Cloning and expression of a novel human profilin variant, profilin II

Upregulation of Profilin 2 on HDAC6 overexpression in mouse GC-1 cells and its putative role in germ cell migration in the testis

Previous reports from this laboratory have demonstrated the involvement of histone deacetylase 6 (HDAC6) in sperm motility. As the presence of HDAC6 has also been reported in the earlier stage germ cells, studies were undertaken to explore its role during these stages of spermatogenesis. HDAC6 was overexpressed in GC-1spg cells, which represent the stage between type B spermatogonia and primary spermatocyte, and its effect on germ cell transcriptome was investigated by microarray. Among the many transcripts that were differentially regulated, Profilin 2, reported previously as a neuronal specific isoform, was observed as one of the genes highly upregulated at the transcript level, which was further confirmed by real-time PCR, and the protein confirmed by indirect immunofluorescence (IIF). Profilin 2 colocalized with HDAC6, as seen both in GC-1 cells and sperm. On the sperm, the presence of Profilin 2 was detected throughout the flagella with its colocalization with HDAC6 seen conspicuously in the mid-piece region of the flagella. Co-immunoprecipitation studies confirmed Profilin 2 interaction with HDAC6. Docking studies using Z dock suggested the interaction of 8 residues of HDAC6 with 6 residues of Profilin 2. The novel observation of Profilin 2 in spermatogonial cells, its significant upregulation on HDAC6 overexpression and its interaction with HDAC6 suggests that HDAC6 in collaboration with Profilin 2 may play a role in regulating the movement of germ cells from one stage/compartment to the next.

The transcriptional landscape of Venezuelan equine encephalitis virus (TC-83) infection

Venezuelan Equine Encephalitis Virus (VEEV) is a major biothreat agent that naturally causes outbreaks in humans and horses particularly in tropical areas of the western hemisphere, for which no antiviral therapy is currently available. The host response to VEEV and the cellular factors this alphavirus hijacks to support its effective replication or evade cellular immune responses are largely uncharacterized. We have previously demonstrated tremendous cell-to-cell heterogeneity in viral RNA (vRNA) and cellular transcript levels during flaviviral infection using a novel virus-inclusive single-cell RNA-Seq approach. Here, we used this unbiased, genome-wide approach to simultaneously profile the host transcriptome and vRNA in thousands of single cells during infection of human astrocytes with the live-attenuated vaccine strain of VEEV (TC-83). Host transcription was profoundly suppressed, yet “superproducer cells” with extremely high vRNA abundance emerged during the first viral life cycle and demonstrated an altered transcriptome relative to both uninfected cells and cells with high vRNA abundance harvested at later time points. Additionally, cells with increased structural-to-nonstructural transcript ratio exhibited upregulation of intracellular membrane trafficking genes at later time points. Loss- and gain-of-function experiments confirmed pro- and antiviral activities in both vaccine and virulent VEEV infections among the products of transcripts that positively or negatively correlated with vRNA abundance, respectively. Lastly, comparison with single cell transcriptomic data from other viruses highlighted common and unique pathways perturbed by infection across evolutionary scales. This study provides a high-resolution characterization of the VEEV (TC-83)-host interplay, identifies candidate targets for antivirals, and establishes a comparative single-cell approach to study the evolution of virus-host interactions.

Little is known about the host response to Venezuelan Equine Encephalitis Virus (VEEV) and the cellular factors this alphavirus hijacks to support effective replication or evade cellular immune responses. Monitoring dynamics of host and viral RNA (vRNA) during viral infection at a single-cell level can provide insight into the virus-host interplay at a high resolution. Here, a single-cell RNA sequencing technology that detects host and viral RNA was used to investigate the interactions between TC-83, the vaccine strain of VEEV, and the human host during the course of infection of U-87 MG cells (human astrocytoma). Virus abundance and host transcriptome were heterogeneous across cells from the same culture. Subsets of differentially expressed genes, positively or negatively correlating with vRNA abundance, were identified and subsequently in vitro validated as candidate proviral and antiviral factors, respectively, in TC-83 and/or virulent VEEV infections. In the first replication cycle, “superproducer” cells exhibited rapid increase in vRNA abundance and unique gene expression patterns. At later time points, cells with increased structural-to-nonstructural transcript ratio demonstrated upregulation of intracellular membrane trafficking genes. Lastly, comparing the VEEV dataset with published datasets on other RNA viruses revealed unique and overlapping responses across viral clades. Overall, this study improves the understanding of VEEV-host interactions, reveals candidate targets for antiviral approaches, and establishes a comparative single-cell approach to study the evolution of virus-host interactions.

Integrated miRNA-/mRNA-Seq of the Habenulo-Interpeduncular Circuit During Acute Nicotine Withdrawal

Tobacco use is the leading preventable cause of mortality in the world. The limited number of smoking cessation aids currently available are minimally effective, highlighting the need for novel therapeutic interventions. We describe a genome-wide approach to identify potential candidates for such interventions. Next-generation sequencing was performed using RNA isolated from the habenulo-interpeduncular circuit of male mice withdrawn from chronic nicotine treatment. This circuit plays a central role in the nicotine withdrawal response. Differentially expressed miRNAs and mRNAs were validated using RT-qPCR. Many of the differentially expressed mRNAs are predicted targets of reciprocally expressed miRNAs. We illustrate the utility of the dataset by demonstrating that knockdown in the interpeduncular nucleus of a differentially expressed mRNA, that encoding profilin 2, is sufficient to induce anxiety-related behavior. Importantly, profilin 2 knockdown in the ventral tegmental area did not affect anxiety behavior. Our data reveal wide-spread changes in gene expression within the habenulo-interpeduncular circuit during nicotine withdrawal. This dataset should prove to be a valuable resource leading to the identification of substrates for the design of innovative smoking cessation aids.

Profilin2a-phosphorylation as a regulatory mechanism for actin dynamics

Profilin is a major regulator of actin dynamics in multiple specific processes localized in different cellular compartments. This specificity is not only meditated by its binding to actin but also its interaction with phospholipids such as phosphatidylinositol (4,5)-bisphosphate (PIP₂ ) at the membrane and a plethora of proteins containing poly-L-proline (PLP) stretches. These interactions are fine-tuned by posttranslational modifications such as phosphorylation. Several phospho-sites have already been identified for profilin1, the ubiquitously expressed isoform. However, little is known about the phosphorylation of profilin2a. Profilin2a is a neuronal isoform important for synapse function. Here, we identified several putative profilin2a phospho-sites in silico and tested recombinant phospho-mimetics with regard to their actin-, PLP-, and PIP₂ -binding properties. Moreover, we assessed their impact on actin dynamics employing a pyrene-actin polymerization assay. Results indicate that distinct phospho-sites modulate specific profilin2a functions. We could identify a molecular switch site at serine residue 71 which completely abrogated actin binding-as well as other sites important for fine-tuning of different functions, for example, tyrosine 29 for PLP binding. Our findings suggest that differential profilin2a phosphorylation is a sensitive mechanism for regulating its neuronal functions. Moreover, the dysregulation of profilin2a phosphorylation may contribute to neurodegeneration.

