Identification of plant cytoskeletal, cell cycle-related and polarity-related proteins using Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe has been used to identify Arabidopsis thaliana proteins that may play a role in cell shape maintenance or cell cycle regulation. An Arabidopsis thaliana cDNA library was constructed in pREP5N vector under the control of the inducible nmt1 promoter and transformed into S. pombe. Expression of the A. thaliana sequences was induced and clones showing severe morphological changes were identified and analysed. Comparison of the sequences of the inserts with the sequence data bases revealed that several cDNAs encode proteins known to play a role in function of the cytoskeleton, the cell cycle and establishment of cell polarity. These include alpha-1, alpha-2, alpha-6 and beta-6 tubulins, myosin heavy chain-like protein, ubiquitin conjugating enzymes UBC9 (E2), 26S protease subunits, Ranbinding protein, myb protein, PRL1 gene product and rho protein. Approximately 30% of the clones encode novel sequences. The results suggest that S. pombe phenotypic screening can be used to identify plant proteins involved in cell shape maintenance and regulation during cell cycle and development.

High expression of the focal adhesion- and microfilament-associated protein VASP in vascular smooth muscle and endothelial cells of the intact human vessel wall

The focal adhesion and microfilament-associated protein VASP is a major substrate of both cAMP- and cGMP-dependent protein kinase in intact human platelets. The recent elucidation of the primary VASP structure and identity of VASP binding proteins suggest that VASP is an important component of focal contacts which link signal transduction pathways and elements controlling cell motility. In this study, the high expression of VASP in vascular smooth muscle and endothelial cells of human blood vessels is reported. Western blot and immunofluorescence analysis detected VASP in human heart, femoral artery and a uterine leiomyosarcoma. Within these tissues, smooth muscle cells, small capillaries and the endothelial cell layer were strongly stained by the VASP antiserum. In human heart, an overlapping cellular distribution of the cGMP-dependent protein kinase I (cGK I) and its substrate VASP was noted. Immunoelectron microscopy experiments with vascular smooth muscle cells of the vessel wall revealed that VASP is localized in close proximity to microfilaments, dense plaques and dense bodies. The data of this study and the properties of VASP as a major target of inhibitory vasoactive agents suggest that VASP is an important component which participates in the regulation of cell motility of human vessel wall cells in vivo.

Pollen specific expression of maize genes encoding actin depolymerizing factor-like proteins

In pollen development, a dramatic reorganization of the actin cytoskeleton takes place during the passage of the pollen grain into dormancy and on activation of pollen tube growth. A role for actin-binding proteins is implicated and we report here the identification of a small gene family in maize that encodes actin depolymerizing factor (ADF)-like proteins. The ADF group of proteins are believed to control actin polymerization and depolymerization in response to both intracellular and extracellular signals. Two of the maize genes ZmABP1 and ZmABP2 are expressed specifically in pollen and germinating pollen suggesting that the protein products may be involved in pollen actin reorganization. A third gene, ZmABP3, encodes a protein only 56% and 58% identical to ZmABP1 and ZmABP2, respectively, and its expression is suppressed in pollen and germinated pollen. The fundamental biochemical characteristics of the ZmABP proteins has been elucidated using bacterially expressed ZmABP3 protein. This has the ability to bind monomeric actin (G-actin) and filamentous actin (F-actin). Moreover, it decreases the viscosity of polymerized actin solutions consistent with an ability to depolymerize filaments. These biochemical characteristics, taken together with the sequence comparisons, support the inclusion of the ZmABP proteins in the ADF group.

Fibrillin: evidence that chondroitin sulphate proteoglycans are components of microfibrils and associate with newly synthesised monomers

We have investigated the potential association of proteoglycans with intact fibrillin-containing microfibrils from foetal bovine elastic tissues and with newly synthesised fibrillin in human and bovine cell cultures. Microfibril integrity was disrupted by chondroitinase ABC lyase and chondroitinase AC lyase, but not by keratanase or hyaluronidase. Following chondroitinase treatment, beads were disrupted but the underlying fibrillar scaffold appeared intact. Cuprolinic blue was prominently associated with beaded domains at a critical electrolyte concentration. Electron-dense rods were often associated with cuprolinic blue-treated microfibrils isolated from fixed tissues. Positive staining revealed charged foci at the beads. Newly synthesised fibrillin could be labelled with 35S TransLabel, [3H]glucosamine or 35SO4 but its electrophoretic mobility was not influenced by treatment with chondroitinase ABC or AC lyase. A diffuse 35SO4-labelled chondroitinase-sensitive component with a resistant band (Mr 35000) co-immunoprecipitated with fibrillin. These experiments indicate that chondroitin sulphate proteoglycans associate with fibrillin and contribute to microfibril assembly. This association has major implications for microfibril function in health and disease.

Single amino acid mutations in Drosophila fascin disrupt actin bundling function in vivo

Fascins bundle actin filaments into large, tightly packed hexagonal arrays that support diverse cellular processes including microvillar projections and filopodial extensions. In Drosophila, fascin is encoded by the singed locus. Severe singed mutants have gnarled bristles and are female sterile due to a defect in rapid cytoplasm transport during oogenesis. In this paper, we report the results of a large EMS mutagenesis screen to generate new singed alleles. A mutation that changes glycine 409 to glutamic acid results in partial inactivation of fascin in vivo; singedG409E mutants have kinked bristles and are fertile with a mild nurse cell cytoplasm transport defect. This mutation is in a small conserved domain near the C-terminus of fascin. A mutation that changes serine 289 to asparagine almost completely inactivates fascin in vivo; singedS289N mutants have gnarled bristles and are sterile due to a severe defect in nurse cell cytoplasm transport caused by the absence of nurse cell cytoplasmic actin bundles. A subsequent EMS mutagenesis screen for dominant suppressors of singedS289N sterility revealed an intragenic suppressor mutation that changes serine 251 to phenylalanine and restores much of fascin's function. These two mutations, S289N and S251F, draw attention to a central domain in fascin.

Movement of yeast cortical actin cytoskeleton visualized in vivo

Fusion proteins between the green fluorescent protein (GFP) and the cytoskeleton proteins Act1p (actin), Sac6p (yeast fimbrin homolog), and Abp1p in budding yeast (Saccharomyces cerevisiae) localize to the cortical actin patches. The actin fusions could not function as the sole actin source in yeast, but fusions between the actin-binding proteins Abp1p and Sac6p complement fully the phenotypes associated with their gene deletions. Direct observation in vivo reveals that the actin cortical patches move. Movement of actin patches is constrained to the asymmetric distribution of the patches in growing cells, and this movement is greatly reduced when metabolic inhibitors such as sodium azide are added. Fusion protein-labeled patches are normally distributed during the yeast cell cycle and during mating. In vivo observation made possible the visualization of actin patches during sporulation as well.

Expression of growth arrest-specific (Gas) genes in murine keratinocytes: Gas2 is specifically regulated

In order to elucidate a possible role of growth arrest-specific (gas) genes in the regulation of tissue proliferation, we analyzed their expression in keratinocytes isolated from murine back skin. On the mRNA level gas1, gas5, and gas6 were found to be significantly expressed whereas there was a relatively low expression of gas2, gas3, and gas4. Using keratinocytes fractionated according to their density resulted in subpopulations of cells: differentiating suprabasal cells in fractions I and II; proliferative basal cells in fractions IIIa, III and IV. We found gas2 protein to be expressed more strongly in the proliferative cells than in the differentiating cells. Stimulation of hyperproliferation by 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in a transient increase of gas2 protein content concomitantly with the time of maximal cell renewal. In this respect the murine keratinocyte cell line MSCP5 resembled freshly isolated keratinocytes. There was a higher expression of gas2 protein during exponential growth than during growth arrest, induced either by serum starvation or by TGFbeta treatment. Since, in contrast to the results reported for 3T3 cells, growth arrest within these cells was not accompanied by an elevation of gas2 protein, we suggest a cell-specific regulation of its expression.

