by Chan Wai Ling. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 170-185). / Abstracts in English and Chinese. / Acknowledgements --- p.i / Abstract --- p.iii / 摘要 --- p.v / Table of Content --- p.vii / Abbreviations --- p.xii / for Nucleotides --- p.xii / for Amino acids --- p.xii / for Standard genetic codes --- p.xiii / for Units --- p.xiii / for Prefixes of units --- p.xiv / for Terms commonly used in the report --- p.xiv / List of Figures --- p.xvii / List of Tables --- p.xxiii / Chapter Chapter I --- Introduction --- p.1 / Chapter 1.1 --- Epidemiology of the Severe Acute Respiratory Syndrome --- p.1 / Chapter 1.2 --- The SARS Coronavirus --- p.3 / Chapter 1.3 --- Cell Biology of Coronavirus Infection and Replication and the Role of Nucleocapsid Protein --- p.9 / Chapter 1.4 --- Recent Advances in the SARS-CoV Nucleocapsid Protein --- p.16 / Chapter 1.5 --- The Sumoylation System --- p.24 / Chapter 1.6 --- Objectives of the Present Study --- p.28 / Chapter Chapter II --- SARS-CoV N protein and Fragment Purification --- p.29 / Chapter 2.1 --- INTRODUCTION --- p.29 / Chapter 2.2 --- METHODOLOGY --- p.31 / Materials --- p.31 / Methods --- p.39 / Chapter 2.2.1 --- Construction of the pMAL-c2P vector --- p.39 / Chapter 2.2.2 --- Sub-cloning of the N protein into expression vectors --- p.42 / Chapter 2.2.2.1 --- Design of primers for the cloning of N protein --- p.43 / Chapter 2.2.2.2 --- DNA amplification using Polymerase Chain Reaction (PCR) --- p.44 / Chapter 2.2.2.3 --- DNA extraction from agarose gel --- p.45 / Chapter 2.2.2.4 --- Restriction digestion of purified PCR product and vectors --- p.46 / Chapter 2.2.2.5 --- Ligation of N protein into expression vectors --- p.47 / Chapter 2.2.2.6 --- Preparation of competent cells --- p.48 / Chapter 2.2.2.7 --- Transformation of plasmids into competent Escherichia coli --- p.49 / Chapter 2.2.2.8 --- Preparation of plasmid DNA --- p.49 / Chapter 2.2.2.8.1 --- Mini-preparation of plasmid DNA --- p.49 / Chapter 2.2.2.8.2 --- Midi-preparation of plasmid DNA --- p.51 / Chapter 2.2.3 --- Expression of tagged and untagged N protein --- p.53 / Chapter 2.2.3.1 --- Preparation of E. coli competent cells for protein expression --- p.53 / Chapter 2.2.3.2 --- Expression of N protein --- p.53 / Chapter 2.2.3.3 --- Solubility tests on the fusion proteins expressed --- p.54 / Chapter 2.2.4 --- Purification of N protein Chromatographic methods --- p.55 / Chapter 2.2.4.1 --- Affinity chromatography --- p.55 / Chapter 2.2.4.1.1 --- Ni-NTA affinity chromatography --- p.55 / Chapter 2.2.4.1.2 --- Glutathione affinity chromatography --- p.56 / Chapter 2.2.4.1.3 --- Amylose affinity chromatography --- p.56 / Chapter 2.2.4.2 --- Ion exchange chromatography --- p.57 / Chapter 2.2.4.2.1 --- Cation exchange chromatography --- p.57 / Chapter 2.2.4.2.2 --- Anion exchange chromatography --- p.58 / Chapter 2.2.4.3 --- Heparin affinity chromatography --- p.58 / Chapter 2.2.4.4 --- Size exclusion chromatography Purification strategies --- p.60 / Chapter 2.2.4.5 --- Purification of His6-tagged N proteins --- p.60 / Chapter 2.2.4.6 --- Purification of MBP-tagged N proteins --- p.60 / Chapter 2.2.4.7 --- Purification of GST-tagged N proteins --- p.61 / Chapter 2.2.4.8 --- Purification of untagged N proteins --- p.61 / Chapter 2.2.5 --- Trypsin digestion assay for the design of stable fragment --- p.64 / Chapter 2.2.6 --- Partial purification of the N