Optical trapping in animal and fungal cells using a tunable, near-infrared titanium-sapphire laser

Laser Scissors and Tweezers to Study Chromosomes: A Review

Starting in 1969 laser scissors have been used to study and manipulate chromosomes in mitotic animal cells. Key studies demonstrated that using the “hot spot” in the center of a focused Gaussian laser beam it was possible to delete the ribosomal genes (secondary constriction), and this deficiency was maintained in clonal daughter cells. It wasn’t until 2020 that it was demonstrated that cells with focal-point damaged chromosomes could replicate due to the cell’s DNA damage repair molecular machinery. A series of studies leading up to this conclusion involved using cells expressing different GFP DNA damage recognition and repair molecules. With the advent of optical tweezers in 1987, laser tweezers have been used to study the behavior and forces on chromosomes in mitotic and meiotic cells. The combination of laser scissors and tweezers were employed since 1991 to study various aspects of chromosome behavior during cell division. These studies involved holding chromosomes in an optical while gradually reducing the laser power until the chromosome recovered their movement toward the cell pole. It was determined in collaborative studies with Prof. Arthur Forer from York University, Toronto, Canada, cells from diverse group vertebrate and invertebrates, that forces necessary to move chromosomes to cell poles during cell division were between 2 and 17pN, orders of magnitude below the 700 pN generally found in the literature.

ATP prevents Woronin bodies from sealing septal pores in unwounded cells of the fungus Zymoseptoria tritici

Septa of filamentous ascomycetes are perforated by septal pores that allow communication between individual hyphal compartments. Upon injury, septal pores are plugged rapidly by Woronin bodies (WBs), thereby preventing extensive cytoplasmic bleeding. The mechanism by which WBs translocate into the pore is not known, but it has been suggested that wound‐induced cytoplasmic bleeding “flushes” WBs into the septal opening. Alternatively, contraction of septum‐associated tethering proteins may pull WBs into the septal pore. Here, we investigate WB dynamics in the wheat pathogen Zymoseptoria tritici. Ultrastructural studies showed that 3.4 ± 0.2 WBs reside on each side of a septum and that single WBs of 128.5 ± 3.6 nm in diameter seal the septal pore (41 ± 1.5 nm). Live cell imaging of green fluorescent ZtHex1, a major protein in WBs, and the integral plasma membrane protein ZtSso1 confirms WB translocation into the septal pore. This was associated with the occasional formation of a plasma membrane “balloon,” extruding into the dead cell, suggesting that the plasma membrane rapidly seals the wounded septal pore wound. Minor amounts of fluorescent ZtHex1‐enhanced green fluorescent protein (eGFP) appeared associated with the “ballooning” plasma membrane, indicating that cytoplasmic ZtHex1‐eGFP is recruited to the extending plasma membrane. Surprisingly, in ~15% of all cases, WBs moved from the ruptured cell into the septal pore. This translocation against the cytoplasmic flow suggests that an active mechanism drives WB plugging. Indeed, treatment of unwounded and intact cells with the respiration inhibitor carbonyl cyanide m‐chlorophenyl hydrazone induced WB translocation into the pores. Moreover, carbonyl cyanide m‐chlorophenyl hydrazone treatment recruited cytoplasmic ZtHex1‐eGFP to the lateral plasma membrane of the cells. Thus, keeping the WBs out of the septal pores, in Z. tritici, is an ATP‐dependent process.

Cell Biology of Hyphal Growth

Filamentous fungi are a large and ancient clade of microorganisms that occupy a broad range of ecological niches. The success of filamentous fungi is largely due to their elongate hypha, a chain of cells, separated from each other by septa. Hyphae grow by polarized exocytosis at the apex, which allows the fungus to overcome long distances and invade many substrates, including soils and host tissues. Hyphal tip growth is initiated by establishment of a growth site and the subsequent maintenance of the growth axis, with transport of growth supplies, including membranes and proteins, delivered by motors along the cytoskeleton to the hyphal apex. Among the enzymes delivered are cell wall synthases that are exocytosed for local synthesis of the extracellular cell wall. Exocytosis is opposed by endocytic uptake of soluble and membrane-bound material into the cell. The first intracellular compartment in the endocytic pathway is the early endosomes, which emerge to perform essential additional functions as spatial organizers of the hyphal cell. Individual compartments within septated hyphae can communicate with each other via septal pores, which allow passage of cytoplasm or organelles to help differentiation within the mycelium. This article introduces the reader to more detailed aspects of hyphal growth in fungi.

