Functional control of transplantable human ESC-derived neurons via optogenetic targeting

Optogenetic Stimulation Array for Confocal Microscopy Fast Transient Monitoring

Optogenetics is an emerging discipline with multiple applications in neuroscience, allowing to study neuronal pathways or serving for therapeutic applications such as in the treatment of anxiety disorder, autism spectrum disorders (ASDs), or Parkinson's disease. More recently optogenetics is opening its way also to stem cell-based therapeutic applications for neuronal regeneration after stroke or spinal cord injury. The results of optogenetic stimulation are usually evaluated by immunofluorescence or flow cytometry, and the observation of transient responses after stimulation, as in cardiac electrophysiology studies, by optical microscopy. However, certain phenomena, such as the ultra-fast calcium waves acquisition upon simultaneous optogenetics, are beyond the scope of current instrumentation, since they require higher image resolution in real-time, employing for instance time-lapse confocal microscopy. Therefore, in this work, an optogenetic stimulation matrix controllable from a graphical user interface has been developed for its use with a standard 24-well plate for an inverted confocal microscope use and validated by using a photoactivable adenyl cyclase (bPAC) overexpressed in rat fetal cortical neurons and the consequent calcium waves propagation upon 100 ms pulsed blue light stimulation.

Towards translational optogenetics

Optogenetics is widely used to interrogate the neural circuits underlying disease and has most recently been harnessed for therapeutic applications. The optogenetic toolkit consists of light-responsive proteins that modulate specific cellular functions, vectors for the delivery of the transgenes that encode the light-responsive proteins to targeted cellular populations, and devices for the delivery of light of suitable wavelengths at effective fluence rates. A refined toolkit with a focus towards translational uses would include efficient and safer viral and non-viral gene-delivery vectors, increasingly red-shifted photoresponsive proteins, nanomaterials that efficiently transduce near-infrared light deep into tissue, and wireless implantable light-delivery devices that allow for spatiotemporally precise interventions at clinically relevant tissue depths. In this Review, we examine the current optogenetics toolkit and the most notable preclinical and translational uses of optogenetics, and discuss future methodological and translational developments and bottlenecks.

Optogenetic Control of Human Stem Cell-Derived Neurons

Spontaneous and optogenetically evoked activities of human induced pluripotent stem cell (hiPSC)-derived neurons can be assessed by patch clamp and multi-electrode array (MEA) electrophysiology. Optogenetic activation of these human neurons facilitates the characterization of their functional properties at the single neuron and circuit level. Here we showcase the preparation of hiPSC-derived neurons expressing optogenetic actuators, in vitro optogenetic stimulation and simultaneous functional recordings using patch clamp and MEA electrophysiology.

The state of the art of biomedical applications of optogenetics

BACKGROUND AND OBJECTIVE

Optogenetics has opened new insights into biomedical research with the ability to manipulate and control cellular activity using light in combination with genetically engineered photosensitive proteins. By stimulating with light, this method provides high spatiotemporal and high specificity resolution, which is in contrast to conventional pharmacological or electrical stimulation. Optogenetics was initially introduced to control neural activities but was gradually extended to other biomedical fields.

STUDY DESIGN

In this paper, firstly, we summarize the current optogenetic tools stimulated by different light sources, including lasers, light-emitting diodes, and laser diodes. Second, we outline the variety of biomedical applications of optogenetics not only for neuronal circuits but also for various kinds of cells and tissues from cardiomyocytes to ganglion cells. Furthermore, we highlight the potential of this technique for treating neurological disorders, cardiac arrhythmia, visual impairment, hearing loss, and urinary bladder diseases as well as clarify the mechanisms underlying cancer progression and control of stem cell differentiation.

CONCLUSION

We sought to summarize the various types of promising applications of optogenetics to treat a broad spectrum of disorders. It is conceivable to expect that optogenetics profits a growing number of patients suffering from a range of different diseases in the near future.

In vivo conversion of dopamine neurons in mouse models of Parkinson's disease - a future approach for regenerative therapy?

Recent advances in cell reprogramming have made it possible to form new therapeutic cells within the body itself via a process called direct conversion or lineage reprogramming. A series of studies have shown that it is possible to reprogram resident glia into new neurons within the brain parenchyma. These studies opened up for the targeted attempts to achieve functional brain repair using in vivo conversion. Because of the relatively focal degeneration, Parkinson's Disease (PD) is an attractive target for both transplantation-based and in vivo conversion-based reparative approaches. Fetal cell transplants have provided proof-of-concept and stem cell-based therapies for PD are now on the verge of entering clinical trials. In the future, in vivo conversion may be an alternative to transplantation-based therapies.

Application of Optogenetics for Muscle Cells and Stem Cells

This chapter describes the current progress of basic research, and potential therapeutic applications primarily focused on the optical manipulation of muscle cells and neural stem cells using microbial rhodopsin as a light-sensitive molecule. Since the contractions of skeletal, cardiac, and smooth muscle cells are mainly regulated through their membrane potential, several studies have been demonstrated to up- or downregulate the muscle contraction directly or indirectly using optogenetic actuators or silencers with defined stimulation patterns and intensities. Light-dependent oscillation of membrane potential also facilitates the maturation of myocytes with the development of T tubules and sarcomere structures, tandem arrays of minimum contractile units consists of contractile proteins and cytoskeletal proteins. Optogenetics has been applied to various stem cells and multipotent/pluripotent cells such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) to generate light-sensitive neurons and to facilitate neuroscience. The chronic optical stimulation of the channelrhodopsin-expressing neural stem cells facilitates their neural differentiation. There are potential therapeutic applications of optogenetics in cardiac pacemaking, muscle regeneration/maintenance, locomotion recovery for the treatment of muscle paralysis due to motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Optogenetics would also facilitate maturation, network integration of grafted neurons, and improve the microenvironment around them when applied to stem cells.