protein amino acid residue 214-422 fragment --- p.65 / Chapter 2.2.7 --- Sumoylation of the SARS-CoV N protein --- p.67 / Chapter 2.2.7.1 --- In vitro sumoylation assay --- p.67 / Chapter 2.2.7.2 --- Sample preparation for mass spectrometric analysis --- p.68 / Chapter 2.3 --- RESULTS --- p.70 / Chapter 2.3.1 --- Construction of the vector pMAL-c2P --- p.70 / Chapter 2.3.2 --- "Construction of recombinant N protein-pAC28m, N-protein- pGEX-6P-l,N protein-pMAL-c2E and N protein-pMAL-c2P plasmids" --- p.72 / Chapter 2.3.3 --- Optimization of expression conditions --- p.79 / Chapter 2.3.4 --- Screening of purification strategies --- p.82 / Chapter 2.3.4.1 --- Purification of His6-N protein --- p.82 / Chapter 2.3.4.2 --- Purification of MBP-N protein --- p.84 / Chapter 2.3.4.3 --- Purification of GST-N protein --- p.85 / Chapter 2.3.4.4 --- Purification of untagged N protein --- p.87 / Chapter 2.3.5 --- Limited trypsinolysis for the determination of discrete structural unit --- p.91 / Chapter 2.3.6 --- Partial purification of the N protein 214-422 fragment --- p.94 / Chapter 2.3.7 --- Sumoylation of N protein --- p.97 / Chapter 2.2.7.1 --- Sumoylation site prediction --- p.97 / Chapter 2.2.7.2 --- In vitro sumoylation assay --- p.99 / Chapter 2.2.7.3 --- Mass spectrometric identification of sumoylated SARS-CoV N protein --- p.103 / Chapter 2.4 --- DISCUSSION --- p.109 / Chapter Chapter III --- Characterization of the Nucleic Acid Binding Ability of N protein --- p.119 / Chapter 3.1 --- INTRODUCTION --- p.119 / Chapter 3.2 --- METHODOLOGY --- p.120 / Materials --- p.120 / Methods --- p.124 / Chapter 3.2.1 --- Spectrophotometric Measurement of ratio OD260/ OD280 --- p.124 / Chapter 3.2.2 --- Native gel electrophoresis --- p.124 / Chapter 3.2.3 --- Quantitative determination of nucleic acids content --- p.125 / Chapter 3.2.3.1 --- Dische assay - quantitative determination of DNA content --- p.125 / Chapter 3.2.3.2 --- Orcinol assay - quantitative determination of RNA content --- p.126 / Chapter 3.2.4 --- RNase digestion of the N protein-bound RNA --- p.128 / Chapter 3.2.5 --- Isolation of RNA from purified GST-N proteins --- p.128 / Chapter 3.2.6 --- In vitro transcription of SARS-CoV genomic RNA fragment --- p.129 / Chapter 3.2.7 --- Vero E6 cell line maintenance and total RNA extraction --- p.131 / Chapter 3.2.8 --- Electrophoretic mobility shift assay (EMSA) --- p.131 / Chapter 3.3 --- RESULTS --- p.133 / Chapter 3.3.1 --- Detection of nucleic acids in the purified N proteins byspectrophotometric Measurement of ratio OD260/ OD280 --- p.133 / Chapter 3.3.2 --- Native gel electrophoresis --- p.135 / Chapter 3.3.3 --- Quantitative determination of nucleic acids content in purified GST-N proteins --- p.136 / Chapter 3.3.3.1 --- Dische assay for the determination of DNA --- p.136 / Chapter 3.3.3.2 --- Orcinol assay for the determination of RNA --- p.138 / Chapter 3.3.4 --- RNase digestion treatment --- p.139 / Chapter 3.3.5 --- Extraction of RNA from GST-N proteins --- p.140 / Chapter 3.3.6 --- In vitro transcription of SARS-CoV genomic RNA fragment --- p.142 / Chapter 3.3.7 --- Electrophoretic mobility shift assay (EMSA) --- p.144 / Chapter 3.4 --- DISCUSSION --- p.147 / Chapter Chapter IV --- Discussion --- p.154 / Chapter 4.1 --- "Purity, Aggregation and RNA Binding Property of the SARS-CoV Nucleocapsid Protein" --- p.154 / Chapter 4.2 --- Future perspectives --- p.156 / Chapter 4.2.1 --- Structural study of the SARS-CoV N protein through x-ray crystallography --- p.156 / Chapter 4.2.2 --- Mapping the RNA binding domain in the SARS-CoV N protein --- p.156 / Chapter 4.2.3 --- Determination of aggregation state by lateral turbidimetry analysis --- p.156 / Chapter 4.2.4 --- Exploring protein interacting partners that enhance RNA binding specificity --- p.157 / Appendix --- p.159 / Chapter I. --- Sequence of the SARS-CoV N protein --- p.159 / Chapter II. --- Sequence of the SARS-CoV genome fragment used for RNA binding assay in section 3.37.1 --- p.161 / Chapter III. --- Vector maps --- p.161 / Chapter a) --- Vector map of pACYC177 --- p.161 / Chapter b) --- Vector map and MCS of pET28a --- p.163 / Chapter c) --- Vector map and MCS of pAC28 --- p.164 / Chapter d) --- Vector map and MCS of pGEX-6P-1 / Chapter e) --- Vector map of pMAL-c2X and MCS of pMAL-c2E / Chapter IV. --- Electrophoresis markers --- p.166 / Chapter V. --- SDS-PAGE gel parathion protocol --- p.169 / References --- p.170

Le glioblastome multiforme (GBM) est une tumeur cérébrale extrêmement agressive associée à un mauvais pronostic. C'est pourquoi, il apparaît nécessaire d'identifier les mécanismes moléculaires participant au développement des GBM ainsi qu'à leurs résistances aux traitements afin de développer de nouvelles approches thérapeutiques. Récemment, il a été montré que les régulations traductionnelles jouent un rôle fondamental dans les propriétés agressives du GBM. Les protéines de liaison à l'ARN (RBP) sont des acteurs majeurs de ces régulations dont l'expression/activité est altérée dans les GBM. Les RBP hnRNP HF (HF) font partie des RBP les plus surexprimées dans les GBM et leur contribution dans la régulation traductionnelle des GBM n'a encore jamais été investiguée. Nous avons émis l'hypothèse que hnRNP H et hnRNP F soient au centre d'un réseau de régulations post-transcriptionnelles impactant la machinerie traductionnelle qui contrôle le développement tumoral et la résistance aux traitements des GBM. Nos résultats montrent qu'HF régulent la prolifération et la réponse aux traitements car leur perte d'expression (i) diminue la prolifération des GBM (modèle cellulaire, sphéroïde et xénogreffes in vivo), (ii) active les voies de réponse aux dommages à l'ADN et (iii) sensibilise les cellules de GBM aux irradiations. De plus, nous avons identifié un nouveau rôle pour HF en tant que régulateurs de la traduction. En effet, nos données montrent que les hnRNP HF contrôlent la traduction d'un ensemble d'ARNm en régulant l'expression et l'activité de facteurs d'initiation ainsi qu'en collaborant avec des ARN hélicases partenaires en ciblant des ARNm impliqués dans des processus reliés au développement tumoral et la résistance aux traitements possédant des structures secondaires G-quadruplexe dans leurs 5'UTR. Les données que nous avons générées suggèrent que hnRNP H et hnRNP F sont des régulateurs traductionnels essentiels au développement tumoral et à la résistance aux traitements des GBM. / Glioblastoma multiforme (GBM) is one of the most aggressive brain tumors with poor prognosis. Understanding the molecular mechanisms involved in the development and resistance to treatments of gliomas could improve treatment efficiency. Recently, it has been demonstrated that translational regulations play a key role in the GBM aggressivity. RNA binding proteins (RBP) are major