Structure of the subsynaptic sarcoplasm in the interfolds of the frog neuromuscular junction

Ultrastructural visualization of the transmembranous and cytomatrix-related part of nicotinic acetylcholine receptor of frog motor endplate by means of an immunochemical avidity of IgG for d-tubocurarine

In the present study, a fine ultrastructural localization of nicotinic acetylcholine receptor (nAChR) was attempted, using d-tubocurarine (d-TC), a quaternary ammonium compound binding to nAChR. The localization was based on the binding avidity of immunoglobulin G (IgG) for acetylcholine (ACh) and other quaternary ammonium compounds, such as d-TC. d-TC was applied to the frog neuromuscular preparation and caused a blockade of neuromuscular transmission. Then, d-TC was rendered insoluble in situ by silicotungstic acid (STA), a precipitating agent of soluble proteins and quaternary ammonium compounds. After tissue fixation, a normal rabbit serum was applied to the fine precipitate of the insoluble salt of d-TC silicotungstate (quaternary ammonium radical of d-TC) to form the immunochemical complex d-TC- rabbit IgG at ACh binding sites. The IgG of the complex was revealed by means of the conventional immunoperoxidase procedure used for ultrastructural localization. Under the electron microscope, fine diaminobenzidine (DAB) precipitates appeared as regular rod-like structures oriented to cytoplasmic side of the horizontal part (crest) of the postsynaptic membrane (between the junctional folds) which is known to be endowed with nAChR. The rod-like precipitates were not observed in the postsynaptic junctional folds which are devoid of nAChR. The distance separating the rods each other was rather constant (12 - 15 nm), while the length of the rods was variable and exceeded the usual length of nAChR. The present work indicates that the rod-like structures, already observed in association with sarcoplasmic side of the postsynaptic membrane, did correspond to the intramembranous and intracytoplasmic part of nAChR and related proteins. These cytochemical results confirm that d-TC binds to ACh binding sites in the pore of nAChR, and raise the question of DAB staining of cytoskeletal proteins related to the nAChR complex.

About the morphological relationships of the sarcoplasmic reticulum in the sole plate area of the frog

In the present investigation the sole plate area of motor end plates of the frog is ultrastructurally examined with different postfixation methods. We concentrated in this case on the proof of the smooth and rough sarcoplasmic reticulum of the sole plate. The relations of the smooth and rough sarcoplasmic reticulum to subsynaptic folds and the local T-system and its connections to diads and triads in the sole plate area are represented. The morphological differences between mammal and frog are pointed out. The possible functions of the sarcoplasmic reticulum in the myofibril-free sarcoplasm are discussed.

Syne-1, a dystrophin- and Klarsicht-related protein associated with synaptic nuclei at the neuromuscular junction

We describe a novel protein, Syne-1, that is associated with nuclear envelopes in skeletal, cardiac, and smooth muscle cells. Syne-1 contains multiple spectrin repeats similar to those found in dystrophin and utrophin, as well as a domain homologous to the carboxyl-terminal of Klarsicht, a protein associated with nuclei and required for a subset of nuclear migrations in Drosophila. In adult skeletal muscle fibers, levels of Syne-1 are highest in the nuclei that lie beneath the postsynaptic membrane at the neuromuscular junction. These nuclei are transcriptionally specialized, expressing genes for synaptic components at higher levels than extrasynaptic nuclei in the same cytoplasm. Syne-1 is the first protein found to be selectively associated with synaptic nuclei. Syne-1 becomes concentrated in synaptic nuclei postnatally. It remains synaptically enriched following denervation or degeneration/regeneration, and is also present at high levels in the central nuclei of dystrophic myotubes. The location and structure of Syne-1 suggest that it may participate in the migration of myonuclei in myotubes and/or their anchoring at the postsynaptic apparatus. Finally, we identify a homologous gene, syne-2, that is expressed in an overlapping but distinct pattern.

