Facilitation of axon regeneration by enhancing mitochondrial transport and rescuing energy deficits

Novel multi-drug delivery hydrogel using scar-homing liposomes improves spinal cord injury repair

Proper selection and effective delivery of combination drugs targeting multiple pathophysiological pathways key to spinal cord injury (SCI) hold promise to address the thus far scarce clinical therapeutics for improving recovery after SCI. In this study, we aim to develop a clinically feasible way for targeted delivery of multiple drugs with different physiochemical properties to the SCI site, detail the underlying mechanism of neural recovery, and detect any synergistic effect related to combination therapy.

Methods: Liposomes (LIP) modified with a scar-targeted tetrapeptide (cysteine-alanine-glutamine-lysine, CAQK) were first constructed to simultaneously encapsulate docetaxel (DTX) and brain-derived neurotrophic factor (BDNF) and then were further added into a thermosensitive heparin-modified poloxamer hydrogel (HP) with affinity-bound acidic fibroblast growth factor (aFGF-HP) for local administration into the SCI site (CAQK-LIP-GFs/DTX@HP) in a rat model. In vivo fluorescence imaging was used to examine the specificity of CAQK-LIP-GFs/DTX binding to the injured site. Multiple comprehensive evaluations including biotin dextran amine anterograde tracing and magnetic resonance imaging were used to detect any synergistic effects and the underlying mechanisms of CAQK-LIP-GFs/DTX@HP both in vivo (rat SCI model) and in vitro (primary neuron).

Results: The multiple drugs were effectively delivered to the injured site. The combined application of GFs and DTX supported neuro-regeneration by improving neuronal survival and plasticity, rendering a more permissive extracellular matrix environment with improved regeneration potential. In addition, our combination therapy promoted axonal regeneration via moderation of microtubule function and mitochondrial transport along the regenerating axon.

Conclusion: This novel multifunctional therapeutic strategy with a scar-homing delivery system may offer promising translational prospects for the clinical treatment of SCI.

The Molecular Interplay between Axon Degeneration and Regeneration

Neurons face a series of morphological and molecular changes following trauma and in the progression of neurodegenerative disease. In neurons capable of mounting a spontaneous regenerative response, including invertebrate neurons and mammalian neurons of the peripheral nervous system (PNS), axons regenerate from the proximal side of the injury and degenerate on the distal side. Studies of Wallerian degeneration slow (Wld^S /Ola) mice have revealed that a level of coordination between the processes of axon regeneration and degeneration occurs during successful repair. Here, we explore how shared cellular and molecular pathways that regulate both axon regeneration and degeneration coordinate the two distinct outcomes in the proximal and distal axon segments. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

Aging-associated changes in hippocampal glycogen metabolism in mice. Evidence for and against astrocyte-to-neuron lactate shuttle

Lactate derived from astrocytic glycogen has been shown to support memory formation in hippocampi of young animals, inhibiting it in old animals. Here we show, using quantitative mass spectrometry-based proteomics, immunofluorescence, and qPCR that aging is associated with an increase of glycogen metabolism enzymes concentration and shift in their localization from astrocytes to neurons. These changes are accompanied with reorganization of hippocampal energy metabolism which is manifested by elevated capacity of aging neurons to oxidize glucose in glycolysis and mitochondria, and decreased ability for fatty acids utilization. Our observations suggest that astrocyte-to-neuron lactate shuttle may operate in young hippocampi, however, during aging neurons become independent on astrocytic lactate and the metabolic crosstalk between the brain's cells is disrupted.

Mitochondrial dynamics in adaptive and maladaptive cellular stress responses

Mitochondria sense and respond to many stressors and can support either cell survival or death through energy production and signaling pathways. Mitochondrial responses depend on fusion-fission dynamics that dilute and segregate damaged mitochondria. Mitochondrial motility and inter-organellar interactions, including with the endoplasmic reticulum, also function in cellular adaptation to stress. In this Review, we discuss how stressors influence these components, and how they contribute to the complex adaptive and pathological responses that lead to disease.

Muscarinic Acetylcholine Type 1 Receptor Activity Constrains Neurite Outgrowth by Inhibiting Microtubule Polymerization and Mitochondrial Trafficking in Adult Sensory Neurons

The muscarinic acetylcholine type 1 receptor (M₁R) is a metabotropic G protein-coupled receptor. Knockout of M₁R or exposure to selective or specific receptor antagonists elevates neurite outgrowth in adult sensory neurons and is therapeutic in diverse models of peripheral neuropathy. We tested the hypothesis that endogenous M₁R activation constrained neurite outgrowth via a negative impact on the cytoskeleton and subsequent mitochondrial trafficking. We overexpressed M₁R in primary cultures of adult rat sensory neurons and cell lines and studied the physiological and molecular consequences related to regulation of cytoskeletal/mitochondrial dynamics and neurite outgrowth. In adult primary neurons, overexpression of M₁R caused disruption of the tubulin, but not actin, cytoskeleton and significantly reduced neurite outgrowth. Over-expression of a M₁R-DREADD mutant comparatively increased neurite outgrowth suggesting that acetylcholine released from cultured neurons interacts with M₁R to suppress neurite outgrowth. M₁R-dependent constraint on neurite outgrowth was removed by selective (pirenzepine) or specific (muscarinic toxin 7) M₁R antagonists. M₁R-dependent disruption of the cytoskeleton also diminished mitochondrial abundance and trafficking in distal neurites, a disorder that was also rescued by pirenzepine or muscarinic toxin 7. M₁R activation modulated cytoskeletal dynamics through activation of the G protein (Gα13) that inhibited tubulin polymerization and thus reduced neurite outgrowth. Our study provides a novel mechanism of M₁R control of Gα13 protein-dependent modulation of the tubulin cytoskeleton, mitochondrial trafficking and neurite outgrowth in axons of adult sensory neurons. This novel pathway could be harnessed to treat dying-back neuropathies since anti-muscarinic drugs are currently utilized for other clinical conditions.

