T1-ALPHA PROTEIN IS DEVELOPMENTALLY-REGULATED AND EXPRESSED BY ALVEOLAR TYPE-I CELLS, CHOROID-PLEXUS, AND CILIARY EPITHELIA OF ADULT-RATS

T1 alpha is the first marker gene known to be expressed in the adult l ung solely by the alveolar type I epithelial cell. Previous studies sh owed that T1 alpha transcripts are abundant in early rat embryos where they are found in the nervous system and in the foregut and certain o f its derivatives including the primitive lung. By mid- to late gestat ion T1 alpha messenger RNA (mRNA) expression is lost from neural tissu es but appears to increase in the lung throughout fetal life. To deter mine whether the T1 alpha transcripts are translated into protein, esp ecially in early embryos which sometimes express transcripts that are translationally silent, we performed immunohistochemistry on embryos a nd fetal tissues and analyzed certain tissues by western blotting usin g a monoclonal antibody against T1 alpha protein. T1 alpha protein is present at all sites that have previously been shown to express the mR NA and at similar developmental stages. As estimated from western blot s, T1 alpha protein abundance peaks at about fetal day 16 in the brain and decreases thereafter to a relative level in the adult that is low er than that of the neural tube of the day 13 embryo. Relative protein abundance in the lung is very low, although detectable, on embryonic day 13 but increases slowly until fetal day 20 when there is a dramati c increase. At the time of birth, restriction to the type I cell is no t complete and therefore must occur during postnatal lung development. Immunostaining reveals additional sites of expression in fetal and ad ult rats that had not been clearly visualized in previous in situ hybr idization studies. T1 alpha is present in mesonephric tubules and appa rently in primitive germ cells but is not detectable in specific cells in the adult kidney, ovary, or testis. However, cells of the choroid plexus of the central nervous system and the ciliary epithelium of the eye express T1 alpha in both fetuses and adults. The well-known funct ions of these epithelia are to elaborate cerebrospinal fluid and aqueo us humor respectively by processes of active ion transport and water f luxes, probably through the aquaporin 1 (channel-forming integral memb rane protein [CHIP] 28). We speculate therefore that T1 alpha protein may modulate or participate in these types of cellular functions in th e lung.