Xenopus laevis actin-depolymerizing factor/cofilin: a phosphorylation-regulated protein essential for development

Two cDNAs, isolated from a Xenopus laevis embryonic library, encode proteins of 168 amino acids, both of which are 77% identical to chick cofilin and 66% identical to chick actin-depolymerizing factor (ADF), two structurally and functionally related proteins. These Xenopus ADF/cofilins (XADs) differ from each other in 12 residues spread throughout the sequence but do not differ in charge. Purified GST-fusion proteins have pH-dependent actin-depolymerizing and F-actin-binding activities similar to chick ADF and cofilin. Similarities in the developmental and tissue specific expression, embryonic localization, and in the cDNA sequence of the noncoding regions, suggest that the two XACs arise from allelic variants of the pseudotetraploid X. laevis. Immunofluorescence localization of XAC in oocyte sections with an XAC-specific monoclonal antibody shows it to be diffuse in the cortical cytoplasm. After fertilization, increased immunostaining is observed in two regions: along the membrane, particularly that of the vegetal hemisphere, and at the interface between the cortical and animal hemisphere cytoplasm. The cleavage furrow and the mid-body structure are stained at the end of first cleavage. Neuroectoderm derived tissues, notochord, somites, and epidermis stain heavily either continuously or transiently from stages 18-34. A phosphorylated form of XAC (pXAC) was identified by 2D Western blotting, and it is the only species found in oocytes. Dephosphorylation of >60% of the pXAC occurs within 30 min after fertilization. Injection of one blastomere at the 2 cell stage, either with constitutively active XAC or with an XAC inhibitory antibody, blocked cleavage of only the injected blastomere in a concentration-dependent manner without inhibiting nuclear division. The cleavage furrow of eggs injected with constitutively active XAC completely regressed. Blastomeres injected with neutralized antibody developed normally. These results suggest that XAC is necessary for cytokinesis and that its activity must be properly regulated for cleavage to occur.

Circulating human dendritic cells differentially express high levels of a 55-kd actin-bundling protein

This study was initiated to examine the differential expression of an evolutionary conserved human 55-kd actin-bundling (p55) protein that is induced in B lymphocytes by Epstein-Barr virus infection. Our study demonstrates that p55 is specifically expressed at constitutively high levels in human peripheral blood dendritic cells and lymph node (interdigitating) dendritic cells. Blood dendritic cells constitute a minority (< 2%) of all blood leukocytes but are a distinct population of potent antigen-presenting cells. Immunofluorescence microscopy with a monoclonal antibody specific for p55 showed that 87% of peripheral blood dendritic cells stained brightly in the cytoplasm and in the veiled cytoplasmic extensions. In contrast, monocytes, granulocytes, T cells, and B lymphocytes showed no expression of the p55 protein. Western blot analysis confirmed that only the dendritic cell component of peripheral blood expressed high levels of p55. Staining of human lymph node sections demonstrated selective expression of the p55 antigen by dendritic cells in the T-cell-dependent areas but not in the B cell follicles. p55 is likely to be involved in the organization of a specialized microfilament cytoskeleton in the dendritic cells, and the anti-p55 antibody should be useful for further characterization of this important population of antigen-presenting cells in clinical transplantation, HIV-1 pathogenesis, and autoimmune diseases.

Overexpression of cofilin stimulates bundling of actin filaments, membrane ruffling, and cell movement in Dictyostelium

Cofilin is a low molecular weight actin-modulating protein whose structure and function are conserved among eucaryotes. Cofilin exhibits in vitro both a monomeric actin-sequestering activity and a filamentous actin-severing activity. To investigate in vivo functions of cofilin, cofilin was overexpressed in Dictyostelium discoideum cells. An increase in the content of D. discoideum cofilin (d-cofilin) by sevenfold induced a co-overproduction of actin by threefold. In cells over-expressing d-cofilin, the amount of filamentous actin but not that of monomeric actin was increased. Overexpressed d-cofilin co-sedimented with actin filaments, suggesting that the sequestering activity of d-cofilin is weak in vivo. The overexpression of d-cofilin increased actin bundles just beneath ruffling membranes where d-cofilin was co-localized. The overexpression of d-cofilin also stimulated cell movement as well as membrane ruffling. We have demonstrated in vitro that d-cofilin transformed latticework of actin filaments cross-linked by alpha-actinin into bundles probably by severing the filaments. D. discoideum cofilin may sever actin filaments in vivo and induce bundling of the filaments in the presence of cross-linking proteins so as to generate contractile systems involved in membrane ruffling and cell movement.

Localization of fibrillin-1 in the human term placenta

OBJECTIVE

We determined the level of message for fibrillin-1, a single-stranded glycoprotein found in the elastic microfibrils of the extracellular matrix, in human placenta compared with other adult human tissues and localized the protein in the term human placenta.

METHODS

Northern blot analysis using a cDNA specific for fibrillin-1 was used to compare message levels in several adult human tissues and placenta. Immunohistochemical staining, using a monoclonal antibody to human fibrillin-1, was used to visualize the site of fibrillin-1 in term human placenta.

RESULTS

Larger amounts of message for fibrillin-1 were present in placenta than in any of the other tissues tested. Staining for fibrillin-1 was present in the villous stroma but not in the basement membranes, trophoblast cells, or in blood vessels of stem, mature intermediate, or terminal villi.

CONCLUSIONS

Fibrillin-1 is distributed extensively in the villous stroma of the term human placenta and may contribute elastic properties to the placenta and basal plate required for an active uterus.

Fibrillin abnormalities and prognosis in Marfan syndrome and related disorders

Marfan syndrome (MFS), a multisystem autosomal-dominant disorder, is characterized by mutations of the fibrillin-1 (FBN1) gene and by abnormal patterns of synthesis, secretion, and matrix deposition of the fibrillin protein. To determine the sensitivity and specificity of fibrillin protein abnormalities in the diagnosis of MFS, we studied dermal fibroblasts from 57 patients with classical MFS, 15 with equivocal MFS, 8 with single-organ manifestations, and 16 with other connective tissue disorders including homocystinuria and Ehlers-Danlos syndrome. Abnormal fibrillin metabolism was identified in 70 samples that were classified into four different groups based on quantitation of fibrillin synthesis and matrix deposition. Significant correlations were found for phenotypic features including arachnodactyly, striae distensae, cardiovascular manifestations, and fibrillin groups II and IV, which included 70% of the MFS patients. In addition, these two groups were associated with shortened "event-free" survival and more severe cardiovascular complications than groups I and III. The latter included most of the equivocal MFS/single manifestation patients with fibrillin abnormalities. Our results indicate that fibrillin defects at the protein level per se are not specific for MFS, but that the drastically reduced fibrillin deposition, caused by a dominant-negative effect of abnormal fibrillin molecules in individuals defined as groups II and IV, is of prognostic and possibly diagnostic significance.

Immunological identification of candidate proteins involved in regulating active shape changes of outer hair cells

By employing immunological methods, it has been demonstrated that myosin, myosin light chain (MLC) and myosin light chain kinase (MLCK) proteins in outer hair cells (OHC) are immunologically different from isoforms in platelets, smooth muscle and heart muscle, and are probably more related to isoforms found in red blood cells (RBC). Moreover, proteins related to band 3 protein (b3p) and protein 4.1 (p 4.1), ankyrin as well as fodrin and spectrin, but not glycophorin, have been identified in isolated OHCs. Both OHCs and RBC differ from other motile non-muscle cells in their lack of smooth muscle isoforms of actin, their common high levels of spectrin-, ankyrin- and band 3-like proteins, as well as the expression of the 80 kDa protein 4.1 isoform. The data support the notion that motility of OHC may be based upon regulation of the b3p/p 4.1/ankyrin complex, and thus may be reminiscent to the active shape changes in RBC.

