Misexpression of the cyclin-dependent kinase inhibitor ICK1/KRP1 in single-celled Arabidopsis trichomes reduces endoreduplication and cell size and induces cell death

A positive correlation between cell size and DNA content has been recognized in many plant cell types. Conversely, misexpression of a dominant-negative cyclin-dependent kinase (CDK) or CDK inhibitor proteins (ICK/KRPs) in Arabidopsis and tobacco leaves has revealed that cell growth can be uncoupled from cell cycle progression and DNA content. However, cell growth also appears to be controlled in a non-cell-autonomous manner by organ size, making it difficult in a ubiquitous expression assay to judge the cell-autonomous function of putative cell growth regulators. Here, we investigated the function of the CDK inhibitor ICK1/KRP1 on cell growth and differentiation independent of any compensatory influence of an organ context using Arabidopsis trichomes as a model system. By analyzing cell size with respect to DNA content, we dissected cell growth in a DNA-dependent and a DNA-independent process. We further found that ICK1/KRP1 misexpression interfered with differentiation and induced cell death, linking cell cycle progression, differentiation, and cell death in plants. The function of ICK1/KRP1 in planta was found to be dependent on a C-terminal domain and regulated negatively by an N-terminal domain. Finally, we identified CDKA;1 and a D-type cyclin as possible targets of ICK1/KRP1 expression in vivo.     

Targeted degradation of the cyclin-dependent kinase inhibitor ICK4/KRP6 by RING-type E3 ligases is essential for mitotic cell cycle progression during Arabidopsis gametogenesis

Following meiosis, plant gametophytes develop through two or three rounds of mitosis. Although the ontogeny of gametophyte development has been defined in Arabidopsis thaliana, the molecular mechanisms regulating mitotic cell cycle progression are not well understood. Here, we report that RING-H2 group F 1a (RHF1a) and RHF2a, two RING-finger E3 ligases, play an important role in Arabidopsis gametogenesis. The rhf1a rhf2a double mutants are defective in the formation of male and female gametophytes due to interphase arrest of the mitotic cell cycle at the microspore stage of pollen development and at female gametophyte stage 1 of embryo sac development. We demonstrate that RHF1a directly interacts with and targets a cyclin-dependent kinase inhibitor ICK4/KRP6 (for Interactors of Cdc2 Kinase 4/Kip-related protein 6) for proteasome-mediated degradation. Inactivation of the two redundant RHF genes leads to the accumulation of ICK4/KRP6, and reduction of ICK4/KRP6 expression largely rescues the gametophytic defects in rhf1a rhf2a double mutants, indicating that ICK4/KRP6 is a substrate of the RHF E3 ligases. Interestingly, in situ hybridization showed that ICK4/KRP6 was predominantly expressed in sporophytes during meiosis. Our findings indicate that RHF1a/2a-mediated degradation of the meiosis-accumulated ICK4/KRP6 is essential to ensure the progression of subsequent mitoses to form gametophytes in Arabidopsis.     

Analysis of the subcellular localization, function, and proteolytic control of the Arabidopsis cyclin-dependent kinase inhibitor ICK1/KRP1

Recent studies have shown that cyclin-dependent kinase (CDK) inhibitors can have a tremendous impact on cell cycle progression in plants. In animals, CDK inhibitors are tightly regulated, especially by posttranslational mechanisms of which control of nuclear access and regulation of protein turnover are particularly important. Here we address the posttranslational regulation of INHIBITOR/INTERACTOR OF CDK 1 (ICK1)/KIP RELATED PROTEIN 1 (KRP1), an Arabidopsis (Arabidopsis thaliana) CDK inhibitor. We show that ICK1/KRP1 exerts its function in the nucleus and its presence in the nucleus is controlled by multiple nuclear localization signals as well as by nuclear export. In addition, we show that ICK1/KRP1 localizes to different subnuclear domains, i.e. in the nucleoplasm and to the chromocenters, hinting at specific actions within the nuclear compartment. Localization to the chromocenters is mediated by an N-terminal domain, in addition we find that this domain may be involved in cyclin binding. Further we demonstrate that ICK1/KRP1 is an unstable protein and degraded by the 26S proteasome in the nucleus. This degradation is mediated by at least two domains indicating the presence of at least two different pathways impinging on ICK1/KRP1 protein stability.     

