Targeting the mDia Formin-Assembled Cytoskeleton Is an Effective Anti-Invasion Strategy in Adult High-Grade Glioma Patient-Derived Neurospheres.

High-grade glioma (HGG, WHO Grade III⁻IV) accounts for the majority of adult primary malignant brain tumors. Failure of current therapies to target invasive glioma cells partly explains the minimal survival advantages: invasive tumors lack easily-defined surgical margins, and are inherently more chemo- and radioresistant. Much work centers upon Rho GTPase-mediated glioma invasion, yet downstream Rho effector roles are poorly understood and represent potential therapeutic targets. The roles for the mammalian Diaphanous (mDia)-related formin family of Rho effectors have emerged in invasive/metastatic disease. mDias assemble linear F-actin to promote protrusive cytoskeletal structures underlying tumor cell invasion. Small molecule mDia intramimic (IMM) agonists induced mDia functional activities including F-actin polymerization. mDia agonism inhibited polarized migration in Glioblastoma (WHO Grade IV) cells in three-dimensional (3D) in vitro and rat brain slice models. Here, we evaluate whether clinically-relevant high-grade glioma patient-derived neuro-sphere invasion is sensitive to formin agonism. Surgical HGG samples were dissociated, briefly grown as monolayers, and spontaneously formed non-adherent neuro-spheres. IMM treatment dramatically inhibited HGG patient neuro-sphere invasion, both at neuro-sphere embedding and mid-invasion assay, inducing an amoeboid morphology in neuro-sphere edge cells, while inhibiting actin- and tubulin-enriched tumor microtube formation. Thus, mDia agonism effectively disrupts multiple aspects of patient-derived HGG neuro-sphere invasion.