Truncated profibrillin of a Marfan patient is of apparent similar size as fibrillin: intracellular retention leads to over-N-glycosylation

We studied profibrillin-1 (proFib) synthesis and microfibril formation in cultured fibroblasts from an individual with severe Marfan syndrome harboring a premature stop codon (W2756ter) in one FBN1 allele. Rotary shadowing analysis of extracellular matrix produced by these cells revealed the presence of only a very few intact microfibrils which showed marked disorganisation within the interbeaded domains. Metabolic pulse-chase studies identified intracellularly a population of truncated proFib molecules which were secreted more slowly than the normal proFib derived from the normal allele. Culture media contained strikingly reduced amounts of wild-type proFib in comparison to fibrillin (Fib). Our findings imply that (1) the truncated proFib is secreted and disturbs microfibril assembly; (2) the mutation is probably close to a putative cleavage site in the proFib C terminus necessary for the conversion of proFib to Fib; (3) the truncated proFib is over-N-glycosylated due to intracellular retention rather than incomplete cleavage of proFib with persistence of N-glycosylated sites; (4) not all potential N-glycosylation sites in proFib seem to be normally used, since we could produce over-N-glycosylated proFib in normal cells by brefeldin A mediated intracellular captivation and subsequent appearance of over-glycosylated Fib in culture medium upon removal of the compound. It is conceivable that post-translational over-modification might be important for modulating the phenotype of FBN1 mutations in Marfan syndrome.

Developmental expression of fibrillin genes suggests heterogeneity of extracellular microfibrils

Extracellular microfibrils, alone or in association with elastin, confer critical biomechanical properties on a variety of connective tissues. Little is known about the composition of the microfibrils or the factors responsible for their spatial organization into tissue-specific macroaggregates. Recent work has revealed the existence of two structurally related microfibrillar components, termed fibrillin-1 and fibrillin-2. The functional relationships between these glycoproteins and between them and other components of the microfibrils and elastic fibers are obscure. As a first step toward elucidating these important points, we compared the expression pattern of the fibrillin genes during mammalian embryogenesis. The results revealed that the two genes are differentially expressed, in terms of both developmental stages and tissue distribution. In the majority of cases, fibrillin-2 transcripts appear earlier and accumulate for a shorter period of time than fibrillin-1 transcripts. Synthesis of fibrillin-1 correlates with late morphogenesis and the appearance of well-defined organ structures; fibrillin-2 synthesis, on the other hand, coincides with early morphogenesis and, in particular, with the beginning of elastogenesis. The findings lend indirect support to our original hypothesis stating that fibrillins contribute to the compositional and functional heterogeneity of the microfibrils. The available evidence is also consistent with the notion that the fibrillins might have distinct, but related roles in microfibril physiology. Accordingly, we propose that fibrillin-1 provides mostly force-bearing structural support, whereas fibrillin-2 predominantly regulates the early process of elastic fiber assembly.

A mutation in FBN1 disrupts profibrillin processing and results in isolated skeletal features of the Marfan syndrome

Dermal fibroblasts from a 13-yr-old boy with isolated skeletal features of the Marfan syndrome were used to study fibrillin synthesis and processing. Only one half of the secreted profibrillin was proteolytically processed to fibrillin outside the cell and deposited into the extracellular matrix. Electron microscopic examination of rotary shadowed microfibrils made by the proband's fibroblasts were indistinguishable from control cells. Sequencing of the FBN1 gene revealed a heterozygous C to T transition at nucleotide 8176 resulting in the substitution of a tryptophan for an arginine (R2726W), at a site immediately adjacent to a consensus sequence recognized by a cellular protease. Six other individuals in the proband's family had the FBN1 mutation that segregated with tall stature. None of the affected individuals have cardiac or ocular manifestations of the Marfan syndrome. This mutation identifies a putative site for profibrillin to fibrillin processing, and is associated with isolated skeletal features of the Marfan syndrome, indicating that the FBN1 gene is one of the genes that determines height in the general population. The cellular effect of the mutation may be equivalent to a "null" FBN1 allele and may define the phenotype associated with FBN1 "null" alleles.

Purification, characterization, and cDNA cloning of profilin from Phaseolus vulgaris

Profilin from common bean (Phaseolus vulgaris L.) was purified to homogeneity by poly-L-Pro affinity chromatography and gel filtration. The hypocotyl and symbiotic root nodule protein was detected as a single isoform with a 14.4-kD molecular mass and an isoelectric point of 5.3. Partial amino acid and DNA sequencing of a full-length cDNA clone confirmed its identity as profilin. An antibody generated against the purified protein binds to a protein with the same molecular mass in leaves and nodules. Immunolocalization of the protein showed a diffuse distribution in the cytoplasm of hypocotyls and nodules but enhanced staining at the vascular bundles. The strong identity of the sequence among the profilins of birch, maize, and bean suggests that it may play an important role in the signal transduction mechanism of plant cells and plant-bacterial symbioses.

Effects of CapG overexpression on agonist-induced motility and second messenger generation

Actin modulating proteins that bind polyphosphoinositides, such as phosphatidylinositol 4, 5-bisphosphate (PIP2), can potentially participate in receptor signaling by restructuring the membrane cytoskeleton and modulating second messenger generation through the phosphoinositide cycle. We examined these possibilities by overexpressing CapG, an actin filament end capping, Ca(2+)- and polyphosphoinositide-binding protein of the gelsolin family. High level transient overexpression decreased actin filament staining in the center of the cells but not in the cell periphery. Moderate overexpression in clonally selected cell lines did not have a detectible effect on actin filament content or organization. Nevertheless, it promoted a dose-dependent increase in rates of wound healing and chemotaxis. The motile phenotype was similar to that observed with gelsolin overexpression, which in addition to capping, also severs and nucleates actin filaments. CapG overexpressing clones are more responsive to platelet-derived growth factor than control-transfected clones. They form more circular dorsal membrane ruffles, have higher phosphoinositide turnover, inositol 1,4,5-trisphosphate generation and Ca2+ signaling. These responses are consistent with enhanced PLC gamma activity. Direct measurements of PIP2 mass showed that the CapG effect on PLC gamma was not due primarily to an increase in the PIP2 substrate concentration. The observed changes in cell motility and membrane signaling are consistent with the hypothesis that PIP(2)-binding actin regulatory proteins modulate phosphoinositide turnover and second messenger generation in vivo. We infer that CapG and related proteins are poised to coordinate membrane signaling with actin filament dynamics following cell stimulation.

Molecular cloning, expression, and mapping of the high affinity actin-capping domain of chicken cardiac tensin

Tensin, an actin filament capping protein first purified from chicken gizzard, is localized to various types of adherens junctions in muscle and nonmuscle cells. In this paper, we describe the isolation and sequencing of tensin cDNA from a chicken cardiac library. The 6.3-kb chicken cardiac tensin cDNA encodes an open reading frame of 1,792 amino acids. Mammalian cells transfected with the chicken tensin cDNA expressed a polypeptide of approximately 200 kD recognizable by antibodies to chicken gizzard tensin. The expressed protein was incorporated into focal adhesions and other actin-containing structures in the transfected cells. To map the domain associated with tensin's high affinity, barbed-end F-actin-capping activity, bacterially expressed recombinant fusion proteins containing various segments of tensin were prepared and assayed for activity. The results of these experiments show that the high affinity capping domain (kD = 1.3 nM) lies within amino acid residues R1037-V1169. Additional studies on a shorter construct, S1061-H1145, showed that these 85 residues were sufficient for producing complete inhibition of actin polymerization and depolymerization. While this active domain is located within that of the "insertin" sequence (Weigt, C., A. Gaertner, A. Wegner, H. Korte, and H. E. Meyer. 1992. J. Mol. Biol. 227:593-595), our data showing complete inhibition of polymerization and shift in critical concentration are consistent with a simple barbed-end capping mechanism rather than the "insertin model." Our results also differ from those of a recent report (Lo, S. H., P. A. Janmey, J. H. Hartwig, and L. B. Chen. 1994. J. Cell Biol. 125:1067-1075), which concluded that their recombinant tensin has an "insertin-like" inhibitory effect on barbed-end actin polymerization, and that this activity is attributed to residues T936-R1037 (residues 888-989 in their numbering system). In our study, a fusion construct (N790-K1060) encompassing T936-R1037 had no significant effect on actin polymerization and depolymerization, even at high concentrations.

