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Previous Article | Next Article
The Journal of Neuroscience, May 15, 2000, 20(10):3663-3675
From Plaque to Pretzel: Fold Formation and Acetylcholine Receptor Loss at the Developing Neuromuscular Junction
Maria Julia Marques, José-Angel Conchello, and Jeff W. Lichtman Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
Although there has been progress in understanding the initial steps in the formation of synapses, less is known about their subsequent maturation (Sanes and Lichtman, 1999
). Two alterations on the postsynaptic side of the mammalian neuromuscular junction occur during early postnatal life: acetylcholine receptors (AChRs) disappear from parts of the developing junction as all but one axonal inputs are removed, and the topography of the postsynaptic membrane becomes more complicated as gutters and folds are established. We have studied the maturation of the AChR distribution and postsynaptic topography simultaneously by imaging labeled AChRs at the mouse neuromuscular junction in a new way, using reflected light confocal microscopy. At birth postsynaptic receptors were localized in irregular patches within a spoon-shaped plaque. Beginning several days later, receptor regions within a single endplate were divided into differentiated and less organized compartments. Folds generally oriented orthogonal to the long axis of the muscle fiber were seen in developing gutters, although the orientation of the gutters seemed to be imposed by the branching pattern of the nerve. Eventually, superficial regions lacking AChR labeling were apparent in all junctions. In junctions denervated in the neonatal period both gutter formation and the disappearance of superficial receptors regions were prevented. We suggest that tension between growing muscle fibers and the relatively inelastic synaptic terminals that adhere to them causes the topographic features of the postsynaptic membrane. This view provides a mechanical explanation for gutters, folds, and the location of folds at sites of neurotransmitter release.
Key words: reflected light confocal microscopy; postsynaptic membrane topography; acetylcholine receptor distribution; neuromuscular junction development; synapse elimination; active zones
INTRODUCTION
The use of fluorescently tagged
-bungarotoxin to label acetylcholine receptors (AChRs) at the neuromuscular junction has shown that there is a dramatic maturational alteration in receptor distribution in early postnatal life. At birth, AChRs are arranged in an oval plaque in what appears to be a relatively uniform density. Over several weeks this plaque perforates (Nyström, 1968
Techniques that are useful for studying three-dimensional topography of synapses, such as scanning (Fahim et al., 1983
MATERIALS AND METHODS
Animals and muscle preparation. Neonatal mice were obtained from breeding colonies established in our animal care facility. Pregnant females were isolated and checked daily. The date of birth was designated postnatal day 0 (P0), and the pups were weaned on P21. Embryos [embryonic day 15 (E15) and E19] were obtained from timed pregnant females (CF1B strain, Harlan Sprague Dawley, Indianapolis, IN), and their prenatal age was confirmed from external physical characteristics. Forty animals (six adults and 34 young animals ranging in age from E15-E20 postpartum) were used.
Animals were anesthetized by an intraperitoneal injection of sodium pentobarbital, and the muscle was fixed. The protocol for fixation involved cardiac perfusion with Ringer's solution, followed by cold fixative freshly prepared (2% paraformaldehyde and 0.1% glutaraldehyde in PBS, pH 7.4). Embryos were removed from the mother after fixation. Both sternomastoid muscles were removed, pinned out on Sylgard-coated (Dow, Midland, MI) dishes, and washed for 3-4 min with Ringer's solution. In some animals the sternomastoid muscle was denervated on P10 by cutting the muscle nerve; 7 d later the animals were killed, and the muscles were removed as above.
AChR staining. Muscles were incubated with biotin-
Frog neuromuscular junctions also were stained and observed with confocal reflected light. Frogs were cold-anesthetized and pithed. Cutaneous pectoris and sartorius muscles were dissected out from Rana pipiens in Ringer's-Frog solution and pinned out in a Sylgard Petri dish. The muscles were incubated in b-BTX (1 µg/ml; Molecular Probes) for 4 hr at room temperature and after a washing in PBS incubated in a-HRP (1 µg/ml; Molecular Probes) overnight at 4°C. After several washes with Ringer's-frog, HRP was visualized by using the Vector SG kit. Muscles were fixed with a cold solution of 2% paraformaldehyde and 0.1% glutaraldehyde in PBS for 1 hr, washed with PBS for 4-5 min, dehydrated, and mounted in Permount for observation. The mouse and frog preparations maintained their AChR labeling for >1 year.
