REST suppression mediates neural conversion of adult human fibroblasts via microRNA-dependent and -independent pathways

The use of a multi-metric readout screen to identify EHMT2/G9a-inhibition as a modulator of cancer-associated fibroblast activation state

Cancer-associated fibroblasts (CAFs) play a pivotal role in cancer progression, including mediating tumour cell invasion via their pro-invasive secretory profile and ability to remodel the extracellular matrix (ECM). Given that reduced CAF abundance in tumours correlates with improved outcomes in various cancers, we set out to identify epigenetic targets involved in CAF activation in regions of tumour-stromal mixing with the goal of reducing tumour aggressiveness. Using the GLAnCE (Gels for Live Analysis of Compartmentalized Environments) platform, we performed an image-based, phenotypic screen that enabled us to identify modulators of CAF abundance and the capacity of CAFs to induce tumour cell invasion. We identified EHMT2 (also known as G9a), an enzyme that targets the methylation of histone 3 lysine 9 (H3K9), as a potent modulator of CAF abundance and CAF-mediated tumour cell invasion. Transcriptomic and functional analysis of EHMT2-inhibited CAFs revealed EHMT2 participated in driving CAFs towards a pro-invasive phenotype and mediated CAF hyperproliferation, a feature typically associated with activated fibroblasts in tumours. Our study suggests that EHMT2 regulates CAF state within the tumour microenvironment by impacting CAF activation, as well as by magnifying the pro-invasive effects of these activated CAFs on tumour cell invasion through promoting CAF hyperproliferation.

Age- and disease-related autophagy impairment in Huntington disease: New insights from direct neuronal reprogramming

Autophagy impairment in Huntington disease (HD) has been reported for almost two decades. However, the molecular mechanisms underlying this phenomenon are still unclear. This is partially because it is challenging to model the impact of the disease-causing mutation, aging, as well as the selective vulnerability of neurons in a single model. Recently developed direct neuronal reprogramming that allows researchers to induce neurons-of-interest retaining biological aging information made it possible to establish HD cellular models to study more relevant age- and disease-related molecular changes in neurons. We here summarized the findings from a few latest studies utilizing directly reprogrammed HD neurons and discussed the new insights they brought to the understanding of the age- and disease-related autophagy impairment in HD.

From Young to Old: Mimicking Neuronal Aging in Directly Converted Neurons from Young Donors

A substantial challenge in human brain aging is to find a suitable model to mimic neuronal aging in vitro as accurately as possible. Using directly converted neurons (iNs) from human fibroblasts is considered a promising tool in human aging since it retains the aging-associated mitochondrial donor signature. Still, using iNs from aged donors can pose certain restrictions due to their lower reprogramming and conversion efficacy than those from younger individuals. To overcome these limitations, our study aimed to establish an in vitro neuronal aging model mirroring features of in vivo aging by acute exposure on young iNs to either human stress hormone cortisol or the mitochondrial stressor rotenone, considering stress as a trigger of in vivo aging. The impact of rotenone was evident in mitochondrial bioenergetic properties by showing aging-associated deficits in mitochondrial respiration, cellular ATP, and MMP and a rise in glycolysis, mitochondrial superoxide, and mitochondrial ROS; meanwhile, cortisol only partially induced an aging-associated mitochondrial dysfunction. To replicate the in vivo aging-associated mitochondrial dysfunctions, using rotenone, a mitochondrial complex I inhibitor, proved to be superior to the cortisol model. This work is the first to use stress on young iNs to recreate aging-related mitochondrial impairments.

Polydatin and Nicotinamide Rescue the Cellular Phenotype of Mitochondrial Diseases by Mitochondrial Unfolded Protein Response (mtUPR) Activation

Primary mitochondrial diseases result from mutations in nuclear DNA (nDNA) or mitochondrial DNA (mtDNA) genes, encoding proteins crucial for mitochondrial structure or function. Given that few disease-specific therapies are available for mitochondrial diseases, novel treatments to reverse mitochondrial dysfunction are necessary. In this work, we explored new therapeutic options in mitochondrial diseases using fibroblasts and induced neurons derived from patients with mutations in the GFM1 gene. This gene encodes the essential mitochondrial translation elongation factor G1 involved in mitochondrial protein synthesis. Due to the severe mitochondrial defect, mutant GFM1 fibroblasts cannot survive in galactose medium, making them an ideal screening model to test the effectiveness of pharmacological compounds. We found that the combination of polydatin and nicotinamide enabled the survival of mutant GFM1 fibroblasts in stress medium. We also demonstrated that polydatin and nicotinamide upregulated the mitochondrial Unfolded Protein Response (mtUPR), especially the SIRT3 pathway. Activation of mtUPR partially restored mitochondrial protein synthesis and expression, as well as improved cellular bioenergetics. Furthermore, we confirmed the positive effect of the treatment in GFM1 mutant induced neurons obtained by direct reprogramming from patient fibroblasts. Overall, we provide compelling evidence that mtUPR activation is a promising therapeutic strategy for GFM1 mutations.

Direct neuronal reprogramming of NDUFS4 patient cells identifies the unfolded protein response as a novel general reprogramming hurdle

Mitochondria account for essential cellular pathways, from ATP production to nucleotide metabolism, and their deficits lead to neurological disorders and contribute to the onset of age-related diseases. Direct neuronal reprogramming aims at replacing neurons lost in such conditions, but very little is known about the impact of mitochondrial dysfunction on the direct reprogramming of human cells. Here, we explore the effects of mitochondrial dysfunction on the neuronal reprogramming of induced pluripotent stem cell (iPSC)-derived astrocytes carrying mutations in the NDUFS4 gene, important for Complex I and associated with Leigh syndrome. This led to the identification of the unfolded protein response as a major hurdle in the direct neuronal conversion of not only astrocytes and fibroblasts from patients but also control human astrocytes and fibroblasts. Its transient inhibition potently improves reprogramming by influencing the mitochondria-endoplasmic-reticulum-stress-mediated pathways. Taken together, disease modeling using patient cells unraveled novel general hurdles and ways to overcome these in human astrocyte-to-neuron reprogramming.

Unveiling the role of iPLA2β in neurodegeneration: From molecular mechanisms to advanced therapies

Calcium-independent phospholipase A2β (iPLA2β), a member of the phospholipase A2 (PLA2s) superfamily, is encoded by the PLA2G6 gene. Mutations in the PLA2G6 gene have been identified as the primary cause of infantile neuroaxonal dystrophy (INAD) and, less commonly, as a contributor to Parkinson's disease (PD). Recent studies have revealed that iPLA2β deficiency leads to neuroinflammation, iron accumulation, mitochondrial dysfunction, lipid dysregulation, and other pathological changes, forming a complex pathogenic network. These discoveries shed light on potential mechanisms underlying PLA2G6-associated neurodegeneration (PLAN) and offer valuable insights for therapeutic development. This review provides a comprehensive analysis of the fundamental characteristics of iPLA2β, its association with neurodegeneration, the pathogenic mechanisms involved in PLAN, and potential targets for therapeutic intervention. It offers an overview of the latest advancements in this field, aiming to contribute to ongoing research endeavors and facilitate the development of effective therapies for PLAN.

