Michael A. Mancini

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

Abstract. 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.

Chaperone suppression of ataxin-1 aggregation and altered subcellular proteasome localization imply protein misfolding in SCA1

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder caused by expansion of a polyglutamine tract in ataxin-1. In affected neurons of SCA1 patients and transgenic mice, mutant ataxin-1 accumulates in a single, ubiquitin-positive nuclear inclusion. In this study, we show that these inclusions stain positively for the 20S proteasome and the molecular chaperone HDJ-2/HSDJ. Similarly, HeLa cells transfected with mutant ataxin-1 develop nuclear aggregates which colocalize with the 20S proteasome and endogenous HDJ-2/HSDJ. Overexpression of wild-type HDJ-2/HSDJ in HeLa cells decreases the frequency of ataxin-1 aggregation. These data suggest that protein misfolding is responsible for the nuclear aggregates seen in SCA1, and that overexpression of a DnaJ chaperone promotes the recognition of a misfolded polyglutamine repeat protein, allowing its refolding and/or ubiquitin-dependent degradation.

FRAP reveals that mobility of oestrogen receptor-α is ligand- and proteasome-dependent

Here we report the use of fluorescence recovery after photobleaching (FRAP) to examine the intranuclear dynamics of fluorescent oestrogen receptor-α (ER). After bleaching, unliganded ER exhibits high mobility (recovery t1/2 < 1 s). Agonist (oestradiol; E2) or partial antagonist (4-hydroxytamoxifen) slows ER recovery (t1/2 ∼5–6 s), whereas the pure antagonist (ICI 182,780) and, surprisingly, proteasome inhibitors each immobilize ER to the nuclear matrix. Dual FRAP experiments show that fluorescent ER and SRC-1 exhibit similar dynamics only in the presence of E2. In contrast to reports that several nuclear proteins show uniform dynamics, ER exhibits differential mobility depending upon several factors that are linked to its transcription function.

Changes in Androgen Receptor Nongenotropic Signaling Correlate with Transition of LNCaP Cells to Androgen Independence

A cure for prostate cancer (CaP) will be possible only after a complete understanding of the mechanisms causing this disease to progress from androgen dependence to androgen independence. To carry on a careful characterization of the phenotypes of CaP cell lines before and after acquisition of androgen independence, we used two human CaP LNCaP sublines: LNCaPnan, which is androgen dependent (AD), and LNCaP-HP, which is androgen independent (AI). In AD LNCaPnan cells, dihydrotestosterone (DHT) stimulated in an androgen receptor (AR)-dependent way a phosphorylation signaling pathway involving steroid receptor coactivator (Src)–mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase (MEK)-1/2–ERK-1/2–cAMP-response element binding-protein (CREB). Activation of this pathway was associated with increased [3H]thymidine incorporation and resistance to apoptosis. Use of dominant-negative forms of MEK-1/2 and CREB demonstrated in LNCaPnan cells that DHT induced [3H]thymidiine incorporation through a thus far unidentified molecule activated downstream of MEK-1/2, and antiapoptosis through phosphorylation of the transcription factor CREB. In contrast, in AI LNCaP-HP cells, the Src–MEK-1/2–ERK-1/2–CREB pathway was constitutively active. Because it was not further stimulated by addition of DHT, no increase of [3H]thymidine incorporation or apoptosis resistance was demonstrated in LNCaP-HP cells. Additional experiments showed that Src and the scaffold protein MNAR coimmunoprecipitated with AR, indicating a role for Src as an apical molecule in the Src–MEK-1/2–ERK-1/2–CREB pathway. Interestingly, differences between the two cell lines were that in LNCaP-HP cells presence of an AI phenotype and lack of response to DHT were associated with constitutive activation of the protein kinase Src and interaction among Src, AR, and MNAR. In contrast, in LNCaPnan cells, presence of an AD phenotype and ability to respond to DHT were associated with DHT-dependent activation of Src kinase activity and interaction among Src, AR, and MNAR. Intriguingly, in LNCaPnan cells, we found that transcription through the prototypical CREB-responsive promoter c-fos could be induced in a DHT-dependent way, and this action was inhibited by the AR antagonist Casodex and MEK-1 inhibitor PD98059. In contrast, transcription through the PSA P/E promoter, a prototypical AR-dependent promoter directly activated by agonist, was obliterated only by Casodex. Additional experiments with genital skin fibroblasts derived from patients with a variety of AR abnormalities indicated that nongenotropic AR signaling does not depend on an intact DNA-binding domain or on the ability of AR to translocate to the nucleus. The results suggest the following: (1) Constitutive activation of the Src–MEK-1/2–ERK-1/2–CREB pathway is associated with the AI phenotype observed in LNCaP-HP cells. (2) Activation of the Src–MEK-1/2–ERK-1/2–CREB pathway is DHT dependent in AD LNCaPnan cells. (3) DHT activation of this pathway is associated with induction of [3H]thymidine incorporation by a molecule activated downstream of MEK-1/2 and of antiapoptosis through activation of the transcription factor CREB in AD LNCaPnan cells. (4) AR regulates transcription either directly upon ligand binding and nuclear translocation or indirectly through kinase pathways leading to activation of downstream transcription factors. (5) Nuclear translocation and ability of the DNA-binding domain of AR to interact with DNA are not prerequisites for nongenotropic AR activity.

