The kinetics and mechanism of repair of UV induced DNA damage in mammalian cells. The use of 'caged' nucleotides and electroporation to study short time course events in DNA repair

Optimization and evaluation of electroporation delivery of siRNA in the human leukemic CEM cell line

In order to study nucleoside analog activation in the CEM cell line, a transfection protocol had to be optimized in order to silence an enzyme involved in nucleoside analog activation. Hematopoetic cell lines can be difficult to transfect with traditional lipid-based transfection, so the electroporation technique was used. Field strength, pulse length, temperature, electroporation media, siRNA concentration, among other conditions were tested in order to obtain approximately 70-80% mRNA and enzyme activity downregulation of the cytosolic enzyme deoxycytidine kinase (dCK), necessary for nucleoside analog activation. Downregulation was assessed at mRNA and enzyme activity levels. After optimizing the protocol, a microarray analysis was performed in order to investigate whether the downregulation was specific. Additionally two genes were differentially expressed besides the downregulation of dCK. These were however of unknown function. The leakage of intracellular nucleotides was also addressed in the electroporated cells since it can affect the DNA repair mechansism and the efficiency of nucleoside analogs. Three of these pools were increased compared to untreated, unelectroporated cells. The siRNA transfected cells with reduced dCK expression and activity showed reduced sensitivity to several nucleoside analogs as expected. The multidrug resistance to other drugs, as seen in nucleoside analog-induced resistant cells, was not seen with this model.

Nucleolar localization and dynamic roles of flap endonuclease 1 in ribosomal DNA replication and damage repair

Despite the wealth of information available on the biochemical functions and our recent findings of its roles in genome stability and cancer avoidance of the structure-specific flap endonuclease 1 (FEN1), its cellular compartmentalization and dynamics corresponding to its involvement in various DNA metabolic pathways are not yet elucidated. Several years ago, we demonstrated that FEN1 migrates into the nucleus in response to DNA damage and under certain cell cycle conditions. In the current paper, we found that FEN1 is superaccumulated in the nucleolus and plays a role in the resolution of stalled DNA replication forks formed at the sites of natural replication fork barriers. In response to UV irradiation and upon phosphorylation, FEN1 migrates to nuclear plasma to participate in the resolution of UV cross-links on DNA, most likely employing its concerted action of exonuclease and gap-dependent endonuclease activities. Based on yeast complementation experiments, the mutation of Ser(187)Asp, mimicking constant phosphorylation, excludes FEN1 from nucleolar accumulation. The replacement of Ser(187) by Ala, eliminating the only phosphorylation site, retains FEN1 in nucleoli. Both of the mutations cause UV sensitivity, impair cellular UV damage repair capacity, and decline overall cellular survivorship.

An age-related increase in resistance to DNA damage-induced apoptotic cell death is associated with development of DNA repair mechanisms

Neurons in the developing brain die via apoptosis after DNA damage, while neurons in the adult brain are generally resistant to these insults. The basis for this resistance is a matter of conjecture. We report here that cerebellar granule neurons (CGNs) in culture lose their competence to die in response to DNA damage as a function of time in culture. CGNs at either 1 day in vitro (DIV) or 7 DIV were treated with the DNA damaging agents camptothecin, UV or gamma-irradiation and neuronal survival measured. The younger neurons were effectively killed by these agents, while the older neurons displayed a significant resistance to killing. Neuronal survival did not change with time in culture when cells were treated with C2-ceramide or staurosporine, agents which do not target DNA. The resistance to UV irradiation developed over time in culture and was not due to changes in mitotic rate. Increases in DNA strand breakage, up-regulation of the levels of both p53 and its phosphorylated form and nuclear translocation of p53 were equivalent in both older and younger neurons, indicating a comparable p53 stress response. In addition, we show that treatment of older neurons with pharmacological inhibitors of distinct components of the DNA repair machinery promotes the accumulation of DNA damage and sensitizes these cells to the toxic effects of UV exposure. These data demonstrate that older neurons appear to be more proficient in DNA repair in comparison to their younger counterparts, and that this leads to increased survival after DNA damage.

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.

Electroporation-induced damage in mammalian cell DNA

Electroporation induced damage in the DNA of HL60 cells has been investigated by alkaline elution techniques. DNA damage is minimised by reducing the total charge applied (i.e., voltage x capacitance). Reduction of either of these electrical parameters, however, compromises the induced permeability of the cells to small molecules. The data presented concerning the effects of voltage and capacitance on DNA damage and the permeability of cells can be used to specify optimum conditions for electroporation in which DNA damage is minimised. The duration for which the current is applied can be seen to have a significant effect on the level of DNA damage. A modest temperature rise may occur when an electric charge is passed through electroporation buffer, but this event alone does not induce DNA damage in cells. The effect of voltage upon the permeability of HL60 cells to fluorescent-labelled molecules of varying molecular weight is reported.

A stopped-flow apparatus for infrared spectroscopy of aqueous solutions

IR spectroscopy has been widely applied in the study of photo-activated biological processes such as photosynthesis, but has not been applied to the study of reacting systems which require rapid mixing of aqueous solutions. This has been due in part to the high pressure needed to make an aqueous solution flow rapidly through the 50 microns optical pathlength between the plates in an IR cuvette suitable for use with 2H2O and the high viscosity of the concentrated protein solutions required to generate measurable IR signals. An apparatus, based largely on conventional stopped-flow technology, is described which achieves mixing well within the time-resolved performance (approximately 40 ms) of modern Fourier-transform IR (FTIR) spectrometers, since the dead time of the mixing device is approximately 15 ms. It has proved possible to achieve efficient mixing by using a simple six-jet mixing device. This is probably at least in part because of the high back pressure which develops when aqueous fluid is passed rapidly through the short pathlength of the cuvette and which promotes turbulent flow. Several examples of measurements of the deacylation of acylchymotrypsins are provided which demonstrate the operation of the apparatus in conjunction with a spectrometer capable of scanning at four scans/s. For cinnamoyl-chymotrypsin, isotope-edited spectra have been obtained which show somewhat lower resolution than is achieved by conventional scanning methods, since some smoothing has to be applied to the spectra. Difference spectra of the acylation of chymotrypsin by glycylglycine p-nitrophenyl ester have been obtained by averaging ten stopped-flow shots and show good signal-to-noise ratio without smoothing. It is predicted that this apparatus is likely to find a variety of applications in the study of enzyme-catalysed reactions, since the spectra are relatively rich in structural information, and isotope editing greatly enhances the interpretability of the spectra.