Striatal Transplantation of Human Dopaminergic Neurons Differentiated from Induced Pluripotent Stem Cells Derived from Umbilical Cord Blood Using Lentiviral Reprogramming

Abstract

Human induced pluripotent stem cells (hiPSCs) are promising sources for regenerative therapies like the replacement of dopaminergic neurons in Parkinson's disease. They offer an unlimited cell source that can be standardized and optimized to produce applicable cell populations to gain maximal functional recovery. In the present study, human cord blood-derived iPSCs (hCBiPSCs) were differentiated into dopaminergic neurons utilizing two different in vitro protocols for neural induction: (protocol I) by fibroblast growth factor (FGF-2) signaling, (protocol II) by bone morphogenetic protein (BMP)/transforming growth factor (TGF-β) inhibition. After maturation, in vitro increased numbers of tyrosine hydroxylase (TH)-positive neurons (7.4% of total cells) were observed by protocol II compared to 3.5% in protocol I. Furthermore, 3 weeks after transplantation in hemiparkinsonian rats in vivo, a reduced number of undifferentiated proliferating cells was achieved with protocol II. In contrast, proliferation still occurred in protocol I-derived grafts, resulting in tumor-like growth in two out of four animals 3 weeks after transplantation. Protocol II, however, did not increase the number of TH⁺ cells in the striatal grafts of hemiparkinsonian rats. In conclusion, BMP/TGF-β inhibition was more effective than FGF-2 signaling with regard to dopaminergic induction of hCBiPSCs in vitro and prevented graft overgrowth in vivo.

Keywords

Dopaminergic neurons Human cord blood cells Intrastriatal transplantation In vitro differentiation Induced pluripotent stem cells (iPSCs)Parkinson's disease

Introduction

Parkinson's disease (PD) is one of the most common neurodegenerative disorders characterized by the progressive loss of dopaminergic (DA) neurons in the substan-tia nigra pars compacta. Current therapies such as deep brain stimulation (33) or DA substitution (37,39) provide symptomatic relief but do not challenge the cause of the disease. As an alternative strategy for cell-based therapy, intrastriatal transplantations of DA neurons derived from various cell sources have been proposed, showing long-term graft survival, DA fiber outgrowth, and reinnervation of the striatum (57). Although functional improvements have been repeatedly observed after transplantation of DA neurons in various animal models of PD, open-label clinical studies and two double-blind clinical trials did not reach the expected outcome (31,58,65).

For all previous clinical trials, including the ongoing study by the transeuro consortium (www.transeuro.org.uk, www.clinicaltrials.gov), dissociated cells or solid tissue pieces obtained from aborted fetuses containing neuronal progenitor cells (NPCs) have been used. Additional cell sources such as embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), or fibroblasts directly converted into DA neurons have drawn attention for preclinical studies (5,6,40,47,57,64). The major disadvantage of both fetal NPCs and ESCs, however, are their related ethical issues, while iPSCs can be generated by somatic cell reprogramming. Moreover, teratoma formation has been reported for ESC-derived DA neuron grafts (6,41). NPCs derived from the ventral mesencephalon seem not to show this disadvantage (8,61), although NPCs transplanted repeatedly into the brain of a young immunode-prived patient suffering from ataxia telangiectasia gave rise to multiple tumors (1). Tumor formation has been also observed in iPSCs, which were generated by viral reprogramming vectors encoding the c-Myc oncogene (43). Therefore, this factor is nowadays omitted for reprogramming viral vectors, including the iPSCs used in this study (18).

iPSCs hold great promise due to their ethical tenability for treatments of neurodegenerative diseases (10). iPSC-derived DA neurons, for example, have been generated from fibroblasts, stomach cells, lymphocytes, B cells, human blood cells, human cord blood (hCB) cells, and others (42). Moreover, iPSCs derived from human somatic cells or, most notably, human PD patient fibroblasts, have been differentiated into DA neurons in vitro and resulted in functional recovery after transplantation in the 6-hydroxydopamine (6-OHDA) PD rat model (19,60). Such personalized DA neurons could be advantageous for autologous transplantation studies due to the low risk for immunorejections (2,17).

Recently, hCB cells have been shown to be a suitable source for iPSCs. The isolation of endothelial progenitors from CB is 10-fold more efficient than from adult peripheral blood. Furthermore, CB cells have the advantage of higher proliferation potential and less accumulation of nuclear and mitochondrial mutations as in ontogenetic older adult cells, which reduce the risk for tumor formation (24,38). We reported recently on the efficient generation of neurons including DA cells from hCB-derived iPSCs (hCBiPSCs) and confirmed neuronal functional properties by electrophysiological recordings and calcium imaging analyses in vitro (55).

For the generation of neuronal tissue during embryogenesis, the inhibition of bone morphogenetic protein (BMP) signaling is required (29,52). BMPs induce phosphorylation of cytoplasmatic SMAD proteins that translocate to the nucleus and regulate gene expression (3). Many studies recapitulating neurogenesis from iPSCs in vitro use BMP or transforming growth factor-β (TGF-β) pathway inhibitors like noggin, dorsomorphin (DM), or SB 431542 (SB) to prevent phosphorylation of SMADs and promote neural induction (9,34,36). Involvement of fibroblast growth factor (FGF) during neural induction is also likely because studies on Xenopus embryos revealed a block of neural induction when FGF signaling was inhibited (23,28).

Here we compared two protocols shown to efficiently induce neuralization from hCBiPSCs by using FGF-2 [protocol I, used for example by Swistowski et al. (60)] and DM/SB-mediated inhibition of BMP/TGF-β signaling [protocol II; used by Stanslowsky et al. (55)], respectively, regarding their dopaminergic differentiation potential in vitro and in vivo after intrastriatal transplantations into rats.

