Abstract:
The present invention relates to the field of stem cells. More specifically, the present invention provides methods and compositions useful for the highly efficient conversion of human stem cells to lineage-specific neurons. In a specific embodiment, a method of inducing differentiation of human stem cells into dopaminergic (DA) neurons comprises the steps of (a) transfecting human stem cells with a lentiviral vector encoding Atoh1, wherein the vector is Dox inducible; and (b) growing the transfected cells in culture in the presence of Dox, Sonic Hedgehog (SHH) and FGF-8b until DA neurons are induced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/911,238, filed Dec. 3, 2013, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of stem cells. More specifically, the present invention provides methods and compositions useful for the highly efficient conversion of human stem cells to lineage-specific neurons. 
       INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ELECTRONICALLY 
       [0003]    This application contains a sequence listing. It has been submitted electronically via EFS-Web as an ASCII text file entitled “P12596-02_ST25.txt.” The sequence listing is 21,808 bytes in size, and was created on Dec. 3, 2014. It is hereby incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0004]    Optimized differentiation strategies are essential for differentiating human stem cells (SCs) into lineage-specific neuronal progenies in sufficient numbers and purity for transplantation or disease modeling. Current strategies for generating lineage-specific neurons from human pluripotent or multipotent stem cells, including induced pluripotent cells (iPSCs), embryonic stem cells (ESCs) and fetal neural stem cells (NSCs), generally yields incomplete neuronal conversion and lineage specification, which adversely affects in vivo engraftment and function of these neurons and also leads to considerable safety concerns regarding their potential for malignancy formation or neural overgrowth. The present invention provides a novel and optimized strategy for highly efficient differentiation of human SCs to lineage-specific and functional neurons. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is based, at least in part, on the development of methods for using transcription factors including Atoh1, Neurogenin 2, and NeuroD1, to drive highly efficient neuronal differentiation of human SCs into lineage-specific neurons, such as dopaminergic (DA) neurons. 
         [0006]    Cell replacement therapy using human stem cells (SCs) holds great promise for treating neurological diseases in which neuronal loss results in cognitive, extrapyramidal, and/or motor dysfunction. Optimized differentiation strategies are essential to differentiate human stem cells into lineage-specific neuronal progenies in sufficient numbers and purity for transplantation or disease modeling. As described herein, we have established a strategy for differentiating human pluripotent or multipotent SCs into lineage-specific and functional neurons. We found a defined transcription factor that can induce the highly efficient conversion of human SCs to lineage-specific neurons (e.g., dopaminergic (DA) neurons). This strategy can generate dopaminergic neurons with &gt;80% purity. The established strategy is novel and highly applicable for disease modeling and cell replacement therapy for several neurological disorders, including but not limited to Parkinson&#39;s disease, spinal cord injury, amyotrophic lateral sclerosis and hearing loss. 
         [0007]    Current differentiation strategies for converting human SCs to neurons act primarily on cell surface receptors or intracellular signaling proteins to alter the level of multiple downstream transcription factors and in turn to change the gene expression profile and cell fate specification. These strategies lack specificity and efficiency because they activate multiple signaling cascades and transcription factors, only a fraction of which are optimal and necessary to drive lineage-specific neuronal differentiation. In certain embodiments, our differentiation strategy uses a single transcription factor Atoh1 to induce highly specific neuronal differentiation signaling, resulting in rapid and highly efficient lineage-specific neuronal conversion. In other embodiments, the strategy may include NeuroD1 and/or Neurogenin 2. Our differentiation strategy saves the time for generating lineage-specific neurons from human SCs by at least 50%. It also generates lineage-specific neurons with &gt;80% purity, significantly higher than the purity (10-40%) normally achieved by other methods. 
         [0008]    Accordingly, in one aspect, the present invention provides methods for differentiating stem cells into neuronal cells. In one embodiment, a method for inducing differentiation of neuronal cells from human stem cells comprises the steps of (a) transfecting human stem cells with an expression vector encoding Atoh1; and (b) growing the transfected cells in culture until the stem cells are differentiated. In certain embodiments, the human stem cells are induced pluripotent stem cells (iPSCs), embryonic stem cells (ESCs) or neural stem cells (NSCs). In specific embodiments, the neural stem cells are fetal or adult neural stem cells. 
         [0009]    In particular embodiments, Atoh1 comprises SEQ ID NO:2. In other embodiments, Atoh1 comprises an N-terminal flag tag and is shown, for example, in SEQ ID NO:4. 
         [0010]    In other embodiments, the expression vector is a viral vector. In particular embodiments, the viral vector is from a lentivirus, adeno-associated virus, herpes simplex virus, Senai virus or baculovirus. In a specific embodiment, the viral vector is from a lentivirus. In other embodiments, the expression vector is non-viral. In a further embodiment, the expression vector encoding Atoh1 is inducible. In a specific embodiment, the lentiviral expression vector is doxycycline (Dox) inducible. 
         [0011]    In a further embodiment, the cells are grown in the presence of a sufficient concentration of Doxycycline (Dox) in order to induce Atoh1 transgene expression and drive the differentiation of human stem cells into neuronal cells. In a more specific embodiment, step (b) further comprises the steps outlined in Table 1. In yet another embodiment, prior to step (b), the method further comprises the step of transfecting the human stem cells with an expression vector encoding NeuroD1. In a particular embodiment, NeuroD1 comprises SEQ ID NO:8. In an alternative embodiment, NeuroD1 comprises SEQ ID NO:9. In another embodiment, prior to step (b), the method further comprises the step of transfecting the human stem cells with an expression vector encoding Neurogenin 2. In a particular embodiment, Neurogenin 2 comprises SEQ ID NO:6. In alternative embodiments, the expression vector of step (a) also encodes Neurogenin 2 and/or NeuroD1. The present invention further contemplates the use of biologically active or functional fragments of Atoh1, Neurogenin 2 and/or NeuroD1 in the present invention. Furthermore, the methods of the present invention can further comprise exposing the cells to sufficient concentrations of additional growth factors. 
         [0012]    In a specific embodiment, a method of inducing differentiation of human stem cells into dopaminergic (DA) neurons comprises the steps of (a) transfecting human stem cells with a lentiviral vector encoding Atoh1, wherein the vector is Dox inducible; and (b) growing the transfected cells in culture in the presence of Dox, Sonic Hedgehog (SHH) and FGF-8b until DA neurons are induced. In one embodiment, the human stem cells are iPSCs. In another embodiment, the human stem cells are ESCs. In such embodiments, step (b) further comprises the steps outlined in Table 2. In an alternative embodiment, the human stem cells are NSCs. In such embodiment, step (b) further comprises the steps outlined in Table 3. In yet another embodiment, prior to step (b), the method further comprises the step of transfecting the human stem cells with an expression vector encoding NeuroD1. In another embodiment, prior to step (b), the method further comprises the step of transfecting the human stem cells with an expression vector encoding Neurogenin 2. In alternative embodiments, the lentiviral vector of step (a) also encodes NeuroD1 and/or Neurogenin 2. 
         [0013]    In another aspect, the present invention provides methods for treating patients suffering from a neurodegenerative disease. In one embodiment, a method for treating a patient suffering from a neurodegenerative disease comprises the steps of (a) obtaining stem cells from the patient; (b) initiating differentiation of the stem cells into a population of differentiated cells using a method described herein; (c) analyzing the development of differentiated neurons in culture; and (d) transplanting the differentiated cells into the patient&#39;s brain. In certain embodiments, the neurodegenerative disease is Parkinson&#39;s disease and the differentiated cells are DA neurons. 
         [0014]    The present invention also provides a population of neuronal cells prepared using any one of the methods described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]      FIG. 1 . Atoh1 is induced during the differentiation of human pluripotent stem cells into neurons. (A): Human induced pluripotent stem cells (iPSCs) were differentiated into neurons following the dual-SMAD inhibition protocol. By day 10 of differentiation, cells expressed neural lineage markers (PAX6 and NESTIN). By day 40 of differentiation, iPSC-derived neurons expressed neuronal markers (TUJ1 and MAP2). Cell nuclei were counterstained with DAPI. Scale bars=50 μm. (B, C): Markers for pluripotent cells (NANOG), neural (PAX6), and neuronal (Ngn2 and NEUROD1) lineages were analyzed by quantitative real-time polymerase chain reaction (qRT-PCR) during iPSC differentiation at days 0, 10, and 20. Atoh1 expression was analyzed by qRT-PCR (B) and Western blotting (C). The data represent means±SEM. *, p&lt;0.01 compared with day 0. Abbreviation: DAPI, 4′,6-diamidino-2-phenylindole. 
           [0016]      FIG. 2 . Ectopic Atoh1 expression drives neuronal conversion in induced pluripotent stem cells (iPSCs). (A): Diagram of the lentiviral vector for Dox-inducible Atoh1 expression. (B): Dox controls the on/off switch of Atoh1 expression. Human iPSCs were infected with lentivirus harboring Dox-inducible Atoh1. Stable Atoh1-iPSCs after puromycin selection were treated with or without Dox for 48 hours and transferred to Dox-free medium. Whole cell lysates collected on each indicated time point were subjected to immunoblot using anti-FLAG antibody. (C): Atoh1-iPSCs were treated with Dox (+Dox) for 3 days and changed to Dox-free medium (Dox withdrawal) for 3 days. Cells were immunostained with FLAG antibody. (D): Diagram of Atoh1-induced neuron differentiation protocol. Atoh1 is induced by Dox from days 1 to 5. (E): Immunostaining from cell cultures at differentiation day 6 shows TUJ1 expression in Atoh1-induced cells but not in control cells. (F): Bright-field microscope images show cell adhesion and neuronal process formation in Atoh1-induced cells on differentiation day 10. (G): Immunostaining shows the coexpression of TUJ1 and Synapsin in Atoh1-induced neurons on differentiation day 36. (H, I): During a 5-day time period, an equal number of Atoh1-iPSCs received different lengths of Dox treatment (from 1 to 5 days). After being matured for 30 days, cells were immunostained against neuronal marker TUJ1 and MAP2. The percentage of TUJ1+/MAP2+ cells over DAPI+ cells and the total number of TUJ1+/MAP2+ cells were quantified in 10 random-selected microscopic fields (p, p, 0.01 compared with cells that had 4- and 5-day Atoh1 induction). (J, K): Atoh1-iPSCs were treated with Dox for 2 days and returned to Dox-free medium for 3 days (J). The expression of Atoh1, NEUROD1, and Ngn2 was measured by quantitative real-time polymerase chain reaction in control, Atoh1 induction, and Atoh1 silencing samples (*, p&lt;01 compared with control). In (C) and (E-G), cell nuclei were counterstained with DAPI. Scale bars=20 μm. The data represent means±SEM. Abbreviations: Con, control; DAPI, 4′,6-diamidino-2-phenylindole; Dox, doxycycline; FLAG-Atoh1, FLAG-tagged Atoh1; IRES, internal ribosome entry site; Puro r , puromycin selection marker; rtTA3, reverse tet-transactivator; TRE, tet-inducible promoter; UBC, human ubiquitin C promoter. 
