Adapting atomic force microscopy for cell biology

Quantitative membrane electrostatics with the atomic force microscope

The atomic force microscope (AFM) is sensitive to electric double layer interactions in electrolyte solutions, but provides only a qualitative view of interfacial electrostatics. We have fully characterized silicon nitride probe tips and other experimental parameters to allow a quantitative electrostatic analysis by AFM, and we have tested the validity of a simple analytical force expression through numerical simulations. As a test sample, we have measured the effective surface charge density of supported zwitterionic dioleoylphosphatidylcholine membranes with a variable fraction of anionic dioleoylphosphatidylserine. The resulting surface charge density and surface potential values are in quantitative agreement with those predicted by the Gouy-Chapman-Stern model of membrane charge regulation, but only when the numerical analysis is employed. In addition, we demonstrate that the AFM can detect double layer forces at a separation of several screening lengths, and that the probe only perturbs the membrane surface potential by <2%. Finally, we demonstrate 50-nm resolution electrostatic mapping on heterogeneous model membranes with the AFM. This novel combination of capabilities demonstrates that the AFM is a unique and powerful probe of membrane electrostatics.

Nanomechanical properties of individual chondrocytes and their developing growth factor-stimulated pericellular matrix

The nanomechanical properties of individual cartilage cells (chondrocytes) and their aggrecan and collagen-rich pericellular matrix (PCM) were measured via atomic force microscope nanoindentation using probe tips of two length scales (nanosized and micron-sized). The properties of cells freshly isolated from cartilage tissue (devoid of PCM) were compared to cells that were cultured for selected times (up to 28 days) in 3-D alginate gels which enabled PCM assembly and accumulation. Cells were immobilized and kept viable in pyramidal wells microfabricated into an array on silicon chips. Hertzian contact mechanics and finite element analyses were employed to estimate apparent moduli from the force versus depth curves. The effects of culture conditions on the resulting PCM properties were studied by comparing 10% fetal bovine serum to medium containing a combination of insulin growth factor-1 (IGF-1)+osteogenic protein-1 (OP-1). While both systems showed increases in stiffness with time in culture between days 7 and 28, the IGF-1+OP-1 combination resulted in a higher stiffness for the cell-PCM composite by day 28 and a higher apparent modulus of the PCM which is compared to the FBS cultured cells. These studies give insight into the temporal evolution of the nanomechanical properties of the pericellar matrix relevant to the biomechanics and mechanobiology of tissue-engineered constructs for cartilage repair.

Detection and localization of single molecular recognition events using atomic force microscopy

Because of its piconewton force sensitivity and nanometer positional accuracy, the atomic force microscope (AFM) has emerged as a powerful tool for exploring the forces and the dynamics of the interaction between individual ligands and receptors, either on isolated molecules or on cellular surfaces. These studies require attaching specific biomolecules or cells on AFM tips and on solid supports and measuring the unbinding forces between the modified surfaces using AFM force spectroscopy. In this review, we describe the current methodology for molecular recognition studies using the AFM, with an emphasis on strategies available for preparing AFM tips and samples, and on procedures for detecting and localizing single molecular recognition events.

Imaging and probing cell mechanical properties with the atomic force microscope

This chapter describes the use of the atomic force microscope (AFM) to probe and map out regional variations in apparent elastic properties of living cells. The importance of mechanics in the field of cell biology is becoming more widely appreciated, and the AFM has unique advantages for cell mechanics applications. However, care must be taken in the acquisition, analysis, Band interpretation of AFM indentation data. To help make this powerful technique accessible to a broad range of investigators, detailed procedures are provided for all stages of the AFM experiment from sample preparation through data analysis and visualization.

Protein crystals as scanned probes for recognition atomic force microscopy

Lysozyme crystal growth has been localized at the tip of a conventional silicon nitride cantilever through seeded nucleation. After cross-linking with glutaraldehyde, lysozyme protein crystal tips image gold nanoparticles and grating standards with a resolution comparable to that of conventional tips. Force spectra between the lysozyme crystal tips and surfaces covered with antilysozyme reveal an adhesion force that drops significantly upon blocking with free lysozyme, thus confirming that lysozyme crystal tips can detect molecular recognition interactions.

