Cingulin and paracingulin tether myosins-2 to junctions to mechanoregulate the plasma membrane.

The mechanisms that regulate the spatial sorting of nonmuscle myosins-2 (NM2) isoforms and couple them mechanically to the plasma membrane are unclear. Here we show that the cytoplasmic junctional proteins cingulin (CGN) and paracingulin (CGNL1) interact directly with NM2s through their C-terminal coiled-coil sequences. CGN binds strongly to NM2B, and CGNL1 to NM2A and NM2B. Knockout (KO), exogenous expression, and rescue experiments with WT and mutant proteins show that the NM2-binding region of CGN is required for the junctional accumulation of NM2B, ZO-1, ZO-3, and phalloidin-labeled actin filaments, and for the maintenance of tight junction membrane tortuosity and apical membrane stiffness. CGNL1 expression promotes the junctional accumulation of both NM2A and NM2B and its KO results in myosin-dependent fragmentation of adherens junction complexes. These results reveal a mechanism for the junctional localization of NM2A and NM2B and indicate that, by binding to NM2s, CGN and CGNL1 mechanically couple the actomyosin cytoskeleton to junctional protein complexes to mechanoregulate the plasma membrane.

CAP2 deficiency delays myofibril actin cytoskeleton differentiation and disturbs skeletal muscle architecture and function.

Actin filaments (F-actin) are key components of sarcomeres, the basic contractile units of skeletal muscle myofibrils. A crucial step during myofibril differentiation is the sequential exchange of α-actin isoforms from smooth muscle (α-SMA) and cardiac (α-CAA) to skeletal muscle α-actin (α-SKA) that, in mice, occurs during early postnatal life. This "α-actin switch" requires the coordinated activity of actin regulators because it is vital that sarcomere structure and function are maintained during differentiation. The molecular machinery that controls the α-actin switch, however, remains enigmatic. Cyclase-associated proteins (CAP) are a family of actin regulators with largely unknown physiological functions. We here report a function for CAP2 in regulating the α-actin exchange during myofibril differentiation. This α-actin switch was delayed in systemic CAP2 mutant mice, and myofibrils remained in an undifferentiated stage at the onset of the often excessive voluntary movements in postnatal mice. The delay in the α-actin switch coincided with the onset of motor function deficits and histopathological changes including a high frequency of type IIB ring fibers. Our data suggest that subtle disturbances of postnatal F-actin remodeling are sufficient for predisposing muscle fibers to form ring fibers. Cofilin2, a putative CAP2 interaction partner, has been recently implicated in myofibril actin cytoskeleton differentiation, and the myopathies in cofilin2 and CAP2 mutant mice showed striking similarities. We therefore propose a model in which CAP2 and cofilin2 cooperate in actin regulation during myofibril differentiation.

Author Correction: Variants in exons 5 and 6 of ACTB cause syndromic thrombocytopenia.

The original version of this Article contained an error in Figure 4. In panel i, the lower CYA and α-SMA images were inadvertently inverted. This has been corrected in both the PDF and HTML versions of the Article.

Variants in exons 5 and 6 of ACTB cause syndromic thrombocytopenia.

Germline mutations in the ubiquitously expressed ACTB, which encodes ß-cytoplasmic actin (CYA), are almost exclusively associated with Baraitser-Winter Cerebrofrontofacial syndrome (BWCFF). Here, we report six patients with previously undescribed heterozygous variants clustered in the 3'-coding region of ACTB. Patients present with clinical features distinct from BWCFF, including mild developmental disability, microcephaly, and thrombocytopenia with platelet anisotropy. Using patient-derived fibroblasts, we demonstrate cohort specific changes to ß-CYA filament populations, which include the enhanced recruitment of thrombocytopenia-associated actin binding proteins (ABPs). These perturbed interactions are supported by in silico modeling and are validated in disease-relevant thrombocytes. Co-examination of actin and microtubule cytoskeleton constituents in patient-derived megakaryocytes and thrombocytes indicates that these ß-CYA mutations inhibit the final stages of platelet maturation by compromising microtubule organization. Our results define an ACTB-associated clinical syndrome with a distinct genotype-phenotype correlation and delineate molecular mechanisms underlying thrombocytopenia in this patient cohort.

Vascular Smooth Muscle Contractile Function Declines With Age in Skeletal Muscle Feed Arteries.

