Dynamic assembly of surface structures in living cells

Potential biomedical applications of the scanned nanopipette

One grand challenge in current biology is to understand how individual cellular molecules interact together to form a functioning living cell. This requires new methods to image a live cell on the nanoscale. The scanned nanopipette can be used to obtain high resolution noncontact images of the surface of live cells under physiological conditions and has been used to develop a family of related methods that allow mapping of cell function on the nanoscale, and hence allow the relationship between cell structure and function to be probed. This is a powerful method to bridge the current gap between high resolution structures of individual molecular complexes and low resolution imaging of live cell structure and function.

Flexibility of single microvilli on live neutrophils and lymphocytes

We measured the flexural stiffness of single microvilli on live human neutrophils and lymphocytes using 40-nm fluorescent beads. The beads were bound to the tips of the microvilli by anti-L-selectin antibodies. Digital bead images were acquired with an exposure time of 3 s at high magnification. Using a Gaussian point spread function, we obtained an analytical expression that relates the image profile to the flexural stiffness. We found that the flexural stiffnesses were 7 and 4 pN/microm for single microvilli on human neutrophils and lymphocytes, respectively. We also verified with live cells that 75% of neutrophil L-selectin and 72% of lymphocyte L-selectin were on the microvillus tips. Our results indicate that the leukocyte microvilli in contact with the endothelium or other surfaces will bend easily under physiological shear stresses.

Nanopipette delivery of individual molecules to cellular compartments for single-molecule fluorescence tracking

We have developed a new method, using a nanopipette, for controlled voltage-driven delivery of individual fluorescently labeled probe molecules to the plasma membrane which we used for single-molecule fluorescence tracking (SMT). The advantages of the method are 1), application of the probe to predefined regions on the membrane; 2), release of only one or a few molecules onto the cell surface; 3), when combined with total internal reflection fluorescence microscopy, very low background due to unbound molecules; and 4), the ability to first optimize the experiment and then repeat it on the same cell. We validated the method by performing an SMT study of the diffusion of individual membrane glycoproteins labeled with Atto 647-wheat germ agglutin in different surface domains of boar spermatozoa. We found little deviation from Brownian diffusion with a mean diffusion coefficient of 0.79 +/- 0.04 microm(2)/s in the acrosomal region and 0.10 +/- 0.02 microm(2)/s in the postacrosomal region; this difference probably reflects different membrane structures. We also showed that we can analyze diffusional properties of different subregions of the cell membrane and probe for the presence of diffusion barriers. It should be straightforward to extend this new method to other probes and cells, and it can be used as a new tool to investigate the cell membrane.

EPI64 regulates microvillar subdomains and structure

EPI64 is a TBC domain–containing protein that binds the PDZ domains of EBP50, which binds ezrin, a major actin-binding protein of microvilli. High-resolution light microscopy revealed that ezrin and EBP50 localize exclusively to the membrane-surrounded region of microvilli, whereas EPI64 localizes to variable regions in the structures. Overexpressing EPI64 results in its and EBP50's relocalization to the base of microvilli, including to the actin rootlet devoid of ezrin or plasma membrane. Uncoupling EPI64's binding to EBP50, expression of any construct mislocalizing its TBC domain, or knock down of EBP50 results in loss of microvilli. The TBC domain of EPI64 binds directly to Arf6-GTP. Overexpressing the TBC domain increases Arf6-GTP levels, and expressing dominant-active Arf6 results in microvillar loss. These data reveal that microvilli have distinct cytoskeletal subdomains and that EPI64 regulates microvillar structure.

Oocyte CD9 is enriched on the microvillar membrane and required for normal microvillar shape and distribution

Microvilli are found on the surface of many cell types, including the mammalian oocyte, where they are thought to act in initial contact of sperm and oocyte plasma membranes. CD9 is currently the only oocyte protein known to be required for sperm-oocyte fusion. We found CD9 is localized to the oocyte microvillar membrane using transmission electron microscopy (TEM). Scanning electron microscopy (SEM) showed that CD9 null oocytes, which are unable to fuse with sperm, have an altered length, thickness and density of their microvilli. One aspect of this change in morphology was quantified using TEM by measuring the radius of curvature at the microvillar tips. A small radius of curvature is thought to promote fusibility and the radius of curvature of microvillar tips on CD9 wild-type oocytes was found to be half that of the CD9 null oocytes. We found that oocyte CD9 co-immunoprecipitates with two Ig superfamily cis partners, EWI-2 and EWI-F, which could have a role in linking CD9 to the oocyte microvillar actin core. We also examined latrunculin B-treated oocytes, which are known to have reduced fusion ability, and found altered microvillar morphology by SEM and TEM. Our data suggest that microvilli may participate in sperm-oocyte fusion. Microvilli could act as a platform to concentrate adhesion/fusion proteins and/or provide a membrane protrusion with a low radius of curvature. They may also have a dynamic interaction with the sperm that serves to capture the sperm cell and bring it into close contact with the oocyte plasma membrane.

