Functional recovery after facial nerve crush is delayed in severe combined immunodeficient mice

The Role of the IL-4 Signaling Pathway in Traumatic Nerve Injuries

Following traumatic peripheral nerve injury, adequate restoration of function remains an elusive clinical goal. Recent research highlights the complex role that the immune system plays in both nerve injury and regeneration. Pro-regenerative processes in wounded soft tissues appear to be significantly mediated by cytokines of the type 2 immune response, notably interleukin (IL)-4. While IL-4 signaling has been firmly established as a critical element in general tissue regeneration during wound healing, it has also emerged as a critical process in nerve injury and regeneration. In this context of peripheral nerve injury, endogenous IL-4 signaling has recently been confirmed to influence more than leukocytes, but including also neurons, axons, and Schwann cells. Given the role IL-4 plays in nerve injury and regeneration, exogenous IL-4 and/or compounds targeting this signaling pathway have shown encouraging preliminary results to treat nerve injury or other neuropathy in rodent models. In particular, the exogenous stimulation of the IL-4 signaling pathway appears to promote postinjury neuron survival, axonal regeneration, remyelination, and thereby improved functional recovery. These preclinical data strongly suggest that targeting IL-4 signaling pathways is a promising translational therapy to augment treatment approaches of traumatic nerve injury. However, a better understanding of the type 2 immune response and associated signaling networks functioning within the nerve injury microenvironment is still needed to fully develop this promising therapeutic avenue.

Delivery of Interleukin-4-Encoding Lentivirus Using Multiple-Channel Bridges Enhances Nerve Regeneration

OBJECTIVES/HYPOTHESIS

Facial nerve injury is a source of major morbidity. This study investigated the neuroregenerative effects of inducing an anti-inflammatory environment when reconstructing a facial nerve defect with a multichannel bridge containing interleukin-4 (IL-4)-encoding lentivirus.

STUDY DESIGN

Animal study.

METHODS

Eighteen adult Sprague-Dawley rats were divided into three groups, all of which sustained a facial nerve gap defect. Group I had reconstruction performed via an IL-4 multichannel bridge, group II had a multichannel bridge with saline placed, and group III had no reconstruction.

RESULTS

Quantitative histomorphometric data were assessed 10 weeks after injury. On post hoc analysis, the IL-4 bridge group demonstrated superior regeneration compared to bridge alone on fiber density (mean = 2,380 ± 297 vs. 1,680 ± 441 fibers/mm² , P = .05) and latency time (mean = 2.9 ms ± 0.6 ms vs. 3.6 ms ± 0.3 ms, P < .001). There was significantly greater regeneration in the IL-4 bridge group versus unreconstructed defect for total fiber and density measurements (P ≤ .05). Comparison of facial motor-evoked distal latencies between the IL-4 bridge group versus bridge alone revealed significant electrophysiological improvement at week 8 (P = .02).

CONCLUSIONS

Inflammation has been implicated in a variety of otolaryngologic disorders. This study demonstrates that placement of a multichannel bridge with lentivirus encoding IL-4 improves regenerative outcomes following facial nerve gap injury in rodents. This effect is likely mediated by promotion of an anti-inflammatory environment, and these findings may inform future therapeutic approaches to facial nerve injury.

LEVEL OF EVIDENCE

NA Laryngoscope, 2020.

Mechanisms of the Immunomodulation Effects of Bone Marrow-Derived Mesenchymal Stem Cells on Facial Nerve Injury in Sprague-Dawley Rats

Normal facial nerve (FN) function is very important for human being. However, if injured, FN function is difficult to restore completely. Recently, many studies reported the immune regulation function of stem cells (SCs). However, the immunomodulation function of SCs on FN injury is still unclear. Our study aims to explore the mechanism of immunomodulation effect of Sprague-Dawley rat bone marrow-derived SCs (BMSCs) on FN injury and specially focus on the regulation of Th17 and the protection effects of BMSCs on central facial motor neurons (FMNs). First, rat FNs were harvested. FN and BMSCs were cultured together or separately and levels of transforming growth factor (TGF)-β1, interleukin (IL)-6, hepatocyte growth factor (HGF), inducible nitric oxide synthase (iNOS), and prostaglandin E2 (PGE2) in supernatant were detected by enzyme-linked immunosorbent assay (ELISA). Then, after treating with or without local BMSCs injection, the proportion of Th17 in neck lymph nodes (LNs) was investigated in rat FN injury models. Furthermore, the apoptotic index of FMNs was studied in rat FN injury models that were treated with or without BMSCs. We found that BMSCs could secrete high levels of IL-6, HGF, PGE2, iNOS, and TGF-β1 in culture. The percentage of Th17 of neck LNs in BMSCs-treated group was significantly lower than that in the control group. The apoptotic index of FMNs in BMSCs-treated group was significantly lower than that in the control group. In conclusion, our research indicates BMSCs could independently secrete cytokines IL-6, HGF, PGE2, iNOS, and TGF-β1, and these cytokines could regulate the balance among subsets of CD4⁺ T cells and could protect FMNs by inhibiting neuron apoptosis.

