Role of actin-binding proteins in cytoskeletal dynamics

Linking Alpha-Synuclein to the Actin Cytoskeleton: Consequences to Neuronal Function

Alpha-Synuclein (αSyn), a protein highly enriched in neurons where it preferentially localizes at the pre-synapse, has been in the spotlight because its intraneuronal aggregation is a central phenomenon in Parkinson's disease. However, the consequences of αSyn accumulation to neuronal function are not fully understood. Considering the crucial role of actin on synaptic function and the fact that dysregulation of this cytoskeleton component is emerging in neurodegenerative disorders, the impact of αSyn on actin is a critical point to be addressed. In this review we explore the link between αSyn and actin and its significance for physiology and pathology. We discuss the relevance of αSyn-actin interaction for synaptic function and highlight the actin-depolymerizing protein cofilin-1 as a key player on αSyn-induced actin dysfunction in Parkinson's disease.

The role of embryo contact and focal adhesions during maternal recognition of pregnancy

Maternal recognition of pregnancy (MRP) in the mare is an unknown process. In a non-pregnant mare on day 14 post-ovulation (PO), prostaglandin F_2α (PGF) is secreted by the endometrium causing regression of the corpus luteum. Prior to day 14, MRP must occur in order to attenuate secretion of PGF. The embryo is mobile throughout the uterus due to uterine contractions from day of entry to day 14. It is unknown what signaling is occurring. Literature stated that infusing oil or placing a glass marble into the equine uterus prolongs luteal lifespan and that in non-pregnant mares, serum exosomes contain miRNA that are targeting the focal adhesion (FA) pathway. The hypothesis of this study is embryo contact with endometrium causes a change in abundance of focal adhesion molecules (FA) in the endometrium leading to decrease in PGF secretion. Mares (n = 3/day) were utilized in a cross-over design with each mare serving as a pregnant and non-pregnant (non-mated) control on days 9 and 11 PO. Mares were randomly assigned to collection day and endometrial samples and embryos were collected on the specified day. Biopsy samples were divided into five pieces, four for culture for 24 hours and one immediately snap frozen. Endometrial biopsies for culture were placed in an incubator with one of four treatments: [1] an embryo in contact on the luminal side of the endometrium, [2] beads in contact on the luminal side of the endometrium, [3] peanut oil in contact on the luminal side of the endometrium or [4] the endometrium by itself. Biopsies and culture medium were frozen for further analysis. RNA and protein were isolated from biopsies for PCR and Western blot analysis for FA. PGF assays were performed on culture medium to determine concentration of PGF. Statistics were performed using SAS (P ≤ 0.05 indicated significance). The presence of beads on day 9 impacted samples from pregnant mares more than non-pregnant mares and had very little impact on day 11. Presence of oil decreased FA in samples from pregnant mares on day 9. On day 11, oil decreased FA abundance in samples from non-pregnant mares. Embryo contact caused multiple changes in RNA and protein abundance in endometrium from both pregnant and non-pregnant mares. The PGF secretion after 24 hours with each treatment was also determined. On day 9, there was no change in PGF secretion compared to any treatments. On day 11, presence of peanut oil increased PGF secretion in samples from non-pregnant mares. In samples from non-pregnant mares, presence of an embryo decreased PGF secretion compared to control samples from non-pregnant mares. Results revealed that while beads and peanut oil may impact abundance of FA RNA and protein in endometrial samples, it does not appear to impact PGF secretion. Conversely, embryo contact for 24 hours with endometrium from a non-pregnant mare causes a decrease in PGF secretion. These results suggest that it is not just contact of any substance/object causing attenuation of PGF secretion, but the embryo itself is necessary to decrease PGF secretion.

