Science - USA (2020-08-21)

SCIENCE sciencemag.org

GRAPHIC: JOSHUA BIRD/

SCIENCE

By Wanda Kukulski

H

uman urinary tracts are highly

susceptible to bacterial infections.

Pathogenic bacteria initiate infec-

tions by attaching to sugar chains

(glycans) exposed on the surface of

the urinary tract epithelium ( 1 ). It

has long been suspected that uromodu-

lin (UMOD)—the most abundant protein

in human urine—prevents bacteria from

binding to urinary tract glycans, thus de-

fending the organism from such infections

( 2 ). However, the mechanism underly-

ing this protection has remained

elusive. Now, on page 1005 of this

issue, Weiss et al. reveal, at the

molecular level, how UMOD fila-

ments interact with uropathogenic

Escherichia coli cells in human

urine ( 3 ). These results provide a

structural basis for understanding

the protective function of UMOD.

UMOD forms filaments first visu-

alized by electron microscopy more

than 60 years ago ( 4 ). Despite these

early images, the filaments’ struc-

tural organization is unknown,

which is, in part, why the protective

role of UMOD has eluded scien-

tists. An important hint regarding

UMOD function came from the fact

that myriad glycans decorate the

filaments, possibly presenting bac-

teria with binding opportunities

that compete with glycan receptors

on the urinary tract walls ( 5 ).

Weiss et al. deciphered a compre-

hensive map of the glycosylation

pattern of UMOD, the structure of

UMOD filaments, and the nature

of bacteria–filament interaction.

Infective E. coli cells attach to the

urinary tract epithelium through

needlelike structures called pili. At

their tip, E. coli type I pili consist

of the protein FimH (type 1 fimbrin

D-mannose specific adhesin) ( 6 ).

The authors show that the armlike struc-

tures extending from UMOD filaments in-

teract with FimH. The interaction between

UMOD and FimH is biochemically strong

and likely leads to stable binding. Indeed,

Weiss et al. show that through this bind-

ing, UMOD mediates the stable formation

of clumps of bacteria.

The suggested mechanism of UMOD-

based defense is notably simple and robust:

The abundant UMOD filaments outcom-

pete receptors on the urinary tract walls in

binding to bacterial pili. Each flexible fila-

ment has multiple binding sites, and each

bacterium can have several pili. Therefore,

this multitude of interactions causes bac-

terial aggregation, effectively preventing

individual bacterial cells from attaching to

and infecting the urinary tract. In case of

the E. coli strain studied by Weiss et al.,

the interaction between UMOD and bacte-

rial cells occurs through specific binding of

FimH to a glycan at asparagine 275 of the

UMOD protein (see the figure).

However, UMOD contains several other

complex glycosylation sites whose func-

tions have not yet been dissected. A com-

pelling possibility is that these serve as

binding sites for proteins of other uro-

pathogenic bacteria. In line with this idea,

when Weiss et al. imaged urine from pa-

tients infected with different bacteria,

namely Klebsiella, Pseudomonas, and

Streptococcus, the authors found similarly

aggregated bacterial cells embedded in

UMOD filaments. Given its implication

in various aspects of kidney function ( 7 ),

UMOD might have other molecular roles

that rely on its distinct glycosylation pat-

tern or its adoption of a filamentous struc-

ture, besides protection from bac-

terial infections.

What has enabled this break-

through in understanding of the

association between UMOD and

uropathogenic bacteria? The care-

ful and systematic mass spectrom-

etry data for the glycosylation map

laid the foundation for resolving

this mystery. The key, however,

was the integration of these data

with cryo–electron tomography

(cryo-ET). This electron micros-

copy (EM)–based method allows

one to visualize three-dimensional

architectures of near-natively pre-

served samples at a resolution

high enough to see individual

macromolecules. Cryo-ET can be

applied to samples that are too ir-

regular, large, or heterogenous for

cryo-EM, which allows cryo-ET

to span the range from purified

samples to complex reconstitu-

tions with diverse components and

even undisturbed cellular samples.

Similar to cryo-EM data, cryo-ET

data can be processed by averaging

structures in subvolumes, thereby

further increasing the resolution

( 8 ). Whereas cryo-EM is currently

revolutionizing structural biology

by visualizing protein structures

at atomic resolution ( 9 ), cryo-ET

lags behind in terms of resolution,

although for certain structures a

resolution better than 5 Å can be

achieved (10, 11).

Institute of Biochemistry and Molecular Medicine,

University of Bern, Bühlstrasse 28, 3012 Bern,

Switzerland. Email: [email protected]

Infection

Urinary tract

epithelium

Aggregation

Bacteria entangled

by UMOD

UMOD flament

architecture

FimH–UMOD

interaction

Removal

with urine

Competitive

binding

UMOD

flament

Bacterial

pili

Urinary tract

Asn^275 FimH

UMOD

BIOMEDICINE

A glycoprotein in urine binds

bacteria and blocks infections

Direct imaging of a human fluid illuminates the molecular

basis of urinary tract protection from disease

21 AUGUST 2020 • VOL 369 ISSUE 6506 917

Filaments fight infection

Uromodulin (UMOD) forms filaments that compete with the adhesion

of uropathogens to the urinary tract epithelium. By binding to bacterial

pili, UMOD filaments corral uropathogens, block bacterial adhesion

in the urinary tract, and permit pathogen clearance through urination.

FimH, type 1 fimbrin D-mannose specific adhesin.

Published by AAAS