Wlds protection distinguishes axon degeneration following injury from naturally occurring developmental pruning

Synergistic Protection of Retinal Ganglion Cells (RGCs) by SARM1 Inactivation with CNTF in a Rodent Model of Nonarteritic Anterior Ischemic Optic Neuropathy

We evaluated whether inhibiting sterile alpha and (Toll/interleukin receptor (TIR)) motif-containing 1 (SARM1) activity protects retinal ganglion cells (RGCs) following ischemic axonopathy (rodent nonarteritic anterior ischemic optic neuropathy: rNAION) by itself and combined with ciliary neurotrophic factor (CNTF). Genetically modified SARM1(-) rats were rNAION-induced in one eye and compared against equivalently induced wild-type animals of the same background. Optic nerve (ON) diameters were quantified using optical coherence tomography (SD-OCT). RGCs were quantified 30 d post-induction using retinal stereology for Brn3a(+) nuclei. ON sections were analyzed by TEM and immunohistochemistry. SARM1(-)(-) and WT animals were then bilaterally sequentially rNAION-induced. One eye received intravitreal vehicle injection following induction; the contralateral side received CNTF and was analyzed 30 d post-induction. Inhibiting SARM1 activity suppressed axonal collapse following ischemic axonopathy. SARM1(-) animals significantly reduced RGC loss, compared with WT animals (49.4 ± 6.8% RGC loss in SARM1(-) vs. 63.6 ± 3.2% sem RGC loss in WT; Mann-Whitney one-tailed U-test, (p = 0.049)). IVT-CNTF treatment vs. IVT-vehicle in SARM1(-) animals further reduced RGC loss by 24% at 30 d post-induction, but CNTF did not, by itself, improve long-term RGC survival in WT animals compared with vehicle (Mann-Whitney one-tailed t-test; p = 0.033). While inhibiting SARM1 activity is itself neuroprotective, combining SARM1 inhibition and CNTF treatment generated a long-term, synergistic neuroprotective effect in ischemic neuropathy. Combinatorial treatments for NAION utilizing independent neuroprotective mechanisms may thus provide a greater effect than individual treatment modalities.

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.

Glial Draper signaling triggers cross-neuron plasticity in bystander neurons after neuronal cell death in Drosophila

Neuronal cell death and subsequent brain dysfunction are hallmarks of aging and neurodegeneration, but how the nearby healthy neurons (bystanders) respond to the death of their neighbors is not fully understood. In the Drosophila larval neuromuscular system, bystander motor neurons can structurally and functionally compensate for the loss of their neighbors by increasing their terminal bouton number and activity. We term this compensation as cross-neuron plasticity, and in this study, we demonstrate that the Drosophila engulfment receptor, Draper, and the associated kinase, Shark, are required for cross-neuron plasticity. Overexpression of the Draper-I isoform boosts cross-neuron plasticity, implying that the strength of plasticity correlates with Draper signaling. In addition, we find that functional cross-neuron plasticity can be induced at different developmental stages. Our work uncovers a role for Draper signaling in cross-neuron plasticity and provides insights into how healthy bystander neurons respond to the loss of their neighboring neurons.

Glial Draper signaling triggers cross-neuron plasticity in bystander neurons after neuronal cell death

Neuronal cell death and subsequent brain dysfunction are hallmarks of aging and neurodegeneration, but how the nearby healthy neurons (bystanders) respond to the cell death of their neighbors is not fully understood. In the Drosophila larval neuromuscular system, bystander motor neurons can structurally and functionally compensate for the loss of their neighbors by increasing their axon terminal size and activity. We termed this compensation as cross-neuron plasticity, and in this study, we demonstrated that the Drosophila engulfment receptor, Draper, and the associated kinase, Shark, are required in glial cells. Surprisingly, overexpression of the Draper-I isoform boosts cross-neuron plasticity, implying that the strength of plasticity correlates with Draper signaling. Synaptic plasticity normally declines as animals age, but in our system, functional cross-neuron plasticity can be induced at different time points, whereas structural cross-neuron plasticity can only be induced at early stages. Our work uncovers a novel role for glial Draper signaling in cross-neuron plasticity that may enhance nervous system function during neurodegeneration and provides insights into how healthy bystander neurons respond to the loss of their neighboring neurons.

Developmental pruning of sensory neurites by mechanical tearing in Drosophila

Mechanical forces actively shape cells during development, but little is known about their roles during neuronal morphogenesis. Developmental neurite pruning, a critical circuit specification mechanism, often involves neurite abscission at predetermined sites by unknown mechanisms. Pruning of Drosophila sensory neuron dendrites during metamorphosis is triggered by the hormone ecdysone, which induces local disassembly of the dendritic cytoskeleton. Subsequently, dendrites are severed at positions close to the soma by an unknown mechanism. We found that ecdysone signaling causes the dendrites to become mechanically fragile. Severing occurs during periods of increased pupal morphogenetic tissue movements, which exert mechanical forces on the destabilized dendrites. Tissue movements and dendrite severing peak during pupal ecdysis, a period of strong abdominal contractions, and abolishing ecdysis causes non-cell autonomous dendrite pruning defects. Thus, our data establish mechanical tearing as a novel mechanism during neurite pruning.

The making of the Drosophila mushroom body

The mushroom body (MB) is a computational center in the Drosophila brain. The intricate neural circuits of the mushroom body enable it to store associative memories and process sensory and internal state information. The mushroom body is composed of diverse types of neurons that are precisely assembled during development. Tremendous efforts have been made to unravel the molecular and cellular mechanisms that build the mushroom body. However, we are still at the beginning of this challenging quest, with many key aspects of mushroom body assembly remaining unexplored. In this review, I provide an in-depth overview of our current understanding of mushroom body development and pertinent knowledge gaps.

Glial TGFβ activity promotes neuron survival in peripheral nerves

Maintaining long, energetically demanding axons throughout the life of an animal is a major challenge for the nervous system. Specialized glia ensheathe axons and support their function and integrity throughout life, but glial support mechanisms remain poorly defined. Here, we identified a collection of secreted and transmembrane molecules required in glia for long-term axon survival in vivo. We showed that the majority of components of the TGFβ superfamily are required in glia for sensory neuron maintenance but not glial ensheathment of axons. In the absence of glial TGFβ signaling, neurons undergo age-dependent degeneration that can be rescued either by genetic blockade of Wallerian degeneration or caspase-dependent death. Blockade of glial TGFβ signaling results in increased ATP in glia that can be mimicked by enhancing glial mitochondrial biogenesis or suppressing glial monocarboxylate transporter function. We propose that glial TGFβ signaling supports axon survival and suppresses neurodegeneration through promoting glial metabolic support of neurons.

