The lipodystrophic hotspot lamin A p.R482W mutation deregulates the mesodermal inducer T/Brachyury and early vascular differentiation gene networks.

The p.R482W hotspot mutation in A-type nuclear lamins causes familial partial lipodystrophy of Dunnigan-type (FPLD2), a lipodystrophic syndrome complicated by early onset atherosclerosis. Molecular mechanisms underlying endothelial cell dysfunction conferred by the lamin A mutation remain elusive. However, lamin A regulates epigenetic developmental pathways and mutations could perturb these functions. Here, we demonstrate that lamin A R482W elicits endothelial differentiation defects in a developmental model of FPLD2. Genome modeling in fibroblasts from patients with FPLD2 caused by the lamin A R482W mutation reveals repositioning of the mesodermal regulator T/Brachyury locus towards the nuclear center relative to normal fibroblasts, suggesting enhanced activation propensity of the locus in a developmental model of FPLD2. Addressing this issue, we report phenotypic and transcriptional alterations in mesodermal and endothelial differentiation of induced pluripotent stem cells we generated from a patient with R482W-associated FPLD2. Correction of the LMNA mutation ameliorates R482W-associated phenotypes and gene expression. Transcriptomics links endothelial differentiation defects to decreased Polycomb-mediated repression of the T/Brachyury locus and over-activation of T target genes. Binding of the Polycomb repressor complex 2 to T/Brachyury is impaired by the mutated lamin A network, which is unable to properly associate with the locus. This leads to a deregulation of vascular gene expression over time. By connecting a lipodystrophic hotspot lamin A mutation to a disruption of early mesodermal gene expression and defective endothelial differentiation, we propose that the mutation rewires the fate of several lineages, resulting in multi-tissue pathogenic phenotypes.

A lipodystrophy-causing lamin A mutant alters conformation and epigenetic regulation of the anti-adipogenic MIR335 locus.

Mutations in the Lamin A/C (LMNA) gene-encoding nuclear LMNA cause laminopathies, which include partial lipodystrophies associated with metabolic syndromes. The lipodystrophy-associated LMNA p.R482W mutation is known to impair adipogenic differentiation, but the mechanisms involved are unclear. We show in this study that the lamin A p.R482W hot spot mutation prevents adipogenic gene expression by epigenetically deregulating long-range enhancers of the anti-adipogenic MIR335 microRNA gene in human adipocyte progenitor cells. The R482W mutation results in a loss of function of differentiation-dependent lamin A binding to the MIR335 locus. This impairs H3K27 methylation and instead favors H3K27 acetylation on MIR335 enhancers. The lamin A mutation further promotes spatial clustering of MIR335 enhancer and promoter elements along with overexpression of the MIR355 gene after adipogenic induction. Our results link a laminopathy-causing lamin A mutation to an unsuspected deregulation of chromatin states and spatial conformation of an miRNA locus critical for adipose progenitor cell fate.

Chrom3D: three-dimensional genome modeling from Hi-C and nuclear lamin-genome contacts.

Current three-dimensional (3D) genome modeling platforms are limited by their inability to account for radial placement of loci in the nucleus. We present Chrom3D, a user-friendly whole-genome 3D computational modeling framework that simulates positions of topologically-associated domains (TADs) relative to each other and to the nuclear periphery. Chrom3D integrates chromosome conformation capture (Hi-C) and lamin-associated domain (LAD) datasets to generate structure ensembles that recapitulate radial distributions of TADs detected in single cells. Chrom3D reveals unexpected spatial features of LAD regulation in cells from patients with a laminopathy-causing lamin mutation. Chrom3D is freely available on github.

Mapping Nuclear Lamin-Genome Interactions by Chromatin Immunoprecipitation of Nuclear Lamins.

