Cells lacking ß-actin are genetically reprogrammed and maintain conditional migratory capacity.

Vertebrate nonmuscle cells express two actin isoforms: cytoplasmic ß- and γ-actin. Because of the presence and localized translation of ß-actin at the leading edge, this isoform is generally accepted to specifically generate protrusive forces for cell migration. Recent evidence also implicates ß-actin in gene regulation. Cell migration without ß-actin has remained unstudied until recently and it is unclear whether other actin isoforms can compensate for this cytoplasmic function and/or for its nuclear role. Primary mouse embryonic fibroblasts lacking ß-actin display compensatory expression of other actin isoforms. Consistent with this preservation of polymerization capacity, ß-actin knockout cells have unchanged lamellipodial protrusion rates despite a severe migration defect. To solve this paradox we applied quantitative proteomics revealing a broad genetic reprogramming of ß-actin knockout cells. This also explains why reintroducing ß-actin in knockout cells does not restore the affected cell migration. Pathway analysis suggested increased Rho-ROCK signaling, consistent with observed phenotypic changes. We therefore developed and tested a model explaining the phenotypes in ß-actin knockout cells based on increased Rho-ROCK signaling and increased TGFß production resulting in increased adhesion and contractility in the knockout cells. Inhibiting ROCK or myosin restores migration of ß-actin knockout cells indicating that other actins compensate for ß-actin in this process. Consequently, isoactins act redundantly in providing propulsive forces for cell migration, but ß-actin has a unique nuclear function, regulating expression on transcriptional and post-translational levels, thereby preventing myogenic differentiation.

Identification and expression analysis of splice variants of mouse enabled homologue during development and in adult tissues.

BACKGROUND: The Enabled/Vasodilator stimulated phosphoprotein (Ena/VASP) gene family comprises three genes in vertebrates: Vasp, Enabled homologue (Enah) and Ena-VASP like (Evl). Enah has the most complex gene structure. It has extra alternatively included exons compared to Vasp and Evl, and possibly one alternatively excluded intron S. The aim of this mapping study was to probe the occurrence of combinations of exon usage in Enah thereby identifying possible vertebrate ENAH splice variants. We investigated this via an in silico analysis and by performing a reverse transcription-polymerase chain reaction (RT-PCR) screen on mouse samples. We further probed the expression pattern of mouse Enah splice variants during development and in a selection of mouse adult tissues and mouse cell lines. RESULTS: In silico analysis of the vertebrate Ena/VASP gene family reveals that birds do not have Vasp, while fish have two Evl genes. Analysis of expressed sequence tags of vertebrate Enah splice variants confirms that an Enah transcript without alternative exons is ubiquitously expressed, but yields only limited information about the existence of other possible alternatively spliced Enah transcripts. Via a RT-PCR screen, we provide evidence that during mouse development and in adult mice at least eight and maximally sixteen different Enah transcripts are expressed. We also show that tissues and cell lines display specific expression profiles of these different transcripts. Exons previously associated with neuronal expression of Enah splice variants are also present in other tissues, in particular in heart. CONCLUSIONS: We propose a more uniform nomenclature for alternative exons in Enah. We provide an overview of distinct expression profiles of mouse Enah splice variants during mouse development, in adult mouse tissues and in a subset of mouse cell lines.

Actin isoform expression patterns during mammalian development and in pathology: insights from mouse models.

The dynamic actin cytoskeleton, consisting of six actin isoforms in mammals and a variety of actin binding proteins is essential for all developmental processes and for the viability of the adult organism. Actin isoform specific functions have been proposed for muscle contraction, cell migration, endo- and exocytosis and maintaining cell shape. However, these specific functions for each of the actin isoforms during development are not well understood. Based on transgenic mouse models, we will discuss the expression patterns of the six conventional actin isoforms in mammals during development and adult life. Ablation of actin genes usually leads to lethality and affects expression of other actin isoforms at the cell or tissue level. A good knowledge of their expression and functions will contribute to fully understand severe phenotypes or diseases caused by mutations in actin isoforms.

The interaction of proline-rich ligands with profilin probed with an enzyme-linked immunosorbent assay.

To detect interactions of different proline-rich ligands with profilins, the authors developed a simple analytical antibody-based screening method. Profilin I or profilin IIa was coated in microplates, and ligand binding was monitored via antibody detection. Using purified components, the authors show that the assay is very sensitive as nanomolar concentrations of recombinant profilin ligands can be used. They further apply this technique to detect interaction of profilin with various proline-rich partners, either endogenously present or ectopically expressed as tagged fusions, using lysates. With this assay, the authors identify Shootin1 as a novel profilin IIa partner. In addition, they demonstrate that this assay can be used for studying competition or ternary complex formation. In conclusion, they developed a sensitive, easy-to-use, and versatile method for the study of the interaction between profilin and different ligands.