Materials and Methods

Culture Conditions of Undifferentiated hiPSCs

The use of hiPSCs in this study was approved by the local ethics committee of Hannover Medical School (No. 776). Cord blood was obtained from a male donor. The hCBiPSC line K2 has been generated by lentiviral overexpression of octamer-binding transcription factor-4 (OCT4), (sex-determining region-Y)-box 2 (SOX2), NANOG, and LIN28 and was characterized by Haase et al. (18). Cell culture conditions and functional properties have been previously described in detail (55). Briefly, undifferentiated hCBiPSCs were maintained on a gamma ray (60 gray)-inactivated feeder layer of mouse embryonic fibroblasts, derived from E13 CD-1 embryos (Charles River, Cologne, Germany) in hiPSC medium [composed of knockout Dulbecco's modified Eagle medium (DMEM) supplemented with 20% knockout serum replacement, 1% penicillin/streptomycin, 1% nonessential amino acids, 1% glutamax, and 0.1 mM β-mercaptoethanol] supplemented with 20 ng/ml FGF-2. If not otherwise stated, all media and medium supplements were purchased from Life Technologies (Darmstadt, Germany) and all cytokines were obtained from Peprotech (Hamburg, Germany).

Dopaminergic Differentiation of hCBiPSCs

Undifferentiated hCBiPSC colonies were detached from the mouse feeders by incubation with 0.2% collagenase IV (Life Technologies) for 10 min at 37°C and cultured in suspension as embryoid bodies (EBs) in hiPSC medium without FGF-2. The following two protocols were investigated for optimization of dopaminergic neuron differentiation (see Fig. 1E).

Figure 1.

In vitro generation of dopaminergic neurons from hCBiPSCs for transplantation studies comparing two differentiation protocols. (E) Schematic summary of the differentiation procedures. Immunocytochemical stainings during in vitro differentiation using protocol I (A–D) and protocol II (F–H). OCT4⁺ hCBiPSC colonies (A, F) were detached from the feeder layer and cultured in suspension as EBs. Protocol I: Cultivation in the absence of morphogenes for initially 7 days yielded a Pax6⁺ neuronal progenitor population (B). By addition of FGF-2, neural rosettes became visible, which were manually isolated and dissected. Those nestin⁺ neural progenitors could be expanded for several passages in the presence of FGF-2 (C). After midbrain induction by SHH and FGF-8 for 10 days and final maturation for 21 days, some TH⁺/Tuj1⁺ double-labeled neurons were detected (D). Protocol II: Differentiation in the presence of dorsomorphin (DM) and SB 431542 (SB) for 6 days generated a Pax6⁺ and OCT4^– cell population (G). On day 4, PMA and FGF-8 were added for 8 days to initiate regionalization. Nestin⁺ EBs were plated on PLO/laminin-coated cell culture dishes on day 12. After 21 days of final maturation numerous TH⁺/Tuj1⁺ were detected (H). To visualize the proportion of specific cell types (OCT4, Pax6, nestin, Tuj1, or TH) among the total numbers of cells, the nuclear DAPI staining was used. In case of (C) and (G) the weaker DAPI signal in the center of the EB is concealed by the stronger fluorescent signal of the Pax6⁺ cells. Scale bars: 100 μm.

Protocol I

On day 8, hiPSC medium was replaced by DMEM/F12 supplemented with 1% penicillin/ streptomycin, 1% nonessential amino acids, 1% glutamax, 1% N2, and 20 ng/μl FGF-2. EBs were cultured for an additional 2 days in suspension before they were plated onto poly-L-ornithine (20 μg/ml; Sigma-Aldrich, Steinheim, Germany)/laminin (10 μg/ml; Invitrogen/Life Technologies)-coated culture dishes (Greiner Bio-One, Frickenhausen, Germany). After 2–3 days, neural rosettes became visible. At day 12 to 13, neural rosettes were manually isolated, dissociated with 1% accutase (Millipore, Steinheim, Germany), and replated onto poly-L-ornithine/laminin-coated culture dishes in neurobasal medium containing 1% penicillin/streptomycin, 1% nonessential amino acids, 1% glutamax, 20 ng/ml FGF-2, and 1% B27 without vitamin A. This neural precursor cell population could be expanded for several passages before differentiation proceeded by addition of 200 ng/ml sonic hedgehog (SHH) and 100 ng/ml fibroblast growth factor 8 (FGF-8) for 10 days.

Protocol II

EBs were cultured in the presence of 10 μM TGF-β receptor inhibitor SB (Biomol, Hamburg, Germany) and 1 μM of the BMP signaling inhibitor DM (R&D Systems, Wiesbaden-Nordenstadt, Germany) for 6 days. In the first 4 days, SB and DM were applied in hiPSC medium without FGF-2, while thereafter DMEM/ F12 supplemented with 1% penicillin/streptomycin, 1% glutamax, and 1% N2 were used. Between days 4 and 12, regionalization was stimulated by the addition of 0.6 μM purmorphamine (PMA; Biomol), which is an activator of the SHH signaling pathway, and 100 ng/ml FGF-8. EBs grown in suspension for 12 days were replated onto polyl-ornithine/laminin-coated Petri dishes to allow attachment of the cells during the final maturation step.

In both protocols, final maturation toward the DA cell lineage was carried out in neurobasal medium supplemented with 1% penicillin/streptomycin, 1% glutamax, 20 ng/ml brain-derived neurotrophic factor (BDNF), 20 ng/ ml glial cell line-derived neurotorphic factor (GDNF), 25 ng/ml transforming growth factor-β3 (TGF-β3), 200 μM ascorbic acid (Sigma-Aldrich), 1 mM dibutyryl cyclic AMP (cAMP; Sigma-Aldrich), and 1% B27 without vitamin A for 21 days.

