Lack of chromokinesin Klp-19 creates a more rigid midzone and affects force transmission during anaphase in C. elegans

Recent studies have highlighted the significance of the spindle midzone - the region positioned between chromosomes - in ensuring proper chromosome segregation. By combining advanced 3D electron tomography and cutting-edge light microscopy we have discovered a previously unknown role of the regulation of microtubule dynamics within the spindle midzone of C. elegans. Using Fluorescence recovery after photobleaching and a combination of second harmonic generation and two-photon fluorescence microscopy, we found that the length of the antiparallel microtubule overlap zone in the spindle midzone is constant throughout anaphase, and independent of cortical pulling forces as well as the presence of the microtubule bundling protein SPD-1. Further investigations of SPD-1 and the chromokinesin KLP-19 in C. elegans suggest that KLP-19 regulates the overlap length and functions independently of SPD-1. Our data shows that KLP-19 plays an active role in regulating the length and turn-over of microtubules within the midzone as well as the size of the antiparallel overlap region throughout mitosis. Depletion of KLP-19 in mitosis leads to an increase in microtubule length in the spindle midzone, which also leads to increased microtubule - microtubule interaction, thus building up a more robust microtubule network. The spindle is globally stiffer and more stable, which has implications for the transmission of forces within the spindle affecting chromosome segregation dynamics. Our data shows that by localizing KLP-19 to the spindle midzone in anaphase microtubule dynamics can be locally controlled allowing the formation of a functional midzone.

Engineering stability, longevity, and miscibility of microtubule-based active fluids

Microtubule-based active matter provides insight into the self-organization of motile interacting constituents. We describe several formulations of microtubule-based 3D active isotropic fluids. Dynamics of these fluids is powered by three types of kinesin motors: a processive motor, a non-processive motor, and a motor which is permanently linked to a microtubule backbone. Another modification uses a specific microtubule crosslinker to induce bundle formation instead of a non-specific polymer depletant. In comparison to the already established system, each formulation exhibits distinct properties. These developments reveal the temporal stability of microtubule-based active fluids while extending their reach and the applicability.

Developmental Stage Classification of Embryos Using Two-Stream Neural Network with Linear-Chain Conditional Random Field

The developmental process of embryos follows a monotonic order. An embryo can progressively cleave from one cell to multiple cells and finally transform to morula and blastocyst. For time-lapse videos of embryos, most existing developmental stage classification methods conduct per-frame predictions using an image frame at each time step. However, classification using only images suffers from overlapping between cells and imbalance between stages. Temporal information can be valuable in addressing this problem by capturing movements between neighboring frames. In this work, we propose a two-stream model for developmental stage classification. Unlike previous methods, our two-stream model accepts both temporal and image information. We develop a linear-chain conditional random field (CRF) on top of neural network features extracted from the temporal and image streams to make use of both modalities. The linear-chain CRF formulation enables tractable training of global sequential models over multiple frames while also making it possible to inject monotonic development order constraints into the learning process explicitly. We demonstrate our algorithm on two time-lapse embryo video datasets: i) mouse and ii) human embryo datasets. Our method achieves 98.1% and 80.6% for mouse and human embryo stage classification, respectively. Our approach will enable more pro-found clinical and biological studies and suggests a new direction for developmental stage classification by utilizing temporal information.

Inferring simple but precise quantitative models of human oocyte and early embryo development

Macroscopic, phenomenological models are useful as concise framings of our understandings in fields from statistical physics to finance to biology. Constructing a phenomenological model for development would provide a framework for understanding the complicated, regulatory nature of oogenesis and embryogenesis. Here, we use a data-driven approach to infer quantitative, precise models of human oocyte maturation and pre-implantation embryo development, by analysing clinical in-vitro fertilization (IVF) data on 7399 IVF cycles resulting in 57 827 embryos. Surprisingly, we find that both oocyte maturation and early embryo development are quantitatively described by simple models with minimal interactions. This simplicity suggests that oogenesis and embryogenesis are composed of modular processes that are relatively siloed from one another. In particular, our analysis provides strong evidence that (i) pre-antral follicles produce anti-Müllerian hormone independently of effects from other follicles, (ii) oocytes mature to metaphase-II independently of the woman's age, her BMI and other factors, (iii) early embryo development is memoryless for the variables assessed here, in that the probability of an embryo transitioning from its current developmental stage to the next is independent of its previous stage. Our results both provide insight into the fundamentals of oogenesis and embryogenesis and have implications for the clinical IVF.