           [0017]      FIG. 3 . Neuron subtype specification in Atoh1-induced neurons. (A): Atoh1-induced neurons derived from Atoh1-induced pluripotent stem cells were allowed to mature in vitro, and cells at differentiation day 36 were immunostained with antibodies detecting dopaminergic (TH), GABAergic (GAD67), serotonergic (serotonin), and glutamatergic (VGluT1) neuron subtypes. Cell nuclei were counterstained with DAPI. Scale bars=50 μm. (B): Immunostained neurons from 10 random-selected microscopic fields were counted to calculate the percentage of TH+, GAD67+, serotonin+, and VGluT1+ cells over DAPI+ cells. The data represent means±SEM. Abbreviations: DAPI, 4′,6-diamidino-2-phenylindole; N.D., not detected; TH, tyrosine hydroxylase; VGluT, vesicular glutamate transporter 1. 
           [0018]      FIG. 4 . Ectopic Atoh1 expression and cell extrinsic factors induce dopaminergic (DA) neurons from PSCs. (A): Diagram of DA neuron differentiation induced by Atoh1, SHH, and FGF8b. (B): Atoh1-iPSCs were differentiated by Atoh1 induction alone or in combination with SHH and FGF8b. The expression of DA lineage markers was analyzed by quantitative real-time polymerase chain reaction using cells at differentiation day 6. Control cells followed the same differentiation protocol but did not receive Dox treatment. Atoh1 induction in combination with SHH and FGF8b more robustly induced DA lineage markers than Atoh1 alone or untreated cells. The data represent means±SEM. *, p&lt;01 compared with Atoh1 alone; ♦, p&lt;01 compared with control. (C, D): Atoh1-iPSCs were differentiated following the protocol shown in (A). Atoh1-induced neurons at differentiation day 36 were immunostained for neuronal marker (TUJ1) and DA neuron marker (TH). Cell nuclei were counterstained with DAPI. Scale bars=20 μm. TH+/TUJ1+DA neurons derived from Atoh1-iPSCs and Atoh1-ESCs from 10 random-selected microscopic fields were counted to calculate the percentage of TH+/TUJ1+ cells over DAPI+ cells. The data represent means±SEM. (E): Bright-field microscope image of Atoh1-induced iPSC-derived DA neurons 7 days after being recovered from cryopreservation. Scale bars=20 μm. (F): Atoh1-iPSCs (lx 10 6 ) were differentiated following the protocol shown in (A). Cells were counted to calculate the number of NPCs (differentiation day 7), DA neurons (differentiation day 14), and post-thaw DA neurons (frozen at differentiation day 7 and cultured 7 days after cryopreservation). Abbreviations: Atoh1+S/F, Atoh1 induction in combination with SHH and FGF8b; DAPI, 4′,6-diamidino-2-phenylindole; DAT, dopamine transporter; ESC, embryonic stem cell; FGF, fibroblast growth factor; iPSC, induced pluripotent stem cell; NPC, neuron precursor cell; SHH, Sonic Hedgehog; TH, tyrosine hydroxylase. 
           [0019]      FIG. 5 . The expression of midbrain DA neuron markers and dopamine release in Atoh1-induced neurons. (A-E): Atoh1-induced pluripotent stem cells were differentiated following the protocol shown in  FIG. 4A . DA neurons at differentiation day 36 were immunostained for midbrain DA neuron markers (FOXA2, NURR1, EN1, TH, GIRK2, and DAT) and mature neuron marker (Synapsin). Cell nuclei were counterstained with DAPI. The arrows and arrowhead in (E) indicate DAT+ and DAT− neurons, respectively. Scale bars=20 μm. (F, G): Representative HPLC chromatogram (F) and quantification (G) of DA and its metabolites (DOPAC, 3-MT, and HVA) released from Atoh1-induced DA neurons at differentiation day 36 in response to KCl-evoked depolarization for 15 minutes. The data represent means±SEM (n=2). Abbreviations: DA, dopaminergic; DAPI, 4′,6-diamidino-2-phenylindole; DAT, dopamine transporter; DOPAC, 3,4-dihydroxy-phenylacetic acid; GIRK2, G protein-regulated inward-rectifier potassium channel 2; HVA, homovanillic acid; 3-MT, 3-methoxytyramine; TH, tyrosine hydroxylase. 
           [0020]      FIG. 6 . Electrophysiological properties of Atoh1-induced dopaminergic (DA) neurons. (A): Differential interference contrast image of a patched Atoh1-induced DA neuron. Scale bar=20 μm. (B, C): Atoh1-induced DA neurons derived from Atoh1-iPSCs showed spontaneous spiking activity. This cell has a resting membrane potential of 265 mV (B) (a zoomed view is shown in the right panel) and an average spiking frequency of 4.8 Hz (C) (left panel). The spontaneous spiking frequencies from 37 neurons were plotted in the right panel of (C) with the means6 SEM marked inside. (D): Whole cell current-clamp recording of action potentials evoked by 70 pA current injection (top panel). Action potentials were suppressed by sodium channel blocker (TTX) (middle panel). Action potentials recovered after TTX withdrawal (bottom panel). (E): An hyperpolarized injection of current (0.2 nA) evoked hyperpolarization and rebound tonic spiking. A typical hyperpolarization sag was observed in the upper panel, which was dampened by ML252 (5 mM, a KCNQ2 inhibitor, lower panel). (F): Voltage-clamp recording of Atoh1-induced neurons. Depolarized sodium and potassium currents were evoked by elevation the membrane potential to different levels (left panel). Both sodium and potassium currents were attenuated by sodium and potassium channel inhibitors (TTX [0.5 mM] and 4-AP [25 mM], respectively). Abbreviations: ACSF, artificial cerebrospinal fluid; TTX, tetrodotoxin. 
           [0021]      FIG. 7 . Neurotoxicity induced by 6-OHDA in Atoh1-induced dopaminergic (DA) neurons. (A): Atoh1-inducedDAneurons derived from Atoh1-iPSCs at differentiation day 36 were treated with 6-OHDA for 15 minutes. Bright-field microscope images show the morphological signs of neuron death at 24 hours after treatment. Scale bar=100 μm. (B): Cell death was quantified by LDH cytotoxicity assay. The data represent means±SEM (n=3). *, p&lt;01 compared with control. Abbreviations: LDH, lactate dehydrogenase; 6-OHDA, 6-hydroxydopamine. 
           [0022]      FIG. 8 . Neuronal maturation of Atoh1-induced neurons derived from human ESCs. At differentiation day 36, Atoh1-induced neurons derived from ESCs co-expressed the synaptic vesicle protein Synapsin and neuronal marker TUJ1. Cell nuclei were counterstained with DAPI. Bar: 20 μm. 
           [0023]      FIG. 9 . Atoh1 induction for 2-5 days is sufficient for neuronal conversion in iPSCs. During a 5-day time period, equal number of Atoh1-iPSCs received different length of Dox treatment (from 1 to 5 days). Brightfield microscope images show neurons at differentiation day 36, which were also immunostained against neuronal marker TUJ1 and MAP2. Cell nuclei were counterstained with DAPI. Bar: 20 μm. 
           [0024]      FIG. 10 . Atoh1-induced DA neurons from human ESCs at differentiation day 36 were immunostained with neuronal marker (TUJ1) and DA neuron marker (TH). Cell nuclei were counterstained with DAPI. Bar: 20 μm. 
           [0025]      FIG. 11 . Atoh1-induced DA neurons from human iPSCs at differentiation day 36 were immunostained against GAD67, serotonin, SOX2 and OCT4. Cell nuclei were counterstained with DAPI. Bar: 20 μm. 
           [0026]      FIG. 12 . Atoh1 induces highly efficient DA neuron differentiation in human ESCs and NSCs. (A) Atoh1-ESCs were differentiated following the protocol as shown in  FIG. 4A  and Table 2. Atoh1-induced neuron cultures at Day 36 were immunostained with antibody against neuronal lineage marker (TUJ1) and DA neuron marker (TH). Cell nuclei were counterstained with DAPI. (Bar: 20 μm). Cells from 10 random-selected microscopic fields were counted to calculate the percentage of TH+/TUJ1+ cells over DAPI+ cells (left panel, Data represents Mean±SEM). (B) Atoh1-NSCs were differentiated following the protocol as shown in  FIG. 4A  and Table 3. Atoh1-induced neuron cultures at Day 36 were immunostained with antibody against neuronal lineage marker (TUJ1) and DA neuron marker (TH). Cell nuclei were counterstained with DAPI. (Bar: 20 μm). Cells from 10 random-selected microscopic fields were counted to calculate the percentage of TH+/TUJ1+ cells over DAPI+ cells (left panel, Data represents Mean±SEM). 
           [0027]      FIG. 13 . Synthetic mRNA encoding Atoh1 drives the differentiation of human iPSCs into neurons. Ectopic Atoh1 (with N-terminal FLAG tag) expression was detected in &gt;90% of the Atoh1-mRNA-transfected cells ( FIG. 13A ) but not in untransfected control cells ( FIG. 13B ), as determined by FLAG immunofluorencence staining After iPSCs received daily transfection of Atoh1 mRNA for 4 days, cells were replated (3×10 5  cells per cm 2 ) on dishes pre-coated with poly-D-Lysine (1 μg/ml) and laminin (1 μg/ml). Neurons after being matured in vitro for 20 days co-express the neuronal marker (TUJ1) and the mature neuron markers (MAP2 and Synapsin) ( FIGS. 13C and 13D ). Overall, these results demonstrate that Atoh1 can be induced in human SCs by delivering synthetic Atoh1 mRNA, and Atoh1 mRNA delivery drives neuronal conversion of human SCs. 
           [0028]      FIG. 14 . Neurogenin 2 can be induced by Atoh1 and also drives the differentiation of human iPSCs into neurons. (A) Atoh1 induction in human iPSCs for 2 days activated known Atoh1 target NeuroD1 and the proneural transcription factor Neurogenin 2 that has not yet been defined as an Atoh1 target gene ( FIG. 14A ), as determined by quantifying gene expression using quantitative real-time PCR. (B) We further tested if the Atoh1 target gene Neurogenin 2 can also drive lineage-specific neuronal conversion of human iPSCs. We applied the same lentivirus-mediated gene delivery system as that for Atoh1 expression ( FIG. 2A ) to achieve Dox inducible expression of Neurogenin 2 in human SCs. By following the differentiation strategy as shown in  FIG. 14B , we successfully generated TH+/TUJ1+DA neurons from human iPSCs by inducing Neurogenin 2 in combination with two morphogens (sonic hedgehog (SHH) and FGF-8b) ( FIG. 14C ). These results demonstrate that Atoh1-induced gene targets (e.g. Neurogenin 2) can be used to drive lineage-specific neuronal conversion of human SCs. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    It is understood that the present invention is not limited to the particular methods and components, etc., described herein, as these may vary. It is also to be understood that the terminology used herein is used for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. It must be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include the plural reference unless the context clearly dictates otherwise. Thus, for example, a reference to a “protein” is a reference to one or more proteins, and includes equivalents thereof known to those skilled in the art and so forth. 
         [0030]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Specific methods, devices, and materials are described, although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention. 
         [0031]    All publications cited herein are hereby incorporated by reference including all journal articles, books, manuals, published patent applications, and issued patents. In addition, the meaning of certain terms and phrases employed in the specification, examples, and appended claims are provided. The definitions are not meant to be limiting in nature and serve to provide a clearer understanding of certain aspects of the present invention. 
