Publisher's Note: Biological Magnetometry: Torque on Superparamagnetic Beads in Magnetic Fields [Phys. Rev. Lett. 114, 218301 (2015)].

This corrects the article DOI: 10.1103/PhysRevLett.114.218301.

Biological magnetometry: torque on superparamagnetic beads in magnetic fields.

Superparamagnetic beads are widely used in biochemistry and single-molecule biophysics, but the nature of the anisotropy that enables the application of torques remains controversial. To quantitatively investigate the torques experienced by superparamagnetic particles, we use a biological motor to rotate beads in a magnetic field and demonstrate that the underlying potential is π periodic. In addition, we tether a bead to a single DNA molecule and show that the angular trap stiffness increases nonlinearly with magnetic field strength. Our results indicate that the superparamagnetic beads' anisotropy derives from a nonuniform intrabead distribution of superparamagnetic nanoparticles.

Dextran hydrogel scaffolds enhance angiogenic responses and promote complete skin regeneration during burn wound healing.

Neovascularization is a critical determinant of wound-healing outcomes for deep burn injuries. We hypothesize that dextran-based hydrogels can serve as instructive scaffolds to promote neovascularization and skin regeneration in third-degree burn wounds. Dextran hydrogels are soft and pliable, offering opportunities to improve the management of burn wound treatment. We first developed a procedure to treat burn wounds on mice with dextran hydrogels. In this procedure, we followed clinical practice of wound excision to remove full-thickness burned skin, and then covered the wound with the dextran hydrogel and a dressing layer. Our procedure allows the hydrogel to remain intact and securely in place during the entire healing period, thus offering opportunities to simplify the management of burn wound treatment. A 3-week comparative study indicated that dextran hydrogel promoted dermal regeneration with complete skin appendages. The hydrogel scaffold facilitated early inflammatory cell infiltration that led to its rapid degradation, promoting the infiltration of angiogenic cells into the healing wounds. Endothelial cells homed into the hydrogel scaffolds to enable neovascularization by day 7, resulting in an increased blood flow significantly greater than treated and untreated controls. By day 21, burn wounds treated with hydrogel developed a mature epithelial structure with hair follicles and sebaceous glands. After 5 weeks of treatment, the hydrogel scaffolds promoted new hair growth and epidermal morphology and thickness similar to normal mouse skin. Collectively, our evidence shows that customized dextran-based hydrogel alone, with no additional growth factors, cytokines, or cells, promoted remarkable neovascularization and skin regeneration and may lead to novel treatments for dermal wounds.

Valsartan and sacubitril combination treatment enhances collagen production in older adult human skin cells.

Collagen is a major component of the skin's support system, allowing for its firmness, elasticity, and mechanical strength. Skin collagen production decreases as we age and is associated with increased sagging, wrinkling, and thinning. The Renin-Angiotensin System (RAS) is a key hormonal system that changes with age and affects multiple organ systems. The primary health benefits of Angiotensin (Ang) receptor type1 (AT1R) blockers are believed to arise from systemic effects on blood pressure. However, there is also a skin-specific RAS, though this system has been less well characterized. There are eight FDA-approved angiotensin receptor blockers (ARBs) on the market, although the impact of topical ARBs on aging skin is unknown. Here, we evaluated the topical penetration of gel formulations of eight ARBs using human cadaver skin. Our results show that valsartan achieved the highest skin penetration compared to other ARBs. We then treated human skin fibroblasts from 2-year-old and 57-year-old individuals with valsartan alone or in combination with the neprilysin inhibitor sacubitril. Sacubitril works synergistically with valsartan by inhibiting the degradation of angiotensin II, thereby increasing its bioavailability. Treatment of young and older adult human skin cells with valsartan and sacubitril led to a five-fold increase in collagen type-1 production in the young cells and a four-fold increase in collagen type-1 in older adult cells. This study demonstrates a potential novel application for the widely prescribed drug combination sacubitril-valsartan as a topical agent in aged skin.

Corrigendum: Applying torque to the Escherichia coli flagellar motor using magnetic tweezers.

This corrects the article DOI: 10.1038/srep43285.

Applying torque to the Escherichia coli flagellar motor using magnetic tweezers.

