Micromanipulation of chromosomes in PTK2 cells using laser microsurgery (optical scalpel) in combination with laser-induced optical force (optical tweezers)

Acentric chromosome congression and alignment on the metaphase plate via kinetochore-independent forces in Drosophila

Chromosome congression and alignment on the metaphase plate involves lateral and microtubule plus-end interactions with the kinetochore. Here we take advantage of our ability to efficiently generate a GFP-marked acentric X chromosome fragment in Drosophila neuroblasts to identify forces acting on chromosome arms that drive congression and alignment. We find acentrics efficiently align on the metaphase plate, often more rapidly than kinetochore-bearing chromosomes. Unlike intact chromosomes, the paired sister acentrics oscillate as they move to and reside on the metaphase plate in a plane distinct and significantly further from the main mass of intact chromosomes. Consequently, at anaphase onset acentrics are oriented either parallel or perpendicular to the spindle. Parallel-oriented sisters separate by sliding while those oriented perpendicularly separate via unzipping. This oscillation, together with the fact that in monopolar spindles acentrics are rapidly shunted away from the poles, indicates that distributed plus-end directed forces are primarily responsible for acentric migration. This conclusion is supported by the observation that reduction of EB1 preferentially disrupts acentric alignment. In addition, reduction of Klp3a activity, a gene required for the establishment of pole-to-pole microtubules, preferentially disrupts acentric alignment. Taken together these studies suggest that plus-end forces mediated by the outer pole-to-pole microtubules are primarily responsible for acentric metaphase alignment. Surprisingly, we find that a small fraction of sister acentrics are anti-parallel aligned indicating that the kinetochore is required to ensure parallel alignment of sister chromatids. Finally, we find induction of acentric chromosome fragments results in a global reorganization of the congressed chromosomes into a torus configuration.

Article Summary

The kinetochore serves as a site for attaching microtubules and allows for successful alignment, separation, and segregation of replicated sister chromosomes during cell division. However, previous studies have revealed that sister chromosomes without kinetochores (acentrics) often align to the metaphase plate, undergo separation and segregation, and are properly transmitted to daughter cells. In this study, we discuss the forces acting on chromosomes, independent of the kinetochore, underlying their successful alignment in early mitosis.

Optical Force of Bessel Pincer Light-Sheets Beam on a Dielectric Sphere of Arbitrary Size

In the framework of Generalized Lorenz-Mie theory (GLMT), based on the expansion results of electromagnetic field radiation components of Bessel pincer light sheets beam acting on dielectric particles of arbitrary size, the expression of radiation force components in a Cartesian coordinate system is obtained by using the Maxwell stress tensor method. On the one hand, the effects of the refractive index and the equivalent radius of spherical particles on the distribution of radiation force are discussed; On the other hand, the influence of beam scaling parameter and beam order of Bessel pincer light sheets beam on the distribution of radiation force are investigated. The results indicate that the changes of particle's refractive index and effective radius only affect the distribution of radiation force. However, the beam scaling parameter and beam order of Bessel pincer light sheets beam have a very sharp impact on the convergence position, distribution range and bending degree far away from the wave source of the radiation force. Single-beam optical tweezers using the self-focusing and self-bending Bessel pincer light-sheets beam are crucial for applications such as single molecule biophysics, optical manipulation and particle separation/clearing.

Kinetochore-independent mechanisms of sister chromosome separation

Although kinetochores normally play a key role in sister chromatid separation and segregation, chromosome fragments lacking kinetochores (acentrics) can in some cases separate and segregate successfully. In Drosophila neuroblasts, acentric chromosomes undergo delayed, but otherwise normal sister separation, revealing the existence of kinetochore- independent mechanisms driving sister chromosome separation. Bulk cohesin removal from the acentric is not delayed, suggesting factors other than cohesin are responsible for the delay in acentric sister separation. In contrast to intact kinetochore-bearing chromosomes, we discovered that acentrics align parallel as well as perpendicular to the mitotic spindle. In addition, sister acentrics undergo unconventional patterns of separation. For example, rather than the simultaneous separation of sisters, acentrics oriented parallel to the spindle often slide past one another toward opposing poles. To identify the mechanisms driving acentric separation, we screened 117 RNAi gene knockdowns for synthetic lethality with acentric chromosome fragments. In addition to well-established DNA repair and checkpoint mutants, this candidate screen identified synthetic lethality with X-chromosome-derived acentric fragments in knockdowns of Greatwall (cell cycle kinase), EB1 (microtubule plus-end tracking protein), and Map205 (microtubule-stabilizing protein). Additional image-based screening revealed that reductions in Topoisomerase II levels disrupted sister acentric separation. Intriguingly, live imaging revealed that knockdowns of EB1, Map205, and Greatwall preferentially disrupted the sliding mode of sister acentric separation. Based on our analysis of EB1 localization and knockdown phenotypes, we propose that in the absence of a kinetochore, microtubule plus-end dynamics provide the force to resolve DNA catenations required for sister separation.

