Control of acetylcholine receptor mobility and distribution in cultured muscle membranes. A fluorescence study

Structural basis for α-bungarotoxin insensitivity of neuronal nicotinic acetylcholine receptors

The ten types of nicotinic acetylcholine receptor α-subunits show substantial sequence homology, yet some types confer high affinity for α-bungarotoxin, whereas others confer negligible affinity. Combining sequence alignments with structural data reveals three residues unique to α-toxin-refractory α-subunits that coalesce within the 3D structure of the α4β2 receptor and are predicted to fit between loops I and II of α-bungarotoxin. Mutating any one of these residues, Lys189, Ile196 or Lys153, to the α-toxin-permissive counterpart fails to confer α-bungarotoxin binding. However, mutating both Lys189 and Ile196 affords α-bungarotoxin binding with an apparent dissociation constant of 104 nM, while combining mutation of Lys153 reduces the dissociation constant to 22 nM. Analogous residue substitutions also confer high affinity α-bungarotoxin binding upon α-toxin-refractory α2 and α3 subunits. α4β2 receptors engineered to bind α-bungarotoxin exhibit slow rates of α-toxin association and dissociation, and competition by cholinergic ligands typical of muscle nicotinic receptors. Receptors engineered to bind α-bungarotoxin co-sediment with muscle nicotinic receptors on sucrose gradients, and mirror single channel signatures of their α-toxin-refractory counterparts. Thus the inability of α-bungarotoxin to bind to neuronal nicotinic receptors arises from three unique and interdependent residues that coalesce within the receptor's 3D structure.

Lateral mobility of nicotinic acetylcholine receptors on neurons is determined by receptor composition, local domain, and cell type

The lateral mobility of surface receptors can define the signaling properties of a synapse and rapidly change synaptic function. Here we use single-particle tracking with Quantum Dots to follow nicotinic acetylcholine receptors (nAChRs) on the surface of chick ciliary ganglion neurons in culture. We find that both heteropentameric alpha3-containing receptors (alpha3*-nAChRs) and homopentameric alpha7-containing receptors (alpha7-nAChRs) access synaptic domains by lateral diffusion. They have comparable mobilities and display Brownian motion in extrasynaptic space but are constrained and move more slowly in synaptic space. The two receptor types differ in the nature of their synaptic restraints. Disruption of lipid rafts, PDZ-containing scaffolds, and actin filaments each increase the mobility of alpha7-nAChRs in synaptic space while collapse of microtubules has no effect. The opposite is seen for alpha3*-nAChRs where synaptic mobility is increased only by microtubule collapse and not the other manipulations. Other differences are found for regulation of alpha3*-nAChR and alpha7-nAChR mobilities in extrasynaptic space. Most striking are effects on the immobile populations of alpha7-nAChRs and alpha3*-nAChRs. Disruption of either lipid rafts or PDZ scaffolds renders half of the immobile alpha3*-nAChRs mobile without changing the proportion of immobile alpha7-nAChRs. Similar results were obtained with chick sympathetic ganglion neurons, though regulation of receptor mobility differed in at least one respect from that seen with ciliary ganglion neurons. Control of nAChR lateral mobility, therefore, is determined by mechanisms that are domain specific, receptor subtype dependent, and cell-type constrained. The outcome is a system that could tailor nicotinic signaling capabilities to specific needs of individual locations.

Postsynaptic scaffolds for nicotinic receptors on neurons

Complex postsynaptic scaffolds determine the structure and signaling capabilities of glutamatergic synapses. Recent studies indicate that some of the same scaffold components contribute to the formation and function of nicotinic synapses on neurons. PDZ-containing proteins comprising the PSD-95 family co-localize with nicotinic acetylcholine receptors (nAChRs) and mediate downstream signaling in the neurons. The PDZ-proteins also promote functional nicotinic innervation of the neurons, as does the scaffold protein APC and transmembrane proteins such as neuroligin and the EphB2 receptor. In addition, specific chaperones have been shown to facilitate nAChR assembly and transport to the cell surface. This review summarizes recent results in these areas and raises questions for the future about the mechanism and synaptic role of nAChR trafficking.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.

Neurotransmitter receptor dynamics studied in vivo by reversible photo-unbinding of fluorescent ligands

We show that fluorescently tagged ligands with high affinity for their targets can be reversibly unbound by focused laser excitation. By sequential unbinding and relabeling with different colors of alpha-bungarotoxin, we selectively labeled adjacent pools of acetylcholine receptors (AChRs) at neuromuscular junctions of adult mice. Timelapse imaging in vivo revealed that synaptic AChRs completely intermingle over approximately 4 days and many extrasynaptic AChRs are incorporated into the synapse each day. In mice that lacked alpha-dystrobrevin, a component of the dystrophin-glycoprotein complex, rates of AChR turnover, and intermingling were increased approximately 4- to 5-fold. These results demonstrate remarkable molecular dynamism underlying macroscopic stability of the postsynaptic membrane, and establish alpha-dystrobrevin as a key control point for regulation of mobility and turnover.

