Targeted disruption of dual leucine zipper kinase and leucine zipper kinase promotes neuronal survival in a model of diffuse traumatic brain injury

Identification of inhibitors for neurodegenerative diseases targeting dual leucine zipper kinase through virtual screening and molecular dynamics simulations

Neurodegenerative diseases lead to a gradual decline in cognitive and motor functions due to the progressive loss of neurons in the central nervous system. The role of dual leucine zipper kinase (DLK) in regulating stress responses and neuronal death pathways highlights its significance as a target against neurodegenerative diseases. The non-availability of FDA-approved drugs emphasizes a need to identify novel DLK-inhibitors. We screened NPAtlas (Natural products) and MedChemExpress (FDA-approved) libraries to identify potent ATP-competitive DLK inhibitors. ADMET analyses identified four compounds (two natural products and two FDA-approved) with favourable features. Subsequently, we performed molecular dynamics simulations to examine the binding-stability and ligand-induced conformational dynamics. Molecular mechanics Poisson Boltzmann surface area (MM-PBSA) calculations demonstrated CID139591660, dithranol, and danthron having greater affinity, while CID156581477 showed lower affinity than control sunitinib. PCA and network analysis results indicated structural and network alteration post-ligand binding. Furthermore, we identified an analogue of CID156581477 using the deep learning-based web server DeLA Drug which demonstrated a higher affinity than its parent compound and the control and identified several crucial interacting residues. Overall, our study provides significant theoretical guidance for designing potent novel DLK inhibitors and compounds that could emerge as promising drug candidates against DLK following laboratory validation.

DLK-MAPK Signaling Coupled with DNA Damage Promotes Intrinsic Neurotoxicity Associated with Non-Mutated Tau

Alzheimer's disease (AD) is the most prevalent form of neurodegeneration. Despite the well-established link between tau aggregation and clinical progression, the major pathways driven by this protein to intrinsically damage neurons are incompletely understood. To model AD-relevant neurodegeneration driven by tau, we overexpressed non-mutated human tau in primary mouse neurons and observed substantial axonal degeneration and cell death, a process accompanied by activated caspase 3. Mechanistically, we detected deformation of the nuclear envelope and increased DNA damage response in tau-expressing neurons. Gene profiling analysis further revealed significant alterations in the mitogen-activated protein kinase (MAPK) pathway; moreover, inhibitors of dual leucine zipper kinase (DLK) and c-Jun N-terminal kinase (JNK) were effective in alleviating wild-type human tau-induced neurodegeneration. In contrast, mutant P301L human tau was less toxic to neurons, despite causing comparable DNA damage. Axonal DLK activation induced by wild-type tau potentiated the impact of DNA damage response, resulting in overt neurotoxicity. In summary, we have established a cellular tauopathy model highly relevant to AD and identified a functional synergy between the DLK-MAPK axis and DNA damage response in the neuronal degenerative process.

The transcriptional response of cortical neurons to concussion reveals divergent fates after injury

Traumatic brain injury (TBI) is a risk factor for neurodegeneration, however little is known about how different neuron types respond to this kind of injury. In this study, we follow neuronal populations over several months after a single mild TBI (mTBI) to assess long ranging consequences of injury at the level of single, transcriptionally defined neuronal classes. We find that the stress responsive Activating Transcription Factor 3 (ATF3) defines a population of cortical neurons after mTBI. We show that neurons that activate ATF3 upregulate stress-related genes while repressing many genes, including commonly used markers for these cell types. Using an inducible reporter linked to ATF3, we genetically mark damaged cells to track them over time. Notably, we find that a population in layer V undergoes cell death acutely after injury, while another in layer II/III survives long term and retains the ability to fire action potentials. To investigate the mechanism controlling layer V neuron death, we genetically silenced candidate stress response pathways. We found that the axon injury responsive kinase MAP3K12, also known as dual leucine zipper kinase (DLK), is required for the layer V neuron death. This work provides a rationale for targeting the DLK signaling pathway as a therapeutic intervention for traumatic brain injury. Beyond this, our novel approach to track neurons after a mild, subclinical injury can inform our understanding of neuronal susceptibility to repeated impacts.