Optogenetic Modulation of Neural Progenitor Cells Improves Neuroregenerative Potential

Neural progenitor cell (NPC) transplantation possesses enormous potential for the treatment of disorders and injuries of the central nervous system, including the replacement of lost cells or the repair of host neural circuity after spinal cord injury (SCI). Importantly, cell-based therapies in this context still require improvements such as increased cell survival and host circuit integration, and we propose the implementation of optogenetics as a solution. Blue-light stimulation of NPCs engineered to ectopically express the excitatory light-sensitive protein channelrhodopsin-2 (ChR2-NPCs) prompted an influx of cations and a subsequent increase in proliferation and differentiation into oligodendrocytes and neurons and the polarization of astrocytes from a pro-inflammatory phenotype to a pro-regenerative/anti-inflammatory phenotype. Moreover, neurons derived from blue-light-stimulated ChR2-NPCs exhibited both increased branching and axon length and improved axon growth in the presence of axonal inhibitory drugs such as lysophosphatidic acid or chondroitin sulfate proteoglycan. Our results highlight the enormous potential of optogenetically stimulated NPCs as a means to increase neuroregeneration and improve cell therapy outcomes for enhancing better engraftments and cell identity upon transplantation in conditions such as SCI.

New Pioneers of Optogenetics in Neuroscience

Optogenetics have recently increased in popularity as tools to study behavior in response to the brain and how these trends relate back to a neuronal circuit. Additionally, the high demand for human cerebral tissue in research has led to the generation of a new model to investigate human brain development and disease. Human Pluripotent Stem Cells (hPSCs) have been previously used to recapitulate the development of several tissues such as intestine, stomach and liver and to model disease in a human context, recently new improvements have been made in the field of hPSC-derived brain organoids to better understand overall brain development but more specifically, to mimic inter-neuronal communication. This review aims to highlight the recent advances in these two separate approaches of brain research and to emphasize the need for overlap. These two novel approaches would combine the study of behavior along with the specific circuits required to produce the signals causing such behavior. This review is focused on the current state of the field, as well as the development of novel optogenetic technologies and their potential for current scientific study and potential therapeutic use.

BrainPhys neuronal medium optimized for imaging and optogenetics in vitro

The capabilities of imaging technologies, fluorescent sensors, and optogenetics tools for cell biology are advancing. In parallel, cellular reprogramming and organoid engineering are expanding the use of human neuronal models in vitro. This creates an increasing need for tissue culture conditions better adapted to live-cell imaging. Here, we identify multiple caveats of traditional media when used for live imaging and functional assays on neuronal cultures (i.e., suboptimal fluorescence signals, phototoxicity, and unphysiological neuronal activity). To overcome these issues, we develop a neuromedium called BrainPhys™ Imaging (BPI) in which we optimize the concentrations of fluorescent and phototoxic compounds. BPI is based on the formulation of the original BrainPhys medium. We benchmark available neuronal media and show that BPI enhances fluorescence signals, reduces phototoxicity and optimally supports the electrical and synaptic activity of neurons in culture. We also show the superior capacity of BPI for optogenetics and calcium imaging of human neurons. Altogether, our study shows that BPI improves the quality of a wide range of fluorescence imaging applications with live neurons in vitro while supporting optimal neuronal viability and function.

Current media for neuronal cell and organoid cultures are suboptimal for functional imaging and optogenetics experiments, owing to phototoxicity and unphysiological performance. Here the authors formulate an optimised neuronal medium to support live cell imaging and electrophysiological activity.

Modulating electrophysiology of motor neural networks via optogenetic stimulation during neurogenesis and synaptogenesis

Control of electrical activity in neural circuits through network training is a grand challenge for biomedicine and engineering applications. Past efforts have not considered evoking long-term changes in firing patterns of in-vitro networks by introducing training regimens with respect to stages of neural development. Here, we used Channelrhodopsin-2 (ChR2) transfected mouse embryonic stem cell (mESC) derived motor neurons to explore short and long-term programming of neural networks by using optical stimulation implemented during neurogenesis and synaptogenesis. Not only did we see a subsequent increase of neurite extensions and synaptophysin clustering, but by using electrophysiological recording with micro electrode arrays (MEA) we also observed changes in signal frequency spectra, increase of network synchrony, coordinated firing of actions potentials, and enhanced evoked response to stimulation during network formation. Our results demonstrate that optogenetic stimulation during neural differentiation can result in permanent changes that extended to the genetic expression of neurons as demonstrated by RNA Sequencing. To our knowledge, this is the first time that a correlation between training regimens during neurogenesis and synaptogenesis and the resulting plastic responses has been shown in-vitro and traced back to changes in gene expression. This work demonstrates new approaches for training of neural circuits whose electrical activity can be modulated and enhanced, which could lead to improvements in neurodegenerative disease research and engineering of in-vitro multi-cellular living systems.

Optogenetics for neural transplant manipulation and functional analysis

Transplantation of neural stem cells (NSCs) or NSC-derived neurons into the brain is a promising therapeutic approach to restore neuronal function. Rapid progress in the NSCs research field, particularly due to the exploitation of induced pluripotent stem cells (iPSCs), offers great potential and an unlimited source of stem cell-derived neural grafts. Studying the functional integration of these grafts into host brain tissues and their effects on each other have been boosted by the implementation of optogenetic technologies. Optogenetics provides high spatiotemporal functional manipulations of grafted or host neurons in parallel. This review aims to highlight the impact of optogenetics in neural stem cell transplantations.

Neurogenic differentiation of human dental pulp stem cells by optogenetics stimulation

INTRODUCTION

Human dental pulp stem cells (hDPSCs), a promising source for autologous transplantation in regenerative medicine, have been shown to be able to differentiate into neural precursors. Optogenetics is considered as an advanced biological technique in neuroscience which is able to control the activity of genetically modified stem cells by light. The purpose of this study is to investigate the neurogenic differentiation of hDPSCs following optogenetic stimulation.

