Aneural muscle cell cultures make synaptic basal lamina components

16S acetylcholinesterase of the extracellular matrix is assembled within mouse muscle cells in culture

The present paper examines where the extracellular-matrix (ECM) 16S acetylcholinesterase (AChE, EC 3.1.1.7) is assembled in muscle cells in culture. The existence of an internal pool of 16S AChE was detected by using AChE inhibitors of differing membrane permeability. After irreversible inhibition of all cellular esterase, the newly synthesized 16S form appears in an intracellular compartment and is only later detected on the cell surface. Results show that the ECM 16S AChE is assembled within muscle cells.

Appearance of high-molecular-weight acetylcholinesterase in aneural muscle developing in vivo

Acetylcholinesterase was studied in the superior oblique muscle of the duck embryo during the course of in vivo development. Normally developing, paralyzed, and uninnervated muscles were studied using velocity sedimentation for separation of various forms and biochemical determination of enzyme activity, and light and electron microscopy for histochemical and cytochemical localization of enzyme. Results indicate that neither muscle activity nor contact by the motor neurons is essential for the appearance of high-molecular-weight form of acetylcholinesterase on muscle cells developing in vivo. Acetylcholinesterase activity per muscle was considerably lower in the paralyzed and aneural muscles than the normal muscle. The absolute loss of acetylcholinesterase parallels loss of muscle protein in paralyzed and aneural muscles and may be secondary. Paralysis or absence of innervation had no significant effect on the specific activity of acetylcholinesterase.

Differentiation of motor nerve terminals formed in the absence of muscle fibres

During reinnervation of frog skeletal muscle, axons form functional nerve terminals at original synaptic sites on denervated myofibres. When muscle is damaged as well as denervated, myofibres decompose but their sheaths of basal lamina (BL) survive. Despite the absence of myofibres, axons regenerate to contact BL and there acquire clusters of synaptic vesicles and membrane-associated dense patches that resemble active zones; BL regulates this differentiation. We show here that these BL-associated axonal segments appear smaller and contain fewer active zones than terminals on intact myofibres in the same preparation. However, terminals formed on BL sheaths are capable of activity-dependent recycling of synaptic vesicles (demonstrated by tracer uptake), and bear an antigen normally present in terminals but not preterminal axons (demonstrated by immunofluorescence). Thus, axons can acquire functional and biochemical, as well as morphological, characteristics of normal motor nerve terminals in the absence of a postsynaptic cell.

Relationship of collagen-tailed acetylcholinesterase with basal lamina components. Absence of binding with laminin, fibronectin, and collagen types IV and V and lack of reactivity with different anti-collagen sera

In view of their supposed localization in extracellular structures, such as basal lamina, we have investigated the possible interactions of collagen-tailed forms of acetylcholinesterase from Electrophorus and bovine superior cervical ganglion with matrix proteins: laminin, fibronectin and types IV and V collagens. Using binding and sedimentation assays, with iodinated or non-radioactive matrix proteins, we have not observed any significant interaction, in conditions of high or low ionic strength. We also examined whether the collagen tail of acetylcholinesterase asymmetric forms possessed an immunological relationship with known collagen types (I, III, IV, V) from mammalian sources. We found no specific immunoreactivity with any of the 32 sera studied, either with the iodinated Electrophorus or with the native bovine enzyme. We conclude from these negative results that the collagen-like tail of acetylcholinesterase is clearly distinct from the classical types of collagen and that asymmetric forms of the enzyme do not interact specifically with the matrix proteins studied. This does not exclude the possibility of specific interactions with other components, remaining to be identified.