Regulation of the Activity of the Dual Leucine Zipper Kinase by Distinct Mechanisms

The dual leucine zipper kinase (DLK) alias mitogen-activated protein 3 kinase 12 (MAP3K12) has gained much attention in recent years. DLK belongs to the mixed lineage kinases, characterized by homology to serine/threonine and tyrosine kinase, but exerts serine/threonine kinase activity. DLK has been implicated in many diseases, including several neurodegenerative diseases, glaucoma, and diabetes mellitus. As a MAP3K, it is generally assumed that DLK becomes phosphorylated and activated by upstream signals and phosphorylates and activates itself, the downstream serine/threonine MAP2K, and, ultimately, MAPK. In addition, other mechanisms such as protein-protein interactions, proteasomal degradation, dephosphorylation by various phosphatases, palmitoylation, and subcellular localization have been shown to be involved in the regulation of DLK activity or its fine-tuning. In the present review, the diverse mechanisms regulating DLK activity will be summarized to provide better insights into DLK action and, possibly, new targets to modulate DLK function.

NAD+, Axonal Maintenance, and Neurological Disease

Significance: The remarkable geometry of the axon exposes it to unique challenges for survival and maintenance. Axonal degeneration is a feature of peripheral neuropathies, glaucoma, and traumatic brain injury, and an early event in neurodegenerative diseases. Since the discovery of Wallerian degeneration (WD), a molecular program that hijacks nicotinamide adenine dinucleotide (NAD+) metabolism for axonal self-destruction, the complex roles of NAD+ in axonal viability and disease have become research priority. Recent Advances: The discoveries of the protective Wallerian degeneration slow (WldS) and of sterile alpha and TIR motif containing 1 (SARM1) activation as the main instructive signal for WD have shed new light on the regulatory role of NAD+ in axonal degeneration in a growing number of neurological diseases. SARM1 has been characterized as a NAD+ hydrolase and sensor of NAD+ metabolism. The discovery of regulators of nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) proteostasis in axons, the allosteric regulation of SARM1 by NAD+ and NMN, and the existence of clinically relevant windows of action of these signals has opened new opportunities for therapeutic interventions, including SARM1 inhibitors and modulators of NAD+ metabolism. Critical Issues: Events upstream and downstream of SARM1 remain unclear. Furthermore, manipulating NAD+ metabolism, an overdetermined process crucial in cell survival, for preventing the degeneration of the injured axon may be difficult and potentially toxic. Future Directions: There is a need for clarification of the distinct roles of NAD+ metabolism in axonal maintenance as contrasted to WD. There is also a need to better understand the role of NAD+ metabolism in axonal endangerment in neuropathies, diseases of the white matter, and the early stages of neurodegenerative diseases of the central nervous system. Antioxid. Redox Signal. 39, 1167-1184.

BAX activation in mouse retinal ganglion cells occurs in two temporally and mechanistically distinct steps

BACKGROUND

Pro-apoptotic BAX is a central mediator of retinal ganglion cell (RGC) death after optic nerve damage. BAX activation occurs in two stages including translocation of latent BAX to the mitochondrial outer membrane (MOM) and then permeabilization of the MOM to facilitate the release of apoptotic signaling molecules. As a critical component of RGC death, BAX is an attractive target for neuroprotective therapies and an understanding of the kinetics of BAX activation and the mechanisms controlling the two stages of this process in RGCs is potentially valuable in informing the development of a neuroprotective strategy.

METHODS

The kinetics of BAX translocation were assessed by both static and live-cell imaging of a GFP-BAX fusion protein introduced into RGCs using AAV2-mediated gene transfer in mice. Activation of BAX was achieved using an acute optic nerve crush (ONC) protocol. Live-cell imaging of GFP-BAX was achieved using explants of mouse retina harvested 7 days after ONC. Kinetics of translocation in RGCs were compared to GFP-BAX translocation in 661W tissue culture cells. Permeabilization of GFP-BAX was assessed by staining with the 6A7 monoclonal antibody, which recognizes a conformational change in this protein after MOM insertion. Assessment of individual kinases associated with both stages of activation was made using small molecule inhibitors injected into the vitreous either independently or in concert with ONC surgery. The contribution of the Dual Leucine Zipper-JUN-N-Terminal Kinase cascade was evaluated using mice with a double conditional knock-out of both Mkk4 and Mkk7.

