Mechanosensitive mechanisms in transcriptional regulation

Identification of mechanosensitive genes during skeletal development: alteration of genes associated with cytoskeletal rearrangement and cell signalling pathways

Background

Mechanical stimulation is necessary for regulating correct formation of the skeleton. Here we test the hypothesis that mechanical stimulation of the embryonic skeletal system impacts expression levels of genes implicated in developmentally important signalling pathways in a genome wide approach. We use a mutant mouse model with altered mechanical stimulation due to the absence of limb skeletal muscle (Splotch-delayed) where muscle-less embryos show specific defects in skeletal elements including delayed ossification, changes in the size and shape of cartilage rudiments and joint fusion. We used Microarray and RNA sequencing analysis tools to identify differentially expressed genes between muscle-less and control embryonic (TS23) humerus tissue.

Results

We found that 680 independent genes were down-regulated and 452 genes up-regulated in humeri from muscle-less Spd embryos compared to littermate controls (at least 2-fold; corrected p-value ≤0.05). We analysed the resulting differentially expressed gene sets using Gene Ontology annotations to identify significant enrichment of genes associated with particular biological processes, showing that removal of mechanical stimuli from muscle contractions affected genes associated with development and differentiation, cytoskeletal architecture and cell signalling. Among cell signalling pathways, the most strongly disturbed was Wnt signalling, with 34 genes including 19 pathway target genes affected. Spatial gene expression analysis showed that both a Wnt ligand encoding gene (Wnt4) and a pathway antagonist (Sfrp2) are up-regulated specifically in the developing joint line, while the expression of a Wnt target gene, Cd44, is no longer detectable in muscle-less embryos. The identification of 84 genes associated with the cytoskeleton that are down-regulated in the absence of muscle indicates a number of candidate genes that are both mechanoresponsive and potentially involved in mechanotransduction, converting a mechanical stimulus into a transcriptional response.

Conclusions

This work identifies key developmental regulatory genes impacted by altered mechanical stimulation, sheds light on the molecular mechanisms that interpret mechanical stimulation during skeletal development and provides valuable resources for further investigation of the mechanistic basis of mechanoregulation. In particular it highlights the Wnt signalling pathway as a potential point of integration of mechanical and molecular signalling and cytoskeletal components as mediators of the response.

Extracellular matrix: a dynamic microenvironment for stem cell niche

Background

Extracellular matrix (ECM) is a dynamic and complex environment characterized by biophysical, mechanical and biochemical properties specific for each tissue and able to regulate cell behavior. Stem cells have a key role in the maintenance and regeneration of tissues and they are located in a specific microenvironment, defined as niche.

Scope of review

We overview the progresses that have been made in elucidating stem cell niches and discuss the mechanisms by which ECM affects stem cell behavior. We also summarize the current tools and experimental models for studying ECM–stem cell interactions.

Major conclusions

ECM represents an essential player in stem cell niche, since it can directly or indirectly modulate the maintenance, proliferation, self-renewal and differentiation of stem cells. Several ECM molecules play regulatory functions for different types of stem cells, and based on its molecular composition the ECM can be deposited and finely tuned for providing the most appropriate niche for stem cells in the various tissues. Engineered biomaterials able to mimic the in vivo characteristics of stem cell niche provide suitable in vitro tools for dissecting the different roles exerted by the ECM and its molecular components on stem cell behavior.

General significance

ECM is a key component of stem cell niches and is involved in various aspects of stem cell behavior, thus having a major impact on tissue homeostasis and regeneration under physiological and pathological conditions. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

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Stem cells have a key role in the maintenance and regeneration of tissues.

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The extracellular matrix is a critical regulator of stem cell function.

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Stem cells reside in a dynamic and specialized microenvironment denoted as niche.

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The extracellular matrix represents an essential component of stem cell niches.

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Bioengineered niches can be used for investigating stem cell–matrix interactions.