α-Parvin and β-parvin in the rat uterus during decidualisation and uterine receptivity

During early pregnancy, the uterine luminal epithelial cells (UECs) and endometrial stromal cells (ESCs) undergo morphological changes to enable blastocyst implantation. The present study investigates, for the first time, the cytoskeletal-associated proteins and α-actinin superfamily members, α-parvin and β-parvin, during early pregnancy in the rat uterus. These two PARVA proteins are involved in cell adhesion, morphological changes and regulation of other cytoskeletal proteins, through binding with proteins such as actin and integrin-linked kinase. α-parvin is present in UECs at fertilisation and significantly decreases by the time of implantation. β-parvin acts in opposition; significantly increasing in both UECs and ESCs at the time of implantation, suggesting a role in the process of decidualisation. Additionally, the presence of a serine-8 residue-phosphorylated α-parvin, which is associated with cell morphology changes, was found in the nuclear region of both UECs and ESCs during implantation and decidualisation. We also show that the presence of both β-parvin and phosphorylated α-parvin in ESCs is dependent on decidualisation occurring. This study demonstrates that the changing balance and localisation of the two PARVA proteins are dependent on the time of uterine receptivity, suggesting a co-dependent role in the cytoskeletal re-organisation crucial to the changing conditions necessary for implantation and decidualisation.

Global shapes of F-actin depolymerization-competent minimal gelsolins: insight into the role of g2-g3 linker in pH/Ca2+ insensitivity of the first half

Because of its ability to rapidly depolymerize F-actin, plasma gelsolin has emerged as a therapeutic molecule in different disease conditions. High amounts of exogenous gelsolin are, however, required to treat animal models of different diseases. Knowing that the F-actin depolymerizing property of gelsolin resides in its N terminus, we made several truncated versions of plasma gelsolin. The smaller versions, particularly the one composed of the first 28-161 residues, depolymerized the F-actin much faster than the native gelsolin and other truncates at the same molar ratios. Although G1-G3 loses its dependence on Ca(2+) or low pH for the actin depolymerization function, interestingly, G1-G2 and its smaller versions were found to regain this requirement. Small angle x-ray scattering-based shape reconstructions revealed that G1-G3 adopts an open shape in both the presence and the absence of Ca(2+) as well as low pH, whereas G1-G2 and residues 28-161 prefer collapsed states in Ca(2+)-free conditions at pH 8. The mutations in the g2-g3 linker resulted in the calcium sensitivity of the mutant G1-G3 for F-actin depolymerization activity, although the F-actin-binding sites remained exposed in the mutant G1-G3 as well as in the smaller truncates even in the Ca(2+)-free conditions at pH 8. Furthermore, unlike wild type G1-G3, calcium-sensitive mutants of G1-G3 acquired closed shapes in the absence of free calcium, implying a role of g2-g3 linker in determining the open F-actin depolymerizing-competent shape of G1-G3 in this condition. We demonstrate that the mobility of the G1 domain, essential for F-actin depolymerization, is indirectly regulated by the gelsolin-like sequence of g2-g3 linker.

Moesin as a key cytoskeleton regulator in corneal fibrosis

PURPOSE

: Corneal fibrosis is the third leading cause of blindness worldwide. α-Smooth muscle actin (SMA), a marker of fibrosis, is closely regulated through an intermediate group of submembrane molecules - cytoskeleton regulators. The purpose of this study was to elucidate the role of specific cytoskeleton regulators in a mouse model of corneal fibrosis.

METHODS

: A mouse model of corneal fibrosis was developed using anterior keratectomy (AK) and the topical application of transforming growth factor (TGF)-β1 (1 μg/ml). The RT² Profiler™ PCR Array for cytoskeleton regulators was used to assay changes in levels of specific members of this class of proteins. Moesin siRNA was delivered into the corneal stroma by iontophoresis in vivo. Transformation of the corneal keratocyte-to-myofibroblast in corneal fibrosis, as defined by the expression of α-SMA, was determined by Western blot.

RESULTS

: After AK and topical application of TGF-β1, moesin was the most highly upregulated gene among 84 cytoskeleton regulator genes; iontophoresing moesin siRNA into the corneal stroma reduced the expression of α-SMA to 0.22-, 0.52-, and 0.31-fold of control at postoperative (PO) day 1, 3, and 5, respectively; also, upregulation of phospho-Smad 2 induced by TGF-β1 was reduced by moesin siRNA to 0.59-, 0.56-, and 0.31-fold of control and expression of phospho-Smad 3 was reduced to 0.58-, 0.53-, and 0.47-fold of control at the same PO days.

