Zac1 functions through TGFβIIto negatively regulate cell number in the developing retina
© Ma et al; licensee BioMed Central Ltd. 2007
Received: 03 April 2007
Accepted: 08 June 2007
Published: 08 June 2007
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© Ma et al; licensee BioMed Central Ltd. 2007
Received: 03 April 2007
Accepted: 08 June 2007
Published: 08 June 2007
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 Zac1 zinc finger transcription factor and that encoding the cytokine TGFβII, in the developing retina.
Using 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.
We 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.
Tissues and organs are genetically programmed to achieve their optimal, mature size, defined by total cell number and individual cellular dimensions. 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]. In the vertebrate retina, negative feedback pathways are used recurrently for cell number control. The retina is composed of one glial and six neuronal cell types that are present in stereotyped proportions in each vertebrate species [3–5]. Based on lineage tracing, all retinal cell types are derived from multipotent progenitor cells [6–11], although distinct cell lineages likely also exist [1, 12]. In mouse, 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 . Differentiated RGCs, amacrine cells and cones secrete signals negatively regulating production of additional cells of that type [13–16]. However, only signals limiting production of RGCs have been identified, including Sonic hedgehog (Shh) and growth and differentiation factor-11 (GDF11) . GDF11, a transforming growth factor (TGF)β family member, has similar autoregulatory functions in other tissues, including the olfactory epithelium  and pancreas , while a related molecule, GDF8 (myostatin), negatively regulates skeletal muscle mass , suggesting a common role for these cytokines in cell number control.
We identified Zac1 (zinc finger protein that regulates apoptosis and cell cycle arrest or pleiomorphic adenoma gene-like 1 (Plag-l1))  in a screen designed to isolate genes involved in neural fate specification . Zac1 encodes a seven-C2H2 zinc finger protein that acts as a transcriptional activator or repressor . Zac1 is a known tumor suppressor gene and is frequently lost in multiple carcinomas . Zac1 is also maternally repressed through genomic imprinting, a mode of epigenetic control common to many genes regulating embryonic growth. Recently, a Zac1 null mutation was shown to lead to intrauterine growth restriction, consistent with the kinship theory that paternally expressed genes are growth promoting . However, growth retardation was not expected if Zac1 has tumor suppressor properties, promoting cell cycle exit and apoptosis [21, 24]. We therefore examined Zac1 function at the cellular level, focusing on the developing retina, where it is robustly expressed . Notably, in our initial cross-species studies in Xenopus, murine Zac1 unexpectedly promoted proliferation . Herein we describe intra-species loss- and gain-of-function assays in mouse that in contrast reveal tumor suppressor-like properties for Zac1 in the retina. Zac1 is required to induce cell cycle exit and apoptosis at late developmental stages, with Zac1 mutant retinae becoming hypercellular, containing supernumary rod photoreceptors and amacrine cells. Strikingly, Zac1 negatively regulates rod and amacrine cell numbers through distinct autonomous and cell non-autonomous (TGFβII-mediated) inhibitory mechanisms, respectively.
At P2 (not shown), 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 RGC layer (GCL; Figure 1k–o). Double immunolabeling with cell type-specific markers at P7 revealed Zac1 expression in CRALBP+ Müller glia (64.1% ± 6.26% Zac1+cells; n = 3 retinae; Figure 1k,k'), syntaxin+ (not shown) and Pax6+ amacrine cells (17.5% ± 3.6%; Figure 1m,m'), Brn3a+ RGCs (17.2% ± 5.0%; Figure 1o,o') and calbindin+ horizontal cells (1.2% ± 0.7%; Figure 1l,l'). Zac1 was not detected in protein kinase C (PKC)-expressing bipolar cells (Figure 1n,n') or in rod and cone photoreceptors in the outer nuclear layer (ONL).
Zac1 is thus expressed biphasically in the retina, initially in dividing retinal progenitors and later in Müller glia, RGCs, amacrine and horizontal cells.
To investigate the in vivo requirement for Zac1, we analyzed embryos with a Zac1 null allele . Because Zac1 is maternally imprinted, Zac1+m/- heterozygotes inheriting a wild-type allele from their mother are effectively mutant for Zac1. Indeed, imprinting occurs in the gametes, and complete methylation of Zac1 is achieved in 96.8% of mature oocytes . Accordingly, Zac1+m/- retinae were devoid of Zac1 immunolabeling (Additional data file 2) and were thus considered equivalent to null mutants throughout this study.
