p21-activated kinase PAK phosphorylates desmin at sites different from those for Rho-associated kinase

Desmin and its molecular chaperone, the αB-crystallin: How post-translational modifications modulate their functions in heart and skeletal muscles?

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network through protein-protein interactions providing an effective mechanochemical integrator of morphology and function. Through a continuous and complex trans-cytoplasmic network, desmin intermediate filaments ensure this essential role in heart and in skeletal muscle. Besides their role in the maintenance of cell shape and architecture (permitting contractile activity efficiency and conferring resistance towards mechanical stress), desmin intermediate filaments are also key actors of cell and tissue homeostasis. Desmin participates to several cellular processes such as differentiation, apoptosis, intracellular signalisation, mechanotransduction, vesicle trafficking, organelle biogenesis and/or positioning, calcium homeostasis, protein homeostasis, cell adhesion, metabolism and gene expression. Desmin intermediate filaments assembly requires αB-crystallin, a small heat shock protein. Over its chaperone activity, αB-crystallin is involved in several cellular functions such as cell integrity, cytoskeleton stabilization, apoptosis, autophagy, differentiation, mitochondria function or aggresome formation. Importantly, both proteins are known to be strongly associated to the aetiology of several cardiac and skeletal muscles pathologies related to desmin filaments disorganization and a strong disturbance of desmin interactome. Note that these key proteins of cytoskeleton architecture are extensively modified by post-translational modifications that could affect their functional properties. Therefore, we reviewed in the herein paper the impact of post-translational modifications on the modulation of cellular functions of desmin and its molecular chaperone, the αB-crystallin.

P21-Activated Kinase Inhibitors FRAX486 and IPA3: Inhibition of Prostate Stromal Cell Growth and Effects on Smooth Muscle Contraction in the Human Prostate

Prostate smooth muscle tone and hyperplastic growth are involved in the pathophysiology and treatment of male lower urinary tract symptoms (LUTS). Available drugs are characterized by limited efficacy. Patients' adherence is particularly low to combination therapies of 5α-reductase inhibitors and α1-adrenoceptor antagonists, which are supposed to target contraction and growth simultaneously. Consequently, molecular etiology of benign prostatic hyperplasia (BPH) and new compounds interfering with smooth muscle contraction or growth in the prostate are of high interest. Here, we studied effects of p21-activated kinase (PAK) inhibitors (FRAX486, IPA3) in hyperplastic human prostate tissues, and in stromal cells (WPMY-1). In hyperplastic prostate tissues, PAK1, -2, -4, and -6 may be constitutively expressed in catecholaminergic neurons, while PAK1 was detected in smooth muscle and WPMY-1 cells. Neurogenic contractions of prostate strips by electric field stimulation were significantly inhibited by high concentrations of FRAX486 (30 μM) or IPA3 (300 μM), while noradrenaline- and phenylephrine-induced contractions were not affected. FRAX486 (30 μM) inhibited endothelin-1- and -2-induced contractions. In WPMY-1 cells, FRAX486 or IPA3 (24 h) induced concentration-dependent (1-10 μM) degeneration of actin filaments. This was paralleled by attenuation of proliferation rate, being observed from 1 to 10 μM FRAX486 or IPA3. Cytotoxicity of FRAX486 and IPA3 in WPMY-1 cells was time- and concentration-dependent. Stimulation of WPMY-1 cells with endothelin-1 or dihydrotestosterone, but not noradrenaline induced PAK phosphorylation, indicating PAK activation by endothelin-1. Thus, PAK inhibitors may inhibit neurogenic and endothelin-induced smooth muscle contractions in the hyperplastic human prostate, and growth of stromal cells. Targeting prostate smooth muscle contraction and stromal growth at once by a single compound is principally possible, at least under experimental conditions.

PAK signaling in cancer

Transformation of a normal cell to a cancer cell is caused by mutations in genes that regulate proliferation, apoptosis, and invasion. Small GTPases such as Ras, Rho, Rac and Cdc42 orchestrate many of the signals that are required for malignant transformation. The p21-activated kinases (PAKs) are effectors of Rac and Cdc42. PAKs are a family of serine/threonine protein kinases comprised of six isoforms (PAK1–6), and they play important roles in cytoskeletal dynamics, cell survival and proliferation. They act as key signal transducers in several cancer signaling pathways, including Ras, Raf, NFκB, Akt, Bad and p53. Although PAKs are not mutated in cancers, they are overexpressed, hyperactivated or amplified in several human tumors and their role in cell transformation make them attractive therapeutic targets. This review discusses the evidence that PAK is important for cell transformation and some key signaling pathways it regulates. This review primarily discusses Group I PAKs (PAK1, PAK2 and PAK3) as Group II PAKs (PAK4, PAK5 and PAK6) are discussed elsewhere in this issue (by Minden).

