The regulation of mitotic nuclear envelope breakdown: a role for multiple lamin kinases

The LEM-ESCRT toolkit: Repair and maintenance of the nucleus

The eukaryotic genome is enclosed in a nuclear envelope that protects it from potentially damaging cellular activities and physically segregates transcription and translation.Transport across the NE is highly regulated and occurs primarily via the macromolecular nuclear pore complexes.Loss of nuclear compartmentalization due to defects in NPC function and NE integrity are tied to neurological and ageing disorders like Alzheimer’s, viral pathogenesis, immune disorders, and cancer progression.Recent work implicates inner-nuclear membrane proteins of the conserved LEM domain family and the ESCRT machinery in NE reformation during cell division and NE repair upon rupture in migrating cancer cells, and generating seals over defective NPCs. In this review, we discuss the recent in-roads made into defining the molecular mechanisms and biochemical networks engaged by LEM and many other integral inner nuclear membrane proteins to preserve the nuclear barrier.

Asymmetric nuclear division in neural stem cells generates sibling nuclei that differ in size, envelope composition, and chromatin organization

Although nuclei are the defining features of eukaryotes, we still do not fully understand how the nuclear compartment is duplicated and partitioned during division. This is especially the case for organisms that do not completely disassemble their nuclear envelope upon entry into mitosis. In studying this process in Drosophila neural stem cells, which undergo asymmetric divisions, we find that the nuclear compartment boundary persists during mitosis thanks to the maintenance of a supporting nuclear lamina. This mitotic nuclear envelope is then asymmetrically remodeled and partitioned to give rise to two daughter nuclei that differ in envelope composition and exhibit a >30-fold difference in volume. The striking difference in nuclear size was found to depend on two consecutive processes: asymmetric nuclear envelope resealing at mitotic exit at sites defined by the central spindle, and differential nuclear growth that appears to depend on the available local reservoir of ER/nuclear membranes, which is asymmetrically partitioned between the two daughter cells. Importantly, these asymmetries in size and composition of the daughter nuclei, and the associated asymmetries in chromatin organization, all become apparent long before the cortical release and the nuclear import of cell fates determinants. Thus, asymmetric nuclear remodeling during stem cell divisions may contribute to the generation of cellular diversity by initiating distinct transcriptional programs in sibling nuclei that contribute to later changes in daughter cell identity and fate.

Nuclear Egress

During viral replication, herpesviruses utilize a unique strategy, termed nuclear egress, to translocate capsids from the nucleus into the cytoplasm. This initial budding step transfers a newly formed capsid from within the nucleus, too large to fit through nuclear pores, through the inner nuclear membrane to the perinuclear space. The perinuclear enveloped virion must then fuse with the outer nuclear membrane to be released into the cytoplasm for further maturation, undergoing budding once again at the trans-Golgi network or early endosomes, and ultimately exit the cell non-lytically to spread infection. This first budding process is mediated by two conserved viral proteins, UL31 and UL34, that form a heterodimer called the nuclear egress complex (NEC). This review focuses on what we know about how the NEC mediates capsid transport to the perinuclear space, including steps prior to and after this budding event. Additionally, we discuss the involvement of other viral proteins in this process and how NEC-mediated budding may be regulated during infection.

The Nucleoskeleton

SUMMARYThe nucleoskeleton is an important structural feature of the metazoan nucleus and is involved in the regulation of genome expression and maintenance. The nuclear lamins are intermediate filament proteins that form a peripheral nucleoskeleton in concert with other lamin-associated proteins. Several other proteins normally found in the cytoskeleton have also been identified in the nucleus, but, as will be discussed here, their roles in forming a nucleoskeleton have not been elucidated. Nevertheless, mutations in lamins and lamin-associated proteins cause a spectrum of diseases, making them interesting targets for future research.