An ECL may develop due to an overall increase in retinal cell number and/or aberrant cellular migration. To determine if Zac1+m/- retinae were hypercellular, DAPI-labeled nuclei were counted. In ECL-containing Zac1+m/- explants (hereafter designated Zac1+m/-+ECL), there was a 1.34-fold increase in the number of INL cells (p < 0.001; 442.9 ± 17.9 cells/field; n = 9 retinae) compared to wild-type controls (329.5 ± 22.0 cells/field; n = 10) or non-ECL containing mutants (henceforth simply designated Zac1+m/-; 314.0 ± 22.1 cells/field; n = 3; Figure 2i). Strikingly, Zac1+m/-+ECL retinae also exhibited a 1.23-fold increase in ONL cells (p < 0.01; 969.4 ± 46.1 cells/field; n = 9) compared to wild-type controls (790.3 ± 40.7 cells/field; n = 10) or Zac1+m/- (735 ± 106.7 cells/field; n = 3; Figure 2i). In contrast, cellular contents of the GCL were comparable in wild-type (59.2 ± 3.1 cells/field; n = 10), Zac1+m/-+ECL (62.0 ± 4.3 cells/field; n = 9) and Zac1+m/- (55.3 ± 1.8 cells/field; n = 3) explants. Zac1 is, therefore, an essential negative regulator of retinal cell number and is also required to orchestrate appropriate cellular migration.
To identify the expanded cell population(s) in Zac1+m/-+ECL retinae, E18.5→8DIV explants were immunostained with cell type-specific markers. Strikingly, almost all cells in the Zac1+m/- ECL expressed the homeodomain transcription factor Pax6 (Figure 2f,f'), which was also expressed by amacrine cells in the INL and GCL in wild-type (Figure 2e,e',f,f') and Zac1+m/- (data not shown) E18.5→8DIV explants. Although Pax6 labels both amacrine cells and RGCs , RGCs rapidly undergo apoptosis following optic nerve transection (in explants ), allowing us to assign an amacrine cell identity to ECL cells. Accordingly, no other RGC markers (Brn3a/3b, Thy1.2; not shown) were detected in the ECL or GCL of wild-type or Zac1+m/-+ECL explants. Moreover, RGC differentiation is essentially complete by E18.5, and at this stage, equivalent numbers of RGCs were labeled by Brn3a (p = 0.95) and Brn3b (p = 0.23) in wild-type and Zac1+m/- retinae, indicating Zac1 does not regulate RGC number (n = 3 for each; total n = 6; Additional data file 4). Furthermore, syntaxin, which labels amacrine cell membranes and processes in the inner plexiform layer (IPL; Figure 2g,g'), marked duplicated and disorganized synaptic plexi (IPL/IPL') in Zac1+m/-+ECL explants (Figure 2h,h'). Finally, amacrine cell subtype markers, including Bhlhb5, calbindin, GABA and the GlyT1 glycine transporter, were all expressed in Zac1+m/- ECL (Additional data file 3 (k–p)).
Quantitation of Pax6+ nuclei in E18.5→8DIV explants revealed a 1.31-fold increase (p < 0.01) in the percentage of amacrine cells in Zac1+m/-+ECL retinae, while Zac1+m/-explants contained wild-type proportions of these interneurons (wild type: 15.1 ± 0.5%; n = 4; Zac1+m/-: 15.2 ± 1.3%; n = 3; Zac1+m/-+ECL: 19.8 ± 1.3%; n = 3; Figure 2j). In contrast, all other INL cell types were present at equivalent ratios in wild-type and Zac1+m/-+/-ECL retinae, including bipolar cells (Chx10+; wild type: 8.7 ± 0.6%; n = 4; Zac1+m/-: 10.5 ± 0.9%; n = 3; Zac1+m/-+ECL: 7.2 ± 1.3%; n = 3), Müller glia (CRALBP+: wild type: 5.2 ± 0.3%; n = 4; Zac1+m/-: 4.3 ± 0.2%; n = 3; Zac1+m/-+ECL: 3.9 ± 0.5%; n = 3; p27Kip1+: wild type: 5.9 ± 0.5%; n = 4; Zac1+m/-: 5.1 ± 0.7%; n = 3; Zac1+m/-+ECL: 5.8 ± 0.1%; n = 3) and horizontal cells (calbindin+; identified also by morphology and apical location; wild type: 1.1 ± 0.3%; n = 3; Zac1+m/-: 0.8 ± 0.2%; n = 4; Zac1+m/-+ECL: 0.1 ± 0.1%; n = 3; Figure 2j; Additional data file 3 (m,n)).
Cones normally comprise only 3% of the murine photoreceptor pool . In Zac1+m/-+ECL and wild-type retinae, similar numbers of cones were labeled with peanut agglutinin (PNA; p = 0.26; wild type: 50.4 ± 1.2cells/field; n = 3; Zac1+m/-+ECL: 64.6 ± 10.1cells/field; n = 3) and s-opsin (p = 0.70; wild type: 44.22 ± 8.91cells/field; n = 3; Zac1+m/-+ECL: 39.75 ± 6.66cells/field; n = 4; Additional data file 3 (g–j)). Instead, the vast majority of ONL cells in wild-type and Zac1+m/-+ECL explants expressed the rod-specific markers rhodopsin (Figure 2c, d) and Nr2e3 (not shown), indicating that the rod pool is expanded in Zac1+m/-+ECL retinae. Zac1 therefore ensures appropriate numbers of rod photoreceptors and amacrine cells are generated during development.
