Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan :...

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Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein Sabine Andre ´ 1 , Shuji Kojima 2 , Ingo Prahl 3 , Martin Lensch 1 , Carlo Unverzagt 3 and Hans-Joachim Gabius 1 1 Institut fu ¨ r Physiologische Chemie, Tiera ¨ rztliche Fakulta ¨ t, Ludwig-Maximilians-Universita ¨t Mu ¨ nchen, Germany 2 Faculty of Pharmaceutical Sciences, Tokyo University of Science, Japan 3 Bioorganische Chemie, Universita ¨t Bayreuth, Germany N-Glycosylation is the most frequent and structurally most variegated form of post-translational modifica- tion [1–3]. Ironically, it is exactly due to this unsur- passed molecular complexity that progress to assign functional significance to distinct glycan epitope lags behind the advances of work on other types of protein modifications. Taking a step to change this situation was the driving force for our study. At first glance, we consider it reasonable to interpret the enormous struc- tural complexity of the carbohydrate part of glycopro- teins as a wide array of signals; this concept provides direction for research [4,5]. As documented already at the level of nascent glycoproteins, their N-glycan struc- ture is relevant for quality control, underscoring the Keywords drug targeting; galectin; glycosylation; lectin; tumor imaging Correspondence S. Andre ´ , Institut fu ¨ r Physiologische Chemie, Tiera ¨ rztliche Fakulta ¨t, Ludwig- Maximilians-Universita ¨t Mu ¨ nchen, Veterina ¨rstr. 13, 80539 Mu ¨ nchen, Germany Fax: +49 80 2180 2508 Tel: +49 89 2180 2290 E-mail: [email protected] (Received 16 December 2004, revised 23 February 2005, accepted 2 March 2005) doi:10.1111/j.1742-4658.2005.04637.x A series of enzymatic substitutions modifies the basic structure of complex-type biantennary N-glycans. Among them, a b1,2-linked N-ace- tylglucosamine residue is introduced to the central mannose moiety of the core-fucosylated oligosaccharide by N-acetylglucosaminyltransferase VII. This so-called LEC14 epitope can undergo galactosylation at the b1,2- linked N-acetylglucosamine residue. Guided by the hypothesis that struc- tural modifications in the N-glycan alter its capacity to serve as ligand for lectins, we prepared a neoglycoprotein with the extended LEC14 N-glycan and tested its properties in three different assays. In order to allow compar- ison to previous results on other types of biantennary N-glycans the func- tionalization of the glycans for coupling and assay conditions were deliberately kept constant. Compared to the core-fucosylated N-glycan no significant change in affinity was seen when testing three galactoside-speci- fic proteins. However, cell positivity in flow cytofluorimetry was enhanced in six of eight human tumor lines. Analysis of biodistribution in tumor- bearing mice revealed an increase of blood clearance by about 40%, yield- ing a favorable tumor blood ratio. Thus, the extended LEC14 motif affects binding properties to cellular lectins on cell surfaces and organs when com- pared to the core-fucosylated biantennary N-glycan. The results argue in favor of the concept of viewing substitutions as molecular switches for lectin-binding affinity. Moreover, they have potential relevance for glyco- engineering of reagents in tumor imaging. Abbreviations CHO, Chinese hamster ovary; GlcNAc, N-acetylglucosamine; GlcNAc-TVII, N-acetylglucosaminyltransferase VII; LacNAc, N-acetyllactosamine; LCA, Lens culinaris agglutinin; PSA, Pisum sativum agglutinin; TFA, trifluoroacetic acid. 1986 FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS

Transcript of Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan :...

Page 1: Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan : Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein

Introduction of extended LEC14-type branching intocore-fucosylated biantennary N-glycan

Glycoengineering for enhanced cell binding and serum clearanceof the neoglycoprotein

Sabine Andre1, Shuji Kojima2, Ingo Prahl3, Martin Lensch1, Carlo Unverzagt3

and Hans-Joachim Gabius1

1 Institut fur Physiologische Chemie, Tierarztliche Fakultat, Ludwig-Maximilians-Universitat Munchen, Germany

2 Faculty of Pharmaceutical Sciences, Tokyo University of Science, Japan

3 Bioorganische Chemie, Universitat Bayreuth, Germany

N-Glycosylation is the most frequent and structurally

most variegated form of post-translational modifica-

tion [1–3]. Ironically, it is exactly due to this unsur-

passed molecular complexity that progress to assign

functional significance to distinct glycan epitope lags

behind the advances of work on other types of protein

modifications. Taking a step to change this situation

was the driving force for our study. At first glance, we

consider it reasonable to interpret the enormous struc-

tural complexity of the carbohydrate part of glycopro-

teins as a wide array of signals; this concept provides

direction for research [4,5]. As documented already at

the level of nascent glycoproteins, their N-glycan struc-

ture is relevant for quality control, underscoring the

Keywords

drug targeting; galectin; glycosylation; lectin;

tumor imaging

Correspondence

S. Andre, Institut fur Physiologische

Chemie, Tierarztliche Fakultat, Ludwig-

Maximilians-Universitat Munchen,

Veterinarstr. 13, 80539 Munchen, Germany

Fax: +49 80 2180 2508

Tel: +49 89 2180 2290

E-mail: [email protected]

(Received 16 December 2004, revised 23

February 2005, accepted 2 March 2005)

doi:10.1111/j.1742-4658.2005.04637.x

A series of enzymatic substitutions modifies the basic structure of

complex-type biantennary N-glycans. Among them, a b1,2-linked N-ace-

tylglucosamine residue is introduced to the central mannose moiety of the

core-fucosylated oligosaccharide by N-acetylglucosaminyltransferase VII.

