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Astrocytes have been recognized as a class of cells that fill the space between neurons for more than a century. From their humble beginnings in the literature as merely space filling cells, an ever expanding list of functions in the CNS now exceeds the list of functions performed by neurons. In virtually all developmental and pathological conditions in the brain, astrocytes are involved in some capacity that directly affects neuronal function. Today we recognize that astrocytes are involved in the development and function of synaptic communication. Increasing evidence suggests that abnormal synaptic function may be a prominent contributing factor to the learning disability phenotype. With the discovery of FMRP in astrocytes, coupled with a role of astrocytes in synaptic function, research directed to glial neurobiology has never been more important. This chapter highlights the current knowledge of astrocyte function with a focus on their involvement in Fragile X syndrome.

The expression of GABA(A) receptors in the fragile X mouse brain is significantly downregulated. We additionally found that the expression of somatostatin and voltage-sensitive calcium channels (VSCCs) is also reduced. GABA(A) and the VSCCs, through a synergistic interaction, perform a critical role in mediating activity-dependent developmental processes. In the developing brain, GABA is excitatory and its actions are mediated through GABA(A) receptors. Subsequent to GABA-mediated depolarization, the VSCCs are activated and intracellular calcium is increased, which mediates gene transcription and other cellular events. GABAergic excitation mediated through GABA(A) receptors and the subsequent activation of the VSCCs are critically important for the establishment of neuronal connectivity within immature neuronal networks. Data from our laboratories suggest that there is a dysregulation of axonal pathfinding during development in the fragile X mouse brain and that this is likely due to a dysregulation of the synergistic interactions of GABA and VSCC. Thus, we hypothesize that the altered expression of these critical channels in the early stages of brain development leads to altered activity-dependent gene expression that may potentially lead to the developmental delay characteristic of the fragile X syndrome.

Fragile X syndrome patients express a wide array of cognitive and other gender-specific phenotypic features. These manifestations result not only from molecular mechanisms that are altered as a result of the expansion of a CGG-repeat region in the FMR1 promoter, but also genetic factors such as founder effects and mosaicism. In this chapter, I will summarize the many and varied features of fragile X syndrome as they present themselves in a clinical setting and describe the procedures that are used to diagnose patients. Finally, I will briefly touch on recent developments that will affect patient screening in the future.

Stem cells, which can self-renew and produce different cell types, are regulated by both extrinsic signals and intrinsic factors. Fragile X syndrome, one of the most common forms of inherited mental retardation, is caused by the functional loss of fragile X mental retardation protein (FMRP). FMRP is a selective RNA-binding protein that forms a messenger ribonucleoprotein (mRNP) complex that associates with polyribosomes. Recently, the role of Fmrp in stem cell biology has been explored in both Drosophila and the mouse. In this chapter, we discuss the role of FMRP in regulating the proliferation and differentiation of stem cells.

Since fragile X syndrome (FXS) is a typical X-linked mendelian disorder, the protein product associated with the disease (FMRP) is absent or reduced not only in the affected individuals but, in case of full mutation, also in their mothers. Here, by using the mouse model of the disease, we provide evidence that hyperactivity, a typical symptom of FXS, is not wholly induced by the lack of Fmrp in mice but also occurs as a result of its reduced expression in their mother. Genetically wild-type offspring of mutant mothers also had hyperactivity, albeit less pronounced than the mutant offspring. However, other features of FXS reproduced in the mouse model, such as sensory hyperreactivity and seizure susceptibility, were exclusively associated with the absence of Fmrp in the offspring. These data indicate that fmr-1, the gene encoding Fmrp, can be both an offspring genetic and a maternal environmental factor in producing a neurodevelopmental condition.

