Detection and ultrastructural localization of human smooth muscle myosin-like molecules in human non-muscle cells by specific antibodies

The role of βII spectrin in cardiac health and disease

Spectrins are large, flexible proteins comprised of α-β dimers that are connected head-to-head to form the canonical heterotetrameric spectrin structure. Spectrins were initially believed to be exclusively found in human erythrocytic membrane and are highly conserved among different species. βII spectrin, the most common isoform of non-erythrocytic spectrin, is found in all nucleated cells and forms larger macromolecular complexes with ankyrins and actins. Not only is βII spectrin a central cytoskeletal scaffolding protein involved in preserving cell structure but it has also emerged as a critical protein required for distinct physiologic functions such as posttranslational localization of crucial membrane proteins and signal transduction. In the heart, βII spectrin plays a vital role in maintaining normal cardiac membrane excitability and proper cardiac development during embryogenesis. Mutations in βII spectrin genes have been strongly linked with the development of serious cardiac disorders such as congenital arrhythmias, heart failure, and possibly sudden cardiac death. This review focuses on our current knowledge of the role βII spectrin plays in the cardiovascular system in health and disease and the potential future clinical implications.

Disruption of spectrin-like cytoskeleton in differentiating keratinocytes by PKCδ activation is associated with phosphorylated adducin

Spectrin is a central component of the cytoskeletal protein network in a variety of erythroid and non-erythroid cells. In keratinocytes, this protein has been shown to be pericytoplasmic and plasma membrane associated, but its characteristics and function have not been established in these cells. Here we demonstrate that spectrin increases dramatically in amount and is assembled into the cytoskeleton during differentiation in mouse and human keratinocytes. The spectrin-like cytoskeleton was predominantly organized in the granular and cornified layers of the epidermis and disrupted by actin filament inhibitors, but not by anti-mitotic drugs. When the cytoskeleton was disrupted PKCδ was activated by phosphorylation on Thr505. Specific inhibition of PKCδ^(Thr505) activation with rottlerin prevented disruption of the spectrin-like cytoskeleton and the associated morphological changes that accompany differentiation. Rottlerin also inhibited specific phosphorylation of the PKCδ substrate adducin, a cytoskeletal protein. Furthermore, knock-down of endogenous adducin affected not only expression of adducin, but also spectrin and PKCδ, and severely disrupted organization of the spectrin-like cytoskeleton and cytoskeletal distribution of both adducin and PKCδ. These results demonstrate that organization of a spectrin-like cytoskeleton is associated with keratinocytes differentiation, and disruption of this cytoskeleton is mediated by either PKCδ^(Thr505) phosphorylation associated with phosphorylated adducin or due to reduction of endogenous adducin, which normally connects and stabilizes the spectrin-actin complex.

Iron-dextran antibody conjugates: General method for simultaneous staining of two components in high-resolution immunoelectron microscopy

We report the preparation and properties of an electron-dense antibody conjugate, made by the covalent bonding of an iron-dextran (Imposil) particle to an antibody molecule. Transmission electron microscopic experiments with the Imposil-antibody conjugates demonstrate their suitability as specific immunostains at high resolution, particularly for simultaneous double staining experiments in conjunction with ferritin-antibody conjugates.

Non-contractile cells with thin processes resembling interstitial cells of Cajal found in the wall of guinea-pig mesenteric arteries

Arterial interstitial cells of Cajal (ICC)-like cells (AIL cells) with a multipolar, irregular, elongated shape and with numerous thin (often less than 1 microm), sometimes branching, processes with lengths up to approximately 60 microm were isolated enzymatically from 1st to 7th order branches of guinea-pig mesenteric artery. Some of the processes of AIL cells were growing (average speed approximately 0.15 microm min-1) and their growth was blocked by 10 microM latrunculin B, an inhibitor of actin polymerisation. Staining with BODIPY phalloidin, a fluorescent dye selective for F-actin, showed the presence of F-actin in the processes of AIL cells. Voltage clamp of single AIL cells revealed an inward current that was four times more dense than in myocytes and was abolished by 10 microM nicardipine, and an outward current carried exclusively by potassium ions that was reduced by 1 mM 4-aminopyridine and/or 100 nM iberiotoxin but unaffected by 10 nM dendrotoxin-K. Imaging of intracellular ionised calcium with fluo-4 using a laser scanning confocal microscope showed local or global calcium transients lasting several seconds in approximately 28 % of AIL cells. When membrane current was recorded simultaneously, the calcium transients were found to correspond to long-lasting transient outward currents, which occurred at potentials positive to -40 mV. Unlike myocytes, AIL cells did not contract in response to 1 mM caffeine or 5 microM noradrenaline, although they responded with a [Ca2+]i increase. The segments of intact arteries did not stain for c-kit, a marker of ICCs. Single AIL cells stained positive for vimentin, desmin and smooth muscle myosin. The presence of ICC-like cells is demonstrated for the first time in the media of resistance arteries.