Confocal microscopy. Muscles were examined with the reflected light port of a laser-scanning confocal microscope (Odyssey, Noran Instruments, Middleton, WI) linked to an upright microscope (Optiphot-2, Nikon, Tokyo, Japan), using a 100×, 1.4 numerical aperture Plan Apo oil objective (Nikon). The excitation source was the 528.7 line of an argon-ion laser. Reflected light confocal used a small pinhole aperture (diameter of 0.1 µm in specimen space). Optical sections were collected at 0.1 µm intervals, and the number of images in the z-plane varied from 40 to 80. The stacks of images were processed into stereo pairs or movies (using a maximum voxel ray tracing algorithm) with locally modified volume-rendering software on a Sun computer (Sparc 370 with TAAC board). An index of junctional area was measured by taking the product of the maximum length of the junction in the long axis of the muscle fiber and the maximum width of the junction in the orthogonal axis.
RESULTS
Reflected light confocal microscopy: Theory
The principal aim of this work was to study the development of the postsynaptic membrane at the neuromuscular junction. We were interested in using a technique that would highlight the location of AChRs in the postsynaptic membrane, because a number of lines of evidence suggest that the distribution of AChRs undergoes major changes during early development. We also wanted a technique that provided topographic (three-dimensional) views of the postsynaptic membrane.
We took advantage of the high refractive index of the reaction product of HRP deposited at sites containing AChRs (see Materials and Methods), which allowed for visualization with epi-illumination reflected light (i.e., the microscope objective was used both to illuminate and view the specimen). With this technique the postsynaptic membrane of adult mouse neuromuscular junctions shows a bright reflection (Fig. 1). The reflection at high magnification and with high numerical aperture objectives showed alternating bright and dark bands reminiscent of SEM images of postsynaptic folds (see, for example, Fahim et al., 1983
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Figure 1. Visualizing the three-dimensional topography of the AChR distribution in the postsynaptic membrane of an adult mouse neuromuscular junction. Left, Red/green overlay of the three-dimensional topography of the AChR-rich neuromuscular junction (for viewing with red, the right eye, and for viewing with green, the left eye glasses). Middle, Right, Stereo pairs (for crossed eye viewing: left eye views right image) of the same junction (note that subsequent three-dimensional figures are all displayed this same way). In this reconstruction the depth of the junctional gutter (going into the plane of the page) and the pattern of alternating bright and dark bands are visible (especially at the bottom of the gutters). Orientation of the folds shows a tendency that they be orthogonal to the long axis of the muscle fiber (doubleheaded arrow). Thus gutters coursing parallel to the long axis of the muscle fiber have folds that generally run transversely (a), whereas branches that run perpendicular to the long axis often have folds that run parallel to the long axis of the gutter (b). Scale bar, 4 µm.
In distinction to the many points of scattered light induced by HRP reaction product seen in retrogradely HRP-labeled cells viewed in dark field (see, for example, Lichtman et al., 1984
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Figure 2. Principle of reflected light imaging of the postsynaptic membrane as a function of the tilt angle. Top, Shown is the predicted amount of reflected light signal from various parts of a single junctional fold. Although receptors are present partway down the fold, the reflected light signal is collected preferentially from the parts of the membrane that are most orthogonal to the optical axis. This dependence on angle is explained by Snell's law of reflection; the incident light rays (solid lines) give rise to reflected rays (dashed lines) that are diverted from the aperture of the objective when the reflecting surface is tilted (see first section of Results). This effect results in a progressively dimmer signal collected by the objective when the membrane is tilted progressively more. Bottom, A schematic view of the gutter or primary synaptic cleft and junctional folds in cross section. AChRs are highly concentrated at the tops of the folds (dark lines). AChRs on folds that are sitting at the bottom of the gutter reflect much of their light back into the objective, because their tops are oriented orthogonal to the optical axis. On the sides of the gutter the folds are stacked in a way that prevents light from being collected efficiently.
