Atomic force microscopy in biology: technology and techniques

Microfluidic characterization of single-cell biophysical properties and the applications in cancer diagnosis

Single-cell biophysical properties play a crucial role in regulating cellular physiological states and functions, demonstrating significant potential in the fields of life sciences and clinical diagnostics. Therefore, over the last few decades, researchers have developed various detection tools to explore the relationship between the biophysical changes of biological cells and human diseases. With the rapid advancement of modern microfabrication technology, microfluidic devices have quickly emerged as a promising platform for single-cell analysis offering advantages including high-throughput, exceptional precision, and ease of manipulation. Consequently, this paper provides an overview of the recent advances in microfluidic analysis and detection systems for single-cell biophysical properties and their applications in the field of cancer. The working principles and latest research progress of single-cell biophysical property detection are first analyzed, highlighting the significance of electrical and mechanical properties. The development of data acquisition and processing methods for real-time, high-throughput, and practical applications are then discussed. Furthermore, the differences in biophysical properties between tumor and normal cells are outlined, illustrating the potential for utilizing single-cell biophysical properties for tumor cell identification, classification, and drug response assessment. Lastly, we summarize the limitations of existing microfluidic analysis and detection systems in single-cell biophysical properties, while also pointing out the prospects and future directions of their applications in cancer diagnosis and treatment.

Investigating the effect of Er,Cr:YSGG laser agitation of sodium hypochlorite on the removal of mature biofilm in the complex root canal systems using atomic force microscopy

Background

Endodontic infections caused by remaining biofilm following disinfection with chemical fluids encourage secondary bacterial infection; hence, employing laser pulses to activate the fluids is advised to improve microbial biofilm clearance. This study investigated the performance of Er,Cr:YSGG laser in photon-induced photoacoustic streaming (PIPS) agitation of 5.25% sodium hypochlorite (NaOCl) to enhance the removal of mature Enterococcus faecalis (E. faecalis) biofilms in complex root canal systems.

Methods

The mesial roots of the lower first and second molars were separated and inoculated with E. faecalis bacteria for 30 days. The roots were irrigated with 5.25% NaOCl, some of them were agitated with passive ultrasonic irrigation (PUI), and the other roots were agitated by Er,Cr:YSGG laser using PIPS at 60 µs/pulse, 5 Hz, and 0.25, 0.5, 0.75, 1, and 1.25 W. An atomic force microscope (AFM) was used as a new method to obtain the results in the isthmus area; the results that have been obtained from each group were compared with each other. ANOVA was utilized to compare the means of the test groups.

Results

Based on the AFM and SEM analyses, laser agitation and passive ultrasonic activation groups have shown higher antimicrobial efficacy than the conventional syringe irrigation group (P<0.05).

Conclusion

Based on the findings of this investigation, the agitation of 5.25% NaOCl solution by Er,Cr:YSGG laser in PIPS at (60 µs/pulse, 5 Hz, 1.25 W) offers better mature bacterial biofilm removal in the mesial root of lower human molars than the same irrigant with syringe irrigation and passive ultrasonic activation technique.

Experimental Analysis of Tear Fluid and Its Processing for the Diagnosis of Multiple Sclerosis

A pilot analysis of the tear fluid of patients with multiple sclerosis (MS) collected by glass microcapillary was performed using various experimental methods: liquid chromatography-mass spectrometry, Raman spectroscopy, infrared spectroscopy, and atomic-force microscopy. Infrared spectroscopy found no significant difference between the tear fluid of MS patients and the control spectra; all three significant peaks were located at around the same positions. Raman analysis showed differences between the spectra of the tear fluid of MS patients and the spectra of healthy subjects, which indicated a decrease in tryptophan and phenylalanine content and changes in the relative contributions of the secondary structures of the polypeptide chains of tear proteins. Atomic-force microscopy exhibited a surface fern-shaped dendrite morphology of the tear fluid of patients with MS, with less roughness on both oriented silicon (100) and glass substrates compared to the tear fluid of control subjects. The results of liquid chromatography-mass spectrometry showed downregulation of glycosphingolipid metabolism, sphingolipid metabolism, and lipid metabolism. Proteomic analysis identified upregulated proteins in the tear fluid of patients with MS such as cystatine, phospholipid transfer protein, transcobalamin-1, immunoglobulin lambda variable 1-47, lactoperoxidase, and ferroptosis suppressor protein 1; and downregulated proteins such as haptoglobin, prosaposin, cytoskeletal keratin type I pre-mRNA-processing factor 17, neutrophil gelatinase-associated lipocalin, and phospholipase A2. This study showed that the tear proteome in patients with MS is modified and can reflect inflammation. Tear fluid is not a commonly used biological material in clinico-biochemical laboratories. Experimental proteomics has the potential to become a promising contemporary tool for personalized medicine, and it might be applied in clinical practice by providing a detailed analysis of the tear-fluid proteomic profile of patients with MS.

