Bio


My academic training and research experience have provided me a broad background in multiple disciplines, including synthetic chemistry, chemical biology, molecular biology, glycoimmunology. As an undergraduate, I supported Dr. Liu-Zhu Gong’s group (USTC) by developing flexible routes to synthesize chiral amines in alkaloid nature products. As a graduate student, I joined Dr. Peter G. Schultz’s lab (Scripps) where chemical synthetic molecules emerge as novel tools to understand molecular mechanisms of complex biological and chemical systems. I engineered several aminoacyl-tRNA synthetase/tRNA pairs enabling the genetic incorporation of unnatural amino acids with new structural features at a specific position of proteins. Furthermore, I have reported enhanced systems to encode multiple, distinct unnatural amino acids in both prokaryotic and eukaryotic organisms. The utilities of these systems were demonstrated by the preparation of multi-functional therapeutic molecules as well as the ability to explore evolutionary advantage of unnatural amino acids to a host organism. My experience as a postdoctoral fellow at Dr. Carolyn R. Bertozzi’s lab (Stanford) has been focused on the study of the modulating role of glycans in the context of immune therapy. Given the critical role of glycans in modulating the immune response, we aim to develop various therapeutic molecules enabling precision glycans editing in tumor-specific manners. Unlike current cancer immunotherapies targeting a single pathway, enzymatic precision glycans editing is able to simultaneously stimulate immune activating pathways and limit immune inhibitory pathways.

Honors & Awards


  • Good Ventures Postdoctoral Fellowship, Life Science Research Foundation (2016-2019)
  • Travel Award Sialoglyco 2016, Sialoglyco Conference Committee (2016)
  • Aldrich Alfred R. Bader Award for Student Innovation, Sigma-Aldrich Co. LLC (2014)
  • Outstanding Self-Financed Students Abroad, Ministry of Education of the People’s Republic of China (2013)
  • Honors Degree in Physical Science, University of Science and Technology of China (2010)
  • National Scholarship, Ministry of Education of the People’s Republic of China (2008)

Professional Education


  • Doctor of Philosophy, Scripps Research Institute Kellogg School (2015)
  • Bachelor of Chemistry, University of Science and Technology of China (2010)

Stanford Advisors


All Publications


  • Precision glycocalyx editing as a strategy for cancer immunotherapy. Proceedings of the National Academy of Sciences of the United States of America Xiao, H., Woods, E. C., Vukojicic, P., Bertozzi, C. R. 2016

    Abstract

    Cell surface sialosides constitute a central axis of immune modulation that is exploited by tumors to evade both innate and adaptive immune destruction. Therapeutic strategies that target tumor-associated sialosides may therefore potentiate antitumor immunity. Here, we report the development of antibody-sialidase conjugates that enhance tumor cell susceptibility to antibody-dependent cell-mediated cytotoxicity (ADCC) by selective desialylation of the tumor cell glycocalyx. We chemically fused a recombinant sialidase to the human epidermal growth factor receptor 2 (HER2)-specific antibody trastuzumab through a C-terminal aldehyde tag. The antibody-sialidase conjugate desialylated tumor cells in a HER2-dependent manner, reduced binding by natural killer (NK) cell inhibitory sialic acid-binding Ig-like lectin (Siglec) receptors, and enhanced binding to the NK-activating receptor natural killer group 2D (NKG2D). Sialidase conjugation to trastuzumab enhanced ADCC against tumor cells expressing moderate levels of HER2, suggesting a therapeutic strategy for cancer patients with lower HER2 levels or inherent trastuzumab resistance. Precision glycocalyx editing with antibody-enzyme conjugates is therefore a promising avenue for cancer immune therapy.

    View details for DOI 10.1073/pnas.1608069113

    View details for PubMedID 27551071

  • Design of Switchable Chimeric Antigen Receptor T Cells Targeting Breast Cancer. Angewandte Chemie (International ed. in English) Cao, Y., Rodgers, D. T., Du, J., Ahmad, I., Hampton, E. N., Ma, J. S., Mazagova, M., Choi, S. H., Yun, H. Y., Xiao, H., Yang, P., Luo, X., Lim, R. K., Pugh, H. M., Wang, F., Kazane, S. A., Wright, T. M., Kim, C. H., Schultz, P. G., Young, T. S. 2016

