Darwinian molecular imaging

From ugly duckling to swan: unexpected identification from cell-SELEX of an anti-Annexin A2 aptamer targeting tumors

Background

Cell-SELEX is now widely used for the selection of aptamers against cell surface biomarkers. However, despite negative selection steps using mock cells, this method sometimes results in aptamers against undesirable targets that are expressed both on mock and targeted cells. Studying these junk aptamers might be useful for further applications than those originally envisaged.

Methodology/Principal Findings

Cell-SELEX was performed to identify aptamers against CHO-K1 cells expressing human Endothelin type B receptor (ET_BR). CHO-K1 cells were used for negative selection of aptamers. Several aptamers were identified but no one could discriminate between both cell lines. We decided to study one of these aptamers, named ACE4, and we identified that it binds to the Annexin A2, a protein overexpressed in many cancers. Radioactive binding assays and flow cytometry demonstrated that the aptamer was able to bind several cancer cell lines from different origins, particularly the MCF-7 cells. Fluorescence microscopy revealed it could be completely internalized in cells in 2 hours. Finally, the tumor targeting of the aptamer was evaluated in vivo in nude mice xenograft with MCF-7 cells using fluorescence diffuse optical tomography (fDOT) imaging. Three hours after intravenous injection, the aptamer demonstrated a significantly higher uptake in the tumor compared to a scramble sequence.

Conclusions/Significance

Although aptamers could be selected during cell-SELEX against other targets than those initially intended, they represent a potential source of ligands for basic research, diagnoses and therapy. Here, studying such aptamers, we identify one with high affinity for Annexin A2 that could be a promising tool for biomedical application.

Phage display and molecular imaging: expanding fields of vision in living subjects

In vivo molecular imaging enables non-invasive visualization of biological processes within living subjects, and holds great promise for diagnosis and monitoring of disease. The ability to create new agents that bind to molecular targets and deliver imaging probes to desired locations in the body is critically important to further advance this field. To address this need, phage display, an established technology for the discovery and development of novel binding agents, is increasingly becoming a key component of many molecular imaging research programs. This review discusses the expanding role played by phage display in the field of molecular imaging with a focus on in vivo applications. Furthermore, new methodological advances in phage display that can be directly applied to the discovery and development of molecular imaging agents are described. Various phage library selection strategies are summarized and compared, including selections against purified target, intact cells, and ex vivo tissue, plus in vivo homing strategies. An outline of the process for converting polypeptides obtained from phage display library selections into successful in vivo imaging agents is provided, including strategies to optimize in vivo performance. Additionally, the use of phage particles as imaging agents is also described. In the latter part of the review, a survey of phage-derived in vivo imaging agents is presented, and important recent examples are highlighted. Other imaging applications are also discussed, such as the development of peptide tags for site-specific protein labeling and the use of phage as delivery agents for reporter genes. The review concludes with a discussion of how phage display technology will continue to impact both basic science and clinical applications in the field of molecular imaging.

Metastasis-focused cell-based SELEX generates aptamers inhibiting cell migration and invasion

Metastasis, the capacity of tumour cells to disseminate and grow at distant sites, is the main factor in cancer mortality. Compounds inhibiting migration and invasion of cancer cells are promising candidates for anticancer therapy strategies. We have generated nuclease-resistant RNA ligands (aptamers) recognizing highly metastatic cells with high affinity and specificity, and inhibiting their migratory and invasive potentials. Aptamers were generated by a cell-based subtractive SELEX technology using isogenic cell lines with similar tumorigenic potentials but opposite metastatic aggressiveness. Two aptamers, E37 and E10, bound specifically to the metastatically aggressive cell line and altered the phosphorylation of several tyrosine kinases. Fluorescent microscopy showed intracellular uptake of E37, in contrast to membrane binding of E10. Both aptamers inhibited migration of tumour cells in culture (50 and 85% inhibition with respect to control pool for E10 and E37, respectively) while only E10 inhibited cell invasion (-75% with respect to control pool). This proof-of-concept study demonstrates the potential of cell-based SELEX to yield ligands that selectively recognize aggressive metastatic cells and inhibit phenotypes linked to metastatic potential.