Comparison between the gelsolin and adseverin domain structure

Articular Chondrocyte Phenotype Regulation through the Cytoskeleton and the Signaling Processes That Originate from or Converge on the Cytoskeleton: Towards a Novel Understanding of the Intersection between Actin Dynamics and Chondrogenic Function

Numerous studies have assembled a complex picture, in which extracellular stimuli and intracellular signaling pathways modulate the chondrocyte phenotype. Because many diseases are mechanobiology-related, this review asked to what extent phenotype regulators control chondrocyte function through the cytoskeleton and cytoskeleton-regulating signaling processes. Such information would generate leverage for advanced articular cartilage repair. Serial passaging, pro-inflammatory cytokine signaling (TNF-α, IL-1α, IL-1β, IL-6, and IL-8), growth factors (TGF-α), and osteoarthritis not only induce dedifferentiation but also converge on RhoA/ROCK/Rac1/mDia1/mDia2/Cdc42 to promote actin polymerization/crosslinking for stress fiber (SF) formation. SF formation takes center stage in phenotype control, as both SF formation and SOX9 phosphorylation for COL2 expression are ROCK activity-dependent. Explaining how it is molecularly possible that dedifferentiation induces low COL2 expression but high SF formation, this review theorized that, in chondrocyte SOX9, phosphorylation by ROCK might effectively be sidelined in favor of other SF-promoting ROCK substrates, based on a differential ROCK affinity. In turn, actin depolymerization for redifferentiation would “free-up” ROCK to increase COL2 expression. Moreover, the actin cytoskeleton regulates COL1 expression, modulates COL2/aggrecan fragment generation, and mediates a fibrogenic/catabolic expression profile, highlighting that actin dynamics-regulating processes decisively control the chondrocyte phenotype. This suggests modulating the balance between actin polymerization/depolymerization for therapeutically controlling the chondrocyte phenotype.

Deletion of Adseverin in Osteoclasts Affects Cell Structure But Not Bone Metabolism

Adseverin is an actin-severing/capping protein that may contribute to osteoclast differentiation in vitro but its role in bone remodeling of healthy animals is not defined. We analyzed bone and osteoclast structure in adseverin conditional null mice at alveolar and long bone sites. In wild-type and adseverin null mice, as measured by dual-energy X-ray absorptiometry, there were no differences of bone mineral content or bone mineral density, indicating no change of bone metabolism. In tibiae, TRAcP⁺ osteoclasts were formed in comparable numbers in adseverin null and wild-type mice. Ultrastructural analysis showed normal and similar abundance of ruffled borders, sealing zones, and mitochondria, and with no difference of osteoclast nuclear numbers. In contrast, analyses of long bone showed that in the absence of adseverin osteoclasts were smaller (120 ± 13 vs. 274 ± 19 µm²; p < 0.05), as were nuclear size and the surface area of cytoplasm. The nuclei of adseverin null osteoclasts exhibited more heterochromatin (31 ± 3%) than wild-type cells (8 ± 1%), suggesting that adseverin affects cell differentiation. The data indicate that in healthy, developing tissues, adseverin contributes to the regulation of osteoclast structure but not to bone metabolism in vivo.

Adseverin plays a role in osteoclast differentiation and periodontal disease-mediated bone loss

Osteoclast differentiation and function are highly dependent on the assembly and turnover of actin filaments, but little is known about the roles of actin binding proteins in these processes. Adseverin (Ads), a member of the gelsolin superfamily of actin capping and severing proteins, regulates actin filament turnover and can regulate the turnover of cortical actin filaments of chromaffin cells during exocytosis. Using a conditional Ads knockout mouse model, we confirmed our previous finding in cultured cells that Ads plays a role in osteoclastogenesis (OCG) and actin cytoskeletal organization in osteoclasts. Here we show that Ads is required for osteoclast formation and that when alveolar bone resorption is experimentally induced in mice, genetic deletion of Ads prevents osteoclast-mediated bone loss. Further, when Ads-null osteoclasts are cultured, they exhibit defective OCG, disorganized podosome-based actin filament superstructures, and decreased bone resorption. Reintroduction of Ads into Ads-null osteoclast precursor cells restored these osteoclast defects. Collectively, these data demonstrate a unique and osteoclast-specific role for Ads in OCG and osteoclast function.

