Wallerian degeneration in C57BL/6J and A/J mice: differences in time course of neurofilament and myelin breakdown, macrophage recruitment and iNOS expression

The effect of laser radiation on glial fibrillary acidic protein and allograft inflammatory factor-1 expression in severed and surgically repaired sciatic nerve

OBJECTIVE

Determine the effect of different spectrum laser radiations on the expression of Glial Fibrillary Acidic Protein (GFAP) and allograft inflammatory factor-1 (Iba-1) in the sciatic nerve during regeneration.

METHODS

The experiment was performed on 60 lab Wistar rats weighing 200-250 g. The left sciatic nerve was severed and subsequent end-to-end epineural suturing was performed 10, 20, 30, and 45 minutes after neurotomy. Western blot and immunohistochemistry analyses were performed by means of polyclonal anti-GFAP antibodies (Thermo Fisher Scientific, USA) and anti-Iba-1 antibodies (Invitrogen, USA) 90 days after nerve repair.

RESULTS

The use of green and blue spectrum laser radiation significantly increased GFAP protein expression regardless of the time when surgical nerve repair was performed after injury. The expression of Iba-1 and tubulin after blue spectrum laser radiation with a wavelength of 470 nm was significantly higher than the control values by 5.1-11.0 times. An increase in the expression of Iba-1 and tubulin was noted when a green spectrum laser with a wavelength of 560 nm was utilized and nerve suturing was performed 10 and 20 minutes after nerve injury. The green spectrum laser with a wavelength of 520 nm had no significant effect on the expression of Iba-1 and tubulin. Morphologically, the highest proliferative reaction of glia was recorded when using a blue spectrum laser.

CONCLUSIONS

Laser radiation with blue (470 nm) and green (560 nm) spectra, promoted the activation of GFAP-positive Schwann cells and nerve regeneration. Activation of microglia is a necessary component of nerve regeneration and the content of Iba-1 represented the efficiency of regeneration.

Melatonin signalling in Schwann cells during neuroregeneration

It has widely been thought that in the process of nerve regeneration Schwann cells populate the injury site with myelinating, non–myelinating, phagocytic, repair, and mesenchyme–like phenotypes. It is now clear that the Schwann cells modify their shape and basal lamina as to accommodate re–growing axons, at the same time clear myelin debris generated upon injury, and regulate expression of extracellular matrix proteins at and around the lesion site. Such a remarkable plasticity may follow an intrinsic functional rhythm or a systemic circadian clock matching the demands of accurate timing and precision of signalling cascades in the regenerating nervous system. Schwann cells react to changes in the external circadian clock clues and to the Zeitgeber hormone melatonin by altering their plasticity. This raises the question of whether melatonin regulates Schwann cell activity during neurorepair and if circadian control and rhythmicity of Schwann cell functions are vital aspects of neuroregeneration. Here, we have focused on different schools of thought and emerging concepts of melatonin–mediated signalling in Schwann cells underlying peripheral nerve regeneration and discuss circadian rhythmicity as a possible component of neurorepair.

Effect of Botulinum Toxin Injection and Extracorporeal Shock Wave Therapy on Nerve Regeneration in Rats with Experimentally Induced Sciatic Nerve Injury

This study was designed to compare the roles of botulinum neurotoxin A (BoNT/A) and extracorporeal shock wave therapy (ESWT) in promoting the functional recovery and regeneration of injured peripheral nerves. A total of 45 six-week-old rats with sciatic nerve injury were randomly divided into two experimental groups and one control group. The experimental groups received a single session of intranerve BoNT/A or ESWT immediately after a nerve-crushing injury. The control group was not exposed to any treatment. Differentiation of Schwann cells and axonal sprouting were observed through immunofluorescence staining, ELISA, real-time PCR, and Western blot at 3, 6, and 10 weeks post-nerve injury. For clinical assessment, serial sciatic functional index analysis and electrophysiological studies were performed. A higher expression of GFAP and S100β was detected in injured nerves treated with BoNT/A or ESWT. The levels of GAP43, ATF3, and NF200 associated with axonal regeneration in the experimental groups were also significantly higher than in the control group. The motor functional improvement occurred after 7 weeks of clinical observation following BoNT/A and ESWT. Compared with the control group, the amplitude of the compound muscle action potential in the experimental groups was significantly higher from 6 to 10 weeks. Collectively, these findings indicate that BoNT/A and ESWT similarly induced the activation of Schwann cells with the axonal regeneration of and functional improvement in the injured nerve.

