Axon degeneration: Mechanisms and implications of a distinct program from cell death

Neutrophil peptide 1 accelerates the clearance of degenerative axons during Wallerian degeneration by activating macrophages after peripheral nerve crush injury

JOURNAL/nrgr/04.03/01300535-202408000-00036/figure1/v/2023-12-16T180322Z/r/image-tiff Macrophages play an important role in peripheral nerve regeneration, but the specific mechanism of regeneration is still unclear. Our preliminary findings indicated that neutrophil peptide 1 is an innate immune peptide closely involved in peripheral nerve regeneration. However, the mechanism by which neutrophil peptide 1 enhances nerve regeneration remains unclear. This study was designed to investigate the relationship between neutrophil peptide 1 and macrophages in vivo and in vitro in peripheral nerve crush injury. The functions of RAW 264.7 cells were elucidated by Cell Counting Kit-8 assay, flow cytometry, migration assays, phagocytosis assays, immunohistochemistry and enzyme-linked immunosorbent assay. Axonal debris phagocytosis was observed using the CUBIC (Clear, Unobstructed Brain/Body Imaging Cocktails and Computational analysis) optical clearing technique during Wallerian degeneration. Macrophage inflammatory factor expression in different polarization states was detected using a protein chip. The results showed that neutrophil peptide 1 promoted the proliferation, migration and phagocytosis of macrophages, and CD206 expression on the surface of macrophages, indicating M2 polarization. The axonal debris clearance rate during Wallerian degeneration was enhanced after neutrophil peptide 1 intervention. Neutrophil peptide 1 also downregulated inflammatory factors interleukin-1α, -6, -12, and tumor necrosis factor-α in vivo and in vitro. Thus, the results suggest that neutrophil peptide 1 activates macrophages and accelerates Wallerian degeneration, which may be one mechanism by which neutrophil peptide 1 enhances peripheral nerve regeneration.

The presence of anhedonia in individuals with subacute and chronic stroke: an exploratory cohort study

Background

Anhedonia refers to the diminished capacity to experience pleasure. It has been described both as a symptom of depression and an enduring behavioral trait that contributes its development. Specifically, in stroke patients, anhedonia has been closely linked to depression, resulting in reduced sensitivity to everyday pleasures and intrinsic motivation to engage in rehabilitation programs and maintain a healthy active lifestyle. This condition may hinder patients' recovery, diminishing their autonomy, functioning, and quality of life.

Objective

We aimed to explore the prevalence and level of anhedonia and those variables that might be associated in patients with both ischemic and hemorrhagic stroke at subacute and chronic phases of the disease.

Methods

We conducted an exploratory cohort study with a sample of 125 patients with subacute and chronic stroke presenting upper-limb motor deficits. We measured participants' level of anhedonia with four items from the Beck Depression Inventory-II that describe the symptoms of this condition: loss of pleasure, loss of interest, loss of energy, and loss of interest in sex. We also collected demographic and clinical information and evaluated motor and cognitive functions as well as levels of depression, apathy, and various mood states. The results were compared to a sample of 71 healthy participants of similar age, sex, and level of education.

Results

Stroke patients demonstrated a significantly higher prevalence (18.5-19.7%) and level of anhedonia compared to the healthy controls (4.3%), regardless of stroke phase, level of motor impairment, and other clinical variables. Furthermore, post-stroke anhedonia was associated with lower levels of motivation and higher levels of negative mood states such as fatigue and anger in the long term. Importantly, anhedonia level was superior in stroke patients than in healthy controls while controlling for confounding effects of related emotional conditions.

Conclusion

This study provides novel evidence on the prevalence, level and factors related to anhedonia post-stroke. We emphasize the importance of assessing and treating anhedonia in this population, as well as conducting large-scale cohort and longitudinal studies to test its influence on long-term functional and emotional recovery.

