Imaging methods in mechanosensing: a historical perspective and visions for the future

Over the past three decades, as mechanobiology has become a distinct area of study, researchers have developed novel imaging tools to discover the pathways of biomechanical signaling. Early work with substrate engineering and particle tracking demonstrated the importance of cell–extracellular matrix interactions on the cell cycle as well as the mechanical flux of the intracellular environment. Most recently, tension sensor approaches allowed directly measuring tension in cell–cell and cell–substrate interactions. We retrospectively analyze how these various optical techniques progressed the field and suggest our vision forward for a unified theory of cell mechanics, mapping cellular mechanosensing, and novel biomedical applications for mechanobiology.     

Evolutionary consequences of many-to-one mapping of jaw morphology to mechanics in labrid fishes

Many physiological traits consist of two hierarchically related levels: physical structures and the emergent functional properties of those structures. Because selection tends to act on the emergent functional traits, the evolution of structural phenotypes will depend on the nature of the form-function relationship. Complex physiological or biomechanical traits are often characterized by many-to-one mapping: numerous structural phenotypes can yield equivalent functions. We suggest that this redundancy can promote the evolution of phenotypic diversity, and we illustrate this effect with a combination of empirical and analytical studies of a complex biomechanical trait, the four-bar linkage found in the jaws of labrid fishes. We show that labrid jaws are subject to many-to-one mapping of form-to-jaw mechanical properties but that some mechanical types have higher levels of morphological redundancy than others. This variation in redundancy has affected the diversity and distribution of labrid jaw shapes: labrid species are disproportionately concentrated around functional traits with higher potential for redundancy. Many-to-one mapping can also mitigate evolutionary constraints imposed by mechanical trade-offs by allowing a species to simultaneously optimize multiple functional properties. Many-to-one mapping may be an important factor in generating the uneven patterns of diversity in physiological traits.     

Mapping the distribution of the outer hair cell motility voltage sensor by electrical amputation.

The outer hair cell (OHC) possesses a nonlinear charge movement whose characteristics indicate that it represents the voltage sensor responsible for OHC mechanical activity. OHC mechanical activity is known to exist along a restricted extent of the cell's length. We have used a simultaneous partitioning microchamber and whole cell voltage clamp technique to electrically isolate sections of the OHC membrane and find that the nonlinear charge movement is also restricted along the cell's length. Apical and basal portions of the OHC are devoid of voltage sensors, corresponding to regions of the cell where the subsurface cisternae and/or the mechanical responses are absent. We conclude that the physical domain of the motility voltage sensor corresponds to that of the mechanical effector and speculate that sensor and effector reside within one intra membranous molecular species, perhaps an evolved nonconducting or poorly conducting voltage-dependent ion channel.     

Localization algorithm in wireless sensor networks based on semi-supervised manifold learning and its application

Localization of mobile nodes in wireless sensor network gets more and more important, because many applications need to locate the source of incoming measurements as precise as possible. Many previous approaches to the location-estimation problem need know the theories and experiential signal propagation model and collect a large number of labeled samples. So, these approaches are coarse localization because of the inaccurate model, and to obtain such data requires great effort. In this paper, a semi-supervised manifold learning is used to estimate the locations of mobile nodes in a wireless sensor network. The algorithm is used to compute a subspace mapping function between the signal space and the physical space by using a small amount of labeled data and a large amount of unlabeled data. This mapping function can be used online to determine the location of mobile nodes in a sensor network based on the signals received. We use independent development nodes to setup the network in metallurgical industry environment, outdoor and indoor. Experimental results show that we can achieve a higher accuracy with much less calibration effort as compared with RADAR localization systems.     

Many-to-One Mapping of Form to Function: A General Principle in Organismal Design?

