Defining molecular mechanisms that ensure proper patterns of cell: cell connectivity in the developing nervous system has relied in part on genetic studies in vertebrate and invertebrate models, and in part on mapping loci responsible for heritable forms of brain dysfunction in humans. These efforts have identified several RNA binding proteins, whose individual loss alters neuronal morphology and connectivity, suggesting that post- transcriptional mechanisms play an important role in neurodevelopment. We co-discovered a form of heritable intellectual disability caused by mutations in the gene encoding a polyadenosine RNA binding protein termed ZC3H14. Our analysis of a D. melanogaster model of this disease created by deletion of the sole invertebrate ZC3H14 homolog, dNab2, has revealed cell-autonomous defects in neuronal projection in the mushroom bodies (MBs), twin neuropil structures in the brain involved in learning and memory. Axons from wildtype MB neurons project towards the midline of the adult brain, while those from dNab2-deficient MB neurons misproject through the midline and into the contralateral brain hemisphere. Given the shared molecular role of dNab2/ZC3H14 as RNA binding proteins, we hypothesize that the axon misprojection and cognitive defects resulting from loss of functional dNab2/ZC3H14 are the result of defects in post-transcriptional control of target RNAs. Our published finding that human ZC3H14 partially compensates for dNab2 loss when expressed in fly neurons indicates that at least some of these RNA targets are shared. This proposal focuses on identification of dNab2-bound RNAs from fly brain neurons that encode factors involved in axononogenesis.
In Aim 1 we propose to use a validated approach, RNA Tagging, to specifically recover dNab2-bound RNAs from Drosophila neurons in their natural context in situ. A pilot application of this technique has already yielded two RNAs encoding protein with established roles in neuronal development.
Aim 2 will pursue the hypothesis that dNab2 loss alters a key aspect of the post-transcriptional regulation of these dNab2-bound mRNAs, with ultimate effects on expression of their cognate proteins.
Aim 3 completes the cycle by applying genetic tools to assess the in vivo role of each of these factors to wt brain development and their roles in dNab2 null phenotypes in the developing mushroom bodies.
These Aims leverage the strength of our Drosophila dNab2 model to support our long-term goal of defining the mechanistic basis of ZC3H14-associated neuronal defects in vertebrates. Our approach does not discount roles for dNab2 in other neuronal processes or cell types, but rather allows us to focus on one novel function of dNab2 (axonogenesis) in an experimentally accessible group of neurons (MB cells). Insights into molecular roles for dNab2 in MB neurons could be relevant to molecular defects in the neurons of human patients lacking ZC3H14.
The applicants have discovered a new form of heritable human intellectual disability (commonly referred to as mental retardation) caused by a defect in a protein that interacts with ribonucleic acids (RNAs) in brain cells. To study why this protein:RNA interaction might be especially important to our brain cells, the applicants have engineered fruit flies that lack the equivalent protein and found that the brains of the flies develop abnormally. The proposal is focused on studying the brains of these flies in order to learn more about the underlying cause of the human disease and to gain insight into potential diagnostic and therapeutic tools.
|Bienkowski, Rick S; Banerjee, Ayan; Rounds, J Christopher et al. (2017) The Conserved, Disease-Associated RNA Binding Protein dNab2 Interacts with the Fragile X Protein Ortholog in Drosophila Neurons. Cell Rep 20:1372-1384|
|Kelly, Seth M; Bienkowski, Rick; Banerjee, Ayan et al. (2016) The Drosophila ortholog of the Zc3h14 RNA binding protein acts within neurons to pattern axon projection in the developing brain. Dev Neurobiol 76:93-106|
|Fasken, Milo B; Corbett, Anita H (2009) Mechanisms of nuclear mRNA quality control. RNA Biol 6:237-41|