Arturo Alvarez-Buylla and Erica L. Jackson, of the University of California, San Francisco, and colleagues reported their findings in a paper in the July 20, 2006, Neuron, published by Cell Press. In their studies, they sought to discover whether neural stem cells in the brain called B cells carry a receptor--known as platelet-derived growth factor receptor á (PDGFRá)--for the signaling molecule PDGF.
Neural stem cells are immature cells that serve as the continual source of new brain cells in adults, and PDGF is known as an important regulator of such cells. Also, PDGF has been implicated as a key signaling molecule underlying the formation of brain tumors. PDGF triggers such cell response by plugging into the target receptor on the stem cell, like a key inserting into a lock.
There had been indirect evidence that neural stem cells give rise to brain tumors. "However, it has not been shown in vivo that tumor stem cells are derived from normal stem cells or that a specific population of cells with demonstrated stem cell properties is capable of initiating tumor formation," wrote the researchers.
Using tracers, the researchers discovered that PDGFRá is, indeed, found on the stem cells in both mouse and human brain tissue. They also found that the receptor is triggered by PDGF in the stem cells to regulate their production of mature brain cells. The researchers located the PDGFRá-containing cells in the subventricular zone (SVZ) of the brain, which is the center for production of new brain cells in adults.
The researchers also found that infusing PDGF into mouse brain caused abnormal growth--called hyperplasia--of tumor-like nodules that invaded surrounding brain tissue. Their analysis indicated that the PDGF infusion caused the stem cells to halt their normal production of mature brain cells and launch into the abnormal proliferation pathway. Importantly, the researchers found that stopping the PDGF infusion caused a complete regression of the nodules.
The researchers wrote that "these findings are significant due to our limited knowledge of surface markers for neural stem cells. Our data also provide evidence of a link between these PDGFRá B cells and the early changes associated with tumor initiation, suggesting they may be targets of neoplastic transformation. The regression of atypical hyperplasia after PDGF removal described here suggests that inhibition of PDGF signaling could provide a useful therapy for those gliomas in which the pathway is upregulated, especially given the recovery of the normal architecture after regression of the hyperplasia."
In a preview of the paper in the same issue of Neuron, Santosh Kesari and Charles D. Stiles wrote that the new findings "lend weight" to the argument that the stem cells identified by Alvarez-Buylla and his colleagues are the cells or origin for malignant gliomas. They wrote that such work offers "therapeutic opportunities," emphasizing that "the people that matter the most do not have the luxury of time to watch this work unfold. The median interval from diagnosis to death for patients with malignant glioma is currently only 14 months."
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