Colocalization of muscleblind with RNA foci is separable from mis-regulation of alternative splicing in myotonic dystrophy

Myotonic dystrophy type I (DM1), which is caused by a non-coding CTG-repeat expansion in the dystrophia myotonica-protein kinase (DMPK) gene, is an RNA-mediated disease. Expanded CUG repeats in transcripts of mutant DMPK form nuclear foci that recruit muscleblind-like (MBNL) proteins, a family of alternative splicing factors. Although transcripts of mutant DMPK and MBNL proteins accumulate in nuclear RNA foci, it is not clear whether foci formation is required for splicing mis-regulation. Here, we use a co-transfection strategy to show that both CUG and CAG repeats form RNA foci that colocalize with green fluorescent protein (GFP)-MBNL1 and endogenous MBNL1. However, only CUG repeats alter splicing of the two tested pre-mRNAs, cardiac troponin T (cTNT) and insulin receptor (IR). Using FRAP, we demonstrate that GFP-MBNL1 in CUG and CAG foci have similar half-times of recovery and fractions of immobile molecules, suggesting that GFP-MBNL1 is bound by both CUG and CAG repeats. We also find an immobile fraction of GFP-MBNL1 in DM1 fibroblasts and a similar rapid exchange in endogenous CUG RNA foci. Therefore, formation of RNA foci and disruption of MBNL1-regulated splicing are separable events.

Ligand-Mediated Assembly and Real-Time Cellular Dynamics of Estrogen Receptor α-Coactivator Complexes in Living Cells

ABSTRACT Studies with live cells demonstrate that agonist and antagonist rapidly (within minutes) modulate the subnuclear dynamics of estrogen receptor α (ER) and steroid receptor coactivator 1 (SRC-1). A functional cyan fluorescent protein (CFP)-taggedlac repressor-ER chimera (CFP-LacER) was used in live cells to discretely immobilize ER on stably integratedlac operator arrays to study recruitment of yellow fluorescent protein (YFP)-steroid receptor coactivators (YFP–SRC-1 and YFP-CREB binding protein [CBP]). In the absence of ligand, YFP–SRC-1 is found dispersed throughout the nucleoplasm, with a surprisingly high accumulation on the CFP-LacER arrays. Agonist addition results in the rapid (within minutes) recruitment of nucleoplasmic YFP–SRC-1, while antagonist additions diminish YFP–SRC-1–CFP-LacER associations. Less ligand-independent colocalization is observed with CFP-LacER and YFP-CBP, but agonist-induced recruitment occurs within minutes. The agonist-induced recruitment of coactivators requires helix 12 and critical residues in the ER–SRC-1 interaction surface, but not the F, AF-1, or DNA binding domains. Fluorescence recovery after photobleaching indicates that YFP–SRC-1, YFP-CBP, and CFP-LacER complexes undergo rapid (within seconds) molecular exchange even in the presence of an agonist. Taken together, these data suggest a dynamic view of receptor-coregulator interactions that is now amenable to real-time study in living cells.

