Human Cortical Neuron Generation Using Cell Reprogramming: A Review of Recent Advances

Human Induced Pluripotent Stem Cells: Directed Differentiation Methods and Applications in Brain Diseases

Human induced pluripotent stem cells (hiPSCs), similar to embryonic stem cells, are a class of pluripotent stem cells with the potential to differentiate into various kinds of cells. Because the application of hiPSCs obtained by reprogramming patients' somatic cells in the treatment of brain diseases bypasses the ethical constraints on the use of embryonic stem cells and mitigates immune rejection, hiPSCs have profound clinical application prospects. In this review, we first summarized the differentiation methods of hiPSCs into different kinds of neurons, and secondly discussed the application of hiPSCs in several brain disease models, so as to provide a reference for the future application of hiPSCs in the studies and treatment of brain diseases.

Chemical transdifferentiation of somatic cells to neural cells: a systematic review

INTRODUCTION

Transdifferentiation is the conversion of a specific somatic cell into another cell type, bypassing a transient pluripotent state. This implies a faster method to generate cells of interest with the additional benefit of reduced tumorigenic risk for clinical use.

OBJECTIVE

We describe protocols that use small molecules as direct conversion inducers, without the need for exogenous factors, to evaluate the potential of cell transdifferentiation for pharmacological and clinical applications.

METHODS

In this systematic review, using PRISMA guidelines, we conducted a personalized search strategy in four databases (PubMed, Scopus, Embase, and Web Of Science), looking for experimental works that used exclusively small molecules for transdifferentiation of non-neural cell types into neural lineage cells.

RESULTS

We explored the main biological mechanisms involved in direct cell conversion induced by different small molecules used in 33 experimental in vitro and in vitro transdifferentiation protocols. We also summarize the main characteristics of these protocols, such as the chemical cocktails used, time for transdifferentiation, and conversion efficiency.

CONCLUSION

Small molecules-based protocols for neuronal transdifferentiation are reasonably safe, economical, accessible, and are a promising alternative for future use in regenerative medicine and pharmacology.

Current status and new avenues of stem cell-based preclinical and therapeutic approaches in amyotrophic lateral sclerosis

INTRODUCTION

Cell therapy development represents a critical challenge in amyotrophic lateral sclerosis (ALS) research. Despite more than 20 years of basic and clinical research, no definitive safety and efficacy results of cell-based therapies for ALS have been published.

AREAS COVERED

This review summarizes advances using stem cells (SCs) in pre-clinical studies to promote clinical translation and in clinical trials to treat ALS. New technologies have been developed and new experimental in vitro and animal models are now available to facilitate pre-clinical research in this field and to determine the most promising approaches to pursue in patients. New clinical trial designs aimed at developing personalized SC-based treatment with biological endpoints are being defined.

EXPERT OPINION

Knowledge of the basic biology of ALS and on the use of SCs to study and potentially treat ALS continues to grow. However, a consensus has yet to emerge on how best to translate these results into therapeutic applications. The selection and follow-up of patients should be based on clinical, biological, and molecular criteria. Planning of SC-based clinical trials should be coordinated with patient profiling genetically and molecularly to achieve personalized treatment. Much work within basic and clinical research is still needed to successfully transition SC therapy in ALS.

Neurod1 mediates the reprogramming of NG2 glial into neurons in vitro

Neuronal defect and loss are the main pathological processes of many central nervous system diseases. Cellular reprogramming is a promising method to supplement lost neurons. However, study on cellular reprogramming is still limited and its mechanism remains unclear. Herein, the effect of Neurod1 expression on differentiation of NG2 glia into neurons was investigated. In this study, we successfully isolated NG2 glial cells from mice prior to identification with immunofluorescence. Afterwards, AAV-Neurod1 virus was used to construct Neurod1 overexpression vectors in NG2 glia. Later, we detected neuronal markers expression with immunofluorescence and real time quantitative polymerase-chain reaction (qRT-PCR). Besides, expression of MAPK-signaling-pathway-related proteins were detected by western blotting technique. Through immunofluorescence and qRT-PCR techniques, we observed that Neurod1 overexpression contributed to NG2 cells differentiated into neurons. Further experiments also showed that Neurod1 overexpression induced the activation of MAPK pathway, but PD98059 (a selective inhibitor of MAPK pathway) partly inhibited the neuronal differentiation induced by Neurod1 overexpression. These findings suggest that Neurod1 could promote NG2 glia cells differentiating into neurons, wherein the mechanism under the differentiation is related to activation of MAPK pathway.

