Differentiation of stem/progenitor cells into vascular cells in response to fluid mechanical forces

Vascular functions are regulated not only by chemical mediators, such as hormones, cytokines, and neurotransmitters, but by mechanical hemodynamic forces generated by blood flow and blood pressure. The mechanical force-mediated regulation is based on the ability of vascular cells, including endothelial cells and smooth muscle cells, to recognize fluid mechanical forces, i.e., the shear stress produced by flowing blood and the cyclic strain generated by blood pressure, and to transmit the signals into the cell interior, where they trigger cell responses that involve changes in cell morphology, cell function, and gene expression. Recent studies have revealed that immature cells, such as endothelial progenitor cells (EPCs) and embryonic stem (ES) cells, as well as adult vascular cells, respond to fluid mechanical forces. Shear stress and cyclic strain promote the proliferation and differentiation of EPCs and ES cells into vascular cells and enhance their ability to form new vessels. Even more recently, attempts have been made to apply fluid mechanical forces to EPCs and ES cells cultured on polymer tubes and develop tissue-engineered blood vessel grafts that have a structure and function similar to that of blood vessels in vivo. This review summarizes the current state of knowledge concerning the mechanobiological responses of stem/progenitor cells and its potential applications to tissue engineering.     

Mechanotransduction of stem cells for tendon repair

Tendon is a mechanosensitive tissue that transmits force from muscle to bone. Physiological loading contributes to maintaining the homeostasis and adaptation of tendon, but aberrant loading may lead to injury or failed repair. It is shown that stem cells respond to mechanical loading and play an essential role in both acute and chronic injuries, as well as in tendon repair. In the process of mechanotransduction, mechanical loading is detected by mechanosensors that regulate cell differentiation and proliferation via several signaling pathways. In order to better understand the stem-cell response to mechanical stimulation and the potential mechanism of the tendon repair process, in this review, we summarize the source and role of endogenous and exogenous stem cells active in tendon repair, describe the mechanical response of stem cells, and finally, highlight the mechanotransduction process and underlying signaling pathways.     

Can shear stress direct stem cell fate?

Mechanical forces are important signals in the development and function of the heart and lung, growth of skin and muscle, and maintenance of cartilage and bone. The specific mechanical force “shear stress” has been implicated as playing a critical role in the physiological responses of blood vessels through endothelial cell signaling. More recently, studies have shown that shear stress can induce differentiation of stem cells toward both endothelial and bone‐producing cell phenotypes. This review will highlight current data supporting the role of shear stress in stem cell fate and will propose potential mechanisms and signaling cascades for transducing shear stress into a biological signal. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Effects of shear stress on differentiation of stem cells into endothelial cells

Stem cell transplantation is an appealing potential therapy for vascular diseases and an indispensable key step in vascular tissue engineering. Substantial effort has been made to differentiate stem cells toward vascular cell phenotypes, including endothelial cells (ECs) and smooth muscle cells. The microenvironment of vascular cells not only contains biochemical factors that influence differentiation but also exerts hemodynamic forces, such as shear stress and cyclic strain. More recently, studies have shown that shear stress can influence the differentiation of stem cells toward ECs. A deep understanding of the responses and underlying mechanisms involved in this process is essential for clinical translation. This review highlights current data supporting the role of shear stress in stem cell differentiation into ECs. Potential mechanisms and signaling cascades for transducing shear stress into a biological signal are proposed. Further study of stem cell responses to shear stress will be necessary to apply stem cells for pharmacological applications and cardiovascular implants in the realm of regenerative medicine.     

Fluid shear stress and the vascular endothelium: for better and for worse

As blood flows, the vascular wall is constantly subjected to physical forces, which regulate important physiological blood vessel responses, as well as being implicated in the development of arterial wall pathologies. Changes in blood flow, thus generating altered hemodynamic forces are responsible for acute vessel tone regulation, the development of blood vessel structure during embryogenesis and early growth, as well as chronic remodeling and generation of adult blood vessels. The complex interaction of biomechanical forces, and more specifically shear stress, derived by the flow of blood and the vascular endothelium raise many yet to be answered questions:How are mechanical forces transduced by endothelial cells into a biological response, and is there a "shear stress receptor"?Are "mechanical receptors" and the final signaling pathways they evoke similar to other stimulus-response transduction systems?How do vascular endothelial cells differ in their response to physiological or pathological shear stresses?Can shear stress receptors or shear stress responsive genes serve as novel targets for the design of diagnostic and therapeutic modalities for cardiovascular pathologies?The current review attempts to bring together recent findings on the in vivo and in vitro responses of the vascular endothelium to shear stress and to address some of the questions raised above.     