The role of pioneer transcription factors in the induction of direct cellular reprogramming

Regenerative medicine is a highly advanced medical field that aims to restore tissues and organs lost due to diseases and injury using a person's own cells or those of others. Direct cellular reprogramming is a promising technology that can directly induce cell-fate conversion from terminally differentiated cells to other cell types and is expected to play a pivotal role in applications in regenerative medicine. The induction of direct cellular reprogramming requires one or more master transcription factors with the potential to reconstitute cell type-specific transcription factor networks. The set of master transcription factors may contain unique transcription factors called pioneer factors that can open compacted chromatin structures and drive the transcriptional activation of target genes. Therefore, pioneer factors may play a central role in direct cellular reprogramming. However, our understanding of the molecular mechanisms by which pioneer factors induce cell-fate conversion is still limited. This review briefly summarizes the outcomes of recent findings and discusses future perspectives, focusing on the role of pioneer factors in direct cellular reprogramming.

Improving Efficiency of Direct Pro-Neural Reprogramming: Much-Needed Aid for Neuroregeneration in Spinal Cord Injury

Spinal cord injury (SCI) is a medical condition affecting ~2.5-4 million people worldwide. The conventional therapy for SCI fails to restore the lost spinal cord functions; thus, novel therapies are needed. Recent breakthroughs in stem cell biology and cell reprogramming revolutionized the field. Of them, the use of neural progenitor cells (NPCs) directly reprogrammed from non-neuronal somatic cells without transitioning through a pluripotent state is a particularly attractive strategy. This allows to "scale up" NPCs in vitro and, via their transplantation to the lesion area, partially compensate for the limited regenerative plasticity of the adult spinal cord in humans. As recently demonstrated in non-human primates, implanted NPCs contribute to the functional improvement of the spinal cord after injury, and works in other animal models of SCI also confirm their therapeutic value. However, direct reprogramming still remains a challenge in many aspects; one of them is low efficiency, which prevents it from finding its place in clinics yet. In this review, we describe new insights that recent works brought to the field, such as novel targets (mitochondria, nucleoli, G-quadruplexes, and others), tools, and approaches (mechanotransduction and electrical stimulation) for direct pro-neural reprogramming, including potential ones yet to be tested.

Patient-Derived Cellular Models for Polytarget Precision Medicine in Pantothenate Kinase-Associated Neurodegeneration

The term neurodegeneration with brain iron accumulation (NBIA) brings together a broad set of progressive and disabling neurological genetic disorders in which iron is deposited preferentially in certain areas of the brain. Among NBIA disorders, the most frequent subtype is pantothenate kinase-associated neurodegeneration (PKAN) caused by pathologic variants in the PANK2 gene codifying the enzyme pantothenate kinase 2 (PANK2). To date, there are no effective treatments to stop the progression of these diseases. This review discusses the utility of patient-derived cell models as a valuable tool for the identification of pharmacological or natural compounds for implementing polytarget precision medicine in PKAN. Recently, several studies have described that PKAN patient-derived fibroblasts present the main pathological features associated with the disease including intracellular iron overload. Interestingly, treatment of mutant cell cultures with various supplements such as pantothenate, pantethine, vitamin E, omega 3, α-lipoic acid L-carnitine or thiamine, improved all pathophysiological alterations in PKAN fibroblasts with residual expression of the PANK2 enzyme. The information provided by pharmacological screenings in patient-derived cellular models can help optimize therapeutic strategies in individual PKAN patients.

MicroRNAs in Extracellular Vesicles of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease with dysfunction of memory, language and thinking. More than 55 million people were diagnosed with AD or other dementia around the world in 2020. The pathology of AD is still unclear and there are no applicable therapies for AD. MicroRNAs (miRNAs) play key roles in AD pathology and have great potential for the diagnosis and treatment of AD. Extracellular vesicles (EVs) widely exist in body fluids such as blood and cerebrospinal fluid (CSF) and contain miRNAs that are involved in cell-to-cell communication. We summarized the dysregulated miRNAs in EVs derived from the different body fluids of AD patients, as well as their potential function and application in AD. We also compared these dysregulated miRNAs in EVs to those in the brain tissues of AD patients aiming to provide a comprehensive view of miRNAs in AD. After careful comparisons, we found that miR-125b-5p and miR-132-3p were upregulated and downregulated in several different brain tissues of AD and EVs of AD, respectively, suggesting their value in AD diagnosis based on EV miRNAs. Furthermore, miR-9-5p was dysregulated in EVs and different brain tissues of AD patients and had also been tested as a potential therapy for AD in mice and human cell models, suggesting that miR-9-5p could be used to design new therapies for AD.

MicroRNA‑124: an emerging therapeutic target in central nervous system disorders

The central nervous system (CNS) consists of neuron and non-neuron cells including neural stem/precursor cells (NSPCs), neuroblasts, glia cells (mainly astrocyte, oligodendroglia and microglia), which thereby form a precise and complicated network and exert diverse functions through interactions of numerous bioactive ingredients. MicroRNAs (miRNAs), with small size approximately  ~ 21nt and as well-documented post-transcriptional key regulators of gene expression, are a cluster of evolutionarily conserved endogenous non-coding RNAs. More than 2000 different miRNAs has been discovered till now. MicroRNA-124(miR-124), the most brain-rich microRNA, has been validated to possess important functions in the central nervous system, including neural stem cell proliferation and differentiation, cell fate determination, neuron migration, synapse plasticity and cognition, cell apoptosis etc. According to recent studies, herein, we provide a review of this conversant miR-124 to further understand the potential functions and therapeutic and clinical value in brain diseases.

Alpha-lipoic acid supplementation corrects pathological alterations in cellular models of pantothenate kinase-associated neurodegeneration with residual PANK2 expression levels

BACKGROUND

Neurodegeneration with brain iron accumulation (NBIA) disorders are a group of neurodegenerative diseases that have in common the accumulation of iron in the basal nuclei of the brain which are essential components of the extrapyramidal system. Frequent symptoms are progressive spasticity, dystonia, muscle rigidity, neuropsychiatric symptoms, and retinal degeneration or optic nerve atrophy. One of the most prevalent subtypes of NBIA is Pantothenate kinase-associated neurodegeneration (PKAN). It is caused by pathogenic variants in the gene of pantothenate kinase 2 (PANK2) which encodes the enzyme responsible for the first reaction on the coenzyme A (CoA) biosynthesis pathway. Thus, deficient PANK2 activity induces CoA deficiency as well as low expression levels of 4'-phosphopantetheinyl proteins which are essential for mitochondrial metabolism.

METHODS

This study is aimed at evaluating the role of alpha-lipoic acid (α-LA) in reversing the pathological alterations in fibroblasts and induced neurons derived from PKAN patients. Iron accumulation, lipid peroxidation, transcript and protein expression levels of PANK2, mitochondrial ACP (mtACP), 4''-phosphopantetheinyl and lipoylated proteins, as well as pyruvate dehydrogenase (PDH) and Complex I activity were examined.