Dynamics and morphology of microvilli driven by actin polymerization

Many different cell types have dynamic protrusions, called microvilli, on their surface. We model these structures as arising from the balance between the force of actin polymerization and the restoring force of the membrane. From this simple model we calculate the distribution function of microvilli heights for several cells. We further describe the phase diagram and the resulting morphology of the microvilli aggregates on the cell surface.

The scanned nanopipette: a new tool for high resolution bioimaging and controlled deposition of biomolecules

The boundary between the physical and biological sciences has been eroded in recent years with new physical methods applied to biology and biological molecules being used for new physical purposes. We have pioneered the application of a form of scanning probe microscopy based on a scanned nanopipette, originally developed by Hansma and co-workers, for reliable non-contact imaging over the surface of a live cell. We have found that the nanopipette can also be used for controlled local voltage-driven application of reagents or biomolecules and this can be used for controlled deposition and the local delivery of probes for mapping of specific species. In this article we review this progress, focussing on the physical principles and new phenomena that we have observed, and then outline the future applications that are now possible.

Actin microridges characterized by laser scanning confocal and atomic force microscopy

We have characterized the cell surface of zebrafish stratified epithelium using a combined approach of light and atomic force microscopy under conditions which simulate wound healing. Microridges rise on average 100 nm above the surface of living epithelial cells, which correlate to bundles of cytochalasin B-insensitive actin filaments. Time-lapse microscopy revealed the bundles to form a highly dynamic network on the cell surface, in which bundles and junctions were severed and annealed on a time scale of minutes. Atomic force microscopy topographs further indicated that actin bundle junctions identified were of two types: overlaps and integrated end to side T- and Y-junctions. The surface bundle network is found only on the topmost cell layer of the explant, and never on individual locomoting cells. Possible functions of these actin bundles include cell compartmentalization of the cell surface, resistance to mechanical stress, and F-actin storage.

Actin Dynamics: Growth from Dendritic Branches

The dendritic nucleation model was devised to explain the cycle of actin dynamics resulting in actin filament network assembly and disassembly in two contexts--at the leading edge of motile cells and in the actin comet tails of intracellular pathogenic bacteria and viruses. Due to the detailed nature of its biochemical predictions, the model has provided an excellent focus for subsequent experimentation. This review summarizes recent work on actin dynamics in the context of the dendritic nucleation model. One outcome of this research is the possibility that additional proteins, as well as the six proteins included in the original model, might increase the efficiency of dendritic nucleation or modify the resulting actin network. In addition, actin dynamics at the leading edge might be influenced by a second actin filament network, independent of dendritic nucleation.

What is in a filopodium? Starfish versus hedgehogs

Many cell types can generate thin actin-based protrusive structures, which are often classified under the general term of 'filopodia'. However, a range of filopodia-like structures exists that differ both morphologically and functionally. In this brief review, we discuss the different types of filopodial structures, together with the actin-binding proteins and signalling pathways involved in their formation. Specifically, we highlight the differences between the filopodial extensions induced by the Rho GTPases Cdc42 and Rif.

Myosin-XVa is required for tip localization of whirlin and differential elongation of hair-cell stereocilia

Stereocilia are microvilli-derived mechanosensory organelles that are arranged in rows of graded heights on the apical surface of inner-ear hair cells. The 'staircase'-like architecture of stereocilia bundles is necessary to detect sound and head movement, and is achieved through differential elongation of the actin core of each stereocilium to a predetermined length. Abnormally short stereocilia bundles that have a diminished staircase are characteristic of the shaker 2 (Myo15a(sh2)) and whirler (Whrn(wi)) strains of deaf mice. We show that myosin-XVa is a motor protein that, in vivo, interacts with the third PDZ domain of whirlin through its carboxy-terminal PDZ-ligand. Myosin-XVa then delivers whirlin to the tips of stereocilia. Moreover, if green fluorescent protein (GFP)-Myo15a is transfected into hair cells of Myo15a(sh2) mice, the wild-type pattern of hair bundles is restored by recruitment of endogenous whirlin to the tips of stereocilia. The interaction of myosin-XVa and whirlin is therefore a key event in hair-bundle morphogenesis.

