Distinct relations of microtubules and actin filaments with dendritic architecture

Abstract

Neuronal morphology underlies circuit connectivity and signal integration, thus profoundly affecting nervous system function. Although microtubules and F-actin have long been recognized as key determinants of dendritic morphology, their precise and distinct roles are yet to be fully elucidated. The lack of arbor-wide cytoskeletal information registered with neuromorphological measurements has so far limited direct observations of the interrelations between microtubules, F-actin and arbor architecture. Here we pair live confocal imaging of fluorescently labeled dendritic arborization (da) neurons in the fruit fly larva with our recently developed multi-signal neural tracing technique. We discovered that dendritic arbor length is highly dependent on local microtubule quantity, whereas local F-actin enrichment is associated with dendritic branching. Earlier computer simulations used quantifiable morphological features both as model parameters (e.g. branch thickness) and emergent properties (e.g. tree asymmetry). Multi-signal reconstructions allowed us to computationally simulate arbor structure solely constrained by experimentally measured, local subcellular distributions of microtubules and F-actin. The generative model yielded synthetic trees with morphological and molecular patterns statistically indistinguishable from those of real da neurons, demonstrating that these two investigated cytoskeletal components are sufficient to define dendritic architecture. The analysis and modeling outcomes hold true for the simplest (Class I) and the most complex (Class IV) da neuron subclasses. Moreover, local increases in the quantity of stable microtubules in Class IV neurons via Formin3 overexpression further strengthens the correlation between local microtubule quantity and dendritic arbor length as predicted by the model and reproduced in simulations. SUPPORT: NIH R01 NS39600 and NS086082 and BICCN U01 MH114829. SignificanceMicrotubules and F-actin are fundamental molecular factors in neuronal arbor architectural development. However, the elucidation of their exact roles in defining dendritic architecture has been hindered by the lack of arbor-wide cytoskeletal information in existing morphological data. Our recently developed enhanced digital tracing methodology allows the systematic quantification of subcellular protein distributions with overall morphology. These multi-signal reconstructions and the novel analyses reported in this study demonstrate two striking double-dissociations in cytoskeletal behavior: local microtubule is the best predictor of arbor length, while local enrichment in F-actin is strongly associated with branching. A computational model of arbor generation purely driven by local microtubule and F-actin quantities fully reproduces tree topology and dendritic cytoskeletal distributions. Moreover, these findings are robust across cell types and genetic alteration.

Dynamic Instability of Dendrite Tips Generates the Highly Branched Morphologies of Sensory Neurons

“…While several theoretical and computational models can produce dendrite-like branched morphologies, they are not grounded in molecular or development data. Early models, designed to describe and classify neurons, reconstituted morphologies based on the statistical properties of the observed arbors themselves (Ascoli and Krichmar, 2000;Nanda et al, 2018). Optimization-based models that minimize wiring (i.e., the total lengths of the branches) capture key features of neuronal morphology (Baltruschat et al, 2020;Cuntz et al, 2010), but lack connection to the cellular processes, as do models based on more abstract processes such as diffusion-limited aggregation (Luczak, 2006) and Turing-like pattern formation (Sugimura et al, 2007).…”

Achieving functional neuronal dendrite structure through sequential stochastic growth and retraction

“…It also possibly avoids the encoding of a deterministic morphogenetic program that may be more costly to implement genetically (Hiesinger and Hassan, 2018). In the future, it will be interesting to elucidate the mechanisms that control the temporal sequence of distinct stages of branch elaboration for the c1vpda sensory neurons (Grueber et al, 2003a;Jinushi-Nakao et al, 2007;Nanda et al, 2019) and on a higher scale to understand to what extent similar self-organising processes and mechanisms are implicated in the formation of other cell types (Ryglewski et al, 2017;Özel et al, 2015), neuronal networks (Hassan and Hiesinger, 2015) and even in the emergence of non-neuronal branching organs (Hannezo et al, 2017).…”

PP2A phosphatase regulates cell-type specific cytoskeletal organization to drive dendrite diversity

“…dendritic architecture, we used multi-fluorescent Drosophila transgenic lines to simultaneously visualize the F-actin (UAS-GMA) and MT (UAS-mCherry::Jupiter) cytoskeletons in a subtype-specific manner for both control neurons and those with gene-specific mutations (Das et al, 2017). We also built next generation neuroanatomical reconstruction tools for quantitative descriptions of the dendritic effects of gene-specific disruptions on cytoskeletal architecture (Das et al, 2017;Nanda et al, 2018Nanda et al, , 2020.…”