Neuronal Intrinsic Regenerative Capacity: The Impact of Microtubule Organization and Axonal Transport

In the adult vertebrate central nervous system, axons generally fail to regenerate. In contrast, peripheral nervous system axons are able to form a growth cone and regenerate upon lesion. Among the multiple intrinsic mechanisms leading to the formation of a new growth cone and to successful axon regrowth, cytoskeleton organization and dynamics is central. Here we discuss how multiple pathways that define the regenerative capacity converge into the regulation of the axonal microtubule cytoskeleton and transport. We further explore the use of dorsal root ganglion neurons as a model to study the neuronal regenerative ability. Finally, we address some of the unanswered questions in the field, including the mechanisms by which axonal transport might be modulated by injury, and the relationship between microtubule organization, dynamics, and axonal transport. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

Mitochondrial Aspects of Synaptic Dysfunction in Alzheimer's Disease

Alzheimer's disease (AD) is characterized by brain deposition of amyloid plaques and tau neurofibrillary tangles along with steady cognitive decline. Synaptic damage, an early pathological event, correlates strongly with cognitive deficits and memory loss. Mitochondria are essential organelles for synaptic function. Neurons utilize specialized mechanisms to drive mitochondrial trafficking to synapses in which mitochondria buffer Ca2+ and serve as local energy sources by supplying ATP to sustain neurotransmitter release. Mitochondrial abnormalities are one of the earliest and prominent features in AD patient brains. Amyloid-β (Aβ) and tau both trigger mitochondrial alterations. Accumulating evidence suggests that mitochondrial perturbation acts as a key factor that is involved in synaptic failure and degeneration in AD. The importance of mitochondria in supporting synaptic function has made them a promising target of new therapeutic strategies for AD. Here, we review the molecular mechanisms regulating mitochondrial function at synapses, highlight recent findings on the disturbance of mitochondrial dynamics and transport in AD, and discuss how these alterations impact synaptic vesicle release and thus contribute to synaptic pathology associated with AD.

Microfluidic platforms for the study of neuronal injury in vitro

Traumatic brain injury (TBI) affects 5.3 million people in the United States, and there are 12,500 new cases of spinal cord injury (SCI) every year. There is yet a significant need for in vitro models of TBI and SCI in order to understand the biological mechanisms underlying central nervous system (CNS) injury and to identify and test therapeutics to aid in recovery from neuronal injuries. While TBI or SCI studies have been aided with traditional in vivo and in vitro models, the innate limitations in specificity of injury, isolation of neuronal regions, and reproducibility of these models can decrease their usefulness in examining the neurobiology of injury. Microfluidic devices provide several advantages over traditional methods by allowing researchers to (1) examine the effect of injury on specific neural components, (2) fluidically isolate neuronal regions to examine specific effects on subcellular components, and (3) reproducibly create a variety of injuries to model TBI and SCI. These microfluidic devices are adaptable for modeling a wide range of injuries, and in this review, we will examine different methodologies and models recently utilized to examine neuronal injury. Specifically, we will examine vacuum-assisted axotomy, physical injury, chemical injury, and laser-based axotomy. Finally, we will discuss the benefits and downsides to each type of injury model and discuss how researchers can use these parameters to pick a particular microfluidic device to model CNS injury.

Epothilone B Speeds Corneal Nerve Regrowth and Functional Recovery through Microtubule Stabilization and Increased Nerve Beading

The successful restoration of corneal innervation and function after a corneal injury is a clinically challenging issue. Structural and functional recovery after a nerve injury involves a complex series of steps in which microtubules play a key role. The aim of the current study was to investigate the effects of epothilone B (EpoB), a microtubule-stabilizing agent, on corneal innervation and the functional recovery of the corneal nerve in mice after corneal epithelial abrasion. The pretreatment of mice with EpoB has a remarkable effect on the stabilization of beta-III tubulin, as demonstrated by substantial increases in the visualization of beta-III tubulin, nerve beading, corneal reinnervation, and reaction to stimuli. Furthermore, a pharmacokinetic analysis showed that EpoB remains at a high concentration in the cornea and the trigeminal ganglion for at least 6 days after administration. In addition, the administration of EpoB at 24 hours after corneal abrasion has a marked therapeutic effect on nerve regrowth and functional recovery. In conclusion, EpoB treatment may have therapeutic utility for improving corneal reinnervation and restoring sensitivity following corneal injury.

