| The goal of this research was to identify proteins secreted by implanted astrocytes that may play a role in modulating the central nervous system (CNS) wound healing response. These studies were initially motivated for two reasons. First we sought to repeat a classical transplantation experiment in which transplanted immature astrocytes were shown to suppress the formation of scar in the CNS. We repeated this experiment using cell encapsulation technology, in order to determine whether soluble factors alone could alter the CNS wound healing response. Secondly, we wanted to assess whether cells could be used to either enhance (or reduce) the biocompatibility of an implanted biomaterial within the CNS, which ultimately could affect its performance. We found that both immature and mature encapsulated astrocyte populations were able to affect the CNS response to an implanted hollow fiber membrane. Immature astrocytes reduced the degree of astrogliosis and the presence of macrophages, while mature astrocyte implants increased both parameters. Physical isolation of the astrocytes mediated by cell encapsulation suggested that the astrocyte populations were modulating the CNS wound healing response by a mechanism involving soluble factors. In an effort to identify these factors, we examined the expression profiles of the immature and mature astrocyte populations using microarray technology. Three cytokines, Transforming Growth Factor beta 2 (TGFbeta2), Connective Tissue Growth Factor (CTGF), and Monocyte Chemoattractant Protein 1 (MCP-1), were identified with a higher level of expression by the mature astrocytes at both the RNA and protein levels. The functions of these factors may explain the enhanced the degree of astrogliosis and the recruitment of macrophages around the hollow fiber implants that contain mature astrocytes. Closer examination of the expression of these factors by mature astrocytes was not dependent on a maturation process, but rather on the presence of serum in the growth media. Serum also induced a permanent change in the gene expression in astrocytes, in that serum-induced genes continued after its removal. The results of these studies provide important insights into cell encapsulation technology and CNS injury, and also supports the use of a chemically defined, serum-free media for culturing cells prior to transplantation. |
