The GIT-associated kinase PAK targets to the centrosome and regulates Aurora-A

Signaling-dependent phosphorylation of mitotic centromere-associated kinesin regulates microtubule depolymerization and its centrosomal localization

BACKGROUND

Although PAK1 regulates cytoskeleton and microtubule dynamics, its role in controlling the functions of MCAK remains unknown.

RESULTS

PAK1 phosphorylates MCAK and thereby regulates both its localization and function.

CONCLUSION

MCAK is a cognate substrate of PAK1.

SIGNIFICANCE

This study provides a novel mechanistic insight into PAK1 regulation of MCAK functions. Although p21-activated kinase 1 (PAK1) and microtubule (MT) dynamics regulate numerous fundamental processes including cytoskeleton remodeling, directional motility, and mitotic functions, the significance of PAK1 signaling in regulating the functions of MT-destabilizing protein mitotic centromere-associated kinesin (MCAK) remains unknown. Here we found that MCAK is a cognate substrate of PAK1 wherein PAK1 phosphorylates MCAK on serines 192 and 111 both in vivo and in vitro. Furthermore, we found that PAK1 phosphorylation of MCAK on serines 192 and 111 preferentially regulates its microtubule depolymerization activity and localization to centrosomes, respectively, in the mammalian cells.

The adaptor protein and Arf GTPase-activating protein Cat-1/Git-1 is required for cellular transformation

Cat-1/Git-1 is a multifunctional protein that acts as a GTPase-activating protein (GAP) for Arf GTPases, as well as serves as a scaffold for a number of different signaling proteins. Cat-1 is best known for its role in regulating cell shape and promoting cell migration. However, whether Cat-1 might also contribute to cellular transformation is currently unknown. Here we show that ∼95% of cervical tumor samples examined overexpress Cat-1, suggesting that the up-regulation of Cat-1 expression is a frequent occurrence in this type of cancer. We demonstrate further that knocking down Cat-1 from NIH3T3 fibroblasts expressing an activated form of Cdc42 (Cdc42 F28L), or from the human cervical carcinoma (HeLa) cell line, inhibits the ability of these cells to form colonies in soft agar, an in vitro measure of tumorgenicity. The requirement for Cat-1 when assaying the anchorage-independent growth of transformed fibroblasts and HeLa cells is dependent on its ability to bind paxillin, while being negatively impacted by its Arf-GAP activity. Moreover, the co-expression of Cat-1 and an activated form of Arf6 in fibroblasts was sufficient to induce their transformation. These findings highlight novel roles for Cat-1 and its interactions with the Arf GTPases and paxillin in oncogenic transformation.

p21-activated kinase 4 regulates mitotic spindle positioning and orientation

During mitosis, microtubules (MTs) are massively rearranged into three sets of highly dynamic MTs that are nucleated from the centrosomes to form the mitotic spindle. Tight regulation of spindle positioning in the dividing cell and chromosome alignment at the center of the metaphase spindle are required to ensure perfect chromosome segregation and to position the cytokinetic furrow that will specify the two daughter cells. Spindle positioning requires regulation of MT dynamics, involving depolymerase activities together with cortical and kinetochore-mediated pushing and pulling forces acting on astral MTs and kinetochore fibres. These forces rely on MT motor activities. Cortical pulling forces exerted on astral MTs depend upon dynein/dynactin complexes and are essential in both symmetric and asymmetric cell division. A well-established spindle positioning pathway regulating the cortical targeting of dynein/dynactin involves the conserved LGN (Leu-Gly-Asn repeat-enriched-protein) and NuMA (microtubule binding nuclear mitotic apparatus protein) complex.¹ Spindle orientation is also regulated by integrin-mediated cell adhesion² and actin retraction fibres that respond to mechanical stress and are influenced by the microenvironment of the dividing cell.³ Altering the capture of astral MTs or modulating pulling forces affects spindle position, which can impair cell division, differentiation and embryogenesis.

 

In this general scheme, the activity of mitotic kinases such as Auroras and Plk1 (Polo-like kinase 1) is crucial.⁴ Recently, the p21-activated kinases (PAKs) emerged as novel important players in mitotic progression. In our recent article, we demonstrated that PAK4 regulates spindle positioning in symmetric cell division.⁵ In this commentary, and in light of recent published studies, we discuss how PAK4 could participate in the regulation of mechanisms involved in spindle positioning and orientation.

Proteomic analysis of HIV-1 Nef cellular binding partners reveals a role for exocyst complex proteins in mediating enhancement of intercellular nanotube formation

Background

HIV-1 Nef protein contributes to pathogenesis via multiple functions that include enhancement of viral replication and infectivity, alteration of intracellular trafficking, and modulation of cellular signaling pathways. Nef stimulates formation of tunneling nanotubes and virological synapses, and is transferred to bystander cells via these intercellular contacts and secreted microvesicles. Nef associates with and activates Pak2, a kinase that regulates T-cell signaling and actin cytoskeleton dynamics, but how Nef promotes nanotube formation is unknown.

Results

To identify Nef binding partners involved in Pak2-association dependent Nef functions, we employed tandem mass spectrometry analysis of Nef immunocomplexes from Jurkat cells expressing wild-type Nef or Nef mutants defective for the ability to associate with Pak2 (F85L, F89H, H191F and A72P, A75P in NL4-3). We report that wild-type, but not mutant Nef, was associated with 5 components of the exocyst complex (EXOC1, EXOC2, EXOC3, EXOC4, and EXOC6), an octameric complex that tethers vesicles at the plasma membrane, regulates polarized exocytosis, and recruits membranes and proteins required for nanotube formation. Additionally, Pak2 kinase was associated exclusively with wild-type Nef. Association of EXOC1, EXOC2, EXOC3, and EXOC4 with wild-type, but not mutant Nef, was verified by co-immunoprecipitation assays in Jurkat cells. Furthermore, shRNA-mediated depletion of EXOC2 in Jurkat cells abrogated Nef-mediated enhancement of nanotube formation. Using bioinformatic tools, we visualized protein interaction networks that reveal functional linkages between Nef, the exocyst complex, and the cellular endocytic and exocytic trafficking machinery.

Conclusions

Exocyst complex proteins are likely a key effector of Nef-mediated enhancement of nanotube formation, and possibly microvesicle secretion. Linkages revealed between Nef and the exocyst complex suggest a new paradigm of exocyst involvement in polarized targeting for intercellular transfer of viral proteins and viruses.

Calmodulin activation of Aurora-A kinase (AURKA) is required during ciliary disassembly and in mitosis

This study demonstrates for the first time that binding of calcium-activated calmodulin to a minimal interaction site within the disordered N-terminal domain is required for the essential Aurora-A activity in mitosis and in regulation of ciliary disassembly.