These expectations were borne out (see Fig. 1). The bottom and the top of the gutter (both orthogonal to the optical axis) were typically more reflective than the sides. In addition, the alternation of bright and dark bands seemed to be associated with the finer grain topography of the folds; these bands had a spatial frequency of ~1.6 dark bands/µm, which is in line with the frequency of folds from electron microscopic studies of the sternomastoid (Salpeter, 1987
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Figure 3. AChRs in the postsynaptic membrane of an adult frog neuromuscular junction show that the reflected light signal is preferentially from the tops of the folds. Folds are seen at regular intervals as fine dark lines (arrows). Some variability in reflection seen at the tops may represent differential density of AChRs (asterisks). Scale bar, 5 µm.
Close inspection of images of the postsynaptic membrane of mammalian junctions also supported the idea that the tops of the folds were reflecting light preferentially. We found, for example, that the dark bands were all approximately the same width (at the diffraction limit, ~0.2 µm), whereas the bright reflecting bands were wider and more variable in width (0.5-1 µm, on average) (Fig. 4, left). This is consistent with the electron microscopic data indicating that the width of the fold is relatively constant and below 0.2 µm, whereas the distance between folds can vary (Salpeter, 1987
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Figure 4. Comparison of fluorescence and reflected light confocal images of postsynaptic receptor labeling. AChRs in the postsynaptic membrane of two different adult mouse neuromuscular junctions were labeled for reflected light (left) or fluorescence (right). Better resolution of the folds was achieved with reflected light because of the orientation dependence of the reflection. A second difference is that the folds do not reflect light (and appear as dark bands), whereas they are highlighted preferentially in fluorescence. Conversely, the tops of the folds are bright in reflected light but relatively dim in fluorescence imaging.
A comparison of fluorescently labeled and reflected light images of AChRs in the postsynaptic membrane showed the advantages of reflected light imaging (Fig. 4). We found that the orientation dependence of the reflection allowed the reflected images to have greater contrast than fluorescence images of comparable samples. Because fluorescence emission is isotropic, part of the fluorescence signal is always directed into the objective irrespective of the orientation of the labeled membrane. In addition, with fluorescence, regions of the membrane at steep angles give greater signals because the fluorescence from multiple depths adds together. Thus the infolded region (which, because of its steep angle, appears dark in reflected light) is the brightest part of the image with fluorescence because nearby AChR-rich membrane regions sum. This summing cannot be ameliorated by confocal microscopy, because the optical depth of focus even with confocal (~0.75 µm) is much wider than the thickness of a membrane so that the intensity of the fluorescence signal is going to be brighter for any membrane that is tilted.
Features of the adult neuromuscular junction
The reconstructions of adult mouse neuromuscular junctions showed an unexpected relation between the orientation of the folds and the orientation of the muscle fiber. In particular, folds tended to run transverse to the long axis of the muscle fiber. Thus when the gutters were running parallel to the long axis of the muscle fiber, the folds were always transverse to the length of the gutters (see Fig. 1, a; 19 branches in 10 junctions), but when the gutters ran circumferentially, the folds tended to be parallel with the long axis of the gutters (see Fig. 1, b; 10 of 12 branches in six junctions). This relation indicates a role for the muscle fiber in establishing the folding pattern and suggests a mechanism that could lead to fold formation (see Discussion).
Neuromuscular junction development
By taking multiple reflected light confocal optical sections and reconstructing them, we obtained views of the three-dimensional topography of the receptor-rich regions of the postsynaptic membrane. Approximately 740 neuromuscular junctions from 80 sternomastoid muscles in mice ranging in age from E17-P15 from were studied. Forty-two representative examples were chosen for detailed analysis and reconstruction.