RESULTS

ONC induces the translocation of GFP-BAX in RGCs at a slower rate and with less intracellular synchronicity than 661W cells, but exhibits less variability among mitochondrial foci within a single cell. GFP-BAX was also found to translocate in all compartments of an RGC including the dendritic arbor and axon. Approximately 6% of translocating RGCs exhibited retrotranslocation of BAX immediately following translocation. Unlike tissue culture cells, which exhibit simultaneous translocation and permeabilization, RGCs exhibited a significant delay between these two stages, similar to detached cells undergoing anoikis. Translocation, with minimal permeabilization could be induced in a subset of RGCs using an inhibitor of Focal Adhesion Kinase (PF573228). Permeabilization after ONC, in a majority of RGCs, could be inhibited with a broad spectrum kinase inhibitor (sunitinib) or a selective inhibitor for p38/MAPK14 (SB203580). Intervention of DLK-JNK axis signaling abrogated GFP-BAX translocation after ONC.

CONCLUSIONS

A comparison between BAX activation kinetics in tissue culture cells and in cells of a complex tissue environment shows distinct differences indicating that caution should be used when translating findings from one condition to the other. RGCs exhibit both a delay between translocation and permeabilization and the ability for translocated BAX to be retrotranslocated, suggesting several stages at which intervention of the activation process could be exploited in the design of a therapeutic strategy.

Retinal ganglion cell repopulation for vision restoration in optic neuropathy: a roadmap from the RReSTORe Consortium

Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.

BAX activation in mouse retinal ganglion cells occurs in two temporally and mechanistically distinct steps

Background Pro-apoptotic BAX is a central mediator of retinal ganglion cell (RGC) death after optic nerve damage. BAX activation occurs in two stages including translocation of latent BAX to the mitochondrial outer membrane (MOM) and then permeabilization of the MOM to facilitate the release of apoptotic signaling molecules. As a critical component of RGC death, BAX is an attractive target for neuroprotective therapies and an understanding of the kinetics of BAX activation and the mechanisms controlling the two stages of this process in RGCs is potentially valuable in informing the development of a neuroprotective strategy. Methods The kinetics of BAX translocation were assessed by both static and live-cell imaging of a GFP-BAX fusion protein introduced into RGCs using AAV2-mediated gene transfer in mice. Activation of BAX was achieved using an acute optic nerve crush (ONC) protocol. Live-cell imaging of GFP-BAX was achieved using explants of mouse retina harvested 7 days after ONC. Kinetics of translocation in RGCs were compared to GFP-BAX translocation in 661W tissue culture cells. Permeabilization of GFP-BAX was assessed by staining with the 6A7 monoclonal antibody, which recognizes a conformational change in this protein after MOM insertion. Assessment of individual kinases associated with both stages of activation was made using small molecule inhibitors injected into the vitreous either independently or in concert with ONC surgery. The contribution of the Dual Leucine Zipper-JUN-N-Terminal Kinase cascade was evaluated using mice with a double conditional knock-out of both Mkk4 and Mkk7 . Results ONC induces the translocation of GFP-BAX in RGCs at a slower rate and with less intracellular synchronicity than 661W cells, but exhibits less variability among mitochondrial foci within a single cell. GFP-BAX was also found to translocate in all compartments of an RGC including the dendritic arbor and axon. Approximately 6% of translocating RGCs exhibited retrotranslocation of BAX immediately following translocation. Unlike tissue culture cells, which exhibit simultaneous translocation and permeabilization, RGCs exhibited a significant delay between these two stages, similar to detached cells undergoing anoikis. Translocation, with minimal permeabilization could be induced in a subset of RGCs using an inhibitor of Focal Adhesion Kinase (PF573228). Permeabilization after ONC, in a majority of RGCs, could be inhibited with a broad spectrum kinase inhibitor (sunitinib) or a selective inhibitor for p38/MAPK14 (SB203580). Intervention of DLK-JNK axis signaling abrogated GFP-BAX translocation after ONC. Conclusions A comparison between BAX activation kinetics in tissue culture cells and in cells of a complex tissue environment shows distinct differences indicating that caution should be used when translating findings from one condition to the other. RGCs exhibit both a delay between translocation and permeabilization and the ability for translocated BAX to be retrotranslocated, suggesting several stages at which intervention of the activation process could be exploited in the design of a therapeutic strategy.

Deciphering the multifunctional role of dual leucine zipper kinase (DLK) and its therapeutic potential in disease

Dual leucine zipper kinase (DLK, MAP3K12), a serine/threonine protein kinase, plays a key role in neuronal development, as it regulates axon regeneration and degeneration through its downstream kinase. Importantly, DLK is closely related to the pathogenesis of numerous neurodegenerative diseases and the induction of β-cell apoptosis that leads to diabetes. In this review, we summarize the current understanding of DLK function, and then discuss the role of DLK signaling in human diseases. Furthermore, various types of small molecule inhibitors of DLK that have been published so far are described in detail in this paper, providing some strategies for the design of DLK small molecule inhibitors in the future.

