The intracellular organization of actin and tubulin in cultured C-1300 mouse neuroblastoma cells (clone NB41A3)

Neurons: The Interplay between Cytoskeleton, Ion Channels/Transporters and Mitochondria

Neurons are permanent cells whose key feature is information transmission via chemical and electrical signals. Therefore, a finely tuned homeostasis is necessary to maintain function and preserve neuronal lifelong survival. The cytoskeleton, and in particular microtubules, are far from being inert actors in the maintenance of this complex cellular equilibrium, and they participate in the mobilization of molecular cargos and organelles, thus influencing neuronal migration, neuritis growth and synaptic transmission. Notably, alterations of cytoskeletal dynamics have been linked to alterations of neuronal excitability. In this review, we discuss the characteristics of the neuronal cytoskeleton and provide insights into alterations of this component leading to human diseases, addressing how these might affect excitability/synaptic activity, as well as neuronal functioning. We also provide an overview of the microscopic approaches to visualize and assess the cytoskeleton, with a specific focus on mitochondrial trafficking.

Control of Synapse Structure and Function by Actin and Its Regulators

Neurons transmit and receive information at specialized junctions called synapses. Excitatory synapses form at the junction between a presynaptic axon terminal and a postsynaptic dendritic spine. Supporting the shape and function of these junctions is a complex network of actin filaments and its regulators. Advances in microscopic techniques have enabled studies of the organization of actin at synapses and its dynamic regulation. In addition to highlighting recent advances in the field, we will provide a brief historical perspective of the understanding of synaptic actin at the synapse. We will also highlight key neuronal functions regulated by actin, including organization of proteins in the pre- and post- synaptic compartments and endocytosis of ion channels. We review the evidence that synapses contain distinct actin pools that differ in their localization and dynamic behaviors and discuss key functions for these actin pools. Finally, whole exome sequencing of humans with neurodevelopmental and psychiatric disorders has identified synaptic actin regulators as key disease risk genes. We briefly summarize how genetic variants in these genes impact neurotransmission via their impact on synaptic actin.

Nonlymphoid cultured cells possess a system controlling cellular compatibility

We show that various nonlymphoid cultured cells can activate the production of cytotoxic factors in response to direct contact with cells of a different kind. Accumulation of cytotoxic factors in the medium was detected 1 h after contact of K562 and L929 cells or after contact of L929 cells with purified membranes of K562 cells. TNF-alpha or immunologically related proteins, or both, but not Fas-ligand or lymphotoxin, were also accumulated in membranes of K562 and L929 cells shortly after these cells had been allowed to contact each other. The cytotoxic factors expressed by nonlymphoid cells trigger apoptosis of target cells. These observations strongly suggest that nonlymphoid cells possess molecular mechanisms controlling cellular compatibility.

Changes in neuronal cell bodies in N. laminaris during deafferentation-induced dendritic atrophy

N. laminaris dendrites begin to atrophy almost immediately after they are deafferented. Accompanying this rapid change in shape is a loss of microtubules and neurofilaments at the base of the dendrite, and a decrease in the density of the dendritic cytoplasm. However, degenerative changes in the dendritic plasma membrane were not evident until 2 days after deafferentation. Thus it was unknown what happened to the volume and membrane lost from the atrophying dendrites before this time. The soma was investigated in this study as a possible recipient of the volume of the atrophying dendrite. Soma size increased significantly by 2 hours after deafferentation and continued to increase for 1-8 days after deafferentation. The nucleus, which is normally concentric with the soma, moved continuously to the dorsal pole of the soma, toward the innervated side of the cell. The cytoplasm on the ventral side of the soma showed a decrease in density and loss of cytoskeleton similar to what was found in the initial portion of the ventral primary dendrites in the accompanying paper. These changes are interpreted as indicative of a rapid resorption of the ventral dendrite back into the soma following deafferentation.

