Controlled ablation of microtubules using a picosecond laser

Laser nanosurgery of single microtubules reveals location-dependent depolymerization rates

In this study, 532-nm picosecond and 800-nm femtosecond lasers are used in combination with fluorescently labeled tubulin to further elucidate microtubule depolymerization and the effect lasers may have on the resulting depolymerization. Depolymerization rates of targeted single microtubules are dependent on location with respect to the nucleus. Microtubules located near the nucleus exhibit a significantly faster depolymerization rate when compared to microtubule depolymerization rates near the periphery of the cell. Microtubules cut with the femtosecond laser depolymerize at a slower rate than unirradiated controls (p=0.002), whereas those cut with the picosecond laser depolymerize at the same rate as unirradiated controls (p=0.704). Our results demonstrate the ability of both the picosecond and femtosecond lasers to cut individual microtubules. The differences between the two ablation results are discussed.

Multiphoton microscopy for monitoring intratissue femtosecond laser surgery effects

BACKGROUND AND OBJECTIVE

Multiphoton microscopy/tomography has been used as a novel diagnostic method for corneal imaging with subcellular resolution. Here, we used this technique to monitor femtosecond laser intratissue surgery effects.

MATERIALS AND METHODS

Multiphoton microscopy/tomography on rabbits based on intense 90 MHz femtosecond Ti: sapphire laser was realized at intensities of MW-GW/cm(2), whereas the surgical procedures were performed with the same system at a higher light intensity on the order of TW/cm(2).

RESULTS

Multiphoton microscopy/tomography proved capable of determining the target of interest, and of visualizing and optically evaluating the in vivo intrastromal laser surgical outcomes with high efficiency. More interesting, using this technique, activated intrastromal keratocytes (myofibroblasts) were detected in vivo 24 hours after intrastromal surgery.

CONCLUSION

Multiphoton microscopy/tomography is an efficient and convenient non-invasive imaging method which can be used not only to assess intrastromal microsurgical performance but also to perform in vivo follow-up observations on wound repair.

Mechanisms of laser cellular microsurgery

This chapter reviews the optics of pulsed laser microbeams and the use of basic instrumentation to provide pulsed laser microbeam capabilities within a microscope platform. Moreover, we review the principal mechanisms by which laser microbeams produce microsurgical effects in cellular targets. We discuss the principal photothermal, photomechanical, and photochemical damage mechanisms as well as their relationship to critical laser microbeam parameters, including wavelength, pulse duration, and numerical aperture. We relate this understanding of damage mechanisms to laser microbeam applications reported in the literature.

Investigating Relaxation Processes in Cells and Developing Organisms: From Cell Ablation to Cytoskeleton Nanosurgery

Dynamic microscopy of living cells and organisms alone does not reveal the high level of complexity of cellular and subcellular organization. All observable processes rely on the activity of biochemical and biophysical processes and many occur at a physiological equilibrium. Experimentally, it is not trivial to apply a perturbation that targets a specific process without perturbing the overall equilibrium of a cell. Drugs and more recently RNAi certainly have general and undesired effects on cell physiology and metabolism. In particular, they affect the entire cell. Pulsed lasers allow to severe biological tissues with a precision in the range of hundreds of nanometers and to achieve ablation on the level of a single cell or a subcellular compartment. In this chapter, we present an efficient implementation of a picosecond UV-A pulsed laser-based nanosurgery system and review the different mechanisms of ablation that can be achieved at different levels of cellular organization. We discuss the performance of the ablation process in terms of the energy deposited onto the sample and compare our implementation to others recently employed for cellular and subcellular surgery. Above the energy threshold of ionization, we demonstrate how to achieve single-cell ablation through the induction of mechanical perturbation and cavitation in living organisms. Below this threshold, we induce cytoskeleton severing inside live cells. By combining nanosurgery with fast live-imaging fluorescence microscopy, we show how the apparent equilibrium of the cytoskeleton can be perturbed regionally inside a cell.