The nuclear lamina is a meshwork of A- and B-type lamins which interact with chromatin and regulate many nuclear functions. Recent studies have reported the discovery of chromatin domains interacting with nuclear lamins by chromatin immunoprecipitation (ChIP) of lamin A/C or B1 and identification of the associated DNA sequences by microarray or high-throughput sequencing. ChIP has been used over many years to get a snapshot of interactions between DNA and proteins in cells, including modified histones, transcription factors, chromatin remodelers, and recently, structural proteins such as nuclear lamins. We describe here the procedure we have established in our laboratory for ChIP of lamin A/C and lamin B1 from human cultured cells. The protocol is compatible with polymerase chain reaction and high-throughput sequencing analysis of the co-immunoprecipitated DNA.

Prepatterning of differentiation-driven nuclear lamin A/C-associated chromatin domains by GlcNAcylated histone H2B.

Dynamic interactions of nuclear lamins with chromatin through lamin-associated domains (LADs) contribute to spatial arrangement of the genome. Here, we provide evidence for prepatterning of differentiation-driven formation of lamin A/C LADs by domains of histone H2B modified on serine 112 by the nutrient sensor O-linked N-acetylglucosamine (H2BS112GlcNAc), which we term GADs. We demonstrate a two-step process of lamin A/C LAD formation during in vitro adipogenesis, involving spreading of lamin A/C-chromatin interactions in the transition from progenitor cell proliferation to cell-cycle arrest, and genome-scale redistribution of these interactions through a process of LAD exchange within hours of adipogenic induction. Lamin A/C LADs are found both in active and repressive chromatin contexts that can be influenced by cell differentiation status. De novo formation of adipogenic lamin A/C LADs occurs nonrandomly on GADs, which consist of megabase-size intergenic and repressive chromatin domains. Accordingly, whereas predifferentiation lamin A/C LADs are gene-rich, post-differentiation LADs harbor repressive features reminiscent of lamin B1 LADs. Release of lamin A/C from genes directly involved in glycolysis concurs with their transcriptional up-regulation after adipogenic induction, and with downstream elevations in H2BS112GlcNAc levels and O-GlcNAc cycling. Our results unveil an epigenetic prepatterning of adipogenic LADs by GADs, suggesting a coupling of developmentally regulated lamin A/C-genome interactions to a metabolically sensitive chromatin modification.

Distinct features of lamin A-interacting chromatin domains mapped by ChIP-sequencing from sonicated or micrococcal nuclease-digested chromatin.

The nuclear lamina has been shown to interact with the genome through lamina-associated domains (LADs). LADs have been identified by DamID, a proximity labeling assay, and more recently by chromatin immunoprecipitation-sequencing (ChIP-seq) of A- and B-type lamins. LADs form megabase-size domains at the nuclear periphery, they are gene-poor and mostly heterochromatic. Here, we show that the mode of chromatin fragmentation for ChIP, namely bath sonication or digestion with micrococcal nuclease (MNase), leads to the discovery of common but also distinct sets of lamin-interacting domains, or LiDs. Using ChIP-seq, we show the existence of lamin A/C (LMNA) LiDs with distinct gene contents, histone composition enrichment and relationships to lamin B1-interacting domains. The extent of genome coverage of lamin A/C (LMNA) LiDs in sonicated or MNase-digested chromatin is similar (∼730 megabases); however over half of these domains are uniquely detected in sonicated or MNase-digested chromatin. Sonication-specific LMNA LiDs are gene-poor and devoid of a broad panel of histone modifications, while MNase-specific LMNA LiDs are of higher gene density and are enriched in H3K9me3, H3K27me3 and in histone variant H2A.Z. LMNB1 LiDs are gene-poor and show no or little enrichment in these marks. Comparison of published LMNB1 DamID LADs with LMNB1 and LMNA LiDs identified here by ChIP-seq further shows that LMNA can associate with 'open' chromatin domains displaying euchromatin characteristics, and which are not associated with LMNB1. The differential genomic and epigenetic properties of lamin-interacting domains reflect the existence of distinct LiD populations identifiable in different chromatin contexts, including nuclease-accessible regions presumably localized in the nuclear interior.