Nuclear Exodus: Herpesviruses Lead the Way

Most DNA viruses replicate in the nucleus and exit it either by passing through the nuclear pores or by rupturing the nuclear envelope. Unusually, herpesviruses have evolved a complex mechanism of nuclear escape whereby nascent capsids bud at the inner nuclear membrane to form perinuclear virions that subsequently fuse with the outer nuclear membrane, releasing capsids into the cytosol. Although this general scheme is accepted in the field, the players and their roles are still debated. Recent studies illuminated critical mechanistic features of this enigmatic process and uncovered surprising parallels with a novel cellular nuclear export process. This review summarizes our current understanding of nuclear egress in herpesviruses, examines the experimental evidence and models, and outlines outstanding questions with the goal of stimulating new research in this area.

Calcium, a Cell Cycle Commander, Drives Colon Cancer Cell Diffpoptosis

The story of the cell commonder, calcium, reaches into all corners of the cell and controls cell proliferation, differentiation, function, and even death. The calcium-driven eukaryotic revolution is one of the great turning points in the life history, happened about two billion years later when it was converted from a dangerous killer that had to be kept out of cell into the cell master which drives the cell. This review article will take the readers to a tour of tissues chosen to best show the calcium's many faces (proliferator, differentiator, and killer). The reader will first see calcium and its many helpers, such as the calcium-binding signaler protein calmodulin, directing the key events of the cell cycle. Then the tour will move onto the colon to show calcium driving the proliferation of progenitor cells, then the differentiation and ultimately the programmed death of their progeny. Moreover, the reader will learn of the striking disabling and bypassing of calcium-dependent control mechanisms during carcinogenesis. Finally, recommendations should be taken from the underlying mechanisms through which calcium masters the presistance, progression, and even apoptosis of colorectal cancer cells. Thus, this could be of great interest for designing of chemoprevention protocols.

Sumoylated protein tyrosine phosphatase 1B localizes to the inner nuclear membrane and regulates the tyrosine phosphorylation of emerin

Protein tyrosine phosphatase (PTP)1B is an abundant non-transmembrane enzyme that plays a major role in regulating insulin and leptin signaling. Recently, we reported that PTP1B is inhibited by sumoylation, and that sumoylated PTP1B accumulates in a perinuclear distribution, consistent with its known localization in the endoplasmic reticulum (ER) and the contiguous outer nuclear membrane. Here, we report that, in addition to its localization at the ER, PTP1B also is found at the inner nuclear membrane, where it is heavily sumoylated. We also find that PTP1B interacts with emerin, an inner nuclear membrane protein that is known to be tyrosine phosphorylated, and that PTP1B expression levels are inversely correlated with tyrosine phosphorylation levels of emerin. PTP1B sumoylation greatly increases as cells approach mitosis, corresponding to the stage where tyrosine phosphorylation of emerin is maximal. In addition, expression of a non-sumoylatable mutant of PTP1B greatly reduced levels of emerin tyrosine phosphorylation. These results suggest that PTP1B regulates the tyrosine phosphorylation of a key inner nuclear membrane protein in a sumoylation- and cell-cycle-dependent manner.

Breach of the nuclear lamina during assembly of herpes simplex viruses

Beneath the inner nuclear membrane lies the dense meshwork of the nuclear lamina, which provides structural support for the nuclear envelope and serves as an important organizing center for a number of nuclear and cytoplasmic constituents and processes. Herpesviruses have a significant and wide-ranging impact on human health, and their capacity to replicate and cause disease includes events that occur in the host cell nucleus. Herpesviruses begin assembly of progeny virus in the nuclei of infected cells and their capsids must escape the confines of the nucleus by budding through the inner nuclear membrane (INM) to proceed with later stages of virion assembly and egress. Access of viral capsids to the INM thus necessitates disruption of the dense nuclear lamina layer. We review herpesvirus effects on the nuclear lamina and in particular the roles of the herpes simplex virus-encoded nuclear envelope complex and viral kinases on lamin phosphorylation, dissociation, and nucleocapsid envelopment at the INM.