This so-called LEC14 epitope can undergo galactosylation at the b1,2-linked N-acetylglucosamine residue. Guided by the hypothesis that struc-

tural modifications in the N-glycan alter its capacity to serve as ligand for

lectins, we prepared a neoglycoprotein with the extended LEC14 N-glycan

and tested its properties in three different assays. In order to allow compar-

ison to previous results on other types of biantennary N-glycans the func-

tionalization of the glycans for coupling and assay conditions were

deliberately kept constant. Compared to the core-fucosylated N-glycan no

significant change in affinity was seen when testing three galactoside-speci-

fic proteins. However, cell positivity in flow cytofluorimetry was enhanced

in six of eight human tumor lines. Analysis of biodistribution in tumor-

bearing mice revealed an increase of blood clearance by about 40%, yield-

ing a favorable tumor ⁄blood ratio. Thus, the extended LEC14 motif affects

binding properties to cellular lectins on cell surfaces and organs when com-

pared to the core-fucosylated biantennary N-glycan. The results argue in

favor of the concept of viewing substitutions as molecular switches for

lectin-binding affinity. Moreover, they have potential relevance for glyco-

engineering of reagents in tumor imaging.

Abbreviations

CHO, Chinese hamster ovary; GlcNAc, N-acetylglucosamine; GlcNAc-TVII, N-acetylglucosaminyltransferase VII; LacNAc, N-acetyllactosamine;

LCA, Lens culinaris agglutinin; PSA, Pisum sativum agglutinin; TFA, trifluoroacetic acid.

1986 FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS

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notion that the glycan’s intimate interplay with specific

lectins can be central to realization of its functional

potential [6–8]. Because glycans differing in structure

by specific substitutions might react with cell lectins in

a characteristic manner, a route of translating struc-

tural differences into effects, e.g. leading to altered cell

adhesion, growth control or endocytic uptake, is envi-

sioned. Fittingly, the complexity of glycan structures is

matched by expression of lectin families [5,9–11]. Our

aim, in essence, is to systematically measure ligand

properties of N-glycans with different substitutions. In

so doing we combine the emerging technology for

chemoenzymatic tailoring of complex-type N-glycans

on a preparative scale with biochemical ⁄ cell biologicalmethods, starting with the preparation of neoglycopro-

teins with a homogeneous sugar part as suitable test

substances. Our initial studies with complex-type bian-

tennary N-glycans bearing either a bisecting N-acetyl-

glucosamine (GlcNAc) or a core-fucose (Fuc) residue

have lent support to the validity of our hypothesis

[12–14]. These two substitutions act like switches on lig-

and properties. With these data in hand, the description

of the naturally occurring core-fucosylated N-glycan

containing an additional b1,2-linked GlcNAc moiety

attached to the central Man residue (LEC14) ([15,16];

for glycan structures see lower part of Fig. 1) promp-

ted us to take the next step in our program by examin-

ing its properties as ligand.

The approach to track down the LEC14 N-glycan

variant actually exploited an impact of N-glycan sub-

stitutions on lectin binding. Development of resistance

to two agglutinins with dual affinity to the core-fucose

unit and the trimannosyl core region, i.e. Pisum sati-

vum and Lens culinaris agglutinins (PSA, LCA) [17],

led to the selection of the LEC14 mutant of Chinese

hamster ovary cells (CHO) [15,16]. Besides substantial

branching in poly(N-acetyllactosamine) chains implied

by the occurrence of 3,4-disubstituted GlcNAc in

methylation analysis the presence of a b1,2-linked Glc-

NAc moiety attached to the central Man residue in the

trimannoside core was defined [18]. It apparently per-

turbs binding to PSA ⁄LCA, the likely cause for the

enhanced lectin resistance, and contributes to the com-

plex changes in the glycoproteomic profile of the

LEC14 mutant vs. wild-type cells [18]. The enzymatic

introduction of the b1,2-linked GlcNAc moiety into

the biantennary N-glycan by N-acetylglucosaminyl-

transferase VII (GlcNAc-TVII) depends critically on

the presence of the core-fucose unit so that the LEC14

type of glycan will invariably harbor two core substitu-

tions [19]. An immediate question concerns the possi-

bility of further processing.

While the bisecting GlcNAc residue in b1,4-linkageto the central mannose unit has only been described as

an acceptor for chain elongation in glycans of Mgat2-

null mice [20,21], the question on branch elongation of

the LEC14-specific b1,2-linked GlcNAc could initially

not be answered unequivocally. The extensive treat-

ment of glycopeptides by b-galactosidase and N-acetyl-

b-d-glucosaminidase to trim the glycan antennae prior

to structural analysis of the core may well have also

impaired this branch elongation [18]. This situation

was resolved by total synthesis of a complete LEC14

N-glycan. The availability of material derived from

chemical synthesis not only unambiguously confirmed

this particular core structure but also provided suffi-

cient quantities for glycosyltransferase assays [22].

Galactosyltransferase was found to elongate the unu-

sual b1,2-linked GlcNAc residue ([23]; for structure of

the glycan with the new branch see Fig. 1). In accord-

ance with the high level of resistance to glycosidase

treatment of this GlcNAc residue (an indication for

rather poor spatial accessibility compared with the

GlcNAc moieties in the a1,3- and a1,6-arms) its reac-

tivity towards galactosyltransferase was lower than for

terminal GlcNAc residues in the antennae [23]. Thus,

the demonstration of substrate properties of the

LEC14 core for galactosylation suggests, but does not

prove the presence of this type of triantennary N-gly-

can in the complex profile observed for CHO cell

glycans. Its occurrence might account for enhanced

glycopeptide binding to immobilized Ricinus communis

agglutinin I relative to wild-type glycans [18]. Conse-

quently, we addressed the ensuing question whether

the addition of an N-acetyllactosamine (LacNAc) unit

to a core-fucosylated biantennary N-glycan at the cen-

tral Man residue in b1,2-linkage will alter the ligand

properties especially for endogenous receptors. Because

chemical conjugation of the synthetic oligosaccharides

to a carrier protein is feasible for a spacered N-glycan

[23], we prepared a neoglycoprotein from an extended

LEC14 N-glycan after suitable linker design. The lig-

and properties of the resulting neoglycoprotein (A)

were tested in three different systems: (a) tested as lig-

and immobilized to a plastic surface with five sugar re-

ceptors, among them growth-regulatory galectins [24];

(b) tested as ligand for surface receptors of different

types of tumor cells; and (c) injected into circulation

with monitoring of the time course of biodistribution.