Nebulin binding impedes mutant desmin filament assembly

Although a clear picture of the in vitro assembly process is established for vimentin, the role of associated partner proteins and their effect on intermediate filament assembly has not been fully examined. This study finds delayed dynamics of desminopathy-linked mutant desmin in myocytes and hindered assembly when associated to nebulin.

Desmin intermediate filaments (DIFs) form an intricate meshwork that organizes myofibers within striated muscle cells. The mechanisms that regulate the association of desmin to sarcomeres and their role in desminopathy are incompletely understood. Here we compare the effect nebulin binding has on the assembly kinetics of desmin and three desminopathy-causing mutant desmin variants carrying mutations in the head, rod, or tail domains of desmin (S46F, E245D, and T453I). These mutants were chosen because the mutated residues are located within the nebulin-binding regions of desmin. We discovered that, although nebulin M160–164 bound to both desmin tetrameric complexes and mature filaments, all three mutants exhibited significantly delayed filament assembly kinetics when bound to nebulin. Correspondingly, all three mutants displayed enhanced binding affinities and capacities for nebulin relative to wild-type desmin. Electron micrographs showed that nebulin associates with elongated normal and mutant DIFs assembled in vitro. Moreover, we measured significantly delayed dynamics for the mutant desmin E245D relative to wild-type desmin in fluorescence recovery after photobleaching in live-cell imaging experiments. We propose a mechanism by which mutant desmin slows desmin remodeling in myocytes by retaining nebulin near the Z-discs. On the basis of these data, we suggest that for some filament-forming desmin mutants, the molecular etiology of desminopathy results from subtle deficiencies in their association with nebulin, a major actin-binding filament protein of striated muscle.

Desminopathies: pathology and mechanisms

The intermediate filament protein desmin is an essential component of the extra-sarcomeric cytoskeleton in muscle cells. This three-dimensional filamentous framework exerts central roles in the structural and functional alignment and anchorage of myofibrils, the positioning of cell organelles and signaling events. Mutations of the human desmin gene on chromosome 2q35 cause autosomal dominant, autosomal recessive, and sporadic myopathies and/or cardiomyopathies with marked phenotypic variability. The disease onset ranges from childhood to late adulthood. The clinical course is progressive and no specific treatment is currently available for this severely disabling disease. The muscle pathology is characterized by desmin-positive protein aggregates and degenerative changes of the myofibrillar apparatus. The molecular pathophysiology of desminopathies is a complex, multilevel issue. In addition to direct effects on the formation and maintenance of the extra-sarcomeric intermediate filament network, mutant desmin affects essential protein interactions, cell signaling cascades, mitochondrial functions, and protein quality control mechanisms. This review summarizes the currently available data on the epidemiology, clinical phenotypes, myopathology, and genetics of desminopathies. In addition, this work provides an overview on the expression, filament formation processes, biomechanical properties, post-translational modifications, interaction partners, subcellular localization, and functions of wild-type and mutant desmin as well as desmin-related cell and animal models.

Muscle creatine kinase deficiency triggers both actin depolymerization and desmin disorganization by advanced glycation end products in dilated cardiomyopathy

Alterations in the balance of cytoskeleton as well as energetic proteins are involved in the cardiac remodeling occurring in dilated cardiomyopathy (DCM). We used two-dimensional DIGE proteomics as a discovery approach to identify key molecular changes taking place in a temporally controlled model of DCM triggered by cardiomyocyte-specific serum response factor (SRF) knock-out in mice. We identified muscle creatine kinase (MCK) as the primary down-regulated protein followed by α-actin and α-tropomyosin down-regulation leading to a decrease of polymerized F-actin. The early response to these defects was an increase in the amount of desmin intermediate filaments and phosphorylation of the αB-crystallin chaperone. We found that αB-crystallin and desmin progressively lose their striated pattern and accumulate at the intercalated disk and the sarcolemma, respectively. We further show that desmin is a preferential target of advanced glycation end products (AGE) in mouse and human DCM. Inhibition of CK in cultured cardiomyocytes is sufficient to recapitulate both the actin depolymerization defect and the modification of desmin by AGE. Treatment with either cytochalasin D or glyoxal, a cellular AGE, indicated that both actin depolymerization and AGE contribute to desmin disorganization. Heat shock-induced phosphorylation of αB-crystallin provides a transient protection of desmin against glyoxal in a p38 MAPK-dependent manner. Our results show that the strong down-regulation of MCK activity contributes to F-actin instability and induces post-translational modification of αB-crystallin and desmin. Our results suggest that AGE may play an important role in DCM because they alter the organization of desmin filaments that normally support stress response and mitochondrial functions in cardiomyocytes.

