Stem integrity in Arabidopsis thaliana requires a load-bearing epidermis.

Because plant cells are glued to each other via their cell walls, failure to coordinate growth among adjacent cells can create cracks in tissues. Here, we find that the unbalanced growth of inner and outer tissues in the clavata3 de-etiolated3 (clv3 det3) mutant of Arabidopsis thaliana stretched epidermal cells, ultimately generating cracks in stems. Stem growth slowed before cracks appeared along clv3 det3 stems, whereas inner pith cells became drastically distorted and accelerated their growth, yielding to stress, after the appearance of cracks. This is consistent with a key role of the epidermis in restricting growth. Mechanical property measurements recorded using an atomic force microscope revealed that epidermal cell wall stiffness decreased in det3 and clv3 det3 epidermises. Thus, we hypothesized that stem integrity depends on the epidermal resistance to mechanical stress. To formally test this hypothesis, we used the DET3 gene as part of a tissue-specific strategy to complement cell expansion defects. Epidermis-driven DET3 expression restored growth and restored the frequency of stem cracking to 20% of the clv3 det3 mutant, demonstrating the DET3-dependent load-bearing role of the epidermis.

Gradient in cytoplasmic pressure in germline cells controls overlying epithelial cell morphogenesis.

It is unknown how growth in one tissue impacts morphogenesis in a neighboring tissue. To address this, we used the Drosophila ovarian follicle, in which a cluster of 15 nurse cells and a posteriorly located oocyte are surrounded by a layer of epithelial cells. It is known that as the nurse cells grow, the overlying epithelial cells flatten in a wave that begins in the anterior. Here, we demonstrate that an anterior to posterior gradient of decreasing cytoplasmic pressure is present across the nurse cells and that this gradient acts through TGFß to control both the triggering and the progression of the wave of epithelial cell flattening. Our data indicate that intrinsic nurse cell growth is important to control proper nurse cell pressure. Finally, we reveal that nurse cell pressure and subsequent TGFß activity in the stretched cells combine to increase follicle elongation in the anterior, which is crucial for allowing nurse cell growth and pressure control. More generally, our results reveal that during development, inner cytoplasmic pressure in individual cells has an important role in shaping their neighbors.

Variations in basement membrane mechanics are linked to epithelial morphogenesis.

The regulation of morphogenesis by the basement membrane (BM) may rely on changes in its mechanical properties. To test this, we developed an atomic force microscopy-based method to measure BM mechanical stiffness during two key processes in Drosophila ovarian follicle development. First, follicle elongation depends on epithelial cells that collectively migrate, secreting BM fibrils perpendicularly to the anteroposterior axis. Our data show that BM stiffness increases during this migration and that fibril incorporation enhances BM stiffness. In addition, stiffness heterogeneity, due to oriented fibrils, is important for egg elongation. Second, epithelial cells change their shape from cuboidal to either squamous or columnar. We prove that BM softens around the squamous cells and that this softening depends on the TGFß pathway. We also demonstrate that interactions between BM constituents are necessary for cell flattening. Altogether, these results show that BM mechanical properties are modified during development and that, in turn, such mechanical modifications influence both cell and tissue shapes.

Stromal protein ßig-h3 reprogrammes tumour microenvironment in pancreatic cancer.

OBJECTIVE: Pancreatic cancer is associated with an abundant stromal reaction leading to immune escape and tumour growth. This massive stroma drives the immune escape in the tumour. We aimed to study the impact of ßig-h3 stromal protein in the modulation of the antitumoural immune response in pancreatic cancer. DESIGN: We performed studies with p48-Cre;KrasG12D, pdx1-Cre;KrasG12D;Ink4a/Arffl/fl, pdx1-Cre;KrasG12D; p53R172H mice and tumour tissues from patients with pancreatic ductal adenocarcinoma (PDA). Some transgenic mice were given injections of anti-ßig-h3, anti-CD8, anti-PD1 depleting antibodies. Tumour growth as well as modifications in the activation of local immune cells were analysed by flow cytometry, immunohistochemistry and immunofluorescence. Tissue stiffness was measured by atomic force microscopy. RESULTS: We identified ßig-h3 stromal-derived protein as a key actor of the immune paracrine interaction mechanism that drives pancreatic cancer. We found that ßig-h3 is highly produced by cancer-associated fibroblasts in the stroma of human and mouse. This protein acts directly on tumour-specific CD8+ T cells and F4/80 macrophages. Depleting ßig-h3 in vivo reduced tumour growth by enhancing the number of activated CD8+ T cell within the tumour and subsequent apoptotic tumour cells. Furthermore, we found that targeting ßig-h3 in established lesions released the tissue tension and functionally reprogrammed F4/80 macrophages in the tumour microenvironment. CONCLUSIONS: Our data indicate that targeting stromal extracellular matrix protein ßig-h3 improves the antitumoural response and consequently reduces tumour weight. Our findings present ßig-h3 as a novel immunological target in pancreatic cancer.

