Reversible permeabilization using high-intensity femtosecond laser pulses: applications to biopreservation

Femtosecond Laser Pulse Ablation of Sub-Cellular Drusen-Like Deposits

Age-related macular degeneration (AMD) is a condition affecting the retina and is the leading cause of vision loss. Dry AMD is caused by the accumulation of lipid deposits called drusen, which form under the retina. This work demonstrates, for the first time, the removal of drusen-like deposits underneath ARPE-19 cell layers using femtosecond laser pulses. A novel cell culture model was created in response to the limited access to primary cell lines and the absence of animal models that recapitulate all aspects of AMD. In the cell culture model, deposits were identified with fluorescent stains specific to known deposit constituents. Trains of sub-10 femtosecond laser pulses from a Ti:Sapphire laser were used to successfully ablate the deposits without causing damage to surrounding cells. This drusen removal method can be used as a potential treatment for dry-stage AMD.

Me2SO- and serum-free cryopreservation of human umbilical cord mesenchymal stem cells using electroporation-assisted delivery of sugars

Cryopreservation is the universal technology used to enable long-term storage and continuous availability of cell stocks and tissues for regenerative medicine demands. The main components of standard freezing media are dimethyl sulfoxide (hereinafter Me₂SO) and fetal bovine serum (FBS). However, for manufacturing of cells and tissue-engineered products in accordance with the principles of Good Manufacturing Practice (GMP), current considerations in regenerative medicine suggest development of Me₂SO- and serum-free biopreservation strategies due to safety concerns over Me₂SO-induced side effects and immunogenicity of animal serum. In this work, the effect of electroporation-assisted pre-freeze delivery of sucrose, trehalose and raffinose into human umbilical cord mesenchymal stem cells (hUCMSCs) on their post-thaw survival was investigated. The optimal strength of electric field at 8 pulses with 100 μs duration and 1 Hz pulse repetition frequency was determined to be 1.5 kV/cm from permeabilization (propidium iodide uptake) vs. cell recovery data (resazurin reduction assay). Using sugars as sole cryoprotectants with electroporation, concentration-dependent increase in cell survival was observed. Irrespective of sugar type, the highest cell survival (up to 80%) was achieved at 400 mM extracellular concentration and electroporation. Cell freezing without electroporation yielded significantly lower survival rates. In the optimal scenario, cells were able to attach 24 h after thawing demonstrating characteristic shape and sugar-loaded vacuoles. Application of 10% Me₂SO/90% FBS as a positive control provided cell survival exceeding 90%. Next, high glass transition temperatures determined for optimal concentrations of sugars by differential scanning calorimetry (DSC) suggest the possibility to store samples at -80 °C. In summary, using electroporation to incorporate cryoprotective sugars into cells is an effective strategy towards Me₂SO- and serum-free cryopreservation and may pave the way for further progress in establishing clinically safe biopreservation strategies for efficient long-term biobanking of cells.

The Use of Antifreeze Proteins in the Cryopreservation of Gametes and Embryos

The cryopreservation of gametes and embryos is a technique widely used in reproductive biology. This technology helps in the reproductive management of domesticated animals, and it is an important tool for gene banking and for human-assisted reproductive technologies. Antifreeze proteins are naturally present in several organisms exposed to subzero temperatures. The ability for these proteins to inhibit ice recrystallization together with their ability to interact with biological membranes makes them interesting molecules to be used in cryopreservation protocols. This mini-review provides a general overview about the use of antifreeze proteins to improve the short and long term storage of gametes and embryos.

Intracellular Delivery by Membrane Disruption: Mechanisms, Strategies, and Concepts

Intracellular delivery is a key step in biological research and has enabled decades of biomedical discoveries. It is also becoming increasingly important in industrial and medical applications ranging from biomanufacture to cell-based therapies. Here, we review techniques for membrane disruption-based intracellular delivery from 1911 until the present. These methods achieve rapid, direct, and universal delivery of almost any cargo molecule or material that can be dispersed in solution. We start by covering the motivations for intracellular delivery and the challenges associated with the different cargo types-small molecules, proteins/peptides, nucleic acids, synthetic nanomaterials, and large cargo. The review then presents a broad comparison of delivery strategies followed by an analysis of membrane disruption mechanisms and the biology of the cell response. We cover mechanical, electrical, thermal, optical, and chemical strategies of membrane disruption with a particular emphasis on their applications and challenges to implementation. Throughout, we highlight specific mechanisms of membrane disruption and suggest areas in need of further experimentation. We hope the concepts discussed in our review inspire scientists and engineers with further ideas to improve intracellular delivery.