Electrical tweezer for highly parallelized electrorotation measurements over a wide frequency bandwidth

Electrorotation (ROT) is a powerful tool for characterizing the dielectric properties of cells and bioparticles. However, its application has been somewhat limited by the need to mitigate disruptions to particle rotation by translation under positive DEP and by frictional interactions with the substrate. While these disruptions may be overcome by implementing particle positioning schemes or field cages, these methods restrict the frequency bandwidth to the negative DEP range and permit only single particle measurements within a limited spatial extent of the device geometry away from field nonuniformities. Herein, we present an electrical tweezer methodology based on a sequence of electrical signals, composed of negative DEP using 180-degree phase-shifted fields for trapping and levitation of the particles, followed by 90-degree phase-shifted fields over a wide frequency bandwidth for highly parallelized electrorotation measurements. Through field simulations of the rotating electrical field under this wave-sequence, we illustrate the enhanced spatial extent for electrorotation measurements, with no limitations to frequency bandwidth. We apply this methodology to characterize subtle modifications in morphology and electrophysiology of Cryptosporidium parvum with varying degrees of heat treatment, in terms of shifts in the electrorotation spectra over the 0.05-40 MHz region. Given the single particle sensitivity and the ability for highly parallelized electrorotation measurements, we envision its application toward characterizing heterogeneous subpopulations of microbial and stem cells.

A large nonconserved region of the tethering protein Leashin is involved in regulating the position, movement, and function of Woronin bodies in Aspergillus oryzae

The Woronin body is a Pezizomycotina-specific organelle that is typically tethered to the septum, but upon hyphal wounding, it plugs the septal pore to prevent excessive cytoplasmic loss. Leashin (LAH) is a large Woronin body tethering protein that contains highly conserved N- and C-terminal regions and a long (∼2,500-amino-acid) nonconserved middle region. As the involvement of the nonconserved region in Woronin body function has not been investigated, here, we functionally characterized individual regions of the LAH protein of Aspergillus oryzae (AoLAH). In an Aolah disruptant, no Woronin bodies were tethered to the septum, and hyphae had a reduced ability to prevent excessive cytoplasmic loss upon hyphal wounding. Localization analysis revealed that the N-terminal region of AoLAH associated with Woronin bodies dependently on AoWSC, which is homologous to Neurospora crassa WSC (Woronin body sorting complex), and that the C-terminal region was localized to the septum. Elastic movement of Woronin bodies was observed when visualized with an AoLAH N-terminal-region-enhanced green fluorescent protein (EGFP) fusion protein. An N- and C-terminal fusion construct lacking the nonconserved middle region of AoLAH was sufficient for the tethering of Woronin bodies to the septum. However, Woronin bodies were located closer to the septum and exhibited impaired elastic movement. Moreover, expression of middle-region-deleted AoLAH in the Aolah disruptant did not restore the ability to prevent excessive cytoplasmic loss. These findings indicate that the nonconserved middle region of AoLAH has functional importance for regulating the position, movement, and function of Woronin bodies.

An electrorotation technique for measuring the dielectric properties of cells with simultaneous use of negative quadrupolar dielectrophoresis and electrorotation

This paper presents an effective electrorotation technique for measuring the dielectric properties of cells using a superposed electrical signal, which can simultaneously generate negative quadrupolar dielectrophoretic (nQDEP) force and electrorotational (ROT) torque. The proposed technique involves a three-dimensional (3D) octode, which includes four electrodes arranged in a crisscross pattern on the top and bottom of a microchannel, respectively. A single cell was trapped in the center of the 3D octode by the nQDEP force and simultaneously rotated by the ROT torque. Using the proposed electrorotation technique, ROT spectra of human leukocyte subpopulations (T and B lymphocytes, granulocytes, and monocytes) and metastatic human breast (SkBr3) and lung (A549) cancer cell lines were accurately measured without any disturbance. Torque on the cells generated by the ROT signal was analyzed theoretically based on the single-shell dielectric model for the cells. Furthermore, the dielectric properties of the cells, such as area-specific membrane capacitance and cytoplasm conductivity, were extracted using the measured ROT spectra and the analyzed torque.