Aging induces a progressive decline in vasoconstrictor responses in central and peripheral arteries. This study investigated the hypothesis that vascular smooth muscle (VSM) contractile function declines with age in soleus muscle feed arteries (SFA). Contractile function of cannulated SFA isolated from young (4 months) and old (24 months) Fischer 344 rats was assessed by measuring constrictor responses of denuded (endothelium removed) SFA to norepinephrine (NE), phenylephrine (PE), and angiotensin II (Ang II). In addition, we investigated the role of RhoA signaling in modulation of VSM contractile function. Structural and functional characteristics of VSM cells were evaluated by fluorescence imaging and atomic force microscopy (AFM). Results indicated that constrictor responses to PE and Ang II were significantly impaired in old SFA, whereas constrictor responses to NE were preserved. In the presence of a Rho-kinase inhibitor (Y27632), constrictor responses to NE, Ang II, and PE were significantly reduced in young and old SFA. In addition, the age-group difference in constrictor responses to Ang II was eliminated. ROCK1 and ROCK2 content was similar in young and old VSM cells, whereas pROCK1 and pROCK2 were significantly elevated in old VSM cells. Aging was associated with a reduction in smooth muscle α-actin stress fibers and recruitment of proteins to cell-matrix adhesions. Old VSM cells presented an increase in integrin adhesion to the matrix and smooth muscle γ-actin fibers that was associated with increased cell stiffness. In conclusion, our results indicate that VSM contractile function declined with age in SFA. The decrement in contractile function was mediated in part by RhoA/ROCK signaling. Upregulation of pROCK in old VSM cells was not able to rescue contractility in old SFA. Collectively, these results indicate that changes at the VSM cell level play a central role in the reduced contractile function of aged SFA.

Divergent roles of ß- and γ-actin isoforms during spread of vaccinia virus.

Actin is a major component of the cytoskeleton and is present as two isoforms in non-muscle cells: ß- and γ-cytoplasmic actin. These isoforms are strikingly conserved, differing by only four N-terminal amino acids. During spread from infected cells, vaccinia virus (VACV) particles induce localized actin nucleation that propel virus to surrounding cells and facilitate cell-to-cell spread of infection. Here we show that virus-tipped actin comets are composed of ß- and γ-actin. We employed isoform-specific siRNA knockdown to examine the role of the two isoforms in VACV-induced actin comets. Despite the high level of similarity between the actin isoforms, and their colocalization, VACV-induced actin nucleation was dependent exclusively on ß-actin. Knockdown of ß-actin led to a reduction in the release of virus from infected cells, a phenotype dependent on virus-induced Arp2/3 complex activity. We suggest that local concentrations of actin isoforms may regulate the activity of cellular actin nucleator complexes.

Monoclonal antibodies against muscle actin isoforms: epitope identification and analysis of isoform expression by immunoblot and immunostaining in normal and regenerating skeletal muscle.

Higher vertebrates express six different highly conserved actin isoforms that can be classified in three subgroups: 1) sarcomeric actins, α-skeletal (α-SKA) and α-cardiac (α-CAA), 2) smooth muscle actins (SMAs), α-SMA and γ-SMA, and 3) cytoplasmic actins (CYAs), ß-CYA and γ-CYA. The variations among isoactins, in each subgroup, are due to 3-4 amino acid differences located in their acetylated N-decapeptide sequence. The first monoclonal antibody (mAb) against an actin isoform (α-SMA) was produced and characterized in our laboratory in 1986 (Skalli  et al., 1986). We have further obtained mAbs against the 5 other isoforms. In this report, we focus on the mAb anti-α-SKA and anti-α-CAA obtained after immunization of mice with the respective acetylated N-terminal decapeptides using the Repetitive Immunizations at Multiple Sites Strategy (RIMMS). In addition to the identification of their epitope by immunoblotting, we describe the expression of the 2 sarcomeric actins in mature skeletal muscle and during muscle repair after micro-lesions. In particular, we analyze the expression of α-CAA, α-SKA and α-SMA by co-immunostaining in a time course frame during the muscle repair process. Our results indicate that a restricted myocyte population expresses α-CAA and suggest a high capacity of self-renewal in muscle cells. These antibodies may represent a helpful tool for the follow-up of muscle regeneration and pathological changes.

Stable incorporation of α-smooth muscle actin into stress fibers is dependent on specific tropomyosin isoforms.

