Aptamers as Promising Molecular Recognition Elements for Diagnostics and Therapeutics in the Central Nervous System

Address correspondence to: Maria C. DeRosa, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada, E-mail: [email protected]

Introduction

Oligonucleotide aptamers are single-stranded DNA or RNA sequences that bind to molecular targets with remarkable affinity and specificity. They are known to form conformationally unique 3-dimensional structures that allow for a selective interaction with their respective targets. Aptamers have been selected for a wide variety of targets ranging from small molecules to cells [1-3]. While aptamers are functionally similar to antibodies as molecular recognition elements, they do offer some advantages. Aptamers are easier to synthesize, offer little to no batch-to-batch variation, and are typically more chemically stable than antibodies. Furthermore, the binding functionality of oligonucleotide aptamers can be regulated through hybridization with their complementary sequences, providing an extra layer of therapeutic control.

Aptamers are selected by the Systematic Evolution of Ligands by EXponential enrichment (SELEX) process [4-6]. Briefly, a starting library containing ∼10¹⁵ oligonucleotide sequences is incubated with a target of interest. Following incubation, sequences that interact with the target are partitioned from those sequences that show little to no affinity. Sequences of interest are subsequently amplified yielding an enriched library, which can be reintroduced to the target to continue selections. Typically, 8-12 rounds of selection with increasing stringency are required to yield high affinity aptamers. Once the sequences are elucidated, an aptamer's affinity for its target is generally evaluated through the dissociation constant (K_d ) of the aptamer-target interaction, with lower values indicative of greater binding affinity. Aptamers with K_d s in the nM to pM range are typically deemed "high affinity" and appropriate for incorporation into biosensors or other aptamer applications, although μM K_d s are commonly reported for small molecule targets given the inherent challenges of small molecule selection [1,7].

Aptamers have shown particular potential in the field of medical diagnostics, targeted delivery, imaging, and therapeutics [8-11]. While aptamers hold promise for central nervous system (CNS) applications, current research in this area is relatively limited. This is likely...