Kinesin Khc-73/KIF13B modulates retrograde BMP signaling by influencing endosomal dynamics at the Drosophila neuromuscular junction.

Retrograde signaling is essential for neuronal growth, function and survival; however, we know little about how signaling endosomes might be directed from synaptic terminals onto retrograde axonal pathways. We have identified Khc-73, a plus-end directed microtubule motor protein, as a regulator of sorting of endosomes in Drosophila larval motor neurons. The number of synaptic boutons and the amount of neurotransmitter release at the Khc-73 mutant larval neuromuscular junction (NMJ) are normal, but we find a significant decrease in the number of presynaptic release sites. This defect in Khc-73 mutant larvae can be genetically enhanced by a partial genetic loss of Bone Morphogenic Protein (BMP) signaling or suppressed by activation of BMP signaling in motoneurons. Consistently, activation of BMP signaling that normally enhances the accumulation of phosphorylated form of BMP transcription factor Mad in the nuclei, can be suppressed by genetic removal of Khc-73. Using a number of assays including live imaging in larval motor neurons, we show that loss of Khc-73 curbs the ability of retrograde-bound endosomes to leave the synaptic area and join the retrograde axonal pathway. Our findings identify Khc-73 as a regulator of endosomal traffic at the synapse and modulator of retrograde BMP signaling in motoneurons.

The C-terminal region of cytoplasmic polyadenylation element binding protein is a ZZ domain with potential for protein-protein interactions.

Cytoplasmic polyadenylation element binding protein (CPEB) provides temporal and spatial control of protein synthesis required for early development and neuronal synaptic plasticity. CPEB regulates protein expression by inhibiting polyadenylation of selected mRNA transcripts, which prevents binding of the ribosome for protein synthesis. Two RNA recognition motif domains and a C-terminal binuclear zinc-binding domain are required for mRNA binding, but the zinc-binding domain is not required for sequence-specific recognition of the targeted mRNA transcript. The structure and function of the zinc-binding domain of CPEB are unknown. The C-terminal region of CPEB may participate in assembly of the ribonucleoprotein complex that includes the scaffold protein, Symplekin, and the cleavage and polyadenylation specificity factor. Sumoylation of Symplekin is required for polyadenylation, and both cleavage and polyadenylation specificity factor and poly(A) polymerase are sumoylated. The foreshortened poly(A) tail is maintained by poly(A) ribonuclease, which associates with CPEB. While zinc-binding domains are renowned for nucleic acid recognition, binuclear zinc-binding structural motifs, such as LIM (Lin-11, Isl-1, Mec-3), RING (really interesting new gene), PHD (plant homeodomain) and ZZ (ZZ-type zinc finger) domains, participate in protein-protein interactions. Here, we report the solution structure of the C-terminal zinc-binding domain of CPEB1 (CPEB1-ZZ), which has a cross-braced zinc binding topology. The structural similarity to other ZZ domains suggests that the CPEB1-ZZ domain recruits sumoylated proteins during assembly of the ribonucleoprotein complex prior to mRNA export from the nucleus.

TOR is required for the retrograde regulation of synaptic homeostasis at the Drosophila neuromuscular junction.

Homeostatic mechanisms operate to stabilize synaptic function; however, we know little about how they are regulated. Exploiting Drosophila genetics, we have uncovered a critical role for the target of rapamycin (TOR) in the regulation of synaptic homeostasis at the Drosophila larval neuromuscular junction. Loss of postsynaptic TOR disrupts a retrograde compensatory enhancement in neurotransmitter release that is normally triggered by a reduction in postsynaptic glutamate receptor activity. Moreover, postsynaptic overexpression of TOR or a phosphomimetic form of S6 ribosomal protein kinase, a common target of TOR, can trigger a strong retrograde increase in neurotransmitter release. Interestingly, heterozygosity for eIF4E, a critical component of the cap-binding protein complex, blocks the retrograde signal in all these cases. Our findings suggest that cap-dependent translation under the control of TOR plays a critical role in establishing the activity dependent homeostatic response at the NMJ.

The Drosophila miR-310 cluster negatively regulates synaptic strength at the neuromuscular junction.

Emerging data implicate microRNAs (miRNAs) in the regulation of synaptic structure and function, but we know little about their role in the regulation of neurotransmission in presynaptic neurons. Here, we demonstrate that the miR-310-313 cluster is required for normal synaptic transmission at the Drosophila larval neuromuscular junction. Loss of miR-310-313 cluster leads to a significant enhancement of neurotransmitter release, which can be rescued with temporally restricted expression of mir-310-313 in larval presynaptic neurons. Kinesin family member, Khc-73 is a functional target for miR-310-313 as its expression is increased in mir-310-313 mutants and reducing it restores normal synaptic function. Cluster mutants show an increase in the active zone protein Bruchpilot accompanied by an increase in electron dense T bars. Finally, we show that repression of Khc-73 by miR-310-313 cluster influences the establishment of normal synaptic homeostasis. Our findings establish a role for miRNAs in the regulation of neurotransmitter release.

Retrograde BMP signaling controls synaptic growth at the NMJ by regulating trio expression in motor neurons.

Retrograde signaling is essential for coordinating the growth of synaptic structures; however, it is not clear how it can lead to modulation of cytoskeletal dynamics and structural changes at presynaptic terminals. We show that loss of retrograde bone morphogenic protein (BMP) signaling at the Drosophila larval neuromuscular junction (NMJ) leads to a significant reduction in levels of Rac GEF Trio and a diminution of transcription at the trio locus. We further find that Trio is required in motor neurons for normal structural growth. Finally, we show that transgenic expression of Trio in motor neurons can partially restore NMJ defects in larvae mutant for BMP signaling. Based on our findings, we propose a model in which a retrograde BMP signal from the muscle modulates GTPase activity through transcriptional regulation of Rac GEF trio, thereby regulating the homeostasis of synaptic growth at the NMJ.