The role of cyclin-dependent kinase inhibitor p27Kip1 in anti-HER2 antibody-induced G1 cell cycle arrest and tumor growth inhibition

Cyclin-dependent kinase (CDK) inhibitor p27Kip1 binds to the cyclin E.CDK2 complex and plays a major role in controlling cell cycle and cell growth. Our group and others have reported that anti-HER2 monoclonal antibodies exert inhibitory effects on HER2-overexpressing breast cancers through G1 cell cycle arrest associated with induction of p27Kip1 and reduction of CDK2. The role of p27Kip1 in anti-HER2 antibody-induced cell cycle arrest and growth inhibition is, however, still uncertain. Here we have provided several lines of evidence supporting a critical role for p27Kip1 in the anti-HER2 antibody-induced G1 cell cycle arrest and tumor growth inhibition. Induction of p27Kip1 and G1 growth arrest by anti-HER2 antibody, murine 4D5, or humanized trastuzumab (Herceptin) are concentration-dependent, time-dependent, irreversible, and long-lasting. The magnitude of G1 cell cycle arrest induced by trastuzumab or 4D5 is well correlated with the level of p27Kip1 protein induced. Up-regulation of p27Kip1 and G1 growth arrest could no longer be removed with as little as 14 h of treatment with trastuzumab. Anti-HER2 antibody-induced p27Kip1 protein, G1 arrest, and growth inhibition persist at least 5 days after a single treatment. The magnitude of growth inhibition of breast cancer cells induced by anti-HER2 antibody closely parallels the level of p27Kip1 induced. Induced expression of exogenous p27Kip1 results in a p27Kip1 level-dependent G1 cell cycle arrest and growth inhibition similar to that obtained with anti-HER2 antibodies. Reducing p27Kip1 expression using p27Kip1 small interfering RNA blocks anti-HER2 antibody-induced p27Kip1 up-regulation and G1 arrest. Treatment with anti-HER2 antibody significantly increases the half-life of p27Kip1 protein. Inhibition of ubiquitin-proteasome pathway, but not inhibition of calpain and caspase activities, up-regulates p27Kip1 protein to a degree comparable with that obtained with anti-HER2 antibodies. We have further demonstrated that anti-HER2 antibody significantly decreases threonine phosphorylation of p27Kip1 protein at position 187 (Thr-187) and increases serine phosphorylation of p27Kip1 protein at position 10 (Ser-10). Expression of S10A and T187A mutant p27Kip1 protein increases the fraction of cells in G1 and reduces a further antibody-induced G1 arrest. Consequently, p27Kip1 plays an important role in the anti-HER2 antibody-induced G1 cell cycle arrest and tumor growth inhibition through post-translational regulation. Regulation of the phosphorylation of p27Kip1 protein is one of the post-translational mechanisms by which anti-HER2 antibody upregulates the protein.     

The Arabidopsis D-type cyclin CYCD2;1 and the inhibitor ICK2/KRP2 modulate auxin-induced lateral root formation

The integration of cell division in root growth and development requires mediation of developmental and physiological signals through regulation of cyclin-dependent kinase activity. Cells within the pericycle form de novo lateral root meristems, and D-type cyclins (CYCD), as regulators of the G₁-to-S phase cell cycle transition, are anticipated to play a role. Here, we show that the D-type cyclin protein CYCD2;1 is nuclear in Arabidopsis thaliana root cells, with the highest concentration in apical and lateral meristems. Loss of CYCD2;1 has a marginal effect on unstimulated lateral root density, but CYCD2;1 is rate-limiting for the response to low levels of exogenous auxin. However, while CYCD2;1 expression requires sucrose, it does not respond to auxin. The protein Inhibitor-Interactor of CDK/Kip Related Protein2 (ICK2/KRP2), which interacts with CYCD2;1, inhibits lateral root formation, and ick2/krp2 mutants show increased lateral root density. ICK2/KRP2 can modulate the nuclear levels of CYCD2;1, and since auxin reduces ICK2/KRP2 protein levels, it affects both activity and cellular distribution of CYCD2;1. Hence, as ICK2/KRP2 levels decrease, the increase in lateral root density depends on CYCD2;1, irrespective of ICK2/CYCD2;1 nuclear localization. We propose that ICK2/KRP2 restrains root ramification by maintaining CYCD2;1 inactive and that this modulates pericycle responses to auxin fluctuations.     