Compensatory mechanisms exist in the retina to ensure that cellular content remains constant, with excess proliferation often balanced by an increase in apoptosis [34, 35]. Given that Zac1 induces apoptosis when misexpressed in cell lines , we tested if it were also required for the normal program of cell death in the retina, using activated-caspase-3 (ac-3), a downstream effector and early marker of commitment to the cell death pathway . During embryonic retinal development, apoptosis peaks during the optic cup stage (E10–E11) in the presumptive retinal pigmented epithelium (rpe) and optic stalk and again between E15.5–E17.5, primarily in retinal cells adjacent to the optic nerve head [37–40]. We analyzed ac-3 staining in wild-type (n = 6) and Zac1 mutant retinae (n = 6) at E10.5 and E15.5 but did not observe more than a few apoptotic cells per retinal section in either genotype (Additional data file 5 (a-d)). Similarly, at E18.5 (p = 0.14; wild type: 0.4 ± 0.02%; n = 3; Zac1+m/-: 0.5 ± 0.03%; n = 3) and in E18.5→2DIV explants (p = 0.93; wild type: 3.5 ± 0.4%; n = 3; Zac1+m/-: 3.5 ± 0.2%; n = 3), comparable levels of apoptosis were observed in both genotypes (Figure 3q). In contrast, after 4 and 8DIV, there were 3.48-fold (p < 0.01; wild type: 4.5 ± 1.0%; n = 7; Zac1+m/-: 1.3 ± .0.2%; n = 5/6) and 2.02-fold (p < 0.05; wild type: 2.9 ± 0.2%; n = 3; Zac1+m/-: 1.4 ± 0.3%; n = 4) reductions, respectively, in the number of ac-3+ retinal cells in Zac1+m/- explants (Figure 3o–r).
The reduction in cell death in Zac1+m/- explants could contribute to the increase in amacrine and rod cell numbers. However, the number of ac-3/Pax6-double+ amacrine cells was similar in E18.5→4DIV explants from both genotypes (p = 0.15; wild type: 1.6 ± 0.4%; n = 3; Zac1+m/-: 0.9 ± 0.1%; n = 3; Additional data file 5 (e–i)). In contrast, there was a 1.82-fold reduction in ac-3+ ONL photoreceptors in Zac1+m/- E18.5→8DIV explants (p < 0.05; wild type: 2.9 ± 0.2%; n = 3; Zac1+m/-: 1.60 ± 0.4%; n = 4). Zac1 deficiency therefore perturbs pro-apoptotic pathways that adjust cell numbers at late stages of retinogenesis, likely contributing to the increase in rod cell number.
To determine if Zac1 was a direct, negative regulator of rod and/or amacrine fates, we examined the molecular phenotype of retinal cells electroporated at P0 and cultured 8DIV. No differences were observed in the ratio of GFP+ cells that became Pax6+ amacrine cells after electroporation of pCIG2 (p = 0.73; 11.7 ± 3.4%; n = 6; Figure 5i–,k,s) versus pCIG2-Zac1 (10.2 ± 2.3%; n = 6; Figure 5l,s). Similarly, misexpression of Zac1 at E15.5 and E17.5, during the peak of amacrine cell genesis, did not affect amacrine cell number (Additional data file 6). In contrast, Zac1 misexpression at P0 resulted in a 4.49-fold reduction in rhodopsin+ rods (p < 0.01; pCIG2: 52.3 ± 4.5%; n = 3; Zac1: 11.6 ± 5.4%; n = 3; Figure 5m,n,s) and a 2.43-fold reduction in Nr2e3-labeled rods (p < 0.05; pCIG2: 30.8 ± 3.6%; n = 3; Zac1: 12.7 ± 3.1%; n = 3; Figure 5s). Zac1-misexpressing progenitors instead preferentially differentiated into Chx10+ bipolar cells (1.72-fold increase; p < 0.05; pCIG2: 11.1 ± 1.8%; n = 6; Zac1: 19.1 ± 2.2%; n = 6) and p27Kip1+ Müller glia (1.95-fold increase; p < 0.05; pCIG2: 14.4 ± 2.2%; n = 6; Zac1: 28.1 ± 4.5%; n = 6), cells types normally generated along with rods postnatally (Figure 5o–s). Zac1 is thus a potent inhibitor of a rod fate but does not directly suppress amacrine cell genesis.
To understand how Zac1 controls amacrine cell numbers, we next determined when ectopic amacrine cells first appeared in Zac1+m/- retinae. In mouse, amacrine cell genesis normally peaks at E15.5, tapering off before birth  (Figure 4j). At E18.5, genes involved in amacrine fate specification/differentiation, including Math3, Foxn4, NeuroD, Pax6 and Barhl2 [41–43], were expressed in an indistinguishable manner in wild-type and Zac1+m/- retinae, as were several other genes involved in the specification of all other cell types (Additional data file 7). Cell fate specification was thus grossly normal in E18.5 Zac1+m/- retinae. In contrast, in E18.5→4DIV Zac1+m/- explants, Pax6 (Figure 4a,a',b,b',e,f), Six3, Barhl2 and Math3 (Additional data file 8 (i–n)) expression increased, suggesting the amacrine cell population expanded during early postnatal stages in Zac1+m/- retinae.
