Studies of excitable membranes. I. Macromolecular specializations of the neuromuscular junction and the nonjunctional sarcolemma

Identification of Intramembrane Particles by Pre- and Post-Fracture Labelling Techniques: A Progress Report

Conventional freeze-fracture replicas provide for high resolution, three-dimensional replication of the internal macromolecular architecture of cell membranes, but they have the distinct disadvantage that no biologically rele¬vant macromolecules remain for subsequent in situ labelling and positive identification. Recently, several ingenious freeze-fracture techniques have been devised to localize biochemical components before the tissues are diges-ted away. These methods include (a) freeze-fracture autoradiography of the frozen whole mount preparation (1, 2) and (b) deep etching (freeze-drying) to expose immunoglobins or other large protein markers attached to membrane surfaces (3). However, freeze-fracture autoradiography has insufficient resolution to permit localization of labels to individual intramembrane particles (IMPs) exposed by the cleaving process, while deep etching to reveal membrane surfaces does not reveal IMPs. To overcome some of these difficulties, we recently introduced the "sectioned replica technique" to provide a method for direct and unambiguous correlation of freeze-fracture and thin section images within a single electron micrograph (4). The fractured and replicated tissues are thawed, but instead of digesting the tissues, the replica-tissue sandwich is post-fixed in 30% buffered glycerol solution con-taining 1% OsO4, post-stained, embedded in plastic, and sectioned parallel to the replica-tissue interface. Using this sectioned-replica technique, we have been able to obtain (a) pre-fracture radioisotope-labelled mouse neuromuscular junctions and (b) pre- and post-fracture immunoferritin and colloidal gold-labelled membrane fragments and intact neuromuscular junctions.

Active zone structure-function relationships at the neuromuscular junction

The impact of presynaptic transmitter release site organization on synaptic function has been a vibrant area of research for synaptic physiologists. Because there is a highly nonlinear relationship between presynaptic calcium influx and subsequent neurotransmitter release at synapses, the organization and density of calcium sources (voltage-gated calcium channels [VGCCs]) relative to calcium sensors located on synaptic vesicles is predicted to play a major role in shaping the dynamics of neurotransmitter release at a synapse. Here we review the history of structure-function studies within transmitter release sites at the neuromuscular junction across three model preparations in an effort to discern the relationship between VGCC organization and synaptic function, and whether that organizational structure imparts evolutionary advantages for each species.

Presynaptic mechanisms controlling calcium-triggered transmitter release at the neuromuscular junction

Calcium-triggered neurotransmission underlies most communication in the nervous system. Yet, despite the conserved and essential nature of this process, the molecular underpinnings of calcium-triggered neurotransmission have been difficult to study directly and our understanding to this date remains incomplete. Here we frame more recent efforts to understand this process with a historical perspective of the study of neurotransmitter release at the neuromuscular junction. We focus on the role of calcium channel distribution and organization relative to synaptic vesicles, as well as the nature of the calcium sensors that trigger release. Importantly, we provide a framework for understanding how the function of neurotransmitter release sites, or active zones, contributes to the function of the synapse as a whole.

Freeze fracture: new avenues for the ultrastructural analysis of cells in vitro

The ultrastructural analysis of biological membranes by freeze fracture has a 60-year tradition. In this review, we summarize the benefits of the freeze-fracture technique and review special structures analyzed by freeze fracture and by combined freeze-fracture replica immunogold labeling (FRIL) of cell cultures. In principle, every cellular membrane whether of cell suspensions, mono- or bilayers of cell cultures can be analyzed in freeze fracture. The combination of freeze fracture and immunogold labeling of the replica allows the ultrastructural identification of protein assemblies in combination with the molecular identification of their constituent proteins using specific antibodies. The analysis of fractured and labeled intramembrane particles enables determination of the arrangement and organization of proteins within the membrane due to the high resolution of the transmission electron microscope. Because of cell-specific ultrastructural features such as square arrays, identification of cell types can be performed in parallel. This review is aimed at presenting the possibilities of freeze fracture and FRIL in the high-resolution ultrastructural analysis of membrane proteins and their assembly in naïve, transfected or otherwise treated cultured cells. At the interface of molecular approaches and morphology, the application of FRIL in genetically modified cells provides a novel and intriguing aspect for their analysis.

Mechanisms underlying AQP4 accumulation in astrocyte endfeet

The brain-blood interface holds the key to our understanding of how cerebral blood flow is regulated and how water and solutes are exchanged between blood and brain. The highly specialized astrocytic membranes that enwrap brain microvessels are salient constituents of the brain-blood interface. These endfoot membranes contain a distinct set of molecules that is anchored to the subendothelial basal lamina forming an endfoot-basal lamina junctional complex. Here we explore the mechanisms underpinning the formation of this complex. By use of a tailor made model system we show that endothelial cells promote AQP4 accumulation by exerting an inductive effect through extracellular matrix components such as agrin, as well as through a direct mechanical interaction with the endfoot processes. Through the compounds they secrete, the endothelial cells also increase AQP4 expression. The present data suggest that the highly specialized gliovascular interface is established through inductive processes that include both chemical and mechanical factors. GLIA 2015;63:2073-2091.

Three-dimensional reconstruction of skeletal muscle extracellular matrix ultrastructure

The skeletal muscle extracellular matrix (ECM) supports muscle's passive mechanical function and provides a unique environment for extracellular tissues such as nerves, blood vessels, and a cadre of mononuclear cells. Within muscle ECM, collagen is thought to be the primary load-bearing protein, yet its structure and organization with respect to muscle fibers, tendon, and mononuclear cells is unknown. Detailed examination of extracellular collagen morphology requires high-resolution electron microscopy performed over relatively long distances because multinucleated muscle cells are very long and extend from several millimeters to several centimeters. Unfortunately, there is no tool currently available for high resolution ECM analysis that extends over such distances relevant to muscle fibers. Serial block face scanning electron microscopy is reported here to examine skeletal muscle ECM ultrastructure over hundreds of microns. Ruthenium red staining was implemented to enhance contrast and utilization of variable pressure imaging reduced electron charging artifacts, allowing continuous imaging over a large ECM volume. This approach revealed previously unappreciated perimysial collagen structures that were reconstructed via both manual and semi-automated segmentation methods. Perimysial collagen structures in the ECM may provide a target for clinical therapies aimed at reducing skeletal muscle fibrosis and stiffness.

