Quantification of interleukin-6 mRNA in wallerian degeneration by competitive reverse transcription polymerase chain reaction

Spinal activity of interleukin 6 mediates myelin basic protein-induced allodynia

Mechanosensory fibers are enveloped by myelin, a unique multilamellar membrane permitting saltatory neuronal conduction. Damage to myelin is thought to contribute to severe pain evoked by innocuous tactile stimulation (i.e., mechanical allodynia). Our earlier (Liu et al., 2012) and present data demonstrate that a single injection of a myelin basic protein-derived peptide (MBP84-104) into an intact sciatic nerve produces a robust and long-lasting (>30days) mechanical allodynia in female rats. The MBP84-104 peptide represents the immunodominant epitope and requires T cells to maintain allodynia. Surprisingly, only systemic gabapentin (a ligand of voltage-gated calcium channel α2δ1), but not ketorolac (COX inhibitor), lidocaine (sodium channel blocker) or MK801 (NMDA antagonist) reverse allodynia induced by the intrasciatic MBP84-104. The genome-wide transcriptional profiling of the sciatic nerve followed by the bioinformatics analyses of the expression changes identified interleukin (IL)-6 as the major cytokine induced by MBP84-104 in both the control and athymic T cell-deficient nude rats. The intrasciatic MBP84-104 injection resulted in both unilateral allodynia and unilateral IL-6 increase the segmental spinal cord (neurons and astrocytes). An intrathecal delivery of a function-blocking IL-6 antibody reduced the allodynia in part by the transcriptional effects in large-diameter primary afferents in DRG. Our data suggest that MBP regulates IL-6 expression in the nervous system and that the spinal IL-6 activity mediates nociceptive processing stimulated by the MBP epitopes released after damage or disease of the somatosensory nervous system.

Repair of the Peripheral Nerve-Remyelination that Works

In this review we summarize the events known to occur after an injury in the peripheral nervous system. We have focused on the Schwann cells, as they are the most important cells for the repair process and facilitate axonal outgrowth. The environment created by this cell type is essential for the outcome of the repair process. The review starts with a description of the current state of knowledge about the initial events after injury, followed by Wallerian degeneration, and subsequent regeneration. The importance of surgical repair, carried out as soon as possible to increase the chances of a good outcome, is emphasized throughout the review. The review concludes by describing the target re-innervation, which today is one of the most serious problems for nerve regeneration. It is clear, compiling this data, that even though regeneration of the peripheral nervous system is possible, more research in this area is needed in order to perfect the outcome.

Bilateral elevation of interleukin-6 protein and mRNA in both lumbar and cervical dorsal root ganglia following unilateral chronic compression injury of the sciatic nerve

Background

Current research implicates interleukin (IL)-6 as a key component of the nervous-system response to injury with various effects.

Methods

We used unilateral chronic constriction injury (CCI) of rat sciatic nerve as a model for neuropathic pain. Immunofluorescence, ELISA, western blotting and in situ hybridization were used to investigate bilateral changes in IL-6 protein and mRNA in both lumbar (L4-L5) and cervical (C7-C8) dorsal root ganglia (DRG) following CCI. The operated (CCI) and sham-operated (sham) rats were assessed after 1, 3, 7, and 14 days. Withdrawal thresholds for mechanical hyperalgesia and latencies for thermal hyperalgesia were measured in both ipsilateral and contralateral hind and fore paws.

Results

The ipsilateral hind paws of all CCI rats displayed a decreased threshold of mechanical hyperalgesia and withdrawal latency of thermal hyperalgesia, while the contralateral hind and fore paws of both sides exhibited no significant changes in mechanical or thermal sensitivity. No significant behavioral changes were found in the hind and fore paws on either side of the sham rats, except for thermal hypersensitivity, which was present bilaterally at 3 days. Unilateral CCI of the sciatic nerve induced a bilateral increase in IL-6 immunostaining in the neuronal bodies and satellite glial cells (SGC) surrounding neurons of both lumbar and cervical DRG, compared with those of naive control rats. This bilateral increase in IL-6 protein levels was confirmed by ELISA and western blotting. More intense staining for IL-6 mRNA was detected in lumbar and cervical DRG from both sides of rats following CCI. The DRG removed from sham rats displayed a similar pattern of staining for IL-6 protein and mRNA as found in naive DRG, but there was a higher staining intensity in SGC.