Homer, Spikar, and Other Drebrin-Binding Proteins in the Brain

Drebrin is a major F-actin-binding protein in the brain. In the past two decades, many drebrin-binding proteins in addition to F-actin have been identified in several research fields including neuroscience, oncology, and immunology. Among the drebrin-binding proteins, there are various kinds of proteins including scaffold proteins, nuclear proteins, phosphatases, microtubule-binding proteins, G-actin-binding proteins, gap junction proteins, chemokine receptors, and cell-adhesion-related proteins. The interaction between drebrin and its binding partners seems to play important roles in higher brain functions, because drebrin is involved in the pathogenesis of some neurological diseases with cognitive defects. In this chapter, we will first review the interaction of Homer and spikar with drebrin, particularly focusing on spine morphogenesis and synaptic function. Homer contributes to spine morphogenesis by cooperating with shank and activated Cdc42 small GTPase, suggesting a novel signaling pathway comprising Homer, drebrin, shank, and Cdc42 for spine morphogenesis. Drebrin sequesters spikar in the cytoplasm and stabilizes it in dendritic spines, leading to spine formation. Finally, we will introduce some other drebrin-binding proteins including end-binding protein 3 (EB3), profilin, progranulin, and phosphatase and tensin homologue (PTEN). These proteins are involved in Alzheimer's disease and cancer. Therefore, further studies on drebrin and its binding proteins will be of great importance to elucidate the pathologies of various diseases and may contribute to their medical treatment and diagnostics development.

Apoptosis in the Trophoblast—Role of Apoptosis in Placental Morphogenesis

Villous trophoblast is the epithelial cover of the placental villous tree and comes in direct contact with maternal blood. The turnover of villous trophoblast includes proliferation and differentiation of cytotrophoblast, syncytial fusion of cytotrophoblast with the overlying syncytiotrophoblast, differentiation in the syncytiotrophoblast, and finally extrusion of apoptotic material into the maternal circulation. In recent years, it has become clear that apoptosis is a normal constituent of trophoblast turnover and the release of apoptotic material does not lead to an inflammatory response of the mother. During preeclampsia there seems to be an altered balance between proliferation and apoptosis of villous trophoblast leading to a dysregulation of the release from the syncytiotrophoblast. The normal apoptotic release may be reduced in favor of a necrotic release. Since apoptosis is still ongoing in the syncytiotrophoblast, a necrotic release of intrasyncytial and partly apoptotic material lead us to call this type of release "aponecrotic shedding." In this situation, cell-free components such as G-actin and DNA freely floating in maternal blood may trigger damage to the maternal endothelium, thereby triggering preeclampsia. This review highlights the importance of the apoptosis cascade in permitting normal physiologic turnover of villous trophoblast. It will demonstrate the participation of initial stages of this cascade within the cytotrophoblast and of the execution stages within the syncytiotrophoblast. Moreover, this review presents hypotheses of how dysregulation of the apoptosis cascade may be linked to endothelial dysfunction of the maternal vasculature in preeclampsia.

Profilin modulates sarcomeric organization and mediates cardiomyocyte hypertrophy

Aims

Heart failure is often preceded by cardiac hypertrophy, which is characterized by increased cell size, altered protein abundance, and actin cytoskeletal reorganization. Profilin is a well-conserved, ubiquitously expressed, multifunctional actin-binding protein, and its role in cardiomyocytes is largely unknown. Given its involvement in vascular hypertrophy, we aimed to test the hypothesis that profilin-1 is a key mediator of cardiomyocyte-specific hypertrophic remodelling.

Methods and results

Profilin-1 was elevated in multiple mouse models of hypertrophy, and a cardiomyocyte-specific increase of profilin in Drosophila resulted in significantly larger heart tube dimensions. Moreover, adenovirus-mediated overexpression of profilin-1 in neonatal rat ventricular myocytes (NRVMs) induced a hypertrophic response, measured by increased myocyte size and gene expression. Profilin-1 silencing suppressed the response in NRVMs stimulated with phenylephrine or endothelin-1. Mechanistically, we found that profilin-1 regulates hypertrophy, in part, through activation of the ERK1/2 signalling cascade. Confocal microscopy showed that profilin localized to the Z-line of Drosophila myofibrils under normal conditions and accumulated near the M-line when overexpressed. Elevated profilin levels resulted in elongated sarcomeres, myofibrillar disorganization, and sarcomeric disarray, which correlated with impaired muscle function.

Conclusion

Our results identify novel roles for profilin as an important mediator of cardiomyocyte hypertrophy. We show that overexpression of profilin is sufficient to induce cardiomyocyte hypertrophy and sarcomeric remodelling, and silencing of profilin attenuates the hypertrophic response.

Arabidopsis plants deficient in constitutive class profilins reveal independent and quantitative genetic effects

BACKGROUND

The actin cytoskeleton is involved in an array of integral structural and developmental processes throughout the cell. One of actin's best-studied binding partners is the small ubiquitously expressed protein, profilin. Arabidopsis thaliana is known to encode a family of five profilin sequence variants: three vegetative (also constitutive) profilins that are predominantly expressed in all vegetative tissues and ovules, and two reproductive profilins that are specifically expressed in pollen. This paper analyzes the roles of the three vegetative profilin members, PRF1, PRF2, and PRF3, in plant cell and organ development.

RESULTS

Using a collection of knockout or severe knockdown T-DNA single mutants, we found that defects in each of the three variants gave rise to specific developmental deficiencies. Plants lacking PRF1 or PRF2 had defects in rosette leaf morphology and inflorescence stature, while those lacking PRF3 led to plants with slightly elongated petioles. To further examine these effects, double mutants and double and triple gene-silenced RNAi epialleles were created. These plants displayed significantly compounded developmental defects, as well as distinct lateral root growth morphological phenotypes.

CONCLUSION

These results suggest that having at least one vegetative profilin gene is essential to viability. Evidence is presented that combinations of independent function, quantitative genetic effects, and functional redundancy have preserved the three vegetative profilin genes in the Arabidopsis lineage.

Profilin 2 promotes migration, invasion, and stemness of HT29 human colorectal cancer stem cells

We investigated the role of profilin 2 in the stemness, migration, and invasion of HT29 cancer stem cells (CSCs). Increased and decreased levels of profilin 2 significantly enhanced and suppressed the self-renewal, migration, and invasion ability of HT29 CSCs, respectively. Moreover, profilin 2 directly regulated the expression of stemness markers (CD133, SOX2, and β-catenin) and epithelial mesenchymal transition (EMT) markers (E-cadherin and snail). CD133 and β-catenin were up-regulated by overexpression of profilin 2 and down-regulated by depletion of profilin 2. SOX2 was decreased by profilin 2 depletion. E-cadherin was not influenced by profilin 2- overexpression but increased by profilin 2- knockdown. The expression of snail was suppressed by profilin 2- knockdown. We speculated that stemness and the EMT are closely linked through profilin 2-related pathways. Therefore, this study indicates that profilin 2 affects the metastatic potential and stemness of colorectal CSCs by regulating EMT- and stemness-related proteins.

Analysis of the effects of polymorphism on pollen profilin structural functionality and the generation of conformational, T- and B-cell epitopes

An extensive polymorphism analysis of pollen profilin, a fundamental regulator of the actin cytoskeleton dynamics, has been performed with a major focus in 3D-folding maintenance, changes in the 2-D structural elements, surface residues involved in ligands-profilin interactions and functionality, and the generation of conformational and lineal B- and T-cell epitopes variability.

Our results revealed that while the general fold is conserved among profilins, substantial structural differences were found, particularly affecting the special distribution and length of different 2-D structural elements (i.e. cysteine residues), characteristic loops and coils, and numerous micro-heterogeneities present in fundamental residues directly involved in the interacting motifs, and to some extension these residues nearby to the ligand-interacting areas. Differential changes as result of polymorphism might contribute to generate functional variability among the plethora of profilin isoforms present in the olive pollen from different genetic background (olive cultivars), and between plant species, since biochemical interacting properties and binding affinities to natural ligands may be affected, particularly the interactions with different actin isoforms and phosphoinositides lipids species.

Furthermore, conspicuous variability in lineal and conformational epitopes was found between profilins belonging to the same olive cultivar, and among different cultivars as direct implication of sequences polymorphism. The variability of the residues taking part of IgE-binding epitopes might be the final responsible of the differences in cross-reactivity among olive pollen cultivars, among pollen and plant-derived food allergens, as well as between distantly related pollen species, leading to a variable range of allergy reactions among atopic patients. Identification and analysis of commonly shared and specific epitopes in profilin isoforms is essential to gain knowledge about the interacting surface of these epitopes, and for a better understanding of immune responses, helping design and development of rational and effective immunotherapy strategies for the treatment of allergy diseases.

