Aptamer screening methods are crucial for selecting aptamers with high affinity and specificity for a target molecule. The most common and widely used screening technique is Systematic Evolution of Ligands by EXponential enrichment (SELEX), but there are also several variations and alternative methods. Below are the key aptamer screening techniques:
SELEX (Systematic Evolution of Ligands by EXponential enrichment)
Overview
SELEX is the classical method for aptamer selection, developed in the 1990s. It involves iterative rounds of selection, amplification, and enrichment to isolate high-affinity aptamers from a large random library of nucleic acids (DNA or RNA).
Steps
Initial Library Generation: A large random library of single-stranded oligonucleotides (~10^12–10^15 sequences) is synthesized.
Target Binding: The library is incubated with the target (e.g., protein, small molecule, or cell). The nucleic acids that bind to the target are retained.
Washing: Unbound or weakly bound sequences are washed away, while bound sequences are retained.
Elution: Bound aptamers are eluted from the target.
Amplification: The retained aptamers are amplified using PCR (for DNA aptamers) or RT-PCR (for RNA aptamers) to generate a new pool of sequences for the next round of selection.
Iterations: This cycle is repeated multiple times (usually 8–15 rounds) to progressively enrich the pool for high-affinity sequences.
Sequence Identification: After several rounds, the enriched sequences are cloned and sequenced to identify the most promising aptamers.
Variations
Negative SELEX: Involves a counter-selection step to eliminate aptamers that bind to non-specific targets, improving selectivity.
Counter-SELEX: Targets unwanted binding to structurally similar molecules or unrelated molecules.
Cell-SELEX: Used to select aptamers against whole cells, especially in cancer research, to target specific cell surface markers.
Cell-SELEX
Overview: This variant of SELEX is used when the target is a whole cell, typically for identifying aptamers that can recognize specific cell types (e.g., cancer cells, immune cells).
Steps
Incubation with Cells: The random nucleic acid library is incubated with live cells. Sequences that bind to specific cell surface markers are retained.
Counter-Selection: Sometimes, a counter-selection step is performed with control cells (non-target cells) to remove aptamers that bind to common, non-specific markers.
Elution and Amplification: The bound aptamers are eluted, amplified, and used for subsequent selection rounds, similar to SELEX.
Applications: This method is widely used in cancer research to select aptamers that specifically bind to tumor cells.
High-Throughput Sequencing SELEX (HT-SELEX)
Overview: HT-SELEX combines the traditional SELEX process with next-generation sequencing (NGS) to allow for deeper and more accurate identification of high-affinity aptamers.
Steps
After each round of selection, instead of cloning and sequencing a small number of aptamer candidates, the entire pool is sequenced using high-throughput sequencing technologies.
This allows researchers to track the evolution of aptamer sequences over multiple rounds, providing insights into how specific sequences dominate the pool as affinity improves.
HT-SELEX is much faster and more efficient than traditional SELEX in identifying the best candidates.
Capillary Electrophoresis SELEX (CE-SELEX)
Overview
CE-SELEX uses capillary electrophoresis to separate bound aptamer-target complexes from free oligonucleotides based on size or charge differences. This method offers very high-resolution separation.
Steps
The nucleic acid library is incubated with the target, and the bound and unbound sequences are separated using capillary electrophoresis.
Bound sequences are collected, amplified, and used in the next selection round.
Advantages
CE-SELEX requires fewer selection rounds (as few as 3–5), offers high-resolution discrimination between bound and unbound aptamers, and is faster than traditional SELEX.
Applications
It is particularly useful for selecting aptamers against small molecules or targets that are difficult to separate via traditional methods.
Microfluidic SELEX (M-SELEX)
Overview
M-SELEX employs microfluidic technology to perform the SELEX process in a miniaturized, automated, and high-throughput manner.
Steps
Aptamer selection is performed on a microfluidic chip that allows efficient mixing of the aptamer library with the target, followed by separation of bound and unbound sequences.
Amplification and washing steps are also integrated into the chip.
Advantages
M-SELEX is faster, consumes less sample and reagents, and allows for high-throughput selection. It is useful for applications where the target material is scarce.
FluMag-SELEX (Magnetic Beads SELEX)
Overview
FluMag-SELEX uses magnetic beads functionalized with the target molecule for aptamer selection. This method simplifies the separation of bound and unbound sequences.
Steps
The target is immobilized on magnetic beads, and the aptamer library is incubated with the beads.
Bound sequences are separated using a magnetic field, and the unbound sequences are washed away.
Bound aptamers are eluted and amplified for subsequent rounds.
Advantages
FluMag-SELEX offers high efficiency in separating bound and unbound sequences, making the process faster and easier to handle than traditional methods.
Toggle-SELEX
Overview
Toggle-SELEX is used to select aptamers that can bind to homologous targets across different species, such as proteins from both humans and animals.
Steps
The library is alternately exposed to the target proteins from different species in successive rounds of SELEX.
This ensures that selected aptamers can recognize structurally conserved regions in the target across different species, useful for cross-species applications.
Applications
Toggle-SELEX is particularly useful for developing aptamers that work in animal models or in diagnostics that require cross-reactivity between species.
SELEX with Chemical Modifications (Modified SELEX)
Overview
In Modified SELEX, aptamers are selected from libraries containing chemically modified nucleotides to enhance their properties, such as increased stability or binding affinity.
Steps
Libraries are generated using nucleotides modified with functional groups (e.g., 2′-fluoro, 2′-O-methyl) to increase nuclease resistance or improve binding interactions.
The selection process proceeds similarly to traditional SELEX, but the resulting aptamers are more stable and suitable for in vivo applications.
Applications
Modified SELEX is used for therapeutic aptamers or aptamers designed for harsh environmental conditions.
Summary of Advantages of SELEX Variants
HT-SELEX: Allows high-throughput analysis and sequence tracking.
CE-SELEX: Offers high-resolution separation and requires fewer rounds.
M-SELEX: Automates and miniaturizes the process for efficiency.
FluMag-SELEX: Simplifies separation using magnetic beads.
Toggle-SELEX: Enables cross-species target binding.
These methods and their variations enable the development of aptamers for a wide range of applications, from diagnostics to therapeutics.