Diverse cellular and molecular modes of axon degeneration

Neuronal guidance signaling in neurodegenerative diseases: Key regulators that function at neuron-glia and neuroimmune interfaces

The nervous system processes a vast amount of information, performing computations that underlie perception, cognition, and behavior. During development, neuronal guidance genes, which encode extracellular cues, their receptors, and downstream signal transducers, organize neural wiring to generate the complex architecture of the nervous system. It is now evident that many of these neuroguidance cues and their receptors are active during development and are also expressed in the adult nervous system. This suggests that neuronal guidance pathways are critical not only for neural wiring but also for ongoing function and maintenance of the mature nervous system. Supporting this view, these pathways continue to regulate synaptic connectivity, plasticity, and remodeling, and overall brain homeostasis throughout adulthood. Genetic and transcriptomic analyses have further revealed many neuronal guidance genes to be associated with a wide range of neurodegenerative and neuropsychiatric disorders. Although the precise mechanisms by which aberrant neuronal guidance signaling drives the pathogenesis of these diseases remain to be clarified, emerging evidence points to several common themes, including dysfunction in neurons, microglia, astrocytes, and endothelial cells, along with dysregulation of neuron-microglia-astrocyte, neuroimmune, and neurovascular interactions. In this review, we explore recent advances in understanding the molecular and cellular mechanisms by which aberrant neuronal guidance signaling contributes to disease pathogenesis through altered cell-cell interactions. For instance, recent studies have unveiled two distinct semaphorin-plexin signaling pathways that affect microglial activation and neuroinflammation. We discuss the challenges ahead, along with the therapeutic potentials of targeting neuronal guidance pathways for treating neurodegenerative diseases. Particular focus is placed on how neuronal guidance mechanisms control neuron-glia and neuroimmune interactions and modulate microglial function under physiological and pathological conditions. Specifically, we examine the crosstalk between neuronal guidance signaling and TREM2, a master regulator of microglial function, in the context of pathogenic protein aggregates. It is well-established that age is a major risk factor for neurodegeneration. Future research should address how aging and neuronal guidance signaling interact to influence an individual's susceptibility to various late-onset neurological diseases and how the progression of these diseases could be therapeutically blocked by targeting neuronal guidance pathways.

Pyruvate kinase deficiency links metabolic perturbations to neurodegeneration and axonal protection

Neurons rely on tightly regulated metabolic networks to sustain their high-energy demands, particularly through the coupling of glycolysis and oxidative phosphorylation. Here, we investigate the role of pyruvate kinase (PyK), a key glycolytic enzyme, in maintaining axonal and synaptic integrity in the Drosophila melanogaster neuromuscular system. Using genetic deficiencies in PyK, we show that disrupting glycolysis induces progressive synaptic and axonal degeneration and severe locomotor deficits. These effects require the conserved dual leucine zipper kinase (DLK), Jun N-terminal kinase (JNK), and activator protein 1 (AP-1) Fos transcription factor axonal damage signaling pathway and the SARM1 NADase enzyme, a key driver of axonal degeneration. As both DLK and SARM1 regulate degeneration of injured axons (Wallerian degeneration), we probed the effect of PyK loss on this process. Consistent with the idea that metabolic shifts may influence neuronal resilience in context-dependent ways, we find that pyk knockdown delays Wallerian degeneration following nerve injury, suggesting that reducing glycolytic flux can promote axon survival under stress conditions. This protective effect is partially blocked by DLK knockdown and fully abolished by SARM1 overexpression. Together, our findings help bridge metabolism and neurodegenerative signaling by demonstrating that glycolytic perturbations causally activate stress response pathways that dictate the balance between protection and degeneration depending on the system's state. These results provide a mechanistic framework for understanding metabolic contributions to neurodegeneration and highlight the potential of metabolism as a target for therapeutic strategies.

Abstract Figure

SARM1: The Checkpoint of Axonal Degeneration in the Nervous System Disorders

Axons are metabolically active neuronal segments with well-controlled axonal degeneration and regeneration. External stress or injury displaces this equilibrium toward degeneration leading to axonal dysfunction observed in the pathology of several diseases. The demand and supply matrix of energy at the synapses are maintained by the axonal transport. Nicotinamide adenine dinucleotide (NAD+) is a major energy-driving coenzyme of cells that controls mitochondrial, cytoplasmic, and other organellar energy cycles generating high amounts of adenosine triphosphate (ATP). NAD+ participates in various cellular cycles and is consumed by several enzymes. One of the key enzymes targeting NAD+ is Sterile alpha and TIR motif-containing protein 1 (SARM1) which gets activated in response to external noxious stimuli. SARM1 is an octamer consisting of multiple domains of which the TIR domain governs NAD+ hydrolysis which eventually leads to axonal deficits. Besides its localization in neurons, SARM1 is also present in astrocytes, microglia, and macrophages in which it regulates inflammatory responses associated with disease pathology. SARM1 localization in the outer mitochondrial membrane is responsible for its association with mitochondrial dynamics. SARM1-mediated mitochondrial dysfunction further drives the axonal degeneration associated with peripheral and central nervous system disorders. Several genetic and pharmacological studies highlight the role of SARM1 in axonal degeneration. SARM1 is thus becoming a popular target for preventing axonal degeneration. Several small molecules consisting of isoquinoline, isothiazole, pyridine, and tryptoline acrylamide moieties have been tested for their activity against SARM1 with a promising foundation for drug discovery in targeting SARM1. In our review, we highlight the role of SARM1 in axonal degeneration associated with several disease pathologies focusing on genetic and pharmacological evaluation.

Targeting sickle cell pathobiology and pain with novel transdermal curcumin

Several comorbidities of sickle cell disease (SCD) originate from red blood cell (RBC) instability, chronic inflammation, and oxidative stress. Development of scalable, cost-effective therapeutics suitable for chronic administration to prevent, attenuate, and perhaps reverse the consequences of RBC instability is needed. Curcumin has many of these attributes as a safe compound with antisickling, antiinflammatory, and antioxidant properties, but its translational potential has been constrained due to limited bioavailability from oral administration. The present study demonstrates the rapid and high bioavailability of a novel topical/transdermal (TD) curcumin gel formulation in the plasma and blood cells and its effectiveness in humanized sickle cell mice in: (i) ameliorating features of sickle cell pain hypersensitivity and axonal injury; (ii) reducing multiple manifestations of RBC instability including evidence of decreased hemolysis (reduced lactate dehydrogenase levels), enhanced RBC ATP levels along with decreased oxidative damage; (iii) decreasing multiple proinflammatory cytokines including interleukin-6, monocyte chemoattractant protein-1, granulocyte-macrophage colony-stimulating factor, and activation, normal T cell expressed and secreted protein in skin secretome; and (iv) reducing mast cell degranulation and activation. Our data suggest that an easy-to-use novel TD curcumin gel formulation has the potential to ameliorate chronic pain, improve RBC stability, and reduce inflammatory consequences of SCD.

Astrocyte-dependent local neurite pruning in Beat-Va neurons

Developmental neuronal remodeling is extensive and mechanistically diverse across the nervous system. We sought to identify Drosophila pupal neurons that underwent mechanistically new types of neuronal remodeling and describe remodeling Beat-VaM and Beat-VaL neurons. We show that Beat-VaM neurons produce highly branched neurites in the CNS during larval stages that undergo extensive local pruning. Surprisingly, although the ecdysone receptor (EcR) is essential for pruning in all other cell types studied, Beat-VaM neurons remodel their branches extensively despite cell autonomous blockade EcR or caspase signaling. Proper execution of local remodeling in Beat-VaM neurons instead depends on extrinsic signaling from astrocytes converging with intrinsic and less dominant EcR-regulated mechanisms. In contrast, Beat-VaL neurons undergo steroid hormone-dependent, apoptotic cell death, which we show relies on the segment-specific expression of the Hox gene Abd-B. Our work provides new cell types in which to study neuronal remodeling, highlights an important role for astrocytes in activating local pruning in Drosophila independent of steroid signaling, and defines a Hox gene-mediated mechanism for segment-specific cell elimination.