Molecular mechanisms underlying major depressive disorder and post-stroke affective disorders

Two of the most common and incapacitating mental health disorders around the world are major depressive disorder (MDD) and post-stroke depression (PSD). MDD is thought to result from abnormal connectivity between the monoaminergic, glutamatergic, GABAergic, and/or cholinergic pathways. Additional factors include the roles of hormonal, immune, ageing, as well as the influence of cellular, molecular, and epigenetics in the development of mood disorders. This complexity of factors has been anticipated by the Swiss psychiatrists Paul Kielholz and Jules Angst who introduced a multimodal treatment of MDD. Depression is the predominant mood disorder, impacting around one-third of individuals who have experienced a stroke. MDD and PSD share common underlying biological mechanisms related to the disruption of monoaminergic pathways. The major contributor to PSD is the stroke lesion location, which can involve the disruption of the serotoninergic, dopaminergic, glutamatergic, GABAergic, or cholinergic pathways. Additionally, various other disorders such as mania, bipolar disorder, anxiety disorder, and apathy might occur post-stroke, although their prevalence is considerably lower. However, there are differences in the onset of MDD among mood disorders. Some mood disorders develop gradually and can persist for a lifetime, potentially culminating in suicide. In contrast, PSD has a rapid onset because of the severe disruption of neural pathways essential for mood behavior caused by the lesion. However, PSD might also spontaneously resolve several months after a stroke, though it is associated with higher mortality. This review also provides a brief overview of the treatments currently available in medical practice.

NMNAT1 and hereditary spastic paraplegia (HSP): expanding the phenotypic spectrum of NMNAT1 variants

In the NAD biosynthetic network, the nicotinamide mononucleotide adenylyltransferase (NMNAT) enzyme fuels NAD as a co-substrate for a group of enzymes. Mutations in the nuclear-specific isoform, NMNAT1, have been extensively reported as the cause of Leber congenital amaurosis-type 9 (LCA9). However, there are no reports of NMNAT1 mutations causing neurological disorders by disrupting the maintenance of physiological NAD homeostasis in other types of neurons. In this study, for the first time, the potential association between a NMNAT1 variant and hereditary spastic paraplegia (HSP) is described. Whole-exome sequencing was performed for two affected siblings diagnosed with HSP. Runs of homozygosity (ROH) were detected. The shared variants of the siblings located in the homozygosity blocks were selected. The candidate variant was amplified and Sanger sequenced in the proband and other family members. Homozygous variant c.769G>A:p.(Glu257Lys) in NMNAT1, the most common variant of NMNAT1 in LCA9 patients, located in the ROH of chromosome 1, was detected as a probable disease-causing variant. After detection of the variant in NMNAT1, as a LCA9-causative gene, ophthalmological and neurological re-evaluations were performed. No ophthalmological abnormality was detected and the clinical manifestations of these patients were completely consistent with pure HSP. No NMNAT1 variant had ever been previously reported in HSP patients. However, NMNAT1 variants have been reported in a syndromic form of LCA which is associated with ataxia. In conclusion, our patients expand the clinical spectrum of NMNAT1 variants and represent the first evidence of the probable correlation between NMNAT1 variants and HSP.

The Nucleotide Revolution: Immunity at the Intersection of Toll/Interleukin-1 Receptor Domains, Nucleotides, and Ca2

The discovery of the enzymatic activity of the toll/interleukin-1 receptor (TIR) domain protein SARM1 five years ago preceded a flood of discoveries regarding the nucleotide substrates and products of TIR domains in plants, animals, bacteria, and archaea. These discoveries into the activity of TIR domains coincide with major advances in understanding the structure and mechanisms of NOD-like receptors and the mutual dependence of pattern recognition receptor- and effector-triggered immunity (PTI and ETI, respectively) in plants. It is quickly becoming clear that TIR domains and TIR-produced nucleotides are ancestral signaling molecules that modulate immunity and that their activity is closely associated with Ca²⁺ signaling. TIR domain research now bridges the separate disciplines of molecular plant- and animal-microbe interactions, neurology, and prokaryotic immunity. A cohesive framework for understanding the role of enzymatic TIR domains in diverse organisms will help unite the research of these disparate fields. Here, we review known products of TIR domains in plants, animals, bacteria, and archaea and use context gained from animal and prokaryotic TIR domain systems to present a model for TIR domains, nucleotides, and Ca²⁺ at the intersection of PTI and ETI in plant immunity. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

SARM1 is a multi-functional NAD(P)ase with prominent base exchange activity, all regulated bymultiple physiologically relevant NAD metabolites

SARM1 is an NAD(P) glycohydrolase and TLR adapter with an essential, prodegenerative role in programmed axon death (Wallerian degeneration). Like other NAD(P)ases, it catalyzes multiple reactions that need to be fully investigated. Here, we compare these multiple activities for recombinant human SARM1, human CD38, and Aplysia californica ADP ribosyl cyclase. SARM1 has the highest transglycosidation (base exchange) activity at neutral pH and with some bases this dominates NAD(P) hydrolysis and cyclization. All SARM1 activities, including base exchange at neutral pH, are activated by an increased NMN:NAD ratio, at physiological levels of both metabolites. SARM1 base exchange occurs also in DRG neurons and is thus a very likely physiological source of calcium-mobilizing agent NaADP. Finally, we identify regulation by free pyridines, NADP, and nicotinic acid riboside (NaR) on SARM1, all of therapeutic interest. Understanding which specific SARM1 function(s) is responsible for axon degeneration is essential for its targeting in disease.

