The Contribution of Pluripotent Stem Cell (PSC)-Based Models to the Study of Fragile X Syndrome (FXS)

Across Dimensions: Developing 2D and 3D Human iPSC-Based Models of Fragile X Syndrome

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability and autism spectrum disorder. FXS is caused by a cytosine-guanine-guanine (CGG) trinucleotide repeat expansion in the untranslated region of the FMR1 gene leading to the functional loss of the gene's protein product FMRP. Various animal models of FXS have provided substantial knowledge about the disorder. However, critical limitations exist in replicating the pathophysiological mechanisms. Human induced pluripotent stem cells (hiPSCs) provide a unique means of studying the features and processes of both normal and abnormal human neurodevelopment in large sample quantities in a controlled setting. Human iPSC-based models of FXS have offered a better understanding of FXS pathophysiology specific to humans. This review summarizes studies that have used hiPSC-based two-dimensional cellular models of FXS to reproduce the pathology, examine altered gene expression and translation, determine the functions and targets of FMRP, characterize the neurodevelopmental phenotypes and electrophysiological features, and, finally, to reactivate FMR1. We also provide an overview of the most recent studies using three-dimensional human brain organoids of FXS and end with a discussion of current limitations and future directions for FXS research using hiPSCs.

In Vitro Models of the Human Blood-Brain Barrier Utilising Human Induced Pluripotent Stem Cells: Opportunities and Challenges

The blood-brain barrier (BBB) is a component of the neurovascular unit formed by specialized brain microvascular endothelial cells surrounded by astrocytes end-feet processes, pericytes, and a basement membrane. The BBB plays an important role in the maintenance of brain homeostasis and has seen a growing involvement in the pathophysiology of various neurological diseases. On the other hand, the presence of such a barrier remains an important challenge for drug delivery to treat such illnesses.Since the pioneering work describing the isolation and cultivation of primary brain microvascular cells about 50 years ago until now, the development of an in vitro model of the BBB that is scalable, capable to form tight monolayers, and predictive of drug permeability in vivo remained extremely challenging.The recent description of the use of induced pluripotent stem cells (iPSCs) as a modeling tool for neurological diseases raised momentum into the use of such cells to develop new in vitro models of the BBB. This chapter will provide an exhaustive description of the use of iPSCs as a source of cells for modeling the BBB in vitro, describe the advantages and limitations of such model, as well as describe their prospective use for disease modeling and drug permeability screening platforms.

DNA Methylation, Mechanisms of FMR1 Inactivation and Therapeutic Perspectives for Fragile X Syndrome

Among the inherited causes of intellectual disability and autism, Fragile X syndrome (FXS) is the most frequent form, for which there is currently no cure. In most FXS patients, the FMR1 gene is epigenetically inactivated following the expansion over 200 triplets of a CGG repeat (FM: full mutation). FMR1 encodes the Fragile X Mental Retardation Protein (FMRP), which binds several mRNAs, mainly in the brain. When the FM becomes methylated at 10-12 weeks of gestation, the FMR1 gene is transcriptionally silent. The molecular mechanisms involved in the epigenetic silencing are not fully elucidated. Among FXS families, there is a rare occurrence of males carrying a FM, which remains active because it is not methylated, thus ensuring enough FMRPs to allow for an intellectual development within normal range. Which mechanisms are responsible for sparing these individuals from being affected by FXS? In order to answer this critical question, which may have possible implications for FXS therapy, several potential epigenetic mechanisms have been described. Here, we focus on current knowledge about the role of DNA methylation and other epigenetic modifications in FMR1 gene silencing.

Human in vitro models for understanding mechanisms of autism spectrum disorder

Early brain development is a critical epoch for the development of autism spectrum disorder (ASD). In vivo animal models have, until recently, been the principal tool used to study early brain development and the changes occurring in neurodevelopmental disorders such as ASD. In vitro models of brain development represent a significant advance in the field. Here, we review the main methods available to study human brain development in vitro and the applications of these models for studying ASD and other psychiatric disorders. We discuss the main findings from stem cell models to date focusing on cell cycle and proliferation, cell death, cell differentiation and maturation, and neuronal signaling and synaptic stimuli. To be able to generalize the results from these studies, we propose a framework of experimental design and power considerations for using in vitro models to study ASD. These include both technical issues such as reproducibility and power analysis and conceptual issues such as the brain region and cell types being modeled.

Modeling Inflammation on Neurodevelopmental Disorders Using Pluripotent Stem Cells

Neurodevelopmental disorders (ND) are characterized by an impairment of the nervous system during its development, with a wide variety of phenotypes based on genetic or environmental cues. There are currently several disorders grouped under ND including intellectual disabilities (ID), attention-deficit hyperactivity disorder (ADHD), and autism spectrum disorders (ASD). Although NDs can have multiple culprits with varied diagnostics, several NDs present an inflammatory component. Taking advantage of induced pluripotent stem cells (iPSC), several disorders were modeled in a dish complementing in vivo data from rodent models or clinical data. Monogenic syndromes displaying ND are more feasible to be modeled using iPSCs also due to the ability to recruit patients and clinical data available. Some of these genetic disorders are Fragile X Syndrome (FXS), Rett Syndrome (RTT), and Down Syndrome (DS). Environmental NDs can be caused by maternal immune activation (MIA), such as the infection with Zika virus during pregnancy known to cause neural damage to the fetus. Our goal in this chapter is to review the advances of using stem cell research in NDs, focusing on the role of neuroinflammation on ASD and environmental NDs studies.

Towards Mechanism-Based Treatments for Fragile X Syndrome

Fragile X syndrome (FXS) is the most common heritable form of intellectual disability, as well as the most common known monogenic cause of autism spectrum disorder (ASD), affecting 1 in 4000-8000 people worldwide [...].