Constructing polymorphonuclear cells: chromatin folding shapes nuclear morphology.

Immune cell fate decisions are regulated, at least in part, by nuclear architecture. Here, we outline how nuclear architecture instructs mammalian polymorphonuclear cell differentiation. We discuss how in neutrophils loop extrusion mechanisms regulate the expression of genes involved in phagocytosis and shape nuclear morphology. We propose that diminished loop extrusion programs also orchestrate eosinophil and basophil differentiation. We portray a new model in which competitive physical forces, loop extrusion, and phase separation, instruct mononuclear versus polymorphonuclear cell fate decisions. We posit that loop extrusion programs instruct the spatial organization of cytoplasmic organelles, including neutrophil granules, mitochondria, and endoplasmic reticulum. Finally, we suggest that changing loop extrusion programs might allow the engineering of new nuclear shapes and artificial cytoplasmic architectures.

Nuclear F-actin and Lamin A antagonistically modulate nuclear shape.

Nuclear shape influences cell migration, gene expression and cell cycle progression, and is altered in disease states like laminopathies and cancer. What factors and forces determine nuclear shape? We find that nuclei assembled in Xenopus egg extracts in the presence of dynamic F-actin exhibit a striking bilobed nuclear morphology with distinct membrane compositions in the two lobes and accumulation of F-actin at the inner nuclear envelope. The addition of Lamin A (encoded by lmna), which is absent from Xenopus eggs, results in rounder nuclei, suggesting that opposing nuclear F-actin and Lamin A forces contribute to the regulation of nuclear shape. Nuclear F-actin also promotes altered nuclear shape in Lamin A-knockdown HeLa cells and, in both systems, abnormal nuclear shape is driven by formins and not Arp2/3 or myosin. Although the underlying mechanisms might differ in Xenopus and HeLa cells, we propose that nuclear F-actin filaments nucleated by formins impart outward forces that lead to altered nuclear morphology unless Lamin A is present. Targeting nuclear actin dynamics might represent a novel approach to rescuing disease-associated defects in nuclear shape.

Shp2 contributes to the regulation of nuclear shape and cellular viscoelasticity in response to substrate spatial cues.

Cell polarization can be guided by substrate topology through space constraints and adhesion induction, which are part of cellular mechanosensing pathways. Here, we demonstrated that protein tyrosine phosphatase Shp2 plays a crucial role in mediating the response of cells to substrate spatial cues. When compared to cells spreading on surfaces coated uniformly with fibronectin (FN), cells attached to 10 µm-width FN-strip micropattern (MP), which provides spatial cues for uniaxial spreading, exhibited elongated focal adhesions (FAs) and aligned stress fibers in the direction of the MP. As a result of uniaxial cell spreading, nuclei became elongated, dependent on ROCK-mediated actomyosin contractility. Additionally, intracellular viscoelasticity also increased. Shp2-deficient cells did not display elongated FAs mediated by MP, well-aligned stress fibers, or changes in nuclear shape and intracellular viscoelasticity. Overall, our data suggest that Shp2 is involved in regulating FAs and the actin cytoskeleton to modulate nuclear shape and intracellular physical properties in response to substrate spatial cues.

YEATS domain-containing protein GAS41 regulates nuclear shape by working in concert with BRD2 and the mediator complex in colorectal cancer.

The maintenance of nuclear shape is essential for cellular homeostasis and disruptions in this process have been linked to various pathological conditions, including cancer, laminopathies, and aging. Despite the significance of nuclear shape, the precise molecular mechanisms controlling it are not fully understood. In this study, we have identified the YEATS domain-containing protein 4 (GAS41) as a previously unidentified factor involved in regulating nuclear morphology. Genetic ablation of GAS41 in colorectal cancer cells resulted in significant abnormalities in nuclear shape and inhibited cancer cell proliferation both in vitro and in vivo. Restoration experiments revealed that wild-type GAS41, but not a YEATS domain mutant devoid of histone H3 lysine 27 acetylation or crotonylation (H3K27ac/cr) binding, rescued the aberrant nuclear phenotypes in GAS41-deficient cells, highlighting the importance of GAS41's binding to H3K27ac/cr in nuclear shape regulation. Further experiments showed that GAS41 interacts with H3K27ac/cr to regulate the expression of key nuclear shape regulators, including LMNB1, LMNB2, SYNE4, and LEMD2. Mechanistically, GAS41 recruited BRD2 and the Mediator complex to gene loci of these regulators, promoting their transcriptional activation. Disruption of GAS41-H3K27ac/cr binding caused BRD2, MED14 and MED23 to dissociate from gene loci, leading to nuclear shape abnormalities. Overall, our findings demonstrate that GAS41 collaborates with BRD2 and the Mediator complex to control the expression of crucial nuclear shape regulators.

