Dendrites have a rapid program of injury-induced degeneration that is molecularly distinct from developmental pruning

Galectins induced from hemocytes bridge phosphatidylserine and N-glycosylated Drpr/CED-1 receptor during dendrite pruning

During neuronal pruning, phagocytes engulf shed cellular debris to avoid inflammation and maintain tissue homeostasis. How phagocytic receptors recognize degenerating neurites had been unclear. Here, we identify two glucosyltransferases Alg8 and Alg10 of the N-glycosylation pathway required for dendrite fragmentation and clearance through genetic screen. The scavenger receptor Draper (Drpr) is N-glycosylated with complex- or hybrid-type N-glycans that interact specifically with galectins. We also identify the galectins Crouching tiger (Ctg) and Hidden dragon (Hdg) that interact with N-glycosylated Drpr and function in dendrite pruning via the Drpr pathway. Ctg and Hdg are required in hemocytes for expression and function, and are induced during dendrite injury to localize to injured dendrites through specific interaction with exposed phosphatidylserine (PS) on the surface membrane of injured dendrites. Thus, the galectins Ctg and Hdg bridge the interaction between PS and N-glycosylated Drpr, leading to the activation of phagocytosis.

SARM1 is responsible for calpain-dependent dendrite degeneration in mouse hippocampal neurons

Sterile alpha and toll/interleukin receptor motif-containing 1 (SARM1) is a critical regulator of axon degeneration that acts through hydrolysis of NAD+ following injury. Recent work has defined the mechanisms underlying SARM1's catalytic activity and advanced our understanding of SARM1 function in axons, yet the role of SARM1 signaling in other compartments of neurons is still not well understood. Here, we show in cultured hippocampal neurons that endogenous SARM1 is present in axons, dendrites, and cell bodies and that direct activation of SARM1 by the neurotoxin Vacor causes not just axon degeneration, but degeneration of all neuronal compartments. In contrast to the axon degeneration pathway defined in dorsal root ganglia, SARM1-dependent hippocampal axon degeneration in vitro is not sensitive to inhibition of calpain proteases. Dendrite degeneration downstream of SARM1 in hippocampal neurons is dependent on calpain 2, a calpain protease isotype enriched in dendrites in this cell type. In summary, these data indicate SARM1 plays a critical role in neurodegeneration outside of axons and elucidates divergent pathways leading to degeneration in hippocampal axons and dendrites.

Temporal and structural sensitivities of major biomarkers for detecting neuropathology after traumatic brain injury in the mouse

Traumatic brain injury (TBI) is a leading cause of morbidity and mortality, especially in teenagers to young adults. In recent decades, different biomarkers and/or staining protocols have been employed to evaluate the post-injury development of pathological structures, but they have produced many contradictory findings. Since correctly identifying the underlying neuroanatomical changes is critical to advancing TBI research, we compared three commonly used markers for their ability to detect TBI pathological structures: Fluoro-Jade C, the rabbit monoclonal antibody Y188 against amyloid precursor protein and the NeuroSilver kit were used to stain adjacent slices from naïve or injured mouse brains harvested at different time points from 30 min to 3 months after lateral fluid percussion injury. Although not all pathological structures were stained by all markers at all time points, we found damaged neurons and deformed dendrites in gray matter, punctate and perivascular structures in white matter, and axonal blebs and Wallerian degeneration in both gray and white matter. The present study demonstrates the temporal and structural sensitivities of the three biomarkers: each marker is highly effective for a set of pathological structures, each of which in turn emerges at a particular time point. Furthermore, the different biomarkers showed different abilities at detecting identical types of pathological structures. In contrast to previous studies that have used a single biomarker at a single time range, the present report strongly recommends that a combination of different biomarkers should be adopted and different time points need to be checked when assessing neuropathology after TBI.

Mechanism of initiation and regulation of axonal degeneration with special reference to NMNATs and Sarm1

Axonal degeneration is observed in a variety of contexts in both the central and peripheral nervous systems. Pathological signaling to regulate the progression of axonal degeneration has long been studied using Wallerian degeneration, the prototypical axonal degradation observed after injury, as a representative model. Understanding metabolism of nicotinamide adenine dinucleotide (NAD+) and the functional regulation of Sarm1 has generated great progress in this field, but there are a number of remaining questions. Here, in this short review, we describe our current understanding of the axonal degeneration mechanism, with special reference to the biology related to wlds mice and Sarm1. Furthermore, variations of axonal degeneration initiation are discussed in order to address the remaining questions needed for mechanistic clarification.

A nerve-wracking buzz: lessons from Drosophila models of peripheral neuropathy and axon degeneration

The degeneration of axons and their terminals occurs following traumatic, toxic, or genetically-induced insults. Common molecular mechanisms unite these disparate triggers to execute a conserved nerve degeneration cascade. In this review, we will discuss how models of peripheral nerve injury and neuropathy in Drosophila have led the way in advancing molecular understanding of axon degeneration and nerve injury pathways. Both neuron-intrinsic as well as glial responses to injury will be highlighted. Finally, we will offer perspective on what additional questions should be answered to advance these discoveries toward clinical interventions for patients with neuropathy.

The Drosophila chemokine-like Orion bridges phosphatidylserine and Draper in phagocytosis of neurons

Phagocytic clearance of degenerating neurons is triggered by "eat-me" signals exposed on the neuronal surface. The conserved neuronal eat-me signal phosphatidylserine (PS) and the engulfment receptor Draper (Drpr) mediate phagocytosis of degenerating neurons in Drosophila. However, how PS is recognized by Drpr-expressing phagocytes in vivo remains poorly understood. Using multiple models of dendrite degeneration, we show that the Drosophila chemokine-like protein Orion can bind to PS and is responsible for detecting PS exposure on neurons; it is supplied cell-non-autonomously to coat PS-exposing dendrites and to mediate interactions between PS and Drpr, thus enabling phagocytosis. As a result, the accumulation of Orion on neurons and on phagocytes produces opposite outcomes by potentiating and suppressing phagocytosis, respectively. Moreover, the Orion dosage is a key determinant of the sensitivity of phagocytes to PS exposed on neurons. Lastly, mutagenesis analyses show that the sequence motifs shared between Orion and human immunomodulatory proteins are important for Orion function. Thus, our results uncover a missing link in PS-mediated phagocytosis in Drosophila and imply conserved mechanisms of phagocytosis of neurons.

A photo-switchable assay system for dendrite degeneration and repair in Drosophila melanogaster

Neurodegeneration arising from aging, injury, or diseases has devastating health consequences. Whereas neuronal survival and axon degeneration have been studied extensively, much less is known about how neurodegeneration affects dendrites, in part due to the limited assay systems available. To develop an assay for dendrite degeneration and repair, we used photo-switchable caspase-3 (caspase-Light-Oxygen-Voltage-sensing [caspase-LOV]) in peripheral class 4 dendrite arborization (c4da) neurons to induce graded neurodegeneration by adjusting illumination duration during development and adulthood in Drosophila melanogaster. We found that both developing and mature c4da neurons were able to survive while sustaining mild neurodegeneration induced by moderate caspase-LOV activation. Further, we observed active dendrite addition and dendrite regeneration in developing and mature c4da neurons, respectively. Using this assay, we found that the mouse Wallerian degeneration slow (Wld^S) protein can protect c4da neurons from caspase-LOV-induced dendrite degeneration and cell death. Furthermore, our data show that Wld^S can reduce dendrite elimination without affecting dendrite addition. In summary, we successfully established a photo-switchable assay system in both developing and mature neurons and used Wld^S as a test case to study the mechanisms underlying dendrite regeneration and repair.

