Cell cycle-dependent nuclear export of Cdh1p may contribute to the inactivation of APC/C(Cdh1)

Modeling the START transition in the budding yeast cell cycle

Budding yeast, Saccharomyces cerevisiae, is widely used as a model organism to study the genetics underlying eukaryotic cellular processes and growth critical to cancer development, such as cell division and cell cycle progression. The budding yeast cell cycle is also one of the best-studied dynamical systems owing to its thoroughly resolved genetics. However, the dynamics underlying the crucial cell cycle decision point called the START transition, at which the cell commits to a new round of DNA replication and cell division, are under-studied. The START machinery involves a central cyclin-dependent kinase; cyclins responsible for starting the transition, bud formation, and initiating DNA synthesis; and their transcriptional regulators. However, evidence has shown that the mechanism is more complicated than a simple irreversible transition switch. Activating a key transcription regulator SBF requires the phosphorylation of its inhibitor, Whi5, or an SBF/MBF monomeric component, Swi6, but not necessarily both. Also, the timing and mechanism of the inhibitor Whi5's nuclear export, while important, are not critical for the timing and execution of START. Therefore, there is a need for a consolidated model for the budding yeast START transition, reconciling regulatory and spatial dynamics. We built a detailed mathematical model (START-BYCC) for the START transition in the budding yeast cell cycle based on established molecular interactions and experimental phenotypes. START-BYCC recapitulates the underlying dynamics and correctly emulates key phenotypic traits of ~150 known START mutants, including regulation of size control, localization of inhibitor/transcription factor complexes, and the nutritional effects on size control. Such a detailed mechanistic understanding of the underlying dynamics gets us closer towards deconvoluting the aberrant cellular development in cancer.

APCCdh1-mediated degradation of Cdh1 is necessary for faithful meiotic chromosome segregation in S. cerevisiae

The Anaphase-Promoting Complex/Cyclosome (APC/C) is a ubiquitin ligase that promotes the ubiquitination and subsequent degradation of numerous cell cycle regulators during mitosis and in G1. Proteins are recruited to the APC/C by activator proteins such as Cdh1. During the cell cycle, Cdh1 is subject to precise regulation so that substrates are not degraded prematurely. We have explored the regulation of Cdh1 during the developmental transition into meiosis and sporulation in the budding yeast S. cerevisiae. Transition to sporulation medium triggers the degradation of Cdh1. Cdh1 degradation is mediated by the APC/C itself in a "trans" mechanism in which one molecule of Cdh1 recruits a second molecule of Cdh1 to the APC/C for ubiquitination. Degradation requires an intact glucose-sensing SNF1 protein kinase complex (orthologous to the mammalian AMPK nutritional sensor), which directly phosphorylates Cdh1 on Ser-200 within an unstructured N-terminal region. In the absence of phosphorylation, expression of a Cdh1-S200A mutant is fully stabilized, leading to chromosome instability and loss of viability. We hypothesize that Cdh1 degradation is necessary for the preservation of cell cycle regulators and chromosome cohesion proteins between the reductional and equational meiotic divisions, which occur without the intervening Gap or S phases found in mitotic cell cycles.

Astrocyte-neuron metabolic cooperation shapes brain activity

The brain has almost no energy reserve, but its activity coordinates organismal function, a burden that requires precise coupling between neurotransmission and energy metabolism. Deciphering how the brain accomplishes this complex task is crucial to understand central facets of human physiology and disease mechanisms. Each type of neural cell displays a peculiar metabolic signature, forcing the intercellular exchange of metabolites that serve as both energy precursors and paracrine signals. The paradigm of this biological feature is the astrocyte-neuron couple, in which the glycolytic metabolism of astrocytes contrasts with the mitochondrial oxidative activity of neurons. Astrocytes generate abundant mitochondrial reactive oxygen species and shuttle to neurons glycolytically derived metabolites, such as L-lactate and L-serine, which sustain energy needs, conserve redox status, and modulate neurotransmitter-receptor activity. Conversely, early disruption of this metabolic cooperation may contribute to the initiation or progression of several neurological diseases, thus requiring innovative therapies to preserve brain energetics.

From the Nuclear Pore to the Fibrous Corona: A MAD Journey to Preserve Genome Stability

The relationship between kinetochores and nuclear pore complexes (NPCs) is intimate but poorly understood. Several NPC components and associated proteins are relocated to mitotic kinetochores to assist in different activities that ensure faithful chromosome segregation. Such is the case of the Mad1-c-Mad2 complex, the catalytic core of the spindle assembly checkpoint (SAC), a surveillance pathway that delays anaphase until all kinetochores are attached to spindle microtubules. Mad1-c-Mad2 is recruited to discrete domains of unattached kinetochores from where it promotes the rate-limiting step in the assembly of anaphase-inhibitory complexes. SAC proficiency further requires Mad1-c-Mad2 to be anchored at NPCs during interphase. However, the mechanistic relevance of this arrangement for SAC function remains ill-defined. Recent studies uncover the molecular underpinnings that coordinate the release of Mad1-c-Mad2 from NPCs with its prompt recruitment to kinetochores. Here, current knowledge on Mad1-c-Mad2 function and spatiotemporal regulation is reviewed and the critical questions that remain unanswered are highlighted.

A novel human Cdh1 mutation impairs anaphase promoting complex/cyclosome activity resulting in microcephaly, psychomotor retardation, and epilepsy

The Fizzy-related protein 1 (Fzr1) gene encodes Cdh1 protein, a coactivator of the E3 ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C). Previously, we found that genetic ablation of Fzr1 promotes the death of neural progenitor cells leading to neurogenesis impairment and microcephaly in mouse. To ascertain the possible translation of these findings in humans, we searched for mutations in the Fzr1 gene in 390 whole exomes sequenced in trio in individuals showing neurodevelopmental disorders compatible with a genetic origin. We found a novel missense (p.Asp187Gly) Fzr1 gene mutation (c.560A>G) in a heterozygous state in a 4-year-old boy, born from non-consanguineous Spanish parents, who presents with severe antenatal microcephaly, psychomotor retardation, and refractory epilepsy. Cdh1 protein levels in leucocytes isolated from the patient were significantly lower than those found in his parents. Expression of the Asp187Gly mutant form of Cdh1 in human embryonic kidney 293T cells produced less Cdh1 protein and APC/C activity, resulting in altered cell cycle distribution when compared with cells expressing wild-type Cdh1. Furthermore, ectopic expression of the Asp187Gly mutant form of Cdh1 in cortical progenitor cells in primary culture failed to abolish the enlargement of the replicative phase caused by knockout of endogenous Cdh1. These results indicate that the loss of function of APC/C-Cdh1 caused by Cdh1 Asp187Gly mutation is a new cause of prenatal microcephaly, psychomotor retardation, and severe epilepsy. Read the Editorial Highlight for this article on page 8. Cover Image for this issue: doi: 10.1111/jnc.14524.

Hyperphosphorylation of CDH1 in Glioblastoma Cancer Stem Cells Attenuates APC/CCDH1 Activity and Pharmacologic Inhibition of APC/CCDH1/CDC20 Compromises Viability

Glioblastoma (GBM) is the most common and lethal primary brain tumor and remains incurable. This is in part due to the cellular heterogeneity within these tumors, which includes a subpopulation of treatment-resistant cells called cancer stem-like cells (CSC). We previously identified that the anaphase-promoting complex/cylosome (APC/C), a key cell-cycle regulator and tumor suppressor, had attenuated ligase activity in CSCs. Here, we assessed the mechanism of reduced activity, as well as the efficacy of pharmacologically targeting the APC/C in CSCs. We identified hyperphosphorylation of CDH1, but not pseudosubstrate inhibition by early mitotic inhibitor 1 (EMI1), as a major mechanism driving attenuated APC/C^CDH1 activity in the G₁-phase of the cell cycle in CSCs. Small-molecule inhibition of the APC/C reduced viability of both CSCs and nonstem tumor cells (NSTCs), with the combination of proTAME and apcin having the biggest impact. Combinatorial drug treatment also led to the greatest mitotic arrest and chromosomal abnormalities. IMPLICATIONS: Our findings demonstrate how the activity of the APC/C^CDH1 tumor suppressor is reduced in CSCs and also validates small-molecule inhibition of the APC/C as a promising therapeutic target for the treatment of GBM.