O-172 Metabolic imaging of cumulus cells to predict embryo implantation potential

Study question

Can non-invasive metabolic imaging detect variations in cumulus cell metabolic parameters associated with a viable pregnancy of the corresponding embryo?

Summary answer

Noninvasive metabolic imaging can detect differences in the cumulus cell metabolic signatures between embryos that led to a viable pregnancy and those that did not. 

What is known already

Bidirectional metabolic cooperativity between the human oocyte and its surrounding cumulus cells is essential for the oocyte to acquire full developmental competency. However, the relationship between cumulus cell metabolism and oocyte viability is not well established. Metabolic imaging uses Fluorescence Lifetime Imaging Microscopy (FLIM) to non-invasively measure autofluorescence of the endogenous molecules, NADH and FAD+, which are essential coenzymes for cellular respiration and glycolysis. This technique enables quantitative information for these coenzyme concentrations and regarding metabolite enzyme engagement. We have previously shown that this technique is an effective tool for quantitatively measuring metabolic state of mouse embryos.

Study design, size, duration

Cumulus cell clusters (n = 617 from 193 patients) were dissected from cumulus-oocyte complexes prior to insemination or ICSI, vitrified, warmed and their metabolic function assessed. We conducted a prospective observational study to evaluate to what extent cumulus cells from an oocyte that led to a viable pregnancy (presence of a viable fetus >7 weeks gestation) after transfer of the corresponding embryo metabolically differed from those that did not. We also evaluated the associations with embryo morphology.

Participants/materials, setting, methods

Cumulus cell metabolism was assessed non-invasively using FLIM to measure the autofluorescence of NADH and FAD+. Overall a single FLIM measurement provides a total of 8 metabolic parameters (4 for NADH and 4 for FAD+). An additional parameter, the Redox Ratio was also acquired (NADH intensity / FAD+ intensity). We used multilevel models to investigate the association of cumulus cell metabolic parameters with the morphology of the corresponding embryo and clinical outcome.

Main results and the role of chance

Of the cumulus samples analyzed, 75 corresponded to embryos that did not result in a viable pregnancy, and 24 that did so. Significant associations were observed between cumulus cell FAD+ fraction bound to enzyme (p = 0.007), FAD+ long lifetime (p = 0.01) and FAD+ short lifetime (p < 0.001) and the clinical outcome of the corresponding embryo. These significant associations held up after controlling for age. We used a support vector machine algorithm to distinguish between embryos that led to a viable pregnancy and those that did not. The optimum hyperplane derived from a support vector machine algorithm predicted whether a sample with random cumulus cell metabolic parameters will lead to a viable pregnancy or not with an accuracy of 80%. Embryo morphological assessments were stratified as excellent, good, fair and poor. We found no significant associations between cumulus cell metabolic signatures and embryo morphology evaluated on day 3. Significant associations of FAD+ short lifetime (p < 0.001) and day 5 embryo morphology were found. However, these associations were not significant after controlling for age. 

Limitations, reasons for caution

Although we observed significant variations in metabolic parameters, further studies with larger sample sizes are required. Despite our validation studies showing no significant effect of vitrification on cumulus cell metabolic parameters, analyses with fresh clusters are needed to confirm our results.

Wider implications of the findings

Noninvasive FLIM imaging detects metabolic variations of cumulus masses and their association with embryo viability. The ability to correlate metabolic measurements of cumulus clusters, in combination with embryo morphology assessments and patient clinical characteristics, with embryo fate paves the way for this approach to be used in a clinical setting.

Trial registration number

5RO1HD092559-03

Automated Measurements of Key Morphological Features of Human Embryos for IVF

A major challenge in clinical In-Vitro Fertilization (IVF) is selecting the highest quality embryo to transfer to the patient in the hopes of achieving a pregnancy. Time-lapse microscopy provides clinicians with a wealth of information for selecting embryos. However, the resulting movies of embryos are currently analyzed manually, which is time consuming and subjective. Here, we automate feature extraction of time-lapse microscopy of human embryos with a machine-learning pipeline of five convolutional neural networks (CNNs). Our pipeline consists of (1) semantic segmentation of the regions of the embryo, (2) regression predictions of fragment severity, (3) classification of the developmental stage, and object instance segmentation of (4) cells and (5) pronuclei. Our approach greatly speeds up the measurement of quantitative, biologically relevant features that may aid in embryo selection.