Stem cells, vascular smooth muscle cells and atherosclerosis

Stem cells have the ability to differentiate into a variety of cells to replace dead cells or to repair tissue. Recently, accumulating evidence indicates that mechanical forces, cytokines and other factors can influence stem cell differentiation into vascular smooth muscle cells (SMCs). In developmental process, SMCs originate from several sources, which show a great heterogenicity in different vessel walls. In adult vessels, SMCs display a less proliferative nature, but are altered in response to risk factors for atherosclerosis. Traditional view on SMC origins in atherosclerotic lesions is challenged by the recent findings that stem cells and smooth muscle progenitors contribute to the development of atherosclerotic lesions. Vascular progenitor cells circulating in human blood and the presence of adventitia in animals are recent discoveries, but the source of these cells is still unknown. The present review gives an update on the progress of stem cell and SMC research in atherosclerosis, and discusses possible mechanisms of stem/progenitor cell differentiation that contribute to the disease process.     

HDAC3 is crucial in shear- and VEGF-induced stem cell differentiation toward endothelial cells

Reendothelialization involves endothelial progenitor cell (EPC) homing, proliferation, and differentiation, which may be influenced by fluid shear stress and local flow pattern. This study aims to elucidate the role of laminar flow on embryonic stem (ES) cell differentiation and the underlying mechanism. We demonstrated that laminar flow enhanced ES cell-derived progenitor cell proliferation and differentiation into endothelial cells (ECs). Laminar flow stabilized and activated histone deacetylase 3 (HDAC3) through the Flk-1-PI3K-Akt pathway, which in turn deacetylated p53, leading to p21 activation. A similar signal pathway was detected in vascular endothelial growth factor-induced EC differentiation. HDAC3 and p21 were detected in blood vessels during embryogenesis. Local transfer of ES cell-derived EPC incorporated into injured femoral artery and reduced neointima formation in a mouse model. These data suggest that shear stress is a key regulator for stem cell differentiation into EC, especially in EPC differentiation, which can be used for vascular repair, and that the Flk-1-PI3K-Akt-HDAC3-p53-p21 pathway is crucial in such a process.     

干细胞发育的生物力学调控

生物力学是采用力学方法对生物系统的结构和功能进行的研究,与生物化学信号一起是调节胚胎发育、干细胞发育分化和组织器官形成的重要因素。近年来,随着学科交叉的深入,生物力学因素越来越受到研究者的重视。目前的研究表明：在心血管和造血系统,血流产生的流体剪切力对于血管内皮和造血细胞的发育分化至关重要;此外,对于广泛研究的间充质干细胞,由细胞外基质物理特性诱导的细胞张力对于干细胞功能及其向不同子代细胞的分化也扮演了重要的角色;而在肝脏等上皮组织来源的器官,也有研究提示生物力学因素,如基质弹性等在疾病的发生发展过程中起到了不可忽视的作用。总之,在干细胞发育分化过程中,生物力学调控与生物化学信号通路怎样协同发挥作用将成为今后研究的重点。     

Mechanical stress-induced apoptosis in the cardiovascular system

All tissues in the body are subjected to physical forces originating either from tension, created by cells themselves, or from the environment. Particularly, the cardiovascular system is continuously subjected to haemodynamic forces created by blood flow and blood pressure. While biomechanical force at physiological levels is essential to develop and maintain organic structure and function, elevated mechanical stress may result in cell death leading to pathological conditions. In recent years, however, it has been widely recognized that cell death, namely apoptosis, is not just the response to an injury but a highly regulated and controlled process. Therefore, physical stimuli must be sensed by cells and transmitted through intracellular signal transduction pathways to the nucleus, resulting in cell apoptosis. Disturbances in the regulatory mechanisms of apoptosis often precede the development of a disease. Exploration of the molecular signalling mechanisms leading to mechanical stress-induced apoptosis in cardiovascular disorders revealed the crucial role of apoptosis in the pathogenesis of these diseases. For instance, heart failure, hypertension and atherosclerosis are believed to be related to sustained mechanical overloading or stress. In this review we summarize the recent data focusing on molecular mechanisms of mechanical stress-induced apoptosis and highlight the role of apoptosis in the development of cardiovascular disorders, which may lead to new therapeutic strategies for these diseases.     