Karyopherin Msn5 is involved in a novel mechanism controlling the cellular level of cell cycle regulators Cln2 and Swi5

The yeast β-karyopherin Msn5 controls the SBF cell-cycle transcription factor, responsible for the periodic expression of CLN2 cyclin gene at G1/S, and the nuclear export of Cln2 protein. Here we show that Msn5 regulates Cln2 by an additional mechanism. Inactivation of Msn5 causes a severe reduction in the cellular content of Cln2. This occurs by a post-transcriptional mechanism, since CLN2 mRNA level is not importantly affected in asynchronous cultures. Cln2 stability is not significantly altered in msn5 cells and inactivation of Msn5 causes a reduction in protein level even when Cln2 is stabilized. Therefore, the reduced amount of Cln2 in msn5 cells is mainly due not to a higher rate of protein degradation but to a defect in Cln2 synthesis. In fact, analysis of polysome profiles indicated that Msn5 inactivation causes a shift of CLN2 and SWI5 mRNAs from heavy-polysomal to light-polysomal and non-polysomal fractions, supporting a defect in Cln2 and Swi5 protein synthesis in the msn5 mutant. The analysis of truncated versions of Cln2 and of chimeric cyclins combining distinct domains from Cln2 and the related Cln1 cyclin identified an internal region in Cln2 from 181 to 225 residues that when fused to GFP is able to confer Msn5-dependent regulation of protein cellular content. Finally, we showed that a high level of Cln2 is toxic in the absence of Msn5. In summary, we described that Msn5 is required for the proper protein synthesis of specific proteins, introducing a new level of control of cell cycle regulators.

APC/C: current understanding and future perspectives

The separation of sister chromatids at anaphase, which is regulated by an E3 ubiquitin ligase called the anaphase-promoting complex/cyclosome (APC/C), is arguably the most important irrevocable event during the cell cycle. The APC/C and cyclin-dependent kinase 1 (Cdk1) are just two of the many significant cell cycle regulators and exert control through ubiquitylation and phosphorylation, respectively. The temporal and spatial regulation of the APC/C is achieved by multiple mechanisms, including phosphorylation, interaction with the structurally related co-activators Cdc20 and Cdh1, loading of distinct E2 ubiquitin-conjugating enzymes, binding with inhibitors and differential affinities for various substrates. Since the discovery of APC/C 25 years ago, intensive studies have uncovered many aspects of APC/C regulation, but we are still far from a full understanding of this important cellular machinery. Recent high-resolution cryogenic electron microscopy analysis and reconstitution of the APC/C have greatly advanced our understanding of molecular mechanisms underpinning the enzymatic properties of APC/C. In this review, we will examine the historical background and current understanding of APC/C regulation.

APC-Cdh1 Regulates Neuronal Apoptosis Through Modulating Glycolysis and Pentose-Phosphate Pathway After Oxygen-Glucose Deprivation and Reperfusion

Anaphase-promoting complex (APC) with its coactivator Cdh1 is required to maintain the postmitotic state of neurons via degradation of Cyclin B1, which aims to prevent aberrant cell cycle entry that causes neuronal apoptosis. Interestingly, evidence is accumulating that apart from the cell cycle, APC-Cdh1 also involves in neuronal metabolism via modulating the glycolysis promoting enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3 (PFKFB3). Here, we showed that under oxygen-glucose deprivation and reperfusion (OGD/R), APC-Cdh1 was decreased in primary cortical neurons. Likewise, the neurons exhibited enhanced glycolysis when oxygen supply was reestablished during reperfusion, which was termed as the "neuronal Warburg effect." In particular, the reperfused neurons showed elevated PFKFB3 expression in addition to a reduction in glucose 6-phosphate dehydrogenase (G6PD). Such changes directed neuronal glucose metabolism from pentose-phosphate pathway (PPP) to aerobic glycolysis compared to the normal neurons, resulting in increased ROS production and apoptosis during reperfusion. Pretreatment of neurons with Cdh1 expressing lentivirus before OGD could reverse this metabolic shift and attenuated ROS-induced apoptosis. However, the metabolism regulation and neuroprotection by Cdh1 under OGD/R condition could be blocked when co-transfecting neurons with Ken box-mut-PFKFB3 (which is APC-Cdh1 insensitive). Based on these data, we suggest that the Warburg effect may contribute to apoptotic mechanisms in neurons under OGD/R insult, and targeting Cdh1 may be a potential therapeutic strategy as both glucose metabolic regulator and apoptosis suppressor of neurons in brain injuries.

Cdh1 degradation is mediated by APC/C-Cdh1 and SCF-Cdc4 in budding yeast

Cdh1, a substrate-recognition subunit of anaphase-promoting complex/cyclosome (APC/C), is a tumor suppressor, and it is downregulated in various tumor cells in humans. APC/C-Cdh1 is activated from late M phase to G1 phase by antagonizing Cdk1-mediated inhibitory phosphorylation. However, how Cdh1 protein levels are properly regulated is ill-defined. Here we show that Cdh1 is degraded via APC/C-Cdh1 and Skp1-Cullin1-F-box (SCF)-Cdc4 in the budding yeast Saccharomyces cerevisiae. Cdh1 degradation was promoted by forced localization of Cdh1 into the nucleus, where APC/C and SCF are present. Cdk1 promoted APC/C-Cdh1-mediated Cdh1 degradation, whereas polo kinase Cdc5 elicited SCF-Cdc4-mediated degradation. Thus, Cdh1 degradation is controlled via multiple pathways.

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.

Upregulation of Cdh1 Attenuates Isoflurane-Induced Neuronal Apoptosis and Long-Term Cognitive Impairments in Developing Rats

Neonatal exposure to isoflurane can result in neuroapoptosis and persistent cognitive impairments. However, the underlying mechanisms remain elusive. Anaphase-promoting complex/cyclosome (APC/C) and its co-activator Cdh1 are E3 ubiquitin ligases that play important roles in the central nervous system, including in the regulation of neuronal survival, synaptic development, and mammalian learning and memory. However, whether APC/C-Cdh1 is involved in isoflurane-induced neurotoxicity in developing rats remains unclear. In this study, postnatal day-7 (P7) rat pups and primary hippocampal neurons were exposed to 2% isoflurane for 6 h. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was used to detect neuronal apoptosis, and the expression of proteins involved in apoptosis (cleaved caspase-3, Bax and Bcl-2) was assessed by western blot. The level of Cdh1 in the hippocampus was downregulated during isoflurane-induced neuroapoptosis. Cdh1-encoding lentivirus was transfected before isoflurane-treatment to increase the level of Cdh1. Our results showed that Cdh1 overexpression by a recombinant Cdh1-encoding lentivirus reduced isoflurane-induced neuronal apoptosis. Moreover, bilateral intra-hippocampal injection with Cdh1-encoding lentivirus attenuated long-term cognitive deficits after exposure to isoflurane in developing rats. Our study indicates that Cdh1 is an important target to prevent isoflurane-induced developmental neurotoxicity.