RESULTS

Treatment with α-LA was able to correct all pathological alterations in responsive mutant fibroblasts with residual PANK2 enzyme expression. However, α-LA had no effect on mutant fibroblasts with truncated/incomplete protein expression. The positive effect of α-LA in particular pathogenic variants was also confirmed in induced neurons derived from mutant fibroblasts.

CONCLUSIONS

Our results suggest that α-LA treatment can increase the expression levels of PANK2 and reverse the mutant phenotype in PANK2 responsive pathogenic variants. The existence of residual enzyme expression in some affected individuals raises the possibility of treatment using high dose of α-LA.

Somatic Cell Reprogramming for Nervous System Diseases: Techniques, Mechanisms, Potential Applications, and Challenges

Nervous system diseases present significant challenges to the neuroscience community due to ethical and practical constraints that limit access to appropriate research materials. Somatic cell reprogramming has been proposed as a novel way to obtain neurons. Various emerging techniques have been used to reprogram mature and differentiated cells into neurons. This review provides an overview of somatic cell reprogramming for neurological research and therapy, focusing on neural reprogramming and generating different neural cell types. We examine the mechanisms involved in reprogramming and the challenges that arise. We herein summarize cell reprogramming strategies to generate neurons, including transcription factors, small molecules, and microRNAs, with a focus on different types of cells.. While reprogramming somatic cells into neurons holds the potential for understanding neurological diseases and developing therapeutic applications, its limitations and risks must be carefully considered. Here, we highlight the potential benefits of somatic cell reprogramming for neurological disease research and therapy. This review contributes to the field by providing a comprehensive overview of the various techniques used to generate neurons by cellular reprogramming and discussing their potential applications.

The Wisdom in Teeth: Neuronal Differentiation of Dental Pulp Cells

Mesenchymal stem/stromal cells (MSCs) are found in almost all postnatal organs. Under appropriate environmental cues, multipotency enables MSCs to serve as progenitors for several lineage-specific, differentiated cell types. In vitro expansion and differentiation of MSCs give the opportunity to obtain hardly available somatic cells, such as neurons. The neurogenic potential of MSCs makes them a promising, autologous source to restore damaged tissue and as such, they have received much attention in the field of regenerative medicine. Several stem cell pool candidates have been studied thus far, but only a few of them showed neurogenic differentiation potential. Due to their embryonic ontology, stem cells residing in the stroma of the dental pulp chamber are an exciting source for in vitro neural cell differentiation. In this study, we review the key properties of dental pulp stem cells (DPSCs), with a particular focus on their neurogenic potential. Moreover, we summarize the various presently available methods used for neural differentiation of human DPSCs also emphasizing the difficulties in reproducibly high production of such cells. We postulate that because DPSCs are stem cells with very close ontology to neurogenic lineages, they may serve as excellent targets for neuronal differentiation in vitro and even for direct reprogramming.

Direct Conversion of Fibroblast into Neurons for Alzheimer's Disease Research: A Systematic Review

BACKGROUND

Alzheimer's disease (AD) is a prevalent neurodegenerative disorder without a cure. Innovative disease models, such as induced neurons (iNs), could enhance our understanding of AD mechanisms and accelerate treatment development. However, a review of AD human iN studies is necessary to consolidate knowledge.

OBJECTIVE

The objective of this review is to examine the current body of literature on AD human iN cells and provide an overview of the findings to date.

METHODS

We searched two databases for relevant studies published between 2010 and 2023, identifying nine studies meeting our criteria.

RESULTS

Reviewed studies indicate the feasibility of generating iNs directly from AD patients' fibroblasts using chemical induction or viral vectors. These cells express mature neuronal markers, including MAP-2, NeuN, synapsin, and tau. However, most studies were limited in sample size and primarily focused on autosomal dominant familial AD (FAD) rather than the more common sporadic forms of AD. Several studies indicated that iNs derived from FAD fibroblasts exhibited abnormal amyloid-β metabolism, a characteristic feature of AD in humans. Additionally, elevated levels of hyperphosphorylated tau, another hallmark of AD, were reported in some studies.

CONCLUSION

Although only a limited number of small-scale studies are currently available, AD patient-derived iNs hold promise as a valuable model for investigating AD pathogenesis. Future research should aim to conduct larger studies, particularly focusing on sporadic AD cases, to enhance the clinical relevance of the findings for the broader AD patient population. Moreover, these cells can be utilized in screening potential novel treatments for AD.

Age-related pathological impairments in directly reprogrammed dopaminergic neurons derived from patients with idiopathic Parkinson's disease

We have developed an efficient approach to generate functional induced dopaminergic (DA) neurons from adult human dermal fibroblasts. When performing DA neuronal conversion of patient fibroblasts with idiopathic Parkinson's disease (PD), we could specifically detect disease-relevant pathology in these cells. We show that the patient-derived neurons maintain age-related properties of the donor and exhibit lower basal chaperone-mediated autophagy compared with healthy donors. Furthermore, stress-induced autophagy resulted in an age-dependent accumulation of macroautophagic structures. Finally, we show that these impairments in patient-derived DA neurons leads to an accumulation of phosphorylated alpha-synuclein, the classical hallmark of PD pathology. This pathological phenotype is absent in neurons generated from induced pluripotent stem cells from the same patients. Taken together, our results show that direct neural reprogramming can be used for obtaining patient-derived DA neurons, which uniquely function as a cellular model to study age-related pathology relevant to idiopathic PD.

miR-29b Modulates Bone Marrow Mesenchymal Stem Cells (BMSCs) Differentiation and Induces Nerve Repair in Diabetic Retina Rat Model

microRNAs are involved in diabetic retinopathy (DR). This study intends to analyze miR-29b’s role in bone marrow mesenchymal stem cells (BMSCs) differentiation in DR rat models to induce nerve repair. BMSCs from DR rat models were cultured and transfected with miR-29b mimics and inhibitors followed by measuring miR-29b level, cell proliferation and apoptosis. Retinal ganglion cells (RGC) were treated with high glucose for 24 h, and BMSCs and si-miR-29b-BMSC were cocultured for 24 h, respectively followed by assessing cell proliferation and apoptosis, inflammatory cytokines by ELISA, MDA, SOD, brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF) level by ELISA. MiR-29b was up-regulated in BMSCs of DR rats. miR-29b mimics significantly up-regulated miR-29b, inhibited cell proliferation and promoted apoptosis (P < 0.05), which were reversed by miR-29b inhibitor (P < 0.05). Co-culture of BMSCs with si-miR-29b-BMSC promoted RGC proliferation, inhibited apoptosis and IL-6 secretion, decreased MDA, increased SOD, BDNF and CNTF expression (P < 0.05) with more significant changes in si-miR-29b-BMSC group (P < 0.05). In conclusion, down-regulation of miR-29b promotes BMSCs proliferation in DR rat models, inhibits BMSCs apoptosis, and promotes the recovery of retinal ganglion cell function.