The centrosomal Aurora-A kinase (AURKA) regulates mitotic progression, and overexpression and hyperactivation of AURKA commonly promotes genomic instability in many tumors. Although most studies of AURKA focus on its role in mitosis, some recent work identified unexpected nonmitotic activities of AURKA. Among these, a role for basal body–localized AURKA in regulating ciliary disassembly in interphase cells has highlighted a role in regulating cellular responsiveness to growth factors and mechanical cues. The mechanism of AURKA activation involves interactions with multiple partner proteins and is not well understood, particularly in interphase cells. We show here that AURKA activation at the basal body in ciliary disassembly requires interactions with Ca²⁺ and calmodulin (CaM) and that Ca²⁺/CaM are important mediators of the ciliary disassembly process. We also show that Ca²⁺/CaM binding is required for AURKA activation in mitosis and that inhibition of CaM activity reduces interaction between AURKA and its activator, NEDD9. Finally, mutated derivatives of AURKA impaired for CaM binding and/or CaM-dependent activation cause defects in mitotic progression, cytokinesis, and ciliary resorption. These results define Ca²⁺/CaM as important regulators of AURKA activation in mitotic and nonmitotic signaling.

Molecular pathways: targeting p21-activated kinase 1 signaling in cancer--opportunities, challenges, and limitations

The evolution of cancer cells involves deregulation of highly regulated fundamental pathways that are central to normal cellular architecture and functions. p21-activated kinase 1 (PAK1) was initially identified as a downstream effector of the GTPases Rac and Cdc42. Subsequent studies uncovered a variety of new functions for this kinase in growth factor and steroid receptor signaling, cytoskeleton remodeling, cell survival, oncogenic transformation, and gene transcription, largely through systematic discovery of its direct, physiologically relevant substrates. PAK1 is widely upregulated in several human cancers, such as hormone-dependent cancer, and is intimately linked to tumor progression and therapeutic resistance. These exciting developments combined with the kinase-independent role of PAK1-centered phenotypic signaling in cancer cells elevated PAK1 as an attractive drug target. Structural and biochemical studies revealed the precise mechanism of PAK1 activation, offering the possibility to develop PAK1-targeted cancer therapeutic approaches. In addition, emerging reports suggest the potential of PAK1 and its specific phosphorylated substrates as cancer prognostic markers. Here, we summarize recent findings about the PAK1 molecular pathways in human cancer and discuss the current status of PAK1-targeted anticancer therapies.

P21-activated kinase 4 (PAK4) is required for metaphase spindle positioning and anchoring

The oncogenic kinase PAK4 was recently found to be involved in the regulation of the G1 phase and the G2/M transition of the cell cycle. We have also identified that PAK4 regulates Ran GTPase activity during mitosis. Here, we show that after entering mitosis, PAK4-depleted cells maintain a prolonged metaphase-like state. In these cells, chromosome congression to the metaphase plate occurs with normal kinetics but is followed by an extended period during which membrane blebbing and spindle rotation are observed. These bipolar PAK4-depleted metaphase-like spindles have a defective astral microtubule (MT) network and are not centered in the cell but are in close contact with the cell cortex. As the metaphase-like state persists, centrosome fragmentation occurs, chromosomes scatter from the metaphase plate and move toward the spindle poles with an active spindle assembly checkpoint, a phenotype that is reminiscent of cohesion fatigue. PAK4 also regulates the acto-myosin cytoskeleton and we report that PAK4 depletion results in the induction of cortical membrane blebbing during prometaphase arrest. However, we show that membrane blebs, which are strongly enriched in phospho-cofilin, are not responsible for the poor anchoring of the spindle. As PAK4 depletion interferes with the localization of components of the dynein/dynactin complexes at the kinetochores and on the astral MTs, we propose that loss of PAK4 could induce a change in the activities of motor proteins.

PAK1-dependent MAPK pathway activation is required for colorectal cancer cell proliferation

P21-activated protein kinase1 (PAK1), a main downstream effector of small Rho GTPases, Rac1, and Cdc42, plays an important role in the regulation of cell morphogenesis, motility, mitosis, and angiogenesis. Despite its importance, the molecular mechanisms of PAK1 that contributed to colorectal carcinogenesis remain unclear. Our immunohistochemistry showed that PAK1 expression was increased with colorectal cancer (CRC) progression through the adenoma to carcinoma sequence. Furthermore, our results suggested a relationship between PAK1 nuclear localization and the Dukes staging. In the present study, we showed that PAK1 knockdown decreased proliferation and delayed the G1/S cell-cycle transition, and increased apoptosis in vivo and in vitro. In addition, PAK1 knock-down downregulated c-Jun amino terminal kinases (JNK) activity and the levels of cyclinD1, CDK4/6. Inhibition of the JNK activity by chemical inhibitor (SP600125) significantly reduced the effects of PAK1 on CRC proliferation via accumulation of phosphatase and tensin homolog deleted on chromosome 10 (PTEN). In conclusion, our results demonstrate that knockdown of PAK1 could enhance the chemosensitivity of CRCs to 5-fluorouracil through G1 arrest. The mechanism by which PAK1 induced cancer growth might involve activation of JNK as well as downregulation of PTEN. Targeting PAK1 may represent a novel treatment strategy for developing novel chemotherapeutic agents.

A functional cooperativity between Aurora A kinase and LIM kinase1: implication in the mitotic process

Aurora kinase A (Aur-A), a mitotic kinase, regulates initiation of mitosis through centrosome separation and proper assembly of bipolar spindles. LIM kinase 1 (LIMK1), a modulator of actin and microtubule dynamics, is involved in the mitotic process through inactivating phosphorylation of cofilin. Phosphorylated LIMK1 is recruited to the centrosomes during early prophase, where it colocalizes with γ-tubulin. Here, we report a novel functional cooperativity between Aur-A and LIMK1 through mutual phosphorylation. LIMK1 is recruited to the centrosomes during early prophase and then to the spindle poles, where it colocalizes with Aur-A. Aur-A physically associates with LIMK1 and activates it through phosphorylation, which is important for its centrosomal and spindle pole localization. Aur-A also acts as a substrate of LIMK1, and the function of LIMK1 is important for its specific localization and regulation of spindle morphology. Taken together, the novel molecular interaction between these two kinases and their regulatory roles on one another's function may provide new insight on the role of Aur-A in manipulation of actin and microtubular structures during spindle formation.