Building a Regulatory Network with Short Linear Sequence Motifs: Lessons from the Degrons of the Anaphase-Promoting Complex

The anaphase-promoting complex or cyclosome (APC/C) is a ubiquitin ligase that polyubiquitinates specific substrates at precise times in the cell cycle, thereby triggering the events of late mitosis in a strict order. The robust substrate specificity of the APC/C prevents the potentially deleterious degradation of non-APC/C substrates and also averts the cell-cycle errors and genomic instability that could result from mistimed degradation of APC/C targets. The APC/C recognizes short linear sequence motifs, or degrons, on its substrates. The specific and timely modification and degradation of APC/C substrates is likely to be modulated by variations in degron sequence and context. We discuss the extensive affinity, specificity, and selectivity determinants encoded in APC/C degrons, and we describe some of the extrinsic mechanisms that control APC/C-substrate recognition. As an archetype for protein motif-driven regulation of cell function, the APC/C-substrate interaction provides insights into the general properties of post-translational regulatory systems.

Nucleolar asymmetry and the importance of septin integrity upon cell cycle arrest

Cell cycle arrest can be imposed by inactivating the anaphase promoting complex (APC). In S. cerevisiae this arrest has been reported to stabilize a metaphase-like intermediate in which the nuclear envelope spans the bud neck, while chromatin repeatedly translocates between the mother and bud domains. The present investigation was undertaken to learn how other features of nuclear organization are affected upon depletion of the APC activator, Cdc20. We observe that the spindle pole bodies and the spindle repeatedly translocate across the narrow orifice at the level of the neck. Nevertheless, we find that the nucleolus (organized around rDNA repeats on the long right arm of chromosome XII) remains in the mother domain, marking the polarity of the nucleus. Accordingly, chromosome XII is polarized: TelXIIR remains in the mother domain and its centromere is predominantly located in the bud domain. In order to learn why the nucleolus remains in the mother domain, we studied the impact of inhibiting rRNA synthesis in arrested cells. We observed that this fragments the nucleolus and that these fragments entered the bud domain. Taken together with earlier observations, the restriction of the nucleolus to the mother domain therefore can be attributed to its massive structure. We also observed that inactivation of septins allowed arrested cells to complete the cell cycle, that the alternative APC activator, Cdh1, was required for completion of the cell cycle and that induction of Cdh1 itself caused arrested cells to progress to the end of the cell cycle.

Regulation of Mitotic Exit in Saccharomyces cerevisiae

The Mitotic Exit Network (MEN) is an essential signaling pathway, closely related to the Hippo pathway in mammals, which promotes mitotic exit and initiates cytokinesis in the budding yeast Saccharomyces cerevisiae. Here, we summarize the current knowledge about the MEN components and their regulation.

APC/C and retinoblastoma interaction: cross-talk of retinoblastoma protein with the ubiquitin proteasome pathway

The ubiquitin (Ub) ligase anaphase promoting complex/cyclosome (APC/C) and the tumour suppressor retinoblastoma protein (pRB) play key roles in cell cycle regulation. APC/C is a critical regulator of mitosis and G₁-phase of the cell cycle whereas pRB keeps a check on proliferation by inhibiting transition to the S-phase. APC/C and pRB interact with each other via the co-activator of APC/C, FZR1, providing an alternative pathway of regulation of G₁ to S transition by pRB using a post-translational mechanism. Both pRB and FZR1 have complex roles and are implicated not only in regulation of cell proliferation but also in differentiation, quiescence, apoptosis, maintenance of chromosomal integrity and metabolism. Both are also targeted by transforming viruses. We discuss recent advances in our understanding of the involvement of APC/C and pRB in cell cycle based decisions and how these insights will be useful for development of anti-cancer and anti-viral drugs.

An APC/C-Cdh1 Biosensor Reveals the Dynamics of Cdh1 Inactivation at the G1/S Transition

B-type cyclin-dependent kinase activity must be turned off for mitotic exit and G1 stabilization. B-type cyclin degradation is mediated by the anaphase-promoting complex/cyclosome (APC/C); during and after mitotic exit, APC/C is dependent on Cdh1. Cdh1 is in turn phosphorylated and inactivated by cyclin-CDK at the Start transition of the new cell cycle. We developed a biosensor to assess the cell cycle dynamics of APC/C-Cdh1. Nuclear exit of the G1 transcriptional repressor Whi5 is a known marker of Start; APC/C-Cdh1 is inactivated 12 min after Whi5 nuclear exit with little measurable cell-to-cell timing variability. Multiple phosphorylation sites on Cdh1 act in a redundant manner to repress its activity. Reducing the number of phosphorylation sites on Cdh1 can to some extent be tolerated for cell viability, but it increases variability in timing of APC/C-Cdh1 inactivation. Mutants with minimal subsets of phosphorylation sites required for viability exhibit striking stochasticity in multiple responses including budding, nuclear division, and APC/C-Cdh1 activity itself. Multiple cyclin-CDK complexes, as well as the stoichiometric inhibitor Acm1, contribute to APC/C-Cdh1 inactivation; this redundant control is likely to promote rapid and reliable APC/C-Cdh1 inactivation immediately following the Start transition.

Dual control by Cdk1 phosphorylation of the budding yeast APC/C ubiquitin ligase activator Cdh1

The antagonism between cyclin-dependent kinases (Cdks) and the ubiquitin ligase APC/C-Cdh1 is central to eukaryotic cell cycle control. APC/C-Cdh1 targets cyclin B and other regulatory proteins for degradation, whereas Cdks disable APC/C-Cdh1 through phosphorylation of the Cdh1 activator protein at multiple sites. Budding yeast Cdh1 carries nine Cdk phosphorylation sites in its N-terminal regulatory domain, most or all of which contribute to inhibition. However, the precise role of individual sites has remained unclear. Here, we report that the Cdk phosphorylation sites of yeast Cdh1 are organized into autonomous subgroups and act through separate mechanisms. Cdk sites 1-3 had no direct effect on the APC/C binding of Cdh1 but inactivated a bipartite nuclear localization sequence (NLS) and thereby controlled the partitioning of Cdh1 between cytoplasm and nucleus. In contrast, Cdk sites 4-9 did not influence the cell cycle-regulated localization of Cdh1 but prevented its binding to the APC/C. Cdk sites 4-9 reside near two recently identified APC/C interaction motifs in a pattern conserved with the human Cdh1 orthologue. Thus a Cdk-inhibited NLS goes along with Cdk-inhibited APC/C binding sites in yeast Cdh1 to relay the negative control by Cdk1 phosphorylation of the ubiquitin ligase APC/C-Cdh1.

Mapping regions in Ste5 that support Msn5-dependent and -independent nuclear export

Careful control of the available pool of the MAPK scaffold Ste5 is important for mating-pathway activation and the prevention of inappropriate mating differentiation in haploid Saccharomyces cerevisiae. Ste5 shuttles constitutively through the nucleus, where it is degraded by a ubiquitin-dependent mechanism triggered by G1 CDK phosphorylation. Here we narrow-down regions of Ste5 that mediate nuclear export. Four regions in Ste5 relocalize SV40-TAgNLS-GFP-GFP from nucleus to cytoplasm. One region is N-terminal, dependent on exportin Msn5/Ste21/Kap142, and interacts with Msn5 in 2 hybrid assays independently of mating pheromone, Fus3, Kss1, Ptc1, the NLS/PM, and RING-H2. A second region overlaps the PH domain and Ste11 binding site and 2 others are on the vWA domain and include residues essential for MAPK activation. We find no evidence for dependence on Crm1/Xpo1, despite numerous potential nuclear export sequences (NESs) detected by LocNES and NetNES1.1 predictors. Thus, Msn5 (homolog of human Exportin-5) and one or more exportins or adaptor molecules besides Crm1/Xpo1 may regulate Ste5 through multiple recognition sites.