Ascl1 phospho-site mutations enhance neuronal conversion of adult cortical astrocytes in vivo

Direct neuronal reprogramming, the process whereby a terminally differentiated cell is converted into an induced neuron without traversing a pluripotent state, has tremendous therapeutic potential for a host of neurodegenerative diseases. While there is strong evidence for astrocyte-to-neuron conversion in vitro, in vivo studies in the adult brain are less supportive or controversial. Here, we set out to enhance the efficacy of neuronal conversion of adult astrocytes in vivo by optimizing the neurogenic capacity of a driver transcription factor encoded by the proneural gene Ascl1. Specifically, we mutated six serine phospho-acceptor sites in Ascl1 to alanines (Ascl1 SA 6) to prevent phosphorylation by proline-directed serine/threonine kinases. Native Ascl1 or Ascl1 SA 6 were expressed in adult, murine cortical astrocytes under the control of a glial fibrillary acidic protein (GFAP) promoter using adeno-associated viruses (AAVs). When targeted to the cerebral cortex in vivo, mCherry+ cells transduced with AAV8-GFAP-Ascl1 SA 6-mCherry or AAV8-GFAP-Ascl1-mCherry expressed neuronal markers within 14 days post-transduction, with Ascl1 SA 6 promoting the formation of more mature dendritic arbors compared to Ascl1. However, mCherry expression disappeared by 2-months post-transduction of the AAV8-GFAP-mCherry control-vector. To circumvent reporter issues, AAV-GFAP-iCre (control) and AAV-GFAP-Ascl1 (or Ascl1 SA 6)-iCre constructs were generated and injected into the cerebral cortex of Rosa reporter mice. In all comparisons of AAV capsids (AAV5 and AAV8), GFAP promoters (long and short), and reporter mice (Rosa-zsGreen and Rosa-tdtomato), Ascl1 SA 6 transduced cells more frequently expressed early- (Dcx) and late- (NeuN) neuronal markers. Furthermore, Ascl1 SA 6 repressed the expression of astrocytic markers Sox9 and GFAP more efficiently than Ascl1. Finally, we co-transduced an AAV expressing ChR2-(H134R)-YFP, an optogenetic actuator. After channelrhodopsin photostimulation, we found that Ascl1 SA 6 co-transduced astrocytes exhibited a significantly faster decay of evoked potentials to baseline, a neuronal feature, when compared to iCre control cells. Taken together, our findings support an enhanced neuronal conversion efficiency of Ascl1 SA 6 vs. Ascl1, and position Ascl1 SA 6 as a critical transcription factor for future studies aimed at converting adult brain astrocytes to mature neurons to treat disease.

An Improved Method to Generate Human Induced Astrocytes

A major improvement in the generation of astrocytes directly from human fibroblasts will now facilitate the study of how aging impacts on astrocyte function and whether this contributes to neurodegenerative disorders.

Modeling Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-Like Episodes Syndrome Using Patient-Derived Induced Neurons Generated by Direct Reprogramming

Mitochondrial diseases are a heterogeneous group of rare genetic disorders caused by mutations in nuclear or mitochondrial DNA (mtDNA). These diseases are frequently multisystemic, although mainly affect tissues that require large amounts of energy such as the brain. Mutations in mitochondrial transfer RNA (mt-tRNA) lead to defects in protein translation that may compromise some or all mtDNA-encoded proteins. Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-like episodes (MELAS) syndrome is mainly caused by the m.3243A>G mutation in the mt-tRNA^Leu(UUR) (MT-TL1) gene. Owing to the lack of proper animal models, several cellular models have been developed to study the disease, providing insight in the pathophysiological mechanisms of MELAS. In this study, we show a successful direct conversion of MELAS patient-derived fibroblasts into induced neurons (iNs) for the first time, as well as an electrophysiological characterization of iNs cocultured with astrocytes. In addition, we performed bioenergetics analysis to study the consequences of m.3243A>G mutation in this neuronal model of MELAS syndrome.

Disease Modeling of Neurodegenerative Disorders Using Direct Neural Reprogramming

Understanding the pathophysiology of CNS-associated neurological diseases has been hampered by the inaccessibility of patient brain tissue to perform live analyses at the molecular level. To this end, neural cells obtained by differentiation of patient-derived induced pluripotent stem cells (iPSCs) are considerably helpful, especially in the context of monogenic-based disorders. More recently, the use of direct reprogramming to convert somatic cells to neural cells has emerged as an alternative to iPSCs to generate neurons, astrocytes, and oligodendrocytes. This review focuses on the different studies that used direct neural reprogramming to study disease-associated phenotypes in the context of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis.

Transcription factor-based direct conversion of human fibroblasts to functional astrocytes

Astrocytes are emerging key players in neurological disorders. However, their role in disease etiology is poorly understood owing to inaccessibility of primary human astrocytes. Pluripotent stem cell-derived cells fail to mimic age and due to their clonal origin do not mimic genetic heterogeneity of patients. In contrast, direct conversion constitutes an attractive approach to generate human astrocytes that capture age and genetic diversity. We describe efficient direct conversion of human fibroblasts to functional induced astrocytes (iAs). Expression of the minimal combination Sox9 and Nfib generates iAs with molecular, phenotypic, and functional properties resembling primary human astrocytes. iAs could be obtained by conversion of fibroblasts covering the entire human lifespan. Importantly, iAs supported function of induced neurons obtained through direct conversion from the same fibroblast population. Fibroblast-derived iAs will become a useful tool to elucidate the biology of astrocytes and complement current in vitro models for studies of late-onset neurological disorders.

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Effective direct conversion of human fibroblasts to induced astrocytes (iAs)

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iAs resemble human primary astrocytes at molecular, phenotypic, and functional levels

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iAs can be generated from fibroblasts covering the entire human lifespan

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iAs support function of induced neurons obtained from the same starting population

UPRmt activation improves pathological alterations in cellular models of mitochondrial diseases

Background

Mitochondrial diseases represent one of the most common groups of genetic diseases. With a prevalence greater than 1 in 5000 adults, such diseases still lack effective treatment. Current therapies are purely palliative and, in most cases, insufficient. Novel approaches to compensate and, if possible, revert mitochondrial dysfunction must be developed.

Results

In this study, we tackled the issue using as a model fibroblasts from a patient bearing a mutation in the GFM1 gene, which is involved in mitochondrial protein synthesis. Mutant GFM1 fibroblasts could not survive in galactose restrictive medium for more than 3 days, making them the perfect screening platform to test several compounds. Tetracycline enabled mutant GFM1 fibroblasts survival under nutritional stress. Here we demonstrate that tetracycline upregulates the mitochondrial Unfolded Protein Response (UPR^mt), a compensatory pathway regulating mitochondrial proteostasis. We additionally report that activation of UPR^mt improves mutant GFM1 cellular bioenergetics and partially restores mitochondrial protein expression.

