Helen Salz Professor Ph.D. Training Faculty

Department of Genetics School of Medicine Case Western Reserve University Biomedical Research Building 626 2109 Adelbert Road Cleveland, Ohio 44106-4955 Tel: (216) 368-2879 Fax: (216) 368-3432 E-mail: helen.salz@case.edu

About Helen Salz

Helen Salz received her Ph.D. in Genetics from the University of California, Davis in 1983. She was a postdoctoral fellow with Drs. Tom Cline and Paul Schedl at Princeton University. After completing her postdoctoral studies in 1987, she joined the faculty in the Department of Genetics.

She is also a member of the Center for RNA Molecular Biology and Program in Developmental Biology.

Research

My research program is focused on understanding how RNA splicing is controlled in response to specific developmental and tissue-specific cues. Splicing, like transcription, is a highly regulated process. In fact, recent estimates show that a third to a half of all human genes (which on average contain 8 exons) encode pre-mRNAs that undergo alternative splicing to produce multiple mRNAs, each of which is capable of encoding a different protein isoform. Given that alternative splicing is a major source of protein diversity and an important means of regulating gene expression, it is not surprising that errors in RNA splicing have been associated with many different human diseases. Despite the importance of splicing in health and disease, relatively little is known about how splicing is controlled.

We study the control of RNA splicing in the fruit fly, Drosophila melanogaster. Drosophila is an outstanding system for studying splicing regulation because of the remarkable conservation of the splicing machinery between flies and humans. Moreover, we are not limited to analyzing gene function in cell culture because Drosophila has a unique set of genetic, genomic and molecular methods that facilitates the analysis of splicing factors and their target pre-mRNAs in the living animal.

RNA splicing is carried out by the spliceosome, a large catalytic RNA-protein machine that is assembled from the U1, U2, U4, U6 and U5 small nuclear ribonucleoprotein particles (snRNPs). In addition, the spliceosome contains many proteins that are not stably associated with the snRNPs. Although the identity of most of the spliceosome components is known, their individual contribution to the building of a functional spliceosome is still poorly understood. We are currently studying the U1 and U2 snRNP components to understand the role of each protein in pre-mRNA splicing.

A second project in my lab is focused on understanding the mechanism that underlies Sex-lethal (Sxl) splicing autoregulation. Alternative splicing of the Sxl pre-mRNA has long served as a model example of a regulated splicing event because splicing is sex-specific and controls Sxl activity. In females, where Sxl activity is needed, the translation terminating exon #3 is skipped, resulting in protein-encoding mRNAs. In males, exon #3 is included in the mature RNA, no protein is produced, and Sxl remains "OFF". Sxl splicing is controlled by an autoregulatory splicing loop. The female-specific SXL protein is necessary for skipping exon #3, thus insuring its own future production. Although this sequence of events is described in many molecular biology textbooks, the mechanism by which the SXL protein directs the splicing machinery to skip the translation terminating male-exon is unknown. Indeed, recent studies argued for mechanisms as diverse as blocking spliceosome assembly altogether to blocking processing during the second catalytic step of splicing. Using in vivo techniques, we were able to establish that SXL represses male-exon splicing by interacting with the general splicing factors U1 snRNP and U2AF, two factors that are involved in splice site recognition, but are only transiently associated with the pre-mRNA during the course of spliceosome assembly. Thus our studies argue that Sxl male-exon splicing repression occurs during the early steps of spliceosome assembly, after splice site recognition, but before catalysis begins. We are now working to identify all the components necessary for Sxl male-exon skipping. While these studies are giving us a clearer picture about how Sxl splicing is regulated in females, we do not know how, or even if, Sxl splicing is regulated in males. Is the absence of SXL protein all that is needed to guarantee that the male-exon is included in the mature mRNA? We think not. We have recently identified a set of male-specific mutations that suggest that male-exon inclusion is as tightly regulated as the mechanism that drives exon skipping in females.

While the majority of our studies are focused on understanding how Sxl RNA splicing is controlled, we have also “followed our noses” and initiated studies into several different areas of Drosophila development. In a recently established project, we have identified a genetic pathway (regulated by RNA splicing) that plays a determinative role in the fate of dividing germline stem cells.

Selected Publications