Altered chromosomal positioning, compaction, and gene expression with a lamin A/C gene mutation

BACKGROUND

Lamins A and C, encoded by the LMNA gene, are filamentous proteins that form the core scaffold of the nuclear lamina. Dominant LMNA gene mutations cause multiple human diseases including cardiac and skeletal myopathies. The nuclear lamina is thought to regulate gene expression by its direct interaction with chromatin. LMNA gene mutations may mediate disease by disrupting normal gene expression.

METHODS/FINDINGS

To investigate the hypothesis that mutant lamin A/C changes the lamina's ability to interact with chromatin, we studied gene misexpression resulting from the cardiomyopathic LMNA E161K mutation and correlated this with changes in chromosome positioning. We identified clusters of misexpressed genes and examined the nuclear positioning of two such genomic clusters, each harboring genes relevant to striated muscle disease including LMO7 and MBNL2. Both gene clusters were found to be more centrally positioned in LMNA-mutant nuclei. Additionally, these loci were less compacted. In LMNA mutant heart and fibroblasts, we found that chromosome 13 had a disproportionately high fraction of misexpressed genes. Using three-dimensional fluorescence in situ hybridization we found that the entire territory of chromosome 13 was displaced towards the center of the nucleus in LMNA mutant fibroblasts. Additional cardiomyopathic LMNA gene mutations were also shown to have abnormal positioning of chromosome 13, although in the opposite direction.

CONCLUSIONS

These data support a model in which LMNA mutations perturb the intranuclear positioning and compaction of chromosomal domains and provide a mechanism by which gene expression may be altered.

Role of the endoplasmic reticulum chaperone BiP, SUN domain proteins, and dynein in altering nuclear morphology during human cytomegalovirus infection

The process of assembly and egress of human cytomegalovirus (HCMV) virions requires significant morphological alterations of the nuclear and cytoplasmic architecture. In the studies presented we show that the nuclear periphery is dramatically altered, especially near the cytoplasmic assembly compartment, where the nuclear lamina is specifically rearranged, the outer nuclear membrane is altered, and the nucleus becomes permeable to large molecules. In addition, the tethering of the inner and outer nuclear membranes is lost during infection due to a decrease in levels of the SUN domain proteins. We previously demonstrated that the endoplasmic reticulum protein BiP functions as a component of the assembly compartment and disruption of BiP causes the loss of assembly compartment integrity. In this study we show that the depletion of BiP, and the loss of assembly compartment integrity, results in the loss of virally induced lamina rearrangement and morphology of the nucleus that is characteristic of HCMV infection. BiP functions in lamina rearrangement through its ability to affect lamin phosphorylation. Depletion of BiP and disruption of the assembly compartment result in the loss of lamin phosphorylation. The dependency of lamin phosphorylation on BiP correlates with an interaction between BiP and UL50. Finally, we confirm previous data (S. V. Indran, M. E. Ballestas, and W. J. Britt, J. Virol. 84:3162-3177, 2010) suggesting an involvement of dynein in assembly compartment formation and extend this observation by showing that when dynein is inhibited, the nuclear morphology characteristic of an HCMV infection is lost. Our data suggest a highly integrated assembly-egress continuum.

Herpes simplex virus 2 UL13 protein kinase disrupts nuclear lamins

Herpesviruses must cross the inner nuclear membrane and underlying lamina to exit the nucleus. HSV-1 US3 and PKC can phosphorylate lamins and induce their dispersion but do not elicit all of the phosphorylated lamin species produced during infection. UL13 is a serine threonine protein kinase conserved among many herpesviruses. HSV-1 UL13 phosphorylates US3 and thereby controls UL31 and UL34 nuclear rim localization, indicating a role in nuclear egress. Here, we report that HSV-2 UL13 alone induced conformational changes in lamins A and C and redistributed lamin B1 from the nuclear rim to intranuclear granular structures. HSV-2 UL13 directly phosphorylated lamins A, C, and B1 in vitro, and the lamin A1 tail domain. HSV-2 infection recapitulated the lamin alterations seen upon expression of UL13 alone, and other alterations were also observed, indicating that additional viral and/or cellular proteins cooperate with UL13 to alter lamins during HSV-2 infection to allow nuclear egress.

