Pattern formation in the absence of cell proliferation: tissue-specific regulation of cell cycle progression by string (stg) during Drosophila eye development

Cell Cycle Regulation by NF-YC in Drosophila Eye Imaginal Disc: Implications for Synchronization in the Non-Proliferative Region

Cell cycle progression during development is meticulously coordinated with differentiation. This is particularly evident in the Drosophila 3rd instar eye imaginal disc, where the cell cycle is synchronized and arrests at the G1 phase in the non-proliferative region (NPR), setting the stage for photoreceptor cell differentiation. Here, we identify the transcription factor Nuclear Factor-YC (NF-YC) as a crucial player in this finely tuned progression, elucidating its specific role in the synchronized movement of the morphogenetic furrow. Depletion of NF-YC leads to extended expression of Cyclin A (CycA) and Cyclin B (CycB) from the FMW to the NPR. Notably, NF-YC knockdown resulted in decreased expression of Eyes absent (Eya) but did not affect Decapentaplegic (Dpp) and Hedgehog (Hh). Our findings highlight the role of NF-YC in restricting the expression of CycA and CycB in the NPR, thereby facilitating cell-cycle synchronization. Moreover, we identify the transcriptional cofactor Eya as a downstream target of NF-YC, revealing a new regulatory pathway in Drosophila eye development. This study expands our understanding of NF-YC's role from cell cycle control to encompass developmental processes.

Coordinated growth of linked epithelia is mediated by the Hippo pathway

An epithelium in a living organism seldom develops in isolation. Rather, most epithelia are tethered to other epithelial or non-epithelial tissues, necessitating growth coordination between layers. We investigated how two tethered epithelial layers of the Drosophila larval wing imaginal disc, the disc proper (DP) and the peripodial epithelium (PE), coordinate their growth. DP growth is driven by the morphogens Hedgehog (Hh) and Dpp, but regulation of PE growth is poorly understood. We find that the PE adapts to changes in growth rates of the DP, but not vice versa, suggesting a "leader and follower" mechanism. Moreover, PE growth can occur by cell shape changes, even when proliferation is inhibited. While Hh and Dpp pattern gene expression in both layers, growth of the DP is exquisitely sensitive to Dpp levels, while growth of the PE is not; the PE can achieve an appropriate size even when Dpp signaling is inhibited. Instead, both the growth of the PE and its accompanying cell shape changes require the activity of two components of the mechanosensitive Hippo pathway, the DNA-binding protein Scalloped (Sd) and its co-activator (Yki), which could allow the PE to sense and respond to forces generated by DP growth. Thus, an increased reliance on mechanically-dependent growth mediated by the Hippo pathway, at the expense of morphogen-dependent growth, enables the PE to evade layer-intrinsic growth control mechanisms and coordinate its growth with the DP. This provides a potential paradigm for growth coordination between different components of a developing organ.

Quantitative Relationships Between Growth, Differentiation, and Shape That Control Drosophila Eye Development and Its Variation

The size of organs is critical for their function and often a defining trait of a species. Still, how organs reach a species-specific size or how this size varies during evolution are problems not yet solved. Here, we have investigated the conditions that ensure growth termination, variation of final size and the stability of the process for developmental systems that grow and differentiate simultaneously. Specifically, we present a theoretical model for the development of the Drosophila eye, a system where a wave of differentiation sweeps across a growing primordium. This model, which describes the system in a simplified form, predicts universal relationships linking final eye size and developmental time to a single parameter which integrates genetically-controlled variables, the rates of cell proliferation and differentiation, with geometrical factors. We find that the predictions of the theoretical model show good agreement with previously published experimental results. We also develop a new computational model that recapitulates the process more realistically and find concordance between this model and theory as well, but only when the primordium is circular. However, when the primordium is elliptical both models show discrepancies. We explain this difference by the mechanical interactions between cells, an aspect that is not included in the theoretical model. Globally, our work defines the quantitative relationships between rates of growth and differentiation and organ primordium size that ensure growth termination (and, thereby, specify final eye size) and determine the duration of the process; identifies geometrical dependencies of both size and developmental time; and uncovers potential instabilities of the system which might constraint developmental strategies to evolve eyes of different size.

