Loss of Neuropilin2a/b or Sema3fa alters olfactory sensory axon dynamics and protoglomerular targeting

Background Olfactory Sensory Neuron (OSN) axons project from the zebrafish olfactory epithelium to reproducible intermediate target locations in the olfactory bulb called protoglomeruli at early stages in development. Two classes of OSNs expressing either OMP or TRPC2 exclusively target distinct, complementary protoglomeruli. Using RNAseq, we identified axon guidance receptors nrp2a and nrp2b, and their ligand sema3fa, as potential guidance factors that are differentially expressed between these two classes of OSNs. Methods To investigate their role in OSN axon guidance, we assessed the protoglomerular targeting fidelity of OSNs labeled by OMP:RFP and TRPC2:Venus transgenes in nrp2a, nrp2b, or sema3fa mutants. We used double mutant and genetic interaction experiments to interrogate the relationship between the three genes. We used live time-lapse imaging to compare the dynamic behaviors of OSN growth cones during protoglomerular targeting in heterozygous and mutant larvae. Results The fidelity of protoglomerular targeting of TRPC2-class OSNs is degraded in nrp2a, nrp2b, or sema3fa mutants, as axons misproject into OMP-specific protoglomeruli and other ectopic locations in the bulb. These misprojections are further enhanced in nrp2a;nrp2b double mutants suggesting that nrp2s work at least partially in parallel in the same guidance process. Results from genetic interaction experiments are consistent with sema3fa acting in the same biological pathway as both nrp2a and nrp2b. Live time-lapse imaging was used to examine the dynamic behavior of TRPC2-class growth cones in nrp2a mutants compared to heterozygous siblings. Some TRPC2-class growth cones ectopically enter the dorsal-medial region of the bulb in both groups, but in fully mutant embryos, they are less likely to correct the error through retraction. The same result was observed when TRPC2-class growth cone behavior was compared between sema3fa heterozygous and sema3fa mutant larvae. Conclusions Our results suggest that nrp2a and nrp2b expressed in TRPC2-class OSNs help prevent their mixing with axon projections in OMP-specific protoglomeruli, and further, that sema3fa helps to exclude TRPC2-class axons by repulsion from the dorsal-medial bulb.

c-Kit mediates cutaneous sensory axon innervation and multi-kinase inhibitor-induced neurotoxicity

Peripheral somatosensory neurons innervate the skin and sense the environment. Loss of skin innervation, often caused by the dying back of distal somatosensory axons, is a common side effect of drug-induced peripheral neuropathies (DIPNs) and results in pain and sensory dysfunction. Targeted cancer therapies frequently employ multi-kinase inhibitor (MKI) drugs that each block multiple receptor tyrosine kinases. Many MKIs produce DIPNs but the molecular targets and cellular mechanisms underlying these are unknown. We performed live-imaging of cutaneous somatosensory axons in larval zebrafish during treatment with several MKIs known to induce DIPNs, and observed axonal retraction consistent with a dying back pathology. These results were replicated in mouse somatosensory neurons. Genetic knockout of potential MKI targets identified c-Kit receptor as a regulator of sensory axon innervation and a major target of these MKIs mediating loss of axonal density. In both fish and mammals, Kit receptor is expressed in cutaneous somatosensory neurons and its ligand, Kitlg, is expressed in the skin. Mosaic misexpression of Kitlg in the skin induced dramatic increases in local sensory axon density, suggesting an important role for Kit signaling in cutaneous axon growth and maintenance. Immunostaining and structure-function analysis revealed Src, a downstream Kit target, mediates Kits role in cutaneous axon innervation and MKI neurotoxicity. Our data shows that the Kit-Src signaling pathway has a major role in cutaneous sensory axon innervation and is a potential therapeutic target to address DIPNs caused by MKIs and other compounds.

The MAP3Ks DLK and LZK direct diverse responses to axon damage in zebrafish peripheral neurons

The 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 (MN) and touch-sensing neurons in live zebrafish, precisely cut their axons with a laser, and assessed the ability of mutant axons to regenerate. 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.

