Force profiles of protein pulling with or without cytoskeletal links studied by AFM

Probing mechanical interaction of immune receptors and cytoskeleton by membrane nanotube extraction

The role of force application in immune cell recognition is now well established, the force being transmitted between the actin cytoskeleton to the anchoring ligands through receptors such as integrins. In this chain, the mechanics of the cytoskeleton to receptor link, though clearly crucial, remains poorly understood. To probe this link, we combine mechanical extraction of membrane tubes from T cells using optical tweezers, and fitting of the resulting force curves with a viscoelastic model taking into account the cell and relevant molecules. We solicit this link using four different antibodies against various membrane bound receptors: antiCD3 to target the T Cell Receptor (TCR) complex, antiCD45 for the long sugar CD45, and two clones of antiCD11 targeting open or closed conformation of LFA1 integrins. Upon disruption of the cytoskeleton, the stiffness of the link changes for two of the receptors, exposing the existence of a receptor to cytoskeleton link-namely TCR-complex and open LFA1, and does not change for the other two where a weaker link was expected. Our integrated approach allows us to probe, for the first time, the mechanics of the intracellular receptor-cytoskeleton link in immune cells.

Optical tweezers study of membrane fluidity in small cell lung cancer cells

The fluidity of the cell membrane is closely related to cancer metastasis/invasion. To test the relationship of membrane fluidity and invasiveness, we first demonstrated that transfection of small RNA miR-92b-3p can significantly increase invasiveness of the small cell lung cancer cell line SHP77. Then optical tweezers were used to measure membrane fluidity. This study employed continuous and step-like stretching methods to examine fluidity changes in SHP77 cell membranes before and after miR-92b-3p transfection. A newly developed physical model was used to derive the effective viscosity and static tension of the cell membrane from relaxation curves obtained via step-like stretching. Experiments showed that invasiveness and fluidity increased significantly after miR-92b-3p transfection. This study paved the way toward a better understanding of cancer cell invasion and membrane mechanical characteristics.

Myosin VII, USH1C, and ANKS4B or USH1G Together Form Condensed Molecular Assembly via Liquid-Liquid Phase Separation

Hair cell stereocilia tip-links function to sense mechanical forces generated by sound waves and maintain the structure of stereocilia by rooting the tail of cadherins to highly dense structures known as tip-link densities. Although the molecular components are largely known, the mechanisms underlying the tip-link density formation are unknown. Here, we show that Myosin VIIB (MYO7B), USH1C, and ANKS4B, which form a specific complex stabilizing tip-links in intestine microvilli, could form dense condensates via liquid-liquid phase separation in vitro and in cells. The MYO7A, USH1C, and USH1G complex also undergoes phase separation in cells. Formation of the MYO7A/USH1C/USH1G and MYO7B/USH1C/ANKS4B condensates requires strong and multivalent interactions between proteins in both tripartite complexes. Point mutations of MYO7A found in Usher syndrome patients weaken or even disrupt the multivalent interactions of the MYO7A/USH1C/USH1G complex and impair its phase separation. Thus, the stereocilia tip-link densities may form via phase separation of the MYO7A/USH1C/USH1G complex.

Hyaluronic acid selective anchoring to the cytoskeleton: An atomic force microscopy study

The hyaluronic acid component of the glycocalyx plays a role in cell mechanotransduction by selectively transmitting mechanical signals to the cell cytoskeleton or to the cell membrane. The aim of this study was to evaluate the mechanical link between the hyaluronic acid molecule and the cell cytoskeleton by means of atomic force microscopy single molecule force spectroscopy. Hyaluronic acid molecules on live cells were targeted with probes coated with hyaluronic acid binding protein. Two different types of events were observed when the detachment of the target molecule from the probe occurred, suggesting the presence of cytoskeleton- and membrane-anchored molecules. Membrane-anchored molecules facilitated the formation of tethers when pulled. About 15% of the tested hyaluronic acid molecules were shown to be anchored to the cytoskeleton. When multiple molecules bonded to the probe, specific detachment patterns were observed, suggesting that a cytoskeletal bond needed to be broken to improve the ability to pull tethers from the cell membrane. This likely resulted in the formation of tethering structures maintaining a cytoskeletal core similar to the ones observed for cells over-expressing HA synthases. The different observed rupture events were associated with separate mechanotransductive mechanisms in an analogous manner to that previously proposed for the endothelial glycocalyx. Single cytoskeleton anchored rupture events represent HA molecules linked to the cytoskeleton and therefore transmitting mechanical stimuli into the inner cell compartments. Single membrane tethers would conversely represent the glycocalyx molecules connected to areas of the membrane where an abundance of signalling molecules reside.