At the day of transplantation, the medium was removed, and wells were washed with Dulbecco phosphate-buffered saline (PBS; Berlin, Germany). Enzymatic dissolution was performed with 0.05% trypsin/0.02% EDTA (PAA/GE Health Care Europe GmbH, Freiburg, Germany) in PBS per well and stopped with the same amount of DMEM/ F12 plus 1% fetal calf serum (PAA). Afterward, the cell solution was centrifuged at 235 × g for 5 min, and the cell pellet was resuspended in DMEM/F12 plus 0.05% DNase (Roche, Mannheim, Germany). Mechanical dissociation of the cells was performed using fire-polished glass pipettes (Brand, Wertheim, Germany). Finally, cell viability and number was determined by trypan blue dye exclusion assay (Sigma-Aldrich), and the final cell concentration was adjusted to 100,000 cells per μl with DMEM/F12.

Immunocytochemistry

Immunostainings were performed as previously described (55). The following primary antibodies were used: rabbit polyclonal anti-OCT4 (1:500; 2750; Cell Signaling, Danvers, MA, USA), mouse monoclonal anti-Pax6 (1:500; MAB5552; Millipore, Schwalbach, Germany), mouse monoclonal anti-nestin (1:500; MAB1259; R&D Systems, Abingdon, UK), mouse monoclonal anti-β III tubulin (Tuj1, 1:500; MAB1637; Millipore), rabbit polyclonal anti-TH (1:500; sc-14007; Santa Cruz Biotechnology, Heidelberg, Germany), and rabbit polyclonal anti-Ki-67 (1:1,000; NCL-Ki67p; Novocastra, Wetzlar, Germany). The number of cells immunoreactive for Tuj1, TH, and Ki-76 was determined related to the number of 4′,6-diamidino-2-phenylindole (DAPI; Sigma-Aldrich) stained nuclei for three wells per condition from four independent differentiation experiments. Approximately 500 cells were counted within three randomly selected fields in 40× magnification visualized by an Olympus fluorescence microscope (BX61; Olympus, Hamburg, Germany) and CellF software (Olympus).

Experimental Design In Vivo

Twenty female Sprague–Dawley rats (Charles River, Sulzfeld, Germany) were included in this study. Animals were kept on a 14-h light/10-h dark schedule in temperature- and humidity-controlled rooms with food and water available ad libitum and were housed in cages of four rats. All experimental protocols followed the German animal protection act and were approved by the local authorities (10/0098 and advice of amendment 20-08-2012, Bezirksregierung LAVES Hannover, Germany). Cell transplantation surgery was performed in three experiments, and animals were sacrificed 1 to 3 weeks later. After transcardial perfusion with 0.9% saline (Braun, Melsungen, Germany) and 4% paraformaldehyde (PFA; Sigma-Aldrich), brains were removed and processed for immunohistochemistry.

In the first experiment (iPSC1H), eight healthy rats (about 3 months old, weighing 230–270 g on the day of transplantation) were intrastriatally transplanted with iPSCs differentiated according to cell culture protocol I. One and 3 weeks after transplantation surgery, four rats were sacrificed, respectively.

In the second experiment (iPSC2H), four healthy rats (about 11 months old, weighing 320–370 g at transplantation) received striatal transplantation of iPSCs, which were differentiated according to cell culture protocol II. After 2 weeks, all animals were sacrificed.

For the third experiment (iPSC2L), eight rats (about 3 months old on the day of lesion, about 14 months old and weighing 310–360 g at transplantation) were lesioned with 6-OHDA injection into the right medial forebrain bundle. All included animals were successfully lesioned, showing at least six full body turns per minute in the amphetamine-induced rotation test [D-amphetamine sulfate (Sigma-Aldrich) 2.5 mg/kg in saline (Braun) according Ungerstedt and Arbuthnott (63)]. Ten months after lesion, animals were transplanted with iPSCs, which were differentiated according to protocol II. During the surgery, one animal died. Rats were sacrificed after 1 (n = 3) and 3 (n = 4) weeks, respectively.

6-OHDA Lesion Surgery

Animals were deeply anesthetized with chloral hydrate (370 mg/kg, IP; Sigma-Aldrich) and 6-OHDA hydrobro-mide (free base 5.14 mg/μl; Tocris, Bristol, UK) in 0.03% L-ascorbate (Sigma-Aldrich)-saline (Braun) stereotactically injected into the right medial forebrain bundle to generate a complete destruction of the nigrostriatal pathway. The coordinates for the injection site (62) are given in mm according to bregma and dura (45): anterior–posterior (AP) –4.0, lateral (LAT) –1.3, dorsoventral (DV) –7.0, tooth bar (TB) –4.5. The injection volume was 3 μl. A 10-μl Hamilton syringe (Hamilton, Bonaduz, Switzerland) with an injection rate of 1 μl/min was used, and the cannula was left in place for an additional 2 min before being slowly retracted.