Modeling Alzheimer's disease in primary neurons reveals DNA damage response coupled with MAPK-DLK signaling in wild-type tau-induced neurodegeneration

Background

Alzheimer's disease (AD) is the most prevalent form of neurodegeneration. Despite the well-established link between tau aggregation and clinical progression, the major pathways driven by this protein to intrinsically damage neurons are incompletely understood.

Methods

To model AD-relevant neurodegeneration driven by tau, we overexpressed wild-type human tau in primary mouse neurons and characterized the subsequent cellular and molecular changes. RNAseq profiling and functional investigation were performed as well. A direct comparison with a mutant human tau was conducted in detail.

Results

We observed substantial axonal degeneration and cell death associated with wild-type tau, a process accompanied by activated caspase 3. Mechanistically, we detected deformation of the nuclear envelope and increased DNA damage response in tau-expressing neurons. Gene profiling analysis further revealed significant alterations in the mitogen-activated protein kinase (MAPK) pathway; moreover, inhibitors of dual leucine zipper kinase (DLK) and c-Jun N-terminal kinase (JNK) were effective in alleviating wild-type human tau-induced neurodegeneration. In contrast, mutant P301L human tau was less toxic to neurons, despite causing comparable DNA damage. Axonal DLK activation induced by wild-type tau potentiated the impact of DNA damage response, resulting in overt neurotoxicity.

Conclusions

We have established a cellular tauopathy model highly relevant to AD and identified a functional synergy between DNA damage response and the MAPK-DLK axis in the neuronal degenerative process.

Traumatic axonopathy in spinal tracts after impact acceleration head injury: Ultrastructural observations and evidence of SARM1-dependent axonal degeneration

Traumatic axonal injury (TAI) and the associated axonopathy are common consequences of traumatic brain injury (TBI) and contribute to significant neurological morbidity. It has been previously suggested that TAI activates a highly conserved program of axonal self-destruction known as Wallerian degeneration (WD). In the present study, we utilize our well-established impact acceleration model of TBI (IA-TBI) to characterize the pathology of injured myelinated axons in the white matter tracks traversing the ventral, lateral, and dorsal spinal columns in the mouse and assess the effect of Sterile Alpha and TIR Motif Containing 1 (Sarm1) gene knockout on acute and subacute axonal degeneration and myelin pathology. In silver-stained preparations, we found that IA-TBI results in white matter pathology as well as terminal field degeneration across the rostrocaudal axis of the spinal cord. At the ultrastructural level, we found that traumatic axonopathy is associated with diverse types of axonal and myelin pathology, ranging from focal axoskeletal perturbations and focal disruption of the myelin sheath to axonal fragmentation. Several morphological features such as neurofilament compaction, accumulation of organelles and inclusions, axoskeletal flocculation, myelin degeneration and formation of ovoids are similar to profiles encountered in classical examples of WD. Other profiles such as excess myelin figures and inner tongue evaginations are more typical of chronic neuropathies. Stereological analysis of pathological axonal and myelin profiles in the ventral, lateral, and dorsal columns of the lower cervical cord (C6) segments from wild type and Sarm1 KO mice at 3 and 7 days post IA-TBI (n = 32) revealed an up to 90% reduction in the density of pathological profiles in Sarm1 KO mice after IA-TBI. Protection was evident across all white matter tracts assessed, but showed some variability. Finally, Sarm1 deletion ameliorated the activation of microglia associated with TAI. Our findings demonstrate the presence of severe traumatic axonopathy in multiple ascending and descending long tracts after IA-TBI with features consistent with some chronic axonopathies and models of WD and the across-tract protective effect of Sarm1 deletion.

