Repetition rate dependency of reactive oxygen species formation during femtosecond laser-based cell surgery

Femtosecond laser oocyte enucleation as a low-invasive and effective method of recipient cytoplast preparation

Recipient cytoplast preparation, commonly performed by DNA aspiration with a needle, inevitably leads to the loss of reprogramming factors. As an alternative to the traditional enucleation technique, femtosecond laser enucleation can eliminate DNA effectively without loss of reprogramming factors and without oocyte puncturing. In this work we have performed oocyte enucleation by destructing the metaphase plate using a 795 nm femtosecond laser. The disability of the enucleated oocytes to develop after the parthenogenetic activation, as well as the lack of DNA staining luminescence, strongly confirms the efficiency of the femtosecond laser enucleation. The parthenogenetic development of oocytes after the cytoplasm treatment suggests a low-invasive effect of the laser enucleation technique.

Femtosecond Plasmonic Laser Nanosurgery (fs-PLN) mediated by molecularly targeted gold nanospheres at ultra-low pulse fluences

Plasmonic Laser Nanosurgery (PLN) is a novel photomodification technique that exploits the near-field enhancement of femtosecond (fs) laser pulses in the vicinity of gold nanoparticles. While prior studies have shown the advantages of fs-PLN to modify cells, further reduction in the pulse fluence needed to initiate photomodification is crucial to facilitate deep–tissue treatments. This work presents an in-depth study of fs-PLN at ultra-low pulse fluences using 47 nm gold nanoparticles, conjugated to antibodies that target the epithelial growth factor receptor and excited off-resonance using 760 nm, 270 fs laser pulses at 80 MHz repetition rate. We find that fs-PLN can optoporate cellular membranes with pulse fluences as low as 1.3 mJ/cm², up to two orders of magnitude lower than those used at lower repetition rates. Our results, corroborated by simulations of free-electron generation by particle photoemission and photoionization of the surrounding water, shed light on the off-resonance fs-PLN mechanism. We suggest that photo-chemical pathways likely drive cellular optoporation and cell damage at these off-resonance, low fluence, and high repetition rate fs-laser pulses, with clusters acting as local concentrators of ROS generation. We believe that the low fluence and highly localized ROS-mediated fs-PLN approach will enable targeted therapeutics and cancer treatment.

Femtosecond laser-based nanosurgery reveals the endogenous regeneration of single Z-discs including physiological consequences for cardiomyocytes

A highly organized cytoskeleton architecture is the basis for continuous and controlled contraction in cardiomyocytes (CMs). Abnormalities in cytoskeletal elements, like the Z-disc, are linked to several diseases. It is challenging to reveal the mechanisms of CM failure, endogenous repair, or mechanical homeostasis on the scale of single cytoskeletal elements. Here, we used a femtosecond (fs) laser to ablate single Z-discs in human pluripotent stem cells (hPSC) -derived CMs (hPSC-CM) and neonatal rat CMs. We show, that CM viability was unaffected by the loss of a single Z-disc. Furthermore, more than 40% of neonatal rat and 68% of hPSC-CMs recovered the Z-disc loss within 24 h. Significant differences to control cells, after the Z-disc loss, in terms of cell perimeter, x- and y-expansion and calcium homeostasis were not found. Only 14 days in vitro old hPSC-CMs reacted with a significant decrease in cell area, x- and y-expansion 24 h past nanosurgery. This demonstrates that CMs can compensate the loss of a single Z-disc and recover a regular sarcomeric pattern during spontaneous contraction. It also highlights the significant potential of fs laser-based nanosurgery to physically micro manipulate CMs to investigate cytoskeletal functions and organization of single elements.