Genetic Material Manipulation and Modification by Optical Trapping and Nanosurgery-A Perspective

Light can be employed as a tool to alter and manipulate matter in many ways. An example has been the implementation of optical trapping, the so called optical tweezers, in which light can hold and move small objects with 3D control. Of interest for the Life Sciences and Biotechnology is the fact that biological objects in the size range from tens of nanometers to hundreds of microns can be precisely manipulated through this technology. In particular, it has been shown possible to optically trap and move genetic material (DNA and chromatin) using optical tweezers. Also, these biological entities can be severed, rearranged and reconstructed by the combined use of laser scissors and optical tweezers. In this review, the background, current state and future possibilities of optical tweezers and laser scissors to manipulate, rearrange and alter genetic material (DNA, chromatin and chromosomes) will be presented. Sources of undesirable effects by the optical procedure and measures to avoid them will be discussed. In addition, first tentative approaches at cellular-level genetic and organelle surgery, in which genetic material or DNA-carrying organelles are extracted out or introduced into cells, will be presented.

ESCRT-III-mediated membrane fusion drives chromosome fragments through nuclear envelope channels

Mitotic cells must form a single nucleus during telophase or exclude part of their genome as damage-prone micronuclei. While research has detailed how micronuclei arise from cells entering anaphase with lagging chromosomes, cellular mechanisms allowing late-segregating chromosomes to rejoin daughter nuclei remain underexplored. Here, we find that late-segregating acentric chromosome fragments that rejoin daughter nuclei are associated with nuclear membrane but devoid of lamin and nuclear pore complexes in Drosophila melanogaster. We show that acentrics pass through membrane-, lamin-, and nuclear pore-based channels in the nuclear envelope that extend and retract as acentrics enter nuclei. Membrane encompassing the acentrics fuses with the nuclear membrane, facilitating integration of the acentrics into newly formed nuclei. Fusion, mediated by the membrane fusion protein Comt/NSF and ESCRT-III components Shrub/CHMP4B and CHMP2B, facilitates reintegration of acentrics into nuclei. These results suggest a previously unsuspected role for membrane fusion, similar to nuclear repair, in the formation of a single nucleus during mitotic exit and the maintenance of genomic integrity.

Mechanisms driving acentric chromosome transmission

The kinetochore-microtubule association is a core, conserved event that drives chromosome transmission during mitosis. Failure to establish this association on even a single chromosome results in aneuploidy leading to cell death or the development of cancer. However, although many chromosomes lacking centromeres, termed acentrics, fail to segregate, studies in a number of systems reveal robust alternative mechanisms that can drive segregation and successful poleward transport of acentrics. In contrast to the canonical mechanism that relies on end-on microtubule attachments to kinetochores, mechanisms of acentric transmission largely fall into three categories: direct attachments to other chromosomes, kinetochore-independent lateral attachments to microtubules, and long-range tether-based attachments. Here, we review these "non-canonical" methods of acentric chromosome transmission. Just as the discovery and exploration of cell cycle checkpoints provided insight into both the origins of cancer and new therapies, identifying mechanisms and structures specifically involved in acentric segregation may have a significant impact on basic and applied cancer research.

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.

Technique of laser chromosome welding for chromosome repair and artificial chromosome creation

Here we report a technique of laser chromosome welding that uses a violet pulse laser micro-beam for welding. The technique can integrate any size of a desired chromosome fragment into recipient chromosomes by combining with other techniques of laser chromosome manipulation such as chromosome cutting, moving, and stretching. We demonstrated that our method could perform chromosomal modifications with high precision, speed and ease of use in the absence of restriction enzymes, DNA ligases and DNA polymerases. Unlike the conventional methods such as de novo artificial chromosome synthesis, our method has no limitation on the size of the inserted chromosome fragment. The inserted DNA size can be precisely defined and the processed chromosome can retain its intrinsic structure and integrity. Therefore, our technique provides a high quality alternative approach to directed genetic recombination, and can be used for chromosomal repair, removal of defects and artificial chromosome creation. The technique may also have applicability on the manipulation and extension of large pieces of synthetic DNA.

Mitotic tethers connect sister chromosomes and transmit "cross-polar" force during anaphase A of mitosis in PtK2 cells

Originally described in crane-fly spermatocytes, tethers physically link and transmit force between the ends of separating chromosomes. Optical tweezers and laser scissors were used to sever the tether between chromosomes, create chromosome fragments attached to the tether which move toward the opposite pole, and to trap the tethered fragments. Laser microsurgery in the intracellular space between separating telomeres reduced chromosome strain in half of tested chromosome pairs. When the telomere-containing region was severed from the rest of the chromosome body, the resultant fragment either traveled towards the proper pole (poleward), towards the sister pole (cross-polar), or movement ceased. Fragment travel towards the sister pole varied in distance and always ceased following a cut between telomeres, indicating the tether is responsible for transferring a cross-polar force to the fragment. Optical trapping of cross-polar traveling fragments places an upper boundary on the tethering force of ~1.5 pN.