Expression of a mutant human fibrillin allele upon a normal human or murine genetic background recapitulates a Marfan cellular phenotype

The Marfan syndrome (MFS) is a connective tissue disorder inherited as an autosomal dominant trait and caused by mutations in the gene encoding fibrillin, a 350-kD glycoprotein that multimerizes to form extracellular microfibrils. It has been unclear whether disease results from a relative deficiency of wild-type fibrillin; from a dominant-negative effect, in which mutant fibrillin monomers disrupt the function of the wild-type protein encoded by the normal allele; or from a dynamic and variable interplay between these two pathogenetic mechanisms. We have now addressed this issue in a cell culture system. A mutant fibrillin allele from a patient with severe MFS was expressed in normal human and murine fibroblasts by stable transfection. Immunohistochemical analysis of the resultant cell lines revealed markedly diminished fibrillin deposition and disorganized microfibrillar architecture. Pulse-chase studies demonstrated normal levels of fibrillin synthesis but substantially reduced deposition into the extracellular matrix. These data illustrate that expression of a mutant fibrillin allele, on a background of two normal alleles, is sufficient to disrupt normal microfibrillar assembly and reproduce the MFS cellular phenotype. This underscores the importance of the fibrillin amino-terminus in normal microfibrillar assembly and suggests that expression of the human extreme 5' fibrillin coding sequence may be sufficient, in isolation, to produce an animal model of MFS. Lastly, this substantiation of a dominant-negative effect offers mutant allele knockout as a potential strategy for gene therapy.

Primary structure and developmental expression of Fbn-1, the mouse fibrillin gene

Previous studies have reported > 10 kilobases of human fibrillin-1 cDNA sequence, but a consensus regarding the 5' end of the transcript remains to be worked out. One approach to developing a clear consensus would be to search for regions of evolutionary conservation in transcripts from a related species such as mouse. As reported here, the mouse fibrillin-1 transcript encodes a highly conserved polypeptide of 2,871 amino acids. The upstream sequence that flanks the ATG is considerably less well conserved, however. Indeed, the ATG codon (which occurs in the context of a Kozak consensus sequence and is located just upstream of a consensus signal peptide) signals the point where human and mouse fibrillin-1 sequences cease to be nearly identical. Together, these results are consistent with previous efforts by Pereira et al. (Pereira, L., D'Alessio, M., Ramirez, F., Lynch, J. R., Sykes, B., Pangilinan, T., and Bonadio, J. (1993) Human Mol. Genet. 2, 961-968) to identify the human fibrillin-1 translational start site. Sequences immediately upstream of the ATG are GC-rich and devoid of TATA and CCAAT boxes, which suggests that the mouse fibrillin-1 gene will be broadly expressed. A survey of expression in mouse embryo tissues is consistent with this hypothesis and suggests two novel functions for fibrillin-associated microfibrils in non-elastic connective tissues.

Molecular cloning, structural analysis and functional expression of the proline-rich focal adhesion and microfilament-associated protein VASP

The vasodilator-stimulated phosphoprotein (VASP), a substrate for cAMP- and cGMP-dependent protein kinases in vitro and in intact cells, is associated with actin filaments, focal adhesions and dynamic membrane regions. VASP, cloned here from human HL-60 and canine MDCK cells, is organized into three distinct domains. A central proline-rich domain contains a GPPPPP motif as a single copy and as a 3-fold tandem repeat, as well as three conserved phosphorylation sites for cyclic nucleotide-dependent protein kinases. A C-terminal domain contains a repetitive mixed-charge cluster which is predicted to form an alpha-helix. The hydrodynamic properties of purified human VASP together with the calculated molecular mass of cloned VASP suggest that the native protein is a homotetramer with an elongated structure. VASP over-expressed in transiently transfected BHK21 cells was predominantly detected at stress fibres, at focal adhesions and in F-actin-containing cell surface protrusions, whereas truncated VASP lacking the C-terminal domain was no longer concentrated at focal adhesions. These data indicate that the C-terminal domain is required for anchoring VASP at focal adhesion sites, whereas the central domain is suggested to mediate VASP interaction with profilin. Our results provide evidence for the structural basis by which VASP, both a target of the cAMP and cGMP signal transduction pathways and a component of the actin-based cytoskeleton, including the cytoskeleton-membrane interface, may be able to exchange signals between these networks.

Molecular cloning and characterization of profilin from tobacco (Nicotiana tabacum): increased profilin expression during pollen maturation

Profilin has recently been identified as an actin-binding protein in higher plants. A cDNA coding for tobacco profilin, which shared an average sequence identity of 75% with other plant profilins, was isolated from a tobacco pollen cDNA library by antibody screening. Tobacco profilin was expressed in Escherichia coli and purified by affinity to poly-(L-proline) Sepharose. A rabbit antiserum was raised against recombinant tobacco profilin and used to estimate the amount of profilin expressed in different tobacco tissues. Profilin can be detected in different somatic tissues, but the expression is 50-100 fold higher in mature pollen. Immunofluorescence and confocal laser scanning microscopy showed a homogeneous distribution of profilin in the cytoplasm of in vitro cultured pollen grains and pollen tubes of tobacco whereas some growing pollen tubes were stained more intensively a their tip. A possible role of pollen profilin as a developmentally upregulated microfilament precursor in mature pollen is discussed.

Differentiation dependent expression of tensin and cortactin in chicken osteoclasts

The expression and localization of tensin and cortactin were examined in osteoclast precursors in comparison with isolated osteoclasts on various substrates. Initially, the ability of hen monocytes to differentiate into osteoclasts was evaluated on plastic or glass, and compared to differentiation on bone. Specifically, monocytes were isolated from the medullary bones of egg-laying hens maintained on a Ca-deficient diet. Differentiation was monitored morphologically and by quantitation of the ability to form Howship's lacunae in bone slices or resorb radiolabeled bone particles of 20-53 microns diameter. These cells differentiated into tartrate resistant acid phosphatase (TRAP)-positive, bone resorbing, multinucleated syncytia in the presence of cytosine-1-beta-D-arabinofuranoside in a time dependent manner (day 1-6). Differentiation into osteoclast-like cells was similar whether cultured on plastic, on glass, or on bone. When compared to GAP-DH control levels, tensin and cortactin mRNA levels increased by 7- and 10-fold, respectively, by day 6. Tensin and cortactin protein levels also increased by 6- and 15-fold, respectively, by day 6. Immunofluorescence of differentiating precursors showed that tensin localized between regions of cell to cell contact and colocalized with vinculin in podosomes of osteoclast-like cells and of real osteoclasts. Cortactin immunofluorescence was not detectable in monocytes but localized inside tensin/vinculin podosome structures after fusion into osteoclast-like cells and in freshly isolated osteoclasts. Both tensin and cortactin were associated with attachment complexes used by osteoclast-like cells and osteoclasts to resorb bone. Specifically, punctate cortactin staining was observed inside tensin staining which formed a double ring structure at the membrane/bone interface of resorbing osteoclasts. These data showed that tensin and cortactin can be used as osteoclast differentiation markers, that participate in attachment complexes used to resorb bone, and that tensin may participate in the fusion process of osteoclast precursors.