Effects of cell state and staining on femtosecond laser nanosurgery

Femtosecond laser nanosurgery enables precise manipulation of subcellular elements to study regeneration. However, currently it is not frequently employed-probably because of its unknown consequences on the whole cell level. To better understand the associated biological response of the cell, especially in the context of different cell states and cell staining, we manipulated C2C12 myoblasts and myotubes, which were either unstained (nicotinamide adenine dinucleotide signal) or stained with MitoTracker Red. Both signals overlap well and stain similar areas in untreated cells. We chose 3 different cutting lengths and performed surgery in the cytosol along the major cell axis. The cuts resealed within several minutes independent of the cutting length. We analyzed cell area, perimeter, major and minor axis on long term. We observed significant changes in the cell area and perimeter, dependent on the staining and more pronounced in differentiated myotubes. We conclude, that laser parameters must be chosen carefully, depending on the staining of the cell, its (differentiation) state, and the extent of the cut region, such that unwanted cell responses can be avoided. Laser manipulation of C2C12 myotubes with small ablation (0.8 μm) and large ablation (3.0 μm). While small damages recover, larger damages lead to elimination from the syncytium. Scale bar: 20 μm.

Molecular response of mitochondria to a short-duration femtosecond-laser stimulation

The research of mitochondrial dysfunction is of great importance and implicated in a range of neurodegenerative diseases. Traditionally, to investigate mitochondrial dynamics and functions, mitochondria are usually stimulated indirectly by treating cells with exogenous chemicals like oxidative agents. Such treatment lacks precision and controllability, and will simultaneously activate unknown complex cell processes. In this study, we report that two-photon 100-μs line scan by a femtosecond laser can induce restorable fragmentation or swelling of any targeted mitochondria instead of ablation or disruption. It can be defined by a customized two-photon line scan and inserted into any microscopy sequence as a single frame. The mitochondrial response is dependent on the peak power of laser pulses, cellular oxidative environment, and membrane permeability transition pores of mitochondria. The translocation of cytochrome C and Bax can be regulated by the photostimulation. Moreover, significant upregulation of Bcl-2 can be observed if the whole cell is stimulated. Those results suggest the mitochondrial and molecular response to photostimulation is quite complex. This femtosecond-laser stimulation method can thus provide a very noninvasive, precise, and controllable method to stimulate single target mitochondria for related biological research.

Practical 24-fs, 1-μJ, 1-MHz Yb-fiber laser amplification system

We develop a practical femtosecond polarization-maintaining fiber laser amplification system with a standard double-cladding fiber technique, enabling 24-fs transform-limited pulses with 1-μJ pulse energy at a 1-MHz repetition rate. The laser system is based on a hybrid amplification scheme. Chirped-pulse amplification is employed in the pre-amplifier stage to supply high-quality pulses with enough energy for the main-amplifier, where nonlinear amplification is utilized to broaden the output spectrum. To obtain a dechirped pulse with high quality and short duration, a pre-shaper is inserted between the two amplification stages to adjust the pre-chirp, central wavelength, and pulse energy of the signal pulses in the main amplifier for optimizing pulse evolution. As a result, temporal pedestal free sub-ten-cycle high-energy laser pulses can be routinely obtained. In the end, the advantages of this novel laser source are demonstrated in the experiments on enhanced damage effect to cells co-cultured with gold nanorods.

Mitochondrial swelling and restorable fragmentation stimulated by femtosecond laser

Mitochondria play a key role in all cellular physiology, processes, and behaviors. It is very difficult to precisely stimulate single mitochondria noninvasively in traditional biomedical research. In this study, we report that femtosecond laser can stimulate fragmentation or swelling of single mitochondria in human mesenchymal stem cells rather than physical disruption or ablation. In experiments, fragmented mitochondria can recover normal very soon but swelling ones cannot. At the same time, laser-induced generation of mitochondrial reactive oxygen species and opening of mitochondria permeability transition pores are involved in mitochondrial responses to photostimulation. Furthermore, the localized translocation of proapoptotic molecules are found in those stimulated mitochondria. Those results suggest femtosecond-laser photostimulation as a noninvasive and precise method for mitochondrial manipulation and related research.

