Master of Science, Korea University (2001)
Bachelor of Science, Korea University (1999)
Doctor of Philosophy, University of Texas Austin (2011)
Jeffrey Axelrod, Postdoctoral Faculty Sponsor
Clustering and negative feedback by endocytosis in planar cell polarity signaling is modulated by ubiquitinylation of prickle.
2015; 11 (5): e1005259
The core components of the planar cell polarity (PCP) signaling system, including both transmembrane and peripheral membrane associated proteins, form asymmetric complexes that bridge apical intercellular junctions. While these can assemble in either orientation, coordinated cell polarization requires the enrichment of complexes of a given orientation at specific junctions. This might occur by both positive and negative feedback between oppositely oriented complexes, and requires the peripheral membrane associated PCP components. However, the molecular mechanisms underlying feedback are not understood. We find that the E3 ubiquitin ligase complex Cullin1(Cul1)/SkpA/Supernumerary limbs(Slimb) regulates the stability of one of the peripheral membrane components, Prickle (Pk). Excess Pk disrupts PCP feedback and prevents asymmetry. We show that Pk participates in negative feedback by mediating internalization of PCP complexes containing the transmembrane components Van Gogh (Vang) and Flamingo (Fmi), and that internalization is activated by oppositely oriented complexes within clusters. Pk also participates in positive feedback through an unknown mechanism promoting clustering. Our results therefore identify a molecular mechanism underlying generation of asymmetry in PCP signaling.
View details for DOI 10.1371/journal.pgen.1005259
View details for PubMedID 25996914
Prickle/spiny-legs isoforms control the polarity of the apical microtubule network in planar cell polarity.
2014; 141 (14): 2866-2874
Microtubules (MTs) are substrates upon which plus- and minus-end directed motors control the directional movement of cargos that are essential for generating cell polarity. Although centrosomal MTs are organized with plus-ends away from the MT organizing center, the regulation of non-centrosomal MT polarity is poorly understood. Increasing evidence supports the model that directional information for planar polarization is derived from the alignment of a parallel apical network of MTs and the directional MT-dependent trafficking of downstream signaling components. The Fat/Dachsous/Four-jointed (Ft/Ds/Fj) signaling system contributes to orienting those MTs. In addition to previously defined functions in promoting asymmetric subcellular localization of 'core' planar cell polarity (PCP) proteins, we find that alternative Prickle (Pk-Sple) protein isoforms control the polarity of this MT network. This function allows the isoforms of Pk-Sple to differentially determine the direction in which asymmetry is established and therefore, ultimately, the direction of tissue polarity. Oppositely oriented signals that are encoded by oppositely oriented Fj and Ds gradients produce the same polarity outcome in different tissues or compartments, and the tissue-specific activity of alternative Pk-Sple protein isoforms has been observed to rectify the interpretation of opposite upstream directional signals. The control of MT polarity, and thus the directionality of apical vesicle traffic, by Pk-Sple provides a mechanism for this rectification.
View details for DOI 10.1242/dev.105932
View details for PubMedID 25005476
Ral inhibits ligand-independent Notch signaling in Drosophila.
2012; 3 (3): 186-191
We discovered recently that the Drosophila Ral GTPase regulates Notch signaling and thereby affects cell patterning in the eye. Although Ral functions in the ligand signaling cells, Ral does not stimulate ligand signaling directly. Rather, in cells that express both Notch receptor and ligand, Ral activity promotes a cell to become the signaler by inhibiting Notch receptor activation in that cell. Moreover, Ral inhibits a particular pathway of Notch activation-receptor activation that occurs independent of ligand binding. In this Commentary, we discuss the phenomenon of ligand-independent Notch receptor activation and how this event might be regulated by Ral.