The bacterial flagellar motor of Escherichia coli is a nanoscale rotary engine essential for bacterial propulsion. Studies on the power output of single motors rely on the measurement of motor torque and rotation under external load. Here, we investigate the use of magnetic tweezers, which in principle allow the application and active control of a calibrated load torque, to study single flagellar motors in Escherichia coli. We manipulate the external load on the motor by adjusting the magnetic field experienced by a magnetic bead linked to the motor, and we probe the motor's response. A simple model describes the average motor speed over the entire range of applied fields. We extract the motor torque at stall and find it to be similar to the motor torque at drag-limited speed. In addition, use of the magnetic tweezers allows us to force motor rotation in both forward and backward directions. We monitor the motor's performance before and after periods of forced rotation and observe no destructive effects on the motor. Our experiments show how magnetic tweezers can provide active and fast control of the external load while also exposing remaining challenges in calibration. Through their non-invasive character and straightforward parallelization, magnetic tweezers provide an attractive platform to study nanoscale rotary motors at the single-motor level.

Engineered Biopolymeric Scaffolds for Chronic Wound Healing.

Skin regeneration requires the coordinated integration of concomitant biological and molecular events in the extracellular wound environment during overlapping phases of inflammation, proliferation, and matrix remodeling. This process is highly efficient during normal wound healing. However, chronic wounds fail to progress through the ordered and reparative wound healing process and are unable to heal, requiring long-term treatment at high costs. There are many advanced skin substitutes, which mostly comprise bioactive dressings containing mammalian derived matrix components, and/or human cells, in clinical use. However, it is presently hypothesized that no treatment significantly outperforms the others. To address this unmet challenge, recent research has focused on developing innovative acellular biopolymeric scaffolds as more efficacious wound healing therapies. These biomaterial-based skin substitutes are precisely engineered and fine-tuned to recapitulate aspects of the wound healing milieu and target specific events in the wound healing cascade to facilitate complete skin repair with restored function and tissue integrity. This mini-review will provide a brief overview of chronic wound healing and current skin substitute treatment strategies while focusing on recent engineering approaches that regenerate skin using synthetic, biopolymeric scaffolds. We discuss key polymeric scaffold design criteria, including degradation, biocompatibility, and microstructure, and how they translate to inductive microenvironments that stimulate cell infiltration and vascularization to enhance chronic wound healing. As healthcare moves toward precision medicine-based strategies, the potential and therapeutic implications of synthetic, biopolymeric scaffolds as tunable treatment modalities for chronic wounds will be considered.

Patterning microscale extracellular matrices to study endothelial and cancer cell interactions in vitro.

The extracellular matrix (ECM) of the tumor niche provides support to residing and migrating cells and presents instructive cues that influence cellular behaviours. The ECM protein fibronectin (Fn) enables vascular network formation, while hyaluronic acid (HA) is known to facilitate breast tumor development. To recapitulate aspects of the tumor microenvironment, we developed systems of spatially defined Fn and HA for the co-culture of endothelial colony forming cells (ECFCs) and breast cancer cells (BCCs). A micropatterned system was developed using sequential microcontact printing of HA and Fn. This approach supported the preferential adhesion of ECFCs to Fn, but did not support the preferential adhesion of BCCs to HA. Thus, we developed a microstructured analog to spatially organize BCC-laden HA micromolded hydrogels adjacent to ECFCs in fibrin hydrogels. These novel, miniaturized systems allow the analysis of the spatial and temporal mechanisms regulating tumor angiogenesis, and can be applied to mimic other microenvironments of healthy and diseased tissues.

Endothelial cell responses to micropillar substrates of varying dimensions and stiffness.

In the vascular niche, the extracellular matrix (ECM) provides a structural scaffold with a rich ligand landscape of essential matrix proteins that supports the organization and stabilization of endothelial cells (ECs) into functional blood vessels. Many of the physical interactions between ECs and macromolecular components of the ECM occur at both the micron and submicron scale. In addition, the elasticity of the ECM has been shown to be a critical factor in the progress of the angiogenic cascade. Here, we sought to determine the effect of substrate topography and elasticity (stiffness) on EC behavior. Utilizing a unique SiO(2) substrate with an array of micropillars, we first demonstrate that micropillars with heights >3 µm significantly decrease EC adhesion and spreading. Fibronectin (Fn) patterning of 1 µm high micropillars enabled EC adhesion onto the micropillars and promoted alignment in a single-cell chain manner. We then developed a robust method to generate a soft micropillar substrate array made of polydimethylsiloxane (PDMS), similar to the SiO(2) substrate. Finally, we examined the kinetics of EC adhesion and spreading on the soft PDMS substrates compared to the stiff SiO(2) substrates. Culturing cells on the PDMS substrates demonstrated an enhanced EC elongation and alignment when compared to stiff SiO(2) with similar topographical features. We conclude that the elongation and alignment of ECs is coregulated by substrate topography and stiffness and can be harnessed to guide vascular organization.

