Modelling frontotemporal dementia using patient-derived induced pluripotent stem cells

Induced pluripotent stem cell models as a tool to investigate and test fluid biomarkers in Alzheimer's disease and frontotemporal dementia

Neurodegenerative diseases are increasing in prevalence and comprise a large socioeconomic burden on patients and their caretakers. The need for effective therapies and avenues for disease prevention and monitoring is of paramount importance. Fluid biomarkers for neurodegenerative diseases have gained a variety of uses, including informing participant selection for clinical trials, lending confidence to clinical diagnosis and disease staging, determining prognosis, and monitoring therapeutic response. Their role is expected to grow as disease-modifying therapies start to be available to a broader range of patients and as prevention strategies become established. Many of the underlying molecular mechanisms of currently used biomarkers are incompletely understood. Animal models and in vitro systems using cell lines have been extensively employed but face important translatability limitations. Induced pluripotent stem cell (iPSC) technology, where a theoretically unlimited range of cell types can be reprogrammed from peripheral cells sampled from patients or healthy individuals, has gained prominence over the last decade. It is a promising avenue to study physiological and pathological biomarker function and response to experimental therapeutics. Such systems are amenable to high-throughput drug screening or multiomics readouts such as transcriptomics, lipidomics, and proteomics for biomarker discovery, investigation, and validation. The present review describes the current state of biomarkers in the clinical context of neurodegenerative diseases, with a focus on Alzheimer's disease and frontotemporal dementia. We include a discussion of how iPSC models have been used to investigate and test biomarkers such as amyloid-β, phosphorylated tau, neurofilament light chain or complement proteins, and even nominate novel biomarkers. We discuss the limitations of current iPSC methods, mentioning alternatives such as coculture systems and three-dimensional organoids which address some of these concerns. Finally, we propose exciting prospects for stem cell transplantation paradigms using animal models as a preclinical tool to study biomarkers in the in vivo context.

Frontotemporal dementia: from genetics to therapeutic approaches

INTRODUCTION

Frontotemporal dementia (FTD) includes a group of neurodegenerative diseases characterized clinically by behavioral disturbances and by neurodegeneration of brain anterior temporal and frontal lobes, leading to atrophy. Apart from symptomatic treatments, there is, at present, no disease-modifying cure for FTD.

AREAS COVERED

Three main mutations are known as causes of familial FTD, and large consortia have studied carriers of mutations, also in preclinical Phases. As genetic cases are the only ones in which the pathology can be predicted in life, compounds developed so far are directed toward specific proteins or mutations. Herein, recently approved clinical trials will be summarized, including molecules, mechanisms of action and pharmacological testing.

EXPERT OPINION

These studies are paving the way for the future. They will clarify whether single mutations should be addressed rather than common proteins depositing in the brain to move from genetic to sporadic FTD.

Human stem cell transplantation models of Alzheimer's disease

Alzheimer's disease (AD) is the most frequent form of dementia. It is characterized by pronounced neuronal degeneration with formation of neurofibrillary tangles and deposition of amyloid β throughout the central nervous system. Animal models have provided important insights into the pathogenesis of AD and they have shown that different brain cell types including neurons, astrocytes and microglia have important functions in the pathogenesis of AD. However, there are difficulties in translating promising therapeutic observations in mice into clinical application in patients. Alternative models using human cells such as human induced pluripotent stem cells (iPSCs) may provide significant advantages, since they have successfully been used to model disease mechanisms in neurons and in glial cells in neurodegenerative diseases in vitro and in vivo. In this review, we summarize recent studies that describe the transplantation of human iPSC-derived neurons, astrocytes and microglial cells into the forebrain of mice to generate chimeric transplantation models of AD. We also discuss opportunities, challenges and limitations in using differentiated human iPSCs for in vivo disease modeling and their application for biomedical research.