Characterization of femtosecond-laser pulse induced cell membrane nanosurgical attachment

This article provides insight into the mechanism of femtosecond laser nanosurgical attachment of cells. We have demonstrated that during the attachment of two retinoblastoma cells using sub-10 femtosecond laser pulses, with 800 nm central wavelength, the phospholipid molecules of both cells hemifuse and form one shared phospholipid bilayer, at the attachment location. In order to verify the hypothesis that hemifusion takes place, transmission electron microscope images of the cell membranes of retinoblastoma cells were taken. It is shown that at the attachment interface, the two cell membranes coalesce and form one single membrane shared by both cells. Thus, further evidence is provided to support the hypothesis that laser-induced ionization process led to an ultrafast reversible destabilization of the phospholipid layer of the cellular membrane, which resulted in cross-linking of the phospholipid molecules in each membrane. This process of hemifusion occurs throughout the entire penetration depth of the femtosecond laser pulse train. Thus, the attachment between the cells takes place across a large surface area, which affirms our findings of strong physical attachment between the cells. The femtosecond laser pulse hemifusion technique can potentially provide a platform for precise molecular manipulation of cellular membranes. Manipulation of the cellular membrane is an important procedure that could aid in studying diseases such as cancer; where the expression level of plasma proteins on the cell membrane is altered.

All-in-one processing of heterogeneous human cell grafts for gene and cell therapy

Current cell processing technologies for gene and cell therapies are often slow, expensive, labor intensive and are compromised by high cell losses and poor selectivity thus limiting the efficacy and availability of clinical cell therapies. We employ cell-specific on-demand mechanical intracellular impact from laser pulse-activated plasmonic nanobubbles (PNB) to process heterogeneous human cell grafts ex vivo with dual simultaneous functionality, the high cell type specificity, efficacy and processing rate for transfection of target CD3+ cells and elimination of subsets of unwanted CD25+ cells. The developed bulk flow PNB system selectively processed human cells at a rate of up to 100 million cell/minute, providing simultaneous transfection of CD3+ cells with the therapeutic gene (FKBP12(V36)-p30Caspase9) with the efficacy of 77% and viability 95% (versus 12 and 60%, respectively, for standard electroporation) and elimination of CD25+ cells with 99% efficacy. PNB flow technology can unite and replace several methodologies in an all-in-one universal ex vivo simultaneous procedure to precisely and rapidly prepare a cell graft for therapy. PNB’s can process various cell systems including cord blood, stem cells, and bone marrow.

Novel Method for Neuronal Nanosurgical Connection

Neuronal injury may cause an irreversible damage to cellular, organ and organism function. While preventing neural injury is ideal, it is not always possible. There are multiple etiologies for neuronal injury including trauma, infection, inflammation, immune mediated disorders, toxins and hereditary conditions. We describe a novel laser application, utilizing femtosecond laser pulses, in order to connect neuronal axon to neuronal soma. We were able to maintain cellular viability, and demonstrate that this technique is universal as it is applicable to multiple cell types and media.

Dry state preservation of nucleated cells: progress and challenges

Effective stabilization of nucleated cells for dry storage would be a transformative development in the field of cell-based biosensors and biotechnologic devices, as well as regenerative medicine and other areas in which stem cells have clinical utility. Ultimately, the tremendous promise of cell-based products will only be fully realized when stable long-term storage becomes available without the use of liquid nitrogen and bulky, energetically expensive freezers. Significant progress has been made over the last 10 years toward this goal, but obstacles still remain. Loading cells with the protective disaccharide trehalose has been achieved by several different techniques and has been shown to increase cell survival at low water contents. Likewise, the protective effect of heat shock proteins and other compounds have also been explored alone and in combination with trehalose. In some cases, the benefit of these molecules is seen not initially upon rehydration, but over time during cellular recovery. Other considerations, such as inhibiting apoptosis and utilizing isotonic buffer conditions have also provided stepwise increases in cell viability and function following drying and rehydration. In all these cases, however, a low level of residual water is required to achieve viability after rehydration. The most significant remaining challenge is to protect nucleated cells such that this residual water can be safely removed, thus allowing vitrification of intra- and extracellular trehalose and stable dry state storage at room temperature.