CONCLUSIONS

: Moesin may be a potential drug target for inhibiting corneal fibrosis, and the details of moesin-related signaling pathways would be critical for understanding corneal fibrosis.

Immunological responses and actin dynamics in macrophages are controlled by N-cofilin but are independent from ADF

Dynamic changes in the actin cytoskeleton are essential for immune cell function and a number of immune deficiencies have been linked to mutations, which disturb the actin cytoskeleton. In macrophages and dendritic cells, actin remodelling is critical for motility, phagocytosis and antigen presentation, however the actin binding proteins, which control antigen presentation have been poorly characterized. Here we dissect the specific roles of the family of ADF/cofilin F-actin depolymerizing factors in macrophages and in local immune responses.

Macrophage migration, cell polarization and antigen presentation to T-cells require n-cofilin mediated F-actin remodelling. Using a conditional mouse model, we show that n-cofilin also controls MHC class II-dependent antigen presentation. Other cellular processes such as phagocytosis and antigen processing were found to be independent of n-cofilin. Our data identify n-cofilin as a novel regulator of antigen presentation, while ADF on the other hand is dispensable for macrophage motility and antigen presentation.

Direct manipulation of intracellular stress fibres using a hook-shaped AFM probe

Atomic force microscopy (AFM) is a highly successful technique for imaging nanometre-sized samples and measuring pico- to nano-newton forces acting between atoms and molecules. When it comes to the manipulation of larger samples with forces of tens and hundreds of nano-newtons, however, the present chemistry-based modification protocols for functionalizing AFM cantilevers to achieve the formation of covalent/non-covalent linkages between the AFM probe and the sample surface do not produce strong enough bonds. For the purpose of measuring the fracture strength and other mechanical properties of stress fibres (SFs) in living as well as semi-intact fibroblast cells, we fabricated an AFM probe with a hooking function by focused ion beam technology and used the AFM probe hook to capture, pull and eventually sever a chosen SF labelled with green or red fluorescent protein.

Functional consequences of actin nitration: in vitro and in disease states

To link the phenomena of inflammatory-induced increases in protein nitrotyrosine (NO(2)Tyr) derivatives to protein dysfunction and consequent pathological conditions, the evaluation of discrete NO(2)Tyr modifications on specific proteins must be undertaken. Mass spectrometric (MS) proteomics-based strategies allow for the identification of all individual proteins that are nitrated by separating tissue homogenates using 2D gel electrophoresis, detecting the nitrated proteins using an anti-NO(2)Tyr antibody, and then identifying the peptides generated during an in-gel proteolytic digest using matrix-assisted laser desorption ionization/time-of-flight (MALDI-TOF) MS. Actin, one of the most abundant proteins in eukaryotic cells, constitutes 5% or more of cell protein and serves with other cytoskeletal proteins as a critical target for nitration-induced functional impairment. Herein, examples of actin nitration detected under physiological conditions in various models of human disease or in clinically derived tissues are given and the impact that this post-translational protein modification can have on cell and organ function is discussed.

Actin binding protein expression is altered in uterine luminal epithelium by clomiphene citrate, a synthetic estrogen receptor modulator