Nucleus-specific translation and assembly of acetylcholinesterase in multinucleated muscle cells

Multinucleated skeletal muscle fibers synthesize cell surface and secreted oligomeric forms of acetylcholinesterase (AChE) that accumulate at specialized locations on the cell surface, such as sites of nerve-muscle contact. Using allelic variants of the AChE polypeptide chains as genetic markers, we show that nuclei homozygous for either the alpha or beta alleles residing in chimeric myotubes preferentially translate their AChE mRNAs on their respective ERs. These results indicate that the events of transcription, translation, and assembly of this membrane protein are compartmentalized into nuclear domains in multinucleated cells, and provide the structural basis for the possible localized expression and regulation of synaptic components at the neuromuscular junctions of vertebrate skeletal muscle fibers.

Association of cytoskeletal proteins with newly formed acetylcholine receptor aggregates induced by embryonic brain extract

Aggregates of acetylcholine receptors (AChR) in muscle cell membranes are associated with accumulations of certain cytoskeletal and peripheral membrane proteins. We treated cultured rat myotubes briefly with embryonic brain extract (EBX) to promote AChR aggregation and determined the distribution of several of these proteins at early stages of aggregation. EBX-treated and control cultures were stained with tetramethylrhodamine-alpha-bungarotoxin to identify AChR aggregates and were then frozen and sectioned on a cryostat. These sections were stained with primary antibodies and fluoresceinated secondary antibodies to localize cytoskeletal proteins. The distributions of AChRs and cytoskeletal proteins was examined qualitatively and analyzed by a semiquantitative assay. Qualitatively, the 43K protein had a distribution that was virtually identical to that of AChR in both control and EBX-treated cultures, and it always colocalized with early AChR aggregates. The 58K protein similarly colocalized with early AChR aggregates, but it was also in aggregate-free areas of muscle membrane. The association of vinculin with the aggregates was quantitatively similar to that of the 43K and 58K proteins, but, qualitatively, its distribution did not follow that of the AChR as closely. Like the 58K protein and vinculin, alpha-actinin, filamin, and actin were concentrated in AChR aggregates and were also enriched elsewhere. However, they were less closely associated with the aggregates, both quantitatively and qualitatively. These results show that AChR aggregates induced by EBX tend to be enriched in the same cytoskeletal proteins that are present at the neuromuscular junction in vivo and at AChR clusters formed at sites of cell-substrate adhesion in vitro. Semiquantitative analysis also revealed that the fractional area of the cell surface associated with vinculin, alpha-actinin, and the 58K protein was the same in controls and EBX-treated myotubes, although the area enriched in AChR and the 43K protein increased about three-fold upon EBX treatment. These results suggest that AChR aggregates may form preferentially in membrane regions that are already enriched in these proteins.

Synaptic current between neuromuscular junction folds

Measurements of membrane infoldings of vertebrate subsynaptic membranes were taken to evaluate the possible electrophysiological implications. The shapes of standard interfolds of different neuromuscular junctions were established from micrographs available in the literature. Electrical properties were estimated using published fibre membrane and myoplasm electrical values. Models of synaptic current pathways were designed taking into account the small size of the postsynaptic patch activated by a transmitter quantum. This analysis reveals a resistance "in series" between the ACh-sensitive interfold crest and the remainder of the muscle fibre. The calculated cytoplasmic resistance of an interfold is between 0.2 and 3 Mohms which is in the same range as the fibre DC input resistance. The calculated interfold resistance appears to be dependent on the fibre type, the age and the pathology. Functional roles of junctional folds and dendritic spines are discussed.

Scanning electron microscopic study of the neuromuscular junction of dystrophic mice

The structure of the end-plate regions of normal and dystrophic 3-month-old mice were studied by scanning electron microscopy after the presynaptic terminals were removed by hydrochloric acid treatment. Quantitative analysis revealed that the end-plate area correlated positively with the muscle fiber diameter in both the normal and dystrophic animals. However, the motor end-plate area was significantly smaller in the dystrophic mice. The total length of the primary cleft of an end-plate correlated positively with the end-plate area and with the muscle fiber diameter in both normal and dystrophic mice. However, the total length of the primary cleft of an end-plate was significantly shorter in dystrophic mice, especially in large-diameter muscle fibers. Finally, the end-plate of dystrophic mice was characterized by shorter primary clefts with less branching points. These changes of several morphometric characteristics of the postsynaptic membrane suggest that the functional denervation of the mouse dystrophic neuromuscular junction has a postsynaptic origin.

