Transport and Secretion of the Wnt3 Ligand by Motor Neuron-like Cells and Developing Motor Neurons

Abstract

The vertebrate neuromuscular junction (NMJ) is formed by a presynaptic motor nerve terminal and a postsynaptic muscle specialization. Cumulative evidence reveals that Wnt ligands secreted by the nerve terminal control crucial steps of NMJ synaptogenesis. For instance, the Wnt3 ligand is expressed by motor neurons at the time of NMJ formation and induces postsynaptic differentiation in recently formed muscle fibers. However, the behavior of presynaptic-derived Wnt ligands at the vertebrate NMJ has not been deeply analyzed. Here, we conducted overexpression experiments to study the expression, distribution, secretion, and function of Wnt3 by transfection of the motor neuron-like NSC-34 cell line and by in ovo electroporation of chick motor neurons. Our findings reveal that Wnt3 is transported along motor axons in vivo following a vesicular-like pattern and reaches the NMJ area. In vitro, we found that endogenous Wnt3 expression increases as the differentiation of NSC-34 cells proceeds. Although NSC-34 cells overexpressing Wnt3 do not modify their morphological differentiation towards a neuronal phenotype, they effectively induce acetylcholine receptor clustering on co-cultured myotubes. These findings support the notion that presynaptic Wnt3 is transported and secreted by motor neurons to induce postsynaptic differentiation in nascent NMJs.

How Are Synapses Born? A Functional and Molecular View of the Role of the Wnt Signaling Pathway

“…In cerebellar granule cells and mossy fibers, Wnt7a expression increases between P6 and P22 when synaptic contacts are made. Wnt1, Wnt3a, and Wnt5a are expressed in ventral midbrain dopaminergic neurons at the end of the embryonic stage in rats [125], whereas motoneurons of the lateral motor column express Wnt3 as they form synapses [126,127]. A growing body of evidence shows that each Wnt ligand plays a key role in the development and maturation of a given neuronal circuit [105,[128][129][130], promoting specific processes, such as axon guidance [131], dendritic spinogenesis and synapse formation [132,133].…”