Mechanosensing in developing lymphatic vessels

The lymphatic vasculature is responsible for fluid homeostasis, transport of immune cells, inflammatory molecules, and dietary lipids. It is composed of a network of lymphatic capillaries that drain into collecting lymphatic vessels and ultimately bring fluid back to the blood circulation. Lymphatic endothelial cells (LECs) that line lymphatic capillaries present loose overlapping intercellular junctions and anchoring filaments that support fluid drainage. When interstitial fluid accumulates within tissues, the extracellular matrix (ECM) swells and pulls the anchoring filaments. This results in opening of the LEC junctions and permits interstitial fluid uptake. The absorbed fluid is then transported within collecting lymphatic vessels, which exhibit intraluminal valves that prevent lymph backflow and smooth muscle cells that sequentially contract to propel lymph.Mechanotransduction involves translation of mechanical stimuli into biological responses. LECs have been shown to sense and respond to changes in ECM stiffness, fluid pressure-induced cell stretch, and fluid flow-induced shear stress. How these signals influence LEC function and lymphatic vessel growth can be investigated by using different mechanotransduction assays in vitro and to some extent in vivo.In this chapter, we will focus on the mechanical forces that regulate lymphatic vessel expansion during embryonic development and possibly secondary lymphedema. In mouse embryos, it has been recently shown that the amount of interstitial fluid determines the extent of lymphatic vessel expansion via a mechanosensory complex formed by β1 integrin and vascular endothelial growth factor receptor-3 (VEGFR3). This model might as well apply to secondary lymphedema.

Mechanical forces as information: an integrated approach to plant and animal development

Mechanical forces such as tension and compression act throughout growth and development of multicellular organisms. These forces not only affect the size and shape of the cells and tissues but are capable of modifying the expression of genes and the localization of molecular components within the cell, in the plasma membrane, and in the plant cell wall. The magnitude and direction of these physical forces change with cellular and tissue properties such as elasticity. Thus, mechanical forces and the mesoscopic fields that emerge from their local action constitute important sources of positional information. Moreover, physical and biochemical processes interact in non-linear ways during tissue and organ growth in plants and animals. In this review we discuss how such mechanical forces are generated, transmitted, and sensed in these two lineages of multicellular organisms to yield long-range positional information. In order to do so we first outline a potentially common basis for studying patterning and mechanosensing that relies on the structural principle of tensegrity, and discuss how tensegral structures might arise in plants and animals. We then provide some examples of morphogenesis in which mechanical forces appear to act as positional information during development, offering a possible explanation for ubiquitous processes, such as the formation of periodic structures. Such examples, we argue, can be interpreted in terms of tensegral phenomena. Finally, we discuss the hypothesis of mechanically isotropic points as a potentially generic mechanism for the localization and maintenance of stem-cell niches in multicellular organisms. This comparative approach aims to help uncovering generic mechanisms of morphogenesis and thus reach a better understanding of the evolution and development of multicellular phenotypes, focusing on the role of physical forces in these processes.

Mechanobiology and developmental control

Morphogenesis is the remarkable process by which cells self-assemble into complex tissues and organs that exhibit specialized form and function during embryological development. Many of the genes and chemical cues that mediate tissue and organ formation have been identified; however, these signals alone are not sufficient to explain how tissues and organs are constructed that exhibit their unique material properties and three-dimensional forms. Here, we review work that has revealed the central role that physical forces and extracellular matrix mechanics play in the control of cell fate switching, pattern formation, and tissue development in the embryo and how these same mechanical signals contribute to tissue homeostasis and developmental control throughout adult life.

Neutrophil and monocyte recruitment by PECAM, CD99, and other molecules via the LBRC

The recruitment of specific leukocyte subtypes to the site of tissue injury is the cornerstone of inflammation and disease progression. This process has become an intense area of research because it presents several possible steps against which disease-specific therapies could be targeted. Leukocytes are recruited out of the blood stream by a series of events that include their capture, rolling, activation, and migration along the endothelium. In the last step, the leukocytes squeeze between adjacent endothelial cells to gain access to the inflamed tissue through a process referred to as transendothelial migration (TEM). Although many of the molecules, such as PECAM and CD99, that regulate these sequential steps have been identified, much less is understood regarding how they work together to coordinate the complex intercellular communications and dramatic shape changes that take place between the endothelial cells and leukocytes. Several of the endothelial cell proteins that function in TEM are localized to the lateral border recycling compartment (LBRC), an interconnected reticulum of membrane that recycles selectively to the endothelial borders. The recruitment of the LBRC to surround the migrating leukocyte is required for efficient TEM. This review will focus on the proteins and mechanisms that mediate TEM and specifically how the LBRC functions in the context of these molecular interactions and membrane movements.

