Age-dependent migratory behavior of human endothelial cells revealed by substrate microtopography

Evaluation of focal adhesion mediated subcellular curvature sensing in response to engineered extracellular matrix

Fibril curvature is bioinstructive to attached cells. Similar to natural healthy tissues, an engineered extracellular matrix can be designed to stimulate cells to adopt desired phenotypes. To take full advantage of the curvature control in biomaterial fabrication methodologies, an understanding of the response to fibril subcellular curvature is required. In this work, we examined morphology, signaling, and function of human cells attached to electrospun nanofibers. We controlled curvature across an order of magnitude using nondegradable poly(methyl methacrylate) (PMMA) attached to a stiff substrate with flat PMMA as a control. Focal adhesion length and the distance of maximum intensity from the geographic center of the vinculin positive focal adhesion both peaked at a fiber curvature of 2.5 μm-1 (both ∼2× the flat surface control). Vinculin experienced slightly less tension when attached to nanofiber substrates. Vinculin expression was also more affected by a subcellular curvature than structural proteins α-tubulin or α-actinin. Among the phosphorylation sites we examined (FAK397, 576/577, 925, and Src416), FAK925 exhibited the most dependance on the nanofiber curvature. A RhoA/ROCK dependance of migration velocity across curvatures combined with an observation of cell membrane wrapping around nanofibers suggested a hybrid of migration modes for cells attached to fibers as has been observed in 3D matrices. Careful selection of nanofiber curvature for regenerative engineering scaffolds and substrates used to study cell biology is required to maximize the potential of these techniques for scientific exploration and ultimately improvement of human health.

Fucoidan and topography modification improved in situ endothelialization on acellular synthetic vascular grafts

Thrombogenesis remains the primary failure of synthetic vascular grafts. Endothelial coverage is crucial to provide an antithrombogenic surface. However, most synthetic materials do not support cell adhesion, and transanastomotic endothelial migration is limited. Here, a surface modification strategy using fucoidan and topography was developed to enable fast in situ endothelialization of polyvinyl alcohol, which is not endothelial cell-adhesive. Among three different immobilization approaches compared, conjugation of aminated-fucoidan promoted endothelial monolayer formation while minimizing thrombogenicity in both in vitro platelet rich plasma testing and ex vivo non-human primate shunt assay. Screening of six topographical patterns showed that 2 μm gratings increased endothelial cell migration without inducing inflammation responses of endothelial cells. Mechanistic studies demonstrated that fucoidan could attract fibronectin, enabling integrin binding and focal adhesion formation and activating focal adhesion kinase (FAK) signaling, and 2 μm gratings further enhanced FAK-mediated cell migration. In a clinically relevant rabbit carotid artery end-to-side anastomosis model, 60% in situ endothelialization was observed throughout the entire lumen of 1.7 mm inner diameter modified grafts, compared to 0% of unmodified graft, and the four-week graft patency also increased. This work presents a promising strategy to stimulate in situ endothelialization on synthetic materials for improving long-term performance.

Hierarchical micro-/nanotopography for tuning structures and mechanics of cells probed by atomic force microscopy

Extracellular matrix plays an important role in regulating the behaviors of cells, and utilizing matrix physics to control cell fate has been a promising way for cell and tissue engineering. However, the nanoscale situations taking place during the topography-regulated cell-matrix interactions are still not fully understood to the best of our knowledge. The invention of atomic force microscopy (AFM) provides a powerful tool to characterize the structures and properties of living biological systems under aqueous conditions with unprecedented spatial resolution. In this work, with the use of AFM, structural and mechanical dynamics of individual cells grown on micro-/nanotopographical surface were revealed. First, the microgroove patterned silicon substrates were fabricated by photolithography. Next, nanogranular topography was formed on microgroove substrates by cell culture medium protein deposition, which was visualized by in situ AFM imaging. The micro-/nanotopographical substrates were then used to grow two types of cells (3T3 cell or MCF-7 cell). AFM morphological imaging and mechanical measurements were applied to characterize the changes of cells grown on the micro-/nanotopographical substrates. The experimental results showed the significant alterations in cellular structures and cellular mechanics caused by micro-/nanotopography. The study provides a novel way based on AFM to unveil the native nanostructures and mechanical properties of cell-matrix interfaces with high spatial resolution in liquids, which will have potential impacts on the studies of topography-tuned cell behaviors.

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.