The membrane skeleton of acetylcholine receptor domains in rat myotubes contains antiparallel homodimers of beta-spectrin in filaments quantitatively resembling those of erythrocytes

I used immunogold labeling and quick-freeze, deep-etch, rotary replication to characterize the membrane skeleton at regions with high concentrations of acetylcholine receptor domains in receptor clusters of cultured rat muscle cells. This membrane skeleton consists of a network of filaments closely applied to the cytoplasmic membrane surface. The filaments are specifically decorated by immunogold labeling with a monoclonal antibody, VIIF7, that recognizes an isoform of beta-spectrin colocalizing with acetylcholine receptors. The filaments are 32 +/- 11 nm in length and three to four filaments (average 3.1-3.3) join at each intersection to form the network. These parameters are nearly identical to those reported previously for the membrane skeleton of erythrocytes. Depending on the amount of platinum coating, filament diameters range from 9 to 11 nm in diameter, and are 1.4 nm larger on average than spectrin filaments of erythrocytes replicated at the same time. Filaments are decorated with gold particles close to one end, consistent with the location of the epitope recognized by the monoclonal antibody. Computer modeling shows that all filament intersections in the membrane skeletal network are equally capable of being labeled by the monoclonal antibody. This pattern of labeling is consistent with a network containing antiparallel homodimers of beta-spectrin.

Subnanosecond polarized fluorescence photobleaching: rotational diffusion of acetylcholine receptors on developing muscle cells

Polarized fluorescence recovery after photobleaching (PFRAP) is a technique for measuring the rate of rotational motion of biomolecules on living, nondeoxygenated cells with characteristic times previously ranging from milliseconds to many seconds. Although very broad, that time range excludes the possibility of quantitatively observing freely rotating membrane protein monomers that typically should have a characteristic decay time of only several microseconds. This report describes an extension of the PFRAP technique to a much shorter time scale. With this new system, PFRAP experiments can be conducted with sample time as short as 0.4 microseconds and detection of possible characteristic times of less than 2 microseconds. The system is tested on rhodamine-alpha-bungarotoxin-labeled acetylcholine receptors (AChRs) on myotubes grown in primary cultures of embryonic rat muscle, in both endogenously clustered and nonclustered regions of AChR distribution. It is found that approximately 40% of the AChRs in nonclustered regions undergoes rotational diffusion fast enough to possibly arise from unrestricted monomer Brownian motion. The AChRs in clusters, on the other hand, are almost immobile. The effects of rat embryonic brain extract (which contains AChR aggregating factors) on the myotube AChR were also examined by the fast PFRAP system. Brain extract is known to abolish the presence of endogenous clusters and to induce the formation of new clusters. It is found here that rotational diffusion of AChR in the extract-induced clusters is as slow as that in endogenous clusters on untreated cells but that rotational diffusion in the nonclustered regions of extract-treated myotubes remains rapid.

Congruity of acetylcholine receptor, acetylcholinesterase, and Dolichos biflorus lectin binding glycoprotein in postsynaptic-like sarcolemmal specializations in noninnervated regenerating rat muscles

Noninnervated regenerating muscles are able to form focal postsynaptic-like sarcolemmal specializations either in places of the former motor endplates ("junctional" specializations) or elsewhere along the muscle fibers (extrajunctional specializations). The triple labeling histochemical method was introduced to analyse the congruity of focalization in such specializations of 3 synaptic components: acetylcholinesterase (AChE), acetylcholine receptor (AChR), and a specific synaptic glycoprotein which binds Dolichos biflorus lectin (DBAR). Noninnervated regenerating soleus and extensor digitorum longus (EDL) muscles of the rat were examined and compared with denervated muscles of neonatal and adult rats. All junctional sarcolemmal specializations in noninnervated regenerating muscles accumulated AChE and AChR. Localization of the 2 components was identical within the limits of resolution of the method. DBAR could not be demonstrated in junctional specializations in 17-day-old regenerating muscles. It seems that an agrin-like inducing substance in the former junctional basal lamina invariably triggers the accumulation of both AChE and AChR in the underlying sarcolemma of the regenerating muscle fiber. However, accumulation of DBAR would probably require the presence of the motor nerve. In most of the extrajunctional sarcolemmal specializations in 8-day-old regenerating soleus and EDL muscles, both AChE and AChR accumulated. However, about 10 percent of AChE accumulations lacked AChR and about 35% of AChR accumulations lacked AChE. Even greater variability was observed in 17-day-old regenerating muscles. The presence of DBAR in the extrajunctional postsynaptic-like sarcolemmal specializations could not be demonstrated. Similar extrajunctional sarcolemmal specializations were observed in denervated postnatal rat muscles. About 70% contained both AChE and AChR, and 30% contained only AChR, but none contained DBAR. In denervated mature muscles, sparse extrajunctional AChR accumulations did not contain detectable amounts of AChE. The ability to form complex postsynaptic-like sarcolemmal specializations in the absence of nerve, which is probably inherent to noninnervated immature muscle fibers, may be reduced with muscle maturation. Variable accumulation of individual components in the postsynaptic-like specializations indicates that different triggering factors may be involved in their accumulation or, at least, the mechanisms of their accumulation can function relatively independently.

