Simultaneous labelling of basal lamina components and acetylcholinesterase at the neuromuscular junction

Regional differences in organization of the extracellular matrix and cytoskeleton at the equator of chicken intrafusal muscle fibres

Equatorial regions of chicken intrafusal fibres were examined with a panel of monoclonal antibodies against intracellular proteins and components of extracellular matrix to identify structural associations at points of contact between sensory terminals and intrafusal fibres, and at points which lacked them. One aspect of this study was to establish whether the known morphological differences between myosensory and neuromuscular junctions also extended to the molecular level. As viewed in cross-sections, myosensory junctions at the equator are restricted to approximately one-half of the intrafusal fibre circumference, a region referred to as the sensory sector. The diametrically opposite region which lacks sensory terminals is referred to as the non-sensory sector. The basal lamina over the sensory sector was positive for chondroitin sulphate, while that part which covered the non-sensory sector was negative. Staining for collagen type IV was very faint at the sensory sector and stronger at the non-sensory sector, but immunoreactivity for heparan sulphate proteoglycan and laminin was moderate to strong in all parts of the basal lamina. Within intrafusal fibres, filamin and alpha-actinin were largely limited to the sensory sector. The major feature of the non-sensory sector was a sharply delineated, narrow intrafibre crescent of vinculin, and colocalized with it, a crescent of talin. The plasmalemma of intrafusal fibres at the non-sensory sector reacted positively for the beta 1 subunit of the integrin family of receptors. Immunolocalization of these receptors was not observed to any significant extent in the sensory sector. Towards the end of the equator and the initial portion of the juxtaequator, chondroitin sulphate, vinculin and the other proteins came gradually to be distributed equally all the way round the intrafusal fibres. This changeover in distribution of connective tissue proteins and structural intracellular proteins parallels the decreasing number of contacts made by sensory terminals.

High resolution immunoelectron microscopic localization of functional domains of laminin, nidogen, and heparan sulfate proteoglycan in epithelial basement membrane of mouse cornea reveals different topological orientations

Thin and ultrathin cryosections of mouse cornea were labeled with affinity-purified antibodies directed against either laminin, its central segments (domain 1), the end of its long arm (domain 3), the end of one of its short arms (domain 4), nidogen, or low density heparan sulfate proteoglycan. All basement membrane proteins are detected by indirect immunofluorescence exclusively in the epithelial basement membrane, in Descemet's membrane, and in small amorphous plaques located in the stroma. Immunoelectron microscopy using the protein A-gold technique demonstrated laminin domain 1 and nidogen in a narrow segment of the lamina densa at the junction to the lamina lucida within the epithelial basement membrane. Domain 3 shows three preferred locations at both the cellular and stromal boundaries of the epithelial basement membrane and in its center. Domain 4 is located predominantly in the lamina lucida and the adjacent half of the lamina densa. The low density heparan sulfate proteoglycan is found all across the basement membrane showing a similar uniform distribution as with antibodies against the whole laminin molecule. In Descemet's membrane an even distribution was found with all these antibodies. It is concluded that within the epithelial basement membrane the center of the laminin molecule is located near the lamina densa/lamina lucida junction and that its long arm favors three major orientations. One is close to the cell surface indicating binding to a cell receptor, while the other two are directed to internal matrix structures. The apparent codistribution of laminin domain 1 and nidogen agrees with biochemical evidence that nidogen binds to this domain.

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

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

Distribution of connective tissue proteins in chick muscle spindles as revealed by monoclonal antibodies: a unique distribution of brachionectin/tenascin

The distribution of chick muscle spindles of eight connective tissue proteins (collagen types I, IV, V, and VI, laminin, heparan sulfate, fibronectin, and brachionectin/tenascin) was examined by immunofluorescent histochemistry. Intrafusal fibers were surrounded by layers of collagen type VI and fibronectin, and by an external lamina containing collagen type IV, laminin, and heparan sulfate. Most of these layers displayed a different pattern of staining at the sensory region of the equator than at the polar region. The crescent-like sheath that caps each intrafusal fiber and sensory terminal at the equator was strongly positive for collagen type I and weakly positive for collagen type V. The outer spindle capsule contained laminin, heparan sulfate, collagen types IV and VI, brachionectin/tenascin, fibronectin, and to a lesser degree also collagen types I and V. Brachionectin/tenascin had the narrowest distribution of any of the connective tissue macromolecules studied. It was found only in the outer capsule and in the coverings of blood vessels and nerves associated with the outer capsule.