FDA-Approved Kinase Inhibitors in Preclinical and Clinical Trials for Neurological Disorders

Cancers and neurological disorders are two major types of diseases. We previously developed a new concept termed "Aberrant Cell Cycle Diseases" (ACCD), revealing that these two diseases share a common mechanism of aberrant cell cycle re-entry. The aberrant cell cycle re-entry is manifested as kinase/oncogene activation and tumor suppressor inactivation, which are hallmarks of both tumor growth in cancers and neuronal death in neurological disorders. Therefore, some cancer therapies (e.g., kinase inhibition, tumor suppressor elevation) can be leveraged for neurological treatments. The United States Food and Drug Administration (US FDA) has so far approved 74 kinase inhibitors, with numerous other kinase inhibitors in clinical trials, mostly for the treatment of cancers. In contrast, there are dire unmet needs of FDA-approved drugs for neurological treatments, such as Alzheimer's disease (AD), intracerebral hemorrhage (ICH), ischemic stroke (IS), traumatic brain injury (TBI), and others. In this review, we list these 74 FDA-approved kinase-targeted drugs and identify those that have been reported in preclinical and/or clinical trials for neurological disorders, with a purpose of discussing the feasibility and applicability of leveraging these cancer drugs (FDA-approved kinase inhibitors) for neurological treatments.

Neuroprotection and Neuroregeneration Strategies Using the rNAION Model: Theory, Histology, Problems, Results and Analytical Approaches

Nonarteritic anterior ischemic optic neuropathy (NAION) is the most common cause of sudden optic nerve (ON)-related vision loss in humans. Study of this disease has been limited by the lack of available tissue and difficulties in evaluating both treatments and the window of effectiveness after symptom onset. The rodent nonarteritic anterior ischemic optic neuropathy model (rNAION) closely resembles clinical NAION in its pathophysiological changes and physiological responses. The rNAION model enables analysis of the specific responses to sudden ischemic axonopathy and effectiveness of potential treatments. However, there are anatomic and genetic differences between human and rodent ON, and the inducing factors for the disease and the model are different. These variables can result in marked differences in lesion development between the two species, as well as in the possible responses to various treatments. These caveats are discussed in the current article, as well as some of the species-associated differences that may be related to ischemic lesion severity and responses.

Protective effects of NAMPT or MAPK inhibitors and NaR on Wallerian degeneration of mammalian axons

Wallerian degeneration (WD) is a conserved axonal self-destruction program implicated in several neurological diseases. WD is driven by the degradation of the NAD+ synthesizing enzyme NMNAT2, the buildup of its substrate NMN, and the activation of the NAD+ degrading SARM1, eventually leading to axonal fragmentation. The regulation and amenability of these events to therapeutic interventions remain unclear. Here we explored pharmacological strategies that modulate NMN and NAD+ metabolism, namely the inhibition of the NMN-synthesizing enzyme NAMPT, activation of the nicotinic acid riboside (NaR) salvage pathway and inhibition of the NMNAT2-degrading DLK MAPK pathway in an axotomy model in vitro. Results show that NAMPT and DLK inhibition cause a significant but time-dependent delay of WD. These time-dependent effects are related to NMNAT2 degradation and changes in NMN and NAD+ levels. Supplementation of NAMPT inhibition with NaR has an enhanced effect that does not depend on timing of intervention and leads to robust protection up to 4 days. Additional DLK inhibition extends this even further to 6 days. Metabolite analyses reveal complex effects indicating that NAMPT and MAPK inhibition act by reducing NMN levels, ameliorating NAD+ loss and suppressing SARM1 activity. Finally, the axonal NAD+/NMN ratio is highly predictive of cADPR levels, extending previous cell-free evidence on the allosteric regulation of SARM1. Our findings establish a window of axon protection extending several hours following injury. Moreover, we show prolonged protection by mixed treatments combining MAPK and NAMPT inhibition that proceed via complex effects on NAD+ metabolism and inhibition of SARM1.

The MAP3Ks DLK and LZK Direct Diverse Responses to Axon Damage in Zebrafish Peripheral Neurons

