Microtubule nucleation and organization in dendrites

Abstract

Dendrite branching is an essential process for building complex nervous systems. It determines the number, distribution and integration of inputs into a neuron, and is regulated to create the diverse dendrite arbor branching patterns characteristic of different neuron types. The microtubule cytoskeleton is critical to provide structure and exert force during dendrite branching. It also supports the functional requirements of dendrites, reflected by differential microtubule architectural organization between neuron types, illustrated here for sensory neurons. Both anterograde and retrograde microtubule polymerization occur within growing dendrites, and recent studies indicate that branching is enhanced by anterograde microtubule polymerization events in nascent branches. The polarities of microtubule polymerization events are regulated by the position and orientation of microtubule nucleation events in the dendrite arbor. Golgi outposts are a primary microtubule nucleation center in dendrites and share common nucleation machinery with the centrosome. In addition, pre-existing dendrite microtubules may act as nucleation sites. We discuss how balancing the activities of distinct nucleation machineries within the growing dendrite can alter microtubule polymerization polarity and dendrite branching, and how regulating this balance can generate neuron type-specific morphologies.

Keywords: Augmin; Golgi outpost; dendrite; microtubule nucleation; microtubule polarity; neuron morphology; pericentriolar material.

Figure 1.

Microtubule organization varies in the dendrites of different neuron types. (A) The dendrites of Drosophila body wall nociceptive class IV neurons contain a sparse microtubule organization (black arrowheads). (B) The dendrites of Drosophila body wall proprioceptive class I neuron contain dense arrays of microtubules (black arrowhead), which are interlinked by bridges (whited arrowheads). In addition, different modes of linkage between the neurons and the body wall also highlight their divergent functions. (C) Class IV neurons have dendrites embedded in the epithelial cells of the body wall. (D) Class I dendrite microtubules are embedded in a dense matrix (green arrowheads), and attach to the surface of the epithelial cells by pads of electron dense material (red arrowheads). This specialized architecture in class I neurons is similar to that found in other cells active in mechanotransduction. Pseudo-coloration in panels C-E: blue – dendrite; yellow – epithelial cell; uncolored – basement membrane. Scale bars: 0.2 μm.

Figure 2.

(A) The centrosome (the structure of interphase centrosome is illustrated⁸⁶) and (B) somatic Golgi / dendritic Golgi outposts share common molecular machinery for microtubule nucleation. (C) Microtubule nucleation from pre-existing microtubules by the Augmin complex. Illustrations adapted from references.

Figure 3.

(A) Anterograde polymerizing microtubule invasion into nascent dendrite branches drives outgrowth. Microtubules nucleated within termini or at the branch site are more likely to drive an extension event. At a lower frequency, similar to the invasion of microtubules into the spines of mature dendrites, both retrograde- or anterograde-oriented microtubules in the main branch enter nascent dendrite branches, becoming anterograde-directed within the termini. (B) The relative contributions of Golgi- and non-Golgi-derived polymerizing microtubules to the dendrite termini. Golgi outpost-associated events contribute approximately equally to both anterograde and retrograde polymerizing microtubules. In cnn mutant neurons, there is an increase in non-Golgi microtubules that polymerize in the anterograde direction; this increase is not accounted for by the loss of Golgi outpost-associated retrograde polymerization events. These data suggest that nucleation machinery at outposts might counteract the activity of additional factors that promote anterograde bias. (C) A model for how arbor complexity can be regulated by modulating the balance between these distinct Golgi- and non-Golgi-associated pathways that orient microtubule polymerization. A single outpost is shown here; it should be noted, however, that throughout the arbor individual Golgi outposts can give rise to either repeated anterograde or retrograde nucleation events. During dendrite outgrowth, Cnn recruits microtubule nucleation to Golgi outposts to suppress an activity (that may be due to Augmin, here illustrated by the Drosophila Augmin component Wac⁶²), which in turn promotes anterograde polymerization.