Gasdermin D in peripheral nerves: the pyroptotic microenvironment inhibits nerve regeneration

Wallerian degeneration (WD) involves the recruitment of macrophages for debris clearance and nerve regeneration, and the cause of the foamy macrophages that are frequently observed in peripheral transection injuries is unknown. Recent studies indicated that these foamy cells are generated by gasdermin D (GSDMD) via membrane perforation. However, whether these foamy cells are pyroptotic macrophages and whether their cell death elicits immunogenicity in peripheral nerve regeneration (PNR) remain unknown. Therefore, we used GSDMD-deficient mice and mice with deficiencies in other canonical inflammasomes to establish a C57BL/6 J mouse model of sciatic nerve transection and microanastomosis (SNTM) and evaluate the role of GSDMD-executed pyroptosis in PNR. In our study, the GSDMD^−/− mice with SNTM showed a significantly diminished number of foamy cells, better axon regeneration, and a favorable functional recovery, whereas irregular axons or gaps in the fibers were found in the wild-type (WT) mice with SNTM. Furthermore, GSDMD activation in the SNTM model was dependent on the NLRP3 inflammasome and caspase-1 activation, and GSDMD-executed pyroptosis resulted in a proinflammatory environment that polarized monocytes/macrophages toward the M1 (detrimental) but not the M2 (beneficial) phenotype. In contrast, depletion of GSDMD reversed the proinflammatory microenvironment and facilitated M2 polarization. Our results suggested that inhibition of GSDMD may be a potential treatment option to promote PNR.

Botulinum Toxin and Neuronal Regeneration after Traumatic Injury of Central and Peripheral Nervous System

Botulinum neurotoxins (BoNTs) are toxins produced by the bacteria Clostridiumbotulinum, the causing agent for botulism, in different serotypes, seven of which (A-G) are well characterized, while others, such as H or FA, are still debated. BoNTs exert their action by blocking SNARE (soluble N-ethylmale-imide-sensitive factor-attachment protein receptors) complex formation and vesicle release from the neuronal terminal through the specific cleavage of SNARE proteins. The action of BoNTs at the neuromuscular junction has been extensively investigated and knowledge gained in this field has set the foundation for the use of these toxins in a variety of human pathologies characterized by excessive muscle contractions. In parallel, BoNTs became a cosmetic drug due to its power to ward off facial wrinkles following the activity of the mimic muscles. Successively, BoNTs became therapeutic agents that have proven to be successful in the treatment of different neurological disorders, with new indications emerging or being approved each year. In particular, BoNT/A became the treatment of excellence not only for muscle hyperactivity conditions, such as dystonia and spasticity, but also to reduce pain in a series of painful states, such as neuropathic pain, lumbar and myofascial pain, and to treat various dysfunctions of the urinary bladder. This review summarizes recent experimental findings on the potential efficacy of BoNTs in favoring nerve regeneration after traumatic injury in the peripheral nervous system, such as the injury of peripheral nerves, like sciatic nerve, and in the central nervous system, such as spinal cord injury.

Acute injury in the peripheral nervous system triggers an alternative macrophage response

BACKGROUND

The activation of the immune system in neurodegeneration has detrimental as well as beneficial effects. Which aspects of this immune response aggravate the neurodegenerative breakdown and which stimulate regeneration remains an open question. To unravel the neuroprotective aspects of the immune system we focused on a model of acute peripheral nerve injury, in which the immune system was shown to be protective.

