Pharmacological rescue of synaptic plasticity, courtship behavior, and mushroom body defects in a Drosophila model of fragile X syndrome

RNA binding proteins and the regulation of neuronal synaptic plasticity

Cognitive function and memory rely on synaptic plasticity - the ability of synapses to modify their strength in response to stimulation. Emerging evidence indicates that post-transcriptional gene regulation is necessary for synaptic plasticity at several levels: by increasing proteome diversity through alternative splicing, or by enabling activity-dependent regulation of mRNA localization, translation or degradation in the dendrite. Mouse knockout studies have linked three RNA-binding proteins, fragile X mental retardation protein, cytoplasmic polyadenylation element binding factor and neuro-oncological ventral antigen to specific aspects of synaptic plasticity. Specificity in the regulation of synaptic plasticity might, in part, relate to the functional coherence of proteins encoded by the RNA targets of each RNA-binding protein.

Drosophilaas a Model for Human Neurodegenerative Disease

Among many achievements in the neurodegeneration field in the past decade, two require special attention due to the huge impact on our understanding of molecular and cellular pathogenesis of human neurodegenerative diseases. First is defining specific mutations in familial neurodegenerative diseases and second is modeling these diseases in easily manipulable model organisms including the fruit fly, nematode, and yeast. The power of these genetic systems has revealed many genetic factors involved in the various pathways affected, as well as provided potential drug targets for therapeutics. This review focuses on fruit fly models of human neurodegenerative diseases, with emphasis on how fly models have provided new insights into various aspects of human diseases.

Advances in X-linked mental retardation

PURPOSE OF REVIEW

Mutations in genes on the X chromosome rival chromosome aberrations as a cause of mental retardation. Progress in the clinical and molecular delineation of X-linked mental retardation has outpaced progress in understanding autosomal mental retardation. This is a result in large part of the identification of large families in which mental retardation has segregated in an X-linked pattern and the greater ease with which molecular technologies can be applied to hemizygosity in males.

RECENT FINDINGS

About one-third of the estimated 165 genes associated with syndromal mutations of genes on the X chromosome and one-fourth of the estimated 100 genes associated with nonsyndromal mutations of genes on the X chromosome have been identified. In a number of instances, the same gene is responsible for syndromal and nonsyndromal mutations of genes on the X chromosome. The molecular delineation of mutations of genes on the X chromosome has allowed certain conditions to be lumped together on the basis of allelism and has caused others that appear clinical similar to remain separate.

SUMMARY

The clinical and molecular advances have allowed X-linked mental retardation to be more clearly delineated, have provided the means of confirmatory laboratory testing, and have ushered in an era of carrier testing, prenatal diagnosis, and prevention strategies.

Diseases of unstable repeat expansion: mechanisms and common principles

The list of developmental and degenerative diseases that are caused by expansion of unstable repeats continues to grow, and is now approaching 20 disorders. The pathogenic mechanisms that underlie these disorders involve either loss of protein function or gain of function at the protein or RNA level. Common themes have emerged within and between these different classes of disease; for example, among disorders that are caused by gain-of-function mechanisms, altered protein conformations are central to pathogenesis, leading to changes in protein activity or abundance. In all these diseases, the context of the expanded repeat and the abundance, subcellular localization and interactions of the proteins and RNAs that are affected have key roles in disease-specific phenotypes.

Therapeutics development for triplet repeat expansion diseases

The underlying genetic mutations for many inherited neurodegenerative disorders have been identified in recent years. One frequent type of mutation is trinucleotide repeat expansion. Depending on the location of the repeat expansion, the mutation might result in a loss of function of the disease gene, a toxic gain of function or both. Disease gene identification has led to the development of model systems for investigating disease mechanisms and evaluating treatments. Examination of experimental findings reveals similarities in disease mechanisms as well as possibilities for treatment.

Therapeutic implications of the mGluR theory of fragile X mental retardation

Evidence is reviewed that the consequences of group 1 metabotropic glutamate receptor (Gp1 mGluR) activation are exaggerated in the absence of the fragile X mental retardation protein, likely reflecting altered dendritic protein synthesis. Abnormal mGluR signaling could be responsible for remarkably diverse psychiatric and neurological symptoms in fragile X syndrome, including delayed cognitive development, seizures, anxiety, movement disorders and obesity.

Recent advances in fragile X: a model for autism and neurodegeneration

PURPOSE OF REVIEW

This review will describe recent developments in the neurobiology of fragile X syndrome (FXS), the association between FXS and autism, and involvement in premutation carriers.

RECENT FINDINGS

Metabotropic glutamate receptor 5 (mGluR5)-coupled pathways are dysregulated in individuals with FXS and this is thought to relate to the FXS phenotype. The mGluR5 model suggests that mGluR5 antagonists, including downstream effectors such as lithium, could be useful for treating FXS. Two forms of clinical involvement associated with the fragile X mental retardation 1 (FMR1) gene, autism and fragile X-associated tremor/ataxia syndrome (FXTAS), have received additional attention during the past year. One study has found that approximately 30% of individuals with FXS have autism; those with autism have lowered cognitive abilities, language problems, and behavioral difficulties compared to those with FXS alone. Furthermore, evidence is mounting that autism also occurs in some young males who have premutation alleles. Finally, males and occasional females with premutation alleles may develop a neurological syndrome with aging that consists of tremor, ataxia, peripheral neuropathy, and cognitive deficits. Significant brain atrophy and white-matter disease is usually seen. This new disorder (FXTAS) is thought to be related to elevated levels of abnormal FMR1 mRNA.

SUMMARY

Full-mutation forms of the gene (> 200 repeats) can cause autism, learning disabilities, anxiety disorders, and mental retardation. Disorders associated with premutation forms of the gene (55-200 repeats) include, in addition to autism, FXTAS in older males and females, and premature ovarian failure.

Come FLY with us: toward understanding fragile X syndrome

The past few years have seen an increased number of articles using Drosophila as a model system to study fragile X syndrome. Phenotypic analyses have demonstrated an array of neuronal and behavioral defects similar to the phenotypes reported in mouse models as well as human patients. The availability of both cellular and molecular tools along with the power of genetics makes the tiny fruit fly a premiere model in elucidating the molecular basis of fragile X syndrome. Here, we summarize the advances made in recent years in the characterization of fragile X Drosophila models and the identification of new molecular partners in neural development.

Courting a Cure for Fragile X

Fragile X syndrome is the most common heritable cause of mental retardation. Previous work has suggested that overactive signaling by group I metabotropic glutamate receptors (mGluRs) may be a mechanism underlying many of the disease symptoms. As a test of this theory, McBride et al. show that in a Drosophila model for Fragile X syndrome, treatment with mGluR antagonists can rescue short-term memory, courtship, and mushroom body defects.