Functional differentiation of midbrain neurons from
human cord blood-derived induced pluripotent stem cells for transplantation in a rat model of Parkinson’s disease
Nancy Stanslowsky
Reichenbach, Germany
University of Veterinary Medicine Hannover Centre for Systems Neuroscience
Department of Neurology, Hannover Medical School
Department of Neurology, Hannover Medical School
Functional differentiation of midbrain neurons from human cord blood-derived induced pluripotent stem cells for
transplantation in a rat model of Parkinson’s disease
THESIS
submitted in partial fulfilment of the requirements for the degree
Doctor rerum naturalium (Dr. rer. nat.)
awarded by the University of Veterinary Medicine Hannover
by
Nancy Stanslowsky
Reichenbach, Germany
Hannover 2014
Supervisor: PD Dr. Florian Wegner
Supervision Group: PD Dr. Florian Wegner Prof. Dr. Claudia Grothe PD Dr. Jörg Ahrens
1stEvaluation: PD Dr. Florian Wegner Department of Neurology,
Hannover Medical School, Germany
Prof. Dr. Claudia Grothe Department of Neuroanatomy, Hannover Medical School, Germany
PD Dr. Jörg Ahrens
Department of Anaesthesiology Hannover Medical School, Germany
2ndEvaluation: Prof. Dr. med. Beate Winner IZKF Junior Research Group and BMBF Research Group Neuroscience,
Friedrich-Alexander University Erlangen-Nuernberg, Germany
Date of final exam: 10th October 2014
Cover:
The cover shows a picture of human cord blood endothelial cell-derived induced pluripotent stem cells (hCBiPSCs) differentiating towards dopaminergic neurons in vitro. Dopaminergic neurons express the neuronal protein βIII tubulin (green) and the dopamine synthesising enzyme tyrosine hydroxylase (red). In one part of this thesis, the dopaminergic differentiation potential of hCBiPSCs was investigated.
Parts of the thesis have been published previously in:
Effenberg A*, Stanslowsky N*, Klein A, Haase A, Martin U, Dengler R, Grothe C, Ratzka A*, Wegner F*. Striatal transplantation of human dopaminergic neurons differentiated from induced pluripotent stem cells derived from umbilical cord blood using lentiviral reprogramming. Cell Transplant. 2014 Nov 07 (epub ahead of print).
* denotes equal contribution
Stanslowsky N, Haase A, Martin U, Naujock M, Leffler A, Dengler R, Wegner F. Functional differentiation of midbrain neurons from human cord blood-derived induced pluripotent stem cells. Stem Cell Res Ther. 2014 Mar 17;5(2):35.
Publications not included in the thesis:
Naujock M, Stanslowsky N, Reinhardt P, Haase A, MartinU, Kim KS, Dengler R, Wegner F*, Petri S*. Molecular and functional analyses of motor neurons generated from human cord blood derived induced pluripotent stem cells. Stem Cells Dev. 2014 Sep 11; In press
* denotes equal contribution
Sun H, Bénardais K, Stanslowsky N, Thau-Habermann N, Hensel N, Huang D, Claus P, Dengler R, Stangel M, Petri S. Therapeutic potential of mesenchymal stromal cells and MSC conditioned medium in Amyotrophic Lateral Sclerosis (ALS) – in vitro evidence from primary motor neuron cultures, NSC-34 cells, astrocytes and microglia. PLoS One. 2013 Sep 12;8(9):e72926.
de la Roche J, Eberhardt MJ, Klinger AB, Stanslowsky N, Wegner F, Koppert W, Reeh PW, Lampert A, Fischer MJ, Leffler A. The molecular basis for species-specific activation of human TRPA1 protein by protons involves poorly conserved residues within transmembrane domains 5 and 6. J Biol Chem. 2013 Jul 12;288(28):20280-92.
Results of the thesis have been presented in form of posters at following congresses:
85th Kongress der Deutschen Gesellschaft für Neurologie – DGN (Hamburg, 2012):
Dopaminergic differentiation of human cord blood-derived induced pluripotent stem cells.
53rd American Society for Cell Biology Annual Meeting (New Orleans, 2013):
Neurons from human cord blood-derived induced pluripotent stem cells exhibit functional properties during maturation in vitro.
12th International Society for Stem Cell Research Annual Meeting (Vancouver, 2014):
Transplantation of human dopaminergic neurons differentiated from umbilical cord blood- derived induced pluripotent stem cells into the rat striatum.
86th Kongress der Deutschen Gesellschaft für Neurologie - DGN (München, 2014):
Transplantation of human dopaminergic neurons differentiated from umbilical cord blood- derived induced pluripotent stem cells into the rat striatum.