Introducing intermediate filaments: from discovery to disease

It took more than 100 years before it was established that the proteins that form intermediate filaments (IFs) comprise a unified protein family, the members of which are ubiquitous in virtually all differentiated cells and present both in the cytoplasm and in the nucleus. However, during the past 2 decades, knowledge regarding the functions of these structures has been expanding rapidly. Many disease-related roles of IFs have been revealed. In some cases, the molecular mechanisms underlying these diseases reflect disturbances in the functions traditionally assigned to IFs, i.e., maintenance of structural and mechanical integrity of cells and tissues. However, many disease conditions seem to link to the nonmechanical functions of IFs, many of which have been defined only in the past few years.

Nuclear lamins: major factors in the structural organization and function of the nucleus and chromatin

Over the past few years it has become evident that the intermediate filament proteins, the types A and B nuclear lamins, not only provide a structural framework for the nucleus, but are also essential for many aspects of normal nuclear function. Insights into lamin-related functions have been derived from studies of the remarkably large number of disease-causing mutations in the human lamin A gene. This review provides an up-to-date overview of the functions of nuclear lamins, emphasizing their roles in epigenetics, chromatin organization, DNA replication, transcription, and DNA repair. In addition, we discuss recent evidence supporting the importance of lamins in viral infections.

Regulation of spontaneous meiosis resumption in mouse oocytes by various conventional PKC isozymes depends on cellular compartmentalization

In this study, we investigated the spatio-temporal distribution of conventional protein kinases C (cPKC) isoforms PKC-alpha, PKC-betaI, PKC-betaII and PKC-gamma in mouse oocytes. The cPKCs were present in the cytoplasm at the start of the process and migrated to the nucleus (or germinal vesicle) before germinal vesicle breakdown, except for PKC-gamma which remained cytoplasmic. In both compartments, the fully phosphorylated form corresponding to the 'mature' enzyme was revealed for PKC-alpha, PKC-betaI and PKC-betaII. Microinjection of specific antibodies against each isozyme in one or the other cell compartment at different times of the meiotic process, permitted us to observe the following: (1) When located in the cytoplasm at the beginning of the process, PKC-alpha is not implicated in germinal vesicle breakdown, PKC-betaI and PKC-gamma are involved in maintaining the meiotic arrest, and PKC-betaII plays a role in meiosis reinitiation. Furthermore, just before germinal vesicle breakdown, these cytoplasmic cPKCs were no longer implicated. (2) When located in the germinal vesicle, PKC-alpha, PKC-betaI and PKC-betaII are involved in meiosis reinitiation. Our data highlight not only the importance of the nuclear pathways in the cell cycle progression, but also their independence of the cytoplasmic ones. Further investigations are however necessary to discover the molecular targets of these cPKCs to better understand the links with the cell cycle progression.

Nuclear lipid signalling

During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide-phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.

Nuclear lipid signaling

Abundant evidence now supports the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope as part of a bilayer membrane, but are actually within the nucleus in the form of proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol (DAG) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet entirely clear. Generation of DAG within the nucleus is believed to recruit protein kinase C (PKC) to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P3 may mobilize Ca2+ from the space between the nuclear membranes and thus increase nucleoplasmic Ca2+. Less well understood are the increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus, (i) two routes of synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]; (ii) two sources of DAG, one from the PI-PLC pathway and the other probably from phosphatidylcholine; (iii) several isoforms of PKC translocating to nuclei; (iv) increases in activity of the PI-PLC pathway at two points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P3, which may have several functions, including a role in the transfer of mRNA out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and, in particular, 3-phosphorylated inositol lipids, which are now emerging as possible major players in nuclear signaling.