Rac1 mediates collapse of microvilli on chemokine-activated T lymphocytes

Lymphocytes circulate in the blood and upon chemokine activation rapidly bind, where needed, to microvasculature to mediate immune surveillance. Resorption of microvilli is an early morphological alteration induced by chemokines that facilitates lymphocyte emigration. However, the antecedent molecular mechanisms remain largely undefined. We demonstrate that Rac1 plays a fundamental role in chemokine-induced microvillar breakdown in human T lymphocytes. The supporting evidence includes: first, chemokine induces Rac1 activation within 5 s via a signaling pathway that involves Galphai. Second, constitutively active Rac1 mediates microvilli disintegration. Third, blocking Rac1 function by cell permeant C-terminal "Trojan" peptides corresponding to Rac1 (but not Rac2, Rho, or Cdc42) blocks microvillar loss induced by the chemokine stromal cell-derived factor 1alpha (SDF-1alpha). Furthermore, we demonstrate that the molecular mechanism of Rac1 action involves dephosphorylation-induced inactivation of the ezrin/radixin/moesin (ERM) family of actin regulators; such inactivation is known to detach the membrane from the underlying actin cytoskeleton, thereby facilitating disassembly of actin-based peripheral processes. Specifically, ERM dephosphorylation is induced by constitutively active Rac1 and stromal cell-derived factor 1alpha-induced ERM dephosphorylation is blocked by either the dominant negative Rac1 construct or by Rac1 C-terminal peptides. Importantly, the basic residues at the C terminus of Rac1 are critical to Rac1's participation in ERM dephosphorylation and in microvillar retraction. Together, these data elucidate new roles for Rac1 in early signal transduction and cytoskeletal rearrangement of T lymphocytes responding to chemokine.

Lymphocyte microvilli are dynamic, actin-dependent structures that do not require Wiskott-Aldrich syndrome protein (WASp) for their morphology

Short microvilli cover the surfaces of circulating mammalian lymphocytes. The surfaces of monocytes and neutrophils are very different, containing ruffles as their predominant structure. In this study, we present the first quantitative characterization of lymphocyte microvilli. From analysis of scanning electron micrographs, we find that median microvillar length and surface density range from 0.3 to 0.4 μm and 2 to 4 microvilli/μm2, respectively, on lymphocytes from a variety of sources. As with similar structures from other cells, lymphocyte microvilli contain parallel bundles of actin filaments. Lymphocyte microvilli rapidly disassemble when exposed to the actin-sequestering molecule, Latrunculin A. This disassembly parallels cellular actin filament depolymerization and is complete within 2 minutes, suggesting that lymphocyte microvilli undergo continuous assembly and disassembly. In contrast to previous reports suggesting lymphocyte microvillar density to be reduced on lymphocytes from Wiskott-Aldrich syndrome (WAS) patient, we find no such deficiency in either mouse or human WAS protein (WASp)–deficient lymphocytes. These results suggest that WASp is either not involved in or is redundant in the rapid dynamics of lymphocyte microvilli.

The effect of raft lipid depletion on microvilli formation in MDCK cells, visualized by atomic force microscopy

We have investigated whether raft lipids of Madin-Darby canine kidney (MDCK) cells play any role in microvilli maintenance using a combination of atomic force microscopy (AFM) and laser scanning confocal microscopy. MDCK cells were treated to reduce the amount of sphingolipids, cholesterol, or both and subsequently imaged, in buffer solution, using AFM. It was observed that inhibition of either sphingolipid or cholesterol biosynthesis led to a reduction in the number of microvilli on the surface of MDCK cells. However, this effect was not uniform across the monolayer, with some cells resembling those in untreated controls. The subsequent extraction of cholesterol from cells grown in the presence of inhibitors led to a further reduction in microvilli on the surface of the cells and, in some cases, resulted in monolayers devoid of full length microvilli. Significantly, smaller spikes were observed on the surface of the smoother cells.

The use of scanning ion conductance microscopy to image A6 cells

BACKGROUND

Continuous high spatial resolution observations of living A6 cells would greatly aid the elucidation of the relationship between structure and function and facilitate the study of major physiological processes such as the mechanism of action of aldosterone. Unfortunately, observing the micro-structural and functional changes in the membrane of living cells is still a formidable challenge for a microscopist.

METHOD

Scanning ion conductance microscopy (SICM), which uses a glass nanopipette as a sensitive probe, has been shown to be suitable for imaging non-conducting surfaces bathed in electrolytes. A specialized version of this microscopy has been developed by our group and has been applied to image live cells at high-resolution for the first time. This method can also be used in conjunction with patch clamping to study both anatomy and function and identify ion channels in single cells.

RESULTS

This new microscopy provides high-resolution images of living renal cells which are comparable with those obtained by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Continuous 24h observations under normal physiological conditions showed how A6 kidney epithelial cells changed their height, volume, and reshaped their borders. The changes in cell area correlated with the density of microvilli on the surface. Surface microvilli density ranged from 0.5 microm(-2) for extended cells to 2.5 microm(2) for shrunk cells. Patch clamping of individual cells enabled anatomy and function to be correlated.

CONCLUSIONS

Scanning ion conductance microscopy provides unique information about living cells that helps to understand cellular function. It has the potential to become a powerful tool for research on living renal cells.