Changes in nano-mechanical properties of human epidermal cornified cells depending on their proximity to the skin surface.

During formation of the stratum corneum (SC) barrier, terminally differentiated keratinocytes continue their maturation process within the dead superficial epidermal layer. Morphological studies of isolated human corneocytes have revealed differences between cornified envelopes purified from the deep and superficial SC. We used atomic force microscopy to measure the mechanical properties of native human corneocytes harvested by tape-stripping from different SC depths. Various conditions of data acquisition have been tested and optimized, in order to obtain exploitable and reproducible results. Probing at 200 nN allowed us to investigate the total stiffness of the cells (at 50 nm indentation) and that of the cornified envelopes (at 10 to15 nm), and lipid envelopes (at 5 to 10 nm). The obtained data indicated statistically significant differences between the superficial (more rigid) and deep (softer) corneocytes, thus confirming the existence of depth and maturation-related morphological changes within the SC. The proposed approach can be potentially used for minimally invasive evaluation of various skin conditions such as aging, skin hydration, and pathologies linked to SC.

Matching Patterns of Gene Expression to Mechanical Stiffness at Cell Resolution through Quantitative Tandem Epifluorescence and Nanoindentation.

Cell differentiation has been associated with changes in mechanical stiffness in single-cell systems, yet it is unknown whether this association remains true in a multicellular context, particularly in developing tissues. In order to address such questions, we have developed a methodology, termed quantitative tandem epifluorescence and nanoindentation, wherein we sequentially determine cellular genetic identity with confocal microscopy and mechanical properties with atomic force microscopy. We have applied this approach to examine cellular stiffness at the shoot apices of Arabidopsis (Arabidopsis thaliana) plants carrying a fluorescent reporter for the CLAVATA3 (CLV3) gene, which encodes a secreted glycopeptide involved in the regulation of the centrally located stem cell zone in inflorescence and floral meristems. We found that these CLV3-expressing cells are characterized by an enhanced stiffness. Additionally, by tracking cells in young flowers before and after the onset of GREEN FLUORESCENT PROTEIN expression, we observed that an increase in stiffness coincides with this onset. This work illustrates how quantitative tandem epifluorescence and nanoindentation can reveal the spatial and temporal dynamics of both gene expression and cell mechanics at the shoot apex and, by extension, in the epidermis of any thick tissue.

Shrinking the hammer: micromechanical approaches to morphogenesis.

Morphogenesis, the remarkable process by which a developing organism achieves its shape, relies on the coordinated growth of cells, tissues, and organs. While the molecular and genetic basis of morphogenesis is starting to be unravelled, understanding shape changes is lagging behind. Actually, shape is imposed by the structural elements of the organism, and the translation of cellular activity into morphogenesis must go through these elements. Therefore, many methods have been developed recently to quantify, at cellular resolution, the properties of the main structural element in plants, the cell wall. As plant cell growth is restrained by the cell wall and powered by turgor pressure, such methods also address the quantification of turgor. These different micromechanical approaches are reviewed here, with a critical assessment of their strengths and weaknesses, and a discussion of how they can help us understand the regulation of growth and morphogenesis.

Stiffness-induced cancer-associated fibroblasts are responsible for immunosuppression in a platelet-derived growth factor ligand-dependent manner.

Pancreatic ductal adenocarcinoma (PDAC) is associated with a vast stromal reaction that arises mainly from cancer-associated fibroblasts (CAFs) and promotes both immune escape and tumor growth. Here, we used a mouse model with deletion of the activin A receptor ALK4 in the context of the KrasG12D mutation, which strongly drives collagen deposition that leads to tissue stiffness. By ligand-receptor analysis of single-cell RNA-sequencing data, we identified that, in stiff conditions, neoplastic ductal cells instructed CAFs through sustained platelet-derived growth factor (PDGF) signaling. Tumor-associated tissue rigidity resulted in the emergence of stiffness-induced CAFs (siCAFs) in vitro and in vivo. Similar results were confirmed in human data. siCAFs were able to strongly inhibit CD8+ T-cell responses in vitro and in vivo, promoting local immunosuppression. More importantly, targeting PDGF signaling led to diminished siCAF and reduced tumor growth. Our data show for the first time that early paracrine signaling leads to profound changes in tissue mechanics, impacting immune responses and tumor progression. Our study highlights that PDGF ligand neutralization can normalize the tissue architecture independent of the genetic background, indicating that finely tuned stromal therapy may open new therapeutic avenues in pancreatic cancer.

