A robust cell cycle control mechanism limits E2F-induced proliferation of terminally differentiated cells in vivo

Noncanonical function of Capicua as a growth termination signal in Drosophila oogenesis

Capicua (Cic) proteins are conserved HMG-box transcriptional repressors that control receptor tyrosine kinase (RTK) signaling responses and are implicated in human neurological syndromes and cancer. While Cic is known to exist as short (Cic-S) and long (Cic-L) isoforms with identical HMG-box and associated core regions but distinct N termini, most previous studies have focused on Cic-S, leaving the function of Cic-L unexplored. Here we show that Cic-L acts in two capacities during Drosophila oogenesis: 1) as a canonical sensor of RTK signaling in somatic follicle cells, and 2) as a regulator of postmitotic growth in germline nurse cells. In these latter cells, Cic-L behaves as a temporal signal that terminates endoreplicative growth before they dump their contents into the oocyte. We show that Cic-L is necessary and sufficient for nurse cell endoreplication arrest and induces both stabilization of CycE and down-regulation of Myc. Surprisingly, this function depends mainly on the Cic-L-specific N-terminal module, which is capable of acting independently of the Cic HMG-box-containing core. Mirroring these observations, basal metazoans possess truncated Cic-like proteins composed only of Cic-L N-terminal sequences, suggesting that this module plays unique, ancient roles unrelated to the canonical function of Cic.

Cell cycle expression of polarity genes features Rb targeting of Vang

Specification of cellular polarity is vital to normal tissue development and function. Pioneering studies in Drosophila and C. elegans have elucidated the composition and dynamics of protein complexes critical for establishment of cell polarity, which is manifest in processes such as cell migration and asymmetric cell division. Conserved throughout metazoans, planar cell polarity (PCP) genes are implicated in disease, including neural tube closure defects associated with mutations in VANGL1/2. PCP protein regulation is well studied; however, relatively little is known about transcriptional regulation of these genes. Our earlier study revealed an unexpected role for the fly Rbf1 retinoblastoma corepressor protein, a regulator of cell cycle genes, in transcriptional regulation of polarity genes. Here we analyze the physiological relevance of the role of E2F/Rbf proteins in the transcription of the key core polarity gene Vang. Targeted mutations to the E2F site within the Vang promoter disrupts binding of E2F/Rbf proteins in vivo, leading to polarity defects in wing hairs. E2F regulation of Vang is supported by the requirement for this motif in a reporter gene. Interestingly, the promoter is repressed by overexpression of E2F1, a transcription factor generally identified as an activator. Consistent with the regulation of this polarity gene by E2F and Rbf factors, expression of Vang and other polarity genes is found to peak in G2/M phase in cells of the embryo and wing imaginal disc, suggesting that cell cycle signals may play a role in regulation of these genes. These findings suggest that the E2F/Rbf complex mechanistically links cell proliferation and polarity.

Misregulation of the Nucleoporins 98 and 96 lead to defects in protein synthesis that promote hallmarks of tumorigenesis

Nucleoporin 98KD (Nup98) is a promiscuous translocation partner in hematological malignancies. Most disease models of Nup98 translocations involve ectopic expression of the fusion protein under study, leaving the endogenous Nup98 loci unperturbed. Overlooked in these approaches is the loss of one copy of normal Nup98 in addition to the loss of Nup96 – a second Nucleoporin encoded within the same mRNA and reading frame as Nup98 – in translocations. Nup98 and Nup96 are also mutated in a number of other cancers, suggesting that their disruption is not limited to blood cancers. We found that reducing Nup98-96 function in Drosophila melanogaster (in which the Nup98-96 shared mRNA and reading frame is conserved) de-regulates the cell cycle. We found evidence of overproliferation in tissues with reduced Nup98-96, counteracted by elevated apoptosis and aberrant signaling associated with chronic wounding. Reducing Nup98-96 function led to defects in protein synthesis that triggered JNK signaling and contributed to hallmarks of tumorigenesis when apoptosis was inhibited. We suggest that partial loss of Nup98-96 function in translocations could de-regulate protein synthesis, leading to signaling that cooperates with other mutations to promote tumorigenesis.

Editor's choice: Compromising Nucleoporins 98 and 96 leads to defects in protein synthesis and stress signaling via JNK that triggers compensatory and apoptosis-induced proliferation.

Endogenous protein tagging in medaka using a simplified CRISPR/Cas9 knock-in approach

The CRISPR/Cas9 system has been used to generate fluorescently labelled fusion proteins by homology-directed repair in a variety of species. Despite its revolutionary success, there remains an urgent need for increased simplicity and efficiency of genome editing in research organisms. Here, we establish a simplified, highly efficient, and precise strategy for CRISPR/Cas9-mediated endogenous protein tagging in medaka (Oryzias latipes). We use a cloning-free approach that relies on PCR-amplified donor fragments containing the fluorescent reporter sequences flanked by short homology arms (30–40 bp), a synthetic single-guide RNA and Cas9 mRNA. We generate eight novel knock-in lines with high efficiency of F0 targeting and germline transmission. Whole genome sequencing results reveal single-copy integration events only at the targeted loci. We provide an initial characterization of these fusion protein lines, significantly expanding the repertoire of genetic tools available in medaka. In particular, we show that the mScarlet-pcna line has the potential to serve as an organismal-wide label for proliferative zones and an endogenous cell cycle reporter.

Intricate Regulatory Mechanisms of the Anaphase-Promoting Complex/Cyclosome and Its Role in Chromatin Regulation

The ubiquitin (Ub)-proteasome system is vital to nearly every biological process in eukaryotes. Specifically, the conjugation of Ub to target proteins by Ub ligases, such as the Anaphase-Promoting Complex/Cyclosome (APC/C), is paramount for cell cycle transitions as it leads to the irreversible destruction of cell cycle regulators by the proteasome. Through this activity, the RING Ub ligase APC/C governs mitosis, G1, and numerous aspects of neurobiology. Pioneering cryo-EM, biochemical reconstitution, and cell-based studies have illuminated many aspects of the conformational dynamics of this large, multi-subunit complex and the sophisticated regulation of APC/C function. More recent studies have revealed new mechanisms that selectively dictate APC/C activity and explore additional pathways that are controlled by APC/C-mediated ubiquitination, including an intimate relationship with chromatin regulation. These tasks go beyond the traditional cell cycle role historically ascribed to the APC/C. Here, we review these novel findings, examine the mechanistic implications of APC/C regulation, and discuss the role of the APC/C in previously unappreciated signaling pathways.