Rotational diffusion of acetylcholine receptors on cultured rat myotubes

The rotational mobility of acetylcholine receptors (AChR) in the plasma membrane of living rat myotubes in culture is measured in this study by polarized fluorescence recovery after photobleaching (PFRAP). These AChR are known to exist in two distinct classes, evident by labeling with rhodamine alpha-bungarotoxin; clustered AChR that are aggregated in a pattern of highly concentrated speckles and streaks, with each cluster occupying an area of approximately 1,000 microns 2; and nonclustered AChR that appear as diffuse labeling. PFRAP results reported here show that: (a) most clustered AChR (approximately 86%) are rotationally immobile within a time scale of at least several seconds; and (b) most nonclustered AChR (approximately 76%) are rotationally mobile with characteristic times ranging from less than 50 ms to 0.1 s. External cross-linking with the tetravalent lectin concanavalin A immobilizes many nonclustered AChR. PFRAP experiments in the presence of carbachol or cytochalasin D show that the restraints to rotational motion in clusters are remarkably immune to treatments that disperse clusters or disrupt cytoplasmic actin. The experiments also demonstrate the feasibility of using PFRAP to measure rotational diffusion on selected microscopic areas of living nondeoxygenated cells labeled with standard fluorescence probes over a very wide range of time scales, and they also indicate what technical improvements would make PFRAP even more practicable.

Topographical rearrangement of a plasma membrane antigen during capacitation of rat spermatozoa in vitro

We have previously described an antigen (termed 2B1) on rat spermatozoa that is present on the plasma membrane overlying the tail domain. The antigen is mobile within the plane of the plasma membrane and a mAb to it blocks fertilization in vitro. In the present study we describe some dynamic properties of this antigen in relation to its topographical distribution. When spermatozoa were incubated in vitro in a capacitation medium and stained with 2B1 mAb/FITC-rabbit anti-mouse F(ab')2, strong fluorescence appeared over the acrosomal domain. Acute exposure of fresh spermatozoa to dissociating reagents (1 M NaCl or 5 mM 2-mercaptoethanol) or inducers of the acrosome reaction (lysolecithin + Ca2+ or A23187 + Ca2+) failed to mimic these effects. Spermatozoa prelabeled with FITC-2B1 IgG and then capacitated in the presence of excess "cold" 2B1 IgG also showed accumulation of fluorescence on the acrosomal domain, suggesting that the antigen had migrated from the tail. Migration was selective and Ca2(+)- and temperature-dependent but was not inhibited by metabolic poisons (NaF or NaN3). Motility was not obligatory for migration. Immunogold-labeling studies at the ultrastructural level showed that 2B1 antigen was restricted to the surface membrane over both the tail and the acrosomal domains and that during migration it did not change the type of membrane into which it was inserted. From a quantitative analysis of fluorescence on spermatozoa prelabeled with FITC-2B1 IgG and then capacitated, the amount of antigen that appeared on the acrosomal domain was approximately equivalent to that lost from the midpiece domain. The Mr of 2B1 antigen extracted from capacitated spermatozoa was 300-500 Da less than that extracted from noncapacitated cells, suggesting that the molecule had undergone processing concomitant with migration. These results are discussed in relation to mechanisms for targeting antigens to sites where they become physiologically active and are correctly positioned to participate in gamete recognition processes.