         [0032]    Human pluripotent stem cells (PSCs), including embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), exhibit unique characteristics such as indefinite self-renewal capacity and multi-lineage differentiation potential. Human PSCs, especially patient-derived iPSCs, hold enormous promise for various applications in regenerative medicine, including disease modeling, drug development and cell replacement therapy. In order to fully utilize these patient-specific iPSCs in regenerative medicine, highly efficient differentiation strategies are required to drive iPSCs into desired lineages and generate functional cell progenies, such as various subtypes of neurons. Current protocols for differentiating human PSCs into lineage-specific neurons (e.g., dopaminergic (DA) neurons) are based on embryoid body formation, stromal feeder co-culture, selective survival conditions or inhibitors of SMAD signaling. These PSC-derived neurons have allowed scientists to study molecular mechanisms underlying various neurological disorders, test potential drugs and optimize strategies for cell replacement therapy. However, current neuron differentiation protocols for PSCs involve months of stem cell culture procedures and multiple reagents, which cause significant variation especially for researchers who have limited practice in PSC culture and desire functional neurons at high purity for disease-in-a-dish models. A broad desire for a robust system to generate human PSC-derived neurons motivated us to develop a highly efficient strategy to generate lineage-specific functional neurons using the proneural transcription factor Atoh1. 
         [0033]    ATOH1 (the mammalian homolog of  Drosophila Atonal ) belongs to the proneural transcription factors of the basic helix-loop-helix (bHLH) family. Proneural transcription factors are crucial in driving the acquisition of a generic neuronal fate and regulating neuronal subtype specification during development. Atoh1 proteins form heterodimers with E proteins, and these heterodimers function as transcriptional activators by binding E box motifs (CANNTG) in the regulatory regions of their target genes. Atoh1 is a key regulator of neurogenesis, governing the differentiation of various neuronal lineages, including cerebellar granule neurons, brainstem neurons, inner ear hair cells, and numerous components of the proprioceptive and interoceptive systems, as well as some nonneuronal cell types. Atoh1 can activate crucial neurogenic transcription factors, such as NeuroD1, 2, 6 and Nhlh1, 2, to initiate a neuronal differentiation program that later becomes self-supporting and Atoh1-independent. Certain members of the proneural transcription factor family, such as ASCL1, Ngn2 and NeuroD1, have been successfully used to generate neurons from both PSCs and somatic cells. However, the role of Atoh1 in the neuronal differentiation and neuron subtype specification of human PSCs is largely unknown, and, as a result, Atoh1-based strategies for the neuronal conversion of human PSCs is still unavailable. 
         [0034]    Here, we show that Atoh1 is induced during the neuronal differentiation of human PSCs. By transiently inducing ectopic Atoh1 expression, we are able to efficiently convert PSCs into neurons. Atoh1 induction, in combination with two neural patterning morphogens (Sonic Hedgehog (SHH) and fibroblast growth factor 8b (FGF8b)), leads to rapid and highly efficient conversion of PSCs into DA neurons that recapitulate key biochemical and electrophysiological features of primary midbrain DA neurons. We also demonstrate that Atoh1-induced DA neurons serve as a reliable model for analyzing 6-OHDA-induced neurotoxicity in human midbrain DA neurons. Since most symptoms of Parkinson&#39;s disease (PD) result from the degeneration of midbrain DA neurons located in the substantia nigra, Atoh1-induced DA neurons provide an in vitro neuron model for mechanistic studies and drug testing for PD. 
         [0035]    Here, we established a method for using Atoh1 to drive highly efficient neuronal differentiation of human SCs into lineage-specific neurons, such as DA neurons. Neuronal differentiation strategies are widely applicable to various kinds of human stem cells, including human embryonic stem cells, induced pluripotent cells, fetal/adult neural stem cells and mesenchymal stem cells. Thus, the present invention can be used in driving the neuronal differentiation of these stem cells. 
         [0036]    Here, we used lentivirus to deliver Atoh1 gene into human stem cells. Atoh1 gene can also be delivered by other virus vectors, including but not limited to adeno-associated virus, herpes simplex virus, Sendai virus and baculovirus. Non-viral Atoh1 delivery is also feasible by using plasmid transfection, mRNA transfection and recombinant cell-penetrating protein. Certain chemical compounds can also be used to activate expression of endogenous Atoh1. See U.S. Patent Publication No. 20090232780. 
         [0037]    Here, we used Atoh1 in the differentiation of human stem cells into dopaminergic neurons. Atoh1 is a key regulator of neurogenesis, governing the differentiation of various neuronal lineages (cerebellar granule and brainstem neurons, inner ear hair cells, and numerous components of the proprioceptive and interoceptive systems), as well as some nonneuronal cell types. We anticipate that Atoh1 can be used to differentiate human stem cells into different neuronal lineages, including but not limited to motor neurons, cerebellar neurons, inner ear hair cells, and numerous neuron lineages of the proprioceptive and interoceptive systems. 
         [0038]    Here we used Atoh1 in combination with sonic hedgehog and FGF-8b as the morphogenes to drive the differentiation of human SCs into DA neurons. Other reagents, such as puromorphamine and CHIR99021, have also been successfully used to drive the DA neuron differentiation of human SCs. Expression of specific genes, including but not limited to FOXA2, LMX1A, PITX3 and NURR1, has also been shown to differentiate human or mouse SCs into DA neurons. We anticipate that these reagents and genes can be incorporated into Atoh1-induced DA neuron differentiation protocol. 
         [0039]    Without further elaboration, it is believed that one skilled in the art, using the preceding description, can utilize the present invention to the fullest extent. The following examples are illustrative only, and not limiting of the remainder of the disclosure in any way whatsoever. 
       EXAMPLES 
       [0040]    The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices, and/or methods described and claimed herein are made and evaluated, and are intended to be purely illustrative and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for herein. Unless indicated otherwise, parts are parts by weight, temperature is in degrees Celsius or is at ambient temperature, and pressure is at or near atmospheric. There are numerous variations and combinations of reaction conditions, e.g., component concentrations, desired solvents, solvent mixtures, temperatures, pressures and other reaction ranges and conditions that can be used to optimize the product purity and yield obtained from the described process. Only reasonable and routine experimentation will be required to optimize such process conditions. 
       Example 1 
     Proneural Transcription Factor Atoh1 Drives Highly Efficient Differentiation of Human Pluripotent Stems Cells into Dopaminergic Neurons 
     Materials and Methods 
       [0041]    Cell Culture. 
         [0042]    Human H1 ESC line was obtained from WiCell Research Resources (WiCell, WI). Human iPSC line ND27760 (passage 25-30) was derived from human skin fibroblasts from a PD patient with a SNCA triplication that were obtained from the Coriell Cell Repositories. Cell reprogramming was performed using non-integrating 4 factor (SOX2/OCT4/KLF4/MYC) Sendai virus system (CytoTune-iPS Reprogramming Kit, Life Technologies). The pluripotency of this iPSC line has been characterized by immunocytochemistry for pluripotent cell markers (NANOG, OCT4, TRA-1-60 and SSEA-3) and embryoid body differentiation. Human ESCs and iPSCs were maintained as feeder-free cultures in Essential 8 medium (Life Technologies) or mTESR1 medium (Stemcell Technologies) in 5% CO 2 /95% air condition at 37° C., and were passaged using dispase (Life Technologies). Karyotype analysis of G-banded metaphase chromosomes was performed to confirm the chromosomal integrity of these ESCs and iPSCs. All experiments involving human stem cells were performed with the approval of the Johns Hopkins Medicine Institutional Review Boards. 
         [0043]    Lentiviral Transduction. 
         [0044]    Human Atoh1 cDNA was constructed using high-fidelity PCR kit (Roche) and cloned into pTRIPZ vector (Thermo Scientific) with AgeI and MluI. The nucleic acid and amino acid sequences for Atoh1 are shown in SEQ ID NOS:1-2, respectively. Trans-Lentiviral Packaging System (Thermo Scientific) was used for lentivirus packaging. Cells were infected by lentivirus at an MOI of 5 for 24 h with the addition of TransDux Virus Infection solution (System Biosciences). Stable cell lines were established by puromycin selection (0.5 μg/ml). All recombinant DNA and lentivirus experiments were performed following the National Institutes of Health guidelines. 
         [0045]    Cell Differentiation and Cryopreservation. 
         [0046]    To measure Atoh1 expression during the neuronal conversion of human PSCs, cells were differentiated following a dual-SMAD inhibition protocol (Chambers et al., 27 NAT. BIOTECHNOL. 275-80 (2009). Noggin in this protocol was replaced by LDN193189 (100 nM, Stemgent). For the Atoh1-induced neuron differentiation protocol, cells were plated (8×10 4  cells per cm 2 ) on matrigel (BD) in Essential 8 Medium (Life Technologies) with the ROCK inhibitor (Y-27632, 10 μM, Stemgent). Atoh1 was induced by Doxycycline (0.5 μg/ml, Sigma-Aldrich) in culture medium from Day 1 to 5. From Day 1 to 3, cell culture medium was changed every day and gradually shifted from Essential 6 Medium (Life Technologies) to N2 Medium (DMEM/F12 medium with N2 supplement, Life Technologies). Cells were cultured in N2 Medium until day 7, dissociated using Accutase (Sigma-Aldrich) and replated (3×10 5  cells per cm 2 ) on dishes pre-coated with poly-D-Lysine (1 μg/ml) and laminin (1 μg/ml) using neuron culture medium (Neurobasal Medium with B27 supplement, BDNF (brain-derived neurotrophic factor, 20 ng/ml, PeproTech), GDNF (Glial cell line-derived neurotrophic factor, 20 ng/ml, PeproTech), TGFβ3 (transforming growth factor type β3, 1 ng/ml, R&amp;D), ascorbic acid (0.2 mM, Sigma-Aldrich), dibutyryl cAMP (0.5 mM, Sigma-Aldrich), and DAPT (10 μM, Stemgent)). From Day 8 to Day 36, half of the cell culture medium was replenished every 3-4 days. For Atoh1-induced DA neuron differentiation protocol, the protocol above was modified by adding SHH (SHH C25II, 100 ng/ml, R&amp;D) and FGF8b (100 ng/ml, PeproTech) from Day 1 to 5. 
         [0047]    Atoh1-induced DA neuron precursors at differentiation Day 7 were dissociated using Accutase. 1×10 6  cells were cryopreserved in 1 ml freezing medium (40% Neurobasal Medium with B27 supplement, 50% fetal bovine serum and 10% DMSO) using a freezing container (Nalgene) in −80° C. for 24 h and stored in liquid nitrogen. 
         [0048]    Atoh1-Mediated Differentiation of Human iPSCs into Mature Neurons. 
         [0049]    As shown in  FIG. 2D , we established protocols for differentiating Atoh1-iPSCs into neurons. The recipes for cell culture media are listed in Table 1. Cells were plated (4×10 4  cells per cm 2 ) on matrigel (BD) in Essential 8 Medium (Life Technologies) with ROCK inhibitor (Y-27632, Stemgent). From Day 1 to Day 7, cell culture medium was changed every day. From Day 8 to Day 36, half of the cell culture medium was changed every 3-4 days. On differentiation day 6, Atoh1 dramatically increases the level of neuronal differentiation marker (TUJ1) when compared with Dox-untreated control ( FIG. 2E ). On day 7, cells were dissociated using Accutase (Sigma-Aldrich) and replated (3×10 5  cells per cm 2 ) on dishes pre-coated with poly-D-Lysine (1 μg/ml) and laminin (1 μg/ml). After cell passaging, cells differentiated by Atoh1 adhered and formed neuronal processes ( FIG. 2F , right panel). Cells without Atoh1 induction failed to attach and grow in neuron culture medium ( FIG. 2F , left panel). Atoh1-induced neurons further matured in vitro, and co-expressed neuronal marker (β-tubulin III, TUJ1) and synaptic resident protein (synapsin), suggesting the establishment of synaptic terminals and neuronal maturation ( FIG. 2G ). 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Neuron differentiation media used in Atoh1- 
               