The p.R482W substitution in A-type lamins deregulates SREBP1 activity in Dunnigan-type familial partial lipodystrophy.

Nuclear lamins are involved in many cellular functions due to their ability to bind numerous partners including chromatin and transcription factors, and affect their properties. Dunnigan type familial partial lipodystrophy (FPLD2; OMIM#151660) is caused in most cases by the A-type lamin R482W mutation. We report here that the R482W mutation affects the regulatory activity of sterol response element binding protein 1 (SREBP1), a transcription factor that regulates hundreds of genes involved in lipid metabolism and adipocyte differentiation. Using in situ proximity ligation assays (PLA), reporter assays and biochemical and transcriptomic approaches, we show that interactions of SREBP1 with lamin A and lamin C occur at the nuclear periphery and in the nucleoplasm. These interactions involve the Ig-fold of A-type lamins and are favored upon SREBP1 binding to its DNA target sequences. We show that SREBP1, LMNA and sterol response DNA elements form ternary complexes in vitro. In addition, overexpression of A-type lamins reduces transcriptional activity of SREBP1. In contrast, both overexpression of LMNA R482W in primary human preadipocytes and endogenous expression of A-type lamins R482W in FPLD2 patient fibroblasts, reduce A-type lamins-SREBP1 in situ interactions and upregulate a large number of SREBP1 target genes. As this LMNA mutant was previously shown to inhibit adipogenic differentiation, we propose that deregulation of SREBP1 by mutated A-type lamins constitutes one underlying mechanism of the physiopathology of FPLD2. Our data suggest that SREBP1 targeting molecules could be considered in a therapeutic context.

Enriched domain detector: a program for detection of wide genomic enrichment domains robust against local variations.

Nuclear lamins contact the genome at the nuclear periphery through large domains and are involved in chromatin organization. Among broad peak calling algorithms available to date, none are suited for mapping lamin-genome interactions genome wide. We disclose a novel algorithm, enriched domain detector (EDD), for analysis of broad enrichment domains from chromatin immunoprecipitation (ChIP)-seq data. EDD enables discovery of genomic domains interacting with broadly distributed proteins, such as A- and B-type lamins affinity isolated by ChIP. The advantages of EDD over existing broad peak callers are sensitivity to domain width rather than enrichment strength at a particular site, and robustness against local variations.

Closing the (nuclear) envelope on the genome: how nuclear lamins interact with promoters and modulate gene expression.

The nuclear envelope shapes the functional organization of the nucleus. Increasing evidence indicates that one of its main components, the nuclear lamina, dynamically interacts with the genome, including the promoter region of specific genes. This seems to occur in a manner that accords developmental significance to these interactions. This essay addresses key issues raised by recent data on the association of nuclear lamins with the genome. We discuss how lamins interact with large chromatin domains and with spatially restricted regions on gene promoters. We address the relationship between these interactions, chromatin modifications and gene expression outcomes. Lamin-genome contacts are redistributed after cell division and during stem cell differentiation, with evidence of lineage specificity. Thus, we also speculate on a developmental role of lamin interactions with specific genes. Finally, we highlight how concepts arising from this recent work lay the foundations of future challenges and investigations.

Deregulation of Fragile X-related protein 1 by the lipodystrophic lamin A p.R482W mutation elicits a myogenic gene expression program in preadipocytes.