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.

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.

TC10 controls human myofibril organization and is activated by the sarcomeric RhoGEF obscurin

The contractile activity of striated muscle depends on myofibrils that are highly ordered macromolecular complexes. The protein components of myofibrils are well characterized, but it remains largely unclear how signaling at the molecular level within the sarcomere and the control of assembly are coordinated. We show that the Rho GTPase TC10 appears during differentiation of human primary skeletal myoblasts and it is active in differentiated myotubes. We identify obscurin, a sarcomere-associated protein, as a specific activator of TC10. Indeed, TC10 binds directly to obscurin via its predicted RhoGEF motif. Importantly, we demonstrate that obscurin is a specific activator of TC10 but not the Rho GTPases Rac and Cdc42. Finally, we show that inhibition of TC10 activity by expression of a dominant-negative mutant or its knockdown by expression of specific shRNA block myofibril assembly. Our findings reveal a novel signaling pathway in human skeletal muscle that involves obscurin and the Rho GTPase TC10 and implicate this pathway in new sarcomere formation.

Intermediate filaments in smooth muscle

The intermediate filament (IF) network is one of the three cytoskeletal systems in smooth muscle. The type III IF proteins vimentin and desmin are major constituents of the network in smooth muscle cells and tissues. Lack of vimentin or desmin impairs contractile ability of various smooth muscle preparations, implying their important role for smooth muscle force development. The IF framework has long been viewed as a fixed cytostructure that solely provides mechanical integrity for the cell. However, recent studies suggest that the IF cytoskeleton is dynamic in mammalian cells in response to various external stimulation. In this review, the structure and biological properties of IF proteins in smooth muscle are summarized. The role of IF proteins in the modulation of smooth muscle force development and redistribution/translocation of signaling partners (such as p130 Crk-associated substrate, CAS) is depicted. This review also summarizes our latest understanding on how the IF network may be regulated in smooth muscle.

Migration of airway smooth muscle cells

Migration of smooth muscle cells is a process fundamental to development of hollow organs, including blood vessels and the airways. Migration is also thought to be part of the response to tissue injury. It has also been suggested to contribute to airways remodeling triggered by chronic inflammation. In both nonmuscle and smooth muscle cells numerous external signaling molecules and internal signal transduction pathways contribute to cell migration. The review includes evidence for the functional significance of airway smooth muscle migration, a summary of promigratory and antimigratory agents, and summaries of important signaling pathways mediating migration. Important signaling pathways and effector proteins described include small G proteins, phosphatidylinositol 3-kinases (PI3-K), Rho activated protein kinase (ROCK), p21-activated protein kinases (PAK), Src family tyrosine kinases, and mitogen-activated protein kinases (MAPK). These signaling modules control multiple critical effector proteins including actin nucleating, capping and severing proteins, myosin motors, and proteins that remodel microtubules. Actin filament remodeling, focal contact remodeling and propulsive force of molecular motors are all coordinated to move cells along gradients of chemical cues, matrix adhesiveness, or matrix stiffness. Airway smooth muscle cell migration can be modulated in vitro by drugs commonly used in pulmonary medicine including beta-adrenergic agonists and corticosteroids. Future studies of airway smooth muscle cell migration may uncover novel targets for drugs aimed at modifying airway remodeling.