PFKFB3-mediated glycolysis is involved in reactive astrocyte proliferation after oxygen-glucose deprivation/reperfusion and is regulated by Cdh1

Reactive astrocyte proliferation is involved in many central degenerative diseases. The enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase isoform 3 (PFKFB3), an allosteric activator of 6-phosphofructo-1-kinase (PFK1), controls glycolytic flux. Furthermore, APC/C-Cdh1 plays a crucial role in brain metabolism by regulating PFKFB3 expression. Previous studies have defined the roles of PFKFB3-mediated glycolysis in pathological angiogenesis, cell autophagy, and amyloid plaque deposition in proliferating cells. However, the role of PFKFB3 in reactive astrocyte proliferation after cerebral ischemia is unknown. In this study, we cultured rat primary cortical astrocytes and established an oxygen-glucose deprivation/reperfusion (OGD/R) model to mimic cerebral ischemia in vivo. Astrocyte proliferation was measured by western blotting for proliferating cell nuclear antigen (PCNA) and by EdU incorporation. We found that OGD/R up-regulated PFKFB3 and PFK1 expression, which was accompanied by reactive astrocyte proliferation. Knockdown of PFKFB3 by siRNA transfection significantly inhibited reactive astrocyte proliferation and lactate release, an indicator of glycolysis. We found that PFKFB3 and PFK1 expression were down-regulated and lactate release was decreased when OGD/R-induced astrocyte proliferation was inhibited by a Cdh1-expressing lentivirus. Thus, reactive astrocyte proliferation can be effectively suppressed by down-regulation of PFKFB3 through control of glycolytic flux, which is downstream of APC/C-Cdh1.

Alleviating neuropathic pain mechanical allodynia by increasing Cdh1 in the anterior cingulate cortex

Background

Plastic changes in the anterior cingulate cortex (ACC) are critical in the pathogenesis of pain hypersensitivity caused by injury to peripheral nerves. Cdh1, a co-activator subunit of anaphase-promoting complex/cyclosome (APC/C) regulates synaptic differentiation and transmission. Based on this, we hypothesised that the APC/C–Cdh1 played an important role in long-term plastic changes induced by neuropathic pain in ACC.

Results

We employed spared nerve injury (SNI) model in rat and found Cdh1 protein level in the ACC was down-regulated 3, 7 and 14 days after SNI surgery. We detected increase in c-Fos expression, numerical increase of organelles, swollen myelinated fibre and axon collapse of neuronal cells in the ACC of SNI rat. Additionally, AMPA receptor GluR1 subunit protein level was up-regulated on the membrane through a pathway that involves EphA4 mediated by APC/C–Cdh1, 3 and 7 days after SNI surgery. To confirm the effect of Cdh1 in neuropathic pain, Cdh1-expressing lentivirus was injected into the ACC of SNI rat. Intra-ACC treatment with Cdh1-expressing lentivirus vectors elevated Cdh1 levels, erased synaptic strengthening, as well as alleviating established mechanical allodynia in SNI rats. We also found Cdh1-expressing lentivirus normalised SNI-induced redistribution of AMPA receptor GluR1 subunit in ACC by regulating AMPA receptor trafficking.

Conclusions

These results provide evidence that Cdh1 in ACC synapses may offer a novel therapeutic strategy for treating chronic neuropathic pain.

Regulation of Bcl-xL-ATP Synthase Interaction by Mitochondrial Cyclin B1-Cyclin-Dependent Kinase-1 Determines Neuronal Survival

The survival of postmitotic neurons needs continuous degradation of cyclin B1, a mitotic protein accumulated aberrantly in the damaged brain areas of Alzheimer's disease and stroked patients. Degradation of cyclin B1 takes place in the proteasome after ubiquitylation by the anaphase-promoting complex/cyclosome (APC/C)-cadherin 1 (Cdh1), an E3 ubiquitin ligase that is highly active in neurons. However, during excitotoxic damage-a hallmark of neurological disorders-APC/C-Cdh1 is inactivated, causing cyclin B1 stabilization and neuronal death through an unknown mechanism. Here, we show that an excitotoxic stimulus in rat cortical neurons in primary culture promotes cyclin B1 accumulation in the mitochondria, in which it binds to, and activates, cyclin-dependent kinase-1 (Cdk1). The cyclin B1-Cdk1 complex in the mitochondria phosphorylates the anti-apoptotic protein B-cell lymphoma extra-large (Bcl-xL), leading to its dissociation from the β subunit of F1Fo-ATP synthase. The subsequent inhibition of ATP synthase activity causes complex I oxidative damage, mitochondrial inner membrane depolarization, and apoptotic neuronal death. These results unveil a previously unrecognized role for mitochondrial cyclin B1 in the oxidative damage associated with neurological disorders.

Cdc14 Early Anaphase Release, FEAR, Is Limited to the Nucleus and Dispensable for Efficient Mitotic Exit

Cdc14 phosphatase is a key regulator of exit from mitosis, acting primarily through antagonism of cyclin-dependent kinase, and is also thought to be important for meiosis. Cdc14 is released from its sequestration site in the nucleolus in two stages, first by the non-essential Cdc Fourteen Early Anaphase Release (FEAR) pathway and later by the essential Mitotic Exit Network (MEN), which drives efficient export of Cdc14 to the cytoplasm. We find that Cdc14 is confined to the nucleus during early mitotic anaphase release, and during its meiosis I release. Proteins whose degradation is directed by Cdc14 as a requirement for mitotic exit (e.g. the B-type cyclin, Clb2), remain stable during mitotic FEAR, a result consistent with Cdc14 being restricted to the nucleus and not participating directly in mitotic exit. Cdc14 released by the FEAR pathway has been proposed to have a wide variety of activities, all of which are thought to promote passage through anaphase. Proposed functions of FEAR include stabilization of anaphase spindles, resolution of the rDNA to allow its segregation, and priming of the MEN so that mitotic exit can occur promptly and efficiently. We tested the model for FEAR functions using the FEAR-deficient mutation net1-6cdk. Our cytological observations indicate that, contrary to the current model, FEAR is fully dispensable for timely progression through a series of anaphase landmarks and mitotic exit, although it is required for timely rDNA segregation. The net1-6cdk mutation suppresses temperature-sensitive mutations in MEN genes, suggesting that rather than activating mitotic exit, FEAR either inhibits the MEN or has no direct effect upon it. One interpretation of this result is that FEAR delays MEN activation to ensure that rDNA segregation occurs before mitotic exit. Our findings clarify the distinction between FEAR and MEN-dependent Cdc14 activities and will help guide emerging quantitative models of this cell cycle transition.

The Ubp15 deubiquitinase promotes timely entry into S phase in Saccharomyces cerevisiae

The anaphase-promoting complex in partnership with its activator, Cdh1, is an E3 ubiquitin ligase responsible for targeting cell cycle proteins during G1 phase. In the budding yeast Saccharomyces cerevisiae, Cdh1 associates with the deubiquitinating enzyme Ubp15, but the significance of this interaction is unclear. To better understand the physiological role(s) of Ubp15, we examined cell cycle phenotypes of cells lacking Ubp15. We found that ubp15∆ cells exhibited delayed progression from G1 into S phase and increased sensitivity to the DNA synthesis inhibitor hydroxyurea. Both phenotypes of ubp15∆ cells were rescued by additional copies of the S-phase cyclin gene CLB5. Clb5 is an unstable protein targeted for proteasome-mediated degradation by several pathways. We found that during G1 phase, the APC(Cdh1)-mediated degradation of Clb5 was accelerated in ubp15∆ cells. Ubp15 interacted with Clb5 independent of Cdh1 and deubiquitinated Clb5 in a reconstituted system. Thus deubiquitination by Ubp15 counteracts APC activity toward cyclin Clb5 to allow Clb5 accumulation and a timely entry into S phase.