Membrane mechanics dictate axonal pearls-on-a-string morphology and function

Axons are ultrathin membrane cables that are specialized for the conduction of action potentials. Although their diameter is variable along their length, how their morphology is determined is unclear. Here, we demonstrate that unmyelinated axons of the mouse central nervous system have nonsynaptic, nanoscopic varicosities ~200 nm in diameter repeatedly along their length interspersed with a thin cable ~60 nm in diameter like pearls-on-a-string. In silico modeling suggests that this axon nanopearling can be explained by membrane mechanical properties. Treatments disrupting membrane properties, such as hyper- or hypotonic solutions, cholesterol removal and nonmuscle myosin II inhibition, alter axon nanopearling, confirming the role of membrane mechanics in determining axon morphology. Furthermore, neuronal activity modulates plasma membrane cholesterol concentration, leading to changes in axon nanopearls and causing slowing of action potential conduction velocity. These data reveal that biophysical forces dictate axon morphology and function, and modulation of membrane mechanics likely underlies unmyelinated axonal plasticity.

Neuronal Cell Rearrangement During Aging: Antioxidant Compounds as a Potential Therapeutic Approach

Aging is a natural process that leads to time-related changes and a decrease in cognitive abilities, executive functions, and attention. In neuronal aging, brain cells struggle to respond to oxidative stress. The structure, function, and survival of neurons can be mediated by different pathways that are sensitive to oxidative stress and age-related low-energy states. Mitochondrial impairment is one of the most noticeable signs of brain aging. Damaged mitochondria are thought to be one of the main causes that feed the inflammation related to aging. Also, protein turnover is involved in age-related impairments. The brain, due to its high oxygen usage, is particularly susceptible to oxidative damage. This review explores the mechanisms underlying neuronal cell rearrangement during aging, focusing on morphological changes that contribute to cognitive decline and increased susceptibility to neurodegenerative diseases. Potential therapeutic approaches are discussed, including the use of antioxidants (e.g., Vitamin C, Vitamin E, glutathione, carotenoids, quercetin, resveratrol, and curcumin) to mitigate oxidative damage, enhance mitochondrial function, and maintain protein homeostasis. This comprehensive overview aims to provide insights into the cellular and molecular processes of neuronal aging and highlight promising therapeutic avenues to counteract age-related neuronal deterioration.

Molsidomine provides neuroprotection against vincristine-induced peripheral neurotoxicity through soluble guanylyl cyclase activation

Peripheral neurotoxicity is a dose-limiting adverse reaction of primary frontline chemotherapeutic agents, including vincristine. Neuropathy can be so disabling that patients drop out of potentially curative therapy, negatively impacting cancer prognosis. The hallmark of vincristine neurotoxicity is axonopathy, yet its underpinning mechanisms remain uncertain. We developed a comprehensive drug discovery platform to identify neuroprotective agents against vincristine-induced neurotoxicity. Among the hits identified, SIN-1-an active metabolite of molsidomine-prevents vincristine-induced axonopathy in both motor and sensory neurons without compromising vincristine anticancer efficacy. Mechanistically, we found that SIN-1's neuroprotective effect is mediated by activating soluble guanylyl cyclase. We modeled vincristine-induced peripheral neurotoxicity in rats to determine molsidomine therapeutic potential in vivo. Vincristine administration induced severe nerve damage and mechanical hypersensitivity that were attenuated by concomitant treatment with molsidomine. This study provides evidence of the neuroprotective properties of molsidomine and warrants further investigations of this drug as a therapy for vincristine-induced peripheral neurotoxicity.

Parasympathetic neurons derived from human pluripotent stem cells model human diseases and development

Autonomic parasympathetic neurons (parasymNs) control unconscious body responses, including "rest-and-digest." ParasymN innervation is important for organ development, and parasymN dysfunction is a hallmark of autonomic neuropathy. However, parasymN function and dysfunction in humans are vastly understudied due to the lack of a model system. Human pluripotent stem cell (hPSC)-derived neurons can fill this void as a versatile platform. Here, we developed a differentiation paradigm detailing the derivation of functional human parasymNs from Schwann cell progenitors. We employ these neurons (1) to assess human autonomic nervous system (ANS) development, (2) to model neuropathy in the genetic disorder familial dysautonomia (FD), (3) to show parasymN dysfunction during SARS-CoV-2 infection, (4) to model the autoimmune disease Sjögren's syndrome (SS), and (5) to show that parasymNs innervate white adipocytes (WATs) during development and promote WAT maturation. Our model system could become instrumental for future disease modeling and drug discovery studies, as well as for human developmental studies.

MoS2 2D materials induce spinal cord neuroinflammation and neurotoxicity affecting locomotor performance in zebrafish

MoS2 nanosheets belong to an emerging family of nanomaterials named bidimensional transition metal dichalcogenides (2D TMDCs). The use of such promising materials, featuring outstanding chemical and physical properties, is expected to increase in several fields of science and technology, with an enhanced risk of environmental dispersion and associated wildlife and human exposures. In this framework, the assessment of MoS2 nanosheets toxicity is instrumental to safe industrial developments. Currently, the impact of the nanomaterial on the nervous tissue is unexplored. In this work, we use as in vivo experimental model the early-stage zebrafish, to investigate whether mechano-chemically exfoliated MoS2 nanosheets reach and affect, when added in the behavioral ambient, the nervous system. By high throughput screening of zebrafish larvae locomotor behavioral changes upon exposure to MoS2 nanosheets and whole organism live imaging of spinal neuronal and glial cell calcium activity, we report that sub-acute and prolonged ambient exposures to MoS2 nanosheets elicit locomotor abnormalities, dependent on dose and observation time. While 25 μg mL-1 concentration treatments exerted transient effects, 50 μg mL-1 ones induced long-lasting changes, correlated to neuroinflammation-driven alterations in the spinal cord, such as astrogliosis, glial intracellular calcium dysregulation, neuronal hyperactivity and motor axons retraction. By combining integrated technological approaches to zebrafish, we described that MoS2 2D nanomaterials can reach, upon water (i.e. ambient) exposure, the nervous system of larvae, resulting in a direct neurological damage.

Deregulated mitochondrial quality control, the heel of Achilles in elucidating the role of autophagy in SARM1-mediated axon degeneration

Autophagic dysfunction in neurodegenerative diseases is being extensively studied, yet the exact mechanism of macroautophagy/autophagy in axon degeneration is still elusive. A recent study by Kim et al. links autophagic stress to the sterile α and toll/interleukin 1 receptor motif containing protein 1 (SARM1)-dependent core axonal degeneration program, providing a new insight into the role of autophagy in axon degeneration. In the classical Wallerian axon degeneration model of axotomy, disruption of axonal transport destroys the coordinated activity of pro-survival and pro-degenerative factors in the axoplasm and activates the NADase activity of SARM1, thus triggering the axonal self-destruction program. However, the mechanism for SARM1 activation in the chronic neurodegenerative disorders is more complex. Mitochondrial defects and oxidative stress contribute to the activation of SARM1, while mitophagy can inhibit mitochondrial dysfunction and promote the clearance of SARM1 on mitochondria, thus protecting against neuronal degeneration. Therefore, in-depth elucidation of the underlying mechanisms of mitophagy during axonal degeneration can help develop promising strategies for the prevention and treatment of various neurodegenerative disorders.