Table of contents
1. Introduction ... 1
1.1. Parkinson’s disease (PD) – pathology, symptoms and treatment ... 1
1.2. Induced pluripotent stem cells (iPSCs) ... 3
1.3. Dopaminergic neuron development during embryogenesis... 4
1.4. Functional maturation and survival of dopaminergic neurons... 7
1.5. Human iPSC-derived dopaminergic neurons for medical applications ... 8
2. Aims of the study ... 9
3. Manuscript I ... 11
4. Manuscript II ... 13
5. Discussion ... 15
6. Summary ... 21
7. Zusammenfassung... 23
8. References ... 25
Acknowledgements ... 33
Declaration ... 34
List of abbreviations
6-OHDA 6-hydroxydopamine
A-P anterior-posterior
BDNF brain-derived neurotrophic factor BMP bone morphogenetic protein cAMP cyclic adenosine monophosphate
D-V dorsal-ventral
DA dopamine
DAPI 4’,6-diamidino-2-phenylindole
EBs embryonic bodies
En1/2 engrailed 1 and 2 ESCs embryonic stem cells FGF fibroblast growth factor
FoxA2 forkhead/winged box protein A2 GABA gamma-aminobutyric acid
GDNF glial cell-derived neurotrophic factor
hCBiPSCs human cord blood-derived induced pluripotent stem cells hESCs human embryonic stem cells
iPSCs induced pluripotent stem cells Klf4 Krueppel-like factor 4
PD Parkinson’s disease
Oct4 octamer 4
SHH sonic hedgehog
TGF-β transforming growth factor-β
TH tyrosin hydroxylase
1 1. Introduction
1.1. Parkinson’s disease (PD) – pathology, symptoms and treatment
PD was first described in 1817 by the British physician James Parkinson and is the second most common neurodegenerative disorder after Alzheimer’s disease with a prevalence of about 0.2 % and a mean age of onset between 50-60 years (DE RIJK et al. 1997). PD is associated with risk factors including heredity, gender, toxin exposure and aging and since it is an age-related disorder, incidences are expected to increase due to the progressive aging of the population. The pathophysiological degeneration of midbrain dopaminergic (DA) neurons in the substantia nigra pars compacta and the formation of abnormal protein aggregates inside the neuronal cells termed Lewy bodies (SPILLANTINI et al. 1997) are characteristic for PD, resulting in striatal dopamine deficiency and in a subsequent alteration of basal ganglia physiology. Although the ultimate causes for the neurodegeneration remain elusive, factors like mitochondrial dysfunction, oxidative stress, glutamate excitotoxicity or inflammation are proposed to be involved (BLANDINI 2013). Characterized by both, motor and non-motor symptoms, PD patients classically display rest tremor, rigidity and bradykinesia (BLANDINI et al. 2000; PARKINSON 2002). The various non-motor symptoms often encompass psychiatric symptoms (depression, anxiety), cognitive impairment (dementia), and autonomic dysfunction (orthostasis and hyperhidrosis) (GALLAGHER et al. 2010).
Even five decades after its introduction into clinics, L-3,4-dihydroxyphenylalanine (L-DOPA) is still the most effective therapy for the treatment of motor symptoms in PD. L-DOPA represents the precursor of the deficient neurotransmitter dopamine and after it had crossed the blood-brain barrier it is taken up by surviving nigrostriatal neurons, converted into dopamine and released into the synaptic cleft. Long-term treatment with L-DOPA, however, will provoke complications in most of the patients. The consistency of response declines over
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time and there is a high risk for L-DOPA-induced dyskinesias (POEWE et al. 2010).
Especially for younger patients or patients with milder symptoms alternative therapies with dopamine agonists that directly activate dopamine receptors or monoamine oxidase type B (MAO-B) inhibitors that reduce the oxidative degradation of dopamine, may be used in the first place. For patients whose symptoms cannot be adequately controlled with medications surgical interventions like deep brain stimulation are used to attenuate motor symptoms.
Thereby an impulse generator, which delivers electrical stimuli to brain areas that control movements is implanted, blocking the abnormal nerve signals that cause PD symptoms (BENABID et al. 2006; PERLMUTTER u. MINK 2006). Symptomatic treatments are usually not able to slow down the neurodegeneration that underlies PD progression. To enhance the survival of DA neurons neuroprotective strategies are under investigation. MAO-B inhibitors possibly slow the rate of PD progression due to their antioxidative effects (OAKES 1993;
PARKINSON STUDY GROUP 2002; BOLL et al. 2011). Neurotrophic factors like brain- derived neurotrophic factor (BDNF), glial cell-derived neurotrophic factor (GDNF), fibroblast growth factor-2 (FGF-2) or transforming growth factor-β (TGF-β) have proven to possess antiapoptotic and neuroprotective effects in animal models of PD (WANG et al. 2002;
FARKAS et al. 2003; GROTHE u. TIMMER 2007). In first clinical trials, GDNF showed promising benefits regarding PD progression (GILL et al. 2003; SLEVIN et al. 2005), which unfortunately could not be verified in a subsequent randomised control trial (LANG et al.
2006).