PAK4 is required for regulation of the cell-cycle regulatory protein p21, and for control of cell-cycle progression

The serine/threonine kinase PAK4 regulates cytoskeletal architecture, and controls cell proliferation and survival. In most adult tissues PAK4 is expressed at low levels, but overexpression of PAK4 is associated with uncontrolled proliferation, inappropriate cell survival, and oncogenic transformation. Here we have studied for the first time, the role for PAK4 in the cell cycle. We found that PAK4 levels peak dramatically but transiently in the early part of G1 phase. Deletion of Pak4 was also associated with an increase in p21 levels, and PAK4 was required for normal p21 degradation. In serum-starved cells, the absence of PAK4 led to a reduction in the amount of cells in G1, and an increase in the amount of cells in G2/M phase. We propose that the transient increase in PAK4 levels at early G1 reduces p21 levels, thereby abrogating the activity of CDK4/CDK6 kinases, and allowing cells to proceed with the cell cycle in a precisely coordinated way.

Prostate-derived sterile 20-like kinases (PSKs/TAOKs) are activated in mitosis and contribute to mitotic cell rounding and spindle positioning

Prostate-derived sterile 20-like kinases (PSKs) 1-α, 1-β, and 2 are members of the germinal-center kinase-like sterile 20 family of kinases. Previous work has shown that PSK 1-α binds and stabilizes microtubules whereas PSK2 destabilizes microtubules. Here, we have investigated the activation and autophosphorylation of endogenous PSKs and show that their catalytic activity increases as cells accumulate in G(2)/M and declines as cells exit mitosis. PSKs are stimulated in synchronous HeLa cells as they progress through mitosis, and these proteins are activated catalytically during each stage of mitosis. During prophase and metaphase activated PSKs are located in the cytoplasm and at the spindle poles, and during telophase and cytokinesis stimulated PSKs are present in trans-Golgi compartments. In addition, small interfering RNA (siRNA) knockdown of PSK1-α/β or PSK2 expression inhibits mitotic cell rounding as well as spindle positioning and centralization. These results show that PSK catalytic activity increases during mitosis and suggest that these proteins can contribute functionally to mitotic cell rounding and spindle centralization during cell division.

Investigation of POPX2 phosphatase functions by comparative phosphoproteomic analysis

Identifying the substrates and biochemical pathway regulated by phosphatases has always been more challenging than finding those regulated by kinases. Here, we report the use of phosphoproteomic methods to analyse the pathways regulated by POPX2 (partner of PIX 2) phosphatase. POPX2 is a serine/threonine phosphatase, found in many cancer types. The levels of the POPX2 have been found to be up-regulated in the more invasive breast cancer cells compared with non-invasive ones. Our observations also suggest that POPX2 level is positively correlated with cell motility. Thus, finding substrates or pathways regulated by POPX2 will help to elucidate the regulatory mechanism of cancer cell motility and invasiveness. We have also developed and validated a protocol using electrostatic repulsion-hydrophilic interaction chromatography (ERLIC) to enrich the phosphopeptides followed by LC-MS/MS to allow comparison between the phosphoproteomes of control and POPX2 overexpressing cells. With this approach, we were able to identify a biochemical pathway through which POPX2 exerts its apparent cellular function: the regulation of activity of glycogen synthase kinase-3, which in turn modulates extracellular signal-regulated kinase and cell motility.

How protein kinases co-ordinate mitosis in animal cells

Mitosis is associated with profound changes in cell physiology and a spectacular surge in protein phosphorylation. To accomplish these, a remarkably large portion of the kinome is involved in the process. In the present review, we will focus on classic mitotic kinases, such as cyclin-dependent kinases, Polo-like kinases and Aurora kinases, as well as more recently characterized players such as NIMA (never in mitosis in Aspergillus nidulans)-related kinases, Greatwall and Haspin. Together, these kinases co-ordinate the proper timing and fidelity of processes including centrosomal functions, spindle assembly and microtubule-kinetochore attachment, as well as sister chromatid separation and cytokinesis. A recurrent theme of the mitotic kinase network is the prevalence of elaborated feedback loops that ensure bistable conditions. Sequential phosphorylation and priming phosphorylation on substrates are also frequently employed. Another important concept is the role of scaffolds, such as centrosomes for protein kinases during mitosis. Elucidating the entire repertoire of mitotic kinases, their functions, regulation and interactions is critical for our understanding of normal cell growth and in diseases such as cancers.

A translational regulator, PUM2, promotes both protein stability and kinase activity of Aurora-A

Aurora-A, a centrosomal serine-threonine kinase, orchestrates several key aspects of cell division. However, the regulatory pathways for the protein stability and kinase activity of Aurora-A are still not completely understood. In this study, PUM2, an RNA-binding protein, is identified as a novel substrate and interacting protein of Aurora-A. Overexpression of the PUM2 mutant which fails to interact with Aurora-A, and depletion of PUM2 result in a decrease in the amount of Aurora-A. PUM2 physically binds to the D-box of Aurora-A, which is recognized by APC/C^Cdh1. Overexpression of PUM2 prevents ubiquitination and enhances the protein stability of Aurora-A, suggesting that PUM2 protects Aurora-A from APC/C^Cdh1-mediated degradation. Moreover, association of PUM2 with Aurora-A not only makes Aurora-A more stable but also enhances the kinase activity of Aurora-A. Our study suggests that PUM2 plays two different but important roles during cell cycle progression. In interphase, PUM2 localizes in cytoplasm and plays as translational repressor through its RNA binding domain. However, in mitosis, PUM2 physically associates with Aurora-A to ensure enough active Aurora-A at centrosomes for mitotic entry. This is the first time to reveal the moonlight role of PUM2 in mitosis.

Protein phosphatase 6 regulates mitotic spindle formation by controlling the T-loop phosphorylation state of Aurora A bound to its activator TPX2

Many protein kinases are activated by a conserved regulatory step involving T-loop phosphorylation. Although there is considerable focus on kinase activator proteins, the importance of specific T-loop phosphatases reversing kinase activation has been underappreciated. We find that the protein phosphatase 6 (PP6) holoenzyme is the major T-loop phosphatase for Aurora A, an essential mitotic kinase. Loss of PP6 function by depletion of catalytic or regulatory subunits interferes with spindle formation and chromosome alignment because of increased Aurora A activity. Aurora A T-loop phosphorylation and the stability of the Aurora A-TPX2 complex are increased in cells depleted of PP6 but not other phosphatases. Furthermore, purified PP6 acts as a T-loop phosphatase for Aurora A-TPX2 complexes in vitro, whereas catalytically inactive mutants cannot dephosphorylate Aurora A or rescue the PPP6C depletion phenotype. These results demonstrate a hitherto unappreciated role for PP6 as the T-loop phosphatase regulating Aurora A activity during spindle formation and suggest the general importance of this form of regulation.