Atomic force microscope as a nano- and micrometer scale biological manipulator: A short review

The amazing capacity of atomic force microscope to let us touch the molecular and cellular level samples with a sharp probe stimulated its application to bio-medical field among others. In addition to topographical imaging of the sample surface, a direct mechanical manipulation has attracted innovative minds to develop new methodologies aiming at direct handling of proteins, DNA/RNA, and cells. Measurement of their mechanical properties brought about a vivid picture of their physical nature. Direct handling of individual molecules and cells prompted development of nano-medical applications. This short review summarized recent application of AFM for measurement of mechanical properties of biological samples and attempts to perform direct manipulations of nano-medicine.

The effect of growth temperature on the nanoscale biochemical surface properties of Yersinia pestis

Yersinia pestis, the causative agent of plague, has been responsible for several recurrent, lethal pandemics in history. Currently, it is an important pathogen to study owing to its virulence, adaptation to different environments during transmission, and potential use in bioterrorism. Here, we report on the changes to Y. pestis surfaces in different external microenvironments, specifically culture temperatures (6, 25, and 37 °C). Using nanoscale imaging coupled with functional mapping, we illustrate that changes in the surfaces of the bacterium from a morphological and biochemical standpoint can be analyzed simultaneously using atomic force microscopy. The results from functional mapping, obtained at a single cell level, show that the density of lipopolysaccharide (measured via terminal N-acetylglucosamine) on Y. pestis grown at 37 °C is only slightly higher than cells grown at 25 °C, but nearly three times higher than cells maintained at 6 °C for an extended period of time, thereby demonstrating that adaptations to different environments can be effectively captured using this technique. This nanoscale evaluation provides a new microscopic approach to study nanoscale properties of bacterial pathogens and investigate adaptations to different external environments.

Evaluation of whole cell fixation methods for the analysis of nanoscale surface features of Yersinia pestis KIM

Manipulation of viable Yersinia pestis (etiologic agent of plague) in the laboratory usually necessitates elevated biosafety and biocontainment procedures, even with avirulent or vaccine strains. To facilitate downstream biochemical or physical analyses in a Biosafety Level 1 laboratory environment, effective inactivation without affecting its intrinsic properties is critical. Here, we report on the morphological and biochemical changes to Y. pestis surfaces following four different fixation methods that render the cells nonviable. The results, obtained at the single cell level, demonstrate that methanol inactivation is best able to preserve bacterial morphology and bioactivity, enabling subsequent analysis. This nanoscale evaluation of the effects of inactivation on cell morphology and surface bioactivity may provide a crucial preparatory approach to study virulent pathogens in the lab setting using high-resolution microscopic techniques such as atomic force microscopy.

Nanoscale Surface Characterization of Human Erythrocytes by Atomic Force Microscopy: A Critical Review

Erythrocytes (red blood cells, RBCs), the most common type of blood cells in humans are well known for their ability in transporting oxygen to the whole body through hemoglobin. Alterations in their membrane skeletal proteins modify shape and mechanical properties resulting in several diseases. Atomic force microscopy (AFM), a new emerging technique allows non-invasive imaging of cell, its membrane and characterization of surface roughness at micrometer/nanometer resolution with minimal sample preparation. AFM imaging provides direct measurement of single cell morphology, its alteration and quantitative data on surface properties. Hence, AFM studies of human RBCs have picked up pace in the last decade. The aim of this paper is to review the various applications of AFM for characterization of human RBCs topology. AFM has been used for studying surface characteristics like nanostructure of membranes, cytoskeleton, microstructure, fluidity, vascular endothelium, etc., of human RBCs. Various modes of AFM imaging has been used to measure surface properties like stiffness, roughness, and elasticity. Topological alterations of erythrocytes in response to different pathological conditions have also been investigated by AFM. Thus, AFM-based studies and application of image processing techniques can effectively provide detailed insights about the morphology and membrane properties of human erythrocytes at nanoscale.

