Starvation at birth impairs germ cell cyst breakdown and increases autophagy and apoptosis in mouse oocytes

Yong-Yong Wang,Yuan-Chao Sun,Massimo De Felici,Xiao-Feng Sun,Xi-Feng Zhang,Wei Shen,Bo Li,Shun-Feng Cheng

doi:10.1038/cddis.2017.3

Abstract

The female reproductive lifespan is largely determined by the size of primordial follicle pool, which is established following germ cell cyst breakdown around birth. Almost two-third of oocytes are lost during germ cell cysts breakdown, following autophagic and apoptosis mechanisms. To investigate a possible relationship between germ cell cyst breakdown and nutrition supply, we established a starvation model in mouse pups at birth and evaluated the dynamics of cyst breakdown during nutrient deprivation. Our results showed that after 36 h of starvation between 1.5 and 3 d.p.p., indicators of metabolism both at systemic and ovarian level were significantly altered and the germ cell cyst breakdown markedly decreased. We also found that markers of oxidative stress, autophagy and apoptosis were increased and higher number of oocytes in cyst showing autophagic markers and of TUNEL-positive oocytes and somatic cells were present in the ovaries of starved pups. Moreover, the proliferation of pre-granulosa cells and the expression of the oocyte-specific transcription factor Nobox were decreased in such ovaries. Finally, we observed that the ovaries of the starved pups could recover a normal number of follicles after about 3 weeks from re-feeding. In conclusion, these data indicate that nutrient deficiency at birth can generate a number of adaptive metabolic and oxidative responses in the ovaries causing increased apoptosis both in the somatic cells and oocyte and autophagy mainly in these latter and leading to a delay of germ cell cyst breakdown and follicle assembly.

Highlights

Germ cell cysts refer to a cluster of interconnected germ cells resulting from incomplete mitotic cytokinesis.[1,2,3] In the mouse, germ cells form cysts from 10.5 to 13.5 days post coitum (d.p.c.) during the period of the primordial germ cell (PGC) proliferation in both sexes.[4,5] In female mice, cysts partially fragment prior to meiosis and gradually decrease in oocyte number throughout 17.5 d.p.c. to 4.0 days post partum (d.p.p.).[6]
Another study showed that cyst breakdown and apoptosis do not precisely coincide indicating that apoptosis is unlikely to be the major cause of cyst breakdown and that most of the oocytes die after cyst breakdown.[6]
By counting the number of oocytes in cysts or enveloped individually into follicles in tissue sections of ovaries at 3 d.p.p., we found that the normal dynamics of cyst breakdown and primordial follicles (PFs) formation was significantly impaired in starved pups (Figure 5a)

Summary

Introduction

Germ cell cysts refer to a cluster of interconnected germ cells resulting from incomplete mitotic cytokinesis.[1,2,3] In the mouse, germ cells form cysts from 10.5 to 13.5 days post coitum (d.p.c.) during the period of the primordial germ cell (PGC) proliferation in both sexes.[4,5] In female mice, cysts partially fragment prior to meiosis and gradually decrease in oocyte number throughout 17.5 d.p.c. to 4.0 days post partum (d.p.p.).[6]. Several lines of evidence demonstrate that inducers of oxidative stress may act as signaling molecules in the pathway of apoptosis in several, if not all, cells and tissues.[16,17] Oxidative stress occurs when the normal cellular redox balance is disturbed, resulting in dysregulation of redox-regulated processes and/or oxidative damage to cellular structures.[18,19] As a matter of fact, oxidative stress can impair the ovary’s function and normal development of follicles and has been associated with polycystic ovary syndrome (PCOS).[20,21] in rodent models, increased ROS levels induce rapid primordial follicle loss and apoptosis in antral follicles.[22] Whatever the exact mechanisms of oocyte apoptosis, in the mouse, more than twothird of oocytes die before and a few days after birth At this time, cyst disappearance is concomitant with the formation of the primordial follicles (PFs). Mouse ovaries exposed to another TGFβ family member, activin A, showed an increased number of PFs.[40]

Methods

Results

Conclusion