regulators of these processes and have altered expression / activity in GBM. The RBP hnRNP H and hnRNP F (HF) are among the most overexpressed RBP in GBM and their role in GBM translational regulation has never been investigated yet. We hypothesize that HF are at the core of a post-transcriptional regulation network which impacts the translational machinery that controls GBM tumor development and resistance to treatment. We have demonstrated that hnRNP H and hnRNP F regulate proliferation and response to treatment because their depletion (i) decreases the GBM proliferation (cell line model, spheroid and in vivo xenografts), (ii) activates the DNA damage response pathways and (iii) sensitizes the GBM cells to irradiation. We have identified HF as new regulators of GBM translation. Indeed, our data show that hnRNP H and hnRNP F control mRNA translation by regulating expression/activity of initiation factors and in collaboration with RNA helicases by targeting mRNA involved in oncogenic processes and containing secondary structures called G-quadruplex in their 5'UTR. The data that we have generated suggest that HF are essential translational regulators involved in tumor development and resistance to treatment in GBM.

Borrelia burgdorferi, the causative agent of Lyme disease, exists in a defined enzootic cycle involving Ixodes scapularis ticks and various vertebrates. Humans can serve as an accidental host, if a tick colonized with B. burgdorferi happens to feed on a human. B. burgdorferi are also accidental pathogens: they do not make toxins, or destroy host tissue by other mechanisms. They merely transmit between vector and host to survive. In order to do this, they must effectively sense their current environment, and appropriately alter cellular processes. Understanding the regulatory mechanisms of how B. burgdorferi manages to do this has been a focus of the Stevenson lab for many years. Previous work identified SpoVG as a DNA-binding protein. Although a homologue of this protein had been implicated to serve a regulatory role in other bacteria, the Stevenson lab was the first to demonstrate a function for the protein, both for B. burgdorferi and two other bacteria. Studies contained in this body of work aim to provide insight into regulation of SpoVG by B. burgdorferi as well the impact that it has on gene regulation. By using genetic mutants, we determined that SpoVG is regulated at the levels of transcription and translation in culture by growth rate, temperature, and other regulatory factors. Additionally, we provide evidence that SpoVG regulates its own expression. Numerous genes are under control of SpoVG. Biochemical analyses revealed that SpoVG specifically interacts with DNAs and RNAs associated with genes found to be under its regulatory control. Finally, we provide evidence for SpoVG acting in concert with other known regulatory factors such as other DNA-binding proteins and the cyclic di-nucleotide second messengers cyclic-di-GMP and cyclic-di-AMP. All together, these studies provide insight into how B. burgdorferi broadly regulates cellular processes during different stages of the enzootic cycle. We hypothesize that SpoVG does this through globally manipulating the three-dimensional structure of the bacterial chromosome, and that exactly how SpoVG acts at any given point will be dependent on the other regulatory factors that are also present in the cell.