Thrombin modulates and reverses neuroblastoma neurite outgrowth

Previous studies have shown that neuroblastoma cells and several types of primary neuronal cells in culture rapidly extend neurites when switched from serum-containing to serum-free medium. The present studies on cloned neuroblastoma cells show that thrombin blocked this spontaneous differentiation at 2 nM with a half-maximal potency of 50 pM. This required the catalytic activity of thrombin and was reversed upon thrombin removal. Thrombin also caused cells in serum-free medium to retract their neurites at equally low concentrations. Two other serine proteases, urokinase and plasmin, did not block or reverse neurite extension even at 100-fold higher concentrations. A specific assay for thrombin indicated that thrombin detected in serum-containing medium from neuroblastoma cultures was derived from serum and that it was likely responsible for much of the known capacity of serum to maintain neuroblastoma cells in a nondifferentiated state. This was supported by the finding that heparin addition reduced the thrombin concentration in serum-containing medium and stimulated neurite outgrowth from neuroblastoma cells in serum-containing medium. Studies on the ability of thrombin to modulate neurite outgrowth by other agents showed that it blocked and reversed the neurite outgrowth activity of two thrombin inhibitors: protease nexin-1 (which is identical to glial-derived neurite-promoting factor) and hirudin. Thrombin, however, did not block the neurite-promoting activity of dibutyryl cAMP or prostaglandin E1. These results suggest a specific role for thrombin in control of neurite outgrowth.

Nerve growth cones isolated from fetal rat brain. IV. Preparation of a membrane subfraction and identification of a membrane glycoprotein expressed on sprouting neurons

This study describes the preparation of a membrane subfraction from isolated nerve growth cone particles (GCPs) (see Pfenninger, K. H., L. Ellis, M. P. Johnson, L. B. Friedman, and S. Somlo, 1983, Cell, 35:573-584) and the identification in this fraction of a glycoprotein expressed during neurite growth. While approximately 40 major polypeptides are visible in Coomassie Blue-stained SDS polyacrylamide gels of pelleted (partially disrupted) GCPs, a salt-washed membrane fraction prepared from lysed, detergent-permeabilized GCPs contains only 14% of this protein and has an unusually simple polypeptide pattern of seven major bands. Monoclonal antibodies have been generated to GCP membranes isolated from fetal rat brain. These antibodies have been screened differentially with synaptosomes from adult rat brain in order to identify those which recognize antigens expressed selectively during neurite growth. One such antibody (termed 5B4) recognizes a developmentally regulated membrane glycoprotein that is enriched in GCP membranes and expressed in fetal neurons sprouting in vitro. The 5B4 antigen in fetal brain migrates in SDS polyacrylamide gels as a diffuse band of approximately 185-255 kD, is rich in sialic acid, and consists of a small family of isoelectric variants. Freezing-thawing and neuraminidase digestion result in the cleavage of the native antigen into two new species migrating diffusely around 200 and 160 kD. Prolonged neuraminidase digestion sharpens these bands at about 180 and 135 kD, respectively. In the mature brain, antibody 5B4 recognizes a sparse polypeptide migrating at approximately 140 kD. As shown in the following paper (Wallis, I., L. Ellis, K. Suh, and K. H. Pfenninger, 1985, J. Cell Biol., 101:1990-1998), the fetal antigen is specifically associated with regions of neuronal sprouting and, therefore, can be used as a molecular marker of neurite growth.

Cytoskeleton and adhesion patterns of cultured chick embryo chondrocytes during cell spreading and Rous sarcoma virus transformation

The cytoskeleton and the adhesion complex of chick embryo chondrocytes maintained in vitro have been studied by fluorescence and interference reflection microscopy during the process of cell spreading. The pattern of actin-containing microfilaments and the distribution of vinculin speckles on adhesion plaques have been found to change as a function of the culture time. Newly plated chondrocytes adhere to the substratum mostly around a peripheral ring-like region and show a complex tridimensional array of microfilaments. When chondrocytes flatten, they develop stress fibres and show a diffuse system of vinculin-containing adhesion plaques scattered over the entire ventral side of the cells. Upon infection with Rous sarcoma virus (RSV) chondrocytes display one or more actin-containing ruffles located on the dorsal side similar to the 'actin flowers' earlier described in other cell types. These structures have been found to accumulate vinculin too. In chondrocytes infected with two td-ts mutants of RSV, 'actin flowers' have been found to persist at the restrictive temperature. At this temperature, however, in the majority of cells, stress fibres and adhesion plaques reappear.