Dynamic morphology imaging of cardiomyocytes based on AFM

A cardiomyocyte is the basic structural and functional unit of the heart, which is the actual executor of the systolic function. The study of the contraction and relaxation characteristics of cardiomyocyte is of great significance to the physiological behavior and pathology of the heart. How to dynamically express its contraction and relaxation behaviors in 3D has become a challenging issue. Although the video analysis method under the optical microscope can observe the changes in the horizontal direction, it is difficult to describe the changes in the vertical direction. The atomic force microscope (AFM) can accurately express the mechanical and morphological characteristics of the changes in the vertical direction, but it cannot be fully expressed in real time because it is acquired by scanning with a single probe. In order to express the contraction and relaxation characteristics of cardiomyocyte accurately and three dimensionally, a dynamic imaging method in this study is proposed using the periodicity of AFM acquisition and the periodicity of cardiomyocyte contraction. Compared with the optical experiment, it is proven that this method can dynamically represent the contraction and relaxation processes of cardiomyocyte and solve the problem of how to express it in 3D. It brings a new way for the study of physiological characteristics of cardiomyocytes and dynamic imaging by AFM.

Data-driven dynamics-based optimal filtering of acoustic noise at arbitrary location in atomic force microscope imaging

This paper presents a post-filtering approach to eliminate distortions in atomic force microscope (AFM) images caused by acoustic noise from an unknown location. AFM operations are sensitive to external disturbances including acoustic noise, as disturbances to the probe-sample interaction directly results in distortions in the sample images obtained. Although conventional passive noise cancellation has been employed, limitation exists and residual noise still persists. Advanced online control techniques face difficulty in capturing the complex noise dynamic and limited system bandwidth imposed by robustness requirement. In this work, we propose a dynamics-based optimal filtering technique to remove the acoustic-caused distortions in AFM images. A dictionary-approach is integrated with time-delay measurement to localize the noise source and estimate the corresponding acoustic dynamics. Then a noise-to-image coherence minimization approach is proposed to minimize the acoustic-caused image distortion via a gradient-based optimization to seek an optimal modulator to the acoustic dynamics. Finally, the filter is obtained as the finite-impulse response of the optimized acoustic dynamics. Experimental implementation is presented and discussed to illustrate the proposed technique.

Assessing Collagen D-Band Periodicity with Atomic Force Microscopy

The collagen superfamily includes more than fifty collagen and/or collagen-like proteins with fibril-forming collagen type I being the most abundant protein within the extracellular matrix. Collagen type I plays a crucial role in a variety of functions, it has been associated with many pathological conditions and it is widely used due to its unique properties. One unique nano-scale characteristic of natural occurring collagen type I fibers is the so-called D-band periodicity, which has been associated with collagen natural structure and properties, while it seems to play a crucial role in the interactions between cells and collagen and in various pathological conditions. An accurate characterization of the surface and structure of collagen fibers, including D-band periodicity, on collagen-based tissues and/or (nano-)biomaterials can be achieved by Atomic Force Microscopy (AFM). AFM is a scanning probe microscope and is among the few techniques that can assess D-band periodicity. This review covers issues related to collagen and collagen D-band periodicity and the use of AFM for studying them. Through a systematic search in databases (PubMed and Scopus) relevant articles were identified. The study of these articles demonstrated that AFM can offer novel information concerning D-band periodicity. This study highlights the importance of studying collagen D-band periodicity and proves that AFM is a powerful tool for investigating a number of different properties related to collagen D-band periodicity.

Biosensing, Characterization of Biosensors, and Improved Drug Delivery Approaches Using Atomic Force Microscopy: A Review