Insights into the cellular mechanism of the yeast ubiquitin ligase APC/C-Cdh1 from the analysis of in vivo degrons

The anaphase-promoting complex/cyclosome (APC/C) controls a variety of cellular processes through its ability to target numerous protein substrates for timely degradation. Substrate selection by this ubiquitin ligase depends on related activator proteins, Cdc20 and Cdh1, which bind and activate the APC/C at distinct cell cycle stages. Biochemical and structural studies revealed that Cdc20 and Cdh1 carry conserved receptor domains to recognize specific sequence motifs in substrates, such as D and KEN boxes. The mechanisms for ordered degradation of APC/C substrates, however, remain incompletely understood. Here we describe minimal degradation sequences (degrons) sufficient for rapid APC/C-Cdh1-specific in vivo degradation. The polo kinase Cdc5-derived degron contained an essential KEN motif, whereas a single RxxL-type D box was the relevant signal in the Cdc20-derived degradation domain, indicating that either motif may support specific recognition by Cdh1. In both degrons, the APC/C recognition motif was flanked by a nuclear localization sequence. Forced localization of the degron constructs revealed that proteolysis mediated by APC/C-Cdh1 is restricted to the nucleus and maximally active in the nucleoplasm. Levels of Iqg1, a cytoplasmic Cdh1 substrate, decreased detectably later than the nucleus-localized Cdh1 substrate Ase1, indicating that confinement to the nucleus may allow for temporal control of APC/C-Cdh1-mediated proteolysis.

Timely activation of budding yeast APCCdh1 involves degradation of its inhibitor, Acm1, by an unconventional proteolytic mechanism

Regulated proteolysis mediated by the ubiquitin proteasome system is a fundamental and essential feature of the eukaryotic cell division cycle. Most proteins with cell cycle-regulated stability are targeted for degradation by one of two related ubiquitin ligases, the Skp1-cullin-F box protein (SCF) complex or the anaphase-promoting complex (APC). Here we describe an unconventional cell cycle-regulated proteolytic mechanism that acts on the Acm1 protein, an inhibitor of the APC activator Cdh1 in budding yeast. Although Acm1 can be recognized as a substrate by the Cdc20-activated APC (APC^Cdc20) in anaphase, APC^Cdc20 is neither necessary nor sufficient for complete Acm1 degradation at the end of mitosis. An APC-independent, but 26S proteasome-dependent, mechanism is sufficient for complete Acm1 clearance from late mitotic and G1 cells. Surprisingly, this mechanism appears distinct from the canonical ubiquitin targeting pathway, exhibiting several features of ubiquitin-independent proteasomal degradation. For example, Acm1 degradation in G1 requires neither lysine residues in Acm1 nor assembly of polyubiquitin chains. Acm1 was stabilized though by conditional inactivation of the ubiquitin activating enzyme Uba1, implying some requirement for the ubiquitin pathway, either direct or indirect. We identified an amino terminal predicted disordered region in Acm1 that contributes to its proteolysis in G1. Although ubiquitin-independent proteasome substrates have been described, Acm1 appears unique in that its sensitivity to this mechanism is strictly cell cycle-regulated via cyclin-dependent kinase (Cdk) phosphorylation. As a result, Acm1 expression is limited to the cell cycle window in which Cdk is active. We provide evidence that failure to eliminate Acm1 impairs activation of APC^Cdh1 at mitotic exit, justifying its strict regulation by cell cycle-dependent transcription and proteolytic mechanisms. Importantly, our results reveal that strict cell-cycle expression profiles can be established independent of proteolysis mediated by the APC and SCF enzymes.

SCF(FBXW7α) modulates the intra-S-phase DNA-damage checkpoint by regulating Polo like kinase-1 stability

The intra-S-checkpoint is essential to control cell progression through S phase under normal conditions and in response to replication stress. When DNA lesions are detected, replication fork progression is blocked allowing time for repair to avoid genomic instability and the risk of cancer. DNA replication initiates at many origins of replication in eukaryotic cells, where a series of proteins form pre-replicative complexes (pre-RCs) that are activated to become pre-initiation complexes and ensure a single round of replication in each cell cycle. PLK1 plays an important role in the regulation of DNA replication, contributing to the regulation of pre-RCs formation by phosphorylating several proteins, under both normal and stress conditions. Here we report that PLK1 is ubiquitinated and degraded by SCFFBXW7α/proteasome. Moreover, we identified a new Cdc4 phosphodegron in PLK1, conserved from yeast to humans, whose mutation prevents PLK1 destruction. We established that endogenous SCFFBXW7α degrades PLK1 in the G1 and S phases of an unperturbed cell cycle and in S phase following UV irradiation. Furthermore, we showed that FBXW7α overexpression or UV irradiation prevented the loading of proteins onto chromatin to form pre-RCs and, accordingly, reduced cell proliferation. We conclude that PLK1 degradation mediated by SCFFBXW7α modulates the intra-S-phase checkpoint.

Phosphorylation adjacent to the nuclear localization signal of human dUTPase abolishes nuclear import: structural and mechanistic insights

Phosphorylation adjacent to nuclear localization signals (NLSs) is involved in the regulation of nucleocytoplasmic transport. The nuclear isoform of human dUTPase, an enzyme that is essential for genomic integrity, has been shown to be phosphorylated on a serine residue (Ser11) in the vicinity of its nuclear localization signal; however, the effect of this phosphorylation is not yet known. To investigate this issue, an integrated set of structural, molecular and cell biological methods were employed. It is shown that NLS-adjacent phosphorylation of dUTPase occurs during the M phase of the cell cycle. Comparison of the cellular distribution of wild-type dUTPase with those of hyperphosphorylation- and hypophosphorylation-mimicking mutants suggests that phosphorylation at Ser11 leads to the exclusion of dUTPase from the nucleus. Isothermal titration microcalorimetry and additional independent biophysical techniques show that the interaction between dUTPase and importin-α, the karyopherin molecule responsible for `classical' NLS binding, is weakened significantly in the case of the S11E hyperphosphorylation-mimicking mutant. The structures of the importin-α-wild-type and the importin-α-hyperphosphorylation-mimicking dUTPase NLS complexes provide structural insights into the molecular details of this regulation. The data indicate that the post-translational modification of dUTPase during the cell cycle may modulate the nuclear availability of this enzyme.

Dual personality of Mad1: regulation of nuclear import by a spindle assembly checkpoint protein

Nuclear transport is a dynamic process that can be modulated in response to changes in cellular physiology. We recently reported that the transport activity of yeast nuclear pore complexes (NPCs) is altered in response to kinetochore-microtubule (KT-MT) interaction defects. Specifically, KT detachment from MTs activates a signaling pathway that prevents the nuclear import of cargos by the nuclear transport factor Kap121p. This loss of Kap121p-mediated import is thought to influence the nuclear environment, including the phosphorylation state of nuclear proteins. A key regulator of this process is the spindle assembly checkpoint protein Mad1p. In response to unattached KTs, Mad1p dynamically cycles between NPCs and KTs. This cycling appears to induce NPC molecular rearrangements that prevent the nuclear import of Kap121p-cargo complexes. Here, we discuss the underlying mechanisms and the physiological relevance of Mad1p cycling and the inhibition of Kap121p-mediated nuclear import, focusing on outstanding questions within the pathway.

Nuclear PP2A-Cdc55 prevents APC-Cdc20 activation during the spindle assembly checkpoint

Cdc55, a regulatory B-subunit of protein phosphatase 2A (PP2A) complex, is essential for the spindle assembly checkpoint (SAC) in budding yeast, but the regulation and molecular targets of PP2A-Cdc55 have not been clearly defined or are controversial. Here, we show that an important target of Cdc55 in the SAC is the anaphase-promoting complex (APC) coupled with Cdc20 and that APC-Cdc20 is kept inactive by dephosphorylation by nuclear PP2A-Cdc55 when spindle is damaged. By isolating a new class of Cdc55 mutants specifically defective in the SAC and by artificially manipulating nucleocytoplasmic distribution of Cdc55, we further show that nuclear Cdc55 is essential for the SAC. Because the Cdc55-binding proteins Zds1 and Zds2 inhibit both nuclear accumulation of Cdc55 and SAC activity, we propose that spatial control of PP2A by Zds1 family proteins is important for tight control of SAC and mitotic progression.