Two types of focal accumulations of acetylcholinesterase appear in noninnervated regenerating skeletal muscles of the rat

Muscle fibers in the soleus muscle of the rat, injured by bupivacaine and free autografting, were allowed to regenerate within their old basal laminae. Histochemical and cytochemical analysis of newly synthesized acetylcholinesterase (AChE) revealed that two kinds of focal accumulations of AChE appeared in regenerating myotubes. First, AChE gets concentrated at the sites of the former motor endplates. Accumulation of AChE starts in places where a tight contact between the remnants of the old junctional basal lamina and the budding surface of the myotube engulf the extracellular material. Appearance of these AChE accumulations can be prevented by papain treatment of the soleus muscle before autografting but not by predenervating it for 1 month. Focalization of AChE is probably induced by a component of the junctional basal lamina, possibly a protein, the existence of which is not dependent upon continuous presence of the motor nerve and may be produced by the muscle. This view is corroborated by the fact that an additional kind of AChE accumulation appeared in regenerating muscles in regions remote from the sites where motor endplates were located in the muscles of origin. Although differing in localization, size, and appearance, both kinds of AChE accumulations ultrastructurally resemble the postsynaptic specialization of the motor endplate: they consist of tubelike sarcolemmal invaginations containing AChE. The extrajunctional AChE accumulations seem to arise spontaneously and are usually located more than 750 micron away from the junctional ones as if some local inhibitory mechanism prevents their formation in the immediate vicinity.

Acetylcholine receptor clustering and triton solubility: neural effect

Previous studies by Prives et al. (1980, 1982a and b) have shown that acetylcholine receptors (AchRs) are extracted from muscle cells in vitro by Triton X-100 at different rates, and that clustered receptors extract most slowly. The present study was aimed at comparing the relative extractability of receptors in clusters with those in intercluster regions and the role of neural factors in regulating this extractability. Using primary rat muscle cells in vitro we confirmed that receptor extraction with Triton X-100 does not fit a single exponential but has more than one rate, and that in control cells clustered receptors extract more slowly than do receptors in intercluster regions. The major new observation in this study was that neural extract lowered the overall Triton extraction rate of intercluster receptors to that of clustered receptors. Additional new observations include the findings that (1) both clustered and intercluster receptors show multiphasic extraction rates; (2) stabilization of AchRs against Triton extraction increases with time in the surface membrane; (3) the effect of neural extract on Triton extractability of AChR is dependent on factors that control RNA synthesis, cytoskeletal elements, and collagen; (4) fixation and/or buffer washes accelerate receptor extraction only in cells that are treated with Triton, but not in control cells; (5) in control cells (not exposed to neural factors) Triton X-100 causes new clusters to form. From experiments using Con A we suggest that the Triton-induced new clusters may not be formed by a redistribution of receptors but are, most likely, due to the presence of groups of intercluster receptors with extraction rates lower than those of surrounding receptors.

Correlation of clustering peak density and total amount of acetylcholine receptor on cultured mouse myotubes

The development of acetylcholine receptor (AChR) and its clustering were studied quantitatively on mouse myotubes in nerve-muscle co-cultures. AChR was visualized by fluorescence-labeled antibodies (F-Ab) against crude AChR or fluorescence-labeled alpha-bungarotoxin (F-alpha-BuTX). The F-Ab stain was observed throughout the entire surface of the myotube at day 8 and appeared clustered at day 13. Both peak density and total amount of fluorescence in F-Ab stained myotubes were plotted against days in culture. Both fluorescence indices markedly increased from days 8 to 13 of culture were greater in extent in myotubes incubated in the presence of spinal cord explant, as compared with its absence. Similar results were observed in myotubes stained with F-alpha-BuTX. D-Tubocurarine (D-TC, 0.1 mg/ml) and native alpha-BuTX (1 microgram/ml) clearly inhibited both the total amount of fluorescence and the development of peak fluorescence density in the F-Ab stained myotubes. But the inhibition by D-TC appeared at the later day in culture than alpha-BuTX did. Low temperature (28 degrees C) and cholesterol (1 microgram/ml) treatment inhibited peak fluorescence density without affecting total amount of fluorescence. These results show that the development of ACh can be characterized both by clustering peak density (indicating the lateral mobility of AChR) and by total amount of fluorescence.

Formation of the vertebrate neuromuscular junction

The sequence of events leading to the formation of the NMJ based on the data presented in this chapter from rat, chick, and Xenopus muscle can be divided into three developmental stages, as shown in Table I. The essential components of the NMJ are acquired early. Acetylcholine is present and can be released from the growing nerve. Acetylcholine receptors are present in the muscle membrane and are functional even at the earliest times. These components of the junction--ACh release and functional ACh receptors--can develop independently of each other; i.e., cell culture studies have shown that nerve cells are capable of releasing ACh before their growing tips have come into contact with the postsynaptic muscle membrane. Conversely, muscle cells grown without nerve synthesize and incorporate in their membranes functional ACh receptors. This situation ensures that functional (table; see text) contacts can occur even at the earliest times. Local accumulation of ACh receptors is also detected at the earliest times of junction formation. Although cell culture studies have demonstrated that receptors can aggregate in the absence of nerve, it would appear that the nerve plays an important role in directing where the highest density of receptors will be localized. Acetylcholinesterase, identified both histochemically and electrophysiologically, occurs at the presumptive NMJ shortly after synaptic transmission and receptor clustering have begun, suggesting that these events may play a role in localizing cholinesterase. Although the studies on rat and chick muscle support this view, development of AChE on Xenopus muscle does not require prior exposure to nerve or muscle activity. The ultrastructural features characteristic of the adult NMJ also do not become apparent until after synaptic transmission and receptor clustering have been seen. However, detection of small regions of specialization could be easily overlooked at the ultrastructural level, particularly if the tissue has not been serially sectioned. The young tissue is more fragile (Gordon et al., 1974) and may be more susceptible to mechanical damage or alterations from the fixation procedures (Kullberg et al., 1977). For these reasons, results pertaining to when the ultrastructural specializations occur are difficult to interpret and must await identification of these structures by other means. A number of other changes occur at the NMJ late in development: (1) ACh receptors become metabolically more stable, (2) there is a conversion in the kinetics of the ACh receptor channel, and (3) junctional folds become apparent. The extent to which these changes occur varies among the different organisms discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Specificity of neuronal factors which aggregate acetylcholine receptors on cultured myotubes