Clonal analysis of the late stages of erythroleukemia induced by two distinct strains of Friend leukemia virus

We observed striking differences between the tumorigenic colony-forming cells present in the spleens of mice late after infection with the anemia-inducing strain of Friend leukemia virus (strain FV-A) and those present after infection with the polycythemia-inducing strain (strain FV-P). Cells within primary colonies derived from FV-A- and FV-P-transformed cells (CFU-FV-A and CFU-FV-P, respectively) contained hemoglobin and spectrin, indicating that the CFU-FV-A and CFU-FV-P were transformed erythroid progenitor cells. The proportion of cells containing hemoglobin was relatively high (> 25%) in newly isolated cell lines derived from CFU-FV-P colonies, whereas cell lines derived from CFU-FV-A colonies had only low levels (0 to 2%) of hemoglobin-containing cells. A high proportion of the cell lines derived from CFU-FV-A colonies responded to pure erythropoietin and accumulated spectrin and hemoglobin, whereas the cell lines derived from CFU-FV-P colonies did not. A cytogenetic analysis indicated that primary CFU-FV-P colony cells were diploid, whereas chromosomal aberrations were observed in the immediate progeny of CFU-FV-A. The presence of unique chromosomal markers in the majority of the cells within individual colonies derived from CFU-FV-A suggested that these colonies originated from single cells. Finally, leukemic progenitor cells transformed by strain FV-A appeared to have an extensive capacity to self-renew (i.e., form secondary colonies in methylcellulose), whereas a significant proportion of the corresponding cells transformed by strain FV-P did not. In addition, the self-renewal capacity of both CFU-FV-A and CFU-FV-P increased as the disease progressed. From these observations, we propose a model for the multistage nature of Friend disease; this model involves clonal evolution and expansion from a differentiating population with limited proliferative capacity to a population with a high capacity for self-renewal and proliferation.

Alpha-spectrin immunoanalog in Acanthamoeba cells

A monospecific, affinity purified antibody was prepared against chicken erythrocyte alpha-spectrin. The antibody cross-reacted with only one high molecular weight polypeptide (235 kDa) from whole Acanthamoeba cells. The localization of alpha-spectrin-related antigen in Acanthamoeba cells was examined using immunofluorescence and postembedding cytochemical techniques. Three patterns of distribution of alpha-spectrin immunoanalog were distinguished: as submembranous layer, cytoplasmic aggregates and uniform dispersion through the cytoplasm. Immunoelectron microscopic studies showed that the colloidal gold label was located in the cytoplasm in the vicinity of the plasma membrane. The gold particles were also aggregated around unidentified cytoplasmic filamentous structures. The presence of spectrin-related protein in protozoan cells of Acanthamoeba is in accordance with previous assumptions of the widespread occurrence of spectrin-related proteins. The heterogenous distribution of the immunoanalog of alpha-spectrin protein in Acanthamoeba cells is discussed.

Spectrin and related molecules

This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.

Comparison of spectrin isolated from erythroid and non-erythroid sources

Spectrin from erythrocytes and two other tissues (brain and intestine) were isolated from two distant species, pig and chicken; some structural and functional properties were compared. A quantitative antibody inhibition assay was used to determine that antibodies to mammalian red cell spectrin cross-react very poorly, if at all, with their non-erythroid (brain) counterpart and similarly antibodies to pig brain spectrin (fodrin) cross-react very weakly with erythroid spectrin. By contrast, antibodies which were directed against the 240000-Mr subunit of avian fodrin were completely inhibited with avian spectrin and vice versa. To analyze the structural relatedness of these molecules further we compared the chymotryptic iodinated peptide maps generated from each individual subunit. Consistent with the antibody results, we find little (less than 10%) homology between peptides derived from mammalian fodrin and spectrin, but complete homology (100%) of the peptides derived from the 240000-Mr subunits of chicken fodrin, spectrin and another related molecule from intestine, TW260/240. Whereas the peptide maps of fodrin (brain spectrin) revealed striking similarity between divergent species, suggesting a high degree of structural conservation, the peptide maps of erythrocyte spectrin was highly variable between species, indicating that it has diverged considerably in mammalian evolution. In addition we have compared a functional activity of mammalian spectrins, the ability to bind calmodulin, using two different assays. Both results show that, whereas fodrin-calmodulin interaction can be readily demonstrated, the binding to mammalian erythroid spectrin is negligible. This suggests that the high-affinity calmodulin site present on fodrin has been lost from spectrin in mammalian evolution.

