The small organic compound HMN-176 delays satisfaction of the spindle assembly checkpoint by inhibiting centrosome-dependent microtubule nucleation.

HMN-176 is a potential new cancer therapeutic known to retard the proliferation of tumor cell lines. Here, we show that this compound inhibits meiotic spindle assembly in surf clam oocytes and delays satisfaction of the spindle assembly checkpoint in human somatic cells by inducing the formation of short and/or multipolar spindles. HMN-176 does not affect centrosome assembly, nuclear envelope breakdown, or other aspects of meiotic or mitotic progression, nor does it affect the kinetics of Spisula or mammalian microtubule (MT) assembly in vitro. Notably, HMN-176 inhibits the formation of centrosome-nucleated MTs (i.e., asters) in Spisula oocytes and oocyte extracts, as well as from isolated Spisula or mammalian centrosomes in vitro. Together, these results reveal that HMN-176 is a first-in-class anticentrosome drug that inhibits proliferation, at least in part, by disrupting centrosome-mediated MT assembly during mitosis.

Characterization of Cep135, a novel coiled-coil centrosomal protein involved in microtubule organization in mammalian cells.

By using monoclonal antibodies raised against isolated clam centrosomes, we have identified a novel 135-kD centrosomal protein (Cep135), present in a wide range of organisms. Cep135 is located at the centrosome throughout the cell cycle, and localization is independent of the microtubule network. It distributes throughout the centrosomal area in association with the electron-dense material surrounding centrioles. Sequence analysis of cDNA isolated from CHO cells predicted a protein of 1,145-amino acid residues with extensive alpha-helical domains. Expression of a series of deletion constructs revealed the presence of three independent centrosome-targeting domains. Overexpression of Cep135 resulted in the accumulation of unique whorl-like particles in both the centrosome and the cytoplasm. Although their size, shape, and number varied according to the level of protein expression, these whorls were composed of parallel dense lines arranged in a 6-nm space. Altered levels of Cep135 by protein overexpression and/or suppression of endogenous Cep135 by RNA interference caused disorganization of interphase and mitotic spindle microtubules. Thus, Cep135 may play an important role in the centrosomal function of organizing microtubules in mammalian cells.

Using centrosome fragments in the directed assembly of microtubules.

In animal cells, microtubules are organized by centrosomes, which are 1-2 microm diameter organelles. The generation of functional centrosome fragments in-vitro through ultrasonication is presented along with microtubule assembly directed by the patterned centrosome fragments. While centrosome fragments are smaller than the fully constituted centrosomes, their microtubule organization function is retained. The centrosome fragment templates offer greater flexibility and better coverage in both patterning and assembly of microtubules when compared with intact centrosomes. This work provides the rationale and potential for the large-area assembly of microtubules and should expand the application of centrosomes and centrosome components for the creation of microtubule-based nanoscale devices.

Laser intensity dependence of femtosecond near-infrared optoinjection.

We report an experimental study on transient membrane permeabilization of single living bovine aortic endothelial cells (BAEC) by tightly focused femtosecond near-infrared laser pulses. The membrane permeabilization of the BAEC cells was studied as a function of the incident laser intensity. The rate of dye uptake by the cells was analyzed using time-lapse imaging. We found that membrane permeabilization occurs for laser intensities higher than 4.0 x 10(12) W/cm(2). For laser intensity above 3.3 x 10(13) W/cm(2) the cell disintegrates. Within these two limits the rate of dye uptake increases logarithmically with increasing laser intensity. This functional dependence is explained by considering the Gaussian intensity distribution across the laser focal spot. Cell membrane permeabilization is explained by the creation of a plasma within the laser focal spot. The physical understanding of the relationship between dye uptake, pore characteristics, and laser intensity allows control of the concentrations of molecules delivered into cells through the control of pore characteristics.

Cyclin E/Cdk2 is required for sperm maturation, but not DNA replication, in early sea urchin embryos.

