Mammalian CAP (Cyclase-associated protein) in the world of cell migration: Roles in actin filament dynamics and beyond

Cell migration is essential for a variety of fundamental biological processes such as embryonic development, wound healing, and immune response. Aberrant cell migration also underlies pathological conditions such as cancer metastasis, in which morphological transformation promotes spreading of cancer to new sites. Cell migration is driven by actin dynamics, which is the repeated cycling of monomeric actin (G-actin) into and out of filamentous actin (F-actin). CAP (Cyclase-associated protein, also called Srv2) is a conserved actin-regulatory protein, which is implicated in cell motility and the invasiveness of human cancers. It cooperates with another actin regulatory protein, cofilin, to accelerate actin dynamics. Hence, knockdown of CAP1 slows down actin filament turnover, which in most cells leads to reduced cell motility. However, depletion of CAP1 in HeLa cells, while causing reduction in dynamics, actually led to increased cell motility. The increases in motility are likely through activation of cell adhesion signals through an inside-out signaling. The potential to activate adhesion signaling competes with the negative effect of CAP1 depletion on actin dynamics, which would reduce cell migration. In this commentary, we provide a brief overview of the roles of mammalian CAP1 in cell migration, and highlight a likely mechanism underlying the activation of cell adhesion signaling and elevated motility caused by depletion of CAP1.     

Impaired spermatogenic ability of testicular germ cells in mice deficient in the LIM-kinase 2 gene.

LIM-Kinase (LIMK), including LIMK1 and LIMK2, is the only known catalytic protein among LIM-family molecules. It is well known that LIMK phosphorylates and inactivates cofilin, an actin-depolymerizing factor regulating actin reorganization, while in vivo functions have remained to be elucidated. In the present study, we generated Limk2 gene-deficient mice in which three LIMK2 isoforms were disrupted in a Cre-mediated fashion. Impaired cofilin phosphorylation was clearly observed in Limk2-/- fibroblasts stimulated with bradykinin or lysophosphatidic acid, thereby suggesting that Cdc42 or Rho-dependent LIMK activation did not occur. However, Limk2-/- mice did not exhibit embryonic lethality or any phenotypic abnormalities in postnatal growth and development, except for spermatogenesis in the testis. The testes of Limk2-/- mice were smaller in size and partial degeneration of spermatogenic cells in the seminiferous tubules was apparent in association with increased apoptosis. In addition, the viability of Limk2-/- spermatogenic cells, when cultured under stressed conditions, was diminished. Furthermore, the potential for germ cells to differentiate in a regenerative state was severely impaired in Limk2-/- testis. Experimental hyperthermia induced impairment of ADF/cofilin phosphorylation and the formation of intranuclear cofilin inclusions in mutant germ cells. Based on these findings, we propose that LIMK2, especially the testis-specific isoform tLIMK2, plays an important role in proper progression of spermatogenesis by regulation of cofilin activity and/or localization in germ cells.     

Conservation and amyloid formation: a study of the gelsolin-like family

The mechanism through which globular proteins transform into amyloid fibrils is still not understood. Here we analyze the structure and sequence conservation to assess the differential stability of segments from two structurally related protein families: the amyloidogenic gelsolin-like and its structurally related cofilin-like. The two families belong to the actin depolymerizing proteins, with a central beta-sheet stacked between 2 and 4 alpha-helices. Although sequentially remote, the two families share regions of high and low conservation and stability. Our results show a highly conserved hydrophobic and aromatic cluster, located at a central buried beta-hairpin. The geometry of the aromatic residues with respect to each other is strictly conserved, suggesting involvement in strand registering and beta-sheet stabilization. Consistent with experiment, we find a region of weak conservation and stability at one of the exposed beta-strands (strand B in the gelsolin-like family). This region was recently found to be affected by a point mutation-mediated destabilization of the human gelsolin domain 2, which facilitates the first proteolytic event in the formation of the amyloidogenic fragment. Thus, both experimental and computational conservation analyses suggest that this unstable region may constitute a first step in amyloid formation. Our analysis uses a recently developed multiple-structure comparison algorithm in which molecules are aligned simultaneously.     

Mechanism of oleic acid-induced myofibril disassembly in rat cardiomyocytes

This study investigated the mechanism of oleic acid (OA)-induced disassembly of myofibrils in cardiomyocytes. OA treatment disrupted myofibrils, as revealed by the disorganization of several sarcomeric proteins. Since focal adhesions (FAs) are implicated in myofibril assembly, we examined structural changes in FAs after OA treatment. Immunofluorescence studies with antibodies against FA proteins (vinculin, integrin beta1D, and paxillin) showed that FAs and costameres disintegrated or disappeared after OA treatment and that the changes in FA proteins occurred prior to myofibril disassembly. The effects of OA on FAs and myofibrils were reversed after removal of OA. OA decreased expression of integrin beta1D, paxillin, vinculin, and actin, and induced tyrosine dephosphorylation of FA kinase (FAK) and paxillin. These effects were blocked by pretreatment with sodium orthovanadate, a protein tyrosine phosphatase (PTP) inhibitor. This inhibitor also prevented OA-induced myofibril disassembly, indicating the involvement of PTP in myofibril disassembly. Furthermore, OA increased protein levels of PTP-PEST. The upregulation of this phosphatase correlated with the tyrosine dephosphorylation of paxillin and FAK, which are targets for PTP-PEST. In addition, OA decreased RhoA activity and the phosphorylation of cofilin, a downstream target of RhoA. Cofilin dephosphorylation increased its actin-severing activity and led to the depolymerization of F-actin, which might provide another potential mechanism for OA-induced myofibril disassembly.     

