RESEARCH ARTICLE
◥CELL BIOLOGY
Mechanism of spindle pole organization
and instability in human oocytes
Chun So^1 , Katerina Menelaou1,2†, Julia Uraji1,2†, Katarina Harasimov^1 , Anna M. Steyer^3 ,
K. Bianka Seres1,2, Jonas Bucevicˇius^4 , Gražvydas Lukinavicˇius^4 , Wiebke Möbius3,5, Claus Sibold^6 ,
Andreas Tandler-Schneider^6 , Heike Eckel^7 , Rüdiger Moltrecht^7 , Martyn Blayney^2 ,
Kay Elder^2 , Melina Schuh1,5*
Human oocytes are prone to assembling meiotic spindles with unstable poles, which can favor
aneuploidy in human eggs. The underlying causes of spindle instability are unknown. We found that
NUMA (nuclear mitotic apparatus protein)Ðmediated clustering of microtubule minus ends focused the
spindle poles in human, bovine, and porcine oocytes and in mouse oocytes depleted of acentriolar
microtubule-organizing centers (aMTOCs). However, unlike human oocytes, bovine, porcine, and aMTOC-
free mouse oocytes have stable spindles. We identified the molecular motor KIFC1 (kinesin superfamily
protein C1) as a spindle-stabilizing protein that is deficient in human oocytes. Depletion of KIFC1
recapitulated spindle instability in bovine and aMTOC-free mouse oocytes, and the introduction of
exogenous KIFC1 rescued spindle instability in human oocytes. Thus, the deficiency of KIFC1 contributes
to spindle instability in human oocytes.
A
neuploidy in human eggs is a leading
cause of aberrant embryonic develop-
ment, resulting in miscarriages and ge-
netic disorders such as Down syndrome.
Around 25 to 50% of human eggs are
aneuploid, and the aneuploidy rate increases
with maternal age ( 1 ). Until recently, it has re-
mained unclear why human eggs are so prone
to aneuploidy, even in young women. Recent
studies of meiosis and chromosome segre-
gation in live human oocytes have revealed
that human oocytes often assemble highly
unstable meiotic spindles during meiosis I ( 2 ).
Bipolar spindles drive chromosome segrega-
tion in mitosis and meiosis, and their correct
assembly is a prerequisite for accurate chro-
mosome segregation. In human oocytes, the
two spindle poles frequently widen and frag-
ment, forming apolar and multipolar spindle
intermediates ( 2 ). These unstable spindles
often misalign and missegregate chromosomes
( 2 ) and have been linked to tridirectional divi-
sion at anaphase I ( 3 ). However, the causes of
spindle instability in human oocytes are still
unknown. Also, human fertilized eggs (zygotes)
are prone to assembling multipolar spindles
and undergo multipolar divisions ( 4 – 14 ). Iden-
tifying the causes of spindle instability may
lead to therapeutic strategies that reduce chro-
mosome segregation errors in human eggs
and improve outcomes of assisted reproduc-
tive technology.
Spindle instability is extremely rare in normal
mitotic cells, whose aneuploidy rates are much
lower ( 15 ). Mitotic cells and oocytes assemble
spindles differently. In mitosis, two centro-
somes act as the main microtubule-organizing
centers (MTOCs) ( 16 ) and stabilize the two
spindle poles ( 17 , 18 ). By contrast, oocytes of
most species, including mammals, lack centro-
somes ( 19 – 21 ). How spindle poles are organized
in the absence of centrosomes has been studied
mostly in nonmammalian oocytes and in mouse
oocytes. In nonmammalian oocytes, spindle
poles are focused by motor and nonmotor
proteins that cross-link microtubules ( 22 – 29 ).
In mouse oocytes, canonical centrosomes are
functionally replaced by acentriolar MTOCs
(aMTOCs) ( 30 ). The aMTOCs lack centrioles
but contain many other components of centro-
somes ( 31 ). They form ring-shaped clusters at
the two spindle poles and represent major
sites of microtubule nucleation and anchoring
in mouse oocytes ( 30 , 32 – 35 ).