The Krüppel-like factor Cabut has cell cycle regulatory properties similar to E2F1

Using a gain-of-function screen in Drosophila, we identified the Krüppel-like factor Cabut (Cbt) as a positive regulator of cell cycle gene expression and cell proliferation. Enforced cbt expression is sufficient to induce an extra cell division in the differentiating fly wing or eye, and also promotes intestinal stem cell divisions in the adult gut. Although inappropriate cell proliferation also results from forced expression of the E2f1 transcription factor or its target, Cyclin E, Cbt does not increase E2F1 or Cyclin E activity. Instead, Cbt regulates a large set of E2F1 target genes independently of E2F1, and our data suggest that Cbt acts via distinct binding sites in target gene promoters. Although Cbt was not required for cell proliferation during wing or eye development, Cbt is required for normal intestinal stem cell divisions in the midgut, which expresses E2F1 at relatively low levels. The E2F1-like functions of Cbt identify a distinct mechanism for cell cycle regulation that may be important in certain normal cell cycles, or in cells that cycle inappropriately, such as cancer cells.

In silico APC/C substrate discovery reveals cell cycle-dependent degradation of UHRF1 and other chromatin regulators

The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase and critical regulator of cell cycle progression. Despite its vital role, it has remained challenging to globally map APC/C substrates. By combining orthogonal features of known substrates, we predicted APC/C substrates in silico. This analysis identified many known substrates and suggested numerous candidates. Unexpectedly, chromatin regulatory proteins are enriched among putative substrates, and we show experimentally that several chromatin proteins bind APC/C, oscillate during the cell cycle, and are degraded following APC/C activation, consistent with being direct APC/C substrates. Additional analysis revealed detailed mechanisms of ubiquitylation for UHRF1, a key chromatin regulator involved in histone ubiquitylation and DNA methylation maintenance. Disrupting UHRF1 degradation at mitotic exit accelerates G1-phase cell cycle progression and perturbs global DNA methylation patterning in the genome. We conclude that APC/C coordinates crosstalk between cell cycle and chromatin regulatory proteins. This has potential consequences in normal cell physiology, where the chromatin environment changes depending on proliferative state, as well as in disease.

Defined factors to reactivate cell cycle activity in adult mouse cardiomyocytes

Adult mammalian cardiomyocytes exit the cell cycle during the neonatal period, commensurate with the loss of regenerative capacity in adult mammalian hearts. We established conditions for long-term culture of adult mouse cardiomyocytes that are genetically labeled with fluorescence. This technique permits reliable analyses of proliferation of pre-existing cardiomyocytes without complications from cardiomyocyte marker expression loss due to dedifferentiation or significant contribution from cardiac progenitor cell expansion and differentiation in culture. Using this system, we took a candidate gene approach to screen for fetal-specific proliferative gene programs that can induce proliferation of adult mouse cardiomyocytes. Using pooled gene delivery and subtractive gene elimination, we identified a novel functional interaction between E2f Transcription Factor 2 (E2f2) and Brain Expressed X-Linked (Bex)/Transcription elongation factor A-like (Tceal) superfamily members Bex1 and Tceal8. Specifically, Bex1 and Tceal8 both preserved cell viability during E2f2-induced cell cycle re-entry. Although Tceal8 inhibited E2f2-induced S-phase re-entry, Bex1 facilitated DNA synthesis while inhibiting cell death. In sum, our study provides a valuable method for adult cardiomyocyte proliferation research and suggests that Bex family proteins may function in modulating cell proliferation and death decisions during cardiomyocyte development and maturation.

Changes in chromatin accessibility ensure robust cell cycle exit in terminally differentiated cells

During terminal differentiation, most cells exit the cell cycle and enter into a prolonged or permanent G0 in which they are refractory to mitogenic signals. Entry into G0 is usually initiated through the repression of cell cycle gene expression by formation of a transcriptional repressor complex called dimerization partner (DP), retinoblastoma (RB)-like, E2F and MuvB (DREAM). However, when DREAM repressive function is compromised during terminal differentiation, additional unknown mechanisms act to stably repress cycling and ensure robust cell cycle exit. Here, we provide evidence that developmentally programmed, temporal changes in chromatin accessibility at a small subset of critical cell cycle genes act to enforce cell cycle exit during terminal differentiation in the Drosophila melanogaster wing. We show that during terminal differentiation, chromatin closes at a set of pupal wing enhancers for the key rate-limiting cell cycle regulators Cyclin E (cycE), E2F transcription factor 1 (e2f1), and string (stg). This closing coincides with wing cells entering a robust postmitotic state that is strongly refractory to cell cycle reactivation, and the regions that close contain known binding sites for effectors of mitogenic signaling pathways such as Yorkie and Notch. When cell cycle exit is genetically disrupted, chromatin accessibility at cell cycle genes remains unaffected, and the closing of distal enhancers at cycE, e2f1, and stg proceeds independent of the cell cycling status. Instead, disruption of cell cycle exit leads to changes in accessibility and expression of a subset of hormone-induced transcription factors involved in the progression of terminal differentiation. Our results uncover a mechanism that acts as a cell cycle–independent timer to limit the response to mitogenic signaling and aberrant cycling in terminally differentiating tissues. In addition, we provide a new molecular description of the cross talk between cell cycle exit and terminal differentiation during metamorphosis.

The longer a cell remains in G0, the more refractory it becomes to re-entering the cell cycle. This study shows that in terminally differentiated cells in vivo, regulatory elements at genes encoding just three key cell cycle regulators (cycE, e2f1 and stg) become inaccessible, limiting their aberrant activation and maintaining a prolonged, robust G0.

Wound-induced polyploidization is driven by Myc and supports tissue repair in the presence of DNA damage

Tissue repair usually requires either polyploid cell growth or cell division, but the molecular mechanism promoting polyploidy and limiting cell division remains poorly understood. Here, we find that injury to the adult Drosophila epithelium causes cells to enter the endocycle through the activation of Yorkie-dependent genes (Myc and E2f1). Myc is even sufficient to induce the endocycle in the uninjured post-mitotic epithelium. As result, epithelial cells enter S phase but mitosis is blocked by inhibition of mitotic gene expression. The mitotic cell cycle program can be activated by simultaneously expressing the Cdc25-like phosphatase String (stg), while genetically depleting APC/C E3 ligase fizzy-related (fzr). However, forcing cells to undergo mitosis is detrimental to wound repair as the adult fly epithelium accumulates DNA damage, and mitotic errors ensue when cells are forced to proliferate. In conclusion, we find that wound-induced polyploidization enables tissue repair when cell division is not a viable option.

