Modeling of Frontotemporal Dementia Using iPSC Technology

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.

Advances in iPSC Technology in Neural Disease Modeling, Drug Screening, and Therapy

Neurodegenerative disorders (NDs) including Alzheimer's Disease, Parkinson's Disease, Amyotrophic Lateral Sclerosis (ALS), and Huntington's disease are all incurable and can only be managed with drugs for the associated symptoms. Animal models of human illnesses help to advance our understanding of the pathogenic processes of diseases. Understanding the pathogenesis as well as drug screening using appropriate disease models of neurodegenerative diseases (NDs) are vital for identifying novel therapies. Human-derived induced pluripotent stem cell (iPSC) models can be an efficient model to create disease in a dish and thereby can proceed with drug screening and identifying appropriate drugs. This technology has many benefits, including efficient reprogramming and regeneration potential, multidirectional differentiation, and the lack of ethical concerns, which open up new avenues for studying neurological illnesses in greater depth. The review mainly focuses on the use of iPSC technology in neuronal disease modeling, drug screening, and cell therapy.

The Breakthroughs and Caveats of Using Human Pluripotent Stem Cells in Modeling Alzheimer's Disease

Modeling Alzheimer's disease (AD) using human-induced pluripotent stem cells (iPSCs) is a field now spanning 15 years. Developments in the field have shown a shift in using simple 2D cortical neuron models to more advanced tri-cultures and 3D cerebral organoids that recapitulate more features of the disease. This is largely due to development and optimization of new cell protocols. In this review, we highlight recent major breakthroughs in the AD field and the implications this has in modeling AD using iPSCs (AD-iPSCs). To date, AD-iPSCs have been largely used to recapitulate and study impaired amyloid precursor protein (APP) processing and tau phosphorylation in both familial and sporadic AD. AD-iPSCs have also been studied for varying neuronal and glial dysfunctions. Moreover, they have been useful for discovering new molecular mechanisms, such as identifying proteins that bridge APP processing with tau phosphorylation and for identifying molecular pathways that bridge APP processing dysfunction with impaired cholesterol biosynthesis. Perhaps the greatest use of AD-iPSCs has been in discovering compounds via drug screening, that reduce amyloid beta (Aβ) in neurons, such as the anti-inflammatory compound, cromolyn, and antiparasitic drugs, avermectins. In addition, high content screening using AD-iPSCs has led to the identification of statins that can reduce levels of phosphorylated tau (p-Tau) in neurons. Some of these compounds have made it through to testing in human clinical trials. Improvements in omic technologies including single cell RNA sequencing and proteomics as well as advances in production of iPSC-cerebral organoids and tri-cultures is likely to result in the further discovery of new drugs and treatments for AD. Some caveats remain in the field, including, long experimental conditions to create mature neurons, high costs of media that limit research capabilities, and a lack of reproducibility using current iPSC-cerebral organoid protocols. Despite these current limitations, AD-iPSCs remain an excellent cellular model for studying AD mechanisms and for drug discovery.

Defective proteostasis in induced pluripotent stem cell models of frontotemporal lobar degeneration

Impaired proteostasis is associated with normal aging and is accelerated in neurodegeneration. This impairment may lead to the accumulation of protein, which can be toxic to cells and tissue. In a subset of frontotemporal lobar degeneration with tau pathology (FTLD-tau) cases, pathogenic mutations in the microtubule-associated protein tau (MAPT) gene are sufficient to cause tau accumulation and neurodegeneration. However, the pathogenic events triggered by the expression of the mutant tau protein remain poorly understood. Here, we show that molecular networks associated with lysosomal biogenesis and autophagic function are disrupted in brains from FTLD-tau patients carrying a MAPT p.R406W mutation. We then used human induced pluripotent stem cell (iPSC)-derived neurons and 3D cerebral organoids from patients carrying the MAPT p.R406W mutation and CRISPR/Cas9, corrected controls to evaluate proteostasis. MAPT p.R406W was sufficient to induce morphological and functional deficits in the lysosomal pathway in iPSC-neurons. These phenotypes were reversed upon correction of the mutant allele with CRISPR/Cas9. Treatment with mTOR inhibitors led to tau degradation specifically in MAPT p.R406W neurons. Together, our findings suggest that MAPT p.R406W is sufficient to cause impaired lysosomal function, which may contribute to disease pathogenesis and serve as a cellular phenotype for drug screening.

