Science - USA (2022-04-15)

CELL BIOLOGY

Epithelial monitoring through ligand-receptor

segregation ensures malignant cell elimination

Geert de Vreede†, Stephan U. Gerlach†, David Bilder*

Animals have evolved mechanisms, such as cell competition, to remove dangerous or nonfunctional cells
from a tissue. Tumor necrosis factor signaling can eliminate clonal malignancies fromDrosophila
imaginal epithelia, but why this pathway is activated in tumor cells but not normal tissue is unknown. We
show that the ligand that drives elimination is present in basolateral circulation but remains latent
because it is spatially segregated from its apically localized receptor. Polarity defects associated with
malignant transformation cause receptor mislocalization, allowing ligand binding and subsequent
apoptotic signaling. This process occurs irrespective of the neighboring cellsÕgenotype and is thus
distinct from cell competition. Related phenomena at epithelial wound sites are required for efficient
repair. This mechanism of polarized compartmentalization of ligand and receptor can generally monitor
epithelial integrity to promote tissue homeostasis.

E

pithelial architecture is the fundamental
organizing principle of animal tissues.
Polarized epithelial sheets provide a
contiguous barrier that allows an organ
to function in a milieu distinct from the
external environment. To maintain the bar-
rier, epithelia must detect threats to their
integrity and resolve them. Integrity can be
compromised by both physical damage and

the production of structurally defective cells.

The latter is a frequent feature of oncogenic

transformation, and it is important to elimi-

nate such cells before a tumor can form. De-

leterious cells can be removed through cell

competition, a broadly used mechanism in

which“winner”cells of one genotype often

induce apoptosis in neighboring“loser”cells

( 1 , 2 ). InDrosophilaimaginal discs, cells mu-

tant for the conserved apicobasal polarity reg-

ulatorsscribble (scrib)ordiscs-large (dlg)form

malignant,“neoplastic”tumors that kill the

animal ( 3 , 4 ). However, prior to tumor growth,

small clones of these polarity-deficient cells

are efficiently eliminated, allowing a healthy

organ to develop. The mechanisms involved

have been described as cell competition, during

which theDrosophilatumor necrosis factor

(TNF) ligand Eiger (Egr) binds the TNF re-

ceptor (TNFR) Grindelwald (Grnd) in mutant

cells, activating the c-Jun N-terminal kinase

(JNK) Basket (Bsk), which induces apoptosis

[reviewed in ( 5 – 7 )]. How polarity loss is coupled

to TNF pathway activation to remove onco-

genic clones is not known.

We investigated TNF-TNFR interactions

during polarity-deficient cell elimination by

coculturing imaginal discs ex vivo alongside

Egr-Venus (EgrV)–expressing fat bodies (a

major endocrine organ) (Fig. 1A) ( 8 ). EgrV

secreted into media associated strongly with

clones ofdlg-depleted cells (Fig. 1, B, C, and

E, and fig. S1, A, B, K, and L). Increased EgrV

SCIENCEscience.org 15 APRIL 2022•VOL 376 ISSUE 6590 297

Department of Molecular and Cell Biology, University of

California, Berkeley, Berkeley, CA 94720, USA.

*Corresponding author. Email: [email protected]

These authors contributed equally to this work.

Fig. 1. Heightened binding of Egr
to polarity-deficient cells.
(A) Diagram of EgrV-expressing fat
bodies cocultured ex vivo with
imaginal discs. (BtoF) Binding of
EgrV and elimination ofdlg-depleted
clones (C) are dependent on cell-
autonomous Grnd (D). Control in
(B). Dotted lines demarcate clones.
Quantitation is shown in (E) and
(F). (GandH) Depletion ofdlg
withptc-Gal4enhances cell death
[(H), DCP-1]. Dotted lines indicate
stripe of expression. Control in (G).
Quantitation is shown in Fig. 2D.
(ItoL) EgrV binding, absent in
control (I), is increased in the
dlg-depleted stripe (J), and this
requires Grnd (K). Quantitation is
shown in (L). Scale bars, 100mm in
(B); 10mm in (I). Information on
statistical tests is provided in the
supplementary materials, materials
and methods, and table S3.

EgrV

EgrV EgrV EgrV

ptcts> dlg KD

DAPIDCP-1 DCP-1

ptcts> dlg KD ptcts> dlg KD

grnd KD

I ptcts> J

G H H′

K

DAPIDCP-1

ptcts>

n.s.

***

***

n.s.

***

***

E

L

ctrl dlg KD dlg KD + grnd KD

***

n.s.

***

F

n.s.

n.s.

RFP+ area / pouch area

EgrV EgrV

EgrV fluorescence

(clones / WT)

EgrV fluorescence (Ptc stripe / WT)

BCD

A Co-culture

Fat Body Wing Disc

expressing EgrV

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