Identification of 22q13 genes most likely to contribute to Phelan McDermid syndrome

Chromosome 22q13.3 deletion (Phelan McDermid) syndrome (PMS) is a rare genetic neurodevelopmental disorder resulting from deletions or other genetic variants on distal 22q. Pathological variants of the SHANK3 gene have been identified, but terminal chromosomal deletions including SHANK3 are most common. Terminal deletions disrupt up to 108 protein-coding genes. The impact of these losses is highly variable and includes both significantly impairing neurodevelopmental and somatic manifestations. The current review combines two metrics, prevalence of gene loss and predicted loss pathogenicity, to identify likely contributors to phenotypic expression. These genes are grouped according to function as follows: molecular signaling at glutamate synapses, phenotypes involving neuropsychiatric disorders, involvement in multicellular organization, cerebellar development and functioning, and mitochondrial. The likely most impactful genes are reviewed to provide information for future clinical and translational investigations.

Reinforcing mitochondrial functions in aging brain: An insight into Parkinson's disease therapeutics

Mitochondria, the powerhouse of the neural cells in the brain, are also the seat of certain essential gene signaling pathways that control neuronal functions. Deterioration of mitochondrial functions has been widely reported in normal aging as well as in a spectrum of age-associated neurological diseases, including Parkinson's disease (PD). Evidences accumulated in the recent past provide not only advanced information on the causes of mitochondrial bioenergetics defects and redox imbalance in PD brains, but also much insight into mitochondrial biogenesis, quality control of mitochondrial proteins, and genes, which regulate intra- and extra-mitochondrial signaling that control the general health of neural cells. The mitochondrial quality control machinery is affected in aging and especially in PD, thus affecting intraneuronal protein transport and degradation, which are primarily responsible for accumulation of misfolded proteins and mitochondrial damage in sporadic as well as familial PD. Essentially we considered in the first half of this review, mitochondria-based targets such as mitochondrial oxidative stress and mitochondrial quality control pathways in PD, relevance of mitochondrial DNA mutations, mitophagy, mitochondrial proteases, mitochondrial flux, and finally mitochondria-based therapies possible for PD. Therapeutic aspects are considered in the later half and mitochondria-targeted antioxidant therapy, mitophagy enhancers, mitochondrial biogenesis boasters, mitochondrial dynamics modulators, and gene-based therapeutic approaches are discussed. The present review is a critical assessment of this information to distinguish some exemplary mitochondrial therapeutic targets, and provides a utilitarian perception of some avenues for therapeutic designs on identified mitochondrial targets for PD, a very incapacitating disorder of the geriatric population, world over.

MicroRNA-133b Negatively Regulates Zebrafish Single Mauthner-Cell Axon Regeneration through Targeting tppp3 in Vivo

Axon regeneration, fundamental to nerve repair, and functional recovery, relies on rapid changes in gene expression attributable to microRNA (miRNA) regulation. MiR-133b has been proved to play an important role in different organ regeneration in zebrafish, but its role in regulating axon regeneration in vivo is still controversial. Here, combining single-cell electroporation with a vector-based miRNA-expression system, we have modulated the expression of miR-133b in Mauthner-cells (M-cells) at the single-cell level in zebrafish. Through in vivo imaging, we show that overexpression of miR-133b inhibits axon regeneration, whereas down-regulation of miR-133b, promotes axon outgrowth. We further show that miR-133b regulates axon regeneration by directly targeting a novel regeneration-associated gene, tppp3, which belongs to Tubulin polymerization-promoting protein family. Gain or loss-of-function of tppp3 experiments indicated that tppp3 was a novel gene that could promote axon regeneration. In addition, we observed a reduction of mitochondrial motility, which have been identified to have a positive correlation with axon regeneration, in miR-133b overexpressed M-cells. Taken together, our work provides a novel way to study the role of miRNAs in individual cell and establishes a critical cell autonomous role of miR-133b in zebrafish M-cell axon regeneration. We propose that up-regulation of the newly founded regeneration-associated gene tppp3 may enhance axonal regeneration.

The axonal endoplasmic reticulum: One organelle-many functions in development, maintenance, and plasticity

The endoplasmic reticulum (ER) is highly conserved in eukaryotes and neurons. Indeed, the localization of the organelle in axons has been known for nearly half a century. However, the relevance of the axonal ER is only beginning to emerge. In this review, we discuss the structure of the ER in axons, examining the role of ER-shaping proteins and highlighting reticulons. We analyze the multiple functions of the ER and their potential contribution to axonal physiology. First, we examine the emerging roles of the axonal ER in lipid synthesis, protein translation, processing, quality control, and secretory trafficking of transmembrane proteins. We also review the impact of the ER on calcium dynamics, focusing on intracellular mechanisms and functions. We describe the interactions between the ER and endosomes, mitochondria, and synaptic vesicles. Finally, we analyze available proteomic data of axonal preparations to reveal the dynamic functionality of the ER in axons during development. We suggest that the dynamic proteome and a validated axonal interactome, together with state-of-the-art methodologies, may provide interesting research avenues in axon physiology that may extend to pathology and regeneration. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 181-208, 2018.

