Evolutionarily conserved role of calcineurin in phosphodegron-dependent degradation of phosphodiesterase 4D.

Calcineurin is a widely expressed and highly conserved Ser/Thr phosphatase. Calcineurin is inhibited by the immunosuppressant drug cyclosporine A (CsA) or tacrolimus (FK506). The critical role of CsA/FK506 as an immunosuppressant following transplantation surgery provides a strong incentive to understand the phosphatase calcineurin. Here we uncover a novel regulatory pathway for cyclic AMP (cAMP) signaling by the phosphatase calcineurin which is also evolutionarily conserved in Caenorhabditis elegans. We found that calcineurin binds directly to and inhibits the proteosomal degradation of cAMP-hydrolyzing phosphodiesterase 4D (PDE4D). We show that ubiquitin conjugation and proteosomal degradation of PDE4D are controlled by a cullin 1-containing E(3) ubiquitin ligase complex upon dual phosphorylation by casein kinase 1 (CK1) and glycogen synthase kinase 3beta (GSK3beta) in a phosphodegron motif. Our findings identify a novel signaling process governing G-protein-coupled cAMP signal transduction-opposing actions of the phosphatase calcineurin and the CK1/GSK3beta protein kinases on the phosphodegron-dependent degradation of PDE4D. This novel signaling system also provides unique functional insights into the complications elicited by CsA in transplant patients.

The role of conserved region 1 of Escherichia coli sigma-70 in transcription initiation

The sigma (σ) subunit of eubacterial RNA polymerase is essential for initiation of transcription at promoter sites. σ factor directs the RNA polymerase core subunits ( a2bb′ ) to the promoter consensus elements and thereby confers selectivity for transcription initiation. The N-terminal domain (region 1.1) of Escherichia coli σ70 has been shown to inhibit DNA binding by the C-terminal DNA recognition domains when σ is separated from the core subunits. Since DNA recognition by RNA polymerase is the first step in transcription, it seemed plausible that region 1 might also influence initiation processes subsesquent to DNA binding. This study explores the functional roles of regions 1.1 and 1.2 of σ70 in transcription initiation. Analysis in vitro of the transcriptional properties of a series of N-terminally truncated σ70 derivates revealed a critical role for region 1.1 at several key stages of initiation. Deletion of the first 75 to 100 amino acids of σ70 (region 1.1) resulted in both a slow rate of transition from a closed promoter complex to a DNA-strand-separated open complex, as well as a reduced efficiency of transition from the open complex to a transcriptionally active open complex. These effects were partially reversed by addition of a polypeptide containing region 1.1 in trans. Therefore, region 1.1 not only modulates DNA binding but is important for efficient transcription initiation, once a closed complex has formed. A deletion of the first 133 amino acids which removes both regions 1.1 and 1.2 resulted in arrest of initiation at the earliest closed complex, suggesting that region 1.2 is required for open complex formation. Mutagenesis of region 1.1 uncovered a mechanistically important role for isoleucine at position 53 (I53). Substitution of I53 with alanine created a σ factor that associated with the core subunits to form holoenzyme, but the holoenzyme was severely deficient for promoter binding. The I53A phenotype was suppressed in vivo by truncation of five amino acids from the C-terminus of σ 70. These observations are consistent with a model in which σ 70I53A fails to undergo a critical conformational change upon association with the core subunits, which is needed to expose the DNA-binding domains and confer promoter recognition capability upon holoenzyme. To understand the basis of the autoinhibitory properties of the σ70 N-terminal domain, in the absence of core RNA polymerase, a preliminary physical assessment of the interdomain interactions within the σ70 subunit was launched. Results support a model in which N-terminal amino acids are in close proximity to residues in the C-terminus of the σ 70 polypeptide. ^

The role of the conserved N -terminal domain of STAT3 in STAT3 dephosphorylation and nuclear export