Differentiation of cardiomyocytes requires functional serine residues within the amino-terminal domain of desmin

Desmin contributes to the stability of the myocardium and its amino-terminal domain influences intermediate filament formation and interacts with a variety of proteins and DNAs. Specific serine residues located in this domain are reversibly phosphorylated in a cell cycle and developmental stage-dependent manner as has been demonstrated also for other cytoplasmic type III intermediate filament proteins. Although absence of desmin apparently does not affect cardiomyogenesis, homozygous deletion of the amino-terminal domain of desmin severely inhibited in vitro cardiomyogenesis. To demonstrate the significance of phosphorylation of this domain in cardiomyogenic commitment and differentiation, we inhibited phosphorylation of serine residues 6, 7, and 8 by mutation to alanine, and investigated early cardiomyogenesis in heterozygous embryoid bodies. As control, serine residues 31 and 32, which are not phosphorylated by kinases mutating serine residues 6, 7, and 8, were mutated to alanine in a second set. Desmin(S6,7,8A) interfered with cardiomyogenesis and myofibrillogenesis in a dominant negative fashion, whereas desmin(S31,32A) produced only a mild phenotype. Desmin(S6,7,8A) led to the down-regulation of the transcription factor genes brachyury, goosecoid, nkx2.5, and mef2C and increased apoptosis of presumptive mesoderm and differentiating cardiomyocytes. Surviving cardiomyocytes which were few in number had no myofibrils. Demonstration that some but not any mutant desmin interfered with the very beginning of cardiomyogenesis suggests an important function of temporarily phosphorylated serine residues 6, 7, and 8 in the amino-terminal domain of desmin in cardiomyogenic commitment and differentiation.

Mechanisms of vascular smooth muscle cell migration

Smooth muscle cell migration occurs during vascular development, in response to vascular injury, and during atherogenesis. Many proximal signals and signal transduction pathways activated during migration have been identified, as well as components of the cellular machinery that affect cell movement. In this review, a summary of promigratory and antimigratory molecules belonging to diverse chemical and functional families is presented, along with a summary of key signaling events mediating migration. Extracellular molecules that modulate migration include small biogenic amines, peptide growth factors, cytokines, extracellular matrix components, and drugs used in cardiovascular medicine. Promigratory stimuli activate signal transduction cascades that trigger remodeling of the cytoskeleton, change the adhesiveness of the cell to the matrix, and activate motor proteins. This review focuses on the signaling pathways and effector proteins regulated by promigratory and antimigratory molecules. Prominent pathways include phosphatidylinositol 3-kinases, calcium-dependent protein kinases, Rho-activated protein kinase, p21-activated protein kinases, LIM kinase, and mitogen-activated protein kinases. Important downstream targets include myosin II motors, actin capping and severing proteins, formins, profilin, cofilin, and the actin-related protein-2/3 complex. Actin filament remodeling, focal contact remodeling, and molecular motors are coordinated to cause cells to migrate along gradients of chemical cues, matrix adhesiveness, or matrix stiffness. The result is recruitment of cells to areas where the vessel wall is being remodeled. Vessel wall remodeling can be antagonized by common cardiovascular drugs that act in part by inhibiting vascular smooth muscle cell migration. Several therapeutically important drugs act by inhibiting cell cycle progression, which may reduce the population of migrating cells.

Big roles for small GTPases in the control of directed cell movement

Small GTPases are involved in the control of diverse cellular behaviours, including cellular growth, differentiation and motility. In addition, recent studies have revealed new roles for small GTPases in the regulation of eukaryotic chemotaxis. Efficient chemotaxis results from co-ordinated chemoattractant gradient sensing, cell polarization and cellular motility, and accumulating data suggest that small GTPase signalling plays a central role in each of these processes as well as in signal relay. The present review summarizes these recent findings, which shed light on the molecular mechanisms by which small GTPases control directed cell migration.

Phosphorylation of cortactin by p21-activated kinase

Cortactin is an F-actin binding protein that is enriched in dynamic cytoskeletal organelles such as podosomes, membrane ruffles, and lamellipodia. We have shown previously that Src-phosphorylation of cortactin is not required for its translocation to phorbol-ester induced podosomes in A7r5 aortic smooth muscle cells, but may be important for stability and turnover of podosomes. However, little is known of the role of Ser/Thr kinases in the regulation of cortactin. Here, we report that p21-associated kinase (PAK), which plays a crucial role in the formation of podosome and membrane ruffles, is able to phosphorylate cortactin in vitro. The predominant phosphorylation site is located at Ser113 in the first actin-binding repeat. Phosphorylation by PAK is not required for the translocation of cortactin to podosomes, lamellipodia, or membrane ruffles in A7r5 smooth muscle cells. However, binding of cortactin to F-actin is significantly reduced by PAK-phosphorylation. Taken together, these results suggest a role for PAK-phosphorylation of cortactin in the regulation of the dynamics of branched actin filaments in dynamic cytoskeletal organelles.