Conclusions

Bilateral elevation of IL-6 protein and mRNA is not limited to DRG homonymous to the injured nerve, but also extended to DRG that are heteronymous to the injured nerve. The results for IL-6 suggest that the neuroinflammatory reaction of DRG to nerve injury is propagated alongside the neuroaxis from the lumbar to the remote cervical segments. This is probably related to conditioning of cervical DRG neurons to injury.

Wallerian degeneration: gaining perspective on inflammatory events after peripheral nerve injury

In this review, we first provide a brief historical perspective, discussing how peripheral nerve injury (PNI) may have caused World War I. We then consider the initiation, progression, and resolution of the cellular inflammatory response after PNI, before comparing the PNI inflammatory response with that induced by spinal cord injury (SCI).In contrast with central nervous system (CNS) axons, those in the periphery have the remarkable ability to regenerate after injury. Nevertheless, peripheral nervous system (PNS) axon regrowth is hampered by nerve gaps created by injury. In addition, the growth-supportive milieu of PNS axons is not sustained over time, precluding long-distance regeneration. Therefore, studying PNI could be instructive for both improving PNS regeneration and recovery after CNS injury. In addition to requiring a robust regenerative response from the injured neuron itself, successful axon regeneration is dependent on the coordinated efforts of non-neuronal cells which release extracellular matrix molecules, cytokines, and growth factors that support axon regrowth. The inflammatory response is initiated by axonal disintegration in the distal nerve stump: this causes blood-nerve barrier permeabilization and activates nearby Schwann cells and resident macrophages via receptors sensitive to tissue damage. Denervated Schwann cells respond to injury by shedding myelin, proliferating, phagocytosing debris, and releasing cytokines that recruit blood-borne monocytes/macrophages. Macrophages take over the bulk of phagocytosis within days of PNI, before exiting the nerve by the circulation once remyelination has occurred. The efficacy of the PNS inflammatory response (although transient) stands in stark contrast with that of the CNS, where the response of nearby cells is associated with inhibitory scar formation, quiescence, and degeneration/apoptosis. Rather than efficiently removing debris before resolving the inflammatory response as in other tissues, macrophages infiltrating the CNS exacerbate cell death and damage by releasing toxic pro-inflammatory mediators over an extended period of time. Future research will help determine how to manipulate PNS and CNS inflammatory responses in order to improve tissue repair and functional recovery.

Axotomy induces axonogenesis in hippocampal neurons through STAT3

After axotomy of embryonic hippocampal neurons in vitro, some of the axotomized axons lose their identity, and new axons arise and grow. This axotomy-induced axonogenesis requires importin, suggesting that some injury-induced signals are transported via axons to elicit axonogenesis after axotomy. In this study, we show that STAT3 is activated in response to axotomy. Because STAT3 was co-immunoprecipitated with importin β in the axotomized neurons, we suggest that STAT3 is retrogradely transported as molecular cargo of importin α/β heterodimers. Indeed, inhibition of importin α binding with STAT3 resulted in the attenuation of axonogenesis. Silencing STAT3 blocked the axonogenesis, demonstrating that STAT3 is necessary for axotomy-induced axonogenesis. Furthermore, the overexpression of STAT3 enhanced axotomy-induced axonogenesis. Taken together, these results demonstrate that activation and retrograde transport of STAT3 in injured axons have key roles in the axotomy-induced axonogenesis of hippocampal neurons.