G-protein-coupled receptors in drug discovery: nanosizing using cell-free technologies and molecular biology approaches

Signal transduction by G-protein-coupled receptors (GPCRs) underpins a multitude of physiological processes. Ligand recognition by the receptor leads to activation of a generic molecular switch involving heterotrimeric G-proteins and guanine nucleotides. Signal transduction has been studied extensively with both cell-based systems and assays comprising isolated signaling components. Interest and commercial investment in GPCRs in areas such as drug targets, orphan receptors, high throughput screening, biosensors, and so on will focus greater attention on assay development to allow for miniaturization, ultra-high throughput and, eventually, microarray/biochip assay formats. Although cell-based assays are adequate for many GPCRs, it is likely that these formats will limit the development of higher density GPCR assay platforms mandatory for other applications. Stable, robust, cell-free signaling assemblies comprising receptor and appropriate molecular switching components will form the basis of future GPCR assay platforms adaptable for such applications as microarrays. The authors review current cell-free GPCR assay technologies and molecular biological approaches for construction of novel, functional GPCR assays.

AFM study of hippocampal cells cultured on silicon wafers with nano-scale surface topograph

The rat hippocampal cells were selected as model to study the interaction between the neural cells and silicon substrates using atomic force microscopy (AFM). The hippocampal cells show tight adherence on silicon wafers with nano-scale surface topograph. The lateral friction force investigated by AFM shows significant increase on the boundary around the cellular body. It is considered to relate to the cytoskeleton and cellular secretions. After ultrasonic wash in ethanol and acetone step by step, the surface of silicon wafers was observed by AFM sequentially. We have found that the culture leftovers form tight porous networks and a monolayer on the silicon wafers. It is concluded that the leftovers overspreading on the silicon substrates are the base of cell adherence on such smooth inert surfaces.

Nanoscale mapping and functional analysis of individual adhesins on living bacteria

Although much progress has been made in the identification and characterization of adhesins borne by pathogenic bacteria, the molecular details underlying their interaction with host receptors remain largely unknown owing to the lack of appropriate probing techniques. Here we report a method, based on atomic force microscopy (AFM) with tips bearing biologically active molecules, for measuring the specific binding forces of individual adhesins and for mapping their distribution on the surface of living bacteria. First, we determined the adhesion forces between the heparin-binding haemagglutinin adhesin (HBHA) produced by Mycobacterium tuberculosis and heparin, used as a model sulphated glycoconjugate receptor. Both the adhesion frequency and adhesion force increased with contact time, indicating that the HBHA-heparin complex is formed via multiple intermolecular bridges. We then mapped the distribution of single HBHA molecules on the surface of living mycobacteria and found that the adhesin is not randomly distributed over the mycobacterial surface, but concentrated into nanodomains.

Estimating the sensitivity of mechanosensitive ion channels to membrane strain and tension

Bone adapts to its environment by a process in which osteoblasts and osteocytes sense applied mechanical strain. One possible pathway for the detection of strain involves mechanosensitive channels and we sought to determine their sensitivity to membrane strain and tension. We used a combination of experimental and computational modeling techniques to gain new insights into cell mechanics and the regulation of mechanosensitive channels. Using patch-clamp electrophysiology combined with video microscopy, we recorded simultaneously the evolution of membrane extensions into the micropipette, applied pressure, and membrane currents. Nonselective mechanosensitive cation channels with a conductance of 15 pS were observed. Bleb aspiration into the micropipette was simulated using finite element models incorporating the cytoplasm, the actin cortex, the plasma membrane, cellular stiffening in response to strain, and adhesion between the membrane and the micropipette. Using this model, we examine the relative importance of the different cellular components in resisting suction into the pipette and estimate the membrane strains and tensions needed to open mechanosensitive channels. Radial membrane strains of 800% and tensions of 5 10(-4) N.m(-1) were needed to open 50% of mechanosensitive channels. We discuss the relevance of these results in the understanding of cellular reactions to mechanical strain and bone physiology.

Simultaneous topography and recognition imaging using force microscopy

We present a method for simultaneously recording topography images and localizing specific binding sites with nm positional accuracy by combining dynamic force microscopy with single molecule recognition force spectroscopy. For this we used lysozyme adsorbed to mica, the functionality of which was characterized by enzyme immunoassays. The topography and recognition images were acquired using tips that were magnetically oscillated during scanning and contained antibodies directed against lysozyme. For cantilevers with low Q-factor (approximately 1 in liquid) driven at frequencies below resonance, the surface contact only affected the downward deflections (minima) of the oscillations, whereas binding of the antibody on the tip to lysozyme on the surface only affected the upwards deflections (maxima) of the oscillations. The recognition signals were therefore well separated from the topographic signals, both in space (Delta z approximately 5 nm) and time (approximately 0.1 ms). Topography and recognition images were simultaneously recorded using a specially designed electronic circuit with which the maxima (U(up)) and the minima (U(down)) of each sinusoidal cantilever deflection period were depicted. U(down) was used for driving the feedback loop to record the height (topography) image, and U(up) provided the data for the recognition image.