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Base exchange is a prominent, and sometimes completely dominant, SARM1 activity

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Physiologically relevant NMN:NAD ratios may regulate all of SARM1's multiple activities

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Physiological NADP may inhibit SARM1 more potently than NAD and via a distinct site

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NaR and VR both selectively inhibit SARM1 and are thus possible effectors or drug leads

Amyloid-Driven Tau Accumulation on Mitochondria Potentially Leads to Cognitive Deterioration in Alzheimer's Disease

Despite the well-accepted role of the two main neuropathological markers (β-amyloid and tau) in the progression of Alzheimer’s disease, the interaction and specific contribution of each of them is not fully elucidated. To address this question, in the present study, an adeno-associated virus (AAV9) carrying the mutant P301L form of human tau, was injected into the dorsal hippocampi of APP/PS1 transgenic mice or wild type mice (WT). Three months after injections, memory tasks, biochemical and immunohistochemical analysis were performed. We found that the overexpression of hTauP301L accelerates memory deficits in APP/PS1 mice, but it did not affect memory function of WT mice. Likewise, biochemical assays showed that only in the case of APP/PS1-hTauP301L injected mice, an important accumulation of tau was observed in the insoluble urea fraction. Similarly, electron microscopy images revealed that numerous clusters of tau immunoparticles appear at the dendrites of APP/PS1 injected mice and not in WT animals, suggesting that the presence of amyloid is necessary to induce tau aggregation. Interestingly, these tau immunoparticles accumulate in dendritic mitochondria in the APP/PS1 mice, whereas most of mitochondria in WT injected mice remain free of tau immunoparticles. Taken together, it seems that amyloid induces tau aggregation and accumulation in the dendritic mitochondria and subsequently may alter synapse function, thus, contributing to accelerate cognitive decline in APP/PS1 mice.

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.

Atg7 Knockout Alleviated the Axonal Injury of Neuro-2a Cells Induced by Tri-Ortho-Cresyl Phosphate

Autophagy is believed to be essential for the maintenance of axonal homeostasis in neurons. However, whether autophagy is causally related to the axon degeneration in organophosphorus-induced delayed neuropathy (OPIDN) still remains unclear. This research was designed to investigate the role of autophagy in axon degeneration following tri-ortho-cresyl phosphate (TOCP) in an in vitro model. Differentiated wild-type and Atg7-/- neuro-2a (N2a) cells were treated with TOCP for 24 h. Axonal degeneration in N2a cells was quantitatively analyzed; the key molecules responsible for axon degeneration and its upstream signaling pathway were determined by Western blotting and real-time PCR. The results found that Atg7-/- cells exhibited a higher resistance to TOCP insult than wild-type cells. Further study revealed that TOCP caused a significant decrease in pro-survival factors NMNATs and SCG10 and a significant increase in pro-degenerative factor SARM1 in both cells. Notably, Atg7-/- cells presented a higher level of pro-survival factors and a lower level of pro-degenerative factors than wild-type cells in the same setting of TOCP administration. Moreover, DLK-MAPK pathway was activated following TOCP. Altogether, our results suggest that autophagy is able to affect TOCP-induced axonal injury via regulating the balance between pro-survival and pro-degenerative factors, providing a promising avenue for the potential therapy for OPIDN patients.

Nmnat1 Modulates Mitochondrial Oxidative Stress by Inhibiting Caspase-3 Signaling in Alzheimer's Disease

Nigrostriatal pathway disturbance is one of the major pathogenic factors in Alzheimer's disease (AD). Dopaminergic neuron dysfunction results in bradykinesia and akinesia (inability to initiate movement), indicating a significant risk factor for substantia nigra pars compacta lesions. Furthermore, the nicotinamide adenine dinucleotide (NAD⁺) is associated with Aβ toxicity decline in AD therapy. Nicotinamide mononucleotide adenylyltransferase 1 (Nmnat1) is an essential enzyme that preserves normal neuronal function and protects neurons from insult. This study aimed to investigate the potential therapeutic effects of Nmnat1 and its underlying mechanisms in a triple-transgenic mouse model of AD (3xTgAD). Results showed that Nmnat1 improved the substantial behavioral measures of cognitive impairments compared with the 3xTgAD control. Additionally, Nmnat1 overexpression significantly increased tyrosine hydroxylase-positive neurons and anti-apoptotic protein Bcl2 and caspase-3 expression levels in 3xTgAD mice. Nmnat1 also effectively controlled SOD1 activation. In conclusion, Nmnat1 substantially decreases multiple AD-associated pathological characteristics at least partially by the increase of caspase-3 activation.