Marfan syndrome: fibrillin expression and microfibrillar abnormalities in a family with predominant ocular defects

We have found abnormal fibrillin microfibrils isolated from tissues and cell cultures from two cousins with Marfan syndrome whose major clinical abnormality is bilateral ectopia lentis, but who also have skeletal involvement but no cardiovascular defects. Ultrastructural analysis of ciliary zonules showed the presence of abundant loose microfibril bundles which in many places appeared disorganised. Microfibrils isolated from ciliary zonules and vitreous were highly fragmented when examined by rotary shadowing electron microscopy. Investigation of microfibrils elaborated by patient dermal fibroblasts showed remarkable variations in periodicity and packing. The synthesis and secretion of fibrillin by these cells was confirmed electrophoretically with the identification of metabolically labelled immunoprecipitated fibrillin (M(r) 300,000) in medium and cell layer compartments. These data show that fibrillin expression is normal but that assembled microfibrils are manifestly abnormal both morphologically and functionally. The occurrence of microfibrils with variable periodicities and susceptibility to fragmentation suggests that structural weakness is probably the primary cause of lens dislocation in these patients.

Maintenance of an open reading frame as an additional level of scrutiny during splice site selection

Although nonsense mutations have been associated with the skipping of specific constitutively spliced exons in selected genes, notably the fibrillin gene, the basis for this association is unclear. Now, using chimaeric constructs in a model in vivo expression system, premature termination codons are identified as determinants of splice site selection. Nonsense codon recognition prior to RNA splicing necessitates the ability to read the frame of precursor mRNA in the nucleus. We propose that maintenance of an open reading frame can serve as an additional level of scrutiny during exon definition. This process may have pathogenic and evolutionary significance.

Analyses of truncated fibrillin caused by a 366 bp deletion in the FBN1 gene resulting in Marfan syndrome

We studied fibrillin synthesis in cultured fibroblasts from 11 members of a three-generation family with Marfan syndrome, caused by a large in-frame deletion in FBN1 (the fibrillin gene) leading to a loss of 366 bases in the corresponding fibrillin mRNA. Metabolic labelling with [35S]Met/Cys and SDS/PAGE allowed unequivocal identification of normal and truncated fibrillin in all cell strains harbouring the deletion. In culture medium, fibrillin and its truncated counterpart were predominant, whereas their respective larger precursors were found only in traces. This proportion, however, was markedly shifted towards the normal and truncated precursors by EGTA and reversed by the addition of calcium, which confirmed the existence of profibrillin and its probably calcium-dependent conversion into fibrillin. Tunicamycin caused increased electrophoretic mobility of normal and truncated molecules without changing their apparent size differences. Intracellularly, only profibrillin was found; in the mutant cells truncated and normal profibrillin molecules were present in similar amounts and both populations were secreted and deposited simultaneously into the extracellular matrix; there, however, truncated profibrillin only became easily detectable after treatment of cells with dextran sulphate, which increased the amount of extractable profibrillin. Immunofluorescence microscopy in patients' cultures identified fibrillin-containing microfibrils which appeared to be moderately reduced both in amount and diameter. Ultrastructural analysis by rotary-shadowing and immunogold electron microscopy demonstrated the presence of numerous beaded domains reacting with fibrillin antibodies, but no intact fibrillin microfibrils in patient's cell-layer extracts, in contrast with the extensive microfibrils elaborated by control cultures. Our findings suggest, that in the patients' cell cultures all microfibrils contained the truncated fibrillin molecules.

The potential role of the elastic fiber system in adolescent idiopathic scoliosis

To assess its possible role in the etiology of adolescent idiopathic scoliosis, the elastic fiber system of the ligamentum flavum was examined in twenty-three patients who had scoliosis and in five age-matched individuals who did not. Elastic fibers are composed of two components: the amorphous core of elastin and microfibrils, of which fibrillin is the primary element. Fresh-frozen histological specimens of ligamentum flavum removed at the time of an operation were examined by Verhoeff staining for elastic fibers and by immunohistochemical staining with use of a monoclonal antibody to fibrillin. Additionally, cultures of fibroblast cells from the ligamentum flavum were used to study the biosynthesis and secretion of fibrillin and its incorporation into the extracellular matrix in vitro. The specimens from one patient did not provide sufficient material for the histological studies; however, fibroblasts were harvested from this specimen. In five (23 percent) of the remaining twenty-two specimens from patients who had adolescent idiopathic scoliosis, Verhoeff staining of elastic fibers showed a marked decrease in fiber density (the number of fibers per unit area) and a non-uniform distribution of fibers throughout the ligament. Eighteen specimens (82 percent) exhibited abnormalities on immunohistochemical staining, including a marked disarrangement of the fibers and a difference in the density of staining, when compared with the control specimens from individuals who did not have adolescent idiopathic scoliosis. Studies of the biosynthesis and secretion of fibrillin and its incorporation into the extracellular matrix in vitro demonstrated that fibroblasts from four (17 percent) of the twenty-three specimens produced normal amounts of fibrillin and secreted it from the cell, but the fibrillin failed to bind to other macromolecules, to form a sedimentable complex, and to incorporate into the extracellular matrix. Collectively, the results suggest the potential role of the elastic fiber system as a component in the pathogenesis of adolescent idiopathic scoliosis in some individuals.

The sea urchin profilin gene is specifically expressed in mesenchyme cells during gastrulation

Eggs and embryos of the purple sea urchin (Strongylocentrotus purpuratus) contain profilin that is partly supplied from maternal sources and partly produced by the gastrula. The maternal profilin protein content is about 13 microM and it persists in the embryo at least through gastrulation. Transcript quantitation from probe excess titrations show that very few profilin gene transcripts are present in the embryo during cleavage, but that they increase at the onset of gastrulation. By in situ hybridization, the newly synthesized profilin transcripts are localized in mesenchyme cells. Profilin gene expression increases when mesenchyme cells initiate migration and filopodial extension and retraction. We show that there are three isoforms of maternal profilin protein produced from the single copy gene during oogenesis. However, the blastula stage embryo only produces the major isoform, whereas the acidic isoform is produced in the early stages of gastrulation and the basic isoform appears by the end of gastrulation. Based on transcript prevalence and protein production rates, our calculations indicate that the amount of new protein produced in the mesenchyme cells in 12 hr is at maximum < 2% of that supplied from maternal sources. Because of the large amount of maternally supplied profilin present in the egg and embryo, we suggest that it may be used in the cytokinetic processes of cleavage. Alternatively, because of the small amount of embryonically produced profilin, we suggest that it may function in the cytoskeletal shape changes required for filopodial extension and motility in the mesenchyme cells during gastrulation.

The protein tyrosine kinase substrate cortactin is differentially expressed in murine B lymphoid tumors

Src family protein tyrosine kinases (PTKs) actively participate in signal transduction during lymphocyte activation. However, little is known about the roles of PTKs and their substrates in lymphocyte differentiation. To identify PTK substrates that may be differentially expressed during B lymphopoiesis, we screened a panel of murine B lymphoid tumor cell lines representing various developmental stages using monoclonal antibodies (MAbs) specific for pp60src substrates. A MAb specific for cortactin, a filamentous-actin binding pp60src substrate, immunoprecipitated proteins from murine plasma-cytoma cell lines but not from pre-B cell lymphoma or B cell lymphoma cell lines. We have cloned a murine cortactin cDNA which encodes a member of a family of proteins distinguished by amino-terminal repeat domains and carboxy-terminal Src Homology 3 domains. Two members of this family (cortactin and HS1) were differentially expressed in murine B lymphoid tumor cell lines; both were detected in plasmacytoma cell lines, however HS1 was additionally detected in pre-B lymphoma and B lymphoma cell lines. Cortactin RNA was detected in most murine tissues, but was not detected in B lymphocytes or plasma cells. We hypothesize that cortactin expression is associated with transformed plasma cells and not with the terminal differentiation of normal B lymphocytes to plasma cells.