Advanced Cellular Models for Rare Disease Study: Exploring Neural, Muscle and Skeletal Organoids

Organoids are self-organized, three-dimensional structures derived from stem cells that can mimic the structure and physiology of human organs. Patient-specific induced pluripotent stem cells (iPSCs) and 3D organoid model systems allow cells to be analyzed in a controlled environment to simulate the characteristics of a given disease by modeling the underlying pathophysiology. The recent development of 3D cell models has offered the scientific community an exceptionally valuable tool in the study of rare diseases, overcoming the limited availability of biological samples and the limitations of animal models. This review provides an overview of iPSC models and genetic engineering techniques used to develop organoids. In particular, some of the models applied to the study of rare neuronal, muscular and skeletal diseases are described. Furthermore, the limitations and potential of developing new therapeutic approaches are discussed.

Small molecules targeting different cellular pathologies for the treatment of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease in which the motor neuron circuitry displays progressive degeneration, affecting mostly the motor neurons in the brain and in the spinal cord. There are no effective cures, albeit three drugs, riluzole, edaravone, and AMX0035 (a combination of sodium phenylbutyrate and taurursodiol), have been approved by the Food and Drug Administration, with limited improvement in patients. There is an urgent need to build better and more effective treatment strategies for ALS. Since the disease is very heterogenous, numerous approaches have been explored, such as targeting genetic mutations, decreasing oxidative stress and excitotoxicity, enhancing mitochondrial function and protein degradation mechanisms, and inhibiting neuroinflammation. In addition, various chemical libraries or previously identified drugs have been screened for potential repurposing in the treatment of ALS. Here, we review previous drug discovery efforts targeting a variety of cellular pathologies that occur from genetic mutations that cause ALS, such as mutations in SOD1, C9orf72, FUS, and TARDP-43 genes. These mutations result in protein aggregation, which causes neuronal degeneration. Compounds used to target cellular pathologies that stem from these mutations are discussed and comparisons among different preclinical models are presented. Because the drug discovery landscape for ALS and other motor neuron diseases is changing rapidly, we also offer recommendations for a novel, more effective, direction in ALS drug discovery that could accelerate translation of effective compounds from animals to patients.

Frontotemporal Dementia, Where Do We Stand? A Narrative Review

Frontotemporal dementia (FTD) is a neurodegenerative disease of growing interest, since it accounts for up to 10% of middle-age-onset dementias and entails a social, economic, and emotional burden for the patients and caregivers. It is characterised by a (at least initially) selective degeneration of the frontal and/or temporal lobe, generally leading to behavioural alterations, speech disorders, and psychiatric symptoms. Despite the recent advances, given its extreme heterogeneity, an overview that can bring together all the data currently available is still lacking. Here, we aim to provide a state of the art on the pathogenesis of this disease, starting with established findings and integrating them with more recent ones. In particular, advances in the genetics field will be examined, assessing them in relation to both the clinical manifestations and histopathological findings, as well as considering the link with other diseases, such as amyotrophic lateral sclerosis (ALS). Furthermore, the current diagnostic criteria will be explored, including neuroimaging methods, nuclear medicine investigations, and biomarkers on biological fluids. Of note, the promising information provided by neurophysiological investigations, i.e., electroencephalography and non-invasive brain stimulation techniques, concerning the alterations in brain networks and neurotransmitter systems will be reviewed. Finally, current and experimental therapies will be considered.

Improved Protocol for Reproducible Human Cortical Organoids Reveals Early Alterations in Metabolism with MAPT Mutations

Cerebral cortical-enriched organoids derived from human pluripotent stem cells (hPSCs) are valuable models for studying neurodevelopment, disease mechanisms, and therapeutic development. However, recognized limitations include the high variability of organoids across hPSC donor lines and experimental replicates. We report a 96-slitwell method for efficient, scalable, reproducible cortical organoid production. When hPSCs were cultured with controlled-release FGF2 and an SB431542 concentration appropriate for their TGFBR1 / ALK5 expression level, organoid cortical patterning and reproducibility were significantly improved. Well-patterned organoids included 16 neuronal and glial subtypes by single cell RNA sequencing (scRNA-seq), frequent neural progenitor rosettes and robust BCL11B+ and TBR1+ deep layer cortical neurons at 2 months by immunohistochemistry. In contrast, poorly-patterned organoids contain mesendoderm-related cells, identifiable by negative QC markers including COL1A2 . Using this improved protocol, we demonstrate increased sensitivity to study the impact of different MAPT mutations from patients with frontotemporal dementia (FTD), revealing early changes in key metabolic pathways.