Previous work has shown that there is variability between junctions during early postnatal life because of a developmental "wave," such that junctions at several different stages of maturation coexist at the same time (Balice-Gordon and Lichtman, 1993
Embryonic period (E17-E19)
The receptor aggregates seen on fibers of 17-d-old embryos appeared as loose accumulations of small patches of receptors, and three-dimensional reconstructions showed that they were mainly in one plane (Fig. 5). We found, as expected, that in the late embryonic period the postsynaptic AChR plaques were oval or round. The reconstructions indicated that these plaques were slightly concave relative to the nearby muscle fiber membrane, typically in the shape of a spoon (see Discussion). Within the plaque the receptors were arranged in patches that resembled cobblestones (2-3 µm in diameter) that were separated by well defined dark bands, presumably because of nascent folding of the postsynaptic membrane (Fig. 5).
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Figure 5. AChR distribution at neuromuscular junctions in the late embryonic period, E17 and E19. Before birth the postsynaptic AChR plaque on each muscle fiber is typically oval. Often the plaque is indented relative to the muscle fiber membrane. Within the plaque, patches of AChRs are arranged as cobblestones (2-3 µm in diameter) separated by dark bands that may represent nascent folding of the postsynaptic membrane. Left panels, Red/green stereo pair. Middle, Right panels, Stereo pairs (left eye views right image). Scale bar, 4 µm.
Early postnatal period (P01-P04)
At birth, all of the endplates studied were still oval. However, with regard to the membrane topography, some junctions resembled embryonic junctions (~40%; n = 60), whereas others showed more complexity. One trend was that the AChR-positive regions tended to break apart into smaller regions than seen at earlier ages. Rather than appearing as cobblestones, in most junctions during the first few postnatal days the receptors were grouped into hundreds of small elongated patches. The elongated receptor patches were aligned approximately in a radial pattern from one point near the side of the junction (Fig. 6, arrows), which may be related to the point of nerve entry. At most junctions the continuous outline of the plaque was interrupted by indentations at one of its edges, leading for the first time to obvious asymmetry in the overall shape. Given that in adult neuromuscular junctions the site of nerve entry contains few AChRs (see for example, Balice-Gordon and Lichtman, 1993
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Figure 6. AChR distribution at the neuromuscular junction during the early postnatal period, P01 and P04. At birth (P01) AChRs were distributed in many small elongated regions that had a radial pattern from one point in the endplate, which we presume is the point of nerve entry (*). At later periods (P04) AChRs started to show a topographical segregation so that coexisting within the same junction were AChRs in creases (c) and in bulges (b). Left panels, Red/green stereo pair. Middle, Right panels, Stereo pairs (left eye views right image). Scale bar, 4 µm.
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Figure 7. Site of nerve entry associated with frayed appearance of AChRs in the postsynaptic membrane. The left panels show the AChR distribution labeled with rhodamine-tagged
First postnatal week (P06)
During the first postnatal week the junctions increase in area (see Table 1). In addition to the increase in length and width, the receptor-rich areas became sorted into two categories. Some parts of the plaque showed clear evidence of organized folding, whereas other regions were more disorganized and less reflective than the areas with obvious folds (Fig. 8). Three-dimensional reconstructions showed that sometimes these two kinds of receptor regions were at different depths. The sites with organized folds generally were arranged as elongated depressions or channels, suggesting that they were the precursors of the primary synaptic clefts. The disorganized receptor regions were, in contrast, located either on more superficial or in deeper regions of the muscle fiber membrane. Interestingly, in some junctions the two kinds of areas were segregated into separate contiguous regions of the neuromuscular junction (see Fig. 8, dashed line). Because competing axons segregate their synapses to separate regions during synapse elimination at this same stage (Gan and Lichtman, 1998
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Table 1. Changes in AChR area of the neuromuscular junction during postnatal development
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Figure 8. AChR distribution at the end of the first postnatal week, P06. At many neuromuscular junctions the topographical areas of the postsynaptic membrane become segregated into two well defined categories; some parts of the receptor pattern show organized folding, whereas other regions have more disorganized and less intensely reflecting receptor labeling. The smoothness of the edges is more obvious. Based on previous work (Balice-Gordon and Lichtman, 1993
At many junctions the process by which organized folds become established in the deeper regions was suggested. We observed that such receptor-rich areas showed some dimpling by small dark circles (Fig. 9). It is likely that these circles represent the "pits" described by Desaki and Uehara (1987)
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Figure 9. High-magnification image of a P06 neuromuscular junction suggests the steps in fold formation. At this age small dark pits (P) are seen. In addition, oblong dark pits appear to be coalescing into contiguous folds (asterisks). At some sites in which AChR reflection is high there is an unusually large distance between adjacent folds (brackets). Because such large interfold distances are rarely seen at adult mammalian neuromuscular junctions, these sites presumably will undergo fold formation soon. Indeed, a faint, dim broken line sometimes was observed in the middle of such interfold regions, which may be the earliest stage in fold formation.
