1. Core Concept: What is Capture-SELEX? Capture-SELEX (Systematic Evolution of Ligands by EXponential Enrichment) is an advanced selection technique used to discover single-stranded DNA or RNA aptamers that bind to a specific target molecule. The key innovation is that the target molecule is immobilized (or "captured") on a solid support via a short, known oligonucleotide sequence that is part of the initial random library. This makes it exceptionally powerful for selecting aptamers against small molecules or targets without natural immobilization sites. 2. The Key Differentiator: How It Differs from Classical SELEX Classical SELEX: The target itself is immobilized directly on a surface (e.g., a bead or plate). This can sometimes lead to aptamers that bind to the surface or the immobilized region of the target, which may not function well in solution. Capture-SELEX: The library itself is immobilized via a complementary "capture sequence." Only sequences that bind to the free, unmodified target in solution undergo a conformational change that releases them from the capture strand for collection. 3. Step-by-Step Process of a Capture-SELEX Service A service provider will typically manage this entire pipeline: Step 1: Project Design & Library Synthesis You define the target (e.g., a small molecule, protein, cell). The service designs a custom single-stranded DNA (ssDNA) library: [5' Fixed Primer Sequence - RANDOM Region…
What is SELEX? First, a quick recap: SELEX (Systematic Evolution of Ligands by EXponential enrichment) is the gold-standard process for discovering aptamers (single-stranded DNA or RNA molecules that bind to a specific target with high affinity and specificity). It involves iterative cycles of binding, partitioning, and amplification. "Free Solution SELEX" Explained Free Solution SELEX (also known as non-immobilized SELEX) is a specific technical approach. Its key characteristic is that neither the target molecule nor the library is fixed to a solid support (like beads, a column, or a chip) during the binding step. How it works: Binding: The random oligonucleotide library is mixed with the free, soluble target in solution. Partitioning: The key challenge is separating the bound sequences from the unbound ones without using immobilization. Common methods include: Nitrocellulose Filter Binding: Aptamer-target complexes are trapped on a filter, while free sequences pass through. Gel Filtration/Size Exclusion: Separates complexes (larger) from unbound sequences (smaller). Capture Techniques: Using a brief, weak tag on the target (like biotin) to pull down complexes after binding in solution. Amplification: The bound sequences are eluted, amplified by PCR (for DNA) or RT-PCR (for RNA), and purified for the next round. Advantages of Free Solution SELEX: Native Target Conformation: The target is in its natural, free state. There's no risk of…
Core Principle Nitrocellulose membrane filter binding exploits a simple but powerful property: nitrocellulose avidly binds proteins and protein-nucleic acid complexes, but does not efficiently bind free, single-stranded DNA or RNA. By passing a mixture of the target protein and a random oligonucleotide library through the membrane, sequences that bind to the protein are retained (as a complex), while unbound sequences are washed away. Typical Workflow of a Service Provider A professional service will manage this complex, iterative process for you: 1. Project Design & Library Synthesis Consultation: Defining your target (purified protein is essential), desired aptamer properties (affinity, specificity, buffer conditions), and format (DNA or RNA). Library Design: A synthetic library of up to 10^15 random sequences (e.g., 40-60 nt random core, flanked by constant primer regions) is prepared. 2. The SELEX Cycles (Iterative Screening) Incubation: The target protein is incubated with the nucleic acid library under optimized conditions (buffer, temperature, time). Positive Selection (Binding & Capture): The mixture is passed through a nitrocellulose membrane. Protein-aptamer complexes stick to the membrane. Washing: Mild washing removes weakly bound or non-specific sequences. Elution: Bound sequences are recovered by denaturing the protein (e.g., using heat, phenol-chloroform, or high-concentration urea). Amplification: For DNA SELEX: The eluted DNA is directly amplified by PCR. For…
What is Whole-cell SELEX? Whole-cell SELEX (Systematic Evolution of Ligands by EXponential enrichment) is a technique used to discover aptamers (single-stranded DNA or RNA molecules) that bind specifically to a target living cell. Unlike traditional SELEX that uses a purified protein target, whole-cell SELEX presents the target in its native, complex cellular environment. This allows for the selection of aptamers against: Native cell-surface proteins in their proper folding and post-translational modifications. Complex targets like transmembrane receptors in their natural lipid environment. Unknown surface biomarkers without prior knowledge of the cell's molecular makeup. Specific cell states (e.g., activated, cancerous, infected) based on differences in surface expression. The Core Process: How Whole-cell SELEX Works A professional service will manage this complex, iterative pipeline: Library & Design: Starting with a vast, random synthetic oligonucleotide library (10^14 - 10^15 unique sequences). Positive Selection: Incubating the library with the target cells (e.g., cancer cells, stem cells, bacteria). Aptamers that bind to any surface structure are retained. Counter-Selection (Critical Step): The bound pool is then exposed to non-target or control cells (e.g., healthy cells, a different cell line). Sequences that bind to these are discarded. This step is crucial for generating specificity. Elution & Amplification: Aptamers specifically bound to the target cells are recovered, amplified by PCR…