METHODS

The hDPSCs were isolated by mechanical enzymatic digestion from an impacted third molar and cultured in DMEM/F12. The cells were infected with lentiviruses carrying CaMKIIa-hChR2 (H134R). Opsin-expressing hDPSCs were plated at the density of 5 × 10⁴ cells/well in 6-well plates and optical stimulation was conducted with blue light (470 nm) pulsing at 15 Hz, 90 % Duty Cycle and 10 mW power for 10 s every 90 minutes, 6 times a day for 5 days. Two control groups including non-opsin-expressing hDPSCs and opsin-expressing hDPSCs with no optical stimulation were also included in the study. A day after last light stimulation, the viability of cells was analyzed by the MTT assay and the morphological changes were examined by phase contrast microscopy. The expression of Nestin, Microtubule-Associated protein 2 (MAP2) and Doublecortin (DCX) were examined by immunocytochemistry.

RESULTS

Human DPSCs expressed the reporter gene, mCherry, 72 hours after lentiviral infection. The result of MTT assay revealed a significant more viability in optical stimulated opsin-expressing hDPSCs as compared with two control groups. Moreover, optical stimulation increased the expression of Nestin, Doublecortin and MAP2 along with morphological changes from spindle shape to neuron-like shape.

CONCLUSION

Optogenetics stimulation through depolarizing the hDPSCs can increase the cells viability and/or proliferation and also promote the differentiation toward neuron-like cells.

Emerging technologies to study glial cells

Development, physiological functions, and pathologies of the brain depend on tight interactions between neurons and different types of glial cells, such as astrocytes, microglia, oligodendrocytes, and oligodendrocyte precursor cells. Assessing the relative contribution of different glial cell types is required for the full understanding of brain function and dysfunction. Over the recent years, several technological breakthroughs were achieved, allowing "glio-scientists" to address new challenging biological questions. These technical developments make it possible to study the roles of specific cell types with medium or high-content workflows and perform fine analysis of their mutual interactions in a preserved environment. This review illustrates the potency of several cutting-edge experimental approaches (advanced cell cultures, induced pluripotent stem cell (iPSC)-derived human glial cells, viral vectors, in situ glia imaging, opto- and chemogenetic approaches, and high-content molecular analysis) to unravel the role of glial cells in specific brain functions or diseases. It also illustrates the translation of some techniques to the clinics, to monitor glial cells in patients, through specific brain imaging methods. The advantages, pitfalls, and future developments are discussed for each technique, and selected examples are provided to illustrate how specific "gliobiological" questions can now be tackled.

Optogenetically transduced human ES cell-derived neural progenitors and their neuronal progenies: Phenotypic characterization and responses to optical stimulation

Optogenetically engineered human neural progenitors (hNPs) are viewed as promising tools in regenerative neuroscience because they allow the testing of the ability of hNPs to integrate within nervous system of an appropriate host not only structurally, but also functionally based on the responses of their differentiated progenies to light. Here, we transduced H9 embryonic stem cell-derived hNPs with a lentivirus harboring human channelrhodopsin (hChR2) and differentiated them into a forebrain lineage. We extensively characterized the fate and optogenetic functionality of hChR2-hNPs in vitro with electrophysiology and immunocytochemistry. We also explored whether the in vivo phenotype of ChR2-hNPs conforms to in vitro observations by grafting them into the frontal neocortex of rodents and analyzing their survival and neuronal differentiation. Human ChR2-hNPs acquired neuronal phenotypes (TUJ1, MAP2, SMI-312, and synapsin 1 immunoreactivity) in vitro after an average of 70 days of coculturing with CD1 astrocytes and progressively displayed both inhibitory and excitatory neurotransmitter signatures by immunocytochemistry and whole-cell patch clamp recording. Three months after transplantation into motor cortex of naïve or injured mice, 60–70% of hChR2-hNPs at the transplantation site expressed TUJ1 and had neuronal cytologies, whereas 60% of cells also expressed ChR2. Transplant-derived neurons extended axons through major commissural and descending tracts and issued synaptophysin⁺ terminals in the claustrum, endopiriform area, and corresponding insular and piriform cortices. There was no apparent difference in engraftment, differentiation, or connectivity patterns between injured and sham subjects. Same trends were observed in a second rodent host, i.e. rat, where we employed longer survival times and found that the majority of grafted hChR2-hNPs differentiated into GABAergic neurons that established dense terminal fields and innervated mostly dendritic profiles in host cortical neurons. In physiological experiments, human ChR2⁺ neurons in culture generated spontaneous action potentials (APs) 100–170 days into differentiation and their firing activity was consistently driven by optical stimulation. Stimulation generated glutamatergic and GABAergic postsynaptic activity in neighboring ChR2^- cells, evidence that hChR2-hNP-derived neurons had established functional synaptic connections with other neurons in culture. Light stimulation of hChR2-hNP transplants in vivo generated complicated results, in part because of the variable response of the transplants themselves. Our findings show that we can successfully derive hNPs with optogenetic properties that are fully transferrable to their differentiated neuronal progenies. We also show that these progenies have substantial neurotransmitter plasticity in vitro, whereas in vivo they mostly differentiate into inhibitory GABAergic neurons. Furthermore, neurons derived from hNPs have the capacity of establishing functional synapses with postsynaptic neurons in vitro, but this outcome is technically challenging to explore in vivo. We propose that optogenetically endowed hNPs hold great promise as tools to explore de novo circuit formation in the brain and, in the future, perhaps launch a new generation of neuromodulatory therapies.

Optogenetic stimulation inhibits the self-renewal of mouse embryonic stem cells

Modulation of the embryonic stem cell state is beneficial for elucidating the innate mechanisms of development and regenerative medicine. Ion flux plays important roles in modulating the transition between stemness and differentiation in mouse embryonic stem cells (mESCs). Optogenetics is a novel tool for manipulating ion flux. To investigate the impact of optical stimulation on embryonic stem cells, optogenetically engineered V6.5 mESCs were used to measure the depolarization mediated by ChR2 on the proliferation, self-renewal, and differentiation of mESCs. Blue light stimulation significantly inhibited ChR2-GFP-V6.5 ESC proliferation and disrupted the cell cycle progression, reducing the proportion of cells in the S phase. Interestingly, optical stimulation could inhibit ChR2-GFP-V6.5 ESC self-renewal and trigger differentiation by activating the extracellular regulated protein kinase (ERK) signaling pathway. Our data suggest that membrane potential changes play pivotal roles in regulating the proliferation, self-renewal and initiation of differentiation of mESCs.

