RESEARCH ARTICLE SUMMARY
◥CELL BIOLOGY
Mechanism of spindle pole organization
and instability in human oocytes
Chun So, Katerina Menelaou†, Julia Uraji†, Katarina Harasimov, Anna M. Steyer, K. Bianka Seres,
Jonas Bucevicˇius, Gražvydas Lukinavicˇius, Wiebke Möbius, Claus Sibold, Andreas Tandler-Schneider,
Heike Eckel, Rüdiger Moltrecht, Martyn Blayney, Kay Elder, Melina Schuh*
INTRODUCTION:Many human eggs carry an
incorrect number of chromosomes, a con-
dition known as aneuploidy. Aneuploidy in
human eggs is the leading cause of aberrant
embryonic development, resulting in preg-
nancy loss and genetic disorders such as Down
syndrome. Most aneuploidy results from chro-
mosome segregation errors during the matu-
ration of oocytes into fertilizable eggs. Unlike
somatic cells, human oocytes segregate chromo-
somes with a specialized microtubule spindle
that lacks centrosomes. Previous live-imaging
studies revealed that human oocytes often
assemble spindles with unstable poles, favoring
chromosome segregation errors. The causes of
high spindle instability in human oocytes were
unknown.
RATIONALE:Identifying the causes of spindle
instability may lead to therapeutic strategies
that reduce chromosome segregation errors in
human eggs and improve outcomes of assisted
reproductive technology. We thus set out to
investigate how spindle poles are organized in
the absence of centrosomes and why spindles
are unstable in human oocytes. To this end, we
systematically studied the localization and
function of proteins that are required for
spindle pole assembly or spindle stability in
oocytes of different mammalian species. In
stark contrast to human oocytes, the spindles
of other mammalian oocytes were stable. We
thus carried out a comparative analysis to in-
vestigate whether differences in molecular
composition can explain the high degree of
spindle instability in human oocytes.RESULTS:Spindle pole assembly requires the
bundling of parallel microtubules by micro-
tubule cross-linking proteins as well as sta-
bilization and/or anchoring of microtubule
minus ends in the spindle pole region by minus
end–binding proteins. We found that the
microtubule cross-linking protein NUMA
(nuclear mitotic apparatus protein) localized
to microtubule minus ends, where it recruited
the molecular motor dynein for spindle pole
focusing. Depletion of NUMA or inhibition
of dynein splayed microtubule minus ends,
demonstrating that NUMA and dynein orga-
nize the spindle poles in human oocytes.
NUMA was similarly enriched at the spindle
poles in bovine and porcine oocytes, which
naturally lack centrosomes, as well as in
mouse oocytes that we artificially depleted
of acentriolar microtubule organizing centers
(aMTOCs). We thus asked whether spindle
instability is a general feature of mamma-
lian oocytes that use NUMA for spindle pole
organization. Live imaging, however, revealed
that bovine, porcine, and aMTOC-free mouse
oocytes did not assemble unstable spindles,
indicating that additional mechanisms sta-
bilize spindles in these oocytes.Using an RNA interference screen of pro-
teins with diverse functions in spindle orga-
nization, we identified the molecular motor
KIFC1 (kinesin superfamily protein C1) as a
spindle-stabilizing factor that is present in
other mammalian oocytes but deficient in
human oocytes. Depletion of KIFC1 in other
mammalian oocytes recapitulated the spin-
dle instability of human oocytes, resulting in
spindles with unstable poles and an increase
in aneuploidy. To investigate further if the
spindle instability in human oocytes was a
result of KIFC1 deficiency, we injected recom-
binant KIFC1 protein into human oocytes and
performed live imaging of spindle assembly.
Introduction of exogenous KIFC1 stabilized
the spindles and reduced chromosome seg-
regation errors, confirming that KIFC1 defi-
ciency contributes to spindle instability in
human oocytes.CONCLUSION:Our data reveal notable differ-
ences in spindle pole organization in different
systems. In somatic cells, two centrosomes
act as the main MTOCs and promote bipolar
spindle assembly. In mouse oocytes, centro-
somes are functionally replaced by aMTOCs.
In other mammalian oocytes, including hu-
mans, NUMA is enriched at microtubule minus
ends. NUMA primarily engages the motor
activity of dynein but can also cross-link micro-
tubules itself. These activities allow NUMA to
cluster microtubule minus ends, and to thereby
organize the spindle poles in the absence of
centrosomes or aMTOCs.
Our data also elucidate a cause of spindle
instability in human oocytes: Human oocytes
are deficient in KIFC1, a key spindle-stabilizing
protein in other mammalian oocytes and in
cancer cells. KIFC1 stabilizes the spindle poles
and prevents their fragmentation. This is likely
achieved through the formation of static cross-
links along parallel microtubules at the poles
and the alignment of antiparallel microtubules
in the central region of the spindle. Because
human oocytes are deficient in KIFC1, we pro-
pose that the deficiency of these activities
renders their spindles unstable.
By delivering a defined amount of KIFC1
protein into human oocytes, we were able to
reduce spindle instability and the risk of aneu-
ploidy in human oocytes. Thus, our data also
reveal a potential method for increasing the
fidelity of spindle assembly and chromo-
some segregation in human oocytes.
▪RESEARCHSCIENCEscience.org 11 FEBRUARY 2022•VOL 375 ISSUE 6581 631
The list of author affiliations is available in the full article online.
*Corresponding author. Email: [email protected]
These authors contributed equally to this work.
Cite this article as C. Soet al.,Science 375 , eabj3944
(2022). DOI: 10.1126/science.abj3944READ THE FULL ARTICLE AT
https://doi.org/10.1126/science.abj3944Spindle instability in mammalian oocytes.Depletion of the molecular motor KIFC1 in aMTOC-free mouse
oocytes (left) and bovine oocytes (middle) resulted in multipolar spindles (gray) and chromosome (magenta)
segregation errors, recapitulating the spindle instability of human oocytes (right), as shown in these
immunofluorescence images. Conversely, introduction of exogenous KIFC1 stabilized the spindles in human
oocytes, which are naturally deficient in KIFC1. Yellow arrowheads highlight multiple spindle poles.