The protein repertoire of excitatory synapses controls dendritic spine morphology, synaptic plasticity and higher brain functions. In brain neurons, the RNA-associated fragile X mental retardation protein (FMRP) binds in vivo to various transcripts encoding key postsynaptic components and may thereby substantially regulate the molecular composition of dendritic spines. In agreement with this notion functional loss of FMRP in patients affected by the fragile X syndrome (FXS) causes cognitive impairment. Here we address our current understanding of the functional role of individual postsynaptic proteins. We discuss how FMRP controls the abundance of select proteins at postsynaptic sites, which signaling pathways regulate the local activity of FMRP at synapses, and how altered levels of postsynaptic proteins may contribute to FXS pathology.

Work in recent years has revealed an abundance of possible new treatment targets for fragile X syndrome (FXS). The use of animal models, including the fragile X knockout mouse which manifests a phenotype very similar to FXS in humans, has resulted in great strides in this direction of research. The lack of Fragile X Mental Retardation Protein (FMRP) in FXS causes dysregulation and usually overexpression of a number of its target genes, which can cause imbalances of neurotransmission and deficits in synaptic plasticity. The use of metabotropic glutamate receptor (mGluR) blockers and gamma amino-butyric acid (GABA) agonists have been shown to be efficacious in reversing cellular and behavioral phenotypes, and restoring proper brain connectivity in the mouse and fly models. Proposed new pharmacological treatments and educational interventions are discussed in this chapter. In combination, these various targeted treatments show promising preliminary results in mitigating or even reversing the neurobiological abnormalities caused by loss of FMRP, with possible translational applications to other neurodevelopmental disorders including autism.

This chapter will briefly tie together a captivating string of scientific discoveries that began in the 1800s and catapulted us into the current state of the field where trials are under way in humans that have arisen directly from the discoveries made in model organisms such as Drosophila (fruit flies) and mice. The hope is that research efforts in the field of fragile X currently represent a roadmap that demonstrates the utility of identifying a mutant gene responsible for human disease, tracking down the molecular underpinnings of pathogenic phenotypes, and utilizing model organisms to identify and validate potential pharmacologic targets for testing in afflicted humans. Indeed, in fragile X this roadmap has already yielded successful trials in humans (J. Med. Genetic 46(4) 266-271; Jacquemont et al. Sci Transl Med 3(64):64ra61), although the work in studying these interventions in humans is just getting underway as the work in model organisms continues to generate new potential therapeutic targets.

Fragile X syndrome (FXS) is the most common cause of inherited intellectual disability and presents with markedly atypical speech-language, likely due to impaired vocal learning. Although current models have been useful for studies of some aspects of FXS, zebra finch is the only tractable lab model for vocal learning. The neural circuits for vocal learning in the zebra finch have clear relationships to the pathways in the human brain that may be affected in FXS. Further, finch vocal learning may be quantified using software designed specifically for this purpose. Knockdown of the zebra finch FMR1 gene may ultimately enable novel tests of therapies that are modality-specific, using drugs or even social strategies, to ameliorate deficits in vocal development and function. In this chapter, we describe the utility of the zebra finch model and present a hypothesis for the role of FMRP in the developing neural circuitry for vocalization.

The use of mutant mouse models of neurodevelopmental and neurodegenerative disease is essential in order to understand the pathogenesis of many genetic diseases such as fragile X syndrome and fragile X-associated tremor/ataxia syndrome (FXTAS). The choice of which animal model is most suitable to mimic a particular disease depends on a range of factors, including anatomical, physiological, and pathological similarities; presence of orthologs of genes of interest; and conservation of basic cell biological and metabolic processes. In this chapter, we will discuss two mouse models of the fragile X premutation which have been generated to study the pathogenesis of FXTAS and the effects of potential therapeutic interventions. Behavioral, molecular, neuropathological, and endocrine features of the mouse models and their relation to human FXTAS are discussed.