Core Concept The central idea is "Target-based Drug Discovery." Instead of screening compounds on whole cells or organisms (phenotypic screening), you start with a specific protein (e.g., a kinase, receptor, ion channel) implicated in a disease. Services then help you understand that target and find molecules that modulate it. Categories of Protein Target Services These services typically follow the drug discovery pipeline: 1. Target Identification & Validation Bioinformatics & Omics Analysis: Mining genomic, proteomic, and clinical data to identify novel disease-associated targets. Genetic Validation: CRISPR/Cas9 gene editing (knock-out/knock-in), siRNA/shRNA knockdown to confirm the target's role in disease pathways. Functional Validation: Cell-based assays to see if modulating the target affects disease-relevant phenotypes. 2. Protein Expression & Purification Recombinant Protein Production: Cloning, expressing (in E. coli, insect, or mammalian cells), and purifying milligram to gram quantities of the target protein. This is essential for structural studies and biochemical assays. Membrane Protein Expertise: Specialized services for difficult-to-express targets like GPCRs and ion channels. Tagging & Labeling: Adding tags (His, GST, FLAG) for purification or fluorescent/isotopic labels for assays. 3. Structural Biology & Biophysics X-ray Crystallography: Determining high-resolution 3D structures of protein-ligand complexes. Cryo-Electron Microscopy (Cryo-EM): For large complexes or membrane proteins unsuitable for crystallography. Nuclear Magnetic Resonance (NMR) Spectroscopy: For studying dynamics and ligand binding in solution. Surface…
Traditional SELEX (Systematic Evolution of Ligands by EXponential enrichment) is a method to select high-affinity, specific nucleic acid aptamers from a vast random library (10¹³-10¹⁵ sequences). The bottleneck has always been the final cloning and Sanger sequencing of only a few dozen candidates, which often misses rare, high-performance aptamers. NGS-assisted SELEX integrates Next-Generation Sequencing at multiple rounds of the SELEX process. This provides a massive, data-rich view of the entire evolutionary landscape, enabling intelligent selection and identification of the best aptamers. Typical Workflow of an NGS-Assisted SELEX Service A professional service provider will manage this entire pipeline: Project Design & Library Synthesis: Collaboration to define target (protein, small molecule, cell), counter-selection requirements, and library design (random region length, fixed primers for NGS). Parallel SELEX Execution: Performing the iterative selection process (binding, partitioning, amplification) across multiple rounds (usually 8-12). Key NGS Integration Points: Initial Library Analysis: Sequencing the naive library to confirm diversity and complexity. Monitoring Rounds (e.g., Rounds 3, 6, 9): Taking small samples from intermediate rounds for NGS. This is the critical advantage. It tracks: Sequence Enrichment: Which families are becoming more abundant. Diversity Collapse: When to stop selection before losing good candidates. Informed Decision-Making: Data guides adjustments in selection stringency for subsequent rounds. Final Round Deep Sequencing: Comprehensive NGS of…
What are Aptamers? Aptamers are short, single-stranded DNA or RNA oligonucleotides (typically 20-80 nucleotides) that fold into specific three-dimensional shapes, enabling them to bind to target molecules with high affinity and specificity. They are often called "chemical antibodies." The process of creating them is called SELEX (Systematic Evolution of Ligands by EXponential enrichment), which iteratively selects aptamers from vast random-sequence libraries against a desired target (e.g., a protein, small molecule, or even a whole cell). Key Advantages of Aptamers as Therapeutics Compared to traditional protein-based biologics like antibodies, aptamers offer several compelling benefits: High Specificity & Affinity: Can distinguish between closely related targets (e.g., different protein isoforms). Small Size: Typically 8-25 kDa, much smaller than antibodies (~150 kDa). This can improve tissue penetration. Full Chemical Synthesis: Produced in vitro via chemical synthesis, eliminating batch-to-batch variability and the need for biological systems (cells or animals). This makes manufacturing scalable and consistent. Low Immunogenicity: Being nucleic acids, they are generally less likely to trigger immune reactions than foreign proteins. Excellent Stability: DNA aptamers, in particular, are thermally stable and can be stored easily. Stability in biological fluids can be engineered. Ease of Modification: Can be chemically modified to enhance stability (e.g., resist nucleases), prolong half-life (e.g., PEGylation), or add functional groups…
The unique secondary and tertiary structures of aptamers provide the specificity to detect even small structural changes in the target molecule, including the presence or absence of methyl or hydroxyl groups or differences in enantiomeric configurations. Aptamers that bind specific targets are identified through a process known as Systematic Evolution of Ligands by Exponential enrichment (SELEX) in which binding molecules are selected from a large and diverse library of nucleic acids (either DNAs or RNAs). In this process, the nucleic acid library is incubated with the target molecule. Non-binding nucleic acids are then washed away, leaving behind only the molecules that have a capacity to bind to the target molecule. The nucleic acids that are not washed away are then used to create a new library of nucleic acids that is enriched for the subset that binds the desired target. Repeating this selection-cycle on each subsequent library with increasing stringency of binding (e.g. lower concentration of target), ensures that nucleic acids that bind to the target with both high specificity and high affinity are enriched. Aptamers are short, single-stranded oligonucleotides (DNA or RNA) that bind to specific target molecules with high affinity and specificity. They are often called "chemical antibodies."…