Centrosomal protein of 192 kDa (Cep192) promotes centrosome-driven spindle assembly by engaging in organelle-specific Aurora A activation

Centrosomes are primary microtubule (MT)-organizing centers (MTOCs). During mitosis, they dramatically increase their size and MT-nucleating activity and participate in spindle assembly from spindle poles. These events require the serine/threonine kinase, Aurora A (AurA), and the centrosomal protein of 192 kDa (Cep192)/spindle defective 2 (Spd-2), but the underlying mechanism remains unclear. We have found that Cep192, unlike targeting protein for Xklp2 (TPX2), a known MT-localizing AurA activator, is an AurA cofactor in centrosome-driven spindle assembly. Cep192, through a direct interaction, targets AurA to mitotic centrosomes where the locally accumulating AurA forms homodimers or oligomers. The dimerization of endogenous AurA, in the presence of bound Cep192, triggers potent kinase activation that, in turn, drives MT assembly. Depletion of Cep192 or specific interference with AurA-Cep192 binding did not prevent AurA oligomerization on MTs but abrogated AurA recruitment to centrosomes and its activation by either sperm nuclei or anti-AurA antibody (αAurA)-induced dimerization. In these settings, MT assembly by both centrosomes and αAurA-coated beads was also abolished or severely compromised. Hence, Cep192 activates AurA by a mechanism different from that previously described for TPX2. The Cep192-mediated mechanism maximizes AurA activity at centrosomes and appears essential for the function of these organelles as MTOCs.

p21-Activated kinases are required for transformation in a cell-based model of neurofibromatosis type 2

Background

NF2 is an autosomal dominant disease characterized by development of bilateral vestibular schwannomas and other benign tumors in central nervous system. Loss of the NF2 gene product, Merlin, leads to aberrant Schwann cell proliferation, motility, and survival, but the mechanisms by which this tumor suppressor functions remain unclear. One well-defined target of Merlin is the group I family of p21-activated kinases, which are allosterically inhibited by Merlin and which, when activated, stimulate cell cycle progression, motility, and increased survival. Here, we examine the effect of Pak inhibition on cells with diminished Merlin function.

Methodology/Principal Findings

Using a specific peptide inhibitor of group I Paks, we show that loss of Pak activity restores normal cell movement in cells lacking Merlin function. In addition, xenografts of such cells form fewer and smaller tumors than do cells without Pak inhibition. However, in tumors, loss of Pak activity does not reduce Erk or Akt activity, two signaling proteins that are thought to mediate Pak function in growth factor pathways.

Conclusions/Significance

These results suggest that Pak functions in novel signaling pathways in NF2, and may serve as a useful therapeutic target in this disease.

Golgi partitioning controls mitotic entry through Aurora-A kinase

At the onset of mitosis, the Golgi complex undergoes a multistep fragmentation process that is required for its correct partitioning into the daughter cells. Inhibition of this Golgi fragmentation results in cell cycle arrest at the G2 stage, suggesting that correct inheritance of the Golgi complex is monitored by a "Golgi mitotic checkpoint." However, the molecular basis of this G2 block is not known. Here, we show that the G2-specific Golgi fragmentation stage is concomitant with centrosome recruitment and activation of the mitotic kinase Aurora-A, an essential regulator for entry into mitosis. We show that a block of Golgi partitioning impairs centrosome recruitment and activation of Aurora-A, which results in the G2 block of cell cycle progression. Overexpression of Aurora-A overrides this cell cycle block, indicating that Aurora-A is a major effector of the Golgi checkpoint. Our findings provide the basis for further understanding of the signaling pathways that coordinate organelle inheritance and cell duplication.

Purification and proteomic identification of putative upstream regulators of polo-like kinase-1 from mitotic cell extracts

Polo-like kinase-1 (Plk1) is phosphorylated on Thr210 for activation during mitosis. Here, we investigated the question of which kinase(s) is the specific upstream kinase of mitotic Plk1. Upstream kinases of Plk1 were purified from mitotic cell extracts through column chromatography procedures, and identified by mass spectrometry. Candidates for Plk1 kinase included p21-activated kinase, aurora A, and mammalian Ste20-like kinases. Immunoprecipitates of these proteins from mitotic cell extracts phosphorylated Plk1 on Thr210. Even if the activity of Aurora A was blocked with a specific inhibitor, Plk1 phosphorylation still occurred, suggesting that function of Plk1 could be controlled by these kinases for proper mitotic progression, as well as by Aurora A in very late G2 phase for the beginning of mitosis.

The WW-HECT protein Smurf2 interacts with the Docking Protein NEDD9/HEF1 for Aurora A activation

The multi-functional adaptor protein NEDD9/HEF1/Cas-L regulates cell motility, invasion and cell cycle progression, and plays key roles in cancer progression and metastasis. NEDD9 is localized to the centrosome and is required for activation of Aurora A kinase in mitosis. Here we demonstrate that the HECT-WW protein Smurf2 physically associates with NEDD9 and is required for the stability of NEDD9 protein. Smurf2 depletion results in a marked decrease in NEDD9 protein levels, by facilitating polyubiquitination and proteasomal degradation of NEDD9. Conversely, forced overexpression of Smurf2 results in upregulation of endogenous NEDD9 protein, confirming the role for Smurf2 in NEDD9 stability. Cells with Smurf2 depletion fail to activate Aurora A at the G₂/M boundary, leading to a marked delay in mitotic entry. These observations suggest that the stable complex of Smurf2 and NEDD9 is required for timely entry into mitosis via Aurora A activation.

Rapid calcium-dependent activation of Aurora-A kinase

Oncogenic hyperactivation of the mitotic kinase Aurora-A (AurA) in cancer is associated with genomic instability. Increasing evidence indicates that AurA also regulates critical processes in normal interphase cells, but the source of such activity has been obscure. We report here that multiple stimuli causing release of Ca²⁺ from intracellular endoplasmic reticulum stores rapidly and transiently activate AurA, without requirement for second messengers. This activation is mediated by direct Ca²⁺-dependent calmodulin (CaM) binding to multiple motifs on AurA. On the basis of structure–function analysis and molecular modelling, we map two primary regions of CaM-AurA interaction to unfolded sequences in the AurA N- and C-termini. This unexpected mechanism for AurA activation provides a new context for evaluating the function of AurA and its inhibitors in normal and cancerous cells.

Aurora-A kinase localizes to centrosomes, is involved in the progression through mitosis and is overexpressed in certain cancers. Here, calcium is shown to induce Aurora-A auto-phosphorylation in a calmodulin-dependent manner, suggesting a novel role for Aurora-A in non-mitotic cells.

Development of small-molecule inhibitors of the group I p21-activated kinases, emerging therapeutic targets in cancer

The p21-activated kinases (PAKs), immediate downstream effectors of the small G-proteins of the Rac/cdc42 family, are critical mediators of signaling pathways regulating cellular behaviors and as such, have been implicated in pathological conditions including cancer. Recent studies have validated the requirement for PAKs in promoting tumorigenesis in breast carcinoma and neurofibromatosis. Thus, there has been considerable interest in the development of inhibitors to the PAKs, as biological markers and leads for the development of therapeutics. While initial approaches were based on screening for competitive organic inhibitors, more recent efforts have focused on the identification of allosteric inhibitors, organometallic ATP-competitive inhibitors and the use of PAK1/inhibitor crystal structures for inhibitor optimization. This has led to the identification of highly selective and potent inhibitors, which will serve as a basis for further development of inhibitors for therapeutic applications.

