Down-regulation of UNC5 homologue expression after the spinal cord injury in the adult rat

Unraveling Axon Guidance during Axotomy and Regeneration

During neuronal development and regeneration axons extend a cytoskeletal-rich structure known as the growth cone, which detects and integrates signals to reach its final destination. The guidance cues “signals” bind their receptors, activating signaling cascades that result in the regulation of the growth cone cytoskeleton, defining growth cone advance, pausing, turning, or collapse. Even though much is known about guidance cues and their isolated mechanisms during nervous system development, there is still a gap in the understanding of the crosstalk between them, and about what happens after nervous system injuries. After neuronal injuries in mammals, only axons in the peripheral nervous system are able to regenerate, while the ones from the central nervous system fail to do so. Therefore, untangling the guidance cues mechanisms, as well as their behavior and characterization after axotomy and regeneration, are of special interest for understanding and treating neuronal injuries. In this review, we present findings on growth cone guidance and canonical guidance cues mechanisms, followed by a description and comparison of growth cone pathfinding mechanisms after axotomy, in regenerative and non-regenerative animal models.

Increased expression of Netrin-4 is associated with allodynia in a trigeminal neuropathic pain model rats by infraorbital nerve injury

Neuropathic pain refers to pain caused by lesions or diseases of the somatosensory nervous system that is characteristically different from nociceptive pain. Moreover, neuropathic pain occurs in the maxillofacial region due to various factors and is treated using tricyclic antidepressants and nerve block therapy; however, some cases do not fully recover. Netrin is a secreted protein crucially involved in neural circuit formation during development, including cell migration, cell death, neurite formation, and synapse formation. Recent studies show Netrin-4 expressed in the dorsal horn of the spinal cord is associated with chronic pain. Here we found involvement of Netrin-4 in neuropathic pain in the maxillofacial region. Netrin-4, along with one of its receptors, Unc5B, are expressed in the caudal subnucleus of the trigeminal spinal tract nucleus. Inhibition of its binding by anti-Netrin-4 antibodies not only shows a behavioral analgesic effect but also neuronal activity suppression. There was increased Netrin-4 expression at 14 days after infraorbital nerve injury. Our findings suggest that Netrin-4 induced by peripheral nerve injury causes neuropathic pain via Unc5B.

Distal axotomy enhances retrograde presynaptic excitability onto injured pyramidal neurons via trans-synaptic signaling

Injury of CNS nerve tracts remodels circuitry through dendritic spine loss and hyper-excitability, thus influencing recovery. Due to the complexity of the CNS, a mechanistic understanding of injury-induced synaptic remodeling remains unclear. Using microfluidic chambers to separate and injure distal axons, we show that axotomy causes retrograde dendritic spine loss at directly injured pyramidal neurons followed by retrograde presynaptic hyper-excitability. These remodeling events require activity at the site of injury, axon-to-soma signaling, and transcription. Similarly, directly injured corticospinal neurons in vivo also exhibit a specific increase in spiking following axon injury. Axotomy-induced hyper-excitability of cultured neurons coincides with elimination of inhibitory inputs onto injured neurons, including those formed onto dendritic spines. Netrin-1 downregulation occurs following axon injury and exogenous netrin-1 applied after injury normalizes spine density, presynaptic excitability, and inhibitory inputs at injured neurons. Our findings show that intrinsic signaling within damaged neurons regulates synaptic remodeling and involves netrin-1 signaling.

Spinal cord injury can induce synaptic reorganization and remodeling in the brain. Here the authors study how severed distal axons signal back to the cell body to induce hyperexcitability, loss of inhibition and enhanced presynaptic release through netrin-1.

Role of Netrin-1 Signaling in Nerve Regeneration

Netrin-1 was the first axon guidance molecule to be discovered in vertebrates and has a strong chemotropic function for axonal guidance, cell migration, morphogenesis and angiogenesis. It is a secreted axon guidance cue that can trigger attraction by binding to its canonical receptors Deleted in Colorectal Cancer (DCC) and Neogenin or repulsion through binding the DCC/Uncoordinated (Unc5) A–D receptor complex. The crystal structures of Netrin-1/receptor complexes have recently been revealed. These studies have provided a structure based explanation of Netrin-1 bi-functionality. Netrin-1 and its receptor are continuously expressed in the adult nervous system and are differentially regulated after nerve injury. In the adult spinal cord and optic nerve, Netrin-1 has been considered as an inhibitor that contributes to axon regeneration failure after injury. In the peripheral nervous system, Netrin-1 receptors are expressed in Schwann cells, the cell bodies of sensory neurons and the axons of both motor and sensory neurons. Netrin-1 is expressed in Schwann cells and its expression is up-regulated after peripheral nerve transection injury. Recent studies indicated that Netrin-1 plays a positive role in promoting peripheral nerve regeneration, Schwann cell proliferation and migration. Targeting of the Netrin-1 signaling pathway could develop novel therapeutic strategies to promote peripheral nerve regeneration and functional recovery.