Second postnatal week (P08-P15)
Measurements of endplate area (Table 1) show, as had been described previously (Balice-Gordon et al., 1993
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Figure 10. AChR distribution at neuromuscular junctions at the beginning of the second postnatal week, P08-P10. By the second postnatal week it was common to see receptor-rich and receptor-poor areas within the neuromuscular junction. The two endplates shown here, still mainly oval, represent the early stages of this process. Areas lacking reflection appear as small holes and strips of different sizes (arrows). With subsequent growth the receptor-rich areas increase in area, and the size of the receptor-poor areas also increases. Left panels, Red/green stereo pair. Middle, Right panels, Stereo pairs (left eye views right image). Scale bar, 4 µm.
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Figure 11. AChR distribution at neuromuscular junctions during the middle of the second postnatal week, P10-P11. Continued loss of postsynaptic sites causes perforations in the receptor density to enlarge and coalesce. Gutters containing well developed folds (presumably associated with nerve branches) are quite obvious (arrows). Areas that lose AChRs are superficial and appear as crests between adjacent deeper gutters. At the neuromuscular junction shown from P10, the bottom part of the junction appears to be terraced with receptor staining at two or more distinct depth levels. At the endplate from P11 it is clear the idea that the loss of AChR staining sculpts the outside edges, and these points are connected to the interior holes by channels lacking reflection (dashed arrow). Left panels, Red/green stereo pair. Middle, Right panels, Stereo pairs (left eye views right image). Scale bar, 4 µm.
Three-dimensional reconstructions showed that the regions that lost AChR staining are (or become) superficial (Fig. 11). The gutters typically contain organized folds, and they become more obvious as receptor staining outside these regions becomes less common (Fig. 11, solid arrows).
We have noticed that in nearly all junctions the radial pattern mentioned above (see Fig. 6) also is associated with an emerging asymmetry between the two sides of the oval plaque. It was noted previously that the multiple axons entering a neuromuscular junction invariably entered from the same side (Balice-Gordon and Lichtman, 1993
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Figure 12. Asymmetry in postsynaptic AChR distribution. Shown is the rhodamine-bungarotoxin-labeled AChR distribution at a P06 (top) and P15 (bottom) neuromuscular junction. In both cases one side (lower; Smooth) maintains its smooth oval shape whereas the opposite side is fuzzier (Irregular); as suggested by numerous observations of in vivo endplates stained for both nerve and receptors, the nerve entry side is irregular (Balice-Gordon and Lichtman, 1993
By the end of the second postnatal week most of the junctions appear to have made the transformation from plaque to fully branched form (Fig. 13). Although the branches are crowded together, each appears as a well defined gutter depressed below the surrounding muscle fiber membrane.