Mitostasis in Neurons: Maintaining Mitochondria in an Extended Cellular Architecture

Neurons have more extended and complex shapes than other cells and consequently face a greater challenge in distributing and maintaining mitochondria throughout their arbors. Neurons can last a lifetime, but proteins turn over rapidly. Mitochondria, therefore, need constant rejuvenation no matter how far they are from the soma. Axonal transport of mitochondria and mitochondrial fission and fusion contribute to this rejuvenation, but local protein synthesis is also likely. Maintenance of a healthy mitochondrial population also requires the clearance of damaged proteins and organelles. This involves degradation of individual proteins, sequestration in mitochondria-derived vesicles, organelle degradation by mitophagy and macroautophagy, and in some cases transfer to glial cells. Both long-range transport and local processing are thus at work in achieving neuronal mitostasis-the maintenance of an appropriately distributed pool of healthy mitochondria for the duration of a neuron's life. Accordingly, defects in the processes that support mitostasis are significant contributors to neurodegenerative disorders.

Pharmacological augmentation of nicotinamide phosphoribosyltransferase (NAMPT) protects against paclitaxel-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) arises from collateral damage to peripheral afferent sensory neurons by anticancer pharmacotherapy, leading to debilitating neuropathic pain. No effective treatment for CIPN exists, short of dose-reduction which worsens cancer prognosis. Here, we report that stimulation of nicotinamide phosphoribosyltransferase (NAMPT) produced robust neuroprotection in an aggressive CIPN model utilizing the frontline anticancer drug, paclitaxel (PTX). Daily treatment of rats with the first-in-class NAMPT stimulator, P7C3-A20, prevented behavioral and histologic indicators of peripheral neuropathy, stimulated tissue NAD recovery, improved general health, and abolished attrition produced by a near maximum-tolerated dose of PTX. Inhibition of NAMPT blocked P7C3-A20-mediated neuroprotection, whereas supplementation with the NAMPT substrate, nicotinamide, potentiated a subthreshold dose of P7C3-A20 to full efficacy. Importantly, P7C3-A20 blocked PTX-induced allodynia in tumored mice without reducing antitumoral efficacy. These findings identify enhancement of NAMPT activity as a promising new therapeutic strategy to protect against anticancer drug-induced peripheral neurotoxicity.

Mitochondria on the Road to Power Axonal Regeneration

In this issue of Neuron, Han et al. (2016) and Cartoni et al. (2016) define a critical role of mitochondrial transport for successful axon regeneration after injury and provide new insights into intrinsic mechanisms controlling neuronal regeneration capacity in worms and mice.

The role of mitochondria in axon development and regeneration

Mitochondria are dynamic organelles that undergo transport, fission, and fusion. The three main functions of mitochondria are to generate ATP, buffer cytosolic calcium, and generate reactive oxygen species. A large body of evidence indicates that mitochondria are either primary targets for neurological disease states and nervous system injury, or are major contributors to the ensuing pathologies. However, the roles of mitochondria in the development and regeneration of axons have just begun to be elucidated. Advances in the understanding of the functional roles of mitochondria in neurons had been largely impeded by insufficient knowledge regarding the molecular mechanisms that regulate mitochondrial transport, stalling, fission/fusion, and a paucity of approaches to image and analyze mitochondria in living axons at the level of the single mitochondrion. However, technical advances in the imaging and analysis of mitochondria in living neurons and significant insights into the mechanisms that regulate mitochondrial dynamics have allowed the field to advance. Mitochondria have now been attributed important roles in the mechanism of axon extension, regeneration, and axon branching. The availability of new experimental tools is expected to rapidly increase our understanding of the functions of axonal mitochondria during both development and later regenerative attempts. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 221-237, 2018.

Can injured adult CNS axons regenerate by recapitulating development?

In the adult mammalian central nervous system (CNS), neurons typically fail to regenerate their axons after injury. During development, by contrast, neurons extend axons effectively. A variety of intracellular mechanisms mediate this difference, including changes in gene expression, the ability to form a growth cone, differences in mitochondrial function/axonal transport and the efficacy of synaptic transmission. In turn, these intracellular processes are linked to extracellular differences between the developing and adult CNS. During development, the extracellular environment directs axon growth and circuit formation. In adulthood, by contrast, extracellular factors, such as myelin and the extracellular matrix, restrict axon growth. Here, we discuss whether the reactivation of developmental processes can elicit axon regeneration in the injured CNS.

Human mesenchymal stromal cells transplanted into mice stimulate renal tubular cells and enhance mitochondrial function

Mesenchymal stromal cells (MSCs) are renoprotective and drive regeneration following injury, although cellular targets of such an effect are still ill-defined. Here, we show that human umbilical cord (UC)-MSCs transplanted into mice stimulate tubular cells to regain mitochondrial mass and function, associated with enhanced microtubule-rich projections that appear to mediate mitochondrial trafficking to create a reparative dialogue among adjacent tubular cells. Treatment with UC-MSCs in mice with cisplatin-induced acute kidney injury (AKI) regulates mitochondrial biogenesis in proximal tubuli by enhancing PGC1α expression, NAD⁺ biosynthesis and Sirtuin 3 (SIRT3) activity, thus fostering antioxidant defenses and ATP production. The functional role of SIRT3 in tubular recovery is highlighted by data that in SIRT3-deficient mice with AKI, UC-MSC treatment fails to induce renoprotection. These data document a previously unrecognized mechanism through which UC-MSCs facilitate renal repair, so as to induce global metabolic reprogramming of damaged tubular cells to sustain energy supply.

Mesenchymal stromal cells drive renal regeneration following injury. Here, the authors show that human mesenchymal stromal cells, when transplanted into mice with acute kidney injury, stimulate renal tubular cell growth and enhance mitochondrial function via SIRT3.