Deficiency of adaptive immunity does not interfere with Wallerian degeneration

Following injury, distal axons undergo the process of Wallerian degeneration, and then cell debris is cleared to create a permissive environment for axon regeneration. The innate and adaptive immune systems are believed to be critical for facilitating the clearance of myelin and axonal debris during this process. However, immunodeficient animal models are regularly used in transplantation studies investigating cell therapies to modulate the degenerative/regenerative response. Given the importance of the immune system in preparing a permissive environment for regeneration by clearing debris, animals lacking, in part or in full, a functional immune system may have an impaired ability to regenerate due to poor myelin clearance, and may, thus, be poor hosts to study modulators of regeneration and degeneration. To study this hypothesis, three different mouse models with impaired adaptive immunity were compared to wild type animals in their ability to degenerate axons and clear myelin debris one week following sciatic nerve transection. Immunofluorescent staining for axons and quantitation of axon density with nerve histomorphometry of the distal stump showed no consistent discrepancy between immunodeficient and wild type animals, suggesting axons tended to degenerate equally between the two groups. Debris clearance was assessed by macrophage density and relative myelin basic protein expression within the denervated nerve stump, and no consistent impairment of debris clearance was found. These data suggested deficiency of the adaptive immune system does not have a substantial effect on axon degeneration one week following axonal injury.

Electrical stimulation and testosterone enhance recovery from recurrent laryngeal nerve crush

OBJECTIVE

This study investigated the effects of a combinatorial treatment, consisting of a brief period of nerve electrical stimulation (ES) and systemic supraphysiologic testosterone, on functional recovery following a crush of the recurrent laryngeal nerve (RLN).

STUDY DESIGN

Prospective, controlled animal study.

METHODS

After a crush of the left RLN, adult male Sprague-Dawley rats were divided into four treatment groups: 1) no treatment, 2) ES, 3) testosterone propionate (TP), and 4) ES + TP. Each group was subdivided into 1, 2, 3, or 4 weeks post-operative survival time points. Groups had an n of 4- 9. Recovery of vocal fold mobility (VFM) was assessed.

RESULTS

Brief ES of the proximal nerve alone or in combination with TP accelerated the initiation of functional recovery. TP administration by itself also produced increased VFM scores compared to controls, but there were no statistical differences between the ES-treated and TP-treated animals. Treatment with brief ES alone was sufficient to decrease the time required to recover complete VFM. Animals with complete VFM were seen in treatment groups as early as 1 week following injury; in the untreated group, this was not observed until at least 3 weeks post-injury, translating into a 66% decrease in time to complete recovery.

CONCLUSIONS

Brief ES, alone or in combination with TP, promise to be effective therapeutic interventions for promoting regeneration following RLN injury.

Facial nerve axotomy in mice: a model to study motoneuron response to injury

The goal of this surgical protocol is to expose the facial nerve, which innervates the facial musculature, at its exit from the stylomastoid foramen and either cut or crush it to induce peripheral nerve injury. Advantages of this surgery are its simplicity, high reproducibility, and the lack of effect on vital functions or mobility from the subsequent facial paralysis, thus resulting in a relatively mild surgical outcome compared to other nerve injury models. A major advantage of using a cranial nerve injury model is that the motoneurons reside in a relatively homogenous population in the facial motor nucleus in the pons, simplifying the study of the motoneuron cell bodies. Because of the symmetrical nature of facial nerve innervation and the lack of crosstalk between the facial motor nuclei, the operation can be performed unilaterally with the unaxotomized side serving as a paired internal control. A variety of analyses can be performed postoperatively to assess the physiologic response, details of which are beyond the scope of this article. For example, recovery of muscle function can serve as a behavioral marker for reinnervation, or the motoneurons can be quantified to measure cell survival. Additionally, the motoneurons can be accurately captured using laser microdissection for molecular analysis. Because the facial nerve axotomy is minimally invasive and well tolerated, it can be utilized on a wide variety of genetically modified mice. Also, this surgery model can be used to analyze the effectiveness of peripheral nerve injury treatments. Facial nerve injury provides a means for investigating not only motoneurons, but also the responses of the central and peripheral glial microenvironment, immune system, and target musculature. The facial nerve injury model is a widely accepted peripheral nerve injury model that serves as a powerful tool for studying nerve injury and regeneration.