Clomiphene citrate (CC), a synthetic oestrogen, is often prescribed as a superovulator in treating infertility. Although CC works efficiently, pregnancy rates following CC treatment are approximately 10 times lower than "natural" rates. This study investigates how a dose of 1.25 mg CC given to ovariectomized rats before the implantation priming hormones (a single dose of progesterone for 3 days and a dose of estradiol-17beta on d3, P-P-PE), alters the expression and distribution of alpha-actinin, gelsolin and vinculin. Actin binding proteins show a specific distribution within the uterine epithelium during implantation, linking the actin cytoskeleton to integrin expression on the uterine surface and in this way aiding "adhesiveness" for blastocyst apposition to the uterine epithelium. In this study, immunocytochemistry on frozen uterine sections using mouse monoclonal antibodies against alpha-actinin, gelsolin and vinculin and peroxidase-conjugated secondary antibodies, show that CC, administered before the P-P-PE regimen, down-regulates the expression of vinculin, does not alter the expression of gelsolin and up-regulates alpha-actinin on the uterine apical surface, when compared to P-P-PE treated animals. All three proteins are down-regulated on the apical surface of the luminal epithelium and glands in all groups when compared to pregnant controls. Vinculin was only localized in the basolateral compartment of the uterine epithelial cells in the CC treated groups. By down-regulating these proteins on the uterine surface and up-regulating vinculin on the basolateral membrane of the epithelium, CC may impede adhesion and invasion of blastocysts at implantation. These results may aid the exogenous manipulation of uterine tissue to control fertility and improve assisted reproductive out-comes.

Identification of interactive gene networks: a novel approach in gene array profiling of myometrial events during guinea pig pregnancy

OBJECTIVE

The transition from myometrial quiescence to activation is poorly understood, and the analysis of array data is limited by the available data mining tools. We applied functional analysis and logical operations along regulatory gene networks to identify molecular processes and pathways underlying quiescence and activation.

STUDY DESIGN

We analyzed some 18,400 transcripts and variants in guinea pig myometrium at stages corresponding to quiescence and activation, and compared them to the nonpregnant (control) counterpart using a functional mapping tool, MetaCore (GeneGo, St Joseph, MI) to identify novel gene networks composed of biological pathways during mid (MP) and late (LP) pregnancy.

RESULTS

Genes altered during quiescence and or activation were identified following gene specific comparisons with myometrium from nonpregnant animals, and then linked to curated pathways and formulated networks. The MP and LP networks were subtracted from each other to identify unique genomic events during those periods. For example, changes 2-fold or greater in genes mediating protein biosynthesis, programmed cell death, microtubule polymerization, and microtubule based movement were noted during the transition to LP.

CONCLUSION

We describe a novel approach combining microarrays and genetic data to identify networks associated with normal myometrial events. The resulting insights help identify potential biomarkers and permit future targeted investigations of these pathways or networks to confirm or refute their importance.

Polyphosphoinositides-dependent regulation of the osteoclast actin cytoskeleton and bone resorption

BACKGROUND

Gelsolin, an actin capping protein of osteoclast podosomes, has a unique function in regulating assembly and disassembly of the podosome actin filament. Previously, we have reported that osteopontin (OPN) binding to integrin alphavbeta3 increased the levels of gelsolin-associated polyphosphoinositides, podosome assembly/disassembly, and actin filament formation. The present study was undertaken to identify the possible role of polyphosphoinositides and phosphoinositides binding domains (PBDs) of gelsolin in the osteoclast cytoskeletal structural organization and osteoclast function.

RESULTS

Transduction of TAT/full-length gelsolin and PBDs containing gelsolin peptides into osteoclasts demonstrated: 1) F-actin enriched patches; 2) disruption of actin ring; 3) an increase in the association polyphosphoinositides (PPIs) with the transduced peptides containing PBDs. The above-mentioned effects were more pronounced with gelsolin peptide containing 2 tandem repeats of PBDs (PBD (2)). Binding of PPIs to the transduced peptides has resulted in reduced levels of PPIs association with the endogenous gelsolin, and thereby disrupted the actin remodeling processes in terms of podosome organization in the clear zone area and actin ring formation. These peptides also exhibited a dominant negative effect in the formation of WASP-Arp2/3 complex indicating the role of phosphoinositides in WASP activation. The TAT-PBD gelsolin peptides transduced osteoclasts are functionally defective in terms of motility and bone resorption.

CONCLUSIONS

Taken together, these data demonstrate that transduction of PBD gelsolin peptides into osteoclasts produced a dominant negative effect on actin assembly, motility, and bone resorption. These findings indicate that phosphoinositide-mediated signaling mechanisms regulate osteoclast cytoskeleton, podosome assembly/disassembly, actin ring formation and bone resorption activity of osteoclasts.

