Use of laser microdissection in the investigation of facial motoneuron and neuropil molecular phenotypes after peripheral axotomy

Testosterone treatment restores vestibular function by enhancing neuronal survival in an experimental closed-head repetitive mild traumatic brain injury model

Repetitive mild traumatic brain injury (rmTBI) results in a myriad of symptoms, including vestibular impairment. The mechanisms underlying vestibular dysfunction in rmTBI patients remain poorly understood. Concomitantly, acute hypogonadism occurs following TBI and can persist chronically in many patients. Using a repetitive mild closed-head animal model of TBI, the role of testosterone on vestibular function was tested. Male Long Evans Hooded rats were randomly divided into sham or rmTBI groups. Significant vestibular deficits were observed both acutely and chronically in the rmTBI groups. Systemic testosterone was administered after the development of chronic vestibular dysfunction. rmTBI animals given testosterone showed improved vestibular function that was sustained for 175 days post-rmTBI. Significant vestibular neuronal cell loss was, however, observed in the rmTBI animals compared to Sham animals at 175 days post-rmTBI and testosterone treatment significantly improved vestibular neuronal survival. Taken together, these data demonstrate a critical restorative role of testosterone in vestibular function following rmTBI. This study has important clinical implications because it identifies testosterone treatment as a viable therapeutic strategy for the long-term recovery of vestibular function following TBI.

Identification of a resilient mouse facial motoneuron population following target disconnection by injury or disease

BACKGROUND

When nerve transection is performed on adult rodents, a substantial population of neurons survives short-term disconnection from target, and the immune system supports this neuronal survival, however long-term survival remains unknown. Understanding the effects of permanent axotomy on cell body survival is important as target disconnection is the first pathological occurrence in fatal motoneuron diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA).

OBJECTIVE

The goal of this study was to determine if facial motoneurons (FMN) could survive permanent target disconnection up to 26 weeks post-operation (wpo) after facial nerve axotomy (FNA). In addition, the potentially additive effects of immunodeficiency and motoneuron disease on post-axotomy FMN survival were examined.

METHODS

This study included three wild type (WT) mouse strains (C57BL/6J, B6SJL, and FVB/NJ) and three experimental models (RAG-2-/-: immunodeficiency; mSOD1: ALS; Smn-/-/SMN2+/+: SMA). All animals received a unilateral FNA, and FMN survival was quantified at early and extended post-operative timepoints.

RESULTS

In the C57BL/6J WT group, FMN survival significantly decreased at 10 wpo (55±6%), and then remained stable out to 26 wpo (47±6%). In the RAG-2-/- and mSOD1 groups, FMN death occurred much earlier at 4 wpo, and survival plateaued at approximately 50% at 10 wpo. The SMA model and other WT strains also exhibited approximately 50% FMN survival after FNA.

CONCLUSION

These results indicate that immunodeficiency and motoneuron disease accelerate axotomy-induced neuron death, but do not increase total neuron death in the context of permanent target disconnection. This consistent finding of a target disconnection-resilient motoneuron population is prevalent in other peripheral nerve injury models and in neurodegenerative disease models as well. Characterization of the distinct populations of vulnerable and resilient motoneurons may reveal new therapeutic approaches for injury and disease.

Impact of peripheral immune status on central molecular responses to facial nerve axotomy