A tetracycline expression system in combination with Sox9 for cartilage tissue engineering

Cartilage tissue engineering using controllable transcriptional therapy together with synthetic biopolymer scaffolds shows higher potential for overcoming chondrocyte degradation and constructing artificial cartilages both in vivo and in vitro. Here, the potential regulating tetracycline expression (Tet-on) system was used to express Sox9 both in vivo and in vitro. Chondrocyte degradation was measured in vitro and overcome by Soxf9 expression. Experiments confirmed the feasibility of the combined use of Sox9 and Tet-on system in cartilage tissue engineering. Engineered poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) scaffolds were seeded with recombinant chondrocytes which were transfected with Tet-induced Sox9 expression; the scaffolds were implanted under the skin of 8-week-old rats. The experimental group was injected with Dox in the abdomen, while the control group was injected with normal saline. After 4 or 8 days of implantation in vivo, the newly formed pieces of articular chondrocytes were taken out and measured. Dox injection in vivo showed positive effect on recombinant chondrocytes, in which Sox9 expression was up-regulated by an inducible system with specific matrix proteins. The results demonstrate this controllable transcriptional therapy is a potential approach for tissue engineering.

The effects of artificial E-cadherin matrix-induced embryonic stem cell scattering on paxillin and RhoA activation via α-catenin

Mechanical forces have been shown to affect stem cell behavior in a large array of ways. However, our understanding of how these mechanical cues may regulate the behavior of embryonic stem cells (ESCs) remains in its infancy. Here, we aim to clarify the effect of cell scattering on the regulation of Rho family GTPases Rac1 and RhoA as well as paxillin. Allowing ESCs to spread and scatter on a synthetically designed E-cadherin substratum causes phosphorylation of paxillin on consensus phosphorylation sites leading to activation of Rac1 and inactivation of RhoA. By culturing cells in presence of RhoA activator or growing cells to a highly confluent state reverses the effect of cell scattering phenotype. Knockdown of E-cadherin-adapter protein α-catenin revealed that it negatively affects paxillin phosphorylation and up-regulates RhoA activity in compact cellular aggregates. Collectively these results indicate that cell scattering might cause a conformational change of α-catenin limiting its capacity to inhibit paxillin phosphorylation that causes an increase in Rac1 activation and RhoA deactivation. Understanding how synthetically designed extracellular matrix affect ESC signaling through mechanical cues brings a new aspect for stem cell engineers to develop technologies for controlling cell function.

Saponin monomer 13 of dwarf lilyturf tuber (DT-13) protects serum withdrawal-induced apoptosis through PI3K/Akt in HUVEC

Dwarf lilyturf tuber is widely used in clinics to prevent cardiovascular diseases. DT-13, the saponin monomer 13 of dwarf lilyturf tuber, shows protective activities in anti-thrombosis, anti-inflammation, and cardioprotective. However, little is known about the cellular function of DT-13 in cardiovascular system. Vascular endothelial cells (EC) are important to maintain the integrity of the vasculature throughout entire body. Dysregulation of EC may lead to pathophysiological processes of numerous cardiovascular diseases. We thus tested the function of DT-13 in EC. In the present study, we are the first to report that DT-13 has anti-apoptosis activity on human umbilical vein endothelial cells (HUVEC), potentially through down regulation of cleaved caspase-3 and cleaved PARP expression. DT-13 also increased mitochondrial membrane potential. To explore the potential mechanism, we investigated the effect of DT-13 on Akt and MAPK pathways and found that DT-13 was involved in Akt signaling confirmed by using PI3K/Akt inhibitor LY294002. Thus, DT-13 could improve survival of EC and therefore be a potential clinical use in the treatment of cardiovascular diseases.

Engineering stem cell fate with biochemical and biomechanical properties of microcarriers

Microcarriers have been widely used for various biotechnology applications because of their high scale-up potential, high reproducibility in regulating cellular behavior, and well-documented compliance with current Good Manufacturing Practices (cGMP). Recently, microcarriers have been emerging as a novel approach for stem cell expansion and differentiation, enabling potential scale-up of stem cell-derived products in large bioreactors. This review summarizes recent advances of using microcarriers in mesenchymal stem cell (MSC) and pluripotent stem cell (PSC) cultures. From the reported data, efficient expansion and differentiation of stem cells on microcarriers rely on their ability to modulate cell shape (i.e. round or spreading) and cell organization (i.e. aggregate size). Nonetheless, current screening of microcarriers remains empirical, and accurate understanding of how stem cells interact with microcarriers still remains unknown. This review suggests that accurate characterization of biochemical and biomechanical properties of microcarriers is required to fully exploit their potential in regulating stem cell fate decision. Due to the variety of microcarriers, such detailed analyses should lead to the rational design of application-specific microcarriers, enabling the exploitation of reproducible effects for large scale biomedical applications.

