Mutant Caldesmon lacking cdc2 phosphorylation sites delays M-phase entry and inhibits cytokinesis

Glucocorticoid receptor-induced non-muscle caldesmon regulates metastasis in castration-resistant prostate cancer

Lethal prostate cancer (PCa) is characterized by the presence of metastases and development of resistance to therapies. Metastases form in a multi-step process enabled by dynamic cytoskeleton remodeling. An actin cytoskeleton regulating gene, CALD1, encodes a protein caldesmon (CaD). Its isoform, low-molecular-weight CaD (l-CaD), operates in non-muscle cells, supporting the function of filaments involved in force production and mechanosensing. Several factors, including glucocorticoid receptor (GR), have been identified as regulators of l-CaD in different cell types, but the regulation of l-CaD in PCa has not been defined. PCa develops resistance in response to therapeutic inhibition of androgen signaling by multiple strategies. Known strategies include androgen receptor (AR) alterations, modified steroid synthesis, and bypassing AR signaling, for example, by GR upregulation. Here, we report that in vitro downregulation of l-CaD promotes epithelial phenotype and reduces spheroid growth in 3D, which is reflected in vivo in reduced formation of metastases in zebrafish PCa xenografts. In accordance, CALD1 mRNA expression correlates with epithelial-to-mesenchymal transition (EMT) transcripts in PCa patients. We also show that CALD1 is highly co-expressed with GR in multiple PCa data sets, and GR activation upregulates l-CaD in vitro. Moreover, GR upregulation associates with increased l-CaD expression after the development of resistance to antiandrogen therapy in PCa xenograft mouse models. In summary, GR-regulated l-CaD plays a role in forming PCa metastases, being clinically relevant when antiandrogen resistance is attained by the means of bypassing AR signaling by GR upregulation.

Revisiting the multisite phosphorylation that produces the M-phase supershift of key mitotic regulators

The term M-phase supershift denotes the phosphorylation-dependent substantial increase in the apparent molecular weight of numerous proteins of varied biological functions during M-phase induction. Although the M-phase supershift of multiple key mitotic regulators has been attributed to the multisite phosphorylation catalyzed by the Cdk1/cyclin B/Cks complex, this view is challenged by multiple lines of paradoxical observations. To solve this problem, we reconstituted the M-phase supershift of Xenopus Cdc25C, Myt1, Wee1A, APC3 and Greatwall in Xenopus egg extracts and characterized the supershift-producing phosphorylations. Our results demonstrate that their M-phase supershifts are each due to simultaneous phosphorylation of a considerable portion of S/T/Y residues in a long intrinsically disordered region that is enriched in both S/T residues and S/TP motifs. Although the major mitotic kinases in Xenopus egg extracts, Cdk1, MAPK, Plx1 and RSK2, are able to phosphorylate the five mitotic regulators, they are neither sufficient nor required to produce the M-phase supershift. Accordingly, inhibition of the four major mitotic kinase activities in Xenopus oocytes did not inhibit the M-phase supershift in okadaic acid-induced oocyte maturation. These findings indicate that the M-phase supershift is produced by a previously unrecognized category of mitotic phosphorylation that likely plays important roles in M-phase induction.

Age-Related Alterations in the Testicular Proteome of a Non-Human Primate

Aging of human testis and associated cellular changes is difficult to assess. Therefore, we used a translational, non-human primate model to get insights into underlying cellular and biochemical processes. Using proteomics and immunohistochemistry, we analyzed testicular tissue of young (age 2 to 3) and old (age 10 to 12) common marmosets (Callithrix jacchus). Using a mass spectrometry-based proteomics approach, we identified 63,124 peptides, which could be assigned to 5924 proteins. Among them, we found proteins specific for germ cells and somatic cells, such as Leydig and Sertoli cells. Quantitative analysis showed 31 differentially abundant proteins, of which 29 proteins were more abundant in older animals. An increased abundance of anti-proliferative proteins, among them CDKN2A, indicate reduced cell proliferation in old testes. Additionally, an increased abundance of several small leucine rich repeat proteoglycans and other extracellular matrix proteins was observed, which may be related to impaired cell migration and fibrotic events. Furthermore, an increased abundance of proteins with inhibitory roles in smooth muscle cell contraction like CNN1 indicates functional alterations in testicular peritubular cells and may mirror a reduced capacity of these cells to contract in old testes.

Cell adhesion is regulated by CDK1 during the cell cycle

In most tissues, anchorage-dependent growth and cell cycle progression are dependent on cells engaging extracellular matrices (ECMs) via integrin-receptor adhesion complexes. In a highly conserved manner, cells disassemble adhesion complexes, round up, and retract from their surroundings before division, suggestive of a primordial link between the cell cycle machinery and the regulation of cell adhesion to the ECM. In this study, we demonstrate that cyclin-dependent kinase 1 (CDK1) mediates this link. CDK1, in complex with cyclin A2, promotes adhesion complex and actin cytoskeleton organization during interphase and mediates a large increase in adhesion complex area as cells transition from G1 into S. Adhesion complex area decreases in G2, and disassembly occurs several hours before mitosis. This loss requires elevated cyclin B1 levels and is caused by inhibitory phosphorylation of CDK1-cyclin complexes. The inactivation of CDK1 is therefore the trigger that initiates remodeling of adhesion complexes and the actin cytoskeleton in preparation for rapid entry into mitosis.

Leupaxin stimulates adhesion and migration of prostate cancer cells through modulation of the phosphorylation status of the actin-binding protein caldesmon

The focal adhesion protein leupaxin (LPXN) is overexpressed in a subset of prostate cancers (PCa) and is involved in the progression of PCa. In the present study, we analyzed the LPXN-mediated adhesive and cytoskeletal changes during PCa progression. We identified an interaction between the actin-binding protein caldesmon (CaD) and LPXN and this interaction is increased during PCa cell migration. Furthermore, knockdown of LPXN did not affect CaD expression but reduced CaD phosphorylation. This is known to destabilize the affinity of CaD to F-actin, leading to dynamic cell structures that enable cell motility. Thus, downregulation of CaD increased migration and invasion of PCa cells. To identify the kinase responsible for the LPXN-mediated phosphorylation of CaD, we used data from an antibody array, which showed decreased expression of TGF-beta-activated kinase 1 (TAK1) after LPXN knockdown in PC-3 PCa cells. Subsequent analyses of the downstream kinases revealed the extracellular signal-regulated kinase (ERK) as an interaction partner of LPXN that facilitates CaD phosphorylation during LPXN-mediated PCa cell migration. In conclusion, we demonstrate that LPXN directly influences cytoskeletal dynamics via interaction with the actin-binding protein CaD and regulates CaD phosphorylation by recruiting ERK to highly dynamic structures within PCa cells.