La nanochirurgie laser en biologie cellulaire

Since their first use in the early 60's, pulsed lasers have become increasingly popular for their ability to ablate biological tissue. Short laser pulses allow high precision surgery for biological and medical applications with minimal invasiveness. Performing highly targeted manipulation and ablation allows experiments impossible so far in development biology, cellular biology or even assisted reproductive technologies and laser surgery has been increasingly used over the last five years to answer key questions in Biology. Recently, picosecond UV and femtosecond IR laser pulses have been used to cleave microtubules and to severe actin stress fibers in vivo with a spatial precision in the submicrometer range to study their dynamics without affecting cell viability. We review recent findings on the underlying principles of pulsed laser nanosurgery mechanisms showing how the use of ultra short laser pulses increases precision and non-invasiveness of laser surgery. We show how the understanding of the surgical process allows one to distinguish between single cell ablation in living organisms or intracellular nanosurgery in living cells and we review recent applications to the study of forces and the quantification of cytoskeleton dynamics.

Mechanical forces alter zyxin unbinding kinetics within focal adhesions of living cells

The formation of focal adhesions that mediate alterations of cell shape and movement is controlled by a mechanochemical mechanism in which cytoskeletal tensional forces drive changes in molecular assembly; however, little is known about the molecular biophysical basis of this response. Here, we describe a method to measure the unbinding rate constant k(OFF) of individual GFP-labeled focal adhesion molecules in living cells by modifying the fluorescence recovery after photobleaching (FRAP) technique and combining it with mathematical modeling. Using this method, we show that decreasing cellular traction forces on focal adhesions by three different techniques--chemical inhibition of cytoskeletal tension generation, laser incision of an associated actin stress fiber, or use of compliant extracellular matrices--increases the k(OFF) of the focal adhesion protein zyxin. In contrast, the k(OFF) of another adhesion protein, vinculin, remains unchanged after tension dissipation. Mathematical models also demonstrate that these force-dependent increases in zyxin's k(OFF) that occur over seconds are sufficient to quantitatively predict large-scale focal adhesion disassembly that occurs physiologically over many minutes. These findings demonstrate that the molecular binding kinetics of some, but not all, focal adhesion proteins are sensitive to mechanical force, and suggest that force-dependent changes in this biophysical parameter may govern the supramolecular events that underlie focal adhesion remodeling in living cells.

Viscoelastic retraction of single living stress fibers and its impact on cell shape, cytoskeletal organization, and extracellular matrix mechanics

Cells change their form and function by assembling actin stress fibers at their base and exerting traction forces on their extracellular matrix (ECM) adhesions. Individual stress fibers are thought to be actively tensed by the action of actomyosin motors and to function as elastic cables that structurally reinforce the basal portion of the cytoskeleton; however, these principles have not been directly tested in living cells, and their significance for overall cell shape control is poorly understood. Here we combine a laser nanoscissor, traction force microscopy, and fluorescence photobleaching methods to confirm that stress fibers in living cells behave as viscoelastic cables that are tensed through the action of actomyosin motors, to quantify their retraction kinetics in situ, and to explore their contribution to overall mechanical stability of the cell and interconnected ECM. These studies reveal that viscoelastic recoil of individual stress fibers after laser severing is partially slowed by inhibition of Rho-associated kinase and virtually abolished by direct inhibition of myosin light chain kinase. Importantly, cells cultured on stiff ECM substrates can tolerate disruption of multiple stress fibers with negligible overall change in cell shape, whereas disruption of a single stress fiber in cells anchored to compliant ECM substrates compromises the entire cellular force balance, induces cytoskeletal rearrangements, and produces ECM retraction many microns away from the site of incision; this results in large-scale changes of cell shape (> 5% elongation). In addition to revealing fundamental insight into the mechanical properties and cell shape contributions of individual stress fibers and confirming that the ECM is effectively a physical extension of the cell and cytoskeleton, the technologies described here offer a novel approach to spatially map the cytoskeletal mechanics of living cells on the nanoscale.

