolution of nonvertebrate and vertebrate organisms [29]. The role of QKI in the adult organism is less well-appreciated, although there is evidence that it could have a role in vascular smooth muscle cell phenotypic plasticity and can ameliorate pathogenic, fibroproliferative responses to vascular injury [52].
This study provides strong evidence that QKI-5 is capable of inducing differentiation toward ECs and that, in parallel, regulates angiogenesis in both in vitro and in vitro Matrigel plus assays, even in the absence of exogenous VEGF. At the moment, the potential of iPSCs to differentiate toward therapeutic cells is only based on directed empiricism, while they are totally dependent on combinations of growth factors, media, and matrices to favor the desired lineage. In regards to vascular regeneration, it is important to understand the key regulatory pathways such as epigenetic alterations, transcriptional activity, and RNA-binding patterns associated with the differentiation processes. In particular, although there has been significant progression in the field [53], there are not any fully defined protocols to generate high-fidelity and stable ECs from human pluripotent stem cells at the moment. This is a major limitation of generating pure populations of rejuvenated EC cells to be used for drug screening and cell-based therapies. Indeed, QKI-5 is a very interesting candidate, which holds the potential to derive stable populations of functional ECs. Specifically, the transcription factor ETS1 is implicated in the induction of the expression of QKI-5 during early stages of EC differentiation. QKI-5 is also acting as a splicing factor involved in the regulation of numerous signaling pathways. Data from our laboratory also showed that QKI-5 induced the splicing factor SF3B1 during EC differentiation (Supporting Information Fig. S9). Importantly, SF3B1 is induced in a timedependent manner during EC differentiation from iPSCs (Supporting Information Fig. S10), indicating that QKI-5 is likely to be an important splicing regulator of EC differentiation. Notably, a number of QKI splicing isoforms have been reported, although their function(s) are not clearly defined. Additional studies are clearly required to shed light on the precise functions of the different QKI splicing isoforms. Interestingly, a recent paper has reported that QKI plays a remarkably dynamic role in regulating hundreds of circular RNAs (circRNAs) during human epithelial to mesenchymal transition (EMT), while QKI itself is regulated during EMT [16]. Therefore, it is tempting to speculate that precise regulation of QKI-5 during EC differentiation will provide fully defined experimental protocols, which could reproducibly guide iPSCs to a vascular lineage [54,55] and, thereby, enable clinical application [56][57][58]. In addition, it would be very interesting to investigate the role of QKI and its mRNA splicing isoforms in disease models, such as atherosclerosis and diabetes, where the endothelial function is impaired.

SUMMARY
In summary, our data strongly support the notion that QKI-5 is induced during EC differentiation, resulting in stabilization of STAT3 expression through direct binding to the 3 0 UTR region of STAT3, modulation of VEGFR transcriptional activation and VEGF secretion. Markedly, iPS-ECs OE-QKI-5 significantly improved angiogenesis and neovascularization and blood flow recovery in experimental hind limb ischemia (Supporting Information Fig. S9) highlighting a clear functional benefit. This study provides support to the growing consensus that elucidating the molecular mechanisms underlying EC differentiation will ultimately advance stem cell regenerative therapy toward treating cardiovascular disease.

ACKNOWLEDGMENTS
This work was supported by Grants from British Heart Foundation and BBSRC.

AUTHOR CONTRIBUTIONS
A.C. and S.K.: conception and design, collection and/or assembly of data, data analysis and interpretation, manuscript writing; M.T., J.B., M.V.-G., D.D., R.C., C.M., M.E., and Y.H.: collection and/or assembly of data; D.G., A.W.S., L.Z., and Q.X.: provision of study material, final approval of manuscript; A.M.: conception and design, collection and/or assembly of data, data analysis and interpretation, manuscript writing, financial support, final approval of manuscript. A.C. and S.K. contributed equally to this article.

DISCLOSURE OF POTENTIAL CONFLICTS OF INTEREST
The authors indicate no potential conflicts of interest.