Tubedown-1 in remodeling of the developing vitreal vasculature in vivo and regulation of capillary outgrowth in vitro

Tubedown-1 (tbdn-1) is a mammalian homologue of the N-terminal acetyltransferase subunit NAT1 of Saccharomyces cerevisiae and copurifies with an acetyltransferase activity. Tbdn-1 expression in endothelial cells becomes downregulated during the formation of capillary-like structures in vitro and is regulated in vivo in a manner which suggests a functional role in dampening blood vessel development. Here we show that tbdn-1 is expressed highly in the vitreal vascular network (tunica vasculosa lentis and vasa hyaloidea propria) during the pruning and remodeling phases of this transient structure. The vitreal blood vessels of mice harboring a targeted inactivation of TGF-β2 fail to remodel and abnormally accumulate, a phenomenon reminiscent of the ocular pathology resembling persistent fetal vasculature (PFV) in humans. Since suppression of normal tbdn-1 expression has been previously observed in retinal vessel proliferation, we analyzed vitreal vascular changes and tbdn-1 expression in TGF-β2^-/- eyes. The nuclei of vitreal vessel endothelial cells in TGF-β2^-/- eyes express proliferating cell nuclear antigen (PCNA) and exhibit increased levels of active ^P42/44mitogen-activated protein kinase (phospho-^P42/44MAPK), characteristics consistent with proliferative endothelial cells. In contrast to normal vitreal vessels, collagen IV expression exhibited a disorganized pattern in the TGF-β2^-/- vitreal vessels, suggesting vessel disorganization and possibly a breakdown of vessel basal laminae. Moreover, vitreal vessels of TGF-β2^-/- mice lack expression of pericyte markers (CD13, alpha smooth muscle actin) and show ultrastructural changes consistent with pericyte degeneration. The accumulating vitreal blood vessels of TGF-β2^-/- mice, while maintaining expression of the endothelial marker von Willebrand Factor, show a significant decrease in the expression of tbdn-1. We addressed the functional role of tbdn-1 in the regulation of vitreal blood vessels using an in vitro model of choroid-retina capillary outgrowth. Clones of the RF/6A fetal choroid-retina endothelial cell line showing suppression of tbdn-1 levels after overexpression of an antisense TBDN-1 cDNA display a significant increase in the formation of capillary-like structures in vitro compared with controls. These findings suggest that tbdn-1 inhibits capillary-like formation in vitro and may serve to dampen vitreal blood vessel formation preceding the regression of the vitreal vasculature during development. Our results also suggest that tbdn-1 may participate with TGF-β2 in regulating normal development of the vitreal vasculature.