Active zones and the readily releasable pool of synaptic vesicles at the neuromuscular junction of the mouse

Synchronous neurotransmitter release is a highly regulated process that takes place at specializations at the presynaptic membrane called active zones (AZs). The relationships between AZs, quantal release, and vesicle replenishment are not well understood in a mature synapse. We have measured the number, distribution, and other properties of AZs in mouse motor nerve terminals and combined these observations with electrophysiological estimates of the size of the readily releasable pool (RRP) of synaptic vesicles. On average, we counted 850 AZs per terminal. Assuming two primary docked vesicles per AZ, we predict a total of ∼1700 vesicles optimally positioned for exocytosis. Electrophysiological estimates of the size of the RRP, using a simple kinetic model that assumes exponential depletion of the initial pool and refilling by recruitment, gave an average value of 1730 quanta during 100 Hz stimulation, in satisfying agreement with the morphology. At lower stimulus frequencies, however, the model revealed that the estimated RRP size is smaller, suggesting that not all AZs participate in release at low stimulation frequencies.

Structure and functions of aquaporin-4-based orthogonal arrays of particles

Orthogonal arrays or assemblies of intramembranous particles (OAPs) are structures in the membrane of diverse cells which were initially discovered by means of the freeze-fracturing technique. This technique, developed in the 1960s, was important for the acceptance of the fluid mosaic model of the biological membrane. OAPs were first described in liver cells, and then in parietal cells of the stomach, and most importantly, in the astrocytes of the brain. Since the discovery of the structure of OAPs and the identification of OAPs as the morphological equivalent of the water channel protein aquaporin-4 (AQP4) in the 1990s, a plethora of morphological work on OAPs in different cells was published. Now, we feel a need to balance new and old data on OAPs and AQP4 to elucidate the interrelationship of both structures and molecules. In this review, the identity of OAPs as AQP4-based structures in a diversity of cells will be described. At the same time, arguments are offered that under pathological or experimental circumstances, AQP4 can also be expressed in a non-OAP form. Thus, we attempt to project classical work on OAPs onto the molecular biology of AQP4. In particular, astrocytes and glioma cells will play the major part in this review, not only due to our own work but also due to the fact that most studies on structure and function of AQP4 were done in the nervous system.

Aquaporin expression in normal and pathological skeletal muscles: a brief review with focus on AQP4

Freeze-fracture electron microscopy enabled us to observe the molecular architecture of the biological membranes. We were studying the myofiber plasma membranes of health and disease by using this technique and were interested in the special assembly called orthogonal arrays (OAs). OAs were present in normal myofiber plasma membranes and were especially numerous in fast twitch type 2 myofibers; while OAs were lost from sarcolemmal plasma membranes of severely affected muscles with dystrophinopathy and dysferlinopathy but not with caveolinopathy. In the mid nineties of the last century, the OAs turned out to be a water channel named aquaporin 4 (AQP4). Since this discovery, several groups of investigators have been studying AQP4 expression in diseased muscles. This review summarizes the papers which describe the expression of OAs, AQP4, and other AQPs at the sarcolemma of healthy and diseased muscle and discusses the possible role of AQPs, especially that of AQP4, in normal and pathological skeletal muscles.

Macromolecular connections of active zone material to docked synaptic vesicles and presynaptic membrane at neuromuscular junctions of mouse

Electron tomography was used to view macromolecules composing active zone material (AZM) in axon terminals at mouse neuromuscular junctions. Connections of the macromolecules to each other, to calcium channels in the presynaptic membrane, and to synaptic vesicles docked on the membrane prior to fusing with it during synaptic transmission were similar to those of AZM macromolecules at frog neuromuscular junctions previously examined by electron tomography and support the hypothesis that AZM regulates vesicle docking and fusion. A species difference in the arrangement of AZM relative to docked vesicles may help account for a greater vesicle-presynaptic membrane contact area during docking and a greater probability of fusion during synaptic transmission in mouse. Certain AZM macromolecules in mouse were connected to synaptic vesicles contacting the presynaptic membrane at sites where fusion does not occur. These secondary docked vesicles had a different relationship to the membrane and AZM macromolecules than primary docked vesicles, consistent with their having a different AZM-regulated behavior.

Aquaporin 4 mRNA levels in neuromuscular tissues of wild-type and dystrophin-deficient mice

Aquaporin (AQP) 4 is a water-specific channel protein and is abundant in central nervous tissues and skeletal muscles. Recently, the AQP4 molecule has been increasingly highlighted in its pathophysiological role of several neurological diseases, such as stroke, muscular dystrophy and neuromyelitis optica. We therefore measured the levels of AQP4 mRNA and glyceraldehyde-3 phosphate dehydrogenase mRNA (an internal control) in muscle and brain tissues of wild-type mice (C57BL10/ScSn) and age-matched dystrophin-deficient mdx mice (C57BL10/ScSn mdx) by real-time quantitative RT-PCR. The relative AQP4 mRNA level was highest in the spinal cord among the neuromuscular tissues examined in wild-type mice. Among the muscle tissues of wild-type mice, the relative AQP4 mRNA level was higher in extensor digitorum longus (EDL) muscles, and its descending order was EDL, quadriceps femoris, soleus and heart muscles. It is noteworthy that there was no difference in the relative AQP4 mRNA levels in the brain tissues between wild-type mice and age-matched mdx mice. In contrast, the AQP4 mRNA level in the quadriceps femoris muscle was significantly lower in mdx mice than in wild-type mice. The fact that the spinal cord contains the highest AQP4 mRNA may be related to the pathogenesis of neuromyelitis optica, in which AQP4 protein is the target antigen. In addition, the low expression level of AQP4 mRNA in the mdx mouse muscle suggests a functional link between AQP4 and dystrophin in the muscle tissue. We suggest that a similar pathomechanism may underlie the phenotypic consequences of the mdx mouse and Duchenne muscular dystrophy.

