Comparative analysis of vertebrates reveals that mouse primordial oocytes do not contain a Balbiani body

Oocytes spend the majority of their lifetime in a primordial state. The cellular and molecular biology of primordial oocytes is largely unexplored; yet, studying these is necessary to understand the mechanisms through which oocytes maintain cellular fitness for decades, and why they eventually fail with age. Here, we develop enabling methods for live-imaging based comparative characterization of Xenopus, mouse and human primordial oocytes. We show that primordial oocytes in all three vertebrate species contain active mitochondria, Golgi apparatus and lysosomes. We further demonstrate that human and Xenopus oocytes have a Balbiani body characterized by a dense accumulation of mitochondria in their cytoplasm. However, despite previous reports, we did not find a Balbiani body in mouse oocytes. Instead, we demonstrate what was previously used as a marker for the Balbiani body in mouse primordial oocytes is in fact a ring-shaped Golgi apparatus that is not functionally associated with oocyte dormancy. Our work provides the first insights into the organisation of the cytoplasm in mammalian primordial oocytes, and clarifies relative advantages and limitations of choosing different model organisms for studying oocyte dormancy.

Mouse oocytes do not contain a conventional Balbiani body

ABSTRACTPoor oocyte quality accounts for majority of female fertility problems, however, we know little about how oocytes can remain healthy for many years or why they eventually decline with age. An oocyte spends majority of its lifetime in primordial state. Unlike many somatic cell types or mature oocytes, the cellular and molecular biology of primordial oocytes are largely unexplored. Yet, studying these aspects is necessary to understand the remarkable lifespan of oocytes and their eventual decline.A hallmark of primordial oocytes in many species is the Balbiani body, a non-membrane bound compartment that contains majority of mitochondria in oocyte cytoplasm. The Balbiani body was shown to be held together by an amyloid-like matrix in Xenopus oocytes. It has been proposed to be essential for maintaining mitochondria in a healthy state during long-lasting dormancy.Here, we develop enabling methods that allow live-imaging based comparative characterisation of Xenopus, mouse and human primordial oocytes. We show that human and Xenopus oocytes have a Balbiani body, characterised by an intense accumulation of mitochondria. Our results suggest that amyloid-like features of Balbiani bodies are conserved in humans and Xenopus. However, despite previous reports, we could not find a conventional Balbiani body in mouse oocytes. We demonstrate what was mistaken for a Balbiani body in mouse primordials is an unconventionally shaped Golgi apparatus. Our work provides the first insights into the organisation of the cytoplasm in mammalian primordial oocytes, and clarifies relative advantages and limitations of different model systems for studying female (in)fertility.Significance StatementWorld-wide data suggest that >25% of female fertility problems are undiagnosed, pointing to a huge gap in our understanding of female reproduction. Our findings help fill this gap by studying the cell biology of primordial oocytes. Having optimised methods for isolation and live-imaging of oocytes, we show that oocytes of evolutionary more distant species, Xenopus and human have a conserved super-organelle, a Balbiani body, in their cytoplasm whereas mouse oocytes do not have one. We propose that the shorter lifespan in mice obviates the need for a Balbiani body. Our results also suggest that primordial oocytes have active organelles, and thus call for a re-examination of the concept of dormancy in these long-lived cells.

Remnants of the Balbiani body are required for formation of RNA transport granules in Xenopus oocytes

Fragmentation of the Balbiani body in Xenopus oocytes engenders a region extending from the germinal vesicle (GV) towards the vegetal pole that is enriched in mitochondria. This area is later transversed by RNA that is being localized to the vegetal cortex. Inhibition of mitochondrial ATP synthesis prevents the perinuclear formation of these RNA transport granules that can be reversed by the nonhydrolyzable ATP analog, adenosine 5’-(βγ-imido) triphosphate. The protein composition and sensitivity of the transport granules to hexanediol indicate that they are liquid phase condensates. Mitochondria in the remnants of the Balbiani body produce a region of elevated ATP that appears to act as a hydrotrope to support the perinuclear phase transition leading to granule formation.SummaryMitochondria in the remnants of the Balbiani body produce elevated levels of ATP required for the formation of liquid phase condensates containing RNA transport granules.