Molecular modifications during nerve-muscle synapse formation

Towards understanding skeletal muscle regeneration

Factors which effect proliferation and fusion of muscle precursor cells have been studied extensively in tissue culture, although little is known about these events in vivo. This review assesses the tissue culture derived data with a view to understanding factors which may control the regeneration of mature skeletal muscle in vivo. The following topics are discussed in the light of recent developments in cell and molecular biology: 1) Injury and necrosis of mature skeletal muscle fibres 2) Phagocytosis of myofibre debris 3) Revascularisation of injured muscle 4) Activation and proliferation of muscle precursor cells (mpc) in vivo Identification of mpcs; Satellite cell relationships; Extracellular matrix; Growth factors; Hormones; Replication. 5) Differentiation and fusion of muscle precursor cells in vivo Differentiation; Fusion; Extracellular matrix; Cell surface molecules: Growth factors and prostaglandins 6) Myotubes and innervation.

Microinjection of a monoclonal antibody against a 37-kD protein (tropomyosin 2) prevents the formation of new acetylcholine receptor clusters

We have shown previously that chick muscle cells transformed with Rous sarcoma virus are unable to form clusters of acetylcholine receptors (AChRs) (Anthony, D. T., S. M. Schuetze, and L. L. Rubin. 1984. Proc. Natl. Acad. Sci. USA. 81:2265-2269) and are missing a 37-KD tropomyosin-like protein (TM-2) (Anthony, D. T., R. J. Jacobs-Cohen, G. Marazzi, and L. L. Rubin. 1988. J. Cell Biol. 106:1713-1721). In an attempt to clarify the role of TM-2 in the formation and/or maintenance of AChR clusters, we have microinjected a monoclonal antibody specific for TM-2 (D3-16) into normal chick muscle cells in culture. D3-16 injection blocks the formation of new clusters but does not affect the preexisting ones. In addition, TM-2 is concentrated at rat neuromuscular junctions. These data suggest that TM-2 may play an important role in promoting the formation of AChR clusters.

A molecular defect in virally transformed muscle cells that cannot cluster acetylcholine receptors

Muscle cells infected at the permissive temperature with temperature-sensitive mutants of Rous sarcoma virus and shifted to the non-permissive temperature form myotubes that are unable to cluster acetylcholine receptors (Anthony, D. T., S. M. Schuetze, and L. L. Rubin. 1984. Proc. Natl. Acad. Sci. USA. 81:2265-2269). Work described in this paper demonstrates that the virally-infected cells are missing a 37-kD peptide which reacts with an anti-tropomyosin antiserum. Using a monoclonal antibody specific for the missing peptide, we show that this tropomyosin is absent from fibroblasts and is distinct from smooth muscle tropomyosins. It is also different from the two previously identified striated muscle myofibrillar tropomyosins (alpha and beta). We suggest that, in normal muscle, this novel, non-myofibrillar, tropomyosin-like molecule is an important component of a cytoskeletal network necessary for cluster formation.

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.