In vivo analysis of local wall stiffness at the shoot apical meristem in Arabidopsis using atomic force microscopy.

Whereas the morphogenesis of developing organisms is relatively well understood at the molecular level, the contribution of the mechanical properties of the cells to shape changes remains largely unknown, mainly because of the lack of quantified biophysical parameters at cellular or subcellular resolution. Here we designed an atomic force microscopy approach to investigate the elastic modulus of the outer cell wall in living shoot apical meristems (SAMs). SAMs are highly organized structures that contain the plant stem cells, and generate all of the aerial organs of the plant. Building on modeling and experimental data, we designed a protocol that is able to measure very local properties, i.e. within 40-100 nm deep into the wall of living meristematic cells. We identified three levels of complexity at the meristem surface, with significant heterogeneity in stiffness at regional, cellular and even subcellular levels. Strikingly, we found that the outer cell wall was much stiffer at the tip of the meristem (5 ± 2 MPa on average), covering the stem cell pool, than on the flanks of the meristem (1.5 ± 0.7 MPa on average). Altogether, these results demonstrate the existence of a multiscale spatialization of the mechanical properties of the meristem surface, in addition to the previously established molecular and cytological zonation of the SAM, correlating with regional growth rate distribution.

Nucleosome positioning by genomic excluding-energy barriers.

Recent genome-wide nucleosome mappings along with bioinformatics studies have confirmed that the DNA sequence plays a more important role in the collective organization of nucleosomes in vivo than previously thought. Yet in living cells, this organization also results from the action of various external factors like DNA-binding proteins and chromatin remodelers. To decipher the code for intrinsic chromatin organization, there is thus a need for in vitro experiments to bridge the gap between computational models of nucleosome sequence preferences and in vivo nucleosome occupancy data. Here we combine atomic force microscopy in liquid and theoretical modeling to demonstrate that a major sequence signaling in vivo are high-energy barriers that locally inhibit nucleosome formation rather than favorable positioning motifs. We show that these genomic excluding-energy barriers condition the collective assembly of neighboring nucleosomes consistently with equilibrium statistical ordering principles. The analysis of two gene promoter regions in Saccharomyces cerevisiae and the human genome indicates that these genomic barriers direct the intrinsic nucleosome occupancy of regulatory sites, thereby contributing to gene expression regulation.

ßig-h3-structured collagen alters macrophage phenotype and function in pancreatic cancer.

Macrophages play an important role in immune and matrix regulation during pancreatic adenocarcinoma (PDAC). Collagen deposition massively contributes to the physical and functional changes of the tissue during pathogenesis. We investigated the impact of thick collagen fibers on the phenotype and function of macrophages. We recently demonstrated that the extracellular protein ßig-h3/TGFßi (Transforming growth factor-ß-induced protein) plays an important role in modulating the stiffness of the pancreatic stroma. By using atomic force microscopy, we show that ßig-h3 binds to type I collagen and establishes thicker fibers. Macrophages cultured on ßig-h3-structured collagen layers display a different morphology and a pro-tumoral M2 phenotype and function compared to those cultured on non-structured collagen layers. In vivo injection of those instructed CD206+CD163+ macrophages was able to suppress T cell responses. These results reveal for the first time that the collagen structure impacts the phenotype and function of macrophages by potentiating their immunosuppressive features.

NanoIndentation, an ImageJ Plugin for the Quantification of Cell Mechanics.

Growth and morphogenesis in plants depend on cell wall mechanics and on turgor pressure. Nanoindentation methods, such as atomic force microscopy (AFM), enable measurements of mechanical properties of a tissue at subcellular resolution, while confocal microscopy of tissues expressing fluorescent reporters indicates cell identity. Associating mechanical data with specific cells is essential to reveal the links between cell identity and cell mechanics. Here we describe an image analysis protocol that allows us to segment AFM scans containing information on tissue topography and/or mechanics, to stitch several scans in order to reconstitute an entire region of the tissue investigated, to segment the scans and label cells, and to associate labeled cells to the projection of confocal images. Thus all mechanical data can be mapped to the corresponding cells and to their identity. This protocol is implemented using NanoIndentation, a plugin that we are developing in the Fiji distribution of ImageJ.