We introduce the concept of many-to-one mapping of form to functionand suggest that this emergent property of complex systems promotesthe evolution of physiological diversity. Our work has focusedon a 4-bar linkage found in labrid fish jaws that transmitsmuscular force and motion from the lower jaw to skeletal elementsin the upper jaws. Many different 4-bar shapes produce the sameamount of output rotation in the upper jaw per degree of lowerjaw rotation, a mechanical property termed Maxillary KT. Weillustrate three consequences of many-to-one mapping of 4-barshape to Maxillary KT. First, many-to-one mapping can partiallydecouple morphological and mechanical diversity within clades.We found with simulations of 4-bars evolving on phylogeniesof 500 taxa that morphological and mechanical diversity wereonly loosely correlated (R² = 0.25). Second, redundant mappingpermits the simultaneous optimization of more than one mechanicalproperty of the 4-bar. Labrid fishes have capitalized on thisflexibility, as illustrated by several species that have MaxillaryKT = 0.8 but have different values of a second property, NasalKT. Finally, many-to-one mapping may increase the influenceof historical factors in determining the evolution of morphology.Using a genetic model of 4-bar evolution we exerted convergentselection on three different starting 4-bar shapes and foundthat mechanical convergence only created morphological convergencein simulations where the starting forms were similar. Many-to-onemapping is widespread in physiological systems and operatesat levels ranging from the redundant mapping of genotypes tophenotypes, up to the morphological basis of whole-organismperformance. This phenomenon may be involved in the uneven distributionof functional diversity seen among animal lineages.     

An optofluidic mechanical system for elasticity measurement of thin biological tissues

As dura mater has an anisotropic fibrous structure and exists under wet and dynamic stretching conditions in the brain, its mechanical properties have not yet been properly investigated. Here we developed a fluid-assisted mechanical system integrated with a photonic sensor and a pressure sensor in order to measure the elasticity of the dura mater. Porcine dura mater sample was loaded as a stretched diaphragm into a liquid chamber to mimic the in vivo condition. Increasing the flow rate of saline solution into the chamber swelled and deformed the dura mater. The micron-scale deflection of the dura mater was optically detected by the photonic sensor. Fluid pressure and deflection values were then used to calculate the elastic modulus. The average elastic modulus of the porcine dura mater was 31.14 MPa. We further measured the elasticity of a well-known material to further validate the system. We expect that this optofluidic system developed in this study will be useful to measure the elasticity of a variety of thin biological tissues.     

Imaging of electrochemical enzyme sensor on gold electrode using surface plasmon resonance

Three types of imaging, namely layer structure, electrochemical reaction, and enzyme sensor response, were achieved by applying surface plasmon resonance (SPR) measurement to an electrochemical biosensor. We constructed glucose oxidase based mediator type sensors on a gold electrode by spotting the mediator that contained horseradish peroxidase and spin coating the glucose oxidase film. The layer structure of the sensor was imaged by means of angle scanning SPR measurement. The single sensor spot (about 1 mm in diameter) consisted of about 100 x 100 pixels and its spatial structure was imaged. The multilayer structure of the enzyme sensor had a complex reflectance-incident angle curve and this required us to choose a suitable incident angle for mapping the redox state. We chose an incident angle that provided the most significant reflection intensity difference by using data obtained from two angle scanning SPR measurements at different electrode potentials. At this incident angle, we controlled the electrochemical states of the spotted mediator in cyclic voltammetry and imaged the degree to which the charged site density changed. Finally, we mapped the enzymatic activity around the mediator spot by the enzymatic reoxidation of pre-reduced mediator in the presence of glucose.     