The sea lamprey UNC5 receptors: cDNA cloning, phylogenetic analysis and expression in reticulospinal neurons at larval and adult stages of development

UNC5 receptors mediate repulsive signaling of netrin on neurons. Although only one UNC5 receptor has been identified in invertebrates, four members of the UNC5 family have been identified in gnathostomes. Lampreys, together with mixynes, belong to the oldest branch of extant vertebrates, and their phylogenetic position near to the vertebrate root makes them an interesting model for understanding molecular evolution. Here, we cloned three sea lamprey UNC5 (UNC5L) receptors, and phylogenetic analyses indicated that the first two duplications of the ancestral UNC5 gene occurred before the separation of jawless and jawed vertebrates. UNC5 receptors play important roles during early development, but expression studies have also suggested that UNC5 receptors play roles in the mature nervous system. Here, we report the expression of the different UNC5L receptor transcripts in identified reticulospinal neurons of mature larval or adult sea lampreys detected by in situ hybridization in wholemounted brain preparations. In addition, an extensive expression of the UNC5 receptors was also observed in most brain regions of the adult lamprey. An increase in the types of identifiable reticulospinal neurons expressing the UNC5L receptors was observed in adults compared with larvae. Expression of UNC5 receptors at late developmental stages appears to be a shared characteristic of lampreys and mammals. In larvae, expression of UNC5L receptors was observed in reticulospinal neurons that when axotomized are known to be "bad regenerators." Results in lampreys and mammals suggest that the UNC5-Netrin axonal guidance system may play a role in limiting axonal regeneration after spinal cord injury.

Intrinsic response of thoracic propriospinal neurons to axotomy

Background

Central nervous system axons lack a robust regenerative response following spinal cord injury (SCI) and regeneration is usually abortive. Supraspinal pathways, which are the most commonly studied for their regenerative potential, demonstrate a limited regenerative ability. On the other hand, propriospinal (PS) neurons, with axons intrinsic to the spinal cord, have shown a greater regenerative response than their supraspinal counterparts, but remain relatively understudied in regards to spinal cord injury.

Results

Utilizing laser microdissection, gene-microarray, qRT-PCR, and immunohistochemistry, we focused on the intrinsic post-axotomy response of specifically labelled thoracic propriospinal neurons at periods from 3-days to 1-month following T9 spinal cord injury. We found a strong and early (3-days post injury, p.i) upregulation in the expression of genes involved in the immune/inflammatory response that returned towards normal by 1-week p.i. In addition, several regeneration associated and cell survival/neuroprotective genes were significantly up-regulated at the earliest p.i. period studied. Significant upregulation of several growth factor receptor genes (GFRa1, Ret, Lifr) also occurred only during the initial period examined. The expression of a number of pro-apoptotic genes up-regulated at 3-days p.i. suggest that changes in gene expression after this period may have resulted from analyzing surviving TPS neurons after the cell death of the remainder of the axotomized TPS neuronal population.

Conclusions

Taken collectively these data demonstrate that thoracic propriospinal (TPS) neurons mount a very dynamic response following low thoracic axotomy that includes a strong regenerative response, but also results in the cell death of many axotomized TPS neurons in the first week after spinal cord injury. These data also suggest that the immune/inflammatory response may have an important role in mediating the early strong regenerative response, as well as the apoptotic response, since expression of all of three classes of gene are up-regulated only during the initial period examined, 3-days post-SCI. The up-regulation in the expression of genes for several growth factor receptors during the first week post-SCI also suggest that administration of these factors may protect TPS neurons from cell death and maintain a regenerative response, but only if given during the early period after injury.

Host biomarkers and biological pathways that are associated with the expression of experimental cerebral malaria in mice

Cerebral malaria (CM) is a primary cause of malaria-associated deaths among young African children. Yet no diagnostic tools are available that could be used to predict which of the children infected with Plasmodium falciparum malaria will progress to CM. We used the Plasmodium berghei ANKA murine model of experimental cerebral malaria (ECM) and high-density oligonucleotide microarray analyses to identify host molecules that are strongly associated with the clinical symptoms of ECM. Comparative expression analyses were performed with C57BL/6 mice, which have an ECM-susceptible phenotype, and with mice that have ECM-resistant phenotypes: CD8 knockout and perforin knockout mice on the C57BL/6 background and BALB/c mice. These analyses allowed the identification of more than 200 host molecules (a majority of which had not been identified previously) with altered expression patterns in the brain that are strongly associated with the manifestation of ECM. Among these host molecules, brain samples from mice with ECM expressed significantly higher levels of p21, metallothionein, and hemoglobin alpha1 proteins by Western blot analysis than mice unaffected by ECM, suggesting the possible utility of these molecules as prognostic biomarkers of CM in humans. We suggest that the higher expression of hemoglobin alpha1 in the brain may be associated with ECM and could be a source of excess heme, a molecule that is considered to trigger the pathogenesis of CM. Our studies greatly enhance the repertoire of host molecules for use as diagnostics and novel therapeutics in CM.

Netrin-1 induces proliferation of Schwann cells through Unc5b receptor

Netrin and its receptors, DCC (Deleted in Colorectal Cancer) and Unc5, are proposed to be involved in the axon guidance and neuroglial migration during development. However, accumulating evidence implies that they may also participate in the cell survival and apoptosis. Here, we show that netrin-1 induces proliferation of Schwann cells. Unc5b is the sole receptor expressed in RT4 schwannoma cells and adult primary Schwann cells, and netrin-1 and Unc5b are found to be expressed in the injured sciatic nerve. It was also found that the netrin-1-induced Schwann cell proliferation was blocked by the specific inhibition of Unc5b expression with RNAi. These data suggest that netrin-1 could be an endogenous trophic factor for Schwann cells in the injured peripheral nerves.