Mitogen-activated protein kinase kinase kinases (MAP3Ks) dual leucine kinase (DLK) and leucine zipper kinase (LZK) are essential mediators of axon damage responses, but their responses are varied, complex, and incompletely understood. To characterize their functions in axon injury, we generated zebrafish mutants of each gene, labeled motor neurons (MNs) and touch-sensing neurons in live zebrafish, precisely cut their axons with a laser, and assessed the ability of mutant axons to regenerate in larvae, before sex is apparent in zebrafish. DLK and LZK were required redundantly and cell autonomously for axon regeneration in MNs but not in larval Rohon-Beard (RB) or adult dorsal root ganglion (DRG) sensory neurons. Surprisingly, in dlk lzk double mutants, the spared branches of wounded RB axons grew excessively, suggesting that these kinases inhibit regenerative sprouting in damaged axons. Uninjured trigeminal sensory axons also grew excessively in mutants when neighboring neurons were ablated, indicating that these MAP3Ks are general inhibitors of sensory axon growth. These results demonstrate that zebrafish DLK and LZK promote diverse injury responses, depending on the neuronal cell identity and type of axonal injury.SIGNIFICANCE STATEMENT The MAP3Ks DLK and LZK are damage sensors that promote diverse outcomes to neuronal injury, including axon regeneration. Understanding their context-specific functions is a prerequisite to considering these kinases as therapeutic targets. To investigate DLK and LZK cell-type-specific functions, we created zebrafish mutants in each gene. Using mosaic cell labeling and precise laser injury we found that both proteins were required for axon regeneration in motor neurons but, unexpectedly, were not required for axon regeneration in Rohon-Beard or DRG sensory neurons and negatively regulated sprouting in the spared axons of touch-sensing neurons. These findings emphasize that animals have evolved distinct mechanisms to regulate injury site regeneration and collateral sprouting, and identify differential roles for DLK and LZK in these processes.

The role of the meningeal lymphatic system in local meningeal inflammation and trigeminal nociception

A system of lymphatic vessels has been recently characterized in the meninges, with a postulated role in ‘cleaning’ the brain via cerebral fluid drainage. As meninges are the origin site of migraine pain, we hypothesized that malfunctioning of the lymphatic system should affect the local trigeminal nociception. To test this hypothesis, we studied nociceptive and inflammatory mechanisms in the hemiskull preparations (containing the meninges) of K14-VEGFR3-Ig (K14) mice lacking the meningeal lymphatic system. We recorded the spiking activity of meningeal afferents and estimated the local mast cells population, calcitonin gene-related peptide (CGRP) and cytokine levels as well as the dural trigeminal innervation in freshly-isolated hemiskull preparations from K14-VEGFR3-Ig (K14) or wild type C57BL/6 mice (WT). Spiking activity data have been confirmed in an acquired model of meningeal lymphatic dysfunction (AAV-mVEGFR3(1–4)Ig induced lymphatic ablation). We found that levels of the pro-inflammatory cytokine IL12-p70 and CGRP, implicated in migraine, were reduced in the meninges of K14 mice, while the levels of the mast cell activator MCP-1 were increased. The other migraine-related pro-inflammatory cytokines (basal and stimulated), did not differ between the two genotypes. The patterns of trigeminal innervation in meninges remained unchanged and we did not observe alterations in basal or ATP-induced nociceptive firing in the meningeal afferents associated with meningeal lymphatic dysfunction. In summary, the lack of meningeal lymphatic system is associated with a new balance between pro- and anti-migraine mediators but does not directly trigger meningeal nociceptive state.

Stress-induced vesicular assemblies of dual leucine zipper kinase are signaling hubs involved in kinase activation and neurodegeneration

Mitogen-activated protein kinases (MAPKs) drive key signaling cascades during neuronal survival and degeneration. The localization of kinases to specific subcellular compartments is a critical mechanism to locally control signaling activity and specificity upon stimulation. However, how MAPK signaling components tightly control their localization remains largely unknown. Here, we systematically analyzed the phosphorylation and membrane localization of all MAPKs expressed in dorsal root ganglia (DRG) neurons, under control and stress conditions. We found that MAP3K12/dual leucine zipper kinase (DLK) becomes phosphorylated and palmitoylated, and it is recruited to sphingomyelin-rich vesicles upon stress. Stress-induced DLK vesicle recruitment is essential for kinase activation; blocking DLK-membrane interaction inhibits downstream signaling, while DLK recruitment to ectopic subcellular structures is sufficient to induce kinase activation. We show that the localization of DLK to newly formed vesicles is essential for local signaling. Inhibition of membrane internalization blocks DLK activation and protects against neurodegeneration in DRG neurons. These data establish vesicular assemblies as dynamically regulated platforms for DLK signaling during neuronal stress responses.