Melamine Exacerbates Neurotoxicity in D-Galactose-Induced Neuronal SH-SY5Y Cells

Numerous studies have depicted the role of diet and environmental toxins in aging. Melamine (Mel) is a globally known notorious food adulterant, and its toxicity has been shown in several organs including the brain. However, till now, there are no reports regarding Mel neurotoxicity in aging neurons. So, this study examined the in vitro neurotoxicity caused by Mel in the D-galactose (DG)-induced aging model of neuronal SH-SY5Y cells. In the present study, the neuronal SH-SY5Y cells were treated with DG and Mel separately and in combination to assess the neurotoxicity potential using MTT assay and neurite length measurement. Further, the superoxide dismutase (SOD), catalase (CAT), and total antioxidant activities were evaluated followed by the determination of the intracellular reactive oxygen species (ROS), mitochondrial membrane potential (MMP), and caspase3 (Casp3) activity. The cotreatment of Mel and DG in neuronal SH-SY5Y cells showed maximum cell death than the cells treated with DG or Mel individually and untreated control cells. The neurite length shrinkage and ROS production were maximum in the DG and Mel cotreated cells showing exacerbated toxicity of Mel. The activity of SOD, CAT, and total antioxidants was also found to be lowered in the cotreatment group (Mel + DG) than in Mel- or DG-treated and untreated cells. Further, the combined toxicity of Mel and DG also elevated the Casp3 activity more than any other group. This is the first study showing the increased neurotoxic potential of Mel in an aging model of neuronal SH-SY5Y cells which implicates that Mel consumption by the elderly may lead to increased incidences of neurodegeneration like Alzheimer's disease and Parkinson's disease.

The Molecular Mechanisms Involved in Axonal Degeneration and Retrograde Retinal Ganglion Cell Death

Axonal degeneration is a pathologic change common to multiple retinopathies and optic neuropathies. Various pathologic factors, such as mechanical injury, inflammation, and ischemia, can damage retinal ganglion cell (RGC) somas and axons, eventually triggering axonal degeneration and RGC death. The molecular mechanisms of somal and axonal degeneration are distinct but also overlap, and axonal degeneration can result in retrograde somal degeneration. While the mitogen-activated protein kinase pathway acts as a central node in RGC axon degeneration, several newly discovered molecules, such as sterile alpha and Toll/interleukin-1 receptor motif-containing protein 1 and nicotinamide mononucleotide adenylyltransferase 2, also play a critical role in this pathological process following different types of injury. Therefore, we summarize the types of injury that cause RGC axon degeneration and retrograde RGC death and important underlying molecular mechanisms, providing a reference for the identification of targets for protecting axons and RGCs.

Chemokine-like Orion is involved in the transformation of glial cells into phagocytes in different developmental neuronal remodeling paradigms

During animal development, neurons often form exuberant or inappropriate axons and dendrites at early stages, followed by the refinement of neuronal circuits at late stages. Neural circuit refinement leads to the production of neuronal debris in the form of neuronal cell corpses, fragmented axons and dendrites, and pruned synapses requiring disposal. Glial cells act as predominant phagocytes during neuronal remodeling and degeneration, and crucial signaling pathways between neurons and glia are necessary for the execution of phagocytosis. Chemokine-like mushroom body neuron-secreted Orion is essential for astrocyte infiltration into the γ axon bundle leading to γ axon pruning. Here, we show a role of Orion in debris engulfment and phagocytosis in Drosophila. Interestingly, Orion is involved in the overall transformation of astrocytes into phagocytes. In addition, analysis of several neuronal paradigms demonstrates the role of Orion in eliminating both peptidergic vCrz+ and PDF-Tri neurons via additional phagocytic glial cells like cortex and/or ensheathing glia. Our results suggest that Orion is essential for phagocytic activation of astrocytes, cortex and ensheathing glia, and point to Orion as a trigger of glial infiltration, engulfment and phagocytosis.

Loss of the extracellular matrix protein Perlecan disrupts axonal and synaptic stability during Drosophila development

Heparan sulfate proteoglycans (HSPGs) form essential components of the extracellular matrix (ECM) and basement membrane (BM) and have both structural and signaling roles. Perlecan is a secreted ECM-localized HSPG that contributes to tissue integrity and cell-cell communication. Although a core component of the ECM, the role of Perlecan in neuronal structure and function is less understood. Here, we identify a role for Drosophila Perlecan in the maintenance of larval motoneuron axonal and synaptic stability. Loss of Perlecan causes alterations in the axonal cytoskeleton, followed by axonal breakage and synaptic retraction of neuromuscular junctions. These phenotypes are not prevented by blocking Wallerian degeneration and are independent of Perlecan's role in Wingless signaling. Expression of Perlecan solely in motoneurons cannot rescue synaptic retraction phenotypes. Similarly, removing Perlecan specifically from neurons, glia, or muscle does not cause synaptic retraction, indicating the protein is secreted from multiple cell types and functions non-cell autonomously. Within the peripheral nervous system, Perlecan predominantly localizes to the neural lamella, a specialized ECM surrounding nerve bundles. Indeed, the neural lamella is disrupted in the absence of Perlecan, with axons occasionally exiting their usual boundary in the nerve bundle. In addition, entire nerve bundles degenerate in a temporally coordinated manner across individual hemi-segments throughout larval development. These observations indicate disruption of neural lamella ECM function triggers axonal destabilization and synaptic retraction of motoneurons, revealing a role for Perlecan in axonal and synaptic integrity during nervous system development.

Assessment of Lumbosacral Nerve Roots in Patients with Type 2 Diabetic Peripheral Neuropathy Using Diffusion Tensor Imaging

BACKGROUND

Diffusion tensor imaging (DTI) has found clinical applications in the evaluation of the central nervous system and has been extensively used to image peripheral neuropathy. However, few studies have focused on lumbosacral nerve root fiber damage in diabetic peripheral neuropathy (DPN). The aim of the study was to evaluate whether DTI of the lumbosacral nerve roots can be used to detect DPN.

METHODS

Thirty-two type 2 diabetic patients with DPN and thirty healthy controls (HCs) were investigated with a 3T MRI scanner. DTI with tractography of the L4, L5, and S1 nerve roots was performed. Anatomical fusion with the axial T2 sequences was used to provide correlating anatomical information. Mean fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values were measured from tractography images and compared between groups. Diagnostic value was assessed using receiver operating characteristic (ROC) analysis. The Pearson correlation coefficient was used to explore the correlation between DTI parameters and clinical data and the nerve conduction study (NCS) in the DPN group.

RESULTS

In the DPN group, FA was decreased (p < 0.001) and ADC was increased (p < 0.001) compared with the values of the HC group. FA displayed the best diagnostic accuracy, with an area under the ROC curve of 0.716. ADC was positively correlated with HbA1c level (r = 0.379, p = 0.024) in the DPN group.

CONCLUSIONS

DTI of lumbosacral nerve roots demonstrates appreciable diagnostic accuracy in patients with DPN.

Teneurin trans-axonal signaling prunes topographically missorted axons

Building precise neural circuits necessitates the elimination of axonal projections that have inaccurately formed during development. Although axonal pruning is a selective process, how it is initiated and controlled in vivo remains unclear. Here, we show that trans-axonal signaling mediated by the cell surface molecules Glypican-3, Teneurin-3, and Latrophilin-3 prunes misrouted retinal axons in the visual system. Retinotopic neuron transplantations revealed that pioneer ventral axons that elongate first along the optic tract instruct the pruning of dorsal axons that missort in that region. Glypican-3 and Teneurin-3 are both selectively expressed by ventral retinal ganglion cells and cooperate for correcting missorted dorsal axons. The adhesion G-protein-coupled receptor Latrophilin-3 signals along dorsal axons to initiate the elimination of topographic sorting errors. Altogether, our findings show an essential function for Glypican-3, Teneurin-3, and Latrophilin-3 in topographic tract organization and demonstrate that axonal pruning can be initiated by signaling among axons themselves.