Myotubularin-related protein protects against neuronal degeneration mediated by oxidative stress or infection

Microbial infections have been linked to the onset and severity of neurodegenerative diseases such as amyotrophic lateral sclerosis, multiple sclerosis, Alzheimer's disease, but the underlying mechanisms remain largely unknown. Here, we used a genetic screen for genes involved in protection from infection-associated neurodegeneration and identified the gene mtm-10. We then validated the role of the encoded myotubularin-related protein, MTM-10, in protecting the dendrites of Caenorhabditis elegans from degeneration mediated by oxidative stress or Pseudomonas aeruginosa infection. Further experiments indicated that mtm-10 is expressed in the AWC neurons of C. elegans, where it functions in a cell-autonomous manner to protect the dendrite degeneration caused by pathogen infection. We also confirm that the changes observed in the dendrites of the animals were not because of premature death or overall sickness. Finally, our studies indicated that mtm-10 functions in AWC neurons to preserve chemosensation after pathogen infection. These results reveal an essential role for myotubularin-related protein 10 in the protection of dendrite morphology and function against the deleterious effects of oxidative stress or infection.

Phagocytosis and self-destruction break down dendrites of Drosophila sensory neurons at distinct steps of Wallerian degeneration

After injury, severed dendrites and axons expose the "eat-me" signal phosphatidylserine (PS) on their surface while they break down. The degeneration of injured axons is controlled by a conserved Wallerian degeneration (WD) pathway, which is thought to activate neurite self-destruction through Sarm-mediated nicotinamide adenine dinucleotide (NAD+) depletion. While neurite PS exposure is known to be affected by genetic manipulations of NAD+, how the WD pathway coordinates both neurite PS exposure and self-destruction and whether PS-induced phagocytosis contributes to neurite breakdown in vivo remain unknown. Here, we show that in Drosophila sensory dendrites, PS exposure and self-destruction are two sequential steps of WD resulting from Sarm activation. Surprisingly, phagocytosis is the main driver of dendrite degeneration induced by both genetic NAD+ disruptions and injury. However, unlike neuronal Nmnat loss, which triggers PS exposure only and results in phagocytosis-dependent dendrite degeneration, injury activates both PS exposure and self-destruction as two redundant means of dendrite degeneration. Furthermore, the axon-death factor Axed is only partially required for self-destruction of injured dendrites, acting in parallel with PS-induced phagocytosis. Lastly, injured dendrites exhibit a unique rhythmic calcium-flashing that correlates with WD. Therefore, both NAD+-related general mechanisms and dendrite-specific programs govern PS exposure and self-destruction in injury-induced dendrite degeneration in vivo.

Mitochondrial dysfunction as a trigger of programmed axon death

Mitochondrial failure has long been associated with programmed axon death (Wallerian degeneration, WD), a widespread and potentially preventable mechanism of axon degeneration. While early findings in axotomised axons indicated that mitochondria are involved during the execution steps of this pathway, recent studies suggest that in addition, mitochondrial dysfunction can initiate programmed axon death without physical injury. As mitochondrial dysfunction is associated with disorders involving early axon loss, including Parkinson's disease, peripheral neuropathies, and multiple sclerosis, the findings that programmed axon death is activated by mitochondrial impairment could indicate the involvement of druggable mechanisms whose disruption may protect axons in such diseases. Here, we review the latest developments linking mitochondrial dysfunction to programmed axon death and discuss their implications for injury and disease.

In vitro, In vivo and Ex vivo Models for Peripheral Nerve Injury and Regeneration

Peripheral Nerve Injuries (PNI) frequently occur secondary to traumatic injuries. Recovery from these injuries can be expectedly poor, especially in proximal injuries. In order to study and improve peripheral nerve regeneration, scientists rely on peripheral nerve models to identify and test therapeutic interventions. In this review, we discuss the best described and most commonly used peripheral nerve models that scientists have and continue to use to study peripheral nerve physiology and function.

Scrap and Build for Functional Neural Circuits: Spatiotemporal Regulation of Dendrite Degeneration and Regeneration in Neural Development and Disease

Dendrites are cellular structures essential for the integration of neuronal information. These elegant but complex structures are highly patterned across the nervous system but vary tremendously in their size and fine architecture, each designed to best serve specific computations within their networks. Recent in vivo imaging studies reveal that the development of mature dendrite arbors in many cases involves extensive remodeling achieved through a precisely orchestrated interplay of growth, degeneration, and regeneration of dendritic branches. Both degeneration and regeneration of dendritic branches involve precise spatiotemporal regulation for the proper wiring of functional networks. In particular, dendrite degeneration must be targeted in a compartmentalized manner to avoid neuronal death. Dysregulation of these developmental processes, in particular dendrite degeneration, is associated with certain types of pathology, injury, and aging. In this article, we review recent progress in our understanding of dendrite degeneration and regeneration, focusing on molecular and cellular mechanisms underlying spatiotemporal control of dendrite remodeling in neural development. We further discuss how developmental dendrite degeneration and regeneration are molecularly and functionally related to dendrite remodeling in pathology, disease, and aging.

Homeostatic Roles of the Proteostasis Network in Dendrites

Cellular protein homeostasis, or proteostasis, is indispensable to the survival and function of all cells. Distinct from other cell types, neurons are long-lived, exhibiting architecturally complex and diverse multipolar projection morphologies that can span great distances. These properties present unique demands on proteostatic machinery to dynamically regulate the neuronal proteome in both space and time. Proteostasis is regulated by a distributed network of cellular processes, the proteostasis network (PN), which ensures precise control of protein synthesis, native conformational folding and maintenance, and protein turnover and degradation, collectively safeguarding proteome integrity both under homeostatic conditions and in the contexts of cellular stress, aging, and disease. Dendrites are equipped with distributed cellular machinery for protein synthesis and turnover, including dendritically trafficked ribosomes, chaperones, and autophagosomes. The PN can be subdivided into an adaptive network of three major functional pathways that synergistically govern protein quality control through the action of (1) protein synthesis machinery; (2) maintenance mechanisms including molecular chaperones involved in protein folding; and (3) degradative pathways (e.g., Ubiquitin-Proteasome System (UPS), endolysosomal pathway, and autophagy. Perturbations in any of the three arms of proteostasis can have dramatic effects on neurons, especially on their dendrites, which require tightly controlled homeostasis for proper development and maintenance. Moreover, the critical importance of the PN as a cell surveillance system against protein dyshomeostasis has been highlighted by extensive work demonstrating that the aggregation and/or failure to clear aggregated proteins figures centrally in many neurological disorders. While these studies demonstrate the relevance of derangements in proteostasis to human neurological disease, here we mainly review recent literature on homeostatic developmental roles the PN machinery plays in the establishment, maintenance, and plasticity of stable and dynamic dendritic arbors. Beyond basic housekeeping functions, we consider roles of PN machinery in protein quality control mechanisms linked to dendritic plasticity (e.g., dendritic spine remodeling during LTP); cell-type specificity; dendritic morphogenesis; and dendritic pruning.