血流剪切力在动脉粥样硬化形成中的作用

血管内皮位于血管壁和血液的界面,直接与血流接触而持续受血流剪切力的作用。血管内皮细胞能感受血流机械力的变化,通过激活相应的信号通路调节血管内皮和平滑肌的结构和功能。研究发现,血液流动力的形式与动脉粥样硬化的发生发展有密切的关系。本综述将就血流剪切力与动脉粥样硬化的相互关系及作用机制的最新研究进展作简要介绍。     

Molecular basis of the effects of shear stress on vascular endothelial cells

Blood vessels are constantly exposed to hemodynamic forces in the form of cyclic stretch and shear stress due to the pulsatile nature of blood pressure and flow. Endothelial cells (ECs) are subjected to the shear stress resulting from blood flow and are able to convert mechanical stimuli into intracellular signals that affect cellular functions, e.g., proliferation, apoptosis, migration, permeability, and remodeling, as well as gene expression. The ECs use multiple sensing mechanisms to detect changes in mechanical forces, leading to the activation of signaling networks. The cytoskeleton provides a structural framework for the EC to transmit mechanical forces between its luminal, abluminal and junctional surfaces and its interior, including the cytoplasm, the nucleus, and focal adhesion sites. Endothelial cells also respond differently to different modes of shear forces, e.g., laminar, disturbed, or oscillatory flows. In vitro studies on cultured ECs in flow channels have been conducted to investigate the molecular mechanisms by which cells convert the mechanical input into biochemical events, which eventually lead to functional responses. The knowledge gained on mechano-transduction, with verifications under in vivo conditions, will advance our understanding of the physiological and pathological processes in vascular remodeling and adaptation in health and disease.     

ATF4在内质网应激调控成骨分化中的作用

为避免内质网中未折叠蛋白质的过度累积,真核细胞能激活一系列信号通路来维持内质网稳态,这个过程称为内质网应激。在骨生长发育中,适宜的内质网应激有助于成骨细胞、破骨细胞和软骨细胞的生长,可以促进骨髓间充质干细胞向成骨细胞分化。而过度的内质网应激会抑制成骨分化,严重的甚至导致骨质疏松、成骨不全等相关骨病的发生。内质网应激时可激活未折叠蛋白质反应,其主要是通过PERK/eIF2α/ATF4信号通路,上调转录激活因子4(ATF4)的表达。ATF4位于许多成骨分化调节因子的下游,是促进成骨分化的关键因子,在内质网应激对成骨分化的调节中发挥重要作用。在成骨分化过程中,适宜的内质网应激能通过激活PERK信号通路,诱导ATF4表达增加,进而上调骨钙素、骨涎蛋白等成骨所必需基因的表达,促进成骨分化。过度的内质网应激会激活ATF4/CHOP促凋亡途径,并导致Bax、胱天蛋白酶等凋亡信号分子的大量产生,进而导致细胞凋亡,抑制成骨分化。由于ATF4在ERS和成骨分化中的重要作用,ATF4在骨质疏松、成骨不全等骨相关疾病的治疗中具有重要意义。本文通过综述ATF4在内质网应激调控成骨分化中的作用机制,为相关骨性疾病治疗提供理论依据。     

ATF4在内质网应激调控成骨分化中的作用

为避免内质网中未折叠蛋白质的过度累积,真核细胞能激活一系列信号通路来维持内质网稳态,这个过程称为内质网应激。在骨生长发育中,适宜的内质网应激有助于成骨细胞、破骨细胞和软骨细胞的生长,可以促进骨髓间充质干细胞向成骨细胞分化。而过度的内质网应激会抑制成骨分化,严重的甚至导致骨质疏松、成骨不全等相关骨病的发生。内质网应激时可激活未折叠蛋白质反应,其主要是通过PERK/eIF2α/ATF4信号通路,上调转录激活因子4(ATF4)的表达。ATF4位于许多成骨分化调节因子的下游,是促进成骨分化的关键因子,在内质网应激对成骨分化的调节中发挥重要作用。在成骨分化过程中,适宜的内质网应激能通过激活PERK信号通路,诱导ATF4表达增加,进而上调骨钙素、骨涎蛋白等成骨所必需基因的表达,促进成骨分化。过度的内质网应激会激活ATF4/CHOP促凋亡途径,并导致Bax、胱天蛋白酶等凋亡信号分子的大量产生,进而导致细胞凋亡,抑制成骨分化。由于ATF4在ERS和成骨分化中的重要作用,ATF4在骨质疏松、成骨不全等骨相关疾病的治疗中具有重要意义。本文通过综述ATF4在内质网应激调控成骨分化中的作用机制,为相关骨性疾病治疗提供理论依据。     