Adseverin knockdown inhibits osteoclastogenesis in RAW264.7 cells

Osteoclastogenesis is a complex process that is highly dependent on the dynamic regulation of the actin cytoskeleton. Adseverin (Ads), a member of the gelsolin superfamily of actin-binding proteins, regulates actin remodeling by severing and capping actin filaments. The objective of the present study was to characterize the role of Ads during osteoclastogenesis by assessing Ads expression and using a knockdown strategy. Immunoblot analyses were used to examine Ads expression during osteoclastogenesis. A stable Ads knockdown macrophage cell line was generated using a retroviral shRNA construct. Osteoclast differentiation was morphologically examined via cell staining with osteoclast specific markers and light microscopy. The results showed that Ads expression was significantly increased in response to receptor activator of nuclear factor-κB ligand during osteoclastogenesis, and Ads was highly expressed in mature osteoclasts. Ads-knockdown macrophages showed major osteoclastogenesis defects, most likely caused by a pre-osteoclast fusion defect. These results indicate that Ads deficiency in monocytes inhibits osteoclastogenesis. Thus, in future studies it could be noteworthy to investigate the function of Ads in bone marrow monocytes during osteoclastogenesis.

The actin binding protein adseverin regulates osteoclastogenesis

Adseverin (Ads), a member of the Gelsolin superfamily of actin binding proteins, regulates the actin cytoskeleton architecture by severing and capping existing filamentous actin (F-actin) strands and nucleating the assembly of new F-actin filaments. Ads has been implicated in cellular secretion, exocytosis and has also been shown to regulate chondrogenesis and megakaryoblastic leukemia cell differentiation. Here we report for the first time that Ads is involved in regulating osteoclastogenesis (OCG). Ads is induced during OCG downstream of RANK-ligand (RANKL) stimulation and is highly expressed in mature osteoclasts. The D5 isoform of Ads is not involved in regulating OCG, as its expression is not induced in response to RANKL. Three clonal Ads knockdown RAW264.7 (RAW) macrophage cell lines with varying degrees of Ads expression and OCG deficiency were generated. The most drastic OCG defect was noted in the clonal cell line with the greatest degree of Ads knockdown as indicated by a lack of TRAcP staining and multinucleation. RNAi mediated knockdown of Ads in osteoclast precursors resulted in distinct morphological changes characterized by altered F-actin distribution and increased filopodia formation. Ads knockdown precursor cells experienced enhanced migration while fusion of knockdown precursors cells was limited. Transient reintroduction of de novo Ads back into the knockdown system was capable of rescuing TRAcP expression but not osteoclast multinucleation most likely due to the transient nature of Ads expression. This preliminary study allows us to conclude that Ads is a RANKL induced early regulator of OCG with a potential role in pre-osteoclast differentiation and fusion.

Duplicated gelsolin family genes in zebrafish: a novel scinderin-like gene (scinla) encodes the major corneal crystallin

We have previously identified a gelsolin-like protein (C/L-gelsolin) as a corneal crystallin in zebrafish. Here we show by phylogenetic analysis that there are at least six genes encoding gelsolin-like proteins based on their gelsolin domains in zebrafish: gsna and gsnb group with the vertebrate gelsolin gene, scina and scinb group with the scinderin (adseverin) gene, and scinla (C/L-gelsolin) and scinlb are novel scinderin-like genes. RT-PCR showed that scinla, scinlb, and gsnb are preferentially expressed in the adult cornea whereas gsna is expressed to a similar extent in cornea, lens, brain, and heart; scina and scinb expression were detectable only in whole zebrafish and not in these adult tissues. Quantitative RT-PCR and 2-dimensional polyacrylamide gel electrophoresis followed by MALDI/TOF mass spectroscopy confirmed high expression of beta-actin and scinla, moderate expression of scinlb, and very low expression of gsna and gsnb in the cornea. Finally, transgenic zebrafish carrying a green fluorescent protein reporter transgene driven by a 4 kb scinla promoter fragment showed expression in the cornea, snout, dorsal fin, and tail fin of 3-day-old zebrafish larvae. Our data suggest that scinla and scinlb are diverged paralogs of the vertebrate scinderin gene and show that scinla encodes the zebrafish corneal crystallin previously called C/L-gelsolin.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