To verify that amacrine genesis increased postnatally in Zac1+m/- retinae, we performed birthdating. E18.5 retinal explants were labeled with BrdU after 1, 2 and 4DIV and then cultivated for 8DIV (Figure 4g,g',h,h'). More BrdU+/Pax6+ amacrine cells were born at 1DIV (1.76-fold increase; p < 0.05; wild type: 22.7 ± 3.4%; n = 3; Zac1+m/-+ECL: 39.9 ± 3.8%; n = 4; Zac1+m/-: 29.8 ± 3.0%; n = 3), 2DIV (2.34-fold increase; p < 0.05; wild type: 8.6 ± 1.3%; n = 7; Zac1+m/-+ECL: 20.0 ± 4.4%; n = 4; Zac1+m/-: 10.1 ± 2.8%; n = 6) and 4DIV (5.42-fold increase; p < 0.05; wild type: 1.7 ± 1.0%; n = 6; Zac1+m/-+ECL: 9.2 ± 2.4%; n = 3; Zac1+m/-: 4.3 ± 0.4%; n = 2; Figure 4g,g',h,h',i) in Zac1+m/-+ECL explants compared to wild type, confirming that the period of amacrine cell genesis was prolonged.
In Zac1 mutants, a notable reduction in TGFβII expression was observed in onbl progenitors and in Pax6+ amacrine cells in the INL, while GCL levels were similar to wild type (Figure 7j–l). An overall reduction in TGFβII levels was confirmed by western blot, demonstrating that the 25 kDa isoform (note: labile 12 kDa mature form not detected) was reduced in most (n = 8/12) Zac1+m/- explants (p < 0.05; signal normalized to β-actin; wild type: 1.5 ± 0.04; n = 4; Zac1+m/-: 0.9 ± 0.2; n = 3/4; Figure 7o,p). To confirm that TGFβ signaling was indeed reduced in Zac1+m/- retinae, we examined expression of the downstream effector, pSmad 2/3. In E18.5→4 DIV wild-type explants, pSmad2/3 was expressed at diffuse levels throughout the retinae, but at significantly higher levels in the GCL and the basal half of the INL, where differentiated amacrine cells reside, as well as at lower levels in dividing progenitor cells in the onbl (Figure 7m). In contrast, pSmad2/3 levels were decreased in the INL and onbl progenitors in Zac1+m/- explants (Figure 7n). Accordingly, western blot analysis revealed a significant reduction in pSmad2/3 protein levels in Zac1+m/- versus wild-type E18.5 > 4 DIV explants when normalized to β-actin (p < 0.01; n = 6/8 mutants analyzed), while total Smad2/3 protein levels were similar in both genotypes (n = 4 for each genotype; Figure 7o,q). TGFβ signaling was thus attenuated in Zac1+m/-retinae.
While the analysis of TGFβRII mutants supported a role for this signaling pathway in regulating amacrine cell number, we were precluded from analyzing the effects of mutating TGFβRII at postnatal stages as the mutants unexpectedly died at early postnatal stages. We therefore used a complementary pharmacological approach to mimic the late reduction in TGFβ signaling observed in Zac1 mutant retinae. The pharmacological inhibition of TGFβII in the early postnatal rat retina increases proliferation and cell number , but specific effects on amacrine cell genesis were not analyzed. In accordance with experiments in rat , addition of 0.5 μg/ml soluble TGFβRII-Fc receptor to E18.5→8DIV retinal explants resulted in a 1.55-fold increase in INL/GCL cell number compared to vehicle controls (p < 0.01; control: 387.1 ± 35.0 cells/field; n = 3; TGFβRII-Fc: 601.6 ± 62.1 cells/field; n = 3; Figure 8k,l), while 0.1 μg/ml had no effect (not shown). Moreover, the inhibition of TGFβII signaling resulted in a 1.50-fold increase in the absolute number of amacrine cells (p < 0.01; vehicle control: 220.2 ± 5.9 cells/field; n = 3; TGFβRII-Fc: 331.2 ± 15.3 cells/field; n = 3; Figure 8k–m). These results are consistent with a requirement for TGFβ signaling to negatively regulate amacrine cell number during development.
Next, to show that attenuation of TGFβ signaling underlies amacrine cell expansion in Zac1+m/- retinae, we performed a rescue experiment. Recombinant TGFβII (or vehicle control) was added to wild-type and Zac1+m/- E18.5→8DIV explants. In control explants, the percentage of Pax6+ amacrine cells was elevated 1.38-fold in Zac1+m/-+ECL versus wild-type explants (p < 0.01; wild type: 53.6 ± 4.2% INL/GCL cells; n = 4; Zac1+m/-+ECL; 68.7 ± 4.8% INL/ECL/GCL cells; n = 3; Figure 8p). In contrast, following exposure to TGFβII for 8DIV, the percentage of amacrine cells was equivalent in wild-type and Zac1+m/-+ECL explants (wild type: 60.4 ± 2.5% INL/GCL cells; n = 3; Zac1+m/-+ECL: 61.5 ± 2.9% INL/ECL/GCL cells; n = 3; Figure 8n–p). Strikingly, however, an ECL still formed in TGFβII-treated Zac1+m/- explants (Figure 8o), suggesting that an alternative, non-TGFβ-mediated pathway underlies amacrine cell migration defects. This is also consistent with the inability of TGFβRII-Fc to induce an ECL (Figure 8l). These studies implicate attenuated TGFβII signaling in amacrine cell expansion in Zac1+m/- retinae.
