NONNEURAL AGRIN CODISTRIBUTES WITH ACETYLCHOLINE-RECEPTORS DURING EARLY DIFFERENTIATION OF TORPEDO ELECTROCYTES

Agrin, an extracellular matrix protein synthesized by nerves and muscl es is known to promote the clustering of acetylcholine receptors and o ther synaptic proteins in cultured myotubes, This observation suggests that agrin may provide at least part of the signal for synaptic speci alization in vivo. The extracellular matrix components agrin, laminin and merosin bind to alpha-dystroglycan, a heavily glycosylated periphe ral protein part of the dystrophin-glycoprotein complex, previously ch aracterized in the sarcolemma of skeletal and cardiac muscles and at t he neuromuscular junction. In order to understand further the function of agrin and alpha DG in the genesis of the acetylcholine receptor-ri ch membrane domain, the settling of components of the dystrophin-glyco protein complex and agrin was followed by immunofluorescence localizat ion in developing Torpedo marmorata electrocytes. In 40-45 mm Torpedo embryos, a stage of development at which the electrocytes exhibit a de finite structural polarity, dystrophin, alpha/beta-dystroglycan and ag rin accumulated concomitantly with acetylcholine receptors at the vent ral pole of the cells, Among these components, agrin appeared as the m ost intensely concentrated and sharply localized, The scarcity of the nerve-electrocyte synaptic contacts at this stage of development, moni tored by antibodies against synaptic vesicles, further indicates that before innervation, the machinery for acetylcholine receptor clusterin g is provided by electrocyte-derived agrin rather than by neural agrin . These observations suggest a two-step process of acetylcholine recep tor clustering involving: (i) an instructive role of electrocyte-deriv ed agrin in the formation of a dystrophin-based membrane scaffold upon which acetylcholine receptor molecules would accumulate according to a diffusion trap model; and (ii) a maturation and/or stabilization ste p controlled by neural agrin. In the light of these data, the existenc e of more than one agrin receptor is postulated to account for the act ion of agrin variants at different stages of the differentiation of th e postsynaptic membrane in Torpedo electrocytes.