The nuclear lamina is implicated in the regulation of various nuclear functions. Several laminopathy-causing mutations in the LMNA gene, notably the p.R482W substitution linked to familial partial lipodystrophy type 2 (FPLD2), are clustered in the immunoglobulin fold of lamin A. We report a functional association between lamin A and fragile X-related protein 1 (FXR1P), a protein of the fragile X-related family involved in fragile X syndrome. Searching for proteins differentially interacting with the immunoglobulin fold of wild-type and R482W mutant lamin A, we identify FXR1P as a novel component of the lamin A protein network. The p.R482W mutation abrogates interaction of FXR1P with lamin A. Fibroblasts from FPLD2 patients display elevated levels of FXR1P and delocalized FXR1P. In human adipocyte progenitors, deregulation of lamin A expression leads to FXR1P up-regulation, impairment of adipogenic differentiation and induction of myogenin expression. FXR1P overexpression also stimulates a myogenic gene expression program in these cells. Our results demonstrate a cross-talk between proteins hitherto implicated in two distinct mesodermal pathologies. We propose a model where the FPLD2 lamin A p.R482W mutation elicits, through up-regulation of FXR1P, a remodeling of an adipogenic differentiation program into a myogenic program.

A hyper-dynamic nature of bivalent promoter states underlies coordinated developmental gene expression modules.

BACKGROUND: Chromatin remodeling is crucial for proper programing of developmental gene expression. Recent work provides a dynamic view of post-translational histone modifications during differentiation; however there is little insight on the evolution of combinatorial genome-wide patterns of chromatin marks, excluding an essential aspect of developmental gene regulation. RESULTS: We report here a 15-chromatin state Hidden Markov Model which describes changes in chromatin signatures in relation to transcription profiles during differentiation of human pre-adipocytes into adipocytes. We identify nineteen modules of gene expression reflecting multiple waves of transcriptional up- and down-regulation which characterize adipogenic differentiation. From our model, we developed chromatin state matrices fitting each of these transcription modules to show how the complexity and dynamic nature of chromatin signatures relate to expression patterns. Spatial relationships between chromatin states underlie a high-order chromatin organization in differentiating adipocytes. We show the importance of gene expression level in generating diversity in chromatin signatures, and show that the hyper-dynamic nature of H3K4me2/H3K27me3-marked 'bivalent' promoter states underlies many of the gene expression patterns associated with adipogenic differentiation. CONCLUSIONS: Our results reveal the highly dynamic nature of bivalent promoter states within the adipogenic lineage. The data constitute a valuable resource enabling the assessment of possibilities to alter the adipogenic program.

Lamin A/C-promoter interactions specify chromatin state-dependent transcription outcomes.

The nuclear lamina is implicated in the organization of the eukaryotic nucleus. Association of nuclear lamins with the genome occurs through large chromatin domains including mostly, but not exclusively, repressed genes. How lamin interactions with regulatory elements modulate gene expression in different cellular contexts is unknown. We show here that in human adipose tissue stem cells, lamin A/C interacts with distinct spatially restricted subpromoter regions, both within and outside peripheral and intra-nuclear lamin-rich domains. These localized interactions are associated with distinct transcriptional outcomes in a manner dependent on local chromatin modifications. Down-regulation of lamin A/C leads to dissociation of lamin A/C from promoters and remodels repressive and permissive histone modifications by enhancing transcriptional permissiveness, but is not sufficient to elicit gene activation. Adipogenic differentiation resets a large number of lamin-genome associations globally and at subpromoter levels and redefines associated transcription outputs. We propose that lamin A/C acts as a modulator of local gene expression outcome through interaction with adjustable sites on promoters, and that these position-dependent transcriptional readouts may be reset upon differentiation.

Hepatic transit time analysis using contrast-enhanced ultrasound with BR1: A prospective study comparing patients with liver metastases from colorectal cancer with healthy volunteers.