    Abstract

    Chimeric antigen receptor T (CAR-T) cells have demonstrated promising results against hematological malignancies, but have encountered significant challenges in translation to solid tumors. To overcome these hurdles, we have developed a switchable CAR-T cell platform in which the activity of the engineered cell is controlled by dosage of an antibody-based switch. Herein, we apply this approach to Her2-expressing breast cancers by engineering switch molecules through site-specific incorporation of FITC or grafting of a peptide neo-epitope (PNE) into the anti-Her2 antibody trastuzumab (clone 4D5). We demonstrate that both switch formats can be readily optimized to redirect CAR-T cells (specific for the corresponding FITC or PNE) to Her2-expressing tumor cells, and afford dose-titratable activation of CAR-T cells ex vivo and complete clearance of the tumor in rodent xenograft models. This strategy may facilitate the application of immunotherapy to solid tumors by affording comparable efficacy with improved safety owing to switch-based control of the CAR-T response.

    View details for DOI 10.1002/anie.201601902

    View details for PubMedID 27145250

  • Enhancing protein stability with extended disulfide bonds. Proceedings of the National Academy of Sciences of the United States of America Liu, T., Wang, Y., Luo, X., Li, J., Reed, S. A., Xiao, H., Young, T. S., Schultz, P. G. 2016; 113 (21): 5910-5

    Abstract

    Disulfide bonds play an important role in protein folding and stability. However, the cross-linking of sites within proteins by cysteine disulfides has significant distance and dihedral angle constraints. Here we report the genetic encoding of noncanonical amino acids containing long side-chain thiols that are readily incorporated into both bacterial and mammalian proteins in good yields and with excellent fidelity. These amino acids can pair with cysteines to afford extended disulfide bonds and allow cross-linking of more distant sites and distinct domains of proteins. To demonstrate this notion, we preformed growth-based selection experiments at nonpermissive temperatures using a library of random β-lactamase mutants containing these noncanonical amino acids. A mutant enzyme that is cross-linked by one such extended disulfide bond and is stabilized by ∼9 °C was identified. This result indicates that an expanded set of building blocks beyond the canonical 20 amino acids can lead to proteins with improved properties by unique mechanisms, distinct from those possible through conventional mutagenesis schemes.

    View details for DOI 10.1073/pnas.1605363113

    View details for PubMedID 27162342

  • At the Interface of Chemical and Biological Synthesis: An Expanded Genetic Code. Cold Spring Harbor perspectives in biology Xiao, H., Schultz, P. G. 2016; 8 (9)

    Abstract

    The ability to site-specifically incorporate noncanonical amino acids (ncAAs) with novel structures into proteins in living cells affords a powerful tool to investigate and manipulate protein structure and function. More than 200 ncAAs with diverse biological, chemical, and physical properties have been genetically encoded in response to nonsense or frameshift codons in both prokaryotic and eukaryotic organisms with high fidelity and efficiency. In this review, recent advances in the technology and its application to problems in protein biochemistry, cellular biology, and medicine are highlighted.

    View details for DOI 10.1101/cshperspect.a023945

    View details for PubMedID 27413101

  • Genetic Incorporation of epsilon-N-2-Hydroxyisobutyryl-lysine into Recombinant Histones ACS CHEMICAL BIOLOGY Xiao, H., Xuan, W., Shao, S., Liu, T., Schultz, P. G. 2015; 10 (7): 1599-1603

    Abstract

    Here, we report the evolution of an orthogonal amber suppressor pyrrolysyl-tRNA synthetase (PylRS)/tRNACUA(Pyl) pair that genetically encodes the post-translationally modified amino acid, ε-N-2-hydroxyisobutyryl-lysine (HibK), in bacteria and mammalian cells. HibK is a new type of histone mark that is widely distributed in histone proteins. The ability to site-specifically incorporate HibK into proteins provides a useful tool to probe the biological function of this newly identified post-translational modification.

    View details for DOI 10.1021/cb501055h

    View details for Web of Science ID 000358395300003

    View details for PubMedID 25909834

  • Exploring the potential impact of an expanded genetic code on protein function PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Xiao, H., Nasertorabi, F., Choi, S., Han, G. W., Reed, S. A., Stevens, R. C., Schultz, P. G. 2015; 112 (22): 6961-6966