The Ran GTPase as a marker of chromosome position in spindle formation and nuclear envelope assembly

The small GTPase Ran is a key regulator of nucleocytoplasmic transport during interphase. The asymmetric distribution of the GTP-bound form of Ran across the nuclear envelope--that is, large quantities in the nucleus compared with small quantities in the cytoplasm--determines the directionality of many nuclear transport processes. Recent findings that Ran also functions in spindle formation and nuclear envelope assembly during mitosis suggest that Ran has a general role in chromatin-centred processes. Ran functions in these events as a signal for chromosome position.

Nuclear apoptotic changes: an overview

Apoptosis is a form of active cell death essential for morphogenesis, development, differentiation, and homeostasis of multicellular organisms. The activation of genetically controlled specific pathways that are highly conserved during evolution results in the characteristic morphological features of apoptosis that are mainly evident in the nucleus. These include chromatin condensation, nuclear shrinkage, and the formation of apoptotic bodies. The morphological changes are the result of molecular alterations, such as DNA and RNA cleavage, post-translational modifications of nuclear proteins, and proteolysis of several polypeptides residing in the nucleus. During the last five years our understanding of the process of apoptosis has dramatically increased. However, the mechanisms that lead to apoptotic changes in the nucleus have been only partially clarified. Here, we shall review the most recent findings that may explain why the nucleus displays these striking modifications. Moreover, we shall take into consideration the emerging evidence about apoptotic events as a trigger for the generation of autoantibodies to nuclear components.

Fate of the inner nuclear membrane protein lamin B receptor and nuclear lamins in herpes simplex virus type 1 infection

During herpesvirus egress, capsids bud through the inner nuclear membrane. Underlying this membrane is the nuclear lamina, a meshwork of intermediate filaments with which it is tightly associated. Details of alterations to the lamina and the inner nuclear membrane during infection and the mechanisms involved in capsid transport across these structures remain unclear. Here we describe the fate of key protein components of the nuclear envelope and lamina during herpes simplex virus type 1 (HSV-1) infection. We followed the distribution of the inner nuclear membrane protein lamin B receptor (LBR) and lamins A and B(2) tagged with green fluorescent protein (GFP) in live infected cells. Together with additional results from indirect immunofluorescence, our studies reveal major morphologic distortion of nuclear-rim LBR and lamins A/C, B(1), and B(2). By 8 h p.i., we also observed a significant redistribution of LBR-GFP to the endoplasmic reticulum, where it colocalized with a subpopulation of cytoplasmic glycoprotein B by immunofluorescence. In addition, analysis by fluorescence recovery after photobleaching reveals that LBR-GFP exhibited increased diffusional mobility within the nuclear membrane of infected cells. This is consistent with the disruption of interactions between LBR and the underlying lamina. In addition to studying stably expressed GFP-lamins by fluorescence microscopy, we studied endogenous A- and B-type lamins in infected cells by Western blotting. Both approaches reveal a loss of lamins associated with virus infection. These data indicate major disruption of the nuclear envelope and lamina of HSV-1-infected cells and are consistent with a virus-induced dismantling of the nuclear lamina, possibly in order to gain access to the inner nuclear membrane.

Presence and activation of nuclear phosphoinositide 3-kinase C2beta during compensatory liver growth

Highly purified liver nuclei incorporated radiolabeled phosphate into phosphatidylinositol 4-phosphate (PtdIns(4)P), PtdIns(4,5)P(2), and PtdIns(3,4,5)P(3). When nuclei were depleted of their membrane, no radiolabeling of PtdIns(3,4,5)P(3) could be detected showing that within the intranuclear region there are no class I phosphoinositide 3-kinases (PI3K)s. In membrane-depleted nuclei harvested 20 h after partial hepatectomy, the incorporation of radiolabel into PtdIns(3)P was observed together with an increase in immunoprecipitable PI3K-C2beta activity, which is sensitive to wortmannin (10 nm) and shows strong preference for PtdIns over PtdIns(4)P as a substrate. On Western blots PI3K-C2beta revealed a single immunoreactive band of 180 kDa, whereas 20 h after partial hepatectomy gel shift of 18 kDa was noticed, suggesting that observed activation of enzyme is achieved by proteolysis. When intact membrane-depleted nuclei were subjected to short term (20 min) exposure to micro-calpain, similar gel shift together with an increase in PI3K-C2beta activity was observed, when compared with the nuclei harvested 20 h after partial hepatectomy. Moreover, the above-mentioned gel shift and increase in PI3K-C2beta activity could be prevented by the calpain inhibitor calpeptin. The data presented in this report show that, in the membrane-depleted nuclei during the compensatory liver growth, there is an increase in PtdIns(3)P formation as a result of PI3K-C2beta activation, which may be a calpain-mediated event.