Rapid redistribution of STAT subcellular localization is an essential feature of cytokine signaling. To elucidate the molecular basis of STAT3 function, which plays a critical role in controlling innate immune responses in vivo, we initiated studies to determine the mechanisms controlling STAT3 nuclear trafficking. We found that STAT3 is transported to the nucleus in the absence of cytokine treatment, as judged by indirect immunofluorescence studies in the presence of leptomycin B, an inhibitor of CRM1-dependent nuclear export, suggesting that the non-phosphorylated STAT3 protein contains a functional nuclear import signal. An isoform lacking the STAT3 N-terminal domain (Δ133STAT3) retains the ability to undergo constitutive nuclear localization, indicating that this region is not essential for cytokine-independent nuclear import. Δ133STAT3 is also transported to the nucleus following stimulation with interleukin-6 (IL-6). Interestingly, IL-6-dependent tyrosine phosphorylation of Δ133STAT3 appears to be prolonged and the nuclear export of the protein delayed in cells expressing endogenous STAT3, consistent with defective Δ133STAT3 dephosphorylation. Endogenous STAT3 does not promote the nuclear export of Δ133STAT3, although dimerization between endogenous Stat3 and Δ133STAT3 is detected readily. Thus, the STAT3 N-terminal domain is not required for dimerization with full-length STAT3, yet appears to play a role in proper export of Stat3 from the nucleus following cytokine stimulation. STAT3-deficient cells reconstituted with Δ133STAT3 show enhanced and prolonged Stat1 signaling in response to IL-6, suggesting that induction of the STAT3-dependent negative regulator SOCS3 is impaired. In fact, Δ133STAT3 fails to induce SOCS3 mRNA efficiently. These studies collectively indicate that the STAT3 N-terminal region may be important for IL-6-dependent target gene activation and nuclear dephosphorylation, while dispensable for nuclear import. STAT3 is an oncogene. STAT3 is constitutively activated in primary tumors of many types. Thus far, research in the design of STAT3 protein inhibitors has focused on the SH2 and DNA-binding domains of STAT3. Interference with these domains eliminates all signaling through STAT3. If the N-terminal domain is involved in tetramerization on a subset of target genes, inhibition of this region may lead to a more selective inhibition of some STAT3 functions while leaving others intact. ^

Mechanism for sortase localization and the role of sortase localization in efficient pilus assembly in Enterococcus faecalis.

Pathogenic streptococci and enterococci primarily rely on the conserved secretory (Sec) pathway for the translocation and secretion of virulence factors out of the cell. Since many secreted virulence factors in gram-positive organisms are subsequently attached to the bacterial cell surface via sortase enzymes, we sought to investigate the spatial relationship between secretion and cell wall attachment in Enterococcus faecalis. We discovered that sortase A (SrtA) and sortase C (SrtC) are colocalized with SecA at single foci in the enterococcus. The SrtA-processed substrate aggregation substance accumulated in single foci when SrtA was deleted, implying a single site of secretion for these proteins. Furthermore, in the absence of the pilus-polymerizing SrtC, pilin subunits also accumulate in single foci. Proteins that localized to single foci in E. faecalis were found to share a positively charged domain flanking a transmembrane helix. Mutation or deletion of this domain in SrtC abolished both its retention at single foci and its function in efficient pilus assembly. We conclude that this positively charged domain can act as a localization retention signal for the focal compartmentalization of membrane proteins.