    Abstract

    With few exceptions, all living organisms encode the same 20 canonical amino acids; however, it remains an open question whether organisms with additional amino acids beyond the common 20 might have an evolutionary advantage. Here, we begin to test that notion by making a large library of mutant enzymes in which 10 structurally distinct noncanonical amino acids were substituted at single sites randomly throughout TEM-1 β-lactamase. A screen for growth on the β-lactam antibiotic cephalexin afforded a unique p-acrylamido-phenylalanine (AcrF) mutation at Val-216 that leads to an increase in catalytic efficiency by increasing kcat, but not significantly affecting KM. To understand the structural basis for this enhanced activity, we solved the X-ray crystal structures of the ligand-free mutant enzyme and of the deacylation-defective wild-type and mutant cephalexin acyl-enzyme intermediates. These structures show that the Val-216-AcrF mutation leads to conformational changes in key active site residues-both in the free enzyme and upon formation of the acyl-enzyme intermediate-that lower the free energy of activation of the substrate transacylation reaction. The functional changes induced by this mutation could not be reproduced by substitution of any of the 20 canonical amino acids for Val-216, indicating that an expanded genetic code may offer novel solutions to proteins as they evolve new activities.

    View details for DOI 10.1073/pnas.1507741112

    View details for Web of Science ID 000355832200059

    View details for PubMedID 26038548

  • A Bacterial Strain with a Unique Quadruplet Codon Specifying Non-native Amino Acids CHEMBIOCHEM Chatterjee, A., Lajoie, M. J., Xiao, H., Church, G. M., Schultz, P. G. 2014; 15 (12): 1782-1786

    Abstract

    The addition of noncanonical amino acids to the genetic code requires unique codons not assigned to the 20 canonical amino acids. Among the 64 triplet codons, only the three nonsense "stop" codons have been used to encode non-native amino acids. Use of quadruplet "frame-shift" suppressor codons provides an abundant alternative but suffers from low suppression efficiency as a result of competing recognition of their first three bases by endogenous host tRNAs or release factors. Deletion of release factor 1 in a genomically recoded strain of E. coli (E. coli C321), in which all endogenous amber stop codons (UAG) are replaced with UAA, abolished UAG mediated translation termination. Here we show that a Methanocaldococcus jannaschii-derived frame-shift suppressor tRNA/aminoacyl-tRNA synthetase pair enhanced UAGN suppression efficiency in this recoded bacterial strain. These results demonstrate that efficient quadruplet codons for encoding non-native amino acids can be generated by eliminating competing triplet codon recognition at the ribosome.

    View details for DOI 10.1002/cbic.201402104

    View details for Web of Science ID 000340506200015

    View details for PubMedID 24867343

  • Genetic Incorporation of Histidine Derivatives Using an Engineered Pyrrolysyl-tRNA Synthetase ACS CHEMICAL BIOLOGY Xiao, H., Peters, F. B., Yang, P., Reed, S., Chittuluru, J. R., Schultz, P. G. 2014; 9 (5): 1092-1096

    Abstract

    A polyspecific amber suppressor aminoacyl-tRNA synthetase/tRNA pair was evolved that genetically encodes a series of histidine analogues in both Escherichia coli and mammalian cells. In combination with tRNACUA(Pyl), a pyrrolysyl-tRNA synthetase mutant was able to site-specifically incorporate 3-methyl-histidine, 3-pyridyl-alanine, 2-furyl-alanine, and 3-(2-thienyl)-alanine into proteins in response to an amber codon. Substitution of His66 in the blue fluorescent protein (BFP) with these histidine analogues created mutant proteins with distinct spectral properties. This work further expands the structural and chemical diversity of unnatural amino acids (UAAs) that can be genetically encoded in prokaryotic and eukaryotic organisms and affords new probes of protein structure and function.

    View details for DOI 10.1021/cb500032c

    View details for Web of Science ID 000336199400004

    View details for PubMedID 24506189

  • Genetic Incorporation of Multiple Unnatural Amino Acids into Proteins in Mammalian Cells ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Xiao, H., Chatterjee, A., Choi, S., Bajjuri, K. M., Sinha, S. C., Schultz, P. G. 2013; 52 (52): 14080-14083

    View details for DOI 10.1002/anie.201308137

    View details for Web of Science ID 000328531100034

    View details for PubMedID 24353230

  • Efficient viral delivery system for unnatural amino acid mutagenesis in mammalian cells PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chatterjee, A., Xiao, H., Bollong, M., Ai, H., Schultz, P. G. 2013; 110 (29): 11803-11808

    Abstract

    Here we report the development of a baculovirus-based delivery system that enables the efficient incorporation of unnatural amino acids into proteins in mammalian cells. We have exploited the large cargo-capacity (>30 kb) and stability of the double-stranded DNA genome of baculovirus to deliver to a variety of cell types all of the components required to genetically incorporate novel amino acids. These include the engineered tRNA/aminoacyl-tRNA synthetase pair and the nonsense mutant of the target gene. Mammalian cell transduction efficiency of baculovirus was significantly improved by incorporating genetic elements from mammalian viruses. Two polyspecific tRNA/aminoacyl-tRNA synthetase pairs were inserted into this expression system, enabling the site-specific incorporation of a variety of unnatural amino acids with novel chemical and biological properties into proteins.