Optical manipulation from the microscale to the nanoscale: fundamentals, advances and prospects

Since the invention of optical tweezers, optical manipulation has advanced significantly in scientific areas such as atomic physics, optics and biological science. Especially in the past decade, numerous optical beams and nanoscale devices have been proposed to mechanically act on nanoparticles in increasingly precise, stable and flexible ways. Both the linear and angular momenta of light can be exploited to produce optical tractor beams, tweezers and optical torque from the microscale to the nanoscale. Research on optical forces helps to reveal the nature of light-matter interactions and to resolve the fundamental aspects, which require an appropriate description of momenta and the forces on objects in matter. In this review, starting from basic theories and computational approaches, we highlight the latest optical trapping configurations and their applications in bioscience, as well as recent advances down to the nanoscale. Finally, we discuss the future prospects of nanomanipulation, which has considerable potential applications in a variety of scientific fields and everyday life.

The chromokinesin Klp3a and microtubules facilitate acentric chromosome segregation

Although chromosome fragments lacking a centromere would be expected to show severe defects in their segregation during anaphase, they do exhibit poleward movement by an unclear mechanism. Karg et al. now show how microtubules and the chromokinesin Klp3a can work together to successfully segregate chromosome fragments to daughter nuclei.

Although poleward segregation of acentric chromosomes is well documented, the underlying mechanisms remain poorly understood. Here, we demonstrate that microtubules play a key role in poleward movement of acentric chromosome fragments generated in Drosophila melanogaster neuroblasts. Acentrics segregate with either telomeres leading or lagging in equal frequency and are preferentially associated with peripheral bundled microtubules. In addition, laser ablation studies demonstrate that segregating acentrics are mechanically associated with microtubules. Finally, we show that successful acentric segregation requires the chromokinesin Klp3a. Reduced Klp3a function results in disorganized interpolar microtubules and shortened spindles. Normally, acentric poleward segregation occurs at the periphery of the spindle in association with interpolar microtubules. In klp3a mutants, acentrics fail to localize and segregate along the peripheral interpolar microtubules and are abnormally positioned in the spindle interior. These studies demonstrate an unsuspected role for interpolar microtubules in driving acentric segregation.

Damage induced in red blood cells by infrared optical trapping: an evaluation based on elasticity measurements

We evaluated the damage caused to optically trapped red blood cells (RBCs) after 1 or 2 min of exposure to near-infrared (NIR) laser beams at 785 or 1064 nm. Damage was quantified by measuring cell elasticity using an automatic, real-time, homemade, optical tweezer system. The measurements, performed on a significant number (hundreds) of cells, revealed an overall deformability decrease up to ∼104% after 2 min of light exposure, under 10 mW optical trapping for the 785-nm wavelength. Wavelength dependence of the optical damage is attributed to the light absorption by hemoglobin. The results provided evidence that RBCs have their biomechanical properties affected by NIR radiation. Our findings establish limits for laser applications with RBCs.

Measurements of forces produced by the mitotic spindle using optical tweezers

An optical trap is used to stop chromosome movement in spermatocytes from an insect and a flatworm and to stop pole movement in PtK cells. The forces required are much smaller than previously believed.

We used a trapping laser to stop chromosome movements in Mesostoma and crane-fly spermatocytes and inward movements of spindle poles after laser cuts across Potorous tridactylus (rat kangaroo) kidney (PtK2) cell half-spindles. Mesostoma spermatocyte kinetochores execute oscillatory movements to and away from the spindle pole for 1–2 h, so we could trap kinetochores multiple times in the same spermatocyte. The trap was focused to a single point using a 63× oil immersion objective. Trap powers of 15–23 mW caused kinetochore oscillations to stop or decrease. Kinetochore oscillations resumed when the trap was released. In crane-fly spermatocytes trap powers of 56–85 mW stopped or slowed poleward chromosome movement. In PtK2 cells 8-mW trap power stopped the spindle pole from moving toward the equator. Forces in the traps were calculated using the equation F = Q′P/c, where P is the laser power and c is the speed of light. Use of appropriate Q′ coefficients gave the forces for stopping pole movements as 0.3–2.3 pN and for stopping chromosome movements in Mesostoma spermatocytes and crane-fly spermatocytes as 2–3 and 6–10 pN, respectively. These forces are close to theoretical calculations of forces causing chromosome movements but 100 times lower than the 700 pN measured previously in grasshopper spermatocytes.