The development of a functional retina requires that appropriate numbers of each cell type be generated. Hence, the molecular events that guide cell fate specification and differentiation must be tightly coordinated with those that govern cell number control. Here we demonstrate that the Zac1 tumor suppressor is an essential negative regulator of retinal size, controlling the absolute number of rod photoreceptors and amacrine cells generated during development. Strikingly, Zac1 regulates rod and amacrine cell genesis through distinct cell autonomous and non autonomous mechanisms, respectively (Figure 9). While Zac1 is a direct negative regulator of a rod photoreceptor fate, it regulates amacrine cell genesis by controlling the expression of TGFβII, which serves as an amacrine cell negative feedback signal. Zac1 and TGFβII thus join a growing list of tumor suppressor genes with established roles in retinogenesis (for example, Rb, p53, p27Kip1 [33, 35, 40, 44, 47–52]), but are the first tumor surveillance molecules shown to control neuronal number through a negative feedback or 'cell sensing' mechanism.
The widespread expression of Zac1 in dividing progenitor cells in the retina (this study) and throughout the developing neural tube [25, 53–55] suggested that it would have an early role in neural development. Unexpectedly, we found that in the murine retina, Zac1 function is restricted to the early postnatal period. While we cannot rule out the possibility that Zac1 functions redundantly with other factors to regulate early events in retinal development, we would predict that the tumor suppressor-like properties of Zac1 would have to be actively suppressed during early retinal development as most cells that express Zac1 at these stages continue to divide for some time. Indeed, we show here that Zac1 is required to promote cell cycle exit only at late stages of retinogenesis, a context dependency that is also characteristic of other tumor suppressor genes and oncogenes . Specifically, we show that, in Zac1 mutants, retinal progenitor cells divide excessively, similar to p27Kip1 mutants [35, 52] and in contrast to Rb mutants, where committed precursors instead fail to exit the cell cycle [33, 47, 48]. Our demonstration that cyclin D1 expression is upregulated in Zac1+m/- retinae provides some insight into the molecular mechanisms underlying Zac1-mediated control of the cell cycle. However, several observations make it unlikely that Zac1 functions directly through p27Kip1 or the related cyclin dependent kinase (CDK) inhibitor (CDKI) p57Kip2 to regulate cell cycle exit. Firstly, p27Kip1 is not required in a temporally restricted manner in the retina, and p57Kip2 is only required at early stages of retinal development [35, 52, 57], which contrasts with the late temporal requirement for Zac1. Furthermore, expression of the Kip family CDKIs was not altered in Zac1 mutants, and while there was an increase in p27Kip1 expression following Zac1 misexpression, it was specific to Müller glia, where this CDKI is normally expressed, and not observed in other cell types. Moreover, a previous cell culture study reported that Zac1 promoted cell cycle exit independently of Kip-family CDKIs or other classic cell cycle regulators such as Rb .
The requirement for Zac1 to promote cell cycle exit and apoptosis at late stages of retinal development likely contributes to the formation of hypercellular retinae in mutants, but does not explain why rod photoreceptors and amacrine cells are the only two cell types that are specifically expanded. Strikingly, misexpression of Zac1 robustly inhibited rod differentiation, implicating Zac1 as a bona fide negative regulator of this cell fate. Accordingly, Zac1 expression declines in progenitor cells at P0 when rod photoreceptor genesis begins to peak. Zac1 is also not expressed in differentiated ONL photoreceptors. However, we cannot rule out the possibility that cell non-autonomous mechanisms may also underlie the expansion of the rod pool in Zac1+m/- retinae. Indeed, we found that the generation of excess rods is directly linked to the formation of an ECL, both occuring in the same approximately 55% of Zac1+m/- retinae. Notably, we implicated attenuated TGFβ signaling , a proapoptotic pathway, in the amacrine cell expansion. However, reduced TGFβ signaling may also underlie the decreased apoptosis we observed in Zac1+m/- ONLs, consequently contributing to the expansion of the rod pool.