Cytoskeletal proteins in uterine epithelial cells only partially return to the pre-receptive state after the period of receptivity

Immunohistochemical staining of 5 cytoskeletal proteins (actin, alpha-actinin, gelsolin, plectin and plakoglobin) was used to investigate changes in distribution patterns of these proteins after the period of uterine receptivity for blastocyst implantation in the rat. Actin was found throughout the cytoplasm but it was concentrated along the apical plasma membrane on day 1 of pregnancy, decreased by day 6 and then increased again at day 9. Alpha-actinin and gelsolin were localized in distinctive bands along the apical plasma membrane at day 6 of pregnancy but became diffusely distributed at day 9. Plectin was localized along the apical and basal plasma membranes at day 6 but in higher amounts apically and at day 9, it was concentrated in apical and basal zones in the cells. Plakoglobin was found along the lateral and basal membranes with increased intensity along the apical third of the lateral plasma membrane from day 6 to day 9 of pregnancy. These results show that all 5 cytoskeletal proteins redistributed after the period of uterine receptivity: some exhibited a similar pattern of labelling to that found during the prereceptive state, whereas others only partially returned to the pre-receptive state. This change in distribution patterns may reflect differences in the epithelial barrier function before and after the period of receptivity.

Nitric oxide-dependent generation of reactive species in sickle cell disease. Actin tyrosine induces defective cytoskeletal polymerization

The intermittent vascular occlusion occurring in sickle cell disease (SCD) leads to ischemia-reperfusion injury and activation of inflammatory processes including enhanced production of reactive oxygen species and increased expression of inducible nitric-oxide synthase (NOS2). Appreciating that impaired nitric oxide-dependent vascular function and the concomitant formation of oxidizing and nitrating species occur in concert with increased rates of tissue reactive oxygen species production, liver and kidney NOS2 expression, tissue 3-nitrotyrosine (NO(2)Tyr) formation and apoptosis were evaluated in human SCD tissues and a murine model of SCD. Liver and kidney NOS2 expression and NO(2)Tyr immunoreactivity were significantly increased in SCD mice and humans, but not in nondiseased tissues. TdT-mediated nick end-label (TUNEL) staining showed apoptotic cells in regions expressing elevated levels of NOS2 and NO(2)Tyr in all SCD tissues. Gas chromatography mass spectrometry analysis revealed increased plasma protein NO(2)Tyr content and increased levels of hepatic and renal protein NO(2)Tyr derivatives in SCD (21.4 +/- 2.6 and 37.5 +/- 7.8 ng/mg) versus wild type mice (8.2 +/- 2.2 and 10 +/- 1.2 ng/mg), respectively. Western blot analysis and immunoprecipitation of SCD mouse liver and kidney proteins revealed one principal NO(2)Tyr-containing protein of 42 kDa, compared with controls. Enzymatic in-gel digestion and MALDI-TOF mass spectrometry identified this nitrated protein as actin. Electrospray ionization and fragment analysis by tandem mass spectrometry revealed that 3 of 15 actin tyrosine residues are nitrated (Tyr(91), Tyr(198), and Tyr(240)) at positions that significantly modify actin assembly. Confocal microscopy of SCD human and mouse tissues revealed that nitration led to morphologically distinct disorganization of filamentous actin. In aggregate, we have observed that the hemoglobin point mutation of sickle cell disease that mediates hemoglobin polymerization defects is translated, via inflammatory oxidant reactions, into defective cytoskeletal polymerization.

Binding of gelsolin domain 2 to actin. An actin interface distinct from that of gelsolin domain 1 and from ADF/cofilin

It is generally assumed that of the six domains that comprise gelsolin, domain 2 is primarily responsible for the initial contact with the actin filament that will ultimately result in the filament being severed. Other actin-binding regions within domains 1 and 4 are involved in gelsolin's severing and subsequent capping activity. The overall fold of all gelsolin repeated domains are similar to the actin depolymerizing factor (ADF)/cofilin family of actin-binding proteins and it has been proposed that there is a similarity in the actin-binding interface. Gelsolin domains 1 and 4 bind G-actin in a similar manner and compete with each other, whereas domain 2 binds F-actin at physiological salt concentrations, and does not compete with domain 1. Here we investigate the domain 2 : actin interface and compare this to our recent studies of the cofilin : actin interface. We conclude that important differences exist between the interfaces of actin with gelsolin domains 1 and 2, and with ADF/cofilin. We present a model for F-actin binding of domain 2 with respect to the F-actin severing and capping activity of the whole gelsolin molecule.