Microtubule dynamics in healthy and injured neurons

Most neurons must last a lifetime and their microtubule cytoskeleton is an important contributor to their longevity. Neurons have some of the most stable microtubules of all cells, but the tip of every microtubule remains dynamic and, although requiring constant GTP consumption, microtubules are always being rebuilt. While some ongoing level of rebuilding always occurs, overall microtubule stability can be modulated in response to injury and stress as well as the normal developmental process of pruning. Specific microtubule severing proteins act in different contexts to increase microtubule dynamicity and promote degeneration and pruning. After axon injury, complex changes in dynamics occur and these are important for both neuroprotection induced by injury and subsequent outgrowth of a new axon. Understanding how microtubule dynamics is modulated in different scenarios, as well as the impact of the changes in stability, is an important avenue to explore for development of strategies to promote neuroprotection and regeneration.

Proteomic mapping of Drosophila transgenic elav.L-GAL4/+ brain as a tool to illuminate neuropathology mechanisms

Drosophila brain has emerged as a powerful model system for the investigation of genes being related to neurological pathologies. To map the proteomic landscape of fly brain, in a high-resolution scale, we herein employed a nano liquid chromatography-tandem mass spectrometry technology, and high-content catalogues of 7,663 unique peptides and 2,335 single proteins were generated. Protein-data processing, through UniProt, DAVID, KEGG and PANTHER bioinformatics subroutines, led to fly brain-protein classification, according to sub-cellular topology, molecular function, implication in signaling and contribution to neuronal diseases. Given the importance of Ubiquitin Proteasome System (UPS) in neuropathologies and by using the almost completely reassembled UPS, we genetically targeted genes encoding components of the ubiquitination-dependent protein-degradation machinery. This analysis showed that driving RNAi toward proteasome components and regulators, using the GAL4-elav.L driver, resulted in changes to longevity and climbing-activity patterns during aging. Our proteomic map is expected to advance the existing knowledge regarding brain biology in animal species of major translational-research value and economical interest.

Phosphatidylserine Externalization Results from and Causes Neurite Degeneration in Drosophila

Phagocytic clearance of degenerating dendrites or axons is critical for maintaining tissue homeostasis and preventing neuroinflammation. Externalized phosphatidylserine (PS) has been postulated to be an ‘‘eat-me’’ signal allowing recognition of degenerating neurites by phagocytes. Here we show that in Drosophila, PS is dynamically exposed on degenerating dendrites during developmental pruning and after physical injury, but PS exposure is suppressed when dendrite degeneration is genetically blocked. Ectopic PS exposure via phospholipid flippase knockout and scramblase overexpression induced PS exposure preferentially at distal dendrites and caused distinct modes of neurite loss that differ in larval sensory dendrites and in adult olfactory axons. Surprisingly, extracellular lactadherin that lacks the integrin-interaction domain induced phagocyte-dependent degeneration of PS-exposing dendrites, revealing an unidentified bridging function that potentiates phagocytes. Our findings establish a direct causal relationship between PS exposure and neurite degeneration in vivo.

Using in vivo phosphatidylserine (PS) sensors, Sapar et al. reveal dynamic patterns of PS exposure on degenerating dendrites in Drosophila. Flippase knockout and scramblase overexpression lead to ectopic PS exposure on distal dendrites and context-dependent neurite degeneration. Lactadherin potentiates phagocytes to destruct PS-exposing dendrites, independent of its integrin-interaction domain.

Longitudinal multimodal assessment of neurodegeneration and vascular remodeling correlated with signal degradation in chronic cortical silicon microelectrodes

Significance: Cortically implanted microelectrode arrays provide a direct interface with neuronal populations and are used to restore movement capabilities and provide sensory feedback to patients with paralysis or amputation. Penetrating electrodes experience high rates of signal degradation within the first year that limit effectiveness and lead to eventual device failure. Aim: To assess vascular and neuronal changes over time in mice with implanted electrodes and examine the contribution of the brain tissue response to electrode performance. Approach: We used a multimodal approach combining in vivo electrophysiology and subcellular-level optical imaging. Results: At acute timescales, we observed structural damage from the mechanical trauma of electrode insertion, evidenced by severed dendrites in the electrode path and local hypofluorescence. Superficial vessel growth and remodeling occurred within the first few weeks in both electrode-implanted and window-only animals, but the deeper capillary growth evident in window-only animals was suppressed in electrode-implanted animals. After longer implantation periods, there was evidence of degeneration of transected dendrites superficial to the electrode path and localized neuronal cell body loss, along with deep vascular velocity changes near the electrode. Total spike rate (SR) across all animals reached a peak between 3 and 9 months postimplantation, then decreased. The local field potential signal remained relatively constant for up to 6 months, particularly in the high-gamma band, indicating long-term electrode viability and neuronal functioning at further distances from the electrode, but it showed a reduction in some animals at later time points. Most importantly, we found that progressive high-gamma and SR reductions both correlate positively with localized cell loss and decreasing capillary density within 100    μ m of the electrode. Conclusions: This multifaceted approach provided a more comprehensive picture of the ongoing biological response at the brain-electrode interface than can be achieved with postmortem histology alone and established a real-time relationship between electrophysiology and tissue damage.

An Antimicrobial Peptide and Its Neuronal Receptor Regulate Dendrite Degeneration in Aging and Infection

Infections have been identified as possible risk factors for aging-related neurodegenerative diseases, but it remains unclear whether infection-related immune molecules have a causative role in neurodegeneration during aging. Here, we reveal an unexpected role of an epidermally expressed antimicrobial peptide, NLP-29 (neuropeptide-like protein 29), in triggering aging-associated dendrite degeneration in C. elegans. The age-dependent increase of nlp-29 expression is regulated by the epidermal tir-1/SARM-pmk-1/p38 MAPK innate immunity pathway. We further identify an orphan G protein-coupled receptor NPR-12 (neuropeptide receptor 12) acting in neurons as a receptor for NLP-29 and demonstrate that the autophagic machinery is involved cell autonomously downstream of NPR-12 to transduce degeneration signals. Finally, we show that fungal infections cause dendrite degeneration using a similar mechanism as in aging, through NLP-29, NPR-12, and autophagy. Our findings reveal an important causative role of antimicrobial peptides, their neuronal receptors, and the autophagy pathway in aging- and infection-associated dendrite degeneration.

Risks of cognitive detriments after low dose heavy ion and proton exposures

Purpose: Heavy ion and proton brain irradiations occur during space travel and in Hadron therapy for cancer. Heavy ions produce distinct patterns of energy deposition in neuron cells and brain tissues compared to X-rays leading to large uncertainties in risk estimates. We make a critical review of findings from research studies over the last 25 years for understanding risks at low dose. Conclusions: A large number of mouse and rat cognitive testing measures have been reported for a variety of particle species and energies for acute doses. However, tissue reactions occur above dose thresholds and very few studies were performed at the heavy ion doses to be encountered on space missions (<0.04 Gy/y) or considered dose-rate effects, such that threshold doses are not known in rodent models. Investigations of possible mechanisms for cognitive changes have been limited by experimental design with largely group specific and not subject specific findings reported. Persistent oxidative stress and activated microglia cells are common mechanisms studied, while impairment of neurogenesis, detriments in neuron morphology, and changes to gene and protein expression were each found to be important in specific studies. Future research should focus on estimating threshold doses carried out with experimental designs aimed at understating causative mechanisms, which will be essential for extrapolating rodent findings to humans and chronic radiation scenarios, while establishing if mitigation are needed.