Molecular events in synaptogenesis: nerve-muscle adhesion and postsynaptic differentiation

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane of newly innervated muscle fibers is one of the earliest events in the development of the vertebrate neuromuscular junction. Here, we describe two hypotheses that can account for AChR clustering in response to innervation. The "trophic factor" hypothesis proposes that the neuron releases a soluble factor that interacts with the muscle cell in a specific manner and that this interaction results in the local accumulation of AChR. The "contact and adhesion" hypothesis proposes that the binding of the nerve to the muscle cell surface is itself sufficient to induce AChR clustering, without the participation of soluble factors. We present a model for the molecular assembly of AChR clusters based on the contact and adhesion hypothesis. The model involves the sequential assembly of three distinct membrane domains. The first domain to form serves to attach microfilaments to the cytoplasmic surface of the muscle cell membrane at sites of muscle-nerve adhesion. The second domain to form is clathrin-coated membrane; it serves as a site of insertion of additional membrane elements, including AChR. Upon insertion of AChR into the cell surface, a membrane skeleton assembles by anchoring itself to the AChR. The skeleton, composed in part of actin and spectrin, binds and immobilizes significant numbers of AChR, thereby forming the third membrane domain of the AChR cluster. We make several predictions that should distinguish this model of AChR clustering from one that invokes soluble, trophic factors.

300-kD subsynaptic protein copurifies with acetylcholine receptor-rich membranes and is concentrated at neuromuscular synapses

Acetylcholine receptor-rich membranes from the electric organ of Torpedo californica are enriched in the four different subunits of the acetylcholine receptor and in two peripheral membrane proteins at 43 and 300 kD. We produced monoclonal antibodies against the 300-kD protein and have used these antibodies to determine the location of the protein, both in the electric organ and in skeletal muscle. Antibodies to the 300-kD protein were characterized by Western blots, binding assays to isolated membranes, and immunofluorescence on tissue. In Torpedo electric organ, antibodies to the 300-kD protein stain only the innervated face of the electrocytes. The 300-kD protein is on the intracellular surface of the postsynaptic membrane, since antibodies to the 300-kD protein bind more efficiently to saponin-permeabilized, right side out membranes than to intact membranes. Some antibodies against the Torpedo 300-kD protein cross-react with amphibian and mammalian neuromuscular synapses, and the cross-reacting protein is also highly concentrated on the intracellular surface of the post-synaptic membrane.

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)

Ultracytochemical localization of acetylcholine-like cations in excited motor end-plates by means of ionic fixation

Using rapid ionic fixation with molybdic or tungstic heteropolyanions (strong precipitating agents of quaternary ammonium cations such as choline and acetylcholine), acetylcholine-like cations were localized as point-like precipitates in the synaptic vesicles of resting (electrically nonstimulated) motor nerve terminals. When performed at low temperature, the same procedure revealed spot-like precipitates (presumed to be exocytotically released acetylcholine-like cations) in the synaptic cleft in the vicinity of the active zone. These precipitates were often seen in paired forms. Unlike resting motor-nerve terminals, excited terminals (electrical stimulation with occasional 4-aminopyridine pretreatment) after ionic fixation exhibited, at first, laminar precipitates both in the vicinity of the active zone inside the nerve terminals and in the synaptic space. In the vicinity of the active zone, the laminar precipitates were directed towards the synaptic membrane, while those in the synaptic space showed no orientation. Ionic fixation also revealed diffused precipitates both around the synaptic vesicles and on the axoplasmic side of the presynaptic membrane. Finally, the same fixation procedure demonstrated the presence of empty synaptic vesicles (without point-like precipitates) in close contact with the presynaptic membrane. The laminar and diffused precipitates are presumed to be two different forms of the same salts of acetylcholine-like cations that are insolubilized by ionic fixation in both the nerve terminals and the synaptic space of excited motor end-plates.