Structure, development and function of cytoskeletal elements in non-neuronal cells of the human eye

The cytoskeleton, of which the main components in the human eye are actin microfilaments, intermediate filaments and microtubules with their associated proteins, is essential for the normal growth, maturation, differentiation, integrity and function of its cells. These components interact with intra- and extracellular environment and each other, and their profile frequently changes during development, according to physiologic demands, and in various diseases. The ocular cytoskeleton is unique in many ways. A special pair of cytokeratins, CK 3 and 12, has apparently evolved only for the purposes of the corneal epithelium. However, other cytokeratins such as CK 4, 5, 14, and 19 are also important for the normal ocular surface epithelia, and other types may be acquired in keratinizing diseases. The intraocular tissues, which have a relatively simple cytoskeleton consisting mainly of vimentin and simple epithelial CK 8 and 18, differ in many details from extraocular ones. The iris and lens epithelium characteristically lack cytokeratins in adults, and the intraocular muscles all have a cytoskeletal profile of their own. The dilator of the iris contains vimentin, desmin and cytokeratins, being an example of triple intermediate filament expression, but the ciliary muscle lacks cytokeratin and the sphincter of the iris is devoid even of vimentin. Conversion from extraocular-type cytoskeletal profile occurs during fetal life. It seems that posttranslational modification of cytokeratins in the eye may also differ from that of extraocular tissues. So far, it has not been possible to reconcile the cytoskeletal profile of intraocular tissues with their specific functional demands, but many theories have been put forward. Systematic search for cytoskeletal elements has also revealed novel cell populations in the human eye. These include transitional cells of the cornea that may represent stem cells on migration, myofibroblasts of the scleral spur and juxtacanalicular tissue that may modulate aqueous outflow, and subepithelial matrix cells of the ciliary body and myofibroblasts of the choroid that may both participate in accommodation. In contrast to the structure and development of the ocular cytoskeleton, changes that take place in ocular disease have not been analysed systematically. Nevertheless, potentially meaningful changes have already been observed in corneal dystrophies (Meesmann's dystrophy, posterior polymorphous dystrophy and iridocorneal endothelial syndrome), degenerations (pterygium) and inflammatory diseases (Pseudomonas keratitis), in opacification of the lens (anterior subcapsular and secondary cataract), in diseases characterized by proliferation of the retinal pigment epithelium (macular degeneration and proliferative vitreoretinopathy), and in intraocular tumours (uveal melanoma). In particular, upregulation of alpha-smooth muscle actin seems to be a relatively general response typical of spreading and migrating corneal stromal and lens epithelial cells, trabecular cells and retinal pigment epithelial cells.

Caspase-3-generated fragment of gelsolin: effector of morphological change in apoptosis

The caspase-3 (CPP32, apopain, YAMA) family of cysteinyl proteases has been implicated as key mediators of apoptosis in mammalian cells. Gelsolin was identified as a substrate for caspase-3 by screening the translation products of small complementary DNA pools for sensitivity to cleavage by caspase-3. Gelsolin was cleaved in vivo in a caspase-dependent manner in cells stimulated by Fas. Caspase-cleaved gelsolin severed actin filaments in vitro in a Ca2+-independent manner. Expression of the gelsolin cleavage product in multiple cell types caused the cells to round up, detach from the plate, and undergo nuclear fragmentation. Neutrophils isolated from mice lacking gelsolin had delayed onset of both blebbing and DNA fragmentation, following apoptosis induction, compared with wild-type neutrophils. Thus, cleaved gelsolin may be one physiological effector of morphologic change during apoptosis.