Ancient genetic redundancy of eyeless and twin of eyeless in the arthropod ocular segment

Pax6 transcription factors are essential upstream regulators in the developing anterior brain and peripheral visual system of most bilaterian animals. While a single homolog is in charge of these functions in vertebrates, two Pax6 genes are in Drosophila: eyeless (ey) and twin of eyeless (toy). At first glance, their co-existence seems sufficiently explained by their differential involvement in the specification of two types of insect visual organs: the lateral compound eyes (ey) and the dorsal ocelli (toy). Less straightforward to understand, however, is their genetic redundancy in promoting defined early and late growth phases of the precursor tissue to these organs: the eye-antennal imaginal disc. Drawing on comparative sequence, expression, and gene function evidence, I here conclude that this gene regulatory network module dates back to the dawn of arthropod evolution, securing the embryonic development of the ocular head segment. Thus, ey and toy constitute a paradigm to explore the organization and functional significance of longterm conserved genetic redundancy of duplicated genes. Indeed, as first steps in this direction, recent studies uncovered the shared use of binding sites in shared enhancers of target genes that are under redundant (string) and, strikingly, even subfunctionalized control by ey and toy (atonal). Equally significant, the evolutionarily recent and paralog-specific function of ey to repress the transcription of the antenna fate regulator Distal-less offers a functionally and phylogenetically well-defined opportunity to study the reconciliation of shared, partitioned, and newly acquired functions in a duplicated developmental gene pair.

Eye selector logic for a coordinated cell cycle exit

Organ-selector transcription factors control simultaneously cell differentiation and proliferation, ensuring the development of functional organs and their homeostasis. How this is achieved at the molecular level is still unclear. Here we have investigated how the transcriptional pulse of string/cdc25 (stg), the universal mitotic trigger, is regulated during Drosophila retina development as an example of coordinated deployment of differentiation and proliferation programs. We identify the eye specific stg enhancer, stg-FMW, and show that Pax6 selector genes, in cooperation with Eya and So, two members of the retinal determination network, activate stg-FMW, establishing a positive feed-forward loop. This loop is negatively modulated by the Meis1 protein, Hth. This regulatory logic is reminiscent of that controlling the expression of differentiation transcription factors. Our work shows that subjecting transcription factors and key cell cycle regulators to the same regulatory logic ensures the coupling between differentiation and proliferation programs during organ development.

Organs develop from groups of undifferentiated cells that proliferate and differentiate into specific cell types. During development, the coupling between proliferation and differentiation programs ensures that enough cells of the different cell types are generated. This is critical for proper organ formation and function. Here, we use the developing Drosophila eye to examine how the coupling between these two programs is achieved. During eye development, progenitors are amplified before they exit the cell cycle and enter the differentiation program. This amplification step depends on an expression burst of the mitotic trigger string/cdc25, which, by forcing cells into mitosis, synchronizes cells in G1 just before differentiation onset. Thus string regulation acts as a hub where differentiation and proliferation programs are integrated. We identify a DNA element that controls the burst of string expression prior to differentiation, and show that it is regulated by the same gene network that triggers eye development. The transcription factor Pax6/Eyeless is a key regulator in this network. Eyeless acts cooperatively with Sine oculis and Eyes absent to regulate string, through a positive feed-forward loop. This loop is negatively modulated by the progenitor-specific transcription factor Homothorax/Meis1. This work shows that transcription factors that instruct cells to acquire an eye fate also control their proliferation regime, thus guaranteeing the coupling between proliferation and differentiation.

My what big eyes you have: how the Drosophila retina grows

The compound eye of the fruit fly, Drosophila melanogaster, has for decades been used extensively to study a number of critical developmental processes including tissue development, pattern formation, cell fate specification, and planar cell polarity. To a lesser degree it has been used to examine the cell cycle and tissue proliferation. Discovering the mechanisms that balance tissue growth and cell death in developing epithelia has traditionally been the realm of those using the wing disc. However, over the last decade a series of observations has demonstrated that the eye is a suitable and maybe even preferable tissue for studying tissue growth. This review will focus on how growth of the retina is controlled by the genes and pathways that govern the specification of tissue fate, the division of the epithelium into dorsal-ventral compartments, the initiation, and progression of the morphogenetic furrow and the second mitotic wave.

Nemo phosphorylates Eyes absent and enhances output from the Eya-Sine oculis transcriptional complex during Drosophila retinal determination

The retinal determination gene network comprises a collection of transcription factors that respond to multiple signaling inputs to direct Drosophila eye development. Previous genetic studies have shown that nemo (nmo), a gene encoding a proline-directed serine/threonine kinase, can promote retinal specification through interactions with the retinal determination gene network, although the molecular point of cross-talk was not defined. Here, we report that the Nemo kinase positively and directly regulates Eyes absent (Eya). Genetic assays show that Nmo catalytic activity enhances Eya-mediated ectopic eye formation and potentiates induction of the Eya-Sine oculis (So) transcriptional targets dachshund and lozenge. Biochemical analyses demonstrate that Nmo forms a complex with and phosphorylates Eya at two consensus mitogen-activated protein kinase (MAPK) phosphorylation sites. These same sites appear crucial for Nmo-mediated activation of Eya function in vivo. Thus, we propose that Nmo phosphorylation of Eya potentiates its transactivation function to enhance transcription of Eya-So target genes during eye specification and development.