Myocardial subproteomic analysis of a constitutively active Rac1-expressing transgenic mouse with lethal myocardial hypertrophy

A two-dimensional gel electrophoresis (2-DE)-based proteomic approach was used to study a transgenic mouse model of acerbated dilated cardiomyopathy in which the small monomeric GTPase, Rac1, was constitutively expressed exclusively in the myocardium. A subfractionation procedure allowed for the focused analysis of both cytoplasmic and myofilament protein-enriched extracts of ventricular tissue from Rac1 transgenic and age-matched nontransgenic (NTG) mice. The majority of these mice displayed severe hypertrophy (heart-to-body weight ratios >2-fold greater in the Rac1 mice) and died from overt heart failure between days 14 and 17. Comparative 2-DE analysis (pH 3-10, 12% SDS-PAGE) derived from Rac1 (n = 4) and NTG (n = 4) groups revealed differences in mean protein spot intensities. Twelve proteins from the cytoplasmic protein-enriched extract met our criteria for robustness and spot resolution and were identified. These proteins represent a broad distribution of cellular functions with only some previously implicated in myocardial hypertrophy. The myofilament subproteome displayed no change in posttranslational modification, but further analysis by one-dimensional Western blot showed increased quantities of myofilament proteins in the Rac1 mouse ventricles. Additionally, three proteins with different functionality that were altered in the cytoplasmic protein-enriched subproteome, tubulin beta-chain, manganese superoxide dismutase, and malate dehydrogenase, were analyzed at days 7, 9, and 11 to assess their role in the development of the dilated cardiomyopathic phenotype. The quantity of all three proteins peaked at day 9, suggesting an early response in cardiac hypertrophic failure.

Sertoli-germ cell anchoring junction dynamics in the testis are regulated by an interplay of lipid and protein kinases

When Sertoli and germ cells were co-cultured in vitro in serum-free chemically defined medium, functional anchoring junctions such as cell-cell intermediate filament-based desmosome-like junctions and cell-cell actin-based adherens junctions (e.g. ectoplasmic specialization (ES)) were formed within 1-2 days. This event was marked by the induction of several protein kinases such as phosphatidylinositol 3-kinase (PI3K), phosphorylated protein kinase B (PKB; also known as Akt), p21-activated kinase-2 (PAK-2), and their downstream effector (ERK) as well as an increase in PKB intrinsic activity. PI3K, phospho (p)-PKB, and PAK were co-localized to the site of apical ES in the seminiferous epithelium of the rat testis in immunohistochemistry studies. Furthermore, PI3K also co-localized with p-PKB to the same site in the epithelium as determined by fluorescence microscopy, consistent with their localization at the ES. These kinases were shown to associate with ES-associated proteins such as beta1-integrin, phosphorylated focal adhesion kinase, and c-Src by co-immunoprecipitation, suggesting that the integrin.laminin protein complex at the apical ES likely utilizes these protein kinases as regulatory proteins to modulate Sertoli-germ cell adherens junction dynamics via the ERK signaling pathway. To validate this hypothesis further, an in vivo model using AF-2364 (1-(2,4-dichlorobenzyl)-1H-indazole-3-carbohydrazide) to perturb Sertoli-germ cell anchoring junction function, inducing germ cell loss from the epithelium in adult rats, was used in conjunction with specific inhibitors. Interestingly, the event of germ cell loss induced by AF-2364 in vivo was also associated with induction of PI3K, p-PKB, PAK-2, and p-ERK as well as a surge in intrinsic PKB activity. Perhaps the most important of all, pretreatment of rats with wortmannin (a PI3K inhibitor) or anti-beta1-integrin antibody via intratesticular injection indeed delayed AF-2364-induced spermatid loss from the epithelium. In summary, these results illustrate that Sertoli-germ cell anchoring junction dynamics in the testis are regulated, at least in part, via the beta1-integrin/PI3K/PKB/ERK signaling pathway.

