Inhibition of PI3K/Akt Pathway Impairs G2/M Transition of Cell Cycle in Late Developing Progenitors of the Avian Embryo Retina

PI3K/Akt is an important pathway implicated in the proliferation and survival of cells in the CNS. Here we investigated the participation of the PI3K/Akt signal pathway in cell cycle of developing retinal progenitors. Immunofluorescence assays performed in cultures of chick embryo retinal cells and intact tissues revealed the presence of phosphorylated Akt and 4E-BP1 in cells with typical mitotic profiles. Blockade of PI3K activity with the chemical inhibitor LY 294002 (LY) in retinal explants blocked the progression of proliferating cells through G2/M transition, indicated by an expressive increase in the number of cells labeled for phosphorylated histone H3 in the ventricular margin of the retina. No significant level of cell death could be detected at this region. Retinal explants treated with LY for 24 h also showed a significant decrease in the expression of phospho-Akt, phospho-GSK-3 and the hyperphosphorylated form of 4E-BP1. Although no change in the expression of cyclin B1 was detected, a significant decrease in CDK1 expression was noticed after 24 h of LY treatment both in retinal explants and monolayer cultures. Our results suggest that PI3K/Akt is an active pathway during proliferation of retinal progenitors and its activity appears to be required for proper CDK1 expression levels and mitosis progression of these cells.     

The enhancement of histone H4 and H2A serine 1 phosphorylation during mitosis and S-phase is evolutionarily conserved

Histone phosphorylation has long been associated with condensed mitotic chromatin; however, the functional roles of these modifications are not yet understood. Histones H1 and H3 are highly phosphorylated from late G2 through telophase in many organisms, and have been implicated in chromatin condensation and sister chromatid segregation. However, mutational analyses in yeast and biochemical experiments with Xenopus extracts have demonstrated that phosphorylation of H1 and H3 is not essential for such processes. In this study, we investigated additional histone phosphorylation events that may have redundant functions to H1 and H3 phosphorylation during mitosis. We developed an antibody to H4 and H2A that are phosphorylated at their respective serine 1 (S1) residues and found that H4S1/H2AS1 are highly phosphorylated in the mitotic chromatin of worm, fly, and mammals. Mitotic H4/H2A phosphorylation has similar timing and localization as H3 phosphorylation, and closely correlates with the chromatin condensation events during mitosis. We also detected a lower level of H4/H2A phosphorylation in 5-bromo-2-deoxyuridine-positive S-phase cells, which corroborates earlier studies that identified H4S1 phosphorylation on newly synthesized histones during S-phase. The evolutionarily conserved phosphorylation of H4/H2A during the cell cycle suggests that they may have a dual purpose in chromatin condensation during mitosis and histone deposition during S-phase.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00412-004-0281-9Communicated by G. Almouzni     

Phosphorylation-induced rearrangement of the histone H3 NH2-terminal domain during mitotic chromosome condensation

The NH2-terminal domain (N-tail) of histone H3 has been implicated in chromatin compaction and its phosphorylation at Ser10 is tightly correlated with mitotic chromosome condensation. We have developed one mAb that specifically recognizes histone H3 N-tails phosphorylated at Ser10 (H3P Ab) and another that recognizes phosphorylated and unphosphorylated H3 N-tails equally well (H3 Ab). Immunocytochemistry with the H3P Ab shows that Ser10 phosphorylation begins in early prophase, peaks before metaphase, and decreases during anaphase and telophase. Unexpectedly, the H3 Ab shows stronger immunofluorescence in mitosis than interphase, indicating that the H3 N-tail is more accessible in condensed mitotic chromatin than in decondensed interphase chromatin. In vivo ultraviolet laser cross-linking indicates that the H3 N-tail is bound to DNA in interphase cells and that binding is reduced in mitotic cells. Treatment of mitotic cells with the protein kinase inhibitor staurosporine causes histone H3 dephosphorylation and chromosome decondensation. It also decreases the accessibility of the H3 N-tail to H3 Ab and increases the binding of the N-tail to DNA. These results indicate that a phosphorylation-dependent weakening of the association between the H3 N-tail and DNA plays a role in mitotic chromosome condensation.     