Femtosecond optoinjection of intact tobacco BY-2 cells using a reconfigurable photoporation platform

A tightly-focused ultrashort pulsed laser beam incident upon a cell membrane has previously been shown to transiently increase cell membrane permeability while maintaining the viability of the cell, a technique known as photoporation. This permeability can be used to aid the passage of membrane-impermeable biologically-relevant substances such as dyes, proteins and nucleic acids into the cell. Ultrashort-pulsed lasers have proven to be indispensable for photoporating mammalian cells but they have rarely been applied to plant cells due to their larger sizes and rigid and thick cell walls, which significantly hinders the intracellular delivery of exogenous substances. Here we demonstrate and quantify femtosecond optical injection of membrane impermeable dyes into intact BY-2 tobacco plant cells growing in culture, investigating both optical and biological parameters. Specifically, we show that the long axial extent of a propagation invariant ("diffraction-free") Bessel beam, which relaxes the requirements for tight focusing on the cell membrane, outperforms a standard Gaussian photoporation beam, achieving up to 70% optoinjection efficiency. Studies on the osmotic effects of culture media show that a hypertonic extracellular medium was found to be necessary to reduce turgor pressure and facilitate molecular entry into the cells.

Femtosecond optical transfection of individual mammalian cells

Laser-mediated gene transfection into mammalian cells has recently emerged as a powerful alternative to more traditional transfection techniques. In particular, the use of a femtosecond-pulsed laser operating in the near-infrared (NIR) region has been proven to provide single-cell selectivity, localized delivery, low toxicity and consistent performance. This approach can easily be integrated with advanced multimodal live-cell microscopy and micromanipulation techniques. The efficiency of this technique depends on an understanding by the user of both biology and physics. Therefore, in this protocol we discuss the subtleties that apply to both fields, including sample preparation, alignment and calibration of laser optics and their integration into a microscopy platform. The entire protocol takes ~5 d to complete, from the initial setup of the femtosecond optical transfection system to the final stage of fluorescence imaging to assay for successful expression of the gene of interest.

Cell-specific multifunctional processing of heterogeneous cell systems in a single laser pulse treatment

Current methods of cell processing for gene and cell therapies use several separate procedures for gene transfer and cell separation or elimination, because no current technology can offer simultaneous multifunctional processing of specific cell subsets in highly heterogeneous cell systems. Using the cell-specific generation of plasmonic nanobubbles of different sizes around cell-targeted gold nanoshells and nanospheres, we achieved simultaneous multifunctional cell-specific processing in a rapid single 70 ps laser pulse bulk treatment of heterogeneous cell suspension. This method supported the detection of cells, delivery of external molecular cargo to one type of cells and the concomitant destruction of another type of cells without damaging other cells in suspension, and real-time guidance of the above two cellular effects.

Plasma mediated off-resonance plasmonic enhanced ultrafast laser-induced nanocavitation

The generation of nanobubbles around plasmonic nanostructures is an efficient approach for imaging and therapy, especially in the field of cancer research. We show a novel method using infrared femtosecond laser that generates ≈800 nm bubbles around off-resonance gold nanospheres using 200 mJ/cm(2) 45 fs pulses. We present experimental and theoretical work that demonstrate that the nanobubble formation results from the generation of a nanoscale plasma around the particle due to the enhanced near-field rather than from the heating of the particle. Energy absorbed in the nanoplasma is indeed more than 11 times the energy absorbed in the particle. When compared to the usual approach that uses nanosecond laser to induce the extreme heating of in-resonance nanoparticles to initiate bubble formation, our off-resonance femtosecond technique is shown to bring many advantages, including avoiding the particles fragmentation, working in the optical window of biological material and using the deposited energy more efficiently.