α-Smooth Muscle Actin (α-SMA), a widely characterized cytoskeletal protein, represents the hallmark of myofibroblast differentiation. Transforming growth factorß1 (TGFß1) stimulates α-SMA expression and incorporation into stress fibers, thus providing an increased myofibroblast contractile force that participates in tissue remodeling. We have addressed the molecular mechanism by which α-SMA is stably incorporated into stress fibers in human myofibroblasts following exposure to TGFß1. The unique N-terminal sequence AcEEED, which is critical for α-SMA incorporation into stress fibers, was used to screen for AcEEED binding proteins. Tropomyosins were identified as candidate binding proteins. We find that after TGFß1 treatment elevated levels of the Tpm1.6/7 isoforms, and to a lesser extent Tpm2.1, precede the increase in α-SMA. RNA interference experiments demonstrate that α-SMA fails to stably incorporate into stress fibers of TGFß1 treated fibroblasts depleted of Tpm1.6/7, but not other tropomyosins. This does not appear to be due to exclusive interactions between α-SMA and just the Tpm1.6/7 isoforms. We propose that an additional AcEEED binding factor may be required to generate α-SMA filaments containing just Tpm1.6/7 which result in stable incorporation of the resulting filaments into stress fibers.

Tumor promotion by γ and suppression by ß non-muscle actin isoforms.

Here we have shown that ß-cytoplasmic actin acts as a tumor suppressor, inhibiting cell growth and invasion in vitro and tumor growth in vivo. In contrast, γ-cytoplasmic actin increases the oncogenic potential via ERK1/2, p34-Arc, WAVE2, cofilin1, PP1 and other regulatory proteins. There is a positive feedback loop between γ-actin expression and ERK1/2 activation. We conclude that non-muscle actin isoforms should not be considered as merely housekeeping proteins and the ß/γ-actins ratio can be used as an oncogenic marker at least for lung and colon carcinomas. Agents that increase ß- and/or decrease γ-actin expression may be useful for anticancer therapy.

Loss of γ-cytoplasmic actin triggers myofibroblast transition of human epithelial cells.

Transdifferentiation of epithelial cells into mesenchymal cells and myofibroblasts plays an important role in tumor progression and tissue fibrosis. Such epithelial plasticity is accompanied by dramatic reorganizations of the actin cytoskeleton, although mechanisms underlying cytoskeletal effects on epithelial transdifferentiation remain poorly understood. In the present study, we observed that selective siRNA-mediated knockdown of γ-cytoplasmic actin (γ-CYA), but not ß-cytoplasmic actin, induced epithelial-to-myofibroblast transition (EMyT) of different epithelial cells. The EMyT manifested by increased expression of α-smooth muscle actin and other contractile proteins, along with inhibition of genes responsible for cell proliferation. Induction of EMyT in γ-CYA-depleted cells depended on activation of serum response factor and its cofactors, myocardial-related transcriptional factors A and B. Loss of γ-CYA stimulated formin-mediated actin polymerization and activation of Rho GTPase, which appear to be essential for EMyT induction. Our findings demonstrate a previously unanticipated, unique role of γ-CYA in regulating epithelial phenotype and suppression of EMyT that may be essential for cell differentiation and tissue fibrosis.

Severe protein aggregate myopathy in a knockout mouse model points to an essential role of cofilin2 in sarcomeric actin exchange and muscle maintenance.

Mutations in the human actin depolymerizing factor cofilin2 result in an autosomal dominant form of nemaline myopathy. Here, we report on the targeted ablation of murine cofilin2, which leads to a severe skeletal muscle specific phenotype within the first two weeks after birth. Apart from skeletal muscle, cofilin2 is also expressed in heart and CNS, however the pathology was restricted to skeletal muscle. The two close family members of cofilin2 - ADF and cofilin1 - were co-expressed in muscle, but unable to compensate for the loss of cofilin2. While primary myofibril assembly and muscle development were unaffected in cofilin2 mutant mice, progressive muscle degeneration was observed between postnatal days 3 and 7. Muscle pathology was characterized by sarcoplasmic protein aggregates, fiber size disproportion, mitochondrial abnormalities and internal nuclei. The observed muscle pathology differed from nemaline myopathy, but showed combined features of actin-associated myopathy and myofibrillar myopathy. In cofilin2 mutant mice, the postnatal expression pattern and turnover of sarcomeric α-actin isoforms were altered. Levels of smooth muscle α-actin were increased and remained high in developing muscles, suggesting that cofilin2 plays a crucial role during the exchange of α-actin isoforms during the early postnatal remodeling of the sarcomere.

Smooth muscle actin isoforms: a tug of war between contraction and compliance.