DETRIMENTS IN NEURON MORPHOLOGY FOLLOWING HEAVY ION IRRADIATION: WHAT'S THE TARGET?

Neuron cells consist of the soma or cell body, axons, dendritic arbor with multiple branches, and dendritic spines which are the substrates for memory storage and synaptic transmission. Detriments in neuron morphology are suggested to play a key role in cognitive impairments following brain irradiation. Multiple molecular mechanisms are involved in the regulation and stability of neuron morphology, while the effects of radiation on these processes have not been studied extensively. In this report, we consider possible biological targets in neurons for energy deposition (ED) by charged particles that could lead to neuron morphology detriments, and the resulting dose and radiation quality dependence of such detriments. The track structures of heavy ions including high charge and energy (HZE) particles consists of core of high-ED events and a penumbra of sparse ED from δ-ray electrons produced in ionization of target molecules. We consider the role of track structure relative to possible targets causative in the degradation of morphology.

Die in pieces: How Drosophila sheds light on neurite degeneration and clearance

Dendrites and axons are delicate neuronal membrane extensions that undergo degeneration after physical injuries. In neurodegenerative diseases, they often degenerate prior to neuronal death. Understanding the mechanisms of neurite degeneration has been an intense focus of neurobiology research in the last two decades. As a result, many discoveries have been made in the molecular pathways that lead to neurite degeneration and the cell-cell interactions responsible for the subsequent clearance of neuronal debris. Drosophila melanogaster has served as a prime in vivo model system for identifying and characterizing the key molecular players in neurite degeneration, thanks to its genetic tractability and easy access to its nervous system. The knowledge learned in the fly provided targets and fuel for studies in other model systems that have further enhanced our understanding of neurodegeneration. In this review, we will introduce the experimental systems developed in Drosophila to investigate injury-induced neurite degeneration, and then discuss the biological pathways that drive degeneration. We will also cover what is known about the mechanisms of how phagocytes recognize and clear degenerating neurites, and how recent findings in this area enhance our understanding of neurodegenerative disease pathology.

Docosanoids and elovanoids from omega-3 fatty acids are pro-homeostatic modulators of inflammatory responses, cell damage and neuroprotection

The functional significance of the selective enrichment of the omega-3 essential fatty acid docosahexaenoic acid (DHA; 22C and 6 double bonds) in cellular membrane phospholipids of the nervous system is being clarified by defining its specific roles on membrane protein function and by the uncovering of the bioactive mediators, docosanoids and elovanoids (ELVs). Here, we describe the preferential uptake and DHA metabolism in photoreceptors and brain as well as the significance of the Adiponectin receptor 1 in DHA retention and photoreceptor cell (PRC) survival. We now know that this integral membrane protein is engaged in DHA retention as a necessary event for the function of PRCs and retinal pigment epithelial (RPE) cells. We present an overview of how a) NPD1 selectively mediates preconditioning rescue of RPE and PR cells; b) NPD1 restores aberrant neuronal networks in experimental epileptogenesis; c) the decreased ability to biosynthesize NPD1 in memory hippocampal areas of early stages of Alzheimer's disease takes place; d) NPD1 protection of dopaminergic circuits in an in vitro model using neurotoxins; and e) bioactivity elicited by DHA and NPD1 activate a neuroprotective gene-expression program that includes the expression of Bcl-2 family members affected by Aβ42, DHA, or NPD1. In addition, we highlight ELOVL4 (ELOngation of Very Long chain fatty acids-4), specifically the neurological and ophthalmological consequences of its mutations, and their role in providing precursors for the biosynthesis of ELVs. Then we outline evidence of ELVs ability to protect RPE cells, which sustain PRC integrity. In the last section, we present a summary of the protective bioactivity of docosanoids and ELVs in experimental ischemic stroke. The identification of early mechanisms of neural cell survival mediated by DHA-synthesized ELVs and docosanoids contributes to the understanding of cell function, pro-homeostatic cellular modulation, inflammatory responses, and innate immunity, opening avenues for prevention and therapeutic applications in neurotrauma, stroke and neurodegenerative diseases.

Identification of Proteins Required for Precise Positioning of Apc2 in Dendrites

In Drosophila neurons, uniform minus-end-out polarity in dendrites is maintained in part by kinesin-2-mediated steering of growing microtubules at branch points. Apc links the kinesin motor to growing microtubule plus ends and Apc2 recruits Apc to branch points where it functions. Because Apc2 acts to concentrate other steering proteins to branch points, we wished to understand how Apc2 is targeted. From an initial broad candidate RNAi screen, we found Miro (a mitochondrial transport protein), Ank2, Axin, spastin and Rac1 were required to position Apc2-GFP at dendrite branch points. YFP-Ank2-L8, Axin-GFP and mitochondria also localized to branch points suggesting the screen identified relevant proteins. By performing secondary screens, we found that energy production by mitochondria was key for Apc2-GFP positioning and spastin acted upstream of mitochondria. Ank2 seems to act independently from other players, except its membrane partner, Neuroglian (Nrg). Rac1 likely acts through Arp2/3 to generate branched actin to help recruit Apc2-GFP. Axin can function in a variety of wnt signaling pathways, one of which includes heterotrimeric G proteins and Frizzleds. Knockdown of Gαs, Gαo, Fz and Fz2, reduced targeting of Apc2 and Axin to branch points. Overall our data suggest that mitochondrial energy production, Nrg/Ank2, branched actin generated by Arp2/3 and Fz/G proteins/Axin function as four modules that control localization of the microtubule regulator Apc2 to its site of action in dendrite branch points.

Biophysics Model of Heavy-Ion Degradation of Neuron Morphology in Mouse Hippocampal Granular Cell Layer Neurons

Exposure to heavy-ion radiation during cancer treatment or space travel may cause cognitive detriments that have been associated with changes in neuron morphology and plasticity. Observations in mice of reduced neuronal dendritic complexity have revealed a dependence on radiation quality and absorbed dose, suggesting that microscopic energy deposition plays an important role. In this work we used morphological data for mouse dentate granular cell layer (GCL) neurons and a stochastic model of particle track structure and microscopic energy deposition (ED) to develop a predictive model of high-charge and energy (HZE) particle-induced morphological changes to the complex structures of dendritic arbors. We represented dendrites as cylindrical segments of varying diameter with unit aspect ratios, and developed a fast sampling method to consider the stochastic distribution of ED by δ rays (secondary electrons) around the path of heavy ions, to reduce computational times. We introduce probabilistic models with a small number of parameters to describe the induction of precursor lesions that precede dendritic snipping, denoted as snip sites. Predictions for oxygen (16O, 600 MeV/n) and titanium (48Ti, 600 MeV/n) particles with LET of 16.3 and 129 keV/μm, respectively, are considered. Morphometric parameters to quantify changes in neuron morphology are described, including reduction in total dendritic length, number of branch points and branch numbers. Sholl analysis is applied for single neurons to elucidate dose-dependent reductions in dendritic complexity. We predict important differences in measurements from imaging of tissues from brain slices with single neuron cell observations due to the role of neuron death through both soma apoptosis and excessive dendritic length reduction. To further elucidate the role of track structure, random segment excision (snips) models are introduced and a sensitivity study of the effects of the modes of neuron death in predictions of morphometric parameters is described. An important conclusion of this study is that δ rays play a major role in neuron morphological changes due to the large spatial distribution of damage sites, which results in a reduced dependence on LET, including modest difference between 16O and 48Ti, compared to damages resulting from ED in localized damage sites.