An Ex Vivo Analysis of Sertoli Cell Actin Dynamics Following Gonadotropic Hormone Withdrawal

The receptors for the steroid hormone testosterone and the peptide hormone follicle-stimulating hormone are localized to the somatic Sertoli cell in the seminiferous epithelium. In the rat, prolonged gonadotrophic hormone withdrawal has been shown to result in substantial germ cell apoptosis. Previous studies have shown that, coincident with the loss of germ cells following hypophysectomy, the actin cytoskeleton of the Sertoli cell becomes disorganized and diffuse throughout the cell's cytoplasm. The molecular mechanisms that govern Sertoli cell actin filament dynamics in response to the loss of gonadotrophic hormones remain undefined. It was therefore hypothesized that hypophysectomy brings about a decrease in the amount of polymerized actin (F-actin) within the Sertoli cell and that this decrease is associated with changes in the expression of genes known to govern Sertoli actin dynamics. To this end, Sertoli cells were isolated from adult control and hypophysectomized rats. Sertoli cells from hypophysectomized rats were found to contain significantly less (72%) F-actin relative to untreated controls, although overall, beta-actin protein and mRNA expression remained constant. The expression levels of genes known to directly influence the amount of F-actin in cells were then examined by Northern blot analysis. Cofilin and profilin I gene expression was unaffected by hypophysectomy, whereas the expression of profilin II and espin both decreased significantly (47% and 42%, respectively). Taken together, these results suggest that, following hypophysectomy, the actin cytoskeleton of the Sertoli cell shifts to a predominantly depolymerized state, perhaps in part because of decreases in profilin II and espin gene products.

Differential remodeling of actin cytoskeleton architecture by profilin isoforms leads to distinct effects on cell migration and invasion

Dynamic actin cytoskeletal reorganization is integral to cell motility. Profilins are well-characterized regulators of actin polymerization; however, functional differences among coexpressed profilin isoforms are not well defined. Here, we demonstrate that profilin-1 and profilin-2 differentially regulate membrane protrusion, motility, and invasion; these processes are promoted by profilin-1 and suppressed by profilin-2. Compared to profilin-1, profilin-2 preferentially drives actin polymerization by the Ena/VASP protein, EVL. Profilin-2 and EVL suppress protrusive activity and cell motility by an actomyosin contractility-dependent mechanism. Importantly, EVL or profilin-2 downregulation enhances invasion in vitro and in vivo. In human breast cancer, lower EVL expression correlates with high invasiveness and poor patient outcome. We propose that profilin-2/EVL-mediated actin polymerization enhances actin bundling and suppresses breast cancer cell invasion.

Role of the actin-binding protein profilin1 in radial migration and glial cell adhesion of granule neurons in the cerebellum

Profilins are small G-actin-binding proteins essential for cytoskeletal dynamics. Of the four mammalian profilin isoforms, profilin1 shows a broad expression pattern, profilin2 is abundant in the brain, and profilin3 and profilin4 are restricted to the testis. In vitro studies on cancer and epithelial cell lines suggested a role for profilins in cell migration and cell-cell adhesion. Genetic studies in mice revealed the importance of profilin1 in neuronal migration, while profilin2 has apparently acquired a specific function in synaptic physiology. We recently reported a mouse mutant line lacking profilin1 in the brain; animals display morphological defects that are typical for impaired neuronal migration. We found that during cerebellar development, profilin1 is specifically required for radial migration and glial cell adhesion of granule neurons. Profilin1 mutants showed cerebellar hypoplasia and aberrant organization of cerebellar cortex layers, with ectopically arranged granule neurons. In this commentary, we briefly introduce the profilin family and summarize the current knowledge on profilin activity in cell migration and adhesion. Employing cerebellar granule cells as a model, we shed some light on the mechanisms by which profilin1 may control radial migration and glial cell adhesion. Finally, a potential implication of profilin1 in human developmental neuropathies is discussed.

Purkinje cell loss and motor coordination defects in profilin1 mutant mice

Profilin1 is an actin monomer-binding protein, essential for cytoskeletal dynamics. Based on its broad expression in the brain and the localization at excitatory synapses (hippocampal CA3-CA1 synapse, cerebellar parallel fiber (PF)-Purkinje cell (PC) synapse), an important role for profilin1 in brain development and synapse physiology has been postulated. We recently showed normal physiology of hippocampal CA3-CA1 synapses in the absence of profilin1, but impaired glial cell binding and radial migration of cerebellar granule neurons (CGNs). Consequently, brain-specific inactivation of profilin1 by exploiting conditional mutants and Nestin-mediated cre expression resulted in a cerebellar hypoplasia, aberrant organization of cerebellar cortex layers, and ectopic CGNs. Apart from these findings we noted a loss of PCs and an irregularly shaped PC layer in adult mutants. In this study, we show that PC migration and development are not affected in profilin1 mutants, suggesting cell type-specific functions for profilin1 in PCs and CGNs. PC loss begins during the second postnatal week and progresses until adulthood with no further impairment in aged mutants. In Nestin-cre profilin1 mutants, defects in cerebellar cortex cytoarchitecture are associated with impaired motor coordination. However, in L7-cre mutants, lacking profilin1 specifically in PCs, the cerebellar cortex cytoarchitecture is unchanged. Thereby, our results demonstrate that the loss of PCs is not caused by cell-autonomous defects, but presumably by impaired CGN migration. Finally, we show normal functionality of PF-PC synapses in the absence of profilin1. In summary, we conclude that profilin1 is crucially important for brain development, but dispensable for the physiology of excitatory synapses.

Characterization of profilin polymorphism in pollen with a focus on multifunctionality

Profilin, a multigene family involved in actin dynamics, is a multiple partners-interacting protein, as regard of the presence of at least of three binding domains encompassing actin, phosphoinositide lipids, and poly-L-proline interacting patches. In addition, pollen profilins are important allergens in several species like Olea europaea L. (Ole e 2), Betula pendula (Bet v 2), Phleum pratense (Phl p 12), Zea mays (Zea m 12) and Corylus avellana (Cor a 2). In spite of the biological and clinical importance of these molecules, variability in pollen profilin sequences has been poorly pointed out up until now. In this work, a relatively high number of pollen profilin sequences have been cloned, with the aim of carrying out an extensive characterization of their polymorphism among 24 olive cultivars and the above mentioned plant species. Our results indicate a high level of variability in the sequences analyzed. Quantitative intra-specific/varietal polymorphism was higher in comparison to inter-specific/cultivars comparisons. Multi-optional posttranslational modifications, e.g. phosphorylation sites, physicochemical properties, and partners-interacting functional residues have been shown to be affected by profilin polymorphism. As a result of this variability, profilins yielded a clear taxonomic separation between the five plant species. Profilin family multifunctionality might be inferred by natural variation through profilin isovariants generated among olive germplasm, as a result of polymorphism. The high variability might result in both differential profilin properties and differences in the regulation of the interaction with natural partners, affecting the mechanisms underlying the transmission of signals throughout signaling pathways in response to different stress environments. Moreover, elucidating the effect of profilin polymorphism in adaptive responses like actin dynamics, and cellular behavior, represents an exciting research goal for the future.

Profilin 1 overexpression in renal cell carcinoma

OBJECTIVES

To gain information about overexpressed antigens in renal cell carcinoma (RCC) by using a chemical proteomics approach.

METHODS

RCC cell line 769P was cultured and proteome analysis was subsequently carried out in the culture supernatants. By using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and tandem mass spectrometry (LC-MS/MS), proteins in the culture supernatants were searched. A MEDLINE search to define the functions of the identified proteins was carried out.