Actin-binding proteins undergo major alterations during the plasma membrane transformation in uterine epithelial cells

BACKGROUND

The apical surface of uterine epithelial cells undergoes a dramatic transformation during early pregnancy. Previous studies have shown that cytoskeletal actin microfilaments are associated with this transformation, but little is known of the role played by actin-binding proteins or which of the many described in other cell types are present in uterine epithelial cells.

METHODS

Immunohistochemical staining using monoclonal antibodies raised against four different actin-binding proteins (alpha-actinin, tropomyosin, gelsolin, and vinculin) was used to study the changing distribution of these proteins in uterine epithelium during early pregnancy in the rat.

RESULTS

Findings indicated the presence of all four of the actin-binding proteins in the uterine epithelium. The distribution of tropomyosin remained unchanged over the period of early pregnancy. Gelsolin and alpha-actinin displayed similarity in distribution. Day 1 showed an apicobasal localisation of reaction product, which by day 6 of pregnancy had concentrated into a thick band across the luminal surface of the cells. Vinculin staining was a diffuse band at the level of the basal plasma membrane at day 1 and became a diffuse faint band across the apical part of the cells on day 6.

CONCLUSIONS

This study confirms the presence of actin-binding proteins in uterine epithelial cells, and these findings are discussed in light of known ultrastructural alterations in the uterine epithelium during early pregnancy. Elucidation of the role of the actin-based cytoskeleton in the uterine epithelium may further our understanding of the dynamics of this unique environment that allows the implantation of a blastocyst.

Multiple pathways for denaturation of horse plasma gelsolin

Gelsolin purified from horse plasma carries a surface charge distribution that greatly influences how the protein unfolds, aggregates, or precipitates as a function of temperature or concentration of chemical denaturant. Modification of gelsolin with fluorescein isothiocyanate replaces positive charges on amine groups with bulky, negatively charged fluorescein moieties. This postpones thermally induced precipitation by about 10 degrees C [Koepf, E.K., and Burtnick, L.D. 1993. Eur. J. Biochem. 212: 713-718]. Interaction with cations such as Ca2+ or guanidinium+ also alters the surface charge on gelsolin. This affects the structure of the protein in solution, modifies the pathway for unfolding, and moderates the onset of precipitation induced by chemical denaturants or heat. Denaturation of gelsolin is not interpretable in terms of a simple two-state cooperative mechanism. The pathway to a denatured state and intermediate structures present along the way depend upon the agent used to unfold the protein.

Functions of [His321]gelsolin isolated from a flat revertant of ras-transformed cells

A mutant gelsolin, [His321]gelsolin, was isolated from R1, a flat revertant of human activated c-Ha-ras oncogene-transformed NIH/3T3 cells (EJ-NIH/3T3) produced by ethylmethanesulfonate treatment. [His321]Gelsolin has a histidine instead of a proline residue at position 321 and suppresses the tumorigenicity of EJ-NIH/3T3 cells when it is constitutively expressed [Müllauer, L., Fujita, H., Ishizaki, A. & Kuzumaki, N. (1993) Oncogene 8, 2531-2536]. To investigate the biochemical consequences of the amino acid substitution of His321, we expressed the [His321]gelsolin and wild-type gelsolin in Escherichia coli, purified them, and analyzed their effects on actin, polyphosphoinositol lipids and phospholipase C. [His321]Gelsolin has decreased actin-filament-severing activity and increased nucleating activity compared with wild-type gelsolin in vitro. Furthermore, compared to wild-type gelsolin both nucleation and severing by [His321]gelsolin are inhibited more strongly by the phosphoinositol lipids phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). In addition, [His321]gelsolin inhibits PtdInsP2 hydrolysis by phospholipase C gamma 1 more strongly than wild-type gelsolin in vitro because of its higher binding capacity for phosphoinositol lipid. Gelsolin has six homologous amino acid repeats called S1-S6. Our results suggest that the segment S3 which contains the mutation is functionally relevant for regulation of gelsolin's activities even though the relevant actin-binding domains are in segments 1, 2, and 4-6, and that the region around the residue 321 may contain a phosphoinositol-lipid-binding site. Altered functions of [His321]gelsolin might be important for the loss of tumorigenicity of the ras-transformed cells.