Transplantation Surgery

Rats received injections of cyclosporine (0.1 ml/100 g, IP; Sandimmun, Novartis, Nürnberg, Germany) for immunosuppression 1 day prior to the surgery and then every day. Animals were deeply anesthetized with chloral hydrate (370 mg/kg, IP; Sigma-Aldrich), and cells were stereotactically injected into the right striatum using a microtransplantation approach with a glass capillary (Drummond Scientific, Broomall, PA, USA) with an outer diameter of 50–70 μm attached to a 2-μl Hamilton syringe (Hamilton). The transplantation surgery was performed similarly as previously described (48,50,61). The injection procedure was modified in terms of only one tract with two deposits of 1 μl cell suspension and modified injection coordinates [in mm according to bregma and dura (45)]: AP +1.0, LAT -3.3, DV -5 and -4, TB 0.0. One microliter cell suspension encompassed 100,000 cells. Injection rates were 1 min/ deposit. In order to allow diffusion and to prevent retraction of the cells, the capillary was left in place for an additional 3 min after injection.

Perfusion

Rats were deeply anesthetized with carbon dioxide and transcardially perfused with 250 ml 0.9% saline (Braun) followed by 250 ml 4% PFA (Sigma-Aldrich). Brains were removed and postfixed in 4% PFA for 24 h. Subsequently, brains were placed in 30% sucrose (Roth, Karlsruhe, Germany) for cryoprotection, embedded in Tissue-Tek OCT Compound (Sakura Finetek, Alphen aan den Rijn, Netherlands), and then frozen. Brains were coronally sectioned on a freezing stage microtome (Leica CM3050S; Leica Microsystems, Wetzlar, Germany) at 40 μm thickness in a series of six slices.

Tyrosine Hydroxylase (TH)-Immunohistochemistry

Free-floating anti-TH immunohistochemistry was performed on every sixth section with an avidin–biotin complex ABC kit (Vector Labs, Burlingame, CA, USA), as previously described (48), using anti-TH antibody (1:5,000; T1299; Sigma-Aldrich), biotinylated rabbit anti-mouse antibody (1:200; E0464; Dako, Hamburg, Germany), and 3′,3-diaminobenzidime (DAB; Sigma-Aldrich) with ammonium nickel sulfate (Riedel-de Häen, Seelze, Germany) intensification for visualization. After the staining, sections were dehydrated and mounted (Corbit-Balsam; Hecht, Hamburg, Germany) on gelatinized SuperFrost slides (Thermo Scientific, Braunschweig, Germany; Merck, Darmstadt, Germany). The TH⁺ cell number per graft was estimated by counting all TH⁺ cells inside the graft in one TH-DAB-stained section series multiplied by the total number of six sections.

Fluorescence Immunohistochemistry

Sections were treated with blocking buffer consisting of 3% normal goat serum (Gibco/Life Technologies) and 0.3% Triton X-100 (Roche) in PBS for 60 min. The following primary antibodies were applied in 1% bovine serum albumin (PAA/GE Health Care Europe GmbH), 0.3% Triton X-100 (Roche) in PBS overnight at 4°C: Ki-67 (1:500; AB15580; Abcam, Cambridge, UK), hNuc (1:500; MAB1281; Upstate/Millipore), TH (1:1,000; AB152; Millipore). Correspondent anti-mouse or anti-rabbit secondary antibodies conjugated to fluorescent dyes Alexa 488 (1:500; A11029; Invitrogen/ Life Technologies) and Alexa 555 (1:500; A21424; Invitrogen/Life Technologies) were applied for 60 min each. Furthermore, all sections were stained with DAPI (1:1,500; D9564; Sigma-Aldrich). Finally, sections were dehydrated and mounted (Mounting Medium; Dako North America, Carpinteria, CA, USA) on gelatinized SuperFrost slides (Thermo Scientific, Merck).

Graft Volume

On one series of hNuc-stained sections, the graft volume was measured for evaluation of the proliferation and graft survival/integration. Multi-image alignment pictures were taken under bright field illumination and 4× magnification with an automated microscope (Olympus BX51; Olympus, Ballerup, Denmark) and CellSens Dimension software (Olympus). The graft border was manually demarcated at high magnification to distinguish bright yellow-stained nuclei from background fluorescence. The area of the graft in each section was measured with CellSens Dimension software and then summed up to obtain the total graft area in one section series in μm². The graft volume in mm³ was calculated by multiplying area value with section thickness of 40 μm and the total number of six series.

Analysis of Proliferating Cells

Proliferating cells were visualized by anti-Ki-67 staining. Since cell density of the grafted hNuc⁺ cells was too high to determine proliferating and total cell numbers on the single cell level, we calculated the area of the Ki-67 signal in proportion to the hNuc area (area Ki-67/ area hNuc × 100 = % area Ki-67) (Fig. 3G–Q). Gray leveled pictures were taken in 20× magnification under an Olympus fluorescent microscope (Olympus BX 60) and with CellP software (Olympus). For each brain, three sections containing the largest part of the graft were chosen. Of each section, four pictures were taken at the lower and upper graft border of the left and right graft side. In total 12 pictures for each brain were taken. Using ImageJ software (National Institute of Health, Bethesda, MD, USA) black and white pictures were created, thereby eliminating Ki-67 or hNuc background stainings, respectively.

Figure 2.

Improved dopaminergic neurogenesis in vitro using protocol II. Immunostainings with neuronal marker Tuj1 (A, B), dopaminergic marker TH (A, B), and cell proliferation marker Ki-67 (C, D) counterstained with the nuclear marker DAPI (A–D) were quantified after 21 days of final maturation in vitro (E). Using protocol I, 42.0% of the total cell numbers were positive for Tuj1 and 3.5% expressed TH. Utilization of protocol II yielded similar numbers of Tuj1⁺ neurons (49.3%), but significantly increased numbers of TH⁺ neurons/DAPI (7.4%, *p < 0.05). Using protocol II, a slight, but not significant (p = 0.23), reduction of the number of Ki-67⁺ proliferating cells (41.4%) was observed compared to protocol I (52.3%). Scale bars: 50 μm.

