Using a soluble, extracellular, hgand-binding fragm ent of r a t PD G F-aR to sequester endogenous PDGF in th e developing retina, I have found evidence t h a t PD G F is im p o r ta n t for th e p ro p e r fo rm a tio n of th e r e tin a l astrocyte/capillary netw ork. The tru n ca te d PD GF-aR was delivered into the v itreo u s of th e eye by im p lan tin g Cos cells th a t were tran sfe cte d w ith an
expression vector encoding th e reagent. The Cos cells rem ained viable because of the suitable environm ent provided by the vitreous and proliferated over th e 5 day duration of th e experiment.
To m y knowledge, th is is th e first tim e a tru n c a te d g ro w th factor receptor of th is type h as been successfully delivered in vivo. T his approach m ight be of g en eral use; o th er tru n c a te d receptors analogous to ou r PDGF- aR 17 have been show n to bin d th e ir cognate ligand(s) an d , in some cases, efficiently seq u ester its activity in vitro, for example, PDGF-BR (D uan e t al., 1990). The tru n c a te d PD G F -aR inhibited th e m igration of re tin a l astrocytes from th e ir g e n erativ e zone in th e optic nerve across th e su rface of th e developing r a t re tin a . I h av e previously show n t h a t m ig ra tin g re tin a l astro cy tes ex p ress m RNA encoding PD G F-aR (M udhar e t al., 1993 an d C h a p te r 3) a n d t h a t th e optic fibre la y e r over w hich th e y m ig ra te is im m unoreactive for PDGF-A. I also showed th a t cells in im m a tu re blood vessels in th e optic fibre layer and elsewhere express mRNA encoding PDGF- B an d PDGF-BR. These observations suggested th a t th e PD G F signalling p ath w ay m ig h t be im p o rta n t for astrocyte m igration across th e optic fibre layer. The re su lts from th ese sets of studies support th is idea, a lth o u g h th ey do not distinguish betw een a direct or an indirect effect of PDGF on astrocyte m igration. R etin al astrocytes are th o u g h t to m igrate along cords of spindle cells, th e precu rso rs of blood vessels, th a t a re laid down in th e developing re tin a p rio r to astrocyte m ig ratio n (Shakib e t al., 1968; Stone a n d D reher, 1987; C han-Ling and Stone, 1991). The spindle cells become capillaries as the astrocytes m ig rate out over th e in n er surface of th e retin a. PDGF m ig h t act through PD G F-aR on astrocytes to stim u late astrocyte m otility directly, or m ight act in d ire ctly th ro u g h PDGF-BR on spindle cells to en h an ce th e ir property as a su b stra te for astrocyte m igration. Since th e spindle cells are in place prior to astrocyte m igration, th e experim ental regim en m ig h t in terfere w ith p o s tn a ta l d ev elo p m en t of th e p re v a sc u la tu re , th u s a ffe ctin g th e astrocytes.
PDGF m ig h t also enhance th e survival of astrocytes, act as a m itogen for astro cy te p recu rso rs or for astro cy tes directly. T he l a tte r I th in k is un lik ely since p relim in ary B rdU incorporation ex p erim en ts show ed th a t astrocytes w ere not actively engaged in DNA synthesis in norm al re tin a e as
well as in those th a t h ad been injected w ith Cos cells (d ata not shown). This suggests th a t astrocytes do n o t proliferate as th ey m igrate. F u rth e r studies w ith th e tru n c a te d PD G F -aR an d specific blocking ag en ts to PD G F a n d its receptors are required to clarify these issues.