Myonuclear alterations associated with exercise are independent of age in humans.

Age-related decline in skeletal muscle structure and function can be mitigated by regular exercise. However, the precise mechanisms that govern this are not fully understood. The nucleus plays an active role in translating forces into biochemical signals (mechanotransduction), with the nuclear lamina protein lamin A regulating nuclear shape, nuclear mechanics and ultimately gene expression. Defective lamin A expression causes muscle pathologies and premature ageing syndromes, but the roles of nuclear structure and function in physiological ageing and in exercise adaptations remain obscure. Here, we isolated single muscle fibres and carried out detailed morphological and functional analyses on myonuclei from young and older exercise-trained individuals. Strikingly, myonuclei from trained individuals were more spherical, less deformable, and contained a thicker nuclear lamina than those from untrained individuals. Complementary to this, exercise resulted in increased levels of lamin A and increased myonuclear stiffness in mice. We conclude that exercise is associated with myonuclear remodelling, independently of age, which may contribute to the preservative effects of exercise on muscle function throughout the lifespan. KEY POINTS: The nucleus plays an active role in translating forces into biochemical signals. Myonuclear aberrations in a group of muscular dystrophies called laminopathies suggest that the shape and mechanical properties of myonuclei are important for maintaining muscle function. Here, striking differences are presented in myonuclear shape and mechanics associated with exercise, in both young and old humans. Myonuclei from trained individuals were more spherical, less deformable and contained a thicker nuclear lamina than untrained individuals. It is concluded that exercise is associated with age-independent myonuclear remodelling, which may help to maintain muscle function throughout the lifespan.

The nuclear lamina is required for proper development and nuclear shape distortion in tomato.

The nuclear lamina in plant cells is composed of plant-specific proteins, including nuclear matrix constituent proteins (NMCPs), which have been postulated to be functional analogs of lamin proteins that provide structural integrity to the organelle and help stabilize the three-dimensional organization of the genome. Using genomic editing, we generated alleles for the three genes encoding NMCPs in cultivated tomato (Solanum lycopersicum) to determine if the consequences of perturbing the nuclear lamina in this crop species were similar to or distinct from those observed in the model Arabidopsis thaliana. Loss of the sole NMCP2-class protein was lethal in tomato but is tolerated in Arabidopsis. Moreover, depletion of NMCP1-type nuclear lamina proteins leads to distinct developmental phenotypes in tomato, including leaf morphology defects and reduced root growth rate (in nmcp1b mutants), compared with cognate mutants in Arabidopsis. These findings suggest that the nuclear lamina interfaces with different developmental and signaling pathways in tomato compared with Arabidopsis. At the subcellular level, however, tomato nmcp mutants resembled their Arabidopsis counterparts in displaying smaller and more spherical nuclei in differentiated cells. This result argues that the plant nuclear lamina facilitates nuclear shape distortion in response to forces exerted on the organelle within the cell.

Nuclear morphology in breast lesions: refining its assessment to improve diagnostic concordance.

AIMS: Although evaluation of nuclear morphology is important for the diagnosis and categorisation of breast lesions, the criteria used to assess nuclear atypia rely upon the subjective evaluation of several features that may result in inter- and intraobserver variation. This study aims to refine the definitions of cytonuclear features in various breast lesions. METHODS AND RESULTS: ImageJ was used to assess the nuclear morphological features including nuclear diameter, axis length, perimeter, area, circularity and roundness in 160 breast lesions comprising ductal carcinoma in situ (DCIS), invasive breast carcinoma of no special type (IBC-NST), tubular carcinoma, usual ductal hyperplasia (UDH), columnar cell change (CCC) and flat epithelial atypia (FEA). Reference cells included normal epithelial cells, red blood cells (RBCs) and lymphocytes. Reference cells showed size differences not only between normal epithelial cells and RBCs but also between RBCs in varied-sized blood vessels. Nottingham grade nuclear pleomorphism scores 1 and 3 cut-offs in IBC-NST, compared to normal epithelial cells, were < ×1.2 and > ×1.4 that of mean maximum Feret's diameter and < ×1.6 and > ×2.4 that of mean nuclear area, respectively. Nuclear morphometrics were significantly different in low-grade IBC-NST versus tubular carcinoma, low-grade DCIS versus UDH and CCC versus FEA. No differences in the nuclear features between grade-matched DCIS and IBC-NST were identified. CONCLUSION: This study provides a guide for the assessment of nuclear atypia in breast lesions, refines the comparison with reference cells and highlights the potential diagnostic value of image analysis tools in the era of digital pathology.