RESULTS

Four differentially regulated proteins (profilin 1, amyloid beta A4 protein [APP], proprotein convertase subtilisin/kexin type 1 inhibitor [ProSAAS], galectin-3-binding protein [LGALS3BP]) were selected. These were not overexpressed in normal kidney tissue or reported in RCC. Their levels were measured through western blotting of normal kidney and RCC tissues. No differences were observed in the expression levels of APP, ProSAAS or LGALS3BP between RCC and normal kidney tissues. Profilin 1 was overexpressed in RCC tissue. On the basis of this observation, an immunohistochemical analysis of profilin 1 in normal kidney and RCC tissues was carried out. In normal tissues, tubules that were sources of RCC stained positive for profilin 1. In RCC tissue, in contrast, the stromal cells in the tumors stained positive.

CONCLUSIONS

Profilin 1 can be a key element in the pathological processes of RCC, such as tumorigenesis and/or tumor growth. Thus, it has the potential to serve as a diagnostic or progression biomarker and therapeutic target in RCC.

Dynamic regulation of sarcomeric actin filaments in striated muscle

In striated muscle, the actin cytoskeleton is differentiated into myofibrils. Actin and myosin filaments are organized in sarcomeres and specialized for producing contractile forces. Regular arrangement of actin filaments with uniform length and polarity is critical for the contractile function. However, the mechanisms of assembly and maintenance of sarcomeric actin filaments in striated muscle are not completely understood. Live imaging of actin in striated muscle has revealed that actin subunits within sarcomeric actin filaments are dynamically exchanged without altering overall sarcomeric structures. A number of regulators for actin dynamics have been identified, and malfunction of these regulators often result in disorganization of myofibril structures or muscle diseases. Therefore, proper regulation of actin dynamics in striated muscle is critical for assembly and maintenance of functional myofibrils. Recent studies have suggested that both enhancers of actin dynamics and stabilizers of actin filaments are important for sarcomeric actin organization. Further investigation of the regulatory mechanism of actin dynamics in striated muscle should be a key to understanding how myofibrils develop and operate.

Proteomic profiling of the epileptic dentate gyrus

The development of epilepsy is often associated with marked changes in central nervous system cell structure and function. Along these lines, reactive gliosis and granule cell axonal sprouting within the dentate gyrus of the hippocampus are commonly observed in individuals with temporal lobe epilepsy (TLE). Here we used the pilocarpine model of TLE in mice to screen the proteome and phosphoproteome of the dentate gyrus to identify molecular events that are altered as part of the pathogenic process. Using a two-dimensional gel electrophoresis-based approach, followed by liquid chromatography-tandem mass spectrometry, 24 differentially expressed proteins, including 9 phosphoproteins, were identified. Functionally, these proteins were organized into several classes, including synaptic physiology, cell structure, cell stress, metabolism and energetics. The altered expression of three proteins involved in synaptic physiology, actin, profilin 1 and α-synuclein was validated by secondary methods. Interestingly, marked changes in protein expression were detected in the supragranular cell region, an area where robust mossy fibers sprouting occurs. Together, these data provide new molecular insights into the altered protein profile of the epileptogenic dentate gyrus and point to potential pathophysiologic mechanisms underlying epileptogenesis.

Profilin oligomerization and its effect on poly (L-proline) binding and phosphorylation

Profilin is a cytoskeletal protein that interacts specifically with actin, phosphoinositides and poly (l-proline). Experimental results and in silico studies revealed that profilin exists as dimer and tetramer. Profilin oligomers possess weak affinity to poly (l-proline) due to unavailability of binding sites in dimers and tetramers. Phosphorylation studies indicate that profilin dimers are not phosphorylated while teramers are preferentially phosphorylated over monomers. In silico studies revealed that PKC phosphorylation site, S137 is buried in dimer while it is accessible in tetramer.

Profilin, a multi-modal regulator of neuronal plasticity

Thirty years after its initial characterization and more than 1000 publications listed in PubMed describing its properties, the small (ca 15 kDa) protein profilin continues to surprise us with new, recently discovered functions. Originally described as an actin-binding protein, profilin has now been shown to interact with more than a dozen proteins in mammalian cells. Some of the more recently described and intriguing interactions are within neurons involving a neuronal profilin family member. Profilin is now regarded as a regulator of various cellular processes such as cytoskeletal dynamics, membrane trafficking and nuclear transport. Profilin is a necessary element in key steps of neuronal differentiation and synaptic plasticity, and embodies properties postulated for a synaptic tag. These findings identify profilin as an important factor linking cellular and behavioural plasticity in neural circuits.

Acid-Induced Equilibrium Folding Intermediate of Human Platelet Profilin

The acid-induced unfolding of human platelet profilin (HPP) can be minimally modeled as a three-state process. Equilibrium unfolding studies have been performed on human platelet profilin1 (HPP) and monitored by far-UV circular dichroism, tryptophan fluorescence, ANS binding, and NMR spectroscopy. Far-UV CD measurements obtained by acid titration demonstrate that HPP unfolds via a three-state mechanism (N --> I --> U), with a highly populated intermediate between pH 4 and 5. Approximately 80% of native helical secondary structural content remains at pH 4, as indicated by monitoring the CD signal at 222 nm. The stability (DeltaGH2O) of the native conformation at pH 7.0 (obtained by monitoring the change in tryptophan signal as a function of urea concentration) is 5.56 +/- 0.51 kcal mol-1; however, the DeltaGH2O for the intermediate species at pH 4 is 2.01 +/- 0.47 kcal mol-1. The calculated m-values for the pH 7.0 and pH 4.0 species were 1.64 +/- 0.15 and 1.34 +/- 0.17 kcal mol-1 M-1, respectively, which is an indication that the native and intermediate species are similarly compact. Additionally, translational diffusion measurements obtained by NMR spectroscopy and ANS binding studies are consistent with a globular and compact conformation at both pH 7.0 and 4.0. The pKa values for the two histidine (His) residues located on helix 4 of HPP were determined to be 5.6 and 5.7 pH units. These pKa values coincide with the midpoint of the far-UV CD acid titration curve and suggest that the protonation of one or both His residues may play a role in the formation of the unfolding intermediate. Stable intermediate species populate the 2D 1H-15N HSQC NMR spectra between pH 4 and 5. A number of backbone and side-chain resonances show significant perturbations relative to the native spectrum; however, considerable nativelike tertiary contacts remain. Interestingly, the residues on HPP that are significantly altered at low pH coincide with segments of the G-actin binding surface and poly-l-proline binding interface. The earlier reports that a decrease in pH below 6.0 induces structural alterations in profilin, favoring dissociation of the profilin-actin complex, corresponds with the structural alterations observed in the partially unfolded species. Our findings suggest that a novel mechanism for pH induced disruption of the profilin-G-actin complex involve a nativelike unfolding intermediate of profilin.

Identification of novel protein biomarkers of preterm birth in human cervical-vaginal fluid

Spontaneous preterm birth (SPTB) is a major contributor to perinatal morbidity and mortality. However, the diagnosis of preterm labor (PTL) that leads to preterm birth is difficult, and there is a pressing need for improved diagnosis. We utilized multidimensional liquid chromatography-tandem mass spectrometry (LC/LC-MS/MS; MudPIT) and Fluorescence two-dimensional differential in-gel electrophoresis (2D-DIGE) to identify potential biomarkers of PTL and SPTB. MudPIT analysis identified 205 proteins in cervical-vaginal fluid (CVF), 28 of which exhibited significant differences in pairwise and progressive comparisons. Calgranulins, annexins, S100 calcium-binding protein A7, and epidermal fatty acid binding protein were abundant in CVF and differentially present in PTL and SPTB samples, as were the serum proteins alpha-1-antitrypsin, alpha1-acid glycoprotein, haptoglobin, serotransferrin, and vitamin D binding protein. 2D-DIGE identified 17 proteins that were significantly differentially present in PTL and SPTB. Immunoblotting with specific antibodies confirmed the differences and trends of selected markers. Further characterization and quantification of these markers in a larger cohort of subjects may provide the basis for new tests for the early, noninvasive positive prediction of SPTB.