Monoclonal antibodies detect a spectrin-like protein in normal and dystrophic human skeletal muscle

Spectrin is the major protein of the erythrocyte membrane skeleton, which is bound to the cytoplasmic surface of the membrane's lipid bilayer and is responsible for cell shape and membrane elasticity. Inability to identify spectrin in other cell types led to the assumption that this protein was unique to erythrocytes. However, spectrin-like proteins have been demonstrated recently in a variety of cell types, including skeletal and cardiac muscle, in several species. We used monoclonal antibodies against human erythrocyte spectrin subunits in an immunocytochemical study to detect related proteins in normal and diseased human skeletal muscle. Six of seven monoclonal antibodies against beta-spectrin determinants were bound at the cytoplasmic surface of muscle fiber plasma membranes, whereas none of six monoclonal antibodies against alpha-spectrin determinants was bound. Muscle fibers of patients with neuromuscular diseases showed similar distribution and specificity of antibody binding to those of normal subjects, but the intensity of binding was increased. In contrast, probable regenerating fibers in muscle of patients with muscular dystrophies showed reduced binding of antibodies, but reduced binding was not seen in fetal muscle fibers nor in those of a patient with a myotubular myopathy. We conclude that human skeletal muscle fibers possess a spectrin-related protein associated with their plasma membrane that shows extensive beta-chain similarities to erythrocyte spectrin but differs significantly with respect to the alpha-subunit. Its function may be associated with the maintenance of membrane and myofibril integrity during contraction, and the increased antibody binding in diseased muscle may reflect a structural rearrangement of spectrin or a compensatory increase in spectrin abundance in response to increased stress on these systems.

Spectrin, fodrin, and TW260/240: a family of related proteins lining the plasma membrane

Recently, molecules highly related to erythrocyte spectrin have been identified in nonerythroid cells. Here we summarize our current understanding of these molecules and suggest a model for their organization. Significant differences exist between this family of proteins isolated from mammalian cells and avian cells, and this may explain the variability in antibody preparations as well as differences in peptide maps of these subunits which have been reported. We have prepared antibodies specific for the variant subunits of the spectrinlike proteins fodrin, spectrin, and TW260/240 and analyzed the distribution of these variant subunits in different chicken cell types as well as their developmental distribution in the intestine. The results suggest that fodrin is the general member of this family of proteins and can even coexist with other spectrinlike proteins in the same cells.

A spectrin-like protein from mouse brain membranes: immunological and structural correlations with erythrocyte spectrin

Membrane-associated mouse brain spectrin is a 972,000 Mr, 10.5S, (alpha beta)2 tetramer containing two approximately 240,000 Mr subunits and two approximately 235,000 Mr subunits. Two-dimensional [125I]tryptic peptide mapping indicates that these subunits share only limited and equivalent overlap with the alpha- and beta-subunits of red blood cell (RBC) spectrin. Both the 220,000 Mr beta-subunit of RBC spectrin and the 235,000 Mr beta-subunit of brain spectrin are phosphorylated in the intact mouse. In vitro analysis suggests that both are phosphorylated by a cAMP-independent protein kinase. Antibodies against pure native mouse red blood cell spectrin cross-react with brain spectrin, and antibodies against pure brain spectrin cross-react with both the alpha- and beta-subunits of mouse RBC spectrin. Both antibodies have been utilized to localize brain spectrin within distinct cellular entities of the mouse cerebellum. Granule cell neurons of the internal granule layer and Purkinje cell neurons demonstrated intense fluorescence of the cortical cytoplasm immediately adjacent to the plasma membrane and unstained nuclei, when either RBC or brain spectrin antibodies were utilized for staining. The molecular layer of the cerebellum stained only lightly, and oligodendrocytes and astrocytes appeared to have little fluorescence. Therefore, while brain is a tissue rich in nonerythroid spectrin, the concentration of these immunoreactive analogues is quite variable within distinct cellular entities of the cerebellum.

Expression of the beta subunit of spectrin in nonerythroid cells

Antibodies raised against electrophoretically purified chicken erythrocyte beta subunit of spectrin, called "beta-spectrin," have been used to demonstrate the presence of an immunoreactive form of this polypeptide in nonerythroid tissues. Immunoautoradiography shows that, in chicken erythrocytes, this antiserum reacts with beta-spectrin (Mr 220,000) and another polypeptide (Mr 230,000) that, by two-dimensional tryptic peptide analysis, shows extensive homology with beta-spectrin but not with the alpha subunit of spectrin, called "alpha-spectrin." Immunoautoradiography and immunoprecipitation of various chicken tissues with this antiserum shows that either one variant or both variants of beta-spectrin are expressed. Indirect immunofluorescence reveals that the antiserum reacts with a plasma membrane-associated component of erythroid and some nonerythroid cells. Particularly strong fluorescence is observed in skeletal and cardiac muscle cells where beta-spectrin appears to form a grid-like network along the inner surface of the sarcolemma. The noncoordinated distribution of alpha- and beta-spectrin variants indicates that their expression may be tailored to the functional requirements of the plasma membrane in different cells.

Actin polymerization induced by calspectin, a calmodulin-binding spectrin-like protein

We have purified from a membrane fraction of bovine brain a calmodulin-binding protein (calspectin) that shares a number of properties with erythrocyte spectrin: It has a heterodimeric structure with Mr 240 000 and 235 000 and binds to (dimeric form) or crosslinks (tetrameric form) F-actin. We show that calspectin (tetramer) is capable of inducing the polymerization of G-actin to actin filaments by increasing nucleation under conditions where actin alone polymerizes at a much slower rate. Thus, brain calspectin behaves in the same manner as erythrocyte spectrin, supporting the idea that, in conjunction with actin oligomers it comprises the cytoskeletal meshwork underlying the cytoplasmic surface of the nerve cell.

