The intrinsic electrophysiological properties of neurons derived from mouse embryonic stem cells overexpressing neurogenin-1

Transcription Factor-Based Fate Specification and Forward Programming for Neural Regeneration

Traditionally, in vitro generation of donor cells for brain repair has been dominated by the application of extrinsic growth factors and morphogens. Recent advances in cell engineering strategies such as reprogramming of somatic cells into induced pluripotent stem cells and direct cell fate conversion have impressively demonstrated the feasibility to manipulate cell identities by the overexpression of cell fate-determining transcription factors. These strategies are now increasingly implemented for transcription factor-guided differentiation of neural precursors and forward programming of pluripotent stem cells toward specific neural subtypes. This review covers major achievements, pros and cons, as well as future prospects of transcription factor-based cell fate specification and the applicability of these approaches for the generation of donor cells for brain repair.

Electrophysiological properties of neurosensory progenitors derived from human embryonic stem cells

In severe cases of sensorineural hearing loss where the numbers of auditory neurons are significantly depleted, stem cell-derived neurons may provide a potential source of replacement cells. The success of such a therapy relies upon producing a population of functional neurons from stem cells, to enable precise encoding of sound information to the brainstem. Using our established differentiation assay to produce sensory neurons from human stem cells, patch-clamp recordings indicated that all neurons examined generated action potentials and displayed both transient sodium and sustained potassium currents. Stem cell-derived neurons reliably entrained to stimuli up to 20 pulses per second (pps), with 50% entrainment at 50 pps. A comparison with cultured primary auditory neurons indicated similar firing precision during low-frequency stimuli, but significant differences after 50 pps due to differences in action potential latency and width. The firing properties of stem cell-derived neurons were also considered relative to time in culture (31-56 days) and revealed no change in resting membrane potential, threshold or firing latency over time. Thus, while stem cell-derived neurons did not entrain to high frequency stimulation as effectively as mammalian auditory neurons, their electrical phenotype was stable in culture and consistent with that reported for embryonic auditory neurons.

Challenges for stem cells to functionally repair the damaged auditory nerve

INTRODUCTION

In the auditory system, a specialized subset of sensory neurons are responsible for correctly relaying precise pitch and temporal cues to the brain. In individuals with severe-to-profound sensorineural hearing impairment these sensory auditory neurons can be directly stimulated by a cochlear implant, which restores sound input to the brainstem after the loss of hair cells. This neural prosthesis therefore depends on a residual population of functional neurons in order to function effectively.

AREAS COVERED

In severe cases of sensorineural hearing loss where the numbers of auditory neurons are significantly depleted, the benefits derived from a cochlear implant may be minimal. One way in which to restore function to the auditory nerve is to replace these lost neurons using differentiated stem cells, thus re-establishing the neural circuit required for cochlear implant function. Such a therapy relies on producing an appropriate population of electrophysiologically functional neurons from stem cells, and on these cells integrating and reconnecting in an appropriate manner in the deaf cochlea.

EXPERT OPINION

Here we review progress in the field to date, including some of the key functional features that stem cell-derived neurons would need to possess and how these might be enhanced using electrical stimulation from a cochlear implant.

Combining topographical and genetic cues to promote neuronal fate specification in stem cells

There is little remedy for the devastating effects resulting from neuronal loss caused by neural injury or neurodegenerative disease. Reconstruction of damaged neural circuitry with stem cell-derived neurons is a promising approach to repair these defects, but controlling differentiation and guiding synaptic integration with existing neurons remain significant unmet challenges. Biomaterial surfaces can present nanoscale topographical cues that influence neuronal differentiation and process outgrowth. By combining these scaffolds with additional molecular biology strategies, synergistic control over cell fate can be achieved. Here, we review recent progress in promoting neuronal fate using techniques at the interface of biomaterial science and genetic engineering. New data demonstrates that combining nanofiber topography with an induced genetic program enhances neuritogenesis in a synergistic fashion. We propose combining patterned biomaterial surface cues with prescribed genetic programs to achieve neuronal cell fates with the desired sublineage specification, neurochemical profile, targeted integration, and electrophysiological properties.

