TGFβ2 activation status during cardiac morphogenesis

Abstract

Transforming growth factor beta (TGFβ) is secreted as a biologically inactive complex by many cell types in vitro, but little is known of TGFβ's activation status in vivo. This study examined the in vivo expression of active and total (active + acid-activatable) TGFβ2 in embryonic chicken hearts during cardiac morphogenesis (Hamburger-Hamilton stage 10-24). The concentration of TGFβ2 was measured by an enzyme-linked immunoassay that recognized active TGFβ2. Whole heart homogenates were either left untreated to measure active TGFβ2 or treated with acid before assay to measure total (active + acid-activatable) TGFβ2. Total TGFβ2 concentration increased more than 16-fold between stage 10/11 and stage 24. Active TGFβ2 concentration was highest at stage 14/15, but overall remained relatively constant varying at most by 2.8-fold. When expressed relative to total TGFβ2, the amount of active TGFβ2 progressively declined from 70% in stage 10/11 hearts to 7% in stage 24 hearts. The distribution of active and total TGFβ2 was examined by immunostaining with an antibody against active TGFβ2. Before immunostaining, sections were either treated with acid or left untreated to determine the distribution of total and active TGFβ2, respectively. Active TGFβ2 immunostaining was first detected in the endothelium, myocardium, and cardiac jelly of stage 14 hearts. Acid treatment had no effect on the distribution or intensity of immunostaining at this stage. Faint, active TGFβ2 immunostaining was restricted to the ventricular myocardium in stage 18 hearts. Acid treatment resulted in a marked increase in staining intensity in the ventricle, but no staining was observed in the atrium or outflow tract. In stage 24 hearts, faint active TGFβ2 staining was detected in the ventricle before acid treatment. After acid treatment, patches of intense punctate stain were found in all regions of the embryonic heart. Increases in TGFβ2 concentration and immunostaining intensity after acidification suggest that a significant amount of TGFβ2 is in the latent form. Stage-dependent differences in activation status suggest that activation may be a developmentally regulated process in the chick heart and support the notion that activation is an important step in regulating TGFβ actions in vivo. © 2001 Wiley-Liss, Inc.