View details for DOI 10.4161/sgtp.19802
View details for PubMedID 22750761
Drosophila Epsin's role in Notch ligand cells requires three Epsin protein functions: The lipid binding function of the ENTH domain, a single Ubiquitin interaction motif, and a subset of the C-terminal protein binding modules
2012; 363 (2): 399-412
Epsin is an endocytic protein that binds Clathrin, the plasma membrane, Ubiquitin, and also a variety of other endocytic proteins through well-characterized motifs. Although Epsin is a general endocytic factor, genetic analysis in Drosophila and mice revealed that Epsin is essential specifically for internalization of ubiquitinated transmembrane ligands of the Notch receptor, a process required for Notch activation. Epsin's mechanism of function is complex and context-dependent. Consequently, how Epsin promotes ligand endocytosis and thus Notch signaling is unclear, as is why Notch signaling is uniquely dependent on Epsin. Here, by generating Drosophila lines containing transgenes that express a variety of different Epsin deletion and substitution variants, we tested each of the five protein or lipid interaction modules for a role in Notch activation by each of the two ligands, Serrate and Delta. There are five main results of this work that impact present thinking about the role of Epsin in ligand cells. First, we discovered that deletion or mutation of both UIMs destroyed Epsin's function in Notch signaling and had a greater negative impact on Epsin activity than removal of any other module type. Second, only one of Epsin's two UIMs was essential. Third, the lipid-binding function of the ENTH domain was required only for maximal Epsin activity. Fourth, although the C-terminal Epsin modules that interact with Clathrin, the adapter protein complex AP-2, or endocytic accessory proteins were necessary collectively for Epsin activity, their functions were highly redundant; most unexpected was the finding that Epsin's Clathrin binding motifs were dispensable. Finally, we found that signaling from either ligand, Serrate or Delta, required the same Epsin modules. All of these observations are consistent with a model where Epsin's essential function in ligand cells is to link ubiquitinated Notch ligands to Clathrin-coated vesicles through other Clathrin adapter proteins. We propose that Epsin's specificity for Notch signaling simply reflects its unique ability to interact with the plasma membrane, Ubiquitin, and proteins that bind Clathrin.
View details for DOI 10.1016/j.ydbio.2012.01.004
View details for Web of Science ID 000300961600006
View details for PubMedID 22265678
Ral GTPase promotes asymmetric Notch activation in the Drosophila eye in response to Frizzled/PCP signaling by repressing ligand-independent receptor activation
2011; 138 (7): 1349-1359
Ral is a small Ras-like GTPase that regulates membrane trafficking and signaling. Here, we show that in response to planar cell polarity (PCP) signals, Ral modulates asymmetric Notch signaling in the Drosophila eye. Specification of the initially equivalent R3/R4 photoreceptor precursor cells in each developing ommatidium occurs in response to a gradient of Frizzled (Fz) signaling. The cell with the most Fz signal (R3) activates the Notch receptor in the adjacent cell (R4) via the ligand Delta, resulting in R3/R4 cell determination and their asymmetric positions within the ommatidium. Two mechanisms have been proposed for ensuring that the cell with the most Fz activation sends the Delta signal: Fz-dependent transcriptional upregulation in R3 of genes that promote Delta signaling, and direct blockage of Notch receptor activation in R3 by localization of an activated Fz/Disheveled protein complex to the side of the plasma membrane adjacent to R4. Here, we discover a distinct mechanism for biasing the direction of Notch signaling that depends on Ral. Using genetic experiments in vivo, we show that, in direct response to Fz signaling, Ral transcription is upregulated in R3, and Ral represses ligand-independent activation of Notch in R3. Thus, prevention of ligand-independent Notch activation is not simply a constitutive process, but is a target for regulation by Ral during cell fate specification and pattern formation.