In higher vertebrates, smooth muscle (SM) contains two tissue-specific actin isoforms: α-SMA and γ-SMA, which predominate in vascular and visceral SM, respectively. Whether α-SMA has been extensively studied and recognized for its contractile activity in SM and SM-like cells such as myofibroblasts, myoepithelial and myoid cells, the distribution and role of γ-SMA remained largely unknown. We developed a new specific monoclonal antibody against γ-SMA and confirmed that γ-SMA predominates in the visceral system and is minor in the vascular system, although more expressed in highly compliant veins than in stiff arteries. Contrary to α-SMA, γ-SMA is absent from myofibroblasts in vitro, and in fibrotic diseases in vivo. We raised the hypothesis that, whereas α-SMA is responsible for the "contractile" activity, γ-SMA would be involved in the "compliance" of SM and SM-like cells. Several models support this hypothesis, namely veins vs. arteries and the physiological modifications occurring in the uterus and mammary glands during pregnancy and lactation. Our results suggest that, in addition to enteric smooth muscles, γ-SMA is expressed in all the tissues submitted to an important dilation including veins, gravid uterus, and lactating mammary glands. The hypothesis of two complementary mechanical roles for the two SMA isoforms is sustained by their different intracellular distributions and by functional assays.

Smooth muscle hyperplasia due to loss of smooth muscle α-actin is driven by activation of focal adhesion kinase, altered p53 localization and increased levels of platelet-derived growth factor receptor-ß.

Mutations in ACTA2, encoding the smooth muscle cell (SMC)-specific isoform of α-actin (α-SMA), cause thoracic aortic aneurysms and dissections and occlusive vascular diseases, including early onset coronary artery disease and stroke. We have shown that occlusive arterial lesions in patients with heterozygous ACTA2 missense mutations show increased numbers of medial or neointimal SMCs. The contribution of SMC hyperplasia to these vascular diseases and the pathways responsible for linking disruption of α-SMA filaments to hyperplasia are unknown. Here, we show that the loss of Acta2 in mice recapitulates the SMC hyperplasia observed in ACTA2 mutant SMCs and determine the cellular pathways responsible for SMC hyperplasia. Acta2(-/-) mice showed increased neointimal formation following vascular injury in vivo, and SMCs explanted from these mice demonstrated increased proliferation and migration. Loss of α-SMA induced hyperplasia through focal adhesion (FA) rearrangement, FA kinase activation, re-localization of p53 from the nucleus to the cytoplasm and increased expression and ligand-independent activation of platelet-derived growth factor receptor beta (Pdgfr-ß). Disruption of α-SMA in wild-type SMCs also induced similar cellular changes. Imatinib mesylate inhibited Pdgfr-ß activation and Acta2(-/-) SMC proliferation in vitro and neointimal formation with vascular injury in vivo. Loss of α-SMA leads to SMC hyperplasia in vivo and in vitro through a mechanism involving FAK, p53 and Pdgfr-ß, supporting the hypothesis that SMC hyperplasia contributes to occlusive lesions in patients with ACTA2 missense mutations.

Cooperation between ß- and γ-cytoplasmic actins in the mechanical regulation of endothelial microparticle formation.

Elevated endothelial microparticle (MP) levels are observed in numerous diseases, increasingly supporting roles as effectors and valuable markers of vascular dysfunction. While a contractile role for the actin cytoskeleton has been implicated in vesiculation, i.e., MP production, the precise interactions and mechanisms of its constituents, ß- and γ-cytoplasmic actins, is unknown. Human cerebral microvascular endothelial cells were stimulated with known agonists, and vesiculation development was monitored by scanning electron microscopy (SEM) and flow cytometry. These data in combination provide new insight into the kinetics, patterns of vesiculating cell recruitment, and degrees of response specific to stimuli. Reorganization of ß- and γ-actins, F-actin, vinculin, and talin accompanied significant MP release. ß-Actin redistribution into basal stress fibers following stimulation was associated with increased apically situated actin-rich particulate structures, which in turn directly correlated with electron-lucent membrane protrusions observed by SEM. Y-27632 Rho-kinase inhibition abolished basal ß-actin fiber formation, minimizing apically associated actin-rich structures, significantly reducing membrane protrusions and MP release to near basal levels. Cytoskeletal protein expression and distribution varied between MPs and mother cells, as determined by Western blot. These data strongly suggest that ß-actin plays an active facilitative role in agonist-induced protuberance formation, through mechanical interactions with newly described actin-rich structures.