SOD1(G93A) transgenic mouse CD4(+) T cells mediate neuroprotection after facial nerve axotomy when removed from a suppressive peripheral microenvironment

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease involving motoneuron (MN) axonal withdrawal and cell death. Previously, we established that facial MN (FMN) survival levels in the SOD1(G93A) transgenic mouse model of ALS are reduced and nerve regeneration is delayed, similar to immunodeficient RAG2(-/-) mice, after facial nerve axotomy. The objective of this study was to examine the functionality of SOD1(G93A) splenic microenvironment, focusing on CD4(+) T cells, with regard to defects in immune-mediated neuroprotection of injured MN. We utilized the RAG2(-/-) and SOD1(G93A) mouse models, along with the facial nerve axotomy paradigm and a variety of cellular adoptive transfers, to assess immune-mediated neuroprotection of FMN survival levels. We determined that adoptively transferred SOD1(G93A) unfractionated splenocytes into RAG2(-/-) mice were unable to support FMN survival after axotomy, but that adoptive transfer of isolated SOD1(G93A) CD4(+) T cells could. Although WT unfractionated splenocytes adoptively transferred into SOD1(G93A) mice were able to maintain FMN survival levels, WT CD4(+) T cells alone could not. Importantly, these results suggest that SOD1(G93A) CD4(+) T cells retain neuroprotective functionality when removed from a dysfunctional SOD1(G93A) peripheral splenic microenvironment. These results also indicate that the SOD1(G93A) central nervous system microenvironment is able to re-activate CD4(+) T cells for immune-mediated neuroprotection when a permissive peripheral microenvironment exists. We hypothesize that a suppressive SOD1(G93A) peripheral splenic microenvironment may compromise neuroprotective CD4(+) T cell activation and/or differentiation, which, in turn, results in impaired immune-mediated neuroprotection for MN survival after peripheral axotomy in SOD1(G93A) mice.

Inflammation and axon regeneration

PURPOSE OF REVIEW

The inflammatory response that accompanies neural injury involves multiple cell types and effector molecules with both positive and negative effects. Inflammation is essential for normal regeneration in the peripheral nervous system, and here we review evidence that augmenting inflammation can enhance regeneration in areas of the central nervous system in which it normally does not occur.

RECENT FINDINGS

Within the spinal cord, inflammation enables transplanted sensory neurons to regenerate lengthy axons and enhances the ability of a trophic factor to promote corticospinal tract sprouting. Induction of inflammation in the eye supports survival of retinal ganglion cells and enables them to regenerate injured axons through the optic nerve. These effects are linked to an atypical trophic factor, oncomodulin, along with other, better known molecules. Induction of inflammation within dorsal root ganglia, when combined with other treatments, enables peripheral sensory neurons to regenerate axons into the spinal cord. However, inflammation also has negative effects that impede recovery.

SUMMARY

In light of the importance of inflammation for neural repair, it is important to identify the specific cell types and molecules responsible for the positive and negative effects of inflammation and to develop treatments that tip the balance to favor repair.

Rett syndrome and other autism spectrum disorders--brain diseases of immune malfunction?

Neuroimmunology was once referred to in terms of its pathological connotation only and was generally understood as covering the deleterious involvement of the immune system in various diseases and disorders of the central nervous system (CNS). However, our conception of the function of the immune system in the structure, function, and plasticity of the CNS has undergone a sea change after relevant discoveries over the past two decades, and continues to be challenged by more recent studies of neurodevelopment and cognition. This review summarizes the recent advances in understanding of immune-system participation in the development and functioning of the CNS under physiological conditions. Considering as an example Rett syndrome a devastating neurodevelopmental disease, we offer a hypothesis that might help to explain the part played by immune cells in its etiology, and hence suggests that the immune system might be a feasible therapeutic target for alleviation of some of the symptoms of this and other autism spectrum disorders.

Functional recovery and facial motoneuron survival are influenced by immunodeficiency in crush-axotomized mice