Xenopus p21-activated kinase 5 regulates blastomeres' adhesive properties during convergent extension movements

The p21-activated kinase (PAK) proteins regulate many cellular events including cell cycle progression, cell death and survival, and cytoskeleton rearrangements. We previously identified X-PAK5 that binds the actin and microtubule networks, and could potentially regulate their coordinated dynamics during cell motility. In this study, we investigated the functional importance of this kinase during gastrulation in Xenopus. X-PAK5 is mainly expressed in regions of the embryo that undergo extensive cell movements during gastrula such as the animal hemisphere and the marginal zone. Expression of a kinase-dead mutant inhibits convergent extension movements in whole embryos and in activin-treated animal cap by modifying behavior of cells. This phenotype is rescued in embryo by adding back X-PAK5 catalytic activity. The active kinase decreases cell adhesiveness when expressed in animal hemisphere and inhibits the calcium-dependent reassociation of cells, while dead X-PAK5 kinase localizes to cell-cell junctions and increases cell adhesion. In addition, endogenous X-PAK5 colocalizes with adherens junction proteins and its activity is regulated by extracellular calcium. Taken together, our results suggest that X-PAK5 regulates convergent extension movements in vivo by modulating the calcium-mediated cell-cell adhesion.

Biology of the p21-activated kinases

The p21-activated kinases (PAKs) 1-3 are serine/threonine protein kinases whose activity is stimulated by the binding of active Rac and Cdc42 GTPases. Our understanding of the regulation and biology of these important signaling proteins has increased tremendously since their discovery in the mid-1990s. PAKs 1-3 are activated by a variety of GTPase-dependent and -independent mechanisms. This complexity reflects the contributions of PAK function in many cellular signaling pathways and the need to carefully control PAK action in a highly localized manner. PAKs serve as important regulators of cytoskeletal dynamics and cell motility, transcription through MAP kinase cascades, death and survival signaling, and cell-cycle progression. Consequently, PAKs have also been implicated in a number of pathological conditions and in cell transformation. We propose here a key role for PAK action in coordinating the dynamics of the actin and microtubule cytoskeletons during directional motility of cells, as well as in other functions requiring cytoskeletal polarization.

Retaining of the assembly capability of vimentin phosphorylated by mitogen-activated protein kinase-activated protein kinase-2

Intermediate filament (IF) networks can be regulated by phosphorylation of unit proteins, such as vimentin, by specific kinases leading to reorganization of the IF filamentous structure. Recently, we identified mitogen-activated protein kinase-activated protein kinase-2 (MAPKAP kinase-2) as a vimentin kinase (Cheng and Lai [1998] J. Cell. Biochem. 71:169-181). Herein we describe the results of further in vitro studies investigating the effects of MAPKAP kinase-2 phosphorylation on vimentin and the effects of the phosphorylation on the filamentous structure. We show that MAPKAP kinase-2 mainly phosphorylates vimentin at Ser-38, Ser-50, Ser-55, and Ser-82, residues all located in the head domain of the protein. Surprisingly, and in stark contrast to phosphorylation by most other kinases, phosphorylation of vimentin by MAPKAP kinase-2 has no discernable effect on its assembly. It suggested that structure disassembly is not the only obligated consequence of phosphorylated vimentin as regulated by other kinases. Finally, a mutational analysis of each of the phosphorylated serine residues in vimentin suggested that no single serine site was primarily responsible for structure maintenance, implying that the retention of filamentous structure may be the result of the coordinated action of several phosphorylated serine sites. This also shed new lights on the functional task(s) of vimentin that is intermediate filament proteins might provide a phosphate reservoir to accommodate the phosphate surge without any structural changes.

Actin filament formation, reorganization and migration are impaired in hepatic stellate cells under influence of trichostatin A, a histone deacetylase inhibitor

BACKGROUND/AIMS

Previously, trichostatin A (TSA), a histone deacetylase inhibitor, has been shown to exhibit strong antifibrotic characteristics in hepatic stellate cells (HSC), which are known to play a central role in chronic liver diseases. TSA retained a more quiescent phenotype in spite of culture conditions that favor transdifferentiation into activated HSC.

METHODS

To identify TSA-sensitive genes, differential mRNA display, Northern and Western blot analysis were used and genes were functionally validated by using contraction and motility assays.

RESULTS

TSA prevented new actin filament formation by down-regulation of two nucleating proteins, actin related protein 2 (Arp2) and Arp3, and by up-regulation of adducin like protein 70 (ADDL70) and gelsolin, two capping proteins. RhoA, a key mediator in the development of the actin cytoskeleton, decreased following TSA exposure. Expression of proteins of Class III intermediate filaments was affected by TSA. Furthermore, F-actin and G-actin were expressed heterogeneously under influence of TSA. Functionally, TSA treatment abrogated migration of quiescent HSC, while migration was reduced in transitional HSC. The endothelin-1-induced contractility properties of HSC was not affected by TSA.

