Formin-like 2 (FMNL2) Regulates Actin Assembly and Organelle Distribution to Control Spindle Migration and Asymmetric Division in Mouse Oocyte Meiosis

Core Concepts

FMNL2, an actin nucleating factor, is essential for actin-based spindle migration, asymmetric division, and the distribution and function of endoplasmic reticulum and mitochondria in mouse oocyte meiosis.

Abstract

The study investigated the roles of the actin nucleating factor formin-like 2 (FMNL2) during mouse oocyte meiosis. The key findings are:

FMNL2 localizes at the oocyte cortex and spindle periphery, suggesting its involvement in actin-related processes during oocyte maturation.

Depletion of FMNL2 leads to defects in polar body extrusion, with oocytes either failing to extrude the polar body or extruding large polar bodies. This indicates FMNL2 is essential for asymmetric division.

Live-cell imaging reveals that FMNL2 knockdown disrupts spindle migration to the oocyte cortex, likely due to decreased polymerization of cytoplasmic actin. FMNL2 regulates actin assembly by associating with Arp2 and Formin2.

Mass spectrometry analysis shows FMNL2 interacts with proteins related to endoplasmic reticulum (ER) and mitochondria. FMNL2 depletion disrupts the distribution and function of ER and mitochondria, leading to ER stress and altered mitochondrial membrane potential.

Exogenous expression of FMNL2 rescues the defects in spindle migration, polar body extrusion, and organelle distribution caused by FMNL2 knockdown.

In summary, FMNL2 plays critical roles in regulating actin assembly, spindle migration, and organelle dynamics, which are essential for the successful asymmetric division of mouse oocytes during meiosis.

Stats

FMNL2 protein expression was significantly decreased in FMNL2-knockdown oocytes compared to control oocytes (1 vs. 0.48 ± 0.08, P < 0.01).
The rate of polar body extrusion was significantly lower in FMNL2-knockdown oocytes compared to control oocytes (59.5 ± 2.82%, n = 398 vs. 74.26 ± 1.44%, n = 439, P < 0.001).
The rate of large polar body formation was significantly higher in FMNL2-knockdown oocytes compared to control oocytes (37.16 ± 1.87%, n = 257 vs. 19.05 ± 1.97%, n = 311, P < 0.0001).
The rate of spindle migration to the cortex was significantly lower in FMNL2-knockdown oocytes compared to control oocytes (38.97 ± 6.34%, n = 64 vs. 59.94 ± 3.42%, n = 78, P < 0.05).
Grp78 and Chop protein expression, markers of ER stress, were significantly increased in FMNL2-knockdown oocytes compared to control oocytes (Grp78: 1 vs. 1.42 ± 0.12, P < 0.05; Chop: 1 vs. 1.53 ± 0.16, P < 0.05).
The JC-1 red/green fluorescence ratio, indicating mitochondrial membrane potential, was significantly reduced in FMNL2-knockdown oocytes compared to control oocytes.
Cofilin protein expression was significantly decreased in FMNL2-knockdown oocytes compared to control oocytes (1 vs. 0.81 ± 0.03, P < 0.01).

Quotes

"FMNL2 is essential for the actin assembly, which further involves into meiotic spindle migration and ER/mitochondria functions in mouse oocytes."
"Our results showed that FMNL2 mainly localized at the oocyte cortex and periphery of spindle. Depletion of FMNL2 led to the failure of polar body extrusion and large polar bodies in oocytes."
"Mass spectrometry analysis indicated that FMNL2 was associated with mitochondria and endoplasmic reticulum-related proteins, and FMNL2 depletion disrupted the function and distribution of mitochondria and endoplasmic reticulum, showing with decreased mitochondrial membrane potential and the occurrence of endoplasmic reticulum stress."

Key Insights Distilled From

FMNL2 regulates actin for ER and mitochondria distribution in oocyte meiosis

by Pan,M.-H., P... at www.biorxiv.org 10-06-2023

https://www.biorxiv.org/content/10.1101/2023.10.05.561058v2

Deeper Inquiries

How might the insights from this study on the role of FMNL2 in oocyte meiosis be applied to improve assisted reproductive technologies or address fertility issues?

The insights from this study on the role of FMNL2 in oocyte meiosis can have significant implications for improving assisted reproductive technologies (ART) and addressing fertility issues. By understanding the critical role of FMNL2 in regulating actin assembly, spindle migration, and organelle dynamics during oocyte maturation, researchers and clinicians can potentially develop new strategies to enhance the success rates of ART procedures such as in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI).