Genetically Engineered Triple MAPT-Mutant Human-Induced Pluripotent Stem Cells (N279K, P301L, and E10+16 Mutations) Exhibit Impairments in Mitochondrial Bioenergetics and Dynamics

Pathological abnormalities in the tau protein give rise to a variety of neurodegenerative diseases, conjointly termed tauopathies. Several tau mutations have been identified in the tau-encoding gene MAPT, affecting either the physical properties of tau or resulting in altered tau splicing. At early disease stages, mitochondrial dysfunction was highlighted with mutant tau compromising almost every aspect of mitochondrial function. Additionally, mitochondria have emerged as fundamental regulators of stem cell function. Here, we show that compared to the isogenic wild-type triple MAPT-mutant human-induced pluripotent stem cells, bearing the pathogenic N279K, P301L, and E10+16 mutations, exhibit deficits in mitochondrial bioenergetics and present altered parameters linked to the metabolic regulation of mitochondria. Moreover, we demonstrate that the triple tau mutations disturb the cellular redox homeostasis and modify the mitochondrial network morphology and distribution. This study provides the first characterization of disease-associated tau-mediated mitochondrial impairments in an advanced human cellular tau pathology model at early disease stages, ranging from mitochondrial bioenergetics to dynamics. Consequently, comprehending better the influence of dysfunctional mitochondria on the development and differentiation of stem cells and their contribution to disease progression may thus assist in the potential prevention and treatment of tau-related neurodegeneration.

Molecular Insights into Cell Type-specific Roles in Alzheimer's Disease: Human Induced Pluripotent Stem Cell-based Disease Modelling

Alzheimer's disease (AD) is the most common cause of dementia resulting in widespread degeneration of the central nervous system with severe cognitive impairment. Despite the devastating toll of AD, the incomplete understanding of the complex molecular mechanisms hinders the expeditious development of effective cures. Emerging evidence from animal studies has shown that different brain cell types play distinct roles in the pathogenesis of AD. Glutamatergic neurons are preferentially affected in AD and pronounced gliosis contributes to the progression of AD in both a cell-autonomous and a non-cell-autonomous manner. Much has been discovered through genetically modified animal models, yet frequently failed translational attempts to clinical applications call for better disease models. Emerging evidence supports the significance of human-induced pluripotent stem cell (iPSC) derived brain cells in modeling disease development and progression, opening new avenues for the discovery of molecular mechanisms. This review summarizes the function of different cell types in the pathogenesis of AD, such as neurons, microglia, and astrocytes, and recognizes the potential of utilizing the rapidly growing iPSC technology in modeling AD.

Stem Cell Therapies in Movement Disorders: Lessons from Clinical Trials

Stem cell-based therapies (SCT) to treat neurodegenerative disorders have promise but clinical trials have only recently begun, and results are not expected for several years. While most SCTs largely lead to a symptomatic therapeutic effect by replacing lost cell types, there may also be disease-modifying therapeutic effects. In fact, SCT may complement a multi-drug, subtype-specific therapeutic approach, consistent with the idea of precision medicine, which matches molecular therapies to biological subtypes of disease. In this narrative review, we examine published and ongoing trials in SCT in Parkinson's Disease, atypical parkinsonian disorders, Huntington's disease, amyotrophic lateral sclerosis, and spinocerebellar ataxia in humans. We discuss the benefits and pitfalls of using this treatment approach within the spectrum of disease-modification efforts in neurodegenerative diseases. SCT may hold greater promise in the treatment of neurodegenerative disorders, but much research is required to determine the feasibility, safety, and efficacy of these complementary aims of therapeutic efforts.