THE ROLE OF TAK1 IN PANCREATIC CANCER DEVELOPMENT

Pancreatic cancer is the fourth leading cause of cancer-related mortality in the United States and the fifth leading cause of cancer-related mortality worldwide. Pancreatic cancer is a big challenge in large due to the lack of early symptoms. In addition, drug resistance is a major obstacle to the success of chemotherapy in pancreatic cancer. The underlying mechanism of drug resistance in human pancreatic cancers is not well understood. Better understanding of the mechanism of molecular pathways in human pancreatic cancers can help to identify the novel therapeutic target candidates, and develop the new preventive and clinic strategies to improve patient survival. We discovered that TAK1 is overexpressed in pancreatic cancer cell lines and patient tumor tissues. We demonstrated that the elevated activity of TAK1 is caused by its binding partner TAB1. Knocking down of TAK1 in pancreatic cancer cells with RNAi technique resulted in cell apoptosis and significantly reduces the size of tumors in mice and made a chemotherapy drug more potent. Targeting the kinase activity of TAK1 with the selective inhibitor LY2610956 strongly synergized in vitro with the antitumor activity of gemcitabine, oxaliplatin, or irinotecan on pancreatic cancer cells. These findings highlighted that TAK1 could be a potential therapeutic target for pancreatic cancer. We also demonstrated that TAK activity is regulated by its binding protein TAB1. We defined a minimum TAB1 sequence which is required and sufficient for TAK1 kinase activity. We created a recombinant TAK1-TAB1 C68 fusion form which has highly kinase activity. This active form could is used for screening TAK1 inhibitors. In addition, several posttranslational modifications were identified in our study. The acetylation of lysine 158 on TAK1 is required for kinase activity. This site is conserved throughout all of kinase. Our findings may reveal a new mechanism by which kinase activity is regulated.

A conserved motif at the 3$\sp\prime$ end of genomic RNA of mouse hepatitis virus required for host protein binding and viral RNA replication

The initial step in coronavirus-mouse hepatitis virus (MHV) replication is the synthesis of negative strand RNA from a positive strand genomic RNA template. Our approach to studying MHV RNA replication is to identify the cis-acting signals for RNA synthesis and the protein(s) which recognizes these signals at the 3$\sp\prime$ end of genomic RNA of MHV. To determine whether host cellular and/or virus-specific proteins interact with the 3$\sp\prime$ end of the coronavirus genome, an RNase T$\sb1$ protection/gel mobility shift electrophoresis assay was used to examine cytoplasmic extracts from either mock- or MHV-JHM-infected 17Cl-1 murine cells for the ability to form complexes with defined regions of the genomic RNA. A conserved 11 nucleotide sequence UGAAUGAAGUU at nucleotide positions 36 to 26 from the 3$\sp\prime$ end of genomic RNA was identified to be responsible for the specific binding of host proteins, by using a series of RNA probes with deletions and mutations in this region. The RNA probe containing the 11 nucleotide sequence bound approximately four host cellular proteins with a highly labeled 120 kDa and three minor species with sizes of 103, 81 and 55 kDa, assayed by UV-induced covalent cross-linking. Mutation of the 11 nucleotide motif strongly inhibited cellular protein binding, and decreased the amount of the 103 and 81 kDa proteins in the complex to undetectable levels and strongly reduced the binding of the 120 kDa protein. Less extensive mutations within this 11 nucleotide motif resulted in variable decreases in RNA-protein complex formation depending on each probe tested. The RNA-protein complexes observed with cytoplasmic extracts from MHV-JHM-infected cells in both RNase protection/gel mobility shift and UV cross-linking assays were indistinguishable to those observed with extracts from uninfected cells.^ To investigate the possible role of this 3$\sp\prime$ protein binding element in viral RNA replication in vivo, defective interfering RNA molecules with complete or partial mutations of the 11 nucleotide conserved sequence were transcribed in vitro, transfected to host 17Cl-1 cells in the presence of helper virus MHV-JHM and analyzed by agarose gel electrophoresis, competitive RT-PCR and direct sequencing of the RT-PCR products. Both negative strand synthesis and positive strand replication of DI RNA were affected by mutation that disrupts RNA-protein complex formation, even though the 11 mutated nucleotides were converted to wild type sequence, presumably by recombination with helper virus. Kinetic analysis indicated that recombination between DI RNA and helper virus occurred 5.5 to 7.5 hours post infection when replication of positive strand DI RNA was barely observed. Replication of positive strand DI RNAs carrying partial mutations within the 11 nucleotide motif was dependent upon recombination events after transfection. Replication was strongly inhibited when reversion to wild type sequence did not occur, and after recombination, reached similar levels as wild type DI RNA. A DI RNA with mutation upstream of the protein binding motif replicated as efficiently as wild type without undergoing recombination. Thus the conserved 11 nucleotide host protein binding motif appears to play an important role in viral RNA replication. ^

The role of the amino terminus of Bacillus subtilis sigmaK in transcription initiation

