Mechanobiology and force transduction in scars developed in darker skin types

BACKGROUND: Scarring is a complex process involving many cell types, cytokines and biological pathways including mechanobiology. Some subtle mechanical properties of skin can be assessed by measuring the speed of ultrasound shear wave propagation. The orientation of abnormal skin tension forces can be visualized, particularly in darker skin types, using dermoscopy showing distinct patterns of rete ridges' conformation. AIM: To assess some mechanobiological features of scars in darker skin types. PATIENTS AND METHODS: Large atrophic and hypertrophic surgical scars were examined on the trunk of 35 darker skin subjects. The surrounding skin was used as a comparator. Dermoscopic aspects were recorded. Resonance running time measurements (RRTM) were performed using a shear wave propagation device (Reviscometer). They were performed in four specific directions at given angles with regard to the long axis of the scar. The minimum, maximum and mean RRTM values were recorded at each site. RESULTS: Dermoscopy revealed patterns of melanin deposits in scars distinct from the normal honeycomb network seen in the surrounding skin. Hypertrophic scars showed a patchy pattern of large macular melanoderma dispersed in a lighter background. In these cases, low RRTM values were obtained with little variations according to the orientation of the measurements. By contrast, atrophic scars showed a streaky laddering melanotic pattern under dermoscopy. Higher RRTM values were often obtained, particularly in the transversal direction of the scars. Mechanical anisotropy was greater in the atrophic scars compared with the normal skin. DISCUSSION: Darker skin types represent a model for visualizing the main orientation of the epidermal rete ridges. A correlation was found between the pattern of melanized rete ridges of scars and the main orientation of the intrinsic forces in the skin.     

空间失重环境力学生物学2023年度研究进展

航空航天活动过程中特殊力学环境导致的生理变化一直是力学生物学研究的重要组成部分。本文综述了2023年度空间失重环境力学生物学研究进展,主要聚焦失重的生物学效应,包括在细胞、模式动物和人体水平在空间真实环境和地面模拟失重条件下得到的研究结果,以期助力航空航天力学生物学的发展研究,并为航天员或地面相关人群的健康防护或对抗措施研究提供帮助。     

Controlling Fibroblast Proliferation with Dimensionality-Specific Response by Stiffness of Injectable Gelatin Hydrogels

Abstract

We report an injectable hydrogel system with tunable stiffness for controlling the proliferation rate of human fibroblasts (HFF-1) in both two-dimensional (2D) and three-dimensional (3D) culture environments for potential use as a wound dressing material. The hydrogel composed of gelatin–hydroxyphenylpropionic acid (Gtn–HPA) conjugate was formed by the oxidative coupling of HPA moieties catalyzed by hydrogen peroxide (H₂O₂) and horseradish peroxidase (HRP). The stiffness of the hydrogels was controlled well by varying the H₂O₂ concentration. The effects of hydrogel stiffness on the proliferation rate of HFF-1 in both 2D and 3D were investigated. We found that the proliferation rate of HFF-1 using Gtn–HPA hydrogels was strongly dependent on the hydrogel stiffness, with a dimensionality-specific response. In the 2D studies, the HFF-1 exhibited a higher proliferation rate when the stiffness of the hydrogel was increased. In contrast, the HFF-1 cultured inside the hydrogel remained non-proliferative for 12 days before a stiffness-dependent proliferation profile was shown. The proliferation rate decreased with an increase in stiffness of the hydrogel in a 3D culture environment, unlike in a 2D environment.     

Reverse engineering the mechanical and molecular pathways in stem cell morphogenesis

The formation of relevant biological structures poses a challenge for regenerative medicine. During embryogenesis, embryonic cells differentiate into somatic tissues and undergo morphogenesis to produce three‐dimensional organs. Using stem cells, we can recapitulate this process and create biological constructs for therapeutic transplantation. However, imperfect imitation of nature sometimes results in in vitro artifacts that fail to recapitulate the function of native organs. It has been hypothesized that developing cells may self‐organize into tissue‐specific structures given a correct in vitro environment. This proposition is supported by the generation of neo‐organoids from stem cells. We suggest that morphogenesis may be reverse engineered to uncover its interacting mechanical pathway and molecular circuitry. By harnessing the latent architecture of stem cells, novel tissue‐engineering strategies may be conceptualized for generating self‐organizing transplants. Copyright © 2013 John Wiley & Sons, Ltd.     

Implanted adipose progenitor cells as physicochemical regulators of breast cancer

Multipotent adipose-derived stem cells (ASCs) are increasingly used for regenerative purposes such as soft tissue reconstruction following mastectomy; however, the ability of tumors to commandeer ASC functions to advance tumor progression is not well understood. Through the integration of physical sciences and oncology approaches we investigated the capability of tumor-derived chemical and mechanical cues to enhance ASC-mediated contributions to tumor stroma formation. Our results indicate that soluble factors from breast cancer cells inhibit adipogenic differentiation while increasing proliferation, proangiogenic factor secretion, and myofibroblastic differentiation of ASCs. This altered ASC phenotype led to varied extracellular matrix (ECM) deposition and contraction thereby enhancing tissue stiffness, a characteristic feature of breast tumors. Increased stiffness, in turn, facilitated changes in ASC behavior similar to those observed with tumor-derived chemical cues. Orthotopic mouse studies further confirmed the pathological relevance of ASCs in tumor progression and stiffness in vivo. In summary, altered ASC behavior can promote tumorigenesis and, thus, their implementation for regenerative therapy should be carefully considered in patients previously treated for cancer.     