Mechanisms of acetylcholine receptor loss from the neuromuscular junction

At the normal mammalian neuromuscular junction the half-life of the acetylcholine receptor (AChR) ranges from 6 to 13 days (estimates from seven different laboratories). Indirect evidence suggests that the internalized receptor is degraded by a lysosomal mechanism. We have now traced the fate of the AChR labelled in vivo with peroxidase-alpha-bungarotoxin. Segments of junctional folds bearing AChRs are internalized by endocytosis. The endocytosed vesicles are engulfed by tubules and larger vesicles which, by electron cytochemical criteria, represent secondary lysosomes. Pathological mechanisms increased AChR loss from the end-plate. These include destruction of junctional folds, formation of immature junctions with a few or no junctional folds, accelerated internalization of AChR, impaired membrane insertion of new AChR and, possibly decreased AChR synthesis. The common mechanism for destruction of the junctional folds is an altered subsynaptic ionic milieu, and especially focal calcium excess. This can be induced by antibody and complement, too frequent or prolonged openings of the acetylcholine (ACh)-induced ion channel, and other membrane defects. In acquired autoimmune myasthenia gravis there is (a) antibody-dependent complement-mediated lysis of the junctional folds, (b) accelerated internalization of AChR cross-linked by antibody and (c) decreased insertion of AChR into the postsynaptic membrane. The last mechanism is attributed to lack of membrane patches available for tight packing and secure anchoring of the receptor. In acute, but not in chronic, experimental autoimmune myasthenia gravis, and infrequently in human myasthenia gravis, macrophages destroy junctional folds opsonized by antibody and C3. In a recently recognized congenital syndrome attributed to a prolonged open time of the ACh-induced ion channel, and to a lesser extent in congenital end-plate acetylcholinesterase deficiency, AChR is lost with degradation of junctional folds. In other, less well-defined, congenital syndromes there is deficiency or abnormal function of AChR. This could arise from decreased synthesis or membrane insertion or accelerated degradation of AChR, or from a structurally abnormal AChR with reduced affinity for ACh or with a diminished conductance or open time of its ion channel.

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.

Freeze-fracture and immunogold analysis of aquaporin-4 (AQP4) square arrays, with models of AQP4 lattice assembly

Each day, approximately 0.5-0.9 l of water diffuses through (primarily) aquaporin-1 (AQP1) channels in the human choroid plexus, into the cerebrospinal fluid of the brain ventricles and spinal cord central canal, through the ependymal cell lining, and into the parenchyma of the CNS. Additional water is also derived from metabolism of glucose within the CNS parenchyma. To maintain osmotic homeostasis, an equivalent amount of water exits the CNS parenchyma by diffusion into interstitial capillaries and into the subarachnoid space that surrounds the brain and spinal cord. Most of that efflux is through AQP4 water channels concentrated in astrocyte endfeet that surround capillaries and form the glia limitans. This report extends the ultrastructural and immunocytochemical characterizations of the crystalline aggregates of intramembrane proteins that comprise the AQP4 "square arrays" of astrocyte and ependymocyte plasma membranes. We elaborate on recent demonstrations in Chinese hamster ovary cells of the effects on AQP4 array assembly resulting from separate vs. combined expression of M1 and M23 AQP4, which are two alternatively spliced variants of the AQP4 gene. Using improved shadowing methods, we demonstrate sub-molecular cross-bridges that link the constituent intramembrane particles (IMPs) into regular square lattices of AQP4 arrays. We show that the AQP4 core particle is 4.5 nm in diameter, which appears to be too small to accommodate four monomeric proteins in a tetrameric IMP. Several structural models are considered that incorporate freeze-fracture data for submolecular "cross-bridges" linking IMPs into the classical square lattices that characterize, in particular, naturally occurring AQP4.

Effects of exposure to low-dose pyridostigmine on neuromuscular junctions in vitro

During the Persian Gulf War, pyridostigmine bromide (PB), a reversible inhibitor of acetylcholinesterase, was used as prophylaxis against exposure to nerve gas. Exposure to PB has been suggested as a potential cause of the persistent fatigue reported among Gulf War veterans. The aim of this study was to evaluate the effects of acute and continuous exposure to low doses of PB on the neuromuscular junction. Organotypic spinal cord-muscle cocultures were used to examine in vitro the effects of PB under controlled conditions. Acute exposure to PB potentiated neuromuscular activity. Continuous exposure to PB produced a progressive decrease in the contractile activity of muscle fibers. Ultrastructural examination by electron microscopy revealed no abnormalities in the neuromuscular junctions after 1 week of exposure. Nerve terminal degeneration and atrophy of the postjunctional folds were evident after 2-week exposure to low-dose PB. The effects of PB were reversible following withdrawal. The reversibility of the PB-induced changes in vitro suggests that such changes are causally unrelated to the fatigue reported by Persian Gulf War veterans years after exposure to PB.

Immunocytochemical studies of aquaporin 4 in the skeletal muscle of mdx mouse

Immunostainability of anti aquaporin 4 antiserum was investigated in the muscles of dystrophin deficient mdx mice. Western blot analysis showed that the rabbit antiserum against aquaporin 4 reacted with a 28 kDa protein in extracts of normal mouse quadriceps femoris muscles but did not react with the protein in extracts of quadriceps femoris muscles of mdx mice. Immunoperoxidase staining of the muscles from normal and mdx mice revealed the positive immunoreaction at the myofiber surface of normal mice and the negative, or the faint and discontinuous immunostaining at the surface of mdx myofibers. Immunogold electron microscopy disclosed the localization of aquaporin 4 molecules at the myofiber plasma membranes of normal mice and the localization was consistent with that of orthogonal array particles in the protoplasmic face of normal muscle plasma membrane seen in freeze fracture replicas. This study demonstrated that the density of aquaporin 4 molecules was decreased in the muscle plasma membranes of mdx mice, resulting in the faulty function of mdx myofibers.