Latrunculins: novel marine toxins that disrupt microfilament organization in cultured cells

Two toxins, latrunculins A and B, which contain a new class of 16- and 14-membered marine macrolides attached to the rare 2-thiazolidinone moiety, were purified recently from the Red Sea sponge Latrunculia magnifica. The effects of these toxins on cultured mouse neuroblastoma and fibroblast cells have been evaluated. In both types of cells, submicromolar toxin concentrations rapidly induce striking changes in cell morphology that are reversible upon removal of the toxin. Immunofluorescence studies with antibodies specific for cytoskeletal proteins reveal that the toxins cause major alterations in the organization of microfilaments without obvious effects on the organization of the microtubular system.

Microtubular organization in flat epitheloid and stellate process-bearing astrocytes in culture

Microtubules and microfilament patterns in cultured astrocytes were revealed by using indirect immunofluorescent microscopy in conjunction with anti-tubulin immune serum and anti-actin immunoglobulins respectively. In flat epitheloid astroglial cells (either polygonal or elongated) colchicine-sensitive immunofluorescent fibres, which correspond to bundles of microtubules, extend from the perinuclear cytoplasm into the cell periphery by running for long distances through the different focal planes. These patterns of organization differ markedly from the patterns of organization of microfilaments which are arranged in fibres parallel to each other and often oriented along the cell boundary. In response to the combined treatments of serum withdrawal and administration of dBcAMP, flat epitheloid astrocytes adopt a morphology similar to that of the mature astrocytes in situ in the CNS, that is of stellate process-bearing cells. This is prevented or is reverted by the administration of colchicine at the appropriate times. There are strong suggestions indicating that during cell processes formation the microtubular network is reorganized and microtubules assembled into dense bundles which are oriented along the axis of the cell processes. In view of these results, we suggest that, in contrast to microfilaments, microtubules are not determinant for the maintenance of cellular shape in elongated or polygonal flat epitheloid astroglial cells but they are required for both the formation and maintenance of processes in stellate astrocytes.

Actin filament capping protein from bovine brain

An actin filament capping protein has been purified from bovine brain. The protein has a native mol. wt. of 63 kilodaltons (kd) with subunits of 36 kd and 31 kd and is globular in shape. It nucleates actin polymerization, inhibits filament elongation and filament interactions, and decreases the steady state viscosity of F-actin in substoichiometric amounts (molar ration 1:1000). In addition, the protein increases the critical concentration for actin polymerization. Neither Ca2+ nor calmodulin affects it action. All these effects can be explained by the binding of the protein to the 'barbed' end of actin filaments leading to a blockade of actin monomer addition at the preferred growing end. This is directly demonstrated by electron microscopy. Concerning the polypeptide composition, Ca2+-independence, mode, and stoichiometry of actin interaction, the protein is similar to the capping protein, previously isolated from Acanthamoeba.

Immobilization of concanavalin A receptors during differentiation of neuroblastoma cells

Neuroblastoma cells serve as a useful model of neuronal development because compounds such as dimethyl sulphoxide (DMSO) and dibutyryl cyclic AMP cause them to undergo a process of controlled differentiation in tissue culture, during which they can extend long processes, develop characteristic excitability mechanisms, synthesize neurotransmitters and form synapses. We have used the technique of fluorescence photobleaching recovery to study the lateral mobility of cell-surface constituents during the differentiation of neuroblastoma clone N1E-115 cells. The concanavalin A (Con A) binding sites appear as discrete patches distributed over the entire cell surface and exhibit lateral mobility in undifferentiated cells comparable with that of surface glycoproteins of other cells. After induction of differentiation, however, the vast majority of Con A binding sites become immobilized, and we present data which suggest that the mechanism of this immobilization may involve linkage to the internal actin network.

