Site Directed Mutagenesis (SDM) Human Point mutation HeLa

Cell cytotoxicity assays measure the ability of certain compounds or chemical mediators to reduce the viability of the cells. The term cell cytotoxicity assay can sometimes be used interchangeably with cell proliferation assay. Healthy living cells can be identified by the use of formazan dyes, protease biomarkers or by measuring ATP content. The formazan dyes are chromogenic products formed by the reduction of tetrazolium salts by dehydrogenases, such as lactate dehydrogenase (LDH) and reductases that are released during cell death. Common tetrazolium salts include INT, MTT, MTS and XTT. Cell cytotoxicity can also be measured by using the SRB and WST-1 assays. These assays can usually be used in a high-throughput fashion and can be quantitated by measuring absorbance, colorimetry or luminescence. All these assays require similar numbers of cell plating at the initiation, a time course of treatment with the cytotoxic agent and at least triplicates for each condition at every point of analysis. Cell shrinkage, plasma membrane blebbing, cell detachment, externalization of phosphatidylserine, nuclear condensation and ultimately DNA fragmentation are well-described features of apoptosis. The assays that rely on cell membrane integrity for their function, may not be able to quantify early apoptosis. Therefore, in order to distinguish early apoptotic vs. late apoptotic or necrotic cells, additional flow cytometry techniques can be used. A combination of Annexin V and PI (propidium iodide) can be used to distinguish early (Annexin V+/PI-) and late apoptotic (Annexin V+/PI+) cells. Sometimes, caspase assays are used in order to differentiate the stages of apoptosis.

A key signature for necrotic cells is the permeabilization of the plasma membrane. Necrosis can be quantified by several cellular and biochemical assays. When studied minutely, it reveals the difficulty in confirmation in secondary induction of necrosis in apoptotic cells. Apoptotic cells are being analyzed to shift to necrotic status owing to membrane permeability at later stages, and thus, discrimination of two cell death becomes critical. Therefore, it is crucial to use a necrosis detection kit or a defined procedure to analyze this unprogrammed form of death in response to immense chemical and physical insults.

Protein isolation is a technique that involves isolation and/ or purification of protein from cells or tissues via chromatography or electrophoresis. The major challenges in protein isolation include: 1. The concentration of proteins in cells is variable and tends to be small for some intracellular proteins. Unlike nucleic acids, proteins cannot be amplified. 2. Proteins are more unstable than nucleic acids. They are easily denatured under suboptimal temperature, pH or salt concentrations. 3. Finally, no generalized technique/protocol can be applied for protein isolation. Proteins may have different electrostatic (number of positively or negatively charged amino acids) or hydrophobic properties. Therefore, protein purification requires multiple steps depending on their charge (a negatively charged resin/column for positively charged proteins and vice-versa), dissolution (using detergents) and unlike in the case of DNA and RNA, instead of using salts, proteins should be isolated by isoelectric precipitation.

The process of RNA extraction from bacteria, in general, involves an RNA-protective, effective lysis of bacterial cell wall (which may pose difficulties). EDTA promotes loss of outer membrane to provide lysozyme with access to peptidoglycan. Another common method for cell wall lysis is mechanical disruption using a homogenizer (applied for gram-positive bacteria and some strains of gram-negative bacteria). Following lysis, it is necessary to disrupt protein-nucleic acid interactions, which can be achieved by adding sodium dodecyl sulfate (SDS). Next step involves using phenol-chloroform-isoamyl alcohol extraction, where RNA can be obtained from the bottom organic phase, the top phase consists of DNA and the interphase contains proteins. Isoamyl alcohol is an inert and optional addition to this mixture and is added as an anti-foaming reagent to reduce the interphase. Following RNA extraction, the samples should be checked for its quality by gel electrophoresis (23S and 16S rRNAs and 5s rRNA and tRNA bands) or UV spectrophotometric or fluorescence methods.

DNA damage assay is a standard method for determining in-vivo/in-vitro genotoxicity by measuring the breaks in the DNA chain of animal and plant cells. Initial DNA damage leads to cell cycle arrest and, at the final stages, leads to induction of senescence or cell death (apoptosis, necrosis, autophagy, or mitotic catastrophe). Detection of DNA damage from mild to moderate to severe is challenging when studying genotoxicity in the pool of cells. It is favorable to use DNA damage assay kits available for prominent identification of the extent of damage in the analysis.

ROS has a very short half-lives in biological environment as they are influenced by exposure to ambient oxygen. As it is highly reactive and hard to measure care should be taken to ensure the stability of the sample during isolation, preparation, storage, and analysis.

As autophagy is a multi-step process which includes not just the formation of autophagosomes, but most importantly, flux through the entire system, including the degradation upon fusion with lysosomes, which makes it quite challenging for detection. There are several methods for detection in mammalian cells, including immunoblotting analysis of LC3 and p62 and detection of autophagosome formation/maturation by fluorescence microscopy, Currently, there is no single “gold standard” for determining the autophagic activity that is applicable in every experimental context, hence it is recommended to go for the combined use of multiple methods to accurately assess the autophagic activity in any given biological setting.

The estimation of DNA methylation level heavily depends on the complete conversion of non-methylated DNA cytosines. It is crucial to ensure complete conversion of non-methylated cytosines in DNA. Therefore, it is important to incorporate controls for bisulfite reactions, as well as to pay attention to the appearance of cytosines in non-CpG sites after sequencing, which is an indicator of incomplete conversion.

The estimation of DNA methylation level heavily depends on the complete conversion of non-methylated DNA cytosines. It is crucial to ensure complete conversion of non-methylated cytosines in DNA. Therefore, it is important to incorporate controls for bisulfite reactions, as well as to pay attention to the appearance of cytosines in non-CpG sites after sequencing, which is an indicator of incomplete conversion.