               
                 induced protocol as shown in FIG. 2D 
               
             
          
           
               
                   
                 Day 
                 Medium 
                 Additives 
               
               
                   
                   
               
               
                   
                 0 
                 E8 
                 Y-27632 
               
               
                   
                 1 
                 E6 (75%) + N2 (25%) 
                 Doxycyline 
               
               
                   
                 2 
                 E6 (50%) + N2 (50%) 
                 Doxycyline 
               
               
                   
                 3 
                 E6 (25%) + N2 (75%) 
                 Doxycyline 
               
               
                   
                 4 
                 N2 
                 Doxycyline 
               
               
                   
                 5 
                 N2 
                 Doxycyline 
               
               
                   
                 6 
                 N2 (50%) + B27 (50%) 
               
               
                   
                 7 
                 B27 
               
               
                   
                 8-36 
                 B27 
                 B/G/C/A/D 
               
               
                   
                   
               
               
                   
                 E8: Essential 8 Medium 
               
               
                   
                 E6: Essential 6 Medium 
               
               
                   
                 N2: DMEM/F12 medium with N2 supplement (1:100) 
               
               
                   
                 B27: neurobasal medium with B27 supplement (1:50) 
               
               
                   
                 Doxycyline (0.5 μg/ml) 
               
               
                   
                 Y-27632 (10 μM) 
               
               
                   
                 B/G/C/A/D: BDNF(20 ng/ml)/GDNF(10 ng/ml)/Dibutyryl cAMP(0.5 mM)/AscorbicAcid(0.2 mM)/DAPT(10 μM) 
               
             
          
         
       