Involvement of the anucleate primary sterigmata protein FgApsB in vegetative differentiation, asexual development, nuclear migration, and virulence in Fusarium graminearum

The protein ApsB has been shown to play critical roles in the migration and positioning of nuclei and in the development of conidiophores in Aspergillus nidulans. The functions of ApsB in Fusarium graminearum, a causal agent of Fusarium head blight in China, are largely unknown. In this study, we used the blastp program at the Broad Institute to identify FgApsB, an F. graminearum homolog of A. nidulansApsB. The functions of FgApsB were evaluated by constructing a deletion mutant of FgApsB, designated ΔFgApsB-28. Conidiation and mycelial growth rate are reduced in ΔFgApsB-28. The hyphae of ΔFgApsB-28 are thinner than those of the wild type and have a different branching angle. ΔFgApsB-28 exhibited reduced aerial hyphae formation, but increased production of rubrofusarin. Whereas nuclei are evenly distributed in germ tubes and hyphae of the wild type, they are clustered and irregularly distributed in ΔFgApsB-28. The mutant exhibited increased resistance to cell wall-damaging agents, but reduced virulence on flowering wheat heads, which is consistent with its reduced production of the toxin deoxynivalenol. All of the defects in ΔFgApsB-28 were restored by genetic complementation with the parental FgApsB gene. Taken together, the results indicate that FgApsB is important for vegetative differentiation, asexual development, nuclear migration, and virulence in F. graminearum.

Multiple modes for gatekeeping at fungal cell-to-cell channels

Cell-to-cell channels appear to be indispensable for successful multicellular organization and arose independently in animals, plants and fungi. Most of the fungi obtain nutrients from the environment by growing in an exploratory and invasive manner, and this ability depends on multicellular filaments known as hyphae. These cells grow by tip extension and can be divided into compartments by cell walls that typically retain a central pore that allows intercellular transport and cooperation. In the major clade of filamentous Ascomycota, integrity of this coenocytic organization is maintained by Woronin body organelles, which function as emergency patches of septal pores. In this issue of Molecular Microbiology, Bleichrodt and co-workers show that Woronin bodies can also form tight reversible associations with the pore and further link this to variation in levels of compartmental gene expression. These data define an additional modality of Woronin body-dependent gatekeeping. This commentary focuses on the implications of this work and the potential role of different modes of pore gating in controlling the growth and development of fungal tissues.

Cell-to-cell communication in plants, animals, and fungi: a comparative review

Cell-to-cell communication is a prerequisite for differentiation and development in multicellular organisms. This communication has to be tightly regulated to ensure that cellular components such as organelles, macromolecules, hormones, or viruses leave the cell in a precisely organized way. During evolution, plants, animals, and fungi have developed similar ways of responding to this biological challenge. For example, in higher plants, plasmodesmata connect adjacent cells and allow communication to regulate differentiation and development. In animals, two main general structures that enable short- and long-range intercellular communication are known, namely gap junctions and tunneling nanotubes, respectively. Finally, filamentous fungi have also developed specialized structures called septal pores that allow intercellular communication via cytoplasmic flow. This review summarizes the underlying mechanisms for intercellular communication in these three eukaryotic groups and discusses its consequences for the regulation of differentiation and developmental processes.

Optical traps to study properties of molecular motors

In vitro motility assays enabled the analysis of coupling between ATP hydrolysis and movement of myosin along actin filaments or kinesin along microtubules. Single-molecule assays using laser trapping have been used to obtain more detailed information about kinesins, myosins, and processive DNA enzymes. The combination of in vitro motility assays with laser-trap measurements has revealed detailed dynamic structural changes associated with the ATPase cycle. This article describes the use of optical traps to study processive and nonprocessive molecular motor proteins, focusing on the design of the instrument and the assays to characterize motility.

Fungal evo-devo: organelles and multicellular complexity

Peroxisome-derived Woronin bodies of the Ascomycota phyla, and the endoplasmic reticulum (ER)-derived septal pore cap (SPC) of the Basidiomycota, are both fungal organelles that prevent cytoplasmic bleeding when multicellular hyphal filaments are wounded. Analysis of Woronin body constituent proteins suggests that these organelles evolved in part through gene duplication and co-opting of non-essential genes for new functions, indicating that new organelles can arise through typical evolutionary mechanisms. Interestingly, clades possessing the Woronin body and SPC also produce the largest and most complex multicellular fungal reproductive structures. Certain Woronin body and SPC mutants have defects in growth and development, suggesting functions beyond cellular wound healing. I argue that studying these specialized systems will help to reveal the basis for fungal diversity and provide general principles for co-evolution of organelles and multicellular complexity.

Overview of laser microbeam applications as related to antibody targeting

This chapter reviews several techniques which combine the use of laser microbeams with antibodies to study molecular and cellular biology. An overview of the basic properties of lasers and their integration with microscopes and computers is provided. Biophysical applications, such as fluorescence recovery after photobleaching to measure molecular mobility and fluorescence resonance energy transfer to measure molecular distances, as well as ablative applications for the selective inactivation of proteins or the selective killing of cells are described. Other techniques, such as optical trapping, that do not rely on the interaction of the laser with the targeting antibody, are also discussed.