The challenge of staying in shape: nuclear size matters.

Cellular organelles have unique morphology and the organelle size to cell size ratio is regulated. Nucleus is one of the most prominent, usually round in shape, organelle of a eukaryotic cell that occupies 8-10% of cellular volume. The shape and size of nucleus is known to undergo remodeling during processes such as cell growth, division and certain stresses. Regulation of protein and lipid distribution at the nuclear envelope is crucial for preserving the nuclear morphology and size. As size and morphology are interlinked, altering one influences the other. In this perspective, we discuss the relationship between size and shape regulation of the nucleus.

The nuclear envelope protein KAKU4 determines the migration order of the vegetative nucleus and sperm cells in pollen tubes.

A putative component protein of the nuclear lamina, KAKU4, modulates nuclear morphology in Arabidopsis thaliana seedlings, but its physiological significance is unknown. KAKU4 was highly expressed in mature pollen grains, each of which has a vegetative cell and two sperm cells. KAKU4 protein was highly abundant on the envelopes of vegetative nuclei and less abundant on the envelopes of sperm cell nuclei in pollen grains and elongating pollen tubes. Vegetative nuclei are irregularly shaped in wild-type pollen. However, KAKU4 deficiency caused them to become more spherical. After a pollen grain germinates, the vegetative nuclei and sperm cells enter and move along the pollen tube. In the wild type, the vegetative nucleus preceded the sperm cell nuclei in >90% of the pollen tubes, whereas, in kaku4 mutants, the vegetative nucleus preceded the sperm cell nuclei in only about half of the pollen tubes. kaku4 pollen was less competitive for fertilization than wild-type pollen after pollination. These results led us to hypothesize that the nuclear shape in vegetative cells of pollen grains affects the orderly migration of the vegetative nucleus and sperm cells in pollen tubes.

Apical cell protrusions cause vertical deformation of the soft cancer nucleus.

Breast cancer nuclei have highly irregular shapes, which are diagnostic and prognostic markers of breast cancer progression. The mechanisms by which irregular cancer nuclear shapes develop are not well understood. Here we report the existence of vertical, apical cell protrusions in cultured MDA-MB-231 breast cancer cells. Once formed, these protrusions persist over time scales of hours and are associated with vertically upward nuclear deformations. They are absent in normal mammary epithelial cells (MCF-10A cells). Microtubule disruption enriched these protrusions preferentially in MDA-MB-231 cells compared with MCF-10A cells, whereas inhibition of nonmuscle myosin II (NMMII) abolished this enrichment. Dynamic confocal imaging of the vertical cell and nuclear shape revealed that the apical cell protrusions form first, and in response, the nucleus deforms and/or subsequently gets vertically extruded into the apical protrusion. Overexpression of lamin A/C in MDA-MB-231 cells reduced nuclear deformation in apical protrusions. These data highlight the role of mechanical stresses generated by moving boundaries, as well as abnormal nuclear mechanics in the development of abnormal nuclear shapes in breast cancer cells.

Effect of PGC1-beta ablation on myonuclear organisation.

Skeletal muscle fibres are large, elongated multinucleated cells. Each nucleus within a myofibre is responsible for generating gene products for a finite volume of cytoplasm-the myonuclear domain (MND). Variation in MND sizes during atrophy, hypertrophy and disease states, are common. The factors that contribute to definitive MND sizes are not yet fully understood. Previous work has shown that peroxisome proliferator-activated receptor gamma coactivator 1α (PGC1-α) modulates MND volume, presumably to support increased biogenesis of mitochondria. The transcriptional co-regulator peroxisome proliferator-activated receptor gamma coactivator 1ß (PGC1-ß) is a homologue of PGC1-α with overlapping functions. To investigate the role of this protein in MND size regulation, we studied a mouse skeletal muscle specific knockout (cKO). Myofibres were isolated from the fast twitch extensor digitorum longus (EDL) muscle, membrane-permeabilised and analysed in 3 dimensions using confocal microscopy. PGC1-ß ablation resulted in no significant difference in MND size between cKO and wild type (WT) mice, however, subtle differences in nuclear morphology were observed. To determine whether these nuclear shape changes were associated with alterations in global transcriptional activity, acetyl histone H3 immunostaining was carried out. We found there was no significant difference in nuclear fluorescence intensity between the two genotypes. Overall, the results suggest that PGC-1α and PGC-1ß play different roles in regulating nuclear organisation in skeletal muscle; however, further work is required to pinpoint their exact functions.