Development of a dual joystick-controlled laser trapping and cutting system for optical micromanipulation of chromosomes inside living cells

A multi-joystick robotic laser microscope system used to control two optical traps (tweezers) and one laser scissors has been developed for subcellular organelle manipulation. The use of joysticks has provided a "user-friendly" method for both trapping and cutting of organelles such as chromosomes in live cells. This innovative design has enabled the clean severing of chromosome arms using the laser scissors as well as the ability to easily hold and pull the severed arm using the laser tweezers.

Automatic transportation of biological cells with a robot-tweezer manipulation system

The positioning of biological cells has become increasingly important in biomedical research such as drug discovery, cell-to-cell interaction, and tissue engineering. Significant demand for both accuracy and productivity in cell manipulation highlights the need for automated cell transportation with integrated robotics and micro/nano-manipulation technologies. Optical tweezers, which use highly focused low-power laser beams to trap and manipulate particles at the micro/nanoscale, can be treated as special robot ‘end-effectors’ to manipulate biological objects in a noninvasive way. In this paper, we propose to use a robot-tweezer manipulation system for automatic transportation of biological cells. A dynamics equation of the cell in an optical trap is analyzed. Closed-loop controllers are designed for positioning single cells as well as multiple cells. A synchronization control technology is utilized for multicell transportation with maintained cell pattern. Experiments are performed on transporting live cells to demonstrate the effectiveness of the proposed approach.

Analysis of DNA double-strand break response and chromatin structure in mitosis using laser microirradiation

In this study the femtosecond near-IR and nanosecond green lasers are used to induce alterations in mitotic chromosomes. The subsequent double-strand break responses are studied. We show that both lasers are capable of creating comparable chromosomal alterations and that a phase paling observed within 1–2 s of laser exposure is associated with an alteration of chromatin as confirmed by serial section electron microscopy, DAPI, γH2AX and phospho-H3 staining. Additionally, the accumulation of dark material observed using phase contrast light microscopy (indicative of a change in refractive index of the chromatin) ∼34 s post-laser exposure corresponds spatially to the accumulation of Nbs1, Ku and ubiquitin. This study demonstrates that chromosomes selectively altered in mitosis initiate the DNA damage response within 30 s and that the accumulation of proteins are visually represented by phase-dark material at the irradiation site, allowing us to determine the fate of the damage as cells enter G1. These results occur with two widely different laser systems, making this approach to study DNA damage responses in the mitotic phase generally available to many different labs. Additionally, we present a summary of most of the published laser studies on chromosomes in order to provide a general guide of the lasers and operating parameters used by other laboratories.

Light at work: the use of optical forces for particle manipulation, sorting, and analysis

We review the combinations of optical micro-manipulation with other techniques and their classical and emerging applications to non-contact optical separation and sorting of micro- and nanoparticle suspensions, compositional and structural analysis of specimens, and quantification of force interactions at the microscopic scale. The review aims at inspiring researchers, especially those working outside the optical micro-manipulation field, to find new and interesting applications of these methods.

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.

Exploring mechanochemical processes in the cell with optical tweezers

Force and torque, stress and strain or work are examples of mechanical and elastic actions which are intimately linked to chemical reactions in the cell. Optical tweezers are a light-based method which allows the real-time manipulation of single molecules and cells to measure their interactions. We describe the technique, briefly reviewing the operating principles and the potential capabilities to the study of biological processes. Additional emphasis is given to the importance of fluctuations in biology and how single-molecule techniques allow access to them. We illustrate the applications by addressing experimental configurations and recent progresses in molecular and cell biology.

Fs‐Laser Scissors for Photobleaching, Ablation in Fixed Samples and Living Cells, and Studies of Cell Mechanics

The use of ultrashort laser pulses for microscopy has steadily increased over the past years. In this so-called multiphoton microscopy, laser pulses with pulse duration around 100 femtoseconds (fs) are used to excite fluorescence within the samples. Due to the high peak powers of fs lasers, the absorption mechanism of the laser light is based on nonlinear absorption. Therefore, the fluorescence signal is highly localized within the bulk of biological materials, similar to a confocal microscope. However, this nonlinear absorption mechanism can not only be used for imaging but for selective alteration of the material at the laser focus: The absorption can on one hand lead to the excitation of fluorescent molecules of fluorescently tagged cells by the simultaneous absorption of two or three photons or on the other hand, in case of higher order processes, to the creation of free-electron plasmas and, consequently, plasma-mediated ablation. Typical imaging powers are in the range of tens of milliwatts using 100-fs pulses at a repetition rate of 80-90 MHz, while pulse energies needed for ablation powers are as low as a few nanojoules when using high numerical aperture microscope objectives for focusing the laser radiation into the sample. Since the first demonstration of this technique, numerous applications of fs lasers have emerged within the field of cellular biology and microscopy. As the typical wavelengths of ultrashort laser systems lie in the near infrared between 800 and 1000 nm, high penetration depth can be achieved and can provide the possibility of imaging and manipulating the biological samples with one single laser system.