Bone Regeneration Induced by Strontium Folate Loaded Biohybrid Scaffolds

Nowadays, regenerative medicine has paid special attention to research (in vitro and in vivo) related to bone regeneration, specifically in the treatment of bone fractures or skeletal defects, which is rising worldwide and is continually demanding new developments in the use of stem cells, growth factors, membranes and scaffolds based on novel nanomaterials, and their applications in patients by using advanced tools from molecular biology and tissue engineering. Strontium (Sr) is an element that has been investigated in recent years for its participation in the process of remodeling and bone formation. Based on these antecedents, this is a review about the Strontium Folate (SrFO), a recently developed non-protein based bone-promoting agent with interest in medical and pharmaceutical fields due to its improved features in comparison to current therapies for bone diseases.

Cellular Models: HD Patient-Derived Pluripotent Stem Cells

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by expanded polyglutamine (polyQ)-encoding repeats in the Huntingtin (HTT) gene. Traditionally, HD cellular models consisted of either patient cells not affected by disease or rodent neurons expressing expanded polyQ repeats in HTT. As these models can be limited in their disease manifestation or proper genetic context, respectively, human HD pluripotent stem cells (PSCs) are currently under investigation as a way to model disease in patient-derived neurons and other neural cell types. This chapter reviews embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) models of disease, including published differentiation paradigms for neurons and their associated phenotypes, as well as current challenges to the field such as validation of the PSCs and PSC-derived cells. Highlighted are potential future technical advances to HD PSC modeling, including transdifferentiation, complex in vitro multiorgan/system reconstruction, and personalized medicine. Using a human HD patient model of the central nervous system, hopefully one day researchers can tease out the consequences of mutant HTT (mHTT) expression on specific cell types within the brain in order to identify and test novel therapies for disease.

Optogenetic control of iPS cell-derived neurons in 2D and 3D culture systems using channelrhodopsin-2 expression driven by the synapsin-1 and calcium-calmodulin kinase II promoters

Development of an optogenetically controllable human neural network model in three-dimensional (3D) cultures can provide an investigative system that is more physiologically relevant and better able to mimic aspects of human brain function. Light-sensitive neurons were generated by transducing channelrhodopsin-2 (ChR2) into human induced pluripotent stem cell (hiPSC) derived neural progenitor cells (Axol) using lentiviruses and cell-type specific promoters. A mixed population of human iPSC-derived cortical neurons, astrocytes and progenitor cells were obtained (Axol-ChR2) upon neural differentiation. Pan-neuronal promoter synapsin-1 (SYN1) and excitatory neuron-specific promoter calcium-calmodulin kinase II (CaMKII) were used to drive reporter gene expression in order to assess the differentiation status of the targeted cells. Expression of ChR2 and characterisation of subpopulations in differentiated Axol-ChR2 cells were evaluated using flow cytometry and immunofluorescent staining. These cells were transferred from 2D culture to 3D alginate hydrogel functionalised with arginine-glycine-aspartate (RGD) and small molecules (Y-27632). Improved RGD-alginate hydrogel was physically characterised and assessed for cell viability to serve as a generic 3D culture system for human pluripotent stem cells (hPSCs) and neuronal cells. Prior to cell encapsulation, neural network activities of Axol-ChR2 cells and primary neurons were investigated using calcium imaging. Results demonstrate that functional activities were successfully achieved through expression of ChR2- by both the CaMKII and SYN1 promoters. The RGD-alginate hydrogel system supports the growth of differentiated Axol-ChR2 cells whilst allowing detection of ChR2 expression upon light stimulation. This allows precise and non-invasive control of human neural networks in 3D.

Human Models Are Needed for Studying Human Neurodevelopmental Disorders

The analysis of animal models of neurological disease has been instrumental in furthering our understanding of neurodevelopment and brain diseases. However, animal models are limited in revealing some of the most fundamental aspects of development, genetics, pathology, and disease mechanisms that are unique to humans. These shortcomings are exaggerated in disorders that affect the brain, where the most significant differences between humans and animal models exist, and could underscore failures in targeted therapeutic interventions in affected individuals. Human pluripotent stem cells have emerged as a much-needed model system for investigating human-specific biology and disease mechanisms. However, questions remain regarding whether these cell-culture-based models are sufficient or even necessary. In this review, we summarize human-specific features of neurodevelopment and the most common neurodevelopmental disorders, present discrepancies between animal models and human diseases, demonstrate how human stem cell models can provide meaningful information, and discuss the challenges that exist in our pursuit to understand distinctively human aspects of neurodevelopment and brain disease. This information argues for a more thoughtful approach to disease modeling through consideration of the valuable features and limitations of each model system, be they human or animal, to mimic disease characteristics.

Fully implantable, battery-free wireless optoelectronic devices for spinal optogenetics

The advent of optogenetic tools has allowed unprecedented insights into the organization of neuronal networks. Although recently developed technologies have enabled implementation of optogenetics for studies of brain function in freely moving, untethered animals, wireless powering and device durability pose challenges in studies of spinal cord circuits where dynamic, multidimensional motions against hard and soft surrounding tissues can lead to device degradation. We demonstrate here a fully implantable optoelectronic device powered by near-field wireless communication technology, with a thin and flexible open architecture that provides excellent mechanical durability, robust sealing against biofluid penetration and fidelity in wireless activation, thereby allowing for long-term optical stimulation of the spinal cord without constraint on the natural behaviors of the animals. The system consists of a double-layer, rectangular-shaped magnetic coil antenna connected to a microscale inorganic light-emitting diode (μ-ILED) on a thin, flexible probe that can be implanted just above the dura of the mouse spinal cord for effective stimulation of light-sensitive proteins expressed in neurons in the dorsal horn. Wireless optogenetic activation of TRPV1-ChR2 afferents with spinal μ-ILEDs causes nocifensive behaviors and robust real-time place aversion with sustained operation in animals over periods of several weeks to months. The relatively low-cost electronics required for control of the systems, together with the biocompatibility and robust operation of these devices will allow broad application of optogenetics in future studies of spinal circuits, as well as various peripheral targets, in awake, freely moving and untethered animals, where existing approaches have limited utility.