Facial nerve axotomy is a well-described injury paradigm for peripheral nerve regeneration and facial motoneuron (FMN) survival. We have previously shown that CD4(+) T helper (Th) 1 and 2 effector subsets develop in the draining cervical lymph node, and that the IL-4/STAT-6 pathway of Th2 development is critical for FMN survival after transection axotomy. In addition, delayed behavioral recovery time in immunodeficient mice may be due to the absence of T and B cells. This study utilized a crush axotomy paradigm to evaluate FMN survival and functional recovery in WT, STAT-6 KO (impaired Th2 response), T-Bet KO (impaired Th1 response), and RAG-2 KO (lacking mature T and B cells) mice to elucidate the contributions of specific CD4(+) T cell subsets in motoneuron survival and recovery mechanisms. STAT-6 KO and RAG-2 KO mice exhibited decreased FMN survival after crush axotomy compared to WT, supporting a critical role for the Th2 effector cell in motoneuron survival before target reconnection. Long term FMN survival was sustained through 10 wpo after crush axotomy in both WT and RAG-2 KO mice, indicating that target derived neurotrophic support maintains FMN survival after target reconnection. In addition, RAG-2 KO mice exhibited delayed functional recovery compared to WT mice. Both STAT-6 and T-Bet KO mice exhibited partially delayed functional recovery compared to WT, though not to the extent of RAG-2 KO mice. Collectively, our findings indicate that both pro- and anti-inflammatory CD4(+) T cell responses contribute to optimal functional recovery from axotomy-induced facial paralysis, while FMN survival is supported by the anti-inflammatory Th2 response alone.

Immune senescence and brain aging: can rejuvenation of immunity reverse memory loss?

The factors that determine brain aging remain a mystery. Do brain aging and memory loss reflect processes occurring only within the brain? Here, we present a novel view, linking aging of adaptive immunity to brain senescence and specifically to spatial memory deterioration. Inborn immune deficiency, in addition to sudden imposition of immune malfunction in young animals, results in cognitive impairment. As a corollary, immune restoration at adulthood or in the elderly results in a reversal of memory loss. These results, together with the known deterioration of adaptive immunity in the elderly, suggest that memory loss does not solely reflect chronological age; rather, it is an outcome of the gap between an increasing demand for maintenance (age-related risk-factor accumulation) and the reduced ability of the immune system to meet these needs.

Protective autoimmunity in the nervous system

The immune system can play both detrimental and beneficial roles in the nervous system. Multiple arms of the immune system, including T cells, B cells, NK cells, mast cells, macrophages, dendritic cells, microglia, antibodies, complement and cytokines participate in limiting damage to the nervous system during toxic, ischemic, hemorrhagic, infective, degenerative, metabolic and immune-mediated insults and also assist in the process of repair after injury has occurred. Immune cells have been shown to produce neurotrophic growth factors and interact with neurons and glial cells to preserve them from injury and stimulate growth and repair. The immune system also appears to participate in proliferation of neural progenitor stem cells and their migration to sites of injury. Neural stem cells can also modify the immune response in the central and peripheral nervous system to enhance neuroprotective effects. Evidence for protective and reparative functions of the immune system has been found in diverse neurologic diseases including traumatic injury, ischemic and hemorrhagic stroke, multiple sclerosis, infection, and neurodegenerative diseases (Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis). Existing therapies including glatiramer acetate, interferon-beta and immunoglobulin have been shown to augment the protective and regenerative aspects of the immune system in humans, and other experimental interventions such as vaccination, minocycline, antibodies and neural stem cells, have shown promise in animal models of disease. The beneficent aspects of the immune response in the nervous system are beginning to be appreciated and their potential as pharmacologic targets in neurologic disease is being explored.

Immunity and cognition: what do age-related dementia, HIV-dementia and 'chemo-brain' have in common?

Until recently, dogma dictated that the immune system and the central nervous system (CNS) live mostly separate, parallel lives, and any interactions between the two were assumed to be limited to extreme cases of pathological insult. It was only a decade ago that T cells in the injured brain were shown to play a protective rather than a destructive role. In this article, we explore the role of the immune system in the healthy brain, focusing on the key function that T lymphocytes have in the regulation of cognition. We discuss candidate mechanisms underlying T cell-mediated control of cognitive function in human cognitive diseases associated with immune decline, such as age- and HIV-related dementias, 'chemo-brain' and others.

T cell memory in the injured facial motor nucleus: relation to functional recovery following facial nerve crush

T cells have the ability to mount a memory response to a previously encountered antigen such that re-exposure to the antigen results in a response that is greater in magnitude and function. Following facial nerve transection, T cells have been shown to traffic to injured motor neurons in the facial motor nucleus (FMN) and may have the ability to promote neuronal survival and functional recovery. Previously, we demonstrated that early exposure to neuronal injury on one side of the brain during young adulthood elicited a T cell response that was greater in magnitude following exposure to the same form of injury on the contralateral side later in adulthood. Whether the T cell memory response to neuronal injury influenced functional recovery following nerve crush injury was unknown. In the current study, we tested the hypotheses that (1) transection of the right facial nerve in sensitized mice would result in faster recovery of the whisker response when the contralateral facial nerve is crushed 10 weeks later, and (2) the early recovery would be associated with an increase in the magnitude of the T cell response in the contralateral FMN following crush injury in sensitized mice. The onset of modest recovery in sensitized mice occurred between 3 and 5 days following crush injury of the contralateral facial nerve, approximately 1.5 days earlier than naïve mice, and was associated with more than a two-fold increase in the magnitude of the T cell response in the contralateral FMN following crush injury. There was no difference between groups in the number of days to full recovery. Further study of how T cell memory influences neuroregeneration may have important implications for translational research.