The Autism Protein Ube3A/E6AP Remodels Neuronal Dendritic Arborization via Caspase-Dependent Microtubule Destabilization

UBE3A gene copy number variation and the resulting overexpression of the protein E6AP is directly linked to autism spectrum disorders (ASDs). However, the underlying cellular and molecular neurobiology remains less clear. Here we report the role of ASD-related increased dosage of Ube3A/E6AP in dendritic arborization during brain development. We show that increased E6AP expression in primary cultured neurons leads to a reduction in dendritic branch number and length. The E6AP-dependent remodeling of dendritic arborization results from retraction of dendrites by thinning and fragmentation at the tips of dendrite branches, leading to shortening or removal of dendrites. This remodeling effect is mediated by the ubiquitination and degradation of XIAP (X-linked inhibitors of aptosis protein) by E6AP, which leads to activation of caspase-3 and cleavage of microtubules. In vivo, male and female Ube3A 2X ASD mice show decreased XIAP levels, increased caspase-3 activation, and elevated levels of tubulin cleavage. Consistently, dendritic branching and spine density are reduced in cortical neurons of Ube3A 2X ASD mice. In revealing an important role for Ube3A/E6AP in ASD-related developmental alteration in dendritic arborization and synapse formation, our findings provide new insights into the pathogenesis of Ube3A/E6AP-dependent ASD.SIGNIFICANCE STATEMENT Copy number variation of the UBE3A gene and aberrant overexpression of the gene product E6AP protein is a common cause of autism spectrum disorders (ASDs). During brain development, dendritic growth and remodeling play crucial roles in neuronal connectivity and information integration. We found that in primary neurons and in Ube3A transgenic autism mouse brain, overexpression of E6AP leads to significant loss of dendritic arborization. This effect is mediated by the ubiquitination of XIAP (X-linked inhibitor of aptosis protein) by E6AP, subsequent activation of caspases, and the eventual cleavage of microtubules, leading to local degeneration and retraction at the tips of dendritic branches. These findings demonstrate dysregulation in neuronal structural stability as a major cellular neuropathology in ASD.

Intrinsic mechanisms for axon regeneration: insights from injured axons in Drosophila

Axonal damage and loss are common and negative consequences of neuronal injuries, and also occur in some neurodegenerative diseases. For neurons to have a chance to repair their connections, they need to survive the damage, initiate new axonal growth, and ultimately establish new synaptic connections. This review discusses how recent work in Drosophila models have informed our understanding of the cellular pathways used by neurons to respond to axonal injuries. Similarly to mammalian neurons, Drosophila neurons appear to be more limited in their capacity regrow (regenerate) damaged axons in the central nervous system, but can undergo axonal regeneration to varying extents in the peripheral nervous system. Conserved cellular pathways are activated by axonal injury via mechanisms that are specific to axons but not dendrites, and new unanticipated inhibitors of axon regeneration can be identified via genetic screening. These findings, made predominantly via genetic and live imaging methods in Drosophila, emphasize the utility of this model organism for the identification and study of basic cellular mechanisms used for neuronal repair.

Mitochondria and Caspases Tune Nmnat-Mediated Stabilization to Promote Axon Regeneration

Axon injury can lead to several cell survival responses including increased stability and axon regeneration. Using an accessible Drosophila model system, we investigated the regulation of injury responses and their relationship. Axon injury stabilizes the rest of the cell, including the entire dendrite arbor. After axon injury we found mitochondrial fission in dendrites was upregulated, and that reducing fission increased stabilization or neuroprotection (NP). Thus axon injury seems to both turn on NP, but also dampen it by activating mitochondrial fission. We also identified caspases as negative regulators of axon injury-mediated NP, so mitochondrial fission could control NP through caspase activation. In addition to negative regulators of NP, we found that nicotinamide mononucleotide adenylyltransferase (Nmnat) is absolutely required for this type of NP. Increased microtubule dynamics, which has previously been associated with NP, required Nmnat. Indeed Nmnat overexpression was sufficient to induce NP and increase microtubule dynamics in the absence of axon injury. DLK, JNK and fos were also required for NP. Because NP occurs before axon regeneration, and NP seems to be actively downregulated, we tested whether excessive NP might inhibit regeneration. Indeed both Nmnat overexpression and caspase reduction reduced regeneration. In addition, overexpression of fos or JNK extended the timecourse of NP and dampened regeneration in a Nmnat-dependent manner. These data suggest that NP and regeneration are conflicting responses to axon injury, and that therapeutic strategies that boost NP may reduce regeneration.

Unlike many other cell types, most neurons last a lifetime. When injured, these cells often activate survival and repair strategies rather than dying. One such response is regeneration of the axon after it is injured. Axon regeneration is a conserved process activated by the same signaling cascade in worms, flies and mammals. Surprisingly we find that this signaling cascade first initiates a different response. This first response stabilizes the cell, and its downregulation by mitochondrial fission and caspases allows for maximum regeneration at later times. We propose that neurons respond to axon injury in a multi-step process with an early lock-down phase in which the cell is stabilized, followed by a more plastic state in which regeneration is maximized.

The microtubule-severing protein fidgetin acts after dendrite injury to promote their degeneration

After being severed from the cell body, axons initiate an active degeneration program known as Wallerian degeneration. Although dendrites also seem to have an active injury-induced degeneration program, no endogenous regulators of this process are known. Because microtubule disassembly has been proposed to play a role in both pruning and injury-induced degeneration, we used a Drosophila model to identify microtubule regulators involved in dendrite degeneration. We found that, when levels of fidgetin were reduced using mutant or RNA interference (RNAi) strategies, dendrite degeneration was delayed, but axon degeneration and dendrite pruning proceeded with normal timing. We explored two possible ways in which fidgetin could promote dendrite degeneration: (1) by acting constitutively to moderate microtubule stability in dendrites, or (2) by acting specifically after injury to disassemble microtubules. When comparing microtubule dynamics and stability in uninjured neurons with and without fidgetin, we could not find evidence that fidgetin regulated microtubule stability constitutively. However, we identified a fidgetin-dependent increase in microtubule dynamics in severed dendrites. We conclude that fidgetin acts after injury to promote disassembly of microtubules in dendrites severed from the cell body.

The effects of amphetamine exposure on juvenile rats on the neuronal morphology of the limbic system at prepubertal, pubertal and postpubertal ages

Amphetamines (AMPH) are psychostimulants widely used for therapy as well as for recreational purposes. Previous results of our group showed that AMPH exposure in pregnant rats induces physiological and behavioral changes in the offspring at prepubertal and postpubertal ages. In addition, several reports have shown that AMPH are capable of modifying the morphology of neurons in some regions of the limbic system. These modifications can cause some psychiatric conditions. However, it is still unclear if there are changes to behavioral and morphological levels when low doses of AMPH are administered at a juvenile age. The aim of this study was to assess the effect of AMPH administration (1mg/kg) in Sprague-Dawley rats (postnatal day, PD21-PD35) on locomotor activity in a novel environment and compare the neuronal morphology of limbic system areas at three different ages: prepubertal (PD 36), pubertal (PD50) and postpubertal (PD 62). We found that AMPH altered locomotor activity in the prepubertal group, but did not have an effect on the other two age groups. The Golgi-Cox staining method was used to describe the neural morphology of five limbic regions: (Layers 3 and 5) the medial prefrontal cortex (mPFC), the dorsal and ventral hippocampus, the nucleus accumbens and the amygdala, showing that AMPH induced changes at pubertal ages in arborization and spine density of these neurons, but interestingly these changes did not persist at postpubertal ages. Our findings suggest that even early-life AMPH exposure does not induce long-term behavioral and morphological changes, however it causes alterations at pubertal ages in the limbic system networks, a stage of life strongly associated with the development of substance abuse behaviors.