Mutually dependent degradation of Ama1p and Cdc20p terminates APC/C ubiquitin ligase activity at the completion of meiotic development in yeast

Background

The execution of meiotic nuclear divisions in S. cerevisiae is regulated by protein degradation mediated by the anaphase promoting complex/cyclosome (APC/C) ubiquitin ligase. The correct timing of APC/C activity is essential for normal chromosome segregation. During meiosis, the APC/C is activated by the association of either Cdc20p or the meiosis-specific factor Ama1p. Both Ama1p and Cdc20p are targeted for degradation as cells exit meiosis II with Cdc20p being destroyed by APC/C^Ama1. In this study we investigated how Ama1p is down regulated at the completion of meiosis.

Findings

Here we show that Ama1p is a substrate of APC/C^Cdc20 but not APC/C^Cdh1 in meiotic cells. Cdc20p binds Ama1p in vivo and APC/C^Cdc20 ubiquitylates Ama1p in vitro. Ama1p ubiquitylation requires one of two degradation motifs, a D-box and a “KEN-box” like motif called GxEN. Finally, Ama1p degradation does not require its association with the APC/C via its conserved APC/C binding motifs (C-box and IR) and occurs simultaneously with APC/C^Ama1-mediated Cdc20p degradation.

Conclusions

Unlike the cyclical nature of mitotic cell division, meiosis is a linear pathway leading to the production of quiescent spores. This raises the question of how the APC/C is reset prior to spore germination. This and a previous study revealed that Cdc20p and Ama1p direct each others degradation via APC/C-dependent degradation. These findings suggest a model that the APC/C is inactivated by mutual degradation of the activators. In addition, these results support a model in which Ama1p and Cdc20p relocate to the substrate address within the APC/C cavity prior to degradation.

Mitosis-specific regulation of nuclear transport by the spindle assembly checkpoint protein Mad1p

Nuclear pore complexes (NPCs) and kinetochores perform distinct tasks, yet their shared ability to bind several proteins suggests their functions are intertwined. Among these shared proteins is Mad1p, a component of the yeast spindle assembly checkpoint (SAC). Here we describe a role for Mad1p in regulating nuclear import that employs its ability to sense a disruption of kinetochore-microtubule interactions during mitosis. We show that kinetochore-microtubule detachment arrests nuclear import mediated by the transport factor Kap121p through a mechanism that requires Mad1p cycling between unattached, metaphase kinetochores and binding sites at the NPC. This signaling pathway requires the Aurora B-like kinase Ipl1p, and the resulting transport changes inhibit the nuclear import of Glc7p, a phosphatase that acts as an Ipl1p antagonist. We propose that a distinct branch of the SAC exists in which Mad1p senses unattached kinetochores and, by altering NPC transport activity, regulates the nuclear environment of the spindle.

Molecular basis of the functional distinction between Cln1 and Cln2 cyclins

Cln1 and Cln2 are very similar but not identical cyclins. In this work, we tried to describe the molecular basis of the functional distinction between Cln1 and Cln2. We constructed chimeric cyclins containing different fragments of Cln1 and Cln2 and performed several functional analysis that make it possible to distinguish between Cln1 or Cln2. We identified that region between amino acids 225 and 299 of Cln2 is not only necessary but also sufficient to confer Cln2 specific functionality compared with Cln1. We also studied Cln1 and Cln2 subcellular localization identifying additional differences between them. Both cyclins are distributed between the nucleus and the cytoplasm, but Cln1 shows stronger nuclear accumulation. Nuclear import of both cyclins is mediated by the classical nuclear import pathway and by sequences in the N-terminal end of the proteins. For Cln2, but not for Cln1, a nuclear export mechanism mediated by karyopherin Msn5 has been identified. Strikingly, Cln2 export depends on a Msn5-dependent NES between amino acids 225 and 299. In fact, the introduction of this region confers to Cln1 an export mechanism dependent on Msn5; importantly, this causes the gain of Cln2-specific cytosolic functions and the impairment of nuclear function. In short, a region from Cln2 controlling an Msn5-dependent nuclear export mechanism confers a specific functionality to Cln2 compared with Cln1.

Regulation of APC/C-Cdh1 and its function in neuronal survival

Neurons are post-mitotic cells that undergo an active downregulation of cell cycle-related proteins to survive. The activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that regulates cell cycle progression in proliferating cells, plays a relevant role in post-mitotic neurons. Recent advances in the study of the regulation of APC/C have documented that the APC/C-activating cofactor, Cdh1, is essential for the function(s) of APC/C in neuronal survival. Here, we review the normal regulation of APC/C activity in proliferating cells and neurons. We conclude that in neurons the APC/C-Cdh1 complex actively downregulates the stability of the cell cycle protein cyclin B1 and the glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3. Keeping these proteins destabilized is critical both for preventing the aberrant reentry of post-mitotic neurons into the cell cycle and for maintaining their reduced antioxidant status. Further understanding of the pathophysiological regulation of these proteins by APC/C-Cdh1 in neurons will be important for the search for novel therapeutic targets against neurodegeneration.

Regulation of cell cycle transcription factor Swi5 by karyopherin Msn5

Inactivation of S. cerevisiae β-karyopherin Msn5 causes hypersensitivity to the overexpression of mitotic cyclin Clb2 and aggravates growth defects of many mutant strains in mitotic exit, suggesting a connection between Msn5 and mitotic exit. We determined that Msn5 controlled subcellular localization of the mitotic exit transcription factor Swi5, since it was required for Swi5 nuclear export. Msn5 physically interacted with the N-terminal end of Swi5. Inactivation of Msn5 caused a severe reduction in cellular levels of Swi5 protein. This effect occurred by a post-transcriptional mechanism, since SWI5 mRNA levels were not affected. The reduced amount of Swi5 in msn5 mutant cells was not due to an increased protein degradation rate, but to a defect in Swi5 synthesis. Despite the change in localization and protein level, Swi5-regulated transcription was not defective in the msn5 mutant strain. However, a high level of Swi5 was toxic in the absence of Msn5. This deleterious effect was eliminated when Swi5 nuclear import was abrogated, suggesting that nuclear export by Msn5 is important for cell physiology, because it prevents toxic Swi5 nuclear accumulation.

APC/C-Cdh1-dependent anaphase and telophase progression during mitotic slippage

Background

The spindle assembly checkpoint (SAC) inhibits anaphase progression in the presence of insufficient kinetochore-microtubule attachments, but cells can eventually override mitotic arrest by a process known as mitotic slippage or adaptation. This is a problem for cancer chemotherapy using microtubule poisons.

Results

Here we describe mitotic slippage in yeast bub2Δ mutant cells that are defective in the repression of precocious telophase onset (mitotic exit). Precocious activation of anaphase promoting complex/cyclosome (APC/C)-Cdh1 caused mitotic slippage in the presence of nocodazole, while the SAC was still active. APC/C-Cdh1, but not APC/C-Cdc20, triggered anaphase progression (securin degradation, separase-mediated cohesin cleavage, sister-chromatid separation and chromosome missegregation), in addition to telophase onset (mitotic exit), during mitotic slippage. This demonstrates that an inhibitory system not only of APC/C-Cdc20 but also of APC/C-Cdh1 is critical for accurate chromosome segregation in the presence of insufficient kinetochore-microtubule attachments.