Caldesmon regulates axon extension through interaction with myosin II

To begin the process of forming neural circuits, new neurons first establish their polarity and extend their axon. Axon extension is guided and regulated by highly coordinated cytoskeletal dynamics. Here we demonstrate that in hippocampal neurons, the actin-binding protein caldesmon accumulates in distal axons, and its N-terminal interaction with myosin II enhances axon extension. In cortical neural progenitor cells, caldesmon knockdown suppresses axon extension and neuronal polarity. These results indicate that caldesmon is an important regulator of axon development.

Caldesmon regulates actin dynamics to influence cranial neural crest migration in Xenopus

A nonmuscle caldesmon (CaD) is highly expressed in premigratory and migrating Xenopus cranial neural crest cells. A loss-of-function approach shows that CaD is critical for neural crest migration. The results further suggest that CaD influences cell morphology and motility by modulating actin dynamics in neural crest cells.

Caldesmon (CaD) is an important actin modulator that associates with actin filaments to regulate cell morphology and motility. Although extensively studied in cultured cells, there is little functional information regarding the role of CaD in migrating cells in vivo. Here we show that nonmuscle CaD is highly expressed in both premigratory and migrating cranial neural crest cells of Xenopus embryos. Depletion of CaD with antisense morpholino oligonucleotides causes cranial neural crest cells to migrate a significantly shorter distance, prevents their segregation into distinct migratory streams, and later results in severe defects in cartilage formation. Demonstrating specificity, these effects are rescued by adding back exogenous CaD. Interestingly, CaD proteins with mutations in the Ca²⁺-calmodulin–binding sites or ErK/Cdk1 phosphorylation sites fail to rescue the knockdown phenotypes, whereas mutation of the PAK phosphorylation site is able to rescue them. Analysis of neural crest explants reveals that CaD is required for the dynamic arrangements of actin and, thus, for cell shape changes and process formation. Taken together, these results suggest that the actin-modulating activity of CaD may underlie its critical function and is regulated by distinct signaling pathways during normal neural crest migration.

Caldesmon over-expression in type 1 diabetic nephropathy

Substantial evidence supports a genetic susceptibility to develop nephropathy in type 1 diabetes and a key pathogenic role of actin cytoskeleton dysfunction in this complication. We previously reported that many cytoskeletal proteins were either up- or down-regulated in fibroblast cells from type 1 diabetic (T1DM) patients with nephropathy. The gene of one of these proteins, caldesmon, lies in a chromosomal region linked to nephropathy and its promoter region contains a single nucleotide polymorphism that is associated with nephropathy. Hence, we analyzed caldesmon gene and protein expression in cultured fibroblasts from T1DM patients with and without nephropathy and from control subjects. Caldesmon gene was studied in cells cultured under normal glucose levels by quantitative real-time RT-PCR. Caldesmon protein isoforms were quantified both under normal and high glucose conditions by two-dimensional electrophoresis. Caldesmon gene was over-expressed in fibroblasts from diabetic patients with nephropathy, in comparison to both those from diabetic patients without nephropathy and those from controls. We quantified six caldesmon protein isoforms, two of them were increased whereas another one was decreased only in fibroblasts from diabetic patients with nephropathy. None of these isoforms showed any difference in their relative abundance in response to high glucose. Variable results in response to high glucose were observed in the expression of other proteins in the three experimental groups. Our data lend further support to an involvement of caldesmon in the susceptibility to diabetic nephropathy in type 1 diabetes, independently from environmental glucose levels.

Diversification of caldesmon-linked actin cytoskeleton in cell motility

The actin cytoskeleton plays a key role in regulating cell motility. Caldesmon (CaD) is an actin-linked regulatory protein found in smooth muscle and non-muscle cells that is conserved among a variety of vertebrates. It binds and stabilizes actin filaments, as well as regulating actin-myosin interaction in a calcium (Ca2+)/calmodulin (CaM)- and/or phosphorylation-dependent manner. CaD function is regulated qualitatively by Ca2+/CaM and by its phosphorylation state and quantitatively at the mRNA level, by three different transcriptional regulation of the CALD1 gene. CaD has numerous functions in cell motility, such as migration, invasion, and proliferation, exerted via the reorganization of the actin cytoskeleton. Here we will outline recent findings regarding CaD's structural features and functions.

Mechanics and regulation of cell shape during the cell cycle

Many cell types undergo dramatic changes in shape throughout the cell cycle. For individual cells, a tight control of cell shape is crucial during cell division, but also in interphase, for example during cell migration. Moreover, cell cycle-related cell shape changes have been shown to be important for tissue morphogenesis in a number of developmental contexts. Cell shape is the physical result of cellular mechanical properties and of the forces exerted on the cell. An understanding of the causes and repercussions of cell shape changes thus requires knowledge of both the molecular regulation of cellular mechanics and how specific changes in cell mechanics in turn effect global shape changes. In this chapter, we provide an overview of the current knowledge on the control of cell morphology, both in terms of general cell mechanics and specifically during the cell cycle.

Phosphorylation of Caldesmon by PFTAIRE1 kinase promotes actin binding and formation of stress fibers

Caldesmon (CaD) is an actin-binding protein that is capable of stabilizing actin filaments. Phosphorylation of CaD is widely accepted in the actin cytoskeletal modeling and promotion of cell migration. In this study, we show that CaD is a downstream phosphorylation substrate of PFTK1, a novel Cdc-2-related ser/thr protein kinase. Our study stemmed from an earlier investigation where we demonstrated that PFTK1 kinase conferred cell migratory advantages in human hepatocellular carcinoma (HCC) cells. Here, we showed that PFTK1-knockdown cells exhibited much reduced CaD phosphorylation and consequently caused dissociation of CaD from the F-actin fibers. The cellular localization of CaD was also altered in the absence of PFTK1. Immunofluorescence analysis revealed that PFTK1-abrogated cells exhibited a diffused and blurred appearance of CaD localization, whereas intact co-localization with F-actins was apparent in PFTK1-expressing cells. Without the binding of CaD to actin, disappearance of actin stress fibers was also evident in PFTK1-abrogated cells. In addition, we found that CaD is also commonly up-regulated in HCC tumors when compared to adjacent non-malignant liver (P = 0.022). Taken together, our results highlight a novel biological cascade that involved the phosphorylation activation of CaD by PFTK1 kinase in promoting formation of actin stress fibers.

Activation of cyclin B1-Cdk1 synchronizes events in the nucleus and the cytoplasm at mitosis

The cyclin B-Cdk1 kinase triggers mitosis in most eukaryotes. In animal cells, cyclin B shuttles between the nucleus and cytoplasm in interphase before rapidly accumulating in the nucleus at prophase, which promotes disassembly of the nuclear lamina and nuclear envelope breakdown (NEBD). What triggers the nuclear accumulation of cyclin B1 is presently unclear, although the prevailing view is that the Plk1 kinase inhibits its nuclear export. In this study, we use a biosensor specific for cyclin B1-Cdk1 activity to show that activating cyclin B1-Cdk1 immediately triggers its rapid accumulation in the nucleus through a 40-fold increase in nuclear import that remains dependent on Cdk1 activity until NEBD. Nevertheless, a substantial proportion of cyclin B1-Cdk1 remains in the cytoplasm. The increase in nuclear import is driven by changes in the nuclear import machinery that require neither Plk1 nor inhibition of nuclear export. Thus, the intrinsic link between cyclin B1-Cdk1 activation and its rapid nuclear import inherently coordinates the reorganization of the nucleus and the cytoplasm at mitotic entry.