Neuritogenic effects of T cell-derived IL-3 on mouse splenic sympathetic neurons in vivo

To determine the role played by lymphocytes and cytokines in the growth of sympathetic neurons in vivo, the innervation and cytokine levels were examined in the spleens of SCID mice that lack T and B cells. Splenic noradrenaline, nerve growth factor (NGF), and IL-1beta levels were elevated in SCID mice. Immunohistochemical examination revealed that the density of tyrosine hydroxylase-positive (TH(+)) fibers of splenic central arteries in SCID mice was increased compared with wild-type C.B-17 mice, while SCID mice had significantly fewer TH(+) fibers in their periarteriolar lymphatic sheaths (PALS). Two weeks after SCID mice were injected with C.B-17 splenic T cells, their TH(+) fiber staining increased in the PALS. IL-3 levels increased significantly in SCID mice following T cell reconstitution, and the administration of anti-IL-3 Ab blocked the above T cell-induced increase in innervation in the PALS. Anti-IL-3 treatment also inhibited the regeneration of splenic sympathetic neurons in C.B-17 mice after they were chemically sympathetomized with 6-hydroxydopamine. Depletion of NK cells by anti-asialo GM1 promoted the splenic innervation in SCID mice, while there were no significant changes in the innervation between CD8(+) T cell-deficient beta(2)-microglobulin knockout mice and their wild type. Our results suggest that T cells (probably CD4(+) Th cells but not CD8(+) CTLs) play a role in regulating the sympathetic innervation of the spleen; this effect appeared to be mediated, at least in part, by IL-3. On the contrary, NK cells may exert an inhibitory effect on the sympathetic innervation.

Phenotype of CD4+ T cell subsets that develop following mouse facial nerve axotomy

We have previously shown that CD4(+) T helper (Th) 2 cells, but not Th1 cells, participate in the rescue of mouse facial motoneurons (FMN) from axotomy-induced cell death. Recently, a number of other CD4(+) T cell subsets have been identified in addition to the Th1 and Th2 effector subsets, including Th17, inducible T regulatory type 1 (Tr1), and naturally thymus-born Foxp3(+) regulatory (Foxp3(+) Treg) cells. These subsets regulate the nature of a T cell-mediated immune response. Th1 and Th17 cells are pro-inflammatory subsets, while Th2, Tr1, and Foxp3(+) Treg cells are anti-inflammatory subsets. Pro-inflammatory responses in the central nervous system are thought to be neurodestructive, while anti-inflammatory responses are considered neuroprotective. However, it remains to be determined if another CD4(+) T cell subset, other than the Th2 cell, develops after peripheral nerve injury and participates in FMN survival. In the present study, we used FACS analysis to determine the temporal frequency of Th1, Th17, Th2, Tr1 and Foxp3(+) Treg CD4(+) T cell subset development in C57BL/6 wild type mice after facial nerve transection at the stylomastoid foramen in the mouse. The results indicate that all of the known CD4(+) T cell subsets develop and expand in number within the draining lymph node, with a peak in number primarily at 7 days postoperative (dpo), followed by a decline at 9 dpo. In addition to the increase in subset frequency over time, FACS analysis of individual cells showed that the level of cytokine expressed per cell also increased for interferon-gamma (IFN-gamma), interleukin (IL)-10 and IL-17, but not IL-4. Additional control double-cytokine labeling experiments were done which indicate that, at 7dpo, the majority of cells indeed have committed to a specific phenotype and express only 1 cytokine. Collectively, our findings indicate for the first time that there is no preferential activation and expansion of any single CD4(+) T cell subset after peripheral nerve injury but, rather, that both pro-inflammatory and anti-inflammatory CD4(+) T cells develop.