Intramembrane structure of the sensory axon terminals in bullfrog muscle spindles

Much physiologic and morphologic research has been done into the sensory mechanism of the frog muscle spindle. However, no freeze-fracture study has described in detail the shape and intramembrane structure of the nonmyelinated sensory axon terminals of the frog muscle spindle. In this study, muscle spindles were isolated from the red part of bullfrog semitendinous muscles. Chemically fixed spindles were subjected to freeze fracturing. The sensory axon endings were reconstructed, and the size and density of intramembrane particles (IMPs) were measured along the sensory nerve endings. The axon terminals had four distinctive parts: parent trunks (>0.5 microm in diameter), primary branches (0.15-0.5 microm), terminal branches (<0.1 pm), and varicosities (0.02-0.5 microm). IMPs ranged from 5 nm to 21 nm in diameter and were present in the intramembrane space of the plasma membrane all throughout the nonmyelinated sensory nerve endings. Mean IMP sizes in the protoplasmic face (PF) and the external face (EF), respectively, were 8.1 nm and 8.4 nm in the parent trunks, 8.8 nm and 8.8 nm in the primary branches, 9.4 nm and 9.0 nm in the varicosities, and 8.7 nm and 8.7 nm in the terminal branches. Mean IMP size in the PF was smallest in the parent trunk and largest in the varicosity. Mean IMP densities (numbers of IMPs per microm2) in the PF and the EF, respectively, were 2,500 and 700 in the parent trunks, 2,200 and 500 in the primary branches, 1,700 and 400 in the varicosities, and 1,000 and 300 in the terminal branches. Density decreased with the tapering of the axon terminal, with IMPs distributed evenly in the PF and the EF. The characteristic intramembrane structure of sensory nerve endings is discussed.

Direct immunogold labeling of aquaporin-4 in square arrays of astrocyte and ependymocyte plasma membranes in rat brain and spinal cord

Aquaporin (AQP) water channels are abundant in the brain and spinal cord, where AQP1 and AQP4 are believed to play major roles in water metabolism and osmoregulation. Immunocytochemical analysis of the brain recently revealed that AQP4 has a highly polarized distribution, with marked expression in astrocyte end-feet that surround capillaries and form the glia limitans; however, the structural organization of AQP4 has remained unknown. In freeze-fracture replicas, astrocyte end-feet contain abundant square arrays of intramembrane particles that parallel the distribution of AQP4. To determine whether astrocyte and ependymocyte square arrays contain AQP4, we employed immunogold labeling of SDS-washed freeze-fracture replicas and stereoscopic confirmation of tissue binding. Antibodies to AQP4 directly labeled approximately 33% of square arrays in astrocyte and ependymocyte plasma membranes in rat brain and spinal cord. Overall, 84% of labels were present beneath square arrays; 11% were beneath particle clusters that resembled square arrays that had been altered during fixation or cleaving; and 5% were beneath the much larger areas of glial plasma membrane that were devoid of square arrays. Based on this evidence that AQP4 is concentrated in glial square arrays, freeze-fracture methods may now provide biophysical insights regarding neuropathological states in which abnormal fluid shifts are accompanied by alterations in the aggregation state or the molecular architecture of square arrays.

Grid-mapped freeze-fracture analysis of gap junctions in gray and white matter of adult rat central nervous system, with evidence for a "panglial syncytium" that is not coupled to neurons

In white matter regions of the brain and spinal cord of adult mammals, gap junctions previously were observed linking astrocytes to astrocytes, as well as to oligodendrocytes and ependymacytes. The resulting "functional syncytium" was proposed to modulate the ion fluxes that occur during electrical activity of the associated axons. Gap junctions also have been reported linking neurons with glia, and functional neuronal-glial coupling has been postulated. To investigate the glial syncytium and the neuron-to-glial coupling hypotheses, we used "grid-mapped freeze fracture," conventional thin-section electron microscopy, and light microscope immunocytochemistry to examine and characterize neurons and glia in gray and white matter of adult rat brain and spinal cord. We have obtained quantitative evidence for the abundance and widespread distribution of gap junctions interlinking the three primary types of macroglia throughout both gray and white matter of the mammalian central nervous system (CNS), thereby extending the concept to that of a functional panglial syncytium. In contrast to previous reports, we show that of more than 400 gap junctions in which both participating cells were identified, none were between neurons and glia. Thus, neuronal coupling and glial coupling involved separate and distinct pathways. Finally, putative water channels (i.e., "square arrays") were confirmed to be abundant and in close association with gap junctions in astrocytes and ependymacytes. Because the astrocyte "intermediaries" extend cytoplasmic conduits throughout gray and white matter of brain and spinal cord, from the ependymal layer to the pia-glial limitans, and from oligodendrocytes surrounding axons to astrocyte endfeet surrounding capillaries, the proposed panglial syncytium, with its abundance of water channels and intercellular ion channels, is optimally positioned and equipped to modulate water and ion fluxes across broad regions of the CNS.

Freeze-fracture analysis of muscle plasma membrane in Becker's muscular dystrophy

The intramembranous particle (IMP), orthogonal array (OA) and orthogonal array subunit particle (OASP) densities in skeletal muscle plasma membranes from eight patients with Becker's muscular dystrophy (BMD) were analysed by the freeze-fracture technique. The results showed almost normal IMP density with the significant decrease of OA and OASP densities in BMD. The group mean densities +/- SE of IMPs on the protoplasmic faces with and without OASPs, and on extracellular faces/microns 2 were 2137 +/- 207, 1839 +/- 68 and 895 +/- 108, respectively in controls; whereas those of BMD were 1989 +/- 259, 1837 +/- 203 and 900 +/- 239, respectively (P > 0.1 by two-tailed t-test). The group median density of OAs and their pits/microns 2 was 4.89 with mid-ranges (25-75% values of the counts) of 2.66-10.18 in controls; whereas that in BMD was 2.15 with mid-ranges of 1.14-4.31 (P < 0.01 by Wilcoxon rank-sum test). The group mean density +/- SE of OASPs in controls was 15.99 +/- 1.83; whereas that in BMD was 13.47 +/- 1.07 (P < 0.01 by two-tailed t-test). However, the diminution of OA and OASP densities in BMD muscle plasma membranes was not as severe as in Duchenne's muscular dystrophy. There was a relationship between OA density and clinical severity in BMD patients; the decrease of OA density in a severe BMD patient was more marked than that in mildly affected BMD patients. Therefore, it seems that marked depletion of OA density may lead to the severe disability in muscular dystrophies.