The efficacy of chronic zinc oxide nanoparticles using on testicular damage in the streptozotocin-induced diabetic rat model

Testicular impairment is a common complication of Diabetes mellitus (DM). Zinc Oxide Nanoparticles (ZnO NPs) are a novel agent for Zn delivery with antidiabetic and antioxidant activities. However, few reports were recorded on it. The current study aimed to investigate the possible ameliorating effect of ZnO NPs treatment on testicular tissues alterations in streptozotocin (STZ)-induced diabetic rats. Therefore, thirty mature male Wistar rats were divided into three main groups: Control group (n = 18) was subdivided equally into three subgroups (negative control, vehicle and ZnO NPs), Diabetic group (n = 6) and ZnO NPs-treated diabetic group (n = 6). Induction of diabetes was done by a single intraperitoneal injection of STZ (60 mg/kg bw). The rats were orally treated by ZnO NPs (10 mg/kg bw) for 30 constitutive days. At the end of the experiment, blood glucose and serum testosterone levels were measured. Also, testicular tissues were obtained for histopathological investigations and immunohistochemical staining with anti-PCNA (proliferating cell marker), anti-ssDNA (apoptotic cell marker), anti-SOX9 (Sertoli cell marker), anti-Stella (spermatogonia marker), anti-STRA8 (preleptotene and early-leptotene spermatocytes marker), anti-DMC1 (leptotene and zygotene spermatocytes marker), anti-Dnmt3a (a marker for cells under DNA methylation) and anti-α-SMA (peritubular myoid cell marker). The biochemical analysis revealed that diabetes resulted in a significant elevation in blood glucose level and a reduction in serum testosterone level. Moreover, histopathological investigations revealed disorganized seminiferous epithelium and sever hyalinization with vacuolization of the testicular interstitium containing Leydig cells. The immunohistochemical findings support spermatogenesis impairment in the diabetic group. However, ZnO NPs treatment restores architecture of seminiferous epithelium and Leydig cells. Furthermore, more PCNA, SOX9, Stella, STRA8, DMC1 and Dnmt3a immunopositive cells with an improvement of peritubular α-SMA immunopositive expression, as well as few ssDNA-immunopositive cells were detected in the seminiferous epithelium. This study suggested the possible protective role of orally administered ZnO NPs on testicular alterations in the STZ-induced diabetic group via steroidogenesis and spermatogenesis enhancement. In addition, further researches are acquired for evaluation mechanism of ZnO NPs treatment via oral or parenteral routes in a dose-dependent manner to identify the more effective route and dose in the treatment of testicular diabetic complications.

Stem cell factor and NSC87877 synergism enhances c-Kit mediated proliferation of human erythroid cells

The biological mechanisms underlying the effects of stem cell factor (SCF) and an inhibitor, NSC87877 (N) of the c-Kit negative regulator (SHP-1 and SHP-2) on cell proliferation are different. Therefore, we compared the cell's response to these two either alone or in combination in K562 cells. Binding of SCF (S) to c-Kit induces dimerization that activates its kinase activity. The activated c-Kit undergoes autophosphorylation at tyrosine residues that serve as a docking site for signal transduction molecules containing SH2 domains. Predominantly, the phosphotyrosine 568 (pY568) in Juxtamembrane (JM) region of c-Kit interacts with adaptor protein APS, Src family kinase, and SHP-2, while phosphotyrosine 570 (pY570) interacts with the SHP-1 and the adaptor protein Shc. The dephosphorylation of phosphotyrosine residues by SHP-1/SHP-2 leads to inhibition of c-Kit proliferative signaling. A chemical molecule, N is reported to inhibit the enzymatic activity of SHP-1/SHP-2, but its effect on c-Kit-mediated proliferation has not been studied yet. Thus, this work aims at examining the effect of the combination of S and N on cells growth as compared to individual treatment. The present study is performed with erythroleukemic K562 cells, chosen for its mRNA expression concerning the c-Kit, and SHP-1/SHP-2. Interestingly, proliferation assay showed that combination significantly increased proliferation when G₁ sorted K562 cells were used. These changes were significantly higher when K562 cells were initially treated with N followed by S treatment. Collectively, these results give mechanistic insight into the proliferation enhancement of bone marrow transplantation through the synergistic effect of S and N by inhibiting SHP-1/SHP-2. The study gives solid evidence that S and N combination can be used to enhance cell proliferation/growth.

Who guards the guardian? Mechanisms that restrain APC/C during the cell cycle

The cell cycle is principally controlled by Cyclin Dependent Kinases (CDKs), whose oscillating activities are determined by binding to Cyclin coactivators. Cyclins exhibit dynamic changes in abundance as cells pass through the cell cycle. The sequential, timed accumulation and degradation of Cyclins, as well as many other proteins, imposes order on the cell cycle and contributes to genome maintenance. The destruction of many cell cycle regulated proteins, including Cyclins A and B, is controlled by a large, multi-subunit E3 ubiquitin ligase termed the Anaphase Promoting Complex/Cyclosome (APC/C). APC/C activity is tightly regulated during the cell cycle. Its activation state increases dramatically in mid-mitosis and it remains active until the end of G1 phase. Following its mandatory inactivation at the G1/S boundary, APC/C activity remains low until the subsequent mitosis. Due to its role in guarding against the inappropriate or untimely accumulation of Cyclins, the APC/C is a core component of the cell cycle oscillator. In addition to the regulation of Cyclins, APC/C controls the degradation of many other substrates. Therefore, it is vital that the activity of APC/C itself be tightly guarded. The APC/C is most well studied for its role and regulation during mitosis. However, the APC/C also plays a similarly important and conserved role in the maintenance of G1 phase. Here we review the diverse mechanisms counteracting APC/C activity throughout the cell cycle and the importance of their coordinated actions on cell growth, proliferation, and disease.

Cezanne/OTUD7B is a cell cycle-regulated deubiquitinase that antagonizes the degradation of APC/C substrates

The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase and key regulator of cell cycle progression. Since APC/C promotes the degradation of mitotic cyclins, it controls cell cycle-dependent oscillations in cyclin-dependent kinase (CDK) activity. Both CDKs and APC/C control a large number of substrates and are regulated by analogous mechanisms, including cofactor-dependent activation. However, whereas substrate dephosphorylation is known to counteract CDK, it remains largely unknown whether deubiquitinating enzymes (DUBs) antagonize APC/C substrate ubiquitination during mitosis. Here, we demonstrate that Cezanne/OTUD7B is a cell cycle-regulated DUB that opposes the ubiquitination of APC/C targets. Cezanne is remarkably specific for K11-linked ubiquitin chains, which are formed by APC/C in mitosis. Accordingly, Cezanne binds established APC/C substrates and reverses their APC/C-mediated ubiquitination. Cezanne depletion accelerates APC/C substrate degradation and causes errors in mitotic progression and formation of micronuclei. These data highlight the importance of tempered APC/C substrate destruction in maintaining chromosome stability. Furthermore, Cezanne is recurrently amplified and overexpressed in numerous malignancies, suggesting a potential role in genome maintenance and cancer cell proliferation.