Engineered micromechanical cues affecting human pluripotent stem cell regulations and fate

The survival, growth, self-renewal, and differentiation of human pluripotent stem cells (hPSCs) are influenced by their microenvironment, or so-called "niche," consisting of particular chemical and physical cues. Previous studies on mesenchymal stem cells and other stem cells have collectively uncovered the importance of physical cues and have begun to shed light on how stem cells sense and process such cues. In an attempt to support similar progress in mechanobiology of hPSCs, we review mechanosensory machinery, which plays an important role in cell-extracellular matrix interactions, cell-cell interactions, and subsequent intracellular responses. In addition, we review recent studies on the mechanobiology of hPSCs, in which engineered micromechanical environments were used to investigate effects of specific physical cues. Identifying key physical cues and understanding their mechanism will ultimately help in harnessing the full potential of hPSCs for clinical applications.

Role of collagen matrix in tumor angiogenesis and glioblastoma multiforme progression

Glioblastoma is a highly vascularized brain tumor, and antiangiogenic therapy improves its progression-free survival. However, current antiangiogenic therapy induces serious adverse effects including neuronal cytotoxicity and tumor invasiveness and resistance to therapy. Although it has been suggested that the physical microenvironment has a key role in tumor angiogenesis and progression, the mechanism by which physical properties of extracellular matrix control tumor angiogenesis and glioblastoma progression is not completely understood. Herein we show that physical compaction (the process in which cells gather and pack together and cause associated changes in cell shape and size) of human glioblastoma cell lines U87MG, U251, and LN229 induces expression of collagen types IV and VI and the collagen crosslinking enzyme lysyl oxidase and up-regulates in vitro expression of the angiogenic factor vascular endothelial growth factor. The lysyl oxidase inhibitor β-aminopropionitrile disrupts collagen structure in the tumor and inhibits tumor angiogenesis and glioblastoma multiforme growth in a mouse orthotopic brain tumor model. Similarly, d-penicillamine, which inhibits lysyl oxidase enzymatic activity by depleting intracerebral copper, also exhibits antiangiogenic effects on brain tumor growth in mice. These findings suggest that tumor microenvironment controlled by collagen structure is important in tumor angiogenesis and brain tumor progression.

The challenging environment on board the International Space Station affects endothelial cell function by triggering oxidative stress through thioredoxin interacting protein overexpression: the ESA-SPHINX experiment

Exposure to microgravity generates alterations that are similar to those involved in age-related diseases, such as cardiovascular deconditioning, bone loss, muscle atrophy, and immune response impairment. Endothelial dysfunction is the common denominator. To shed light on the underlying mechanism, we participated in the Progress 40P mission with Spaceflight of Human Umbilical Vein Endothelial Cells (HUVECs): an Integrated Experiment (SPHINX), which consisted of 12 in-flight and 12 ground-based control modules and lasted 10 d. Postflight microarray analysis revealed 1023 significantly modulated genes, the majority of which are involved in cell adhesion, oxidative phosphorylation, stress responses, cell cycle, and apoptosis. Thioredoxin-interacting protein was the most up-regulated (33-fold), heat-shock proteins 70 and 90 the most down-regulated (5.6-fold). Ion channels (TPCN1, KCNG2, KCNJ14, KCNG1, KCNT1, TRPM1, CLCN4, CLCA2), mitochondrial oxidative phosphorylation, and focal adhesion were widely affected. Cytokine detection in the culture media indicated significant increased secretion of interleukin-1α and interleukin-1β. Nitric oxide was found not modulated. Our data suggest that in cultured HUVECs, microgravity affects the same molecular machinery responsible for sensing alterations of flow and generates a prooxidative environment that activates inflammatory responses, alters endothelial behavior, and promotes senescence.

From cell phenotype to epigenetic mechanisms: new insights into regenerating myocardium

The self-regenerating property of the adult myocardium is not a new discovery. Even though we could not confirm that the adult myocardium is a post-mitotic tissue, we should consider that its plasticity is extremely low. Studies are still in progress to decipher the mechanisms underlying the abovementioned potential fetal features of the adult heart. The modest results of several clinical trials based on the transplantation of millions of autologous stem cells into the dysfunctional heart have confirmed that the cross-talk of different signals, such as the microenvironment, promotes the regeneration of adult myocardium. Recent scientific evidence has revealed that cellular cross-talk does not depend on the action of a single cell phenotype. It is conceivable that the limited turnover of cardiomyocytes is ensured by the interplay of adult cardiac cells in response to environmental changes. The epigenetic state of a cell serves as a dynamic interface between the environment and phenotype. The epigenetic modulation of the adult cardiac cells by natural active compounds encourages further studies to improve myocardial plasticity. In this review, we will highlight the most relevant studies demonstrating the epigenetic modulation of myocardial regeneration without the use of stem cell transplantation.