Impairments in dendrite morphogenesis as etiology for neurodevelopmental disorders and implications for therapeutic treatments

Dendrite morphology is pivotal for neural circuitry functioning. While the causative relationship between small-scale dendrite morphological abnormalities (shape, density of dendritic spines) and neurodevelopmental disorders is well established, such relationship remains elusive for larger-scale dendrite morphological impairments (size, shape, branching pattern of dendritic trees). Here, we summarize published data on dendrite morphological irregularities in human patients and animal models for neurodevelopmental disorders, with focus on autism and schizophrenia. We next discuss high-risk genes for these disorders and their role in dendrite morphogenesis. We finally overview recent developments in therapeutic attempts and we discuss how they relate to dendrite morphology. We find that both autism and schizophrenia are accompanied by dendritic arbor morphological irregularities, and that majority of their high-risk genes regulate dendrite morphogenesis. Thus, we present a compelling argument that, along with smaller-scale morphological impairments in dendrites (spines and synapse), irregularities in larger-scale dendrite morphology (arbor shape, size) may be an important part of neurodevelopmental disorders' etiology. We suggest that this should not be ignored when developing future therapeutic treatments.

Cdk5 regulates developmental remodeling of mushroom body neurons in Drosophila

BACKGROUND

During metamorphosis, axons and dendrites of the mushroom body (MB) in the Drosophila central brain are remodeled extensively to support the transition from larval to adult behaviors.

RESULTS

We show here that the neuronal cyclin-dependent kinase, Cdk5, regulates the timing and rate of mushroom body remodeling: reduced Cdk5 activity causes a delay in pruning of MB neurites, while hyperactivation accelerates it. We further show that Cdk5 cooperates with the ubiquitin-proteasome system in this process. Finally, we show that Cdk5 modulates the first overt step in neurite disassembly, dissolution of the neuronal tubulin cytoskeleton, and provide evidence that it also acts at additional steps of MB pruning.

CONCLUSIONS

These data show that Cdk5 regulates the onset and extent of remodeling of the Drosophila MB. Given the wide phylogenetic conservation of Cdk5, we suggest that it is likely to play a role in developmental remodeling in other systems, as well. Moreover, we speculate that the well-established role of Cdk5 in neurodegeneration may involve some of the same cellular mechanisms that it uses during developmental remodeling.

The Docosanoid Neuroprotectin D1 Induces TH-Positive Neuronal Survival in a Cellular Model of Parkinson's Disease

Parkinson's disease (PD) does not manifest clinically until 80 % of striatal dopamine is reduced, thus most neuronal damage takes place before the patient presents clinical symptoms. Therefore, it is important to develop preventive strategies for this disease. In the experimental models of PD, 1-methyl-4-phenylpyridinium ion (MPP+) and rotenone induce toxicity in dopaminergic neurons. Neuroprotectin D1 (NPD1) displays neuroprotection in cells undergoing proteotoxic and oxidative stress. In the present report, we established an in vitro model using a primary neuronal culture from mesencephalic embryonic mouse tissue in which 17-20 % of neurons were TH-positive when differentiated in vitro. NPD1 (100 nM) rescued cells from apoptosis induced by MPP+ and rotenone, and the dendritic arbor of surviving neurons was examined using Sholl analysis. Rotenone, as well as MPP+ and its precursor 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), severely promoted retraction of dendritic arbor distal segments, thus decreasing the maximum branch order reached. On average, NPD1 counteracted the effects of MPP+ on the dendritic arborization, but failed to do so in the rotenone-treated neurons. However, rotenone did decrease the Sholl intersection number from radii 25 to 125 µm, and NPD1 did restore the pattern to control levels. Similarly, NPD1 partially reverted the dendrite retraction caused by MPP+ and MPTP. These results suggest that the apoptosis occurring in mesencephalic TH-positive neurons is not a direct consequence of mitochondrial dysfunction alone and that NPD1 signaling may be counteracting this damage. These findings lay the groundwork for the use of the in vitro model developed for future studies and for the search of specific molecular events that NPD1 targets to prevent early neurodegeneration in PD.

Drosophila models of neuronal injury

Neurite degeneration is a hallmark feature of nearly all neurodegenerative diseases, occurs after most brain trauma, and is thought to be the underlying cause of functional loss in patients. Understanding the genetic basis of neurite degeneration represents a major challenge in the neuroscience field. If it is possible to define key signaling pathways that promote neurite destruction, their blockade represents an exciting new potential therapeutic approach to suppressing neurological loss in patients. This review highlights recently developed models that can be used to study fundamental aspects of neuronal injury using the fruit fly Drosophila. The speed, precision, and powerful molecular-genetic tools available in the fruit fly make for an attractive system in which to dissect neuronal signaling after injury. Their use has led to the identification of some of the first molecules whose endogenous function includes promoting axonal degeneration after axotomy, and these signaling pathways appear functionally well conserved in mammals.

Mechanisms of developmental neurite pruning

The precise wiring of the nervous system is a combined outcome of progressive and regressive events during development. Axon guidance and synapse formation intertwined with cell death and neurite pruning sculpt the mature circuitry. It is now well recognized that pruning of dendrites and axons as means to refine neuronal networks, is a wide spread phenomena required for the normal development of vertebrate and invertebrate nervous systems. Here we will review the arising principles of cellular and molecular mechanisms of neurite pruning. We will discuss these principles in light of studies in multiple neuronal systems, and speculate on potential explanations for the emergence of neurite pruning as a mechanism to sculpt the nervous system.

Diverse cellular and molecular modes of axon degeneration

The elimination of large portions of axons is a widespread event in the developing and diseased nervous system. Subsets of axons are selectively destroyed to help fine-tune neural circuit connectivity during development. Axonal degeneration is also an early feature of nearly all neurodegenerative diseases, occurs after most neural injuries, and is a primary driver of functional impairment in patients. In this review we discuss the diversity of cellular mechanisms by which axons degenerate. Initial molecular characterization highlights some similarities in their execution but also argues that unique genetic programs modulate each mode of degeneration. Defining these pathways rigorously will provide new targets for therapeutic intervention after neural injury or in neurodegenerative disease.

Signaling mechanisms regulating Wallerian degeneration

Wallerian degeneration (WD) occurs after an axon is cut or crushed and entails the disintegration and clearance of the severed axon distal to the injury site. WD was initially thought to result from the passive wasting away of the distal axonal fragment, presumably because it lacked a nutrient supply from the cell body. The discovery of the slow Wallerian degeneration (Wld(s)) mutant mouse, in which distal severed axons survive intact for weeks rather than only one to two days, radically changed our thoughts on the autonomy of axon survival. Wld(s) taught us that under some conditions the axonal compartment can survive for weeks after axotomy without a cell body. The phenotypic and molecular characterization of Wld(S) and current models for Wld(S) molecular function are reviewed herein-the mechanism(s) by which Wld(S) spares severed axons remains unresolved. However, recent studies inspired by Wld(s) have led to the identification of the first 'axon death' signaling molecules whose endogenous activities promote axon destruction during WD.

