No abstract

Chromosome segregation defects in cancer cells lead to encapsulation of chromosomes in micronuclei (MN), small nucleus-like structures within which dangerous DNA rearrangements termed chromothripsis can occur. Here we uncover a strikingly different consequence of MN formation in preimplantation development. We find that chromosomes from within MN become damaged and fail to support a functional kinetochore. MN are therefore not segregated, but are instead inherited by one of the two daughter cells. We find that the same MN can be inherited several times without rejoining the principal nucleus and without altering the kinetics of cell divisions. MN motion is passive, resulting in an even distribution of MN across the first two cell lineages. We propose that perpetual unilateral MN inheritance constitutes an unexpected mode of chromosome missegregation, which could contribute to the high frequency of aneuploid cells in mammalian embryos, but simultaneously may serve to insulate the early embryonic genome from chromothripsis.A ccurate chromosome segregation is achieved by correct attachment of spindle microtubules to kinetochores, complex proteinaceous structures that assemble on centromeric DNA. Misattachment can cause so-called lagging chromosomes during anaphase (1), which are a hallmark of chromosomally unstable cells (2), and can result in micronuclei (MN)-small nucleus-like bodies that form if a chromosome remains separate from the main group of chromosomes at the time of nuclear envelope reformation. MN have long been used as a marker of genetic fidelity. For example, MN may be predictive of tumorigenicity and can be used to screen chemicals for genotoxicity (3-5). However, the cellular impact of MN formation is poorly understood. Recent studies found that chromosomes within MN become heavily damaged, causing DNA rearrangements (6-9). Paired with reincorporation of the MN chromosome into the main nucleus (principal nucleus; PN) during the next cell cycle (7, 9, 10), this provided an elegant explanation for chromosome-specific extensive rearrangements seen in many cancer cells, termed chromothripsis (11,12). In the present study we show that the outcome of MN formation is markedly different in the preimplantation mammalian embryo.Chromosomal mosaisicm is common in mammalian preimplantation embryos, up to 50% of human embryos produced in fertility clinics containing some aneuploid cells resulting from early mitotic errors (13-16), for which there is currently no clear cellular explanation. Simultaneously, early embryos frequently exhibit MN, the causes and consequences of which are unknown (17, 18). Here we pair long-term live 4D microscopy with high-resolution fixed embryo analysis to analyze the causes and consequences of naturally occurring MN in mouse embryos. Our experiments reveal an unexpected series of events in which chromosomes from within MN do not rejoin the principal nucleus, but are repeatedly inherited by only one daughter cell. We propose that this mechanism should generate a cascade of aneuploi...

Throughout development, activin A signaling stimulates proliferation and inhibits differentiation of testicular Sertoli cells. A decline in activin levels at puberty corresponds with the differentiation of Sertoli cells that is required to sustain spermatogenesis. In this study, we consider whether terminally differentiated Sertoli cells can revert to a functionally immature phenotype in response to activin A. To increase systemic activin levels, the right tibialis anterior muscle of 7-wk-old C57BL/6J mice was transduced with an adeno-associated virus (rAAV6) expressing activin A. We show that chronic activin signaling reduces testis mass by 23.5% compared with control animals and induces a hypospermatogenic phenotype, consistent with a failure of Sertoli cells to support spermatogenesis. We use permeability tracers and transepithelial electrical resistance measurements to demonstrate that activin potently disrupts blood-testis-barrier function in adult mice and ablates tight junction formation in differentiated primary Sertoli cells, respectively. Furthermore, increased activin signaling reinitiates a program of cellular proliferation in primary Sertoli cells as determined by 5-ethynyl-2'-deoxyuridine incorporation. Proliferative cells reexpress juvenile markers, including cytokeratin-18, and suppress mature markers, including claudin-11. Thus, activin A is the first identified factor capable of reprogramming Sertoli cells to an immature, dedifferentiated phenotype. This study indicates that activin signaling must be strictly controlled in the adult in order to maintain Sertoli cell function in spermatogenesis.

This study was funded by grants from Fondation Jean-Louis Lévesque (Canada), CIHR (MOP142334) and CFI (32711) to GF. JH is supported by Postdoctoral Fellowships from The Lalor Foundation and CIHR (146703). The authors have no conflict of interest.