The Wnt and BMP families of signaling morphogens at the vertebrate neuromuscular junction

Abstract

The neuromuscular junction has been extensively employed in order to identify crucial determinants of synaptogenesis. At the vertebrate neuromuscular synapse, extracellular matrix and signaling proteins play stimulatory and inhibitory roles on the assembly of functional synapses. Studies in invertebrate species have revealed crucial functions of early morphogens during the assembly and maturation of the neuromuscular junction. Here, we discuss growing evidence addressing the function of Wnt and Bone morphogenetic protein (BMP) signaling pathways at the vertebrate neuromuscular synapse. We focus on the emerging role of Wnt proteins as positive and negative regulators of postsynaptic differentiation. We also address the possible involvement of BMP pathways on motor neuron behavior for the assembly and/or regeneration of the neuromuscular junction.

Keywords: BMP; Wnt; neuromuscular junction.

Figure 1

Wnt ligands modulate postsynaptic differentiation at the vertebrate NMJ. (A) Wnts induce aneural AChR clustering. In zebrafish, Wnt11r silencing results in strong axonal branching defects associated with impaired AChR pre-patterning, similar to MuSK null mutants (middle panel). MuSK rescue after pre-patterning inhibition (induced by MuSK depletion), resulted in mislocalized, but still functional, neuromuscular synapses (right panel); (B) Wnts as positive cues for postsynaptic differentiation. Wnt3 and agrin released from motoneurons collaborate to promote the formation of AChR clusters. Wnt3-induced AChR microclusters via Rac1 are converted into large clusters by agrin, which promotes the further activation of Rac1 and Rho; (C) Wnt-dependent disaggregation of AChR clusters. Wnt3a, secreted by muscle cells at the stages of NMJ formation, activates a β-catenin pathway that induces the dispersal of AChR clusters through the inhibition of rapsyn expression.

Figure 2

BMP signaling. Smad signaling is initiated upon phosphorylation of Smad-1, -5 or -8 by a heteromeric complex of BMPRII, BMPRI and BMP ligand. Phosphorylated Smads recruit Smad-4 and translocate to the nucleus where, in association with co-repressors or co-activators, regulate the transcription of specific target genes. Non Smad pathways, such as p38 MAP kinase and PI3 kinase, are also initiated by heteromeric complexes. The long cytoplasmic tail of BMPRII binds to cytoskeleton regulators mediating actin remodeling (LIMK1) and microtubule stabilization (JNK).

Figure 3

BMP signaling in motor neurons. (A) In Drosophila, the evidence suggests that a retrograde signal involving homologues of the BMP pathway regulates synaptic growth, assembly and maintenance by pathways related to either Smad or cytoskeleton regulators; (B) In vertebrates, spinal cord injury induces an increase of BMP ligands around the injury site. BMP receptors and Smad1 phosphorylation is also increased in spinal neurons, revealing the ability of BMP to transduce Smad-dependent signaling. However, the target genes induced by Smad pathways or the mechanisms involving local cytoskeletal rearrangement are unknown.