Selleck Laboratory Research Summary
Our laboratory has had a long-standing interest in the molecular mechanisms of nervous system development, from the spatial control of cell division in the brain[1,2] to the assembly of postsynaptic specializations. While most of our work has been in the model system, the fruit fly Drosophila melanogaster, in recent years we have extended our studies into the genetic analysis of autism spectrum disorder in humans. The overlying theme of all this work is an understanding of the assembly, function and developmental plasticity of the nervous system at the molecular level.
Proteoglycans as Regulators of Signaling Pathways that Govern Neural Development
Our entry into this field came through genetics, namely, a screen for mutations affecting the patterning of cell division in the visual system of the fruit fly. This effort lead to the identification of division abnormally delayed (dally), a gene required for the ordered progression through the cell cycle of a specific set of neuronal precursor cells. Cloning and sequencing of dally showed it encoded a heparan sulfate proteoglycan (HSPG) of the glypican family. Glypicans were well-characterized biochemically in vertebrates, but our dally mutants provided the first capability to study the function of this class of molecules in a whole organism. Glypicans are a related set of integral membrane proteins that belong to a larger class of molecules, heparan sulfate-modified proteins, or proteoglycans. Heparan sulfate is a long, linear unbranched sugar polymer that is highly sulfated and attached to specific amino acid residues of the modified protein. At the time of our identification of dally it was not known if HSPGs existed in Drosophila, and some of our work was dedicated to understanding the structural variety of heparan sulfate in Drosophila and the degree of functional conservation with vertebrate systems[6,7].
A decade of work has established that HSPGs, including the glypicans, are important determinants of neural specification, axon guidance and synapse assembly[8,9,10,11,12,13,14,15]. More generally, HSPGs control growth factor signaling and the distribution and function of morphogens, secreted factors that create gradients of signaling required for developmental specification.
Genetic Analysis of Autism Spectrum Disorder and Copy Number Variation
Our initial investigation of autism spectrum disorder genetics came through the identification of an inherited deletion on chromosome 10q associated with autism and a variety of other developmental anomalies. Subsequent analysis of many other affected individuals and their family members showed this to be a dosage-sensitive locus, where either loss or gain of copies affected neural development and behavior. In addition, we discovered that these copy number variations on 10q were associated with a sequence architecture that conferred genomic instability to this region. That realization led to the investigation of copy number variation genome-wide, with a focus on regions where the sequence structure promoted genomic rearrangements at high levels. Our recently published study established that autism is associated with a higher level of DNA duplication than typically-developing controls, suggesting that some level of genomic instability is associated with autism spectrum disorder. In addition, we determined that the severity of the behavioral and developmental phenotypes are associated with increased levels of copy number change, not only for autism, but severe intellectual disability and multiple congenital anomalies as well. These findings beg the question as to the origin of the elevated levels of copy number change, and whether genomic instability is an important predisposing factor toward behavioral disorders in children.
Current work examines a longstanding question in the genetics of complex disorders and phenotypes, the relative contribution of genetic and environmental factors, as well as the interactions between genes (intergenic interactions) and genes and environment. Work in progress suggests that copy number variation-environment interactions can be substantial and contribute significantly to autism susceptibility.
Genetic Analysis of Postsynaptic Specializations
We have continued our work in Drosophila, with a focus on genes affecting specialized features of the postsynaptic cell. These studies derived from our modeling of a human behavioral and growth disorder, tuberous sclerosis complex (TSC), in Drosophila. In the course of this work we discovered that Akt, a conserved and critical signaling molecule in the TSC pathway, is required for two features of the postsynaptic specialization: 1) the selective trafficking of one neurotransmitter receptor subunit, glutamate receptor IIA, and 2) the expansion and elaboration of a complex postsynaptic membrane system, the subsynaptic reticulum. We have conducted a genetic screen to identify other molecules required for postsynaptic development and several of these are being investigated in the lab currently. These studies identify molecules and processes critical for the regulation and development of synapses, information critical to understanding how disruptions of these pathways affect neural function and behavior.
Our analysis of the varied functions of HSPGs has also been expanded into the synapse. We have discovered that HSPGs are critical determinants of synapse function and assembly, and have identified autophagy as a cellular process regulated by HSPGs.
- Selleck SB, Gonzalez C, Glover DM, White K (1992) Regulation of the G1-S transition in post-embryonic neuronal precursors by axon ingrowth. Nature: 253-255.