Neuronal factors from conditioned medium of neuroblastoma X glioma hybrid cells or isolated from embryonic pig brain aggregate acetylcholine receptors (AChR) on cultured chicken and rat myotubes. A membrane surface protein labelled with a fluorescent monospecific antibody was not aggregated with the same treatment. Antibodies against AChR block the action of the aggregating factors but do not produce large aggregates themselves. These findings indicate that the factors specifically react with the AChR on developing myotubes.

Role of the cytoskeleton in the formation, stabilization, and removal of acetylcholine receptor clusters in cultured muscle cells

We have examined the effects of microtubule- and microfilament-disrupting drugs on the stability, formation, and removal of acetylcholine (ACh) receptors and ACh receptor clusters on the surface of aneurally cultured chick embryonic myotubes. (a) In muscle cell cultures, cytochalasin D (0.2 microgram/ml) or B (2.0 micrograms/ml) causes the dispersal of 50-60% of the existing clusters over a 24-h period (visualized with rhodamine-conjugated alpha-bungarotoxin); Colcemid (0.5 micrograms/ml) has no affect on these clusters. The total number of cell surface ACh receptors does not decline during this period (measured by [125I]alpha-bungarotoxin binding) in the presence of either drug. (b) When cells are treated with biotinylated alpha-bungarotoxin and fluorescent avidin, ACh receptors are cross-linked and rapidly internalized (Axelrod, D., 1980, Proc. Natl. Acad. Sci. USA., 77: 4823-4827). Within 6 h, I have found that 0-15% of the existing large clusters remain. Cytochalasin D or B had no effect on this removal of clusters; however, Colcemid completely prevented the removal of clusters from the cell surface. (c) Addition of chick brain extract to chick myotubes causes an increase in the synthesis and clustering of ACh receptors (Jessell et al., 1979, Proc. Natl. Acad. Sci. USA. 76: 5397-5401). Cytochalasin D caused a slight increase in the number of receptors synthesized in the presence of brain extract whereas Colcemid had no effect on the synthesis and insertion of new receptors into the plasma membrane induced by the brain extract. However, both drugs prevented the increase in the number of receptor clusters. These results are consistent with the hypothesis that receptor clusters are stabilized by actin-containing filaments, but that the movement of receptors in the plane of the membrane requires Colcemid-sensitive microtubules.

Temperature effects in the stimulus-secretion process from isolated chromaffin cells

Temperature effects on the stimulus-secretion coupling process was studied by inducing release of catecholamines (CA) from isolated chromaffin cells of the bovine adrenal medulla. Use was made of three different secretagogues: acetylcholine (ACH), high potassium concentration, and the calcium ionophore A23187, at various incubation temperatures. The latter two agents induced a monotonic increase in secretion with rise in temperature, suggesting different regions of the dependence of total release on temperature. The ACH-induced secretion was, however, markedly different and exhibited a maximal release at 30 degrees C. Kinetic experiments using ACH stimulus revealed that this maximum is produced by different temperature dependence in the stages of activation and desensitization. A proposed model for the total release process yields temperature-dependent parameters that can be divided into three regions of initial rates of secretory activity corresponding to the above independent findings using high K+ concentration and the calcium ionophore. The transitions between the various regions indicate possible transitions in the physical properties of the plasma and secretory granule membranes. Elucidation of the interaction between the membranes is of primary importance in the determining mechanism of CA secretion from the isolated adrenal medulla cell.