The sigma (σ) subunit of eubacterial RNA polymerase (RNAP) is required for specific recognition of promoter DNA sequences and transcription initiation. Regulation of bacterial gene expression can be achieved by modulating a factor activity. The Bacillus subtilis sporulation a σ factor, σ K, controls gene expression of the late sporulation regulon. σ K is synthesized as an inactive precursor protein, pro-σ K, with a 20 amino acid pro sequence. Proteolytic processing of the pro sequence produces the active form, σK, which is able to bind to the core subunits of RNAP to direct gene expression. Thus, the pro sequence renders σK inactive in vivo. After processing, the amino terminus of σK consists of region 1.2, which is conserved among various σ factors. To understand the role of the amino terminus of σK, namely the pro sequence and region 1.2, mutagenesis of both regions was pursued. NH 2-terminal truncations of pro-σK were constructed to address how the pro sequence silences σK activity. The work described here shows that the pro sequence inhibits the ability of σ K to associate with the core subunits and that a deletion of only six amino acids of the pro sequence is sufficient to activate pro-σ K for DNA binding and transcription initiation to levels similar to σ K. Additionally, site directed mutagenesis was used to obtain single amino acid substitutions in region 1.2 to address the role of region 1.2 in σ K transcriptional activity. Two mutations were isolated, converting a lysine (K) to an alanine (A) at position three, and an asparagine (N) to a tyrosine (Y) at position five, both of which alter the efficiency of transcription initiation by RNAP containing the mutant σKs. Surprisingly, σ KK3A increased transcript production when compared to wild type. This increase is due to improvement in DNA affinity and increased stability of RNAP-DNA promoter open complexes. σKN5Y showed a decrease in transcription activity that is related to defects in the ability of RNAP to make the transition from the closed to open RNAP-DNA complex. Results of both the pro sequence and region 1.2 analyses indicate that the amino terminus of σK is important for transcription activity and this work adds to the increasing body of evidence that the amino termini of many σ factors modulate transcription initiation by RNAP. ^

14-3-3zeta overexpression in multiple human cancers plays a critical role in cancer progression

14-3-3 is a family of highly conserved and ubiquitously expressed proteins in eukaryotic organisms. 14-3-3 isoforms bind in a phospho-serine/threonine-dependent manner to a host of proteins involved in essential cellular processes including cell cycle, signal transduction and apoptosis. We fortuitously discovered 14-3-3 zeta overexpression in many human primary cancers, such as breast, lung, and sarcoma, and in a majority of cancer cell lines. To determine 14-3-3 zeta involvement in breast cancer progression, we used immunohistochemical analysis to examine 14-3-3 zeta expression in human primary invasive breast carcinomas. High 14-3-3 zeta expression was significantly correlated with poor prognosis of breast cancer patients. Increased expression of 14-3-3 zeta was also significantly correlated with elevated PKB/Akt activation in patient samples. Thus, 14-3-3 zeta is a marker of poor prognosis in breast cancers. Furthermore, up-regulation of 14-3-3 zeta enhanced malignant transformation of cancer cells in vitro. ^ To determine the biological significance of 14-3-3 zeta in human cancers, small interfering RNAs (siRNA) were used to specifically block 14-3-3 zeta expression in cancer cells. 14-3-3 zeta siRNA inhibited cellular proliferation by inducing a G1 arrest associated with up-regulation of p27 KIP1 and p21CIP1 cyclin dependent kinase inhibitors. Reduced 14-3-3 zeta inhibited PKB/Akt activation while stimulating the p38 signaling pathway. Silencing 14-3-3 zeta expression also increased stress-induced apoptosis by caspase activation. Notably, 14-3-3 zeta siRNA inhibited transformation related properties of breast cancer cells in vitro and inhibited tumor progression of breast cancer cells in vivo. 14-3-3 zeta may be a key regulatory factor controlling multiple signaling pathways leading to tumor progression. ^ The data indicate 14-3-3 zeta is a major regulator of cell growth and apoptosis and may play a critical role in the development of multiple cancer types. Hence, blocking 14-3-3 zeta may be a promising therapeutic approach for numerous cancers. ^