Since its invention, atomic force microscopy (AFM) has come forth as a powerful member of the “scanning probe microscopy” (SPM) family and an unparallel platform for high-resolution imaging and characterization for inorganic and organic samples, especially biomolecules, biosensors, proteins, DNA, and live cells. AFM characterizes any sample by measuring interaction force between the AFM cantilever tip (the probe) and the sample surface, and it is advantageous over other SPM and electron micron microscopy techniques as it can visualize and characterize samples in liquid, ambient air, and vacuum. Therefore, it permits visualization of three-dimensional surface profiles of biological specimens in the near-physiological environment without sacrificing their native structures and functions and without using laborious sample preparation protocols such as freeze-drying, staining, metal coating, staining, or labeling. Biosensors are devices comprising a biological or biologically extracted material (assimilated in a physicochemical transducer) that are utilized to yield electronic signal proportional to the specific analyte concentration. These devices utilize particular biochemical reactions moderated by isolated tissues, enzymes, organelles, and immune system for detecting chemical compounds via thermal, optical, or electrical signals. Other than performing high-resolution imaging and nanomechanical characterization (e.g., determining Young’s modulus, adhesion, and deformation) of biosensors, AFM cantilever (with a ligand functionalized tip) can be transformed into a biosensor (microcantilever-based biosensors) to probe interactions with a particular receptors of choice on live cells at a single-molecule level (using AFM-based single-molecule force spectroscopy techniques) and determine interaction forces and binding kinetics of ligand receptor interactions. Targeted drug delivery systems or vehicles composed of nanoparticles are crucial in novel therapeutics. These systems leverage the idea of targeted delivery of the drug to the desired locations to reduce side effects. AFM is becoming an extremely useful tool in figuring out the topographical and nanomechanical properties of these nanoparticles and other drug delivery carriers. AFM also helps determine binding probabilities and interaction forces of these drug delivery carriers with the targeted receptors and choose the better agent for drug delivery vehicle by introducing competitive binding. In this review, we summarize contributions made by us and other researchers so far that showcase AFM as biosensors, to characterize other sensors, to improve drug delivery approaches, and to discuss future possibilities.

A Universal Positioning System for Coupling Characterization of SEM and AFM

Hyphenated techniques, providing comprehensive information in various aspects such as constituent, structure, functional group, and morphology, play an important role in scientific research. Nowadays, coupling characterization of the same position in microscale is in great need in the field of nanomaterial research and exploration. In this article, a new hyphenated technique was developed to facilitate the coupling characterization of atomic force microscope (AFM) and scanning electron microscope (SEM) by designing a universal positioning system. The system consisted of a specimen holder with coordinate grids and a software for converting the coordinate values of the same point to fit SEM, specimen holder, and AFM system. In working condition, the coordinates of the labeled points and target position were firstly extracted from the SEM operation software, then converted into the numerical values adapted to the specimen holder itself, and finally transformed into the coordinates matching the AFM system. The experimental result showed that a retrieving rate of 96% was achieved for a spherical target with a diameter of 1 μm in a 30 μm × 30 μm square. The hyphenated technique is a universal, accurate, efficient, and financially feasible method in microanalysis field and has great application potential.

Nicotine induces morphological and functional changes in astrocytes via nicotinic receptor activity

Nicotine is a highly addictive compound present in tobacco, which causes the release of dopamine in different regions of the brain. Recent studies have shown that astrocytes express nicotinic acetylcholine receptors (nAChRs) and mediate calcium signaling. In this study, we examine the morphological and functional adaptations of astrocytes due to nicotine exposure. Utilizing a combination of fluorescence and atomic force microscopy, we show that nicotine-treated astrocytes exhibit time-dependent remodeling in the number and length of both proximal and fine processes. Blocking nAChR activity with an antagonist completely abolishes nicotine's influence on astrocyte morphology indicating that nicotine's action is mediated by these receptors. Functional studies show that 24-hr nicotine treatment induces higher levels of calcium activity in both the cell soma and the processes with a more substantial change observed in the processes. Nicotine does not induce reactive astrocytosis even at high concentrations (10 μM) as determined by cytokine release and glial fibrillary acidic protein expression. We designed tissue clearing experiments to test whether morphological changes occur in vivo using astrocyte specific Aldh1l1-tdTomato knock in mice. We find that nicotine induces a change in the volume of astrocytes in the prefrontal cortex, CA1 of the hippocampus, and the substantia nigra. These results indicate that nicotine directly alters the functional and morphological properties of astrocytes potentially contributing to the underlying mechanism of nicotine abuse.

How did correlative atomic force microscopy and super-resolution microscopy evolve in the quest for unravelling enigmas in biology?

With the invention of the Atomic Force Microscope (AFM) in 1986 and the subsequent developments in liquid imaging and cellular imaging it became possible to study the topography of cellular specimens under nearly physiological conditions with nanometric resolution. The application of AFM to biological research was further expanded with the technological advances in imaging modes where topographical data can be combined with nanomechanical measurements, offering the possibility to retrieve the biophysical properties of tissues, cells, fibrous components and biomolecules. Meanwhile, the quest for breaking the Abbe diffraction limit restricting microscopic resolution led to the development of super-resolution fluorescence microscopy techniques that brought the resolution of the light microscope comparable to the resolution obtained by AFM. The instrumental combination of AFM and optical microscopy techniques has evolved over the last decades from integration of AFM with bright-field and phase-contrast imaging techniques at first to correlative AFM and wide-field fluorescence systems and then further to the combination of AFM and fluorescence based super-resolution microscopy modalities. Motivated by the many developments made over the last decade, we provide here a review on AFM combined with super-resolution fluorescence microscopy techniques and how they can be applied for expanding our understanding of biological processes.