Frataxin deficiency induces Schwann cell inflammation and death

Mutations in the frataxin gene cause dorsal root ganglion demyelination and neurodegeneration, which leads to Friedreich's ataxia. However the consequences of frataxin depletion have not been measured in dorsal root ganglia or Schwann cells. We knocked down frataxin in several neural cell lines, including two dorsal root ganglia neural lines, 2 neuronal lines, a human oligodendroglial line (HOG) and multiple Schwann cell lines and measured cell death and proliferation. Only Schwann cells demonstrated a significant decrease in viability. In addition to the death of Schwann cells, frataxin decreased proliferation in Schwann, oligodendroglia, and slightly in one neural cell line. Thus the most severe effects of frataxin deficiency were on Schwann cells, which enwrap dorsal root ganglia neurons. Microarray of frataxin-deficient Schwann cells demonstrated strong activations of inflammatory and cell death genes including interleukin-6 and Tumor Necrosis Factor which were confirmed at the mRNA and protein levels. Frataxin knockdown in Schwann cells also specifically induced inflammatory arachidonate metabolites. Anti-inflammatory and anti-apoptotic drugs significantly rescued frataxin-dependent Schwann cell toxicity. Thus, frataxin deficiency triggers inflammatory changes and death of Schwann cells that is inhibitable by inflammatory and anti-apoptotic drugs.

Differential expression and potential role of SOCS1 and SOCS3 in Wallerian degeneration in injured peripheral nerve

Pro-inflammatory chemokines and cytokines play an important role in Wallerian degeneration (WD) after peripheral nerve injury. These pro-inflammatory signals are "turned-off" in a timely manner to ensure that the inflammatory response in the injured nerve is limited. The factors that regulate the turning-off of the pro-inflammatory state are not fully understood. The suppressors of cytokine signaling (SOCS) proteins are potential candidates that could limit the inflammatory response by acting to regulate cytokine signaling at the intracellular level. In this work we show that the expression SOCS1 and SOCS3 proteins differ from each other during WD in the mouse sciatic nerve after cut/ligation and crush injuries. SOCS1 is mainly expressed by macrophages and its expression is inversely correlated with phosphorylation of JAK2 and STAT3 signaling proteins and the expression of pro-inflammatory cytokines IL-1beta and TNFalpha. In addition, treatment of cut/ligated nerves, which express lower levels of SOCS1 as compared to crush injury, with a SOCS1 mimetic peptide leads to a decrease in macrophage numbers at 14 days post-injury and reduces IL-1beta mRNA expression 1 day post-injury. In contrast, SOCS3 expression is restricted mainly to Schwann cells and is negatively correlated with the expression of IL-6 and LIF. These data suggest that SOCS1 and SOCS3 may play different roles in WD and provide a better understanding of some of the potential regulatory mechanisms that may control inflammation and regeneration in the injured peripheral nerve.

Increase in inflammatory cytokines in median nerves in a rat model of repetitive motion injury

We examined cytokines in rat median nerves following performance of a high repetition reaching and grasping task at a rate of 8 reaches/min for up to 8 weeks. IL-1alpha, IL-1beta, TNF-alpha, IL-6 and IL-10 were analyzed by immunohistochemistry. Double-labeling immunohistochemistry for ED1, a marker of phagocytic macrophages, was also performed. We found increased immunoexpression of IL-6 by week 3, increases in all 5 cytokines by week 5. This response was transient as all cytokines returned to control levels by 8 weeks of performance of a high repetition negligible force task. Cytokine sources included Schwann cells, fibroblasts and phagocytic macrophages (ED1-immunopositive). These findings suggest that cytokines are involved in the pathophysiology of repetitive motion injuries in peripheral nerves.