Age-related accumulation of phosphorylated mitofusin 2 protein in retinal ganglion cells correlates with glaucoma progression

Dysregulation of axonal bioenergetics is likely a key mechanism in the initiation and progression of age-related neurodegenerative diseases. Glaucoma is a quintessential neurodegenerative disorder characterized by progressive deterioration of the optic nerve (ON) and eventual death of retinal ganglion cells (RGCs). Age and elevation of intraocular pressure are key risk factors in glaucoma, but the common early hallmarks of decreased axonal transport and increased bioenergetic vulnerability likely underlie disease initiation. We examined the correlation between bioenergetics and axonal transport with mitochondrial mutation frequency and post-translational modifications of mitofusin 2 (Mfn2) in RGCs during glaucoma progression. No increase in the frequency of mtDNA mutations was detected, but we observed significant shifts in mitochondrial protein species. Mfn2 is a fusion protein that functions in mitochondrial biogenesis, maintenance, and mitochondrial transport. We demonstrate that Mfn2 accumulates selectively in RGCs during glaucomatous degeneration, that two novel states of Mfn2 exist in retina and ON, and identify a phosphorylated form that selectively accumulates in RGCs, but is absent in ON. Phosphorylation of Mfn2 is correlated with higher ubiquitination, and failure of the protein to reach the ON. Together, these data suggest that post-translational modification of Mfn2 is associated with its dysregulation during a window of metabolic vulnerability that precedes glaucomatous degeneration. Future work to either manipulate expression of Mfn2 or to prevent its degradation could have therapeutic value in the treatment of neurodegenerative diseases where long-tract axons are vulnerable.

Chondroitin sulfate proteoglycans negatively regulate the positioning of mitochondria and endoplasmic reticulum to distal axons

Chondroitin sulfate proteoglycans (CSPGs) are components of the extracellular matrix that inhibit the extension and regeneration of axons. However, the underlying mechanism of action remains poorly understood. Mitochondria and endoplasmic reticulum (ER) are functionally inter-linked organelles important to axon development and maintenance. We report that CSPGs impair the targeting of mitochondria and ER to the growth cones of chicken embryonic sensory axons. The effect of CSPGs on the targeting of mitochondria is blocked by inhibition of the LAR receptor for CSPGs. The regulation of the targeting of mitochondria and ER to the growth cone by CSPGs is due to attenuation of PI3K signaling, which is known to be downstream of LAR receptor activation. Dynactin is a required component of the dynein motor complex that drives the normally occurring retrograde evacuation of mitochondria from growth cones. CSPGs elevate the levels of p150^Glu dynactin found in distal axons, and inhibition of the interaction of dynactin with dynein increased axon lengths on CSPGs. CSPGs decreased the membrane potential of mitochondria, and pharmacological inhibition of mitochondria respiration at the growth cone independent of manipulation of mitochondria positioning impaired axon extension. Combined inhibition of dynactin and potentiation of mitochondria respiration further increased axon lengths on CSPGs relative to inhibition of dynactin alone. These data reveal that the regulation of the localization of mitochondria and ER to growth cones is a previously unappreciated aspect of the effects of CSPGs on embryonic axons. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1351-1370, 2017.

A high mitochondrial transport rate characterizes CNS neurons with high axonal regeneration capacity

Improving axonal transport in the injured and diseased central nervous system has been proposed as a promising strategy to improve neuronal repair. However, the contribution of each cargo to the repair mechanism is unknown. DRG neurons globally increase axonal transport during regeneration. Because the transport of specific cargos after axonal insult has not been examined systematically in a model of enhanced regenerative capacity, it is unknown whether the transport of all cargos would be modulated equally in injured central nervous system neurons. Here, using a microfluidic culture system we compared neurons co-deleted for PTEN and SOCS3, an established model of high axonal regeneration capacity, to control neurons. We measured the axonal transport of three cargos (mitochondria, synaptic vesicles and late endosomes) in regenerating axons and found that the transport of mitochondria, but not the other cargos, was increased in PTEN/SOCS3 co-deleted axons relative to controls. The results reported here suggest a pivotal role for this organelle during axonal regeneration.

ROS induced distribution of mitochondria to filopodia by Myo19 depends on a class specific tryptophan in the motor domain

The role of the actin cytoskeleton in relation to mitochondria function and dynamics is only recently beginning to be recognized. Myo19 is an actin-based motor that is bound to the outer mitochondrial membrane and promotes the localization of mitochondria to filopodia in response to glucose starvation. However, how glucose starvation induces mitochondria localization to filopodia, what are the dynamics of this process and which enzymatic adaptation allows the translocation of mitochondria to filopodia are not known. Here we show that reactive oxygen species (ROS) mimic and mediate the glucose starvation induced phenotype. In addition, time-lapse fluorescent microscopy reveals that ROS-induced Myo19 motility is a highly dynamic process which is coupled to filopodia elongation and retraction. Interestingly, Myo19 motility is inhibited by back-to-consensus-mutation of a unique residue of class XIX myosins in the motor domain. Kinetic analysis of the purified mutant Myo19 motor domain reveals that the duty ratio (time spent strongly bound to actin) is highly compromised in comparison to that of the WT motor domain, indicating that Myo19 unique motor properties are necessary to propel mitochondria to filopodia tips. In summary, our study demonstrates the contribution of actin-based motility to the mitochondrial localization to filopodia by specific cellular cues.

