Oocyte aging in a marine protostome worm: The roles of maturation‐promoting factor and extracellular signal regulated kinase form of mitogen‐activated protein kinase

Abstract

The roles of maturation-promoting factor (MPF) and an extracellular signal regulated kinase form of mitogenactivated protein kinase (ERK MAPK) are analyzed during oocyte aging in the marine protostome worm Cerebratulus. About a day after removal from the ovary, unfertilized metaphase-I-arrested oocytes of Cerebratulus begin to flatten and swell before eventually lysing, thereby exhibiting characteristics of a necroptotic mode of regulated cell death. Based on immunoblots probed with phospho-specific antibodies, MPF and ERK are initially active in freshly mature specimens. However, as oocytes age, both kinase activities decline, with ERK deactivation occurring well before MPF downregulation. Experiments using pharmacological modulators indicate that oocyte degradation is promoted by the maturation-initiated activation of ERK as well as by the deactivation of MPF that occurs in extensively aged specimens. The potential significance of these findings is discussed relative to previously published results for apoptotic eggs and oocytes of echinoderm and vertebrate deuterostomes.

The potential roles of c-Jun N-terminal kinase (JNK) during the maturation and aging of oocytes produced by a marine protostome worm

“…As noted previously (Stricker et al , 2016), uninseminated mature oocytes began to degrade approximately 1 day after undergoing GVBD. At the onset of degradation, oocytes tended to expand, flatten, and gradually become lighter in colour before eventually lysing without forming noticeable cytoplasmic blebs (Fig.…”

“…2 A ). Collectively, such morphological characteristics suggested a more necroptotic type of cell death (Stricker et al , 2016), rather than the typical apoptotic demise described for other animal oocytes, in which aging specimens typically shrink, exhibit a denser cytoplasm, and generate marked blebs before dying (Sadler et al , 2004; Tiwari et al , 2015).

Figure 2 ( A ) Interspecimen variation in the onset of aging-induced degradation for oocytes produced by five female (fem) worms.

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“…After reaching full size, such GV-containing immature oocytes are usually incapable of being fertilized and instead must first undergo a maturation process that involves nuclear disassembly [= ‘germinal vesicle breakdown’ (GVBD)] and further meiotic progression before becoming fertilizable oocytes or eggs (Stricker, 1999; Deguchi et al , 2015). In starfish and nemertean worms, immature oocytes are able to remain intact for more than 48 h in the absence of maturation-promoting stimuli, whereas non-inseminated mature oocytes begin an aging process that ultimately results in their death within about a day (Yuce & Sadler, 2001; Stricker et al , 2016). Similarly, by ~6–12 h post-ovulation, an aging-induced loss of viability can prevent mature oocytes of many mammals from being properly fertilized (Fissore et al , 2002; Miao et al , 2009).…”

“…Previously, it has been shown that, as in other animals, maturing oocytes of the nemertean Cerebratulus activate MPF during the process of GVBD (Stricker et al , 2013). Accordingly, if MPF levels are kept low, Cerebratulus oocytes can be maintained in an immature state, and such GV-containing specimens remain intact for several days without exhibiting marked signs of cellular degradation (Stricker et al , 2016). Conversely, cohort oocytes that had undergone a GVBD-related increase in MPF during maturation subsequently begin to deactivate MPF and lyse after only 1 day of aging, indicating that MPF deactivation in mature oocytes is correlated with an accelerated onset of death (Stricker et al , 2016).…”

“…Accordingly, if MPF levels are kept low, Cerebratulus oocytes can be maintained in an immature state, and such GV-containing specimens remain intact for several days without exhibiting marked signs of cellular degradation (Stricker et al , 2016). Conversely, cohort oocytes that had undergone a GVBD-related increase in MPF during maturation subsequently begin to deactivate MPF and lyse after only 1 day of aging, indicating that MPF deactivation in mature oocytes is correlated with an accelerated onset of death (Stricker et al , 2016). Given such findings coupled with previous reports that oocyte degradation is potentially mediated by JNK and MPF in deuterostomes, phospho-specific antibodies are used here in conjunction with pharmacological modulators to analyze these kinase activities in maturing and aged oocytes of the protostome worm Cerebratulus .…”