Investigation of an hsp68-lacZ transgene inserted at the dystonia musculorum locus and expressed in the neural tube floor plate

Experiments involving the random insertion of foreign DNA constructs into the genome have frequently had unanticipated but informative consequences. In the Tg4 line of transgenic mice, an hsp68-lacZ transgene caused an insertional mutation of the dystonia musculorum (dt) locus. In the same line, the transgene gained an ectopic expression pattern in the neural tube floor plate. The co-occurence of these events suggested that the dt phenotype of postnatal sensory neuropathy might arise from a primary defect expressed in floor plate cells. To test this hypothesis, the cellular target of the dt mutation was determined by analyzing $dt/dt leftrightarrow +/+$ chimeras, in which wild type neurons and their axons were identified by their expression of a human neurofilament transgene. In these chimeras, axonal pathology was confined to the axons of mutant neurons, thus defining an intrinsic neuronal dt defect, and ruling out an extrinsic influence of floor plate cells. The expression of $ beta$-galactosidase in the Tg4 line was further used to examine, using histochemical staining, the morphology of floor plate cells, which play a role in the guidance of commissural axons. Floor plate cells were found to possess a number of morphological specializations which appeared to establish a high degree of membrane apposition with commissural axons. Unexpectedly, $ beta$-galactosidase activity was also detected adjacent to stained floor plate cells in profiles that could not be distinguished from midline axon segments, suggesting that a process of macromolecular transfer may normally operate between floor plate cells and commissural axons