The data set on the neoglycoprotein prepared from a

core-fucosylated N-glycan devoid of the b1,2-substitu-tion (C) and tested previously under identical condi-

tions [13] allowed direct comparison to pinpoint any

detectable influence of the new b1,2-branch.

S. Andre et al. Extended LEC14-type N-glycan as lectin ligand

FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS 1987

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Extended LEC14-type N-glycan as lectin ligand S. Andre et al.

1988 FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS

Page 4: Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan : Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein

Results

Synthetic background

We have set ourselves the task of systematically deter-

mining ligand properties of N-glycans, here the per-

galactosylated LEC14 dodecasaccharide (Fig. 1,

compound 7), by combining chemoenzymatic synthesis

with biochemical ⁄ cell biological techniques. In order to

imitate the natural presentation of N-glycans on a gly-

coprotein, conjugation of the synthetic product to the

carrier protein, which is otherwise free of ligand prop-

erties, was necessary. Toward this end we started from

the protected nonasaccharide 1 [22] to reach the per-

galactosylated LEC14 epitope [23] as strategically

outlined in Fig. 1. The resulting spacered dodecasac-

charide was conjugated to BSA after the spacer’s

amino function was activated by reaction with thio-

phosgene to its isothiocyanate. N-glycan attachment to

the carrier protein was visualized by gel electrophoretic

analysis revealing the N-glycan-dependent shift of the

molecular mass (lower part of Fig. 1). The protein was

turned totally into a glycan carrier, because no staining

was visible at the position of unsubstituted albumin.

To determine the incorporation yield, additional ana-

lytical procedures were performed. MS gave evidence

for a spectrum of neoglycoproteins with one to four

attached N-glycan chains, and the colorimetric assay

determined an average of 3.2 N-glycans per carrier

molecule [23]. Of note for the intended comparison to

the other so far tested complex-type biantennary

N-glycans without ⁄with substitutions, the linker design

could thus be kept constant. Even more important, the

incorporation yield of this reaction was only slightly

lower than for the N-glycan with a bisecting GlcNAc

moiety (B) or the unsubstituted form (D) at 3.6 N-gly-

cans per carrier protein and the core-fucosylated

N-glycan at 3.9 oligosaccharide chains (C) [12–14].

This result, ensuring rather similar glycan density, was

the prerequisite to proceed to testing the ligand prop-

erties of the extended LEC14 dodecasaccharide using

neoglycoprotein A in three different assay systems

with: (a) purified sugar receptors; (b) tumor cell surfa-

ces in vitro; and (c) organ lectins in vivo.

Affinity to purified lectins/antibodies

The first assay system was designed to simulate proper-

ties of glycoproteins presented on a cell surface by

adsorbing the neoglycoprotein to the plastic surface of

microtiter plate wells. The homogeneity of the structure

of the synthetic N-glycan, rigorously controlled by our

analytical procedures (see Experimental procedures)

and definitively excluding the presence of contaminating

glycoforms, will account for a clear-cut correlation

between structure and ligand properties. Also, the assay

deliberately avoided surface immobilization of the car-

bohydrate-binding proteins, which might affect their

binding properties. Under these conditions, carbohy-

drate-dependent and saturable binding of toxic mistletoe

lectin, the growth ⁄ invasion-regulatory galectin-1 and

the natural autoantibody was invariably detected

(Fig. 2). The calculated Scatchard plots gave straight

lines in all cases, evidence for a single class of binding

sites. Although the different sugar receptors home in on

the same basic unit, i.e. terminal galactose, their affinity

is clearly disparate (Fig. 2A–C). The IgG subfraction

bound with the highest affinity, followed by the plant

agglutinin with two binding sites per B-subunit in the

(AB)2-tetramer and the homodimeric endogenous lectin

with its two binding sites at opposing sides of the pro-

tein. Galectins afford the opportunity to further exam-

ine the relationship between the spatial presentation of

carbohydrate recognition domains and ligand affinity.

We thus tested two further members of the galectin fam-

ily, i.e. galectins-3 and -5. These two monomeric pro-

teins share ligand specificity with galectin-1 but not its

homodimeric cross-linking design. Due to their mono-

meric constitution in solution no affinity enhancement

by ligand clustering through a bivalent module is expec-

ted. Indeed, these two lectins were inferior in terms of

binding affinity to galectin-1, their KD values at

820 ± 71 nm (galectin-3) and 734 ± 157 nm (galectin-

5; Fig. 2D) with about three to fivefold increases in Bmax

Fig. 1. Chemical and enzymatic steps to produce the LEC14-type N-glycan with the LacNAc branch in b1,2-linkage at the central Man unit

starting from the protected nonasaccharide 1 [34,35]. (a) (NH4)2Ce(NO3)6, CH3CN-H2O, 80 �C (71%); (b) 1 Ethylenediamine, n-BuOH, 80 �C;

2. Ac2O, pyridine; 3 MeNH2 (41%) in H2O (96% for steps 1–3); (c) 1 Propanedithiol, NEt3, MeOH; 2 N-carbobenzoxy-6-amino hexanoic acid

4, TBTU, HOBt, N-methylpyrrolidone (31% for steps 1–2 after RP-HPLC); (d) PdO-H2O ⁄H2, MeOH-AcOH (95%); (e) UDP-Gal (4 eq.), b1,4-ga-

lactosyltransferase, alkaline phosphatase (75%); (f) 1 Thiophosgene, CH2Cl2-H2O, NaHCO3; 2 BSA, H2O, NaHCO3; 6 days. The last two

schemes for N-glycan sequences allow structural comparison between the N-glycan of neoglycoprotein A and the previously studied N-gly-

cans in neoglycoproteins B–D (upper panel). Gel documentation is added in the bottom panel for visualization of the gel electrophoretic

mobility of the carbohydrate-free carrier protein BSA (lanes a, c, e; 0.15 lgÆlane)1) relative to that of the neoglycoprotein A (lanes b, d, f;

0.2 lgÆper lane, 0.15 lgÆper lane and 0.1 lgÆper lane, respectively). Positions of two marker proteins for molecular mass designation (in kDa)

are indicated by arrowheads.

