Measuring nucleus mechanics within a living multicellular organism: Physical decoupling and attenuated recovery rate are physiological protective mechanisms of the cell nucleus under high mechanical load.

Nuclei within cells are constantly subjected to compressive, tensile, and shear forces, which regulate nucleoskeletal and cytoskeletal remodeling, activate signaling pathways, and direct cell-fate decisions. Multiple rheological methods have been adapted for characterizing the response to applied forces of isolated nuclei and nuclei within intact cells. However, in vitro measurements fail to capture the viscoelastic modulation of nuclear stress-strain relationships by the physiological tethering to the surrounding cytoskeleton, extracellular matrix and cells, and tissue-level architectures. Using an equiaxial stretching apparatus, we applied a step stress and measured nucleus deformation dynamics within living Caenorhabditis elegans nematodes. Nuclei deformed nonmonotonically under constant load. Nonmonotonic deformation was conserved across tissues and robust to nucleoskeletal and cytoskeletal perturbations, but it required intact linker of nucleoskeleton and cytoskeleton complex attachments. The transition from creep to strain recovery fits a tensile-compressive linear viscoelastic model that is indicative of nucleoskeletal-cytoskeletal decoupling under high load. Ce-lamin (lmn-1) knockdown softened the nucleus, whereas nematode aging stiffened the nucleus and decreased deformation recovery rate. Recovery lasted minutes rather than seconds due to physiological damping of the released mechanical energy, thus protecting nuclear integrity and preventing chromatin damage.

A Unified Linear Viscoelastic Model of the Cell Nucleus Defines the Mechanical Contributions of Lamins and Chromatin.

The cell nucleus is constantly subjected to externally applied forces. During metazoan evolution, the nucleus has been optimized to allow physical deformability while protecting the genome under load. Aberrant nucleus mechanics can alter cell migration across narrow spaces in cancer metastasis and immune response and disrupt nucleus mechanosensitivity. Uncovering the mechanical roles of lamins and chromatin is imperative for understanding the implications of physiological forces on cells and nuclei. Lamin-knockout and -rescue fibroblasts and probed nucleus response to physiologically relevant stresses are generated. A minimal viscoelastic model is presented that captures dynamic resistance across different cell types, lamin composition, phosphorylation states, and chromatin condensation. The model is conserved at low and high loading and is validated by micropipette aspiration and nanoindentation rheology. A time scale emerges that separates between dominantly elastic and dominantly viscous regimes. While lamin-A and lamin-B1 contribute to nucleus stiffness, viscosity is specified mostly by lamin-A. Elastic and viscous association of lamin-B1 and lamin-A is supported by transcriptional and proteomic profiling analyses. Chromatin decondensation quantified by electron microscopy softens the nucleus unless lamin-A is expressed. A mechanical framework is provided for assessing nucleus response to applied forces in health and disease.

Does the transfer of a poor quality embryo together with a good quality embryo affect the In Vitro Fertilization (IVF) outcome?

BACKGROUND: IVF cycles which result in only one good quality embryo, and a second poor quality embryo present a dilemma when the decision involves transferring two embryos. The aim of this study was to evaluate whether a poor quality embryo has a negative effect on a good quality embryo when transferred along with a good quality embryo. METHODS: We retrospectively evaluated in vitro fertilization (IVF) cycles involving single embryo transfers (SET) and double embryo transfers (DET). Embryo quality was divided into poor "P" and good "G" quality. The main outcome measures were: live birth, implantation rate, miscarriage rate, clinical pregnancy rate and multiple pregnancy ratio. RESULTS: Six hundred three women were included. The study group consisted of 180 (29.9%) patients who had a double embryo transfer (DET) with one poor quality embryo and one good quality embryo (P + G). Control 1 group included 303 (50.2%) patients who had DET with two good quality embryos (G + G), and control 2 group consisted of 120 (19.9%) patients who had a single embryo transfer (SET) with one good quality embryo (G). Live birth rates were not significantly different when compared between study groups: 30.8% in the SET group (G), 27.2% in the (G + P) group and 33.7% in the (G + G) group. The SET group had the highest implantation rate (33.9%) compared to the DET groups (21.8% (G + P), 25.4% (G + G)) (P =0.022). The clinical pregnancy rate was 33.3% in the SET group (G), 33.3% in the (G + P) group, and 39.3% in the (G + G) group (P =0.39). The miscarriage rate was comparable in all groups. CONCLUSION: A poor quality embryo does not negatively affect a good quality embryo, when transferred together in a double embryo transfer.