Partial displacement of histone H1 from chromatin is required before it can be phosphorylated by mitotic H1 kinase in vitro.

The massive nonselective and reversible phosphorylation of histone H1 during mitosis is a universal phenomenon among eukaryotes. The growth-associated kinase responsible for this phosphorylation is identical to the maturation promoting factor, a key regulator of the cell cycle. Here we showed that growth-associated kinase, isolated from mitotic HeLa cells which were capable of phosphorylating HeLa H1 in vitro with high activity and mostly at the same sites phosphorylated during mitosis in vivo (assayed by two-dimensional analysis of tryptic phosphopeptides), did not significantly phosphorylate chromatin-bound or nuclear H1 in vitro. Its inability to phosphorylate chromatin-bound H1 did not change when the amount of kinase was increased or the incubation was prolonged. The resistance of chromatin-bound H1 to phosphorylation did not result from chromatin aggregation. Rapid phosphorylation of H1 in vitro, as well as in a nuclear system, was restored when NaCl concentrations were raised above 200 mM where H1:DNA interactions are weakened. At 300 mM NaCl, chromatin-bound H1 was phosphorylated in a subset of the sites observed for free H1 phosphorylated in vitro. These results suggest that active displacement of H1 from chromatin DNA may take place before H1 can be fully phosphorylated during mitosis.     

Phosphorylation of nuclear proteins

Many nuclear proteins are phosphorylated: they range from enzymes to several structural proteins such as histones, non-histone chromosomal proteins and the nuclear lamins. The pattern of phosphorylation varies through the cell cycle. Although histone H1 is phosphorylated during interphase its phosphorylation increases sharply during mitosis. Histone H3, chromosomal protein HMG 14 and lamins A, B and C all show reversible phosphorylation during mitosis. Several nuclear kinases have been characterized, including one that increases during mitosis and phosphorylates H1 in vitro. Factors have been demonstrated in maturing amphibian oocytes and mitotic mammalian cells that induce chromosome condensation and breakdown of the nuclear membrane. The possibility that they are autocatalytic protein kinases is considered. The location of histone phosphorylation sites within the nucleosome is consistent with a role for phosphorylation in modulating chromatin folding.     

Selective involvement of the PI3K/PKB/bad pathway in retinal cell death

The phosphoinositide-3-kinase (PI3K)/protein kinase B (PKB)/Bad signal transduction pathway is engaged in the control of apoptosis in many different cell types, particularly through phosphorylation of the Bcl-2 family protein Bad. We examined the involvement of this pathway in the control of programmed cell death in the retina of developing rats. PKB is constitutively phosphorylated in retinal tissue in vitro, whereas Bad was dephosphorylated both in Ser112 and Ser136. Cell death induced by either the PI3K inhibitor LY294002, or the general kinase inhibitor 2-aminopurine, were followed by PKB dephosphorylation, but PKB was not modulated during cell death induced by the protein synthesis inhibitor anisomycin. Treatment of retinal tissue cultures with forskolin, which increases intracellular levels of cAMP, partially blocked apoptosis induced by both anisomycin and 2-aminopurine, but not by LY294002, whereas forskolin invariably induced phosphorylation of Bad on both Ser112 and Ser136. The data suggest that Bad may be engaged in survival pathways in the immature retina, but pathways other than PI3K/PKB/Bad, and phosphorylation sites other than Ser112 and Ser136 in the Bad protein control cell survival in retinal tissue.     

