Activation of MAP3K DLK and LZK in Purkinje cells causes rapid and slow degeneration depending on signaling strength

Rab11 suppresses neuronal stress signaling by localizing Dual leucine zipper kinase to axon terminals for protein turnover

Dual Leucine Zipper Kinase (DLK) mediates multiple neuronal stress responses, and its expression levels are constantly suppressed to prevent excessive stress signaling. We found that Wallenda (Wnd), the Drosophila ortholog of DLK, is highly enriched in the axon terminals of Drosophila sensory neurons in vivo and that this subcellular localization is necessary for Highwire-mediated Wnd protein turnover under normal conditions. Our structure-function analysis found that Wnd palmitoylation is essential for its axon terminal localization. Palmitoylation-defective Wnd accumulated in neuronal cell bodies, exhibited dramatically increased protein expression levels, and triggered excessive neuronal stress responses. Defective intracellular transport is implicated in neurodegenerative conditions. Comprehensive dominant-negative Rab protein screening identified Rab11 as an essential factor for Wnd localization in axon terminals. Consequently, Rab11 loss-of-function increased the protein levels of Wnd and induced neuronal stress responses. Inhibiting Wnd activity significantly ameliorated neuronal loss and c-Jun N-terminal kinase signaling triggered by Rab11 loss-of-function. Taken together, these suggest that DLK proteins are constantly transported to axon terminals by Rab11 for protein turnover. Our study demonstrates how subcellular protein localization is coupled to protein turnover for neuronal stress signaling.

Highlights

Wnd is highly enriched in axon terminals.Wnd protein turnover by Hiw is restricted in the axon terminals.Protein palmitoylation of Wnd and Rab11 activity is essential for Wnd axonal localization. Rab11 mutations and defective Wnd palmitoylation impair Wnd protein turnover leading to increased Wnd protein levels and neuronal loss. Inhibiting Wnd activity mitigates neuronal stress response caused by Rab11 loss-of-function.

Promoting regeneration while blocking cell death preserves motor neuron function in a model of ALS

Amyotrophic lateral sclerosis (ALS) is a devastating and fatal neurodegenerative disease of motor neurons with very few treatment options. We had previously found that motor neuron degeneration in a mouse model of ALS can be delayed by deleting the axon damage sensor MAP3K12 or dual leucine zipper kinase (DLK). However, DLK is also involved in axon regeneration, prompting us to ask whether combining DLK deletion with a way to promote axon regeneration would result in greater motor neuron protection. To achieve this, we used a mouse line that constitutively expresses ATF3, a master regulator of regeneration in neurons. Although there is precedence for each individual strategy in the SOD1G93A mouse model of ALS, these have not previously been combined. By several lines of evidence including motor neuron electrophysiology, histology and behaviour, we observed a powerful synergy when combining DLK deletion with ATF3 expression. The combinatorial strategy resulted in significant protection of motor neurons with fewer undergoing cell death, reduced axon degeneration and preservation of motor function and connectivity to muscle. This study provides a demonstration of the power of combinatorial therapy to treat neurodegenerative disease.

An induced pluripotent stem cell-based model identifies molecular targets of vincristine neurotoxicity

To model peripheral nerve degeneration and investigate molecular mechanisms of neurodegeneration, we established a cell system of induced pluripotent stem cell (iPSC)-derived sensory neurons exposed to vincristine, a drug that frequently causes chemotherapy-induced peripheral neuropathy. Sensory neurons differentiated from iPSCs exhibit distinct neurochemical patterns according to the immunocytochemical phenotypes, and gene expression of peripherin (PRPH, hereafter referred to as Peri) and neurofilament heavy chain (NEFH, hereafter referred to as NF). The majority of iPSC-derived sensory neurons were PRPH positive/NEFH negative, i.e. Peri(+)/NF(-) neurons, whose somata were smaller than those of Peri(+)/NF(+) neurons. On exposure to vincristine, projections from the cell body of a neuron, i.e. neurites, were degenerated quicker than somata, the lethal concentration to kill 50% (LC50) of neurites being below the LC50 for somata, consistent with the clinical pattern of length-dependent neuropathy. We then examined the molecular expression in the MAP kinase signaling pathways of, extracellular signal-regulated kinases 1/2 (MAPK1/3, hereafter referred to as ERK), p38 mitogen-activated protein kinases (MAPK11/12/13/14, hereafter referred to as p38) and c-Jun N-terminal kinases (MAPK8/9/10, hereafter referred to as JNK). Regarding these three cascades, only phosphorylation of JNK was upregulated but not that of p38 or ERK1/2. Furthermore, vincristine-treatment resulted in impaired autophagy and reduced autophagic flux. Rapamycin-treatment reversed the effect of impaired autophagy and JNK activation. These results not only established a platform to study peripheral degeneration of human neurons but also provide molecular mechanisms for neurodegeneration with the potential for therapeutic targets.

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.

Transcriptomes of Injured Lamprey Axon Tips: Single-Cell RNA-Seq Suggests Differential Involvement of MAPK Signaling Pathways in Axon Retraction and Regeneration after Spinal Cord Injury

Axotomy in the CNS activates retrograde signals that can trigger regeneration or cell death. Whether these outcomes use different injury signals is not known. Local protein synthesis in axon tips plays an important role in axon retraction and regeneration. Microarray and RNA-seq studies on cultured mammalian embryonic or early postnatal peripheral neurons showed that axon growth cones contain hundreds to thousands of mRNAs. In the lamprey, identified reticulospinal neurons vary in the probability that their axons will regenerate after axotomy. The bad regenerators undergo early severe axon retraction and very delayed apoptosis. We micro-aspirated axoplasms from 10 growing, 9 static and 5 retracting axon tips of spinal cord transected lampreys and performed single-cell RNA-seq, analyzing the results bioinformatically. Genes were identified that were upregulated selectively in growing (n = 38), static (20) or retracting tips (18). Among them, map3k2, csnk1e and gtf2h were expressed in growing tips, mapk8(1) was expressed in static tips and prkcq was expressed in retracting tips. Venn diagrams revealed more than 40 components of MAPK signaling pathways, including jnk and p38 isoforms, which were differentially distributed in growing, static and/or retracting tips. Real-time q-PCR and immunohistochemistry verified the colocalization of map3k2 and csnk1e in growing axon tips. Thus, differentially regulated MAPK and circadian rhythm signaling pathways may be involved in activating either programs for axon regeneration or axon retraction and apoptosis.

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.