     
         [0050]    Quantitative Real-Time PCR (qRT-PCR). 
         [0051]    Total RNA was extracted using the RNeasy Mini kit (Qiagen). Reverse transcription was performed using MuLV reverse transcriptase (Applied biosystems) and Oligo(dT) primers. qRT-PCR was performed using SYBR Green PCR Master Mix (Applied Biosystems) and IQ5 RT-PCR detection system (Bio-rad). All primer sequences are listed in Table 1. Relative expression of each gene was normalized to the 18S rRNA. 
         [0052]    Western Blot. 
         [0053]    Total cellular proteins were extracted with RIPA buffer (Sigma-Aldrich) containing a protease and phosphatase inhibitor cocktail (Calbiochem). SDS-PAGE was performed with 50 μg total cellular proteins per lane using 4-12% gradient Tris-glycine gels (Lonza). Western blot was performed using Quantitative Western Blot System (LI-COR Biosciences) following the manufacturer&#39;s instructions. The primary antibodies were: mouse anti-FALG M2 (Sigma-Aldrich), rabbit anti-Atoh1 (Millipore) and mouse anti-β-actin (Sigma-Aldrich). Secondary antibodies were labeled with IRDye infrared dyes and protein levels were quantified with Odyssey Infrared Imaging System (LI-COR Biosciences). 
         [0054]    Immunofluorescence and Cell Counting. 
         [0055]    Differentiated cells were fixed in 4% PFA/1% sucrose in PBS (pH 7.4) at room temperature, and blocked with 5% normal goat serum and 0.2% Triton X-100. Primary antibodies (Table 2) were diluted in 5% normal goat serum and incubated with samples overnight at 4° C. Cy3 and Alexa 488 labeled secondary antibodies were applied for 2 hours. Samples were counterstained with DAPI and mounted on glass slides using ProLong anti-fade kit (Life Technologies). 
         [0056]    The percentage of marker positive cells was determined in samples derived from at least three independent experiments. In Adobe Photoshop software, images from 10 randomly selected fields were used for counting the number of DAPI-positive cells expressing a specific marker. 
         [0057]    Electrophysiological Recordings. 
         [0058]    Voltage-clamp or current-clamp recordings were performed at 35° C. in a chamber perfused with regular Artificial Cerebrospinal Fluid (ACSF; in mM: NaCl, 124; KCl, 2.5; MgCl 2 , 1.3; CaCl 2  2.5; NaH 2 PO 4 , 1; NaHCO 3 , 26.2; glucose, 20; pH 7.4, equilibrated with 95% O 2  and 5% CO 2 , ˜310 mosm) which flowed at 4 ml/min. Patch electrodes were pulled from borosilicate glass and had resistances of 2-4.0 MΩ when filled with an intracellular solution (in mM: KMeSO 4 , 135; KCl, 5; HEPES, 5; EGTA free acid, 0.25; Mg-ATP, 2; GTP, 0.5; phosphocreatine-tris, 10; pH 7.3, ˜290 mosm). 
         [0059]    Neurons were identified using a 10× objective mounted on an upright microscope with transmitted light, and their neuronal somata were then visualized through a 40× water immersion objective using infrared differential interference contrast (DIC) optics. The cell somatic recordings were made using an Axopatch 200B amplifier in combination with pClamp 9.0 software (Molecular Devices). Neurons were voltage-clamped at −80 mV. R series  and R input  were monitored using a 2.5 mV 100 ms depolarizing voltage step in each recording sweep. Current traces were filtered at 5 kHz, and digitized at 10 kHz using a Digidata 1322A interface, and stored for off-line analysis. Leak and capacitative currents were corrected by subtracting a scaled current elicited by a +2.5 mV step from the holding potential. 
         [0060]    For current clamp recording, the same Axopatch 200B amplifier was used; whole cell mode was achieved initially in the voltage clamp configuration. Then, the recording was switched into current clamp mode. The resting membrane potential was monitored for more than five minutes. The experiment was discontinued if the resting membrane potential became more positive than −40 mV. The action potential was continually monitored for five minutes, and if there was no threshold change, the reagent perfusion commenced. All reagents were bought from Sigma (St Louis, Mo.) except TTX (abcam, Cambridge, Mass.) and ML252 (Vanderbilt Center for Neuroscience Drug Discovery). 
         [0061]    High-Performance Liquid Chromatography (HPLC) Analysis. 
         [0062]    On day 36 of differentiation, medium was replaced by HBSS buffer with addition of 56 mM KCl (200 μl per well in 24-well plates) and incubated for 15 min at 37° C. Medium was collected and centrifuged (15,000 g for 15 min at 4° C.) to clear cell debris. Samples were immediately frozen in liquid nitrogen and stored at −80° C. For HPLC analysis, samples were thawed and concentrated using a vacuum (Savant SDP 121P ThermoSci) connected with refrigerated vapor trap (Savant RVT 5105 ThermoSci) and the freeze dried samples were resuspended in 10 mM perchloric acid. Monoamines were analyzed by HPLC-ECD (Electrochemical Detection) by dual channel coulchem III electrochemical detector (Model 5300, ESA, Inc. Chelmsford, Mass., USA), and monoamines were separated by using a reverse phase C18 column (3 mm×150 mm C-18 RP-column, Acclaim Polar advantage II, Thermo Scientific) with a flow rate of 0.600 mL/min. Monoamine concentrations were quantified by comparison of the area under the curve (AUC) to known standard dilutions. 
         [0063]    6-OHDA Treatment in DA Neurons and LDH Analysis. 
         [0064]    Neuron culture medium was changed to neurobasal medium before treatment. 6-OHDA was freshly prepared in vehicle solution (ascorbic acid (0.15%) in H 2 O) and quickly added to the neuron culture. Control cells were treated with vehicle solution alone. After 15 min at 37° C., the medium was removed and neurons were gently washed twice with Neurobasal Medium. 200 μl neuron culture medium (Neurobasal Medium with B27 Supplement) was added to each well and further incubated for 24 h. Cytotoxicity induced by 6-OHDA was measured using LDH Cytotoxicity Detection Kit (Roche) following the manufacturer&#39;s protocol. The percentage cytotoxicity was calculated using the following equation: Cytotoxicity (%)=(Experiment Value−Low Control)/(High Control−Low Control)×100. (Low control: culture medium; High control: total cell lysate). 
         [0065]    Data Analysis and Statistics. 
         [0066]    All results reported here represent at least three independent replications. Statistical analysis was performed using Prizm software (GraphPad). Post-hoc tests included the Students t-test and the Tukey multiple comparison tests as appropriate. All data are represented as mean value±standard error of mean (SEM). 
         [0067]    For neurophysiological recordings, the recorded data were first visualized with Clampfit 9.2, and exported to Matlab (Mathworks, Natick, Mass.) for further analysis and plotting. The recording traces are visualized with Igor 6.0 (WaveMetrics, Portland, Oreg.). All group data are reported as mean±STD except otherwise stated. 
       Results 
       [0068]    Atoh1 is Induced During the Neuronal Differentiation of Human PSCs. 
         [0069]    We followed a dual-SMAD inhibition protocol (Chambers et al., 27 NAT. BIOTECHNOL. 275-80 (2009)) for differentiating human iPSCs into neurons. Differentiated cells first expressed neurectodermal marker (PAX6) and neural rosette marker (NESTIN) at day 10 of differentiation ( FIG. 1A , left). Mature neurons at day 40 of differentiation expressed neuronal marker β-Tubulin III (TUJ1) and MAP2 ( FIG. 1A , right). We further examined the expression of various markers at differentiation day 0, 10 and 20 by quantitative real-time PCR (qRT-PCR), which confirmed the inhibition of pluripotency marker (NANOG) and the induction of neural markers (PAX6, Ngn2 and NEUROD1) ( FIG. 1B ). Next, we examined Atoh1 expression by qRT-PCR and found that ATOH1 was induced at differentiation day 10 and 20 when compared to undifferentiated cells ( FIG. 1B ). Western blotting also confirmed the induction of Atoh1 protein at differentiation day 20 ( FIG. 1C ). These data suggested that Atoh1 is involved in the neuronal conversion of human PSCs, which warranted further study. 
         [0070]    Ectopic Atoh1 Expression Induces Highly Efficient Neuronal Conversion of Human PSCs. 
         [0071]    To address whether Atoh1 induction is sufficient for the neuronal differentiation of human PSCs, we established a lentivirus-mediated gene delivery system to achieve Dox-inducible Atoh1 expression in human PSCs. We constructed a Tet-On lentiviral vector that harbors human Atoh1 transgene with N-terminal FLAG tag (SEQ ID NO:3) ( FIG. 2A ). Human iPSCs and ESCs were infected with Atoh1 lentivirus to establish stable cell lines (Atoh1-iPSC, and Atoh1-ESC) after puromycin selection. Dox treatment for 48 h in Atoh1-iPSCs induced Atoh1 expression as determined by immunoblotting against FLAG tag, and transgenic Atoh1 expression was turned off after Dox withdrawal ( FIG. 2B ). Immunostaining against FLAG tag also confirmed Atoh1 induction after 3-day Dox treatment and the silencing of Atoh1 transgene after Dox withdrawal for 3 days ( FIG. 2C ). 