[Truncated abstract] Regulation of mRNA stability is an important posttranscriptional mechanism involved in the control of gene expression. The rate of mRNA decay can differ greatly from one mRNA to another and may be regulated by RNA-protein interactions. A key determinant of mRNA decay are sequence instability (cis) elements often located in the 3' untranslated region (UTR) of many mRNAs. For example, the AU rich elements (AREs), are such well characterized elements, and most commonly involved in promoting mRNA degradation, and specific binding of proteins to these elements leading to the stabilization of some mRNAs. Other cis-elements have been described for mRNA in which mRNA stability is a critical component of gene regulation. This includes the androgen receptor (AR) UC-rich cis element in its 3'UTR. The AR is a key target for therapeutics in human prostate cancer and thus understanding the mechanism involved in regulating its expression is an important goal. The [alpha]CP1 protein, a KH-domain containing RNA-binding protein has been found to bind this UC-rich region of the AR and is thought to play an important role in regulating AR mRNA expression. [alpha]CP1 protein is a triple KH (hnRNP K homology) domain protein with specificity for Crich tracts of RNA and ssDNA (single stranded DNA). Relatively little is known about the structural interaction of [alpha]CP1 with target RNA cis elements, thus the present study aimed to better understand the nature of interaction between 30 nt 3'UTR UC-rich AR mRNA and [alpha]CP1 protein using various biophysical techniques, in an attempt to determine which [alpha]CP1 domain or combination of domains is involved in RNA-binding. These studies could ultimately provide novel targets for drugs aimed to regulate AR mRNA expression in prostate cancer cells. At the commencement of this study little was known about the structure of the [alpha]CP1- KH domains and their basis for poly (C) binding specificity. ... Additional studies addressed the significance of the four core recognition nucleotides (TCCC) using a series of cytosine to thymine mutants. The findings verified some of the results predicted from structural studies, especially the need for maximum KH binding to a core tetranucleotide recognition sequence. Our mutational studies of the four core bases confirmed the importance of cytosine in positions two and three as no binding was observed, while some binding was observed when the fourth base was mutated. In summary, the work presented in this thesis provides new detailed insight into the molecular interactions between the [alpha]CP1-KH domain and AR mRNA. Furthermore, these studies shed light on the nature of protein/mRNA interactions in general, as well as the specific complex that forms on AR mRNA. These studies have provided new understanding into the mode of [alpha]CP1 binding at a target oligonucleotide binding site and, provide a foundation for future studies to define structure of multiprotein/oligonucleotide complexes involved in AR mRNA gene regulation. Understanding the detailed interaction between the AR mRNA and [alpha]CP1 could provide possible targets for drug development at reducing AR expression in prostate cancer cells by interfering with the interaction of [alpha]CP1 and AR-mRNA.

Doctor of Philosophy (PhD) / Gene expression is known to be regulated at the level of transcription. Recently, however, there has been a growing realisation of the importance of gene regulation at the post-transcriptional level, namely at the level of pre-mRNA processing (5’ capping, splicing and polyadenylation), nuclear export, mRNA localisation and translation. Erythroid krüppel-like factor (Eklf) is the founding member of the Krüppel-like factor (Klf) family of transcription factors and plays an important role in erythropoiesis. In addition to its nuclear presence, Eklf was recently found to localise to the cytoplasm and this observation prompted us to examine whether this protein has a role as an RNA-binding protein, in addition to its well-characterised DNA-binding function. In this thesis we demonstrate that Eklf displays RNA-binding activity in an in vitro and in vivo context through the use of its classical zinc finger (ZF) domains. Furthermore, using two independent in vitro assays, we show that Eklf has a preference for A and U RNA homoribopolymers. These results represent the first description of RNA-binding by a member of the Klf family. We developed a dominant negative mutant of Eklf by expressing its ZF region in murine erythroleukaemia (MEL) cells. We used this to investigate the importance of this protein in haematopoietic lineage decisions by examining its effect on the multipotent K562 cell line. We provide evidence that Eklf appears to be critical not only for the promotion of erythropoiesis, but also for the inhibition of megakaryopoiesis.

