Nuclear morphogenesis: forming a heterogeneous nucleus during embryogenesis

An embryo experiences progressively complex spatial and temporal patterns of gene expression that guide the morphogenesis of its body plan as it matures. Using super-resolution fluorescence microscopy in Drosophila melanogaster embryos, we observed a similar increase in complexity in the nucleus: the spatial distributions of transcription factors became increasingly heterogeneous as the embryo matured. We also observed a similar trend in chromatin conformation with the establishment of specific histone modification patterns. However, transcription sites of specific genes had distinct local preferences for histone marks separate from the average nuclear trend, depending on the time and location of their expression. These results suggest that reconfiguring the nuclear environment is an integral part of embryogenesis and that the physical organization of the nucleus a key element in developmental gene regulation.

First-Principles Study on Al/Al3Hf Heterogeneous Nucleation Interface

Using first-principles method, the work of adhesion (Wad) and electronic structure of Al3Hf (001)/Al (001) interface are studied and the mechanism of Al3Hf as enhanced heterogeneous nucleus of α-Al are discussed. The results indicate that Al + Hf-termination interfaces with same stacking sequence and the HCP (Al atom locating on top of the Al3Hf slab) interface with the same termination have maximum Wad and minimum interface energy (yint), and therefore they are more stable ones. It is noteworthy that Al + Hf-terminated interface with HCP is most steady one. The stacking of Al atoms on Al3Hf substrates tend to occur in this way. Besides, electronic structures indicate that Al + Hf-terminated interfaces have stronger electronic interaction than that of Al-terminated ones and the Al-Hf bonds of Al + Hf-terminated interface with HCP stacking tend to covalent bonds, while Al–Al bonds of Al-terminated one are metal bonds. Al3Hf as enhanced heterogeneous nucleation of α-Al are effective from crystallography and thermodynamics.

Towards Tissue-Specific Stem Cell Therapy for the Intervertebral Disc: PPARδ Agonist Increases the Yield of Human Nucleus Pulposus Progenitor Cells in Expansion

(1) Background: Low back pain (LBP) is often associated with intervertebral disc degeneration (IVDD). Autochthonous progenitor cells isolated from the center, i.e., the nucleus pulposus, of the IVD (so-called nucleus pulposus progenitor cells (NPPCs)) could be a future cell source for therapy. The NPPCs were also identified to be positive for the angiopoietin-1 receptor (Tie2). Similar to hematopoietic stem cells, Tie2 might be involved in peroxisome proliferator-activated receptor delta (PPARδ) agonist-induced self-renewal regulation. The purpose of this study was to investigate whether a PPARδ agonist (GW501516) increases the Tie2+ NPPCs’ yield within the heterogeneous nucleus pulposus cell (NPC) population. (2) Methods: Primary NPCs were treated with 10 µM of GW501516 for eight days. Mitochondrial mass was determined by microscopy, using mitotracker red dye, and the relative gene expression was quantified by qPCR, using extracellular matrix and mitophagy-related genes. (3) The NPC’s group treated with the PPARδ agonist showed a significant increase of the Tie2+ NPCs yield from ~7% in passage 1 to ~50% in passage two, compared to the NPCs vehicle-treated group. Furthermore, no significant differences were found among treatment and control, using qPCR and mitotracker deep red. (4) Conclusion: PPARδ agonist could help to increase the Tie2+ NPCs yield during NPC expansion.

Investigation of cell nucleus heterogeneity

Nucleus deformation has been shown to play a key role in cell mechanotransduction and migration. Therefore, it is of wide interest to accurately characterize nucleus mechanical behavior. In this study we present the first computational investigation of the in-situ deformation of a heterogeneous cell nucleus. A novel methodology is developed to accurately reconstruct a three-dimensional finite element spatially heterogeneous model of a cell nucleus from confocal microscopy z-stack images of nuclei stained for nucleus DNA. The relationship between spatially heterogeneous distributions microscopic imaging-derived greyscale values, shear stiffness and resultant shear strain is explored through the incorporation of the reconstructed heterogeneous nucleus into a model of a chondrocyte embedded in a PCM and cartilage ECM. Externally applied shear deformation of the ECM is simulated and computed intra-nuclear strain distributions are directly compared to corresponding experimentally measured distributions. Simulations suggest that the nucleus is highly heterogeneous in terms of its mechanical behaviour, with a sigmoidal relationship between experimentally measure greyscale values and corresponding local shear moduli (μn). Three distinct phases are identified within the nucleus: a low stiffness phase (0.17 kPa ≤ μn ≤ 0.63 kPa) corresponding to mRNA rich interchromatin regions; an intermediate stiffness phase (1.48 kPa ≤ μn ≤ 2.7 kPa) corresponding to euchromatin; a high stiffness phase (3.58 kPa ≤ μn ≤ 4.0 kPa) corresponding to heterochromatin. Our simulations indicate that disruption of the nucleus envelope associated with lamin-A/C depletion significantly increases nucleus strain in regions of low DNA concentration. A phenotypic shift of chondrocytes to fibroblast-like cells, a signature for osteoarthritic cartilage, results in a 35% increase in peak nucleus strain compared to control. The findings of this study may have broad implications for the current understanding of the role of nucleus deformation in cell mechanotransduction.

Characterization of transgenic mouse models targeting neuromodulatory systems reveals organizational principles of the dorsal raphe

The dorsal raphe (DR) is a heterogeneous nucleus containing dopamine (DA), serotonin (5HT), γ-aminobutyric acid (GABA) and glutamate neurons. Consequently, investigations of DR circuitry require Cre-driver lines that restrict transgene expression to precisely defined cell populations. Here, we present a systematic evaluation of mouse lines targeting neuromodulatory cells in the DR. We find substantial differences in specificity between lines targeting DA neurons, and in penetrance between lines targeting 5HT neurons. Using these tools to map DR circuits, we show that populations of neurochemically distinct DR neurons are arranged in a stereotyped topographical pattern, send divergent projections to amygdala subnuclei, and differ in their presynaptic inputs. Importantly, targeting DR DA neurons using different mouse lines yielded both structural and functional differences in the neural circuits accessed. These results provide a refined model of DR organization and support a comparative, case-by-case evaluation of the suitability of transgenic tools for any experimental application.

Grain Refinement Mechanism and Effective Nucleation Phase of Al-5Ti-1B Master Alloy

The Al-5Ti-1B, Al-10Ti, Al-4B master alloys and TiB2 powder were applied to refine the pure aluminum, respectively. The effects of the TiAl3 phase, TiB2 particle, and AlB2 phase on the grain size of pure aluminum were compared. The grain refinement mechanism of the Al-5Ti-1B grain refiner was studied. The results showed that the TiAl3 phase was an effective heterogeneous nucleus of the α-Al grain. But the TiAl3 phase in the Al-5Ti-1B grain refiner was not the heterogeneous nucleus of the α-Al grain due to its re-melting in the Al melt. The separate TiB2 particle or AlB2 phase was not the heterogeneous nucleus of the α-Al grain. However, the TiB2 coated by the TiAl3 phase can be the effective heterogeneous nucleus of the α-Al grain. The grain refinement mechanism of the Al-5Ti-1B grain refiner can be summarized as follows: when the Al-5Ti-1B grain refiner is added into the Al melt, the TiAl3 phases are re-melted to release the Ti atoms, while the TiB2 particles are remaining in the Al melt. During the solidification of the Al melt, the Ti atoms are segregating on the surface of TiB2 particles to form the TiAl3 phases. The TiB2 particles coated by the TiAl3 phases then reacts with the Al melt to generate α-Al crystal nucleus.