Themechanismofspindlepoleorganization
in nonrodent mammalian oocytes, including
humans, cows, and pigs, remains unclear. Un-
like mouse oocytes, oocytes from these species
lack distinct aMTOC foci at their spindle poles
( 2 , 36 ). Although studies of these oocytes have
reported localization of a few proteins such
asg-tubulin and NUMA (nuclear mitotic ap-
paratus protein) at the spindle poles ( 2 , 37 – 40 ),how these oocytes assemble their spindle poles
and control their spindle polarity without
centrosomes and aMTOCs remains poorly
understood.Results
NUMA organizes the spindle poles in
human oocytes
To investigate how spindle poles are organized
during meiosis I in human oocytes, we selected
eight candidate proteins for analysis. Spin-
dle pole assembly requires the bundling of
parallel microtubules by microtubule cross-
linking proteins ( 41 , 42 ). In addition, micro-
tubule minus ends can be anchored to MTOCs
( 43 , 44 )and/orstabilizedbyminusend–
binding proteins ( 41 , 42 ). We analyzed the
localization of four minus end–binding proteins
[g-tubulin, CAMSAP3 (calmodulin-regulated
spectrin-associated protein 3), KANSL3 (KAT8
regulatory NSL complex subunit 3), and MCRS1
(microspherule protein 1)] and four micro-
tubule cross-linking proteins [ASPM (abnormal
spindle-like microcephaly-associated protein),
KIF11/EG5 (kinesin superfamily protein 11),
NUMA, and TPX2 (targeting protein for Xklp2)]
in human oocytes because these proteins as-
sociate with the spindle poles in mitotic cells
( 41 , 42 ) and oocytes from diverse organisms
( 2 , 31 , 33 , 37 – 40 , 45 – 53 ).
As a general microtubule minus end–capping
protein ( 54 ),g-tubulin was enriched on micro-
tubules at the spindle poles (fig. S1A), in line
with the high abundance of microtubule minus
ends at the poles. However,g-tubulin was not
enriched as distinct, micron-sized foci in hu-
man oocytes (fig. S1A), consistent with the
observation that their spindles lack aMTOCs
( 2 ). CAMSAP3, KANSL3, MCRS1, and ASPM
showed weak or no spindle localization and
were not enriched at the poles (fig. S1A), sug-
gesting that they are unlikely to be important
for the organization of spindle poles in human
oocytes. Whereas KIF11 and TPX2 were de-
tected both along spindle microtubules and
enriched at the poles (fig. S1A), NUMA stain-
ing was confined to the extreme region of
thepoleswhereminusendsofindividual
microtubules converge (Fig. 1A and movie S1).
Imaging of the spindle in a vertical orientation
confirmed that NUMA specifically associated
with microtubule minus ends (Fig. 1B), making
it a good candidate for organizing microtubule
minus ends at the poles in human oocytes.
NUMA was localized at the poles through-
out all stages of meiosis (Fig. 1, A and C). It was
detected early before spindle bipolarization,
suggesting that NUMA is recruited as soon
as the poles are forming in human oocytes.
Whereas two clusters of NUMA were detected
in oocytes undergoing bidirectional anaphase I,
three clusters of NUMA were detected in an
oocyte undergoing tridirectional anaphase I
(Fig. 1D). Thus, NUMA localization is relatedRESEARCH
Soet al.,Science 375 , eabj3944 (2022) 11 February 2022 1of19
(^1) Department of Meiosis, Max Planck Institute for
Multidisciplinary Sciences, Göttingen, Germany.^2 Bourn Hall
Clinic, Cambridge, UK.^3 Electron Microscopy Core Unit,
Department of Neurogenetics, Max Planck Institute for
Multidisciplinary Sciences, Göttingen, Germany.^4 Chromatin
Labeling and Imaging Group, Department of NanoBiophotonics,
Max Planck Institute for Multidisciplinary Sciences, Göttingen,
Germany.^5 Cluster of Excellence“Multiscale Bioimaging: from
Molecular Machines to Networks of Excitable Cells”(MBExC),
University of Göttingen, Göttingen, Germany.^6 Fertility
Center Berlin, Berlin, Germany.^7 Kinderwunschzentrum
Göttingen, Göttingen, Germany.
*Corresponding author. Email: [email protected]
†These authors contributed equally to this work.