Optimizing Oocyte Maturation: Understanding how FMNL2 influences spindle migration and asymmetric division in oocytes can lead to the development of targeted interventions to improve oocyte maturation. By modulating the activity of FMNL2 or its interacting partners, it may be possible to enhance the quality of mature oocytes produced for ART procedures.

Enhancing Fertilization and Embryo Development: Since successful polar body extrusion is crucial for fertilization and embryo implantation, interventions that target FMNL2 could potentially improve these outcomes. By ensuring proper actin assembly and organelle distribution, the chances of successful fertilization and subsequent embryo development could be increased.

Personalized Treatment Approaches: By identifying individuals with specific FMNL2-related issues affecting oocyte maturation, personalized treatment approaches could be developed. This could involve targeted therapies or interventions tailored to address the specific reproductive challenges faced by each individual.

Preventing Age-Related Decline: As oocyte aging is a significant factor in fertility decline, understanding the role of FMNL2 in this process could lead to interventions aimed at preserving oocyte quality and function with advancing age. This knowledge could be leveraged to develop strategies to mitigate the impact of age-related reproductive decline.

What other actin-related factors might interact with FMNL2 to coordinately regulate spindle migration and organelle dynamics during oocyte maturation?

Several actin-related factors may interact with FMNL2 to coordinately regulate spindle migration and organelle dynamics during oocyte maturation. These interactions are crucial for ensuring the proper orchestration of cellular processes involved in meiosis. Some potential actin-related factors that may interact with FMNL2 include:

Arp2/3 Complex: The Arp2/3 complex is known to play a key role in actin nucleation and branching. It interacts with FMNL2 to promote actin assembly and generate forces necessary for spindle migration and organelle dynamics during oocyte maturation.

Formin Family Proteins: Other members of the formin family, such as Formin2 (FMN2) and Formin1 (FMNL1), may interact with FMNL2 to regulate actin polymerization and spindle movement. These interactions are essential for coordinating the dynamics of the actin cytoskeleton during meiosis.

Cofilin: Cofilin is an actin-depolymerizing factor that regulates actin dynamics. It may interact with FMNL2 to modulate actin turnover and contribute to the coordination of spindle migration and organelle distribution in oocytes.

INF2: INF2 is an actin nucleator that is associated with the endoplasmic reticulum (ER) and mitochondria. It may interact with FMNL2 to coordinate actin polymerization at these organelles' interfaces, influencing their dynamics and functions during oocyte maturation.

By forming complexes or interacting with these actin-related factors, FMNL2 can synergistically regulate spindle migration and organelle dynamics, ensuring the successful progression of oocyte meiosis.

Could the disruption of FMNL2-mediated actin assembly and organelle distribution contribute to oocyte aging or other reproductive disorders, and if so, how could this knowledge be leveraged for therapeutic interventions?

The disruption of FMNL2-mediated actin assembly and organelle distribution could indeed contribute to oocyte aging and other reproductive disorders. Oocyte aging is associated with a decline in quality and function, leading to decreased fertility and an increased risk of chromosomal abnormalities. Dysregulation of actin dynamics and organelle distribution, as mediated by FMNL2, could exacerbate these age-related changes and contribute to reproductive disorders.

Therapeutic Interventions: Understanding the role of FMNL2 in oocyte aging and reproductive disorders could pave the way for targeted therapeutic interventions. By developing strategies to modulate FMNL2 activity or its interactions with other factors, it may be possible to mitigate the effects of oocyte aging and improve reproductive outcomes in individuals facing fertility challenges.

Mitigating Reproductive Disorders: Reproductive disorders such as polycystic ovary syndrome (PCOS) or endometriosis may also involve disruptions in actin dynamics and organelle function. By targeting FMNL2 and related pathways, therapeutic interventions could be designed to address these underlying mechanisms and alleviate the symptoms of reproductive disorders.

Preserving Oocyte Quality: Therapeutic approaches aimed at preserving oocyte quality and function could benefit from insights into FMNL2-mediated processes. By enhancing actin assembly and organelle distribution through targeted interventions, it may be possible to slow down the aging process of oocytes and improve their reproductive potential.

Overall, leveraging the knowledge of FMNL2's role in actin assembly and organelle dynamics for therapeutic interventions could offer new avenues for addressing oocyte aging and reproductive disorders, ultimately improving fertility outcomes for individuals experiencing these challenges.