         [0072]    Next, we induced ectopic Atoh1 expression in PSCs for neuronal differentiation following a protocol outlined in  FIG. 2D  (also see details in Materials and Methods). Atoh1-iPSCs were maintained in a feeder-free culture system, and Atoh1 was induced by Dox for 5 days to drive neuronal conversion. After Dox withdrawal, neuronal precursors were passaged and allowed to further mature in vitro. On differentiation day 6, Atoh1 induced robust expression of the neuronal differentiation marker TUJ1, which was not detected in Dox-untreated cells ( FIG. 2E ). On day 7, cells were dissociated and replated on surfaces pre-coated for neuron culture. Two days after cell passaging, Atoh1-induced cells adhered and formed neuronal processes. In contrast, control cells failed to attach or grow in neuron culture medium ( FIG. 2F ). After further maturation in vitro for 30 days, Atoh1-induced neurons co-expressed the synaptic vesicle protein Synapsin and neuronal marker (TUJ1), demonstrating the establishment of synaptic terminals and neuronal maturation ( FIG. 2G ). We also replicated these results in Atoh1-ESC, in which Atoh1 also initiated the neuronal differentiation process and generated mature neurons ( FIG. 8 ). 
         [0073]    To further optimize the Atoh1-mediated differentiation strategy, we asked what is the minimum time of Atoh1 induction for successful neuronal conversion. Equal numbers of Atoh1-iPSCs received different durations of Dox treatment (1 to 5 days), after which cells were replated and allowed to mature in vitro for additional 30 days. Cells with Dox treatment for 1 day failed to attach after replating. In contrast, Dox treatment for 2 to 5 days successfully differentiated iPSCs into neurons expressing TUJ1 and MAP2 ( FIG. 9 ). By quantifying the number of TUJ1+/MAP2+ neurons, we found that longer Atoh1 induction time did not increase the purity of Atoh1-induced neurons ( FIG. 2H ), but did significantly increase the yield of neurons, especially when comparing Atoh1 induction for 4-5 days to 2-day induction ( FIG. 2I ). To compare the level of neurogenic signaling before and after silencing ectopic Atoh1 expression, we treated Atoh1-iPSCs with Dox for 2 days and then withdrew Dox for 3 days ( FIGS. 2J and 2K ). As determined by qRT-PCR, Atoh1 showed 49-fold up-regulation after Dox treatment and decreased after Dox withdrawal. Two neurogenic transcription factors (NEUROD1 and Ngn2) showed 6- and 5-fold induction, respectively, in response to Atoh1 induction. After Dox withdrawal, their expression did not decrease but increased further to 261- and 189-fold higher than control cells, respectively. These results suggest that ectopic Atoh1 expression in PSCs initiates a neurogenic program that becomes self-sustaining after the withdrawal of ectopic Atoh1. 
         [0074]    To determine the subtype specification of Atoh1-induced neurons, we characterized neurons induced from Atoh1-iPSCs with various neuron subtype markers ( FIG. 3 ). By day 36 of differentiation, ˜35% of Atoh1-induced neurons expressed tyrosine hydroxylase (TH), the rate-limiting enzyme in DA synthesis and a widely-used DA neuron marker. Fewer than 10% of total cells expressed glutamate decarboxylase (GAD67) and serotonin, markers for GABAergic and serotonergic neurons, respectively. Glutamatergic neurons expressing Vesicular Glutamate Transporter 1 (VGluT1) were not detected in Atoh1-induced neurons. 
         [0075]    Atoh1-Mediated Differentiation of Human PSCs into DA Neurons. 
         [0076]    We found that ectopic Atoh1 expression preferentially drives the differentiation of human PSCs to TH-expressing neurons, suggesting a DA lineage specification. Two morphogens (SHH and FGF-8b) for neural patterning have been widely used to drive DA lineage specification during the neuronal conversion of human PSCs. We combined Atoh1 induction with these two morphogens to differentiate PSCs into DA neurons, following a protocol outlined in  FIG. 4A . Ectopic Atoh1 expression alone induced multiple DA neuron markers, such as FOXA2, NURR1, LMX1A, OTX2, Ngn2, TH, DAT and VMAT2, most of which were further upregulated significantly by combining Atoh1 induction with SHH and FGF-8b ( FIG. 4B ). At day 36 of differentiation, Atoh1-induced neurons derived from both iPSCs and ESCs co-expressed the neuronal marker (TUJ1) and the DA neuron marker (TH) ( FIG. 4C  and FIG.  10 ). The Atoh1-mediated protocol yielded DA neurons from human iPSCs and ESCs with 82±8% and 84±9% purity, respectively, as determined by the percentage of TH+/TUJ1+ cells over DAPI+ cells ( FIG. 4D ). 
         [0077]    In order to store Atoh1-induced DA neurons, Atoh1-induced DA neuron precursor cells (NPCs) at differentiation day 7 were cryopreserved, and these cells showed high viability and neuronal morphology when being recovered from cryopreservation and cultured for 7 days ( FIG. 4E ). From 1×10 6  iPSCs, the Atoh1-mediated protocol generated 4.7×10 6  DA NPCs, and yielded 2.7×10 6  or 1.4×10 6  DA neurons after direct cell passaging or cryopreservation, respectively ( FIG. 4F ). 
         [0078]    Next, we analyzed the expression of midbrain DA neuron markers in Atoh1-induced neurons. By differentiation day 36, these neurons expressed the midbrain DA neuron markers FOXA2, NURR1, Engrailed 1 (EN1), TH, G-protein-regulated inward-rectifier potassium channel 2 (GIRK2) and dopamine transporter (DAT) ( FIGS. 5A, 5B, 5C, 5D and 5E ), which are also expressed in midbrain DA neurons located in substantia nigra pars compacta (SNpc). These Atoh1-induced DA neurons show extensive TUJ1+ nerve fiber growth and robust expression of synaptic vesicle protein Synapsin ( FIG. 5E ). GABAergic (GAD67+) or serotonergic (serotonin+) neurons were not detected in Atoh1-induced neurons derived from iPSCs, and undifferentiated iPSCs (SOX2+ or OCT4+) were also not detected ( FIG. 11 ). 
         [0079]    Functional Characterization of Atoh1-Induced DA Neurons. 
         [0080]    We asked if Atoh1-induced DA neurons exhibit key physiological properties of mature midbrain DA neurons. Dopamine release was quantified in Atoh1-induced DA neurons at differentiation day 36. HPLC analysis demonstrated the release of dopamine and its metabolites evoked by KCl depolarization ( FIGS. 5F and 5G ). 
         [0081]    It has been well established that midbrain DA neurons are pacemaker neurons that discharge spontaneously at a rate between 1 and 10 Hz with an average rate of 4.5 Hz. To test whether Atoh1-induced DA neurons display similar electrophysiological properties to primary midbrain DA neurons, we performed patch-clamp recording in Atoh1-induced DA neurons (n=57) derived from human iPSCs at differentiation day 36-49 ( FIG. 6A ). In voltage-clamp experiments, the series input resistance of these Atoh1-induced DA neurons was 7.7±3 MΩ; the input resistance was 295.8±174.5 MΩ, and the average resting membrane potential was 75.3±9.9 mV. 64.9% of these neurons showed spontaneous spiking activity ( FIG. 6B , n=37) with an amplitude of 66.1±18.3 mV and a mean frequency of 6.2±4.7 Hz (n=37;  FIG. 6C ). 26.3% of these neurons discharged action potentials during current injection either by depolarization or hyperpolarization (n=15), and only 8.7% (n=5) of these neurons did not have typical action potential either by positive or negative current injection. 
         [0082]    We further investigated the maturation of intrinsic ion channels in Atoh1-induced DA neurons. As shown in  FIG. 6D , we first injected current (70 pA) to a neuron to depolarize the membrane potential. This induced a train of action potentials ( FIG. 6D , up panel), which were completely blocked by the sodium channel blocker TTX (0.5 μM, 5 minutes administration,  FIG. 6D , middle panel). This effect was reversed by TTX washout, after which the action potentials recovered in 17 minutes ( FIG. 6D , bottom panel). 
         [0083]    Midbrain DA neurons have been found to also have KCNQ potassium channels that contribute to their tonic spontaneous activity, and during hyperpolarization these neurons display a typical sag in voltage. Here, we injected negative current to hyperpolarize the membrane of Atoh1-induced neurons ( FIG. 6E ). This negative current injection produced hyperpolarization sag and rebound action potentials that resemble the tonic spontaneous spiking activity. Both events were blocked by ML252 (5 μM, a KCNQ2 inhibitor). We further investigated the voltage-sensitive sodium and potassium currents in voltage-clamp mode by elevating membrane potentials to different levels ( FIG. 6F , left). After the treatment of sodium and potassium channel inhibitors (TTX (0.5 μM) and 4-AP (25 μM), respectively), both sodium and potassium currents are significantly attenuated ( FIG. 6F , right). 
         [0084]    Atoh1-Induced DA Neurons are Sensitive to 6-OHDA Treatment. 
         [0085]    6-hydroxydopamine (6-OHDA) is a neurotoxin widely used to induce neurotoxicity both in vivo and in vitro to model DA neuron loss in PD pathogenesis. In neuron cultures from the substantia nigra of neonatal rat brains, 6-OHDA treatment at 40 μM causes selective DA neuron loss without affecting GABA neurons. Here, we tested the response of Atoh1-induced DA neurons to 6-OHDA treatment. 6-OHDA treatment (40 μM and 100 μM) for 15 minutes caused morphological signs of neuron death, including cell condensation and neurite fragmentation ( FIG. 7A ), and neuron death was also confirmed by LDH Cytotoxicity assay ( FIG. 7B ). Thus, Atoh1-induced DA neurons derived from human iPSCs are sensitive to 6-OHDA treatment at a concentration that selectively damages primary DA neurons isolated from substantia nigra. 
       Discussion 