Despite these concerted efforts, current therapies have been unable to restore function to PD patients. Cell replacement strategies focus on the retrieval of movement control through transplantation of dopamine-producing neurons or their progenitors to substitute for the lost midbrain DA neurons. In the first clinical trials more than 20 years ago human fetal mesencephalic tissue was used for transplantation in PD patients. Initially, the first results
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seemed to be quite promising, showing robust graft survival, striatal reinnervation, dopamine release and functional integration. However, functional outcomes were highly variable. While in several patients functional improvements have been reported, others showed no or only mild benefits or even a worsening of symptoms, the onset of dyskinesias or development of Lewy bodies in the grafted tissue (LI et al. 2008). Retrospectively several factors including tissue quality, implantation technique, patients age, stage of disease progression and immunosuppressive treatment turned out to strongly influence study outcomes (FREED et al.
2001; OLANOW et al. 2003; BJORKLUND u. KORDOWER 2013). The issue of limited availability and variable quality of fetal tissue for regenerative therapies may be overcome by the utilization of pluripotent stem cells, like embryonic or induced pluripotent stem cells. In theory, those cells provide an unlimited source to generate a large amount of the appropriate cell types to gain maximal functional recovery.
1.2. Induced pluripotent stem cells (iPSCs)
Pluripotent stem cells are cells with the innate capacity to self-renew and differentiate in all three germ layers, therefore, producing all cells of an adult organism. Different from other pluripotent cell types, induced pluripotent stem cells (iPSCs) originate from mature somatic cells through molecular reprogramming by overexpression of certain genes or exposure to specific proteins. In the first study reporting about iPSCs from fibroblasts four factors (Oct4, Sox2, c-Myc and Klf4) were introduced by retroviral gene transfer (TAKAHASHI u.
YAMANAKA 2006). In the following years the reprogramming process was successfully applied also to cell types different from fibroblasts, with various gene combinations and delivery methods (OKITA et al. 2007; TAKAHASHI et al. 2007; STADTFELD et al. 2008;
FENG et al. 2009; GIORGETTI et al. 2010). Molecular reprogramming leads to epigenetic modification of the whole genome, resulting in cells that epigenetically resemble embryonic
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stem cells (ESCs) rather than the somatic cells from which they originate. Genomic and chromosomal abnormalities present in somatic cells are not rectified during reprogramming, though. This, on the one hand, predestines iPSCs generated from patients as disease models to investigate underlying pathomechanisms and develop methods for diagnosis and therapy (PARK et al. 2008). On the other hand, mutations that have been accumulated during the person’s lifetime are retained in the iPSCs and their derivatives and could potentially impair cellular function or promote tumor formation when it comes to replacement therapies. This issue may possibly be circumvented by iPSCs created from “young” cell sources such as umbilical cord blood (HAASE et al. 2009). iPSCs hold great potential for future clinical applications by providing an unlimited source of personalized cells for replacement therapies with a diminished risk of immunorejection and without the negative ethical connotation of ESCs (HENNINGSON et al. 2003; ARAKI et al. 2013). Although reprogramming has demonstrated a proof of principle, improvements regarding the efficiency of reprogramming methods and the safety of this cell source for clinical use will be required.
1.3. Dopaminergic neuron development during embryogenesis
Specification of distinct cell types from human iPSCs (hiPSC) is a key to the potential application in regenerative medicine. Neurogenesis is a highly regulated process during embryogenesis involving several signaling pathways and a series of rearrangements in cell shape, division, migration, cell death and contacts (EDLUND u. JESSELL 1999) and is generally recapitulated in in vitro differentiation protocols.
The nervous system develops from the embryonic ectoderm after gastrulation. Neural induction begins when ectodermal cells located along the dorsal midline of the embryo receive signals from the underlying mesendoderm, which later gives rise to the notochord and the prechordal plate, and begin to thicken to form the neural plate. Ectodermal cells acquire
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specific neural fates depending on their initial axial positions that define the exposure to inductive signals and through their gene expression profiles that allow them to respond to those signals, termed the cells competence. Molecular interactions involved in the process of neural induction are the inhibition of bone morphogenetic protein (BMP) signaling by endogenous proteins like follistatin, noggin, chordin, or cerberus secreted by an organizer region, which was first discovered in amphibian embryos by Spemann and Mangold (SPEMANN u. MANGOLD 2001) and was named “Hensen’s node” in mammals. The observation that BMP inhibition directs ectodermal cells to adopt a neural fate was leading to the default model of neural induction, which has been modified and extended and may not apply in all species. Recent studies suggest that neural induction begins prior to the formation of this organizer area, indicating the presence of inductive signals from other anatomical regions. Members of the FGF family produced by the primary endoderm are proposed to act as those early neural inducing factors and suppression of BMP signaling is thought to maintain rather than initiate neural differentiation (STERN 2005; LINKER et al. 2009).
Additionally, the inhibition of Wnt signaling has been implicated in the early specification of neural fate (WILSON et al. 2001). Most strategies reaching for neural induction in ESCs or iPSCs in vitro are based either on BMP inhibition or FGF-2 signaling (CHAMBERS et al.
2009; SWISTOWSKI et al. 2010; MAK et al. 2012; REINHARDT et al. 2013).