On-demand optogenetic activation of human stem-cell-derived neurons

The widespread application of human stem-cell-derived neurons for functional studies is impeded by complicated differentiation protocols, immaturity, and deficient optogene expression as stem cells frequently lose transgene expression over time. Here we report a simple but precise Cre-loxP-based strategy for generating conditional, and thereby stable, optogenetic human stem-cell lines. These cells can be easily and efficiently differentiated into functional neurons, and optogene expression can be triggered by administering Cre protein to the cultures. This conditional expression system may be applied to stem-cell-derived neurons whenever timed transgene expression could help to overcome silencing at the stem-cell level.

Stem cell transplantation therapy for multifaceted therapeutic benefits after stroke

One of the exciting advances in modern medicine and life science is cell-based neurovascular regeneration of damaged brain tissues and repair of neuronal structures. The progress in stem cell biology and creation of adult induced pluripotent stem (iPS) cells has significantly improved basic and pre-clinical research in disease mechanisms and generated enthusiasm for potential applications in the treatment of central nervous system (CNS) diseases including stroke. Endogenous neural stem cells and cultured stem cells are capable of self-renewal and give rise to virtually all types of cells essential for the makeup of neuronal structures. Meanwhile, stem cells and neural progenitor cells are well-known for their potential for trophic support after transplantation into the ischemic brain. Thus, stem cell-based therapies provide an attractive future for protecting and repairing damaged brain tissues after injury and in various disease states. Moreover, basic research on naïve and differentiated stem cells including iPS cells has markedly improved our understanding of cellular and molecular mechanisms of neurological disorders, and provides a platform for the discovery of novel drug targets. The latest advances indicate that combinatorial approaches using cell based therapy with additional treatments such as protective reagents, preconditioning strategies and rehabilitation therapy can significantly improve therapeutic benefits. In this review, we will discuss the characteristics of cell therapy in different ischemic models and the application of stem cells and progenitor cells as regenerative medicine for the treatment of stroke.

Genetically Encoded Photoactuators and Photosensors for Characterization and Manipulation of Pluripotent Stem Cells

Our knowledge of pluripotent stem cell biology has advanced considerably in the past four decades, but it has yet to deliver on the great promise of regenerative medicine. The slow progress can be mainly attributed to our incomplete understanding of the complex biologic processes regulating the dynamic developmental pathways from pluripotency to fully-differentiated states of functional somatic cells. Much of the difficulty arises from our lack of specific tools to query, or manipulate, the molecular scale circuitry on both single-cell and organismal levels. Fortunately, the last two decades of progress in the field of optogenetics have produced a variety of genetically encoded, light-mediated tools that enable visualization and control of the spatiotemporal regulation of cellular function. The merging of optogenetics and pluripotent stem cell biology could thus be an important step toward realization of the clinical potential of pluripotent stem cells. In this review, we have surveyed available genetically encoded photoactuators and photosensors, a rapidly expanding toolbox, with particular attention to those with utility for studying pluripotent stem cells.

Chiro-Optical Modulation for NURR1 Production from Stem Cells

Nuclear receptor related 1 (NURR1) is an essential protein for maintenance of dopaminergic neurons in adult midbrain of which deficiency leads to Parkinson's disease. To enhance the NURR1 production of neural cells, various approaches are under investigation. Here we report that NURR1 is highly expressed in stem cells by exposure to an L-polarized blue light emitting diode (LED). Compared to stem cells cultured in the absence of a LED, under polarized green and red LEDs, the stem cells exposed to a polarized blue LED significantly enhanced neuronal biomarkers such as neurofilament M (NFM) and neuron specific enolase (NSE) at both mRNA and protein levels. In particular, NURR1 was selectively enhanced by the stem cells exposed to the L-polarized blue LED. Stem cells exposed to the L-polarized blue LED increased mitochondrial ATP and intracellular calcium ions, which support neuronal differentiation of the stem cells. This study suggests that chiro-optical treatments by using polarized light with a specific wavelength can be used for engineering of stem cells with enhanced specific biochemicals, which may open a new method for a specific disease.

Predicting the functional states of human iPSC-derived neurons with single-cell RNA-seq and electrophysiology

Human neural progenitors derived from pluripotent stem cells develop into electrophysiologically active neurons at heterogeneous rates, which can confound disease-relevant discoveries in neurology and psychiatry. By combining patch clamping, morphological and transcriptome analysis on single-human neurons in vitro, we defined a continuum of poor to highly functional electrophysiological states of differentiated neurons. The strong correlations between action potentials, synaptic activity, dendritic complexity and gene expression highlight the importance of methods for isolating functionally comparable neurons for in vitro investigations of brain disorders. Although whole-cell electrophysiology is the gold standard for functional evaluation, it often lacks the scalability required for disease modeling studies. Here, we demonstrate a multimodal machine-learning strategy to identify new molecular features that predict the physiological states of single neurons, independently of the time spent in vitro. As further proof of concept, we selected one of the potential neurophysiological biomarkers identified in this study-GDAP1L1-to isolate highly functional live human neurons in vitro.