Up‐regulation of interleukin‐6 induced by prostaglandin E 2 from invading macrophages following nerve injury: an in vivo and in vitro study

The mechanisms underlying neuropathic pain caused by nerve injury are not well understood. Inflammatory responses in injured nerves are likely to be key contributing factors in the generation and maintenance of neuropathic pain. The pro-inflammatory cytokine interleukin-6 (IL-6) is up-regulated in invading macrophages and has been implicated in the development of neuropathic pain. We previously demonstrated that invading macrophages up-regulate cyclooxygenase 2 (COX2) and prostaglandin E2 (PGE2) receptors EP1 and EP4, suggesting that PGE2 may affect macrophage function via autocrine or paracrine mechanisms. This study was undertaken to determine whether PGE2 is involved in the up-regulation of IL-6 in invading macrophages. Two weeks following partial sciatic nerve ligation, numerous IL-6 immunoreactive (IR) cell profiles were present in injured nerves. Colocalization of IL-6 with the invading macrophage marker ED1 or with COX2 was frequently observed. IL-6-IR, COX2-IR and ED1-IR cells were present only in cultures derived from injured nerve segments. PGE2 and IL-6 release from cultured cells derived from injured nerves was increased significantly compared with uninjured nerves. Non-selective and selective COX2 inhibitors suppressed PGE2 and IL-6 release. Treatment with PGE2 further enhanced IL-6 release in a concentration- and time-dependent manner. A selective EP4 receptor antagonist L-161982 was able to suppress IL-6 release, whereas an EP1 receptor antagonist, SC19220, was ineffective. Moreover, a protein kinase C inhibitor, calphostin C, dramatically suppressed IL-6 release, whereas a protein kinase A inhibitor H-89 and a Ca2+ chelator EGTA failed. Taken together, our data suggest that PGE2 is involved in mediating the up-regulation of IL-6 occurring in invading macrophages. This action is mediated through an EP4 receptor and the protein kinase C signaling pathway.

Conditioning injury-induced spinal axon regeneration fails in interleukin-6 knock-out mice

Regeneration of injured adult sensory neurons within the CNS is essentially abortive, attributable in part to lesion-induced or revealed inhibitors such as the chondroitin sulfate proteoglycans and the myelin inhibitors (Nogo-A, MAG, and OMgp). Much of this inhibition may be overcome by boosting the growth status of sensory neurons by delivering a conditioning lesion to their peripheral branches. Here, we identify a key role for the lesion-induced cytokine interleukin-6 (IL-6) in mediating conditioning lesion-induced enhanced regeneration of injured dorsal column afferents. In adult mice, conditioning injury to the sciatic nerve 1 week before bilateral dorsal column crush resulted in regeneration of dorsal column axons up to and beyond the injury site into host CNS tissue. This enhanced growth state was accompanied by an increase in the expression of the growth-associated protein GAP43 in preinjured but not intact dorsal root ganglia (DRGs). Preconditioning injury of the sciatic nerve in IL-6 -/- mice resulted in the total failure in regeneration of dorsal column axons consequent on the lack of GAP43 upregulation after a preconditioning injury. DRGs cell counts and cholera toxin beta subunit labeling revealed that impaired regeneration in knock-out mice was unrelated to cell loss or a deficit in tracer transport. In vitro, exogenous IL-6 boosted sensory neuron growth status as evidenced by enhanced neurite extension. This effect required NGF or NT-3 but not soluble IL-6 receptor as cofactors. Evidence of conditioning lesion-enhanced growth status of DRGs cells can also be observed in vitro as an earlier and enhanced rate of neurite extension; this phenomenon fails in IL-6 -/- mice preinjured 7 d in vivo. We suggest that injury-induced IL-6 upregulation is required to promote regeneration within the CNS. Our results indicate that this is achieved through a boosted growth state of dorsal column projecting sensory neurons.

Denervated Schwann cells attract macrophages by secretion of leukemia inhibitory factor (LIF) and monocyte chemoattractant protein-1 in a process regulated by interleukin-6 and LIF