Figure 3.

Graft volume and proliferation potential of grafted cells in healthy and lesioned rats. Graft volume measurements were performed on hNuc immunohistochemical-stained sections and are given in mm³ (A–H). For both protocol I (iPSC1H) and II (iPSC2L), graft sizes increased from the first to third weeks (F, not significant). Two out of four animals of the 3-week protocol I-derived group showed extensive graft growth (iPSC1H, B corresponds to the 4.1 mm³ specimen). One-way ANOVA of all three groups of the protocol II-derived grafts confirmed a significant increase in graft volumes between the 1- and 2-week groups (*p < 0.05) (F). Protocol I-derived grafts were densely packed and showed a well-defined graft-host border (G), whereas protocol II-derived grafts displayed a reduced cell density and a diffuse graft–host boundary (H). Proliferation potential of grafted cells was determined by measuring the area stained with the cell proliferation marker Ki-67⁺ inside the hNuc⁺ area on representative photographs (see Materials and Methods, I–S). The proliferative area was significantly decreased in the 3-week iPSC2L group compared to the 1-week iPSC2L, but not in the 2-week group (I, one-way ANOVA comparing protocol II-derived groups, **p < 0.01). This decrease was also significant compared to both protocol I-derived groups (I, two-way ANOVA comparing 1- and 3-week groups for both protocols, p < 0.01, #p < 0.001).

Statistics

Data from the in vitro experiments were analyzed with GraphPad Prism 5 (GraphPad Software, San Diego, CA, USA) using the unpaired Student's t-test. The in vivo data were analyzed with GraphPad Prism 6 using two-way ANOVA followed by Tukey's post hoc test for comparison of both differentiation protocols for the 1- and 3-week groups. As the 2-week group was only performed in protocol II-derived cells (iPSC2H), those values were compared to the 1- and 3-week groups of iPSC2L using one-way ANOVA followed by Tukey's post hoc test. In addition, values for the 2-week iPSC2H were compared to 1- and 3-week iPSC1H with one-way ANOVA, but showed no significant differences for graft volume or Ki-67 area (data not shown). All data are presented as means ± SEM, and the significance level was set as p < 0.05, p < 0.01, and p < 0.001.

Results

Comparison of Two DA Differentiation Protocols In Vitro

We investigated two protocols for DA differentiation of hCBiPSCs illustrated in Figure 1. In protocol I, FGF-2, crucial for the maintenance of pluripotency of iPSCs, was withdrawn for the first 7 days of differentiation followed by a differentiation period in the presence of FGF-2 to enhance neurogenesis. In protocol II, we used the small molecules DM and SB, inhibitors of BMP and TGF-β signaling, respectively, during the first 6 days of differentiation for efficient neural conversion (Fig. 1E).

To confirm the neural induction activity, immunostainings for the pluripotency marker OCT4 and neural precursor markers Pax6 and nestin were carried out. During expansion (day 0) hCBiPSC colonies exhibited typical morphological features of undifferentiated iPSCs and uniformly expressed OCT4 (Fig. 1A, F). On day 12 of the in vitro differentiation according to protocol I, most of the cells expressed Pax6, and only single OCT4⁺ cells were detected (Fig. 1B). The utilization of protocol II yielded a Pax6⁺ progenitor population already after 6 days of differentiation in the absence of OCT4⁺ cells (Fig. 1G).

In protocol I, nestin⁺ neural precursors could be expanded as single cells (Fig. 1C) for several passages before mesencephalic dopaminergic differentiation was initiated by adding SHH and FGF-8 for 10 days, followed by incubation in final maturation medium supplemented with BDNF, GDNF, TGF-β3, cAMP, and ascorbic acid for 21 days. In protocol II nestin⁺ EBs (Fig. 1H) were exposed to PMA, a small-molecule agonist of SHH, and FGF-8 already on day 4 of differentiation for 8 days in total, followed by 21 days in final maturation medium, which is identical to the respective medium used in protocol I. Quantification of the cellular composition after 21 days of final maturation (Fig. 2) revealed comparable numbers of Tuj1⁺ neurons for both protocols (42.0 ± 3.8% and 49.3 ± 3.0% of the total cell population using protocol I and protocol II, respectively; p = 0.20) (Fig. 1D, I; Fig. 2A, B, E). The number of TH⁺ DA cells per DAPI-stained cell was significantly elevated to 7.4 ± 1.2% using protocol II compared to protocol I with 3.5 ± 0.8% (p < 0.05) (Fig. 2A, B, E). The number of cells positive for the proliferation marker Ki-67 was not significantly different in both protocols (41.4 ± 3.7% compared to 52.3 ± 7.0%, p = 0.23) (Fig. 2C–E).

Intrastriatal Graft Size Is Influenced by the Preceding In Vitro Differentiation Protocol

At the end of the 21-day final differentiation period in vitro, cells were detached, dissociated, and transplanted into the striatum of immunosuppressed adult rats. Grafted human cells were identified in coronal brain sections using the human cell-specific marker hNuc (Fig. 3A–H). The graft volume of all five transplantation groups was quantified. Cells differentiated with protocol I formed densely packed, well-defined small grafts of 0.56 ± 0.05 mm³ 1 week after transplantation into healthy (nonlesioned) rats (iPSC1H) (Fig. 3A). However, 3 weeks after transplantation those cells (iPSC1H) caused huge tumors with a 10- to 15-fold increased volume (4.1 mm³ or 8.8 mm³) (Fig. 3B, F) in two out of four animals compared to the 1-week group, while the remaining two animals displayed three times enlarged grafts of 1.5 mm³ (Fig. 3F). Volume differences between the 1- and 3-week iPSC1H groups did not reach statistical significance due to the high variance seen in the 3-week group (Fig. 3F). After 3 weeks, in vivo cells derived from protocol I (iPSC1H) formed a well-defined graft–host border with densely packed cells especially in the periphery of the graft (Fig. 3B, G), which was similarly dense as seen in the 1-week iPSC1H group (Fig. 3A). In the center of the iPSC1H grafts at 1 week and more prominent at 3 weeks, hNuc cells were sparsely distributed (Fig. 3A, B).