Conclusions

The sequential activation of APC/C-Cdc20 to APC/C-Cdh1 during mitosis is central to accurate mitosis. Precocious activation of APC/C-Cdh1 in metaphase (pre-anaphase) causes mitotic slippage in SAC-activated cells. For the prevention of mitotic slippage, concomitant inhibition of APC/C-Cdh1 may be effective for tumor therapy with mitotic spindle poisons in humans.

Acm1 contributes to nuclear positioning by inhibiting Cdh1-substrate interactions

The anaphase-promoting complex (APC) is tightly regulated during cell division, often by pseudosubstrate binding to its coactivators Cdh1 and Cdc20. Budding yeast Acm1 is a Cdh1 pseudosubstrate inhibitor whose biological function is unknown. We show here that cells lacking Acm1 have defects in nuclear positioning and spindle morphology during mitosis. However, Cdh1 substrates are not destabilized in the absence of Acm1 and expression of inactive Cdh1 mutants that retain substrate binding is sufficient for the acm1 phenotype. We conclude that Acm1 is not required to inhibit APC(Cdh1) activity but rather prevents untimely Cdh1-substrate interactions. We further provide evidence suggesting that the substrate primarily responsible for the acm1 phenotype is the bud neck-localized kinase, Hsl1. Our results imply that at least some coactivator-substrate interactions require regulation. Several unrelated APC pseudosubstrates have been identified in diverse eukaryotes and their ability to simultaneously inhibit enzymatic activity and substrate binding may partly explain why this regulatory mechanism has been selected repeatedly during evolution.

Targeting the anaphase promoting complex: common pathways for viral infection and cancer therapy

INTRODUCTION

The anaphase promoting complex/cyclosome (APC/C) is a ubiquitin ligase involved in regulation of the cell cycle through ubiquitination-dependent substrate proteolysis. Many viral proteins have been shown to interact with the APC/C, derailing cell cycle progression in order to facilitate their own replication. Induction of G(2)/M arrest by viral APC/C inhibition can lead to apoptotic cell death. Some viral proteins cause cytotoxicity specifically in tumour cells, providing evidence that targeting the APC/C could be exploited to selectively eliminate cancer cells.

AREAS COVERED

In this review, we provide a summary of studies from viral APC/C interactions over the last decade, as well as recent discoveries identifying the APC/C as a promising target in the context of cancer therapy.

EXPERT OPINION

Current therapeutic strategies inducing mitotic arrest rely on activation of the spindle assembly checkpoint (SAC) for their function. Many cancer cells have a weakened SAC and escape apoptosis through mitotic slippage. Recent evidence has demonstrated that targeting the APC/C, particularly the co-activator Cdc20, might be a better alternative. Tumour cells display greater dependency on APC/C function than normal cells and oncogenic transformation can lead to increased mitotic stress, rendering cancer cells more vulnerable to APC/C inhibition.

The karyopherin Kap95 and the C-termini of Rfa1, Rfa2, and Rfa3 are necessary for efficient nuclear import of functional RPA complex proteins in Saccharomyces cerevisiae

Nuclear protein import in eukaryotic cells is mediated by karyopherin proteins, which bind to specific nuclear localization signals on substrate proteins and transport them across the nuclear envelope and into the nucleus. Replication protein A (RPA) is a nuclear protein comprised of three subunits (termed Rfa1, Rfa2, and Rfa3 in Saccharomyces cerevisiae) that binds single-stranded DNA and is essential for DNA replication, recombination, and repair. RPA associates with two different karyopherins in yeast, Kap95, and Msn5/Kap142. However, it is unclear which of these karyopherins is responsible for RPA nuclear import. We have generated GFP fusion proteins with each of the RPA subunits and demonstrate that these Rfa-GFP chimeras are functional in yeast cells. The intracellular localization of the RPA proteins in live cells is similar in wild-type and msn5Δ deletion strains but becomes primarily cytoplasmic in cells lacking functional Kap95. Truncating the C-terminus of any of the RPA subunits results in mislocalization of the proteins to the cytoplasm and a loss of protein-protein interactions between the subunits. Our data indicate that Kap95 is likely the primary karyopherin responsible for RPA nuclear import in yeast and that the C-terminal regions of Rfa1, Rfa2, and Rfa3 are essential for efficient nucleocytoplasmic transport of each RPA subunit.

Efficient nuclear transport of structurally disturbed cargo: mutations in a cargo protein switch its cognate karyopherin

The Karyopherin (Kap) family of nuclear transport receptors enables trafficking of proteins to and from the nucleus in a precise, regulated manner. Individual members function in overlapping pathways, while simultaneously being very specific for their main cargoes. The details of this apparent contradiction and rules governing pathway preference remain to be further elucidated. S. cerevisiae Lhp1 is an abundant protein that functions as an RNA chaperone in a variety of biologically important processes. It localizes almost exclusively to the nucleus and is imported by Kap108. We show that mutation of 3 of the 275 residues in Lhp1 alters its import pathway to a Kap121-dependent process. This mutant does not retain wild-type function and is bound by several chaperones. We propose that Kap121 also acts as a chaperone, one that can act as a genetic buffer by transporting mutated proteins to the nucleus.

Nuclear translocation of Skp2 facilitates its destruction in response to TGFβ signaling

Skp2, a F-box protein that determines the substrate specificity for SCF ubiquitin ligase, has recently been demonstrated to be degraded by Cdh1/APC in response to TGFβ signaling. The TGFβ-induced Skp2 proteolysis results in the stabilization of p27 that is necessary to facilitate TGFβ cytostatic effect. Previous observation from immunocytochemistry indicates that Cdh1 principally localizes in the nucleus while Skp2 mainly localizes in the cytosol, which leaves us a puzzle on how Skp2 is recognized and then ubiquitylated by Cdh1/APC in response to TGFβ stimulation. Here, we report that Skp2 is rapidly translocated from the cytosol to the nucleus upon the cellular stimulation with TGFβ. Using a combinatorial approach of immunocytochemistry, biochemical-fraction-coupled immunoprecipitation, mutagenesis as well as protein degradation assay, we have demonstrated that the TGFβ-induced Skp2 nucleus translocation is critical for TGFβ cytostatic effect that allows physical interaction between Cdh1 and Skp2 and in turn facilitates the Skp2 ubquitylation by Cdh1/APC. Disruption of nuclear localization motifs on Skp2 stabilizes Skp2 in the presence of TGF-β signaling, which attenuates TGFβ-induced p27 accumulation and antagonizes TGFβ-induced growth inhibition. Our finding reveals a cellular mechanism that facilitates Skp2 ubiquitylation by Cdh1/APC in response to TGFβ.

Ama1p-activated anaphase-promoting complex regulates the destruction of Cdc20p during meiosis II

During meiosis, the APC/C is activated by either Cdc20 or the meiosis-specific activator Ama1. Upon exit from meiosis II, APC/C^Ama1 mediates Cdc20 destruction using Db1 and GxEN degrons. The amino terminus of Ama1, which contains the Cdc20-binding domain, is sufficient for Cdc20 degradation but not spore formation.

The execution of meiotic divisions in Saccharomyces cerevisiae is regulated by anaphase-promoting complex/cyclosome (APC/C)–mediated protein degradation. During meiosis, the APC/C is activated by association with Cdc20p or the meiosis-specific activator Ama1p. We present evidence that, as cells exit from meiosis II, APC/C^Ama1 mediates Cdc20p destruction. APC/C^Ama1 recognizes two degrons on Cdc20p, the destruction box and destruction degron, with either domain being sufficient to mediate Cdc20p destruction. Cdc20p does not need to associate with the APC/C to bind Ama1p or be destroyed. Coimmunoprecipitation analyses showed that the diverged amino-terminal region of Ama1p recognizes both Cdc20p and Clb1p, a previously identified substrate of APC/C^Ama1. Domain swap experiments revealed that the C-terminal WD region of Cdh1p, when fused to the N-terminal region of Ama1p, could direct most of Ama1p functions, although at a reduced level. In addition, this fusion protein cannot complement the spore wall defect in ama1Δ strains, indicating that substrate specificity is also derived from the WD repeat domain. These findings provide a mechanism to temporally down-regulate APC/C^Cdc20 activity as the cells complete meiosis II and form spores.