A tether for Woronin body inheritance is associated with evolutionary variation in organelle positioning

Eukaryotic organelles evolve to support the lifestyle of evolutionarily related organisms. In the fungi, filamentous Ascomycetes possess dense-core organelles called Woronin bodies (WBs). These organelles originate from peroxisomes and perform an adaptive function to seal septal pores in response to cellular wounding. Here, we identify Leashin, an organellar tether required for WB inheritance, and associate it with evolutionary variation in the subcellular pattern of WB distribution. In Neurospora, the leashin (lah) locus encodes two related adjacent genes. N-terminal sequences of LAH-1 bind WBs via the WB–specific membrane protein WSC, and C-terminal sequences are required for WB inheritance by cell cortex association. LAH-2 is localized to the hyphal apex and septal pore rim and plays a role in colonial growth. In most species, WBs are tethered directly to the pore rim, however, Neurospora and relatives have evolved a delocalized pattern of cortex association. Using a new method for the construction of chromosomally encoded fusion proteins, marker fusion tagging (MFT), we show that a LAH-1/LAH-2 fusion can reproduce the ancestral pattern in Neurospora. Our results identify the link between the WB and cell cortex and suggest that splitting of leashin played a key role in the adaptive evolution of organelle localization.

In the kingdom Fungi, tubular cells called hyphae grow by tip extension and lateral branching to produce an interconnected multicellular syncytium and this unique cellular architecture is especially suited to foraging, long distance transport, and invasive growth. Major groups of fungi have independently evolved cellular organelles that support this form of multicellularity. Woronin bodies evolved over 400 million years ago in the common ancestor of filamentous Ascomycetes and perform an adaptive function to seal pores that connect hyphal compartments (septal pores) in response to cellular wounding. This study identifies Leashin, a tethering protein that promotes equitable Woronin body inheritance by providing a link to the cell cortex. Patterns of cortex association display systematic variation; in most of the filamentous Ascomycetes, Woronin bodies are tethered to the septal pore. By contrast, a delocalized pattern has recently evolved in a group represented by Neurospora and Sordaria. We present evidence suggesting that the ancestral leashin gene was split into two independent transcription units to permit this evolutionary transition. This work is exemplary of how filamentous Ascomycetes with well-resolved phylogenetic relationships, diverse sequenced genomes and powerful haploid genetics provide model systems for understanding evolutionary innovation within a functional cellular and physiological context.

Optical tweezers: tethers, wavelengths, and heat

This chapter briefly review the four major methods of optical trapping: (1) directly on to single cells or groups of cells, (2) directly on to organelles and structures inside of the cell, (3) on to a bead as a "handle" to apply force, and (4) on to a bead that has been coated with an antigen or antibody that is moved to the cell membrane for the purpose of activation of a chemical response (no force is applied to the cell). In addition, this chapter discusses the issue of optimal wavelength selection for trapping and the potential temperature rise within the trap.

Optical tweezer micromanipulation of filamentous fungi

Optical tweezers have been little used in experimental studies on filamentous fungi. We have built a simple, compact, easy-to-use, safe and robust optical tweezer system that can be used with brightfield, phase contrast, differential interference contrast and fluorescence optics on a standard research grade light microscope. We have used this optical tweezer system in a range of cell biology applications to trap and micromanipulate whole fungal cells, organelles within cells, and beads. We have demonstrated how optical tweezers can be used to: unambiguously determine whether hyphae are actively homing towards each other; move the Spitzenkörper and change the pattern of hyphal morphogenesis; make piconewton force measurements; mechanically stimulate hyphal tips; and deliver chemicals to localized regions of hyphae. Significant novel experimental findings from our study were that germ tubes generated significantly smaller growth forces than leading hyphae, and that both hyphal types exhibited growth responses to mechanical stimulation with optically trapped polystyrene beads. Germinated spores that had been optically trapped for 25min exhibited no deleterious effects with regard to conidial anastomosis tube growth, homing or fusion.

Non-destructive analysis of the nuclei of transgenic living cells using laser tweezers and near-infrared raman spectroscopic technique

Transgenic cell lines of loblolly pine (Pinus taeda L.) were analyzed by a compact laser-tweezers-Raman-spectroscopy (LTRS) system in this investigation. A low power diode laser at 785 nm was used for both laser optical trapping of single transgenic cells and excitation for near-infrared Raman spectroscopy of the nuclei of synchronized cells, which were treated as single organic particles, at the S-phase of the cell cycle. Transgenic living cells with gfp and uidA genes were used as biological samples to test this LTRS technique. As expected, different Raman spectra were observed from the tested biological samples. This technique provides a high sensitivity and enables real-time spectroscopic measurements of transgenic cell lines. It could be a valuable tool for the study of the fundamental cell and molecular biological process by trapping single nucleus and by providing a wealth of molecular information about the nuclei of cells.

