RNA-binding proteins as regulators of transcription and axial patterning during Xenopus embryogenesis
by Caitlin Suzanne DeJong
Doctor of Philosophy in Molecular and Cell Biology
University of California, Berkeley
Professor Richard M. Harland, Chair
The over-arching goal of this thesis is to expand our knowledge of the mechanisms by which one cell, a fertilized egg, develops into an organism composed of multiple cell types, each with different functions and behaviors. RNA-binding proteins have been identified as potent regulators of development and embryogenesis. The studies presented in this thesis illustrate the pleiotropic effects of RNA-binding proteins in Xenopus development and will focus specifically on two RNA-binding proteins that are maternally deposited and zygotically transcribed: TAF15 and DGCR8.
TATA-binding protein-associated factor 15 (TAF15) belongs to the FET family of atypical RNA-binding proteins, which also includes Fused in sarcoma (Fus) and Ewing’s sarcoma (EWS). FET proteins were originally discovered as components of fusion oncogenes and are most noted for their implication in various cancers and neuromuscular degenerative diseases. However, little is known of the endogenous function of FET proteins. The diverse biological activities of the FET family proteins can be likened to a biological Swiss army knife; as these proteins contain domains for transcriptional activation, RNA-binding, DNA-binding, and function in both RNA Polymerase II-mediated transcription and pre-mRNA splicing. An exciting possibility is that the FET proteins may function to connect transcription and splicing. By employing the bioinformatics approach of RNA-sequencing, I generated a list of significant genes that are differentially expressed between uninjected and taf15 depleted embryos. From this analysis I found that TAF15 regulates target genes at both the transcriptional and post-transcriptional level. The studies that focus on the role of TAF15 in Xenopus development are described in chapters two and three of this thesis.
In the second chapter of this thesis I describe studies that illustrate the novel concept that a protein can regulate the same set of target genes but through different molecular mechanisms. Both maternal and zygotic TAF15 regulate the expression of the transcripts fgfr4, isl1, and pax8. Interestingly, maternal TAF15 is required for the post-transcriptional regulation of fgfr4, isl1, and pax8, regulating the splicing of single introns within these transcripts, whereas zygotic TAF15 is required for the transcriptional regulation of these genes. Therefore, the studies described in chapter two demonstrate, for the first time, that a single protein can utilize a different molecular mechanism to control the same target genes and the use of these different mechanisms of action appears to be dependent on whether the protein is maternally deposited or zygotically transcribed. Single intron retention is a known mechanism to retain transcripts in the nucleus, preventing their translation. In chapter two of this thesis I provide evidence for the following model: in the absence of genome activation, before the zygotic genome is transcribed, maternal TAF15 cooperates with a splicing factor, the RNA-binding protein SRSF4, to regulate the splicing of single introns from transcripts. As a result, TAF15 and SRSF4 control the splicing of target genes and thus control the timing of transcript maturation and subsequent translation. This mechanism is logical as it provides a mechanism by which to spatially and temporally regulate gene expression in the absence of the ability to transcriptionally regulate genes. I further show evidence that following zygotic genome is activation, zygotic TAF15 activates target gene transcription, regulating genes at the transcriptional level, likely associating with the core promoter. The findings described in chapter two of this thesis are the first to show that a single protein can regulate the same gene targets but depending on the milieu of maternal of zygotic cofactors, regulates these targets via different underlying mechanisms. The variety of functional domains intrinsic to TAF15 supports the hypothesis that this atypical RNA-binding protein could operate as part of both a splicing and transcriptional complex.
In the third chapter of this thesis I describe studies that illustrate the novel finding that TAF15 is required for dorsoventral patterning via the repression of ventx2.1. Ventx2 and BMP4 function in an autocatalytic positive feedback loop to specify ventral tissue and antagonize organizer function. Following taf15 depletion, ventx2.1 expression is expanded in the neural ectoderm and embryos exhibit a BMP overexpression phenotype: reduction in head, and dorsal, and posterior fin structures, with an increase in ventral tissue. Unlike the findings in chapter two, in this study, both maternal and zygotic TAF15 function to suppress ventx2.1 expression. These findings place TAF15 in the regulatory network of dorsoventral patterning and suggest that maternal and zygotic TAF15 control expression of ventx2.1 in a similar manner but do not rule out differential mechanisms of this control. Currently, it is unknown if TAF15 represses ventx2.1 expression directly or if TAF15 is required to activate a repressor of ventx2.1.
In the fourth chapter of this thesis I describe studies that serve as a resource for future investigations into the role of microRNAs (miRNAs) in Xenopus development. DiGeorge syndrome critical region 8 (DGCR8) is a subunit of the microprocessor complex required for miRNA biogenesis. Unlike most members (e.g. Dicer, Argonaute2) of the RNA interference biogenesis pathway, DGCR8 is required specifically for miRNA biogenesis. Furthermore, unlike previous studies in mice and zebrafish that have depleted maternal dgcr8 throughout oogenesis to look at the role of miRNAs during embryogenesis, the antisense oligodeoxynucleotide (ODN) that I have designed can be used in host transfer assays to assess the effects of maternal dgcr8 depletion once oogenesis is complete, specifically during embryogenesis. Additionally, I have designed a splice-blocking morpholino (MO) antisense oligonucleotide that targets zygotic dgcr8 for depletion. Using these two tools (ODN and MO), the first studies can be performed that tease apart the role of maternal versus zygotic DGCR8 during embryogenesis.
The work presented in this thesis exemplifies the value of carefully assessing biological functions of genes that are both maternally deposited and zygotically transcribed. The surprising finding that TAF15 utilizes distinct molecular mechanisms to control conserved target genes depending on whether this protein is maternally deposited or zygotically expressed demonstrates a new level of molecular complexity that future studies must address. Additionally, these studies further support the motivation to investigate RNA-binding proteins in development and disease as they continually prove to be multifaceted players in molecular biology.