Clustered acetylcholine receptors have two levels of organization in Xenopus muscle cells

We studied the organization of acetylcholine receptor (AChR) clusters by shearing cultured Xenopus muscle cells with a stream of buffer, and preparing rotary replicas of the exposed cytoplasmic surface of the sarcolemma. AChR clusters contained numerous particles that protruded from the sarcolemma and formed an irregular array composed of discrete aggregates. AChR were located within these particle aggregates, as shown by comparison of the replicas to labeling by fluorescent alpha-bungarotoxin, and by immunogold cytochemistry with antibodies specific for the receptor. The aggregates were cross-linked by a dense network of 7 nm filaments that replicated with the banded pattern characteristic of actin microfilaments. The organization of receptors into the small aggregates was independent of the organization of these aggregates into clusters, as alkaline extraction removed the microfilament network and disrupted the irregular array of particle aggregates, but did not disperse individual receptors from the aggregates. We conclude that two levels of interactions organize AChR clusters in Xenopus muscle cells: short-range interactions that assemble individual AChR into small aggregates, and long-range interactions, perhaps mediated by actin microfilaments, that anchor the aggregates into larger clusters.

Effects of a benzodiazepine on the muscle membrane architecture of the rat diaphragm. A freeze-fracture study

1. Midazolam increases, stimulation-independently, the amount of intermembraneous particles on the sarcolemma of the muscle fibres of the diaphragm. 2. Midazolam does not affect the amount of orthogonal arrays of particles on the sarcolemma. 3. Possible mechanisms for the action of midazolam are discussed.

Effect of denervation on regenerating muscle plasma membrane integrity: freeze-fracture and dystrophin immunostaining analyses

We investigated time sequentially, the densities of intramembranous assembly "orthogonal arrays" of regenerating rat extensor digitorum longus muscle after bupivacaine-induced muscle injury. There was no evidence of orthogonal arrays at the early stage, but the densities of orthogonal arrays increased with the maturation of the innervated regenerating myofibers. In contrast, the orthogonal arrays were scarcely observed at any time point examined in denervated regenerating muscles. Therefore, the neural factor may have an important effect on the appearance of orthogonal arrays. Moreover, we studied the immunostainability of Duchenne muscular dystrophy gene product "dystrophin" in the regenerating muscle at the same time points. Positive immunostaining was observed in both innervated and denervated regenerating myofibers even from the early stage of regeneration. On the basis of these data, the relationship between orthogonal array and dystrophin are discussed.

Glycoproteins of rat skeletal muscle sarcolemma: characterization by two-dimensional gel electrophoresis and effect of denervation

Sarcolemmal membrane glycoproteins from rat mixed, fast, and slow muscles were characterized by concanavalin A (ConA) binding following two-dimensional polyacrylamide gel electrophoresis (PAGE). Analysis of electrophoretic profiles revealed that although sarcolemmal membranes prepared from these muscle types contained common glycoprotein species, each had a distinct glycoprotein pattern. In sarcolemma from mixed muscle, three major classes of ConA binding glycoproteins could be distinguished: (i) an acidic species of 110,000-120,000 Da, pI 5.0 to 5.3 (CG-1, ConA binding glycoproteins-1); (ii) a group of highly charged isomers, ranging from 75,000 to 80,000 Da pI 5.2 to 8.2 (CG-2); and (iii) a group of charged isomers of predominantly acidic nature of approximately 50,000 Da pI 5.2 to 5.8 (CG-3). ConA bound exclusively to CG-1 in sarcolemma from a fast muscle (extensor digitorum longus muscle, EDL). In soleus muscle sarcolemma (slow fiber type) both CG-1 and CG-3 were readily detected but CG-2 was markedly diminished. ConA binding to slow muscle sarcolemma revealed as well a glycoprotein species of 66,000-70,000 Da, pI 4.3-5.1 (CG-4), which was unique to this fiber type and as such may be a specific marker for slow fiber type. Denervation had no significant effect on the properties of ConA binding to mixed or slow muscle sarcolemma but dramatically altered the ConA binding to fast muscle sarcolemma, specifically increasing binding to CG-2. These findings demonstrate that denervation differentially affects the metabolism of ConA binding glycoproteins in these muscle types.

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.

Serum influences the differentiation of membrane structure in cultured astrocytes

The membranes of mammalian astrocytic processes apposed to blood vessels or forming the surface of the brain contain high concentrations of a characteristic intramembrane particle aggregate, termed "assemblies." In order to identify developmental processes which contribute to this remarkable regional specialization of membrane structure, we have devised culture conditions which support the differentiation of assemblies in secondary cultures of astrocytes derived from neonatal rat forebrain. We report here that different lots of fetal calf serum vary dramatically in their capacity to support the differentiation of assemblies. Fetal calf serum thus appears to exert two distinct influences on astrocyte development: it promotes the differentiation of type 2 astrocytes from bipotential precursor cells, as shown by others, and it influences the density of assemblies in type 1, flat, GFAP-immunoreactive astrocytes in our secondary cultures. Horse serum and defined media also support the appearance of assemblies in flat, GFAP-immunoreactive astrocytes. The separate effects of serum supplementation upon cell lineage and membrane differentiation have to be carefully considered in studies designed to examine factors influencing astrocytic development in vitro.

Counting and measuring IMPs and pits: why accurate counts are exceedingly rare

Particle counting and measuring techniques are now widely used to characterize normal membranes and to identify molecular changes occurring during development, maturation, and aging during progression of disease and following pharmacological manipulation. However, the use of particle counting and measuring for the identification of molecular changes in membranes has been premature. We show that current procedures rarely yield replicas that are free of cryogenic or mechanical prefractures, and as a result, the "complementarity" of membrane faces is severely compromised. However, with simple alterations of procedure, combined with the resolve to recognize and discard images of pre-fractured membrane faces, a high degree of "complementarity" may be obtained. Criteria for recognizing the occurrence and relative frequency of noncomplementarity are presented and a cleaving method for avoiding a primary source of water vapor contamination is described. In such replicas, membrane pits are found in equivalent numbers and near-identical diameters as the intramembrane particles (IMPs) in the complementary-type membrane faces. When conditions of "cold fracture" and immediate replication are demonstrated, fracture faces are minimally contaminated by frozen water vapor, yielding images where 1) diameters of IMPs vs. pits are very nearly identical, 2) large diameter IMPs are very rare, and 3) the numbers of IMPs and pits are increased substantially over the numbers currently reported. Thus, we reiterate previous proposals that complementarity of membrane faces is the single most important criterion that must be met before accepting the validity of IMP counts or for attributing perceived changes in IMP density or size to conditions of experimental manipulation, to normal developmental processes, or to disease etiology.

