ZacI functions through TGFßII to negatively regulate cell number in the developing retina

Abstract

Background: Organs are programmed to acquire a particular size during development, but the regulatory mechanisms that dictate when dividing progenitor cells should permanently exit the cell cycle and stop producing additional daughter cells are poorly understood. In differentiated tissues, tumor suppressor genes maintain a constant cell number and intact tissue architecture by controlling proliferation, apoptosis and cell dispersal. Here we report a similar role for two tumor suppressor genes, the ZacI zinc finger transcription factor and that encoding the cytokine TGFßII, in the developing retina. Results: Using loss and gain-of-function approaches, we show that ZacI is an essential negative regulator of retinal size. ZacI mutants develop hypercellular retinae due to increased progenitor cell proliferation and reduced apoptosis at late developmental stages. Consequently, supernumerary rod photoreceptors and amacrine cells are generated, the latter of which form an ectopic cellular layer, while other retinal cells are present in their normal number and location. Strikingly, ZacI functions as a direct negative regulator of a rod fate, while acting cell nonautonomously to modulate amacrine cell number. We implicate TGFßII, another tumor suppressor and cytokine, as a ZacI-dependent amacrine cell negative feedback signal. TGFßII and phospho-Smad2/3, its downstream effector, are expressed at reduced levels in ZacI mutant retinae, and exogenous TGFßII relieves the mutant amacrine cell phenotype. Moreover, treatment of wild-type retinae with a soluble TGFß inhibitor and TGFß receptor II (TGFßRII) conditional mutants generate excess amacrine cells, phenocopying the ZacI mutant phenotype. Conclusion: We show here that ZacI has an essential role in cell number control during retinal development, akin to its role in tumor surveillance in mature tissues. Furthermore, we demonstrate that ZacI employs a novel cell non-autonomous strategy to regulate amacrine cell number, acting in cooperation with a second tumor suppressor gene, TGFßII, through a negative feedback pathway. This raises the intriguing possibility that tumorigenicity may also be associated with the loss of feedback inhibition in mature tissues.