Zac1 misexpression also increased bipolar and Müller glial production in our gain-of-function assays, but rather than proposing that Zac1 is instructive for these fates, we favor the interpretation that progenitor cells prevented from adopting a rod fate instead acquire later-born fates by default. Accordingly, in Zac1+m/- retinae, we did not observe compensatory decreases in bipolar and Müller glial cell number. Nevertheless, in Xenopus, murine Zac1 also promoted Müller glial as well as RGC genesis, suggesting it might be instructive for a glial identity in different vertebrate species . However, there are numerous examples whereby misexpression of a murine gene in Xenopus specifies distinct cell fates compared to misexpression in a mouse model (for example, Mash1 promotes a rod fate when misexpressed in mouse and a bipolar fate in Xenopus [59, 60]. Moreover, in a previous study we showed that murine Zac1 unexpectedly promoted proliferation in Xenopus retina , in sharp contrast to its ability to promote cell cycle exit in the murine retina (this study) and cell lines in vitro [21, 24]. To simplify our model of Zac1 retinal function, we therefore consider results obtained in mouse and Xenopus as independent systems where gene function may differ substantively.
Feedback pathways exist in diverse biological systems, including the counting factor in Dictyostelium, which dictates group size , Drosophila miRNA9a, which regulates sensory organ precursor number by downregulating Senseless expression , and the well established role of feedback signals in regulating cell number in vertebrate liver, pancreas, olfactory epithelium and retina . Feedback pathways operate by secreting limiting amounts of extrinsic signals that must reach threshold levels to signal cessation of cell genesis . Our data support the idea Zac1 acts in post-mitotic amacrine cells during the postnatal period to regulate TGFβII expression, which in turn suppresses amacrine cell genesis. However, our analysis of TGFβRII mutants also indicates that deleting TGFβ signaling earlier in development (that is, from E16 to E18.5), during the peak period of amacrine cell genesis, can also influence amacrine cell genesis. Invoking a threshold model for TGFβII could help explain why defects in cell cycle exit and expansion of the amacrine cell population were not completely penetrant phenotypes in Zac1 mutants. Indeed, developmental processes are known to be highly sensitive to levels of signaling molecules, and stochastic differences in signaling often account for phenotypic variability . Moreover, abrogation of the feedback pathway regulating sense organ production in Drosophila, through deletion of miR-9a, similarly results in variable expressivity and penetrance of neuronal overproduction .
Notably, amacrine cell migration defects and the subsequent formation of an ECL were independent of attenuated TGFβ signaling in Zac1 mutant retinae. While we attribute the generation of an ECL to the mutation of Zac1, it remains a possibility that ECL formation requires both this genetic deletion as well as the loss of RGCs that occurs in retinal explant cultures, a possibility we cannot directly address given that Zac1 mutants die at birth. Another possibility is that Zac1 directly regulates cell migration by controlling the expression of cell adhesion genes, an idea based on a meta-analysis of microarray data in which several extracellular matrix molecules that could potentially modulate cell adhesion/migration were found to be co-regulated with Zac1 . The underlying cause of ECL formation is the subject of current investigations.
Here we demonstrate that Zac1 is an essential negative regulator of retinal size, controlling the absolute number of rod and amacrine cells generated during development. Strikingly, while Zac1 acts as a direct negative regulator of a rod fate, it negatively regulates amacrine cell genesis via TGFβII-mediated negative feedback inhibition. Zac1 and TGFβII are thus the first tumor surveillance molecules shown to control neuronal number through a negative feedback, 'cell sensing' mechanism. In summary, Zac1 regulates cell number and migration in the developing retina, highly reminiscent of its function in the prevention of tumor formation, suggesting that similar cellular and molecular mechanisms may underlie these processes.
For embryo staging, the day of the vaginal plug was considered E0.5. Generation of the Zac1 mutant allele was previously described . The Zac1 mutant allele was maintained on a C57BL/6 background. Zac1+m/- heterozygous embryos were generated by crossing Zac1+/- heterozygous males to C57BL/6 females. Primers for PCR genotyping (35 cycles; 94° for 1 minute, 60° for 1 minute, and 72° for 1 minute) of Zac1 were: wild type 5': AGTGACTCCCCACCTTCTTTCTG; wild type 3': CTTGCCACATTTTTGACAGCG; mutant 5': TGACCGCTTCCTCGTGCTTTAC; mutant 3': CCCCCCAGAATAGAATGACACC. Genotyping of GLAST::CreERT2 and R26R reporter mice were previously described . The floxed TGFβRII allele was previously reported  and was genotyped by PCR (38 cycles; 95° for 30 s, 62° for 30 s, and 72° for 40 s) with: primer 1 5': TGG GGATAGAGGTAGAAAGACATA-3'; primer 2 5': TATGGACTGGCT TTTGTATTC. To induce deletion of the TGFβRII gene, 3 mg of tamoxifen was administered by oral gavage at E16.0 as previously described .
For RNA in situ hybridization, tissue preparation and experimental procedures were followed as previously described . Briefly, tissue was fixed in 4% paraformaldehyde (PFA)/1X-phosphate buffered saline (PBS) overnight at 4°C, cryopreserved in 20%sucrose/1X PBS overnight at 4°C and embedded in Cryomatrix™ (Anatomical Pathology USA [Pittsburgh, PA, USA]). Digoxygenin (dig)-labelled probes were generated using a dig-UTP labeling mix and T3, T7 or SP6 RNA polymerases according to the manufacturer's instructions (Roche [Laval, QC, Canada]). Mouse probes included Zac1 , Hes1, Hes5, Mash1 , Ngn2 , Math3 , Math5 , NeuroD , Pax6 , Rx , Crx , Chx10 . Foxn4  and Barhl2 , Six3  and s-opsin .
