Micromanipulation of mitotic chromosomes in PTK2 cells using laser-induced optical forces ("optical tweezers")

Shining Light in Mechanobiology: Optical Tweezers, Scissors, and Beyond

Mechanobiology helps us to decipher cell and tissue functions by looking at changes in their mechanical properties that contribute to development, cell differentiation, physiology, and disease. Mechanobiology sits at the interface of biology, physics and engineering. One of the key technologies that enables characterization of properties of cells and tissue is microscopy. Combining microscopy with other quantitative measurement techniques such as optical tweezers and scissors, gives a very powerful tool for unraveling the intricacies of mechanobiology enabling measurement of forces, torques and displacements at play. We review the field of some light based studies of mechanobiology and optical detection of signal transduction ranging from optical micromanipulation-optical tweezers and scissors, advanced fluorescence techniques and optogenentics. In the current perspective paper, we concentrate our efforts on elucidating interesting measurements of forces, torques, positions, viscoelastic properties, and optogenetics inside and outside a cell attained when using structured light in combination with optical tweezers and scissors. We give perspective on the field concentrating on the use of structured light in imaging in combination with tweezers and scissors pointing out how novel developments in quantum imaging in combination with tweezers and scissors can bring to this fast growing field.

PICH acts as a force-dependent nucleosome remodeler

In anaphase, any unresolved DNA entanglements between the segregating sister chromatids can give rise to chromatin bridges. To prevent genome instability, chromatin bridges must be resolved prior to cytokinesis. The SNF2 protein PICH has been proposed to play a direct role in this process through the remodeling of nucleosomes. However, direct evidence of nucleosome remodeling by PICH has remained elusive. Here, we present an in vitro single-molecule assay that mimics chromatin under tension, as is found in anaphase chromatin bridges. Applying a combination of dual-trap optical tweezers and fluorescence imaging of PICH and histones bound to a nucleosome-array construct, we show that PICH is a tension- and ATP-dependent nucleosome remodeler that facilitates nucleosome unwrapping and then subsequently slides remaining histones along the DNA. This work elucidates the role of PICH in chromatin-bridge dissolution, and might provide molecular insights into the mechanisms of related SNF2 proteins.

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.

Optical Tweezers: Phototoxicity and Thermal Stress in Cells and Biomolecules

For several decades optical tweezers have proven to be an invaluable tool in the study and analysis of myriad biological responses and applications. However, as with every tool, they can have undesirable or damaging effects upon the very sample they are helping to study. In this review the main negative effects of optical tweezers upon biostructures and living systems will be presented. There are three main areas on which the review will focus: linear optical excitation within the tweezers, non-linear photonic effects, and thermal load upon the sampled volume. Additional information is provided on negative mechanical effects of optical traps on biological structures. Strategies to avoid or, at least, minimize these negative effects will be introduced. Finally, all these effects, undesirable for the most, can have positive applications under the right conditions. Some hints in this direction will also be discussed.

Imaging and physically probing kinetochores in live dividing cells

The kinetochore mediates chromosome segregation at cell division. It is the macromolecular machine that links chromosomes to spindle microtubules, and is made of more than 100 protein species in mammalian cells. Molecular tools are presently revealing the biochemical interactions and regulatory mechanisms that ensure proper kinetochore function. Here, we discuss two approaches for imaging and physically probing kinetochores despite mitotic cell rounding and rapid kinetochore dynamics. First, we describe how mild spindle compression can improve kinetochore imaging and how stronger compression can mechanically perturb the spindle and kinetochores. Second, we describe how simultaneously imaging two-colored kinetochore reporter probes at subpixel resolution can report on kinetochore structural dynamics under cellular forces. We hope that the experimental details we provide here will make these two approaches broadly accessible and help move forward our understanding of kinetochore function--and make these approaches adaptable to the study of other cellular structures.

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.

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.

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.

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.

Physiological monitoring of optically trapped cells: assessing the effects of confinement by 1064-nm laser tweezers using microfluorometry

We report the results of microfluorometric measurements of physiological changes in optically trapped immotile Chinese hamster ovary cells (CHOs) and motile human sperm cells under continuous-wave (CW) and pulsed-mode trapping conditions at 1064 nm. The fluorescence spectra derived from the exogenous fluorescent probes laurdan, acridine orange, propidium iodide, and Snarf are used to assess the effects of optical confinement with respect to temperature, DNA structure, cell viability, and intracellular pH, respectively. In the latter three cases, fluorescence is excited via a two-photon process, using a CW laser trap as the fluorescence excitation source. An average temperature increase of < 0.1 +/- 0.30 degrees C/100 mW is measured for cells when held stationary with CW optical tweezers at powers of up to 400 mW. The same trapping conditions do not appear to alter DNA structure or cellular pH. In contrast, a pulsed 1064-nm laser trap (100-ns pulses at 40 microJ/pulse and average power of 40 mW) produced significant fluorescence spectral alterations in acridine orange, perhaps because of thermally induced DNA structural changes or laser-induced multiphoton processes. The techniques and results presented herein demonstrate the ability to perform in situ monitoring of cellular physiology during CW and pulsed laser trapping, and should prove useful in studying mechanisms by which optical tweezers and microbeams perturb metabolic function and cellular viability.

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.

In situ microparticle analysis of marine phytoplankton cells with infrared laser-based optical tweezers

We describe the application of infrared optical tweezers to the in situ microparticle analysis of marine phytoplankton cells. A Nd:YAG laser (λ= 1064 nm) trap is used to confine and manipulate single Nannochloris and Synechococcus cells in an enriched seawater medium while spectral fluorescence and Lorenz-Mie backscatter signals are simultaneously acquired under a variety of excitation and trapping conditions. Variations in the measured fluorescence intensities of chlorophyll a (Chl a) and phycoerythrin pigments in phytoplankton cells are observed. These variations are related, in part, to basic intrasample variability, but they also indicate that increasing ultraviolet-exposure time and infrared trapping power may have short-term effects on cellular physiology that are related to Chl a photobleaching and laser-induced heating, respectively. The use of optical tweezers to study the factors that affect marine cell physiology and the processes of absorption, scattering, and attenuation by individual cells, organisms, and particulate matter that contribute to optical closure on a microscopic scale are also described.

Catch and move--cut or fuse

Still almost unbelievable, but true: light exerts force. With these forces it is indeed possible to catch and move cells or small particles and microsurgically to process them without any mechanical contact. As if by magic, objects are moved via focused laser light.

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.