Quantitative differences in biosynthesis and extracellular deposition of fibrillin in cultured fibroblasts distinguish five groups of Marfan syndrome patients and suggest distinct pathogenetic mechanisms

Pulse-chase studies of [35S]cysteine-labeled fibrillin were performed on fibroblast strains from 55 patients with Marfan syndrome (MFS), including 13 with identified mutations in the fibrillin-1 gene and 10 controls. Quantitation of the soluble intracellular and insoluble extracellular fibrillin allowed discrimination of five groups. Groups I (n = 8) and II (n = 19) synthesize reduced amounts of normal-sized fibrillin, while synthesis is normal in groups III (n = 6), IV (n = 18), and V (n = 4). When extracellular fibrillin deposition is measured, groups I and III deposit between 35 and 70% of control values, groups II and IV < 35%, and group V > 70%. A deletion mutant with a low transcript level from the mutant allele and seven additional patients have the group I protein phenotype. Disease in these patients is caused by a reduction in microfibrils associated with either a null allele, an unstable transcript, or an altered fibrillin product synthesized in low amounts. In 68% of the MFS individuals (groups II and IV), a dominant negative effect is invoked as the main pathogenetic mechanism. Products made by the mutant allele in these fibroblasts are proposed to interfere with microfibril formation. Insertion, deletion, and exon skipping mutations, resulting in smaller fibrillin products, exhibit the group II phenotype. A truncated form of fibrillin of 60 kD was identified with specific fibrillin antibodies in one of the group II cell culture media. Seven of the nine known missense mutations, giving rise to abnormal, but normal-sized fibrillin molecules, are in group IV.

The Schizosaccharomyces pombe cdc3+ gene encodes a profilin essential for cytokinesis

The fission yeast Schizosaccharomyces pombe divides by medial fission and, like many higher eukaryotic cells, requires the function of an F-actin contractile ring for cytokinesis. In S. pombe, a class of cdc- mutants defective for cytokinesis, but not for DNA replication, mitosis, or septum synthesis, have been identified. In this paper, we present the characterization of one of these mutants, cdc3-124. Temperature shift experiments reveal that mutants in cdc3 are incapable of forming an F-actin contractile ring. We have molecularly cloned cdc3 and used the cdc3+ genomic DNA to create a strain carrying a cdc3 null mutation by homologous recombination in vivo. Cells bearing a cdc3-null allele are inviable. They arrest the cell cycle at cytokinesis without forming a contractile ring. DNA sequence analysis of the cdc3+ gene reveals that it encodes profilin, an actin-monomer-binding protein. In light of recent studies with profilins, we propose that Cdc3-profilin plays an essential role in cytokinesis by catalyzing the formation of the F-actin contractile ring. Consistent with this proposal are our observations that Cdc3-profilin localizes to the medial region of the cell where the F-actin contractile ring forms, and that it is essential for F-actin ring formation. Cells overproducing Cdc3-profilin become elongated, dumbbell shaped, and arrest at cytokinesis without any detectable F-actin staining. This effect of Cdc3-profilin overproduction is relieved by introduction of a multicopy plasmid carrying the actin encoding gene, act1+. We attribute these effects to potential sequestration of actin monomers by profilin, when present in excess.

Characterization of a novel cofilin isoform that is predominantly expressed in mammalian skeletal muscle

Cofilin is an actin-modulating protein of 20 kDa, which is widely distributed throughout muscle and non-muscle cells. By means of immunoblotting combined with two-dimensional gel electrophoresis, we found that two cofilin variants, muscle type (M-type) and non-muscle type (NM-type), exist in mammals, while a single isoform exists in chickens. During in vitro myogenesis of mouse C2 cells, expression of the M-type cofilin was upregulated. To better understand the nature of the M-type cofilin, we cloned cDNAs encoding M-type cofilin from the cDNA library of C2 myotubes and determined the entire sequence. The deduced peptide sequence contained a nuclear localization signal and a putative actin-binding sequence as reported in NM-type cofilin. The sequence showed 81% identity in the amino acid residues with the mouse NM-type cofilin sequence and, interestingly, higher homology (96% identity) with that of chicken cofilin. The mRNA encoding M-type cofilin, though it contains two variants that differ in the size of their 3'-non-coding sequences, was detected predominantly in heart, skeletal muscle, C2 myotubes, and testis by Northern blotting, while the mRNA for NM-type cofilin was seen in a variety of non-muscle tissues. The presence of the muscle type isoform of cofilin strongly suggests that cofilin is deeply involved in the regulation of actin function not only in non-muscle cells but also in muscle cells.

Identification of Drosophila cytoskeletal proteins by induction of abnormal cell shape in fission yeast

To clone metazoan genes encoding regulators of cell shape, we have developed a functional assay for proteins that affect the morphology of a simple organism, the fission yeast Schizosaccharomyces pombe. A Drosophila melanogaster cDNA library was constructed in an inducible expression vector and transformed into S. pombe. When expression of the Drosophila sequences was induced, aberrant cell shapes were found in 0.2% of the transformed colonies. Four severe phenotypes representing defects in cytokinesis and/or cell shape maintenance were examined further. Each displayed drastic and specific reorganizations of the actin cytoskeleton. Three of the cDNAs responsible for these defects appear to encode cytoskeletal components: the actin binding proteins profilin and cofilin/actin depolymerizing factor and a membrane-cytoskeleton linker of the ezrin/merlin family. These results demonstrate that a yeast phenotypic screen efficiently identifies conserved genes from more complex organisms and sheds light on their potential in vivo functions.

Axonal transport of actin and actin-binding proteins in the rat sciatic nerve

Actin is one of the major cytoskeletal proteins carried in slow axonal transport. Since more than 50% of actin in the axon was recovered in the high-speed supernatant, we looked for G-actin-binding proteins in slow axonal transport. Two weeks after injection of L-[35S]methionine into the rat spinal cord (L3-L5), labeled proteins in the sciatic nerve were extracted and those with potential abilities to interact with G-actin were detected by two independent methods: (A) DNAase I affinity chromatography and (B) blot overlay with biotinylated actin. By method (A), a 68 kDa Ca(2+)-dependent binding protein and a 45 kDa Ca(2+)-independent binding protein were detected. The 68 kDa protein was also a major protein binding to actin in method (B). The 68 kDa protein was identified with the Ca(2+)-dependent phospholipid binding protein annexin VI by two-dimensional electrophoresis and Western blotting. As annexin VI is a component of slow axonal transport, it does not seem to be bound to membranous organelles in the axon. Our results suggest that annexin VI may play a role in the control of actin assembly and membrane-microfilament interaction.

Comparative analysis of neutral endopeptidase (NEP) and villin gene expression during mouse embryogenesis and enterocyte maturation

Neutral endopeptidase (Endopeptidase 24.11; NEP; neprilysin), an integral membrane protein, and villin, a major microvillar cytoskeletal actin-binding protein, are both typically associated with brush border epithelia. In this study, cRNA probes were hybridized in situ to investigate the expression of NEP and villin genes in embryo and adult mouse enterocytes. During development, villin mRNAs were easily detected in the immature digestive tract well before establishment of the brush border. In 17-day-old embryos, a transient elevation of villin mRNA occurred just prior to a dramatic increase in microvilli length and density. NEP only appeared by day 17 as the embryonic gut began to become functional. It therefore appears that the onset of transcription of specialized cytoskeletal proteins from the brush border preceded that of intrinsic membrane-bound enzyme from microvilli. In the adult intestinal fold, both mRNAs were expressed along the whole length of the villus with maximal expression at its base. In contrast, both proteins were uniformly expressed along the whole crypt-villus axis. Quantitative analysis revealed an asymmetric intracellular distribution of both mRNAs that were differentially polarized in the apical cytoplasm of enterocytes.