Bioelectric Medicine and Devices for the Treatment of Spinal Cord Injury

Recovery of motor control is paramount for patients living with paralysis following spinal cord injury (SCI). While a cure or regenerative intervention remains on the horizon for the treatment of SCI, a number of neuroprosthetic devices have been employed to treat and mitigate the symptoms of paralysis associated with injuries to the spinal column and associated comorbidities. The recent success of epidural stimulation to restore voluntary motor function in the lower limbs of a small cohort of patients has breathed new life into the promise of electric-based medicine. Recently, a number of new organic and inorganic electronic devices have been developed for brain-computer interfaces to bypass the injury, for neurorehabilitation, bladder and bowel control, and the restoration of motor or sensory control. Herein, we discuss the recent advances in neuroprosthetic devices for treating SCI and highlight future design needs for closed-loop device systems.

Microfluidic device for the formation of optically excitable, three-dimensional, compartmentalized motor units

Motor units are the fundamental elements responsible for muscle movement. They are formed by lower motor neurons and their muscle targets, synapsed via neuromuscular junctions (NMJs). The loss of NMJs in neurodegenerative disorders (such as amyotrophic lateral sclerosis or spinal muscle atrophy) or as a result of traumatic injuries affects millions of lives each year. Developing in vitro assays that closely recapitulate the physiology of neuromuscular tissues is crucial to understand the formation and maturation of NMJs, as well as to help unravel the mechanisms leading to their degeneration and repair. We present a microfluidic platform designed to coculture myoblast-derived muscle strips and motor neurons differentiated from mouse embryonic stem cells (ESCs) within a three-dimensional (3D) hydrogel. The device geometry mimics the spinal cord-limb physical separation by compartmentalizing the two cell types, which also facilitates the observation of 3D neurite outgrowth and remote muscle innervation. Moreover, the use of compliant pillars as anchors for muscle strips provides a quantitative functional readout of force generation. Finally, photosensitizing the ESC provides a pool of source cells that can be differentiated into optically excitable motor neurons, allowing for spatiodynamic, versatile, and noninvasive in vitro control of the motor units.

hESC-derived neural progenitors prevent xenograft rejection through neonatal desensitisation

Stem cell therapies for neurological disorders are rapidly moving towards use in clinical trials. Before initiation of clinical trials, extensive pre-clinical validation in appropriate animal models is essential. However, grafts of human cells into the rodent brain are rejected within weeks after transplantation and the standard methods of immune-suppression for the purpose of studying human xenografts are not always sufficient for the long-term studies needed for transplanted human neurons to maturate, integrate and provide functional benefits in the host brain. Neonatal injections in rat pups using human fetal brain cells have been shown to desensitise the host to accept human tissue grafts as adults, whilst not compromising their immune system. Here, we show that differentiated human embryonic stem cells (hESCs) can be used for desensitisation to achieve long-term graft survival of human stem cell-derived neurons in a xenograft setting, surpassing the time of conventional pharmacological immune-suppressive treatments. The use of hESCs for desensitisation opens up for a widespread use of the technique, which will be of great value when performing pre-clinical evaluation of stem cell-derived neurons in animal models.

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Neonatal desensitisation prevents graft rejection in a xenograft rat model.

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Successful neonatal desensitisation using hESC-derived progenitors

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Grafted human neurons survive beyond the time of conventional immune suppression.

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Multiple breaches of the BBB do not affect graft protection.

Probing Neural Transplant Networks In Vivo with Optogenetics and Optogenetic fMRI

Understanding how stem cell-derived neurons functionally integrate into the brain upon transplantation has been a long sought-after goal of regenerative medicine. However, methodological limitations have stood as a barrier, preventing key insight into this fundamental problem. A recently developed technology, termed optogenetic functional magnetic resonance imaging (ofMRI), offers a possible solution. By combining targeted activation of transplanted neurons with large-scale, noninvasive measurements of brain activity, ofMRI can directly visualize the effect of engrafted neurons firing on downstream regions. Importantly, this tool can be used to identify not only whether transplanted neurons have functionally integrated into the brain, but also which regions they influence and how. Furthermore, the precise control afforded over activation enables the input-output properties of engrafted neurons to be systematically studied. This review summarizes the efforts in stem cell biology and neuroimaging that made this development possible and outlines its potential applications for improving and optimizing stem cell-based therapies in the future.

Optogenetic modulation in stroke recovery

Stroke is one of the leading contributors to morbidity, mortality, and health care costs in the United States. Although several preclinical strategies have shown promise in the laboratory, few have succeeded in the clinical setting. Optogenetics represents a promising molecular tool, which enables highly specific circuit-level neuromodulation. Here, the conceptual background and preclinical body of evidence for optogenetics are reviewed, and translational considerations in stroke recovery are discussed.

Restoring motor function using optogenetics and neural engraftment

Controlling muscle function is essential for human behaviour and survival, thus, impairment of motor function and muscle paralysis can severely impact quality of life and may be immediately life-threatening, as occurs in many cases of traumatic spinal cord injury (SCI) and in patients with amyotrophic lateral sclerosis (ALS). Repairing damaged spinal motor circuits, in either SCI or ALS, currently remains an elusive goal. Therefore alternative strategies are needed to artificially control muscle function and thereby enable essential motor tasks. This review focuses on recent advances towards restoring motor function, with a particular focus on stem cell-derived neuronal engraftment strategies, optogenetic control of motor function and the potential future translational application of these approaches.

Strontium folate loaded biohybrid scaffolds seeded with dental pulp stem cells induce in vivo bone regeneration in critical sized defects

Strontium folate loaded biohybrid scaffolds enhance dental pulp stem cells replication and differentiation, promoting complete regeneration of critical bone defects.

Prospects for Optogenetic Augmentation of Brain Function

The ability to optically control neural activity opens up possibilities for the restoration of normal function following neurological disorders. The temporal precision, spatial resolution, and neuronal specificity that optogenetics offers is unequalled by other available methods, so will it be suitable for not only restoring but also extending brain function? As the first demonstrations of optically “implanted” novel memories emerge, we examine the suitability of optogenetics as a technique for extending neural function. While optogenetics is an effective tool for altering neural activity, the largest impediment for optogenetics in neural augmentation is our systems level understanding of brain function. Furthermore, a number of clinical limitations currently remain as substantial hurdles for the applications proposed. While neurotechnologies for treating brain disorders and interfacing with prosthetics have advanced rapidly in the past few years, partially addressing some of these critical problems, optogenetics is not yet suitable for use in humans. Instead we conclude that for the immediate future, optogenetics is the neurological equivalent of the 3D printer: its flexibility providing an ideal tool for testing and prototyping solutions for treating brain disorders and augmenting brain function.