JNK is Associated with Bcl-2 and PP1 in Mitochondria: Paclitaxel Induces Its Activation and Its Association with the Phosphorylated Form of Bcl-2

It has been shown that the activation of JNK after paclitaxel-inducedmicrotubule damage is parallel to Bcl-2 phosphorylation, cell cycle arrest in mitosis andapoptosis. Using subcellular fractionation and immunocytochemistry, we found herethat a pool of activated JNK is located in mitochondria of HeLa cells treated withpaclitaxel. Furthermore, whereas the JNK protein is present in a tripartite complex withthe anti-apoptotic Bcl-2 protein and the PP1 phosphatase in mitochondria isolated fromcontrol cells, the activated form of JNK was associated with the phosphorylated form ofBcl-2, but devoid of PP1, in mitochondria isolated from paclitaxel-treated cells.Moreover, using an original cell-free system, we evidenced a direct involvement of JNKas the kinase responsible for the phosphorylation of mitochondrial Bcl-2 in mitoticarrested cells. Indeed, cytosols prepared from mitotic arrested cells led to a dosedependentphosphorylation of mitochondrial Bcl-2. Bcl-2 phosphorylation was inhibitedby CEP 11004, a JNK pathway inhibitor and by immunodepletion of JNK. Takentogether, these data show that JNK activation provides a molecular linkage frommicrotubule damages to the mitochondrial apoptotic machinery and also point to apivotal role for the JNK/Bcl-2/PP1 complex in the control of apoptosis followingpaclitaxel treatment.     

Mitotic reorganization of the intermediate filament protein nestin involves phosphorylation by cdc2 kinase

The intermediate filament protein nestin is expressed during early stages of development in the central nervous system and in muscle tissues. Nestin expression is associated with morphologically dynamic cells, such as dividing and migrating cells. However, little is known about regulation of nestin during these cellular processes. We have characterized the phosphorylation-based regulation of nestin during different stages of the cell cycle in a neuronal progenitor cell line, ST15A. Confocal microscopy of nestin organization and (32)P in vivo labeling studies show that the mitotic reorganization of nestin is accompanied by elevated phosphorylation of nestin. The phosphorylation-induced alterations in nestin organization during mitosis in ST15A cells are associated with partial disassembly of nestin filaments. Comparative in vitro and in vivo phosphorylation studies identified cdc2 as the primary mitotic kinase and Thr(316) as a cdc2-specific phosphorylation site on nestin. We generated a phosphospecific nestin antibody recognizing the phosphorylated form of this site. By using this antibody we observed that nestin shows constitutive phosphorylation at Thr(316), which is increased during mitosis. This study shows that nestin is reorganized during mitosis and that cdc2-mediated phosphorylation is an important regulator of nestin organization and dynamics during mitosis.     

Negative regulation of condensin I by CK2-mediated phosphorylation

Condensin I, which plays an essential role in mitotic chromosome assembly and segregation in vivo, constrains positive supercoils into DNA in the presence of adenosine triphosphate in vitro. Condensin I is constitutively present in a phosphorylated form throughout the HeLa cell cycle, but the sites at which it is phosphorylated in interphase cells differ from those recognized by Cdc2 during mitosis. Immunodepletion, in vitro phosphorylation, and immunoblot analysis using a phospho-specific antibody suggested that the CK2 kinase is likely to be responsible for phosphorylation of condensin I during interphase. In contrast to the slight stimulatory effect of Cdc2-induced phosphorylation of condensin I on supercoiling, phosphorylation by CK2 reduced the supercoiling activity of condensin I. CK2-mediated phosphorylation of condensin I is spatially and temporally regulated in a manner different to that of Cdc2-mediated phosphorylation: CK2-dependent phosphorylation increases during interphase and decreases on chromosomes during mitosis. These findings are the first to demonstrate a negative regulatory mode for condensin I, a process that may influence chromatin structure during interphase and mitosis.     

