Science - 16.08.2019

SCIENCE sciencemag.org

PHOTO: D. KRNDIJA

ET AL

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agent that blocks cell division when used in

low doses. Intestinal stem cell proliferation

and differentiation occur along well-defined

axes and renewal is rapid, meaning that em-

pirical data can be collected within relatively

short time frames. If cellular migration along

the villus occurs passively, driven by a push-

ing force resulting from mitosis in the crypts,

then blocking cell division with hydroxyurea

should reduce or eliminate cell movement.

The authors found that cell movement along

the villus is not affected by hydroxyurea treat-

ment, which suggests that cell migration up

the villus may instead be actively regulated.

The authors then built a mathematical

model to independently assess the contrib-

uting forces of mitotic pushing and mi-

gratory pulling. The mathematical model

predicted dynamic changes to cell density

and migratory speed as pushing forces be-

gin to decline moving away from the crypt

and pulling forces increase toward the vil-

lus tip. Direct observation of cell density

revealed a bimodal distribution along

intestinal villi in mice, with cell packing

first decreasing along the lower part of the

villus and then increasing toward the vil-

lus tip. To assess differences in migration

speed along the villus, Krndija et al. used

mouse gut explants, tissue slices of small

bowel mucosa that can be maintained ex

vivo for several hours, to assess cell migra-

tion through live imaging in a controlled

environment (see the image). By tracing in-

dividual cells marked by green fluorescent

protein moving along the villus, Krndija et

al. found that cells accelerate as they move

toward the villus tip.

Further analyses using gut explants

in which some cells expressed a reporter

molecule attached to actin, a cytoskeletal

protein that underpins cell shape and mi-

gration, revealed that villus epithelial cells

demonstrate basal protrusions, much like

little feet, that allow intestinal colum-

nar epithelial cells to crawl up the villus.

Chemically blocking these protrusions also

disrupted cell migration and caused cell

crowding toward the villus tip. This rev-

epithelial sheet passively responding to

forces emanating from the crypt seemed

to fit with the coordinated behavior of

intestinal epithelial cells traveling up the

villus in tight ribbons. In an extension to

their model, Krndija et al. reveal that co-

ordinated epithelial cell movement up

the villus is due to strong intercellular

connections that ensure the epithelium

experiences a strong tensile spring force.

Therefore, active migratory pulling of indi-

vidual cells and tight intercellular connec-

tions together ensure that the epithelium

moves as a coordinated sheet.

The study of Krndija et al. suggests

many avenues for further research to in-

vestigate how this active migratory mecha-

nism might be linked to human disease.

For example, the rare pediatric disorder

congenital tufting enteropathy presents

with early-onset severe and intractable

diarrhea, which leads to irreversible in-

testinal failure. The disease is linked to

germline mutations in the epithelial cell

adhesion molecule (EPCAM) gene, and in-

testinal biopsies of affected patients dem-

onstrate characteristic protruding tufts of

epithelial cells, typically at the villus tip.

These defects in villous maturation lead

to malabsorption of nutrients, which pro-

vokes severe diarrhea. EPCAM mutations

are paradoxically linked to increased cell

migration ( 7 ), which, according to the bio-

physical model presented by Krndija et al.,

would further increase crowding at the vil-

lus tip, possibly precipitating the charac-

teristic epithelial tufts associated with this

disease. Similarly, colorectal adenomas,

the benign precursor lesions of colorectal

cancer, often expand their territory by ac-

tively pushing out neighboring epithelium.

The adenomatous epithelium grows top-

down into neighboring crypts, gradually

replacing the normal intestinal crypt epi-

thelium by growing underneath it. Similar

biophysical models could be applied to dis-

entangle the clonal contribution of passive

pushing and active pulling forces in this

tug of war between normal and precancer-

ous epithelium. j

REFERENCES AND NOTES

1. D. Krndija et al., Science 365 , 705 (2019).


2. D. J. Winton, M. A. Blount, B. A. Ponder, Nature 333 ,
   463 (1988).


3. L. Gutierrez-Gonzalez et al., J. Pathol. 217 , 489 (2009).


4. H. J. Snippert et al., Cell 143 , 134 (2010).


5. T. A. Graham et al., Gastroenterology 140 , 1241 (2011).


6. G. T. Eisenhoffer et al., Nature 484 , 546 (2012).


7. J. L. Mueller, M. D. McGeough, C. A. Peña, M. Sivagnanam,
   Am. J. Physiol. Gastrointest. Liver Physiol. 306 ,
   G278 (2014).

ACKNOWLEDGMENTS

Work in my laboratory is supported by Cancer Research UK.

10.1126/science.aay5861

Mouse intestinal villi of a gut explant show scattered epithelial cells marked by green fluorescent protein.

Live imaging of these explants allows the movement of individual epithelial cells to be recorded over time.

16 AUGUST 2019 • VOL 365 ISSUE 6454 643

elation also indicates that in addition to a

cellular polarity axis from the apical part

of the cell that is exposed to the intestinal

lumen and the basal end that is attached to

the basement membrane, intestinal epithe-

lial cells also maintain a front-rear polar-

ity axis from villus to crypt. Together, the

experimental data confirm the theoretical

predictions and show that cell movement

up the villus is dominated by passive push-

ing forces in the crypt and lower villus and

active pulling forces through migration

along the remainder of the villus.

The data of Krndija et al. reveal an un-

expected aspect of intestinal epithelial

homeostasis that had been overlooked

for many years. The long-held view of an