Co-targeting myelin inhibitors and CSPGs enhances sensory axon regeneration within, but not into, the spinal cord

ABSTRACTA major barrier to intraspinal regeneration after dorsal root (DR) injury is the DR entry zone (DREZ), the CNS/PNS interface. DR axons stop regenerating at the DREZ, even if regenerative capacity is increased by a conditioning lesion. This potent blockade has long been attributed to myelin-associated inhibitors and CSPGs, but incomplete lesions and conflicting reports have hampered conclusive agreement. Here we evaluated DR regeneration in adult mice, using novel strategies to facilitate complete lesions and comprehensive analyses, selective tracing of proprio-/mechanoreceptive axons with AAV2, and genetic or viral targeting of Nogo, MAG, OMgp, CSPGs and GDNF. Simultaneously eliminating Nogo/MAG/OMgp elicited little intraspinal penetration of DR axons, even with additional removal of CSPGs and a conditioning lesion. Their absence, however, synergistically enhanced GDNF-elicited intraspinal regeneration. We conclude that myelin inhibitors and CSPGs constrain intraspinal regrowth of DR axons, but that they are not the primary mechanism(s) stopping axons at the DREZ.

N6-methyladenine DNA demethylase ALKBH1 regulates mammalian axon regeneration

Recent studies have shown that DNA N6-methyladenine (N6-mA) modification is emerging to be a novel and important epigenetic regulator of mammalian gene transcription. Several studies demonstrated DNA N6-mA in human or rodents was regulated by methyltransferase N6AMT1 and demethylase ALKBH1. Moreover, studies in mouse brain or human glioblastoma cells showed that reduced level of N6-mA or higher level of ALKBH1 was correlated with up regulated levels of genes associated with neuronal development. We thus investigated the functional roles of ALKBH1 in sensory axon regeneration. Our results showed that ALKBH1 regulated the level of N6-mA in sensory neurons, and upon peripheral nerve injury ALKBH1 was up regulated in mouse sensory neurons. Functionally, knocking down ALKBH1 in sensory neurons resulted in reduced axon regeneration in vitro and in vivo, which could be rescued by simultaneously knocking down N6AMT1. Moreover, knocking down ALKBH1 led to decreased levels of many neurodevelopment regulatory genes, including neuritin that is well known to enhance axon growth and regeneration. Our study not only revealed a novel physiological function of DNA N6-mA, but also identified a new epigenetic mechanism regulating mammalian axon regeneration.Significance StatementThe study demonstrated that DNA N6-methyladenine (N6-mA) modification played important roles in regulation of sensory axon regeneration, likely through controlling the expression of neurodevelopmental associated genes. The results will add new evidence about the physiological function of DNA N6-mA and its regulatory demethylase ALKBH1 in neurons.

Inhibition of PTEN activity promotes IB4-positive sensory neuronal axon growth

Traumatic nerve injuries have become a common clinical problem, and axon regeneration is a critical process in the successful functional recovery of the injured nervous system. In this study, we found that peripheral axotomy reduce total PTEN expression in adult sensory neurons, however, it did not alter the expression level of PTEN in IB4-positive sensory neurons. Additionally, our results indicate that the artificial inhibition of PTEN markedly promotes adult sensory axon regeneration, including IB4-positive neuronal axon growth. Thus, our results provide strong evidence that PTEN is a prominent repressor of adult sensory axon regeneration, especially in IB4-positive neurons.

NeuroHeal Treatment Alleviates Neuropathic Pain and Enhances Sensory Axon Regeneration

Peripheral nerve injury (PNI) leads to the loss of motor, sensory, and autonomic functions, and often triggers neuropathic pain. During the last years, many efforts have focused on finding new therapies to increase axonal regeneration or to alleviate painful conditions. Still only a few of them have targeted both phenomena. Incipient or aberrant sensory axon regeneration is related to abnormal unpleasant sensations, such as hyperalgesia or allodynia. We recently have discovered NeuroHeal, a combination of two repurposed drugs; Acamprosate and Ribavirin. NeuroHeal is a neuroprotective agent that also enhances motor axon regeneration after PNI. In this work, we investigated its effect on sensory fiber regeneration and PNI-induced painful sensations in a rat model of spare nerve injury and nerve crush. The follow up of the animals showed that NeuroHeal treatment reduced the signs of neuropathic pain in both models. Besides, the treatment favored sensory axon regeneration, as observed in dorsal root ganglion explants. Mechanistically, the effects observed in vivo may improve the resolution of cell-protective autophagy. Additionally, NeuroHeal treatment modulated the P2X4-BDNF-KCC2 axis, which is an essential driver of neuropathic pain. These data open a new therapeutic avenue based on autophagic modulation to foster endogenous regenerative mechanisms and reduce the appearance of neuropathic pain in PNI.