Functional dissection and molecular characterization of calcium-sensitive actin-capping and actin-depolymerizing sites in villin

All proteins of the villin superfamily, which includes the actin-capping and -severing proteins such as gelsolin, scinderin, and severin, are calcium-regulated actin-modifying proteins. Like some of these proteins, villin has morphologically distinct effects on actin assembly depending on the free calcium concentrations. At physiological calcium (Ca2+) villin nucleates and bundles actin, whereas at higher concentrations it caps (>50 microm) and severs (>200 microM) actin filaments. Although Ca(2+)-binding sites have been described in villin, the functional characterization of these sites has not been done previously. In the present study we functionally dissect the calcium-dependent actin-capping and -depolymerizing sites in villin. Our analysis reveals that villin binds Ca2+ with a Kd of 80.5 microM, a stoichiometry of 5.97, and a Hill's coefficient of 1.2. Using the NMR structure of villin 14T and the gelsolin-actin/Ca2+ crystal structure, six putative sites that result in Ca(2+)-induced conformational changes were identified in human villin and confirmed by mutational analysis. Molecular dynamics studies support the mutational analysis and provide a model for structural difference in the A93G mutant that prevents the calcium-induced conformational changes in the S1 domain of villin. Furthermore, we determined that villin expresses at least two types of Ca(2+)-sensitive sites that determine separate functional properties; site 1 (Glu-25, Asp-44, and Glu-74) regulates actin-capping, whereas sites 1 and 2 (Asp-86, Ala-93, and Asp-61), together with the intra-domain calcium-sensitive sites in villin, regulate actin depolymerization by villin. This is the first study that employs sequential mutagenesis to biochemically and functionally characterize the calcium-sensitive sites in villin. Such mutational analysis and functional characterization of the actin-capping and -depolymerizing sites are unknown for other proteins of the villin family.

Diverse eukaryotic transcripts suggest short tandem repeats have cellular functions

Previously thought "junk" DNA, short tandem repeats consisting of (GATA)n, or its compliment, were found in varied metazoan eukaryotic genomes but were rare in yeast and bacterial genomes. The (GATA)n sequence was found in cDNAs encoding mRNAs with known functions. At least 16 of 18 such transcripts encode membrane-associated proteins including: plasma membranes, synapses, mitochondrial membranes, nuclear envelopes, and brush border membranes. Flanking sequences were diverse but (GATA)n sequences clustered around 500 bases from stop codons. The (GATA)n sequences occurred in both orientations and showed constrained polymorphism. In sets of splice variants with and without (GAUA)n, the STR containing transcripts were the most abundant. These observations suggest that (GATA)n sequences probably function. In many cases, the function may be to encode post-transcriptional signals for mRNAs encoding membrane-associated proteins.