On the crawling of animal cells

Cells crawl in response to external stimuli by extending and remodeling peripheral elastic lamellae in the direction of locomotion. The remodeling requires vectorial assembly of actin subunits into linear polymers at the lamella's leading edge and the crosslinking of the filaments by bifunctional gelation proteins. The disassembly of the crosslinked filaments into short fragments or monomeric subunits away from the leading edge supplies components for the actin assembly reactions that drive protrusion. Cellular proteins that respond to lipid and ionic signals elicited by sensory cues escort actin through this cycle in which filaments are assembled, crosslinked, and disassembled. One class of myosin molecules may contribute to crawling by guiding sensory receptors to the cell surface, and another class may contribute by imposing contractile forces on actin networks in the lamellae.

Horse plasma gelsolin labelled with fluorescein isothiocyanate responds to calcium and actin

Reaction between horse plasma gelsolin and fluorescein-5-isothiocyanate (FITC) resulted in incorporation of 4.8 +/- 0.6 fluorescein groups/gelsolin molecule. The sites of modification were not clustered in any one portion of the gelsolin polypeptide chain; all major peptides produced by proteolytic digestion with alpha-chymotrypsin exhibited a fluorescence characteristic of fluorescein. FITC-gelsolin has a peptide-backbone circular dichroism spectrum at 20 degrees C that is indistinguishable from that of native gelsolin, but FITC-gelsolin is considerably more resistant than native gelsolin to thermally induced precipitation. FITC-gelsolin is fully able to carry out severing of F-actin filaments, the prime function of gelsolin in plasma. An opening up of the structure of gelsolin on binding Ca2+ is evident from an increased susceptibility of FITC-gelsolin to quenching by I-. Ca2+ dependence of the interaction between gelsolin and actin is evident in titrations both of intensity and polarization of the fluorescence of FITC-gelsolin solutions. A Ca(2+)-sensitive interaction between gelsolin and tropomyosin also is observed.

F-actin capping proteins

Recent research on F-actin capping proteins has concentrated on three main areas. The discovery that controlled actin polymerization is the driving force for intracellular movement suggests an important role for capping proteins in regulating filament number and length. A capping protein from Dictyostelium (related to heat-shock protein HSP70) has been characterized that is activated by external stimuli. This provides a pivotal connection between extracellular signalling, cytoskeletal reorganization and locomotory behaviour. The roles of individual actin-binding sites in the gelsolin family of severing/capping proteins and binding sites for phosphatidylinositol 4,5-bisphosphate have been identified.

Evidence for functional homology in the F-actin binding domains of gelsolin and alpha-actinin: implications for the requirements of severing and capping

The F-actin binding domains of gelsolin and alpha-actinin compete for the same site on actin filaments with similar binding affinities. Both contain tandem repeats of approximately 125 amino acids, the first of which is shown to contain the actin-binding site. We have replaced the F-actin binding domain in the NH2-terminal half of gelsolin by that of alpha-actinin. The hybrid severs filaments almost as efficiently as does gelsolin or its NH2-terminal half, but unlike the latter, requires calcium ions. The hybrid binds two actin monomers and caps the barbed ends of filaments in the presence or absence of calcium. The cap produced by the hybrid binds with lower affinity than that of gelsolin and is not stable: It dissociates from filament ends with a half life of approximately 15 min. Although there is no extended sequence homology between these two different F-actin binding domains, our experiments show that they are functionally equivalent and provide new insights into the mechanism of microfilament severing.

Crystallization of the complex of actin with gelsolin segment 1

Crystals of a 1:1 complex between human gelsolin segment 1 and actin have been grown from solutions containing polyethylene glycol 6000. The crystals are orthorhombic, space group P2(1)2(1)2(1); the axes are a = 57.4 A, b = 70.4 A, c = 184.5 A. They are moderately stable to X-rays and diffract to beyond 2.5 A. There is one molecule of complex in the asymmetric unit.