A strategy for the selective imaging of glycans using caged metabolic precursors

Article abstract of DOI:10.1021/ja101080y

Glycans can be imaged by metabolic labeling with azidosugars followed by chemical reaction with imaging probes; however, tissue-specific labeling is difficult to achieve. Here we describe a strategy for the use of a caged metabolic precursor that is activated for cellular metabolism by enzymatic cleavage. An N-azidoacetylmannosamine derivative caged with a peptide substrate for the prostate-specific antigen (PSA) protease was converted to cell-surface azido sialic acids in a PSA-dependent manner. The approach has applications in tissue-selective imaging of glycans for clinical and basic research purposes.

Full text of DOI:10.1021/ja101080y

Published on Web 06/22/2010
A Strategy for the Selective Imaging of Glycans Using Caged Metabolic
Precursors
Pamela V. Chang, Danielle H. Dube,† Ellen M. Sletten, and Carolyn R. Bertozzi*
Departments of Chemistry and Molecular and Cell Biology and Howard Hughes Medical Institute, UniVersity of
California, Berkeley, California 94720, and The Molecular Foundry, Lawrence Berkeley National Laboratory,
Berkeley, California 94720
Received February 12, 2010; E-mail: crb@berkeley.edu
Abstract: Glycans can be imaged by metabolic labeling with
azidosugars followed by chemical reaction with imaging probes;
however, tissue-specific labeling is difficult to achieve. Here we
describe a strategy for the use of a caged metabolic precursor
that is activated for cellular metabolism by enzymatic cleavage.
An N-azidoacetylmannosamine derivative caged with a peptide
substrate for the prostate-specific antigen (PSA) protease was
converted to cell-surface azido sialic acids in a PSA-dependent
manner. The approach has applications in tissue-selective imag-
ing of glycans for clinical and basic research purposes.
A fundamental challenge in molecular imaging is the design of
reagents that report on cell- or tissue-specific molecular processes. A
popular approach to achieving such selectivity is the design of caged
probes that are activated by enzymes produced by the target cells, a
prototypical example being cancer-associated proteases.1 Alternatively,
probes can be conjugated to targeting elements that bind, noncovalently
or covalently, to cell-surface markers.2 We have been exploring this
latter approach in the context of imaging cell-surface glycans. These
biopolymers, collectively termed the glycome, change in structure and
expression during development and cancer progression as well as many
other physiological processes.3 We previously reported that broad
Figure 1. Strategy for tissue-specific release of Ac3ManNAz via enzymatic
activation. (A) (i) A nonmetabolizable caged azidosugar serves as a substrate
for a secreted, cancer-specific protease, releasing Ac3ManNAz. (ii) This
sectors of the glycome can be imaged in model organisms using a
two-step procedure: (1) metabolic labeling of the glycans with
azidosugar precursors and (2) chemical reaction of azide-labeled cell-
surface glycans in ViVo via Cu-free click chemistry with difluorinated
cyclooctyne (DIFO) probes.4
azidosugar is then metabolized by the cell and incorporated into cell-surface
glycans. (iii) The azide-labeled glycans are detected via Cu-free click chemistry
using DIFO reagents. (B) Caged azidosugar used in this study (1). Cleavage
of the indicated bond (dashed line) by the prostate-specific antigen (PSA)
protease results in the release of a linker, the peptide, and Ac3ManNAz.
The tissue selectivity of this approach is limited by the breadth of
glycans targeted by a single metabolic precursor. For example,
treatment of mice with peracetylated N-azidoacetylmannosamine
(Ac4ManNAz) leads to widespread labeling of glycans with N-
azidoacetyl sialic acid (SiaNAz) in numerous tissues (e.g., liver, heart,
kidney, and intestines) as well as serum glycoproteins.5 The broad
distribution of the azidosugar can undermine experiments that seek to
probe glycomic changes in a single organ as a function of time or
disease. One means to achieve tissue selectivity in glycan imaging is
to restrict azidosugar metabolism to the cells of interest. Tissue-specific
enzyme activities such as those mentioned above might be exploited
to achieve this goal. Here, we describe a strategy for cell-selective
glycan imaging using a caged metabolic precursor that is activated by
a protease (Figure 1A).
cells.6 The enzyme has been widely targeted with caged drugs and
imaging reagents in cell culture and in animal models.6 The hexapep-
tide Mu-HSSKLY (Mu ) morpholino ureidyl) was chosen as a PSA
substrate based on reports of its high selectivity for this enzyme over
other ubiquitous serine proteases.6c,7 Upon cleavage of the peptide,
the p-aminobenzyl alcohol (PABA) linker in 1 spontaneously fragments
to release 1,3,4-tri-O-acetyl-N-azidoacetylmannosamine (Ac3ManNAz),
carbon dioxide, and an iminoquinone methide intermediate that is
subsequently quenched by water (Figure 1B).8 This process is known
to occur rapidly and should therefore limit the diffusion of the released
metabolic substrate from its target cell (for discussion see Figure S1