Differential developmentally regulated expression of gelsolin family members in the mouse

The gelsolin family of actin-modulating proteins contains seven mammalian members of which three have similar domain structure and function: gelsolin, capG, and adseverin. Previous studies have provided some information on the expression of these proteins, but no comprehensive analysis of expression during development has been performed. By in situ hybridization to murine embryo sections, we show that gelsolin expression is widespread but focal from e12.5 onward, with the exception of brain and mucosal epithelium. In contrast, CapG expression is high in mucosal epithelium, inner renal medulla, and adrenal cortex, and seen at much lower levels more broadly. Adseverin expression is even more restricted, being seen at sites of endochondral bone formation during development only, and in developing and adult outer renal medulla and intestine. In parallel analyses the three genes demonstrated patterns of expression that were complementary and non-overlapping in nearly all organs. The observations suggest new functions for these proteins in organ systems and tissues where their expression was not previously recognized. They further suggest that the proteins have distinct tissue-specific functions in modulating the actin cytoskeleton during cellular motile activities, and that such functions have diverged since the genes arose ancestrally by gene duplication. Dev Dyn 1999;215:297-307.

Murine adseverin (D5), a novel member of the gelsolin family, and murine adseverin are induced by interleukin-9 in T-helper lymphocytes

We identified a number of upregulated genes by differential screening of interleukin-9-stimulated T-helper lymphocytes. Interestingly, two of these messengers encode proteins that are similar to proteins of the gelsolin family. The first displays a typical structure of six homologous domains and shows a high level of identity (90%) with bovine adseverin (or scinderin) and may therefore be considered the murine adseverin homolog. The second encodes a protein with only five segments. Sequence comparison shows that most of the fifth segment and a short amino-terminal part of the sixth segment (amino acids 528 to 628 of adseverin) are missing, and thus, this form may represent an alternatively spliced product derived from the same gene. The corresponding protein is called mouse adseverin (D5). We expressed both proteins in Escherichia coli and show that mouse adseverin displays the typical characteristics of all members of the gelsolin family with respect to actin binding (capping, severing, and nucleation) and its regulation by Ca2+. In contrast, mouse adseverin (D5) fails to nucleate actin polymerization, although like mouse adseverin and gelsolin, it severs and caps actin filaments in a Ca2+-dependent manner. Adseverin is present in all of the tissues and most of the cell lines tested, although at low concentrations. Mouse adseverin (D5) was found only in blood cells and in cell lines derived from T-helper lymphocytes and mast cells, where it is weakly expressed. In a gel filtration experiment, we demonstrated that mouse adseverin forms a 1:2 complex with G actin which is stable only in the presence of Ca2+, while no stable complex was observed for mouse adseverin (D5).

Localization by segmental deletion analysis and functional characterization of a third actin-binding site in domain 5 of scinderin

Scinderin is a Ca2+-dependent actin filament severing protein present in a variety of secretory cells. Previous work suggests that scinderin-evoked cortical F-actin disassembly is required for secretion because local disassembly of cortical cytoskeleton allows secretory vesicle exocytosis (Vitale, M. L., Rodríguez Del Castillo, A., Tchakarov, L., and Trifaró, J.-M. (1991) J. Cell Biol. 113, 1057-1067). Scinderin has six domains each containing three internal sequence motifs, two actin, and two phosphatidylinositol disphosphate-binding sites in domains 1 and 2. In this paper we report the presence of another actin-binding site at the NH2-terminal of domain 5 (Sc511-518). This site binds actin in a Ca2+-independent manner and a recombinant fragment (Sc5-6 or Sc502-715) containing this site binds to actin-DNase-I-Sepharose 4B beads, co-sediments with actin and is able to nucleate actin assembly. Recombinant ScL5-6, a fusion protein devoid of the actin-binding site (Sc519-715), did not exhibit these properties. Moreover, Sc-ABP3, a peptide constructed with sequence (RLFQVRRNLASIT) identical to Sc511-523 blocked the binding of Sc5-6 to actin. Sc5-6 and Sc-ABP3 also prevented the actin severing activity of recombinant full-length scinderin (r-Sc) and inhibited the potentiation by r-Sc of Ca2+-evoked release of serotonin from permeabilized platelets. On the other hand, ScL5-6 failed to block the effect of r-Sc on platelet serotonin release. Sc1-4,6, a construct devoid of domain 5, was able to sever but unable to nucleate actin, indicating that an actin nucleation site of scinderin was in domain 5. The results suggest that scinderin, in addition to binding actin on sites present in domains 1 and 2, must bind actin on a third site in domain 5 to sever and nucleate actin effectively.

