Heparin blocks functional innervation of cultured human muscle by rat motor nerve

In vitro innervated human muscle is the only experimental model to study synaptogenesis of the neuromuscular junction in humans. Cultured human muscle never contracts spontaneously but will if innervated and therefore is a suitable model to study the effects of specific neural factors on the formation of functional neuromuscular contacts. Here, we tested the hypothesis that nerve derived factor agrin is essential for the formation of functional synapses between human myotubes and motoneurons growing from the explant of embryonic rat spinal cord. Agrin actions were blocked by heparin and the formation of functional neuromuscular contacts was quantitated. At a heparin concentration of 25 microg/ml, the number of functional contacts was significantly reduced. At higher concentrations, formation of such contacts was blocked completely. Except at the highest heparin concentrations (150 microg/ml) neuronal outgrowth was normal indicating that blockade of neuromuscular junction formation was not due to neuronal dysfunction. Our results are in accord with the concept that binding of neural agrin to the synaptic basal lamina is essential for the formation of functional neuromuscular junctions in the human muscle.