Studies of MEKK2 activation mechanisms and its biological role in LPS/TLR4 signaling

MEKK2 is an evolutionarily conserved mitogen-activated protein kinase (MAPK) kinase kinase (MAP3K) that controls the MAPK and IKK-NF-κB pathways. The MAPK and IKK pathways are intracellular signaling networks that are crucial for the Toll-like receptor (TLR) mediated innate immunity, cellular stress and many other physiological responses. Members of the MAP3K family are central to the activation of these processes. However, the molecular mechanisms underlying stimuli-mediated MAP3K activation remain largely unknown. In this study, we identified a key phosphoserine residue, Ser-519 in MEKK2, and its equivalent site Ser-526 in MEKK3 within their activation loop whose phosphorylation are essential for their optimal activation. Mutation of this regulatory serine to an alanine severely impaired MEKK2 activation and MEKK2 signaling to its downstream targets. To demonstrate that physiological stimuli induce this serine phosphorylation, we generated an antibody that specifically recognizes the phosphorylated serine residue. We found that many, but not all, of the MAPK agonists, including the TLR ligands, growth factors, cytokines and cellular stresses, induced this regulatory serine phosphorylation in MEKK2, suggesting an involvement of MEKK2 in the activation of the MAPK cascade leading to different cellular responses. We further investigated the specific role of MEKK2 in LPS/TLR4 signaling by using MEKK2−/− mice. We found that MEKK2 was selectively required for LPS-induced ERK1/2 activation, but not JNK, p38 or NF-κB activation. We also found that MEKK2 was involved in TLR4 dependent induction of proinflammatory cytokines and LPS-induced septic shock. In conclusion, we identified a key regulatory serine residue in the activation loop of MEKK2 whose phosphorylation is a key sensor of receptor- and cellular stress-mediated signals. We also demonstrated that MEKK2 is crucial for TLR4-mediated innate immunity. ^

Role of telomere dysfunction, DNA damage response and p53 mutations in tumorigenesis and aging

The ends of eukaryotic chromosomes are protected by specialized ribonucleoprotein structures termed telomeres. Telomeres protect chromosomes from end-to-end fusions, inappropriate repair and degradation. Disruption of this complex activates an ATM/ATR DNA damage response (DDR) pathway. One component of the complex is the Protection Of Telomeres 1 (POT1) protein, an evolutionarily conserved protein which binds single-stranded 3' overhang and is required for both chromosomal end protection and telomere length regulation. The mouse contains two POT1 orthologs, Pot1a and Pot1b. Here we show that both proteins colocalize with telomeres through interaction with the adapter protein TPP1. In addition, compared to Pot1a, the OB-folds of Pot1b possess less sequence specificity for telomeres. Disruption of POT1 proteins result in telomere dysfunction and activation of an ATR-dependent DDR at telomeres, suggesting that this response is normally suppressed by POT1 binding to the single-stranded G-overhang. ^ Telomeres are maintained by telomerase, and its absence in somatic cells results in telomere progressive loss that triggers the activation of p53. Telomere dysfunction initiates genomic instability and induces both p53-dependent replicative senescence and apoptosis to suppress tumorigenesis. In the absence of functional p53, this genomic instability promotes cancer. It was previously not known which aspect of the p53 dependent DNA damage response is important to suppress tumorigenesis initiated by dysfunctional telomeres. The p53R172P knock-in mouse, which is unable to induce apoptosis but retains intact cell cycle arrest/cellular senescence pathways, allowed us to examine whether p53-dependent apoptosis is a major tumor suppression pathway initiated in the setting of telomere dysfunction. Spontaneous tumorigenesis remains potently suppressed in late generation telomerase null mice possessing the p53P/P mutation. These results suggest that suppression of spontaneous tumorigenesis initiated by dysfunctional telomeres requires activation of a p53-dependent senescence pathway. In addition, we used another knock-in mouse model with a p53R172H (p53H) point mutation to test the hypothesis that telomere dysfunction promotes chromosomal instability and accelerates the onset of tumorigenesis in vivo in the setting of this most common gain-of-function mutation in the human Li Fraumeni cancer syndrome. We unexpectedly observed that telomerase null mice possessing dysfunctional telomeres in the setting of the p53H/+ mutation develop significantly fewer tumors, die prematurely and exhibit higher level of cellular senescence, apoptosis and elevated genomic instability compared to telomerase intact p53H/+ and telomerase null p53+/+ mice. These contrasting results thus link cancer and aging to the functional status of telomeres and the integrity of the p53 pathway. ^