Traction forces exerted through N-cadherin contacts

BACKGROUND INFORMATION: Mechanical forces play an important role in the organization, growth and function of living tissues. The ability of cells to transduce mechanical signals is governed by two types of microscale structures: focal adhesions, which link cells to the extracellular matrix, and adherens junctions, which link adjacent cells through cadherins. Although many studies have examined forces induced by focal adhesions, there is little known about the role of adherens junctions in force-regulation processes. The present study focuses on the determination of force transduction through cadherins at a single cell level. RESULTS: We characterized for the first time the distribution of forces developed by the cell through cadherin contacts. A N-cadherin (neural cadherin)-Fc chimaera, which mimicks the cell adhesion molecule N-cadherin, was immobilized on a muFSA (micro-force sensor array), comprising a dense array of vertical elastomer pillars, which were used both as a cell culture support for N-cadherin-expressing C2 myogenic cells and as detectors for force mapping. We coated the top of the pillars on which cells adhere and recruit adhesion complexes and F-actin. Individual pillar bending allowed the measurement of forces that mainly developed at the cell edge and directed toward their centre. Similar force distribution and amplitude were detected with an unrelated cell line of neuronal origin. Further comparison with forces applied by cells on pillars coated with fibronectin indicates that mechanical stresses transduced through both types of adhesions were comparable in distribution, orientation and amplitude. CONCLUSIONS: These results present a versatile method to measure and map forces exerted by cell-cell adhesion complexes. They show that cells transduce mechanical stress through cadherin contacts which are of the same order as magnitude of those previously characterized for focal adhesions. Altogether, they emphasize the mechanotransduction role of cytoskeleton-linked adhesion receptors of the cadherin family in tissue cohesion and reshaping.     

Rapid analysis of protein interactions: On-chip micropurification of recombinant protein expressed in Esherichia coli

We describe a rapid analysis of interactions between antibodies and a recombinant protein present in total cell lysates. Using a surface plasmon resonance biosensor, a low concentration of glutathione-S-transferase (GST) fused protein expressed in small scale Esherichia coli culture was purified on an anti-GST antibody immobilized sensor chip. The 'on-chip purification' was verified using matrix-assisted laser desorption/ionization-time of flight mass spectrometry by measuring the molecular masses of recombinant proteins purified on the sensor chip. The specific binding of monoclonal antibodies for the on-chip micropurified recombinant proteins can then be monitored, thus enabling kinetic analysis and epitope mapping of the bound antibodies. This approach reduced time, resources and sample consumption by avoiding conventional steps related to concentration and purification.     

The 35S promoter‐driven mDII auxin control sensor is uniformly distributed in leaf primordia

<正>The DII auxin sensor has been an invaluable tool for mapping the spatiotemporal auxin response and distribution in the model plant Arabidopsis thaliana.The DII sensor and the m DII control sensor are driven by the widely used constitutive 35S promoter. Recently, however, the reliability of the DII sensor has been questioned (Bhatia et al. 2019).     

Discordance between morphological and mechanical diversity in the feeding mechanism of centrarchid fishes

Morphological diversity is routinely used to infer ecological variation among species because differences in form underlie variation in functional performance of ecological tasks like capturing prey, avoiding predators, or defending territories. However, many functions have complex morphological bases that can weaken associations between morphological and functional diversification. We investigate the link between these levels of diversity in a mechanically explicit model of fish suction-feeding performance, where the map of head morphology to feeding mechanics is many-to-one: multiple, alternative forms can produce the same mechanical property. We show that many-to-one mapping leads to discordance between morphological and mechanical diversity in the freshwater fish family, the Centrarchidae, despite close associations between morphological changes and their mechanical effects. We find that each of the model's five morphological variables underlies evolution of suction capacity. Yet, the major centrarchid clades exhibit an order of magnitude range in diversity of suction mechanics in the absence of any clear difference in diversity of the morphological variables. This cryptic pattern of mechanical diversity suggests an evolutionary history for suction performance that is unlike the one inferred from comparisons of morphological diversity. Because many-to-one mapping is likely to be common in functional systems, this property of design may lead to widespread discordance between functional and morphological diversity. Although we focus on the interaction between morphology and mechanics, many-to-one mapping can decouple diversity between levels of organization in any hierarchical system.     