CRISPR and iPSCs: Recent Developments and Future Perspectives in Neurodegenerative Disease Modelling, Research, and Therapeutics

Neurodegenerative diseases are prominent causes of pain, suffering, and death worldwide. Traditional approaches modelling neurodegenerative diseases are deficient, and therefore, improved strategies that effectively recapitulate the pathophysiological conditions of neurodegenerative diseases are the need of the hour. The generation of human-induced pluripotent stem cells (iPSCs) has transformed our ability to model neurodegenerative diseases in vitro and provide an unlimited source of cells (including desired neuronal cell types) for cell replacement therapy. Recently, CRISPR/Cas9-based genome editing has also been gaining popularity because of the flexibility they provide to generate and ablate disease phenotypes. In addition, the recent advancements in CRISPR/Cas9 technology enables researchers to seamlessly target and introduce precise modifications in the genomic DNA of different human cell lines, including iPSCs. CRISPR-iPSC-based disease modelling, therefore, allows scientists to recapitulate the pathological aspects of most neurodegenerative processes and investigate the role of pathological gene variants in healthy non-patient cell lines. This review outlines how iPSCs, CRISPR/Cas9, and CRISPR-iPSC-based approaches accelerate research on neurodegenerative diseases and take us closer to a cure for neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, Amyotrophic Lateral Sclerosis, and so forth.

Human mini brains and spinal cords in a dish: Modeling strategies, current challenges, and prospective advances

Engineered three-dimensional (3D) in vitro and ex vivo neural tissues, also known as "mini brains and spinal cords in a dish," can be derived from different types of human stem cells via several differentiation protocols. In general, human mini brains are micro-scale physiological systems consisting of mixed populations of neural progenitor cells, glial cells, and neurons that may represent key features of human brain anatomy and function. To date, these specialized 3D tissue structures can be characterized into spheroids, organoids, assembloids, organ-on-a-chip and their various combinations based on generation procedures and cellular components. These 3D CNS models incorporate complex cell-cell interactions and play an essential role in bridging the gap between two-dimensional human neuroglial cultures and animal models. Indeed, they provide an innovative platform for disease modeling and therapeutic cell replacement, especially shedding light on the potential to realize personalized medicine for neurological disorders when combined with the revolutionary human induced pluripotent stem cell technology. In this review, we highlight human 3D CNS models developed from a variety of experimental strategies, emphasize their advances and remaining challenges, evaluate their state-of-the-art applications in recapitulating crucial phenotypic aspects of many CNS diseases, and discuss the role of contemporary technologies in the prospective improvement of their composition, consistency, complexity, and maturation.

Genes for RNA-binding proteins involved in neural-specific functions and diseases are downregulated in Rubinstein-Taybi iNeurons