The role of beta-arrestin 2 in lysophosphatidic acid-induced NF-kappaB activation

Lysophosphatidic acid (LPA) is a bioactive phospholipid and binds to its receptors, a family of G protein-coupled receptors (GPCR), which initiates multiple signaling cascades and leads to activation of several transcription factors, including NF-κB. NF-κB critically regulates numerous gene expressions, and is persistently active in many diseases. In our previous studies, we have demonstrated that LPA-induced NF-κB activation is dependent on a novel scaffold protein, CARMA3. However, how CARMA3 is recruited to receptor remains unknown. β-Arrestins are a family of proteins involved in desensitization of GPCR signaling. Additionally, β-arrestins function as signaling adaptor proteins, and mediate multiple signaling pathways. Therefore, we have hypothesized that β-arrestins may link CARMA3 to LPA receptors, and facilitate LPA-induced NF-κB activation. ^ Using β-arrestin-deficient MEFs, we found that β-arrestin 2, but not β-arrestin 1, was required for LPA-induced NF-κB activation. Also, we showed that the expression of NF-κB-dependent cytokines, such as interlukin-6, was impaired in β-arrestin 2-deficient MEFs. Mechanistically, we demonstrated the inducible association of endogenous β-arrestin 2 and CARMA3, and we found the CARD domain of CARMA3 interacted with 60-320 residues of β-arrestin 2. To understand why β-arrestin 2, but not β-arrestin 1, mediated NF-κB activation, we generated β-arrestin mutants. However, some mutants degraded quickly, and the rest did not rescue NF-κB activation in β-arrestin-deficient MEFs, though they had similar binding affinities with CARMA3. Therefore, it indicates that slight changes in residues may determine the different functions of β-arrestins. Moreover, we found β-arrestin 2 deficiency impaired LPA-induced IKK kinase activity, while it did not affect LPA-induced IKKα/β phosphorylation. ^ In summary, our results provide the genetic evidence that β-arrestin 2 serves as a positive regulator in NF-κB signaling pathway by connecting CARMA3 to LPA receptors. Additionally, we demonstrate that β-arrestin 2 is required for IKKα/β activation, but not for the inducible phosphorylation of IKKα/β. Because the signaling pathways around the membrane-proximal region of LPA receptors and GPCRs are quite conserved, our results also suggest a possible link between other GPCRs and CARMA3-mediated NF-κB activation. To fully define the role of β-arrestins in LPA-induced NF-κB signaling pathways will help to identify new drug targets for clinical therapeutics.^