The profile of profilins

Profilins are small proteins involved in actin dynamics. In accordance with this function, they are found in all eukaryotes and are structurally highly conserved. However, their precise role in regulating actin-related functions is just beginning to emerge. This article recapitulates the wealth of information on structure, expression and functions accumulated on profilins from many different organisms in the 30 years after their discovery as actin-binding proteins. Emphasis is given to their interaction with a plethora of many different ligands in the cytoplasm as well as in the nucleus, which is considered the basis for their various activities and the significance of the tissue-specific expression of profilin isoforms.

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.

Mouse profilin 2 regulates endocytosis and competes with SH3 ligand binding to dynamin 1

Mammalian profilins are abundantly expressed actin monomer-binding proteins, highly conserved with respect to their affinities for G-actin, poly-L-proline, and phosphoinositides. Profilins associate with a large number of proline-rich proteins; the physiological significance and regulation of which is poorly understood. Here we show that profilin 2 associates with dynamin 1 via the C-terminal proline-rich domain of dynamin and thereby competes with the binding of SH3 ligands such as endophilin, amphiphysin, and Grb2, thus interfering with the assembly of the endocytic machinery. We also present a novel role for the brain-specific mouse profilin 2 as a regulator of membrane trafficking. Overexpression of profilin 2 inhibits endocytosis, whereas lack of profilin 2 in neurons results in an increase in endocytosis and membrane recycling. Phosphatidylinositol 4,5-bisphosphate releases profilin 2 from the profilin 2-dynamin 1 complex as well as from the profilin 2-actin complex, suggesting that profilin 2 is diverging the phosphoinositide signaling pathway to actin polymerization as well as endocytosis.

The actin-binding protein profilin I is localized at synaptic sites in an activity-regulated manner

Morphological changes at synaptic specializations have been implicated in regulating synaptic strength. Actin turnover at dendritic spines is regulated by neuronal activity and contributes to spine size, shape and motility. The reorganization of actin filaments requires profilins, which stimulate actin polymerization. Neurons express two independent gene products - profilin I and profilin II. A role for profilin II in activity-dependent mechanisms at spine synapses has recently been described. Although profilin I interacts with synaptic proteins, little is known about its cellular and subcellular localization in neurons. Here, we investigated the subcellular distribution of this protein in brain neurons as well as in hippocampal cultures. Our results indicate that the expression of profilin I varies in different brain regions. Thus, in cerebral cortex and hippocampus profilin I immunostaining was associated predominantly with dendrites and was present in a subset of dendritic spines. In contrast, profilin I in cerebellum was associated primarily with presynaptic structures. Profilin I immunoreactivity was partially colocalized with the synaptic molecules synaptophysin, PSD-95 and gephyrin in cultured hippocampal neurons, indicating that profilin I is present in only a subset of synapses. At dendritic spine structures, profilin I was found primarily in protrusions, which were in apposition to presynaptic terminal boutons. Remarkably, depolarization with KCl caused a moderate but significant increase in the number of synapses containing profilin I. These results show that profilin I can be present at both pre- and postsynaptic sites and suggest a role for this actin-binding protein in activity-dependent remodelling of synaptic structure.

The role of profilin complexes in cell motility and other cellular processes

Profilins are small actin-binding proteins that are essential in all organisms that have been examined to date. In vitro, profilins regulate the dynamics of actin polymerization, which is their key role in vivo during cell motility. However, there is growing evidence that, apart from actin binding, profilins function as hubs that control a complex network of molecular interactions. Profilins interact with a plethora of proteins and the importance of this aspect of their function is just beginning to be understood. In this article, I will summarize recent findings in mammalian cells and mice, and discuss the evidence of a role for profilins in cellular processes such as membrane trafficking, small-GTPase signaling and nuclear activities, in addition to neurological diseases and tumor formation.

Novel testis-expressed profilin IV associated with acrosome biogenesis and spermatid elongation

A novel profilin, named profilin IV, was cloned and characterized as a testicular isoform, distinct from the previously described testis-specific profilin III. Profilin IV showed only 30% amino acid identity with the other mammalian profilins; nevertheless, database searches produced significant alignments with the conserved profilin domain. Northern blot analysis and in situ transcript hybridization suggested that profilin IV, like profilin III, is transcribed in the germ cells. However, the timing of their expression during post-natal development of rat testis and in the rat spermatogenetic cycle was distinct. In the human testis, profilin IV mRNA expression correlates with the presence of germ cells suggesting that it may be a suitable molecular diagnostic parameter to supplement conventional histopathological diagnostics in the assessment of testicular biopsies. The predicted profilin IV protein was verified employing an anti-oligopeptide antibody. Western blot analysis detected an immunorelated testicular protein of approximately 14 kDa. Immunohistochemistry revealed an intracellular protein of the rat, the mouse and the human testis accumulating asymmetrically in the cytoplasm of round and elongating spermatids with its perinuclear location coinciding with the position of the developing acrosome-acroplaxome and the manchette. Profilin IV thus may regulate testicular actin cytoskeleton dynamics and play a role in acrosome generation and spermatid nuclear shaping.

Gene Expression of Early Hypertrophic Scar Tissue Screened by Means of cDNA Microarrays

BACKGROUND

Hypertrophic scar is an excessive healing response that often follows thermal injury. The most outstanding morphologic change is the overdeposition of collagen, which is caused by imbalance between synthesis and metabolism of collagen. Previous studies also found that transforming growth factor-beta was the key factor controlling scar formation. However, neither anti-transforming growth factor-beta nor other methods could completely control scar formation and contraction. This fact suggests a multifactorial cause. Fortunately, cDNA microarray throws light on the general alteration at the gene level, and thus could allow us to find some new clues for understanding scar formation and contraction.

METHODS

In this article, we report the results obtained from the scanning of gene expression of hypertrophic scar by means of cDNA microarray. Five cases of early human postburn hypertrophic scars were selected. Total tissue RNA was extracted from each hypertrophic scar sample and the corresponding uninjured region skin tissue; mRNA was further purified by Oligotex and then was reversely transcribed to cDNAs with the incorporation of fluorescent dUTP to prepare the hybridization probes. The mixed probes were hybridized to the cDNA microarray containing 4,096 genes on a type of chemical material-coated glass slide. After high-stringent washing, the hybridized slides were scanned for fluorescent signal detection. Then, the expression and distribution of cytoskeletal genes such as alpha-smooth muscle actin (alpha-SMA) gene; fibroblast tropomyosin TM30(pl) gene; vimentin gene; profilin gene; and BM40 gene of hypertrophic scar at 3, 6, 9, and 12 months age were further quantitatively studied by in situ hybridization or immunohistochemistry.

RESULTS

Our data indicated that there were 94 genes overexpressed and 3 genes down-regulated in early postburn hypertrophic scar. These altered genes were related to proto-oncogenes, apoptosis, immune regulatory genes, cytoskeletal elements, metabolism, and so forth. We also found that the detected cytoskeletal gene expression was much more intense at all time points than the control group. Consistent with clinical observation, cytoskeletal genes reached a peak at an early stage and gradually decreased.

CONCLUSION

Our study implied that multiple genes are involved in scar formation and contraction. Interferon is an autosecreted cytokine that might be responsible for self-control of overgrowth of cells in wounds. The early period of hypertrophic scar formation might be a good time for preventing overgrowth and contraction of hypertrophic scar by gene therapy.