In situ cell-type-specific cell-surface proteomic profiling in mice

Cell-surface proteins (CSPs) mediate intercellular communication throughout the lives of multicellular organisms. However, there are no generalizable methods for quantitative CSP profiling in specific cell types in vertebrate tissues. Here, we present in situ cell-surface proteome extraction by extracellular labeling (iPEEL), a proximity labeling method in mice that enables spatiotemporally precise labeling of cell-surface proteomes in a cell-type-specific environment in native tissues for discovery proteomics. Applying iPEEL to developing and mature cerebellar Purkinje cells revealed differential enrichment in CSPs with post-translational protein processing and synaptic functions in the developing and mature cell-surface proteomes, respectively. A proteome-instructed in vivo loss-of-function screen identified a critical, multifaceted role for Armh4 in Purkinje cell dendrite morphogenesis. Armh4 overexpression also disrupts dendrite morphogenesis; this effect requires its conserved cytoplasmic domain and is augmented by disrupting its endocytosis. Our results highlight the utility of CSP profiling in native mammalian tissues for identifying regulators of cell-surface signaling.

Traumatic brain injury recapitulates developmental changes of axons

During development, half of brain white matter axons are maintained for growth, while the remainder undergo developmental axon degeneration. After traumatic brain injury (TBI), injured axons also appear to follow pathways leading to either degeneration or repair. These observations raise the intriguing, but unexamined possibility that TBI recapitulates developmental axonal programs. Here, we examined axonal changes in the developing brain in young rats and after TBI in adult rat. Multiple shared changes in axonal microtubule (MT) through tubulin post-translational modifications and MT associated proteins (MAPs), tau and MAP6, were found in both development and TBI. Specifically, degenerating axons in both development and TBI underwent phosphorylation of tau and excessive tubulin tyrosination, suggesting MT instability and depolyermization. Conversely, nearby axons without degenerating morphologies, had increased MAP6 expression and maintenance of tubulin acetylation, suggesting enhanced MT stabilization, thereby supporting survival or repair. Quantitative proteomics revealed similar signaling pathways of axon degeneration and growth/repair, including protein clusters and networks. This comparison approach demonstrates how focused evaluation of developmental processes may provide insight into pathways initiated by TBI. In particular, the data suggest that TBI may reawaken dormant axonal programs that direct axons towards either degeneration or growth/repair, supporting further study in this area.

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.

Coordinated NADPH oxidase/hydrogen peroxide functions regulate cutaneous sensory axon de- and regeneration

Tissue wounding induces cutaneous sensory axon regeneration via hydrogen peroxide (H2O2) that is produced by the epithelial NADPH oxidase, Duox1. Sciatic nerve injury instead induces axon regeneration through neuronal uptake of the NADPH oxidase, Nox2, from macrophages. We therefore reasoned that the tissue environment in which axons are damaged stimulates distinct regenerative mechanisms. Here, we show that cutaneous axon regeneration induced by tissue wounding depends on both neuronal and keratinocyte-specific mechanisms involving H2O2 signaling. Genetic depletion of H2O2 in sensory neurons abolishes axon regeneration, whereas keratinocyte-specific H2O2 depletion promotes axonal repulsion, a phenotype mirrored in duox1 mutants. Intriguingly, cyba mutants, deficient in the essential Nox subunit, p22Phox, retain limited axon regenerative capacity but display delayed Wallerian degeneration and axonal fusion, observed so far only in invertebrates. We further show that keratinocyte-specific oxidation of the epidermal growth factor receptor (EGFR) at a conserved cysteine thiol (C797) serves as an attractive cue for regenerating axons, leading to EGFR-dependent localized epidermal matrix remodeling via the matrix-metalloproteinase, MMP-13. Therefore, wound-induced cutaneous axon de- and regeneration depend on the coordinated functions of NADPH oxidases mediating distinct processes following injury.

Distinct developmental and degenerative functions of SARM1 require NAD+ hydrolase activity

SARM1 is the founding member of the TIR-domain family of NAD⁺ hydrolases and the central executioner of pathological axon degeneration. SARM1-dependent degeneration requires NAD⁺ hydrolysis. Prior to the discovery that SARM1 is an enzyme, SARM1 was studied as a TIR-domain adaptor protein with non-degenerative signaling roles in innate immunity and invertebrate neurodevelopment, including at the Drosophila neuromuscular junction (NMJ). Here we explore whether the NADase activity of SARM1 also contributes to developmental signaling. We developed transgenic Drosophila lines that express SARM1 variants with normal, deficient, and enhanced NADase activity and tested their function in NMJ development. We find that NMJ overgrowth scales with the amount of NADase activity, suggesting an instructive role for NAD⁺ hydrolysis in this developmental signaling pathway. While degenerative and developmental SARM1 signaling share a requirement for NAD⁺ hydrolysis, we demonstrate that these signals use distinct upstream and downstream mechanisms. These results identify SARM1-dependent NAD⁺ hydrolysis as a heretofore unappreciated component of developmental signaling. SARM1 now joins sirtuins and Parps as enzymes that regulate signal transduction pathways via mechanisms that involve NAD⁺ cleavage, greatly expanding the potential scope of SARM1 TIR NADase functions.

SARM1 is the central executioner of axon loss, and inhibition of SARM1 is a therapeutic target for many devastating neurodegenerative disorders. SARM1 is the founding member of the TIR-domain family of NAD⁺ cleaving enzymes, destroying the essential metabolite NAD⁺ and inducing an energetic crisis in the axon. This was a surprising finding, as previously studied TIR-domain proteins were characterized as scaffolds that bind signaling proteins to coordinate signal transduction cascades. Indeed, before the discovery of the role of SARM1 in axon degeneration, SARM1 was studied as a regulator of intracellular signaling in immunity and neurodevelopment where it was assumed to act as a scaffold. Here we investigate whether the recently described SARM1 enzymatic activity also regulates such signal transduction pathways. Indeed, we show that a developmental signaling pathway scales with the amount of NADase activity, suggesting an instructive role for NAD⁺ cleavage. While degenerative and developmental SARM1 signaling share a requirement for NAD⁺ cleavage, they utilize distinct upstream and downstream mechanisms. With these findings, SARM1 now joins sirtuins and Parps as enzymes that regulate signal transduction pathways via mechanisms that involve NAD⁺ cleavage, greatly expanding the potential scope of SARM1 TIR NADase functions.

Neuron-glia crosstalk in neuronal remodeling and degeneration: Neuronal signals inducing glial cell phagocytic transformation in Drosophila

Neuronal remodeling is a conserved mechanism that eliminates unwanted neurites and can include the loss of cell bodies. In these processes, a key role for glial cells in events from synaptic pruning to neuron elimination has been clearly identified in the last decades. Signals sent from dying neurons or neurites to be removed are received by appropriate glial cells. After receiving these signals, glial cells infiltrate degenerating sites and then, engulf and clear neuronal debris through phagocytic mechanisms. There are few identified or proposed signals and receptors involved in neuron-glia crosstalk, which induces the transformation of glial cells to phagocytes during neuronal remodeling in Drosophila. Many of these signaling pathways are conserved in mammals. Here, we particularly emphasize the role of Orion, a recently identified neuronal CX₃ C chemokine-like secreted protein, which induces astrocyte infiltration and engulfment during mushroom body neuronal remodeling. Although, chemokine signaling was not described previously in insects we propose that chemokine-like involvement in neuron/glial cell interaction is an evolutionarily ancient mechanism.