CONCLUSIONS

These data indicate that TSA affects the development of the actin cytoskeleton in quiescent HSC and thereby abrogates the process of HSC transdifferentiation.

Synapsin I is phosphorylated at Ser603 by p21-activated kinases (PAKs) in vitro and in PC12 cells stimulated with bradykinin

The function of synapsin I is regulated by phosphorylation of the molecule at multiple sites; among them, the Ser(603) residue (site 3) is considered to be a pivotal site targeted by Ca(2+)/calmodulin-dependent kinase II (CaMKII). Although phosphorylation of the Ser(603) residue responds to several kinds of stimuli, it is unlikely that many or all of the stimuli activate the CaMKII-involved pathway. Among the several stimulants tested in PC12 cells, bradykinin evoked the phosphorylation of Ser(603) without inducing the autophosphorylation of CaMKII, which was determined using phosphorylation site-specific antibodies against phospho-Ser(603)-synapsin I (pS603-Syn I-Ab) and phospho-Thr(286/287)-CaMKII. The bradykinin-evoked phosphorylation of Ser(603) was not suppressed by the CaMKII inhibitor KN62, whereas high KCl-evoked phosphorylation was accompanied by CaMKII autophosphorylation and inhibited by KN62. Thus, we attempted to identify Ser(603) kinase(s) besides CaMKII. We consequently detected four and three fractions with Ca(2+)/calmodulin-independent Ser(603) kinase activity on the DEAE column chromatography of bovine brain homogenate and PC12 cell lysate, respectively, two of which were purified and identified by amino acid sequence of proteolytic fragments as p21-activated kinase (PAK) 1 and PAK3. The immunoprecipitants from bovine brain homogenate with anti-PAK1 and PAK3 antibodies incorporated (32)P into synapsin I in a Cdc42/GTPgammaS-dependent manner, and its phosphorylation site was confirmed as Ser(603) using pS603-Syn I-Ab. Additionally, recombinant GST-PAK2 could phosphorylate the Ser(603) residue in the presence of Cdc42/GTPgammaS. Finally, we confirmed by immunocytochemical analysis that the transfection of constitutively active rat alphaPAK (PAK1) in PC12 cells evokes the phosphorylation of Ser(603) even in the resting mutant cells and enhances it in the bradykinin-stimulated cells, whereas that of dominant-negative alphaPAK quenches the phosphorylation. These results raise the possibility that Ser(603) on synapsin I is alternatively phosphorylated by PAKs, not only by CaMKII, in neuronal cells in response to some stimulants.

p21-activated kinase increases the calcium sensitivity of rat triton-skinned cardiac muscle fiber bundles via a mechanism potentially involving novel phosphorylation of troponin I

Phosphorylation of myofilament proteins by kinases such as cAMP-dependent protein kinase and protein kinase C has been shown to lead to altered thin-filament protein-protein interactions and modulation of cardiac function in vitro. In the present study, we report that a small GTPase-dependent kinase, p21-activated kinase (PAK), increases the calcium sensitivity of Triton-skinned cardiac muscle fiber bundles. Constitutively active PAK3 caused an average 1.25-fold (25.0+/-6.0%, n=6) increase in force at pCa 5.75, 1.44-fold (44.0+/-7.78%, n=6) at pCa 6.25, and 2.41-fold (141.2+/-23.7%, n=4) at pCa 6.5, representing a change in pCa50 value of approximately 0.25. Constitutively active PAK3 produced no change in force under conditions of relaxation (pCa 8.0) or maximal contraction (pCa 4.5). Furthermore, an inactive, kinase-dead form of PAK3 failed to produce any change in force development at any pCa value. The myofilament proteins phosphorylated by PAK3, at pCa 6.5, are desmin, troponin T, troponin I, and an unidentified 70-kDa protein. Importantly, cardiac troponin I was found to be phosphorylated at serine 149 of human cardiac troponin I, representing a novel phosphorylation site. These findings suggest a novel mechanism of modulating the calcium sensitivity of cardiac muscle contraction.