Impaired nerve regeneration and enhanced neuroinflammatory response in mice lacking pituitary adenylyl cyclase activating peptide

Peripheral nerve injury models are used to investigate processes that can potentially be exploited in CNS injury. A consistent change that occurs in injured peripheral neurons is an induction in expression of pituitary adenylyl cyclase activating peptide (PACAP), a neuropeptide with putative neuroprotective and neuritogenic actions. PACAP-deficient mice were used here to investigate actions of endogenous PACAP after facial nerve injury. Although motor neuron survival after axotomy was not significantly different in PACAP deficient vs. wild type mice, recovery of axon regeneration after crush injury was significantly delayed. The impaired regeneration was associated with 8- to 12-fold increases in gene expression of proinflammatory cytokines tumor necrosis factor-alpha, interferon-gamma, interleukin (IL) -6, and a 90% decrease in the anti-inflammatory cytokine IL-4 at the injury site. Similar cytokine changes and an increased microglial response were observed in the brainstem facial motor nucleus. Because immunocompromised animals such as SCID mice are known to exhibit peripheral nerve regeneration defects, the observations raise the novel hypothesis that PACAP is critically involved in a carefully controlled immune response that is necessary for proper nerve regeneration after injury.

The making of successful axonal regeneration: Genes, molecules and signal transduction pathways

Unlike its central counterpart, the peripheral nervous system is well known for its comparatively good potential for regeneration following nerve fiber injury. This ability is mirrored by the de novo expression or upregulation of a wide variety of molecules including transcription factors, growth-stimulating substances, cell adhesion molecules, intracellular signaling enzymes and proteins involved in regulating cell-surface cytoskeletal interactions, that promote neurite outgrowth in cultured neurons. However, their role in vivo is less known. Recent studies using neutralizing antibodies, gene inactivation and overexpression techniques have started to shed light on those endogenous molecules that play a key role in axonal outgrowth and the process of successful functional repair in the injured nervous system. The aim of the current review is to provide a summary on this rapidly growing field and the experimental techniques used to define the specific effects of candidate signaling molecules on axonal regeneration in vivo.

CD4(+) T cell-mediated facial motoneuron survival after injury: Distribution pattern of cell death and rescue throughout the extent of the facial motor nucleus

We have previously demonstrated that CD4(+) T cells transiently rescue facial motoneurons (FMN) from axotomy-induced death in immunodeficient mice. Three subpopulations of motoneurons have been observed within the facial motor nucleus following axotomy: one that always survives axotomy (50%), one that is amenable to rescue from axotomy-induced death through the addition of neurotrophic factors or CD4(+) T cells (30-40%), and one that always dies after axotomy (10-15%). The objective of this study was to anatomically map the extent of axotomy-induced cell death and immune cell rescue in the facial nucleus to study the differential survival capabilities of each subpopulation. Wild-type (WT) mice, recombinase activating gene 2 knockout (RAG-2 KO) mice, and RAG-2 KO mice reconstituted with CD4(+) T cells were subjected to right facial nerve axotomy. At 4 weeks post-axotomy, topographical mapping of axotomy-induced cell death throughout the rostro-caudal extent of the facial nucleus was accomplished in accordance with previously published maps of the subnuclear arrangement of the facial neurons. The results indicate that all 3 subpopulations of FMN can be found in each of the subnuclear groups throughout the entire rostro-caudal extent of the facial nucleus. These data are discussed in context of recent work in amyotrophic lateral sclerosis, a fatal motoneuron disease.

Impairment of axotomy-induced pituitary adenylyl cyclase-activating peptide gene expression in T helper 2 lymphocyte-deficient mice

CD4+ (T helper) lymphocytes appear to play important roles in neuron survival and regeneration after injury, although their functions in regulating gene expression in injured neurons are unknown. Mice with targeted mutations in the STAT4 and STAT6 genes are deficient in T helper (Th)1 and Th2 responses, respectively, and have been used to determine the relative importance of T helper subsets in a variety of inflammatory processes. As pituitary adenylyl cyclase-activating peptide mRNA is normally strongly induced in facial motor neurons after axotomy, we examined this induction in Th1 and Th2 lymphocyte-deficient and control Balb/C wild-type mice. As previously reported, pituitary adenylyl cyclase-activating peptide gene expression was strongly induced in ipsilateral but not contralateral motor neurons in the facial motor nucleus of wild-type mice. The mean number of hybridizing motor neurons in STAT4-deficient mice did not differ from that in wild-type mice, whereas the number in STAT6 mice was reduced by more than 50%. The results indicate that STAT6 plays a key role in the upregulation of pituitary adenylyl cyclase-activating peptide gene expression in facial motor neurons after injury, possibly through its role in regulating T helper cell differentiation to the type 2 phenotype.

Interdisciplinary research: noradrenergic regulation of adaptive immunity

To understand the complexity of mechanisms involved in the regulation of adaptive immunity by the sympathetic neurotransmitter norepinephrine and adrenergic receptor stimulation, there must be a rich history of basic science and clinical findings upon which to form hypotheses for testing, as well as a rich supply of individuals trained in two or more disciplines. This review is intended to offer a tour of the past, present, and future discoveries that have been made in the area of adrenergic regulation of adaptive immunity, as well as share a vision of how our field of study will progress years from now, given that every individual who contributes to the interdisciplinary nature of our research is valued. And finally, this review will discuss how the lessons from the past can help us to attain a vision of interdisciplinary research for the future.