Cofilin increases the torsional flexibility and dynamics of actin filaments

We have measured the effects of cofilin on the conformation and dynamics of actin filaments labeled at Cys374 with erythrosin-iodoacetemide (ErIA), using time-resolved phosphorescence anisotropy (TPA). Cofilin quenches the phosphorescence intensity of actin-bound ErIA, indicating that binding changes the local environment of the probe. The cofilin concentration-dependence of the phosphorescence intensity is sigmoidal, consistent with cooperative actin filament binding. Model-independent analysis of the anisotropies indicates that cofilin increases the rates of the microsecond rotational motions of actin. In contrast to the reduction in phosphorescence intensity, the changes in the rates of rotational motions display non-nearest-neighbor cooperative interactions and saturate at substoichiometric cofilin binding densities. Detailed analysis of the TPA decays indicates that cofilin decreases the torsional rigidity (C) of actin, increasing the thermally driven root-mean-square torsional angle between adjacent filament subunits from approximately 4 degrees (C = 2.30 x 10(-27) Nm2 radian(-1)) to approximately 17 degrees (C = 0.13 x 10(-27) Nm2 radian(-1)) at 25 degrees C. We favor a mechanism in which cofilin binding shifts the equilibrium between thermal ErIA-actin filament conformers, and facilitates two distinct structural changes in actin. One is local in nature, which affects the structure of actin's C terminus and is likely to mediate nearest-neighbor cooperative binding and filament severing. The second is a change in the internal dynamics of actin, which displays non-nearest-neighbor cooperativity and increases the torsional flexibility of filaments. The long-range effects of cofilin on the torsional dynamics of actin may accelerate P(i) release from filaments and modulate interactions with other regulatory actin filament binding proteins.     

Actin Mechanics and Fragmentation

Cell physiological processes require the regulation and coordination of both mechanical and dynamical properties of the actin cytoskeleton. Here we review recent advances in understanding the mechanical properties and stability of actin filaments and how these properties are manifested at larger (network) length scales. We discuss how forces can influence local biochemical interactions, resulting in the formation of mechanically sensitive dynamic steady states. Understanding the regulation of such force-activated chemistries and dynamic steady states reflects an important challenge for future work that will provide valuable insights as to how the actin cytoskeleton engenders mechanoresponsiveness of living cells.     

An MEK-cofilin signalling module controls migration of human T cells in 3D but not 2D environments

T cells infiltrate peripheral tissues to execute immunosurveillance and effector functions. For this purpose, T cells first migrate on the two‐dimensional (2D) surface of endothelial cells to undergo transendothelial migration. Then they change their mode of movement to undergo migration within the three‐dimensional (3D)‐extracellular matrix of the infiltrated tissue. As yet, no molecular mechanisms are known, which control migration exclusively in either 2D or 3D environments. Here, we describe a signalling module that controls T‐cell chemotaxis specifically in 3D environments. In chemotaxing T cells, Ras activity is spatially restricted to the lamellipodium. There, Ras initiates activation of MEK, which in turn inhibits LIM‐kinase 1 activity, thereby allowing dephosphorylation of the F‐actin‐remodelling protein cofilin. Interference with this MEK‐cofilin module by either inhibition of MEK or by knockdown of cofilin reduces speed and directionality of chemotactic migration in 3D‐extracellular matrices, but not on 2D substrates. This MEK‐cofilin module may have an important function in the tissue positioning of T cells during an immune response.     

Changes in cell adhesivity and cytoskeleton‐related proteins during imatinib‐induced apoptosis of leukemic JURL‐MK1 cells

The fusion protein Bcr–Abl, which is the molecular cause of chronic myelogenous leukemia (CML) interacts in multiple points with signaling pathways regulating the cellular adhesivity and cytoskeleton architecture and dynamics. We explored the effects of imatinib mesylate, an inhibitor of Bcr–Abl protein used in front‐line CML therapy, on the adhesivity of JURL‐MK1 cells to fibronectin and searched for underlying changes in the cell proteome. As imatinib induces apoptosis of JURL‐MK1 cells, we used three different caspase inhibitors to discriminate between direct consequences of Bcr–Abl inhibition and secondary changes related to the apoptosis. Imatinib treatment caused a transient increase in JURL‐MK1 cell adhesivity to fibronectin, possibly due to the switch off of Bcr–Abl activity. Subsequently, we observed a number of changes including a decrease in cell adhesivity, F‐actin decomposition, reduction of integrin β1, CD44, and paxillin expression levels and a marked increase in cofilin phophorylation at Ser3. These events were generally related to the proceeding apoptosis but they differed in their sensitivity to the individual caspase inhibitors. J. Cell. Biochem. 111: 1413–1425, 2010. © 2010 Wiley‐Liss, Inc.