Nanoscale Characteristics of Ocular Lipid Thin Films Using Kelvin Probe Force Microscopy

Purpose

To describe the use of Kelvin probe force microscopy (KPFM) to investigate the electrical surface potential of human meibum and to demonstrate successful use of this instrument on both human meibum and a meibum model system (six-lipid stock [6LS]) to elucidate nanoscale surface chemistry and self-assembly characteristics.

Materials and Methods

6LS and meibum were analyzed in this study. Mica-supported thin films were created using the Langmuir-Blodgett trough. Topography and electrical surface potential were quantified using simultaneous atomic force microscopy/KPFM imaging.

Results

Both lipid mixtures formed thin film patches on the surface of the mica substrate, with large aggregates resting atop. The 6LS had aggregate heights ranging from 41 to 153 nm. The range in surface potential was 33.0 to 125.9 mV. The meibum thin films at P = 5 mN/m had aggregates of 170 to 459 nm in height and surface potential ranging from 15.9 to 76.1 mV, while thin films at P = 10 mN/m showed an aggregate size range of 147 to 407 nm and a surface potential range of 11.5 to 255.1 mV.

Conclusions

This study showed imaging of the differences in electrical surface potential of meibum via KPFM and showed similarities in nanoscale topography. 6LS was also successfully analyzed, showing the capabilities of this method for use in both in vitro and ex vivo ocular research.

Translational Relevance

This study describes the use of KPFM for the study of ocular surface lipids for the first time and outlines possibilities for future studies to be carried out using this concept.

Atomic Force Microscopy on Biological Materials Related to Pathological Conditions

Atomic force microscopy (AFM) is an easy-to-use, powerful, high-resolution microscope that allows the user to image any surface and under any aqueous condition. AFM has been used in the investigation of the structural and mechanical properties of a wide range of biological matters including biomolecules, biomaterials, cells, and tissues. It provides the capacity to acquire high-resolution images of biosamples at the nanoscale and allows at readily carrying out mechanical characterization. The capacity of AFM to image and interact with surfaces, under physiologically relevant conditions, is of great importance for realistic and accurate medical and pharmaceutical applications. The aim of this paper is to review recent trends of the use of AFM on biological materials related to health and sickness. First, we present AFM components and its different imaging modes and we continue with combined imaging and coupled AFM systems. Then, we discuss the use of AFM to nanocharacterize collagen, the major fibrous protein of the human body, which has been correlated with many pathological conditions. In the next section, AFM nanolevel surface characterization as a tool to detect possible pathological conditions such as osteoarthritis and cancer is presented. Finally, we demonstrate the use of AFM for studying other pathological conditions, such as Alzheimer's disease and human immunodeficiency virus (HIV), through the investigation of amyloid fibrils and viruses, respectively. Consequently, AFM stands out as the ideal research instrument for exploring the detection of pathological conditions even at very early stages, making it very attractive in the area of bio- and nanomedicine.

AFM assessing of nanomechanical fingerprints for cancer early diagnosis and classification: from single cell to tissue level

Cancer development and progression are closely associated with changes both in the mechano-cellular phenotype of cancer and stromal cells and in the extracellular matrix (ECM) structure, composition, and mechanics. In this paper, we review the use of atomic force microscopy (AFM) as a tool for assessing the nanomechanical fingerprints of solid tumors, so as to be potentially used as a diagnostic biomarker for more accurate identification and early cancer grading/classification. The development of such a methodology is expected to provide new insights and a novel approach for cancer diagnosis. We propose that AFM measurements could be employed to complement standard biopsy procedures, offering an objective, novel and quantitative diagnostic approach with the properties of a blind assay, allowing unbiased evaluation of the sample.

Exploring the Nano-Surface of Collagenous and Other Fibrotic Tissues with AFM

Atomic force microscope (AFM) is a powerful and invaluable tool for imaging and probing the mechanical properties of biological samples at the nanometric scale. The importance of nano-scale characterization and nanomechanics of soft biological tissues is becoming widely appreciated, and AFM offers unique advantages in this direction. In this chapter, we describe the procedure to collect data sets (imaging and mechanical properties measurement) of collagen gels and tumor tissues. We provide step-by-step instructions throughout the procedure, from sample preparation to cantilever calibration, data acquisition, analysis, and visualization, using two commercial AFM systems (PicoPlus and Cypher ES) and software that accompanied the AFM systems and/or are freeware available (WSxM, AtomicJ). Our protocols are written specifically for these two systems and the mentioned software; however, most of the general concepts can be readily translated to other AFM systems and software.