Mitochondrial redox signaling enables repair of injured skeletal muscle cells

Strain and physical trauma to mechanically active cells, such as skeletal muscle myofibers, injures their plasma membranes, and mitochondrial function is required for their repair. We found that mitochondrial function was also needed for plasma membrane repair in myoblasts as well as nonmuscle cells, which depended on mitochondrial uptake of calcium through the mitochondrial calcium uniporter (MCU). Calcium uptake transiently increased the mitochondrial production of reactive oxygen species (ROS), which locally activated the guanosine triphosphatase (GTPase) RhoA, triggering F-actin accumulation at the site of injury and facilitating membrane repair. Blocking mitochondrial calcium uptake or ROS production prevented injury-triggered RhoA activation, actin polymerization, and plasma membrane repair. This repair mechanism was shared between myoblasts, nonmuscle cells, and mature skeletal myofibers. Quenching mitochondrial ROS in myofibers during eccentric exercise ex vivo caused increased damage to myofibers, resulting in a greater loss of muscle force. These results suggest a physiological role for mitochondria in plasma membrane repair in injured cells, a role that highlights a beneficial effect of ROS.

Mitochondrial dynamics, transport, and quality control: A bottleneck for retinal ganglion cell viability in optic neuropathies

Retinal ganglion cells, the neurons that selectively die in glaucoma and other optic neuropathies, are endowed with an exceedingly active metabolism and display a particular vulnerability to mitochondrial dysfunction. Mitochondria are exquisitely dynamic organelles that are continually responding to endogenous and environmental cues to readily meet the energy demand of neuronal networks. The highly orchestrated regulation of mitochondrial biogenesis, fusion, fission, transport and degradation is paramount for the maintenance of energy-expensive synapses at RGC dendrites and axon terminals geared for optimal neurotransmission. The present review focuses on the progress made to date on understanding the biology of mitochondrial dynamics and quality control and how dysregulation of these processes can profoundly affect retinal ganglion cell viability and function in optic nerve diseases.

Nicotinamide riboside, a form of vitamin B3, protects against excitotoxicity-induced axonal degeneration

NAD⁺ depletion is a common phenomenon in neurodegenerative pathologies. Excitotoxicity occurs in multiple neurologic disorders and NAD⁺ was shown to prevent neuronal degeneration in this process through mechanisms that remained to be determined. The activity of nicotinamide riboside (NR) in neuroprotective models and the recent description of extracellular conversion of NAD⁺ to NR prompted us to probe the effects of NAD⁺ and NR in protection against excitotoxicity. Here, we show that intracortical administration of NR but not NAD⁺ reduces brain damage induced by NMDA injection. Using cortical neurons, we found that provision of extracellular NR delays NMDA-induced axonal degeneration (AxD) much more strongly than extracellular NAD⁺ Moreover, the stronger effect of NR compared to NAD⁺ depends of axonal stress since in AxD induced by pharmacological inhibition of nicotinamide salvage, both NAD⁺ and NR prevent neuronal death and AxD in a manner that depends on internalization of NR. Taken together, our findings demonstrate that NR is a better neuroprotective agent than NAD⁺ in excitotoxicity-induced AxD and that axonal protection involves defending intracellular NAD⁺ homeostasis.-Vaur, P., Brugg, B., Mericskay, M., Li, Z., Schmidt, M. S., Vivien, D., Orset, C., Jacotot, E., Brenner, C., Duplus, E. Nicotinamide riboside, a form of vitamin B₃, protects against excitotoxicity-induced axonal degeneration.

Mitochondria Localize to Injured Axons to Support Regeneration

Axon regeneration is essential to restore the nervous system after axon injury. However, the neuronal cell biology that underlies axon regeneration is incompletely understood. Here we use in vivo, single-neuron analysis to investigate the relationship between nerve injury, mitochondrial localization, and axon regeneration. Mitochondria translocate into injured axons so that average mitochondria density increases after injury. Moreover, single-neuron analysis reveals that axons that fail to increase mitochondria have poor regeneration. Experimental alterations to axonal mitochondrial distribution or mitochondrial respiratory chain function result in corresponding changes to regeneration outcomes. Axonal mitochondria are specifically required for growth-cone migration, identifying a key energy challenge for injured neurons. Finally, mitochondrial localization to the axon after injury is regulated in part by dual-leucine zipper kinase 1 (DLK-1), a conserved regulator of axon regeneration. These data identify regulation of axonal mitochondria as a new cell-biological mechanism that helps determine the regenerative response of injured neurons.

Mitochondrial anchors: Positioning mitochondria and more

The shape and position of mitochondria are intimately connected to both mitochondrial and cellular function. Mitochondrial anchors play a central role in mitochondrial positioning by exerting spatial, temporal, and contextual control over the cellular position of the organelle. Investigations into the molecular mechanisms of mitochondrial anchoring are still in the early stages, and we are beginning to appreciate the number and variety of anchors that exist. From the insight gained thus far, it is clear that mitochondrial anchoring has functional and physiological consequences that extend beyond mitochondrial positioning to other critical cellular processes.