Nuclear lipid signaling

There is now abundant evidence for the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope, but are actually within the nucleus, presumably not in a bilayer membrane but instead forming proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol and inositol-1,4,5-trisphosphate [Ins(1,4,5)P(3)]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet clear. Generation of diacylglycerol within the nucleus is believed to recruit protein kinase C to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P(3) may mobilize Ca(2+) from the space between the nuclear membranes and thus increase nucleoplasmic Ca(2+). Less well understood are an increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus: (i) two separate routes of synthesis of phosphatidylinositol-4,5-bisphosphate; (ii) two different sources of diacylglycerol, one being from the PI-PLC pathway, and the other probably from phosphatidylcholine; (iii) several different isoforms of PKC translocating to the nuclei; (iv) increases in activity of the PI-PLC pathway at two different points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P(3), which may have several functions, including a role in the transfer of messenger RNA (mRNA) out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and 3-phosphorylated inositol lipids.

The polo-like kinase PLK-1 is required for nuclear envelope breakdown and the completion of meiosis in Caenorhabditis elegans

The Polo-like kinases are key regulatory molecules required during the cell cycle for the successful completion of mitosis. We have cloned a C. elegans homolog of the Drosophila melanogaster polo gene (designated plk-1 for C. elegans polo-like kinase-1) and present the subcellular localization of the PLK-1 protein during the meiotic and mitotic cell cycles in C. elegans oocytes and embryos, respectively. Disruption of PLK-1 expression by RNA-mediated interference (RNAi) disrupts normal oocyte and embryonic development. Inspection of oocytes revealed a defect in nuclear envelope breakdown (NEBD) before ovulation. This defect in NEBD was also observed in oocytes that were depleted of the cyclin-dependent kinase NCC-1 (C. elegans homolog of Cdc2). The plk-1 RNAi oocytes were fertilized; however the resulting embryos were unable to separate their meiotic chromosomes or form and extrude polar bodies. These defects led to embryonic arrest as single cells. genesis 26:26-41, 2000. Published 2000 Wiley-Liss, Inc.

Mitosis and checkpoints that control progression through mitosis in vertebrate somatic cells

During mitosis in vertebrates the sister kinetochores on each replicated chromosome interact with two separating arrays of astral microtubules to form a bipolar spindle that produces and/or directs the forces for chromosome motion. In order to ensure faithful chromosome segregation cells have evolved mechanisms that delay progress into and out of mitosis until certain events are completed. At least two of these mitotic "checkpoint controls" can be identified in vertebrates. The first prevents nuclear envelope breakdown, and thus spindle formation, when the integrity of some nuclear component(s) is compromised. The second prevents chromosome disjunction and exit from mitosis until all of the kinetochores are attached to the spindle.

Viruses and the cell cycle

Viruses depend on the host's machineries to replicate and express their genome. Actively replicating cells have large pools of deoxynucleotides and high levels of key enzyme activities that viruses exploit to their own needs. Some viruses have developed strategies for driving quiescent cells into the S phase of the cell cycle, e.g. adenovirus, others, such as parvovirus, wait until the host itself begins to replicate. Viruses may also force the host cell to stay in a favourable phase, e.g. Epstein-Barr virus, or, if necessary, they may inhibit apoptotic cell death, e.g. human cytomegalovirus. In this review, we focus on the different strategies that viruses use to create in infected cells an environment favourable to the accomplishment of the viral life cycle through acting on cell cycle regulators.