The cardiac mechanical stretch sensor machinery involves a Z disc complex that is defective in a subset of human dilated cardiomyopathy

Muscle cells respond to mechanical stretch stimuli by triggering downstream signals for myocyte growth and survival. The molecular components of the muscle stretch sensor are unknown, and their role in muscle disease is unclear. Here, we present biophysical/biochemical studies in muscle LIM protein (MLP) deficient cardiac muscle that support a selective role for this Z disc protein in mechanical stretch sensing. MLP interacts with and colocalizes with telethonin (T-cap), a titin interacting protein. Further, a human MLP mutation (W4R) associated with dilated cardiomyopathy (DCM) results in a marked defect in T-cap interaction/localization. We propose that a Z disc MLP/T-cap complex is a key component of the in vivo cardiomyocyte stretch sensor machinery, and that defects in the complex can lead to human DCM and associated heart failure.     

Effects of fusion between tactile and proprioceptive inputs on tactile perception

Tactile perception is typically considered the result of cortical interpretation of afferent signals from a network of mechanical sensors underneath the skin. Yet, tactile illusion studies suggest that tactile perception can be elicited without afferent signals from mechanoceptors. Therefore, the extent that tactile perception arises from isomorphic mapping of tactile afferents onto the somatosensory cortex remains controversial. We tested whether isomorphic mapping of tactile afferent fibers onto the cortex leads directly to tactile perception by examining whether it is independent from proprioceptive input by evaluating the impact of different hand postures on the perception of a tactile illusion across fingertips. Using the Cutaneous Rabbit Effect, a well studied illusion evoking the perception that a stimulus occurs at a location where none has been delivered, we found that hand posture has a significant effect on the perception of the illusion across the fingertips. This finding emphasizes that tactile perception arises from integration of perceived mechanical and proprioceptive input and not purely from tactile interaction with the external environment.     

Targeted regeneration of bone in the osteoporotic human femur

We have recently developed image processing techniques for measuring the cortical thicknesses of skeletal structures in vivo, with resolution surpassing that of the underlying computed tomography system. The resulting thickness maps can be analysed across cohorts by statistical parametric mapping. Applying these methods to the proximal femurs of osteoporotic women, we discover targeted and apparently synergistic effects of pharmaceutical osteoporosis therapy and habitual mechanical load in enhancing bone thickness.     

U-Shaped Photonic Crystal Fiber Based Surface Plasmon Resonance Sensors

A surface plasmon resonance sensor based on a U-shaped photonic crystal fiber with a rectangular lattice has been designed through finite element method. The U-shaped fiber exhibits not only stronger mechanical strength but also better sensor performance than our previous scheme. The upper detection limit extends to higher analyze refractive index, 1.384, for phase interrogation. We introduce a ratio to evaluate the impact of higher order plasmonic mode. For wavelength modulation scheme, the parameter to describe the performance of a sensor is chosen to be the figure of merit, which can be up to 533.8[RIU^?1] around complete coupling condition.     

3D Active Stabilization System with Sub-Micrometer Resolution

Stable positioning between a measurement probe and its target from sub- to few micrometer scales has become a prerequisite in precision metrology and in cellular level measurements from biological tissues. Here we present a 3D stabilization system based on an optoelectronic displacement sensor and custom piezo-actuators driven by a feedback control loop that constantly aims to zero the relative movement between the sensor and the target. We used simulations and prototyping to characterize the developed system. Our results show that 95 % attenuation of movement artifacts is achieved at 1 Hz with stabilization performance declining to ca. 70 % attenuation at 10 Hz. Stabilization bandwidth is limited by mechanical resonances within the displacement sensor that occur at relatively low frequencies, and are attributable to the sensor's high force sensitivity. We successfully used brain derived micromotion trajectories as a demonstration of complex movement stabilization. The micromotion was reduced to a level of ～1 μm with nearly 100 fold attenuation at the lower frequencies that are typically associated with physiological processes. These results, and possible improvements of the system, are discussed with a focus on possible ways to increase the sensor's force sensitivity without compromising overall system bandwidth.     

Relative microelastic mapping of living cells by atomic force microscopy.

