c phosphodiesterases have been generated by mutating an essential histidine in the catalytic site. Using comparative genomics we aligned the sequences of these phosphodiesterases with PDE7B, identified the same essential histidine and generated an analogous catalytically inactive histidine to glutamine mutant. Both the wild type and mutant forms PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19674025 of PDE7B were stably introduced using PiggyBac transposase and hygromycin selection. Verification of the overexpression was determined by qRT-PCR, and Western blot analysis. The effect of wild type and mutant PDE7B on cAMP levels was verified by cAMP ELISA. Overexpression of wild type PDE7B, but not catalytically inactive PDE7B, resulted in significantly lower levels of cAMP. As interactions between ECs and GBM cells are known to maintain the GBM stem-like cell state, we assayed the effect PDE7B in the GBM Perivascular Niche PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19675955 only cultures that contain a clonogenic stem-like cell are 
capable of forming a sphere. The fraction of wells without spheres as a function of cells plated provides a measure of the frequency of cells with clonogenic stem-like properties. When analyzed, the U87GL+PDE7B and the G144+ PDE7B cells had significantly higher frequencies of stem-like cells compared to cells expressing the catalytically inactive form of PDE7B. One out of every nine U87GL+ PDE7B cells had sphere-forming ability compared to one out of every 14 U87GL+PDE7B cells. Similarly, one out of every 11 G144+PDE7B cells had sphere-forming ability compared to one out of every 15 G144+PDE7B cells. In vitro expansion of the stem-like cell population by overexpression of PDE7B, together with the DMXB-A negative prognostic effect of PDE7B expression in human disease, suggested that overexpression of PDE7B might have a pro-tumorigenic effect in vivo. To investigate this possibility, we generated intracranial xenografts in NCR nude mice with 50,000 U87-GL+PDE7B or U87-GL+PDE7B cells engineered to also express firefly luciferase. Tumors were generated in three separate cohorts of mice with 5 mice per group in each cohort. Tumor growth was monitored with weekly bioluminescence imaging and survival was tracked. Tumors expressing wild type PDE7B exhibited a significantly greater rate of growth than those expressing mutant PDE7B. The greater rate of growth resulted in significantly decreased survival as determined by log-rank testing of Kaplan-Meier survival curves. Consistent with the BLI and survival data, histological analysis revealed that tumor size was consistently greater in the PDE7B WT tumor group than the PDE7B H217Q tumor group. Strikingly, PDE7B WT tumors exhibited a more aggressive tumor phenotype than is normally seen with U87 xenografts. Intracranial xenografts of U87 cells typically grow in a simple expansile fashion, without invasion of the surrounding tissue, resulting in compression of the surrounding brain. This pattern of growth is evident in the analysis of PDE7B H217Q tumors where a clear boundary is evident between the tumor and the surrounding brain. In contrast, PDE7B WT tumors grew with extensive infiltration. This was clearly evident upon examination of the leading edge of the xenografts where abundant tumor cells can be seen invading the surrounding brain parenchyma accompanied by robust neovascularization. Discussion The complexity of the tumor microenvironment presents obstacles to progress in both understanding molecular oncology and in developing therapeutics to effectively target contextualized tu