How integrins control breast biology

This article explores new ideas about how the ECM-integrin axis controls normal and malignant breast biology. We discuss the role of integrins in mammary stem cells, and how cell-matrix interactions regulate ductal and alveolar development and function. We also examine the contribution of integrins to tissue disorganisation and metastasis, and how an altered stromal and ECM tumour microenvironment affects the cancer cell niche both within primary tumours and at distant sites. Finally, we mention novel strategies for integrin-directed breast cancer treatment.

Combating Adaptation to Cyclic Stretching By Prolonging Activation of Extracellular Signal-Regulated Kinase

In developing implantable tissues based on cellular remodeling of a fibrin scaffold, a key indicator of success is high collagen content. Cellular collagen synthesis is stimulated by cyclic stretching but is limited by cellular adaptation. Adaptation is mediated by deactivation of extracellular signal-regulated kinase (ERK); therefore inhibition of ERK deactivation should improve mechanically stimulated collagen production and accelerate the development of strong engineered tissues. The hypothesis of this study is that p38 mitogen activated protein kinase (p38) activation by stretching limits ERK activation and that chemical inhibition of p38/isoforms with SB203580 will increase stretching-induced ERK activation and collagen production. Both p38 and ERK were activated by 15 minutes of stretching but only p38 remained active after 1 hour. After an effective dose of inhibitor was identified using cell monolayers, 5 M SB203580 was found to increase ERK activation by two-fold in cyclically stretched fibrin-based tissue constructs. When 5 M SB203580 was added to the culture medium of constructs exposed to three weeks of incremental amplitude cyclic stretch, 2.6 fold higher stretching-induced total collagen was obtained. In conclusion, SB203580 circumvents adaptation to stretching induced collagen production and may be useful in engineering tissues where mechanical strength is a priority.

Role of the extracellular matrix in regulating stem cell fate

The field of stem cells and regenerative medicine offers considerable promise as a means of delivering new treatments for a wide range of diseases. In order to maximize the effectiveness of cell-based therapies - whether stimulating expansion of endogenous cells or transplanting cells into patients - it is essential to understand the environmental (niche) signals that regulate stem cell behaviour. One of those signals is from the extracellular matrix (ECM). New technologies have offered insights into how stem cells sense signals from the ECM and how they respond to these signals at the molecular level, which ultimately regulate their fate.

Transcriptional regulation of endothelial cell and vascular development

The establishment and maintenance of the vascular system is critical for embryonic development and postnatal life. Defects in endothelial cell development and vessel formation and function lead to embryonic lethality and are important in the pathogenesis of vascular diseases. Here, we review the underlying molecular mechanisms of endothelial cell differentiation, plasticity, and the development of the vasculature. This review focuses on the interplay among transcription factors and signaling molecules that specify the differentiation of vascular endothelial cells. We also discuss recent progress on reprogramming of somatic cells toward distinct endothelial cell lineages and its promise in regenerative vascular medicine.

Mechanical regulation of skeletal development

Development of the various components of a normal skeleton requires highly regulated signalling systems that co-ordinate spatial and temporal patterns of cell division, cell differentiation, and morphogenesis. Much work in recent decades has revealed cascades of molecular signalling, acting through key transcription factors to regulate, for example, organized chondrogenic and osteogenic differentiation. It is now clear that mechanical stimuli are also required for aspects of skeletogenesis but very little is known about how the mechanical signals are integrated with classic biochemical signalling. Spatially organized differentiation is vital to the production of functionally appropriate tissues contributing to precise, region specific morphologies, for example transient chondrogenesis of long bone skeletal rudiments, which prefigures osteogenic replacement of the cartilage template, compared with the production of permanent cartilage at the sites of articulation. Currently a lack of understanding of how these tissues are differentially regulated hampers efforts to specifically regenerate stable bone and cartilage. Here, we review current research revealing the influence of mechanical stimuli on specific aspects of skeletal development and refer to other developing systems to set the scene for current and future work to uncover the molecular mechanisms involved. We integrate this with a brief overview of the effects of mechanical stimulation on stem cells in culture bringing together developmental and tissue engineering aspects of mechanoregulation of cell behavior. A better understanding of the molecular mechanisms that link mechanical stimuli to transcriptional control guiding cell differentiation will lead to new ideas about how to effectively prime stem cells for tissue engineering and regenerative therapies.

