Oocyte-secreted factor activation of SMAD 2/3 signaling enables initiation of mouse cumulus cell expansion

Area of Science:

• Reproductive Biology
• Cell Signaling
• Molecular Endocrinology

Background:

• Cumulus expansion in mice is regulated by oocyte-secreted paracrine factors.
• Transforming Growth Factor beta (TGFB) superfamily molecules are implicated as cumulus expansion-enabling factors (CEEFs).
• Growth differentiation factor 9 (GDF9) alone has been shown not to be the sole CEEF.

Purpose of the Study:

• To investigate oocyte paracrine factors and their signaling pathways regulating mouse cumulus expansion.
• To identify the specific TGFB superfamily members involved in cumulus expansion.
• To elucidate the role of the SMAD 2/3 signaling pathway in this process.

Main Methods:

• Reverse transcription polymerase chain reaction (RT-PCR) to detect gene expression in oocytes.
• Use of oocytectomized (OOX) complexes to assess FSH-induced cumulus expansion.
• Pharmacological inhibition of ALK 4/5/7 receptors using SB-431542.
• Real-time RT-PCR to quantify the expression of key cumulus expansion genes.

Main Results:

• Oocytes express activin subunits Inhba and Inhbb; activin A and B promote FSH-induced cumulus expansion.
• Follistatin blocked activin-induced expansion but not oocyte-induced expansion.
• The ALK 4/5/7 inhibitor SB-431542 completely inhibited FSH-, GDF9-, activin A-, activin B-, and oocyte-induced cumulus expansion.
• SB-431542 also antagonized epidermal growth factor-stimulated oocyte-induced expansion and attenuated the expression of key cumulus expansion genes.

Conclusions:

• The CEEF is composed of TGFB superfamily molecules.
• These factors signal through the SMAD 2/3 pathway to initiate mouse cumulus expansion.
• Activins, acting via ALK4/5/7 and SMAD 2/3, are key mediators of cumulus expansion.