aptamer

An aptamer is a short, single-stranded DNA or RNA sequence that is synthesized artificially. It can fold into a specific three-dimensional structure (such as a hairpin, bulge, G-quadruplex, etc.), enabling it to bind to a target molecule with high specificity and high affinity.

You can think of it as a “chemical antibody,” but its essence is not protein—it is nucleic acid.

Key Characteristics and Advantages (Compared to Traditional Antibodies)

How Are They Produced? – SELEX Technology

Aptamers are not designed; they are “selected.” The core screening technology is called SELEX.

Basic Steps of SELEX:

1. Library Construction: Synthesize a single-stranded nucleic acid library containing up to 10^15 different random sequences.

2. Binding: Incubate the library with the immobilized target molecule.

3. Washing: Wash away weakly bound or non-binding sequences.

4. Elution: Elute the nucleic acid sequences that specifically bind to the target.

5. Amplification: Amplify these binding sequences in large quantities using PCR (for DNA) or RT-PCR (for RNA).

6. Iteration: Use the amplified product as the library for the next round of selection. Repeat steps 2-5 multiple times (typically 8-15 rounds) to enrich the few sequences with the highest affinity and strongest specificity.

7. Sequencing & Validation: Sequence the final enriched pool, then synthesize individual sequences to validate their binding properties.

Main Application Areas

Due to their unique advantages, aptamers show great potential in numerous fields:

1. Therapeutic Applications:

   • Targeted Drugs: As drugs themselves (e.g., the marketed drug Pegaptanib for age-related macular degeneration), or as “targeting heads” to deliver drugs precisely to diseased cells (e.g., targeted cancer therapy).

   • Antagonists: Bind to and inhibit the function of harmful proteins (e.g., viral proteins, pathogenic cytokines).

2. Diagnostics & Detection:

   • Biosensors: Immobilize aptamers on sensor surfaces; binding to the target generates a detectable signal (electrical, optical, etc.) for rapid detection of pathogens, toxins, biomarkers, etc.

   • In Vitro Diagnostics: Replace antibodies in assays like ELISA, lateral flow assays (e.g., pregnancy test strips), improving stability and sensitivity.

3. Targeted Delivery Systems:

   • Conjugating aptamers to the surface of liposomes, nanoparticles, or exosomes to achieve cell-specific delivery of genes, drugs, or imaging agents.

4. Basic Research Tools:

   • As molecular probes for studying protein function, isolating specific cell types, or real-time imaging within living cells.

5. Environmental & Food Safety Monitoring:

   • Detecting small molecule contaminants like antibiotic residues, heavy metal ions, and toxins in water and food.

Challenges and Limitations

• Nuclease Degradation: Especially for RNA aptamers, they can be rapidly degraded by nucleases in biological systems. Chemical modifications (e.g., 2′-fluoro, 2′-O-methyl) are often required to enhance stability.

• Rapid Renal Clearance: Due to their small size, unmodified aptamers are quickly filtered and cleared by the kidneys in the bloodstream. Conjugation to molecules like polyethylene glycol (PEG) is often used to increase size and extend half-life.

• Complex Selection Process: The SELEX process can be time-consuming and labor-intensive, and it does not guarantee successful selection of high-quality aptamers for every target.

Summary

An aptamer is an artificially synthesized nucleic acid molecule with antibody-like functions. With its advantages of high specificity, high stability, ease of modification and production, it is emerging as a highly promising novel tool and therapeutic agent in fields such as life science research, disease diagnosis, targeted therapy, and drug development. It serves as a crucial foundation at the intersection of chemical biology and nanomedicine.