The cell cycle is driven by the activity of cyclin/cdk complexes. In somatic cells, cyclin E/cdk2 oscillates throughout the cell cycle and has been shown to promote S-phase entry and initiation of DNA replication. In contrast, cyclin E/cdk2 activity remains constant throughout the early embryonic development of the sea urchin and localizes to the sperm nucleus following fertilization. We now show that cyclin E localization to the sperm nucleus following fertilization is not unique to the sea urchin, but also occurs in the surf clam, and inhibition of cyclin E/cdk2 activity by roscovitine inhibits the morphological changes indicative of male pronuclear maturation in sea urchin zygotes. Finally, we show that inhibition of cyclin E/cdk2 activity does not block DNA replication in the early cleavage cycles of the sea urchin. We conclude that cyclin E/cdk2 activity is required for male pronuclear maturation, but not for initiation of DNA replication in early sea urchin development.

Aurora-A kinase regulates breast cancer associated gene 1 inhibition of centrosome-dependent microtubule nucleation.

Breast cancer-associated gene 1 (BRCA1) regulates the duplication and the function of centrosomes in breast cells. We have previously shown that BRCA1 ubiquitin ligase activity directly inhibits centrosome-dependent microtubule nucleation. However, there is a paradox because centrosome microtubule nucleation potential is highest during mitosis, a phase when BRCA1 is most abundant at the centrosome. In this study, we resolve this conundrum by testing whether centrosomes from cells in M phase are regulated differently by BRCA1 when compared with other phases of the cell cycle. We observed that BRCA1-dependent inhibition of centrosome microtubule nucleation was high in S phase but was significantly lower during M phase. The cell cycle-specific effects of BRCA1 on centrosome-dependent microtubule nucleation were detected in living cells and in cell-free experiments using centrosomes purified from cells at specific stages of the cell cycle. We show that Aurora-A kinase modulates the BRCA1 inhibition of centrosome function by decreasing the E3 ubiquitin ligase activity of BRCA1. In addition, dephosphorylation of BRCA1 by protein phosphatase 1 alpha enhances the E3 ubiquitin ligase activity of BRCA1. These observations reveal that the inhibition of centrosome microtubule nucleation potential by the BRCA1 E3 ubiquitin ligase is controlled by Aurora-A kinase and protein phosphatase 1 alpha-mediated phosphoregulation through the different phases of the cell cycle.

BRCA1 regulates gamma-tubulin binding to centrosomes.

Centrosomes are the cellular organelles that nucleate microtubules (MTs) via the activity of gamma-tubulin ring complex(s) (gammaTuRC) bound to the pericentriolar material of the centrosomes. BRCA1, the breast and ovarian cancer specific tumor suppressor, inhibits centrosomal MT nucleation via its ubiquitin ligase activity, and one of the known BRCA1 substrates is the key gammaTuRC component, gamma-tubulin. We analyzed the mechanism by which BRCA1 regulates centrosome function using an in vitro reconstitution assay, which includes separately staged steps. Our results are most consistent with a model by which the BRCA1 ubiquitin ligase modifies both gamma-tubulin plus a second centrosomal protein that controls localization of gammaTuRC to the centrosome. We suggest that this second protein is an adapter protein or protein complex that docks gamma-TuRC to the centrosome. By controlling gamma-TuRC localization, BRCA1 appropriately inhibits centrosome function, and loss of BRCA1 would result in centrosome hyperactivity, supernumerary centrosomes and, possibly, aneuploidy.

Centrosome-associated RNA in surf clam oocytes.