Activity-dependent accumulation of basal lamina by cultured rat myotubes

Myoblasts from 20-day rat embryos fuse and differentiate in culture to form spontaneously active myotubes. The myotubes acquire an extracellular matrix that includes a patchy basal lamina (BL) and a layer of fibrils that runs among and above the cells. Several antibodies that bind to muscle fiber basement membrane in vivo were used to study the organization of the extracellular matrix and the effect of muscle activity on the accumulation of its components. Light and electron microscopic immunohistochemical methods showed that the composition and organization of myotube BL in vitro resemble those seen in vivo. Antibodies that bind to both synaptic and extrasynaptic muscle fiber BL, in vivo stain the entire myotube BL in vitro, while antisera that bind preferentially to synaptic BL in vivo stain small patches of myotube BL, which are usually associated with regions rich in acetylcholine receptors. The effects of activity on accumulation of BL were studied by comparing control myotubes to myotubes paralyzed with tetrodotoxin or lidocaine. Immunohistochemical and 125I-antibody binding experiments with three antisera that stain the entire BL showed that paralyzed myotubes accumulate less BL than active myotubes. The effects of activity and inactivity are reversible: new BL forms if toxin is removed from cultures and BL is lost if active myotubes are paralyzed. Thus, accumulation of BL by myotubes is dependent, at least in part, on activity. In contrast, the number of patches stained by synapse-specific BL antibodies is increased in inactive cultures. Thus, immunologically distinguishable components of BL are differentially affected by activity.

The A12 acetylcholinesterase and polypeptide composition of electric organ basal lamina of Electrophorus and some Torpedinae fishes

Basal lamina (BL) of Torpedo, Discopyge and Electrophorus electric organs was purified in order to establish polypeptide composition and association with acetylcholinesterase (AChE). Results indicate that BL presents a distinct peptide pattern and that the A12 form of AChE is directly attached to it. Comparison of the species studied demonstrated similarities both in polypeptide composition and AChE content of the purified BL. Extractions of BL with solutions of high ionic strength, guanidine-HCl and acetic acid indicated the differential solubilization of various domains of BL polypeptides.

The myasthenic (Eaton-Lambert) syndrome associated with carcinoma. Enzyme induction as a possible mechanism of paraneoplastic syndromes

A myasthenic syndrome associated with small-cell tumours of the bronchus and with autoimmune diseases (Eaton-Lambert syndrome) has been attributed to diminished probability that a nerve action potential will release acetylcholine (ACh) from terminals of cholinergic nerves (somatic motor and autonomic). This model derives from evidence for reduced quantal content of the transmitter released by a nerve impulse. The test procedure implies certain constancies of postsynaptic response. Abnormal responses to ACh-agonists indicate that receptor response is not normal. It is suggested that all previously described neuromuscular responses are compatible with new model: the subsynaptic apparatus produces excess acetylcholinesterase (AChE) which limits the endplate conductance changes produced by normal output of ACh. This model is supported by earlier ultramicroscopic studies which cannot be accounted for by the contemporary model. It is proposed that enzyme induction by peptide or immunoglobulin may also be responsible for other paraneoplastic syndromes.

Association of the synaptic form of acetylcholinesterase with extracellular matrix in cultured mouse muscle cells

Myotubes of a mouse muscle-cell line (C2) synthesize in culture a 16S form of acetylcholinesterase that is normally found only in regions of adult mouse muscle that contain endplates. The 16S enzyme in C2 cell extracts has the properties expected of acetylcholinesterase forms that have a collagen-like tail. In intact cells, the active site of the 16S acetylcholinesterase is protected by a membrane-impermeable inhibitor, and this form of the enzyme can be removed by treatment of the cells with collagenase. Thus the enzyme is extracellular. Its extraction by high ionic strength solutions lacking detergent suggests that the 16S form is associated with the extracellular matrix by ionic interactions. Histochemical staining shows focal patches of acetylcholinesterase activity on the cell surface. Collagenase treatment, which removes only the 16S form, abolishes this staining pattern, indicating that the patches consist of the 16S enzyme. We conclude that the 16S enzyme in C2 myotubes occurs in focal patches on the cell surface, where it is associated with the extracellular matrix.