Recovery of mimic function after facial nerve transection is poor. The successful regrowth of regenerating motor nerve fibers to reinnervate their targets is compromised by (i) poor axonal navigation ...and excessive collateral branching, (ii) abnormal exchange of nerve impulses between adjacent regrowing axons, namely axonal crosstalk, and (iii) insufficient synaptic input to the axotomized facial motoneurons. As a result, axotomized motoneurons become hyperexcitable but unable to discharge. We review our findings, which have addressed the poor return of mimic function after facial nerve injuries, by testing the hypothesized detrimental component, and we propose that intensifying the trigeminal sensory input to axotomized and electrophysiologically silent facial motoneurons improves the specificity of the reinnervation of appropriate targets. We compared behavioral, functional, and morphological parameters after single reconstructive surgery of the facial nerve (or its buccal branch) with those obtained after identical facial nerve surgery, but combined with direct or indirect stimulation of the ipsilateral infraorbital nerve. We found that both methods of trigeminal sensory stimulation, i.e., stimulation of the vibrissal hairs and manual stimulation of the whisker pad, were beneficial for the outcome through improvement of the quality of target reinnervation and recovery of vibrissal motor performance.
Microglial cells play an important role in neuroinflammation and secondary damages after spinal cord injury (SCI). Progressive microglia/macrophage inflammation along the entire spinal axis follows ...SCI, and various factors may determine the microglial activation profile. Neurotrophin-3 (NT-3) is known to control the survival of neurons, the function of synapses, and the release of neurotransmitters, while also stimulating axon plasticity and growth. We examined the effects of whole-body vibration (WBV) and forms of assisted locomotor therapy, such as passive flexion-extension (PFE) therapy, at the neuronal level after SCI, with a focus on changes in NT-3 expression and on microglia/macrophage reaction, as they play a major role in the reconstitution of CNS integrity after injury and they may critically account for the observed structural and functional benefits of physical therapy. More specifically, the WBV therapy resulted in the best overall functional recovery when initiated at day 14, while inducing a decrease in Iba1 and the highest increase in NT-3. Therefore, the WBV therapy at the 14th day appeared to be superior to the PFE therapy in terms of recovery. Functional deficits and subsequent rehabilitation depend heavily upon the inflammatory processes occurring caudally to the injury site; thus, we propose that increased expression of NT-3, especially in the dorsal horn, could potentially be the mediator of this favorable outcome.
It is well-known that, after nerve transection and surgical repair, misdirected regrowth of regenerating motor axons may occur in three ways. The first way is that the axons enter into endoneurial ...tubes that they did not previously occupy, regenerate through incorrect fascicles and reinnervate muscles that they did not formerly supply. Consequently the activation of these muscles results in inappropriate movements. The second way is that, in contrast with the precise target-directed pathfinding by elongating motor nerves during embryonic development, several axons rather than a single axon grow out from each transected nerve fiber. The third way of misdirection occurs by the intramuscular terminal branching (sprouting) of each regenerating axon to culminate in some polyinnervation of neuromuscular junctions, i.e. reinnervation of junctions by more than a single axon. Presently, “fascicular” or “topographic specificity” cannot be achieved and hence target-directed nerve regeneration is, as yet, unattainable. Nonetheless, motor and sensory reinnervation of appropriate endoneurial tubes does occur and can be promoted by brief nerve electrical stimulation.
This review considers the expression of neurotrophic factors in the neuromuscular system and how this expression can promote functional recovery, with emphasis on the whisking of vibrissae on the rat face in relationship to the expression of the factors. Evidence is reviewed for a role of neurotrophic factors as short-range diffusible sprouting stimuli in promoting complete functional recovery of vibrissal whisking in blind Sprague Dawley (SD)/RCS rats but not in SD rats with normal vision, after facial nerve transection and surgical repair. Briefly, a complicated time course of growth factor expression in the nerves and denervated muscles include (1) an early increase in FGF2 and IGF2, (2) reduced NGF between 2 and 14days after nerve transection and surgical repair, (3) a late rise in BDNF and (4) reduced IGF1 protein in the denervated muscles at 28days. These findings suggest that recovery of motor function after peripheral nerve injury is due, at least in part, to a complex regulation of nerve injury-associated neurotrophic factors and cytokines at the neuromuscular junctions of denervated muscles. In particular, the increase of FGF2 and concomittant decrease of NGF during the first week after facial nerve-nerve anastomosis in SD/RCS blind rats may prevent intramuscular axon sprouting and, in turn, reduce poly-innervation of the neuromuscular junction.
In unilateral facial palsy, cross-face nerve grafts are used for emotional facial reanimation. Facial nerve regeneration through the grafts takes several months, and the functional results are ...sometimes inadequate. Chronic denervation of the cross-face nerve graft results in incomplete nerve regeneration. The authors hypothesize that donor axons from regional sensory nerves will enhance facial motoneuron regeneration, improve axon regeneration, and improve the amplitude of facial muscle movement.
