SELEX stands for Systematic Evolution of Ligands by Exponential Enrichment.
The evolutionary nature of the process is characterized by the way that non-binding nucleotides are culled from the binding nucleotides, in an essentially evolutionary process at a molecular level.
Exponential Enrichment refers to the exponential nature of the amplification stage of aptamer selection.
SELEX is an in vitro, or test tube-based combination of chemical reactions that synthetically (non-naturally) isolate, rather then create, aptamers.
Although there are many different SELEX processes in existence today, they all follow the basic outline developed by Gold and Tuerk.
Process: SELEX
Basic Steps:
1. Library Generation: A library is created, containing around 1x10 oligonucleotides. These are single-strand nucleic acids consisting of a random sequence region flanked by a binding site.
2. Binding and Separation: The library is incubated with the target molecule. A few nucleic acids will bind to this target and then be considered aptamers. Unbound nucleic acids are filtered out of the solution, and the bound nucleic acids are separated from the target - this is called elution.
3. Amplification: The binding nucleic acids are then copied using Polymerase Chain Reaction (PCR) to create a new library. This new library will be used in a new round of SELEX to further optimize the quality of the aptamers.
In the over 20 years since SELEX first emerged, there have been many modifications to the original version of SELEX, all in an effort to streamline and perfect this aptamer-isolation process. Most modifications simply improve efficiency/optimization to result in the best possible aptamers, or increase the speed of the process. Additional notable SELEX developments include the addition of non-nucleic functional units and nucleotide modifications that drastically improve aptamer performance.
15
"Classic" SELEX cycle with steps.
"Classic" SELEX cycle with steps.
Timeline of SELEX development
Timeline of SELEX development
SELEX Overview
Video Citation 2: Challenges to aptamer development include refinement of the SELEX process and overcoming nuclease degradation. Smaller fragments have also been faced with renal filtration - one of several traits of aptamers that are beneficial in some applications, but detrimental in others.
Classic SELEX process showing stages at which modified variants deviate (Click to Expand).
Classic SELEX process showing stages at which modified variants deviate (Click to Expand).