S. Andre et al. Extended LEC14-type N-glycan as lectin ligand

FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS 1989

Page 5: Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan : Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein

values. These results confirm a striking effect of receptor

topology at constant spatial features of the ligand. They

also enable us to set the ligand properties of the exten-

ded LEC14 dodecasaccharide in relation to the so far

studied N-glycans, especially the N-glycans without any

substitution or with core-fucosylation.

The presence of the new glycan branch in the exten-

ded LEC14 neoglycoprotein appeared to enhance

affinities toward human proteins and reduce affinity

toward the plant lectin relative to the properties of the

complex-type biantennary N-glycan lacking a substitu-

tion. However, the strict dependence of GlcNAc-TVII

activity on core-fucosylation in its substrate [19], and

therefore the presence of this substitution in the

LEC14 epitope, must not be neglected. The ensuing

comparison between the properties of the LEC14

dodecasaccharide epitope and the core-fucosylated

decasaccharide clearly revealed that the b1,2-linkedglycan branch did not significantly affect affinity to the

three tested types of sugar receptor in this assay sys-

tem. In contrast, the consideration of the data for the

N-glycan with bisecting GlcNAc (B) with reductions in

affinity underscores the sensitivity of this parameter to

other structural alterations in the N-glycan. Because

the nature of the carbohydrate-binding protein matters

notably in this respect, it is mandatory to proceed to

test the pergalactosylated LEC14 N-glycan against a

complex panel of binding partners. In order to extend

mapping the ligand properties of the LEC14 dodeca-

saccharide, we thus moved in our analysis from a test

system with purified sugar receptors to cell surfaces

with an array of lectins. To add potential clinical rele-

vance we selected tumor cells of different histogenesis

representing common cancer types. In the same way as

isolated lectins these established cell models also offer

the advantage for comparative analysis when ade-

quately controlled for constant surface properties.

Affinity to tumor cell surfaces

The first step in the cytofluorimetric analysis was dedi-

cated to documenting the carbohydrate-dependent and

saturable binding of the labeled neoglycoprotein to cell

surfaces (Fig. 3). Biotinylated carrier protein without

the N-glycan failed to produce a signal above back-

ground, excluding interactions by the protein part or

its label. Mimicking the situation when a glycoprotein

encounters a cell surface, the neoglycoprotein A reacted

with cell surfaces in a cell type-specific manner

(Fig. 4). As highlighted by these results, distinct pre-

ference of binding was determined for the B- and

T-lymphoblastoid cells among the set of leukemia ⁄lymphoma lines and to the mammary carcinoma cells

among the carcinoma lines, when measuring staining

Fig. 2. Illustration of Scatchard plot analysis of carbohydrate-dependent interaction between the carrier-immobilized N-glycan (A) and the

mistletoe lectin (VAA; A), human galectin-1 (B), the lactoside-binding IgG subfraction (C) and rat galectin-5 (D) in a representative experimen-

tal series. KD values (mean ± SD) are given for the complete set of analytical data with at least four different experimental series for each

sugar receptor. The extent of total binding (s) was reduced by that of binding which was not inhibitable by glycoinhibitors (h, 75 mM lactose

and 1 mg asialofetuinÆmL)1) to calculate the level of carbohydrate-dependent binding (n) (see inset).

Extended LEC14-type N-glycan as lectin ligand S. Andre et al.

1990 FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS

Page 6: Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan : Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein

intensity. Extending the data of the solid-phase assay,

the LEC14 dodecasaccharide is a ligand for cellular

lectins. To give potential reason to the presence of the

GlcNAc-TVII, as detected in the LEC14 mutant [19],

the comparison between binding data of this neoglyco-

protein (A) and that presenting the core-fucosylated

N-glycan (C) without the b1,2-branch is expedient.

In general, comparison to the other, so far tested

100 101 102 103 104 100 101 102 103 104

100

012

8

num

ber

of e

vent

s

012

80

128

012

8

101 102 103 104 100 101 102 103 104

A B

C D

Fig. 3. Semilogarithmic representation of

the fluorescent surface staining of cells of

the human T-lymphoblastoid line CCRF-CEM

in the absence of incubation with the biotin-

ylated neoglycoprotein (negative control;

shaded) and after incubation with increasing

concentrations of neoglycoprotein in two

steps: up to 2 lgÆmL)1 (lines with

0.5 lgÆmL)1, 1 lgÆmL)1 and 2 lgÆmL)1 from

left to right); (A) and up to 50 lgÆmL)1 (lines

with 2 lgÆmL)1, 5 lgÆmL)1, 10 lgÆmL)1,

25 lgÆmL)1 and 50 lgÆmL)1 from left to

right); (B). Controls with an incubation step

using biotinylated carrier protein

(25 lgÆmL)1); (C) instead of the neoglyco-

protein and an inhibition of staining using

glycoinhibitors (75 mM lactose and 1 mg

asialofetuinÆmL)1); (D) document lack of

label ⁄ carrier protein-dependent binding and

the carbohydrate dependence of binding.

num

ber

of e

vent

s

012

8

100 101 102 103 104 012

8

100 101 102 103 104 012

8

100 101 102 103 104 012

8

100 101 102 103 104

012

8

100 101 102 103 104

70.3 %/252.2 98.2 %/177.8 16.3 %/15.3 55.0 %/45.1

64.4 %/62.388.4 %/56.174.4 %/29.483.3 %/420.6

012

8

100 101 102 103 104 012

8

100 101 102 103 104 012

8

100 101 102 103 104

A B C D

E F G H

Fig. 4. Semilogarithmic representation of the binding of the fluorescent indicator (streptavidin ⁄R-phycoerythrin conjugate) in the absence of

the probe during processing (negative control; shaded) and after the incubation step with the biotinylated neoglycoprotein (25 lgÆmL)1; black

line) for the B-lymphoblastoid line Croco II (A), the T-lymphoblastoid line CCRF-CEM (B), the erythroleukemia line K-562 (C), the acute myelo-

genous leukemia line KG-1 (D), the mammary carcinoma line DU4475 (E) and the colon adenocarcinoma lines C205 (F), SW480 (G) and

SW620 (H). Quantitative data on percentage of positive cells (%) and mean channel fluorescence are given in each panel.