The E3 Ubiquitin Ligase SCF(Cyclin F) Transmits AKT Signaling to the Cell-Cycle Machinery

The oncogenic AKT kinase is a key regulator of apoptosis, cell growth, and cell-cycle progression. Despite its important role in proliferation, it remains largely unknown how AKT is mechanistically linked to the cell cycle. We show here that cyclin F, a substrate receptor F-box protein for the SCF (Skp1/Cul1/F-box) family of E3 ubiquitin ligases, is a bona fide AKT substrate. Cyclin F expression oscillates throughout the cell cycle, a rare feature among the 69 human F-box proteins, and all of its known substrates are involved in proliferation. AKT phosphorylation of cyclin F enhances its stability and promotes assembly into productive E3 ligase complexes. Importantly, expression of mutant versions of cyclin F that cannot be phosphorylated by AKT impair cell-cycle entry. Our data suggest that cyclin F transmits mitogen signaling through AKT to the core cell-cycle machinery. This discovery has potential implications for proliferative control in malignancies where AKT is activated.

Quantitative Cell Cycle Analysis Based on an Endogenous All-in-One Reporter for Cell Tracking and Classification

Cell cycle kinetics are crucial to cell fate decisions. Although live imaging has provided extensive insights into this relationship at the single-cell level, the limited number of fluorescent markers that can be used in a single experiment has hindered efforts to link the dynamics of individual proteins responsible for decision making directly to cell cycle progression. Here, we present fluorescently tagged endogenous proliferating cell nuclear antigen (PCNA) as an all-in-one cell cycle reporter that allows simultaneous analysis of cell cycle progression, including the transition into quiescence, and the dynamics of individual fate determinants. We also provide an image analysis pipeline for automated segmentation, tracking, and classification of all cell cycle phases. Combining the all-in-one reporter with labeled endogenous cyclin D1 and p21 as prime examples of cell-cycle-regulated fate determinants, we show how cell cycle and quantitative protein dynamics can be simultaneously extracted to gain insights into G1 phase regulation and responses to perturbations.

•

Endogenous mRuby-PCNA as an all-in-one cell cycle reporter

•

Automated image analysis pipeline to segment, track, and classify based on PCNA

•

Quantitative analysis of cyclin oscillations during a complete cell cycle

•

Cyclin D1 maintains G1 phase and prevents the transition into quiescence

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.

•

RBF/E2F1 regulates the Hippo pathway by modulating formation of Yki/Sd complexes

•

E2F1 releases Yki:Sd association and suppresses a set of Yki target expression

•

Human E2F1 competes with YAP for TEAD1 binding and affects YAP activity

Chromatin organization changes during the establishment and maintenance of the postmitotic state

Background

Genome organization changes during development as cells differentiate. Chromatin motion becomes increasingly constrained and heterochromatin clusters as cells become restricted in their developmental potential. These changes coincide with slowing of the cell cycle, which can also influence chromatin organization and dynamics. Terminal differentiation is often coupled with permanent exit from the cell cycle, and existing data suggest a close relationship between a repressive chromatin structure and silencing of the cell cycle in postmitotic cells. Heterochromatin clustering could also contribute to stable gene repression to maintain terminal differentiation or cell cycle exit, but whether clustering is initiated by differentiation, cell cycle changes, or both is unclear. Here we examine the relationship between chromatin organization, terminal differentiation and cell cycle exit.

Results

We focused our studies on the Drosophila wing, where epithelial cells transition from active proliferation to a postmitotic state in a temporally controlled manner. We find there are two stages of G₀ in this tissue, a flexible G₀ period where cells can be induced to reenter the cell cycle under specific genetic manipulations and a state we call “robust,” where cells become strongly refractory to cell cycle reentry. Compromising the flexible G₀ by driving ectopic expression of cell cycle activators causes a global disruption of the clustering of heterochromatin-associated histone modifications such as H3K27 trimethylation and H3K9 trimethylation, as well as their associated repressors, Polycomb and heterochromatin protein 1 (HP1). However, this disruption is reversible. When cells enter a robust G₀ state, even in the presence of ectopic cell cycle activity, clustering of heterochromatin-associated modifications is restored. If cell cycle exit is bypassed, cells in the wing continue to terminally differentiate, but heterochromatin clustering is severely disrupted. Heterochromatin-dependent gene silencing does not appear to be required for cell cycle exit, as compromising the H3K27 methyltransferase Enhancer of zeste, and/or HP1 cannot prevent the robust cell cycle exit, even in the face of normally oncogenic cell cycle activities.

Conclusions

Heterochromatin clustering during terminal differentiation is a consequence of cell cycle exit, rather than differentiation. Compromising heterochromatin-dependent gene silencing does not disrupt cell cycle exit.

Electronic supplementary material

The online version of this article (10.1186/s13072-017-0159-8) contains supplementary material, which is available to authorized users.

APC/CFzr/Cdh1-Dependent Regulation of Planar Cell Polarity Establishment via Nek2 Kinase Acting on Dishevelled

The Anaphase-Promoting Complex/Cyclosome (APC/C) is an E3 ubiquitin ligase, well known for its role in cell-cycle progression. However, it has been linked to additional functions, mainly in neuronal contexts, when using the co-activator Cdh1/Fzr. Here, our data indicate a post-mitotic requirement for the APC/C^Fzr/Cdh1 in epithelial cell patterning and planar cell polarity (PCP) in Drosophila. PCP signaling is critical for development by establishing cellular asymmetries and orientation within the plane of an epithelium, via differential localization of distinct complexes of core PCP factors. Loss of APC/C function leads to reduced levels of Dishevelled (Dsh), a core PCP factor. The effect of APC/C on Dsh is mediated by Nek2 kinase, which can phosphorylate Dsh and is a direct APC/C^Fzr/Cdh1 substrate. We have thus uncovered a pathway of regulation whereby APC/C^Fzr/Cdh1 negatively regulates Nek2, which negatively regulates Dsh, to ensure its proper stoichiometric requirement and localization during PCP establishment.