The Interplay of Axonal Energy Homeostasis and Mitochondrial Trafficking and Anchoring

Mitochondria are key cellular power plants essential for neuronal growth, survival, function, and regeneration after injury. Given their unique morphological features, neurons face exceptional challenges in maintaining energy homeostasis at distal synapses and growth cones where energy is in high demand. Efficient regulation of mitochondrial trafficking and anchoring is critical for neurons to meet altered energy requirements. Mitochondrial dysfunction and impaired transport have been implicated in several major neurological disorders. Thus, research into energy-mediated regulation of mitochondrial recruitment and redistribution is an important emerging frontier. In this review, I discuss new insights into the mechanisms regulating mitochondrial trafficking and anchoring, and provide an updated overview of how mitochondrial motility maintains energy homeostasis in axons, thus contributing to neuronal growth, regeneration, and synaptic function.

A Select Subset of Electron Transport Chain Genes Associated with Optic Atrophy Link Mitochondria to Axon Regeneration in Caenorhabditis elegans

The role of mitochondria within injured neurons is an area of active interest since these organelles are vital for the production of cellular energy in the form of ATP. Using mechanosensory neurons of the nematode Caenorhabditis elegans to test regeneration after neuronal injury in vivo, we surveyed genes related to mitochondrial function for effects on axon regrowth after laser axotomy. Genes involved in mitochondrial transport, calcium uptake, mitophagy, or fission and fusion were largely dispensable for axon regrowth, with the exception of eat-3/Opa1. Surprisingly, many genes encoding components of the electron transport chain were dispensable for regrowth, except for the iron-sulfur proteins gas-1, nduf-2.2, nduf-7, and isp-1, and the putative oxidoreductase rad-8. In these mutants, axonal development was essentially normal and axons responded normally to injury by forming regenerative growth cones, but were impaired in subsequent axon extension. Overexpression of nduf-2.2 or isp-1 was sufficient to enhance regrowth, suggesting that mitochondrial function is rate-limiting in axon regeneration. Moreover, loss of function in isp-1 reduced the enhanced regeneration caused by either a gain-of-function mutation in the calcium channel EGL-19 or overexpression of the MAP kinase DLK-1. While the cellular function of RAD-8 remains unclear, our genetic analyses place rad-8 in the same pathway as other electron transport genes in axon regeneration. Unexpectedly, rad-8 regrowth defects were suppressed by altered function in the ubiquinone biosynthesis gene clk-1. Furthermore, we found that inhibition of the mitochondrial unfolded protein response via deletion of atfs-1 suppressed the defective regrowth in nduf-2.2 mutants. Together, our data indicate that while axon regeneration is not significantly affected by general dysfunction of cellular respiration, it is sensitive to the proper functioning of a select subset of electron transport chain genes, or to the cellular adaptations used by neurons under conditions of injury.

Meat Intake and the Dose of Vitamin B3 - Nicotinamide: Cause of the Causes of Disease Transitions, Health Divides, and Health Futures?

Meat and vitamin B3 - nicotinamide - intake was high during hunter-gatherer times. Intake then fell and variances increased during and after the Neolithic agricultural revolution. Health, height, and IQ deteriorated. Low dietary doses are buffered by 'welcoming' gut symbionts and tuberculosis that can supply nicotinamide, but this co-evolved homeostatic metagenomic strategy risks dysbioses and impaired resistance to pathogens. Vitamin B3 deficiency may now be common among the poor billions on a low-meat diet. Disease transitions to non-communicable inflammatory disorders (but longer lives) may be driven by positive 'meat transitions'. High doses of nicotinamide lead to reduced regulatory T cells and immune intolerance. Loss of no longer needed symbiotic 'old friends' compounds immunological over-reactivity to cause allergic and auto-immune diseases. Inhibition of nicotinamide adenine dinucleotide consumers and loss of methyl groups or production of toxins may cause cancers, metabolic toxicity, or neurodegeneration. An optimal dosage of vitamin B3 could lead to better health, but such a preventive approach needs more equitable meat distribution. Some people may require personalised doses depending on genetic make-up or, temporarily, when under stress.

Releasing Syntaphilin Removes Stressed Mitochondria from Axons Independent of Mitophagy under Pathophysiological Conditions

Chronic mitochondrial stress is a central problem associated with neurodegenerative diseases. Early removal of defective mitochondria from axons constitutes a critical step of mitochondrial quality control. Here we investigate axonal mitochondrial response to mild stress in wild-type neurons and chronic mitochondrial defects in Amytrophic Lateral Sclerosis (ALS)- and Alzheimer's disease (AD)-linked neurons. We show that stressed mitochondria are removed from axons triggered by the bulk release of mitochondrial anchoring protein syntaphilin via a new class of mitochondria-derived cargos independent of Parkin, Drp1, and autophagy. Immuno-electron microscopy and super-resolution imaging show the budding of syntaphilin cargos, which then share a ride on late endosomes for transport toward the soma. Releasing syntaphilin is also activated in the early pathological stages of ALS- and AD-linked mutant neurons. Our study provides new mechanistic insights into the maintenance of axonal mitochondrial quality through SNPH-mediated coordination of mitochondrial stress and motility before activation of Parkin-mediated mitophagy. VIDEO ABSTRACT.

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.