In vitro Response of the Bone Marrow-Derived Mesenchymal Stem Cells Seeded in a Type-I Collagen-Glycosaminoglycan Scaffold for Skin Wound Repair Under the Mechanical Loading Condition

In order to achieve successful wound repair by regenerative tissue engineering using mesenchymal stem cells (MSCs), it is important to understand the response of stem cells in the scaffold matrix to mechanical stress.
To investigate the clinical effects of mechanical stress on the behavior of cells in scaffolds, bone marrow-derived mesenchymal stem cells (MSCs) were grown on a type-I collagen-glycosaminoglycan (GAG) scaffold matrix for one week under cyclic stretching loading conditions.
The porous collagen-GAG scaffold matrix for skin wound repair was prepared, the harvested canine MSCs were seeded on the scaffold, and cultured under three kinds of cyclic stretching loading conditions ( 0%: control, 5% strain, 15% strain ). After 7 days incubation, MSCs were evaluated histologically and immunohistochemically regarding the proliferation and differentiation.
Cultured MSCs in the high strain (15% strain) group showed activea-smooth muscle actin (α-SMA) expression and poor differentiation into type-I collagen-positive cells, whereas enhanced differentiation into type-I collagen positive cells and a lack ofa-SMA expression where shown in the lower stress (5% strain) group. These results suggest that mechanical stress may affect the proliferation and differentiation of stem cells, and subsequently the wound healing process, through attachment interactions between the stem cells and scaffold matrix. Our findings provide an additional consideration for clinical treatment of wound repair using regenerative tissue engineering.     

Arteriogenesis versus angiogenesis: similarities and differences

Cardiovascular diseases account for more than half of total mortality before the age of 75 in industrialized countries. To develop therapies promoting the compensatory growth of blood vessels could be superior to palliative surgical interventions. Therefore, much effort has been put into investigating underlying mechanisms. Depending on the initial trigger, growth of blood vessels in adult organisms proceeds via two major processes, angiogenesis and arteriogenesis. While angiogenesis is induced by hypoxia and results in new capillaries, arteriogenesis is induced by physical forces, most importantly fluid shear stress. Consequently, chronically elevated fluid shear stress was found to be the strongest trigger under experimental conditions. Arteriogenesis describes the remodelling of pre-existing arterio-arteriolar anastomoses to completely developed and functional arteries. In both growth processes, enlargement of vascular wall structures was proposed to be covered by proliferation of existing wall cells. Recently, increasing evidence emerges, implicating a pivotal role for circulating cells, above all blood monocytes, in vascular growth processes. Since it has been shown that monocytes/ macrophage release a cocktail of chemokines, growth factors and proteases involved in vascular growth, their contribution seems to be of a paracrine fashion. A similar role is currently discussed for various populations of bone-marrow derived stem cells and endothelial progenitors. In contrast, the initial hypothesis that these cells -after undergoing a (trans-)differentiation- contribute by a structural integration into the growing vessel wall, is increasingly challenged.     

Mechanostasis in apoptosis and medicine

Mechanostasis describes a complex and dynamic process where cells maintain equilibrium in response to mechanical forces. Normal physiological loading modes and magnitudes contribute to cell proliferation, tissue growth, differentiation and development. However, cell responses to abnormal forces include compensatory apoptotic mechanisms that may contribute to the development of tissue disease and pathological conditions. Mechanotransduction mechanisms tightly regulate the cell response through discrete signaling pathways. Here, we provide an overview of links between pro- and anti-apoptotic signaling and mechanotransduction signaling pathways, and identify potential clinical applications for treatments of disease by exploiting mechanically-linked apoptotic pathways.