Roles for the Histone Modifying and Exchange Complex NuA4 in Cell Cycle Progression in Drosophila melanogaster

Robust and synchronous repression of E2F-dependent gene expression is critical to the proper timing of cell cycle exit when cells transition to a postmitotic state. Previously NuA4 was suggested to act as a barrier to proliferation in Drosophila by repressing E2F-dependent gene expression. Here we show that NuA4 activity is required for proper cell cycle exit and the repression of cell cycle genes during the transition to a postmitotic state in vivo However, the delay of cell cycle exit caused by compromising NuA4 is not due to additional proliferation or effects on E2F activity. Instead NuA4 inhibition results in slowed cell cycle progression through late S and G2 phases due to aberrant activation of an intrinsic p53-independent DNA damage response. A reduction in NuA4 function ultimately produces a paradoxical cell cycle gene expression program, where certain cell cycle genes become derepressed in cells that are delayed during the G2 phase of the final cell cycle. Bypassing the G2 delay when NuA4 is inhibited leads to abnormal mitoses and results in severe tissue defects. NuA4 physically and genetically interacts with components of the E2F complex termed D: rosophila, R: bf, E: 2F A: nd M: yb/ M: ulti-vulva class B: (DREAM/MMB), and modulates a DREAM/MMB-dependent ectopic neuron phenotype in the posterior wing margin. However, this effect is also likely due to the cell cycle delay, as simply reducing Cdk1 is sufficient to generate a similar phenotype. Our work reveals that the major requirement for NuA4 in the cell cycle in vivo is to suppress an endogenous DNA damage response, which is required to coordinate proper S and G2 cell cycle progression with differentiation and cell cycle gene expression.

Regulation of cell polarity determinants by the Retinoblastoma tumor suppressor protein

In addition to their canonical roles in the cell cycle, RB family proteins regulate numerous developmental pathways, although the mechanisms remain obscure. We found that Drosophila Rbf1 associates with genes encoding components of the highly conserved apical-basal and planar cell polarity pathways, suggesting a possible regulatory role. Here, we show that depletion of Rbf1 in Drosophila tissues is indeed associated with polarity defects in the wing and eye. Key polarity genes aPKC, par6, vang, pk, and fmi are upregulated, and an aPKC mutation suppresses the Rbf1-induced phenotypes. RB control of cell polarity may be an evolutionarily conserved function, with important implications in cancer metastasis.

Coordinating cell proliferation and differentiation: Antagonism between cell cycle regulators and cell type-specific gene expression

Cell proliferation and differentiation show a remarkable inverse relationship. Precursor cells continue division before acquiring a fully differentiated state, while terminal differentiation usually coincides with proliferation arrest and permanent exit from the division cycle. Mechanistic insight in the temporal coordination between cell cycle exit and differentiation has come from studies of cells in culture and genetic animal models. As initially described for skeletal muscle differentiation, temporal coordination involves mutual antagonism between cyclin-dependent kinases that promote cell cycle entry and transcription factors that induce tissue-specific gene expression. Recent insights highlight the contribution of chromatin-regulating complexes that act in conjunction with the transcription factors and determine their activity. In particular SWI/SNF chromatin remodelers contribute to dual regulation of cell cycle and tissue-specific gene expression during terminal differentiation. We review the concerted regulation of the cell cycle and cell type-specific transcription, and discuss common mutations in human cancer that emphasize the clinical importance of proliferation versus differentiation control.

Protein phosphatase 2A promotes the transition to G0 during terminal differentiation in Drosophila

Protein phosphatase type 2A complex (PP2A) has been known as a tumor suppressor for over two decades, but it remains unclear exactly how it suppresses tumor growth. Here, we provide data indicating a novel role for PP2A in promoting the transition to quiescence upon terminal differentiation in vivo. Using Drosophila eyes and wings as a model, we find that compromising PP2A activity during the final cell cycle prior to a developmentally controlled cell cycle exit leads to extra cell divisions and delays entry into quiescence. By systematically testing the regulatory subunits of Drosophila PP2A, we find that the B56 family member widerborst (wdb) is required for the role of PP2A in promoting the transition to quiescence. Cells in differentiating tissues with compromised PP2A retain high Cdk2 activity when they should be quiescent, and genetic epistasis tests demonstrate that ectopic Cyclin E/Cdk2 activity is responsible for the extra cell cycles caused by PP2A inhibition. The loss of wdb/PP2A function cooperates with aberrantly high Cyclin E protein levels, allowing cells to bypass a robust G0 late in development. This provides an example of how loss of PP2A can cooperate with oncogenic mutations in cancer. We propose that the PP2A complex plays a novel role in differentiating tissues to promote developmentally controlled quiescence through the regulation of Cyclin E/Cdk2 activity.

Scalloped and Yorkie are required for cell cycle re-entry of quiescent cells after tissue damage

Regeneration of damaged tissues typically requires a population of active stem cells. How damaged tissue is regenerated in quiescent tissues lacking a stem cell population is less well understood. We used a genetic screen in the developing Drosophila melanogaster eye to investigate the mechanisms that trigger quiescent cells to re-enter the cell cycle and proliferate in response to tissue damage. We discovered that Hippo signaling regulates compensatory proliferation after extensive cell death in the developing eye. Scalloped and Yorkie, transcriptional effectors of the Hippo pathway, drive Cyclin E expression to induce cell cycle re-entry in cells that normally remain quiescent in the absence of damage. Ajuba, an upstream regulator of Hippo signaling that functions as a sensor of epithelial integrity, is also required for cell cycle re-entry. Thus, in addition to its well-established role in modulating proliferation during periods of tissue growth, Hippo signaling maintains homeostasis by regulating quiescent cell populations affected by tissue damage.

Regulatory system for the G1-arrest during neuronal development in Drosophila

Neuronal network consists of many types of neuron and glial cells. This diversity is guaranteed by the constant cell proliferation of neuronal stem cells following stop cell cycle re-entry, which leads to differentiation during development. Neuronal differentiation occurs mainly at the specific cell cycle phase, the G1 phase. Therefore, cell cycle exit at the G1 phase is quite an important issue in understanding the process of neuronal cell development. Recent studies have revealed that aberrant S phase re-entry from the G1 phase often links cellular survival. In this review we discuss the different types of G1 arrest on the process of neuronal development in Drosophila. We also describe the issue that aberrant S phase entry often causes apoptosis, and the same mechanism might contribute to sensory organ defects, such as deafness.