Activity-induced targeting of profilin and stabilization of dendritic spine morphology

Morphological changes in dendritic spines have been implicated in connective plasticity in brain circuitry, but the underlying pathway leading from synaptic transmission to structural change is unknown. Using primary neurons expressing GFP-tagged proteins, we found that profilin, a regulator of actin polymerization, is targeted to spine heads when postsynaptic NMDA receptors are activated and that actin-based changes in spine shape are concomitantly blocked. Profilin targeting was triggered by electrical stimulation patterns known to induce the long-term changes in synaptic responsiveness associated with memory formation. These results suggest that, in addition to electrophysiological changes, NMDA receptor activation initiates changes in the actin cytoskeleton of dendritic spines that stabilize synaptic structure.

Complex formation between the postsynaptic scaffolding protein gephyrin, profilin, and Mena: a possible link to the microfilament system

Gephyrin is an essential component of the postsynaptic cortical protein network of inhibitory synapses. Gephyrin-based scaffolds participate in the assembly as well as the dynamics of receptor clusters by connecting the cytoplasmic domains of glycine and GABA(A) receptor polypeptides to two cytoskeletal systems, microtubules and microfilaments. Although there is evidence for a physical linkage between gephyrin and microtubules, the interaction between gephyrin and microfilaments is not well understood so far. Here, we show that neuronal gephyrin interacts directly with key regulators of microfilament dynamics, profilin I and neuronal profilin IIa, and with microfilament adaptors of the mammalian enabled (Mena)/vasodilator stimulated phosphoprotein (VASP) family, including neuronal Mena. Profilin and Mena/VASP coprecipitate with gephyrin from tissue and cells, and complex formation requires the E-domain of gephyrin, not the proline-rich central domain. Consequently, gephyrin is not a ligand for the proline-binding motif of profilins, as suspected previously. Instead, it competes with G-actin and phospholipids for the same binding site on profilin. Gephyrin, profilin, and Mena/VASP colocalize at synapses of rat spinal cord and cultivated neurons and in gephyrin clusters expressed in transfected cells. Thus, Mena/VASP and profilin can contribute to the postulated linkage between receptors, gephyrin scaffolds, and the microfilament system and may regulate the microfilament-dependent receptor packing density and dynamics at inhibitory synapses.

Interactions between Piccolo and the actin/dynamin-binding protein Abp1 link vesicle endocytosis to presynaptic active zones

Piccolo is a high molecular weight multi-domain protein shown to be a structural component of the presynaptic CAZ (cytoskeletal matrix assembled at active zones). These features indicate that Piccolo may act to scaffold proteins involved in synaptic vesicle endo- and exocytosis near their site of action. To test this hypothesis, we have utilized a functional cell-based endocytosis assay and identified the N-terminal proline-rich Q domain in Piccolo as a region that interferes with clathrin-mediated endocytosis. Utilizing the Piccolo Q domain as bait in a yeast two-hybrid screen, we have identified the F-actin-binding protein Abp1 (also called SH3P7 or HIP-55) as a potential binding partner for this domain. The physiological relevance of this interaction is supported by in vitro binding studies, colocalization in nerve terminals, in vivo recruitment studies, and immunoprecipitation experiments. Intriguingly, Abp1 binds to both F-actin and the GTPase dynamin and has been implicated in linking the actin cytoskeleton to clathrin-mediated endocytosis. Our results suggest that Piccolo, as a structural protein of the CAZ, may serve to localize Abp1 at active zones where it can actively participate in creating a functional connection between the dynamic actin cytoskeleton and synaptic vesicle recycling.

Endocytosis and the cytoskeleton

In this review we describe the potential roles of the actin cytoskeleton in receptor-mediated endocytosis in mammalian cells and summarize the efforts of recent years in establishing a relationship between these two cellular functions. With molecules such as dynamin, syndapin, HIP1R, Abp1, synaptojanin, N-WASP, intersectin, and cortactin a set of molecular links is now available and it is likely that their further characterization will reveal the basic principles of a functional interconnection between the membrane cytoskeleton and the vesicle-budding machinery. We will therefore discuss proteins involved in endocytic clathrin coat formation and accessory factors to control and regulate coated vesicle formation but we will also focus on actin cytoskeletal components such as the Arp2/3 complex, spectrin, profilin, and motor proteins involved in actin dynamics and organization. Additionally, we will discuss how phosphoinositides, such as PI(4,5)P2, small GTPases thought to control the actin cytoskeleton, such as Rho, Rac, and Cdc42, or membrane trafficking, such as Rab GTPases and ARF proteins, and different kinases may participate in the functional connection of actin and endocytosis. We will compare the concepts and different molecular mechanisms involved in mammalian cells with yeast as well as with specialized cells, such as epithelial cells and neurons, because different model organisms often offer complementary advantages for further studies in this thriving field of current cell biological research.

Genomic organization of profilin-III and evidence for a transcript expressed exclusively in testis

Profilins are small, widely expressed actin binding proteins, thought to be key regulators of actin dynamics in living cells. So far, three profilin-genes have been described: profilin-I (PFN1), profilin-II (PFN2) with two splice variants and the recently identified profilin-III (PFN3). Here we describe the genomic organization of the genes encoding human and mouse profilin-III. Both are single exon genes and lie in close vicinity to the renal sodium-phosphate transport gene 2 (SLC34A1, NPT2) which is highly expressed in kidney. Northern hybridization to rat tissues has previously demonstrated expression of an approximately 4.5 kb long profilin-III mRNA transcript in kidney and a mRNA transcript of approximately 1 kb in length in testis. Here we show that mouse profilin-III expression is restricted to testis and that the 4.2 kb profilin-III mRNA in kidney is the result of a slc34a1 transcript which includes the antisense profilin-III open reading frame in its 3'-untranslated region. Finally, we demonstrate by in situ hybridization that profilin-III mRNA is localized to cells in the late stage of spermatogenesis.

Actin dynamics in platelets

The human blood platelet circulates in the blood as a non-adherent disk. Upon receiving signals of blood vessel damage, the platelet reorganizes its actin cytoskeleton which transforms it into a spiky dynamic adherent glue. This transformation involves a temporal sequence of four morphologically distinct steps which is reproducible in vitro. The actin dynamics underlying these shape changes depend on a large number of actin-binding proteins. Maintenance of the discoid shape requires actin-binding proteins that inhibit these reorganizations, whereas transformation involves other proteins, some to disassemble old filaments and others to polymerize new ones. F-Actin-affinity chromatography identified a large set of actin-binding proteins including VASP, Arp2 and 2E4/kaptin. Recent discoveries show that VASP inhibits filament disassembly and Arp2/3 is required to polymerize new filaments. Morphological analysis of the distribution of these actin-binding proteins in spread platelets together with biochemical measurements of their interactions with actin lead to a model of interactions with actin that mediate shape change.

Molecular cloning and characterization of profilin-3: a novel cytoskeleton-associated gene expressed in rat kidney and testes

Profilin is a small actin-binding protein that is involved in diverse functions such as maintaining cell structure integrity, cell mobility, tumor cell metastasis, as well as growth factor signal transduction. In this paper, we describe the molecular cloning and characterization of a novel form of profilin, termed profilin-3, from rat kidney using a PCR-based procedure for isolating tissue-specific genes. The profilin-3 cDNA encoded 137 amino acids, and it shared extensive homology to profilin-1 and profilin-2 from mice and humans. More strikingly, the expression of profilin-3 was highly selective, and its mRNA was only found in the kidney and to a much lesser extent in the testis. The size of the mRNA for profilin-3 in the testis was 1.2 kb, while in kidney it was approximately 4.4-5 kb, suggesting the presence of tissue-specific transcription from different promoters. In addition, we also found that the expression of profilin-3 mRNA was significantly elevated in two types of renal diseases: diabetic nephropathy (db/db mice) and polycystic kidney diseases (cpk mice). Similar to profilin-1 and profilin-2, profilin-3 was localized in the cytoplasmic domain. Furthermore, it was capable of interacting with actin and poly-L-proline in an in vitro assay as well as in transfected cells. These results strongly suggest that, contrary to the ubiquitous presence of profilin-1 and profilin-2, there is a tissue-specific form of these cytoskeleton-regulatory proteins and that the kidney/testes-specific profilin-3 may play a unique role in renal and/or reproductive functions.