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Figure 13. AChR distribution during the late second postnatal week, P15. By the end of the second postnatal week, despite their compact size, most of the endplates have attained the adult form. Well defined gutters containing receptors arranged in distinct patterns of folds are seen. Folds are oriented orthogonal to the long axis of the muscle (arrow). Because the sides of the gutter are steep, only the folds at the bottom of the gutters reflect much light back into the objective. Left panels, Red/green stereo pair. Middle, Right panels, Stereo pairs (left eye views right image). Scale bar, 4 µm.
In addition, by the end of the second postnatal week most superficial areas no longer show evidence of AChRs, suggesting that, albeit small, junctions have attained their mature receptor distribution.
Denervation in the second postnatal week
To determine, once gutter formation and AChR loss had begun, how autonomous these processes were, we denervated the sternomastoid muscle during the second postnatal week and then viewed the junctions either 3 d later (denervations on P07, n = 19) or 1 week later at a time when junctions normally appear to be relatively mature (denervations on P10, n = 10). We found that denervated junctions appeared not to progress at all from the time of their denervation. Furthermore, in the case of the longer denervations the junctions appeared to regress. Thus junctions denervated at P07 and viewed 3 d later retained characteristics seen at P07. For example, the denervated junctions showed relatively few perforations as compared with contralateral control muscles and were often still oval-shaped. In addition, there were less obvious gutters. In junctions denervated at P10 and viewed at P17 we found evidence of gutter formation, and fold elaboration was only partially present (Fig. 14). Last, there were only hints of areas that were in the process of losing AChRs (Fig. 14).
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Figure 14. AChR topography and distribution do not mature at denervated neonatal neuromuscular junctions. At P17, 10 d after the sternomastoid muscle is denervated, the postsynaptic AChR distribution is oval, and well defined gutters are not present. Both of these features are seen commonly in junctions of much younger ages. Left panels, Red/green stereo pair. Middle, Right panels, Stereo pairs (left eye views right image). Scale bar, 4 µm.
DISCUSSION
This work adds to the extensive literature on neuromuscular junction structure by providing a topographical representation of location of AChRs. Additionally, this is the first optical technique capable of generating a high-resolution topography of the junctional folding pattern; it requires no sectioning or nerve removal procedure as is needed in electron microscopical approaches.
We found that during the first few postnatal weeks the postsynaptic membrane at individual neuromuscular junctions is a mosaic of two different regions. Some regions of the membrane are less reflective and have less organized folding, whereas other areas show folds and are associated with newly forming gutters. The transition from polyneuronal to single innervation is associated with a disappearance of AChRs at the underlying postsynaptic sites (Balice-Gordon and Lichtman, 1993
This work provides insight into the mechanism that generates the patterns of branches within a neuromuscular junction and the arrangement of folds within each branch. We saw that the nascent gutters of the future primary synaptic clefts fan out in a radial pattern in the oval plaque of AChRs (see, for example, Figs. 6, 10). The entry site also is associated with postsynaptic remodeling, because virtually all junctions became asymmetric; the outlines of the original oval AChR plaque remained evident at those edges of the junction that are not near the site of nerve entry, whereas the loss of AChRs disrupted the oval outline at the entry zone. Because the multiple inputs always approach the plaque from the same side (Balice-Gordon and Lichtman, 1993
The muscle, however, is also apparently responsible for dictating some aspects of synaptic organization. We observed that mammalian junctions often have folds that run transverse to the long axis of the muscle fiber irrespective of the orientation of the gutters (see Fig. 1). Thus when junctional gutters were running in the long axis of the muscle fiber, the folds were transverse to the gutters, but when the gutters were running transverse to the long axis, the folds often were parallel to the gutter orientation. Previous scanning electron microscopical studies based on smaller sample sizes had noted that folds often were transverse to gutters (Ogata and Yamasaki, 1985