Conclusions

Overall, we provide compelling evidence to propose the activation of intrinsic cellular compensatory mechanisms as promising therapeutic strategy for mitochondrial diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02331-8.

Vitamin E prevents lipid peroxidation and iron accumulation in PLA2G6-Associated Neurodegeneration

BACKGROUND

PLA2G6-Associated Neurodegeneration (PLAN) is a rare neurodegenerative disease with autosomal recessive inheritance, which belongs to the NBIA (Neurodegeneration with Brain Iron Accumulation) group. Although the pathogenesis of the disease remains largely unclear, lipid peroxidation seems to play a central role in the pathogenesis. Currently, there is no cure for the disease.

OBJECTIVE

In this work, we examined the presence of lipid peroxidation, iron accumulation and mitochondrial dysfunction in two cellular models of PLAN, patients-derived fibroblasts and induced neurons, and assessed the effects of α-tocopherol (vitamin E) in correcting the pathophysiological alterations in PLAN cell cultures.

METHODS

Pathophysiological alterations were examined in fibroblasts and induced neurons generated by direct reprograming. Iron and lipofuscin accumulation were assessed using light and electron microscopy, as well as biochemical analysis techniques. Reactive Oxygen species production, lipid peroxidation and mitochondrial dysfunction were measured using specific fluorescent probes analysed by fluorescence microscopy and flow cytometry.

RESULTS

PLAN fibroblasts and induced neurons clearly showed increased lipid peroxidation, iron accumulation and altered mitochondrial membrane potential. All these pathological features were reverted with vitamin E treatment.

CONCLUSIONS

PLAN fibroblasts and induced neurons reproduce the main pathological alterations of the disease and provide useful tools for disease modelling. The main pathological alterations were corrected by Vitamin E supplementation in both models, suggesting that blocking lipid peroxidation progression is a critical therapeutic target.

Neurodegenerative brain models vs. cell replacement or restoration therapy: A review on promises and pitfalls

Disease models have proven useful tools for gaining deeper mechanistic insights into neurodegenerative diseases. In this context, stem cell technology is effective, especially induced pluripotent stem cell (iPSC)-derived brain organoids and cell replacement/restoration which can be used for personalized medicine, allowing physicians to test the efficacy of drugs in vitro before delivering them to patients, enabling more precise and personalized treatment. Nonetheless, it offers the potential to minimize (or even eliminate) the use of animals, provides important clues for disease processes, and accelerates therapeutic strategies. Perhaps in the not-too-distant future, organoid models of the human brain will be able to link blood-brain barrier cultures with other liver cultures, simulating blood flow across organs and as a method of testing medicines, giving crucial pharmacokinetics and pharmacodynamics data. Simultaneously, stem cell interventions for cell replacements or restoration therapy would enable us to realize efficacious and realistic therapeutic options for Neurodegenerative diseases.

Distinct subcellular autophagy impairments in induced neurons from Huntington’s disease patients

Huntington's disease is a neurodegenerative disorder caused by CAG expansions in the huntingtin (HTT) gene. Modelling Huntington's disease is challenging, as rodent and cellular models poorly recapitulate the disease as seen in ageing humans. To address this, we generated induced neurons through direct reprogramming of human skin fibroblasts, which retain age-dependent epigenetic characteristics.

Huntington's disease induced neurons (HD-iNs) displayed profound deficits in autophagy, characterized by reduced transport of late autophagic structures from the neurites to the soma. These neurite-specific alterations in autophagy resulted in shorter, thinner and fewer neurites specifically in HD-iNs. CRISPRi-mediated silencing of HTT did not rescue this phenotype but rather resulted in additional autophagy alterations in control induced neurons, highlighting the importance of wild-type HTT in normal neuronal autophagy.

In summary, our work identifies a distinct subcellular autophagy impairment in adult patient derived Huntington's disease neurons and provides a new rationale for future development of autophagy activation therapies.

Direct neuronal reprogramming: Fast forward from new concepts toward therapeutic approaches

Differentiated cells have long been considered fixed in their identity. However, about 20 years ago, the first direct conversion of glial cells into neurons in vitro opened the field of "direct neuronal reprogramming." Since then, neuronal reprogramming has achieved the generation of fully functional, mature neurons with remarkable efficiency, even in diseased brain environments. Beyond their clinical implications, these discoveries provided basic insights into crucial mechanisms underlying conversion of specific cell types into neurons and maintenance of neuronal identity. Here we discuss such principles, including the importance of the starter cell for shaping the outcome of neuronal reprogramming. We further highlight technical concerns for in vivo reprogramming and propose a code of conduct to avoid artifacts and pitfalls. We end by pointing out next challenges for development of less invasive cell replacement therapies for humans.

Reprogramming Human Adult Fibroblasts into GABAergic Interneurons

Direct reprogramming is an appealing strategy to generate neurons from a somatic cell by forced expression of transcription factors. The generated neurons can be used for both cell replacement strategies and disease modelling. Using this technique, previous studies have shown that γ-aminobutyric acid (GABA) expressing interneurons can be generated from different cell sources, such as glia cells or fetal fibroblasts. Nevertheless, the generation of neurons from adult human fibroblasts, an easily accessible cell source to obtain patient-derived neurons, has proved to be challenging due to the intrinsic blockade of neuronal commitment. In this paper, we used an optimized protocol for adult skin fibroblast reprogramming based on RE1 Silencing Transcription Factor (REST) inhibition together with a combination of GABAergic fate determinants to convert human adult skin fibroblasts into GABAergic neurons. Our results show a successful conversion in 25 days with upregulation of neuronal gene and protein expression levels. Moreover, we identified specific gene combinations that converted fibroblasts into neurons of a GABAergic interneuronal fate. Despite the well-known difficulty in converting adult fibroblasts into functional neurons in vitro, we could detect functional maturation in the induced neurons. GABAergic interneurons have relevance for cognitive impairments and brain disorders, such as Alzheimer’s and Parkinson’s diseases, epilepsy, schizophrenia and autism spectrum disorders.

Leigh Syndrome: A Tale of Two Genomes

Leigh syndrome is a rare, complex, and incurable early onset (typically infant or early childhood) mitochondrial disorder with both phenotypic and genetic heterogeneity. The heterogeneous nature of this disorder, based in part on the complexity of mitochondrial genetics, and the significant interactions between the nuclear and mitochondrial genomes has made it particularly challenging to research and develop therapies. This review article discusses some of the advances that have been made in the field to date. While the prognosis is poor with no current substantial treatment options, multiple studies are underway to understand the etiology, pathogenesis, and pathophysiology of Leigh syndrome. With advances in available research tools leading to a better understanding of the mitochondria in health and disease, there is hope for novel treatment options in the future.