Alterations in the morphology of rabbit skeletal muscle plasma membrane during membrane isolation

This study describes changes in morphology of plasmalemma from fast skeletal muscle in the course of tissue disruption and isolation. We find that conditions used to solubilize muscle contractile elements, in the isolation of plasmalemma, including the use of 0.6 M KCl or 0.4 M LiBr in the cold (0-4 degrees C), lead to altered plasmalemma morphology. The intramembrane particles, as revealed by freeze-fracture electron microscopy, become aggregated, leaving large domains devoid of particles. The square arrays in the P face and the complementary "pits" in the E face also become aggregated, sometimes forming sizeable aggregates of square arrays. Thin-section electron microscopy using tannic acid enhancement reveals plasma membrane associated components, on both cytoplasmic and extracellular faces, are largely reduced by the salt treatment. Pyrophosphate and magnesium at lower concentrations, sometimes used instead of high salt, also resulted in particle aggregation, although less pronounced than with concentrated salt solutions. The plasma membrane-associated proteins on both plasma membrane surfaces were likewise decreased by this treatment. Pyrophosphate treatment also separated the basal lamina from the plasma membrane. Incubation of muscle in isoosmotic sucrose does not alter the morphology of the plasmalemma with regard to particle aggregation, diminution of membrane associated components, or separation of the basal lamina. Our observations suggest that membrane-associated protein and/or cytoskeleton constrains the mobility of components in the plane of the membrane and that removal of this constraint leads to aggregation of intramembrane particles.

Organization of acetylcholine receptor clusters in cultured rat myotubes is calcium dependent

The effect of extracellular Ca2+ concentration and myasthenic globulin on the distribution and appearance of acetylcholine receptor (AChR) clusters on rat myotubes was studied with tetramethyl-rhodamine-labeled alpha BTX. Low Ca2+ medium (2.5 X 10(-5) M) caused a time-dependent loss of AChR clusters, and a concomitant increase in small punctate areas of fluorescence. High Ca2+ concentrations (1.5 X 10(-2) M) increased the size of AChR clusters without altering AChR synthesis. These changes were not observed with other divalent ions. In the presence of myasthenic globulin, the rate of AChR turnover increases, and AChR clusters are rapidly dispersed. High Ca2+ concentration partially protects the AChR clusters from dispersal and decreases the rate of receptor turnover.

Clonal myoblasts and myotubes show differences in lectin-binding patterns

To determine changes in distribution or mobility of cell-surface glycoconjugates during myogenesis the binding of fluorescein-conjugated plant lectins to myoblasts and myotubes of the L6 rat skeletal muscle cell line has been studied. Binding has been carried out at 4 degrees C on either live or glutaraldehyde-fixed cells. Fluorescein conjugates of soybean agglutinin (Fl-SBA), wheat germ agglutinin (Fl-WGA), concanavalin A (Fl-conA) and Lens culinaris agglutinin (Fl-LCA) produced predominantly uniform fluorescence on both live and fixed myoblasts. On fixed myotubes, Fl-LCA, Fl-conA and Fl-SBA again produced predominantly uniform fluorescence, whereas Fl-WGA showed a pattern of diffuse, irregular spots in addition to uniform fluorescence. Fl-conA, Fl-LCA and Fl-WGA binding to live myotubes resulted in patterns quite similar to those on fixed myotubes; the only differences being the presence of weak patterns of diffuse spots with Fl-LCA and Fl-conA and an enhanced pattern of diffuse spots with Fl-WGA. Fl-SBA, however, showed a unique pattern on live myotubes which consisted of discrete, round spots and minimal uniform fluorescence. With shorter labeling times, Fl-SBA produced relatively more prominent uniform fluorescence on live myotubes. It appears, therefore, that the native distribution of SBA, conA and LCA-binding sites is similar and predominantly random on L6 myoblasts and myotubes, whereas some WGA-binding sites may be aggregated on myotubes. The results also suggest that SBA-binding sites readily cluster at 4 degrees C on myotubes but not myoblasts, whereas the other lectin sites undergo little or no redistribution on either cell type. Thus the mobility of SBA-binding sites may increase with differentiation.

Diffusely distributed acetylcholine receptors can participate in cluster formation on cultured rat myotubes

On aneurally cultured rat primary myotubes, 10% of the acetylcholine receptors (AcChoR) are found to be aggregated and immobilized in endogenous clusters while the remainder are diffusely distributed and partially mobile. This paper reports that AcChoR in clusters can be gathered from AcChoR in diffuse areas during the course of normal myotube development. AcChoR were fluorescently labeled with rhodamine-conjugated alpha-bungarotoxin, and all existing clusters in a circumscribed region of the culture dish were irreversibly photobleached by a slightly defocused laser beam, the movement of which was controlled by a lens mounted on a joystick translator. This procedure leaves intact only the fluorescent label on the diffusely distributed AcChoR. Observation of the myotubes after several hours of incubation revealed cluster fluorescence redevelopment. This cluster fluorescence must have consisted of AcChoR that previously were diffusely distributed. The majority (but not all) of cluster fluorescence redevelopment occurred in the location of a previously bleached cluster. About half of the redeveloped clusters have an annular shape. The major conclusions of this study are (i) diffusely distributed AcChoR can become clustered; (ii) endogenous clusters appear to form, at least in part, by "trapping" receptors as they diffuse in from surrounding regions; (iii) cluster formation is an ongoing process in cultured rat myotubes; and (iv) colchicine (a microtubule-disrupting agent) inhibits cluster formation.