Microfluidics for Neuronal Cell and Circuit Engineering

The widespread adoption of microfluidic devices among the neuroscience and neurobiology communities has enabled addressing a broad range of questions at the molecular, cellular, circuit, and system levels. Here, we review biomedical engineering approaches that harness the power of microfluidics for bottom-up generation of neuronal cell types and for the assembly and analysis of neural circuits. Microfluidics-based approaches are instrumental to generate the knowledge necessary for the derivation of diverse neuronal cell types from human pluripotent stem cells, as they enable the isolation and subsequent examination of individual neurons of interest. Moreover, microfluidic devices allow to engineer neural circuits with specific orientations and directionality by providing control over neuronal cell polarity and permitting the isolation of axons in individual microchannels. Similarly, the use of microfluidic chips enables the construction not only of 2D but also of 3D brain, retinal, and peripheral nervous system model circuits. Such brain-on-a-chip and organoid-on-a-chip technologies are promising platforms for studying these organs as they closely recapitulate some aspects of in vivo biological processes. Microfluidic 3D neuronal models, together with 2D in vitro systems, are widely used in many applications ranging from drug development and toxicology studies to neurological disease modeling and personalized medicine. Altogether, microfluidics provide researchers with powerful systems that complement and partially replace animal models.

Small Molecules Enhance Reprogramming of Adult Human Dermal Fibroblasts to Dorsal Forebrain Precursor Cells

The development of human cell-based platforms for disease modelling, drug discovery and regenerative therapy rely on robust and practical methods to derive high yields of relevant neuronal subtypes. Direct reprogramming strategies have sought to provide a means of deriving human neurons that mitigate the low conversion efficiencies and protracted timing of human embryonic stem cell (hESC) and induced pluripotent stem cell (iPSC)-derived neuron specification in vitro. However, few studies have demonstrated the direct conversion of adult human fibroblasts into multipotent neural precursors with the capacity to differentiate into cortical neurons with high efficiency. In this study, we demonstrate a direct reprogramming strategy using chemically modified mRNA (cmRNA) encoding the pro-neural genes SOX2 and PAX6 coupled with small molecule supplementation to enhance the derivation of human induced dorsal forebrain precursors directly from adult human fibroblasts (aHDFs). Through transcriptional and phenotypic analysis of lineage-specific precursor and cortical neuron markers, we have demonstrated that this combined strategy significantly enhances the direct derivation of dorsal forebrain precursors from aHDFs which, following timely exposure to defined differentiation media gives rise to high yields of functional glutamatergic neurons. We propose this combined strategy provides a highly tractable and efficient human cell-based platform for disease modelling and drug discovery.

Modeling schizophrenia with iPS cell technology and disease mouse models

Induced pluripotent stem cell (iPSC) technology, which enables the direct analysis of neuronal cells with the same genetic background as patients, has recently garnered significant attention in schizophrenia research. This technology is important because it enables a comprehensive interpretation using mice and human clinical research and cross-species verification. Here I review recent advances in modeling schizophrenia using iPSC technology, alongside the utility of disease mouse models.

[Pharmacological studies using iPSC-derived neurons from patients with schizophrenia]

Schizophrenia is characterized by positive symptoms, negative symptoms and cognitive dysfunction. Although the abnormal neuronal development, impaired synaptic functions and impaired neural circuit functions are suggested to be the causes of psychiatric disorders, the molecular and cellular etiology of schizophrenia remains largely unclear. iPS-related technologies can be powerful for not only understanding the molecular and cellular etiology of schizophrenia but also drug discovery research. In 2011, the first iPS cells derived from patients with schizophrenia harboring a DISC1 mutation were generated. Subsequently, many iPS cells from patients with schizophrenia were established for understanding the molecular and cellular disease phenotypes of the differentiated neuronal cells. For replicating disease phenotypes with iPSC-derived neuronal cells, it is important to develop the differentiation strategies for generating cell-type specific cultures of various types of neurons, astrocytes and oligodendrocytes. Especially, scalable cultures of iPSC-derived neuronal cells are valuable platforms for drug discovery research. In this review, the focus has been made on the iPSC differentiation technology, pharmacological and drug discovery studies with iPSC-derived neurons from patients with schizophrenia. Continued advancement of the iPSC-related technologies and research will help the success in central nervous system drug discovery and development.