Cell micropatterning reveals the modulatory effect of cell shape on proliferation through intracellular calcium transients.

The mechanism by which cell shape regulates the function of the cell is one of the most important biological issues, but it remains unclear. Here, we investigated the effect of the regulation of cell shape on proliferation by using a micropatterning approach to confine MC3T3-E1 cells into specific shapes. Our results show that the proliferation rate for rectangle-, triangle-, square- and circle-shaped osteoblasts increased sequentially and was related to the nuclear shape index (NSI) but not the cell shape index (CSI). Interestingly, intracellular calcium transients also displayed different patterns, with the number of Ca2+ peaks increasing with the NSI in shaped cells. Further causal investigation revealed that the gene expression levels of the inositol 1,4,5-triphosphate receptor 1 (IP3R1) and sarco/endoplasmic reticulum Ca2+-ATPase 2 (SERCA2), two major calcium cycling proteins in the endoplasmic reticulum (ER), were increased with an increase in NSI as a result of nuclear volume changes. Moreover, the down-regulation of IP3R1 and/or SERCA2 using shRNAs in circle-shaped or control osteoblasts resulted in changes in intracellular calcium transient patterns and cell proliferation rates towards that of smaller-NSI-shaped cells. Our results indicate that changes in cell shape changed nuclear morphology and then the gene expression of IP3R1 and SERCA2, which produced different intracellular calcium transient patterns. The patterns of intracellular calcium transients then determined the proliferation rate of the shaped osteoblasts.

The nucleus is irreversibly shaped by motion of cell boundaries in cancer and non-cancer cells.

Actomyosin stress fibers impinge on the nucleus and can exert compressive forces on it. These compressive forces have been proposed to elongate nuclei in fibroblasts, and lead to abnormally shaped nuclei in cancer cells. In these models, the elongated or flattened nuclear shape is proposed to store elastic energy. However, we found that deformed shapes of nuclei are unchanged even after removal of the cell with micro-dissection, both for smooth, elongated nuclei in fibroblasts and abnormally shaped nuclei in breast cancer cells. The lack of shape relaxation implies that the nuclear shape in spread cells does not store any elastic energy, and the cellular stresses that deform the nucleus are dissipative, not static. During cell spreading, the deviation of the nucleus from a convex shape increased in MDA-MB-231 cancer cells, but decreased in MCF-10A cells. Tracking changes of nuclear and cellular shape on micropatterned substrata revealed that fibroblast nuclei deform only during deformations in cell shape and only in the direction of nearby moving cell boundaries. We propose that motion of cell boundaries exert a stress on the nucleus, which allows the nucleus to mimic cell shape. The lack of elastic energy in the nuclear shape suggests that nuclear shape changes in cells occur at constant surface area and volume.

Direct force probe reveals the mechanics of nuclear homeostasis in the mammalian cell.

How cells maintain nuclear shape and position against various intracellular and extracellular forces is not well understood, although defects in nuclear mechanical homeostasis are associated with a variety of human diseases. We estimated the force required to displace and deform the nucleus in adherent living cells with a technique to locally pull the nuclear surface. A minimum pulling force of a few nanonewtons--far greater than typical intracellular motor forces--was required to significantly displace and deform the nucleus. Upon force removal, the original shape and position were restored quickly within a few seconds. This stiff, elastic response required the presence of vimentin, lamin A/C, and SUN (Sad1p, UNC-84)-domain protein linkages, but not F-actin or microtubules. Although F-actin and microtubules are known to exert mechanical forces on the nuclear surface through molecular motor activity, we conclude that the intermediate filament networks maintain nuclear mechanical homeostasis against localized forces.

GNG11 (G-protein subunit γ 11) suppresses cell growth with induction of reactive oxygen species and abnormal nuclear morphology in human SUSM-1 cells.

Enforced expression of GNG11, G-protein subunit γ 11, induces cellular senescence in normal human diploid fibroblasts. We here examined the effect of the expression of GNG11 on the growth of immortalized human cell lines, and found that it suppressed the growth of SUSM-1 cells, but not of HeLa cells. We then compared these two cell lines to understand the molecular basis for the action of GNG11. We found that expression of GNG11 induced the generation of reactive oxygen species (ROS) and abnormal nuclear morphology in SUSM-1 cells but not in HeLa cells. Increased ROS generation by GNG11 would likely be caused by the down-regulation of the antioxidant enzymes in SUSM-1 cells. We also found that SUSM-1 cells, even under normal culture conditions, showed higher levels of ROS and higher incidence of abnormal nuclear morphology than HeLa cells, and that abnormal nuclear morphology was relevant to the increased ROS generation in SUSM-1 cells. Thus, SUSM-1 and HeLa cells showed differences in the regulation of ROS and nuclear morphology, which might account for their different responses to the expression of GNG11. Thus, SUSM-1 cells may provide a unique system to study the regulatory relationship between ROS generation, nuclear morphology, and G-protein signaling.

