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Glioblastoma (GBM) remains the most pervasive and lethal of all brain malignancies. One factor that contributes to this poor prognosis is the highly invasive character of the tumor. GBM is characterized by microscopic infiltration of tumor cells throughout the brain, whereas non-neural metastases, as well as select lower grade gliomas, develop as self-contained and clearly delineated lesions. Illustrated by rodent xenograft tumor models as well as pathological human patient specimens, we present evidence that one fundamental switch between these two distinct pathologies--invasion and noninvasion--is mediated through the tumor extracellular matrix. Specifically, noninvasive lesions are associated with a rich matrix containing substantial amounts of glycosylated chondroitin sulfate proteoglycans (CSPGs), whereas glycosylated CSPGs are essentially absent from diffusely infiltrating tumors. CSPGs, acting as central organizers of the tumor microenvironment, dramatically influence resident reactive astrocytes, inducing their exodus from the tumor mass and the resultant encapsulation of noninvasive lesions. Additionally, CSPGs induce activation of tumor-associated microglia. We demonstrate that the astrogliotic capsule can directly inhibit tumor invasion, and its absence from GBM presents an environment favorable to diffuse infiltration. We also identify the leukocyte common antigen-related phosphatase receptor (PTPRF) as a putative intermediary between extracellular glycosylated CSPGs and noninvasive tumor cells. In all, we present CSPGs as critical regulators of brain tumor histopathology and help to clarify the role of the tumor microenvironment in brain tumor invasion.

An important milestone in brain development is the transition of neuroprogenitor cells to postmitotic neurons. We report that the bZIP transcription factor ATF5 plays a major regulatory role in this process. In developing brain ATF5 expression is high within ventricular zones containing neural stem and progenitor cells and is undetectable in postmitotic neurons. In attached clonal neurosphere cultures ATF5 is expressed by neural stem/progenitor cells and is undetectable in tau-positive neurons. In PC12 cell cultures nerve growth factor (NGF) dramatically downregulates endogenous ATF5 protein and transcripts, whereas exogenous ATF5 suppresses NGF-promoted neurite outgrowth. Such inhibition requires the repression of CRE sites. In contrast, loss of function conferred by dominant-negative ATF5 accelerates NGF-promoted neuritogenesis. Exogenous ATF5 also suppresses, and dominant-negative ATF5 and a small-interfering RNA targeted to ATF5 promote, neurogenesis by cultured nestin-positive telencephalic cells. These findings indicate that ATF5 blocks the differentiation of neuroprogenitor cells into neurons and must be downregulated to permit this process to occur.

Extracellular matrix (ECM) molecules, including chondroitin-4 or chondroitin-6 sulfate proteoglycans (CSPGs) and tenascin, are upregulated in and around wounds and transplants to the adult CNS. In the present study, striatal wounds from adult mice were used in a novel in vitro paradigm to assess the effects of these wound-associated molecules on embryonic dopamine cell attachment and neurite outgrowth. Light and electron microscopic immunocytochemistry studies have shown that astroglial scar constituents persist in cultured explants for at least 1 week in vitro, and despite the loss of neurons from adult striatal explants, there is a retention of certain structural features suggesting that the wound explant-neuron coplant is a viable model for analysis of graft-scar interactions. Explants from the wounded striatum taken at different times after a penetrating injury in vivo were used as substrates for embryonic ventral mesencephalon neurons that were plated on their surfaces. Dopamine cell attachment is increased significantly in relation to the expression of both CSPG and tenascin. The increase in neuronal attachment in this paradigm, however, is accompanied by a postlesion survival time-dependent significant decrease in neuritic growth from these cells. In vitro ECM antibody treatment suggests that CSPG may be responsible for heightened dopamine cell attachment and that tenascin simultaneously may support cell attachment while inhibiting neurite growth. The present study offers a new approach for the in vitro analysis of cell and molecular interactions after brain injury and brain grafting, in essence acting as a nigrostriatal transplant-in-a-dish.