Vascular endothelial and smooth muscle cell galvanotactic response and differential migratory behavior

Chronic disease or injury of the vasculature impairs the functionality of vascular wall cells particularly in their ability to migrate and repair vascular surfaces. Under pathologic conditions, vascular endothelial cells (ECs) lose their non-thrombogenic properties and decrease their motility. Alternatively, vascular smooth muscle cells (SMCs) may increase motility and proliferation, leading to blood vessel luminal invasion. Current therapies to prevent subsequent blood vessel occlusion commonly mechanically injure vascular cells leading to endothelial denudation and smooth muscle cell luminal migration. Due to this dichotomous migratory behavior, a need exists for modulating vascular cell growth and migration in a more targeted manner. Here, we examine the efficacy of utilizing small direct current electric fields to influence vascular cell-specific migration ("galvanotaxis"). We designed, fabricated, and implemented an in vitro chamber for tracking vascular cell migration direction, distance, and displacement under galvanotactic influence of varying magnitude. Our results indicate that vascular ECs and SMCs have differing responses to galvanotaxis; ECs exhibit a positive correlation of anodal migration while SMCs exhibit minimal change in directional migration in relation to the electric field direction. SMCs exhibit less motility response (i.e. distance traveled in 4 h) compared to ECs, but SMCs show a significantly higher motility at low electric potentials (80 mV/cm). With further investigation and translation, galvanotaxis may be an effective solution for modulation of vascular cell-specific migration, leading to enhanced endothelialization, with coordinate reduced smooth muscle in-migration.

The Research Advance of Cell Bridges in vitro

The microenvironment in which cells reside in vivo dictates their biological and mechanical functioning is associated with morphogenetic and regenerative processes and may find implications in regenerative medicine and tissue engineering. The development of nano- and micro-fabricated technologies, three-dimensional (3D) printing technique, and biomimetic medical materials have enabled researchers to prepare novel advanced substrates mimicking the in vivo microenvironment. Most of the novel morphologies and behaviors of cells, including contact guidance and cell bridges which are observed in vivo but are not perceived in the traditional two-dimensional (2D) culture system, emerged on those novel substrates. Using cell bridges, cell can span over the surface of substrates to maintain mechanical stability and integrity of tissue, as observed in physiological processes, such as wound healing, regeneration and development. Compared to contact guidance, which has received increased attention and is investigated extensively, studies on cell bridges remain scarce. Therefore, in this mini-review, we have comprehensively summarized and classified different kinds of cell bridges formed on various substrates and highlighted possible biophysical mechanisms underlying cell bridge formation for their possible implication in the fields of tissue engineering and regenerative medicine.

Lipoconstruct surface topography grating size influences vascularization onset in the dorsal skinfold chamber model

After skin tissue injury or pathological removal, vascularization timing is paramount in graft survival. As full thickness skin grafts often fail to become perfused over larger surfaces, split-thickness grafts are preferred and can be used together with biomaterials, which themselves are non-angiogenic. One way of promoting vascular ingrowth is to "pre-vascularize" an engineered substitute by introducing endothelial cells (ECs). Since it has been previously demonstrated that surface structured biomaterials have an effect on wound healing, skin regeneration, and fibrosis reduction, we proposed that a microvascular-rich lipoconstruct with anisotropic topographical cues could be a clinically translatable vascularization approach. Murine lipofragments were formed with three polydimethylsiloxane molds (flat, 5 µm, and 50 µm parallel gratings) and implanted into the dorsal skinfold chamber of male C57BL/6 mice. Vascular ingrowth was observed through intravital microscopy over 21 days and further assessed by histology and protein identification. Our investigation revealed that topographical feature size influenced the commencement of neovascular ingrowth, with 5 µm gratings exhibiting early construct perfusion at 3 days post-operation, and 50 µm being delayed until day 5. We therefore postulate that surface structured lipoconstructs may serve as an easily obtained and produced construct suitable for providing soft tissue and ECs to tissue defects. STATEMENT OF SIGNIFICANCE: Skin graft failures due to inadequate or uneven perfusion frequently occur and can be even more complicated in deep, difficult to heal wounds, or bone coverage. In complex injuries, biomaterials are often used to cover bone structures with a standard split thickness graft; however, perfusion can take up to 3 weeks. Thus, any means to promote faster and uniform vascularization could significantly reduce healing time, as well as lower patient down-time. As pre-vascularized constructs have reported success in research, we created a cost-efficient, translatable method with no additional laboratory time as adipose tissue can be harvested and used immediately. We further used surface topography as an aspect to modulate construct perfusion, which has been reported for the first time here.

Natural killer cell migration control in microchannels by perturbations and topography

Natural killer (NK) cells are lymphocytes which play an important role in the immune system by recognizing and killing potentially malignant cells without antigen sensitization, and could be utilized in cancer therapy. NK cell migration is an essential process to find and kill target cells, which is well known to be driven by the chemotaxis effect. NK cells also experience a topographical effect induced by the extracellular matrix (ECM) during their migration. However, topographical effects on NK cell locomotion in three dimensional (3D) environments are not well studied yet. In this work, polydimethylsiloxane based platforms containing microchannels with different types of perturbations and decorated with various surface patterns were fabricated to systematically study the topographical effect on NK cell migration with and without the chemotaxis effect. The results showed that perturbation sites in channels induced pauses and reversals in chemotaxis driven NK cell migration. Surface topography such as gratings in confined environments could introduce directional preference to NK cell movement even without chemoattractants. These findings showed that NK cell migration could be controlled by contact guidance, which provides future possibility to manipulate NK cell migration in controlled in vitro bioengineering systems. Results in this study showed that the complex topography of 3D microenvironments in the ECM could have significant effects on NK cell migration in different tissues and organs, and provided insight for explaining the dynamics of NK cell activities in clinical experiments.