Jay Z. Parrish,
Published: August 7, 2020
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AbstractTo remodel functional neuronal connectivity, neurons often alter dendrite arbors through elimination and subsequent regeneration of dendritic branches. However, the intrinsic mechanisms underlying this developmentally programmed dendrite regeneration and whether it shares common machinery with injury-induced regeneration remain largely unknown. Drosophila class IV dendrite arborization (C4da) sensory neurons regenerate adult-specific dendrites after eliminating larval dendrites during metamorphosis. Here we show that the microRNA miR-87 is a critical regulator of dendrite regeneration in Drosophila. miR-87 knockout impairs dendrite regeneration after developmentally-programmed pruning, whereas miR-87 overexpression in C4da neurons leads to precocious initiation of dendrite regeneration. Genetic analyses indicate that the transcriptional repressor Tramtrack69 (Ttk69) is a functional target for miR-87-mediated repression as ttk69 expression is increased in miR-87 knockout neurons and reducing ttk69 expression restores dendrite regeneration to mutants lacking miR-87 function. We further show that miR-87 is required for dendrite regeneration after acute injury in the larval stage, providing a mechanistic link between developmentally programmed and injury-induced dendrite regeneration. These findings thus indicate that miR-87 promotes dendrite regrowth during regeneration at least in part through suppressing Ttk69 in Drosophila sensory neurons and suggest that developmental and injury-induced dendrite regeneration share a common intrinsic mechanism to reactivate dendrite growth.
Dendrites are the primary sites for synaptic and sensory inputs. To remodel or repair neuronal connectivity, dendrites often exhibit large-scale structural changes that can be triggered by developmental signals, alterations in sensory inputs, or injury. Despite the importance of dendritic remodeling to nervous system function, the molecular basis for this remodeling is largely unknown. Here we used an unbiased genetic screen and in vivo imaging in Drosophila sensory neurons to demonstrate that the microRNA miR-87 is a critical factor required in neurons to reactivate dendritic growth both in developmental remodeling and following injury. Our work supports the model that miR-87 promotes dendrite regeneration by blocking expression of the transcriptional repressor Tramtrack69 in neurons. This study thus establishes a role for miRNAs in temporal control of dendrite regeneration.
Citation: Kitatani Y, Tezuka A, Hasegawa E, Yanagi S, Togashi K, Tsuji M, et al. (2020) Drosophila miR-87 promotes dendrite regeneration by targeting the transcriptional repressor Tramtrack69. PLoS Genet 16(8):
https://doi.org/10.1371/journal.pgen.1008942Editor: Bing Ye, University of Michigan, UNITED STATESReceived: March 3, 2020; Accepted: June 17, 2020; Published: August 7, 2020Copyright: © 2020 Kitatani et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Data Availability: All relevant data are within the manuscript and its supporting information files.Funding: This work was supported by MEXT Grants-in-Aid for Scientific Research on Innovative Areas ‘‘Dynamic regulation of brain function by Scrap & Build system’’ (KAKENHI 16H06456), JSPS (KAKENHI 16H02504), WPI-IRCN, AMED-CREST (JP18gm0610014), JST-CREST, the Strategic Research Program for Brain Sciences, Toray Foundation, Naito Foundation, Takeda Science Foundation, and Uehara Memorial Foundation to KE; and by grants from the National Institutes of Health (NINDS R01 NS076614) and a JSPS invitational fellowship to JZP. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Competing interests: The authors declare that no conflicts of interest exist.
IntroductionDuring critical periods of brain development, neurons exhibit juvenile plasticity in which connectivity can be modified in response to sensory inputs. To achieve these changes in connectivity, neurons often remodel their dendrite shape by elimination and subsequent regeneration of dendritic branches. For instance, Purkinje cells in the mouse cerebellum initially eliminate all perisomatic dendrites followed by regenerating single stem dendritic branches to form mature dendritic trees during postnatal development . Likewise, during early postnatal development, layer 4 neurons in the mouse barrel cortex refine their connectivity with thalamocortical axons by biased elimination and regeneration of preexisting dendritic branches [2,3]. Over time, many types of neurons progressively reduce dynamics and stabilize their dendritic arbors as they mature [4–6]. However, dendritic arbors of mature neurons can undergo dramatic regeneration under pathological conditions such as epilepsy and after injury [7–9]. Therefore, understanding the mechanisms that underlie dendrite regeneration has important implications for understanding normal development of functional dendrite arbors and functional repair of injured neural circuits.
Drosophila class IV dendrite arborization (C4da) neurons exhibit both developmentally programmed and damage-induced dendrite regeneration and therefore present a genetically tractable and optically accessible model to study cellular and molecular mechanisms underlying dendrite remodeling [10–12]. During metamorphosis, dendrites which elaborate during larval stages are completely pruned away and subsequently replaced with adult-specific dendritic arbors [13–18] (Fig 1A). This developmental dendrite regeneration after pruning requires intrinsic factors including transcriptional factors  as well as extrinsic mechanisms such as remodeling of the extracellular matrix [17, 18]. Recent studies indicate that, in addition to this developmental dendrite regeneration, removal of a part of dendritic branches during larval stages triggers robust dendrite regeneration in C4da neurons [20–23]. In the course of injury-induced dendrite regeneration, a new dendritic process initiates growth at the severed stump by ~24 hrs after injury and then further elongates and elaborates dendritic arbors by ~72 hrs after injury [20, 21]. This progression observed in the injury-induced dendrite regeneration is morphologically similar to what has been reported during developmental dendrite regeneration, but it is unknown whether these dendrite regrowth programs share common mechanisms.
Fig 1. miR-87 is required for dendrite regeneration during metamorphosis.(A) A schematic model of developmental dendrite regeneration in C4da neurons. (B) Dendrite regeneration in wild-type control (WT) and miR-87 knockout (miR-87) C4da neurons at the indicated time points. WP, white pupa; APF, after pupa formation; Adult 1 day, 1 day post-eclosion. Scale bar=100 μm. (C-D) Quantitative comparison of total dendrite length in wild-type (WT) and miR-87 KO (miR-87) C4da neurons during pupal (C) and adult (D) stages. Points depict mean values, error bars indicate standard deviation values. n=30. *p