- Selleck SB, Steller H (1991) The influence of retinal innervation on neurogenesis in the first optic ganglion of Drosophila. Neuron 6: 83-99.
- Rawson JM, Lee M, Kennedy EL, Selleck SB (2003) Drosophila neuromuscular synapse assembly and function require the TGF-beta type I receptor saxophone and the transcription factor Mad. J Neurobiol 55: 134-150.
- Balciuniene J, Feng N, Iyadurai K, Hirsch B, Charnas L, et al. (2007) Recurrent 10q22-q23 deletions: a genomic disorder on 10q associated with cognitive and behavioral abnormalities. American Journal of Human Genetics 80: 938-947.
- Nakato H, Futch TA, Selleck SB (1995) The division abnormally delayed (dally) gene: a putative integral membrane proteoglycan required for cell division patterning during postembryonic development of the nervous system in Drosophila. Development 121: 3687-3702.
- Toyoda H, Kinoshita-Toyoda A, Fox B, Selleck SB (2000) Structural analysis of glycosaminoglycans in animals bearing mutations in sugarless, sulfateless, and tout-velu. Drosophila homologues of vertebrate genes encoding glycosaminoglycan biosynthetic enzymes. J Biol Chem 275: 21856-21861.
- Toyoda H, Kinoshita-Toyoda A, Selleck SB (2000) Structural analysis of glycosaminoglycans in Drosophila and Caenorhabditis elegans and demonstration that tout-velu, a Drosophila gene related to EXT tumor suppressors, affects heparan sulfate in vivo. J Biol Chem 275: 2269-2275.
- Fujise M, Izumi S, Selleck SB, Nakato H (2001) Regulation of dally, an integral membrane proteoglycan, and its function during adult sensory organ formation of Drosophila. Dev Biol 235: 433-448.
- Jackson SM, Nakato H, Sugiura M, Jannuzi A, Oakes R, et al. (1997) dally, a Drosophila glypican, controls cellular responses to the TGF-beta-related morphogen, Dpp. Development 124: 4113-4120.
- Kirkpatrick CA, Dimitroff BD, Rawson JM, Selleck SB (2004) Spatial Regulation of Wingless Morphogen Distribution and Signaling by Dally-like Protein. Dev Cell 7: 513-523.
- Kirkpatrick CA, Knox SM, Staatz WD, Fox B, Lercher DM, et al. (2006) The function of a Drosophila glypican does not depend entirely on heparan sulfate modification. Dev Biol 300: 570-582.
- Nakato H, Fox B, Selleck SB (2002) dally, a Drosophila member of the glypican family of integral membrane proteoglycans, affects cell cycle progression and morphogenesis via a Cyclin A-mediated process. J Cell Sci 115: 123-130.
- Rawson JM, Dimitroff B, Johnson KG, Ge X, Van Vactor D, et al. (2005) The Heparan Sulfate Proteoglycans Dally-like and Syndecan Have Distinct Functions in Axon Guidance and Visual-System Assembly in Drosophila. Curr Biol 15: 833-838.
- Ren Y, Kirkpatrick CA, Rawson JM, Sun M, Selleck SB (2009) Cell type-specific requirements for heparan sulfate biosynthesis at the Drosophila neuromuscular junction: effects on synapse function, membrane trafficking, and mitochondrial localization. J Neuroscience 29: 8539-8550.
- Tsuda M, Kamimura K, Nakato H, Archer M, Staatz W, et al. (1999) The cell-surface proteoglycan Dally regulates Wingless signalling in Drosophila. Nature 400: 276-280.
- van Bon BW, Balciuniene J, Fruhman G, Nagamani SC, Broome DL, et al. (2011) The phenotype of recurrent 10q22q23 deletions and duplications. Eur J Hum Genet 19: 400-408.
- Girirajan S, Johnson RL, Tassone F, Balciuniene J, Katiyar N, et al. (2013) Global increases in both common and rare copy number load associated with autism. Human Molecular Genetics 22: 2870-2880.
- Dimitroff B, Howe K, Watson A, Campion B, Lee HG, et al. (2012) Diet and energy-sensing inputs affect TorC1-mediated axon misrouting but not TorC2-directed synapse growth in a Drosophila model of tuberous sclerosis. PLoS One 7: e30722.
- Lee HG, Zhao N, Campion BK, Nguyen MM, Selleck SB (2013) Akt regulates glutamate receptor trafficking and postsynaptic membrane elaboration at the Drosophila neuromuscular junction. Developmental neurobiology 73: 723-743.