Injury to peripheral nerves results in the infiltration of immune cells, which remove axonal- and myelin-derived material. Schwann cells could play a key role in this process by regulating macrophage infiltration. We show here that medium conditioned by primary denervated Schwann cells or the Schwannoma cell line RN22 produces chemotactic activity for macrophages. The presence of blocking antibodies to macrophage chemoattractant protein-1 (MCP-1) or leukemia inhibitory factor (LIF) reduced this activity to approximately 35 and 65% of control levels, respectively, and only 15% remained in the presence of both antibodies. The presence of chemotactic LIF in Schwann cell-conditioned medium was confirmed by using cells from lif-/- mice. Although interleukin-6 (IL-6) is not itself a chemotactic factor, we found that medium from il-6-/- nerves showed only 40% of the activity secreted by wild-type nerves. Furthermore, IL-6 rapidly induced LIF mRNA in primary Schwann cells, and LIF rapidly induced MCP-1 mRNA expression. Treatment of RN22 Schwannoma cells with IL-6 or LIF enhanced the secretion of the chemotactic activity of these cells. These observations show that Schwann cells attract macrophages by secreting MCP-1 and LIF. They also provide evidence for an autocrine-signaling cascade involving IL-6, LIF, and MCP-1, which amplifies the Schwann cell-derived chemotactic signals gradually, in agreement with the delayed entry of macrophages to injured nerves.

Enhanced B7 costimulatory molecule expression in inflammatory human sural nerve biopsies

OBJECTIVES

To define the role of the costimulatory molecules B7-1 and B7-2 in inflammatory disorders of the peripheral nervous system. B7 molecules are essential for effective antigen presentation and may determine the differentiation of T cells into a Th-1 or Th-2 phenotype, thus modulating immune response and disease course.

METHODS

Forty nine sural nerve biopsies from patients with neuroborreliosis, Guillain-Barré syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), CIDP variants and hereditary neuropathies, and those with no detectable abnormality were investigated. The expression of B7-1 and B7-2 mRNA and protein was investigated by polymerase chain reaction (PCR) and immunocytochemistry.

RESULTS

B7-1 mRNA was strongly upregulated in both cases of neuroborreliosis, in two cases of GBS and one case of variant CIDP. Moderate to low levels were detected in the remaining GBS and CIDP biopsies and were rarely found in a non-inflammatory control group consisting of hereditary neuropathy and normal nerves. At the immunocytochemical level, strong expression of B7-1 protein was found in both neuroborreliosis cases, and moderate or low expression in six of eight GBS cases and seven of 17 CIDP cases investigated, whereas only one of five non-inflammatory control nerves showed staining, which was very weak. In neuroborreliosis, B7-1 protein was found very pronounced in epineurial infiltrates, whereas in GBS and CIDP, labelling was predominantly endoneurial and localised to putative macrophages. B7-2 mRNA and protein were expressed only at low levels in neuroborreliosis and selected autoimmune neuropathy cases, and were essentially absent from non-inflammatory controls.

CONCLUSIONS

B7 molecules are expressed in the peripheral nervous system and regulated during disease, and their presence in macrophages underlines the putative function of endoneurial macrophages as local antigen presenting cells in the immunopathology of peripheral nerve. B7-1 rather than B7-2 is preferentially upregulated, possibly promoting the induction of a Th-1-type T cell response within the nerve.

Potential novel uses of thalidomide: focus on palliative care

Thalidomide, after being banned from the market in the early 1960s because of the worldwide teratogenesis disaster, is currently being rediscovered because of its multiple therapeutic effects in various serious diseases and symptoms. Original studies examined the anxiolytic, mild hypnotic, anti-emetic and adjuvant analgesic properties of this drug. Subsequently, thalidomide was found to be highly effective in managing the cutaneous manifestations of leprosy (erythema nodosum leprosum) and even to be superior to aspirin (acetylsalicylic acid) in controlling leprosy-associated fever. Recent research shows promising results with thalidomide in patients with progressive bodyweight loss related to advanced cancer and HIV infection. Thalidomide therapy of diseases such as tuberculosis, sarcoidosis, aphthous ulcers in HIV syndrome and Behcet's disease, rheumatoid arthritis, multiple myeloma, graft-versus-host disease, pyoderma gangrenosum, inflammatory bowel disease, Sjögren's syndrome, lupus erythematosus and a variety of solid tumours is currently being explored. Furthermore, in preliminary studies, thalidomide has been found to be effective in several syndromes related to advanced cancer, such as the cancer cachexia syndrome, chronic nausea, insomnia, profuse sweating and pain. Whether thalidomide has a therapeutic effect on neoplastic fever has yet to be elucidated. These intriguing features make the use of the drug potentially attractive for palliative care. In addition, by a distinct mechanism of action compared with most other drugs, thalidomide offers the possibility of combined treatment with other agents with non-overlapping toxicities. The mechanism of action of thalidomide is probably based on the suppression of tumour necrosis factor-alpha and the modulation of interleukins. However, it is not possible to identify a single dominant mechanism, since the action of cytokines and the effect of thalidomide appear to be complex. This review article discusses the original uses and teratogenic effects of thalidomide within its historical context and, linking recent research at the molecular level with clinical findings, aims to provide the reader with insight into the current understanding of its biological actions, toxicities and potential benefits.