Epidermal cells are the primary phagocytes in the fragmentation and clearance of degenerating dendrites in Drosophila

During developmental remodeling, neurites destined for pruning often degenerate on-site. Physical injury also induces degeneration of neurites distal to the injury site. Prompt clearance of degenerating neurites is important for maintaining tissue homeostasis and preventing inflammatory responses. Here we show that in both dendrite pruning and dendrite injury of Drosophila sensory neurons, epidermal cells rather than hemocytes are the primary phagocytes in clearing degenerating dendrites. Epidermal cells act via Draper-mediated recognition to facilitate dendrite degeneration and to engulf and degrade degenerating dendrites. Using multiple dendritic membrane markers to trace phagocytosis, we show that two members of the CD36 family, croquemort (crq) and debris buster (dsb), act at distinct stages of phagosome maturation for dendrite clearance. Our finding reveals the physiological importance of coordination between neurons and their surrounding epidermis, for both dendrite fragmentation and clearance.

New approaches for studying synaptic development, function, and plasticity using Drosophila as a model system

The fruit fly Drosophila melanogaster has been established as a premier experimental model system for neuroscience research. These organisms are genetically tractable, yet their nervous systems are sufficiently complex to study diverse processes that are conserved across metazoans, including neural cell fate determination and migration, axon guidance, synaptogenesis and function, behavioral neurogenetics, and responses to neuronal injury. For several decades, Drosophila neuroscientists have taken advantage of a vast toolkit of genetic and molecular techniques to reveal fundamental principles of neuroscience illuminating to all systems, including the first behavioral mutants from Seymour Benzer's pioneering work in the 1960s and 1970s, the cloning of the first potassium channel in the 1980s, and the identification of the core genes that orchestrate axon guidance and circadian rhythms in the 1990s. Over the past decade, new tools and innovations in genetic, imaging, and electrophysiological technologies have enabled the visualization, in vivo, of dynamic processes in synapses with unprecedented resolution. We will review some of the fresh insights into synaptic development, function, and plasticity that have recently emerged in Drosophila with an emphasis on the unique advantages of this model system.

Neurite pruning and neuronal cell death: spatial regulation of shared destruction programs

During development, neurons are initially overproduced and excess neurons are eliminated later on by programmed cell death. In a more refined developmental process termed pruning, excess axons and dendritic branches are removed while the cell body remains intact. In mature animals, axons that become disconnected as a result of injury are eliminated through a series of events collectively known as Wallerian degeneration. Recent evidence points to unexpected similarities between these three types of obliterative processes, as they share common regulators. These findings provide new ideas on how cellular destruction programs are spatially regulated in neurons.

Axonal pruning is actively regulated by the microtubule-destabilizing protein kinesin superfamily protein 2A

Extensive axonal pruning and neuronal cell death are critical events for the development of the nervous system. Like neuronal cell death, axonal elimination occurs in discrete steps; however, the regulators of these processes remain mostly elusive. Here, we identify the kinesin superfamily protein 2A (KIF2A) as a key executor of microtubule disassembly and axonal breakdown during axonal pruning. Knockdown of Kif2a, but not other microtubule depolymerization or severing proteins, protects axonal microtubules from disassembly upon trophic deprivation. We further confirmed and extended this result to demonstrate that the entire degeneration process is delayed in neurons from the Kif2a knockout mice. Finally, we show that the Kif2a-null mice exhibit normal sensory axon patterning early during development, but abnormal target hyperinnervation later on, as they compete for limited skin-derived trophic support. Overall, these findings reveal a central regulatory mechanism of axonal pruning during development.

Spatiotemporal resolution of BDNF neuroprotection against glutamate excitotoxicity in cultured hippocampal neurons

Brain-derived neurotrophic factor (BDNF) protects hippocampal neurons from glutamate excitotoxicity as determined by analysis of chromatin condensation, through activation of extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3-K) signaling pathways. However, it is still unknown whether BDNF also prevents the degeneration of axons and dendrites, and the functional demise of synapses, which would be required to preserve neuronal activity. Herein, we have studied the time-dependent changes in several neurobiological markers, and the regulation of proteolytic mechanisms in cultured rat hippocampal neurons, through quantitative western blot and immunocytochemistry. Calpain activation peaked immediately after the neurodegenerative input, followed by a transient increase in ubiquitin-conjugated proteins and increased abundance of cleaved-caspase-3. Proteasome and calpain inhibition did not reproduce the protective effect of BDNF and caspase inhibition in preventing chromatin condensation. However, proteasome and calpain inhibition did protect the neuronal markers for dendrites (MAP-2), axons (Neurofilament-H) and the vesicular glutamate transporters (VGLUT1-2), whereas caspase inhibition was unable to mimic the protective effect of BDNF on neurites and synaptic markers. BDNF partially prevented the downregulation of synaptic activity measured by the KCl-evoked glutamate release using a Förster (Fluorescence) resonance energy transfer (FRET) glutamate nanosensor. These results translate a time-dependent activation of proteases and spatial segregation of these mechanisms, where calpain activation is followed by proteasome deregulation, from neuronal processes to the soma, and finally by caspase activation in the cell body. Moreover, PI3-K and PLCγ small molecule inhibitors significantly blocked the protective action of BDNF, suggesting an activity-dependent mechanism of neuroprotection. Ultimately, we hypothesize that neuronal repair after a degenerative insult is initiated at the synaptic level.

Diapause formation and downregulation of insulin-like signaling via DAF-16/FOXO delays axonal degeneration and neuronal loss

Axonal degeneration is a key event in the pathogenesis of neurodegenerative conditions. We show here that mec-4d triggered axonal degeneration of Caenorhabditis elegans neurons and mammalian axons share mechanistical similarities, as both are rescued by inhibition of calcium increase, mitochondrial dysfunction, and NMNAT overexpression. We then explore whether reactive oxygen species (ROS) participate in axonal degeneration and neuronal demise. C. elegans dauers have enhanced anti-ROS systems, and dauer mec-4d worms are completely protected from axonal degeneration and neuronal loss. Mechanistically, downregulation of the Insulin/IGF-1-like signaling (IIS) pathway protects neurons from degenerating in a DAF-16/FOXO–dependent manner and is related to superoxide dismutase and catalase-increased expression. Caloric restriction and systemic antioxidant treatment, which decrease oxidative damage, protect C. elegans axons from mec-4d-mediated degeneration and delay Wallerian degeneration in mice. In summary, we show that the IIS pathway is essential in maintaining neuronal homeostasis under pro-degenerative stimuli and identify ROS as a key intermediate of neuronal degeneration in vivo. Since axonal degeneration represents an early pathological event in neurodegeneration, our work identifies potential targets for therapeutic intervention in several conditions characterized by axonal loss and functional impairment.

Axonal degeneration and neuronal loss are currently considered crucial pathological factors in neurodegenerative diseases. Therefore, delaying or blocking these procesess is key for neuroprotection. In this work, we used an in vivo approach combining invertebrate (C. elegans) and vertebrate (mice) model systems to identify a novel and unexpected player in the mechanisms of axonal degeneration. Here, we demonstrate that both neuronal somas and axons degenerate through a step dependent on oxidative stress that can be efficiently delayed by genetic downregulation of a pathway controlling oxidative stress resistance. Impressively, we discovered that diapause formation, which is a state related to hibernating conditions, fully prevents neuronal degeneration. We uncovered new players in the degenerative mechanisms of neurons with relevance for several conditions associated to axonal degeneration, such as multiple sclerosis, motoneuron, and Parkinson diseases, offering novel potential targets for neuroprotection.

