Heart-specific activation of LTK results in cardiac hypertrophy, cardiomyocyte degeneration and gene reprogramming in transgenic mice

Leukocyte tyrosine kinase (LTK) is a receptor-type protein tyrosine kinase belonging to the insulin receptor superfamily. To elucidate its biological role, we generated transgenic mice expressing LTK under the control of cyto megarovirus enhancer and beta-actin promoter. The transgenic mice exhibited growth retardation and most of the transgenic mice died within several mon ths after birth, Interestingly, although LTK was expressed in several major organs, the activation (tyrosine-phosphorylation, kinase activity, and mul timerization) of LTK was observed selectively in the heart, where LTK was l ocalized on intracellular membrane, presumably on endoplasmic reticulum. Ec hocardiography showed that the transgenic heart underwent severe concentric hypertrophy, which resulted in reduced cardiac output, low blood pressure, and increased heart rate. Histological examination of the heart exhibited focal degeneration of cardiomyocytes. These histological changes were consi dered to be due to apoptosis, based on the finding that the sarcolemmas of the degenerative cardiomyocytes were well preserved. In addition, expressio n of fetal genes, such as atrial natriuretic peptide and skeletal alpha-act in, was markedly induced in the transgenic heart. These results indicate th at a certain tissue-specific mechanism of activating LTK exists in the hear t and that the activated LTK resulted in cardiac hypertrophy, cardiomyocyte degeneration and gene reprogramming. These findings will provide novel ins ights into the activating mechanism and biological role of LTK in vivo.