Clearing Your Mind: Mechanisms of Debris Clearance After Cell Death During Neural Development

Neurodevelopment and efferocytosis have fascinated scientists for decades. How an organism builds a nervous system that is precisely tuned for efficient behaviors and survival and how it simultaneously manages constant somatic cell turnover are complex questions that have resulted in distinct fields of study. Although neurodevelopment requires the overproduction of cells that are subsequently pruned back, very few studies marry these fields to elucidate the cellular and molecular mechanisms that drive nervous system development through the lens of cell clearance. In this review, we discuss these fields to highlight exciting areas of future synergy. We first review neurodevelopment from the perspective of overproduction and subsequent refinement and then discuss who clears this developmental debris and the mechanisms that control these events. We then end with how a more deliberate merger of neurodevelopment and efferocytosis could reframe our understanding of homeostasis and disease and discuss areas of future study. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Lessons from Injury: How Nerve Injury Studies Reveal Basic Biological Mechanisms and Therapeutic Opportunities for Peripheral Nerve Diseases

Since Waller and Cajal in the nineteenth and early twentieth centuries, laboratory traumatic peripheral nerve injury studies have provided great insight into cellular and molecular mechanisms governing axon degeneration and the responses of Schwann cells, the major glial cell type of peripheral nerves. It is now evident that pathways underlying injury-induced axon degeneration and the Schwann cell injury-specific state, the repair Schwann cell, are relevant to many inherited and acquired disorders of peripheral nerves. This review provides a timely update on the molecular understanding of axon degeneration and formation of the repair Schwann cell. We discuss how nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) and sterile alpha TIR motif containing protein 1 (SARM1) are required for axon survival and degeneration, respectively, how transcription factor c-JUN is essential for the Schwann cell response to nerve injury and what each tells us about disease mechanisms and potential therapies. Human genetic association with NMNAT2 and SARM1 strongly suggests aberrant activation of programmed axon death in polyneuropathies and motor neuron disorders, respectively, and animal studies suggest wider involvement including in chemotherapy-induced and diabetic neuropathies. In repair Schwann cells, cJUN is aberrantly expressed in a wide variety of human acquired and inherited neuropathies. Animal models suggest it limits axon loss in both genetic and traumatic neuropathies, whereas in contrast, Schwann cell secreted Neuregulin-1 type 1 drives onion bulb pathology in CMT1A. Finally, we discuss opportunities for drug-based and gene therapies to prevent axon loss or manipulate the repair Schwann cell state to treat acquired and inherited neuropathies and neuronopathies.

Axon morphogenesis and maintenance require an evolutionary conserved safeguard function of Wnk kinases antagonizing Sarm and Axed

The molecular and cellular mechanisms underlying complex axon morphogenesis are still poorly understood. We report a novel, evolutionary conserved function for the Drosophila Wnk kinase (dWnk) and its mammalian orthologs, WNK1 and 2, in axon branching. We uncover that dWnk, together with the neuroprotective factor Nmnat, antagonizes the axon-destabilizing factors D-Sarm and Axundead (Axed) during axon branch growth, revealing a developmental function for these proteins. Overexpression of D-Sarm or Axed results in axon branching defects, which can be blocked by overexpression of dWnk or Nmnat. Surprisingly, Wnk kinases are also required for axon maintenance of adult Drosophila and mouse cortical pyramidal neurons. Requirement of Wnk for axon maintenance is independent of its developmental function. Inactivation of dWnk or mouse Wnk1/2 in mature neurons leads to axon degeneration in the adult brain. Therefore, Wnk kinases are novel signaling components that provide a safeguard function in both developing and adult axons.

Sarm1-mediated neurodegeneration within the enteric nervous system protects against local inflammation of the colon

Axonal degeneration is one of the key features of neurodegenerative disorders. In the canonical view, axonal degeneration destructs neural connections and promotes detrimental disease defects. Here, we assessed the enteric nervous system (ENS) of the mouse, non-human primate, and human by advanced 3D imaging. We observed the profound neurodegeneration of catecholaminergic axons in human colons with ulcerative colitis, and similarly, in mouse colons during acute dextran sulfate sodium-induced colitis. However, we unexpectedly revealed that blockage of such axonal degeneration by the Sarm1 deletion in mice exacerbated the colitis condition. In contrast, pharmacologic ablation or chemogenetic inhibition of catecholaminergic axons suppressed the colon inflammation. We further showed that the catecholaminergic neurotransmitter norepinephrine exerted a pro-inflammatory function by enhancing the expression of IL-17 cytokines. Together, this study demonstrated that Sarm1-mediated neurodegeneration within the ENS mitigated local inflammation of the colon, uncovering a previously-unrecognized beneficial role of axonal degeneration in this disease context.

Phagocytosis by Peripheral Glia: Importance for Nervous System Functions and Implications in Injury and Disease

The central nervous system (CNS) has very limited capacity to regenerate after traumatic injury or disease. In contrast, the peripheral nervous system (PNS) has far greater capacity for regeneration. This difference can be partly attributed to variances in glial-mediated functions, such as axon guidance, structural support, secretion of growth factors and phagocytic activity. Due to their growth-promoting characteristic, transplantation of PNS glia has been trialed for neural repair. After peripheral nerve injuries, Schwann cells (SCs, the main PNS glia) phagocytose myelin debris and attract macrophages to the injury site to aid in debris clearance. One peripheral nerve, the olfactory nerve, is unique in that it continuously regenerates throughout life. The olfactory nerve glia, olfactory ensheathing cells (OECs), are the primary phagocytes within this nerve, continuously clearing axonal debris arising from the normal regeneration of the nerve and after injury. In contrast to SCs, OECs do not appear to attract macrophages. SCs and OECs also respond to and phagocytose bacteria, a function likely critical for tackling microbial invasion of the CNS via peripheral nerves. However, phagocytosis is not always effective; inflammation, aging and/or genetic factors may contribute to compromised phagocytic activity. Here, we highlight the diverse roles of SCs and OECs with the focus on their phagocytic activity under physiological and pathological conditions. We also explore why understanding the contribution of peripheral glia phagocytosis may provide us with translational strategies for achieving axonal regeneration of the injured nervous system and potentially for the treatment of certain neurological diseases.

Spatiotemporal regulation of developmental neurite pruning: Molecular and cellular insights from Drosophila models

Developmental neurite pruning is a process by which neurons selectively eliminate unnecessary processes of axons and/or dendrites without cell death, which shapes the mature wiring of nervous systems. In this sense, developmental neurite pruning requires spatiotemporally precise control of local degradation of cellular components including cytoskeletons and membranes. The Drosophila nervous system undergoes large-scale remodeling, including axon/dendrite pruning, during metamorphosis. In addition to this unique phenomenon in the nervous system, powerful genetic tools make the Drosophila nervous system a sophisticated model to investigate spatiotemporal regulation of neural remodeling. This article reviews recent advances to our understanding of the molecular and cellular mechanisms of developmental axon/dendrite pruning, mainly focusing on studies in Drosophila sensory neurons and mushroom body neurons.