Early Patterning of the developing embryo along an anterior-posterior (A-P) and dorsal- ventral (D-V) axis is triggered by signals from the mesendoderm and is closely connected to the neural induction. Initially, the early neural tissue appears to acquire anterior (forebrain) characteristics by default through the inhibition of Wnt signaling (WATANABE et al. 2005).
One inhibitory signal with a strong forebrain inducing activity is thought to be the multifunctional inhibitor cerberus. For the generation of more posterior tissue (midbrain, hindbrain and spinal cord) additional signaling is required. Signaling molecules involve
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factors including WNTs, FGFs and retinoic acid secreted by the mesoderm (MUGURUMA u.
SASAI 2012).
In the following process of neurulation, the neural plate rolls up towards the dorsal midline of the embryo to form the neural tube. The anterior neural tube further compartmentalizes into several vesicles representing the future prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain) while the posterior region forms the spinal cord (VIEIRA et al. 2010). Cell differentiation in both the dorsal and ventral halves of the neural tube is controlled by inductive signals. Ventral patterning is regulated by a single protein, sonic hedgehog (SHH), which is secreted by the Hensen’s node and later also by its derivatives, the prechordal plate and the notochord. SHH generates distinct neuronal cell types in a concentration dependent manner. The concentration gradient, controlled by the rate of diffusion from the organizer region and its derivatives, is leading to the formation of forkhead/winged helix transcriptin factor 2A (FoxA2) positive floor plate cells in the overlying neural plate. Later the floor plate cells contribute to the gradient maintenance (ROELINK et al. 1995). Dorsal patterning involves BMP signaling which originates from epidermal ectoderm flanking the neural plate and after neural tube closure from the roof plate and the adjacent dorsal neural tube (KUDOH et al. 2004). Contrasting the inhibition of neurulation by BMPs during earlier developmental stages, at later stages of differentiation BMPs are responsible for generating neural crest cells, giving rise to mesodermal and peripheral nervous system derivatives. Along the A-P axis the neural tube further subdivides into smaller domains by the establishment of secondary organizer areas and partitioning into small modules or segments. At the midbrain-hindbrain boundary, the isthmus organizer secretes FGF8 which is involved in maintaining the expression of characteristic midbrain transcription factors like engrailed 1 and 2 (En1/2) (JOYNER et al. 2000). Dopaminergic neurons develop at the intersection of SHH signals from the floorplate and FGF8 signals from
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the isthmus organizer, indicating that the combined action of both factors is essential for their formation (YE et al. 1998). Both factors are routinely used for the induction of midbrain identity in in vitro differentiation protocols of ESCs and iPSCs.
1.4. Functional maturation and survival of dopaminergic neurons
Following differentiation into DA neurons and establishment of connections with the target area, the striatum, the nigrostriatal pathway is thought to be further refined during naturally occurring cell death (BURKE 2003). During this period pro-survival molecules secreted by the striatum might actively control survival of neurons in the substantia nigra. These neurotrophic factors include BDNF, GDNF and TGF-β3 and have been shown to control multiple neuronal processes ranging from cell proliferation, differentiation to axon guidance, dentritic growth and regulation of synaptic plasticity and function (KRIEGLSTEIN 2004;
SMIDT u. BURBACH 2007). BDNF promotes survival of cultured midbrain DA neurons and stimulates their activity (HYMAN et al. 1991; KNUSEL et al. 1991). It also has been shown to exert neuroprotective effects on midbrain DA neurons in animal models of PD (HYMAN et al. 1991; LEVIVIER et al. 1995). GDNF was initially identified as a potent survival factor for cultured embryonic DA neurons (AIRAKSINEN u. SAARMA 2002). In animal models of PD, GDNF has shown to increase the survival of midbrain DA neurons, prevent cell death and induce axonal sprouting (SAUER et al. 1995; ROSENBLAD et al. 1999). The protective role of TGF-β has been described extensively (FARKAS et al. 2003) and recent in vivo experiments have underlined the significance of TGF-β signaling for DA neuron survival (ROUSSA et al. 2006). Several of these neurotrophic factors as well as substances like cyclic adenosine monophosphate (cAMP) or the antioxidant ascorbic acid (BRANTON et al. 1998;
SUTACHAN et al. 2012) are routinely used in cell culture to protect and improve the survival of DA neurons.
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1.5. Human iPSC-derived dopaminergic neurons for medical applications
Human iPSCs have the potential to become a versatile research and clinical tool to model and study the mechanisms of diseases, perform drug screenings and toxicity testing, and deliver cells for replacement therapies. IPSCs from patients with a variety of disorders including amyotrophic lateral sclerosis, Alzheimer’s disease, Huntington’s disease and also PD have already been generated, showing distinct disease-related phenotypes that could be corrected by drug treatment or gene delivery methods (SEIBLER et al. 2011; AN et al. 2012; EGAWA et al. 2012; ISRAEL et al. 2012). The reprogramming technology offers the potential of creating patient-specific cells for putative applications in regenerative medicine. IPSC-derived cells could serve as the basis for autologous cell transplantation. Because the donor cells originate from the patient, immune rejection of the differentiated derivatives would be minimized. As a result, the need for immunosuppression would be lessened and may even be completely eliminated. However, to generate personalized iPSCs for routine clinical applications the reprogramming methods are currently too inefficient. A major concern regarding the usage of hiPSC-derived cells is the risk of tumor formation. Because life expectancy is virtually normal in PD patients, even a minor risk of tumor growth associated with stem cell therapy is unacceptable in this disorder (BJORKLUND u. KORDOWER 2013).