We prospectively compared hepatic transit time (HTT) measurements in subjects with liver metastases from colorectal cancer (group a) and healthy volunteers (group b) using contrast-enhanced ultrasound with BR1. The purpose of this study was to verify our hypothesis that the hemodynamic changes of the liver, which occur during metastasis seeding, would shorten the HTT, and we expect that such changes could be used for the detection of occult liver metastases from colorectal cancer in the future. The study had institutional review board approval and all subjects gave informed written consent. Group a and group b consisted of 22 subjects each. Baseline and post contrast images were acquired starting 10 s before and ending 40 s after administration of BR1, using nonlinear imaging at a frame rate of 5/s. The baseline images were used to determine the signal intensity without contrast enhancement as the reference signal. Arrival times (AT) of the contrast agent for the hepatic artery, the portal vein and one hepatic vein were determined using (i) quantitative analysis and (ii) subjective analysis by two blinded readers. HTT was calculated based on arrival time measurements. Quantitative and subjective analysis showed significantly shorter arterial to venous and portal to venous HTT in group a compared with group b (p < 0.001). Arterial to venous HTT (quantitative analysis) was < or = 9 s in 19 of 22 subjects of group a and >9 s in 18 of 22 subjects of group b (sensitivity 86%, specificity 82%, positive predictive value 83%, negative predictive value 86%, area under the curve [AUC] 0.87). Portal to venous HTT (quantitative analysis) was < 7 s in 21 of 22 subjects of group a and > 7s in 15 of 22 subjects of group b (sensitivity 95%, specificity 68%, PPV 75%, NPV 94%, AUC 0.85). There was an inverse relation with number of liver segments involved for arterial to venous and portal to venous HTT in group a (p < 0.05), but no correlation between HTT and overall volume of metastases (group a) or subject age (group b). From the results of our study, we conclude that HTT measurements using contrast-enhanced ultrasound with BR1 can detect hemodynamic changes caused by metastatic liver disease from colorectal cancer. However, comparison with the literature suggests that the use of other contrast agents might provide better results. Comparison of different contrast agents for the purpose of transit time analysis would therefore be useful before embarking on a prospective trial looking at the detection of occult liver metastases in patients with colorectal cancer. (E-mail: jhohmann@uhbs.ch).

Local delivery of paclitaxel to inhibit restenosis during angioplasty of the leg.

BACKGROUND: Drug-eluting stents reduce restenosis in coronary arteries, but clinical trials have failed to prove their efficacy in peripheral arteries. We investigated the use of paclitaxel-coated angioplasty balloons and paclitaxel dissolved in the angiographic contrast medium during angioplasty of the leg. METHODS: In a small, multicenter trial, we randomly assigned 154 patients with stenosis or occlusion of a femoropopliteal artery to treatment with standard balloon catheters coated with paclitaxel, uncoated balloons with paclitaxel dissolved in the contrast medium, or uncoated balloons without paclitaxel (control). The primary end point was late lumen loss at 6 months. RESULTS: The mean (+/-SD) age of the patients was 68+/-8 years, 24% were smokers, and 49% had diabetes. Twenty-seven percent of the lesions were total occlusions, and 36% were restenotic lesions. The mean lesion length was 7.4+/-6.5 cm. There were no significant differences in baseline characteristics between the groups. There were no adverse events attributable to the paclitaxel-coated balloons. At 6 months, the mean late lumen loss was 1.7+/-1.8 mm in the control group, as compared with 0.4+/-1.2 mm (P<0.001) in the group treated with paclitaxel-coated balloons and 2.2+/-1.6 mm (P=0.11) in the group treated with paclitaxel in the contrast medium. The rate of revascularization of target lesions at 6 months was 20 of 54 (37%) in the control group, 2 of 48 (4%) in the group treated with paclitaxel-coated balloons (P<0.001 vs. control), and 15 of 52 (29%) in the group treated with paclitaxel in the contrast medium (P=0.41 vs. control); at 24 months, the rates increased to 28 of 54 (52%), 7 of 48 (15%), and 21 of 52 (40%), respectively. CONCLUSIONS: Use of paclitaxel-coated angioplasty balloons during percutaneous treatment of femoropopliteal disease is associated with significant reductions in late lumen loss and target-lesion revascularization. No significant benefit is seen with the use of a paclitaxel-containing contrast medium. (ClinicalTrials.gov number, NCT00156624 [ClinicalTrials.gov].).

