Systematic evolution of a DNA aptamer binding to rat brain tumor microvessels - Selective targeting of endothelial regulatory protein pigpen

Tumor microvessels differ in structure and metabolic function from normal v asculature, and neoangiogenesis is associated with quantitative and qualita tive changes in expression of endothelial proteins. Such molecules could se rve as molecular addresses differentiating the tumor vasculature from those of the normal brain. We have applied Systematic Evolution of Ligands by EX ponential enrichment (SELEX) against transformed endothelial cells as a com plex target to select single-stranded DNA-ligands (aptamers) that function as histological markers to detect microvessels of rat experimental glioma, a fatal brain tumor that is highly vascularized. Both the SELEX selection p rocedure as well as subsequent deconvolution-SELEX were analyzed by fluores cence based methods (flow cytometry and fluorescence microscopy), Of 25 apt amers analyzed, one aptamer was selected that selectively bound microvessel s of rat brain glioblastoma but not the vasculature of the normal rat brain including peritumoral areas. The molecular target protein of aptamer III.1 was isolated from endothelial cells by ligand-mediated magnetic DNA affini ty purification. This protein was identified by mass spectrometry as rat ho mologue of mouse pigpen, a not widely known endothelial protein the express ion of which parallels the transition from quiescent to angiogenic phenotyp es in vitro, Because neoangiogenesis, the formation of new blood vessels, i s a key feature of tumor development, the presented aptamer can be used as a probe to analyze pathological angiogenesis of glioblastoma, The presented data show that pigpen is highly expressed in tumor microvessels of experim ental rat brain glioblastoma and may play an important role in warranting b lood supply, thus growth of brain tumors.