Gene Expression in the Brain

The goal of the laboratory is to understand the nervous system through knowledge of the structure and properties of proteins that mediate its functions. Our general experimental approach is to use selective cloning and other molecular techniques to identify molecules of interest, to define their structures, and, ultimately, to test their functions. In particular, we are interested in molecules that are selectively expressed in neural tissues or in specific neural cell types, on the assumption that such molecules are likely to mediate functions of importance to the nervous system. These experimental strategies have lead to the characterization of two proteins, myelin-associated glycoprotein and proteolipid protein, that compose myelin, the insulating sheath that is wrapped around axons; the proteolipid protein gene was shown to be disrupted by a single base mutation in the mouse mutant jimpy, which is unable to synthesize myelin. Other studies have focused on proteins expressed in astrocytes, the predominant cell type in the vertebrate brain; for example, glial fibrillary acidic protein (see figure) is a structural protein found almost exclusively in astrocytes. A third area of interest has been genes expressed in neurons at critical stages of development, such as axonal growth or synaptogenesis; this work has included the analysis of the expression of isotypes of tubulin and the synapse-associated protein SNAP-25.

More recently, we have begun to analyze gene expression in cells that play key roles in the neural response to injury. Several cell types are recruited following injury to the central nervous system: microglia are activated from a quiescent state, circulating macrophages invade the injury site, and astrocytes undergo a characteristic proliferative and morphological change ("reactive gliosis"). Our goals are to identify genes that encode products essential for the injury response and to characterize markers for these cell populations. Using cultures of purified glia cells as a source of mRNAs, we have applied two complementary approaches to identify cDNA clones encoding molecules of interest: subtractive cloning and differential display techniques. In particular, initial studies have focused on the protein tyrosine kinase and tyrosine phosphatase gene families, which have been implicated in signal transduction and cell differentiation pathways. These studies are the starting point for characterization of the corresponding mRNAs and functional studies of the encoded proteins. In addition to providing potential marker proteins for the various cell populations, these results indicate diversity among the signaling pathways utilized by the cells involved in the neural response to injury.