Optical Micromanipulation Methods for Controlled Rotation, Transportation, and Microinjection of Biological Objects

The use of laser microtools for rotation and controlled transport of microscopic biological objects and for microinjection of exogenous material in cells is discussed. We first provide a brief overview of the laser tweezers-based methods for rotation or orientation of microscopic objects. Particular emphasis is placed on the methods that are more suitable for the manipulation of biological objects, and the use of these for two-dimensional (2D) and 3D rotations/orientations of intracellular objects is discussed. We also discuss how a change in the shape of a red blood cell (RBC) suspended in hypertonic buffer leads to its rotation when it is optically tweezed. The potential use of this approach for the diagnosis of malaria is also illustrated. The use of a line tweezers having an asymmetric intensity distribution about the center of its major axis for simultaneous transport of microscopic objects, and the successful use of this approach for induction, enhancement, and guidance of neuronal growth cones is presented next. Finally, we describe laser microbeam-assisted microinjection of impermeable drugs into cells and also briefly discuss possible adverse effects of the laser trap or microbeams on cells.

Evaluation of trapping efficiency of optical tweezers by dielectrophoresis

A relatively new method for measuring optically induced forces on microparticles and cells, different from the conventional Brownian motion and viscous drag force calibration methods widely used, is introduced. It makes use of the phenomenon of dielectrophoresis for the calibration of optical tweezers through the dielectrophoretic force calculations. A pair of microelectrodes is fabricated by photolithography on a microscope slide and it is connected to a high-frequency generator. The calibration of the optical tweezers setup is performed by the manipulation of polystyrene beads and yeast cells. Calibration diagrams of the transverse forces versus power are deduced for different cell radii and numerical apertures of the objective lenses. The optical system and the related technique provide a fast and easy method for optical tweezers calibration.

Generation of ROS in cells on exposure to CW and pulsed near-infrared laser tweezers

We report the results of a study on generation of reactive oxygen species (ROS) and changes in the membrane potential of mitochondria of carcinoma of cervix (HeLa) and Chinese hamster ovary (CHO) cells following exposure to continuous wave (cw) or pulsed Nd: YAG laser (1064 nm). For a given laser irradiation, the generation of ROS and induced changes in the membrane potential of mitochondria were more pronounced for HeLa cells as compared to CHO cells. However, in both the cells the laser dose required to elicit a given change was much lower with pulsed laser exposure compared to that required with a cw laser exposure. This suggests involvement of photothermal effects in the laser irradiation induced changes. Mechanistic studies using quenchers for ROS suggest that laser irradiation leads to generation of hydroxyl radicals.

Micromanipulation of chromosomes reveals that cohesion release during cell division is gradual and does not require tension

In mitosis, cohesion appears to be present along the entire length of the chromosome, between centromeres and along chromosome arms. By metaphase, sister chromatids appear as two adjacent but visibly distinct rods. Sister chromatids separate from one another in anaphase by releasing all chromosome cohesion. This is different from meiosis I, in which pairs of sister chromatids separate from one another, moving to each spindle pole by releasing cohesion only between sister chromatid arms. Then, in anaphase II, sister chromatids separate by releasing centromere cohesion. Our objective was to find where cohesion is present or absent on chromosomes in mitosis and meiosis and when and how it is released. We determined cohesion directly by pulling on chromosomes with two micromanipulation needles. Thus, we could distinguish for the first time between apparent doubleness as seen in the microscope and physical separability. We found that apparent doubleness can be deceiving: Visibly distinct sister chromatids often cannot be separated. We also demonstrated that cohesion is released gradually in anaphase, with chromosomes looking as if they were unzipped or pulled apart. This implied that tension from spindle forces was required, but we showed directly that no tension was necessary to pull chromatids apart.

Controlled ablation of microtubules using a picosecond laser

The use of focused high-intensity light sources for ablative perturbation has been an important technique for cell biological and developmental studies. In targeting subcellular structures many studies have to deal with the inability to target, with certainty, an organelle or large macromolecular complex. Here we demonstrate the ability to selectively target microtubule-based structures with a laser microbeam through the use of enhanced yellow fluorescent protein (EYFP) and enhanced cyan fluorescent protein (ECFP) variants of green fluorescent protein fusions of tubule. Potorous tridactylus (PTK2) cell lines were generated that stably express EYFP and ECFP tagged to the alpha-subunit of tubulin. Using microtubule fluorescence as a guide, cells were irradiated with picosecond laser pulses at discrete microtubule sites in the cytoplasm and the mitotic spindle. Correlative thin-section transmission electron micrographs of cells fixed one second after irradiation demonstrated that the nature of the ultrastructural damage appeared to be different between the EYFP and the ECFP constructs suggesting different photon interaction mechanisms. We conclude that focal disruption of single cytoplasmic and spindle microtubules can be precisely controlled by combining laser microbeam irradiation with different fluorescent fusion constructs. The possible photon interaction mechanisms are discussed in detail.