Role of ovarian cancer antigen CA125/MUC16 in development and cancer

Cancer antigen 125 (CA125) is a tumor antigen that is routinely used to monitor the disease progress and the outcome of treatment in ovarian cancer patients. Elevated serum levels of CA125 are detected in over 80% of epithelial ovarian cancer patients. CA125 is a high molecular weight (>1M Dalton) mucin-type glycoprotein encoded by the MUC16 gene on human chromosome 19. Although MUC16 has served as the best serum marker for monitoring growth of ovarian cancer, roles for MUC16 in normal physiology and ovarian cancer are largely unknown. To understand the biological functions of MUC16, I characterized a mouse Muc16 homolog on chromosome 9 by means of expression pattern profiling, phenotype analysis of Muc16 knockout mice, and in vitro and in vivo studies of Muc16 null transformed ovarian surface epithelial (OSE) cells. ^ The mouse Muc16 homolog shares a conserved genomic structure with human MUC16. In addition to being expressed in mouse ovarian cancer, mouse Muc16 mRNA and protein were expressed in the mesothelia covering the heart, lung, ovary, oviduct, spleen, testis, and uterus. The conserved genomic structure and expression pattern of mouse Muc16 to human MUC16 suggests that mouse Muc16 is the ortholog of human MUC16. To understand the biological functions of Muc16, I generated Muc16 knockout mice. Muc16 knockout mice were viable, fertile and normal by one year of age. However, between 18 and 24 months of age, Muc16 knockout mice developed various tissue abnormalities such as ovarian cysts and tumors of the liver and other peritoneal organs. To determine the role of MUC16 in ovarian cancer progression, I established Muc16 null transformed ovarian surface epithelial (OSE) cell lines, following the same method to develop mouse model of epithelial ovarian cancer (Orsulic et al., 2002). Loss of Muc16 did not affect cell morphology, cell proliferation rate, or tumorigenic potential. However, Muc16-null OSE cells showed decreased attachment to extracellular matrix proteins as well as to primary mouse peritoneal mesothelial cells. Peritoneal mesothelia are the most frequent implantation sites of ovarian cancer. Furthermore, a pilot transplantation assay suggests that Muc16 null transformed OSE cells formed less disseminated tumors in the peritoneal cavity compared to wild-type OSE cells. ^ In conclusion, these results demonstrate that MUC16 is not required for normal mouse development or reproduction, but plays important roles in tissue homeostasis, ovarian cancer cell adhesion and dissemination. This study provides the first in vivo evidence of the roles of MUC16 in development, as well as ovarian cancer progression and dissemination. These studies offer valuable insights into possible mechanisms of ovarian cancer development and potential molecular targets for ovarian cancer treatment. ^

Identification of a conserved cluster in the RH domain of GRK critical for activation by GPCRs

One of the most critical aspects of G Protein Coupled Receptors (GPCRs) regulation is their rapid and acute desensitization following agonist stimulation. Phosphorylation of these receptors by GPCR kinases (GRK) is a major mechanism of desensitization. Considerable evidence from studies of rhodopsin kinase and GRK2 suggests there is an allosteric docking site for the receptor distinct from the GRK catalytic site. While the agonist-activated GPCR appears crucial for GRK activation, the molecular details of this interaction remain unclear. Recent studies suggested an important role for the N- and C-termini and domains in the small lobe of the kinase domain in allosteric activation; however, neither the mechanism of action of that site nor the RH domain contributions have been elucidated. To search for the allosteric site, we first indentified evolutionarily conserved sites within the RH and kinase domains presumably deterministic of protein function employing evolutionary trace (ET) methodology and crystal structures of GRK6. Focusing on a conserved cluster centered on helices 3, 9, and 10 in the RH domain, key residues of GRK5 and 6 were targeted for mutagenesis and functional assays. We found that a number of double mutations within helices 3, 9, and 10 and the N-terminus markedly reduced (50–90%) the constitutive phosphorylation of the β-2 Adrenergic Receptor (β2AR) in intact cells and phosphorylation of light-activated rhodopsin (Rho*) in vitro as compared to wild type (WT) GRK5 or 6. Based on these results, we designed peptide mimetics of GRK5 helix 9 both computationally and through chemical modifications with the goal of both confirming the importance of helix 9 and developing a useful inhibitor to disrupt the GPCR-GRK interaction. Several peptides were found to block Rho* phosphorylation by GRK5 including the native helix 9 sequence, Peptide Builder designed-peptide preserving only the key ET residues, and chemically locked helices. Most peptidomimetics showed inhibition of GRK5 activity greater than 80 % with an IC50 of ∼ 30 µM. Alanine scanning of helix 9 has further revealed both essential and non-essential residues for inhibition. Importantly, substitution of Arg 169 by an alanine in the native helix 9-based peptide gave an almost complete inhibition at 30 µM with an IC50 of ∼ 10 µM. In summary we report a previously unrecognized crucial role for the RH domain of GRK5 and 6, and the subsequent identification of a lead peptide inhibitor of protein-protein interaction with potential for specific blockade of GPCR desensitization. ^