Aurora kinases orchestrate mitosis; who are the players?

The Aurora are a conserved family of serine/threonine kinases with essential functions in cell division. In mitosis, Aurora kinases are required for chromosome segregation, condensation and orientation in the metaphase plate, spindle assembly, and the completion of cytokinesis. This review presents the Aurora kinases, their partners and how their interactions impact on the different mitotic functions.

Arpc1b, a centrosomal protein, is both an activator and substrate of Aurora A

In addition to its function as an Arp2/3 complex subunit, Arp1cb interacts with and stimulates Aurora A at centrosomes, functioning in cell cycle progression.

Here we provide evidence in support of an inherent role for Arpc1b, a component of the Arp2/3 complex, in regulation of mitosis and demonstrate that its depletion inhibits Aurora A activation at the centrosome and impairs the ability of mammalian cells to enter mitosis. We discovered that Arpc1b colocalizes with γ-tubulin at centrosomes and stimulates Aurora A activity. Aurora A phosphorylates Arpc1b on threonine 21, and expression of Arpc1b but not a nonphosphorylatable Arpc1b mutant in mammalian cells leads to Aurora A kinase activation and abnormal centrosome amplification in a Pak1-independent manner. Together, these findings reveal a new function for Arpc1b in centrosomal homeostasis. Arpc1b is both a physiological activator and substrate of Aurora A kinase and these interactions help to maintain mitotic integrity in mammalian cells.

PAK1 as a therapeutic target

IMPORTANCE OF THE FIELD

P21-activated kinases (PAKs) are involved in multiple signal transduction pathways in mammalian cells. PAKs, and PAK1 in particular, play a role in such disorders as cancer, mental retardation and allergy. Cell motility, survival and proliferation, the organization and function of cytoskeleton and extracellular matrix, transcription and translation are among the processes affected by PAK1.

AREAS COVERED IN THIS REVIEW

We discuss the mechanisms that control PAK1 activity, its involvement in physiological and pathophysiological processes, the benefits and the drawbacks of the current tools to regulate PAK1 activity, the evidence that suggests PAK1 as a therapeutic target and the likely directions of future research.

WHAT THE READER WILL GAIN

The reader will gain a better knowledge and understanding of the areas described above.

TAKE HOME MESSAGE

PAK1 is a promising therapeutic target in cancer and allergen-induced disorders. Its suitability as a target in vascular, neurological and infectious diseases remains ambiguous. Further advancement of this field requires progress on such issues as the development of specific and clinically acceptable inhibitors, the choice between targeting one or multiple PAK isoforms, elucidation of the individual roles of PAK1 targets and the mechanisms that may circumvent inhibition of PAK1.

PAK as a therapeutic target in gastric cancer

IMPORTANCE OF THE FIELD

Gastric cancer is one of the most common causes of cancer death worldwide. P21-activated kinases (PAKs), regulators of cancer-cell signalling networks, play fundamental roles in a range of cellular processes through their binding partners or kinase substrates.

AREAS COVERED IN THIS REVIEW

The complex regulation of PAKs through their upstream or downstream effectors in human cancers, especially in gastric cancer, are described and the identified inhibitors of PAKs are summarized.

WHAT THE READERS WILL GAIN

The structural differences and activation mechanisms between two subgroups of PAK are described. Both groups of PAKs play complicated and important roles in human gastric cancer, which indicated a possible way for us to identify the specific inhibitors targeting PAKs for gastric cancer.

TAKE HOME MESSAGE

PAKs play important roles in progression of many cancer types, the full mechanisms of PAKs in gastric cancer are still unclear. It seems there are different roles for two groups of PAKs in cancers. Group I PAKs play their functions mostly through their specific substrates, however, many binding partners that are independent of phosphorylation by group II PAKs were identified. Finding specific inhibitors of PAKs will help us discover the roles of PAKs and target these kinases in human gastric cancer.

Aurora kinase inhibitors as anti-cancer therapy

Aurora kinases are serine and threonine kinases that function as key regulators of the mitosis process. There are three distinct human aurora kinases known as Aurora A, Aurora B, and Aurora C. Aurora A and Aurora B are overexpressed in a number of human cancers including non-small cell lung cancer, glioblastomas, and upper gastrointestinal adenocarcinomas. Given their association with tumorigenesis, both Aurora A and Aurora B have been targeted for cancer therapy. Currently, a number of selective and nonselective aurora kinase inhibitors are being tested in preclinical and clinical settings as anti-tumor agents. We review the biology of human aurora kinases, followed by an overview of inhibitors undergoing current clinical investigations.

Conventional myosins - unconventional functions

While the discovery of unconventional myosins raised expectations that their actions were responsible for most aspects of actin-based cell motility, few anticipated the wide range of cellular functions that would remain the purview of conventional two-headed myosins. The three nonsarcomeric, cellular myosins-M2A, M2B and M2C-participate in diverse roles including, but not limited to: neuronal dynamics, axon guidance and synaptic transmission; endothelial cell migration; cell adhesion, polarity, fusion and cytokinesis; vesicle trafficking and viral egress. These three conventional myosins each take on specific, differing functional roles during development and maturity, characteristic of each cell lineage; exact roles depend on the developmental stage of the cell, cellular location, upstream regulatory controls, relative isoform expression, orientation and associated state of the actin cytoscaffolds in which these myosins operate. Here, we discuss the separate yet related roles that characterise the actions of M2A, M2B and M2C in various cell types and show that these conventional myosins are responsible for functions as unconventional as any performed by unconventional myosins.

Actin cytoskeleton dynamics and the cell division cycle

The network of actin filaments is one of the crucial cytoskeletal structures contributing to the morphological framework of a cell and which participates in the dynamic regulation of cellular functions. In adherent cell types, cells adhere to the substratum during interphase and spread to assume their characteristic shape supported by the actin cytoskeleton. This actin cytoskeleton is reorganized during mitosis to form rounded cells with increased cortical rigidity. The actin cytoskeleton is re-established after mitosis, allowing cells to regain their extended shape and attachment to the substratum. The modulation of such drastic changes in cell shape in coordination with cell cycle progression suggests a tight regulatory interaction between cytoskeleton signalling, cell-cell/cell-matrix adhesions and mitotic events. Here, we review the contribution of the actin cytoskeleton to cell cycle progression with an emphasis on the effectors responsible for the regulation of the actin cytoskeleton and integration of their activities with the cell cycle machinery.