The Application of Brain Organoid Technology in Stroke Research: Challenges and Prospects

Stroke is a neurological disease responsible for significant morbidity and disability worldwide. However, there remains a dearth of effective therapies. The failure of many therapies for stroke in clinical trials has promoted the development of human cell-based models, such as brain organoids. Brain organoids differ from pluripotent stem cells in that they recapitulate various key features of the human central nervous system (CNS) in three-dimensional (3D) space. Recent studies have demonstrated that brain organoids could serve as a new platform to study various neurological diseases. However, there are several limitations, such as the scarcity of glia and vasculature in organoids, which are important for studying stroke. Herein, we have summarized the application of brain organoid technology in stroke research, such as for modeling and transplantation purposes. We also discuss methods to overcome the limitations of brain organoid technology, as well as future prospects for its application in stroke research. Although there are many difficulties and challenges associated with brain organoid technology, it is clear that this approach will play a critical role in the future exploration of stroke treatment.

Enhanced efficiency of nonviral direct neuronal reprogramming on topographical patterns

Nonviral direct neuronal reprogramming holds significant potential in the fields of tissue engineering and regenerative medicine. However, the issue of low reprogramming efficiency poses a major barrier to its application. We propose that topographical cues, which have been applied successfully to enhance lineage-directed differentiation and multipotent stem cell transdifferentiation, could improve nonviral direct neuronal reprogramming efficiency. To investigate, we used a polymer-BAM (Brn2, Ascl1, Myt1l) factor transfection polypex to reprogram primary mouse embryonic fibroblasts. Using a multiarchitecture chip, we screened for patterns that may improve transfection and/or subsequent induced neuron reprogramming efficiency. Selected patterns were then investigated further by analyzing β-tubulin III (TUJ1) and microtubule-associated protein 2 (MAP2) protein expression, cell morphology and electrophysiological function of induced neurons. Certain hierarchical topographies, with nanopatterns imprinted on micropatterns, significantly improved the percentage of TUJ1+ and MAP2+ cells. It is postulated that the microscale base pattern enhances initial BAM expression while the nanoscale sub-pattern promotes subsequent maturation. This is because the base pattern alone increased expression of TUJ1 and MAP2, while the nanoscale pattern was the only pattern yielding induced neurons capable of firing multiple action potentials. Nanoscale patterns also produced the highest fraction of cells showing spontaneous synaptic activity. Overall, reprogramming efficiency with one dose of polyplex on hierarchical patterns was comparable to that of five doses without topography. Thus, topography can enhance nonviral direct reprogramming of fibroblasts into induced neurons.

Depression patient-derived cortical neurons reveal potential biomarkers for antidepressant response

Major depressive disorder is highly prevalent worldwide and has been affecting an increasing number of people each year. Current first line antidepressants show merely 37% remission, and physicians are forced to use a trial-and-error approach when choosing a single antidepressant out of dozens of available medications. We sought to identify a method of testing that would provide patient-specific information on whether a patient will respond to a medication using in vitro modeling. Patient-derived lymphoblastoid cell lines from the Sequenced Treatment Alternatives to Relieve Depression study were used to rapidly generate cortical neurons and screen them for bupropion effects, for which the donor patients showed remission or non-remission. We provide evidence for biomarkers specific for bupropion response, including synaptic connectivity and morphology changes as well as specific gene expression alterations. These biomarkers support the concept of personalized antidepressant treatment based on in vitro platforms and could be utilized as predictors to patient response in the clinic.

Comparative Transcriptomic Analysis of Cerebral Organoids and Cortical Neuron Cultures Derived from Human Induced Pluripotent Stem Cells

Human induced pluripotent stem cells (iPSCs) can be differentiated along various neuronal lineages to generate two-dimensional neuronal cultures as well as three-dimensional brain organoids. Such iPSC-derived cellular models are being utilized to study the basic biology of human neuronal function and to interrogate the molecular underpinnings of disease biology. The different cellular models generated from iPSCs have varying properties in terms of the diversity and organization of the cells as well as the cellular functions that are present. To understand transcriptomic differences in iPSC-derived monolayer neuronal cultures and three-dimensional brain organoids, we differentiated eight human iPSC lines from healthy control subjects to generate cerebral organoids and cortical neuron monolayer cultures from the same set of iPSC lines. We undertook RNA-seq experiments in these model systems and analyzed the gene expression data to identify genes that are differentially expressed in cerebral organoids and two-dimensional cortical neuron cultures. In cerebral organoids, gene ontology analysis showed enrichment of genes involved in tissue development, response to stimuli, and the interferon-γ pathway, while two-dimensional cortical neuron cultures showed enrichment of genes involved in nervous system development and neurogenesis. We also undertook comparative analysis of these gene expression profiles with transcriptomic data from the human fetal prefrontal cortex (PFC). This analysis showed greater overlap of the fetal PFC transcriptome with cerebral organoid gene expression profiles compared to monolayer cortical neuron culture profiles. Our studies delineate the transcriptomic differences between cortical neuron monolayer cultures and three-dimensional cerebral organoids and can help inform the appropriate use of these model systems to address specific scientific questions.