View details for DOI 10.1242/dev.056002
View details for Web of Science ID 000288116500013
View details for PubMedID 21350007
The Functions of Auxilin and Rab11 in Drosophila Suggest That the Fundamental Role of Ligand Endocytosis in Notch Signaling Cells Is Not Recycling
2011; 6 (3)
Notch signaling requires ligand internalization by the signal sending cells. Two endocytic proteins, epsin and auxilin, are essential for ligand internalization and signaling. Epsin promotes clathrin-coated vesicle formation, and auxilin uncoats clathrin from newly internalized vesicles. Two hypotheses have been advanced to explain the requirement for ligand endocytosis. One idea is that after ligand/receptor binding, ligand endocytosis leads to receptor activation by pulling on the receptor, which either exposes a cleavage site on the extracellular domain, or dissociates two receptor subunits. Alternatively, ligand internalization prior to receptor binding, followed by trafficking through an endosomal pathway and recycling to the plasma membrane may enable ligand activation. Activation could mean ligand modification or ligand transcytosis to a membrane environment conducive to signaling. A key piece of evidence supporting the recycling model is the requirement in signaling cells for Rab11, which encodes a GTPase critical for endosomal recycling. Here, we use Drosophila Rab11 and auxilin mutants to test the ligand recycling hypothesis. First, we find that Rab11 is dispensable for several Notch signaling events in the eye disc. Second, we find that Drosophila female germline cells, the one cell type known to signal without clathrin, also do not require auxilin to signal. Third, we find that much of the requirement for auxilin in Notch signaling was bypassed by overexpression of both clathrin heavy chain and epsin. Thus, the main role of auxilin in Notch signaling is not to produce uncoated ligand-containing vesicles, but to maintain the pool of free clathrin. Taken together, these results argue strongly that at least in some cell types, the primary function of Notch ligand endocytosis is not for ligand recycling.
View details for DOI 10.1371/journal.pone.0018259
View details for Web of Science ID 000288810500049
View details for PubMedID 21448287
Promoter hypomethylation of a novel cancer/testis antigen gene CAGE is correlated with its aberrant expression and is seen in premalignant stage of gastric carcinoma
BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS
2003; 307 (1): 52-63
Previously, we reported the identification and characterization of a novel cancer/testis antigen gene, CAGE(4), that was expressed in various histological types of tumors, but not in normal tissues, with the exception of the testis. To date, molecular mechanisms for the expression of CAGE have never been studied. In our expression analysis, we found that some cancer cell lines did not express CAGE. The expression of CAGE could be restored in these cell lines by treatment with 5(')-aza-2(')-deoxycytidine, suggesting that the expression of CAGE is mainly suppressed by hypermethylation. Bisulfite sequencing analysis of the 16 CpG sites of the CAGE promoter in various cancer cell lines and tissues revealed a close relationship between the methylation status of the CAGE promoter and the expression of CAGE. The transient transfection experiments displayed that the methylation of CpG sites inhibited the CAGE promoter activity in luciferase reporter assays. The methylation of the CpG sites inhibited the binding of transcription factors, shown by a mobility shift assay. A methylation-specific PCR analysis revealed that hypomethylation of the CAGE promoter was present at frequencies of more than 60% in breast, gastric, and lung cancers, and hepatocellular carcinomas, and at frequencies of less than 40% in prostate, uterine cervical, and laryngeal cancers. Promoter hypomethylation was found in chronic gastritis (19/55, 34.5%) and liver cirrhosis (13/22, 59%), but not in normal prostate, normal colon, or chronic hepatitis. These results suggest that the methylation status of the CpG sites of CAGE determines its expression, that the hypomethylation of CAGE precedes the development of gastric cancer and hepatocellular carcinoma, and that the high frequencies of hypomethylation of CAGE, in various cancers would be valuable as a cancer diagnostic marker.
View details for DOI 10.1016/S0006-291X(03)01121-5
View details for Web of Science ID 000184083000008
View details for PubMedID 12849980
Identification and characterization of a novel cancer/testis antigen gene CAGE-1
BIOCHIMICA ET BIOPHYSICA ACTA-GENE STRUCTURE AND EXPRESSION
2003; 1625 (2): 173-182
Serological analysis of cDNA expression library (SEREX) was employed to identify cancer-associated genes. By screening cDNA expression libraries with sera of patients with lung cancers, we identified a total of 49 genes that specifically reacted with the sera of patients with lung cancers. Among these, we characterized a novel gene with expression pattern similar to that of cancer/testis antigens. Its open reading frame is 1920 bp in size and encodes for putative protein composed of 639 amino acids. Southern blot analysis reveals that this gene exists as single copy. In vitro transcription/translation and Western blot analysis confirm that this gene encodes a protein of 73 kDa in size. The comparison of cDNA and genomic sequences reveals that it is composed of 11 exons and 10 introns. This gene displays testis-specific expression among normal tissues, and wide expression among various cancer tissues and cancer cell lines. A study using GFP fusion construct reveals mainly nuclear localization of CAGE-1 protein. The expression of this clone is relatively higher in cancer tissues compared with their surrounding non-cancerous tissues. This suggests that overexpression of CAGE-1 may be associated with the progression of tumor. Because of its association with cancer, this gene was named cancer-associated gene-1 (CAGE-1). Given the fact that several cancer/testis antigens reportedly induce cytolytic T lymphocyte (CTL) reactions, it is reasonable that this gene will be a valuable target for cancer immunotherapy. The exact functional role of CAGE-1 in tumorigenesis remains to be seen.