Activity disruption causes degeneration of entorhinal neurons in a mouse model of Alzheimer's circuit dysfunction

Neurodegenerative diseases are characterized by selective vulnerability of distinct cell populations; however, the cause for this specificity remains elusive. Here, we show that entorhinal cortex layer 2 (EC2) neurons are unusually vulnerable to prolonged neuronal inactivity compared with neighboring regions of the temporal lobe, and that reelin + stellate cells connecting EC with the hippocampus are preferentially susceptible within the EC2 population. We demonstrate that neuronal death after silencing can be elicited through multiple independent means of activity inhibition, and that preventing synaptic release, either alone or in combination with electrical shunting, is sufficient to elicit silencing-induced degeneration. Finally, we discovered that degeneration following synaptic silencing is governed by competition between active and inactive cells, which is a circuit refinement process traditionally thought to end early in postnatal life. Our data suggests that the developmental window for wholesale circuit plasticity may extend into adulthood for specific brain regions. We speculate that this sustained potential for remodeling by entorhinal neurons may support lifelong memory but renders them vulnerable to prolonged activity changes in disease.

GPBAR1 preserves neurite and synapse of dopaminergic neurons via RAD21-OPCML signaling: Role in preventing Parkinson's disease in mouse model and human patients

Parkinson's disease (PD) exhibits systemic impacts on the metabolism, while metabolic alteration contributes to the risk and progression of PD. Bile acids (BA) metabolism disturbance has been linked to PD pathology. Membrane-bound G protein-coupled bile acid receptor 1 (GPBAR1) is expressed in the brain and thought to be neuroprotective; however, the role of GPBAR1 in PD remains unknown. The current study aimed to explore the effect of GPBAR1 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice with dopaminergic (DA) neuron-specific Gpbar1 knockdown or central GPBAR1 activation. The underlying mechanisms were investigated using mesencephalic primary neurons analyzed. Our study found that GPBAR1 was reduced in the substantia nigra of PD patients and MPTP-PD mice, and its expression was negatively correlated with the severity of PD-related features. Genetic downregulation of Gpbar1 in mouse mesencephalic DA neurons exacerbated MPTP-induced neurobehavioral and neuropathological deficits, whereas activation of central GPBAR1 with INT-777 (INT) relieved it. Moreover, in vivo and in vitro experiments showed the neurite- and synapse-protective effects of GPBAR1 activation in PD model. Mechanistically, by promoting the nuclear localization of cohesin subunit RAD21, GPBAR1 activation increased opioid-binding cell adhesion molecule (Opcml) expression, thereby inhibiting neurite and synapse degeneration of DA neurons in PD model. Collectively, our findings demonstrate that GPBAR1 is implicated in PD pathogenesis and activation of central GPBAR1 with INT antagonizes neurodegenerative pathology in PD model. This neuroprotection, at least in part, is attributed to the RAD21-OPCML signaling in neurons. Hence, GPBAR1 may serve as a promising candidate target for PD treatment.

APP Genetic Deficiency Alters Intracellular Ca2+ Homeostasis and Delays Axonal Degeneration in Dorsal Root Ganglion Sensory Neurons

The activation of self-destructive cellular programs helps sculpt the nervous system during development, but the molecular mechanisms used are not fully understood. Prior studies have investigated the role of the APP in the developmental degeneration of sensory neurons with contradictory results. In this work, we sought to elucidate the impact of APP deletion in the development of the sensory nervous system in vivo and in vitro. Our in vivo data show an increase in the number of sciatic nerve axons in adult male and female APP-null mice, consistent with the hypothesis that APP plays a pro-degenerative role in the development of peripheral axons. In vitro, we show that genetic deletion of APP delays axonal degeneration triggered by nerve growth factor deprivation, indicating that APP does play a pro-degenerative role. Interestingly, APP depletion does not affect caspase-3 levels but significantly attenuates the rise of axoplasmic Ca2+ that occurs during degeneration. We examined intracellular Ca2+ mechanisms that could be involved and found that APP-null DRG neurons had increased Ca2+ levels within the endoplasmic reticulum and enhanced store-operated Ca2+ entry. We also observed that DRG axons lacking APP have more mitochondria than their WT counterparts, but these display a lower mitochondrial membrane potential. Finally, we present evidence that APP deficiency causes an increase in mitochondrial Ca2+ buffering capacity. Our results support the hypothesis that APP plays a pro-degenerative role in the developmental degeneration of DRG sensory neurons, and unveil the importance of APP in the regulation of calcium signaling in sensory neurons.SIGNIFICANCE STATEMENT The nervous system goes through a phase of pruning and programmed neuronal cell death during development to reach maturity. In such context, the role played by the APP in the peripheral nervous system has been controversial, ranging from pro-survival to pro-degenerative. Here we present evidence in vivo and in vitro supporting the pro-degenerative role of APP, demonstrating the ability of APP to alter intracellular Ca2+ homeostasis and mitochondria, critical players of programmed cell death. This work provides a better understanding of the physiological function of APP and its implication in developmental neuronal death in the nervous system.

The CARM1 transcriptome and arginine methylproteome mediate skeletal muscle integrative biology

OBJECTIVE

Coactivator-associated arginine methyltransferase 1 (CARM1) catalyzes the methylation of arginine residues on target proteins to regulate critical processes in health and disease. A mechanistic understanding of the role(s) of CARM1 in skeletal muscle biology is only gradually emerging. The purpose of this study was to elucidate the function of CARM1 in regulating the maintenance and plasticity of skeletal muscle.

METHODS

We used transcriptomic, methylproteomic, molecular, functional, and integrative physiological approaches to determine the specific impact of CARM1 in muscle homeostasis.

RESULTS

Our data defines the occurrence of arginine methylation in skeletal muscle and demonstrates that this mark occurs on par with phosphorylation and ubiquitination. CARM1 skeletal muscle-specific knockout (mKO) mice displayed altered transcriptomic and arginine methylproteomic signatures with molecular and functional outcomes confirming remodeled skeletal muscle contractile and neuromuscular junction characteristics, which presaged decreased exercise tolerance. Moreover, CARM1 regulates AMPK-PGC-1α signalling during acute conditions of activity-induced muscle plasticity.

CONCLUSIONS

This study uncovers the broad impact of CARM1 in the maintenance and remodelling of skeletal muscle biology.

Pivotal roles for membrane phospholipids in axonal degeneration

Membrane phospholipids are critical components of several signaling pathways. Maintained in a variety of asymmetric distributions, their trafficking across the membrane can be induced by intra-, extra-, and intercellular events. A familiar example is the externalization of phosphatidylserine from the inner leaflet to the outer leaflet in apoptosis, inducing phagocytosis of the soma. Recently, it has been recognized that phospholipids in the axonal membrane may be a signal for axonal degeneration, regeneration, or other processes. This review focuses on key recent developments and areas for ongoing investigations. KEY FACTS: Phosphatidylserine externalization propagates along an axon after axonal injury and is delayed in the Wallerian degeneration slow (Wld^S) mutant. The ATP8A2 flippase mutant has spontaneous axonal degeneration. Microdomains of axonal degeneration in spheroid bodies have differential externalization of phosphatidylserine and phosphatidylethanolamine. Phospholipid trafficking could represent a mechanism for coordinated axonal degeneration and elimination, i.e. axoptosis, analogous to apoptosis of the cell body.

