• Developmental brain disorders: genetics and drug discovery
• Genetics of brain development and neuronal plasticity
• Brain remodeling during metamorphosis

Overview

Our goal is to understand the genetic bases of normal brain development, and the alterations of brain development that cause neurocognitive disorders such as mental retardation and autism. Our mission is to make developmental brain disorders treatable with safe and effective drug therapies. Our methodological approaches combine the power of a premiere genetic model organism with that of primary neuron culture.

The genetic model organism

We are using the fruit fly, Drosophila melanogaster, as a model system to study genetic pathways that control brain development. Abnormalities of brain morphogenesis, neuronal differentiation, and neuronal plasticity are likely to underlie many human developmental brain disorders. Because of the remarkable phylogenetic conservation of genes that cause hereditary mental retardation or autism in humans, and because many brain-development mechanisms are shared by mammals and insects, we can use the fruit fly to better understand these human conditions. Moreover, we believe that the Drosophila system can serve as a valuable stepping stone for discovery of drugs that will benefit human patients with cognitive disabilities.

We focus on the metamorphosis period of Drosophila development because during that stage, the central nervous system (CNS) undergoes dramatic changes, including neuronal remodeling. These changes are controlled by a steroid hormone, 20-hydroxyecdysone (20E), whose receptor subunits are members of the nuclear receptor superfamily. At the cellular level, steroid hormone-induced changes in neuronal structure and function are very similar in mammals and insects. Many of our studies deal with a fascinating brain region, the mushroom bodies, which are remodeled during metamorphosis and which mediate complex adult behaviors, including many forms of associative learning and memory.

Mutations in hundreds of human genes can cause mental retardation phenotype, either in isolation or as part of a syndrome. Using bioinformatics methods, we showed that ~70% of human mental retardation genes have a candidate functional ortholog in Drosophila. At least seven of the Drosophila genes have been shown by others to have learning or memory phenotypes, and we predict that this will be true for many more of them. We discovered that mutations in Drosophila fragile X mental retardation 1 (dfmr1)—the fruit fly counterparts of the mutations that cause fragile X syndrome in humans—have defects in mushroom body development during metamorphosis. A major focus of our current and future research is the use of neuron culture to identify in vitro cellular phenotypes caused by mutations in mental retardation genes.

The power of primary neuron cell culture

We used dissociated cell culture methods to demonstrate that 20E promotes neurite outgrowth of mushroom body intrinsic neurons harvested early in metamorphosis. We also discovered striking neuronal morphology phenotypes in vitro, caused by genetic mutations or exposure to neurotoxins. To facilitate analysis of cultured-neuron phenotypes, we developed NeuronMetrics™ software, which we are distributing through iBridge. Our results indicate that cell culture can provide a sensitive bioassay for identifying and quantifying neuronal defects due to hereditary or acquired conditions.

A proof-of-concept drug screen

We have recently completed a first-of-its-kind drug screen, using known compounds in the NINDS Custom Collection II library, to identify pharmacological modifiers of the "filagree" phenotype of fascin-deficient neurons cultured in vitro. Because of the biology of fascin, a highly conserved actin-bundling protein, the drugs from our screen could be beneficial to either children with developmental brain disorders (filagree normalizers) or to adults with carcinomas and gliomas (filagree enhancers).

Broad Complex (BRC) and CNS metamorphosis

We determined that BRC transcription factors play a pivotal role in mediating 20E-regulated nervous system metamorphosis. This family of BTB-zinc-finger proteins (BRC-Z1 through -Z4) is generated by alternative splicing of transcripts from a large gene directly activated by 20E in the CNS and other tissues. Our phylogenetic and developmental genetic analyses revealed that the BRC's duplicated zinc-finger exons have undergone subfunctionalization, even when they contribute to the same developmental events. Transgenic-rescue and spatial expression studies support a model of BRC function in coordinating cell-cell interactions that underlie CNS morphogenetic movements. We are currently analyzing a remarkable set of CNS microarray-based experiments in which we track genome-wide CNS expression profiles across the entire metamorphosis interval in wild-type D. melanogaster. We also used microarray studies to identify candidate BRC target genes involved in CNS remodeling during metamorphosis.