The spatial and temporal changes of the mechanical properties of living cells reflect complex underlying physiological processes. Following these changes should provide valuable insight into the biological importance of cellular mechanics and their regulation. The tip of an atomic force microscope (AFM) can be used to indent soft samples, and the force versus indentation measurement provides information about the local viscoelasticity. By collecting force-distance curves on a time scale where viscous contributions are small, the forces measured are dominated by the elastic properties of the sample. We have developed an experimental approach, using atomic force microscopy, called force integration to equal limits (FIEL) mapping, to produce robust, internally quantitative maps of relative elasticity. FIEL mapping has the advantage of essentially being independent of the tip-sample contact point and the cantilever spring constant. FIEL maps of living Madine-Darby canine kidney (MDCK) cells show that elasticity is uncoupled from topography and reveal a number of unexpected features. These results present a mode of high-resolution visualization in which the contrast is based on the mechanical properties of the sample.     

Utility and feasibility of the CartoUnivu™ system for atrial flutter ablation in patients with mechanical prosthetic valves

Ablation of macroreentrant atrial tachycardia in patients with mechanical prosthetic valves represents a challenge for electrophysiologists, because of the complexity of the procedure and the potential complications. Moreover, the need for fluoroscopy in this type of procedure is greater, due to the risk of interference between the prosthetic valve and the ablation or mapping catheter. We present two cases of patients with mechanical prosthetic valves and atrial flutter who underwent successful ablation with no complications using the CartoUnivu? tool, which integrates the electroanatomical map and the fluoroscopy image.     

The design and use of an optical mapping system for the study of intracardiac electrical signaling

Fluorescent optical mapping of electrically active cardiac tissues provides a unique method to examine the excitation wave dynamics of underlying action potentials. Such mapping can be viewed as a bridge between cellular level and organ systems physiology, e.g., by facilitating the development of advanced theoretical concepts of arrhythmia. We present the design and use of a high-speed, high-resolution optical mapping system composed entirely of "off the shelf" components. The electrical design integrates a 256 element photodiode array with a 16 bit data acquisition system. Proper grounding and shielding at various stages of the design reduce electromagnetic interference. Our mechanical design provides flexibility in terms of mounting positions and applications (use for whole heart or tissue preparations), while maintaining precise alignment between all optical components. The system software incorporates a user friendly graphical user interface, e.g., spatially recorded action potentials can be represented as intensity graphs or in strip chart format. Thus, this system is capable of displaying cardiac action potentials with high spatiotemporal resolution. Results from cardiac action potential mapping with intact mouse hearts are provided. It should be noted that this system could be readily configured to study isolated myocardial biopsies (e.g., isolated ventricular trabeculae). We describe the details of a versatile, user-friendly system that could be employed for a magnitude of study protocols.     

Nano-mechanical Compliance of Müller Cells Investigated by Atomic Force Microscopy

It has been known that a single Müller cell displays a large variation in the cytoskeletal compositions along its cell body, suggesting different mechanical properties in different segments. Müller cells are thought to be involved in many retinal diseases such as retinoschisis, which can be facilitated by a mechanical stress. Thus, mapping of mechanical properties on localized nano-domains of Müller cells could provide essential information for understanding their structural functions in the retina and roles in their pathological progresses. Using Atomic Force Microscopy (AFM) - based bio-nano-mechanics, we have investigated the local variations of the mechanical properties of Müller cells in vitro. We have a particular interest in identifying elastic moduli in regions closer to three distinctive segments of the cells - process, endfoot, and soma. Using the modified spherical AFM probes, we were able to accurately determine mechanical properties, i.e., elastic moduli from the obtained force-distance curves. We found that the regions closer to soma were mechanically more compliant than regions closer to endfoot and process of Müller cells. We found that this lateral heterogeneity of the mechanical compliance within a single Müller cell is consistent with reports from other cell types. The local variation in mechanical compliances along a single Müller cell may support their diverse mechanical functions in the retina such as a soft mechanical embedding, mechanosensing, and neurotrophic functions for neurons.