Mouse retinal development: Role of cell adhesion and mechanism of gene regulation

Cell differentiation and pattern formation are fundamental processes in animal development that are under intense investigation. The mouse retina is a good model to study these processes because it has seven distinct cell types, and three well-laminated nuclear layers that form during embryonic and postnatal life. β-catenin functions as both the nuclear effector for the canonical Wnt pathway and a cell adhesion molecule, and is required for the development of various organs. To study the function of β-catenin in retinal development, I used a Cre-loxP system to conditionally ablate β-catenin in the developing retina. Deletion of β-catenin led to disrupted laminar structure but did not affect the differentiation of any of the seven cell types. Eliminating β-catenin did not reduce progenitor cell proliferation, although enhanced apoptosis was observed. Further analysis showed that disruption of cell adhesion was the major cause of the observed patterning defects. Overexpression of β-catenin during retinal development also disrupted the normal retinal lamination and caused a transdifferentiation of neurons into pigmented cells. The results indicate that β-catenin functions as a cell adhesion molecule but not as a Wnt pathway component during retinal neurogenesis, and is essential for lamination but not cell differentiation. The results further imply that retinal lamination and cell differentiation are genetically separable processes. ^ Sonic hedgehog (shh) is expressed in retinal ganglion cells under the control of transcription factor Pou4f2 during retinal development. Previous studies identified a phylogenetically conserved region in the first intron of shh containing a Pou4f2 binding site. Transgenic reporter mice in which reporter gene expression was driven by this region showed that this element can direct gene expression specifically in the retina, but expression was not limited to the ganglion cells. From these data I hypothesized that this element is required for shh expression in the retina but is not sufficient for specific ganglion cell expression. To further test this hypothesis, I created a conditional allele by flanking this region with two loxP sites. Lines carrying this allele will be crossed with retinal-specific Cre lines to remove this element in the retina. My hypothesis predicts that alteration in shh expression and subsequent retinal defects will occur in the retinas of these mice. ^

Characterization of a Novel Role for the Tousled- like Kinase in Kinetochore Assembly and Function in Caenorhabditis elegans

Chromosome segregation is a critical step during cell division to avoid aneuploidy and promote proper organismal development. Correct sister chromatid positioning and separation during mitosis helps to achieve faithful transmission of genetic material to daughter cells. This prevents improper chromosome partitioning that can potentially result in extrachromosomal fragments, increasing the tumorigenic potential of the cells. The kinetochore is a protenaicious structure responsible for the initiation and orchestration of chromosome movement during mitosis. This highly conserved structure among eukaryotes is required for chromosome attachment to the mitotic spindle and failure to assemble the kinetochore results in aberrant chromosome segregation. Thus elucidating the mechanism of kinetochore assembly is important to have a better understanding of the regulation that controls chromosome segregation. Our previous work identified the C. elegans Tousled-like kinase (TLK-1) as a mitotic kinase and depletion of TLK-1 results in embryonic lethality, characterized by nuclei displaying poor mitotic chromosome alignment, lagging chromosome, and chromosome bridges during anaphase. Additionally, previous studies from our group revealed that TLK-1 is phosphorylated independently by Aurora B at serine 634, and by CHK-1 at threonine T610. The research presented herein reveals that both phosphorylated forms of TLK-1 associate with the kinetochore during mitosis. Moreover, by systematic depletion of kinetochore proteins, I uncovered that pTLK-1 is bona fide kinetochore component that is located at the outer kinetochore layer, influencing the microtubule-binding interface. I also demonstrated that TLK-1 is necessary for the kinetochore localization of the microtubule interacting proteins CLS-2 and LIS-1 and I show that embryos depleted of TLK-1 presented an aberrant twisted kinetochore pattern. Furthermore, I established that the inner kinetochore protein KNL-2 is an in vitro substrate of TLK-1 indicating a possible role of TLK-1 in regulating centromeric assembly. Collectively, these results suggest a novel role for the Tousled-like kinase in regulation of kinetochore assembly and microtubule dynamics and demonstrate the necessity of TLK-1 for proper chromosome segregation in C. elegans.