Use of muscle fibrillation for tracking nerve regeneration

Individual muscle fibers in denervated muscle demonstrate repetitive, spontaneous contraction. Such fibrillation activity disappears in denervated muscle if reinnervation occurs, but this relationship has not been formally studied. To test whether the disappearance of fibrillation can be used to track nerve regeneration, we quantified the presence and subsequent disappearance of electromyographic (EMG) fibrillation potentials and fibrillation-related movement in the rat tongue after unilateral hypoglossal nerve crush at two locations. In mice, fibrillation movement of vibrissae were monitored after unilateral facial nerve crush and compared with the return of symmetrical vibrissae sweeping movements. In both of these rodent cranial motor systems, there was a conspicuous loss of fibrillation at a time when reinnervation is known to take place, suggesting that the visual appearance of fibrillation-related movement can be used as a simple, noninvasive means of tracking nerve regeneration in these popular experimental motor systems.

Functional Recovery After Facial and Sciatic Nerve Crush Injury in the Rat

OBJECTIVES

To systematically record rat facial nerve recovery following crush injury to the main trunk with respect to ocular and vibrissial function and to compare the rates of facial and sciatic nerve recovery from crush injury in the same animals. This serves as a means of validating the functional parameters of facial nerve recovery against the well-known measure of hind limb function, the Sciatic Function Index.

METHODS

The main trunk of the facial nerve and the proximal segment of the sciatic nerve were exposed in all animals. Both nerves were subjected to standardized crush injury and subsequent daily functional testing. After a plateau of functional recovery was achieved, the animals were killed, and the distances between the sites of injury and the end musculature were measured, which allowed determination and comparison of recovery rates in both systems.

RESULTS

All crush injuries resulted in loss of electrical conductivity, as proven by intraoperative proximal nerve stimulation. Recovery of ocular and vibrissial motor function occurred starting at postoperative day (POD) 9 and continuing through POD 20. Hind limb function returned later (POD 14-34); however, when corrected for distance, the sciatic recovery rate (2.26 mm/d) appeared to match that of the facial nerve (1.5-2.4 mm/d).

CONCLUSIONS

Recovery after facial nerve crush injury follows a predictable time course, and the rate of recovery is consistent with that of sciatic nerve injury. Return of the blink reflex, loss of vibrissial fibrillations, and return of vibrissial sweeping function appear to be internally consistent functional measures of facial recovery. These quantitative measures will be useful for future facial nerve manipulation studies.

Galectin-1 in regenerating motoneurons

The exogenous application of recombinant galectin-1 has recently been shown to promote the rate of peripheral nerve regeneration. Endogenous neuronal galectin-1 expression has recently been demonstrated to increase after axotomy. Here we demonstrate a significant increase in the endogenous neuronal expression of galectin-1 mRNA in facial motoneurons after either a nerve resection or crush injury in mice. This increase in galectin-1 expression was due in part to the loss of target-derived factor(s) as indicated by both the return of galectin-1 expression to control levels following target re-innervation and the increase in galectin-1 expression after blockade of axonal transport by an interneuronal colchicine injection. Furthermore, interneuronal injections of glial-derived neurotrophic factor into the uninjured nerve also increased galectin-1 mRNA expression within facial motoneurons suggesting that positive signals may also be involved in the regulation of galectin-1 expression. Galectin-1 null mutant mice showed an attenuated rate of functional recovery of whisking movement after a facial nerve crush.

Restoration of axotomy-induced PACAP gene induction in SCID mice with CD4+ T-lymphocytes

PACAP is a neuropeptide with putative neuroprotective, regenerative, and immunomodulatory actions. PACAP mRNA is up-regulated in motor neurons following facial nerve axotomy in wild type, but not immunodeficient SCID mice. Because CD4+ lymphocytes appear to be neuroprotective in facial nerve and other injury models, we studied PACAP gene expression in SCID mice preinfused with CD4+ enriched splenocytes. Whereas the mean number of PACAP hybridizing neurons after axotomy was reduced by 75% in uninfused SCID mice, infusion of CD4+ enriched splenocytes restored the number to a value not significantly different than controls. The CD4+ cell-dependent induction of PACAP in motor neurons may thus be a factor in the cascade of events triggered by immune cells that ultimately lead to nerve regeneration.