Fly-FUCCI: A versatile tool for studying cell proliferation in complex tissues

One promising approach for in vivo studies of cell proliferation is the FUCCI system (fluorescent ubiquitination-based cell cycle indicator). Here, we report the development of a Drosophila-specific FUCCI system (Fly-FUCCI) that allows one to distinguish G1, S, and G2 phases of interphase. Fly-FUCCI relies on fluorochrome-tagged degrons from the Cyclin B and E2F1 proteins, which are degraded by the ubiquitin E3-ligases APC/C and CRL4(Cdt2), during mitosis or the onset of S phase, respectively. These probes can track cell-cycle patterns in cultured Drosophila cells, eye and wing imaginal discs, salivary glands, the adult midgut, and probably other tissues. To support a broad range of experimental applications, we have generated a toolkit of transgenic Drosophila lines that express the Fly-FUCCI probes under control of the UASt, UASp, QUAS, and ubiquitin promoters. The Fly-FUCCI system should be a valuable tool for visualizing cell-cycle activity during development, tissue homeostasis, and neoplastic growth.

Temporal regulation of Dpp signaling output in the Drosophila wing

BACKGROUND

The Decapentaplegic (Dpp) signaling pathway is used in many developmental and homeostatic contexts, each time resulting in cellular responses particular to that biological niche. The flexibility of Dpp signaling is clearly evident in epithelial cells of the Drosophila wing imaginal disc. During larval stages of development, Dpp functions as a morphogen, patterning the wing developmental field and stimulating tissue growth. A short time later, however, as wing-epithelial cells exit the cell cycle and begin to differentiate, Dpp is a critical determinant of vein-cell fate. It is likely that the Dpp signaling pathway regulates different sets of target genes at these two developmental time points.

RESULTS

To identify mechanisms that temporally control the transcriptional output of Dpp signaling in this system, we have taken a gene expression profiling approach. We identified genes affected by Dpp signaling at late larval or early pupal developmental time points, thereby identifying patterning- and differentiation-specific downstream targets, respectively.

CONCLUSIONS

Analysis of target genes and transcription factor binding sites associated with these groups of genes revealed potential mechanisms by which target-gene specificity of the Dpp signaling pathway is temporally regulated. In addition, this approach revealed novel mechanisms by which Dpp affects the cellular differentiation of wing-veins.

Hunting complex differential gene interaction patterns across molecular contexts

Heterogeneity in genetic networks across different signaling molecular contexts can suggest molecular regulatory mechanisms. Here we describe a comparative chi-square analysis (CPχ²) method, considerably more flexible and effective than other alternatives, to screen large gene expression data sets for conserved and differential interactions. CPχ² decomposes interactions across conditions to assess homogeneity and heterogeneity. Theoretically, we prove an asymptotic chi-square null distribution for the interaction heterogeneity statistic. Empirically, on synthetic yeast cell cycle data, CPχ² achieved much higher statistical power in detecting differential networks than alternative approaches. We applied CPχ² to Drosophila melanogaster wing gene expression arrays collected under normal conditions, and conditions with overexpressed E2F and Cabut, two transcription factor complexes that promote ectopic cell cycling. The resulting differential networks suggest a mechanism by which E2F and Cabut regulate distinct gene interactions, while still sharing a small core network. Thus, CPχ² is sensitive in detecting network rewiring, useful in comparing related biological systems.

Functional overlap among distinct G1/S inhibitory pathways allows robust G1 arrest by yeast mating pheromones

Multiple, functionally overlapping regulatory circuits control the sensitivity of the G1/S transition to yeast pheromones. In the absence of the Cdk inhibitor Far1, pheromone-induced G1 arrest depends on the p27 analogue Sic1, transcriptional repression by the Rb analogues Whi5 and Stb1, and induced degradation of the CLN1 transcription factor Tec1.

In budding yeast, mating pheromones arrest the cell cycle in G1 phase via a pheromone-activated Cdk-inhibitor (CKI) protein, Far1. Alternate pathways must also exist, however, because deleting the cyclin CLN2 restores pheromone arrest to far1∆ cells. Here we probe whether these alternate pathways require the G1/S transcriptional repressors Whi5 and Stb1 or the CKI protein Sic1, whose metazoan analogues (Rb or p27) antagonize cell cycle entry. Removing Whi5 and Stb1 allows partial escape from G1 arrest in far1∆ cln2∆ cells, along with partial derepression of G1/S genes, which implies a repressor-independent route for inhibiting G1/S transcription. This route likely involves pheromone-induced degradation of Tec1, a transcriptional activator of the cyclin CLN1, because Tec1 stabilization also causes partial G1 escape in far1∆ cln2∆ cells, and this is additive with Whi5/Stb1 removal. Deleting SIC1 alone strongly disrupts Far1-independent G1 arrest, revealing that inhibition of B-type cyclin-Cdk activity can empower weak arrest pathways. Of interest, although far1∆ cln2∆ sic1∆ cells escaped G1 arrest, they lost viability during pheromone exposure, indicating that G1 exit is deleterious if the arrest signal remains active. Overall our findings illustrate how multiple distinct G1/S-braking mechanisms help to prevent premature cell cycle commitment and ensure a robust signal-induced G1 arrest.

The molecular chaperone Hsp90 is required for cell cycle exit in Drosophila melanogaster

The coordination of cell proliferation and differentiation is crucial for proper development. In particular, robust mechanisms exist to ensure that cells permanently exit the cell cycle upon terminal differentiation, and these include restraining the activities of both the E2F/DP transcription factor and Cyclin/Cdk kinases. However, the full complement of mechanisms necessary to restrain E2F/DP and Cyclin/Cdk activities in differentiating cells are not known. Here, we have performed a genetic screen in Drosophila melanogaster, designed to identify genes required for cell cycle exit. This screen utilized a PCNA-miniwhite⁺ reporter that is highly E2F-responsive and results in a darker red eye color when crossed into genetic backgrounds that delay cell cycle exit. Mutation of Hsp83, the Drosophila homolog of mammalian Hsp90, results in increased E2F-dependent transcription and ectopic cell proliferation in pupal tissues at a time when neighboring wild-type cells are postmitotic. Further, these Hsp83 mutant cells have increased Cyclin/Cdk activity and accumulate proteins normally targeted for proteolysis by the anaphase-promoting complex/cyclosome (APC/C), suggesting that APC/C function is inhibited. Indeed, reducing the gene dosage of an inhibitor of Cdh1/Fzr, an activating subunit of the APC/C that is required for timely cell cycle exit, can genetically suppress the Hsp83 cell cycle exit phenotype. Based on these data, we propose that Cdh1/Fzr is a client protein of Hsp83. Our results reveal that Hsp83 plays a heretofore unappreciated role in promoting APC/C function during cell cycle exit and suggest a mechanism by which Hsp90 inhibition could promote genomic instability and carcinogenesis.

