Integration Site‐Dependent Transgene Expression Used to Mark Subpopulations of Cells in vivo : An Example from the Neuromuscular Junction

Abstract

To produce transgenic mice, an exogenous gene is inserted into the germ line, usually by injection of the DNA construct into a pronucleus of a fertilized egg. In most cases the transgene is integrated into the genome at a single random site. Frequently, the transgenes are combinations of regulatory elements from one gene and protein coding sequences from another gene, the reporter. As expected, the promoter in the construct usually controls the expression pattern of the reporter. In some cases, however, transgenes have been constructed with regulatory elements that are not able to direct transcription on their own. In animals containing such transgenes, the expression of the reporter is dependent on endogenous regulatory elements near the chromosomal site of transgene integration. In the present study, an Escherischia coli beta‐galactosidase (lacZ) reporter gene linked to a weak promoter was selectively expressed in discrete subpopulations of cells in each of eight independently derived lines of mice. In one line (line 42), which we analyzed in detail, a subset of cells in skeletal muscle were lacZ‐positive. Specifically, fibroblasts close to neuromuscular junctions expressed the lacZ‐protein, whereas skeletal muscle fibroblasts far from synaptic sites and fibroblasts in other organs were lacZ‐negative. Moreover, Schwann cells at nerve terminals were lacZ‐positive, whereas Schwann cells in extramuscular nerves were lacZ‐negative. These results indicate the existence of differences between perisynaptic and extrasynaptic fibroblasts in normal skeletal muscle. They also demonstrate how such transgenic mice can be used to identify and mark discrete cell populations.