Molecular chaperones protect against JNK- and Nmnat-regulated axon degeneration in Drosophila

Axon degeneration is observed at the early stages of many neurodegenerative conditions and this often leads to subsequent neuronal loss. We previously showed that inactivating the c-Jun N-terminal kinase (JNK) pathway leads to axon degeneration in Drosophila mushroom body (MB) neurons. To understand this process, we screened candidate suppressor genes and found that the Wallerian degeneration slow (Wld(S)) protein blocked JNK axonal degeneration. Although the nicotinamide mononucleotide adenylyltransferase (Nmnat1) portion of Wld(S) is required, we found that its nicotinamide adenine dinucleotide (NAD(+)) enzyme activity and the Wld(S) N-terminus (N70) are dispensable, unlike axotomy models of neurodegeneration. We suggest that Wld(S)-Nmnat protects against axonal degeneration through chaperone activity. Furthermore, ectopically expressed heat shock proteins (Hsp26 and Hsp70) also protected against JNK and Nmnat degeneration phenotypes. These results suggest that molecular chaperones are key in JNK- and Nmnat-regulated axonal protective functions.

High-resolution in vivo imaging of regenerating dendrites of Drosophila sensory neurons during metamorphosis: local filopodial degeneration and heterotypic dendrite-dendrite contacts

Neuronal circuits that are formed in early development are reorganized at later developmental stages to support a wide range of adult behaviors. At Drosophila pupal stages, one example of this reorganization is dendritic remodeling of multidendritic neurons, which is accomplished by pruning and subsequent regeneration of branches in environments quite distinct from those in larval life. Here, we used long-term in vivo time-lapse recordings at high spatiotemporal resolution and analyzed the dynamics of two adjacent cell types that remodel dendritic arbors, which eventually innervate the lateral plate of the adult abdomen. These neurons initially exhibited dynamic extension, withdrawal and local degeneration of filopodia that sprouted from all along the length of regenerating branches. At a midpupal stage, branches extending from the two cell types started fasciculating with each other, which prompted us to test the hypothesis that this heterotypic contact may serve as a guiding scaffold for shaping dendritic arbors. Unexpectedly, our cell ablation study gave only marginal effects on the branch length and the arbor shape. This result suggests that the arbor morphology of the adult neurons in this study can be specified mostly in the absence of the dendrite–dendrite contact.

Normal spastin gene dosage is specifically required for axon regeneration

Axon regeneration allows neurons to repair circuits after trauma; however, most of the molecular players in this process remain to be identified. Given that microtubule rearrangements have been observed in injured neurons, we tested whether microtubule-severing proteins might play a role in axon regeneration. We found that axon regeneration is extremely sensitive to levels of the microtubule-severing protein spastin. Although microtubule behavior in uninjured neurons was not perturbed in animals heterozygous for a spastin null allele, axon regeneration was severely disrupted in this background. Two types of axon regeneration-regeneration of an axon from a dendrite after proximal axotomy and regeneration of an axon from the stump after distal axotomy-were defective in Drosophila with one mutant copy of the spastin gene. Other types of axon and dendrite outgrowth, including regrowth of dendrites after pruning, were normal in heterozygotes. We conclude that regenerative axon growth is uniquely sensitive to spastin gene dosage.

Critical pathogenic events underlying progression of neurodegeneration in glaucoma

Glaucoma is a common optic neuropathy with a complex etiology often linked to sensitivity to intraocular pressure. Though the precise mechanisms that mediate or transduce this sensitivity are not clear, the axon of the retinal ganglion cell appears to be vulnerable to disease-relevant stressors early in progression. One reason may be because the axon is generally thin for both its unmyelinated and myelinated segment and much longer than the thicker unmyelinated axons of other excitatory retinal neurons. This difference may predispose the axon to metabolic and oxidative injury, especially at distal sites where pre-synaptic terminals form connections in the brain. This idea is consistent with observations of early loss of anterograde transport at central targets and other signs of distal axonopathy that accompany physiological indicators of progression. Outright degeneration of the optic projection ensues after a critical period and, at least in animal models, is highly sensitive to cumulative exposure to elevated pressure in the eye. Stress emanating from the optic nerve head can induce not only distal axonopathy with aspects of dying back neuropathy, but also Wallerian degeneration of the optic nerve and tract and a proximal program involving synaptic and dendritic pruning in the retina. Balance between progressive and acute mechanisms likely varies with the level of stress placed on the unmyelinated axon as it traverses the nerve head, with more acute insult pushing the system toward quicker disassembly. A constellation of signaling factors likely contribute to the transduction of stress to the axon, so that degenerative events along the length of the optic projection progress in retinotopic fashion. This pattern leads to well-defined sectors of functional depletion, even at distal-most sites in the pathway. While ganglion cell somatic drop-out is later in progression, some evidence suggests that synaptic and dendritic pruning in the retina may be a more dynamic process. Structural persistence both in the retina and in central projection sites offers the possibility that intrinsic self-repair pathways counter pathogenic mechanisms to delay as long as possible outright loss of tissue.

Axon degeneration and regeneration: insights from Drosophila models of nerve injury

Axon degeneration is the pivotal pathological event of acute traumatic neural injury as well as many chronic neurodegenerative diseases. It is an active cellular program and yet molecularly distinct from cell death. Much effort is devoted toward understanding the nature of axon degeneration and promoting axon regeneration. However, the fundamental mechanisms of self-destruction of damaged axons remain unclear, and there are still few treatments for traumatic brain injury (TBI) or spinal cord injury (SCI). Genetically approachable model organisms such as Drosophila melanogaster, the fruit fly, have proven exceptionally successful in modeling human neurodegenerative diseases. More recently, this success has been extended into the field of acute axon injury and regeneration. In this review, we discuss recent findings, focusing on how these models hold promise for accelerating mechanistic insight into axon injury and identifying potential therapeutic targets for TBI and SCI.

Axon injury and stress trigger a microtubule-based neuroprotective pathway

Axon injury elicits profound cellular changes, including axon regeneration. However, the full range of neuronal injury responses remains to be elucidated. Surprisingly, after axons of Drosophila dendritic arborization neurons were severed, dendrites were more resistant to injury-induced degeneration. Concomitant with stabilization, microtubule dynamics in dendrites increased. Moreover, dendrite stabilization was suppressed when microtubule dynamics was dampened, which was achieved by lowering levels of the microtubule nucleation protein γ-tubulin. Increased microtubule dynamics and global neuronal stabilization were also activated by expression of expanded polyglutamine (poly-Q) proteins SCA1, SCA3, and huntingtin. In all cases, dynamics were increased through microtubule nucleation and depended on JNK signaling, indicating that acute axon injury and long-term neuronal stress activate a common cytoskeleton-based stabilization program. Reducing levels of γ-tubulin exacerbated long-term degeneration induced by SCA3 in branched sensory neurons and in a well established Drosophila eye model of poly-Q-induced neurodegeneration. Thus, increased microtubule dynamics can delay short-term injury-induced degeneration, and, in the case of poly-Q proteins, can counteract progressive longer-term degeneration. We conclude that axon injury or stress triggers a microtubule-based neuroprotective pathway that stabilizes neurons against degeneration.