    View details for DOI 10.1073/pnas.1309584110

    View details for Web of Science ID 000322086100042

    View details for PubMedID 23818609

  • Synthesis of Indolines via Pd(II)-Catalyzed Amination of C-H Bonds Using Phl(OAc)(2) as the Bystanding Oxidant ORGANIC LETTERS Mei, T., Leow, D., Xiao, H., Laforteza, B. N., Yu, J. 2013; 15 (12): 3058-3061

    Abstract

    The Pd(II)-catalyzed intramolecular C-H amination of 2-pyridinesulfonyl-protected phenethylamine derivatives has been achieved using PhI(OAc)2 as a bystanding oxidant, providing access to a variety of substituted indoline derivatives in good yields. The use of the 2-pyridinesulfonyl protecting group allows for facile deprotection following C-H functionalization.

    View details for DOI 10.1021/ol401246u

    View details for Web of Science ID 000320979000043

    View details for PubMedID 23725020

  • A Versatile Platform for Single- and Multiple-Unnatural Amino Acid Mutagenesis in Escherichia coli BIOCHEMISTRY Chatterjee, A., Sun, S. B., Furman, J. L., Xiao, H., Schultz, P. G. 2013; 52 (10): 1828-1837

    Abstract

    To site-specifically incorporate an unnatural amino acid (UAA) into target proteins in Escherichia coli, we use a suppressor plasmid that provides an engineered suppressor tRNA and an aminoacyl-tRNA synthetase (aaRS) specific for the UAA of interest. The continuous drive to further improve UAA incorporation efficiency in E. coli has resulted in several generations of suppressor plasmids. Here we describe a new, highly efficient suppressor plasmid, pUltra, harboring a single copy each of the tRNA and aaRS expression cassettes that exhibits higher suppression activity than its predecessors. This system is able to efficiently incorporate up to three UAAs within the same protein at levels up to 30% of the level of wild-type protein expression. Its unique origin of replication (CloDF13) and antibiotic resistance marker (spectinomycin) allow pUltra to be used in conjunction with the previously reported pEVOL suppressor plasmid, each encoding a distinct tRNA/aaRS pair, to simultaneously insert two different UAAs into the same protein. We demonstrate the utility of this system by efficiently incorporating two bio-orthogonal UAAs containing keto and azido side chains into ketosteroid isomerase and subsequently derivatizing these amino acid residues with two distinct fluorophores, capable of Förster resonance energy transfer interaction. Finally, because of its minimal composition, two different tRNA/aaRS pairs were encoded in pUltra, allowing the generation of a single plasmid capable of dual suppression. The high suppression efficiency and the ability to harbor multiple tRNA/aaRS pairs make pUltra a useful system for conducting single- and multiple-UAA mutagenesis in E. coli.

    View details for DOI 10.1021/bi4000244

    View details for Web of Science ID 000316168500018

    View details for PubMedID 23379331

  • A Tryptophanyl-tRNA Synthetase/tRNA Pair for Unnatural Amino Acid Mutagenesis in E.coli ANGEWANDTE CHEMIE-INTERNATIONAL EDITION Chatterjee, A., Xiao, H., Yang, P., Soundararajan, G., Schultz, P. G. 2013; 52 (19): 5106-5109

    View details for DOI 10.1002/anie.201301094

    View details for Web of Science ID 000318799800020

    View details for PubMedID 23554007

  • Evolution of multiple, mutually orthogonal prolyl-tRNA synthetase/tRNA pairs for unnatural amino acid mutagenesis in Escherichia coli PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chatterjee, A., Xiao, H., Schultz, P. G. 2012; 109 (37): 14841-14846