Basal c-Jun N-terminal kinases promote mitotic progression through histone H3 phosphorylation

Phosphorylation of histone H3 at serine 10 (S10) is essential for the onset of mitosis. Here, we show that basal c-Jun N-terminal kinases (JNKs) are required for mitotic histone H3 S10 phosphorylation in human primary fibroblast IMR90 cells. Inhibition of JNKs by specific pharmacologic inhibitors, expression of dominant-negative JNK1 and 2 mutants, or RNAi of JNK1 and 2 prevented phosphorylation of histone H3 at S10 in vivo. The JNK-specific inhibitor SP600125 blocked mitotic entry, as shown by its ability to prevent CDK1 dephosphorylation and cyclin A degradation. Basal JNK phosphorylation increased at G2/M-phase, although total JNK protein levels remained unchanged. In addition, basal JNKs were localized in nuclei and centrosomes during this time, suggesting that the nuclear localization of JNKs during G2/M is tightly coupled with histone H3 phosphorylation. Basal JNKs were able to phosphorylate histone H3 in vitro and co-precipitation of histone H3 and JNKs was only detected at G2/M. Taken together, these data strongly suggest that basal JNKs play a key role in controlling histone H3 phosphorylation for mitotic entry at G2/M-phase.     

Immunolocalization of phospho-S6 kinases: a new way to detect mitosis in tissue sections and in cell culture

During a study on the mTor pathway in the rat kidney we observed a striking increase of the phosphorylation of the S6 kinase in mitosis. In cryostat sections of perfusion-fixed tissue mitotic cells appeared as bright spots in immunofluorescence using an antibody specific for the phosphorylation site Thr421/Ser424. They were easily spotted in overviews with the objective 4× and 10×. Immunofluorescence was weak during the interphase. During the prophase it increased in both the nucleus and the cytoplasm and it remained bright during the subsequent phases of mitosis. All mitotic cells which were found in tubules and in the interstitium of the kidney using a chromatin stain displayed the bright immunofluorescence for phospho-S6 kinase. The same phenomenon was observed in rat liver and in mouse kidney as well as in a human cell line. Provided a rapid fixation, mitotic cells could be identified with the immunoperoxidase technique in paraffin sections of immersion-fixed tissue. This is the first report of phosphorylation of S6 kinase during mitosis in vivo. Thus, immunohistochemistry with anti-phospho-S6 kinase (Thr421/Ser424) appears to provide a convenient way to detect mitotic cells at low magnification.     

Relationship between histone phosphorylation and premature chromosome condensation

The relationship between histone phosphorylation and chromosome condensation was investigated by determining changes in phosphorylation levels of histones H1 and H3 following fusion between mitotic and interphase cells and subsequent premature chromosome condensation. We detected significant increases in the levels of phosphorylation of H1 and H3 from interphase chromatin in which a majority of nuclei had undergone premature chromosome condensation. In addition, we noted significant decreases in the phosphate content of the highly phosphorylated mitotic H1 and H3, presumably resulting from phosphatase activity contributed by the interphase component of mitotic/interphase fused cells. These observations further strengthen the correlation between histone phosphorylation and the changes in chromosome condensation associated with the initiation of mitosis. This study also suggests that maintenance of the mitotic chromosomes in a highly condensed state does not require the continued presence of histones in a highly phosphorylated form.     

MAP kinases mediate UVB-induced phosphorylation of histone H3 at serine 28

Histone H3 phosphorylation is related closely to chromatin remodeling and chromosome condensation. H3 phosphorylation at serine 28 is coupled with mitotic chromosome condensation in diverse mammalian cell lines. However, the pathway that mediates phosphorylation of H3 at serine 28 is unknown. In the present study, ERK1, ERK2, or p38 kinase strongly phosphorylated H3 at serine 28 in vitro. JNK1 or JNK2 was able also to phosphorylate H3 at serine 28 in vitro but to a lesser degree. UVB irradiation markedly induced phosphorylation of H3 at serine 28 in JB6 Cl 41 cells. PD 98059, a MEK1 inhibitor, and SB 202190, a p38 kinase inhibitor, efficiently repressed UVB-induced H3 phosphorylation at serine 28. Expression of dominant negative mutant (DNM) ERK2 in JB6 Cl 41 cells totally blocked UVB-induced phosphorylation of H3 at serine 28. Additionally, DNM p38 kinase or DNM JNK1 partially blocked UVB-induced H3 phosphorylation at serine 28. Furthermore, UVB-induced H3 phosphorylation at serine 28 was inhibited in Jnk1(-/-) cells but not in Jnk2(-/-) cells. These results suggest that UVB-induced H3 phosphorylation at serine 28 may be mediated by mitogen-activated protein kinases.     

