Zac1 functions through TGFβIIto negatively regulate cell number in the developing retina

Lin Ma,Magdalena Götz,Carol Schuurmans,Sarah Mcfarlane,Natalia Klenin,Laurent Journot,Robert Cantrup,Annie Varrault,Dilek Colak

doi:10.1186/1749-8104-2-11

Lin Ma, Magdalena Götz + Show 7 more

Open Access

https://doi.org/10.1186/1749-8104-2-11

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Abstract

BackgroundOrgans 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 Zac1 zinc finger transcription factor and that encoding the cytokine TGFβII, in the developing retina.ResultsUsing loss and gain-of-function approaches, we show that Zac1 is an essential negative regulator of retinal size. Zac1 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, Zac1 functions as a direct negative regulator of a rod fate, while acting cell non-autonomously to modulate amacrine cell number. We implicate TGFβII, another tumor suppressor and cytokine, as a Zac1-dependent amacrine cell negative feedback signal. TGFβII and phospho-Smad2/3, its downstream effector, are expressed at reduced levels in Zac1 mutant retinae, and exogenous TGFβII relieves the mutant amacrine cell phenotype. Moreover, treatment of wild-type retinae with a soluble TGFβ inhibitor and TGFβ receptor II (TGFβRII) conditional mutants generate excess amacrine cells, phenocopying the Zac1 mutant phenotype.ConclusionWe show here that Zac1 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 Zac1 employs a novel cell non-autonomous strategy to regulate amacrine cell number, acting in cooperation with a second tumor suppressor gene, TGFβII, 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.

Highlights

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
Retinal ganglion cells (RGCs), horizontal cells, cone photoreceptors and amacrine cells are primarily generated during the second half of the embryonic period, while rod photoreceptor, bipolar and Müller glial cell production ends on postnatal days (P) 5–6 in the central retina [3]
At P2, P7 (Figure 1e, j) and P21 (Additional data file 1 (g,h)), Zac1 transcripts and protein were detected in scattered postmitotic cells in the inner nuclear layer (INL) and retinal ganglion cells (RGCs) layer (GCL; Figure 1k–o)

Summary

Introduction

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. Several regulatory strategies are employed to control cell number, including: direct negative regulators, which inhibit alternative cell fates but permit (or instruct) a primary fate; negative feedback pathways, acting as cell sensors that halt the continued genesis of specific cell types once a feedback signal reaches threshold levels; and cell counting mechanisms, whereby the number of times a progenitor divides before differentiating is genetically determined [1,2]. Differentiated RGCs, amacrine cells and cones secrete signals negatively regulating production of additional cells of that type [13,14,15,16]. GDF11, a transforming growth factor (TGF)β family member, has similar autoregulatory functions in other tissues, including the olfactory epithelium [18] and pancreas [19], while a related molecule, GDF8 (myostatin), negatively regulates skeletal muscle mass [20], suggesting a common role for these cytokines in cell number control

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