METHODS

To determine the type of immune response triggered after axotomy of the sciatic nerve, a model for Wallerian degeneration in the peripheral nervous system, we evaluated markers representing the two extremes of a type I and type II immune response (classical vs. alternative) using real-time quantitative polymerase chain reaction (RT-qPCR), western blot, and immunohistochemistry.

RESULTS

Our results showed that acute peripheral nerve injury triggers an anti-inflammatory and immunosuppressive response, rather than a pro-inflammatory response. This was reflected by the complete absence of classical macrophage markers (iNOS, IFN γ, and IL12p40), and the strong up-regulation of tissue repair markers (arginase-1, Ym1, and Trem2). The signal favoring the alternative macrophage environment was induced immediately after nerve damage and appeared to be established within the nerve, well before the infiltration of macrophages. In addition, negative regulators of the innate immune response, as well as the anti-inflammatory cytokine IL-10 were induced. The strict regulation of the immune system dampens the potential tissue damaging effects of an over-activated response.

CONCLUSIONS

We here demonstrate that acute peripheral nerve injury triggers an inherent protective environment by inducing the M2 phenotype of macrophages and the expression of arginase-1. We believe that the M2 phenotype, associated with a sterile inflammatory response and tissue repair, might explain their neuroprotective capacity. As such, shifting the neurodegeneration-induced immune responses towards an M2/Th2 response could be an important therapeutic strategy.

Botulinum neurotoxin for pain management: insights from animal models

The action of botulinum neurotoxins (BoNTs) at the neuromuscular junction has been extensively investigated and knowledge gained in this field laid the foundation for the use of BoNTs in human pathologies characterized by excessive muscle contractions. Although much more is known about the action of BoNTs on the peripheral system, growing evidence has demonstrated several effects also at the central level. Pain conditions, with special regard to neuropathic and intractable pain, are some of the pathological states that have been recently treated with BoNTs with beneficial effects. The knowledge of the action and potentiality of BoNTs utilization against pain, with emphasis for its possible use in modulation and alleviation of chronic pain, still represents an outstanding challenge for experimental research. This review highlights recent findings on the effects of BoNTs in animal pain models.

Botulinum neurotoxin type A counteracts neuropathic pain and facilitates functional recovery after peripheral nerve injury in animal models

A growing interest was recently focused on the use of Botulinum neurotoxin serotype A (BoNT/A) for fighting pain. The aim of this study was to investigate the effects of BoNT/A on neuropathic pain. It was observed that BoNT/A is able to counteract neuropathic pain induced by chronic constriction injury (CCI) to the sciatic nerve both in mice and in rats. This effect is already present after a single intraplantar (i.pl.) or intrathecal (i.t.) neurotoxin administration that significantly reduces the sciatic nerve ligation-induced mechanical allodynia in mice and rats and thermal hyperalgesia in rats. This effect was evident starting 24 h after the administration of BoNT/A and it was long-lasting, being present 81 or 25 days after i.pl. injection of the higher dose in mice (15 pg/paw) and rats (75 pg/paw), respectively, and 35 days after i.t. injection in rats (75 pg/rat). Moreover, BoNT/A-injected mice showed a quicker recovery of the walking pattern and weight bearing compared to control groups. The behavioral improvement was accompanied by structural modifications, as revealed by the expression of cell division cycle 2 (Cdc2) and growth associated protein 43 (GAP-43) regeneration associated proteins, investigated by immunofluorescence and Western blotting in the sciatic nerve, and by the immunofluorescence expression of S100β and glial fibrillary acidic protein (GFAP) Schwann cells proteins. In conclusion, the present research demonstrate long-lasting anti-allodynic and anti-hyperalgesic effects of BoNT/A in animal models of neuropathic pain together with an acceleration of regenerative processes in the injured nerve, as evidenced by both behavioral and immunohistochemistry/blotting analysis. These results may have important implications in the therapy of neuropathic pain.