       [0086]    Human iPSCs provide a unique cell resource for establishing patient-specific disease models and for testing potential therapies. Due to the limited resource of human neurons, lineage-specific neurons derived from human iPSCs are the most desirable cells for modeling various neurological disorders, such as midbrain DA neurons for PD, striatal GABAergic neurons for Huntington&#39;s disease, and cholinergic motor neurons for amyotrophic lateral sclerosis. It is critical to develop highly efficient protocols for neuronal conversion in PSCs, in order to translate current iPSC-derived neuron models from small-scale laboratory applications to large-scale personalized drug testing platforms. Proneural transcription factors are core drivers of neurogenesis, and multiple members in this family (e.g., ASCL1, Ngn2 and NeuroD1) have been used to differentiate PSCs into neurons and more recently transdifferentiate somatic cells into neurons. We now show that Atoh1 is a highly efficient driver for neuronal conversion in PSCs, and Atoh1 induction in combination with cell extrinsic factors rapidly differentiates human PSC to functional DA neurons at high purity. 
         [0087]    Multiple proneural transcription factors, e.g., ASCL1, Ngn2, and NeuroD1, are activated during the neuronal conversion of human PSCs, which initiate and sustain a neurogenic transcriptional network. We identified Atoh1 as a proneural transcription factor that is also upregulated during this neuronal conversion process. By using a Tet-On gene expression system to transiently induce ectopic Atoh1 expression in PSCs, we found that Atoh1 induction alone is sufficient for highly efficient neuronal conversion in PSCs. We further determined that 2 days are the minimal amount of time and that 4-5 days are ideal for transient Atoh1 induction in order to achieve successful neuronal conversion in PSCs. Several studies have suggested that Atoh1 and other proneural transcription factors are able to activate a neurogenic transcription factor network that over time becomes self-supporting. We also found that the neurogenic transcription factor NeuroD1 and Ngn2 were induced by ectopic Atoh1 and their expression was sustained after the silencing of exogenous Atoh1. This demonstrates that Atoh1-induced neuronal differentiation program in PSCs can become self-supporting and independent of exogenous Atoh1. It is noteworthy to mention that although exogenous Atoh1 was not detectable by western blotting after Dox withdrawal, Atoh1 expression did not return to baseline. This result is consistent with our previous result in  FIG. 1B  showing that endogenous Atoh1 is upregulated during the neuronal conversion of PSCs. The persistence of endogenous Atoh1 expression can be explained by the evidence that Atoh1 protein binds to its own enhancer to establish an autoregulation loop for maintaining its expression. Overall, our results support the mechanism that transient Atoh1 expression in PSCs can activate a cell-intrinsic program for neuronal commitment (a neuro-programming process). This process might share similar features to somatic cell reprogramming, where transient expression of reprogramming transcription factors induces the remodeling of epigenetic markers and drives cells into a self-sustaining pluripotent status. The epigenetic dynamics during this neuro-programming process warrants further studies, and a deep understanding of this process might lead to more potent approaches for converting both PSCs and somatic cells into neurons. 
         [0088]    Proneural transcription factors have been shown to coordinately control the acquisition of a generic neuronal fate and the neuron subtype specification [9]. The functions of proneural transcription factors during neural development are strongly influenced by the spatial and temporal context including multiple modifiers such as transcriptional cofactors and cell extrinsic factors. Atoh1 has been found to drive the differentiation of numerous neuronal populations (e.g., cerebellar granule neurons, spinal cord neurons and inner ear hair cells), as well as diverse nonneuronal cell types (e.g., Merkel cells and intestinal secretory lineages), suggesting that the functions of Atoh1 depend on specific developmental contexts. When ectopic Atoh1 was expressed in the context of human PSCs, we detected a high percentage of neurons expressing the DA marker TH, and multiple DA neuron markers were induced in response to ectopic Atoh1 expression. Moreover, SHH and FGF8b, two neural patterning morphogens for DA specification, further promoted the expression of DA neuron markers and increased the efficiency of Atoh1-induced DA neuron conversion. These results suggest that Atoh1-induced neurons respond to extrinsic factors for generating lineage-specific neurons. A recent report shows that, in embryonic bodies derived from mouse ESCs, Atoh1 induction in combination with extrinsic factors promotes the generation of cerebellar granule neurons. Atoh1 induction has also been found to induce inner ear hair cell-like cells from mouse ESC-derived embryonic bodies. Overall, it is possible to derive different neuron subtypes from human or mouse PSCs by controlling the temporal induction of Atoh1 in various differentiation stages of PSCs and combining Atoh1 induction with different cell intrinsic and extrinsic factors (e.g., neuron-subtype-specific transcription factors or morphogens). Other proneural transcription factors also show this plasticity in specifying various neuron subtypes. For example, ASCL1 has been used to generate glutamatergic/GABAergic, DA and cholinergic neurons. 
         [0089]    We established a highly efficient Atoh1-mediated approach for generating lineage-specific functional neurons from human PSCs. Inducible Atoh1 transgene was delivered using a single-vector Tet-On lentivirus and is stable in PSCs after &gt;15 passages with puromycin selection (data not shown). Atoh1-induced DA neuron cultures derived from human iPSCs and ESCs showed &gt;80% pan-neuronal purity and &gt;80% DA subtype purity. This protocol yields DA neurons from PSCs with a rate of return of &gt;250% or &gt;100% after the cyropreservation of Atoh1-induced NPCs. The cryopreservation of Atoh1-induced DA NPCs will enable us to establish patient-specific DA neuron banks After in vitro maturation, Atoh1-induced DA neurons expressed midbrain DA neuron markers (such as GIRK2, NURR1, FOXA2 and DAT) and exhibited robust synapse formation. The functional maturation of these DA neurons was further confirmed by DA release and spontaneous spiking activity. Overall, Atoh1-induced DA neurons derived from human iPSCs recapitulate key features of primary midbrain DA neurons, making this Atoh1-mediated approach particularly applicable for PD modeling using patient-derived iPSCs. It has been reported that primary DA neurons but not GABAergic neurons, from the substantia nigra of neonatal rat brains, are sensitive to 6-OHDA treatment at low concentration (40 μM). We also demonstrated that Atoh1-induced DA neurons showed similar 6-OHDA sensitivity to primary midbrain DA neurons, supporting that Atoh1-induced DA neurons can serve as a reliable neurotoxicity model for PD. 
         [0090]    Due to the use of genome-integrating lentivirus for ectopic Atoh1 expression, our Atoh1-induced DA neurons will not be optimal for cell replacement therapy. However, we found that transient Atoh1 expression for 3-5 days is sufficient for highly efficient neuronal conversion in PSCs. Thus, non-integrating viruses (e.g., adenovirus and Sendai virus) should be suitable to overcome the limitation due to using lentivirus. More recently, multiple virus-free systems based on mRNA or protein delivery have been established to generate transgene-free human iPSCs [42-44], and these approaches can be applied to generate transplant-ready Atoh1-induced DA neurons. It is also noteworthy that chemical compounds that induce Atoh1 expression have been identified and patented (Patent Publication Number: US20090232780A1), thus providing another potential method for Atoh1 induction in PSCs that warrants further testing. 
       Conclusion 
       [0091]    Atoh1 is a potent driver for highly efficient neuronal conversion in human PSCs. Atoh1 induction in combination with cell extrinsic factors differentiates PSCs into functional DA neurons in high purity. Atoh1-induced DA neurons derived from human iPSCs recapitulate key features of primary midbrain DA neurons and provide a useful cell model for studying the pathogenesis of both familial PD and, more importantly, sporadic PD, and testing potential PD therapies. 
       Example 2 
     Atoh1-Mediated Differentiation of Human ESCs and NSCs into DA Neurons 
       [0092]    Atoh1-ESCs were differentiated following the protocol as shown in  FIG. 4A  and Table 2. Atoh1-induced neuron cultures at Day 36 co-expressed neuronal and DA neuron markers, β-tubulin III (TUJ1) and tyrosine hydroxylase (TH), respectively ( FIG. 12A ). The Atoh1 induction protocol yielded DA neurons from human ESCs with 84±9% purity as determined by the percentage of TUJ+/TH+ cells ( FIG. 12A ). Atoh1-NSCs were differentiated following the protocol as shown in  FIG. 4A  and Table 3. Atoh1-induced neuron cultures at Day 36 co-expressed neuronal marker (β-tubulin III, TUJ1) and DA neuron marker (tyrosine hydroxylase, TH) ( FIG. 12B ). The Atoh1 induction protocol yielded DA neurons from human NSCs with 82±8% purity as determined by the percentage of TUJ+/TH+ cells ( FIG. 12B ). 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 DA neuron differentiation media used in Atoh1- 
               