Transcriptomic investigation of wound healing and regeneration in the cnidarian Calliactis polypus

Wound healing and regeneration in cnidarian species, a group that forms the sister phylum to Bilateria, remains poorly characterised despite the ability of many cnidarians to rapidly repair injuries, regenerate lost structures, or re-form whole organisms from small populations of somatic cells. Here we present results from a fully replicated RNA-Seq experiment to identify genes that are differentially expressed in the sea anemone Calliactis polypus following catastrophic injury. We find that a large-scale transcriptomic response is established in C. polypus, comprising an abundance of genes involved in tissue patterning, energy dynamics, immunity, cellular communication, and extracellular matrix remodelling. We also identified a substantial proportion of uncharacterised genes that were differentially expressed during regeneration, that appear to be restricted to cnidarians. Overall, our study serves to both identify the role that conserved genes play in eumetazoan wound healing and regeneration, as well as to highlight the lack of information regarding many genes involved in this process. We suggest that functional analysis of the large group of uncharacterised genes found in our study may contribute to better understanding of the regenerative capacity of cnidarians, as well as provide insight into how wound healing and regeneration has evolved in different lineages.

In vivo Imaging of Mitochondrial Transport in Single-Axon Regeneration of Zebrafish Mauthner Cells

Mitochondrial transport is essential for neuronal function, but the evidence of connections between mitochondrial transport and axon regeneration in the central nervous system (CNS) of living vertebrates remains limited. Here, we developed a novel model to explore mitochondrial transport in a single Mauthner axon (M axon) of zebrafish with non-invasive in vivo imaging. To confirm the feasibility of using this model, we treated labeled zebrafish with nocodazole and demonstrated that it could disrupt mitochondrial transport. We also used two-photon laser axotomy to precisely axotomize M axons and simultaneously recorded their regeneration and the process of mitochondrial transport in living zebrafish larvae. The findings showed that the injured axons with stronger regenerative capability maintain greater mitochondrial motility. Furthermore, to stimulate axon regeneration, treatment with dibutyryl cyclic adenosine monophosphate (db-cAMP) could also augment mitochondrial motility. Taken together, our results provide new evidence that mitochondrial motility is positively correlated with axon regeneration in the living vertebrate CNS. This promising model will be useful for further studies on the interaction between axon regeneration and mitochondrial dynamics, using various genetic and pharmacological techniques.

Selective antagonism of muscarinic receptors is neuroprotective in peripheral neuropathy

Sensory neurons have the capacity to produce, release, and respond to acetylcholine (ACh), but the functional role of cholinergic systems in adult mammalian peripheral sensory nerves has not been established. Here, we have reported that neurite outgrowth from adult sensory neurons that were maintained under subsaturating neurotrophic factor conditions operates under cholinergic constraint that is mediated by muscarinic receptor-dependent regulation of mitochondrial function via AMPK. Sensory neurons from mice lacking the muscarinic ACh type 1 receptor (M1R) exhibited enhanced neurite outgrowth, confirming the role of M1R in tonic suppression of axonal plasticity. M1R-deficient mice made diabetic with streptozotocin were protected from physiological and structural indices of sensory neuropathy. Pharmacological blockade of M1R using specific or selective antagonists, pirenzepine, VU0255035, or muscarinic toxin 7 (MT7) activated AMPK and overcame diabetes-induced mitochondrial dysfunction in vitro and in vivo. These antimuscarinic drugs prevented or reversed indices of peripheral neuropathy, such as depletion of sensory nerve terminals, thermal hypoalgesia, and nerve conduction slowing in diverse rodent models of diabetes. Pirenzepine and MT7 also prevented peripheral neuropathy induced by the chemotherapeutic agents dichloroacetate and paclitaxel or HIV envelope protein gp120. As a variety of antimuscarinic drugs are approved for clinical use against other conditions, prompt translation of this therapeutic approach to clinical trials is feasible.

Spatial and temporal arrangement of neuronal intrinsic and extrinsic mechanisms controlling axon regeneration

Axon regeneration and neuronal tissue repair varies across animal lineages as well as in the mammalian central and peripheral nervous systems. While the peripheral nervous system retains the ability to self-repair, the majority of axons in the adult mammalian central nervous system (CNS) fail to reactivate intrinsic growth programs after injury. Recent findings, however, suggest that long-distance axon regeneration, neuronal circuit assembly and recovery of functions in the adult mammalian CNS are possible. Here, we discuss our current knowledge of the cell signaling pathways and networks controlling axon regeneration. In addition, we outline a number of combinatorial strategies that include among others microtubule-based treatments to foster regeneration and functional connectivity after CNS trauma.

The Mammalian-Specific Protein Armcx1 Regulates Mitochondrial Transport during Axon Regeneration

Mitochondrial transport is crucial for neuronal and axonal physiology. However, whether and how it impacts neuronal injury responses, such as neuronal survival and axon regeneration, remain largely unknown. In an established mouse model with robust axon regeneration, we show that Armcx1, a mammalian-specific gene encoding a mitochondria-localized protein, is upregulated after axotomy in this high regeneration condition. Armcx1 overexpression enhances mitochondrial transport in adult retinal ganglion cells (RGCs). Importantly, Armcx1 also promotes both neuronal survival and axon regeneration after injury, and these effects depend on its mitochondrial localization. Furthermore, Armcx1 knockdown undermines both neuronal survival and axon regeneration in the high regenerative capacity model, further supporting a key role of Armcx1 in regulating neuronal injury responses in the adult central nervous system (CNS). Our findings suggest that Armcx1 controls mitochondrial transport during neuronal repair.