Tau, XMAP215/Msps and Eb1 co-operate interdependently to regulate microtubule polymerisation and bundle formation in axons

Abstract

The formation and maintenance of microtubules requires their polymerisation, but little is known about how this polymerisation is regulated in cells. Focussing on the essential microtubule bundles in axons of Drosophila and Xenopus neurons, we show that the plus-end scaffold Eb1, the polymerase XMAP215/Msps and the lattice-binder Tau co-operate interdependently to promote microtubule polymerisation and bundle organisation during axon development and maintenance. Eb1 and XMAP215/Msps promote each other’s localisation at polymerising microtubule plus-ends. Tau outcompetes Eb1-binding along microtubule lattices, thus preventing depletion of Eb1 tip pools. The three factors genetically interact and show shared mutant phenotypes: reductions in axon growth, comet sizes, comet numbers and comet velocities, as well as prominent deterioration of parallel microtubule bundles into disorganised curled conformations. This microtubule curling is caused by Eb1 plus-end depletion which impairs spectraplakin-mediated guidance of extending microtubules into parallel bundles. Our demonstration that Eb1, XMAP215/Msps and Tau co-operate during the regulation of microtubule polymerisation and bundle organisation, offers new conceptual explanations for developmental and degenerative axon pathologies.

Re-evaluating the actin-dependence of spectraplakin functions during axon growth and maintenance

“…Shot also plays major roles in maintaining MT bundles in axon shafts. We confirmed here the importance of F-actin/MT/Eb1 cross-linkage for MT guidance into parallel bundles (Alves-Silva et al, 2012;Hahn et al, 2021;Figs.6, 8). We believe that these roles are merely permissive and not subject to F-actin dependent regulation, because F-actin networks in axon shafts appear sparse and far less dynamic when compared to GCs.…”

“…Previous work has demonstrated that Shot prevents MT curling through a F-actin/Eb1/MT guidance mechanism: via its N-terminus it binds cortical F-actin and via its C-terminus to MTs and Eb1 -thus guiding the extension of polymerising MTs along the axonal cortex into parallel bundles; this Factin/Eb1/MT guidance mechanism is supported by numerous structure-function, loss-of-function, pharmacological and genetic interaction studies (details in Fig.5;Alves-Silva et al, 2012;Hahn et al, 2021;Qu et al, 2019;Sánchez-Soriano et al, 2009).…”

“…In Shot-deficient neurons, MT bundles in axon shafts and GCs frequently disintegrate into disorganised, curled, criss-crossing arrangements (from now on referred to as MT curling). This dramatic MT phenotype can be rescued when reinstating actin-MT cross-linking activity of Shot, through a mechanism where Shot guides the extension of polymerising MTs along the axonal cortex into parallel bundles (Alves-Silva et al, 2012;Hahn et al, 2021;Sánchez-Soriano et al, 2010). Whereas the necessary C-terminal interaction of Shot with MTs is quite well described, we have little knowledge of the N-terminal interaction with neuronal F-actin networks, especially when considering that these can be of very different nature: presenting as sparse cortical F-actin rings in the axon shaft (Leterrier et al, 2017) or dense F-actin networks in GCs (Dent et al, 2011).…”

Tubulinopathy mutations in TUBA1A that disrupt neuronal morphogenesis and migration override XMAP215/Stu2 regulation of microtubule dynamics

“…Tight control over microtubule dynamics is essential for cells, and particularly in neuron morphogenesis and migration as the cytoskeletal must nimbly respond to cues throughout the migratory process (Kapitein and Hoogenraad, 2015). XMAP215/Stu2 works in cells as an extrinsic regulator to finely tune microtubule dynamics (Brouhard et al, 2008;Hahn et al, 2021;Lee et al, 2001;van der Vaart et al, 2011;Vasquez et al, 1994;Zanic et al, 2013). The role of XMAP215 in neurons has been explored primarily in the axon and growth cone.…”

With the Permission of Microtubules: An Updated Overview on Microtubule Function During Axon Pathfinding

“…Despite significant advances, experimental designs performed in non-neuronal cells or limited to few cytoskeleton-regulatory proteins and guidance cues, may not reflect the full scope of cytoskeletal changes triggered by extracellular guidance signaling during axon pathfinding. As a sign of the complex regulation of physiological MT dynamics in cells, recent data has demonstrated that MAP combinations exert collective effects on MTs and MAPs must follow certain hierarchies in their MT recruitment to achieve specific functions ( Niu et al, 2019 ; Hahn et al, 2021 ). Besides, in addition to stereotyped mechanisms of guidance signal transduction - including regulated guidance receptor expression, dimerization or trafficking - other molecular mechanisms underlying axon guidance decisions are being characterized ( Harada et al, 2020 ; Klein and Pasterkamp, 2021 ).…”