Erythroid spectrin, brain fodrin, and intestinal brush border proteins (TW-260/240) are related molecules containing a common calmodulin-binding subunit bound to a variant cell type-specific subunit

Spectrin, fodrin, and TW-260/240 form a group of structurally and functionally similar but not identical high molecular weight actin-binding proteins from chicken erythrocytes, brain tissue, or intestinal epithelial brush borders. Immunological data and one-dimensional peptide maps of the separated subunits suggest that a common (Mr 240,000) and a variant (Mr 220,000, 235,000, or 260,000) subunit account for the three different heterodimers. These results are in line with the related but distinct morphology of the three proteins observed in micrographs of rotary-shadowed molecules and the finding that the common (Mr 240,000) subunit seems to account for the calcium-dependent calmodulin-binding activity displayed by the three proteins. The possible functions of spectrin-like molecules in nonerythroid cells are discussed.

Distribution of actin and myosin in a rat neuronal cell line infected with herpes simplex virus

The indirect immunofluorescence technique was used to study alterations in the distribution of actin and myosin filaments in a rat B 103 neuronal cell line infected with herpes simplex virus type 1 (HSV-1). In uninfected cells, actin filaments were arranged in parallel and ran lengthwise from one end of the cell to the other; although myosin filaments were closely associated with actin filaments, additional myosin formed a netlike distribution which did not stain for actin. In infected cells, actin filaments became more randomly aligned and were concentrated along with myosin in close association with rosette-like formations of nuclei in syncytial cells; structural organization of actin and myosin within these intensely staining areas was no longer evident. The possible role of contractile proteins (actin and myosin) in viral infections of neural tissue is raised.

Identification of a spectrin-like protein in nonerythroid cells

We have demonstrated the existence of a spectrin-like protein in a variety of nonerythroid cultured cells. Indirect immunofluorescence studies with monospecific antispectrin IgG indicated the presence of proteins that have common antigenic determinants to spectrin in embryonic chicken cardiac myocytes, mouse fibroblast lines (3T3, simian virus 4-transformed 3T3), and rat hepatoma lines (HTC, HMOA). Two spectrin-like peptides of 240,000 and 230,000 daltons were immunoprecipitated from octyl glucoside-solubilized embryonic chicken cardiac myocytes, along with associated cytoskeletal proteins. Immunoautoradiographic characterization of the myocyte immunoprecipitate showed that only the spectrin-like 240,000- and 230,000-dalton peptides were stained with monospecific antispectrin IgG and 125I-labeled protein A. One-dimensional partial proteolytic mapping of the myocyte 240,000- and 230,000-dalton peptides showed that these peptides share substantial sequence homology with embryonic chicken erythrocyte spectrin 240,000- and 220,000-dalton peptides.

Clonal analysis of the late stages of erythroleukemia induced by two distinct strains of Friend leukemia virus

We observed striking differences between the tumorigenic colony-forming cells present in the spleens of mice late after infection with the anemia-inducing strain of Friend leukemia virus (strain FV-A) and those present after infection with the polycythemia-inducing strain (strain FV-P). Cells within primary colonies derived from FV-A- and FV-P-transformed cells (CFU-FV-A and CFU-FV-P, respectively) contained hemoglobin and spectrin, indicating that the CFU-FV-A and CFU-FV-P were transformed erythroid progenitor cells. The proportion of cells containing hemoglobin was relatively high (> 25%) in newly isolated cell lines derived from CFU-FV-P colonies, whereas cell lines derived from CFU-FV-A colonies had only low levels (0 to 2%) of hemoglobin-containing cells. A high proportion of the cell lines derived from CFU-FV-A colonies responded to pure erythropoietin and accumulated spectrin and hemoglobin, whereas the cell lines derived from CFU-FV-P colonies did not. A cytogenetic analysis indicated that primary CFU-FV-P colony cells were diploid, whereas chromosomal aberrations were observed in the immediate progeny of CFU-FV-A. The presence of unique chromosomal markers in the majority of the cells within individual colonies derived from CFU-FV-A suggested that these colonies originated from single cells. Finally, leukemic progenitor cells transformed by strain FV-A appeared to have an extensive capacity to self-renew (i.e., form secondary colonies in methylcellulose), whereas a significant proportion of the corresponding cells transformed by strain FV-P did not. In addition, the self-renewal capacity of both CFU-FV-A and CFU-FV-P increased as the disease progressed. From these observations, we propose a model for the multistage nature of Friend disease; this model involves clonal evolution and expansion from a differentiating population with limited proliferative capacity to a population with a high capacity for self-renewal and proliferation.