Optical induction of muscle contraction at the tissue scale through intrinsic cellular amplifiers

The smooth muscle cell is the principal component responsible for involuntary control of visceral organs, including vascular tonicity, secretion, and sphincter regulation. It is known that the neurotransmitters released from nerve endings increase the intracellular Ca(2+) level in smooth muscle cells followed by muscle contraction. We herein report that femtosecond laser pulses focused on the diffraction-limited volume can induce intracellular Ca(2+) increases in the irradiated smooth muscle cell without neurotransmitters, and locally increased intracellular Ca(2+) levels are amplified by calcium-induced calcium-releasing mechanisms through the ryanodine receptor, a Ca(2+) channel of the endoplasmic reticulum. The laser-induced Ca(2+) increases propagate to adjacent cells through gap junctions. Thus, ultrashort-pulsed lasers can induce smooth muscle contraction by controlling Ca(2+), even with optical stimulation of the diffraction-limited volume. This optical method, which leads to reversible and reproducible muscle contraction, can be used in research into muscle dynamics, neuromuscular disease treatment, and nanorobot control.

Femtosecond laser axotomy in Caenorhabditis elegans and collateral damage assessment using a combination of linear and nonlinear imaging techniques

In this work highly localized femtosecond laser ablation is used to dissect single axons within a living Caenorhabditis elegans (C. elegans). We present a multimodal imaging methodology for the assessment of the collateral damage induced by the laser. This relies on the observation of the tissues surrounding the targeted region using a combination of different high resolution microscopy modalities. We present the use of Second Harmonic Generation (SHG) and Polarization Sensitive SHG (PSHG) to determine damage in the neighbor muscle cells. All the above is done using a single instrument: multimodal microscopy setup that allows simultaneous imaging in the linear and non-linear regimes and femtosecond-laser ablation.

Exploring the ultrashort pulse laser parameter space for membrane permeabilisation in mammalian cells

The use of ultrashort femtosecond pulsed lasers to effect membrane permeabilisation and initiate both optoinjection and transfection of cells has recently seen immense interest. We investigate femtosecond laser-induced membrane permeabilisation in mammalian cells as a function of pulse duration, pulse energy and number of pulses, by quantifying the efficiency of optoinjection for these parameters. Depending on pulse duration and pulse energy we identify two distinct membrane permeabilisation regimes. In the first regime a nonlinear dependence of order 3.4-9.6 is exhibited below a threshold peak power of at least 6 kW. Above this threshold peak power, the nonlinear dependence is saturated resulting in linear behaviour. This indicates that the membrane permeabilisation mechanism requires efficient multiphoton absorption to produce free electrons but once this process saturates, linear absorption dominates. Our experimental findings support a previously proposed theoretical model and provide a step towards the optimisation of laser-mediated gene delivery into mammalian cells.

Mechanisms of nanoparticle-mediated photomechanical cell damage

Laser-assisted killing of gold nanoparticle targeted macrophages was investigated. Using pressure transient detection, flash photography and transmission electron microscopy (TEM) imaging, we studied the mechanism of single cell damage by vapor bubble formation around gold nanospheres induced by nanosecond laser pulses. The influence of the number of irradiating laser pulses and of particle size and concentration on the threshold for acute cell damage was determined. While the single pulse damage threshold is independent of the particle size, the threshold decreases with increasing particle size when using trains of pulses. The dependence of the cell damage threshold on the nanoparticle concentration during incubation reveals that particle accumulation and distribution inside the cell plays a key role in tissue imaging or cell damaging.

Local small airway epithelial injury induces global smooth muscle contraction and airway constriction

Small airway epithelial cells form a continuous sheet lining the conducting airways, which serves many functions including a physical barrier to protect the underlying tissue. In asthma, injury to epithelial cells can occur during bronchoconstriction, which may exacerbate airway hyperreactivity. To investigate the role of epithelial cell rupture in airway constriction, laser ablation was used to precisely rupture individual airway epithelial cells of small airways (<300-μm diameter) in rat lung slices (∼250-μm thick). Laser ablation of single epithelial cells using a femtosecond laser reproducibly induced airway contraction to ∼70% of the original cross-sectional area within several seconds, and the contraction lasted for up to 40 s. The airway constriction could be mimicked by mechanical rupture of a single epithelial cell using a sharp glass micropipette but not with a blunt glass pipette. These results suggest that soluble mediators released from the wounded epithelial cell induce global airway contraction. To confirm this hypothesis, the lysate of primary human small airway epithelial cells stimulated a similar airway contraction. Laser ablation of single epithelial cells triggered a single instantaneous Ca(2+) wave in the epithelium, and multiple Ca(2+) waves in smooth muscle cells, which were delayed by several seconds. Removal of extracellular Ca(2+) or decreasing intracellular Ca(2+) both blocked laser-induced airway contraction. We conclude that local epithelial cell rupture induces rapid and global airway constriction through release of soluble mediators and subsequent Ca(2+)-dependent smooth muscle shortening.