This ability of the muscle to determine the orientation of the junctional folds has motivated us to consider an explanation for the origin of junctional folds based on the forces exerted on the postsynaptic membrane as a consequence of the growth of muscle fibers and adhesion to the nerve. Analogous arguments have related tension forces that act in the developing brain to the origin of sulci and gyri and other structural features of the CNS (Van Essen, 1997
We consider the consequences for the topography of the synapse that result from acknowledging that sites of synaptic contact are tightly adhesive for the pre- and postsynaptic membranes. Evidence for this strong adhesion comes from the requirement of harsh acid or proteolytic treatments to strip nerve terminals from muscle fibers (Kuffler and Yoshikami, 1975
The intercalary membrane addition in muscle fibers thus may cause the established sites of adhesion with the nerve terminal via the intervening basal lamina to be pushed apart by new membrane insertion in the growing muscle fiber. If the connective tissue between the nerve and muscle is relatively inelastic, the adhesion between nerve terminals and muscle fibers will exert forces on the muscle fiber membrane associated with muscle growth. The direction of the tension forces will require either that the muscle fiber curl around the nerve and embrace it and/or that the nerve stretch to keep the adhesive sites in the basal lamina from breaking (Fig. 15). The concave shape of the AChR plaque in neonatal animals thus may be a consequence of the inability of the nerve to enlarge as fast as the expansion of the muscle fiber membrane. Disproportionate growth between muscle and nerve also may be the explanation for the subsequent sinking of the synaptic gutters as the muscle continues to add membrane at sites near contact with less quickly growing nerve terminal branches (Fig. 15).
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Figure 15. Hypothesis for the role of adhesion in generating three-dimensional topography of the neuromuscular junction. Shown are three stages in the development of the mammalian neuromuscular junction. Left, When nerve first contacts the muscle, sites of adhesion are established between the nerve terminal plasmalemma and the membrane of the myofiber via molecules that extend into the basal lamina (red and blue lines). Three sites of synaptic adhesion are labeled a-a', b-b', and c-c'. Middle, as the muscle fiber grows to keep pace with the enlargement of the body, new membrane (green) is inserted in the membrane. This insertion causes an intercalary expansion of the postsynaptic site (Balice-Gordon and Lichtman, 1990
If preferential muscle fiber growth is not matched by an equivalently rapid growth of the nerve terminal, then the nerve will prevent the elongation of the postsynaptic territory attached to it. In the presence of this restraining force any newly inserted muscle fiber membrane would have to fold to accommodate the constraints imposed by attachment to the nerve terminal. One location in which such infolding might occur is at sites in which there are few adhesive connections between the nerve and the underlying muscle. Adhesive connections are thought to occur around, but not at, release sites of central synapses (Uchida et al., 1996
This conceptual framework also provides a potential explanation for the loss of folds seen in denervated adult muscles although AChRs are maintained (Loring and Salpeter, 1980
The fact that the depths of folds vary between muscle types and species also may be explained by differences in the growth potential or elasticity of nerve terminals. For example, human neuromuscular junctions are typically smaller than mouse junctions, although often they are located on muscle fibers that are much larger. Human junctions, however, do have much longer junctional infoldings and deeper gutters (Engel and Santa, 1971
FOOTNOTES Received Nov. 3, 1999; revised Feb. 18, 2000; accepted Feb. 28, 2000.
Financial support was received from Fundo de Amparo × Pesquisa do Estado de Sao Paulo (93/3419-7; 95/6110-2), Fundo de Apoio ao Ensino e à Pesquisa, and Conselho Nacional de Pesquisas (to M.J.M.) and the Muscular Dystrophy Association, National Institutes of Health, and Bakewell NeuroImaging Fund.
Correspondence should be addressed to Dr. Jeff W. Lichtman, Department of Anatomy and Neurobiology, Box 8108, Washington University School of Medicine, 660 South Euclid, St. Louis, MO 63110. E-mail: jeff{at}thalamus.wustl.edu.
Dr. Marques's present address: Departamento de Anatomia, Instituto de Biologia, Universidade Estadual de Campinas, UNICAMP, Campinas Sao Paulo, Brasil 13083-970. E-mail: marques{at}obelix.unicamp.br.
Dr. Conchello's present address: Institute of Biomedical Computing, Washington University, Old Shriner's 2229, Box 8036, St. Louis, MO 63110.
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