Cell-specific transmembrane injection of molecular cargo with gold nanoparticle-generated transient plasmonic nanobubbles

Optimal cell therapies require efficient, selective and rapid delivery of molecular cargo into target cells without compromising their viability. Achieving these goals ex vivo in bulk heterogeneous multi-cell systems such as human grafts is impeded by low selectivity and speed of cargo delivery and by significant damage to target and non-target cells. We have developed a cell level approach for selective and guided transmembrane injection of extracellular cargo into specific target cells using transient plasmonic nanobubbles (PNB) as cell-specific nano-injectors. As a technical platform for this method we developed a laser flow cell processing system. The PNB injection method and flow system were tested in heterogeneous cell suspensions of target and non-target cells for delivery of Dextran-FITC dye into squamous cell carcinoma HN31 cells and transfection of human T-cells with a green fluorescent protein-encoding plasmid. In both models the method demonstrated single cell type selectivity, high efficacy of delivery (96% both for HN31 cells T-cells), speed of delivery (nanoseconds) and viability of treated target cells (96% for HN31 cells and 75% for T-cells). The PNB injection method may therefore be beneficial for real time processing of human grafts without removal of physiologically important cells.

Improved cellular specificity of plasmonic nanobubbles versus nanoparticles in heterogeneous cell systems

The limited specificity of nanoparticle (NP) uptake by target cells associated with a disease is one of the principal challenges of nanomedicine. Using the threshold mechanism of plasmonic nanobubble (PNB) generation and enhanced accumulation and clustering of gold nanoparticles in target cells, we increased the specificity of PNB generation and detection in target versus non-target cells by more than one order of magnitude compared to the specificity of NP uptake by the same cells. This improved cellular specificity of PNBs was demonstrated in six different cell models representing diverse molecular targets such as epidermal growth factor receptor, CD3 receptor, prostate specific membrane antigen and mucin molecule MUC1. Thus PNBs may be a universal method and nano-agent that overcome the problem of non-specific uptake of NPs by non-target cells and improve the specificity of NP-based diagnostics, therapeutics and theranostics at the cell level.

Transient photothermal spectra of plasmonic nanobubbles

The photothermal efficacy of near-infrared gold nanoparticles (NP), nanoshells, and nanorods was studied under pulsed high-energy optical excitation in plasmonic nanobubble (PNB) mode as a function of the wavelength and duration of the excitation laser pulse. PNBs, transient vapor nanobubbles, were generated around individual and clustered overheated NPs in water and living cells. Transient PNBs showed two photothermal features not previously observed for NPs: the narrowing of the spectral peaks to 1 nm and the strong dependence of the photothermal efficacy upon the duration of the laser pulse. Narrow red-shifted (relative to those of NPs) near-infrared spectral peaks were observed for 70 ps excitation laser pulses, while longer sub- and nanosecond pulses completely suppressed near-infrared peaks and blue shifted the PNB generation to the visual range. Thus, PNBs can provide superior spectral selectivity over gold NPs under specific optical excitation conditions.

Selective gene transfection of individual cells in vitro with plasmonic nanobubbles

Gene delivery and transfection of eukaryotic cells are widely used for research and for developing gene cell therapy. However, the existing methods lack selectivity, efficacy and safety when heterogeneous cell systems must be treated. We report a new method that employs plasmonic nanobubbles (PNBs) for delivery and transfection. A PNB is a novel, tunable cellular agent with a dual mechanical and optical action due to the formation of the vapor nanobubble around a transiently heated gold nanoparticle upon its exposure to a laser pulse. PNBs enabled the mechanical injection of the extracellular cDNA plasmid into the cytoplasm of individual target living cells, cultured leukemia cells and human CD34+ CD117+ stem cells and expression of a green fluorescent protein (GFP) in those cells. PNB generation and lifetime correlated with the expression of green fluorescent protein in PNB-treated cells. Optical scattering by PNBs additionally provided the detection of the target cells and the guidance of cDNA injection at single cell level. In both cell models PNBs demonstrated a gene transfection effect in a single pulse treatment with high selectivity, efficacy and safety. Thus, PNBs provided targeted gene delivery at the single cell level in a single pulse procedure that can be used for safe and effective gene therapy.