Remodeling Components of the Tumor Microenvironment to Enhance Cancer Therapy

Solid tumor pathophysiology is characterized by an abnormal microenvironment that guides tumor progression and poses barriers to the efficacy of cancer therapies. Most common among tumor types are abnormalities in the structure of the tumor vasculature and stroma. Remodeling the tumor microenvironment with the aim to normalize any aberrant properties has the potential to improve therapy. In this review, we discuss structural abnormalities of the tumor microenvironment and summarize the therapeutic strategies that have been developed to normalize tumors as well as their potential to enhance therapy. Finally, we present different in vitro models that have been developed to analyze and better understand the effects of the tumor microenvironment on cancer cell behavior.

The quality assessment of stored red blood cells probed using atomic-force microscopy

At the moment the suitability of stored red blood cells (sRBC) for transfusion is checked by routine methods such as haemoglobin estimation and the level of haemolysis. These methods cannot characterize directly the quality of the membranes of sRBC. The aim of this work is to assess the quality of sRBC based on such criteria as the membrane's stiffness and the size and the form of sRBC. Materials and Methods. We have investigated 5 series of dry cytosmears of the sRBC which had been kept in blood bank in a period from 1 to 35 days. After AFM imaging, in every specimen, 5 RBC were chosen at random; the diameter, the height, and the stiffness were measured on each of them. Results. The present study shows high increase of the mean values of YM and height of RBC after 35 days of storage and decrease of the mean values of their diameter. Conclusion. Statistically significant high increase of the mean values of YM indicates the decrease of the elasticity of the cells in the course of storing of the RBC. This parameter along with the morphological characteristics can be used as criterion for assessment of applicability of the sRBC for blood transfusion.

Techniques for physicochemical characterization of nanomaterials

Advances in nanotechnology have opened up a new era of diagnosis, prevention and treatment of diseases and traumatic injuries. Nanomaterials, including those with potential for clinical applications, possess novel physicochemical properties that have an impact on their physiological interactions, from the molecular level to the systemic level. There is a lack of standardized methodologies or regulatory protocols for detection or characterization of nanomaterials. This review summarizes the techniques that are commonly used to study the size, shape, surface properties, composition, purity and stability of nanomaterials, along with their advantages and disadvantages. At present there are no FDA guidelines that have been developed specifically for nanomaterial based formulations for diagnostic or therapeutic use. There is an urgent need for standardized protocols and procedures for the characterization of nanoparticles, especially those that are intended for use as theranostics.

The basement membrane of the isolated rat colonic mucosa. A light, electron and atomic force microscopy study

Basement membranes (BM) are structures of the extracellular matrix (ECM), which are involved in epithelial barriers, but also play an important role in processes such as cell adhesion, cell growth and tissue healing. The aim of this study was to investigate possible effects of cell removal on the structure of the BM of the colonic mucosa. The superficial epithelium was removed with EDTA and the samples were then mechanically fixed for immunohistochemistry, TEM, SEM and AFM. For SEM and AFM, some samples were also prepared according to the OTO method. BM marker proteins were detected after cell removal by immunohistochemistry, indicating that BM remains. However, a lamina lucida (LL) was no longer visible in TEM, it disappeared and the BM became slightly thinner. The surface topography of the BM is characterized by the presence of globules, fenestrations and pore-like structures, which were visualized with SEM and AFM. Noteworthy is the visualization for the first time with AFM of a 3D network of fine fibers and filaments ("cords"), which very much resembled that described with TEM by Inoue (1994). An unresolved question is whether the pore-like structures observed in this study, especially with SEM, actually correspond to the pores of the BM whose existence has been demonstrated functionally. In conclusion, the structural patterns and changes described could be considered as a reference to evaluate the effects of other decellularization protocols on BMs, such as those used in tissue engineering.

Investigating bioconjugation by atomic force microscopy

Nanotechnological applications increasingly exploit the selectivity and processivity of biological molecules. Integration of biomolecules such as proteins or DNA into nano-systems typically requires their conjugation to surfaces, for example of carbon-nanotubes or fluorescent quantum dots. The bioconjugated nanostructures exploit the unique strengths of both their biological and nanoparticle components and are used in diverse, future oriented research areas ranging from nanoelectronics to biosensing and nanomedicine. Atomic force microscopy imaging provides valuable, direct insight for the evaluation of different conjugation approaches at the level of the individual molecules. Recent technical advances have enabled high speed imaging by AFM supporting time resolutions sufficient to follow conformational changes of intricately assembled nanostructures in solution. In addition, integration of AFM with different spectroscopic and imaging approaches provides an enhanced level of information on the investigated sample. Furthermore, the AFM itself can serve as an active tool for the assembly of nanostructures based on bioconjugation. AFM is hence a major workhorse in nanotechnology; it is a powerful tool for the structural investigation of bioconjugation and bioconjugation-induced effects as well as the simultaneous active assembly and analysis of bioconjugation-based nanostructures.