Arabidopsis CDKA;1, a cdc2 homologue, controls proliferation of generative cells in male gametogenesis

The protein kinase cdc2 is conserved throughout eukaryotes and acts as a key regulator of the cell cycle. In plants, A-type cyclin-dependent kinase (CDKA), a homologue of cdc2, has a role throughout the cell cycle. Here we show that a loss-of-function mutation in CDKA;1, encoding the only Arabidopsis CDKA, results in lethality of the male gametophyte. Heterozygous plants produced mature siliques containing about 50% aborted seeds, and segregation distortion was observed in paternal inheritance. Microspores normally undergo an asymmetric cell division, pollen mitosis I (PMI), to produce bicellular pollen grains. The larger vegetative cell does not divide, but the smaller generative cell undergoes mitosis, PMII, to form the two sperm cells, thereby generating tricellular pollen grains. The cdka-1 mutant, however, produces mature bicellular pollen grains, consisting of a single sperm-like cell and a vegetative cell, due to failure of PMII. The mutant sperm-like cell is fertile, and preferentially fuses with the egg cell to initiate embryogenesis. As the central cell nucleus remains unfertilized, however, double fertilization does not occur. In heterozygous plants, the embryo is arrested at the globular stage, most likely because of loss of endosperm development, whereas it is arrested at the one- or two-cell stage in presumptive homozygous plants. Thus, CDKA;1 is essential for cell division of the generative cell in male gametogenesis.     

The BLADE-ON-PETIOLE 1 gene controls leaf pattern formation through the modulation of meristematic activity in Arabidopsis

The plant leaf provides an ideal system to study the mechanisms of organ formation and morphogenesis. The key factors that control leaf morphogenesis include the timing, location and extent of meristematic activity during cell division and differentiation. We identified an Arabidopsis mutant in which the regulation of meristematic activities in leaves was aberrant. The recessive mutant allele blade-on-petiole1-1 (bop1-1) produced ectopic, lobed blades along the adaxial side of petioles of the cotyledon and rosette leaves. The ectopic organ, which has some of the characteristics of rosette leaf blades with formation of trichomes in a dorsoventrally dependent manner, was generated by prolonged and clustered cell division in the mutant petioles. Ectopic, lobed blades were also formed on the proximal part of cauline leaves that lacked a petiole. Thus, BOP1 regulates the meristematic activity of leaf cells in a proximodistally dependent manner. Manifestation of the phenotypes in the mutant leaves was dependent on the leaf position. Thus, BOP1 controls leaf morphogenesis through control of the ectopic meristematic activity but within the context of the leaf proximodistality, dorsoventrality and heteroblasty. BOP1 appears to regulate meristematic activity in organs other than leaves, since the mutation also causes some ectopic outgrowths on stem surfaces and at the base of floral organs. Three class I knox genes, i.e., KNAT1, KNAT2 and KNAT6, were expressed aberrantly in the leaves of the bop1-1 mutant. Furthermore, the bop1-1 mutation showed some synergistic effect in double mutants with as1-1 or as2-2 mutation that is known to be defective in the regulation of meristematic activity and class I knox gene expression in leaves. The bop1-1 mutation also showed a synergistic effect with the stm-1 mutation, a strong mutant allele of a class I knox gene, STM. We, thus, suggest that BOP1 promotes or maintains a developmentally determinate state in leaf cells through the regulation of class I knox genes.     

Defects in leaf epidermis of Arabidopsis thaliana plants with CDKA;1 activity reduced in the shoot apical meristem

In Arabidopsis thaliana, like in other dicots, the shoot epidermis originates from protodermis, the outermost cell layer of shoot apical meristem. We examined leaf epidermis in transgenic A. thaliana plants in which CDKA;1.N146, a negative dominant allele of A-type cyclin-dependent kinase, was expressed from the SHOOTMERISTEMLESS promoter, i.e., in the shoot apical meristem. Using cleared whole mount preparations of expanding leaves and sequential in vivo replicas of expanding leaf surface, we show that dominant-negative CDKA;1 expression results in defects in epidermis continuity: loss of individual cells and occurrence of gaps between anticlinal walls of neighboring pavement cells. Another striking feature is ingrowth-like invaginations of anticlinal cell walls of pavement cells. Their formation is related to various processes: expansion of cells surrounding the sites of cell loss, defected cytokinesis, and presumably also, the actual ingrowth of an anticlinal cell wall. The mutant exhibits also increased variation in cell size and locally reduced waviness of anticlinal walls of pavement cells. These unusual features of leaf epidermis phenotype may shed a new light on our knowledge on morphogenesis of jigsaw puzzle-shaped pavement cells and on the CDKA;1 role in regulation of plant development via influence on cytoskeleton and plant cell wall.     

