Efa6 protects axons and regulates their growth and branching by inhibiting microtubule polymerisation at the cortex

The microtubule regulator EFA-6 forms cortical foci dependent on its intrinsically disordered region and interactions with tubulins

The EFA6 protein family, originally identified as Sec7 guanine nucleotide exchange factors, has also been found to regulate cortical microtubule (MT) dynamics. Here, we find that in the mature C. elegans epidermal epithelium, EFA-6 forms punctate foci in specific regions of the apical cortex, dependent on its intrinsically disordered region (IDR). The EFA-6 IDR can form biomolecular condensates in vitro. In genetic screens for mutants with altered GFP::EFA-6 localization, we identified a gain-of-function (gf) mutation in α-tubulin tba-1 that induces ectopic EFA-6 foci in multiple cell types. Lethality of tba-1(gf) is partially suppressed by loss of function in efa-6. The ability of TBA-1(gf) to trigger ectopic EFA-6 foci requires β-tubulin TBB-2 and the chaperon EVL-20/Arl2. tba-1(gf)-induced EFA-6 foci display slower turnover, contain the MT-associated protein TAC-1/TACC, and require the EFA-6 MT elimination domain (MTED). Our results reveal functionally important crosstalk between cellular tubulins and cortical MT regulators in vivo.

KLP-7/Kinesin-13 orchestrates axon-dendrite checkpoints for polarized trafficking in neurons

The polarized nature of neurons depends on their microtubule dynamics and orientation determined by both microtubule-stabilizing and destabilizing factors. The role of destabilizing factors in developing and maintaining neuronal polarity is unclear. We investigated the function of KLP-7, a microtubule depolymerizing motor of the Kinesin-13 family, in axon-dendrite compartmentalization using PVD neurons in Caenorhabditis elegans. Loss of KLP-7 caused a mislocalization of axonal proteins, including RAB-3, SAD-1, and their motor UNC-104, to dendrites. This is rescued by cell-autonomous expression of the KLP-7 or colchicine treatment, indicating the involvement of KLP-7-dependent microtubule depolymerization. The high mobility of KLP-7 is correlated to increased microtubule dynamics in the dendrites, which restricts the enrichment of UNC-44, an integral component of Axon Initial Segment (AIS) in these processes. Due to the loss of KLP-7, ectopic enrichment of UNC-44 in the dendrite potentially redirects axonal traffic into dendrites that include plus-end out microtubules, axonal motors, and cargoes. These observations indicate that KLP-7-mediated depolymerization defines the microtubule dynamics conducive to the specific enrichment of AIS components in dendrites. This further compartmentalizes dendritic and axonal microtubules, motors, and cargoes, thereby influencing neuronal polarity.

A methodology for specific disruption of microtubule polymerization into dendritic spines

Dendritic spines, the mushroom-shaped extensions along dendritic shafts of excitatory neurons, are critical for synaptic function and are one of the first neuronal structures disrupted in neurodevelopmental and neurodegenerative diseases. Microtubule (MT) polymerization into dendritic spines is an activity-dependent process capable of affecting spine shape and function. Studies have shown that MT polymerization into spines occurs specifically in spines undergoing plastic changes. However, discerning the function of MT invasion of dendritic spines requires the specific inhibition of MT polymerization into spines, while leaving MT dynamics in the dendritic shaft, synaptically connected axons and associated glial cells intact. This is not possible with the unrestricted, bath application of pharmacological compounds. To specifically disrupt MT entry into spines we coupled a MT elimination domain (MTED) from the Efa6 protein to the actin filament-binding peptide LifeAct. LifeAct was chosen because actin filaments are highly concentrated in spines and are necessary for MT invasions. Temporally controlled expression of this LifeAct-MTED construct inhibits MT entry into dendritic spines, while preserving typical MT dynamics in the dendrite shaft. Expression of this construct will allow for the determination of the function of MT invasion of spines and more broadly, to discern how MT-actin interactions affect cellular processes.