A vertebrate-specific Chp-PAK-PIX pathway maintains E-cadherin at adherens junctions during zebrafish epiboly

Background

In early vertebrate development, embryonic tissues modulate cell adhesiveness and acto-myosin contractility to correctly orchestrate the complex processes of gastrulation. E-cadherin (E-cadh) is the earliest expressed cadherin and is needed in the mesendodermal progenitors for efficient migration [1], [2]. Regulatory mechanisms involving directed E-cadh trafficking have been invoked downstream of Wnt11/5 signaling [3]. This non-canonical Wnt pathway regulates RhoA-ROK/DAAM1 to control the acto-myosin network. However, in this context nothing is known of the intracellular signals that participate in the correct localization of E-cadh, other than a need for Rab5c signaling [3].

Methodology/Principal Findings

By studying loss of Chp induced by morpholino-oligonucleotide injection in zebrafish, we find that the vertebrate atypical Rho-GTPase Chp is essential for the proper disposition of cells in the early embryo. The underlying defect is not leading edge F-actin assembly (prominent in the cells of the envelope layer), but rather the failure to localize E-cadh and β-catenin at the adherens junctions. Loss of Chp results in delayed epiboly that can be rescued by mRNA co-injection, and phenocopies zebrafish E-cadh mutants [4], [5]. This new signaling pathway involves activation of an effector kinase PAK, and involvement of the adaptor PAK-interacting exchange factor PIX. Loss of signaling by any of the three components results in similar underlying defects, which is most prominent in the epithelial-like envelope layer.

Conclusions/Significance

Our current study uncovers a developmental pathway involving Chp/PAK/PIX signaling, which helps co-ordinate E-cadh disposition to promote proper cell adhesiveness, and coordinate movements of the three major cell layers in epiboly. Our data shows that without Chp signaling, E-cadh shifts to intracellular vesicles rather than the adhesive contacts needed for directed cell movement. These events may mirror the requirement for PAK2 signaling essential for the proper formation of the blood-brain barrier [6], [7].

SP600125 suppresses Cdk1 and induces endoreplication directly from G2 phase, independent of JNK inhibition

Cell cycle controls ensure that DNA replication (S phase) follows mitosis resulting in two precise copies of the genome. A failure of the control mechanisms can result in multiple rounds of DNA replication without cell division. In endoreplication, cells with replicated genomes bypass mitosis, then replicate their DNA again, resulting in polyploidy. Endoreplication from G2 phase lacks all hallmarks of mitosis. Using synchronized cells, we show that the c-Jun N-terminal kinase (JNK) inhibitor, SP600125, prevents the entry of cells into mitosis and leads to endoreplication of DNA from G2 phase. We show that cells proceed from G2 phase to replicate their DNA in the absence of mitosis. This effect of SP600125 is independent of its suppression of JNK activity. Instead, the inhibitory effect of SP600125 on mitotic entry predominantly occurs upstream of Aurora A kinase and Polo-like kinase 1, resulting in a failure to remove the inhibitory phosphorylation of Cdk1. Importantly, our results directly show that the inhibition of Cdk1 activity and the persistence of Cdk2 activity in G2 cells induces endoreplication without mitosis. Furthermore, endoreplication from G2 phase is independent of p53 control.

Making the Auroras glow: regulation of Aurora A and B kinase function by interacting proteins

The conserved Aurora family of protein kinases have emerged as crucial regulators of mitosis and cytokinesis. Despite their high degree of homology, Aurora A and B have very distinctive localisations and functions: Aurora A associates with the spindle poles to regulate entry into mitosis, centrosome maturation and spindle assembly; Aurora B is a member of the Chromosomal Passenger Complex (CPC) that transfers from the inner centromere in early mitosis to the spindle midzone, equatorial cortex and midbody in late mitosis and cytokinesis. Aurora B functions include regulation of chromosome–microtubule interactions, cohesion, spindle stability and cytokinesis. This review will focus on how interacting proteins make this functional diversity possible by targeting the kinases to different subcellular locations and regulating their activity.

Biology of Aurora A kinase: implications in cancer manifestation and therapy

The Aurora A kinase belongs to serine/threonine group of kinases, well known for its role in cell cycle, especially in the regulation of mitosis. Numerous substrates of Aurora A kinase have been identified, which are predominantly related to cell cycle progression while some of them are transcription factors. Aurora A-mediated phosphorylation can either directly or indirectly regulate the function of its substrates. There are overwhelming evidences which report overexpression and gene amplification of Aurora A in several human cancers, and suggest that Aurora A could be a bona fide oncogene involved in tumorigenesis. Hence, Aurora A plays wide-ranging roles in both mitosis and its deregulation manifests in cancer progression. These observations have favored the choice of Aurora kinases as a target for cancer therapy. Recently, numerous small molecules have been discovered against Aurora kinases and many have entered clinical trials. Most of these small-molecule modulators designed are specific against either Aurora A or Aurora B, but some are dual inhibitors targeting the ATP-binding site which is highly conserved among the three human homologues of Aurora kinase. In this review, we discuss the physiological functions of Aurora A, interactions between Aurora A kinase and its cellular substrates, tumorigenesis mediated by Aurora A kinase upon overexpression, and small-molecule modulators of Aurora kinase as targets for cancer therapy.

Physiological and oncogenic Aurora-A pathway

Aurora family of protein kinases have emerged as crucial factors of, not only mitosis and cytokinesis, but also human carcinogenesis. Among these family members is Aurora-A that is frequently overexpressed in varieties of human cancer. Both in vitro and in vivo studies demonstrated that Aurora-A induces tumorigenesis through genome instability. These studies have further shown that cell signaling cross-talk between Aurora-A and other cellular proteins are essential for fully-transformed phenotypes. This review summarizes recent progress of Aurora-A-associated carcinogenesis.

Downregulation of p21-activated kinase-1 inhibits the growth of gastric cancer cells involving cyclin B1

p21-Activated kinase 1 (Pak1), a serine/threonine kinase, has been implicated in cytoskeletal remodelling, cell motility, apoptosis and transformation. However, the role of Pak1 in gastric cancer remains unclear. In this study, we detected Pak1 expression in gastric cancer tissues from 40 patients by western blot. Overexpression of Pak1 was associated with progression, metastasis and prognosis of gastric cancer. In addition, we found that knockdown of Pak1 expression significantly inhibited anchorage-dependent and anchorage-independent growth in gastric cancer cells, and markedly inhibited gastric cancer cell xenograft tumor growth. In further study, data showed that Pak1 could regulate the expression of cyclin B1 at the mRNA and protein levels, and impact the subcellular distribution and the promoter activity of cyclin B1. Results from deletion and mutant analysis supplied a new NF-kappaB binding sites at position -321 of cyclin B1 promoter, and indicated that Pak1 regulated the transcription of cyclin B1 in gastric cancer through NF-kappaB. In conclusion, Pak1 may be a potential prognostic marker and therapeutic target in gastric cancer.