S. Andre et al. Extended LEC14-type N-glycan as lectin ligand

FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS 1991

Page 7: Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan : Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein

carrier-immobilized N-glycans revealed rather favora-

ble ligand properties (Fig. 5). A tendency for enhanced

binding relative to the core-fucosylated N-glycan indi-

cated that the new branch is not an inert modification

at this level of testing. The decrease of cell positivity

for KG-1 cells and rather constant results for K-562

cells can be judged as internal controls for cell-type

specificity in the direct comparison to the core-fucosyl-

ated N-glycan. Thus, this assay system revealed several

cases with an improvement of ligand properties with

cell type-dependent characteristics. Because the N-gly-

can profile of glycoproteins not only governs cell

surface binding in vitro but also serum survival in

circulation, a parameter of interest for prolonging

bioavailability of pharmaproteins, we next tested the

influence of this N-glycan in biodistribution analysis

in vivo.

Biodistribution in vivo

Organ retention and blood content of the iodinated

neoglycoprotein (A) were monitored after intravenous

injection. Six time points from 15 min to 12 h were set

to determine the time course of these parameters. Hep-

atic uptake was rapid and the major route of blood

clearance (Table 1). When comparing blood ⁄organretention of this neoglycoprotein for the four major

sites to the cases of the so far tested N-glycans (B–D),

blood clearance was best for the neoglycoprotein bear-

ing the LEC14 dodecasaccharide (A) (Fig. 6). We have

deliberately run these experiments in tumor-bearing

mice to look at tumor uptake relative to blood back-

ground, a factor with impact on sensitivity of tumor

imaging. The tumor ⁄blood ratio after 1 h was 0.7 for

neoglycoprotein (A) but 0.53 for C bearing a core-

fucosylated N-glycan with 3.11 ± 0.17% in blood and

1.65 ± 0.06% in the tumor. The detected difference

adds to the evidence for modulation of ligand proper-

ties by the extended LEC14 motif. Placing this glycan

at best position in this respect, the ratio for the unsub-

stituted nonasaccharide N-glycan (D) was 0.38 (tumor:

1.16 ± 0.09; blood: 3.07 ± 0.06%) and 0.47 (tumor:

1.47 ± 0.09; blood: 3.11 ± 0.14%) for the decasac-

charide with the bisecting GlcNAc moiety (B). Regard-

ing the individual organ sites no major alteration of

uptake and retention after 1 h was detectable except

for the N-glycan with bisecting GlcNAc (Fig. 6).

Discussion

The basic complex-type biantennary N-glycan is sub-

ject to enzymatic substitutions. Structural aspects have

been mostly clarified but the functional significance of

their presence is still a matter of debate. Our hypothe-

sis interprets occurrence of substitutions as a means to

modulate ligand properties in interactions with endo-

genous lectins. The versatile potential for fine-tuning a

respective information transfer would then clearly out-

weigh the required investment in coding for the diver-

sity of glycosyltransferases and regulation of their

activity. In other words, glycosyltransferase activities

produce lectin-binding epitopes. By virtue of adding

substitutions, which may not even directly participate

in binding, they might also affect the affinity of the

binding sites. To demonstrate that a structural alter-

ation in an N-glycan changes its binding parameters

requires experimental evidence difficult to collect with

natural glycoproteins. They generally present more

than one type of glycan chain and exhibit microhetero-

geneity, confounding efforts to establish a direct struc-

ture ⁄activity profile. To address this issue, we turned

Fig. 5. Comparison of the percentage of positive cells (upper panel)

and mean channel fluorescence (bottom panel) in flow cytofluori-

metric analysis using the LEC14-dodecasaccharide-bearing neogly-

coprotein A and neoglycoproteins with complex-type biantennary

N-glycan ligand parts substituted by bisecting GlcNAc (B) or core-

fucosylation (C) or without any substitution (D) (see Fig. 1 for struc-

tural comparison). Data for neoglycoproteins B, C, D have

previously been published [12–14] and are shown for comparison.

The standard deviations within experimental series are generally

below 7.5%.

Extended LEC14-type N-glycan as lectin ligand S. Andre et al.

1992 FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS

Page 8: Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan : Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein

to the synthesis of neoglycoproteins. In contrast to free

N-glycans they harbor a homogeneous sugar part and

favorably maintain a local density akin to natural gly-

coproteins. In fact, affinity to galectins is sensitive to

changes in local density of glycan chains and improved

by certain modes of clustering [25–27].