Serial block face SEM visualization of unusual plant nuclear tubular extensions in a carnivorous plant (Utricularia, Lentibulariaceae).

Background and Aims: In Utricularia nelumbifolia , the nuclei of placental nutritive tissue possess unusually shaped projections not known to occur in any other flowering plant. The main aim of the study was to document the morphology and ultrastructure of these unusual nuclei. In addition, the literature was searched to find examples of nuclear tubular projections in other plant groups, and the nuclei of closely related species of Utricularia (i.e. sects Iperua , Orchidioides , Foliosa and Utricularia ) were examined. Methods: To visualize the complexity of the nuclear structures, transmission electron microscopy (TEM) was used, and 3-D ultrastructural reconstructions were made using the serial block face scanning electron microscopy (SBEM) technique. The nuclei of 11 Utricularia species, i.e. U. nelumbifolia , U. reniformis , U. cornigera , U. nephrophylla (sect. Iperua ), U. asplundii , U. alpina , U. quelchii (sect. Orchidioides ), U. longifolia (sect. Foliosa ), U. intermedia , U. minor and U. gibba (sect. Utricularia ) were examined. Key Results: Of the 11 Utricularia species examined, the spindle-like tubular projections (approx. 5 µm long) emanating from resident nuclei located in placental nutritive tissues were observed only in U. nelumbifolia . These tubular nuclear extensions contained chromatin distributed along hexagonally shaped tubules. The apices of the projections extended into the cell plasma membrane, and in many cases also made contact at the two opposing cellular poles, and with plasmodesmata via a short cisterna of the cortical endoplasmic reticulum. Images from the SBEM provide some evidence that the nuclear projections are making contact with those of neighbouring cells. Conclusions: The term chromatubules (chromatin-filled tubules) for the nuclear projections of U. nelumbifolia placental tissue was proposed here. Due to the apparent association with the plasma membrane and plasmodesmata, it was also speculated that chromatubules are involved in nucleus-cell-cell communication. However, further experimental evidence is required before any functional hypothesis can be entertained.

New approaches for understanding the nuclear force balance in living, adherent cells.

Cytoskeletal forces are transmitted to the nucleus to position and shape it. Linkages mediated by the LINC (linker of nucleoskeleton and cytoskeleton) complex transfer these forces to the nuclear envelope. Nuclear position and shape can be thought to be determined by a balance of cytoskeletal forces generated by microtubule motors that shear the nuclear surface, actomyosin forces that can pull, push and shear the nucleus, and intermediate filaments that may passively resist nuclear decentering and deformation. Parsing contributions of these different forces to nuclear mechanics is a very challenging task. Here we review new approaches that can be used in living cells to probe and understand the nuclear force balance.

Insights into blood cell formation from hemogenic endothelium in lesser-known anatomic sites.

BACKGROUND: Hematopoietic stem and progenitor cells (HSPCs) are generated de novo in the embryo in a process termed the endothelial to hematopoietic transition (EHT). EHT is most extensively studied in the yolk sac and dorsal aorta. Recently new sites of hematopoiesis have been described, including the heart, somites, head, and venous plexus of the yolk sac. RESULTS: We examined sites of HSPC formation in well-studied and in less well-known sites by mapping the expression of the key EHT factor Runx1 along with several other markers by means of confocal microscopy. We identified sites of HSPC formation in the head, heart and somites. We also identified sites of HSPC formation in both the arterial and venous plexuses of the yolk sac, and show that progenitors with lymphoid potential are enriched in hematopoietic clusters in close proximity to arteries. Furthermore, we demonstrate that many of the cells in hematopoietic clusters resemble monocytes or granulocytes based on nuclear shape. CONCLUSIONS: We identified sites of HSPC formation in the head, heart, and somites, confirming that embryonic hematopoiesis is less spatially restricted than previously thought. Furthermore, we show that HSPCs in the yolk sac with lymphoid potential are located in closer proximity to arteries than to veins. Developmental Dynamics 245:1011-1028, 2016. © 2016 Wiley Periodicals, Inc.