Mechanical Shielding in Plant Nuclei.

In animal single cells in culture, nuclear geometry and stiffness can be affected by mechanical cues, with important consequences for chromatin status and gene expression. This calls for additional investigation into the corresponding physiological relevance in a multicellular context and in different mechanical environments. Using the Arabidopsis root as a model system, and combining morphometry and micro-rheometry, we found that hyperosmotic stress decreases nuclear circularity and size and increases nuclear stiffness in meristematic cells. These changes were accompanied by enhanced expression of touch response genes. The nuclear response to hyperosmotic stress was rescued upon return to iso-osmotic conditions and could even lead to opposite trends upon hypo-osmotic stress. Interestingly, nuclei in a mutant impaired in the functions of the gamma-tubulin complex protein 3 (GCP3) interacting protein (GIP)/MZT1 proteins at the nuclear envelope were almost insensitive to such osmotic changes. The gip1gip2 mutant exhibited constitutive hyperosmotic stress response with stiffer and deformed nuclei, as well as touch response gene induction. The mutant was also resistant to lethal hyperosmotic conditions. Altogether, we unravel a stereotypical geometric, mechanical, and genetic nuclear response to hyperosmotic stress in plants. Our data also suggest that chromatin acts as a gel that stiffens in hyperosmotic conditions and that the nuclear-envelope-associated protein GIPs act as negative regulators of this response.

Atomic force microscopy of DNA in solution and DNA modelling show that structural properties specify the eukaryotic replication initiation site.

The replication origins (ORIs) of Schizosaccharomyces pombe, like those in most eukaryotes, are long chromosomal regions localized within A+T-rich domains. Although there is no consensus sequence, the interacting proteins are strongly conserved, suggesting that DNA structure is important for ORI function. We used atomic force microscopy in solution and DNA modelling to study the structural properties of the Spars1 origin. We show that this segment is the least stable of the surrounding DNA (9 kb), and contains regions of intrinsically bent elements (strongly curved and inherently supercoiled DNAs). The pORC-binding site co-maps with a superhelical DNA region, where the spatial arrangement of adenine/thymine stretches may provide the binding substrate. The replication initiation site (RIP) is located within a strongly curved DNA region. On pORC unwinding, this site shifts towards the apex of the curvature, thus potentiating DNA melting there. Our model is entirely consistent with the sequence variability, large size and A+T-richness of ORIs, and also accounts for the multistep nature of the initiation process, the specificity of pORC-binding site(s), and the specific location of RIP. We show that the particular DNA features and dynamic properties identified in Spars1 are present in other eukaryotic origins.

TBP binding capacity of the TATA box is associated with specific structural properties: AFM study of the IL-2R alpha gene promoter.

DNA is not only a nucleotide sequence which allows the binding of regulators but its intrinsic structural properties such as curvature and flexibility are also viewed as playing an active role in the regulation of transcription. Our combination of computer modelling and AFM imaging allow direct access to DNA curvature and flexibility. We have searched for these DNA structural features involved in transcription regulation within the IL-2Ralpha gene promoter. Investigation of these structural characteristics shows concordant results. First, in the core promoter, the region containing the functional TATA box shows intrinsic curvature associated with a peculiar distribution of flexibility. Both these inherent properties are characteristic of this region as compared with the other parts of the promoter. Second, the proximal promoter exhibits two important regions: a first one flexible and curved, followed by a segment of rigid linear DNA, each localised within one of the two Positive Regulatory Regions PRRI and PRRII respectively. Based on these observations, we propose different roles for DNA curvature and/or flexibility in promoter sequences.

Gradual melting of a replication origin (Schizosaccharomyces pombe ars1): in situ atomic force microscopy (AFM) analysis.

Local DNA melting is integral to fundamental processes such as replication or transcription. In vivo, these two processes do not occur on molecules free in solution but, instead, involve DNA molecules which are organized into DNA/proteins complexes. Atomic force microscopy imaging offers a possibility to look at individual molecules. It allowed us to follow the progress of local denaturation in liquid, but with the added constraints of DNA lying on a surface. We present a kinetic analysis of the mapping of the temperature-driven melting seen at a replication origin (Schizosaccharomyces pombe ars1). The results indicate an expected base composition dependency, but also a strong extremity effect. Noteworthy, a "structural" effect is clearly occurring - which is shown by the greater susceptibility of the strongly curved region present in the sequence to unwind. DNA melting, at this place, is seen to occur after an increase in the curvature amplitude and a simultaneous shift of the nucleotide sequence positioned at the apex. Because this may determine the position of the Replication Initiation (R.I.) site, the result suggests that eukaryotic replication origins, although described as possessing no consensus sequences, may well have their mechanics sustained by the properties of common structural features. Our analysis may, therefore, provide new information that will give genuine insights on how DNA molecules behave when organized into primosomes, replisomes, promoter initiation complexes, etc. and thus, be essential to better understanding the way genes function.