Role of the spindle-pole-body protein ApsB and the cortex protein ApsA in microtubule organization and nuclear migration in Aspergillus nidulans

Nuclear migration and positioning in Aspergillus nidulans depend on microtubules, the microtubule-dependent motor protein dynein, and auxiliary proteins, two of which are ApsA and ApsB. In apsA and apsB mutants nuclei are clustered and show various kinds of nuclear navigation defects, although nuclear migration itself is still possible. We studied the role of several components involved in nuclear migration through in vivo fluorescence microscopy using fluorescent-protein tagging. Because ApsA localizes to the cell cortex and mitotic spindles were immobile in apsA mutants, we suggest that astral microtubule-cortex interactions are necessary for oscillation and movement of mitotic spindles along hyphae, but not for post-mitotic nuclear migration. Mutation of apsA resulted in longer and curved microtubules and displayed synthetic lethality in combination with the conventional kinesin mutation DeltakinA. By contrast, ApsB localized to spindle-pole bodies (the fungal centrosome), to septa and to spots moving rapidly along microtubules. The number of cytoplasmic microtubules was reduced in apsB mutants in comparison to the wild type, indicating that cytoplasmic microtubule nucleation was affected, whereas mitotic spindle formation appeared normal. Mutation of apsB suppressed dynein null mutants, whereas apsA mutation had no effect. We suggest that nuclear positioning defects in the apsA and apsB mutants are due to different effects on microtbule organisation. A model of spindle-pole body led nuclear migration and the roles of dynein and microtubules are discussed.

Optical micromanipulations inside yeast cells

We present a combination of nonlinear microscopy and optical trapping applied to three-dimensional imaging and manipulation of intracellular structures in living cells. We use Titanium-sapphire laser pulses for nonlinear microscopy of the nuclear envelope and the microtubules marked with green fluorescent protein in fission yeast. The same laser source is also used to trap small lipid granules naturally present in the cell. The trapped granule is used as a handle to exert a pushing force on the cell nucleus. The granule is moved in a raster-scanning fashion to cover the area of the nucleus and hence displace the nucleus away from its normal position in the center of the cell. Such indirect manipulations of an organelle (e.g., nucleus) can be useful when direct trapping of the chosen organelle is disadvantageous or inefficient. We show that nonlinear microscopy and optical manipulation can be performed without substantial damage or heating of the cell. We present this method as an important tool in cell biology for manipulation of specific structures, as an alternative to genetic and biochemical methods. This technique can be applied to several fundamental problems in cell biology, including the mechanism of nuclear positioning and the spatial coordination of nuclear and cell division.

Preparation of single rice chromosome for construction of a DNA library using a laser microbeam trap

We report the development of a laser micromanipulation system and its application in the isolation of individual rice chromosomes directly from a metaphase cell. Microdissection and flow sorting are two major methods for the isolation of single chromosome. These methods are dependent on the techniques of chromosome spread and chromosome suspension, respectively. In the development of this system, we avoided using chromosome spread and cell suspension was used instead. The cell wall of metaphase rice cell was cut by optical scissors. The released single chromosome was captured by an optical trap and transported to an area without cell debris. The isolated single chromosome was then collected and specific library was constructed by linker adaptor PCR. The average insert size of the library was about 300 bp. Two hundred inserts of chromosome 4 library were sequenced, and 96.5% were aligned to the corresponding sequences of rice chromosome 4. These results suggest the possible application of this method for the preparation of other subcellular structures and for the cloning of single macromolecule through a laser microbeam trap.

Mitosis and motor proteins in the filamentous ascomycete, Nectria haematococca, and some related fungi

Among filamentous fungi, mitosis has been studied in-depth in just a few species. The mitotic apparatuses in the ascomycetous Fusarium spp. are the most clearly and readily visualized in vivo within this group; fluorescent labeling is unnecessary. This superior cytological tractability has enabled detailed studies and revealing experiments that have led the way toward a more complete understanding of fungal mitosis. Some of the most important discoveries include the role of half-spindles in development of the bipolar spindle, the existence of true kinetochores in fungi, the unorthodox chromosome configurations and movements comprising metaphase and anaphase A, the attachment of astral microtubules to the plasmalemma, the role of the astral pulling force in elongating the spindle, an inwardly directed force within the spindle, and microtubule cross-bridging in both spindle and asters. Recent research has focused on the roles of microtubuleassociated motor proteins in Fusarium solani f. sp. pisi (anamorph of Nectria haematococca). Cytoplasmic dynein was shown to be involved in the development and/or maintenance of mitotic asters and necessary for motility and functionality of the interphase spindle pole body. The inwardly directed force within the anaphase spindle was shown to be produced by a kinesin-related protein, NhKRP1. Because of its superior cytological tractability, the considerable and unique knowledge we have of many aspects of its mitosis, and its genetic tractability, Fusarium solani f. sp. pisi is a good choice for further investigations of mitosis in filamentous fungi.