Direct Neuronal Reprogramming: Bridging the Gap Between Basic Science and Clinical Application

Direct neuronal reprogramming is an innovative new technology that involves the conversion of somatic cells to induced neurons (iNs) without passing through a pluripotent state. The capacity to make new neurons in the brain, which previously was not achievable, has created great excitement in the field as it has opened the door for the potential treatment of incurable neurodegenerative diseases and brain injuries such as stroke. These neurological disorders are associated with frank neuronal loss, and as new neurons are not made in most of the adult brain, treatment options are limited. Developmental biologists have paved the way for the field of direct neuronal reprogramming by identifying both intrinsic cues, primarily transcription factors (TFs) and miRNAs, and extrinsic cues, including growth factors and other signaling molecules, that induce neurogenesis and specify neuronal subtype identities in the embryonic brain. The striking observation that postmitotic, terminally differentiated somatic cells can be converted to iNs by mis-expression of TFs or miRNAs involved in neural lineage development, and/or by exposure to growth factors or small molecule cocktails that recapitulate the signaling environment of the developing brain, has opened the door to the rapid expansion of new neuronal reprogramming methodologies. Furthermore, the more recent applications of neuronal lineage conversion strategies that target resident glial cells in situ has expanded the clinical potential of direct neuronal reprogramming techniques. Herein, we present an overview of the history, accomplishments, and therapeutic potential of direct neuronal reprogramming as revealed over the last two decades.

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.

Cell Reprogramming to Model Huntington's Disease: A Comprehensive Review

Huntington’s disease (HD) is a neurodegenerative disorder characterized by the progressive decline of motor, cognitive, and psychiatric functions. HD results from an autosomal dominant mutation that causes a trinucleotide CAG repeat expansion and the production of mutant Huntingtin protein (mHTT). This results in the initial selective and progressive loss of medium spiny neurons (MSNs) in the striatum before progressing to involve the whole brain. There are currently no effective treatments to prevent or delay the progression of HD as knowledge into the mechanisms driving the selective degeneration of MSNs has been hindered by a lack of access to live neurons from individuals with HD. The invention of cell reprogramming provides a revolutionary technique for the study, and potential treatment, of neurological conditions. Cell reprogramming technologies allow for the generation of live disease-affected neurons from patients with neurological conditions, becoming a primary technique for modelling these conditions in vitro. The ability to generate HD-affected neurons has widespread applications for investigating the pathogenesis of HD, the identification of new therapeutic targets, and for high-throughput drug screening. Cell reprogramming also offers a potential autologous source of cells for HD cell replacement therapy. This review provides a comprehensive analysis of the use of cell reprogramming to model HD and a discussion on recent advancements in cell reprogramming technologies that will benefit the HD field.

REST Inhibits Direct Reprogramming of Pancreatic Exocrine to Endocrine Cells by Preventing PDX1-Mediated Activation of Endocrine Genes

The emerging appreciation of plasticity among pancreatic lineages has created interest in harnessing cellular reprogramming for β cell replacement therapy of diabetes. Current reprogramming methodologies are inefficient, largely because of a limited understanding of the underlying mechanisms. Using an in vitro reprogramming system, we reveal the transcriptional repressor RE-1 silencing transcription factor (REST) as a barrier for β cell gene expression in the reprogramming of pancreatic exocrine cells. We observe that REST-bound loci lie adjacent to the binding sites of multiple key β cell transcription factors, including PDX1. Accordingly, a loss of REST function combined with PDX1 expression results in the synergistic activation of endocrine genes. This is accompanied by increased histone acetylation and PDX1 binding at endocrine gene loci. Collectively, our data identify a mechanism for REST activity involving the prevention of PDX1-mediated activation of endocrine genes and uncover REST downregulation and the resulting chromatin alterations as key events in β cell reprogramming.

Dopamine Neuron Diversity: Recent Advances and Current Challenges in Human Stem Cell Models and Single Cell Sequencing

Human midbrain dopamine (DA) neurons are a heterogeneous group of cells that share a common neurotransmitter phenotype and are in close anatomical proximity but display different functions, sensitivity to degeneration, and axonal innervation targets. The A9 DA neuron subtype controls motor function and is primarily degenerated in Parkinson’s disease (PD), whereas A10 neurons are largely unaffected by the condition, and their dysfunction is associated with neuropsychiatric disorders. Currently, DA neurons can only be reliably classified on the basis of topographical features, including anatomical location in the midbrain and projection targets in the forebrain. No systematic molecular classification at the genome-wide level has been proposed to date. Although many years of scientific efforts in embryonic and adult mouse brain have positioned us to better understand the complexity of DA neuron biology, many biological phenomena specific to humans are not amenable to being reproduced in animal models. The establishment of human cell-based systems combined with advanced computational single-cell transcriptomics holds great promise for decoding the mechanisms underlying maturation and diversification of human DA neurons, and linking their molecular heterogeneity to functions in the midbrain. Human pluripotent stem cells have emerged as a useful tool to recapitulate key molecular features of mature DA neuron subtypes. Here, we review some of the most recent advances and discuss the current challenges in using stem cells, to model human DA biology. We also describe how single cell RNA sequencing may provide key insights into the molecular programs driving DA progenitor specification into mature DA neuron subtypes. Exploiting the state-of-the-art approaches will lead to a better understanding of stem cell-derived DA neurons and their use in disease modeling and regenerative medicine.

Directly reprogrammed Huntington's disease neural precursor cells generate striatal neurons exhibiting aggregates and impaired neuronal maturation

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by the progressive loss of striatal medium spiny neurons. Using a highly efficient protocol for direct reprogramming of adult human fibroblasts with chemically modified mRNA, we report the first generation of HD induced neural precursor cells (iNPs) expressing striatal lineage markers that differentiated into DARPP32+ neurons from individuals with adult-onset HD (41-57 CAG). While no transcriptional differences between normal and HD reprogrammed neurons were detected by NanoString nCounter analysis, a subpopulation of HD reprogrammed neurons contained ubiquitinated polyglutamine aggregates. Importantly, reprogrammed HD neurons exhibited impaired neuronal maturation, displaying altered neurite morphology and more depolarized resting membrane potentials. Reduced BDNF protein expression in reprogrammed HD neurons correlated with increased CAG repeat lengths and earlier symptom onset. This model represents a platform for investigating impaired neuronal maturation and screening for neuronal maturation modifiers to treat HD.

Down regulation of the expression of mitochondrial phosphopantetheinyl-proteins in pantothenate kinase-associated neurodegeneration: pathophysiological consequences and therapeutic perspectives

Background

Neurodegeneration with brain iron accumulation (NBIA) is a group of genetic neurological disorders frequently associated with iron accumulation in the basal nuclei of the brain characterized by progressive spasticity, dystonia, muscle rigidity, neuropsychiatric symptoms, and retinal degeneration or optic nerve atrophy. Pantothenate kinase-associated neurodegeneration (PKAN) is the most widespread NBIA disorder. It is caused by mutations in the gene of pantothenate kinase 2 (PANK2) which catalyzes the first reaction of coenzyme A (CoA) biosynthesis. Thus, altered PANK2 activity is expected to induce CoA deficiency as well as low levels of essential metabolic intermediates such as 4′-phosphopantetheine which is a necessary cofactor for critical proteins involved in cytosolic and mitochondrial pathways such as fatty acid biosynthesis, mitochondrial respiratory complex I assembly and lysine and tetrahydrofolate metabolism, among other metabolic processes.