As stated above, it is our aim to delineate struc-

ture ⁄ activity profiles for N-glycans. Toward this end, we

have so far tested three types of biantennary N-glycans

in different assay systems [12–14]. They were confronted

with different situations in vitro and in vivo, i.e. the N-

glycan as ligand in a matrix simulating a cell membrane

or in solution ⁄ serum confronted with lectin-presenting

cell surfaces. Evidently, monitoring tumor cells and bio-

distribution has relevance to the glycan’s suitability for

drug targeting or imaging [28–30]. Our previous results

with the neoglycoprotein probes, which were kept rather

constant in all relevant features (nature of carrier pro-

tein, linker chemistry, yield of glycan incorporation),

supported the hypothesis given above [12–14]. In this

report, we examined the impact of the pergalactosylated

LEC14 motif, a b1,2-linked LacNAc disaccharide emer-

ging from the central bMan unit of a core-fucosylated

N-glycan [7] (Fig. 1). The bMan moiety of a

LEC14 N-glycan is substituted in positions 2, 3 and 6, a

remarkable illustration of the capacity of glycan units to

engender branching, in contrast with amino acids and

nucleotides. As outlined in the introduction, a dis-

tinct N-acetylglucosaminyltransferase (GlcNAc-TVII) is

responsible for the introduction of the b1,2-GlcNAc

into mammalian N-glycans [19]. Of note, a b1,2-substi-tution at this site of the core is also found in nonmam-

malian N-glycan chains, here with xylose as added sugar

unit [31–33]. This position is thus apparently predis-

posed for enzymatic modification. Immunologically, this

residue is relevant due to its allergenic activity, an indi-

cation for accessibility to interactions with immunoglo-

bulin E [33]. Likewise, the b1,2-GlcNAc residue at this

position of a mammalian-type core-fucosylated N-gly-

can is a contact point, as shown by its acceptor capacity

in enzymatic galactosylation [23]. Moreover, the selec-

tion process to isolate the LEC14 mutant cells exploited

the detrimental effect of this core substitution on an

interaction with a receptor protein, i.e. reduction of

binding of the plant agglutinins PSA ⁄LCA [15,16].

Our results with purified lectins ⁄ antibodies reveal

no major influence of this enzymatically elongated

branch on ligand properties, when compared with

the core-fucosylated N-glycan lacking this branch.

Table 1. Biodistribution of LEC14-dodecasaccharide-bearing neoglycoprotein A in Ehrlich-solid-tumor-bearing mice (% injected doseÆg)1

tissue). Each value indicates the mean ± SD for four to five mice.

Time (h) 1 ⁄ 4 h 1 ⁄ 2 h 1 h 3 h 6 h 12 h

Blood 5.15 ± 0.36 4.49 ± 0.22 2.19 ± 0.33 0.92 ± 0.13 0.62 ± 0.06 0.45 ± 0.10

Liver 34.45 ± 4.49 6.51 ± 0.74 2.52 ± 0.14 1.29 ± 0.17 0.95 ± 0.07 0.71 ± 0.03

Kidneys 1.53 ± 0.23 4.60 ± 0.56 2.96 ± 0.64 1.45 ± 0.33 0.81 ± 0.28 0.51 ± 0.22

Spleen 0.73 ± 0.10 2.37 ± 0.27 1.23 ± 0.20 0.58 ± 0.10 0.33 ± 0.04 0.25 ± 0.05

Heart 0.24 ± 0.02 1.50 ± 0.11 0.69 ± 0.13 0.29 ± 0.04 0.18 ± 0.02 0.15 ± 0.01

Lungs 0.55 ± 0.07 1.99 ± 0.14 1.10 ± 0.17 0.48 ± 0.09 0.38 ± 0.08 0.26 ± 0.01

Thymus 0.08 ± 0.01 1.71 ± 0.14 1.01 ± 0.16 0.47 ± 0.16 0.23 ± 0.07 0.27 ± 0.12

Pancreas 0.36 ± 0.07 1.87 ± 0.21 1.05 ± 0.20 0.40 ± 0.06 0.20 ± 0.04 0.17 ± 0.12

Lymph node 0.09 ± 0.02 1.25 ± 0.12 0.69 ± 0.13 0.38 ± 0.09 0.18 ± 0.07 0.19 ± 0.03

Muscle 0.28 ± 0.02 0.59 ± 0.06 0.36 ± 0.04 0.16 ± 0.04 0.11 ± 0.01 0.07 ± 0.01

Brain 0.07 ± 0.01 0.18 ± 0.02 0.10 ± 0.01 0.04 ± 0.01 0.02 ± 0.01 0.03 ± 0.00

Vertebrae 0.21 ± 0.03 1.04 ± 0.09 0.67 ± 0.15 0.32 ± 0.10 0.16 ± 0.03 0.16 ± 0.03

Tumor 0.36 ± 0.02 1.97 ± 0.22 1.54 ± 0.21 0.58 ± 0.07 0.37 ± 0.08 0.31 ± 0.02

Fig. 6. Comparison of aspects of the biodistribution patterns of iodi-

nated neoglycoproteins with the following complex-type N-glycan

ligand parts 1 h afer injection: LEC14 dodecasaccharide (A) and

complex-type biantennary N-glycans substituted by bisecting Glc-

NAc (B) or core-fucosylation (C) or without any substitution (D) (see

Fig. 1 for structural comparison). Data for neoglycoproteins B, C, D

have previously been published [12–14] and are shown for compar-

ison. The range of the standard deviation shown for each result by

bars was between 1.95 and 21.6%.

S. Andre et al. Extended LEC14-type N-glycan as lectin ligand

FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS 1993

Page 9: Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan : Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein

Looking at the growth ⁄ invasion-regulatory galectin-1,

it might be that the necessary contact to the subter-

minal GlcNAc residue during binding, a factor contri-

buting to ligand selection [34], is spatially hindered.

No affinity enhancement relative to the core-fucosyl-

ated decasaccharide was detectable. The affinity of

binding of the monomeric galectins-3 and -5 was con-

siderably reduced, arguing in favor of an influence of

the strong cross-linking activity of galectin-1 as a clue

for the functional divergence noted in a tumor cell

system [35,36]. No indication for positive cooperati-

vity of galectin-3 binding was observed. This binding

mode was operative with laminin as substratum for

this generally monomeric lectin which can form a

small extent of pentamer in solution [37,38]. When

testing cell surfaces with their full array of carbohy-

drate-binding proteins, a clear impact of presence of

the new branch was determined. This effect hinged

on the cell type, preferentially leading to increased

binding relative to the core-fucosylated decasaccharide

as ligand. In addition to its principal value to delin-

eate evidence for a structure ⁄ activity correlation this

result signifies that cell-presented lectins in most of

these tumor lines do not share the core specificity

with the plant agglutinins PSA ⁄LCA which would

have been impaired by introducing the b1,2-branch.Our result underscores differences between plant and

mammalian lectins and recommends using endo-

genous lectins for functional glycoproteomic profiling

of clinical samples [39].