Caldesmon regulates the motility of vascular smooth muscle cells by modulating the actin cytoskeleton stability

Background

Migration of vascular smooth muscle cells (SMCs) from the media to intima constitutes a critical step in the development of proliferative vascular diseases. To elucidate the regulatory mechanism of vacular SMC motility, the roles of caldesmon (CaD) and its phosphorylation were investigated.

Methods

We have performed Transwell migration assays, immunofluorescence microscopy, traction microscopy and cell rounding assays using A7r5 cells transfected with EGFP (control), EGFP-wtCaD or phosphomimetic CaD mutants, including EGFP-A1A2 (the two PAK sites Ser452 and Ser482 converted to Ala), EGFP-A3A4 (the two Erk sites Ser497 and Ser527 converted to Ala), EGFP-A1234 (both PAK- and Erk-sites converted to Ala) and EGFP-D1234 (both PAK- and Erk-sites converted to Asp).

Results

We found that cells transfected with wtCaD, A1A2 or A3A4 mutants of CaD migrated at a rate approximately 50% more slowly than those EGFP-transfected cells. The migration activity for A1234 cells was only about 13% of control cells. Thus it seems both MAPK and PAK contribute to the motility of A7r5 cells and the effects are comparable and additive. The A1234 mutant also gave rise to highest strain energy and lowest rate of cell rounding. The migratory and contractile properties of these cells are consistent with stabilized actin cytoskeletal structures. Indeed, the A1234 mutant cells exhibited most robust stress fibers, whereas cells transfected with wtCaD or A3A4 (and A1A2) had moderately reinforced actin cytoskeleton. The control cells (transfected with EGFP alone) exhibited actin cytoskeleton that was similar to that in untransfected cells, and also migrated at about the same speed as the untransfected cells.

Conclusions

These results suggest that both the expression level and the level of MAPK- and/or PAK-mediated phosphorylation of CaD play key roles in regulating the cell motility by modulating the actin cytoskeleton stability in dedifferentiated vascular SMCs such as A7r5.

Abnormal cytoskeletal protein expression in cultured skin fibroblasts from type 1 diabetes mellitus patients with nephropathy: A proteomic approach

Diabetic nephropathy (DN) develops in about 40% of insulin-dependent type 1 diabetes mellitus (T1DM) patients, and is associated not only with diabetes duration and metabolic control, but also with a genetic predisposition. Constitutive alterations of cytoskeletal proteins may play a role in the development of DN. We investigated the expression of these proteins in cultured skin fibroblasts, obtained from long-term T1DM patients with and without DN but comparable metabolic control, and from matched healthy subjects, by means of 2-DE electrophoresis and MS-MALDI analyses. In T1DM with DN, compared to the other two groups, quantitative analyses revealed an altered expression of 17 spots (p<0.05-p<0.01), corresponding to 12 unique proteins. In T1DM with DN, beta-actin and three isoforms of tubulin beta-2 chain, tropomodulin-3, and LASP-1 were decreased, whereas two tubulin beta-4 chain isoforms, one alpha actinin-4 isoform, membrane-organizing extension spike protein (MOESIN), FLJ00279 (corresponding to a fragment of myosin heavy chain, non-muscle type A), vinculin, a tropomyosin isoform, and the macrophage capping protein were increased. A shift in caldesmon isoforms was also detected. These results demonstrate an association between DN and the constitutive expression of cytoskeleton proteins in cultured skin fibroblasts from T1DM with DN, which may retain pathophysiologycal implications.

Caldesmon is necessary for maintaining the actin and intermediate filaments in cultured bladder smooth muscle cells

Caldesmon (CaD), a component of microfilaments in all cells and thin filaments in smooth muscle cells, is known to bind to actin, tropomyosin, calmodulin, and myosin and to inhibit actin-activated ATP hydrolysis by smooth muscle myosin. Thus, it is believed to regulate smooth muscle contraction, cell motility and the cytoskeletal structure. Using bladder smooth muscle cell cultures and RNA interference (RNAi) technique, we show that the organization of actin into microfilaments in the cytoskeleton is diminished by siRNA-mediated CaD silencing. CaD silencing significantly decreased the amount of polymerized actin (F-actin), but the expression of actin was not altered. Additionally, we find that CaD is associated with 10 nm intermediate-sized filaments (IF) and in vitro binding assay reveals that it binds to vimentin and desmin proteins. Assembly of vimentin and desmin into IF is also affected by CaD silencing, although their expression is not significantly altered when CaD is silenced. Electronmicroscopic analyses of the siRNA-treated cells showed the presence of myosin filaments and a few surrounding actin filaments, but the distribution of microfilament bundles was sparse. Interestingly, the decrease in CaD expression had no effect on tubulin expression and distribution of microtubules in these cells. These results demonstrate that CaD is necessary for the maintenance of actin microfilaments and intermediate-sized filaments in the cytoskeletal structure. This finding raises the possibility that the cytoskeletal structure in smooth muscle is affected when CaD expression is altered, as in smooth muscle de-differentiation and hypertrophy seen in certain pathological conditions.

Caldesmon and the regulation of cytoskeletal functions

Caldesmon (CaD) is an extraordinary actin-binding protein, because in addition to actin, it also bindsmyosin, calmodulin and tropomyosin. As a component of the smoothmuscle and nonmuscle contractile apparatus CaD inhibits the actomyosin ATPase activity and its inhibitory action is modulated by both Ca2+ and phosphorylation. The multiplicity of binding partners and diverse biochemical properties suggest CaD is a potent and versatile regulatory protein both in contractility and cell motility. However, after decades ofinvestigation in numerous laboratories, hard evidence is still lacking to unequivocally identify its in vivo functions, although indirect evidence is mounting to support an important role in connection with the actin cytoskeleton. This chapter reviews the highlights of the past findings and summarizes the current views on this protein, with emphasis of its interaction with tropomyosin.