Elongated membrane tethers, individually anchored by high affinity α4β1/VCAM-1 complexes, are the quantal units of monocyte arrests

The α4β1 integrin facilitates both monocyte rolling and adhesion to the vascular endothelium and is physiologically activated by monocyte chemoattractant protein (MCP-1). The current study investigated the initial events in the adhesion of THP-1 cells to immobilized Vascular Cell Adhesion Molecule 1 (VCAM-1). Using AFM force measurements, cell adhesion was shown to be mediated by two populations of α4β1/VCAM-1 complexes. A low affinity form of α4β1 was anchored to the elastic elements of the cytoskeleton, while a higher affinity conformer was coupled to the viscous elements of the cell membrane. Within 100 ms of contact, THP-1 cells, stimulated by co-immobilized MCP-1, exhibited a tremendous increase in adhesion to VCAM-1. Enhanced cell adhesion was accompanied by a local decoupling of the cell membrane from the cytoskeleton and the formation of long membrane tethers. The tethers were individually anchored by multiple α4β1/VCAM-1 complexes that prolonged the extension of the viscous tethers. In vivo, the formation of these membrane tethers may provide the quantal structural units for the arrest of rolling monocytes within the blood vessels.

Demonstration of specific binding of heparin to Plasmodium falciparum-infected vs. non-infected red blood cells by single-molecule force spectroscopy

Glycosaminoglycans (GAGs) play an important role in the sequestration of Plasmodium falciparum-infected red blood cells (pRBCs) in the microvascular endothelium of different tissues, as well as in the formation of small clusters (rosettes) between infected and non-infected red blood cells (RBCs). Both sequestration and rosetting have been recognized as characteristic events in severe malaria. Here we have used heparin and pRBCs infected by the 3D7 strain of P. falciparum as a model to study GAG-pRBC interactions. Fluorescence microscopy and fluorescence-assisted cell sorting assays have shown that exogenously added heparin has binding specificity for pRBCs (preferentially for those infected with late forms of the parasite) vs. RBCs. Heparin-pRBC adhesion has been probed by single-molecule force spectroscopy, obtaining an average binding force ranging between 28 and 46 pN depending on the loading rate. No significant binding of heparin to non-infected RBCs has been observed in control experiments. This work represents the first approach to quantitatively evaluate GAG-pRBC molecular interactions at the individual molecule level.

Amyloid-β peptide on sialyl-Lewis(X)-selectin-mediated membrane tether mechanics at the cerebral endothelial cell surface

Increased deposition of amyloid-β peptide (Aβ) at the cerebral endothelial cell (CEC) surface has been implicated in enhancement of transmigration of monocytes across the brain blood barrier (BBB) in Alzheimer's disease (AD). In this study, quantitative immunofluorescence microscopy (QIM) and atomic force microscopy (AFM) with cantilevers biofunctionalized by sialyl-Lewis(x) (sLe(x)) were employed to investigate Aβ-altered mechanics of membrane tethers formed by bonding between sLe(x) and p-selectin at the CEC surface, the initial mechanical step governing the transmigration of monocytes. QIM results indicated the ability for Aβ to increase p-selectin expression at the cell surface and promote actin polymerization in both bEND3 cells (immortalized mouse CECs) and human primary CECs. AFM data also showed the ability for Aβ to increase cell stiffness and adhesion probability in bEND3 cells. On the contrary, Aβ lowered the overall force of membrane tether formation (Fmtf ), and produced a bimodal population of Fmtf , suggesting subcellular mechanical alterations in membrane tethering. The lower Fmtf population was similar to the results obtained from cells treated with an F-actin-disrupting drug, latrunculin A. Indeed, AFM results also showed that both Aβ and latrunculin A decreased membrane stiffness, suggesting a lower membrane-cytoskeleton adhesion, a factor resulting in lower Fmtf . In addition, these cerebral endothelial alterations induced by Aβ were abrogated by lovastatin, consistent with its anti-inflammatory effects. In sum, these results demonstrated the ability for Aβ to enhance p-selectin expression at the CEC surface and induce cytoskeleton reorganization, which in turn, resulted in changes in membrane-cytoskeleton adhesion and membrane tethering, mechanical factors important in transmigration of monocytes through the BBB.