Regulation of tissue fibrosis by the biomechanical environment

The biomechanical environment plays a fundamental role in embryonic development, tissue maintenance, and pathogenesis. Mechanical forces play particularly important roles in the regulation of connective tissues including not only bone and cartilage but also the interstitial tissues of most organs. In vivo studies have correlated changes in mechanical load to modulation of the extracellular matrix and have indicated that increased mechanical force contributes to the enhanced expression and deposition of extracellular matrix components or fibrosis. Pathological fibrosis contributes to dysfunction of many organ systems. A variety of in vitro models have been utilized to evaluate the effects of mechanical force on extracellular matrix-producing cells. In general, application of mechanical stretch, fluid flow, and compression results in increased expression of extracellular matrix components. More recent studies have indicated that tissue rigidity also provides profibrotic signals to cells. The mechanisms whereby cells detect mechanical signals and transduce them into biochemical responses have received considerable attention. Cell surface receptors for extracellular matrix components and intracellular signaling pathways are instrumental in the mechanotransduction process. Understanding how mechanical signals are transmitted from the microenvironment will identify novel therapeutic targets for fibrosis and other pathological conditions.

Force nanoscopy of cell mechanics and cell adhesion

Cells are constantly exposed to mechanical stimuli in their environment and have several evolved mechanisms to sense and respond to these cues. It is becoming increasingly recognized that many cell types, from bacteria to mammalian cells, possess a diverse set of proteins to translate mechanical cues into biochemical signalling and to mediate cell surface interactions such as cell adhesion. Moreover, the mechanical properties of cells are involved in regulating cell function as well as serving as indicators of disease states. Importantly, the recent development of biophysical tools and nanoscale methods has facilitated a deeper understanding of the role that physical forces play in modulating cell mechanics and cell adhesion. Here, we discuss how atomic force microscopy (AFM) has recently been used to investigate cell mechanics and cell adhesion at the single-cell and single-molecule levels. This knowledge is critical to our understanding of the molecular mechanisms that govern mechanosensing, mechanotransduction, and mechanoresponse in living cells. While pushing living cells with the AFM tip provides a means to quantify their mechanical properties and examine their response to nanoscale forces, pulling single surface proteins with a functionalized tip allows one to understand their role in sensing and adhesion. The combination of these nanoscale techniques with modern molecular biology approaches, genetic engineering and optical microscopies provides a powerful platform for understanding the sophisticated functions of the cell surface machinery, and its role in the onset and progression of complex diseases.

Platelet rich plasma extract promotes angiogenesis through the angiopoietin1-Tie2 pathway

Development and regeneration of tissues and organs require precise coordination among endothelial, epithelial and mesenchymal morphogenesis. Angiogenesis plays key roles in normal development, wound healing, recovery from ischemic disease, and organ regeneration. It has been recognized that the combination of various angiogenic factors in an appropriate physiological ratio is critical for long-term functional blood vessel formation. Here we show that mouse soluble platelet-rich-plasma (PRP) extract, which includes abundant angiopoetin-1 (Ang1) and other angiogenic factors, stimulates endothelial cell growth, migration and differentiation in cultured human dermal microvascular endothelial cells in vitro and neonatal mouse retinal angiogenesis in vivo. Mouse platelet rich fibrin (PRF) matrix, the three-dimensional fibrin matrix that releases angiogenic factors with similar concentrations and proportions to the PRP extract, also recapitulates robust angiogenesis inside the matrix when implanted subcutaneously on the living mouse. Inhibition of Ang1-Tie2 signaling suppresses PRP extract-induced angiogenesis in vitro and angiogenic ability of the PRF matrix in vivo. Since human PRP extract and PRF matrix can be prepared from autologous peripheral blood, our findings may lead to the development of novel therapeutic interventions for various angiogenesis-related diseases as well as to the improvement of strategies for tissue engineering and organ regeneration.