The Advance on Frontotemporal Dementia (FTD)’s Neuropathology and Molecular Genetics

The morbidity of frontotemporal dementia (FTD), one of the most prevalent dementias praccox, is second to Alzheimer disease (AD). It is different with AD that FTD has a rapider course and a higher mortality. FTD has not yet been fully understood in terms of etiology or pathogenesis, but genetic factors are believed to be involved. In this paper, we were committed to providing a comprehensive overview to FTD in aspects of the neuropathology features and the relevant molecular genetics advances, so that there would be insights to those researchers in search of novel approaches in FTD diagnosis and treatment.

Genetic Screening in Korean Patients with Frontotemporal Dementia Syndrome

Background: Frontotemporal dementia (FTD) syndrome is a genetically heterogeneous group of diseases. However, pathogenic variants in the chromosome 9 open reading frame 72 (C9orf72), microtubule-associated protein tau (MAPT), and progranulin (GRN) genes are mainly associated with genetic FTD in Caucasian populations. Objective: To understand the genetic background of Korean patients with FTD syndrome. Methods: We searched for pathogenic variants of 52 genes related to FTD, amyotrophic lateral sclerosis, familial Alzheimer’s disease, and other dementias, and hexanucleotide repeats of the C9orf72 gene in 72 Korean patients with FTD using whole exome sequencing and the repeat-primed polymerase chain reaction, respectively. Results: One likely pathogenic variant, p.G706R of MAPT, in a patient with behavioral variant FTD (bvFTD) and 13 variants of uncertain significance (VUSs) in nine patients with FTD were identified. Of these VUSs, M232R of the PRNP gene, whose role in pathogenicity is controversial, was also found in two patients with bvFTD. Conclusions: These results indicate that known pathogenic variants of the three main FTD genes (MAPT, GRN, and C9orf72) in Western countries are rare in Korean FTD patients.

Rapid in vitro quantification of TDP-43 and FUS mislocalisation for screening of gene variants implicated in frontotemporal dementia and amyotrophic lateral sclerosis

Identified genetic mutations cause 20% of frontotemporal dementia (FTD) and 5-10% of amyotrophic lateral sclerosis (ALS) cases: however, for the remainder of patients the origin of disease is uncertain. The overlap in genetic, clinical and pathological presentation of FTD and ALS suggests these two diseases are related. Post-mortem, ~ 95% of ALS and ~ 50% of FTD patients show redistribution of the nuclear protein TDP-43 to the cytoplasm within affected neurons, while ~ 5% ALS and ~ 10% FTD show mislocalisation of FUS protein. We exploited these neuropathological features to develop an unbiased method for the in vitro quantification of cytoplasmic TDP-43 and FUS. Utilising fluorescently-tagged cDNA constructs and immunocytochemistry, the fluorescence intensity of TDP-43 or FUS was measured in the nucleus and cytoplasm of cells, using the freely available software CellProfiler. Significant increases in the amount of cytoplasmic TDP-43 and FUS were detectable in cells expressing known FTD/ALS-causative TARDBP and FUS gene mutations. Pharmacological intervention with the apoptosis inducer staurosporine and mutation in a secondary gene (CYLD) also induced measurable cytoplasmic mislocalisation of endogenous FUS and TDP-43, respectively. These findings validate this methodology as a novel in vitro technique for the quantification of TDP-43 or FUS mislocalisation that can be used for initial prioritisation of predicted FTD/ALS-causative mutations.

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.