The significance of vasoactive intestinal peptide in immunomodulation

First identified by Said and Mutt some 30 years ago, the vasoactive intestinal peptide (VIP) was originally isolated as a vasodilator peptide. Subsequently, its biochemistry was elucidated, and within the 1st decade, their signature features as a neuropeptide became consolidated. It did not take long for these insights to permeate the field of immunology, out of which surprising new attributes for VIP were found in the last years. VIP is rapidly transforming into something more than a mere hormone. In evolving scientifically from a hormone to a novel agent for modifying immune function and possibly a cytokine-like molecule, VIP research has engaged many physiologists, molecular biologists, biochemists, endocrinologists, and pharmacologists and it is a paradigm to explore mutual interactions between neural and neuroendocrine links in health and disease. The aim of this review is firstly to update our knowledge of the cellular and molecular events relevant to VIP function on the immune system and secondly to gather together recent data that support its role as a type 2 cytokine. Recognition of the central functions VIP plays in cellular processes is focusing our attention on this "very important peptide" as exciting new candidates for therapeutic intervention and drug development.

Single-chip mechatronic microsystem for surface imaging and force response studies

We report on a stand-alone single-chip (7 x 10 mm) atomic force microscopy unit including a fully integrated array of cantilevers, each of which has an individual actuation, detection, and control unit so that standard atomic force microscopy operations can be performed by means of the chip only without any external controller. The system offers drastically reduced overall size and costs as well as increased scanning speed and can be fabricated with standard complementary metal oxide semiconductor technology with some subsequent micromachining steps to form the cantilevers. Full integration of microelectronic and micromechanical components on the same chip allows for the controlling and monitoring of all system functions. The on-chip circuitry, which includes analog signal amplification and filtering stages with offset compensation, analog-to-digital converters, a powerful digital signal processor, and an on-chip digital interface for data transmission, notably improves the overall system performance. The microsystem characterization evidenced a vertical resolution of < 1 nm and a force resolution of < 1 nN as shown in the measurement results. The monolithic system represents a paradigm of a mechatronic microsystem that allows for precise and fully controlled mechanical manipulation in the nanoworld.

Control Issues in High-speed AFM for Biological Applications: Collagen Imaging Example

This article considers the precision positioning problem associated with high-speed operation of the Atomic Force Microscope (AFM), and presents an inversion-based control approach to achieve precision positioning. Although AFMs have high (nanoscale) spatial resolution, a problem with current AFM systems is that they have low temporal resolution, i.e., AFM imaging is slow. In particular, current AFM imaging cannot be used to provide three-dimensional, time-lapse images of fast processes when imaging relatively-large, soft samples. For instance, current AFM imaging of living cells takes 1-2 minutes (per image frame) - such imaging speeds are too slow to study rapid biological processes that occur in seconds, e.g., to investigate the rapid movement of cells or the fast dehydration and denaturation of collagen. This inability, to rapidly image fast biological processes, motivates our current research to increase the operating speed of the AFM. We apply an inversion-based feedback/feedforward control approach to overcome positioning problems that limit the operating speed of current AFM systems. The efficacy of the method, to achieve high-speed AFM operation, is experimentally evaluated by applying it to image collagen samples.

Development and validation of vertical scanning interferometry as a novel method for acquiring chondrocyte geometry

Chondrocytes are sensitive to changes in shape which depend on the type of substrate, mechanical factors, or biochemical stimuli. Shape changes can cause metabolic and phenotypic alterations. Cell geometry is also important for mechanical models, determination of mechanical properties, and the study of cell attachment and spreading. In this study, a novel method called vertical scanning interferometry (VSI) was developed to allow rapid and straightforward determination of the height, diameter, surface area, volume, and curvature of single chondrocytes. The dimensions of single chondrocytes at 4 and 18 h were obtained and validated. Differences in the heights of zonal chondrocytes were found to be statistically significant. This method was also used to capture the geometry of a cell dehydrating as it was exposed to air. VSI has advantages over confocal microscopy and atomic force microscopy in terms of speed, ease of use, field of view, and precision. VSI quickly obtains and graphically represents the three-dimensional geometry of chondrocytes in a simple format. These methods could be expanded to image many types of cells on various biomaterials to assess biocompatibility and attachment and cell spreading characteristics.