Prospects and developments in cell and embryo laser nanosurgery

Recently, there has been increasing interest in the application of femtosecond (fs) laser pulses to the study of cells, tissues and embryos. This review explores the developments that have occurred within the last several years in the fields of cell and embryo nanosurgery. Each of the individual studies presented in this review clearly demonstrates the nondestructiveness of fs laser pulses, which are used to alter both cellular and subcellular sites within simple cells and more complicated multicompartmental embryos. The ability to manipulate these model systems noninvasively makes applied fs laser pulses an invaluable tool for developmental biologists, geneticists, cryobiologists, and zoologists. We are beginning to see the integration of this tool into life sciences, establishing its status among molecular and genetic cell manipulation methods. More importantly, several studies demonstrating the versatility of applied fs laser pulses have established new collaborations among physicists, engineers, and biologists with the common intent of solving biological problems.

Single cell optical transfection

The plasma membrane of a eukaryotic cell is impermeable to most hydrophilic substances, yet the insertion of these materials into cells is an extremely important and universal requirement for the cell biologist. To address this need, many transfection techniques have been developed including viral, lipoplex, polyplex, capillary microinjection, gene gun and electroporation. The current discussion explores a procedure called optical injection, where a laser field transiently increases the membrane permeability to allow species to be internalized. If the internalized substance is a nucleic acid, such as DNA, RNA or small interfering RNA (siRNA), then the process is called optical transfection. This contactless, aseptic, single cell transfection method provides a key nanosurgical tool to the microscopist-the intracellular delivery of reagents and single nanoscopic objects. The experimental possibilities enabled by this technology are only beginning to be realized. A review of optical transfection is presented, along with a forecast of future applications of this rapidly developing and exciting technology.

Laser selection significantly affects cell viability following single-cell nanosurgery

This paper compares the viability of over 700 NG108 cells after membrane disruption either with a single 3 ns pulse at 337 nm or with a 5 ms train of 110 fs pulses (80 MHz) at 770 nm. Cell viability was monitored over a period of 12 h so as to understand the effect of laser ablation-induced cell apoptosis. The use of one-photon membrane disruption with the UV-laser resulted in approximately 36% cell viability after 12 h while the use of two-photon ablation with the femtosecond laser resulted in a much higher viability of approximately 79% after 12 h, which was the same within error of the approximately 79% viability of cells in the control group. Changing the laser power to achieve a 90% probability of membrane disruption (PMD) from 50% PMD did not change the percentage of viable cells after 12 h, regardless of whether one- or two-photon ablation was employed. A systematic comparison between different methods of cellular ablation and their effect upon the viability of single cells has not been done before over such a long time frame. These results show the importance of laser choice when cell viability postsurgery is a concern.

Fibre based cellular transfection

Optically assisted transfection is emerging as a powerful and versatile method for the delivery of foreign therapeutic agents to cells at will. In particular the use of ultrashort pulse lasers has proved an important route to transiently permeating the cell membrane through a multiphoton process. Though optical transfection has been gaining wider usage to date, all incarnations of this technique have employed free space light beams. In this paper we demonstrate the first system to use fibre delivery for the optical transfection of cells. We engineer a standard optical fibre to generate an axicon tip with an enhanced intensity of the remote output field that delivers ultrashort (~ 800 fs) pulses without requiring the fibre to be placed in very close proximity to the cell sample. A theoretical model is also developed in order to predict the light propagation from axicon tipped and bare fibres, in both air and water environments. The model proves to be in good agreement with the experimental findings and can be used to establish the optimum fibre parameters for successful cellular transfection. We readily obtain efficiencies of up to 57 % which are comparable with free space transfection. This advance paves the way for optical transfection of tissue samples and endoscopic embodiments of this technique.