View details for DOI 10.1016/S0167-4781(02)00620-6
View details for Web of Science ID 000180692700006
View details for PubMedID 12531476
Identification of autoantibodies associated with systemic lupus erythematosus
BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS
2002; 295 (1): 119-124
Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the presence of antinuclear antibodies. We performed serological analysis of cDNA expression library (SEREX) to identify autoantibodies associated with SLE. The screening of three different cDNA expression libraries with pooled sera of patients with SLE yielded 11 independent clones that reacted with pooled sera of patients with SLE. In this screening, autoantibodies to poly(ADP-ribose) polymerase (PARP), U1snRNP, and galectin-3 were prevalent in the sera of patients with SLE (26/68, 25/68, 12/63, respectively). The frequency of autoantibody to PARP was significantly higher in SLE than that of healthy donors (0/76) (38.2% vs 0%, p<0.00001). The autoantibody to PARP was infrequently detected in the serum of patients with RA (1/50). However, autoantibody to PARP was not found in the sera of patients with other rheumatic diseases including Sjogren's syndrome (0/19), systemic sclerosis (0/18), and polymyositis/myositis (0/37). The frequency of autoantibody to human galectin-3 (12/63) was significantly higher in SLE than that of healthy donors (0/56) (19% vs 0%, p=0.0006). Autoantibody to galectin-3 was not found in the sera of patients with rheumatoid arthritis (0/50), Sjogren's syndrome (0/18), and systemic sclerosis (0/19). Interestingly, autoantibody to galectin-3 was also prevalent in the sera of patients with polymyositis/dermatomyositis (16/37, 43.2%). Further functional characterization of these autoantibodies would be necessary to determine their value as diagnostic markers or to define clinical subsets of patients with SLE. Statistical analysis revealed that the presence of autoantibody to PARP was inversely related with pleurisy, and the presence of autoantibody to galectin-3 related with renal disease.
View details for Web of Science ID 000176815700021
View details for PubMedID 12083777
Identification and characterization of a novel cancer/testis antigen gene CAGE
BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS
2002; 292 (3): 715-726
We applied serological analysis of cDNA expression library technique to identify cancer-associated genes. We screened cDNA expression libraries of human testis and gastric cancer cell lines with sera of patients with gastric cancers. We identified a gene whose expression is testis-specific among normal tissues. We cloned and characterized this novel gene. It contains D-E-A-D box domain and encodes a putative protein of 630 amino acids with possible helicase activity. It showed wide expression in various cancer tissues and cancer cell lines. The corresponding gene was named cancer-associated gene (CAGE). PCR of human x hamster Radiation Hybrids showed localization of CAGE on the human chromosome Xp22. Transient transfection of CAGE showed predominantly nuclear localization. Both Western blot and plaque assay indicated seroreactivity of CAGE protein. We found that demethylation played a role in the activation of CAGE in some cancer cell lines that do not express it. Cell synchronization experiments showed that the expression of CAGE was related with cell cycle. This suggests that CAGE might play a role in cellular proliferation. Because CAGE is expressed in a variety of cancers but not in normal tissues except testis, this gene can be a target of antitumor immunotherapy.
View details for DOI 10.1006/bbrc.2002.6701
View details for Web of Science ID 000174842600019
View details for PubMedID 11922625