Centrosomes are the major microtubule-organizing center in animal cells. They are composed of a pair of [9(3) + 0] centrioles surrounded by a relatively ill-defined pericentriolar matrix, provide the ciliary centriole-kinetosome (basal body) progenitor, and organize the assembly of microtubules into the mitotic spindle during cell division. Despite >100 years of microscopic observation and their obvious significance, our understanding of centrosome composition, dynamic organization, and mechanism of action is limited when compared with that of other cellular organelles. Centrosomes duplicate only once per cell cycle to ensure development of a normal bipolar spindle. The initial event in centrosome duplication is centriole replication, which is generative, semiconservative, and independent of the nucleus. Such observations led to the proposal that centrosomes contain their own complement of nucleic acids, possibly representative of an organellar genome comparable with those described for mitochondria and chloroplasts. The consensus in the field is that centrosomes lack DNA but may contain RNA. We isolated centrosomes from oocytes of the surf clam, Spisula solidissima, and purified from them a unique set of RNAs. We show here by biochemical means and subcellular in situ hybridization that the first transcript we analyzed is intimately associated with centrosomes. Sequence analysis reveals that this centrosome-associated RNA encodes a conserved RNA-directed polymerase domain. The hypothesis that centrosomes contain an intrinsic complement of specific RNAs suggests new opportunities to address the century-old problem of centrosome function, heredity, and evolution.

Direct patterning of centrosome arrays as templates for the assembly of microtubules.

We have achieved, for the first time, the selective patterning of centrosomes onto solid substrates. The use of such patterned centrosome arrays as templates for the directed polymerization of microtubules was also demonstrated. Centrosomes are small organelles in animal cells that serve as nucleation and organization centers of microtubules. Directed assembly of microtubules on the patterned centrosome arrays provides a new route to control the positions and directions of microtubules on surfaces. Combining the patterning of the isolated centrosomes and the directed growth of microtubules may lead to the generation of desired microtubule networks for bio-based nanodevices.

Rapid tau aggregation and delayed hippocampal neuronal death induced by persistent thrombin signaling.

Tau hyperphosphorylation, leading to self-aggregation, is widely held to underlie the neurofibrillary degeneration found in Alzheimer's disease (AD) and other tauopathies. However, it is unclear exactly what environmental factors may trigger this pathogenetic tau hyperphosphorylation. From several perspectives, the coagulation serine protease, thrombin, has been implicated in AD and activates several different protein kinase pathways but has not previously been shown how it may contribute to AD pathogenesis. Here we report that nanomolar thrombin induced rapid tau hyperphosphorylation and aggregation in murine hippocampal neurons via protease-activated receptors, which was followed by delayed synaptophysin reduction and apoptotic neuronal death. Mechanistic study revealed that a persistent thrombin signaling via protease-activated receptor 4 and prolonged downstream p44/42 mitogenactivated protein kinase activation are at least in part responsible. These results pathogenetically linked thrombin to subpopulations of AD and other tauopathies associated with cerebrovascular damage. Such knowledge may be instrumental in transforming therapeutic paradigms.

Effect of gamma-synuclein overexpression on matrix metalloproteinases in retinoblastoma Y79 cells.

gamma-Synuclein is a small cytoplasmic protein implicated in neurodegenerative diseases and cancer. However, the mechanism of its involvement in diseases is not clear. We studied the role of gamma-synuclein in the regulation of matrix metalloproteinases in retinoblastoma cell culture. Matrix metalloproteinases play important roles in the remodeling of extracellular matrix implicated in tumor progression and in the neurodegenerative diseases. Western blot and zymography data demonstrated a moderate elevation of matrix metalloproteinases-2 and significant upregulation of matrix metalloproteinases-9 in stable cell lines overexpressing gamma-synuclein. No effect of gamma-synuclein overexpression on matrix metalloproteinases-1 level or activity was found. Chloramphenicol-acetyltransferase assay demonstrated that overexpression of gamma-synuclein increases the efficiency of the matrix metalloproteinases-9 promoter. This increment of promoter activity may be mediated by the AP-1 binding site(s), since point mutations in one of these sites (Pr18 or Pr19) and elimination of the distal AP-1 site (Pr14) reduced the increment of promoter activity.