Aurora A, centrosome structure, and the centrosome cycle

The centrosome, also known as the microtubule organizing center of the cell, is a membrane-less organelle composed of a pair of barrel-shaped centrioles surrounded by electron-dense pericentriolar material. The centrosome progresses through the centrosome cycle in step with the cell cycle such that centrosomes are duplicated in time to serve as the spindle poles during mitosis and that each resultant daughter cell contains a single centrosome. Regulation of the centrosome cycle with relation to the cell cycle is an essential process to maintain the ratio of one centrosome per new daughter cell. Numerous mitosis-specific kinases have been implicated in this regulation, and phosphorlyation plays an important role in coordinating the centrosome and cell cycles. Centrosome amplification can occur when the cycles are uncoupled, and this amplification is associated with cancer and with an increase in the levels of chromosomal instability. The aurora kinases A, B, and C are serine/threonine kinases that are active during mitosis. Aurora A is associated with centrosomes, being localized at the centrosome just prior to the onset of mitosis and for the duration of mitosis. Overexpression of aurora A leads to centrosome amplification and cellular transformation. The activity of aurora A is regulated by phosphorlyation and proteasomal degradation.

Inhibition of histone deacetylase suppresses EGF signaling pathways by destabilizing EGFR mRNA in ER-negative human breast cancer cells

Estrogen receptor alpha (ER)-negative human breast cancer cells frequently overexpress epidermal growth factor receptor (EGFR) and respond poorly to endocrine therapies. Our previous studies demonstrate that histone deacetylation plays a key role in ER gene silencing, and ER expression can be restored with histone deacetylase (HDAC) inhibitors in ER-negative human breast cancer cells. Whether inhibition of HDAC also alters epidermal growth factor (EGF) signaling pathways is not defined. Here we present evidence that reexpression of ER protein by a clinically available HDAC inhibitor, suberoylanilide hydroxamic acid (SAHA or vorinostat), is coupled with loss of EGFR in ER-negative human breast cancer cells. Consistent with this observation, MDA-MB-231 cells, which are ER-negative and overexpress EGFR, that are engineered to express ER show a decrease in EGFR protein expression. Down-regulation of EGFR by SAHA results from attenuation of its mRNA stability. We also confirm that new protein synthesis is required for maintaining EGFR mRNA stability. Further experiments indicate that a decrease in EGFR abolished EGF-initiated signaling pathways including phosphorylated PAK1, p38MAPK and AKT. Thus, SAHA may not only reactivate silenced ER, but also simultaneously deplete EGFR expression. These data suggest that inhibition of HDAC is a promising epigenetic therapy for ER-negative human breast cancer.

PAKing up to the endothelium

Angiogenesis recapitulates the growth of blood vessels that progressively expand and remodel into a highly organized and stereotyped vascular network. During adulthood, endothelial cells that formed the vascular wall retain their plasticity and can be engaged in neo-vascularization in response to physiological stimuli, such as hypoxia, wound healing and tissue repair, ovarian cycle and pregnancy. In addition, numerous human diseases and pathological conditions are characterized by an excessive, uncontrolled and aberrant angiogenesis. The signalling pathways involving the small Rho GTPase, Rac and its downstream effector the p21-activated serine/threonine kinase (PAK) had recently emerged as pleiotropic modulators in these processes. Indeed, Rac and PAK were found to modulate endothelial cell biology, such as sprouting, migration, polarity, proliferation, lumen formation, and maturation. Elucidating the Rac/PAK molecular circuitry will provide essential information for the development of new therapeutic agents designed to normalize the blood vasculature in human diseases.

Pak protein kinases and their role in cancer

Some of the characteristics of cancer cells are high rates of cell proliferation, cell survival, and the ability to invade surrounding tissue. The cytoskeleton has an essential role in these processes. Dynamic changes in the cytoskeleton are necessary for cell motility and cancer cells are dependent on motility for invasion and metastasis. The signaling pathways behind the reshaping and migrating properties of the cytoskeleton in cancer cells involve a group of Ras-related small GTPases and their effectors, including the p21-activated kinases (Paks). Paks are a family of serine/threonine protein kinases comprised of six isoforms (Pak 1-6), all of which are direct targets of the small GTPases Rac and Cdc42. Besides their role in cytoskeletal dynamics, Paks have recently been shown to regulate various other cellular activities, including cell survival, mitosis, and transcription. Paks are overexpressed and/or hyperactivated in several human tumors and their role in cell transformation makes them attractive therapeutic targets. Pak-targeted therapeutics may efficiently inhibit certain types of tumors and efforts to identify selective Pak-inhibitors are underway.

Centrosomes and cancer: how cancer cells divide with too many centrosomes

Precise control of centrosome number is crucial for bipolar spindle assembly and accurate transmission of genetic material to daughter cells. Failure to properly control centrosome number results in supernumerary centrosomes, which are frequently found in cancer cells. This presents a paradox: during mitosis, cells with more than two centrosomes are prone to multipolar mitoses and cell death, however, cancer cells possessing extra centrosomes usually divide successfully. One mechanism frequently utilized by cancer cells to escape death caused by multipolar mitoses is the clustering of supernumerary centrosomes into bipolar arrays. An understanding of the molecular mechanisms by which cancer cells can suppress multipolar mitoses is beginning to emerge. Here, we review what's currently known about centrosome clustering mechanisms and discuss potential strategies to target these mechanisms for the selective killing of cancer cells.

PAK signaling in oncogenesis

The p21-activated kinase (PAK) family of serine/threonine kinases is important in physiological processes including motility, survival, mitosis, transcription and translation. PAKs are evolutionally conserved and widely expressed in a variety of tissues and are often overexpressed in multiple cancer types. Depending on structural and functional similarities, the six members of PAK family are divided into two groups with three members in each group. Group I PAKs are activated by extracellular signals through GTPase-dependent and GTPase-independent mechanisms. In contrast, group II PAKs are constitutively active. Over the years, accumulating data from tissue culture models and human tumors has increased our understanding about the biology of PAK family members. In this review, we have summarized the complex regulation of PAK and its downstream diverse myriads of effectors, which in turn are responsible for the biological effects of PAK family of kinases in cancer cells.

Coordinating cellular events during spermatogenesis: a biochemical model

Throughout spermatogenesis, a select pool of germ cells, the leptotene spermatocytes, must traverse the blood-testis barrier (BTB) to enter the adluminal compartment of the seminiferous epithelium. This event requires extensive restructuring of cell junctions, and it must also coincide with germ cell cycle progression in preparation for primary spermatocyte meiosis. Recent findings show that cell-cycle-associated kinases and phosphatases, including mitogen-activated protein kinases (MAPKs), participate in the pathways that also direct germ cell adhesion and movement. Our new biochemical model explains, in part, how two distinct cellular events, BTB restructuring and spermiation, are coordinated to maintain spermatogenesis and fertility. In this way, MAPKs would synchronize cell cycle progression in primary spermatocytes with junction remodeling and cell migration across the BTB.