Mechanotransduction of fluid stresses governs 3D cell migration

Solid tumors are characterized by high interstitial fluid pressure, which drives fluid efflux from the tumor core. Tumor-associated interstitial flow (IF) at a rate of ∼3 µm/s has been shown to induce cell migration in the upstream direction (rheotaxis). However, the molecular biophysical mechanism that underlies upstream cell polarization and rheotaxis remains unclear. We developed a microfluidic platform to investigate the effects of IF fluid stresses imparted on cells embedded within a collagen type I hydrogel, and we demonstrate that IF stresses result in a transcellular gradient in β1-integrin activation with vinculin, focal adhesion kinase (FAK), FAK^PY397, F actin, and paxillin-dependent protrusion formation localizing to the upstream side of the cell, where matrix adhesions are under maximum tension. This previously unknown mechanism is the result of a force balance between fluid drag on the cell and matrix adhesion tension and is therefore a fundamental, but previously unknown, stimulus for directing cell movement within porous extracellular matrix.Integrins and associated focal adhesion (FA) proteins form a tension-sensitive mechanical link between the extracellular matrix (ECM) and the cytoskeleton, and serve as key components in the signaling cascade by which cells transduce mechanical signals into biological responses (mechanotransduction) (1, 2). Contractile stresses generated by the cell are balanced by tractions at cell–substrate adhesions, and the FA protein vinculin accumulates at regions of high substrate stress (3, 4). The FA protein paxillin colocalizes with vinculin (4) and mediates β1-integrin FA turnover through interaction with FA kinase (FAK) (5). The FAK–paxillin signaling axis recruits vinculin to β1 integrins at regions of high matrix adhesion tension (6), and paxillin—a key mechanosensor (7)—mediates protrusion formation at regions of high stress on 2D substrates (8), and FAK–paxillin–vinculin signaling is required for mechanosensing and durotaxis (9).The tumor microenvironment imparts mechanical and chemical signals on tumor and stromal cells (10), and advanced breast carcinomas are characterized by high interstitial fluid pressure (11), an indicator of poor prognosis (12). This elevated fluid pressure drives interstitial flow (IF) and alters chemical transport within the tumor (13), and IF influences tumor cell migration through the generation of autocrine chemokine gradients (14). Equally important, although not as well understood, is the physical drag imparted on the ECM and constitutive cells (15) by IF, which is analogous to the FA-activating shear stresses generated on endothelial cells by hemodynamic forces (16). With endothelial cells, shear stress can be the dominant mechanical stimulus that induces FAK activation and cytoskeletal remodeling; however, for cells embedded within a porous matrix scaffold, the ratio of the force due to the pressure drop across the cell to the total shear force is inversely proportional to hydrogel permeability (SI Appendix, Eq. S5). In this study, we recapitulate physiologically relevant IF through collagen gel within a microfluidic device. Because the permeability of the collagen I hydrogel used in this study is small (1 × 10⁻¹³ m²), the integrated pressure force is more than 30× the integrated shear force for a 20-μm-diameter cell (17) (SI Appendix, Eq. S5). To maintain static equilibrium, all fluid stresses imparted on the cell must be balanced by tension in matrix adhesions. In 2D, the adhesions balancing the fluid drag on the cell are confined to the basal cell surface, whereas in porous media, such as breast stromal ECM, matrix adhesions are distributed across the full cell surface. Consequently, maintaining static equilibrium requires greater adhesion tension on the upstream side of the cell to balance fluid stresses. From the reference frame of the cell, the effect of IF is mechanically equivalent to applying a net outward force at matrix adhesions on the upstream side of the cell, similar to the net tensile stresses applied by use of optical tweezers to study the molecular mechanisms underlying mechanotransduction (4, 18).Here, we demonstrate that the forces required to balance drag imparted on the cell by IF induce a transcellular gradient in matrix adhesion tension, and the tensile stresses at the upstream side of the cell induce FA reorganization and polarization of FA-plaque proteins including vinculin, paxillin, FAK, FAK^PY397, and α-actinin. FA polarization leads to paxillin-dependent actin localization, the formation of protrusions upstream, and rheotaxis. Consistent with the governing mechanism of durotaxis on 2D substrates, this 3D mechanotransduction occurs through FAK and requires paxillin. Importantly, silencing paxillin does not affect cell migration speed but does attenuate rheotaxis. IF is present in many tissues in vivo (19), and because FA polarization and rheotaxis result from a mechanical force balance, this 3D mechanotransduction mechanism may be fundamental to all cells embedded within porous ECM.     

Effect of altered mechanical load conditions on the structure and function of cultured tendon fascicles

We have developed an in vitro model system to investigate the relationships between mechanical unloading and tendon matrix remodeling. Remodeling was characterized by changes in the functional and structural characteristics of rat tail tendon fascicles (RTTF) subjected to no load conditions for 1 week in vitro. We hypothesized that the absence of load will: (I) maintain cross‐sectional area (CSA), with decreased elastic modulus and increased stress‐relaxation; (II) cause an increase in denatured collagen and a decrease in water and total glycosaminoglycan (GAG) content. Fascicles cultured under a nominal static stress were used as control for culture conditions effects. Unloading resulted in a decrease of approximately 23% in the elastic modulus of cultured fascicles, consistent with previous stress‐deprivation studies. Contrary to our hypothesis, a nominal static stress caused an increase in elastic modulus (～30%) and a significant decrease in stress‐relaxation when compared to fresh fascicles at 1% strain. Mechanical changes were associated with changes in the GAG content of the fascicles, but not their CSA, water, or collagen content. Furthermore, we did not find evidence of measurable denatured collagen in the cultured fascicles. Together these results suggest a role for GAG but not collagen or water in the elastic and viscoelastic changes measured in tendon fascicles cultured for 1 week under altered load conditions. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:364–373, 2008