Human pluripotent stem cell tools for cardiac optogenetics

It is likely that arrhythmias should be avoided for therapies based on human pluripotent stem cell (hPSC)-derived cardiomyocytes (CM) to be effective. Towards achieving this goal, we introduced light-activated channelrhodopsin-2 (ChR2), a cation channel activated with 480 nm light, into human embryonic stem cells (hESC). By using in vitro approaches, hESC-CM are able to be activated with light. ChR2 is stably transduced into undifferentiated hESC via a lentiviral vector. Via directed differentiation, hESC(ChR2)-CM are produced and subjected to optical stimulation. hESC(ChR2)-CM respond to traditional electrical stimulation and produce similar contractility features as their wild-type counterparts but only hESC(ChR2)-CM can be activated by optical stimulation. Here it is shown that a light sensitive protein can enable in vitro optical control of hESC-CM and that this activation occurs optimally above specific light stimulation intensity and pulse width thresholds. For future therapy, in vivo optical stimulation along with optical inhibition could allow for acute synchronization of implanted hPSC-CM with patient cardiac rhythms.

Modeling Huntington׳s disease with patient-derived neurons

Huntington׳s Disease (HD) is a fatal neurodegenerative disorder caused by expanded polyglutamine repeats in the Huntingtin (HTT) gene. While the gene was identified over two decades ago, it remains poorly understood why mutant HTT (mtHTT) is initially toxic to striatal medium spiny neurons (MSNs). Models of HD using non-neuronal human patient cells and rodents exhibit some characteristic HD phenotypes. While these current models have contributed to the field, they are limited in disease manifestation and may vary in their response to treatments. As such, human HD patient MSNs for disease modeling could greatly expand the current understanding of HD and facilitate the search for a successful treatment. It is now possible to use pluripotent stem cells, which can generate any tissue type in the body, to study and potentially treat HD. This review covers disease modeling in vitro and, via chimeric animal generation, in vivo using human HD patient MSNs differentiated from embryonic stem cells or induced pluripotent stem cells. This includes an overview of the differentiation of pluripotent cells into MSNs, the established phenotypes found in cell-based models and transplantation studies using these cells. This review not only outlines the advancements in the rapidly progressing field of HD modeling using neurons derived from human pluripotent cells, but also it highlights several remaining controversial issues such as the 'ideal' series of pluripotent lines, the optimal cell types to use and the study of a primarily adult-onset disease in a developmental model. This article is part of a Special Issue entitled SI: Exploiting human neurons.

Optogenetic LED array for perturbing cardiac electrophysiology

Optogenetics is the targeted genetic introduction of light-sensitive channels, such as Channelrhodopsin, and pumps, such as Halorhodopsin, into electrically-excitable cells that enables high spatiotemporal electrical stimulation and inhibition by optical actuation. Technologies for inducing optogenetically-based electrical stimulation for investigating in vitro and in vivo neural perturbations have been described. However, modification of existing technologies or creation of new ones has not been described for chronic cardiac applications. Here, an LED array system for optogenetically perturbing cardiac electrophysiology is described. The overall layout of the system consists of an LED holder containing six LED's that deliver pulsed ∼470 nm light to pluripotent stem cell-derived cardiomyocytes cultured in a 6-well tissue culture plate. The response of the cardiomyocytes is monitored by microscopy and the system is enclosed within a standard incubator. This system is relatively simple to create and uses mostly off-the-shelf components. The overall function of the system is to deliver chronic light stimulation over days to weeks to differentiating stem cell-derived cardiomyocytes in order to investigate perturbations in their electrophysiology.

Restoring nervous system structure and function using tissue engineered living scaffolds

Neural tissue engineering is premised on the integration of engineered living tissue with the host nervous system to directly restore lost function or to augment regenerative capacity following nervous system injury or neurodegenerative disease. Disconnection of axon pathways – the long-distance fibers connecting specialized regions of the central nervous system or relaying peripheral signals – is a common feature of many neurological disorders and injury. However, functional axonal regeneration rarely occurs due to extreme distances to targets, absence of directed guidance, and the presence of inhibitory factors in the central nervous system, resulting in devastating effects on cognitive and sensorimotor function. To address this need, we are pursuing multiple strategies using tissue engineered “living scaffolds”, which are preformed three-dimensional constructs consisting of living neural cells in a defined, often anisotropic architecture. Living scaffolds are designed to restore function by serving as a living labeled pathway for targeted axonal regeneration – mimicking key developmental mechanisms– or by restoring lost neural circuitry via direct replacement of neurons and axonal tracts. We are currently utilizing preformed living scaffolds consisting of neuronal clusters spanned by long axonal tracts as regenerative bridges to facilitate long-distance axonal regeneration and for targeted neurosurgical reconstruction of local circuits in the brain. Although there are formidable challenges in preclinical and clinical advancement, these living tissue engineered constructs represent a promising strategy to facilitate nervous system repair and functional recovery.

Moving stem cells to the clinic: potential and limitations for brain repair

Stem cell-based therapies hold considerable promise for many currently devastating neurological disorders. Substantial progress has been made in the derivation of disease-relevant human donor cell populations. Behavioral data in relevant animal models of disease have demonstrated therapeutic efficacy for several cell-based approaches. Consequently, cGMP grade cell products are currently being developed for first in human clinical trials in select disorders. Despite the therapeutic promise, the presumed mechanism of action of donor cell populations often remains insufficiently validated. It depends greatly on the properties of the transplanted cell type and the underlying host pathology. Several new technologies have become available to probe mechanisms of action in real time and to manipulate in vivo cell function and integration to enhance therapeutic efficacy. Results from such studies generate crucial insight into the nature of brain repair that can be achieved today and push the boundaries of what may be possible in the future.