A Quantitative Model of Sporadic Axonal Degeneration in the Drosophila Visual System

In human neurodegenerative diseases, neurons undergo axonal degeneration months to years before they die. Here, we developed a system modeling early degenerative events in Drosophila adult photoreceptor cells. Thanks to the stereotypy of their axonal projections, this system delivers quantitative data on sporadic and progressive axonal degeneration of photoreceptor cells. Using this method, we show that exposure of adult female flies to a constant light stimulation for several days overcomes the intrinsic resilience of R7 photoreceptors and leads to progressive axonal degeneration. This was not associated with apoptosis. We furthermore provide evidence that loss of synaptic integrity between R7 and a postsynaptic partner preceded axonal degeneration, thus recapitulating features of human neurodegenerative diseases. Finally, our experiments uncovered a role of postsynaptic partners of R7 to initiate degeneration, suggesting that postsynaptic cells signal back to the photoreceptor to maintain axonal structure. This model can be used to dissect cellular and circuit mechanisms involved in the early events of axonal degeneration, allowing for a better understanding of how neurons cope with stress and lose their resilience capacities.SIGNIFICANCE STATEMENT Neurons can be active and functional for several years. In the course of aging and in disease conditions leading to neurodegeneration, subsets of neurons lose their resilience and start dying. What initiates this turning point at the cellular level is not clear. Here, we developed a model allowing to systematically describe this phase. The loss of synapses and axons represents an early and functionally relevant event toward degeneration. Using the ordered distribution of Drosophila photoreceptor axon terminals, we assembled a system to study sporadic initiation of axon loss and delineated a role for non-cell-autonomous activity regulation in the initiation of axon degeneration. This work will help shed light on key steps in the etiology of nonfamilial cases of neurodegenerative diseases.

Microtubule Dynamics Following Central and Peripheral Nervous System Axotomy

Disturbance in the neuronal network leads to instability in the microtubule (MT) railroad of axons, causing hindrance in the intra-axonal transport and making it difficult to re-establish the broken network. Peripheral nervous system (PNS) neurons can stabilize their MTs, leading to the formation of regeneration-promoting structures called "growth cones". However, central nervous system (CNS) neurons lack this intrinsic reparative capability and, instead, form growth-incompetent structures called "retraction bulbs", which have a disarrayed MT network. It is evident from various studies that although axonal regeneration depends on both cell-extrinsic and cell-intrinsic factors, any therapy that aims at axonal regeneration ultimately converges onto MTs. Understanding the neuronal MT dynamics will help develop effective therapeutic strategies in diseases where the MT network gets disrupted, such as spinal cord injury, traumatic brain injury, multiple sclerosis, and amyotrophic lateral sclerosis. It is also essential to know the factors that aid or inhibit MT stabilization. In this review, we have discussed the MT dynamics postaxotomy in the CNS and PNS, and factors that can directly influence MT stability in various diseases.

NAD+-dependent mechanism of pathological axon degeneration

Pathological axon degeneration is broadly observed in neurodegenerative diseases. This unique process of axonal pathology could directly interfere with the normal functions of neurocircuitries and contribute to the onset of clinical symptoms in patients. It has been increasingly recognized that functional preservation of axonal structures is an indispensable part of therapeutic strategies for treating neurological disorders. In the past decades, the research field has witnessed significant breakthroughs in understanding the stereotyped self-destruction of axons upon neurodegenerative insults, which is distinct from all the known types of programmed cell death. In particular, the novel NAD+-dependent mechanism involving the WLDs, NMNAT2, and SARM1 proteins has emerged. This review summarizes the landmark discoveries elucidating the molecular pathway of pathological axon degeneration and highlights the evolving concept that neurodegeneration would be intrinsically linked to NAD+ and energy metabolism.

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.

Quantification of Neurite Degeneration with Enhanced Accuracy and Efficiency in an In Vitro Model of Parkinson's Disease

Neurite degeneration is associated with early stages of neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease (PD), and amyotrophic lateral sclerosis. One method that is commonly used to analyze neurite degeneration involves calculation of a Degeneration Index (DI) following utilization of the Analyze Particles tool of ImageJ to detect neurite fragments in micrographs of cultured cells. However, DI analyses are prone to several types of measurement error, can be time consuming to perform, and are limited in application. Here, we describe an improved method for performing DI analyses. Accuracy of measurements was enhanced through modification of selection criteria for detecting neurite fragments, removal of image artifacts and non-neurite materials from images, and optimization of image contrast. Such enhancements were implemented into an ImageJ macro that enables rapid and fully automated DI analysis of multiple images. The macro features operations for automated removal of cell bodies from micrographs, thus expanding the application of DI analyses to use in experiments involving dissociated cultures. We present experimental findings supporting that, compared with the conventional method, the enhanced analysis method yields measurements with increased accuracy and requires significantly less time to perform. Furthermore, we demonstrate the utility of the method to investigate neurite degeneration in a cell culture model of PD by conducting an experiment revealing the effects of c-Jun N-terminal kinase (JNK) on neurite degeneration induced by oxidative stress in human mesencephalic cells. This improved analysis method may be used to gain novel insight into factors underlying neurite degeneration and the progression of neurodegenerative disorders.

Motoneuron Diseases

Motoneuron diseases (MNDs) represent a heterogeneous group of progressive paralytic disorders, mainly characterized by the loss of upper (corticospinal) motoneurons, lower (spinal) motoneurons or, often both. MNDs can occur from birth to adulthood and have a highly variable clinical presentation, even within gene-positive forms, suggesting the existence of environmental and genetic modifiers. A combination of cell autonomous and non-cell autonomous mechanisms contributes to motoneuron degeneration in MNDs, suggesting multifactorial pathogenic processes.

The NAD+ precursor NMN activates dSarm to trigger axon degeneration in Drosophila

Axon degeneration contributes to the disruption of neuronal circuit function in diseased and injured nervous systems. Severed axons degenerate following the activation of an evolutionarily conserved signaling pathway, which culminates in the activation of SARM1 in mammals to execute the pathological depletion of the metabolite NAD⁺. SARM1 NADase activity is activated by the NAD⁺ precursor nicotinamide mononucleotide (NMN). In mammals, keeping NMN levels low potently preserves axons after injury. However, it remains unclear whether NMN is also a key mediator of axon degeneration and dSarm activation in flies. Here, we demonstrate that lowering NMN levels in Drosophila through the expression of a newly generated prokaryotic NMN-Deamidase (NMN-D) preserves severed axons for months and keeps them circuit-integrated for weeks. NMN-D alters the NAD⁺ metabolic flux by lowering NMN, while NAD⁺ remains unchanged in vivo. Increased NMN synthesis by the expression of mouse nicotinamide phosphoribosyltransferase (mNAMPT) leads to faster axon degeneration after injury. We also show that NMN-induced activation of dSarm mediates axon degeneration in vivo. Finally, NMN-D delays neurodegeneration caused by loss of the sole NMN-consuming and NAD⁺-synthesizing enzyme dNmnat. Our results reveal a critical role for NMN in neurodegeneration in the fly, which extends beyond axonal injury. The potent neuroprotection by reducing NMN levels is similar to the interference with other essential mediators of axon degeneration in Drosophila.

Assessment of peripheral neuropathy in type 2 diabetes by diffusion tensor imaging: A case-control study

OBJECTIVES

This study aimed to evaluate diabetes peripheral neuropathy (DPN) by diffusion tensor imaging (DTI) and explore the correlation between DTI parameters and electrophysiological parameters.

METHODS

We examined tibial nerve (TN) and common peroneal nerve (CPN) of 32 DPN patients and 23 healthy controls using T1-weighted magnetic resonance imaging and DTI. Fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD) of TN and CPN were measured and compared between groups. Spearman correlation coefficient was used to explore the relationship between DTI parameters and electrophysiology parameters in the DPN group. Diagnostic value was assessed by receiver operating characteristic (ROC) analysis.