Emerging functions of p21-activated kinases in human cancer cells

The p21 activated kinases (Paks), an evolutionarily conserved family of serine/threonine kinases, are important for a variety of cellular functions including cell morphogenesis, motility, survival, mitosis, and angiogenesis. Paks are widely expressed in numerous tissues and are activated by growth factors and extracellular signals through GTPase-dependent and -independent mechanisms. Overexpression of Paks in epithelial cancer cells has been shown to increase migration potential, increase anchorage independent growth, and cause abnormalities in mitosis. Dysregulation of Paks has been reported in several human tumors and neurodegenerative diseases. A growing list of novel Pak interacting proteins has opened up exciting avenues of investigation by which to understand the functions of Paks in tumorigenesis. In this review, we will summarize the current knowledge of the Paks family with respect to emerging cellular functions and possible contributions to cancer.

Amino-terminally truncated desmin rescues fusion of des(-/-) myoblasts but negatively affects cardiomyogenesis and smooth muscle development

Desmin fulfils important functions in maintenance of muscle cells and mutations in the desmin gene have been linked to a variety of myopathies. To ascertain the role of desmin's amino-terminal domain in muscle cells we generated embryonic stem cells constitutively expressing desmin(Delta1-48) in a null background and investigated muscle cell development in vitro. Desmin(Delta1-48) lacking the first 48 amino acid residues promotes fusion of myoblasts, rescues myogenesis and down-regulates vimentin expression in embryoid bodies, but hampers cardiomyogenesis and blocks smooth muscle development. These results demonstrate that desmin's amino-terminus has different roles in skeletal, cardiac, and smooth muscle cell development and function.

Phosphorylation and reorganization of vimentin by p21-activated kinase (PAK)

BACKGROUND

Intermediate filament (IF) is one of the three major cytoskeletal filaments. Vimentin is the most widely expressed IF protein component. The Rho family of small GTPases, such as Cdc42, Rac and Rho, are thought to control the organization of actin filaments as well as other cytoskeletal filaments.

RESULTS

We determined if the vimentin filaments can be regulated by p21-activated kinase (PAK), one of targets downstream of Cdc42 or Rac. In vitro analyses revealed that vimentin served as an excellent substrate for PAK. This phosphorylated vimentin lost the potential to form 10 nm filaments. We identified Ser25, Ser38, Ser50, Ser65 and Ser72 in the amino-terminal head domain as the major phosphorylation sites on vimentin for PAK. The ectopic expression of constitutively active PAK in COS-7 cells induced vimentin phosphorylation. Fibre bundles or granulates of vimentin were frequent in these transfected cells. However, the kinase-inactive mutant induced neither vimentin phosphorylation nor filament reorganization.

CONCLUSION

Our observations suggest that PAK may regulate the reorganization of vimentin filaments through direct vimentin phosphorylation.

A novel p21-activated kinase binds the actin and microtubule networks and induces microtubule stabilization

Coordination of the different cytoskeleton networks in the cell is of central importance for morphogenesis, organelle transport, and motility. The Rho family proteins are well characterized for their effects on the actin cytoskeleton, but increasing evidence indicates that they may also control microtubule (MT) dynamics. Here, we demonstrate that a novel Cdc42/Rac effector, X-p21-activated kinase (PAK)5, colocalizes and binds to both the actin and MT networks and that its subcellular localization is regulated during cell cycle progression. In transfected cells, X-PAK5 promotes the formation of stabilized MTs that are associated in bundles and interferes with MTs dynamics, slowing both the elongation and shrinkage rates and inducing long paused periods. X-PAK5 subcellular localization is regulated tightly, since coexpression with active Rac or Cdc42 induces its shuttling to actin-rich structures. Thus, X-PAK5 is a novel MT-associated protein that may communicate between the actin and MT networks during cellular responses to environmental conditions.

The mechanism of PAK activation. Autophosphorylation events in both regulatory and kinase domains control activity

The p21-activated kinases (PAKs), in common with many kinases, undergo multiple autophosphorylation events upon interaction with appropriate activators. The Cdc42-induced phosphorylation of PAK serves in part to dissociate the kinase from its partners PIX and Nck. Here we investigate in detail how autophosphorylation events affect the catalytic activity of PAK by altering the autophosphorylation sites in both alpha- and betaPAK. Both in vivo and in vitro analyses demonstrate that, although most phosphorylation events in the PAK N-terminal regulatory domain play no direct role in activation, a phosphorylation of alphaPAK serine 144 or betaPAK serine 139, which lie in the kinase inhibitory domain, significantly contribute to activation. By contrast, sphingosine-mediated activation is independent of this residue, indicating a different mode of activation. Thus two autophosphorylation sites direct activation while three others control association with focal complexes via PIX and Nck.