Immobilization of pseudorabies virus in porcine tracheal respiratory mucus revealed by single particle tracking

Pseudorabies virus (PRV) initially replicates in the porcine upper respiratory tract. It easily invades the mucosae and submucosae for subsequent spread throughout the body via blood vessels and nervous system. In this context, PRV developed ingenious processes to overcome different barriers such as epithelial cells and the basement membrane. Another important but often overlooked barrier is the substantial mucus layer which coats the mucosae. However, little is known about how PRV particles interact with porcine respiratory mucus. We therefore measured the barrier properties of porcine tracheal respiratory mucus, and investigated the mobility of nanoparticles including PRV in this mucus. We developed an in vitro model utilizing single particle tracking microscopy. Firstly, the mucus pore size was evaluated with polyethylene glycol coupled (PEGylated) nanoparticles and atomic force microscope. Secondly, the mobility of PRV in porcine tracheal respiratory mucus was examined and compared with that of negative, positive and PEGylated nanoparticles. The pore size of porcine tracheal respiratory mucus ranged from 80 to 1500 nm, with an average diameter of 455±240 nm. PRV (zeta potential: −31.8±1.5 mV) experienced a severe obstruction in porcine tracheal respiratory mucus, diffusing 59-fold more slowly than in water. Similarly, the highly negatively (−49.8±0.6 mV) and positively (36.7±1.1 mV) charged nanoparticles were significantly trapped. In contrast, the nearly neutral, hydrophilic PEGylated nanoparticles (−9.6±0.8 mV) diffused rapidly, with the majority of particles moving 50-fold faster than PRV. The mobility of the particles measured was found to be related but not correlated to their surface charge. Furthermore, PEGylated PRV (-13.8±0.9 mV) was observed to diffuse 13-fold faster than native PRV. These findings clearly show that the mobility of PRV was significantly hindered in porcine tracheal respiratory mucus, and that the obstruction of PRV was due to complex mucoadhesive interactions including charge interactions rather than size exclusion.

Compliance profile of the human cornea as measured by atomic force microscopy

The ability to accurately determine the elastic modulus of each layer of the human cornea is a crucial step in the design of better corneal prosthetics. In addition, knowledge of the elastic modulus will allow design of substrates with relevant mechanical properties for in vitro investigations of cellular behavior. Previously, we have reported elastic modulus values for the anterior basement membrane and Descemet's membrane of the human cornea, the surfaces in contact with the epithelial and endothelial cells, respectively. We have completed the compliance profile of the stromal elements of the human cornea by obtaining elastic modulus values for Bowman's layer and the anterior stroma. Atomic force microscopy (AFM) was used to determine the elastic modulus, which is a measure of the tissue stiffness and is inversely proportional to the compliance. The elastic response of the tissue allows analysis with the Hertz equation, a model that provides a relationship between the indentation force and depth and is a function of the tip radius and the modulus of the substrate. The elastic modulus values for each layer of the cornea are: 7.5±4.2 kPa (anterior basement membrane), 109.8±13.2 kPa (Bowman's layer), 33.1±6.1 kPa (anterior stroma), and 50±17.8 kPa (Descemet's membrane). These results indicate that the biophysical properties, including elastic modulus, of each layer of the human cornea are unique and may play a role in the maintenance of homeostasis as well as in the response to therapeutic agents and disease states. The data will also inform the design and fabrication of improved corneal prosthetics.

The applications of atomic force microscopy to vision science

The atomic force microscope (AFM) is widely used in materials science and has found many applications in biological sciences but has been limited in use in vision science. The AFM can be used to image the topography of soft biological materials in their native environments. It can also be used to probe the mechanical properties of cells and extracellular matrices, including their intrinsic elastic modulus and receptor-ligand interactions. In this review, the operation of the AFM is described along with a review of how it has been thus far used in vision science. It is hoped that this review will serve to stimulate vision scientists to consider incorporating AFM as part of their research toolkit.