A revolution in optical manipulation

Optical tweezers use the forces exerted by a strongly focused beam of light to trap and move objects ranging in size from tens of nanometres to tens of micrometres. Since their introduction in 1986, the optical tweezer has become an important tool for research in the fields of biology, physical chemistry and soft condensed matter physics. Recent advances promise to take optical tweezers out of the laboratory and into the mainstream of manufacturing and diagnostics; they may even become consumer products. The next generation of single-beam optical traps offers revolutionary new opportunities for fundamental and applied research.

Laser-assisted optoporation of single cells

The plasma membrane of Chinese hamster ovary cells was made permeable using the focused beam of an argon ion laser (488 nm) and phenol red as a light absorbing dye. Small circular dark spots on the cell surface appeared immediately after laser irradiation and disappeared within about 5 min. They were related to transient changes in membrane properties, which could be visualized using the fluorescent marker laurdan, and were probably due to a local increase in temperature. According to a colony forming assay, cell viability was maintained by using light doses up to 2.5 MJ/cm(2) applied for 1 s. In addition to measurements of the efflux of the cytoplasmic marker calcein, cell transfection using a green fluorescent protein (GFP) coding plasmid was studied: brightly fluorescent GFP with an emission maximum around 510 nm was observed within part of the cells after 24 h. The transfection rates after laser irradiation were around 30% for younger subcultures and less than 10% for aging cells. This may be due to age dependent changes in the phase transition of membrane lipids from gel phase to liquid crystalline phase. High transfection rates, visual control and universality towards various cell lines are possibly the main advantages of laser-assisted optoporation in comparison with presently existing methods of cell transfection.

Comet assay measurements of DNA damage in cells by laser microbeams and trapping beams with wavelengths spanning a range of 308 nm to 1064 nm

DNA damage induced in NC37 lymphoblasts by optical tweezers with a continuous-wave Ti:sapphire laser and a continuous-wave Nd:YAG laser (60-240 mW; 10-50 TJ/m2; 30-120 s irradiation) was studied with the comet assay, a single-cell technique used to detect DNA fragmentation in genomes. Over the wavelength range of 750-1064 nm, the amount of damage in DNA peaks at around 760 nm, with the fraction of DNA damage within the range of 750-780 nm being a factor of two larger than the fraction of DNA damage within the range of 800-1064 nm. The variation in DNA damage was not significant over the range of 800-1064 nm. When the logarithm of damage thresholds measured in the present work, as well as values reported previously in the UV range, was plotted as a function of wavelength, a dramatic wavelength dependence became apparent. The damage threshold values can be fitted on two straight lines, one for continuous-wave sources and the other for pulsed sources, irrespective of the type of source used (e.g. classical lamp or laser). The damage threshold around 760 nm falls on the line extrapolated from values for UV-radiation-induced damage, while the data for 800-1064 nm fall on a line that has a different slope. The change in the slope between 320 and 340 nm observed earlier is consistent with a well-known change in DNA-damaging mechanisms. The change observed around 780 nm is therefore suggestive of a further change in the mechanism(s). The data from this work together with our previous measurements provide, to the best of our knowledge, the most comprehensive view available of the DNA damage produced by microfocused light.

Controlled rotation of optically trapped microscopic particles

We demonstrate controlled rotation of optically trapped objects in a spiral interference pattern. This pattern is generated by interfering an annular shaped laser beam with a reference beam. Objects are trapped in the spiral arms of the pattern. Changing the optical path length causes this pattern, and thus the trapped objects, to rotate. Structures of silica microspheres, microscopic glass rods, and chromosomes are set into rotation at rates in excess of 5 hertz. This technique does not depend on intrinsic properties of the trapped particle and thus offers important applications in optical and biological micromachines.

Automated single-cell sorting system based on optical trapping

We provide a basis for automated single-cell sorting based on optical trapping and manipulation using human peripheral blood as a model system. A counterpropagating dual-beam optical-trapping configuration is shown theoretically and experimentally to be preferred due to a greater ability to manipulate cells in three dimensions. Theoretical analysis performed by simulating the propagation of rays through the region containing an erythrocyte (red blood cell) divided into numerous elements confirms experimental results showing that a trapped erythrocyte orients with its longest axis in the direction of propagation of the beam. The single-cell sorting system includes an image-processing system using thresholding, background subtraction, and edge-enhancement algorithms, which allows for the identification of single cells. Erythrocytes have been identified and manipulated into designated volumes using the automated dual-beam trap. Potential applications of automated single-cell sorting, including the incorporation of molecular biology techniques, are discussed.