Induction of neuropeptide gene expression and blockade of retrograde transport in facial motor neurons following local peripheral nerve inflammation in severe combined immunodeficiency and BALB/C mice

Peripheral nerve inflammation is a common clinical problem that accompanies nerve injury and several diseases including Guillain-Barre syndrome and acute and chronic inflammatory demyelinating polyneuropathy. To determine if neuropeptides are induced in motor neurons after inflammation and to study the mechanisms involved, a nerve cuff soaked in complete Freund's adjuvant (CFA) was applied locally to the facial nerve of Balb/C mice. This procedure resulted in an influx of lymphocytes and macrophages to the affected area and a blockade of retrograde axonal transport distal, but not proximal, to the site of application. The same treatment resulted in a strong ipsilateral induction of pituitary adenylyl cyclase activating peptide (PACAP) gene expression in motor neurons in the facial motor nucleus. Because the changes could have occurred due to the loss of target-derived factors or to the production of new factors by immune cells, we studied the effect of the inflammatory stimulus on PACAP mRNA in mice with severe combined immunodeficiency (SCID). As expected, SCID mice showed a severely reduced influx of T-lymphocytes but not macrophages to the peripheral nerve. Moreover, although retrograde transport distal to the inflammation site was blocked similarly in control and SCID mice, the number of motor neurons expressing PACAP mRNA after CFA application was significantly reduced in SCID mice. The data indicate that the induction of PACAP mRNA during nerve inflammation requires the involvement of lymphocytes. However, because the induction of PACAP gene expression was only partially blocked in SCID mice, macrophages, loss of target-derived factors, or other mechanisms may also contribute to the upregulation of PACAP gene expression in motor neurons after nerve inflammation.

Lymphocyte regulation of neuropeptide gene expression after neuronal injury

The neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase-activating peptide (PACAP) are induced strongly in neurons after several types of injury, and exhibit neuroprotective actions in vitro and in vivo. It is thought that changes in expression of neuropeptides and other molecules in injured neurons are mediated by new factors produced in Schwann and immune cells at the injury site, a loss of target-derived factors, or a combination of mediators. To begin to determine the role of the inflammatory mediators, we investigated axotomy-induced changes in VIP and PACAP gene expression in the facial motor nucleus in severe combined immunodeficient (SCID) mice, and in mice with targeted mutations in specific cytokine genes. In normal mice, VIP and PACAP mRNA was induced strongly in facial motor neurons 4 days after axotomy. The increase in PACAP mRNA was blocked selectively in SCID mice, indicating that mechanisms responsible for VIP and PACAP gene induction are not identical. The loss of PACAP gene expression in SCID mice after axotomy was fully reversed by an infusion of normal splenocytes, suggesting that PACAP mRNA induction requires inflammatory mediators. PACAP and VIP mRNA inductions, however, were maintained in mice lacking leukemia inhibitory factor (LIF) and interleukin-6 (IL-6), and in mice lacking both receptors for tumor necrosis factor alpha (TNFalpha). The data suggest that an inflammatory response, most likely involving T lymphocytes, is necessary for the axotomy-induced increase in PACAP but not in VIP. LIF, IL-6, and TNFalpha, however, are not required for this response to injury.

CD4+ T, but not CD8+ or B, lymphocytes mediate facial motoneuron survival after facial nerve transection

The capacity of facial motor neurons (FMN) to survive injury and successfully regenerate is substantially compromised in immunodeficient mice, which lack T and B lymphocytes (). The goal of the present study was to determine which T cell subset (CD4+ and/or CD8+), and whether the B lymphocyte, is involved in FMN survival after nerve injury. All mice were subjected to a right facial nerve axotomy, with the left (uncut) side serving as an internal control. FMN survival, of the right (cut) side, was measured 4 weeks post-operative, and expressed as a percentage of the left (uncut) control side. FMN survival in wild-type mice was 86%+/-1.5. In contrast, FMN survival in CD4 KO mice was 60%+/-2.0. Reconstitution of either CD4 KO mice, or recombinase activating gene-2 knockout (RAG-2 KO) mice (which lack functional T and B cells) with CD4+ T cells alone restored FMN survival to wild-type levels (85%+/-1.2 and 84%+/-2.5, respectively). There was no difference in FMN survival between wild-type, CD8 KO and MmuMT (B cell deficient) mice. Reconstitution of RAG-2 KO mice with CD8+ T cells alone, or B cells alone, failed to restore FMN survival levels (65%+/-1.5 and 63%+/-1.0, respectively). It is concluded that, of the population of FMN that do not survive injury, CD4+ T lymphocytes, but not CD8+ T lymphocytes or B cells, mediate FMN survival after peripheral nerve injury.