RESULTS

In the DPN group, FA was decreased (p < 0.0001) and MD and RD were increased (p < 0.05, p < 0.001) in the TN and CPN compared with the values of healthy control group. Moreover, in the DPN group, FA was positively correlated with motor nerve conduction velocity (MCV) (p < 0.0001), and both MD and RD were negatively correlated with MCV (p < 0.05, p < 0.001). However, there was no correlation between AD and any electrophysiological parameters. Among all DTI parameters, FA displayed the best diagnostic accuracy, with an area under the ROC curve of 0.882 in TN and 0.917 in CPN.

CONCLUSION

FA and RD demonstrate appreciable diagnostic accuracy. Furthermore, they both have a moderate correlation with MCV.

Axonal spheroids in neurodegeneration

Axonal spheroids are bubble-like biological features that form on most degenerating axons, yet little is known about their influence on degenerative processes. Their formation and growth has been observed in response to various degenerative triggers such as injury, oxidative stress, inflammatory factors, and neurotoxic molecules. They often contain cytoskeletal elements and organelles, and, depending on the pathological insult, can colocalize with disease-related proteins such as amyloid precursor protein (APP), ubiquitin, and motor proteins. Initial formation of axonal spheroids depends on the disruption of axonal and membrane tension governed by cytoskeleton structure and calcium levels. Shortly after spheroid formation, the engulfment signal phosphatidylserine (PS) is exposed on the outer leaflet of spheroid plasma membrane, suggesting an important role for axonal spheroids in phagocytosis and debris clearance during degeneration. Spheroids can grow until they rupture, allowing pro-degenerative factors to exit the axon into extracellular space and accelerating neurodegeneration. Though much remains to be discovered in this area, axonal spheroid research promises to lend insight into the etiologies of neurodegenerative disease, and may be an important target for therapeutic intervention. This review summarizes over 100 years of work, describing what is known about axonal spheroid structure, regulation and function.

Deep Learning to Decipher the Progression and Morphology of Axonal Degeneration

Axonal degeneration (AxD) is a pathological hallmark of many neurodegenerative diseases. Deciphering the morphological patterns of AxD will help to understand the underlying mechanisms and develop effective therapies. Here, we evaluated the progression of AxD in cortical neurons using a novel microfluidic device together with a deep learning tool that we developed for the enhanced-throughput analysis of AxD on microscopic images. The trained convolutional neural network (CNN) sensitively and specifically segmented the features of AxD including axons, axonal swellings, and axonal fragments. Its performance exceeded that of the human evaluators. In an in vitro model of AxD in hemorrhagic stroke induced by the hemolysis product hemin, we detected a time-dependent degeneration of axons leading to a decrease in axon area, while axonal swelling and fragment areas increased. Axonal swellings preceded axon fragmentation, suggesting that swellings may be reliable predictors of AxD. Using a recurrent neural network (RNN), we identified four morphological patterns of AxD (granular, retraction, swelling, and transport degeneration). These findings indicate a morphological heterogeneity of AxD in hemorrhagic stroke. Our EntireAxon platform enables the systematic analysis of axons and AxD in time-lapse microscopy and unravels a so-far unknown intricacy in which AxD can occur in a disease context.

SARM1-mediated wallerian degeneration: A possible mechanism underlying organophosphorus-induced delayed neuropathy

Some organophosphorus compounds (OPs) can cause a type of delayed neurotoxicity in human being, which is known as organophosphorus-induced delayed neuropathy (OPIDN). Signs and symptoms of the patients include tingling and sensory loss of the hands and feet, followed by progressive muscle weakness in the lower and upper limbs, and ataxia. Pathologically, OPIDN are characterized by distal sensorimotor axonopathy due to the distal axonal degeneration of nerve tracts located in central and peripheral nervous systems. The morphological pattern of the distal axonopathy is similar to Wallerian degeneration that occurs after nerve injury in vitro. It is generally acknowledged that inhibition and subsequent aging of neuropathy target esterase (NTE) is required for the occurrence of OPIDN. However, the underlying mechanisms through which NTE triggers axonal degeneration in OPIDN is still largely unclear. Recently, sterile alpha and toll/interleukin receptor motif-containing protein 1（SARM1) has been identified as a key player in Wallerian degeneration. In physical and chemical transection of axons, SARM1 was found to promotes axon degeneration by hydrolyzing NAD+. By contrast, SARM1 deficiency could prevent neuron degeneration in response to a wide range of insults. Furthermore, SARM1 can also translocate to mitochondria and cause mitochondrial damage, thus triggering axon degeneration and neuron death. These findings suggested the existence of a pathway in axonal degeneration that might be targeted therapeutically. Here, we hypothesize that SARM1 activation after NTE inhibition and aging might be an etiological factor in OPIDN that regulates Wallerian-like degeneration. Analysing SARM1 mediated NAD degeneration pathway and its upstream activators in OPIDN could contribute to the development of novel therapies to treat OPIDN.

Deuterium double quantum-filtered NMR studies of peripheral and optic nerves

OBJECTIVE

Characterization of the nerve components by deuterium double quantum-filtered magnetization transfer (DQF-MT) NMR.

METHODS

Nerves were equilibrated in deuterated saline and ²H single-pulse and ²H DQF-MT NMR spectra were measured, enabling the separation of the different water compartments, according to their quadrupolar splittings.

RESULTS

Rat sciatic and brachial nerves and porcine optic nerve immersed in deuterated saline yielded ²H DQF spectra composed of three pairs of quadrupolar-split signals assigned to the water in the collagenous compartments and the myelin bilayer and one narrow signal assigned to the axonal water. Stretching of the nerves, application of osmotic stress and incubation in collagenase did not affect the quadrupolar splitting of the myelin water. The signals of myelin and axonal water were shown to decay during Wallerian degeneration and to rise during maturation. The chemical exchange between the myelin and the intra-axonal water was measured for optic nerve during maturation. The quadrupolar splitting of the signal of myelin water was not sensitive to its orientation relative to the magnetic field. This resembles liquid crystalline behavior, but leaves its mechanism open for interpretation.

CONCLUSIONS

²H DQF-MT NMR characterizes the different components of nerves, the water exchange between them and their changes during processes such as nerve maturation and Wallerian degeneration.

Silencing of circIgf1r plays a protective role in neuronal injury via regulating astrocyte polarization during epilepsy

Epilepsy is a common brain disorder, repeated seizures of epilepsy may lead to a series of brain pathological changes such as neuronal or glial damage. However, whether circular RNAs are involved in neuronal injury during epilepsy is not fully understood. Here, we screened circIgf1r in the status epilepticus model through circRNA sequencing, and found that it was upregulated after the status epilepticus model through QPCR analysis. Astrocytes polarizing toward neurotoxic A1 phenotype and neurons loss were observed after status epilepticus. Through injecting circIgf1r siRNA into the lateral ventricle, it was found that knocking down circIgf1r in vivo would induce the polarization of astrocytes to phenotype A2 and reduce neuronal loss. The results in vitro further confirmed that inhibiting the expression of circIgf1r in astrocytes could protect neurons by converting reactive astrocytes from A1 to the protective A2. In addition, knocking down circIgf1r in astrocytes could functionally promote astrocyte autophagy and relieve the destruction of 4-AP-induced autophagy flux. In terms of mechanism, circIgf1r promoted the polarization of astrocytes to phenotype A1 by inhibiting autophagy. Taken together, our results reveal circIgf1r may serve as a potential target for the prevention and treatment of neuron damage after epilepsy.

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.

