Professional Education

  • Doctor of Philosophy, Ohio State University (2010)

Stanford Advisors


All Publications

  • Fidelity of tRNA 5 '-maturation: a possible basis for the functional dependence of archaeal and eukaryal RNase P on multiple protein cofactors NUCLEIC ACIDS RESEARCH Chen, W., Singh, D., Lai, L. B., Stiffler, M. A., Lai, H. D., Foster, M. P., Gopalan, V. 2012; 40 (10): 4666-4680


    RNase P, which catalyzes tRNA 5'-maturation, typically comprises a catalytic RNase P RNA (RPR) and a varying number of RNase P proteins (RPPs): 1 in bacteria, at least 4 in archaea and 9 in eukarya. The four archaeal RPPs have eukaryotic homologs and function as heterodimers (POP5•RPP30 and RPP21•RPP29). By studying the archaeal Methanocaldococcus jannaschii RPR's cis cleavage of precursor tRNA(Gln) (pre-tRNA(Gln)), which lacks certain consensus structures/sequences needed for substrate recognition, we demonstrate that RPP21•RPP29 and POP5•RPP30 can rescue the RPR's mis-cleavage tendency independently by 4-fold and together by 25-fold, suggesting that they operate by distinct mechanisms. This synergistic and preferential shift toward correct cleavage results from the ability of archaeal RPPs to selectively increase the RPR's apparent rate of correct cleavage by 11,140-fold, compared to only 480-fold for mis-cleavage. Moreover, POP5•RPP30, like the bacterial RPP, helps normalize the RPR's rates of cleavage of non-consensus and consensus pre-tRNAs. We also show that archaeal and eukaryal RNase P, compared to their bacterial relatives, exhibit higher fidelity of 5'-maturation of pre-tRNA(Gln) and some of its mutant derivatives. Our results suggest that protein-rich RNase P variants might have evolved to support flexibility in substrate recognition while catalyzing efficient, high-fidelity 5'-processing.

    View details for DOI 10.1093/nar/gks013

    View details for Web of Science ID 000304535500043

    View details for PubMedID 22298511

  • Molecular Imaging of Stem Cells: Tracking Survival, Biodistribution, Tumorigenicity, and Immunogenicity THERANOSTICS Gu, E., Chen, W., Gu, J., Burridge, P., Wu, J. C. 2012; 2 (4): 335-345


    Being able to self-renew and differentiate into virtually all cell types, both human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have exciting therapeutic implications for myocardial infarction, neurodegenerative disease, diabetes, and other disorders involving irreversible cell loss. However, stem cell biology remains incompletely understood despite significant advances in the field. Inefficient stem cell differentiation, difficulty in verifying successful delivery to the target organ, and problems with engraftment all hamper the transition from laboratory animal studies to human clinical trials. Although traditional histopathological techniques have been the primary approach for ex vivo analysis of stem cell behavior, these postmortem examinations are unable to further elucidate the underlying mechanisms in real time and in vivo. Fortunately, the advent of molecular imaging has led to unprecedented progress in understanding the fundamental behavior of stem cells, including their survival, biodistribution, immunogenicity, and tumorigenicity in the targeted tissues of interest. This review summarizes various molecular imaging technologies and how they have advanced the current understanding of stem cell survival, biodistribution, immunogenicity, and tumorigenicity.

    View details for DOI 10.7150/thno.3666

    View details for Web of Science ID 000304031200002

    View details for PubMedID 22509197

  • Cooperative RNP Assembly: Complementary Rescue of Structural Defects by Protein and RNA Subunits of Archaeal RNase P JOURNAL OF MOLECULAR BIOLOGY Chen, W., Xu, Y., Cho, I., Oruganti, V., Foster, M. P., Gopalan, V. 2011; 411 (2): 368-383


    Ribonuclease P (RNase P) is a ribonucleoprotein complex that utilizes a Mg(2+)-dependent RNA catalyst to cleave the 5' leader of precursor tRNAs (pre-tRNAs) and generate mature tRNAs. The bacterial RNase P protein (RPP) aids RNase P RNA (RPR) catalysis by promoting substrate binding, Mg(2+) coordination and product release. Archaeal RNase P comprises an RPR and at least four RPPs, which have eukaryal homologs and function as two binary complexes (POP5·RPP30 and RPP21·RPP29). Here, we employed a previously characterized substrate-enzyme conjugate [pre-tRNA(Tyr)-Methanocaldococcus jannaschii (Mja) RPR] to investigate the functional role of a universally conserved uridine in a bulge-helix structure in archaeal RPRs. Deletion of this bulged uridine resulted in an 80-fold decrease in the self-cleavage rate of pre-tRNA(Tyr)-Mja?U RPR compared to the wild type, and this defect was partially ameliorated upon addition of either RPP pair. The catalytic defect in the archaeal mutant RPR mirrors that reported in a bacterial RPR and highlights a parallel in their active sites. Furthermore, an N-terminal deletion mutant of Pyrococcus furiosus (Pfu) RPP29 that is defective in assembling with its binary partner RPP21, as assessed by isothermal titration calorimetry and NMR spectroscopy, is functional when reconstituted with the cognate Pfu RPR. Collectively, these results indicate that archaeal RPPs are able to compensate for structural defects in their cognate RPR and vice-versa, and provide striking examples of the cooperative subunit interactions critical for driving archaeal RNase P toward its functional conformation.