Direct in vivo assessment of human stem cell graft-host neural circuits

Despite the potential of stem cell-derived neural transplants for treating intractable neurological diseases, the global effects of a transplant's electrical activity on host circuitry have never been measured directly, preventing the systematic optimization of such therapies. Here, we overcome this problem by combining optogenetics, stem cell biology, and neuroimaging to directly map stem cell-driven neural circuit formation in vivo. We engineered human induced pluripotent stem cells (iPSCs) to express channelrhodopsin-2 and transplanted resulting neurons to striatum of rats. To non-invasively visualize the function of newly formed circuits, we performed high-field functional magnetic resonance imaging (fMRI) during selective stimulation of transplanted cells. fMRI successfully detected local and remote neural activity, enabling the global graft-host neural circuit function to be assessed. These results demonstrate the potential of a novel neuroimaging-based platform that can be used to identify how a graft's electrical activity influences the brain network in vivo.

Optogenetics for retinal disorders

Optogenetics is the use of genetic methods combined with optical technology to achieve gain or loss of function within neuronal circuits. The field of optogenetics has been rapidly expanding in efforts to restore visual function to blinding diseases such as retinitis pigmentosa (RP). Most work in the field includes a group of light-sensitive retinaldehyde-binding proteins known as opsins. Opsins couple photon absorption to molecular signaling chains that control cellular ion currents. Targeting of opsin genes to surviving retinal cells is fundamental to the success of optogenetic therapy. Viral delivery, primarily adeno-associated virus, using intravitreal injection for inner retinal cells and subretinal injection for outer retinal cells, has proven successful in many models. Challenges in bioengineering remain for optogenetics including relative insensitivity of opsins to physiologic light levels of stimulation and difficulty with viral delivery in primate models. However, targeting optogenetic therapy may present an even greater challenge. Neural and glial remodeling seen in advanced stages of RP result in reorganization of remaining neural retina, and optogenetic therapy may not yield functional results. Remodeling also poses a challenge to the selection of cellular targets, with bipolar, amacrine and ganglion cells all playing distinct physiologic roles, and affected by remodeling differently. Although optogenetics has drawn closer to clinical utility, advances in opsin engineering, therapeutic targeting and ultimately in molecular inhibition of remodeling will play critical roles in the continued clinical advancement of optogenetic therapy.

Optogenetics enables functional analysis of human embryonic stem cell-derived grafts in a Parkinson's disease model

Recent studies have shown evidence of behavioral recovery after transplantation of human pluripotent stem cell (PSC)-derived neural cells in animal models of neurological disease. However, little is known about the mechanisms underlying graft function. Here we use optogenetics to modulate in real time electrophysiological and neurochemical properties of mesencephalic dopaminergic (mesDA) neurons derived from human embryonic stem cells (hESCs). In mice that had recovered from lesion-induced Parkinsonian motor deficits, light-induced selective silencing of graft activity rapidly and reversibly re-introduced the motor deficits. The re-introduction of motor deficits was prevented by the dopamine agonist apomorphine. These results suggest that functionality depends on graft neuronal activity and dopamine release. Combining optogenetics, slice electrophysiology and pharmacological approaches, we further show that mesDA-rich grafts modulate host glutamatergic synaptic transmission onto striatal medium spiny neurons in a manner reminiscent of endogenous mesDA neurons. Thus, application of optogenetics in cell therapy can link transplantation, animal behavior and postmortem analysis to enable the identification of mechanisms that drive recovery.

Living scaffolds for neuroregeneration

Neural tissue engineers are exploiting key mechanisms responsible for neural cell migration and axonal path finding during embryonic development to create living scaffolds for neuroregeneration following injury and disease. These mechanisms involve the combined use of haptotactic, chemotactic, and mechanical cues to direct cell movement and re-growth. Living scaffolds provide these cues through the use of cells engineered in a predefined architecture, generally in combination with biomaterial strategies. Although several hurdles exist in the implementation of living regenerative scaffolds, there are considerable therapeutic advantages to using living cells in conjunction with biomaterials. The leading contemporary living scaffolds for neurorepair are utilizing aligned glial cells and neuronal/axonal tracts to direct regenerating axons across damaged tissue to appropriate targets, and in some cases to directly replace the function of lost cells. Future advances in technology, including the use of exogenous stimulation and genetically engineered stem cells, will further the potential of living scaffolds and drive a new era of personalized medicine for neuroregeneration.

In vivo imaging of transplanted stem cells in the central nervous system

In vivo imaging is increasingly being utilized in studies investigating stem cell-based treatments for neurological disorders. Direct labeling is used in preclinical and clinical studies to track the fate of transplanted cells. To further determine cell viability, experimental studies are able to take advantage of reporter gene technologies. Structural and functional brain imaging can also be used alongside cell imaging as biomarkers of treatment efficacy. Furthermore, it is possible that new imaging techniques could be used to monitor functional integration of stem cell-derived cells with the host nervous system. In this review, we examine recent developments in these areas and identify promising directions for future research at the interface of stem cell therapies and neuroimaging.

How to make spinal motor neurons

All muscle movements, including breathing, walking, and fine motor skills rely on the function of the spinal motor neuron to transmit signals from the brain to individual muscle groups. Loss of spinal motor neuron function underlies several neurological disorders for which treatment has been hampered by the inability to obtain sufficient quantities of primary motor neurons to perform mechanistic studies or drug screens. Progress towards overcoming this challenge has been achieved through the synthesis of developmental biology paradigms and advances in stem cell and reprogramming technology, which allow the production of motor neurons in vitro. In this Primer, we discuss how the logic of spinal motor neuron development has been applied to allow generation of motor neurons either from pluripotent stem cells by directed differentiation and transcriptional programming, or from somatic cells by direct lineage conversion. Finally, we discuss methods to evaluate the molecular and functional properties of motor neurons generated through each of these techniques.