Neuronal autophagy and axon degeneration

Axon degeneration is a pathophysiological process of axonal dying and breakdown, which is characterized by several morphological features including the accumulation of axoplasmic organelles, disassembly of microtubules, and fragmentation of the axonal cytoskeleton. Autophagy, a highly conserved lysosomal-degradation machinery responsible for the control of cellular protein quality, is widely believed to be essential for the maintenance of axonal homeostasis in neurons. In recent years, more and more evidence suggests that dysfunctional autophagy is associated with axonal degeneration in many neurodegenerative diseases. Here, we review the core machinery of autophagy in neuronal cells, and provide several major steps that interfere with autophagy flux in neurodegenerative conditions. Furthermore, this review highlights the potential role of neuronal autophagy in axon degeneration, and presents some possible molecular mechanisms by which dysfunctional autophagy leads to axon degeneration in pathological conditions.

Wide resection of inguinal nerves versus simple section to prevent postoperative pain after prosthetic inguinal hernioplasty: our experience

BACKGROUND

In the literature, chronic groin pain (i.e. lasting >3 months) occurs in about 10 % of patients who undergo inguinal hernioplasty with prosthesis; it is characterized by a broad range of symptoms, and is relative to individual perceptions of pain. In 2-5 % of cases, the painful symptomatology is so intense that it interferes with daily activities, and can be debilitating in 0.5-6 % of cases. The best known cause of inguinodynia is neuropathy, due to implication of one or more inguinal nerves (iliohypogastric, ilioinguinal, and genitofemoral nerves) into fibroblastic processes; or from nervous stimulation caused by prosthetic material on adjacent nervous trunks. Many therapeutic strategies have been proposed to treat chronic groin pain, including intra-operative prophylactic neurectomy.

OBJECTIVE

The purpose of our study was to perform a comparative analysis between outcomes from wide resections of inguinal nerves versus those from simple nervous section (or minimal resection).

PATIENTS AND METHODS

We considered 350 patients who had undergone inguinal prosthetic hernioplasty with Trabucco's technique between 2004 and 2010. Wide nervous resection (removal of nerve segments 3-8 cm in length) was performed in 180. The other 170 patients underwent simple section or minimal resection. All patients were checked 1 week, 1 month, and 1 year after surgery.

RESULTS

Group 1: At 1-week follow-up, 63 patients (35 %) reported no pain, 113 (63 %) reported moderate pain, and 4 (2 %) intense pain; 1 month after the procedure, 152 patients (84.4 %) reported no pain, 25 (14 %) complained of moderate pain, and 3 (1.6 %) of severe pain; 1 year after surgery, only 1 patient (0.5 %) complained of constant pain. Group 2: At 1 week follow-up, 48 patients (28 %) reported no pain, 101 (59 %) reported moderate pain, and 21 (13 %) intense pain; 1 month after the procedure, 81 patients (47.6 %) had no pain, 72 (42.4 %) complained of moderate pain, and 17 (10 %) of severe pain; 1 year after surgery, 11 patients (6.5 %) had constant pain, and two of them were re-admitted for surgery. The lower incidence of chronic pain after long nervous resection is statistically significant (0.5 vs. 6.5 %; p = 0.006); the incidence of moderate pain 1 month after operation is also lower (14 vs. 42.4 %; p < 0.0001); patients who underwent a long resection experienced faster resolution of pain symptomatology, during a month. Also noteworthy is the lower incidence of intense pain in the short and medium term (after 1 week, 13 vs. 2 %, p = 0.0005; after 1 month, 10 vs. 1.6 %, p = 0.0018).

CONCLUSIONS

The prophylactic wide resection of selected segments of inguinal nerves, despite the apparent paradox of greater tissue damage, appears more effective than simple section at preventing postoperative inguinodynia, given both the lower incidence and the faster resolution of painful symptomatology.