Cells must permanently stop dividing when they terminally differentiate for development to occur normally. Maintenance of this postmitotic state is also important, as unscheduled proliferation of differentiated cells can result in cancer. To identify genes important for restraining cell proliferation during terminal differentiation, we performed a genetic screen in Drosophila and found that mutation of Hsp90 caused ectopic cell proliferation in differentiating tissues. Hsp90 is a molecular chaperone that is essential for viability in all eukaryotes and has been shown to facilitate the activity of hundreds of “client” proteins. Indeed, several inhibitors of Hsp90 are currently being tested in clinical trials for use as anti-cancer therapeutics due to their ability to silence multiple client oncoproteins simultaneously. Our data suggest that Hsp90 is necessary to halt cell proliferation during differentiation because the protein Cdh1, which is required for normal cell cycle exit, may be a client of Hsp90. As reduced Cdh1 function results in genomic instability and tumorigenesis, our work highlights the need to design more precisely targeted Hsp90 inhibitors for use as cancer treatments.

Mitosis in neurons: Roughex and APC/C maintain cell cycle exit to prevent cytokinetic and axonal defects in Drosophila photoreceptor neurons

The mechanisms of cell cycle exit by neurons remain poorly understood. Through genetic and developmental analysis of Drosophila eye development, we found that the cyclin-dependent kinase-inhibitor Roughex maintains G1 cell cycle exit during differentiation of the R8 class of photoreceptor neurons. The roughex mutant neurons re-enter the mitotic cell cycle and progress without executing cytokinesis, unlike non-neuronal cells in the roughex mutant that perform complete cell divisions. After mitosis, the binucleated R8 neurons usually transport one daughter nucleus away from the cell body into the developing axon towards the brain in a kinesin-dependent manner resembling anterograde axonal trafficking. Similar cell cycle and photoreceptor neuron defects occurred in mutants for components of the Anaphase Promoting Complex/Cyclosome. These findings indicate a neuron-specific defect in cytokinesis and demonstrate a critical role for mitotic cyclin downregulation both to maintain cell cycle exit during neuronal differentiation and to prevent axonal defects following failed cytokinesis.

Neurons generally differentiate and never divide again. One barrier to understanding the mechanisms has been the paucity of genetic mutations that result in neuronal cell cycles. Here we show that mutation in three genes lead to cell cycle re-entry by a particular class of developing photoreceptor neurons in the fly retina. Strikingly, these neurons do not complete cell division but only divide their nuclei. The binucleated neurons then typically retain one nucleus in its normal location in the cell body, while transporting the other into the growing axon like other axonal material. Our findings identify Cyclin A regulation as crucial to maintaining cell cycle exit by at least some neurons and identify a neuron-specific defect in cell division as a further barrier to neuron proliferation. Because defects in transporting axonal material have been implicated in the origin of multiple neurodegenerative diseases, our findings also suggest a possible connection between defective cell cycle regulation and neuronal cell death.

Combinatorial control of temporal gene expression in the Drosophila wing by enhancers and core promoters

Background

The transformation of a developing epithelium into an adult structure is a complex process, which often involves coordinated changes in cell proliferation, metabolism, adhesion, and shape. To identify genetic mechanisms that control epithelial differentiation, we analyzed the temporal patterns of gene expression during metamorphosis of the Drosophila wing.

Results

We found that a striking number of genes, approximately 50% of the Drosophila transcriptome, exhibited changes in expression during a time course of wing development. While cis-acting enhancer sequences clearly correlated with these changes, a stronger correlation was discovered between core-promoter types and the dynamic patterns of gene expression within this differentiating tissue. In support of the hypothesis that core-promoter type influences the dynamics of expression, expression levels of several TATA-box binding protein associated factors (TAFs) and other core promoter-associated components changed during this developmental time course, and a testes-specific TAF (tTAF) played a critical role in timing cellular differentiation within the wing.

Conclusions

Our results suggest that the combinatorial control of gene expression via cis-acting enhancer sequences and core-promoter types, determine the complex changes in gene expression that drive morphogenesis and terminal differentiation of the Drosophila wing epithelium.

Co-ordination of cell cycle and differentiation in the developing nervous system

During embryonic development, cells must divide to produce appropriate numbers, but later must exit the cell cycle to allow differentiation. How these processes of proliferation and differentiation are co-ordinated during embryonic development has been poorly understood until recently. However, a number of studies have now given an insight into how the cell cycle machinery, including cyclins, CDKs (cyclin-dependent kinases), CDK inhibitors and other cell cycle regulators directly influence mechanisms that control cell fate and differentiation. Conversely, examples are emerging of transcriptional regulators that are better known for their role in driving the differentiated phenotype, which also play complementary roles in controlling cell cycle progression. The present review will summarise our current understanding of the mechanisms co-ordinating the cell cycle and differentiation in the developing nervous system, where these links have been, perhaps, most extensively studied.

Quiescence: early evolutionary origins and universality do not imply uniformity

Cell cycle investigations have focused on relentless exponential proliferation of cells, an unsustainable situation in nature. Proliferation of cells, whether microbial or metazoan, is interrupted by periods of quiescence. The vast majority of cells in an adult metazoan lie quiescent. As disruptions in this quiescence are at the foundation of cancer, it will be important for the field to turn its attention to the mechanisms regulating quiescence. While often presented as a single topic, there are multiple forms of quiescence each with complex inputs, some of which are tied to conceptually challenging aspects of metazoan regulation such as size control. In an effort to expose the enormity of the challenge, I describe the differing biological purposes of quiescence, and the coupling of quiescence in metazoans to growth and to the structuring of tissues during development. I emphasize studies in the organism rather than in tissue culture, because these expose the diversity of regulation. While quiescence is likely to be a primitive biological process, it appears that in adapting quiescence to its many distinct biological settings, evolution has diversified it. Consideration of quiescence in different models gives us an overview of this diversity.