The microtubule regulator EFA-6 forms spatially restricted cortical foci dependent on its intrinsically disordered region and interactions with tubulins

Microtubules (MTs) are dynamic components of the cytoskeleton and play essential roles in morphogenesis and maintenance of tissue and cell integrity. Despite recent advances in understanding MT ultrastructure, organization, and growth control, how cells regulate MT organization at the cell cortex remains poorly understood. The EFA-6/EFA6 proteins are recently identified membrane-associated proteins that inhibit cortical MT dynamics. Here, combining visualization of endogenously tagged C. elegans EFA-6 with genetic screening, we uncovered tubulin-dependent regulation of EFA-6 patterning. In the mature epidermal epithelium, EFA-6 forms punctate foci in specific regions of the apical cortex, dependent on its intrinsically disordered region (IDR). We further show the EFA-6 IDR is sufficient to form biomolecular condensates in vitro. In screens for mutants with altered GFP::EFA-6 localization, we identified a novel gain-of-function (gf) mutation in an α-tubulin tba-1 that induces ectopic EFA-6 foci in multiple cell types. tba-1(gf) animals exhibit temperature-sensitive embryonic lethality, which is partially suppressed by efa-6(lf), indicating the interaction between tubulins and EFA-6 is important for normal development. TBA-1(gf) shows reduced incorporation into filamentous MTs but has otherwise mild effects on cellular MT organization. The ability of TBA-1(gf) to trigger ectopic EFA-6 foci formation requires β-tubulin TBB-2 and the chaperon EVL-20/Arl2. The tba-1(gf)-induced EFA-6 foci display slower turnover, contain the MT-associated protein TAC-1/TACC, and require the EFA-6 MTED. Our results reveal a novel crosstalk between cellular tubulins and cortical MT regulators in vivo.

A Methodology for Specific Disruption of Microtubules in Dendritic Spines

Dendritic spines, the mushroom-shaped extensions along dendritic shafts of excitatory neurons, are critical for synaptic function and are one of the first neuronal structures disrupted in neurodevelopmental and neurodegenerative diseases. Microtubule (MT) polymerization into dendritic spines is an activity-dependent process capable of affecting spine shape and function. Studies have shown that MT polymerization into spines occurs specifically in spines undergoing plastic changes. However, discerning the function of MT invasion of dendritic spines requires the specific inhibition of MT polymerization into spines, while leaving MT dynamics in the dendritic shaft, synaptically connected axons and associated glial cells intact. This is not possible with the unrestricted, bath application of pharmacological compounds. To specifically disrupt MT entry into spines we coupled a MT elimination domain (MTED) from the Efa6 protein to the actin filament-binding peptide LifeAct. LifeAct was chosen because actin filaments are highly concentrated in spines and are necessary for MT invasions. Temporally controlled expression of this LifeAct-MTED construct inhibits MT entry into dendritic spines, while preserving typical MT dynamics in the dendrite shaft. Expression of this construct will allow for the determination of the function of MT invasion of spines and more broadly, to discern how MT-actin interactions affect cellular processes.

Neuronal ageing is promoted by the decay of the microtubule cytoskeleton

Natural ageing is accompanied by a decline in motor, sensory, and cognitive functions, all impacting quality of life. Ageing is also the predominant risk factor for many neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease. We need to therefore gain a better understanding of the cellular and physiological processes underlying age-related neuronal decay. However, gaining this understanding is a slow process due to the large amount of time required to age mammalian or vertebrate animal models. Here, we introduce a new cellular model within the Drosophila brain, in which we report classical ageing hallmarks previously observed in the primate brain. These hallmarks include axonal swellings, cytoskeletal decay, a reduction in axonal calibre, and morphological changes arising at synaptic terminals. In the fly brain, these changes begin to occur within a few weeks, ideal to study the underlying mechanisms of ageing. We discovered that the decay of the neuronal microtubule (MT) cytoskeleton precedes the onset of other ageing hallmarks. We showed that the MT-binding factors Tau, EB1, and Shot/MACF1, are necessary for MT maintenance in axons and synapses, and that their functional loss during ageing triggers MT bundle decay, followed by a decline in axons and synaptic terminals. Furthermore, genetic manipulations that improve MT networks slowed down the onset of neuronal ageing hallmarks and confer aged specimens the ability to outperform age-matched controls. Our work suggests that MT networks are a key lesion site in ageing neurons and therefore the MT cytoskeleton offers a promising target to improve neuronal decay in advanced age.

