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During development, mammalian neuromuscular junctions (NMJs) transit from multiple-innervation to single-innervation through axonal competition via unknown molecular mechanisms. Previously, using an in vitro model system, we demonstrated that the postsynaptic secretion of pro-brain-derived neurotrophic factor (proBDNF) stabilizes or eliminates presynaptic axon terminals, depending on its proteolytic conversion at synapses. Here, using developing mouse NMJs, we obtained in vivo evidence that proBDNF and mature BDNF (mBDNF) play roles in synapse elimination. We observed that exogenous proBDNF promoted synapse elimination, whereas mBDNF infusion substantially delayed synapse elimination. In addition, pharmacological inhibition of the proteolytic conversion of proBDNF to mBDNF accelerated synapse elimination via activation of p75 neurotrophin receptor (p75(NTR)). Furthermore, the inhibition of both p75(NTR) and sortilin signaling attenuated synapse elimination. We propose a model in which proBDNF and mBDNF serve as potential "punishment" and "reward" signals for inactive and active terminals, respectively, in vivo.

Protein synthesis inhibitor antibiotics are widely used to produce amnesia, and have been recognized to inhibit general or global mRNA translation in the basic translational machinery. For instance, anisomycin interferes with protein synthesis by inhibiting peptidyl transferase or the 80S ribosomal function. Therefore, de novo general or global protein synthesis has been thought to be necessary for long-term memory formation. However, it is unclear which mode of translation-gene-specific translation or general/global translation-is actually crucial for the memory consolidation process in mammalian brains. Here, we generated a conditional transgenic mouse strain in which double-strand RNA-dependent protein kinase (PKR)-mediated phosphorylation of eIF2alpha, a key translation initiation protein, was specifically increased in hippocampal CA1 pyramidal cells by the chemical inducer AP20187. Administration of AP20187 significantly increased activating transcription factor 4 (ATF4) translation and concomitantly suppressed CREB-dependent pathways in CA1 cells; this led to impaired hippocampal late-phase LTP and memory consolidation, with no obvious reduction in general translation. Conversely, inhibition of general translation by low-dose anisomycin failed to block hippocampal-dependent memory consolidation. Together, these results indicated that CA1-restricted genetic manipulation of particular mRNA translations is sufficient to impair the consolidation and that consolidation of memories in CA1 pyramidal cells through eIF2alpha dephosphorylation depends more on transcription/translation of particular genes than on overall levels of general translation. The present study sheds light on the critical importance of gene-specific translations for hippocampal memory consolidation.

Although neurotrophins elicit both acute and long-term effects, it is unclear whether the two modes of action are mediated by the same or different mechanisms. Using neuromuscular junction (NMJ) as a model system, we identified three characteristic features required for long-term, but not acute, forms of synaptic modulation by neurotrophin-3 (NT-3): endocytosis of NT-3-receptor complex, activation of the PI3 kinase substrate Akt, and new protein synthesis. Long-term effects were eliminated when NT-3 was conjugated to a bead that was too large to be endocytosed or when dominant-negative dynamin was expressed in presynaptic neurons. Presynaptic inhibition of Akt also selectively prevented NT-3-mediated long-term effects. Blockade of protein translation by the mammalian target of rapamycin inhibitor rapamycin prevented the long-term structural and functional changes at the NMJ, without affecting the acute potentiation of synaptic transmission by NT-3. These results reveal fundamental differences between acute and long-term modulation by neurotrophins.