Fudan University Science published an article to break the traditional cognition: the mystery of human oocyte spindle assembly

The correct assembly of the spindle is a key event for the smooth progress of mammalian mitomy classification and meiosis, and only the precise assembly of the spindle can ensure the correct separation of chromosomes and the normal division and development
of subsequent cells and embryos.
The spindle assembly of human and mouse somatic cells is mediated by centrosomes, which are not present in human and mouse oocytes
.
It has been found that the spindles of mouse oocytes are mediated
by acentriolar microtubule organizing centers (aMTOCs).
However, since the 80s of the last century, the presence of MTOC has not been observed in human oocytes, so the academic view has long been that MTOC does not exist in human eggs (Science, 2015; Annu.
Rev.
Cell Dev.
Biol, 2018).

So how exactly is the human oocyte spindle formed? The answer to this scientific question has been unknown
.

On November 18, 2022, the team of Wang Lei and Sang Qing of the Institute of Biomedical Sciences of Fudan University and the team of Sun Xiaoxi of the Jiai Genetics and Infertility Diagnosis and Treatment Center of Fudan University Obstetrics and Gynecology Hospital published a research article entitled "The mechanism of acentrosomal spindle assembly in human oocytes" in the journal Science The article "The Mechanism of Non-central Spindle Assembly in Human Oocytes") found that there is a previously unknown and distinctive microtubule tissue center in human oocytes, which the researchers named huoMTOC (Human Oocyte Microtubule Organizing Center).
The relevant molecular composition was clarified, the physiological mechanism of initiating spindle assembly in human oocytes was elucidated, and finally the gene mutation of huoMTOC component was identified in patients with abnormal spindle assembly of oocytes, revealing a new mechanism
of spindle assembly of human oocytes from the perspective of physiology and pathology.

It is worth mentioning that in 2016, the Wang Lei/Sang Qing team identified human oocyte maturation disorder as a neo-Mendelian genetic disease, discovered the first pathogenic gene TUBB8 and revealed the pathological mechanism (N Eng J Med, 2016).

TUBB8 is a primate-specific gene that constitutes the main member
of the human oocyte spindle.
Combined with the discovery that the unique huoMTOC in human oocytes is closely related to microtubule formation and spindle assembly, these evidence suggests that human oocytes have many unique features in development and evolution compared to other mammals
.

First, the researchers used immunofluorescence and high-resolution imaging techniques to observe human oocytes after nuclear membrane rupture, and the results showed that the spindle microtubules formed by them mainly gathered at the kinetochore (kinetochore) location
of chromosomes.
Furthermore, the dynamic process
of kinematic initiation microtubule assembly was recorded in detail by 3D time-lapse imaging.
It is speculated that human oocyte kinetodons may have special microtubule tissue center initiation spindle microtubule polymerization
.
Subsequently, the researchers performed localization analysis on 86 major centrosome or microtubule-associated proteins in human oocytes in the middle stage of first meiosis, and finally determined that 43 proteins had human oocyte spindle-related localization
.
Among them, four proteins (CCP110, CKAP5, DISC1, and TACC3) are localized to both kinetogranes and microtubules, which is very different
from their localization during mitosis.
The researchers then located the above four proteins in GV (Geminal vesical) stage human oocytes, and surprisingly, these four proteins formed a special structure that had never been discovered before near the nuclear membrane of the GV phase, and newborn microtubule aggregation was observed around this structure, but not in mouse and pig oocytes
.
Because this structure is responsible for polymerizing microtubules in human oocytes, the researchers named it the human oocyte microtubule organizing center (huoMTOC).

Further, the researchers tracked this structure using live-cell tracking imaging and found that huoMTOC formed in the cortex of the oocytes and migrated near
the nuclear membrane before NEBD (Nuclear envelope breakdown).
As NEBD occurs, huoMTOC begins to fragment and gradually localize to the kinetograne, while initiating spindle microtubule aggregation and growth
near the kinemat.
When the huoMTOC structure is disrupted (by laser ablation or down-regulation of the main member protein level), the spindle microtubules in human oocytes are blocked from polymerization, eventually preventing spindle
formation.
The above results suggest that huoMTOC plays an important role
in the spindle assembly process of human oocytes.

Finally, in order to further determine whether huoMTOC is clinically significant, the researchers performed mutation screening in the whole exome sequencing data of more than 1400 patients with oocyte maturation disorders, and found that two patients carried complex co-pathogenic mutations
of TACC3.
Immunofluorescence detection showed that the huoMTOC structure of GV-stage oocytes in both patients was completely destroyed, and there was no spindle formation
in MI-stage oocytes.
It was confirmed that abnormal huoMTOC would directly lead to impaired oocyte maturation in women
.

In summary, the researchers discovered for the first time a new submicrostructure huoMTOC for spindle microtubule assembly in human oocytes, and elucidated the unique physiological mechanism of huoMTOC regulating spindle assembly in human oocyte, and revealed that abnormal huoMTOC causes oocytes maturation disorders in patients, contributing to a new understanding
of the pathological mechanism of the disease.

Wang Lei, Sang Qing, Institute of Biomedical Research, Fudan University, and Sun Xiaoxi, Shanghai Jiai Genetics and Infertility Diagnosis and Treatment Center, are the corresponding authors
.
Takeda Yu, Institute of Biomedical Research, Fudan University, doctoral student Dong Jie (graduated), Fu Jing of Shanghai Jiai Genetics and Infertility Diagnosis and Treatment Center, and Kuang Yanping of the Reproductive Center of the Ninth People's Hospital affiliated to Shanghai Jiao Tong University are the co-first authors of this paper.
In addition, Li Wen and Zhang Meiling of Shanghai Jiao Tong University International Peace Maternity and Child Health Hospital and Dong Xi of Zhongshan Hospital affiliated to Fudan University also participated in the study
.

Original link: https://doi.
org/10.
1126/science.
abq7361