Intracellular localization of the cyclin-dependent kinase inhibitor p21<Superscript>CDKN1A</Superscript>-GFP fusion protein during cell cycle arrest

The cyclin-dependent kinase (CDK) inhibitor p21^CDKN1A is known to induce cell cycle arrest by inhibiting CDK activity and by interfering with DNA replication through binding to proliferating cell nuclear antigen. Although the molecular mechanisms have been elucidated, the temporal dynamics, as well as the intracellular sites of the activity of p21 bound to cyclin/CDK complexes during cell cycle arrest, have not been fully investigated. In this study we have induced the expression of p21^CDKN1A fused to green fluorescent protein (GFP) in HeLa cells, in order to visualize the intracellular localization of the inhibitor during the cell cycle arrest. We show that p21-GFP is preferentially expressed in association with cyclin E in cells arrested in G1 phase, and with cyclin A more than with cyclin B1 in cells arrested in the G2/M compartment. In addition, we show for the first time that p21-GFP colocalizes with cyclin E in the nucleolus of HeLa cells during the G1 phase arrest.O. Cazzalini and P. Perucca contributed equally to this work     

Expression of CKS1At in Arabidopsis thaliana indicates a role for the protein in both the mitotic and the endoreduplication cycle

Although endoreduplication is common in plants, little is known about the mechanisms regulating this process. Here, we report the patterns of endoreduplication at the cellular level in the shoot apex of Arabidopsis thaliana L. Heynh. plants grown under short-day conditions. We show that polyploidy is developmentally established in the pith, maturing leaves, and stipules. To investigate the role of the cell cycle genes CDC2aAt, CDC2bAt, CYCB1;1, and CKS1At in the process of endoreduplication, in-situ hybridizations were performed on the vegetative shoot apices. Expression of CDC2aAt, CDC2bAt, and CYCB1;1 was restricted to mitotically dividing cells. In contrast, CKS1At expression was present in both mitotic and endoreduplicating tissues. Our data indicate that CDC2aAt, CDC2bAt, and CYCB1;1 only operate during mitotic divisions, whereas CKS1At may play a role in both the mitotic and endoreduplication cycle. Received: 11 May 1998 / Accepted: 29 September 1998     

Characterization of microstructure and cell wall components of Arabidopsis thaliana overexpressing cyclin-dependent kinase inhibitor 2

Overexpression of a cyclin-dependent kinase inhibitor (KRP2) caused changes in the general morphology in the leaves of Arabidopsis thaliana. The wild type plant had obovate leaves with entire margins whereas the transgenic line had leaves with denticulate margins. The epidermal cells and stomata of the adult transgenic leaves were significantly larger than those of the wild-type plants and the number of stomata was in proportion to the number of epidermal cells. No apparent differences in thickness and structure of cell walls of the mesophyll cells between the two samples were observed. The smaller amount of cell wall material in the transgenic leaves caused by the larger cell size was also apparent in the lower dry weight of the transgenic leaves. The chemical analysis revealed the main differences to be in pectin and neutral sugar contents, and especially in the amounts of glucose, all being higher in the leaves of the KRP2 transgenic plants. p-Coumaric acid content varied more in the transgenic leaf material than in the control one reflecting possibly fewer cross-links in the cell walls of transgenic plants.     

Cip/Kip cyclin-dependent kinase inhibitors: brakes of the cell cycle engine during development

Precise control of cell-cycle progression is believed to be critical for normal development, while oncogenesis may be a direct result of its disturbance. Cell-cycle progression is regulated predominantly by a series of serine/threonine kinases, the cyclin-dependent kinases (CDKs). The activities of the CDKs are controlled by a variety of mechanisms, and a group of molecules that inhibit CDK activity, CDK inhibitors (CKIs), has recently become the focus of interest, particularly in the fields of development and tumorigenesis. To date, seven CKIs have been identified in mammals and categorized into two families, the Cip/Kip and Ink4 families. The Cip/Kip family is well conserved phylogenetically, suggesting that it is biologically important. Despite the structural and biochemical similarities among the Cip/Kip members, the phenotypes of knockout mice of each Cip/Kip member are surprisingly different, which suggests that the Cip/Kip CKIs have a variety of physiological functions. In this review, the biological roles of Cip/Kip CKIs in development and tumor suppression are discussed.