ARL4D recruits cytohesin-2/ARNO to modulate actin remodeling

ARL4D is a developmentally regulated member of the ADP-ribosylation factor/ARF-like protein (ARF/ARL) family of Ras-related GTPases. Although the primary structure of ARL4D is very similar to that of other ARF/ARL molecules, its function remains unclear. Cytohesin-2/ARF nucleotide-binding-site opener (ARNO) is a guanine nucleotide-exchange factor (GEF) for ARF, and, at the plasma membrane, it can activate ARF6 to regulate actin reorganization and membrane ruffling. We show here that ARL4D interacts with the C-terminal pleckstrin homology (PH) and polybasic c domains of cytohesin-2/ARNO in a GTP-dependent manner. Localization of ARL4D at the plasma membrane is GTP- and N-terminal myristoylation-dependent. ARL4D(Q80L), a putative active form of ARL4D, induced accumulation of cytohesin-2/ARNO at the plasma membrane. Consistent with a known action of cytohesin-2/ARNO, ARL4D(Q80L) increased GTP-bound ARF6 and induced disassembly of actin stress fibers. Expression of inactive cytohesin-2/ARNO(E156K) or small interfering RNA knockdown of cytohesin-2/ARNO blocked ARL4D-mediated disassembly of actin stress fibers. Similar to the results with cytohesin-2/ARNO or ARF6, reduction of ARL4D suppressed cell migration activity. Furthermore, ARL4D-induced translocation of cytohesin-2/ARNO did not require phosphoinositide 3-kinase activation. Together, these data demonstrate that ARL4D acts as a novel upstream regulator of cytohesin-2/ARNO to promote ARF6 activation and modulate actin remodeling.

Reorganization of the actin cytoskeleton via transcriptional regulation of cytoskeletal/focal adhesion genes by myocardin-related transcription factors (MRTFs/MAL/MKLs)

RhoA is a crucial regulator of stress fiber and focal adhesion formation through the activation of actin nucleation and polymerization. It also regulates the nuclear translocation of myocardin-related transcription factor-A and -B (MRTF-A/B, MAL or MKL 1/2), which are co-activators of serum response factor (SRF). In dominant-negative MRTF-A (DN-MRTF-A)-expressing NIH 3T3 cell lines, the expressions of several cytoskeletal/focal adhesion genes were down-regulated, and the formation of stress fiber and focal adhesion was severely diminished. MRTF-A/B-knockdown cells also exhibited such cytoskeletal defects. In reporter assays, both RhoA and MRTF-A enhanced promoter activities of these genes in a CArG-box-dependent manner, and DN-MRTF-A inhibited the RhoA-mediated activation of these promoters. In dominant-negative RhoA (RhoA-N19)-expressing NIH 3T3 cell lines, the nuclear translocation of MRTF-A/B was predominantly prevented, resulting in the reduced expression of cytoskeletal/focal adhesion proteins. Further, constitutive-active MRTF-A/B increased the expression of endogenous cytoskeletal/focal adhesion proteins, and thereby rescued the defective phenotype of stress fibers and focal adhesions in RhoA-N19 expressing cells. These results indicate that MRTF-A/B act as pivotal mediators of stress fiber and focal adhesion formation via the transcriptional regulation of a subset of cytoskeletal/focal adhesion genes.

Cyclin B1/Cdk1 binds and phosphorylates Filamin A and regulates its ability to cross-link actin

Substantial actin remodelling occurs prior to mitosis as cells alter their shape in preparation for cytokinesis. In mammalian cells, mitosis is initiated by a heterodimer of cyclin B1 and the cyclin dependent kinase 1 (Cdk1) serine/threonine kinase. In this report. we show that human cyclin B1 binds the actin cross-linking protein Filamin-A (FLNa). The proteins co-immunoprecipitate and co-localize in mitotic human cells. We find that cyclin B1/Cdk1 can phosphorylate FLNa in vitro and reduce its ability to gelate actin. We have also identified serine 1436 as one FLNa residue phosphorylated by cyclin B1/Cdk1 in vitro. Our results suggest a role for cyclin B1/Cdk1 in FLNa-dependent actin remodelling.

Cdc2-mediated Schwann cell migration during peripheral nerve regeneration

Schwann cell migration facilitates peripheral nerve regeneration after injury. We have recently found increased activation of Cdc2 kinase in regenerating sciatic nerves. Here we show that Cdc2 phosphorylation of caldesmon regulates Schwann cell migration and nerve regeneration. A robust but transient increase in Cdc2 expression was found in cultured Schwann cells prepared from the sciatic nerve in rats that had undergone crush injury for 7 days. These `injury-preconditioned' Schwann cells exhibited enhanced migration compared with non-preconditioned control cells and treatment with the cdk inhibitor roscovitine prevented cell migration. After transduction with recombinant Cdc2 DNA adenoviral vectors, Schwann cells were implanted into sciatic nerves; those expressing wild-type Cdc2 migrated further in the distal direction than those expressing dominant-negative Cdc2. We identified caldesmon as a downstream substrate of Cdc2 in Schwann cells and its phosphorylation by Cdc2 changed its subcellular localization. Overexpression of dominant-negative caldesmon significantly counteracted the migration effect caused by Cdc2. Finally, neurite outgrowth of cultured DRG sensory neurons, facilitated by co-culture with injury-preconditioned Schwann cells, was suppressed by roscovitine treatment. The results indicate that activation of the Cdc2-caldesmon pathway is necessary for Schwann cell migration and suggest a role for this pathway in peripheral axonal growth.

Tropomyosin and caldesmon regulate cytokinesis speed and membrane stability during cell division

The contractile ring and the cell cortex generate force to divide the cell while maintaining symmetrical shape. This requires temporal and spatial regulation of the actin cytoskeleton at these areas. We force-expressed misregulated versions of actin-binding proteins, tropomyosin and caldesmon, into cells and analyzed their effects on cell division. Cells expressing proteins that increase actomyosin ATPase, such as human tropomyosin chimera (hTM5/3), significantly speed up division, whereas cells expressing proteins that inhibit actomyosin, such as caldesmon mutants defective in Ca(2+)/calmodulin binding (CaD39-AB) and in cdk1 phosphorylation sites (CaD39-6F), divide slowly. hTM5 and hTM5/3-expressing cells lift one daughter cell off the substrate and twist. Furthermore, CaD39-AB- and CaD39-6F-expressing cells are sensitive to hypotonic swelling and show severe blebbing during division, whereas hTM5/3-expressing cells are resistant to hypotonic swelling and produce membrane bulges. These results support a model where Ca(2+)/calmodulin and cdk1 dynamically control caldesmon inhibition of tropomyosin-activated actomyosin to regulate division speed and to suppress membrane blebs.

