Herman Gordon

Agrin-independent activation of the agrin signal transduction pathway.

The neural factor agrin induces the aggregation of acetylcholine receptors (AChRs) and other synaptic molecules on cultured myotubes. This aggregating activity can be mimicked by experimental manipulations that include treatment with neuraminidase or elevated calcium. We report evidence that neuraminidase and calcium act through the agrin signal transduction pathway. The effects of neuraminidase and calcium on AChR clustering are additive with that of agrin at low concentrations and cosaturating at high concentrations. In addition, like agrin, both neuraminidase and calcium cause rapid tyrosine phosphorylation of the muscle-specific kinase (MuSK) and the AChR-beta subunit. Our results argue that all three agents act directly on components of the same signal transduction complex. We suggest that sialic acids on components of the complex inhibit interactions necessary for signal transduction and that disinhibition can result in activation. In such a model, agrin could activate signal transduction by disinhibition or by circumventing the inhibition.

A Mechanism for Acetylcholine Receptor Clustering Distinct from Agrin Signaling

Acetylcholine receptors (AChRs) and other postsynaptic molecules cluster spontaneously on cultured C2 myotubes. The frequency of clustering is enhanced by neural agrin, neuraminidase, or calcium through a signaling pathway which includes tyrosine phosphorylation of a muscle-specific kinase (MuSK) and the AChR β-subunit. Vicia villosa agglutinin (VVA) lectin, previously shown to potentiate agrin-induced clustering on C2 myotubes, is shown here to also potentiate neuraminidase- and calcium-induced clustering of AChRs, while having no effect on the level of tyrosine phosphorylation of MuSK or the AChR β-subunit. We propose that VVA lectin increases the frequency of AChR clustering through a mechanism that is distinct from agrin signaling, and that may involve α-dystroglycan.

Acetylcholine receptors are required for postsynaptic aggregation driven by the agrin signalling pathway.

To investigate the role of acetylcholine receptors (AChRs) in the aggregation of postsynaptic molecules on muscle cells, we utilized the 1R– genetic variant of C2 muscle cells which has very little expression of AChRs in its cell membrane. On C2 myotubes, AChRs cluster spontaneously, with the frequency of clustering greatly enhanced by motor neuron‐derived agrin. Signal transduction events driven by agrin, including the tyrosine phosphorylation of muscle‐specific kinase (MuSK) and the AChR β subunit, have been implicated as requirements of postsynaptic scaffold assembly. We show here that some molecules of the postsynaptic scaffold spontaneously aggregate and colocalize on 1R– myotubes at very low frequency, including an as yet unidentified agrin binding molecule, β‐dystroglycan and MuSK. Agrin is unable to increase the frequency of these aggregations, but does cause tyrosine phosphorylation of MuSK. We conclude that free molecules can associate into aggregates independently of AChRs, but AChRs are required for high‐frequency molecular aggregation driven by the agrin signalling pathway. MuSK tyrosine phosphorylation appears to precede a requisite event involving AChRs that aggregates postsynaptic molecules.

Altered Glycosaminoglycan Chain Structure in a Variant of the C2 Mouse Muscle Cell Line

Abstract: Experiments on the S27 cell line, a variant of the C2 mouse muscle cell line that shows reduced incorporation of 35SO4 into proteoglycans, suggest that proteoglycans play a role in the clustering of acetylcholine receptors, an early step in synaptogenesis. Thus, unlike the C2 line, S27 myotubes do not form acetylcholine receptor clusters on their surface in aneural cultures and form few clusters in response to agrin. We have examined the proteoglycans synthesized by S27 myotubes to define further the biochemical defect in these cells. Gel filtration analysis of radiolabeled proteoglycans synthesized by C2 and S27 myotubes shows that both cell types express a similarly polydisperse complement of proteoglycans. Both radiolabeled heparan sulfate proteoglycans and chondroitin/dermatan sulfate proteoglycans are reduced in S27 myotubes, with the chondroitin/dermatan sulfate proteoglycans showing a distinct reduction in size. The core protein of perlecan, a major proteoglycan species in muscle, was present in S27 cells and unaltered in electrophoretic mobility. Thus a principal deficiency in S27 cells appears to be a defect in glycosaminoglycan chain elongation.

Binding of Asymmetric (Al2) Acetylcholinesterase to C2 Muscle Cells and to Cho Mutants Defective in Glycosaminoglycan Synthesis

The principal biological role of acetylcholinesterase (AChE) is termination of impulse transmission at cholinergie synapses by rapid hydrolysis of the neurotransmitter acetylcholine (Katz, 1966). AChE is a polymprphic enzyme which may be classified in globular and asymmetric forms (Bon et al., 1979). Of these, the tailed asymmetric Al2 form is found in high concentrations at endplates (Hall, 1973). The occurrence of a collagenous domain in the tailed enzyme suggests that it interacts with the extracellular matrix (Lwebuga-Mukasa et al., 1976; Inestrosa et al., 1982). It has been postulated that this interaction occurs through attachment to heparin-like glycosaminoglycans (GAGs) present in proteoglycans (PGs) (Brandan et al., 1985). In the present study we have directly evaluated the binding of purified Al2 AChE to cultured wild-type and mutant mouse C2 skeletal muscle cells. Also, to investigate the potential role of heparan sulfate proteoglycans (HSPGs) as Al2 AChE cell surface receptors, we studied the interactions of Al2 AChE with chinese hamster ovary (CHO) cells that synthesize varying amounts of cell surface heparan sulfate (HS) and other GAGs (Esko, 1991).

Acetylcholine receptor clustering associates with proteoglycan biosynthesis in C2 variant and heterkaryon muscle cells

Several lines of evidence have suggested roles for proteoglycans (PGs) in acetylcholine receptor (AChR) clustering on muscle cells. One line of evidence comes from the correlation between a defect in the biosynthesis of glycosaminoglycans (GAGs), the defining carbohydrates of PGs, and the failure of spontaneous AChR clustering in the S27 cell line, a genetic variant of the C2 muscle cell line. Two approaches were used in the present study to investigate whether GAG and AChR clustering defects are causally linked. First, the formation of AChR clusters was examined in two more variant lines, S11 and S26, also isolated from the C2 muscle cell line on the basis of deficiencies in GAG biosynthesis. S11 and S26, like S27, are also defective in AChR clustering. Ion exchange analysis of the GAGs made by the S11, S26, and S27 lines revealed that the defects in GAG biosynthesis differ between the three lines. Second, heterokaryon myotubes formed between pairs of the GAG defective variants were tested for complementation in both AChR clustering and GAG biosynthesis. AChR clusters were conspicuous on individual heterokaryon myotubes, and GAG biosynthesis was restored to near wild type levels in the heterokaryon cultures. Complementation in GAG biosynthesis corroborates the biochemical data that the relevant mutations in the genetic variants are in different genes and establishes that the defects are not dominant. The consistent correlation between GAG defects and the failure of AChR clustering across three independent genetic variants and the complementary association of GAG biosynthesis with AChR clustering in heterokaryon myotubes argues against a chance association of the two phenotypes and for a causal relationship between PGs and AChR clustering. A prominent chondroitin sulfate peak correlated with AChR clustering in the heterokaryon cultures. This is consistent with earlier results suggesting that chondroitin sulfate in general is required for the spontaneous clustering of AChRs in C2 cultures and further suggests that a particular chondroitin sulfate proteoglycan may be essential for the clustering process.