CHIQUITA1 maintains the temporal transition between proliferation and differentiation in Arabidopsis thaliana.

Body size varies widely among species, populations and individuals, depending on the environment. Transitioning between proliferation and differentiation is a crucial determinant of final organ size, but how the timing of this transition is established and maintained remains unknown. Using cell proliferation markers and genetic analysis, we show that CHIQUITA1 (CHIQ1) is required to maintain the timing of the transition from proliferation to differentiation in Arabidopsis thaliana. Combining kinematic and cell lineage-tracking studies, we found that the number of actively dividing cells in chiquita1-1 plants decreases prematurely compared with wild-type plants, suggesting CHIQ1 maintains the proliferative capacity in dividing cells and ensures that cells divide a specific number of times. CHIQ1 belongs to a plant-specific gene family of unknown molecular function and genetically interacts with three close members of its family to control the timing of proliferation exit. Our work reveals the interdependency between cellular and organ-level processes underlying final organ size determination.

Control of Drosophila wing size by morphogen range and hormonal gating.

The stereotyped dimensions of animal bodies and their component parts result from tight constraints on growth. Yet, the mechanisms that stop growth when organs reach the right size are unknown. Growth of the Drosophila wing-a classic paradigm-is governed by two morphogens, Decapentaplegic (Dpp, a BMP) and Wingless (Wg, a Wnt). Wing growth during larval life ceases when the primordium attains full size, concomitant with the larval-to-pupal molt orchestrated by the steroid hormone ecdysone. Here, we block the molt by genetically dampening ecdysone production, creating an experimental paradigm in which the wing stops growing at the correct size while the larva continues to feed and gain body mass. Under these conditions, we show that wing growth is limited by the ranges of Dpp and Wg, and by ecdysone, which regulates the cellular response to their signaling activities. Further, we present evidence that growth terminates because of the loss of two distinct modes of morphogen action: 1) maintenance of growth within the wing proper and 2) induced growth of surrounding "pre-wing" cells and their recruitment into the wing. Our results provide a precedent for the control of organ size by morphogen range and the hormonal gating of morphogen action.

Syd/JIP3 controls tissue size by regulating Diap1 protein turnover downstream of Yorkie/YAP.

How organisms control organ size is not fully understood. We found that Syd/JIP3 is required for proper wing size in Drosophila. JIP3 mutations are associated with organ size defects in mammals. The underlying mechanisms are not well understood. We discovered that Syd/JIP3 inhibition results in a downregulation of the inhibitor of apoptosis protein 1 (Diap1) in the Drosophila wing. Correspondingly, Syd/JIP3 deficient tissues exhibit ectopic cell death and yield smaller wings. Syd/JIP3 inhibition generated similar effects in mammalian cells, indicating a conserved mechanism. We found that Yorkie/YAP stimulates Syd/JIP3 in Drosophila and mammalian cells. Notably, Syd/JIP3 is required for the full effect of Yorkie-mediated tissue growth. Thus Syd/JIP3 regulation of Diap1 functions downstream of Yorkie/YAP to control growth. This study provides mechanistic insights into the recent and perplexing link between JIP3 mutations and organ size defects in mammals, including in humans where de novo JIP3 variants are associated with microcephaly.

Organ size control via hydraulically gated oscillations.

Hollow vesicular tissues of various sizes and shapes arise in biological organs such as ears, guts, hearts, brains and even entire organisms. Regulating their size and shape is crucial for their function. Although chemical signaling has been thought to play a role in the regulation of cellular processes that feed into larger scales, it is increasingly recognized that mechanical forces are involved in the modulation of size and shape at larger length scales. Motivated by a variety of examples of tissue cyst formation and size control that show simultaneous growth and size oscillations, we create a minimal theoretical framework for the growth and dynamics of a soft, fluid-permeable, spherical shell. We show that these shells can relieve internal pressure by bursting intermittently, shrinking and re-growing, providing a simple mechanism by which hydraulically gated oscillations can regulate size. To test our theory, we develop an in vitro experimental set-up to monitor the growth and oscillations of a hollow tissue spheroid growing freely or when confined. A simple generalization of our theory to account for irreversible deformations allows us to explain the time scales and the amplitudes of oscillations in terms of the geometry and mechanical properties of the tissue shells. Taken together, our theory and experimental observations show how soft hydraulics can regulate the size of growing tissue shells.

Kupffer's vesicle size threshold for robust left-right patterning of the zebrafish embryo.

BACKGROUND: Motile cilia in the "organ of asymmetry" create directional fluid flows that are vital for left-right (LR) asymmetric patterning of vertebrate embryos. Organ function often depends on tightly regulated organ size control, but the role of organ of asymmetry size in LR patterning has remained unknown. Observations of the organ of asymmetry in the zebrafish, called Kupffer's vesicle (KV), have suggested significant variations in KV size in wild-type embryos, raising questions about the impact of KV organ size on LR patterning. RESULTS: To understand the relationship between organ of asymmetry size and its function, we characterized variations in KV at several developmental stages and in several different zebrafish strains. We found that the number of KV cilia and the size of the KV lumen were highly variable, whereas the length of KV cilia showed less variation. These variabilities were similar among different genetic backgrounds. By specifically modulating KV size and analyzing individual embryos, we identified a size threshold that is necessary for KV function. CONCLUSIONS: Together these results indicate the KV organ of asymmetry size is not tightly controlled during development, but rather must only exceed a threshold to direct robust LR patterning of the zebrafish embryo.