Analysis of the behaviour of erythrocytes in an optical trapping system

We present a theoretical analysis of the behaviour of erythrocytes in an optical trapping system. We modeled erythrocyte behaviour in an optical trap by an algorithm which divided the cell surface into a large number of elements and recursively summed the force and torque on each element. We present a relationship between the torque and angle of orientation of the cell, showing that stable equilibrium orientations are at angles of 0 o , 180 o and 360 o and unstable equilibrium orientations are at 90 o and 270 o relative to the axis of beam propagation. This is consistent with our experimental observations and with results described in the literature. We also model behaviour of the erythrocyte during micromanipulation by calculating the net force on it. Such theoretical analysis is practical as it allows for the optimization of the optical parameters of a trapping system prior to performing a specific optical micromanipulation application, such as cell sorting or construction of a cell pattern for lab-on-a-chip applications.

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.

Cell viability in optical tweezers: high power red laser diode versus Nd:YAG laser

Viability of cultivated Chinese hamster ovary cells in optical tweezers was measured after exposure to various light doses of red high power laser diodes (lambda = 670-680 nm) and a Nd:yttrium-aluminum-garnet laser (lambda = 1064 nm). When using a radiant exposure of 2.4 GJ/cm2, a reduction of colony formation up to a factor 2 (670-680 nm) or 1.6 (1064 nm) as well as a delay of cell growth were detected in comparison with nonirradiated controls. In contrast, no cell damage was found at an exposure of 340 MJ/cm2 for both wavelengths, and virtually no lethal damage at 1 GJ/cm2 applied at 1064 nm. Cell viabilities were correlated with fluorescence excitation spectra and with literature data of wavelength dependent cloning efficiencies. Fluorescence excitation maxima of the coenzymes NAD(P)H and flavins were detected at 365 and 450 nm, respectively. This is half of the wavelengths of the maxima of cell inactivation, suggesting that two-photon absorption by these coenzymes may contribute to cellular damage. Two-photon excitation of NAD(P)H and flavins may also affect cell viability after exposure to 670-680 nm, whereas one-photon excitation of water molecules seems to limit cell viability at 1064 nm.

Microinstrument gradient-force optical trap

A micromachined fiber-optic trap is presented. The trap consists of four single-mode, 1064-nm optical intersection. The beam fibers mounted in a micromachined silicon and glass housing. Micromachining provides the necessary precision to align the four optical fibers so that the outputs have a common intersection forms a strong three-dimensional gradient-force trap with trapping forces comparable with that of optical tweezers. Characterization of the multibeam fiber trap is illustrated for capture of polystyrene microspheres, computer simulations of the trap stiffness, and experimental determination of the trapping forces.

The centromeric sister chromatid cohesion site directs Mcd1p binding to adjacent sequences

Cohesion between sister chromatids occurs along the length of chromosomes, where it plays essential roles in chromosome segregation. We show here that the centromere, a cis-acting cohesion factor, directs the binding of Mcd1p, a cohesin subunit, to at least 2 kb regions flanking centromeres in a sequence-independent manner. The centromere is essential for the maintenance as well as the establishment of this cohesin domain. The efficiency of Mcd1p binding within the cohesin domain is independent of the primary nucleotide sequence of the centromere-flanking DNA but correlates with high A + T DNA content. Thus, the function of centromeres in the cohesion of centromere-proximal regions may be analogous to that of enhancers, nucleating cohesin complex binding over an extended chromosomal domain of A + T-rich DNA.

Separating sister chromatids

Loss of cohesion between sister chromatids triggers their segregation during anaphase. Recent work has identified both a cohesin complex that holds sisters together and a sister-separating protein, separin, that destroys cohesion. Separins are bound by inhibitory proteins whose proteolysis at the metaphase-anaphase transition is mediated by the anaphase-promoting complex and its activator protein CDC20 (APCCDC20). When chromosomes are misaligned, a surveillance mechanism (checkpoint) blocks sister separation by inhibiting APCCDC20. Defects in this apparatus are implicated in causing aneuploidy in human cells.

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.

Insertion of microscopic objects through plant cell walls using laser microsurgery

A detailed protocol is presented for precisely inserting microscopic objects into the periplasmic region of plant callus cells using laser microsurgery. Ginkgo biloba and Agrobacterium rhizogenes were used as the model system for developing the optical tweezers and scalpel techniques using a single laser. We achieved better than 95% survival after plasmolyzing G. biloba cells, ablating a 2-4-μm hole through the cell wall using a pulsed UV laser beam, trapping and translating bacteria into the periplasmic region using a pulsed infrared laser beam, and then deplasmolyzing the cells. Insertion of bacteria is also described. A thermal model for temperature changes of trapped bacteria is included. Comparisons with other methods, such as a reverse-pressure gradient technique, are discussed and additional experiments on plants using laser microsurgery are suggested. Copyright 1998 John Wiley & Sons, Inc.