Dynamics of nucleic acid/cationic polymer complexation and disassembly under biologically simulated conditions using in situ atomic force microscopy

Elucidating dynamic morphological changes of gene-carrying vectors and their nucleic acid release under varying intracellular conditions has been a technical challenge. Atomic force microscopy (AFM) was used to observe nucleic acid/polymer polyplexes under endosomal and reducible cytosolic conditions. Both ketalized (acid-degradable) and unmodified (nondegradable) polyethylenimine (PEI) in linear and branched forms were used to prepare plasmid DNA- or siRNA-complexing polyplexes. Then, the polyplexes' complexation and disassembly were observed by in situ AFM in various differentially changing buffers that represent intracellular conditions. Results demonstrated obvious morphological destruction of DNA/ketalized linear PEI (KL-PEI) polyplexes under mildly acidic endosomal conditions, whereas no morphological changes were observed by DNA/ketalized branched PEI (KB-PEI) under the same conditions. In addition, siRNA was more efficiently dissociated from KL-PEI than KB-PEI under the same conditions. Nondegradable PEI did not show any evidence that DNA or siRNA was released. Anionic biomacromolecules (e.g., heparan sulfate), which was hypothesized to dissociate nucleic acids from cationic polymers, did not successfully disassemble polyplexes but appeared to be adsorbed on cationic polymers. The in situ AFM results combined with in vitro cellular transfection and gene silencing indicated that efficient endosomal escape of plasmid DNA in a compact polyplex form is required for efficient gene expression. On the contrary, rapid dissociation of siRNA from its cationic carrier is crucial for efficient gene silencing. The findings of this study may provide new insightful information for designing stimuli-responsive nonviral gene vectors as well as expanding tools for investigating nonviral vectors in nanoscales under biologically inspired conditions.

Molecular imaging of membrane proteins and microfilaments using atomic force microscopy

Atomic force microscopy (AFM) is an emerging technique for a variety of uses involving the analysis of cells. AFM is widely applied to obtain information about both cellular structural and subcellular events. In particular, a variety of investigations into membrane proteins and microfilaments were performed with AFM. Here, we introduce applications of AFM to molecular imaging of membrane proteins, and various approaches for observation and identification of intracellular microfilaments at the molecular level. These approaches can contribute to many applications of AFM in cell imaging.

Effects of a surface topography composite with puerariae radix on human STRO-1-positive stem cells

Human skeletal stem cells (STRO-1 positive/STRO-1+) respond to different topographical features in various ways. On a flat surface these cells spread and tend to develop a fibroblast-like morphology. On a microgrooved surface enriched skeletal stem cell populations prefer to stretch along the grooves, which affects their cellular structure and differentiation, a phenomenon known as contact guidance. Growth factors, hormones and chemicals can also stimulate cell differentiation. A traditional Chinese medicine, puerariae radix, has previously been observed to stimulate bone formation. The active ingredients have been identified as isoflavones with estrogen-like bioactivity. This study combined the effects of microgrooved topology and hormone-like isoflavones in the biodegradable polymer polycaprolactone (PCL). Human osteogenic cells (STRO-1+) were cultured on flat PCL, grooved PCL and puerariae powder-impregnated grooved PCL for 5 weeks. Coomassie staining indicated that cell growth and survival was similar on flat PCL, grooved PCL and grooved PCL impregnated with 1 wt.% or 2 wt.% puerariae powder. Grooved PCL impregnated with 2 wt.% puerariae powder was observed to have an influence on protein expression, as observed by positive osteocalcin staining. Protein expression profiles were analyzed by difference gel electrophoresis to identify proteins that showed modulation of expression in response to these different environments. Overall, our results suggest that puerariae powder has an additive effect, along with microgrooved topographical stimulation, to promote changes in the STRO-1+ proteome that affect cell phenotype.

Distinct defects in collagen microarchitecture underlie vessel-wall failure in advanced abdominal aneurysms and aneurysms in Marfan syndrome

An aneurysm of the aorta is a common pathology characterized by segmental weakening of the artery. Although it is generally accepted that the vessel-wall weakening is caused by an impaired collagen metabolism, a clear association has been demonstrated only for rare syndromes such as the vascular type Ehlers-Danlos syndrome. Here we show that vessel-wall failure in growing aneurysms of patients who have aortic abdominal aneurysm (AAA) or Marfan syndrome is not related to a collagen defect at the molecular level. On the contrary our findings indicate similar (Marfan) or even higher collagen concentrations (AAA) and increased collagen cross-linking in the aneurysms. Using 3D confocal imaging we show that the two conditions are associated with profound defects in collagen microarchitecture. Reconstructions of normal vessel wall show that adventitial collagen fibers are organized in a loose braiding of collagen ribbons. These ribbons encage the vessel, allowing the vessel to dilate easily but preventing overstretching. AAA and aneurysms in Marfan syndrome show dramatically altered collagen architectures with loss of the collagen knitting. Evaluations of the functional characteristics by atomic force microscopy showed that the wall has lost its ability to stretch easily and revealed a second defect: although vascular collagen in normal aortic wall behaves as a coherent network, in AAA and Marfan tissues it does not. As result, mechanical forces loaded on individual fibers are not distributed over the tissue. These studies demonstrate that the mechanical properties of tissue are strongly influenced by collagen microarchitecture and that perturbations in the collagen networks may lead to mechanical failure.