When facial nerve axotomy (FNA) is performed on immunodeficient recombinase activating gene-2 knockout (RAG-2^-/-) mice, there is greater facial motoneuron (FMN) death relative to wild type (WT) mice. Reconstituting RAG-2^-/- mice with whole splenocytes rescues FMN survival after FNA, and CD4+ T cells specifically drive immune-mediated neuroprotection. Evidence suggests that immunodysregulation may contribute to motoneuron death in amyotrophic lateral sclerosis (ALS). Immunoreconstitution of RAG-2^-/- mice with lymphocytes from the mutant superoxide dismutase (mSOD1) mouse model of ALS revealed that the mSOD1 whole splenocyte environment suppresses mSOD1 CD4+ T cell-mediated neuroprotection after FNA. The objective of the current study was to characterize the effect of CD4+ T cells on the central molecular response to FNA and then identify if mSOD1 whole splenocytes blocked these regulatory pathways. Gene expression profiles of the axotomized facial motor nucleus were assessed from RAG-2^-/- mice immunoreconstituted with either CD4+ T cells or whole splenocytes from WT or mSOD1 donors. The findings indicate that immunodeficient mice have suppressed glial activation after axotomy, and cell transfer of WT CD4+ T cells rescues microenvironment responses. Additionally, mSOD1 whole splenocyte recipients exhibit an increased astrocyte activation response to FNA. In RAG-2^-/- + mSOD1 whole splenocyte mice, an elevation of motoneuron-specific Fas cell death pathways is also observed. Altogether, these findings suggest that mSOD1 whole splenocytes do not suppress mSOD1 CD4+ T cell regulation of the microenvironment, and instead, mSOD1 whole splenocytes may promote motoneuron death by either promoting a neurotoxic astrocyte phenotype or inducing Fas-mediated cell death pathways. This study demonstrates that peripheral immune status significantly affects central responses to nerve injury. Future studies will elucidate the mechanisms by which mSOD1 whole splenocytes promote cell death and if inhibiting this mechanism can preserve motoneuron survival in injury and disease.

Molecular Assessment of Neuroregenerative Response in the Pudendal Nerve: A Useful Tool in Regenerative Urology

AIMS

Assessing pudendal nerve neuroregenerative response provides valuable insight into injuries and regenerative treatments related to urinary incontinence. This project developed and validated a cost-effective, expedient, and adoptable method of assessing pudendal nerve neuroregenerative response.

METHODS

Sprague Dawley rats underwent unilateral pudendal nerve crush prior to spinal cord harvest and laser microdissection for separate collection of the injured and uninjured Onuf's nuclei (pudendal motor neuron cell bodies). Commercially available kits were used to extract and isolate RNA, as well as reverse transcribe and amplify cDNA from cells. Utilizing standard quantitative polymerase chain reaction (Q-PCR), expression of β_II-Tubulin, a cytoskeletal protein indicative of nerve growth and neuroregenerative response, was determined in the injured side relative to the uninjured side 1 week after injury.

RESULTS

Injury upregulated β_II-Tubulin 2.36±0.46 times via Q-PCR, which was not significantly (p=0.508) different from the 2.49±0.38 times increase noted with in-situ hybridization previously. Starting with tissue collection, results are available within 1 day using PCR, while in-situ hybridization requires 4-weeks.

CONCLUSIONS

An easily adoptable PCR-based method of assessing the neuroregenerative response of the pudendal nerve successfully reproduced results obtained with a previous radioisotope-based in-situ hybridization technique.

CD4 + T Cells and Neuroprotection: Relevance to Motoneuron Injury and Disease

We have established a physiologically relevant mechanism of CD4+ T cell-mediated neuroprotection involving axotomized wildtype (WT) mouse facial motoneurons (FMN) with significance in the treatment of amyotrophic lateral sclerosis (ALS), a fatal MN disease. Use of the transgenic mouse model of ALS involving expression of human mutant superoxide dismutase genes (SOD1(G93A); abbreviated here as mSOD1) has accelerated basic ALS research. Superimposition of facial nerve axotomy (FNA) on the mSOD1 mouse during pre-symptomatic stages indicates that they behave like immunodeficient mice in terms of increased FMN loss and decreased functional recovery, through a mechanism that, paradoxically, is not inherent within the MN itself, but, instead, involves a defect in peripheral immune: CNS glial cell interactions. Our goal is to utilize our WT mouse model of immune-mediated neuroprotection after FNA as a template to elucidate how a malfunctioning peripheral immune system contributes to motoneuron cell loss in the mSOD1 mouse. This review will discuss potential immune defects in ALS, as well as provide an up-to-date understanding of how the CD4+ effector T cells provide neuroprotection to motoneurons through regulation of the central microglial and astrocytic response to injury. We will discuss an IL-10 cascade within the facial nucleus that requires a functional CD4+ T cell trigger for activation. The review will discuss the role of T cells in ALS, and our recent reconstitution experiments utilizing our model of T cell-mediated neuroprotection in WT vs mSOD1 mice after FNA. Identification of defects in neural:immune interactions could provide targets for therapeutic intervention in ALS.

