Expression of the Elastin Promoter in Novel Tissue Sites in Transgenic Mouse Embryos

Abstract

This paper is dedicated to the 80th birthday of Dr. Ines Mandl, our dynamic friend and colleague for many years. Ines made ground breaking advances and was one of the founders of the modern era in connective tissue biochemistry. One of the earliest memories of one of us (JR, although she may not remember!) is of a time in 1958 when Ines visited the Surgical Unit of the Walter Reed Army Hospital in Washington to consult on the possibility of debriding severe burns with the newly described collagenase isolated from Clostridium. Strikingly, this is a procedure in use today. This visit was typical of her intensity and desire to rapidly bring new discoveries in basic science to clinical application. In collaboration with her associates at Columbia University, she continued in this productive path in seminal studies on the critical role of elastase and elastic fiber destruction in the pathogenesis of pulmonary emphysema. It is likely that this crucial observation will ultimately lead to effective therapy. For many years, Ines was Editor of this Journal, providing leadership and direction to the emerging field of matrix biology. In addition to the debt we owe Ines for her pioneering work, we are grateful to her for the high personal standards she set and her marvelous enthusiasm.We have previously shown in a transgenic mouse line, in which 5.2 kb of the elastin promoter was linked to the reporter enzyme chloramphenicol acetyltransferase (CAT), that the highest levels of expression were found in embryonic lungs and aorta, while lower levels were detected in other elastin-containing tissues. Furthermore, in general, expression of the transgene showed developmental regulation similar to that of the endogenous gene. However, the precise location of cellular expression could not be determined in this model. To overcome this limitation, we have developed a similar model, but replaced CAT with the reporter enzyme β-galactosidase. Enzyme activity was readily detected in the transgenic mouse embryos in expected regions of tissue forming elastic fibers, including the dermis and elastic cartilage. Of considerable interest, however, was the novel finding of expression in specific areas of neuroepithelium of the brain and in the perichondrium surrounding areas destined to form hyaline cartilage in endochondral bone formation. These latter areas included all the bones of the limbs, the spine and rib cage. It appeared that these segments of elastin expression demarcated the border between the developing cartilage and the surrounding mesenchymal tissue. Elastin promoter expression was also found in developing somites, in the mesenchymal layer of the forming cornea of the eye, in the genital tubercle and in the epithelium destined to form the olfactory epithelium. These findings indicate that the elastin promoter is activated during embryonic development in a variety of tissues, suggesting that elastin gene expression may play a role in organizing cutaneous, skeletal and neural structures.