Targeting of biotinylated compounds to its target tissue using a low-density lipoprotein receptor–avidin fusion protein

The very high binding affinity of avidin to biotin is one of the highest to occur in nature. We constructed a fusion protein composed of avidin and the endocytotic LDL receptor in order to target biotinylated molecules to cells of the desired tissues. In addition to the native avidin, charge-mutated and nonglycosylated avidins were utilized as part of the fusion proteins, in order to modify its properties. All of the fusion protein versions retained the biotin-binding capacity. Although the specificity was not increased, however, fusion proteins composed of natural avidin and nonglycosylated avidin bound most efficiently to the biotinylated ligands. Fluorescence microscopy and atomic force microscopy studies revealed the expression of the fusion protein on cell membranes, and demonstrated specific and high-affinity binding of biotin to the low-density lipoprotein receptor (LDLR)-avidin fusion protein in vitro. Additionally, systemically administered biotinylated ligand targeted with high specificity the intracerebral tumors of rats that were expressing fusion protein after the virus-mediated gene transfer. These results suggest that local gene transfer of the fusion protein to target tissues may offer a novel tool for the delivery of biotinylated molecules in vitro and in vivo for therapeutic and imaging purposes.

mRNA analysis of single living cells

Analysis of specific gene expression in single living cells may become an important technique for cell biology. So far, no method has been available to detect mRNA in living cells without killing or destroying them. We have developed here a novel method to examine gene expression of living cells using an atomic force microscope (AFM). AFM tip was inserted into living cells to extract mRNAs. The obtained mRNAs were analyzed with RT-PCR, nested PCR, and quantitative PCR. This method enabled us to examine time-dependent gene expression of single living cells without serious damage to the cells.

Use of atomic force microscopy for morphological and morphometric analyses of acrosome intact and acrosome-reacted human sperm

The objective of this study was to use atomic force microscopy (AFM), with submicron resolution, for morphophologic and morphometric analyses of acrosome intact and acrosome-reacted human sperm heads. A mixed population of acrosome intact and reacted sperm was produced by treating capacitated sperm with A23187, which induced the acrosome reaction in approximately 50% of total sperm population. This A23187-treated sperm suspension was then plated onto a coverslip and acrosome reacted sperm were preidentified by their specific staining with rhodamine-conjugated Concanavalin A. The sperm coverslip was then air-dried and scanned by a Nanoscope IIIa atomic force microscope, using the contact mode. Top and side view images processed through the illuminate mode revealed three dimensional sperm head contour, with the highest point situated in the head posterior in both acrosome intact and acrosome reacted sperm. Maximum height, length, and width measured in 50 acrosome intact and 50 acrosome-reacted sperm were the same in both populations. However, head length at half maximum height was significantly decreased in acrosome reacted sperm (2.99 +/- 0.24 microm vs. 3.56 +/- 0.32 microm of acrosome intact sperm), due to the sudden change of the height contour from the maximum peak to the anterior tip of acrosome-reacted sperm. Our results described here can therefore be used to differentiate acrosome intact and reacted sperm from each other. This would allow future studies on subcellular changes, related to the acrosome reaction, at the submicron resolution level under more physiological conditions, since AFM does not require fixing or staining of the samples.

Determination of cellular strains by combined atomic force microscopy and finite element modeling

Many organs adapt to their mechanical environment as a result of physiological change or disease. Cells are both the detectors and effectors of this process. Though many studies have been performed in vitro to investigate the mechanisms of detection and adaptation to mechanical strains, the cellular strains remain unknown and results from different stimulation techniques cannot be compared. By combining experimental determination of cell profiles and elasticities by atomic force microscopy with finite element modeling and computational fluid dynamics, we report the cellular strain distributions exerted by common whole-cell straining techniques and from micromanipulation techniques, hence enabling their comparison. Using data from our own analyses and experiments performed by others, we examine the threshold of activation for different signal transduction processes and the strain components that they may detect. We show that modulating cell elasticity, by increasing the F-actin content of the cytoskeleton, or cellular Poisson ratio are good strategies to resist fluid shear or hydrostatic pressure. We report that stray fluid flow in some substrate-stretch systems elicits significant cellular strains. In conclusion, this technique shows promise in furthering our understanding of the interplay among mechanical forces, strain detection, gene expression, and cellular adaptation in physiology and disease.