Diffusion rates of cell surface antigens of mouse-human heterokaryons. III. Regulation of lateral diffusion rates by calcium ions

In mouse-human heterokaryons, the lateral diffusion of major histocompatibility (MHC) antigens in the plasma membrane is enhanced by treatment of parent cells with ouabain. Ouabain treatment is ineffective if the medium lacks calcium ion, or if Verapamil, a blocker of calcium channels, is present. The divalent ionophore A23187 also enhances lateral diffusion of MHC antigens, to the same extent as ouabain, A23187 is effective only if calcium is present in the medium. Thus it appears that increased levels of cell calcium release constraints to lateral diffusion of MHC antigens.

Cholinesterase localization at sites of nerve contact on embryonic amphibian muscle cells in culture

Cholinesterase (ChE), detected histochemically, was found to be localized at many sites of nerve-muscle contact in cultures of spinal cord and muscle cells derived from Xenopus laevis embryos. Such contacts were often characterized by a corresponding localization of acetylcholine receptors and by synaptic ultrastructure, including aggregates of clear vesicles in the nerve fibre and an 80-100 nm wide intercellular cleft. The ChE reaction product was localized in the cleft. When cultures were grown in the presence of curare many of the nerve-contacted muscle cells still exhibited ChE at the sites of contact. It is concluded that ChE accumulates at synaptic contacts in these cultures even in the absence of muscle action potentials and contraction.

Enhanced molecular diffusibility in muscle membrane blebs: release of lateral constraints

Measurements of lateral molecular diffusion on blebs formed on the surfaces of isolated muscle cells and myoblasts are reported. These blebbed membranes retain integral proteins but apparently separate from the detectable cytoskeleton. On blebs, acetylcholine receptors, concanavalin A receptors, and stearoyldextran extrinsic model receptor molecules are free to diffuse with a diffusion coefficient (D) approximately 3 x 10(-9) cm2/s, which is close to the value predicted for hydrodynamic drag in the lipid membrane. In contrast, diffusion of these typical receptors in intact cell membranes is constrained to D approximately less than 10(-10) cm2/s with substantial fractions virtually nondiffusible (D less than 10(-12) cm2/s). Lipid analog diffusion is also slightly enhanced in blebs as expected of evanescent lipid protein interaction. This strong enhancement of membrane protein diffusion is attributed to release from unidentified natural constraints that is induced in some way by detachment of the bleb membrane.

Matrix control of protein diffusion in biological membranes

Lateral diffusion coefficients of fluorescently labeled lipids and integral membrane proteins were determined in the membranes of normal and spectrin-deficient spherocytic mouse erythrocytes by the technique of fluorescence redistribution after photobleaching. The results were used to generate a mathematical description of a matrix-control model of membrane protein diffusion. In the spherocytic cells, which lack the principal components of the cytoskeletal matrix of normal cells, the diffusion coefficients of lipid (1.5 +/- 0.5 X 10(-8) cm2/s) and protein (2.5 +/- 0.6 X 10(-9) cm2/s) differ only by a factor of 6, close to the difference predicted on the basis of size by the two-dimensional bilayer continuum model of Saffman and Delbrück [Saffman, P. G. l& Delbrück, M. (1975) Proc. Natl. Acad. Sci. USA 72, 3111-3113]. In contrast, the membranes of normal cells show a lipid diffusion coefficient (1.4 +/- 0.5 X 10(-8) cm2/s) that is some 300-fold greater than that of the membrane proteins (4.5 +/- 0.8 X 10(-11) cm2/s). Analysis of these results, based on the hypothesis that protein diffusion in normal membranes is sterically hindered by a labile matrix, yields an effective matrix surface viscosity consistent with the viscoelastic mechanical properties of the membranes. Thus, a relationship is established between the deformation characteristics of the membrane and the lateral mobility of proteins suspended in the membrane.

Membrane-associated cytoskeletal proteins in squid giant axons

Cytoskeletal proteins (e.g., tubulin, actin, and neurofilament proteins) in the squid giant axon are separable into KF-soluble and -insoluble forms. The KF-insoluble cytoskeletal components appear to constitute the major proteins in the subaxolemmal fibrous network on the inner surface of the axon. These cytoskeletal proteins and the subaxolemmal network are both highly soluble in KI solutions. Whereas giant axons tolerate prolonged perfusions in KF solutions with no loss of excitable properties, a relatively short perfusion with KI solution completely eliminates the excitability of the axon. The loss of this excitability correlates with the simultaneous dissolution of the subaxolemmal network of cytoskeletal proteins and the release of its proteins into the perfusate. These data support the hypothesis that cytoskeletal proteins associated with the inner surface of the axolemma are involved in the regulation of axonal excitability.