Although first promising studies are reporting transplantable amounts of hiPSC-derived DA neurons which could treat animal models of PD, large variations in respect to yield, survival and functional recovery have been observed (CAI et al. 2010; HARGUS et al. 2010;
SWISTOWSKI et al. 2010; RHEE et al. 2011). To move hiPSCs closer to clinical application, in vitro differentiation protocols have to be optimized and standardized and in vivo long-term stability and long-lasting functional recovery need to be addressed.
9 2. Aims of the study
IPSCs are capable of unlimited proliferation and can generate each cell type of the human body. Therefore, iPSCs represent a promising source for potential cell replacement therapies.
During reprogramming genetic mutations that accumulate in somatic cells over lifetime and contribute to aging and cancer formation are not rectified and may influence the safety and functionality of iPSCs in respect to clinical applications. This thesis examines human cord blood-derived iPSCs as a juvenile cell source for DA neuron differentiation in vitro and investigates their functionality as well as in vivo survival and integration after transplantation in an animal model of PD.
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11 3. Manuscript I
Published in Stem Cell Research & Therapy, 2014 Mar 17, Volume 5(2): 35.
Functional differentiation of midbrain neurons from human cord blood-derived induced pluripotent stem cells
Nancy Stanslowsky1,2,*, Alexandra Haase3,4, Ulrich Martin3,4, Maximilian Naujock1,2, Andreas Leffler5, Reinhard Dengler1,2 and Florian Wegner1,2
1 Department of Neurology, Hannover Medical School, Hannover, Germany
2 Center for Systems Neuroscience, Hannover, Germany
3 Department of Cardiac, Thoracic, Transplantation, and Vascular Surgery, Leibnitz Research Laboratories for Biotechnology and Artificial Organs (LEBAO), Hannover Medical School, Hannover, Germany
4 REBIRTH-Cluster of Excellence, Hannover, Germany
5 Department of Anaesthesia and Critical Care Medicine, Hannover Medical School, Hannover, Germany
* Corresponding author
Preface- about this manuscript
Human induced pluripotent stem cells (hiPSCs) are promising sources for regenerative therapies like the replacement of dopaminergic neurons in Parkinson`s disease (PD). In the present study human cord blood-derived iPSCs (hCBiPSCs) served as a juvenescent cell source for the differentiation into dopaminergic neurons utilizing inhibition of transforming growth factor-β and bone morphogenetic protein signaling by the small molecules dorsomorphin and SB 431542 for neural induction. Differentiated hCBiPSCs expressed neuronal and dopaminergic markers as seen by immunocytochemistry and quantitative real time PCR. Voltage- and ligand-gated ion currents, action potential firing and synaptic activity as characteristics for essential functional maturation were demonstrated by patch-clamp recordings and calcium imaging experiments.
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13 4. Manuscript II
Accepted for publication in Cell Transplantation, 2014 Nov 07. DOI:
10.3727/096368914X685591 (epub ahead of print).
Striatal transplantation of human dopaminergic neurons differentiated from induced pluripotent stem cells derived from umbilical cord blood using lentiviral
reprogramming
Anna Effenberg a,1, Nancy Stanslowsky b,c,1, Alexander Klein a, Alexandra Haase d,e, Ulrich Martin d,e, Reinhard Dengler b,c, Claudia Grothe a,c, Andreas Ratzka a,2,*, Florian Wegner b,c,2
a Institute of Neuroanatomy, Hannover Medical School, Hannover, Germany
b Department of Neurology, Hannover Medical School, Hannover, Germany
c Center for Systems Neuroscience (ZSN), Hannover Medical School, Hannover, Germany
d Department of Cardiac, Thoracic, Transplantation, and Vascular Surgery, Leibnitz Research Laboratories for Biotechnology and Artificial Organs (LEBAO), Hannover Medical School, Hannover, Germany
e REBIRTH-Cluster of Excellence, Hannover, Germany
1 Both authors contributed equally to this work.
2 Both senior authors contributed equally to this work.
* Corresponding author
Preface- about this manuscript
Human cord blood derived induced pluripotent stem cells (hCBiPSCs) were differentiated into dopaminergic neurons utilizing two different in vitro protocols for neural induction:
protocol I by fibroblast growth factor (FGF-2) signaling and protocol II by bone morphogenetic protein (BMP)/transforming growth factor (TGFβ) inhibition. After maturation in vitro increased numbers of tyrosine hydroxylase (TH) positive neurons were present in protocol II- compared to protocol I-derived cells. A reduced number of proliferating cells was observed after transplantation in hemiparkinsonian rats with protocol II, whereas proliferation still occurred in protocol I derived grafts, resulting in tumor-like growth in some cases. In conclusion, BMP/TGFβ inhibition was more effective than FGF-2 signaling with regard to dopaminergic induction of hCBiPSCs in vitro and prevented graft overgrowth in vivo.