A Critical Role for DLK and LZK in Axonal Repair in the Mammalian Spinal Cord

The limited ability for axonal repair after spinal cord injury underlies long-term functional impairment. Dual leucine-zipper kinase [DLK; MAP kinase kinase kinase 12; MAP3K12] is an evolutionarily conserved MAP3K implicated in neuronal injury signaling from Caenorhabditis elegans to mammals. However, whether DLK or its close homolog leucine zipper kinase (LZK; MAP3K13) regulates axonal repair in the mammalian spinal cord remains unknown. Here, we assess the role of endogenous DLK and LZK in the regeneration and compensatory sprouting of corticospinal tract (CST) axons in mice of both sexes with genetic analyses in a regeneration competent background provided by PTEN deletion. We found that inducible neuronal deletion of both DLK and LZK, but not either kinase alone, abolishes PTEN deletion-induced regeneration and sprouting of CST axons, and reduces naturally-occurring axon sprouting after injury. Thus, DLK/LZK-mediated injury signaling operates not only in injured neurons to regulate regeneration, but also unexpectedly in uninjured neurons to regulate sprouting. Deleting DLK and LZK does not interfere with PTEN/mTOR signaling, indicating that injury signaling and regenerative competence are independently controlled. Together with our previous study implicating LZK in astrocytic reactivity and scar formation, these data illustrate the multicellular function of this pair of MAP3Ks in both neurons and glia in the injury response of the mammalian spinal cord.SIGNIFICANCE STATEMENT Functional recovery after spinal cord injury is limited because of a lack of axonal repair in the mammalian CNS. Dual leucine-zipper kinase (DLK) and leucine zipper kinase (LZK) are two closely related protein kinases that have emerged as regulators of neuronal responses to injury. However, their role in axonal repair in the mammalian spinal cord has not been described. Here, we show that DLK and LZK together play critical roles in axonal repair in the mammalian spinal cord, validating them as potential targets to promote repair and recovery after spinal cord injury. In addition to regulating axonal regeneration from injured neurons, both kinases also regulate compensatory axonal growth from uninjured neurons, indicating a more pervasive role in CNS repair than originally anticipated.

Raloxifene Modulates Microglia and Rescues Visual Deficits and Pathology After Impact Traumatic Brain Injury

Mild traumatic brain injury (TBI) involves widespread axonal injury and activation of microglia, which initiates secondary processes that worsen the TBI outcome. The upregulation of cannabinoid type-2 receptors (CB2) when microglia become activated allows CB2-binding drugs to selectively target microglia. CB2 inverse agonists modulate activated microglia by shifting them away from the harmful pro-inflammatory M1 state toward the helpful reparative M2 state and thus can stem secondary injury cascades. We previously found that treatment with the CB2 inverse agonist SMM-189 after mild TBI in mice produced by focal cranial blast rescues visual deficits and the optic nerve axon loss that would otherwise result. We have further shown that raloxifene, which is Food and Drug Administration (FDA)-approved as an estrogen receptor modulator to treat osteoporosis, but also possesses CB2 inverse agonism, yields similar benefit in this TBI model through its modulation of microglia. As many different traumatic events produce TBI in humans, it is widely acknowledged that diverse animal models must be used in evaluating possible therapies. Here we examine the consequences of TBI created by blunt impact to the mouse head for visual function and associated pathologies and assess raloxifene benefit. We found that mice subjected to impact TBI exhibited decreases in contrast sensitivity and the B-wave of the electroretinogram, increases in light aversion and resting pupil diameter, and optic nerve axon loss, which were rescued by daily injection of raloxifene at 5 or 10 mg/ml for 2 weeks. Raloxifene treatment was associated with reduced M1 activation and/or enhanced M2 activation in retina, optic nerve, and optic tract after impact TBI. Our results suggest that the higher raloxifene dose, in particular, may be therapeutic for the optic nerve by enhancing the phagocytosis of axonal debris that would otherwise promote inflammation, thereby salvaging less damaged axons. Our current work, together with our prior studies, shows that microglial activation drives secondary injury processes after both impact and cranial blast TBI and raloxifene mitigates microglial activation and visual system injury in both cases. The results thus provide a strong basis for phase 2 human clinical trials evaluating raloxifene as a TBI therapy.

The internal limiting membrane: Roles in retinal development and implications for emerging ocular therapies

Basement membranes help to establish, maintain, and separate their associated tissues. They also provide growth and signaling substrates for nearby resident cells. The internal limiting membrane (ILM) is the basement membrane at the ocular vitreoretinal interface. While the ILM is essential for normal retinal development, it is dispensable in adulthood. Moreover, the ILM may constitute a significant barrier to emerging ocular therapeutics, such as viral gene therapy or stem cell transplantation. Here we take a neurodevelopmental perspective in examining how retinal neurons, glia, and vasculature interact with individual extracellular matrix constituents at the ILM. In addition, we review evidence that the ILM may impede novel ocular therapies and discuss approaches for achieving retinal parenchymal targeting of gene vectors and cell transplants delivered into the vitreous cavity by manipulating interactions with the ILM.