It is well-established that the organization of nuclear components influences gene expression processes, yet little is known about the mechanisms that contribute to the spatial co-ordination of nuclear activities. The salivary gland cells of Chironomus tentans provide a suitable model system for studying gene expression in situ, as they allow for direct visualization of the synthesis, processing and export of a specific protein-coding transcript, the Balbiani ring (BR) pre-mRNA, in a nuclear environment in which chromatin and non-chromatin structures can easily be distinguished. The RNAbinding protein Hrp65 has been identified in this model system as a protein associated with non-chromatin nucleoplasmic fibers, referred to as connecting fibers (CFs). The CFs associate with BR RNP particles in the nucleoplasm, suggesting that Hrp65 is involved in mRNA biogenesis at the post-transcriptional level. However, the function of Hrp65 is not known, nor is the function or the composition of CFs. In the work described in this thesis, we have identified by yeast two-hybrid screening and characterized different proteins that bind to Hrp65. These proteins include a novel hnRNP protein in C. tentans named Hrp59, various isoforms of Hrp65, the splicing- and mRNA export factor HEL/UAP56, and a RING-domain protein of unknown function. Immuno-electron microscopy experiments showed that Hrp59 and HEL are present in CFs, and in larger structures in the nucleoplasm of C. tentans salivary gland cells. Hrp59 is a C. tentans homologue of human hnRNP M, and it associates cotranscriptionally with a subset of pre-mRNAs, including its own transcript, in a manner that does not depend quantitatively on the amount of synthesized RNA. Hrp59 accompanies the BR pre-mRNA from the gene to the nuclear envelope, and is released from the BR mRNA at the nuclear pore complex. We have identified the preferred RNA targets of Hrp59 in Drosophila cells, and we have shown that Hrp59 binds preferentially to exonic splicing enhancer sequences. Hrp65 self-associates through an evolutionarily conserved domain that can also mediate heterodimerization of Hrp65 homologues. Different isoforms of Hrp65 interact with each other in all possible combinations, and Hrp65 can oligomerize into complexes of at least six molecules. The interaction between different Hrp65 isoforms is crucial for their intracellular localization, and we have discovered a mechanism by which Hrp65-2 is imported into the nucleus through binding to Hrp65-1. Hrp65 binds to HEL/UAP56 in C. tentans cells. We have analyzed the distribution of the two proteins on polytene chromosomes and in the nucleoplasm of salivary gland cells, and our results suggest that Hrp65 and HEL become associated during posttranscriptional gene expression events. HEL binds to the BR pre-mRNP cotranscriptionally, and incorporation of HEL into the pre-mRNP does not depend on the location of introns along the BR pre-mRNA. HEL accompanies the BR mRNP to the nuclear pore and is released from the BR mRNP during translocation into the cytoplasm.

Spinal muscular atrophy is an autosomal recessive genetic disease that results from the loss and/or degeneration of alpha motor neurons in the lower part of the spinal cord. With ~ 1 in 6000 live births per year being affected, this disease is the second leading cause of infant death and is caused by the loss or decrease of the Survival of Motor Neuron protein (SMN). While a lot is known about the role that SMN plays in the cytoplasmic assembly of spliceosomal small nuclear ribonucleoproteins (snRNPs), it remains a crucial question in the field to gain a better understanding of what specific/distinct function(s) SMN might have in motor neurons. We have identified novel interactions between SMN and two RNA-binding proteins (RBPs) known to be components of axonal RNA granules. More specifically, we demonstrated that SMN interacts with HuD and SERBP1 in a direct fashion in foci-like structures along neurites of motor neuron-derived cells. We have also demonstrated that the SMN/HuD interaction is required for the localization of HuD into RNA granules in neurites of motor neuron-derived cells. Furthermore, I have shown that SERBP1 is down-regulated in the absence of normal levels of SMN and, most importantly, that over-expression of SERBP1 can rescue SMA-like neuronal defects using a cell culture model of the disease. These findings may help shed light on the non-canonical molecular pathway(s) involving SMN and RBPs in motor neurons and underscores the possible therapeutic benefits of targeting these RBPs in the treatment of SMA.