Size matters: the contribution of cell proliferation to the progression of the specification Drosophila eye gene regulatory network

Organ development is a complex process in which the activity of scores of interacting transcription factors and signaling pathways need to be integrated with proliferative growth. Developmental gene regulatory networks (GRNs) allow capturing essential regulatory pathways at a systems-level and provide an effective way of approaching such complexity. However typical GRNs studies focus on very early embryonic stages (usually pre-gastrulation) or late stages, when there is little or no cell proliferation, and therefore do not consider how organ growth is integrated in the developmental process. This can be conveniently investigated in the Drosophila melanogaster eye primordium. Here we present a working model meant to illustrate how during a critical period, the second larval stage, changes in cells' proliferative pattern are coordinated with the initiation of the Retinal Determination (RD) gene program. Such changes are regulated in response to two different sources of signal (Wnt1/wg and BMP2/4/dpp) produced by the anterior and posterior ends of the primordium, respectively. The dpp signaling is necessary to trigger the RD program. However in order for it to be effective, cells receiving Dpp have to be out of the wg signaling range. This is obtained thanks to the proliferative growth that precedes the onset of RD expression. With this network model many of the gene regulatory steps previously known to participate in growth and patterning are linked. Analysis of the model highlights a few essential regulatory principles, as well as poses new questions. In addition, these principles might operate during the growth and patterning of other organs.

Smoothened, thickveins and the genetic control of cell cycle and cell fate in the developing Drosophila eye

The Hedgehog and Decapentaplegic pathways have several well-characterized functions in the developing Drosophila compound eye, including initiation and progression of the morphogenetic furrow. Other functions involve control of cell cycle and cell survival as well as cell type specification. Here we have used the mosaic clone analysis of null mutations of the smoothened and thickveins genes (which encode the receptors for these two signals) both alone and in combination, to study cell cycle and cell fate in the developing eye. We conclude that both pathways have several, but differing roles in furrow induction and cell fate and survival, but that neither directly affects cell type specification.

Cell-cycle control during development: taking it up a notch

The signals that coordinate cellular proliferation with G1 arrest and differentiation have long been of interest. Two papers in this issue of Developmental Cell show that the conserved Hedgehog and Notch signaling pathways regulate cell division during development of the Drosophila compound eye.

Dominant Drop mutants are gain-of-function alleles of the muscle segment homeobox gene (msh) whose overexpression leads to the arrest of eye development

Dominant Drop (Dr) mutations are nearly eyeless and have additional recessive phenotypes including lethality and patterning defects in eye and sensory bristles due to cis-regulatory lesions in the cell cycle regulator string (stg). Genetic analysis demonstrates that the dominant small eye phenotype is the result of separate gain-of-function mutations in the closely linked muscle segment homeobox (msh) gene, encoding a homeodomain transcription factor required for patterning of muscle and nervous system. Reversion of the Dr(Mio) allele was coincident with the generation of lethal loss-of-function mutations in msh in cis, suggesting that the dominant eye phenotype is the result of ectopic expression. Molecular genetic analysis revealed that two dominant Dr alleles contain lesions upstream of the msh transcription start site. In the Dr(Mio) mutant, a 3S18 retrotransposon insertion is the target of second-site mutations (P-element insertions or deletions) which suppress the dominant eye phenotype following reversion. The pattern of 3S18 expression and the absence of msh in eye imaginal discs suggest that transcriptional activation of the msh promoter accounts for ectopic expression. Dr dominant mutations arrest eye development by blocking the progression of the morphogenetic furrow leading to photoreceptor cell loss via apoptosis. Gal4-mediated ubiquitous expression of msh in third-instar larvae was sufficient to arrest the morphogenetic furrow in the eye imaginal disc and resulted in lethality prior to eclosion. Dominant mutations in the human msx2 gene, one of the vertebrate homologs of msh, are associated with craniosynostosis, a disease affecting cranial development. The Dr mutations are the first example of gain-of-function mutations in the msh/msx gene family identified in a genetically tractible model organism and may serve as a useful tool to identify additional genes that regulate this class of homeodomain proteins.

Cell cycle and cell fate in the nervous system

Recently, a number of molecules originally thought to have a primary role in cell determination have been shown to affect the cell cycle at specific check points, while other molecules discovered for their roles in the cell cycle progression are known to affect the determination and differentiation of neurons. These discoveries have led to a more detailed investigation of the complex molecular machinery that co-ordinates proliferation and differentiation.