Actomyosin cross-linking by caldesmon in non-muscle cells

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.

Mechanisms of Vascular Smooth Muscle Contraction and the Basis for Pharmacologic Treatment of Smooth Muscle Disorders

The smooth muscle cell directly drives the contraction of the vascular wall and hence regulates the size of the blood vessel lumen. We review here the current understanding of the molecular mechanisms by which agonists, therapeutics, and diseases regulate contractility of the vascular smooth muscle cell and we place this within the context of whole body function. We also discuss the implications for personalized medicine and highlight specific potential target molecules that may provide opportunities for the future development of new therapeutics to regulate vascular function.

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 is a cytoskeletal target for PKC in endothelium

We have previously shown that treatment of bovine endothelial cell (EC) monolayers with phorbol myristate acetate (PMA) leads to the thinning of cortical actin ring and rearrangement of the cytoskeleton into a grid-like structure, concomitant with the loss of endothelial barrier function. In the current work, we focused on caldesmon, a cytoskeletal protein, regulating actomyosin interaction. We hypothesized that protein kinase C (PKC) activation by PMA leads to the changes in caldesmon properties such as phosphorylation and cellular localization. We demonstrate here that PMA induces both myosin and caldesmon redistribution from cortical ring into the grid-like network. However, the initial step of PMA-induced actin and myosin redistribution is not followed by caldesmon redistribution. Co-immunoprecipitation experiments revealed that short-term PMA (5 min) treatment leads to the weakening of caldesmon ability to bind actin and, to the lesser extent, myosin. Prolonged incubation (15-60 min) with PMA, however, strengthens caldesmon complexes with actin and myosin, which correlates with the grid-like actin network formation. PMA stimulation leads to an immediate increase in caldesmon Ser/Thr phosphorylation. This process occurs at sites distinct from the sites specific for ERK1/2 phosphorylation and correlates with caldesmon dissociation from the actomyosin complex. Inhibition of ERK-kinase MEK fails to abolish grid-like structure formation, although reducing PMA-induced weakening of the cortical actin ring, whereas inhibition of PKC reverses PMA-induced cytoskeletal rearrangement. Our results suggest that PKC-dependent phosphorylation of caldesmon is involved in PMA-mediated complex cytoskeletal changes leading to the EC barrier compromise.

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.

The role of caldesmon in the regulation of endothelial cytoskeleton and migration

The actin- and myosin-binding protein, caldesmon (CaD) is an essential component of the cytoskeleton in smooth muscle and non-muscle cells and is involved in the regulation of cell contractility, division, and assembly of actin filaments. CaD is abundantly present in endothelial cells (EC); however, the contribution of CaD in endothelial cytoskeletal arrangement is unclear. To examine this contribution, we generated expression constructs of l-CaD cloned from bovine endothelium. Wild-type CaD (WT-CaD) and truncated mutants lacking either the N-terminal myosin-binding site or the C-terminal domain 4b (containing actin- and calmodulin-binding sites) were transfected into human pulmonary artery EC. Cell fractionation experiments and an actin overlay assay demonstrated that deleting domain 4b, but not the N-terminal myosin-binding site, resulted in decreased affinity to both the detergent-insoluble cytoskeleton and soluble actin. Recombinant WT-CaD co-localized with acto-myosin filaments in vivo, but neither of CaD mutants did. Thus both domain 4b and the myosin-binding site are essential for proper localization of CaD in EC. Overexpression of WT-CaD led to cell rounding and formation of a thick peripheral subcortical actin rim in quiescent EC, which correlated with decreased cellular migration. Pharmacological inhibition of p38 MAPK, but not ERK MAPK, caused disassembly of this peripheral actin rim in CaD-transfected cells and decreased CaD phosphorylation at Ser531 (Ser789 in human h-CaD). These results suggest that CaD is critically involved in the regulation of the actin cytoskeleton and migration in EC, and that p38 MAPK-mediated CaD phosphorylation may be involved in endothelial cytoskeletal remodeling.

