Secreted midbody remnants are a class of extracellular vesicles molecularly distinct from exosomes and microparticles

During the final stages of cell division, newly-formed daughter cells remain connected by a thin intercellular bridge containing the midbody (MB), a microtubule-rich organelle responsible for cytokinetic abscission. Following cell division the MB is asymmetrically inherited by one daughter cell where it persists as a midbody remnant (MB-R). Accumulating evidence shows MB-Rs are secreted (sMB-Rs) into the extracellular medium and engulfed by neighbouring non-sister cells. While much is known about intracellular MB-Rs, sMB-Rs are poorly understood. Here, we report the large-scale purification and biochemical characterisation of sMB-Rs released from colon cancer cells, including profiling of their proteome using mass spectrometry. We show sMB-Rs are an abundant class of membrane-encapsulated extracellular vesicle (200-600 nm) enriched in core cytokinetic proteins and molecularly distinct from exosomes and microparticles. Functional dissection of sMB-Rs demonstrated that they are engulfed by, and accumulate in, quiescent fibroblasts where they promote cellular transformation and an invasive phenotype.

Dynamics of hydraulic and contractile wave-mediated fluid transport during Drosophila oogenesis

From insects to mice, oocytes develop within cysts alongside nurse-like sister germ cells. Prior to fertilization, the nurse cells’ cytoplasmic contents are transported into the oocyte, which grows as its sister cells regress and die. Although critical for fertility, the biological and physical mechanisms underlying this transport process are poorly understood. Here, we combined live imaging of germline cysts, genetic perturbations, and mathematical modeling to investigate the dynamics and mechanisms that enable directional and complete cytoplasmic transport in Drosophila melanogaster egg chambers. We discovered that during “nurse cell (NC) dumping” most cytoplasm is transported into the oocyte independently of changes in myosin-II contractility, with dynamics instead explained by an effective Young–Laplace law, suggesting hydraulic transport induced by baseline cell-surface tension. A minimal flow-network model inspired by the famous two-balloon experiment and motivated by genetic analysis of a myosin mutant correctly predicts the directionality, intercellular pattern, and time scale of transport. Long thought to trigger transport through “squeezing,” changes in actomyosin contractility are required only once NC volume has become comparable to nuclear volume, in the form of surface contractile waves that drive NC dumping to completion. Our work thus demonstrates how biological and physical mechanisms cooperate to enable a critical developmental process that, until now, was thought to be mainly biochemically regulated.

Non-genetic inheritance restraint of cell-to-cell variation

Heterogeneity in physical and functional characteristics of cells (e.g. size, cycle time, growth rate, protein concentration) proliferates within an isogenic population due to stochasticity in intracellular biochemical processes and in the distribution of resources during divisions. Conversely, it is limited in part by the inheritance of cellular components between consecutive generations. Here we introduce a new experimental method for measuring proliferation of heterogeneity in bacterial cell characteristics, based on measuring how two sister cells become different from each other over time. Our measurements provide the inheritance dynamics of different cellular properties, and the 'inertia' of cells to maintain these properties along time. We find that inheritance dynamics are property-specific, and can exhibit long-term memory (~10 generations) that works to restrain variation among cells. Our results can reveal mechanisms of non-genetic inheritance in bacteria and help understand how cells control their properties and heterogeneity within isogenic cell populations.

Short stop is a gatekeeper at the ring canals of Drosophila ovary

SUMMARYMicrotubules and actin filaments are two major cytoskeletal components essential for a variety of cellular functions. Spectraplakins are a family of large cytoskeletal proteins cross-linking microtubules and actin filaments among other components. In this study, we aim to understand how Short stop (Shot), the single Drosophila spectraplakin, coordinates microtubules and actin filaments for oocyte growth. The oocyte growth completely relies on the acquisition of cytoplasmic materials from the interconnected sister cells (nurse cells), through ring canals, cytoplasmic bridges that remained open after incomplete germ cell division. Given the open nature of the ring canals, it is unclear how the direction of transport through the ring canal is controlled. Here we show that Shot controls the directionality of flow of material from the nurse cells towards the oocyte. Knockdown of shot changes the direction of transport of many types of cargo through the ring canals from unidirectional (toward the oocyte) to bidirectional, resulting in small oocytes that fail to grow over time. In agreement with this flow-directing function of Shot, we find that it is localized at the asymmetric actin fibers adjacent to the ring canals at the nurse cell side, and controls the uniform polarity of microtubules located in the ring canals connecting the nurse cells and the oocyte. Together, we propose that Shot functions as a gatekeeper directing the material flow from the nurse cells to the oocyte, via organization of microtubule tracks.