Central-spindle microtubules are strongly coupled to chromosomes during both anaphase A and anaphase B

Spindle microtubules, whose dynamics vary over time and at different locations, cooperatively drive chromosome segregation. Measurements of microtubule dynamics and spindle ultrastructure can provide insight into the behaviors of microtubules, helping elucidate the mechanism of chromosome segregation. Much work has focused on the dynamics and organization of kinetochore microtubules, that is, on the region between chromosomes and poles. In comparison, microtubules in the central-spindle region, between segregating chromosomes, have been less thoroughly characterized. Here, we report measurements of the movement of central-spindle microtubules during chromosome segregation in human mitotic spindles and Caenorhabditis elegans mitotic and female meiotic spindles. We found that these central-spindle microtubules slide apart at the same speed as chromosomes, even as chromosomes move toward spindle poles. In these systems, damaging central-spindle microtubules by laser ablation caused an immediate and complete cessation of chromosome motion, suggesting a strong coupling between central-spindle microtubules and chromosomes. Electron tomographic reconstruction revealed that the analyzed anaphase spindles all contain microtubules with both ends between segregating chromosomes. Our results provide new dynamical, functional, and ultrastructural characterizations of central-spindle microtubules during chromosome segregation in diverse spindles and suggest that central-spindle microtubules and chromosomes are strongly coupled in anaphase.

Dynein pulling forces counteract lamin-mediated nuclear stability during nuclear envelope repair

Transient nuclear envelope (NE) ruptures in the Caenorhabditis elegans zygote are caused by a weakened nuclear lamina during nuclear positioning. Dynein-pulling forces enhance the severity of ruptures, while lamin restricts nucleocytoplasmic mixing and allows stable NE repair. This work is the first mechanistic analysis of NE rupture and repair in an organism.

Recent work done exclusively in tissue culture cells revealed that the nuclear envelope (NE) ruptures and repairs in interphase. The duration of NE ruptures depends on lamins; however, the underlying mechanisms and relevance to in vivo events are not known. Here, we use the Caenorhabditis elegans zygote to analyze lamin’s role in NE rupture and repair in vivo. Transient NE ruptures and subsequent NE collapse are induced by weaknesses in the nuclear lamina caused by expression of an engineered hypomorphic C. elegans lamin allele. Dynein-generated forces that position nuclei enhance the severity of transient NE ruptures and cause NE collapse. Reduction of dynein forces allows the weakened lamin network to restrict nucleo–cytoplasmic mixing and support stable NE recovery. Surprisingly, the high incidence of transient NE ruptures does not contribute to embryonic lethality, which is instead correlated with stochastic chromosome scattering resulting from premature NE collapse, suggesting that C. elegans tolerates transient losses of NE compartmentalization during early embryogenesis. In sum, we demonstrate that lamin counteracts dynein forces to promote stable NE repair and prevent catastrophic NE collapse, and thus provide the first mechanistic analysis of NE rupture and repair in an organismal context.

Cytoplasmic flows as signatures for the mechanics of mitotic positioning

The proper positioning of mitotic spindle in the single-cell Caenorhabditis elegans embryo is achieved initially by the migration and rotation of the pronuclear complex (PNC) and its two associated astral microtubules (MTs). Pronuclear migration produces global cytoplasmic flows that couple the mechanics of all MTs, the PNC, and the cell periphery with each other through their hydrodynamic interactions (HIs). We present the first computational study that explicitly accounts for detailed HIs between the cytoskeletal components and demonstrate the key consequences of HIs for the mechanics of pronuclear migration. First, we show that, because of HIs between the MTs, the cytoplasm-filled astral MTs behave like a porous medium, with its permeability decreasing with increasing the number of MTs. We then directly study the dynamics of PNC migration under various force-transduction models, including the pushing or pulling of MTs at the cortex and the pulling of MTs by cytoplasmically bound force generators. Although achieving proper position and orientation on reasonable time scales does not uniquely choose a model, we find that each model produces a different signature in its induced cytoplasmic flow. We suggest that cytoplasmic flows can be used to differentiate between mechanisms.