How neurons maintain their axons long-term: an integrated view of axon biology and pathology

Axons are processes of neurons, up to a metre long, that form the essential biological cables wiring nervous systems. They must survive, often far away from their cell bodies and up to a century in humans. This requires self-sufficient cell biology including structural proteins, organelles, and membrane trafficking, metabolic, signalling, translational, chaperone, and degradation machinery-all maintaining the homeostasis of energy, lipids, proteins, and signalling networks including reactive oxygen species and calcium. Axon maintenance also involves specialised cytoskeleton including the cortical actin-spectrin corset, and bundles of microtubules that provide the highways for motor-driven transport of components and organelles for virtually all the above-mentioned processes. Here, we aim to provide a conceptual overview of key aspects of axon biology and physiology, and the homeostatic networks they form. This homeostasis can be derailed, causing axonopathies through processes of ageing, trauma, poisoning, inflammation or genetic mutations. To illustrate which malfunctions of organelles or cell biological processes can lead to axonopathies, we focus on axonopathy-linked subcellular defects caused by genetic mutations. Based on these descriptions and backed up by our comprehensive data mining of genes linked to neural disorders, we describe the 'dependency cycle of local axon homeostasis' as an integrative model to explain why very different causes can trigger very similar axonopathies, providing new ideas that can drive the quest for strategies able to battle these devastating diseases.

Maintaining essential microtubule bundles in meter-long axons: a role for local tubulin biogenesis?

Axons are the narrow, up-to-meter long cellular processes of neurons that form the biological cables wiring our nervous system. Most axons must survive for an organism's lifetime, i.e. up to a century in humans. Axonal maintenance depends on loose bundles of microtubules that run without interruption all along axons. The continued turn-over and the extension of microtubule bundles during developmental, regenerative or plastic growth requires the availability of α/β-tubulin heterodimers up to a meter away from the cell body. The underlying regulation in axons is poorly understood and hardly features in past and contemporary research. Here we discuss potential mechanisms, particularly focussing on the possibility of local tubulin biogenesis in axons. Current knowledge might suggest that local translation of tubulin takes place in axons, but far less is known about the post-translational machinery of tubulin biogenesis involving three chaperone complexes: prefoldin, CCT and TBC. We discuss functional understanding of these chaperones from a range of model organisms including yeast, plants, flies and mice, and explain what is known from human diseases. Microtubules across species depend on these chaperones, and they are clearly required in the nervous system. However, most chaperones display a high degree of functional pleiotropy, partly through independent functions of individual subunits outside their complexes, thus posing a challenge to experimental studies. Notably, we found hardly any studies that investigate their presence and function particularly in axons, thus highlighting an important gap in our understanding of axon biology and pathology.

Activating RAC1 variants in the switch II region cause a developmental syndrome and alter neuronal morphology

RAC1 is a highly conserved Rho GTPase critical for many cellular and developmental processes. De novo missense RAC1 variants cause a highly variable neurodevelopmental disorder. Some of these variants have previously been shown to have a dominant negative effect. Most previously reported patients with this disorder have either severe microcephaly or severe macrocephaly. Here, we describe eight patients with pathogenic missense RAC1 variants affecting residues between Q61 and R68 within the switch II region of RAC1. These patients display variable combinations of developmental delay, intellectual disability, brain anomalies such as polymicrogyria and cardiovascular defects with normocephaly or relatively milder micro- or macrocephaly. Pulldown assays, NIH3T3 fibroblast spreading assays and staining for activated PAK1/2/3 and WAVE2 suggest that these variants increase RAC1 activity and over-activate downstream signalling targets. Axons of neurons isolated from Drosophila embryos expressing the most common of the activating variants are significantly shorter, with an increased density of filopodial protrusions. In vivo, these embryos exhibit frequent defects in axonal organization. Class IV dendritic arborization neurons expressing this variant exhibit a significant reduction in the total area of the dendritic arbour, increased branching and failure of self-avoidance. RNAi knock down of the WAVE regulatory complex component Cyfip significantly rescues these morphological defects. These results establish that activating substitutions affecting residues Q61-R68 within the switch II region of RAC1 cause a developmental syndrome. Our findings reveal that these variants cause altered downstream signalling, resulting in abnormal neuronal morphology and reveal the WAVE regulatory complex/Arp2/3 pathway as a possible therapeutic target for activating RAC1 variants. These insights also have the potential to inform the mechanism and therapy for other disorders caused by variants in genes encoding other Rho GTPases, their regulators and downstream effectors.