In the rat model, a 30-mm nerve graft (right common peroneal nerve) was used as a cross-face nerve graft. The graft was coapted to the proximal stump of the transected right buccal branch of the facial nerve and the distal stumps of the transected left buccal and marginal mandibular branches. In one group, sensory occipital nerves were coapted end-to-side to the cross-face nerve graft. Regeneration of green fluorescent protein-positive axons was imaged in vivo in transgenic Thy1-green fluorescent protein rats, in which all neurons express green fluorescence. After 16 weeks, retrograde labeling of regenerated neurons and histomorphometric analysis of myelinated axons was performed. Functional outcomes were assessed with video analysis of whisker motion.
"Pathway protection" with sensory axons significantly enhanced motoneuron regeneration, as assessed by retrograde labeling, in vivo fluorescence imaging, and histomorphometry, and significantly improved whisker motion during video analysis.
Sensory pathway protection of cross-face nerve grafts counteracts chronic denervation in nerve grafts and improves regeneration and functional outcomes.
After peripheral nerve injury, recovery of motor performance negatively correlates with the poly-innervation of neuromuscular junctions (NMJ) due to excessive sprouting of the terminal Schwann cells. ...Denervated muscles produce short-range diffusible sprouting stimuli, of which some are neurotrophic factors. Based on recent data that vibrissal whisking is restored perfectly during facial nerve regeneration in blind rats from the Sprague Dawley (SD)/RCS strain, we compared the expression of brain derived neurotrophic factor (BDNF), fibroblast growth factor-2 (FGF2), insulin growth factors 1 and 2 (IGF1, IGF2) and nerve growth factor (NGF) between SD/RCS and SD-rats with normal vision but poor recovery of whisking function after facial nerve injury. To establish which trophic factors might be responsible for proper NMJ-reinnervation, the transected facial nerve was surgically repaired (facial-facial anastomosis, FFA) for subsequent analysis of mRNA and proteins expressed in the levator labii superioris muscle. A complicated time course of expression included (1) a late rise in BDNF protein that followed earlier elevated gene expression, (2) an early increase in FGF2 and IGF2 protein after 2days with sustained gene expression, (3) reduced IGF1 protein at 28days coincident with decline of raised mRNA levels to baseline, and (4) reduced NGF protein between 2 and 14days with maintained gene expression found in blind rats but not the rats with normal vision. These findings suggest that recovery of motor function after peripheral nerve injury is due, at least in part, to a complex regulation of lesion-associated neurotrophic factors and cytokines in denervated muscles. The increase of FGF-2 protein and concomittant decrease of NGF (with no significant changes in BDNF or IGF levels) during the first week following FFA in SD/RCS blind rats possibly prevents the distal branching of regenerating axons resulting in reduced poly-innervation of motor endplates.
•Blind rats from the SD/RCS strain recover whisking after facial nerve injury.•We studied the expression of BDNF, FGF2 IGF1, IGF2 and NGF in denervated muscles.•We compared the amounts to those obtained in sighted rats (no functional recovery).•In the blind rats we found an early increase in FGF2 and a concomitant decrease in NGF.•FGF2-increase and NGF-decrease prevent the distal branching of regenerating axons.•This in turn reduces poly-innervation of NMJ and promotes functional recovery.
Injury to a peripheral nerve induces a complex cellular and molecular response required for successful axon regeneration. Proliferating Schwann cells organize into chains of cells bridging the lesion ...site, which is invaded by macrophages. Approximately half of the injured neuron population sends out axons that enter the glial guidance channels in response to secreted neurotrophic factors and neuropoietic cytokines. These lesion-associated polypeptides create an environment that is highly supportive for axon regrowth, particularly after acute injury, and ensure that the vast majority of regenerating axons are directed toward the distal nerve stump. Unfortunately, most neurotrophic factors and neuropoietic cytokines are also strong stimulators of axonal sprouting. Although some of the axonal branches will withdraw at later stages, the sprouting effect contributes to the misdirection of reinnervation that results in the lack of functional recovery observed in many patients with peripheral nerve injuries. Here, we critically review the role of neuronal growth factors and cytokines during axon regeneration in the peripheral nervous system. Their differential effects on axon elongation and sprouting were elucidated in various studies on intraneuronal signaling mechanisms following nerve lesion. The present data define a goal for future therapeutic strategies, namely, to selectively stimulate a Ras/Raf/ERK-mediated axon elongation program over an intrinsic PI3K-dependent axonal sprouting program in lesioned motor and sensory neurons. Instead of modulating growth factor or cytokine levels at the lesion site, targeting specific intraneuronal molecules, such as the negative feedback inhibitors of ERK signaling, has been shown to promote long-distance regeneration while avoiding sprouting of regenerating axons until they have reached their target areas.