Deciphering the evolutionary history of open and closed mitosis

The origin of the nucleus at the prokaryote-to-eukaryote transition represents one of the most important events in the evolution of cellular organization. The nuclear envelope encircles the chromosomes in interphase and is a selectively permeable barrier between the nucleoplasm and cytoplasm and an organizational scaffold for the nucleus. It remains intact in the 'closed' mitosis of some yeasts, but loses its integrity in the 'open' mitosis of mammals. Instances of both types of mitosis within two evolutionary clades indicate multiple evolutionary transitions between open and closed mitosis, although the underlying genetic changes that influenced these transitions remain unknown. A survey of the diversity of mitotic nuclei that fall between these extremes is the starting point from which to determine the physiologically relevant characteristics distinguishing open from closed mitosis and to understand how they evolved and why they are retained in present-day organisms. The field is now poised to begin addressing these issues by defining and documenting patterns of mitotic nuclear variation within and among species and mapping them onto a phylogenic tree. Deciphering the evolutionary history of open and closed mitosis will complement cell biological and genetic approaches aimed at deciphering the fundamental organizational principles of the nucleus.

Automated segmentation of the first mitotic spindle in differential interference contrast microcopy images of C. elegans embryos

Differential interference contrast (DIC) microscopy is a non-fluorescent microscopy technique that is commonly used to visualize the first mitotic spindle in C. elegans embryos. DIC movies are easy to acquire and provide data with high spatial and temporal resolution, allowing detailed investigations of the dynamics of the spindle-which elongates, oscillates, and is positioned asymmetrically. Despite the immense amount of information such movies provide, they are normally only used to draw qualitative conclusion based on manual inspection. We have developed an algorithm to automatically segment the mitotic spindle in DIC movies of C. elegans embryos, determine the position of centrosomes, quantify the morphology and motions of the spindle, and track these features over time. This method should be widely useful for studying the first mitotic spindle in C. elegans.

Developing cell biology

Experiments in Xenopus embryo extracts reveal that changes in cellular biochemistry cause mitotic spindles to decrease in size over the course of early development.

A novel small-molecule inhibitor reveals a possible role of kinesin-5 in anastral spindle-pole assembly

The tetrameric plus-end-directed motor, kinesin-5, is essential for bipolar spindle assembly. Small-molecule inhibitors of kinesin-5 have been important tools for investigating its function, and some are currently under evaluation as anti-cancer drugs. Most inhibitors reported to date are ;non-competitive' and bind to a specific site on the motor head, trapping the motor in an ADP-bound state in which it has a weak but non-zero affinity for microtubules. Here, we used a novel ATP-competitive inhibitor, FCPT, developed at Merck (USA). We found that it induced tight binding of kinesin-5 onto microtubules in vitro. Using Xenopus egg-extract spindles, we found that FCPT not only blocked poleward microtubule sliding but also selectively induced loss of microtubules at the poles of bipolar spindles (and not asters or monoasters). We also found that the spindle-pole proteins TPX2 and gamma-tubulin became redistributed to the spindle equator, suggesting that proper kinesin-5 function is required for pole assembly.

Treatment of patients with advanced cancer using multiple long-term cultured lymphokine-activated killer (LAK) cell infusions and recombinant human interleukin-2

Escalating doses of recombinant human interleukin-2 (rIL-2) were combined with long-term cultured rIL-2 activated killer cells to treat patients with disseminated melanoma, renal cell cancer, and colon cancer. Twenty-four patients were entered, 12 with renal cell cancer, 8 with colon cancer, and 4 with melanoma; 23 were evaluable for efficacy and toxicity. The (dose-related) toxicities were moderate to severe and consisted of fever, chills, rigors, weight gain, hypotension, mild confusion, elevation of liver enzymes and serum creatinine, thrombocytopenia, and eosinophilia. No cardiac events (arrhythmias or myocardial infarction) were recorded. None of the patients were admitted to the intensive care unit, and no deaths occurred. Two partial responses were observed, one at relatively low doses of rIL-2 in a patient with renal cell carcinoma and one at the highest dose level in a patient with malignant melanoma. The maximally tolerated dose level of rIL-2 for this study was 6 X 10(6) U/m2 i.v./day. The recommended dose for further studies is 3 X 10(6) U/m2 i.v./day in three divided doses.

The interaction of melittin with calmodulin and its tryptic fragments

Melittin has been found to interact with both the N- and C-terminal half-molecules of calmodulin, as well as the intact molecule, in the presence of Ca2+. The interaction results in a major change in the microenvironment of Trp-19, which is in a more nonpolar, solvent-shielded, and immobilized microenvironment in the complex. The properties of Tyr-99 and Tyr-138 of calmodulin are altered by complex formation. From measurements of the efficiencies of radiationless energy transfer from Trp-19 to the nitro derivatives of Tyr-99 and/or Tyr-138, it is concluded that Trp-19 is located in proximity to the C-terminal lobe of calmodulin in the complex.