Caldesmon effects on the actin cytoskeleton and cell adhesion in cultured HTM cells

Caldesmon is a multifunctional ubiquitous regulator of the actin cytoskeleton, which can affect both actomyosin contractility and actin polymerization. Previous studies showed that caldesmon over-expression in cultured fibroblasts produces effects that resemble those of chemical inhibitors of cellular contractility. Since these inhibitors (H-7, Y-27632, etc.) have been shown to lower intraocular pressure and increase outflow facility from the anterior chamber of the eye, we proposed that caldesmon might be used for gene therapy of glaucoma. In the present study we examined the effects of expression of adenovirus-delivered rat non-muscle caldesmon fused with green fluorescent protein (AdCaldGFP) on the actin cytoskeleton and matrix adhesions in cultured human trabecular meshwork (HTM) cells. In addition, we assessed the effect of caldesmon on the stability of cell-cell junctions in kidney epithelial MDCK cells. Cultured HTM cells demonstrate a well-developed actin cytoskeleton, comprising mainly arrays of parallel actomyosin bundles (stress fibers). Lamellipodial protrusions containing dense actin networks are also observed. Cell-matrix adhesions are dominated by focal adhesions (FAs) associated with the ends of the stress fibers, focal complexes in lamellipodia, and fibrillar adhesions in the central part of the spread cells. Treatment of HTM cells with AdCaldGFP resulted in dose-dependent morphological changes within 24-48 hr post-infection. Cells expressing moderate levels of caldesmon exhibited straight bundles containing actin and myosin II, which were considerably shorter than those in control cells. Short filament bundles in caldesmon over-expressing cells formed arrays consisting of triangular actin structures with small vinculin-positive FAs at their vertices. In addition, the fraction of cells displaying large lamellipodia increased. About 40-50% of the population of caldesmon-expressing cells demonstrated high levels of GFP-caldesmon expression and severe changes in the actin cytoskeleton, manifested by the disappearance of stress fibers and the formation of curved actin- and myosin-containing bundles. These bundles formed together a dynamic network consisting of pulsating loops filling the entire cytoplasm. Addition of thapsigargin, which increases intracellular Ca++ concentration, resulted in a straightening of the curved bundles. Another type of novel actin structures induced by caldesmon over-expression were highly dynamic circular waves that propagated over the affected cells with a velocity about 10 microm min. In cells with disrupted stress fibers, vinculin-containing FAs and tensin-rich fibrillar adhesions had also essentially vanished. However, phosphotyrosine-positive focal complexes were still prominent throughout the lamellipodia of these cells. Over-expression of caldesmon in MDCK cells reduced, in a dose dependent manner, the beta-catenin content at cell-cell adherens junctions and in some cases led to physical disruption of adherens junctions. Thus, caldesmon over-expression induces unique reorganization of the actin cytoskeleton in affected cells, accompanied by disruption of focal and fibrillar cell-matrix adhesions, and destabilization of cell-cell adherens junctions. Inducing such changes in the contractility and actin cytoskeleton of HTM cells in glaucomatous eyes in vivo could produce a therapeutically useful increase in outflow facility.

Phosphorylation of caldesmon during smooth muscle contraction and cell migration or proliferation

The actin-binding protein caldesmon (CaD) exists both in smooth muscle (the heavy isoform, h-CaD) and non-muscle cells (the light isoform, l-CaD). In smooth muscles h-CaD binds to myosin and actin simultaneously and modulates the actomyosin interaction. In non-muscle cells l-CaD binds to actin and stabilizes the actin stress fibers; it may also mediate the interaction between actin and non-muscle myosins. Both h- and l-CaD are phosphorylated in vivo upon stimulation. The major phosphorylation sites of h-CaD when activated by phorbol ester are the Erk-specific sites, modification of which is attenuated by the MEK inhibitor PD98059. The same sites in l-CaD are also phosphorylated when cells are stimulated to migrate, whereas in dividing cells l-CaD is phosphorylated more extensively, presumably by cdc2 kinase. Both Erk and cdc2 are members of the MAPK family. Thus it appears that CaD is a downstream effector of the Ras signaling pathways. Significantly, the phosphorylatable serine residues shared by both CaD isoforms are in the C-terminal region that also contains the actin-binding sites. Biochemical and structural studies indicated that phosphorylation of CaD at the Erk sites is accompanied by a conformational change that partially dissociates CaD from actin. Such a structural change in h-CaD exposes the myosin-binding sites on the actin surface and allows actomyosin interactions in smooth muscles. In the case of non-muscle cells, the change in l-CaD weakens the stability of the actin filament and facilitates its disassembly. Indeed, the level of l-CaD modification correlates very well in a reciprocal manner with the level of actin stress fibers. Since both cell migration and cell division require dynamic remodeling of actin cytoskeleton that leads to cell shape changes, phosphorylation of CaD may therefore serve as a plausible means to regulate these processes. Thus CaD not only links the smooth muscle contractility and non-muscle motility, but also provides a common mechanism for the regulation of cell migration and cell proliferation.

Caldesmon is an integral component of podosomes in smooth muscle cells

Podosomes are highly dynamic actin-based structures commonly found in motile and invasive cells such as macrophages, osteoclasts and vascular smooth muscle cells. Here, we have investigated the role of caldesmon, an actin-binding protein, in the formation of podosomes in aortic smooth muscle A7r5 cells induced by the phorbol ester PDBu. We found that endogenous low molecular weight caldesmon (l-caldesmon), which was normally localised to actin-stress fibres and membrane ruffles, was recruited to the actin cores of PDBu-induced podosomes. Overexpression of l-caldesmon in A7r5 cells caused dissociation of actin-stress fibres and disruption of focal adhesion complexes, and significantly reduced the ability of PDBu to induce podosome formation. By contrast, siRNA interference of caldesmon expression enhanced PDBu-induced formation of podosomes. The N-terminal fragment of l-caldesmon, CaD40, which contains the myosin-binding site, did not label stress fibres and was not translocated to PDBu-induced podosomes. Cad39, the C-terminal fragment housing the binding sites for actin, tropomyosin and calmodulin, was localised to stress fibres and was translocated to podosomes induced by PDBu. The caldesmon mutant, CadCamAB, which does not interact with Ca2+/calmodulin, was not recruited to PDBu-induced podosomes. These results show that (1) l-caldesmon is an integral part of the actin-rich core of the podosome; (2) overexpression of l-caldesmon suppresses podosome formation, whereas siRNA knock-down of l-caldesmon facilitates its formation; and (3) the actin-binding and calmodulin-binding sites on l-caldesmon are essential for the translocation of l-caldesmon to the podosomes. In summary, this data suggests that caldesmon may play a role in the regulation of the dynamics of podosome assembly and that Ca2+/calmodulin may be part of a regulatory mechanism in podosome formation.

Requirement of reversible caldesmon phosphorylation at P21-activated kinase-responsive sites for lamellipodia extensions during cell migration

Caldesmon is believed to be one of the key regulators for actin dynamics and thereby cell polarity, membrane extension, and cell motility. We have shown previously that stress fiber formation and cell movement are severely impaired in the cells expressing human fibroblast caldesmon fragment defective in Ca2+/CaM binding sites. Both Ser458 and Ser489, adjacent to the Ca2+/CaM-binding sites, are phosphorylated by p21-activated kinase (PAK) in vitro. Here we report that Ser458 is phosphorylated in response to cell movement. We substituted Ser458 and Ser489 on C-terminal caldesmon (CaD39) with alanine or glutamic acid to mimic under-phosphorylated (CaD39-PAKA) or constitutively phosphorylated (CaD39-PAKE) caldesmon. In vitro, CaD39-PAKE, but not CaD39-PAKA, fails to inhibit myosin ATPase activity and exhibits reduced binding to Ca2+/CaM. When stably expressed in Chinese Hamster Ovary cells, both CaD39-PAKA and CaD39-PAKE incorporate into stress fibers and localize to the leading edge of the migrating cell. Expression of CaD39-PAKE, but not CaD39-PAKA, fails to protect stress fibers from cytochalasin depolymerization. However, both mutations inhibit cell polarization and lead to defects in membrane extension and cell migration. We conclude that phosphorylation of caldesmon by PAK is a dynamic process required to regulate actin dynamics and membrane protrusions in wound-induced cell migration.