Grb2 regulation of the actin-based cytoskeleton is required for ligand-independent EGF receptor-mediated oncogenesis

Mutations within members of the EGF/ErbB receptor family frequently release the oncogenic potential of these receptors, resulting in the activation of downstream signaling events independent of ligand regulatory constraints. We previously have demonstrated that the signal transduction events originating from S3-v-ErbB, a ligand-independent, oncogenic EGF receptor mutant, are qualitatively distinct from the ligand-dependent mitogenic signaling pathways associated with the wild-type EGF receptor. Specifically, expression of S3-v-ErbB in primary fibroblasts results in anchorage-independent growth, increased invasive potential, and the formation of a transformation-specific phosphoprotein signaling complex, all in a Ras-independent manner. Here we demonstrate the transformation-specific interaction between two components of this complex: the adaptor protein Grb2 and the cytoskeletal regulatory protein caldesmon. This interaction is mediated via both the amino-terminal SH3 and central SH2 domains of Grb2, and the amino-terminal (myosin-binding) domain of caldesmon. Expression of a dominant-negative Grb2 deletion mutant, which lacks the carboxy-terminal SH3 domain, in fibroblasts expressing S3-v-ErbB results in a reduction in phosphoprotein complex formation, the loss of anchorage-independent growth, and a reduction in invasive potential. Together, these results demonstrate a Ras-independent role for Grb2 in modulating cytoskeletal function during ligand-independent EGF receptor-mediated transformation, and provide further support for the hypothesis that ligand-independent oncogenic signaling is qualitatively distinct from ligand-dependent mitogenic signaling by the EGF receptor.

Smooth muscle myosin filament assembly under control of a kinase-related protein (KRP) and caldesmon

Kinase-related protein (KRP) and caldesmon are abundant myosin-binding proteins of smooth muscle. KRP induces the assembly of unphosphorylated smooth muscle myosin filaments in the presence of ATP by promoting the unfolded state of myosin. Based upon electron microscopy data, it was suggested that caldesmon also possessed a KRP-like activity (Katayama et al., 1995, J Biol Chem 270: 3919-3925). However, the nature of its activity remains obscure since caldesmon does not affect the equilibrium between the folded and unfolded state of myosin. Therefore, to gain some insight into this problem we compared the effects of KRP and caldesmon, separately, and together on myosin filaments using turbidity measurements, protein sedimentation and electron microscopy. Turbidity assays demonstrated that KRP reduced myosin filament aggregation, while caldesmon had no effect. Additionally, neither caldesmon nor its N-terminal myosin binding domain (N152) induced myosin polymerization at subthreshold Mg2+ concentrations in the presence of ATP, whereas the filament promoting action of KRP was enhanced by Mg2+. Moreover, the amino-terminal myosin binding fragment of caldesmon, like the whole protein, antagonizes Mg(2+)-induced myosin filament formation. In electron microscopy experiments, caldesmon shortened myosin filaments in the presence of Mg2+ and KRP, but N152 failed to change their appearance from control. Therefore, the primary distinction between caldesmon and KRP appears to be that caldesmon interacts with myosin to limit filament extension, while KRP induces filament propagation into defined polymers. Transfection of tagged-KRP into fibroblasts and overlay of fibroblast cytoskeletons with Cy3KRP demonstrated that KRP colocalizes with myosin structures in vivo. We propose a new model that through their independent binding to myosin and differential effects on myosin dynamics, caldesmon and KRP can, in concert, control the length and polymerization state of myosin filaments.

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.

Activation of p38 MAP-kinase and caldesmon phosphorylation are essential for urokinase-induced human smooth muscle cell migration

We have explored intracellular pathways involved in the urokinase type plasminogen activator (urokinase or uPA)-stimulated migration of human airway smooth muscle cells (hAWSMC). Using a set of uPA mutants we found that protease activity, growth factor-like and kringle domains of uPA differentially contribute to activation of p42/p44erk1,2 and p38 MAP-kinases. Consistent with our earlier data [Mukhina et al., J. Biol. Chem. 275 (2000), 16450-16458], the kringle domain of uPA was sufficient and required to stimulate cell motility. Here we report that uPA mutants containing the kringle domain specifically activate the p38 MAP-kinase pathway and actomyosin by increasing phosphorylation of the critical Ser-19 on the myosin regulatory light chain and MAP-kinase sites of the actin-associated regulatory protein caldesmon. While pharmacological inhibition of p38 MAP-kinase activation did not affect myosin light chain phosphorylation, it blocked the increase in caldesmon phosphorylation and uPA-stimulated migration of hAWSMC on a collagen-coated surface. We conclude that activation of p38 MAP-kinase and downstream phosphorylation of non-muscle caldesmon is essential for urokinase-stimulated smooth muscle cell migration.