Presynaptic protein synthesis and brain plasticity: From physiology to neuropathology

To form and maintain extremely intricate and functional neural circuitry, mammalian neurons are typically endowed with highly arborized dendrites and a long axon. The synapses that link neurons to neurons or to other cells are numerous and often too remote for the cell body to make and deliver new proteins to the right place in time. Moreover, synapses undergo continuous activity-dependent changes in their number and strength, establishing the basis of neural plasticity. The innate dilemma is then how a highly complex neuron provides new proteins for its cytoplasmic periphery and individual synapses to support synaptic plasticity. Here, we review a growing body of evidence that local protein synthesis in discrete sites of the axon and presynaptic terminals plays crucial roles in synaptic plasticity, and that deregulation of this local translation system is implicated in various pathologies of the nervous system.

Axonal chemokine-like Orion induces astrocyte infiltration and engulfment during mushroom body neuronal remodeling

The remodeling of neurons is a conserved fundamental mechanism underlying nervous system maturation and function. Astrocytes can clear neuronal debris and they have an active role in neuronal remodeling. Developmental axon pruning of Drosophila memory center neurons occurs via a degenerative process mediated by infiltrating astrocytes. However, how astrocytes are recruited to the axons during brain development is unclear. Using an unbiased screen, we identify the gene requirement of orion, encoding for a chemokine-like protein, in the developing mushroom bodies. Functional analysis shows that Orion is necessary for both axonal pruning and removal of axonal debris. Orion performs its functions extracellularly and bears some features common to chemokines, a family of chemoattractant cytokines. We propose that Orion is a neuronal signal that elicits astrocyte infiltration and astrocyte-driven axonal engulfment required during neuronal remodeling in the Drosophila developing brain.

Astrocytes can engulf axonal debris in the developing brain. However, the mechanisms regulating astrocyte recruitment to the proper axons is unclear. Here, the authors identify Orion as a signal for astrocyte infiltration and engulfment to the mushroom bodies in the Drosophila developing brain.

The influence of BACE1 on macrophage recruitment and activity in the injured peripheral nerve

Following peripheral nerve injury, multiple cell types, including axons, Schwann cells, and macrophages, coordinate to promote nerve regeneration. However, this capacity for repair is limited, particularly in older populations, and current treatments are insufficient. A critical component of the regeneration response is the network of cell-to-cell signaling in the injured nerve microenvironment. Sheddases are expressed in the peripheral nerve and play a role in the regulation if this cell-to-cell signaling through cleavage of transmembrane proteins, enabling the regulation of multiple pathways through cis- and trans-cellular regulatory mechanisms. Enhanced axonal regeneration has been observed in mice with deletion of the sheddase beta-secretase (BACE1), a transmembrane aspartyl protease that has been studied in the context of Alzheimer’s disease. BACE1 knockout (KO) mice display enhanced macrophage recruitment and activity following nerve injury, although it is unclear whether this plays a role in driving the enhanced axonal regeneration. Further, it is unknown by what mechanism(s) BACE1 increases macrophage recruitment and activity. BACE1 has many substrates, several of which are known to have immunomodulatory activity. This review will discuss current knowledge of the role of BACE1 and other sheddases in peripheral nerve regeneration and outline known immunomodulatory BACE1 substrates and what potential roles they could play in peripheral nerve regeneration. Currently, the literature suggests that BACE1 and substrates that are expressed by neurons and Schwann cells are likely to be more important for this process than those expressed by macrophages. More broadly, BACE1 may play a role as an effector of immunomodulation beyond the peripheral nerve.

AAV-mediated inhibition of ULK1 promotes axonal regeneration in the central nervous system in vitro and in vivo

Axonal damage is an early step in traumatic and neurodegenerative disorders of the central nervous system (CNS). Damaged axons are not able to regenerate sufficiently in the adult mammalian CNS, leading to permanent neurological deficits. Recently, we showed that inhibition of the autophagic protein ULK1 promotes neuroprotection in different models of neurodegeneration. Moreover, we demonstrated previously that axonal protection improves regeneration of lesioned axons. However, whether axonal protection mediated by ULK1 inhibition could also improve axonal regeneration is unknown. Here, we used an adeno-associated viral (AAV) vector to express a dominant-negative form of ULK1 (AAV.ULK1.DN) and investigated its effects on axonal regeneration in the CNS. We show that AAV.ULK1.DN fosters axonal regeneration and enhances neurite outgrowth in vitro. In addition, AAV.ULK1.DN increases neuronal survival and enhances axonal regeneration after optic nerve lesion, and promotes long-term axonal protection after spinal cord injury (SCI) in vivo. Interestingly, AAV.ULK1.DN also increases serotonergic and dopaminergic axon sprouting after SCI. Mechanistically, AAV.ULK1.DN leads to increased ERK1 activation and reduced expression of RhoA and ROCK2. Our findings outline ULK1 as a key regulator of axonal degeneration and regeneration, and define ULK1 as a promising target to promote neuroprotection and regeneration in the CNS.

p53 is required for nuclear but not mitochondrial DNA damage-induced degeneration

While the consequences of nuclear DNA damage have been well studied, the exact consequences of acute and selective mitochondrial DNA (mtDNA) damage are less understood. DNA damaging chemotherapeutic drugs are known to activate p53-dependent apoptosis in response to sustained nuclear DNA damage. While it is recognized that whole-cell exposure to these drugs also damages mtDNA, the specific contribution of mtDNA damage to cellular degeneration is less clear. To examine this, we induced selective mtDNA damage in neuronal axons using microfluidic chambers that allow for the spatial and fluidic isolation of neuronal cell bodies (containing nucleus and mitochondria) from the axons (containing mitochondria). Exposure of the DNA damaging drug cisplatin selectively to only the axons induced mtDNA damage in axonal mitochondria, without nuclear damage. We found that this resulted in the selective degeneration of only the targeted axons that were exposed to DNA damage, where ROS was induced but mitochondria were not permeabilized. mtDNA damage-induced axon degeneration was not mediated by any of the three known axon degeneration pathways: apoptosis, axon pruning, and Wallerian degeneration, as Bax-deficiency, or Casp3-deficiency, or Sarm1-deficiency failed to protect the degenerating axons. Strikingly, p53, which is essential for degeneration after nuclear DNA damage, was also not required for degeneration induced with mtDNA damage. This was most evident when the p53-deficient neurons were globally exposed to cisplatin. While the cell bodies of p53-deficient neurons were protected from degeneration in this context, the axons farthest from the cell bodies still underwent degeneration. These results highlight how whole cell exposure to DNA damage activates two pathways of degeneration; a faster, p53-dependent apoptotic degeneration that is triggered in the cell bodies with nuclear DNA damage, and a slower, p53-independent degeneration that is induced with mtDNA damage.

Axon Degeneration and Inflammation in Neuropathy: The Role of Acupuncture

Chemotherapy-induced peripheral neuropathy (CIPN) induces debilitating long-term side-effects in breast cancer-survivors. This article describes some of the more-recent research in this area including a randomized controlled pilot trial the current author and colleagues performed to assess feasibility, safety, and effectiveness of acupuncture for treating CIPN in this population. In this randomized pilot trial of 40 breast-cancer survivors with CIPN after adjuvant taxane therapy, an 8-week acupuncture intervention (versus usual care) led to a statistically and clinically significant reduction in subjective sensory symptoms, including neuropathic pain and paresthesia. Given the lack of effective therapies and given the established safety profile of acupuncture, clinicians may consider acupuncture as a treatment option for mild-to-moderate CIPN in practice. Additional larger studies are needed to confirm these findings.