For immunohistochemistry, fixation in 4% PFA/1 × PBS was shortened to 1–2 h at 4°C. Primary antibodies were incubated on slides overnight at 4°C or 1 h at room temperature. The following primary antibodies were used: rabbit active-caspase 3 (1/500; Promega [Madison, WI, USA]), mouse Brn3a (1/500; Chemicon [Temecula, CA, USA]), goat anti-Brn3b (1/250; Santa Cruz [Santa Cruz, CA, USA]), mouse anti-BrdU (5-bromo-2'-deoxyuridine, 1/500; Roche), rat-anti-BrdU (1/10; Oxford Biotech [now Antibodies by Design, Raleigh, NC, USA]), rabbit anti-calbindin (1/1,000; SWANT [Bellinzona, Switzerland]), mouse anti-cyclinD1 (1/100; Santa Cruz), rabbit anti-Chx10 (1/50; Rod McInnes), mouse anti-CRALBP (1/5,000; Jack Saari), rabbit anti-GFP (1/500, Chemicon), goat anti-Math3 (1/100, Santa Cruz), mouse anti-neurofilament 200 (1/500; NF200, Sigma [Oakville, ON, Canada]), rabbit anti-Nr2e3 (1/100; Chemicon), rabbit anti-Pax6 (1/500; Babco [Richmond, CA, USA]]), mouse anti-Pax6 (1/4, Developmental Studies Hybridoma Bank [Iowa City, IA, USA]), rabbit anti-p27Kip1 (1/500; NeoMarker Lab Vision, [Freemont, CA, USA] ]), mouse anti-protein kinase C (PKC; 1/500; Sigma), mouse anti-rhodopsin (1/500; Chemicon), mouse anti-syntaxin (1/2,000; Sigma), rabbit anti-TGFβII (1/100; Santa Cruz), rabbit anti-TGFβRI (1/100; Santa Cruz), rabbit anti-TGFβRII (1/100; Santa Cruz), rabbit anti-phospho-Smad2/3 (1/100; Santa Cruz), guinea pig anti-GLAST (1/8,000; Chemicon) and rabbit anti-Zac1 (1/1,000 ). Primary antibodies were washed 3 times in PBS with 0.1% triton X-100 (PBT) and detected using secondary antibodies conjugated with Cy3- (1/500; Jackson ImmunoResearch Laboratories, Inc. [West Grove, PA, USA]) or Alexa488 (1/500; Molecular Probes [Invitrogen, Eugene, OR, USA]). Secondary antibodies were diluted in PBT and left on the slides for 1 h prior to 3–10 minute washes with PBT. Note that the TSA™ Tyramide-Fluorescein Immunostaining Kit (NEL701, Perkin-Elmer [Shelton, CT, USA]) was used to amplify anti-TGFβII, TGFβRI, TGFβRII and phospho-Smad2/3 immunostaining as per the manufacturer's instructions. Peanut Agglutinin (PNA) staining was carried out using a 1:200 dilution of the PNA lectin incubated at 37°C for 30 minutes. Sections were then stained for five minutes with DAPI, washed an additional three times with PBS, and mounted with AquaPolymount. β-Galactosidase activity was detected using X-gal as a substrate as previously described .
To label S-phase progenitors, pregnant females were injected intraperitoneally with 100 μg/g body weight BrdU (Sigma) 30 minutes prior to sacrifice. For birthdating studies, BrdU was added to the culture media at a final concentration of 10 μM. Embryos were processed for anti-BrdU staining as above except for the addition of a pretreatment with 2N HCl for 30 minutes at 37°C. BrdU immunolabeling after RNA in situ hybridization was carried out using 3,3'-diaminobenzidine (DAB) as a substrate using the Vectastain kit (Vector Laboratories Inc. [Burlingame, CA, USA]).
Retinae were lysed for 15 minutes on ice in RIPA buffer (1% SDS, 1% sodium deoxycholate, 0.1% Nonidet P-40 in 50 mM Tris-HCl (pH 7.6)/150 mM NaCl) plus protease (Complete inhibitor tablet, Roche) and phosphatase (5 mM NaF and 1 mM orthovanadate) inhibitors. Cell lysates were cleared and protein concentrations determined via Bradford analysis. Cell free extract (25 μg) was loaded per lane on a 12% (Smad/phospho-Smad) or 15% (TGFβII) SDS-PAGE gel. Protein was then transferred to PVDF membrane at 80 V for 1 h. Membranes were blocked in 5% skim milk powder or 5% bovine serum albumin (for phospho-Smad) in tris-buffered saline with 0.1% tween 20 (TBST) and then incubated with anti-phospho-Smad2/3 (1/200; Santa Cruz), Smad2/3 (1/200; Santa Cruz), TGFβII (1/200; Santa Cruz) or anti-β-actin (1/5,000, AbCam [Cambridge, MA, USA]) overnight at 4°C. Membranes were washed three times for ten minutes each prior to incubation in horse radish peroxidase (HRP)-conjugated secondary antibodies and development with ECL (Roche).