    Abstract

    The site-specific incorporation of unnatural amino acids (UAAs) into proteins in living cells relies on an engineered tRNA/aminoacyl-tRNA synthetase (tRNA/aaRS) pair, orthogonal to the host cell, to deliver the UAA of choice in response to a unique nonsense or frameshift codon. Here we report the generation of mutually orthogonal prolyl-tRNA/prolyl-tRNA synthase (ProRS) pairs derived from an archaebacterial ancestor for use in Escherichia coli. By reprogramming the anticodon-binding pocket of Pyrococcus horikoshii ProRS (PhProRS), we were able to identify synthetase variants that recognize engineered Archaeoglobus fulgidus prolyl-tRNAs (Af-tRNA(Pro)) with three different anticodons: CUA, AGGG, and CUAG. Several of these evolved PhProRSs show specificity toward a particular anticodon variant of Af-tRNA(Pro), whereas others are promiscuous. Further evolution of the Af-tRNA(Pro) led to a variant exhibiting significantly improved amber suppression efficiency. Availability of a prolyl-tRNA/aaRS pair should enable site-specific incorporation of proline analogs and other N-modified UAAs into proteins in E. coli. The evolution of mutually orthogonal prolyl-tRNA/ProRS pairs demonstrates the plasticity of the tRNA-aaRS interface and should facilitate the incorporation of multiple, distinct UAAs into proteins.

    View details for DOI 10.1073/pnas.1212454109

    View details for Web of Science ID 000309208000034

    View details for PubMedID 22927411

  • Enantioselective concomitant creation of vicinal quaternary stereogenic centers via cyclization of alkynols triggered addition of azlactones TETRAHEDRON LETTERS Han, Z., Guo, R., Wang, P., Chen, D., Xiao, H., Gong, L. 2011; 52 (45): 5963-5967
  • Organocatalytic Synthesis of Spiro[pyrrolidin-3,3 '-oxindoles] with High Enantiopurity and Structural Diversity JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Chen, X., Wei, Q., Luo, S., Xiao, H., Gong, L. 2009; 131 (38): 13819-13825

    Abstract

    The privileged spiro[pyrrolidin-3,3'-oxindole] derivatives exhibit important biological activities. An enantioselective organocatalytic approach to the rapid synthesis of spiro[pyrrolidin-3,3'-oxindole] derivatives with high enantiopurity and structural diversity is described. The asymmetric catalytic three-component 1,3-dipolar cycloaddition of a broad range of methyleneindolinones with aldehydes and amino esters in the presence of chiral phosphoric acid provides spirooxindole derivatives in high yield with unusual regiochemistry and excellent stereoselectivities (up to 98% ee) under mild conditions. The straightforward construction of spirooxindole skeletons with high stereo- and regioselectivity suggests a new avenue to medicinal chemistry and diversity-oriented synthesis. Theoretical calculations disclosed that both the azomethine ylide and the methyleneindolinone are hydrogen-bonded with the phosphoric acid, which accounted for the high enantio- and regioselectivity and indicated that the unusual regioselectivity results from the stabilization stemming from the favorable pi-pi stacking interaction between the oxo-indole ring and the conjugated esters.

    View details for DOI 10.1021/ja905302f

    View details for Web of Science ID 000270186600053

    View details for PubMedID 19736987

  • Dynamic kinetic asymmetric transfer hydrogenation of racemic 2,4-diaryl-2,3-dihydrobenzo[b][1,4]diazepines catalyzed by chiral phosphoric acids BIOORGANIC & MEDICINAL CHEMISTRY LETTERS Han, Z., Xiao, H., Gong, L. 2009; 19 (14): 3729-3732

    Abstract

    Dynamic kinetic Transfer hydrogenation reaction of 2-methyl-2,4-diaryl-2,3-dihydrobenzo[b][1,4]diazepines, using phosphoric acids as catalysts and Hantzsch ester as hydride source, has been studied. A 3,3'-H8-binol derived phosphoric acid has been identified the optimal chiral catalyst for this transformation, affording 1,3-diamine derivatives with up to 8/1 dr, 86% ee and 94% ee for the major and minor diastereomers, respectively.

    View details for DOI 10.1016/j.bmcl.2009.05.039

    View details for Web of Science ID 000267762600005

    View details for PubMedID 19481933

  • Consecutive Intramolecular Hydroamination/Asymmetric Transfer Hydrogenation under Relay Catalysis of an Achiral Gold Complex/Chiral Bronsted Acid Binary System JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Han, Z., Xiao, H., Chen, X., Gong, L. 2009; 131 (26): 9182-?

    Abstract

    Consecutive hydroamination/asymmetric transfer hydrogenation under relay catalysis of an achiral gold complex/chiral Brønsted acid binary system has been described for the direct transformation of 2-(2-propynyl)aniline derivatives into tetrahydroquinolines with high enantiomeric purity.

    View details for DOI 10.1021/ja903547q

    View details for Web of Science ID 000267633300025

    View details for PubMedID 19518048