Single cell mechanotransduction and its modulation analyzed by atomic force microscope indentation

The skeleton adapts to its mechanical usage, although at the cellular level, the distribution and magnitude of strains generated and their detection are ill-understood. The magnitude and nature of the strains to which cells respond were investigated using an atomic force microscope (AFM) as a microindentor. A confocal microscope linked to the setup enabled analysis of cellular responses. Two different cell response pathways were identified: one, consequent upon contact, depended on activation of stretch-activated ion channels; the second, following stress relaxation, required an intact microtubular cytoskeleton. The cellular responses could be modulated by selectively disrupting cytoskeletal components thought to be involved in the transduction of mechanical stimuli. The F-actin cytoskeleton was not required for responses to mechanical strain, whereas the microtubular and vimentin networks were. Treatments that reduced membrane tension, or its transmission, selectively reduced contact reactions. Immunostaining of the cell cytoskeleton was used to interpret the results of the cytoskeletal disruption studies. We provide an estimate of the cellular strain magnitude needed to elicit intracellular calcium responses and propose a model that links single cell responses to whole bone adaptation. This technique may help to understand adaptation to mechanical usage in other organs.

Analysis of ligand-receptor interactions in cells by atomic force microscopy

Atomic force microscopy (AFM) increasingly has been used to analyse "receptor" function, either by using purified proteins ("molecular recognition microscopy") or, more recently, in situ in living cells. The latter approach has been enabled by the use of a modified commercial AFM, linked to a confocal microscope, which has allowed adhesion forces between ligands and receptors in cells to be measured and mapped, and downstream cellular responses analysed. We review the application of AFM to cell biology and, in particular, to the study of ligand-receptor interactions and draw examples from our own work and that of others to show the utility of AFM, including for the exploration of cell surface functionalities. We also identify shortcomings of AFM in comparison to "standard" methods, such as receptor auto-radiography or immuno-detection, that are widely applied in cell biology and pharmacological analysis.

Cell biology beyond the diffraction limit: near-field scanning optical microscopy

Throughout the years, fluorescence microscopy has proven to be an extremely versatile tool for cell biologists to study live cells. Its high sensitivity and non-invasiveness, together with the ever-growing spectrum of sophisticated fluorescent indicators, ensure that it will continue to have a prominent role in the future. A drawback of light microscopy is the fundamental limit of the attainable spatial resolution – ∼250 nm – dictated by the laws of diffraction. The challenge to break this diffraction limit has led to the development of several novel imaging techniques. One of them, near-field scanning optical microscopy (NSOM), allows fluorescence imaging at a resolution of only a few tens of nanometers and, because of the extremely small near-field excitation volume, reduces background fluorescence from the cytoplasm to the extent that single-molecule detection sensitivity becomes within reach. NSOM allows detection of individual fluorescent proteins as part of multimolecular complexes on the surface of fixed cells, and similar results should be achievable under physiological conditions in the near future.

Early detection of cytotoxic events between hepatic natural killer cells and colon carcinoma cells as probed with the atomic force microscope

The atomic force microscope (AFM) is a powerful tool to investigate surface and submembranous structures of living cells under physiological conditions at high resolution. These properties enabled us to study the interaction between live hepatic natural killer (NK) cells, also called pit cells, and colon carcinoma cells in vitro by AFM. In addition, the staining for filamentous actin and DNA was performed and served as a reference, because actin and nuclear observations at the light microscopic level during the cytotoxic interaction between these two cell types have been presented earlier. In this study, we collected evidence that conjugation of hepatic NK cells with CC531s colon carcinoma cells results in a decreased binding of CC531s cells to the substratum as probed with the AFM in contact mode as early as 10 min after cell contact (n = 11). To avoid the lateral forces and smearing artefacts of contact mode AFM, non-contact imaging was performed on hepatic NK/CC531s cell conjugates, resulting in identical observations (n = 3). In contrast, the first cytotoxic signs, as determined with the nuclear staining dye Hoechst 33342, could be observed 3 h after the start of the co-culture. This study illustrates that the AFM can be used to probe early cytotoxic effects of effector to target cell contact in nearby physiological conditions. Other routine cytotoxicity tests detect the first cytotoxic effects after 1.5-3 h co-incubation at the earliest.