Secretion from permeabilised mast cells is enhanced by addition of gelsolin: contrasting effects of endogenous gelsolin

Permeabilised rat mast cells were exposed to gelsolin and its N-terminal half (S1-3), proteins that sever actin filaments in a calcium-dependent and independent manner, respectively. Gelsolin and S1-3 induced a decrease in cellular F-actin content and an increase in the extent of the secretory response. The calcium sensitivities of both these effects were consistent with the differential calcium requirements of the two proteins. Segment 1 (S1), which binds G-actin and caps filaments but does not sever them, did not show these effects. Thus, secretion of mast cells is promoted as a consequence of the severing activity of exogenous gelsolin or S1-3. Most of the endogenous gelsolin remained within permeabilised, washed mast cells and its distribution in resting state was predominantly cortical. Addition of calcium in the absence of MgATP did not reduce the F-actin content; by contrast, calcium with MgATP induced F-actin loss that was unaffected by the presence of anti-gelsolin. Because this antibody inhibits the severing activity of gelsolin, these results indicate that in permeabilised mast cells the severing activity of the remaining endogenous gelsolin is not involved in cortical actin filaments disassembly. Upon exposure to GTP-gamma-S in the absence of calcium, the content of cortical gelsolin was reduced. This parallels our previous observation of a GTP-gamma-S induced reduction of cortical actin filaments followed by their relocation to the cell's interior (Norman et al. (1994) J. Cell Biol. 126, 1005–1015) and suggests that actin redistribution may be a consequence of dissociation of gelsolin caps brought about by activation of a GTP-binding protein.

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.

Scinderin and chromaffin cell actin network dynamics during neurotransmitter release

It has been demonstrated that filamentous actin (F-A) is mainly localized in the cortical surface of the chromaffin cell. This F-A network acts as a barrier to the chromaffin granules impeding their contact with the plasma membrane. Stimulation of chromaffin cells with either nicotine or a depolarizing concentration of K+ induces the disassembly of cortical F-A in focal areas underneath the plasma membrane. Sites of exocytosis are localised to these areas with low concentration of F-A. The cortical surface of the chromaffin cell also contains scinderin, a Ca(2+)-dependent actin filament-severing protein recently isolated in our laboratory. Nicotine and high K+ stimulation also induce redistribution of cortical scinderin. Both nicotine and high K(+)-induced scinderin redistribution and F-A disassembly are Ca(2+)-dependent events which seem to precede neurotransmitter secretion. A possible target for protein kinase C in the modulation of secretion is the cortical F-A network. Treatment of chromaffin cells with phorbol esters prior to secretion induced scinderin redistribution, F-A disassembly and enhanced the initial rate of subsequent nicotine-evoked catecholamine release. The present results strongly indicate the involvement of the cortical cytoskeleton in the regulation of neurotransmitter release.

The amino-terminal fragment of gelsolin is cross-linked to Cys-374 of actin in the EGTA-resistant actin-gelsolin complex

It has been shown that the EGTA-resistant actin, one of the two actin molecules associated to gelsolin, can be predominantly cross-linked to gelsolin by benzophenone-4-maleimide (BPM), a photoaffinity-labeling reagent, which was conjugated to Cys-374 of actin prior to cross-linking (Doi, Y., Banba, M. and Vertut-Doï, A. (1991) Biochemistry 30, 5769-5777). When a chymotryptic digest of gelsolin containing the amino-terminal 15-kDa fragment was mixed with BPM-actin (42 kDa) and irradiated for cross-linking, a band of 58 kDa appeared on SDS-PAGE which was shown to contain actin molecule by using fluorescently labeled actin. The amino-terminal sequence of the 58-kDa complex was identical to that of gelsolin, confirming that the amino-terminal segment (residues 1-133) of pig plasma gelsolin lies closely to Cys-374 of actin in the EGTA-resistant complex.