Stress response in Caenorhabditis elegans caused by optical tweezers: wavelength, power, and time dependence

Optical tweezers have emerged as a powerful technique for micromanipulation of living cells. Although the technique often has been claimed to be nonintrusive, evidence has appeared that this is not always the case. This work presents evidence that near-infrared continuous-wave laser light from optical tweezers can produce stress in Caenorhabditis elegans. A transgenic strain of C. elegans, carrying an integrated heat-shock-responsive reporter gene, has been exposed to laser light under a variety of illumination conditions. It was found that gene expression was most often induced by light of 760 nm, and least by 810 nm. The stress response increased with laser power and irradiation time. At 810 nm, significant gene expression could be observed at 360 mW of illumination, which is more than one order of magnitude above that normally used in optical tweezers. In the 700-760-nm range, the results show that the stress response is caused by photochemical processes, whereas at 810 nm, it mainly has a photothermal origin. These results give further evidence that the 700-760-nm wavelength region is unsuitable for optical tweezers and suggest that work at 810 nm at normal laser powers does not cause stress at the cellular level.

Investigation of the molecular extraction process in single subpicoliter droplets using a near-infrared laser Raman trapping system

The near-infrared (NIR) laser Raman trapping system was applied to study liquid-liquid extraction in a single droplet in a subpicoliter range. The system trapped a single subpicoliter toluene droplet in water using the NIR laser beam and provided time-dependent optical images of the droplet during liquid-liquid extraction. The size of the trapped droplet gradually increased afterp-nonylphenol solution was added in water. The Raman spectra of the droplet prove that the increase of the droplet size was caused by the absorption of p-nonylphenol from the water. The distribution coefficient of p-nonylphenol in the sub-picoliter droplet was much higher than that in bulk solution.

Giant cell formation in cells exposed to 740 nm and 760 nm optical traps

BACKGROUND AND OBJECTIVE

Optical trapping is becoming a useful and widespread technique for the micromanipulation of cells and organelles. Giant cell formation following optical trapping was studied to detect the potential adverse effects.

STUDY DESIGN/MATERIALS AND METHODS

The nuclei of preselected single CHO cells were exposed to 740 nm and 760 nm laser microbeam generated by a titanium-sapphire tunable laser at 88 and 176 mW and different time exposures. The irradiated single cells were recorded and observed morphologically following exposure. Giant cells were tabulated and photographed.

RESULTS

The irradiated cells either failed to divide, or they underwent nuclear proliferation to form giant cells through endoreduplication.

CONCLUSION

Giant cells were induced by both 740 nm and 760 nm. The frequency of giant cell formation was higher for the longer time exposures and at the higher power densities. The use of an optical etalon to remove intracavity mode beating and high peak powers of the titanium-sapphire laser caused a significant reduction in the formation of giant cells.

Perillyl alcohol inhibits TCR-mediated [Ca(2+)](i) signaling, alters cell shape and motility, and induces apoptosis in T lymphocytes

Perillyl alcohol (POH) inhibits isoprenylation and has shown anticancer and chemopreventive properties in rodent models. The mechanism that underlies the anticancer activity of POH and other isoprenylation inhibitors is unknown but has been postulated to involve decreased levels of isoprenylated Ras and Ras-related proteins. Previously we demonstrated that POH effectively inhibits human T cell proliferation in vitro and can prevent acute and chronic rejection in a rat cardiac transplant model. In this report, we investigate the effects of POH on T lymphocytes at the single-cell level. POH disrupts the polarized shape and motility of antigen-specific murine 1E5 T cells. Using an optical trap to position anti-CD3-coated beads in contact with 1E5 T cells, we demonstrate that POH inhibits their TCR-mediated calcium response. Furthermore, we show that POH preferentially induces apoptosis in PHA-activated human T cells as well as in 1E5 T cells.