Large protein assemblies formed by multivalent interactions between cadherin23 and harmonin suggest a stable anchorage structure at the tip link of stereocilia

Stereocilia tip links of inner ear hair cells are subjected to constant stretching during hair-bundle deflection, and accordingly are well designed to prevent from being broken by mechanical tensions. The roots of tip links, which couple tip links with the cytoskeleton, supposedly play important roles in withstanding large forces under stimulated conditions. The upper root of the tip link is mainly formed by the cytoplasmic tail of cadherin23 and its actin-anchoring protein harmonin. However, the detailed organization mode of the two proteins that gives rise to a strong upper root remains unclear. Here we show that the exon68-encoded peptide of cadherin23 can either interact with the N-terminal domain (NTD) of harmonin or form a homodimer. We demonstrate that the three harmonin binding sites of cadherin23, namely the NTD-binding motif, the exon68 peptide, and the C-terminal PDZ binding motif, do not synergize with each other in binding to harmonin, instead they facilitate formation of polymeric cadherin23/harmonin complexes. The exon68 peptide can promote the cadherin23/harmonin polymer formation via either binding to harmonin NTD or self-dimerization. We propose that the polymeric cadherin23/harmonin complex formed beneath the upper tip link membranes may serve as part of the stable rootlet structure for anchoring the tip links of stereocilia.

Direct detection of cellular adaptation to local cyclic stretching at the single cell level by atomic force microscopy

The cellular response to external mechanical forces has important effects on numerous biological phenomena. The sequences of molecular events that underlie the observed changes in cellular properties have yet to be elucidated in detail. Here we have detected the responses of a cultured cell against locally applied cyclic stretching and compressive forces, after creating an artificial focal adhesion under a glass bead attached to the cantilever of an atomic force microscope. The cell tension initially increased in response to the tensile stress and then decreased within ∼1 min as a result of viscoelastic properties of the cell. This relaxation was followed by a gradual increase in tension extending over several minutes. The slow recovery of tension ceased after several cycles of force application. This tension-recovering activity was inhibited when cells were treated with cytochalasin D, an inhibitor of actin polymerization, or with (-)-blebbistatin, an inhibitor of myosin II ATPase activity, suggesting that the activity was driven by actin-myosin interaction. To our knowledge, this is the first quantitative analysis of cellular mechanical properties during the process of adaptation to locally applied cyclic external force.

Interaction mechanism of cortisol and catecholamines with structural components of erythrocyte membranes

Nonspecific mechanisms of the stress hormones interaction with erythrocyte membranes were studied by means of atomic force microscopy, fluorescence analysis, and IR spectroscopy. It was shown that stress hormones (cortisol, adrenaline, noradrenaline) can bind to erythrocyte membranes with high affinity (K(b) approximately 10(6) M(-1)). The binding mechanism involves hydrogen bonds and hydrophobic and electrostatic interactions. Active groups of the hormones (NH(2), NHCH(3), keto, and hydroxy groups) interact simultaneously with CO and NH groups both of proteins and phospholipids. This leads to the formation of complex protein-lipid domains that distort the surface of the erythrocyte membrane. Water dipoles are displaced from the domains to adjacent regions and facilitate membrane loosening. The interaction of hormones with the membrane is accompanied by structural transitions of disorder --> order (tangle --> alpha-helix, tangle --> beta-structure) in membrane proteins and structural transitions of order --> order in phospholipids. Formation of large domains (clusters) of the lipid-protein and lipid nature leads to distortion of membranes and deteriorates their elasticity and rheological properties.

