Title Non-invasive quantification of brain tumor-induced astrogliosis Permalink

Background: CNS injury including stroke, infection, and tumor growth lead to astrogliosis, a process that involves upregulation of glial fibrillary acidic protein (GFAP) in astrocytes. However, the kinetics of astrogliosis that is related to these insults (i.e. tumor) is largely unknown. Results: Using transgenic mice expressing firefly luciferase under the regulation of the GFAP promoter (GFAP-luc), we developed a model system to monitor astrogliosis upon tumor growth in a rapid, non-invasive manner. A biphasic induction of astrogliosis was observed in our xenograft model in which an early phase of activation of GFAP was associated with inflammatory response followed by a secondary, long-term upregulation of GFAP. These animals reveal GFAP activation with kinetics that is in parallel with tumor growth. Furthermore, a strong correlation between astrogliosis and tumor size was observed. Conclusions: Our results suggest that non-invasive, quantitative bioluminescent imaging using GFAP-luc reporter animal is a useful tool to monitor temporal-spatial kinetics of host-mediated astrogliosis that is associated with glioma and metastatic brain tumor growth. Background The tumor microenvironment is a dynamic niche for tissue remodeling because of its production of tumor celland host stromal cell-derived growth factors, cytokines and matrix proteins. Historically, the study of such host-stromal interactions has generally relied on classical histological methods such as immunohistochemistry, in situ hybridization or biochemical techniques such as immunoblotting and enzyme assays. Unfortunately, the analytical power of these techniques is limited by the ability of reagents to distinguish between tumor and host compartments and by the need for terminal harvest of tissues for analysis. For example, in malignant gliomas, tumor cells co-opt the functions of the surrounding brain to support their growth and invasion. However, gliomas fail to completely compromise an otherwise tight blood brain barrier of normal vessels, based on the wide range of drugs and small molecules that fail to cross the BBB and target brain tumors [1]. GFAP expression has been widely used as a marker for astrogliosis and the host response to injury [2-4] and its analysis has generally relied on immunohistochemistry rather than quantification. Recently, however, bioluminescent imaging of GFAP activity using transgenic GFAP-luc mice has been reported to measure astrogliosis in animal models of kainic lesions [5], prion infection [6], ischemic injury [7], and experimental autoimmune encephalomyelitis (EAE)[8], but has not been described in a tumor model. In this study, we used immunodeficient (i.e., Rag2) transgenic GFAP-luc mice [5] to grow orthotopic brain tumors and to monitor the coactivation of the GFAP promoter with tumor development. GFAP promoter activation was used as a surrogate marker for host compartment astrogliosis to assess tumor progression. Here we show that these GFAP-luc; Rag2 mice injected with malignant glioma cells can be used to monitor and quantify tumor-induced astrogliosis response of the host. Analysis of the serial imaging supports a model in which intracranial tumor injection induces an early GFAP response, which is likely a consequence of the local wounding of the stereotactic injection. This early response resolves and is followed by a secondary astrogliosis response correlating with tumor progression both in terms of kinetics and localization. * Correspondence: beliceiri@ucsd.edu Department of Surgery, University of California San Diego, San Diego, CA, USA Lee et al. BMC Neuroscience 2011, 12:9 http://www.biomedcentral. com/1471-2202/12/9 © 2011 Lee et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Results and Discussion Immunohistochemical analysis of GFAP activity in tumor-