In vivo selective cytoskeleton dynamics quantification in interphase cells induced by pulsed ultraviolet laser nanosurgery

We report on the manipulation of intracellular filaments using a nanosurgery system based on a subnanosecond pulsed UV laser optimized for the localized severing of biological polymers. By inducing artificial catastrophe of selected microtubules (MTs), we perform shrinkage-rate measurements in interphase Ptk-2 cells throughout the entire cell. We quantify the impact of two labeling methods and three fluorescent markers, showing a 25% faster depolymerization with Alexa-488 tubulin compared with Rhodamine and yellow fluorescent protein (YFP) tubulins and a 20% higher variability induced by microinjection compared with stable transfection. Using EB3-GFP as a tip marker, we establish a new protocol to measure shrinkage rate, growth rate and rescue frequency simultaneously with high temporal and spatial specificity in live cells. As our analysis shows, laser-induced MT dynamics are physiologically relevant. The high statistical efficiency that the method offers in terms of numbers of measured events and therefore reduced standard deviations represents an important quantitative improvement in the measurement of dynamic instability parameters in vivo. We extend the application of the method by demonstrating induced dynamic behavior of actin-stress fibers after severing. This new method enables the quantitative investigation of cytoskeleton dynamics in a local confinement.

Internet-based robotic laser scissors and tweezers microscopy

We have engineered a robotic laser ablation and tweezers microscope that can be operated via the internet using most internet accessible devices, including laptops, desktop computers, and personal data assistants (PDAs). The system affords individual investigators the ability to conduct micromanipulation experiments (cell surgery or trapping) from remote locations (i.e., between the US and Australia). This system greatly expands the availability of complex and expensive research technologies via investigator-networking over the internet. It serves as a model for other "internet-friendly" technologies leading to large scale networking and data-sharing between investigators, groups, and institutions on a global scale. The system offers three unique features: (1) the freedom to operate the system from any internet-capable computer, (2) the ability to image, ablate, and/or trap cells and their organelles by "remote-control," and (3) the security and convenience of controlling the system in the laboratory on the user's own personal computer and not on the host machine. Four "proof of principle" experiments were conducted: (1) precise control of microscope movement and live cell visualization, (2) subcellular microsurgery on the microtubule organizing center of live cells viewed under phase contrast and fluorescence microscopy, (3) precise targeting of multiple sites within single red blood cells, and (4) optical trapping of 10 microm diameter polystyrene microspheres.

Within the cell: analytical techniques for subcellular analysis

This review covers recent developments in the preparation, manipulation, and analyses of subcellular environments. In particular, it highlights approaches for (1) separation and detection of individual organelles, (2) preparation of ultra-pure organelle fractions, and (3) utilization of novel labeling strategies. These approaches, based on innovative technologies such as microfluidics, immunoisolation, mass spectrometry and electrophoresis, suggest that subcellular analyses will soon become as commonplace as single cell and bulk cellular assays.

Cell cycle dependence of DNA-dependent protein kinase phosphorylation in response to DNA double strand breaks

DNA-dependent protein kinase (DNA-PK), consisting of Ku and DNA-PKcs subunits, is the key component of the non-homologous end-joining (NHEJ) pathway of DNA double strand break (DSB) repair. Although the kinase activity of DNA-PKcs is essential for NHEJ, thus far, no in vivo substrate has been conclusively identified except for an autophosphorylation site on DNA-PKcs itself (threonine 2609). Here we report the ionizing radiation (IR)-induced autophosphorylation of DNA-PKcs at a novel site, serine 2056, the phosphorylation of which is required for the repair of DSBs by NHEJ. Interestingly, IR-induced DNA-PKcs autophosphorylation is regulated in a cell cycle-dependent manner with attenuated phosphorylation in the S phase. In contrast, DNA replication-associated DSBs resulted in DNA-PKcs autophosphorylation and localization to DNA damage sites. These results indicate that although IR-induced DNA-PKcs phosphorylation is attenuated in the S phase, DNA-PKcs is preferentially activated by the physiologically relevant DNA replication-associated DSBs at the sites of DNA synthesis.