Laser surgery of zebrafish (Danio rerio) embryos using femtosecond laser pulses: optimal parameters for exogenous material delivery, and the laser's effect on short- and long-term development

BACKGROUND

Femtosecond (fs) laser pulses have recently received wide interest as an alternative tool for manipulating living biological systems. In various model organisms the excision of cellular components and the intracellular delivery of foreign exogenous materials have been reported. However, the effect of the applied fs laser pulses on cell viability and development has yet to be determined. Using the zebrafish (Danio rerio) as our animal model system, we address both the short- and long-term developmental changes following laser surgery on zebrafish embryonic cells.

RESULTS

An exogenous fluorescent probe, fluorescein isothiocyanate (FITC), was successfully introduced into blastomere cells and found to diffuse throughout all developing cells. Using the reported manipulation tool, we addressed whether the applied fs laser pulses induced any short- or long-term developmental effects in embryos reared to 2 and 7 days post-fertilization (dpf). Using light microscopy and scanning electron microscopy we compared key developmental features of laser-manipulated and control samples, including the olfactory pit, dorsal, ventral and pectoral fins, notochord, pectoral fin buds, otic capsule, otic vesicle, neuromast patterning, and kinocilia of the olfactory pit rim and cristae of the lateral wall of the ear.

CONCLUSION

In our study, no significant differences in hatching rates and developmental morphologies were observed in laser-manipulated samples relative to controls. This tool represents an effective non-destructive technique for potential medical and biological applications.

An alternative method for delivering exogenous material into developing zebrafish embryos

Non-invasive manipulation of multicellular systems is important for medical and biological research. The ability to introduce, remove, or modify molecules in the intracellular environment is pivotal to our understanding of cellular structure and function. Herein, we report on an alternative method for introducing foreign material into developing embryos using the application of femtosecond (fs) laser pulses. When intense fs laser pulses are focused to a sub-micron spot, transient pores are formed, providing a transport pathway for the delivery of exogenous material into embryonic cells. In this study, zebrafish embryos were used as a model system to demonstrate the non-invasiveness of this applied delivery tool. Utilizing optically induced transient pores chorionated and dechorionated zebrafish embryos were successfully loaded with a fluorescent reporter molecule (fluorescein isothiocyanate), Streptavidin-conjugated quantum dots or DNA (Simian-CMV-EGFP). Pore formation was independent of the targeted location, with both blastomere-yolk interface and blastomere pores competent for delivery. Long-term survival of laser manipulated embryos to pec-fin stage was 89% and 100% for dechorionated and chorionated embryos, respectively. To our knowledge, this is the first report of DNA delivery into zebrafish embryos utilizing fs laser pulses.

Spectrophotometric measurement of intraliposomal trehalose

Trehalose, a non-reducing glucose disaccharide found at high concentrations in many species of anhydrobiotic organisms, shows significant promise in protecting cellular viability and structural integrity during freezing and desiccation. As mammalian cell membranes are impermeable to trehalose, extensive efforts have been taken to introduce trehalose into mammalian cells. In this study, we report on the characterization of trehalose-containing liposomes, with focus on the entrapment of trehalose inside liposomes, as the first step in establishing liposomes as a delivery system in the biopreservation field. Liposomes were synthesized by hydrating a phospholipid/cholesterol lipid bilayer with 200-400 mM trehalose buffer and repeatedly extruding the lipid suspension to form unilamellar vesicles. The trehalose content of the liposomal lysate was determined spectrophotometrically using a commercial kit Megazyme and confirmed with HPLC measurements. The number of liposomes was calculated from the phosphate content of the liposomal preparation and an estimated number of lipid molecules in a 401+/-8 nm liposome. Based on an intraliposomal trehalose content, the calculated liposomal encapsulation efficiency of 200 mM trehalose liposomes was of 92+/-0.7%. This value was in agreement with the 300 and 400 mM trehalose liposomes (91.1+/-8.2% and 102.1+/-9.4%, respectively). The Megazyme method for trehalose measurement is an inexpensive and sensitive technique that does not require specialized instrumentation or extensive technical expertise. Therefore, it can be used to enhance current efforts in the development of alternative strategies for the cryo- and lyoprotection of mammalian cells.