Dual function of Yap in the regulation of lens progenitor cells and cellular polarity.

Hippo-Yap signaling has been implicated in organ size determination via its regulation of cell proliferation, growth and apoptosis (Pan, 2007). The vertebrate lens comprises only two major cell types, lens progenitors and differentiated fiber cells, thereby providing a relatively simple system for studying size-controlling mechanisms. In order to investigate the role of Hippo-Yap signaling in lens size regulation, we conditionally ablated Yap in the developing mouse lens. Lens progenitor-specific deletion of Yap led to near obliteration of the lens primarily due to hypocellularity in the lens epithelium (LE) and accompanying lens fiber (LF) defects. A significantly reduced LE progenitor pool resulted mainly from failed self-renewal and increased apoptosis. Additionally, Yap-deficient lens progenitor cells precociously exited the cell cycle and expressed the LF marker, ß-Crystallin. The mutant progenitor cells also exhibited multiple cellular and subcellular alterations including cell and nuclear shape change, organellar polarity disruption, and disorganized apical polarity complex and junction proteins such as Crumbs, Pals1, Par3 and ZO-1. Yap-deficient LF cells failed to anchor to the overlying LE layer, impairing their normal elongation and packaging. Furthermore, our localization study results suggest that, in the developing LE, Yap participates in the cell context-dependent transition from the proliferative to differentiation-competent state by integrating cell density information. Taken together, our results shed new light on Yap's indispensable and novel organizing role in mammalian organ size control by coordinating multiple events including cell proliferation, differentiation, and polarity.

Sterol carrier protein 2 regulates proximal tubule size in the Xenopus pronephric kidney by modulating lipid rafts.

The kidney is a homeostatic organ required for waste excretion and reabsorption of water, salts and other macromolecules. To this end, a complex series of developmental steps ensures the formation of a correctly patterned and properly proportioned organ. While previous studies have mainly focused on the individual signaling pathways, the formation of higher order receptor complexes in lipid rafts is an equally important aspect. These membrane platforms are characterized by differences in local lipid and protein compositions. Indeed, the cells in the Xenopus pronephric kidney were positive for the lipid raft markers ganglioside GM1 and Caveolin-1. To specifically interfere with lipid raft function in vivo, we focused on the Sterol Carrier Protein 2 (scp2), a multifunctional protein that is an important player in remodeling lipid raft composition. In Xenopus, scp2 mRNA was strongly expressed in differentiated epithelial structures of the pronephric kidney. Knockdown of scp2 did not interfere with the patterning of the kidney along its proximo-distal axis, but dramatically decreased the size of the kidney, in particular the proximal tubules. This phenotype was accompanied by a reduction of lipid rafts, but was independent of the peroxisomal or transcriptional activities of scp2. Finally, disrupting lipid micro-domains by inhibiting cholesterol synthesis using Mevinolin phenocopied the defects seen in scp2 morphants. Together these data underscore the importance for localized signaling platforms in the proper formation of the Xenopus kidney.

Size control during organogenesis: Development of the lateral line organs in zebrafish.

Many genes that play essential roles in organ growth have been identified across a range of organisms. However, the mechanisms by which growing organs can sense their sizes and stop growing when they reach their proper sizes remain poorly understood. The mechanosensory organs of the fish lateral line system (neuromasts) provide an ideal system to address this question for the following reasons. First, each superficial neuromast is composed of a small number of cells situated on the body surface, making it relatively easy to quantify organ size throughout development. Second, while the sensory cells of superficial neuromasts are continuously renewed, overall organ size is homeostatically maintained. Third, there is another type of neuromast showing an opposite mode of growth: that is, canal neuromasts increase in size in proportion to organism body size. Here, we review recent findings regarding the mechanisms that control organ size in the zebrafish lateral line.

Yes-associated protein promotes early hepatocyte cell cycle progression in regenerating liver after tissue loss.

The ability of the liver to restore its original volume following tissue loss has been associated with the Hippo-YAP1 pathway, a key controller of organ size. Yes-associated protein 1 (YAP1)-a growth effector usually restrained by Hippo signaling-is believed to be of particular importance; however, its role in liver regeneration remains ill-defined. To explore its function, we knocked down YAP1 prior to standard 70%-hepatectomy (sHx) using a hepatocyte-specific nanoformulation. Knockdown was effective during the major parenchymal growth phase (S-phase/M-phase peaks at 32 hours/48 hours post-sHx). Liver weight gain was completely suppressed by the knockdown at 32 hours, but was reaccelerated toward 48 hours. Likewise, proliferative markers, Ccna2/b2 and YAP1 target gene expression were downregulated at 32 hours, but re-elevated at 48 hours post-sHx. Nonetheless, knockdown slightly compromised survival after sHx. When assessing a model of resection-induced liver failure (extended 86%-hepatectomy, eHx) featuring deficient S- and M-phase progression, YAP1 was not induced at 32 hours, but upregulated at 48 hours post-eHx, confirming its dissociation from M-phase regulation. Therefore, YAP1 is vital to push hepatocytes into cycle and through the S-phase, but is not required for further cell cycle progression during liver regeneration. The examination of YAP1 in human livers suggested its function is conserved in the regenerating mammalian liver.