APC/C-Cdh1: from cell cycle to cellular differentiation and genomic integrity

Anaphase-promoting complex/cyclosome (APC/C) is a multifunctional ubiquitin-protein ligase that targets various substrates for proteolysis inside and outside of the cell cycle. The activation of APC/C is dependent on two WD-40 domain proteins, Cdc20 and Cdh1. While APC/Cdc20 principally regulates mitotic progression, APC/Cdh1 shows a broad spectrum of substrates in and beyond cell cycle. In the past several years, numerous biochemical and mouse genetic studies have greatly attracted our attention to the emerging role of APC/Cdh1 in genomic integrity, cellular differentiation and human diseases. This review will aim to summarize the recently expanded understanding of APC/Cdh1 in regulating biological function and how its dysfunction may lead to diseases.

Requirements and reasons for effective inhibition of the anaphase promoting complex activator CDH1

Inhibitory phosphorylation of Cdh1 by CDK and Polo kinase has been proposed to inactivate APC-Cdh1. Through an exact gene replacement approach, we find CDK, but not Polo, phosphorylation of Cdh1 to be a critical regulatory mechanism. APC-Cdh1 inhibits multiple aspects of spindle morphogenesis, and its activity is modulated by endogenous ACM1.

Anaphase promoting complex (APC)-Cdh1 targets multiple mitotic proteins for degradation upon exit from mitosis into G1; inhibitory phosphorylation of Cdh1 by cyclin-dependent kinase (CDK) and Polo kinase has been proposed to prevent the premature degradation of substrates in the ensuing cell cycle. Here, we demonstrate essentiality of CDK phosphorylation of Cdh1 in Saccharomyces cerevisiae by exact endogenous gene replacement of CDH1 with CDK-unphosphorylatable CDH1-m11; in contrast, neither Cdh1 polo kinase sites nor polo interaction motifs are required. CDH1-m11 cells arrest in the first cycle with replicated DNA and sustained polarized growth; most cells have monopolar spindles. Blocking proteolysis of the Cin8 kinesin in CDH1-m11 cells does not promote spindle pole body (SPB) separation. In contrast, expression of undegradable mitotic cyclin results in both SPB separation and the restoration of isotropic growth. A minority of CDH1-m11 cells arrest with short bipolar spindles that fail to progress to anaphase; this can be accounted for by a failure to accumulate Cdc20 and consequent failure to cleave cohesin. Bipolar spindle assembly in CDH1-m11 cells is strikingly sensitive to gene dosage of the stoichiometric Cdh1 inhibitor ACM1. Thus, different spindle-regulatory pathways have distinct sensitivities to Cdh1, and ACM1 may buffer essential CDK phosphorylation of Cdh1.

E3 ubiquitin ligase APC/C-Cdh1 accounts for the Warburg effect by linking glycolysis to cell proliferation

Cell proliferation is known to be accompanied by activation of glycolysis. We have recently discovered that the glycolysis-promoting enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, isoform 3 (PFKFB3), is degraded by the E3 ubiquitin ligase APC/C-Cdh1, which also degrades cell-cycle proteins. We now show in two different cell types (neoplastic and nonneoplastic) that both proliferation and aerobic glycolysis are prevented by overexpression of Cdh1 and enhanced by its silencing. Furthermore, we have coexpressed Cdh1 with PFKFB3--either wild-type or a mutant form resistant to ubiquitylation by APC/C-Cdh1--or with the glycolytic enzyme 6-phosphofructo-1-kinase and demonstrated that whereas glycolysis is essential for cell proliferation, its initiation in the presence of active Cdh1 does not result in proliferation. Our experiments indicate that the proliferative response, regardless of whether it occurs in normal or neoplastic cells, is dependent on a decrease in the activity of APC/C-Cdh1, which activates both proliferation and glycolysis. These observations have implications for cell proliferation, neoplastic transformation, and the prevention and treatment of cancer.

The emerging role of APC/CCdh1 in controlling differentiation, genomic stability and tumor suppression

Deregulation of the G1/G0 phase of the cell cycle can lead to cancer. During G1, most cells commit alternatively to DNA replication and division, or to cell-cycle exit and differentiation. The anaphase-promoting complex or cyclosome (APC/C) activated by Cdh1 coordinately eliminates positive cell-cycle regulators as well as inhibitors of differentiation, thereby coupling cell-cycle exit and differentiation. Misregulation of Cdh1 thus has the potential to promote both cell-cycle re-entry and either perturbed differentiation or dedifferentiation. In addition, APC/C(Cdh1) is required to maintain genomic stability. As a result, loss of Cdh1 can contribute to tumorigenesis in the form of proliferation of poorly differentiated and genetically unstable cells.

The mitotic arrest deficient protein MAD2B interacts with the small GTPase RAN throughout the cell cycle

BACKGROUND

Previously, we identified the mitotic arrest deficient protein MAD2B (MAD2L2) as a bona fide interactor of the renal cell carcinoma (RCC)-associated protein PRCC. In addition, we found that fusion of PRCC with the transcription factor TFE3 in t(X;1)(p11;q21)-positive RCCs results in an impairment of this interaction and, concomitantly, an abrogation of cell cycle progression. Although MAD2B is thought to inhibit the anaphase promoting complex (APC) by binding to CDC20 and/or CDH1(FZR1), its exact role in cell cycle control still remains to be established.

METHODOLOGY/PRINCIPAL FINDINGS

Using a yeast two-hybrid interaction trap we identified the small GTPase RAN, a well-known cell cycle regulator, as a novel MAD2B binding protein. Endogenous interaction was established in mammalian cells via co-localization and co-immunoprecipitation of the respective proteins. The interaction domain of RAN could be assigned to a C-terminal moiety of 60 amino acids, whereas MAD2B had to be present in its full-length conformation. The MAD2B-RAN interaction was found to persist throughout the cell cycle. During mitosis, co-localization at the spindle was observed.

CONCLUSIONS/SIGNIFICANCE

The small GTPase RAN is a novel MAD2B binding protein. This novel protein-protein interaction may play a role in (i) the control over the spindle checkpoint during mitosis and (ii) the regulation of nucleocytoplasmic trafficking during interphase.

APC/C-CCS52A complexes control meristem maintenance in the Arabidopsis root

Plant organs originate from meristems where stem cells are maintained to produce continuously daughter cells that are the source of different cell types. The cell cycle switch gene CCS52A, a substrate specific activator of the anaphase promoting complex/cyclosome (APC/C), controls the mitotic arrest and the transition of mitotic cycles to endoreduplication (ER) cycles as part of cell differentiation. Arabidopsis, unlike other organisms, contains 2 CCS52A isoforms. Here, we show that both of them are active and regulate meristem maintenance in the root tip, although through different mechanisms. The CCS52A1 activity in the elongation zone of the root stimulates ER and mitotic exit, and contributes to the border delineation between dividing and expanding cells. In contrast, CCS52A2 acts directly in the distal region of the root meristem to control identity of the quiescent center (QC) cells and stem cell maintenance. Cell proliferation assays in roots suggest that this control involves CCS52A2 mediated repression of mitotic activity in the QC cells. The data indicate that the CCS52A genes favor a low mitotic state in different cell types of the root tip that is required for meristem maintenance, and reveal a previously undescribed mechanism for APC/C mediated control in plant development.