SUMMARY Glioblastomas (GBMs) are highly vascular and lethal brain tumors that display cellular hierarchies containing self-renewing tumorigenic glioma stem cells (GSCs). As GSCs often reside in perivascular niches and may undergo mensenchymal differentiation, we interrogated GSC potential to generate vascular pericytes. Here we show that GSCs give rise to pericytes to support vessel function and tumor growth. In vivo cell lineage tracing with constitutive and lineage specific fluorescent reporters demonstrated that GSCs generate the majority of vascular pericytes. Selective elimination of GSC-derived pericytes disrupts neovasculature and potently inhibits tumor growth. Analysis of human GBM specimens showed that most pericytes are derived from neoplastic cells. GSCs are recruited toward endothelial cells via the SDF-1/CXCR4 axis and induced to become pericytes predominantly by TGF-β. Thus, GSCs contribute to vascular pericytes that may actively remodel perivascular niches. Therapeutic targeting of GSC-derived pericytes may effectively block tumor progression and improve the anti-angiogenic therapy.

Glioblastoma is the most common and most aggressive primary brain tumour. Standard of care consists of surgical resection followed by radiotherapy and concomitant and maintenance temozolomide (temozolomide/radiotherapy→temozolomide). Corticosteroids are commonly used perioperatively to control cerebral oedema and are frequently continued throughout subsequent treatment, notably radiotherapy, for amelioration of side effects. The effects of corticosteroids such as dexamethasone on cell growth in glioma models and on patient survival have remained controversial. We performed a retrospective analysis of glioblastoma patient cohorts to determine the prognostic role of steroid administration. A disease-relevant mouse model of glioblastoma was used to characterize the effects of dexamethasone on tumour cell proliferation and death, and to identify gene signatures associated with these effects. A murine anti-VEGFA antibody was used in parallel as an alternative for oedema control. We applied the dexamethasone-induced gene signature to The Cancer Genome Atlas glioblastoma dataset to explore the association of dexamethasone exposure with outcome. Mouse experiments were used to validate the effects of dexamethasone on survival in vivo Retrospective clinical analyses identified corticosteroid use during radiotherapy as an independent indicator of shorter survival in three independent patient cohorts. A dexamethasone-associated gene expression signature correlated with shorter survival in The Cancer Genome Atlas patient dataset. In glioma-bearing mice, dexamethasone pretreatment decreased tumour cell proliferation without affecting tumour cell viability, but reduced survival when combined with radiotherapy. Conversely, anti-VEGFA antibody decreased proliferation and increased tumour cell death, but did not affect survival when combined with radiotherapy. Clinical and mouse experimental data suggest that corticosteroids may decrease the effectiveness of treatment and shorten survival in glioblastoma. Dexamethasone-induced anti-proliferative effects may confer protection from radiotherapy- and chemotherapy-induced genotoxic stress. This study highlights the importance of identifying alternative agents such as vascular endothelial growth factor antagonists for managing oedema in glioblastoma patients. Beyond the established adverse effect profile of protracted corticosteroid use, this analysis substantiates the request for prudent and restricted use of corticosteroids in glioblastoma.

SUMMARY The tumor vascular microenvironment supports tumorigenesis by supplying not only oxygen and diffusible nutrients but also by secreting soluble factors that promote tumorigenesis. Here we identify a feed-forward mechanism in which endothelial cells (EC), in response to tumor-derived mediators, release angiocrines driving aberrant vascularization and glioblastoma multiforme (GBM) progression through a hypoxia-independent induction of hypoxia-inducible factor (HIF)-1α. Phosphorylation of profilin-1 (Pfn-1) at Tyr129 in EC induces binding to tumor suppressor protein von Hippel-Lindau (VHL), prevents VHL-mediated degradation of prolyl-hydroxylated HIF-1α, culminating in HIF-1α accumulation even in normoxia. Elevated HIF-1α induces expression of multiple angiogenic factors, leading to vascular abnormality and tumor progression. In a genetic model of GBM, mice with an EC-specific defect in Pfn-1 phosphorylation exhibit reduced tumor angiogenesis, normalized vasculature, and improved survival. Moreover, EC-specific Pfn-1 phosphorylation is associated with tumor aggressiveness in human glioma. These findings suggest that targeting Pfn-1 phosphorylation may offer a selective strategy for therapeutic intervention of malignant solid tumors.

Injectable Magnetic Resonance Imaging (MRI) contrast agents have been widely used to provide critical assessments of disease for both clinical and basic science imaging research studies. The scope of available MRI contrast agents has expanded over the years with the emergence of molecular imaging contrast agents specifically targeted to biological markers. Unfortunately, synergistic application of more than a single molecular contrast agent has been limited by MRI's ability to only dynamically measure a single agent at a time. In this study, a new Dual Contrast -Magnetic Resonance Fingerprinting (DC -MRF) methodology is described that can detect and independently quantify the local concentration of multiple MRI contrast agents following simultaneous administration. This "multi-color" MRI methodology provides the opportunity to monitor multiple molecular species simultaneously and provides a practical, quantitative imaging framework for the eventual clinical translation of molecular imaging contrast agents.Over the past 3 decades, Magnetic Resonance Imaging (MRI) has become an essential medical imaging modality due to its exceptional soft tissue contrast and lack of ionizing radiation. Along with a wide variety of endogenous tissue contrast mechanisms, many MRI applications utilize an intravenous injection of an MRI contrast agent (e.g., gadolinium chelates or iron oxides) to enable sensitive identification of numerous pathologies such as tumors 1 , vascular abnormalities 2 , and cardiac infarcts 3 through local alterations in the tissue's magnetic properties (T1 and T2 relaxation times). Clinical use of these contrast-enhanced MRI scans has further expanded as multiple contrast agents have been approved for specific clinical imaging applications (e.g., blood pool contrast agents 4 , hepatobiliary contrast agents 5 ).With the emergence of the field of molecular imaging, there has been a dramatic increase in the number of MRI contrast agents targeted to proteins 6-8 , cell receptors 9, 10 , and other molecular species 11,12 . In addition, a number of activatable agents have been described that have different relaxivities based on the local tissue environment in vivo 13,14 . In a typical preclinical molecular MRI study, the T1 or T2 relaxation time (or MRI signal intensity) is

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