P7C3 neuroprotective chemicals block axonal degeneration and preserve function after traumatic brain injury

The P7C3 class of neuroprotective aminopropyl carbazoles has been shown to block neuronal cell death in models of neurodegeneration. We now show that P7C3 molecules additionally preserve axonal integrity after injury, before neuronal cell death occurs, in a rodent model of blast-mediated traumatic brain injury (TBI). This protective quality may be linked to the ability of P7C3 molecules to activate nicotinamide phosphoribosyltransferase, the rate-limiting enzyme in nicotinamide adenine dinucleotide salvage. Initiation of daily treatment with our recently reported lead agent, P7C3-S243, 1 day after blast-mediated TBI blocks axonal degeneration and preserves normal synaptic activity, learning and memory, and motor coordination in mice. We additionally report persistent neurologic deficits and acquisition of an anxiety-like phenotype in untreated animals 8 months after blast exposure. Optimized variants of P7C3 thus offer hope for identifying neuroprotective agents for conditions involving axonal damage, neuronal cell death, or both, such as occurs in TBI.

Calcium-dependent proteasome activation is required for axonal neurofilament degradation

Even though many studies have identified roles of proteasomes in axonal degeneration, the molecular mechanisms by which axonal injury regulates proteasome activity are still unclear. In the present study, we found evidence indicating that extracellular calcium influx is an upstream regulator of proteasome activity during axonal degeneration in injured peripheral nerves. In degenerating axons, the increase in proteasome activity and the degradation of ubiquitinated proteins were significantly suppressed by extracellular calcium chelation. In addition, electron microscopic findings revealed selective inhibition of neurofilament degradation, but not microtubule depolymerization or mitochondrial swelling, by the inhibition of calpain and proteasomes. Taken together, our findings suggest that calcium increase and subsequent proteasome activation are an essential initiator of neurofilament degradation in Wallerian degeneration.

Preserve and protect: maintaining axons within functional circuits

During development, neural circuits are initially generated by exuberant innervation and are rapidly refined by selective preservation and elimination of axons. The establishment and maintenance of functional circuits therefore requires coordination of axon survival and degeneration pathways. Both developing and mature circuits rely on interdependent mitochondrial and cytoskeletal components to maintain axonal health and homeostasis; injury or diseases that impinge on these components frequently cause pathologic axon loss. Here, we review recent findings that identify mechanisms of axonal preservation in the contexts of development, injury, and disease.

The relationship between frontal lobe lesions, course of post-stroke depression, and 1-year prognosis in patients with first-ever ischemic stroke

BACKGROUND AND PURPOSE

Most studies on post-stroke depression (PSD) have focused on a certain time point after stroke instead of the time course of PSD. The aim of this study was to determine the relationship between frontal lobe lesions, course of PSD over a year following the stroke onset, and the 1-year prognosis in patients with first-ever ischemic stroke.

METHODS

A total of 1067 patients from the prospective cohort study on the incidence and outcome of patients with post stroke depression in China who were diagnosed with first-ever ischemic stroke and attended 4 follow-up visits at 14±2 days, 3 months, 6 months, and 1 year after stroke onset, were enrolled in the study. PSD was diagnosed according to DSM-IV. The course of PSD was divided into the following two categories: persistent/recurrent depression and no/transient depression. Patients with any ischemic lesion responsible for the indexed stroke event located in the frontal lobe were defined as patients with frontal lobe lesions. Modified Rankin Scale (mRS) ≥2 at 1-year was considered to be poor prognosis.

RESULTS

There were 109 patients with and 958 patients without frontal lobe lesions that formed the frontal lobe (FL) and no-frontal lobe (NFL) groups, respectively. After adjusting for confounding variables, frontal lobe lesion was significantly associated with persistent/recurrent PSD (OR 2.025, 95%CI 1.039-3.949). Overall, 32.7% of patients in the FL group had poor prognosis at 1- year compared with 22.7% in the NFL group (P = 0.021). Compared with no/transient depression, persistent/recurrent depression was found to be an independent predictor of poor prognosis at 1-year both in FL and NFL groups.

CONCLUSIONS

Long-term and periodical screening, evaluation and treatment are needed for PSD after the onset of ischemic stroke, particularly for patients with frontal lobe infarction.

