Dynamic Microtubules Drive Circuit Rewiring in the Absence of Neurite Remodeling

Abstract

A striking neuronal connectivity change in C. elegans involves the coordinated elimination of existing synapses and formation of synapses at new locations, without altering neuronal morphology. Here, we investigate the tripartite interaction between dynamic microtubules (MTs), kinesin-1, and vesicular cargo during this synapse remodeling. We find that a reduction in the dynamic MT population in motor neuron axons, resulting from genetic interaction between loss of function in the conserved MAPKKK dlk-1 and an α-tubulin mutation, specifically blocks synapse remodeling. Using live imaging and pharmacological modulation of the MT cytoskeleton, we show that dynamic MTs are increased at the onset of remodeling and are critical for new synapse formation. DLK-1 acts during synapse remodeling, and its function involves MT catastrophe factors including kinesin-13/KLP-7 and spastin/SPAS-1. Through a forward genetic screen, we identify gain-of-function mutations in kinesin-1 that can compensate for reduced dynamic MTs to promote synaptic vesicle transport during remodeling. Our data provide in vivo evidence supporting the requirement of dynamic MTs for kinesin-1 dependent axonal transport and shed insight on the role of the MT cytoskeleton in facilitating neural circuit plasticity.

Neural circuit rewiring: insights from DD synapse remodeling

“…21 Much to our surprise, we found that while DD neurites switch polarity after synapse remodeling, with the ventral neurite assuming a dendritic and dorsal neurite an axonal identity, their MT polarity remained unchanged. 22 This observation shows that neurite identity could be uncoupled from MT polarity, and suggests that the specificity of axonal and dendritic cargo might be determined by factors besides the orientation of MTs.…”

“…One possible factor regulating cargo trafficking is the number of dynamic MTs, as we observed a dramatic increase in dynamic MTs during DD remodeling. 22 Indeed, DD remodeling was dependent on this increase in growing MTs, such that mutant animals with fewer dynamic MTs during remodeling failed to rewire their synapses to the dorsal neurite. 22 The increase in number of dynamic MTs facilitates synaptic vesicle transport mediated by plus-end directed motors UNC-104/Kinesin-3 and UNC-116/Kinesin-1 22 along the DD neuron commissure during remodeling.…”

“…22 Indeed, DD remodeling was dependent on this increase in growing MTs, such that mutant animals with fewer dynamic MTs during remodeling failed to rewire their synapses to the dorsal neurite. 22 The increase in number of dynamic MTs facilitates synaptic vesicle transport mediated by plus-end directed motors UNC-104/Kinesin-3 and UNC-116/Kinesin-1 22 along the DD neuron commissure during remodeling. There is also evidence pointing to the involvement of the cyclin-dependent kinase CDK-5 in modulating UNC-104 activity during remodeling.…”

“…23 MT polarity remains plus-end out in all DD neuron processes during remodeling, including those in the dorsal neurites. 22 Thus a combination of UNC-104 and the minus end directed DHC-1/dynein is required to pattern the newly formed synapses along the dorsal DD neurite. 23 Concurrent to new synapse formation to dorsal muscles, existing synapses are eliminated from the ventral neurite of DD neurons, and some synaptic vesicle components appear to be recycled during new synapse formation.…”

“…In conditions where MT structure is compromised due to a genetic alteration in a-tubulin, 25 loss of dlk-1 completely blocks DD remodeling. 22 The DLK-1 promoter (~2 kb upstream from the TSS) contains binding sites for LIN-14, 26 UNC-30 8 and UNC-55, 8 suggesting possible regulation by these transcription factors. DLK-1 transcript levels are globally upregulated from mid L1-L2 stage, 27 and a pulse of DLK-1 at the onset of DD remodeling facilitates new synapse formation.…”