Science - USA (2020-08-21)

sciencemag.org SCIENCE

INSIGHTS | PERSPECTIVES

A singular asset of cryo-ET, however, is

its ability to seamlessly investigate a struc-

ture across multiple scales of complexity.

The power of this approach is demon-

strated impressively in this study. The

authors used cryo-ET followed by subto-

mogram averaging to determine the archi-

tecture of purified native UMOD filaments

and the interaction region between UMOD

filaments and FimH. They also used cryo-

ET to image the b inding of bacterial cells

to UMOD. The visualization of entangled

bacteria is particularly notable, as it in-

volved direct imaging of unprocessed urine

from patients diagnosed with urinary tract

infections.

Although c ryo-ET continues to provide

unprecedented views of large macromolec-

ular assemblies and cellular architecture

(12, 13), its application to primary samples

of human origin is thus far scarce ( 14 ). The

approach taken by Weiss et al.—to assess

the molecular basis of disease by d irectly

imaging a human fluid—is conceptually

simple, yet represents a milestone by dem-

onstrating the potential of cryo-ET for bio-

medical imaging. Future studies likely will

expand the use of cryo-ET to explore fun-

damental questions on the role of supra-

molecular architecture in human health

and disease. j

REFERENCES AND NOTES

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2. F. Serafini-Cessi, A. Monti, D. Cavallone, Glycoconj. J. 22 ,
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3. G. L. Weiss et al., Science 369 , 1005 (2020).


4. K. R. Porter, I. Tamm, J. Biol. Chem. 212 , 135 (1955).


5. J. Pak, Y. Pu, Z. T. Zhang, D. L. Hasty, X. R. Wu, J. Biol.
   Chem. 276 , 9924 (2001).


6. E. Hahn et al., J. Mol. Biol. 323 , 845 (2002).


7. O. Devuyst, E. Olinger, L. Rampoldi, Nat. Rev. Nephrol. 13 ,
   525 (2017).


8. S. Pfeffer, J. Mahamid, Trends Cell Biol. 21 , 11 (2018).


9. E. Callaway, Nature 582 , 156 (2020).


10. F. K. Schur et al., Science 353 , 506 (2016).


11. A. von Kügelgen et al., Cell 180 , 348 (2020).
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12. M. A. Jordan, D. R. Diener, L. Stepanek, G. Pigino, Nat. Cell
    Biol. 20 , 1250 (2018).


13. A. Al-Amoudi, D. C. Díez, M. J. Betts, A. S. Frangakis,
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ACKNOWLEDGMENTS

W.K. is supported by the Nat ional Centre of Competence in

Research (NCCR) TransCure and the University of Bern.

10.1126/science.abd7124

LIQUID CRYSTALS

When the smallest details count

T he type of liquid crystals formed by smooth colloidal rods

depends on their degree of curvature

By Maria Helena Godinho

N

atural and synthetic micro- and

nanoparticles—in an appropriate

solvent and within a given range of

concentration, pressure, and tempera-

ture—can form colloidal liquid crystal-

line systems that combine the optical

properties of crystals (anisotropy) and the

fluidity of liquids. The particles are largely

anisotropic, with one or two characteristic

dimensions much larger than the third. The

particles can also be bent or curved or, if de-

rived from natural materials, can have chi-

ral interactions, all of which can affect how

the particles self-assemble and form liquid

crystalline phases. On page 950 of this issue,

Fernández-Rico et al. ( 1 ) report on a simple

method allowing the production of large

quantities of polydisperse colloidal synthetic

rods from a viscous photoresin. They im-

posed a well-controlled curvature on these

rods by fine-tuning the cross-link density of

the resin and the temperature. They show

that curvature has pronounced effects on the

liquid crystalline phase behavior.

Bawden et al. ( 2 ) first reported the forma-

tion of colloidal liquid crystals in aqueous

solutions of rodlike tobacco mosaic virus.

Later, Onsager ( 3 ) used entropic arguments

to explain the formation of parallel align-

ment (nematic phase) of long, hard rods from

a disorder phase. For ellipsoidal particles, in

addition to the nematic phase, helicoidal

structures (chiral nematic order), which are

characterized by the existence of successive

pseudonematic layers that are rotated by a

small angle about an axis perpendicular to

the plane of the layers, were also reported.

Colloidal solutions of cellulose nanorods ( 4 )

have a chiral nematic structure that can be

frozen-in (see the figure, top), by removing

the solvent so that scanning electron micros-

copy can reveal the layers of the precursor

liquid crystalline phase ( 5 ). This helicoidal

structure is often found in animals and plants

and is interpreted by a twisted plywood

model proposed by Bouligand ( 6 ). Similarly,

for spherocylinders (cylinders capped with

a hemisphere on both ends), smectic phases

with layered structures more ordered than

nematics were also reported. Bent colloidal

particles have also been reported to produce

liquid crystalline solutions. Yang et al. ( 7 )

generated suspensions of silica particles that

exhibited different smectic structures, includ-

ing a twisted smectic phase, by controlling

the bending angle and aspect ratio of the par-

ticles, which were different from the curved

systems Fernández-Rico et al. produced.

Curved filaments are common in nature

and have inspired theoretical investiga-

tions and functional applications. A straight

filament converted into a curved shape can

sometimes coil into a helix even when the

filament lacks chirality ( 8 , 9 ). The intrinsic

curvature was attributed to the existence

across the filament of materials with differ-

ent mechanical characteristics that contract

asymmetrically. The resulting shapes of the

filaments depend on diameter, length, and

boundary conditions at extremities. Long fil-

aments tend to generate spirals, if supported

by one end, or helical structures, if clamped

at both ends. In the latter case, left- and right-

handed helices, separated by straight seg-

ments, were observed, so the overall system

was still achiral (see the figure, bottom left).

The tuning of the curvature can be pre-

cisely controlled by varying the asymmetric

characteristics of the materials existing along

the filament. Similar mechanisms imposed

by asymmetric cross-linking should be at

work in the formation of the curved sphero-

cylinders that Fernández-Rico et al. made by

cross-linking and heating photoresin rods.

The particles were stiffened by further cross-

linking, which fixed their curved shapes be-

fore creating colloidal suspensions.

Fernández-Rico et al. used confocal mi-

croscopy to image a series of different liquid

crystalline phases obtained from three types

of curved particles. The most interesting

finding was the first experimental evidence

of the nematic splay-bend phase, in which

the less-curved polydisperse particles were

organized in a serpentine undulated struc-

ture (see the figure, bottom right). Only the

less curved particles are at the origin of the

splay-bend phase. More curved particles

generate liquid crystalline phases described

previously for molecular banana-shaped mol-

ecules ( 10 ) and colloidal rods. Thus, small

details had large effects on the packing of

the particles, in accord with theoretical pre-

dictions ( 11 ). Indeed, the smectic phase was

“...directly imaging a human

fluid...represents a milestone by

demonstrating the potential

of cryo–electron tomography

for biomedical imaging.”

Centro de Investigação de Materiais–I3N (CENIMAT/I3N),

Department of Materials Science, Faculty of Science and

Technology, University NOVA of Lisbon, Campus da Caparica,

2829-516 Caparica, Portugal. Email: [email protected]

918 21 AUGUST 2020 • VOL 369 ISSUE 6506

Published by AAAS