The evidence for a contribution of this b1,2-branchin the LEC14-type dodecasaccharide to overall ligand

properties was supported by the biodistribution analy-

sis, revealing rapid clearance elicited by pergalactosyl-

ated LEC14 epitope. In contrast to galectins the

C-type endocytic receptor of hepatocytes accommo-

dates galactose as central contact point [40]. This result

can be relevant for an application. Actually, tailoring

of the glycan part of pharmaproteins (glycoengineer-

ing) has become a fertile field of research in order to

manipulate cellular uptake and serum half-life [41–47].

The measured rapid clearance of the respective neogly-

coprotein bearing a b1,2-branch constituted by a Lac-

NAc disaccharide can be advantageous when using an

iodinated glycoprotein for imaging, as it lowers the

background. The detection of this property immedi-

ately raises the question of how this parameter will be

altered when the b1,2-branch is shifted away from the

central Man unit to the Man residues in the branch

extensions by GlcNAc-TIV or GlcNAc-TV. Indeed,

the consequences of hereby generating the two natural

versions of triantennary N-glycans as part of neoglyco-

proteins have not yet been rigorously determined using

our panel of assays. Thus, it is our next challenge to

address this issue.

Experimental procedures

Synthetic and analytical procedures

NMR spectra were recorded on a Bruker AMX 500 spectro-

meter (Karlsruhe, Germany). HPLC separations were per-

formed on a Pharmacia LKB gradient system 2249 equipped

with a Pharmacia LKB Detector VWM 2141 (Freiburg,

Germany). For size exclusion chromatography a Pharmacia

Hi Load Superdex 30 column (600 · 16 mm) was used,

RP-HPLC was performed on a Macherey-Nagel Nucleogel

RP 100–10 column (Duren, Germany, 300 · 25 mm).

Carbohydrate-free BSA and bovine b1,4-galactosyltrans-ferase were purchased from Sigma (Munich, Germany), alka-

line phosphatase (calf intestine, molecular biology grade)

from Roche Diagnostics (Heidelberg, Germany). UDP-

galactose was a generous donation from Roche Diagnostics.

ESI-TOF mass spectra were recorded with methanol ⁄wateras solvent on a Micromass LCT spectrometer connected to

an Agilent HP 1100 HPLC apparatus. MALDI-TOF mass

spectra were recorded on a Bruker Reflex III using linear

mode and an acceleration voltage of 20 kV. For sample

preparation in MALDI-TOF-MS a solution of the neogly-

coprotein (1 lL, 7 mgÆmL)1) in 0.1% (v ⁄ v) trifluoroacetic

acid (TFA) was mixed with 1.5 lL of 33% acetonitrile in

0.1% (v ⁄ v) TFA and 2.5 lL of a saturated solution of

sinapinic acid in 0.1% (v ⁄ v) TFA and dried in high vacuum.

The structures of the synthetic N-glycans were routinely

confirmed by the following 2D-NMR-experiments: TOCSY,

NOESY, HMQC, HMQC-COSY, HMQC-DEPT, and

HMQC-TOCSY. Signals of NMR spectra were assigned

according to the following convention including designation

of spacer atoms illustrated for compound 7 in Fig. 1.

Preparation of neoglycoprotein A

For conjugation of the derivatized dodecasaccharide to the

carrier protein the amino group was transformed into its

isothiocyanate. In a 1.5 mL plastic vial 6-aminohexanoyl-

N-glycan 7 (0.77 mg, 0.34 lmol) was dissolved in sodium

hydrogencarbonate (200 lL, 10 mgÆmL)1) followed by addi-

tion of dichloromethane (200 lL) and thiophosgene (5 lL,19.7 lmol). The biphasic mixture was vigorously stirred.

After 4 h (TLC: isopropanol ⁄ 1 m ammonium acetate, 2 : 1)

the mixture was centrifuged and the aqueous phase was sep-

arated. Subsequently, the organic phase was extracted twice

with sodium hydrogencarbonate (100 lL, 10 mgÆmL)1). The

combined aqueous phases were extracted twice with dichlo-

romethane (500 lL). Carbohydrate-free BSA (2 mg) was dis-

solved in the aqueous solution of the isothiocyanate, and the

reaction vial was kept for 6 days at ambient temperature.

Extended LEC14-type N-glycan as lectin ligand S. Andre et al.

1994 FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS

Page 10: Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan : Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein

The reaction mixture was centrifuged, and the clear superna-

tant was fractionated by gel filtration (Pharmacia Hi Load

Superdex 30 (600 · 16 mm); eluent: 0.1 m ammonium hydro-

gencarbonate; flow rate 0.75 mLÆmin)1; detection: 214 and

254 nm). Further quality controls were performed by gel

electrophoretic analysis and colorimetric determination of

the average glycan content as described previously [12–14]:

yield, 2.07 mg; Rf amine 7 ¼ 0.14 (i-propanol ⁄ 1 m ammo-

nium acetate, 2 : 1); Rf isothiocyanate ¼ 0.50 (i-propanol ⁄1 m ammonium acetate, 2 : 1). MALDI-MS: Mcalcd ¼ 68694,

70957, 73220, 75484 (1, 2, 3, 4 N-glycans per BSA molecule);

Mfound ¼ 68649, 70971, 73242, 75475 (1, 2, 3, 4 N-glycans

per BSA molecule).

The neoglycoprotein A was then used in solid-phase and

cell-binding assays either free of label or for labeling with

the N-hydroxysuccinimide ester derivative of biotin under

conditions identical to the preparation of the other N-gly-

can-bearing probes tested previously [12–14].