               
                 induced protocol as shown in FIG. 4A 
               
             
          
           
               
                   
                 Day 
                 Medium 
                 Additives 
               
               
                   
                   
               
               
                   
                 0 
                 E8 
                 Y-27632 
               
               
                   
                 1 
                 E6 (75%) + N2 (25%) 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 2 
                 E6 (50%) + N2 (50%) 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 3 
                 E6 (25%) + N2 (75%) 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 4 
                 N2 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 5 
                 N2 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 6 
                 N2 (50%) + B27 (50%) 
               
               
                   
                 7 
                 B27 
               
               
                   
                 8-36 
                 B27 
                 B/G/T/C/A/D 
               
               
                   
                   
               
               
                   
                 E8: Essential 8 Medium 
               
               
                   
                 E6: Essential 6 Medium 
               
               
                   
                 N2: DMEM/F12 medium with N2 supplement(1:100) 
               
               
                   
                 B27: neurobasal medium with B27 supplement(1:50) 
               
               
                   
                 Doxycyline (0.5 μg/ml); Y-27632 (10 μM) 
               
               
                   
                 SHH: Sonic Hedgehog (200 ng/ml); FGF-8b (100 ng/ml) 
               
               
                   
                 B/G/T/C/A/D: BDNF(20 ng/ml)/GDNF(10 ng/ml)/TGF-β3(1 ng/ml)/Dibutyryl cAMP(0.5 mM)/AscorbicAcid(0.2 mM)/DAPT(10 μM) 
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 DA neuron differentiation media used in 
               
               
                 Atoh1-induced protocol as shown in FIG. 4A 
               
               
                 NSC Differentiation 
               
             
          
           
               
                   
                 Day 
                 Medium 
                 Additives 
               
               
                   
                   
               
               
                   
                 0 
                 N2 
                 bFGF 
               
               
                   
                 1 
                 N2 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 2 
                 N2 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 3 
                 N2 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 4 
                 N2 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 5 
                 N2 
                 Doxycyline/SHH/FGF-8b 
               
               
                   
                 6 
                 N2 (50%) + B27 (50%) 
               
               
                   
                 7 
                 B27 
               
               
                   
                 8-36 
                 B27 
                 B/G/T/C/A/D 
               
               
                   
                   
               
               
                   
                 E8: Essential 8 Medium 
               
               
                   
                 E6: Essential 6 Medium 
               
               
                   
                 N2: DMEM/F12 medium with N2 supplement(1:100) 
               
               
                   
                 B27: neurobasal medium with B27 supplement(1:50) 
               
               
                   
                 bFGF(10 ng/ml); SHH: Sonic Hedgehog (200 ng/ml); FGF-8b (100 ng/ml) 
               
               
                   
                 B/G/T/C/A/D: BDNF(20 ng/ml)/GDNF(10 ng/ml)/TGF-β3(1 ng/ml)/Dibutyryl cAMP(0.5 mM)/AscorbicAcid(0.2 mM)/DAPT(10 μM) 
               
             
          
         
       
     
       Example 3 
     Atoh1-Mediated Differentiation of Human iPSCs into Dopaminergic (DA) Neurons 
       [0093]    As shown in  FIG. 4A , we established protocols for differentiating human iPSCs into DA neurons by Atoh1 and other additives (sonic hedgehog and FGF-8b). The recipes for cell culture media are listed in Table 2. Cells were plated (4×10 4  cells per cm 2 ) on matrigel (BD) in Essential 8 Medium (Life Technologies) with ROCK inhibitor (Y-27632, Stemgent). From Day 1 to Day 7, cell culture medium was changed every day. From Day 8 to Day 36, half of the cell culture medium was changed every 3-4 days. At day 36 of differentiation, these neurons co-expressed neuronal marker (β-tubulin III, TUJ1) and DA neuron marker (tyrosine hydroxylase, TH), the rate-limiting enzyme in the synthesis of dopamine ( FIG. 4C ). The Atoh1 induction protocol yielded DA neurons with 89±6% purity as determined by the percentage of TUJ+/TH+ cells ( FIG. 4D ). Moreover, the Atoh1-induced DA neurons also co-expressed other midbrain DA neuron markers, such as G-protein-regulated inward-rectifier potassium channel 2 (GIRK2), forkhead box protein A2 (FOXA2) and dopamine transporter (DAT) ( FIGS. 6A, 6B and 6C ), which are also expressed in substantia nigra pars compacta (SNPC) DA neurons. By day 60, these DA neurons show extensive TUJ1+ nerve fiber growth and robustly express mature neuron marker (synapsin) ( FIG. 6C ). 
       Example 4 
     Synthetic mRNA Encoding Atoh1 Drives the Differentiation of Human iPSCs into Neurons 
       [0094]    We further tested Atoh1 induction in iPSCs by using synthetic mRNAs encoding Atoh1. A cDNA encoding Atoh1 protein with an N-terminal FLAG tag was cloned into a DNA vector with T7 promoter for in vitro mRNA synthesis. The synthesis of Atoh1 mRNA was performed by using the mMESSAGE mMACHINE® T7 ULTRA Transcription Kit (Ambion). Synthetic Atoh1 mRNA was transfected into human iPSCs cultured in the 24-well plate (0.25 μg mRNA per well) using a lipid-based mRNA transfection reagent. Ectopic Atoh1 expression was detected in &gt;90% of the Atoh1-mRNA-transfected cells ( FIG. 13A ) but not in untransfected control cells ( FIG. 13B ), as determined by FLAG immunofluorencence staining After iPSCs received daily transfection of Atoh1 mRNA for 4 days, cells were replated (3×10 5  cells per cm 2 ) on dishes pre-coated with poly-D-Lysine (1 μg/ml) and laminin (1 μg/ml). Neurons after being matured in vitro for 20 days co-express the neuronal marker (TUJ1) and the mature neuron markers (MAP2 and Synapsin) ( FIGS. 13C and 13D ). Overall, these results demonstrate that Atoh1 can be induced in human SCs by delivering synthetic Atoh1 mRNA, and Atoh1 mRNA delivery drives neuronal conversion of human SCs. 
       Example 5 
     Neurogenin 2 can be Induced by Atoh1 and Also Drives the Differentiation of Human iPSCs into Neurons 
       [0095]    Atoh1 can activate crucial neurogenic transcription factors, such as NeuroD1, 2, 6 and Nhlh1, 2, to initiate a neuronal differentiation program that later becomes self-supporting and Atoh1-independent. Here, we found that Atoh1 induction in human iPSCs for 2 days activated known Atoh1 target NeuroD1 and the proneural transcription factor Neurogenin 2 that has not yet been defined as an Atoh1 target gene ( FIG. 14A ), as determined by quantifying gene expression using quantitative real-time PCR. We further tested if the Atoh1 target gene Neurogenin 2 can also drive lineage-specific neuronal conversion of human iPSCs. We applied the same lentivirus-mediated gene delivery system as that for Atoh1 expression ( FIG. 2A ) to achieve Dox inducible expression of Neurogenin 2 in human SCs. By following the differentiation strategy as shown in  FIG. 14B , we successfully generated TH+/TUJ1+DA neurons from human iPSCs by inducing Neurogenin 2 in combination with two morphogens (sonic hedgehog (SHH) and FGF-8b) ( FIG. 14C ). These results demonstrate that Atoh1-induced gene targets (e.g. Neurogenin 2) can be used to drive lineage-specific neuronal conversion of human SCs.