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15 5. Discussion
To date, several studies proved DA differentiation capability and functionality of iPSCs that have been generated from aged human cells of various origins (YU et al. 2007; LOWRY et al.
2008; LOH et al. 2009; LIU et al. 2010). We investigated hiPSCs generated from cord blood endothelial cells as potential cell source for DA neurons because they are expected to have advantages over adult fibroblast-derived iPSCs in respect to genetic alterations that tend to accumulate over an organisms lifetime. Such mutations are not rectified during reprogramming and may contribute to aging and cancer formation, influence the quality of iPSCs and cellular function. Moreover, cord blood is a cell source that can easily be collected without surgical interventions and is readily available due to cord blood banks what predestines hCBiPSCs for clinical use.
In the first study of this PhD project we determined whether hCBiPSCs hold the potential to generate functional dopaminergic neurons. Therefore, hCBiPSCs were differentiated using BMP/TGF-β inhibition, a procedure shown to induce dopaminergic cell fate in cultures from dermal fibroblast-derived iPSCs (CHAMBERS et al. 2009; MORIZANE et al. 2011; MAK et al. 2012). Neural induction by BMP/TGF-β inhibition produced nearly the double amount of dopaminergic neurons in differentiated hCBiPSC cultures to what was reported from fibroblast-derived iPSCs (MAK et al. 2012). In general, the differentiation capacity strongly varies between different iPSC lines. This diversity may be attributed to the cell types of origin, reprogramming factors, and factor delivery methods used (MIURA et al. 2009). In particular, iPSCs may retain the epigenetic memory of donor cells which favours their differentiation toward donor-like cell types (KIM et al. 2010). The genetic background of the donor cell, as well as the different culture conditions may also influence differentiation potential (HIROSE et al. 2012; KAJIWARA et al. 2012).
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We focused more than any previous study on the functional characterization of differentiated hCBiPSCs, since especially in respect to potential clinical applications but also for biological research those cell properties are essentially important. By whole-cell patch clamp recordings we showed that differentiated hCBiPSCs exhibited voltage-gated ion currents, action potentials and spontaneous synaptic activity. Additionally, calcium imaging experiments revealed the presence of functional ligand-gated ion currents. Compared to other studies on fibroblast-derived hiPSCs (ZENG et al. 2010) the amplitudes of voltage-gated currents in differentiated hCBiPSCs were smaller and also multiple action potential firing could not be demonstrated. Accordingly, the recorded resting membrane potentials were more positive than described for fully mature neurons. In calcium imaging measurements we observed an excitatory action of the inhibitory neurotransmitter GABA, which naturally is the case only in the prenatal state of development (GE et al. 2007). Taken together those results indicated a not yet fully mature neuronal phenotype, even though differentiated hCBiPSCs displayed all essential functional characteristics of neuronal cells.
For the assessment of in vivo survival, differentiation and striatal integration in the second part of the thesis, we transplanted the in vitro pre-differentiated hCBiPSCs in healthy rats and in the 6-OHDA rat model of PD. We decided to graft pre-differentiated hCBiPSC-derived cells that had undergone 21 days of terminal maturation in vitro, because shorter differentiation periods resulted in extensive, tumor-like graft growth and with longer pre- differentiation nearly no cells survived after transplantation (data not shown). The same was observed in a comparative study on human ESCs (BREDERLAU et al. 2006), where the total number of cells present in the grafts declined with differentiation time in vitro, presumably because mature neurons survive transplantation poorer than immature neurons. Moreover, Brederlau et al. showed that the majority of rats transplanted with cells differentiated for shorter periods (16 days) developed tumors, whereas none of the rats transplanted with cells
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differentiated for longer periods (23 days) showed teratoma formation (BREDERLAU et al.
2006). This suggests that in vitro differentiation reduces the proliferative capability and pluripotency of the cells and, therefore, the risk of tumor formation.
In addition to BMP/TGF-β inhibition (referred to as protocol II) we tested another protocol for transplantation studies, based on FGF-2 signaling (termed protocol I). Protocol I was developed by Swistowski et al. (2010) and yielded about 30% TH+ cells in their fibroblast- and mesenchymal stem cell-derived hiPSC cultures, which is so far the highest percentage reported by in vitro differentiation of hiPSCs. The in vitro comparison of both protocols in differentiated hCBiPSCs, however, revealed the twofold amount of TH+ cells in protocol II- (7.4% TH+/Dapi) over protocol I-derived cells (3.5% TH+/Dapi) after three weeks of final maturation while the overall number of neurons was similar. The direct comparison of these protocols might indicate that the initial neural induction method has an effect on subsequent DA differentiation, even though similar neural conversion efficiencies have been reported for both methods (SWISTOWSKI et al. 2010; MAK et al. 2012). Possibly, cell densities during midbrain patterning and final maturation might have influenced differentiation outcomes.