Introduction/Aims
Daily intramuscular injections of fibroblast growth factor 2 (FGF2) but not of brain‐derived neurotrophic factor (BDNF) significantly improve whisking behavior and mono‐innervation ...of the rat levator labii superioris (LLS) muscle 56 days after buccal nerve transection and suture (buccal–buccal anastomosis, BBA). We explored the dose–response of BDNF, FGF2, and insulin growth factor 2 (IGF2) on the same parameters, asking whether higher doses of BDNF would promote recovery.
Methods
After BBA, growth factors were injected (30 μL volume) daily into the LLS muscle over 14, 28, or 56 days. At 56 days, video‐based motion analysis of vibrissal whisking was performed and the extent of mono‐ and poly‐reinnervation of the reinnervated neuromuscular junctions (NMJs) of the muscle determined with immunostaining of the nerve with β‐tubulin and histochemical staining of the endplates with Alexa Fluor 488‐conjugated α‐bungarotoxin.
Results
The dose–response curve demonstrated significantly higher whisking amplitudes and corresponding increased mono‐innervation of the NMJ in the reinnervated LLS muscle at concentrations of 20–30 μg/mL BDNF administered daily for 14–28 days after BBA surgery. In contrast, high doses of IGF2 and FGF2, or doses of 20 and 40 μg/mL of BDNF administered for 14–56 days had no effect on either whisking behavior or in reducing poly‐reinnervation of endplates in the muscle.
Discussion
These data suggest that the re‐establishment of mono‐innervation of whiskerpad muscles and the improved motor function by injections of BDNF into the paralyzed vibrissal musculature after facial nerve injury have translation potential and promote clinical application.
Abstract
The relationships between various parameters of tissue damage and subsequent functional recovery after spinal cord injury (SCI) are not well understood. Patients may regain micturition ...control and walking despite large postinjury medullar cavities. The objective of this study was to establish possible correlations between morphological findings and degree of functional recovery after spinal cord compression at vertebra Th8 in rats. Recovery of motor (Basso, Beattie, Bresnahan, foot-stepping angle, rump-height index, and ladder climbing), sensory (withdrawal latency), and bladder functions was analyzed at 1, 3, 6, 9, and 12 weeks post-SCI. Following perfusion fixation, spinal cord tissue encompassing the injury site was cut in longitudinal frontal sections. Lesion lengths, lesion volumes, and areas of perilesional neural tissue bridges were determined after staining with cresyl violet. The numbers of axons in these bridges were quantified after staining for class III β-tubulin. We found that it was not the area of the spared tissue bridges, which is routinely determined by magnetic resonance imaging (MRI), but the numbers of axons in them that correlated with functional recovery after SCI (Spearman’s ρ > 0.8; p < 0.001). We conclude that prognostic statements based only on MRI measurements should be considered with caution.
SCI is followed by dramatic upregulation of chondroitin sulfate proteoglycans (CSPGs) which limit axonal regeneration, oligodendrocyte replacement and remyelination. The recent discovery of the ...specific CSPGs signaling receptor protein tyrosine phosphatase sigma (RPTPσ) provided an opportunity to refine the therapeutic approach to overcome CSPGs inhibitory actions. In previously published work, subcutaneous (s.c.) delivery of 44 μg/day of a peptide mimetic of PTPσ called intracellular sigma peptide (ISP), which binds to PTPσ and blocks CSPG-mediated inhibition, facilitated recovery after contusive SCI. Since this result could be of great interest for clinical trials, we independently repeated this study, but modified the method of injury as well as peptide application and the dosage. Following SCI at the Th10-segment, 40 rats were distributed in 3 groups. Animals in group 1 (20 rats) were subjected to SCI, but received no treatment. Rats in group 2 were treated with intraperitoneal (i.p.) injections of 44 μg/day ISP (SCI + ISP44) and animals of group 3 with s.c. injections of 500 μg/day ISP (SCI + ISP500) for 7 weeks after lesioning. Recovery was analyzed at 1, 3, 6, 9 and 12 weeks after SCI by determining (i) BBB-score, (ii) foot-stepping angle, (iii) rump-height index, (iv) number of correct ladder steps, (v) bladder score and (vi) sensitivity (withdrawal latency after thermal stimulus). Finally, we determined the amount of serotonergic fibers in the preserved neural tissue bridges (PNTB) around the lesion site. Our results show that, systemic therapy with ISP improved locomotor, sensory and vegetative recovery which correlated with more spared serotonergic fibers in PNTB.
•ISP was applied two modes: 44 μg/day i.p. and 500 μg/day s.c for 7 weeks after SCI.•Recovery was analyzed at 1, 3, 6, 9 and 12 weeks after SCI.•We determined the spared serotonergic axons in preserved neural tissue bridges (PNTB).•Systemic therapy with ISP improved locomotor, sensory and vegetative recovery.•This correlated with more spared serotonergic fibers in PNTB
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