Drosophila GSK3β promotes microtubule disassembly and dendrite pruning in sensory neurons

The evolutionarily conserved Glycogen Synthase Kinase 3β (GSK3β), a negative regulator of microtubules, is crucial for neuronal polarization, growth and migration during animal development. However, it remains unknown whether GSK3β regulates neuronal pruning, which is a regressive process. Here, we report that the Drosophila GSK3β homologue Shaggy (Sgg) is cell-autonomously required for dendrite pruning of ddaC sensory neurons during metamorphosis. Sgg is necessary and sufficient to promote microtubule depolymerization, turnover and disassembly in the dendrites. Although Sgg is not required for the minus-end-out microtubule orientation in dendrites, hyperactivated Sgg can disturb the dendritic microtubule orientation. Moreover, our pharmacological and genetic data suggest that Sgg is required to promote dendrite pruning at least partly via microtubule disassembly. We show that Sgg and Par-1 kinases act synergistically to promote microtubule disassembly and dendrite pruning. Thus, Sgg and Par-1 might converge on and phosphorylate a common downstream microtubule-associated protein(s) to disassemble microtubules and thereby facilitate dendrite pruning.

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

Axons are the long and slender processes of neurons constituting the biological cables that wire the nervous system. The growth and maintenance of axons require loose microtubule bundles that extend through their entire length. Understanding microtubule regulation is therefore an essential aspect of axon biology. Key regulators of neuronal microtubules are the spectraplakins, a well-conserved family of cytoskeletal cross-linkers that underlie neuropathies in mouse and humans. Spectraplakin deficiency in mouse or Drosophila causes severe decay of microtubule bundles and reduced axon growth. The underlying mechanisms are best understood for Drosophila's spectraplakin Short stop (Shot) and believed to involve cytoskeletal cross-linkage: Shot's binding to microtubules and Eb1 via its C-terminus has been thoroughly investigated, whereas its F-actin interaction via N-terminal calponin homology (CH) domains is little understood. Here, we have gained new understanding by showing that the F-actin interaction must be finely balanced: altering the properties of F-actin networks or deleting/exchanging Shot's CH domains induces changes in Shot function-with a Lifeact-containing Shot variant causing remarkable remodeling of neuronal microtubules. In addition to actin-microtubule (MT) cross-linkage, we find strong indications that Shot executes redundant MT bundle-promoting roles that are F-actin-independent. We argue that these likely involve the neuronal Shot-PH isoform, which is characterized by a large, unexplored central plakin repeat region (PRR) similarly existing also in mammalian spectraplakins.

Drosophila Primary Neuronal Cultures as a Useful Cellular Model to Study and Image Axonal Transport

The use of primary neuronal cultures generated from Drosophila tissue provides a powerful model for studies of transport mechanisms. Cultured fly neurons provide similarly detailed subcellular resolution and applicability of pharmacology or fluorescent dyes as mammalian primary neurons. As an experimental advantage for the mechanistic dissection of transport, fly primary neurons can be combined with the fast and highly efficient combinatorial genetics of Drosophila, and genetic tools for the manipulation of virtually every fly gene are readily available. This strategy can be performed in parallel to in vivo transport studies to address relevance of any findings. Here we will describe the generation of primary neuronal cultures from Drosophila embryos and larvae, the use of external fluorescent dyes and genetic tools to label cargo, and the key strategies for live imaging and subsequent analysis.

A systematic analysis of microtubule-destabilizing factors during dendrite pruning in Drosophila

It has long been thought that microtubule disassembly, one of the earliest cellular events, contributes to neuronal pruning and neurodegeneration in development and disease. However, how microtubule disassembly drives neuronal pruning remains poorly understood. Here, we conduct a systematic investigation of various microtubule-destabilizing factors and identify exchange factor for Arf6 (Efa6) and Stathmin (Stai) as new regulators of dendrite pruning in ddaC sensory neurons during Drosophila metamorphosis. We show that Efa6 is both necessary and sufficient to regulate dendrite pruning. Interestingly, Efa6 and Stai facilitate microtubule turnover and disassembly prior to dendrite pruning without compromising the minus-end-out microtubule orientation in dendrites. Moreover, our pharmacological and genetic manipulations strongly support a key role of microtubule disassembly in promoting dendrite pruning. Thus, this systematic study highlights the importance of two selective microtubule destabilizers in dendrite pruning and substantiates a causal link between microtubule disassembly and neuronal pruning.

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

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.