Nonredundant roles of cytoplasmic ß- and γ-actin isoforms in regulation of epithelial apical junctions.

Association with the actin cytoskeleton is critical for normal architecture and dynamics of epithelial tight junctions (TJs) and adherens junctions (AJs). Epithelial cells express ß-cytoplasmic (ß-CYA) and γ-cytoplasmic (γ-CYA) actins, which have different cellular localization and functions. This study elucidates the roles of cytoplasmic actins in regulating structure and remodeling of AJs and TJs in model intestinal epithelia. Immunofluorescence labeling and latrunculin B treatment reveal affiliation of dynamic ß-CYA filaments with newly assembled and mature AJs, whereas an apical γ-CYA pool is composed of stable perijunctional bundles and rapidly turning-over nonjunctional filaments. The functional effects of cytoplasmic actins on epithelial junctions are examined by using isoform-specific small interfering RNAs and cell-permeable inhibitory peptides. These experiments demonstrate unique roles of ß-CYA and γ-CYA in regulating the steady-state integrity of AJs and TJs, respectively. Furthermore, ß-CYA is selectively involved in establishment of apicobasal cell polarity. Both actin isoforms are essential for normal barrier function of epithelial monolayers, rapid AJ/TJ reassembly, and formation of three-dimensional cysts. Cytoplasmic actin isoforms play unique roles in regulating structure and permeability of epithelial junctions.

Resveratrol treatment reduces cardiac progenitor cell dysfunction and prevents morpho-functional ventricular remodeling in type-1 diabetic rats.

Emerging evidence suggests that both adult cardiac cell and the cardiac stem/progenitor cell (CSPC) compartments are involved in the patho-physiology of diabetic cardiomyopathy (DCM). We evaluated whether early administration of Resveratrol, a natural antioxidant polyphenolic compound, in addition to improving cardiomyocyte function, exerts a protective role on (i) the progenitor cell pool, and (ii) the myocardial environment and its impact on CSPCs, positively interfering with the onset of DCM phenotype. Adult Wistar rats (n = 128) with streptozotocin-induced type-1 diabetes were either untreated (D group; n = 54) or subjected to administration of trans-Resveratrol (i.p. injection: 2.5 mg/Kg/day; DR group; n = 64). Twenty-five rats constituted the control group (C). After 1, 3 or 8 weeks of hyperglycemia, we evaluated cardiac hemodynamic performance, and cardiomyocyte contractile properties and intracellular calcium dynamics. Myocardial remodeling and tissue inflammation were also assessed by morphometry, immunohistochemistry and immunoblotting. Eventually, the impact of the diabetic "milieu" on CSPC turnover was analyzed in co-cultures of healthy CSPCs and cardiomyocytes isolated from D and DR diabetic hearts. In untreated animals, cardiac function was maintained during the first 3 weeks of hyperglycemia, although a definite ventricular remodeling was already present, mainly characterized by a marked loss of CSPCs and adult cardiac cells. Relevant signs of ventricular dysfunction appeared after 8 weeks of diabetes, and included: 1) a significant reduction in ±dP/dt in comparison with C group, 2) a prolongation of isovolumic contraction/relaxation times, 3) an impaired contraction of isolated cardiomyocytes associated with altered intracellular calcium dynamics. Resveratrol administration reduced atrial CSPC loss, succeeded in preserving the functional abilities of CSPCs and mature cardiac cells, improved cardiac environment by reducing inflammatory state and decreased unfavorable ventricular remodeling of the diabetic heart, leading to a marked recovery of ventricular function. These findings indicate that RSV can constitute an adjuvant therapeutic option in DCM prevention.

γ-Actin regulates cell migration and modulates the ROCK signaling pathway.

Cell migration plays a crucial role in numerous cellular functions, and alterations in the regulation of cell migration are required for invasive transformation of a tumor cell. While the mechanistic process of actin-based migration has been well documented, little is known as to the specific function of the nonmuscle actin isoforms in mammalian cells. Here, we present a comprehensive examination of γ-actin's role in cell migration using an RNAi approach. The partial suppression of γ-actin expression in SH-EP neuroblastoma cells resulted in a significant decrease in wound healing and transwell migration. Similarly, the knockdown of γ-actin significantly reduced speed of motility and severely affected the cell's ability to explore, which was, in part, due to a loss of cell polarity. Moreover, there was a significant increase in the size and number of paxillin-containing focal adhesions, coupled with a significant decrease in phosphorylated paxillin in γ-actin-knockdown cells. In addition, there was a significant increase in the phosphorylation of cofilin and myosin regulatory light chain, suggesting an overactivated Rho-associated kinase (ROCK) signaling pathway in γ-actin-knockdown cells. The alterations in the phosphorylation of paxillin and myosin regulatory light chain were unique to γ-actin and not ß-actin knockdown. Inhibition of the ROCK pathway with the inhibitor Y-27632 restored the ability of γ-actin-knockdown cells to migrate. This study demonstrates γ-actin as a potential upstream regulator of ROCK mediated cell migration.

