UC San Diego

Glioblastoma is the deadliest form of brain cancer, yet treatment remains largely palliative. Recurrence and resistance to conventional radiation therapies are largely due to subsets of radioresistant glioma stem cells (GSCs) which survive and can repropagate the tumor. Phage display is a well-established high-throughput screening technique which can identify high-affinity, surface-binding peptides against targets of interest. We used phage display to identify a single peptide sequence, YPHKWHEFKQRV, which was highly enriched in irradiated patient-derived GSC neurospheres compared to both non-tumor populations and unirradiated GSCs. Irradiation significantly increased surface binding of the target peptide in a number of different patient-derived GSC neurospheres in vitro and in vivo. This binding increase was abrogated by dissociation into single-cell suspensions prior to peptide binding, suggesting a possible cell-cell adhesion-related peptide binding partner. We identified four candidate genes which preferentially bind the peptide, and we observed aberrant plasma membrane localization in both Ezrin (EZR) and Metadherin (MTDH) following irradiation. All four candidate genes were essential for GSC proliferation based on shRNA knockdowns, while only EZR knockdown significantly impaired unirradiated proliferation, post-irradiation proliferation, and survival to ionizing radiation. Together, these data suggest a possible radiation-induced translocation mechanism for EZR supporting radioresistance uniquely in GSCs. Additionally, the specificity of the phage display-identified for irradiated GSCs highlight the power of this technique and the potential for this and other tumor-targeting peptides in GBM immunotherapy.