Mechanics in neuronal development and repair

Biological cells are well known to respond to a multitude of chemical signals. In the nervous system, chemical signaling has been shown to be crucially involved in development, normal functioning, and disorders of neurons and glial cells. However, there are an increasing number of studies showing that these cells also respond to mechanical cues. Here, we summarize current knowledge about the mechanical properties of nervous tissue and its building blocks, review recent progress in methodology and understanding of cellular mechanosensitivity in the nervous system, and provide an outlook on the implications of neuromechanics for future developments in biomedical engineering to aid overcoming some of the most devastating and currently incurable CNS pathologies such as spinal cord injuries and multiple sclerosis.

Cell patterns emerge from coupled chemical and physical fields with cell proliferation dynamics: the Arabidopsis thaliana root as a study system

A central issue in developmental biology is to uncover the mechanisms by which stem cells maintain their capacity to regenerate, yet at the same time produce daughter cells that differentiate and attain their ultimate fate as a functional part of a tissue or an organ. In this paper we propose that, during development, cells within growing organs obtain positional information from a macroscopic physical field that is produced in space while cells are proliferating. This dynamical interaction triggers and responds to chemical and genetic processes that are specific to each biological system. We chose the root apical meristem of Arabidopsis thaliana to develop our dynamical model because this system is well studied at the molecular, genetic and cellular levels and has the key traits of multicellular stem-cell niches. We built a dynamical model that couples fundamental molecular mechanisms of the cell cycle to a tension physical field and to auxin dynamics, both of which are known to play a role in root development. We perform extensive numerical calculations that allow for quantitative comparison with experimental measurements that consider the cellular patterns at the root tip. Our model recovers, as an emergent pattern, the transition from proliferative to transition and elongation domains, characteristic of stem-cell niches in multicellular organisms. In addition, we successfully predict altered cellular patterns that are expected under various applied auxin treatments or modified physical growth conditions. Our modeling platform may be extended to explicitly consider gene regulatory networks or to treat other developmental systems.

The emergence of tumors results from altered cell differentiation and proliferation during organ and tissue development. Understanding how such altered or normal patterns are established is still a challenge. Molecular genetic approaches to understanding pattern formation have searched for key central genetic controllers. However, biological patterns emerge as a consequence of coupled complex genetic and non-genetic sub-systems operating at various spatial and temporal scales and levels of organization. We present a two-dimensional model and simulation benchmark that considers the integrated dynamics of physical and chemical fields that result from cell proliferation. We aim at understanding how the cellular patterns of stem-cell niches emerge. In these, organizer cells with very low rates of proliferation are surrounded by stem cells with slightly higher proliferation rates that transit to a domain of active proliferation and then of elongation and differentiation. We quantified such cellular patterns in the Arabidopsis thaliana root to test our theoretical propositions. The results of our simulations closely mimic observed root cellular patterns, thus providing a proof of principle that coupled physical fields and chemical processes under active cell proliferation give rise to stem-cell patterns. Our framework may be extended to other developmental systems and to consider gene regulatory networks.

Mechanical control of epithelial lumen formation

Epithelial cells differentiate and polarize to build complete epithelial organs during development. The study of epithelial morphogenesis is instrumental to the understanding of disease processes where epithelial polarity is disrupted. Recently, we demonstrated that matrix-induced cell confinement controls the acquisition of three-dimensional epithelial polarity, by modulating the initiation of the apical membrane to form a central lumen (J Cell Biol 2012; 198:1011-1026). Cell confinement can be achieved by use of micropatterned culture chips that allow precise micrometric-scale control of the cell adhesion surface and its composition. Using micropattern chips, we demonstrated that polarizing epithelial cells require high confinement conditions to properly position the centrosome and the trafficking machinery toward the cell-cell contacts and to initiate lumen morphogenesis. Low confinement induces LKB1 and RhoA-mediated cell contractility, which inhibits this mechanism for lumen formation. Deactivation of Myosin-II-mediated contractility rescued normal lumen initiation in low confinement conditions. Our results indicate that a mechanotransduction pathway coordinates nuclear and centrosome positioning to initiate epithelial morphogenesis. Here we discuss the potential candidates that control this process, specifically the polarized activation of Rho and Rab-family GTPases, and also a group of recently characterized nuclear transcription factors.