Stable isotopic labeling in proteomics.

Labeling of proteins and peptides with stable heavy isotopes (deuterium, carbon-13, nitrogen-15, and oxygen-18) is widely used in quantitative proteomics. These are either incorporated metabolically in cells and small organisms, or postmetabolically in proteins and peptides by chemical or enzymatic reactions. Only upon measurement with mass spectrometers holding sufficient resolution, light, and heavy labeled peptide ions or reporter peptide fragment ions segregate and their intensity values are subsequently used for quantification. Targeted use of these labels or mass tags further leads to specific monitoring of diverse aspects of dynamic proteomes. In this review article, commonly used isotope labeling strategies are described, both for quantitative differential protein profiling and for targeted analysis of protein modifications.

The actin propulsive machinery: the proteome of Listeria monocytogenes tails.

Actin-based comet tails produced by Listeria monocytogenes are considered as representative models for cellular force-producing machineries crucial for cell migration. We here present a proteomic picture of these tails formed in extracts from brain and platelets. This provides a comprehensive view, revealing high molecular complexity and novel host cell proteins as tail components, and suggests the participation of specific multicomponent regulatory complexes. This work forms a new basis to expand current models of cellular protrusion.

On the origin and evolution of vertebrate and viral profilins.

The three dimensional structures of profilins from invertebrates and vertebrates are remarkably similar despite low sequence similarity. Their evolutionary relationship remains thus enigmatic. A phylogenetic analysis of profilins from Deuterostoma indicates that profilin III and IV isoforms each form distinct groups. Profilin IV is most related to invertebrate profilins and originated prior to vertebrate evolution whereas separation of profilin I, II and III isoforms occurred early in vertebrate evolution. Viral profilins are most similar to profilin III. In silico analysis of representative profilin gene structures corroborates the phylogenetic result and we discuss this in terms of biochemical differences.

The actin cytoskeleton in normal and pathological cell motility.

Cell motility is crucial for tissue formation and for development of organisms. Later on cell migration remains essential throughout the lifetime of the organism for wound healing and immune responses. The actin cytoskeleton is the cellular engine that drives cell motility downstream of a complex signal transduction cascade. The basic molecular machinery underlying the assembly and disassembly of actin filaments consists of a variety of actin binding proteins that regulate the dynamic behavior of the cytoskeleton in response to different signals. The multitude of proteins and regulatory mechanisms partaking in this system makes it vulnerable to mutations and alterations in expression levels that ultimately may cause diseases. The most familiar one is cancer that in later stages is characterized by active aberrant cell migration. Indeed tumor invasion and metastasis are increasingly being associated with deregulation of the actin system.

Mutational analysis of human profilin I reveals a second PI(4,5)-P2 binding site neighbouring the poly(L-proline) binding site.

BACKGROUND: Profilin is a small cytoskeletal protein which interacts with actin, proline-rich proteins and phosphatidylinositol 4,5-bisphosphate (PI(4,5)-P2). Crystallography, NMR and mutagenesis of vertebrate profilins have revealed the amino acid residues that are responsible for the interactions with actin and poly(L-proline) peptides. Although Arg88 of human profilin I was shown to be involved in PI(4,5)-P2-binding, it was suggested that carboxy terminal basic residues may be involved as well. RESULTS: Using site directed mutagenesis we have refined the PI(4,5)-P2 binding site of human profilin I. For each mutant we assessed the stability and studied the interactions with actin, a proline-rich peptide and PI(4,5)-P2 micelles. We identified at least two PI(4,5)-P2-binding regions in human profilin I. As expected, one region comprises Arg88 and overlaps with the actin binding site. The second region involves Arg136 in the carboxy terminal helix and neighbours the poly(L-proline) binding site. In addition, we show that adding a small protein tag to the carboxy terminus of profilin strongly reduces binding to poly(L-proline), suggesting local conformational changes of the carboxy terminal alpha-helix may have dramatic effects on ligand binding. CONCLUSIONS: The involvement of the two terminal alpha-helices of profilin in ligand binding imposes important structural constraints upon the functions of this region. Our data suggest a model in which the competitive interactions between PI(4,5)-P2 and actin and PI(4,5)-P2 and poly(L-proline) regulate profilin functions.

Listeria comet tails: the actin-based motility machinery at work.

Listeria monocytogenes is a master of mimicry that uses the host cell actin system both to move within the cytoplasm of infected cells and for cell-to-cell spread. Recent studies of Listeria and similarly acting pathogens have generated leaps in our understanding of the actin-based force producing machinery. This machinery is essential for most motile properties of cells, not least for cell migration. In a minimal configuration, it consists of the Arp2/3-complex, Ena-VASP proteins, cofilin, capping protein and a nucleation-promoting factor. In this review, we discuss current models of pseudopodial protrusions and describe how the road to more complex models lies open and is already paved by recent studies using Listeria-based biomimetic motility assays.