BDNF profoundly and specifically increases KCNQ4 expression in neurons derived from embryonic stem cells

Neurons resembling the spiral ganglion neurons (SGNs) of the auditory nerve can be generated from embryonic stem cells through induced overexpression of the transcription factor Neurogenin-1 (Neurog1). While recapitulating this developmental pathway produces glutamatergic, bipolar neurons reminiscent of SGNs, these neurons are functionally immature, being characterized by a depolarized resting potential and limited excitability. We explored the effects of two neurotrophins known to be present in the inner ear, brain derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), on the electrophysiology of neurons following Neurog1 induction. Our data reveal a significant reduction in resting membrane potential (RMP) following neurotrophin exposure, with BDNF producing a more robust effect than NT-3. This effect was accompanied by a profound and specific upregulation of the KCNQ4 subtype, where a 9-fold increase was observed with quantitative PCR. The other neuronally expressed KCNQ subtypes (2, 3, and 5) exhibited upregulation which was 3-fold or less in magnitude. Quantitative immunohistochemistry confirmed the increase in KCNQ4 expression at the protein level. The present data show a novel link between BDNF and KCNQ4 expression, yielding insight into the restricted expression pattern of a channel known to play special roles in setting the resting potential of auditory cells and in the etiology of progressive high-frequency hearing loss.

Netrin-1-mediated axon guidance in mouse embryonic stem cells overexpressing neurogenin-1

Stem cell therapy holds great promise for treating neurodegenerative disease, but major barriers to effective therapeutic strategies remain. A complete understanding of the derived phenotype is required for predicting cell response once introduced into the host tissue. We sought to identify major axonal guidance cues present in neurons derived from the transient overexpression of neurogenin-1 (Neurog1) in mouse embryonic stem cells (ESCs). Neurog1 upregulated the netrin-1 axon guidance receptors DCC (deleted in colorectal cancer) and neogenin (NEO1). Quantitative polymerase chain reaction results showed a 2-fold increase in NEO1 mRNA and a 36-fold increase in DCC mRNA in Neurog1-induced compared with control ESCs. Immunohistochemistry indicated that DCC was primarily expressed on cells positive for the neuronal marker TUJ1. DCC was preferentially localized to the cell soma and growth-cones of induced neurons. In contrast, NEO1 expression showed less specificity, labeling both TUJ1-positive and TUJ1-negative cells as well as uninduced control cells. Axonal outgrowth was directed preferentially toward aggregates of HEK293 cells secreting a recombinant active fragment of netrin-1. These data indicate that DCC and NEO1 are downstream products of Neurog1 and may guide the integration of Neurog1-induced ESCs with target cells secreting netrin-1. Differential expression profiles for netrin receptors could indicate different roles for this guidance cue on neuronal and non-neuronal cells.

Intraspinal transplantation of neurogenin-expressing stem cells generates spinal cord neural progenitors

Embryonic stem (ES) cells and their derivatives are an important resource for developing novel cellular therapies for disease. Controlling proliferation and lineage selection, however, are essential to circumvent the possibility of tumor formation and facilitate the safe translation of ES-based therapies to man. Expression of appropriate transcription factors is one approach to direct the differentiation of ES cells towards a specific lineage and stop proliferation. Neural differentiation can be initiated in ES cells by expression of Neurogenin1 (Ngn1). In this study we investigate the effects of controlled Ngn1 expression on mouse ES (mES) cell differentiation in vitro and following grafting into the rat spinal cord. In vitro, Ngn1 expression in mES cells leads to rapid and specific neural differentiation, and a concurrent decrease in proliferation. Similarly transplantation of Ngn1-expressing mES cells into the spinal cord lead to in situ differentiation and spinal precursor formation. These data demonstrate that Ngn1 expression in mES cells is sufficient to promote neural differentiation and inhibit proliferation, thus establishing an approach to safely graft ES cells into the spinal cord.