Accelerated Dystrophy and Decay of Oligodendrocyte Precursor Cells in the APP/PS1 Model of Alzheimer’s-Like Pathology

Myelin disruption is a feature of natural aging and Alzheimer’s disease (AD). In the CNS, myelin is produced by oligodendrocytes, which are generated throughout life by oligodendrocyte progenitor cells (OPCs). Here, we examined age-related changes in OPCs in APP/PS1 mice, a model for AD-like pathology, compared with non-transgenic (Tg) age-matched controls. The analysis was performed in the CA1 area of the hippocampus following immunolabeling for NG2 with the nuclear dye Hoescht, to identify OPC and OPC sister cells, a measure of OPC replication. The results indicate a significant decrease in the number of OPCs at 9 months in APP/PS1 mice, compared to age-matched controls, without further decline at 14 months. Also, the number of OPC sister cells declined significantly at 14 months in APP/PS1 mice, which was not observed in age-matched controls. Notably, OPCs also displayed marked morphological changes at 14 months in APP/PS1 mice, characterized by an overall shrinkage of OPC process domains and increased process branching. The results indicate that OPC disruption is a pathological sign in the APP/PS1 mouse model of AD.

Accelerated dystrophy and decay of oligodendrocyte precursor cells in the APP/PS1 model of Alzheimer’s-like pathology

Myelin disruption is a feature of natural aging and of Alzheimer’s disease (AD). In the CNS, myelin is produced by oligodendrocytes, which are generated throughout life by oligodendrocyte progenitor cells (OPCs). Here, we examined age-related changes in OPCs in APP/PS1 mice, a model for AD-like pathology, compared with non-transgenic (Tg) age-matched controls. Analysis was performed in the CA1 area of the hippocampus following immunolabelling for NG2 with the nuclear dye Hoescht, to identify OPC and OPC sister cells, a measure of OPC replication, together with Gpr17 and Olig2 for oligodendrocytes and myelin basic protein (MBP) immunostaining as a measure of myelination. The results indicate a decrease in the number of OPCs between 9 and 14 months in natural ageing and this occurred earlier at 9 months in APP/PS1 mice, without further decline at 14 months. The number of OPC sister cells was unaltered in natural aging, but declined significantly at 14-months in APP/PS1 mice. The number of GPR17+ and Olig2+ oligodendrocytes was not altered in APP/PS1, whereas MBP immunostaining increased between 9 and 14 months in natural ageing, but not in APP/PS1 mice. Notably, OPCs displayed marked morphological changes at 14 months in APP/PS1 mice, characterized by an overall shrinkage of OPC process domains and increased process branching, characteristic of reactive pathological changes. The results indicate that OPC and myelin disruption are pathological signs in the APP/PS1 mouse model of AD.

Binary decision between asymmetric and symmetric cell division is defined by the balance of PAR proteins in C. elegans embryos

ABSTRACTCoordination between cell differentiation and proliferation during development requires the balance between asymmetric and symmetric modes of cell division. However, the cellular intrinsic cue underlying the binary choice between these two division modes remains elusive. Here we show evidence in Caenorhabditis elegans that the invariable lineage of the division modes is programmed by the balance between antagonizing complexes of partitioning-defective (PAR) proteins. By uncoupling unequal inheritance of PAR proteins from that of fate determinants during zygote division, we demonstrated that changes in the balance between PAR-2 and PAR-6 are sufficient to re-program the division modes from symmetric to asymmetric and vice versa in two-cell stage embryos. The division mode adopted occurs independently of asymmetry in cytoplasmic fate determinants, cell-size asymmetry, and cell-cycle asynchrony between the sister cells. We propose that the balance between antagonizing PAR proteins represents an intrinsic self-organizing cue for binary specification of the division modes during development.