Member-associated changes during erythropoiesis. On the mechanism of maturation of reticulocytes to erythrocytes

The mature mammalian erythrocyte has a unique membranoskeleton, the spectrin-actin complex, which is responsible for many of the unusual membrane properties of the erythrocyte. Previous studies have shown that in successive stages of differentiation of the erythropoietic series leading to the mature erythrocyte there is a progressive increase in the density of spectrin associated with the membranes of these cells. An important stage of this progression occurs during the enucleation of the late erythroblast to produce the incipient reticulocyte, when all of the spectrin of the former cell is sequestered to the membrane of the reticulocyte. The reticulocyte itself, however, does not exhibit a fully formed membranoskeleton. In particular, the in vitro binding of multivalent ligands to specific membrane receptors on the reticulocyte was shown to cause a clustering of some fractions of these ligand-receptor complexes into special mobile domains on the cell surface. These domains of clustered ligand-receptor complexes became invaginated and endocytosed as small vesicles. By immunoelectron microscopic experiments, these invaginations and endocytosed vesicles were found to be specifically free of spectrin on their cytoplasmic surfaces. These earlier findings then raised the possibility that the maturation of reticulocytes to mature erythrocytes in vivo might involve a progressive loss of reticulocyte membrane free of spectrin, thereby producing a still more concentrated spectrin-actin membranoskeleton in the erythrocyte than in the reticulocyte. This proposal is tested experimentally in this paper. In vivo reticulocytes were observed in ultrathin frozen sections of spleens from rabbits rendered anemic by phenylhydrazine treatment. These sections were indirectly immunolabeled with ferritin-antibody reagents directed to rabbit spectrin. Most reticulocytes in a section had one or more surface invaginations and one or more intracellular vesicles that were devoid of spectrin labeling. The erythrocytes in the same sections did not exhibit these features, and their membranes were everywhere uniformly labeled for spectrin. Spectrin-free surface invaginations and intracellular vesicle were also observed with reticulocytes within normal rabbit spleens. Based on these results, a scheme for membrane remodeling during reticulocyte maturation in vivo is proposed.

Appearance of cytoskeletal components on the surface of leukemia cells and of lymphocytes transformed by mitogens and Epstein-Barr virus

Lactoperoxidase iodination and two-dimensional electrophoresis of the labeled proteins have demonstrated well-characterized cytoskeletal proteins (actin and tubulins) on the surface of human lymphocytes undergoing blastogenic transformation and of certain malignant human cells. Such proteins could not be detected on the surface of normal resting human lymphocytes. The most prominent cytoskeletal protein identified on the surface membrane of mitogen-transformed T and B lymphocytes was actin. In Epstein-Barr virus genome-positive Burkitt's lymphoma and lymphoblastoid cell lines and in two leukemia cells, the major iodinated membrane protein components were actin and alpha 1-, alpha 2-, and beta-tubulins. These proteins were firmly connected to the cytoplasmic skeleton and could not be removed by Triton X-100. Concurrent immunofluorescence studies with specific antibodies and F(ab')2 fragments confirmed the appearance of cytoskeletal components on the biochemical data, and indicated that such cytoskeletal proteins formed distinctive patterns on the cell surface, ranging from small patches to large projections. Five-hour labeling with [35S]methionine indicates that all such cells released large quantities of labeled actin and tubulins into the culture medium. These materials were not readsorbed to the membrane surfaces of the cells.

Erythroleukemia induction by replication-competent type C viruses cloned from the anemia- and polycythemia-inducing isolates of Friend leukemia virus

In this study, the biological properties of the replication-competent viruses, F-MuLVA, present in the anemia-inducing isolate of Friend leukemia virus complex (FV-A); and F-MuLVP, present in the polycythemia-inducing isolate of Friend leukemia virus complex (FV-P) have been examined. BALB/c mice infected as newborns with clonal isolates of F-MuLVA or F-MuLVP become anemic and show splenic enlargement characterized by an increased proportion of cells that resemble immature nucleated erythroid cells. In addition, the spleens of these F-MuLVA- or F-MuLVP-infected mice contain a markedly increased proportion of both erythropoietin-dependent erythroid progenitor cells and spectrin-containing erythroid cells. These results suggest that Friend murine leukemia virus (F-MuLV) by itself can induce an erythroleukemic transformation in newborn BALB/c mice similar to that induced by the anemia-inducing spleen focus-forming virus (SFFVA) in newborn or adult mice. Kinetic studies indicated that the alterations in hemopoietic cell populations induced by F-MuLVA or F-MuLVP in newborn BALB/c mice occurred more slowly than the rapid changes observed after infection with FV-A. In addition, adult BALB/c mice were fully susceptible to the erythroleukemic transformation induced by either SFFVA or SFFVP, whereas only newborn mice were susceptible to F-MuLV. Taken together, these results suggest that, although the replication-defective Friend spleen focus-forming viruses appear to be the major determinant of erythroleukemia induction in adults, the replication-competent helper F-MuLV also have erythroleukemic potential when assayed in newborn animals.