In contrast, cells differentiated with protocol II and transplanted into healthy (nonlesioned) rats (iPSC2H) displayed a more diffuse graft-host border and less densely packed cells inside the grafts (Fig. 3C) compared to protocol I-derived iPSC1H grafts (Fig. 3A, B). After 2 weeks in vivo, the mean graft volume was 1.9 mm³ (iPSC2H) (Fig. 3F). Since cell density and graft volume of protocol II-derived cells did not point toward a graft overgrowth, and those cells showed subtle TH⁺ fiber outgrowth after 2 weeks in vivo (see below), we selected protocol II-derived cells for additional transplantation experiments into the 6-OHDA-lesioned PD rat model (iPSC2L). One week after grafting into lesioned animals, the iPSC2L grafts had a volume of 0.3 ± 0.1 mm³ (Fig. 3D, F), which was a fourfold increase in the 3-week iPSC2L grafts (1.3 ± 0.2 mm³) (Fig. 3E, F). Growth of the 3-week iPSC2L grafts was, however, not as strong as in the biggest grafts of the protocol I-derived 3-week iPSC1H group (Fig. 3F). Comparison of graft-type (protocol I or II derived) and time after grafting (1 or 3 weeks) by two-way ANOVA revealed no significant differences between the graft volumes (Fig. 3F). Comparison of all three protocol II-derived grafts using one-way ANOVA revealed a significant increase in graft volumes only between the 1- and 2-week groups (Fig. 3F). Irrespective of whether lesioned or nonlesioned animals were used for transplantation of protocol II-derived cells, all three groups displayed a diffuse graft-host boundary and reduced cell density (Fig. 3C–E, H), compared to both protocol I-derived groups (Fig. 3A, B, G). In summary, our morphometric analysis revealed that the in vitro differentiation protocol influences the later appearance and size of the graft in vivo. Importantly, protocol I-derived cells (using FGF-2) did show tumorigenic properties, while protocol II-derived cells (using SB and DM for differentiation) did not show graft overgrowth (as determined by graft volume and cell density).

Decreased Cell Proliferation in Protocol II-Derived Cells 3 Weeks After Transplantation

Cell proliferation was analyzed using Ki-67 immunohistochemistry in the peripheral region of the graft on selected brain sections (see Materials and Methods) (Fig. 3I–S). As cells were too densely packed to be countable as individual cells, we measured the Ki-67⁺ area relative to the hNuc⁺ area. Protocol I-derived cells showed a high “proliferative area” of 45.9 ± 4.4% and 34.3 ± 4.2% in the 1- and 3-week iPSC1H groups, respectively (Fig. 3I–K, O, P). Protocol II-derived grafts, however, showed a highly significant reduction of the Ki-67⁺ area between 1 and 3 weeks (Fig. 3I, L–N, Q–S), from 44.5 ± 1.5% to 3.7 ± 0.8%, respectively (one-way ANOVA comparing 1-, 2-, and 3-week iPSC2 groups). The large standard deviation seen in the intermitted 2-week point of time (Fig. 3I) indicates that two out of four animals displayed a diminished proliferation rate similar to that seen in the 3-week iPSC2L group, while the remaining two animals of the 2-week iPSC2H group still showed a high proliferation level similarly to the 1-week iPSC2L group (Fig. 3I). The reduced proliferative area of the 3-week protocol II-derived iPSC2L group was also significantly lower compared to both 1- and 3-week protocol I-derived iPSC1H groups (Fig. 3I, two-way ANOVA).

Analysis of Transplanted DA Cells

The in vitro analyses of the differentiated iPSCs revealed that protocol II yielded twofold more TH⁺ dopaminergic neurons than protocol I at the end of the 21-day maturation period in vitro (corresponding to the day of transplantation). Therefore, we expected that already 1 week after transplantation, DA neurons should be present in the grafts, especially in the protocol II-derived ones. However, TH immunohistochemistry (dark staining in Fig. 4A–F) revealed only a few DA neurons in 1-week-old grafts derived from both in vitro protocols (iPSC1H, iPSC2L) (Fig. 4).

Figure 4.