Protocol I implies the dissociation of neural precursors and maturation as single cells whereas protocol II relies on the outgrowth of mature dopaminergic neurons from dense EBs. It has been reported that higher cell seeding densities improve neural differentiation (EL- AKABAWY et al. 2011) and especially dopaminergic neurons are often found in dense cell aggregates during differentiation (BELINSKY et al. 2011). After transplantation of in vitro pre-differentiated hCBiPSCs into the striatum of immunosuppressed rats, only low numbers of surviving TH+ cells were observed in grafts derived from both protocols. It is not clear whether the DA cells present in the cell suspension either died or survived but ceased to express TH after being grafted. This important difference might be addressed in future studies.
However, it is known that DA neurons are very susceptible to transplantation procedures and
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most of the hiPSC transplantation studies have shown only modest cell survival and behavioural improvements of PD rats (HARGUS et al. 2010; SWISTOWSKI et al. 2010). In contrast, a recent study by Rhee et al. (2011) reported a high number of surviving TH+ neurons with prominent behavioural effects 8 weeks after transplantation. The difference to our study and the studies done by others was that Rhee and colleagues grafted hiPSC-derived neural progenitor cells which were not subjected to in vitro terminal differentiation at all. In their study, a pre-differentiation of just 5 days resulted in a lack of functional recovery in the transplanted animals. They further showed that functional outcomes depend on the number of cells that were transplanted. Grafting large numbers of neural progenitor cells resulted in irregular proliferating cell masses and tumor formation. In our protocol I-derived grafts we also observed continuous proliferation and tumor-like growth in half of the animals, even though we transplanted moderate concentrations of pre-differentiated cells. By in vitro differentiation according to protocol II a similar amount of immature, proliferating cells remained as with protocol I, but still no tumor formation was observed and we found a profound depletion of proliferating cells in vivo over time. This indicates that the type of differentiation protocol contributes to the transplantation outcomes. The reason for the observed tumor formation might be attributed to the presence of a few undifferentiated Oct4+ iPSCs at the end of the in vitro differentiation period in protocol I but not protocol II, which underlines the importance of complete removal of undifferentiated cells for safe clinical applications (MIURA et al. 2009).
A potential limitation of this work is that, for reasons of feasibility, we used rats of different ages for cell transplantation which can affect the functional outcome. Thus, a more straightforward comparison could be made using age-matched animals. In most publications no details on the age of the utilized animals were provided at all, although Collier et al.
demonstrated 1999 a correlation of the age of the animals at the time of grafting and the
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behavioural responses (COLLIER et al. 1999). In this comparative study fetal DA neurons were transplanted in the 6-OHDA rat model of PD, showing that the number of surviving TH+ neurons and neurite outgrowth significantly decreased in older animals (>24 months) compared to younger ones (3 and 17 months) and that a reduction of rotational behaviour as seen in younger rats was not observed in old animals. All animals used in our study aged between 3 and 11-14 months and thereby were within the age range of the well performing younger groups in the Collier study.
The next logical step would be to test the behavioral effect of transplanted hCBiPSC-derived DA neurons in the rat model of PD. We decided not to perform behavioral tests in the current set of experiments because the low numbers of surviving DA neurons spotted by immunohistochemical analyzes after transplantation predicted no functional benefits. In the literature nearly no functional recovery with similar low amounts of TH+ cells in the grafts has been stated (RHEE et al. 2011). Nevertheless, our data demonstrated the proof of principle of survival and integration of hCBiPSC-derived DA neurons in healthy rats and an animal model of PD and encourage further development of differentiation protocols to enhance the amount and function of implanted hCBiPSC-derived DA neurons in regard to potential therapeutic applications.
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21 6. Summary
Nancy Stanslowsky
Functional differentiation of midbrain neurons from human cord blood-derived induced pluripotent stem cells for transplantation in a rat model of Parkinson’s disease
Parkinson’s disease (PD) is a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Present treatment strategies provide symptomatic relief but do not slow the progression of the disease or lead to regeneration of dopaminergic cells. Human induced pluripotent stem cells (hiPSCs) reprogrammed from somatic cells hold great promise for replacement therapies, as they represent an unlimited source for the generation of patient-specific cells that can be standardized and optimized for research and regenerative medicine. Currently, cell sources for the generation of hiPSCs are mostly somatic cells obtained from aged individuals. A critical issue concerning their potential clinical use are mutations that accumulate over lifetime and are transfered onto iPSCs. In this thesis we investigated the in vitro and in vivo dopaminergic differentiation capacity of iPSCs generated from human cord blood endothelial cells (hCB) as a juvenile cell source.
In the first study we showed the efficient differentiation of hCBiPSCs into functional neurons including dopaminergic cells in vitro by inhibition of bone morphogenetic protein (BMP) and transforming growth factor-β (TGF-β) signaling. Since the functionality of neuronal cells generated in vitro is of high relevance for preclinical and clinical studies, we characterized the cells electrophysiologically and by calcium imaging. Differentiated hCBiPSCs displayed functional neuronal properties including voltage- and ligand-gated ion currents, action potential firing and synaptic activity (Stanslowsky et al., 2014).