An integrated view of cyclin E function and regulation

Cancers of diverse cell lineages express high levels of cyclin E, and in various studies, cyclin E overexpression correlates with increased tumor aggression. One way that normal control of cyclin E expression is disabled in cancer cells is via loss-of-function mutations sustained by FBXW7. This gene encodes the Fbw7 tumor suppressor protein that provides substrate specificity for a ubiquitin ligase complex that targets multiple oncoproteins for degradation. Numerous other mechanisms besides Fbw7 mutations can deregulate cyclin E expression and activity in cancer cells. Recent reports demonstrate that inappropriate cyclin E expression may have far-reaching biological consequences for cell physiology, including altering gene expression programs governing proliferation, differentiation, survival and senescence. In this review, we discuss the function of mammalian cyclin E in the context of these new data as well as the complex network that connects cyclin E functions to the cellular controls regulating its expression and activity.

dp53 Restrains ectopic neural stem cell formation in the Drosophila brain in a non-apoptotic mechanism involving Archipelago and cyclin E

Accumulating evidence suggests that tumor-initiating stem cells or cancer stem cells (CSCs) possibly originating from normal stem cells may be the root cause of certain malignancies. How stem cell homeostasis is impaired in tumor tissues is not well understood, although certain tumor suppressors have been implicated. In this study, we use the Drosophila neural stem cells (NSCs) called neuroblasts as a model to study this process. Loss-of-function of Numb, a key cell fate determinant with well-conserved mammalian counterparts, leads to the formation of ectopic neuroblasts and a tumor phenotype in the larval brain. Overexpression of the Drosophila tumor suppressor p53 (dp53) was able to suppress ectopic neuroblast formation caused by numb loss-of-function. This occurred in a non-apoptotic manner and was independent of Dacapo, the fly counterpart of the well-characterized mammalian p53 target p21 involved in cellular senescence. The observation that dp53 affected Edu incorporation into neuroblasts led us to test the hypothesis that dp53 acts through regulation of factors involved in cell cycle progression. Our results show that the inhibitory effect of dp53 on ectopic neuroblast formation was mediated largely through its regulation of Cyclin E (Cyc E). Overexpression of Cyc E was able to abrogate dp53's ability to rescue numb loss-of-function phenotypes. Increasing Cyc E levels by attenuating Archipelago (Ago), a recently identified transcriptional target of dp53 and a negative regulator of Cyc E, had similar effects. Conversely, reducing Cyc E activity by overexpressing Ago blocked ectopic neuroblast formation in numb mutant. Our results reveal an intimate connection between cell cycle progression and NSC self-renewal vs. differentiation control, and indicate that p53-mediated regulation of ectopic NSC self-renewal through the Ago/Cyc E axis becomes particularly important when NSC homeostasis is perturbed as in numb loss-of-function condition. This has important clinical implications.

Caenorhabditis elegans cyclin D/CDK4 and cyclin E/CDK2 induce distinct cell cycle re-entry programs in differentiated muscle cells

Cell proliferation and differentiation are regulated in a highly coordinated and inverse manner during development and tissue homeostasis. Terminal differentiation usually coincides with cell cycle exit and is thought to engage stable transcriptional repression of cell cycle genes. Here, we examine the robustness of the post-mitotic state, using Caenorhabditis elegans muscle cells as a model. We found that expression of a G1 Cyclin and CDK initiates cell cycle re-entry in muscle cells without interfering with the differentiated state. Cyclin D/CDK4 (CYD-1/CDK-4) expression was sufficient to induce DNA synthesis in muscle cells, in contrast to Cyclin E/CDK2 (CYE-1/CDK-2), which triggered mitotic events. Tissue-specific gene-expression profiling and single molecule FISH experiments revealed that Cyclin D and E kinases activate an extensive and overlapping set of cell cycle genes in muscle, yet failed to induce some key activators of G1/S progression. Surprisingly, CYD-1/CDK-4 also induced an additional set of genes primarily associated with growth and metabolism, which were not activated by CYE-1/CDK-2. Moreover, CYD-1/CDK-4 expression also down-regulated a large number of genes enriched for catabolic functions. These results highlight distinct functions for the two G1 Cyclin/CDK complexes and reveal a previously unknown activity of Cyclin D/CDK-4 in regulating metabolic gene expression. Furthermore, our data demonstrate that many cell cycle genes can still be transcriptionally induced in post-mitotic muscle cells, while maintenance of the post-mitotic state might depend on stable repression of a limited number of critical cell cycle regulators.

During development, cells face the important decision whether to continue to proliferate, or to exit the cell-division cycle and fully differentiate. Improved insight into the molecular mechanisms that arrest the cell cycle during terminal differentiation is important for our understanding of normal development, as well as for cancer research and regenerative medicine. To investigate the arrested state of terminally differentiated cells, we examined muscle cells in the model organism C. elegans, which is known for its reproducible cell-division pattern. We found that expression of a single cell cycle kinase with its regulatory partner (Cyclin) induced many cell division genes in muscle. While Cyclin D and E kinases often act similarly, only Cyclin D with CDK-4 triggered DNA replication in muscle, and this combination induced a much broader transcriptional response than Cyclin E/CDK-2. Despite activation of a substantial cell cycle program, Cyclin/CDK expression did not induce complete muscle cell division and failed to induce some key cell cycle regulators. Our results highlight distinct activities of Cyclin D and Cyclin E kinases, and they indicate that cell-cycle gene expression remains remarkably flexible in differentiated cells. We propose that the post-mitotic state of differentiated cells is maintained by tight control of a few regulatory genes.

N-cadherin-mediated cell adhesion restricts cell proliferation in the dorsal neural tube

Neural progenitors are organized as a pseudostratified epithelium held together by adherens junctions (AJs), multiprotein complexes composed of cadherins and α- and β-catenin. Catenins are known to control neural progenitor division; however, it is not known whether they function in this capacity as cadherin binding partners, as there is little evidence that cadherins themselves regulate neural proliferation. We show here that zebrafish N-cadherin (N-cad) restricts cell proliferation in the dorsal region of the neural tube by regulating cell-cycle length. We further reveal that N-cad couples cell-cycle exit and differentiation, as a fraction of neurons are mitotic in N-cad mutants. Enhanced proliferation in N-cad mutants is mediated by ligand-independent activation of Hedgehog (Hh) signaling, possibly caused by defective ciliogenesis. Furthermore, depletion of Hh signaling results in the loss of junctional markers. We therefore propose that N-cad restricts the response of dorsal neural progenitors to Hh and that Hh signaling limits the range of its own activity by promoting AJ assembly. Taken together, these observations emphasize a key role for N-cad-mediated adhesion in controlling neural progenitor proliferation. In addition, these findings are the first to demonstrate a requirement for cadherins in synchronizing cell-cycle exit and differentiation and a reciprocal interaction between AJs and Hh signaling.