DLK signaling in axotomized neurons triggers complement activation and loss of upstream synapses

Axotomized spinal motoneurons (MNs) lose presynaptic inputs following peripheral nerve injury; however, the cellular mechanisms that lead to this form of synapse loss are currently unknown. Here, we delineate a critical role for neuronal kinase dual leucine zipper kinase (DLK)/MAP3K12, which becomes activated in axotomized neurons. Studies with conditional knockout mice indicate that DLK signaling activation in injured MNs triggers the induction of phagocytic microglia and synapse loss. Aspects of the DLK-regulated response include expression of C1q first from the axotomized MN and then later in surrounding microglia, which subsequently phagocytose presynaptic components of upstream synapses. Pharmacological ablation of microglia inhibits the loss of cholinergic C boutons from axotomized MNs. Together, the observations implicate a neuronal mechanism, governed by the DLK, in the induction of inflammation and the removal of synapses.

Increased IP3R-3 degradation induced by acrylamide promoted Ca2+-dependent calpain activation and axon damage in rats

Occupational and environmental exposure to acrylamide (ACR) can cause selective peripheral and central nerve fiber degeneration. IP3R-3 is an important transmembrane Ca²⁺ channel on the endoplasmic reticulum (ER), previous studies have found that ACR could induce Ca²⁺-dependent calpain activation and axon injury, but the exact role of IP3R-3 in ACR neuropathy is still unclear. Here we show that ACR exposure (40mg/kg) markedly increased the ubiquitination of IP3R-3 in rat spinal cords, and promoted the degradation of IP3R-3 through the ubiquitin-proteasome pathway. Furthermore, the normal structure of ER, especially the mitochondrial associated membranes (MAMs) component, was significantly impaired in ACR neuropathy, and the ER stress pathway was activated, which indicated that the aberrant increase of cytoplasmic Ca²⁺ could be attributed the destruction of IP3R-3. Further investigation demonstrated that the proteasome inhibitor MG-132 effectively rescued the IP3R-3 loss, attenuated the intracellular Ca²⁺ increase, and reduced the axon loss of Neuron 2a (N2a) cells following ACR exposure. Moreover, the calpain inhibitor ALLN also reduced the loss of IP3R-3 and axon injury in N2a cells, but did not alleviate the Ca²⁺ increase in cytosol, supporting that the abnormal ubiquitination of IP3R-3 was the upstream of the cellular Ca²⁺ rise and axon damage in ACR neuropathy. Taken together, our results suggested that the aberrant IP3R-3 degradation played an important role in the disturbance of Ca²⁺ homeostasis and the downstream axon loss in ACR neuropathy, thus providing a potential therapeutic target for ACR neurotoxicity.

Repetitive Blast Exposure Produces White Matter Axon Damage without Subsequent Myelin Remodeling: In Vivo Analysis of Brain Injury Using Fluorescent Reporter Mice

The potential effects of blast exposure on the brain health of military personnel have raised concerns and led to increased surveillance of blast exposures. Neuroimaging studies have reported white matter abnormalities in brains of service members with a history of blast exposure. However, blast effects on white matter microstructure remain poorly understood. As a novel approach to screen for white matter effects, transgenic mice that express fluorescent reporters to sensitively detect axon damage and myelin remodeling were exposed to simulated repetitive blasts (once/day on 5 consecutive days). Axons were visualized using Thy1-YFP-16 reporter mice that express yellow fluorescent protein (YFP) in a broad spectrum of neurons. Swelling along damaged axons forms varicosities that fill with YFP. The frequency and size of axonal varicosities were significantly increased in the corpus callosum (CC) and cingulum at 3 days after the final blast exposure, versus in sham procedures. CC immunolabeling for reactive astrocyte and microglial markers was also significantly increased. NG2CreER;mTmG mice were given tamoxifen (TMX) on days 2 and 3 after the final blast to induce fluorescent labeling of newly synthesized myelin membranes, indicating plasticity and/or repair. Myelin synthesis was not altered in the CC over the intervening 4 or 8 weeks after repetitive blast exposure. These experiments show the advantages of transgenic reporter mice for analysis of white matter injury that detects subtle, diffuse axon damage and the dynamic nature of myelin sheaths. These results show that repetitive low-level blast exposures produce infrequent but significant axon damage along with neuroinflammation in white matter.

Functional Effects of Cuprizone-Induced Demyelination in the Presence of the mTOR-Inhibitor Rapamycin

Persistent demyelination has been implicated in axon damage and functional deficits underlying neurodegenerative diseases such as multiple sclerosis. The cuprizone diet model of demyelination allows for the investigation of mechanisms underlying timed and reproducible demyelination and remyelination. However, spontaneous oligodendrocyte (OL) progenitor (OPC) proliferation, OPC differentiation, and axon remyelination during cuprizone diet may convolute the understanding of remyelinating events. The Akt (a serine/threonine kinase)/mTOR (the mammalian target of rapamycin) signaling pathway in OLs regulates intermediate steps during myelination. Thus, in an effort to inhibit spontaneous remyelination, the mTOR inhibitor rapamycin has been administered during cuprizone diet. Intrigued by the potential for rapamycin to optimize the cuprizone model by producing more complete demyelination, we sought to characterize the effects of rapamycin on axonal function and myelination. Functional remyelination was assessed by callosal compound action potential (CAP) recordings along with immunohistochemistry in mice treated with rapamycin during cuprizone diet. Rapamycin groups exhibited similar myelination, but significantly increased axonal damage and inflammation compared to non-rapamycin groups. There was minimal change in CAP amplitude between groups, however, a significant decrease in conduction velocity of the slower, non-myelinated CAP component was observed in the rapamycin group relative to the non-rapamycin group. During remyelination, rapamycin groups showed a significant decrease in OPC proliferation and mature OLs, suggesting a delay in OPC differentiation kinetics. In conclusion, we question the use of rapamycin to produce consistent demyelination as rapamycin increased inflammation and axonal damage, without affecting myelination.