Potential value of contrast-enhanced intraoperative ultrasonography during partial hepatectomy for metastases: an essential investigation before resection?

OBJECTIVE: The aim of the study was to assess the clinical value of contrast-enhanced intraoperative ultrasound (CE-IOUS) as a novel tool in the hepatic staging of patients undergoing liver resection. METHODS: Sixty patients scheduled to undergo liver resection for metastatic disease were studied. Preoperative staging with contrast-enhanced CT and/or MR scans was performed within 2 to 6 weeks of operation. Following exploration, intraoperative ultrasound (IOUS) was performed using an HDI-5000 scanner (Philips) and a finger-probe with pulse inversion harmonic (PIH) capability. CE-IOUS in the PIH mode was performed in a standardized protocol (low MI: 0.02-0.04) after intravenous injection of 3-4 mL of SonoVue (Bracco spa, Milan); all detected lesions on precontrast and postcontrast scans were counted and mapped. Any alteration in surgical management was documented following CE-IOUS compared with IOUS. RESULTS: Three patients were excluded due to disseminated disease on exploration. CE-IOUS was significantly more sensitive than CT/MR and IOUS in detecting liver metastases (96.1% versus 76.7% and 81.5%, respectively) (P<0.05); it altered surgical management in 29.8% (17 of 57) of cases, due to 1) additional metastases in 19.3% (11 of 57), 2) less metastases in 3.5% (2 of 57), 3) benign lesions wrongly diagnosed as metastasis on IOUS/CT in 5.3% (3 of 57), and 4) vascular proximity in 1.8% (1 of 57). Management was unchanged in 70.2% (40 of 57) despite additional lesions detected in 3.5% (2 of 57) and benign lesion wrongly diagnosed on IOUS and CT as metastasis in 1.8% (1 of 57). CE-IOUS altered combined IOUS/CT/MR staging in 35.1%. CONCLUSION: These preliminary results suggest CE-IOUS is an essential tool prior to liver resection for metastases.

Post-translational generation of constitutively active cores from larger phosphatases in the malaria parasite, Plasmodium falciparum: implications for proteomics.

BACKGROUND: Although the complete genome sequences of a large number of organisms have been determined, the exact proteomes need to be characterized. More specifically, the extent to which post-translational processes such as proteolysis affect the synthesized proteins has remained unappreciated. We examined this issue in selected protein phosphatases of the protease-rich malaria parasite, Plasmodium falciparum. RESULTS: P. falciparum encodes a number of Ser/Thr protein phosphatases (PP) whose catalytic subunits are composed of a catalytic core and accessory domains essential for regulation of the catalytic activity. Two examples of such regulatory domains are found in the Ca+2-regulated phosphatases, PP7 and PP2B (calcineurin). The EF-hand domains of PP7 and the calmodulin-binding domain of PP2B are essential for stimulation of the phosphatase activity by Ca+2. We present biochemical evidence that P. falciparum generates these full-length phosphatases as well as their catalytic cores, most likely as intermediates of a proteolytic degradation pathway. While the full-length phosphatases are activated by Ca+2, the processed cores are constitutively active and either less responsive or unresponsive to Ca+2. The processing is extremely rapid, specific, and occurs in vivo. CONCLUSIONS: Post-translational cleavage efficiently degrades complex full-length phosphatases in P. falciparum. In the course of such degradation, enzymatically active catalytic cores are produced as relatively stable intermediates. The universality of such proteolysis in other phosphatases or other multi-domain proteins and its potential impact on the overall proteome of a cell merits further investigation.

Multiple cerebral aneurysms in factor VII deficiency.

