Science: Chinese scientists revealed the mechanism of spindle assembly without centrosomes in human oocytes

Spindle assembly is essential
to ensure accurate transmission of chromosomes in meiosis and mitosis.
In somatic cells, mitotic spindle assembly is mediated by repeated centrosomes, but many species do not have typical centrosomes
in oocytes 。 In rodents, acentriolar microtubule organizing centers (aMTOC) are responsible for meiotic spindle assembly, but it has long been thought that human oocytes lack significant aMTOC on meiotic spindles, and the exact mechanism of centrosome-free spindle assembly in human oocytes has been unclear
.

Microtubule nucleation and ensuring spindle assembly are central events
that regulate oocyte nuclear maturation.
In a new study, to identify the underlying proteins that drive spindle microtubule nucleation in human oocytes, researchers from Fudan University and Shanghai Jiao Tong University in China systematically mapped 86 human centrosome and microtubule-associated proteins
in more than 2,000 human oocytes by immunofluorescence or three-dimensional high-resolution live-cell imaging 。 They then tracked the dynamic migration
of microtubule nucleators identified at different time points before and after nuclear envelope breakdown (NEBD).
They further downregulated the corresponding proteins to confirm their role in
microtubule nuclei and spindle assembly.
Given that defects in spindle microtubule nucleation can lead to impaired spindle assembly and abnormal oocyte maturation, they screened a cohort of 1394 infertile women with oocyte maturation arrest for mutations in genes encoding microtubule nucleation components
.
The findings were published in the Nov.
18, 2022 issue of Science as "The mechanism of acentrosomal spindle assembly in human oocytes.
"

First, these authors found that in human oocytes, spindle microtubule nucleation begins with kinetozore 2 to 4 hours after the nuclear membrane is decomposed
.
They demonstrated the process of
spindle microtubule conesced granular nucleation in human oocytes.
They then found that 43 proteins were localized to meiotic spindles, of which 4 proteins --- CCP110, CKAP5, DISC1, and TACC3--- exhibiting kinematical and spindle microtubule localization
.
The localization of these four proteins is markedly different
from their localization in human mitotic cells and mouse oocytes.
Just before the nuclear membrane breaks down, these four proteins together form an unusual structure
surrounded by microtubules in human germinal vesicle (GV) oocytes.
They called this potential nucleogenic structure the human oocyte microtubule organizing center (huoMTOC).

They found that a single huoMTOC forms in the cortex of human germinal foaming oocytes and migrates to the nuclear membrane
before it breaks down.
After the nuclear membrane breaks down, huoMTOCs become fragmented and recruited onto the kinemates to initiate spindle microtubule nucleation
.
The down-regulated expression of huoMTOC components leads to significant impairment
of spindle microtubule nucleation and spindle assembly in human oocytes.
This structure has not been detected
in oocytes of other mammalian species such as mice and pigs.
They finally identified two oocytes maturation arrest patients with compound heterozygous mutations
in TACC3, a key component of huoMTOC.
All mutations disrupted the normal function of TACC3, resulting in the absence of huoMTOC structure in the oocytes of these two patients and complete impaired
spindle assembly.

HuoMTOC structure
in human oocytes.
Image from Science, 2022, doi:10.
1126/science.
abq7361
.

Taken together, the new study shows that human oocytes possess an aMTOC-like structure, or huoMTOC, which is the main site for microtubule nucleation and required for
spindle assembly.
Compared to aMTOCs in mouse oocytes, huoMTOCs show distinct characteristics
in terms of number, localization, and composition.
These findings suggest that human oocytes have evolved a different initiation mechanism
for microtubule nucleation and spindle assembly.
These authors found that TACC3 mutations lead to defects in spindle assembly by disrupting the structure of huoMTOC, leading to clinically delayed oocyte maturation
.
This suggests that huoMTOC may be an important biomarker for assessing the quality of human oocytes
.

The findings of huoMTOC by these authors provide insight into the physiological mechanisms of microtubule nucleation and spindle assembly in human oocytes
.
These findings also enhance scientists' understanding
of the pathological mechanism of oocyte maturation arrest.
(Biovalley Bioon.
com)

Resources:

Tianyu Wu et al.
 The mechanism of acentrosomal spindle assembly in human oocytes, Science, 2022, doi:10.
1126/science.
abq7361.