Cell death in animal development

Cell death is an important facet of animal development. In some developing tissues, death is the ultimate fate of over 80% of generated cells. Although recent studies have delineated a bewildering number of cell death mechanisms, most have only been observed in pathological contexts, and only a small number drive normal development. This Primer outlines the important roles, different types and molecular players regulating developmental cell death, and discusses recent findings with which the field currently grapples. We also clarify terminology, to distinguish between developmental cell death mechanisms, for which there is evidence for evolutionary selection, and cell death that follows genetic, chemical or physical injury. Finally, we suggest how advances in understanding developmental cell death may provide insights into the molecular basis of developmental abnormalities and pathological cell death in disease.

Models of Axon Degeneration in Drosophila Larvae

The fruit fly Drosophila melanogaster has been a powerful model to study axonal biology including axon degeneration and regeneration (Brace et al., J Neurosci 34:8398-8410, 2014; Valakh et al. J Neurosci 33:17863-17,873, 2013; Xiong and Collins J Neurosci 32:610-615, 2012; Xiong et al. 191:211-223, 2010). Both adult and larval injury models have been developed in the fruit fly. This chapter focuses on in vivo and ex vivo methods developed for studying axon degeneration in Drosophila larvae. Additional models have been developed in the adult fly including injury models of olfactory receptor neurons in the brain and a model of axonal degeneration of sensory axons in the wing (Fang and Bonini, Annu Rev. Cell Dev Biol 28:575-597, 2012; Hoopfer et al. Neuron 50:883-895, 2006; Neukomm et al. Proc Natl Acad Sci U S A 111:9965-9970, 2014).

Phagocytic glia are obligatory intermediates in transmission of mutant huntingtin aggregates across neuronal synapses

Emerging evidence supports the hypothesis that pathogenic protein aggregates associated with neurodegenerative diseases spread from cell to cell through the brain in a manner akin to infectious prions. Here, we show that mutant huntingtin (mHtt) aggregates associated with Huntington disease transfer anterogradely from presynaptic to postsynaptic neurons in the adult Drosophila olfactory system. Trans-synaptic transmission of mHtt aggregates is inversely correlated with neuronal activity and blocked by inhibiting caspases in presynaptic neurons, implicating synaptic dysfunction and cell death in aggregate spreading. Remarkably, mHtt aggregate transmission across synapses requires the glial scavenger receptor Draper and involves a transient visit to the glial cytoplasm, indicating that phagocytic glia act as obligatory intermediates in aggregate spreading between synaptically-connected neurons. These findings expand our understanding of phagocytic glia as double-edged players in neurodegeneration-by clearing neurotoxic protein aggregates, but also providing an opportunity for prion-like seeds to evade phagolysosomal degradation and propagate further in the brain.

Modeling Neurodegenerative Disorders in Drosophila melanogaster

Drosophila melanogaster provides a powerful genetic model system in which to investigate the molecular mechanisms underlying neurodegenerative diseases. In this review, we discuss recent progress in Drosophila modeling Alzheimer's Disease, Parkinson's Disease, Amyotrophic Lateral Sclerosis (ALS), Huntington's Disease, Ataxia Telangiectasia, and neurodegeneration related to mitochondrial dysfunction or traumatic brain injury. We close by discussing recent progress using Drosophila models of neural regeneration and how these are likely to provide critical insights into future treatments for neurodegenerative disorders.

Programmed axon degeneration: from mouse to mechanism to medicine

Wallerian degeneration is a widespread mechanism of programmed axon degeneration. In the three decades since the discovery of the Wallerian degeneration slow (Wld^S) mouse, research has generated extensive knowledge of the molecular mechanisms underlying Wallerian degeneration, demonstrated its involvement in non-injury disorders and found multiple ways to block it. Recent developments have included: the detection of NMNAT2 mutations that implicate Wallerian degeneration in rare human diseases; the capacity for lifelong rescue of a lethal condition related to Wallerian degeneration in mice; the discovery of 'druggable' enzymes, including SARM1 and MYCBP2 (also known as PHR1), in Wallerian pathways; and the elucidation of protein structures to drive further understanding of the underlying mechanisms and drug development. Additionally, new data have indicated the potential of these advances to alleviate a number of common disorders, including chemotherapy-induced and diabetic peripheral neuropathies, traumatic brain injury, and amyotrophic lateral sclerosis.

Sleep Regulates Glial Plasticity and Expression of the Engulfment Receptor Draper Following Neural Injury

Chronic sleep disturbance is associated with numerous health consequences, including neurodegenerative disease and cognitive decline [1]. Neurite damage due to apoptosis, trauma, or genetic factors is a common feature of aging, and clearance of damaged neurons is essential for maintenance of brain function. In the central nervous system, damaged neurites are cleared by Wallerian degeneration, in which activated microglia and macrophages engulf damaged neurons [2]. The fruit fly Drosophila melanogaster provides a powerful model for investigating the relationship between sleep and Wallerian degeneration [3]. Several lines of evidence suggest that glia influence sleep duration, sleep-mediated neuronal homeostasis, and clearance of toxic substances during sleep, raising the possibility that glial engulfment of damaged axons is regulated by sleep [4]. To explore this possibility, we axotomized olfactory receptor neurons and measured the effects of sleep loss or gain on the clearance of damaged neurites. Mechanical and genetic sleep deprivation impaired the clearance of damaged neurites. Conversely, treatment with the sleep-promoting drug gaboxadol accelerated clearance, while genetic induction of sleep promotes Draper expression. In sleep-deprived animals, multiple markers of glial activation were delayed, including activation of the JAK-STAT pathway, upregulation of the cell corpse engulfment receptor Draper, and innervation of the antennal lobe by glial membranes. These markers were all enhanced following genetic and pharmacological sleep induction. Taken together, these findings reveal a critical association between sleep and glial activation following neural injury, providing a platform for further investigations of the molecular mechanisms underlying sleep-dependent modulation of glial function and neurite clearance.