Membrane lipid heterogeneity associated with acetylcholine receptor particle aggregates in Xenopus embryonic muscle cells

Filipin, digitonin, and saponin react with membrane cholesterol to produce unique membrane alterations (sterol-specific complexes) that are easily discernible in freeze-fracture replicas. We have treated both noninnervated and innervated Xenopus embryonic muscle cells in culture with these agents. Freeze-fracture of these treated muscle cells showed that most areas of the muscle plasma membrane contain sterol-specific complexes (19- to 40-nm protuberances and dimples with filipin, a scalloped appearance with digitonin, or an irregular, rough appearance with saponin). However, these complexes were virtually absent from membrane areas of junctional and nonjunctional aggregates of acetylcholine receptor particles. This result suggests that the membrane matrix of these aggregates is low in cholesterol and that this membrane lipid heterogeneity may be linked to the mechanisms involved in their formation and stabilization on muscle cells in culture.

A factor from neurons induces partial immobilization of nonclustered acetylcholine receptors on cultured muscle cells

A factor or factors released by cultured NG108-15 neuroblastoma X glioma hybrid cells and added to the medium of rat myotube primary cultures was found to immobilize some of the previously mobile acetylcholine receptors in the myotube membrane. Partial receptor immobilization occurred within 3 h after the beginning of treatment with the NG108-15-conditioned medium factor and persisted for at least 24 h of continuous treatment. A similarly derived conditioned medium concentrate from the non-neuronal parent glioma cell line did not immobilize receptors, relative to untreated controls. Acetylcholine receptors were visualized by fluorescent alpha-bungarotoxin and their lateral motion was observed by the technique of fluorescence photobleaching recovery.

Absence of filipin-sterol complexes from the membranes of active zones and acetylcholine receptor aggregates at frog neuromuscular junctions

The polyene antibiotic filipin reacts specifically with membrane cholesterol and produces distinctive membrane lesions. We treated frog cutaneous and sartorius muscles with 0.04% filipin in a glutaraldehyde solution with or without prefixation with glutaraldehyde. Freeze-fracture of these muscles revealed numerous 19 to 38-nm protuberances and depressions (filipin-sterol complexes) in most areas of muscle, axon, and Schwann cell membranes. In the presynaptic membrane, however, these filipin-sterol complexes were absent from active zones consisting of ridges bordered with double rows of particles. In the postsynaptic membrane, filipin-sterol complexes were also virtually absent from the areas occupied by aggregates of large particles representing acetylcholine receptors. These results suggest that the membrane regions of active zones and acetylcholine receptor aggregates have a low cholesterol content.

Reduced lateral mobility of a fluorescent lipid probe in cholesterol-depleted erythrocyte membrane

The effect of cholesterol depletion of the human erythrocyte membrane on the lateral diffusion rate of a fluorescent lipid probe is reported. At low temperature (-5 to 5 degrees C), the diffusion of the probe is 50% slower in the cholesterol-depleted membrane than in non-depleted membrane. At high temperatures (30 to 40 degrees C), probe mobility is not affected by cholesterol depletion. These results suggest that cholesterol suppresses aspects of phospholipid phase changes in animal cells in a manner consistent with its behavior in artificial bilayers and multilayers. Whole erythrocytes were depleted of 30--50% of their cholesterol by incubation with a sonicated dispersion of dipalmitoyl phosphatidylcholine. Cells were then labeled with 3,3'-dioctadecylindocarbocyanine (diI), a phospholipid-like fluorescent dye, and hemolyzed into spherical ghosts. The rate of lateral motion of diI was measured by observing the fluorescence recovery after local photobleaching with a focused laser spot. The diffusion rate of the lipid probe in both control and cholesterol-depleted erythrocyte membrane is substantially smaller than in any cell or model membrane previously measured.

Lateral mobility in reconstituted membranes--comparisons with diffusion in polymers

The diffusion coefficients (D) of lipopolysaccharide, phospholipid, and Escherichia coli matrix protein were determined in reconstituted multibilayer membranes. Over a range of protein concentration of 0--60% by weight, D for lipopolysaccharide decreased 10-fold, whereas D for phospholipid remained essentially constant. The diffusion coefficient of matrix protein at a concentration of 50% was less than or equal to 10(-12) cm2 s-1. These results are discussed in terms of a model for diffusion in polymeric networks.