Visualization of the peripheral weave of microfilaments in glia cells

A peripheral weave of microfilaments is visualized in human glia cells. In this weave small numbers of microfilaments converge to structures in the cell edge. Similar assemblies of microfilaments seem to be attached to structures on the surface of microspikes. Together with filaments splaying from the paracrystalline arrangement in microspikes, these units make up the peripheral weave. The filaments of the weave come in close contact with each other and with filaments of internal actin fibres.

Immunohistochemical and immunocytochemical localization of myosin, chromogranin A and dopamine-beta-hydroxylase in nerve cells in culture and in adrenal glands

Dopamine-beta-hydroxylase, chromogranin A and myosin were purified from bovine adrenal medulla and antibodies prepared against these proteins. Indirect immunocytochemical methods were used to localize dopamine-beta-hydroxylase, chromogranin A and myosin in bovine adrenal medulla and myosin in rat adrenal glands and cells from rat C.N.S. maintained in primary culture. Dopamine-beta-hydroxylase and chromogranin A were found in chromaffin granules, in agreement with biochemical data and, using electron microscopy, dopamine-beta-hydroxylase was found within the matrix and in the surrounding membrane of the storage granule, whereas chromogranin A was confined to the granule matrix. Myosin was localized in the vascular system irrigating adrenal glands, fibroblasts lining the vessels and chromaffin cells. In chromaffin cells, staining was found at the cell boundaries and electron microscopy showed myosin to be associated with the plasma membrane. Faint immunocytochemical staining by antimyosin antibodies was observed around certain exocytotic profiles but particular association with such structures was not demonstrable. Myosin localization was also studied in bovine adrenal cortex, where it was found in vascular channels and faintly in adrenal cortical cells, as in rat adrenal cortex and medulla, where identical patterns were obtained. In neuronal and glial cells dissociated from 13 day rat embryo cerebral hemispheres and cultured for 48 h, localization of myosin was studied using immunohistochemistry. The neuritic expansions and growth cones of neurons were fluorescent, whereas in glial cells, filamentous networks were visualized enclosing the nucleus and as long fibres traversing the entire cytoplasm.

Palate morphogenesis. I. Immunological and ultrastructural analyses of mouse palate

Midpalate was analyzed for the presence of nonmuscle contractile systems. The results indicate that increased amounts of actin and myosin are present in cells of regions 2 and 3. A localization of the contractile proteins in cellular projections (filopodia) and in the peripheral cytoplasm of the cell body was confirmed by indirect immunofluorescence studies, using antibodies directed against smooth muscle myosin and against skeletal muscle actin. Specificity of the immunofluorescence reactions was ascertained by immunoabsorption studies using purified myosin and actin. Electron microscopic observations of the mesenchymal cells in region 2 revealed 70A microfilaments along the cell periphery and packed in fliopodia-like projections which course between the cells. These cells, which surround a small ossification center, show no orientation, but extend up to the cranial base perichondrium and down into the shelf between the tongue side epithelium and the ossification center. The cells and projections are attached to each other by adherens and tight-like junctions, forming a putative cohesive contractile network. Putative contractile cells in region 3 are strikingly aligned perpendicular to the oral epithelium and extend one-third of the distance into the shelf. Projections from region 3 cells are contiguous with basement membrane material of the oral epithelium. Axonal bundles and single axons were commonly observed coursing through regions 2 and 3, often seen in close association with the mesenchymal cells. Both clear and dense-core vesicles were found in the axons and cells of these regions. The possible role of these putative nonmuscle contractile cells in palate morphogenesis is discussed.

Reconstitution of spectrin-deficient, spherocytic mouse erythrocyte membranes

To study directly the role of spectrin in erythrocyte membrane function, we have designed a reconstituted membrane system using erythrocyte membranes from spectrin-deficient mice and purified spectrin from normal mice. The normal spectrin is inserted into the spectrin-deficient spherocytes by exchange hemolysis. Thereafter, raising the ionic strength and temperature reseals the cells and, with time, facilitates binding of the spectrin to the spectrin-deficient membranes. The binding is apparently specific as shown by its dependence upon the concentration of undenatured spectrin and the concentration of salt used, as well as by the immunofluorescent appearance of the reconstituted cells after treatment with specific antispectrin antibody. In terms of in vitro cellular behavior, the reconstituted preparations show marked changes in comparison to the untreated spherocytes. In particular, membrane stability, as measured by the reduction of myelin figure formation and lipid loss, is considerably enhanced. In addition, membrane fusion, which occurs readily with the untreated spherocytes, is virtually eliminated. Finally, the osmotic behavior of the native spherocytes is appreciably altered, such that the early phase of osmotically induced swelling, as measured in a high-speed stop-flow apparatus, is delayed and modified. Taken together, these findings indicate specific roles for spectrin in the stabilization of the erythrocyte membrane, in the limitation of membrane fusion, and in the modulation of the membrane's response to osmotic stress.