Antenatal antioxidant treatment with melatonin to decrease newborn neurodevelopmental deficits and brain injury caused by fetal growth restriction

Fetal intrauterine growth restriction (IUGR) is a serious pregnancy complication associated with increased rates of perinatal morbidity and mortality, and ultimately with long-term neurodevelopmental impairments. No intervention currently exists that can improve the structure and function of the IUGR brain before birth. Here, we investigated whether maternal antenatal melatonin administration reduced brain injury in ovine IUGR. IUGR was induced in pregnant sheep at 0.7 gestation and a subset of ewes received melatonin via intravenous infusion until term. IUGR, IUGR + melatonin (IUGR + MLT) and control lambs were born naturally, neonatal behavioral assessment was used to examine neurological function and at 24 hr after birth the brain was collected for the examination of neuropathology. Compared to control lambs, IUGR lambs took significantly longer to achieve normal neonatal lamb behaviors, such as standing and suckling. IUGR brains showed widespread cellular and axonal lipid peroxidation, and white matter hypomyelination and axonal damage. Maternal melatonin administration ameliorated oxidative stress, normalized myelination and rescued axonopathy within IUGR lamb brains, and IUGR + MLT lambs demonstrated significant functional improvements including a reduced time taken to attach to and suckle at the udder after birth. Based on these observations, we began a pilot clinical trial of oral melatonin administration to women with an IUGR fetus. Maternal melatonin was not associated with adverse maternal or fetal effects and it significantly reduced oxidative stress, as evidenced by reduced malondialdehyde levels, in the IUGR + MLT placenta compared to IUGR alone. Melatonin should be considered for antenatal neuroprotective therapy in human IUGR.

Concepts for regulation of axon integrity by enwrapping glia

Long axons and their enwrapping glia (EG; Schwann cells (SCs) and oligodendrocytes (OLGs)) form a unique compound structure that serves as conduit for transport of electric and chemical information in the nervous system. The peculiar cytoarchitecture over an enormous length as well as its substantial energetic requirements make this conduit particularly susceptible to detrimental alterations. Degeneration of long axons independent of neuronal cell bodies is observed comparatively early in a range of neurodegenerative conditions as a consequence of abnormalities in SCs and OLGs . This leads to the most relevant disease symptoms and highlights the critical role that these glia have for axon integrity, but the underlying mechanisms remain elusive. The quest to understand why and how axons degenerate is now a crucial frontier in disease-oriented research. This challenge is most likely to lead to significant progress if the inextricable link between axons and their flanking glia in pathological situations is recognized. In this review I compile recent advances in our understanding of the molecular programs governing axon degeneration, and mechanisms of EG’s non-cell autonomous impact on axon-integrity. A particular focus is placed on emerging evidence suggesting that EG nurture long axons by virtue of their intimate association, release of trophic substances, and neurometabolic coupling. The correction of defects in these functions has the potential to stabilize axons in a variety of neuronal diseases in the peripheral nervous system and central nervous system (PNS and CNS).

Proteoglycan-mediated axon degeneration corrects pretarget topographic sorting errors

Proper arrangement of axonal projections into topographic maps is crucial for brain function, especially in sensory systems. An important mechanism for map formation is pretarget axon sorting, in which topographic ordering of axons appears in tracts before axons reach their target, but this process remains poorly understood. Here, we show that selective axon degeneration is used as a correction mechanism to eliminate missorted axons in the optic tract during retinotectal development in zebrafish. Retinal axons are not precisely ordered during initial pathfinding but become corrected later, with missorted axons selectively fragmenting and degenerating. We further show that heparan sulfate is required non-cell-autonomously to correct missorted axons and that restoring its synthesis at late stages in a deficient mutant is sufficient to restore topographic sorting. These findings uncover a function for developmental axon degeneration in ordering axonal projections and identify heparan sulfate as a key regulator of that process.