Dynamics of hydraulic and contractile wave-mediated fluid transport during Drosophila oogenesis

From insects to mice, oocytes develop within cysts alongside nurse-like sister germ cells. Prior to fertilization, the nurse cells’ cytoplasmic contents are transported into the oocyte, which grows as its sister cells regress and die. Although critical for fertility, the biological and physical mechanisms underlying this transport process are poorly understood. Here, we combined live imaging of germline cysts, genetic perturbations, and mathematical modeling to investigate the dynamics and mechanisms that enable directional and complete cytoplasmic transport in Drosophila melanogaster egg chambers. We discovered that during ‘nurse cell (NC) dumping’, most cytoplasm is transported into the oocyte independently of changes in myosin-II contractility, with dynamics instead explained by an effective Young-Laplace’s law, suggesting hydraulic transport induced by baseline cell surface tension. A minimal flow network model inspired by the famous two-balloon experiment and genetic analysis of a myosin mutant correctly predicts the directionality of transport time scale, as well as its intercellular pattern. Long thought to trigger transport through ‘squeezing’, changes in actomyosin contractility are required only once cell volume is reduced by ∼75%, in the form of surface contractile waves that drive NC dumping to completion. Our work thus demonstrates how biological and physical mechanisms cooperate to enable a critical developmental process that, until now, was thought to be a mainly biochemically regulated phenomenon.

Sperm DNA damage causes genomic instability in early embryonic development

Genomic instability is common in human embryos, but the underlying causes are largely unknown. Here, we examined the consequences of sperm DNA damage on the embryonic genome by single-cell whole-genome sequencing of individual blastomeres from bovine embryos produced with sperm damaged by γ-radiation. Sperm DNA damage primarily leads to fragmentation of the paternal chromosomes followed by random distribution of the chromosomal fragments over the two sister cells in the first cell division. An unexpected secondary effect of sperm DNA damage is the induction of direct unequal cleavages, which include the poorly understood heterogoneic cell divisions. As a result, chaotic mosaicism is common in embryos derived from fertilizations with damaged sperm. The mosaic aneuploidies, uniparental disomies, and de novo structural variation induced by sperm DNA damage may compromise fertility and lead to rare congenital disorders when embryos escape developmental arrest.

X-chromosome dosage compensation dynamics in human early embryos

In mammals, female cells are obliged to inactivate one of two X chromosomes to achieve dosage parity with the single X chromosome in male cells, and it is also thought that the single active X chromosome is increased 2-fold to achieve dosage balance with two sets of autosomes (X:A ratio = 1, or Ohno’s hypothesis). However, the ontogeny of X-chromosome inactivation and augmentation of the single active X remains unclear during human embryogenesis. Here, we perform single-cell RNA-seq analysis to examine the timing of X:A balancing and X-inactivation (XCI) in pre- and peri-implantation human embryos up to fourteen days in culture. We find that X-chromosome gene expression in both male and female preimplantation embryos is approximately balanced with autosomes (X:A ratio = 1) after embryonic genome activation (EGA) and persists through fourteen days in vitro. Cross-species analysis of preimplantation embryo also show balanced X:A ratio within the first few days of development. By single-cell mRNA SNP profiling, we find XCI beginning in day 6-7 blastocyst embryos, but does not affect X:A dosage balance. XCI is most evident in trophoectoderm (TE) cells, but can also be observed in a small number of inner cell mass (ICM)-derived cells including primitive endoderm (PE) and epiblast (EPI) cells. Analysis between individual XaXa and XaXi sister cells from the same embryo reveals random XCI and persistently balanced X:A ratio, including sister cells transitioning between XaXa and XaXi states. We therefore conclude that the male X-chromosome undergoes X chromosome augmentation prior to the simultaneous X-chromosome inactivation and augmentation in females. Together, our data demonstrate an evolutionally conserved model of X chromosome dosage compensation in humans and other mammalian species.