The Drosophila genome contains most genes known to be involved in heritable disease. The extraordinary genetic malleability of Drosophila, coupled to sophisticated imaging, electrophysiology, and behavioral paradigms, has paved the way for insightful mechanistic studies on the causes of developmental and neurological disease as well as many possible interventions. Here, we focus on one of the most advanced examples of Drosophila genetic disease modeling, the Drosophila model of Fragile X Syndrome, which for the past decade has provided key advances into the molecular, cellular, and behavioral defects underlying this devastating disorder. We discuss the multitude of RNAs and proteins that interact with the disease-causing FMR1 gene product, whose function is conserved from Drosophila to human. In turn, we consider FMR1 mechanistic relationships in non-neuronal tissues (germ cells and embryos), peripheral motor and sensory circuits, and central brain circuits involved in circadian clock activity and learning/memory.

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an adult-onset neurodegenerative disorder clinically characterized by intention tremor and gait ataxia, in addition to other conditions including hypothyroidism, autonomic dysfunction, hypertension, peripheral neuropathy, and cognitive decline. FXTAS affects some males (approximately 40%) and in less degree female premutation carriers (8-16%) older than 50 years with an age-dependent symptomatology and penetrance. The CGG repeat number appears to influence the severity and the age of onset of the disorder. The neuropathological hallmark of FXTAS is the presence of eosinophillic, ubiquitin-positive intranuclear inclusions in both neurons and astroglia throughout brain. FXTAS is due to RNA toxicity caused by elevated levels of CGG-expanded mRNA containing 55-200 CGG repeats, which is found in the intranuclear inclusions that sequester various proteins including ubiquitin, αB-crystallin, lamin A/C, hnRNP A2, myelin basic protein, and Sam68. The expression of the expanded CGG repeat FMR1 mRNA also induces a cellular stress response and leads to a disruption of the nuclear lamin A/C architecture. These alterations are observable even in early development, suggesting that the expanded-repeat mRNA triggers pathogenic mechanisms that can provide a molecular basis for the neurodevelopmental abnormalities observed in some children who are carriers of an FMR1 premutation allele. Finally, the presence of cellular dysregulation in older adults who do not present clinical features of FXTAS may suggest that additional genetic or environmental protective factors may play a role in the pathogenesis of FXTAS.

A priority of fragile X syndrome (FXS) research is to determine the molecular mechanisms underlying the functional, behavioral, and structural deficits in humans and in the FXS mouse model. Given that metabotropic glutamate receptor (mGluR) long-term depression (LTD) is exaggerated in FXS mice, considerable effort has focused on proteins that regulate this form of synaptic plasticity. STriatal-Enriched protein tyrosine Phosphatase (STEP) is a brain-specific phosphatase implicated as an "LTD protein" because it mediates AMPA receptor internalization during mGluR LTD. STEP also promotes NMDA receptor endocytosis and inactivates ERK1/2 and Fyn, thereby opposing synaptic strengthening. We hypothesized that dysregulation of STEP may contribute to the pathophysiology of FXS. We review how STEP's expression and activity are regulated by dendritic protein synthesis, ubiquitination, proteolysis, and phosphorylation. We also discuss implications for STEP in FXS and other disorders, including Alzheimer's disease. As highlighted here, pharmacological interventions targeting STEP may prove successful for FXS.

Fragile X syndrome (FXS) is associated with a complex but relatively consistent psychiatric phenotype. Recent research has suggested neural substrates for the behavioral abnormalities typically seen in FXS, and enhanced treatment strategies for managing disabling psychiatric comorbidity. While disease-specific, and possibly disease-modifying, therapeutics are being developed for FXS, currently available psychiatric medications can provide significant symptomatic relief of the hyperactivity, anxiety disorders, and affective disturbances often seen in the course of FXS. However, patients with fragile X may be especially susceptible to the psychiatric side effects of these medications, requiring particular care in prescribing. Recent findings concerning disease mechanisms and treatment strategies are reviewed from the perspective of a clinical psychiatrist, in an effort to enhance conventional pharmacotherapy of FXS.