Post-stroke depression: mechanisms, translation and therapy

The interaction between depression and stroke is highly complex. Post-stroke depression (PSD) is among the most frequent neuropsychiatric consequences of stroke. Depression also negatively impacts stroke outcome with increased morbidity, mortality and poorer functional recovery. Antidepressants such as the commonly prescribed selective serotonin reuptake inhibitors improve stroke outcome, an effect that may extend far beyond depression, e.g., to motor recovery. The main biological theory of PSD is the amine hypothesis. Conceivably, ischaemic lesions interrupt the projections ascending from midbrain and brainstem, leading to a decreased bioavailability of the biogenic amines – serotonin (5HT), dopamine (DA) and norepinephrine (NE). Acetylcholine would also be involved. So far, preclinical and translational research on PSD is largely lacking. The implementation and characterization of suitable animal models is clearly a major prerequisite for deeper insights into the biological basis of post-stroke mood disturbances. Equally importantly, experimental models may also pave the way for the discovery of novel therapeutic targets. If we cannot prevent stroke, we shall try to limit its long-term consequences. This review therefore presents animal models of PSD and summarizes potential underlying mechanisms including genomic signatures, neurotransmitter and neurotrophin signalling, hippocampal neurogenesis, cellular plasticity in the ischaemic lesion, secondary degenerative changes, activation of the hypothalamo-pituitary-adrenal (HPA) axis and neuroinflammation. As stroke is a disease of the elderly, great clinical benefit may especially accrue from deciphering and targeting basic mechanisms underlying PSD in aged animals.

Protection of mouse retinal ganglion cell axons and soma from glaucomatous and ischemic injury by cytoplasmic overexpression of Nmnat1

PURPOSE

The Wlds mutation affords protection of retinal ganglion cell (RGC) axons in retinal ischemia and in inducible and hereditary preclinical models of glaucoma. We undertook the present study to determine whether the Nmnat1 portion of the chimeric protein provides axonal and somatic protection of RGCs in models of ischemia and glaucoma, particularly when localized to nonnuclear regions of the cell.

METHODS

The survival and integrity of RGC axons and soma from transgenic mice with confirmed cytoplasmic overexpression of Nmnat1 in retina and optic nerve (cytNmnat1-Tg mice) were examined in the retina and postlaminar optic nerve 4 days following acute retinal ischemia, and 3 weeks following the chronic elevation of intraocular pressure.

RESULTS

Ischemia- and glaucoma-induced disruptions of proximal segments of RGC axons that comprise the nerve fiber layer in wild-type mice were both robustly abrogated in cytNmnat1-Tg mice. More distal portions of RGC axons within the optic nerve were also protected from glaucomatous disruption in the transgenic mice. In both disease models, Nmnat1 overexpression in extranuclear locations significantly enhanced the survival of RGC soma.

CONCLUSIONS

Overexpression of Nmnat1 in the cytoplasm and axons of RGCs robustly protected against both ischemic and glaucomatous loss of RGC axonal integrity, as well as loss of RGC soma. These findings reflect the more pan-cellular protection of CNS neurons that is realized by cytoplasmic Nmnat1 expression, and thus provide a therapeutic strategy for protecting against retinal neurodegeneration, and perhaps other CNS neurodegenerative diseases as well.

Epigenetic regulation of axonal growth of Drosophila pacemaker cells by histone acetyltransferase tip60 controls sleep

Tip60 is a histone acetyltransferase (HAT) enzyme that epigenetically regulates genes enriched for neuronal functions through interaction with the amyloid precursor protein (APP) intracellular domain. However, whether Tip60-mediated epigenetic dysregulation affects specific neuronal processes in vivo and contributes to neurodegeneration remains unclear. Here, we show that Tip60 HAT activity mediates axonal growth of the Drosophila pacemaker cells, termed "small ventrolateral neurons" (sLNvs), and their production of the neuropeptide pigment-dispersing factor (PDF) that functions to stabilize Drosophila sleep-wake cycles. Using genetic approaches, we show that loss of Tip60 HAT activity in the presence of the Alzheimer's disease-associated APP affects PDF expression and causes retraction of the sLNv synaptic arbor required for presynaptic release of PDF. Functional consequence of these effects is evidenced by disruption of the sleep-wake cycle in these flies. Notably, overexpression of Tip60 in conjunction with APP rescues these sleep-wake disturbances by inducing overelaboration of the sLNv synaptic terminals and increasing PDF levels, supporting a neuroprotective role for dTip60 in sLNv growth and function under APP-induced neurodegenerative conditions. Our findings reveal a novel mechanism for Tip60 mediated sleep-wake regulation via control of axonal growth and PDF levels within the sLNv-encompassing neural network and provide insight into epigenetic-based regulation of sleep disturbances observed in neurodegenerative diseases like Alzheimer's disease.