Silencing profilin-1 inhibits endothelial cell proliferation, migration and cord morphogenesis.

Expression of several actin-binding proteins including profilin-1 is up-regulated during capillary morphogenesis of endothelial cells, the biological significance of which remains unknown. Specifically, we hypothesized that profilin-1 is important for endothelial migration and proliferation. In this study, we suppressed profilin-1 expression in human umbilical vein endothelial cells by RNA-interference. Gene silencing of profilin-1 led to significant reduction in the formation of actin filaments and focal adhesions. Loss of profilin-1 expression was also associated with reduced dynamics of cell-cell adhesion. Data from both wound-healing experiments and time-lapse imaging of individual cells showed inhibition of cell migration when profilin-1 expression was suppressed. Cells lacking profilin-1 exhibited defects in membrane protrusion, both in terms of its magnitude and directional persistence. Furthermore, loss of profilin-1 expression inhibited cell growth without compromising cell survival, at least in the short-term, thus suggesting that profilin-1 also plays an important role in endothelial proliferation as hypothesized. Finally, silencing profilin-1 expression suppressed matrigel-induced early cord morphogenesis of endothelial cells. Taken together, our data suggest that profilin-1 may play important role in biological events that involve endothelial proliferation, migration and morphogenesis.

Profilin-I-ligand interactions influence various aspects of neuronal differentiation.

Differentiating neurons extend membrane protrusions that develop into growing neurites. The driving force for neurite outgrowth is the dynamic actin cytoskeleton, which is regulated by actin-binding proteins. In this study, we describe for the first time, the role of profilin I and its ligand interactions in neuritogenesis of PC12 cells. High-level overexpression of wild-type profilin I had an inhibitory effect on neurite outgrowth. Low levels of profilin I did not disturb this process, but these cells developed many more filopodia along the neurite shafts. Low-level overexpression of mutant forms of profilin I changed one or more aspects of PC12 differentiation. Expression of a profilin I mutant that is defective in actin binding (profilin I(R74E)) decreased neurite length and strongly inhibited filopodia formation. Cells expressing mutants defective in binding proline-rich ligands (profilin I(W3A) and profilin I(R136D)) differentiated faster, developed more and longer neurites and more branches. The profilin I(R136D) mutant, which is also defective in phosphatidylinositol 4,5-bisphosphate binding, enhanced neurite outgrowth even in the absence of NGF. Parental PC12 cells treated with the ROCK inhibitor Y27632, differentiate faster and display longer neurites and more branches. Similar effects were seen in cells expressing profilin I(WT), profilin I(W3A) and profilin I(R74E). By contrast, the profilin I(R136D)-expressing cells were insensitive to the ROCK inhibitor, suggesting that regulation of profilin I by phosphatidylinositol 4,5-bisphosphate metabolism is crucial for proper neurite outgrowth. Taken together, our data show the importance of the interaction of profilin I with actin, proline-rich proteins and phosphatidylinositol 4,5-bisphosphate in neuronal differentiation of PC12 cells.

A role for complexes of survival of motor neurons (SMN) protein with gemins and profilin in neurite-like cytoplasmic extensions of cultured nerve cells.

Spinal muscular atrophy (SMA) is caused by reduced levels of SMN (survival of motor neurons protein) and consequent loss of motor neurons. SMN is involved in snRNP transport and nuclear RNA splicing, but axonal transport of SMN has also been shown to occur in motor neurons. SMN also binds to the small actin-binding protein, profilin. We now show that SMN and profilin II co-localise in the cytoplasm of differentiating rat PC12 cells and in neurite-like extensions, especially at their growth cones. Many components of known SMN complexes were also found in these extensions, including gemin2 (SIP-1), gemin6, gemin7 and unrip (unr-interacting protein). Coilin p80 and Sm core protein immunoreactivity, however, were seen only in the nucleus. SMN is known to associate with beta-actin mRNA and specific hnRNPs in axons and in neurite extensions of cultured nerve cells, and SMN also stimulates neurite outgrowth in cultures. Our results are therefore consistent with SMN complexes, rather than SMN alone, being involved in the transport of actin mRNPs along the axon as in the transport of snRNPs into the nucleus by similar SMN complexes. Antisense knockdown of profilin I and II isoforms inhibited neurite outgrowth of PC12 cells and caused accumulation of SMN and its associated proteins in cytoplasmic aggregates. BIAcore studies demonstrated a high affinity interaction of SMN with profilin IIa, the isoform present in developing neurons. Pathogenic missense mutations in SMN, or deletion of exons 5 and 7, prevented this interaction. The interaction is functional in that SMN can modulate actin polymerisation in vitro by reducing the inhibitory effect of profilin IIa. This suggests that reduced SMN in SMA might cause axonal pathfinding defects by disturbing the normal regulation of microfilament growth by profilins.