Dystrophin immunostaining and freeze-fracture studies of muscles of patients with early stage amyotrophic lateral sclerosis and Duchenne muscular dystrophy

We used polyclonal antibodies against dystrophin for the immunohistochemical localization of this protein in human skeletal muscle. Dystrophin was localized in the sarcolemma of the myofibers in 8 infantile and 11 adult normal control muscles and in 10 early stage patient muscles with amyotrophic lateral sclerosis (ALS). The protein was absent or markedly decreased in 8 early stage patients with Duchenne muscular dystrophy (DMD). Moreover the densities of sarcolemmal plasma membrane assemblies, orthogonal arrays and their pits were estimated by freeze-fracture electron microscopy studies in the same number of muscle samples in each disease and control case. The group median densities of orthogonal arrays and their pits in the ALS group and adult control group were 4.8 with a midrange of 1.1-13.5 (25-75%) and 7.5 with a midrange of 2.3-12.9, respectively (P greater than 0.1, Wilcoxon rank-sum test), whereas those of the DMD group and child control group were 0 with a midrange of 0-1.1 and 10.8 with a midrange of 5.4-16.7 respectively (P less than 0.01). The skeletal muscles of mdx mice and their controls were also investigated by the same techniques. In mdx mice, the absence or marked deficiency of dystrophin was also noted; however, the decrease of orthogonal arrays was not as severe as in DMD, which might relate to the milder clinical features in mdx mice as compared with those in DMD.

Quantitation of 'junctional feet' content in two types of muscle fiber from hind limb muscles of the rat

1. Transverse tubules in fibers from rat soleus and extensor digitorum longus (EDL) muscles of the rat were infiltrated with silver dichromate (black reaction of Golgi). This provides a faithful, high-contrast outline of the tubules, which allows distinction between segments involved in junction formation with the sarcoplasmic reticulum and segments that are free. 2. Electron micrographs of semithin transverse sections were used to quantitate T tubule parameters and to measure cross-sectional area and perimeter of individual fibers. Thin sections and data from the literature were used to obtain the contribution of caveolae to external surface area and the frequency of junctional feet along the junctional T tubule membrane. 3. From the above data we calculate the ratio of number of feet to total external surface area for a given fiber segment. The ratio is compared with data in the literature on the total amount of 'charge movement' (in nC/uF of total external surface area). 4. The average feet/surface area ratio is twice as large in EDL than in soleus fibers, while the charge movement is up to five-fold larger. Probably some of the total charge movement is not directly associated with events related to the turning on of the SR permeability to calcium.

Orthogonal arrays of intramembrane particles in the supporting cells of the guinea-pig vestibular sensory epithelium

Membrane specializations of the contact region between afferent nerve endings and supporting cells of the sensory epithelia of guinea-pig vestibular endorgans were examined by thin-section and freeze-fracture electron microscopy. The calyx-type nerve endings (C-endings) are separated from supporting cells (SC) by a 25-30 nm space. At irregular intervals along the upper lateral surface of supporting cells, the intercellular space narrows markedly to form special close contacts between the C-ending and SC plasma membranes. Freeze-fracture replicas reveal membrane specializations--orthogonal arrays of particulate units--in the region where the close intercellular contacts were found in sections. Orthogonal arrays consisting of from 5 to 20 units were observed on the cytoplasmic (P) fracture face of the lateral SC plasma membrane. These particulate units from a 12 x 12-nm square, and each unit is composed of four 6-nm subunits. Possible roles of the orthogonal arrays are discussed.

Membrane and cytoplasmic structure at synaptic junctions in the mammalian central nervous system

Application of rapid freezing, freeze substitution fixation, and freeze fracture techniques to the study of synaptic junctions in the mammalian central nervous system has revealed new aspects of synaptic structure that are consistent with and partially explicate advances in synaptic biochemistry and physiology. In the axoplasm adjacent to the presynaptic active zone, synaptic vesicles are linked to large spectrin-like filamentous proteins by shorter proteins that resemble synapsin I in morphology. This mesh of presynaptic filamentous proteins serves to concentrate synaptic vesicles in the vicinity of the active zone. The affinity with which the vesicles are bound by the mesh is probably modulated by the extent of phosphorylation at specific sites on the constituent filamentous proteins, and changes in the binding affinity result in changes in transmitter release. The structural organization of the postsynaptic density in Purkinje cell dendritic spines consists of very fine strands with adherent, heterogeneous globular proteins. Some of these globular proteins probably correspond to protein kinases and their substrates. The postsynaptic density, positioned at the site of the maximal depolarization caused by synaptic currents, apparently serves as a supporting framework for a variety of proteins, which respond to and transduce postsynaptic depolarization. At least two classes of filamentous protein fill the cytoplasm of spines with a complex mesh, which presumably contributes to maintenance of the spine shape. Membrane bound cisterns are a ubiquitous feature of Purkinje cell dendritic spines. Studies of rapidly frozen tissue with electron probe microanalysis and elemental imaging reveal that these cisterns take up and sequester calcium, which is derived from the extracellular space, and which probably enters the spine as part of the synaptic current.

Structural and functional correlates of synaptic transmission in the vertebrate neuromuscular junction

Because vertebrate neuromuscular junctions are readily accessible for experimental manipulation, they have provided a superb model in which to examine and test functional correlates of chemical synaptic transmission. In the neuromuscular synapse, acetylcholine receptors have been localized to the crests of the junctional folds and visualized by a variety of ultrastructural techniques. By using ultrarapid freezing techniques with a temporal resolution of less than 1 msec, quantal transmitter release has been correlated with synaptic vesicle exocytosis at discrete sites called "active zones." Mechanisms for synaptic vesicle membrane retrieval and recycling have been identified by using immunological approaches and correlated with endocytosis via coated pits and coated vesicles. In this review, available ultrastructural, physiological, immunological, and biochemical data have been used to construct an ultrastructural model of neuromuscular synaptic transmission that correlates structure and function at the molecular level.

