VEGF deprivation-induced apoptosis is a component of programmed capillary regression

The pupillary membrane (PM) is a transient ocular capillary network, which can serve as a model system in which to study the mechanism of capillary re gression. Previous work has shown that there is a tight correlation between the cessation of blood flow in a capillary segment and the appearance of a poptotic capillary cells throughout the segment, This pattern of cell death is referred to as synchronous apoptosis (Lang, R, A., Lustig, M,, Francois , F,, Sellinger, M, and Plesken, H. (1994) Development 120, 3395-3404; Mees on, A., Palmer, M,, Calfon, M, and Lang, R, A. (1996) Development 122, 3929 -3938), In the present study, we have investigated whether the cause of syn chronous apoptosis might be a segmental deficiency of either oxygen or a su rvival factor. Labeling with the compound EF5 in a normal PM indicated no s egmental hypoxia; this argued that oxygen deprivation was unlikely to be th e cause of synchronous apoptosis, When rat plasma was used as a source of s urvival factors in an in vitro PM explant assay, inhibition of vascular end othelial growth factor (VEGF) all but eliminated the activity of plasma in suppressing apoptosis, This argued that VEGF was an important plasma surviv al factor. Furthermore, inhibition of VEGF in vivo using fusion proteins of the human Flk-1/KDR receptor resulted in a significantly increased number of capillaries showing synchronous apoptosis. This provides evidence that V EGF is necessary for endothelial cell survival in this system and in additi on, that VEGF deprivation mediated by flow cessation is a component of sync hronous apoptosis.