Alternative splicing of the mouse profilin II gene generates functionally different profilin isoforms

Profilins are a conserved family of proteins participating in actin dynamics and cell motility. In the mouse, two profilin genes are known. Profilin I is expressed universally at high levels, while profilin II is expressed mainly in the brain. Here we describe the occurrence of two mouse profilin II isoforms, A and B, which are derived by alternative splicing. They are identical through residue 107 of the protein, but then have distinct C-terminal sequences. Profilin IIA binds to poly-L-proline and actin with high affinity similar to profilin I. Profilin IIB on the other hand does not bind to actin and the affinity for poly-L-proline is greatly diminished. However, tubulin was found to bind to GST-profilin IIB, and in vivo GFP-profilin IIB was recruited to spindles and asters during mitosis in HeLa cells. Our results indicate unexpected diversity in the functions of the profilin family of proteins, and suggest that in mouse profilin IIA is intimately involved in actin dynamics, while profilin IIB associates with other cytoskeletal components.

Profilin II is alternatively spliced, resulting in profilin isoforms that are differentially expressed and have distinct biochemical properties

We deduced the structure of the mouse profilin II gene. It contains five exons that can generate four different transcripts by alternative splicing. Two transcripts encode different profilin II isoforms (designated IIa and IIb) that have similar affinities for actin but different affinities for polyphosphoinositides and proline-rich sequences. Profilins IIa and IIb are also present in humans, suggesting that all mammals have three profilin isoforms. Profilin I is the major form in all tissues, except in the brain, where profilin IIa is most abundant. Profilin IIb appears to be a minor form, and its expression is restricted to a limited number of tissues, indicating that the alternative splicing is tightly regulated. Western blotting and whole-mount in situ hybridization show that, in contrast to the expression of profilin I, the expression level of profilin IIa is developmentally regulated. In situ hybridization of adult brain sections reveals overlapping expression patterns of profilins I and IIa.

Identification of candidate endothelial cell autoantigens in systemic lupus erythematosus using a molecular cloning strategy: a role for ribosomal P protein P0 as an endothelial cell autoantigen

OBJECTIVE

To attempt to characterize the diversity and nature of antigens recognized by anti-endothelial cell antibodies (AECA) in patients with systemic lupus erythematosus (SLE) using a molecular cloning strategy.

METHODS

AECA in sera of 15 SLE patients were measured by ELISA and Western blot analysis was used to examine the diversity of autoantigen targets in two clinically active patients. A human umbilical vein endothelial cell cDNA expression library was immunoscreened with sera from these two patients to identify their autoantigen targets. An anti-ribosomal P peptide antibody ELISA was used to assess the clinical significance of anti-ribosomal P protein antibodies in the sera of one patient.

RESULTS

Significantly higher AECA levels were found in five patients with active disease and nephritis than in five patients with clinically inactive disease. Sera from two clinically active patients were found to recognize distinct spectra of autoantigens. The candidate autoantigens that were identified included (1) endothelial cell-specific plasminogen activator inhibitor; (2) the classical lupus antigen, i.e. ribosomal P protein P0; and (3) proteins never before described as putative autoantigens in SLE, including ribosomal protein L6, elongation factor 1alpha, adenyl cyclase-associated protein, DNA replication licensing factor, profilin II and the novel proteins HEAPLA 1 and HEAPLA 2 (human endothelial associated putative lupus autoantigens 1 and 2). In one patient, antibodies against ribosomal P protein P0 were predominant and levels of these antibodies correlated with total AECA levels, anti-DNA antibody titres, overall clinical score and renal disease in a longitudinal study.

CONCLUSIONS

A panel of candidate endothelial autoantigens in SLE, which includes previously described autoantigens and novel targets, has been identified by a molecular cloning strategy. This novel molecular approach could also be applied to the identification of autoantigens in other autoimmune vascular diseases.

Profilin is required for sustaining efficient intra- and intercellular spreading of Shigella flexneri

The ability of Shigella to mediate actin-based motility within the host cell is a prominent pathogenic feature of bacillary dysentery. The ability is dependent on the interaction of VirG with neural Wiskott-Aldrich syndrome protein (N-WASP), which in turn mediates recruitment of Arp2/3 complex and several actin-related proteins. In the present study, we show that profilin I is essential to the rapid movement of Shigella in epithelial cells, for which the capacity of profilin to interact with G-actin and N-WASP is critical. In COS-7 cells overexpressing either mutated profilin H119E, which failed to bind G-actin, or H133S, which is unable to interact with poly-l-proline, Shigella motility was significantly inhibited. Similarly, depletion of profilin from Xenopus egg extracts resulted in a decrease in bacterial motility that was completely rescued by adding back profilin I but not H119E or H133S. In COS-7 cells overexpressing a N-WASP mutant lacking the proline-rich domain (Deltap) unable to interact with profilin, the actin tail formation of intracellular Shigella was inhibited. In N-WASP-depleted extracts, addition of Deltap but not full-length N-WASP was unable to restore the bacterial motility. Furthermore, in a plaque formation assay with Madin-Darby canine kidney cell monolayers infected by Shigella, Madin-Darby canine kidney cells stably expressing H119E, H133S, or Deltap reduced the bacterial cell-to-cell spreading. These results indicate that profilin I associated with N-WASP is an essential host factor for sustaining efficient intra- and intercellular spreading of Shigella.

Vasodilator-stimulated phosphoprotein is involved in stress-fiber and membrane ruffle formation in endothelial cells

Vasodilator-stimulated phosphoprotein (VASP) is highly expressed in vascular endothelial cells, where it has been implicated in cellular reorganization during angiogenesis, as well as in endothelial retraction and changes in vessel permeability. However, the cellular functions of VASP are not known. In this study, we have expressed wild-type and mutant forms of VASP in endothelial cells to determine in what aspects of cytoskeletal behavior this protein participates. Expression of wild-type VASP induces marked membrane ruffling and formation of prominent stress fibers in bovine aortic endothelial cells. Deletion of the proline-rich domain of VASP abolishes its ability to bind profilin but does not affect ruffling or stress fiber formation. Further deletions reveal a sequence within the carboxy-terminal domain that is responsible for in vivo bundle formation. Ruffling occurs only on the expression of forms of VASP that possess bundling activity and the capacity to bind zyxin/vinculin-derived peptide. The ability of distinct subdomains within VASP to bind adhesion proteins and induce F-actin bundling in vivo suggests that this protein could function in the aggregation and tethering of actin filaments during the formation of endothelial cell-substrate and cell-cell contacts. These data provide a mechanism whereby VASP can influence endothelial migration and organization during capillary formation and modulate vascular permeability via effects on endothelial cell contractility.

