Distinct Target-Derived Signals Organize Formation, Maturation, and Maintenance of Motor Nerve Terminals

Abstract

Target-derived factors organize synaptogenesis by promoting differentiation of nerve terminals at synaptic sites. Several candidate organizing molecules have been identified based on their bioactivities in vitro, but little is known about their roles in vivo. Here, we show that three sets of organizers act sequentially to pattern motor nerve terminals: FGFs, beta2 laminins, and collagen alpha(IV) chains. FGFs of the 7/10/22 subfamily and broadly distributed collagen IV chains (alpha1/2) promote clustering of synaptic vesicles as nerve terminals form. beta2 laminins concentrated at synaptic sites are dispensable for embryonic development of nerve terminals but are required for their postnatal maturation. Synapse-specific collagen IV chains (alpha3-6) accumulate only after synapses are mature and are required for synaptic maintenance. Thus, multiple target-derived signals permit discrete control of the formation, maturation, and maintenance of presynaptic specializations.

Acetylcholine negatively regulates development of the neuromuscular junction through distinct cellular mechanisms

“…MNs were treated overnight with FGF9 or FGF22 to induce aggregation of synaptic vesicles, washed with medium to remove FGFs, and then treated with the ACh agonist carbachol (CCh). Consistent with previous studies (43,44), FGFs induce formation of Syn-and SV2-immunoreactive varicosities (Fig. 5A, arrows), but treatment with CCh markedly reduces the number of these varicosities (Fig.…”

“…According to this model, presynaptic specialization occurs only in nerve terminals closely opposed to these agrin-stabilized postsynaptic AChR clusters and thus, is downstream of postsynaptic differentiation (16). This bottom-up model is also consistent with the idea that agrin-stabilized AChR clusters release a retrograde factor that specializes presynaptic terminals (43,44). However, the current results suggest that this model may be too simple and that a parallel top-down pathway also exists to regulate NMJ development.…”

“…The presence of an alternative pathway by which ACh negatively regulates synaptic differentiation prompted us to examine if and how agrin might antagonize this pathway. Recent studies show that musclederived organizing molecules, including selective members of the fibroblast growth factor (FGF) family, play essential roles in regulating presynaptic specialization (43,44). To investigate if the retrograde signal induced by agrin was a member of this family, RNA was isolated from C2C12 myotubes treated with Fig.…”

The MuSK activator agrin has a separate role essential for postnatal maintenance of neuromuscular synapses

“…Therefore, agrin may evoke signals other than MuSK activation by binding to laminins and/or Lrp4. Because the laminin β2-subunit acts as an essential presynaptic organizer after, but not before, birth by binding to the presynaptic voltage-gated calcium channels at NMJs (33)(34)(35), agrin might promote the presynaptic specialization essential for NMJ maintenance via interaction with the laminins containing the β2-subunit. Notably, this hypothetical role for agrin could explain our finding that agrin deficiency impaired motor innervation as judged by the coverage of presynaptic nerve terminals over postsynaptic AChR clusters (Fig.…”

“…Given that the exogenous Dok-7-mediated MuSK activation was maintained at a high level ( Only a few elements that play indispensable roles in the postnatal maintenance, but not prenatal formation, of NMJs have been suggested (32). The laminin β2-subunit is one such element; mice lacking it show no defects in NMJ formation during embryogenesis and the first few postnatal days, but develop presynaptic defects by 1 wk after birth, leading to lethality during the third postnatal week (33)(34)(35). Interestingly, agrin-deficient mice are rescued by muscle-specific expression of "mini-agrin," which retains only agrin's laminin-binding and MuSK-activating (Lrp4-binding) regions (36).…”