TH immunohistochemistry of grafts. Dopaminergic TH⁺ cells and fibers were identified by TH immunohistochemistry using DAB (dark staining, A–F) or green fluorescence dye (G–J). High magnification regions are indicated by a square. Differentiation with protocol I (iPSC1H) revealed TH⁺ cells with barely detectable fiber outgrowth (arrowheads in A, B). After 3 weeks in vivo, two out of four grafts of iPSC1H grew distinctly in their size and compressed the surrounding host tissue (B, arrow indicates compacted striosomes/patches and matrix of the nonlesioned TH⁺ host striatum). Cells that were differentiated with protocol II showed a few TH⁺ cells with faint fiber outgrowth after 2 weeks in healthy rats (iPSC2H, C, arrowhead points to TH⁺ fiber). Transplantation of protocol II-derived cells into lesioned rats yielded almost no TH⁺ cells with fiber outgrowth after 1 week (D, iPSC2L 1 week, *graft). In two animals of the 3-week iPSC2L group, the grafts showed only few TH⁺ cells and fibers (E, arrowhead points to TH⁺ fiber, *graft), while the other two grafts showed a TH⁺ halo (arrow in F, arrowhead points to TH⁺ cell). Human origin of the TH⁺ cells was demonstrated by double-labeled fluorescence stainings for TH (green signal, G–J), which overlapped with human cell marker hNuc (red signal, arrowheads, H–J) in both 3-week iPSC1H and iPSC2L groups, respectively. The high density of TH⁺ cells shown in (H) was only seen in both tumorigenic 3-week iPSC1H grafts, but not in the two smaller grafts, which contained lower TH⁺ cell numbers similar to that seen in the 3-week iPSC2L group (I). In the fluorescent TH staining, TH⁺ cells and fibers were clearly visible (H, J arrowheads). The 3-week iPSC2L group showed some hNuc⁺/TH⁺ cells in the periphery of the graft with axons reaching into the host striatum (J, arrowhead on the right side); however, the TH-fluorescent staining did not confirm a substantial TH⁺ fiber outgrowth, suggested by the TH⁺ halo in the TH-DAB staining of two iPSC2L 3-week animals (F, arrow).

Protocol I-derived 1-week grafts contained low numbers of TH⁺ cells (243 ± 7) that were mostly located in the periphery of the graft (arrowhead in Fig. 4A), which was similarly seen in both smaller grafts of the 3-week iPSC1H group (246 ± 36). The two larger (tumorigenic) grafts of the 3-week iPSC1H group contained 80-fold more TH⁺ cells (19,821 ± 416). The average of all 3-week iPSC1H animals was 10,034 ± 5,654 TH⁺ cells per graft. Those cells showed almost no fiber outgrowth in the TH-DAB staining (arrowhead in Fig. 4B), while fiber outgrowth of hNuc⁺/TH⁺ cells was clearly visible in the TH-fluorescence staining (Fig. 4H, arrowheads). Moreover, the compact cell mass of both huge 3-week iPSC1H grafts compressed the surrounding host tissue, illustrated by an oval instead of a round shape of striosomes/patches and a denser striatal matrix close to the graft–host border (arrow in Fig. 4B).

Cells derived from protocol II showed variable TH⁺ cell numbers (1,800 ± 1,506) and displayed a faint TH⁺ fiber outgrowth inside the grafts after 2 weeks in the TH-DAB staining (arrowhead, Fig. 4C). To allow a better discrimination between host- and graft-derived TH⁺ fibers, we used for subsequent protocol II-derived transplantations the 6-OHDA-lesioned PD rat model, which is characterized by a complete degeneration of host-derived ipsilateral TH⁺ fibers. Note the absent staining throughout the ipsilateral striatum compared to the nonlesioned contralateral striatum (Fig. 4D). Protocol II-derived cells grafted into 6-OHDA-lesioned animals showed few TH⁺ cells (1-week group 70 ± 4, 3-week group 107 ± 21). Comparison of graft type (protocol I or II derived) and time after grafting (1 or 3 weeks) by two-way ANOVA revealed no significant differences in the TH⁺ cell numbers between groups; likewise one-way ANOVA between all three protocol II-derived grafts or between 2-week iPSC2H and 1- and 3-week iPSC1H revealed no differences. In the TH-DAB staining no graft-derived TH⁺ fibers were detected in the 1-week iPSC2L group (Fig. 4D, * indicates the graft). After 3 weeks, iPSC2L showed in two animals a weak TH⁺ staining (arrowhead in Fig. 4E, * indicates the graft) resembling the faint staining also seen in the 2-week nonlesioned iPSC2H group (Fig. 4C), while the other two 3-week iPSC2L grafts showed a distinctly stained TH⁺ region surrounding the grafts (arrows and arrowhead in Fig. 4F). In the TH fluorescence staining, few hNuc⁺/TH⁺ cells and fibers could be identified in the 3-week iPSC2L grafts (Fig. 4J, arrowhead on the right); however, no broad fiber outgrowth was detectable as suggested by the TH-DAB staining (Fig. 4F). The identity of the TH⁺ cells as graft-derived cells was confirmed by double label immunohistochemistry using TH and human-specific hNuc antibodies for both protocol I- (iPSC1H, Fig. 4G, H, arrowhead) and protocol II (iPSC2L) (Fig. 4I, J, arrowhead)-derived 3-week grafts.

Discussion

In this study, we adopted two iPSC neural differentiation protocols (9,55,60) for hCBiPSCs to investigate the impact on striatal cell transplantations.

In Vitro Pretreatment Influences Differentiation of DA Neurons

Both in vitro protocols, I (FGF-2 signaling) and II (BMP/TGF-β inhibition) (Fig. 1), showed similar neural conversion efficiencies seen by the number of generated Tuj1⁺ cells. Endogenous BMP/TGF-β inhibition and FGF signaling are both reported to be essential for neural induction (56), whereas exogenous administration of BMP/TGF-β antagonists or FGF-2 during differentiation seemed to be dispensable, due to intrinsic signaling pathways that account for efficient neural conversion (11,30). Nevertheless, many laboratories including ours observed improved neural differentiation by application of BMP/ TGF-β antagonists in vitro (9,34,36,49). This could be attributed to elevated levels of neural differentiation inhibiting molecules including BMPs due to high-density cultures or coculturing on stromal cells, which can only be sufficiently antagonized by exogenous BMP inhibition (30). Similarly, exogenous administration of FGF-2 augments the derivation of neural precursors from hESCs and hiPSCs (11,30,59). The neuralizing effect of both signaling cascades has been suggested to converge on SMAD1. While BMP-phosphorylated SMAD1 accumulates in the nucleus and inhibits neural induction, ERK1/2-mediated SMAD1 phosphorylation (induced for example by FGF-2 signaling) is proposed to suppress nuclear translocation, thereby promoting neuralization (27,46,51).