Microinjection of fluorescently labeled alpha-actinin into living fibroblasts

alpha-Actinin from chicken gizzard labeled with tetramethylrhodamine isothiocyanate has been incorporated into living fibroblast cells by microinjection. Fluorescent labeling of alpha-actinin was carried out such that the conjugated protein was functional in vitro as shown by its ability to bind to F-actin. Within 1-2 hr after injection, diffuse fluorescence was observed throughout the cytoplasm and only faint fluorescence was apparently associated with the stress fibers. During the ensuing 2-15 hr, however, most of the fluorescence was seen as periodicities along the stress fibers and as foci of the microfilament polygonal networks. This distribution of alpha-actinin in the living cells was strikingly similar to that found by indirect immunofluorescence localization of endogenous alpha-actinin in fixed samples of the same cell type. Control studies in which heat-treated (100 degrees C, 2 min) fluorescent alpha-actinin or tetramethylrhodamine isothiocyanate alone was injected into the cells indicated that the stress fiber and polygonal network labeling was specific for "native" fluorescently labeled alpha-actinin. These results suggest that the dynamic properties of proteins and structures in cultured mammalian cells can be studied with the use of microinjection and fluorescence microscopic techniques.

The regional association of actin and myosin with sites of particle phagocytosis

Contractile proteins are thought to play a causative role in motile processes such as phagocytosis. In order to investigate their role in phagocytosis further, simultaneous immunofluorescence localization of F-actin and myosin was carried out in resident mouse peritoneal macrophages after phagocytosis of opsonized zymosan particles. Both actin and myosin appeared to concentrate rapidly at sites of particle phagocytosis. The observed concentration of both proteins at such sites preceded ultimate particle engulfment. Cytochalasin B, a drug which was shown to block pseudopod extensions around the particle, did not prevent the concentration of the two congth effects as an explanation for the observed concentration of actin and myosin at phagocytic sites. Kinetic analysis showed that actin rapidly concentrates at particle-cell binding sites within minutes (or less) of contact with cell surface. The two proteins are present throughout the engulfment phase until and after ingestion is complete. Finally, at later times the particles become clustered over the cell nucleus and the particle-associated actin-myosin seen earlier is no longer evident.

Presence of spectrin in untreated Friend erythroleukemic cells. Its accumulation upon treatment of the cells with dimethyl sulfoxide

Friend leukemia cells (FLC) are nucleated erythroid precursors, and are markedly stimulated towards more advanced stages of differentiation by treatment with dimethyl sulfoxide (DMSO). The presence of spectrin, an erythrocyte membrane protein, has been investigated in untreated and in DMSO-treated FLC by indirect immunofluorescence and by analysis in SDS-polyacrylamide gel electrophoresis of low-ionic-strength cell extracts immunoprecipitated with a monospecific anti-spectrin serum. Spectrin is detectable in significant amounts in the "inducible" clones prior to DMSO stimulation, and accumulates 4- to 5-fold upon addition of this compound to the cultures. Spectrin accumulation occurs rather early (24 hours after cell seeding) and reaches its peak on the third day to decline thereafter. Semiquantitative determinations of spectrin amounts present in DMSO-stimulated 745A and A degree 1 cells on the third day after treatment were 2.4 X 10(5) and 3.0 X 10(5) molecules/cell, respectively. Spectrin is also detectable in very low amounts in an "uninducible" line of FLC, and is not accumulated upon DMSO treatment thereof, whereas treatment with hemin does cause a significant increase of spectrin-positive cells. These data indicate that spectrin is a convenient "early" marker for in vitro studies of erythropoiesis.

Subaxolemmal filamentous network in the giant nerve fiber of the squid (Loligo pealei L.) and its possible role in excitability

A new technique utilizing the squid giant nerve fiber has been developed which permits direct examination of the inner face of the axolemma by scanning electron microscopy. The axoplasm was removed sequentially in a 15-mm long segment of the fiber by intracellular perfusion with a solution of KF, KCl, Ca++-containing seawater, or with pronase. The action potential of the fibers was monitored during these treatments. After brief prefixation in 1% paraformaldehyde and 1% glutaraldehyde, the perfused segment was opened by a lne could be related to information on the detailed morphology of the cytoplasmic face of the axolemma and the ectoplasm. The results obtained by scanning electron microscopy were further substantiated by transmission electron microscopy of thin sections. In addition, living axons were studied with polarized light during axoplasm removal, and the identification of actin by heavy meromyosin labeling and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis was accomplished. These observations demonstrate that a three-dimensional network of interwoven filaments, consisting partly of an actinlike protein, is firmly attached to the axolemma. The axoplasmic face of fibers in which the filaments have been removed partially after perfusion with pronase displays smooth membranous blebs and large profiles which sppose the axolemma. In fibers where the excitability has been suppressed by pronase perfusion, approximately one-third of the inner face of the axolemma in the perfusion zone is free of filaments. It is hypothesized that the attachment of axoplasm filaments to the axolemma may have a role in the maintenance of the normal morphology of the axolemma, and, thus, in some aspect of excitability.

An efficient method to produce specific anti-actin

The production of affinity column purified (specific) anti-actin is described. With the immunization scheme employed, all rabbits produced precipitating antibodies over several months, so that 30 mg specific anti-actin per rabbit could be isolated in 6 months. The antibodies against native and detergent denatured smooth muscle actin are characterized by immunodiffusion tests, staining of the I-band of isolated myofibrils and stress fibers in tissue culture cells, using indirect immunofluorescence.