Acetylcholine receptor: an allosteric protein

The nicotine receptor for the neurotransmitter acetylcholine is an allosteric protein composed of four different subunits assembled in a transmembrane pentamer alpha 2 beta gamma delta. The protein carries two acetylcholine sites at the level of the alpha subunits and contains the ion channel. The complete sequence of the four subunits is known. The membrane-bound protein undergoes conformational transitions that regulate the opening of the ion channel and are affected by various categories of pharmacologically active ligands.

Structure and distribution of neuromuscular junctions on slow muscle fibers in the frog

Neuromuscular junctions on slow fibers in frog cruralis muscle have been examined with light and electron microscopy. The slow fiber bundle of this muscle is approximately half slow fibers and half fast twitch fibers. Each slow fiber has two-five clusters of nerve terminals in its central region. In contrast, fast fibers in the same bundle have single anastomosing neuromuscular junctions. The average length of terminal clusters on slow fibers is half that of the terminals on fast fibers. Less cholinesterase activity is associated with nerve terminals on slow muscle fibers. The two types of muscle fiber were identified in freeze-fracture replicas by characteristic patterns of the sarcolemmal caveolae and square arrays. Presynaptic membranes of terminals on fast fibers have long, paired double rows of intramembrane particles which lie along the sides of ridges aligned above each fold in the muscle. Ridges are less prominent or absent in terminals on slow fibers and the associated membrane particles are more often in single rows. The lengths of the particle rows also tend to be shorter and rows tend to branch and to lie at various orientations with respect to the longitudinal axis of the terminal. The average length of the particle rows per unit length of nerve terminal on slow fiber terminals is half that of particle rows on fast fiber terminals. It is concluded that the total length as well as the concentration of these active zone specializations is less at terminals on slow fibers. The lack of parallel orientation of the presynaptic active zones at slow fiber terminals corresponds to a lack of postsynaptic folds. Postsynaptic specializations seen in thin sections through slow fibers are either on flat expanses of sarcolemma or on low sarcolemmal bulges. Patches of large intramembrane particles, similar to those on fast fibers, mark regions of postsynaptic specialization in freeze-fracture replicas of slow fibers.

Internal and external differentiations of the postsynaptic membrane at the neuromuscular junction

Frog, snake and rat neuromuscular junctions were prepared for electron microscopy by the quick-freeze, deep-etch, rotary replication procedure. The postsynaptic membrane was exposed by treating muscles with 1 mg/ml collagenase to remove the basal lamina. Present on the apices of the postsynaptic folds are regular arrays of 8-9 nm protrusions. These are not seen in the depths of the folds nor elsewhere on the muscle surface, thus they presumably represent the heads of cholinergic receptor molecules. These protrusions tend to be arranged in parallel rows two-abreast. Their high concentration (10 000/microns2) and their orderly arrangement is basically similar to the receptors seen in Torpedo postsynaptic membrane. Their distribution did not appear to change after denervation. Efforts were made to expose possible anchoring structures of these receptors, by treating muscles with 0.1% Saponin immediately before and/or during fixation in 1% formaldehyde, or by homogenizing muscles after brief formaldehyde fixation. This washed most soluble protein out of the cytoplasm and exposed a submembraneous meshwork just beneath the postsynaptic membrane. This meshwork appears to connect the membrane to underlying bundles of intermediate filaments which course through the postsynaptic processes that border each fold. This meshwork is presumably equivalent to the postsynaptic 'density' seen in thin sections. Its three-dimensional structure suggests that it could anchor receptor molecules to underlying cytoskeletal elements and thus immobilize receptors in the plane of the postsynaptic membrane.