<p>In Drosophila melanogaster, where males are XY and females are XX, dosage compensation is acheived by approximately two-fold upregulation of transcription of the single male X chromosome. This upregulation is mediated by the dosage compensation complex (DCC), which is assembled in a sequential manner on the male X chromosome and is composed of the two noncoding roX (RNA on the X) RNAs and at least five proteins, including the RNA helicase Maleless (MLE). MLE contains two highly conserved double stranded RNA binding domains (DRBDs) at the N terminus. We investigated the roles of the roX RNAs and MLE helicase through experiments using classical genetic methods to generate and analyze the effects of mutants and mutant transgenes, immunolocalization experiments to study MSL protein and roX RNA to chromosomes. For the first time in vivo, we demonstrate that MLE associates with double stranded RNA in a sequence non-specific manner that is independent of other DCC components. Importantly, we find that the DSRBDs of MLE are essential for dosage compensation but are not required for MLE localization to the male X chromosome. We propose that although the DSRBDs are not essential for ds RNA binding, they may act synergistically with other domains of MLE or other MSLs to associate with RNA in vivo. We propose that a MLE/ roX RNA association involving secondary structure formed by the roX RNAs may be involved in the assembly, stabilization, or function of the DCC.</p> / Dissertation

<p>Germ cells represent the immortal line: they are guardians of a totipotent genome and are essential for the genetic survival of an individual organism and ultimately a species. An error at any stage in development (specification, migration, colonization, differentiation, adult maintenance) can lead to one of two disastrous outcomes: (1) germ cell death or (2) unchecked growth and proliferation leading to tumorigenesis. The work in this dissertation utilizes a classic mouse model (<italic>Ter</italic>) resulting in both of these phenotypes to further explore the molecular mechanisms important for development of germ cells. </p> <p>A homozygous nonsense mutation (<italic>Ter</italic>) in murine <italic>Dnd1</italic> (<italic>Dnd1<super>Ter/Ter</super></italic>) results in a significant (but not complete) early loss of primordial germ cells (PGCs) prior to colonization of the gonad in both sexes and all genetic backgrounds tested. The same mutation also leads to testicular teratomas only on the 129/SvJ background. Male mutants on other genetic backgrounds ultimately lose all PGCs with no incidence of teratoma formation. It is not clear how these PGCs are lost, develop into teratomas, or what factors directly control the strain-specific phenotype variation. </p> <p>Work here demonstrates that <italic>Dnd1</italic> expression is restricted to germ cells and that the <italic>Ter</italic> mutant defect is cell autonomous. The early loss of germ cells is due in part to BAX&ndash;mediated apoptosis which also affects the incidence of tumorigenesis on a mixed genetic background. Moreover, tumor formation is-specific to the male developmental pathway and not dependent on sex chromosome composition of the germ cell (XX vs. XY). Despite normal initiation of the male somatic pathway, mutant germ cells fail to differentiate as pro&ndash;spermatogonia and instead prematurely enter meiosis.</p> <p>Results here also reveal that, on a 129/SvJ background, many mutant germ cells fail to commit to the male differentiation pathway, instead maintain expression of the pluripotency markers, NANOG, SOX2, and OCT4, and initiate teratoma formation at the stage when male germ cells normally enter mitotic arrest. RNA immunoprecipitation experiments reveal that mouse DND1 directly binds a group of transcripts that encode negative regulators of the cell cycle, including <italic>p27Kip1</italic>, which is not translated in <italic>Dnd1<super>Ter/Ter</super></italic> germ cells. Additionally, overexpression of DND1 in a teratocarcinoma cell line leads to significant alteration of pathways controlling the G1/S checkpoint and the RB tumor suppressor protein. This strongly suggests that DND1 regulates mitotic arrest in male germ cells through regulation of cell cycle genes, serving as a gatekeeper to prevent the activation of a pluripotent program leading to teratoma formation. Furthermore, strain&ndash;specific morphological and expression level differences possibly account for sensitivity to tumor development.</p> / Dissertation