Models of dynamic extraction of lipid tethers from cell membranes

When a ligand that is bound to an integral membrane receptor is pulled, the membrane and the underlying cytoskeleton can deform before either the membrane delaminates from the cytoskeleton or the ligand detaches from the receptor. If the membrane delaminates from the cytoskeleton, it may be further extruded and form a membrane tether. We develop a phenomenological model for this process by assuming that deformations obey Hooke's law up to a critical force at which the cell membrane locally detaches from the cytoskeleton and a membrane tether forms. We compute the probability of tether formation and show that tethers can be extruded only within an intermediate range of force loading rates and pulling velocities. The mean tether length that arises at the moment of ligand detachment is computed as are the force loading rates and pulling velocities that yield the longest tethers.

Fusion of biomimetic stealth probes into lipid bilayer cores

Many biomaterials are designed to regulate the interactions between artificial and natural surfaces. However, when materials are inserted through the cell membrane itself the interface formed between the interior edge of the membrane and the material surface is not well understood and poorly controlled. Here we demonstrate that by replicating the nanometer-scale hydrophilic-hydrophobic-hydrophilic architecture of transmembrane proteins, artificial "stealth" probes spontaneously insert and anchor within the lipid bilayer core, forming a high-strength interface. These nanometer-scale hydrophobic bands are readily fabricated on metallic probes by functionalizing the exposed sidewall of an ultrathin evaporated Au metal layer rather than by lithography. Penetration and adhesion forces for butanethiol and dodecanethiol functionalized probes were directly measured using atomic force microscopy (AFM) on thick stacks of lipid bilayers to eliminate substrate effects. The penetration dynamics were starkly different for hydrophobic versus hydrophilic probes. Both 5- and 10 nm thick hydrophobically functionalized probes naturally resided within the lipid core, while hydrophilic probes remained in the aqueous region. Surprisingly, the barrier to probe penetration with short butanethiol chains (E(o,5 nm) = 21.8k(b)T, E(o,10 nm) = 15.3k(b)T) was dramatically higher than longer dodecanethiol chains (E(o,5 nm) = 14.0k(b)T, E(o,10 nm) = 10.9k(b)T), indicating that molecular mobility and orientation also play a role in addition to hydrophobicity in determining interface stability. These results highlight a new strategy for designing artificial cell interfaces that can nondestructively penetrate the lipid bilayer.

A review on: atomic force microscopy applied to nano-mechanics of the cell

Since its introduction in 1986, AFM has been applied to biological studies along with its widespread use in physics, chemistry and engineering fields. Due to its dual capabilities of imaging nano-materials with an atomic level resolution and of directly manipulating samples with high precision, AFM is now considered an indispensable instrument for nano-technological researchers especially in physically oriented fields. In biology in general, however, and in biotechnology in particular, its usefulness must be critically examined and, if necessary as it certainly is, further explored from a practical point of view. In this review, a new trend of applying AFM based technology to elucidate the mechanical basis of the cellular structure and its interaction with the extracellular matrix including cell to cell interaction is reviewed. Some of the recent studies done by using other force measuring or force exerting methods are also covered in the hope that all the nano-mechanical work on the cellular level will eventually contribute to the emergence of the mechano-chemical view of the cell in a unified manner.

Force spectroscopy of the interaction between mycobacterial adhesins and heparan sulphate proteoglycan receptors

Understanding the molecular interactions between bacterial adhesion proteins (adhesins) and their receptors is essential for elucidating the molecular mechanisms of bacterial pathogenesis. Here, atomic force microscopy (AFM) is used to explore the specific interactions between the heparin-binding hemagglutinin (HBHA) from Mycobacterium tuberculosis, and heparan sulphate proteoglycan (HSPG) receptors on live A549 pneumocytes. First, we show that the specific binding forces between single HBHA-HSPG pairs, 57+/-16 pN, are similar to the forces measured earlier between HBHA and heparin molecules. Second, we mapped the distribution of single HSPG receptors on the surface of A549 cells, revealing that the proteins are widely and homogeneously exposed. Third, we observed force curves with constant force plateaus at large pulling velocities, reflecting the extraction of membrane tethers or nanotubes. These single-molecule measurements provide new avenues in pathogenesis research, particularly for elucidating the molecular basis of pathogen-host interactions.