Sensory impairments and delayed regeneration of sensory axons in interleukin-6-deficient mice

Interleukin-6 (IL-6) is a multifunctional cytokine mediating inflammatory or immune reactions. Here we investigated the possible role of IL-6 in the intact or lesioned peripheral nervous system using adult IL-6 gene knockout (IL-6(-/-)) mice. Various sensory functions were tested by applying electrophysiological, morphological, biochemical, and behavioral methods. There was a 60% reduction of the compound action potential of the sensory branch of IL-6(-/-) mice as compared with the motor branch in the intact sciatic nerve. Cross sections of L5 DRG of IL-6(-/-) mice showed a shift in the relative size distribution of the neurons. The temperature sensitivity of IL-6(-/-) mice was also significantly reduced. After crush lesion of the sciatic nerve, its functional recovery was delayed in IL-6(-/-) mice as analyzed from a behavioral footprint assay. Measurements of compound action potentials 20 d after crush lesion showed that there was a very low level of recovery of the sensory but not of the motor branch of IL-6(-/-) mice. Similar results of sensory impairments were obtained with mice showing slow Wallerian degeneration (Wlds) and a delayed lesion-induced recruitment of macrophages. However, in contrast to WldS mice, in IL-6(-/-) mice we observed the characteristic lesion-induced invasion of macrophages and the upregulation of low-affinity neurotrophin receptor p75 (p75LNTR) mRNA levels identical to those of IL-6(+/+) mice. Thus, the mechanisms leading to the common sensory deficiencies were different between IL-6(-/-) and WldS mice. Altogether, the results suggest that interleukin-6 is essential to modulate sensory functions in vivo.

Nature of the retrograde signal from injured nerves that induces interleukin-6 mRNA in neurons

In previous studies, interleukin-6 was shown to be synthesized in approximately one-third of lumbar dorsal root ganglion neurons during the first week after nerve transection. In present studies, interleukin-6 mRNA was found to be induced also in axotomized facial motor neurons and sympathetic neurons. The nature of the signal that induces interleukin-6 mRNA in neurons after nerve injury was analyzed. Blocking of retrograde axonal transport by injection of colchicine into an otherwise normal nerve did not induce interleukin-6 mRNA in primary sensory neurons, but injection of colchicine into the nerve stump prevented induction of interleukin-6 mRNA by nerve transection. Therefore, it was concluded that interleukin-6 is induced by an injury factor arising from the nerve stump rather than by interruption of normal retrograde trophic support from target tissues or distal nerve segments. Next, injection into the nerve of a mast cell degranulating agent was shown to stimulate interleukin-6 mRNA in sensory neurons and systemic administration of mast cell stabilizing agents to mitigate the induction of interleukin-6 mRNA in sensory neurons after nerve injury. These data implicate mast cells as one possible source of the factors that lead to induction of interleukin-6 mRNA after nerve injury. In search of a possible function of inducible interelukin-6, neuronal death after nerve transection was assessed in mice with null deletion of the interleukin-6 gene. Retrograde death of neurons in the fifth lumbar dorsal root ganglion was 45% greater in knockout than in wild-type mice. Thus, endogenous interleukin-6 contributes to the survival of axotomized neurons.