Taking advantage of the fast-growing knowledge of RNA-binding proteins (RBPs) we review the signature of downregulated genes for RBPs in the transcriptome of induced pluripotent stem cell neurons (iNeurons) modelling the neurodevelopmental Rubinstein Taybi Syndrome (RSTS) caused by mutations in the genes encoding CBP/p300 acetyltransferases. We discuss top and functionally connected downregulated genes sorted to “RNA processing” and “Ribonucleoprotein complex biogenesis” Gene Ontology clusters. The first set of downregulated RBPs includes members of hnRNHP (A1, A2B1, D, G, H2-H1, MAGOHB, PAPBC), core subunits of U small nuclear ribonucleoproteins and Serine-Arginine splicing regulators families, acting in precursor messenger RNA alternative splicing and processing. Consistent with literature findings on reduced transcript levels of serine/arginine repetitive matrix 4 (SRRM4) protein, the main regulator of the neural-specific microexons splicing program upon depletion of Ep300 and Crebbp in mouse neurons, RSTS iNeurons show downregulated genes for proteins impacting this network. We link downregulated genes to neurological disorders including the new HNRNPH1-related intellectual disability syndrome with clinical overlap to RSTS. The set of downregulated genes for Ribosome biogenesis includes several components of ribosomal subunits and nucleolar proteins, such NOP58 and fibrillarin that form complexes with snoRNAs with a central role in guiding post-transcriptional modifications needed for rRNA maturation. These nucleolar proteins are “dual” players as fibrillarin is also required for epigenetic regulation of ribosomal genes and conversely NOP58-associated snoRNA levels are under the control of NOP58 interactor BMAL1, a transcriptional regulator of the circadian rhythm. Additional downregulated genes for “dual specificity” RBPs such as RUVBL1 and METTL1 highlight the links between chromatin and the RBP-ome and the contribution of perturbations in their cross-talk to RSTS. We underline the hub position of CBP/p300 in chromatin regulation, the impact of its defect on neurons’ post-transcriptional regulation of gene expression and the potential use of epidrugs in therapeutics of RBP-caused neurodevelopmental disorders.

Neuroinflammation in Amyotrophic Lateral Sclerosis and Frontotemporal Dementia and the Interest of Induced Pluripotent Stem Cells to Study Immune Cells Interactions With Neurons

Inflammation is a shared hallmark between amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). For long, studies were conducted on tissues of post-mortem patients and neuroinflammation was thought to be only bystander result of the disease with the immune system reacting to dying neurons. In the last two decades, thanks to improving technologies, the identification of causal genes and the development of new tools and models, the involvement of inflammation has emerged as a potential driver of the diseases and evolved as a new area of intense research. In this review, we present the current knowledge about neuroinflammation in ALS, ALS-FTD, and FTD patients and animal models and we discuss reasons of failures linked to therapeutic trials with immunomodulator drugs. Then we present the induced pluripotent stem cell (iPSC) technology and its interest as a new tool to have a better immunopathological comprehension of both diseases in a human context. The iPSC technology giving the unique opportunity to study cells across differentiation and maturation times, brings the hope to shed light on the different mechanisms linking neurodegeneration and activation of the immune system. Protocols available to differentiate iPSC into different immune cell types are presented. Finally, we discuss the interest in studying monocultures of iPS-derived immune cells, co-cultures with neurons and 3D cultures with different cell types, as more integrated cellular approaches. The hope is that the future work with human iPS-derived cells helps not only to identify disease-specific defects in the different cell types but also to decipher the synergistic effects between neurons and immune cells. These new cellular tools could help to find new therapeutic approaches for all patients with ALS, ALS-FTD, and FTD.

Directly Reprogrammed Human Neurons to Understand Age-Related Energy Metabolism Impairment and Mitochondrial Dysfunction in Healthy Aging and Neurodegeneration

Brain aging is characterized by several molecular and cellular changes grouped as the hallmarks or pillars of aging, including organelle dysfunction, metabolic and nutrition-sensor changes, stem cell attrition, and macromolecular damages. Separately and collectively, these features degrade the most critical neuronal function: transmission of information in the brain. It is widely accepted that aging is the leading risk factor contributing to the onset of the most prevalent pathological conditions that affect brain functions, such as Alzheimer's, Parkinson's, and Huntington's disease. One of the limitations in understanding the molecular mechanisms involved in those diseases is the lack of an appropriate cellular model that recapitulates the “aged” context in human neurons. The advent of the cellular reprogramming of somatic cells, i.e., dermal fibroblasts, to obtain directly induced neurons (iNs) and induced pluripotent stem cell- (iPSC-) derived neurons is technical sound advances that could open the avenues to understand better the contribution of aging toward neurodegeneration. In this review, we will summarize the commonalities and singularities of these two approaches for the study of brain aging, with an emphasis on the role of mitochondrial dysfunction and redox biology. We will address the evidence showing that iNs retain age-related features in contrast to iPSC-derived neurons that lose the aging signatures during the reprogramming to pluripotency, rendering iNs a powerful strategy to deepen our knowledge of the processes driving normal cellular function decline and neurodegeneration in a human adult model. We will finally discuss the potential utilization of these novel technologies to understand the differential contribution of genetic and epigenetic factors toward neuronal aging, to identify and develop new drugs and therapeutic strategies.