Structure and expression of two plant genes encoding chromoplast-specific proteins: occurrence of partially spliced transcripts

The genes encoding fibrillin and capsanthin-capsorubin synthase are specifically expressed during fruit ripening in Capsicum annuum, leading to the accumulation of these two proteins in chromoplasts. Here, we report for the first time the cloning of genomic DNA fragments encoding these two enzymes, as well as DNA fragments containing upstream regions which are potentially involved in the regulation of the expression of these genes. While the capsanthin-capsorubin synthase gene is uninterrupted, the fibrillin gene is interrupted by two introns, the first one being inefficiently spliced. Occurrence of unspliced transcripts is apparently not related to a post-transcriptional mechanism controlling the synthesis of fibrillin or an alternative polypeptide. This work provides tools for studies on gene activation and intron splicing in plants.

Abnormal fibrillin assembly by dermal fibroblasts from two patients with Marfan syndrome

The microfibrillar glycoprotein fibrillin is linked to the Marfan syndrome, an autosomal dominant connective tissue disorder. In this study, fibrillin synthesis, deposition and assembly has been investigated in Marfan dermal fibroblast lines from two unrelated patients for whom distinct mutations in the fibrillin gene FBN1 have been identified. In patient NB, a point mutation has occurred which causes an amino acid substitution and the other patient (GK) has a deletion in one allele. The two cell lines were broadly comparable with respect to de novo fibrillin synthesis and its distribution between medium and cell layer compartments. Electrophoresis of fibrillin immunoprecipitates confirmed the presence of fibrillin in medium and cell layers. GK cells secreted an additional higher relative molecular mass fibrillin-immunoreactive component. The time-course of fibrillin secretion was similar for the two lines, but differences in fibrillin aggregation were apparent. Rotary shadowing electron microscopy of extracted cell layers demonstrated the presence of abundant and extensive microfibrils in NB cell layers. These were abnormal in their gross morphology in comparison to microfibrils isolated from control cultures. No periodic microfibrillar structures were isolated from GK cell layers. These studies underline the need to classify fibrillin defects in terms of biochemical and ultrastructural criteria. Examination of the effects of individual mutations on microfibril organization will be particularly informative in elucidating the relationship between microfibril dysfunction and the complex clinical manifestations of Marfan patients.

Severe neonatal Marfan syndrome resulting from a de novo 3-bp insertion into the fibrillin gene on chromosome 15

Severe neonatal Marfan syndrome has features of the Marfan syndrome and congenital contractural arachnodactyly present at birth, along with unique features such as loose, redundant skin and pulmonary emphysema. Since the Marfan syndrome and congenital contractural arachnodactyly are due to mutations in different genes, it has been uncertain whether neonatal Marfan syndrome is due to mutations in the fibrillin gene on chromosome 15 or in another gene. We studied an infant with severe neonatal Marfan syndrome. Dermal fibroblasts were metabolically labeled and found to secret fibrillin inefficiently when compared with control cells. Reverse transcription and amplification of the proband's fibroblast RNA was used to identify a 3-bp insertion between nucleotides 480-481 or 481-482 of the fibrillin cDNA. The insertion maintains the reading frame of the protein and inserts a cysteine between amino acids 160 and 161 in an epidermal growth-factor-like motif of fibrillin. This 3-bp insertion was not found in the fibrillin gene in 70 unrelated, unaffected individuals and 11 unrelated individuals with the Marfan syndrome. We conclude that neonatal Marfan syndrome is the result of mutations in the fibrillin gene on chromosome 15 and is part of the Marfan syndrome spectrum.

Fibrillin secretion and microfibril assembly by Marfan dermal fibroblasts

The Marfan syndrome has been linked to the FBN1 gene encoding the microfibrillar glycoprotein fibrillin. To date, there have been no descriptions of microfibrillar abnormalities characteristic of this connective tissue disorder, although biochemical analyses have highlighted apparent abnormalities in fibrillin synthesis, secretion and processing. We have conducted a biochemical and ultrastructural investigation of fibrillin expression and assembly by a panel of dermal fibroblast lines from patients with Marfan syndrome and related diseases. The study has highlighted marked differences between cells in terms of secretion and aggregation of newly-synthesised fibrillin. In addition, electron microscopic visualization of fibrillin assemblies has clearly demonstrated for the first time the plethora of microfibrillar abnormalities that underlie this heterogeneous disorder. These data emphasize the molecular complexity that is a feature of the diverse clinical phenotypes exhibited by Marfan patients.

Structure and expression of fibrillin-2, a novel microfibrillar component preferentially located in elastic matrices

During the previous cloning of the fibrillin gene (FBN1), we isolated a partial cDNA coding for a fibrillin-like peptide and mapped the corresponding gene (FBN2) to human chromosome 5. (Lee, B., M. Godfrey, E. Vitale, H. Hori, M. G. Mattei, M. Sarfarazi, P. Tsipouras, F. Ramirez, and D. W. Hollister. 1991. Nature [Lond.]. 352:330-334). The study left, however, unresolved whether or not the FBN2 gene product is an extracellular component structurally related to fibrillin. Work presented in this report clarifies this important point. Determination of the entire primary structure of the FBN2 gene product demonstrated that this polypeptide is highly homologous to fibrillin. Immunoelectron microscopy localized both fibrillin proteins to elastin-associated extracellular microfibrils. Finally, immunohistochemistry revealed that the fibrillins co-distribute in elastic and non-elastic connective tissues of the developing embryo, with preferential accumulation of the FBN2 gene product in elastic fiber-rich matrices. These results support the original hypothesis that the fibrillins may have distinct but related functions in the formation and maintenance of extracellular microfibrils. Accordingly, we propose to classify the FBN1 and FBN2 gene products as a new family of extracellular proteins and to name its members fibrillin-1 and fibrillin-2, respectively.

Differential expression of bovine adseverin in adrenal gland revealed by in situ hybridization. Cloning of a cDNA for adseverin

Adseverin is a Ca(2+)-dependent actin filament-severing protein that is presumed to have a regulatory function in exocytosis by affecting the organization of the microfilament network underneath the plasma membrane. As the first step to investigate whether adseverin has the expected function in vivo, we have cloned a cDNA for bovine adseverin. Analysis of the nucleotide sequence revealed the following points. 1) Adseverin consisted of 715 amino acid residues, and its predicted molecular mass was 80.5 kDa. 2) Adseverin belonged to the gelsolin family of proteins, which consists of six homologous segments. 3) Adseverin had two putative polyphosphoinositide binding sequences, very similar to but distinct from those in gelsolin. Adseverin was produced in Escherichia coli and purified to homogeneity. The adseverin reacted with adseverin-specific antibody and showed Ca(2+)-dependent actin filament severing activity like authentic adseverin. In situ hybridization demonstrated that adseverin mRNA was differentially expressed in the bovine adrenal gland; it was expressed in the adrenal medulla but not in most parts of the adrenal cortex, although both have a secretory function. The observation strongly suggested that adseverin is not involved in secretory processes in general but is specifically involved in exocytosis.

Dynamic actin structures stabilized by profilin

We describe the production and analysis of clonal cell lines in which we have overexpressed human profilin, a small ubiquitous actin monomer binding protein, to assess the role of profilin on actin function in vivo. The concentration of filamentous actin is increased in cells with higher profilin levels, and actin filament half-life measured in these cells is directly proportional to the steady-state profilin concentration. The distribution of actin filaments is altered by profilin overexpression. While parallel actin bundles crossing the cells are virtually absent in cells overexpressing profilin, the submembranous actin network of these cells is denser than in control cells. These results suggest that in vivo profilin regulates the stability, and thereby distribution, of specific dynamic actin structures.

