Expression of Thy-1/lacZ fusion genes in the CNS of transgenic mice.

Thy-1 is a cell surface glycoprotein of unknown function that is found on nerve cells and mature T-lymphocytes. To study the regulation of Thy-1 gene expression, mouse Thy-1.2 genomic sequences were joined to various marker sequences and the resulting chimeric constructs were used to produce nearly three dozen independent lines of transgenic mice. The starting point for our studies was an 8.2 kb EcoRI fragment that begins 1.7 kb 5' to the transcription start site and ends with 1.3 kb of 3' flanking sequences. Addition of a small marker oligonucleotide to the 3' untranslated region of this fragment had little or no effect on gene regulation. All of the lines derived from injection of this construct expressed the transgene in the appropriate tissues. Thus, as expected, the Thy-1.2 genomic fragment contains all of the information necessary for tissue-specific, position-independent expression of the modified transgene. Unexpectedly, Thy-1/lacZ hybrid genes did not mimic this behavior. Using either mRNA or histochemical detection of lacZ protein, these constructs were expressed in patterns that varied dramatically from line to line. This behavior suggests that integration site-specific effects dominate the cis-active Thy-1 regulatory elements leading to wide variability of expression. This is further emphasized by the observation that the bacterial reporter protein was found in a few non-neuronal cell-types, in contrast to the known pattern of native Thy-1 expression. These results suggest that either the Thy-1.2 sequences which are necessary for appropriate brain-specific expression are not contained solely within the proposed CNS enhancer in the first intron, or that fusion of the Thy-1.2 sequences with the lacZ coding region may disrupt normal Thy-1 regulatory signals (or result in the creation of new regulatory elements).