Mechano-sensing and transduction by endothelial surface glycocalyx: composition, structure, and function

The endothelial cells (ECs) lining every blood vessel wall are constantly exposed to the mechanical forces generated by blood flow. The EC responses to these hemodynamic forces play a critical role in the homeostasis of the circulatory system. To ensure proper EC mechano-sensing and transduction, there are a variety of mechano-sensors and transducers that have been identified on the EC surface, intra- and trans-EC membrane and within the EC cytoskeleton. Among them, the most recent candidate is the endothelial surface glycocalyx (ESG), which is a matrix-like thin layer covering the luminal surface of the EC. It consists of various proteoglycans, glycosaminoglycans, and plasma proteins, and is close to other prominent EC mechano-sensors and transducers. The ESG thickness was found to be in the order of 0.1-1 µm by different visualization techniques and in different types of vessels. Detailed analysis on the electron microscopy (EM) images of the microvascular ESG revealed a quasi-periodic substructure with the ESG fiber diameter of 10-12 and 20 nm spacing between adjacent fibers. Atomic force microscopy and optical tweezers were applied to investigate the mechanical properties of the ESG on the cultured EC monolayers and in solutions. Enzymatic degradation of specific ESG glycosaminoglycan components was used to directly elucidate the role of the ESG in EC mechano-sensing and transduction by measuring the shear-induced productions of nitric oxide and prostacyclin, two characteristic responses of the ECs to the flow. The unique location, composition, and structure of the ESG determine its role in EC mechano-sensing and transduction.

Development of a novel approach to safely couple the intestine to a distraction-induced device for intestinal growth: use of reconstructive tissue matrix

BACKGROUND

Distraction-induced intestinal growth may be a novel treatment for short bowel syndrome. Longitudinal, distractive tension created by the application of force creates a significant challenge: to produce adequate force, yet not cause perforation at the fixation points. This paper describes our development of a coupling strategy to allow for successful bowel lengthening.

METHODS

A curvilinear hydraulic device was implanted in an isolated Roux limb of small bowel in young Yorkshire pigs. Bowel was lengthened over a 2-week period. Study groups included: Group 1: Twelve silk transmural anchoring sutures into an engineered-coupling ring at the ends of each device. Group 2: Addition of felt pledgets to the coupling rings on the serosal surface of the small bowel. Group 3: Extraluminal use of either thin AlloDerm(®), thick AlloDerm(®), or Strattice™ mesh to anchor the device.

RESULTS

Group 1 (suture-only) resulted in a gradual pulling through the suture with increasing tension and no lengthening. Felt pledgets eroded in a similar fashion, causing abdominal sepsis. Thin AlloDerm(®) failed to prevent erosion; however, it protected against gross contamination. Animals in which either thick AlloDerm(®) or Strattice™ mesh was used survived complication free to the study endpoint. Both thick AlloDerm(®) and Strattice™ prevented erosion and perforation allowing for an average of 10.85 cm expansion.

CONCLUSION

This study demonstrates the use of either thick AlloDerm(®) or Strattice™ reconstructive tissue matrix which allows for safe and effective coupling. Further, we suggest this approach could be an adjunct to esophageal lengthening procedures.

Mechanical cues in cellular signalling and communication

Multicellular organisms comprise an organized array of individual cells surrounded by a meshwork of biomolecules and fluids. Cells have evolved various ways to communicate with each other, so that they can exchange information and thus fulfil their specified and unique functions. At the same time, cells are also physical entities that are subjected to a variety of local and global mechanical cues arising in the microenvironment. Cells are equipped with several different mechanisms to sense the physical properties of the microenvironment and the mechanical forces arising within it. These mechanical cues can elicit a variety of responses that have been shown to play a crucial role in vivo. In this review, we discuss the current views and understanding of cell mechanics and demonstrate the emerging evidence of the interplay between physiological mechanical cues and cell-cell communication pathways.

Stiffness-controlled three-dimensional extracellular matrices for high-resolution imaging of cell behavior

Regulation of cell functions by the physical properties of the extracellular matrix (ECM) has emerged as a crucial contributor to development and disease. Two specific physical properties of the ECM, stiffness and dimensionality, each influence cell signaling and function. As these ECM physical properties are linked to other properties that also regulate cell behavior, e.g., integrin ligand density, parsing the specific contributions of ECM stiffness and dimensionality has proven difficult. Here we detail a simple protocol, which can be completed in 1-2 d, for combining three-dimensional (3D) ECM engagement with controlled underlying ECM stiffness. In these 'sandwich gels', cells are sandwiched between a 3D fibrillar ECM and an ECM-coupled polyacrylamide gel of defined compliance, allowing the study of the specific effects of ECM compliance on cell function in physiologically relevant 3D ECMs. This type of system enables high-resolution time-lapse imaging and is suitable for a wide range of cell types and molecular perturbations.