Methods

In this manuscript, we examined the effect of PANK2 mutations on the expression levels of proteins with phosphopantetheine cofactors in fibroblast derived from PKAN patients. These proteins include cytosolic acyl carrier protein (ACP), which is integrated within the multifunctional polypeptide chain of the fatty acid synthase involved in cytosolic fatty acid biosynthesis type I (FASI); mitochondrial ACP (mtACP) associated with mitocondrial fatty acid biosynthesis type II (FASII); mitochondrial alpha-aminoadipic semialdehyde synthase (AASS); and 10-formyltetrahydrofolate dehydrogenases (cytosolic, ALD1L1, and mitochondrial, ALD1L2).

Results

In PKAN fibroblasts the expression levels of cytosolic FAS and ALD1L1 were not affected while the expression levels of mtACP, AASS and ALD1L2 were markedly reduced, suggesting that 4′-phosphopantetheinylation of mitochondrial but no cytosolic proteins were markedly affected in PKAN patients. Furthermore, the correction of PANK2 expression levels by treatment with pantothenate in selected mutations with residual enzyme content was able to correct the expression levels of mitochondrial phosphopantetheinyl-proteins and restore the affected pathways. The positive effects of pantothenate in particular mutations were also corroborated in induced neurons obtained by direct reprograming of mutant PANK2 fibroblasts.

Conclusions

Our results suggest that the expression levels of mitochondrial phosphopantetheinyl-proteins are severely reduced in PKAN cells and that in selected mutations pantothenate increases the expression levels of both PANK2 and mitochondrial phosphopantetheinyl-proteins associated with remarkable improvement of cell pathophysiology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-021-01823-3.

Plasma metabolomics of presymptomatic PSEN1-H163Y mutation carriers: a pilot study

BACKGROUND AND OBJECTIVE

PSEN1-H163Y carriers, at the presymptomatic stage, have reduced ¹⁸ FDG-PET binding in the cerebrum of the brain (Scholl et al., Neurobiol Aging 32:1388-1399, 2011). This could imply dysfunctional energy metabolism in the brain. In this study, plasma of presymptomatic PSEN1 mutation carriers was analyzed to understand associated metabolic changes.

METHODS

We analyzed plasma from noncarriers (NC, n = 8) and presymptomatic PSEN1-H163Y mutation carriers (MC, n = 6) via untargeted metabolomics using gas and liquid chromatography coupled with mass spectrometry, which identified 1199 metabolites. All the metabolites were compared between MC and NC using univariate analysis, as well as correlated with the ratio of Aβ_{1-42/A β 1-40} , using Spearman's correlation. Altered metabolites were subjected to Ingenuity Pathway Analysis (IPA).

RESULTS

Based on principal component analysis the plasma metabolite profiles were divided into dataset A and dataset B. In dataset A, when comparing between presymptomatic MC and NC, the levels of 79 different metabolites were altered. Out of 79, only 14 were annotated metabolites. In dataset B, 37 metabolites were significantly altered between presymptomatic MC and NC and nine metabolites were annotated. In both datasets, annotated metabolites represent amino acids, fatty acyls, bile acids, hexoses, purine nucleosides, carboxylic acids, and glycerophosphatidylcholine species. 1-docosapentaenoyl-GPC was positively correlated, uric acid and glucose were negatively correlated with the ratio of plasma Aβ_1-42 /Aβ_1-40 (P < 0.05).

INTERPRETATION

This study finds dysregulated metabolite classes, which are changed before the disease symptom onset. Also, it provides an opportunity to compare with sporadic Alzheimer's Disease. Observed findings in this study need to be validated in a larger and independent Familial Alzheimer's Disease (FAD) cohort.

A quantitative model of cellular decision making in direct neuronal reprogramming

The direct reprogramming of adult skin fibroblasts to neurons is thought to be controlled by a small set of interacting gene regulators. Here, we investigate how the interaction dynamics between these regulating factors coordinate cellular decision making in direct neuronal reprogramming. We put forward a quantitative model of the governing gene regulatory system, supported by measurements of mRNA expression. We found that nPTB needs to feed back into the direct neural conversion network most likely via PTB in order to accurately capture quantitative gene interaction dynamics and correctly predict the outcome of various overexpression and knockdown experiments. This was experimentally validated by nPTB knockdown leading to successful neural conversion. We also proposed a novel analytical technique to dissect system behaviour and reveal the influence of individual factors on resulting gene expression. Overall, we demonstrate that computational analysis is a powerful tool for understanding the mechanisms of direct (neuronal) reprogramming, paving the way for future models that can help improve cell conversion strategies.

Advances in mt-tRNA Mutation-Caused Mitochondrial Disease Modeling: Patients' Brain in a Dish

Mitochondrial diseases are a heterogeneous group of rare genetic disorders that can be caused by mutations in nuclear (nDNA) or mitochondrial DNA (mtDNA). Mutations in mtDNA are associated with several maternally inherited genetic diseases, with mitochondrial dysfunction as a main pathological feature. These diseases, although frequently multisystemic, mainly affect organs that require large amounts of energy such as the brain and the skeletal muscle. In contrast to the difficulty of obtaining neuronal and muscle cell models, the development of induced pluripotent stem cells (iPSCs) has shed light on the study of mitochondrial diseases. However, it is still a challenge to obtain an appropriate cellular model in order to find new therapeutic options for people suffering from these diseases. In this review, we deepen the knowledge in the current models for the most studied mt-tRNA mutation-caused mitochondrial diseases, MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) and MERRF (myoclonic epilepsy with ragged red fibers) syndromes, and their therapeutic management. In particular, we will discuss the development of a novel model for mitochondrial disease research that consists of induced neurons (iNs) generated by direct reprogramming of fibroblasts derived from patients suffering from MERRF syndrome. We hypothesize that iNs will be helpful for mitochondrial disease modeling, since they could mimic patient’s neuron pathophysiology and give us the opportunity to correct the alterations in one of the most affected cellular types in these disorders.

Prototypical oncogene family Myc defines unappreciated distinct lineage states of small cell lung cancer

Comprehensive genomic analyses of small cell lung cancer (SCLC) have revealed frequent mutually exclusive genomic amplification of MYC family members. Hence, it has been long suggested that they are functionally equivalent; however, more recently, their expression has been associated with specific neuroendocrine markers and distinct histopathology. Here, we explored a previously undescribed role of L-Myc and c-Myc as lineage-determining factors contributing to SCLC molecular subtypes and histology. Integrated transcriptomic and epigenomic analyses showed that L-Myc and c-Myc impart neuronal and non-neuroendocrine-associated transcriptional programs, respectively, both associated with distinct SCLC lineage. Genetic replacement of c-Myc with L-Myc in c-Myc-SCLC induced a neuronal state but was insufficient to induce ASCL1-SCLC. In contrast, c-Myc induced transition from ASCL1-SCLC to NEUROD1-SCLC characterized by distinct large-cell neuroendocrine carcinoma-like histopathology. Collectively, we characterize a role of historically defined general oncogenes, c-Myc and L-Myc, for regulating lineage plasticity across molecular and histological subtypes.