22
In the second study hCBiPSCs differentiated according to an established dopaminergic differentiation protocol encompassing FGF-2 signaling (protocol I) and neurons generated by BMP/TGF-β inhibition (protocol II) were transplanted in an animal model of PD to investigate their in vivo survival and integration. Compared to protocol I, in vitro maturation yielded the twofold amount of dopaminergic neurons using hCBiPSCs differentiated according to protocol II. After striatal transplantation survival of the grafted hCBiPSC- derived cells from both protocols could be demonstrated, however, only the grafts generated by protocol II revealed several TH+ cells with fiber outgrowth and reinnervation of the lesioned striatum in half of the animals analyzed. Reduced numbers of undifferentiated proliferating cells were observed within protocol II-derived grafts after 3 weeks in vivo. In contrast, proliferation still occurred in protocol I-derived grafts, resulting in tumor-like growth in two out of four animals (Effenberg et al., accepted for publication).
23 7. Zusammenfassung
Nancy Stanslowsky
Funktionelle Differenzierung von Mittelhirn-Neuronen aus induzierten pluripotenten Stammzellen aus Nabelschnurblut für die Transplantation in ein Rattenmodell der Parkinson’schen Krankheit.
Morbus Parkinson ist eine neurodegenerative Erkrankung, die sich durch ein fortschreitendes Absterben dopaminerger Neurone in der Substantia nigra pars compacta auszeichnet.
Gegenwärtige Behandlungsmethoden erbringen zwar symptomatische Linderung, verlangsamen aber weder den Krankheitsverlauf noch führen sie zu einer Regeneration dopaminerger Zellen. Humane induzierte pluripotente Stammzellen (hiPS-Zellen), die durch Reprogrammierung aus somatischen Zellen erzeugt werden, bergen großes Potential für Zellersatztherapien, da sie eine unerschöpfliche Quelle zur Herstellung patienten-spezifischer Zellen darstellen und für Forschung und regenerative Medizin standardisiert und optimiert werden können. Die momentan häufigste Zellquelle zur Erzeugung von hiPS-Zellen sind somatische Zellen von Erwachsenen. Ein kritischer Aspekt bezüglich deren potentieller klinischer Nutzung sind Mutationen, die sich im Laufe des Lebens ansammeln und an die iPS-Zellen weitergegeben werden. In dieser Arbeit wurde das dopaminerge Differenzierungspotential von iPS-Zellen, die aus juvenilen, humanen Nabelschnurblut- Endothelzellen hergestellt wurden, in vitro und in vivo untersucht.
Im ersten Teil des Dissertationsprojektes erfolgte der Nachweis einer effizienten in vitro Differenzierung von Nabelschnurblut-iPS-Zellen in funktionelle Neurone einschließlich dopaminerger Zellen durch Inhibierung des bone morphogenetic protein (BMP) und transforming growth factor-β (TGF-β) Signalwegs. Da die Funktionalität in vitro kultivierter
24
Neurone von präklinischer und klinischer Bedeutung ist, charakterisierten wir die Zellen elektrophysiologisch und mittels Kalzium-Imaging. Differenzierte Nabelschnurblut-iPS- Zellen wiesen funktionelle neuronale Eigenschaften einschließlich spannungs- und ligandenabhängiger Ionenströme, Aktionspotentialen und synaptischer Aktivität auf (Stanslowsky et al., 2014).
In der zweiten, im Rahmen der Dissertation durchgeführten Studie wurden Nabelschnurblut- iPS-Zellen, die entsprechend eines auf FGF-2-Signalübertragung (Protokoll I) basierenden, dopaminergen Differenzierungsprotokolls generiert wurden sowie die, durch BMP/TGF-β Inhibierung (Protokoll II) erzeugten Neurone in ein Tiermodel der Parkinson’schen Krankheit transplantiert, um deren Überleben und Integration in vivo zu beurteilen. Verglichen mit Protokoll I brachte die in vitro Reifung die doppelte Anzahl an dopaminergen Neuronen aus Nabelschnurblut-iPS-Zellen hervor, die nach Protokoll II differenziert wurden. Nach striataler Transplantation konnten zwar überlebende Zellen mit beiden Protokollen nachgewiesen werden, allerdings zeigten nur Protokoll II-Transplantate TH+-Zellen mit Neuritenaussprossung und Reinnervation des läsionierten Striatums in der Hälfte der transplantierten Tiere. In Protokoll II-Transplantaten konnte nach 3 Wochen in vivo eine reduzierte Anzahl an undifferenzierten, proliferierenden Zellen beobachtet werden. Im Gegensatz dazu proliferierten die Zellen in Protokoll I-Transplantaten vermehrt weiter, was zu einem tumorähnlichen Transplantatwachstum in zwei aus vier untersuchten Tieren führte (Effenberg et al., accepted for publication).
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