Functionally significant plasticity of synaptic morphology: studies on the ribbon synapse of the ampullae of Lorenzini

Changes in electrophysiological properties measured in vitro were correlated with ultrastructural differences at synapses between sense cells and the primary afferent neurons in electrosensory organs of the thornback ray (the ampullae of Lorenzini). Variation in synaptic structure was classified into four synaptic morphotypes, which appear to represent stages in a cyclic pattern of ultrastructural modification associated with changes in synaptic efficacy. Synapses with deeper postsynaptic troughs, and active zone regions located at the "narrow point" of the presynaptic evagination, and other morphological differences, were associated with greater sensitivity and spontaneous activity. Furthermore, the morphology of synapses was different in organs that had shown increasing, decreasing or stable trends in sensitivity prior to fixation, suggesting that changes in synaptic physiology and morphology are interrelated, and providing evidence for the sequence of ultrastructural modifications represented by the four synaptic morphotypes. These results support the conclusion that synaptic morphology is plastic and that this plasticity has functional significance in terms of the threshold sensitivity and spontaneous activity monitored from the afferent nerves. Plasticity of synaptic morphology which is associated with changes in the efficacy of transmitter release at chemically mediated synapses could be important in relatively long-term phenomena.

Structural alterations in the membrane of the slow muscle fiber of Rana temporaria after denervation

Mixed fiber bundles as well as separated slow and fast fibers from normal and denervated muscles of Rana temporaria were freeze-fractured. The membranes of both fiber types are distinguished in this species by the presence of fairly regularly distributed particle aggregates or arrays of different shapes and sizes; the number per unit area of the membrane is six times higher in fast than in slow fibers. The intramembrane particle (IMP) density is higher in slow than in fast fibers. After denervation, the fast fiber membrane structure does not change whereas the slow fiber membrane acquires the characteristics of the fast fiber, i.e., an increase in the density of particle arrays and a decrease in IMP density. These changes in the slow fiber membrane are compared to the altered physiological properties of this fiber type after denervation.

The perinodal astrocyte

Several studies have demonstrated the presence of perinodal astrocyte processes at nodes of Ranvier in the central nervous system, suggesting that, in addition to the axon and oligodendrocyte, astrocytes participate in the formation of mature central nodes. The specific association between perinodal astrocyte processes and nodal membrane develops at the time of, or soon after, the appearance of relatively differentiated nodes of Ranvier. This interaction is likely to be mediated by cell adhesion molecules. J1 is a member of a family of glycoproteins that share a common carbohydrate epitope, designated L2/HNK-1, and that have been implicated in cell-cell interactions. This glycoprotein is concentrated at the interface between perinodal astrocyte processes and the nodal region of the axon. Moreover, N-CAM, which is a member of the same family as J1, and cytotactin, an extracellular matrix component produced by glia, are localized at the interface between the axon and perinodal astrocyte processes at nodes of Ranvier. The association of perinodal astrocyte processes with nodal membrane in the central nervous system is similar to that exhibited by perinodal Schwann cell processes at peripheral nodes, and similar functional properties have been suggested for these two glial cell processes, including production of nodal gap substance, buffering of perinodal extracellular ion concentration, and development and/or maintenance of nodal specializations in the axon membrane. Perinodal astrocyte and Schwann cell processes may also function as extraneuronal sites for the synthesis of voltage-sensitive sodium channels, to complement neuronal perikaryal synthesis and axonal transport. Ultrastructural studies on specialized patches of axon membrane within some unmyelinated, demyelinated, and dysmyelinated axons support the hypothesis of a specific role for perinodal astrocyte processes in the assembly, stabilization, and/or maintenance of axolemma with nodal characteristics. These observations suggest a multiplicity of functions for perinodal astrocyte processes at central nodes and implicate the astrocyte as an important component of the node of Ranvier.

Small vesicle bouton synapses on the distal half of the lateral dendrite of the goldfish Mauthner cell: freeze-fracture and thin section study

To understand principles of synaptic integration, it is necessary to define the types of synapses on a particular neuron and their distribution. Thin sectioning and double replica freeze-fracture techniques were employed to characterize the small vesicle bouton (SVB) synapses on the distal half of the Mauthner (M) cell lateral dendrite, which probably mediate a remote dendritic inhibition. Three morphologically distinct SVB synapses, types A, B, and C, were found. These three SVB synapses form roughly 90% of the synapses on the distal half of the lateral dendrite, with types A and B being most common. The SVB A synapse is characterized by mostly oval and round synaptic vesicles, a discrete presynaptic active zone with a highly variable shape, and a postsynaptic active zone with no apparent particle aggregate in either the E or P face. At the SVB B synapse, most of the synaptic vesicles are flat. A very high particle density is present throughout the presynaptic P face, and vesicle attachment sites are dispersed over much of the presynaptic membrane. Postsynaptic P face particle aggregates are subjacent to the presynaptic vesicle attachment sites, and are often large and anastomosing. The SVB C synapse is characterized by synaptic vesicle profiles that vary from flattened to round. The SVB C cytoplasm was unclouded by the flocculent material that characterized SVBs A and B. The presynaptic active zones at the SVB C synapse are discrete, and macular or oblong. No particle aggregates are apparent in the postsynaptic active zone. Small, macular particle aggregates were found in nonactive zone regions of the postsynaptic E face of all three types of SVBs. Small subsurface cisterns were also observed underlying the M cell membrane at all three types of SVB synapses. Neither the postsynaptic E face aggregates nor the subsurface cisterns were ever observed directly subjacent to presynaptic active zones, but were often seen adjacent to active zones. Short, straight rows of particles and short cylinders were often seen in both pre- and postsynaptic surrounding zone regions of SVB A and C synapses. These structures are thought to represent tight junctions.