Mapping the sensitivity of T cells with an optical trap: polarity and minimal number of receptors for Ca(2+) signaling

Contact with antigen-presenting cells (APCs) initiates an activation cascade within T lymphocytes, including a rise in cytosolic calcium, lymphokine production, and cell division. Although T cell-APC physical contact is required for an immune response, little is known about the patterns of cellular interactions and their relation to activation. Calcium imaging combined with an optical trap enabled the T cell contact requirements and polarity to be investigated at the single-cell level. APCs or anti-CD3 mAb-coated beads were trapped with a laser and placed at different locations along the T cell, which has a polarized appearance defined by the shape and direction of crawling. T cells were 3-fold more sensitive to APC contact made at the leading edge of the T cell than with contact made at the tail. Anti-CD3 mAb-coated 6-micrometer beads induced calcium signaling with approximately 10-fold higher frequency and approximately 4-fold shorter latency on contact with the leading edge of the T cell than on contact with the trailing edge. Alterations in antibody density (2 to 500 per micrometer(2)) and bead size (1 to 6 micrometer in diameter) were used to determine the spatial requirements and the minimal number of receptors which must be engaged to transmit a positive signal. T cell response percentage, latency, and calcium-signaling pattern (transient vs. sustained or oscillatory) depended on antibody density on the bead. The presence of approximately 170 anti-CD3 mAb within the contact area elicited a detectable T cell calcium response. We propose here that engagement of no more than 340 T cell receptors (approximately 1% of the total on the cell) is sufficient to initiate Ca(2+) signaling. The minimal contact area was approximately 3 micrometer(2).

The micro-robotic laboratory: optical trapping and scissing for the biologist

With the addition of tightly focused laser beams, microscopes have been turned into elaborate preparative tools that permit not only allow detailed observation of a specimen but also the capture, displacement, and microdissection of biological samples in vitro with astonishing ease and accuracy. Laser-Tweezers are used to capture and manipulate cells and organelles. LaserScissors are used to perform microdissections at the submicrometer level. After a short technical description of the instrumentation and its principles of operation, several examples of applications are given relevant to the field of clinical research that could only be achieved using such modern technology. For instance, LaserTweezers and LaserScissors offer a unprecedented means to study the immune response to cancer, to control the growth of nerve cells, or expand the significance of assisted reproductive technologies. It is suggested that newly developing procedures and assays using laser-assisted technologies will prove beneficial for future clinical laboratory testing.

Role of fungal dynein in hyphal growth, microtubule organization, spindle pole body motility and nuclear migration

Cytoplasmic dynein is a microtubule-associated motor protein with several putative subcellular functions. Sequencing of the gene (DHC1) for cytoplasmic dynein heavy chain of the filamentous ascomycete, Nectria haematococca, revealed a 4,349-codon open reading frame (interrupted by two introns) with four highly conserved P-loop motifs, typical of cytoplasmic dynein heavy chains. The predicted amino acid sequence is 78.0% identical to the cytoplasmic dynein heavy chain of Neurospora crassa, 70.2% identical to that of Aspergillus nidulans and 24.8% identical to that of Saccharomyces cerevisiae. The genomic copy of DHC1 in N. haematococca wild-type strain T213 was disrupted by inserting a selectable marker into the central motor domain. Mutants grew at 33% of the wild-type rate, forming dense compact colonies composed of spiral and highly branched hyphae. Major cytological phenotypes included (1) absence of aster-like arrays of cytoplasmic microtubules focused at the spindle pole bodies of post-mitotic and interphase nuclei, (2) limited post-mitotic nuclear migration, (3) lack of spindle pole body motility at interphase, (4) failure of spindle pole bodies to anchor interphase nuclei, (5) nonuniform distribution of interphase nuclei and (6) small or ephemeral Spitzenkorper at the apices of hyphal tip cells. Microtubule distribution in the apical region of tip cells of the mutant was essentially normal. The nonuniform distribution of nuclei in hyphae resulted primarily from a lack of both post-mitotic nuclear migration and anchoring of interphase nuclei by the spindle pole bodies. The results support the hypothesis that DHC1 is required for the motility and functions of spindle pole bodies, normal secretory vesicle transport to the hyphal apex and normal hyphal tip cell morphogenesis.

Design for fully steerable dual-trap optical tweezers

A design for complete beam steering (in three dimensions) of one or two optical tweezers traps is presented. The two most important requirements for efficient and stable movement of an optical trap are identified. A detailed recipe for the construction of a movable optical tweezers trap that fulfills these requirements is given (exemplified with an inverted microscope). The system has been found to allow for precise and free movements of both traps in all three dimensions in a dual-trap optical tweezers configuration and to be robust and reliable, as well as forgiving of small misalignments in the optical system.