Current and future applications of induced pluripotent stem cell-based models to study pathological proteins in neurodegenerative disorders

Neurodegenerative disorders emerge from the failure of intricate cellular mechanisms, which ultimately lead to the loss of vulnerable neuronal populations. Research conducted across several laboratories has now provided compelling evidence that pathogenic proteins can also contribute to non-cell autonomous toxicity in several neurodegenerative contexts, including Alzheimer's, Parkinson's, and Huntington's diseases as well as Amyotrophic Lateral Sclerosis. Given the nearly ubiquitous nature of abnormal protein accumulation in such disorders, elucidating the mechanisms and routes underlying these processes is essential to the development of effective treatments. To this end, physiologically relevant human in vitro models are critical to understand the processes surrounding uptake, release and nucleation under physiological or pathological conditions. This review explores the use of human-induced pluripotent stem cells (iPSCs) to study prion-like protein propagation in neurodegenerative diseases, discusses advantages and limitations of this model, and presents emerging technologies that, combined with the use of iPSC-based models, will provide powerful model systems to propel fundamental research forward.

The application of in vitro-derived human neurons in neurodegenerative disease modeling

The development of safe and effective treatments for age-associated neurodegenerative disorders is an on-going challenge faced by the scientific field. Key to the development of such therapies is the appropriate selection of modeling systems in which to investigate disease mechanisms and to test candidate interventions. There are unique challenges in the development of representative laboratory models of neurodegenerative diseases, including the complexity of the human brain, the cumulative and variable contributions of genetic and environmental factors over the course of a lifetime, inability to culture human primary neurons, and critical central nervous system differences between small animal models and humans. While traditional rodent models have advanced our understanding of neurodegenerative disease mechanisms, key divergences such as the species-specific genetic background can limit the application of animal models in many cases. Here we review in vitro human neuronal systems that employ stem cell and reprogramming technology and their application to a range of neurodegenerative diseases. Specifically, we compare human-induced pluripotent stem cell-derived neurons to directly converted, or transdifferentiated, induced neurons, as both model systems can take advantage of patient-derived human tissue to produce neurons in culture. We present recent technical developments using these two modeling systems, as well as current limitations to these systems, with the aim of advancing investigation of neuropathogenic mechanisms using these models.

Precision Medicine in Rare Diseases

Rare diseases are those that have a low prevalence in the population (less than 5 individuals per 10,000 inhabitants). However, infrequent pathologies affect a large number of people, since according to the World Health Organization (WHO), there are about 7000 rare diseases that affect 7% of the world’s population. Many patients with rare diseases have suffered the consequences of what is called the diagnostic odyssey, that is, extensive and prolonged serial tests and clinical visits, sometimes for many years, all with the hope of identifying the etiology of their disease. For patients with rare diseases, obtaining the genetic diagnosis can mean the end of the diagnostic odyssey, and the beginning of another, the therapeutic odyssey. This scenario is especially challenging for the scientific community, since more than 90% of rare diseases do not currently have an effective treatment. This therapeutic failure in rare diseases means that new approaches are necessary. Our research group proposes that the use of precision or personalized medicine techniques can be an alternative to find potential therapies in these diseases. To this end, we propose that patients’ own cells can be used to carry out personalized pharmacological screening for the identification of potential treatments.

Direct Conversion of Human Stem Cell-Derived Glial Progenitor Cells into GABAergic Interneurons

Glial progenitor cells are widely distributed in brain parenchyma and represent a suitable target for future therapeutic interventions that generate new neurons via in situ reprogramming. Previous studies have shown successful reprogramming of mouse glia into neurons whereas the conversion of human glial cells remains challenging due to the limited accessibility of human brain tissue. Here, we have used a recently developed stem cell-based model of human glia progenitor cells (hGPCs) for direct neural reprogramming by overexpressing a set of transcription factors involved in GABAergic interneuron fate specification. GABAergic interneurons play a key role in balancing excitatory and inhibitory neural circuitry in the brain and loss or dysfunction of these have been implicated in several neurological disorders such as epilepsy, schizophrenia, and autism. Our results demonstrate that hGPCs successfully convert into functional induced neurons with postsynaptic activity within a month. The induced neurons have properties of GABAergic neurons, express subtype-specific interneuron markers (e.g. parvalbumin) and exhibit a complex neuronal morphology with extensive dendritic trees. The possibility of inducing GABAergic interneurons from a renewable in vitro hGPC system could provide a foundation for the development of therapies for interneuron pathologies.

How to reprogram human fibroblasts to neurons

Destruction and death of neurons can lead to neurodegenerative diseases. One possible way to treat neurodegenerative diseases and damage of the nervous system is replacing damaged and dead neurons by cell transplantation. If new neurons can replace the lost neurons, patients may be able to regain the lost functions of memory, motor, and so on. Therefore, acquiring neurons conveniently and efficiently is vital to treat neurological diseases. In recent years, studies on reprogramming human fibroblasts into neurons have emerged one after another, and this paper summarizes all these studies. Scientists find small molecules and transcription factors playing a crucial role in reprogramming and inducing neuron production. At the same time, both the physiological microenvironment in vivo and the physical and chemical factors in vitro play an essential role in the induction of neurons. Therefore, this paper summarized and analyzed these relevant factors. In addition, due to the unique advantages of physical factors in the process of reprogramming human fibroblasts into neurons, such as safe and minimally invasive, it has a more promising application prospect. Therefore, this paper also summarizes some successful physical mechanisms of utilizing fibroblasts to acquire neurons, which will provide new ideas for somatic cell reprogramming.

Direct Reprogramming of Human Fetal- and Stem Cell-Derived Glial Progenitor Cells into Midbrain Dopaminergic Neurons

Human glial progenitor cells (hGPCs) are promising cellular substrates to explore for the in situ production of new neurons for brain repair. Proof of concept for direct neuronal reprogramming of glial progenitors has been obtained in mouse models in vivo, but conversion using human cells has not yet been demonstrated. Such studies have been difficult to perform since hGPCs are born late during human fetal development, with limited accessibility for in vitro culture. In this study, we show proof of concept of hGPC conversion using fetal cells and also establish a renewable and reproducible stem cell-based hGPC system for direct neural conversion in vitro. Using this system, we have identified optimal combinations of fate determinants for the efficient dopaminergic (DA) conversion of hGPCs, thereby yielding a therapeutically relevant cell type that selectively degenerates in Parkinson's disease. The induced DA neurons show a progressive, subtype-specific phenotypic maturation and acquire functional electrophysiological properties indicative of DA phenotype.

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Human glial progenitors (hGPCs) can be directly converted into functional neurons

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Specific transcription factor combinations result in dopaminergic conversion

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Reprogrammed neurons show subtype-specific and functional maturation over time