Inhibition of GCK-IV kinases dissociates cell death and axon regeneration in CNS neurons

Axonal injury plays a major role in many neurodegenerative diseases. The dual leucine zipper kinase (DLK) signaling pathway is a key regulator of axonal injury-induced neuronal cell death; however, DLK also has an important role in promoting axonal outgrowth. Therefore, inhibiting DLK as a therapeutic approach for neurodegenerative diseases is limited to a neuroprotective outcome without axon regeneration, prohibiting restoration of function. In fact, there are currently no strategies that provide long-term neuroprotection and axonal regeneration after injury. Here, we identified the germinal cell kinase four (GCK-IV) family of kinases as targets to maximize neuroprotection while promoting axon regeneration, making it an attractive therapeutic approach for a subset of neurodegenerative diseases.

Axon injury is a hallmark of many neurodegenerative diseases, often resulting in neuronal cell death and functional impairment. Dual leucine zipper kinase (DLK) has emerged as a key mediator of this process. However, while DLK inhibition is robustly protective in a wide range of neurodegenerative disease models, it also inhibits axonal regeneration. Indeed, there are no genetic perturbations that are known to both improve long-term survival and promote regeneration. To identify such a neuroprotective target, we conducted a set of complementary high-throughput screens using a protein kinase inhibitor library in human stem cell-derived retinal ganglion cells (hRGCs). Overlapping compounds that promoted both neuroprotection and neurite outgrowth were bioinformatically deconvoluted to identify specific kinases that regulated neuronal death and axon regeneration. This work identified the role of germinal cell kinase four (GCK-IV) kinases in cell death and additionally revealed their unexpected activity in suppressing axon regeneration. Using an adeno-associated virus (AAV) approach, coupled with genome editing, we validated that GCK-IV kinase knockout improves neuronal survival, comparable to that of DLK knockout, while simultaneously promoting axon regeneration. Finally, we also found that GCK-IV kinase inhibition also prevented the attrition of RGCs in developing retinal organoid cultures without compromising axon outgrowth, addressing a major issue in the field of stem cell-derived retinas. Together, these results demonstrate a role for the GCK-IV kinases in dissociating the cell death and axonal outgrowth in neurons and their druggability provides for therapeutic options for neurodegenerative diseases.

JNK/c-Jun-driven NLRP3 inflammasome activation in microglia contributed to retinal ganglion cells degeneration induced by indirect traumatic optic neuropathy

BACKGROUND

Indirect traumatic optic neuropathy (ITON) is a major cause of permanent loss of vision after blunt head trauma. Neuroinflammation plays a crucial role in neurodegenerative diseases. The present study concentrated on JNK/c-Jun-driven NLRP3 inflammasome activation in microglia during the degeneration of retinal ganglion cells (RGCs) in ITON.

METHODS

An impact acceleration (IA) model was employed to induce ITON, which could produce significant neurodegeneration in the visual system. Pharmacological approaches were employed to disrupt JNK and to explore whether JNK and the microglial response contribute to RGC death and axonal degeneration.

RESULTS

Our results indicated that the ITON model induced significant RGC death and axonal degeneration and activated JNK/c-Jun signaling, which could further induce the microglial response and NLRP3 inflammasome activation. Moreover, JNK disruption is sufficient to suppress NLRP3 inflammasome activation in microglia and to prevent RGC death and axonal degeneration.

CONCLUSIONS

ITON could promote JNK/c-Jun signaling, which further activates the NLRP3 inflammasome in microglia and contributes to the degeneration of axons and death of RGCs. JNK inhibition is able to suppress the inflammatory reaction and improve RGC survival. Although further work is needed to determine whether pharmacological inhibition of the NLRP3 inflammasome can prevent ITON, our findings indicated that such intervention could be promising for translational work.

The Behavioral Neuroscience of Traumatic Brain Injury

Traumatic brain injury is a calamity of various causes, pathologies, and extremely varied and often complex clinical presentations. Because of its predilection for brain systems underlying cognitive and complex behavioral operations, it may cause chronic and severe psychiatric illness that requires expert management. This is more so for the modern epidemic of athletic and military brain injuries which are dominated by psychiatric symptoms. Past medical, including psychiatric, history, and comorbidities are important and relevant for formulation and management. Traumatic brain injury is a model for other neuropsychiatric disorders and may serve as an incubator of new ideas for neurodegenerative disease.