Characterization of the wheat germ agglutinin binding to self-assembled monolayers of neoglycoconjugates by AFM and SPR

Carbohydrate-protein interactions govern many crucial life processes involved in cell recognition events, but are often difficult to study because the interactions are weak, and multivalent exposure appears to be crucial for their biological function. We have used self-assembled monolayers (SAMs) of neoglycoconjugates as a model system to probe the specific interactions between the lectin wheat germ agglutinin (WGA) and monosaccharides by surface plasmon resonance (SPR) and atomic force microscopy (AFM) force measurements. SAMs presenting N-acetyl-D-glucosamine (GlcNAc) as a neoglycoconjugate were produced on gold surfaces, where the SAM formation was monitored using a quartz crystal microbalance (QCM) and shown to be a very rapid process. In the AFM force measurements WGA was covalently coupled to flexible polyethylene glycol (PEG) molecules at a probe surface using amine coupling. GlcNAc-specific binding events were detected with a WGA-modified probe on the GlcNAc-neoglycoconjugate SAM at bond rupture forces of 47 +/- 15 pN. Additionally, less frequent GlcNAc-specific unbinding events were detected at higher forces (120 +/- 20 pN) which are believed to originate from simultaneous detachment of multiple binding sites from the SAM surface. SPR measurements confirmed that WGA has higher affinity toward the immobilized GlcNAc-SAM than toward the soluble free monosaccharide. The binding constants obtained for soluble chitinoligosaccharides suggested up to three subsites within one carbohydrate-binding site of the WGA molecule and also provided further evidence of the multivalent binding character of the WGA dimer.

TPA primes alpha2beta1 integrins for cell adhesion

Integrin avidity is regulated by changes in the conformation of the heterodimer and cluster formation. We measured cell adhesion by integrin alpha2beta1 (CHO-alpha2) to collagen at short contact times (0.5-60s) by single cell force spectroscopy (SCFS). The adhesion increased rapidly with contact time and was further strengthened by the addition of 12-O-tetradecanoylphorbol-13-acetate (TPA), a protein kinase C (PKC) and integrin activator. TPA also improved the strength of adhesive units. Furthermore, changes in membrane nanotube properties indicated better coupling of integrins to the cell cytoskeleton. We conclude that in addition to increasing integrin avidity TPA strengthens integrin-cytoskeletal linkage.

Nanobiomechanics of proteins and biomembrane

A review of the work done in the Laboratory of Biodynamics of Tokyo Institute of Technology in the last decade has been summarized in this article in relation to the results reported from other laboratories. The emphasis here is the application of nanomechanics based on the force mode of atomic force microscopy (AFM) to proteins and protein-based biological structures. Globular proteins were stretched in various ways to detect the localized rigidity inside of the molecule. When studied by this method, bovine carbonic anhydrase II (BCA II), calmodulin and OspA protein all showed the presence of localized rigid structures inside the molecules. Protein compression experiments were done on BCA II to obtain an estimate of the Young modulus and its change in the process of denaturation. Then, the AFM probe method was turned on to cell membranes and cytoplasmic components. Force curves accompanying the extraction process of membrane proteins from intact cells were analysed in relation to their interaction with the cytoskeletal components. By pushing the AFM probe further into the cytoplasm, mRNAs were recovered from a live cell with minimal damage, and multiplied using PCR technology for their identification. Altogether, the work introduced here forms the basis of nanomechanics of protein and protein-based biostructures and application of the nanomechanical technology to cell biology.