A rare case of multiple cerebral aneurysms and factor VII deficiency is presented. The authors hypothesize a possible combined genetic defect similar to that of other conditions with clotting disorders. Different treatment options are discussed for factor VII deficiency in particular and multiple cerebral aneurysms in general. The authors advise treatment of all detected aneurysms in case of a subarachnoid hemorrhage rather than only treatment of the ruptured aneurysms in order to immediately start the so-called triple-H therapy.

A zinc-binding dual-specificity YVH1 phosphatase in the malaria parasite, Plasmodium falciparum, and its interaction with the nuclear protein, pescadillo.

Biochemical evidence revealed protein tyrosine kinase and phosphatase activities in the human malarial parasite Plasmodium falciparum, a member of the Apicomplexa. A novel cDNA sequence of a dual-specificity phosphatase was identified in both sexual and asexual stages of P. falciparum, and named PfYVH1, since the predicted primary structure of the 278-amino acid polypeptide showed significant similarity to the human and yeast YVH1 phosphatases. The N-terminal half of PfYVH1 contained a conserved tyrosine phosphatase catalytic domain within a dual-specificity phosphatase domain. The C-terminal region, consisting of one histidine and eight cysteines, represented a zinc-binding domain with a potentially unconventional architecture. Recombinant PfYVH1 contained 2mol of zinc per mol protein and dephosphorylated both phosphoserine and phosphotyrosine residues. Mutation of specific Cys residues in the putative zinc finger region abolished zinc binding and drastically reduced phosphatase activity, suggesting an allosteric role of zinc in catalysis. PfYVH1 was expressed in essentially all erythrocytic stages of the parasite, and shuttled between the nucleus and the cytoplasm in a stage-specific manner. A Plasmodium ortholog of the nuclear pescadillo protein (PfPES) was also characterized and shown to interact with PfYVH1, thus implicating PfYVH1 in the regulation of parasitic development. PfYVH1 represents the first dual-specificity zinc-finger phosphatase characterized in the protozoan kingdom.

Imaging of paediatric splenic injury with contrast-enhanced ultrasonography.

We report two children who sustained traumatic parenchymal splenic injury and were monitored with contrast-enhanced ultrasound (CEUS). In both cases, unenhanced US failed to diagnose splenic haematoma, but the injury was well demonstrated after injection of contrast agent. In one case agreement with CT was excellent; in the other, CT was not performed due to the comprehensive information provided by CEUS.

Profilin is required for viral morphogenesis, syncytium formation, and cell-specific stress fiber induction by respiratory syncytial virus.

BACKGROUND: Actin is required for the gene expression and morphogenesis of respiratory syncytial virus (RSV), a clinically important Pneumovirus of the Paramyxoviridae family. In HEp-2 cells, RSV infection also induces actin stress fibers, which may be important in the immunopathology of the RSV disease. Profilin, a major regulator of actin polymerization, stimulates viral transcription in vitro. Thus, we tested the role of profilin in RSV growth and RSV-actin interactions in cultured cells (ex vivo). RESULTS: We tested three cell lines: HEp-2 (human), A549 (human), and L2 (rat). In all three, RSV grew well and produced fused cells (syncytium), and two RSV proteins, namely, the phosphoprotein P and the nucleocapsid protein N, associated with profilin. In contrast, induction of actin stress fibers by RSV occurred in HEp-2 and L2 cells, but not in A549. Knockdown of profilin by RNA interference had a small effect on viral macromolecule synthesis but strongly inhibited maturation of progeny virions, cell fusion, and induction of stress fibers. CONCLUSIONS: Profilin plays a cardinal role in RSV-mediated cell fusion and viral maturation. In contrast, interaction of profilin with the viral transcriptional proteins P and N may only nominally activate viral RNA-dependent RNA polymerase. Stress fiber formation is a cell-specific response to infection, requiring profilin and perhaps other signaling molecules that are absent in certain cell lines. Stress fibers per se play no role in RSV replication in cell culture. Clearly, the cellular architecture controls multiple steps of host-RSV interaction, some of which are regulated by profilin.