<p>SR proteins are a family of splicing factors involved in the regulation of both constitutive and alternative splicing of pre-mRNAs. Despite years of studies, several big questions still remain: how the expression levels of SR proteins are regulated; what are the underlying mechanisms responsible for SR proteins-mediated gene regulation; what are the physiological targets of SR proteins in vivo. In my dissertation study, I am focusing on two members of the family, SF2/ASF and SRp20, to study their functional involvement in regulating microRNA/mRNA biogenesis and their own expression. </p><p>Negative feedback regulation is a common mechanism maintaining the steady-state level of SR proteins (i.e. SC35 and SRp20), and several mechanism may be involved. In order to test if miRNAs are also involved in such negative feedbacks, small RNA sequencing was used to identify differentially expressed miRNAs after SF2/ASF overexpression in an inducible stable cell line system. Among the 40 differentially expressed miRNAs, miR-7 is particularly interesting, because it is also predicted to target SF2/ASF, which forms a negative feedback regulation. This is indeed the case as shown by luciferase reporter assay and overexpression/knocking down of miR-7 in vivo. To our knowledge, this is the first identified negative feedback circuit between a SR protein and a miRNA, which may be a general mechanism in regulating SR protein homeostasis.</p><p>To characterize the mechanism underlying SF2/ASF-enhanced miRNA biogenesis, I have employed a series of molecular and biochemical approaches to pinpoint the key molecular interactions in a minigene system, which is consist of miR-7 embedded intron and the flanking exons of its host gene. By manipulating the splicing pattern of such minigene, I have uncovered a splicing-independent function of SF2/ASF in regulating miRNA biogenesis. Directly binding between SF2/ASF protein and pri-miR-7 was demonstrated by Cross-linking and immunoprecipitation assay (CLIP) and RNA affinity purification. The precise binding site was then pinpointed by combining computational prediction and mutagenesis assay. Finally, by using in vitro pri-miRNA processing assay, I showed that SF2/ASF can promote the Drosha cleavage step of pri-miR-7 through directly association with the predicted binding site. So far, this is the first SR protein discovered, which is directly involved in miRNA biogenesis. Moreover, our preliminary data also suggested that SF2/ASF may promote miRNA biogenesis in other steps after Drosha cleavage; and different SR proteins can regulate miRNA biogenesis in a substrate-specific manner. Taken together, SR family of splicing factors may be broadly involved in miRNA biogenesis through direct interactions.</p><p>In order to study the general involvement of SR proteins in RNA biogenesis, one important step stone is to have a better profile of their targets in vivo. To achieve this, I focused on SRp20, another classic SR protein. Photoactivatable-Ribonucleoside-Enhanced Cross-linking and immunoprecipitation assay combined with deep sequencing (PAR-CLIP-seq) was used to identify the binding partners of SRp20 globally, which is subsidized by candidate gene validations. Consistent with the literature, I found that SRp20 primarily targets exonic regions for splicing regulation, and such interactions are likely to be sequence dependent on the CWWCW motif. Surprisingly, I also observed extensive binding between SRp20 and the 3' UTRs of mRNA, which may affect the choice of alternative polyadenylation sites. The underlying mechanisms are being investigated by a variety of molecular methods. </p><p>In summary, I have identified a subset of miRNAs, the expression of which can be regulated by SF2/ASF in a splicing independent manner. This is the first SR protein identified in regulating miRNA biogenesis. One of the upregulated miRNAs, miRNA-7 can form a negative feedback with SF2/ASF by negatively regulating the expression of SF2/ASF on translational level. By using PAR-CLIP method, I have identified the genome-wide binding partners of SRp20 in vivo. When SRp20 binds to the exonic regions, it potentially affects the alternative splicing patterns of nearby introns. Interestingly, the 3' end choices for a subset of genes may be regulated by SRp20 through directly binding, which may be a new mechanism for the regulation of 3' end processing.</p> / Dissertation