Abstract Glioblastoma (GBM) remains the most lethal primary brain tumor, with a median survival of under 15 months, underscoring the critical need for novel therapies. The RNA-binding protein Musashi-1 (MSI1) is a pivotal oncoprotein that drives tumorigenesis and is associated with a poor prognosis. However, direct targeting of MSI1 is hampered by its high homology with MSI2, a lack of specific inhibitors, and the biochemical challenges posed by its C-terminal intrinsically disordered region (IDR), which has obscured key regulatory mechanisms. We employed a comprehensive strategy using GBM cell lines, orthotopic mouse models, and patient-derived tissues. A custom phospho-specific antibody detected MSI1 phosphorylation at Serine 347 (pS347). Functional roles were delineated using tetracycline-inducible shRNA and site-directed mutagenesis. Metastatic potential was assessed via multiplex immunohistochemistry, gelatin zymography for MMP activity, and invasion assays. A cell-penetrating decoy peptide was synthesized by fusing the S347-mimic sequence to the TAT protein transduction domain. We identified Serine 347 as a novel, functionally critical phosphorylation switch within the C-terminal IDR of MSI1. Clinically, pS347-MSI1 levels were significantly elevated in GBM patient samples and correlated with reduced survival in preclinical models. Mechanistically, preventing S347 phosphorylation via the S347A mutation profoundly suppressed GBM tumor formation and invasive potential. For the signaling axis, phosphorylation at S347 is essential for the expression and pericellular localization of key metalloproteinases MMP-2 and MMP-9 in vivo. The subsequent MMP activity drives an Epithelial-Mesenchymal Transition (EMT), as evidenced by the upregulation of vimentin, Twist, Slug, and Snail, which in turn facilitates the F-actin reorganization necessary for effective motility. Based on this mechanism, we engineered a strategic, cell-penetrating decoy peptide that mimics the S347 epitope. This peptide acts as a highly specific competitive inhibitor, selectively reducing MSI1 phosphorylation because MSI2 lacks a homologous serine. Treatment with this decoy peptide successfully recapitulated the anti-tumor effects of genetic dephosphorylation, significantly reducing MMP-2/9 levels, suppressing EMT markers, inducing disruptive morphological changes, and potently inhibiting cellular invasion. Our work establishes MSI1-S347 phosphorylation as a previously unrecognized druggable master switch that drives GBM metastasis via an MMP-EMT signaling axis. The development of a first-in-class MSI1-S347 decoy peptide that effectively neutralizes this switch and curbs invasion offers a highly specific, mechanistically grounded therapeutic strategy with significant potential to combat GBM progression. Citation Format: Xian Liu, Liang-Ting Lin. A novel MMP-targeting peptide suppresses glioblastoma metastasis by reducing MSI1 activity abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3077.
Liu et al. (Fri,) studied this question.