Tissue-specific enhancement and restriction of galanin gene expression in transgenic mice by 5' flanking sequences

Galanin (GAL) is a 29/30 amino acid residue neuropeptide that regulates a wide variety of neuroendocrine functions. Galanin is expressed in specific populations of neurons in the hypothalamus and other regions of the brain and in numerous peripheral sites. Previous studies in which galanin-reporter genes were transfected into neural crest-derived neuroblastoma and other tumor cells indicated that cell-specific galanin expression is controlled by gene elements on the 5' flanking sequence which enhance and restrict transcriptional activity. To determine how the gene sequences act in vivo, we first determined the distribution of endogenous galanin gene expression in normal mice. Galanin mRNA was detected in several parts of the central nervous system (CNS), and in several peripheral organs, including the pituitary, pancreas, small and large intestine, adrenal gland, lung, tongue, testes, ovary-fallopian tubes, and uterus, but not at detectable levels in the heart, liver, kidney, urinary bladder or skeletal muscle. We then created several lines of transgenic mice which contained either 5 or 0.131 kilobases (kb) of the bovine galanin gene 5' flanking sequence fused to the luciferase (luc) reporter gene (5GAL-luc vs. 0.1GAL-luc mice, respectively) and compared luciferase activity in these and other organs. In some regions of the CNS that expressed high amounts of galanin mRNA, such as the spinal cord, hypothalamus, thalamus, and medulla, transgene expression was significantly higher in 5GAL-luc vs. 0.1GAL-luc mice, whereas in certain other regions of the brain and in all peripheral organs, the ratio was strikingly reversed. It is concluded that 5 kb of flanking sequence contains elements that mediate basal transcriptional activity in certain parts of the CNS, but also contains sequences that restrict expression in many tissues. However, because the larger transgene was expressed at very low levels in some peripheral sites of high galanin expression such as the pituitary, pancreas, adrenal gland, and intestine, it is concluded that sequences on the 5 kb transgene are not sufficient to direct expression to these peripheral tissues in mice.