Differential in-gel electrophoresis (DIGE) analysis of human bone marrow osteoprogenitor cell contact guidance

We have used a recent comparative proteomics technique, differential in-gel electrophoresis (DIGE), to study osteoprogenitor cell response to contact guidance in grooves. In order to increase protein output from small sample sizes, we used bioreactor culture before protein extraction and gel electrophoresis. Mass spectroscopy was used for protein identification. A number of distinct proteins were observed to exhibit significant changes in expression. These changes in protein expression suggest that the cells respond to tailored grooved topographies, with alterations in their proteome concurrent with changes in osteoprogenitor phenotype.

Acute hyperoxia increases lipid peroxidation and induces plasma membrane blebbing in human U87 glioblastoma cells

Atomic force microscopy (AFM), malondialdehyde (MDA) assays, and amperometric measurements of extracellular hydrogen peroxide (H(2)O(2)) were used to test the hypothesis that graded hyperoxia induces measurable nanoscopic changes in membrane ultrastructure and membrane lipid peroxidation (MLP) in cultured U87 human glioma cells. U87 cells were exposed to 0.20 atmospheres absolute (ATA) O(2), normobaric hyperoxia (0.95 ATA O(2)) or hyperbaric hyperoxia (HBO(2), 3.25 ATA O(2)) for 60 min. H(2)O(2) (0.2 or 2 mM; 60 min) was used as a positive control for MLP. Cells were fixed with 2% glutaraldehyde immediately after treatment and scanned with AFM in air or fluid. Surface topography revealed ultrastructural changes such as membrane blebbing in cells treated with hyperoxia and H(2)O(2). Average membrane roughness (R(a)) of individual cells from each group (n=35 to 45 cells/group) was quantified to assess ultrastructural changes from oxidative stress. The R(a) of the plasma membrane was 34+/-3, 57+/-3 and 63+/-5 nm in 0.20 ATA O(2), 0.95 ATA O(2) and HBO(2), respectively. R(a) was 56+/-7 and 138+/-14 nm in 0.2 and 2 mM H(2)O(2). Similarly, levels of MDA were significantly elevated in cultures treated with hyperoxia and H(2)O(2) and correlated with O(2)-induced membrane blebbing (r(2)=0.93). Coapplication of antioxidant, Trolox-C (150 microM), significantly reduced membrane R(a) and MDA levels during hyperoxia. Hyperoxia-induced H(2)O(2) production increased 189%+/-5% (0.95 ATA O(2)) and 236%+/-5% (4 ATA O(2)) above control (0.20 ATA O(2)). We conclude that MLP and membrane blebbing increase with increasing O(2) concentration. We hypothesize that membrane blebbing is an ultrastructural correlate of MLP resulting from hyperoxia. Furthermore, AFM is a powerful technique for resolving nanoscopic changes in the plasma membrane that result from oxidative damage.

Study of the sensitivity of the first four flexural modes of an AFM cantilever with a sidewall probe

The resonant frequency and sensitivity of flexural vibration for an atomic force microscope (AFM) cantilever with a sidewall probe have been analyzed. A closed-form expression for the sensitivity of vibration modes has been obtained using the relationship between the resonant frequency and contact stiffness of cantilever and sample. The results show that a sidewall scanning AFM is more sensitive when the contact stiffness is lower and that the first mode is the most sensitive. However, the high-order modes become more sensitive than the low-order modes as the contact stiffness increases. The resonance frequency of an AFM cantilever is low when contact stiffness is small. However, the frequency rapidly increases as contact stiffness increases. In addition, it can be found that the effects of the vertical extension on the sensitivity and the resonant frequency of an AFM cantilever are significant. Decreasing the length of vertical extension can increase the resonance frequency and sensitivity of mode 1 when the contact stiffness is small. However, the situation is reverse when the contact stiffness becomes large.

A mechanical spike accompanies the action potential in Mammalian nerve terminals

Large and rapid changes in light scattering accompany secretion from nerve terminals of the mammalian neurohypophysis (posterior pituitary). In the mouse, these intrinsic optical signals are intimately related to the arrival of the action potential E-wave and the release of arginine vasopressin and oxytocin (S-wave). Here we have used a high bandwidth atomic force microscope to demonstrate that these light-scattering signals are associated with changes in terminal volume that are detected as nanometer-scale movements of a cantilever positioned on top of the neurohypophysis. The most rapid mechanical response ("spike"), having a duration shorter than the action potential but comparable to that of the E-wave, represents a transient increase in terminal volume due to water movement associated with Na(+)-influx. The slower mechanical event ("dip"), on the other hand, depends upon Ca(2+)-entry as well as on intraterminal Ca(2+)-transients and, analogously to the S-wave, seems to monitor events associated with secretion.