Retinae were dissected and grown as explants as previously described . Briefly, the retinal pigmented epithelium (RPE) and lens were removed from dissected eyes, and the retina was flattened and cultured GCL-up on a Nucleopore Track-Etch membrane (13 mm; Whatman [Maldstone, England]) in explant media (50% MEM, 25% Hanks Solution, 25% horse serum, 6.75 mg/ml glucose, 200 μM L-glutamine, 2.5 mM HEPES) at 37°C in 5% CO2. The TGFβRII-Fc soluble receptor inhibitor (R&D systems [Burlington, ON, Canada]) was added at 0.5 μg/ml dissolved in PBS (vehicle control) every second day as described . Recombinant TGFβII (R&D systems) was added to explants every second day at 1 ng/ml.
Retinae were dissected, dissociated into single cell suspensions and cultured as aggregates essentially as described [13, 75]. Briefly, E14.5 wild-type retinae were dissociated in trypsin (10 min/37°C) and triturated in DMEM/10% fetal calf serum with 100 μl DNAseI (2 mg/ml). Dissociated progenitors were labeled in media with 10 μM BrdU for 1 h. BrdU was washed out and cells were resuspended in culture media at 5 × 105 cells/ml. For co-cultures, 100 μl (5 × 104 cells) of labeled E14.5 progenitors were added to a 20-fold excess (1 × 106 cells) of dissociated E18.5 wild-type or Zac1 mutant cells. Aggregated cells were pelleted by centrifuging for 8 minutes at 2,200 rpm and pellets were transferred after 1 h onto Nucleopore membranes and cultured 8DIV. Pellets were then dissociated and plated on poly-D-lysine-coated slides for immunostaining.
For misexpression, full-length Zac1 cDNA  was cloned into a pCIG2 expression vector containing a CMV-enhancer/chicken β-actin promoter and IRES-EGFP cassette (gift from Franck Polleux) . For electroporation, eyes were dissected and the RPE removed prior to immersion in 10 μl DNA (3 μg/μl) on a 3% agarose gel plug. Platinum electrodes were placed on either side of the eye (E15.5, 4 mm spacing; and E18.5, 5 mm spacing) and seven 20 ms pulses of 25 V were applied. Electroporated retinae were then cultured as explants.
Immunoreactive cells were counted in sections adjacent to the optic nerve or site of optic nerve transection in explants. In all experiments, cells were counted from a minimum of three embryos (or explants) and three sections per embryo (or explant). The total number of individual retinae analyzed per experiment (n values) is presented in the results section and the total number of cells counted per experiment is presented in the figure legends. All quantification was done from photomicrographs representing a 0.33 mm × 0.25 mm counting field. Statistical variation was determined using the standard error of the mean (SEM). Statistical significance was calculated using a Student's t-test, individually comparing experimental bars against wild-type or control counts.
The following additional data are available with the online version of this paper. Additional data file 1 is a figure showing that Zac1 is expressed in dividing progenitors at embryonic stages and differentiated cells at postnatal stages. Additional data file 2 is a figure showing Zac1 genotyping and verification of maternal imprinting in the embryonic retina. Additional data file 3 is a figure showing that equivalent numbers of bipolar cells, Müller glia, horizontal cells and cone photoreceptors develop in wild-type and Zac1 mutant retinal explants, while the number of amacrine cells increased in Zac1 mutant retinae. Additional data file 4 is a figure showing that RGC differentiation is unperturbed in Zac1-deficient retinae at E18.5. Additional data file 5 is a figure showing that amacrine cell precursors do not undergo more apoptosis or divide ectopically in Zac1 mutant retinae. Additional data file 6 is a figure showing that misexpression of Zac1 in the retina does not affect amacrine cells genesis. Additional data file 7 is a figure showing that the molecular profile of Zac1-deficient retinal progenitors is unperturbed at E18.5. Additional data file 8 is a figure showing that amacrine cell marker expression domains are expanded in E18.5 Zac1 mutant retinal explants cultured 4 DIV.
We thank C Logan, K Markham, R Wevrick, D Eisenstat, M Vetter, V Wallace, F Polleux, D Zinyk, P Mattar, R Slack, R Kageyama, C Cepko, R McInnes, J Wigle, M Xiang, J Saari, T Doetschmann, Y Wang, J Cross, S Hill, S Rawn, R Dixit and T Glaser for reagents, technical assistance and/or critical reading of the manuscript. CS and SM are Alberta Heritage Foundation for Medical Research (AHFMR) Senior Scholars. This work was supported by CIHR (MOP-44094), March of Dimes (FY05-107) and Networks of Centers of Excellence (NCE; Stem Cell Network) grants to CS, by CNRS and European Commission grants (CT-1999-00602) to LJ and by CIHR grant MOP-14138 to SM. LM and RC were supported by CIHR Training Grant in Genetics, Child Development & Health and LM is a William H Davies Scholar.
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