Role of kinesin-1 in the pathogenesis of SPG10, a rare form of hereditary spastic paraplegia

Molecular protein motors play key roles in processes such as intracellular cargo transport and brain wiring, and failure of function can give rise to serious diseases. Kinesin-1, a member of the kinesin superfamily (also known as KIFs) is a two-headed motor protein that uses energy derived from ATP hydrolysis to transport diverse types of intracellular cargo toward the plus-ends of microtubules within axons. Recent studies at the level of a single molecule have provided extensive knowledge on how kinesin-1 moves along microtubules. Further elucidation of kinesin-1 movement may shed light on its influence on axon generation, thereby leading to therapies for diseases such as spastic paraplegia type 10 (SPG10), the subject of this review. SPG10 is an autosomal dominant form of hereditary spastic paraplegia caused by mutations in KIF5A, which encodes one of the isoforms of kinesin-1 (KIF5A, KIF5B, and KIF5C). Although little is known about the cargo of KIF5A, a recent study revealed an axonal transport defect of mitochondria in a KIF5A (-/-) mouse model. This review discusses the consensus moving model of kinesin-1 and the pathogenicity of SPG10 caused by defective KIF5A function.

Chemical genetic-mediated spatial regulation of protein expression in neurons reveals an axonal function for wld(s)

The degeneration of axons is the underlying pathological process of several neurological disorders. The Wallerian degeneration (Wld(S)) slow protein, which is primarily nuclear, markedly inhibits axonal degeneration. Contradictory models have been proposed to explain its mechanism, including a role in the nucleus, where it affects gene transcription, and roles outside the nucleus, where it regulates unknown effectors. To determine which pool of Wld(S) accounts for its axon-protective effects, we developed a strategy to control the spatial expression of proteins within neurons. This strategy couples a chemical genetic method to control protein stability with microfluidic culturing. Using neurons that are selectively deficient in Wld(S) in axons, we show that the axonal pool of Wld(S) is necessary for protection from axon degeneration. These results implicate an axonal pathway regulated by Wld(S) that controls axon degeneration.

Wallerian degeneration: the innate-immune response to traumatic nerve injury

Traumatic injury to peripheral nerves results in the loss of neural functions. Recovery by regeneration depends on the cellular and molecular events of Wallerian degeneration that injury induces distal to the lesion site, the domain through which severed axons regenerate back to their target tissues. Innate-immunity is central to Wallerian degeneration since innate-immune cells, functions and molecules that are produced by immune and non-immune cells are involved. The innate-immune response helps to turn the peripheral nerve tissue into an environment that supports regeneration by removing inhibitory myelin and by upregulating neurotrophic properties. The characteristics of an efficient innate-immune response are rapid onset and conclusion, and the orchestrated interplay between Schwann cells, fibroblasts, macrophages, endothelial cells, and molecules they produce. Wallerian degeneration serves as a prelude for successful repair when these requirements are met. In contrast, functional recovery is poor when injury fails to produce the efficient innate-immune response of Wallerian degeneration.

Abeta oligomers cause localized Ca(2+) elevation, missorting of endogenous Tau into dendrites, Tau phosphorylation, and destruction of microtubules and spines

Aggregation of amyloid-beta (Abeta) and Tau protein are hallmarks of Alzheimer's disease (AD), and according to the Abeta-cascade hypothesis, Abeta is considered toxic for neurons and Tau a downstream target of Abeta. We have investigated differentiated primary hippocampal neurons for early localized changes following exposure to Abeta oligomers. Initial events become evident by missorting of endogenous Tau into the somatodendritic compartment, in contrast to axonal sorting in normal neurons. In missorted dendritic regions there is a depletion of spines and local increase in Ca(2+), and breakdown of microtubules. Tau in these regions shows elevated phosphorylation at certain sites diagnostic of AD-Tau (e.g., epitope of antibody 12E8, whose phosphorylation causes detachment of Tau from microtubules, and AT8 epitope), and local elevation of certain kinase activities (e.g., MARK/par-1, BRSK/SADK, p70S6K, cdk5, but not GSK3beta, JNK, MAPK). These local effects occur without global changes in Tau, tubulin, or kinase levels. Somatodendritic missorting occurs not only with Tau, but also with other axonal proteins such as neurofilaments, and correlates with pronounced depletion of microtubules and mitochondria. The Abeta-induced effects on microtubule and mitochondria depletion, Tau missorting, and loss of spines are prevented by taxol, indicating that Abeta-induced microtubule destabilization and corresponding traffic defects are key factors in incipient degeneration. By contrast, the rise in Ca(2+) levels, kinase activities, and Tau phosphorylation cannot be prevented by taxol. Incipient and local changes similar to those of Abeta oligomers can be evoked by cell stressors (e.g., H(2)O(2), glutamate, serum deprivation), suggesting some common mechanism of signaling.