Identification of B6SJL mSOD1(G93A) mouse subgroups with different disease progression rates

Disease progression rates among patients with amyotrophic lateral sclerosis (ALS) vary greatly. Although the majority of affected individuals survive 3-5 years following diagnosis, some subgroups experience a more rapidly progressing form, surviving less than 1 year, and other subgroups experience slowly progressing forms, surviving nearly 50 years. Genetic heterogeneity and environmental factors pose significant barriers in investigating patient progression rates. Similar to the case for humans, variation in survival within the mSOD1 mouse has been well documented, but different progression rates have not been investigated. The present study identifies two subgroups of B6SJL mSOD1(G93A) mice with different disease progression rates, a fast progression group (FPG) and slow progression group, as evidenced by differences in the rate of motor function decline. In addition, increased disease-associated gene expression within the FPG facial motor nucleus confirmed the presence of a more severe phenotype. We hypothesize that a more severe disease phenotype could be the result of 1) an earlier onset of axonal disconnection with a consistent degeneration rate or 2) a more severe or accelerated degenerative process. We performed a facial nerve transection axotomy in both mSOD1 subgroups prior to disease onset as a method to standardize the axonal disconnection. Instead of leading to comparable gene expression in both subgroups, this standardization did not eliminate the severe phenotype in the FPG facial nucleus, suggesting that the FPG phenotype is the result of a more severe or accelerated degenerative process. We theorize that these mSOD1 subgroups are representative of the rapid and slow disease phenotypes often experienced in ALS.

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.

Axotomy-induced target disconnection promotes an additional death mechanism involved in motoneuron degeneration in amyotrophic lateral sclerosis transgenic mice

The target disconnection theory of amyotrophic lateral sclerosis (ALS) pathogenesis suggests that disease onset is initiated by a peripheral pathological event resulting in neuromuscular junction loss and motoneuron (MN) degeneration. Presymptomatic mSOD1(G93A) mouse facial MN (FMN) are more susceptible to axotomy-induced cell death than wild-type (WT) FMN, which suggests additional CNS pathology. We have previously determined that the mSOD1 molecular response to facial nerve axotomy is phenotypically regenerative and indistinguishable from WT, whereas the surrounding microenvironment shows significant dysregulation in the mSOD1 facial nucleus. To elucidate the mechanisms underlying the enhanced mSOD1 FMN loss after axotomy, we superimposed the facial nerve axotomy model on presymptomatic mSOD1 mice and investigated gene expression for death receptor pathways after target disconnection by axotomy vs. disease progression. We determined that the TNFR1 death receptor pathway is involved in axotomy-induced FMN death in WT and is partially responsible for the mSOD1 FMN death. In contrast, an inherent mSOD1 CNS pathology resulted in a suppressed glial reaction and an upregulation in the Fas death pathway after target disconnection. We propose that the dysregulated mSOD1 glia fail to provide support the injured MN, leading to Fas-induced FMN death. Finally, we demonstrate that, during disease progression, the mSOD1 facial nucleus displays target disconnection-induced gene expression changes that mirror those induced by axotomy. This validates the use of axotomy as an investigative tool in understanding the role of peripheral target disconnection in the pathogenesis of ALS.

Up-regulation of matrix metallopeptidase 12 in motor neurons undergoing synaptic stripping

Axotomy of the rodent facial nerve represents a well-established model of synaptic plasticity. Post-traumatic "synaptic stripping" was originally discovered in this system. We report upregulation of matrix metalloproteinase MMP12 in regenerating motor neurons of the mouse and rat facial nucleus. Matrix metalloproteinases (matrix metallopeptidases, MMPs) are zinc-binding proteases capable of degrading components of the extracellular matrix and of regulating extracellular signaling networks including within synapses. MMP12 protein expression in facial motor neurons was enhanced following axotomy and peaked at day 3 after the operation. The peak of neuronal MMP12 expression preceded the peak of experimentally induced synaptic plasticity. At the same time, MMP12 redistributed intracellularly and became predominantly localized beneath the neuronal somatic cytoplasmic membrane. Both findings point to a role of MMP12 in the neuronal initiation of the synaptic stripping process. MMP12 is the first candidate molecule for such a trigger function and has potential as a therapeutic target. Moreover, since statins have been shown to increase the expression of MMP12, interference with synaptic stability may represent one mechanism by which these widely used drugs exert their side effects on higher CNS functions.