Influence of laser parameters and staining on femtosecond laser-based intracellular nanosurgery

Femtosecond (fs) laser-based intracellular nanosurgery has become an important tool in cell biology, albeit the mechanisms in the so-called low-density plasma regime are largely unknown. Previous calculations of free-electron densities for intracellular surgery used water as a model substance for biological media and neglected the presence of dye and biomolecules. In addition, it is still unclear on which time scales free-electron and free-radical induced chemical effects take place in a cellular environment. Here, we present our experimental study on the influence of laser parameters and staining on the intracellular ablation threshold in the low-density plasma regime. We found that the ablation effect of fs laser pulse trains resulted from the accumulation of single-shot multiphoton-induced photochemical effects finished within a few nanoseconds. At the threshold, the number of applied pulses was inversely proportional to a higher order of the irradiance, depending on the laser repetition rate and wavelength. Furthermore, fluorescence staining of subcellular structures before surgery significantly decreased the ablation threshold. Based on our findings, we propose that dye molecules are the major source for providing seed electrons for the ionization cascade. Consequently, future calculations of free-electron densities for intracellular nanosurgery have to take them into account, especially in the calculations of multiphoton ionization rates.

Quantitative phase study of the dynamic cellular response in femtosecond laser photoporation

We use Digital Holographic Microscopy to study dynamic responses of live cells to femtosecond laser cellular membrane photoporation. Temporal and spatial characteristics of morphological changes as well as dry mass variation are analyzed and compared with conventional fluorescent assays for viability and photoporation efficiency. With the latter, the results provide a new insight into the efficiency and toxicity of this novel optical method of drug delivery. In addition, quantitative phase maps reveal photoporation related sub-cellular dynamics of cytoplasmic vesicles.

Monitoring of laser micromanipulated optically trapped cells by digital holographic microscopy

For a precise manipulation of particles and cells with laser light as well as for the understanding and the control of the underlying processes it is important to visualize and quantify the response of the specimens. Thus, we investigated if digital holographic microscopy (DHM) can be used in combination with microfluidics to observe optically trapped living cells in a minimally invasive fashion during laser micromanipulation. The obtained results demonstrate that DHM multi-focus phase contrast provides label-free quantitative monitoring of optical manipulation with a temporal resolution of a few milliseconds.

Fs-laser-induced Ca2+ concentration change during membrane perforation for cell transfection

Fs-laser based opto-perforation is a gentle method for gene transfer into sensitive cells such as stem cells or primary cells. The high selectivity and the low damage to the cell lead to a high efficiency of transfection. However, there are side effects which induce stress to the cell due to the exchange of intra- and extracellular media as well as the disintegration of the structure of biomolecules resulting from the laser exposure. Moreover, the mechanisms of the optical transfection are still unclear. In this paper, we present our study on calcium (Ca(2+)) homeostasis during cell surgery, especially during laser induced membrane perforation. We show that the manipulation of cells can induce an increase in the cytosolic Ca(2+) concentration. This increase was not observed if the manipulation of the cells was performed in absence of the extracellular calcium indicating the importance of the Ca(2+) uptake. We found, that the uptake of extracellular Ca(2+) strongly depends on the repetition rate and the irradiation time of the laser pulses. The exposure for several seconds to kHz pulses even induces Ca(2+) induced Ca(2+) release. Dependent on the location of perforation, probably in the vicinity of an intracellular Ca(2+) stock, an instantaneous intracellular Ca(2+) release can be induced. Since Ca(2+) could be involved in negative side effect by cell surgery, we propose an application of the optoperforation technique in nominal Ca(2+)-free external solution.