    View details for DOI 10.1016/j.jmb.2011.05.012

    View details for Web of Science ID 000293938400006

    View details for PubMedID 21683084

  • Dissecting functional cooperation among protein subunits in archaeal RNase P, a catalytic ribonucleoprotein complex NUCLEIC ACIDS RESEARCH Chen, W., Pulukkunat, D. K., Cho, I., Tsai, H., Gopalan, V. 2010; 38 (22): 8316-8327


    RNase P catalyzes the Mg(2)(+)-dependent 5'-maturation of precursor tRNAs. Biochemical studies on the bacterial holoenzyme, composed of one catalytic RNase P RNA (RPR) and one RNase P protein (RPP), have helped understand the pleiotropic roles (including substrate/Mg(2+) binding) by which a protein could facilitate RNA catalysis. As a model for uncovering the functional coordination among multiple proteins that aid an RNA catalyst, we use archaeal RNase P, which comprises one catalytic RPR and at least four RPPs. Exploiting our previous finding that these archaeal RPPs function as two binary RPP complexes (POP5•RPP30 and RPP21•RPP29), we prepared recombinant RPP pairs from three archaea and established interchangeability of subunits through homologous/heterologous assemblies. Our finding that archaeal POP5•RPP30 reconstituted with bacterial and organellar RPRs suggests functional overlap of this binary complex with the bacterial RPP and highlights their shared recognition of a phylogenetically-conserved RPR catalytic core, whose minimal attributes we further defined through deletion mutagenesis. Moreover, single-turnover kinetic studies revealed that while POP5•RPP30 is solely responsible for enhancing the RPR's rate of precursor tRNA cleavage (by 60-fold), RPP21•RPP29 contributes to increased substrate affinity (by 16-fold). Collectively, these studies provide new perspectives on the functioning and evolution of an ancient, catalytic ribonucleoprotein.

    View details for DOI 10.1093/nar/gkq668

    View details for Web of Science ID 000285420300047

    View details for PubMedID 20705647

  • Rb is critical in a mammalian tissue stem cell population GENES & DEVELOPMENT Wenzel, P. L., Wu, L., de Bruin, A., Chong, J., Chen, W., Dureska, G., Sites, E., Pan, T., Sharma, A., Huang, K., Ridgway, R., Mosaliganti, K., Sharp, R., Machiraju, R., Saltz, J., Yamamoto, H., Cross, J. C., Robinson, M. L., Leone, G. 2007; 21 (1): 85-97


    The inactivation of the retinoblastoma (Rb) tumor suppressor gene in mice results in ectopic proliferation, apoptosis, and impaired differentiation in extraembryonic, neural, and erythroid lineages, culminating in fetal death by embryonic day 15.5 (E15.5). Here we show that the specific loss of Rb in trophoblast stem (TS) cells, but not in trophoblast derivatives, leads to an overexpansion of trophoblasts, a disruption of placental architecture, and fetal death by E15.5. Despite profound placental abnormalities, fetal tissues appeared remarkably normal, suggesting that the full manifestation of fetal phenotypes requires the loss of Rb in both extraembryonic and fetal tissues. Loss of Rb resulted in an increase of E2f3 expression, and the combined ablation of Rb and E2f3 significantly suppressed Rb mutant phenotypes. This rescue appears to be cell autonomous since the inactivation of Rb and E2f3 in TS cells restored placental development and extended the life of embryos to E17.5. Taken together, these results demonstrate that loss of Rb in TS cells is the defining event causing lethality of Rb(-/-) embryos and reveal the convergence of extraembryonic and fetal functions of Rb in neural and erythroid development. We conclude that the Rb pathway plays a critical role in the maintenance of a mammalian stem cell population.

    View details for DOI 10.1101/gad.1485307

    View details for Web of Science ID 000243382700009

    View details for PubMedID 17210791

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