An ESP1/PDS1 complex regulates loss of sister chromatid cohesion at the metaphase to anaphase transition in yeast

Cohesion between sister chromatids during G2 and M phases depends on the "cohesin" protein Scc1p (Mcd1p). Loss of cohesion at the metaphase to anaphase transition is accompanied by Scc1p's dissociation from chromatids, which depends on proteolysis of Pds1p mediated by a ubiquitin protein ligase called the anaphase promoting complex (APC). We show that destruction of Pds1p is the APC's sole role in triggering Scc1p's dissociation from chromatids and that Pds1p forms a stable complex with a 180 kDa protein called Esp1p, which is essential for the dissociation of Scc1p from sister chromatids and for their separation. We propose that the APC promotes sister separation not by destroying cohesins but instead by liberating the "sister-separating" Esp1 protein from its inhibitor Pds1p.

Neocentromere-mediated chromosome movement in maize

Neocentromere activity is a classic example of nonkinetochore chromosome movement. In maize, neocentromeres are induced by a gene or genes on Abnormal chromosome 10 (Ab10) which causes heterochromatic knobs to move poleward at meiotic anaphase. Here we describe experiments that test how neocentromere activity affects the function of linked centromere/kinetochores (kinetochores) and whether neocentromeres and kinetochores are mobilized on the spindle by the same mechanism. Using a newly developed system for observing meiotic chromosome congression and segregation in living maize cells, we show that neocentromeres are active from prometaphase through anaphase. During mid-anaphase, normal chromosomes move on the spindle at an average rate of 0.79 micron/min. The presence of Ab10 does not affect the rate of normal chromosome movement but propels neocentromeres poleward at rates as high as 1.4 micron/min. Kinetochore-mediated chromosome movement is only marginally affected by the activity of a linked neocentromere. Combined in situ hybridization/immunocytochemistry is used to demonstrate that unlike kinetochores, neocentromeres associate laterally with microtubules and that neocentromere movement is correlated with knob size. These data suggest that microtubule depolymerization is not required for neocentromere motility. We argue that neocentromeres are mobilized on microtubules by the activity of minus end-directed motor proteins that interact either directly or indirectly with knob DNA sequences.

Laser scissors and tweezers

In summary, we described the use of laser scissors and tweezers from three perspectives: (a) the historical background from which these two techniques evolved, (b) an understanding and lack of understanding of the mechanisms of interaction with the biological systems, and (c) the applications of the scissors and tweezers alone and in combination. As the technology improves and we gain a better understanding of how these two tools operate they will become even more useful in probing cell structure and function, as well as practically manipulating cells in genetics, oncology, and developmental biology.

Optical trapping and manipulation of neutral particles using lasers

The techniques of optical trapping and manipulation of neutral particles by lasers provide unique means to control the dynamics of small particles. These new experimental methods have played a revolutionary role in areas of the physical and biological sciences. This paper reviews the early developments in the field leading to the demonstration of cooling and trapping of neutral atoms in atomic physics and to the first use of optical tweezers traps in biology. Some further major achievements of these rapidly developing methods also are considered.

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.

Cut out or poke in--the key to the world of single genes: laser micromanipulation as a valuable tool on the look-out for the origin of disease

The optical micromanipulation systems UV(ultraviolet)-Laser Microbeam and Optical Tweezers Trap, already proven to be powerful tools for 'non-contact' micro-manipulation of gametes, cells and organelles, have now made their way into the nanocosmos of genes and molecules. Force measurements of DNA transcription have been performed and selective DNA molecule micromanipulation gives insight into single molecule behaviour. Retrievement of selected single cells without contamination is an import prerequisite for further processing with modern methods of molecular biology. Laser micro-dissection allows to precisely eliminate any unwanted material or to isolate pieces of chromosomes or single cells of interest with high accuracy and efficiency. This enables the cell or chromosome specific molecular analysis of genes and genetic defects underlying disease, such as cancer or infection. This review article gives an overview of current topics of laser microbeam application in biological or medical research and advanced molecular diagnosis.

Forces on chromosomal DNA during anaphase

In the course of anaphase, the chromosomal DNA is submitted to the traction of the spindle. Several physical problems are associated with this action. In particular, the sister chromatids are generally topologically intertwined at the onset of anaphase, and the removal of the intertwinings results from a coupling between the enzymatic action of type II DNA topoisomerases and the force exerted by the spindle. We propose a physical analysis of some of these problems: 1) We compare the maximum force the spindle can produce with the force required to break a DNA molecule, and define the conditions compatible with biological safety during anaphase. 2) We show that the behavior of the sister chromatids in the absence of type II DNA topoisomerases can be described by two distinct models: a chain pullout model accounts for the experimental observations made in the budding yeast, and a model of the mechanical rupture of rubbers accounts for the nondisjunction in standard cases. 3) Using the fluctuation-dissipation theorem, we introduce an effective protein friction associated with the strand-passing activity of type II DNA topoisomerases. We show that this friction can be used to describe the situation in which one chromosome passes entirely through another one. Possible experiments that could test these theoretical analyses are discussed.