Human-induced pluripotent stem cells as a model for studying sporadic Alzheimer's disease

The discovery of induced pluripotent stem cell (iPSC) technology has the potential to accelerate scientific research for Alzheimer's disease (AD). iPSCs are therefore increasingly considered for AD modeling and drug development. Nevertheless, most of the work conducted so far has mainly focused on iPSC models from patients with familial AD (fAD), while actually sporadic AD (sAD) is more prevalent and represents over 90% of the AD cases in the population. The development of more sAD models is therefore key for studying this multifactorial disorder. In fact, probing the unique genomes of sAD patients and their interaction with AD-associated environmental factors could contribute to a better understanding of this disease. However, initial iPSC-based models for sAD have shown a high degree of variability and inconsistencies in terms of AD hallmarks. In this review, we provide an overview of the studies that have been conducted for sAD so far. In addition, we critically assess important sources of variability related to the model in addition to those that might be explained by the heterogeneous nature of sAD. These considerations might aid in developing more consistent iPSC models of sAD, which could help in developing a better understanding of the molecular mechanisms underlying the disease.

Neuronal Reprogramming for Tissue Repair and Neuroregeneration

Stem cell and cell reprogramming technology represent a rapidly growing field in regenerative medicine. A number of novel neural reprogramming methods have been established, using pluripotent stem cells (PSCs) or direct reprogramming, to efficiently derive specific neuronal cell types for therapeutic applications. Both in vitro and in vivo cellular reprogramming provide diverse therapeutic pathways for modeling neurological diseases and injury repair. In particular, the retina has emerged as a promising target for clinical application of regenerative medicine. Herein, we review the potential of neuronal reprogramming to develop regenerative strategy, with a particular focus on treating retinal degenerative diseases and discuss future directions and challenges in the field.

Taking central nervous system regenerative therapies to the clinic: curing rodents versus nonhuman primates versus humans

The central nervous system is known to have limited regenerative capacity. Not only does this halt the human body's reparative processes after central nervous system lesions, but it also impedes the establishment of effective and safe therapeutic options for such patients. Despite the high prevalence of stroke and spinal cord injury in the general population, these conditions remain incurable and place a heavy burden on patients' families and on society more broadly. Neuroregeneration and neural engineering are diverse biomedical fields that attempt reparative treatments, utilizing stem cells-based strategies, biologically active molecules, nanotechnology, exosomes and highly tunable biodegradable systems (e.g., certain hydrogels). Although there are studies demonstrating promising preclinical results, safe clinical translation has not yet been accomplished. A key gap in clinical translation is the absence of an ideal animal or ex vivo model that can perfectly simulate the human microenvironment, and also correspond to all the complex pathophysiological and neuroanatomical factors that affect functional outcomes in humans after central nervous system injury. Such an ideal model does not currently exist, but it seems that the nonhuman primate model is uniquely qualified for this role, given its close resemblance to humans. This review considers some regenerative therapies for central nervous system repair that hold promise for future clinical translation. In addition, it attempts to uncover some of the main reasons why clinical translation might fail without the implementation of nonhuman primate models in the research pipeline.

Screening of pure synthetic coating substrates for induced pluripotent stem cells and iPSC-derived neuroepithelial progenitors with short peptide based integrin array

Simple and pure synthetic coating substrates are needed to overcome the disadvantages of traditional coating products like animal derived Matrigel in stem cell research. Since integrins are of great importance in cell adhesion and cell-ECM communication, in this study, a commercially available integrin array established by synthetic integrin binding peptides is used to screen coating substrates for iPSCs and NEPs. The results showed that binding peptides of integrin α₅β₁, α_Vβ₁, α_Mβ₂ and α_IIbβ₃ supported cell adhesion of iPSCs, while α₅β₁, α_Vβ₁ and α_IIbβ₃ binding peptides supported NEPs adhesion. Additionally, integrin α5β1 binding peptide was revealed to support rapid expansion of iPSCs and iPSC-derived NEPs, as well as the process of NEPs generation, with equal efficiency as Matrigel. In this work, we demonstrated that by supporting stem cell growth in an integrin dependent manner, the integrin array and coating system has the potential to develop more precise and efficient systems in neurological disease modeling.