Mechanochemical Polarization of Contiguous Cell Walls Shapes Plant Pavement Cells.

The epidermis of aerial plant organs is thought to be limiting for growth, because it acts as a continuous load-bearing layer, resisting tension. Leaf epidermis contains jigsaw puzzle piece-shaped pavement cells whose shape has been proposed to be a result of subcellular variations in expansion rate that induce local buckling events. Paradoxically, such local compressive buckling should not occur given the tensile stresses across the epidermis. Using computational modeling, we show that the simplest scenario to explain pavement cell shapes within an epidermis under tension must involve mechanical wall heterogeneities across and along the anticlinal pavement cell walls between adjacent cells. Combining genetics, atomic force microscopy, and immunolabeling, we demonstrate that contiguous cell walls indeed exhibit hybrid mechanochemical properties. Such biochemical wall heterogeneities precede wall bending. Altogether, this provides a possible mechanism for the generation of complex plant cell shapes.

Subcellular and supracellular mechanical stress prescribes cytoskeleton behavior in Arabidopsis cotyledon pavement cells.

Although it is a central question in biology, how cell shape controls intracellular dynamics largely remains an open question. Here, we show that the shape of Arabidopsis pavement cells creates a stress pattern that controls microtubule orientation, which then guides cell wall reinforcement. Live-imaging, combined with modeling of cell mechanics, shows that microtubules align along the maximal tensile stress direction within the cells, and atomic force microscopy demonstrates that this leads to reinforcement of the cell wall parallel to the microtubules. This feedback loop is regulated: cell-shape derived stresses could be overridden by imposed tissue level stresses, showing how competition between subcellular and supracellular cues control microtubule behavior. Furthermore, at the microtubule level, we identified an amplification mechanism in which mechanical stress promotes the microtubule response to stress by increasing severing activity. These multiscale feedbacks likely contribute to the robustness of microtubule behavior in plant epidermis. DOI: http://dx.doi.org/10.7554/eLife.01967.001.

An auxin-mediated shift toward growth isotropy promotes organ formation at the shoot meristem in Arabidopsis.

To control morphogenesis, molecular regulatory networks have to interfere with the mechanical properties of the individual cells of developing organs and tissues, but how this is achieved is not well known. We study this issue here in the shoot meristem of higher plants, a group of undifferentiated cells where complex changes in growth rates and directions lead to the continuous formation of new organs. Here, we show that the plant hormone auxin plays an important role in this process via a dual, local effect on the extracellular matrix, the cell wall, which determines cell shape. Our study reveals that auxin not only causes a limited reduction in wall stiffness but also directly interferes with wall anisotropy via the regulation of cortical microtubule dynamics. We further show that to induce growth isotropy and organ outgrowth, auxin somehow interferes with the cortical microtubule-ordering activity of a network of proteins, including AUXIN BINDING PROTEIN 1 and KATANIN 1. Numerical simulations further indicate that the induced isotropy is sufficient to amplify the effects of the relatively minor changes in wall stiffness to promote organogenesis and the establishment of new growth axes in a robust manner.

Mechanics of the IL2RA gene activation revealed by modeling and atomic force microscopy.

Transcription implies recruitment of RNA polymerase II and transcription factors (TFs) by DNA melting near transcription start site (TSS). Combining atomic force microscopy and computer modeling, we investigate the structural and dynamical properties of the IL2RA promoter and identify an intrinsically negative supercoil in the PRRII region (containing Elf-1 and HMGA1 binding sites), located upstream of a curved DNA region encompassing TSS. Conformational changes, evidenced by time-lapse studies, result in the progressive positioning of curvature apex towards the TSS, likely facilitating local DNA melting. In vitro assays confirm specific binding of the General Transcription Factors (GTFs) TBP and TFIIB over TATA-TSS position, where an inhibitory nucleosome prevented preinitiation complex (PIC) formation and uncontrolled DNA melting. These findings represent a substantial advance showing, first, that the structural properties of the IL2RA promoter are encoded in the DNA sequence and second, that during the initiation process DNA conformation is dynamic and not static.