legend). Compound 1 was synthesized as a mixture of sugar anomers
analogously to the route developed by Jones et al. (Scheme S1).6c
Given the importance of hydroxy group acylation for the cellular uptake
of Ac4ManNAz,9 we verified that Chinese hamster ovary (CHO) and
prostate cancer (PC-3) cells incubated with Ac3ManNAz could produce
SiaNAz residues in their cell-surface glycans (Figure S1).
We synthesized a variant of Ac4ManNAz in which the 6-hydroxy
group was conjugated through a self-immolating linker to a peptide
substrate for the prostate-specific antigen (PSA) protease (1, Figure
1B). PSA is a serine protease that is secreted at low levels by normal
prostatic glandular cells but is highly upregulated by prostate cancer
To confirm that 1 can serve as a substrate for PSA, the compound
was incubated in Vitro with active enzyme or, as negative controls,
buffer only or heat-killed (HK) PSA. These enzymatic reactions were
analyzed by reversed-phase HPLC and mass spectrometry. Incubation
† Current address: Department of Chemistry, Bowdoin College, Brunswick, ME
04011.
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C O M M U N I C A T I O N S
Figure 4. Selective imaging of cells using 1 in the presence of PSA.
Fluorescence microscopy analysis of CHO cells treated with 1 (100 µM) and
(A) PSA (50 µg/mL) or (B) HK PSA (50 µg/mL), followed by DIFO-biotin
(100 µM) and a quantum dot 605-streptavidin conjugate. Green ) Texas Red
channel; Blue ) DAPI channel. Scale bar ) 20 µm.
in PSA-dependent metabolic labeling (Figure S3). In the absence of
PSA or with HK PSA, both CHO and PC-3 cells exhibited modest
background labeling that likely reflects low levels of Ac3ManNAz
produced by nonenzymatic carbonate hydrolysis (Figures 3B and S3).
Importantly, we verified that 1 did not cause any cytotoxicity by
incubating CHO cells labeled as above with phycoerythrin-conjugated
annexin V, a marker of apoptosis (Figure S4).
Finally, we tested 1 as an enzyme-activatable metabolic substrate
for glycan imaging. CHO cells were incubated with 1 in the presence
of PSA or HK PSA for 12 h at 37 °C. The cells were then washed
and labeled with DIFO-biotin, followed by quantum-dot-conjugated
streptavidin. We observed substantial cell-surface labeling of cells
treated with 1 and PSA (Figure 4A) and minimal fluorescence on cells
treated with 1 and HK PSA (Figure 4B).
Figure 2. Compound 1 is a substrate for PSA in Vitro. Shown are HPLC
traces of in Vitro 6 h enzymatic reactions of 1 (500 µM) in 50 mM Tris, 0.1 M
NaCl, pH 7.8, with (A) active PSA (50 µg/mL), (B) buffer only, or (C) HK
PSA (50 µg/mL). The identities of the various species based on mass
spectrometry are indicated on the traces. *Monoacetylated Mu-HSSKLY-
PABA, **Monodeacetylated 1, and ***Isomers of 1.
In conclusion, we have developed a strategy for targeted metabolism
of azidosugars using an enzymatically activated substrate. While we
chose PSA to demonstrate proof-of-concept, it should be noted that
the concentrations of PSA employed in our studies are physiologically
relevant; i.e., they are similar to the levels of PSA secreted by both
prostate cancer xenografts in mice and prostate tumor tissue obtained
from human patients.10 In addition, many cancers, including prostate
cancer, are known to express elevated levels of sialic acid compared
to surrounding tissue.11 Thus, clinical imaging applications may be
worth pursuing. More generally, however, the approach has promise
for use in tissue-specific glycan imaging, a major future direction.
Acknowledgment. This work was supported by NIH Grant
GM058867. We thank A. Lo for technical assistance and J. Baskin
for critical reading of the manuscript. P.V.C. and D.H.D. were
supported by NSF predoctoral fellowships. P.V.C. was also supported
by an ACS Division of Medicinal Chemistry predoctoral fellowship.
E.M.S. was supported by an ACS Division of Organic Chemistry
predoctoral fellowship.
Figure 3. Cell-selective metabolic labeling of glycans using 1 and PSA. Flow
cytometry analysis of CHO cells treated with (A) various concentrations of 1
(0-100 µM) and PSA (50 µg/mL, squares) or buffer only (circles) or (B) 1
(100 µM) and either buffer only (-), HK PSA (50 µg/mL, HK), or PSA (50
µg/mL, +). Cells were then labeled with DIFO-biotin (100 µM) and FITC-
avidin. Error bars represent the standard deviation from the mean of three
replicate samples. MFI ) mean fluorescence intensity in arbitrary units (AU).
Supporting Information Available: Synthetic procedures and ad-
ditional data. This material is available free of charge via the Internet at
http://pubs.acs.org.
of 1 with PSA resulted in the release of Mu-HSSKLY as the major
peptide product, along with Ac3ManNAz (Figure 2A). Minor products
were also observed. These included Mu-HSSKLY-PABA, presumably
produced by nonenzymatic carbonate hydrolysis, as well as mono-
deacetylated 1 and products formed by migration of acetyl groups
within 1. Control reactions lacking PSA (Figure 2B) or with HK PSA
(Figure 2C) produced only nonenzymatic hydrolysis and acetyl
migration products.
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