Phosphorylation of the lysine-rich histones throughout the cell cycle.

The phosphorylating of the lysine-rich histone at various stages in the cell cycle has been studied. In rapidly dividing cell populations the lysine-rich histone is phosphorylated rapidly after synthesis and more slowly once bound to the chromosome. The half-life of hydrolysis of such interphase phosphorylation in 5 hr except during mitosis when the phosphata hydrolysis increases almost three-fold. During mitosis there is extensive phosphorylation at sites different from those phosphorylated during interphase and a smaller measure of sites common to both mitotic and interphase cells. The sites of mitotic phosphorylation are most critically distinguished from those phosphorylated in interphase by the rapidly hydrolysis of M-phase phosphohistone when the cells divide and enter the G1 phase of the cell cycle.     

Mitosis-specific phosphorylation of histone H3 initiates primarily within pericentromeric heterochromatin during G2 and spreads in an ordered fashion coincident with mitotic chromosome condensation

We have generated and characterized a novel site-specific antibody highly specific for the phosphorylated form of the amino-terminus of histone H3 (Ser10). In this study, we used this antibody to examine in detail the relationship between H3 phosphorylation and mitotic chromosome condensation in mammalian cells. Our results extend previous biochemical studies by demonstrating that mitotic phosphorylation of H3 initiates nonrandomly in pericentromeric heterochromatin in late G2 interphase cells. Following initiation, H3 phosphorylation appears to spread throughout the condensing chromatin and is complete in most cell lines just prior to the formation of prophase chromosomes, in which a phosphorylated, but nonmitotic, chromosomal organization is observed. In general, there is a precise spatial and temporal correlation between H3 phosphorylation and initial stages of chromatin condensation. Dephosphorylation of H3 begins in anaphase and is complete immediately prior to detectable chromosome decondensation in telophase cells. We propose that the singular phosphorylation of the amino-terminus of histone H3 may be involved in facilitating two key functions during mitosis: (1) regulate protein-protein interactions to promote binding of trans-acting factors that “drive” chromatin condensation as cells enter M-phase and (2) coordinate chromatin decondensation associated with M-phase. Received: 4 September 1997; in revised form: 14 September 1997 /Accepted: 14 September 1997     

Serine phosphorylation regulates disabled-1 early isoform turnover independently of Reelin

The Reelin-Disabled 1 (Dab1) signaling pathway plays an important role in neuronal cell migration during brain development. Dab1, an intracellular adapter protein which is tyrosine phosphorylated upon Reelin stimulation, has been directly implicated in the transmission and termination of Reelin-mediated signaling. Two main forms of Dab1 have been identified in the developing chick retina, an early isoform (Dab1-E) expressed in progenitor cells and a late isoform (Dab1-L, a.k.a. Dab1) expressed in differentiated cells. Dab1-E is missing two Src family kinase (SFK) phosphorylation sites that are critical for Reelin-Dab1 signaling and is not tyrosine phosphorylated. We have recently demonstrated a role for Dab1-E in the maintenance of retinal progenitor cells. Here, we report that Dab1-E is phosphorylated at serine/threonine residues independent of Reelin. Cdk2, highly expressed in retinal progenitor cells, mediates Dab1-E phosphorylation at serine 475 which in turn promotes ubiquitination-triggered proteasome degradation of Dab1-E. Inhibition of protein phosphatase 1 and/or protein phosphatase 2A leads to increased Dab1-E instability. We propose that Dab1 turnover is regulated by both Reelin-independent serine/threonine phosphorylation and Reelin-dependent tyrosine phosphorylation.