Cell-type-specific, multicolor labeling of endogenous proteins with split fluorescent protein tags in Drosophila

The impact of the Drosophila experimental system on studies of modern biology cannot be understated. The ability to tag endogenously expressed proteins is essential to maximize the use of this model organism. Here, we describe a method for labeling endogenous proteins with self-complementing split fluorescent proteins (split FPs) in a cell-type-specific manner in Drosophila A short fragment of an FP coding sequence is inserted into a specific genomic locus while the remainder of the FP is expressed using an available GAL4 driver line. In consequence, complementation fluorescence allows examination of protein localization in particular cells. Besides, when inserting tandem repeats of the short FP fragment at the same genomic locus, we can substantially enhance the fluorescence signal. The enhanced signal is of great value in live-cell imaging at the subcellular level. We can also accomplish a multicolor labeling system with orthogonal split FPs. However, other orthogonal split FPs do not function for in vivo imaging besides split GFP. Through protein engineering and in vivo functional studies, we report a red split FP that we can use for duplexed visualization of endogenous proteins in intricate Drosophila tissues. Using the two orthogonal split FP systems, we have simultaneously imaged proteins that reside in distinct subsynaptic compartments. Our approach allows us to study the proximity between and localization of multiple proteins endogenously expressed in essentially any cell type in Drosophila.

EFA6 in Axon Regeneration, as a Microtubule Regulator and as a Guanine Nucleotide Exchange Factor

Axon regeneration after injury is a conserved biological process that involves a large number of molecular pathways, including rapid calcium influx at injury sites, retrograde injury signaling, epigenetic transition, transcriptional reprogramming, polarized transport, and cytoskeleton reorganization. Despite the numerous efforts devoted to understanding the underlying cellular and molecular mechanisms of axon regeneration, the search continues for effective target molecules for improving axon regeneration. Although there have been significant historical efforts towards characterizing pro-regenerative factors involved in axon regeneration, the pursuit of intrinsic inhibitors is relatively recent. EFA6 (exchange factor for ARF6) has been demonstrated to inhibit axon regeneration in different organisms. EFA6 inhibition could be a promising therapeutic strategy to promote axon regeneration and functional recovery after axon injury. This review summarizes the inhibitory role on axon regeneration through regulating microtubule dynamics and through affecting ARF6 (ADP-ribosylation factor 6) GTPase-mediated integrin transport.

Fidgetin-like 2 negatively regulates axonal growth and can be targeted to promote functional nerve regeneration

The microtubule (MT) cytoskeleton plays a critical role in axon growth and guidance. Here, we identify the MT-severing enzyme fidgetin-like 2 (FL2) as a negative regulator of axon regeneration and a therapeutic target for promoting nerve regeneration after injury. Genetic knockout of FL2 in cultured adult dorsal root ganglion neurons resulted in longer axons and attenuated growth cone retraction in response to inhibitory molecules. Given the axonal growth-promoting effects of FL2 depletion in vitro, we tested whether FL2 could be targeted to promote regeneration in a rodent model of cavernous nerve (CN) injury. The CNs are parasympathetic nerves that regulate blood flow to the penis, which are commonly damaged during radical prostatectomy (RP), resulting in erectile dysfunction (ED). Application of FL2-siRNA after CN injury significantly enhanced functional nerve recovery. Remarkably, following bilateral nerve transection, visible and functional nerve regeneration was observed in 7 out of 8 animals treated with FL2-siRNA, while no control-treated animals exhibited regeneration. These studies identify FL2 as a promising therapeutic target for enhancing regeneration after peripheral nerve injury and for mitigating neurogenic ED after RP - a condition for which, at present, only poor treatment options exist.

A common theme for axonopathies? The dependency cycle of local axon homeostasis

The number of acquired or inherited conditions leading to axon degeneration (from now on referred to as axonopathies) is vast. To diagnose patients, clinicians use a range of indicators including physiology, morphology, family and patient history, as well as genetics, with the specific location of the lesion within the nervous system being a prominent feature. For the neurobiologist, these criteria are often unsatisfactory, and key questions remain unanswered. For example, does it make sense that different axonopathies affect distinct neuron groups through distinct mechanisms? Would it not be more likely that there are common routes to axon degeneration? In this opinion piece, I shall pose this fundamental question and try to find answers that are hopefully thought-provoking and trigger some conceptual rethinking in the field. I will conclude by describing the 'dependency cycle of axon homeostasis' as a new approach to make sense of the intricate connections of axon biology and physiology, also suggesting that different axonopathies might share common paths to axon degeneration.

