Horabin Laboratory Jamila I. Horabin, Ph.D. Duke University Florida State University College of Medicine 1115 West Call Street Tallahassee, FL 32306-4300 Office: (850) 645-2820, COM 3300-G Dr. Horabin's Faculty Profile
Research Interests
LAB RESEARCH FOCUS: RNA silencing and its regulation of Sex Determination in Drosophila   While the simple fruit fly seems a far cry from humans, the surprising degree of conservation between flies and humans of both developmental regulators and proteins tells us that the information we gather from analyzing fruit flies will be directly applicable to understanding human development. For several years, our major focus of interest has been the mechanism of sex determination in the fruit fly, Drosophila melanogaster. How do flies know what sex to develop? The ratio of X chromosomes to the sets of autosomes (the X:A ratio) is the primary sex determination signal in D. melanogaster. An X:A ratio of 1, as in XX individuals, is read as a female signal while a ratio of 1/2, as in XY individuals, is read as a male signal. The outcome of reading this ratio is imparted to the binary switch gene, Sex-lethal (Sxl) which serves as the masterswitch of sex determination. Sxl has two modes of operation; it is turned on in females while it remains off in males. Sxl controls sexual development as a splicing and translational regulator. It regulates dosage compensation (the process which corrects the level of gene expression from the sex chromosomes), turning off the system by both splicing regulation and translational repression. In regulating somatic sexual differentiation, Sxl promotes female differentiation by controlling the female specific splicing of transformer (tra). When tra is on female differentiation is driven through the female form of doublesex (dsx). The male form of dsx is the default mode, resulting in male differentiation. Recently, we discovered that the RNA silencing system regulates the sex determination process. Genes from all three major arms of RNA silencing (germline piRNAs, and both the microRNA and silencing RNA dicers) influence the expression Sxl. Our challenge now is to unravel how these various RNA silencing pathways impact the sex determination process and elucidate what the RNA silencing players do.
Current Projects
We investigated the role of dcr-2 in development, without addition of exogenous double stranded RNA. We find dcr-2 impacts female sex determination by altering the transcription of key sex determination genes, a role for dcr-2 not previously described. Dcr-2 influences the timing as well as strength of expression of the specialized X:A ratio sensing promoter of Sxl. Surprisingly, the ability to dice and silence shows a poor correlation with its sex determination role, indicating RISC loading and the interaction of Dcr-2 with other components in the complex, as the more important activity. Our results suggest Dcr-2 alters RISC function at a level beyond canonical silencing, expanding its functions in gene regulation. (Top) Viability of females relative to males from mating of wild type (Ore R) or dcr-2- homozygous mothers to fathers mutant for numerator genes (sis-a1, sis-bsc3-1) or Sxl (SxlfP7B0). Dcr-2 alleles on the X-axis placed relative to their position in the protein. (Bottom) Expression of Sxl X:A ratio sensing promoter in Ore R, dcr-2G31R and dcr-2E1237A embryos. Analysis of chromatin markers in early embryos suggest that Dcr-2 may be affecting the chromatin around the X:A ratio sensing promoter of Sxl. Since the dicing activity of Dcr-2 does not appear to be important, it is not the endogenous silencing RNAs dependent on Dcr-2 that are involved. Future challenges include identifying the small RNAs involved, how they are generated and how they regulate expression of the X:A ratio sensing promoter of Sxl. The strong understanding we have of the sex determination process will provide a useful backdrop towards understanding how RNA silencing regulates development.
Selected References
**Vied, C. and Horabin, J. I. The Sex Determination Master Switch, Sex-lethal, Responds to Hedgehog Signaling in the Drosophila Germline. (2001). Development 128: 2649-2660. Horabin, J. I., Walthall, S., Vied, C and Moses, M. A Positive role for Patched in Hedgehog signaling revealed by the intracellular trafficking of Sex-lethal, the Drosophila Sex Determination Master Switch (2003). Development. 130: 6101-6109. Horabin, J. I. Splitting the Hedgehog signal: Sex and Patterning in Drosophila. (2005) Development 132: 4801-4810. Walthall, S. L., Moses, M. and Horabin, J. I. A large complex containing both Patched and Smoothened initiates Hedgehog signaling in Drosophila. (2007) J. Cell Science 120: 826-837. Horabin, J. I. Kleinman, E. and Olcese, U. RISC loading and a role of Drosophila Dicer-2 in transcription as revealed by the sex determination pathway. (2008) Submitted. ** Article covered in ‘News and Views’ of Nature Cell Biology see: Greaves, S. The lethality of sex (2001). Nature Cell Biology 3: E208