Actin cytoskeleton. Through the bottleneck

Like serendipity-alpha and nullo, the newly characterized gene bottleneck is involved in organizing the actin cytoskeleton of the Drosophila embryo to achieve the transition from a syncytium to a cellular blastoderm.

Different temporal patterns of expression result in the same type, amount, and distribution of filamin (ABP) in cardiac and skeletal myofibrils

The morphogenesis of functional myofibrils in chick skeletal and cardiac muscle occurs in greatly different time spans, in about 7 and 2 days, respectively. In chick skeletal myogenic cells, one isoform of the 250 kD actin-binding protein (ABP) filamin is associated with stress fiber-like structures of myoblasts and early myotubes, then disappears for approximately 4 days, whereupon a second filamin isoform reappears at the Z-disc periphery. We sought to determine if cardiac myogenesis involves this sequence of appearance, disappearance, and reappearance of a new filamin isoform in a compressed time scale. It was known that in mature heart, filamin is localized at the Z-disc periphery as in mature (fast) skeletal muscle, and is also associated with intercalated discs. We find that myocardial filamin has an apparent molecular weight similar to that of adult skeletal muscle filamin and lower than that of smooth muscle filamin, and that both skeletal and cardiac muscle contain roughly 200 filamin monomers per sarcomere. Two-dimensional peptide mapping shows that myocardial filamin is very similar to skeletal muscle filamin. Myocardial, slow skeletal, and fast skeletal muscle filamins are all phosphorylated, as previously shown for filamin of non-striated muscle. Using immunofluorescence, we found that filamin could not be detected in the developing heart until the 14-somite stage, when functional myofibrils exist and the heart has been beating for 3 to 4 hours. We conclude that in cardiac and skeletal myogenesis, different sequences of filamin gene expression result in myofibrils with similar filamin distributions and isoforms.

Missense mutations impair intracellular processing of fibrillin and microfibril assembly in Marfan syndrome

Dermal fibroblasts from nine Marfan syndrome patients with missense mutations in the fibrillin-1 gene (FBN1) produced nearly normal amounts of fibrillin as determined by quantitative pulse-chase experiments. However, six of the seven mutations involving substitutions of highly conserved cysteine residues exhibited lower rates of intracellular transport and secretion. This effect is likely due to improper folding, since intracellular fibrillin processing was also affected by the reducing agent dithiothreitol. Normal secretion patterns were seen in three mutations that either change the conformation of EGF-like domains or change consensus amino acids required for Ca(++)-binding. In all nine fibroblasts strains, however, the deposition of fibrillin in the extracellular matrix was reduced to 50% of normal in two and to less than 30% in seven of the nine samples studied. The protein alterations caused by these missense mutations are associated with moderate to severe features of Marfan syndrome and a dominant negative mechanism is suggested to play a major role in their pathogenesis.

Identification of profilin as an actin-binding protein in higher plants

Profilin is a low molecular weight protein involved in the organization of the mammalian and protozoan cytoskeleton as well as in signal transduction. In this study, profilin is identified as an actin-binding protein in higher plants which is present in monocot and dicot angiosperms. Birch pollen profilin and actin can be copurified as a complex, and purified recombinant birch profilin can be used as an affinity matrix to obtain birch pollen actin. The binding of 125I-labeled recombinant birch pollen profilin to plant and animal actins can be blocked by profilin-specific antibodies that react with different epitopes of birch profilin. One of the blocking antibodies was raised against the 25 COOH-terminal amino acids indicating the importance of this region in the profilactin complex formation.

Modulation of tensin and vimentin expression in chick embryo developing cartilage and cultured differentiating chondrocytes

It has been proposed that tensin, in association with several other proteins, mediates the micro-filament-integrin link. Here we describe the isolation of clones spanning about 5 kb from the 3' end of tensin mRNA from cultured chick embryo chondrocyte and embryonic heart cDNA libraries. Tensin expression was investigated in cultured chick embryo cells. It was observed that tensin expression is dependent upon substrate adhesion and it is turned off after 7 days of suspension culture. This process is reversible. Tensin expression is also regulated during cartilage cell differentiation in vivo; at Hamburger and Hamilton stage 39-40, non-hypertrophic tibial chondrocytes express both RNA and protein while hypertrophic chondrocytes do not. In the culture system the expression of vimentin, a major component of intermediate filaments, showed an opposite behaviour since the suspension culture enhances the accumulation of both vimentin and its mRNAs. Therefore in chick embryo cultured chondrocytes and in vivo, during cartilage development, cell shape changes and/or integrin-extracellular matrix protein interactions may be involved in the regulation of these two genes coding for cytoskeletal proteins.

Synthesis and assembly of fibrillin by fibroblasts and smooth muscle cells

The expression and assembly of the microfibrillar glycoprotein fibrillin has been investigated in cultures of nuchal ligament fibroblasts, skin fibroblasts and vascular smooth muscle cells. The level of fibrillin expression varied with the cell type and growth conditions. Higher levels of synthesis were recorded in quiescent post-confluent cells than in actively dividing subconfluent cultures. Nuchal ligament fibroblasts consistently synthesized the highest levels of fibrillin. Growth of cells in the presence of ascorbate resulted in an increased proportion of newly synthesized fibrillin retained within cell layers. Fibrillin was immunoprecipitated from medium and cell layer extracts in the form of monomers and high-M(r) disulphide-bonded aggregates. Rotary shadowing electron microscopy of cell layer extracts and collagen gels provided direct evidence for the assembly of extensive intact microfibrils by smooth muscle cells and fibroblast cultures. Gel filtration chromatography of medium and cell layer extracts, in combination with immunoprecipitation of column fractions, provided a means of analysing the size distribution and assembly of newly synthesized fibrillin. This cell culture approach provides an opportunity to evaluate normal and aberrant synthesis and assembly of fibrillin in a wide range of cell types.

[A study on alterations of gene expression in a flat revertant R1 from ras-oncogene transformed NIH/3T3 cells]

The flat revertant cell line R1, isolated from human activated Ha-ras oncogene transformed NIH/3T3 cells (EJ-NIH/3T3) by mutagen treatment, expresses a variant form of the actin-regulatory protein gelsolin, designated p92-5.7. To clone the gene encoding p92-5.7, gelsolin cDNAs were isolated from a cDNA library of R1 cells. In vitro transcription-translation and nucleotide sequence analyses of the cloned cDNAs identified a point mutation in codon 321 at the cause for the expression of p92-5.7. Considering gelsolin's function as an actin binding protein, the expression of alpha-actin, which is downregulated in many transformed fibroblasts, was analyzed. In EJ-NIH/3T3 cells no alpha-actin transcript was detected, whereas in R1 cells alpha-actin mRNA expression was restored to a level similar to NIH/3T3 cells. Immunofluorescence staining of the cells with an alpha-actin specific monoclonal antibody did not detect any alpha-actin containing microfilaments in EJ-NIH/3T3 cells, but revealed an ordered microfilament pattern in R1 and NIH/3T3 cells. In order to identify other genetic alterations that may also contribute to the revertant phenotype, genes with an elevated expression in R1 cells compared with the parental EJ-NIH/3T3 cells were isolated by using a differential hybridization approach. The identified sequences represented mitochondrial (cytochrome b, cytochrome c oxidase subunit II, NADH dehydrogenase subunits 1 and 4) and alpha 2 (type I) collagen genes. In summary, these results suggest that a complex alteration of the expression of cytoskeletal, mitochondrial and extracellular matrix components is closely associated with the flat reversion of R1 cells.