Solid-phase assay

The matrix for the assay was established by adsorption of

neoglycoprotein to the surface of plastic microtiter plate

wells under conditions used previously [12–14]. Controls for

standardizing coating density were performed with biotinyl-

ated neoglycoprotein using streptavidin–peroxidase conju-

gate as indicator. Ligand properties of the N-glycan were

probed with different types of carbohydrate-binding pro-

teins. The galactoside-specific agglutinin from mistletoe

(Viscum album L. agglutinin, VAA), human galectin-1,

murine galectin-3 and rat galectin-5 as well as the immuno-

globulin G subfraction with preferential affinity to b-gal-actosides from human serum were isolated and checked for

purity and quaternary structure by gel electrophoresis and

filtration, electrospray ionization MS, ultracentrifugation

and haemagglutination [48–54]. Biotinylation was carried

out under activity-preserving conditions, and label incor-

poration was assessed by binding assays with streptavidin–

peroxidase conjugate or a proteomics protocol [48,55].

Binding studies of the sugar receptors to the glycan-present-

ing matrix were performed by stepwise increases of probe

concentration up to saturation with duplicates at each con-

centration and at least four independent series including

controls to determine extent of carbohydrate-dependent

binding by its inhibition using a mixture of 75 mm lactose

and 1 mg asialofetuinÆmL)1, and the data sets were algebra-

ically transformed to obtain KD values and the number of

bound sugar receptor molecules at saturation (Bmax), fol-

lowing the protocol of our previous reports on neoglyco-

proteins with synthetic N-glycans [12–14].

Cell-binding assay

Using the biotinylated neoglycoprotein as probe, automated

flow cytofluorimetric analysis of carbohydrate-dependent

cell surface binding was performed with the following

human tumor lines: Croco II (B-lymphoblastoid cell line),

CCRF-CEM (T-lymphoblastoid cell line), K-562 (erythro-

leukemia cell line), KG-1 (acute myelogenous leukemia cell

line), DU4475 (mammary carcinoma cell line) as well as

C205, SW480 and SW620 (colon adenocarcinoma cell

lines). Except for the Croco II line established in our labor-

atory [56] the cells were commercially available (American

Type Culture Collection, Rockville, MD, USA) and rou-

tinely cultured under the recommended conditions. The

adherent colon carcinoma cells were detached by exposing

them to NaCl ⁄Pi containing 2 mm EDTA. Prior to the ana-

lysis cells were routinely washed carefully with Dulbeccos’s

NaCl ⁄Pi solution containing 0.1% (w ⁄ v) carbohydrate-free

BSA to remove any inhibitory serum glycoproteins and to

saturate nonspecific protein-binding sites. For this purpose,

an incubation step with ligand-free carrier protein for

30 min at 4 �C was added prior to the incubation with the

labeled neoglycoprotein at this temperature to minimize

uptake by endocytosis. Carbohydrate-dependent binding of

the neoglycoprotein (25 lgÆmL)1) to the cells (8 · 106

cellsÆmL)1) was assessed in a FACScan instrument (Becton-

Dickinson, Heidelberg, Germany) with the fluorescent

indicator conjugate streptavidin ⁄R-phycoerythrin (1 : 40;

Sigma). Controls to assess carbohydrate-independent bind-

ing of the carrier via its protein part or label and to docu-

ment sugar inhibition were run in each series, as previously

described [12–14].

Analysis of in vivo biodistribution

Radiolabeling of the neoglycoprotein was performed by the

chloramine-T method [57]. Briefly, fresh chloramine-T and

sodium metabisulfite solutions were prepared, and the neo-

glycoprotein was dissolved in NaCl ⁄Pi (pH 7.2) at a con-

centration of 1 mg proteinÆmL)1. A 10 lL portion of125I-labelled NaI (74 MBqÆmL)1 NaCl ⁄Pi) solution was

added to 50 lL of the neoglycoprotein-containing solution,

subsequently 10 lL of chloramine-T (3 mgÆmL)1 H2O) solu-

tion were added, and the mixture was incubated at room

temperature for 3 min. Thereafter, chloramine-T solution

was pipetted to the above mixture in two further portions at

intervals of 3 min, and then the reaction was stopped by add-

ing 30 lL of freshly prepared sodium metabisulfite solution

(5 mgÆmL)1). Label-free neoglycoprotein (50 lg) was added

as a carrier prior to the separation step by Sephadex G-50

(Pharmacia Biotech, Freiburg, Germany) gel permeation

chromatography to remove any reagents from the radioiodi-

nated product. The specific radioactivity of batches of125I-labeled neoglycoprotein was in the range between 8 and

10 MBqÆmg)1 protein. To monitor biodistribution of the

iodinated product tumor-bearing mice were used [58,59].

Approximately 5 · 106 Ehrlich ascites tumor (EAT) cells had

been injected subcutaneously into the right rear leg of male

ddY mice of the age of 7 weeks for tumor inoculation. On

S. Andre et al. Extended LEC14-type N-glycan as lectin ligand

FEBS Journal 272 (2005) 1986–1998 ª 2005 FEBS 1995

Page 11: Introduction of extended LEC14-type branching into core-fucosylated biantennary N-glycan : Glycoengineering for enhanced cell binding and serum clearance of the neoglycoprotein

the sixth to eighth day after inoculation, when the tumor had

grown to 0.3–0.6 g in weight, radioiodinated neoglycoprotein

was injected intravenously at a dose of 80–100 kBq (equival-

ent to 10 lg of protein) via the tail vein. Tissues including

blood samples were obtained after the indicated periods,

weighed, and the radioactivity level was assessed with an

Auto Well gamma System (Aloka ARC 300, Tokyo, Japan).

The percentage of injected dose per gram of wet tissue or per

ml of blood was calculated in each case as described previ-

ously [12].

Acknowledgements

We express our gratitude to B. Hofer and L. Mantel

for skillful technical assistance, to Dr S. Namirha for

helpful discussion and to the Deutsche Forschungsge-

meinschaft, the Dr M.-Scheel-Stiftung fur Krebs-

forschung, the Fonds der Deutschen Chemischen

Industrie, Roche Diagnostics and the Mizutani

Foundation for Glycoscience for generous financial

support.

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