Altered mitochondrial bioenergetics are responsible for the delay in Wallerian degeneration observed in neonatal mice

Neurodegenerative and neuromuscular disorders can manifest throughout the lifespan of an individual, from infant to elderly individuals. Axonal and synaptic degeneration are early and critical elements of nearly all human neurodegenerative diseases and neural injury, however the molecular mechanisms which regulate this process are yet to be fully elucidated. Furthermore, how the molecular mechanisms governing degeneration are impacted by the age of the individual is poorly understood. Interestingly, in mice which are under 3 weeks of age, the degeneration of axons and synapses following hypoxic or traumatic injury is significantly slower. This process, known as Wallerian degeneration (WD), is a molecularly and morphologically distinct subtype of neurodegeneration by which axons and synapses undergo distinct fragmentation and death following a range of stimuli. In this study, we first use an ex-vivo model of axon injury to confirm the significant delay in WD in neonatal mice. We apply tandem mass-tagging quantitative proteomics to profile both nerve and muscle between P12 and P24 inclusive. Application of unbiased in silico workflows to relevant protein identifications highlights a steady elevation in oxidative phosphorylation cascades corresponding to the accelerated degeneration rate. We demonstrate that inhibition of Complex I prevents the axotomy-induced rise in reactive oxygen species and protects axons following injury. Furthermore, we reveal that pharmacological activation of oxidative phosphorylation significantly accelerates degeneration at the neuromuscular junction in neonatal mice. In summary, we reveal dramatic changes in the neuromuscular proteome during post-natal maturation of the neuromuscular system, and demonstrate that endogenous dynamics in mitochondrial bioenergetics during this time window have a functional impact upon regulating the stability of the neuromuscular system.

Axon death signalling in Wallerian degeneration among species and in disease

Axon loss is a shared feature of nervous systems being challenged in neurological disease, by chemotherapy or mechanical force. Axons take up the vast majority of the neuronal volume, thus numerous axonal intrinsic and glial extrinsic support mechanisms have evolved to promote lifelong axonal survival. Impaired support leads to axon degeneration, yet underlying intrinsic signalling cascades actively promoting the disassembly of axons remain poorly understood in any context, making the development to attenuate axon degeneration challenging. Wallerian degeneration serves as a simple model to study how axons undergo injury-induced axon degeneration (axon death). Severed axons actively execute their own destruction through an evolutionarily conserved axon death signalling cascade. This pathway is also activated in the absence of injury in diseased and challenged nervous systems. Gaining insights into mechanisms underlying axon death signalling could therefore help to define targets to block axon loss. Herein, we summarize features of axon death at the molecular and subcellular level. Recently identified and characterized mediators of axon death signalling are comprehensively discussed in detail, and commonalities and differences across species highlighted. We conclude with a summary of engaged axon death signalling in humans and animal models of neurological conditions. Thus, gaining mechanistic insights into axon death signalling broadens our understanding beyond a simple injury model. It harbours the potential to define targets for therapeutic intervention in a broad range of human axonopathies.

Axon injury signaling and compartmentalized injury response in glaucoma

Axonal degeneration is an active, highly controlled process that contributes to beneficial processes, such as developmental pruning, but also to neurodegeneration. In glaucoma, ocular hypertension leads to vision loss by killing the output neurons of the retina, the retinal ganglion cells (RGCs). Multiple processes have been proposed to contribute to and/or mediate axonal injury in glaucoma, including: neuroinflammation, loss of neurotrophic factors, dysregulation of the neurovascular unit, and disruption of the axonal cytoskeleton. While the inciting injury to RGCs in glaucoma is complex and potentially heterogeneous, axonal injury is ultimately thought to be the key insult that drives glaucomatous neurodegeneration. Glaucomatous neurodegeneration is a complex process, with multiple molecular signals contributing to RGC somal loss and axonal degeneration. Furthermore, the propagation of the axonal injury signal is complex, with injury triggering programs of degeneration in both the somal and axonal compartment. Further complicating this process is the involvement of multiple cell types that are known to participate in the process of axonal and neuronal degeneration after glaucomatous injury. Here, we review the axonal signaling that occurs after injury and the molecular signaling programs currently known to be important for somal and axonal degeneration after glaucoma-relevant axonal injuries.

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.

Axonal Degeneration Is Mediated by Necroptosis Activation

Axonal degeneration, which contributes to functional impairment in several disorders of the nervous system, is an important target for neuroprotection. Several individual factors and subcellular events have been implicated in axonal degeneration, but researchers have so far been unable to identify an integrative signaling pathway activating this self-destructive process. Through pharmacological and genetic approaches, we tested whether necroptosis, a regulated cell-death mechanism implicated in the pathogenesis of several neurodegenerative diseases, is involved in axonal degeneration. Pharmacological inhibition of the necroptotic kinase RIPK1 using necrostatin-1 strongly delayed axonal degeneration in the peripheral nervous system and CNS of wild-type mice of either sex and protected in vitro sensory axons from degeneration after mechanical and toxic insults. These effects were also observed after genetic knock-down of RIPK3, a second key regulator of necroptosis, and the downstream effector MLKL (Mixed Lineage Kinase Domain-Like). RIPK1 inhibition prevented mitochondrial fragmentation in vitro and in vivo, a typical feature of necrotic death, and inhibition of mitochondrial fission by Mdivi also resulted in reduced axonal loss in damaged nerves. Furthermore, electrophysiological analysis demonstrated that inhibition of necroptosis delays not only the morphological degeneration of axons, but also the loss of their electrophysiological function after nerve injury. Activation of the necroptotic pathway early during injury-induced axonal degeneration was made evident by increased phosphorylation of the downstream effector MLKL. Our results demonstrate that axonal degeneration proceeds by necroptosis, thus defining a novel mechanistic framework in the axonal degenerative cascade for therapeutic interventions in a wide variety of conditions that lead to neuronal loss and functional impairment.SIGNIFICANCE STATEMENT We show that axonal degeneration triggered by diverse stimuli is mediated by the activation of the necroptotic programmed cell-death program by a cell-autonomous mechanism. This work represents a critical advance for the field since it identifies a defined degenerative pathway involved in axonal degeneration in both the peripheral nervous system and the CNS, a process that has been proposed as an early event in several neurodegenerative conditions and a major contributor to neuronal death. The identification of necroptosis as a key mechanism for axonal degeneration is an important step toward the development of novel therapeutic strategies for nervous-system disorders, particularly those related to chemotherapy-induced peripheral neuropathies or CNS diseases in which axonal degeneration is a common factor.

Benefits of Enhancing Nicotinamide Adenine Dinucleotide Levels in Damaged or Diseased Nerve Cells

Three unbiased lines of research have commonly pointed to the benefits of enhanced levels of nicotinamide adenine dinucleotide (NAD⁺) to diseased or damaged neurons. Mice carrying a triplication of the gene encoding the culminating enzyme in NAD⁺ salvage from nicotinamide, NMNAT, are protected from a variety of insults to axons. Protection from Wallerian degeneration of axons is also observed in flies and mice bearing inactivating mutations in the SARM1 gene. Functional studies of the SARM1 gene product have revealed the presence of an enzymatic activity directed toward the hydrolysis of NAD⁺ Finally, an unbiased drug screen performed in living mice led to the discovery of a neuroprotective chemical designated P7C3. Biochemical studies of the P7C3 chemical show that it can enhance recovery of NAD⁺ from nicotinamide by activating NAMPT, the first enzyme in the salvage pathway. In combination, these three unrelated research endeavors offer evidence of the benefits of enhanced NAD⁺ levels to damaged neurons.

