16 Scientific American, June 2022B I O L O G YBody Invader
Placenta genetics offer ideas for anticancer drugs
To obtain nutrients for a growing fetus, a placenta embeds itself
into the uterus—an “invasion” that resembles the way a tumor
takes over healthy tissue. Now researchers have identified
genes that help to regulate placental embedding and may prove
instructive in developing anticancer drugs, according to a new
study in the Proceedings of the National Academy of Sciences USA.
Scientists knew that the amount a placenta embeds varies
across species. In some, such as humans and apes, placenta cells
push deep into the uterine wall with relative ease. But in animals
like cows and horses, the uterus has evolved to resist such intru-
sions more strongly. This capability may help protect the mater-
nal immune system and lessen uterus damage when giving birth.
In 2019 cellular biologists Günter Wagner of Yale University
and Kshitiz of the University of Connecticut Health found that this
cellular resistance extends beyond the uterus. They observed a
direct correlation between how deeply a species’ placenta embed-
ded and the rate of tumors that spread beyond their primary site in
that species’ body. Species with highly embedded placentas were
associated with higher rates of metastatic cancer; cellular material
connecting tissues and organs in these species was less resistant
to invasion by both placentas and tumors. The question was why.
For the new study, Kshitiz, Wagner and their colleagues exam-
ined nine mammal species for differences in protein production
that might explain how some species’ tissues resist invasion more
strongly. The team identified two proteins that, when produced in
abundance, made tissues more susceptible to cellular intrusion—
whether from a placenta or a tumor. When the protein-producing
genes were removed, cells blocked invasion more effectively.
“A mutation that helps the uterus keep the placenta out also
[could affect] the biology of cancer in, say, the skin,” Wagner says.
To Wellcome Sanger Institute cancer biologist Sam Behjati,
who was not involved in the study, this finding suggests new ways
to target tumor growth and spread. “This is a hard-core, compar-
ative biological study,” he says. Scientists know a lot about the
molecular steps required for implantation—and “it would be nice
to employ that line of pharmacological thinking for metastases.”
Still, Amy Boddy, a comparative oncologist at the University of
California, Santa Barbara, who was also not involved, cautions that
this specific invasion process probably does not tell the whole story.
Cancers have a variety of causes and contributors. “Everything
that is multicellular is vulnerable to cancer,” Boddy says. “We’ve
just started probing the potential mechanisms.” — Carrie Arnold
E C O L O G YInto the Mucosphere
Tiny organisms trap prey with carbon-rich slime
Ocean oddities called mixoplankton are organisms that can
get energy both through photosynthesis and by eating other
microbes. Now new research published in Nature Communications
suggests that one such species, Prorocentrum cf. balticum, displays
a bizarre and clever hunting technique —one that significantly
contributes to the crucial cycling of carbon through land, atmo-
sphere and oceans.
Study lead author Michaela Larsson, a marine biologist at the
University of Technology Sydney, and her colleagues were studying
marine mixoplankton in the laboratory when they noticed the crea-
tures twisting and turning as they exuded mucus. The researchers
found that beyond photosynthesizing in the daytime, these organ-
isms also form a carbon-rich “mucosphere” around themselves at
night—then use chemical cues to lure other microbes into it. The
mixoplankton then eat the microbes and shed the mucus package,
which sinks to the ocean floor and deposits a rich store of carbon.
The study “is evidence that our thinking around how carbon is
cycled in the ocean must be revised to include the sophisticated
behaviors of microbes,” Larsson says. Identifying P. balticum DNA
in a global marine microbe data set shows the organisms’ preva-
lence across the ocean, suggesting they contribute considerably
to the planetary carbon cycle. The authors estimate these plank-
ton could sequester up to 0.15 gigaton of carbon every year—
about 0.5 percent of the world’s annual carbon emissions.
According to Aditee Mitra, a marine systems modeler at Car-
diff University in Wales, who was not involved in the research,
the paper “is yet another indication of how little we know about
[ocean] organisms that collectively have generated around half of
the oxygen in Earth’s atmosphere and continue to play a pivotal
role in planetary biogeochemical cycling.”
Since the study’s publication, scientists have observed that other
mixoplankton species can form and release mucospheres. Research-
ers are also investigating how microbe behaviors might vary in dif-
ferent marine conditions. “Discovering this microbial behavior and
evaluating the broader implications for ocean biogeochemical
cycling really are just the beginning,” Larsson says. — Susan CosierGetty ImagesPlacenta after birthMixoplanktonMucosphereMicrobesGraphic by Elena Hartley