CDK11p58 phosphorylation of PAK1 Ser174 promotes DLC2 binding and roles on cell cycle progression

CDK11(p58), a CDK11 family Ser/Thr kinase, is a G2/M specific protein and contributed to regulation of cell cycle, transcription and apoptotic signal transduction. Recently, CDK11(p58) has been reported to exert important functions in mitotic process, such as the regulation of bipolar spindle formation and sister chromatid cohesion. Here, we identified p21 activated kinase 1 (PAK1) as a new CDK11(p58) substrate and we mapped a new phosphorylation site of Ser174 on PAK1. By mutagenesis, we created PAK1(174A) and PAK1(174E), which mimic the dephosphorylated and phosphorylated form of PAK1; further analysis showed PAK1(174E) could be recruited to myosin V motor complex through binding to dynein light chain 2 (DLC2). PAK1(174E) could accelerate the mitosis progression in a nocodazole blocked cell model, while PAK1(174A) exhibited an opposite role. Our results indicated PAK1 may serve as a downstream effector of CDK11(p58) during mitosis progression.

Dual role of Cdc42 in spindle orientation control of adherent cells

The spindle orientation is regulated by the interaction of astral microtubules with the cell cortex. We have previously shown that spindles in nonpolarized adherent cells are oriented parallel to the substratum by an actin cytoskeleton- and phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3]-dependent mechanism. Here, we show that Cdc42, a Rho family of small GTPases, has an essential role in this mechanism of spindle orientation by regulating both the actin cytoskeleton and PtdIns(3,4,5)P3. Knockdown of Cdc42 suppresses PI(3)K activity in M phase and induces spindle misorientation. Moreover, knockdown of Cdc42 disrupts the cortical actin structures in metaphase cells. Our results show that p21-activated kinase 2 (PAK2), a target of Cdc42 and/or Rac1, plays a key role in regulating actin reorganization and spindle orientation downstream from Cdc42. Surprisingly, PAK2 regulates spindle orientation in a kinase activity-independent manner. BetaPix, a guanine nucleotide exchange factor for Rac1 and Cdc42, is shown to mediate this kinase-independent function of PAK2. This study thus demonstrates that spindle orientation in adherent cells is regulated by two distinct pathways downstream from Cdc42 and uncovers a novel role of the Cdc42-PAK2-betaPix-actin pathway for this mechanism.

Aurora A is essential for early embryonic development and tumor suppression

Aurora A is a serine/threonine kinase that functions in various stages of mitosis. Accumulating evidence has demonstrated that gene amplification and overexpression of Aurora A are linked to tumorigenesis, suggesting that Aurora A is an oncogene. In addition, Aurora A overexpression has been used as a negative prognostic marker, because it is associated with resistance to anti-mitotic agents commonly used for cancer therapy. To understand the physiological functions of Aurora A, we generated Aurora A knock-out mice. Aurora A null mice die early during embryonic development before the 16-cell stage. These Aurora A null embryos have defects in mitosis, particularly in spindle assembly, supporting critical functions of Aurora A during mitotic transitions. Interestingly, Aurora A heterozygosity results in a significantly increased tumor incidence in mice, suggesting that Aurora A may also act as a haploinsufficient tumor suppressor. Consistently, Aurora A heterozygous mouse embryonic fibroblasts have higher rates of aneuploidy. We further discovered that VX-680, an Aurora kinase inhibitor currently in phase II clinical trials for cancer treatment, could induce aneuploidy in wild type mouse embryonic fibroblasts. We conclude that a balanced Aurora A level is critical for maintaining genomic stability and one needs to be fully aware of the potential side effects of anti-cancer therapy based on the use of Aurora A-specific inhibitors.

PAK signalling in neuronal physiology

Group I p21-activated kinases are a family of key effectors of Rac1 and Cdc42 and they regulate many aspects of cellular function, such as cytoskeleton dynamics, cell movement and cell migration, cell proliferation and differentiation, and gene expression. The three genes PAK1/2/3 are expressed in brain and recent evidence indicates their crucial roles in neuronal cell fate, in axonal guidance and neuronal polarisation, and in neuronal migration. Moreover they are implicated in neurodegenerative diseases and play an important role in synaptic plasticity, with PAK3 being specifically involved in mental retardation. The main goal of this review is to describe the molecular mechanisms that govern the different functions of group I PAK in neuronal signalling and to discuss the specific functions of each isoform.

A genome-wide RNAi screen to dissect centriole duplication and centrosome maturation in Drosophila

Centrosomes comprise a pair of centrioles surrounded by an amorphous pericentriolar material (PCM). Here, we have performed a microscopy-based genome-wide RNA interference (RNAi) screen in Drosophila cells to identify proteins required for centriole duplication and mitotic PCM recruitment. We analysed 92% of the Drosophila genome (13,059 genes) and identified 32 genes involved in centrosome function. An extensive series of secondary screens classified these genes into four categories: (1) nine are required for centriole duplication, (2) 11 are required for centrosome maturation, (3) nine are required for both functions, and (4) three genes regulate centrosome separation. These 32 hits include several new centrosomal components, some of which have human homologs. In addition, we find that the individual depletion of only two proteins, Polo and Centrosomin (Cnn) can completely block centrosome maturation. Cnn is phosphorylated during mitosis in a Polo-dependent manner, suggesting that the Polo-dependent phosphorylation of Cnn initiates centrosome maturation in flies.

A major goal of the cell cycle is to accurately separate the duplicated chromosomes between two daughter cells. To achieve this, a pair of centrosomes organise a bipolar spindle made of microtubules; the chromosomes line up on the spindle and are then separated to the two spindle poles. Centrosomes are also required for the formation of cilia and flagella, which are present in many eukaryotic cells; centrosome dysfunction is a common feature of many human cancers and several neurological disorders, whereas mutations in genes that affect cilia function give rise to several human diseases. Here, we perform a genome-wide screen using RNA interference to try to identify all of the proteins required for centrosome function in the model organism Drosophila melanogaster (a fruitfly). We identified all 16 of the centrosomal proteins that were already known to be required for centrosome function in Drosophila, as well as 16 new centrosomal components or regulators. We confirmed the centrosomal location of several of the components and performed some analysis of their functions. We believe we are approaching a complete inventory of the proteins required for centrosome function in flies.

An RNAi screen identifies 16 new centrosomal components or regulators inDrosophila, and molecular dissection of their function addresses the role of Polo kinase in the maturation of pericentriolar material.