NMNAT Proteins that Limit Wallerian Degeneration Also Regulate Critical Period Plasticity in the Visual Cortex

Many brain regions go through critical periods of development during which plasticity is enhanced. These critical periods are associated with extensive growth and retraction of thalamocortical and intracortical axons. Here, we investigated whether a signaling pathway that is central in Wallerian axon degeneration also regulates critical period plasticity in the primary visual cortex (V1). Wallerian degeneration is characterized by rapid disintegration of axons once they are separated from the cell body. This degenerative process is initiated by reduced presence of cytoplasmic nicotinamide mononucleotide adenylyltransferases (NMNATs) and is strongly delayed in mice overexpressing cytoplasmic NMNAT proteins, such as Wld^S mutant mice producing a UBE4b-NMNAT1 fusion protein or NMNAT3 transgenic mice. Here, we provide evidence that in Wld^S mice and NMNAT3 transgenic mice, ocular dominance (OD) plasticity in the developing visual cortex is reduced. This deficit is only observed during the second half of the critical period. Additionally, we detect an early increase of visual acuity in the V1 of Wld^S mice. We do not find evidence for Wallerian degeneration occurring during OD plasticity. Our findings suggest that NMNATs do not only regulate Wallerian degeneration during pathological conditions but also control cellular events that mediate critical period plasticity during the physiological development of the cortex.

NGF-Dependent Changes in Ubiquitin Homeostasis Trigger Early Cholinergic Degeneration in Cellular and Animal AD-Model

Basal forebrain cholinergic neurons (BFCNs) depend on nerve growth factor (NGF) for their survival/differentiation and innervate cortical and hippocampal regions involved in memory/learning processes. Cholinergic hypofunction and/or degeneration early occurs at prodromal stages of Alzheimer's disease (AD) neuropathology in correlation with synaptic damages, cognitive decline and behavioral disability. Alteration(s) in ubiquitin-proteasome system (UPS) is also a pivotal AD hallmark but whether it plays a causative, or only a secondary role, in early synaptic failure associated with disease onset remains unclear. We previously reported that impairment of NGF/TrkA signaling pathway in cholinergic-enriched septo-hippocampal primary neurons triggers "dying-back" degenerative processes which occur prior to cell death in concomitance with loss of specific vesicle trafficking proteins, including synapsin I, SNAP-25 and α-synuclein, and with deficit in presynaptic excitatory neurotransmission. Here, we show that in this in vitro neuronal model: (i) UPS stimulation early occurs following neurotrophin starvation (-1 h up to -6 h); (ii) NGF controls the steady-state levels of these three presynaptic proteins by acting on coordinate mechanism(s) of dynamic ubiquitin-C-terminal hydrolase 1 (UCHL-1)-dependent (mono)ubiquitin turnover and UPS-mediated protein degradation. Importantly, changes in miniature excitatory post-synaptic currents (mEPSCs) frequency detected in -6 h NGF-deprived primary neurons are strongly reverted by acute inhibition of UPS and UCHL-1, indicating that NGF tightly controls in vitro the presynaptic efficacy via ubiquitination-mediated pathway(s). Finally, changes in synaptic ubiquitin and selective reduction of presynaptic markers are also found in vivo in cholinergic nerve terminals from hippocampi of transgenic Tg2576 AD mice, even from presymptomatic stages of neuropathology (1-month-old). By demonstrating a crucial role of UPS in the dysregulation of NGF/TrkA signaling on properties of cholinergic synapses, these findings from two well-established cellular and animal AD models provide novel therapeutic targets to contrast early cognitive and synaptic dysfunction associated to selective degeneration of BFCNs occurring in incipient early/middle-stage of disease.

Axonal fusion: An alternative and efficient mechanism of nerve repair

Injuries to the nervous system can cause lifelong morbidity due to the disconnect that occurs between nerve cells and their cellular targets. Re-establishing these lost connections is the ultimate goal of endogenous regenerative mechanisms, as well as those induced by exogenous manipulations in a laboratory or clinical setting. Reconnection between severed neuronal fibers occurs spontaneously in some invertebrate species and can be induced in mammalian systems. This process, known as axonal fusion, represents a highly efficient means of repair after injury. Recent progress has greatly enhanced our understanding of the molecular control of axonal fusion, demonstrating that the machinery required for the engulfment of apoptotic cells is repurposed to mediate the reconnection between severed axon fragments, which are subsequently merged by fusogen proteins. Here, we review our current understanding of naturally occurring axonal fusion events, as well as those being ectopically produced with the aim of achieving better clinical outcomes.

Time course of traumatic neuroma development

This study was designed to characterize morphologic stages during neuroma development post amputation with an eye toward developing better treatment strategies that intervene before neuromas are fully formed. Right forelimbs of 30 Sprague Dawley rats were amputated and limb stumps were collected at 3, 7, 28, 60 and 90 Days Post Amputation (DPA). Morphology of newly formed nerves and neuromas were assessed via general histology and neurofilament protein antibody staining. Analysis revealed six morphological characteristics during nerve and neuroma development; 1) normal nerve, 2) degenerating axons, 3) axonal sprouts, 4) unorganized bundles of axons, 5) unorganized axon growth into muscles, and 6) unorganized axon growth into fibrotic tissue (neuroma). At early stages (3 & 7 DPA) after amputation, normal nerves could be identified throughout the limb stump and small areas of axonal sprouts were present near the site of injury. Signs of degenerating axons were evident from 7 to 90 DPA. From day 28 on, variability of nerve characteristics with signs of unorganized axon growth into muscle and fibrotic tissue and neuroma formation became visible in multiple areas of stump tissue. These pathological features became more evident on days 60 and 90. At 90 DPA frank neuroma formation was present in all stump tissue. By following nerve regrowth and neuroma formation after amputation we were able to identify 6 separate histological stages of nerve regrowth and neuroma development. Axonal regrowth was observed as early as 3 DPA and signs of unorganized axonal growth and neuroma formation were evident by 28 DPA. Based on these observations we speculate that neuroma treatment and or prevention strategies might be more successful if targeted at the initial stages of development and not after 28 DPA.

Neuronal integrity and complement control synaptic material clearance by microglia after CNS injury

Norris et al. show that microglia are the key phagocytes in removal of synaptic debris in the dorsal lateral geniculate nucleus after optic nerve injury. This microglial function is dependent on recognition of neurodegeneration and is mediated by the complement system.

Phagocytosis of synaptic material by microglia is critical for central nervous system development. Less well understood is this microglial function in the injured adult brain. Assay of microglial phagocytosis is challenging, because peripheral myeloid cells engraft the site of injury, which could obscure interpretation of microglial roles. The model used here, optic nerve crush injury, results in degeneration of synapses in the dorsal lateral geniculate nucleus (dLGN), which stimulates rapid activation and engulfment of synaptic material by resident microglia without myeloid cell engraftment. Pharmacological depletion of microglia causes postinjury accumulation of synaptic debris, suggesting that microglia are the dominant postinjury phagocytes. Genetic or pharmacological manipulations revealed that neuronal activity does not trigger microglia phagocytosis after injury. RNA sequencing reveals C1q and CD11b/CR3 involvement in clearance of debris by dLGN-resident microglia. Indeed, C1qa^−/− and Itgam^−/− mice exhibit impaired postinjury debris clearance. Our results show how neurodegenerative debris is cleared by microglia and offers a model for studying its mechanisms and physiological roles.