Biochemistry of actomyosin-dependent cell motility (a review)

Actins and myosins similar to the major proteins of muscle are the major molecular components of intricate mechanochemical systems that perform numerous vital motility and structural functions in all eukaryotic cells. In this article, after a brief summary of the morphological distribution and ultrastructure of actin, myosin, and interrelated proteins of nonmuscle cells, our present knowledge of their biochemistry is critically appraised from the perspective that understanding complex cellular processes depends ultimately on the identification, purification, and biochemical characterization of the proteins involved. Although few conclusions are reached, possible molecular mechanisms for cellular regulation of actin polymerization, filament association, actomyosin ATPase activity, and mechanochemical coupling are discussed and a number of potentially fruitful directions for further research are suggested. These include comparative biochemical investigations and the study of the interaction of heterologous proteins, but particular emphasis is given to the need for quantitative studies at the molecular level of motility proteins purified from a single cellular source.

The identification of two intra-axonally transported polypeptides resembling myosin in some respects in the rabbit visual system

Two polypeptides (M1 and M2) which co-sediment with F-actin in an ATP-reversible way have been detected in extracts of tissue from the rabbit visual system. Both polypeptides resemble skeletal muscle myosin in their ATP-sensitive co-sedimentation with actin, while they resemble the heavy chain of myosin and the lighter polypeptide of erythrocyte spectrin in their electrophoretic mobilities. (The estimated molecular weights are: MI congruent to 195,000; myosin congruent 200,000; M2 and spectrin congruent to 220,000). M1 and M2 were labeled in the cell bodies of the retinal ganglion cells with a radioactive amino acid and subsequently recovered in tissues (optic nerve, optic tract, lateral geniculate nucleus, and superior colliculus) containing segments of the retinal ganglion cell axons. The temporal sequence of labeling M1 and M2 in these tissues indicated that both polypeptides were synthesized in the cell bodies of retinal ganglion cells and subsequently transported down their axons at different maximum velocities. The estimated velocities were: M1, 4-8 mm per day; and M2, 2-4 mm per day.

The binding sites of cytochalasin D. I. Evidence that they may be peripheral membrane proteins

Binding sites for tritiated cytochalasin D (3H-CD) on the isolated plasma membrane from HEp-2 cells were reversibly inactivated, but not dissociated from the membrane, by dialysis in 0.6 M KCl. Activity was restored by subsequent dialysis in 0.06 M KCl. Treatment with 0.2 mM ATP at low ionic strength also inactivated these sites, apparently irreversibly. Extraction of the membrane with 6% Triton X-100 removed 75% of its protein, resulting in a two-fold increase in specific binding activity for 3H-CD. Both high and low affinity binding sites were retained by the detergent-extracted membrane; at least 60% of the high affinity sites were resistant to this treatment. Evidence is presented for the attachment to the HEp-2 plasma membrane of both actin and myosin. The results support the tentative conclusion that plasma membrane binding sites for 3H-CD are peripheral proteins on the cytoplasmic face of the membrane. They are consistent with the hypothesis that myosin may be the location of the high affinity binding site and actomyosin may be the low affinity site. Comparison of these observations with those reported for the congeneric drug, cytochalasin B, suggests that CD binding sites differ from the high affinity site for cytochalasin B.

Smooth muscle antibody in Burkitt's lymphoma and in nasopharyngeal carcinoma

Smooth muscle antibodies (SMA) with specificity for actin, were found with a higher frequency in sera from Burkitt's lymphoma (BL) and nasopharyngeal carcinoma (NPC) patients than in sera from matched controls. No correlation could be found between SMA and anti-Epstein-Barr virus (EBV) antibody titres. There was no parallelism, in individual sera, between the finding of SMA and the occurrence of cold lymphocytotoxins, aother antibody activity found with an abnormally high frequency among BL and NPC patients. The reason why actin, a weak antigen in experimental animals, may become immunogenic in humans remains unexplained.

Biochemical approaches to problems of cellular patterning

The expression of intracellular patterning is perhaps nowhere more impressive than in the arrangements of structural elements associated with the cell surface in protozoa. The view is proposed that biochemical studies of protozoan plasma membranes and associated surface structures represent important contributions of potential significance for the understanding of the perpetuation and expression of positional information at the intracellular level. Some recent work dealing with the isolation, identification, and metabolism of pellicular proteins in Tetrahymena is presented and discussed. Some integral membrane proteins have been identified by iodination and polyacrylamide gel electrophoresis. Labeling studies suggest heterogeneous turnover rates within the group of presently identified membrane proteins. High molecular weight proteins with some similarity to spectrin have been isolated from Tetrahymena epiplasm. It is suggested that the ciliate epiplasm is one example of membrane-associated, actomyosin-like systems found in a variety of cell types. The epiplasm may play a role in the positioning of surface-associated structures and in the control of cell shape.