Glioma-propagating cells as an in vitro screening platform: PLK1 as a case study

Gliomas are the most devastating of primary adult malignant brain tumors. These tumors are highly infiltrative and can arise from cells with extensive self-renewal capability and chemoresistance, frequently termed glioma-propagating cells (GPCs). GPCs are thus the plausible culprits of tumor recurrence. Treatment strategies that eradicate GPCs will greatly improve disease outcome. Such findings support the use of GPCs as in vitro cellular systems for small-molecule screening. However, the nuances in using GPCs as a cellular screening platform are not trivial. These slow-growing cells are typically cultured as suspension, spheroid structures in serum-free condition supplemented with growth factors. Consequently, replenishment of growth factors throughout the screening period must occur to maintain cells in their undifferentiated state, as the more lineage-committed, differentiated cells are less tumorigenic. We present a case study of a small-molecule screen conducted with GPCs and explain how unique sphere activity assays were implemented to distinguish drug efficacies against the long-term, self-renewing fraction, as opposed to transient-amplifying progenitors, the latter of which are detected in conventional viability assays. We identified Polo-like kinase 1 as a regulator of GPC survival. Finally, we leveraged on public glioma databases to illustrate GPC contribution to disease progression and patient survival outcome.