Experimental Traumatic Brain Injury Identifies Distinct Early and Late Phase Axonal Conduction Deficits of White Matter Pathophysiology, and Reveals Intervening Recovery

Traumatic brain injury (TBI) patients often exhibit slowed information processing speed that can underlie diverse symptoms. Processing speed depends on neural circuit function at synapses, in the soma, and along axons. Long axons in white matter (WM) tracts are particularly vulnerable to TBI. We hypothesized that disrupted axon–myelin interactions that slow or block action potential conduction in WM tracts may contribute to slowed processing speed after TBI. Concussive TBI in male/female mice was used to produce traumatic axonal injury in the corpus callosum (CC), similar to WM pathology in human TBI cases. Compound action potential velocity was slowed along myelinated axons at 3 d after TBI with partial recovery by 2 weeks, suggesting early demyelination followed by remyelination. Ultrastructurally, dispersed demyelinated axons and disorganized myelin attachment to axons at paranodes were apparent within CC regions exhibiting traumatic axonal injury. Action potential conduction is exquisitely sensitive to paranode abnormalities. Molecular identification of paranodes and nodes of Ranvier detected asymmetrical paranode pairs and abnormal heminodes after TBI. Fluorescent labeling of oligodendrocyte progenitors in NG2CreER;mTmG mice showed increased synthesis of new membranes extended along axons to paranodes, indicating remyelination after TBI. At later times after TBI, an overall loss of conducting axons was observed at 6 weeks followed by CC atrophy at 8 weeks. These studies identify a progression of both myelinated axon conduction deficits and axon–myelin pathology in the CC, implicating WM injury in impaired information processing at early and late phases after TBI. Furthermore, the intervening recovery reveals a potential therapeutic window.

SIGNIFICANCE STATEMENT Traumatic brain injury (TBI) is a major global health concern. Across the spectrum of TBI severities, impaired information processing can contribute to diverse functional deficits that underlie persistent symptoms. We used experimental TBI to exploit technical advantages in mice while modeling traumatic axonal injury in white matter tracts, which is a key pathological feature of human TBI. A combination of approaches revealed slowed and failed signal conduction along with damage to the structure and molecular composition of myelinated axons in the white matter after TBI. An early regenerative response was not sustained yet reveals a potential time window for intervention. These insights into white matter abnormalities underlying axon conduction deficits can inform strategies to improve treatment options for TBI patients.

Chronic demyelination-induced seizures

Multiple sclerosis (MS) patients are three to six times more likely to develop epilepsy compared to the rest of the population. Seizures are more common in patients with early onset or progressive forms of the disease and prognosticate rapid progression to disability and death. Gray matter atrophy, hippocampal lesions, interneuron loss, and elevated juxtacortical lesion burden have been identified in MS patients with seizures; however, translational studies aimed at elucidating the pathophysiological processes underlying MS epileptogenesis are limited. Here, we report that cuprizone-mediated chronically demyelinated (9-12weeks) mice exhibit marked changes to dorsal hippocampal electroencephalography (EEG) and evidence of overt seizure activity. Immunohistochemical (IHC) analyses within the hippocampal CA1 region revealed extensive demyelination, loss of parvalbumin (PV⁺) interneurons, widespread gliosis, and changes in aquaporin-4 (AQP4) expression. Our results suggest that chronically demyelinated mice are a valuable model with which we may begin to understand the mechanisms underlying demyelination-induced seizures.

Myelin basic protein associates with AβPP, Aβ1-42, and amyloid plaques in cortex of Alzheimer's disease brain

The goal of this study was to show that myelin and axons in cortical gray matter are damaged in Alzheimer's disease (AD) brain. Superior temporal gyrus gray matter of AD patients (9 male, 14 female) was compared to cognitively normal controls (8 male, 7 female). Myelin basic protein (MBP) and a degraded myelin basic protein complex (dMBP) were quantified by Western blot. Brain sections were immunostained for MBP, dMBP, axonal neurofilament protein (NF), autophagy marker microtubule-associated proteins 1A/B light chain 3B precursor (LC3B), amyloid-β protein precursor (AβPP), and amyloid markers amyloid β1-42 (Aβ1-42) and FSB. Co-immunoprecipitation and mass spectroscopy evaluated interaction of AβPP/Aβ1-42 with MBP/dMBP. Evidence of axonal injury in AD cortex included appearance of AβPP in NF stained axons, and NF at margins of amyloid plaques. Evidence of myelin injury in AD cortex included (1) increased dMBP in AD gray matter compared to control (p < 0.001); (2) dMBP in AD neurons; and (3) increased LC3B that co-localized with MBP. Evidence of interaction of AβPP/Aβ1-42 with myelin or axonal components included (1) greater binding of dMBP with AβPP in AD brain; (2) MBP at the margins of amyloid plaques; (3) dMBP co-localized with Aβ1-42 in the core of amyloid plaques in AD brains; and (4) interactions between Aβ1-42 and MBP/dMBP by co-immunoprecipitation and mass spectrometry. We conclude that damaged axons may be a source of AβPP. dMBP, MBP, and NF associate with amyloid plaques and dMBP associates with AβPP and Aβ1-42. These molecules could be involved in formation of amyloid plaques.