Neuroprotective Effects of Curcumin-Loaded Emulsomes in a Laser Axotomy-Induced CNS Injury Model

PURPOSE

Curcumin, a polyphenol isolated from the rhizomes of turmeric, holds great potential as a neuroprotective agent in addition to its anti-inflammatory and antioxidant characteristics. The poor bioavailability and low stability of curcumin are the greatest barriers to its clinical use. This study aims to investigate the neuroprotective effect of curcumin on axonal injury, by delivering the lipophilic polyphenol to a primary hippocampal neuron culture by means of a lipid-based drug delivery system, named emulsomes.

METHODS

To study neuroregeneration ex vivo, an injury model was established through single-cell laser axotomy on hippocampal neurites. Upon treatment with curcumin-loaded emulsomes (CurcuEmulsomes), curcumin and CurcuEmulsome uptake into neurons was verified by three-dimensional Z-stack images acquired with confocal microscopy. Neuron survival after axonal injury was tracked by propidium iodide (PI) and Hoechst staining. Alterations in expression levels of physiological markers, such as anti-apoptotic marker Bcl2, apoptotic marker cleaved caspase 3, neuroprotective marker Wnt3a and the neuronal survival marker mTOR, were investigated by immunocytochemistry analyses.

RESULTS

The results indicated significant improvement in the survival rate of injured neurons upon CurcuEmulsome treatment. Bcl2 expression was significantly higher for injured neurons treated with curcumin or CurcuEmulsome. Reduction in caspase 3 expression was seen in both curcumin and CurcuEmulsome treatment, whereas there were no significant changes in Wnt3a and mTOR expression.

CONCLUSION

The established laser-axotomy model was proven as a reliable methodology to study neurodegenerative models ex vivo. CurcuEmulsomes delivered curcumin to primary hippocampal neurons successfully. Treated with CurcuEmulsomes, injured hippocampal neurons benefit from the neuroprotective effects of curcumin, exhibiting a higher survival rate and increased anti-apoptotic marker levels.

Protective Effects of (E)-β-Caryophyllene (BCP) in Chronic Inflammation

(E)-β-caryophyllene (BCP) is a bicyclic sesquiterpene widely distributed in the plant kingdom, where it contributes a unique aroma to essential oils and has a pivotal role in the survival and evolution of higher plants. Recent studies provided evidence for protective roles of BCP in animal cells, highlighting its possible use as a novel therapeutic tool. Experimental results show the ability of BCP to reduce pro-inflammatory mediators such as tumor necrosis factor-alfa (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), thus ameliorating chronic pathologies characterized by inflammation and oxidative stress, in particular metabolic and neurological diseases. Through the binding to CB2 cannabinoid receptors and the interaction with members of the family of peroxisome proliferator-activated receptors (PPARs), BCP shows beneficial effects on obesity, non-alcoholic fatty liver disease/nonalcoholic steatohepatitis (NAFLD/NASH) liver diseases, diabetes, cardiovascular diseases, pain and other nervous system disorders. This review describes the current knowledge on the biosynthesis and natural sources of BCP, and reviews its role and mechanisms of action in different inflammation-related metabolic and neurologic disorders.

Trans-Axonal Signaling in Neural Circuit Wiring

The development of neural circuits is a complex process that relies on the proper navigation of axons through their environment to their appropriate targets. While axon–environment and axon–target interactions have long been known as essential for circuit formation, communication between axons themselves has only more recently emerged as another crucial mechanism. Trans-axonal signaling governs many axonal behaviors, including fasciculation for proper guidance to targets, defasciculation for pathfinding at important choice points, repulsion along and within tracts for pre-target sorting and target selection, repulsion at the target for precise synaptic connectivity, and potentially selective degeneration for circuit refinement. This review outlines the recent advances in identifying the molecular mechanisms of trans-axonal signaling and discusses the role of axon–axon interactions during the different steps of neural circuit formation.

Beading of injured axons driven by tension- and adhesion-regulated membrane shape instability

The formation of multiple beads along an injured axon will lead to blockage of axonal transport and eventually neuron death, and this has been widely recognized as a hallmark of nervous system degeneration. Nevertheless, the underlying mechanisms remain poorly understood. Here, we report a combined experimental and theoretical study to reveal key factors governing axon beading. Specifically, by transecting well-developed axons with a sharp atomic force microscope probe, significant beading of the axons was triggered. We showed that adhesion was not required for beading to occur, although when present strong axon–substrate attachments seemed to set the locations for bead formation. In addition, the beading wavelength, representing the average distance between beads, was found to correlate with the size and cytoskeleton integrity of axon, with a thinner axon or a disrupted actin cytoskeleton both leading to a shorter beading wavelength. A model was also developed to explain these observations which suggest that axon beading originates from the shape instability of the membrane and is driven by the release of work done by axonal tension as well as the reduction of membrane surface energy. The beading wavelength predicted from this theory was in good agreement with our experiments under various conditions. By elucidating the essential physics behind axon beading, the current study could enhance our understanding of how axonal injury and neurodegeneration progress as well as provide insights for the development of possible treatment strategies.

Enhanced autophagic retrograde axonal transport by dynein intermediate chain upregulation improves Aβ clearance and cognitive function in APP/PS1 double transgenic mice

Autophagosome accumulation is observed in the distal axons of Alzheimer disease (AD) patients and AD animal models, suggesting that deficient retrograde transport and impaired autophagic clearance of beta-amyloid (A β) contribute to AD pathogenesis. Expression of the retrograde axonal transport-related protein dynein intermediate chain (DIC) is also reduced in AD patients, but the contributions of DIC to AD pathology remain elusive. This study investigated the effects of DIC expression levels on cognitive function, autophagosome axonal transport, and A β clearance in the APP/PS1 double transgenic mouse model of AD. Autophagic activity was enhanced in the hippocampus of young (3-month-old) AD mice, as evidenced by greater expression of autophagosome markers, lysosome markers, axonal transport motors (including DIC), and dynein regulatory proteins. The expression levels of autophagosome markers remained elevated, whereas those of autophagic and axonal transport proteins decreased progressively with age, accompanied by spatial learning and memory deficits, axonal autophagosome accumulation, and A β deposition. Knockdown of DIC exacerbated while overexpression improved axonal transport, autophagosome maturation, Aβ clearance, and spatial learning and memory in aged AD mice. Our study provides evidence that age-dependent failure of axonal autophagic flux contributes to AD-associated neuropathology and cognitive deficits, suggesting DIC as a potential therapeutic target for AD.

The human motor cortex microcircuit: insights for neurodegenerative disease

The human motor cortex comprises a microcircuit of five interconnected layers with different cell types. In this Review, we use a layer-specific and cell-specific approach to integrate physiological accounts of this motor cortex microcircuit with the pathophysiology of neurodegenerative diseases affecting motor functions. In doing so we can begin to link motor microcircuit pathology to specific disease stages and clinical phenotypes. Based on microcircuit physiology, we can make future predictions of axonal loss and microcircuit dysfunction. With recent advances in high-resolution neuroimaging we can then test these predictions in humans in vivo, providing mechanistic insights into neurodegenerative disease.

Loss of highwire Protects Against the Deleterious Effects of Traumatic Brain Injury in Drosophila Melanogaster

Traumatic brain injury is a major global cause of death and disability. Axonal injury is a major underlying mechanism of TBI and could represent a major therapeutic target. We provide evidence that targeting the axonal death pathway known as Wallerian degeneration improves outcome in a Drosophila Melanogaster model of high impact trauma. This cell-autonomous neurodegenerative pathway is initiated following axon injury, and in Drosophila, involves activity of the E3 ubiquitin ligase highwire. We demonstrate that a loss-of-function mutation in the highwire gene rescues deleterious effects of a traumatic injury, including-improved functional outcomes, lifespan, survival of dopaminergic neurons, and retention of synaptic proteins. This data suggests that highwire represents a potential therapeutic target in traumatic injury.