Beta- and gamma-cytoplasmic actins are required for meiosis in mouse oocytes.

In mammals, female meiosis consists of two asymmetric cell divisions, which generate a large haploid oocyte and two small polar bodies. Asymmetric partitioning of the cytoplasm results from migration of the meiotic spindle toward the cortex and requires actin filaments. However, the subcellular localization and the role of the existing two cytoplasmic actin (CYA) isoforms, beta and gamma, have not been characterized. We show that beta- and gamma-CYA are differentially distributed in the maturing oocyte from late metaphase I as well as in preimplantation embryos. Gamma-CYA is preferentially enriched in oocyte cortices and is absent from all cell-cell contact areas from metaphase II until the blastocyst stage. Beta-CYA is enriched in contractile structures, at cytokinesis, at cell-cell contacts, and around the forming blastocoel. Alteration of beta- or gamma-CYA function by isoform-specific antibody microinjection suggests that gamma-CYA holds a major and specific role in the establishment and/or maintenance of asymmetry in meiosis I and in the maintenance of overall cortical integrity. In contrast, beta- and gamma-CYA, together, appear to participate in the formation and the cortical anchorage of the second meiotic spindle in waiting for fertilization. Finally, differences in gamma-CYA expression are amongst the earliest markers of cell fate determination in development.

Tropomyosin variants describe distinct functional subcellular domains in differentiated vascular smooth muscle cells.

Tropomyosin (Tm) is known to be an important gatekeeper of actin function. Tm isoforms are encoded by four genes, and each gene produces several variants by alternative splicing, which have been proposed to play roles in motility, proliferation, and apoptosis. Smooth muscle studies have focused on gizzard smooth muscle, where a heterodimer of Tm from the α-gene (Tmsm-α) and from the ß-gene (Tmsm-ß) is associated with contractile filaments. In this study we examined Tm in differentiated mammalian vascular smooth muscle (dVSM). Liquid chromatography-tandem mass spectrometry (LC MS/MS) analysis and Western blot screening with variant-specific antibodies revealed that at least five different Tm proteins are expressed in this tissue: Tm6 (Tmsm-α) and Tm2 from the α-gene, Tm1 (Tmsm-ß) from the ß-gene, Tm5NM1 from the γ-gene, and Tm4 from the δ-gene. Tm6 is by far most abundant in dVSM followed by Tm1, Tm2, Tm5NM1, and Tm4. Coimmunoprecipitation and coimmunofluorescence studies demonstrate that Tm1 and Tm6 coassociate with different actin isoforms and display different intracellular localizations. Using an antibody specific for cytoplasmic γ-actin, we report here the presence of a γ-actin cortical cytoskeleton in dVSM cells. Tm1 colocalizes with cortical cytoplasmic γ-actin and coprecipitates with γ-actin. Tm6, on the other hand, is located on contractile bundles. These data indicate that Tm1 and Tm6 do not form a classical heterodimer in dVSM but rather describe different functional cellular compartments.

Sendai virus induced cytoplasmic actin remodeling correlates with efficient virus particle production.

Cytoplasmic actins have been found interacting with viral proteins and identified in virus particles. We analyzed by confocal microscopy the cytoplasmic ß- and γ-actin patterns during the course of Sendai virus infections in polarized cells. We observed a spectacular remodeling of the ß-cytoplasmic actin which correlated with productive viral multiplication. Conversely, suppression of M during the course of a productive infection resulted in the decrease of particle production and the absence of ß-actin remodeling. As concomitant suppression of ß- and γ-actins resulted as well in reduction of virus particle production, we propose that Sendai virus specifically induces actin remodeling in order to promote efficient virion production. Beta- and γ-cytoplasmic actin recruitment could substitute for that of the endosomal sorting complex required for transport (ESCRT) mobilized by other enveloped viruses but apparently not used by Sendai virus.