The dynamic mobility of histone H1 is regulated by cyclin/CDK phosphorylation

ABSTRACT The linker histone H1 is involved in maintaining higher-order chromatin structures and displays dynamic nuclear mobility, which may be regulated by posttranslational modifications. To analyze the effect of H1 tail phosphorylation on the modulation of the histones nuclear dynamics, we generated a mutant histone H1, referred to as M1-5, in which the five cyclin-dependent kinase phosphorylation consensus sites were mutated from serine or threonine residues into alanines. Cyclin E/CDK2 or cyclin A/CDK2 cannot phosphorylate the mutant in vitro. Using the technique of fluorescence recovery after photobleaching, we observed that the mobility of a green fluorescent protein (GFP)-M1-5 fusion protein is decreased compared to that of a GFP-wild-type H1 fusion protein. In addition, recovery of H1 correlated with CDK2 activity, as GFP-H1 mobility was decreased in cells with low CDK2 activity. Blocking the activity of CDK2 by p21 expression decreased the mobility of GFP-H1 but not that of GFP-M1-5. Finally, the level and rate of recovery of cyan fluorescent protein (CFP)-M1-5 were lower than those of CFP-H1 specifically in heterochromatic regions. These data suggest that CDK2 phosphorylates histone H1 in vivo, resulting in a more open chromatin structure by destabilizing H1-chromatin interactions.

Differential effects of prolactin and src/abl kinases on the nuclear translocation of STAT5B and STAT5A.

In this study, DNA binding and tyrosine phosphorylation of STAT5A and STAT5B were compared with their subcellular localization determined using indirect immunofluorescence microscopy. Following prolactin activation, both STAT5A and STAT5B were rapidly translocated into the nucleus and displayed a detergent-resistant, punctate nuclear staining pattern. Similar to prolactin induction, src activation resulted in tyrosine phosphorylation and DNA binding of both STAT5A and STAT5B. However, nuclear translocation of only STAT5B but not STAT5A was observed. This selective nuclear translocation appears to be mediated via the carboxyl-terminal sequences in STAT5B. Furthermore, overexpression of a dominant negative kinase-inactive mutant of JAK2 prevented prolactin-induced tyrosine phosphorylation and nuclear translocation of STAT5A and STAT5B but did not block src kinase activation and nuclear translocation of STAT5B. In co-transfection assays, prolactin-mediated activation but not src kinase-mediated activation of STAT5B resulted in the induction of a β-casein promoter-driven reporter construct. These results suggest that STAT5 activation by src may occur by a mechanism distinct from that employed in cytokine activation of the JAK/STAT pathway, resulting in the selective nuclear translocation of STAT5B.

The cenpB gene is not essential in mice

Abstract. Centromere protein B (CENP-B) is a centromeric DNA-binding protein that binds to α-satellite DNA at the 17 bp CENP-B box sequence. The binding of CENP-B, along with other proteins, to α-satellite DNA sequences at the centromere, is thought to package the DNA into heterochromatin subjacent to the kinetochore of mitotic chromosomes. To determine the importance of CENP-B to kinetochore assembly and function, we generated a mouse null for the cenpB gene. The deletion removed part of the promoter and the entire coding sequence except for the carboxyl-terminal 35 amino acids of the CENP-B polypeptide. Mice heterozygous or homozygous for the cenpB null mutation are viable and healthy, with no apparent defect in growth and morphology. We have established mouse embryo fibroblasts from heterozygous and homozygous cenpB null littermates. Microscopic analysis, using immunofluorescence and electron microscopy of the cultured cells, indicated that the centromere-kinetochore complex was intact and identical to control cells. Mitosis was identical in fibroblasts derived from cenpB wild-type, heterozygous and null animals. Our studies demonstrate that CENP-B is not required for the assembly of heterochromatin or the kinetochore, or for completion of mitosis.

Intranuclear ataxin1 inclusions contain both fast- and slow-exchanging components.

A hallmark of neurodegenerative diseases caused by polyglutamine expansion is the abnormal accumulation of mutant proteins into ubiquitin-positive inclusions. The local build-up of these ubiquitinated proteins suggests that the proteasome machinery inadequately clears misfolded proteins, resulting in their increase to potentially toxic levels. Inclusions may disrupt normal cell homeostasis by sequestering vital cellular factors, such as chaperones, proteasomes and transcription components. Here, we used fluorescence recovery after photobleaching (FRAP) to examine the intranuclear dynamics of polyglutamine-expanded ataxin1 and inclusion-associated proteins. These experiments demonstrated that at least two types of ataxin1 inclusions exist; those that undergo rapid and complete exchange with a nucleoplasmic pool and those that contain varying levels of slow-exchanging ataxin1. Slow-exchanging inclusions contain high ubiquitin levels, but surprisingly low proteasome levels, suggesting an impairment in the ability of proteasomes to recognize ubiquitinated substrates. Proteasomes and CBP remained highly dynamic components of inclusions, indicating that although enriched with ataxin1, they are not irreversibly trapped. These results redefine our perception of polyglutamine inclusions and demonstrate the usefulness of FRAP and live cell imaging to study factors that modulate their behaviour.