Acetylcholinesterase in neuroblastoma and neuroblastoma x glioma hybrid cells: cellular localization and molecular forms

The cellular localization of acetylcholinesterase (AChE) was investigated at the electron microscope (E.M.) in a neuroblastoma and neuroblastoma x glioma hybrid line, which differ for their ability to establish synaptic contacts. Only cells of the latter line show association of AChE to the plasmamembrane, while in the former the activity is mainly intracellular. Sucrose sedimentation analysis of AChE molecular forms has shown no significant differences in the distribution of the two forms, G2 and G4, between the two cell lines. On the contrary a marked difference is observed in the ability of the cell to release the enzyme in the culture medium. In fact the cells lacking AChE on their surface release in the medium a much higher proportion of their enzyme, than the cells showing AChE association to their plamamembrane. The possible role of two alternative fates for AChE, secretion or membrane insertion, in determining the observed differences of enzyme localization is discussed.

Caveolae preservation in the characterization of human neuromuscular disease

We have examined freeze-fracture replicas and conventional thin-section images of rat myofibers prepared by perfusion and by conventional immersion fixation protocols, and myofibers of normal and dystrophic human myofibers prepared by similar immersion fixation methods. In both rat and human myofibers, the size and distribution of caveolae was found to differ substantially according to the method of glutaraldehyde exposure, the depth of the myofiber from the surface exposed to the fixative, and if surgically bisected, the distance from the cut end of the myofiber. Conventional immersion fixation resulted in unavoidable but predictable alterations in sarcolemmal caveolae. These reproducible artifacts of fixation technique substantially complicate the use of caveolae as reliable markers for the characterization of human neuromuscular disease.

Effect of denervation on sarcolemmal proteins and glycoproteins of fast and slow mammalian skeletal muscle

We compared the protein and glycoprotein composition of a sarcolemmal membrane fraction isolated from normal and denervated rat extensor digitorum longus (EDL) and soleus muscles. Membranes from EDL and soleus muscles showed significantly different protein compositions. A relatively small number of glycoproteins, which were all minor proteins, accounted for the majority of concanavalin A (ConA) and Ricinus communis agglutinin (RCA120) binding. These glycoproteins appear to be common to EDL and soleus but bound different relative amounts of lectin in the two muscles. A large proportion of the ConA binding sites in EDL, but not soleus, were cryptic (not accessible by ConA unless the membrane structure was disrupted). Denervation had a differential effect on sarcolemma from the two muscles with EDL exhibiting large changes and soleus changing little if at all. Several major proteins changed their relative concentrations after denervation and the relative amount of RCA120 bound to the major glycoproteins also changed. The major ConA-binding glycoproteins did not change in either membrane but denervation resulted in the exposure of most of the cryptic ConA-binding sites in EDL membranes. Endogenous sialyl- and galactosyl-transferase activities in the membrane fractions significantly increased in EDL, but did not change in soleus, suggesting that the turnover of the glycoproteins is increased in EDL after denervation.

Small size of orthogonal array in muscle plasma membrane of Fukuyama type congenital muscular dystrophy

Freeze fracture analysis was carried out on the density of orthogonal array subunit particles in the plasma membrane of skeletal muscle of six patients with Fukuyama type congenital muscular dystrophy and seven control cases. The group mean density of orthogonal array subunit particles per one orthogonal array was significantly lower in the plasma membrane of Fukuyama patients. The results suggested the possible impairment of orthogonal array function in the plasma membrane of muscle fibers in congenital muscular dystrophy of the Fukuyama type.

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)

Comparison of the ultrastructural myopathy induced by anticholinesterase agents at the end plates of rat soleus and extensor muscles

Rats were treated with single subcutaneous injections of the irreversible AChE inhibitors, sarin (90 to 100 micrograms/kg) or soman (55 micrograms/kg), and with chronic doses of the reversible carbamate inhibitor, pyridostigmine. In surviving animals with severe behavioral symptoms, we examined the end-plate regions of the slow-twitch soleus and the fast-twitch extensor digitorum longus muscles, using the electron microscope. Within 30 min, sarin administration caused a recognizable subjunctional myopathy. The progress of morphologic damage was followed for 7 days, during which time the occurrence of damage diminished. The initial swelling of subjunctional organelles and vacuole generation progressed to the point where nerve terminals and attached postjunctional folds were lifted away from the muscle surface. This appeared to be caused by a combination of enlarging vacuoles and insertion of Schwann and macrophage cells into the lesions, and was followed by degeneration of the postjunctional folds. A new component of anti-AChE myopathy was recognized: progressive swelling of chromatin in subjunctional muscle nuclei. The soleus muscle was considerably more sensitive to these effects than the extensor muscle. Soman had a much less prominent ultrastructural effect on the muscle end plates. Chronic pyridostigmine treatment had effects similar to those of a single sarin injection on the soleus as well as a pronounced effect on the extensor muscle.

A freeze-fracture study of postembryonic differentiation of latissimus dorsi muscles of the chicken

The differentiation of fiber type characteristics in the anterior (ALD) and posterior (PLD) latissimus dorsi muscles is examined by the freeze-fracture technique in 1-, 7- and 30-day-old chicks. Several characteristics of plasma membrane (caveolae, rectilinear arrays, intramembranous particles) and sarcoplasmic reticulum which show fiber type differences in the adult ALD and PLD muscles are compared in the developmental stages. The caveolar density in the ALD fibers is about 20/microns2 at 1 day increasing to about 37/microns2 at 30 days, whereas in the PLD fibers it remains at about 20/microns2 during this period. The distribution of the caveolae in the two muscles is different from the beginning; in the ALD fibers the caveolae are distributed throughout the plasma membrane and in PLD fibers they are patterned into clusters overlying the I band regions. The density of intramembranous particles of 1-day ALD and PLD plasma membranes appears similar, but by 7 days the particle counts in the sarcolemma of the ALD muscle are about twice as numerous as those in the PLD muscle. The rectilinear arrays are virtually absent in the ALD muscle, whereas in the PLD muscle their density is about 10/microns2 at 1 day and about 20/microns2 at 7 days. Already at 1 day posthatching the SR in ALD and PLD fibers has the adult configuration, i.e., an open irregular network in ALD fibers and periodically arranged tubules with triadic expansions in the PLD fibers.(ABSTRACT TRUNCATED AT 250 WORDS)