Polarity of T cell shape, motility, and sensitivity to antigen

T cell activation requires contact with APCs. We used optical techniques to demonstrate T cell polarity on the basis of shape, motility, and localized sensitivity to antigen. An intracellular Ca2+ clamp showed that T cell shape and motility are extremely sensitive to changes in [Ca2+]i (Kd = 200 nM), with immobilization and rounding occurring via a calcineurin-independent pathway. Ca2+ dependent immobilization prolonged T cell contact with the antigen-presenting B cell; buffering the [Ca2+]i signal prevented the formation of stable cell pairs. Optical tweezers revealed spatial T cell sensitivity to antigen by controlling placement on the T cell surface of either B cells or alpha-CD3 MAb-coated beads. T cells were 4-fold more sensitive to contact made at the leading edge of the T cell compared with the tail. We conclude that motile T cells are polarized antigen sensors that respond physically to [Ca2+]i signals to stabilize their interaction with APCs.

Wavelength dependence of cell cloning efficiency after optical trapping

A study on clonal growth in Chinese hamster ovary (CHO) cells was conducted after exposure to optical trapping wavelengths using Nd:YAG (1064 nm) and tunable titanium-sapphire (700-990 nm) laser microbeam optical traps. The nuclei of cells were exposed to optical trapping forces at various wavelengths, power densities, and durations of exposure. Clonal growth generally decreased as the power density and the duration of laser exposure increased. A wavelength dependence of clonal growth was observed, with maximum clonability at 950-990 nm and least clonability at 740-760 nm and 900 nm. Moreover, the most commonly used trapping wavelength, 1064 nm from the Nd:YAG laser, strongly reduced clonability, depending upon the power density and exposure time. The present study demonstrates that a variety of optical parameters must be considered when applying optical traps to the study of biological problems, especially when survival and viability are important factors. The ability of the optical trap to alter either the structure or biochemistry of the process being probed with the trapping beam must be seriously considered when interpreting experimental results.

Independent nuclear motility and hyphal tip growth

Independent nuclear motility is involved in many important aspects of fungal life cycles, including the following: nuclear division; population of hyphal tip cells, branches, and spores with nuclei; dikaryotization; and karyogamy. Spindle pole bodies are almost constantly in motion during all phases of the nuclear cycle, and they have been linked to most instances of independent nuclear motility. A role for microtubules in this process is now well established, and research is being focused on which set of them, astral or cytoplasmic, is utilized as well as on the microtubule-associated motor proteins that may generate the force. In some cases, F-actin may interact with the microtubules or even provide an alternative cytoskeleton supporting nuclear migration. Hyphal tip growth and independent nuclear motility are coordinated and interrelated processes, making the elucidation of the signals, processes, and structures involved an attractive area for further research. Key words: actin, microtubule, microtubule associated protein, microtubule organizing center, motility, nucleus.

Laser micromanipulators for biotechnology and genome research

The use of lasers for complete micromanipulation of metaphase chromosomes, cells and subcellular structures is reviewed. DNA probes from single microdissected chromosome segments can be prepared using Alu or Adaptor PCR. In plant biotechnology, laser microsurgery can be used to prepare non-enzymatically protoplasts from Medicago sativa. Microgravity can be simulated in the alga Chara by lifting intracellular gravity transmitting elements with the optical tweezers.

Optical trapping for chromosome manipulation: a wavelength dependence of induced chromosome bridges

Using a tunable titanium-sapphire laser, we have compared different wavelengths (from 700 to 840 nm) for their utility in optical trapping of chromosomes in mitotic rat kangaroo Potorous tridactylus (PtK2) cells. It was found that irradiation with a near-infrared light induces the sticking together of chromosome shoulders. The attached chromatids failed to separate, or separated with significant delay and formed a chromosome bridge during anaphase. Using this bridge (and induced c-mitosis) as a reference, we compared the action of different wavelengths (from 700 to 840 nm). Chromosomes were irradiated at metaphase and the cells were observed until the end of cytokinesis. Chromosomes were irradiated for different periods of time, using 130 mW of power at the objective focal plane. The biological responses observed after optical trapping were: (1) normal cell division, (2) formation of a temporary chromosome bridge, (3) formation of a permanent chromosome bridge, (4) complete blockage of chromosome separation (c-mitosis). The chromosomes were found to have a maximal sensitivity to 760-765 nm light and minimal sensitivity to 700 and 800-820 nm light. Cells with chromosomes irradiated for a long time, using wavelength 760-765 nm, generally were incapable of going through anaphase and remained in c-mitosis. We conclude that the optimal wavelengths for optical trapping are 700 and 800-820 nm.

Making light work with optical tweezers

Microscopic objects, including biological material, can be remotely manipulated with tightly focused beams of infrared laser light. The use of optical traps, or 'optical tweezers', holds great promise for noninvasive micromanipulation and mechanical measurement in cell biology. Optical tweezers are the 'tractor beams' of today's technology.