The Heterochronic Gene lin-14 Controls Axonal Degeneration in C. elegans Neurons

The disproportionate length of an axon makes its structural and functional maintenance a major task for a neuron. The heterochronic gene lin-14 has previously been implicated in regulating the timing of key developmental events in the nematode C. elegans. Here, we report that LIN-14 is critical for maintaining neuronal integrity. Animals lacking lin-14 display axonal degeneration and guidance errors in both sensory and motor neurons. We demonstrate that LIN-14 functions both cell autonomously within the neuron and non-cell autonomously in the surrounding tissue, and we show that interaction between the axon and its surrounding tissue is essential for the preservation of axonal structure. Furthermore, we demonstrate that lin-14 expression is only required during a short period early in development in order to promote axonal maintenance throughout the animal's life. Our results identify a crucial role for LIN-14 in preventing axonal degeneration and in maintaining correct interaction between an axon and its surrounding tissue.

Role of SARM1 and DR6 in retinal ganglion cell axonal and somal degeneration following axonal injury

Optic neuropathies such as glaucoma are characterized by the degeneration of retinal ganglion cells (RGCs) and the irreversible loss of vision. In these diseases, focal axon injury triggers a propagating axon degeneration and, eventually, cell death. Previous work by us and others identified dual leucine zipper kinase (DLK) and JUN N-terminal kinase (JNK) as key mediators of somal cell death signaling in RGCs following axonal injury. Moreover, others have shown that activation of the DLK/JNK pathway contributes to distal axonal degeneration in some neuronal subtypes and that this activation is dependent on the adaptor protein, sterile alpha and TIR motif containing 1 (SARM1). Given that SARM1 acts upstream of DLK/JNK signaling in axon degeneration, we tested whether SARM1 plays a similar role in RGC somal apoptosis in response to optic nerve injury. Using the mouse optic nerve crush (ONC) model, our results show that SARM1 is critical for RGC axonal degeneration and that axons rescued by SARM1 deficiency are electrophysiologically active. Genetic deletion of SARM1 did not, however, prevent DLK/JNK pathway activation in RGC somas nor did it prevent or delay RGC cell death. These results highlight the importance of SARM1 in RGC axon degeneration and suggest that somal activation of the DLK/JNK pathway is activated by an as-yet-unidentified SARM1-independent signal.

Cell death in neural precursor cells and neurons before neurite formation prevents the emergence of abnormal neural structures in the Drosophila optic lobe

Programmed cell death is a conserved strategy for neural development both in vertebrates and invertebrates and is recognized at various developmental stages in the brain from neurogenesis to adulthood. To understand the development of the central nervous system, it is essential to reveal not only molecular mechanisms but also the role of neural cell death (Pinto-Teixeira et al., 2016). To understand the role of cell death in neural development, we investigated the effect of inhibition of cell death on optic lobe development. Our data demonstrate that, in the optic lobe of Drosophila, cell death occurs in neural precursor cells and neurons before neurite formation and functions to prevent various developmental abnormalities. When neuronal cell death was inhibited by an effector caspase inhibitor, p35, multiple abnormal neuropil structures arose during optic lobe development-e.g., enlarged or fused neuropils, misrouted neurons and abnormal neurite lumps. Inhibition of cell death also induced morphogenetic defects in the lamina and medulla development-e.g., failures in the separation of the lamina and medulla cortices and the medulla rotation. These defects were reproduced in the mutant of an initiator caspase, dronc. If cell death was a mechanism for removing the abnormal neuropil structures, we would also expect to observe them in mutants defective for corpse clearance. However, they were not observed in these mutants. When dead cell-membranes were visualized with Apoliner, they were observed only in cortices and not in neuropils. These results suggest that the cell death occurs before mature neurite formation. Moreover, we found that inhibition of cell death induced ectopic neuroepithelial cells, neuroblasts and ganglion mother cells in late pupal stages, at sites where the outer and inner proliferation centers were located at earlier developmental stages. Caspase-3 activation was observed in the neuroepithelial cells and neuroblasts in the proliferation centers. These results indicate that cell death is required for elimination of the precursor cells composing the proliferation centers. This study substantiates an essential role of early neural cell death for ensuring normal development of the central nervous system.

Regulation of Adult CNS Axonal Regeneration by the Post-transcriptional Regulator Cpeb1

Adult mammalian central nervous system (CNS) neurons are unable to regenerate following axonal injury, leading to permanent functional impairments. Yet, the reasons underlying this regeneration failure are not fully understood. Here, we studied the transcriptome and translatome shortly after spinal cord injury. Profiling of the total and ribosome-bound RNA in injured and naïve spinal cords identified a substantial post-transcriptional regulation of gene expression. In particular, transcripts associated with nervous system development were down-regulated in the total RNA fraction while remaining stably loaded onto ribosomes. Interestingly, motif association analysis of post-transcriptionally regulated transcripts identified the cytoplasmic polyadenylation element (CPE) as enriched in a subset of these transcripts that was more resistant to injury-induced reduction at the transcriptome level. Modulation of these transcripts by overexpression of the CPE binding protein, Cpeb1, in mouse and Drosophila CNS neurons promoted axonal regeneration following injury. Our study uncovered a global evolutionarily conserved post-transcriptional mechanism enhancing regeneration of injured CNS axons.

Common and Divergent Mechanisms in Developmental Neuronal Remodeling and Dying Back Neurodegeneration

Cell death is an inherent process that is required for the proper wiring of the nervous system. Studies over the last four decades have shown that, in a parallel developmental pathway, axons and dendrites are eliminated without the death of the neuron. This developmentally regulated 'axonal death' results in neuronal remodeling, which is an essential mechanism to sculpt neuronal networks in both vertebrates and invertebrates. Studies across various organisms have demonstrated that a conserved strategy in the formation of adult neuronal circuitry often involves generating too many connections, most of which are later eliminated with high temporal and spatial resolution. Can neuronal remodeling be regarded as developmentally and spatially regulated neurodegeneration? It has been previously speculated that injury-induced degeneration (Wallerian degeneration) shares some molecular features with 'dying back' neurodegenerative diseases. In this opinion piece, we examine the similarities and differences between the mechanisms regulating neuronal remodeling and those being perturbed in dying back neurodegenerative diseases. We focus primarily on amyotrophic lateral sclerosis and peripheral neuropathies and highlight possible shared pathways and mechanisms. While mechanistic data are only just beginning to emerge, and despite the inherent differences between disease-oriented and developmental processes, we believe that some of the similarities between these developmental and disease-initiated degeneration processes warrant closer collaborations and crosstalk between these different fields.