Although the neural conversion efficiency (see above) and the 21-day final maturation period were identical for both protocols, a twofold amount of TH⁺ dopaminergic neurons was obtained with protocol II in vitro. This might be attributed to the persistent influence of FGF-2 signaling or BMP/TGF-β inhibition on subsequent DA differentiation or to different biological activities of midbrain inducers (SHH, PMA) used in both protocols. However, comparative studies report similar efficiencies for differentiation of TH⁺ neurons by SHH or PMA (14,34). Another possibility influencing the percentage of TH⁺ cell numbers in vitro might be the cell density during midbrain patterning and final maturation steps. While in protocol I cells were plated as single cells prior to midbrain induction and final maturation, cells were cultured and plated as highly dense EBs in protocol II. It has been reported previously that increased seeding densities promote neural differentiation (14), and especially dopaminergic neurons are often found in dense cell aggregates during differentiation (4). Further studies need to be done to optimize the dopamine neuron differentiation efficiency of iPSCs.

In Vitro Pretreatment Influences Graft Characteristics

One week after intrastriatal transplantation into immunosuppressed rats, grafts derived from both protocols contained low to absent numbers of TH⁺ cells, which is in line with other studies showing that DA neurons are very vulnerable during the transplantation procedure with an estimated survival rate of 3–20% of the original cell number (7). Three weeks after transplantation, however, TH⁺ cells could be detected in both graft types, indicating that DA differentiation of the transplanted cells continued in the host brain. Moreover, proliferating cells were observed in both protocols after 21 days of final in vitro maturation (52% and 41% in protocols I and II, respectively) and 1 week after intrastriatal transplantation. More pronounced 3 weeks after grafting, the type of in vitro pretreatment had long-lasting influence on the future properties of the graft, producing either continuously growing tumorigenic protocol I-derived grafts (in two out of four animals) or in case of protocol II yielding average-sized grafts characterized by a profound depletion of proliferating cells after 3 weeks. These differences might be attributed to a few undifferentiated OCT4⁺ iPSCs still present at the end of the in vitro differentiation period in protocol I, but not protocol II. Additionally, it is known that the genomic integration site may lead to genetic or epigenetic modifications, which can cause tumor formation (16). Therefore, alternative approaches have been investigated in order to generate iPSCs without retroviral vectors, such as reprogramming proteins, nonviral expression plasmids or nonintegrating adenoviruses (25,44,54,66).

Other Parameters Influencing Graft Morphology

As the striatal DA content and striatal trophic activity decreases in older rats, the age of the host has been discussed for transplantation studies (12,32). Transplantation of dissociated embryonic nigral tissue into aged (21–23 months) nonlesioned rats improved motor coordination (15). In a comparative study, however, transplantation into old (24–26 months) lesioned rats resulted in the poorest outcome with respect to TH⁺ cell numbers, fiber density, and rotational behavior compared to middle-aged (17 months) or young (4 months) lesioned rats (12). We used for transplantation 3- (protocol I) and 11- to 14-month (protocol II) old rats, which were even younger than the well-performing middle-aged group of the Collier et al. (12) study. In a sophisticated study (13) using two sequential 6-OHDA lesions pre- and posttransplantation, the influence of a lesioned and nonlesioned environment on the DA graft was analyzed. Importantly, overall graft size and number of surviving DA neurons was not influenced, whereas TH⁺ fiber outgrowth was increased in the pretransplantation lesioned hemisphere.

Comparison of Both Differentiation Protocols

Protocol I has been previously used to generate 30% DA neurons from iPSCs obtained from human fetal fibroblasts (60), while this protocol generated only 3% DA neurons from our hCB-derived iPSCs. Variable differentiation propensities between different iPSC lines have been frequently reported (21) and might be attributed to the cell types of origin, reprogramming factors, and factor delivery methods used (35). In particular, iPSCs may retain the epigenetic memory of donor cells, which favors their differentiation toward donor-like cell types (26). Line-specific genetic and epigenetic aberrations as well as the different culture conditions may also influence differentiation potential (20,22,53). Transplantation of protocol I-differentiated cells into 6-OHDA-lesioned rats has been shown to improve amphetamine-induced rotation 12 weeks after transplantation (60), but no data were reported on graft size after this duration. Our comparative analysis of both protocols, however, favored, at least for hCB-derived iPSCs, protocol II over protocol I. Not only was the number of TH⁺ neurons twofold increased after in vitro differentiation with protocol II, in addition, it is more convenient, less time-consuming, and therefore more cost-effective than protocol I. Furthermore, protocol II-derived grafts displayed no tumor formation (at least after 3 weeks) and reduced cell proliferation. Given that 3.5% or 7.4% of the cells were TH⁺ in vitro, and in most cases only 100–300 TH⁺ cells survived the transplantation of 200,000 total cells, further approaches are needed to optimize the survival rate of the vulnerable TH⁺ cells. On the basis of the present data, further studies will be performed applying protocol II in order to reduce the in vitro and in vivo proliferation potential and to optimize the TH⁺ cell survival in the grafts. These upcoming long-term studies will include behavioral and electrophysiological analyses of the graft function.

Footnotes

Acknowledgments

The authors wish to thank Silke Fischer and Natascha Heidrich for excellent technical assistance (Institute of Neuroanatomy, MHH). This work was supported by a Ph.D. scholarship from Cusanuswerk to Anna Effenberg. The authors declare no conflicts of interest.

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