A role for galectin-1 in the immune response to peripheral nerve injury

Galectin-1 (Gal1) is a multi-functional protein that has key roles in organismal growth and survival. In the adult nervous system, Gal1 promotes axonal regeneration following peripheral nerve injury. Although the mechanism by which Gal1 promotes regeneration is unclear, previous reports suggested that Gal1 acts indirectly by activating macrophages. An appropriate response of macrophages is crucial for repair of injured nerves: these immune cells remove obstructive axon and myelin debris in the distal nerve. Here we establish a role for Gal1 in the accumulation of immune cells following peripheral axotomy. We used immunohistochemistry to visualize macrophages (F4/80) in wild-type (Lgals1(+/+)) and knockout (Lgals1(-/-)) mouse sciatic nerves following injury and/or manipulation of Gal1 levels. Density of F4/80 immunoreactivity, which peaks around 3 days post-injury, was decreased in Lgals1(+/+) nerves injected with Gal1 antibody. The typical injury-induced peak of macrophage/microglial density was delayed in the sciatic nerves and fifth lumbar dorsal root ganglia of Lgals1(-/-) mice relative to control mice. Injection of oxidized Gal1 into uninjured sciatic nerve promoted the accumulation of macrophages in Lgals1(+/+) nerves. Finally, we used transplants of sciatic nerve to uncover a compensatory mechanism in Lgals1(-/-) mice that allows for macrophage accumulation (albeit delayed and diminished) following axotomy. We conclude that Gal1 is necessary to direct the typical accumulation of macrophages in the injured peripheral nerve, and that Gal1 is sufficient to promote macrophage accumulation in the uninjured nerve of wild-type mice.

Hyperbaric oxygenation in peripheral nerve repair and regeneration

Peripheral nerves are essential connections between the central nervous system and muscles, autonomic structures and sensory organs. Their injury is one of the major causes for severe and longstanding impairment in limb function. Acute peripheral nerve lesion has an important inflammatory component and is considered as ischemia-reperfusion (IR) injury. Surgical repair has been the standard of care in peripheral nerve lesion. It has reached optimal technical development but the end results still remain unpredictable and complete functional recovery is rare. Nevertheless, nerve repair is not primarily a mechanical problem and microsurgery is not the only key to success. Lately, there have been efforts to develop alternatives to nerve graft. Work has been carried out in basal lamina scaffolds, biologic and non-biologic structures in combination with neurotrophic factors and/or Schwann cells, tissues, immunosuppressive agents, growth factors, cell transplantation, principles of artificial sensory function, gene technology, gangliosides, implantation of microchips, hormones, electromagnetic fields and hyperbaric oxygenation (HBO). HBO appears to be a beneficial adjunctive treatment for surgical repair in the acute peripheral nerve lesion, when used at lower pressures and in a timely fashion (<6 hours).

Increased sensory neuron apoptotic death 2 weeks after peripheral axotomy in C57BL/6J mice compared to A/J mice

Peripheral nerve transection results in a disconnection of the neuron from its target. As a result, a series of metabolic changes occur in the cell body that may cause neuronal death, mainly by apoptotic mechanisms. Although neurons from neonatal animals are the most susceptible, peripheral, lesion-induced, neuronal loss also occurs in adults, and is particularly evident in mouse sensory neurons. However, differences in genetic background cause particular isogenic strains of mice to react unevenly to peripheral nerve lesion. In this work, we investigated the occurrence of apoptosis as well as the ultrastructural changes in the dorsal root ganglion sensory neurons and satellite cells of C57BL/6J and A/J mice 2 weeks after ipsilateral sciatic nerve transection at the mid-thigh level. C57BL/6J mice displayed a stronger sensory neuron chromatolytic reaction that resulted in an increased loss of neurons when compared with isogenic A/J mice (p<0.01). Additionally, most of the degenerating neurons displayed the classic features of apoptosis. These findings reinforced previous data obtained by the terminal-deoxynucleotidyl transferase nick-end labeling (TUNEL) technique.