Human Induced Pluripotent Stem Cell Models of Frontotemporal Dementia With Tau Pathology

Neurodegenerative dementias are the most common group of neurodegenerative diseases affecting more than 40 million people worldwide. One of these diseases is frontotemporal dementia (FTD), an early onset dementia and one of the leading causes of dementia in people under the age of 60. FTD is a heterogeneous group of neurodegenerative disorders with pathological accumulation of particular proteins in neurons and glial cells including the microtubule-associated protein tau, which is deposited in its hyperphosphorylated form in about half of all patients with FTD. As for other patients with dementia, there is currently no cure for patients with FTD and thus several lines of research focus on the characterization of underlying pathogenic mechanisms with the goal to identify therapeutic targets. In this review, we provide an overview of reported disease phenotypes in induced pluripotent stem cell (iPSC)-derived neurons and glial cells from patients with tau-associated FTD with the aim to highlight recent progress in this fast-moving field of iPSC disease modeling. We put a particular focus on genetic forms of the disease that are linked to mutations in the gene encoding tau and summarize mutation-associated changes in FTD patient cells related to tau splicing and tau phosphorylation, microtubule function and cell metabolism as well as calcium homeostasis and cellular stress. In addition, we discuss challenges and limitations but also opportunities using differentiated patient-derived iPSCs for disease modeling and biomedical research on neurodegenerative diseases including FTD.

Regenerative Stem Cell Therapy for Neurodegenerative Diseases: An Overview

Neurodegenerative diseases resulting from the progressive loss of structure and/or function of neurons contribute to different paralysis degrees and loss of cognition and sensation. The lack of successful curative therapies for neurodegenerative disorders leads to a considerable burden on society and a high economic impact. Over the past 20 years, regenerative cell therapy, also known as stem cell therapy, has provided an excellent opportunity to investigate potentially powerful innovative strategies for treating neurodegenerative diseases. This is due to stem cells' capability to repair injured neuronal tissue by replacing the damaged or lost cells with differentiated cells, providing a conducive environment that is in favor of regeneration, or protecting the existing healthy neurons and glial cells from further damage. Thus, in this review, the various types of stem cells, the current knowledge of stem-cell-based therapies in neurodegenerative diseases, and the recent advances in this field are summarized. Indeed, a better understanding and further studies of stem cell technologies cause progress into realistic and efficacious treatments of neurodegenerative disorders.

Mass spectrometry analysis of tau and amyloid-beta in iPSC-derived models of Alzheimer's disease and dementia

Induced pluripotent stem cell (iPSC) technology enables the generation of human neurons in vitro, which contain the precise genome of the cell donor, therefore permitting the generation of disease models from individuals with a disease-associated genotype of interest. This approach has been extensively used to model inherited forms of Alzheimer's disease and frontotemporal dementia. The combination of iPSC-derived neuronal models with targeted mass spectrometry analysis has provided unprecedented insights into the regulation of specific proteins in human neuronal physiology and pathology. For example enabling investigations into tau and APP/Aβ, specifically: protein isoform expression, relative levels of cleavage fragments, aggregated species and functionally critical post-translational modifications. The use of mass spectrometry has enabled a determination of how closely iPSC-derived models recapitulate disease profiles observed in the human brain. This review will highlight the progress to date in studies using iPSCs and mass spectrometry to model Alzheimer's disease and dementia. We go on to convey our optimism, as studies in the near future will make use of this precedent, together with novel techniques such as genome editing and stable isotope labelling, to provide real progress towards an in depth understanding of early neurodegenerative processes and development of novel therapeutic agents.