Differential colocalization of profilin with microfilaments in PtK2 cells

Profilins are thought to be involved in the control of actin dynamics in eukaryotic cells. In accordance with this concept, profilin was found to be colocalized with the cortical microfilament webs in leading lamellae of locomoting and spreading fibroblasts. However, so far, there is little information on the distribution of profilin in other cell types. In this study, we report on the colocalization of profilin with various microfilament suprastructures in the epithelial cell line PtK2. This cell line, which is derived from rat kangaroo, contains a profilin sharing an N-terminal epitope with bovine and human profilin I, as seen by immunoblotting with monoclonal antibodies. By using immunofluorescence in conjunction with conventional fluorescence microscopy and confocal laser-scanning microscopy, we found profilin in ruffling areas of the peripheral lamellae and nascent stress fibers of spreading cells, whereas the peripheral belts of stationary cells growing in epithelioid sheets lacked profilin staining. In these cells, profilin was primarily distributed in a fine reticular or vesicular network that was not related to the microfilament system. Conspicuously low levels of profilins was not related to the contractile ring of mitotic cells. This was found for different fixation protocols and antibodies of the IgG and IgM type, respectively, indicating that lack of staining of the cleavage furrow was not due to antibody penetration problems. Depending on the fixation protocol, the nuclear matrix appeared strongly positive or negative for profilin. Cells microinjected with birch pollen profilin and labeled with a birch profilin-specific monoclonal antibody corroborated the results obtained with the endogeneous protein: The injected profilin was targeted to the cortical web and to nascent stress fibers of spreading cells but not to the cleavage ring of mitotic cells. These results suggest that high concentrations of a profilin I homologue are preferentially located with those microfilament suprastructures in PtK2 cells that are subject to rapid modulation by external signals.

Accumulation of profilin II at the surface of Listeria is concomitant with the onset of motility and correlates with bacterial speed

The spatial and temporal activity of the actin cytoskeleton is precisely regulated during cell motility by several microfilament-associated proteins of which profilin plays an essential role. We have analysed the distribution of green fluorescent protein (GFP)-tagged profilins in cultured and in Listeria-infected cells. Among the different GFP-profilin fusion proteins studied, only the construct in which the GFP moiety was fused to the carboxy terminus of profilin II (profilin II-GFP) was recruited by intracellular Listeria. The in vitro ligand-binding properties of this construct, e.g. the binding to monomeric actin, poly-L-proline and phosphatidylinositol 4,5-bisphosphate (PIP2), were unaffected by GFP. Profilin II-GFP co-localised with vinculin and Mena to the focal adhesions in REF-52 fibroblasts and was distributed as a thin line at the front of protruding lamellipodia in B16-F1 mouse melanoma cells. In Listeria-infected cells, profilin II-GFP was recruited, in an asymmetric fashion, to the surface of Listeria at the onset of motility whereas it was not detectable on non-motile bacteria. In contrast to the vasodilator-stimulated phosphoprotein (VASP), profilin II-GFP localised at the bacterial surface only on motile Listeria. Moreover, the fluorescence intensity of profilin II-GFP directly correlated with the speed of the bacteria. Thus, the use of GFP-tagged profilin II provides new insights into the role of profilins in cellular motility.

X-ray structure determination of human profilin II: A comparative structural analysis of human profilins

Human profilins are multifunctional, single-domain proteins which directly link the actin microfilament system to a variety of signalling pathways via two spatially distinct binding sites. Profilin binds to monomeric actin in a 1:1 complex, catalyzes the exchange of the actin-bound nucleotide and regulates actin filament barbed end assembly. Like SH3 domains, profilin has a surface-exposed aromatic patch which binds to proline-rich peptides. Various multidomain proteins including members of the Ena/VASP and formin families localize profilin:actin complexes through profilin:poly-L-proline interactions to particular cytoskeletal locations (e.g. focal adhesions, cleavage furrows). Humans express a basic (I) and an acidic (II) isoform of profilin which exhibit different affinities for peptides and proteins rich in proline residues. Here, we report the crystallization and X-ray structure determination of human profilin II to 2.2 A. This structure reveals an aromatic extension of the previously defined poly-L-proline binding site for profilin I. In contrast to serine 29 of profilin I, tyrosine 29 in profilin II is capable of forming an additional stacking interaction and a hydrogen bond with poly-L-proline which may account for the increased affinity of the second isoform for proline-rich peptides. Differential isoform specificity for proline-rich proteins may be attributed to the differences in charged and hydrophobic residues in and proximal to the poly-L-proline binding site. The actin-binding face remains nearly identical with the exception of five amino acid differences. These observations are important for the understanding of the functional and structural differences between these two classes of profilin isoforms.

Dimerization of profilin II upon binding the (GP5)3 peptide from VASP overcomes the inhibition of actin nucleation by profilin II and thymosin beta4

Profilin II dimers bind the (GP5)3 peptide derived from VASP with an affinity of approximately 0.5 microM. The resulting profilin II-peptide complex overcomes the combined capacity of thymosin beta4 and profilin II to inhibit actin nucleation and restores the extent of filament formation. We do not observe such an effect when barbed filament ends are capped. Neither can profilin I, in the presence of the peptide, promote actin polymerization during its early phase consistent with a lower affinity. Since a Pro17 peptide-profilin II complex only partially restores actin polymerization, the glycine residues in the VASP peptide appear important.

Profilin is associated with the plasma membrane in microspores and pollen

In higher plants, a large number of isoforms for the actin monomer-binding protein profilin have been identified, whereas other organisms express only few profilins. Furthermore, plant profilin isoforms are expressed in a tissue-specific manner. These observations raise questions concerning functional and locational differences between isoforms of plant profilins. In this paper, we introduce three polyclonal antisera and one monoclonal antibody developed against purified pollen profilins from Zea mays and against recombinant maize profilin. Immunoblot analyses of native profilins and four recombinant maize pollen profilin isoforms show that three of the antibodies display a preference for certain isoforms. In situ immunofluorescence of pollen of Zea mays and two developmental stages of microspores of Betula pumila indicates that all antibodies label plasma membrane-associated domains. Thus, we show that at least some profilin isoforms are located at a distinct subcellular domain within developing microspores and, less distinctly, in mature pollen. This contrasts previously reported uniform distributions throughout the cytoplasm of mature pollen and pollen tubes. The results are discussed in light of the large number of profilins co-expressed in plants and with reference to accumulating evidence for functional differences between profilin isoforms.

The essential role of profilin in the assembly of actin for microspike formation

Profilin was first identified as an actin monomer binding protein; however, recent reports indicate its involvement in actin polymerization. To date, there is no direct evidence of a functional role in vivo for profilin in actin cytoskeletal reorganization. Here, we prepared a profilin mutant (H119E) defective in actin binding, but retaining the ability to bind to other proteins. This mutant profilin I suppresses actin polymerization in microspike formation induced by N-WASP, the essential factor in microspike formation. Profilin associates both in vivo and in vitro with N-WASP at proline-rich sites different from those to which Ash/Grb2 binds. This association between profilin and N-WASP is required for N-WASP-induced efficient microspike elongation. Moreover, we succeeded in reconstituting microspike formation in permeabilized cells using profilin I combined with N-WASP and its regulator, Cdc42. These findings provide the first evidence that profilin is a key molecule linking a signaling network to rapid actin polymerization in microspike formation.

In mouse brain profilin I and profilin II associate with regulators of the endocytic pathway and actin assembly

Profilins are thought to be essential for regulation of actin assembly. However, the functions of profilins in mammalian tissues are not well understood. In mice profilin I is expressed ubiquitously while profilin II is expressed at high levels only in brain. In extracts from mouse brain, profilin I and profilin II can form complexes with regulators of endocytosis, synaptic vesicle recycling and actin assembly. Using mass spectrometry and database searching we characterized a number of ligands for profilin I and profilin II from mouse brain extracts including dynamin I, clathrin, synapsin, Rho-associated coiled-coil kinase, the Rac-associated protein NAP1 and a member of the NSF/sec18 family. In vivo, profilins co-localize with dynamin I and synapsin in axonal and dendritic processes. Our findings strongly suggest that in brain profilin I and profilin II complexes link the actin cytoskeleton and endocytic membrane flow, directing actin and clathrin assembly to distinct membrane domains.

In vivo functions of actin-binding proteins

Actin and actin-binding proteins have been identified in eukaryotic organisms across the evolutionary spectrum. Although many actin-binding proteins have been purified and studied in vitro, our understanding of the in vivo functions of these proteins has, until recently, lagged behind. In the past year, in vivo studies, especially those using genetic approaches, have led to significant advances in our understanding of how actin-binding proteins function in a cellular environment.