Reconstructing the differentiation niche of embryonic stem cells using biomaterials.

The biochemical cues and topographical architecture of the extracellular environment extensively influence ES cell fate. The microenvironment surrounding the developing embryo presents these instructive cues in a complex and interactive manner in order to guide cell fate decisions. Current stem cell research aims to reconstruct this multifaceted embryonic niche to recapitulate development in vitro. This review focuses on 2D and 3D differentiation niches created from natural and synthetic biomaterials to guide the differentiation of ES cells toward specific lineages. Biomaterials engineered to present specific physical constraints are also reviewed for their role in differentiation.

Micropatterned surfaces to study hyaluronic acid interactions with cancer cells.

Cancer invasion and progression involves a motile cell phenotype, which is under complex regulation by growth factors/cytokines and extracellular matrix (ECM) components within the tumor microenvironment. Hyaluronic acid (HA) is one stromal ECM component that is known to facilitate tumor progression by enhancing invasion, growth, and angiogenesis(1). Interaction of HA with its cell surface receptor CD44 induces signaling events that promote tumor cell growth, survival, and migration, thereby increasing metastatic spread(2-3). HA is an anionic, nonsulfated glycosaminoglycan composed of repeating units of D-glucuronic acid and D-N-acetylglucosamine. Due to the presence of carboxyl and hydroxyl groups on repeating disaccharide units, native HA is largely hydrophilic and amenable to chemical modifications that introduce sulfate groups for photoreative immobilization (4-5). Previous studies involving the immobilizations of HA onto surfaces utilize the bioresistant behavior of HA and its sulfated derivative to control cell adhesion onto surfaces(6-7). In these studies cell adhesion preferentially occurs on non-HA patterned regions. To analyze cellular interactions with exogenous HA, we have developed patterned functionalized surfaces that enable a controllable study and high-resolution visualization of cancer cell interactions with HA. We utilized microcontact printing (uCP) to define discrete patterned regions of HA on glass surfaces. A "tethering" approach that applies carbodiimide linking chemistry to immobilize HA was used (8). Glass surfaces were microcontact printed with an aminosilane and reacted with a HA solution of optimized ratios of EDC and NHS to enable HA immobilization in patterned arrays. Incorporating carbodiimide chemistry with mCP enabled the immobilization of HA to defined regions, creating surfaces suitable for in vitro applications. Both colon cancer cells and breast cancer cells implicitly interacted with the HA micropatterned surfaces. Cancer cell adhesion occurred within 24 hours with proliferation by 48 hours. Using HA micropatterned surfaces, we demonstrated that cancer cell adhesion occurs through the HA receptor CD44. Furthermore, HA patterned surfaces were compatible with scanning electron microscopy (SEM) and allowed high resolution imaging of cancer cell adhesive protrusions and spreading on HA patterns to analyze cancer cell motility on exogenous HA.

Functional surfaces for high-resolution analysis of cancer cell interactions on exogenous hyaluronic acid.

Hyaluronic acid, a nonsulfated, linear glycosaminoglycan, is ubiquitously distributed in the extracellular matrix and is known to facilitate tumor progression by enhancing invasion, growth, and angiogenesis. Native HA has been attached to substrates to create patterned surfaces resistant to cell adhesion, and has been utilized in a variety of cell adhesion studies using either non covalently bound layers patterned by soft lithography or related methods. We use a new approach to study cell interactions with HA-presenting regions, by covalently linking HA adjacent to PEG-ylated regions, which resist cell adhesion. Colon and breast cancer cells seeded on the patterned HA surfaces adhere preferentially on HA-presenting regions and proliferate there. Furthermore, we demonstrate that cell adhesion is inhibited with the blocking of HA receptor, CD44, and that cellular adhesive processes, through protrusions spreading onto the HA surface, enhance spreading and movement outside the HA-presenting regions. Overall, this approach allows high-resolution analysis of cancer cell attachment, growth, and migration on exogenous native HA.