Atomic force microscopy can be used to mechanically stimulate osteoblasts and evaluate cellular strain distributions

In this study, atomic force microscopy (AFM) was used to mechanically stimulate primary osteoblasts. In response to mechanical force applied by the AFM, the indented cell increased its intracellular calcium concentration. The material properties of the cell could be estimated and the membrane strains calculated. We proceeded to validate this technique experimentally and a 20% error was found between the predicted and the measured diameter of indentation. We also determined the strain distributions within the cell that result from AFM indentation using a simple finite element model. This enabled us to formulate hypotheses as to the mechanism through which cells may sense the applied mechanical strains. Finally, we report the effect of the Poisson ratio and the cell thickness on the strain distributions. Varying the Poisson ratio did not change the order of magnitude of the strains; whereas the cellular thickness dramatically changed the order of magnitude of the cellular strains. We conclude that AFM can be used for controlled mechanical stimulation of osteoblasts and that cellular strain distributions can be computed with a good accuracy when the cell is indented in its highest part.

Vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide receptor subtypes in mouse calvarial osteoblasts: presence of VIP-2 receptors and differentiation-induced expression of VIP-1 receptors

Three distinct complementary DNAs for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP) receptors have been cloned and designated VIP-1 receptor (VIP-1R), VIP-2 receptor (VIP-2R), and PACAP receptor (PACAP-R). In the present study, we have characterized the binding sites on primary mouse calvarial osteoblasts for VIP and related peptides. By analyzing the cAMP response, the rank order of response observed was PACAP 38 > PACAP 27 > helodermin > VIP > helospectin > glucagon > PHI >>> secretin. The VIP-2R/PACAP-R antagonist, PACAP 6-38, inhibited both VIP- and PACAP-stimulated cAMP formation. Binding studies using an atomic force microscopy (AFM) technique showed high affinity binding for VIP and PACAP 38, but not for secretin. Radioligand binding studies using (125)I-VIP and (125)I-PACAP 38 demonstrated a more specific and higher affinity binding for PACAP 38 than for VIP. Secretin failed to inhibit both (125)I-VIP and (125)I-PACAP 38 binding. RT-PCR demonstrated that undifferentiated mouse calvarial osteoblasts express messenger RNA for VIP-2R, but not for VIP-1R or PACAP-R. When the osteoblasts were cultured for 20 days to induce bone noduli formation, VIP-1R, in addition to VIP-2R, were expressed when the nodules started to mineralize at 12 days. Taken together, these data demonstrate that mouse calvarial osteoblasts express functional VIP-2R with higher affinity binding for PACAP than for VIP and that the VIP-1R expression is induced during osteoblastic differentiation.

Vasoactive intestinal peptide regulates osteoclast activity via specific binding sites on both osteoclasts and osteoblasts

Clinical and experimental observations, together with immunohistochemical findings, suggest that neuro-osteogenic interactions may occur in the skeleton. In this study, we have examined the effect of vasoactive intestinal peptide (VIP), one of the neuropeptides present in bone, on the activity of the bone-resorbing osteoclast. Effects on bone resorption were assessed by counting the number of pits formed by rat osteoclasts incubated on devitalized slices of bovine cortical bone. Under conditions with an initially sparse density of stromal cells/osteoblasts, VIP caused a rapid cytoplasmic contraction and decreased motility of osteoclasts. This was coupled with a decrease in the number of resorption lacunae and a decrease in the total area resorbed by the osteoclasts in 48-h cultures. Time-course experiments revealed that the inhibitory effects on contraction and motility were transient and that the cells gradually regained their activity, such that, when culture time was prolonged to 120 h, a stimulatory effect by VIP on bone resorption was observed. When osteoclasts were incubated on bone slices, in the presence of an initially large number of stromal cells/osteoblasts, VIP treatment increased the number of resorption pits and total bone area resorbed in 48-h cultures. Using atomic force microscopy, we provide direct evidence that both osteoclasts and stromal cells/osteoblasts bind VIP. Also, VIP was shown to cause a rapid rise of intracellular calcium in osteoclasts and in a proportion (20%) of stromal cells/osteoblasts. Taken together, these data suggest that differentiated osteoclasts are equipped with receptors for VIP that are linked to a transient inhibition of osteoclast activity and, in addition, that stromal cells/osteoblasts have VIP receptors coupled to a delayed stimulation of osteoclastic resorption.