Engineering a light-activated caspase-3 for precise ablation of neurons in vivo

The circuitry of the brain is characterized by cell heterogeneity, sprawling cellular anatomy, and astonishingly complex patterns of connectivity. Determining how complex neural circuits control behavior is a major challenge that is often approached using surgical, chemical, or transgenic approaches to ablate neurons. However, all these approaches suffer from a lack of precise spatial and temporal control. This drawback would be overcome if cellular ablation could be controlled with light. Cells are naturally and cleanly ablated through apoptosis due to the terminal activation of caspases. Here, we describe the engineering of a light-activated human caspase-3 (Caspase-LOV) by exploiting its natural spring-loaded activation mechanism through rational insertion of the light-sensitive LOV2 domain that expands upon illumination. We apply the light-activated caspase (Caspase-LOV) to study neurodegeneration in larval and adult Drosophila Using the tissue-specific expression system (UAS)-GAL4, we express Caspase-LOV specifically in three neuronal cell types: retinal, sensory, and motor neurons. Illumination of whole flies or specific tissues containing Caspase-LOV-induced cell death and allowed us to follow the time course and sequence of neurodegenerative events. For example, we find that global synchronous activation of caspase-3 drives degeneration with a different time-course and extent in sensory versus motor neurons. We believe the Caspase-LOV tool we engineered will have many other uses for neurobiologists and others for specific temporal and spatial ablation of cells in complex organisms.

A novel Drosophila injury model reveals severed axons are cleared through a Draper/MMP-1 signaling cascade

Neural injury triggers swift responses from glia, including glial migration and phagocytic clearance of damaged neurons. The transcriptional programs governing these complex innate glial immune responses are still unclear. Here, we describe a novel injury assay in adult Drosophila that elicits widespread glial responses in the ventral nerve cord (VNC). We profiled injury-induced changes in VNC gene expression by RNA sequencing (RNA-seq) and found that responsive genes fall into diverse signaling classes. One factor, matrix metalloproteinase-1 (MMP-1), is induced in Drosophila ensheathing glia responding to severed axons. Interestingly, glial induction of MMP-1 requires the highly conserved engulfment receptor Draper, as well as AP-1 and STAT92E. In MMP-1 depleted flies, glia do not properly infiltrate neuropil regions after axotomy and, as a consequence, fail to clear degenerating axonal debris. This work identifies Draper-dependent activation of MMP-1 as a novel cascade required for proper glial clearance of severed axons.

The Drosophila LC8 homolog cut up specifies the axonal transport of proteasomes

Because of their functional polarity and elongated morphologies, microtubule-based transport of proteins and organelles is critical for normal neuronal function. The proteasome is required throughout the neuron for the highly regulated degradation of a broad set of protein targets whose functions underlie key physiological responses, including synaptic plasticity and axonal degeneration. Molecularly, the relationship between proteasome transport and the transport of the targets of proteasomes is unclear. The dynein motor complex is required for the microtubule-based motility of numerous proteins and organelles in neurons. Here, we demonstrate that microtubule-based transport of proteasomes within the neuron in Drosophila utilizes a different dynein light chain to that used by synaptic proteins. Live imaging of proteasomes and synaptic vesicle proteins in axons and synapses finds that these cargoes traffic independently, and that proteasomes exhibit significantly reduced retrograde transport velocities compared to those of synaptic vesicle proteins. Genetic and biochemical analyses reveals that the Drosophila homolog of the LC8 dynein light chains (mammalian DYNLL1 and DYNLL2), called Cut up, binds proteasomes and functions specifically during their transport. These data support the model that Cut up functions to specify the dynein-mediated transport of neuronal proteasomes.

Axon Death Pathways Converge on Axundead to Promote Functional and Structural Axon Disassembly

Axon degeneration is a hallmark of neurodegenerative disease and neural injury. Axotomy activates an intrinsic pro-degenerative axon death signaling cascade involving loss of the NAD⁺ biosynthetic enzyme Nmnat/Nmnat2 in axons, activation of dSarm/Sarm1, and subsequent Sarm-dependent depletion of NAD⁺. Here we identify Axundead (Axed) as a mediator of axon death. axed mutants suppress axon death in several types of axons for the lifespan of the fly and block the pro-degenerative effects of activated dSarm in vivo. Neurodegeneration induced by loss of the sole fly Nmnat ortholog is also fully blocked by axed, but not dsarm, mutants. Thus, pro-degenerative pathways activated by dSarm signaling or Nmnat elimination ultimately converge on Axed. Remarkably, severed axons morphologically preserved by axon death pathway mutations remain integrated in circuits and able to elicit complex behaviors after stimulation, indicating that blockade of axon death signaling results in long-term functional preservation of axons.

Nicotinamide and WLDS Act Together to Prevent Neurodegeneration in Glaucoma

Glaucoma is a complex neurodegenerative disease characterized by progressive visual dysfunction leading to vision loss. Retinal ganglion cells are the primary affected neuronal population, with a critical insult damaging their axons in the optic nerve head. This insult is typically secondary to harmfully high levels of intraocular pressure (IOP). We have previously determined that early mitochondrial abnormalities within retinal ganglion cells lead to neuronal dysfunction, with age-related declines in NAD (NAD⁺ and NADH) rendering retinal ganglion cell mitochondria vulnerable to IOP-dependent stresses. The Wallerian degeneration slow allele, Wld^S, decreases the vulnerability of retinal ganglion cells in eyes with elevated IOP, but the exact mechanism(s) of protection from glaucoma are not determined. Here, we demonstrate that Wld^S increases retinal NAD levels. Coupled with nicotinamide administration (an NAD precursor), it robustly protects from glaucomatous neurodegeneration in a mouse model of glaucoma (94% of eyes having no glaucoma, more than Wld^S or nicotinamide alone). Importantly, nicotinamide and Wld^S protect somal, synaptic, and axonal compartments, prevent loss of anterograde axoplasmic transport, and protect from visual dysfunction as assessed by pattern electroretinogram. Boosting NAD production generally benefits major compartments of retinal ganglion cells, and may be of value in other complex, age-related, axonopathies where multiple neuronal compartments are ultimately affected.

Metabolic Vulnerability in the Neurodegenerative Disease Glaucoma

Axons can be several orders of magnitude longer than neural somas, presenting logistical difficulties in cargo trafficking and structural maintenance. Keeping the axon compartment well supplied with energy also presents a considerable challenge; even seemingly subtle modifications of metabolism can result in functional deficits and degeneration. Axons require a great deal of energy, up to 70% of all energy used by a neuron, just to maintain the resting membrane potential. Axonal energy, in the form of ATP, is generated primarily through oxidative phosphorylation in the mitochondria. In addition, glial cells contribute metabolic intermediates to axons at moments of high activity or according to need. Recent evidence suggests energy disruption is an early contributor to pathology in a wide variety of neurodegenerative disorders characterized by axonopathy. However, the degree to which the energy disruption is intrinsic to the axon vs. associated glia is not clear. This paper will review the role of energy availability and utilization in axon degeneration in glaucoma, a chronic axonopathy of the retinal projection.