Caldesmon phosphorylation in actin cytoskeletal remodeling

Caldesmon is an actin-binding protein that is capable of stabilizing actin filaments against actin-severing proteins, inhibiting actomyosin ATPase activity, and inhibiting Arp2/3-mediated actin polymerization in vitro. Caldesmon is a substrate of cdc2 kinase and Erk1/2 MAPK, and phosphorylation by either of these kinases reverses the inhibitory effects of caldesmon. Cdc2-mediated caldesmon phosphorylation and the resulting dissociation of caldesmon from actin filaments are essential for M-phase progression during mitosis. Cells overexpressing the actin-binding carboxyterminal fragment of caldesmon fail to release the fragment completely from actin filaments during mitosis, resulting in a higher frequency of multinucleated cells. PKC-mediated MEK/Erk/caldesmon phosphorylation is an important signaling cascade in the regulation of smooth muscle contraction. Furthermore, PKC activation has been shown to remodel actin stress fibers into F-actin-enriched podosome columns in cultured vascular smooth muscle cells. Podosomes are cytoskeletal adhesion structures associated with the release of metalloproteases and degradation of extracellular matrix during cell invasion. Interestingly, caldesmon is one of the few actin-binding proteins that is associated with podosomes but excluded from focal adhesions. Caldesmon also inhibits the function of gelsolin and Arp2/3 complex that are essential for the formation of podosomes. Thus, caldesmon appears to be well positioned for playing a modulatory role in the formation of podosomes. Defining the roles of actin filament-stabilizing proteins such as caldesmon and tropomyosin in the formation of podosomes should provide a more complete understanding of molecular systems that regulate the remodeling of the actin cytoskeleton in cell transformation and invasion.

Regulation of microfilament organization by Kaposi sarcoma-associated herpes virus-cyclin.CDK6 phosphorylation of caldesmon

Kaposi sarcoma-associated herpes virus (KSHV) encodes a D-like cyclin (K-cyclin) that is thought to contribute to the viral oncogenicity. K-cyclin activates cellular cyclin-dependent kinases (CDK) 4 and 6, generating enzymes with a substrate selectivity deviant from CDK4 and CDK6 activated by D-type cyclins, suggesting different biochemical and biological functions. Here we report the identification of the actin- and calmodulin-binding protein caldesmon (CALD1) as a novel K-cyclin.CDK substrate, which is not phosphorylated by D.CDK. CALD1 plays a central role in the regulation of microfilament organization, consequently controlling cell shape, adhesion, cytokinesis and motility. K-cyclin.CDK6 specifically phosphorylates four Ser/Thr sites in the human CALD1 carboxyl terminus, abolishing CALD1 binding to its effector protein, actin, and its regulator protein, calmodulin. CALD1 is hyperphosphorylated in cells following K-cyclin expression and in KSHV-transformed lymphoma cells. Moreover, expression of exogenous K-cyclin results in microfilament loss and changes in cell morphology; both effects are reliant on CDK catalysis and can be reversed by the expression of a phosphorylation defective CALD1. Together, these data strongly suggest that K-cyclin expression modulates the activity of caldesmon and through this the microfilament functions in cells. These results establish a novel link between KSHV infection and the regulation of the actin cytoskeleton.

Specific disruption of smooth muscle caldesmon expression in mice

Caldesmon (CaD) is an actin-binding protein that is capable of inhibiting the actomyosin ATPase activity in vitro. CaD has a single gene that is alternatively spliced to generate the smooth muscle-specific form, h-CaD, and a shorter isoform, l-CaD, that is present only in non-muscle cells. The difference between h- and l-CaD is a highly charged repeating sequence, corresponding to a 35 nm-long single helical region that separates the N-terminal domain from the C-terminal domain of h-CaD. To test whether such an elongated h-CaD is essential for smooth muscles to function properly, we have specifically abrogated its expression in the mouse by targeting h-CaD without affecting the expression of l-CaD. After genotyping, we have obtained homozygous knockout mice that indeed lack h-CaD, but nevertheless express varying amounts of l-CaD in a tissue-dependent fashion. The contractility of smooth muscles isolated from the knockout animals is currently under investigation.

Caldesmon tethers myosin V to actin and facilitates in vitro motility

The current study examines the hypothesis that caldesmon can facilitate the interaction of myosin V with actin filaments by tethering myosin V to actin. Myosin V, purified from bovine brain stem, translocated actin filaments in an in vitro motility assay at a velocity of 0.30+/-0.05 microm/s in the absence of caldesmon at a myosin concentration of 50 microg/ml (ionic strength=110 mM). Filament binding and motility was absent when the myosin concentration applied to the coverslip was reduced to 5 microg/ml, however, the addition of 0.4 microM caldesmon restored binding and motility (0.28+/-0.06 microm/s). This restoration of motility in the presence of caldesmon was blocked by an N-terminal fragment of caldesmon that competitively inhibits the binding of intact caldesmon to myosin. Similar to previous findings with both smooth muscle myosin and platelets (Haeberle et al., 1992; Hemric et al., 1994), these results demonstrate that caldesmon can form a mobile tether that maintains the proximity of myosin V with actin filaments without restricting filament sliding.

Phosphorylation of actopaxin regulates cell spreading and migration

Actopaxin is an actin and paxillin binding protein that localizes to focal adhesions. It regulates cell spreading and is phosphorylated during mitosis. Herein, we identify a role for actopaxin phosphorylation in cell spreading and migration. Stable clones of U2OS cells expressing actopaxin wild-type (WT), nonphosphorylatable, and phosphomimetic mutants were developed to evaluate actopaxin function. All proteins targeted to focal adhesions, however the nonphosphorylatable mutant inhibited spreading whereas the phosphomimetic mutant cells spread more efficiently than WT cells. Endogenous and WT actopaxin, but not the nonphosphorylatable mutant, were phosphorylated in vivo during cell adhesion/spreading. Expression of the nonphosphorylatable actopaxin mutant significantly reduced cell migration, whereas expression of the phosphomimetic increased cell migration in scrape wound and Boyden chamber migration assays. In vitro kinase assays demonstrate that extracellular signal-regulated protein kinase phosphorylates actopaxin, and treatment of U2OS cells with the MEK1 inhibitor UO126 inhibited adhesion-induced phosphorylation of actopaxin and also inhibited cell migration.