Distribution of nerve growth factor in chick embryo sympathetic neurons in vitro

The distribution of nerve growth factor (NGF) in chick embryo sympathetic neurons has been followed by two distinct procedures, that is, indirect immunofluorescence microscopy employing purified NGF antibodies and autoradiography after exposure of cells to [125I]NGF. This study shows that NGF bound to sympathetic neurons is not uniformly distributed but appears in spots over the surface of perikarya and along processes. If the same cells, after incubation with NGF and fixation, are treated with methanol-acetone to allow permeation of immunoglobulins across the plasma membrane, NGF antibody immunoreactive material is also found within the cytoplasm and notably in the paranuclear area only in sympathetic neurons. Analogous findings are obtained when sympathetic neurons are incubated in the presence of [125I]NGF, fixed, sectioned and processed for autoradiography. Also with this technique NGF appears to be localized in the cytoplasmic compartment and is found around the nucleus. These studies are discussed in connection with the results of similar experiments performed on a clonal line of NGF target cells known as PC12.

Basal lamina glycoproteins are produced by neuroblastoma cells

Murine neuroblastoma cells have been widely used as a model system for neuronal cells as they can be induced to differentiate in culture by various stimuli, such as dibutyryl cyclic AMP (dbcAMP), prostaglandin, and serum starvation. The cells respond with assembly of microtubules, leading to neurite outgrowth, with increased activity of neuronal-specific enzymes, tyrosine hydroxylase, choline acetyltransferase and acetylcholine-esterase, and synthesis of neurotransmitters. The differentiated cells lose tumorigenicity. Cell-to-substratum adhesion is evidently crucial for neurone extension in vitro. Neurite outgrowth is induced by treatments that increase cell-to-substratum adhesion in some neuronal cell cultures. We have now identified the major high molecular weight proteins synthesized and secreted by murine C1300 neuroblastoma cells as fibronectin, laminin and type IV procollagen, of which the latter two were also found to be deposited in pericellular matrix form.

Growth cones in differentiated neuroblastoma: a time-lapse cinematographic and electron microscopic study

Growth cones of 'differentiating' neuroblastoma cells in monolayer culture were studied by time-lapse cinematography and electron microscopy. Morphological differentiation, and thus growth cone formation, was induced by the glucocorticoid dexamethasone. Growth cones lengthened gradually at an average rate of 30 microns/h, advancing in stages that involved alternating extensions and retractions of the filopodia and lamellar sheets. During neurite growth the cell body usually remained stationary. The ultrastructure of growth cones was typified by several filopodia, each containing a bundle of microfilaments, agranular endoplasmic reticulum, aggregates of large agranular vesicles lying adjacent to filopodia (previously termed vesicle-filled mounds), many dense-cored vesicles, 100-140 nm in diameter, microtubules, bizarre and distorted mitochondria, and scattered from ribosomes. Comparing the findings with previous ultrastructural accounts of growth cones of cultured ganglion cells, similarities outnumbered differences. The organization of the microfilament bundles and the abundance of free ribosomes were remarkable in the neuroblastoma cell as was the profusion of dense-cored vesicles which were most numerous in the proximal portion of the growth cone.

Tubulin immunohistochemistry. Fixation methods affect the response of spinal cord cells in vitro

We report here the results observed when tubulin fluorescence immunohistochemistry is performed upon dissociated cultures of nervous tissue, principally of chick embryo spinal cord. When fixation includes nonpolar solvents or detergents, a uniform fluorescence is seen in neuron perikarya (with the exception of their nuclei), and the processes to which they give rise. Fixation with formaldehyde or glutaraldehyde alone, however, results in a discontinuous staining of neurites, and a less regular staining of their perikarya. The former pattern of response can be elicited if aldehyde fixation is followed by exposure to non-polar solvents. Such results are obtained both in thinly spread regions of the cultures, where neurons and their processes can easily be seen, and in the cell aggregates that also characterise them. Possible interpretations of these results are discussed.