A Balance of Yki/Sd Activator and E2F1/Sd Repressor Complexes Controls Cell Survival and Affects Organ Size.

The Hippo/Yki and RB/E2F pathways both regulate tissue growth by affecting cell proliferation and survival, but interactions between these parallel control systems are poorly defined. In this study, we demonstrate that interaction between Drosophila E2F1 and Sd disrupts Yki/Sd complex formation and thereby suppresses Yki target gene expression. RBF modifies these effects by reducing E2F1/Sd interaction. This regulation has significant effects on apoptosis, organ size, and progenitor cell proliferation. Using a combination of DamID-seq and RNA-seq, we identified a set of Yki targets that play a diversity of roles during development and are suppressed by E2F1. Further, we found that human E2F1 competes with YAP for TEAD1 binding, affecting YAP activity, indicating that this mode of cross-regulation is conserved. In sum, our study uncovers a previously unknown mechanism in which RBF and E2F1 modify Hippo signaling responses to modulate apoptosis, organ growth, and homeostasis.

Growth suppressor lingerer regulates bantam microRNA to restrict organ size.

The evolutionarily conserved Hippo signaling pathway plays an important role in organ size control by regulating cell proliferation and apoptosis. Here, we identify Lingerer (Lig) as a growth suppressor using RNAi modifying screen in Drosophila melanogaster. Loss of lig increases organ size and upregulates bantam (ban) and the expression of the Hippo pathway target genes, while overexpression of lig results in diminished ban expression and organ size reduction. We demonstrate that Lig C-terminal exhibits dominant-negative function on growth and ban expression, and thus plays an important role in organ size control and ban regulation. In addition, we provide evidence that both Yki and Mad are essential for Lig-induced ban expression. We also show that Lig regulates the expression of the Hippo pathway target genes partially via Yorkie. Moreover, we find that Lig physically interacts with and requires Salvador to restrict cell growth. Taken together, we demonstrate that Lig functions as a critical growth suppressor to control organ size via ban and Hippo signaling.

Nitrogen dioxide is a positive regulator of plant growth.

Atmospheric nitric oxide (NO) and nitrogen dioxide (NO2) have long been recognized as either detrimental or beneficial for plant development. Recent research has established that NO is a phytohormone. Our present knowledge of the physiological role of NO2 is incomplete. We do know, however, that exogenous NO2 positively regulates the vegetative and reproductive growth of plants. We may therefore postulate that NO2 is a positive growth regulator for plants. We are now in a position to coherently summarize what is known of NO2 physiology; collated information on the topic is presented here.

Combining growth-promoting genes leads to positive epistasis in Arabidopsis thaliana.

Several genes positively influence final leaf size in Arabidopsis when mutated or overexpressed. The connections between these growth regulators are still poorly understood although such knowledge would further contribute to understand the processes driving leaf growth. In this study, we performed a combinatorial screen with 13 transgenic Arabidopsis lines with an increased leaf size. We found that from 61 analyzed combinations, 39% showed an additional increase in leaf size and most resulted from a positive epistasis on growth. Similar to what is found in other organisms in which such an epistasis assay was performed, only few genes were highly connected in synergistic combinations as we observed a positive epistasis in the majority of the combinations with samba, BRI1(OE) or SAUR19(OE). Furthermore, positive epistasis was found with combinations of genes with a similar mode of action, but also with genes which affect distinct processes, such as cell proliferation and cell expansion.DOI: http://dx.doi.org/10.7554/eLife.02252.001.

Genetic relationship between angustifolia3 and extra-small sisters highlights novel mechanisms controlling leaf size.

Coordination between cell proliferation and cell expansion is pivotal in leaf size determination. A group of mutants that are impaired in cell proliferation such as the angustifolia3 (an3) has provided a clue to understanding how these cellular processes are coordinated. In these mutants, impaired cell proliferation is accompanied by enhanced cell enlargement. We propose to call this phenomenon "compensated cell enlargement." Previously, we isolated ten extra-small sisters (xs) mutants that are specifically impaired in post-mitotic cell expansion and found that several xs mutations are able to suppress compensated cell enlargement in an3. Thus, the enhanced cell expansion observed in an3 results from the hyperactivation of post-mitotic cell expansion involving specific members of the XS gene family. These results suggested that cell proliferation process(es) and post-mitotic cell expansion process(es) are somehow linked in an as yet unknown fashion in leaf primordia. In this addendum, we propose possible models for the linking mechanisms that coordinate AN3-dependent cell proliferation and XS-dependent cell expansion in leaf development.