Caldesmon mutant defective in Ca2+-calmodulin binding interferes with assembly of stress fibers and affects cell morphology, growth and motility

Despite intensive in vitro studies, little is known about the regulation of caldesmon (CaD) by Ca2+-calmodulin (Ca2+-CaM) in vivo. To investigate this regulation, a mutant was generated of the C-terminal fragment of human fibroblast CaD, termed CaD39-AB, in which two crucial tryptophan residues involved in Ca2+-CaM binding were each replaced with alanine. The mutation abolished most CaD39-AB binding to Ca2+-CaM in vitro but had little effect on in vitro binding to actin filaments and the ability to inhibit actin/tropomyosin-activated heavy meromyosin ATPase. To study the functional consequences of these mutations in vivo, we transfected an expression plasmid carrying CaD39-AB cDNA into Chinese hamster ovary (CHO) cells and isolated several clones expressing various amounts of CaD39-AB. Immunofluorescence microscopy revealed that mutant CaD39-AB was distributed diffusely throughout the cytoplasm but also concentrated at membrane ruffle regions. Stable expression of CaD39-AB in CHO cells disrupted assembly of stress fibers and focal adhesions, altered cell morphology, and slowed cell cycle progression. Moreover, CaD39-AB-expressing cells exhibited motility defects in a wound-healing assay, in both velocity and the persistence of translocation, suggesting a role for CaD regulation by Ca2+-CaM in cell migration. Together, these results demonstrate that CaD plays a crucial role in mediating the effects of Ca2+-CaM on the dynamics of the actin cytoskeleton during cell migration.

Association between variation in the actin-binding gene caldesmon and diabetic nephropathy in type 1 diabetes

Dysfunction of the actin cytoskeleton is a key event in the pathogenesis of diabetic nephropathy. We previously reported that certain cytoskeletal genes are upregulated in mesangial cells exposed to a high extracellular glucose concentration. One such gene, caldesmon, lies on chromosome 7q35, a region linked to nephropathy in family studies, making it a candidate susceptibility gene for diabetic nephropathy. We screened all exons, untranslated regions, and a 5-kb region upstream of the gene for variation using denaturing high-performance liquid chromatography technology. An A>G single nucleotide polymorphism (SNP) at position -579 in the promoter region was associated with nephropathy in a case-control study using 393 type 1 diabetic patients from Northern Ireland (odds ratio [OR] 1.38, 95% CI 1.02-1.86, P = 0.03). A similar trend was found in an independent sample from a second center. When the sample groups were combined (n = 606), the association between the -579G allele and nephropathy remained significant (OR 1.35, 1.07-1.70, P = 0.01). The haplotype structure in the surrounding 7-kb region was determined. No single haplotype was more strongly associated with nephropathy than the -579A>G SNP. These results suggest a role for the caldesmon gene in susceptibility to diabetic nephropathy in type 1 diabetes.

Myosin function in nervous and sensory systems

Development of the nervous system requires remarkable changes in cell structure that are dependent upon the cytoskeleton. The importance of specific components of the neuronal cytoskeleton, such as microtubules and neurofilaments, to neuronal function and development has been well established. Recently, increasing focus has been put on understanding the functional role of the actin cytoskeleton in neurons. Important modulators of the actin cytoskeleton are the large family of myosins, many of which (classes I, II, III, V, VI, VII, IX, and XV; Fig. 1) are expressed in developing neurons or sensory cells. Myosins are force-producing proteins that have been implicated in a wide variety of cellular functions in the developing nervous system, including neuronal migration, process outgrowth, and growth cone motility, as well as other aspects of morphogenesis, axonal transport, and synaptic and sensory functions. We review the roles that neuronal myosins play in these functions with particular focus on the first three events listed above, as well as sensory function.

Postnatal changes in caldesmon expression and localization in cardiac myocytes

Caldesmon is a heat-stable protein found in both muscle and non-muscle tissue. It binds to a number of contractile and cytoskeletal proteins and may be involved in regulating acto-myosin interaction in smooth muscle cells and/or the assembly of microfilaments in muscle and non-muscle cells. We have shown previously that caldesmon is localized at the Z-lines in adult cardiac myocytes and that both the low- and high-molecular-weight forms (/-caldesmon and h-caldesmon, respectively) are present in atrial and ventricular myocytes. Here we examined the expression of caldesmon and its localization in freshly isolated cardiac myocytes during postnatal development and when these myocytes were grown in culture. We found that /-caldesmon is expressed in both neonatal and adult rat ventricular myocytes. The expression of h-caldesmon, however, was not detected in myocytes from newborn animals but increased during the first 2 weeks of postnatal development. Caldesmon was generally not co-localized with alpha-actinin at the Z-lines in neonatal myocytes but became increasingly more so during the first 2 weeks of postnatal development. When myocytes from 5- and 10-day-old rats were grown in primary culture, h-caldesmon expression decreased and caldesmon could not be detected at the Z-lines in the cultured cells. These results indicate that caldesmon plays a role at the Z-lines in adult cardiac myocytes; however, its localization at the Z-lines is not necessary for the prenatal development that occurs at these sites or for the establishment of a contractile phenotype in cultured cardiac myocytes.

Role of CaM kinase II and ERK activation in thrombin-induced endothelial cell barrier dysfunction

We have previously shown that thrombin-induced endothelial cell barrier dysfunction involves cytoskeletal rearrangement and contraction, and we have elucidated the important role of endothelial cell myosin light chain kinase and the actin- and myosin-binding protein caldesmon. We evaluated the contribution of calmodulin (CaM) kinase II and extracellular signal-regulated kinase (ERK) activation in thrombin-mediated bovine pulmonary artery endothelial cell contraction and barrier dysfunction. Similar to thrombin, infection with a constitutively active adenoviral alpha-CaM kinase II construct induced significant ERK activation, indicating that CaM kinase II activation lies upstream of ERK. Thrombin-induced ERK-dependent caldesmon phosphorylation (Ser789) was inhibited by either KN-93, a specific CaM kinase II inhibitor, or U0126, an inhibitor of MEK activation. Immunofluorescence microscopy studies revealed phosphocaldesmon colocalization within thrombin-induced actin stress fibers. Pretreatment with either U0126 or KN-93 attenuated thrombin-mediated cytoskeletal rearrangement and evoked declines in transendothelial electrical resistance while reversing thrombin-induced dissociation of myosin from nondenaturing caldesmon immunoprecipitates. These results strongly suggest the involvement of CaM kinase II and ERK activities in thrombin-mediated caldesmon phosphorylation and both contractile and barrier regulation.