The viscoelasticity of membrane tethers and its importance for cell adhesion

Cell adhesion mechanically couples cells to surfaces. The durability of individual bonds between the adhesive receptors and their ligands in the presence of forces determines the cellular adhesion strength. For adhesive receptors such as integrins, it is a common paradigm that the cell regulates its adhesion strength by altering the affinity state of the receptors. However, the probability distribution of rupture forces is dependent not only on the affinity of individual receptor-ligand bonds but also on the mechanical compliance of the cellular anchorage of the receptor. Hence, by altering the anchorage, the cell can regulate its adhesion strength without changing the affinity of the receptor. Here, we analyze the anchorage of the integrin VLA-4 with its ligand VCAM-1. For this purpose, we develop a model based on the Kelvin body, which allows one to quantify the mechanical properties of the adhesive receptor's anchorage using atomic force microscopy on living cells. As we demonstrate, the measured force curves give valuable insight into the mechanics of the cellular anchorage of the receptor, which is described by the tether stiffness, the membrane rigidity, and the membrane viscosity. The measurements relate to a tether stiffness of k(t) = 1.6 microN/m, an initial membrane rigidity of k(i) = 260 microN/m, and a viscosity of mu = 5.9 microN x s/m. Integrins exist in different activation states. When activating the integrin with Mg(2+), we observe altered viscoelastic parameters of k(t) = 0.9 microN/m, k(i) = 190 microN/m, and mu = 6.0 microN x s/m. Based on our model, we postulate that anchorage-related effects are common regulating mechanisms for cellular adhesion beyond affinity regulation.

mRNA detection of individual cells with the single cell nanoprobe method compared with in situ hybridization

BACKGROUND

The localization of specific mRNA generates cell polarity by controlling the translation sites of specific proteins. Although most of these events depend on differences in gene expression, no method is available to examine time dependent gene expression of individual living cells. In situ hybridization (ISH) is a powerful and useful method for detecting the localization of mRNAs, but it does not allow a time dependent analysis of mRNA expression in single living cells because the cells have to be fixed for mRNA detection. To overcome these issues, the extraction of biomolecules such as mRNAs, proteins, and lipids from living cells should be performed without severe damage to the cells. In previous studies, we have reported a single cell nanoprobe (SCN) method to examine gene expression of individual living cells using atomic force microscopy (AFM) without killing the cells.

RESULTS

In order to evaluate the SCN method, we compared the SCN method with in situ hybridization (ISH). First, we examined spatial beta-actin mRNA expression in single living cells with the SCN method, and then the same cells were subjected to ISH for beta-actin mRNA. In the SCN method, quantity of beta-actin mRNA were analysed by quantitative PCR, and in ISH we used intensity of ISH as a parameter of concentration of beta-actin mRNA. We showed that intensity of ISH is higher; quantity of beta-actin mRNA detected by the SCN method increased more.

CONCLUSION

In this study, we compare the SCN method with the ISH. We examined beta-actin mRNA expression in single cells using both methods. We picked up beta-actin mRNA from several loci of a single living cell using an AFM nanoprobe, and identical cells were subjected to ISH. The results showed a good correlation between the SCN method and ISH. The SCN method is suitable and reliable to examine mRNAs at medium or higher expression level.

Exploring transferrin-receptor interactions at the single-molecule level

Interaction between the iron transporter protein transferrin (Tf) and its receptor at the cell surface is fundamental for most living organisms. Tf receptor (TfR) binds iron-loaded Tf (holo-Tf) and transports it to endosomes, where acidic pH favors iron release. Iron-free Tf (apo-Tf) is then brought back to the cell surface and dissociates from TfR. Here we investigated the Tf-TfR interaction at the single-molecule level under different conditions encountered during the Tf cycle. An atomic force microscope tip functionalized with holo-Tf or apo-Tf was used to probe TfR. We tested both purified TfR anchored to a mica substrate and in situ TfR at the surface of living cells. Dynamic force measurements showed similar results for TfR on mica or at the cell surface but revealed striking differences between holo-Tf-TfR and apo-Tf-TfR interactions. First, the forces necessary to unbind holo-Tf and TfR are always stronger compared to the apo-Tf-TfR interaction. Second, dissociation of holo-Tf-TfR complex involves overcoming two energy barriers, whereas the apo-Tf-TfR unbinding pathway comprises only one energy barrier. These results agree with a model that proposes differences in the contact points between holo-Tf-TfR and apo-Tf-TfR interactions.