Dynamic sex chromosome expression in Drosophila male germ cells

Given their copy number differences and unique modes of inheritance, the evolved gene content and expression of sex chromosomes is unusual. In many organisms the X and Y chromosomes are inactivated in spermatocytes, possibly as a defense mechanism against insertions into unpaired chromatin. In addition to current sex chromosomes, Drosophila has a small gene-poor X-chromosome relic (4th) that re-acquired autosomal status. Here we use single cell RNA-Seq on fly larvae to demonstrate that the single X and pair of 4th chromosomes are specifically inactivated in primary spermatocytes, based on measuring all genes or a set of broadly expressed genes in testis we identified. In contrast, genes on the single Y chromosome become maximally active in primary spermatocytes. Reduced X transcript levels are due to failed activation of RNA-Polymerase-II by phosphorylation of Serine 2 and 5.

Profilin 1 delivery tunes cytoskeletal dynamics toward CNS axon regeneration

After trauma, regeneration of adult CNS axons is abortive, causing devastating neurologic deficits. Despite progress in rehabilitative care, there is no effective treatment that stimulates axonal growth following injury. Using models with different regenerative capacities, followed by gain- and loss-of-function analysis, we identified profilin 1 (Pfn1) as a coordinator of actin and microtubules (MTs), powering axonal growth and regeneration. In growth cones, Pfn1 increased actin retrograde flow, MT growth speed, and invasion of filopodia by MTs, orchestrating cytoskeletal dynamics toward axonal growth. In vitro, active Pfn1 promoted MT growth in a formin-dependent manner, whereas localization of MTs to growth cone filopodia was facilitated by direct MT binding and interaction with formins. In vivo, Pfn1 ablation limited regeneration of growth-competent axons after sciatic nerve and spinal cord injury. Adeno-associated viral (AAV) delivery of constitutively active Pfn1 to rodents promoted axonal regeneration, neuromuscular junction maturation, and functional recovery of injured sciatic nerves, and increased the ability of regenerating axons to penetrate the inhibitory spinal cord glial scar. Thus, we identify Pfn1 as an important regulator of axonal regeneration and suggest that AAV-mediated delivery of constitutively active Pfn1, together with the identification of modulators of Pfn1 activity, should be considered to treat the injured nervous system.

Co-expression of C9orf72 related dipeptide-repeats over 1000 repeat units reveals age- and combination-specific phenotypic profiles in Drosophila

A large intronic hexanucleotide repeat expansion (GGGGCC) within the C9orf72 (C9orf72-SMCR8 Complex Subunit) locus is the most prevalent genetic cause of both Frontotemporal Dementia (FTD) and Motor Neuron Disease (MND). In patients this expansion is typically hundreds to thousands of repeat units in length. Repeat associated non-AUG translation of the expansion leads to the formation of toxic, pathological Dipeptide-Repeat Proteins (DPRs). To date there remains a lack of in vivo models expressing C9orf72 related DPRs with a repeat length of more than a few hundred repeats. As such our understanding of how physiologically relevant repeat length DPRs effect the nervous system in an ageing in vivo system remains limited. In this study we generated Drosophila models expressing DPRs over 1000 repeat units in length, a known pathological length in humans. Using these models, we demonstrate each DPR exhibits a unique, age-dependent, phenotypic and pathological profile. Furthermore, we show co-expression of specific DPR combinations leads to distinct, age-dependent, phenotypes not observed through expression of single DPRs. We propose these models represent a unique, in vivo, tool for dissecting the molecular mechanisms implicated in disease pathology, opening up new avenues in the study of both MND and FTD.

Cofilin regulates axon growth and branching of Drosophila γ-neurons

The mechanisms that control intrinsic axon growth potential, and thus axon regeneration following injury, are not well understood. Developmental axon regrowth of Drosophila mushroom body γ-neurons during neuronal remodeling offers a unique opportunity to study the molecular mechanisms controlling intrinsic growth potential. Motivated by the recently uncovered developmental expression atlas of γ-neurons, we here focus on the role of the actin-severing protein cofilin during axon regrowth. We show that Twinstar (Tsr), the fly cofilin, is a crucial regulator of both axon growth and branching during developmental remodeling of γ-neurons. tsr mutant axons demonstrate growth defects both in vivo and in vitro, and also exhibit actin-rich filopodial-like structures at failed branch points in vivo Our data is inconsistent with Tsr being important for increasing G-actin availability. Furthermore, analysis of microtubule localization suggests that Tsr is required for microtubule infiltration into the axon tips and branch points. Taken together, we show that Tsr promotes axon growth and branching, likely by clearing F-actin to facilitate protrusion of microtubules.