Alpha(v)beta3 integrin expression up-regulates cdc2, which modulates cell migration

The alphavbeta3 integrin has been shown to promote cell migration through activation of intracellular signaling pathways. We describe here a novel pathway that modulates cell migration and that is activated by alphavbeta3 and, as downstream effector, by cdc2 (cdk1). We report that alphavbeta3 expression in LNCaP (beta3-LNCaP) prostate cancer cells causes increased cdc2 mRNA levels as evaluated by gene expression analysis, and increased cdc2 protein and kinase activity levels. We provide three lines of evidence that increased levels of cdc2 contribute to a motile phenotype on integrin ligands in different cell types. First, increased levels of cdc2 correlate with more motile phenotypes of cancer cells. Second, ectopic expression of cdc2 increases cell migration, whereas expression of dominant-negative cdc2 inhibits migration. Third, cdc2 inhibitors reduce cell migration without affecting cell adhesion. We also show that cdc2 increases cell migration via specific association with cyclin B2, and we unravel a novel pathway of cell motility that involves, downstream of cdc2, caldesmon. cdc2 and caldesmon are shown here to localize in membrane ruffles in motile cells. These results show that cdc2 is a downstream effector of the alphavbeta3 integrin, and that it promotes cell migration.

Caldesmon inhibits Arp2/3-mediated actin nucleation

The Arp2/3 complex greatly accelerates actin polymerization, which is thought to play a major role in cell motility by inducing membrane protrusions including ruffling movements. Membrane ruffles contain a variety of actin-binding proteins, which would modulate Arp2/3-dependent actin polymerization. However, their exact roles in actin polymerization remain to be established. Because caldesmon is present in membrane ruffles, as well as in stress fibers, it may alter Arp2/3-mediated actin polymerization. We have found that caldesmon greatly retards Arp2/3-induced actin polymerization. Kinetic analyses have revealed that caldesmon inhibits the nucleation process, whereas it does not largely reduce elongation. Caldesmon is found to inhibit binding of Arp2/3 to F-actin, which apparently reduces the ability of F-actin as a secondary activator of Arp2/3-mediated nucleation. We also have found that the inhibition of the binding between actin and caldesmon either by Ca(2+)/calmodulin or by phosphorylation with cdc2 kinase reverses the inhibitory effect of caldesmon on Arp2/3-induced actin polymerization. Our results suggest that caldesmon may be a key protein that modulates membrane ruffling and that this may involve changes in caldesmon phosphorylation and/or intracellular calcium concentrations during signal transduction.

Calcium regulation of actin crosslinking is important for function of the actin cytoskeleton in Dictyostelium

The actin cytoskeleton is sensitive to changes in calcium, which affect contractility, actin-severing proteins, actin-crosslinking proteins and calmodulin-regulated enzymes. To dissect the role of calcium control on the activity of individual proteins from effects of calcium on other processes, calcium-insensitive forms of these proteins were prepared and introduced into living cells to replace a calcium-sensitive form of the same protein. Crosslinking and bundling of actin filaments by the Dictyostelium 34 kDa protein is inhibited in the presence of micromolar free calcium. A modified form of the 34 kDa protein with mutations in the calcium binding EF hand (34 kDa deltaEF2) was prepared using site-directed mutagenesis and expressed in E. coli. Equilibrium dialysis using [(45)Ca]CaCl(2) revealed that the wild-type protein is able to bind one calcium ion with a Kd of 2.4 microM. This calcium binding is absent in the 34 kDa deltaEF2 protein. The actin-binding activity of the 34 kDa deltaEF2 protein was equivalent to wildtype but calcium insensitive in vitro. The wild-type and 34 kDa deltaEF2 proteins were expressed in 34-kDa-null and 34 kDa/alpha-actinin double null mutant Dictyostelium strains to test the hypothesis that calcium regulation of actin crosslinking is important in vivo. The 34 kDa deltaEF2 failed to supply function of the 34 kDa protein important for control of cell size and for normal growth to either of these 34-kDa-null strains. Furthermore, the distribution of the 34 kDa protein and actin were abnormal in cells expressing 34 kDa deltaEF2. Thus, calcium regulation of the formation and/or dissolution of crosslinked actin structures is required for dynamic behavior of the actin cytoskeleton important for cell structure and growth.

A genetic screen for suppressors and enhancers of the Drosophila cdk1-cyclin B identifies maternal factors that regulate microtubule and microfilament stability

Coordination between cell-cycle progression and cytoskeletal dynamics is important for faithful transmission of genetic information. In early Drosophila embryos, increasing maternal cyclin B leads to higher Cdk1-CycB activity, shorter microtubules, and slower nuclear movement during cycles 5-7 and delays in nuclear migration to the cortex at cycle 10. Later during cycle 14 interphase of six cycB embryos, we observed patches of mitotic nuclei, chromosome bridges, abnormal nuclear distribution, and small and large nuclei. These phenotypes indicate disrupted coordination between the cell-cycle machinery and cytoskeletal function. Using these sensitized phenotypes, we performed a dosage-sensitive genetic screen to identify maternal proteins involved in this process. We identified 10 suppressors classified into three groups: (1) gene products regulating Cdk1 activities, cdk1 and cyclin A; (2) gene products interacting with both microtubules and microfilaments, Actin-related protein 87C; and (3) gene products interacting with microfilaments, chickadee, diaphanous, Cdc42, quail, spaghetti-squash, zipper, and scrambled. Interestingly, most of the suppressors that rescue the astral microtubule phenotype also reduce Cdk1-CycB activities and are microfilament-related genes. This suggests that the major mechanism of suppression relies on the interactions among Cdk1-CycB, microtubule, and microfilament networks. Our results indicate that the balance among these different components is vital for normal early cell cycles and for embryonic development. Our observations also indicate that microtubules and cortical microfilaments antagonize each other during the preblastoderm stage.

Actopaxin is phosphorylated during mitosis and is a substrate for cyclin B1/cdc2 kinase

Prior to cell division, normal adherent cells adopt a round morphology that is associated with a loss of actin stress fibres and disassembly of focal adhesions. In this study, we investigate the mitotic phosphorylation of the recently described paxillin and actin-binding focal-adhesion protein actopaxin [Nikolopoulos and Turner (2000) J. Cell Biol. 151, 1435-1448]. Actopaxin is comprised of an N-terminus containing six putative cdc2 phosphorylation sites and a C-terminus consisting of tandem calponin homology domains. Here we show that the N-terminus of actopaxin is phosphorylated by cyclin B1/cdc2 kinase in vitro and that this region of actopaxin precipitates cdc2 kinase activity from mitotic lysates. Actopaxin exhibits reduced electrophoretic mobility during mitosis that is dependent on phosphorylation within the first two consensus cdc2 phosphorylation sites. Finally, as cells progress from mitosis to G(1) there is an adhesion-independent dephosphorylation of actopaxin, suggesting that actopaxin dephosphorylation precedes cell spreading and the reformation of focal adhesions. Taken together, these results suggest a role for cyclin B1/cdc2-dependent phosphorylation of actopaxin in regulating actin cytoskeleton reorganization during cell division.