Delayed functional recovery in presymptomatic mSOD1G93A mice following facial nerve crush axotomy

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease involving progressive loss of motoneurons (MN). Axonal pathology and presynaptic deaf-ferentation precede MN degeneration during disease progression in patients and the ALS mouse model (mSOD1). Previously, we determined that a functional adaptive immune response is required for complete functional recovery following a facial nerve crush axotomy in wild-type (WT) mice. In this study, we investigated the effects of facial nerve crush axotomy on functional recovery and facial MN survival in presymptomatic mSOD1 mice, relative to WT mice. The results indicate that functional recovery and facial MN survival levels are significantly reduced in presymptomatic mSOD1, relative to WT, and similar to what has previously been observed in immunodeficient mice. It is concluded that a potential immune system defect exists in the mSOD1 mouse that negatively impacts neuronal survival and regeneration following target disconnection associated with peripheral nerve axotomy.

Differential gene expression in the axotomized facial motor nucleus of presymptomatic SOD1 mice

Previously, we compared molecular profiles of one population of wild-type (WT) mouse facial motoneurons (FMNs) surviving with FMNs undergoing significant cell death after axotomy. Regardless of their ultimate fate, injured FMNs respond with a vigorous pro-survival/regenerative molecular response. In contrast, the neuropil surrounding the two different injured FMN populations contained distinct molecular differences that support a causative role for glial and/or immune-derived molecules in directing contrasting responses of the same cell types to the same injury. In the current investigation, we utilized the facial nerve axotomy model and a presymptomatic amyotrophic lateral sclerosis (ALS) mouse (SOD1) model to experimentally mimic the axonal die-back process observed in ALS pathogenesis without the confounding variable of disease onset. Presymptomatic SOD1 mice had a significant decrease in FMN survival compared with WT, which suggests an increased susceptibility to axotomy. Laser microdissection was used to accurately collect uninjured and axotomized facial motor nuclei of WT and presymptomatic SOD1 mice for mRNA expression pattern analyses of pro-survival/pro-regeneration genes, neuropil-specific genes, and genes involved in or responsive to the interaction of FMNs and non-neuronal cells. Axotomized presymptomatic SOD1 FMNs displayed a dynamic pro-survival/regenerative response to axotomy, similar to WT, despite increased cell death. However, significant differences were revealed when the axotomy-induced gene expression response of presymptomatic SOD1 neuropil was compared with WT. We propose that the increased susceptibility of presymptomatic SOD1 FMNs to axotomy-induced cell death and, by extrapolation, disease progression, is not intrinsic to the motoneuron, but rather involves a dysregulated response by non-neuronal cells in the surrounding neuropil.

Müllerian inhibiting substance is anterogradely transported and does not attenuate avulsion-induced death of hypoglossal motor neurons

Müllerian Inhibiting Substance (MIS, Anti-Müllerian hormone) is a gonadal hormone that contributes to the subtle sex-biases in the nervous system. Mature neurons of both sexes also produce MIS, suggesting that MIS may be a paracrine regulator of adult neural networks. We report here that murine hypoglossal motor neurons produce MIS and its receptors, MISRII and bone morphogenetic protein receptor 1A (BMPR1A, ALK3), but differentially transport them, with only MIS being detectable in axons. The production of MIS and its receptors were rapidly down regulated after axonal damage, which is a characteristic of genes involved in mature neuronal function. MIS is a survival factor for embryonic spinal motor neurons, but the rate of cell loss after hypoglossal nerve avulsion was normal in Mis(-/-) mice and was not attenuated by intraventricular administration of MIS. These observations suggest that MIS may be involved in anterograde rather than autocrine or retrograde regulation of neurons.